JPH05323265A - Polarization non-dependency type wavelength variable filter - Google Patents

Abstract

PURPOSE:To provide the polarization non-independence type wavelength variable filter which can be easily miniaturized and has a high yield. CONSTITUTION:Incident light is separated to an ordinary light component and extraordinary light component by calcite 31. The ordinary light component and exctraordinary light component are passed through a phase difference plate 10A, by which the polarization directions thereof are aligned to the direction coinciding with the orientation direction of the liquid crystal molecules of a Fabry-Perot resonator type wavelength variable liquid crystal filter 20. The ordinary light component and extraordinary light component are passed through a phase difference plate 10B after passing these components through the liquid crystal filter 20 to change the wavelength thereof or without changing the wavelength, by which the polarization directions thereof are put into the state before the incidence on the phase difference plate 10A; further the two components are synthesized by calcite 32 to one light beam.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization independent wavelength tunable filter used in an optical demultiplexer of an optical communication line for wavelength multiplexing.

[0002]

2. Description of the Related Art FIG. 2 shows an example of a conventional polarization independent wavelength tunable filter of this type, for example, JSPatel and MWMaeda.
“Tunable Polarization Diversity Liquid-Crystal Wa
velength Filter ”(Photonics Technology Letters, v
ol.3, No.8, 1991, pp739-740).
In the figure, the incident light 2 from a collimator 1 composed of an optical fiber and a lens is separated by a calcite 3 into an ordinary light component 4a and an extraordinary light component 4b, and two liquid crystal molecule orientations matching the respective polarization directions are obtained. The light passes through the Fabry-Perot resonator type wavelength tunable liquid crystal filters 5a and 5b. At this time, the wavelength thereof is changed or not changed, and is again synthesized by the calcite 6 to become the polarization-independent emitted light 7, and the collimator 8 Is emitted.

[0003]

However, the alignment films of the liquid crystal molecules constituting the Fabry-Perot resonator type wavelength tunable liquid crystal filters 5a and 5b described above are separated in their alignment directions by the photolithography technique. Especially, it is difficult to control the alignment with high precision at the boundary between regions where the alignment directions of the liquid crystal molecules are different, and it is difficult to miniaturize the alignment region. Therefore, the size required to spatially separate the polarized waves becomes large, and it is difficult to configure the polarization independent wavelength tunable filter in a small size. In addition, in the process of manufacturing the alignment film, despite the highly accurate photoresist pattern, a rubbing process that mechanically causes friction is performed after the pattern is manufactured. It was difficult to fabricate the device.

In view of the above conventional problems, it is an object of the present invention to realize a polarization independent wavelength tunable filter which can be easily miniaturized and has a high yield.

[0005]

In order to achieve the above-mentioned object in the present invention, in claim 1, two transparent substrates provided with a transparent electrode, a semitransparent mirror and an alignment film are separated from each other by a certain distance. The cells are opposed to each other to form a cell, and the liquid crystal is encapsulated in the cell so that the molecular orientation at the interface between the two transparent substrates is parallel to the substrates and all the molecules are oriented in the same direction. A polarization-independent wavelength equipped with a Fabry-Perot resonator type tunable liquid crystal filter that changes the resonant state of transmitted light by changing the alignment state of liquid crystal molecules by applying a voltage to the In the tunable filter, the Fabry-Perot resonator type tunable liquid crystal filter functions as a half-wave plate and two polarization routing elements for separating or combining light into an ordinary light component and an extraordinary light component. And, a region where the polarization direction is not rotated is sandwiched between two spatially separated retardation plates, and the incident light is separated into an ordinary light component and an extraordinary light component by the first polarization routing element, and the ordinary light component or the abnormal light component is separated. One of the light components is allowed to pass through a region that functions as a half-wave plate of the first retardation plate, and the other is passed through a region of the first retardation plate that does not rotate the polarization direction to change its polarization direction. Alignment is passed through the Fabry-Perot resonator type wavelength tunable liquid crystal filter, and then either the ordinary light component or the extraordinary light component is passed through a region functioning as a half-wave plate of the second retardation plate, The other is passed through a region in which the polarization direction of the second retardation plate is not rotated, and the polarization direction is set to the state before the incidence of the first retardation plate or the state rotated by 90 degrees from each, and both components are Polarization of 2 It proposes one polarization-independent tunable filter having no to synthesize the light beams in the computing elements.

