JP2009265195A - Wavelength variable filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength variable filter which reduces a number of components and prevent the increase in length/size of a device and is equal to a Lyot filter having a multistage structure exhibiting characteristics of a steep transmission spectrum and a narrow band in a wide wavelength range including a short-wavelength range. <P>SOLUTION: A wavelength filter 31 includes a PEM element 32 comprising an optical modulation unit 33 made of flat plate type quartz having a constant thickness and a crystal oscillator 34 connected to one end thereof. First to fourth polarizers 37 to 40 supported by a transparent glass plate 43 and upper reflecting mirrors 41 and 42 are provided above an upper principal surface of the PEM element, and a lower reflecting mirror 44 supported by a support plate 45 is provided below a lower principal surface. Light incident on the light modulator is transmitted through the optical modulation unit while reflected repeatedly between the first to fourth polarizers with the upper and lower reflecting mirrors to be projected. The phase difference between two polarizers which are adjacent to each other along an optical path is set to 2<SP>n-1</SP>×2π (n: 1 to 3). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a wavelength tunable filter for controlling or variably controlling the wavelength of transmitted light at a specific value and / or range.

Conventionally, various band-pass filters are used to transmit light in a desired wavelength region. For example, a rio filter is known as a bandpass filter that transmits only narrow-band light using a birefringent plate (see, for example, Patent Documents 1 to 3). FIG. 12 shows a basic configuration of the Rio filter. As shown in the figure, the Rio filter 1 includes birefringent plates 3a to 3c between polarizers 2a to 2d having parallel transmission axes, and crystal optical axes 3a1 to 3c1 are transmission axes 2a1 of the polarizers. Arranged at 45 ° with 2d1. For the birefringent plates 3a to 3c, uniaxial crystals such as calcite and quartz are used, and their thicknesses di.
Is configured to be 2 ^i-1 d (i = 1 to 3). In Patent Document 1, a rio filter is stacked on the surface of a liquid crystal display panel in order to increase the color purity of the three primary colors of RGB in a liquid crystal display panel having a reflecting mirror on the back. The plastic film constituting the rio filter determines its optical phase difference so that the peak of the transmission spectrum of light reflected from the back side of the liquid crystal display panel corresponds to RGB.

  The rio filter has its transmission spectrum fixed at a specific wavelength by the birefringent plate used. Therefore, a wavelength tunable optical bandpass filter in which a liquid crystal cell is sandwiched between two polarizers instead of a birefringent plate is known (see, for example, Patent Documents 2 and 3). The configuration of this bandpass filter is shown in FIG. In the figure, the band pass filter 11 changes the transmission spectrum wavelength by appropriately setting the voltage applied to the liquid crystal cells 13a to 13c sandwiched between the polarizers 12a to 12d. In particular, the band-pass filter shown in FIG. 13 is a so-called parallel Nicol liquid crystal cell 13a between so-called crossed Nicols polarizers 12a and 12b arranged so that transmission axes 12a1 and 12b1 are orthogonal to each other, and transmission axes 12b1 to 12d1 arranged in parallel. By combining the two liquid crystal cells 13b and 13c sandwiched between the polarizers 12b to 12d, one of the two monochromatic lights remaining in the visible region is removed.

  Furthermore, a wavelength tunable filter having a configuration similar to that in FIG. 13 is known in which a composite layer composed of a liquid crystal cell and a retardation film is sandwiched between polarizers (see, for example, Patent Document 4). The configuration of this wavelength tunable filter is shown in FIG. In the figure, the wavelength tunable filter 21 is configured such that retardation films 24a to 24c are overlapped on liquid crystal cells 23a to 23c sandwiched between polarizers 22a to 22d, respectively. This retardation film suppresses the cell thickness of the liquid crystal cell to the necessary minimum, supplements the decrease in retardation value thereby, and generates a transmittance peak with a narrow half-value width while avoiding a decrease in the response of the liquid crystal. Also, by setting the cell thickness of the liquid crystal cell to d, 2d, and 1.5d in the order of transmission, the same control voltage is applied to all the liquid crystal cells by sharing one voltage application device, and the wavelength of the transmission spectrum is adjusted. be able to.

  In addition, a photoelastic modulation (PEM) element is known as an optical element that periodically modulates the phase of incident light. A PEM element is configured by attaching a piezoelectric vibrator such as quartz to one end of an optical modulation unit made of an isotropic medium such as quartz and mechanically abutting the other end to a stopper (for example, Patent Documents). 5). When the light modulator excites the crystal resonator at the resonance frequency of the light modulator, elastic distortion occurs and the refractive index of light changes in a predetermined direction. Thereby, the polarization state of the transmitted light is periodically modulated between linearly polarized light and elliptically polarized light, and the phase difference between ordinary light and abnormal light changes at a constant frequency. In general, a PEM element is used for polarization modulation spectroscopy in which a thin film or a sample is measured in an ellipsometer or the like (see, for example, Patent Documents 6 and 7).

Japanese Patent No. 3000669 Japanese Patent No. 3102012 JP 2000-267127 A JP 2005-115208 A JP 2000-28441 A Japanese Patent Laid-Open No. 5-196502 JP-A-7-134068

  However, the liquid crystal cell has a property of absorbing light in a short wavelength range from ultraviolet to blue in addition to causing a loss in transmitted light amount due to reflection on an ITO (indium tin oxide) film or the like. Therefore, as described above, the wavelength tunable filter using the liquid crystal cell has a problem that the transmittance in a short wavelength region is significantly reduced.

  Furthermore, in order to precisely control the voltage applied to the plurality of liquid crystal cells, an independent drive control circuit is required for each liquid crystal cell, which causes a problem that the configuration and control of the entire apparatus are complicated. Moreover, the liquid crystal cell has problems such as low heat resistance and large transmitted wavefront aberration. Moreover, since the liquid crystal cell has a slow response to the applied voltage of the refractive index, it may be practically difficult to continuously change the transmission wavelength by changing the applied voltage continuously.

