JP3074940B2 - Tunable optical filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable optical filter used for a wavelength division multiplexing optical communication line demultiplexer or an optical signal processing device.

[0002]

2. Description of the Related Art Conventionally, a liquid crystal is sealed in a Fabry-Perot etalon, and by changing the voltage applied to the liquid crystal, the refractive index of the liquid crystal in the etalon is changed, and this is used to provide wavelength selectivity. Known tunable optical filters are known (for example, Katsuhiko Hirabayashi, Yo
shitaka Ohiso, and Takashi Kurokawa, “Polarization
-Independent Tunable Wavelength-Selective Filter U
sing a Liquid Cystal ”IEEE Trans. Photonics Techno
logy Letters, vol.3, No.12, pp.1091-1093 (1991)).

FIG. 2 shows a wavelength tunable optical filter described in the above document. In the above device, the incident light 1 from the optical fiber is separated into two linearly polarized lights 3a and 3b whose polarization directions are orthogonal by a polarization beam splitter 2,
One linearly polarized light 3b is rotated by 90 degrees by a mirror 4 and a half-wave plate 5, and the other linearly polarized light 3a is rotated.
And is incident on the liquid crystal etalon 6. The beam interval between the two linearly polarized light beams 3a and 3b is about 5 mm, and passes through separate optical paths spatially separated in the liquid crystal etalon 6. The two light beams 3a and 3b transmitted through the liquid crystal etalon 6 are polarized light beam splitters 7,
The mirror 8 and the half-wave plate 9 receive the opposite operation to that on the incident side and are combined into one light beam to become outgoing light 10.
This functions as a polarization independent wavelength tunable optical filter.

The liquid crystal etalon 6 for performing the wavelength changing operation of the optical filter includes a glass substrate 14 on which a dielectric mirror 11, a transparent electrode 12, and an alignment film 13 are provided, and a dielectric mirror 1
5. A liquid crystal 20 is sealed between a transparent electrode 16, 17 and a glass substrate 19 provided with an alignment film 18. The transmission wavelength of the liquid crystal etalon 6 is determined by the product of the refractive index of the medium in the etalon and the gap between the mirrors 11 and 15 (here, called the optical path length). When an electric field is applied to the liquid crystal 20, the optical path length is changed by utilizing the phenomenon that the molecular orientation changes and the refractive index changes. That is, by controlling the voltage applied to the liquid crystal 20, it is possible to function as a wavelength variable optical filter.

[0005] However, the gap of the etalon (the gap between the two glass substrates 14 and 19) varies spatially due to warpage or deformation of the glass substrates and is not constant. In addition, the alignment state of the liquid crystal molecules that determines the refractive index of the liquid crystal also varies spatially depending on the state of the alignment film and the like, and is not constant. Therefore, even when the distance between the two linearly polarized beams is as small as about 5 mm as in the above-described apparatus, the optical path lengths are different from each other, and the transmission wavelengths are also different. In the optical filter, two linearly polarized waves are combined into one light beam on the output side. However, since the transmission bands of the two light beams are slightly different, the combined spectrum of the two light beams is separated into two. However, there is a problem that the half width of the transmission band of the entire optical filter cannot be reduced.

To remedy this problem, the device of FIG. 2 uses two independent electrodes 16, 17 for two different optical paths.
And an AC voltage source 21 is connected to the electrodes 16 and 17.
By applying different voltages from the variable resistor 22 and the variable resistor 22, respectively, the refractive indexes of the liquid crystal 20 in the etalon 6 are adjusted to make both optical path lengths uniform, thereby avoiding separation of the transmission band into two peaks. However, the above-mentioned document does not describe a specific control circuit and a control method for the present optical filter. In addition, the liquid crystal is in an intermediate phase state between the liquid and the crystal, and its optical characteristics have essentially large temperature dependence. Therefore, temperature control is necessary to operate the optical filter stably. However, there is no description about temperature control in the aforementioned documents.

As a filter similar to the tunable optical filter, a fiber Fabry-Perot (FFP) tunable filter and a control method thereof are known. This FFP wavelength tunable filter is a filter that changes a transmission wavelength by changing an optical path length using a piezoelectric effect actuator made of PZT. For a method of controlling the transmission wavelength of the FFP wavelength tunable filter to be constant, for example,
IPKaminow, PPIannone, J. Stone and LWStulz,
“FDMA-FSK Star Network with a Tunable Optical Fil
ter Demultiplexr ”IEEE Journal of lightwave techno
logy, vol.6, No.9, pp.1406-1414 (1988).

