JPH1048425A - Variable Bandwidth Transmission Optical Filter - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To easily change a transmission bandwidth,
A transmission optical filter with excellent wavelength selectivity is realized. SOLUTION: Light incident from an optical fiber 1a is converted into a fiber grating 6 by a 4-port optical circulator 7.
a. Only light in a certain wavelength band is reflected by the fiber grating 6a, and guided to the fiber grating 6b by the 4-port optical circulator 7.
Only light in a certain wavelength band is reflected by the fiber grating 6b, and guided to the optical fiber 1b by the 4-port optical circulator 7. Light of other wavelengths is radiated out of the fiber by the non-reflection end portions 8a and 8b, and is not guided to the optical fiber 1b. That is, from the optical fiber 1b,
The reflection wavelength of the fiber grating 6b is adjusted by the wavelength adjusting device 9, and the fiber gratings 6a and 6b are adjusted.
By adjusting the width of the overlapping portion of the two reflection wavelength bands, the wavelength bandwidth of the light emitted from the optical fiber 1b is varied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable band transmission optical filter having excellent wavelength selectivity and capable of changing a transmission bandwidth.

[0002]

2. Description of the Related Art FIG. 10 shows an example of a conventional transmission type optical filter. In the figure, reference numerals 1a and 1b denote optical fibers, 2a, and 2a.
2b is a semi-transparent mirror, 3a and 3b are collimators, 4 is a micrometer head, and 5 is a holder.

In the above-mentioned configuration, light incident from the optical fiber 1a is guided to a Fabry-Perot interferometer comprising semi-transparent mirrors 2a and 2b opposed to each other so that mirror surfaces are parallel via a collimator 3a. The light wavelength is selected in the Fabry-Perot interferometer, and only the light of the desired wavelength passes through the Fabry-Perot interferometer, and the collimator 3b.
Through the optical fiber 1b. The bandwidth of the transmitted light is determined by the finesse of the Fabry-Perot interferometer, and the wavelength of the transmitted light is determined by the spacing of the transflectors.

The band of the transmitted light is adjusted by operating the micrometer head 4 connected to the holder 5 fixed to the semi-transparent mirrors 2a and 2b placed opposite to each other. Is controlled by controlling the parallelism and adjusting the finesse.

The wavelength of the transmitted light is adjusted by operating the micrometer head 4 to adjust the distance between the mirror surfaces of the semi-transparent mirrors 2a and 2b.

[0006]

However, the adjustment of the parallelism of the semi-transparent mirror is delicate, so that it is difficult to control the bandwidth.
Further, when the finesse is deteriorated in order to expand the transmission band, there is a problem that the wavelength selectivity of the filter is reduced and the transmitted light intensity is also changed.

SUMMARY OF THE INVENTION An object of the present invention is to realize a band variable transmission type optical filter which can easily change a transmission bandwidth and has excellent wavelength selectivity.

[0008]

According to the present invention, there is provided a four-port optical circulator, comprising: a first optical port connected to the four-port optical circulator for transmitting light to the four-port optical circulator; The optical fiber has a periodic refractive index change formed in the longitudinal direction of the optical fiber, and has a function of reflecting light having a wavelength corresponding to the pitch of the period of the refractive index change. Light incident from the first optical fiber is guided by being connected to the port optical circulator, and the other end is formed in the longitudinal direction of the optical fiber with the first fiber grating terminated by the non-reflection terminal. It has a periodic refractive index change and has a function of reflecting light of a wavelength corresponding to the pitch of the period of the refractive index change, and one end is connected to the four-port optical circulator and The light reflected by the fiber grating is guided, and the other end is connected to the second fiber grating terminated by a non-reflection terminator and the four-port optical circulator, and is reflected by the second fiber grating. A second optical fiber that guides the emitted light and emits this light, and at least one of the first fiber grating and the second fiber grating, wherein the pitch of the refractive index change in the mounted fiber grating. A wavelength adjusting device for changing
It is characterized by having.

