JPH085976A - Variable wavelength optical filter - Google Patents

Abstract

PURPOSE:To provide a variable wavelength optical filter which is capable of eliminating dependency on polarization by using respectively one piece each of a double refractive plate and a half-wave plate, uses liquid crystal and is inexpensive. CONSTITUTION:A light beam A is separated to a polarized light component P which is made incident on the double refractive plate 23 and advances rectilinearly and a polarized light component S which is refracted. The polarized light component P advances rectilinearly as it is, transmits a liquid crystal cell 32, is turned back by a right-angle prism 91 and is emitted after advancing rectilinearly again in the liquid crystal cell 32 and the double refractive plate 23. On the other hand, the polarized light component S is converted in the polarization state to P at the time of transmitting the half-wave plate 13, transmits the liquid crystal cell 32 in the state of the P polarized light, is turned back by the right-angle prism 91, again transmits the liquid crystal cell 32, is admitted into the half-wave plate 13 to restore the state of the S polarized light and is emitted as a light beam B after joining with the P component turned back rectilinearly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable wavelength 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 configuration, the light beams that pass through the liquid crystal cell 31 have the same polarization, so no polarization dependence occurs, but because of polarization separation, conversion, and multiplexing, There is a drawback that it is necessary to arrange two half-wave plates and two birefringent plates, which are expensive and have uniform characteristics.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to use a birefringent plate and a half-wave plate, one each, and an inexpensive variable wavelength using a liquid crystal capable of eliminating polarization dependence. It is to provide an optical filter.

[0009]

In order to achieve the above object, the invention according to claim 1 is a variable wavelength optical filter, wherein incident light is divided into a first light beam and a second light beam having different polarization states.
One birefringent plate that separates the second light beam, one half-wave plate that transmits the second light beam, and the first half-wave plate that transmits the first light beam and the half-wave plate. The liquid crystal cell having one Fabry-Perot etalon structure on which the two light beams are incident, and the traveling directions of the first light beam and the second light beam that have passed through the liquid crystal cell are both changed by 180 degrees, and again. An optical component arranged to be incident on the liquid crystal cell, the first light beam passing through the liquid crystal cell again through the optical component is incident on the birefringent plate again, and the optical component is After passing through the liquid crystal cell, the second light beam again passes through the half-wave plate, enters the birefringent plate again, and reenters the birefringent plate. And the second light beam is combined and emitted. Sea urchin wherein the one of the birefringent plate wherein one half-wave plate and the liquid crystal cell optical component, characterized in that disposed.

Here, the invention according to claim 2 is the invention according to claim 1.
In the tunable wavelength optical filter described above, the liquid crystal cell may be a nematic liquid crystal cell having a homogeneous arrangement.

That is, according to the present invention, a light beam that has once passed through a birefringent plate, a half-wave plate and a Fabry-Perot etalon structure liquid crystal cell changes its traveling direction by 180 degrees by a right-angle prism as the optical component, Liquid crystal cell and 1
The main feature is that the / 2 wave plate and the birefringent plate are made to be transmitted again. What is conventional technology?
The difference is that the light beam is transmitted through the birefringent plate, the half-wave plate and the liquid crystal cell twice, one each.

[0012]

According to the present invention, since the light beams passing through the liquid crystal cell are in the same polarization state, the resonance wavelength does not have polarization dependency, and the wavelength selection depends on the magnitude of the voltage applied to the liquid crystal cell. It can be done by Therefore, the birefringent plate, 1 /
By using each of the two wave plates and the liquid crystal cell one by one, and by using the right-angle prism, the same operation as that of the optical filter having the conventional configuration can be obtained, and the number of times of transmission of the light beam to the liquid crystal cell is reduced to two. Narrowing can be realized by setting the number of times.

[0013]

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

FIGS. 3A and 3B are views for explaining the basic principle of the tunable wavelength optical filter of the present invention, wherein FIG. 3A is a plan view and FIG. 3B is a side view. Is. Reference numeral 32 in the figure is a nematic liquid crystal cell having a Fabry etalon structure having the same configuration as the liquid crystal cell shown in FIGS. The liquid crystal cell 32 is arranged in the space between one birefringent plate 23 and one right-angle prism 91. The half-wave plate 13 is attached to a part of one surface (the surface on the liquid crystal cell 32 side) of the birefringent plate 23. The right-angle prism 91 is an optical component that changes the traveling direction of the light beam transmitted through the birefringent plate 23 and the liquid crystal cell 32 by 180 degrees and returns it to the same birefringent plate 23 through the same liquid crystal cell 32.

In the tunable wavelength optical filter having such a structure, the light beam A is incident on the birefringent plate 23 and is separated into a polarization component P which goes straight and a polarization component S which is refracted. The polarization component P goes straight through as it is, passes through the liquid crystal cell 32, is folded back by the right-angle prism 91, and goes straight through the liquid crystal cell 32 and the birefringent plate 23 again to be emitted. On the other hand, when the polarized component S is transmitted through the half-wave plate 13, the polarized state is converted into P, transmitted through the liquid crystal cell 32 in the P polarized state, folded back by the right-angle prism 91, and then again the liquid crystal. After passing through the cell 32, it enters the half-wave plate 13 and returns to the S-polarized state,
It merges with the P component that has returned straight ahead and is emitted as a light beam B.

