Method and system for stabilizing optical parameters of a fiber grating - Patents.com

Abstract

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 includes a longitudinal groove. The system also includes a fiber anchor coupled to the mechanical mount, and a fiber including an FBG mechanically attached to the fiber anchor. The FBG of the fiber is disposed in the longitudinal groove.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002]

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 140,709, filed January 22, 2021, the contents of which are incorporated by reference in their entirety for all purposes. [Background technology]

[0003]

[0002] Fiber Bragg gratings (FBGs) are characterized by a reflection or transmission bandwidth of, for example, about 10-300 GHz, and a center frequency of, for example, about 283,000 GHz. Although FBGs can be designed to provide a desired reflection or transmission, many detrimental effects occur when operating in realistic environments and under high optical intensities.

[0004]

[0003] Despite the advances made in the development of FBGs, there remains a need in the art for improved methods and systems relating to the stabilization and control of FBGs. Summary of the Invention [Means for solving the problem]

[0005]

[0004] The present disclosure generally relates to methods and systems related to FBGs operating at specific frequencies. More specifically, embodiments of the present invention provide methods and systems that enable FBGs to reflect or transmit at specific frequencies in a realistic operating environment. In certain embodiments, the FBGs are controlled using a thermo-mechanical housing that minimizes the effects of optical nonlinearities as well as temperature, stress, and vibration from the surrounding environment.

[0006] 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 includes a longitudinal groove. The system also includes a fiber anchor coupled to the mechanical mount, and a fiber including an FBG mechanically attached to the fiber anchor. The FBG of the fiber is disposed in the longitudinal groove.

[0007]

[0006] The system may further include a fiber guide coupled to the mechanical mount. The fiber guide may include a longitudinal channel, and a portion of the fiber may be disposed in the longitudinal channel. The lubricant compound may at least partially surround the portion of the fiber disposed in the longitudinal channel. The thermal compound may at least partially surround the FBG of the fiber. A low friction sheath, such as a graphite wrap, may at least partially surround the FBG of the fiber. The mechanical mount may be attached to the housing by a shock absorbing member. The system may further include a cover plate coupled to the base plate and a thermal sensor coupled to the cover plate. The thermal sensor may include a thermistor embedded in the cover plate. The optical parameter may include a center frequency of the FBG or a bandwidth of the FBG.

[0008]

[0007] According to another embodiment of the present invention, a method for stabilizing an FBG disposed in a thermo-mechanical housing is provided. The method includes performing an initialization process including setting the temperature of the FBG to an initial value, changing the temperature of the FBG to a final value, and measuring the transmittance at a number of FBG temperatures between the initial value and the final value. The initialization process also includes determining a maximum transmittance, setting the temperature of the FBG to the initial value, and repeatedly changing the temperature of the FBG and measuring the transmittance until the transmittance is equal to a predetermined percentage of the maximum transmittance. The method also includes performing an operation process including (a) measuring the transmittance, (b) adjusting the FBG temperature to reduce the measured transmittance and the predetermined percentage of the maximum transmittance, and (c) repeating (a) and (b).

[0009]

[0008] Repeatedly varying the temperature of the FBG may include increasing the temperature of the FBG or decreasing the temperature of the FBG. The predetermined percentage may be between 80% and 95%. The initial value may be greater than the final value. Changing the temperature of the FBG may include repeatedly varying the temperature of the FBG and measuring the transmittance until the transmittance is equal to the target transmittance.

[0010]

[0009] According to a specific embodiment of the present invention, a system for stabilizing optical parameters of a fiber Bragg grating (FBG) portion 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 includes a longitudinal groove and an FBG portion of an optical fiber disposed in the longitudinal groove. The system also includes a first fiber guide coupled to the mechanical mount. The first fiber guide includes a first longitudinal channel and a first portion of the optical fiber disposed in the first longitudinal channel. The system further includes a second fiber guide coupled to the mechanical mount. The second fiber guide includes a second longitudinal channel and a second portion of the optical fiber disposed in the second longitudinal channel.

[0011]

[0010] The system may further include a lubricant compound at least partially surrounding the first portion of the optical fiber disposed in the first longitudinal channel and the second portion of the optical fiber disposed in the second longitudinal channel. The system may also include a thermal compound at least partially surrounding the FBG portion of the optical fiber. A low friction sheath, such as a graphite wrap, may at least partially surround the FBG portion of the optical fiber. The mechanical mount may be attached to the housing by a shock absorbing member. The system may further include a cover plate coupled to the base plate and a thermal sensor coupled to the cover plate. The thermal sensor may include a thermistor embedded in the cover plate.

[0012]

[0011] 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 includes 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 mounting element mechanically coupled to a first end of the fiber and a second mounting element mechanically coupled to a second end of the fiber.

[0013]

[0012] The optical system may also include a first bend at a first end of the fiber disposed between the base plate and the first mounting element, and a second bend at a second end of the fiber disposed between the base plate and the second mounting element. The first bend and the second bend may have different values. The optical system may further include a first fiber guide coupled to the mechanical mount, the first fiber guide including a first longitudinal channel, the first end of the fiber disposed in the first longitudinal channel, and a second fiber guide coupled to the mechanical mount, the second fiber guide including a second longitudinal channel, the second end of the fiber disposed in the second longitudinal channel.

