Method and system for stabilizing fiber grating optical parameters

A system for stabilizing optical parameters of a fiber Bragg grating (FBG) includes a mechanical mount, a heating element coupled to the mechanical mount, and a base plate coupled to the heating element. The base plate comprises a longitudinal groove. The system also includes a fiber anchor coupled to the mechanical mount and a fiber including the FBG mechanically attached to the fiber anchor. The FBG of the fiber is disposed in the longitudinal groove.

BACKGROUND OF THE INVENTION

Fiber Bragg gratings (FBGs) are characterized by a reflection or transmission bandwidth, for example, on the order of 10-300 GHz, and a central frequency, for example, on the order of 283,000 GHz. Although FBGs can be designed to provide the desired reflection or transmission, a number of detrimental effects are encountered when operating in realistic environments and under high optical intensities.

Despite the progress made in the development of FBGs, there is a need in the art for improved methods and systems related to stabilizing and controlling FBGs.

SUMMARY OF THE INVENTION

The present disclosure relates generally to methods and systems related to FBGs that operate at specific frequencies. More particularly, embodiments of the present invention provide methods and systems that allow FBGs to reflect or transmit at specific frequencies in realistic operating environments. In a particular embodiment, the FBGs are controlled using a thermo-mechanical housing that minimizes the impact of optical nonlinearities and temperature, stress, and vibrations from the surrounding environment.

According to an embodiment of the present invention, a system for stabilizing optical parameters of a fiber Bragg grating (FBG) is provided. The system includes a mechanical mount, a heating element coupled to the mechanical mount, and a base plate coupled to the heating element. The base plate comprises a longitudinal groove. The system also includes a fiber anchor coupled to the mechanical mount and a fiber including the FBG mechanically attached to the fiber anchor. The FBG of the fiber is disposed in the longitudinal groove.

According to another embodiment of the present invention, a method of stabilizing an FBG disposed in a thermomechanical housing is provided. The method includes performing an initialization process including setting a temperature of the FBG to an initial value, changing the temperature of the FBG to a final value, and measuring a transmission ratio at a plurality of FBG temperatures between the initial value and the final value. The initialization process also includes determining a maximum transmission ratio, setting the temperature of the FBG to the initial value, and iteratively, changing the temperature of the FBG and measuring the transmission ratio until the transmission ratio equals a predetermined fraction of the maximum transmission ratio. The method also includes performing an operation process including (a) measuring the transmission ratio, (b) adjusting the FBG temperature to reduce the measured transmission ratio and the predetermined fraction of the maximum transmission ratio, and (c) iteratively performing (a) and (b).

According to a specific embodiment of the present invention, a system for stabilizing optical parameters of a fiber Bragg grating (FBG) section of an optical fiber is provided. The system includes a mechanical mount, a heating element coupled to the mechanical mount, and a base plate coupled to the heating element. The base plate comprises a longitudinal groove and the FBG section of the optical fiber is disposed in the longitudinal groove. The system also includes a first fiber guide coupled to the mechanical mount. The first fiber guide comprises a first longitudinal channel and a first portion of the optical fiber is disposed in the first longitudinal channel. The system further includes a second fiber guide coupled to the mechanical mount. The second fiber guide comprises a second longitudinal channel and a second portion of the optical fiber is disposed in the second longitudinal channel.

According to another specific embodiment of the present invention, an optical system is provided. The optical system includes a mechanical mount, a heating element coupled to the mechanical mount, and a base plate coupled to the heating element. The base plate comprises a longitudinal groove. The optical system also includes a fiber including a fiber Bragg grating (FBG) disposed in the longitudinal groove. The FBG of the fiber is disposed in the longitudinal groove. The optical system further includes a first attachment element mechanically coupled to a first end of the fiber and a second attachment element mechanically coupled to a second end of the fiber.

Numerous benefits are achieved by way of the present disclosure over conventional techniques. For example, embodiments of the present disclosure, the fiber Bragg grating (FBG) is located in close proximity to a thermo-electric cooler, allowing accurate temperature control of the FBG. Moreover, the effects of stress and vibration are reduced by securing the FBG at only one location on the fiber.

