JP2013047772A - Optical function element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow an optical function element to be constituted in such a state that the optical function element can be miniaturized and can be easily manufactured.SOLUTION: The optical function element includes at least an optical waveguide 101 and a polarization splitting part 102 disposed on the output side of the optical waveguide 101. A phase modulation signal 121 which has polarization inclined at 45° with respect to an intrinsic polarization plane of the optical waveguide 101 or being in a circularly polarized state is inputted to the optical waveguide 102, and the optical waveguide 101 has such an optical waveguide length that the difference in propagation time between a first polarized wave 122 parallel with the intrinsic polarization plane and a second polarized wave 123 vertical to the intrinsic polarization plane is a time corresponding to one bit of the phase modulation signal.

Description

  The present invention relates to an optical functional element that can be used as a 1-bit delayed optical interferometer, an optical frequency filter, or the like used when demodulating a differential phase modulation signal.

  In the field of optical communication, in recent years, a method of performing phase modulation as signal modulation has attracted attention in order to cope with a high bit rate of 40 Gb / s or more. In particular, a differential phase shift keying system that splits signal-modulated light into two, shifts the timing by one bit of signal modulation, combines and interferes, converts a phase change into a light intensity signal, and demodulates it. Since it is not necessary to synchronize the frequencies of the signal light and the locally transmitted light, it is attracting attention as a method close to practical use because it is relatively easy to mount.

  A 1-bit delay optical interferometer is used for demodulation of the optical differential phase modulation signal by the differential phase shift keying method. The 1-bit delay optical interferometer currently in general use is a Michelson interferometer or a Mach-Zehnder interferometer, and obtains interference by shifting the time ΔT of 1 bit of the modulation signal by a spatial optical path difference. Yes.

  For example, as shown in FIG. 4, a 1-bit delay optical interferometer using a Michelson interferometer uses a beam splitter 401, two reflecting mirrors 402, and a reflecting mirror 403. A certain optical path is formed (see Non-Patent Document 1). The incident light (Input) passes through the beam splitter 401 and is reflected by the reflecting mirror 402 and is output through a delayed optical path (Output1), and is reflected by the beam splitter 401 and is not reflected by the reflecting mirror 403. The light is output separately from the light (Output2) that is output through the optical path.

  Since the phase difference from both optical paths changes depending on the wavelength, the output spectrum of each optical path has an interference pattern that repeats light and dark at a modulation bit rate frequency = 1 / ΔT. This confirms that the 1-bit interferometer is configured with the configuration shown in FIG.

JP 2001-208933 A

J. Hsieh et al., "Athermal Demodulator for 42.7-Gb / s DPSK Signals", Proceedings of the 31st European Conference on Optical Communication 2005 (ECOC 2005), vol.4, pp.827-828, ISBN: 0-86341 -543-1. D. Lockwood, L. Pavesi (Eds.), "Silicon Photonic Wire Waveguides: Fundamentals and Applications" in "Silicon Photonics II," Springer, 2011. H. Fukuda et al., "Polarization rotator based on silicon wire waveguides", Optics Express, vol.16, pp.2628-2635, 2008. H. Fukuda et al., "Ultrasmall polarization splitter based on silicon wire waveguides", Optics Express, vol.14, pp.12401-12408, 2006. Y. Vlasov et al., "Losses in single-mode silicon-on-insulator strip waveguides and bends", Optics Express, vol.12, pp.1622-1631, 2004.

  However, the configuration described above has a problem that the delayed optical interferometer becomes large. A delay optical interferometer using the spatial optical system as described above requires a mirror constituting the interferometer and a support / adjustment mechanism for the mirror in addition to the spatial optical path difference necessary for the delay. For this reason, for example, the size of the interferometer adapted to the demodulation operation of the modulation signal of about 40 Gbps is very large, 27 × 27 × 10 (mm), and is not suitable for multi-channel integration. In addition, the mirror and the mirror support / adjustment mechanism are mechanically operated, have high costs for parts and assembly, and are still unsuitable for multi-channel integration.

  The present invention has been made to solve the above-described problems, and it is an object of the present invention to make it possible to configure an optical functional element in a state that can be reduced in size and can be more easily manufactured. .

  An optical waveguide comprising a core having a rectangular or trapezoidal cross section perpendicular to the waveguide direction according to the present invention, and polarization separation means disposed on the output side of the optical waveguide. An optical signal in a linearly polarized state or a circularly polarized state in which the polarization is inclined by 45 ° with respect to the intrinsic polarization plane is input, and the first polarization parallel to the intrinsic polarization plane and the perpendicular to the intrinsic polarization plane The difference in propagation time from the second polarization is the optical waveguide length that takes a desired value, and the polarization reference plane of the polarization separation means has an angle of 45 ° with respect to the intrinsic polarization plane. .

  In the optical functional element, the optical signal is a phase modulation signal, and the optical waveguide has an optical waveguide length in which a difference in propagation time between the first polarization and the second polarization is a time corresponding to one bit of the phase modulation signal. It is said that. The optical signal is an optical signal to be filtered, and the optical waveguide is an optical waveguide whose propagation time difference is the time indicated by the reciprocal of the desired free spectrum range. It is the waveguide length.

