JP5090783B2 - Variable optical attenuator, variable optical attenuator built-in receiver and optical attenuation method - Google Patents

Description

  The present invention relates to an optical attenuation method, a variable optical attenuator, and a receiver with a built-in variable optical attenuator, and more particularly, to an optical attenuation method suitable for an optical communication network, a variable optical attenuator, and a receiver with a built-in variable optical attenuator. .

  Optical technology and information processing technology have been developed and are becoming indispensable elements in daily life. Many parts of the communication network represented by the Internet and in-house communication networks are constructed by optical communication networks using the most advanced optical technology.

  Optical technology has facilitated multiplexing and demultiplexing optical signals of different wavelengths. As a result, in recent optical communication networks, the transmission capacity per optical fiber is expanded by a wavelength division multiplexing method called WDM (Wavelength Division Multiplexing) that multiplexes and separates optical signals having different wavelengths. It is composed.

  In this WDM system, the line is switched using the difference in wavelength. However, since the path of the optical signal is also changed, there is a problem that the received light intensity varies. In order to solve this problem, an attempt has been made to insert a variable optical attenuator immediately before the optical signal receiver to control the intensity of the optical signal incident on the receiver to be constant.

  The variable optical attenuators that have been used so far are roughly classified into the following two by the variable attenuation mechanism. One is an attenuator using a micromachine called MEMS (Micro Electro Mechanical Systems), and the other is an attenuator using a dielectric optical element. A typical example of the former is an attenuator that controls the rotation angle of a Si mirror (see, for example, Non-Patent Documents 1 and 2). Also, as a representative example of the latter, an attenuator that combines a magneto-optical element that rotates a polarization plane of light and a polarizer is known (see, for example, Non-Patent Document 3).

Furukawa Electric 111th issue, pp 25-30, 2003 Journal of Japan Institute of Electronics Packaging, vol.9, No. 4, 2006 “Variable Optical Attenuator C-band: YS-5010-155”, http://www.fdk.co.jp/cyber-j/opt/YDE101.htm, April 2007 search

  Since the MEMS optical attenuator uses a simple phenomenon of reflection by a total reflection type flat mirror, there is an advantage that the insertion loss is small and the polarization of incident light does not depend on the optical attenuation. On the other hand, with MEMS type optical attenuators, the amount of attenuation fluctuates due to vibration, the relationship between the mirror drive voltage and the amount of attenuation cannot be reproduced, and it is difficult to maintain the mirror angle constant over the long term. There was a problem.

  In addition, a variable optical attenuator (VOA) that combines the magneto-optical element and the polarizer does not use dynamic control by a movable part, and thus has excellent vibration resistance and retention of attenuation. Yes. On the other hand, it is necessary to modulate the magnetic field in order to control the polarization state, and there is a problem that the size of the attenuator and the power consumption increase.

  On the other hand, there is a liquid crystal optical element as a small-sized and low-power-consumption device having a function of rotating the plane of polarization like a magneto-optical element. Liquid crystal optical elements typified by twisted nematic liquid crystal are widely applied to displays such as displays because they are inexpensive, have low power consumption, and can be easily miniaturized. The obstacle in applying this liquid crystal optical element to the configuration of the optical attenuator is the polarization dependence of the optical characteristics. For example, in optical fiber communication, since the optical signal propagating through an optical fiber having a circular cross section is handled, the polarization state of the signal light changes with time, and if an optical element having polarization dependence is used, the optical attenuation amount changes with time. Change is triggered. For this reason, in applying the liquid crystal optical element to a variable attenuator for optical communication, a device for removing the polarization dependence of the liquid crystal optical element is required.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a small, low power consumption, simple and inexpensive variable optical attenuator using a polarization-dependent liquid crystal optical element. And providing a receiver including the variable optical attenuator.

  Another object of the present invention is to provide a variable optical attenuator in which the polarization dependence of a liquid crystal optical element is suppressed, and a receiver including the variable optical attenuator.

