JP2000111840A - Polarizing beam splitter and optical communication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber type polarizing beam splitter excellent in a polarization separation degree. SOLUTION: This polarizing beam splitter is composed of an optical fiber 1b which is connected with a 3-terminal optical circulator 5 and its input/output terminal 2b and has double refraction ability forming a grating 4, and the grating 4 has such characteristic as it reflects one of the two directionally and orthogonally polarized components of the light inputted from an input terminal 2a and terminals the other polarized component, and as result, it separates the two directionally and orthogonally polarized components of the light inputted from the terminal 2a and outputs them from the input/output terminal 2b and an output terminal 2c.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization beam splitter having a function of separating two orthogonal polarization components of an optical signal input from one terminal and outputting the separated components to different terminals. .

[0002]

2. Description of the Related Art A beam splitter has a function of separating input light and outputting the separated light to two terminals, and more specifically, a function of separating input light into a predetermined light intensity ratio and outputting the separated light. , A wavelength splitting beam splitter having a function of separating input light containing two different wavelength components for each wavelength and outputting the separated light, a polarization component in two directions orthogonal to the input light ( There is a polarization separation type beam splitter that has a function of separating and outputting a polarization component in the X direction and a polarization component in the Y direction. The polarization splitting type beam splitter is usually called a polarization beam splitter.

[0003] A polarization beam splitter normally receives light having a mixture of an X-direction polarization component and a Y-direction polarization component.
There are a total of three input terminals and two output terminals for separating and outputting the polarization component in the X direction and the polarization component in the Y direction. An optical fiber type polarization beam splitter capable of connecting optical fibers to these input terminals and output terminals is a component constituting a wavelength division multiplexing communication network using optical fibers,
Alternatively, it is important as a component of an optical fiber sensor. As an optical fiber type polarization beam splitter, one having a configuration shown in Japanese Patent Application Laid-Open No. 6-59154 is known. The optical fiber type polarization beam splitter disclosed in this publication is obtained by heating and melting and integrating two constant polarization optical fibers 62 and 63 with a burner as shown in FIG. The wave optical fiber has a pair of stress applying portions 62b and 63b at positions facing each other with the cores 62a and 63a serving as signal transmission portions therebetween.

Due to the presence of the stress applying portions 62b and 63b, the stresses applied to the constant polarization optical fibers 62 and 63 in the X direction and the Y direction are different, and the photoelastic effect of the material forming the core 62a and the core 63a causes , The effective refractive index difference between the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction is different. As a result, a difference occurs in the propagation speed between the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction, and a constant polarization optical fiber is generated in the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction. A difference occurs in the amount of mode coupling between 62 and 63. The mode coupling amount is equal to the fusion unit 67.
When the two constant polarization optical fibers are fused and integrated, the cross-sectional shape of the fused integrated part 67, specifically, the aspect ratio (b / a in FIG. 7) is used. By controlling, the fiber end 66 of the polarization maintaining optical fiber 62 is controlled.
The component of the propagation mode polarized in the X direction of the optical signal incident from the
The propagation mode component polarized in the Y direction can be output from the fiber end 65 of the constant polarization optical fiber 63.

[0005]

In the polarization beam splitter of the fixed polarization optical fiber fusion-integrated type, ideally, the Y from the fiber end 64 of the constant polarization optical fiber 62 is Y.
The component of the propagation mode polarized in the direction is not output. However, in practice, variations in the manufacturing conditions of the polarization beam splitter, that is, the constant polarization optical fiber 62 and the constant polarization fiber 63
The component of the propagation mode polarized in the Y direction is also detected from the fiber end 64 of the constant-polarization optical fiber 62 due to the variation in the cross-sectional shape of the fusion-integration section 67 when the fusion-integration is performed.
From the fiber end 65 of the constant polarization optical fiber 63, not only the component of the propagation mode polarized in the Y direction but also the component of the propagation mode polarized in the X direction are detected.

