CA2133849C - Method for making stable optical devices employing radiation-induced index changes - Google Patents
Method for making stable optical devices employing radiation-induced index changesInfo
- Publication number
- CA2133849C CA2133849C CA002133849A CA2133849A CA2133849C CA 2133849 C CA2133849 C CA 2133849C CA 002133849 A CA002133849 A CA 002133849A CA 2133849 A CA2133849 A CA 2133849A CA 2133849 C CA2133849 C CA 2133849C
- Authority
- CA
- Canada
- Prior art keywords
- decay
- index changes
- induced
- glass
- temperature
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
- G02B6/124—Geodesic lenses or integrated gratings
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/02171—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes
- G02B6/02176—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes due to temperature fluctuations
- G02B6/02185—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes due to temperature fluctuations based on treating the fibre, e.g. post-manufacture treatment, thermal aging, annealing
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/13—Integrated optical circuits characterised by the manufacturing method
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B2006/12166—Manufacturing methods
- G02B2006/12169—Annealing
Abstract
The present invention is predicated upon the discovery by applicants of a relationship describing thermal decay of radiation-induced index changes and a mechanism which permits stabilization by accelerated aging, Specifically, the induced index change decays in proportion to 1/(1 + At .alpha.) where A and .alpha. are functions of temperature, and the decay can be accelerated by heat treatment.
As a consequence, the extent of decay can be determined for arbitrary time and temperature and, significantly, an appropriate heat treatment can be scheduled for making a device stable within predeterminable limits.
As a consequence, the extent of decay can be determined for arbitrary time and temperature and, significantly, an appropriate heat treatment can be scheduled for making a device stable within predeterminable limits.
Description
21334-~
METHOD FOR MAKING STABLE OPTICAL DEVICES
EMPLOYING RADIATION-INDUCED INDEX CHANGES
Field of the Invention This invention relates to optical devices and, in particular, to optical devices employing radiation induced changes in the index of refraction of a glass medium.
Background of the Invention The application of high intensity radiation to certain glasses, such as germanium doped silicate, gamma irradiated silicate, and europium doped glass, is known to produce a change in the index of refraction in the irradiated region.
This phenomenon has application to a wide variety of optical devices. For example, ' interfering beams of ultraviolet light have been used to induce Bragg phase gratings in optical fibers, and such uv-induced gratings have been employed in fiber lasers, optical filters, polarization converters, mode converters and fiber sensors.
Another potential application is fabrication of optical memories.
An impediment to the deployment of devices containing radiation-induced index changes is that not much is known concerning their long term reliability. It is known that the induced index changes diminish with time.
But not enough is known to predict the decay at arbitrary time or temperature. Some applications, such as fiber lasers for undersea cable, require high tolerances over long periods of time. Some applications also require operation at elevated temperatures, posing the additional threat of environmental stress. Accordingly, there is a need for making such devices of assured reliability.
Summary of the Invention The present invention is predicated upon the discovery by applicants of a relationship describing thermal decay of radiation-induced index changes and a mechanism which permits stabilization by accelerated aging. Empirically, the induced index change decays in proportion to 1 / ( 1 + At°' ) where A
and a are functions of temperature, and the decay can be accelerated by heat treatment.
As a consequence, the extent of decay can be determined for arbitrary time and temperature and, significantly, an appropriate heat treatment can be scheduled for making a device stable within predeterminable limits.
~~ Q38 4g - la -In accordance with one aspect of the present invention there is provided a method for making an optical device employing radiation induced refractive index changes, said induced changes subject to decay with time and described for an operating temperature by a characteristic decay curve, said device designed to S operate at a maximum operating temperature ToP for an operating period toP
with decay of the induced index changes less than BnoP over the period of operation, comprising the steps of: providing a body of glass; exposing said glass to radiation to induce said refractive index changes; determining the characteristic decay curve of said induced index changes in said glass for at least one temperature T>ToP and extrapolating therefrom the characteristic decay curve for the induced index changes in said glass for ToP; determining a point P on said decay curve for the induced index changes in said glass for TaP for which the passage of toP will produce a decay of less than BnoP wherein point P
represents a value of decay over a period of time; and heating said glass at an anneal temperature T~,>ToP for an anneal time t~,<toP to achieve a value of decay equivalent to that value represented by point P on said extrapolated curve.
a n.
~~.~3$~-~
METHOD FOR MAKING STABLE OPTICAL DEVICES
EMPLOYING RADIATION-INDUCED INDEX CHANGES
Field of the Invention This invention relates to optical devices and, in particular, to optical devices employing radiation induced changes in the index of refraction of a glass medium.
Background of the Invention The application of high intensity radiation to certain glasses, such as germanium doped silicate, gamma irradiated silicate, and europium doped glass, is known to produce a change in the index of refraction in the irradiated region.
This phenomenon has application to a wide variety of optical devices. For example, ' interfering beams of ultraviolet light have been used to induce Bragg phase gratings in optical fibers, and such uv-induced gratings have been employed in fiber lasers, optical filters, polarization converters, mode converters and fiber sensors.
Another potential application is fabrication of optical memories.
An impediment to the deployment of devices containing radiation-induced index changes is that not much is known concerning their long term reliability. It is known that the induced index changes diminish with time.
But not enough is known to predict the decay at arbitrary time or temperature. Some applications, such as fiber lasers for undersea cable, require high tolerances over long periods of time. Some applications also require operation at elevated temperatures, posing the additional threat of environmental stress. Accordingly, there is a need for making such devices of assured reliability.
Summary of the Invention The present invention is predicated upon the discovery by applicants of a relationship describing thermal decay of radiation-induced index changes and a mechanism which permits stabilization by accelerated aging. Empirically, the induced index change decays in proportion to 1 / ( 1 + At°' ) where A
and a are functions of temperature, and the decay can be accelerated by heat treatment.
