EP0960349A1

EP0960349A1 - Optical devices and methods

Info

Publication number: EP0960349A1
Application number: EP98902853A
Authority: EP
Inventors: Martin Charles Elias; Stephen John Madden
Original assignee: AOFR Pty Ltd
Current assignee: AOFR Pty Ltd
Priority date: 1997-02-14
Filing date: 1998-02-13
Publication date: 1999-12-01
Also published as: WO1998036302A1; JP2001511905A; AUPO511697A0; CA2280964A1; EP0960349A4

Abstract

An optical device (1) for dividing input light having a mix of wavelengths (μ1 to μ16) into discrete output wavelength bands (μ1 to μ16) and combining discrete wavelength bands (μ1 to μ16) into an output of mixed wavelengths (μ1 to μ16). The device (1) includes at least three optical wavelength selective devices (2, 3, 10) of the kind having a first port (23), a second port (25) and a third port (26) optically interconnected at least partly by one or more waveguides (18, 19). A first of the selective devices (2) has the second port (25) optically connected by waveguide to the first port (23) of a second of the devices (3) and the third port (26) optically connected by waveguide to the first port (23) of a third of the devices (10).

Description

OPTICAL DEVICES AND METHODS

FIELD OF THE INVENTION

This invention relates to optical devices for dividing light having a mix of wavelengths and a number of discrete wavelength bands. It also relates to optical devices for combining light in a number of discrete wavelength bands into light having a mix of wavelengths. Devices of this type have application in optical fibre communication systems using wavelength division multiplexing (WDM) and demultiplexing (WDD).

BACKGROUND ART

In known WDM/WDD techniques the filters or other wavelength selective devices are used to extract discrete wavelength bands from a signal comprising a mix of wavelengths. Sequential use of such devices is required to obtain the required number of discrete wavelength channels. Known devices can be divided into those which utilise optical components to which the light travels in free space and those in which the light is divided whilst propagating in an optical waveguide such as an optical fibre or planar optical device. In practice, optical waveguides are required for the transmission of light signals over any significant distance and normally optical fibres are used for this purpose. If a WDM/WDD device uses discrete optical components and free space propagation then the light must be extracted from the optical fibre, focused into the optical system and usually relaunched into another optical fibre. This procedure creates various problems including difficulties in alignment, a device of inconveniently large size, and a lack of ruggedness.

United States Patent 4,900, 119 describes a narrow band wavelength selective optical device in which the separation of a narrow band of wavelengths can be achieved wholly within an optical fibre. A series of such devices can be used to sequentially tap narrow bands of wavelength from a light signal to provide for WDM and WDD. A difficulty with these prior art devices is that the insertion losses for each of the narrow band wavelength selective devices are cumulative and thus provide increasing attenuation of the outputs in the order in which they are tapped from the input signal. It is not uncommon for a 16 channel device of this type to attenuate the signal by as much as 8dB.

United States Patent 5,583,683 describes an optical multiplexing/demultiplexing device which utilises a multiport optical block with variable thickness multi-cavity interference filters extending over the multiple ports. Input light is separated into individual bands by the transmission selectivity of the filter at each port. The wavelengths not transmitted are reflected to other ports with different filters. By using multiple reflections several bandwidth channels can be removed from a multiplexed signal. In common with one of the class of devices described above this device performs the division of the input light after it has been extracted from the transmitting wave guide and requires the separated channels to be relaunched for subsequent transmission. Additionally the device requires the use of a multilayer filter of complex construction to provide the desired filtering at each port.

DISCLOSURE OF THE INVENTION

It is one object of this invention to provide a wavelength dividing optical device and a wavelength combining optical device as well as methods for dividing and combining optical signals.

Another object of this invention is to provide a wavelength selective optical device for use in wavelength dividing of optical signals.

The present invention proceeds from the recognition that an optical wavelength selective device can be constructed to separate from light having a mix of wavelengths an intermediate wavelength band narrower than the input wavelength band but broader than the desired discrete output wavelength bands.

Accordingly, in a first aspect this invention provides an optical device for dividing input light having a mix of wavelengths into at least two output wavelength bands, said device including two optical wavelength selective devices of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides, in which wavelength selective means in the waveguide(s) route input light of selected wavelength band at said first port to said third port and route input light of other wavelengths at said first port to said second port, a first of said selective devices having the third port optically connected by waveguide to the first port of a second of said devices or the second port optically connected by waveguide to the first port of said second device, wherein said wavelength selective means of said first device route to the corresponding second or third port light in a first wavelength band significantly broader than a second wavelength band of light routed to the corresponding second or third port by the wavelength selective means of the second selective device.

Preferably the first wavelength band is at least 10% to 20% broader than the second wavelength band.

Preferably, the other wavelengths include a third wavelength band having a width substantially equal to the selected wavelength band.

In a second aspect this invention provides an optical device for dividing input light having a mix of wavelengths into a plurality of discrete output wavelength bands said device including at least three optical wavelength selective devices of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides, in which wavelength selective means in the waveguide(s) route input light of selected wavelength band at said first port to said third port and route input light of other wavelengths at said first port to said second port, a first of said selective devices having the second port optically connected by waveguide to the first port of a second of said devices and the third port optically connected by waveguide to the first port of a third of said devices, wherein said wavelength selective means of said first device route to the corresponding third port light in a first intermediate wavelength band broader than any of said discrete bands and route to the corresponding second port light having a wavelength outside said first intermediate band including a second intermediate band broader than any of said discrete bands, said wavelength selective means of said second selective device route to at least one of the corresponding second port or third ports light in a wavelength band narrower than said second intermediate wavelength band and said wavelength selective means of said third selective device route to at least one of the corresponding second port or third port light in a wavelength band narrower than said first intermediate wavelength band.

