JP2006047678A - Variable band type optical filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable band type optical filter which largely increases a variable range of the band and copes with an optical network in which a lot of wave channels are used. <P>SOLUTION: The optical filter is provided with three-port type optical circulators 9a and 9b, a chirp fiber grating 10 of short wavelength side connection connected to the three-port type optical circulator 9a, a chirp fiber grating 11 of long wavelength side connection having a wavelength dispersion value which is the same as that of the chirp fiber grating 10 and connected to a three-port type optical circulator 11. An optical switch 12 is provided on a single area of the chirp fiber gratings 10 and 11. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a band-variable optical filter, and more particularly to a band-variable optical filter capable of controlling a light transmission band.

  The amount of communication information in the future is expected to increase further due to the advancement of advanced information technology in recent years, and as a large-capacity communication method, wavelength multiplexing transmission technology that transmits light of a plurality of different wavelengths through a single optical fiber attracts attention. ing. The development of such wavelength division multiplexing technology has been actively carried out, and along with its progress, in addition to the function of the conventional wavelength tunable optical filter, an optical filter having a variable bandwidth function will be required. Conceivable. This is because it is assumed that optical signals of several different wavelengths are processed together according to the traffic situation.

  Heretofore, several devices have been proposed as variable bandwidth optical filters. For example, an apparatus using a planar lightwave circuit (PLC) has been proposed (see Non-Patent Document 1). In this apparatus, the amount of change in the band is extremely small and the practicality is poor. In addition, a method of controlling the overlapping state of reflection spectra of two uniform fiber gratings has been proposed (see Patent Document 1). This method is a method for controlling the difference between the Bragg wavelengths of two fiber gratings. However, since the bandwidth of the fiber grating itself is limited, this also has a small variable width.

  On the other hand, as another method using a fiber grating, use of a chirp variable fiber grating by imparting strain is also conceivable (see Patent Document 2). In the filter described in Patent Document 2, as shown in FIG. 1, for example, a fiber grating 4 and a metal rod 2 are wrapped in a polyethylene sleeve 3 and heated to integrate them to produce a beam. As shown in FIG. 2, it is held by two types of clamps (first clamp 5 and second clamp 6). With such a configuration, distortion is applied to the filter described in Patent Document 2 by moving the second clamp 6 in a direction perpendicular to the initial direction of the beam (displacement direction of the displacement z).

  Since the second clamp 6 does not restrain the beam in the first direction of the beam, the beam is bent into an S shape, and the strain distribution changes linearly from negative to positive in the longitudinal direction. As a result, the Bragg wavelength is distributed in the longitudinal direction and becomes a chirped fiber Bragg grating, so that the reflection band is widened while the center wavelength remains constant. Originally, the distortion control is for controlling the chromatic dispersion (group delay dispersion) of the fiber grating. However, the filter described in Patent Document 2 also changes the band at the same time by adjusting the distortion. It is an optical filter. However, in this case, the incident light is chirped and emitted. In the case of an optical signal having a narrow band, there is no practical problem, but in the case of a signal having a wide band, that is, a high speed, it becomes a problem.

  In order to avoid this problem, a filter that gives an inverse chirp to an optical signal has been proposed (see Non-Patent Document 2). Specifically, two beams are produced using fiber gratings of the same specification, and the same chirped fiber grating is realized by applying distortion to the two beams in exactly the same manner. As shown in FIG. 3, if these fiber gratings 8 are connected via an optical circulator 7 so that the long wavelength side and the short wavelength side are coupled, the incident light is finally emitted with the chirp canceled. It will be. When the system composed of these two fiber gratings 8 is viewed as one optical filter, the incident light is always emitted without a chirp as long as each distortion amount is controlled to be the same. .

  FIG. 4 is a diagram illustrating a band variable characteristic of a conventional band variable optical filter by applying distortion. Here, distortion is given by changing the value of the displacement z shown in FIG. The length l is 160 mm. FIG. 4 shows that even if the displacement z is changed, the center wavelength is constant and only the band is changed. When z = 0 mm, the 3 dB band is 0.3 nm, and when z = 5 mm, the 3 dB band is 1.0 nm. By further increasing the value of z, a maximum bandwidth of 3 to 4 nm is possible (see Non-Patent Document 2). However, there is a limit to the amount of variable bandwidth due to the strength limit of the beam, and it is difficult to process optical signals with a large number of wavelengths, so the applications are very limited.

Japanese Patent No. 3348601 Japanese Patent No. 3337408 E.Pawlowski and 6 others, “Variable bandwidth and tunable center frequency filter using transversal-form programmable optical tilter”, British Electrical Engineers Association, January 16, 1996, vol.32, no.2, p.113- 114 Chee S. Goh and three others, “A Dispersion Free Filter with Bandwidth Tunability based on Simultaneous S-bend Chirping of Linearly Chirped Fiber Bragg Gratings”, 2003 IEICE Society Conference Proceedings 1, September 2003, C-3-69, p.202 Tetsuro Komukai and three others, “Application of Fiber Grating to Spectral Filtering”, January 1997, IEICE Transactions, C-I, vol.J80-C-I, p.32-40 S.Savin and three others, “Tunable mechanically induced long-period fiber gratings”, May 15, 2000, American Optical Society, vol.25, no.10, p.710-712 AKAhuja and 3 others, “Tunable single phase-shifted and superstructure gratings using microfabricated on-fiber thin film heaters”, Optics Communications (Elsevier Science, Netherlands), October 1, 2000, vol.184, p.119- 125 Edited by M.J.F.Digonnet, “Rare earth doped fiber lasers and amplifiers”, Marcel Dekker (USA), p.442 Michiko Harumoto and two others, "Tension dependence of loss characteristics of long-period fiber gratings", Advancement Technical Report, April 27, 1998, vol.98, no.37, OFT-98-9, p.47 -51

