JP3137162B2 - Tunable wavelength optical filter - Google Patents

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 a basic configuration of a conventional optical filter of this type.

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

[0004] A light beam having various wavelengths mixed passes through the optical fiber 11 and is emitted from the collimator lens 21 as a parallel beam toward the liquid crystal cell 31. The emitted light beam propagates through the twisted nematic liquid crystal cell 31 having the Fabry-Perot etalon structure and the collimating lens 22 for light reception to the optical fiber 12.

Here, the liquid crystal cell 31 has a pair of transparent substrates 71 and 72 facing each other with a certain gap, and ITO (oxidized) as a transparent electrode formed on each opposing surface of the transparent electrodes 71 and 72. indium·
(Tin compound) films 41 and 42, dielectric mirror films 51 and 52 provided on these ITO films 41 and 42, and liquid crystal alignment films 61 and 62 provided on these mirror films 51 and 52. , These alignment films 61
And a liquid crystal 81 of a nematic type filled in a gap between the liquid crystal and the liquid crystal. Further, 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. Further, the ITO films 41 and 4
A power supply 40 for applying 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 Only an optical signal of a specific wavelength of the passing light beam 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 of the liquid crystal cell 31 is slightly different even at the same voltage depending on the polarization state of the light beam passing therethrough. For example, the orthogonal polarizations are P and S, respectively.
FIG. 2 shows the resonance characteristics of the liquid crystal cell 31 when. The horizontal axis in FIG. 2 shows the voltage applied to the liquid crystal cell, and the vertical axis shows 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 a voltage of a 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)).
s (λ)), there is a maximum deviation of several nm, and there is a drawback that polarization dependence is large.

[0009]

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

[0010]

In order to achieve the above object, the present invention relates to a liquid crystal cell having a Fabry-Perot etalon structure, wherein an optical signal transmitted through the twisted nematic liquid crystal cell passes through a half-wave plate, and then the same liquid crystal cell is again formed. The most important feature is to have an optical system that transmits light. In the conventional technique, an optical signal passes through the same liquid crystal cell twice, and in the middle of the first and second times, 1
The difference is that the light passes through a half-wave plate.

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

According to a second aspect of the present invention, there is provided a variable wavelength 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 that passes through the liquid crystal cell in parallel to 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, An optical system arranged to transmit the liquid crystal cell.

[0013]

[0014]

According to a third aspect of the present invention, there is provided a tunable optical filter having a Fabry-Perot etalon structure, wherein a plurality of transparent electrode pairs are electrically insulated from each other. A half-wave plate disposed perpendicular to the optical axis of the liquid crystal cell, and a plurality of light beams transmitted through the liquid crystal cell in parallel to the optical axis after the half-wave plate passes through the liquid crystal cell. An optical system that passes through the wave plate, changes the traveling direction of the light beam by 180 degrees, and is again disposed so as to transmit through the liquid crystal cell.

[0016]

Here, the invention described in claim 4 is based on claim 1.
4. The variable wavelength optical filter according to claim 3, wherein a temperature control element is attached to the liquid crystal cell so as to be thermally coupled.

According to a fifth aspect of the present invention, in the variable wavelength optical filter according to any one of the first to fourth aspects, a part of the light beam whose traveling direction has been changed by 180 degrees and passed through the liquid crystal cell again. A half mirror that transmits light and reflects the remainder is disposed, and a photodiode on which the light beam reflected by the half mirror is incident is disposed, and the liquid crystal cell is driven such that the amount of light received by the photodiode is always maximum. A circuit for adjusting the voltage may be provided.

Further, the invention described in claim 6 is the first invention.
6. The variable wavelength optical filter according to claim 5, wherein the direction of the light beam incident on the liquid crystal cell is changed by 90 degrees before the light beam is incident on the liquid crystal cell, and the direction of the light beam is changed with respect to the optical axis of the liquid crystal cell. The liquid crystal display device may further include a first optical system that is incident in parallel, and a second optical system that changes the direction of the light beam that has passed through the liquid crystal cell twice by 90 degrees.