  In addition, the Rio filter and the above-described conventional wavelength tunable filter have two polarizers and a phase shifter such as a birefringent plate or a liquid crystal cell sandwiched between them as one block, which are arranged in series along the optical path. Configure in multiple stages. For this reason, the transmission spectrum becomes steeper and narrow band transmission characteristics can be obtained, but there is a problem that the number of parts increases and the entire filter becomes longer and larger. In particular, a multi-wavelength variable wavelength filter requires a plurality of liquid crystal cells having different optical thicknesses and other specifications, which increases costs.

  Furthermore, since the conventional wavelength filter uses an absorptive polarizer such as a retardation film as the polarizer, the optical loss is large. On the other hand, reflective polarizers such as wire grid polarizers, photonic crystal polarizers, and brightness enhancement films generally have low loss and broadband optical characteristics, and are excellent in heat resistance. However, if a reflective polarizer is used in a wavelength filter having a multi-stage configuration in which conventional optical elements are arranged in series along the optical path, unnecessary reflected light is generated and stray light cannot be obtained. .

  As described above, the inventors of the present application paid attention to a PEM element used for measuring a phase difference exclusively in polarization modulation spectroscopy such as an ellipsometer. In view of the above-described conventional problems, the present invention has been devised as a result of various studies on wavelength filters using PEM elements.

  Accordingly, an object of the present invention is to reduce the number of parts as much as possible and suppress the increase in the length and size of the entire apparatus, and can exhibit a narrow band characteristic with a sharp transmission spectrum, similar to a multistage rio filter. By using a PEM element, a wavelength tunable filter that can variably control the transmission wavelength of light in a wide wavelength range including a short wavelength range from ultraviolet to blue can be realized more easily.

  According to the present invention, in order to achieve the above object, a plate-shaped light modulation unit disposed between the first main surface and the second main surface, and a piezoelectric vibrator connected to one end of the light modulation unit, And a plurality of polarizers and reflection mirrors provided on the first main surface and the second main surface, and light incident on the light modulation unit is reflected by the polarizer inside the light modulation unit There is provided a wavelength tunable filter that multiple-reflects and transmits with a certain angle with respect to the normal direction of the main surface with respect to the mirror and emits from the light modulation unit.

  The phase difference between two polarizers adjacent to each other along the optical path of light that passes through the inside of the light modulation unit of the PEM element while being multiple-reflected is determined by the optical path length between the two polarizers. Correspondingly, the peak wavelength of the transmission spectrum is set. The PEM element, as described above, applies a stress to the quartz by exciting a piezoelectric vibrator such as quartz attached to one end of a transparent isotropic material such as quartz with a predetermined voltage. Optical anisotropy due to an elastic phenomenon is induced, thereby causing birefringence and giving a phase difference to transmitted light. This phase difference periodically changes with the oscillation frequency of the piezoelectric vibrator, but can be controlled by changing the applied voltage of the piezoelectric vibrator according to the thickness of the quartz. Therefore, a wavelength tunable filter capable of freely changing the transmission spectrum wavelength is realized with the same structure as the conventional rio filter.

  In addition, by increasing the number of polarizers provided on the main surface of one common light modulation section, a configuration equivalent to a multistage rio filter can be obtained, so the number of components is reduced and the overall length and size of the apparatus are increased. Can be suppressed. In particular, it is extremely advantageous from the viewpoint that the influence of the manufacturing process and used waste on the environment is reduced when the number of parts is significantly reduced as compared with the prior art.

  In addition, since the PEM element has good response characteristics with respect to changes in the applied voltage, the transmission wavelength region can be changed at high speed and stably. Further, the light modulation part made of the transparent isotropic substance of the PEM element is different from the conventional wavelength tunable filter using a liquid crystal cell by selecting a material having extremely low light absorption such as quartz. A sufficient phase modulation amount and high transmittance can be obtained for light in a wide wavelength range including the short wavelength range from blue to blue. Moreover, since the PEM element has a large amount of phase modulation, the degree of freedom in designing the wavelength tunable filter can be increased.

In some embodiments, a phase difference gamma _i between two polarizers which are adjacent to each other along the optical path of the light transmitted through the interior of the light modulation unit of PEM devices, with respect to the wavelength of light transmitted through the optical path gamma _i = 2 ⁱ⁻¹ × 2π (where i = 1 to n, n: an integer equal to or greater than 2), so that it is always an integer multiple of 2π. Therefore, a variable band similar to the conventional rio filter Transmission characteristics as a pass filter can be obtained.

Here Furthermore, at least one phase difference gamma _j between two polarizers which are adjacent to each other along the optical path of the light transmitted through the interior of the light modulation unit of PEM devices, with respect to the wavelength of light transmitted through the optical path gamma _j = 2 ^j−1 × 2π−π (where j = 1 to m, m: natural number), the variable wavelength region can be expanded.

  In yet another embodiment, two adjacent polarizers are arranged in a parallel Nicol or crossed Nicol relationship along the optical path of light that passes through the inside of the light modulation unit of the PEM element. Similar transmission characteristics as a variable bandpass filter can be obtained.

  In a certain embodiment, the incident light and the emitted light can be arranged in-line by providing the light incident port and the light emitting port on different main surfaces. In another embodiment, an incident port and an exit port for light to the light modulation unit of the PEM element are provided on one main surface, so that the incident light and the emitted light can be arranged to face each other.

  In one embodiment, when the polarizer provided on the main surface is a wire grid polarizer, light incident on the polarizer can be selectively reflected depending on the polarization direction, and thus a separate reflecting means is added. There is no need, the number of parts can be reduced, and high transmittance can be obtained for both p-polarized light and s-polarized light, so that high light utilization efficiency can be obtained.

  In another embodiment, all the polarizers, which are wire grid polarizers, are provided on one main surface, and the light exit is provided on the other main surface, so that unnecessary light transmitted from each polarizer to the outside is provided. Can be reliably excluded from the desired outgoing light.

  Further, in an embodiment, a vertical reflection mirror provided on the end face of the light modulation unit of the PEM element is further provided, and the light transmitted through the light modulation unit of the PEM element is reflected by the vertical reflection mirror and reversely directed. Thus, the entire filter can be shortened and downsized.

  Exemplary embodiments of a tunable filter according to the present invention will be described below in detail with reference to the accompanying drawings. In each drawing, similar components are denoted by the same or similar reference numerals.

