JPH06500408A - Tunable liquid crystal etalon filter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Tunable liquid crystal etalon filter field of invention The present invention relates to optical filters, particularly liquid crystal optical filters.

Background of the invention There is currently a growing need for inexpensive tunable optical filters. for example The proposed architecture for future telephone networks based on optical fibers is WD M (wavelength multiplexing) is used. In WDM, the design of various communication channels is data (e.g. multiple audio channels, video channels, high-speed data channels) Modulating optical carriers of different wavelengths, all optical carriers are packed into one optical fiber. I get sucked into it. All multiplexed optical signals are sent to multiple receivers, each with its own receiver. each receiver picks up one of these multiplexed signals. It must be something that can be produced. In the direct detection method, an optical filter allows only selected photocarriers to pass through, so the photodetector is filtered. The time-varying (data-modulated) intensity of the optical carriers is detected. . These filters allow easy selection of separate data channels. Preferably, it is tunable. Channel spacing as narrow as lnm or 1 ．． It has been proposed for the infrared region of 3 to 1.5 μm.

Diffraction gratings are either a solution or are too expensive and user-intensive. There is also a risk of damage when intended for use. Also, is synchrony required? Preferably, this is purely electrical and does not involve any mechanical movement. So the acoustic light A filter was suggested. The filter has high resolution, tuning range and robustness. ing. However, its price is relatively high, and the filter requires expensive RF power. It is also something that is consumed in large quantities.

Liquid Crystal Light Modulator U.S. Pat. No. 4,779,959 issued to 5unders and In 5unders et al., ``For liquid crystal devices capable of sub-millisecond switching. 0ptical and Quantum El Published in electronic, 1986, volume 18, pages 426-430. written on the item. A modulator is something that blocks or passes incident light; A router is a filter that passes certain components by selecting frequencies from a wide input vector. It functions in a more complex way, allowing chemicals to pass through while simultaneously blocking other components. 5u nderS creates a 10 μm optical cavity between two partially reflective metal mirrors. , this resonator is filled with nematic liquid crystal. The mirror has an applied bias serves as an electrode for standard LCD device configurations, reorienting the liquid crystal direction. It also works. However, in a 5unders etalon configuration, the applied bias As the effective refractive index of the liquid crystal changes, the resonance conditions of the resonator also change.

This relationship indicates bias-dependent optical filtering. There are two 5unders. This effect can be used to intensity modulate a beam of well-defined frequencies between the intensities of ing.

5unders' liquid crystal modulator can be used as a tunable filter for wideband signals. It can be changed to suit. However, its operability will be low. Easy fabrication For the Perot etalon, for light incident perpendicularly to the surface, at a given wavelength λ, Transmittance is calculated using the following formula. However, δ-φ+kQdn0 where τ and ρ are transmittance and reflectance, and φ is The phase shift that changes due to reflection, d is the thickness of the uniaxial material, and n is the refraction on the director axis. The coefficient, ko, is the strength of the wave vector outside that layer. Equation (1) is the transmission peak width Δλ essentially depends on the reflectance of that surface, while the overall transmission is dominated by absorption losses. This indicates that the 5unders is a silver mirror with a reflectance of 85-90%. I am using ra. The transmission peak at about 50% that he drew is limited to Although this value is applicable, this peak is relatively broad. This width is , it poses little problem when used as a modulator for well-defined wavelengths. The distance between the peaks of 5unders is only several times the peak width. . His design therefore filters a very small number of channels. or not valid. For example, for practical devices that can be used in multichannel systems. For this reason, the reflectance of the mirror must be maintained at 95% or higher.

The peak width of 5unders can be reduced by increasing the thickness of the silver mirror, In this case, the reflectance of the mirror can be improved to 90% or more. but increased The thickness is such that the absorption loss of the mirror is reduced to the point where the peak transmittance is reduced beyond an acceptable range. There is a danger that it may increase.

In the liquid crystal optical filters that have been proposed so far, the incident light directed to the filter is It must be linearly polarized in a specific direction related to the direction of the liquid crystal molecules. It was. Otherwise the filter cannot be electrically tuned. Depending on the application, This polarization requirement imposed on the light directed to the filter is easily met or In other applications, such as fiber optic communication systems where the polarization is approximately elliptically polarized, The requirement constitutes an unfavorable and disadvantageous limitation.

In many applications, especially in fiber optic communication networks, optical filters or polarization It is required that the spectral characteristics of the filter It is necessary to be unaffected by the polarization of the light passing through it.

Summary of the invention The first embodiment of the present invention is an optical resonator in which a liquid crystal is sandwiched between two dielectric multilayer mirrors. It can be summarized as a liquid crystal etalon optical filter. on each mirror The connecting electrodes apply a bias to the liquid crystal, thereby changing its dielectric properties. , thus changing the resonance conditions of the optical resonator. Preferably these electrodes are It is better to place it behind the mirror to avoid absorption loss.

Changes in the bias applied to the electrodes electrically change the filter passing frequency.

According to a second embodiment of the invention, a nematic liquid crystal in a Fabry-Bérot etalon The orientation of the material's molecules is such that it forms an electrically tunable, polarization-insensitive optical filter. It has been rejected in a manner detailed below. Specifically, it is attached to the molecules of liquid crystal. The deviation is set to nπ/2. Here, n is a positive odd integer.

A third embodiment of the present invention is a Fabry-Perot mirror formed between two end mirrors. A liquid crystal etalon filter consisting of a liquid crystal that fills a resonator] 7 modulators I can do something. The alignment layer [aligna+ent1a] at the end of this resonator one of the two areas so that the liquid crystal is aligned at right angles within the plane of the alignment layer. Parallel alignment treatment agent for patterning [homogeneous alignin] g agent].

The other alignment layer is also a similarly patterned parallel alignment layer, or preferably, A vertical alignment layer in which liquid crystals are aligned perpendicular to the alignment layer [homeotropic ignment 1 ayer]. The incident beam is split between two regions of the liquid crystal and both of its polarization states are filtered. Preferably, Polarization splits the incident beam into the lateral sides of the filter with the corresponding polarization. In addition, it is recommended to fix a birefringent crystal such as calcite on the input side of the filter.

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

FIG. 1 is a sectional view of a first embodiment of a liquid crystal etalon filter of the present invention.