FIG. 3 shows a control circuit described in the above document. In the circuit, first, the switch SW
1 and SW2 are connected to the solid line side. At this time, a ramp wave is applied from the ramp waveform generator 23 to the PZT driver 24, the optical path length of the FFP variable wavelength filter 25 changes at a fixed rate, and the transmission wavelength is swept. When the transmission wavelength of the FFP variable wavelength filter 25 matches the wavelength of the incident light 26 by the wavelength sweep, the signal of the transmission light 28 is detected by the photodetector 27. Based on this signal, the switches SW1 and SW2 are connected to the broken line side, and the operation shifts to a closed loop operation.

In this closed loop operation, the perturbation signal generator 2
9 superimposes a minute sine wave signal on the PZT drive voltage,
The transmission wavelength of the P variable wavelength filter 25 is slightly changed. Thereby, the intensity of the transmitted light 28 of the FFP variable wavelength filter 25 also fluctuates in synchronization with the minute fluctuation signal applied to the PZT. Therefore, the fluctuation component of the transmitted light 28 is
7 and synchronously detected by the demodulator 30 to obtain P
The differential coefficient of the amount of transmitted light with respect to the drive voltage of the ZT can be obtained. The PZT drive voltage can be controlled by the magnitude and sign of the differential coefficient obtained in this closed loop circuit. By controlling the differential coefficient to be always 0, the transmission of the FFP variable wavelength filter 25 can be controlled. Control can be performed so that the wavelength always matches the wavelength of the incident light 26.

[0010]

As described above, in the conventional wavelength tunable optical filter, since the characteristics of the liquid crystal have a substantially large temperature dependency, it is difficult to control the driving voltage of the liquid crystal filter (etalon) to a constant value. The selected wavelength was not constant, but fluctuated unstablely depending on the temperature. In addition, in order to eliminate the polarization dependence, when a polarization separation method is adopted in which incident light is separated into two orthogonal linear polarizations and transmitted through separate optical paths, the thickness of the liquid crystal filter and the reflectivity of the mirror are reduced. Due to the spatial dispersion of the two polarized waves, the transmission wavelengths are different for the two separated polarized waves. When the two beams are combined, the transmission band is separated, and narrow band wavelength separation is difficult as a characteristic of the entire filter. there were. Furthermore, when the liquid crystal filters are spatially arrayed to increase the capacity, it is difficult to adjust the individual liquid crystal filters, and it is difficult to form a large-scale array.

The present invention has been made in view of the above circumstances, and provides a tunable optical filter capable of automatically and stably performing a wavelength selecting operation by eliminating a factor of instability due to a variation in manufacturing and a temperature change. The purpose is to:

[0012]

According to the present invention, in order to achieve the above object, according to the present invention, a cell is constructed by opposing two transparent substrates provided with a transparent electrode and a mirror, and in the cell, sealing liquid crystal, liquid crystal filter der which forms a transmission wavelength variable by varying the voltage applied to the liquid crystal
Thus, at least one of the transparent electrodes on the two transparent substrates
One separates the incident light into two orthogonal polarization directions,
Rotate one of the polarization directions by 90 degrees to the same polarization direction
Spatially separated two linearly polarized lights aligned
Liquids that are two transparent electrodes respectively corresponding to different optical paths
In the tunable optical filter having a crystal filter, the light intensity of the transmitted through the liquid crystal filter two linearly polarized light it
A light detecting means for detecting each of them independently, and a light detecting means in front of the light detecting means.
Using the detection signals respectively corresponding to the light intensities of the two linearly polarized lights, the liquid crystal filter for the two linearly polarized lights is used .
Make sure that the transmission wavelength always matches the wavelength of the light source you want to select
Independent voltage applied to the two transparent electrodes of the liquid crystal filter
Tunable optical filter and control means for controlling the,
According to a second aspect of the present invention, there is provided a wavelength tunable optical filter according to the first aspect, further comprising: a temperature detection unit configured to detect a temperature around the liquid crystal filter; and a temperature control unit configured to control a temperature around the liquid crystal filter. Item 3 proposes the tunable optical filter according to claim 1, wherein the liquid crystal filters are arranged in a two-dimensional array in parallel.

