JP4071184B2 - Grating filter and reciprocating optical modulator - Google Patents

Description

  The present invention relates to a grating filter for an optical modulator used in an optical fiber radio system or the like, and more particularly to a grating filter for a reciprocating optical modulator that obtains an optical signal modulated at a high frequency in the millimeter wave band by a low frequency electric signal.

  Optical modulation in the millimeter wave and microwave bands is a key technology in fiber optic wireless systems, but the modulation efficiency is low in the high frequency band due to the effects of electrode loss and phase mismatch, and large amplitude is required to drive the modulator. RF signal is required. In order to solve this problem, a reciprocating optical modulator has been proposed in which an optical filter is placed at each of an input port and an output port of one optical phase modulator to efficiently generate optical harmonics. The reciprocating optical modulator has an advantage that the cost of the drive circuit can be reduced because an optical signal modulated at a frequency several times higher than the electric signal supplied to the modulator can be obtained. In the conventional reciprocating optical modulator, a fiber grating is directly connected to the optical modulator chip as an optical filter used therefor. The distance between the optical filters is 16 cm. When the reciprocating optical modulator is modulated with an electrical signal having a frequency of 5.5 GHz, a millimeter wave signal of 49.5 GHz is generated by the reciprocating optical modulator. (For example, refer nonpatent literature 1.)

  Also, the intensity of output light from a reciprocating optical modulator using a fiber grating as an optical filter generally fluctuates, and the fluctuation of the output light is converted into an electric signal by a photodiode, for example, and fed back to the phase modulator. Thus, the output light can be kept stable. The reason for this is that the fluctuation of the output light changes the phase of the light due to the fluctuation of the optical path. (For example, refer to Patent Document 1.)

2003 IEICE General Conference Proceedings C-14-11 JP 2002-6277 A

  In the reciprocating optical modulator as described above, since the distance between the fiber gratings provided on the input / output side of the optical modulator is long, the temperature and stress changes due to slight environmental changes cause a change between the two fiber gratings. The optical path length changes and the output light is not stable. For this reason, it is necessary to monitor the output light and stabilize it by feedback control. However, there is a problem that this feedback mechanism is complicated and high cost cannot be avoided. In addition, since the distance between the two fiber gratings on the input / output side sandwiching the optical modulator is long, there is a problem that it becomes an obstacle when miniaturizing the device.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a grating filter for a reciprocating multiplying optical modulator, in which the intensity of output light is stable even when the ambient temperature changes, and the size can be reduced.

The grating filter according to the present invention is:
A fiber grating in which a grating having a predetermined length is formed in a core of an optical fiber, wherein the grating is a fiber grating provided in the vicinity of one end face of the optical fiber;
The fiber grating includes both ends of the grating, and at least one substrate having an end face that is substantially flush with the end face of the fiber grating.

  According to the grating filter of the present invention, the grating is provided in the vicinity of one end face of the optical fiber. Further, the end face of the fiber grating and the end face of the substrate for fixing the fiber grating constitute substantially the same plane. As a result, when this grating filter is used in a reciprocating optical modulator, the optical axis can be aligned without going through other members, and can be directly connected to the input / output end of the optical modulator. The distance between the input / output terminals of the grating and the grating filter can be shortened. Further, since the grating filter is stably fixed by the substrate, the optical path length change is small and the light output is stabilized.

  A grating filter and a manufacturing method thereof according to an embodiment of the present invention, and a reciprocating optical modulator using the same will be described with reference to the accompanying drawings. In the drawings, substantially the same members are denoted by the same reference numerals.

Embodiment 1 FIG.
A grating filter according to Embodiment 1 of the present invention will be described. FIG. 1 is a perspective view showing a configuration of a grating filter 1 of the present invention. FIG. 2 is an exploded perspective view illustrating the configuration of the grating filter. In this grating filter 1, the fiber grating 2 is housed in a V-shaped groove 3 of the first quartz substrate 4 and is fixed between the V-shaped groove 3 and the second quartz substrate 5. In the fiber grating 2, a grating having a length of 15 mm is formed in the core of the optical fiber. The first quartz substrate 4 is formed with a V-shaped groove 3 that accommodates the grating portion of the grating fiber 2. The fiber grating 2 is housed in the V-shaped groove 3 of the first quartz substrate 4, and the V-shaped groove 3 and the second quartz substrate 5 of the first quartz substrate 4 are slightly separated from both ends of the grating portion. And fixed with an adhesive 6. The fiber grating 2 is a fiber grating in the coating-removed optical fiber portion 8 in which a portion of the optical fiber cable 7 having a diameter of 250 μm or a diameter of 900 μm is removed by coating the periphery of a single mode fiber having a diameter of 125 μm with a resin or the like. 2 is formed. The diameter and type of the optical fiber forming the fiber grating 2, the diameter of the coated optical fiber cable, etc. are shown as examples, and other specifications may be used as long as the grating can be formed. The length of the fiber grating 2 on which the grating is formed is 15 mm, but may be any length designed according to the required specifications. Furthermore, although the V-shaped groove 3 is formed in the first quartz substrate 4, if the positional accuracy when the fiber grating 2 is installed in the groove is not important, the U-shaped groove may be a groove of another shape. May be.

  Next, the optical fiber fixing portion 9 that fixes the fiber grating 2 at one end of the optical fiber to the first quartz substrate will be described. One end of the first quartz substrate 4 is provided with an optical fiber fixing portion 9 in which a part of the surface where the V-shaped groove 3 is formed is cut to reduce the substrate thickness. The optical fiber fixing portion 9 is a portion for fixing the optical fiber cable 7 coated with resin with the fixing adhesive 10 so that the coating-removed optical fiber portion 8 is not subjected to bending stress depending on the thickness of the coating resin. In addition, a part of the first quartz substrate 4 is cut and thinned in advance. The quartz substrate 5 is slightly longer than the grating length of the fiber grating 2, but may be any length shorter than the length of the coating removal optical fiber portion 8, for example, 17 mm here. Since the second quartz substrate 5 is for fixing and protecting the fiber grating 2, it is sufficient that the length is about 1 mm at both ends from the portion where the grating is formed. The region above the optical fiber fixing portion 9 of the coating removal optical fiber portion 8 and the optical fiber cable 7 outside the second quartz substrate 5 is fixed to the optical fiber fixing portion 9 of the first quartz substrate 4 by the fixing adhesive 10. Fixed. At this time, the fixing adhesive 10 may protrude to the top of the quartz substrate 5 or the end surface of the second quartz substrate 5, but the covering removal optical fiber portion 8 is covered so as not to be exposed. However, it is also good for ensuring reliability. The length of the optical fiber fixing portion 9 may be any number, but about 2 to 5 mm is sufficient, and about 3 to 4 mm is appropriate considering the downsizing of the grating filter and the reliability of fiber fixing. It is.

