JP2005157299A - Fiber grating module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber grating module capable of obtaining a stable Bragg wavelength shift.
SOLUTION: An optical fiber 6 in which a V-groove 2 is formed in a polyethylene substrate 1 having a very large linear expansion coefficient, a core 3 having a fiber grating 4 is provided at the center of a clad 5, and the fiber grating 4 is a polyethylene substrate 1 is provided. The optical fiber 6 is bonded to the V-groove 2 with a polymer-based adhesive so as to be positioned above and integrated. Then, a temperature control mechanism for controlling the temperature and expansion / contraction of the polyethylene material is provided to constitute the module. When the polyethylene substrate 1 is heated (cooled), the polyethylene substrate 1 having a large linear expansion coefficient expands (shrinks) greatly. At this time, the fiber grating 4 bonded and integrated with the polyethylene substrate 1 is also stretched (shrinked), and the Bragg wavelength is shifted to the long wavelength side (short wavelength side).
[Selection] Figure 1

Description

  The present invention relates to a fiber grating module, and more particularly to a fiber grating module capable of obtaining a stable Bragg wavelength shift.

  A fiber grating is a distributed feedback structure by periodically modulating the refractive index of the core of an optical fiber. In the vicinity of a wavelength called “Bragg wavelength”, a part of incident light is converted into reflected light. To convert. With such a function of the fiber grating, it also functions as a “reflective optical filter” that reflects light having a wavelength in the vicinity of the Bragg wavelength.

The Bragg wavelength λ _B depends on the modulation period Λ of the refractive index of the core and the effective core refractive index n _eff, and is given by the following equation (1).
λ _B = 2n _eff Λ (1)

In general, since it is desirable that the optical filter has a variable operating wavelength, as can be seen from Equation (1), in order to make the Bragg wavelength of the fiber grating variable, at least the modulation period Λ or the effective core refractive index n _eff What is necessary is just to change one side.

  In order to change the modulation period Λ, a method of applying strain to the fiber grating by applying mechanical extension or compression can be considered.

The relationship between the distortion and the shift amount Δλ _B of the Bragg wavelength is given by the following equation (2) (see Non-Patent Document 1), and the distortion amount ε is given by the following equation (3).
(Δλ _B / λ _B ) = 0.78ε (2)
ε = (△ l _FG / l _FG ) (3)
Here, l _FG is the length of the fiber grating, and Δl _FG is the expansion amount (positive value) or the compression amount (negative value). The reason why the coefficient does not become 1 is due to the effect of slightly changing the segregation rate due to the application of strain and shifting the Bragg wavelength in the reverse direction. According to equation (2), for example, if the Bragg wavelength is in the vicinity of 1550 nm, it can be seen that when the strain amount ε is ± 0.1%, the shift amount of the Bragg wavelength is about ± 1 nm.

On the other hand, in order to change the effective core diffraction rate n _eff , it is only necessary to change the temperature of the core. The relationship between the temperature change ΔT and the Bragg wavelength shift amount Δλ _B is given by the following equation (4) (see Non-Patent Document 1).
(Δλ _B / λ _B ) = 6.67 × 10 ⁻⁶ ΔT (4)
Note that the change in Bragg wavelength due to the temperature change in the core is caused not only by the above-described change in the refractive index but also by thermal expansion. According to equation (4), for example, assuming that the Bragg wavelength is in the vicinity of 1550 nm, it can be seen that a shift amount of the Bragg wavelength of about ± 1 nm finally occurs with a temperature change of ± 100 ° C.

  The wavelength tuning range of the fiber grating is generally considered to be about several nanometers. However, if wavelength tuning at the level of several nanometers is to be executed due to the temperature change applied to the core, a temperature change exceeding 100 ° C. is required. Therefore, it is not a practical method. Therefore, as a general wavelength tuning method, a method of imparting distortion is employed. As a specific method, a method of compressing or expanding a fiber grating with an actuator using an electrically controlled piezo element is known. By compressing a fiber grating, a 2.5 mm long fiber grating is compressed. An example in which the Bragg wavelength is shifted by 45 nm to the short wavelength side (Non-patent Document 2) and by extending the fiber grating, the reflection band of the 10 mm long chirped fiber grating is shifted to the long wavelength side by 7 nm as a whole. (Non-Patent Document 3).

A. D. Kersey et al., “Fiber grating sensors”, IEEE / OSA Journal of Lightwave Technology, vol.15, no.8, pp.1442-1463 (1997). A. Iocco et al., “Bragg grating fast tunable fiber for wavelength division multiplexing”, IEEE / OSA Journal of Lightwave Technology, vol.17, no.7, pp.1217-1221 (1999). T. Inui et al., “Highly efficient tunable fiber Bragg grating filters using multilayer piezoelectric transducers”, Optics Communications, vol.190, no.7, pp.1-4 (2001). National Astronomical Observatory of Japan "Science Chronology 2003 Edition" (Maruzen, pp.399-400, (2003). T. Yamamoto et al., “270-360 GHz tunable beat signal light generator for photonic local oscillator”, Electronic Letters, vol.38, no.15, pp.795-797 (2002).