As a consequence, the extent of decay can be determined for arbitrary time and temperature and, significantly, an appropriate heat treatment can be scheduled for making a device stable within predeterminable limits.
~~ Q38 4g - la -In accordance with one aspect of the present invention there is provided a method for making an optical device employing radiation induced refractive index changes, said induced changes subject to decay with time and described for an operating temperature by a characteristic decay curve, said device designed to S operate at a maximum operating temperature ToP for an operating period toP
with decay of the induced index changes less than BnoP over the period of operation, comprising the steps of: providing a body of glass; exposing said glass to radiation to induce said refractive index changes; determining the characteristic decay curve of said induced index changes in said glass for at least one temperature T>ToP and extrapolating therefrom the characteristic decay curve for the induced index changes in said glass for ToP; determining a point P on said decay curve for the induced index changes in said glass for TaP for which the passage of toP will produce a decay of less than BnoP wherein point P
represents a value of decay over a period of time; and heating said glass at an anneal temperature T~,>ToP for an anneal time t~,<toP to achieve a value of decay equivalent to that value represented by point P on said extrapolated curve.
a n.
~~.~3$~-~
Brief Description of the Drawings In the drawings:
FIG. 1 is a block diagram of the steps in making an optical device having stable uv-induced index changes.
- FIG. 2 is a schematic view of an exemplary optical device employing uv-induced index changes comprising a fiber optic laser;
FIG. 3 is a graphical display showing the decay of uv-induced index changes as a function of time; and FIG. 4 plots the time at various temperatures required for aging an exemplary glass.
Detailed Description This description is divided into three parts. Part I qualitatively describes the ' application of the inventive method to making an exemplary device employing uv-induced index changes, specifically a stable optical fiber laser. Part II
describes quantitative aspects of the invention; part III sets forth a theoretical physical model of radiation-induced index changes believed to underlie the invention; and part IV
derives from the physical model a quantitative model for device stabilization consistent with the empirical results of Part II.
I. The Method of the Invention (Qualitative Description) Referring to the drawings, FIG. 1 is a block diagram of the steps involved in making a stable optical device having uv-induced index variations.
As shown in block A of FIG. 1, the first step is to provide a body of uv-sensitive glass such as germanium-doped silicate. Advantageously the glass body can include other dopants as needed for its intended operation. For example, the glass body for making an optical fiber laser can be a conventionally clad optical fiber having a silicate core doped with 14 mole % germanium and 600 ppm erbium, co-doped with aluminum to minimize concentration-quenching effects. After application of overcladding, the fiber can be drawn to a 2.6p.m core diameter corresponding to a cutoff wavelength of 880 nm, all in accordance with techniques well known in the art.
The next step shown in block B of FIG. 1 is to expose a portion of the glass body to ultraviolet light in order to produce localized changes in the index of refraction. The degree of exposure can be spatially modulated by variation of exposure time, use of phase masks or use of interfering beams. For example, one or more Bragg phase gratings can be induced in the body by exposing it to interfering beams of ultraviolet light. High intensity interfering uv laser beams produce variations of the local index of refraction directly related to variations in the local beam intensity. The interfering beams thus produce a pattern of index variations having spacing corresponding to the spacing between interference peaks. These periodic variations can act as a Bragg grating to light incident upon them.
- Test gratings were formed in the core of the previously described glass fiber by interfering two beams of ultraviolet light (242 nm) on the side of the fiber.
The uv light was generated by frequency doubling a dye laser pumped by a XeF
excimer laser producing 15 ns pulses at 30 Hz repetition rate. The interfering beams were focussed onto the fiber to a spot size of 6 mm by SOp.m with a nearly gaussian time-averaged profile. The average power was 15-20 mW, and the fiber was exposed until the gratings reached peak reflectivity of 90% (typical exposure times between 30s and 60s). The peak uv-induced index change was estimated to be 2x 10-4.
The next step shown in block C of FIG. 1 is to heat the exposed body for an anneal time t a~ at an anneal temperature T ~, such that the device will operate for a time top at an operating temperature Top with a loss of only 8nop in the induced index change. Because the decay in the induced index On is proportional to 1 / ( 1 + At°' ) and because the device is amenable to accelerated aging, it is always possible to age the device to a state where the prescribed conditions can be met. This can be seen by reference to FIG. 3 which plots a curve proportional to 1 /( 1 +At°' ).
The experimental data were taken over a range of induced indices from 98% to 35%
of the initial induced grating strength. As can be seen from FIG. 3, the amount of decay per unit time diminishes with increasing time. And because aging can be accelerated by using a temperature Ta" >Top, the required aging can be effected in a time t ~, « t op, i.e., t ~, can be 10% of t op or considerably less. An algorithm for calculating appropriate annealing times and temperatures is set forth in part II, infra, and a physical model explaining the observed behavior is set forth in part III. In essence, the device is heat treated to a point t 1 on the curve of FIG. 3 such that the decay between t 1 and t 1 + t op is less than the required tolerance.
II. Method of the Invention (Quantitative Aspects) Applicants have determined that the uv-induced index change Vin; of a glass body subjected to an operating temperature Top will decay in proportion to ' a where A is a decay prefactor which is a function of Top and a is a decay 1 + At constant which is also a function of T op. Applicants have further realized that with this model of decay, one can now prescribe a course of preoperational heat treatment which will maintain the device within a prescribed variation in index 8nop for a r prescribed period of time top at an operating temperature Top.
The ability to prescribe such treatment can be understood by reference to FIG. 3 which is a schematic graphical illustration of the decay at T op ~of the normalized uv-induced index change rl as a function of time. If t 1 is the time when device operation commences, then t 1 + t op is the time when the device completes its anticipated operating life. If D n op is the induced index change at t 1, then the requirement that the index decay no further than 8nop during its operating life requires that 0n be no less than Onop -8nop at t 1 +top.