  Further, the end face of the grating filter will be described. In this grating filter, end faces of the fiber grating 2, the first quartz substrate 4, and the second quartz substrate 5 are optically polished, and an optical polishing surface 11 that is substantially the same surface is provided. As a result, when the grating filter 1 is connected to an optical modulator or the like, the optical axis can be aligned without using other members, so that it can be directly connected to the input / output end of the optical modulator. The interval between the input / output end and the grating filter 1 can be shortened. Here, optical polishing refers to polishing substantially equivalent to a polishing step used in a normal connector for optical fiber communication. The optical polishing surface 11 may be a surface perpendicular to the light propagation direction in the fiber grating 2, but it is 7 ° from the vertical surface in order to suppress reflection loss at the connection surface when connected to the optical modulator. It may be a surface having an arbitrary angle. At this time, the direction of the angle may be any direction. In this way, the grating filter 1 is configured.

  Next, a method for bonding the fiber grating 2, the first quartz substrate 4 and the second quartz substrate 5 will be described in detail. The fiber grating 2, the first quartz substrate 4 and the second quartz substrate 5 are fixed by an adhesive 6. Although an ultraviolet curable adhesive mainly composed of an epoxy resin is used as the adhesive 6, a thermosetting adhesive may be used, or an adhesive of another material may be used. The adhesive 6 was not provided in the portion of the fiber grating where the grating was formed, but was provided outside the both ends of the grating and between the quartz substrate 4 and the quartz substrate 5 at a location away from the V-shaped groove 3. Since the adhesive is applied while avoiding the grating portion in this way, no stress is applied to the grating, and good filter characteristics can be obtained. When an adhesive is applied to the portion of the fiber grating 2 where the grating is formed, when the adhesive is cured, random expansion and contraction occurs in the portion where the adhesive is applied, and a random force is applied. This is because the random stress causes random pitch disturbances such as a pitch extending in a certain portion of the grating and a pitch shrinking in a certain portion, and the optical characteristics of the grating change. In addition, the grating portion to which the adhesive 6 is not applied may be left without being provided (the surroundings remain air). For example, a soft filler such as silicone gel or silicone rubber that does not apply a load to the grating portion may be used. You may provide around. Examples of such materials include high-strength, translucent addition type liquid silicone rubber TSE3476T manufactured by GE Toshiba Silicone, and one-component heat-curable adhesive liquid silicone rubber TSE3282-G.

  FIG. 3 is a diagram showing the reflection characteristics of the grating before and after bonding when an adhesive is also applied to the grating portion. FIG. 4 shows the grating before and after bonding when no adhesive is applied to the part where the grating is formed, and an adhesive is provided between the quartz substrate outside the both ends of the grating and away from the V-shaped groove. It is a figure which shows the reflection characteristic. As can be seen from FIG. 3, when the adhesive is applied also to the portion where the grating is formed, the reflection characteristic of the grating changes greatly before and after the adhesion, and the reflection characteristic changes from a steep reflection characteristic to a slow reflection characteristic. This is because the steep reflection characteristics were obtained by manufacturing with a uniform grating pitch over the entire length of the grating, but the grating pitch was disturbed due to random expansion and contraction caused by curing of the adhesive, and reflection This is because the characteristics have become sluggish. Such disturbance of the grating pitch caused by the curing of the adhesive is called a phase error, and is described in, for example, Japanese Patent Application Laid-Open No. 2002-196158. On the other hand, as can be seen from FIG. 4, when the adhesive is not applied to the portion where the grating is formed, the reflection characteristic of the grating hardly changes before and after the bonding, indicating a steep reflection characteristic. When this grating filter is provided at both ends of the optical modulator to form a reciprocating optical modulator, a grating is formed because a steep reflection characteristic is required for the grating filter to obtain a high optical modulation signal of several tens GHz. It is preferable that no adhesive is provided on the part. Also, when using a sampled grating or a superstructure grating that reflects the light of a plurality of wavelengths by controlling the phase of the grating pitch of the fiber grating, it is better not to provide an adhesive on the part where the grating is formed. . Sampled gratings and superstructure gratings are described in, for example, Japanese translations of PCT publication No. 2003-510627 and US Published Patent Publication US2003 / 0021532A1.

Next, a reciprocating optical modulator using the grating filter 1 will be described. FIG. 5 shows a perspective view of the reciprocating optical modulator 20. The reciprocating optical modulator 20 includes an optical modulator 12 and first and second grating filters 1 a and 1 b connected to input / output terminals of the optical modulator 12. In the figure, the optical modulator 12 and the grating filters 1a and 1b are shown separately from each other for easy understanding. However, in practice, the input / output terminals of the optical modulator 12 and the first and second grating filters are shown. The cores 1a and 1b are bonded to each other by an adhesive with the respective cores aligned. The optical modulator 12 may be an intensity modulator or a phase modulator, but is a phase modulator in this embodiment. The modulator 12 is an LN modulator formed on a niobium lithium oxide (LiNbO ₃ ) substrate, and modulation electrodes 13a and 13b are formed on two optical waveguides branched in two between input and output ends, respectively. Yes. A high-frequency electrical signal of 4.4 GHz is applied from the high-frequency electrical signal source 14 to the modulation electrodes 13a and 13b, and the optical signal propagating through the two optical waveguides is modulated.

  Next, the connection between the optical signal input / output terminals at both ends of the optical modulator 12 and the optical polishing surfaces 11 of the first and second grating filters 1a and 1b will be described. The optical signal input / output end and the optical polishing surface 11 of the first and second grating filters 1a and 1b are connected by an adhesive. This adhesive is a commercially available adhesive for optical communication devices used for connecting a normal LN modulator and a connector, and is designed and manufactured so as to reduce light propagation loss. Although the grating filters 1a and 1b are shown in a simplified manner in FIG. 5, they have the same structure as that shown in FIG. 1, and their characteristics are almost the same as those after bonding shown in FIG. Further, as shown in FIG. 5, the input / output end faces of the optical modulator 12 and the end faces of the grating filters 1a and 1b are each polished by being inclined by 7 ° from a plane perpendicular to the light propagation direction. . As a result, reflection at the connection surface between the optical modulator 12 and the grating filters 1a and 1b can be suppressed.