  As described above, although a technology for electrically controlling the Bragg wavelength of a fiber grating having a length of several millimeters has been proposed, the actual length of the fiber grating is not limited to several millimeters. It is. In general, in order to narrow the band of the fiber grating, it is necessary to lengthen the length, and in the case of the chirp fiber grating for dispersion compensation, it is necessary to lengthen the length in order to increase the dispersion compensation amount. is there. For this reason, a fiber grating having a length of 100 mm is actually produced.

  As can be seen from Equation (3), when the grating length is increased, the displacement amount of the actuator needs to be increased in order to obtain the same wavelength shift amount. However, in practice, the displacement amount of the actuator using the piezo element is about several tens of μm, so that the wavelength shift amount decreases as the grating length increases. For example, even if a shift amount of 45 nm is obtained with a 2.5 mm long fiber grating as in the above example, the shift amount is about 1 nm when the grating length is 100 mm. Further, when the grating length is 100 mm, the compression itself becomes difficult. For these reasons, it is difficult to tune the Bragg wavelength of a long fiber grating over a wide range.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a fiber grating module capable of obtaining a stable Bragg wavelength shift.

  In order to achieve such an object, the present invention provides a fiber grating module in which the fiber grating is provided in thermal contact with a holder made of polyethylene or a polyethylene-based material. And a temperature control means having at least one of a heating means and a cooling means, and the temperature control means tunes the Bragg wavelength of the fiber grating.

  According to a second aspect of the present invention, in the fiber grating module according to the first aspect, the holding body is a plate provided with a groove for placing the fiber grating on one surface.

  According to a third aspect of the present invention, in the fiber grating module according to the first aspect, the holding body includes first and second plates each having a groove on one surface, and the fiber grating includes the two It is characterized by being sandwiched and held in a groove provided on a plate.

  According to a fourth aspect of the present invention, in the fiber grating module according to the first aspect, the holding body is a cylinder having a hollow portion for storing the fiber grating.

  According to a fifth aspect of the present invention, in the fiber grating module according to the first to fourth aspects, the heating means is a metal thin film that is provided on the outer surface of the holding body and generates heat when energized. And

  According to a sixth aspect of the present invention, in the fiber grating module according to the third aspect, the temperature control means includes first and second Peltier elements each having a heat absorbing portion and a heat generating portion, and the first Peltier element. The element is provided in such a manner that the other surface of the first plate and the heat absorbing part are in contact with each other, while the second Peltier element is provided in contact with the other surface of the second plate and the heat generating part. It is characterized by being.

  The invention according to claim 7 is the fiber grating module according to claim 1, wherein the holding body is a ring having a groove for winding the fiber grating around an outer periphery. .

  The invention according to claim 8 is the fiber grating module according to any one of claims 1 to 7, wherein the fiber grating is fixed to the holding body with a polymer adhesive.

  According to a ninth aspect of the present invention, in the fiber grating module according to any one of the first to seventh aspects, the fiber grating closely contacts the fiber grating with the holder softened or melted by heat and cooled. It is fixed to the holding body by being cured again.

  According to the present invention, a sufficiently large Bragg wavelength shift can be stably obtained even with a long fiber grating. In addition, by using the fiber grating module of the present invention, it is possible to realize a high-performance optical system that was difficult to realize with the prior art.

Embodiments of the present invention will be described below with reference to the drawings.
The fiber grating module of the present invention integrates a polyethylene material having an extremely large linear expansion coefficient and a fiber grating (or an optical fiber provided with a fiber grating) to control the temperature and expansion / contraction of the polyethylene material. A module is configured by providing a control mechanism.

The polyethylene-based material used in the present invention has an extremely large linear thermal expansion coefficient α among general materials, and has a linear expansion coefficient α that is 10 times or more that of glass as an optical fiber material, and l (T) is The length at the temperature T is given by the following equation (5) (see Non-Patent Document 4).
α = (1 / l) · (dl / dT) (5)

  The integration of the fiber grating and the polyethylene-based material can be freely changed and designed as desired. For example, the fiber grating is integrated by forming a groove in a polyethylene substrate and bonding the fiber grating to the groove. When the polyethylene substrate is heated in this state, the polyethylene substrate having a large linear expansion coefficient expands greatly. At this time, the fiber grating bonded and integrated with the polyethylene substrate is also expanded and contracted, and the Bragg wavelength is shifted to the long wavelength side. On the contrary, when the polyethylene substrate is cooled, the polyethylene substrate having a large linear expansion coefficient is greatly contracted, and the fiber grating is also contracted to shift the Bragg wavelength to the short wavelength side.