While t 1 might be an impractically long period for treating the device, applicants have further discovered that accelerated aging is possible, i.e, that by treating the device at a higher temperature T ~ > T op for a shorter period of time t an « t l , the same effect can be achieved to ensure device stability.
A problem confronting a designer of an optical device employing uv-induced index change can be described as follows: "How do I process a device so that I can use it for a time at least top at a temperature Top? The device requires that the uv-induced index change not decay more than 8nop over top. Specifically:
what are the ranges of anneal times t a~ and anneal temperatures T ~ which will give the desired result".
To obtain these values for a particular glass, one should plot the empirical decay curves for at least two temperatures, e.g., T 1 and T2, and by conventional curve fitting techniques, calculate a 1, a 2 and A 1, A 2 for fitting the observed curves into the form O n/ ( 1 + Ata ). It is advantageous to choose T
1 and T 2 greater than the decay anticipated operating temperature T op so that the parameters A and a can be quickly measured. The corresponding parameters at Top can be readily calculated by the relations:
(i) Aop =Ao eaT°P ,aop = Tip where Ln(A2)-Ln(Al) -aT _ T T
a= ,Ao=Ale '=A2e aT2,andTo= 1 - z Tz _Tl al _ a2 Analytically, t 1 can now be found by solving the equality:
( la a (11) Onop Aop I tl +topJ ~ -Aoptl av _ a~
= 8nop 1+Aop f tl+top, -s-Once t 1 is calculated, accelerated aging conditions can be prescribed to yield a decay equivalent to that of the device at t 1 in FIG. 3. Specifically, we can use any combination of T a" ,t ~, that solves:
T~ T~
(111) ~t~~ T° eaTa° - (t 1 ~ T° eaT°P
s Specifically, we can use an annealing temperature ? Tao for an annealing time >_ ta~.
After the device is subject to heat treatment at Tao for time tan, it is in the same state on the curve of FIG. 3 as if it had been operated at T
°p for a period t 1.
Its future operation at temperature Top for a period t°p will produce only the small residual decay shown on the curve.
Applying these teachings to the specific example of the erbium fiber optic laser with uv-induced gratings, it should be noted that such lasers are contemplated as single mode lasers in undersea cable where economy dictates they should operate without service for many years. With only a 7% decrease in the uv-induced index changes, such a laser could switch from single mode to multiple mode is operation with serious consequences to the cable system.
FIG. 4 plots the time required at various temperatures for aging the glass of the exemplary laser by an amount equivalent to l00 years decay at 300 K.
After such aging, the decrease in index for the next 100 years at 300 K will be less than 1%.
To test these predictions, a fiber laser was annealed 48 hours at 120°C
and then operated single mode at 100°C for a time greater than 1800 hours (equivalent to s0 years at 6s°C). The laser showed no signs of two mode operation or mode hopping during this time.
III. Physical Model of UV -Induced Index Changes 2s While not necessary for the invention, applicants' best current belief is that in uv-sensitive glasses electrons are excited by uv light from photosensitive defects. These excited electrons are then trapped in a continuous distribution of energy states rather than at a single trap level. The detrapping rate from any state in the continuum via thermal excitation is dependent on the temperature of the excitation and the depth of the trap state. Upon thermal release from the traps, electrons repopulate the original deep level occupied prior to uv excitation.
~~ ~~84~~
At a time t after trapping, the distribution of states can be separated into two groups, divided by a demarcation energy Ed. The first group, for which E<Ed, is in thermal equilibrium with the conducting states above E=0. The second group, for which E > E d, is too deep to have been thermally excited by time t.
~ There is a unique demarcation energy Ed associated with each value of induced index change. Removing a certain fraction of the induced index change corresponds to thermally depleting the traps with energies less than E d, while leaving the traps with energies greater than E d unchanged.
By adjusting the temperature, the rate of index change decay can be controlled. Also, the progression of decay at any given temperature becomes much slower as time progresses. Thus accelerated aging involves inducing quick decay by increasing the temperature and then reducing the temperature once the decay process progresses to a point where further decay is sufficiently slow. Thus it is possible to stabilize a device by annealing, wiping out the unstable trap states that would cause decay by a substantial amount over the operating lifetime of the device and retaining only the stable trap states that cause only slow decay of the index change over the prescribed device lifetime.
IV. Implications of Physical Model For Device Stabilization This model provides a general procedure for device stabilization which is applicable whenever the decay is governed by a distribution of independent activation energies. By definition, an activated process with an activation energy E
occurs at a rate vo exp(-E/kT), where T is the temperature and k is Boltzmann's constant. The attempt frequency v o is typically very rapid, in the range 101° -1016 sec -1 for atomic processes. Thus the demarcation energy E d (at which the rate is equal to 1/t) is simply E d = kTLn ( v o t ). The basis of the procedure is that the remaining population of occupied traps is just the number initially created with E > E d ) and thus depends only E d, not on the particular combination of T
and t that produce that Ed.
The procedure is:
( 1 ) The empirical decay is determined for a representative specimen of the device at two temperatures, say T 1 and T 2. The measured decays should be of sufficient duration and be close enough in temperature that they have some values of On(t) in common.
..~ ~~338~~9 (2) A value of On re f in common between the two decays is chosen.
Since the decay depends only on Ed, the time t 1 at which the decay at T 1 reaches D n re f and the corresponding time t 2 for the decay at T 2 are related by w kTl Ln(votl )=kT2Ln(vo t2 ). The value of vo is thus determined by TILn(tl)-TzLn(t2) vo =exp T i -T2 (3) With this value of vo, all decay data should obey the same functional dependence O n = f ( E d ), where E d = kTLn ( v o t ). The accuracy of this assertion can be checked by comparing data at other values of On or other temperatures.