Furthermore, embodiments of the present invention include a novel architecture including a single region of rigid contact between elements of the thermo-mechanical housing and the fiber, vibration isolation between the housing and the fiber, and a thermal control section adjacent to and thermally coupled to the FBG that enables the implementation of a feedback control system using optical input and output power measurements. These and other embodiments of the disclosure, along with many of its advantages and features, are described in more detail in conjunction with the text below and corresponding figures.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present disclosure relates generally to methods and systems related to FBGs that are stabilized at an optical frequency. More particularly, embodiments of the present invention provide methods and systems provide control and stabilization of the reflection and transmission spectra of FBGs. In a particular embodiment, the reflection and transmission spectra of FBGs is stabilized against environmental perturbations, for example due to temperature, stress, or vibrations. The disclosure is applicable to a variety of applications in telecommunications, signal processing, lasers and optics, including fiber laser implementations.

FIG.1Ais a plot illustrating a typical output spectrum of a diode laser and an FBG spectral response. The diode laser has a desired signal peak and an undesirable noise floor that includes multiple longitudinal modes120of the laser cavity that are not completely suppressed. In this example, the laser has a longitudinal mode spacing of 15 GHz and a noise floor that is −38 dB below the signal peak. The FBG spectrum110illustrated inFIG.1Ais shown for an “ideal” case in which it is assumed that no manufacturing errors are present, no temperature variations or stress is experienced, and the optical intensity is sufficiently low to avoid nonlinear optical effects. In this example, the FBG has a central frequency that is centered on the desired peak of the diode laser and has a bandwidth of 10 GHz.

The diode laser light can be directed through the FBG to reduce the noise floor and undesirable longitudinal modes. Mathematically, this is represented by a multiplication of the two spectra.FIG.1Bshows the resultant filtered optical spectrum130, which has eliminated the multiple longitudinal modes and reduced the noise floor over all frequencies except a narrow band around the central peak. This function is desirable in many applications including telecommunications and signal processing.

FIG.2Ais a plot illustrating the output spectrum220of a diode laser and an FBG spectral response210with fabrication errors. When typical fabrication errors of ±0.02 nm in the periodicity of the FBG used, the reflection spectrum for the FBG and the central peak of the diode laser spectrum are shifted in central frequency related to one another, resulting in reduced spectral overlap. This leads to a significant 75% loss of optical power in the resultant filtered optical spectrum230as shown inFIG.2B. Thus, small variations in manufacturing tolerances can impact the effectiveness of the filtering operation.

FIG.3Ais a plot illustrating the output spectrum320of a diode laser and an FBG spectral response310with temperature variation. When the temperature of the FBG used as the filter is varied, for example, by 3° C. as illustrated inFIG.3A, the reflection spectrum for the FBG central peak of the diode laser spectrum are shifted in central frequency relative to one another, resulting in reduced spectral overlap. This leads to a significant 99% loss of optical power in the resultant filtered optical spectrum330as shown inFIG.3B. Thus, in addition to variations in manufacturing tolerances, modification of the operating temperature of the oscillator can impact the lasing bandwidth. The inventors have determined that environmental temperatures may change by +/−5° C. during normal operation.

Accordingly, embodiments of the present invention utilize precise control over the temperature and physical properties of the FBG to enable the laser oscillator, despite the narrow spectral bandwidth, for example, on the order of 10 GHz, to retain a desired central frequency.

AlthoughFIGS.1A-3Billustrate a system in which an FBG functions as a spectral filter on the optical output of a diode laser, embodiments of the present invention are not limited to this implementation. In other embodiments, for example, the FBGs are used as cavity mirrors in fiber lasers.