In order to achieve such an object, the variable optical attenuator according to the present invention includes a polarization separation unit that separates an optical path of incident light into polarized light having different optical paths according to the polarization state of the incident light. And at least one polarization-dependent liquid crystal optical element that is arranged for each polarized light separated by the polarized light separating means and that reflects and changes the polarization state of the polarized light incident according to the applied electric field; And polarization multiplexing means for multiplexing each polarized light whose polarization state has been changed by the liquid crystal optical element.

  With this configuration, the amount of attenuation can be changed by controlling the electric field applied to the liquid crystal optical element to control the amount of change with respect to the polarization.

In this onset bright, the polarization separating means and the polarization multiplexing unit of the variable optical attenuator is constituted by a single polarizing beam splitter element (PBS element), each polarized light separated by the polarization beam splitter element, at least one liquid crystal The light is reflected by the optical element, is incident on the polarization beam splitter element again, and is multiplexed. The polarization beam splitter element separates incident light into two linearly polarized lights (also referred to as S-polarized light and P-polarized light in this specification) having optical paths orthogonal to each other.

Further, in this onset bright, liquid crystal optical element of the variable optical attenuator, the polarized light separated by the polarization beam splitter element is distribution to be incident perpendicular to the reflective surface of the liquid crystal optical element.

Further, in this onset bright, further comprising an optical isolator for preventing an incidence of the optical path of incident light polarized light reflected by the liquid crystal optical element in front of the polarization beam splitter element.

Furthermore, in this onset bright, branches into a plurality of at least one polarization of each polarized light separated by the polarization separating means comprises a branching means for entering branched polarization to each one of the liquid crystal optical element.

  Furthermore, it goes without saying that the present invention can be implemented as a receiver with a built-in variable optical attenuator that incorporates at least one of the above variable optical attenuators.

  In the variable optical attenuating method according to the present invention, in the variable optical attenuator, the step of monitoring the emitted light from the polarization multiplexing means and the monitor value is applied to the liquid crystal optical element so as to become a desired value. Controlling the electric field.

  As described above, according to the present invention, a small, low power consumption, simple and inexpensive variable optical attenuator using a polarization-dependent liquid crystal optical element and a receiver including the variable optical attenuator Can be provided. Further, according to the present invention, it is possible to provide a variable optical attenuator that suppresses the polarization dependence of the liquid crystal optical element and a receiver including the variable optical attenuator.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following detailed description is intended to illustrate specific embodiments of the present invention and effects thereof, and it goes without saying that various modifications can be made without departing from the spirit of the present invention.

  1 and 2 are diagrams showing a configuration example of a variable optical attenuator (VOA) provided by the present invention. The VOA shown in FIGS. 1 and 2 has a polarization separating element 3 that separates an optical path of incident light into different optical paths according to a polarization state (polarization component) of the incident light, and an applied electric field (voltage). Two liquid crystal optical elements 4 and 5 having polarization dependency that change and reflect the polarization state of incident light are provided to cancel the polarization dependency.

  1 and 2, the polarization separation element 3 also serves as the polarization separation means and the polarization multiplexing means of the present invention. 1 and 2 show examples in which two reflection-type rotators having polarization dependency are used as the liquid crystal optical elements 4 and 5 and a polarization beam splitter element (PBS element) is used as the polarization separation element 3. ing.

  S-polarized light and P-polarized light separated from the incident light by the PBS element 3 are incident on different reflective rotators 4 and 5, respectively.

  The reflection type rotors 4 and 5 have a characteristic of reflecting as it is without affecting the polarization state of incident light when an electric field exceeding a threshold value is applied.

  At this time, the variable light depending on whether the S-polarized light and the P-polarized light separated by the PBS element 3 are perpendicularly incident on the reflecting surface of the reflective rotator or incident at an angle of about 3 to 8 degrees from the vertical. The configuration of the attenuator is different.