According to a second aspect of the present invention, a polarization independent wavelength tunable filter according to the first aspect, wherein calcite is used as either one or both of the first and second polarization routing elements, and In 3, the first or second
The polarization independent wavelength tunable filter according to claim 1 or 2, wherein a unit in which a polarization beam splitter and a mirror are combined is used as either one or both of the polarization routing elements.

According to a fourth aspect of the present invention, the polarization independent wavelength tunable filter according to any one of the first to third aspects, wherein the twisted nematic liquid crystal cell having a plurality of electrodes is used as the retardation plate, and the fifth aspect is also provided. Then, a ferroelectric liquid crystal cell having a plurality of electrodes is used as the retardation plate.
A polarization independent wavelength tunable filter according to any of the above,
The polarization independent wavelength tunable filter according to any one of claims 1 to 3, wherein a plurality of ferroelectric liquid crystal cells having a plurality of electrodes are stacked and used as the retardation plate. Then, the polarization independent wavelength tunable filter according to any one of claims 4 to 6 is proposed in which spatial phase characteristics can be arbitrarily changed by applying different voltages to a plurality of electrodes.

In the eighth aspect, the phase difference plate is made of quartz, uniaxial crystal, or optical rotatory crystal.
A polarization independent wavelength tunable filter according to any one of claims 1 to 3, which uses a unit in which a transparent medium having a refractive index close to that of a two-wave plate and the half-wave plate is used.

In the ninth aspect, a plurality of polarization independent wavelength tunable filters according to any one of the first to eighth aspects are arranged in an array, and the transmission wavelength of each of them can be controlled independently. We propose a polarization independent wavelength tunable filter.

[0010]

[Function] Since a Fabry-Perot resonator type tunable liquid crystal filter using homeotropically aligned nematic liquid crystal has a polarization dependence in principle, a polarizer has heretofore been used to polarize the liquid crystal molecules in the alignment direction of the liquid crystal molecule. Only the component was passed and used, and the remaining polarized component was lost. Further, the incident light of the liquid crystal filter is polarized light in a certain direction, and in the case where the polarization direction changes with time, the intensity of light passing through the polarizer changes with time,
It was difficult to obtain a constant intensity optical filter output.

According to the first aspect of the present invention, the polarization component, which has conventionally been acting as a loss, is also spatially separated from the optical path, and the polarization direction is aligned by the retardation plate so that the Fabry-Perot resonator type tunable liquid crystal filter. Since it is configured to pass through the inside, there is no loss, and a low loss, polarization independent wavelength tunable filter can be configured.

Further, according to the polarization routing element of claim 2, it can be made compact by using calcite, and according to the polarization routing element of claim 3, the cost can be reduced by using the polarization beam splitter. Can be configured.

According to the fourth aspect of the present invention, the retardation plate has a plurality of electrodes, and the electrode intervals are patterned at a certain pitch. Therefore, if the respective electrode voltages are controlled, at least the electrodes are controlled. It is possible to easily realize a retardation plate in which spatially arranged half-wave plates having a double pitch of the patterned electrode pitch are provided, and, for example, ordinary light that is spatially separated and incident at a pitch that matches the electrode pitch of the retardation plate. The polarization directions of the component and the extraordinary light component can be easily aligned in one direction. Therefore, it is possible to inject polarized light in the optimum polarization direction into the Fabry-Perot resonator type tunable liquid crystal filter and restore the ordinary and extraordinary light components to the original polarization state from the liquid crystal filter. is there.
Further, since it is easy to realize that the electrode pitch of the retardation plate is about 10 μm, the wavelength tunable filter of the present invention can spatially multiplex 1000 channels per 1 cm, and the size thereof can be significantly reduced. Can be converted. Further, the phase difference plate can arbitrarily change the spatial phase characteristic by changing the patterned electrode voltage, and can realize excellent phase characteristic for light in a wide wavelength range. further,
Since liquid crystal is used, power consumption is low, yield is high,
It has a feature that it can be constructed at low cost.