[0014]

[0013] The optical system may further include a lubricant compound at least partially surrounding the first end of the fiber disposed in the first longitudinal channel and the second end of the fiber disposed in the second longitudinal channel. The first mounting element may include a first fiberized component, and the second mounting element may include a second fiberized component. The first mounting element may include a first fiber anchor, and the second mounting element may include a second fiber anchor. The thermal compound may at least partially surround the FBG of the fiber. A low friction sheath, such as a graphite wrap, may at least partially surround the FBG of the fiber. The mechanical mount may be attached to the housing by a shock absorbing member. The optical system may also include a cover plate coupled to the base plate and a thermal sensor coupled to the cover plate. The thermal sensor may include a thermistor embedded in the cover plate.

[0015]

[0014] The present disclosure achieves many benefits over the prior art. For example, in embodiments of the present disclosure, a fiber Bragg grating (FBG) is placed in close proximity to a thermoelectric cooler, allowing precise temperature control of the FBG. Furthermore, the FBG is fixed to only one location on the fiber, reducing the effects of stress and vibration.

[0016]

[0015] Additionally, embodiments of the present invention include a novel architecture that includes a single area of ​​fixed contact between thermo-mechanical housing and fiber elements, vibration isolation between the housing and fiber, and a thermal control section thermally coupled adjacent to the FBG that allows for the implementation of a feedback control system using optical input and output power measurements. These and other embodiments of the present disclosure, along with many of their advantages and features, are described in more detail in conjunction with the following text and corresponding figures. [Brief description of the drawings]

[0017]

[0016] Aspects of the present disclosure are more fully described below with reference to the accompanying drawings, which are intended to be read in conjunction with both this summary, the detailed description, and any preferred and / or specific embodiments specifically discussed or otherwise disclosed. However, the various aspects may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided by way of example only, so that this disclosure will be thorough and complete, and will fully convey its full scope to those skilled in the art.

[0018] [Figure 1A] 1 is a plot showing the spectrum of diode laser emission and FBG reflection according to one embodiment of the present invention. [Figure 1B] 1 is a plot showing the output of a diode laser after being filtered by an FBG in accordance with an embodiment of the present invention. [Figure 2A] 1 is a plot showing spectra of diode laser emission and FBG reflection with manufacturing errors according to an embodiment of the present invention. [Figure 2B] 1 is a plot showing the output of a diode laser after being filtered by an FBG in accordance with an embodiment of the present invention. [Figure 3A] 1 is a plot showing the spectrum of diode laser emission and FBG reflection with temperature change according to an embodiment of the present invention. [Figure 3B] 1 is a plot showing the output of a diode laser after being filtered by an FBG in accordance with an embodiment of the present invention. [Figure 4A] FIG. 2 is a simplified schematic diagram illustrating a thermo-mechanical housing according to one embodiment of the present invention. [Figure 4B] 1 is a simplified schematic diagram illustrating a thermo-mechanical housing according to an alternative embodiment of the present invention. [Figure 4C] FIG. 2 is a simplified schematic diagram of an FBG assembly integrated with fiberization components according to an embodiment of the present invention. [Figure 4D] FIG. 2 is a simplified schematic diagram of an FBG assembly incorporating a fixed fiber component according to one embodiment of the present invention. [Figure 5A] FIG. 2 is a simplified schematic diagram of an FBG stabilization architecture for a conventional FBG according to an embodiment of the present invention. [Figure 5B] FIG. 2 is a simplified schematic diagram of an FBG stabilised architecture of phase-shifted FBGs according to one embodiment of the present invention. [Figure 6] 4 is a simplified flow chart illustrating a method for initializing and operating a spectrally controlled FBG according to one embodiment of the present invention. [Figure 7] 1 is a plot showing transmission as a function of temperature for a grating according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019]

[0031] The present disclosure generally relates to methods and systems related to optical frequency stabilized FBGs. More specifically, embodiments of the present invention provide methods and systems that provide control and stabilization of the reflection and transmission spectra of FBGs. In certain embodiments, the reflection and transmission spectra of FBGs are stabilized against environmental variations due to, for example, temperature, stress, or vibration. The present disclosure is applicable to a variety of applications in telecommunications, signal processing, lasers, and optics, including fiber laser implementations.

[0020]

[0032] FIG. 1A is a plot showing a typical output spectrum of a diode laser and an FBG spectral response. The diode laser has a desired signal peak and an undesired noise floor that includes multiple longitudinal modes 120 of the laser cavity that are not fully suppressed. In this example, the longitudinal mode spacing of the laser is 15 GHz, and the noise floor is −38 dB below the signal peak. The FBG spectrum 110 shown in FIG. 1A shows an “ideal” case that assumes no manufacturing errors, no temperature variations or stress, and low enough optical intensity to avoid nonlinear optical effects. In this example, the FBG has a center frequency centered on the desired peak of the diode laser and has a bandwidth of 10 GHz.

[0021]

[0033] To reduce the noise floor and unwanted longitudinal modes, diode laser light can be guided through an FBG. Mathematically, this is represented by the multiplication of two spectra. Figure 1B shows the filtered optical spectrum 130, where several longitudinal modes have been removed and the noise floor has been reduced across all frequencies except for a narrow band around the central peak. This feature is desirable in many applications, including telecommunications and signal processing.