[0020]

According to the present invention, the light beam is caused to pass twice through the same twisted nematic liquid crystal cell having the Fabry-Perot etalon structure via the half-wave plate, so that the output level depends on the polarization state of the light beam. Even if it changes, the transmission characteristics can be kept constant, so that the polarization dependence can be eliminated.

[0021]

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

Embodiment 1 FIG. 4 is a schematic plan view showing an embodiment of the tunable optical filter according to the present invention. In this embodiment, a liquid crystal cell 31 having the same configuration as the liquid crystal cell shown in FIG.
The light beam that has passed through is turned back by the right-angle prism 101 and changes its traveling direction by 180 degrees.
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. For this reason, when the incident light beam is P-polarized light and when the incident light beam is S-polarized light, the peak shifts by a maximum of several nm, and the polarization dependence appears greatly.

However, since the optical filter having the configuration shown in FIG. 4 has an optical system in which the light beam passes twice through the same liquid crystal cell 31 via the half-wave plate, for example, the P-polarized light is used. When a light beam consisting of only a wave is incident, the transmission characteristic of the P polarization is Tp (λ) in the first pass, and the transmission characteristic Ts (λ) of the S polarization in the second pass. Ts (λ) × Tp shown in FIG.
(Λ) transmission characteristic. On the other hand, when a light beam of only S-polarized light is incident, the transmission characteristic of Ts (λ) in the first pass is Tp (λ) in the second pass, so that Ts (λ) as a whole is also obtained. × Tp (λ), which is consistent with the transmission characteristics when a light beam of only P polarization 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 similarly Ts (λ) × Tp.
(Λ), and the transmission characteristics match in any case, so that there is no polarization dependency.

In this embodiment, the light beam may be transmitted through the half-wave plate before being turned back by the prism 101. Even in the case of having such an optical system, there is an effect that the polarization dependence of the light beam can be eliminated as in the above case.

[0026]

[0027]

[0028]

Embodiment 2 FIG. 5 is a schematic plan view showing still another embodiment of the variable wavelength optical filter of the present invention. This embodiment is based on the first embodiment shown in FIG. In FIG. 5, reference numeral 13 denotes an optical fiber for inputting an optical signal, 14 denotes an optical fiber for outputting an optical signal, 23 and 24 denote collimating lenses, 102 and 104 denote prisms for bending the optical path at right angles, and 34 denotes a Fabry-Perot twisted structure. A liquid crystal cell filled with nematic liquid crystal in a state; 103, a prism for folding the optical path; 92, a half-wave plate;
1 is a half mirror, 301 is a photodiode for receiving a light beam reflected from the half mirror 211, 401 is a detection circuit for detecting the light receiving level of the photodiode 301, and 501 is a circuit that keeps the level detected by the detection circuit 401 constant. APC circuit for performing the
1 liquid crystal drive circuit. Reference numeral 701 denotes a power supply terminal for operating the light amount detection circuit 401, the APC circuit 501, and the liquid crystal drive circuit 601, and 801 denotes a Peltier element or a heater, which is thermally coupled to the liquid crystal cell. The thermistor 901 is thermally coupled to the liquid crystal cell 34, and its output is supplied to a temperature control controller (not shown) via a terminal 702, and a terminal 703 is connected 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. 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, an anti-reflection treatment is applied to all the light beam entrance / exit surfaces.

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 through the prism 102, and then passes through the prism 103.
After passing through the half-wave plate 92 and passing through the liquid crystal cell 34 again, the light 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 apparatus of the present invention. A signal can be obtained.

When filtering an optical signal according to this embodiment, the setting of the voltage corresponding to the wavelength to be transmitted is as follows.
A transmission window Ts (λ) × Tp (λ) obtained by crossing the transmission windows of the two polarizations P and S in FIG.
The wavelength of the output light signal is maximized at this time.

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 and the liquid crystal cell is set so that the intensity of the output light always becomes the maximum value. In addition to automatically adjusting the voltage applied to the liquid crystal 34, the temperature of the liquid crystal cell 34 is kept as constant as possible.