  The optical functional element includes a polarization adjusting unit disposed on the input side of the optical waveguide, and the polarization adjusting unit is configured to change the polarization of the input light to the optical waveguide to 45 ° with respect to the intrinsic polarization plane of the optical waveguide. Linear polarization or circular polarization may be used.

  In the optical functional element, the polarization separation means and the polarization adjustment means are integrated on the substrate on which the optical waveguide is formed, and the polarization separation means on the output side of the optical waveguide is connected to the polarization adjustment element and the polarization The polarization adjustment element is composed of a 45-degree linearly polarized wave that is horizontal or vertical to the eigenpolarization plane, or a circularly polarized wave that is horizontal or vertical to the eigenpolarization plane. The polarization reference plane of the polarization separation element may be parallel or perpendicular to the intrinsic polarization plane.

  In the above-described optical functional element, the core may be made of silicon, and the clad of the optical waveguide may be made of air, vacuum, silicon oxide, or silicon oxynitride. Moreover, the width and height of the cross section of the core should just be formed in the dimension of the range of 200-500 nm.

  As described above, according to the present invention, for example, the difference in propagation time between the first polarization parallel to the intrinsic polarization plane and the second polarization perpendicular to the intrinsic polarization plane is Since an optical waveguide having an optical waveguide length corresponding to a time corresponding to one bit of the phase modulation signal to be guided is used, the optical functional element can be configured in a state where it can be downsized and can be manufactured more easily. An excellent effect is obtained.

FIG. 1 is a configuration diagram (a) showing the configuration of the optical functional element according to Embodiment 1 of the present invention, and a sectional view (b) showing a cross section of a part of the configuration. FIG. 2 is a characteristic diagram showing the result of measuring the wavelength spectrum of the optical output of the optical functional element in the first embodiment. FIG. 3 is a configuration diagram showing the configuration of the optical functional element according to Embodiment 2 of the present invention. FIG. 4 is a configuration diagram showing a configuration of an optical functional element using a Michelson interferometer. FIG. 5: is the block diagram (a) which shows the structure of the optical functional element in Embodiment 3 of this invention, and (b) which shows the cross section of a partial structure. FIG. 6 is a characteristic diagram showing the results of measuring the wavelength spectrum of the optical output of the optical functional element in the third embodiment. FIG. 7 is a configuration diagram showing the configuration of the optical functional element according to Embodiment 4 of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
First, Embodiment 1 of the present invention will be described with reference to FIG. FIG. 1 is a configuration diagram (a) showing the configuration of the optical functional element according to Embodiment 1 of the present invention, and a sectional view (b) showing a cross section of a part of the configuration. Hereinafter, a 1-bit delay optical interferometer will be described as an example of the optical functional element.

  This 1-bit delay optical interferometer includes an optical waveguide 101 and a polarization separation unit 102 disposed on the output side of the optical waveguide 101. The optical waveguide 101 includes, for example, a substrate 111 serving as a lower clad, a core 112 formed on the substrate 111 and having a rectangular or trapezoidal cross section perpendicular to the waveguide direction, and an upper portion formed on the core 112. And a clad layer 113. The lower cladding and the substrate may be configured separately.

  In addition, the optical waveguide 101 has a difference in propagation time between the first polarization 122 parallel to the intrinsic polarization plane and the second polarization 123 perpendicular to the intrinsic polarization plane, corresponding to one bit of the phase modulation signal. It is assumed that the length of the optical waveguide becomes the time. Note that a phase modulation signal 121 in a state where the polarization is inclined by 45 ° with respect to the intrinsic polarization plane of the optical waveguide 101 or a circularly polarized state is input to the optical waveguide 101. Further, the polarization reference plane of the polarization separation unit 102 has an angle of 45 ° with respect to the intrinsic polarization plane. The polarization separation unit 102 is, for example, a polarization beam splitter. This can be constituted, for example, by bonding two right-angle prisms having a dielectric multilayer film or metal thin film formed on the surface.

  The intrinsic polarization plane is a plane that is parallel or perpendicular to a pair of opposing sides of the core 112 that are parallel to each other. As shown in FIG. 1B, when the core 112 having a rectangular cross section is formed on the substrate 111, a plane parallel to the plane of the substrate 111 can be the intrinsic polarization plane of the core 112. . As shown in FIG. 1B, in the core 112 having a cross section in which the width in the plane direction of the substrate 111 is different from the height in the direction perpendicular to the plane of the substrate 111, the group refraction of light that is guided (propagated). The rate differs between a component parallel to the natural polarization plane (TE component: first polarization) and a component perpendicular to the natural polarization plane (TM component: second polarization).

  Here, by providing the polarization adjustment unit 103 on the input side of the optical waveguide 101, a phase modulation signal in a state where the polarization is inclined by 45 ° with respect to the intrinsic polarization plane with respect to the optical waveguide 101 or in a circular polarization state May be input. For example, the polarization adjusting unit 103 can be configured by a polarization controller that generates a phase difference between polarized light by applying stress or heat to the fiber.

Hereinafter, the optical waveguide length of the optical waveguide 101 will be described in more detail. First, assuming that the group refractive index for the TE component of the optical waveguide 101 is _ngTE and the group refractive index for the TM component is _ngTM , the light of each polarization of the first polarization and the second polarization is of length L. time through the optical waveguide 101 of the _WG is n _GTE L _WG / c and _n gTM L _WG / c. In this case, the optical waveguide length L _WG of the optical waveguide 101 can be determined by "| L _WG = cΔT, where c is the speed of light in vacuum _{··· (1) | n gTE -n} gTM ".

