CA2216793A1 - Optical multiplexing device and method - Google Patents

Abstract

An optical multiplexing device spatially disburses collimated light from a fiber optic waveguide into individual wavelength bands, or multiplexes such individual wavelength bands to a common fiber optic waveguide or other destination. The optical multiplexing device has application for dense channel wavelength division multiplexing (WDM) systems for fiber optic telecommunications, as well as compact optical instrument design. Multiple wavelength light traveling in a fiber optic waveguide is separated into multiple narrow spectral bands directed to individual fiber optic carriers or detectors. An optical block has an optical port for passing the afor esaid multiple wavelength collimated light, and multiple ports arrayed in spaced relation to each other along a multiport surface of the optical block. A continuous, variable thickness, multicavity interference filter (22) extends on the multiport surface (20) of the optical bl ock over the aforesaid multiple ports. At each of the multiple ports the continuous interference filter transmits a different sub-rang e of the multiple wavelength light passed by the optical port, and reflects other wavelengths. Multicolor light passed to the opt ical block from the optical port is directed to a first one of the multiple ports on an opposite surface of the optical block. The wavelengt h sub-range which is "in-band" of such first one of the multiple ports is transmitted through that port by the local portion of the cont inuous, variable thickness interference filter (22) there, and all other wavelengths are reflected. The light not transmitted through the first port (16) is reflected to strike a second port, at which a second (different) wavelength band is transmitted and all other light again re flected. The reflected optical signals thus cascades in a "multiple-bounce" sequence down the optical block (10) of the multiplexing devic e, sequentially removing each channel of the multiplexed signal. In reverse operation, individual channels are combined in the optical bl ock and transmitted through the optical port.

Description

W O 97/00458 PCT~US~'037~7 OPTICAL MULTIPLEXING DEV~CE AND METHOD

Related Application This application is a con~ on-in-part of copending United States patent application of Michael A. Scobey, Serial No 08/490,829, filed on June 15, 1995 and entitled Optical Mulli~ g Device, and a co..l;..~ l;Qn-in-part of copending United States patent al~pli~ .AI;-~n of Michael A. Scobey, Serial No. 08/300,741 filed on SepLel,lbel 2, 1994 and entitled Low Pressure Reactive Magnetron Sputtering Apparatus And Method.

INTRODUCTION
The present .n~llliol1 is directed to an optical multiplexing device which spatially di~w~es collim~tecl multi-wavelength light from a fiber optic waveguide into individual wavelength bands, each of which can be dht;-;Led to an individual fiber optic waveguide output line, light detector, etc., or multiplexes such individual wavelength bands to a con~non fiber optic waveguide or other ~ n In certain pl~re~led embo~im~nt~, theimproved multiplexing devices of the present invention are particularly well suited for dense channel wavelength division multiplexing (DWDM) ~y~Lellls for fiber optic telecomm-lnic~tions systems.

BACKGROUND
While fiber optic cable is finding widespread use for data tr~n~mi~ion and othertelecnmml-nication applications, the cost of new in~t~lled fiber optic cable pl~sellL~ a barrier to increased carrying capacity. Wavelength division multiplexing (WDM) would :

