JP2009036903A - Optical wavelength filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve problems associated with: significant power consumption in a heat control mechanism due to utilization of a thermooptic effect in a conventional optical wavelength variable filter; long response time of wavelength variation of 2-60 ms; and moreover reliability damaged from hygroscopicity caused by use of a polymer waveguide from a standpoint of thermooptic characteristics. <P>SOLUTION: The optical wavelength variable filter materializes a wavefront conversion control for varying the wavelength with a spatial optical device other than an AWG. A wavefront control element utilizes a micromachine type wavefront controller by an electrostatic drive method. A magnetic drive method, a thermal drive method, and a piezoelectric drive method are optionally utilized. A similar result is also obtained by utilizing an electro-optical crystal. An LCOS (Liquid Crystal On Silicon) and a liquid crystal element of vertical alignment are also utilized by making light polarization independent. A wavelength variation control is conducted with extremely low power consumption, and response time for wavelength switching is very short. The optical wavelength filter with a simple optical arrangement configuration and excellent in stability and in implementation is materialized. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical wavelength filter. More specifically, the present invention relates to an optical wavelength tunable filter used in optical fiber communication.

  Along with the explosive spread of the Internet, wavelength division multiplexing (WDM) transmission method has been introduced, and multiple wavelength channels are multiplexed and transmitted by a single fiber. Has been. In recent years, WDM communication is moving from a conventional point-to-point system to a ring-type configuration. Such a ring-type configuration system can flexibly respond to changes in communication demand between nodes by using ROADM (Reconfigurable Optical Add Drop Multiplexer) or the like. In a ring network, a plurality of nodes are connected in a ring shape. Transmission of a signal from one node to another is realized by specifying a wavelength. The number of wavelengths used between nodes and the optical wavelength used change with changes in communication demand. The optical wavelength tunable filter is very important as a basic element device for flexibly realizing such a network.

  On the other hand, from the viewpoint of miniaturization and integration of an optical signal processing device, research and development of a waveguide type optical circuit (PLC: Planar Lightwave Circuit) is in progress. In a PLC, for example, a core made of quartz glass is formed on a silicon substrate, and various functions are integrated in one chip. An optical communication device using a PLC is widely used for components constituting the above-described ROADM because it is excellent in reliability and mass productivity and has excellent optical properties. Furthermore, a complex optical signal processing component (apparatus) that combines a plurality of PLC chips and other optical functional components has also appeared.

FIG. 15 is a conceptual diagram of a wavelength tunable filter using a conventional arrayed waveguide grating. The variable wavelength filter uses the thermo-optic effect of the waveguide in the arrayed waveguide. In FIG. 15, the optical signal input to the input waveguide 102 on the arrayed waveguide grating 101 is distributed to the arrayed waveguide 104 via the input slab waveguide 103. Array waveguide 104
Has a constant path length difference between adjacent waveguides. On the incident surface to the output slab waveguide 106, a different phase corresponding to the wavelength is imparted to the optical signal. Since the output slab waveguide 106 operates as a condensing lens, the optical signal is condensed at different positions for each wavelength at the boundary between the output slab waveguide 106 and the output waveguide 107. Accordingly, an optical signal having a specific wavelength collected at the boundary between the output slab waveguide 106 and the output waveguide 107 is propagated through the output waveguide 107 and output. That is, since only an optical signal having a specific wavelength can be transmitted, it operates as a wavelength filter.

  Here, by passing a current through the heater 105a or 105b arranged on the arrayed waveguide 104, the equivalent refractive index of the arrayed waveguide 104 can be changed through the thermo-optic effect by the generated heat. Due to the change in the equivalent refractive index, the phase of the light wave passing through the arrayed waveguide 104 changes. The heaters 105 a and 105 b are arranged with a certain length difference for each waveguide of the arrayed waveguide 105. The amount of phase shift given to the optical signal with the thermo-optic effect varies linearly for each arrayed waveguide. Therefore, the light wavefront at the boundary between the arrayed waveguide 104 and the output slab waveguide 106 can be tilted. That is, the wavelength of the optical signal condensed on the output waveguide 107 can be changed by changing the value of the current flowing through the heater 105a or 105b. That is, an optical wavelength variable filter can be realized. As another configuration example of the optical wavelength filter, for example, Non-Patent Document 2 discloses a configuration using a galvano scanner and a bulk diffraction grating in a spatial system.

Japanese Patent Laid-Open No. 2002-250828 (pages 16, 19, 20, 20, 27, 29D, etc.) S. Toyoda, N. Ooba, A, Kaneko, M. Hikita, T. Kurihara and T. Maruno, "Wideband polymer thermo-optic wavelength tunable filter with fast response for WDM systems", Electronics Letters, US, 2000, 36 ( 7), pp.658-660 WY Oh, SH Yun, GJ Tearney, and BE Bouma, "115 kHz tuning repetition rate ultrahigh-speed wavelength-swept semiconductor laser", OPTICS LETTERS, United States America, December 1, 2005, Vol. 30, No. 23, pp .3159-3161.

  However, the conventional optical wavelength tunable filter shown in FIG. 15 has a problem that power consumption for energizing the heater is large in order to use the thermo-optic effect. Since it is necessary to heat each of several hundreds of arrayed waveguides, a polymer material having a large thermo-optic constant is used as the waveguide material. However, the power consumption for energizing the heater is still large as 3.9 W.

  As described above, the conventional optical wavelength tunable filter consumes a large amount of power and requires a heat control mechanism for the optical wavelength filter module. Furthermore, since the thermo-optic effect is used, the time response of the wavelength variable is as slow as 2-60 ms. Furthermore, since a polymer waveguide is used from the viewpoint of thermo-optical characteristics, it has a drawback in terms of reliability such as moisture absorption.

