JPH04248515A - Variable wavelength filter - Google Patents

Abstract

PURPOSE:To obtain the variable wavelength filter having no dependency on polarized waves. CONSTITUTION:Incident light is separated to two pieces of polarized beams p, s by a double refractive prism DRP 1. The p polarized light is made incident on a liquid crystal LCb parallel in orientation direction with the electric field vector thereof and the s polarized light is similarly made incident on the liquid crystal layer LCa, where the impressed voltages by AC power sources ASa, ASb are so controlled as to match the resonance wavelengths of the respective cavities to obtain the same resonance wavelength to two pieces of the polarized beams p, s. The p polarized and s polarized past the respective liquid crystal layers LCb, LCa are synthesized by the double refractive prism DRP 1 and are outputted as one piece of the light beam.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable wavelength filter that selectively and variably extracts optical signals of arbitrary wavelengths from wavelength-multiplexed optical signals.

0002

[Background Art] Optical communication using optical fibers has been rapidly put into practical use because it is capable of transmitting large amounts of information at high speed. There are only Ideally, it would be possible to transmit light pulses of many different frequencies.
Furthermore, a large amount of information can be transmitted. This is called frequency division multiplexing (FDM), and is currently being actively researched. This frequency multiplexing communication requires a variable wavelength filter that selectively selects only light of an arbitrary frequency from among optical pulses of a large number of frequencies.

[0003] Hereinafter, a plurality of conventional examples of this type of filter will be explained in order with reference to the drawings.

FIG. 2 is a configuration diagram of a first conventional example, which is a variable wavelength filter module using a conventional variable wavelength filter that changes the resonator wavelength by changing the resonator length of an etalon using a piezo element. It shows. In Figure 2, F
1, F2 are optical fibers, L1, L2 are lenses, M1, M
2 is a mirror, PZ is a piezo element, and two mirrors M
1 and M2 constitute an etalon.

[0005] In this module, by moving mirror M2 in the light traveling direction by piezo element PZ, the resonator length of the etalon consisting of mirrors M1 and M2 is changed, and the resonator wavelength is changed. .

FIG. 3 is a configuration diagram of a second conventional example.
This figure shows a variable wavelength filter module using a conventional variable wavelength filter that changes the resonator wavelength by mechanically rotating an etalon. In the figure, the ETL is an etalon, which is rotated by a rotation mechanism (not shown).

FIG. 4 is a configuration diagram of a third conventional example.
A tunable wavelength filter using a semiconductor optical waveguide with a Bragg reflector is shown. In Figure 4, ARC is an anti-reflection coating film, S is a substrate, and G is a Bragg reflector.
ACT is the active layer, E1 to E
3 is an electrode.

[0008] This wavelength filter is
, Z to control the transmission wavelength band.

[0009]

[Problems to be Solved by the Invention] However, in the first conventional example, a high voltage is required to drive the piezo element PZ, a large drive power source is required, and it is difficult to make the device smaller and lighter. There was a drawback.

[0010] Also, in the case of the second conventional example, since the etalon is mechanically driven, it is difficult to reduce the size and weight of the device.

Furthermore, the third conventional example has the disadvantage that the wavelength sweep width is as small as 5 nm.

Therefore, in order to solve the drawbacks of the mechanical filters shown in the first and second conventional examples and the semiconductor optical waveguide filter shown in the third conventional example, an etalon is filled with liquid crystal and a voltage is applied to it. A variable wavelength filter has been proposed in which the optical gap of an etalon is changed by applying a voltage.

As a fourth conventional example, a variable wavelength filter using an etalon filled with liquid crystal is shown in FIG.
The principle of operation will be explained with reference to FIGS. 6 and 7.

FIG. 5 is a block diagram of a tunable wavelength filter having an etalon filled with liquid crystal. In Figure 5, GS
1, GS2 is a glass substrate, ARC1, ARC2 are anti-reflection coating films, TEL1, TEL2 are transparent electrodes, M1, M2
is a dielectric mirror, ORL1 and ORL2 are liquid crystal alignment films, and L
C is liquid crystal, SP1 and SP2 are spacers, EW1 and EW2
is a lead wire for applying voltage to the liquid crystal. mirror
The glass substrates GS1 and GS2 on which M1 and M2 have been deposited (non-reflective coatings ARC1 and ARC2 are applied to the back surfaces) are separated by a distance L.
When arranged in parallel, the wavelength dependence of transmittance is expressed by the following equation.