When the components of the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction input from the fiber end 66 of the constant polarization optical fiber 62 are equal, the fiber end 64 is
The component of the propagation mode polarized in the X direction output from
Assuming that x is the propagation mode component polarized in the Y direction output from the fiber end 64, Py is the degree of polarization separation at the fiber end 64 defined as 10 × log {Px / Py}. The larger the value, the better the characteristics as a polarization beam splitter, but the dispersion of the cross-sectional shape of the fusion-integrated portion, especially the polarization separation degree is degraded due to the deviation of the aspect ratio from the optimum value, is disclosed in No. 6,59,154.

In the polarization beam splitter of the fixed polarization optical fiber fusion type, the direction of a straight line formed by a pair of stress applying portions 62b sandwiching the core 62a of the constant polarization optical fiber 62, If the directions of the straight lines formed by the pair of stress applying portions 63b sandwiching the core 63a do not match, the constant polarization optical fiber 62 and the constant polarization optical fiber 6
3, the axes of the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction do not coincide with each other, so that the degree of polarization separation deteriorates.

Further, in the polarization beam splitter of the fixed polarization optical fiber fusion-integrated type, the fiber
The light output from 4, 65 is fixed to the component of the propagation mode polarized in the X direction or the component of the propagation mode polarized in the Y direction by setting the processing conditions for fusion and integration,
Output the propagation mode component polarized in the X direction or the propagation mode component polarized in the Y direction from the fiber end 64 as necessary, that is, to provide the polarization beam splitter with a function of switching the polarization optical signal. Can not.

The present invention solves the above-mentioned problems and provides a polarization beam splitter having an improved degree of polarization separation as compared with a polarization beam splitter of a fixed polarization optical fiber fusion-integrated type. The wave state is variable, and the polarization beam splitter has a function of a multiplexer / demultiplexer and a switch of a polarization optical signal, which are essential components for configuring a wavelength division multiplexing communication network and an optical fiber sensor.

[0010]

A polarization beam splitter according to the present invention has three terminals, an input terminal, an input / output terminal, and an output terminal, and outputs an optical signal input from the input terminal to the input / output terminal. A three-terminal optical circulator for outputting an optical signal input from the input / output terminal to an output terminal, and a birefringence connected to the input / output terminal of the three-terminal optical circulator and having a grating formed in the core in the longitudinal direction. The effective refractive index differs between the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction of the optical fiber having birefringence. This is based on the fact that the reflection wavelength of a grating formed on a fiber is different between a propagation mode polarized in the X direction and a propagation mode polarized in the Y direction.

In the case where the grating formed on the optical fiber having birefringence is a Bragg grating in which several thousand layers of refractive index changes having the same period are formed, the period (pitch) of the refractive index change of the grating is Δ, and the grating is formed. Assuming that the effective refractive index of the core of the optical fiber is Neff, the center wavelength (Bragg wavelength) λb of the reflected light of the grating is given by 2ΛNeff. The Bragg grating has a sharp reflection characteristic near the Bragg wavelength, and has a characteristic of transmitting an optical signal deviating from the Bragg wavelength.

In the present invention, since the grating is formed on the birefringent optical fiber, Neff is a polarization component in two directions propagating through the core of the birefringent optical fiber, that is, polarized in the X direction. The wave propagation mode differs from the propagation mode polarized in the Y direction. As a result, the effective refractive index corresponding to the propagation mode polarized in the X direction is Neff-x, and the effective refractive index corresponding to the propagation mode polarized in the Y direction is Neff-.
Assuming y, the center wavelength of the light reflected by the Bragg grating is 2ΛNeff-x for the propagation mode polarized in the X direction, and 2２Neff-x for the propagation mode polarized in the Y direction.
ΛNeff-y. That is, in the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction, the central wavelength of the light reflected by the Bragg grating is 2Λ (Neff-x−
Neff-y).

Therefore, when the Bragg grating is designed and manufactured so as to reflect only the propagation mode of the specific wavelength polarized in the X direction, the propagation mode polarized in the Y direction of the wavelength is the Bragg grating. Is transmitted and output, and the propagation mode polarized in the X direction reflected by the Bragg grating is input again to the three-terminal optical circulator and connected to the terminal to which the optical fiber forming the Bragg grating is connected. Are output from different terminals to separate and extract a propagation mode polarized in the X direction and a propagation mode polarized in the Y direction, respectively, from different terminals, that is, realize a function of a polarization beam splitter. Things.