  Further, since the optical wavelength tunable filter disclosed in Non-Patent Document 2 uses a galvano scanner or a polygon mirror, it is difficult to realize a function of fixing the mirror at a certain angle. That is, as required in ROADM, the wavelength cannot be semi-fixedly selected and functions as a wavelength sweeper. Further, in adapting as a component constituting the ROADM, the optical arrangement is complicated, lacks stability and mountability, and is functionally limited. There has been a demand for an optical wavelength filter that has a simple optical arrangement and is highly stable and mountable.

  In order to achieve such an object, the present invention provides a waveguide-type spectroscopic unit having at least one input waveguide and at least one output waveguide, and an output from the spectroscopic unit. A cylindrical lens for collimating the optical signal, and wavefront conversion means for inverting the traveling direction of the optical wavefront of the optical signal from the cylindrical lens, the wavefront conversion means, depending on the transmitted light wavelength to be selected, The optical wavelength tunable filter is characterized in that the direction of travel of the inverted light wavefront can be tilted in the direction of the spectral axis of the spectroscopic means.

  According to a second aspect of the present invention, in the optical wavelength tunable filter according to the first aspect, the wavefront conversion unit is configured to perform an electrostatic drive method, a magnetic drive method, or a magnetic drive method based on a drive signal corresponding to the selected transmitted light wavelength. It is a micromachine type spatial phase modulation element driven by a piezoelectric drive system.

  According to a third aspect of the present invention, in the optical wavelength tunable filter according to the first aspect, the optical signal from the cylindrical lens is converted into a TM mode optical signal traveling on the first optical path and a TE mode signal traveling on the second optical path. And a λ / 2 wavelength plate disposed between the polarization separation element and the wavefront conversion means and in either the first optical path or the second optical path The wavefront converting means is an electro-optic crystal to which a drive signal corresponding to the selected transmitted light wavelength is applied.

  According to a fourth aspect of the present invention, in the optical wavelength tunable filter according to the first aspect, the optical signal from the cylindrical lens includes a TM mode optical signal that travels along a first optical path and a TE mode signal that travels along a second optical path. And a λ / 2 wavelength plate disposed between the polarization separation element and the wavefront conversion means and in either the first optical path or the second optical path The wavefront converting means includes a plurality of liquid crystal element elements arranged along a line of intersection with the demultiplexing wavefront of the spectroscopic means, and a predetermined phase difference is provided between the adjacent element elements. It is a spatial phase modulation element.

  According to a fifth aspect of the present invention, in the optical wavelength tunable filter according to any one of the first to fourth aspects, an optical signal generator that outputs a reference optical signal and the reference optical signal are input and connected to the spectroscopic unit. A second input waveguide, a plurality of reference light signal output waveguide groups connected to the spectroscopic means and outputting the reference light signal from the wavefront conversion means, and the plurality of reference light signal output waveguide groups A light receiving means for detecting the reference light signal intensity from each of the reference light signal output waveguides, an analysis means for analyzing the intensity signal of the reference light signal output from the light receiving means, and an analysis result of the analysis means Determining the reflection direction or reflection direction setting sensitivity of the traveling direction of the light wavefront by the wavefront conversion means, and outputting the drive signal such that an optical signal having a desired transmitted light wavelength is output from the at least one output waveguide. Generate And further comprising a dynamic signal control means.

  According to a sixth aspect of the present invention, in the optical wavelength tunable filter according to the fifth aspect, a predetermined variable drive signal is supplied from the drive signal control unit to the wavefront conversion unit, and each reference light is synchronized with the variable drive signal. The reflection direction or the reflection direction setting sensitivity is determined by the analysis unit based on a change in signal intensity of the reference light signal sequentially output from the signal output waveguide.

  According to a seventh aspect of the present invention, in the optical wavelength tunable filter according to the sixth aspect, the signal level of the variable drive signal varies linearly on a time axis, and the reference light is synchronized with the variable drive signal. The reflection direction or the reflection direction setting sensitivity is determined by the analysis unit based on information on the peak signal intensity of the reference light signal sequentially output from the signal output waveguide.

  According to an eighth aspect of the present invention, in the optical wavelength tunable filter according to the sixth aspect, the variable drive signal has a minute size such that the reference optical signal is output from any one of the reference optical signal output waveguides. It is a repetitive waveform signal having an amplitude, and the reflection direction or the reflection direction setting sensitivity is determined by the analysis means by synchronously detecting the signal intensity of the reference light signal.

  The invention of claim 9 is the optical wavelength tunable filter according to any one of claims 1 to 8, wherein the spectroscopic means is a slab waveguide to which the at least one input waveguide and the at least one output waveguide are connected. And an arrayed waveguide grating including an arrayed waveguide.

  The invention of claim 10 is the optical wavelength tunable filter according to any one of claims 2 to 9, wherein the at least one input waveguide, the second input waveguide, the at least one output waveguide, and the reference light. It is an arrayed waveguide grating including a slab waveguide and an arrayed waveguide to which signal output waveguide groups are connected.

  As described above, according to the optical wavelength filter of the present invention, the optical wavelength can be variably controlled with very little power consumption. There is no need to form a heater electrode on the AWG. An optical wavelength filter having an excellent effect that the wavelength switching speed is extremely fast, a simple optical arrangement, and high stability and mountability can be realized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all the drawings for explaining the embodiments of the invention, the same reference numerals are given to those having the same function. The repeated description is omitted. The optical wavelength filter of the present invention is characterized in that AWG is used as a spectroscopic means, and further, wavefront conversion control for variable wavelength is realized by a spatial optical element arranged outside the AWG. The arrangement and fixing of the spatial optical element is easy, and a configuration that is rich in mountability is realized. The selection wavelength can be set stably and anti-fixedly, and the number of wavelengths used between nodes and the optical wavelength to be used are suitable for changing with changes in communication demand.