[0013]

According to the first aspect of the present invention, the light detecting means is provided.
And separates the incident light into two orthogonal polarization directions.
Rotate one of the polarization directions by 90 degrees to the same polarization direction
Detects light intensity of two aligned linearly polarized lights independently
Then, using this detection signal, two straight
The transmission wavelength of the liquid crystal filter for linearly polarized light is always selected
Of the liquid crystal filter to match the wavelength of the
By independently controlling the voltage applied to the bright electrode,
For spatial variations in filter thickness and mirror reflectivity
Therefore, the transmission wavelengths are different for the two separated polarizations.
When the two beams are combined, the transmission band is separated.
As a result, narrow-band wavelength separation is a characteristic of the entire filter.
Can solve the conventional disadvantage it was difficult, also possible to reduce variation in the transmission wavelength band due to temperature changes, stable and
Ru can achieve narrowband polarization-independent optical tunable filter. According to the second aspect, the temperature detecting means and the temperature controlling means maintain a constant temperature environment with respect to a temperature change accompanied by a large change in the use environment, thereby enabling a stable wavelength selecting operation. According to the third aspect, a large-capacity tunable optical filter that does not require adjustment for each liquid crystal filter can be realized.

[0014]

FIG. 1 shows a tunable optical filter according to a first embodiment of the present invention. Wavelength multiplexed light of wavelengths λ _{1 to} λ _N (where N is an integer) from the optical fiber 31 on the incident side is converted into a parallel space propagation light 33 by a collimator lens 32, and a polarization beam splitter 34, mirrors 35 and 1/2
The light is separated into two linearly polarized light beams having the same polarization direction by the wavelength plate 36. These two beams are made incident on a tunable liquid crystal filter 37, the wavelength of which is selected by the liquid crystal filter 37, and a target wavelength λ _i (where i = 1, 2, 3,...).
N) is selected. The two transmitted lights transmitted through the liquid crystal filter 37 are polarized beam splitter 38, mirrors 39 and 1
The half-wave plate 40 receives the opposite action to that on the incident side to be combined into one light beam 41, and further exits through the collimator lens 42 and the optical fiber 43 on the exit side. This functions as a polarization independent wavelength tunable optical filter.

The intensity of the two linearly polarized lights transmitted through the liquid crystal filter 37 is detected independently by the photodetectors 44 and 45. By using, for example, a transmission type photodetector that detects a part of incident light as the photodetectors 44 and 45, a small module integrated with the liquid crystal filter 37 can be configured. The signals obtained from the photodetectors 44 and 45 are input to the control circuit 46, respectively. Control circuit 46
There are a wavelength sweep circuit that gradually changes the transmission wavelength of the liquid crystal filter 37 and a wavelength lock circuit that locks the transmission wavelength of the liquid crystal filter 37 to the wavelength of the light source. The transmission wavelength of the liquid crystal filter 37 was locked to that wavelength, and the transmission wavelength of the liquid crystal filter 37 was always made to coincide with the wavelength of the light source. In the wavelength locking operation, signals obtained from the photodetectors 44 and 45 are processed by the control circuit 46, and the liquid crystal filter driving circuit 4
7, the transparent electrodes 48 and 49 of the liquid crystal filter 37 were independently driven. Reference numeral 50 denotes a transparent electrode facing the liquid crystal filter 37.

The liquid crystal filter 37, photodetectors 44 and 4
5 and a control circuit 46,
The variation in the spatial characteristics of the liquid crystal filter 37 and the time variation in the transmission characteristics due to the temperature change can be automatically absorbed, and the variation in the transmission wavelength band of the liquid crystal filter 37 due to polarization separation can be compensated. In the control circuit 46, a memory for setting the transmission wavelength of the liquid crystal filter 37, a memory for setting the temperature, and an interface circuit with the host controller 51 are provided. The setting can be made from the host controller 51 via the host.

The liquid crystal filter 37 can operate stably without temperature control by the control circuit 46 in an environment where the temperature change is less than several degrees, but it is necessary to control the temperature in an environment where the temperature change is large. Become. LCD filter 3
7 is controlled by a temperature sensor 53 and a heating / cooling element 54 composed of a heater and a cooling element (such as a Peltier element).
And the temperature control circuit 55. That is, the temperature sensor 5
3, the temperature of the liquid crystal filter 37 could be kept constant by detecting the temperature and controlling the heat generation / cooling element 54 by the temperature control circuit 55. Thereby, the liquid crystal filter 3
7 was able to stabilize the transmission wavelength.