Further, according to the present invention, there is provided a first three-port optical circulator, and a first optical fiber connected to the first three-port optical circulator for making light incident on the first three-port optical circulator. And has a function of reflecting light having a wavelength corresponding to the pitch of the period of the change in the refractive index, the light having a periodic refractive index change formed in the longitudinal direction of the optical fiber. Is connected to the first three-port optical circulator, the light incident from the first optical fiber is guided, and the other end is terminated by a non-reflection terminator. A second three-port optical circulator that is connected and guides light reflected by the first fiber grating; and a periodic flexure formed in the longitudinal direction of the optical fiber. Having the rate change, it has a function of reflecting light having a wavelength corresponding to the pitch of the period of refractive index change,
One end is connected to the second three-port optical circulator to guide the light reflected by the first fiber grating, and the other end is terminated by a non-reflection terminator.
A second optical fiber, which is connected to the second three-port optical circulator and guides the light reflected by the second fiber grating and emits the light, and a first fiber. A wavelength adjusting device that is mounted on at least one of the grating and the second fiber grating, and that changes a pitch of a refractive index change in the mounted fiber grating.

Further, according to the present invention, there is provided a four-port optical circulator, a first optical fiber connected to the four-port optical circulator for inputting light to the four-port optical circulator, And has a function of reflecting light having a wavelength corresponding to the pitch of the cycle of the refractive index change. One end is connected to the four-port optical circulator and the first The first fiber grating, in which light incident from the optical fiber is guided and the other end is terminated by a non-reflection terminal, has a periodic refractive index change formed in the longitudinal direction of the optical fiber, and has a refractive index. It has a function of reflecting light having a wavelength corresponding to the pitch of the cycle of the rate change. One end is connected to the four-port optical circulator, and the light is reflected by the first fiber grating. And the other end is connected to the four-port optical circulator, and the light reflected by the second fiber grating is guided. A second optical fiber for emitting the light,
And the second fiber grating is sandwiched between the two fiber gratings. In order to simultaneously change the pitch of the refractive index change between the two fiber gratings, one fiber grating is provided with a tensile force that increases the pitch. At the same time, the other fiber grating is provided with one wavelength adjusting device for applying a compressive force for reducing the pitch.

[Action]

According to the present invention, light incident on the first optical fiber is incident on the first fiber grating, and only light of a specific wavelength is reflected, and the reflected light is transmitted to the second fiber grating. The guided light reflects only light of a specific wavelength, and the light reflected twice by the two fiber gratings is emitted from the second optical fiber. The light emitted from the second optical fiber is light in a wavelength band where both reflection wavelength bands of the two fiber gratings overlap. Moreover, at least one of the two fiber gratings is changed in the pitch of the period of the refractive index change by the wavelength adjusting device.
The width of the overlapping portion of the two reflection wavelength bands can be adjusted.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings.

<First Embodiment> FIG. 1 shows a first embodiment of the present invention. In the drawing, the same components as those of the conventional example are denoted by the same reference numerals. That is, 1a and 1b are optical fibers, 6
Reference numerals a and 6b denote fiber gratings, 7 denotes a four-port optical circulator, 8a and 8b denote non-reflection terminations of optical fibers, and 9 denotes a wavelength adjusting device.

The fiber gratings 6a and 6b have a periodic refractive index change formed in the longitudinal direction of the optical fiber, and reflect light having a wavelength corresponding to the pitch of the period of the refractive index change in a certain wavelength band. One end is connected to the four-port optical circulator 7 and the other end is terminated by anti-reflection terminators 8a and 8b. The fiber grating is manufactured by irradiating the core of an optical fiber with ultraviolet rays to induce a refractive index modulation, and is a reflection filter having a narrow band and excellent wavelength selectivity.

The four-port optical circulator 7 transmits light only in a certain direction. The four-port optical circulator 7 transmits the light incident from the optical fiber 1a to the fiber grating 6a, the light reflected by the fiber grating 6a to the fiber grating 6b, and the fiber grating. The light reflected by 6b is installed to be guided to the optical fiber 1b.

The wavelength adjusting device 9 adjusts the reflection wavelength of the fiber grating by changing the pitch of the refractive index change in the fiber grating, and is mounted on the fiber grating 6b in this embodiment. As a method of adjusting the wavelength of the fiber grating, a method of applying tension in the longitudinal direction of the fiber grating using a piezoelectric vibrator or the like, or a method of attaching a fiber grating to a flat plate or rod and bending the flat plate or rod There are a method of applying tension in the longitudinal direction of the grating and a method of applying heat to the fiber grating using a heating wire or the like.

The reflection wavelengths of the fiber gratings 6a and 6b are close to each other so that the size of the overlapping region of the reflection wavelength bands can be adjusted by the wavelength adjusting device 9.

