JPH0695164A - Variable wavelength filter - Google Patents

Abstract

PURPOSE:To form arbitrary transmission spectra with the multichannel variable wavelength filter. CONSTITUTION:Transparent electrodes 1-5 of the liquid crystal variable wavelength filter are previously patterned to a stripe shape and many filters are arranged in an array form within one element. The light of one wavelength is selected by the on filter and the remaining reflected light is made incident again on another part of the liquid crystal filters by mirrors 1-2-1 adhered to the incident side of the liquid crystal filters. The wavelengths are variably selected in the respective electrodes by repeating the above-mentioned selection. The emitted light repeats the reflection by the mirrors 1-2-2 adhered to the exit side of the liquid crystal filters and is emitted as one beam. The reflectivity of the mirrors is previously varied in correspondence to the transparent electrodes 1-5 patterned to the stripe form and further current is passed in the electrode direction of the respective transparent electrodes 105, by which the temp. of the liquid crystals is varied and the loss thereof is arbitrarily changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to wavelengths in optical fibers.
The present invention relates to a variable wavelength filter capable of selectively and variably extracting optical signals of arbitrary plural wavelengths / frequencies of a frequency-multiplexed optical signal and arbitrarily setting the shape of a transmission spectrum.

[0002]

2. Description of the Related Art Optical communication using an optical fiber has recently been rapidly put into practical use because it can transmit a large amount of information at high speed. However, at present, only optical pulses of a specific wavelength / frequency are transmitted. If a large number of optical pulses having different wavelengths / frequencies can be transmitted, a large amount of information can be further transmitted. This is the wavelength division multiplexing (WDM) / frequency division communication (FD)
M) and is currently under active research. In wavelength / frequency multiplex communication, a variable wavelength filter is a key device that selectively selects only light of an arbitrary wavelength and frequency from optical pulses of many wavelengths and frequencies. Therefore, various variable wavelength filters have been studied and put into practical use. However, most of these can variably extract only one wavelength and frequency from the wavelength and frequency multiplexed optical signals. Further, the shape of the transmission spectrum is determined by the principle of the filter, and a light filter having an arbitrary transmission spectrum cannot be formed like an electric filter. In future wavelength-multiplexed / frequency-multiplexed communication, a tunable wavelength filter that can variably extract a plurality of wavelengths / frequencies or set an arbitrary transmission spectrum will be very important.

For example, it is expected that a future switch will be an optical switch that switches light as it is, but among them, a multi-channel variable wavelength filter capable of selecting a plurality of wavelengths and frequencies becomes very important. . FIG. 12 shows a conceptual diagram of wavelength light exchange. 8-1 is an incident optical fiber, 8-2 is a demultiplexer, 8
-3 is a variable wavelength filter, 8-4 is a wavelength conversion element, 8-5
Is a coupler, and 8-6 is a multi-channel variable wavelength filter.

The wavelength-multiplexed optical signal from the subscriber is demultiplexed by the demultiplexer 8-2, further selected by the variable wavelength filter 8-3, and wavelength-converted into the light of the wavelength corresponding to each subscriber.
The wavelength is multiplexed by the coupler 8-5. Optical switching in the wavelength region can be realized by selecting a plurality of wavelengths from the wavelength-multiplexed optical signal and repeating the subsequent wavelength conversion.

Further, a filter having an arbitrary transmission spectrum shape is very important for correcting the wavelength dependence of optical components. For example, as shown in FIG. 13, a fiber amplifier has wavelength dependence in its optical amplification characteristic. Therefore, there is only a 2 nm band at 1.536 μm, and 1.
There is only a 4 nm band at 552 μm. By placing an optical filter that corrects this amplification characteristic after the fiber amplifier, it is possible to multiplex more wavelengths if the band can be expanded to several tens of nm.

There is a photoacoustic filter as a filter capable of variably extracting a plurality of wavelengths and frequencies. The photoacoustic filter can variably select wavelengths by changing the frequency of the applied high-frequency voltage, but it is possible to variably select a plurality of wavelengths by applying a plurality of high frequencies at once.