  As is clear from the above, in the wavelength division multiplexing transmission technology, a variable bandwidth optical filter is effective as a means of high-speed and large-capacity information transmission. In order to increase the amount, there are still problems that must be improved. In particular, the band-variable optical filters proposed so far have a small band-variable amount and are not practical for use in an actual optical network.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a variable bandwidth type that greatly increases the variable amount of bandwidth and is compatible with an optical network in which a large number of wavelength channels are used. It is to provide an optical filter.

  In order to achieve the above object, according to the present invention, in the first aspect of the present invention, at least one optical circulator and a first refractive index period varying from the first end toward the second end are provided. A first chirped fiber grating coupled to the at least one optical circulator at the first end, and having the same chromatic dispersion value as the first chirped fiber grating; The refractive index period changes from the third end toward the fourth end so as to be the same as the change direction of the refractive index period from the first end to the second end of the first chirped fiber grating. A second chirped fiber grating coupled to the at least one optical circulator at the fourth end. And blocking means for blocking incident light from the at least one optical circulator for a predetermined time in a single region or a plurality of regions on the first and second chirped fiber gratings. .

  The invention according to claim 2 is at least one optical circulator and a first chirped fiber grating formed in a planar lightwave circuit, the refractive index period of which varies from the first end toward the second end. A first chirped fiber grating coupled to the at least one optical circulator at the first end, and the first chirped fiber formed in a planar lightwave circuit that is the same as or different from the planar lightwave circuit. Having the same chromatic dispersion value as the grating, and from the third end so as to be the same as the changing direction of the refractive index period from the first end to the second end of the first chirped fiber grating. A second chirped fiber grating with a refractive index period changing toward the end of 4, wherein the at least one optical circular is at the fourth end. The incident light from the at least one optical circulator is blocked for a predetermined time in the second chirped fiber grating coupled to the first and second or single regions on the first and second chirped fiber gratings. And a blocking means.

  According to a third aspect of the present invention, there is provided at least one optical circulator and a first chirped fiber grating having a refractive index period changing from a first end toward a second end, wherein the first end A first chirped fiber grating coupled to the at least one optical circulator, and having the same chromatic dispersion value as the first chirped fiber grating, from the first end of the first chirped fiber grating A second chirped fiber grating in which the refractive index period is changed from the third end toward the fourth end so as to be the same as the direction of change of the refractive index period toward the second end, A second chirped fiber grating coupled to the at least one optical circulator at a fourth end; and the first and second chirped fibers. In a bug rating, attenuation means for attenuating the incident light for a predetermined time in a region from a first point on the incident side of the incident light from the optical circulator to a second point on the opposite side of the incident side; It is characterized by providing.

  According to a fourth aspect of the present invention, there is provided at least one optical circulator and a first chirped fiber grating formed in a planar lightwave circuit, the refractive index period of which changes from the first end toward the second end. A first chirped fiber grating coupled to the at least one optical circulator at the first end, and the first chirped fiber formed in a planar lightwave circuit that is the same as or different from the planar lightwave circuit. Having the same chromatic dispersion value as the grating, and from the third end so as to be the same as the changing direction of the refractive index period from the first end to the second end of the first chirped fiber grating. A second chirped fiber grating with a refractive index period changing toward the end of 4, wherein the at least one optical circular is at the fourth end. In the second chirped fiber grating coupled to the first and second chirped fiber gratings, the first point on the incident side of the incident light from the optical circulator from the first point opposite to the incident side Attenuating means for attenuating the incident light for a predetermined time is provided for a region up to two points.

  The invention according to claim 5 is the invention according to claim 3 or 4, wherein the region contains rare earth ions or transition metal ions, and the rare earth ions or transition metals are arranged on the side surfaces of the first and second chirped fiber gratings. It further comprises a light source for irradiating excitation light for ions and means for preventing the excitation light from being irradiated to a predetermined region of the region.

  The invention according to claim 6 is the invention according to claim 3 or 4, wherein the attenuating means is means for applying a lateral pressure to the first and second chirped fiber gratings at a linearly changing pitch. It is characterized by being.

  The invention according to claim 7 is the invention according to claim 3 or 4, wherein the attenuation means forms a temperature distribution with a linearly changing pitch with respect to the first and second chirped fiber gratings. It is characterized by being.