According to the retardation plate of claim 8, 1 /
Since the two-wave plate includes a natural crystal, the structure can be simplified.

According to the present invention, the polarization independent wavelength tunable filter is arranged in an array, so that the optical demultiplexer having a large number of channels can be realized in a small size.

[0016]

FIG. 1 shows the first embodiment of the present invention.
In the figure, 10A and 10B are retardation plates, 20 is a Fabry-Perot resonator type tunable liquid crystal filter, and 31 and 32 are polarization routing elements, here calcite.

Incident light 33 is an ordinary light component 3 due to calcite 31.
4a and the extraordinary light component 34b. The separated ordinary light component 34a and extraordinary light component 34b are incident on the phase difference plate 10A, undergo different phase changes, respectively, and immediately before entering the Fabry-Perot resonator type wavelength tunable liquid crystal filter 20, the polarization directions of both are changed. The Fabry-Perot resonator type tunable liquid crystal filter 20 is aligned in a direction that coincides with the alignment direction of liquid crystal molecules.

Here, the phase difference plate 1 for giving the phase change
OA has the same structure as the twisted nematic liquid crystal cell,
It is a substrate in which a common electrode 12 and an alignment film 13 made of transparent electrodes are deposited on a glass substrate 11, and a patterned transparent electrode 15 and an alignment film 16 to be drive electrodes are deposited on another glass substrate 14. And are subjected to alignment treatment so that the liquid crystal alignment directions at the substrate interface are orthogonal to each other, and they are opposed to each other at a certain fixed interval.
(Note that 18 represents the alignment state of the liquid crystal molecules.). In addition, antireflection coating 19 is applied to both sides of the cell of the retardation plate 10A,
The loss due to reflection of transmitted light is reduced.

By applying an AC or DC voltage between the drive electrode 15 and the common electrode 12 of the retardation plate 10A, it is possible to change the alignment state of the liquid crystal molecules sandwiched between the electrodes. In the retardation plate, regions having different optical anisotropies can be spatially separated by the number of patterned electrodes. In the phase difference plate 10A, the liquid crystal in the region to which the voltage is not applied is in the twist alignment state and functions as a ½ wavelength plate that rotates the polarization direction of the incident polarized light by 90 degrees, and the liquid crystal in the region to which the voltage is applied is homeotropic. It becomes an oriented state, its optical anisotropy does not appear for incident polarized light, and it functions as a transparent medium. Further, by controlling the voltage between the drive electrode 15 and the common electrode 12, the phase characteristics of the liquid crystal sandwiched between the electrodes can be easily and arbitrarily controlled, and the incident light in a wide wavelength range can be controlled. It can function as an optimum retardation plate.

The ordinary light component 35a and the extraordinary light component 35b whose polarization directions are aligned by the retardation plate 10A are incident on the Fabry-Perot resonator type tunable liquid crystal filter 20.

