JP2010278906A - Delay interferometer and optical receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a delay interferometer and an optical receiver by which responsivity for adjustment of an optical path length is improved. <P>SOLUTION: The delay interferometer which has a first optical path and a second optical path is provided with: a first optical path length varying means and a second optical path length varying means both of which reside on the first optical path and have a variable optical path length; and an optical path length control means for controlling the optical path lengths of the first and second optical path length varying means. A response time constant related with the optical path length variation of the first optical path length varying means is smaller than the response time constant related to the optical path length variation of the second optical path length varying means. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a delay interferometer and an optical receiver.

  In an optical communication system using a differential phase modulation transmission system, a modulated optical signal is branched by a delay interferometer and combined after a predetermined delay amount is provided. For example, in a delay interferometer using a differential phase modulation method, a delay amount is provided by providing a difference in the geometric length of the branched optical path. Patent Document 1 discloses a technique for adjusting an optical path length by a temperature change of a medium on the optical path.

JP 2007-67955 A

  However, high-speed response cannot be obtained by temperature control using heat. In this case, it takes time to repeat the control and converge to a stable point.

  The present invention has been made in view of the above problems, and an object thereof is to provide a delay interferometer and an optical receiver capable of improving the response of adjustment of the optical path length.

  In order to solve the above problems, a delay interferometer disclosed in the specification is a delay interferometer having a first optical path and a second optical path, and is arranged on the first optical path and has a variable optical path length. And a second optical path length variable means, and an optical path length control means for controlling the optical path lengths of the first optical path length variable means and the second optical path length variable means, and relating to the optical path length change of the first optical path length variable means The response time constant is smaller than the response time constant related to the change in the optical path length of the second optical path length varying means.

  In order to solve the above problems, an optical receiver disclosed in the specification has a first optical path and a second optical path, and is arranged on the first optical path and has a variable optical path length. An optical path length control means for controlling the optical path lengths of the length variable means, the first optical path length variable means, and the second optical path length variable means, and the response time constant related to the optical path length change of the first optical path length variable means is the second optical path. A delay interferometer, which is smaller than the response time constant related to the change in the optical path length of the length variable means, and a light receiver for receiving the optical signal output from the delay interferometer are provided.

  According to the delay interferometer and the optical receiver disclosed in the specification, the response of adjustment of the optical path length can be improved.

1 is a block diagram for explaining an overall configuration of a differential phase modulation optical transmission system including a delay interferometer according to Embodiment 1. FIG. It is a block diagram for demonstrating the whole structure of an optical receiver. It is a perspective view for demonstrating the detail of a 1st medium and a 2nd medium. It is a figure for demonstrating the relationship between the wavelength and transmittance | permeability in a delay interferometer. It is a perspective view for demonstrating the medium provided with the heater. It is a figure for demonstrating the example of calculation of the thickness and response time constant of a 1st medium and a 2nd medium. It is a figure for demonstrating the amount of phase delays of a 1st medium and a 2nd medium. FIG. 10 is a diagram for explaining an example of a flowchart executed when controlling the first medium and the second medium in Case 1 and Case 2. FIG. 6 is a block diagram for explaining an overall configuration of an optical receiver according to a second embodiment. FIG. 9 is a schematic diagram for explaining an overall configuration of a delay interference unit according to a third embodiment.

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

  FIG. 1 is a block diagram for explaining an overall configuration of a differential phase modulation (DPSK) optical transmission system 1000 including a delay interferometer 100 according to the first embodiment. Referring to FIG. 1, an optical transmission system 1000 includes an optical transmitter 200, an optical transmission path 300, and an optical receiver 400.

  The optical transmitter 200 includes a phase modulator 500 and an intensity modulator 600. The phase modulator 500 generates a phase modulation signal from the input unmodulated optical signal. The intensity modulator 600 converts the optical signal from the phase modulator 500 into an RZ pulse and outputs it. The optical signal output from the optical transmitter 200 is input to the optical receiver 400 via the optical repeater 700 and the wavelength filter 800 of the optical transmission line 300.