Figure 2 shows the spectral dependence of the transmittance of the filter in Figure 1 at two bias voltages. This is shown in Figure 7.

FIG. 3 is a graph showing the bias dependence of the transmission peak of the filter of FIG.

FIG. 4 is a sectional view of the second embodiment.

FIG. 5 is a plan view of the alignment layer of the third embodiment.

FIG. 6 is a diagrammatic overview of an optical filter made according to a fourth embodiment of the present invention. FIG.

FIG. 7 schematically depicts the outline of the long axes of the liquid crystal molecules included in the arrangement of FIG.

FIG. 8 is a sectional view of a fifth embodiment of the liquid crystal etalon filter of the present invention.

FIG. 9 shows the patterned alignment layer at section line 9-9 in FIGS. 8 and 11. FIG.

FIG. 10 is a diagram illustrating transmission wavelength as a function of voltage for an example of the invention.

FIG. 11 is a sectional view of a sixth embodiment of the present invention.

Figure 12 is a schematic diagram of an optical drop circuit using the polarization diversity filter of the present invention. be.

FIG. 13 is a plan view of the aligned alignment layers taken along section line 9-9 in FIGS. 8 and 11. be.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The optical filter according to the first embodiment of the present invention uses an end mirror of an optical resonator or an extremely high reflectance mirror. A liquid crystal etalon filter, preferably a dielectric interference mirror.

Example 1 A first example of the first embodiment of the present invention is shown in cross-sectional view in FIG. From soda lime crow Two glass plates 10 and 12 each having a thickness of 1° and 62 mm are used as substrates. Oxidation transparent electrodes 1 and 1.16 of tin-tin are deposited on these substrates 10.12. Ru. A dielectric multilayer mirror 18.20 is formed on the electrode 14.16. mirror 18 ．． 20 is from Virgo 0ptics, In0 of Port Rickey, Florida. You can purchase it. In this case, it will be installed alongside the existing equipment. Or spatter It may be deposited on the electrodes 14.16 by evaporation. The deposited mirror 18.20 is 1 ．． Made for 5μm band infrared filters. However, the inventors We also manufacture products in the 1.9 μm band. The mirrors manufactured by these inventors are different. Specifically, the refractive index is 4 pairs of A1203 (insulator) and Si with a quarter wavelength thickness. Consists of layers. For a mirror in the 1.5 μm band, the Al2O3 layer is ~240 nm thick and S The i-layer was ~120 nm thick. As will be explained later, the transmittance curve in Figure 2 is based on the reflection of the mirror. fitted to theoretical values including the rate. The reflectance of the mirror so calculated is ~9 8%, and the difference is clear when compared with the maximum reflectance of 90% for the 5unders modulator. It is. In this way, the required 95% is fully satisfied.

These dielectric mirrors are well known. For example, Yoo et al. published on April 19, 1990. No. 0,715,10960 discloses a method for optical communication for surface-emitting semiconductor lasers. Two interference mirrors are disclosed that define the ends of the shaker. one of them is made of alternating layers of Sl and Al2O3, and the other is made of semiconductors AlAs and GaAs. Alternating layers are emerging.

The alignment layers 22,24 are as described in Patel et al. Ferroelectrics, Volume 59, 1984, pp. 137-144, “Reliable alignment for smectic liquid crystals.” It is formed on a dielectric multilayer film 18.20 by the method published under the title . between two combined structures or by the method described below. Can be combined at intervals. Four types of epoxy dots that are cured by ultraviolet rays are Place it at the four corners on one alignment layer 24 of the structure. This epoxy is - Thorn's EM 10μm rod space available from Chemicals It is pre-mixed in a sacer. The second structure consists of two arrays arranged in parallel. The orientation layer 22.24 is placed on the first structure with the epoxy in the orientation direction. monochromatic Gently press these structures with your hand while observing the optical interference pattern under light. The interference fringes are minimized. This structure is UV cured by exposing it to UV radiation. This structure is obtained by hardening the epoxy. Approximately 10 combined structures It is heated to 0° C. and the liquid crystal 26 is poured into the space by capillary action. EM　C A nematic liquid crystal, E7 (trademark) available from heI [llcals, Inc. Used in isotropic conditions. The gap is approximately 11 μm between the dielectric multilayer films 18 and 20. It is estimated that this creates a Fabry-Perot cavity length.

The alignment layer 22.24 aligns the liquid crystal molecules 28 in the liquid crystal material 26 with respect to the mirror 18.20. Align the long axes so that they are parallel to each other, and align one set of short axes parallel to the mirrors 18 and 20. and at the same time perpendicular to the long axis. Such orientation can be achieved by applying a bias or It will only be done to those who have not been. In this example, from one mirror to another There is no significant twisting of the liquid crystal molecules.

Electrical leads are connected between the electrodes and a power source 29. The power supply 29 used in this experiment is , a controller such as Wavetek Model 175 (trademark) that creates a square wave at 1kHz. computer-controlled programmable voltage source.

The polarizing plate 30 is attached to the glass substrate 10 or 1 with its polarization direction aligned with the long sleeve of the molecules 28. It can be formed outside either of the two. However, the filter used in this experiment used unpolarized light or external means to control polarization. more fill The device can also be made to operate in the radiation band, eliminating the need for polarizers. .

] Testing the filter of the example shown in Figure 1 on the light-emitting guioto that emits light in the -15 μm band. I used it as a light source 32 to do this. In the first test, we did not use any polarizers, as shown in Figure 2. As shown in the graph, the transmission spectrum is applied to the AC bias of Ov and 4■. Measured per as. The transmission unit is arbitrary, and the baseline is deleted. .

Some peaks shift with bias and some others remain approximately constant. It is observed that the person is being held. Another experiment showed that these constant peaks were caused by liquid crystal content. corresponds to light polarized parallel to the short axis of the beam 28, and the tunable peak is parallel to the long axis. It turned out that it corresponds to polarized light. As a result, this filter has a narrow band bias. It can also be thought of as a corrugated sheet.

Due to the high reflectivity of the dielectric multilayer film 18.20, the width of the transmission peak Δλ is relatively small. The full width at half maximum is ~11-2n.

