JP2888372B2 - Tunable wavelength filter module - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of selectively and variably extracting an optical signal of any desired wavelength from a wavelength-multiplexed optical signal propagating in an optical fiber. The present invention relates to a variable wavelength filter module that can be used.

[Conventional technology]

2. Description of the Related Art Optical communication using an optical fiber can rapidly transmit a large amount of information, and has recently been rapidly put into practical use. However, at present, only an optical pulse of a specific wavelength is transmitted. If a large number of light pulses of different wavelengths can be transmitted, a much larger amount of information can be transmitted. This is called wavelength division multiplexing and is being actively studied at present. In wavelength division multiplexing communication, a variable wavelength filter is required which selectively selects only light having an arbitrary desired wavelength from among light pulses having a large number of wavelengths.

As a prior art of this type of filter, for example,
As shown in the drawing, there is a variable wavelength filter module using a variable wavelength filter that changes the resonator wavelength by changing the resonator length of the etalon with a piezo element.
Here, F1 and F2 are optical fibers, L1 and L2 are lenses, M1 and M2 are mirrors, and PZ is a piezo element. An etalon is constituted by two mirrors M1 and M2, and one of the mirrors M2 is driven by a piezo element PZ to change the resonator length of the etalon.

However, in this conventional example, a high voltage is required to drive the piezo element PZ, and it is difficult to reduce the size and weight of the device.

As another example of the prior art, as shown in FIG. 8, there is a variable wavelength filter module using a variable wavelength filter that changes the resonator wavelength by mechanically rotating an etalon ETL. Here, the same parts as those in FIG. 7 are denoted by the same reference numerals.

However, in this method, since the etalon is driven mechanically, it is also difficult to reduce the size and weight of the device.

As another example of the prior art, as shown in FIG. 9, there is a variable wavelength filter using a semiconductor optical waveguide having a Bragg reflector. Here, ARC is an anti-reflection coating, S is a substrate, and G is a grating, and these constitute a Bragg reflector. ACT is an active layer, and E1 to E3 are electrodes. The transmission wavelength band is controlled by controlling the injection current into the regions I to III.

However, this filter does not have a wide wavelength sweep width.
Has a shortcoming of 5nm.

In order to solve the above-mentioned disadvantages of the mechanical filter and the semiconductor optical waveguide filter, there is a filter in which an etalon is filled with liquid crystal and a voltage is applied to change an optical gap of the etalon.

However, when connecting a variable wavelength filter using this liquid crystal to an optical fiber, a special optical fiber such as a polarization maintaining fiber or the like is connected to the input side because the transmittance greatly depends on the polarization direction of the light. There was a drawback that it had to be provided.

[Problems to be Solved by the Invention] Accordingly, an object of the present invention is to eliminate the above-mentioned drawbacks of the variable wavelength filters, to be able to be driven at a low voltage, to have no mechanical movable parts, to be small and light, An object of the present invention is to provide a variable wavelength filter module having a wide wavelength sweep width and not requiring a polarization maintaining fiber.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides a first method for receiving a light input and outputting first and second polarization outputs of a first and second polarization state, respectively. A birefringent prism, a first half-wave plate that receives the second polarization output and outputs a third polarization output in the first polarization state, and receives the first and third polarization outputs and is selected. A tunable wavelength filter that outputs fourth and fifth polarization outputs of the first polarization state corresponding to the first and third polarization outputs at different wavelengths,
A second half-wave plate that receives the fifth polarization output and outputs a sixth polarization output of the second polarization state, and a second half that receives the fourth and sixth polarization outputs and extracts one output light A birefringent prism, and an electronic circuit that controls the tunable wavelength filter and specifies the wavelength of light transmitted through the filter, further comprising a first birefringent prism and a first half-wave plate separated from each other. And a second birefringent prism and a second 1 /
It is characterized in that it is separated from the two-wavelength plate.