  The optical receiver 400 includes a delay interferometer 100 and a photoelectric conversion unit 900. The delay interferometer 100 causes a delay component of 1 bit time and a component subjected to phase control of 0 radians to interfere with each other in the optical signal input to the optical receiver 400 and outputs the interference result as two outputs. The photoelectric conversion unit 900 is configured by a dual pin photodiode or the like, and performs differential photoelectric conversion detection by receiving each output from the delay interferometer 100. Through the above process, DPSK optical transmission is performed.

  FIG. 2 is a block diagram for explaining the overall configuration of the optical receiver 400. Referring to FIG. 2, delay interferometer 100 includes an interference unit 10 and a control unit 20. The interference unit 10 includes a half mirror 11, a mirror 12, a first medium 13, a second medium 14, a mirror 15, a half mirror 16, and a mirror 17. The control unit 20 includes an analog / digital converter (ADC) 21, a digital signal processing unit (DSP) 22, a digital / analog converter (DAC) 23, a first drive circuit (DRV1) 24, and a second drive circuit (DRV2) 25. including. The photoelectric conversion unit 900 includes a balanced receiver 30, a normal phase monitor 40, a reverse phase monitor 50, and a deserializer 60.

  FIG. 3 is a perspective view for explaining the details of the first medium 13 and the second medium 14. With reference to FIG. 3, the first medium 13 includes a medium part 131 and a first heater 132. The second medium 14 includes a medium unit 141 and a second heater 142. The first heater 132 and the second heater 142 are made of resistors that generate heat when electric power is supplied. For example, nichrome or the like can be used as the first heater 132 and the second heater 142.

  The medium part 131 and the medium part 141 are made of a medium whose refractive index changes according to a temperature change. Since the optical path length changes when the refractive index changes, the medium part 131 and the medium part 141 function as an optical path length variable means. As the medium part 131 and the medium part 141, it is preferable to use a medium having a high transmittance and a high refractive index in the optical communication band. For example, silicon can be used for the medium portion 131 and the medium portion 141.

  The medium part 131 has a smaller thickness than the medium part 141. Therefore, the medium part 131 has higher responsiveness with respect to a change in refractive index than the medium part 141. That is, the medium unit 141 has a small response time constant with respect to a change in optical path length. However, the delay amount variable range of the medium unit 141 becomes narrow. On the other hand, the medium part 141 has a larger thickness than the medium part 131. Therefore, the medium unit 141 has a wider variable delay amount range than the medium unit 131. However, the medium unit 141 has a larger response time constant with respect to a change in optical path length than the medium unit 131.

  Next, an outline of the operation of the optical receiver 400 will be described with reference to FIGS. 2 and 3. The half mirror 11 functions as a branching unit that branches input light. The half mirror 11 branches the input light into first branched light that passes through the first optical path and second branched light that passes through the second optical path. The first optical path is an optical path formed by the half mirror 11, the mirror 12, the first medium 13, the second medium 14, the mirror 15, and the half mirror 16. The second optical path is an optical path formed by the half mirror 11 and the half mirror 16. The optical path length of the first optical path and the optical path length of the second optical path are set so that the first branched light is delayed by 1 bit with respect to the second branched light.

  The first branched light is reflected by the mirror 12, passes through the first medium 13 and the second medium 14, is then reflected by the mirror 15, and is input to the half mirror 16. The second branched light passes through the half mirror 11 and is input to the half mirror 16. The first branched light passes through the first medium 13 and the second medium 14 and is delayed by 1 bit with respect to the second branched light.

  In the half mirror 16, the first branched light and the second branched light are combined and interfere with each other. The first branched light output from the half mirror 16 is input to the photodiode on the positive phase side of the balanced receiver 30. The second branched light output from the half mirror 16 is reflected by the mirror 17 and input to the photodiode on the opposite phase side of the balanced receiver 30. The serial signal output from the balanced receiver 30 is parallelized by the deserializer 60 and output.

  The positive phase monitor 40 detects the photocurrent generated by the photodiode on the positive phase side of the balanced receiver 30 and inputs the detected photocurrent to the analog / digital converter 21. The negative phase monitor 50 detects the photocurrent generated by the photodiode on the negative phase side of the balanced receiver 30 and inputs the detected photocurrent to the analog / digital converter 21.