In the preferred form of the invention the first, second and third ports of the wavelength selective devices are optically interconnected by one or more waveguides. In this form the wavelength selective devices are preferably formed by coupled optical waveguides with wavelength selective optical gratings within or adjacent to the coupled region. The wavelength selective devices can, for example, include a grating frustrated coupler or a Mach-Zehnder interferometer. In one preferred form of the invention the waveguides are formed by optical fibres. The wavelength selective devices preferably include a fourth port and input light of the other wavelengths at the third port and input light in the selected band at the second port are routed to the fourth port. Preferably, the fourth port is optically terminated.

The optical device can include further wavelength selective means to either reflect light of the other wavelengths routed for output to the third port as input to the third port or reflect light in the selected wavelength band routed for output to the second port as input to the second port. Preferably, the further wavelength selective means both reflect light of the other wavelengths routed for output to the third port as input to the third port and reflect light in the selected wavelength band routed for output to the second port as input to the second port. The further wavelength selective means preferably includes one or more wavelength selective gratings in the waveguides.

The wavelength selective device can also include a circulator with three optically connected waveguides forming the first second and third ports. In this arrangement the wavelength selective means is preferably formed by a wavelength selective grating in the waveguide forming the second port. In this form of the invention the wavelength selective devices are preferably arranged such that input light of the other wavelengths at the third port is not routed for output at the first or second ports and including further wavelength selective means in the waveguides to reflect light of the other wavelengths routed for output to the third port as input to the third port.

Preferably, the wavelength selective means of said second selective device route to both the corresponding second and third ports light respectively in first and second output wavelength bands each narrower than the second intermediate band and the wavelength selective means of the third selective device route to both the corresponding second and third ports light respectively in third and fourth output wavelength bands each narrower than the first intermediate band. Each of the first, second third and fourth output bands are preferably broader than said discrete bands. In the preferred form of the invention the first and second intermediate wavelength bands are of substantially equal width. More preferably, the first and second output bands are of substantially equal width and the third and fourth output bands are of substantially equal width.

In a preferred form of the invention the selected wavelength band of each of the selective devices is shorter in wavelength than wavelength band included in the other wavelengths routed by the same selective device.

Preferably, the output of a last stage of the selective devices provides the discrete wavelength bands, the selected wavelength band of each of the selective devices not in the last stage is shorter in wavelength than a wavelength band included in the other wavelengths routed by the same selective device and wherein the selected wavelength band of each of the selective devices in the last stage is longer than the wavelength band included in the other wavelengths routed by the same selective device. Filters can preferably be associated with the second port of one or more of the selective devices to inhibit output of light of selected out of band wavelengths.

Preferably, the optical device comprises further wavelength selective devices respectively optically connected to the second and third ports of the second and third wavelength selective devices to further subdivide the wavelength bands. The wavelength selective means of the subsequent wavelength selective devices transmit light of a narrower wavelength band than the wavelength selective means of the preceding wavelength selective device.

It will be apparent by the use of the above described tree configuration of wavelength selective devices an input light signal of mixed wavelengths can be divided into n channels of discrete wavelength bands by the use of n-1 wavelength selective devices.

In a third aspect this invention provides an optical device for combining light of a first wavelength band with light of a second wavelength band into an output light having a mix of wavelengths of said first wavelength band and said second wavelength band said device including two optical wavelength selective devices of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides in which wavelength selective means in the waveguide(s) route input light of a selected wavelength band at said third port to said first port and route input light of other wavelengths at said second port to said first port, a first of said selective devices having the second port optically connected by waveguide to the first port of a second of said devices or the third port optically connected by waveguide to the first port of said second device, wherein said wavelength selective means of said first device route input light to third port for output at the corresponding first port in a first wavelength band significantly broader than a second wavelength band of light routed for output to the corresponding first port from the corresponding third port by the wavelength selective means of said second selective device.

Preferably, the first wavelength band is at least 10% to 20% broader than the second wavelength band. The other wavelengths preferably include a third wavelength band having a width substantially equal to the selected wavelength band.

In a fourth aspect this invention provides an optical device for combining a plurality of discrete wavelength bands of input light into an output light having a mix of wavelengths of said first wavelength band and said second wavelength band said device including at least three optical waveguide wavelength selective devices of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides, in which said wavelength selective means route input light of a selected wavelength band at said third port to said first port and route input light of other wavelengths at said second port to said first port, a first of said selective devices having the second port optically connected by waveguide to the first port of a second of said devices and the third port optically connected by waveguide to the first port of a third of said devices, wherein said wavelength selective means of said first device routed from the corresponding third port to the first port light in a first intermediate wavelength band broader than any of said discrete bands and routed from the corresponding second port to the first port light having a wavelength outside said first intermediate band including a second intermediate band broader than any of said discrete bands, said wavelength selective means of said second selective device route from at least one of the corresponding second or third ports to the first port light in a wavelength band narrower than said second intermediate wavelength band and said wavelength selective means of said third selective device route from at least one of the corresponding second or third port to the first port light in a wavelength band narrower than said first intermediate wavelength band.

Preferably, the wavelength selective means of the second selective device route from both the corresponding second and third ports to the first port light respectively in first and second input wavelength bands each narrower than the second intermediate band and the wavelength selective means of the third selective device route from both the corresponding second and third ports to the first port light respectively in third and fourth input wavelength bands each narrower than the first intermediate band. Each of said first, second third and fourth input bands are preferably broader than the discrete bands. In a preferred form of the invention the first and second intermediate wavelength bands are of substantially equal width. It is also preferred that the first and second input bands are of substantially equal width and the third and fourth input bands are of substantially equal width.

The device preferably includes further wavelength selective devices respectively optically connected by waveguide to the second and third ports of both the second and third wavelength selective devices. In this way n of said discrete wavelength bands can be combined using n-1 wavelength selective devices.

Preferably, each wavelength selective device inputs at the second and third ports light in a wavelength band substantially equal in width and the discrete wavelength bands are of substantially equal width.

In the preferred form of the invention, the first, second and third ports of the wavelength selective devices are optically interconnected by one or more waveguides. In this form of the invention, the wavelength selective devices are formed by coupled optical waveguides with wavelength selective optical gratings within or adjacent to the coupled region.