[0007]

However, when the applied voltage becomes plural, the heat generation of the element becomes large, and the number of wavelengths actually selectable is about 5 (KW Cheung).
et al, ECOC, 89, Gothenburg, Sweeden, Sept., 10-14,
1989.).

There is also a report that a plurality of wavelengths are selected by finely separating electrodes of a liquid crystal variable wavelength filter in which a liquid crystal is filled in a Fabry-Perot etalon and expanding a light beam so as to pass through these electrodes. The number of selected wavelengths is at most 2, and there is also a problem that the loss increases as the number of wavelengths increases (JS Patel and MWMaeda, IEEEPh.
on. Tech. Lett., vol.3, pp.643-644 (1991)).

A multi-channel filter combining a Mach-Zehnder interferometer with an optical waveguide and a phase shifter has been reported (Sasayama, Okuno, Habara, IEICE Tech. There is a problem that the loss increases as the number increases, and the driving of the device is complicated.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a technique capable of forming an arbitrary transmission spectrum in a multi-channel variable wavelength filter. It is in.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[0012]

In order to achieve the above object, the means (1) of the present invention comprises a glass substrate, a transparent electrode,
High-reflection mirror, alignment film, liquid crystal, alignment film, high-reflection mirror,
In a liquid crystal variable wavelength filter having a structure in which a transparent electrode and a glass substrate are sequentially laminated, the transparent electrode is patterned in a stripe shape, and a wiring to which a controlled voltage is applied is made to each stripe electrode. The tunable wavelength filter has a structure sandwiched by two high-reflection mirrors, and the incident optical beam from the optical fiber with an incident collimating lens is incident on the filter at an inclined angle. The filter and the high-reflecting mirror are arranged so that the light beam is repeatedly reflected between the filter and the high-reflecting mirror, hits each transparent electrode in a stripe shape, and the emitted light beam becomes one. It is output to an optical fiber with a collimating lens for emission.

According to the means (2) of the present invention, in the variable wavelength filter according to the means (1), the mirrors are also patterned in stripes corresponding to the respective transparent electrodes, and the reflectance of each transparent electrode is Characterized by being different.

According to a third aspect of the present invention, in the variable wavelength filter according to the first aspect or the second aspect, a birefringent plate is placed after the incident optical fiber and before the outgoing optical fiber to allow the incident light to pass. The beam is divided into two light beams having polarizations orthogonal to each other and divided into two parts so that the alignment directions of the liquid crystal variable wavelength filter are perpendicular to each other so that they have the same alignment directions as those polarizations. It is characterized by being done.

According to a fourth aspect of the present invention, in the variable wavelength filter according to the first aspect, a polarization beam splitter is provided after the incident side optical fiber and before the emission side optical fiber.
It is characterized by having a two-wave plate.

Since the filter of the present invention basically has polarization dependence, as a means for solving this, a birefringent plate or a polarization beam splitter and λ / I have two plates.

[0017]

According to the above-mentioned means (1) to (4), the transparent electrodes of the liquid crystal variable wavelength filter are patterned in stripes, and a large number of filters are arranged in an array in one element. The light of one wavelength is selected by one filter, and the remaining reflected light is made incident on another part of the liquid crystal filter again by the mirror attached to the incident side of the liquid crystal filter, and by repeating this, it is changed at each electrode. The wavelength can be selected.

The emitted light is repeatedly reflected by a mirror attached to the emission side of the liquid crystal filter and emitted as a single beam. Corresponding to the transparent electrodes patterned in stripes, the reflectance of the mirror is changed, and the temperature of the liquid crystal is changed by changing the temperature of the liquid crystal by changing the temperature of the transparent electrodes. Can be formed.

[0019]

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

(Embodiment 1) FIG. 1 is a schematic configuration diagram for explaining the configuration of a liquid crystal variable wavelength filter according to Embodiment 1 of the present invention. 1-1 is an optical fiber with an incident collimating lens, 1- 2-1 and 1-2-2 are mirrors (reflectance 100
%) 1-3 is AR coating, 1-4 is glass substrate, 1-5
Are transparent electrodes patterned in stripes, 1-6
Is a mirror, 1-7 is an alignment film, 1-8 is a parallel-aligned liquid crystal layer, and 1-9 is an optical fiber with a collimating lens for emission.