[0022]

[0034] FIG. 2A is a plot showing the output spectrum 220 of a diode laser and the FBG spectral response 210 with manufacturing tolerances. For a typical manufacturing tolerance of ±0.02 nm in the periodicity of the FBG used, the central peaks of the FBG reflection spectrum and the diode laser spectrum shift in center frequency from each other, resulting in a reduction in the spectral overlap. This results in a significant loss of optical power of 75% in the resulting filtered optical spectrum 230, as shown in FIG. 2B. Thus, small variations in manufacturing tolerances can affect the effectiveness of the filtering operation.

[0023]

[0035] FIG. 3A is a plot of the output spectrum 320 of a diode laser and the FBG spectral response 310 with temperature change. Changing the temperature of the FBG used as a filter, for example by 3° C. as shown in FIG. 3A, causes the reflection spectrum for the FBG central peak of the diode laser spectrum to shift in center frequency with each other, resulting in less spectral overlap. This results in a significant 99% loss of optical power in the resulting filtered optical spectrum 330, as shown in FIG. 3B. Thus, in addition to variations in manufacturing tolerances, changing the operating temperature of the oscillator can affect the oscillation bandwidth. The inventors have determined that the environmental temperature can vary by + / - 5° C. during normal operation.

[0024]

[0036] Thus, embodiments of the present invention utilize precise control over the temperature and physical properties of the FBG to enable a laser oscillator to maintain a desired center frequency, for example around 10 GHz, despite a narrow spectral bandwidth.

[0025]

[0037] 1A-3B show a system in which the FBG acts as a spectral filter on the optical output of a diode laser, embodiments of the invention are not limited to this implementation. In other embodiments, for example, the FBG is used as a cavity mirror in a fiber laser.

[0026]

[0038] FIG. 4A is a simplified schematic diagram of a thermo-mechanical housing 400 according to an embodiment of the present invention. The illustrated thermo-mechanical housing can be used as a thermo-mechanical housing for laser applications, providing a precision temperature controlled platform for the control and manipulation of fiber Bragg gratings. It is noted that while the thermo-mechanical housing 400 can be used in conjunction with signal processing, the thermo-mechanical housing is not limited to these applications and can be used in other optical applications. As an example, narrowband fiber gratings deployed in the thermo-mechanical housing 400 can be used in intra-cavity or extra-cavity filtering in low noise laser systems. The use of a thermo-mechanical housing allows fiber gratings with high quality factors, e.g., Q>2,000,000 (i.e., bandwidth<100 MHz), which corresponds to an order of magnitude higher quality factor than passive laser cavities typically found in semiconductor lasers. Additionally, placing high quality, high reflectivity, and low loss fiber gratings in a thermo-mechanical housing can suppress laser noise in the mid-to-high offset frequency band (offset frequency>100 MHz) when integrated into optical communication systems. Excess noise in the mid-to-high offset frequency bands can significantly affect the signal-to-noise ratio of a communication channel and the sensitivity of an optical sensing system by compromising additive noise and phase noise filtering.

[0027]

[0039] As shown in Figure 4A, embodiments of the present invention provide a mechanical design of a thermo-mechanical housing for a fiber grating that provides mechanical support to the fiber while reducing or minimizing the transfer of mechanical perturbations in the environment to the fiber, specifically to the grating section of the fiber. The new architecture, including a single area of ​​fixed contact between the thermo-mechanical housing and fiber elements, vibration isolation between the housing and the fiber, and a thermal control section thermally coupled adjacent to the FBG, allows for the implementation of a feedback control system using optical input and output power measurements useful for laser architectures and is provided by embodiments of the present invention.

[0028]

[0040] 4A, the thermo-mechanical housing 400 includes a mechanical mount 410 that can be manufactured using a high stiffness material. By way of example, the mechanical mount 410 can be manufactured from Invar or other suitable material and can have dimensions of about 20 mm in length, about 10 mm in width, and about 10 mm in thickness. A number of vias passing through the mechanical mount 410 are used to allow a shock absorbing member 412 to pass through the mechanical mount 410. The shock absorbing member 412 can be manufactured from a resilient material having high elasticity including silicone, rubber, and the like.

[0029]

[0041] Although not shown in FIG. 4A for clarity, in this embodiment, the mechanical mount 410 is joined to the housing (not shown) using fasteners 413, which are illustrated as Phillips head screws. To isolate the mechanical mount 410 from the housing (not shown), a shock absorbing member 412 is attached between the mechanical mount 410 and the housing (not shown) and fasteners 413. In other embodiments, other attachment techniques are used to mechanically isolate the mechanical mount 410 from the surrounding structure. Although the shock absorbing member 412 is shown as a ring-shaped elastic O-ring, this is not required in the present invention and other implementations and may include an elastic gasket having a shape corresponding to the shape of the mechanical mount 410 and may be placed between the mechanical mount 410 and the housing (not shown) to reduce vibrations of the fiber 420, particularly the grating portion 422, which may be referred to as an FBG, at, for example, acoustic frequencies. This allows the thermo-mechanical housing 400, including the housing (not shown), to be utilized as a thermo-mechanical housing for a laser. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0030]

[0042] To support the grating portion 422 of the fiber 420, the fiber suspension unit 424 includes a heating element 426 attached to the mechanical mount 410 and a base plate 428 attached to the heating element 426. In some embodiments, the heating element 426 includes a thermoelectric material that can heat or cool in response to the flow of electrical current. In other embodiments, a resistive element is included in the heating element 426, allowing the temperature of the heating element to increase or decrease in response to the flow of electrical current. Although thermoelectric and resistive elements are described herein, embodiments of the invention are not limited to these particular implementations of heating elements and other devices can be used to provide thermal control of the heating element 426.