  Further, the size of the reciprocating optical modulator 20 will be described. First, the total length of the optical modulator 12 is 38 mm. The length of the grating filter 1a, 1b where the grating is formed is 15 mm, and the distance between one end of the grating and the optically polished end face 11 is 1 mm. Further, since the end faces of the grating filters 1a and 1b are optically polished surfaces, the optical axes can be aligned and connected directly to the input / output ends of the optical modulator 12 without using a connecting member. That is, the distance between the grating filters is 16 cm in the reciprocating optical modulator described in Non-Patent Document 1 described in the prior art, but in the reciprocating optical modulator using the grating filter of the present embodiment, the grating is The distance between the filters was 40 mm, and the optical path length could be greatly shortened.

Next, the operating principle of the reciprocating optical modulator 20 will be described. FIG. 6 is a schematic diagram showing the operation principle of the reciprocating optical modulator 20 of FIG. The first grating filter 1a provided in the optical signal input end of the LN modulator 12, is transmitted through the non-modulated light from the optical frequency f ₀ from the light source to the LN modulator 12, light in a specific wavelength band is reflected . On the other hand, the second grating filter 1b provided at the optical signal output terminal of the LN modulator 12 reflects light within a specific wavelength band, but transmits light of an optical frequency component desired to be obtained as output light. The unmodulated light having the optical frequency f ₀ is phase-modulated by the LN modulator 12. Therefore, the steps from when the unmodulated light having the optical frequency f ₀ is input from the light source to the first grating filter 1a and modulated by the LN modulator 12 until the output light is obtained from the second grating filter 1b are as follows. .
(A) First, unmodulated light having an optical frequency f ₀ is input from the light source to the first grating filter 1 a, passes through, and reaches the LN modulator 12.
(B) A sideband is generated by the LN modulator 12 from unmodulated light having the optical frequency f ₀ .
(C) Among the generated sidebands, a component (referred to as an intermediate component) included in the reflection band of the second grating filter 1b on the output side is reflected by the second grating filter 1b on the output side, and the LN modulator Return to 12. On the other hand, of the sidebands, components that are not included in the reflection band are output.
(D) A sideband is generated by the LN modulator 12 from the reflected intermediate component.
(E) Among the generated sidebands, a component (referred to as an intermediate component) included in the reflection band of the first grating filter 1a on the input side is reflected by the first grating filter 1a on the input side, and the LN modulator Return to 12.
(F) A sideband is generated by the LN modulator 12 from the reflected intermediate component.
(G) Further, among the generated sidebands, a component (referred to as an intermediate component) included in the reflection band of the second grating filter 1b on the output side is reflected by the second grating filter 1b on the output side, and LN Return to the modulator 12. On the other hand, of the sidebands, components that are not included in the reflection band are output.

As described above, the intermediate component reciprocates between the two grating filters 1a and 1b and is modulated by the LN modulator 12 a plurality of times. As a result, it is possible to generate high-order sidebands with high efficiency. The reflection bandwidth of the grating filter is set so that the first and second grating filters 1a and 1b on the input side and the output side reflect light of optical frequency components f ₁ , f ₂ ,..., F _n . Then, the optical frequency component f _{(n + 1)} which is the _{(n + 1) th} order harmonic component can be obtained as an output. 6, a case of generating an optical frequency component f ₅ are mentioned as examples, the optical output of the modulation frequency 5f _m of 5 times the modulation frequency f _m of the LN modulator 12 is obtained. Note that by increasing the reflection bandwidth of the grating filter and increasing the number of reflections at the grating filters 1a and 1b at both ends, it is possible to obtain a light output with a higher frequency component.

  FIG. 7 is a diagram showing the time change of the light output intensity of the reciprocating optical modulator 20. Here, the stabilization of the light output by the feedback control, which is essential in the conventional reciprocating optical modulator, is not performed. An optical signal modulated to a millimeter wave frequency of 61.6 GHz was obtained by performing a reciprocating multiplication operation with a 4.4 GHz electrical signal by the reciprocating optical modulator 20 having the structure shown in FIG. As shown in FIG. 7, the reciprocating optical modulator 20 using the grating filters 1a and 1b can obtain a stable light output without using feedback control. This is because when the conventional grating filter is used, the distance between the grating filters is as long as 16 cm, so that the refractive index of the optical fiber or the optical waveguide changes due to a slight temperature change, and the optical path length changes. However, in the reciprocating optical modulator 20 of the present embodiment, the distance between the first and second grating filters 1a and 1b is significantly shortened to 40 mm, so that the change in the optical path length due to the temperature change becomes extremely small. This is because. In the present embodiment, feedback control for stabilizing the light output is not provided, but feedback control may be further provided. Thereby, more stable light output characteristics can be realized. Even in this case, since the output stability of the reciprocating optical modulator 20 itself is excellent, a simpler and cheaper control mechanism than the conventional feedback control mechanism is sufficient.

An example of a multiplying optical modulator having a reduced overall size is an integrated optical multiplying modulator described in Japanese Patent Application Laid-Open No. 2002-148572. In this method, a grating filter is also formed on a semiconductor substrate on which an optical modulator is formed, and the overall size is as very small as 10 mm × 10 mm × 5 mm. In general, in order to obtain an optical modulation signal having a high frequency with a reciprocating optical modulator, the reflection characteristic of the grating filter is required to be steep. In order to obtain steep reflection characteristics, the grating needs to have a length of at least 10 mm. However, it is difficult to obtain a steep reflection characteristic by forming a grating having a length of 10 mm or more on an optical waveguide formed on such a substrate. The reason is as follows. The wavelength λ of the light reflected by the grating is expressed by the following equation.
λ = 2 · N _eff · Λ (1)
Here, N _eff is an effective refractive index, and is an average refractive index of the optical waveguide determined by the interaction between the core and the cladding of the optical waveguide. Λ is a grating pitch.

As can be seen from the above equation (1), as N _eff and Λ are constant over the entire grating, a stronger reflection at the wavelength λ is obtained, and a steep reflection characteristic is obtained. Since N _eff is determined by the interaction between the core and the clad, for example, when the refractive index of the core or the core width is disturbed in the light propagation direction, N _eff is obtained uniformly over the entire length of the grating. It becomes difficult. When an optical waveguide manufactured on a semiconductor substrate or a planar optical waveguide (PLC) including quartz as a main component on a substrate is manufactured, it is difficult to manufacture a refractive index and a core width uniformly. It is difficult to obtain a uniform N _eff over the entire length of the grating. For this reason, it is difficult to obtain a grating filter having a reflection characteristic necessary for a reciprocating optical modulator for obtaining a high frequency optical modulation signal. On the other hand, in this embodiment, since the optical waveguide forming the grating is an optical fiber, the uniformity of the refractive index and the core diameter is very excellent, and it is suitable for obtaining a grating filter having steep reflection characteristics. . Therefore, by using this grating filter, it is possible to obtain a high-performance reciprocating optical modulator higher than that using a grating filter formed on a conventional optical waveguide fabricated on a substrate.

