WO2013140476A1

WO2013140476A1 - Optical transmitter, optical transmission/reception system, and drive circuit

Info

Publication number: WO2013140476A1
Application number: PCT/JP2012/007109
Authority: WO
Inventors: 栄実野口
Original assignee: 日本電気株式会社
Priority date: 2012-03-22
Filing date: 2012-11-06
Publication date: 2013-09-26
Also published as: JP6032275B2; JPWO2013140476A1

Abstract

The present invention provides an optical transmitter, an optical transmission/reception system, and a drive circuit which are capable of adjusting the linearity of output light. An optical modulator (11) includes optical transmission paths (111, 112) in which phase modulation areas (PM11_1 to PM11_4, PM12_1 to PM12_4) are formed and through which optical signals propagate. A decoder (12) decodes an input digital signal (D[3:0]) and outputs signals (D1 to D4) corresponding to decoded values. On the basis of said signals (D1 to D4), a drive circuit (13) outputs drive signals having three or more tones, including signals having different numbers of tones, to each of said phase modulation areas (PM11_1 to PM11_4, PM12_1 to PM12_4). A control circuit (14) designates the correlation between the decoded values and the respective drive signals by controlling the drive circuit (13).

Description

Optical transmitter, optical transmission / reception system, and drive circuit

The present invention relates to an optical transmitter, an optical transmission / reception system, and a drive circuit, and more particularly to an optical transmitter, an optical transmission / reception system, and a drive circuit that perform multilevel modulation.

With the explosive demand for broadband multimedia communication services such as the Internet and video distribution, long-distance, large-capacity and high-reliability high-density wavelength-division-multiplexed optical fiber communication systems are being introduced in trunk lines and metro lines. In addition, optical fiber access services are rapidly spreading in subscriber systems. In a communication system using such an optical fiber, it is important to reduce the installation cost of an optical fiber that is an optical transmission line and to increase the transmission band utilization efficiency per optical fiber. For this reason, a wavelength multiplexing technique that multiplexes and transmits optical signals having different wavelengths is widely used.

Optical transmitters for WDM optical fiber communication systems are capable of high-speed optical modulation, have small optical signal wavelength dependency, and unnecessary optical phase modulation components (modulation) that cause deterioration of the received optical waveform during long-distance signal transmission There is a need for an optical modulator in which the light intensity modulation component (when the method is a light intensity modulation method) or the light intensity modulation component (when the modulation method is an optical phase modulation method) is minimized. For such applications, an MZ light intensity modulator incorporating an optical waveguide type optical phase modulator similar to an optical waveguide type Mach-Zehnder (hereinafter, MZ) interferometer is practical.

In addition, when expanding the transmission capacity per wavelength channel, from the viewpoint of spectrum utilization efficiency and resistance to chromatic dispersion and polarization mode dispersion of the optical fiber, the optical modulation spectrum is higher than that of the normal binary light intensity modulation method. A multilevel optical modulation signaling scheme with a narrower bandwidth is advantageous. This multi-level optical modulation signal system is considered to become mainstream particularly in a trunk optical fiber communication system exceeding 40 Gb / s, where future demand is expected to increase. Currently, a monolithic integrated multilevel optical modulator combining two MZ optical intensity modulators and an optical multiplexer / demultiplexer has been developed for such applications.

When such an optical modulator is used to perform high-speed optical modulation particularly in a high-frequency region where the frequency of the modulated electrical signal exceeds 1 GHz, the length of the electrode provided in the optical phase modulator region of the optical modulator is set. On the other hand, the propagation wavelength of the modulated electric signal is shortened to a level that cannot be ignored. For this reason, the potential distribution of the electrode structure, which is a means for applying an electric field to the optical phase modulator, cannot be regarded as uniform in the optical signal propagation axis direction. Therefore, in order to accurately estimate the light modulation characteristics, it is necessary to treat the electrode itself as a distributed constant line and a modulated electric signal propagating through the optical phase modulator region as a traveling wave. In this case, in order to obtain the effective interaction length between the modulated optical signal and the modulated electrical signal as much as possible, the phase velocity vo of the modulated optical signal and the phase velocity vm of the modulated electrical signal are made as close as possible (phase velocity). A so-called traveling wave type electrode structure is required which is devised.

