JPH07325281A - Variable wavelength optical filter - Google Patents

Abstract

PURPOSE:To obtain a variable wavelength optical filter which has lower dependency on polarization and has a steep filter characteristic by constituting an optical system in which the light signal past a twisted nematic liquid crystal cell having a Fabry-Perot etalon structure passes the same liquid crystal cell again after passing a wavelength plate of a specific wavelength. CONSTITUTION:This variable wavelength optical filter has the optical system in which the light beam past the twisted nematic liquid crystal cell 31 having the Fabry-Perot etalon structure is turned back by a right-angle prism 101, is changed 180 deg. in the progressing direction by the prism and is emitted by passing the half-wave plate 91, then passing the same liquid crystal cell 31 again. The optical filter is so formed that the light beam is passed twice the same liquid crystal cell via the half-wave plate 91 in such a manner and, therefore, the specified transmission characteristic is obtd. and the dependency on the polarization is eliminated even if the output level is changed by the polarization state 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 optical filter using a liquid crystal capable of extracting only an optical signal of an arbitrary wavelength from optical signals of various wavelengths.

[0002]

2. Description of the Related Art FIG. 1 shows the basic structure of a conventional optical filter of this type.

The structure of the optical filter shown in FIG. 1 will be described along the propagation of a light beam.

A light beam mixed with various wavelengths passes through the optical fiber 11 and becomes a parallel beam from the collimator lens 21 and is emitted toward the liquid crystal cell 31. The emitted light beam is propagated to the optical fiber 12 via the twisted nematic liquid crystal cell 31 having the Fabry-Perot etalon structure and the collimating lens 22 for receiving light.

Here, the liquid crystal cell 31 has a pair of transparent substrates 71 and 72 facing each other with a certain gap, and ITO (oxidation) as transparent electrodes formed on the respective facing surfaces of the transparent electrodes 71 and 72. indium·
Tin compound) films 41 and 42, mirror films 51 and 52 made of a dielectric material provided on the ITO films 41 and 42, and liquid crystal alignment films 61 and 62 provided on the mirror films 51 and 52. , These alignment films 61
And a nematic type liquid crystal 81 filled in the gap between 62 and 62. The alignment films 61 and 62 are arranged so that the respective alignment directions are orthogonal to each other, whereby the liquid crystal 81 is arranged in a twist type twisted by 90 degrees. Furthermore, the ITO films 41 and 4
A power supply device 40 that applies a predetermined voltage is connected between the two.

In the liquid crystal cell 31 having such a structure, since the refractive index of the liquid crystal 81 changes according to the magnitude of the voltage V applied to the ITO films 41 and 42, the resonance wavelength changes and the liquid crystal cell 31 is changed. Only the optical signal of a specific wavelength in the light beam passing through is selected and transmitted. 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 in the liquid crystal cell 31 is slightly different depending on the polarization state of the light beam passing therethrough. For example, polarized waves orthogonal to each other are P and S, respectively.
2 shows the resonance characteristic of the liquid crystal cell 31 in such a case. The horizontal axis of FIG. 2 represents the voltage applied to the liquid crystal cell, and the vertical axis represents the resonance wavelength of the liquid crystal cell.

The conventional optical filter having the configuration shown in FIG.
Even if the liquid crystal cell 31 is set to the voltage of the required wavelength, the output level changes depending on the polarization state of the light beam. That is, as shown in FIG. 3, when the light beam incident on the liquid crystal cell 31 is P-polarized (Tp (λ)) and when it is S-polarized (Tp (λ)).
The difference between s (λ) and s (λ) is a maximum of several nm, which is a drawback that polarization dependence is large.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a tunable wavelength optical filter having a small polarization dependence and a steep filter characteristic.

[0010]

In order to achieve the above object, according to the present invention, an optical signal transmitted through a twisted nematic liquid crystal cell having a Fabry-Perot etalon structure passes through a ½ wavelength plate or a ¼ wavelength plate. After that, the most main feature is to have an optical system that transmits the same liquid crystal cell again. It differs from the conventional technique in that the optical signal passes through the same liquid crystal cell twice and passes through the ½ wavelength plate or the ¼ wavelength plate in the middle of the first and second times.

That is, the invention according to claim 1 is a variable wavelength optical filter, which is a twisted nematic liquid crystal cell having a Fabry-Perot etalon structure and a half-wavelength arranged perpendicular to the optical axis of the liquid crystal cell. A plate and a light beam passing through the liquid crystal cell in parallel to the optical axis change the traveling direction of the light beam by 180 degrees after passing through the liquid crystal cell,
And an optical system arranged so as to pass through the half-wave plate and again pass through the liquid crystal cell.