  In each of the embodiments described in detail below, the case where AWG is used is shown in terms of good optical characteristics and ease of manufacturing, but a stepped diffraction grating or the like made on an optical waveguide substrate is used. However, the same effect can be obtained. Any waveguide type spectral means can be used to reduce the size and thickness of the apparatus by using the waveguide type spectral means.

  FIG. 1 is a conceptual diagram showing the configuration of an optical wavelength filter according to a first embodiment of the present invention. In this embodiment, a reflection type structure is used in which optical signals are input and output using one arrayed waveguide grating and one waveguide. First, the configuration and operation around the arrayed waveguide grating of the optical wavelength filter will be described.

  The optical signal to the wavelength filter is input from the input optical fiber 10 and output from the output optical fiber 12. The input optical fiber 10 and the output optical fiber 12 are connected to the connection fiber 13 via the optical circulator 11. An optical signal is input to the input waveguide 2 on the arrayed waveguide grating (hereinafter referred to as AWG) 1 through the connection fiber 13. The input optical signal is distributed to the arrayed waveguide 4 by the slab waveguide 3. The optical signal distributed by the arrayed waveguide 4 is emitted from the end face 7 of the arrayed waveguide grating 1 as it is. The emitted optical signal is collimated by the cylindrical lens 5 in the substrate thickness direction (y direction) of the AWG 1.

  On the other hand, the emitted optical signal is already collimated light due to the lens effect of the slab waveguide 3 in the direction (x direction) including the demultiplexing surface and perpendicular to the emitting direction of the optical signal. For this reason, the optical signal emitted from the cylindrical lens 5 becomes parallel light in both the x-axis and y-axis directions. Further, the optical signal emitted from the arrayed waveguide grating 1 is emitted in a different direction for each wavelength at the end face 7 due to the dispersion effect of the AWG 1. The emitted optical signal propagates through free space and enters the micromachine type wavefront controller 6.

  FIG. 2 is a diagram illustrating the configuration of the micromachine type wavefront controller. The micromachine type wavefront controller 6 has a mechanism for tilting the mirror 30 about the y axis as a center of rotation by an electrostatic drive system. In FIG. 2, only the mirror 30 is shown, and a known divided electrode or the like for tilting the mirror 30 by electrostatic attraction is arranged on the opposite side of the reflection surface of the optical signal from the AWG 1. Here, a structural example of a micromachine type wavefront controller by an electrostatic drive system is shown, but the present invention is not limited to this. It goes without saying that the same wavefront conversion control action can be obtained even with the magnetic drive system or the piezoelectric drive system. Further, as will be described later in the second embodiment or the third embodiment, the same effect can be obtained even when an electro-optic crystal is used. Further, by making polarization independence, a similar wavefront conversion control function can be realized even when LCOS (Liquid Crystal on Silicon) or a vertically aligned liquid crystal element is used as shown in the fourth embodiment. .

  The optical signal reflected by the micromachine type wavefront controller 6 is propagated in the direction of the AWG 1 by inverting the direction of the optical path. The optical signal further propagates in the direction of the input waveguide 2 via the cylindrical lens 5, the arrayed waveguide 4, and the slab waveguide 3. Here, it should be noted that, of the optical signal reflected by the micromachine wavefront controller 6, only the wavelength component perpendicularly incident on the mirror 30 is collected at the position of the input waveguide 2. That is, only an optical signal having a wavelength perpendicularly incident on the mirror 30 is reflected by the mirror 30, and the reflected light returns to the AWG through the same return path as the forward path. On the other hand, optical signals of other wavelengths are condensed at a position different from the connection position between the input waveguide 2 and the slab waveguide 3 due to the lens effect of the slab waveguide 3. Therefore, only an optical signal having a specific wavelength selectively propagates back through the input waveguide 2, and the band-pass filter characteristic is realized.

  Here, in the micromachine wavefront controller 6, the transmission wavelength of the optical wavelength filter can be controlled by changing the angle of the mirror 30. The optical signal having the selected wavelength component is output via the connection fiber 13, the optical circulator 11, and the output fiber 12.

  In this example, AWG1 was fabricated as a quartz-based planar lightwave circuit with a relative refractive index difference of 1.5%, and the optical parameters were set as follows to realize an optical wavelength filter. That is, the path length difference of the arrayed waveguide 4 is 33.8 μm, the diffraction order is 31, the center wavelength is 1.59 μm, the focal length of the slab waveguide is 8240 μm, the pitch at the output end 7 of the arrayed waveguide 4 is 10 μm, and the cylindrical The focal length of the lens was 1 mm.

  FIG. 3 is a diagram showing the relationship between the mirror angle and the transmission wavelength in this example. The horizontal axis represents the wavelength (μm), and the vertical axis represents the transmission spectrum (dB) of the wavelength tunable filter. Each transmission characteristic is shown using the mirror tilt angle as a parameter. As can be seen from FIG. 3, when the tilt of the mirror is changed from −1.6 ° to 1.6 °, the central wavelength of the transmission band spectrum can be varied from 1565 nm to 1606 nm. Further, an insertion loss of 4 dB was obtained at each central wavelength.

  Further, the electric power necessary for setting the inclination of the mirror 30 to a predetermined angle was a very small value of 0.1 W or less. This is because the micromachine type wavefront controller 7 is controlled by an electrostatic drive system and essentially does not require drive power. This is because there is no need to drive the current as in the conventional method using the thermo-optic effect using a heater. Compared with the conventional wavelength tunable filter that warms the waveguide and uses the thermo-optic effect, the wavelength tunable filter of the present invention can obtain the same function only by rotating the mirror that is the wavefront control element. That is, the energy required for wavelength selection is only necessary for rotating the mirror, and its power consumption is extremely small. As shown in this embodiment, a tunable filter with very low power consumption can be configured by using AWG and a micromachine wavefront controller.