Therefore, if an optical waveguide length L _WG determined using Equation (1), in the optical waveguide 101, the difference in propagation time between the TE and TM components is, the phase-modulated signal is input to one bit It will be time. Note that the optical waveguide 101 formed on the flat substrate 111 is generally a flat core 112 in a direction parallel to the substrate 111 in order to facilitate manufacture. In this case, usually _ngTE > _ngTM , and the TE component has a slower propagation speed and arrives at the exit of the optical waveguide 101 later.

  In such a system, when 45-degree linearly polarized light or circularly polarized light is input to the optical waveguide 101 with respect to its intrinsic polarization plane, the TE component and TM component having the same strength are input to the optical waveguide 101. Is excited. As described above, these propagation components arrive at the exit of the optical waveguide 101 with a shift of 1 bit of the modulation signal, and the polarization state at the time of arrival interferes according to the phase difference between the TE component and the TM component, A polarization state of circular polarization is taken from linear polarization that forms an angle of ± 45 degrees with respect to the intrinsic polarization plane.

  Specifically, when the TE component and the TM component are in-phase interference, the polarization is linearly polarized with an angle of 45 degrees with respect to the natural polarization plane, similar to the entrance of the optical waveguide 101. Further, when the TE component and the TM component are π interference, the above-described in-phase interference is −45 degrees linearly polarized wave. Further, when the TE component and the TM component are π / 2 and 3π / 2 interference, they are circularly polarized waves that are reversely rotated.

  As described above, two polarization components having different propagation speeds travel (waveguide) in one optical waveguide 101 to obtain interference. The TE component having a slow propagation speed is delayed by a spatial optical delay interferometer. This is equivalent to the case where the TM component having a fast propagation component is made to correspond to the optical path without delay of the spatial optical system delay interferometer. Therefore, the polarization separation unit 102 having a polarization reference plane of 45 degrees with respect to the intrinsic polarization plane of the optical waveguide 101 is disposed at the exit of the optical waveguide 101, and the 45-degree linear polarization component and the −45 degree straight line are arranged. If each of the polarization components is extracted, an output with 1-bit delay interference can be obtained at the exit (output) of the polarization separation unit 102.

  Further, even if the polarization separation unit 102 has a function of separating and extracting the circularly polarized waves that are reversely rotated from each other, the TE component and the TM component can be extracted as an interference output that is shifted by π. It functions as a total.

  Further, from equation (1), the larger the difference in the group index of refraction between the TE component and the TM component, the shorter the optical waveguide length of the optical waveguide 101, which is preferable for miniaturization and integration of the 1-bit delayed optical interferometer. In particular, FIG. As described in 1.1 (a), the core 112 is made of a silicon fine wire optical waveguide made of silicon, so that the size can be further reduced.

  For example, if an SOI (Silicon on Insulator) substrate is used and this SOI layer is processed by a well-known photolithography technique and etching technique, a core of a silicon fine wire can be formed. Thereafter, an upper clad layer can be formed by depositing silicon oxide by a well-known CVD method or the like. In this case, the buried insulating layer becomes the lower cladding layer.

  Silicon optical waveguides are optical waveguides composed of a silicon core with a cross-sectional width and height of several hundreds of nanometers and a clad made of silicon oxide, silicon oxynitride, air, vacuum, etc. To have a flat core in the horizontal direction to the substrate.

This flat structure causes a large polarization dependence in the group refractive index. Further, by increasing the ratio (flatness ratio) between the width direction and the height direction of the cross section of the core, the difference in group refractive index between two orthogonal polarizations can be further increased. FIG. 1.4 (a) shows the calculation result of the group refractive index in the silicon fine wire optical waveguide. When the core has a cross section of 400 × 200 nm, a difference in group refractive index of about 1.6 can be obtained. I understand. Substituting this into equation (1) and determining the optical waveguide length required for the 1-bit delay optical interferometer, L _WG = 4.7 mm when the bit rate is 40 Gbps.

According to Non-Patent Document 3 and Non-Patent Document 4, the polarization adjustment element and the polarization separation element can be manufactured with a size of 100 μm or less in length and 20 μm or less in width, and finally these elements are integrated. The area occupied by one interferometer is approximately 0.1 mm ² . This exclusive area is about 1/7000 compared with a delay interferometer based on an existing spatial optical system, and is found to be very suitable for multi-channel integration.

  The configuration of the 1-bit delay optical interferometer in the present embodiment can be realized by using a conductive medium (quartz optical waveguide) such as a polarization maintaining fiber instead of the core made of a semiconductor such as silicon. However, the group refractive index difference of the polarization maintaining fiber is about 0.0005, which is about 1 / 30,000 of that of silicon, and a fiber length of 140 m is required. Further, the bending radius of the fiber is limited to 1 cm, and further, the polarization adjustment element and the polarization separation element cannot be integrated. For this reason, the configuration using the polarization maintaining fiber cannot be downsized.

  Non-Patent Document 5 describes examples of various research institutes regarding the core size of the silicon fine wire optical waveguide. According to this, the dimension (width) of the cross section of the core is approximately 200 nm to 500 nm in the infrared light around the wavelength of 1550 nm for communication. However, in this dimension range, the single mode condition is ensured and the propagation by the higher order mode is performed. Since deterioration of characteristics and interference characteristics can be prevented, the present invention is also preferably applied in the present invention.