Here, the variable wavelength filter includes a first glass substrate,
The first transparent electrode, the first high reflection mirror, the first liquid crystal alignment film, the liquid crystal, the second liquid crystal alignment film, the second high reflection mirror, the second transparent electrode, and the second glass substrate are arranged in this order. Can be.

Further, it is preferable that the main surfaces of the first and second glass substrates in the variable wavelength filter are arranged so as to be inclined from a direction perpendicular to the optical axis of light incident on the variable wavelength filter.

(Operation)

In the present invention, by arranging a birefringent prism and a half-wave plate on the input side and the output side of the etalon type variable wavelength filter filled with liquid crystal, light of the same polarization can be incident on the filter. Even if a normal optical fiber is used for the input / output end without using a polarization maintaining fiber on the side, the transmittance of the tunable wavelength filter can always be kept constant.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing the configuration of one embodiment of the variable wavelength filter module of the present invention. Here, PF1 and PF2 are optical fibers, L1 is a collimating lens, L2 is a fiber coupling lens, VF1 is a variable wavelength filter, EC1 is an electronic circuit for controlling a variable wavelength filter, P1, P2 are birefringent prisms, λ1, λ2
Is a half-wave plate.

The light beam emitted from the optical fiber PF1 through the collimator lens L1 enters the birefringent prism P1 almost in parallel. In the birefringent prism P1, the traveling directions of the p and s polarized lights are divided, so that the incident light is divided into two rays, pl and sl. Two light beams pl, s exiting the birefringent prism P1
l proceed parallel to each other. Only the light beam sl is a half-wave plate λ1
The half-wave plate λ1 is set so as to pass through. As a result, the s-polarized light becomes the p-polarized light. Therefore, two p
Polarized light enters the variable wavelength filter VF1 filled with liquid crystal.

Here, in the variable wavelength filter VF1, the electronic circuit E
When a voltage is applied from C1, the liquid crystal is oriented so that the refractive index changes with respect to the p-polarized light, and thus the transmission peak of the etalon changes with the applied voltage with respect to the p-polarized light.

The two light beams emitted from the tunable wavelength filter VF1 become one light by the half-wave plate λ2 and the birefringent prism P2 symmetrically arranged on the input side, and further pass through the condenser lens L2 to the output side. To the optical fiber PF2.

As can be seen from the above, in the present invention, since the light propagating between the tunable wavelength filters VF1 is light having the same polarization state,
Whatever polarization light enters the input side, the wavelength can be varied according to the voltage from the electronic circuit EC1, without changing the characteristics of the variable wavelength filter VF1.

FIG. 2 shows a specific example of the variable wavelength filter VF1 composed of an etalon filled with liquid crystal. In FIG. 2, GS
Is a glass substrate, ARC is a non-reflective coating film, TEL transparent electrode, M
Is a dielectric mirror, ORL is a liquid crystal alignment film, LC is a liquid crystal, SP is a spacer, and EW is a lead wire for applying a voltage to the liquid crystal. Here, a transparent electrode TEL is provided on one main surface of the glass substrate GS.
Is formed by a sputtering method, and a dielectric mirror M is disposed thereon by vapor deposition. A liquid crystal alignment film OR is provided on the exposed surface of the mirror M.
Apply L. On the other main surface of the glass substrate GS, an antireflection coat film ARC is deposited. The lead wire EW is connected to the transparent electrode TEL.

As described above, when the two glass substrates SG in which the respective layers are stacked are arranged in parallel at the interval L by the spacer SP, the wavelength dependence of the transmittance is expressed by the following equation.

T = 1 / [1 + Fsin ² (2πnL / λ)] (1) F = 4r / (1-r) ² (2) where n is the refractive index of the liquid crystal, λ is the wavelength, and r is the mirror M
Is the reflectance.