The Fabry-Perot resonator type tunable liquid crystal filter 20 has a transparent electrode 2 on a glass substrate 21.
2. Two substrates on which the semitransparent mirror 23 and the alignment film 24 are deposited, and the alignment film 24 is aligned so that the liquid crystal molecules at the interface of the substrates are parallel to each other. And a nematic liquid crystal 25 having a refractive index anisotropy (26 indicates the alignment state of the liquid crystal molecules) is enclosed therein. Further, the Fabry-Perot resonator type wavelength tunable liquid crystal filter 20 is provided with antireflection coatings 27 on both sides to reduce loss due to reflection of transmitted light. In the Fabry-Perot resonator type tunable liquid crystal filter 20, the transparent electrodes 22,
By applying a voltage across 22, the molecular alignment of the liquid crystal is changed from the homogeneous alignment state to the homeotropic alignment state, the refractive index of the liquid crystal medium is changed, and the resonant wavelength to be transmitted is variable.

The ordinary light component 36a and the extraordinary light component 36 transmitted through the Fabry-Perot resonator type tunable liquid crystal filter 20.
Since b has the same polarization direction, the phase difference plate 10B restores the polarization state before entering the phase difference plate 10A. The retardation plate 10B has the same structure as the retardation plate 10A, and its spatial phase characteristic is also the same as that of the retardation plate 10A.

The ordinary light component 37a and the extraordinary light component 37b whose polarization state is restored to the original state by the phase difference plate 10B are combined by the calcite 32 to become the emitted light 38.

FIG. 3 shows a second embodiment of the present invention. Here, instead of the retardation plate 10B in the first embodiment, its spatial phase characteristic is opposite to that of the retardation plate 10B. An example in which the retardation plate 10C configured as described above is arranged is shown. That is, according to this configuration, the ordinary light component 37a 'and the extraordinary light component 37b' that have passed through the phase difference plate 10C have polarization directions different by 90 degrees from those in the first embodiment, and thus pass through the calcite 32. The optical path at that time is opposite to that at the time of passing through the retardation plate 10A, and the optical path length of the ordinary light component and the optical path length of the extraordinary light component between the incident light 33 and the outgoing light 39 become equal to each other. The phase difference due to the optical path difference can be reduced, and good filter characteristics can be realized even for incident light modulated at high speed.

FIG. 4 shows a third embodiment of the present invention. Here, an example using a retardation plate having another structure in the second embodiment is shown. That is, in the figure, 40A and 40B are phase difference plates, which are a half-wave plate 41 made of quartz and
Glass plate 4 having a refractive index close to that of the two-wave plate 41
2 are adhered to each other, and antireflection coating 43 is applied to both surfaces thereof. Although the spatial phase characteristics of the phase difference plates 40A and 40B are fixed, the phase difference plates 40A and 40B can be satisfactorily operated in a region where the wavelength variable range of transmitted light is small. In addition to quartz, a material such as uniaxial crystal represented by mica or calcite, optical rotatory crystal, or the like can be used as the half-wave plate 41.

Also, in the first, second and third embodiments,
The incident position of the incident light 33, the emitting position of the emitted light 38 or 39, the phase difference plate 10A or 40A, and the phase difference plate 10
B or 10C or 40B must be aligned with each other.
Positioning can be realized with an accuracy of m. Furthermore, the first,
In the second and third embodiments, the ordinary light component and the extraordinary light component pass through the spatially separated optical paths, but since the optical paths can be made close to each other, the phase difference plates 10A, 10B, 1
It is possible to minimize variations in optical characteristics such as 0C cell thickness and liquid crystal molecule alignment unevenness, and to realize a polarization independent wavelength tunable filter that operates stably.

FIG. 5 shows another example of the structure of the retardation plate, here an example realized by using a surface alignment type ferroelectric liquid crystal cell. FIG. 5A is its front view and FIG. ) Shows the side view. That is, the present retardation plate is a substrate in which a common electrode 52 and an alignment film 53 made of transparent electrodes are deposited on a glass substrate 51, and a patterned transparent electrode 55 and an orientation which is a drive electrode on another glass substrate 54. The substrate on which the film 56 is deposited is aligned so that the liquid crystal alignment direction at the substrate interface becomes parallel, and the substrates are opposed to each other at a certain interval, and the ferroelectric liquid crystal 57 (58a and 58b are It represents the alignment state of the liquid crystal molecules) and is enclosed. Further, antireflection coating 59 is applied to both surfaces of the cell of the present retardation plate to reduce the loss due to reflection of transmitted light.