  In order to confirm the effect of the 1-bit delay optical interferometer in the present embodiment described above, the optical waveguide length is set for the phase modulation signal having a bit rate of 10 Gbps, and the spectrum of the output on one side of the polarization separation unit 102 is calculated. It was measured. In this measurement, the cross-sectional dimensions of the core 112 were 460 nm in width, 200 nm in height, and the optical waveguide length was 28 mm. With this configuration, the difference between the TE component and the TM component through the optical waveguide 101 is 1 bit time (100 ps) of the 10 Gbps modulated signal. As shown in FIG. 2, an interference ripple having a period of 10 GHz was observed in the wavelength spectrum of the output. From this measurement result, it was confirmed that the 1-bit delay optical interferometer in the present embodiment performs the intended operation.

[Embodiment 2]
Next, Embodiment 2 of the present invention will be described with reference to FIG. FIG. 3 is a configuration diagram showing the configuration of the optical functional element according to Embodiment 2 of the present invention. Hereinafter, a 1-bit delay optical interferometer will be described as an optical functional element.

  This 1-bit delay optical interferometer includes an optical waveguide 301 and a polarization adjustment element 303 disposed on the input side of the optical waveguide 301. Further, a polarization adjustment element 304 and a polarization separation element 302 are provided on the output side of the optical waveguide 301. In the second embodiment, the polarization separation element 302, the polarization adjustment element 303, and the polarization adjustment element 304 are monolithically integrated on the substrate 311 on which the optical waveguide 301 is formed.

  In the optical waveguide 301, the difference in propagation time between the first polarization parallel to the intrinsic polarization plane and the second polarization perpendicular to the intrinsic polarization plane is 1 bit of the input phase modulation signal. The optical waveguide length is a minute time. This optical waveguide length is the waveguide length between the polarization adjusting element 303 and the polarization adjusting element 304. Further, the polarization reference plane of the polarization separation element 302 is parallel or perpendicular to the intrinsic polarization plane. The polarization separation element 302 is, for example, a polarization beam splitter.

  The polarization adjusting element 303 gives a 45 ° polarization rotation to the horizontal or vertical polarization on the intrinsic polarization plane of the optical waveguide 301. Alternatively, the polarization adjusting element 303 converts the polarization that is horizontal or perpendicular to the intrinsic polarization plane of the optical waveguide 301 into a circular polarization.

  The polarization adjustment element 304 converts a linearly polarized wave of 45 ° with respect to the intrinsic polarization plane of the optical waveguide 301 into a polarization that is horizontal or perpendicular to the intrinsic polarization plane. Alternatively, the polarization adjustment element 304 converts the circularly polarized wave into a polarized wave that is horizontal or perpendicular to the intrinsic polarization plane. In the second embodiment, the polarization adjustment element 304 and the polarization separation element 302 are used to separate the polarization reference means whose polarization reference plane is at an angle of 45 ° with respect to the intrinsic polarization plane, or the reverse rotation circular polarization separation means. Is configured.

  As described above, when integrating on the substrate 311, it is not easy to set the polarization reference plane of the polarization separation element 302 to 45 ° with respect to the plane of the substrate 311. Parallel or vertical. On the other hand, the optical waveguide 301 is polarized by 45 ° with respect to the intrinsic polarization plane. With this combination, the polarized light guided through the optical waveguide 301 cannot be separated by the polarization separation element 302. On the other hand, if the polarization adjusting element 304 is provided and the light output therefrom is in a state parallel or perpendicular to the intrinsic polarization plane, the polarization can be separated by the polarization separating element 302.

Also in the second embodiment, an optical waveguide 301, if an optical waveguide length L _WG determined using Equation (1) described above, in the optical waveguide 301, the difference in propagation time between the TE and TM components This is the time for one bit of the input phase modulation signal. In such a system, when linearly polarized light (phase modulation signal) that is horizontal or vertical with respect to the specific polarization plane of the optical waveguide 301 enters the polarization adjusting element 303, the polarization at the entrance of the optical waveguide 301 is A linearly polarized wave or a circularly polarized wave of 45 degrees with respect to the intrinsic polarization plane of the optical waveguide 301 is excited, and this polarized wave excites the TE component and the TM component of the same strength in the optical waveguide 301. As described above, these propagation components arrive at the exit of the optical waveguide 301 with a shift of 1 bit of the modulation signal, and the polarization state at the time of arrival interferes according to the phase difference between the TE component and the TM component, A polarization state of circular polarization is taken from linear polarization that forms an angle of ± 45 degrees with respect to the intrinsic polarization plane.

  Specifically, when the TE component and the TM component are in-phase interference, the polarization is linearly polarized with an angle of 45 degrees with respect to the natural polarization plane as with the entrance of the optical waveguide 301. Further, when the TE component and the TM component are π interference, the above-described in-phase interference is −45 degrees linearly polarized wave. Further, when the TE component and the TM component are π / 2 and 3π / 2 interference, circular polarization is obtained.