In this operation, first, light incident from the optical fiber 1a is guided to the fiber grating 6a by the four-port optical circulator 7. Only light in a certain wavelength band is reflected by the fiber grating 6a,
The light is guided to the fiber grating 6 b by the port optical circulator 7. Light of other wavelengths is the non-reflection terminal 8
a is radiated out of the fiber and is not guided to the fiber grating 6b.

With respect to the light reflected by the fiber grating 6a and guided to the fiber grating 6b, only light in a certain wavelength band is reflected by the fiber grating 6b, and guided to the optical fiber 1b by the 4-port optical circulator 7. Light of other wavelengths is radiated out of the fiber by the non-reflection termination 8b and is not guided to the optical fiber 1b.

That is, from the optical fiber 1b, light of a wavelength band in a portion where the reflection wavelength bands of both the fiber gratings 6a and 6b are reflected, which is reflected by both of the fiber gratings 6a and 6b, is emitted. Therefore, by adjusting the reflection wavelength of the fiber grating 6b and adjusting the width of the overlapping portion of the reflection wavelength bands of the fiber gratings 6a and 6b, the wavelength band of the light emitted from the optical fiber 1b can be adjusted. it can.

This will be described with reference to FIG. FIG.
(A) shows the relationship between the reflection wavelength bands of the fiber gratings 6a and 6b and the transmission band of the filter according to the present embodiment, where the vertical axis represents light intensity and the horizontal axis represents light wavelength. In the figure, reference numerals 6a and 6b denote fiber gratings 6
a and 6b represent the reflection wavelength bands. Here, it is assumed that the fiber grating 6a is on the longer wavelength side than the fiber grating 6b. The hatched portions where the reflection wavelength bands overlap each other represent the transmission band of the present filter.

FIG. 2B is a diagram in which the reflection wavelength of the fiber grating 6b is shifted to a longer wavelength side than in FIG. 2A, and is closer to the reflection wavelength of the fiber grating 6a. As compared with FIG. 2A, there are more overlapping portions of the reflection wavelength band, that is, the width of the transmission band is wider.

FIG. 2C is a view in which the wavelength of the fiber grating 6b is set farther away from the fiber grating 6a than in FIG. 2A. As compared with FIG. 2A, the overlapping portion of the reflection wavelength band is small, that is, the width of the transmission band is narrow.

From this, it can be seen that the transmission bandwidth of the present filter can be varied by adjusting the reflection wavelength of the fiber grating 6b.

Although the wavelength selection characteristics of the present optical filter are caused by the wavelength selection characteristics of the fiber grating, changing the wavelength bandwidth of the light emitted from the present optical filter is dependent on the wavelength selection characteristics of the fiber grating. Is not accompanied by a large change in the wavelength selection characteristic, so that the deterioration of the wavelength selection characteristic due to the control of the wavelength band is small.

According to the above configuration, it is possible to realize a band variable transmission type optical filter having excellent wavelength selectivity, which can easily change the transmission band by changing the reflection wavelength of the fiber grating 6b.

FIG. 3 shows fiber gratings 6a and 6b.
The transmission spectrum of the present filter when the wavelength of the fiber grating 6b is sequentially changed using the full width at half maximum of 1.3 nm and 1.5 nm respectively. The horizontal axis is the wavelength of the transmission spectrum, and the vertical axis is the transmittance. The band of the transmission spectrum changes according to the change of the wavelength of the fiber grating 6b. Further, even if the band of the transmission spectrum changes, no extreme change in the slope of the side lobe of the spectrum and the peak value of the transmission spectrum is observed, and it can be seen that there is no deterioration in the wavelength selection characteristics.

FIG. 4 is a graph showing the relationship between the full width at half maximum of the transmission spectrum and the transmittance of the present filter. The horizontal axis represents the full width at half maximum of the transmission spectrum, and the vertical axis represents the transmittance. Even if the full width at half maximum of the transmission spectrum is changed from 1.3 nm to 0.7 nm, there is no significant change in transmittance, and even if the full width at half maximum of the transmission spectrum is changed to 0.25 nm, the loss increase is 2.5.
It is about dB. FIG. 4 shows that the bandwidth can be reduced to 0.15 nm if the loss increase is 3 dB.

<Second Embodiment> FIG. 5 shows a second embodiment of the present invention. In this embodiment, two 3-port optical circulators are used instead of the 4-port optical circulator in the first embodiment. Here is an example. That is, 10a and 10b represent three-port optical circulators. Other configurations are first
This is the same as the embodiment.