T=1/{1+F si
n2 (2πnL/λ)} …(1)

F=4r/(1-r)2
...(2) Here, n is the refractive index of the etalon cavity,
λ is the wavelength.

Further, FIG. 6 shows a transmission spectrum, and as shown in FIG. 6, many sharp peaks appear in the variable wavelength filter of FIG. The half-width of this peak is the mirror
It depends on the reflectance of M1 and M2, and in the case of a mirror with a normal reflectance of 99%, it will have a sharp peak with a finesse of 200 or more. The peak wavelength λres is λres = m/nL
...It is expressed as (3).

[0017] An alignment film is applied on the mirrors M1 and M2,
When antiparallel rubbing treatment is performed and nematic liquid crystal is filled, the liquid crystal LC is oriented as shown in FIG. 7(a). Note that LCM is a liquid crystal molecule. Liquid crystal molecules LCM have large dielectric anisotropy (ne, no), and when the polarization direction of incident light matches the orientation direction of liquid crystal LC, this light has ne
Feel the refractive index. When a voltage is applied in this state, the liquid crystal molecules LCM rise as shown in FIG. 7(b).
We feel that the refractive index of polarized light changes from ne to no. Therefore, according to the above equation (3), the peak wavelength is shifted by applying a voltage to the liquid crystal layer LC.

However, when trying to connect a variable wavelength filter using this liquid crystal to an optical fiber, two peaks appear for p and s polarized light, and the transmittance changes in the direction of the polarization of the light. This has the disadvantage that special fibers such as polarization-maintaining fibers must be used.

The present invention has been made in view of the above circumstances, and its object is to provide a variable wavelength filter without polarization dependence.

[0020]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a transparent substrate, a transparent electrode, a mirror, an alignment film for liquid crystal, a liquid crystal layer, an alignment film for liquid crystal, a mirror, a transparent electrode, a transparent substrate. In the variable wavelength filter laminated in the order indicated, the liquid crystal layer is composed of two layers that are homogeneously oriented and arranged in parallel with the orientation directions perpendicular to each other, and the liquid crystal layer is arranged on both sides of the liquid crystal layer. A transparent electrode is separately provided corresponding to each liquid crystal layer, and is arranged opposite to the light input/output surface of each transparent substrate, and separates the incident light into two polarized lights whose electric field directions are orthogonal to each other. , a means for making each polarized light incident on the liquid crystal layer in an alignment direction that corresponds to the electric field vector thereof, and a means for synthesizing the two polarized lights emitted from each liquid crystal layer, and an AC power source for individually driving each of the liquid crystal layers. Ta.

[0021]

According to the present invention, the light incident on the variable wavelength filter is separated into two polarized beams whose electric field directions are perpendicular to each other. Each separated polarized beam is guided to an optical path to the liquid crystal layer whose electric field vector and alignment direction match, and then passes through a transparent substrate, a transparent electrode, a mirror, and a liquid crystal alignment film to reach each liquid crystal layer. is incident on the

[0022] Here, the voltage applied by the AC power supply to each liquid crystal layer via each transparent electrode is controlled so that the resonance wavelengths of the respective cavities coincide. This makes the resonance wavelengths of the two polarized beams exactly the same.

After passing through each liquid crystal layer, the polarized beam passes through a liquid crystal alignment film, a mirror, a transparent electrode, and a transparent substrate.
The light beams are combined and output from the variable wavelength filter as a single light beam.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing a first embodiment of a variable wavelength filter according to the present invention. In Figure 1, GS1
, GS2 is a glass substrate, TEL1a, TEL2a, TE
L1b, TEL2b are transparent electrodes, M1, M2 are dielectric mirrors, ORL1a, ORL2a, ORL1b, ORL2
b is a liquid crystal alignment film, LCa and LCb are liquid crystal layers made of, for example, nematic liquid crystal, SPa and SPb are spacers, and DRP.
1, DRP2 is a birefringent prism, ASa, ASb are driving AC power supplies, EWa, EWb are AC power supplies ASa, ASb
and transparent electrodes TEL1a, TEL2a and TEL1b,
This is a lead wire that connects to TEL2b. Further, the black circle "●" in the figure indicates p-polarized light, and the vertical line "|" indicates s-polarized light. Note that, in FIG. 1, hatching indicating a cross section is omitted for simplification of the drawing.