FIG.4Ais a simplified schematic diagram illustrating a thermo-mechanical housing400according to an embodiment of the present invention. The illustrated thermo-mechanical housing can be utilized as a thermo-mechanical housing utilized in a laser and provides a precision, temperature-controlled platform for control and operation of a fiber Bragg grating. It should be noted that although thermo-mechanical housing400can be utilized in conjunction with signal processing, the thermo-mechanical housing is not limited to these applications and can be utilized in other optical applications. As an example, narrow-band fiber gratings deployed in thermo-mechanical housing400can be utilized in intra-cavity or extra-cavity filtering in low-noise laser systems. The use of the thermo-mechanical housing enables fiber gratings with high quality factors, for example, Q>2,000,000 (i.e., bandwidth <100 MHz) to be implemented, representing an order of magnitude higher quality factor than passive laser cavities typically found in semiconductor lasers. Moreover, when integrated into an optical communication system, high-Q, high-reflectivity, and low-loss fiber gratings deployed in a thermo-mechanical housing can enable suppression of laser noise in the mid-high offset frequency band (offset frequency >100 MHz), where the excess noise in this band significantly affects the signal to noise ratio of communication channels and the sensitivity of optical sensing systems due to compromise between additive noise and phase noise filtering.

As illustrated inFIG.4A, embodiments of the present invention provide a mechanical design for a thermo-mechanical housing for the fiber grating that provides mechanical support for the fiber while reducing or minimizing the transfer of mechanical perturbations in the environment to the fiber and specifically, to the grating section of the fiber. A novel architecture including a single region of rigid contact between elements of the thermo-mechanical housing and the fiber, vibration isolation between the housing and the fiber, and a thermal control section adjacent to and thermally coupled to the FBG enables the implementation of a feedback control system using optical input and output power measurements useful for laser architectures is provided by embodiments of the present invention.

Referring toFIG.4A, thermo-mechanical housing400includes mechanical mount410, which can be fabricated using materials with high rigidity. As an example, mechanical mount410can be fabricated from invar or other suitable materials and can have dimensions on the order of 20 mm in length, 10 mm in width, and 10 mm in thickness. A plurality of vias passing through mechanical mount410are used to enable shock absorbing members412to pass through mechanical mount410. Shock absorbing members412can be fabricated from elastic materials with high elasticity, including silicone, rubber, and the like.

Although not illustrated inFIG.4Afor purposes of clarity, mechanical mount410is joined to a housing (not shown) using fasteners413, which are illustrated as Phillips head screws in this embodiment. Shock absorbing members412are mounted between mechanical mount410and the housing (not shown) and fasteners413in order to isolate mechanical mount410from the housing (not shown). In other embodiments, other mounting techniques are utilized to mechanically isolate mechanical mount410from surrounding structures. Although shock absorbing members412are illustrated as ring-shaped elastic O-rings, this is not required by the present invention and other implementations, including an elastic gasket having a shape corresponding to that of mechanical mount410can be positioned between mechanical mount410and the housing (not shown) in order to reduce vibration of fiber420, particularly grating section422, which can be referred to as the FBG, for example, at acoustic frequencies. Thus, thermo-mechanical housing400, including the housing (not shown), can be utilized as a thermo-mechanical housing for a laser. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

In order to support grating section422of fiber420, fiber suspension unit424includes heating element426mounted on mechanical mount410and base plate428mounted on heating element426. In some embodiments, heating element426includes a thermoelectric material that can be heated or cooled in response to current flow. In other embodiments, a resistive element is included in heating element426to enable the temperature of the heating element to be increased and decreased in response to current flow. Although thermoelectric and resistive elements are described herein, embodiments of the present invention are not limited to these particular implementations of a heating element and other devices can be used to provide thermal control for heating element426.

In embodiments in which heating element426is implemented as a thermoelectric element, it can have a thickness on the order of ˜2-3 mm can be mounted directly to mechanical mount410or positioned between a shock absorbing pad (now shown) and base plate428. In this embodiment, base plate428can be positioned on the cold side and mechanical mount410can be positioned on the hot side of the thermoelectric element. A membrane heater can also be used, for example, with a thermal insulator (e.g., a Teflon plate) positioned between the membrane heater and mechanical mount410to reduce heat flow to mechanical mount410. Thus, in this embodiment, the membrane heater in combination with the thermal insulator can result in a combined thickness >5 mm. As will be evident to one of skill in the art, the response time associated with thermal control roughly scales with (total thermal capacity of the holder)/(maximum thermal output power of thermal element−heat loss to external elements), so reductions in size of the various elements and better thermal insulation around the elements will increase responsivity.