According to a second aspect of the present invention, there is provided a tunable optical filter comprising a twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, and a half-wave plate arranged perpendicular to the optical axis of the liquid crystal cell. A light beam passing through the liquid crystal cell in parallel with the optical axis passes through the half-wave plate after passing through the liquid crystal cell, and then changes the traveling direction of the light beam by 180 degrees, and again And an optical system arranged so as to pass through the liquid crystal cell.

The invention according to claim 3 is a variable wavelength optical filter, comprising a twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, and a quarter-wave plate arranged perpendicularly to the optical axis of the liquid crystal cell. , A light beam that passes through the liquid crystal cell in parallel to the optical axis passes through the liquid crystal cell and then passes through the quarter wavelength plate, and the traveling direction of the light beam is set to 1
The optical system is arranged so as to be changed by 80 degrees, pass through the quarter wavelength plate again, and pass through the liquid crystal cell again.

According to a fourth aspect of the present invention, there is provided a variable wavelength optical filter having a Fabry-Perot etalon structure, and a twisted nematic liquid crystal cell in which a plurality of transparent electrode pairs are electrically insulated from each other. A half-wave plate arranged perpendicularly to the optical axis of the liquid crystal cell, and a plurality of light beams that pass through the liquid crystal cell in parallel with the optical axis are transmitted after passing through the liquid crystal cell. Change the direction of travel by 180 degrees
An optical system arranged so as to pass through the / 2 wave plate and pass through the liquid crystal cell again.

A fifth aspect of the present invention is a variable wavelength optical filter, which has a Fabry-Perot etalon structure, and a twisted nematic liquid crystal cell in which a plurality of transparent electrode pairs are electrically insulated from each other. A half-wave plate arranged perpendicularly to the optical axis of the liquid crystal cell, and a plurality of light beams passing through the liquid crystal cell in parallel to the optical axis are passed through the liquid crystal cell and then The optical system is arranged so as to pass through the wave plate, then change the traveling direction of the light beam by 180 degrees, and again pass through the liquid crystal cell.

A sixth aspect of the present invention is a variable wavelength optical filter, which has a Fabry-Perot etalon structure, and a twisted nematic liquid crystal cell in which a plurality of transparent electrode pairs are electrically insulated from each other. A quarter-wave plate arranged perpendicular to the optical axis of the liquid crystal cell, and a quarter-wave plate after a light beam passing through the liquid crystal cell parallel to the optical axis passes through the liquid crystal cell The direction of travel of the light beam
The optical system is arranged so as to be changed by 80 degrees, pass through the quarter wavelength plate again, and pass through the liquid crystal cell again.

Here, the invention according to claim 7 is the same as claim 1.
In the variable wavelength optical filter according to any one of items 1 to 6, a temperature adjusting element may be attached to the liquid crystal cell so as to be thermally coupled.

The invention according to claim 8 is the variable wavelength optical filter according to any one of claims 1 to 7, wherein a part of the light beam whose traveling direction is changed by 180 degrees and which has passed through the liquid crystal cell again. A half mirror that transmits light and reflects the rest is arranged, and a photodiode on which the light beam reflected by the half mirror is incident is arranged, and the liquid crystal cell is driven so that the amount of light received by the photodiode is always maximized. A circuit for adjusting the voltage may be arranged.

Further, the invention according to claim 9 is the same as claim 1.
In the variable wavelength optical filter according to any one of items 1 to 8, the direction of the light beam incident on the liquid crystal cell is changed to
A first optical system which is changed by 90 degrees before being incident on the liquid crystal cell and which is incident parallel to the optical axis of the liquid crystal cell, and a direction of a light beam which has passed through the liquid crystal cell by two degrees is changed by 90 degrees. Two optical systems may be provided.

[0020]

According to the present invention, the light beam is made to pass through the twisted nematic liquid crystal cell of the same Fabry-Perot etalon structure twice through the half-wave plate or the quarter-wave plate. Even if the output level changes depending on the wave state, the transmission characteristic can be made constant, so that the polarization dependence can be eliminated.

[0021]

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

[Embodiment 1] FIG. 4 is a schematic plan view showing an embodiment of the variable wavelength optical filter of the present invention. In this embodiment, a liquid crystal cell 31 having the same structure as the liquid crystal cell shown in FIG.
The light beam that has passed through is folded back by the right-angle prism 101, and after changing the traveling direction by 180 degrees, the half-wave plate 91
It has an optical system which is emitted through the liquid crystal cell 31 again.