[0018] Figure 4 shows the manner of change in the transmission wavelength band of the wavelength tunable optical filter, FIG. 4 (a) lambda ₁ ~
₄ shows a spectrum of incident light obtained by wavelength multiplexing four waves of λ4 at a constant wavelength interval. The wavelength of the light source here is λ ₁ <λ ₂ <λ ₃
<Λ ₄ a relationship of. When the applied voltage (the voltage between the transparent electrode 48 or 49 and the transparent electrode 50) of the liquid crystal filter 37 is gradually increased, the liquid crystal molecules gradually rise with respect to the glass substrate, so that the refractive index of the liquid crystal decreases, The transmission band of the filter 37 shifts to the shorter wavelength side. By this operation, the wavelength scanning shown in FIG. 4B can be realized.

For a light source having a spectrum as shown in FIG. 4A, the output of the photodetector (the amount of transmitted light) with respect to the applied voltage (effective value) of the liquid crystal filter 37 is, as shown in FIG. In a region where is low, the wavelength interval of transmitted light with respect to the applied voltage becomes narrow. Generally, when the applied voltage of the liquid crystal is equal to or lower than the threshold voltage _Vth of the liquid crystal, the alignment state of the liquid crystal molecules does not change, and since the change in the refractive index is very small, the transmission wavelength of the liquid crystal filter does not change and the threshold voltage is exceeded. This is because the alignment state of the liquid crystal molecules changes rapidly in the vicinity, and the refractive index also changes rapidly.

Further, as the voltage applied to the liquid crystal increases, the change in the orientation of the liquid crystal molecules becomes gentler, and the change in the refractive index becomes saturated, so that the transmission wavelength shift of the liquid crystal filter 37 becomes gentler. That is, in order to perform the wavelength sweeping at a constant speed in the liquid crystal filter 37, the voltage applied to the liquid crystal, as shown in curve 56 of FIG. 6, firstly, the liquid crystal threshold voltage V _th
A voltage waveform having characteristics inverse to the change in the refractive index of the liquid crystal is effective, in which the voltage is increased immediately after that, then gradually increased, and finally sharply increased. Of course, even if the voltage is linearly increased as shown by a broken line 57 in order to facilitate the voltage control, there is no operational problem. After the end of the wavelength scanning, the applied voltage is returned to 0 V, but it takes several ms to several tens of meters before the next wavelength scanning.
s time is required. This is the time required for the liquid crystal to return to the molecular alignment state before voltage application.

Since the operating speed of the liquid crystal used in the wavelength tunable liquid crystal filter 37 is several milliseconds to several tens of milliseconds, the applied voltage change (effective value change) of the liquid crystal is performed at a speed lower than the operating speed of the liquid crystal. The applied voltage varies depending on the viscosity of the liquid crystal material used for the liquid crystal filter 37, the dielectric anisotropy, the gap between cells, and the like, but is approximately in the range of several volts to several tens of volts. Depending on the voltage applied to the liquid crystal, a wavelength of 1.33 μm or
Wavelength sweeping in the range of several nm to several tens of nm was achieved in the 1.55 μm band. Further, in this embodiment, an AC voltage of several tens Hz to several tens kHz is used as the liquid crystal application voltage to prevent the life degradation due to the electrochemical change of the liquid crystal and the alignment film.

Next, the wavelength locking operation of the liquid crystal filter in the present variable wavelength optical filter will be described with reference to FIG. Here, the example of wavelength multiplexing of four waves shown in FIG. 4A is assumed. In particular, the operation of wavelength locking to λ ₃ will be described. Note that the wavelength can be locked for wavelengths other than λ _{3 by} the same operation.

First, the selected wavelength data (λ _{3 in} this case) and a wavelength lock operation start command are transmitted from the host controller 51 to the control circuit 46. Upon receiving this command, the control circuit 46 increases the voltage applied to the liquid crystal as shown in FIG. 7, and starts wavelength scanning. Then, the transmission wavelength band of the liquid crystal filter 37 shifts to the shorter wavelength side as shown in FIG. 4 (b), and first coincides with the wavelength λ ₄ of the light source. At this time, the photodetector 44 (or 45) outputs the peak “4” in FIG. When the output of the photodetector 44 (or 45) exceeds a preset threshold for determining the presence or absence of a light source, the control circuit 46 counts the peak and stores it.