A Bragg grating formed on a fiber core usually has a frequency of 99.99% near the Bragg wavelength.
The above-mentioned reflectance can be obtained, and the half width of the reflection spectrum is as steep as about 0.5 nm. In the wavelength range excluding the vicinity of the Bragg wavelength, the transmittance is almost 100%. On the other hand, when the difference between the effective refractive index (Neff-x) corresponding to the propagation mode polarized in the X direction and the effective refractive index (Neff-y) corresponding to the propagation mode polarized in the Y direction is 1%. If the Bragg wavelength is 1.5 μm, the Bragg wavelength differs by 15 nm between the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction. This value is compared with the half-width of the reflection spectrum of the Bragg grating. Is a sufficiently large value. Therefore, when an optical signal having a Bragg wavelength with respect to the polarization component in the X direction is incident on a Bragg grating connected to an optical circulator, the propagation mode polarized in the X direction is totally reflected, but is polarized in the Y direction. The waved propagation mode is totally transmitted by the Bragg grating, and a function of a polarization beam splitter having excellent polarization separation can be realized.

Further, in the polarization beam splitter according to the present invention, a tension is applied to a grating forming portion of a birefringent fiber constituting the polarization beam splitter, and the temperature around the grating is made variable. By changing the period (pitch) of the change in the refractive index of the grating and making the reflection wavelength of the grating variable, the output light from the terminal connected to the optical fiber having birefringence forming the grating, and the grating Provided is a polarization beam splitter capable of mutually switching polarization states of output light from a terminal that outputs a reflected optical signal.

That is, by changing the pitch Λ to ΛNeff-x / Neff-y by expanding and contracting the Bragg grating, the propagation mode initially polarized in the X direction is reflected and the propagation mode polarized in the Y direction is transmitted. The Bragg grating reflects the propagation mode polarized in the Y direction and transmits the propagation mode polarized in the X direction. Since the optical fiber having the birefringence forming the Bragg grating is connected to the optical circulator, the output terminal of the propagation mode polarized in the X direction due to the change in the pitch of the Bragg grating and the propagation terminal polarized in the Y direction. The mode output terminal will be replaced. That is, the function of the switch of the polarized light signal is realized.

By realizing the above-described functions, optical fibers are connected to the polarization beam splitter according to the present invention, respectively, and the polarization beam splitter is used as a component of an optical communication network. The function of multiplexing / demultiplexing and switching between the wave propagation mode and the propagation mode polarized in the Y direction is realized.

A polarization beam splitter can be realized by forming a Bragg grating on an optical fiber having birefringence. However, since the Bragg grating has a narrow reflection wavelength band, different optical signals are transmitted using a plurality of wavelengths. In wavelength-division multiplexing communication, signal light of a plurality of wavelengths cannot be collectively separated into a propagation mode polarized in the X direction and a propagation mode polarized in the Y direction. in this case,
By forming a chirped grating in which the pitch of the grating is continuously changed in the longitudinal direction instead of the Bragg grating, and by expanding the reflection wavelength bandwidth of the grating, the signal light of a plurality of wavelengths in the wavelength multiplex communication can be changed in the X direction. It is possible to collectively separate the propagation mode polarized in the Y direction and the propagation mode polarized in the Y direction.

[0019]

FIG. 1 is a diagram showing the configuration of a polarization beam splitter according to the present invention. The three-terminal optical circulator 5 has an input terminal 2a, an input / output terminal 2b, and an output terminal 2c. Birefringent optical fiber 1 connected to input / output terminal 2b of terminal optical circulator 5
b. Optical fiber 1b having birefringence
Is a constant polarization optical fiber, and a grating 4 is formed in the core thereof. Optical fibers 1a and 1c are connected to an input terminal 2a and an output terminal 2c of the three-terminal optical circulator 5 in order to input / output to / from a polarization beam splitter using an optical fiber. However, in the polarization beam splitter according to the present invention, the spatially propagating light is
When the optical fiber 1a is used as the input light, the optical fiber 1a is unnecessary. When the output light from the output terminal 2c is extracted as spatially propagated light, the optical fiber 1c is unnecessary.