[0031]

[0043] In an embodiment in which the heating element 426 is implemented as a thermoelectric element, the thermoelectric element has a thickness on the order of 2-3 mm and can be attached directly to the mechanical mount 410 or can be placed between a shock absorbing pad (not shown) and a base plate 428. In this embodiment, the base plate 428 can be placed on the cold side and the mechanical mount 410 can be placed on the hot side of the thermoelectric element. The membrane heater can also be used with, for example, insulation (e.g., a Teflon plate) placed between the membrane heater and the mechanical mount 410 to reduce heat flow to the mechanical mount 410. Thus, in this embodiment, the membrane heater can be >5 mm thick in combination with the insulation. As will be apparent to one skilled in the art, the response time associated with thermal control is roughly proportional to (total thermal capacity of the holder) / (maximum thermal output power of the thermal element-heat loss to external elements), so reducing the size of the various elements and improving insulation around the elements will increase response.

[0032]

[0044] The combination of mechanical mount 410 supported by shock absorbing member 412, fiber guide 430 and fiber suspension unit 424 that suspend fiber 420 with a low friction and high thermal conductivity material, and fiber anchor 434 attached to mechanical mount 410 using shock absorbing pad 436 allows fiber 420 to be isolated from both external vibrations and mechanical stresses through the damped suspension of fiber 420, particularly grating portion 422, by these elements utilizing thermal grease.

[0033]

[0045] The base plate 428 is attached to the heating element 426 and is thermally coupled to the heating element 426 for injecting or extracting thermal energy into or from the interior or exterior of the grating portion 422 of the fiber 420. The base plate 428 includes a groove running along the length of the base plate. The groove may be referred to as a flute. In the illustrated embodiment, the groove is laterally centered in the base plate 428 and has a depth approximately equal to the thickness of the base plate 428. The grating portion 422 of the fiber 420 is suspended in a thermal compound (not shown). The thermal compound, for example, a silicone or graphite infused paste, provides a high level of thermal conductivity between the grating portion 422 of the fiber 420, the base plate 428, and the heating element 426. Various thermal compounds, also referred to as thermal greases, may be utilized to provide a low friction, high thermal conductivity support for the grating portion 422 of the fiber 420. Thus, thermally induced stresses that may occur in the grating portion 422 of the fiber 420 when the temperature of the grating portion 422 is changed without a change in length can be reduced or prevented.

[0034]

[0046] In some embodiments, the grating section 422 of the fiber 420 as well as other portions of the fiber 420 can be encased in a low friction sheath, such as a graphite wrap that circumscribes and contacts the fiber, allowing the fiber to move longitudinally relative to the low friction sheath. Such a graphite wrapped fiber surface would be characterized by reduced friction against the side walls of the groove in the base plate 428. In addition to reducing friction, a low friction sheath that allows the fiber to translate longitudinally can be characterized by high thermal conductivity, improving thermal uniformity of the fiber and thermal compound along the grating section 422. In embodiments using a graphite wrap, the graphite sheet has high thermal conductivity (>>100 W / (mK)) parallel to the surface of the graphite sheet, i.e., along the longitudinal axis of the fiber, so that the temperature of the fiber, and particularly the grating section, can be maintained at a uniform temperature, thereby preventing passband broadening. As will be apparent to one skilled in the art, if the grating section is characterized by a non-uniform temperature as a function of length, the grating section will chirp because either the refractive index and / or the grating spacing will vary with length. This chirp will result in passband broadening because different portions of the fiber at different temperatures will be characterized by different reflectivity profiles. In addition to graphite traps, other approaches to increase longitudinal heat transfer are included within the scope of the present invention and may be applied as appropriate, including embedding a heat transfer filament in a thermal compound. Thus, utilizing embodiments of the present invention, thermal control via heating element 426 and thermal sensor 416, e.g., a thermistor embedded in cover plate 414, an element of fiber suspension unit 424, allows control of the center frequency of the FBG, and passband broadening is reduced by uniform temperature as a function of the length of the FBG.