  In the grating filter of the present embodiment, the groove 3 is provided only on one of the first quartz substrates 4. However, as shown in FIGS. 8 and 9, both the first and second substrates 4 and 5 are provided. Grooves 3a and 3b for housing the fiber grating 2 may be provided, and the fiber grating 2 may be housed therein and fixed between the grooves 3a and 3b. Also in this case, an adhesive is applied to the grooves 3a and 3b at portions slightly outside from both ends of the grating portion and fixed. Thus, it is preferable not to apply an adhesive to the grating portion. The material of the substrate is not limited to quartz, and may be a semiconductor substrate such as a silicon substrate, a metal substrate such as a copper alloy, or a resin substrate such as polyimide.

Embodiment 2. FIG.
A grating filter according to Embodiment 2 of the present invention will be described with reference to FIGS. FIG. 10 is a perspective view of the grating filter. FIG. 11 is an exploded perspective view for showing the configuration of the grating filter. In the present embodiment, a thin film heater 15 is provided on the surface of the second quartz substrate 5 constituting the grating filter 1 that is in contact with the fiber grating 2. The second quartz substrate 5 is wider than the first quartz substrate 4 in which the V-shaped groove 3 is formed, and electrodes 16a and 16b for supplying power to the thin film heater 15 are provided in the widened portion. Yes. The first quartz substrate 4, the second quartz substrate 5, and the fiber grating 2 are fixed by an adhesive 6, and the adhesive 6 is outside the both ends of the grating and the first quartz substrate 4, as in the first embodiment. It is provided between the second quartz substrates 5 and is not provided in the portion where the grating of the fiber grating 2 is formed. There is no need to provide anything in the portion of the fiber grating 2 where the grating is formed, but in order to make it easier for the heat of the thin film heater 15 to be transferred to the fiber grating 2, a heat transfer silicone rubber 17 is provided. By providing the thin film heater 15 in this way, the reflection wavelength of the grating of the fiber grating 2 can be controlled. As a result, the reflection wavelength can be varied as the grating filter 1. In addition, it is possible to adjust the variation in the reflected wavelength in manufacturing. Furthermore, since the thin film heater 15 and the fiber grating 2 are brought into close contact with each other, the reflection wavelength of the grating can be controlled with small power consumption.

Next, the operation will be described. When electric power is supplied from the electrodes 16a and 16b to the thin film heater 15, the thin film heater 15 generates heat, the heat is transmitted to the fiber grating 2, and the temperature of the fiber grating 2 rises. Quartz, which is a material for fiber gratings, has an increased refractive index due to temperature rise, and this is called the thermo-optic effect. Since the wavelength of the light reflected by the grating is proportional to the effective refractive index N _eff , as shown in Equation (1), the wavelength reflected can be controlled by controlling the temperature of the fiber grating. The rate of change of the reflected wavelength is about 10 pm / ° C.

  As shown in FIG. 4, even when the adhesive 6 is not provided on the portion of the fiber grating 2 where the grating is formed, the reflection characteristics of the grating slightly change before and after bonding. Although there is no change in the steepness of the reflection characteristics, the reflection characteristics are shifted to the long wavelength side as a whole. This is because although the grating pitch is not disturbed by the curing of the adhesive, tension is generated at both ends of the grating, and the grating pitch extends at a uniform rate. As described above, the wavelength slightly changes before and after bonding, and advanced manufacturing techniques are required to accurately control the wavelength. Further, when high accuracy is required for the reflection wavelength, the yield may be remarkably deteriorated. Therefore, the reflection wavelength can be controlled by providing temperature control means such as a thin film heater in a part of the grating filter as in the present embodiment and controlling the temperature of the fiber grating 2. This makes it possible to compensate for reflection characteristics that change after bonding.

  Further, when the fiber grating is manufactured, it may be prepared in advance so as to have a shorter reflection wavelength. After the fiber grating is fixed between the first and second substrates 4 and 5 and the grating filter 1 is manufactured, the power supplied to the thin film heater 15 is adjusted to adjust the reflection wavelength to the required wavelength. Good. Also, since the reflection wavelength varies depending on the temperature of the environment in which the grating filter is used, there is a problem when the reciprocating optical modulator is configured by connecting the grating filter to the input / output end of the optical modulator. Therefore, when the environmental temperature changes, the reflection wavelength can be kept constant by controlling the grating temperature of the fiber grating 2 to be always constant by the thin film heater 15. Further, it is possible to adjust the temperature of the grating so that the characteristics such as the light output intensity of the reciprocating optical modulator are the best.

  Although the thin film heater 15 is provided on the surface of the second quartz substrate 5 in contact with the fiber grating, it is provided on the upper surface of the second quartz substrate 5 or the lower surface of the first quartz substrate 4 to control the temperature of the entire grating filter. However, as shown in FIG. 10, it is preferable to provide it on the surface in contact with the fiber grating 2 because the power consumption of the thin film heater can be kept small.

  In this embodiment, the quartz substrate is used as the substrate. However, the present invention is not limited to this, and an insulating substrate made of another material such as alumina or a resin substrate such as polyimide may be used. In addition, a conductive substrate such as a metal substrate or a semiconductor substrate can be used by providing an insulating layer between the thin film heater and the substrate.

Embodiment 3 FIG.
A grating filter according to an embodiment of the present invention will be described with reference to FIGS. In the second embodiment, one thin film heater is provided in the grating filter. However, in this embodiment, a plurality of thin film heaters 15 ₁ , 15 ₂ ,... 15 _N are provided to control the temperature gradient of the fiber grating. It is. By providing the plurality of thin film heaters 15 ₁ , 15 ₂ ,... 15 _N , the temperature gradient of the fiber grating 2 can be controlled to control the chirp amount of the grating. The optical output can be controlled by adjusting the phase of the optical signal by the chirped grating formed thereby.