  FIG. 1 shows the configuration of a variable optical attenuator when S-polarized light and P-polarized light are perpendicularly incident on a reflective rotator. As shown in FIG. 1 (a), in the case of normal incidence, the light reflected by the reflective rotator goes back as it is in the incident optical path, so that the optical isolator is located before the PBS element 3, that is, between the PBS element 3 and the optical fiber. Insert 2 to quench the reflected light. In this configuration, when the electric field applied to the reflective rotator is set to no electric field, the reflective rotator 4 reflects S-polarized light after converting it to P-polarized light, and the reflective rotator 5 reflects the light after converting P-polarized light to S-polarized light. These reflected lights are combined by the PBS element 3 as shown in FIG. 1B and coupled to the output side optical fiber.

  FIG. 2 shows the configuration of the variable optical attenuator when it is incident at an angle of about 3 to 8 degrees from the vertical to the reflection surface of the reflection type rotor. In this case, the optical isolator 2 shown in FIG. 1 is unnecessary. In this case, when an electric field exceeding the threshold value is applied to the reflective rotator, there is no influence on the polarization state of the incident light, and there is an inclination corresponding to the incident angle as shown in FIG. Reflect. Therefore, the reflected light passes through a different path from the incident light, does not form an image on both the incident side and the outgoing side optical fibers, and is in a quenched state. In this configuration, when the electric field applied to the reflective rotator is set to no electric field, the reflective rotator 4 reflects S-polarized light into P-polarized light, and the reflective rotator 5 reflects P-polarized light after being converted into S-polarized light. These reflected lights are combined by the PBS element 3 as shown in FIG. 2B, coupled to the output side optical fiber, and become non-attenuated (0.5 to 1 dB insertion loss remains).

  In the two configurations described above, since both S-polarized light and P-polarized light separated from the incident light by the PBS 3 are coupled to the output side fiber, there is no difference in attenuation even if the polarization state of the incident light changes. In addition, when an electric field below the threshold is applied to the liquid crystal reflective rotor, an analog intermediate state occurs when there is no electric field and when an electric field above the threshold is applied, and the amount of light coupled to the output side optical fiber is controlled by the electric field strength. it can.

  As described above, the variable optical attenuator of the present invention combines the polarization-dependent liquid crystal optical element (liquid crystal reflection type rotator) and the polarization separation element (PBS element) to obtain the polarization dependence of the liquid crystal optical element. It cancels the sex.

  FIGS. 1 and 2 show an example in which the light separated from the incident light by the polarization separation element is reflected by changing the polarization state by one liquid crystal optical element, but will be described below with reference to FIG. As described above, a part or all of the separated light can be further branched by the multiplexing / demultiplexing element, and the polarization state can be changed and reflected by the plurality of liquid crystal optical elements.

  Alternatively, as will be described below with reference to FIG. 10, a plurality of variable optical attenuators using two liquid crystal optical elements are arranged in parallel, and incident light is demultiplexed and supplied to each variable optical attenuator. A variable optical attenuator provided with a multiplexing / demultiplexing element that multiplexes output light from each variable optical attenuator can also be provided.

( Reference Example 1)
A first reference example will be described with reference to FIGS. In this reference example, the configuration of a variable optical attenuator of the type that cancels the polarization dependence by combining two polarization-dependent liquid crystal reflective rotators and one polarization beam splitter is clarified. The effect will be specifically described.

3A to 3D show an outline of the assembly process of the variable optical attenuator described in this reference example. First, the polarization-independent optical isolator 2 and the PBS element 3 are set so that incident light incident from the incident port 6 of the housing 1 passes through the polarization-independent optical isolator 2 and enters the PBS element 3. It fixes to the housing | casing 1 with solder (FIG. 3 (a)).

As shown in FIG. 4, the PBS element 3 used in this reference example separates incident light perpendicularly incident on the incident surface into S-polarized light and P-polarized light, and bends the optical path of S-polarized light by 90 ° on the reflective surface. It has an optical characteristic that transmits the light path as it is.

In this reference example, the PBS element 3 bends the optical path of S-polarized light incident from the incident surface on the polarization-independent optical isolator 2 side by 90 °, and emits S-polarized light to a surface (A surface) perpendicular to the incident surface. have. The PBS element 3 has an optical characteristic that allows the P-polarized light to pass through the surface (B surface) facing the incident surface as it is without bending the optical path of the P-polarized light incident from the incident surface.