Also, the Fries Vector Range (FSR) is the wavelength separation between successive transmission peaks. ) is relatively large, ~75 nm. It is the optical thickness of the etalon n- d and the spectral region to be operated on. This region is the area where the interferometer moves. It depends on the order created and is given by the following equation.

Here m is the order. The refractive index of liquid crystal is usually in the range of 1.5 to 1.7. like this 1.5 μm light is transmitted over 11 μm when the etalon is used in the 22nd order. Pass through a spaced etalon.

In this wavelength range, the fleece vector range of the 11 μm etalon is approximately 75 μm. It would be nm. Choosing to use 11 μm thickness is simply a matter of actual device fabrication. It's just for architectural convenience. This thickness ideally covers the wavelength range in question. It should be chosen to be the same as the vector range. The tuning range is It is estimated to be on the order of 200 nm, corresponding to a change in refractive index of 0.2. refractive index The maximum change in is equal to the birefringence of the axle liquid crystal, and the tunability of the filter in Figure 1 is equal to the graph in Figure 3. This is indicated by the bias dependence of the peak wavelength, as shown in Fig. The dashed line is liquid crystal molecule 2 The polarization along the axle of 8 represents the transmission of light. These peaks have essentially There is no tunability at all, and this polarization must be filtered to be tunable. It won't happen. The solid line represents the transmission of polarized light along the long sleeve of the liquid crystal molecules 28 . There is a threshold below which no wavelength shift is observed. This plateau is Fr This is due to the eedricks effect, which was also observed by 5unders et al. It is. This is observed when the orientation vectors of both sides are parallel to those planes. child It is possible to remove the threshold of , and by changing the plane tilt of the molecule, its properties can be controlled. For example, the liquid crystal molecules in one of these planes are A hive that exists parallel to one plane, and liquid crystal molecules in other planes are perpendicular. No threshold is observed for lid-like oriented samples. vertical orientation can be obtained by using a vertical alignment agent such as octadecyltriethoxysilane. can be done. In such a structure under low voltage, the refractive index changes with the applied voltage. It changes almost linearly with the field.

The tuning operation of the filter can be understood from FIG. If the bias voltage If the pressure is varied between approximately 1.6v and 2.6v, interference from other interference peaks will occur. One transmission peak varies between approximately 1.528 μm and 1.472 μm without any and the tunability is 56 nm. Over the range of 1.529μm to 1.532μm In order to obtain the tunability of It would be necessary. These tuning ranges can be shifted and optimized for manufacturing design. It can be expanded through optimization.

The calculated transmission spectrum is fitted to the transmission data in FIG.

For zero electric field, the normal refractive index is no=1.5 and the extraordinary refractive index is n,=1. It is 7. On the other hand, when 4V was applied at intervals of 11.32μm, no = 1.5 and n; (E) = 1.536. However, ne! f is the effective refractive index Ru.

The power required for filter operation is estimated to be in the microwatt range. Sui The cutting speed is on the order of milliseconds.

The filter of the present invention is described in Electronics Letters Vol. 27, 10- “A new polarization-insensitive method based on fiber polarization scrambling”, p. 12, 1991. We conducted an experiment on the polarization scrambler by Maeda et al. It was used to

The liquid crystal etalon filter of FIG. 1 is not laterally patterned, and the present invention There is no such limitation.

What is shown in Figure 4 is a second implementation of a liquid crystal etalon with a one-dimensional or two-dimensional filter array. This is an example. This means that at least one of the electrodes in the pixel 40 extends beyond the substrate 12. This filter differs from the filter shown in FIG. 1 in that it is patterned into a pattern. Pixel 40 is It is possible to touch each side of the filter structure. If combined dielectric If the multilayer film 20 is deposited over the electrode pixel 40, the planarization layer 42 is a dielectric multilayer film 20. To ensure the optical smoothness of the layer 20, it must be deposited first.

In the third embodiment, one of the alignment layers 22 and 24 is slightly patterned. I'm here. The orientation procedure is as described by Patel et al. After applying the alignment material on the liquid crystal molecules, rubbing the alignment material in the direction in which the liquid crystal molecules are oriented. By row・),,.

FIG. 5 shows the alignment layer 44, with the first portion 46 rubbed in the first direction and the second portion 46 rubbed in the first direction. 48 in a different direction, preferably perpendicular to the direction of the cut. distinguish The lever is made of nylon.

This is easily achieved with oriented materials such as 1.4 polybutylene terephthalate.

The entire alignment layer 44 is rubbed in a defined direction, ie in the initial direction. Then The first portion 46 is correctly oriented. Then, the entire structure was made of photoresist. covered, to keep the first portion 46 covered with photoresist material. processed using lithographic techniques. The second exposed area should be thoroughly rubbed. The first portion 46 of the photoresist is then rubbed in the second direction while protecting it with photoresist.

Removal of photoresist affects the orientation of the first and second rubbed locations 46.48 do not.

The second alignment layer 22 or 24 has an alignment similar to the patterned alignment layer 44. It can be created by patterning. When the two structures are then combined, The orientation directions of the opposing parts are parallel to each other. This requires the alignment of two alignment layers 22 and 24. Precise physical location is required. Alternatively, the second alignment layer is in the vertical direction of the liquid crystal. can be made to guide uniformly, ie oriented perpendicular to the alignment layer. All mentioned The vertical alignment treatment agent provides this effect. Vertical alignment process that does not require patterning Physical medicine is required.

The third embodiment of FIG. 5 is a polarization-independent tunable soil for a single mode optical fiber. It is convenient for Ta. The filter cannot be tuned outside the desired wavelength tuning range. It is assumed that the design is such that a passband (dotted line in FIG. 3) is arranged. fiber The distributed index lens on the output of the paired first and second parts 46.48 placed on the border between Then, each part 46.48 has half the light intensity. receive. One of the sections 46 or 48 disturbs the polarization of the first light, but due to tuning the polarization of the second light is Pass the th polarized wave. Similarly, the other portion 46 or 48 blocks the second polarization. However, due to tuning, the first polarized wave passes through. A single photodetector detects the light passing through both parts. Detect intensity. Alternatively, the polarized waves passing through two frequency filters can be It can be recombined by a gradient index lens onto a single mode fiber. both The intensity transmitted through portions 46 and 48 is independent of the polarization of the light. 3dB loss In this polarization changing method, it is received.

