JPH03273217A - Variable wavelength filter module - Google Patents

Abstract

PURPOSE:To allow sweeping over a wide wavelength range by providing a circuit which controls a transmission wavelength and an electronic circuit which maintains a variable wavelength filter at a specified temp. in the variable wavelength filter. CONSTITUTION:Wavelength multiplexed optical signals are guided to a polarization maintaining optical fiber PF1. This light is converted to collimated beams of light of about 100 mum diameter by a lens L1 and is made incident on the variable wavelength filter VF1. Only the arbitrary one wave among the multiplex light signals is transmitted through the VF1. The selected light is coupled by a lens L2 to a polarization maintaining optical fiber PF2 and is outputted therefrom. The transmitted wavelength of the variable wavelength filter VF1 is controlled by the voltage impressed from an electronic circuit EC2 for controlling the variable wavelength filter to a transparent electrode layer TEL. Since the transmitted wavelength of the variable wavelength filter VF1 depends on the ambient temp., an electronic cooler CL is mounted and the specified temp. is maintained by the electronic circuit EC2 for controlling the electronic cooler. The sweeping of the wide wavelength range with the low voltage is executed in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a variable wavelength filter that selectively and variably extracts an optical signal of an arbitrary wavelength from a wavelength-multiplexed optical signal.

(Prior Art) As a conventional technology, there is a variable wavelength filter module using a variable wavelength filter that changes the resonant wavelength by changing the resonator length of an etalon using a piezo element as shown in FIG. 12(a). Here, PL, F2 are optical fibers, Ll
, L2 is a lens, M is a mirror, and PZ is a piezo element, and the two mirrors M constitute an etalon. However, this method requires a high voltage to drive the piezo element, making it difficult to reduce the size and weight of the device.

Another conventional technique is a variable wavelength filter module using a variable wavelength filter that changes the resonant wavelength by mechanically rotating an etalon as shown in FIG. 12(b). Here, ETL is an etalon.

However, in this method, the etalon is mechanically driven, making it difficult to reduce the size and weight of the device. Another conventional technique is a variable wavelength filter using a semiconductor optical waveguide having a Bragg reflector as shown in FIG. 12(c). Here, ARC is a non-reflective coating, S is a substrate, G is a grating and constitutes a Bragg reflector, ACT is an active layer, and E1 to E3 are electrodes. This variable wavelength filter controls the transmission wavelength band by controlling the current injected into the regions I to ■ shown in the figure. However, it has the disadvantage that the wavelength sweep width is only 5 na+ at most.

(Problems to be Solved by the Invention) An object of the present invention is to provide a tunable wavelength filter module that has no mechanically moving parts, is easily miniaturized and weighed, and is capable of sweeping a wide wavelength range with low voltage. It is in.

(Means for Solving the Problems) The tunable wavelength filter module of the present invention includes a collimating lens adjacent to a polarization-maintaining optical fiber for optical input, and a fiber coupling lens adjacent to a polarization-maintaining optical fiber for optical output. Between the lens, one or more variable wavelength filters are formed by sequentially laminating a glass substrate, a transparent electrode, a high-reflection mirror, an alignment film, a liquid crystal, an alignment film, a high-reflection mirror, a transparent electrode, and a glass substrate. The variable wavelength filter is provided with a variable wavelength filter control electronic circuit that applies an AC or DC voltage to control the transmission wavelength, and an electronic circuit that maintains the temperature of the variable wavelength filter constant.

Further, a variable wavelength filter module is configured in which an isolator is inserted into the polarization-maintaining optical fiber for optical input.

Further, a variable wavelength filter module is constructed in which the mirror plane of the variable wavelength filter is arranged tilted from perpendicular to the optical axis.

Further, the input and output optical fibers are non-polarization-maintaining optical fibers, and a polarization separation element is added to the output side of the collimating lens and a coupling element is added to the input side of the fiber coupling lens.
A variable wavelength filter module is constructed in which a variable wavelength filter is arranged for each separated polarized light.