FIG. 12 is a perspective view of the grating filter 1. FIG. 13 is an exploded perspective view for showing the configuration of the grating filter. In the present embodiment, a plurality of N thin film heaters 15 ₁ , 15 ₂ ,..., 15 _N are provided on the surface in contact with the fiber grating 2. The second quartz substrate 5 is wider on both sides than the first quartz substrate 4 in which the V-shaped groove 3 is formed, and electrodes 16a ₁ for supplying power to each of the N thin film heaters in the widened portion. , 16a ₂ ,..., 16a _N, and an electrode 16b common to the N electrodes. Other components such as the adhesive 6 and the heat conductive silicone rubber 17 are the same as those in the second embodiment.

Next, the operation of the grating filter 1 will be described. Electric power is controlled and applied to each of the _N thin film heaters 15 ₁ , 15 ₂ ,..., 15 _N from a power source (not shown). The grating portion of the fiber grating 2 by supplying power so that the power supplied to the individual thin film heaters 15 ₁ , 15 ₂ ,..., 15 _N increases or decreases linearly along the fiber grating 2. Is applied with a linear temperature gradient, and the temperature gradient can be controlled by controlling the power of each thin film heater. As described in the second embodiment, since the refractive index of the optical fiber can be controlled by the thermo-optic effect, the equivalent refractive index is changed by providing a temperature gradient as described above, and reflected by the grating. The wavelength of light can be controlled. By applying a linear temperature gradient to the grating portion of the fiber grating 2 as shown in the present embodiment, it can be reflected at different positions of the grating depending on the wavelength. For example, it is assumed that light is incident from the direction of the optical polishing surface 11 in FIG. When a temperature gradient is applied to the grating portion of the fiber grating 2 by the individual thin film heaters such that the temperature on the optical polishing surface 11 side is higher and the temperature on the coating portion side of the optical fiber cable 7 is lower, the grating The refractive index increases toward the optical polishing surface 11 and decreases as the distance from the optical polishing surface 11 increases. As a result, light having a longer wavelength is reflected closer to the optical polishing surface 11, and light having a shorter wavelength is reflected farther away. On the other hand, when a reverse temperature gradient is applied, light having a shorter wavelength is reflected closer to the optical polishing surface 11, and light having a longer wavelength is reflected farther away. That is, the chirp characteristic can be given to the grating by controlling the temperature gradient. The technique for controlling the chirp of the grating by this temperature gradient is used in a tunable dispersion compensator used in optical communication, and is described in detail in, for example, Japanese Patent Application Laid-Open No. 2002-196159. By controlling the chirp characteristics of the grating in this way, it is possible to compensate for the dispersion generated during the reciprocating optical modulation, so that it is possible to perform the reciprocating optical modulation without reducing the efficiency.

  Note that the uniform grating whose grating pitch is constant over the entire grating is shown as the grating, but a chirped grating may be used instead of the uniform grating.

Further, the plurality of thin film heaters may be provided at both ends of the grating outside the first quartz substrate 4 or the second quartz substrate 5 as shown in FIG. In this case, the two thin film heaters 15 ₁ and 15 ₂ can be controlled to apply a temperature gradient to the entire second quartz substrate 5, thereby applying a temperature gradient to the fiber grating. In this case, power consumption increases because the entire second quartz substrate 5 is heated. However, since the number of thin film heaters is small and the structure and control are simple, the cost reduction is excellent.

Embodiment 4 FIG.
A method for manufacturing a grating filter according to Embodiment 4 of the present invention will be described with reference to FIGS. 15 to 20 are schematic views showing the respective steps of the method for manufacturing a grating filter. In this method for manufacturing a grating filter, first, one end of an optical fiber is sandwiched and fixed between the first and second quartz substrates 4 and 5, and then a grating is formed in the vicinity of the end face of the optical fiber. As shown in the first embodiment, the fiber grating may be manufactured first, and then the fiber grating may be fixed between the first and second substrates 4 and 5. By forming the grating after placing the optical fiber in the substrate as shown in the embodiment mode, the grating can be manufactured with high accuracy, and productivity can be improved. The details such as the size of each member are the same as those shown in the first embodiment.

Below, each process of the manufacturing method of this grating filter 1 is demonstrated.
(A) First, as shown in FIG. 15, the coating-removed optical fiber portion 8 from which a portion of the coating of the optical fiber cable 7 is removed is placed in the V-shaped groove 3 of the first quartz substrate 4 in which the V-shaped groove 3 is formed. Install in. The coating removal optical fiber portion is longer than the length of the V-shaped groove 3 and is disposed so as to protrude from the V-shaped groove 3 as shown in FIG. 15 or FIG. The V-shaped groove 3 may be a U-shaped groove or a groove having another shape, but the V-shaped groove is optimal for forming a highly accurate grating. As described in the first embodiment, the first quartz substrate 4 has the fixing portion 9 whose thickness is thinned at one end of the substrate.
(B) The fixing portion 9 is arranged so that the boundary between the portion where the coating of the optical fiber cable 7 is coated and the portion where the coating is removed comes.
(C) Next, outside both ends of the portion of the V-shaped groove 3 where the grating of the coating-removed optical fiber portion is formed, and on the surface of the first quartz substrate 4 where the V-shaped groove 3 is formed, other than the V-shaped groove 3 A fixing adhesive 6 is applied.
(D) After that, the second quartz substrate 5 is placed on the surface of the first quartz substrate 4 where the V-shaped groove 3 is formed, and the adhesive is cured, so that both ends of the portion where the grating is formed are first. And fixed between the second quartz substrates 4 and 5.

  Here, the adhesive 6 may be a thermosetting type, an ultraviolet curable type, or a natural curable type. In addition, the ultraviolet curable type most often used for the production of PLC optical connectors and the like is most preferable because of its high reliability. Further, since the adhesive 6 does not flow into the V-shaped groove 3, it is not necessary to apply the adhesive 6 to the immediate vicinity of the V-shaped groove 3. The fixing method is not limited to the adhesive, and other methods such as screwing may be used as long as the coating removal optical fiber portion 8 can be irradiated with the ultraviolet laser light through the second quartz substrate 5. Further, since the grating is formed by irradiating interference fringes of ultraviolet laser light through the second quartz substrate 5, the thickness of the second quartz substrate 5 is preferably 1 mm or less, preferably about 0.1 to 0.3 mm. Is desirable.

  In this way, the first quartz substrate 4, the second quartz substrate 5 and the optical fiber cable 7 are fixed, and finally the optical fiber cable 7 is fixed to the fixing portion 9 with the fixing adhesive 10. FIG. 16 shows a state assembled in this way. An end face 18 that is not optically polished is provided on the end face opposite to the fixed portion 9, and a part of the optical fiber from which the coating has been removed protrudes. Thereafter, the protruding optical fiber is cut, and the end surface 18 is optically polished to form the optically polished surface 11. In the optical polishing surface 11, the end surfaces of the first and second quartz substrates 4 and 5 and the end surface of the optical fiber constitute substantially the same surface. This state is shown in FIG. The optical polishing step may be performed after the next grating forming process.