The filter of the present invention can be used as a short optical pulse generator. like a laser A narrow band light source 32 is a liquid crystal etalonfon which can be tuned as a monochromatic continuous light of wavelength λ. Illuminate the filter. For example, the filter shown in FIG. 1 has at least a limited tuning range. Design it so that λ widens from one side to the other. The bandwidth of light source 32 is preferably In other words, C should be narrower than the width of the filter's transmission peak. to create light pulses The DC bias of the step function is set to cover a tuning range limited around 2 As is applied to the electrode from a power source 29. Filter or narrow band light to pass through, λ is required to pass through a transmission peak of short or finite time or finite width.

In a fourth embodiment of the present invention, the liquid crystal between the two alignment layers is tilted at 90 degrees (or an odd multiple thereof). It is possible to make the liquid crystal filter insensitive to polarization by giving it a twist of Ru.

The optical filter 60 specifically shown in FIG. 6 is based on the principle of the fourth embodiment of the present invention. It is something. The optical signal from light source 62 is, as indicated by dotted arrow 64, Shown to the left of filter 60. For example, light source 62 may be a standard light emitting diode. an optical fiber that is combined to provide a signal to the card and filter 60.

The light source 62 of FIG. 6 simultaneously supports multiple input wavelengths, e.g. Provides a wavelength of μm. Only one wavelength selected from these wavelengths is filtered 60 and is transmitted to the usage device 68 in the direction of arrow 66. device 68 is used in optical fibers, including, for example, traditional wavelength division multiplexing (WDM) communication systems. be.

In the present invention, a control signal source 70 shown in FIG. 6 applies an electrical control signal to the filter 60. It is used to add When no control signal is applied, the filter 60 is determined by the characteristics and arrangement of the components of the filter 60. It allows optical signals of specific wavelengths to pass through. Above the specified operating threshold, the light Each associated wavelength provided from a source 62 passes through a filter 60 to a device 68. As shown, the control voltage applied to the filter 60 changes its electro-optical characteristics. It is particularly effective. In this way, an electrically tunable optical filter is realized.

The left glass plate 72 transmits the input optical signal and passes through, and the glass plate 72 transmits the output optical signal and passes through. A fourth embodiment of the filter 60 having another glass plate 74 on the right side, shown in FIG. It is. an optically transparent material placed inside or on the surface of the glass plate 72.74; Each electrode 76,78 may be made of, for example, a standard indium-tin oxide film. Ru. As shown in FIG. 6, wires 80,81 control electrodes 76,78, respectively. It is connected to the signal source 70.

Filter 60 of FIG. 6 also includes mirrors 82,83. On the drawing, mirror Each has multiple layers of known dielectric materials. For example, mirror 82.83 is Approximately 94% to 99.99% in the range of wavelengths transmitted by the filter 60, respectively. It is designed to have a reflectance of

The filter 60 of FIG. 6, with spaced mirrors 82, 83, It has the effect of a Li-Perot etalon.

As is well known, the alignment of these devices depends on a particular wavelength (and its multiples). It can be designed to resonate at certain wavelengths. In this way, this device It can be used to output only at specific frequencies.

According to the invention, the filter 60 of FIG. By changing the electrical properties of the layer 84, it is possible to tune to the electric curve. In addition, LCD Depending on the initial state of the material's molecules in a specific direction (described later), the tuning range may be exceeded. The operation of the filter 60 is independent of the polarization of the input optical signal. ·In other words, The wavelength selected by the control signal source 70 to pass through the filter 60 is the wavelength of the input light. Regardless of the polarization of the signal, it is transmitted to the utilization device 68 with the same output strength.

The layer 84 of liquid crystal material shown in FIG. (not shown) is held in place as shown. Specifically, for example, the layer 84 is Consists of a standard nematic liquid crystal material of elongated rod-shaped molecules with positive dielectric anisotropy . - As a specific example, the thickness of the layer 84 in the Z direction in the drawing is approximately 10 μm at most. be.

The portion sandwiched between the liquid crystal layer 84 and the mirrors 82 and 83 in FIG. 6 is a so-called alignment layer [a The lignment is 1ayersl. Various types suitable for use with LCD Oriented materials are well known to those skilled in the art. Specifically, the mirror 82 has a known arrangement on its right side. The mirror 83 has on its left side a layer 88 of a known alignment material. ing.

Each of the alignment layers 86 and 88 has a molecularly constant state in the portion adjacent to the liquid crystal layer 84. It is effective in giving direction. Specifically, each of alignment layers 86 and 88 First, the liquid crystal molecules are rubbed in a certain direction to localize them in the direction corresponding to the adjacent liquid crystal molecules. It will be done.

Rubbing the alignment layer to control the molecular orientation of such liquid crystal materials is well known to those skilled in the art. This is the technology of

According to the invention, the alignment layer 86,88 of FIG. is given to the liquid crystal molecules in the liquid crystal layer 84. The nature of this twisting is schematically shown in Figure 7. . A portion of rod-shaped liquid crystal molecules oriented between alignment layers 86 and 88 is depicted.　Ingredients Physically, the liquid crystal molecules of the liquid crystal layer 84 in FIG. It is twisted by π/2. In this twisted orientation, the portion on the surface of the left alignment layer 86 is The child has its long axis defined parallel to the X-axis shown or on the surface of the right alignment layer 88. Some molecules have defined long axes parallel to the illustrated Y-axis. As shown, these two The long axes of the molecules on the two surfaces are oriented 90° to each other. this Between these planes, the long axis of the liquid crystal molecules gradually changes from the X-axis direction to the Y-axis direction as it advances in the Z direction. It is changing rapidly.

According to the present invention, the amount of twist given to the liquid crystal molecules in the optical filter is determined by nπ/2. will be determined. In the specific example shown in FIG. 7, n is 1. more generally To be clear, n may be any positive odd integer. In this way, for example, if n=3 , the long axis of the liquid crystal molecules is in the Z direction between the planes of the alignment layers 86 and 88 in the liquid crystal layer 84 in FIG. undergoes a rotation of 270°.

No electrical control voltage is applied to the electrodes 76, 78 of the optical filter 60 in FIGS. 6 and 7. When both components of the orthogonal polarized waves are Identification of resonance by the Fabry-Bérot etalon determined by electro-optical properties There will be no wavelength (and multiples thereof) of Is it only a certain wavelength that resonates? , appear in the output of the filter 60, or have different wavelengths for either polarization condition. Resonance is selected.