This operation is performed by the fiber grating 6
a except that the reflected light from a is emitted to the three-port optical circulator 10b using the three-port optical circulator 10a.
This is the same as the first embodiment. That is, the three-port optical circulator 10a transmits the light incident from the optical fiber 1a to the fiber grating 6a,
The light reflected by a is emitted to the three-port optical circulator 10b. The three-port optical circulator 10b transmits the light incident from the three-port optical circulator 10a to the fiber grating 6b and the fiber grating 6b.
The light reflected by the light guide b is guided to the optical fiber 1b. Each fiber grating 6a,
Light not reflected by 6b is processed by the non-reflection terminations 8a and 8b.

The wavelength adjusting device 9 is mounted on the fiber grating 6b, and can adjust the wavelength band of light emitted from the optical fiber 1b by adjusting its reflection wavelength band. At this time, the optical fiber 1a
And the optical fiber 1b are separate optical circulators 10a, 10a.
0b, it is optically separated.
That is, part of the light from the optical fiber 1a
No part of the light directly emitted to the optical fiber 1b or, conversely, part of the light emitted to the optical fiber 1b is emitted to the optical fiber 1a.

According to the above configuration, stray light in the filter can be reduced as compared with the case of the first embodiment.

<Third Embodiment> FIG. 6 shows a third embodiment of the present invention. Here, an example in which both wavelengths of the fiber gratings 6a and 6b in the first embodiment are adjusted is shown. That is, 9a and 9b represent wavelength adjusting devices. Other configurations are the same as those of the first embodiment.

This operation is performed by the fiber grating 6
This is the same as the first embodiment except that the reflection wavelength of a is adjusted by the wavelength adjusting device 9a. That is, the optical fiber 1a
The incident light is guided by the 4-port optical circulator 7 to the fiber grating 6a. Only light in a certain wavelength band is reflected by the fiber grating 6a, and guided to the fiber grating 6b by the 4-port optical circulator 7. Fiber grating 6a
As for the light reflected by the fiber grating 6b, only the light in a certain wavelength band is reflected by the fiber grating 6b, and the four-port optical circulator 7
Is guided to the optical fiber 1b. Light of other wavelengths is processed by the non-reflection terminations 8a and 8b.

From the optical fiber 1b, light of a wavelength band in a portion where the reflection wavelength bands of both fiber gratings 6a and 6b overlap is emitted. That is, the wavelength of the light emitted from the optical fiber 1b can be adjusted by adjusting the wavelength of the overlapping portion of the reflection wavelength bands of both the fiber gratings 6a and 6b.

According to the above configuration, as compared with the case of the first embodiment, not only the wavelength band can be changed, but also the wavelength can be adjusted.

<Fourth Embodiment> FIG. 7 shows a fourth embodiment of the present invention. In this embodiment, both wavelengths of the fiber gratings 6a and 6b in the first embodiment are adjusted by one wavelength adjusting device. An example in which the adjustment is performed at the same time is shown. That is, the wavelength adjusting device 9c includes two fiber gratings 6
a, 6b. Other configurations are the same as those of the first embodiment.

This operation is performed by the fiber grating 6
This is the same as the first embodiment except that the reflection wavelengths a and 6b are adjusted by the wavelength adjusting device 9c. That is, light incident from the optical fiber 1a is guided to the fiber grating 6a by the 4-port optical circulator 7. The light reflected by the fiber grating 6a and guided to the fiber grating 6b is a four-port optical circulator 7
Is guided to the optical fiber 1b. Light of other wavelengths is processed by the non-reflection terminations 8a and 8b. From the optical fiber 1b, light of a wavelength band in a portion where the reflection wavelength bands of both the fiber gratings 6a and 6b overlap is emitted.

Here, the wavelength adjusting device 9c is a device that simultaneously applies a positive tension to one of the fiber gratings 6a and 6b, that is, tension, and applies a negative tension to the other, that is, compression. is there.

When a positive tension is applied to the fiber grating, the pitch of the change in the refractive index in the fiber grating increases, and the reflection wavelength of the fiber grating shifts to the longer wavelength side. When a negative tension is applied to the fiber grating, the pitch of the change in the refractive index in the fiber grating becomes smaller, and the reflection wavelength of the fiber grating moves to the shorter wavelength side. Fiber grating 6
When a positive tension is applied to one of the a and 6b and a negative tension is applied to the other, the reflection wavelengths of the fiber gratings rapidly approach each other to increase the bandwidth of the filter,
Alternatively, the bandwidths of the present filter are reduced apart from each other.

