JPH085977A - Variable wavelength liquid crystal optical filter - Google Patents

Abstract

PURPOSE:To obtain a variable wavelength liquid crystal optical filter which has no dependency on polarization even if a liquid crystal cell having versatile accuracy is used by transmitting two light beams separated by a polarization splitter from opposite directions in the same position of the liquid crystal cell. CONSTITUTION:A light beam A is separated to an S1 component and a P2 component by the polarization splitter 91. The separated Si component is rotated +pi/4 by passing a magneto-optical element 101 and is further rotated +pi/4 by passing an azimuth rotator 201 and is converted to the orthogonal polarized waves, by which the P1 component is obtd. The progressing direction of this P1 component is directed to the liquid crystal cell 31 by a rectangular prism. The P1 component is subjected to wavelength selection by transmitting the liquid crystal cell 31 and is made into P1' component. This component passes the right-angle prism 302 and enters the azimuth rotator 202, by which the component is rotated -pi/4 and is then admitted into the magneto-optical element 102. Since the transmission of the magneto-optical element 102 is the transmission in an opposite direction, the component is rotated -pi/ 2 and is again converted to the orthogonal polarized waves, by which the S1' component is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable wavelength liquid crystal optical filter using a liquid crystal for passing only an optical signal of an arbitrary wavelength among wavelength multiplexed optical signals.

[0002]

2. Description of the Related Art FIG. 1 is a plan view showing the basic structure of a conventional optical filter of this type. Reference numerals 21 and 22 in the figure
Is a pair of birefringent plates which are arranged at a position orthogonal to the optical axis and facing each other at a constant interval.
A part of the facing surface of each of the birefringent plates 21 and 22 has 1
The half wave plates 11 and 12 are attached. A liquid crystal cell 31 having a Fabry-Perot etalon structure is arranged in the space between the birefringent plates 21 and 22 perpendicularly to the optical axis.

FIG. 2 is a schematic sectional view showing the structure of the liquid crystal cell 31. As shown in FIG. 2, the liquid crystal cell 31 is
A pair of transparent substrates 41 and 42 facing each other with a constant gap, ITO (indium tin oxide compound) films 51 and 52 as transparent electrodes formed on the respective facing surfaces of the transparent substrates 41 and 42, and these. IT
Mirror films 61 and 62 made of a dielectric material provided on the O films 51 and 52, and the mirror films 61 and 62
It is roughly composed of liquid crystal alignment films 71 and 72 provided above and liquid crystal 81 filled in a gap between the alignment films 71 and 72. Alignment films 71 and 72
Are formed so that their respective orientation directions are parallel to each other, and the liquid crystal 81 is filled in a homogeneous arrangement so as to be in an electric field control birefringence effect (ECB) mode. Also,
A power supply device 40 for applying a predetermined voltage is connected between the ITO films 51 and 52.

When the light beam passes through the nematic liquid crystal cell 31 having the above structure, the ITO films 51 and 5 are formed.
Since the refractive index of the liquid crystal 81 changes according to the magnitude of the voltage applied to 2, the resonance wavelength changes, and only light of a specific wavelength is selected and transmitted from the light beam. Therefore, the liquid crystal cell 31 functions as a variable optical filter.

However, in reality, due to the birefringence of the liquid crystal, the resonance wavelength greatly differs depending on the polarization state of the transmitted light beam even at the same voltage.

The optical filter shown in FIG. 1 has a liquid crystal cell 31.
The purpose is to eliminate the polarization dependence of. In this optical filter, the light beam A incident on the birefringent plate 21 is divided into a polarization component P that goes straight and a polarization component S that refracts. The polarization component P enters the liquid crystal cell 31 as it is,
The polarization component S passes through the half-wave plate 11 and is changed to P polarization before entering the liquid crystal cell 31, and all the light beams A are transmitted through the liquid crystal cell 31 with the same polarization and are wavelength-selected. The light beam is emitted as one light beam B by passing through the half-wave plate 12 and the birefringent plate 22.

With such a structure, the light beams passing through the liquid crystal cell 31 have the same polarization, so that polarization dependency does not occur. However, since the positions in the liquid crystal cell 31 through which the two light beams are transmitted are different, if the gaps of the liquid crystal cells 31 at the respective transmission positions do not exactly match, the selected wavelengths will differ, and the input will occur. There was a problem that the loss varied depending on the polarization state of light. As a result, problems such as precise control of the uniformity of the gap of the liquid crystal cell used for the optical filter occur in cell fabrication, and adjustment during assembly is technically difficult.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a variable wavelength liquid crystal optical filter having no polarization dependence even when using a liquid crystal cell manufactured with general precision in which the liquid crystal cell gap is not strictly controlled. Especially.