As the birefringent optical fiber 1b,
The mode birefringence (Neff-x-Neff-y), which is the difference between the effective refractive index of the propagation mode polarized in the X direction and the effective refractive index of the propagation mode polarized in the Y direction, is 2 × 10 ^-2 . A polarized optical fiber was used, and a chirped grating whose period Λ was uniformly changed from 534.1 nm to 538.3 nm was formed as a grating 4 in the core region thereof. The grating length is 20 mm.

FIG. 2 shows the transmission characteristics of the chirped grating alone. FIG. 2A is a diagram showing the wavelength dependence of the transmission / reflection characteristics of the chirped grating with respect to the propagation mode polarized in the X direction. FIG. 2B is a diagram showing the propagation mode of the propagation mode polarized in the Y direction. FIG. 4 is a diagram illustrating wavelength dependence of transmission / reflection characteristics. The vertical axes of FIGS. 2A and 2B show the transmittance and the reflectance of the chirped grating in decibels.

FIG. 2A shows that the propagation mode polarized in the X direction has a wavelength of 1550 nm to 1560 nm.
, A transmission characteristic of -40 dB or less, that is, a reflectance of 99.99% or more is obtained if there is no contribution of other loss elements. On the other hand, with respect to the propagation mode polarized in the Y direction of the chirped grating, as shown in FIG. 2B, -4 in the wavelength range of 1572 nm to 1583 nm.
If the transmission characteristic is 0 dB or less, that is, if there is no contribution from other loss elements, a reflectance of 99.99% or more is obtained, and -25 in the wavelength range of 1550 nm to 1560 nm.
If the reflection characteristics are not more than dB, that is, if there is no contribution from other loss elements, a transmittance of 99% or more is obtained.

FIG. 3 shows the characteristics of the polarization beam splitter of FIG. 1 in which the birefringent optical fiber 1b on which the chirped grating is formed as the grating 4 is connected to the input / output terminal 2b of the three-terminal optical circulator 5. . Optical fibers 1a and 1c were connected to the input terminal 2a and the output terminal 2c of the three-terminal optical circulator, respectively.
A propagation mode in which the output light of the wavelength tunable laser is incident from the open end 3a of the optical fiber 1a and is polarized in the X direction from the output light of the open end 3b of the birefringent optical fiber 1b and the open end 3c of the optical fiber 1c. And the component of the propagation mode polarized in the Y direction were determined. Wavelength range from 1550nm to 15
60 nm.

FIG. 3A shows the output of the open end 3b based on the output of the open end 3c when a propagation mode polarized in the X direction is input from the open end 3a. On the other hand, FIG.
(B) shows the output of the open end 3c based on the output of the open end 3b when the propagation mode polarized in the Y direction is input from the open end 3a. The extinction ratio with respect to the propagation mode polarized in the X direction is obtained by dividing the output component of the propagation mode polarized in the X direction at the open end 3c by Px (c) and the output component of the propagation mode polarized by the open end 3b in the X direction. Since Px (b) is given by 10 × log {Px (b) / Px (c)}, a value of −40 dB or less is obtained as the erasure ratio for the component of the propagation mode polarized in the X direction. I have. Similarly, for the component of the propagation mode polarized in the Y direction,
An erase ratio of 25 dB or less was obtained. Since the components of the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction input from the open end 3a are equal to each other, when expressed by the degree of polarization separation, at the open end 3b, the polarization separation of 40 dB or more is obtained. The degree of polarization separation of 25 dB or more is obtained at the open end 3c.