W O 97ioo4s8 PCT~US96/09727 allow di~ wav~,lF..-gll..e to be carried over a cc,ll~ - fiber optic wave&uide. Plesell~ly pl~;rell~d wavelength bands for fiber optic trAn~mi~ion media include those centered at 1.3~1 and 1.55~1. The latter is e~speri~lly ple~ll~d because of its minimAl absorption and the collllllelcial availability of erbium doped fiber amplifiers. It has a useful band width S of ap,olc,~illlale1y 10 to 40 nm, depP~ on application. Wavelength divisionmultiplexing can separate this band width into multiple çhAnn~lc Ideally, the 1.55~1 wa~ ,n~ band, for example, would be divided into multiple discreet ~hAnnPl~, such as 8, 16 or even as many as 32 ~hAnn~l~, through a techniq~e referred to as dense channel wavelength division multiplexing (DWDM), as a low cost method of subst~ntiAlly hlcl~asing long-haul tP1eco.. ;cation capacity over ~x;~l;.. g fiber optic trAn~mi~.~ion lines. Wavelength division mulLip'~ hlg may be used to supply video-on-d~mAnd and other PYi~ting or planned mllltime~i~ interactive services. Techniques and devices are required, however, for .,...l~ illg the di~el~llL discreet carrier wavel~ngth.s That is, the individual optic signals must be colllbil~ed onto a common fiber optic waveguide and then later sepa, aLed again into the individual signals or ~.hAnnPl~ at the opposite end of the fiber optic cable. Thus, the ability to ~ iv~l~ combine and then separate individual wa~ . .gll .~; (or w~ ,L,ngLll bands) from a broad spectral source is of growing importance to the fiber optic tclec~ n.~ field and other fields employing optical instruments.
Optical mulLi~ el~ are kno-wn for use in spectroscopic analysis e~uil,.,le"L andfor the col"l,i"aLion or separation of optical signals in wavelength division multiplexed fiber optic t~leco ~Ation~ systems. Known devices for this purpose have employed, for example, diffraction gldLillg~, prisms and various types of fixed or tunable filters.
Gratings and prisms typically require complicated and bulky AlignmPnt systems and have been found to provide poor Pffic;~ncy and poor stability under ~hAI1P;IIg ambient W O 97/00458 PCTAUS9G1~3/ 7 conditions. Fixed wavelength filters, such as inte,rt:,ence co~ting~, can be made ~ubs~ lly more stable, but llal~slllil only a single wavelength or wavelength band. In this regard, highly improved illLelr~l~llce coalil~s of metal oxide materials, such as niobia and silica, can be produced by COllllllCl ~,;ally known plasma deposition te~ ues, such as ion ~ ted electron beam evaporation, ion beam sl.ulle~illg~ reactive m~n~trone.g., as disclosed in U.S. patent No. 4,851,095 to Scobey et al. Such coating methods can produce intelrt;l~ ce cavity filters formed of stacked dielectric optical coatings which are advantageously dense and stable, with low film scatter and low al~sol~lio4 as well as low sensitivity to telnp~ re cl~An~es and ambient humidity. The lllec,l c~Li~al spectral p~. ru., "~ e of a stable, three-cavity filter (tilted 12 ~) prod~ced using any of such advanced, deposition methods is shown in Fig. 1 ofthe appended d~vings.
The spectral profile is seen to be suitable to meet stringent application specifications.
To overcome the aforesaid d~ficiency of such il,l~:,relence filters, that is, that they ll~lllil only a single wa~_lenglll or range of wavelengths, it has been su~gested to gang or join together multiple filter units to a co.lllllon parallelogram prism or other common substrate. Optical filters are joined together, for c,~alllple, in the multiplexing device of U.K. patent application GB 2,014,752A to sepal~l~ light of difrelc~llL wav~
ed down a common optical waveguide. At least two ~ -n filters, each of which Ll~llliLS light of a di~l~llL predeterrni~-~i wavelength and reflects light of other wavelengths, are ~tt~c.h~d ~dj~cent each other to a Ll~lspal~lll dielectric substrate. The optical filters are arranged so that an optical beam is partially Ll~ e~1 and partially reflected by each optical filter in tum, producing a zigzag light path. Light of a particular w~,le.~lh is subtracted or added at each filter (depending upon whether the element is being used as a m~lLi~l or de~, ...Il;pl_,.el)~ Similarly, in the device of European patent W O 97/00458 PCT~US96/09727 applir~l;Qn No. 85102054.5 by Oki Eleetric Industry Co., Ltd., a so-called hybrid optical wavelength division multiplexer-deml~ltiplexer is suggested, wheleill mllltiple separate inlklrel~lce filters of dirrt;lel,- tln~ ities are applied to the side surfaces of a glass block. A somewhat related approach is suggf~,sted in U.S. patent 5,005,935 to Klmilrzlni et al, whelein a wavelength division mllltipl~ ~ ~ optical lln~ ;on system for use in bi-directional optical fiber comml~n:~~tione b1lweell a central telephone ~ hA~ e and a subsc,iber employs multiple separate filter e lemf~nte applied to various surfaces of a parallelogram prism. Alternative approaches for tapping selective wavf~lf~ngthe from a main trunk line carrying optical signals on a plurality of wavelength bands is sll~gf~ste-l, for ~ ~ p'e, in U.S. patent 4,768,849 to Hicks, Jr. In that patent, mllltiple filter taps, each employing ~lielf~tnc mirrors and lenses for directing optical signals, are shown in an al~n.~gk.. .~ for removing a series of wavelength bands or Ç~ F1C from a main trunk line.
Applying ml '~i, le st~ le filter f ~ - ~I X to the surface of a prism or other optical substrate involves ~;~.;l~c~.. l disadvantages in assembly cost and complexity. In addition, a si~ l pr~ associated with wa~ ,.~LIl division multiplexing devices and the like employing ml-ltiple discreet intelrel ei1ce filter elements, arises from unct;, l~illly as to the precise wavelength of a filter element as it is m~n--f~ct--red. That is, in the m~n--f~c.t--re of multiplexing devices, wherein b~n-lp~ee filter f~lf mf~nte are produced separately, a device employing eight individual b~ntlp~ee fikers, for example, typically will require considerably more than eight coating lots to produce the necf ss~ry eight suitable filter elements. R~ntlp~es filters (particularly in the infrared range) are c Al,~-.,ely thick and require complicated and eA~ensive vacuum deposition eq--ipmf~nt and teçhni~ es Accol.lil,~ly, each coating lot can be expensive and difflcult to produce. For this reason, W O 97/00458 PCT~US~G/'~3/~7 devices ~ 10yill~, for example, eight separate h~l~lr~lence filter 1~ ; to produce an eight channel WDM device, have been relatively costly and have not enjoyed full C(~lllllle;l~;;al acc~ ce.
Another pr~'~~n ~c~i~ted with optical multiplexing devices employing multiple S individual b~dl)~cs filter f~l-.. "~."l~i involves the need to mount the elements in nearly perfect parallelism on an optical ~ubsllale. The filter ~l~m~-nts are quite small, typically being on the order of l to 5 mm in ~ m~t~r, and are, accoldillgly, .liffi.,l.lt to handle with precision. Illlplopel m~ ntin~ ofthe filter elements can significantly decrease the optical accuracy and thermal stability of the device. A related problem is the n~cçccity of an a&esive m~li~lm bt;lw~n the filter el~m~nt and the surface of the optical :i~s~ . The optical signal path travels through the adhesive, with consequent system degr~d~tion In optical mulli~,l ~ g devices inf~nrled for the telecc~.. ;c~tions industry, preferably there is as little as possible epoxy adhesive in the optical signal path.
It is an object of the present invention to provide improved optical mllltiplçxin~
devices which reduce or wholly overcome some or all of the afol~said ~liffiallti~s inherent in prior known devices. Particular objects and advantages of the invention will be a~p~ to those skilled in the art, that is, those who are knowledgeable and experienced in this field ofte~hn~lo y, in view ofthe following disclosure of the invention and det~iled description of certain plt;r~lled embo-lim~ntc SUMMARY OF TEIE INVENTION
In accordance with a first aspect, an optical multiplexing device comprises an optical block which may be either a solid optical substrate, such as glass or fused silica or the like, or an enclosed chamber which is hollow, m~ning either ev~c. -~ted or filled with WO 97/00458 PCTnJS9G/~5/~7 air or other optically ~ alelll ...e~ The optical block has an optical port for passing multiple wavelength collimAted light. Depending upon the application of the optical mlllt. I g device, such mllltiple wavelength collim~ted light may be passed through the optical port into the optical block to be dlomllltiplexed, or from the optical S block as a mullipl ~d signal to a fiber optic l~ .. ;ex;c-n line or other destinAtion ' ~ ports are arrayed in spaced relation to each other along a multiport surface of the optical block. As illu~ led below in co~ F~I;Qn with certain l,lerwled embo-lim.o.nts, the optical block may have more than one such multiport surface. Each of these mllltiple ports is ll~ls~enl to the optical signal of one ch~nnel Thus, each 11a~IS1niI~ a wavelength sub-range of the multiple wavclen~ collimAted light passed by the optical port. In an application of the optical multiplexing device in a multi-channel to'o~ ~ ~n system, each ofthe m~ o ports In lt~y would pass a single discreet channel and, in collll~hlalion, the ..hA~ form the aforesaid multiple wavelength collim~tecl light ~ e(l by the optical port. A continuous, variable thic~l~ness illle;lrelt;nce filter, plt;r~l~bly a multi-cavity interference filter, is carried on the multiport surface of the optical block to provide the aforesaid multiple ports. Because this c ntimlous illlelrt;l~llce filter e~en~iing over the multiport surface has a dirr~;lel~ optical thickness at each of the multiple ports, the wavelength (or wavelength range) passed by the Blter at each such port will differ from that passed at the other ports. Thus, a single film"~ r~ bly deposited directly onto the surface of the optical block, separately passes optical signals for each of a number of cllA~ F.l.~ at s~ e locations, while reflecting other wav~l~.n~h~ As noted above, the optical block may be either solid or a hollow chamber. In the case of a solid optical block, the multiport surface ca.lyhlg the c~ ntimloll~, variable th~ n~ intelrt;i c;nce filter would typically be an exterior surface of the block. As ~liecllesed further below, the individual ports of the multiport surface may be bA~ )Aee filters, preferably narrow b~n-lp~es filters l~ ~t;n~ to a wavelength sub-range ~p~ ed from the sub-range ofthe next ~Ij~ port(s) by as little as 2 nm or even less for DWD~ AltelllaLivt;ly, some or all of the mllltiple ports could be dichroic, i.e., S a long wavepass or short wavepass edge filter, preferably with a very sharp trAneition point. The transition point of each port would be set at a slightly (e.g., 2 nm) longer (or shorter~ boundary wav~l~n~h In a dÇmllltirl g operation, each port, in turn, would pass or Ll~sllliL only optic signals in the h~ l range beyond the boundary w~velen~;LII ofthe previous port, since all light at shorter (or longer) wavol~n~he would already have been removed. Light beyond the boundary wavelength of the new port would be reflected, in accol dallce with the above described principles of operation.
The optical mlll~ B device further co.-.l..;~e me~ns for c~ec~ling light within the optical block along a multi-point travel path from one to another of the multiple ports.
In a demultiplexing operation, the optical signals would enter the optical block at the ~ole~d optical port and travel to the multiple ports (acting in this case as output ports) along the ~olesaid multi-point travel path. The signal for each individual channel is L~ ed out of the optical block at its corresponding port; other wavelength are reflected, or bounced, back to cA.ecAde further along the optical travel path within the optical block. It will be understood that at the last output port(s) there may be no rçm~in-lçr light to be reflected. It will also be understood from this disclosure, that the optical multiplexing device can operate in the reverse or both directions. The çAeç~-ling means plere ~bly comprises a reflective film carried on a second surface of the optical block, either as a contimlolle coating s~A.~..;. .g the multi-point travel path of the ~ec~,~ing light signals, or as multiple discreet reflector elements. The optical block would most W O 97/00458 PCT~US~GI'~7~7 typically be rectiline~r, having the reflective film on or at a second surface of the optical block opposite and parallel to the multiport surface carrying the aforesaid continuous intelrt;rellce filter. This second film can be a bro~db~n~ high reflector, that is, a film coating which is highly reflective of all wavPl~n~h~ of the çh~nnPI~ which colllbille to S form the multiple wavelength collim~ted light, or can act as a second illl~lÇ~l~nce filter cllL at spaced locations (i.e., at some or each ofthe bounce points) to the optical signal of one or more ofthe 1~l1AI~ IC In either case, the illl~lrel~ilce filter and reflective film on spaced s--rf~ces ofthe optical block operate to cascade optical signals through the optical block in a multiple-bounce seql-~n~e starting at (or fini~hing at) the optical port through which the multiple wavelellglll c-~llim~tecl light passes. This mllltiple-bou~ice ling will be further described below in connection with certain pl~rt;lled embot1im~nt~