  Next, grating formation is performed on the coating removal optical fiber portion 8. The grating forming method is described in, for example, Japanese Patent Application Laid-Open No. 2002-196158. In general, when forming a grating, high-pressure hydrogen treatment is performed in which an optical fiber is left in high-pressure hydrogen or deuterium at about 100 atm for several days. The high-pressure hydrogen treatment is performed on the optical fiber cable before assembling in FIG. It may be performed in a state, or may be performed in a state where the optical fiber and the substrate are fixed as shown in FIG. 16 or FIG.

  18 and 19 show a process for forming a grating in the coating-removed optical fiber portion 8. When the ultraviolet laser light is irradiated through the phase mask 19 as shown in the figure, the ultraviolet laser light is diffracted by the phase mask 18 in the ± first-order directions. The diffracted ultraviolet laser light generates interference fringes in the core of the coating removal optical fiber portion 8, the refractive index of the core changes according to the distribution of interference fringes, and a grating is formed. As the ultraviolet laser light, a KrF excimer laser (wavelength 248 nm), a fourth harmonic (wavelength 266 nm) of an Nd: YAG laser, a second harmonic (wavelength 244 nm) of an Ar ion laser, or the like is used. Ultraviolet laser light may be radiated to the part where the grating is formed at once, but since a reciprocating optical modulator requires a grating filter with a sharp reflection characteristic, the beam diameter is usually 2 mm or less, preferably 1 mm or less. It is preferable to irradiate the ultraviolet laser beam while scanning so as to have a predetermined irradiation amount distribution along the longitudinal direction of the coating removal optical fiber portion 8. When a desired grating is formed in this way, the grating filter 1 is completed as shown in FIG.

  In the manufacturing method of the grating filter shown in the present embodiment, the optical fiber is fixed in the substrate, and then the grating is formed. The position can be formed with high accuracy, and there is an advantage that a grating filter having stable characteristics can be mass-produced.

  In this embodiment, only the quartz substrate is used. However, by using a substrate having good thermal conductivity as a substrate for forming the V-shaped groove, a local temperature increase of the optical fiber due to irradiation with ultraviolet laser light can be suppressed. Can do. As a result, local thermal expansion can be suppressed and a highly accurate grating filter can be manufactured. As such a substrate, a metal substrate or a semiconductor substrate may be used, and aluminum nitride or the like is an insulator but is a suitable substrate material because it has high thermal conductivity.

Embodiment 5. FIG.
The grating filter 21a and the reciprocating optical modulator 30 according to the embodiment of the present invention will be described with reference to FIGS. In the reciprocating optical modulator according to the first embodiment, the band limiting filter that reflects light of a specific wavelength is used for the first and second grating filters 1a and 1b on the input side and the output side. In this embodiment, as shown in FIG. 21, a case where a narrow band pass filter that transmits a specific wavelength is used for the first grating filter 21a on the input side will be described. The structure of the grating filter is the structure shown in FIG. 1, FIG. 2, FIG. 8, or FIG. 9 shown in the above embodiment, but the fiber grating is different from the above embodiment. Further, a thin film heater as shown in FIG. 10 may be provided to control the reflection and transmission wavelengths of the fiber grating by temperature control.

  FIG. 22 is a schematic diagram of a fiber grating used in the first grating filter 21a on the input side according to the present embodiment. The fiber grating is formed by forming a refractive index modulation at a period of about 535 nm in the core of the optical fiber. In the fiber grating used in the first embodiment, the period of refractive index modulation regularly changes over the entire length of the grating as shown in FIG. On the other hand, in the narrow band pass fiber grating used in the first grating filter 21a on the input side according to the present embodiment, the phase of the refractive index modulation is shifted by 90 ° at almost the center of the grating as shown in FIG. In other words, the grating pitch is shifted by a quarter pitch (about 134 nm when the grating pitch is 535 nm) at the center of the grating. Note that the phase shift angle is the same whether it is 90 ° or 270 °, and in general, it may be shifted by (90 ° + 180 ° × n). Further, an ideal phase shift of 90 ° is ideal, but substantially 90 ° ± 30 ° is sufficient. Such a fiber grating can be manufactured using a phase mask in which the phase mask pitch is just shifted from the center of the grating, but it can also be manufactured using a phase mask that does not have a phase shift that is used for normal fiber grating manufacturing. can do.

FIG. 24 to FIG. 26 are schematic views showing respective steps of forming a grating having a phase shift of 90 ° ± 30 ° at a substantially midpoint using a phase mask having no phase shift.
(A) First, as shown in FIG. 24, the phase mask is placed close to the optical fiber.
(B) Next, ultraviolet laser light is irradiated along the optical axis direction of the optical fiber to half of the entire length of the irradiation range to form a grating up to half.
(C) Thereafter, as shown in FIG. 25, the phase mask is slightly shifted along the optical axis direction. At this time, it is difficult to shift the phase mask so that the phase shift is exactly 90 ° because it is necessary to control the shift of only 100 nm. Therefore, what is necessary is just to shift | deviate substantially substantially.
(D) Thereafter, as shown in FIG. 26, the remaining half of the irradiation range is irradiated with ultraviolet laser light to form a grating in the remaining half. By appropriately shifting the phase mask, the angle of phase shift produced is determined by the probability. If the allowable range of phase shift is 90 ° ± 30 °, it can be manufactured with a probability of 1/3.