The combination of mechanic mount410being supported by shock absorbing members412, fiber guide430and fibers suspension unit424suspending fiber420in a low friction and high thermal conductivity material, and fiber anchor434mounted to mechanical mount410using shock absorbent pad436enables the isolation of fiber420from both external vibrations and mechanical stress via the damped suspension of fiber420, particularly grating section422, by these elements utilizing the thermal grease.

Base plate428is mounted to heating element426and is thermally coupled to heating element426in order to inject or extract thermal energy into our out of grating section422of fiber420. Base plate428includes a groove running along the length of the base plate. The groove can be referred to as a longitudinal groove. In the illustrated embodiment, the groove is laterally centered on base plate428and has a depth approximately equal to the thickness of base plate428. Grating section422of fiber420is suspended in a thermal compound (not shown). The thermal compound, for example, silicone or graphite-infused paste, provides a high level of thermal conductivity between grating section422of fiber420, base plate428, and heating element426. A variety of thermal compounds, also referred to as thermal grease, can be utilized to provide low friction, high thermal conductivity support for grating section422of fiber420. Thus, thermally-induced stress that would arise in grating section422of fiber420if the temperature of grating section422varied without an accompanying variation in length can be reduced or prevented.

In some embodiments, grating section422of fiber420, as well as other sections of fiber420, can be encased in a low friction sheath, for example, a graphite wrap, that surrounds and contacts the periphery of the fiber, thereby allowing for motion of the fiber longitudinally with respect to the low friction sheath. Such a graphite-wrapped fiber surface will be characterized by reduced friction with respect to the side walls of the groove of base plate428. In addition to a reduction in friction, enabling the fiber to translate longitudinally, the low friction sheath can be characterized by high thermal conductivity, improving the thermal uniformity of the fiber and the thermal compound along grating section422. In embodiments using a graphite wrap, since graphite sheets possess high thermal conductivity (>>100 W/(m-K)) parallel to the surface of the graphite sheet, i.e., along the longitudinal axis of the fiber, the temperature of the fiber, particularly, the grating section, can be maintained at a uniform temperature, thereby preventing pass-band broadening. As will be evident to one of skill in the art, if the grating section were characterized by a non-uniform temperature as a function of length, the grating section will be chirped as either the index of refraction and/or the grating spacing will vary with length. This chirp will result in broadening of the pass-band as the different sections of the fiber at different temperatures are characterized by different reflectivity profiles. In addition to graphite wraps, other approaches to increase longitudinal heat transfer, including embedding heat transfer filaments in the thermal compound, are included within the scope of the present invention and are applicable as appropriate. Thus, utilizing embodiments of the present invention, thermal control via heating element426and a thermal sensor416, for example, a thermistor, embedded in cover plate414, which are elements of fiber suspension unit424, enables control of the center frequency of the FBG and uniform temperature as a function of length of the FBG reduces pass-band broadening.

Fiber guide430supports fiber420, but allows fiber420to translate longitudinally in response to temperature variation and thermal expansion or contraction of the fiber in the longitudinal direction. As discussed in relation to base plate428, fiber guide430includes a groove running along the length of the fiber guide and fiber420is suspended in a lubricant compound (not shown) filling the groove. In some embodiments, to distinguish from the groove present in base plate428, the groove in fiber guide430is referred to as a longitudinal channel. The lubricant compound, for example, silicone or graphite-infused paste, in conjunction with the groove in fiber guide430enables fiber420to be supported while still enabling translation along the longitudinal direction. In some implementations, the portion of fiber420supported by fiber guide430is encased in a low friction sheath as discussed above. Thus, as the temperature of the fiber varies, mechanically-induced stress that would arise if fiber420were not able to expand or contract in length, can be reduced or prevented. As illustrated inFIG.4A, a cap432is attached, for example, bonded, bolted, attached with a screw attachment, or otherwise affixed, to fiber guide430to enclose the corresponding portion of fiber420and the lubricant compound in the groove in fiber guide430. Accordingly, fiber420is supported in the lateral directions while enabling a degree of longitudinal motion resulting from thermal changes.