Here, apply an electrical control voltage to the filter shown in the figure to give an electric field in the Z direction. Assume that Then, the magnitude of the electric field in the liquid crystal layer 84 is determined by the well-known Freedr icks Until the L threshold is reached, the direction of the molecules shown in Figure 7 does not substantially change. stay without. For example, if the liquid crystal layer 84 is 10 μm thick, Outside? , Ov or more), the long axes of molecules in the center of the liquid crystal layer 84 begin to align in the Z direction. Melt. However, the molecules on the surface of the alignment layer 86,88 are , remains unaffected by the applied electric field. Not affected at all or almost The thickness of the area of the surface of the liquid crystal layer 84 that is not exposed to water is a function of the magnitude of the applied electric field.

Thus, in response to an applied control voltage above the Freedrieks threshold, The alignment layer 86, 88 has a field-dependent variable thickness on each surface and in its vicinity. The liquid crystal layer 84 has two regions of birefringence. What is important is that these areas The main optical axis in one of the regions is 90° to the main optical axis in the other region. This means that they are arranged so that they intersect with each other. Therefore, the polarization of the orthogonal components of the input optical signal Each wave state is subject to an effect of the same magnitude regardless of the input polarization state. Ru.

Freedricks threshold or higher (for example, in the liquid crystal layer 84 with a thickness of 10 μm) , EM Chemicals, Hawthorne, N.Y. Kiru ET (ilffi mark) nematic liquid crystal material, at 1kHz, approx. 2. The strength of the operating electric field (over OV RMS) and the thickness of the birefringence region identified previously and the electro-optical properties are virtually the same, further increasing the applied control or tuning voltage Once I decided to raise it, I followed each one. As a result, the operating voltage or In the region of high electric field values, the filter structure described here does not change the phase of the input optical signal. to the orthogonal polarization of the signal. The wavelength that passes through this filter structure in this way is It is determined by the specific value of the applied voltage and is independent of the state of input polarization.

In one of the birefringent regions, one of the components of the input polarization is shadowed by the other birefringent region. parallel to the major axes of the liquid crystal molecules in an unaffected or affected region. Approximately oriented. The other, or orthogonally polarized, component of the input optical signal is Since it is affected in the refractive region, it is essentially unaffected in the first region. Tunable polarization-insensitive optical filters. filter like this , the selected wavelength and corresponding specific applied voltage are the same regardless of its polarization conditions. Transparent intensity levels.

The embodiment of the invention shown here is based on a twisted nematic film of the type shown in Figure 7 that is 10 μm thick. The liquid crystal layer 84 includes a liquid crystal layer 84. The refractive index of such a layer is high, ranging from 2V to 10V. The applied electric field operating voltage varies approximately between 1.5 and 1.7. (This implementation In the example Freedricks threshold voltage is approximately 2v). selected Each input wavelength has a filter that allows that particular input wavelength to pass through. There is a corresponding operating voltage applied to the filter. By changing the control voltage by a specific amount Therefore, a different input wavelength may be selected to be transmitted. As a specific example, 15 The wavelengths separated by 2 nm in the r+m tuning range are each controlled by a control voltage. It is selected by changing it by 0.5V. Wavelength passed through such a filter is characterized by a spectral passband width of approximately inm.

In the above types of optical filters, the conditions for passing one wavelength change to the condition for passing another wavelength. conditions can be changed relatively quickly. Its switching speed is initially It depends on how fast the liquid crystal molecules in the liquid crystal layer 84 change direction. next , this depends on factors such as the dielectric anisotropy of the liquid crystal material, the value of the control voltage, and the thickness of the liquid crystal layer 84. It also depends on various factors. In fact, switching speeds on the order of milliseconds are possible. It is Noh. Additionally, the power required for switching is For filters, it is typically less than a microwatt.

Finally, the arrangement described above illustrates the principle of the fourth embodiment of the invention presented herein. It must be understood that this is nothing more than Numerous variations according to this principle An example can be devised. For example, one modification of the polarization insensitive filter is This is achieved by combining two eyebrows of liquid crystal material, each of which is physically separated. . In this variant, the two layers are formed by the previously mentioned mirror and electrode structures. In a sandwich structure containing an optical cavity and containing three spaced glass plates Therefore, it is kept in a fixed place. The optical axes of the two layers are orthogonal to each other. , so that light of any polarization is given the same phase change.

This modification ensures that the two liquid crystal layers have the same thickness or polarization-insensitive operation. It is important to

Additionally, the device described here can be used as a polarization-insensitive spatial light modulator. It is clear that it can be done. A voltage propagates through a selected wavelength or device Another operating voltage is selected to prevent the selected wavelength from passing through. Design devices to stop

One variation of the third embodiment described above is a polarization-independent tunable liquid crystal filter. requires vertical and horizontal alignment of liquid crystals with opposing alignment layers. P Atel et al. Applied Physics Letters, volume 57.1990. pI), 1718-1720. A similar emission filter was developed as an "infrared wavelength optical filter using liquid crystal in the Burri-Perot etalon." I'm doing a table. This concept is based on a polarization-independent tunable liquid whose cross section is shown in Figure 8. 5 is a further enlargement of the fifth embodiment of the crystal filter 110. Filter 110 is transparent two glass substrates 11 on which indium tin oxide electrodes 116 and 118 are attached; Created on 2.114. Dielectric multilayer mirror 120.122 is electrode 116.1 18, each serving as an interference mirror for the wavelength of interest. It is made up of a multilayer set of layers with a thickness of a quarter wavelength, which is a refractive index.

The parallel alignment layer 124 and the vertical alignment layer 126 are connected to the respective dielectric multilayer mirrors 120, 122 and rubbed. The alignment layers 124 and 126 are composed of liquid crystal molecules, respectively. oriented in a particular direction so as to be placed next to the orientation layer. In particular, nematic Liquid crystals are designed so that the average direction of the long axes of liquid crystal molecules points in a direction called the orientation vector n. Characterize sea urchins. The alignment layers 124 and 126 set the alignment vector n on the surface of the liquid crystal. Determine. The orientation vector n then smoothly moves between the two orientation layers 124 and 126. changes in the liquid crystal. If an electric field is applied across the liquid crystal, the orientation vector n gradually becomes oriented in the electric field in the gap between the alignment layers 124 and 126.