Further, a variable wavelength filter module is configured in which a liquid crystal cell for polarization compensation is added, which is connected to the electronic circuit for controlling the variable wavelength filter.

In addition, there is an internal light source for precise wavelength sweep position control connected to the polarization-maintaining optical fiber for optical input and the electronic circuit for controlling the tunable wavelength filter, and an internal light source for controlling the wavelength sweep position precisely connected to the polarization-maintaining optical fiber for optical output and the electronic circuit for controlling the tunable wavelength filter. A variable wavelength filter module is constructed by adding a connected optical receiver.

(Example) FIG. 1 shows the configuration of a basic wavelength tunable filter module, which is the most basic configuration of the present invention.

Here, PFI and PP2 are polarization maintaining optical fibers, Ll
is a collimating lens, L2 is a fiber coupling lens,
VFI is a variable wavelength filter, ECI is an electronic circuit for controlling the variable wavelength filter, CL is an electronic cooler, and EC2 is an electronic circuit for controlling the electronic cooler.

FIG. 2 shows an enlarged view of the variable wavelength filter.

ARC is an anti-reflection coating layer, GS is a glass substrate, ITO is a transparent electrode layer such as indium tin oxide (ITO) or tin oxide, M is a mirror, ORL is an alignment film such as a solid metal, LC is a liquid crystal layer, and E- is the wiring.

First, the operating principle of the variable wavelength filter will be explained.

FIGS. 3(a) and 3(b) illustrate the orientation of liquid crystal molecules when no electric field is applied to the liquid crystal and when it is applied.

Here, LCM is a liquid crystal molecule, and the arrow indicates the alignment direction. etc.) or using an obliquely deposited SiO film of similar thickness, liquid crystal molecules are aligned parallel to the substrate in the alignment direction of the alignment film in the absence of an applied electric field. This is because the energy obtained by adding the interaction energy between the liquid crystal and the alignment film to the potential energy of the liquid crystal during alignment treatment becomes minimum when the liquid crystal molecules are aligned in the alignment direction. However, when an electric field is applied perpendicular to the substrate, the liquid crystal molecules have a dipole moment, which interacts with the electric field, and the system becomes energetically stable as the liquid crystal rotates in the direction of the electric field.

At this time, since the alignment directions of the opposing alignment films are opposite, the rotation direction of the liquid crystal molecules is uniquely determined.
Disclination, which is a factor of light scattering, can be avoided. Here, consider the case where parallel light polarized in the alignment direction of the liquid crystal is incident perpendicularly on the substrate. As inferred from the highly anisotropic shape, liquid crystal molecules have extremely large optical anisotropy.For this reason, when an electric field is applied, the curvature index n in the direction of the polarization plane changes greatly as the liquid crystal molecules rotate. do. The change in flexural index reaches a maximum of several ten percent. n is a function of the electric field E.

n(E)−nO+Δn(E)(1
) Here, no is the bending index when no electric field is applied.

In the variable wavelength filter, an optical resonator is constructed by sandwiching the liquid crystal layer and alignment film between opposing panels. When the reflectance of the mirror is r, the wavelength of the incident light is λ, and the distance between the mirrors is L, the transmittance T of the resonator and the resonant wavelength λres are expressed by the following equation.

T= 1/ (1+Fsin”(2rn(E)L/λ)
) (2) F = 4r/(1-r)2(3) λres=m/2n(E)L (m=1,2'
) (4) Figure 4 shows a transmission spectrum focusing on one resonance peak. The solid line is the transmission spectrum when no electric field is applied, and the broken line is the transmission spectrum when the electric field is applied. As is clear from equation (4), the resonance peak moves as the curvature of the liquid crystal layer changes when an electric field is applied. That is, by controlling the potential difference between the transparent electrodes on the outside of the resonator, the resonant wavelength of the resonator can be controlled and the resonator can be operated as a variable wavelength filter. Let λ-axλ■in be the upper and lower limits of the operating wavelength of the variable wavelength filter, respectively, and K be the allowable crosstalk. The upper limit of the value of m is given by the following equation. (Condition that only one resonance peak exists in the operating wavelength range) ■
1+λvaax/(λwax-λ-1n)
(5) If the minimum channel spacing of a wavelength-multiplexed optical signal is Δλ, the crosstalk is λ■in<λ×λ
-ax is the sum of crosstalk from different channels, and is expressed by the following equation.