Fiber anchor434is positioned at the end of mechanical mount410opposite fiber guide430, and supports fiber420. Fiber420is anchored at a substantially fixed longitudinal position using fiber anchor434. As an example, fiber suspension unit424can be fabricated using a plastic/metal cube that is divided in half and has a v-groove engraved onto the top surface of the bottom half. The two halves are then bonded together after fiber420has been positioned between them. In the illustrated embodiment, fiber anchor434, which can be fabricated using a rigid material such as metal or a hard plastic, is mounted on shock absorbent pad436, however, in some embodiments, fiber anchor434is fabricated using an elastic material such as rubber that provides both mechanical support and flexibility. In other embodiments, an elastic material is utilized for both fiber anchor434and shock absorbent pad436. In one embodiment, fiber420is positioned on shock absorbent pad436and fiber anchor434is pressed down to hold fiber420between shock absorbent pad436and fiber anchor434. In other embodiments, a bonding material such as epoxy is used to join the fiber to these elements. Thus, fiber420is mechanically supported at a substantially single longitudinal position by fiber anchor434in a substantially rigid manner while still providing some mechanical isolation with respect to mechanical mount410.

In another embodiment, fiber anchor434is attached to mechanical mount410using fasteners (e.g., screws) that pass through shock absorbent pad436, preloading shock absorbent pad436. In another embodiment, fiber420can be sandwiched between shock absorbent pads, for example, with one shock absorbent pad positioned on top of the fiber and the other shock absorbent pad positioned below the fiber. In this embodiment, a clamp, for example, a bent metal clamp can be used to press the shock absorbent pads against the metal support (the gray pad in this case).

By anchoring fiber420at a single longitudinal region that is spatially separated from grating section422, grating section422is free to expand and contract longitudinally in response to thermal changes. This structure differs from conventional approaches in which the grating section is anchored at both ends or along portions or all of the length of the grating section. If, for example, the grating section were anchored at both ends and the temperature of the thermo-mechanical housing increased, the elements of the thermo-mechanical housing including mechanical mount410would expand, stretching the fiber, thereby increasing the grating spacing and changing the reflection profile of the grating.

Cover plate414is mounted to base plate428to enclose grating section422fiber420in the groove formed in base plate428. In some embodiments, a temperature sensor, for example, a thermistor, is embedded in cover plate414in order to provide data related to the temperature of grating section422of fiber420. In contrast with approaches that would measure the temperature of elements more remote from grating section422, for example, the temperature of mechanical mount410, the close proximity between grating section422of fiber420and the temperature sensor embedded in cover plate414as well as the thermal conduction between these elements enables the temperature measured at the temperature sensor to closely match the actual temperature of grating section422of fiber420. In other embodiments, the temperature sensor is attached to cover plate414, embedded in or attached to base plate428, or the like.

FIG.4Bis a simplified schematic diagram illustrating a thermo-mechanical housing according to an alternative embodiment of the present invention. The thermo-mechanical housing illustrated inFIG.4Bshares common components with the thermo-mechanical housing illustrated inFIG.4Aand the description provided in relation toFIG.4Ais application toFIG.4Bas appropriate.

Referring toFIG.4B, thermo-mechanical housing450utilizes components found in thermo-mechanical housing400, but replaces fiber anchor434and shock absorbent pad436with fiber guide430and cap432. This, in this embodiment, rather than fiber420being anchored at one end, both ends of the fiber are free to translate longitudinally in response to temperature variation and thermal expansion or contraction of the fiber in the longitudinal direction. In this embodiment, cap432can be attached to fiber guide430in manners other than bonding, including a screw attachment.

In the embodiment illustrated inFIG.4B, although fiber420is free to move in the longitudinal direction, the fiber will remain in the longitudinal groove in base plate428for several reasons. For example, fiber position in the base plate will occur due to contact with the thermal material (e.g., thermal paste) placed in the longitudinal groove along with the fiber and/or in the groove running along the length of the fiber guide. Additionally, contact between the fiber and the lubricant compound (not shown) filling the groove will aid in maintaining the fiber in the base plate.