  The invention according to claim 8 is at least one optical circulator and a first chirped fiber grating having a refractive index period changing from the first end toward the second end, wherein the first end A first chirped fiber grating coupled to the at least one optical circulator, and having the same chromatic dispersion value as the first chirped fiber grating, from the first end of the first chirped fiber grating A second chirped fiber grating in which the refractive index period is changed from the third end toward the fourth end so as to be the same as the direction of change of the refractive index period toward the second end, A second chirped fiber grating coupled to the at least one optical circulator at the end of 4, and the at least one optical circulator With respect to the incident light to et the first and second chirped fiber grating, characterized in that it comprises a means for changing the wavelength band of the reflected light by the first and second chirped fiber grating.

  As described above, according to the present invention, it is possible to realize a band-variable optical filter having a variable amount greatly increased as compared with the prior art, which is useful in an actual optical network.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof is omitted.
In an embodiment of the present invention, the bandwidth of an optical filter is reduced by blocking or attenuating light propagating through each chirped fiber grating for each of two chirped fiber gratings connected to each other via an optical circulator. Control. In this specification, “chirped fiber grating” refers to an optical fiber grating in which the refractive index period in the longitudinal direction of the core portion of the optical fiber is changed.

  The optical filter of FIG. 3 described above is a combination of two linear chirped fiber gratings having the same absolute value of chromatic dispersion and different signs. The optical filter having such a configuration has already been studied as a method for obtaining a broadband characteristic (see Non-Patent Document 3). This is devised for the purpose of obtaining a filter having a wider band because a single uniform fiber grating can only obtain a band of up to about 3 nm. According to the optical filter described in Non-Patent Document 3, a chirp characteristic is not premised on realization by imparting distortion, so a band of 10 nm or more can be expected. FIG. 5 shows an example of the configuration. Or the structure of FIG. 6 may be sufficient. One embodiment of the present invention also has a basic configuration illustrated in FIGS. 5 and 6.

  FIG. 5 is a diagram showing a configuration of a filter using a three-port optical circulator according to an embodiment of the present invention. In FIG. 5, an optical fiber is connected to the first end of the three-port optical circulator 9a, from which incident light is incident. Further, a short wavelength side coupled chirped fiber grating 10 is connected to the second end of the three-port optical circulator, and the first end of the three-port optical circulator 9b is connected to the third end via the optical fiber. The ends of are connected. Similarly to the above, a chirped fiber grating 11 coupled to the long wavelength side is connected to the second end of the three-port optical circulator 9b, and an optical fiber is connected to the third end. Outgoing light is emitted.

  FIG. 6 is a diagram showing a configuration of a filter using a 4-port optical circulator according to an embodiment of the present invention. In FIG. 6, an optical fiber is connected to the first end of the four-port optical circulator 7, and incident light enters from here. A short-wavelength-side chirped fiber grating 10 is connected to the second end of the 4-port optical circulator 7, and a long-wavelength-side-coupled chirped fiber grating 11 is connected to the third end. An optical fiber is connected to the end, and outgoing light is emitted from this optical fiber.

  Each of the optical circulators described above transmits light only in a certain direction. That is, for the three-port optical circulators 9a and 9b, the light incident from the first end is transmitted to the second end, and the light reflected from the chirped fiber grating 10 or 11 is transmitted from the second end to the third end. Communicate to the edge. Similarly, for the 4-port optical circulator 7, light incident from the first end is reflected to the second end, light reflected from the chirped fiber grating 10 is reflected to the third end, and reflected from the chirped fiber grating 11. The transmitted light is transmitted to the fourth end.

Here, the chirped fiber gratings 10 and 11 are linear chirped fiber gratings having the same chromatic dispersion value, a length of L, and a reflection wavelength region from λ _S to λ _L. In the present specification, the “same chromatic dispersion value” refers to a transmission band of a desired optical filter. Therefore, the chirped fiber gratings 10 and 11 may have both desired bands even if the reflection wavelength ranges are different.

  In one embodiment of the present invention, the light entrance is different from that of the chirped fiber gratings 10 and 11, that is, whether the light is incident from the short wavelength side or the long wavelength side. . 5 and 6, for convenience, the wavelength band of the reflected light is next to the chirped fiber grating (also referred to as a short-wavelength-side coupled chirped fiber grating) in which light enters from the short wavelength side. It is assumed that a chirped fiber grating (also referred to as a long wavelength side coupled chirped fiber grating) in which light enters from the long wavelength side is arranged.

  In one embodiment of the present invention, the same effect can be obtained by arranging a chirped fiber grating with short wavelength side coupling next to a chirped fiber grating with long wavelength side coupling, which is the reverse of the above. can get.

  In one embodiment of the present invention, in the chirped fiber gratings 10 and 11, as long as any chirped fiber grating has a desired band in the reflection wavelength range, the dispersion values may not be the same.

Next, the operation of the optical filter having the above configuration will be described.
In FIG. 5, the light incident from the first end of the three-port optical circulator 9a is first transmitted to the second end and reflected by the chirped fiber grating 10 with short wavelength side coupling. At this time, in the chirped fiber grating 10, the location where the light is reflected differs depending on the wavelength of the incident light. Light having a wavelength shorter than λ _S and light longer than λ _L pass without being reflected and are emitted outside the fiber. The light reflected from the chirped fiber grating 10 is transmitted from the second end to the third end of the three-port optical circulator and emitted from the third end. The emitted light enters the circulator 9b from the first end of the three-port optical circulator 9b, is transmitted to the second end, and is reflected by the long wavelength side coupled chirped fiber grating 11 in the same manner as described above. . The reflected light is transmitted from the second end of the three-port optical circulator 9b to the third end and is emitted from the third end.