  Therefore, the polarization adjustment element 304 disposed at the exit of the optical waveguide 301 converts the 45 ° linear polarization with respect to the natural polarization plane into a horizontal or vertical polarization with respect to the natural polarization plane, or reverse rotations to each other. If each circularly polarized wave is converted into a horizontal or vertical polarized wave, and a polarization separating element 302 extracts a linearly polarized wave component parallel to the converted polarized wave and a vertical linearly polarized wave component, the polarized wave is obtained. At the exit (output) of the separation element 302, an output with 1-bit delay interference can be obtained.

  As described above, in the above-described embodiment of the present invention, an optical waveguide composed of a core having a rectangular or trapezoidal cross section orthogonal to the waveguide direction is used. The difference in propagation time between the first polarized wave and the second polarized wave perpendicular to the natural polarization plane is used as a state corresponding to a time of one bit of the phase modulation signal. As a result, compared with the conventional 1-bit delay optical interferometer, for example, the area ratio can be reduced to about 1/7000. Further, each part can be monolithically integrated on the same substrate. In the case of such integration, the optical circuit may be increased in scale, but according to the present invention, the optical waveguide can be reduced in size, which is particularly effective. Thus, according to the present invention, inexpensive multi-channel integration of an optical differential phase modulation signal demodulator is possible.

  In addition, in order to use a 1-bit delay optical interferometer as a demodulator, as is well known, a detector that outputs a difference between the optical intensities of polarization-separated signal light is required. For example, a balanced photodiode may be used as such a detection unit. The balanced photodiode is composed of two photodiodes that receive two polarization components of TE and TM, respectively. If a differential signal is generated by a subtracting circuit from an output (electric signal) photoelectrically converted by each photodiode, the differential phase-modulated light can be demodulated. If this photodiode is integrated on the same substrate, it is advantageous for miniaturization.

  By the way, as an optical functional element, there is an optical frequency filter as well known. In an optical device using an optical frequency filter, a large-scale monolithic integration technique of an optical / electronic device using a group III-V or group IV semiconductor material typified by indium phosphorus or silicon has attracted attention. By using such a technology, it has excellent advantages such as mass production and easy miniaturization, and therefore, a wide range of applications are expected from inter-chip optical interconnection to medium / long-distance optical transmission.

  On the other hand, wavelength division multiplexing (WDM) technology is generally used to cope with increasing transmission traffic. Since erbium-doped fiber amplifiers developed in the 1990s have been commercialized, WDM has become widespread as a device for performing relay amplification because it can perform a collective amplification over a wide band. However, since there are still concerns about the increase in size, complexity, and cost of devices and systems, attempts have been made to solve these problems by applying the above-described monolithic integration technology using semiconductor materials.

  When the above-described monolithic integration technique is applied to a transceiver compatible with WDM, an optical frequency filter generally called an arrayed waveguide grating (AWG) is required. However, in the case of manufacturing with the existing planar lightwave circuit (PLC) technology using quartz glass, it is several mm square to several tens mm square. However, the device size is extremely small. From the above, conversely, the use of AWG may cause a problem in the precision of microfabrication technology. In particular, when the channel frequency interval is 50 GHz or 100 GHz, the above problem becomes significant.

  Therefore, there is a need for a finer optical frequency filter that can be adapted to the correction technique described above. Below, the optical frequency filter which can be reduced in size and can be manufactured more easily as an optical functional element is demonstrated.

[Embodiment 3]
Hereinafter, Embodiment 3 of the present invention will be described with reference to FIG. FIG. 5: is the block diagram (a) which shows the structure of the optical functional element in Embodiment 3 of this invention, and (b) which shows the cross section of a partial structure. Hereinafter, an optical frequency filter will be described as an example of the optical functional element.

  This optical frequency filter includes an optical waveguide 501 and a polarization separation unit 502 disposed on the output side of the optical waveguide 501. The optical waveguide 501 is, for example, a substrate 511 serving as a lower clad, a core 512 formed on the substrate 511 and having a rectangular or trapezoidal cross section orthogonal to the waveguide direction, and an upper portion formed on the core 512. And a clad layer 513. The lower cladding and the substrate may be configured separately.

  In addition, the optical waveguide 501 has a desired free spectral range in which the difference in propagation time between the first polarization 522 parallel to the intrinsic polarization plane and the second polarization 523 perpendicular to the intrinsic polarization plane is desired. The optical waveguide length is the time indicated by the reciprocal of. An optical signal 521 in a state where the polarization is inclined by 45 ° with respect to the intrinsic polarization plane of the optical waveguide 501 or a circularly polarized state is input to the optical waveguide 501. The polarization reference plane of the polarization separation unit 502 has an angle of 45 ° with respect to the intrinsic polarization plane.

  The intrinsic polarization plane is a plane parallel to or perpendicular to a pair of opposing sides of the core 512 that are parallel to each other. As shown in FIG. 5B, when the core 512 having a rectangular cross section is formed on the substrate 511, a plane parallel to the plane of the substrate 511 can be the intrinsic polarization plane of the core 512. . As shown in FIG. 5B, in the core 512 having a cross section in which the width in the planar direction of the substrate 511 is different from the height in the direction perpendicular to the plane of the substrate 511, the group refraction of light guided (propagated). The rate differs between a component parallel to the natural polarization plane (TE component: first polarization) and a component perpendicular to the natural polarization plane (TM component: second polarization).