The drive electrode 55 and the common electrode 5 of this phase difference plate
By applying a positive or negative pulse voltage between the two electrodes, the alignment state of the liquid crystal molecules sandwiched between the electrodes can be changed, and in one retardation plate, regions with different optical anisotropy can be formed. It can be spatially separated. In this retardation plate, the liquid crystal in the region to which the positive pulse voltage is applied is indicated by reference numeral 58b.
The orientation becomes as shown in, and the polarization direction of the incident polarized light is changed to 9
It functions as a half-wave plate rotated by 0 degree, and the liquid crystal in the region to which a negative pulse voltage is applied has an alignment state as shown by reference numeral 58a, and its optical anisotropy does not appear for incident polarized light. , Functions as a transparent medium.

In this embodiment, one ferroelectric liquid crystal cell as a retardation plate is shown as an example. However, a plurality of retardation plates are stacked according to the wavelength of the transmitted light of the optical filter. Can also be configured.

FIG. 6 shows calcite 31, in the first embodiment.
32 shows another configuration example of 32, and here an example configured by using polarization beam splitters 61 and 62 and mirrors 63 and 64 is shown. Incident light 65 is polarized beam splitter 61
Is separated into an ordinary light component 66a and an extraordinary light component 66b, and the extraordinary light component 66b is reflected by the mirror 63,
It goes through an optical path parallel to a. Further, the extraordinary light component 66b is reflected by the mirror 64, is combined with the ordinary light component 66a by the polarization beam splitter 62, and becomes the emitted light 67.

FIG. 7 shows another example of the structure of the calcite 31, 32 in the second or third embodiment. Here, an example of the structure using the polarization beam splitters 71, 72 and the mirrors 73, 74 is shown. .. The incident light 75 is separated by the polarization beam splitter 71 into an ordinary light component 76a and an extraordinary light component 76b, the extraordinary light component 76b is reflected by a mirror 73, and passes through an optical path parallel to the ordinary light component 76a. Phase difference plate 10C or 4
0B (however, not shown in FIG. 7) causes the polarization direction to be 9
The ordinary light component 77a rotated by 0 degrees is reflected by the mirror 74, is combined with the extraordinary light component 77b by the polarization beam splitter 72, and becomes the emitted light 78.

When the polarization routing element shown in FIG. 6 or 7 is used, the polarization beam splitters 61 and 62 are used.
Or 71, 72 and mirrors 63, 64 or 73, 74
, The incident position of the incident light 65 or 75, the emitting position of the emitted light 67 or 78, the retardation plate 10A or 40A and the retardation plate 10B or 10C or 40B (however, not shown in FIG. 6 or FIG. 7). However, by providing alignment marks on all of them, alignment can be realized with an accuracy of about several μm.

FIG. 8 shows a fourth embodiment of the present invention. In this example, the polarization independent wavelength tunable filter shown in the first embodiment is arranged in parallel with 4 (2 × 2) channels. Indicates. That is, this wavelength tunable filter is composed of calcite 81, 82.
And retardation plate modules 83 and 84 using liquid crystal, Fabry-Perot resonator type tunable liquid crystal filter module 85, and optical fiber array modules 86 and 87 composed of an optical fiber array and a collimator lens. ..

The light incident from the optical fiber array module 86 is separated by the calcite 81 into an ordinary light component 88a and an extraordinary light component 88b. The phase characteristic of the phase difference plate module 83 can be controlled by controlling the applied voltage to the drive electrodes 831 and 832 provided for each channel, and the ordinary light component 88a
And the polarization direction of the extraordinary light component 88b is aligned before entering the Fabry-Perot resonator type tunable liquid crystal filter module 85. In the Fabry-Perot resonator type wavelength tunable liquid crystal filter module 85, each channel is independently controlled by changing the voltage of the wavelength control electrode 851 provided for each channel. The ordinary light component 89a and the extraordinary light component 89b emitted from the Fabry-Perot resonator type tunable liquid crystal filter module 85 are returned to the original polarization by the retardation plate module 84 as in the case of the first embodiment, and the calcite 82 And is emitted from the optical fiber array module 87.