Since the resonance characteristic when a voltage is applied to the liquid crystal cell 31 is as shown in FIG. 2, the filter characteristic when the light beam passes through the liquid crystal cell 31 only once is as described above. It is represented by Tp (λ) or Ts (λ) in FIG. Therefore, when the incident light beam is P-polarized light and when it is S-polarized light, a peak shift of several nm occurs, and the polarization dependence appears significantly.

However, the optical filter having the structure shown in FIG. 4 has an optical system in which the light beam passes through the same liquid crystal cell 31 twice through the half-wave plate, and therefore, for example, P polarization is used. When a light beam of only waves is incident, the transmission characteristic of the P polarization is Tp (λ) in the first pass, and the transmission characteristic of the S polarization is Ts (λ) in the second pass. Ts (λ) × Tp shown in FIG. 3 multiplied by the transmission characteristics of
The transmission characteristic is (λ). On the contrary, when a light beam of only S-polarized light is incident, on the contrary, the transmission characteristic of Ts (λ) at the first passage is Tp (λ) at the second passage, so that Ts (λ) is also overall as a whole. It becomes × Tp (λ), which coincides with the transmission characteristic when the light beam of only P-polarized light is passed twice. Of course, even if the polarization state of the light beam is a mixture of P polarization and S polarization, the transmission characteristic is Ts (λ) × Tp.
(Λ), and the transmission characteristics match in any case, so that the polarization dependence disappears.

Further, in the present embodiment, the light beam may be transmitted through the half-wave plate before being reflected by the prism 101. Even if such an optical system is provided, it is possible to eliminate the polarization dependence of the light beam as in the above case.

[Embodiment 2] FIG. 5 is a schematic plan view showing another embodiment of the variable wavelength optical filter of the present invention. The feature of the configuration of the present embodiment is that after the first passage of the light beam through the liquid crystal cell, instead of passing the half-wave plate only once, the quarter-wave plate is divided into quarters.
The point is that it has an optical system that allows the wave plate 201 to pass twice.

Using a twisted nematic liquid crystal cell, T
Only the peaks of p (λ) and Ts (λ) are shifted by a maximum of several nm, the spectra of Tp (λ) and Ts (λ) sufficiently overlap, and Tp (λ) × Ts (λ) has a finite value. However, Tp (λ) and Ts using other types of liquid crystal cells
Since the peak of (λ) is too far apart, Tp (λ) × Ts
(Λ) has only a small value.

Therefore, in the structure of the present embodiment, the influence of the polarization state of the input light beam is reduced, the polarization dependence can be eliminated, and the light beam is reflected by the liquid crystal cell.
Since the light passes through a number of times, it is possible at the same time to obtain a filter having a sharp and narrow band transmission characteristic.

[Embodiment 3] FIG. 6 is a schematic plan view showing still another embodiment of the tunable wavelength optical filter of the present invention.
This embodiment has a basic configuration of the first embodiment shown in FIG. In FIG. 6, reference numeral 13 is an optical fiber for inputting an optical signal, 14 is an optical fiber for outputting an optical signal, 23 and 24 are collimating lenses, 102 and 104 are prisms for bending an optical path at right angles, and 34 is a Fabry-Perot structure and twisted. A liquid crystal cell filled with nematic liquid crystal in the state, 103 is a prism for folding the optical path,
Reference numeral 92 is a half-wave plate, 211 is a half mirror, 301 is a photodiode for receiving the light beam reflected from the half mirror 211, 401 is a detection circuit of the light receiving level of the photodiode 301, and 501 is a detection circuit 4.
This is an APC circuit for holding the level detected at 01 constant, and its output is connected to the liquid crystal drive circuit 601. 701 is a light amount detection circuit 401 and an APC circuit 50.
1 and a power supply terminal for operating the liquid crystal drive circuit 601.
Reference numeral 801 denotes a Peltier element or a heater, which is the liquid crystal cell 3
Thermally coupled with 4. The thermistor 901 is thermally coupled to the liquid crystal cell 34, and its output is supplied to a temperature controller (not shown) via the terminal of 702, and the terminal of 703 is controlled so that the liquid crystal cell 34 has a constant temperature. The current supplied to the Peltier element or the heater 801 is fed back through the device. 100 is a case.

In the collimating lenses 23 and 24, the half mirror 211, the photodiode 301, the Peltier element or the heater 801 are firmly fixed to the case 1000, respectively. In addition, the entrance and exit surfaces of the light beam are all subjected to antireflection treatment.