The wavelength selective devices can include a grating frustrated coupler or a Mach-Zehnder interferometer.

In one form of the invention the waveguides are formed by optical fibres. In other forms of the invention the waveguides can be formed in planar optical devices, for example.

The wavelength selective devices can in one form of the invention include a circulator with three optically connected waveguides forming the first second and third ports. In this form of the invention the wavelength selective means is formed by a wavelength selective grating in the waveguide forming the second port. The wavelength selective device is preferably an interferometer arrangement formed by at least two optical waveguides optically coupled together at spaced apart locations to form the arms of the interferometer. The wavelength selective means preferably comprises a Bragg grating formed in each of the arms of the interferometer. The optical couplers between the waveguides and the path lengths of the arms of the interferometer are arranged to provide the desired output, or absence of output, at the various ports formed by the waveguides extending from the optical couplers away from the interferometer formed between the couplers.

In the case of two optical waveguides, for example, the optical couplers have 3dB coupling ratios so that light is divided equally between the two arms of the interferometer. A Bragg grating is formed in each of the arms and provides for reflection of light of a selected wavelength band. In the preferred form of the invention the waveguides are formed by optical fibres and the coupling between the fibres is achieved using a fused biconic taper. In other forms of the invention 3x3 or 4x4 optical fibre couplers of known type can be used to provide multiple outputs of the reflected band widths for input to corresponding multiple wavelength selective devices.

In a fifth aspect this invention provides a wavelength selective optical device of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides and in which wavelength selective means in said waveguide(s) route input light of selected wavelength band at said first port for output at said third port and route input light of other wavelengths for output at said second port, said device being arranged such that input light of said other wavelengths at said third port is not routed for output at said first or second ports and including further wavelength selective means in said waveguide(s) to reflect light of said other wavelengths routed for output to said third port as input to said third port.

In a sixth aspect this invention provides a wavelength selective optical waveguide device of the kind having a first port, a second port and a third port optically interconnected by one or more waveguides and in which wavelength selective means in said waveguide(s) route input light of selected wavelength band at said first port for output at said third port and route input light of other wavelengths for output at said second port, said device being arranged such that input light of said other wavelengths at said third port is not routed for output at said first or second ports and input light in said selected wavelength band at second port is not routed for output at said first or third ports and including further wavelength selective means in said waveguide(s) to either reflect light of said other wavelengths routed for output to said third port as input to said third port or reflect light in said selected wavelength band routed for output to said second port as input to said second port.

Preferably, the further wavelength selective means in the waveguide(s) both reflect light of the other wavelengths routed for output to the third port as input to the third port and reflect light in the selected wavelength band routed for output to the second port as input to the second port.

In accordance with a further aspect this invention also provides a method for dividing input light having a mix of wavelengths into a plurality of discrete output wavelength bands said method including the steps of dividing said input light into light in a first intermediate wavelength band broader than any of said discrete bands and light having a wavelength range outside said first intermediate band including a second intermediate band broader than any of said discrete bands, and subsequently dividing light in said first intermediate band into light in a first output wavelength band narrower than said first intermediate wavelength band and light of a wavelength outside said first output wavelength band, and dividing light in said second intermediate band into at least light in a second output wavelength band narrower than said second intermediate wavelength band and light outside said second output wavelength band.

Preferably, light in the first intermediate band is divided into first and third output wavelength bands each narrower than the first intermediate wavelength band and light in the second intermediate band is divided into light in second and fourth output wavelength bands each narrower than the second intermediate band. In accordance with yet another aspect this invention also provides a method for combining a plurality of discrete wavelength bands of input light into an output light band having a mix of wavelengths including the steps of combining a first light input including light in a first wavelength band with a second light input including light in a second wavelength band to produce light in a first intermediate wavelength band broader than said discrete wavelength bands and narrower than said output light band, and combining a third light input including light in a third wavelength band with a fourth light input including light in a fourth wavelength band to produce light in a second intermediate wavelength band broader than said discrete wavelength bands and narrower than said output light band and combining light in said first and second intermediate wavelength bands to produce light in an output band broader than either than said first and second intermediate wavelength bands.

Preferably, the first, second, third and fourth wavelength bands are each broader than the discrete bands. In the preferred form of the invention the first and second intermediate wavelength bands are of substantially equal width. It is further preferred that the first and second output wavelength bands are of substantially equal width and third and fourth output wavelength bands are of substantially equal width or that all are of equal width.

It will be apparent that the optical device of this invention provides for the division of light of mixed wavelengths into a number of channels or combination of a number of bandwidth channels with a lesser number of insertion losses than in the prior art devices.

Additionally, if the numbers of channels are provided in powers of two then the outputs or components in the composite all experience the same insertion losses.

The invention is readily scalable to provide for any required number of channels. The discrete channels can be provided in any spacing or width by appropriate selection of the characteristics of each frequency selective device. This has particular application in long links where non linear mixing can be avoided by using unequal channel spacings.

The present invention also provides for greater attenuation of out of band wavelength light because each of the discrete channels undergoes several stages of division or combination rather than a single stage by selective filtering of the prior art.

In some embodiments the present invention also allows the division to be achieved wholly within the optical waveguide and thus avoids the losses and alignment difficulties associated with prior art devices that achieve division using discrete optical elements and free space propagation. An additional advantage of the signals staying within an optical waveguide, and in particular an optical fibre, is that different parts of the dividing tree do not have to be in the same physical location. This can be useful in the layout of typical broadband communications networks such as hybrid fibre coax systems, for example.

The optical waveguide device of this invention also has the advantage of allowing upgrading without replacing existing components. A change to narrower channel bandwidths can be easily accommodated by the addition of a further stage or stages of wavelength selective devices. Similarly, extra channels can be accommodated by providing an additional stage or stages of wavelength selective devices at the head of the tree. This feature is of considerable practical importance in allowing accommodation in changes of telecommunication standards and allowing users to initially install a system readily capable of upgrade to meet increased load.

The invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a 16 channel wavelength dividing device according to this invention;

Figure 2 is a schematic diagram of a wavelength selective device forming part of the invention shown in Figure 1. Figure 3 is a schematic diagram of an alternative wavelength selective device forming part of the invention shown in Figure 1 ;

Figure 4 is a schematic diagram showing a modification to reduce cross talk; and

Figure 5 is a graph of transmission against wavelength illustration showing cross talk between adjacent wavelength channels.

BEST MODE FOR CARRYING OUT THE INVENTION The following description relates specifically to the use of the invention for dividing light having a mix of wavelengths into a number of discrete output wavelength bands. It will be apparent to those skilled in the art that the described devices can equally be used to combine a number of discrete wavelength bands of light.

Figure 1 shows a 16 channel wavelength dividing device 1 according to this invention.

The device 1 comprises 15 wavelength selective devices 2 to 16 interconnected by optical fibres 17. Figure 2 shows one wavelength selective device 2. The device 2 comprises two lengths of standard telecommunications grade optical fibre optically coupled at spaced apart locations 20 and 21 by means of a fused tapered coupler of known type produced by the applicant. The coupling ratio of the couplers is 3dB. Identical Bragg gratings 22 are formed in each of the optical fibres 18,19 between the couplers 20,21. The Bragg gratings are formed using known techniques such as by the selective irradiation of the fibre material using ultraviolet light to change the refractive index of the fibre. This can be effectively achieved using the standard telecommunications grade fibre although in some instances a pretreatment with high pressure hydrogen is used to improve the effectiveness of the UV irradiation. The Bragg gratings can be written into the fibre to provide for reflection of light of selected wavelength. Gratings as fine as 0.02 nm and as coarse as 150 nm have been formed in optical fibres. These gratings readily provide for reflection of light having a wavelength as short as 400 nm to the upper wavelength transmission limit for optical fibres. By using a grating of composite spacing a band of selected wavelengths can be reflected by the Bragg grating. In practice very sharply defined reflection characteristics can be achieved and a discrimination of 27dB over 0.2 nm has been found to be possible.

The device 2 has a first port 23, a second port 25, a third port 26 and a fourth port 24 optically interconnected by the optical fibres 18,19. As described in detail below the couplers 20,21 and Bragg gratings 22 form wavelength selective means which route input light of selected wavelength band at the first port 23 to the third port 26 and route light of other wavelengths at the first port 23 to the second port 25. More particularly, device 2 operates in the known manner of a Mach-Zehnder interferometer. If an input light signal of wavelength mix λ_t to λ₁₆ is launched into fibre 18 at input port 23 it is divided at coupler 20 between fibres 18 and 19 in equal amounts. A phase shift of π/2 exists between the optical signals in fibres 18 and 19. The signal in each of fibres 18 and 19 encounters Bragg gratings 22. In the case of the first wavelength selective device 2 (Figure 1) the reflection characteristics of the grating are set such that light having wavelengths λ₁ to λ₈ are reflected. Light having wavelengths λ₉ to λ₁₆ are transmitted and reach coupler 21. At coupler 21 half of the light from each of fibres 18 and 19 is respectively coupled to the other of the fibres. The coupled light again has a π/2 phase shift. As a consequence the two components of light in fibre 18 are out of phase by π and thus complete cancellation occurs. That is, there is no output at output port 24 of fibre 18. The light components in fibre 19 are however in phase and constructive interference results in output of light having wavelengths λ₉ to λ₁₆ at output port 25 of fibre 19.

The components of light reflected by Bragg gratings 22 return to coupler 20 and are again coupled to the other fibre 18,19 with a π/2 phase shift. As a consequence the light components in fibre 18 are out of phase by π and thus total cancellation occurs to provide no output at port 23. The components in fibre 19 are in phase and provide an output at third port

26 of light having wavelengths λ_t to λ_g.

Conversely, an input of light having selected wavelength bands λ_x to λ₈ at third port 26 will result in an output of light having the same wavelength bands λ_t to λ_g at first port 23 and nil output at second port 25 or fourth port 24. Similarly an input of light having other wavelength bands λ₉ to λ₁₆ at second port 25 will result in an output of light having the same wavelength bands λ₉ to λ₁₆ at first port 23 and nil output at third port 26 and fourth port 24.

Further, input light at the third port 26 outside the selected wavelength bands λj to λ₈ is routed to fourth port 24 with nil output to first port 23 and second port 25. Input in the wavelength bands λ_t to λ₈ at the second port 25 is routed to the fourth port 24 with nil output to first port 23 and third port 26.

It will be apparent that the foregoing description assumes that the path lengths for the light between the gratings and the couplers are equal so that the appropriate phase relationships are maintained. It will also be apparent to those skilled in the art that these path lengths can be adjusted as required for example by modifying the refractive index of the fibre using ultraviolet light to appropriately tune the phase matching.

The device 2 allows a light input to the first port 23 having wavelengths λ, to λ₁₆ to be divided into an intermediate component having wavelengths λ, to λ₈ and another intermediate component having wavelengths λ₉ to λ₁₆. It will be apparent that the component having wavelengths λ₉ to λ₁₆ also includes any out of band wavelengths since they will not be reflected by the Bragg grating.

Referring to Figures 1 and 2 the output at port 26 of selective device 2 becomes the input to selective device 10. Similarly the output at port 25 of selective device 2 becomes the input to port 23 of selective device 3. Selective device 3 again divides the light into components having wavelengths λ₉ to λ₁₂ and λ ₁₃ to λ ₁₆ (plus any of out of band signal). Similarly, wavelength selective device 10 divides wavelengths λ_t to λ₈ into components of λ_t to λ₄ and λ₅ to λ₈. This process is repeated in each of the remaining wavelength selective devices to divide the input signal into discrete wavelength bands λ, to λ₁₆ as shown.