In wavelength division multiplexing communication, for example, 0.8n
The wavelength of the signal light is set at m intervals,
The transmission is carried out using a single mode optical fiber, but about 10 waves in the range of 1550 nm to 1560 nm are obtained by the polarization beam splitter of FIG. 1 using a chirped grating having the characteristics of FIG. It is possible to collectively separate the propagation mode polarized in the orthogonal X direction and the propagation mode polarized in the Y direction of the wavelength multiplexed optical signal.

FIG. 1 shows a chirped grating.
When the wavelength of the input signal light to the polarization beam splitter is a specific wavelength used in the wavelength division multiplexing communication, the component of the propagation mode polarized in the X direction and the polarization in the Y direction are used. When separating the wave propagation mode component, a Bragg grating having a narrow reflection wavelength bandwidth can be used in place of the chirped grating.

FIG. 4 shows a grating 4 having a grating length of 4 m as a grating 4 constituting the polarization beam splitter of FIG.
It is a figure which shows the measurement result at the time of forming a Bragg grating whose m and (psi) are 534.1 nm, and the Bragg wavelength of the formed Bragg grating is 1550.1 nm with respect to the propagation mode polarized in the X direction, Y It is 1572 nm for the propagation mode polarized in the direction.
The measurement wavelength range is 1549.6 nm to 1550.6 n.
m. Other measurement conditions are the same as the measurement conditions when the measurement result of FIG. 3 is obtained.

FIG. 4A shows an open end 3c when a propagation mode component polarized in the X direction is input from the open end 3a.
The output of the open end 3b is displayed based on the output of the open end 3b. On the other hand, FIG. 4B shows the output of the open end 3c based on the output of the open end 3b when the component of the propagation mode polarized in the Y direction is input from the open end 3a. , FIG.
From (A), it can be seen that a value of -40 dB or less is obtained as the erasure ratio of the propagation mode polarized in the X direction. Also, from FIG. 4B, it can be seen that a value of −30 dB or less is obtained as the erasure ratio of the propagation mode polarized in the Y direction.
Since the components of the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction input from the open end 3a are equal to each other, the polarization separation degree of the open end 3b is 40 dB or more. A value of 30 dB or more is obtained as the degree of wave separation.

Since the half-width of the output wavelength of a DFB laser used for wavelength-division multiplexing communication is about 0.4 nm, the propagation mode component obtained by polarizing an optical signal of a specific wavelength in the X-direction and the Y-direction in wavelength-division multiplexing communication. A Bragg grating can be used for the purpose of separating into a component of a propagation mode polarized in a negative direction.

The chirped grating and the Bragg grating are usually formed by removing the coating of the optical fiber and irradiating the core of the optical fiber with ultraviolet interference light generated by a phase grating or the like. In a state where the coating is removed, the mechanical strength of the optical fiber is not sufficient. Therefore, after forming the grating, the relevant portion of the optical fiber is re-coated with a UV resin or the like. The tensile strength of the recoated optical fiber is usually 2 kg or more in the section of about 20 mm where the grating is formed.

In the polarization beam splitter shown in FIG. 1, one end 6a of the re-coated grating forming portion of the birefringent optical fiber 1b is fixed, and the other end 6a.
b is fixed to a movable stage moved by a manual manipulator in a state where the slack of the optical fiber 1b having birefringence is removed. Thereafter, by operating a manual manipulator and applying tension to the birefringent optical fiber 1b, the pitch Λ of the grating can be increased and the transmission wavelength band can be shifted to the longer wavelength side.

To confirm the above operation, a mode birefringence of 7
A chirped grating was formed on a × 10 ⁻³ constant polarization optical fiber. Chirped grating pitch is 5
It changes uniformly from 34.5 nm to 536.2 nm, and its grating length is 20 mm. The transmission and reflection characteristics of the grating were measured without applying tension to the constant polarization optical fiber on which the grating was formed.
FIG. 5 shows the results. FIG. 5A shows the transmission / reflection characteristics of the propagation mode polarized in the X direction in decibels as a function of the measurement wavelength. FIG. 5B shows the transmission / reflection characteristics of the propagation mode polarized in the Y direction in decibels as a function of the measurement wavelength.