  Next, the liquid crystal reflective rotator 4 and the liquid crystal reflective rotator 5 have their respective reflecting surfaces facing the A and B surfaces of the PBS element 3, and the polarized light is vertically incident or an angle of about 3 to 8 degrees from the vertical. Is fixed to the PBS element 3 so as to be incident (FIG. 3B). For example, the liquid crystal reflective rotator 4 and the liquid crystal reflective rotator 5 can be fixed to the A and B surfaces of the PBS element 3 using an optical adhesive, for example.

In this reference example, the configuration in which the liquid crystal reflective rotor is fixed on the PBS element is adopted. However, the light from the appropriate position in the housing 1, that is, the A surface and the B surface of the PBS element 3, is reflected by the liquid crystal. The liquid crystal reflective type is placed at a position where it is incident on the reflecting surfaces of the mold rotor 4 and the liquid crystal reflective rotor 5 at a normal incidence or an angle of about 3 to 8 ° from the vertical, and is reflected and incident again on the PBS element 3. The rotor may be fixed.

  After the liquid crystal reflective rotator 4 and the liquid crystal reflective rotator 5 are fixed, the liquid crystal reflective rotator 4, the electrodes of the liquid crystal reflective rotator 5 and the pins 18 of the housing 1 are connected by wire pumping. An electric field control device (not shown) for controlling the electric field (voltage) applied to the liquid crystal reflective rotor 4 and the liquid crystal reflective rotor 5 is connected to the pin 18.

  The liquid crystal reflective rotator 4 has a polarization plane of S-polarized light whose optical path is bent by 90 ° by the PBS element 3 in the direction of the dotted line in FIG. 3B (the direction from the PBS element 3 to the liquid crystal reflective rotator 4). It has a function of reflecting after rotating clockwise as an axis, and its rotation angle can be controlled by an electric field applied to the liquid crystal reflective rotor 4.

  Similarly, the liquid crystal reflective rotator 5 has the polarization plane of P-polarized light that has passed through the PBS element 3 in the direction of the dotted line in FIG. 3B (clockwise around the direction from the PBS element 3 to the liquid crystal reflective rotator B). There is a function to reflect after rotating. The rotation angle can also be controlled by the electric field applied to the liquid crystal reflective rotor 5.

Subsequently, the lid 10 is seam welded to the housing 1 in a dry nitrogen atmosphere and hermetically sealed. Thereafter, the incident side fiber collimator 7 is aligned and fixed to the incident port 6 side of the casing 1 optical signal. Since the liquid crystal reflective rotors 4 and 5 used in this reference example have the characteristics described with reference to FIGS. 1 and 2, the incident light output to the exit port 8 is observed and adjusted without applying an electric field. I can core.

  Specifically, the fiber collimator 7 is mounted at a position where the reflected light from the liquid crystal reflective rotator 4 and the reflected light from the liquid crystal reflective rotator 5 observed from the exit port 8 with an IR (infrared camera) camera are superimposed. The body 1 is YAG (Yittrium Aluminum Garnet) laser welded (FIG. 3C).

  Finally, the output side fiber collimator 9 is aligned and fixed to the output port 8. At the time of alignment, an optical signal is incident from the incident-side fiber collimator 7 and an electric field is applied to the liquid crystal reflective rotor 4 and the liquid crystal reflective rotor 5 so that the attenuation is minimized. In this state, alignment is performed so that the intensity of the optical signal coupled to the output-side fiber collimator 9 is maximized, and the output-side fiber collimator 9 is welded to the housing 1 with a YAG laser (FIG. 3D).

  FIG. 5 is a diagram showing the relationship between the light attenuation amount of the variable optical attenuator completed through the above steps and the voltage applied to the liquid crystal reflective rotor. The attenuation decreases from 0.8 dB at no voltage with the applied voltage, decreases to 36 dB near 5 V, and then saturates. Thus, a dynamic range exceeding 30 dB can be obtained with an easy-to-handle voltage value of about 0 to 5 V, and the performance is extremely good.