1A to 1C schematically show the configuration of a first embodiment of a wavelength filter according to the present invention. The wavelength filter 31 of the present embodiment includes a phaser having a certain optical thickness and a sufficient length for the thickness. The phase shifter includes a PEM element 32, and includes a light modulator 33 made of flat-plate quartz having a constant thickness, and a crystal resonator 34 bonded to one end thereof. The light modulator is formed of various known transparent isotropic materials such as CaF ₂ and Ge in addition to quartz. The quartz resonator can be replaced with various known piezoelectric resonators using piezoelectric ceramics such as lead zirconate titanate in addition to quartz.

  The PEM element is fixed so that the other end of the light modulator 33 does not abut against a stopper (not shown) and is not displaced. The crystal unit 34 has a pair of electrodes 35, 35 formed on opposite surfaces, and each electrode is connected to a power source 36. When a predetermined voltage is applied from the power source, the crystal resonator 34 is excited at a predetermined frequency so as to expand and contract in the direction indicated by the arrow in FIG. 1A along the direction toward the stopper. The light modulator 33 is distorted by elastic deformation with the stopper due to the vibration of the crystal resonator 34, and the refractive index of light periodically changes in a predetermined direction. Therefore, the light transmitted through the light modulator is periodically modulated in polarization state between linearly polarized light and elliptically polarized light, and the phase difference between ordinary light and extraordinary light changes at a constant frequency. The applied voltage from the power source 36 is controlled so as to synchronize the vibrations of the crystal units.

  In the present embodiment, one end (left side in the figure) 33a side of the upper main surface of the light modulator 33 is a light incident port, and the other end (right side) 33b side is a light exit port. The light is caused to travel from the left to the right in the drawing in the light modulation section. Above the light modulation unit, first to fourth polarizers 37 to 40 and upper reflection mirrors 41 and 42 are disposed. The first to fourth polarizers and the upper reflection mirror are supported on a transparent glass plate 43 that is arranged slightly spaced from the upper main surface of the light modulator. The first and second polarizers 37 and 38 are continuously arranged along the traveling direction of the light, and then the third polarizer 39 and the upper reflection mirror 42 are sandwiched with the upper reflection mirror 41 interposed therebetween. A fourth polarizer 40 is arranged. A lower reflection mirror 44 is provided below the light modulation unit so as to be separated from the lower main surface and over the entire length range thereof. The lower reflection mirror 44 is also supported on a support plate 43 that is arranged slightly spaced from the lower main surface of the light modulator.

  The first to fourth polarizers 37 to 40 are wire grid polarizers. Wire grid polarizers are arranged on a transparent substrate surface in the form of a lattice of metal thin wires periodically with a constant period shorter than the transmission wavelength, reflect the light of vibration components perpendicular to the periodic direction of the lattice, and are parallel to each other It has the characteristic of transmitting light of vibration components. In the present embodiment, a black dot in the figure is defined as linearly polarized light that is perpendicular to a plane (the paper surface of FIG. 1B) including the incident light L1 to the wavelength filter 31 and the normal line of the upper surface of the light modulator. ●, parallel linearly polarized light is represented by p-polarized light with a short double-ended arrow in the figure. The first to fourth polarizers orient the grating in the direction of 45 ° with respect to the vibration direction Q of the PEM element 32. The orientation of the grating of each polarizer is represented by a number of parallel thin lines in the plan view of FIG.

  As each of the polarizers, various known polarizers other than the wire grid polarizer can be used as long as they split the incident light so as to be transmitted or reflected depending on the polarization direction. Examples of such a polarizer include a photonic crystal polarizer and a brightness enhancement film made of a resin material. Each of the reflection mirrors is formed by attaching a metal film such as Al, Ag, Au, or the like to the surface of the glass plate or the support plate by a method such as vapor deposition. The reflection mirror is formed by laminating dielectric multilayer films, and is configured to selectively reflect only a desired linearly polarized light component and not to reflect other linearly polarized light components. Can be less.

  Incident light L1 to the wavelength filter 31 passes through the light modulation section 33 from the light incident port, and is reflected by the lower reflection mirror 44 at a predetermined angle θ with respect to the normal direction of the main surface of the light modulation section. The reflected light enters the first polarizer 37 and is split into an s-polarized component and a p-polarized component. The s-polarized light component passes through the first polarizer 37 and is emitted to the outside as unnecessary light. The p-polarized component is reflected by the first polarizer with the same reflection angle θ with respect to the normal direction of the main surface of the light modulation unit, and passes through the light modulation unit 33 while being subjected to multiple reflections. The two adjacent polarizers are arranged in a parallel Nicol relationship in which the transmission axes are parallel to each other by aligning the periodic direction of the grating of the wire grid polarizer as described above.

  The p-polarized light reflected from the first polarizer 37 is transmitted through the light modulating unit 33, converted into s-polarized light in the meantime, reflected by the lower reflecting mirror 44, and transmitted again through the light modulating unit. It is converted into polarized light, and is incident on the second polarizer 38 and reflected. Next, the p-polarized light reflected from the second polarizer is reflected three times by the lower reflection mirror and the upper reflection mirror 41, and reciprocates twice through the light modulation unit, and enters the third polarizer 39. Incident and reflected. In the meantime, every time the light modulation unit reciprocates through the light modulator, the p-polarized light is repeatedly converted into s-polarized light, and further s-polarized light to p-polarized light twice. The p-polarized light reflected from the third polarizer 39 is reflected seven times by the lower reflecting mirror 44 and the upper reflecting mirror 42, passes through the light modulator four times, and enters the fourth polarizer 40. reflect. In the meantime, every time the light modulator 33 is reciprocally transmitted, the p-polarized light is similarly converted into s-polarized light, and further converted four times from s-polarized light to p-polarized light. Finally, the p-polarized light reflected from the fourth polarizer 40 is reflected again by the lower reflection mirror 44 and once passes back and forth through the light modulation unit, during which time it is converted from p-polarized light to s-polarized light, and further from the s-polarized light. The light is returned to p-polarized light and is emitted to the outside from the light exit port.