BRIEF DESCRIPTION OF TEE DRAWINGS
Certain ~l~re;lled embodiments ofthe invention are ~icc~ below with reference to the accolllp~lyillg drawings in which:
Fig. 1 is a graph showing the theoretical pelr~,lll,allce of a high quality multi-cavity, di~lectric., optical inte,r~ ce filter.
Fig. 2 is a s~ ;c illustration of a first plert;lle~d embodiment of an optical m~ Yinp device, ~rerific~lly, a dense channel wavelength division multiplexing device for an eight channel fiber optic telecomml~nic~tions system or like application;Fig. 3 is a sl~hPm~tic illustration of an alternative prc;r~ ;d embodiment of anoptical If, 1 ~ ~ device in ~ldance with the invention, specifically, a dense channel W O 97/00458 PCT~US~G/'~3/17 wavelength division multiplexing device for an eight channel fiber optic tcleco................................................ lnic~tions system or like application;
Fig. 4 is a s~l-e~ ;c illustration of another allel-~ali~re pler~; -ed embodiment of an optical m~ device in acco.dallce with the invention, specifically, a dense ch~nnPl wavelength division mllltip'-Ying device for an eight channel fiber optic tclecc................................................ mications system or like applic~tic-n;
Figs. 5, 6, and 7 are sçh~ ;c illustrations, in cross-section, ofthe c~ntim~oll~variable i' ' nP~, three cavity illl~;lre:lt;llce filter of the optical multiplexing device of Fig.
2;
Fig. 8 is a cross-section~l s~hPm~tic illustration of appal~ s in acco.dance with a pr~rell ed embodiment of this invention;
Fig. 9 is a s~he...,.l;c lepleselll~lion in cross-section of a m~gnPtron assembly inclu~ling a source or target and an inert gas shroud in accordance with a plerellcid embodiment of this invention;
Fig. 10 is a cross-sectional sçh~m~tic illustration of appal~LIls in accoldallce with an alternative plerell~d embodiment of this invention, having multiple m~gnetron elill~3, assemblies;
Fig. 11 is a graph ~ willg the r~l~ti-)rl~hir between ch~lll)el pressure and chamber pumping speed ~sllming the m~gn~tron pressure of 0.7 microns and a m~gnP,tron assembly contlllct~n~e (CM) of 3000 l/sec.;
Fig. 12 is a graph showillg the rPl~tiQn~hir between chamber pl~ule and chamber pllmring speed ~sllm; ng a m~nPtron pressure of 0.4 microns and a m~gnetron assembly con~iurt~n~.e (CM) of 3000 l/sec; and W O 97/00458 PCT~US96/09727 Figs. 13 and 14 are sch~ Al;c elevation and plan views, respectively, illu~ i,lgapp~lus in accordance with an ~ltprn~tive prcrcll ed embodiment of this invention.
It should be understood that the optical m~ g devices and hllclrel~llce filter illu~ ed in the dl~wi~ are not nece~es~rily to scale, either in their various dimPn.eic-ne or angular relationships. It will be well within the ability of those skilled in the art to select suitable ~lim~nei~me and angular r~l~tionehiI s for such devices in view of the r.,l t;gOillg disclosure and the following detailed description of plcrellcd embo~limPnte DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
The optical multiplexing device, as disclosed above, has numerous applications inçh--ling, for example, in fiber optic telecc,mlllu~ialion systems. Optical mulli~lc~g devices ofthis design are particularly useful, for ~mple, in test equipment and the like, as well as laboratory i.~L.. ~ ;on. For ~,u.~oses of illustration, the pl t;rt;.. ed embodiments dPS~;~ il ~ below in detail are dense channel wavelength division mukiplexing devices which can solve or reduce the above described problems of individually mounting multiple filter ~ to an optical substrate for each individual signal f h~nn~l, the problems of cost and complexity involved in multiple coating lots for plcp~hlg such individual filter elements, and the associated problems of filter wavelength unce~ ly.
As 11iealeeesl below in connection with the appended dl~win~,s, a graded wavelength all-dielectric narrow bandpass filter is placed on at least one side of an optical block, prerc.~ly a polished parallel plate of specific thickness. The filter, forming a contin~lo~s coating over at least a portion of the surface of the optical block, preferably is a multi-cavity, most p~rtl~ly three cavity, film stack coating modeled a~er a Fabry-Perot etalon, and may be referred to simply as a cavity filter. Two dielectric thin film stacks which by W O 9i/00458 PCTAUS96/09727 th~nn! f~Jves form a reflector for the optical wavçlength~ in question, are sepal~Led by a thicker cavity layer. This structure is then repeated one or more times to produce a filter with ~l~hA~ ed blocking and improved in-band trAn~mi~ion fl~tn~eS The net effect is to produce a l~lo~dl~d Ll,-~ e filter where in-band light is Ll~ Ied and out-of-band S light is r~-flçcted As noted above, dichroic filters may also be used. This iLlpl~Jved filter pelr~,ll.lal1ce provides co-ll-llelcially acce~l~ble dense channel wavelength division mllltipl~Ying for fiber optic telecomm- ons applications of the optical mllltip1~Ying device. It provides low cross-talk beLween çh~nneJc and permits a suitably high number of within a given band width. An excessive number of cavities will adversely affectthe ll;.n~ ity of even in-band wav~ n~h!. and may increase production costs for the optical multiplexing device beyond col-----e- ~,;ally accept~ble levels.
The contin-loll~, variable i' ~ multi-cavity hlLelrelel1ce filter can be produced with dense, stable metal oxide film stackes using the deposition teçhn;q~les mentioned above. Such filters have been demonstrated to have eXc~ nt thermal stability, e.g., 0.004 nm/~C at 1550 nm, and ultra-narrow band widths, separated by as little as 2 nm, or even as little as 1 nm. Suitable variable ' ~ ' - filters have been used in other applications such as, for c rl-~ in U.S. patent 4,957,371 to Pellicori et al. Stable Ultra-Narrowband Filters also are shown in SPIE Proceeding~ 7/1994. In particular, high-quality interference filters comprising stacked layers of metal oxide materials, such as niobia and silica, can be produced by col--l--elc;ally known plasma deposition techniques, such as ion a~ tecl electron beam evaporation, ion beam sputtering, and reactive m~gn~tron sputtering, for , as ~ ,sed in U.S. patent No. 4,851,095 to Scobey et al., the disclosure of which is hereby incorporated by lert;l~nce. Such coating methods can produce intelrt;.ence cavity filters formed of stacked dielectric optical coatings which are advantageously dense and W O 97/00458 PCT~US96/09727 stable, with low film scatter and low abso,~lion, as well as low sensitivity to ten~t;l~Lu~e ch~ngee and ambient humidity. The spectral profile of such co~ting.e is suitable to meet ~I. ;ng~ p~li., lion specifiç~tione Multi-cavity narrow b~n~p~e~e filters can be produced using such techniques, which are L,~l~al~; lL to a wavelength range separated from an ~ c~-l wavelength range by as little as two nano~wle~ or less. One suitable deposition technique is low plt:S~ulc: m~gnetron sputtering in which the vacuum ~I,~"ber of a .ull ~uLlt;lillg system, which can be otherwise conventional, is e-luipped with high speed vacuum ~ ;~g A gas m~nif~l1 around the . . ~ . Oil and target material co~- ~. .e~e the inert wwking gas, IYl ~ / argon, in the vicinity of the m~gnetron As the gas diffuses and e-Yr~n-le from the area of the m~nPtron, the l-mlell~lly high pumping speed vacuum removes the c,~p~ P gas from the . h*. . .l.~, at a high speed. The inert gas pressure in the Clla~ ,er~ in the range of S x 10-5 Torr to 1. 5 x 10 ' Torr, is then a function of the p~ g speed of the vacuum pump and the confin-om~nt ~offi~ oncy of the m~gnetron baffle.
Reactive gas enters the chamber through an ion gun which ionizes the gas and directs it toward the substrate. This has the effect of reducing the amount of gas required to provide the film with proper stoichiometry as well as re~hlt~in~ the reactive gas at the magnto.tron Throw diet~nce of 16 inch and longer can be achieved. Such deposition te~hniqlles are ~1ie~le~d in co-pending U.S. patent applit ~tion~ Serial No. 07/791,773 filed November 13, 1991 and Serial No. 07/300,741 filed September 21, 1994, the disclosures of which are hereby incorporated by reference.
As noted above, the filter preferably comprises a multi-cavity coating in which two dielectric thin film stacks which by themselves form a reflector for the u"wa"Led wavelengths are separ~Led by a cavity layer. This structure is then repeated one or more times to produce the aforesaid multi-cavity filters with enh~nced blocking and improved in--W O 97/00458 PCTrUS9~1~57 7 band t~ ,;on fl~tness The net effect is to produce a narrowband tr~n~mi~sive filter where in-band light is l,~ ed and out-of-band light is reflected. In plt;r~ ;d three-cavity embo~limrnte produced by the deposition terhni~ es mentioned above, with dense, stable metal oxide film stacks, eYr~ ont thermal ~L~bilily has been achieved, for ~Y~mple~
0.004 nm per degree centigrade or better at 1550 ~ and ultra-narrow band widths se~ ed by as little as 2 nm or even as little as 1 nm. Preferably, the h~le~rwellce filter is continuously linearly variable in thickn.o~L, Optionally, however, the ~ ..Ps~ of the continuous filter may be variable non-continuously for example, having a subsl~llially unir~ thir~n~ss over each of the multiple ports of the optical block ~L~sor.i~ted with the separate çh~nnrk of the fiber optic system.
The inte,rt:lence filter typically comprises two materia!s, one of a high refractive index such as niobium pentoxide, tit~ni~m dioxide, t~nt~ m p~nto,Yide and/or mixtures thereofe.g., niobia and titania, etc. At 1.5 microns wav~ ngth~ the refractive index value for these materials is roughly 2.1 to 2.3. The low refractive index material is typically silica, having a ~t;fi~cli~e index of about 1.43. An interference filter has an "optical i ' ' - " which is the numerical product of its physical thirl~ness times its refractive index.
The optical i' -L-nr~.L ofthe continuous, variable thirL-ne~L:, multi-cavity interference filters used in the optical multiplexing devices disclosed here varies, of course, with the physical thickness of the filter at various points along the surface of the optical block. At each of the multiple ports of the optical block associated with an individual signal r.h~nnrl, the optical ' -' ~s~ ofthe interference filter is tuned to llansnliL the desired wavelength sub-range(s). It will be appalt;ll~ to those skilled in the art in view of this disclosure that the i' ' - and composition of the layers of the continuous filter can be selected to suit the spectral profile l~uil~d for any given apl)licalion of the optical mulliplc~ilg device. It will W O 97/00458 PCT~US~G/'~7~7 be apparent also, that the continuous filter can be continuously variable in its thi~L neee lineally or otherwise, or ~liec~ntin-lol~ely ~,aliablc in its thickness. In certain prerel,ed embc-' s, the thickness ofthe filter at each port is subs~ l;qlly CQn~ incleasillg (or decreasing) in thiel~n~ee only between one port and the next.
The contim~ol~e, variable i' ' -, multi-cavity intelre;lellce filters used in the optical ' il ' ~ devices t1ie~ lse~ here have many advantages over prior known ~hf-.rin~
devices. They can be produced to coat the entire operative portion of a surface of the optical block in a single coating step, tunable at each "bounce point" ~e.g., by applopliale pl~-~.ment of-q-eeo~iqted lens apparatus, ct~llimeters, etc.) to exact wav~l~n~he of ' 0.1 nm.
When m~qnllfq~tllred with durable materials to form dense layers of near unity p,q,~L-ing density, they are stable over time and with respect to humidity. A large number of optical blocks can be coated eimllltqneously with the hllelrelellce filters in a single coating run, thereby s~ lly reducing the cost of the optical multiplexing device. They are readily mqnllfqctllred cOI~ isill3 mllltiple cavities, which are coherently coupled using a quarter wave Ll. cl n~ee layer in accordance with known techniques. The effect of using mllltiple cavities, as described above, is to produce a filter with an increased slope of the spectral skirts, along with a wider L. ~ - zone. As described above, both of these effects offer advantages over other types of filtering devices, such as etalons and diffraction gratings.
Since the filters can be formed by deposition directly onto a surface of the optical block, no epoxy need be used in the mounting of the filter so as to be in the path traveled by the optical signals. The stability of the filter is el~h~. .ce~l since it is formed on the optical block, and need not be po.e;linl-eA and aligned in a separate mounting operation. As noted above, the center wavelength for each of the multiple signal ~hqnnele can be tuned by simply moving a GRIN lens collimqtor or the like associated with each of the signal rhqnnPle a W O 97/00458 PCTAJS~ 7 slight measure in the direction of the varying thic~neee of the continuous filter. By so moving the associated lens appa,~lus, it is aligned with the desired signal wav~l~.~Lh. In this fashion the unce~ l~llly of achieving the correct center wavcle~lh in the mAmlfActllre ~ of discl eeL filter element$ is ~ubsl ~ ;Ally overcome.
S A dense channel wavelength division mllll;pl~ g device is illustrated in Fig. 1, lg a contimlolle variable i' ' ., multi-cavity inte~re.t;ilce filter to form an ultra-narrow b~ JA~ filter at each of eight s~al~le ports on an optical block. This multiplexing device has the ability to multiplex individual, separate wavelength signals into a common fiber optic carrier line and/or to demllltir)lex such signals. For simplicity of c ~lan~lion only the deml ' . ' g ~ ;o~ y is described here in detail, since those skilled in the art will readily understand the correlative mullipl~illg functionality. That is those skilled in the art will recogni7e that the same device can be employed in reverse to multiplex optical signals from the individual r.hAnnele Typical spe~.ificAtions for an optical mlllfirle~ing device in accoldal-ce with the p,ere;"ed embodiment illustrated in Fig. 2 include those provided in Table A.