  At this time, the optical signal is chirped when reflected by the short-wavelength side-coupled chirped fiber grating 10, but is also reflected by the long-wavelength-side-coupled chirped fiber grating 11. Return to state.

Here, coordinates are set on the chirped fiber gratings 10 and 11 as shown in FIG.
FIG. 7 is a diagram showing the coordinates on the chirped fiber grating set to explain the principle of the present invention. In FIG. 7, in the chirped fiber gratings 10 and 11, the Bragg wavelength is distributed in the longitudinal direction. In the case of the chirped fiber grating 10 with short wavelength side coupling, it is assumed that the shortest wavelength λ _{S of} reflected light is at z = 0 and the longest wavelength λ _L is at z = L. In the case of the chirped fiber grating 11 coupled to the long wavelength side, it is assumed that the longest wavelength λ _L of the reflected light is at z = 0 and the shortest wavelength λ _S is at z = L. The Bragg wavelength at a point z = _{L a} on the chirped fiber grating 10 on the short wavelength side coupling and lambda _'L, the Bragg wavelength at z = _{L b} point on the chirped fiber grating 11 on the long wavelength side coupling lambda ' _S.

In chirped fiber grating 10 on the short wavelength side binding, the light of wavelength longer than lambda _'L is reflected at a point on the z> L _a, the light before and after reflection (z> light reflected by the L _a) is If it is lost due to attenuation or radiation to the outside, as a result, the effective band of the short-wavelength side-coupled chirped fiber grating 10 becomes λ _S <λ <λ ′ _L. On the other hand, even in the chirped fiber grating 11 on the long wavelength side binding, the light having a shorter wavelength than lambda _'S is reflected at a point on the z> L _b, it is reflected by the light (z> L _b back and forth reflection If the light is lost due to attenuation or radiation to the outside, the effective band of the long-wavelength-side chirped fiber grating 11 is λ ′ _S <λ <λ _L , as with the short-wavelength-side chirped fiber grating 10. It becomes.

Therefore, the effective band of the entire optical filter shown in FIG. 5 or 6 is λ ′ _S <λ <λ ′ _L. That is, the band is changing. The shortest wavelength of the effective band is determined by the value of L _b, the longest wavelength will be determined by the value of L _a. Here, if L _a + L _b ≧ L, the effective band will be lost. That is, all signal light is blocked or attenuated.

  Thus, if the light (signal light) propagating through each chirped fiber grating is blocked or attenuated by some means at a desired position of each chirped fiber grating, the band of the wideband optical filter can be changed. That is, in one embodiment of the present invention, in the chirped fiber grating according to one embodiment of the present invention, which has the basic configuration illustrated in FIGS. 5 and 6, the chirped fiber grating in the chirped fiber grating according to a desired band. It is characterized by blocking or attenuating light propagating to a certain area. Further, by changing the area where the propagation light is blocked or attenuated, the bandwidth of the optical filter can be increased, and the bandwidth can be changed as desired. Further, the bandwidth can be switched at a desired time by controlling the blocking or attenuation time of the propagation light.

In order to block or attenuate the light propagating here, several methods may be appropriately selected and used as shown below.
First, when a discontinuous change in the band is sufficient, a plurality of optical switches may be inserted into each chirped fiber grating. The band of the optical filter is determined depending on which optical switch is used to block light.
When continuous variation is required for the band, it is necessary to attenuate the signal light by some means in an arbitrary region in the chirped fiber grating.

  As means for attenuating the signal light, the ground level absorption (around 1.5 μm) of rare earth ions or transition metal ions may be used. For this purpose, it is necessary to form a chirped fiber grating in an optical fiber to which rare earth ions or transition metal ions used for optical amplification are added. Usually, the entire chirped fiber grating is irradiated with excitation light from the side surface of the chirped fiber grating so that all the ions are in an excited state and are made transparent.

  When it is desired to generate the attenuation region, it is only necessary to temporarily stop the excitation for only the portion that should be the attenuation region, thereby forming an absorption medium. That is, with respect to the chirped fiber grating, the light irradiation amount is reduced to a dose at which the region where the attenuation region is to be generated is blocked or the region becomes an absorption medium. As such specific means, for example, a mask or the like that blocks light, or an ND filter that reduces the amount of light such as an ND filter for the wavelength of the excitation light may be formed in a desired attenuation region.

  Alternatively, several lasers are arranged along the chirped fiber grating to irradiate the chirped fiber grating with excitation light. By arranging the lasers in this way, each of the arranged lasers takes charge of irradiating the chirped fiber grating with excitation light. When it is desired to generate an attenuation region, it is only necessary to stop the output of the laser that is responsible for irradiating the portion to be the attenuation region.

As the rare earth ions, Er ³⁺ , Tm ³⁺ , Sm ^{3+ and the} like may be selected. As the transition metal ion, Co ²⁺ , Cr ²⁺ or the like may be selected. However, in order to efficiently excite the ions added to the core, it is necessary to devise such as irradiating the laser beam from the excitation light source by converging the core with a cylindrical lens.