The light modulator 33 is configured so that a phase difference of 180 ° is given while light is once transmitted between the upper and lower main surfaces. Accordingly, each of the polarizers is disposed between two polarizers adjacent to each other along the optical path traveling in the light modulator, that is, between the first polarizer 37 and the second polarizer 38, and between the second polarizer and the third polarizer. The phase differences between the polarizers 39 and between the third polarizer and the fourth polarizer 40 are sequentially 360 °, 720 °, 1440 °, that is, 2 ^n-1 × 2π, (n: 1 to 3). It is arranged to become. Since the phase difference between the two adjacent polarizers is always an integral multiple of 2π in this way, the wavelength tunable filter 31 can obtain transmission characteristics as a bandpass filter similar to a conventional rio filter.

  In another embodiment, the phase difference given while light is once transmitted between the upper and lower main surfaces of the light modulator 33 can be set to 90 °. In that case, the number of reflections of the light transmitted through the light modulator is doubled between the two adjacent polarizers so that a desired phase difference is obtained. As a result, the wavelength tunable filter 31 similarly sets the phase difference between two adjacent polarizers to an integral multiple of 2π to obtain transmission characteristics as a bandpass filter similar to the conventional rio filter. Can do.

  The wavelength tunable filter 31 of the present embodiment changes the wavelength of light transmitted through the PEM element 32 by changing the resonance frequency by changing the voltage applied to the crystal resonator 34, so that the transmission spectrum wavelength can be freely set. Can be changed. However, since the light modulation unit 33 vibrates in accordance with the resonance frequency of the crystal resonator 34, the light transmitted through the light modulation unit changes the phase difference between ordinary light and abnormal light at a constant frequency. Correspondingly, the wavelength of the peak of the transmission spectrum changes periodically.

  Further, in the wavelength tunable filter 31 of this embodiment, the light modulator 33 is shared in this way, and only one is required regardless of the number of polarizers arranged along the optical path. It is possible to significantly reduce the length and size of the entire apparatus. Further, by increasing the number of polarizers to be arranged, a tunable filter having a multistage structure can be obtained. Furthermore, the material such as quartz constituting the light modulation unit 33 exhibits high transmittance even in a short wavelength range from ultraviolet to blue. Therefore, a wavelength tunable filter that can variably control the transmission wavelength of light in a wide wavelength range including a short wavelength range is realized.

2A to 2D show the results of simulating the wavelength characteristics of the wavelength tunable filter 31, that is, the transmittance of the peak of the transmission spectrum. 2A to 2D show changes in the transmittance with respect to the wavelength of light at the respective positions P1 to P4 shown in FIG. 1B along the optical path of the light transmitted through the light modulator 33 and emitted. Show. Here, λ ₀ is a specific wavelength, that is, a wavelength of incident light, and λ is a wavelength of transmitted light or outgoing light at each of the positions P1 to P4. In the figure, the solid line indicates the transmittance when the voltage applied to the PEM element is 0, and shifts at a constant frequency from side to side as indicated by the broken line.

  After the incidence, at the position P1 of the optical path after being reflected from the first polarizer 37, the light transmittance is constant 50% in the entire wavelength range (FIG. 2A). The position P2 of the optical path after being reflected from the second polarizer 38 first shows the transmission characteristics between the adjacent polarizers, and has a peak at a wavelength that is an integral multiple of the specific wavelength (FIG. 2B). ). Next, the transmission characteristic between the adjacent polarizers has a peak at a wavelength that is an integral multiple of ½ wavelength of the specific wavelength. Therefore, at the position P3 of the optical path after being reflected from the third polarizer 39, And FIG. 2B show the transmission characteristics (FIG. 2C). Finally, since the transmission characteristic between the adjacent polarizers has a peak at a wavelength that is an integral multiple of a quarter wavelength of the specific wavelength, this is further illustrated at the position P4 emitted from the light modulator 33 as shown in FIG. ) Shows the superimposed transmission characteristics (FIG. 2D). Since the peaks of the transmission spectra between the adjacent polarizers all overlap at an integer multiple of the specific wavelength, the wavelength filter 31 is an integer of the specific wavelength as shown in FIG. Transmission characteristics having double and steep peaks can be obtained.

3A to 3C schematically show the configuration of a modification of the first embodiment. Tunable filter 31 ₁ of this embodiment, the entrance of light and the exit port, and one end portion of the lower principal surface of the optical modulation unit 33 (the left side in the drawing) side, the other end (right side in the drawing ) Side and the first embodiment is different from the first embodiment. Therefore, the lower side reflection mirror 44 ₁ is provided on the lower surface under the birefringent plate 34 at a portion of the entrance and exit of the light. Other configurations are the same as those of the wavelength filter 31 of FIG.

Incident light L1 to the wavelength tunable filter 31 ₁ is first incident on the polarizer 35 passes through the optical modulating section 33 from the light entrance, is separated into the s-polarized light component and p-polarized light component. The s-polarized light component passes through the first polarizer 37 and is emitted to the outside as unnecessary light. The p-polarized component is reflected by the first polarizer with the same reflection angle θ with respect to the normal direction of the phase plate main surface, and is transmitted while being subjected to multiple reflections within the light modulator 33 as described in the embodiment of FIG. Then, the light enters the last fourth polarizer 40. The p-polarized light reflected from the fourth polarizer is transmitted through the light modulator and is emitted to the outside from the lower light exit.

  In the present embodiment, the light emission port is provided on the side opposite to the first to fourth polarizers. Therefore, the emitted light L2 is completely separated from unnecessary light emitted from the respective polarizers to the outside, and the mixing thereof can be reliably prevented.

4A to 4C schematically show the configuration of another modification of the first embodiment. Wavelength tunable filter 31 ₂ of the present embodiment, in that the exit port of the light is provided in the vicinity of the other end portion of the lower principal surface (the right side in the drawing) of the light modulation unit 33, different from the first embodiment. Lower reflection mirrors 44 ₂ is provided at a portion of the exit port of the light. Other configurations are the same as those of the wavelength filter 31 of FIG.