  Furthermore, the incident-side fiber collimator 7 was removed, collimated light was incident using a spatial optical system, and the polarization plane was rotated by a rotator to measure the polarization dependence of light attenuation.

  FIG. 6 is a diagram showing the results. Even if the polarization state of the incident light is changed from S-polarized light to P-polarized light, the variation in the amount of attenuation in the emitted light is within ± 0.1 dB, and according to the configuration of the present invention, the polarization dependence of each liquid crystal rotator It was confirmed that the sex was cancelled. Further, the amount of light attenuation was constant over a wide range of wavelengths from 1500 to 1600 nm without depending on the wavelength of incident light.

In this reference example, parallel-aligned nematic liquid crystal was used as the reflective liquid crystal rotating element, but if vertical-aligned nematic liquid crystal is used, the change in optical characteristics with respect to the magnitude of the electric field can be reversed, and a normally closed type variable. Functions as an optical attenuator.

( Reference Example 2)
A second reference example will be described with reference to FIG. In this reference example, two liquid crystal reflective rotators and one polarization beam splitter are combined to change the output side fiber collimator of a variable optical attenuator that cancels the polarization dependence. An optical receiving element with a VOA will be described.

The assembly process of the optical receiving element with a built-in optical variable attenuator described in this reference example is the same as that of Reference Example 1 up to the YAG laser welding process of the incident side fiber collimator shown in FIGS. Therefore, detailed description is omitted. As in Reference Example 1, the polarization-independent optical isolator 2, the PBS element 3, the liquid crystal reflective rotator 4 and the liquid crystal reflective rotator 5 are fixed in the housing 1 and hermetically sealed. The fiber collimator 7 is aligned and fixed in the same manner as in Reference Example 1.

  After that, as shown in FIG. 7, a light receiving element with a lens sealed in a cylindrical housing is inserted into the emission port 8 of the housing 1 through a collar 11 for fixing the alignment, and the light receiving current is After aligning to the maximum, the light receiving element 12 is fixed by YAG welding.

  In the receiver with VOA completed by the above steps, the receiving element is detected from the excessive light input at the time of instability immediately after the power is turned on by controlling the light attenuation amount gradually while monitoring the received photocurrent value after the power is turned on. Can be protected.

  Also, the correspondence between the received photocurrent value and the electric field of the liquid crystal reflection type rotors 4 and 5 is stored in advance in the memory of the electric field control device, and the monitor value of the received photocurrent value is stored in the electric field connected to the pin 18. Supplied to a control device (not shown), and the electric field control device controls the electric field of the liquid crystal reflective rotors 4 and 5 based on the monitor value, so that the current value of the received light is attenuated to a desired value. The amount can be adjusted.

( Reference Example 3)
A third reference example will be described with reference to FIG. In this reference example, an avalanche multiplication optical receiver (hereinafter referred to as APD) 14, a received signal amplifier circuit (hereinafter referred to as TIA) 15, two liquid crystal reflective rotators 4 and 5, the configuration of a variable optical attenuator built-in receiver equipped with one polarization beam splitter 3 and one polarization independent optical isolator 2 will be described. Further, in this reference example, a case where the variable optical attenuator is normally open will be described as an example.

The outline of the assembly process of the variable optical attenuator described in this reference example is shown in FIGS. First, the APD chip 14 is fixed by soldering to a predetermined position of the housing 1 via the ceramic carrier 13, and the TIA chip 15 is fixed with a conductive adhesive (FIG. 8A). Thereafter, necessary locations of the APD chip 14, the TIA chip 15, and the pins 18 of the housing terminal 1 are connected by wire bonding. Further, collimated light is irradiated from the direction perpendicular to the light receiving surface of the APD chip 14, the ball lens 16 is aligned at a position where the light receiving current is maximized, and is fixed by welding with a YAG laser (FIG. 8B).