For this reason, the fiber gratings 6a,
6b, the amount of shift of the center wavelength of each fiber grating for controlling the size of the overlapping portion of both reflection wavelength bands is the same as that of the first embodiment when obtaining the size of the overlapping portion of the same reflection wavelength band in the first embodiment. It is smaller than that, and has a high wavelength controllability.

FIG. 8 shows an example of the wavelength adjusting device 9c. 1
1 denotes an actuator, and 12 denotes a fixed block. The actuator 11 is an element that bends in the vertical direction in the plane of the paper, and one end is fixed by a fixing block 12.
The fiber gratings 6a and 6b are the actuator 1
1 is tightly attached to the surface in the direction in which the bending occurs, that is, the surface in the vertical direction in the paper surface, and bends integrally with the actuator 11.

The operation will be described with reference to FIG. FIG.
FIG. 3A is a diagram showing a state in which no bending occurs in the actuator 11. Here, the fiber grating 6a is provided on the upper surface of the actuator 11, and the fiber grating 6b
Are in close contact with the lower surfaces, respectively. FIG. 9B shows the reflection wavelength band of the fiber gratings 6a and 6b at this time, and the hatched portion represents the transmission band of the filter according to the present embodiment. Here, it is assumed that the fiber grating 6a is on the longer wavelength side than the fiber grating 6b. Similarly, FIGS. 9C and 9D show a state in which the actuator 11 is bent downward in the drawing.
FIGS. 9E and 9F respectively show states in which the actuator 11 is bent upward in the plane of the drawing.

As shown in FIG. 9C, the actuator 1
When a downward bending is applied to 1, the fiber grating 6a located outside the bend is subjected to a tensile force, and the fiber grating 6b located inside the bend is subjected to a compressive force. For this reason, as shown in FIG. 9D, the reflection wavelength band of the fiber grating 6a moves to the longer wavelength side and the reflection wavelength band of the fiber grating 6b moves to the shorter wavelength side. Part)
Is narrower than that of FIG. When an upward bending is applied to the actuator 11 as shown in FIG. 9E, a compressive force acts on the fiber grating 6a and a tensile force acts on the fiber grating 6b.
As shown in FIG. 9F, the reflection wavelength band of the fiber grating 6a is shifted to the shorter wavelength side.
b moves to the longer wavelength side, and the width of the transmission wavelength band of the present filter becomes wider than that in FIG. 9B.

According to the above configuration, compared to the first embodiment, the amount of movement of the center wavelength of the fiber grating for obtaining the same size of the overlapping portion of the reflection wavelength band can be reduced, and the controllability is high.

[0046]

As described above, according to the first aspect of the present invention,
According to the variable band transmission optical filter, the transmission bandwidth can be easily varied by changing the reflection wavelength of the optical fiber grating, and a transmission optical filter having excellent wavelength selectivity can be realized.

At present, an optical filter having a variable transmission bandwidth has been desired for controlling the pulse width in high-speed optical pulse communication. However, according to the present invention, an optical filter capable of easily controlling the transmission band is provided. A filter can be realized.

According to the band variable transmission type optical filter of the second aspect of the present invention, the transmission bandwidth can be easily changed by changing the reflection wavelength of the optical fiber grating, and the wavelength selectivity can be improved. Not only can an excellent transmission optical filter be realized, but also stray light in the filter can be reduced.

According to the band variable transmission optical filter of the third aspect of the present invention, since the pitch of the refractive index change of the two fiber gratings is changed simultaneously, the amount of movement of the center wavelength of each fiber grating can be small. High wavelength controllability.

Further, the band variable filter according to the present invention can be constructed at a price that is less than half the price of the conventional Fabry-Perot filter. In addition, since delicate adjustment such as control of the parallelism of a semi-transparent mirror in a Fabry-Perot filter is not required to change the transmission bandwidth, the operability is excellent.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a variable band transmission optical filter according to the present invention.

FIG. 2 is a characteristic diagram showing an operation of the band variable transmission type optical filter according to the first embodiment.

FIG. 3 is a characteristic diagram illustrating a transmission spectrum of the variable-bandwidth transmission optical filter according to the first embodiment.

FIG. 4 is a characteristic diagram showing a relationship between a transmittance and a band of the variable-bandwidth optical filter according to the first embodiment.