Any out of band light can be dealt with by introducing another wavelength selective device (not shown) prior to wavelength selective device 2. This device is configured to provide a reflected output of wavelengths λ_x to λ₁₆ which is provided as the input to wavelength selective device 2. The non reflected (or passed) component from the wavelength selective device contains only out of band wavelengths and would not be used. Alternatively in the arrangement shown in Figure 1 only the output λ₁₆ of selective device 5 contains out of band light. Thus another filtering device (not shown) can be added to separate light of wavelength λ₁₆ from out of band light. To further improve crosstalk filters can be added to all the remaining even numbered wavelength outputs.

Figure 3 shows an alternative wavelength selective device 30 for use in this invention.

The wavelength selective device 30 comprises a circulator 31 of known type optically connected to optical fibres 32, 33, 34. Optical fibres 32, 33, 34 respectively form first, second and third ports of the device 30. Optical fibre 33 has a Bragg grating 35 written into the fibre in the manner described above. The Bragg grating is written to reflect light in a wavelength band λ_a. In this way the selective device operates to route input light containing wavelengths in the band λ_a+b to the other two ports 33 and 34. The circulator 31 outputs to the optical fibre 33 the input to optical fibre 32. The grating 35 reflects the selected wavelength band λ_a as input to the second port 33 and hence into the circulator 31. Other wavelengths λ_b are transmitted by the grating to form the output of other wavelengths at second port 33. The circulator 31 outputs light in the wavelength band λ_a at the third port 24. It will be apparent that in this way the selective device 30 can be used to perform the same division as the selective device 2 described above.

A further filter can also be used in the output of the selective devices to reflect the unwanted wavelength component and reduce crosstalk. The further filter is usually an additional Bragg grating written into the optical fibre to reflect the unwanted component. Where the selective devices are Mach-Zehnder interferometers the wavelengths reflected by the extra grating are directed to the fourth port which can be optically terminated. This avoids the resonance condition caused by having more than one grating of the same wavelength in the same optical path and which results in increased noise. If a circulator of the kind described in relation to Figure 3 is used as the selective device a further filter can only be used on the third port since an additional filter on the second port would lead to an optical resonance condition. In the case of a circulator the reflected unwanted wavelengths 5 are input at the third port and consequently are not output at the first and second ports. In practice the input to the third port is lost as radiation from the circulator.

The extra filters usually in the form of gratings provide exceptionally high cross talk attenuation for the adjacent bands.

10

Figure 4 shows an arrangement used to remove crosstalk incorporating the above technique. Two wavelength selective devices 41,42 of the optical fibre interferometer type described above are arranged at the last stage of the tree structure shown in Figure 1. That is, selective device 41 corresponds to each of devices 5, 6, 8, 9, 12, 13, 15, and 16. The first port 23 of

15 device 41 is connected to either output of a preceding device and receives as input a signal containing two adjacent discrete wavelength bands λ_a ,λ_b . Device 41 has gratings selected to reflect the shorter wavelength band λ_a which is output at third port 26 as previously described. A further filter in the form of a Bragg grating 43 is provided in the output. The grating 43 is selected to reflect wavelengths in the band λ_b . Any signal in the λ_b band

20 reflected as input to port 26 is output to fourth port 24 which is optically terminated. This removes any crosstalk from the λ_b band appearing at the λ_a output. Only the narrow band grating 43 is required because the other wavelength bands have been removed by preceding selective devices. The longer wavelength band λ_b is routed through device 41 and becomes output at second port 25. The λ_b band signal is input to first port 23 of device 42. Device

25 42 has gratings selected to reflect wavelengths in the λ_b band which is output at third port 26. The second and fourth ports 25,24 of the device 42 are optically terminated.

Figure 5 graphically illustrates what happens at the last separation formed by any one of devices 5, 6, 8, 9 12, 13, 15 or 16 of Figure 1. The device described in relation to Figure

30 1 is constructed by using the Mach-Zehnder interferometers in each of the selective devices to reflect the shorter wavelength band and transmit the longer wavelength band. In Figure 4 the trace marked "blue" is the transfer function versus wavelength from the input port 23 of selective device 2 to the output port 26 of any one of the selective devices 5,6,8,9,12,13,15,16 in the last stage of the device 1 shown in Figure 1. This corresponds to the odd numbered discrete wavelengths λ_t> λ₃ ... . The trace marked "red" is the transfer function versus wavelength from the input port 23 of selective device 2 to the output port 25 of any one of the selective devices 5,6,8,9,12,13,15,16 in die last stage of the device 1 shown in Figure 1 including the extra filter 42 but without the grating 43 shown in Figure 4. This corresponds to the even numbered discrete wavelengths λ₂ λ₄... but not including λ₁₆ which includes any out of band component. The only significant cross talk is represented by the part of the blue trace between 1548 and 1549 nm and gives a relative amount of power in this wavelength range that exits port 26 of device 41. The grating 43 removes this crosstalk. The other cross talk term, the relative amount of power in the 1547 to 1548 nm range that exits port 25 is much lower because it combines the effects of the selectivity of the red filter with the fact that most of the input power in the blue or shorter wavelength range has been removed by the preceding blue or shorter wavelength range filter.

In order to improve the cross talk, the filtering can be reversed at the last stage. That is, the longer wavelength component becomes the selected component that is reflected and the shorter wavelength component is transmitted. This leaves the residual cross talk term being a side lobe on the short wavelength side of the red filter. These tend to be lower than the side lobe levels on the long wavelength side. It is possible to change the order of the wavelength filtering at the last stage because the cladding mode effects that increase the insertion loss of gratings on the short wavelength side are far enough away in wavelength.