Incident light L1 to the wavelength tunable filter 31 _2, similar to the embodiment of FIG. 1, it is transmitted through the upper birefringent plate 33 upper surface optical modulating section 33 from the light entrance of, s-polarized light component by the first polarizer 37 And p-polarized component. The s-polarized component is transmitted through the first polarizer 37 and emitted to the outside as unnecessary light, and the p-polarized component is reflected by the first polarizer and transmitted through the inside of the light modulator while being multiple-reflected, The light enters the last fourth polarizer 40. The p-polarized light reflected from the fourth polarizer is transmitted through the light modulator and is emitted to the outside from the lower light exit.

  Also in this embodiment, the light emission port is provided opposite to the first to fourth polarizers. Therefore, the emitted light L2 is completely separated from unnecessary light emitted from the respective polarizers to the outside, and the mixing thereof can be reliably prevented.

5A to 5C schematically show the configuration of still another modified example of the first embodiment. Wavelength tunable filter 31 ₃ of this embodiment, in that the entrance of the light is provided on one end portion of the lower surface of the lower surface or lower birefringence plate 34 of the optical modulation unit 33 (the left side in the drawing) side, the first Different from the embodiment. Therefore, the lower reflection mirror 44 _3, is provided at a portion of the entrance of the light. Other configurations are the same as those of the wavelength filter 31 of FIG.

Incident light L1 to the wavelength-variable filter ₃₁₃ transmits light modulating unit 33 from the light entrance of the lower, is split by the first polarizer 37 and the s-polarized light component and p-polarized light component. The s-polarized component is transmitted through the first polarizer 37 and emitted to the outside as unnecessary light, and the p-polarized component is reflected by the first polarizer and transmitted through the light modulation unit while being subjected to multiple reflections. Is incident on the fourth polarizer 40. P-polarized reflected from the fourth polarizer, the light modulating unit and forth transmitted is reflected by the lower reflecting mirror 44 ₃ and is emitted from the upper side of the light exit opening to the outside.

6A to 6C schematically show the configuration of still another modification of the first embodiment. Wavelength tunable filter 31 ₄ of the present embodiment, in the modification of FIG. 5, a fourth polarizer 40 ₁ which is disposed above the light modulation unit 33, the direction of periodicity of the grating of the wire grid polarizer of FIG. 1 The fourth embodiment differs from the first embodiment in that it is oriented in a direction orthogonal to that of the four polarizers 40.

Incident light L1 to the wavelength tunable filter 31 ₄ is transmitted through the optical modulating section 33 from the lower side the light entrance, is split incident on the first polarizer 37 and the s-polarized light component and p-polarized light component. The s-polarized light component is transmitted through the first polarizer 37 and emitted to the outside as unnecessary light, and the p-polarized light component is reflected by the first polarizer and is transmitted through the light modulator while being multiple-reflected. P polarized light reflected on the lower side reflection mirrors 44 ₃ and enters the fourth polarizer 40 ₁ of the last is emitted to the outside is transmitted through the fourth polarizer. Other configurations are the same as those of the wavelength filter 31 of FIG.

FIGS. 7A to 7C schematically show the configuration of still another modification of the first embodiment. Wavelength tunable filter 31 ₅ of this embodiment, first to fourth polarizer 37 to 40 and the upper reflecting mirror 41 and 42, instead of the transparent glass plate, on the fixed phase plate 46 consisting of an optical crystal material such as quartz This is different from the first embodiment in that it is supported. The optical axis 46 a of the fixed phase plate 46 is oriented in a direction orthogonal to the traveling direction of the light transmitted through the light modulation unit 33.

Thus, the wavelength tunable filter 31 ₅ of the present embodiment, the transmission spectrum of the light applied voltage to the PEM element 32 is transmitted through the optical modulation unit 33 in the state in which the crystal oscillator 34 is not excited at 0, fixed phase A value other than 0 is set according to the phase difference of the plate 46. Therefore, it is possible to eliminate the zero point from the transmission characteristics of the wavelength tunable filter 31 ₅ using a PEM element.

  FIGS. 8A to 8C schematically show the configuration of the second embodiment of the wavelength filter according to the present invention. The wavelength filter 51 of the present embodiment is different from the first embodiment in that an additional polarizer 52 made of a wire grid polarizer is provided below the light modulation section 33. The additional polarizer 52 is disposed on the support plate 45 made of a transparent glass plate that supports the lower reflection mirror 44, immediately upstream of the first polarizer 37 along the optical path. The additional polarizer 52 is oriented so that the periodic direction of the grating of the wire grid polarizer is orthogonal to that of the first polarizer of FIG. Therefore, the additional polarizer 52 and the first polarizer 37 are arranged in a crossed Nicols relationship. Other configurations are the same as those of the wavelength filter 31 of FIG.

  Incident light L1 to the wavelength filter 51 passes through the light modulator 33 from the upper light entrance, enters the additional polarizer 52, and is split into an s-polarized component and a p-polarized component. The p-polarized light component passes through the additional polarizer and the support plate 45 and is emitted to the outside as unnecessary light. The s-polarized light component is reflected by the additional polarizer and passes through the inside of the light modulation unit while being multiple-reflected.

  The s-polarized light reflected from the additional polarizer 52 is transmitted through the light modulation unit 33, converted into p-polarized light in the meantime, and incident on the first polarizer 37 to be reflected. The p-polarized light reflected from the first polarizer 37 is transmitted through the light modulating unit 33, converted into s-polarized light in the meantime, reflected by the lower reflecting mirror 44, and transmitted again through the light modulating unit. The light is converted into polarized light and is incident on the second polarizer 37 and reflected. Next, the p-polarized light reflected from the first polarizer is reflected by the lower reflecting mirror and the upper reflecting mirror 41 three times, and passes back and forth twice through the light modulator, and enters the third polarizer 39. Incident and reflected. In the meantime, every time the light modulation unit reciprocates through the light modulator, the p-polarized light is repeatedly converted into s-polarized light, and further s-polarized light to p-polarized light twice. The p-polarized light reflected from the third polarizer is reflected seven times by the lower reflection mirror and the upper reflection mirror 42, passes back and forth through the light modulation unit four times, and enters the fourth polarizer 40. reflect. In the meantime, every time the light modulator is reciprocally transmitted, the p-polarized light is similarly converted into s-polarized light, and further converted from s-polarized light to p-polarized light four times. Finally, the p-polarized light reflected from the fourth polarizer is reflected again by the lower reflecting mirror 44 and once passes through the light modulator once. During that time, the p-polarized light is converted from p-polarized light to s-polarized light. The light is returned to p-polarized light and is emitted to the outside from the light exit port.