K= Σ (1/ (1+Fsin”(mz λ+pΔ J))) (6) Here,
p is an integer (≠O), and λ−in <λ+pΔλ×λ■a
x and sum over P from 6 or more λ*ax=1
．． 6B/m, λ1n=1.5μs, m=1
6 (maximum value), R=0.99. K=0.1
Assuming that, it is possible to demultiplex approximately 100 wavelength multiplexed optical signals with a minimum channel spacing Δλ=0.92n−.

Next, a high-precision voltage control circuit used in the wavelength tunable filter module will be explained. If we use a variable wavelength filter to separate 100 wavelengths as described above, since the driving voltage of the liquid crystal is several square meters, the voltage will be 2 mV to 3 mV.
must be controlled with high precision.

The configuration of the electronic circuit ECI for controlling the variable wavelength filter is shown in FIGS. 5(a), (b), (c), and (d).
FIG. 5(a) shows a built-in battery type configuration that is less susceptible to external disturbances, where BA is a battery and VR is a variable resistor.

Figure 5(b) shows an external power supply type configuration, in which the GE
N is a reference voltage generator using a Zener diode, etc., and V
R is a variable resistor, AMP is an amplifier, and DEVI.

DEV2 is an amplification factor setting element. Figure 5(c) shows a configuration in which the same Keyaki is externally powered and can digitally vary the applied voltage; GEN is a reference voltage generator using a Zener diode, etc., 5II4 is a switch for setting the applied voltage, OA is a digital/analog converter. Figure 5 (
d) is an alternating current application type configuration to extend the life of the liquid crystal layer, GEN is a reference voltage generator using a Zener diode, etc., ROM is a memory element that stores the waveform of an alternating current signal, and CP
U is a one-chip microcomputer for generating digital signals, and DA is a digital/analog converter.For example, if the supply voltage is 15V and a 16-bit digital circuit is used, the output voltage can be controlled with an accuracy of about 200μV.

Next, the operation of the basic type wavelength tunable filter module will be explained with reference to FIG. Polarization maintaining optical fiber PFI
A wavelength-multiplexed optical signal is guided in the waveguide.

This light is converted into parallel light with a diameter of about 100 us by the lens L1, and is input to the variable wavelength filter VFI. In vpl, one selected light that transmits only one wave of the multiplexed optical signal is coupled by lens L2 to polarization maintaining optical fiber PF2 and output. The transmission wavelength of the variable wavelength filter VFI is controlled by the voltage applied to the transparent electrode device from the variable wavelength filter control electronic circuit EC2. Since the transmission wavelength of the variable wavelength filter VFI also depends on the surrounding temperature, an electronic cooler CL is attached and the temperature is maintained at a constant temperature by an electronic cooler control circuit EC2.

FIG. 6 shows the configuration of an isolator-inserted variable wavelength filter module according to another embodiment of the present invention.
Here, the basic type embodiment is an isolator IS on the input side.
The difference is that the reflected light is reduced by inserting a The operation is the same as the basic type.

FIG. 7 shows the configuration of an oblique incidence type variable wavelength filter module, which is another embodiment of the present invention. here,
This embodiment differs from the basic embodiment in that the normal line of the variable wavelength filter is arranged at an angle with respect to the optical axis. In this case as well, unnecessary reflected light can be reduced.

FIG. 8(a) shows the configuration of a multi-stage connection type variable wavelength filter module which is another embodiment of the present invention.