  Such heating may be performed using a commercially available small heater or small cooler, but for example, resistance heating with a nichrome wire or the like and Peltier element is performed, and cooling is performed with a Peltier element. The temperature can be easily controlled by the value of the flowing current.

  In principle, the module configuration described above can provide the effects of the present invention only by preparing a polyethylene-based material corresponding to the length of the fiber grating. Therefore, the module configuration corresponds to the amount of temperature change regardless of the fiber grating length. There is a significant advantage that a Bragg wavelength shift amount can be obtained. Further, the present invention is not limited to the uniform type fiber grating having a constant Bragg wavelength distribution, but can be applied to a so-called chirped fiber Bragg grating in which the Bragg wavelength changes in the longitudinal direction.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  FIG. 1 is a diagram for explaining a first configuration example of a fiber grating module according to the present invention. A V-groove 2 is formed on a polyethylene substrate 1 having a length of 150 mm, a width of 10 mm, and a thickness of 2 mm, and the length is 100 mm. The optical fiber 6 in which the core 3 having the fiber grating 4 is provided at the center of the clad 5 is bonded to the V-groove 2 with a polymer-based adhesive so that the fiber grating 4 is positioned on the polyethylene substrate 1. doing.

  FIG. 2 is a diagram for explaining the temperature dependence of the Bragg wavelength when the polyethylene substrate 1 on which the fiber grating 4 is mounted as shown in FIG. 1 is placed on a small heater and heated. The temperature dependence characteristic of the single fiber grating calculated based on (4) is shown simultaneously.

  The Bragg wavelength of the present example in which the fiber grating 4 is bonded to the polyethylene substrate 4 is changed so that the Bragg wavelength monotonically increases at a rate of about 0.15 nm / ° C., whereas the single fiber grating is heated. In this case, the temperature dependence is about 0.01 nm / ° C., and the change in Bragg wavelength is extremely small.

  Thus, since the fiber grating module of the present invention can obtain a large Bragg wavelength shift even with a slight temperature change, a stable Bragg wavelength shift is possible.

  In addition, the shape of the groove | channel provided in the polyethylene substrate 1 does not need to be V shape, For example, shapes, such as a U groove | channel, may be sufficient. Needless to say, the optical fiber 6 provided with the fiber grating 4 may be resin-coated.

  In the configuration shown in FIG. 1, the portion corresponding to the fiber grating 4 of the optical fiber 6 is bonded to the polyethylene substrate 1. There is a risk of becoming bigger only. If such anisotropic distortion occurs, birefringence may occur and polarization dispersion may occur.

  FIG. 3 is a diagram for explaining a second configuration example of the fiber grating module of the present invention for solving such a problem. In this configuration, the module having the configuration shown in FIG. 1 is further provided with a second polyethylene substrate 1 ′ having a V-groove 2 ′, and the optical fiber 6 (ie, two polyethylene substrates (1 and 1 ′)) is used. The fiber grating 4) is closely attached so as to cover the fiber grating 4). The second polyethylene substrate 1 ′ is also fixed in close contact with a heating / cooling unit such as a small heater, similarly to the polyethylene substrate 1.

  FIG. 4 is a diagram for explaining a third configuration example of the fiber grating module of the present invention, which is another configuration for avoiding the above-described birefringence induction, and in this configuration, around the fiber grating 4. A polyethylene layer 7 is provided concentrically, and this polyethylene layer 7 is coated with a titanium thin film 8. The optical fiber and the polyethylene layer 7 are in close contact with an adhesive or the like.

  In the case of this configuration, a current is passed through the titanium thin film 8 provided on the outer surface of the polyethylene layer 7 to generate heat, the polyethylene layer 7 is expanded in the longitudinal direction (extending direction of the optical fiber 6), and a fiber grating. 4 can be stretched.

  In addition, the film | membrane provided in the outer surface of the polyethylene layer 7 does not need to be a titanium thin film, and should just be a material which generate | occur | produces heat by electricity supply.

  FIG. 5 is a diagram for explaining a fourth configuration example of the fiber grating module of the present invention. This configuration is different from the module configuration of the second embodiment shown in FIG. 9 is provided on each of the upper and lower polyethylene substrates (1 and 1 '). The Peltier element 9 has a heat absorbing portion (surface) 10 and a heat radiating portion (surface) 11. The heat absorbing portion (surface) 10 is in contact with the surface of one substrate, and the surface of the other substrate is The Peltier element 9 is pasted so that the heat radiating part (surface) 11 contacts.