FIG. 9 shows a fifth embodiment of the present invention. Here, an example in which the polarization independent wavelength tunable filter shown in the second embodiment is configured in 4 (2 × 2) channels in parallel is shown. Indicates. That is, in the figure, 90 is a retardation plate module using liquid crystal, and its phase characteristic is configured to be opposite to that of the retardation plate module 84 in the fourth embodiment.
The control is performed by controlling the voltage applied to the drive electrodes 901 and 902 provided for each channel. According to this configuration, the ordinary light component 8 that has passed through the retardation plate module 90.
9a ′ and the extraordinary light component 89b ′ have the fourth polarization direction.
This is different from the case of the example of 90 degrees, and as a result, calcite 8
The optical path when passing through 2 is opposite to when passing through the retardation plate 83, and the optical path length of the ordinary light component and the optical path length of the extraordinary light component between the incident light and the outgoing light become equal to each other, and The phase difference due to the optical path difference between the components can be reduced, and good filter characteristics can be realized even for incident light modulated at high speed. The other structure and operation are similar to those of the fourth embodiment.

In the above-mentioned fourth and fifth embodiments,
Retardation plate modules 83, 84, 90 using liquid crystal,
In the Fabry-Perot resonator type wavelength conversion liquid crystal filter module 85, since each control electrode can be manufactured by using the photolithography technique, a large number of electrodes can be realized in one cell at the same time, which is suitable for multiple channels. Further, since these can be realized with a thickness of several mm, they can be easily laminated, and a multichannel wavelength tunable filter can be configured in a small size.

In the fourth and fifth embodiments, a configuration example of 2 × 2 channels is shown. However, a polarization independent wavelength tunable filter of M × N channels (M and N are natural numbers) with the same configuration is shown. It goes without saying that can be configured.

[0038]

As described above, according to the first aspect of the present invention, the Fabry-Perot resonator type wavelength tunable liquid crystal filter in which the spatially separated ordinary and extraordinary light components are aligned in the polarization direction by the phase difference plate. Since it is configured to pass through the inside, it is not necessary to separate the Fabry-Perot resonator type tunable liquid crystal filter according to the ordinary light component and the extraordinary light component as in the conventional structure, and therefore, the miniaturization is facilitated and The yield can be increased.

Further, according to claim 2 of the present invention, by using calcite, the polarization routing element can be made compact, and according to claim 3, by using the polarization beam splitter, the cost can be reduced. Can be configured.

According to claims 4 to 7 of the present invention,
By using a retardation plate with a liquid crystal structure that can be microfabricated, its size can be significantly reduced.
Power consumption can also be reduced.

According to claim 8 of the present invention,
The structure can be simplified by using a retardation plate containing a natural crystal as the wave plate.

According to the ninth aspect of the present invention, by arraying the polarization independent wavelength tunable filter in an array, an optical demultiplexer having a large number of channels can be realized in a small size.

[Brief description of drawings]

FIG. 1 is a first polarization-independent wavelength tunable filter of the present invention.
Configuration diagram showing an example of

FIG. 2 is a configuration diagram showing an example of a conventional polarization independent wavelength tunable filter.

FIG. 3 is a second polarization-independent wavelength tunable filter according to the present invention.
Configuration diagram showing an example of

FIG. 4 is a third polarization-independent wavelength tunable filter of the present invention.
Configuration diagram showing an example of

FIG. 5 is a diagram showing another configuration example of the retardation plate of the present invention.