[0035]

[0047] The fiber guide 430 supports the fiber 420 but allows the fiber 420 to move longitudinally in response to temperature changes and longitudinal thermal expansion or contraction of the fiber. As discussed in relation to the base plate 428, the fiber guide 430 includes grooves that run along the length of the fiber guide and the fiber 420 suspended in a lubricant compound (not shown) that fills the grooves. In some embodiments, the grooves in the fiber guide 430 are referred to as longitudinal channels to distinguish them from the grooves present in the base plate 428. The lubricant compound, e.g., a silicone or graphite infusion paste, in conjunction with the grooves in the fiber guide 430 allows the fiber 420 to be supported while allowing translational movement along the longitudinal direction. In some implementations, the portion of the fiber 420 supported by the fiber guide 430 is encased in a low friction sheath as discussed above. Thus, mechanically induced stresses that may result if the length of the fiber 420 is not able to expand or contract as the temperature of the fiber changes can be reduced or prevented. 4A, cap 432 is attached, for example, glued, bolted, screwed, or otherwise affixed to fiber guide 430 to enclose the corresponding portion of fiber 420 and the lubricant compound within the groove of fiber guide 430. Thus, fiber 420 is supported laterally while allowing some vertical movement resulting from thermal changes.

[0036]

[0048] A fiber anchor 434 is disposed on the end of the mechanical mount 410 opposite the fiber guide 430 and supports the fiber 420. The fiber 420 is secured in a substantially fixed vertical position using the fiber anchor 434. As an example, the fiber suspension unit 424 can be manufactured using a plastic / metal cube that is split in half and has a V-groove cut into the top surface of the lower half. The halves are then bonded together after the fiber 420 is placed between them. In the illustrated embodiment, the fiber anchor 434, which can be manufactured using a hard material such as metal or hard plastic, is mounted on a shock absorbing pad 436, although in some embodiments, the fiber anchor 434 is manufactured using a resilient material such as rubber that provides both mechanical support and flexibility. In other embodiments, a resilient material is utilized for both the fiber anchor 434 and the shock absorbing pad 436. In one embodiment, the fiber 420 is placed on the shock absorbing pad 436 and the fiber anchor 434 is pressed down to hold the fiber 420 between the shock absorbing pad 436 and the fiber anchor 434. In other embodiments, a bonding material such as epoxy is used to bond the fiber to these elements. Thus, fiber 420 is mechanically supported in a substantially rigid manner at a substantially single longitudinal location by fiber anchor 434, while providing some mechanical isolation with respect to mechanical mount 410.

[0037]

[0049] In another embodiment, the fiber anchor 434 is attached to the mechanical mount 410 using the shock absorbing pad 436, a fastener (e.g., a screw) passing through the preloaded shock absorbing pad 436. In another embodiment, the fiber 420 can be sandwiched between shock absorbing pads, for example, one shock absorbing pad placed above the fiber and the other shock absorbing pad placed below the fiber. In this embodiment, a clamp, for example a bent metal clamp, can be used to press the shock absorbing pad against the metal support (in this case the grey pad).

[0038]

[0050] By anchoring the fiber 420 in a single longitudinal region that is spatially separated from the grating section 422, the grating section 422 is free to expand and contract longitudinally in response to thermal changes. This structure differs from conventional approaches in which the grating section is fixed at both ends, along a portion of, or the entire length of the grating section. For example, if the grating section is fixed at both ends and the temperature of the thermo-mechanical housing increases, elements of the thermo-mechanical housing, including the mechanical mount 410, will expand and stretch the fiber, thereby increasing the grating spacing and changing the grating's reflection profile.

[0039]

[0051] A cover plate 414 is attached to the base plate 428 to surround the fiber 420 at the grating portion 422 with a groove formed in the base plate 428. In some embodiments, a thermal sensor, e.g., a thermistor, is embedded in the cover plate 414 to provide data regarding the temperature of the grating portion 422 of the fiber 420. In contrast to approaches that measure the temperature of elements more distant from the grating portion 422, for example, the temperature of the mechanical mount 410, the proximity between the grating portion 422 of the fiber 420 and the thermal sensor embedded in the cover plate 414, and thermal conduction between these elements allows the temperature measured by the thermal sensor to closely match the actual temperature of the grating portion 422 of the fiber 420. In other embodiments, the thermal sensor is attached to the cover plate 414 and embedded in or attached to the base plate 428.

[0040]

[0052] Figure 4B is a simplified schematic diagram illustrating a thermo-mechanical housing according to an alternative embodiment of the present invention. The thermo-mechanical housing shown in Figure 4B has components in common with the thermo-mechanical housing shown in Figure 4A, and the description provided in connection with Figure 4A applies to Figure 4B as appropriate.

[0041]

[0053] 4B, thermo-mechanical housing 450 utilizes the components included in thermo-mechanical housing 400, but replaces fiber anchor 434 and shock absorbing pad 436 with fiber guide 430 and cap 432. In this embodiment, rather than the fiber 420 being fixed at one end, both ends of the fiber are free to translate longitudinally in response to temperature changes and longitudinal thermal expansion or contraction of the fiber. In this embodiment, cap 432 can be attached to fiber guide 430 in a manner other than adhesive, including a screw attachment.

[0042]

[0054] 4B, the fiber 420 is free to move longitudinally, but the fiber will remain in the longitudinal grooves of the base plate 428 for several reasons. For example, the fiber position within the base plate is caused by contact with a thermal material (such as thermal paste) that is placed in the longitudinal grooves with the fiber and / or by contact with grooves that run along the length of the fiber guide. Additionally, contact between the fiber and a lubricant compound (not shown) that fills the grooves maintains the fiber within the base plate.