The foregoing describes only one embodiment of this invention and modifications can be made without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. An optical device for dividing input light having a mix of wavelengths into at least two output wavelength bands, said device including two optical wavelength selective devices of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides, in which wavelength selective means in the waveguide(s) route input light of selected wavelength band at said first port to said third port and route input light of other wavelengths at said first port to said second port, a first of said selective devices having the third port optically connected by waveguide to the first port of a second of said devices or the second port optically connected by waveguide to the first port of said second device, wherein said wavelength selective means of said first device route to the corresponding second or third port light in a first wavelength band significantly broader than a second wavelength band of light routed to the corresponding second or third port by the wavelength selective means of the second selective device.

2. An optical device as claimed in claim 1 wherein said first wavelength band is at least 10% to 20% broader than the second wavelength band.

3. An optical device as claimed in claim 1 or claim 2 wherein said other wavelengths include a third wavelength band having a width substantially equal to said selected wavelength band.

4. An optical device as claimed in any one of claims 1 to 3 wherein the first, second and third ports of said wavelength selective devices are optically interconnected by one or more waveguides.

5. An optical device as claimed in claim 4 wherein said wavelength selective devices are formed by coupled optical waveguides with wavelength selective optical gratings within or adjacent to the coupled region.

6. An optical device as claimed in claim 5 wherein said wavelength selective devices include a grating frustrated coupler.

7. An optical device as claimed in claim 5 wherein said wavelength selective devices include a Mach-Zehnder interferometer.

5

8. An optical device as claimed any one of claims 4 to 7 wherein said waveguides are formed by optical fibres.

9. An optical device as claimed in any one of claims 4 to 8 wherein said wavelength 10 selective devices include a fourth port and wherein input light of said other wavelengths at said third port and input light in said selected band at said second port are routed to said fourth port.

10. An optical device as claimed in claim 9 wherein said fourth port is optically 15 terminated.

11. An optical device as claimed in claim 9 or claim 10 including further wavelength selective means to either reflect light of said other wavelengths routed for output to said third port as input to said third port or reflect light in said selected wavelength band routed for 0 output to said second port as input to said second port.

12. An optical device as claimed in claim 11 wherein said further wavelength selective means reflect light of said other wavelengths routed for output to said third port as input to said third port and reflect light in said selected wavelength band routed for output to said

25 second port as input as input to said second port.

13. An optical device as claimed in claim 11 or claim 12 wherein said further wavelength selective means includes one or more wavelength selective gratings in the waveguides.

30 14. An optical device as claimed in any one of claims 1 to 3 wherein said wavelength selective devices are arranged such that input light of said other wavelengths at said third port is not routed for output at said first or second ports and including further wavelength selective means in said waveguides to reflect light of said other wavelengths routed for output to said third port as input to said third port. 5

15. An optical device as claimed in any one of claims 1 to 3 or 14 wherein said wavelength selective device includes a circulator with three optically connected waveguides forming said first second and third ports.

10 16. An optical device as claimed in claim 15 wherein said wavelength selective means is formed by a wavelength selective grating in the waveguide forming said second port.

17. An optical device for dividing input light having a mix of wavelengths into a plurality of discrete output wavelength bands said device including at least three optical wavelength

15 selective devices of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides, in which wavelength selective means in the waveguide(s) route input light of selected wavelength band at said first port to said third port and route input light of other wavelengths at said first port to said second port, a first of said selective devices having the second port optically connected by waveguide to

20 the first port of a second of said devices and the third port optically connected by waveguide to the first port of a third of said devices, wherein said wavelength selective means of said first device route to the corresponding third port light in a first intermediate wavelength band broader than any of said discrete bands and route to the corresponding second port light having a wavelength outside said first intermediate band including a second intermediate band

25 broader than any of said discrete bands, said wavelength selective means of said second selective device route to at least one of the corresponding second port or third ports light in a wavelength band narrower than said second intermediate wavelength band and said wavelength selective means of said third selective device route to at least one of the corresponding second port or third port light in a wavelength band narrower than said first

30 intermediate wavelength band.

18. An optical device as claimed in claim 17 wherein said wavelength selective means of said second selective device route to both the corresponding second and third ports light respectively in first and second output wavelength bands each narrower than said second intermediate band and said wavelength selective means of the third selective device route to

5 both the corresponding second and third ports light respectively in third and fourth output wavelength bands each narrower than said first intermediate band.

19. An optical device as claimed in claim 18 wherein each of said first, second third and fourth output bands are broader than said discrete bands.

10

20. An optical device as claimed in claim 18 wherein said first and second intermediate wavelength bands are of substantially equal width.

21. An optical device as claimed in claim 20 wherein said first and second output bands 15 are of substantially equal width and said third and fourth output bands are of substantially equal width.

22. An optical device as claimed in any one of claims 17 to 21 including further wavelength selective devices respectively optically connected by waveguide to the second and

20 third ports of both the second and third wavelength selective devices.

23. An optical device as claimed in any one of claims 17 to 22 including n-1 wavelength selective devices arranged to divide said input light into n of said discrete wavelength bands.

25 24. An optical device as claimed in claim 23 wherein each said wavelength selective device outputs at said second and third ports light in a wavelength band substantially equal in width and said n discrete wavelength bands are of substantially equal width.

25. An optical device as claimed in any one of claims 17 to 24 wherein the first, second 30 and third ports of said wavelength selective devices are optically interconnected by one or more waveguides.

26. An optical device as claimed in claim 25 wherein said wavelength selective devices are formed by coupled optical waveguides with wavelength selective optical gratings within

5 or adjacent to the coupled region.

27. An optical device as claimed in claim 26 wherein said wavelength selective devices include a grating frustrated coupler.

10 28. An optical device as claimed in claim 26 wherein said wavelength selective devices include a Mach-Zehnder interferometer.

29. An optical device as claimed in any one of claims 25 to 28 wherein said waveguides are formed by optical fibres.

15

30. An optical device as claimed in any one of claims 25 to 29 wherein said wavelength selective devices include a fourth port and wherein input light of said other wavelengths at said third port and input light in said selected band at said second port are routed to said fourth port.