According to a fifth embodiment of the present invention, the parallel alignment layers 124 shown in FIG. It is further divided into two parts 130 and 132. Tochira part 130.132 is also flat Row alignment agents, such as nylon or 1.4 polybutylene terephthalate Made of polyester. It determines the orientation vector n at that location and Adjacent liquid crystal molecules are aligned horizontally to the surface of the alignment layer 124. However However, the two parts 130 and 132 are rubbed in the perpendicular direction, and one parallel oriented part 130 aligns the liquid crystal perpendicularly to the boundary m1134, and the other parallel alignment part 132 orients it horizontally to interface 134. Details about the different scrubbing steps are already available. As discussed in . On the other hand, the other alignment layer 126 contains a vertical alignment treatment agent, e.g. For example, it is formed from octadecyltriethoxysilane, which determines the orientation vector n. , aligning adjacent liquid crystal molecules perpendicularly to the surface of the alignment layer 126.

After constructing the important alignment layers 124, 126, the two substrates 112, 114 are A nematic liquid crystal 1 is formed in a component with a gap of about 10 μm between two alignment layers. 36 are filled during this interval.

When a voltage source 138 is connected to the electrodes 116, 118, the applied electric field The effective refractive index of the liquid crystal 136 of the light polarized in the direction of n is changed. Do it like this The effective value of the Fabry-Perot resonator formed between two mirrors 120 and 122 is This affects the optical path length. The direction of the orientation vector n, which is horizontal to the direction in which the light travels, is flat. It is determined by the direction of orientation of the row orientation portions 130 or 132. On the other hand, the electric field There is no effect on the refractive index for light polarized in the direction perpendicular to the direction of the orientation vector. stomach. Therefore, the change in effective refractive index strongly depends on the polarization of light. Orientation layer 124 Due to the different rubbing directions, the light passing through section 130 is perpendicular to interface 134. is influenced only by certain electrical polarization components, while passing through section 132 The light is influenced only by electrical polarization components parallel to interface 134. Each The polarized wave components pass through portions 130 and 132 of , and the corresponding portions of the liquid crystal 136. When, due to symmetry, both polarization components are equally affected by the applied voltage. It will be done.

A tunable liquid crystal etalon filter 110 enters the interface 134 of the parallel alignment layer 124. It can be used by optically orienting the optical fiber 140. figure 8, the loft type graded index lens 142 is Spread the beam so that an amount of light energy falls on the two parts 130.132 Ru. On the output side, a corresponding lens 144 is filtered into an output optical fiber 146. recombining the light that was lost.

Example 2 A dual polarization liquid crystal etalon filter 110 was manufactured according to the procedure described above. The cell spacing is It is 10 μm. The lateral dimensions are 1cm×1, C[T1. Poly 1.4b Aligning treatment agent using thiol non-terephthalate and ophthalate-toxysilane It is used as The Nematzuk liquid crystal is the previously mentioned E7 (trademark). liquid The crystals were filled isotropically into the cell spacing of 1-0 μm by vacuum filling technique. orientation The layer cannot absolutely determine the alignment direction of the liquid crystal molecules. That is, the parallel orientation part Parallel orientation where multiple regions are formed in each of 130 and 132 This is because parallel and non-parallel are degenerate in processing agents. Multiple regions are parallel This can be avoided by breaking the symmetry with a finite tilt angle of the alignment layer 124. death However, in this example, after filling the gap with the liquid crystal 136 in the isotropic phase, the electrode 1 16. By cooling while applying an electric field in the orthogonal direction, the parallel parts A single area is obtained at each of minutes 130 and 132.

The device consists of an optical spectrum analyzer and a 1.5 μm light emitting diode as a light source. We also measured the characteristics of the device at room temperature using a multimode optical fiber. . The multimode fiber and graded index loft lens have a diameter of approximately 400 μm. Remate. The use of single mode fiber produces a beam with a diameter of approximately 250 μm. produce. 1kHz square with programmable computer-controlled power supply make waves Spectral position of maximum transmission peak as a function of applied voltage This is shown in the graph of FIG. The transmission peak is approximately 0.9 nm wide or passband have. Two bands P and Q are visible in the transmission spectrum.

The P band corresponds to something that does not change with voltage and is polarized perpendicular to the orientation vector n.

Q-band, however, varies significantly with voltage. This is the orientation vector corresponds to polarization parallel to n. The position of the P band is determined by the following resonance conditions. Ru.

mλ=n, d, (3) Here ■ is the mode number, λ is the wavelength of light, n is the refractive index in the short axis direction of the liquid crystal, d is the physical thickness of the Fabry-Perot cavity. Applied voltage changes ns by 12% let From the figure, the tuning range of the Q peak is approximately 1100n, and the fleece vector level is approximately 1100n. The wavelength is approximately 70 nm. Insert a polarizing plate on the input side and measure the spectrum again. Ta. When the light is polarized perpendicularly to the interface 134 and when is it parallel to the interface 134? The spectra in both cases were not significantly different from those in FIG.

Since the P-band is not a tunable filter, the filter should not be able to remove this band. It needs to be designed with a desirable fleece vector range.

The filter of Figure 8 is 3 dB relative to the unfiltered polarization or P-band. The disadvantage is that there is a loss. That is, the lens 142 has two parts 130.1 Equal amounts of both polarizations are distributed to 32. However, each part 13 0 or 132, only the polarized light corresponding to the selectively rubbed direction will have that wavelength. is allowed. The other polarization is unaffected by the applied electric field and therefore of course It will be blocked.

A common way to avoid this 3 dB loss is to use lens 142 or a polarization-insensitive spatial beam. act as a beam splitter.If you want the input beam to be polarized, There are many types of polarization beam splitters that split into two beams. There is.

For example, a Wollaston prism. Polarized beam splinter like this are sections 130 that receive light from an input optical fiber 140 and filter its polarization. and 132, transmit the respective polarization components.