[0009]

In order to achieve the above object, the invention according to claim 1 is a variable wavelength liquid crystal optical filter, wherein incident light is incident on a first light beam having a different polarization state. A polarization splitter that splits the light beam into two light beams, a first magneto-optical element that transmits the first light beam split by the polarization splitter, and a first magneto-optical element that transmits the first magneto-optical element. A first optical rotator that transmits one light beam;
A first optical component that changes the traveling direction of the first light beam that has passed through the first optical rotator and the first light beam that changes the traveling direction by the first optical component are incident. Fabry-Perot etalon structure liquid crystal cell, second magneto-optical element transmitting the second light beam separated by the polarization splitter, and second light beam transmitting the second magneto-optical element Of the second optical rotator that transmits the second optical rotator and the second light beam that transmits the second optical rotator and has the same polarization state as the first light beam. The second light beam is incident on the liquid crystal cell at the same position as the first light beam in a direction opposite to the first light beam.
And a second optical component that changes the traveling direction of the light beam.

That is, according to the present invention, the two light beams divided by the polarization splitter are made to have the same polarization state by transmitting the magneto-optical element and the optical rotator, respectively, and then the two light beams are mutually made by the right-angle prism. The most main feature is that light is made incident on the same position of the liquid crystal cell from opposite directions. In the conventional technique, two light beams are incident on the liquid crystal cell from the same direction and are incident on different positions of the liquid crystal cell. However, in the present invention, the two light beams after polarization separation are incident on the liquid crystal cell. When entering, the liquid crystal cells are different in that they enter the same position instead of different positions and from opposite directions.

[0011]

In the present invention, the two light beams separated by the polarization splitter are transmitted to the same position of the liquid crystal cell from the opposite directions in the same polarization state.
The exact same wavelength selection can be performed for each light beam. Therefore, even if a general-purpose liquid crystal cell in which the parallel accuracy of the gap is not strictly controlled is used, the spread of the emitted light beam can be suppressed and the separation can be eliminated.

Further, since the two beams separated by the polarization splitter pass through the same path although the directions are opposite to each other, the same beam passes through the same optical element once, respectively.
It has a function of compensating for the phase shift of the light beam caused by the variation in the characteristics of individual optical components.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 3 is a plan view for explaining the basic principle of the variable wavelength liquid crystal optical filter of the present invention. Reference numeral 9 in the figure
1 is a polarization splitter, 101 and 102 are + π
/ 4 rotation magneto-optical element, 201 is + π / 4 rotation optical rotator, 202 is −π / 4 rotation optical rotator, 301 and 302 are right-angle prisms, 31 is FIGS. The liquid crystal cell has the same structure as the liquid crystal cell shown in FIG. These respective constituent elements are arranged so as to be orthogonal to the optical axes shown by the dotted lines in FIG.

In the variable wavelength liquid crystal optical filter having such a structure, when a light beam composed of orthogonal polarization components S1 and P2 is incident from the collimator lens 501, the polarization splitter 91 first converts the light beam into the S1 component. It is separated into P2 component. The separated S1 component is the magneto-optical element 10
Since the light is transmitted by 1 to be rotated by + π / 4, and further transmitted by the optical rotator 201, is further rotated by + π / 4. Therefore, a total of + π / 2 rotation is obtained, and the polarized light is converted into orthogonal polarized waves and becomes P1. The traveling direction of the P1 component is directed to the liquid crystal cell 31 by the rectangular prism 301. The P1 component is wavelength-selected by passing through the liquid crystal cell 31 to become the P1 ′ component, and the right angle prism 3
After passing through 02, it enters the optical rotator 202, is rotated by −π / 4, and then enters the magneto-optical element 102. Since the transmission of the magneto-optical element 102 is the transmission in the opposite direction, it is rotated by -π / 4, and as a result, rotated by -π / 2, is converted again into the orthogonal polarized waves and becomes the S1 ′ component. , Return to the polarization splitter 91 and collimate lens 50
The light is emitted from the optical fiber 401 via 2

On the other hand, P separated by the polarization splitter 91
The two components pass through the magneto-optical element 102, and thus + π
Although it is rotated by / 4 rotation, the original polarization state is obtained because it is rotated by -π / 4 by the optical rotator 202. The P2 component is directed by the right-angle prism 302 in its traveling direction. P
The two components are wavelength-selected by the liquid crystal cell 31 to become P2 ′ components, which pass through the rectangular prism 301 and enter the optical rotator 201. This optical rotator 201 causes the light beam of the P2 ′ component to be +
It is rotated by π / 4, and then enters the magneto-optical element 101, but in this case as well, it is rotated by -π / 4 due to the transmission in the opposite direction, and the polarization splitter 91 is in the original polarization state P2 ′.
To the optical fiber 40 through the collimating lens 502.
It is emitted from 2.