  As another means, a long-period fiber grating may be temporarily formed in a portion to be an attenuation region, and most of the signal light may be converted into a radiation mode in that portion. In this case, a long-period fiber grating is superimposed on a part of the chirped fiber grating, and a radiation loss band can be formed in the 1.5 μm band by setting the period to about several hundred μm.

  As a method for forming a long-period fiber grating, it can be formed by a conventional ultraviolet irradiation method, but when the band is controlled as desired, means other than the normal ultraviolet irradiation method are used to ensure reversibility. There is a need. Specifically, the following two methods may be performed.

  One is to apply a periodic strain to the radiation region by periodically applying a lateral pressure to the portion to be the radiation region (see Non-Patent Document 4). Due to the distortion caused by the side pressure, refractive index modulation is induced in the portion to be the radiation region, and a long-period fiber grating is formed.

  As another method, a metal thin film whose film pressure is periodically changed is coated on a portion to be a radiation region, and a current is supplied to a portion on which a long-period fiber grating is to be formed to generate heat. A long-period fiber grating may be formed by forming a periodic temperature distribution due to a difference in resistance (see Non-Patent Document 5). This is a method using a change in refractive index due to a temperature change.

  In order to emit the signal light in the region where the signal light of the chirped fiber grating propagates, as another means, the chirped fiber grating may be bent to generate bending loss. At that time, a base optical fiber that is likely to cause bending loss is selected. For this purpose, the cutoff wavelength is set to less than 1 μm.

  In order to improve the wavelength selectivity of the obtained band-variable optical filter, it is necessary to sufficiently suppress the base, and for that purpose, it is necessary to increase the light attenuation effect in the chirped fiber grating. If the attenuation rate per unit length is small, the chirped fiber grating must be lengthened. In that case, the optical filter can be miniaturized by arranging the chirped fiber grating on the circumference.

  The chirped fiber grating is generally formed of a silica glass-based optical fiber, but in one embodiment of the present invention, it is not necessary to be limited to this. An optical fiber made of fluoride glass, tellurite glass, or chalcogenite glass can also be applied to one embodiment of the present invention.

  Further, an optical waveguide other than the optical fiber may be used. For example, a planar lightwave circuit may be used. In the planar lightwave circuit, a quartz glass film having a thickness of about 100 μm is deposited on a silicon substrate (silicon wafer), and the film contains 10 μm containing germanium (Ge) parallel to the substrate and in the vicinity of several tens of μm from the substrate. A long corner line is formed, and this portion corresponds to the core of the optical fiber. Therefore, if a grating structure is formed in this part, the same effect as an optical fiber can be expected. When two chirped fiber gratings are formed on the planar lightwave circuit, each chirped fiber grating may be formed in the same planar lightwave circuit or in separate planar lightwave circuits. Also good.

Further, if the optical waveguide can be manufactured and a Bragg grating (chirped fiber grating) by refractive index modulation can be formed on the waveguide, the constituent material is not necessarily glass. For example, a polymer or LiNbO ₃ crystal may be used.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited to the following Example, It cannot be overemphasized that it can change variously in the range which does not deviate from the summary.

Example 1
The first embodiment according to the present invention shows a configuration example of a band-variable optical filter in which an optical switch is provided at each of the chirped fiber gratings 10 and 11 in the optical filter having the basic configuration as shown in FIG.

  FIG. 8 is a diagram illustrating a configuration of the band-variable optical filter according to the first embodiment of the present invention. In FIG. 8, the chirped fiber gratings 10 and 11 have a reflection band from a wavelength of 1500 nm to 1600 nm and a bandwidth of 100 nm. The optical switch 12 is inserted at a point where the Bragg wavelength is 1575 nm for the short-wavelength side-coupled chirped fiber grating 10 and at the point where the Bragg wavelength is 1525 nm for the long-wavelength-side-coupled chirped fiber grating 11.

  Since the signal light is blocked by the two optical switches 12, as described above, the signal light having a wavelength of 1575 nm to 1600 nm is not reflected for the chirped fiber grating 10, and the wavelength of 1500 nm to 1525 nm for the chirped fiber grating 11. Wavelength signal light is not reflected. Accordingly, the bandwidth is from 1525 nm to 1575 nm, and the bandwidth is 50 nm. In this embodiment, the bandwidth is halved, but an optical filter with a variable amount of bandwidth can be obtained depending on the insertion location of the optical switch. Further, since the signal light is blocked by the optical switch, reflection of signal light having an undesired wavelength can be suppressed, and the side flow of the band can be suppressed. Furthermore, the bandwidth can be switched in a desired time by controlling the on / off of the optical switch.

(Example 2)
In the second embodiment according to the present invention, the band variable using the chirped fiber gratings 10 and 11 produced by adding erbium ions (Er ³⁺ ), which are rare earth ions, in the optical filter having the basic configuration as shown in FIG. The structural example of a type | mold optical filter is shown.