  By using a micromachine type wavefront controller or a MEMS mirror, precise angle control is possible, so that it can be stably fixed to a specific mirror position. Therefore, as required in ROADM, semi-fixed wavelength selection with stability becomes possible. The mirror control is also simpler than the conventional AWG temperature control.

Second embodiment:
In the second embodiment, a configuration example using potassium tantalate niobate (KTa _1-x Nb _x O) as a wavefront control element is shown. In the present embodiment and the following third embodiment, since the wavefront control is performed using the electro-optic effect, it is possible to configure an optical wavelength filter capable of a high-speed switching operation. In general, since the electro-optic crystal has polarization dependency, a method for solving it is also described in this embodiment.

  FIG. 4 is a conceptual diagram showing the configuration of the optical wavelength filter according to the second embodiment of the present invention. In this embodiment, a reflection type configuration is used in which optical signals are input and output using one arrayed waveguide grating. Compared with the configuration of the first embodiment shown in FIG. 1, it is different in that it has independent input waveguides and output waveguides. Another difference is that a means for eliminating the polarization dependence of the electro-optic crystal is provided. Hereinafter, the configuration and operation of the optical wavelength filter according to the present embodiment will be described while focusing on differences from the first embodiment.

  The optical signal incident on the input waveguide 2 formed on the AWG 1 is distributed to the arrayed waveguide 4 via the slab waveguide 3. As in the first embodiment, the cylindrical lens 5 is arranged so that the parallel beam is in the x direction and the beam waist is in the y direction at the position of a mirror 19 described later. Here, the emitted light from the end face 7 is emitted in different directions in the xz plane corresponding to the wavelength due to the dispersion effect of the AWG 1. A broken line represents reflected light reflected by a mirror described later.

Referring to FIG. 4b, the outgoing light from the AWG 1 represented by the solid line is transmitted by the polarization splitting crystal 14 in the yz plane, the TM polarization component traveling on the optical path 18a and the TE polarization component traveling on the optical path 18b. Are separated into optical signals. Here, the TM polarization component and the TE polarization component relate to the polarization mode of the waveguide of AWG1, respectively. A λ / 2 wavelength plate 15 is inserted in the optical path 18a through which the TM polarized wave propagates. The TM-polarized optical signal passes through the λ / 2 wavelength plate 15, rotates its polarization axis by 90 °, and becomes the same TE-polarized optical signal as the optical signal passing through the optical path 18b. The optical signal incident on the potassium tantalate niobate (KTa _1-x Nb _x O) crystal 16 has the same polarization (both the component traveling in the optical path 18a and the component traveling in the optical path 18b) due to the configuration including the deflection separation crystal. TE). Therefore, it is not affected by the polarization dependence of the potassium tantalate niobate crystal 16.

  In the potassium tantalate niobate crystal 16, electrodes 17a and 17b are arranged so as to sandwich the crystal 16 at two points on the x-axis. By applying a voltage between the electrodes 17a and 17b, a refractive index distribution in which the refractive index changes linearly in the x-axis direction according to the applied voltage is obtained. Therefore, the wavefront of the optical signal passing through the potassium tantalate niobate crystal 16 is inclined by the deflection angle θ in the x-axis direction, for example, the + x direction. Here, it goes without saying that if the polarity of the voltage applied between the electrodes 17a and 17b is reversed, it can be tilted in the -x direction.

  FIG. 5a is a diagram showing an example of a refractive index distribution in the x-axis direction in an electro-optic crystal. By applying a voltage between the electrodes 17a and 17b, a distribution in which the refractive index changes linearly in the x-axis direction including the spectral plane of the AWG 1 can be formed. FIG. 5b is a diagram for explaining how the optical signal is reflected in the vicinity of the electro-optic crystal. The optical path shown in FIG. 5b represents the center line of the optical path. The outgoing light from the AWG is incident from the left side of FIG. 5b perpendicularly to the acquisition surface of the potassium tantalate niobate crystal 16, reflected by a mirror disposed in contact with the opposite surface of the incident surface, and then emitted again. The case is shown by way of example.

  On the opposite side of the optical signal incident surface of the potassium tantalate niobate crystal 16, a mirror 19 is provided in contact with the crystal, and the optical signal is reflected by this mirror 19. The reflected optical signal propagates again through the potassium tantalate niobate crystal 16 in the opposite direction, that is, in the −z direction, as represented by a broken line. Even during this reverse propagation, the optical signal passes through the potassium tantalate niobate crystal 16 in which a refractive index distribution in which the refractive index changes linearly in the x-axis direction is set. For this reason, the reflected light traveling in the crystal is inclined in the same direction as when propagating in the + z direction, that is, in the + x direction. Therefore, when the optical signal is emitted again from the potassium tantalate niobate crystal 16, it is inclined by 2θ with respect to the optical axis of the original incident light.

  Referring again to FIG. 4b, the component of the back-propagating optical signal that returns through one optical path 18a passes through the λ / 2 plate 15 once again and the component that passes through the other optical path 18b remains as it is. Enter the crystal 14. The components traveling along the optical path 18a and the optical path 18b are combined by the polarization separation crystal 14. The light enters the arrayed waveguide 4 from the end face 7 of the AWG 1 via the cylindrical lens 5. Further, the signal is output via the slab waveguide 3 and the output waveguide 22.

  Here, it should be noted that the condensing point position So to the output waveguide 22 is different from the incident point position Si of the input waveguide 2. In FIG. 4, the outgoing light represented by the solid line and the reflected light from the mirror 19 represented by the broken line differ in the outgoing angle and the incident angle at the end face 7 of the AWG 1. For this reason, the optical signal is input from the point Si on the boundary with the slab waveguide 3, and is output from the point So at a position different from Si. In this respect, it should be noted that the configuration and operation of the first embodiment that inputs and outputs optical signals at the same point of the slab waveguide 3 are different.