In the optical filter having such a structure,
The optical signal incident on the optical fiber 13 first passes through the liquid crystal cell 34 via the prism 102, and then the prism 103.
Then, the light passes through the half-wave plate 92, passes through the liquid crystal cell 34 again, and is emitted from the optical fiber 14 via the prism 104 and the collimator lens 24. As a result, since the light beam passes through the optical path shown in FIG. 4, the light of the required wavelength is adjusted by adjusting the voltage applied to the liquid crystal cell 34 without depending on the polarization of the light beam incident by the device of the present invention. You can get a signal.

When filtering an optical signal according to this embodiment, the setting of the voltage corresponding to the wavelength to be transmitted is as follows.
Transmission windows Ts (λ) × Tp (λ) obtained by multiplying the respective transmission windows of the two polarizations P and S in FIG.
Is performed by adjusting to the wavelength of the peak position of, and at this time, the output light signal intensity becomes maximum.

Further, in this embodiment, in order to prevent the output fluctuation due to the wavelength fluctuation of the optical signal, a part of the output light beam is taken out by the half mirror 211 so that the intensity of the output light always becomes the maximum value. In addition to being able to automatically adjust the voltage applied to 34, the liquid crystal cell 34 is made to have a constant temperature as much as possible.

In this embodiment, wavelengths of 1540 to 1560n
An optical signal of 20 waves with a 1 nm interval between m and a loss of 4 dB,
It was confirmed that the crosstalk was -20 dB or less and the polarization dependence was 0.5 dB or less. In addition, Ts in this case
The shift between the peaks of (λ) and Tp (λ) was about 1.5 nm.

[Fourth Embodiment] FIG. 7 is a schematic perspective view showing another embodiment of the tunable wavelength optical filter of the present invention, and FIG. 8 shows a Fabry-Perot etalon structure which can be used in the optical filter shown in FIG. It is a schematic perspective view which shows the internal structure of an example of the twisted nematic liquid crystal cell which it has. The liquid crystal cell of FIG. 8 has the transparent substrates 73 and 74 which are essentially parallel to each other.
For the sake of explanation, the figure is shown in a composition in which the front side of the drawing is opened.

The feature of this embodiment is that the liquid crystal cells are in the form of an array having four pairs of optical signal input / output sections. That is, the liquid crystal cell 35 of this embodiment has the basic configuration of the liquid crystal cell of the first embodiment shown in FIG. 4, and as shown in FIG. 8, of the ITO films 43 and 44 on the pair of transparent substrates 73 and 74. Only one ITO film 43 is divided into four stripes by a strip-shaped electrically insulating portion.
On the input side of the liquid crystal cell 35, as shown in FIG.
3, 23 ′, 23 ″ and 23 ″ ″ and optical fibers 13, 13 ′, 13 ″ and 13 ″ ″ are connected, and collimator lenses 24, 24 ′, 2 are provided on the output side.
4 ″ and 24 ″ ″ and optical fibers 14, 1
4 ', 14 "and 14"' are connected. 9
Reference numeral 2 is a half-wave plate, and 103 is a rectangular prism.

The liquid crystal cell 35 having such a structure constitutes an optical filter in the form of an array having four pairs of optical signal input / output sections independent of each other. A four-output variable wavelength optical filter was produced.

In this embodiment, since the voltage applied to the four stripe electrodes can be changed independently, it was confirmed that each filter has the same filter characteristics as those of the previous embodiment 3 without mutual interference. did it.

[0039]

As described above, according to the tunable wavelength optical filter of the present invention, when the wavelength division multiplexed optical signal is transmitted using the optical fiber system, it is installed when the signal separation is performed on the receiving side. Even if the polarization state of the optical signal changes due to a change in environmental temperature or bending stress due to an external force applied to the optical fiber, stable wavelength separation and signal level can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic plan view showing a configuration of a conventional optical filter.

FIG. 2 is a graph showing a resonance wavelength characteristic with respect to an applied voltage of the liquid crystal cell shown in FIG.

FIG. 3 is a graph showing filter characteristics of the liquid crystal cell shown in FIG.

FIG. 4 is a schematic plan view showing an embodiment of the variable wavelength optical filter of the present invention.

FIG. 5 is a schematic plan view showing another embodiment of the variable wavelength optical filter of the present invention.

FIG. 6 is a schematic plan view showing still another embodiment of the tunable wavelength optical filter of the present invention.

FIG. 7 is a schematic perspective view showing another embodiment of the tunable wavelength optical filter of the present invention.