FIG. 6A shows the transmission and reflection characteristics of the propagation mode polarized in the X direction when a tension of 400 g is applied to the constant polarization optical fiber having the chirped grating.
FIG. 6 shows the transmission and reflection characteristics of the propagation mode polarized in the Y direction.
It is shown in (B). Regarding the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction, it is understood that the reflection wavelength band of the chirped grating is shifted to the longer wavelength side by the application of tension. In the case where a signal light having a wavelength of 1559 nm is used, when no tension is applied, the transmission mode polarized in the X direction exhibits total transmission characteristics, and the propagation mode polarized in the Y direction exhibits total reflection characteristics. When 400 g is applied, the propagation mode polarized in the X direction shows total reflection characteristics and the propagation mode polarized in the Y direction shows total transmission characteristics. This chirped grating is used as the grating 4 in FIG. As a result, the function of switching between the propagation mode in which the optical signal of the wavelength is polarized in the X direction and the propagation mode in which the optical signal is polarized in the Y direction can be realized by applying the tension.

In order to make the reflection wavelength band of the grating variable, the pitch or modulation refractive index may be changed. In addition to applying tension to the fiber grating forming portion to make the pitch variable, the fiber grating is made variable. It is also possible to adopt a method of changing the environmental temperature in which the is stored.

In the above embodiment, a constant polarization optical fiber is used as the birefringent optical fiber shown in FIG. 1. However, even when the grating is formed in a waveguide whose core cross section is rectangular, the waveguide may be used. Since the effective refractive index differs between the major axis direction and the minor axis direction of the waveguide, the major axis direction of the waveguide is defined as X
When the direction and the minor axis direction are Y direction, the effective refractive index, that is, the reflection wavelength is different between the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction, and a grating is formed in the core of the polarized optical fiber. It is possible to obtain the same effect as described above.

[0036]

As described above, by forming a grating on an optical fiber having a predetermined birefringence, the effective refractive index can be reduced between the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction. No, in a given wavelength band,
A function of reflecting only the propagation mode polarized in the X direction and transmitting the propagation mode polarized in the Y direction can be achieved. A birefringent optical fiber having a grating formed thereon is used as an input / output terminal 2 with its longitudinal direction as an axis.
When rotated and fixed at 90 degrees relative to b, it goes without saying that only the propagation mode polarized in the Y direction can be reflected and the propagation mode polarized in the X direction can be transmitted.

The optical fiber having the birefringence formed with the grating is connected to the input / output terminal of the three-terminal circulator, and the component of the propagation mode polarized in the X direction from the input terminal of the optical circulator and the polarization in the Y direction. An optical signal mixed with the propagation mode component is incident. By properly selecting the mounting direction angle of the birefringent optical fiber having the grating formed on the three-terminal optical circulator, the input / output terminal of the three-terminal optical circulator can be changed to Y.
The component of the propagation mode polarized in the direction and the component of the propagation mode polarized in the X direction can be extracted from the output terminal, and the function of the polarization beam splitter is realized.

By increasing the modulation refractive index and the number of layers, the transmittance of the grating can be reduced to -40 dB or less (99.99% or more in reflectance), and the gratings can be connected in series. Thereby, the reflectance can be improved. Further, by using the apodizing method of changing the modulation refractive index of the grating in the longitudinal direction of the grating, it is also possible to reduce reflected light in wavelength bands other than the reflection wavelength band of the grating, and as a result, the polarization beam splitter Can be dramatically improved.

In wavelength-division multiplexing communication and optical fiber sensors, when transmission is performed with independent information placed on the X-direction polarization component and the Y-direction polarization component, the propagation mode component polarized in the X direction and Y component An element for combining and demultiplexing the components of the propagation mode polarized in the direction with a sufficient erasure ratio is required. The polarization beam splitter according to the present invention is suitable for this purpose. On the other hand, it is practically difficult to reduce the erasing ratio to -20 dB or less in a conventional polarization-polarized optical fiber fusion-integrated polarization beam splitter. If it is used, the S / N is deteriorated, which is not suitable.