As described above, the light modulation unit 33 is given a phase difference of 180 ° while the light is once transmitted between the upper and lower main surfaces. Therefore, the phase difference between the two polarizers 52 and 35 that are initially adjacent to each other along the optical path traveling inside the light modulator is 180 °. Subsequent to this subsequent stage, the phase differences between the two adjacent polarizers 37, 38, 38, 39, and 39, 40 are sequentially 360 °, 720 °, 1440 °, that is, 2 ⁿ⁻¹ × 2π. , (N: 1 to 3).

In this way, in the wavelength tunable filter 51 in which the phase difference between adjacent polarizers is an integral multiple of 2π, by setting the one phase difference to 180 °, transmission as a bandpass filter similar to the conventional rio filter is possible. Characteristics can be obtained, and the variable wavelength band can be expanded to widen the band. In another embodiment, the number of additional polarizers is increased to two or more, and the phase difference Γ _j between each additional polarizer and the polarizer adjacent to the additional polarizer is Γ _j = for the wavelength of light transmitted through the optical path. 2 ^j−1 × 2π−π (where j = ^{1 to} m, m: natural number) By satisfying the relationship, it is configured to be an odd multiple of π to further expand the variable wavelength range. Can do.

  In another embodiment, the additional polarizer 52 can be disposed after the fourth polarizer 40. In this case, the fourth polarizer 40 and the additional polarizer 52 are arranged in a crossed Nicols relationship, and the phase difference between them is 180 °. Therefore, similarly, transmission characteristics as a band-pass filter similar to the conventional rio filter can be obtained, and the variable wavelength range can be expanded to widen the band.

9A to 9E show the results of simulating the wavelength characteristics of the wavelength tunable filter 51, that is, the transmittance of the peak of the transmission spectrum. FIGS. 9A to 9E show changes in the transmittance with respect to the wavelength of the light at the respective positions P1 to P5 shown in FIG. 8 along the optical path of the light transmitted through the light modulator 33 and emitted. . Here, λ ₀ is a specific wavelength, that is, a wavelength of incident light, and λ is a wavelength of transmitted light or outgoing light at each of the positions P1 to P5. In the figure, the solid line indicates the transmittance when the voltage applied to the PEM element is 0, and shifts at a constant frequency from side to side as indicated by the broken line.

  At the position P1 of the optical path after passing through the additional polarizer 52 after incidence, the light transmittance is constant 50% in the entire wavelength range (FIG. 9A). At the position P2 of the optical path after being reflected from the first polarizer 37, the transmission characteristic between the adjacent polarizers is shown first, and has a peak at a wavelength that is an integral multiple of the specific wavelength (FIG. 9B). ). Next, the transmission characteristic between the adjacent polarizers has a peak at a wavelength that is an integral multiple of ½ wavelength of the specific wavelength. Therefore, at the position P3 of the optical path after being reflected from the second polarizer 38, And FIG. 9B show transmission characteristics (FIG. 9C). Further, since the transmission characteristics between the adjacent polarizers have a peak at a wavelength that is an integral multiple of a quarter wavelength of the specific wavelength, at the position P4 of the optical path after being reflected from the third polarizer 39, The transmission characteristics are shown in FIG. 9C (FIG. 9D). Finally, since the transmission characteristic between the adjacent polarizers has a peak at a wavelength that is an integral multiple of 1/8 wavelength of the specific wavelength, this is shown in FIG. D) shows the superimposed transmission characteristics (FIG. 9E). In this way, the peaks of the transmission spectra between the adjacent polarizers all overlap at a wavelength that is an integral multiple of the specific wavelength. Therefore, as shown in FIG. A transmission characteristic having a sharp peak at an integral multiple is obtained.

FIGS. 10A to 10C schematically show the configuration of a modification of the second embodiment. The wavelength tunable filter 511 of the present embodiment is different from the _first embodiment in that the additional polarizer 52 ₁ is disposed below the light modulator 33 and immediately downstream of the second polarizer 36 along the optical path. And different. Add polarizer 52 ₁ is oriented in a direction in which the periodic direction of the grating of the wire grid polarizer is perpendicular to that of the first polarizer of FIG. Therefore, the second polarizer 38 and the additional polarizer 52 ₁ is disposed in crossed Nicols relationship, the phase difference between them is 180 °. The third and fourth polarizers 39 ₁ and 40 ₁ on the downstream side of the additional polarizer 52 ₁ are arranged in a parallel Nicols relationship with the additional polarizer so that the periodic direction of the grating of the wire grid polarizer is illustrated. It is oriented in a direction orthogonal to that of one of the third and fourth polarizers.

Incident light L1 to the wavelength tunable filter 51 ₁ is transmitted through the light modulator portion 33 from the upper side of the light entrance, and enters the first polarizer 37 is reflected by the lower reflecting mirror 44, s-polarized light component and p The light is split into polarized components. The s-polarized light component passes through the first polarizer 37 and is emitted to the outside as unnecessary light. The p-polarized component is reflected by the first polarizer with the same reflection angle θ with respect to the normal direction of the phase plate main surface, and passes through the inside of the light modulator 33 while being subjected to multiple reflection.

The p-polarized light reflected from the first polarizer 37 is transmitted through the light modulating unit 33, converted into s-polarized light in the meantime, reflected by the lower reflecting mirror 44, and transmitted again through the light modulating unit. Is incident on the second polarizer 38 and reflected. Then, p-polarized light reflected from said first polarizer is transmitted through the optical modulation unit 33, is converted into s-polarized light between them incident to add the polarizer 52 _1. Add polarizer 52 ₁ reflects the s-polarized light, and transmits light of other wavelengths, emitted to the outside as unwanted light.

S-polarized reflected from the additional polarizer 52 _1, is three times reflected by the upper reflecting mirror 41 and the lower side reflection mirror 44, the light modulation unit twice reciprocates transmitted, and incident on the third polarizer 39 ₁ Reflect. In the meantime, every time the light modulator is reciprocally transmitted through the light modulator, the s-polarized light is converted into p-polarized light, and further converted from p-polarized light to s-polarized light twice. S-polarized reflected from said third polarizer, the upper reflection mirror and is 7 degrees reflected by the lower reflecting mirror 4 _4, the light modulating unit 4 degrees reciprocally transmits, enters the fourth polarizer 40 ₁ To do. In the meantime, every time the light modulator is reciprocally transmitted, the s-polarized light is similarly converted into p-polarized light and further converted from p-polarized light to s-polarized light four times. The fourth polarizer 40 ₁ reflects the s-polarized light, and transmits light of other wavelengths, emitted to the outside as unwanted light. The 4 s-polarized reflected from the polarizer 40 ₁ is transmitted through the light modulator is converted into p-polarized light therebetween is emitted to the outside from the light exit opening.

Also in this embodiment, in the wavelength tunable filter 51 ₁ the phase difference between the adjacent polarizer is an integral multiple of 2 [pi, by that one of the phase difference and 180 °, conventional Rio filter similar to the bandpass Transmission characteristics as a filter can be obtained, and the variable wavelength range can be expanded to widen the band. In yet another embodiment, increasing the additional polarizer into two or more, and the phase difference gamma _j between the polarizer adjacent thereto and each additional polarizer, gamma _j with respect to the wavelength of light transmitted through the optical path = 2 ^j−1 × 2π−π (where j = 1 to m, m: natural number), so that the variable wavelength region is further expanded by being configured to be an odd multiple of π. be able to.

  11A to 11C schematically show the configuration of a third embodiment of the wavelength filter according to the present invention. In the wavelength tunable filter 61 of this embodiment, the length of the PEM element 62 is shorter than that of each of the above embodiments. Similar to the above embodiments, the PEM element 62 includes a light modulator 63 made of flat-plate quartz having a constant thickness, and a crystal resonator 64 bonded to one end thereof.

  The PEM element is fixed so that the other end of the light modulator 63 does not abut against a stopper (not shown) and is not displaced. When a predetermined voltage is applied, the crystal resonator 34 is excited at a predetermined frequency so as to expand and contract in the direction indicated by the arrow in FIG. 11A along the direction toward the stopper. In the light modulation unit 63, the refractive index of light periodically changes due to the vibration of the crystal resonator 64. Therefore, the light transmitted through the light modulation unit has a polarization state periodically between linearly polarized light and elliptically polarized light. The phase difference between ordinary light and extraordinary light changes at a constant frequency.

  In the present embodiment, one end (left side in the figure) on the lower side and upper side of the light modulation unit 63 is used as the light entrance and exit. Above the light modulation section, first to fourth polarizers 65 to 68 and an upper reflection mirror 69 are provided on a transparent glass plate 70 arranged slightly spaced from the upper main surface of the light modulation section. It is done. The first, fourth, and second polarizers 65, 68, 66 are continuously arranged along the light traveling direction, and then the third polarizer 37 is arranged with the upper reflection mirror 69 interposed therebetween. A lower reflection mirror 70 is provided below the light modulation unit 63 so as to be spaced from the lower main surface of the light modulation unit 63 and leave the light emission port part over the entire length of the other part. The lower reflection mirror 71 is also supported on a support plate 72 arranged slightly spaced from the lower main surface of the light modulator. Furthermore, in this embodiment, a vertical reflection mirror 73 is provided on the end surface of the light modulation unit 63 opposite to the light incident port. The first to fourth polarizers 65 to 68 are wire grid polarizers. The first to fourth polarizers orient the wire grid polarizer in a direction of 45 ° with respect to the vibration direction of the PEM element 62.

  Incident light L1 to the wavelength filter 61 passes through the light modulator 63 from the lower light entrance and enters the first polarizer 65, where it is split into an s-polarized component and a p-polarized component. The s-polarized light component passes through the first polarizer 65 and is emitted to the outside as unnecessary light. The p-polarized light component is reflected by the first polarizer with the same reflection angle θ with respect to the normal direction of the main surface of the light modulation unit, and passes through the light modulation unit 63 while performing multiple reflection as will be described later.

  The p-polarized light reflected from the first polarizer 65 is transmitted through the light modulation unit 63, converted into s-polarized light in the meantime, reflected by the lower reflection mirror 71, and transmitted again through the light modulation unit. The light is converted into polarized light and is incident on the second polarizer 66 and reflected. Next, the p-polarized light reflected from the second polarizer is reflected three times by the lower reflection mirror and the upper reflection mirror 69, and then reciprocates twice through the light modulation unit. Incident and reflected. In the meantime, every time the light modulation unit reciprocates through the light modulator, the p-polarized light is repeatedly converted into s-polarized light, and further s-polarized light to p-polarized light twice. The p-polarized light reflected from the third polarizer is reflected by the lower reflection mirror, the vertical reflection mirror 73, and the upper reflection mirror 69 by 8 degrees, and reciprocally transmits through the light modulation unit by 4 degrees. It is incident on 68 and reflected. In the meantime, every time the light modulator is reciprocally transmitted, the p-polarized light is similarly converted into s-polarized light, and further converted from s-polarized light to p-polarized light four times. Finally, the p-polarized light reflected from the fourth polarizer is again reflected by the lower reflecting mirror 71 and once passes through the light modulator once. During that time, the p-polarized light is converted from p-polarized light to s-polarized light, and from the s-polarized light to p. The light is returned to the polarized light and is emitted to the outside from the light exit port.

The light modulation unit 63 is configured such that a phase difference of 180 ° is given while light is once transmitted between the upper and lower main surfaces. Accordingly, each of the polarizers has a phase difference between two polarizers adjacent to each other along an optical path traveling in the light modulation unit in order of 360 °, 720 °, and 1440 °, that is, 2 ⁿ⁻¹ ×. It arrange | positions so that it may become 2 (pi) and (n: 1-3). Thus, since the phase difference between the two polarizers is always an integer multiple of 2π, the wavelength tunable filter 61 can obtain transmission characteristics as a bandpass filter similar to a conventional rio filter. Furthermore, in this embodiment, the light transmitted through the light modulation section 63 is folded back at the end face and travels in both directions. Therefore, the number of parts can be significantly reduced as compared with the above embodiments, and the length of the apparatus is particularly significant. The dimensions can be reduced.