[0043]

[0055] 4C is a simplified schematic diagram of an FBG assembly assembled with fiberized components according to an embodiment of the present invention. The FBG assembly 460 can be implemented as the thermo-mechanical housing 400 shown in FIG. 4A or as the thermo-mechanical housing 450 shown in FIG. 4B. Furthermore, in some embodiments, only certain components of the thermo-mechanical housing 450 are utilized to fabricate the FBG assembly 460, such as the elements shown in FIG. 4B, including the base plate with longitudinal grooves, the heating elements, such as one or more thermoelectric coolers (TECs), the cover plate with embedded thermal sensors, other thermal sensors, etc., but with the fiber guide 430 and cap 432 removed at both ends of the base plate 428.

[0044]

[0056] 4C, the fiber 420 extending from the thermo-mechanical housing 400 / 450 is mechanically attached at a first end to a first mounting element, implemented in this embodiment as fiberizing component 462, and at a second end to a second mounting element, implemented in this embodiment as fiberizing component 464. By providing an overall anchor that secures in place the fiber portion adjacent the fiberizing component, fiberizing components 462 and 464 help maintain the fiber in the longitudinal groove in the base plate. Examples of fiberizing components include, but are not limited to, isolators, couplers, filters, taps, and wavelength division multiplexers.

[0045]

[0057] Additionally, bends 463, shown in this example as a 90° bend, and bends 465, shown in this example as a 180° bend, act in a spring-like manner to provide flexibility along the length of the fiber to cushion temperature induced changes in fiber length and vibrations. Although 90° and 180° bends are shown in FIG. 4C, this is merely exemplary and other bend angles can be utilized by embodiments described herein to secure the fiber in place via the support provided by the fiberization components and the bends in the fiber.

[0046]

[0058] 4D is a simplified schematic diagram of an FBG assembly incorporating fixed fiber components according to one embodiment of the present invention. The FBG assembly 470 can be implemented as the thermo-mechanical housing 400 shown in FIG. 4A or as the thermo-mechanical housing 450 shown in FIG. 4B. Furthermore, in some embodiments, only certain components of the thermo-mechanical housing 450 are utilized to fabricate the FBG assembly 470, such as those shown in FIG. 4B, but the fiber guides 430 and caps 432 at both ends of the base plate 428 are removed.

[0047]

[0059] 4D, the fiber 420 extending from the thermo-mechanical housing 400 / 450 is mechanically attached at a first end to a first mounting element, implemented in this embodiment as fiber anchor 472, and at a second end to a second mounting element, implemented in this embodiment as fiber anchor 474. The fiber anchor may be implemented using fiber anchor 434 shown in FIG. 4A. In other embodiments, the fiber anchor may be implemented using a suitable adhesive and / or any of a permanent or temporary adhesive material. By providing an overall anchor that secures in place the fiber portion adjacent to the fiber anchor, fiber anchors 472 and 474 help maintain the fiber in the longitudinal groove in the base plate.

[0048]

[0060] Additionally, bend 473, shown in this example as a 90° bend, and bend 475, shown in this example as a 180° bend, act in a spring-like manner to provide flexibility along the length of the fiber to cushion temperature induced changes in fiber length and vibrations. Although 90° and 180° bends are shown in FIG. 4D, this is merely exemplary and other bend angles can be utilized by embodiments described herein to secure the fiber in place via the support provided by the fiber anchor and the bend in the fiber.

[0049]

[0061] Figures 5A and 5B show two implementations of using FBGs as optical filters. A standard FBG has a narrow reflection spectrum. To use an FBG as a filter and generate a filtered output 518, an optical circulator 514 is placed between the signal of interest, generated using, for example, a diode laser 510, and the FBG 515, as shown in Figure 5A. This allows the reflected light to be sent in a different direction than the signal origin. As is commonly known to those skilled in the art, a phase-shifted FBG has a narrow transmission peak within a wider reflection band. In this case, the transmitted light is the filtered light 528, and no optical circulator is needed to separate the filtered light from the input signal path, as shown in Figure 5B. In this filtering application, two optical taps are used to measure the optical power. For a conventional FBG, the input optical tap 512 is placed in the optical circulator 514 before the signal input, and the output optical tap 516 is placed after the reflected output of the optical circulator 514. For a phase-shifted FBG, an input optical tap 522 is placed before the signal input to the FBG 525, and an output optical tap 526 is placed after the transmit output of the FBG 525. The ratio of the power measured at the input and output taps gives the transmission coefficient θ = I TapOUT / I TapIN . is obtained.

[0050]

[0062] FIG. 6 is a simplified flow chart illustrating a method for initializing and operating a spectrally controlled FBG according to an embodiment of the present invention. The method 600 illustrated in FIG. 6 can be applied to an FBG disposed in a thermo-mechanical housing as illustrated in FIG. 4A. The method 600 illustrated in the flow chart includes two parts or sections: initialization and operation. In both the initialization and operation sections, the temperature T G 6, in the initialization section 605 including steps 610, 612, 614, 616, 618, and 620, the grating temperature T G is the initial value T Start (610). This initial temperature T Start Starting from the grating temperature TG is the initial temperature T Start to the final temperature T End At each temperature in the range, the transmittance θ is measured (612). Based on the measured transmittance, the maximum transmittance θ Max is calculated (614).