8 is a schematic perspective view showing an internal structure of an example of a liquid crystal cell that can be used in the optical filter shown in FIG.

[Explanation of symbols]

 11, 12, 13, 14 Optical fiber 21, 22, 23, 24 Collimating lens 31, 34, 35 Liquid crystal cell 41, 42, 43, 44 ITO film 51, 52 Mirror film 61, 62 Alignment film 71, 72, 73, 74 Transparent Substrate 81 Liquid Crystal 91, 92 1/2 Wave Plate 102, 103, 104 Right Angle Prism 201 1/4 Wave Plate 211 Half Mirror 301 Photodiode 401 Light Quantity Detection Circuit 501 APC Circuit 601 Liquid Crystal Drive Circuit 701, 702, 703 Electric Terminal 801 Peltier element or heater 901 Thermistor 1000 Case

Claims (9)

[Claims] 1. A twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, a half-wave plate arranged perpendicularly to the optical axis of the liquid crystal cell, and transmitting through the liquid crystal cell parallel to the optical axis. After the light beam passes through the liquid crystal cell, the traveling direction of the light beam is changed by 180 degrees,
An optical system arranged so as to pass through the wave plate and pass through the liquid crystal cell again. 2. A twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, a half-wave plate arranged perpendicularly to the optical axis of the liquid crystal cell, and transmitting through the liquid crystal cell parallel to the optical axis. An optical system arranged so that the light beam passes through the liquid crystal cell and then passes through the half-wave plate, then changes the traveling direction of the light beam by 180 degrees, and again passes through the liquid crystal cell. A tunable wavelength optical filter characterized by: 3. A twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, a quarter-wave plate arranged perpendicular to the optical axis of the liquid crystal cell, and transmitting the liquid crystal cell parallel to the optical axis. The light beam passes through the liquid crystal cell and then passes through the quarter wavelength plate, changes the traveling direction of the light beam by 180 degrees, passes through the quarter wavelength plate again, and passes through the liquid crystal cell again. A tunable wavelength optical filter, comprising: 4. A twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, in which a plurality of transparent electrode pairs are electrically insulated from each other, and 1 arranged perpendicular to the optical axis of the liquid crystal cell. 1/2 wave plate and a plurality of light beams that pass through the liquid crystal cell in parallel to the optical axis change the traveling direction of the light beam by 180 degrees after passing through the liquid crystal cell. A tunable wavelength optical filter, wherein the optical system is arranged so as to pass through the liquid crystal cell and pass through the liquid crystal cell again. 5. A twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, in which a plurality of transparent electrode pairs are electrically insulated from each other, and 1 arranged perpendicular to the optical axis of the liquid crystal cell. A half wave plate and a plurality of light beams passing through the liquid crystal cell in parallel with the optical axis pass through the half wave plate after passing through the liquid crystal cell, and then the traveling direction of the light beam is changed. A variable wavelength optical filter, comprising: an optical system which is changed by 180 degrees and is arranged so as to pass through the liquid crystal cell again. 6. A twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, in which a plurality of transparent electrode pairs are electrically insulated from each other, and 1 arranged perpendicular to the optical axis of the liquid crystal cell. A quarter-wave plate and a light beam that passes through the liquid crystal cell parallel to the optical axis pass through the liquid crystal cell and then pass through the quarter-wave plate, and the traveling direction of the light beam is changed by 180 degrees. And an optical system arranged so as to pass through the quarter-wave plate again and pass through the liquid crystal cell again. 7. The variable wavelength optical filter according to claim 1, wherein a temperature adjusting element is attached to the liquid crystal cell so as to be thermally coupled. Wavelength optical filter. 8. The variable wavelength optical filter according to claim 1, wherein a part of the light beam whose traveling direction is changed by 180 degrees and which has passed through the liquid crystal cell again is transmitted and the rest is reflected. A half mirror is arranged, a photodiode on which the light beam reflected by the half mirror is incident is arranged, and a circuit for adjusting the drive voltage of the liquid crystal cell is arranged so that the amount of light received by the photodiode is always maximized. A tunable wavelength optical filter including: 9. The variable wavelength optical filter according to claim 1, wherein the direction of the light beam incident on the liquid crystal cell is set to 9 before entering the liquid crystal cell.
It comprises a first optical system which is changed by 0 degree and is incident parallel to the optical axis of the liquid crystal cell, and a second optical system which changes the direction of the light beam which has passed through the liquid crystal cell by 2 degrees by 90 degrees. A tunable wavelength optical filter characterized in that