  The wavelength of the optical signal that is input from the input waveguide connected to the Si point and can be output as it is from the output waveguide connected to the So point depends on the deflection angle θ of the potassium tantalate niobate crystal 16. . Therefore, by varying the change angle θ according to the voltage applied to the potassium tantalate niobate crystal 16, the wavelength of the optical signal output from the output waveguide at the So point can be changed. That is, since the center wavelength of the transmission band can be controlled, a variable optical wavelength filter can be realized.

  FIG. 6 is a diagram showing the relationship between the voltage applied to the electro-optic crystal and the center wavelength of the transmission band. The applied voltage (V) is shown on the horizontal axis, and the center wavelength (nm) of the transmission band is shown on the vertical axis. Here, the center wavelength of the AWG 1 is 1.587 μm, the path length difference of the arrayed waveguide 4 is 65.4 μm, the waveguide interval at the end surface 7 of the arrayed waveguide 4 is 11 μm, and the arrayed waveguide 4 with the slab waveguide 3 The waveguide interval at the boundary 8 is 10.5 μm, the slab waveguide length is 3280 μm, the focal length of the cylindrical lens is 1 mm, the width Wk of the potassium tantalate niobate crystal is 1 mm, and the electrode length Lk is 5 mm. It can be seen that by changing the applied voltage from −2500 V to 2500 V, the center wavelength of the transmission band can be adjusted from 1567 nm to 1605 nm so as to cover the entire L band.

  FIG. 7 is a diagram illustrating dynamic response characteristics of the optical wavelength filter according to the second embodiment. The horizontal axis represents time (ns), and the vertical axis represents normalized light intensity. Initially, the center wavelength of the transmission band was set to 1580 nm, and the control voltage was changed stepwise at a high speed so that the center wavelength was 1595 nm. The control voltage itself applied between the electrodes 17a and 17b switches at a sufficiently high speed. The center wavelength of the transmission band can be variably controlled from 1580 nm to 1595 nm with a control time of about 18 ns. As described above, according to the present invention, a high-speed wavelength switching operation that cannot be realized by a conventional wavelength tunable filter is possible. Not only when the potassium tantalate niobate crystal 16 is used as an optical wavefront controller, but also when a crystal exhibiting the secondary electro-optic effect shown in the third embodiment is used, the high-speed switching of the wavelength is similarly achieved. Realized.

Third embodiment:
In the third embodiment, an electro-optic crystal having a secondary electro-optic effect is used instead of the potassium tantalate niobate (KTa _1-x Nb _x O) crystal 16 in the second embodiment. . As a crystal having a secondary electro-optic effect, for example, K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, <y <1), LiNbO ₃ , LiTaO ₃ , LiIO ₃ , KNbO ₃ , KTiOPO ₄ , BaTiO ₃ , SrTiO ₃ , Ba _1-x Sr _x TiO ₃ (0 <x <1), Ba _1-x Sr _x Nb ₂ O ₆ (0 <x <1), Sr _0.75 Ba _{0.25 Nb 2 O 6, Pb 1-} y La y Ti 1-x Zr x O 3 (0 <x <1, <y <1), Pb (Mg 1/3 Nb 2/3) O 3 -PbTiO 3, KH ₂ PO ₄ , KD ₂ PO ₄ , (NH ₄ ) H ₂ PO ₄ , BaB ₂ O ₄ , LiB ₃ O ₅ , CsLiB ₆ O ₁₀ , GaAs, CdTe, GaP, ZnS, ZnSe, ZnTe, CdS, CdSe, ZnO and so on. Here, a case where lithium niobate LiNbO ₃ is used as an electro-optic crystal is shown.

  FIG. 8 is a conceptual diagram showing the configuration of the optical wavelength filter according to the third embodiment of the present invention. Since the configuration shown in FIG. 8 is almost the same as the configuration shown in FIG. 4, only differences from FIG. 4 will be described here. In this embodiment, lithium niobate crystal 20 is arranged at the same position instead of potassium tantalate niobate crystal. The lithium niobate crystal 20 is provided with electrodes 21a and 21b so as to sandwich the crystal 20 at two points on the y-axis. Only the arrangement of electrodes for applying a voltage to the lithium niobate crystal 20 is different, and other configurations and operations as the optical wavelength filter are the same as those of the second embodiment, and the description thereof is omitted.

  FIG. 9 is a diagram showing the relationship between the voltage applied to the secondary electro-optic crystal and the center wavelength of the transmission band. The AWG waveguide interval at the end face 7 was set to 30 μm, and the other parameters of the arrayed waveguide grating 1 and the cylindrical lens 5 were set to the same values as in the second embodiment. Here, the angle Φ formed by the acute angle of the electrodes 21a and 21b was set to 30 °.

  As shown in this embodiment, since the AWG can be a diffraction grating having different grating periods on the input side and the output side, a flexible optical design that is not possible with a normal bulk diffraction grating is possible. The angular dispersion of the arrayed waveguide grating is expressed by the following equation, where n is the group index of the waveguide, ΔL is the path length difference of the arrayed waveguide, d is the pitch of the arrayed waveguide, and λ is 0 as the center wavelength.

Here, d can be set arbitrarily. Therefore, even when an electro-optic crystal having a small electro-optic constant is used, the shift sensitivity of the center wavelength of the transmission band with respect to the deflection angle θ by the electro-optic crystal can be set large. This is a great advantage not in the case of using a bulk diffraction grating disclosed in Non-Patent Document 2 or the like.

Fourth embodiment:
In the embodiments described so far, the wavefront angle of the optical signal is continuously and smoothly changed by controlling the mirror tilt angle of the micromachine type wavefront controller or the refractive index of the electro-optic crystal as the wavefront control element. However, the present invention is not limited to this, and a phase modulation element composed of a plurality of element elements can also be used. By giving a phase difference little by little between the adjacent element elements, the wavefront angle of the optical signal can be changed smoothly as a whole.