Among the rare earth ions, erbium ions (Er ³⁺ ) added to the glass have ground level absorption at 1.48 to 1.6 μm, and the 1.48 μm band, 0.98 μm band, 0.8 μm band, 0. It becomes transparent by being excited with a 67 μm band laser beam. For example, an optical fiber doped with 10,000 ppm of erbium ions has a loss of approximately 2 dB / cm at 1.50 to 1.55 μm (see Non-Patent Document 6). Using this optical fiber, two linear chirped fiber gratings having a length of 100 cm and a reflection band of wavelengths from 1500 nm to 1550 nm are produced, and the optical filter of FIG. 5 is constructed. In the second embodiment, a plurality of lasers in charge of irradiation of excitation light to each chirped fiber grating are arranged along the chirped fiber gratings 10 and 11.

  The entire region of the chirped fiber gratings 10 and 11 to which erbium ions were added was irradiated with light (excitation light) having a wavelength excited by the erbium ions by a laser arrayed along each chirped fiber grating. In this case, the transmission spectrum is as shown by the solid line in FIG. 9, and the flat bandwidth is 50 nm. Next, with respect to the short-wavelength-side coupled chirp fiber grating 10, the output of the laser responsible for irradiating the 1530 to 1550 nm region with the excitation light is stopped to stop the excitation of the region, and the long-wavelength-side coupled chirp With respect to the fiber grating 11, the output of the laser responsible for the irradiation of the excitation light to the 1500 to 1520 nm region is stopped to stop the excitation of the region.

  At this time, the transmission spectrum of the optical filter is as shown by the dotted line in FIG. 9, and the flat region changes to 10 nm and the 20 dB bandwidth changes to 20 nm. In this way, the excitation region can be selected by stopping the output of the laser responsible for the region where excitation is desired to be stopped, and a transmission spectrum different from that in FIG. 9 can be obtained. Further, by controlling on / off of the output of each laser, the bandwidth can be switched in a desired time.

(Example 3)
In the method using rare earth ions for the attenuation of the signal light as in the second embodiment, the desired characteristics may not always be designed because the absorption spectrum of the ions is determined. The same applies to the case where transition metal ions are used. If ground level absorption of rare earth ions or transition metal ions cannot be used, a long-period fiber grating that can be formed reversibly may be used. In this case, the signal light is attenuated by radiating most of the propagation light out of the core.

  A normal long-period fiber grating is not reversible because it is formed by refractive index modulation of the core using ultraviolet irradiation. However, in the type formed by applying a periodic side pressure, when the application of the side pressure is stopped, the long-period fiber grating formed by the side pressure returns to the original optical fiber characteristics (see Non-Patent Document 4). Therefore, in the third embodiment of the present invention, in the optical filter having the basic configuration as shown in FIG. 5, a periodic lateral pressure is applied to a region where the signal light propagating to the chirped fiber gratings 10 and 11 is to be attenuated. 2 shows a configuration example of a band-variable optical filter having a long-period fiber grating formed by the above.

In general, in a long-period fiber grating having a period Λ, the wavelength λ _{m at} which LP ₀₁ waveguide mode light is coupled to the m-th order forward cladding mode
λ _m = (n _eff −n _cl ^m ) Λ
(See Non-Patent Document 4). Here, n _eff and n _cl ^m are effective refractive indexes of the LP ₀₁ waveguide mode and the m-th order cladding mode at the wavelength λ _m , respectively. With this wavelength as a peak, a few nm around it becomes a loss band. For this reason, if the lateral pressure is applied with a uniform period Λ, only signal light of a specific wavelength is attenuated. Therefore, in order to form a loss band in a wide wavelength band, it is necessary to gradually change the period of the side pressure.

  For example, in the optical filter of FIG. 5, it is assumed that the reflection bands of chirped fiber gratings 10 and 11 made of an optical fiber corresponding to a normal 1.3 μm zero-dispersion single mode fiber are 1550 to 1600 nm. As a method of applying a side pressure to the chirped fiber gratings 10 and 11, when the optical fiber is compressed from the side surface with the chirped groove plate 13 shown in FIG. 11, the period of the unevenness (pitch) of the groove of the groove plate is as shown in FIG. It needs to be chirped like this. Specifically, when the lowest-order cladding mode is selected, the period needs to be linearly changed from 700 μm to 720 μm in the region corresponding to the chirped fiber grating.

  FIG. 10 is a diagram illustrating a state in which a lateral pressure is applied to the chirped fiber grating according to the third embodiment of the present invention. In FIG. 10, chirped fiber gratings 10 and 11 are sandwiched between a chirped groove plate 13 and a flat plate 14. With this configuration, by pressing the chirped groove plate 13 from the chirped groove plate 13 toward the flat plate 14 by a pressing control device (not shown), a lateral pressure is applied to the chirped fiber gratings 10 and 11, and the portion to be a radiation region is applied. Distortion occurs. Refractive index modulation is induced by the distortion, and a long-period fiber grating can be formed. At this time, the formation time of the long-period fiber grating can be controlled by controlling the pressing control device.

  Note that it is desirable that the chirped groove plate 15 is separated into several parts as shown in FIG. By separating the chirped groove plate into a plurality of pieces as described above, it is possible to selectively apply a lateral pressure to the chirped fiber grating at the time of pressing by the pressing control device.