TABLE A
Number of Channels 8 Channel wavelength 1544-1560 Channel spacing 2 nm ~ 0.2 nm ~inimllm Isolation 20 dB to 35 dB
Insertion loss (total) less than 6 dB
Fiber type single mode, 1 meter pigtail Operating le"")e,~ re range -20~C to +50~C

The optical multiplexing device of Fig. 2 meeting the specifications of Table A is seen to include an optical block 10 which, p,t;~,~bly, is a stable glass substrate. A means for projecting collimAted light, such as a fiber optic GRIN lens collimator 12 or the like, W O 97/00458 PCT~US96/09727 couples highly c~ llimAted light 14 to the optical block at a slight angle through a hole or facet in surface 16 ofthe optical block. In acco~ance with one plerelled embodim~nt the optical block has a thi~n~cs "a" of 5 mm, and a length "b" of 14.1 mm or more, and a refractive index of about 1.5. The collimAted light p~r~lably has a div~ ellce of not more than about 0.15 ~ and the tilt angle "c" at which the collimAted light enters the optical block is about 15~. Thus, multicolor or multi-wavelength light carried by an optical fiber ;r~l~ly a single mode fiber) carrier is collimAt d by lens means 12 and directed through an optical port 18 in surface 16 ofthe optical block 10, from which it passes within the optical block to the opposite surface 20. A graded wavelength all-dielectric narrow bA~--lp~ filter 22 is carried on surface 20 ofthe optical block. Specifically, filter 22 is a contin--o -e, variable 1~ , multi-cavity illl~;lr~l~nce filter as described above, and, most pler~;l~ly, is a contin--o~-s linearly variable filter. Light entering the optical block at optical port 18 first strikes oppo~ile surface 20 at output port 24. Filter 22 is l-a l~pal~ at output port 24 to a sub-range ofthe wavel~ngth~ inçlllded in the collimAted light 14. Spe~ifically~
light 26 passes through port 24 ofthe optical block preferably to a cc-ll;.. ~l;.. ~a lens means 28 associated with a first signal ~hAnn~l The optical signal passed by port 24 is thereby Ll~ ed to optical fiber, pl~r~lably single mode fiber 30, as a d~mllltiplexed signal.
The continuous filter 22 at port 24 is reflective of wavel~n~h~ which are not "in-band" ofthe filter at that location. This reflected light 32 is reflected from surface 20 of the optical block back to surface 16. Surface 16 carries a broaflban<l high reflector film or coating 34. High reflector film 34 does not cover optical port 18, so as to avoid ~ el relillg with the passage of ~11;. . .i.l ~l light 14 into the optical block at that location. The reflected light 32 from the first output port 24 is reflected at surface 16 by reflector film 34 back to surface 20 of the optical block. The collimAted light 14 enters the optical block at optical W O 97/00458 PCT~US96~7~7 port 18 at a tilt angle of about 15 ~, where it refracts acco,dil,g to Snell's Law to an angle of app,~ y 9.9~ and then bounces between the opposite parallel surfaces 16 and 20 ofthe optical block. Thus, light 32 is reflected by reflector film 34 to strike surface 20 of the optical block at a second location 36 corresponding to a second output port of the -optical block. At the locatiQn of output port 36, the contim~oUS, variable th;cL-nes~, multi-cavity i"l~,rt;,~"ce filter 22 is ll~l~alellL to a dirrérelll wavelel1gll, or sub-range of wavelengths than it is at output port 24. For dense channel wavelength division multiplexing applications, the wavelength separation between each of the multiple ports linearly spaced along surface 20 of the optical block is preferably about 2nm or less. Thus, at outport port 36 an optical signal corresponding to a second channel is l~ ed through the filter 22 to a co~ g lens 38 and from there to fiber optic carrier 40. As at the first output port 24, the il,Lelre;~ ce filter 22 at output port 36 reflects light which is not in-band at that location. Thus, the ,~ h.i.,~. portion 42 ofthe collim~ted light 14 which first entered the optical block at optical port 18 is reflected back from port 36 to the high reflector 34 on opposite surface 16 ofthe optical block, and from there it is reflected or bounced back to a third output port 44. In similar fashion, the reflected wavelengths then contin--e to cascade in a zigzag or "m~ bounce" path down the optical block, with the optical signal for each individual channel being removed by sl1cces~ive bounces at surface 20 of the optical block.
As seen in Fig. 2, lhe,t;rore, the zigzag path of light travel through optical block 10 causes the ,t:ne~;Led wav~ l t.~ to strike, in turn, the aCl~1ition~l dow,l~L,e~ll output ports 46, 48, 50, 52 and 54. At each ofthese multiple ports, the dçml~ltiplexed optical signal is passed to an associated collim~ting lens, each c- mml-nicating with a corresponding signal carrier line or other des~in~tion. While p-c~rt;l~ly the filter 22 is reflective of all wavf~len~h~

W O 97t00458 PCTAUS96/09727 which are not in-band at each of the multiple output ports, in certain applications it would necess53rily be reflective only of the wavel~n~h~ of optical signals which had not been extracted at u~ ealll output ports, that is, at output ports encountered previously in the multi-bounce ç~cade sequence. Also, those skilled in the art will understand from this S description that the optical m~-ltir'~Yin~ device of Fig. 2 is equally suitable for use in collll)illillg the optical signals ofthe eight individual çh~nn~l~ Thus, the mllltirle ports in surface 20 would be input ports and optical port 18 would be an output port. Thec~ec~-ling would then proceed duw-~Lleanl from the bottom (as viewed in Fig. 2) ofthe optical block toward the top.
For an optical block of 5 mm ' ~ -' n~c, as recited above for optical block 10 of Fig.
2, with a tilt angle of 15 ~ leading to a bounce angle of 9.9~ within the optical block, the linear spacing of the individual output ports (TAN [9.9] x 2 x Smm) would be 1.76 mm.
Thus, continlloll~ in~ relellce filter 22 on surface 20 of the optical block should be at least 14.1 mm in length (8 x 1.76 mm). The total ~ t~n~e traveled by the optical signal ~co~ te~ with the last ofthe eight rh~nn~l~ (5 mm x 8 r.h~nn~l~ x 2 bounces) would be 80 mm. Thetotalbeamspread(80mmTAN- 1 [SIN- 1] [SIN] [0.15/1.5])wouldbeabout 0.138 mm. The total loss, therefore, for a 0.5 mm beam would be about 1.9 dB.
Accordingly, it will be appl eciaLed by those skilled in the art that the optical multiplexing device illustrated in Fig. 2 as described above, is suitable to demllltirlex numerous individual wav_lengLll cl~ c out of an in~ nt lightbeam in a very ~ffi(~ nt manner due to the minim~l beam divelgellce incurred. The total beam spreading for the pl~;r~"ed embodiment described above would be appluxi"-~(ely 40% for a half millimet~r beam, which produces the alore~d loss of only l.9 dB or less than 0.25 dB per channel c~c~-led through the device. More specifically, those skilled in the art will recognize that the W O 97/00458 PCT~US961'~7~7 ' . le bounce c~ .g t~ e achieved with a continuous, variable thir~n~c~, multi-cavity hlLelrel ellce filter deposited directly on the surface of an optical block provides an optical multiplexing device having pelrc,llllallce characteristics, in~ lin~ cost and silllplicily of construction, reliability of pe-r~ allce, c~mp~ctnese, etc., which are S ~i~nificz~ntly improved over prior known devices.
In the ~ n~tive pr~r~;l-ed embodiment illustrated in Fig. 3, collim~ted light 60 from a lens &-i ~,e~ .l 62 ~u ~~~tin~ with a single mode optical fiber 64 passes into optical block 66 at optical port 68 ~Ub~ ly in accoldance with the embodiment of Fig. 2 de&;lil,ed above. Thus, the light passes through optical block 66 to the opposite, multi-port surface 70 ofthe optical block, striking it first at output port 72. A contin-loll~, variable thirl~nP~ multi-cavity intelrelellce filter 74 extends over surface 70 to provide a narrow b~n-1pass filter at each of the multiple output ports 72, 76, 78 and 80. As in the embodiment of Fig. 2, the fiker 74 is Ll a~ L to a dirr~lt;ll~ wavelength at each such port, wLelel~y the single optical signal associated with rh~nnP.l~ 1, 3, 5 and 7, respectively, are ~ ed to cc,ll~olldi--g lens apparatus and fiber optic wave~liclPs On surface 82 ofthe optical block a refiective film 84 is provided to cooperate with illLt;lre;l~ilce filter 74 on surface 70 to achieve the multi-bounce c~c~-ling within the optical block. Inaccol~ ce with this ple~lled embodiment, however, reflective film 84 also forms a wballd filter at each bounce location. Thus, each bounce location at surface 82 of the optical block is an additional output port at which the optical signal associated with an additional channel is passed to an ~ o~ ted lens arri~n~Pmpnt and fiber optic carrier line.
More specifically, reflective film 84, which preferably is also a continllo~ls~ variable thickness, multi-cavity inlelr~lt;llce filter, and most plt;r~l~bly a contimlollely linearly variable i IL~;lrt;l~nce filter, is Ll~l~alenL to the wavelength of the optical signal of channel WO 97/00458 PCTnJS96/09727 2 at output port 86 and reflective of the other wav~l~n~h.c Similarly, it is l~ are,lL to the optical signal of channel 4 at output port 8B and, again, reflective at that location to other wavelengths. Output port 90 is Ll~lsp~ellL to the optical signal of channel 6 and, finally, output port 92 is Ll~1spal~l~L to the optical signal of channel 8.
It will be recognized by those skilled in the art that the optical mu~ e~illg device illustrated in Fig. 3 can provide highly Pffil ient and compact mnltir~ ~B and d~ fi~ ,l;ol-Al;ly. Forcr~ P.~Iighthavingadivelgenceof0.15~andentering optical port 68 at a tilt angle of about 12 ~, the optical block may advantageously be formed of fused silica and have a width of about 10.361 mm. Linear spacing of the output ports on each of surfaces 70 and 82 is preferably about 3.067 mm, yielding an overall linear ~limPneion of ~plox;.l~nl~ly 15 to 20 mm for the optical block. Generally, it is p-ler~lled in devices of the type ~~iecl-eeed here, to have a low entry angle or tilt angle (where zero degrees would be normal to the surface of the optical block) at which light passes through the optical port (measuring the angle of the light outside the optical block). A low entry angle reduces pol~liol1 dependent effects. It also reduces adverse effects of collim~ted light divergence on filter p~lrollll~ce, since a lower entry angle results in more closely spaced bounce points within the optical block and a shorter travel path for the light.
Typically, the entry angle is less than 30~, being pl~rel~ly from 4~ to 15~, more plt;rel~bly 6~ to 10~, most plere,~bly about 8~.
Fig. 4 illustrates another pl ~rel I c~d embodiment, wherehl the reflective film on the second surface 82 ofthe optical block 66 comprises multiple separate elements 120 - 126.
The other r~aLures and ~lemente are the same as the corresponding features and elements of the embodiment of Fig. 3, and are given the same l efel ence numbers. The individual reflective film ~ ; 120 - 126 can be deposited, e.g., by a sputtering process or the like, W O 97/00458 PCTrUS~6,'~57~7 directly onto the surface 82 of the optical block or onto separate carrier sub~ es to be individually positioned and att~ched to the optical block.
Epoxy or other adhesive may be used to attach the reflector r1~-"~ The individual reflector films can be bro~1b~n~l reflectors, opel~lh~g sub~ ly as reflector film 34 in the embodiment of Fig. 1. Alternatively, they may operate as mllltiple additional ports, i.e., as bA~ filters or dichroic filters ~ub~ lly in acccildance with thep.iilciples of reflective film 84 of the optical m~ ,k.~ g device of Fig. 3 .
Additional alternative embodiments will be appal~llL to those skilled in the art in view ofthis disclosure, in~ lrling, for example, optical multiplexing devices wheleill two (or more) solid optical sul~LIi~les are coated, one or both (or all) with continuous, variable thickness interference filters to form multiple ports on a single mono-planer surface as illustrated and described above, and then joined together to form the optical block.
The film stack structure for the contimlolls, variable thickness, multi-cavity illlelrelt;nce filter 22 in the ple~lled embodiment illustrated in Fig. 2 is illustrated in Figs.
5 and 6. Preferably, the thickness of each alternating layer (for eY~mrle, of niobium pf~.ntoxifle and silicon dioxide), as well as the total thickness of the film stack, is precisely LIolled, most preferably within 0.01% or 0.2 nm over several square inches of area. In addition, the film stack should be deposited with very low film absorption and scatter, and with a bulk density near unity to prevent water-ind~lced filter shi~ing Such ultra-narrow, multi-cavity b~n-lpa~.~ filters have excellent performance characteristics in~.llltling temperature and envho~ l stability; narrow bandwidth; high Ll~ ce of the desired optical signal and high r~fl~ n~e of other wavelengths; steep edges, that is, highly selective Ll,.~ ivity (particularly in designs employing three cavities or more); and relatively low cost and simple construction. As shown in Fig. 5, the filter is a three cavity W O 97/00458 PCT~U',GI~/ 7 filter, wl~l~l one cavity, the "first cavity," is imm~ t~ly ~ c~nt the glass substrate. A
second cavity ;....~ l. Iy overlies the first cavity and the third cavity immerli~t~ly overlies the second cavity and, typically, is exposed to the ambient atmosphere. In Fig. 6 the structure ofthe "first cavity" is further illustrated. A seq~l~n~e of stacked films, preferably S about S to 15 films of alLel,-a~ g high and low refractive index materials, are del~osiled to form a first reflector. Preferably, the first film imme~ tFly acljacçnt the substrate surface is a layer of high index m~t~n~l, followed by a layer of low index m~tP.ri~l, etc. Each of the high index layers 90 is an odd integer of quarter waV~l~on~h~ optlcal th;cl~n~c~ (QWOT), p.ere.~bly one or three quarter wave1F~-gll..~ or other odd number of QWOTs The low l~ ;Iiv~ index layers 92 which are interleaved with the high refractive index layers 90 are similarly one quarter wavel~,l.glll optical th:~lrn~cs or other odd number of QWOTs in thickness. There may be, for; , 1~, about six sets of high and low refractive index layers forming the bottom-most dielectric reflector 94. Cavity spacer 96, ~lthollgh shown s. .hf~ lly as a single layer, typically col-lpl ises one to four allel llalhlg films of high and low index materials, wherein each of the films is an even number of QWOTs in thic.knes~, that is, an integral number of half wa~,le.~Llls optical thickness. The second dielectric reflector 98 preferably is subst~nti~lly identical to dielectric reflector 94 described above.
The second and third cavities are deposited, in turn, immer1i~t~1y upon the first cavity and plt;rel~ly are .Y~b~ lly i~ .ntir.~l in form. The thi~L-n~ ofthe hll~lreLellce filter layers varies along the length of the multi-port surface of the optical block, as described above.
Thus, the physical thickness of a QWOT will vary along the multi-port surface. Various alternative suitable film stack structures are possible, and will be app~llL to those skilled in the art in view of this disclosure.

WO 97/00458 PCT~US96/09727 One alternative film stack is illustrated in Fig. 7, wherein the upper and lowerreflectors 94, 98 are as described above for the embodiment of Figs. 5 and 6. The cavity spacer 97 is shown to be formed offour films, two high index films 97a alte--lalii,g with two low index films 97b. Each such film is 2 QWOTs thick or one half wavelength. Various other alternative suitable film stack structures are possible, and will be appalel.l to those skilled in the art in view of this disclosure.
~C~r~llc;d embo~ ic~rlosed here differ and dl~ ly illlplUVe over the prior systems in several key areas. The ion gun used to direct ionized reactive gas towards the ~ull~tl~le during de,oo~ilioll acts to lower overall reactive gas pressure to prevent poisoning and arcing at the target, and the ioni7~tion i,l-"~ses the reactivity of the gas to improve film stoichiometry. The energy imparted by the ion source helps densify and improve film quality. P~c;rt;ll~d emb~im~ntc. also require no manifold in the substrate plane and do not require the target or sputter sources to be tilted to improve coating rate. Rather than Pond's ,..~n~ y "gas separation process" between the substrate and m~gn~.tron, plc:r~ d embc~im~ntc ~Iic(~l~sed here rely on high speed p Imping systems to reduce inert gas as well as reactive gas levels to further reduce arcing. High pllmping speeds allow greater inert gas flows at the m~gnetron without increasing background pressure. This in turn allows increased sputtering rates at correspondingly higher power levels. Coating rates are typically 3 - 6 ang/sec, with throw tlict~nces of 30 to 35 inches. (Effective coating rate or ll"ù~ t decrecases by the square of the ~lict~nce ) Films are fully reacted and possess a fully dense packing structure even at this surprisingly high deposition rate due to the energy and reactivity of the ionized reactive gas.
The variable thickness filter coating on the multi-port surface can be plt;~ d in accoldance with the following p~c:rel~ed embo~im~-ntc The variable thickness can be WO 97/00458 PCTnJS96/09727 achieved by po~;l io~ the substrate to be coated in the vacuum chamber at an angle to the magnetron. Partial and/or interrnittent ~hi~l~ing of the substrate also can be used. The shield or .~ means can include a .~ g lllellll>el positioned in the vacuum chamber b~;lween the multi-port surface of the optical block (that is, the surface to be coated) and S the source of sputter material at the m~gn~tron Preferably the "~-I;"g lll~:lllhel is subst~nti~lly closer to the multi-port surface than to the target source material at the ms-Enetron. For ~ unplç, the mask or shield can take the form of a ~b~ lly planar L;~ lllLc~l positioned less than .5" from the multi-port surface. The planar m~kinE
llwllll~el can be spun, where the term spill~ g or spun in~ des both spinning on its axis or orbital motion sub~ ly in the plane of the planar member. Typically, multiple optical blocks are formed as a unitary coated substrate to be diced or divided up following the coating process. Most typically, the ~ul~Ll~le being coated colll~lises a unitary optical glass disk which spins in an opposite rotational direction from the planar m~1,ing member.
Alternatively, the optical block, again me~ning the circular disk substrate which eventually will be divided into a number of optical blocks, can be positioned stationary in the vacuum chamber at a position laterally offset from the m~gnP,tron. As tliecllssed further below, . m~ in~ means also can be used to achieve the variable thickness desired of the optical block.
The pl'~rt;ll~;d embodiments described below are capable of producing high quality co~ on sul,~ es, e.g. to form mirrors which are usable in fiber optic systems, in ring laser gyroscopes, etc., using a DC reactive m~gnPtron sputtering system instead of IBS.
Such films have conlp~ble properties to IBS coatings in that they have extremely high p~çl~ing density, as well as smooth snrf~ces and low scatter. Total losses for a high W O 97ioo4s8 PCT~US9G~ / 7 reflector laser mirror made in accordance with plert;.lt;d embodim~-nt~ of the method disclosed here, for example, are well less than 0.01% or 100 ppm.
Figs. 8 and 9 show the method and app~ s of plert;llc;d embo~liment~ It should be understood that the substrate rerell ~d to here is typically a flat disk of optical glass or S the like, having a ~ m~,tçr of, for ~x~mpl~, 8", 20", etc. One or both surfaces of the disk are coated to ~im~ eo. .~l~ form many (pelllaps hundreds) of optical blocks. That is, the coated disk is diced or cut into many individual optical blocks, each having on one surface a variable thickness filter as described above to form the desired multi-port surface. The housing 110 forms a vacuum chamber 111 co..l~;..;..g a low pressure m~gnetron assembly 1 12 and a pl~l-el~.y substrate holder 113 with a plurality of rotatable planets 114. Each planet 114 holds a substrate facing the m~netron assembly 112. In this embodiment, the distance between the top of the m~gnetron assembly 112 and the planets is 16". The el - ul1 assembly 1 12 is co~ ;Led to a source of working gas 1 16 by conduit 1 17. In this embodiment, the housing 110 is shown spherical with a radius of 48", but other configurations are equally applùp,iate.
The housing 11 0 has a lower sleeve 11 8 which opens into the vacuum rh~mh~r 111and cc...l~ a high speed vacuum pump 120 with a gate valve 121 located between it and the vacuum chamber 111. The vacuum pump is of course used to lower and . . .~ i. . the pl es~u[e in the vacuum challll)er at a very low level in the inert gas pl t:;S:julc~ range of S x 10-5 Torr to 2.0 x 10 ' Torr.
In this regard the invention ~ tin~ hes sharply from the known prior m~gn~Sron sputtering ter.hniq~les and from conventional ion beam tec hni(l~les It is characterized by extremely low chamber pressures, inr.l~ltling ~ lllely low reactive gas ples~ule and extremely low inert gas pressure. The reactive gas pressure, such as ~2, N2, NO, etc.

W O 97/00458 PCT~US96/09727 (measured at the substratesurface being coated) is preferably in the range of 2.0 x 10-5 to 1.5 x 10 ~ Torr, more preferably 3 x 10-5 to 9 x 10-5 Torr. This advantageously reduces or ~ I;...;-.i.lc~s arcing at the magnetron and "poisoning'~ ofthe source by the reactive gas. The inert gas, such as argon, krypton, xenon, etc., is in ~ler~llt;d embodim~nte introduced S primarily at the m~netron. A sharp ple~ule drop is est~hliehe~ for the inert gas, p.ert;l~l~ having a p.e~ule (measured at the ~ubsl-~le surface being coated) in the range of 5.0 x 10-5 Torr to 2.0 x 10 1 Torr, more p- ~re~ly 5 x 10-5 Torr to 1.5 x 10~ Torr. Such low chculll,el gas ~ iUl~S provide long mean free path, (~P) and correspondingly allow adv~nt~geollely long throw (l;~ ~e without undue collisions between the chamber gasses and the ~ ulle t:d material. Advantageously good coating uniro---lily is achieved via long throw lliet~n~; pr~;;r~ c~ly greater than 12", more 1~ ~r~-~Lbly 20" or longer. The extremely low ~ h~l pl~S:~ul~S enable the use of long throw distances. That is, noLw;l~ tling the use of such long throw rliet~n~ee advantageously high coating deposition rates can be achieved with c(j.. e~ondingly high magnetron power levels. The loss of films or coating quality normally expected to result from higher m~n~tron power levels and longer throw e is avoided by the novel use of extremely low chamber pie~ult:s. Thus, pl~;relled embollimente ofthis invention duplicate several key process conditions of IBS (which, for . ~e, opel~les in the same pressure range as d~scrihed above), but uses a DC m~gnetron sputtering system. This novel system, based on m~gnetron sputtering substantially improves the coating speed and colle~ondillgly cost and throughput of depositing high film quality co~tin~e Typical high speed vacuum pumps in this invention are 16" cryopumps or 16"
rliffil~;on pumps. Pumping speeds with these pumps are on the order of 5000 liters/second (nitrogen) for a 16" cryopump and 10000 liters/second for a 16" diffusion pump (ref.