  The analog / digital converter 21 performs analog / digital conversion on the input photocurrent. The digital signal processing unit 22 performs arithmetic processing according to the digital signal from the analog / digital converter 21. The digital / analog converter 23 performs digital / analog conversion of the digital signal from the digital signal processing unit 22 and inputs the converted analog signal to the first drive circuit 24 and the second drive circuit 25. The first drive circuit 24 supplies power to the first heater 132 in accordance with the input analog signal. The second drive circuit 25 supplies power to the second heater 142 in accordance with the input analog signal.

  FIG. 4 is a diagram for explaining the relationship between the wavelength and the transmittance in the delay interferometer 100. In FIG. 4, the horizontal axis is the wavelength, and the vertical axis is the transmittance. Referring to FIG. 4, a transmission spectrum appears in delay interferometer 100. This transmission spectrum can be shifted by controlling the power supplied to the first heater 132 and the second heater 142. In the present embodiment, the digital signal processing unit 22 performs arithmetic processing so that the wavelength of light received by the optical receiver 400 matches one of the transmission spectrum peaks. The first drive circuit 24 and the second drive circuit 25 control the power supplied to the first heater 132 and the second heater 142 according to the calculation result of the digital signal processing unit 22.

  By the way, since the response of the medium to heat is generally low, when trying to match the wavelength of the received light and the peak wavelength of the transmission spectrum of the delay interferometer using a single medium, the response time constant of the medium Controls the overall response speed. Therefore, it takes time to converge the transmission spectrum to a stable point. Thus, it is conceivable to make the medium thinner in order to increase the thermal response speed of the medium. However, in that case, there arises a problem that the phase adjustment range becomes small.

In order to simplify the description, considering a configuration in which a heater is attached to one side of a plate-like medium as shown in FIG. 5, the response time constant τ of the medium and the change width Δx of the transfer adjustment range are expressed by the following formulas ( 1) and (2).
τ = (C × W × L) / (K × H) (1)
Δx = L × (dn / dT) × ΔT (2)

  Here, C: heat capacity of medium, W: width of medium, L: thickness of medium, K: thermal conductivity of medium, H: height of medium, dn / dT: amount of change in refractive index of medium, L: medium ΔT: the amount of change in temperature of the medium.

  As derived from the equations (1) and (2), the response speed and the phase adjustment range have a trade-off relationship with the thickness L of the medium as a parameter. Therefore, the phase adjustment range is narrowed if the medium is thinned for high speed. As a result, the wavelength range that can be used in the optical receiver is narrowed. Therefore, in this embodiment, the response speed of the delay interferometer is improved by individually controlling the refractive indexes of a plurality of media.

  In this embodiment, first, it is determined whether the first medium 13 and the second medium 14 are heated or dissipated. For example, the determination can be made based on whether the output current value of the positive-phase side photodiode of the balanced receiver 30 is increased or decreased by minutely changing heat in one of the first medium 13 and the second medium 14. The time required for this determination is determined by the response time constant of the medium.

  FIG. 6 is a diagram for explaining a calculation example of the thicknesses and response time constants of the first medium 13 and the second medium 14 using the relationship of the above formula (1). In FIG. 6, the amount of phase delay necessary to detect the direction to be controlled is 1 au (arbitrary unit: arbitrary scale). As will be described with reference to FIG. 6, it can be seen that the phase delay amount with respect to time is large (the response time constant is small) by reducing the thickness. Therefore, the time for detecting the direction to be controlled can be shortened by using the first medium 13. Next, by controlling the refractive index of the second medium 14 or both the first medium 13 and the second medium 14, a necessary phase delay amount can be realized.

  The time required for feedback control of the phase delay amount is the sum of the time (A) required for detecting the control direction and the time (B) for realizing the required delay amount according to the result. Therefore, by detecting the direction to be controlled first using the first medium 13 having a small response time constant, the time (A) is shortened, and as a result, the feedback control can be speeded up. Since the above two controls are repeated to converge the phase delay amount to the optimum point, the effect of improving the overall convergence time by reducing the time (A) is great.