At this time, since the wavelength for which the wavelength is to be locked is λ ₃ , the control circuit 46 determines that the wavelength is different from the selected wavelength, continues the wavelength sweep mode, and further increases the voltage applied to the liquid crystal. . Then, the liquid crystal filter 3
The transmission wavelength band 7 further shifts to the shorter wavelength side and approaches the wavelength of λ ₃ . At this time, the photodetector 44 (or 45) outputs the peak “3” in FIG. When this output exceeds a threshold value for determining the presence or absence of the light source, the control circuit 46 determines that the wavelength of the light source matches λ ₃ , and switches its operation mode to the wavelength lock mode.

In the wavelength lock mode, the control circuit 46
Stops increasing the applied voltage of the liquid crystal, adds a signal obtained by superimposing a sine wave signal having a small amplitude shown in FIG. 7 or 8 to the applied voltage at that time, and outputs the signal. Further, the control circuit 46 performs synchronous detection of the signal from the photodetector 44 (or 45), and detects the derivative of the output of the photodetector with respect to the applied voltage. The control circuit 46 implements the wavelength lock operation by performing feedback control so that the differential coefficient is always zero. Specifically, the voltage lock operation was realized by controlling the applied voltage _{VLC of the} liquid crystal using the following recurrence formula. V _LC ^{(ν + 1)} = V _LC ^(ν) + ΔV _LC ^(ν) ΔV _LC ^(ν) = η × (dV _PD / dV _LC ) ^(ν) where ν is the number of repetitions and dV _PD / dV _LC is a photodetector. The derivative of the voltage applied to the liquid crystal with respect to the output, η, is a constant (here, a positive constant).

This wavelength locking operation is performed by the liquid crystal filter 37.
, The photodetector 4 through which components having different polarization directions pass
4 and 45, which are performed independently and simultaneously, the transmission wavelengths of the two liquid crystal filters can always be made to coincide with the wavelength λ ₃ of the light source, and variations in characteristics of the liquid crystal filter 37 and fluctuations due to temperature changes can be compensated. Wavelength selection operation. In the above description, an example of wavelength multiplexing of four waves has been described, but it is needless to say that wavelength multiplexing of N waves (N is an integer) can be similarly realized.

FIG. 9 shows a second embodiment of the tunable optical filter according to the present invention.
Here, an example is shown in which the liquid crystal filter shown in the first embodiment is formed into a 2 × 2 two-dimensional array.

The liquid crystal filter array 61 is constructed not by simply arranging four polarization-independent liquid crystal filters but by patterning eight electrodes in one liquid crystal filter. As the photodetector array 62, a transmission type photodetector that detects a part of the light at the foot of the incident light beam was used. This allows
The entire optical filter can be stacked in an in-line form, can be made compact, suppress the spread of a light beam, and reduce crosstalk between adjacent channels. The polarization separation element is the first
In this embodiment, a polarizing beam splitter and a mirror are used, but in this embodiment, calcites 63 and 64 are used. Even if calcite is used, a polarization-independent optical filter can be similarly formed. Electrodes are taken out from each photodetector (eight) of the photodetector array 62 and the control circuit 65
Entered. Further, the output from the control circuit 65 was amplified by the drive circuit 66, and the eight electrodes of the liquid crystal filter array 61 were independently controlled. The control method in the control circuit 65 is the same as in the first embodiment. The input and output of the light beam used the collimator lens arrays 67 and 68. Also, 69 and 70 are multi-core optical fiber cables, 71 and 72 are optical fibers, 7
Reference numeral 3 denotes glass, 74 denotes a half-wave plate, 75 denotes a host controller, 76a denotes ordinary light, and 76b denotes extraordinary light.

By forming the liquid crystal filter in a two-dimensional array, a large capacity can be easily realized. In this embodiment, a 2 × 2 two-dimensional array is shown.
Needless to say, a two-dimensional array of × M2 (where M1 and M2 are integers) can be realized by a similar configuration.