Here, the difference from the basic type embodiment is that variable wavelength filters are arranged in multiple stages. For example, when two identical variable wavelength filters are connected in series, the transmittance is Tt in equation (2), so The half width of the filter's transmission band is approximately 6
4%, and the 1/10 value width is reduced to 49%, so the resolution improves and the number of channels increases significantly. Also the 8th
Filters with different resonator lengths can be used as shown in Figure (b). (The transmittance of each filter is TI.

Assuming T2, the transmittance when combined is TI
It becomes T2. ) In this case, it is not necessarily necessary to satisfy the condition of equation (5) for one etalon. Therefore, a variable wavelength filter with a long resonator length and high finesse can be used, and the number of channels increases.

Further, in order to prevent interference between the filters, the filters are arranged in different directions with respect to the optical axis.

FIG. 9 shows the configuration of an embodiment of a polarization-independent variable wavelength filter module, which is another embodiment of the present invention, where Fl and F2 are ordinary optical fibers, PBSI, and P.
H10 is a polarizing beam splitter, PI, and F2 is a right angle prism. In this case, the light emitted from the lens L1 is separated into two polarized light components by a polarizing beam splitter PBSI, and a variable wavelength filter is arranged so that the plane of polarization matches the alignment direction of the liquid crystal of the variable wavelength filter. The light is input to a wavelength filter, demultiplexed, superimposed again by a polarization beam splitter PBS2, and then output. This configuration has the advantage that the wavelength-multiplexed signal light does not necessarily have to be linearly polarized light.

FIG. 10 shows the configuration of another embodiment of the polarization compensation type variable wavelength filter module of the present invention, where COM is a liquid crystal cell for polarization compensation and serves as a variable retardation plate. . In general, the orientation direction of liquid crystals is not clear, as is the crystal axis of the crystals, so the light transmitted through the variable wavelength filter is elliptically polarized light. For this reason, the liquid crystal cell COM was configured to always output linearly polarized light.

FIG. 11 shows the configuration of a precisely controlled variable wavelength filter module according to another embodiment of the present invention, where CPI, CF2 are optical cans 1, os is a light source, and DE
T is a photoreceiver. In this configuration, monochromatic light with a narrow spectrum is incident on the variable wavelength filter from the light source os, and the transmitted light is monitored by the light receiver DET, allowing accurate tuning to the wavelength transmitted by the filter.

Needless to say, it is possible to create a tank bottom that combines the construction methods of several types of variable wavelength filter modules explained above.Also, in the above embodiments, nematic liquid crystal with a homogeneous arrangement was used as the liquid crystal, but other arrangements (e.g. It goes without saying that other liquid crystals (such as cholesteric liquid crystals) or homeotropic liquid crystals (eg, homeotropic liquid crystals) can also be used.

(Effects of the Invention) As explained above, the variable wavelength filter module of the present invention has the following effects.

(1) It can be driven with low voltage, has no mechanically moving parts, can be easily miniaturized and weighed, and can sweep a wide wavelength range.

(2) Reflected return light can be reduced by inserting an isolator or diagonally arranging a variable wavelength filter.

(3) Higher resolution can be achieved by arranging variable wavelength filters in multiple stages.

(4) By separating the signal light into orthogonal polarization components and demultiplexing them independently, polarization independence can be achieved.

(5) By inserting a liquid crystal cell for polarization compensation, it is possible to always output linearly polarized light.

(6) By monitoring the transmitted light, it is possible to precisely tune to the desired wavelength for demultiplexing.

(7) The electronic circuit for controlling the variable wavelength filter can stably and selectively extract specific wavelengths using a high-precision DC or AC voltage control circuit.

(8) Since each part is integrated and modularized, it is easy to handle and can be incorporated into other devices.