  In addition, when controlling the refractive index of both the 1st medium 13 and the 2nd medium 14, it is preferable to stabilize the refractive index of the 1st medium 13 in the center vicinity of the phase adjustable range. This is because it becomes difficult to exceed the phase adjustable range of the first medium 13 when further phase adjustment is required thereafter.

  There are two cases where control of the phase delay amount is required. One case is a case where the phase delay amount is controlled so that the transmittance of the delay interferometer is maximized according to the reception wavelength at the time of activation (case 1). The other case is a case (case 2) in which control is performed in response to minute wavelength fluctuations after the initial phase adjustment is completed.

  In order to deal with Case 1, it is only necessary to detect the direction to be controlled using the first medium 13 and then realize the necessary amount of phase delay using the second medium 14. In order to cope with case 2, it is possible to cope with the first medium 13 and the second medium 14 by cooperative control. The operation will be described with reference to FIG.

  FIG. 7A is a diagram for explaining the phase delay amount due to the change in the refractive index of the first medium 13. FIG. 7B is a diagram for explaining the phase delay amount due to the change in the refractive index of the second medium 14. FIG. 7C is a diagram for explaining the total phase delay amount of the first medium 13 and the second medium 14.

  As illustrated in FIG. 7A, when the wavelength variation occurs, the first medium 13 generates a phase delay at high speed following the wavelength variation. However, when the wavelength variation exceeds the phase adjustment range of the first medium 13, the wavelength variation cannot be followed. On the other hand, as will be described with reference to FIG. 7B, it takes time until the phase delay amount is generated even if the phase adjustment is performed by the second medium 14. Therefore, as illustrated in FIG. 7C, the control unit 20 causes the total delay amount of the first medium 13 and the second medium 14 to reach the target delay amount after the phase adjustment by the first medium 13. In addition, the delay amount of the second medium 14 is increased while the phase delay amount of the first medium 13 is decreased.

  In this embodiment, since the response time constant is determined according to the thickness of the medium, the power or voltage applied to the first heater 132 of the first medium 13 is controlled in accordance with the known response time constant of the second medium 14. To do. Thereby, it can respond to said case 2. FIG.

  FIG. 8 is a diagram for explaining an example of a flowchart executed when controlling the first medium 13 and the second medium 14 in the case 1 and the case 2. As illustrated in FIG. 8, the digital signal processing unit 22 controls the first driving circuit 24 to drive the first heater 132 of the first medium 13 (step S1). Next, the digital signal processing unit 22 determines whether or not the required amount of phase delay is within the phase adjustment range of the first medium 13 (step S2).

  If it is determined as “Yes” in step S2, the digital signal processing unit 22 controls the first drive circuit 24 to drive the first heater 132 of the first medium 13 (step S3). Thereafter, the digital signal processing unit 22 controls the first drive circuit 24 until the phase delay amount of the delay interferometer 100 reaches a target value (step S4). Next, the digital signal processing unit 22 controls the first drive circuit 24 and the second drive circuit 25 to increase the power to the second heater 142 while maintaining the phase delay amount, and to the first heater 132. The power is reduced (step S5).

  Next, the digital signal processing unit 22 supplies the first heater 132 with the refractive index control point of the first medium 13 near the center of the phase adjustable range of the first medium 13 while maintaining the phase delay amount. Is controlled (step S6). Thereafter, the digital signal processing unit 22 ends the execution of the flowchart.

  If it is determined “No” in step S2, the digital signal processing unit 22 controls the first drive circuit 24 and the second drive circuit 25 to drive both the first heater 132 and the second heater 142 ( Step S7). Thereafter, the digital signal processing unit 22 controls the first drive circuit 24 and the second drive circuit 25 until the phase delay amount of the delay interferometer 100 reaches a target value (step S8). Next, the digital signal processing unit 22 controls the first drive circuit 24 and the second drive circuit 25 to increase the power to the second heater 142 while maintaining the phase delay amount, and to the first heater 132. The power is reduced (step S9).

  Next, the digital signal processing unit 22 supplies the first heater 132 with the refractive index control point of the first medium 13 near the center of the phase adjustable range of the first medium 13 while maintaining the phase delay amount. Is controlled (step S10). Thereafter, the digital signal processing unit 22 ends the execution of the flowchart.