  In another embodiment, the phase difference given while light is once transmitted between the upper and lower main surfaces of the light modulator 63 can be set to 90 °. In that case, the number of reflections of the light transmitted through the light modulator is doubled between the two adjacent polarizers so that a desired phase difference is obtained. As a result, the wavelength tunable filter 61 can similarly set the phase difference between the two polarizers to an integral multiple of 2π to obtain transmission characteristics as a bandpass filter similar to the conventional rio filter. .

  In another embodiment, the tunable filter 61 may include the additional polarizer of the second embodiment. Thereby, similarly, the variable wavelength region can be widened.

  The present invention is not limited to the above embodiments, and can be implemented with various modifications or changes within the technical scope thereof. For example, each polarizer and reflection mirror can be arranged at various positions other than the above embodiments with respect to the light modulation unit. Furthermore, the number of the polarizers and the reflection mirrors can be increased to configure a multistage wavelength tunable filter. Moreover, each polarizer can also be comprised combining an absorption type polarizing plate and a reflective mirror except for the incident end and the output end.

(A) is a plan view schematically showing a first embodiment of a wavelength tunable filter according to the present invention, (B) is a side view showing the traveling direction of light, and (C) is an end face on the incident side. Figure. (A)-(D) are diagrams each showing the transmission characteristics of the first embodiment. (A) is a plan view schematically showing a modification of the first embodiment, (B) is a side view along the light traveling direction, and (C) is an end view on the incident side. FIG. 5A is a plan view schematically showing another modification of the first embodiment, FIG. 5B is a side view showing the traveling direction of light, and FIG. 5C is an end view on the incident side. (A) is a plan view schematically showing still another modification of the first embodiment, (B) is a side view showing along the traveling direction of light, and (C) is an end view on the incident side. . (A) is a plan view schematically showing still another modification of the first embodiment, (B) is a side view showing along the traveling direction of light, and (C) is an end view on the incident side. . (A) is a plan view schematically showing still another modification of the first embodiment, (B) is a side view showing along the traveling direction of light, and (C) is an end view on the incident side. . (A) is a plan view schematically showing a second embodiment of a wavelength tunable filter according to the present invention, (B) is a side view showing the light traveling direction, and (C) is an incident side thereof. End view. (A)-(E) are diagrams each showing the transmission characteristics of the second embodiment. (A) is a plan view schematically showing a modification of the second embodiment, (B) is a side view along the light traveling direction, and (C) is an end view on the incident side. (A) is a plan view schematically showing a third embodiment of a wavelength tunable filter according to the present invention, (B) is a side view showing the light traveling direction, and (C) is an incident side thereof. End view. The figure which shows the basic composition of a Rio filter. The block diagram of the prior art example using a liquid crystal cell. The block diagram of another prior art example using a liquid crystal cell.

Explanation of symbols

1 ... Rio _{filter, 2a~2d, 12a~12d, 22a~22d, 35~38,37} 1, 38 1, 52,52 1, 65~68 ... polarizer, 2A1～2d1,12a1～12d11 ... transmission shaft, 3a-3c: birefringent plate, 3a1-3c1: optical axis, 4: optical axis, 11: band pass filter, 13a-13c, 23a-23c ... liquid crystal cell, 21, 31, 31 ₁ -31 ₅ , 51,51 _1, 61 ... tunable filter, 24a-24c ... retardation film, 32, 62 ... PEM devices, 33,34,63,64 ... optical modulator portion, 34, 64 ... crystal resonator, 37～40,39 _1, 65-68 ... Polarizer, 41, 42, 44, 69, 71, 73 ... Reflecting mirror, 43, 70 ... Transparent glass plate, 45, 72 ... Support plate.

Claims (9)

A photoelastic polarization (PEM) element having a flat plate-like light modulation unit disposed between the first main surface and the second main surface and a piezoelectric vibrator connected to one end of the light modulation unit; A plurality of polarizers and reflecting mirrors provided on the main surface and the second main surface;
The light incident on the light modulator is transmitted through the light modulator by multiple reflection between the polarizer and the reflection mirror at a certain angle with respect to the normal direction of the main surface. A wavelength tunable filter that emits light from a portion. The phase difference Γ _i between the two polarizers adjacent to each other along the optical path of the light passing through the inside of the light modulator is Γ _i = 2 ⁱ⁻¹ × with respect to the wavelength of the light passing through the optical path. The wavelength tunable filter according to claim 1, wherein a relationship of 2π (where i = 1 to n, n: an integer of 2 or more) is satisfied. At least one phase difference Γ _j between the two polarizers adjacent to each other along the optical path of the light passing through the inside of the light modulator is Γ _j = 2 ^{j with} respect to the wavelength of the light passing through the optical path. The wavelength tunable filter according to claim 2, wherein a relationship of ⁻¹ × 2π−π (where j = 1 to m, m: natural number) is satisfied.   4. The two polarizers adjacent to each other along the optical path of the light transmitted through the light modulation unit are arranged in a parallel Nicol or crossed Nicol relationship. Tunable filter.   The wavelength tunable filter according to claim 1, wherein an entrance and an exit of the light to the light modulation unit are provided on different main surfaces.   The wavelength tunable filter according to claim 1, wherein an entrance and an exit of the light to the light modulator are provided on one main surface.   The wavelength tunable filter according to claim 1, wherein the polarizer is a wire grid polarizer.   2. The polarizer according to claim 1, wherein the polarizer is a wire grid polarizer, and all the polarizers are provided on one of the main surfaces, and the light exit is provided on the other main surface. 8. The wavelength tunable filter according to any one of 7 above.   The apparatus further comprises a vertical reflection mirror provided on an end surface of the light modulation unit, and the light transmitted through the light modulation unit is reflected by the vertical reflection mirror and travels in the reverse direction. The wavelength tunable filter according to any one of 1 to 8.

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