  FIG. 27 shows the characteristics of a fiber grating in which the grating length is 5 mm and the phase shift of the grating pitch is produced at a position of 2.5 mm. FIG. 28 shows an enlarged view around the center wavelength. is there. As can be seen from FIG. 28, in the fiber grating in which the phase shift is formed at the center of the grating, a narrow band transmission characteristic can be obtained. In the manufactured grating, a full bandwidth at half maximum (FWHM) of 2.5 GHz was obtained, and the transmission loss was 1.9 dB. Further, as can be seen from the reflection characteristics in FIG. 27, a bandwidth with a reflectance of 90% or more was obtained as 0.527 nm, and a characteristic in which the reflectance decreased at the approximate center of the reflection bandwidth was obtained. Thus, it can be seen that the phase shift is very close to 90 ° because the reflectance is reduced at the center of the reflection bandwidth. When the phase shift is 90 ° or less, the position where the reflectivity decreases is shifted to the short wavelength side of the reflection bandwidth, and when it is larger than 90 °, the position is shifted to the long wavelength side. Even if the wavelength at which the reflectance decreases is shifted from the center wavelength of the reflection bandwidth to the long and short wavelength side, if there is a sufficient wavelength range of 90% or more on both sides, the wavelength can be used as the first grating filter 21a on the input side. . A wavelength range with a reflectance of 90% or more should be 0.15 nm or more in practical use, and a phase shift range obtained by 0.15 nm or more is 90 ° ± 30 °. In the transmission characteristics of FIG. 28, the transmission loss is 1.9 dB. A transmission loss of 2 dB or less is sufficient, but it goes without saying that a smaller transmission loss is better. This is because, in the manufactured grating, the position of the phase shift is not the true center of the grating but is shifted to either one. As this deviation increases, the transmission loss increases and the full width at half maximum of the transmission band increases. In order to obtain a good reciprocating optical modulator, the full width at half maximum is preferably 3 GHz or less. In order to achieve a transmission loss of 2 dB or less and a full width at half maximum of 5 GHz, it is necessary to produce the phase shift at a position ± 10% from the center of the grating. Here, 10% is a percentage of the entire length of the grating, for example, ± 0.5 mm in the case of 5 mm.

Next, the operation principle of the reciprocating optical modulator 30 using the grating filter 21a having such a narrow band transmission characteristic on the input side will be described with reference to FIG. The output side grating filter 1b is the same as that shown in the first embodiment. The structure of the reciprocating optical modulator 30 is the same as that shown in FIG. 5 except that the first grating filter 21a on the input side is a narrow band transmission grating filter. FIG. 21 shows the operation principle of the reciprocating optical modulator 30 of the present embodiment.
(1) for grating filter 21a provided in the optical signal input end of the LN modulator 12 is a narrow band pass filter, it is transmitted through the non-modulated light from the optical frequency f ₀ from the light source to the LN modulator 12, both sides The light of the wavelength of is reflected.
(2) The second grating filter 1b provided at the optical signal output terminal of the LN modulator 12 reflects light within a specific wavelength band, as in the first embodiment, but the light of the optical frequency component desired to be obtained as output light. Is transparent.
(3) Unmodulated light having the optical frequency f ₀ is phase-modulated by the LN modulator 12.
(4) Therefore, the generated sidebands on both sides are reflected by the second grating filter 1b on the output side, modulated by the LN modulator 12, and sidebands are generated for the respective components.
(5) Thereafter, light other than the optical frequency f ₀ is reflected by the first grating filter 21 a on the input side and is input to the LN modulator 12 again. Thus, like the first embodiment, the intermediate component reciprocates between the two grating filters and is modulated a plurality of times.
(6) input and the optical frequency components _f -n at the output side grating _{filter, ···, f -2, f -1} , f 1, f 2, ···, grating to reflect light of _{f n} Since the reflection bandwidth of the filter is set, the optical frequency components f _(n−1) and f _{(n + 1)} can be obtained as outputs.

Figure 21 is an optical frequency component _f -5, and as an example a case of generating the _{f 5,} the light output of the modulation frequency 10f _m of 10 times the modulation frequency _{f m} of the LN modulator 12 is obtained. That is, in the reciprocating optical modulator shown in the present embodiment, a double modulation frequency can be obtained with the same number of reciprocations as in the first embodiment. That is, an optical signal having a higher modulation frequency can be obtained with an electric signal having a lower modulation frequency. Further, when the number of reciprocations increases, the light output decreases due to loss in the grating filter or the LN modulator 12, so that the number of reciprocations may be half that of the first embodiment, and a large light output can be obtained.

Embodiment 6 FIG.
A reciprocating optical modulator 40 according to the sixth embodiment will be described with reference to FIGS. In the fifth embodiment, the narrow-band transmission filter 21a is used only for the first grating filter on the input side. However, in this embodiment, the reciprocating optical modulator 40 using the narrow-band grating filter 21b on the output side is used. This will be described with reference to FIG. The structure of the reciprocating optical modulator 40 is the same as that shown in FIG. 5, and only the characteristics of the fiber grating are different. The first grating filter 21a on the input side is the same as that in the fifth embodiment.

  When a narrow band transmission filter is used for the second grating filter 21b on the output side, it is necessary to transmit light of two wavelengths unlike the input side. This can be produced by providing two grating phase shifts. FIG. 30 is a diagram in which the characteristics of the grating when the range of 5 to 10 mm of the chirped fiber grating having a grating length of 15 mm is 90 ° out of phase with the portions of 0 to 5 mm and 10 to 15 mm are calculated by simulation. is there. FIG. 31 is an enlarged view thereof. For such a fiber grating having two phase shifts, firstly, a first grating portion is produced in the range of 0 to 5 mm and 10 to 15 mm using a chirp phase mask, and then the chirp phase mask is appropriately shifted. This can be realized by producing a second grating part having a phase shift from the first grating part in the range of 5 to 10 mm. The manufacturing process can use the manufacturing process of the fiber grating shown in the fifth embodiment. As the grating, a uniform grating or a chirped grating having a constant grating pitch can be used. By using a chirped fiber grating as the grating, the interval between the two wavelengths to be transmitted can be made wider than in the case of a uniform grating. Further, the wavelength interval can be arbitrarily designed by controlling the chirp rate.

Next, the principle of operation of this reciprocating optical modulator will be described with reference to FIG. As can be seen from the figure, the operating principle is almost the same as in the fifth embodiment, and only the light of the optical frequency components f _(n−1) and f _{(n + 1)} passes through the output-side grating filter 21b and is obtained as output light. It is done. Thus, since only the output light is extracted and all other light is blocked, a reciprocating multiplying optical modulator with small noise light can be obtained.

  When a narrow-band transmission filter is used for the output-side grating filter 21b, the transmission band is a narrow band, so the wavelength interval accuracy between the two transmitted lights is important, and it is desirable that this interval can be adjusted. This can be realized by adopting a structure capable of applying a temperature gradient to the chirped fiber grating as shown in FIG. 12 and FIG. 13 or FIG. FIG. 32 shows changes in transmission characteristics when a temperature gradient of −20 ° C. to + 20 ° C. is applied to the grating filter shown in FIGS. Here, the negative temperature gradient refers to a temperature gradient that tends to decrease the chirp rate of the chirped fiber grating, and the positive temperature gradient refers to a temperature gradient that increases the chirp rate. As can be seen from FIG. 32, the two transmission wavelength intervals can be controlled by controlling the temperature gradient.