W O 97/00458 PCTnJS96~'0~/ 7 Leybold Product and Vacuum Technology reference book, 1993). Larger pumps can beused such as a 20" pump having l,u~ hlg speeds of 10000 liters/second for cryopumps (N2) and 17500 liters/second for ~liffi~eion pumps (N2) (re~ Varian Vacuum Products Catalogue 1991-92). Pumping speeds referenced above are at the throat ofthe pump.
The ~n~ Oll aew"ll~ly l l2 is in vertical ~li8nmçnt with the axis of rotation (main center line 122) of the planetary substrate holder 113 and with a holder for Illvni~olillg witness chip 123. In this embodiment, the throw or the diet~nce bcLwccll the top of the "~nel,vil asw"llWy and the planets is 16". Each planet and its substrate rotate about their own center line 124. Such planetary holders are conventional and need not be described further except to point out that, in this embo~lim~nt, the planets are 15" and the ~ubsll ~les are 15" or any size less than 15 " in ~ met~r, and the center line of each planet is 14" from the center line 122 to accommodate large subsll~Les Larger planets can be used, for e , le, 24" planets, with cwlcspol-dingly increased substrates sizes and throw ~liet~ncçe, whereby even greater throughput improvements can be achieved. Masks can be used,preferably ~ub~ y planar m~QLing members positioned, e.g., about 0.5" from the substrate. The . . .~ lllCmlJCI can be moved progressively during the deposition process, i.e., whLle the ...~ el ~ vn is opel ~L,lg, to achieve the desired variation of film thi~L-nloss for establishing multiple di~c,e,ll wavelength ports.
An ion gun 126 whosw output, lc~lesellled by dashed lines 127, is directed obliquely toward the substrate holder 113 and whose input in conn~cted to a source of reactive gas mixture 128 by conduit 130. The ion gun is positioned such that its output of ions and gas mixture cover the entire substrate holder 113 and in this embodiment the top of the ion gun is 20" from the planets. The pli".,ipal function ofthe ion gun is twofold. The first is to modify and improve film pl vpe~ ties in a manner similar in concept to the Scott et al patent WO 97/00458 PCTAUS~GI'~3/ 7 U.S. No. 4,793,908. The second function may be more important, which is to serve to ln;~l low reactive gas background pressure. With the ion gun, reactive gas is ionized and directed toward the ~LI~le(s). The ~ m ofthe reactive gas then carries it only toward the sul,sll~Le(s) (and not toward the m~gn.otron where it would have the effect of S c~lsing arcing and rate red~lction). The small amount of gas which diffuses toward the m~ n~.tron does not noticeably affect its operation. Typical reactive gas ple~ ult~S are in the range of 2 x 10-5 Torr to 1.5 x 10~ Torr, preferably, 3 x 10-5 Torr to 9 x 10-5 Torr.
A suitable hot cathode pressure gauge 131 is also co, -l l~ led to the vacuum challlbel 111 to measure the pl~s~ule; within the vacuum chamber. Also, vacuum çh~mh~r is provided with a shutter 132 oscillatible about a stem 133 blocking the output of the .Ollassembly 112, representedbydashedlines 134. Thestem 133 isconnected in any suitable manner to a pldLr~ . 135 and to a means for osf ill~ting the stem (not shown).
The shutter is used to pre-sputter the source(s) to remove col~ es from the target which may have con-l~n~l etc., onto the surface ofthe target while the appal~ s was idle bc;Lweell layers being deposited on the substrate.
As shown in Fig. 9, the m~gn~tron assembly 112 comprises a target holder- 136 having a cavity 137 formed by walls 138 and target material 140. Centrally within the cavity 136 are con~llLiollal m~gn~,tc 141 which are water cooled by the cirr,l-l~ting flow of water in and out of the cavity 136 through p~ ges 142 and 143. The metallic target m~t~ri~l 140, cl~"ped by the holder, also is water cooled. A manifold 144, spaced slightly from the holder 136, and sealed by insulators 145, is conn~cted to the source of working gas 116 by conduit 117 (Fig. 8) which enables the gas to flow entirely around the top of the holder and over the metallic target material 140. The manifold 144 has an openh~g 145 subst~ntiQIIy the size of the m~t~llic target material so that sputtered target material and working gas is emitted as represented by the lines 134. The m~gnetron is available from Material Sciences of Boulder, Colorado and is typically 6" to 8" in ~ metPr with high glll m~nets.
When it is realized that this invention has the capability of producing ~ ely high S quality film coaLn~;~ by m~gnetron sputtering without the con~ nls of IBS or other known teçhn:qllçc, it will also be realized that this invention is a major advance over the prior art.
The ro~e~going rlimP~n~ions and pressures ofthis embodiment-- a throw ~list~nce of 16",15" ~ Pf planets, 15" or less ~ metfr substrates and the ~ t~nce. of 20" from the top of the ion gun to the planets along with extremely low reactant gas pressures in the range of 2 x 10-5 Torr to 1.5 x 10~ Torr and extremely low inert gas pressures of 5 x 10-5 to 2 x 10~ Torr--also show the great difference between this invention and the prior art.
Compare also the throughput of pl erel I ~;d embol1im~ nt~ of this invention with the throughput of a typical IBS system in making laser quality mirrors:

This Invention IBS
Coating Rate 2-SA~/sec .2-lA~/sec Substrate Area 800-1200 in Z 50-lOOin 2 (Total a-ea of 5 Planets) From the r.,. ~goi..g it can be seen that the throughput of this invention is 20 to 120 times faster than the throughput ofthe typical IBS system. Coating throughput is a function of coating rate and substrate area.

W O 97/00458 PCT~US96/09727 Fu,llle,.no.~, the method of this invention scales easily to larger appa-~lus ;on~ All ofthe .~ ;o~ above can be easily increased at least by a factor oftwo to allow coating of optical subsl~les of 30" ~ or even large with laser low losscoatings having good ullir~lllllLy. Scaling is a simple linear issue. A larger system uses larger m~gnetrons and more process gas (e.g., argon). The vacuum pumps need to be COIlc;~lldin ~ il~weased to accommodate the larger chamber and the increase in process gas flow.
Thus, as is app~elll, this invention is capable of producing, for ~ ,lç, laser quality mirrors which are many times greater in ~ m~tP.r than those known to be made by current IBS systems.
The long throw of 16" and more pler~l~ly 20" and greater, and low challlbe, pressures of pler~lled embo~lim~ontc~ of this invention allow two or more materials to be concurrently deposited to form high optical films composed of mixtures of materials. Fig.
9 shows two sources, m~gn~tron assembly 112 and m~gnetron a~s~mhly 112a in vacuum chamber 11 1 as an ~"~mple of multiple sources. (The subscripts to the added source and the use of all other reference numerals as in Fig. 8 are to simplify the description herein).
By controlling the level of power of each source which effectively controls the deposition rate, a layer of selected refractive index can be formed as a mixture of two or more materials. The mixture can be homogenous throughout the layer to form a film of selected index, or inhomogeneous where the layer composition and hence the refractive index varies throughout the film. One common form of inhomogeneous film is called a "rugate" filter, where the refractive index varies in a sinusoidal manner which has the effect of forming a narrow notch reflector.

WO 97/00458 PCT/US9G~u57~7 To . . .~ a low pressure for such a multi-source system, the pumping speed must be roughty il~ cased by a factor of two for two con.iull elll deposition sources, or a factor of N for N sources. Given the benefit of the disclosure, adding pul-lpillg speed will be a simple exercise for those skilled in the art, involving generally either increasing the size of the pump or adding more pumps to the chamber. In practice, however, two concurrent sources need not be powered at a level equal to that used for a simple source to ~ ~A;~
coating rate, as the rate from the sources is additive, and hence the sources can be sized to smaller levels which use less gas.
Another device which may be used in this invention is an arc re~lçing èlectronicdevice sold by Advanced Energy of Boulder, Colorado under the trademark SPARC-LE.
In Fig. 8, the SPARC-LE 46 is shown connecte~l to the m~gnetron assemblies 112 by an ele~llical c~ n~1~1ct~ r 147 with its own DC power supply 148. The SPARC-LE is conn~cted similarly to the two ll~leLIu~ lies 112 and 112a as shown in Fig. 9. Such a device helps in recl~lçing arcing but it is not nece~.y in the method and appal~ s of this 1 5 invention.
From the ÇoLego;llg, it can be appreciated that the m~gnetron system operates ately low pressures. The chamber pressure of the inert gas will be a function of the ;Llun pressure. Most impol~ ly in this invention, the low total pressure region 150 (A + O~) is always much less than the higher argon pressure region 152 as depictecl in Fig.
9.
Pressure in the chamber can be modeled using the well known pressure-flow equations (see Leybold Product & Technology Reference Book, page 18 - 5, 1993):

PChamber= FIowAr / Cp W O 97/00458 PCTAJS~ ,7~7 PMagnetron = FbWAr / CM + PCharnber Where:

PChamber is the pressure in the chamber;
FhnvAr is the flow of argon into the challll)er (through the m~nPtron);