  FIG. 10 is a conceptual diagram showing the configuration of an optical wavelength filter according to the fourth embodiment of the present invention. Since the configuration of FIG. 10 is almost the same as the configuration of FIG. 4, only differences from FIG. 4 will be described here. In this embodiment, the phase modulation element 23 is arranged at the same position instead of the potassium tantalate niobate crystal 16 and the mirror 19. As the phase modulation element, an element using MEMS or LCOS of a liquid crystal element can be used. Since the phase modulation element 23 is voltage-controlled, the power consumption is not increased for variable control of the transmission wavelength as in the prior art. The other configurations and operations as the optical wavelength filter are the same as those of the second embodiment, except for the wavefront control means, and detailed description thereof is omitted.

  FIG. 11 is a conceptual diagram illustrating an example of phase setting in the phase modulation element. FIGS. 11a and 11b show examples of different phase settings. FIG. 11c illustrates the relationship between the phase setting value and the arrangement position of the element elements of the phase modulation element. FIG. 11 conceptually illustrates a configuration in which 20 element elements are arranged. The arrangement pitch of the element elements is represented by p. The direction in which the element elements are arranged corresponds to the x-axis direction in FIG. It should be noted that the actual number of element elements is not limited to this.

  For example, in the case of the liquid crystal element, by providing an appropriate phase difference between the individual liquid crystal pixels 24, the incident light wave incident on the phase modulation element 23 is diffracted, and the reflection direction thereof differs from the incident direction. be able to. That is, the overall phase gradient in the arrangement direction of the phase modulation elements given by the phase change between adjacent pixels determines the deflection angle θ of the optical signal. FIG. 11 a is an example in which a phase difference within a maximum range of 2π is set between pixels at both ends. FIG. 11b shows a case where the maximum phase difference to be set exceeds 2π. In this case, the phase may be set by turning back when 2π is exceeded. This is because the light wave is diffracted by feeling only the phase.

Specifically, in the case where the liquid crystal element is configured by setting the pixel pitch p of the phase modulation element to 5 μm, the number of pixels N to 512, and the wavelength of the input optical signal to 1.5 μm, the phase given between the pixels at both ends is 5. If 7 × 10 ⁻¹ [rad], the deflection angle θ can be set to 10 °.

  By using a phase modulation element such as a liquid crystal element, precise phase control becomes possible, so that it can be stably fixed at a specific deflection angle. Therefore, as required in ROADM, semi-fixed wavelength selection with stability becomes possible. The control of the liquid crystal element is also simpler than the temperature control of the conventional AWG.

Fifth embodiment:
FIG. 12 is a conceptual diagram showing the configuration of an optical wavelength filter according to the fifth embodiment of the present invention. The present embodiment is characterized by further including means for setting or calibrating the transmission wavelength of the optical wavelength filter in addition to the optical wavelength filter having the configuration described so far. Since the optical wavelength filter realizes a function of selectively transmitting only a predetermined optical signal, the optical wavelength filter does not actively emit an optical signal. Therefore, it is necessary to confirm whether the central transmission wavelength is set appropriately. This embodiment includes means for calibrating the setting of the transmission wavelength. The configuration shown in FIG. 12 is substantially the same as that of the optical wavelength filter of the third embodiment shown in FIG. 8, and only the characteristic points of this embodiment will be described here.

  In the present embodiment, a second input waveguide 26 and a second output waveguide group 27 connected to predetermined positions are further installed at the boundary 50 of the slab waveguide 3 on the AWG 1. A reference light signal having a narrow line width from the reference light source 28 is input to the second input waveguide 26 via the connection optical fiber 32. On the other hand, the reference light signal output from the second output waveguide group 27 is received by the light receiving element 29. The light reception output signal from the light receiving element 29 is converted into a digital signal by the AD converter 30. The AD converter 30 is further connected to a digital signal processor (DSP) 31, and a control signal from the DSP 31 is applied between the electrodes 21 a and 21 b that sandwich the electro-optic crystal 20.

  The wavelength of the reference light signal output from the reference light source 28 is set so that this reference light signal is output to one of the output waveguides of the second output waveguide group 27 via the optical wavelength filter. The The full width at half maximum of the reference light signal can be set to 100 kHz by using, for example, an external resonator type semiconductor laser. By applying a voltage between the electrodes 21 a and 21 b installed in the electro-optic crystal 20, the waveguides in which the reference light signal is output in the second output waveguide group 27 are sequentially switched.

  FIG. 13 is a diagram illustrating a change in the output signal level of the reference light signal observed in the light receiving element. When the voltage between the electrodes 21a and 21b is linearly changed by the waveguide configuration of this embodiment, the output signal level output from the light receiving element 29 by the reference light signal is a pulse train shape as shown in FIG. Varies with shape. This change in the output signal level can be explained as follows. That is, at the boundary 50 with the slab waveguide 3, the reference light signal is received when the reference light signal from the reference light source 28 is collected at the center of any output waveguide of the second output waveguide group 27. This is because the reference light signal is not guided to the light receiving element 29 when it is coupled to the element 29 and condensed at a position off the center of the waveguide. By observing the change in the output signal in the form of a pulse train as shown in FIG. 13, the control sensitivity of the beam deflection angle by the electro-optic crystal 20 with respect to the control voltage can be known.

  For example, the DSP 31 can know the control signal voltage value to the electro-optic crystal output by itself and the corresponding pulsed light receiving output signal voltage value at the control voltage value. Therefore, the control sensitivity of the deflection angle can be obtained based on the information on the second input waveguide and the second output waveguide group 27 having a known predetermined structure, the AWG design parameter information, and the like.

  Based on the control sensitivity of the deflection angle obtained from the reference light signal and the absolute value of the control signal, the optical signal incident from the input waveguide 2 is output to the output waveguide 22 by the function as a wavelength selection filter. Wavelength components can be determined. In other words, the transmission wavelength of the optical wavelength filter can be precisely set by analyzing the output signal waveform from the pulse train-shaped light receiving element as shown in FIG. Further, a minute modulation signal is superimposed on the control signal voltage applied to the electro-optic crystal from the DSP 31, and the frequency component of the modulation signal is synchronously detected from the light reception output signal waveform output from the light receiving element 29, thereby generating a specific beam. A control signal voltage to the electro-optic crystal may be set so as to have a deflection angle.

  As described above, in the present embodiment, the position of the input point of the optical signal on the boundary 50 of the slab waveguide 3 and the position of the condensing point of the optical signal passing through the optical wavelength filter are in a fixed linear relationship. From the relationship between the second input waveguide using the reference light signal and the second output waveguide group 27, it is to be understood that the deflection sensitivity of the control signal is calibrated. Therefore, on the boundary 50 of the slab waveguide, as long as each waveguide is arranged in a range where the relationship between the light incident point and the condensing point is linear, the input / output waveguides 2, 22 as the filter function and The position and order of the second input / output waveguides 26 and 27 as the calibration function are not limited at all. For example, in FIG. 12, the second input waveguide 26, the input waveguide 2, the second output waveguide group 27, and the output waveguide 22 are arranged in this order on the boundary 50. However, the present invention is not limited to this. The second input waveguide 26, the second output waveguide group 27, the input waveguide 2, and the output waveguide 22 may be arranged in this order.

  FIG. 14 is a diagram illustrating an example of the configuration of each waveguide connected to the slab waveguide. As shown in FIG. 14, the distance S between the second input waveguide 26 and the input waveguide 2 is 278 μm, the waveguide distance d of each of the second output waveguide groups 27 is 14.1 μm, and the second output. In the waveguide group 27, the distance between the output waveguide 27b farthest from the input waveguide 2 and the input waveguide 2 and the distance between the output waveguide 27b and the output waveguide 22 are both set as a distance S (= 278 μm). do it.

  In order to realize the wavelength tunable filter of this embodiment, for example, the center wavelength of the AWG 1 is 1.587 μm, the path length difference of the arrayed waveguide 4 is 16.4 μm, and the waveguide interval at the end face 7 of the arrayed waveguide 4 is 11 μm. The waveguide interval at the boundary 8 between the slab waveguide 3 and the arrayed waveguide 4 is 10.5 μm, the slab waveguide length is 11400 μm, and the focal length of the cylindrical lens 5 is 1 mm.

  Here, the wavelength of the reference light signal of the standard light source 28 is 1587 nm. When the deflection direction of the electro-optic crystal 20 is set so that the reference optical signal is output to the waveguide 27a, the optical signal of 1595 nm among the signals input from the input waveguide 2 is output to the output waveguide 22. Shall be. When the reference optical signal is set to be output from the waveguide 27 b by changing the deflection direction of the electro-optic crystal 20 by the control voltage, an optical signal of 1587 nm is output from the output waveguide 22. As described above, by using the reference light source 28, it is possible to precisely set the transmission wavelength of the wavelength tunable filter.

  Furthermore, even when the deflection characteristics of the electro-optic crystal 20 vary due to changes in the environmental temperature or the like, the reference light signal input to the second input waveguide is used at any time and from the second output waveguide group 27. The change in the optical signal level can be observed, and the control sensitivity of the deflection angle can be recalibrated. The applied voltage to the electrodes 21a and 21b can be set based on the recalibrated control sensitivity, and the temperature of the set wavelength of the wavelength tunable filter can be stabilized. The frequency of recalibration may be arbitrarily determined in consideration of changes in environmental temperature, long-term changes in filter characteristics, and the like. For example, calibration may be performed at equal time intervals, or a constant temperature change may be detected and configured using this as a trigger. The filter characteristics can also be similarly obtained by superimposing a minute modulation signal on the control voltage applied to the electro-optic crystal and synchronously detecting the frequency component of the modulation signal from the light reception output signal waveform output from the light receiving element 29. Can be stabilized.

  In the configuration shown in FIG. 12, the output signal from the light receiving element 29 is observed by the AD converter 30 and the DSP 31, but any method can be used as long as it can analyze and calibrate the control sensitivity of the deflection angle described above. A simple configuration may be used. That is, it is possible to use a more general CPU and to have an independent DA converter or the like for generating a control voltage. Needless to say, the positional relationship between the optical signal input / output waveguides and the calibration waveguides on the end face of the slab waveguide is not limited to the configuration in which the waveguides are arranged at equal intervals S as shown in FIG.

  In any of the above-described embodiments, by using a mirror, an incident system for the input optical signal and an output system for the wavelength-selected output optical signal are shared by one AWG. However, a transmission type configuration in which an AWG for an incident system and an AWG for an output system are provided and two AWGs are arranged to face each other may be used. In this case, the transmission wavelength can be selectively set by controlling the deflection angle by the wavefront control element arranged in the optical path of the space system between the two AWGs. The reflection type configuration described above has advantages over the transmission type due to the simplicity of the configuration.

  In the first embodiment, the output waveguide is shared with the input waveguide. In the second embodiment, one output waveguide is provided independently. However, the present invention is not limited to this. Two or more output waveguides may be provided.

  As described in detail above, according to the optical wavelength filter of the present invention, wavelength tunable control can be performed with extremely low power consumption. There is no need to form a heater electrode on the AWG. An optical wavelength filter having an excellent effect that the wavelength switching speed is extremely fast, a simple optical arrangement, and high stability and mountability can be realized.

  It can be used for an optical signal processing device used for optical communication. Application to an optical wavelength tunable filter is possible.

It is a conceptual diagram which shows the structure of the optical wavelength filter which concerns on the 1st Example of this invention. It is a figure explaining the structure of a micromachine type | mold wavefront controller. It is the figure which showed the relationship between the mirror angle and transmission wavelength of a present Example. It is a conceptual diagram which shows the structure of the optical wavelength filter which concerns on the 2nd Example of this invention. It is a figure explaining an example and refractive operation of the refractive index distribution of the x-axis direction in an electro-optic crystal. It is a figure which shows the relationship between the voltage applied to an electro-optic crystal, and the center wavelength of a transmission band. It is a figure which shows the dynamic response characteristic of the optical wavelength filter which concerns on a 2nd Example. It is a conceptual diagram which shows the structure of the optical wavelength filter which concerns on the 3rd Example of this invention. It is a figure which shows the relationship between the voltage applied to a secondary electro-optic crystal, and the center wavelength of a transmission band. It is a conceptual diagram which shows the structure of the optical wavelength filter which concerns on the 4th Example of this invention. It is a conceptual diagram explaining the example of the phase setting in a phase modulation element. It is a conceptual diagram which shows the structure of the optical wavelength filter which concerns on the 5th Example of this invention. It is a figure which shows the change of the light reception output signal level observed with a light receiving element. It is the figure which showed an example of the structure of each waveguide connected to a slab waveguide. It is a figure which shows the structure of the wavelength tunable filter using the thermo-optic effect by a prior art.

Explanation of symbols

1, 101 AWG (spectral element)
2, 102 Input waveguide 3, 103, 106 Slab waveguide 4, 104 Array waveguide 5 Cylindrical lens 6 Wavefront control element 11 Optical circulator 14 Deflection crystal 15 λ / 2 plate 16, 20 Electro-optic crystal 17a, 17b, 21a , 21b Electrode 19 Mirror 22, 107 Output waveguide 105a, 105b Heater

Claims (10)

Waveguide-type spectroscopic means having at least one input waveguide and at least one output waveguide;
A cylindrical lens for collimating the optical signal emitted from the spectroscopic means;
Wavefront conversion means for inverting the traveling direction of the optical wavefront of the optical signal from the cylindrical lens,
The optical wavelength tunable filter characterized in that the wavefront conversion means can tilt the traveling direction of the inverted light wavefront in the direction of the spectral axis of the spectroscopic means in accordance with the transmitted light wavelength to be selected.   The wavefront conversion means is a micromachine type spatial phase modulation element driven by an electrostatic drive method, a magnetic drive method or a piezoelectric drive method based on a drive signal corresponding to the selected transmitted light wavelength. The optical wavelength tunable filter according to claim 1. A polarization separation element that separates the optical signal from the cylindrical lens into a TM mode optical signal that travels along a first optical path and a TE mode signal that travels along a second optical path, the polarization separation element, and the wavefront conversion means And a λ / 2 wavelength plate disposed in either the first optical path or the second optical path,
2. The optical wavelength tunable filter according to claim 1, wherein the wavefront conversion unit is an electro-optic crystal to which a drive signal corresponding to the selected transmitted light wavelength is applied. A polarization separation element that separates the optical signal from the cylindrical lens into a TM mode optical signal that travels along a first optical path and a TE mode signal that travels along a second optical path, the polarization separation element, and the wavefront conversion means And a λ / 2 wavelength plate disposed in either the first optical path or the second optical path,
The wavefront conversion means is a liquid crystal spatial phase modulation element in which a plurality of liquid crystal element elements are arranged along an intersection line with the branching wavefront of the spectroscopic means, and a predetermined phase difference is given between the adjacent element elements. The optical wavelength tunable filter according to claim 1, wherein the optical wavelength tunable filter is provided. An optical signal generator for outputting a reference optical signal;
A second input waveguide to which the reference light signal is input and connected to the spectroscopic means;
A plurality of reference light signal output waveguide groups connected to the spectroscopic means and outputting the reference light signal from the wavefront conversion means;
A light receiving means for detecting a reference light signal intensity from each reference light signal output waveguide of the plurality of reference light signal output waveguide groups;
Analyzing means for analyzing the intensity signal of the reference light signal output from the light receiving means;
The reflection direction or reflection direction setting sensitivity of the traveling direction of the light wavefront by the wavefront conversion unit is determined according to the analysis result of the analysis unit, and an optical signal having a desired transmitted light wavelength is output from the at least one output waveguide. The optical wavelength tunable filter according to any one of claims 2 to 4, further comprising: a drive signal control unit that generates the drive signal to be output.   A predetermined variable drive signal is supplied from the drive signal control means to the wavefront conversion means, and the signal intensity change of the reference light signal sequentially output from each reference light signal output waveguide in synchronization with the variable drive signal. 6. The optical wavelength tunable filter according to claim 5, wherein the reflection direction or the reflection direction setting sensitivity is determined by the analysis unit.   The variable drive signal linearly changes its signal level on a time axis, and relates to a peak signal intensity of the reference light signal sequentially output from each reference light signal output waveguide in synchronization with the variable drive signal. The optical wavelength tunable filter according to claim 6, wherein the reflection direction or the reflection direction setting sensitivity is determined by the analysis unit based on information.   The variable drive signal is a repetitive waveform signal having a minute amplitude such that the reference light signal is output from any one of the reference light signal output waveguides, and synchronizes the signal intensity of the reference light signal. The optical wavelength tunable filter according to claim 6, wherein the reflection direction or the reflection direction setting sensitivity is determined by the analysis unit by detecting.   9. The arrayed waveguide grating according to claim 1, wherein the spectroscopic means is an arrayed waveguide grating including a slab waveguide and an arrayed waveguide to which the at least one input waveguide and the at least one output waveguide are connected. The optical wavelength tunable filter according to any one of the above.   An arrayed waveguide grating including an arrayed waveguide and a slab waveguide connected to the at least one input waveguide, the second input waveguide, the at least one output waveguide, and the reference optical signal output waveguide group; The optical wavelength tunable filter according to claim 2, wherein the optical wavelength tunable filter is provided.

Images