[0051]

[0063] This method uses the grating temperature T G T Start (616) and the target transmittance θ Target The maximum transmittance θ Max As shown in step 618 of FIG. 6, if the multiplier η is less than 1, then θ Target is ηθ Max Grating T G The temperature of the measured transmittance θ Current is the target transmittance θ Target , i.e., θ Current =θ Target The temperature T 1 corresponding to the maximum transmittance is set to 620. The target transmittance is set to a value less than the peak value, so that the change in transmittance can be corrected clearly. G The temperature T corresponding to the maximum transmittance G Depending on whether the initial temperature is smaller than the temperature T G In larger embodiments, the grating temperature is θ Current =θ Target The initial temperature is reduced to the temperature T G In an embodiment where the grating temperature is less than θ Current =θ Target One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0052]

[0064] The operational portion 625 of the method 600 comprises determining a current transmission ratio θ Current(630) is measured continuously or repeatedly, and the grating temperature T G (632) to adjust the transmittance to its target value θ Current =θ Target In some embodiments, the response of the temperature control is determined by the grating temperature T G When the input power exceeds the input power used for initialization, the target transmission θ is adjusted to prevent feedback / control switching due to an increase in the optical power in the FBG. Target decreases.

[0053]

[0065] It should be understood that the particular steps illustrated in FIG. 6 provide a particular method for stabilizing an FBG according to one embodiment of the present invention. Other sequences of steps may be performed according to alternative embodiments. For example, the steps outlined above may be performed in a different order in alternative embodiments of the present invention. Additionally, the individual steps illustrated in FIG. 6 may include multiple sub-steps that may be performed in various orders as appropriate for the individual steps. Additionally, additional steps may be added or removed depending on the particular application. Those skilled in the art will recognize numerous variations, modifications, and alternatives.

[0054]

[0066] FIG. 7 is a plot showing transmission as a function of grating temperature according to one embodiment of the present invention. An initialization section 605 is shown below the grating temperature axis. (1) The temperature of the grating T G Set the initial value T Start (2) This initial temperature T Start Starting from the grating temperature T G is the initial temperature T Start to the final temperature T End At each temperature in the range, the transmittance θ is measured. Based on the measured transmittance, the maximum transmittance θ Max is calculated.

[0055]

[0067] (3) Grating temperature TG T Start (4) The measured transmittance θ Current is the target transmittance θ Target Grating T to reach G Change the temperature.

[0056]

[0068] As discussed in relation to the operating portion 625, the temperature regulation is Current Measure the grating temperature T G Adjust the transmittance to the target value θ Target In the example of FIG. 7, the current transmission rate θ Current is the target value θ Target Therefore, the grating temperature T G Reduce by ΔT and set the transmittance to the target value θ Target In the embodiment shown in FIG. Target is approximately 90% of the maximum transmittance, but this is merely an example, and in this and other embodiments of the present invention, the target value θ Target is either close to or far from the maximum transmittance. As an example, in some embodiments, the target value θ Target can be approximately half of the maximum transmission, which can be placed close to the maximum slope of the transmission versus grating temperature to achieve high sensitivity. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0057]

[0069] The examples and embodiments described herein are for illustrative purposes only. Various modifications or changes thereto will be apparent to those skilled in the art. These are to be included within the spirit and scope of this application and the following appended claims.

Claims

1. 1. A system for stabilizing optical parameters of a fiber Bragg grating (FBG), comprising: Mechanical mount and a heating element coupled to the mechanical mount; a base plate coupled to the heating element, the base plate comprising a longitudinal groove; A fiber including a first portion, a second portion opposite the first portion, and the FBG, the FBG is disposed between the first portion of the fiber and the second portion of the fiber; the first portion of the fiber is fixed in a longitudinal position; the second portion of the fiber is free to move; a fiber, the FBG of the fiber being disposed in the longitudinal groove and free to move; A system comprising:

2. The system of claim 1 , further comprising a fiber guide coupled to the mechanical mount, the fiber guide including a longitudinal channel, the second portion of the fiber being disposed in the longitudinal channel.

3. The system of claim 2 , further comprising a lubricant compound at least partially surrounding the second portion of the fibers disposed in the longitudinal channel.

4. The system of claim 1 , further comprising a thermal compound at least partially surrounding the FBG of the fiber.

5. The system of claim 1 , further comprising a low-friction sheath at least partially surrounding the FBG of the fiber.

6. The system of claim 5 , wherein the low friction sheath comprises a graphite wrap.

7. The system of claim 1 , wherein the mechanical mount is attached to the housing by a shock absorbing member.

8. a cover plate coupled to the base plate; a thermal sensor coupled to the cover plate; The system of claim 1 further comprising:

9. The system of claim 8 , wherein the thermal sensor comprises a thermistor embedded in the cover plate.

10. The system of claim 1 , wherein the optical parameter comprises a center frequency of the FBG.

11. The system of claim 1 , wherein the optical parameter comprises a bandwidth of the FBG.

12. 1. A method for stabilizing a fiber Bragg grating (FBG) disposed within a thermo-mechanical housing, comprising: setting the temperature of the FBG to an initial value; changing the temperature of the FBG to a final value; measuring the transmission at a number of FBG temperatures between the initial value and the final value; determining a maximum transmittance; setting the temperature of the FBG to the initial value; setting a target transmittance equal to a predetermined percentage of the maximum transmittance; varying the temperature of the FBG until the transmittance is equal to the target transmittance; varying the temperature of the FBG until the transmittance is equal to the target transmittance; performing an initialization process, including: (a) measuring the transmittance; (b) adjusting the FBG temperature to match the measured transmittance to the predetermined percentage of the maximum transmittance; (c) repeatedly executing (a) and (b); performing an operational process, including: A method for providing the above.