(4) The decay of an unannealed device during its lifetime can be determined from 0 n = f ( E op ) at an energy E op = kT op Ln ( v o t op ).
Since the desired lifetime is typically much longer than can be accessed by experiment, the data for this prediction will usually come from data at shorter times but at correspondingly higher temperatures.
FIG. 1 is a block diagram of the steps in making an optical device having stable uv-induced index changes.
- FIG. 2 is a schematic view of an exemplary optical device employing uv-induced index changes comprising a fiber optic laser;
FIG. 3 is a graphical display showing the decay of uv-induced index changes as a function of time; and FIG. 4 plots the time at various temperatures required for aging an exemplary glass.
Detailed Description This description is divided into three parts. Part I qualitatively describes the ' application of the inventive method to making an exemplary device employing uv-induced index changes, specifically a stable optical fiber laser. Part II
describes quantitative aspects of the invention; part III sets forth a theoretical physical model of radiation-induced index changes believed to underlie the invention; and part IV
derives from the physical model a quantitative model for device stabilization consistent with the empirical results of Part II.
I. The Method of the Invention (Qualitative Description) Referring to the drawings, FIG. 1 is a block diagram of the steps involved in making a stable optical device having uv-induced index variations.
As shown in block A of FIG. 1, the first step is to provide a body of uv-sensitive glass such as germanium-doped silicate. Advantageously the glass body can include other dopants as needed for its intended operation. For example, the glass body for making an optical fiber laser can be a conventionally clad optical fiber having a silicate core doped with 14 mole % germanium and 600 ppm erbium, co-doped with aluminum to minimize concentration-quenching effects. After application of overcladding, the fiber can be drawn to a 2.6p.m core diameter corresponding to a cutoff wavelength of 880 nm, all in accordance with techniques well known in the art.
The next step shown in block B of FIG. 1 is to expose a portion of the glass body to ultraviolet light in order to produce localized changes in the index of refraction. The degree of exposure can be spatially modulated by variation of exposure time, use of phase masks or use of interfering beams. For example, one or more Bragg phase gratings can be induced in the body by exposing it to interfering beams of ultraviolet light. High intensity interfering uv laser beams produce variations of the local index of refraction directly related to variations in the local beam intensity. The interfering beams thus produce a pattern of index variations having spacing corresponding to the spacing between interference peaks. These periodic variations can act as a Bragg grating to light incident upon them.
- Test gratings were formed in the core of the previously described glass fiber by interfering two beams of ultraviolet light (242 nm) on the side of the fiber.
The uv light was generated by frequency doubling a dye laser pumped by a XeF
excimer laser producing 15 ns pulses at 30 Hz repetition rate. The interfering beams were focussed onto the fiber to a spot size of 6 mm by SOp.m with a nearly gaussian time-averaged profile. The average power was 15-20 mW, and the fiber was exposed until the gratings reached peak reflectivity of 90% (typical exposure times between 30s and 60s). The peak uv-induced index change was estimated to be 2x 10-4.
The next step shown in block C of FIG. 1 is to heat the exposed body for an anneal time t a~ at an anneal temperature T ~, such that the device will operate for a time top at an operating temperature Top with a loss of only 8nop in the induced index change. Because the decay in the induced index On is proportional to 1 / ( 1 + At°' ) and because the device is amenable to accelerated aging, it is always possible to age the device to a state where the prescribed conditions can be met. This can be seen by reference to FIG. 3 which plots a curve proportional to 1 /( 1 +At°' ).
The experimental data were taken over a range of induced indices from 98% to 35%
of the initial induced grating strength. As can be seen from FIG. 3, the amount of decay per unit time diminishes with increasing time. And because aging can be accelerated by using a temperature Ta" >Top, the required aging can be effected in a time t ~, « t op, i.e., t ~, can be 10% of t op or considerably less. An algorithm for calculating appropriate annealing times and temperatures is set forth in part II, infra, and a physical model explaining the observed behavior is set forth in part III. In essence, the device is heat treated to a point t 1 on the curve of FIG. 3 such that the decay between t 1 and t 1 + t op is less than the required tolerance.
II. Method of the Invention (Quantitative Aspects) Applicants have determined that the uv-induced index change Vin; of a glass body subjected to an operating temperature Top will decay in proportion to ' a where A is a decay prefactor which is a function of Top and a is a decay 1 + At constant which is also a function of T op. Applicants have further realized that with this model of decay, one can now prescribe a course of preoperational heat treatment which will maintain the device within a prescribed variation in index 8nop for a r prescribed period of time top at an operating temperature Top.
The ability to prescribe such treatment can be understood by reference to FIG. 3 which is a schematic graphical illustration of the decay at T op ~of the normalized uv-induced index change rl as a function of time. If t 1 is the time when device operation commences, then t 1 + t op is the time when the device completes its anticipated operating life. If D n op is the induced index change at t 1, then the requirement that the index decay no further than 8nop during its operating life requires that 0n be no less than Onop -8nop at t 1 +top.
While t 1 might be an impractically long period for treating the device, applicants have further discovered that accelerated aging is possible, i.e, that by treating the device at a higher temperature T ~ > T op for a shorter period of time t an « t l , the same effect can be achieved to ensure device stability.
A problem confronting a designer of an optical device employing uv-induced index change can be described as follows: "How do I process a device so that I can use it for a time at least top at a temperature Top? The device requires that the uv-induced index change not decay more than 8nop over top. Specifically:
what are the ranges of anneal times t a~ and anneal temperatures T ~ which will give the desired result".