  Here, by providing the polarization adjustment unit 503 on the input side of the optical waveguide 501, an optical signal in a state where the polarization is inclined by 45 ° with respect to the intrinsic polarization plane or a circularly polarized state is provided with respect to the optical waveguide 501. You may make it input.

  The polarization separation unit 502 and the polarization adjustment unit 503 can take various forms. However, in order to form a small and inexpensive configuration, the polarization separation unit 502 and the polarization adjustment unit 503 can be monolithically integrated based on an optical waveguide on a planar substrate. Is preferred. For example, a polarization rotator structure (see Non-Patent Document 3) can be applied to the polarization adjustment unit 503. In addition, a polarization splitter structure can be applied to the polarization adjustment unit 503 (see Non-Patent Document 4).

  According to Embodiment 3 described above, a periodic optical frequency filter having a desired free spectrum range can be configured. If the temperature of the optical waveguide 101 is controlled by heating the substrate 111 or the like, the wavelength (frequency) of light that can be guided through the optical waveguide 101 can be controlled by the thermo-optic effect.

According to the optical frequency filter in the third embodiment, for example, the polarization separation unit 502 is configured by a polarization separation element having both polarization outputs, so that X: 1 (MUX) or 1: X (DEMUX). As a result, the processing accuracy requirement of ultra-high density AWG containing silicon can be relaxed. In addition, since the polarization separation unit 502 is composed of a unipolar transmission element (polarizer), it can also be used as an ultra-small frequency band limiting element. According to Non-Patent Document 3 and Non-Patent Document 4, the polarization adjustment element and the polarization separation element can be manufactured with a size of 100 μm or less in length and 20 μm or less in width, and finally these elements are integrated. The area occupied by one optical frequency filter is approximately 0.1 mm ² .

In addition, in the optical frequency filter using birefringence described in Patent Document 1, polarization maintaining fiber (PMF), LiNbO ₃ crystal, rutile, calcite, and the like are described. In either case, monolithic light is used. Circuit integration is practically impossible. In addition, since birefringence due to material anisotropy is used, the difference in refractive index is very small (for example, 10 ⁻⁴ to 10 ⁻³ ), which requires a large interaction length. If the optical waveguide 501 in the third embodiment is a silicon fine wire waveguide, this group refractive index difference is> 1, whereas in the case of the PMF group refractive index difference, 10 ⁻⁴ to 10 ⁻³ is very high. These ratios are insignificant and the ratios are> 1000 times.

  For example, the configuration of the optical frequency filter in the third embodiment can be realized by using a conductive medium (quartz optical waveguide) such as a polarization maintaining fiber instead of the core made of a semiconductor such as silicon. However, the group refractive index difference of the polarization maintaining fiber is about 0.0005, which is about 1 / 30,000 of that of silicon, and a fiber length of 140 m is required. Further, the bending radius of the fiber is limited to 1 cm, and further, the polarization adjustment element and the polarization separation element cannot be integrated. For this reason, the configuration using the polarization maintaining fiber cannot be downsized.

Hereinafter, the optical waveguide length of the optical waveguide 501 will be described in more detail. First, the relationship between the phase φ, the group refractive index _ng , the length L, and the wavelength λ can be expressed by the following equation (2). The wavelength λ and the frequency f can be expressed by the following equation (3). Expressions (4) and (5) can be obtained from these expressions. Incidentally, phi is the phase, n _g is the group refractive index, L is the waveguide length, lambda is optical wavelength, f is the optical frequency, [Delta] [phi is a phase shift, [Delta] n _g is the group refractive index change, n g _{· TE} is TE mode _{Ng · TM} is the TM mode group refractive index, and Δf is the optical frequency difference.

As can be seen from these formulas, since the phase shift Δφ required per free spectral range (FSR) is 2 [pi, FSR serve as a frequency filter, the group refractive index difference between the TE and TM [Delta] n _g And depends on the waveguide length L. The time required for TE and TM to pass through the optical waveguide 501 of length L is _{ng · TE} L / c and _{ng · TM} L _WG / c. From Equation (5), the time difference between TE and TM propagating through the waveguide having the waveguide length L is the time indicated by the reciprocal of FSR. Therefore, the optical frequency filter of the desired FSR can be obtained by setting the optical waveguide length at which the difference in propagation time between the first polarization and the second polarization is the time indicated by the reciprocal of the desired FSR. .

As a comparison between Expression (4) and Expression (5), the following Expression (6) and Expression (7) show the operating principle for configuring a conventional frequency filter. Δφ is a phase shift, _ng is a group refractive index, ΔL is a waveguide length difference, L _arm1 and L _arm1 are waveguide lengths of each arm, and Δf is an optical frequency difference. As shown in equations (6) and (7), in the past, instead of using a waveguide with negligible polarization dependence, the frequency is adjusted by adjusting the waveguide length difference ΔL between the arms. An FSR that functions as a filter is obtained.

  Here, as is well known, since the length of the optical waveguide 501 can be shortened as the difference in the group refractive index between the TE component and the TM component increases, it is preferable for miniaturization and integration of the optical frequency filter. As the optical waveguide having such characteristics, it is desirable to use a silicon fine wire waveguide that enables an ultrahigh refractive index difference.