In FIG. 1, the light beam emitted from the optical fiber 1-1 with the collimating lens for incidence is incident on the liquid crystal variable wavelength filter at a certain angle θ and enters the first electrode (first electrode). When the voltage V ₁ is applied to the first electrode (1-5), the light of λ ₁ corresponding to the voltage is emitted and the other light is reflected by the first electrode. The reflected light is reflected again by the mirror 1-2-1 attached to the liquid crystal filter and enters the second second electrode (1-5). By repeating this, different wavelengths can be selected for each electrode (1-5). The light emitted from the first electrode is reflected by the mirror 1-2-2 and enters the second electrode, but since only the light of λ ₂ is transmitted through the second electrode, λ ₁ is reflected. When this is repeated for each electrode, the light beam finally emitted from each electrode (1-5) becomes one and is output to the optical fiber 1-9 with the collimating lens for emission.

FIG. 2 shows the spectrum of incident wavelength-multiplexed light and the spectrum of emitted light. It can be seen that light of a plurality of wavelengths can be selected from among the wavelength-division-multiplexed lights.

(Embodiment 2) FIG. 3 is a schematic cross-sectional configuration diagram for explaining the configuration of a second embodiment of the present invention, and FIG. 4 is a schematic plan configuration view seen from the upper surface of FIG.

3 and 4, 3-1-1, 3-1-
2 is a polarization beam splitter, 3-2-1, 3-2-2 is λ / 2
The plate is the same as that of FIG. 1 except for those.

The variable wavelength filter having the structure of the first embodiment has polarization dependency. That is, the polarization direction of incident light must be parallel to the liquid crystal molecules. Therefore, it is necessary to use a polarization maintaining fiber on the incident side.

The second embodiment is a polarization-independent tunable wavelength filter. As shown in FIGS. 3 and 4, the optical fibers (1-1, 1-9) on the incident side and the emitting side are provided in front of each other. Polarization beam splitter (3-1-1, 3-1-2) and λ / 2 plate (3-2-1,
3-2-2) is provided. The incident light beam is split into two polarized waves parallel and perpendicular to the liquid crystal through a polarization beam splitter (3-1-3-1-3-1-2). Furthermore, λ / 2
Through the plate (3-2-1, 3-2-2), the polarized wave perpendicular to the liquid crystal is converted into the polarized wave parallel to the liquid crystal. In this way, the polarized light parallel to the two liquid crystal molecules enters the liquid crystal variable wavelength filter,
A plurality of wavelengths will be selected as in the first embodiment.
The light emitted from the liquid crystal variable wavelength filter is again transmitted to the λ / 2 plate (3-2-1 and 3-2-2) and the polarization beam splitter (3-
It is converted into a single light beam by 1-1, 3-1-2) and enters the outgoing optical fiber 1-9. In this way
Polarization independence is achieved.

(Third Embodiment) FIG. 5 is a schematic cross-sectional configuration diagram for explaining the configuration of a third embodiment of the present invention, and FIG. 6 is a schematic plan configuration diagram seen from the upper surface of FIG.

5 and 6, 4-1-1, 4-1-
Reference numeral 2 denotes a birefringent plate, and 4-2 and 4-3 denote regions where the liquid crystal alignment directions are perpendicular to each other. Other than those,
It is the same as FIG.

In the second embodiment, the polarization beam splitter and the λ / 2 plate are used to make the polarization independent.

In the third embodiment, similarly to the second embodiment, a birefringent plate is placed instead of these, and the liquid crystal variable wavelength filter is divided into two so that the alignment directions are perpendicular to each other. Wave independence is achieved by providing birefringent plates (4-1-1, 4-1-2) on the incident side and the outgoing side, as shown in FIG. 5 and FIG. The beam is split into two polarized waves that are parallel and perpendicular to the liquid crystal molecules. These beams are incident on the liquid crystal layers 4-2 and 4-3 separated into two. Since the alignment directions are vertical in the two regions, the polarization directions of the two polarized waves match the directions of the liquid crystal molecules. The emitted light again becomes a single light beam by the birefringent plate and enters the optical fiber on the output side. In this way, polarization independence was achieved.