Subsequently, the polarization-independent optical isolator 2 is fixed to a predetermined position in the housing 1 by soldering. In this reference example, since the VOA is normally open, the PBS element 3 is of a type that bends the optical path of P-polarized light at right angles and transmits S-polarized light. Liquid crystal reflection type rotors 4 and 5 equivalent to those of Reference Examples 1 and 2 are bonded and fixed to the PBS element 3 in advance. The PBS element 17 with the reflective liquid crystal element is placed on the housing of the ball lens 16 and aligned with the incident side fiber collimator 7 so as to maximize the light receiving current of the APD. After alignment, the PBS element 17 with a reflective liquid crystal element is fixed to the ball lens casing 16 with an adhesive, and the fiber collimator 7 is fixed to the casing 1 by YAG laser welding. Next, the electrodes of the liquid crystal reflective rotors 4 and 5 and the pin 18 of the housing 1 are connected by wire pumping. Finally, the lid 10 is completed by seam welding (FIG. 8C).

  The receiver with a built-in variable optical attenuator completed by the above process can control the optical attenuation and the M value of the APD 14 while monitoring the received photocurrent value, and can receive at the light receiving level with the best characteristic of the receiver. It becomes possible.

  Further, the monitor value of the received photocurrent value amplified by the TIA 15 is supplied to an electric field control device (not shown) connected to the pin 18, and the electric field control device supplies the liquid crystal reflective rotors 4 and 5 based on the monitor value. By controlling the electric field, the amount of attenuation can be adjusted so that the current value of the received light becomes a desired value.

( Example )
With reference to FIGS. 9 and 10, illustrating the implementation and examples reference configuration of the VOA of the present invention.

  FIG. 9 shows a configuration example in which three reflective liquid crystal optical elements are used as the liquid crystal optical elements in the entire VOA, that is, for the S-polarized light separated by the PBS element 3, the reflective liquid crystal optical element 4 changes the polarization state. The P-polarized light reflected and separated by the PBS element 3 is branched into two by the optical coupler 19 (multiplexing / demultiplexing element) and incident on the reflective liquid crystal optical elements 5-1 and 5-2. A configuration in which the elements 5-1 and 5-2 reflect the polarization state by changing the respective polarization states is shown. The polarized lights reflected by the reflective liquid crystal optical elements 5-1 and 5-2 are combined by the optical coupler 19 and incident on the PBS element 3 again.

  Similarly, S-polarized light can also be configured to be reflected by changing the polarization state with a plurality of reflective liquid crystal optical elements.

  In this way, by using a plurality of reflective liquid crystal optical elements arranged for each polarized light, it is possible to finely control the amount of attenuation, provide an optical attenuator with high response speed and low power consumption. Become.

FIG. 10 shows a reference example of a VOA configuration that uses the configuration shown in FIG. 1 in parallel. The VOA shown in FIG. 10 demultiplexes incident light and supplies it to the isolators 2-1 and 2-2, and outputs from the PBS elements 3-1 and 3-2. An optical coupler (multiplexing / demultiplexing element) 20-2 for multiplexing light is provided.

  With this configuration, it is possible to provide an optical attenuator capable of finely controlling the amount of attenuation, having a high response speed, and low power consumption, as in the VOA shown in FIG.

  As described above in detail, taking advantage of the small size and low power consumption of the attenuator of the present invention, an optical attenuator that can be easily integrated into the casing of an existing optical receiver for optical communication is provided. Can be provided. Furthermore, since it does not have a movable part like the MEMS (macro machine) type, it is possible to provide an optical attenuator that is excellent in vibration resistance and stable for a long period of time as shown by the results of display devices.