The variable wavelength filter shown in FIG. 1 includes a birefringent prism DPR1, a glass substrate GS1, transparent electrodes TEL1 (a, b), a dielectric mirror M1, and a liquid crystal alignment film ORL1 (a) through the manufacturing process described later. ,b), liquid crystal layer LC
(a, b), liquid crystal alignment film ORL2 (a, b), dielectric mirror M2, transparent electrode TEL2 (a, b), glass substrate G
S2 and birefringent prism DRP2 are stacked in the order shown. In addition, the members with subscripts a and b in parentheses, that is, the transparent electrodes TEL1a and T
EL1b, liquid crystal alignment films ORL1a and ORL1b, liquid crystal layers LCa and LCb, liquid crystal alignment films ORL2a and ORL2b
Furthermore, the transparent electrodes TEL2a and TEL2b are arranged in parallel with each other in the light traveling direction.

Further, the liquid crystal layer LC is homogeneously aligned, and the liquid crystal alignment films ORL1a, ORL2a and the liquid crystal alignment film O
Due to the difference in surface treatment between RL1b and ORL2b, they are aligned so that their alignment directions are perpendicular to each other, and are divided into two layers LCa and LCb. Specifically, the direction of liquid crystal molecules in the liquid crystal layer LCa matches the electric field vector direction of s-polarized light, and the direction of liquid crystal molecules in the liquid crystal layer LCb matches the electric field vector direction of p-polarized light. For this reason, p
Polarized light and s-polarized light have the refractive index ne of the liquid crystal when no voltage is applied.
, and as voltage is applied, the refractive index changes from ne → no
I feel it decreasing.

The birefringent prisms DRP1 and DRP2 separate incident light from the outside into p-polarized light and s-polarized light, make the p-polarized light enter the liquid crystal layer LCb, and make the s-polarized light enter the liquid crystal layer LC.
On the other hand, the p-polarized light and the s-polarized light that have passed through each liquid crystal layer LCa and LCb are combined.

[0028] The AC power supplies ASa and ASb are connected to the liquid crystal layer LCa.
, LCb are provided to individually control voltage application. This is the two beams of this variable wavelength filter.
If the cavity gap is exactly the same for the
The resonant wavelengths are exactly the same for the two s- and p-polarized beams, making it possible to create a tunable wavelength filter with no polarization dependence, but in reality, the cavity gap must be uniformly controlled over a wide area. By driving the two liquid crystal layers LCa and LCb individually,
This is provided to match the resonance wavelengths of both.

Next, a method for manufacturing the variable wavelength filter shown in FIG. 1 will be explained.

First, indium tin oxide (ITO) is formed to a thickness of 50 nm over the entire surface of a pair of glass substrates GS1 and GS2 having a surface precision of λ/20 and a thickness of 6 mm by sputtering. Next, through photo processing, IT
The central part of the O layer is etched by 10 .mu.m to form two surfaces of the ITO that are not in electrical contact. Next, 1.52 on the entire surface
A dielectric mirror with a reflectance of 99% in the μm band is formed.

Next, an SiO2 obliquely deposited film is deposited over the entire surface. At this time, when the glass substrates GS1 and GS2 are opposed to each other, the directions of their oblique axes are made to be antiparallel. Next, the S that is placed on top of one ITO by photo processing
Remove iO2 by etching. With the resist film placed thereon, a SiO2 film is further deposited. At this time, the oblique direction of the obliquely deposited film is perpendicular to the previously deposited film, and furthermore, when the glass substrates GS1 and GS2 are opposed, they are made antiparallel. Thereafter, the SiO2 on the resist is removed by lift-off. As a result, two orthorhombic films whose orthorhombic axes are perpendicular to each other are formed on one glass substrate GS1 or GS2.

Next, 15 μm liquid crystal spacer SP1,
Mix SP2 with UV curing adhesive and apply this to glass substrate G.
Apply it to the edges of S1 and GS2 and bond the two glasses together. At this time, the opposing mirror surfaces M1 and M2 are adjusted so that they are parallel to each other, and ultraviolet rays are irradiated to complete the adhesion. Then, it is filled with nematic liquid crystal. then 11
Heat treatment at 0°C. At this point, two liquid crystal layers LCa, L
Cb liquid crystal molecules are homogeneously aligned in each layer, and the directions are orthogonal to each other. Next, birefringent prisms DRP1,D
RP2 is attached to the outer surface (light input/output end surface) of each glass substrate GS1, GS2.

Next, a single mode optical fiber with a collimator lens is attached to the light input side (for example, the birefringent prism DRP1 side) and the output side (for example, the birefringent prism DRP2 side) of the variable wavelength filter, and the intensity of the output light is adjusted. The positions of the birefringent prisms DRP1, DRP2 and the optical fiber are adjusted so that the

The characteristics of the variable wavelength filter manufactured by the method described above were actually measured. At this time, a 1.5 μm band superluminescent diode was used as a light source, and an optical spectrum analyzer was placed on the output side.