In this embodiment, the wavelength is 1540 to 1560n.
m, 20-wave optical signals at 1 nm intervals, with a loss of 4 dB,
It has been confirmed that separation can be performed with a crosstalk of −20 dB or less and a polarization dependency of 0.5 dB or less. In this case, Ts
The deviation between the peaks of (λ) and Tp (λ) was about 1.5 nm.

Embodiment 3 FIG. 6 is a schematic perspective view showing another embodiment of the variable wavelength optical filter of the present invention. FIG. 7 shows a Fabry-Perot etalon structure which can be used in the optical filter shown in FIG. FIG. 2 is a schematic perspective view showing an internal structure of an example of a twisted nematic liquid crystal cell having the same. In the liquid crystal cell of FIG. 7, the transparent substrates 73 and 74 which are originally parallel are shown in a composition in which the front of the drawing is opened for description.

The feature of this embodiment is that the liquid crystal cell has an array shape having four pairs of optical signal input / output sections. That is, the liquid crystal cell 35 of the present embodiment has the basic configuration of the liquid crystal cell of the first embodiment shown in FIG. 4, and as shown in FIG. 7, 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 band-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 collimating lenses 24, 24 'and 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 denotes a half-wave plate, and reference numeral 103 denotes a right-angle prism.

The liquid crystal cell 35 having such a configuration constitutes an arrayed optical filter having four pairs of optical signal input / output sections independent of each other. A four-output tunable wavelength optical filter was manufactured.

In this embodiment, since the voltages applied to the four stripe-shaped electrodes can be changed independently, it was confirmed that each filter exhibited the same filter characteristics as in the third embodiment without mutual interference. did it.

[0039]

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

[Brief description of the drawings]

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

FIG. 2 is a graph showing resonance wavelength characteristics 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 one 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 perspective view showing still another embodiment of the variable wavelength optical filter of the present invention.

7 is a schematic perspective view showing an internal structure of an example of a liquid crystal cell that can be used for 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 wavelength plate 102, 103, 104 Right angle prism 211 Half mirror 301 Photodiode 401 Light intensity detection circuit 501 APC circuit 601 Liquid crystal drive circuit 701, 702, 703 Electric terminal 801 Peltier element or heater 901 Thermistor 1000 case

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/13 500 G02F 1/1335

Claims (6)

(57) [Claims] 1. A twisted nematic liquid crystal cell having a Fabry-Perot etalon structure, a half-wave plate disposed perpendicular 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, and
An optical system disposed so as to pass through the wavelength 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 disposed perpendicular 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 half-wave plate after passing through the liquid crystal cell, then changes the traveling direction of the light beam by 180 degrees, and passes through the liquid crystal cell again. A tunable optical filter. 3. 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 a twisted nematic liquid crystal cell, which is disposed perpendicular to the optical axis of the liquid crystal cell. A half-wave plate and a plurality of light beams that pass through the liquid crystal cell in parallel to the optical axis pass through the liquid crystal cell, pass through the half-wave plate, and then change the traveling direction of the light beam. An optical system which is changed by 180 degrees and arranged so as to transmit the liquid crystal cell again. 4. The variable wavelength optical filter according to claim 1, wherein a temperature control element is attached to the liquid crystal cell so as to be thermally coupled. Wavelength light filter. 5. The variable wavelength optical filter according to claim 1, wherein a part of the light beam whose traveling direction has been changed by 180 degrees and has passed through the liquid crystal cell again is transmitted.
A half mirror that reflects the rest is arranged, a photodiode on which the light beam reflected by the half mirror is incident is arranged, and the drive voltage of the liquid crystal cell is adjusted so that the amount of light received by the photodiode is always maximum. A tunable optical filter, comprising a circuit. 6. The variable wavelength optical filter according to claim 1, wherein a direction of a light beam incident on the liquid crystal cell is adjusted by 90 degrees before entering the liquid crystal cell.
And a second optical system that changes the direction of a light beam that has passed through the liquid crystal cell by 90 degrees and changes the direction of the light beam that has passed through the liquid crystal cell by 90 degrees. A tunable optical filter characterized by the following.