To obtain these values for a particular glass, one should plot the empirical decay curves for at least two temperatures, e.g., T 1 and T2, and by conventional curve fitting techniques, calculate a 1, a 2 and A 1, A 2 for fitting the observed curves into the form O n/ ( 1 + Ata ). It is advantageous to choose T
1 and T 2 greater than the decay anticipated operating temperature T op so that the parameters A and a can be quickly measured. The corresponding parameters at Top can be readily calculated by the relations:
(i) Aop =Ao eaT°P ,aop = Tip where Ln(A2)-Ln(Al) -aT _ T T
a= ,Ao=Ale '=A2e aT2,andTo= 1 - z Tz _Tl al _ a2 Analytically, t 1 can now be found by solving the equality:
( la a (11) Onop Aop I tl +topJ ~ -Aoptl av _ a~
= 8nop 1+Aop f tl+top, -s-Once t 1 is calculated, accelerated aging conditions can be prescribed to yield a decay equivalent to that of the device at t 1 in FIG. 3. Specifically, we can use any combination of T a" ,t ~, that solves:
T~ T~
(111) ~t~~ T° eaTa° - (t 1 ~ T° eaT°P
s Specifically, we can use an annealing temperature ? Tao for an annealing time >_ ta~.
After the device is subject to heat treatment at Tao for time tan, it is in the same state on the curve of FIG. 3 as if it had been operated at T
°p for a period t 1.
Its future operation at temperature Top for a period t°p will produce only the small residual decay shown on the curve.
Applying these teachings to the specific example of the erbium fiber optic laser with uv-induced gratings, it should be noted that such lasers are contemplated as single mode lasers in undersea cable where economy dictates they should operate without service for many years. With only a 7% decrease in the uv-induced index changes, such a laser could switch from single mode to multiple mode is operation with serious consequences to the cable system.
FIG. 4 plots the time required at various temperatures for aging the glass of the exemplary laser by an amount equivalent to l00 years decay at 300 K.
After such aging, the decrease in index for the next 100 years at 300 K will be less than 1%.
To test these predictions, a fiber laser was annealed 48 hours at 120°C
and then operated single mode at 100°C for a time greater than 1800 hours (equivalent to s0 years at 6s°C). The laser showed no signs of two mode operation or mode hopping during this time.
III. Physical Model of UV -Induced Index Changes 2s While not necessary for the invention, applicants' best current belief is that in uv-sensitive glasses electrons are excited by uv light from photosensitive defects. These excited electrons are then trapped in a continuous distribution of energy states rather than at a single trap level. The detrapping rate from any state in the continuum via thermal excitation is dependent on the temperature of the excitation and the depth of the trap state. Upon thermal release from the traps, electrons repopulate the original deep level occupied prior to uv excitation.
~~ ~~84~~
At a time t after trapping, the distribution of states can be separated into two groups, divided by a demarcation energy Ed. The first group, for which E<Ed, is in thermal equilibrium with the conducting states above E=0. The second group, for which E > E d, is too deep to have been thermally excited by time t.
~ There is a unique demarcation energy Ed associated with each value of induced index change. Removing a certain fraction of the induced index change corresponds to thermally depleting the traps with energies less than E d, while leaving the traps with energies greater than E d unchanged.
By adjusting the temperature, the rate of index change decay can be controlled. Also, the progression of decay at any given temperature becomes much slower as time progresses. Thus accelerated aging involves inducing quick decay by increasing the temperature and then reducing the temperature once the decay process progresses to a point where further decay is sufficiently slow. Thus it is possible to stabilize a device by annealing, wiping out the unstable trap states that would cause decay by a substantial amount over the operating lifetime of the device and retaining only the stable trap states that cause only slow decay of the index change over the prescribed device lifetime.
IV. Implications of Physical Model For Device Stabilization This model provides a general procedure for device stabilization which is applicable whenever the decay is governed by a distribution of independent activation energies. By definition, an activated process with an activation energy E
occurs at a rate vo exp(-E/kT), where T is the temperature and k is Boltzmann's constant. The attempt frequency v o is typically very rapid, in the range 101° -1016 sec -1 for atomic processes. Thus the demarcation energy E d (at which the rate is equal to 1/t) is simply E d = kTLn ( v o t ). The basis of the procedure is that the remaining population of occupied traps is just the number initially created with E > E d ) and thus depends only E d, not on the particular combination of T
and t that produce that Ed.
The procedure is:
( 1 ) The empirical decay is determined for a representative specimen of the device at two temperatures, say T 1 and T 2. The measured decays should be of sufficient duration and be close enough in temperature that they have some values of On(t) in common.
..~ ~~338~~9 (2) A value of On re f in common between the two decays is chosen.
Since the decay depends only on Ed, the time t 1 at which the decay at T 1 reaches D n re f and the corresponding time t 2 for the decay at T 2 are related by w kTl Ln(votl )=kT2Ln(vo t2 ). The value of vo is thus determined by TILn(tl)-TzLn(t2) vo =exp T i -T2 (3) With this value of vo, all decay data should obey the same functional dependence O n = f ( E d ), where E d = kTLn ( v o t ). The accuracy of this assertion can be checked by comparing data at other values of On or other temperatures.
(4) The decay of an unannealed device during its lifetime can be determined from 0 n = f ( E op ) at an energy E op = kT op Ln ( v o t op ).
Since the desired lifetime is typically much longer than can be accessed by experiment, the data for this prediction will usually come from data at shorter times but at correspondingly higher temperatures.
(5) Accelerated aging for a time tan at temperature Tan can be represented by the equivalent anneal time t~ at the temperature Top. t~
satisfies kTan Ln(Va tan )=kTop Ln(Vo t~ ), Or t~ =tan (Vo tan )~T~-T°~)lT~.
since t an » v o -1, even a modest fractional increase in T an over T op result in very long teq. The annealing conditions should be then chosen such that f[kTopLn(voTeq )]-f[kTopLn(vo (t~ +top))] <_8nop.