  The silicon thin wire optical waveguide is an optical waveguide composed of a silicon core having a cross-sectional width and height of several hundred nanometers and a clad made of silicon oxide, silicon oxynitride, air, vacuum, or the like. In order to facilitate manufacture, the optical waveguide using the silicon thin wire core usually has a flat core in a horizontal direction on the substrate. For example, if an SOI (Silicon on Insulator) substrate is used and this SOI layer is processed by a well-known photolithography technique and etching technique, a core of a silicon fine wire can be formed. Thereafter, an upper clad layer can be formed by depositing silicon oxide by a well-known CVD method or the like. In this case, the buried insulating layer becomes the lower cladding layer.

  Due to the structure of the flat core as described above, a large polarization dependency occurs in the group refractive index. For example, in the case of a flat core having a cross section of 460 × 200 nm, it is found by optical mode propagation simulation that a difference in group refractive index of about 1.274 occurs between TE and TM at a wavelength of 1.55 μm. Yes. The group refractive index in the TE component and the group refractive index in the TM component were 4.218 and 2.944, respectively.

As a result of substituting these numerical conditions into the equation (5) and obtaining a waveguide length necessary for FSR = 10 GHz, a waveguide length L = 2.4 cm was obtained. By setting this waveguide length, the propagation time difference between the TE component and the TM component of the optical signal passing through the optical waveguide 501 becomes the time indicated by the reciprocal of the FSB. According to Non-Patent Document 3 and Non-Patent Document 4, the polarization adjustment element and the polarization separation element can be manufactured with a size of 100 μm or less in length and 20 μm or less in width, and finally these elements are integrated. However, the area occupied by one element is approximately 0.1 mm ² . This exclusive area is very small and is found to be very suitable for multi-channel integration.

  As for the core size of the silicon wire waveguide, the core size is approximately between 200 nm and 500 nm in the infrared wavelength of about 1.55 μm for communication. Since it is possible to prevent the propagation characteristics and interference characteristics from being deteriorated due to the mode, it is preferable to apply this embodiment as well.

  In order to confirm the effect of the optical frequency filter in the third embodiment, the optical frequency filter in the third embodiment was actually manufactured, and the spectrum of the single output of the polarization separation element 502 was measured using the manufactured device. In this measurement, the silicon thin wire core had a cross section of 460 × 200 nm and a waveguide length of 28 mm. As shown in FIG. 6, an interference spectrum having a 10 GHz period was observed in the output wavelength spectrum. From this measurement result, it was confirmed that the optical frequency filter in the third embodiment operates as an optical frequency filter having an FSR of 10 GHz.

[Embodiment 4]
Next, Embodiment 4 of the present invention will be described with reference to FIG. FIG. 7 is a configuration diagram showing the configuration of the optical functional element according to Embodiment 4 of the present invention. Hereinafter, an optical frequency filter will be described as the optical functional element.

  This optical frequency filter includes an optical waveguide 701 and a polarization adjustment element 703 disposed on the input side of the optical waveguide 701. Further, a polarization adjustment element 704 and a polarization separation element 702 are provided on the output side of the optical waveguide 701. In the fourth embodiment, the polarization separation element 702, the polarization adjustment element 703, and the polarization adjustment element 704 are monolithically integrated on the substrate 711 on which the optical waveguide 701 is formed.

  In the optical waveguide 701, the difference in propagation time between the first polarization parallel to the intrinsic polarization plane and the second polarization perpendicular to the intrinsic polarization plane is the reciprocal of the desired free spectrum range. The length of the optical waveguide is the time indicated. This optical waveguide length is the waveguide length between the polarization adjusting element 703 and the polarization adjusting element 704. The polarization reference plane of the polarization separation element 702 is parallel or perpendicular to the intrinsic polarization plane. The polarization separation element 702 is, for example, a polarization beam splitter.

  The polarization adjusting element 703 gives a 45 ° polarization rotation with respect to the horizontal or vertical polarization on the intrinsic polarization plane of the optical waveguide 701. Alternatively, the polarization adjustment element 703 converts a polarization that is horizontal or perpendicular to the intrinsic polarization plane of the optical waveguide 701 into a circular polarization.

  The polarization adjustment element 704 converts a 45-degree linearly polarized wave with respect to the intrinsic polarization plane of the optical waveguide 701 into a polarization that is horizontal or perpendicular to the intrinsic polarization plane. Alternatively, the polarization adjusting element 704 converts the circularly polarized wave into a polarized wave that is horizontal or perpendicular to the intrinsic polarization plane. In the fourth embodiment, the polarization adjusting element 704 and the polarization separating element 702 are used to separate the polarization reference means whose polarization reference plane is at an angle of 45 ° with respect to the intrinsic polarization plane, or the separating means for counter-rotating circularly polarized waves. Is configured.

  As described above, when integrating on the substrate 711, it is not easy to set the polarization reference plane of the polarization separation element 702 to 45 ° with respect to the plane of the substrate 711. Parallel or vertical. On the other hand, the optical waveguide 701 guides a 45 ° polarized wave with respect to the intrinsic polarization plane. With this combination, the polarization guided through the optical waveguide 701 cannot be separated by the polarization separation element 702. On the other hand, if the polarization adjustment element 704 is provided and the light output from the polarization adjustment element 704 is parallel or perpendicular to the intrinsic polarization plane, the polarization separation element 702 makes an angle of 45 ° with respect to the plane of the substrate 711. The polarized waves can be separated.