In wavelength division multiplexing communication, the separation between the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction of a specific wavelength signal among a plurality of wavelength signals used is narrow in the reflection wavelength bandwidth. This can be achieved by a Bragg grating, but in the case where signal lights of a plurality of wavelengths in wavelength division multiplexing communication are collectively separated into a propagation mode polarized in the X direction and a propagation mode polarized in the Y direction, the reflection wavelength of the grating It is necessary to increase the band width, and it is preferable to use a chirped grating. For example, in wavelength division multiplexing communication, a propagation mode in which the polarization directions of adjacent wavelength channels are made orthogonal to each other for the purpose of removing coherent crosstalk between optical signals of adjacent wavelengths, that is, an optical signal of an even channel is polarized in the X direction. In the case where a method of using a propagation mode polarized in the Y direction as the optical signal of the odd-numbered channel is adopted, the propagation mode polarized in the X direction and the propagation mode polarized in the Y direction are multiplexed together. It is preferable to use the polarization beam splitter according to the present invention that uses a chirped grating for demultiplexing.

The polarization beam splitter according to the present invention is also suitable as a component for realizing the multiplexing / demultiplexing function of an optical fiber sensor. For example, an optical fiber type gyroscope using a constant polarization optical fiber that detects rotation speed with high accuracy requires a polarization beam splitter with a high erasure ratio for the purpose of detecting rotation speed with high accuracy. The polarization beam splitter according to the invention is suitable for this use.

Further, a tension is applied to the grating forming portion of the birefringent optical fiber of the polarization beam splitter according to the present invention, and the pitch of the grating is changed to change its reflection wavelength, and the reflection wavelength is initially changed in the X direction. The characteristics of the grating, which reflected the polarized propagation mode component and transmitted the Y-polarized propagation mode component, are represented by X
It is possible to realize a function of transmitting the component of the propagation mode polarized in the direction and reflecting the component of the propagation mode polarized in the Y direction as appropriate, that is, a function of switching the polarization optical signal. By realizing the function of switching the polarization optical signal, the function of the optical signal switch is realized in the wavelength division multiplexing communication in which the communication signal is carried in each of the propagation modes polarized in the X direction and the Y direction.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an embodiment of a polarization beam splitter according to the present invention.

FIG. 2 is a diagram illustrating characteristics of a chirped grating used in the present invention.

FIG. 3 is a diagram illustrating a polarization separation degree of a polarization beam splitter using a chirped grating.

FIG. 4 is a diagram illustrating a polarization separation degree of a polarization beam splitter using a Bragg grating.

FIG. 5 is a diagram showing characteristics of a chirped grating used in the present invention.

FIG. 6 is a diagram showing characteristics in a state where tension is applied to a chirped grating.

FIG. 7 is a diagram illustrating a polarization beam splitter according to the related art.

[Explanation of symbols]

 1a, 1c: Optical fiber 1b: Optical fiber having birefringence 2a: Input terminal 2b: I / O terminal 2c: Output terminal 3a, 3b, 3c: Open end 4: Grating 5: Three terminal optical circulator 6a, 6b: Ends of grating forming portion 62, 63: constant polarization optical fiber 62a, 63a: core 62b, 63b: stress applying portion 64, 65, 66: fiber end 67: fusion united portion

Claims (6)

[Claims] 1. A polarization beam splitter comprising a three-terminal optical circulator and a birefringent optical waveguide connected to an input / output terminal of the three-terminal optical circulator and having a core formed with a grating in a longitudinal direction. 2. The polarization beam splitter according to claim 1, wherein said optical waveguide is an optical fiber. 3. The polarization beam splitter according to claim 2, wherein a reflection wavelength of a grating formed on the birefringent optical fiber is variable. 4. The polarization beam splitter according to claim 3, wherein a reflection wavelength of the grating is made variable by expanding and contracting a portion of the optical fiber having birefringence where the grating is formed. 5. An optical communication method, wherein the polarization beam splitter according to claim 1 is used as a multiplexer / demultiplexer for polarized optical signals. 6. An optical communication method, wherein the polarization beam splitter according to claim 3 is used as a polarization optical signal switch.