The sixth embodiment of the present invention is a particularly excellent tunable dual-polarization liquid crystal etalon filter 1. Regarding 50. This uses a polarized beam splitter as shown in the cross section in Figure 11. There is. This is a substrate 152.15 of birefringent material that separates the beams according to their polarization state. It is formed on 4. On the input side, a birefringent substrate 152 absorbs the bits from the input filter. The beam is separated into an ordinary ray 156 and an extraordinary ray 158.

If the optical axis of birefringence is perpendicular to the long axis of surface 134 of parallel alignment layer 124, preferably If the input optical axis is set at 45°, even if the extraordinary ray 158 is biased, the ordinary ray 15 6 is not biased. The light of the ordinary ray 156 is electrically polarized parallel to the interface ]-34, Extraordinary ray 158 is vertically polarized. Between the birefringent substrate 152 and the transparent electrode 1-16 At the interface, the two beams 156, 158 are oriented in their respective initial directions The two portions 130 and 132 containing parallel alignment agents that are properly oriented as shown in FIG. Pass through each. Importantly, the two beams 156 and 158 are at this stage. The diameter and location of the incident beam does not matter, as long as the beams are sufficiently far apart. LCD 1 After passing through the birefringent substrate 152, the two beams are directed toward the birefringent optical axis or the first birefringent substrate 152. They are recombined using another birefringent substrate 154 arranged in a mirrored manner. unpolarized beam A coupler could be used instead, but the two birefringent substrates 152 and 154 are They are designed to be easily matched and form an integral structure. Using a birefringent substrate The birefringent layer may be permanently fixed on the glass substrate without bending. LCD dual polarization filter The filter 150 can completely filter all polarization components. 3 dB loss can be avoided. This filter 150 It also has the advantage of eliminating the If the photodetector has a large area If it can be used, the birefringent layer 154 on the output side will not be necessary.

Example 3 An integrated dual-polarization liquid crystal filter with a polarization beam splitter and a coupler is shown in Example 2. 4mm thickness on the outside of the soda lime glass substrate 112, 114 of the polarized liquid crystal filter This can be achieved by fixing calcite plates. This filter is It was possible to inspect by inputting the beam and detecting the output. Polarization of input light When the direction is rotated using a manually operated polarization control device, the output intensity variation is less than 1 dB. Met. On the other hand, if a polarizer is additionally inserted across the input beam, the output intensity There was a variation of 25 dB.

Since the dual polarization filter 150 in FIG. 11 is polarization insensitive, it is useful for optical drop circuits. It can be used for profit. A WDM communication system uses a group of wavelengths for one optical fiber. λ111. It is characteristic that there is a multiplexed optical signal of λ. N multiplexed optical signals are Σλ expressed.

Channel λ2 is selected from the N channels of Σλ using various types of filters.

However, the simple filter discards the other N-1 channels. light mud In a drop circuit, a channel of λ, equivalent to an electrical drop circuit, is removed from the fiber. Conveniently, the other N-1 channels may remain in the fiber.

The optical drop circuit shown in FIG. 12] 70 is the seventh embodiment, which is It is something that fulfills a function.

The input fiber 172 carries a WDM signal Σλ, or this WDM signal Σλ is A polarized beam splitter 174 splits the beam into a first beam 176 and a second beam 178. Can be divided. A polarizing beam splitter 174 of the type shown forms a flat surface. This is a cube of quartz separated along the diagonal so that it looks like this. surface - or multilayer A cube is assembled by covering it with a dielectric layer. Light polarized parallel to the surface is , 90°, and orthogonal polarized waves are transmitted. The first beam 176 initially A second beam 178 carries the polarized component of the WDM signal polarized parallel to the drawing. It carries orthogonal polarization components. The first beam 176 passes through a quarter wave plate '-80. Once out, all frequency components become circularly polarized waves. Circularly polarized beam 176 is shown in FIG. The wavelengths λ, is tuned to select. Filter 182 filters circularly polarized first beam 176. Spatially separate into two linearly polarized components, both λ.

Passes the linearly polarized wave component. It is then recombined into a circularly polarized beam component of λ. Another one The quarter-wave plate 184 splits the channel of the circularly polarized wave, λ, perpendicular to the original polarization. Converts to linear polarization, which is perpendicular to the drawing. Another N-1 optical channels Σλ-λ3 does not pass through filter 182 and is violated with minimal energy loss.

Passing through the first quarter-wave plate 180 in reverse, the linearly polarized wave is perpendicular to the original polarization. Become a wave. Therefore, polarization beam splitter 174 directs them to output fiber 18 unselected output beam 186 received by 8.

The quarter-wave plate J, 80.184 rotates the linear polarization of the light beam by 45° and reverses it. A magnetic field that passes through the beam twice and rotates by 90 degrees, rotating the beam transmitted in the same direction by the same angle. An air optic element may be used instead. Thereby, equal amounts of the two polarizations are transmitted to the filter 18. 2, and the polarization beam splitter 174 separates the beam from the input beam. It will be reflected by sea urchins. The second magneto-optical element returns to the original polarization direction, if necessary. vinegar.

The structure described above is suitable for optical drop when the WDM signal is controlled by linear polarization. Works as a circuit. However, in order to compensate for the absence of such control, the second A similar quadrant filters and reflects orthogonal polarization components of beam 178. One long plate 190, 192 and two polarization filters 194 are required. The reflected component is The first polarized beam is directed to the output fiber 188. It will pass through the splitter 174. In other words, beam splitter 17 4 also acts as a combiner of two linearly polarized components of the non-selected signal Σλ-λ. mirror 196.198 converts the two polarization components of the selected channel λ into the second polarization beam splitter 200, and this splitter 200 connects another output fiber. The selected output beam 202 is coupled to the selected output beam 202 received from the bar 204 . Trombe circuit 1 70 indicates the selected output signal regardless of the polarization state of the input channel group Σλ. A similar selection of A 202 and a similar reflection of the unselected output beam 186 is performed.

In one arrangement embodiment, shown in plan view in FIG. 13, the parallel alignment layers 124 are orthogonal. It is divided into a plurality of parallel oriented sections 130, 132 rubbed on the surface. Bilateral figure 8 When the array of FIG. 13 is combined with the wave filter 110, three bits are formed at the interface between 134 and 160. You can filter the However, the dual polarization filter 150 of FIG. Only two beams can be filtered at the interface 134. another array In the embodiment, multiple beams are distributed between the two alignment layer sections 130 and They are arranged along the interface 134. One electrode 116 or 118 is The electrodes are patterned so that they are divided into electrodes for each arm.