FIG. 5 is a configuration diagram showing a second embodiment of the variable band transmission optical filter of the present invention.

FIG. 6 is a configuration diagram showing a third embodiment of the variable band transmission optical filter of the present invention.

FIG. 7 is a configuration diagram showing a fourth embodiment of the variable band transmission optical filter of the present invention.

FIG. 8 is a configuration diagram illustrating a wavelength control device according to a fourth embodiment.

FIG. 9 is a characteristic diagram illustrating an operation of the wavelength controller according to the fourth embodiment.

FIG. 10 is a configuration diagram illustrating an example of a conventional optical filter.

[Explanation of symbols]

 1a, 1b Optical fiber 2a, 2b Semi-transmissive mirror 3a, 3b Collimator 4 Micrometer head 5 Holder 6a, 6b Fiber grating 7 4-port optical circulator 8a, 8b Non-reflective termination of optical fiber 9a, 9b, 9c Wavelength adjusting device 10a, 10b 3-port optical circulator 11 Actuator 12 Fixed block

Claims (3)

[Claims] 1. A four-port optical circulator, a first optical fiber connected to the four-port optical circulator for making light incident on the four-port optical circulator, and a period formed in a longitudinal direction of the optical fiber. Having a characteristic refractive index change and a function of reflecting light having a wavelength corresponding to the pitch of the period of the refractive index change, and one end of which is connected to the four-port optical circulator and is incident from the first optical fiber. A first fiber grating in which light is guided and the other end of which is terminated by a non-reflection termination; and a periodic refractive index change formed in a longitudinal direction of the optical fiber, wherein a period of the refractive index change is It has a function of reflecting light having a wavelength corresponding to the pitch, and one end is connected to the four-port optical circulator to guide the light reflected by the first fiber grating. A second fiber grating, the other end of which is terminated by a non-reflection terminator; and the second fiber grating is connected to the four-port optical circulator, and the light reflected by the second fiber grating is guided and emitted. A second optical fiber, and a wavelength adjusting device mounted on at least one of the first fiber grating and the second fiber grating, and changing a pitch of a refractive index change in the mounted fiber grating. A variable band transmission optical filter characterized in that: 2. A first three-port optical circulator, a first optical fiber connected to the first three-port optical circulator, and configured to input light to the first three-port optical circulator; It has a periodic refractive index change formed in the longitudinal direction, and has a function of reflecting light of a wavelength corresponding to the pitch of the period of the refractive index change.
A light which is connected to a port optical circulator and guided from a first optical fiber is guided, and the other end is connected to a first fiber grating terminated by a non-reflection terminal, and to the first three-port optical circulator. A second three-port optical circulator through which light reflected by the first fiber grating is guided; and a periodic refractive index change formed in the longitudinal direction of the optical fiber. It has a function of reflecting light having a wavelength corresponding to the pitch of the period, and one end of the second
A second fiber grating connected to a second port optical circulator for guiding light reflected by the first fiber grating and having the other end terminated by a non-reflection terminator; A second optical fiber that is connected and guides the light reflected by the second fiber grating and emits the light, and is attached to at least one of the first fiber grating and the second fiber grating. And a wavelength adjusting device for changing a pitch of a change in the refractive index in the mounted fiber grating. 3. A four-port optical circulator, a first optical fiber connected to the four-port optical circulator for making light incident on the four-port optical circulator, and a period formed in a longitudinal direction of the optical fiber. Having a characteristic refractive index change and a function of reflecting light having a wavelength corresponding to the pitch of the period of the refractive index change, and one end of which is connected to the four-port optical circulator and is incident from the first optical fiber. A first fiber grating in which light is guided and the other end of which is terminated by a non-reflection termination; and a periodic refractive index change formed in a longitudinal direction of the optical fiber, wherein a period of the refractive index change is It has a function of reflecting light having a wavelength corresponding to the pitch, and one end is connected to the four-port optical circulator to guide the light reflected by the first fiber grating. A second fiber grating, the other end of which is terminated by a non-reflection terminator; and the second fiber grating is connected to the four-port optical circulator, and the light reflected by the second fiber grating is guided and emitted. The second optical fiber is mounted so as to be sandwiched between the first fiber grating and the second fiber grating, and one of the fibers is used to simultaneously change the pitch of the refractive index change in both fiber gratings. A wavelength adjusting device that applies a tensile force to increase the pitch to the grating and simultaneously applies a compressive force to reduce the pitch to the other fiber grating; Optical filter.