FIG. 8(a) shows the transmission spectrum when no voltage is applied in this measurement. As shown in the figure, when no voltage was applied, a pair of very sharp peaks A and B were observed at intervals of 50 nm. The A and B peaks correspond to p-polarized light and s-polarized light, respectively.

Next, the voltage of 5V is applied to the AC power supplies ASa, AS
B was applied to the liquid crystal layers LCa and LCb. Figure 8
(b) shows the transmission spectrum at this time. As shown in the figure, when a voltage of 5 V was applied to both layers, each peak shifted to the shorter wavelength side by 15 nm.

Next, the voltage applied by the AC power source ASa was changed so that the peaks of A and B overlapped. The voltage of the AC power supply ASa at this time was 5.3V. FIG. 8(c) shows the transmission spectrum at this time. As shown in the figure, when the voltage applied to one liquid crystal layer LCa is changed, the peaks of A and B overlap, and 1
This is the peak of the book.

At this time, even if the state of polarization was changed by bending the optical fiber to some extent, no change was observed in the transmission spectrum.

As explained above, according to the first embodiment, a variable wavelength filter that does not depend on the direction of polarization can be realized.

FIG. 9 is a block diagram showing a second embodiment of the variable wavelength filter according to the present invention. The difference between the second embodiment and the first embodiment is that instead of using a birefringent prism to separate and combine p-polarized light and s-polarized light, p-polarized light is transmitted and s-polarized light is transmitted at a 90° angle with respect to the incident direction. This is because polarizing beam splitters PBS1, PBS2 which reflect light in the direction of degrees and reflecting mirrors RM1, RM2 are used.

The second embodiment having such a configuration can also provide the same effects as the first embodiment.

[0042]

[Effects of the Invention] As explained above, according to the present invention,
The incident light is separated into two polarized lights whose electric field directions are perpendicular to each other, and two liquid crystal layers are provided in which the electric field vectors and the alignment directions of the liquid crystal molecules match. Light from an optical fiber is made incident on these layers, and each layer is By controlling the voltage, the resonant wavelengths of each light beam are matched, and then the two light beams are combined again, making it possible to create a tunable wavelength filter that does not depend on the direction of polarization. .

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing a first embodiment of a variable wavelength filter according to the present invention.

FIG. 2 is a configuration diagram showing a first conventional example.

FIG. 3 is a configuration diagram showing a second conventional example.

FIG. 4 is a configuration diagram showing a third conventional example.

FIG. 5 is a configuration diagram showing a fourth conventional example.

FIG. 6 is a diagram showing the transmission spectrum of the variable wavelength filter of FIG. 5;

FIG. 7 is an explanatory diagram of the operation of the variable wavelength filter in FIG. 5;

8 is a diagram showing a transmission spectrum of the variable wavelength filter of FIG. 1. FIG.

FIG. 9 is a configuration diagram showing a second embodiment of a variable wavelength filter according to the present invention.

[Explanation of symbols]

GS1, GS2...Glass substrate TEL1a, TEL1b, TEL2a, TEL2b...Transparent electrode M1, M2...Dielectric mirror ORL1a, ORL1b, ORL2a, ORL2b...Liquid crystal alignment film LCa, LCb...Liquid crystal layer DRP1, DRP2...Birefringent prism ASa, ASb...driving AC power supply PBS1, PBS2...polarizing beam splitter RM1, R
M2...Reflection mirror

Claims (1)

[Claims] 1. A variable wavelength filter in which a transparent substrate, a transparent electrode, a mirror, a liquid crystal alignment film, a liquid crystal layer, a liquid crystal alignment film, a mirror, a transparent electrode, and a transparent substrate are laminated in the stated order, wherein the liquid crystal layer is laminated in the stated order. , consists of two layers that are homogeneously oriented and arranged in parallel with the orientation directions perpendicular to each other, and transparent electrodes are arranged on both sides of the liquid crystal layer separately for each liquid crystal layer. A liquid crystal is provided, and is arranged opposite to the light input/output surface of each of the transparent substrates, separates the incident light into two polarized lights whose electric field directions are orthogonal to each other, and each polarized light has an alignment direction that corresponds to its electric field vector. 1. A variable wavelength filter comprising means for combining two polarized lights incident on the liquid crystal layer and emitted from each liquid crystal layer, and an AC power source for individually driving each of the liquid crystal layers.