(Embodiment 4) FIG. 7 is a schematic cross-sectional configuration diagram for explaining the configuration of a fourth embodiment of the present invention, and FIG. 8 is a schematic plan configuration view seen from the upper surface of FIG.

In FIGS. 7 and 8, reference numeral 5-1 is a dielectric mirror, which is patterned similarly to the transparent electrode, and the reflectances thereof are changed to 70%, 80% and 90%. 5-2 is a current source for flowing a controlled current to each transparent electrode, and 5-3 is a variable resistor for controlling the current.

The fourth embodiment is a filter to which the present invention is applied as a filter for correcting the wavelength dependence of an optical fiber amplifier. As shown in FIGS. 7 and 8, the dielectric mirror 5 is used.
A current source 5-2 that passes a current in the direction of the electrodes to each transparent electrode of -1.
Is provided.

In order to correct the amplification characteristic of the optical fiber amplifier shown in FIG. 13, the filter having the transmission spectrum shown in FIG. 9 may be prepared. Basically, it is possible to combine three transmission spectra as shown in FIG. The parameters of each spectrum are λ _1,2,3 , Δλ _1,2,3 , T
It is _1,2,3 . By adjusting these parameters, the synthesized transmission spectrum can be brought close to that in FIG. λ _1,2,3 can be set by adjusting the voltage applied to each electrode. △ λ _1,2,3 , T
_It is difficult to control _{1, 2 and 3} independently, but it is possible to control them by changing the reflectance of the mirror and the liquid crystal loss. The reflectivity of the mirror varies from place to place and can be changed by choosing the place. Furthermore, the loss of liquid crystal is caused by passing an electric current through the transparent electrodes in stripes in the direction of the electrodes.
It is possible to control the loss by raising the temperature and increasing its scattering.

As a result of controlling the above parameters, the wavelength dependence of the optical fiber amplifier with a filter according to the present embodiment 4 can obtain a substantially flat characteristic over about 50 nm as shown in FIG.

Although the polarization dependency is not described in the fourth embodiment, the polarization independence can be achieved by the same method as in the second and third embodiments.

As can be seen from the above explanation, the liquid crystal variable wavelength filter is used as a basic device, two mirrors are further sandwiched, the transparent electrodes are patterned into stripes, and light is incident at an angle with respect to this filter. It becomes possible to select a plurality of wavelengths at the same time by multiple reflection at, and it is possible to obtain an arbitrary transmission spectrum by controlling the plurality of transmission spectra.

Although the present invention has been specifically described based on the embodiments, it is needless to say that the present invention is not limited to the above embodiments and various modifications can be made without departing from the scope of the invention. Absent.

[0039]

As described above, according to the present invention, the transparent electrodes of the liquid crystal variable wavelength filter are patterned in stripes, and a large number of filters are arranged in an array in one element. The light of one wavelength is selected by one filter, and the remaining reflected light is made incident on another part of the liquid crystal filter again by the mirror attached to the incident side of the liquid crystal filter, and by repeating this, it is changed at each electrode. The wavelength can be selected.

In addition, the reflectance of the mirror is changed corresponding to the transparent electrodes patterned in stripes, and a current is applied to each transparent electrode in the direction of the electrodes to change the temperature of the liquid crystal so that the loss can be arbitrarily set. , So that an arbitrary transmission spectrum can be formed.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional configuration diagram for explaining a configuration of a liquid crystal variable wavelength filter according to a first embodiment of the present invention,

FIG. 2 is a diagram showing a spectrum of incident wavelength-multiplexed light and a spectrum of emitted light according to the first embodiment;

FIG. 3 is a schematic cross-sectional configuration diagram for explaining the configuration of Example 2 of the present invention,