It is a perspective view which shows the structure of the grating filter which concerns on Embodiment 1 of this invention. It is a disassembled perspective view which shows the structure of the grating filter which concerns on Embodiment 1 of this invention. It is a figure which shows the change of the reflective characteristic before adhesion | attachment at the time of providing an adhesive agent in a grating part, and after adhesion | attachment. It is a figure which shows the change of the reflective characteristic before adhesion | attachment when not providing an adhesive agent in a grating part, and after adhesion | attachment. 1 is a perspective view showing a configuration of a reciprocating optical modulator according to Embodiment 1 of the present invention. It is the schematic which shows the principle of operation of the reciprocating optical modulator which concerns on Embodiment 1 of this invention. It is a figure which shows the time change of the optical output of the reciprocating optical modulator which concerns on Embodiment 1 of this invention. It is a perspective view which shows another example of the grating filter which concerns on Embodiment 1 of this invention. It is a disassembled perspective view of the grating filter of FIG. It is a perspective view which shows the structure of the grating filter which concerns on Embodiment 2 of this invention. It is a disassembled perspective view which shows the structure of the grating filter of FIG. It is a perspective view which shows the structure of the grating filter which concerns on Embodiment 3 of this invention. It is a disassembled perspective view which shows the structure of the grating filter of FIG. It is a perspective view which shows another example of the grating filter which concerns on Embodiment 3 of this invention. It is a perspective view which shows 1 process of the manufacturing method of the grating filter which concerns on Embodiment 4 of this invention. It is a perspective view which shows 1 process of the manufacturing method of the grating filter which concerns on Embodiment 4 of this invention. It is a perspective view which shows 1 process of the manufacturing method of the grating filter which concerns on Embodiment 4 of this invention. It is a perspective view which shows 1 process of the manufacturing method of the grating filter which concerns on Embodiment 4 of this invention. It is a perspective view which shows 1 process of the manufacturing method of the grating filter which concerns on Embodiment 4 of this invention. It is a perspective view which shows 1 process of the manufacturing method of the grating filter which concerns on Embodiment 4 of this invention. It is the schematic which shows the operation | movement principle of the reciprocating optical modulator based on Embodiment 5 of this invention. It is a figure which shows the refractive index modulation | alteration of the grating | lattice which the phase of refractive index modulation has shifted | deviated 90 degree | times in the approximate middle point of the full length of a grating. It is a figure which shows the refractive index modulation | alteration of the grating which has a constant grating pitch over the full length of a grating, and does not have a phase shift. It is the schematic which shows 1 process of the manufacturing process of the grating filter which concerns on Embodiment 5 of this invention. It is the schematic which shows 1 process of the manufacturing process of the grating filter which concerns on Embodiment 5 of this invention. It is the schematic which shows 1 process of the manufacturing process of the grating filter which concerns on Embodiment 5 of this invention. It is a figure which shows the characteristic of the grating filter which concerns on Embodiment 5 of this invention. It is an enlarged view of FIG. It is the schematic which shows the operation | movement principle of the reciprocating optical modulator based on Embodiment 6 of this invention. It is a figure which shows the characteristic of the grating filter which concerns on Embodiment 6 of this invention. It is an enlarged view of FIG. It is a figure which shows the change of the permeation | transmission characteristic at the time of applying the temperature gradient of -20 degreeC-+20 degreeC to the grating filter which concerns on Embodiment 6 of this invention.

Explanation of symbols

1, 1a, 1b grating filter, 2 fiber grating, 3 V-shaped groove, 4 first quartz substrate, 5 second quartz substrate, 6 adhesive, 7 optical fiber cable, 8 coating removal optical fiber portion, 9 fixing portion, 10 Fixing adhesive, 11 Optical polishing surface, 12 Optical modulator, 13a, 13b Modulating electrode, 14 Electric signal source, 15 Thin film heater, 16a, 16b Electrode, 17 Thermal conductive silicone rubber, 18 End face, 19 Phase mask, 20, 30, 40 Reciprocating optical modulator, 21a 1st grating filter, 21b 2nd grating filter

Claims (6)

A fiber grating in which a grating having a predetermined length is formed in a core of an optical fiber, wherein the grating is a fiber grating provided in the vicinity of one end face of the optical fiber;
A first substrate and a second substrate each having an end face that is substantially flush with the end face of the fiber grating, and is fixed by sandwiching both ends of the grating of the fiber grating;
With
The first substrate is provided with a groove for storing the fiber grating over the longitudinal direction,
The fiber grating is housed in a groove of the first substrate, and fixed between the groove and the second substrate,
The fiber grating is not provided with an adhesive in the grating portion, and is bonded to the first substrate and the second substrate only in the first and second portions that are separated from both ends of the grating by a predetermined distance. A grating filter characterized by being fixed. A fiber grating in which a grating having a predetermined length is formed in a core of an optical fiber, wherein the grating is a fiber grating provided in the vicinity of one end face of the optical fiber;
A first substrate and a second substrate each having an end face that is substantially flush with the end face of the fiber grating, and is fixed by sandwiching both ends of the grating of the fiber grating;
With
The first substrate is provided with a first groove having a depth shallower than the diameter of the grating fiber, which stores the fiber grating in the longitudinal direction.
The second substrate is provided with a second groove having a depth shallower than the diameter of the grating fiber for accommodating the fiber grating in the longitudinal direction.
The fiber grating is fixed with the first groove of the first substrate and the second groove of the second substrate facing each other and sandwiched between the first groove and the second groove,
The fiber grating is not provided with an adhesive in the grating portion, and is bonded to the first substrate and the second substrate only in the first and second portions that are separated from both ends of the grating by a predetermined distance. A grating filter characterized by being fixed.   2. The grating filter according to claim 1, wherein a thin film heater having a length equal to or longer than the length of the fiber grating is provided on the second substrate so as to face the fiber grating.   3. The grating filter according to claim 2, wherein a thin film heater for heating the grating is provided on at least one of the first substrate and the second substrate.   5. The grating filter according to claim 3, wherein the thin film heater includes a plurality of thin film heaters, and controls temperature distribution of the fiber grating by controlling electric power input to the thin film heaters. An optical modulator;
A first grating filter connected to the input end of the optical modulator with the optical axis aligned;
A second grating filter connected to the output end of the optical modulator with the optical axis aligned,
The reciprocating optical modulator using the grating filter according to any one of claims 1 to 5 as the first and second grating filters.

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