[0030]

As described above, according to the present invention, by providing the liquid crystal filter with the light detecting means, the wavelength scanning means and the control means, it is possible to make the transmission wavelength of the liquid crystal filter always coincide with the wavelength of the light source. It is possible to absorb a variation in transmission wavelength due to a variation during manufacturing of the liquid crystal filter or a temperature change, and to stabilize the transmission wavelength for a temperature change of less than several degrees without special temperature control. Further, a filter that operates stably even when the half-width of the transmission band of the liquid crystal filter is 1 nm or less can be realized. Further, even when the incident light of the liquid crystal filter is separated into two orthogonal polarization components in order to make the liquid crystal filter polarization-independent, the wavelength difference between the transmission wavelength bands of the two can be almost eliminated, which is stable and stable. A narrow-band polarization-independent tunable filter can be realized. Furthermore, even in the case of multi-channeling by forming a two-dimensional array of liquid crystal filters, the transmission wavelength band of each liquid crystal filter can be automatically adjusted.
Can be easily realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a wavelength tunable optical filter of the present invention.

FIG. 2 is a configuration diagram showing an example of a conventional tunable optical filter.

FIG. 3 is a configuration diagram showing a control circuit of a conventional fiber Fabry-Perot tunable filter.

FIG. 4 is an explanatory diagram showing how a transmission wavelength band changes in the first embodiment.

FIG. 5 is an explanatory diagram showing a photodetector output with respect to a liquid crystal filter applied voltage.

FIG. 6 is an explanatory diagram showing a state of scanning in a transmission wavelength band.

FIG. 7 is an explanatory diagram showing how the scanning of the transmission wavelength band and the wavelength locking operation are performed.

FIG. 8 is an explanatory diagram showing a photodetector output with respect to a minute change in a voltage applied to a liquid crystal filter.

FIG. 9 is a configuration diagram showing a second embodiment of the wavelength tunable optical filter of the present invention.

[Explanation of symbols]

31, 43 ... optical fiber, 32, 42 ... collimator lens, 33 ... incident light, 34, 38 ... polarizing beam splitter, 35, 39 ... mirror, 36, 40 ... 1/2 wavelength plate,
37: tunable liquid crystal filter, 41: outgoing light, 44, 4
5 photodetector, 46, 65 control circuit, 47, 66 liquid crystal filter drive circuit, 48, 49, 50 transparent electrode, 5
1, 75: host controller, 52: input / output bus, 5
3: Temperature sensor, 54: Heating / cooling element, 55: Temperature control circuit, 61: Liquid crystal filter array, 62: Photodetector array, 63, 64: Calcite, 67, 68: Collimator lens array, 69, 70: Many Fiber optic cable, 7
1, 72 optical fiber, 73 glass, 74 half-wave plate, 76a ordinary light, 76b extraordinary light.

Continuation of the front page (56) References JP-A-58-354224 (JP, A) JP-A-3-246531 (JP, A) JP-A-4-140714 (JP, A) JP-A-3-273217 (JP) JP-A-5-273509 (JP, A) JP-A-5-19690 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/13 505

Claims (3)

(57) [Claims] A cell is constructed by opposing two transparent substrates provided with a transparent electrode and a mirror, a liquid crystal is sealed in the cell, and a transmission voltage is changed by changing a voltage applied to the liquid crystal. A variable liquid crystal filter , wherein
At least one of the transparent electrodes on one of the transparent substrates
Into two orthogonal polarization directions, one of which is
Two that rotate the polarization direction by 90 degrees and align them to the same polarization direction
Spatially separated optical paths through which the linearly polarized light passes
In a wavelength tunable optical filter including a liquid crystal filter which is a corresponding two transparent electrode, the light intensity of the two linearly polarized lights transmitted through the liquid crystal filter is determined.
Light detecting means for detecting each independently, it to the light intensity of the two linearly polarized light from the optical detection means
Using the corresponding detection signals, the two linearly polarized lights are
The wavelength of the light source that you always want to select
The two transparent electrodes of the liquid crystal filter to match the length
And a control means for independently controlling a voltage applied to the wavelength tunable optical filter. 2. The wavelength tunable optical filter according to claim 1, further comprising a temperature detecting means for detecting a temperature around the liquid crystal filter, and a temperature control means for controlling the temperature around the liquid crystal filter. 3. The tunable optical filter according to claim 1, wherein the liquid crystal filters are arranged in a two-dimensional array in parallel.