  Also in the fourth embodiment, if the optical waveguide 701 has an optical waveguide length obtained in the same manner as in the third embodiment, in the optical waveguide 701, the difference in propagation time between the TE component and the TM component. Can be the time indicated by the reciprocal of the desired free spectral range. In such a system, when linearly polarized light (phase modulation signal) that is horizontal or vertical with respect to the intrinsic polarization plane of the optical waveguide 701 is incident on the polarization adjustment element 703, the polarization at the entrance of the optical waveguide 701 is The polarization is 45 ° linear polarization or circular polarization with respect to the intrinsic polarization plane of the optical waveguide 701. This polarization excites the TE and TM components of the same strength in the optical waveguide 701. As described above, these propagation components arrive at the exit of the optical waveguide 701 with a shift by the reciprocal of the free spectrum range, and the polarization state at the time of arrival interferes according to the phase difference between the TE component and the TM component. The polarization state of the circularly polarized wave is taken from the linearly polarized wave having an angle of ± 45 ° with respect to the natural polarization plane.

  Specifically, when the TE component and the TM component are in-phase interference, the polarization is linearly polarized with an angle of 45 ° with respect to the natural polarization plane as with the entrance of the optical waveguide 701. Further, when the TE component and the TM component are anti-phase interference, it becomes −45 ° linearly polarized wave orthogonal to the above-described in-phase interference. Further, when the TE component and the TM component are π / 2 and 3π / 2 interference, circular polarization is obtained.

  Therefore, the polarization adjustment element 704 disposed at the exit of the optical waveguide 701 converts a linear polarization of 45 ° with respect to the natural polarization plane into a horizontal or vertical polarization with respect to the natural polarization plane, or rotates in a reverse direction to each other. If circularly polarized waves are respectively converted into horizontal or vertical polarized waves, and a polarization separation element 702 extracts a linearly polarized wave component parallel to the converted polarized wave and a vertical linearly polarized wave component, respectively, At the exit (output) of the separation element 702, an output in which the optical frequency is filtered in the desired free spectral range can be obtained.

  It should be noted that the present invention is not limited to the embodiment described above, and that many modifications can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, in the above description, the case where the core is mainly made of silicon has been described. However, the present invention is not limited to this, and the core may be made of a compound semiconductor. The core may be made of silicon nitride. With these materials, the difference in the group refractive index between different polarized waves can be increased, and the optical waveguide length can be shortened. Further, with these materials, a large refractive index difference from the cladding can be realized, and the bending radius of the optical waveguide (core) can be reduced, so that it can be integrated in a smaller region.

  DESCRIPTION OF SYMBOLS 101 ... Optical waveguide, 102 ... Polarization separation part, 103 ... Polarization adjustment part, 111 ... Substrate, 112 ... Core, 113 ... Upper clad layer, 121 ... Phase modulation signal, 122 ... First polarization, 123 ... Second Polarization.

Claims (7)

An optical waveguide comprising a core having a rectangular or trapezoidal cross section perpendicular to the waveguide direction;
And polarization separation means disposed on the output side of the optical waveguide,
The optical waveguide receives an optical signal in a linearly polarized state or a circularly polarized state whose polarization is inclined by 45 ° with respect to the intrinsic polarization plane of the optical waveguide, and a first polarization parallel to the intrinsic polarization plane. The difference in propagation time between the wave and the second polarization perpendicular to the natural polarization plane is the optical waveguide length that takes a desired value,
The optical functional element, wherein the polarization reference plane of the polarization separation means has an angle of 45 ° with respect to the intrinsic polarization plane. The optical functional element according to claim 1,
The optical signal is a phase modulation signal;
The optical function is characterized in that the optical waveguide has an optical waveguide length in which a difference in propagation time between the first polarization and the second polarization is a time corresponding to one bit of the phase modulation signal. element. The optical functional element according to claim 1,
The optical signal is an optical signal to be filtered,
The optical waveguide has an optical waveguide length in which a difference in propagation time between the first polarization and the second polarization is a time indicated by a reciprocal of a desired free spectrum range. Optical functional element. The optical functional element according to any one of claims 1 to 3,
Comprising polarization adjusting means disposed on the input side of the optical waveguide;
The optical functional element characterized in that the polarization adjusting means sets the polarization of the input light to the optical waveguide to 45 ° linear polarization or circular polarization with respect to the intrinsic polarization plane of the optical waveguide. The optical functional element according to claim 4,
The polarization separation means and the polarization adjustment means are integrated on a substrate on which the optical waveguide is formed,
The polarization separation means arranged on the output side of the optical waveguide is composed of a polarization adjustment element and a polarization separation element,
The polarization adjusting element converts a linearly polarized wave of 45 ° with respect to the intrinsic polarization plane into a polarization that is horizontal or perpendicular to the intrinsic polarization plane, or a circular polarization into a polarization that is horizontal or perpendicular to the intrinsic polarization plane. Converted,
An optical functional element, wherein a polarization reference plane of the polarization separation element is parallel or perpendicular to the intrinsic polarization plane. In the optical functional element according to any one of claims 1 to 5,
The core is made of silicon,
The clad of the optical waveguide is composed of any one of air, vacuum, silicon oxide, and silicon oxynitride. The optical functional element according to claim 6,
An optical functional element characterized in that the cross-sectional width and height of the core are formed in a size range of 200 to 500 nm.

Images