In the above example, a sufficiently narrow tunability is used to isolate a portion of the broad spectrum. However, the present invention has a relatively wide and tunable passband. The present invention can be similarly applied to modulators for bands with high performance. For example, by 5unders Invented in For purposes of the sixth embodiment, the modulator may be considered as a special case of a filter. cormorant.

In the fifth and sixth embodiments, the parallel alignment layer comprises two parallel alignment parts 130, 132. The interface 134 between them is rubbed in parallel and perpendicular directions. However, the dual polarization effect Obtained by rubbing perpendicularly to the surface in different directions. In the embodiment described above, A parallel alignment treatment agent is used in the second alignment layer 120. However, the second orientation The layer is formed with a vertical alignment agent that is patterned in the same way as the first alignment layer. You can get the same effect even if you use Kokichi. In that case, two alignment layers or It must be precisely assembled to align the interface 134. Parallel orientation treatment The chemical is applied in parallel between the two alignment layers to give a 90° twist to the liquid crystal in the gap. Or it can be oriented vertically.

The liquid crystal etalon optical filter with interfacial mirror of the present invention has wide electrical tunability. Provides a flexible and narrow bass band. They are low power and economical to manufacture. It is durable and sturdy.

F″IG,i Wavelength (μI]) FIG.3 Voltage (AC FIG.5 FIG.7 FIG.8 FIG.9 FIG, igo FIG. 12 FIG, Go3 Procedural amendment June 21, Hei Ti5C5

Claims (12)

[Claims] 1. Two interface mirrors placed with a gap between them, and a mirror placed between these interface mirrors. and two electrodes that change the refractive index of the liquid crystal. tunable optical filter. 2. It further includes means for creating a twist of nπ/2 in the principal axis of the liquid crystal across the interval. The tunable optical filter of claim 1, characterized in that: 3. In order to orient the partial molecules of the liquid crystal adjacent to the two alignment layers, the first alignment layer and a second alignment layer is disposed between the liquid crystal and each interface mirror; The tunable optical filter of claim 1. 4. The first alignment layer and the second alignment layer are arranged so that nπ/2 is aligned with the main axis of the liquid crystal molecules across the gap. means for generating twist, and the main axis of the liquid crystal molecules is aligned with the first alignment layer and the second alignment layer. substantially in the first alignment layer and the second alignment layer in a portion within the interval adjacent to the alignment layer. 3. Tunable optical filter according to claim 2, characterized in that they are arranged in parallel. 5. The first alignment layer has a parallel alignment treatment agent and is separated at the interface to form the first and second parts. the first portion is adjacent to the first portion in a first direction substantially parallel to the plane of the first portion; the liquid crystal is aligned, and the second portion is substantially parallel to the surface of the second portion and in the first direction. It is characterized by aligning adjacent liquid crystals in a second direction that is substantially orthogonal to each other. 3. The tunable optical filter of claim 2. 6. A first substrate on which a first mirror is formed, and a second substrate on which a second mirror is formed. and at least one of the first or second substrate is a layer of birefringent material. 6. The tunable optical filter of claim 5. 7. A second alignment layer aligns the liquid crystal in a third direction substantially perpendicular to the plane of the second alignment layer. 6. The tunable optical fiber of claim 5, further comprising a vertical alignment treatment agent that Luta. 8. The input beam is placed on the optical axis on which is placed an optical filter that receives the input beam and is tunable. a polarization beam splitter that outputs polarization components of the beam; When the above optical filter is tuned to select the frequency components of the input beam , the unselected frequency components are reflected by the tunable optical filter. The polarization beam splitter splits the unselected frequency components into a non-selected frequency component that is different from the optical axis of the input beam. The above splitter and the above optical filter are adapted to output in the selected beam direction. and a polarization converting means placed between the tuning device and the tuning device according to claim 5. Optical drop circuit with possible optical filters. 9. a first substrate having a first mirror, a first electrode and a first alignment layer formed thereon; The first alignment layer has a parallel alignment treatment agent, and the interface separates the first and second portions. The first portion is substantially parallel to the plane of the first portion and substantially perpendicular to the first direction. Adjacent liquid crystals are aligned in a second intersecting direction, and one or more predetermined a second mirror, a second electrode, and a second alignment layer for aligning adjacent liquid crystals in the direction in which the liquid crystal is aligned; a second substrate formed on the substrate, the first substrate and the second substrate having the first orientation; layer and a second alignment layer face-to-face with a predetermined spacing therein; in which at least one of the two substrates constitutes a layer of birefringent material; The LCD is filled with A dual polarization liquid crystal etalon filter characterized by having. 10. receiving an input beam and outputting a first polarization component of the input beam to a first optical path; a first polarized beam splitter that receives the first polarized beam from the first optical path; unselected light that separates a second polarized component orthogonal to the input beam from the optical axis of the input beam; The one that outputs to the path, It has two interface mirrors forming an optical resonator of the filter and is directed toward the first optical path. polarization-insensitive liquid crystal etalon filter, a first polarization converter directed to the first optical path between the splitter and the filter; step by step, An optical drop circuit characterized by having: 11. It has two interface mirrors forming an optical resonator, and an electric current directed to the second optical path. a polarization-insensitive second liquid crystal etalon filter that is mechanically tunable; polarization conversion means directed to a second optical path between the first splitter and the second filter; a polarization beam combiner directed to the first optical path and the second optical path. The first polarization beam splitter splits the second polarization of the input beam onto the second optical path. 11. The optical drop circuit according to claim 10, which outputs a component. 12. A tuning electrode and two end mirrors each having a reflectance of 90% or more are used. A certain optical liquid crystal etalon filter is characterized by a variable voltage applied to the tuning electrode. having a tunable bandpass between the first wavelength and the second wavelength; a light source emitting light to the router, the light source having a radiation wavelength between the first wavelength and the second wavelength; , and has a radiation band substantially smaller than the difference between the first wavelength and the second wavelength. and the variable voltage source continuously shifts the bandpass of the filter to the first wavelength. and a second wavelength, and the light at the emission wavelength radiates through the filter; a variable voltage source that generates the variable voltage; An optical pulse generator characterized by having: