OPTICALLY VARIABLE FILTER APPARATUS AND FILTER CHARACTERISTIC CONTROL METHOD THEREOF
1. Field of the Invention The present invention relates to an optically variable filter apparatus and its filter characteristic control method that are used in optical communication field and spectroscopic analysis field. 2. Discussion of the Related Art Nowadays, an optically variable filter is widely used in various fields represented by the optical communication field and spectroscopic analysis field. Especially in the optical communication field, in order to meet increasing demand for an increase in transmission capacity in recent years, a higher transmission rate and new modification format have been actively researched and developed, and an optical network has become complicated. In such optical network, the optically variable filter capable of changing a light beam having a desired wavelength in optical signal is used. In such optical filter, in order to achieve optimum filtering with respect to the transmission rate and modification format of each optical signal, there is a demand for a function of dynamically controlling a filter center frequency and passband at an optical frequency level in addition to the conventional frequency selective function. For example, US2006/0067611A1 discloses an optically variable filter using a two-dimensional reflection-type LCOS (Liquid Crystal On Silicon)-based liquid crystal element (hereinafter referred to as LCOS element) as a frequency selective element. In an optically variable filter apparatus, there is a demand for a function of controlling a filter center frequency or changing a passband so as to be optimum with respect to the transmission rate and modification format of each optical signal. To change the center frequency or passband, it is considered that light is dispersed in different directions according to the optical frequency, the dispersed light beams are incident on a frequency selective element having a lot of pixels and the transmittance is ON/OFF controlled for each pixel. However, frequency resolution is limited to a frequency assigned to each pixel. The assigned frequency is determined by a product of reciprocal linear dispersion amount D as frequency per unit length on the plane of the frequency selective element and a pixel width d in the frequency dispersion direction (D·d). Thus, in order to achieve higher resolution of frequency selective characteristics, it is needed to reduce the pixel width or reciprocal linear dispersion amount, thereby causing problems that the apparatus is increased in size and the frequency selective element having a lot of pixels is required. Therefore, it has been demanded to enhance the frequency resolution without having to increase the apparatus in size and refine the frequency selective element. In consideration of these problems of the conventional optically variable filter apparatus, a technical object of the present invention is to enhance the frequency selective resolution and continuously change filter characteristics without having to increase the apparatus in size or fine the frequency selective element. In order to solve the problems, a first aspect of the present invention is directed to an optically variable filter apparatus which comprises: an entrance/exit section which receives a light beam and allows exit a light beam of selected frequencies; a frequency dispersion element which spatially disperses the light beam incident on said entrance/exit section according to their frequencies and synthesizes reflected light beams; a light condensing element which condenses light beams dispersed by said frequency dispersion element as parallel light beams; a frequency selective element which has a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams condensed by said light condensing element, and which changes reflection characteristics of each pixel to obtain desired frequency selective characteristics; and a frequency selective element driving unit which drives each pixel of said frequency selective element to gradation-control transmission characteristics according to frequencies of the incident light beam. A second aspect of the present invention is directed to an optically variable filter apparatus which comprises: an entrance section which receives a light beam; a frequency dispersion element which spatially disperses the light beam received by said entrance section according to their frequencies; a first light condensing element which condenses light beams dispersed by said frequency dispersion element; a frequency selective element which has a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams condensed by said light condensing element, and which changes transmission characteristics of each pixel to obtain desired frequency selective characteristics; a frequency selective element driving unit which drives each pixel of said frequency selective element to gradation-control optical transmission characteristics according to frequency of the incident light beams; a second light condensing element which condenses light beams passed through said frequency selective element; a frequency synthesizing element which synthesizes the dispersed light beams condensed by said second light condensing element; and an exit section which allows exit the light beam synthesized by said frequency synthesizing element. In the optically variable filter apparatuses, said frequency selective element driving unit controls the pixels of said frequency selective element at least four gradations. In the optically variable filter apparatuses, a pixel width in the frequency dispersion direction in said frequency selective element is smaller than a beam radius of an incident light beam to said frequency selective element in the frequency dispersion direction. In the optically variable filter apparatuses, said frequency selective element is an LCOS element having a plurality of pixels arranged at least in a one-dimensional manner, and said frequency selective element driving unit controls a voltage applied to each pixel according to the frequency selective characteristics. In the optically variable filter apparatuses said frequency selective element is a liquid crystal element having a plurality of pixels arranged at least in a one-dimensional manner, and said frequency selective element driving unit controls a voltage applied to each pixel according to the frequency selective characteristics. To solve the problems, a third aspect of the present invention is directed to a filter characteristic control method in an optically variable filter apparatus which has a frequency selective element having a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams, comprising the steps of: upon setting a ratio of incident to emitted light beams emitted through pixel group each composed of at least one pixel of said frequency selective element, the pixel corresponding to each frequency of the incident light beam, to transmittance of the pixel group, bringing successive desired pixel groups into an optical transmissive state; and gradually increasing transmittance of at least one first control pixel group adjacent to one end pixel group among pixel groups in a transmission frequency range and transmittance of at least one second control pixel group adjacent to the other end pixel group among the pixel groups in said transmission frequency range, thereby increasing a bandwidth. A forth aspect of the present invention is directed to a filter characteristic control method in an optically variable filter apparatus which has a frequency selective element having a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams, comprising the steps of: upon setting a ratio of incident to emitted light beams emitted through pixel group each composed of at least one pixel of said frequency selective element, the pixel corresponding to each frequency of the incident light beam, to transmittance of the pixel group, bringing successive desired pixel groups into an optical transmissive state; and gradually decreasing transmittance of at least one first control pixel group which is one end pixel group among pixel groups in a transmission frequency range and transmittance of at least one second control pixel group which is the other end pixel group among the pixel groups in said transmission frequency range, thereby decreasing a bandwidth. A fifth aspect of the present invention is directed to a filter characteristic control method in an optically variable filter apparatus which has a frequency selective element having a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams, comprising the steps of: upon setting a ratio of incident to emitted light beams emitted through pixel group each composed of at least one pixel of said frequency selective element, the pixel corresponding to each frequency of the incident light beam, to transmittance of the pixel group, bringing successive desired pixel groups into an optical transmissive state; gradually increasing transmittance of at least one first control pixel group adjacent to one end pixel group among pixel groups in a frequency changing direction in a transmission frequency range; and gradually decreasing transmittance of at least one second control pixel group which is the other end pixel group among the pixel groups in said transmission frequency range, thereby changing a center frequency in said transmission frequency range along a frequency axis. According to the present invention having such features, light beams dispersed according to their frequencies are made correspond to a plurality of pixels arranged in the dispersion direction and the transmittance of the pixels is continuously controlled. Accordingly, the frequency resolution can be enhanced without having to increase the apparatus in size and highly refine the frequency selective element. As a result, it is possible to change a passband width and center frequency of the passband with high resolution. Herein, In the present embodiment, an optical axis of the incident light is separated from that of emitted light. However, a common optical axis may be employed and the incident/emitted light may be guided to an identical fiber and separated by a circulator into incident/emitted light beams. Then, the separated incident/emitted light beams may be guided to respective optical fibers 11, 14. Next, a transmission-type optically variable filter apparatus in accordance with a second embodiment of the present invention will be described. Next, the frequency selective elements 17, 25 used in the optically variable filter apparatuses in accordance with the first and second embodiments will be described below. In the first and second embodiments, when the incident light is dispersed on the xz plane according to frequency and the dispersed light beams are incident on the frequency selective elements 17, 25 as strip-like light beams, an incident region is defined as a rectangular region R shown in Next, specific examples of the frequency selective element 17 will be described. A first example of the frequency selective element 17 is a two-dimensional reflection-type LCOS (Liquid Crystal On Silicon)-based liquid crystal element (hereinafter referred to as LCOS element) 17A1. The two-dimensional LCOS element 17A1 has a built-in liquid crystal modulation driver located at the back of the element. Accordingly, the number of pixels can be increased, and thus, the LCOS element 17A1 can be formed of a lot of pixels arranged, for example, in a 1000×1000 lattice pattern. In the LCOS element 17A1, since light beams are incident separately at different positions according to their frequencies, by bringing pixels corresponding to the incidence position of a target light beam into reflective state, it is possible to select the light beam of a specific frequency. Now, as one of modulation modes applicable to the LCOS element 17A1, a phase modulation mode will be explained. Next, as one of modulation modes applicable to the LCOS element 17A1, an intensity modulation mode will be explained. As a second example of the frequency selective element 17, a liquid crystal element 17A2 having a reflection-type two-dimensional electrode array will be described. The liquid crystal element 17A2 has no LCOS structure. The LCOS element has the built-in liquid crystal driver disposed at the back of each pixel. On the other hand, in the liquid crystal element 17A2, the liquid crystal modulation driver is provided at the outside of the element 17A2. The other configuration of the liquid crystal element 17A2 is the same as that of the LCOS element and can achieve the above-mentioned phase modulation mode and intensity modulation mode. Further, reflectance can be continuously gradation-controlled by changing voltage levels with respect to pixels in an analog manner. As a third example of the frequency selective element 17, the two-dimensional MEMS element 17A3 will be described. The MEMS element in which a lot of MEMS mirrors are two-dimensionally arranged has been put into practical use as a Digital Micromirror Device (DMD). It is assumed that all pixels in one column of the MEMS mirror in the y-axis direction correspond to one optical frequency of the WDM signal. Also in the case of MEMS, since a plurality of pixels of the MEMS element are associated with one frequency band, the reflectance can be varied by controlling voltages applied to many pixels associated with one frequency and performing phase modulation. In addition, as shown in Next, as a fourth example of the frequency selective element 17, a one-dimensional LCOS element 17B1 will be described. This frequency selective element 17B1 is, as shown in As a fifth example of the frequency selective element 17, a liquid crystal element 17B2 having a reflection-type one-dimensional electrode array can be employed. Also in this case, the above-mentioned phase modulation mode is inapplicable, and the frequency is selected according to only the intensity modulation mode. Further, as a sixth example of the frequency selective element 17, a reflection-type one-dimensional MEMS mirror element 17B3 can be employed. Also in this case, the above-mentioned phase modulation mode is inapplicable, and the frequency is selected according to only the intensity modulation mode. Next, the transmission-type frequency selective element 25 used in a wavelength variable filter apparatus in accordance with the second embodiment will be described. As a first example of the frequency selective element 25, a transmission-type two-dimensional LCOS element 25A1 can be employed. Also in the LCOS element 25A1, since light beams are incident separately at different positions according to their frequencies, by bringing pixels corresponding to the incidence position of a target light beam into the transmissive state, it is possible to select optical signals of the frequency. Now, as one of modulation modes applicable to the LCOS element 25A1, a phase modulation mode will be explained. Next, as another modulation mode applicable to the LCOS element, an intensity modulation mode will be explained. As a second example of the frequency selective element 25, a liquid crystal element 25A2 having a transmission-type two-dimensional electrode array can be employed. The liquid crystal element 25A2 has no LCOS structure. The LCOS element has the built-in liquid crystal driver disposed at the back of each pixel. On the other hand, in the liquid crystal element 25A2, the liquid crystal modulation driver is provided at the outside of the two-dimensional electrode array liquid crystal element 25A2. The other configuration of the liquid crystal element 25A2 is the same as that of the LCOS element and can achieve the above-mentioned phase modulation mode and intensity modulation mode. Further, the transmittance can be continuously gradation-controlled by changing the voltage level with respect to pixels in an analog manner. As a third example of the frequency selective element 25, a one-dimensional LCOS element 25B1 will be described. This frequency selective element 25B1 is, as shown in As a fourth example of the frequency selective element 25, an electrode array liquid crystal element 25B2 having a transmission-type one-dimensional electrode array can be employed. Also in this case, the above-mentioned phase modulation mode is inapplicable, and the frequency is selected according to only the intensity modulation mode. Next, frequency control of the optically variable filter apparatus in accordance with the first and second embodiments of the present invention will be described in detail. In the following description, even in the reflection-type optically variable filter apparatus in the first embodiment, part of incident light is reflected on each pixel of the frequency selective element and returns to the output side. The ratio of transmitted light at this time is assumed as transmittance. Further, in the following description, even in the two-dimensional frequency selective element, pixels corresponding to the same frequency, that is, pixels having a common x coordinate along the y axis, are regarded as one pixel group. In other words, in the following description, pixel groups P1to Pmare arranged in the x-axis direction in the frequency selective element. In the one-dimensional frequency selective element, one pixel corresponds to the pixel group. Each of the pixel groups P1to Pmhas respective transmittance. In an initial state, the transmittance of the pixel groups Pi+1to Pi+k(1<=i, k<=m) (i, k, and m are natural numbers) is set to 1 and the transmittance of the other pixel groups is set to 0. This causes a band-pass filter for allowing light to pass through a frequency band having the transmittance of 1. To increase the bandwidth, the transmittance of the pixel groups adjacent to the pixel groups Pi+1and Pi+kis continuously increased at the same time. Such pixel groups whose transmittance is changed are referred to as control pixel groups, and Piand Pi+k+1are referred to as a first control pixel group and second control pixel group, respectively. (1) Linear dispersion amount on the plane of the frequency selective element, that is, pixel width per GHz, is 2.89 μm/GHz;
In the above-mentioned initial state, by gradually decreasing the transmittances of the pair of pixel groups Pi+l, Pi+k(the first and second control pixel groups) at both ends among the pixel groups Pi+1to Pi+kconstituting the band-pass filter from 1 to 0 as shown in By continuously changing the transmittance of the pair of pixel groups on the high-frequency side and low-frequency side of predetermined bandwidth in the same direction in this manner, the bandwidth of the band-pass filter can be increased or decreased. Although the number of control pixel groups at both ends are one herein, the present invention is not limited to this, and a slope characteristic of the band-pass filter can be changed by simultaneously controlling a plurality of pixel groups in both sides. When the variable width of the passband is set to the frequency corresponding to the two pixel groups or greater, the variable width of the passband can be controlled unlimitedly and continuously by sequentially shifting the control pixel groups to adjacent pixel groups. Frequency resolution of the frequency selective element is determined depending on the voltage applied to the element. In the intensity modulation mode, gradations of voltage, that is, the number of bits of D/A converters included in the driver 31 or 33 directly correspond to the resolution. Next, a control method of shifting the center frequency of the band-pass filter will be described. First, it is assumed that the transmittances of the pixel groups P1+1 to Pi+kare 1 and the transmittances of the other pixel groups are 0, thereby the band-pass filter having a frequency range of light incident on the pixel groups Pi+1to Pi+kis formed. Here, when the passband is shifted to the high-frequency side, as shown in Next, a control method of decreasing the center frequency of the filter from the above-mentioned initial state will be described. In this case, the transmittance is continuously controlled so as to increase the transmittance of the pixel group Piadjacent to the pixel group Pi+1(first control pixel group) on the low-frequency side in the changing direction and decrease the transmittance of the pixel group Pi+k(second control pixel group) on the high-frequency side. In both of the above-mentioned cases, when the variable width of the center frequency exceeds frequency for one pixel, the variable width of the center frequency can be controlled unlimitedly and continuously by sequentially shifting the control pixel groups to the adjacent pixel groups. Further, control in the order of MHz can be performed by increasing the number of bits as gradations of voltage applied to the frequency selective element. Next, optical design conditions for the control method will be described. In the present embodiment, since an intermediate value of the transmittance of the pixel groups is reflected on the filter form, conditions that enable such continuous characteristic change are determined depending on a pixel width in the frequency dispersion direction in the control pixel group and input beam diameter. That is, it is assumed that, when the width of the control pixel group is greater than the input beam diameter of each frequency component of a WDM signal light beam, the transmittance designed for the control pixel group is directly reflected on filter waveform, generating distortion. Herein, the pixel width in the frequency dispersion direction in the frequency selective element is defined as d, and in a range of light intensity of 1/e2of a peak or less, the beam radius of a light beam of each frequency component is defined as w. At this time, a parameter γ determined by the pixel width d and beam radius w is introduced. Although incident light is WDM signal light in the first and second embodiments, the incident light is not limited to the WDM signal light. In other words, the present invention can be applied to various applications of filtering a desired light beam, for example, fields of tunable lasers and spectroscopic analysis. As has been described in detail, according to the present invention, by changing reflection characteristics or transmission characteristic of the frequency selective element in units of pixel, the frequency selective characteristics of incident light can be changed. Thereby, the filter apparatus can be used as a main component of a node having an add-drop function of the WDM light or a component of spectroscopic apparatus. It is to be understood that although the present invention has been described with regard to preferred embodiments thereof, various other embodiments and variants may occur to those skilled in the art, which are within the scope and spirit of the invention, and such other embodiments and variants are intended to be covered by the following claims. The text of Japanese application No. 2010-254648 filed on Nov. 15, 2010 is hereby incorporated by reference. Light from an optical fiber is incident on a frequency dispersion element. The frequency dispersion element disperses the incident light into light beams in different directions according to their frequencies and directs the dispersed light beams to a lens. The lens develops the incident light beams over an xy plane according to their frequencies in a strip-like form. A frequency selective element has pixels arranged in a frequency dispersion direction and brings pixels located at positions corresponding to the frequency to be selected into a reflective state. A light beam selected by the frequency selective element is emitted from an optical fiber through the same path. By changing reflection characteristics of the frequency selective element according to each pixel, optical filter characteristics can be desirably changed so as to achieve change of passband width and frequency shift. 1. An optically variable filter apparatus comprising:
an entrance/exit section which receives a light beam and allows exit a light beam of selected frequencies; a frequency dispersion element which spatially disperses the light beam incident on said entrance/exit section according to their frequencies and synthesizes reflected light beams; a light condensing element which condenses light beams dispersed by said frequency dispersion element as parallel light beams; a frequency selective element which has a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams condensed by said light condensing element, and which changes reflection characteristics of each pixel to obtain desired frequency selective characteristics; and a frequency selective element driving unit which drives each pixel of said frequency selective element to gradation-control transmission characteristics according to frequencies of the incident light beam. 2. The optically variable filter apparatus according to said frequency selective element driving unit controls the pixels of said frequency selective element at least four gradations. 3. The optically variable filter apparatus according to a pixel width in the frequency dispersion direction in said frequency selective element is smaller than a beam radius of an incident light beam to said frequency selective element in the frequency dispersion direction. 4. The optically variable filter apparatus according to said frequency selective element is an LCOS element having a plurality of pixels arranged at least in a one-dimensional manner, and said frequency selective element driving unit controls a voltage applied to each pixel according to the frequency selective characteristics. 5. The optically variable filter apparatus according to said frequency selective element is a liquid crystal element having a plurality of pixels arranged at least in a one-dimensional manner, and said frequency selective element driving unit controls a voltage applied to each pixel according to the frequency selective characteristics. 6. The optically variable filter apparatus according to said frequency selective element is an MEMS element having a plurality of pixels arranged at least in a one-dimensional manner, and said frequency selective element driving unit controls a voltage applied to each pixel according to frequency selective characteristics. 7. An optically variable filter apparatus comprising:
an entrance section which receives a light beam; a frequency dispersion element which spatially disperses the light beam received by said entrance section according to their frequencies; a first light condensing element which condenses light beams dispersed by said frequency dispersion element; a frequency selective element which has a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams condensed by said light condensing element, and which changes transmission characteristics of each pixel to obtain desired frequency selective characteristics; a frequency selective element driving unit which drives each pixel of said frequency selective element to gradation-control optical transmission characteristics according to frequency of the incident light beams; a second light condensing element which condenses light beams passed through said frequency selective element; a frequency synthesizing element which synthesizes the dispersed light beams condensed by said second light condensing element; and an exit section which allows exit the light beam synthesized by said frequency synthesizing element. 8. The optically variable filter apparatus according to said frequency selective element driving unit controls the pixels of said frequency selective element at least four gradations. 9. The optically variable filter apparatus according to a pixel width in the frequency dispersion direction in said frequency selective element is smaller than a beam radius of an incident light beam to said frequency selective element in the frequency dispersion direction. 10. The optically variable filter apparatus according to said frequency selective element is an LCOS element having a plurality of pixels arranged at least in a one-dimensional manner, and said frequency selective element driving unit controls a voltage applied to each pixel according to the frequency selective characteristics. 11. The optically variable filter apparatus according to said frequency selective element is a liquid crystal element having a plurality of pixels arranged at least in a one-dimensional manner, and said frequency selective element driving unit controls a voltage applied to each pixel according to the frequency selective characteristics. 12. A filter characteristic control method in an optically variable filter apparatus which has a frequency selective element having a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams, comprising the steps of:
upon setting a ratio of incident to emitted light beams emitted through pixel group each composed of at least one pixel of said frequency selective element, the pixel corresponding to each frequency of the incident light beam, to transmittance of the pixel group, bringing successive desired pixel groups into an optical transmissive state; and gradually increasing transmittance of at least one first control pixel group adjacent to one end pixel group among pixel groups in a transmission frequency range and transmittance of at least one second control pixel group adjacent to the other end pixel group among the pixel groups in said transmission frequency range, thereby increasing a bandwidth. 13. A filter characteristic control method in an optically variable filter apparatus which has a frequency selective element having a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams, comprising the steps of:
upon setting a ratio of incident to emitted light beams emitted through pixel group each composed of at least one pixel of said frequency selective element, the pixel corresponding to each frequency of the incident light beam, to transmittance of the pixel group, bringing successive desired pixel groups into an optical transmissive state; and gradually decreasing transmittance of at least one first control pixel group which is one end pixel group among pixel groups in a transmission frequency range and transmittance of at least one second control pixel group which is the other end pixel group among the pixel groups in said transmission frequency range, thereby decreasing a bandwidth. 14. A filter characteristic control method in an optically variable filter apparatus which has a frequency selective element having a plurality of pixels placed at positions at least in a frequency dispersion direction so as to receive light beams, comprising the steps of:
upon setting a ratio of incident to emitted light beams emitted through pixel group each composed of at least one pixel of said frequency selective element, the pixel corresponding to each frequency of the incident light beam, to transmittance of the pixel group, bringing successive desired pixel groups into an optical transmissive state; gradually increasing transmittance of at least one first control pixel group adjacent to one end pixel group among pixel groups in a frequency changing direction in a transmission frequency range; and gradually decreasing transmittance of at least one second control pixel group which is the other end pixel group among the pixel groups in said transmission frequency range, thereby changing a center frequency in said transmission frequency range along a frequency axis. BACKGROUND OF THE INVENTION
SUMMARY OF THE INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
Second Embodiment
(Configuration of Frequency Selective Element)
(Change of Bandwidth)
(2) Light beam radius w is 22.6 μm;
(3) Pixel width on the frequency selective element in the frequency dispersion direction is 8.5 μm; and
(4) There is one control pixel group on the high-frequency side and one control pixel group on the low-frequency side. In this example, as shown in the curves A to E, the transmission characteristics vary in the range from the bandwidth of ±25 GHz of the center frequency to the bandwidth of ±28 GHz of the center frequency. By continuously increasing the transmittances of the pixel groups at both ends, the bandwidth can be continuously increased.
(Center Frequency Shift)
γ=