  For the band-variable optical filter having the long-period fiber grating manufactured as described above, the transmission spectrum characteristics are the same as those in FIG. 9, but in order to reduce the base, the loss of the long-period fiber grating is reduced. It needs to be bigger. In this embodiment, since it is several dB / cm (see Non-Patent Document 4), when the reflection band is 1550 to 1600 nm, the length needs to be designed to be 50 cm or more in order to sufficiently suppress the skirt.

  As described above, in the band-variable optical filter according to the third embodiment, a long-period fiber grating can be formed by applying a lateral pressure to the region to be attenuated of the chirped fiber grating, and the signal light is attenuated in that region. The bandwidth can be controlled. Moreover, the bandwidth can be switched in a desired time by controlling the side pressure application time.

Example 4
When a long period fiber grating is formed by applying a side pressure as in the third embodiment, the optical fiber may be damaged due to the side pressure. However, if the refractive index change is induced by applying a periodic temperature change, there is no concern about physical damage to the optical fiber. In the fourth embodiment of the present invention, therefore, a long-period fiber grating formed by causing a periodic temperature distribution in the chirped fiber gratings 10 and 11 in the optical filter having the basic configuration as shown in FIG. 1 shows a configuration example of a band-variable optical filter including

  FIG. 13 is a diagram illustrating a state for generating a temperature distribution in the chirped fiber grating according to the fourth embodiment of the present invention. As a method of applying a periodic temperature change, as shown in FIG. 13, a thin film coating (thick film portion 16 and thin film thin film) in which the film pressure is periodically changed is applied to the portion where the chirped fiber grating of the optical fiber is formed. The portion 17 and the portion 17 are alternately formed), and a current may be passed through the portion where the long-period fiber grating is to be formed (see Non-Patent Document 5). A conducting wire 18 is connected to the thick portion 16 of the thin film.

  In Non-Patent Document 5, a superstructure is formed by inducing a refractive index change with a period of 2.5 mm on a normal fiber grating, but a long-period fiber grating can also be formed by the same principle. However, in order to reduce thermal diffusion, the period should be as long as possible, and the lowest cladding mode must be used. As a specific thin film material, for example, gold may be used as described in Non-Patent Document 5.

  Since the electric resistance is large in the region where the film pressure is thin, and the electric resistance is small in the thick region, when a current is passed through the conductive wire 18 connected to the thick portion 16 of the thin film at both ends of the portion where the current is to flow, A difference occurs in the amount of heat generated, and a periodic temperature distribution occurs. When the temperature is high, the refractive index is large, and when the temperature is low, the refractive index remains small, and a long-period fiber grating is formed.

  Therefore, in Example 4, a long-period fiber grating can be formed in this area | region by sending an electric current through the conducting wire 18 connected to the thick part 16 of the thin film of the both ends of the part which should be used as an attenuation | damping area | region. At this time, by selecting a conducting wire through which a current flows, a current can be passed to a desired region, and a long-period fiber grating can be formed in the desired region. In addition, the formation time of the long-period fiber grating can be controlled by controlling the time for flowing the current to the conducting wire.

  In the present invention, it is necessary to form a long-period fiber grating only in a desired region. For this purpose, it is necessary to arrange the conductors from the power source at an appropriate interval ΔL. When forming a long-period fiber grating in a certain region, it is only necessary to select a conducting wire at both ends of the region and pass a current.

  Here, the period Λ needs to change linearly as in the third embodiment. Assuming that the reflection band of the chirped fiber grating is 1550 to 1600 nm, when the lowest order mode is used, it is necessary to change the period linearly from 700 μm to 720 μm as in the third embodiment.

Similarly to Example 3, Example 4 also has a transmission spectrum characteristic similar to that of FIG. 9 for the band-variable optical filter having the long-period fiber grating manufactured as described above, but the base is reduced. In order to achieve this, it is necessary to increase the loss of the long-period fiber grating as in the third embodiment. In Example 4, the temperature difference becomes smaller than the original due to thermal diffusion, and the refractive index difference substantially obtained is about 10 ⁻⁴ (see Non-Patent Document 5). For this reason, the radiation mode loss is about 1 dB / cm (see Non-Patent Document 7). Therefore, if the reflection band is designed to be 1550 to 1600 nm, the chirped fiber grating needs to be about 100 cm in order to sufficiently reduce the base.

  As described above, in the band-variable optical filter according to the fourth embodiment, a long-period fiber grating can be formed by causing a current distribution in the region to be attenuated of the chirped fiber grating to generate a temperature distribution. Since the light can be attenuated, the bandwidth can be controlled. Further, by controlling the time during which the current flows, the bandwidth can be switched in a desired time.

It is a figure explaining the structure of the beam which encloses a fiber grating in the band variable type optical filter by the conventional distortion provision. It is explanatory drawing of the S-shaped bending method of the beam shown in FIG. It is a block diagram of the conventional band variable type optical filter by distortion provision. It is a figure which shows the band variable characteristic of the conventional band variable type optical filter by distortion provision. It is a figure which shows the structure of the filter by the 3 port type | mold optical circulator which concerns on one Embodiment of this invention. It is a figure which shows the structure of the filter by the 4 port type | mold optical circulator which concerns on one Embodiment of this invention. It is a figure which shows the coordinate on the chirped fiber grating set in order to demonstrate the principle of this invention. It is a figure which shows the structure of the band variable type optical filter which concerns on one Embodiment of this invention. It is a figure which shows the transmission spectrum by the band variable type optical filter which concerns on one Embodiment of this invention. It is a figure which shows a mode that a side pressure is provided to the chirped fiber grating which concerns on one Embodiment of this invention. It is a figure which shows the chirp groove board which concerns on one Embodiment of this invention. It is a figure which shows the chirp groove board which concerns on one Embodiment of this invention. It is a figure which shows the mode for producing temperature distribution in the chirped fiber grating which concerns on one Embodiment of this invention.