FIG.4Cis a simplified schematic diagram of an FBG assembly incorporated with fiberized components according to an embodiment of the present invention. The FBG assembly460can be implemented as thermo-mechanical housing400illustrated inFIG.4Aor as thermo-mechanical housing450illustrated inFIG.4B. Additionally in some embodiments, only certain components of thermo-mechanical housing450are utilized to fabricate FBG assembly460, for example, elements illustrated inFIG.4B, including a base plate including a longitudinal groove, a heating element, for example, one or more thermo-electric coolers (TECs), a cover plate with an embedded thermal sensor, other thermal sensors, or the like, but with fiber guide430and cap432removed at both ends of base plate428.

Referring toFIG.4C, fiber420extending from thermo-mechanical housing400/450is mechanically attached at a first end to a first attachment element, implemented as a fiberized component462in this embodiment and at a second end to a second attachment element, implemented as a fiberized component464in this embodiment. Fiberized components462and464assist in maintaining the fiber in the longitudinal groove in the base plate by providing an overall anchor that fixes the portion of the fiber adjacent the fiberized component at a predetermined location. Examples of fiberized components include but are not limited to isolators, couplers, filters, taps, and wavelength division multiplexers.

Additionally, bend463, illustrated as a 90° bend in this example, and bend465, illustrated as a 180° in this example, provide flexibility along the length of the fiber, operating in a manner similar to a spring, in order to mitigate temperature-induced length changes or vibrations in the fiber. Although 90° and 180° bends are illustrated inFIG.4C, this is merely exemplary and other bend angles can be utilized by the embodiments described herein in order to secure the fiber in place via the support provided by the fiberized components and the bends in the fiber.

FIG.4Dis a simplified schematic diagram of an FBG assembly incorporated with fixed fiber components according to an embodiment of the present invention. The FBG assembly470can be implemented as thermo-mechanical housing400illustrated inFIG.4Aor as thermo-mechanical housing450illustrated inFIG.4B. Additionally in some embodiments, only certain components of thermo-mechanical housing450are utilized to fabricate FBG assembly470, for example, elements illustrated inFIG.4B, but with fiber guide430and cap432removed at both ends of base plate428.

Referring toFIG.4D, fiber420extending from thermo-mechanical housing400/450is mechanically attached at a first end to a first attachment element, implemented as a fiber anchor472in this embodiment and at a second end to a second attachment element, implemented as a fiber anchor474in this embodiment. The fiber anchors can be implemented using fiber anchor434illustrated inFIG.4A. In other embodiments, the fiber anchors can be implemented using suitable adhesives and/or either permanent or temporary bonding materials. Fiber anchors472and474assist in maintaining the fiber in the longitudinal groove in the base plate by providing an overall anchor that fixes the portion of the fiber adjacent the fiber anchors at a predetermined location.

Additionally, bend473, illustrated as a 90° bend in this example, and bend475, illustrated as a 180° in this example, provide flexibility along the length of the fiber, operating in a manner similar to a spring, in order to mitigate temperature-induced length changes or vibrations in the fiber. Although 90° and 180° bends are illustrated inFIG.4D, this is merely exemplary and other bend angles can be utilized by the embodiments described herein in order to secure the fiber in place via the support provided by the fiber anchors and the bends in the fiber.

FIGS.5A and5Bshow two example implementations of using FBGs as an optical filter. A standard FBG has a narrow reflection spectrum. In order to use it as a filter and produce a filtered output518, an optical circulator514is placed between the signal of interest generated, for example, using diode laser510, and the FBG515as illustrated inFIG.5A. This allows the reflected light to be sent in a different direction than the signal origin. As is commonly known to one skilled in the art, a phase-shifted FBG has a narrow transmission peak within the broader reflection band. In this case, the transmitted light is the filtered light528, and no optical circulator is necessary to separate the filtered light from the input signal path as illustrated inFIG.5B. In this filtering application, two optical taps are used to measure optical power. In the conventional FBG case, the input optical tap512is located before the signal input to the optical circulator514and the output optical tap516is located after the reflected output of the optical circulator514. In the phase-shifted FBG case, the input optical tap522is located before the signal input to the FBG525and the output optical tap526is located after the transmitted output of the FBG525. The ratio of the power measured at the input and output taps yields a transmission ratio θ=ITapOUT/ITapIN.