In this example, since the light beams passing through the liquid crystal cell 32 have the same polarization state, the resonance wavelength does not have polarization dependence, and the wavelength selection depends on the voltage applied to the liquid crystal cell 32. It can be done by size. Therefore, in the present embodiment, in addition to using one each of the birefringent plate 23, the half-wave plate 13 and the liquid crystal cell 32, the right angle prism 9 is used.
By using 1, the same effect as that of the optical filter having the conventional configuration shown in FIG. 1 can be obtained, and a narrow band can be achieved by transmitting the light beam to the liquid crystal cell 32 twice. .

(Embodiment Example) FIG. 4 is a schematic perspective view for explaining an embodiment example of the variable wavelength optical filter of the present invention. In the figure, reference numeral 101 is a light beam incident fiber, and 102 is a light beam emitting fiber. 201 and 202 are collimating lenses. The collimating lenses 201 and 202 are fixed on one surface of one birefringent plate 24 by a refractive index matching adhesive having an appropriate refractive index in a direction orthogonal to each other. The birefringent plate 24 is fixed to one surface of the liquid crystal cell 33 via one half-wave plate 14, and the right-angled prism 9 is attached to the other surface of the liquid crystal cell 33.
2 is fixed. Such fixation is done by each component,
That is, the birefringent plate 24, the half-wave plate 14, the liquid crystal cell 33, and the right-angled prism 92 are arranged so as to be orthogonal to the incoming and outgoing light beams, respectively, and then the refractive index matching adhesive having mutual appropriate refractive index. This is achieved by sticking with the agent. In addition, each of the above-mentioned components is mounted on the constant temperature controller 301 and fixed by a heat conductive adhesive. The liquid crystal cell 33 in the present embodiment is a nematic liquid crystal cell of Fabry-Perot etalon structure having the same configuration as the liquid crystal cell shown in FIG. 3 as in the first embodiment.

In the optical filter having such a structure, when light in which optical signals of various wavelengths are mixed is introduced into the incident fiber 101, the collimator lens 201 forms a parallel light beam, and the birefringent plate 24 causes the liquid crystal cell 33 to enter. It is separated into a polarization component that travels straight toward and a polarization component that is refracted and enters the half-wave plate 14. Both polarization components pass through the liquid crystal cell 33 and are then reflected by the right-angle prism 92,
A component that passes through the same liquid crystal cell 33 again and directly enters the birefringent plate 24, and a component that passes through the half-wave plate 14 and then enters the birefringent plate 24 merge into the collimator lens 202 and enter the fiber 102. Is emitted from. The optical signal of the required wavelength can be selected by the voltage applied to the liquid crystal cell 33.

With this configuration, an optical signal of 600 waves with a wavelength of 1520 to 1580 nm and an interval of 0.1 nm is lost by 6 dB,
It was confirmed that the crosstalk was -20 dB or less and the polarization dependence was 0.2 dB or less.

In this embodiment, a liquid crystal cell having a homogeneous arrangement is used, but it goes without saying that the same effect can be obtained by using a liquid crystal cell having a twist arrangement.

[0021]

As described above, according to the tunable wavelength optical filter of the present invention, when performing the signal separation of the wavelength division multiplexed signal, the polarization independence is not impaired, and compared with the conventional device. It is also possible to select wavelengths with excellent characteristics in a narrow band at a low price.

[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.

3A and 3B are diagrams for explaining the basic principle of the variable wavelength optical filter of the present invention, in which FIG. 3A is a plan view and FIG.
Is a side view.

FIG. 4 is a perspective view for explaining an example of an embodiment of a tunable wavelength optical filter of the present invention.

[Explanation of symbols]

 11, 12, 13, 14 1/2 wavelength plate 21, 22, 23 Birefringent plate 31, 32, 33 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 91,92 Right angle prism 101,102 Optical fiber 201,202 Collimating lens 301 Constant temperature controller

Claims (2)

[Claims] 1. A birefringent plate for separating incident light into a first light beam and a second light beam having different polarization states,
A half wave plate through which the second light beam is transmitted;
One Fabry-Perot etalon structure liquid crystal cell on which the first light beam and the second light beam transmitted through the half-wave plate are incident, and the first liquid crystal cell transmitted through the liquid crystal cell
Both the light beam and the second light beam change their traveling directions by 180 degrees and are arranged to enter the liquid crystal cell again, and pass through the liquid crystal cell again through the optical component. The first light beam again enters the birefringent plate, and the second light beam that has passed through the liquid crystal cell again through the optical component again passes through the half-wave plate and again The first light beam and the second light beam that have entered the birefringent plate and then re-entered the birefringent plate are combined with each other so that the first light beam and the second light beam are combined and emitted. A tunable optical filter comprising a two-wave plate, the liquid crystal cell, and the optical component. 2. The variable wavelength optical filter according to claim 1, wherein the liquid crystal cell is a nematic liquid crystal cell having a homogeneous arrangement.