Summary Thus an optical device employing ultraviolet light induced refractive index changes in a glass body can be stabilized against decay beyond a specified level by: (1) determining the amount of aging required to place the device in such condition that further aging at the anticipated operating temperature for the anticipated operating time will not produce decay of the induced index changes beyond the specified level, and (2) preannealing the device prior to operation at a higher temperature than the operating temperature for a time sufficient to place the device in the determined condition.
More specifically, if the device is designed to operate at a maximum operating temperature Top for an operating period top with decay of the induced index changes less than 8 n op over the period of operation, the device can be made by: (1) providing a body of glass, (2) exposing the glass to ultraviolet light to induce the refractive index changes, (3) determining the characteristic decay curve of the induced index changes in the glass at Top, (4) determining a point P on the decay _g_ curve for which the passage of top will produce a decay of less than 8n, and (5) heating the glass at an anneal temperature T ~, > Top for an anneal time tan < top equivalent to point F on the curve.
Applicants have found that the characteristic decay curve for these devices is such that a point P can be found for any 8n op > 0 and that the decay mechanism in the devices is such that aging can be accelerated by using higher temperatures.
Even more specifically, the characteristic decay curve for Top can be expeditiously determined from the characteristic decay curves at two temperatures T 1 > T op and T 2 > T op . Specifically, for T op , O n; is proportional to 1 / ( 1 + A op t°' °~ ) where Ln(A -Ln(A _ _ Aop=AleaT, =A2eaTz'aop=Top/To,a= 2) 1) 'Ao=Ale aT~ =A2e aT2 T2-Ti and To= Tl - T2 .
al a2 More generally, in accordance with the physical model presented, an optical device employing an induced refractive index change O n in a medium where decay is governed by a distribution of independent activation energies can be stabilized for operation at Top for top against decay beyond a specified level 8nop by annealing the device at a temperature Tan for a time tan such that f[kTapLn(voteq)]-f[kTopLn(vo(t~+top))]58nop.
The method is particularly useful for devices containing uv-induced Bragg gratings such as optical fiber lasers.
satisfies kTan Ln(Va tan )=kTop Ln(Vo t~ ), Or t~ =tan (Vo tan )~T~-T°~)lT~.
since t an » v o -1, even a modest fractional increase in T an over T op result in very long teq. The annealing conditions should be then chosen such that f[kTopLn(voTeq )]-f[kTopLn(vo (t~ +top))] <_8nop.
Summary Thus an optical device employing ultraviolet light induced refractive index changes in a glass body can be stabilized against decay beyond a specified level by: (1) determining the amount of aging required to place the device in such condition that further aging at the anticipated operating temperature for the anticipated operating time will not produce decay of the induced index changes beyond the specified level, and (2) preannealing the device prior to operation at a higher temperature than the operating temperature for a time sufficient to place the device in the determined condition.
More specifically, if the device is designed to operate at a maximum operating temperature Top for an operating period top with decay of the induced index changes less than 8 n op over the period of operation, the device can be made by: (1) providing a body of glass, (2) exposing the glass to ultraviolet light to induce the refractive index changes, (3) determining the characteristic decay curve of the induced index changes in the glass at Top, (4) determining a point P on the decay _g_ curve for which the passage of top will produce a decay of less than 8n, and (5) heating the glass at an anneal temperature T ~, > Top for an anneal time tan < top equivalent to point F on the curve.
Applicants have found that the characteristic decay curve for these devices is such that a point P can be found for any 8n op > 0 and that the decay mechanism in the devices is such that aging can be accelerated by using higher temperatures.
Even more specifically, the characteristic decay curve for Top can be expeditiously determined from the characteristic decay curves at two temperatures T 1 > T op and T 2 > T op . Specifically, for T op , O n; is proportional to 1 / ( 1 + A op t°' °~ ) where Ln(A -Ln(A _ _ Aop=AleaT, =A2eaTz'aop=Top/To,a= 2) 1) 'Ao=Ale aT~ =A2e aT2 T2-Ti and To= Tl - T2 .
al a2 More generally, in accordance with the physical model presented, an optical device employing an induced refractive index change O n in a medium where decay is governed by a distribution of independent activation energies can be stabilized for operation at Top for top against decay beyond a specified level 8nop by annealing the device at a temperature Tan for a time tan such that f[kTapLn(voteq)]-f[kTopLn(vo(t~+top))]58nop.
The method is particularly useful for devices containing uv-induced Bragg gratings such as optical fiber lasers.
Claims (9)
1. A method for making an optical device employing radiation induced refractive index changes, said induced changes subject to decay with time and described for an operating temperature by a characteristic decay curve, said device designed to operate at a maximum operating temperature T op for an operating period t op with decay of the induced index changes less than .delta.n op over the period of operation, comprising the steps of:
providing a body of glass;
exposing said glass to radiation to induce said refractive index changes;
determining the characteristic decay curve of said induced index changes in said glass for at least one temperature T > T op and extrapolating therefrom the characteristic decay curve for the induced index changes in said glass for T op;
determining a point P on said decay curve for the induced index changes in said glass for T op for which the passage of t op will produce a decay of less than .delta.n op wherein point P represents a value of decay over a period of time;
and heating said glass at an anneal temperature T an > T op for an anneal time t an < t op to achieve a value of decay equivalent to that value represented by point P on said extrapolated curve.
providing a body of glass;
exposing said glass to radiation to induce said refractive index changes;
determining the characteristic decay curve of said induced index changes in said glass for at least one temperature T > T op and extrapolating therefrom the characteristic decay curve for the induced index changes in said glass for T op;
determining a point P on said decay curve for the induced index changes in said glass for T op for which the passage of t op will produce a decay of less than .delta.n op wherein point P represents a value of decay over a period of time;
and heating said glass at an anneal temperature T an > T op for an anneal time t an < t op to achieve a value of decay equivalent to that value represented by point P on said extrapolated curve.