FIG. 6 is a diagram showing another configuration example of calcite in the first embodiment of the present invention.

FIG. 7 is a diagram showing another structural example of calcite in the second or third embodiment of the present invention.

FIG. 8 is a fourth polarization-independent wavelength tunable filter according to the present invention.
Configuration diagram showing an example of

FIG. 9 is a fifth polarization-independent wavelength tunable filter of the present invention.
Configuration diagram showing an example of

[Explanation of symbols]

10A, 10B, 10C, 40A, 40B ... Retardation plate,
20 ... Fabry-Perot resonator type tunable liquid crystal filter,
31, 32, 81, 82 ... Calcite, 83, 84, 90 ...
Retardation plate module, 85 ... Fabry-Perot resonator type tunable liquid crystal filter module.

Claims (9)

[Claims] 1. A cell is constituted by facing two transparent substrates provided with a transparent electrode, a semitransparent mirror and an alignment film with each other with a certain space therebetween, and a liquid crystal is contained in the cell. The orientation of the liquid crystal molecules is changed by applying a voltage to the transparent electrode so that the orientation of molecules at the interface of the transparent substrate becomes parallel to the substrate and the molecules are oriented in the same direction. In the polarization independent wavelength tunable filter provided with a Fabry-Perot resonator type tunable liquid crystal filter in which the resonant wavelength of transmitted light is tunable, the Fabry-Perot resonator type tunable liquid crystal filter,
Two polarization routing elements that separate or combine light into an ordinary light component and an extraordinary light component, and two phase difference plates that spatially separate a region that functions as a ½ wavelength plate and a region that does not rotate the polarization direction. The first polarization routing element separates the incident light into an ordinary light component and an extraordinary light component, and one of the ordinary light component and the extraordinary light component is used as a half-wave plate of the first retardation plate. Pass the area that functions, and pass the other through the area that does not rotate the polarization direction of the first retardation plate to align the polarization direction,
After passing through the Fabry-Perot resonator type tunable liquid crystal filter, either one of the ordinary light component and the extraordinary light component is passed through a region functioning as a half-wave plate of the second retardation plate, and the other is passed. The polarization direction of the second retardation plate is passed through a region that does not rotate, and the polarization direction is set to the state before the incidence of the first retardation plate or the state rotated by 90 degrees from each other, and both components are added to the second phase. A polarization independent wavelength tunable filter characterized in that a polarization routing element combines the light beams into one light beam. 2. The polarization independent wavelength tunable filter according to claim 1, wherein calcite is used as either or both of the first and second polarization routing elements. 3. A polarization non-polarizing unit according to claim 1, wherein a unit in which a polarization beam splitter and a mirror are combined is used as either one or both of the first and second polarization routing elements. Dependent wavelength tunable filter. 4. The polarization independent wavelength tunable filter according to claim 1, wherein a twisted nematic liquid crystal cell having a plurality of electrodes is used as the retardation plate. 5. The polarization independent wavelength tunable filter according to claim 1, wherein a ferroelectric liquid crystal cell having a plurality of electrodes is used as the retardation plate. 6. The polarization independent wavelength tunable filter according to claim 1, wherein a plurality of ferroelectric liquid crystal cells having a plurality of electrodes are stacked and used as the phase difference plate. 7. The polarization independent type according to claim 4, wherein the spatial phase characteristics can be arbitrarily changed by applying different voltages to the plurality of electrodes. Wavelength tunable filter. 8. A half-wave plate made of quartz, a uniaxial crystal, or an optical rotatory crystal as the retardation plate
The polarization independent wavelength tunable filter according to any one of claims 1 to 3, wherein a unit in which a transparent medium having a refractive index close to that of a two-wave plate is combined is used. 9. A plurality of polarization independent wavelength tunable filters according to any one of claims 1 to 8 are arranged in an array, and the transmission wavelength of each of them is independently controllable. Polarization independent tunable filter.