  According to the flowchart of FIG. 8, if the required adjustment amount of the phase is within the phase adjustment range of the first medium 13, the required adjustment amount of the phase can be realized with good response and in a short time only by controlling the power of the first heater 132. can do. When the phase adjustment amount exceeds the phase adjustment possible range of the first medium 13, the phase adjustment amount is realized in a short time by controlling the power of both the first heater 132 and the second heater 142. be able to. Therefore, according to the flowchart of FIG. 8, both the case 1 and the case 2 described above can be handled.

  In this embodiment, one optical path length variable means with a small response time constant and one optical path length variable means with a large delay amount variable width are used, but one or more may be used. In this case, the response speed can be improved. Further, in FIG. 3, the heater is illustrated in a structure in contact with one side of the medium, but the present invention is not limited to this form.

  In the correspondence relationship between the present embodiment and the means for solving the problem, the first medium 13 corresponds to the first optical path length variable means, the second medium 14 corresponds to the second optical path length variable means, and the control unit 20. Corresponds to the optical path length control means.

  A MEMS (Micro Electro Mechanical Systems) mirror or the like may be used as the optical path length variable means having a small response time constant related to the optical path length. The response time constant can be reduced by using the MEMS mirror.

  FIG. 9 is a block diagram for explaining the overall configuration of the optical receiver 400a according to the second embodiment. With reference to FIG. 9, the optical receiver 400 a is different from the optical receiver 400 according to the first embodiment in that an interference unit 10 a is provided instead of the interference unit 10. The interference unit 10a includes a half mirror 31, a medium 32, a MEMS mirror 33, and mirrors 34 to 37. The interference unit 10 according to the first embodiment has a Mach-Zehnder structure, whereas the interference unit 10a according to the second embodiment has a Michelson structure.

  The half mirror 31 functions as a branching unit that branches input light. The half mirror 31 branches the input light into first branched light that passes through the first optical path and second branched light that passes through the second optical path. The first branched light is reflected by the mirror 34 and input to the half mirror 31 again. Therefore, the first optical path is a path that reciprocates between the half mirror 31 and the mirror 34. The second branched light passes through the medium 32, is reflected by the MEMS mirror 33, passes through the medium 32 again, and is input to the half mirror 31. Therefore, the second optical path is a path that reciprocates between the half mirror 31 and the MEMS mirror 33. The optical path length of the first optical path and the optical path length of the second optical path are set so that the first branched light is delayed by 1 bit with respect to the second branched light.

  The half mirror 31 also functions as a multiplexing interference unit. The first split light input to the half mirror 31 via the first optical path is emitted after being combined and interfered with the second optical signal via the second optical path, reflected by the mirror 37, and output from the balanced receiver 30. Input to the positive-phase photodiode. The second branched light input to the half mirror 31 via the second optical path is emitted after being combined and interfered with the first optical signal via the first optical path, reflected by the mirrors 35 and 36, and a balanced receiver. 30 is input to the photodiode on the opposite phase side.

  The movable direction of the MEMS mirror 33 is the same direction as the optical axis incident on the MEMS mirror 33. By designing the resonance frequency of the MEMS mirror 33 to be high, it is possible to configure an optical path length variable means with a small response time constant. Also in this embodiment, the response speed of the delay interferometer 100a is improved by including the optical path length varying means (MEMS mirror 33) having a small response time constant and the optical path length varying means (medium 32) having a large response time constant. be able to.

  In the correspondence between the present embodiment and the means for solving the problem, the MEMS mirror 33 corresponds to the first optical path length variable means, the medium 32 corresponds to the second optical path length variable means, and the control unit 20 This corresponds to the optical path length control means.

  The structure of the delay interferometer according to each of the above embodiments may be applied to a quartz waveguide on a silicon substrate. FIG. 10 is a schematic diagram for explaining the overall configuration of the delay interference unit 10b according to the third embodiment. Referring to FIG. 10, this quartz waveguide layer structure includes an input waveguide 41, a branching section 42, a multiplexing section 43, a connection waveguide 44, a delay waveguide 45, a first output waveguide 46a, and a second output waveguide 46a. Output waveguide 46b.