The transmission spectrum in this case is shown in FIG. As can be seen from FIG. 3, many sharp peaks appear. The half width of these peaks depends on the reflectivity of the mirror M. In the case of the mirror M of 99%, a sharp peak with a finesse of 200 or more is obtained. The peak wavelength λres is expressed by λres = m / 2nL (m = 1, 2,...) (3). When a liquid crystal alignment film ORL is applied on the mirror M, antiparallel rubbing is performed, and a portion of the space L is filled with a nematic liquid crystal, the liquid crystal is aligned as shown in FIG.
LCM is a liquid crystal molecule. Liquid crystal molecules LCM has a large dielectric anisotropy (n _e, n _o), the light feels the refractive index of n _e the polarization direction of the incident light coincides with the orientation direction of the liquid crystal. When a voltage is applied to the liquid crystal, Fig. 4 a liquid crystal rises as (B), the polarization feel to change the refractive index to n _e → n _o. Therefore, the peak wavelength can be shifted by applying a voltage to the liquid crystal layer according to the equation (3). What is important here is that polarized light having the same direction as that of the liquid crystal needs to be incident. For this purpose, the embodiment shown in FIG. 1 uses birefringent prisms P1, P2 and half-wave plates λ1, λ2.

One embodiment of a manufacturing process of the liquid crystal-filled etalon type variable wavelength filter module shown in FIGS. 1 and 2 will be described.

An indium tin oxide (ITO) transparent electrode TEL was formed to a thickness of 50 nm by sputtering on a pair of glass substrates GS having a flatness of λ / 50. Further, TiO ₂ and SiO ₂ were alternately multilayer-deposited on the transparent electrode TEL by an evaporation method to form a mirror M having a reflectivity of 98% in a 1.5 μm band. Next, a polyimide alignment film ORL was applied thereon by a spinner and dried. Next, a rubbing treatment was performed on the alignment films ORL so as to be antiparallel to each other. A spacer SP having a diameter of 10 μm was mixed with an ultraviolet-curing adhesive, and this was applied to the end of the glass substrate GS.
Two glass substrates GS are bonded together. At that time, the cell gap L was adjusted to be uniform, and irradiation was performed with ultraviolet rays to complete the bonding. Thereafter, a nematic liquid crystal was filled in the gap. Here, ZLI3103 manufactured by Merck was used.
Thereafter, heat treatment was performed at 110 ° C. to obtain a variable wavelength filter VF1.

Next, 5 mm square rutile birefringent prisms P1 and P2 and a half-wave plate λ1 and λ2 are arranged on both sides of the variable wavelength filter VF1, and axes of these members are aligned. Glued. The variable wavelength filter module was configured by attaching the optical fibers PF1 and PF2, the collimating lens L1, and the condensing lens L2 to the filter body configured as described above.

The characteristics of the tunable wavelength filter module configured as described above were measured. A 1.5 μm band superluminescent diode was used as a light source. FIG. 5 shows the transmission spectrum in this case. Here, a maximum transmittance of 50% and a peak half width of 0.4 nm were obtained.

Next, a voltage (driving frequency: 20 kHz) was applied to the variable wavelength filter module, and the manner in which the peak shifted was measured. The results are shown in FIG. Applied voltage (AC)
When changing from 0V to 8V, the peak wavelength is from 1.52μm
The shift was about 50 nm to 1.47 μm. Further, the transmittance was constant at 50% regardless of the presence or absence of voltage application. When the state of polarization in the fiber was changed by bending the fiber, no change was observed in the above characteristics.

In the present invention, when the optical axis of the light incident on the variable wavelength filter VF1 and the mirror surface of the variable wavelength filter VF1, that is, the main surface of the glass substrate GS are completely perpendicular, the light reflected without passing through this filter is considered. Is the optical fiber PF1 on the incident side
It happened often to get back to. This causes deterioration of the light source. For this reason, in the present invention, the main surface of the glass substrate GS of the variable wavelength filter VF1 is slightly inclined from the direction perpendicular to the optical axis of the incident light so that the reflected light does not enter the optical fiber PF1 on the input side. Is preferred.