Explanation of symbols

7 4-port optical circulator 9a, 9b 3-port optical circulator 10 Short wavelength side coupled chirped fiber grating 11 Long wavelength side coupled chirped fiber grating 12 Optical switch 13 Chirped groove plate 14 Flat plate 15 Split type chirped groove plate 16 Thin film Thick part 17 Thin part of thin film 18 Conductor

Claims (8)

At least one optical circulator;
A first chirped fiber grating having a refractive index period changing from a first end toward a second end, the first chirp coupled to the at least one optical circulator at the first end Fiber grating,
It has the same chromatic dispersion value as the first chirped fiber grating, and is the same as the change direction of the refractive index period from the first end to the second end of the first chirped fiber grating. A second chirped fiber grating having a refractive index period changing from the third end toward the fourth end, wherein the second chirp is coupled to the at least one optical circulator at the fourth end. Fiber grating,
A band-variable type, comprising: a blocking unit that blocks incident light from the at least one optical circulator for a predetermined time in a single region or a plurality of regions on the first and second chirped fiber gratings. Optical filter. At least one optical circulator;
A first chirped fiber grating formed in a planar lightwave circuit and having a refractive index period changing from a first end toward a second end, wherein the at least one optical circulator is formed at the first end. A first chirped fiber grating coupled to
The same wavelength dispersion value as that of the first chirped fiber grating formed in the same or separate planar lightwave circuit as the planar lightwave circuit, and from the first end of the first chirped fiber grating. A second chirped fiber grating in which the refractive index period changes from the third end toward the fourth end so as to be the same as the direction of change of the refractive index period toward the end of 2. A second chirped fiber grating coupled to the at least one optical circulator at the end of the
A band-variable type, comprising: a blocking unit that blocks incident light from the at least one optical circulator for a predetermined time in a single region or a plurality of regions on the first and second chirped fiber gratings. Optical filter. At least one optical circulator;
A first chirped fiber grating having a refractive index period changing from a first end toward a second end, the first chirp coupled to the at least one optical circulator at the first end Fiber grating,
It has the same chromatic dispersion value as the first chirped fiber grating, and is the same as the change direction of the refractive index period from the first end to the second end of the first chirped fiber grating. A second chirped fiber grating having a refractive index period changing from the third end toward the fourth end, wherein the second chirp is coupled to the at least one optical circulator at the fourth end. Fiber grating,
In the first and second chirped fiber gratings, the incident light is predetermined for a region from a first point on the incident side of incident light from the optical circulator to a second point on the opposite side to the incident side. A band-variable optical filter comprising: attenuating means for attenuating for a period of time. At least one optical circulator;
A first chirped fiber grating formed in a planar lightwave circuit and having a refractive index period changing from a first end toward a second end, wherein the at least one optical circulator is formed at the first end. A first chirped fiber grating coupled to
The same wavelength dispersion value as that of the first chirped fiber grating formed in the same or separate planar lightwave circuit as the planar lightwave circuit, and from the first end of the first chirped fiber grating. A second chirped fiber grating in which the refractive index period changes from the third end toward the fourth end so as to be the same as the direction of change of the refractive index period toward the end of 2. A second chirped fiber grating coupled to the at least one optical circulator at the end of the
In the first and second chirped fiber gratings, the incident light is predetermined for a region from a first point on the incident side of incident light from the optical circulator to a second point on the opposite side to the incident side. A band-variable optical filter comprising: attenuating means for attenuating for a period of time. The region includes rare earth ions or transition metal ions,
A light source for irradiating side surfaces of the first and second chirped fiber gratings with excitation light for the rare earth ions or transition metal ions;
The band-variable optical filter according to claim 3, further comprising means for preventing the excitation light from being irradiated to a predetermined region of the region.   5. The band-variable optical filter according to claim 3, wherein the attenuating means is means for applying a lateral pressure to the first and second chirped fiber gratings at a linearly changing pitch. .   5. The band-variable light according to claim 3, wherein the attenuation means is means for forming a temperature distribution at a linearly changing pitch with respect to the first and second chirped fiber gratings. filter. At least one optical circulator;
A first chirped fiber grating having a refractive index period changing from a first end toward a second end, the first chirp coupled to the at least one optical circulator at the first end Fiber grating,
The first chirped fiber grating has the same chromatic dispersion value, and the first chirped fiber grating has the same refractive index period changing direction from the first end to the second end. A second chirped fiber grating having a refractive index period changing from the end of 3 toward the fourth end, the second chirped fiber coupled to the at least one optical circulator at the fourth end The grating,
And means for changing a wavelength band of light reflected by the first and second chirped fiber gratings with respect to light incident on the first and second chirped fiber gratings from the at least one optical circulator. Band-variable optical filter.

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