FIG.6is a simplified flowchart illustrating a method of initializing and operating a spectrally controlled FBG according to an embodiment of the present invention. The method600illustrated inFIG.6can be applied to the FBG disposed in the thermomechanical housing illustrated inFIG.4A. The method600illustrated in the flowchart includes two parts or portions: initialization and operation. In both the initialization and operation portions, the grating temperature TGof the FBG is adjusted and the transmission ratio θ is measured. Referring toFIG.6, in the initialization portion605, which includes steps610,612,614,616,618, and620, the temperature of the grating TGis set to an initial value TStart(610). Starting from this initial temperature TStart, the temperature of the grating TGis swept over a range from the initial temperature TStartto a final temperature TEnd(612). At each temperature in the range, the transmission ratio θ is measured (612). Based on the measured transmission ratios, the maximum transmission ratio θMaxis calculated (614).

The method also includes resetting the grating temperature TGto TStart(616) and setting a target transmission ratio θTargetto a value less than the maximum transmission ratio θMax. (618). As illustrated in step618inFIG.6, for a multiplier less than one, θTargetis set to ηθMax. The temperature of the grating TGis changed, e.g., gradually changed, until the measured transmission ratio θCurrentreaches the target transmission ratio θTarget, i.e., θCurrent=θTarget(620). It should be noted that by setting the target transmission ratio to a value less than the peak value, any change in the transmission ratio can be corrected for unambiguously. Depending on whether the initial temperature is greater than the temperature TGcorresponding to the maximum transmission ratio or less than the temperature TGcorresponding to the maximum transmission ratio, the temperature changes implemented in step620are either an increase in grating temperature or a decrease in grating temperature. In embodiments in which the initial temperature is greater than the temperature TGcorresponding to the maximum transmission ratio, the grating temperature is decreased until θCurrent=θTarget. In embodiments in which the initial temperature is less than the temperature TGcorresponding to the maximum transmission ratio, the grating temperature is increased until θCurrent=θTarget. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

The operation portion625of the method600includes continuously or iteratively measuring the current transmission ratio θCurrent(630) and adjusting (632) the grating temperature TGto maintain the transmission ratio at its target value θCurrent=θTarget. In some embodiments, the temperature controller response is overdamped to minimize over- and under-shoot in the adjustment of the grating temperature TG. If the input power exceeds the input power used in the initialization, the target transmission ratio θTargetis reduced to prevent feedback/control toggling due to increased optical power within the FBG.

FIG.7is a plot illustrating transmission ratio as a function of grating temperature according to an embodiment of the present invention. The initialization portion605is illustrated below the grating temperature axis. (1) The temperature of the grating TGis set to an initial value TStart. (2) Starting from this initial temperature TStart, the temperature of the grating TGis swept over a range from the initial temperature TStartto a final temperature TEnd. At each temperature in the range, the transmission ratio θ is measured. Based on the measured transmission ratios, the maximum transmission ratio θMaxis calculated.

(3) The grating temperature TGis reset to TStartand (4) and the temperature of the grating TGis changed so that the measured transmission ratio θCurrentreaches the target transmission ratio θTarget.

As discussed in relation to the operation portion625, temperature tuning can be implemented by measuring the current transmission ratio θCurrentand adjusting the grating temperature TGto shift the transmission ratio toward the target value θTarget. In the example shown in inFIG.7, the current transmission ratio θCurrentis less than target value θTarget. Accordingly, the grating temperature TGis decreased by ΔT to shift the transmission ratio toward the target value θTarget. Although in the embodiment illustrated inFIG.7, the target value θTargetis approximately 90% of the maximum transmission ratio, this is merely illustrative and is not required by embodiments of the present invention and in other embodiments, the target value θTargetis either closer to the maximum transmission ratio or farther from the maximum transmission ratio. As an example, in some embodiments, the target value θTargetcan be approximately half the maximum transmission ratio, thereby being position near the maximum of the slope of the transmission ratio to the grating temperature in order to achieve high sensitivity. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

The examples and embodiments described herein are for illustrative purposes only. Various modifications or changes in light thereof will be apparent to persons skilled in the art. These are to be included within the spirit and purview of this application, and the scope of the appended claims, which follow.