  With such a configuration, when a current is passed through the Peltier element 9 in which the heat absorbing portion (surface) 10 is in contact with the substrate surface, the substrate is cooled and the wavelength of the Bragg of the fiber grating 4 is shifted to the short wavelength side. On the other hand, if a current is applied to the Peltier element 9 in which the heat radiating part (surface) 11 is in contact with the substrate surface, the substrate is heated and the wavelength of the Bragg of the fiber grating 4 is shifted to the longer wavelength side.

  As described above, according to the present invention, a sufficiently large Bragg wavelength shift can be stably obtained even with a long fiber grating. In addition, by using the fiber grating module of the present invention, it is possible to realize a high-performance optical system that has been difficult to realize with the prior art. For example, there is known a system that uses two fiber grating modules provided with a wavelength variable function by a piezo actuator and cuts out two arbitrary longitudinal modes from a broadband optical spectrum to generate beat light having a frequency of several hundred GHz. However, since the tuning range of the Bragg wavelength of this system is about 1 nm, the variable range of the frequency is narrow (Non-patent Document 5). However, according to the fiber grating module of the present invention, not only the frequency variable range can be expanded, but also the reliability of the system is improved.

  FIG. 6 is a diagram for explaining a fifth configuration example of the fiber grating module of the present invention. The V ring 2a and the V groove 2b are formed on the outer periphery and the inner periphery of the polyethylene ring 12, respectively, and the fiber grating 4 is provided. The optical fiber 6 is bonded with a polymer adhesive along the V groove 2a on the outer periphery of the polyethylene ring 12, and the nichrome wire 13 is attached along the V groove 2b.

  With such a configuration, the polyethylene ring 12 is expanded by passing a current through the nichrome wire 13, the fiber grating 4 formed in the optical fiber 6 is expanded, and the Bragg wavelength is shifted to the long wavelength side. Become. The shift amount can be controlled by a current flowing through the nichrome wire 13.

  In the above embodiment, the fiber grating is made to adhere to the surface of the holder softened or melted by heat, or to the groove or hollow part formed in the holder, and the part is cooled and cured again. Thus, it may be fixed to the holding body.

  The present invention makes it possible to provide a fiber grating module capable of obtaining a stable Bragg wavelength shift.

FIG. 3 is a diagram for explaining a first configuration example of a fiber grating module of the present invention. It is a figure for demonstrating the temperature dependence of Bragg wavelength at the time of arrange | positioning and heating the polyethylene substrate which mounted a fiber grating on FIG. 1 on a small heater. It is a figure for demonstrating the 2nd structural example of the fiber grating module of this invention. It is a figure for demonstrating the 3rd structural example of the fiber grating module of this invention. It is a figure for demonstrating the 4th structural example of the fiber grating module of this invention. It is a figure for demonstrating the 5th structural example of the fiber grating module of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Polyethylene substrate 2 V groove 3 Core 4 Fiber grating 5 Clad 6 Optical fiber 7 Polyethylene layer 8 Titanium thin film 9 Peltier element 10 Heat absorption part (surface)
11 Heat dissipation part (surface)
12 Polyethylene ring 13 Nichrome wire

Claims (9)

  The fiber grating is provided in thermal contact with a holder made of polyethylene or a polyethylene-based material, and has a temperature control means including at least one of a heating means and a cooling means, and the fiber grating has a temperature control means. A fiber grating module that is tuned to the Bragg wavelength.   2. The fiber grating module according to claim 1, wherein the holding body is a plate provided with a groove on one surface for mounting the fiber grating. 3.   The holding body includes first and second plates each having a groove on one surface, and the fiber grating is sandwiched and held between grooves provided on the two plates. The fiber grating module according to claim 1.   2. The fiber grating module according to claim 1, wherein the holding body is a cylinder having a hollow portion for storing the fiber grating. 3.   The fiber grating module according to any one of claims 1 to 4, wherein the heating means is a metal thin film that is provided on an outer surface of the holding body and generates heat when energized.   The temperature control means includes first and second Peltier elements each having a heat absorption part and a heat generation part. The first Peltier element is in contact with the other surface of the first plate and the heat absorption part. 4. The fiber grating module according to claim 3, wherein the second Peltier element is provided in such a manner that the other surface of the second plate and the heat generating portion are in contact with each other.   The fiber grating module according to claim 1, wherein the holding body is a ring having a groove for winding the fiber grating around an outer periphery.   The fiber grating module according to any one of claims 1 to 7, wherein the fiber grating is fixed to the holding body with a polymer adhesive.   8. The fiber grating is fixed to the holding body by bringing the fiber grating into close contact with the holding body softened or melted by heat, cooling and re-hardening. The fiber grating module according to any one of the above.

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