2. The method of claim 1 wherein said step of determining the characteristic decay curve comprises empirically determining the characteristic decay curves of said glass for at least two temperatures T wherein the first temperature is denoted as T, and the second temperature is denoted as T2 and wherein T1 > T op and T2 > T op and extrapolating therefrom the characteristic decay curve for the induced index changes for said glass for T op.
3. The method of claim 1 wherein said exposing to radiation comprises exposing to ultraviolet light.
4. The method of claim 2 wherein the decay curve at each temperature T i is proportional to 1/(1 + A i t .alpha.i).
5. The method according to any one of claims 1 to 3 wherein said device comprises an optical fiber laser.
6. The method according to any one of claims 1 to 3 wherein said device comprises an optical filter.
7. An optical device employing radiation induced refractive index changes made by the method of claim 1.
8. An optical device employing radiation induced refractive index changes made by the method of claim 3.
9. An optical device employing radiation induced refractive index changes made by the method of claim 4.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US15212093A | 1993-11-12 | 1993-11-12 | |
| US152,120 | 1993-11-12 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2133849A1 CA2133849A1 (en) | 1995-05-13 |
| CA2133849C true CA2133849C (en) | 1999-08-17 |
Family
ID=22541596
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002133849A Expired - Fee Related CA2133849C (en) | 1993-11-12 | 1994-10-05 | Method for making stable optical devices employing radiation-induced index changes |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US5620496A (en) |
| EP (1) | EP0653652B1 (en) |
| JP (1) | JP3153083B2 (en) |
| KR (1) | KR100351218B1 (en) |
| AU (1) | AU683305B2 (en) |
| CA (1) | CA2133849C (en) |
| DE (1) | DE69417687T2 (en) |
Families Citing this family (26)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5943152A (en) * | 1996-02-23 | 1999-08-24 | Ciena Corporation | Laser wavelength control device |
| US5673129A (en) * | 1996-02-23 | 1997-09-30 | Ciena Corporation | WDM optical communication systems with wavelength stabilized optical selectors |
| US6111681A (en) | 1996-02-23 | 2000-08-29 | Ciena Corporation | WDM optical communication systems with wavelength-stabilized optical selectors |
| US6154593A (en) * | 1996-03-18 | 2000-11-28 | Japan Science & Technology Corp | Optical device and formation of optical waveguide using light-induced effect on refractive index |
| JP3649835B2 (en) * | 1996-03-18 | 2005-05-18 | 独立行政法人科学技術振興機構 | Optical waveguide fabrication method |
| US6272886B1 (en) * | 1996-10-23 | 2001-08-14 | 3M Innovative Properties Company | Incremental method of producing multiple UV-induced gratings on a single optical fiber |
| US6292607B1 (en) | 1997-05-01 | 2001-09-18 | Sumitomo Electric Industries | Method for determining an aging condition for an optical waveguide grating and temperature sensor |
| TW434193B (en) * | 1997-05-01 | 2001-05-16 | Sumitomo Electric Industries | Method of producing optical fiber grating |
| US6504962B2 (en) | 1997-05-01 | 2003-01-07 | Sumitomo Electric Industries, Ltd. | Method for determining an aging condition for an optical waveguide grating and temperature sensor |
| US6522797B1 (en) | 1998-09-01 | 2003-02-18 | Input/Output, Inc. | Seismic optical acoustic recursive sensor system |
| KR100322121B1 (en) | 1998-10-13 | 2002-03-08 | 윤종용 | Apparatus for fabricating long-period grating filter |
| US6146713A (en) * | 1999-03-25 | 2000-11-14 | Acme Grating Ventures, Llc | Optical transmission systems and apparatuses including Bragg gratings and methods of making |
| US20010035029A1 (en) * | 1999-07-12 | 2001-11-01 | Akira Ikushima | Method of manufacturing an optical fiber |
| US6588236B2 (en) | 1999-07-12 | 2003-07-08 | Kitagawa Industries Co., Ltd. | Method of processing a silica glass fiber by irradiating with UV light and annealing |
| US6796148B1 (en) | 1999-09-30 | 2004-09-28 | Corning Incorporated | Deep UV laser internally induced densification in silica glasses |
| US6823110B2 (en) * | 2000-06-14 | 2004-11-23 | 3M Innovative Properties Company | Method to stabilize and adjust the optical path length of waveguide devices |
| US6647167B1 (en) | 2000-10-31 | 2003-11-11 | Lightwave Technologies, Inc. | Solid state optical switch and router |
| US6601411B2 (en) | 2001-03-26 | 2003-08-05 | Cidra Corporation | Method for annealing an optical waveguide having a bragg grating to accelerate ageing |
| JP3753975B2 (en) * | 2001-11-29 | 2006-03-08 | 株式会社フジクラ | Single-mode optical fiber manufacturing method and single-mode optical fiber |
| CA2436499C (en) * | 2003-03-21 | 2012-04-17 | Her Majesty In Right Of Canada As Represented By The Minister Of Industry | Bragg grating and method of producing a bragg grating using an ultrafast laser |
| US7689087B2 (en) * | 2003-03-21 | 2010-03-30 | Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of Industry, Through The Communications Research Centre Canada | Method of changing the birefringence of an optical waveguide by laser modification of the cladding |
| US7031571B2 (en) | 2003-03-21 | 2006-04-18 | Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of Industry Through The Communications Research Centre Canada | Bragg grating and method of producing a Bragg grating using an ultrafast laser |