20

31. An optical device as claimed in claim 3.0 wherein said fourth port is optically terminated.

32. An optical device as claimed in claim 30 or claim 31 including further wavelength 25 selective means to either reflect light of said other wavelengths routed for output to said third port as input to said third port or reflect light in said selected wavelength band routed for output to said second port as input to said second port.

33. An optical device as claimed in claim 32 wherein said further wavelength selective 30 means reflect light of said other wavelengths routed for output to said third port as input to said third port and reflect light in said selected wavelength band routed for output to said second port as input as input to said second port.

34. An optical device as claimed in claim 32 or claim 33 wherein said further wavelength 5 selective means includes one or more wavelength selective gratings in the waveguides.

35. An optical device as claimed in any one of claims 17 to 24 wherein said wavelength selective devices are arranged such that input light of said other wavelengths at said third port is not routed for output at said first or second ports and including further wavelength selective

10 means in said waveguides to reflect light of said other wavelengths routed for output to said third port as input to said third port.

36. An optical device as claimed in any one of claims 17 to 24 wherein said wavelength selective device includes a circulator with three optically connected waveguides forming said

15 first second and third ports.

37. An optical device as claimed in claim 36 wherein said wavelength selective means is formed by a wavelength selective grating in the waveguide forming said second port.

20 38. An optical device as claimed in any one of claims 17 to 37 wherein said selected wavelength band of each said selective device is shorter in wavelength than a wavelength band included in said other wavelengths routed by the same selective device.

39. An optical device as claimed in any one of claims 17 to 37 wherein the outputs of a 25 last stage of said selective devices provides said discrete wavelength bands, said selected wavelength band of each said selective device not in the last stage is shorter in wavelength than a wavelength band included in said other wavelengths routed by the same selective device and wherein said selected wavelength band of each said selective device in said last stage is longer than the wavelength band included in said other wavelengths routed by the 30 same selective device.

40. An optical waveguide device as claimed in any one of claims 17 to 39 further including filter means associated with the second port of one or more of said selective devices to inhibit output of light of selected out of band wavelengths.

5 41. An optical device for combining light of a first wavelength band with light of a second wavelength band into an output light having a mix of wavelengths of said first wavelength band and said second wavelength band said device including two optical wavelength selective devices of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides in which wavelength selective means in the

10 waveguide(s) route input light of a selected wavelength band at said third port to said first port and route input light of other wavelengths at said second port to said first port, a first of said selective devices having the second port optically connected by waveguide to the first port of a second of said devices or the third port optically connected by waveguide to the first port of said second device, wherein said wavelength selective means of said first device route

15 input light to the third port for output at the corresponding first port in a first wavelength band significantly broader than a second wavelength band of light routed for output to the corresponding first port from the corresponding third port by the wavelength selective means of said second selective device.

20 42. An optical device as claimed in claim 41 wherein said first wavelength band is at least 10% to 20% broader than the second wavelength band.

43. An optical device as claimed in claim 41 or claim 42 wherein said other wavelengths include a third wavelength band having a width substantially equal to said selected wavelength

25 band.

44. An optical device as claimed in any one of claims 41 to 43 wherein the first, second and third ports of said wavelength selective devices are optically interconnected by one or more waveguides.

30

45. An optical device as claimed in claim 44 wherein said wavelength selective devices are formed by coupled optical waveguides with wavelength selective optical gratings within or adjacent to the coupled region.

46. An optical device as claimed in claim 44 wherein said wavelength selective devices include a grating frustrated coupler.

47. An optical device as claimed in claim 45 wherein said wavelength selective devices include a Mach-Zehnder interferometer.

48. An optical device as claimed in any one of claims 45 to 47 wherein said waveguides are formed by optical fibres.

49. An optical device as claimed in any one of claims 44 to 48 wherein said wavelength selective devices include a fourth port and wherein input light of said other wavelengths at said third port and input light in said selected band at said second port are routed to said fourth port.

50. An optical device as claimed in claim 49 wherein said fourth port is optically terminated.

51. An optical device as claimed in claim 49 or claim 50 including further wavelength selective means to either reflect light of said other wavelengths routed for output to said third port as input to said third port or reflect light in said selected wavelength band routed for output to said second port as input to said second port.

52. An optical device as claimed in claim 51 wherein said further wavelength selective means reflect light of said other wavelengths routed for output to said third port as input to said third port and reflect light in said selected wavelength band routed for output to said second port as input as input to said second port.

53. An optical device as claimed in claim 51 or claim 52 wherein said further wavelength selective means includes one or more wavelength selective gratings in the waveguides.

54. An optical device as claimed in any one of claims 41 to 43 wherein said wavelength 5 selective devices are arranged such that input light of said other wavelengths at said third port is not routed for output at said first or second ports and including further wavelength selective means in said waveguides to reflect light of said other wavelengths routed for output to said third port as input to said third port.

10 55. An optical device as claimed in any one of claims 41 to 43 or 54 wherein said wavelength selective device includes a circulator with three optically connected waveguides forming said first second and third ports.

56. An optical device as claimed in claim 55 wherein said wavelength selective means is 15 formed by a wavelength selective grating in the waveguide forming said second port.

57. An optical device for combining a plurality of discrete wavelength bands of input light into an output light having a mix of wavelengths device including at least three optical wavelength selective devices of the kind having a first port, a second port and a third port

20 optically interconnected at least partly by one or more waveguides, in which said wavelength selective means route input light of a selected wavelength band at said third port to said first port and route input light of other wavelengths at said second port to said first port, a first of said selective devices having the second port optically connected by waveguide to the first port of a second of said devices and the third port optically connected by waveguide to the

25 first port of a third of said devices, wherein said wavelength selective means of said first device route from the corresponding third port to the first port light in a first intermediate wavelength band broader than any of said discrete bands and route from the corresponding second port to the first port light having a wavelength outside said first intermediate band including a second intermediate band broader than any of said discrete bands, said wavelength

30 selective means of said second selective device route from at least one of the corresponding second or third ports to the first port light in a wavelength band narrower than said second intermediate wavelength band and said wavelength selective means of said third selective device route from at least one of the corresponding second or third ports to the first port light in a wavelength band narrower than said first intermediate wavelength band. 5

58. An optical device as claimed in claim 57 wherein said wavelength selective means of said second selective device route from both the corresponding second and third ports to the first port light respectively in first and second input wavelength bands each narrower than said second intermediate band and said wavelength selective means of the third selective device

10 route from both the corresponding second and third ports to the first port light respectively in third and fourth input wavelength bands each narrower than said first intermediate band.