As described above, since the two light beams passing through the liquid crystal cell 31 pass through the same position of the liquid crystal cell 31 in the same polarization state, even if a voltage is applied to the liquid crystal cell 31 by the power supply device 40. The exact same wavelength selection is performed for each light beam. Therefore, as the liquid crystal cell 31, it is possible to use a liquid crystal cell having a parallel precision of a general-purpose display liquid crystal cell gap, and even if such a liquid crystal cell is used,
The spread of the outgoing light beam, which is the object of the present invention, can be suppressed and the separation can be eliminated.

In the arrangement of FIG. 3, the two beams separated by the polarization splitter will pass through the same optical element once through the same path but opposite directions. Therefore, it has a function of compensating for the phase shift of the light beam caused by the variation in the characteristics of the individual optical components.

(Example of Embodiment) FIG. 4 is a schematic perspective view for explaining an example of the embodiment of the variable wavelength liquid crystal optical filter of the present invention. In the figure, reference numeral 401 is a light beam incident fiber,
Reference numeral 402 denotes a light beam emitting fiber, and 501 and 502 denote collimating lenses. In FIG. 4, elements having the same configurations as the elements of the filter shown in FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted. The polarization splitter 91, the magneto-optical elements 101 and 102, the optical rotators 201 and 202, and the right-angle prisms 301 and 302 of this example are arranged so as to be orthogonal to the incoming and outgoing light beams, and the liquid crystal cell 31 receives the reflected light. It is placed slightly tilted (about 1 degree) so as not to interfere. The liquid crystal cell 31 is a liquid crystal cell having the same configuration as the liquid crystal cell shown in FIG.

In the variable wavelength liquid crystal optical filter having such a structure, when light in which optical signals of various wavelengths are mixed is introduced into the incident fiber 401, the collimator lens 501.
To form a parallel light beam, which is split into two polarization components by the polarization splitter 91, one polarization component being the magneto-optical element 1
01, the optical rotator 102, and the right-angled prism 301, then the wavelength is selected by the transmission of the liquid crystal cell 31, and then the right-angled prism 302, the optical rotator 202, and the magneto-optical element 102, and then returns to the polarization splitter 91. , Enters the collimator lens 502 and is emitted from the emission fiber 402. The other polarization component split by the polarization splitter 91 passes through the magneto-optical element 102, the optical rotator 202, and the right-angle prism 302, and is then wavelength-selected by the transmission of the liquid crystal cell 31, and then the right-angle prism 301 and the rotator. 20
1. Polarization splitter 91 passing through magneto-optical element 101
After entering the optical fiber, the other polarization component of which the wavelength is selected merges and enters the collimator lens 502 and the output fiber 402.
Is emitted from. The optical signal of the required wavelength is selected by the voltage applied to the liquid crystal cell 31.

With this configuration, wavelengths of 1520 to 1580 nm
When the filter characteristics were examined by using a liquid crystal cell having a half-width of selected wavelength of 0.3 nm, the insertion loss was 3 dB and the polarization dependence was 0.1 dB or less.

[0022]

As described above, according to the variable wavelength liquid crystal optical filter of the present invention, in the signal separation of the wavelength division multiplexed optical signal, the wavelength of a sharp single light beam is maintained without impairing the polarization independence. The selection can be made by using an inexpensive liquid crystal cell with general precision.

[Brief description of drawings]

FIG. 1 is a plan view showing a basic configuration of a conventional optical filter.

FIG. 2 is a cross-sectional view showing a configuration of a liquid crystal cell having a Fabry-Perot etalon structure.

FIG. 3 is a plan view showing an embodiment of a variable wavelength liquid crystal optical filter of the present invention.

FIG. 4 is a perspective view for explaining another embodiment of the variable wavelength liquid crystal optical filter of the present invention.

[Explanation of symbols]

 11, 12 1/2 wavelength plate 21, 22 birefringent plate 31 liquid crystal cell 40 power supply device 41, 42 transparent substrate 51, 52 transparent electrode 61, 62 mirror film 71, 72 alignment film 81 liquid crystal 91 polarization splitter 101, 102 magnetic Optical element 201,202 Optical rotator 301,302 Right-angle prism 401,402 Optical fiber 501,502 Collimating lens

Claims (1)

[Claims] 1. A polarization splitter for splitting incident light into a first light beam and a second light beam having different polarization states,
A first magneto-optical element that transmits the first light beam separated by the polarization splitter; and a first optical rotator that transmits the first light beam that has passed through the first magneto-optical element. , A first optical component that changes the traveling direction of the first light beam that has passed through the first optical rotator, and the first light beam that has changed the traveling direction by the first optical component are incident. A liquid crystal cell having a Fabry-Perot etalon structure, a second magneto-optical element that transmits the second light beam separated by the polarization splitter, and the second light that transmits the second magneto-optical element. A second optical rotator that transmits a beam and a second optical beam that transmits the second optical rotator and has the same polarization state as the first optical beam From the opposite direction to the first light beam at the same position as the incident position As is incident on serial liquid crystal cell, a variable wavelength liquid crystal optical filter, characterized in that it comprises a second optical component for changing the traveling direction of the second light beam.