〔The invention's effect〕

As described above, according to the present invention, a birefringent prism and a half-wave plate are arranged on the input side and the output side of a liquid crystal-filled etalon type variable wavelength filter, so that light of the same polarization is incident on the filter. The transmission of the tunable wavelength filter can be kept constant even when a normal optical fiber is used for the input and output ends without using a polarization maintaining fiber on the input side, thereby reducing the low voltage. It is possible to provide a variable wavelength filter module that can be driven, has no mechanically movable parts, is easily reduced in size and weight, and can sweep a wide wavelength range.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing one embodiment of the variable wavelength filter module of the present invention, FIG. 2 is a configuration diagram showing an example of the structure of the liquid crystal-filled etalon type variable wavelength filter shown in FIG. 1, and FIG. FIGS. 4A and 4B are diagrams showing transmission spectra of a filled etalon type variable wavelength filter, and FIGS. 4A and 4B show alignment states of liquid crystal molecules of a liquid crystal filled etalon type variable wavelength filter when no voltage is applied and when a voltage is applied, respectively. FIG. 5, FIG. 5 is a diagram showing a transmission spectrum in one embodiment of the variable filter module of the present invention, FIG. 6 is a characteristic diagram showing applied voltage dependence of a transmission peak wavelength in one embodiment of the variable wavelength filter module of the present invention, FIG. 7 is a block diagram showing an example of a conventional variable wavelength filter that changes the resonator wavelength of an etalon using a piezo element, and FIG. FIG. 9 is a configuration diagram illustrating an example of a conventional variable wavelength filter that changes a resonator wavelength. FIG. 9 is a perspective view illustrating an example of a conventional variable wavelength filter using a semiconductor optical waveguide having a Bragg reflector. PF1, PF2: Optical fiber, L1: Collimating lens, L2: Fiber coupling lens, VF1: Variable wavelength filter, EC1: Electronic circuit for controlling variable wavelength filter, P1, P2: Birefringent prism, λ1, λ2: 1/2 wavelength plate, GS: glass substrate, ARC: anti-reflection coating film, TEL: transparent electrode, M: dielectric mirror, ORL: liquid crystal alignment film, LC: liquid crystal, SP: Spacer, EW: Lead wire for applying voltage to liquid crystal, LCM: Liquid crystal molecule, F1, F2: Optical fiber, L1, L2: Lens, M: Mirror, PZ: Piezo element, ETL … Etalon, ARC… non-reflective coating, S… substrate, G… grading, ACT… active layer, E1 to E3… electrodes.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Kurokawa 1-6-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-62-178219 (JP, A) JP-A Sho 58-54322 (JP, A) JP-A-2-132424 (JP, A) JP-A-63-22109 (JP, A)

Claims (2)

(57) [Claims] 1. A first glass substrate, a first transparent electrode, a first high reflection mirror, a first liquid crystal alignment film, a liquid crystal, a second liquid crystal alignment film, a second high reflection mirror, a second transparent electrode, a second glass substrate. And a variable wavelength filter having a configuration arranged in this order. The variable wavelength filter receives a light input on a light input side thereof, and receives a first light in a p-polarized state in the same direction as the liquid crystal. A first birefringent prism that outputs a polarization output and a second polarization output in an s-polarization state perpendicular to the p-polarization state; and a third polarization output in the p-polarization state upon receiving the second polarization output. A variable wavelength filter receiving the first and third polarized light outputs, corresponding to the selected wavelengths and corresponding to the first and third polarized light outputs, respectively. Output fourth and fifth polarization outputs of the p-polarization state A second half-wave plate that receives the fifth polarized light output on the light output side with respect to the variable wavelength filter and outputs a sixth polarized light output in the s-polarized state; A second birefringent prism for receiving one polarized light and extracting one output light; and an electronic circuit for controlling the variable wavelength filter and designating a wavelength of light transmitted through the filter, the first birefringence. A variable wavelength filter, wherein a prism and the first half-wave plate are separated from each other, and the second birefringent prism and the second half-wave plate are separated from each other. module. 2. The variable wavelength filter according to claim 1, wherein the principal surfaces of the first and second glass substrates in the variable wavelength filter are arranged to be inclined from a direction perpendicular to an optical axis of incident light of the variable wavelength filter. Tunable filter module.