59. An optical device as claimed in claim 58 wherein each of said first, second third and fourth input bands are broader than said discrete bands.

15

60. An optical device as claimed in claim 58 wherein said first and second intermediate wavelength bands are of substantially equal width.

61. An optical device as claimed in claim 59 wherein said first and second input bands are 0 of substantially equal width and said third and fourth input bands are of substantially equal width.

62. An optical device as claimed in any one of claims 57 to 61 including further wavelength selective devices respectively optically connected by waveguide to the second and

25 third ports of both the second and third wavelength selective devices.

63. An optical device as claimed in any one of claims 57 to 62 including n-1 wavelength selective devices arranged to combine n of said discrete wavelength bands.

30 64. An optical device as claimed in claim 63 wherein each said wavelength selective device inputs at said second and third ports light in a wavelength band substantially equal in width and said n discrete wavelength bands are of substantially equal width.

65. An optical device as claimed in any one of claims 57 to 64 wherein the first, second 5 and third ports of said wavelength selective devices are optically interconnected by one or more waveguides.

66. An optical device as claimed in claim 65 wherein said wavelength selective devices are formed by coupled optical waveguides with wavelength selective optical gratings within

10 or adjacent to the coupled region.

67. An optical device as claimed in claim 66 wherein said wavelength selective devices include a grating frustrated coupler.

15 68. An optical device as claimed in claim 66 wherein said wavelength selective devices include a Mach-Zehnder interferometer.

69. An optical device as claimed in any one of claims 65 to 68 wherein said waveguides are formed by optical fibres.

20

70. An optical device as claimed in any one of claims 57 to 60 wherein said wavelength selective device includes a circulator with three optically connected waveguides forming said first second and third ports.

25 71. An optical device as claimed in claim 70 wherein said wavelength selective means is formed by a wavelength selective grating in the waveguide forming said second port.

72. A wavelength selective optical device of the kind having a first port, a second port and a third port optically interconnected at least partly by one or more waveguides and in which

30 wavelength selective means in said waveguide(s) route input light of selected wavelength band at said first port for output at said third port and route input light of other wavelengths for output at said second port, said device being arranged such that input light of said other wavelengths at said third port is not routed for output at said first or second ports and including further wavelength selective means in said waveguide(s) to reflect light of said other 5 wavelengths routed for output to said third port as input to said third port.

73. A wavelength selective optical waveguide device of the kind having a first port, a second port and a third port optically interconnected by one or more waveguides and in which wavelength selective means in said waveguide(s) route input light of selected wavelength band

10 at said first port for output at said third port and route input light of other wavelengths for output at said second port, said device being arranged such that input light of said other wavelengths at said third port is not routed for output at said first or second ports and input light in said selected wavelength band at second port is not routed for output at said first or third ports and including further wavelength selective means in said waveguide(s) to either

15 reflect light of said other wavelengths routed for output to said third port as input to said third port or reflect light in said selected wavelength band routed for output to said second port as input to said second port.

74. A method for dividing input light having a mix of wavelengths into a plurality of 0 discrete output wavelength bands said method including the steps of dividing said input light into light in a first intermediate wavelength band broader than any of said discrete bands and light having a wavelength outside said first intermediate band including a second intermediate band broader than any of said discrete bands, and subsequently dividing light in said first intermediate band into light in a first output wavelength band narrower than said first 25 intermediate wavelength band and light of a wavelength outside said first output wavelength band, and dividing light in said second intermediate band into at least light in a second output wavelength band narrower than said second intermediate wavelength band and light outside said second output wavelength band.

30 75. A method as claimed in claim 74 wherein light in the first intermediate band is divided into first and third output wavelength bands each narrower than said first intermediate wavelength band and light in said second intermediate band is divided into light in second and fourth output wavelength bands each narrower than said second intermediate band.

5 76. A method as claimed in claim 75 wherein each of said first, second third and fourth output wavelength band are broader than said discrete bands.

77. A method as claimed in claim 74 wherein said first and second intermediate wavelength bands are of substantially equal width.

10

78. A method as claimed in claim 75 wherein said first and third output wavelength bands are of substantially equal width and said second and fourth output wavelength bands are of substantially equal width.

15 79. A method as claimed in any one of claims 74 to 78 wherein said first, second, third and fourth wavelength bands are each of substantially equal width.

80. A method for combining a plurality of discrete wavelength bands of input light into an output light band having a mix of wavelengths including the steps of combining a first light

20 input including light in a first wavelength band with a second light input including light in a second wavelength band to produce light in a first intermediate wavelength band broader than said discrete wavelength bands and narrower than said output light band, and combining a third light input including light in a third wavelength band with a fourth light input including light in a fourth wavelength band to produce light in a second intermediate wavelength band

25 broader than said discrete wavelength bands and narrower than said output light band and combining light in said first and second intermediate wavelength bands to produce light in an output band broader than either than said first and second intermediate wavelength bands.

81. A method as claimed in claim 80 wherein said first, second, third and fourth 30 wavelength bands are each broader than said discrete bands.

82. A method as claimed in claim 80 or claim 81 wherein said first and second intermediate wavelength bands are of substantially equal width.

83. A method as claimed in claim 80 wherein said first and second wavelength bands are of substantially equal width and said third and fourth wavelength bands are of substantially equal width.

84. A method as claimed in any one of claims 80 to 83 wherein said first, second, third and fourth wavelength bands are each of substantially equal width.