It is a figure which shows the reference structural example of VOA of this invention, (a) is a figure which shows the state at the time of an electric field application, (b) shows the state at the time of no electric field, respectively. It is a figure which shows the reference structural example of VOA of this invention, (a) is a figure which shows the state at the time of an electric field application, (b) shows the state at the time of no electric field, respectively. It is a figure for demonstrating the assembly process of the VOA element of the reference example 1 of this invention. It is a figure which shows the optical characteristic of the PBS element used by the reference example 1 of this invention. It is a figure which shows the relationship between the light attenuation amount of the VOA element produced in the reference example 1 of this invention, and the applied voltage. It is a figure which shows the polarization dependence of the optical attenuation amount of the VOA element produced in the reference example 1 of this invention. It is a figure which shows the structure of the optical receiver with VOA of the reference example 2 of this invention. It is a figure which shows the structure of the optical receiver with a built-in VOA demonstrated in the reference example 3 of this invention. It is a figure which shows the implementation structural example of VOA of this invention. It is a figure which shows the reference structural example of VOA of this invention.

DESCRIPTION OF SYMBOLS 1 Case 2 Polarization-independent optical isolator 3 PBS element 4 Reflective liquid crystal optical element 5 Reflective liquid crystal optical element 6 Incident port 7 Incident side fiber collimator 8 Ejection port 9 Ejection side fiber collimator 10 Lid (lid)
11 Color 12 Receiving Element 13 Ceramic Carrier 14 Avalanche Multiplier Type Optical Receiving Element APD
15 Received signal amplifier circuit TIA
16 ball lens 17 PBS element with reflective liquid crystal optical element 18 pin 19, 20 optical coupler

Claims (4)

Polarization separation means for separating the optical path of the incident light into polarized light having different optical paths according to the polarization state of the incident light;
At least one liquid crystal optical element having a polarization dependency that is arranged for each polarized light separated by the polarization separating means and changes the polarization state of the incident polarized light according to the applied electric field and reflects the polarized light;
A variable optical attenuator and a polarization multiplexing unit for multiplexing the respective polarization the polarization state is changed by the liquid crystal optical element,
The polarization separation means and the polarization multiplexing means are one polarization beam splitter element, which separates the incident light into two linearly polarized lights having orthogonal optical paths and reflected by the at least one liquid crystal optical element. The polarized light is incident again and combined,
The liquid crystal optical element is arranged so that each polarized light separated by the polarization beam splitter element is incident perpendicularly to a reflection surface of the liquid crystal optical element,
An optical isolator that prevents the polarized light reflected by the liquid crystal optical element from entering the optical path of the incident light is provided in the front stage of the polarization beam splitter element,
A variable optical attenuator comprising branching means for branching at least one of the polarized lights separated by the polarized light separating means into a plurality of pieces and for allowing the branched polarized light to enter each of the liquid crystal optical elements . A receiver with a built-in variable optical attenuator, comprising the variable optical attenuator according to claim 1 and a receiving element. A variable optical attenuator built-in receiver comprising the variable optical attenuator according to claim 1, an avalanche multiplication optical receiver, and a reception signal amplifier circuit. A polarization separating means for separating the polarized light having different optical paths according to the polarization state of the incident light the optical path of incident light, the disposed in the polarization separating means polarizing each separated by, in response to an applied electric field At least one liquid crystal optical element having polarization dependency that changes and reflects the polarization state of incident polarized light, and polarization multiplexing means for combining each polarized light whose polarization state has been changed by the liquid crystal optical element. An optical attenuation method in a variable optical attenuator provided,
The variable optical attenuator is
The polarization separation means and the polarization multiplexing means are one polarization beam splitter element that separates the incident light into two linearly polarized light having orthogonal optical paths and reflected by the at least one liquid crystal optical element Each polarized light is incident again and combined,
The liquid crystal optical element is arranged so that each polarized light separated by the polarization beam splitter element is incident perpendicularly to a reflection surface of the liquid crystal optical element,
An optical isolator that prevents the polarized light reflected by the liquid crystal optical element from entering the optical path of the incident light is provided in the front stage of the polarization beam splitter element,
Branching means for branching at least one of the polarized lights separated by the polarized light separating means into a plurality of pieces, and for allowing the branched polarized light to enter each of the liquid crystal optical elements,
The method
Monitoring light from the polarization multiplexing means;
And a step of controlling an electric field applied to the liquid crystal optical element so that the monitor value becomes a desired value.

Images