cp is the con~luct~nce ofthe high vacuum pump (chamber pumping speed);
PM~,tron is the pressure in the m~n~tron;
CM is the c~mtiuct~n~e due to gas co~ )PmPnt at the m~gn~tron (confin~mPnt efflciency ofthe m~ ,tron) ..
slll)x/;lul;i-g terms, the ch~llber pressure can be written as:
PChamber = PM~gnf~tron / (Cp / CM + 1) l~is is an illl~olL~l~ rel ~ion~hir~ because it shows that the pr~s~ule in the cl~ bel is dependent upon the ~Ulllp;llg speed of the l~.h~mb~r (Cp). It also shows that if the pulllpillg speed of the chamber were low, then the pressure in the chamber would be appr~-x;...~ y equal to the pressure in the m~gn~tron. Such low pumping speed type 15systems are known in the prior art, where throttle valve me~ are placed in front of the pump to reduce pumping speed. See Vossen and Kern, "Thin Film Processes,"
.mic Press, New York (1978) above, at page 156. However, if the pumping speed in the ~ is large, as taught by this invention, then the chamber pl es~ul e becomes low relative to the m~gnetron pressure.
20Using the eql~tion~ above, cll~-ll,el- pressures can be determined ap~lu;~il.la~ely for any new ch~,lber with known pumping speed, as shown in Figs. 11 and 12. As is clearly evident from the figures shown, any suitable desired pressure can be achieved by increasing the pllmpi~ speed ofthe chamber. If the opel~Lillg inert gas pressure in the m~gn.o,tron is W O 97/00458 PCT/U~,G~ 17 lowered, it is possible with certain mAgnetron types, then the entire pressure curve is correspondingly lowered. This is shown by the co."p~ison of the pressure curve of Fig.
11 for l"Agi~ Oll p,es~ of 0.7 microns and a mAgn~.tron assembly ct ntluct~nce (CM) of 3000 l/sec. with the pressure curve of Fig. 12 for a ~ ~ ~A~ .~ n pressure of 0.4 microns and a ~--9~-';~0il aSSelll~ Crnd~c,tAnr~ (CM) of 3000 1/sec. The pumping speeds shown on the abscissa are quite achievable--for example, a commonly used 20" diffusion pump is rated at 17500 1/sec, and 32" diffusion pump is rated at 32000 1/sec.
An Al~ Al;~e l"t;r~"ed embodiment is illustrated in Figs. 13 and 14, incorporating a physical mask between the source and the substrate to control and taylor the th~ nrcc of the filter coating. In other ,~e~;Ls, the app~ s of Figs. 13 and 14 is seen to correspond to the emborlimPntc rliccllcsed above. Specifically, a physical mask 150 is positioned between the ...A~ n asse...l~ly 112 and the substrate surface 115. The degree of mA~ing varies with ~1istAnce from the center of the substrate, such that at one radial ~lictAnr,e the resulting filter is tuned to a first particular wavelength and at a second radial dictAn~e from the center the filter is tuned to a di~l ~;nl wavelength. The mask can be fixed or moving, for example, spinning or rotating. For example, a mask typically can rotate about a collllllol1 axis with the substrate surface, although other rotation srllem~c are possible and will be readily appa~ c;"l to those skilled in the art given the benefit of this disclosure. In general, it is desirable to position the mask as close as possible to substrate surface 115, p,~;r~,~bly less than .5 inch, more preferably 0.25 inch, most preferably, for example, about 0.125 inch. Preferably, any rotation or other movement of the film has zero or near zero wobble, or runout, preferably less than .001 inch. In general, a greater degree of wabble is tolerable for filters having looser tolerance sperificAtiQns or wider bandwidth. Preferably, WO 97/00458 PCT~US96/057 7 the mask rotates or spins at a high rate of speed, pleferably several hundred revolutions per layer, for example, about 50 to 100 revollltiQn~ per minllte It will be a~parellt from the above discussion that various additions and mo~lifir~tions can be made to the optical multiplexing devices described here indetail, without departing from the true scope and spirit of this invention. All such modifications and additions are inte.n~le~ to be covered by the following claims.

Claims (34)

I Claim:

1. An optical multiplexing device comprising an optical block having an optical port transparent to multiple wavelength collimated light, a continuous, variable thickness interference filter extending on a multiport surface of the optical block and forming multiple ports arrayed in spaced relation to each other along the multiport surface, the continuous, variable thickness interference filter being transparent at each of the multiple ports to a different wavelength sub-range of the multiple wavelength collimated light and reflective of other wavelengths thereof, and means for cascading light along a multi-point travel path from one to another of the multiple ports.

2. The optical multiplexing device in accordance with claim 1 wherein the interference filter is continuously variable.

3. The optical multiplexing device in accordance with claim 1 wherein the interference filter is continuously linearly variable.

4. The optical multiplexing device in accordance with claim 1 wherein the means for cascading light comprises a reflective coating on a second surface of the optical block.

5. The optical multiplexing device in accordance with claim 4 wherein the second surface of the optical block is spaced from and substantially parallel to the multiport surface.

6. The optical multiplexing device in accordance with claim 4 wherein the reflective coating is continuous over the second surface, being at least co-extensive with said multi-point travel path.

7. The optical multiplexing device in accordance with claim 6 wherein the reflective coating is a broadband high reflector film coating which is substantially uniformly reflective of all of said sub-ranges of the multiple wavelength collimated light.

8. The optical multiplexing device in accordance with claim 6 wherein the reflective coating forms multiple additional ports arrayed in spaced relation to each other along the second surface, the reflective coating being transparent at each of the multiple additional ports to a different wavelength sub-range of the multiple wavelength collimated light, and reflective of other wavelengths thereof.

9. The optical multiplexing device of claim 4 wherein the reflective coating comprises multiple discreet reflective film elements arrayed in spaced relation to each other along said second surface.

10. The optical multiplexing device in accordance with claim 4 wherein the means for cascading light further comprises means for directing multiple wavelength collimated light into the optical block through the optical port at an angle to the multiport surface between 4° and 15°.

11. The optical multiplexing device in accordance with claim 1 wherein each one of the multiple ports has an associated lens means for focusing collimated light passed by that one of the multiple ports.

12. The optical multiplexing device in accordance with claim 11 wherein the lens means comprises a GRIN lens communicating with optic fiber.

13. The optical multiplexing device in accordance with claim 1 wherein the optical block comprises a solid block of material substantially transparent to said multiple wavelength collimated light and selected from the group consisting of glass and fused silica, the continuous, variable thickness interference filter being on an outside surface thereof.

14. The optical multiplexing device in accordance with claim 1 wherein the optical block comprises an enclosed chamber.

15. The optical multiplexing device in accordance with claim 1 wherein the optical block is substantially rectilinear, with the optical port being at a front surface of the optical block which is opposite and parallel the multiport surface of the optical block.

16. The optical multiplexing device in accordance with claim 15 wherein (a) the means for cascading light comprises on the front surface a reflective film coating not extending over the optical port; (b) there are at least eight of said multiple ports, each being a bandpass filter transparent to a discreet wavelength sub-range separated from the wavelength sub-range of adjacent ones of the multiple ports by approximately 2 nm; (c) collimated light passes through the optical port at an angle of approximately 6° - 10° to the plane of the front surface, and (d) the multiple ports are linearly spaced from one another along the multiport surface.

17. The optical multiplexing device in accordance with claim 16 wherein the reflective film on the front surface of the optical block is a broadband high reflector film coating.

18. The optical multiplexing device in accordance with claim 15 wherein the means for cascading light comprises on the front surface a reflective film coating not extending over the optical port, the reflective film coating being a second continuous, variable thickness interference filter extending on said front surface of the optical block forming multiple additional ports, the second interference filter being transparent at each of the multiple additional ports to a different wavelength sub-range and reflective of other wavelengths of the multiple wavelength collimated light.

19. The optical multiplexing device in accordance with claim 18 wherein (a) there are at least four of said multiple ports and at least four of said multiple additional ports.

20. The optical multiplexing device in accordance with claim 1 wherein the continuous, variable thickness interference filter forms at each one of the multiple ports an all-dielectric narrow bandpass filter.

21. The optical multiplexing device in accordance with claim 1 wherein the continuous, variable thickness interference filter is a multi-cavity interference filter.

22. The optical multiplexing device in accordance with claim 21 wherein the continuous, variable thickness interference filter comprises a film stack forming at least three interference cavities.

23. The optical multiplexing device in accordance with claim 1 wherein the continuous, variable thickness interference filter comprises a film stack formed of alternating films of niobium pentoxide and silicon dioxide.

24. A method of producing an optical multiplexing device comprising an optical block having an optical port transparent to multiple wavelength collimated light, a continuous, variable thickness interference filter extending on a multi-port surface of the optical block and forming multiple ports arrayed in spaced relation to each other along the multi-port surface, the continuous, variable thickness interference filter being transparent at each of the multiple ports to a different wavelength sub-range of the multiple wavelength collimated light and reflective of other wavelengths thereof, and means for cascading light along a multi-point travel path from one to another of the multiple ports, said method comprising the steps of:
positioning the optical block in a vacuum chamber having magnetron means and source means for sputtered particles, the multi-port surface of the optical block facing the source means at a long throw distance therefrom;
operating the magnetron means to sputter particles from the source means for coating the multi-port surface, including introducing inert gas to the vicinity of the source means at an enveloping pressure;
rapidly withdrawing and depleting the inert gas from the chamber by high speed, high vacuum pump; and directing ionized reactant gas to the multi-port surface to facilitate reactive coating, whereby the interference filter is obtained as a low-loss optical coating.

25. The method of producing an optical multiplexing device in accordance with claim 24 wherein the long throw distance between the source and the multi-port surface is at least 16".

26. The method of producing an optical multiplexing device in accordance with claim 24 wherein the inert gas pressure in the chamber is maintained less than 2.0 x 10 -4 Torr and greater than 5 x 10 -5 Torr.

27. The method of producing an optical multiplexing device in accordance with claim 24 wherein the masking means is positioned in the vacuum chamber between the multi-port surface and the source means.

28. The method of producing an optical multiplexing device in accordance with claim 27 wherein the masking means is substantially closer to the multi-port surface than to the source means.

29. The method of producing an optical multiplexing device in accordance with claim 28 wherein the masking means comprises a planar masking member positioned less than .5" from the multi-port surface.

30. The method of producing an optical multiplexing device in accordance with claim 29 further comprising the step of spinning the planar masking member concurrently with operating the magnetron means.

31. The method of producing an optical multiplexing device in accordance with claim 30 wherein the optical block and the planar masking member spin in opposite rotational directions.

32. The method of producing an optical multiplexing device in accordance with claim 24 wherein the optical block is positioned stationary in the vacuum chamber at a position laterally offset from the magnetron means.

33. The method of producing an optical multiplexing device in accordance with claim 24 wherein the magnetron means comprises a magnetron and a shield partially shrouding the magnetron for inhibiting diffusion of inert gas away from the source means while allowing diffusion of sputtered particles from the source means to impinge upon the multi-port surface.

34. The method of producing an optical multiplexing device in accordance with claim 24 further comprising the step of rotating the multi-port surface with respect to the vacuum chamber.