13. The method of claim 12 , wherein altering a temperature of the FBG comprises increasing the temperature of the FBG.

14. The method of claim 12 , wherein repeatedly varying the temperature of the FBG comprises decreasing the temperature of the FBG.

15. The method of claim 12 , wherein the predetermined percentage is between 80% and 95%.

16. The method of claim 12 , wherein the initial value is greater than the final value.

17. 13. The method of claim 12, wherein varying the temperature of the FBG comprises repeatedly varying the temperature of the FBG and measuring the transmittance until the transmittance equals the target transmittance.

18. 1. A system for stabilizing optical parameters of a fiber Bragg grating (FBG) portion of an optical fiber, comprising: Mechanical mount and a heating element coupled to the mechanical mount; a base plate coupled to the heating element, the base plate including a longitudinal groove, the FBG portion of the optical fiber being disposed in the longitudinal groove; A fiber including a first portion, a second portion opposite the first portion, and the FBG portion, the FBG section is disposed between the first section of the fiber and the second section of the fiber; the first portion of the fiber is fixed in a longitudinal position; the second portion of the fiber is free to move; a fiber, the FBG portion of the fiber being disposed in the longitudinal groove and free to move; a first fiber guide coupled to the mechanical mount, the first fiber guide including a first longitudinal channel, the first portion of the optical fiber being disposed in the first longitudinal channel; a second fiber guide coupled to the mechanical mount, the second fiber guide including a second longitudinal channel, the second portion of the optical fiber being disposed in the second longitudinal channel; and A system comprising:

19. 20. The system of claim 18, further comprising a lubricant compound at least partially surrounding the first portion of the optical fiber disposed in the first longitudinal channel and the second portion of the optical fiber disposed in the second longitudinal channel.

20. The system of claim 18 , further comprising a thermal compound at least partially surrounding the FBG portion of the optical fiber.

21. 20. The system of claim 18, further comprising a low-friction sheath at least partially surrounding the FBG portion of the optical fiber.

22. 22. The system of claim 21, wherein the low friction sheath comprises a graphite wrap.

23. The system of claim 18 , wherein the mechanical mount is attached to the housing by a shock absorbing member.

24. a cover plate coupled to the base plate; a thermal sensor coupled to the cover plate; 20. The system of claim 18, further comprising:

25. The system of claim 24 , wherein the thermal sensor comprises a thermistor embedded in the cover plate.

26. An assembly comprising: Mechanical mount and a heating element coupled to the mechanical mount; an assembly comprising: a base plate coupled to the heating element, the base plate comprising a longitudinal groove; a fiber including a fiber Bragg grating (FBG) passing through the assembly, the fiber is disposed in the longitudinal groove of the base plate and is configured to be free to move longitudinally; a fiber, the FBG of the fiber being disposed in the longitudinal groove of the base plate; a first attachment element mechanically coupled to a first end of the fiber; a second mounting element mechanically coupled to a second end of the fiber; An optical system comprising:

27. a first bend at the first end of the fiber disposed between the base plate and the first mounting element; a second bend at the second end of the fiber disposed between the base plate and the second mounting element; and 27. The optical system of claim 26, further comprising:

28. 28. The optical system of claim 27, wherein the first bend and the second bend have different values.

29. a first fiber guide coupled to the mechanical mount, the first fiber guide comprising a first longitudinal channel, the front first end of the fiber being disposed in the first longitudinal channel; a second fiber guide coupled to the mechanical mount, the second fiber guide comprising a second longitudinal channel, the second end of the fiber being disposed in the second longitudinal channel; and 27. The optical system of claim 26, further comprising:

30. 30. The optical system of claim 29, further comprising a lubricant compound at least partially surrounding the first ends of the fibers disposed in the first longitudinal channel.

31. 27. The optical system of claim 26, wherein the first mounting element comprises a first fiberizing component and the second mounting element comprises a second fiberizing component.

32. 27. The optical system of claim 26, wherein the first mounting element comprises a first fiber anchor and the second mounting element comprises a second fiber anchor.

33. 27. The optical system of claim 26, further comprising a thermal compound at least partially surrounding the FBG of the fiber.

34. 27. The optical system of claim 26, further comprising a graphite wrap at least partially surrounding the FBG of the fiber.

35. The optical system of claim 31, wherein the first fiberized component comprises an isolator, a coupler, a filter, a tap, or a wavelength division multiplexer.

36. 27. The optical system of claim 26, wherein the mechanical mount is attached to the housing by a shock absorbing member.

37. a cover plate coupled to the base plate; a thermal sensor coupled to the cover plate; 27. The optical system of claim 26, further comprising:

38. 38. The lighting system of claim 37, wherein the thermal sensor comprises a thermistor embedded in the cover plate.

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