FIG. 4 is a schematic plan configuration diagram seen from the top of FIG. 3 for explaining the configuration of Example 2 of the present invention;

FIG. 5 is a schematic cross-sectional configuration diagram for explaining the configuration of Example 3 of the present invention,

FIG. 6 is a schematic plan configuration diagram seen from the upper surface of FIG. 5 for explaining the configuration of a third embodiment of the present invention;

FIG. 7 is a schematic cross-sectional configuration diagram for explaining the configuration of Example 4 of the present invention,

FIG. 8 is a schematic plan configuration diagram viewed from the top of FIG. 7 for explaining the configuration of a fifth embodiment of the present invention;

FIG. 9 is a diagram showing a transmission spectrum of a filter for correcting the wavelength dependence of an optical fiber amplifier,

FIG. 10 is a diagram showing the spectrum of FIG. 9 decomposed into three;

FIG. 11 is a diagram showing the results of measuring the wavelength dependence of the amplification factor of a variable wavelength filter prepared according to Example 4 of the present invention after the optical fiber amplifier.

FIG. 12 is a diagram showing a concept of conventional optical switching using wavelengths;

FIG. 13 is a diagram showing the wavelength dependence of the amplification factor of an optical fiber amplifier.

[Explanation of symbols]

1-1 Optical fiber with collimating lens for incidence 1-2
-1,1-2-2 ... Mirror (100% reflectance), 1-3 ... A
R coat, 1-4, glass substrate, 1-5, transparent electrodes patterned in stripes, 1-6, mirrors, 1-7, ...
Alignment film, 1-8 ... Parallel-aligned liquid crystal layer, 1-9 ... Optical fiber with collimating lens for emission, 3-1-1, 3-1-2
... Polarizing beam splitter, 3-2-1, 3-2-2 ... λ / 2
Plate, 4-1-1, 4-1-2 ... Birefringent plate, 4-2, 4-3 ... Region where liquid crystal alignment directions are perpendicular to each other, 5-1 ... Dielectric mirror, 5-2 ... Each transparent A current source that supplies a controlled current to the electrodes,
5-3 ... Variable resistance for controlling current, 8-1 ... Incident optical fiber, 8-2 ... Demultiplexer, 8-3 ... Variable wavelength filter, 8-4
... Wavelength conversion element, 8-5 ... Coupler, 8-6 ... Multi-channel variable wavelength filter.

Claims (4)

[Claims] 1. A glass substrate, a transparent electrode, a high reflection mirror,
In a liquid crystal variable wavelength filter having a structure in which an alignment film, a liquid crystal, an alignment film, a high-reflection mirror, a transparent electrode, and a glass substrate are sequentially laminated, the transparent electrode is patterned in a stripe shape and is controlled for each stripe electrode. The liquid crystal variable wavelength filter has a structure sandwiched by two high-reflection mirrors, and the incident light beam from the optical fiber with the collimating lens for incidence is inclined with respect to this filter. An optical fiber with a collimating lens for incidence that is incident at an angle is arranged, and this light beam repeats reflection between the filter and the high-reflection mirror, hits each stripe-shaped transparent electrode, and the emitted light beam becomes one. The filter and the high-reflection mirror are arranged so that the light is output to the optical fiber with a collimating lens for emission. Variable wavelength filter to. 2. The variable wavelength filter according to claim 1, wherein the mirror is also patterned in a stripe shape corresponding to each transparent electrode, and the reflectance of each transparent electrode is different. . 3. The variable wavelength filter according to claim 1 or 2, wherein a birefringent plate is placed after the incident optical fiber and in front of the outgoing optical fiber, and the incident light beams have polarized waves orthogonal to each other. The tunable wavelength filter is characterized in that the liquid crystal tunable wavelength filter is divided into two parts so that the liquid crystal tunable wavelength filters have the same orientation directions as those of the polarized light beams. 4. The variable wavelength filter according to claim 1, further comprising a polarization beam splitter and a 1/2 wavelength plate after the incident side optical fiber and before the emission side optical fiber.