| US8272236B2 (en) | 2008-06-18 | 2012-09-25 | Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of Industry, Through The Communications Research Centre Canada | High temperature stable fiber grating sensor and method for producing same |
| CN102105772B (en) * | 2009-04-09 | 2014-10-08 | 株式会社藤仓 | Method and device for measuring hole diameter of optical fiber with hole, and method and device for manufacturing optical fiber with hole |
| EP3677936A1 (en) | 2017-08-31 | 2020-07-08 | Fujikura Ltd. | Method for setting heating condition, method for manufacturing fiber bragg grating, and method for manufacturing fiber laser system |
| EP3499281B1 (en) * | 2017-12-13 | 2020-09-23 | IMEC vzw | A method for manufacturing of a photonic circuit |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2746193A (en) * | 1954-08-18 | 1956-05-22 | Owens Illinois Glass Co | Decorating glassware by high energy radiation |
| US3542536A (en) * | 1967-09-01 | 1970-11-24 | Hazeltine Research Inc | Method of forming optical waveguide by irradiation of dielectric material |
| US4777146A (en) * | 1987-02-24 | 1988-10-11 | American Telephone And Telegraph Company, At&T Bell Laboratories | Fabrication process involving semi-insulating material |
| US5066133A (en) * | 1990-10-18 | 1991-11-19 | United Technologies Corporation | Extended length embedded Bragg grating manufacturing method and arrangement |
| US5235659A (en) * | 1992-05-05 | 1993-08-10 | At&T Bell Laboratories | Method of making an article comprising an optical waveguide |
| US5287427A (en) * | 1992-05-05 | 1994-02-15 | At&T Bell Laboratories | Method of making an article comprising an optical component, and article comprising the component |
| US5218655A (en) * | 1992-05-29 | 1993-06-08 | At&T Bell Laboratories | Article comprising an optical waveguide with in-line refractive index grating |
-
1994
- 1994-10-05 CA CA002133849A patent/CA2133849C/en not_active Expired - Fee Related
- 1994-11-02 DE DE69417687T patent/DE69417687T2/en not_active Expired - Lifetime
- 1994-11-02 EP EP94308070A patent/EP0653652B1/en not_active Expired - Lifetime
- 1994-11-03 AU AU77639/94A patent/AU683305B2/en not_active Ceased
- 1994-11-03 KR KR1019940028716A patent/KR100351218B1/en not_active IP Right Cessation
- 1994-11-11 JP JP30159694A patent/JP3153083B2/en not_active Expired - Fee Related
-
1995
- 1995-12-04 US US08/566,650 patent/US5620496A/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| EP0653652B1 (en) | 1999-04-07 |
| US5620496A (en) | 1997-04-15 |
| JP3153083B2 (en) | 2001-04-03 |
| AU683305B2 (en) | 1997-11-06 |
| KR100351218B1 (en) | 2002-12-16 |
| DE69417687T2 (en) | 1999-09-09 |
| KR960017551A (en) | 1996-06-17 |
| CA2133849A1 (en) | 1995-05-13 |
| EP0653652A1 (en) | 1995-05-17 |
| AU7763994A (en) | 1995-05-18 |
| DE69417687D1 (en) | 1999-05-12 |
| JPH07187695A (en) | 1995-07-25 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA2133849C (en) | Method for making stable optical devices employing radiation-induced index changes | |
| US5287427A (en) | Method of making an article comprising an optical component, and article comprising the component | |
| Baker et al. | Thermal decay of fiber Bragg gratings written in boron and germanium codoped silica fiber | |
| KR100382442B1 (en) | Method for producing photoinduced bragg gratings | |
| Kannan et al. | Thermal stability analysis of UV-induced fiber Bragg gratings | |
| US5235659A (en) | Method of making an article comprising an optical waveguide | |
| US6160944A (en) | Optical waveguide device | |
| CA2768718C (en) | Method for writing high power resistant bragg gratings using short wavelength ultrafast pulses | |
| EP0731364A1 (en) | Radiolytic modification of birefringence in silica planar waveguide structures | |
| US8737780B2 (en) | Method for writing high power resistant Bragg gratings using short wavelength ultrafast pulses | |
| US8402789B2 (en) | High temperature stable fiber grating sensor and method for producing same | |
| CA2750461A1 (en) | A process and system for manufacturing stable fiber bragg gratings (fbgs) | |
| Malo et al. | Photosensitivity in optical fiber and silica-on-substrate waveguides | |
| Ahmed et al. | Femtosecond laser based in-fiber long period grating fabrication for improved solution sensing | |
| Lemaire et al. | Hydrogen-enhanced UV photosensitivity of optical fibers: Mechanisms and reliability | |
| US20220120964A1 (en) | Photobleaching of fiber bragg gratings written through the fiber coating | |
| Yu et al. | Temperature sensor using a long period fiber grating fabricated by 800 nm femtosecond laser pulses | |
| Viswanathan et al. | Accelerated-aging studies of chirped Bragg gratings written in deuterium-loaded germano-silicate fibers | |
| Morana et al. | Temperature Influence on the Inscription Efficiency of Type I-UV Bragg Gratings | |
| Sørensen et al. | Thermal stability of UV-written gratings in low-and high Ge content fibers | |
| Gunawardena | Photosensitivity, Grating Strength and Thermal Endurance of Fibre Bragg Gratings | |
| CA3107452A1 (en) | Photobleaching of fiber bragg gratings written through the fiber coating | |
| Yu et al. | Transmission spectra dependence on the grating lengths of long period fiber grating fabricated by 800nm femtosecond laser pulses | |
| CA2669437C (en) | High temperature stable fiber grating sensor and method for producing same | |
| Dianov et al. | Germanosilicate glass refractive-index change induced by direct excitation of GODC triplet state |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKLA | Lapsed |