[Brief explanation of drawings]

Figure 1 is the most basic configuration of the present invention, Figure 2 is an explanatory diagram of a variable wavelength filter using liquid crystal, Figure 3(a) is a configuration diagram of liquid crystal molecules when no electric field is applied,
Figure (b) is a configuration diagram of liquid crystal molecules when an electric field is applied, Figure 4 is a diagram showing changes in the transmission spectrum of a variable wavelength filter as an electric field is applied, and Figures 5 (a), (b), (c). ), (d) is a configuration diagram of an electronic circuit for controlling a variable wavelength filter, FIG. 6 is a diagram showing the configuration of an isolator-inserted variable wavelength filter according to another embodiment of the present invention, and FIG. 7 is a diagram of another embodiment of the present invention. Component countries of the diagonally arranged variable wavelength filter. FIG. 8(a) shows the component countries of the multi-stage connected variable wavelength filter according to another embodiment of the present invention. FIG. An explanatory diagram of the operation of a multi-stage connection type variable wavelength filter, FIG. 9 shows the configuration of a polarization-independent variable wavelength filter according to another embodiment of the present invention, and FIG. 10 illustrates a polarization-compensated variable wavelength filter according to another embodiment of the present invention. Component countries of the wavelength filter. FIG. 11 shows the component countries of a precisely controlled variable wavelength filter according to another embodiment of the present invention. FIG. It is an explanatory diagram. Fl, F2... Optical fiber Ll, L2...
- Lens M... Mirror PZ... Piezo element ETL... Etalon ARC... Anti-reflection coating S... Substrate G... Grating ACT... Active layer El, F2. F3
...Electrode PFI, PF2...Polarization maintaining optical fiber VFI, VF2...Variable wavelength filter ECI...
Electronic circuit for controlling variable wavelength filter EC2...Electronic circuit for controlling electronic cooler CL...Electronic cooler GS...Glass substrate Placement...Transparent electrode layer OR
L...Alignment film LC...Liquid crystal layer E-...
Wiring LCM...Liquid crystal molecule BA...Battery VR...Variable resistor GEN...Reference voltage generator AMP...Amplifier ROM...AC waveform storage elements DEVI, DEV2...For setting amplification factor Element S...
・Applied voltage setting switch DA...Digital/analog converter IS...Isolator CPU...One-chip microcomputer PBSI
, PBS2...Polarizing beam splitter PL, P2
...Right-angle prism COM...Liquid crystal cell for polarization compensation CPI, CF2...Optical coupler O5...Light source DET...Photoreceiver Fig. 1

Claims (1)

[Claims] 1. A glass substrate, a transparent One or more tunable wavelength filters are installed, each of which is formed by sequentially laminating an electrode, a high-reflection mirror, an alignment film, a liquid crystal, an alignment film, a high-reflection mirror, a transparent electrode, and a glass substrate, and the tunable wavelength filter A tunable wavelength filter module characterized in that the tunable wavelength filter module is provided with a tunable wavelength filter control electronic circuit that controls a transmitted wavelength and an electronic circuit that keeps the temperature of the tunable wavelength filter constant. 2. The tunable wavelength filter module according to claim 1, wherein an isolator is inserted into a polarization-maintaining optical fiber for optical input. 3. In the variable wavelength filter module according to claim 1 or 2, the input and output optical fibers are non-polarization-maintaining optical fibers, and a polarization splitting element and a fiber coupling lens are incident on the output side of the collimating lens. A tunable wavelength filter module characterized by adding a coupling element on the side and arranging a tunable wavelength filter for each separated polarized light. 4. The tunable wavelength filter module according to claim 1, 2, or 3, further comprising a liquid crystal cell for polarization compensation connected to the tunable wavelength filter control electronic circuit. Wavelength filter module. 5. In the tunable wavelength filter module according to any one of claims 1 to 4, an internal section for precise wavelength sweep position control connected to the polarization maintaining optical fiber for optical input and the electronic circuit for controlling the tunable wavelength filter. A tunable wavelength filter module comprising a light source, a polarization-maintaining optical fiber for light output, and a light receiver connected to an electronic circuit for controlling the tunable wavelength filter.