  The input waveguide 41 is a waveguide that guides incident light that is differential phase-modulated light. The branching unit 42 is a branching unit for branching incident light into first branched light and second branched light. The first branched light is input to the delay waveguide 45. The second branched light is input to the connection waveguide 44. The optical path lengths of the connection waveguide 44 and the delay waveguide 45 are set so that the first branched light is delayed by 1 bit with respect to the second branched light. The multiplexing unit 43 combines the two branched lights separated by the branching unit 42 with each other.

  In this embodiment, the first heater 47 and the second heater 48 larger than the first heater are pasted in the middle of the delay waveguide 45 so as to be separated from each other. The first heater 47 and the second heater 48 are, for example, thin film heaters. The first heater 47 has a smaller heat capacity than the second heater 48. Therefore, the waveguide provided with the first heater 47 has a narrow delay amount variable range and a small response time constant as compared with the waveguide provided with the second heater 48. On the other hand, the second heater 48 has a larger heat capacity than the first heater 47. Therefore, the waveguide provided with the second heater 48 has a wide delay amount variable range and a large response time constant as compared with the waveguide provided with the first heater 47.

  Also in the present embodiment, the response speed of the interference unit 10b can be improved by providing the optical path length varying means having a small response time constant and the optical path length varying means having a large response time constant.

  In each of the above embodiments, the binary differential phase modulation transmission system (DPSK) is adopted, but the present invention is not limited to this. By integrating a plurality of delay interferometers whose phases are shifted, a multi-level differential phase modulation transmission system such as DQPSK can be employed.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

DESCRIPTION OF SYMBOLS 10 Interfering part 13 1st medium 14 2nd medium 20 Control part 22 Digital signal processing part 24 1st drive circuit 25 2nd drive circuit 30 Balanced receiver 33 MEMS mirror 40 Positive phase monitor 50 Reversed phase monitor 100 Delay interferometer 131, 141 Media Unit 132 First Heater 142 Second Heater 200 Optical Transmitter 400 Optical Receiver 1000 Optical Transmission System

Claims (9)

A delay interferometer having a first optical path and a second optical path,
A first optical path length variable means and a second optical path length variable means arranged on the first optical path and having a variable optical path length;
Optical path length control means for controlling the optical path length of the first optical path length variable means and the second optical path length variable means,
A delay interferometer, wherein a response time constant related to a change in optical path length of the first optical path length variable means is smaller than a response time constant related to a change in optical path length of the second optical path length variable means.   2. The delay interferometer according to claim 1, wherein the variable optical path length of the second optical path length variable means is larger than the variable optical path length of the first optical path length variable means.   The optical path length control means controls the optical path length of the first optical path using the first optical path length variable means to determine whether to increase or decrease the optical path length of the first optical path, and then determines the second optical path. 3. The delay interferometer according to claim 1, wherein the optical path length of the first optical path is controlled using a length variable means.   The optical path length control means controls the optical path length of the first optical path using the first optical path length variable means and determines whether to increase or decrease the optical path length of the first optical path, and then determines the first optical path. 3. The delay interferometer according to claim 1, wherein the optical path length of the first optical path is controlled by using a variable length means and the second optical path length variable means.   5. The delay interferometer according to claim 1, wherein the first optical path length varying unit and the second optical path length varying unit are media whose refractive index changes according to a temperature change.   6. The delay interferometer according to claim 5, wherein the first optical path length varying means and the second optical path length varying means are media provided with a heater.   The delay interferometer according to claim 1, wherein the first optical path length varying unit is a MEMS mirror. The delay interferometer is provided as a quartz waveguide on a substrate;
6. The delay interferometer according to claim 5, wherein the first optical path length varying means and the second optical path length varying means are regions where a heater is provided in the optical waveguide. First optical path length varying means and second optical path length varying means having a first optical path and a second optical path, the optical path length being arranged on the first optical path and variable, the first optical path length varying means and the second optical path length Optical path length control means for controlling the optical path length of the optical path length variable means, and the response time constant related to the optical path length change of the first optical path length variable means is the response related to the optical path length change of the second optical path length variable means A delay interferometer that is small compared to the time constant,
A light receiver for receiving an optical signal output from the delay interferometer.

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