An optical modulator module having a split electrode structure for realizing such a traveling wave electrode structure and a multilevel optical modulation signal system has already been proposed (Patent Documents 1 to 4). In addition, there has been proposed an optical modulator module capable of multilevel control of the phase change of the modulated optical signal in each of the divided electrodes. This optical modulator module is a compact, wideband, and capable of generating an arbitrary multilevel optical modulation signal by inputting a digital signal while maintaining phase velocity matching and impedance matching required for traveling wave structure operation. This is an optical modulator module with a low driving voltage.

JP 7-13112 A Japanese Patent Laid-Open No. 5-289033 JP-A-5-257102 International Publication No. 2011/043079

However, the inventor has found that the above-described optical modulator module has the following problems. The divided electrode structure has a problem that large-scale multi-level modulation is difficult because the number of divided electrodes is limited.

In the divided electrode structure, theoretically, the number of divided electrodes can be increased by increasing the number of divided electrodes. Further, if the phase change of the modulated optical signal in each of the divided electrodes is controlled in multiple values, further multiple values can be achieved. However, the number of divided electrodes that can be mounted on an actually manufactured optical modulator module is limited by the size of the optical modulator. For this reason, the number of gradation levels of multilevel modulation is actually limited, and it is difficult to realize an optical modulator module capable of large-scale multilevel modulation.

Therefore, it is conceivable to multi-value the signal given to each divided electrode. However, if the phase is changed at equal intervals, the gradation interval of the signal intensity of the modulated output light becomes non-uniform. That is, the linearity of the output light with respect to the input digital signal cannot be ensured. That is, in order to operate the multilevel modulated optical transmitter having the above-described divided electrode structure, it is necessary to provide a function capable of adjusting the linearity of the output light.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an optical transmitter, an optical transmission / reception system, and a drive circuit capable of adjusting the linearity of output light. .

An optical transmitter according to one embodiment of the present invention includes an optical modulator having a plurality of phase modulation regions and an optical transmission path through which an optical signal propagates, a signal corresponding to a decoded value, and an input digital signal Based on a signal corresponding to the decode value, a drive circuit for outputting a drive signal of three or more gradations including those having different numbers of gradations in each of the plurality of phase modulation regions, and the drive And a control circuit that specifies a correspondence relationship of the plurality of drive signals with respect to the decode value by controlling the circuit.

An optical transmission / reception system according to an aspect of the present invention includes: an optical transmitter that transmits an optical signal; a transmission path through which the optical signal propagates; and an optical receiver that receives the optical signal through the transmission path. An optical modulator having an optical transmission path through which an optical signal is propagated, and a decoder that decodes an input digital signal and outputs a signal corresponding to the decoded value And a drive circuit for outputting a drive signal of three or more gradations including those having different numbers of gradations to each of the plurality of phase modulation regions based on a signal corresponding to the decode value, and controlling the drive circuit And a control circuit for designating the correspondence relationship of the plurality of drive signals with respect to the decode value.

In the driver circuit which is one embodiment of the present invention, the number of gradations is set in each of the plurality of phase modulation regions formed in the optical waveguide provided in the optical modulator based on a signal corresponding to the decoded value of the input digital signal. It comprises a plurality of DACs that output drive signals of three or more gradation levels including different ones and that can specify an operation range for the decode value by an external control circuit.

According to the present invention, it is possible to provide an optical transmitter, an optical transmission / reception system, and a drive circuit that can adjust the linearity of output light.

It is a block diagram which shows typically the structure of the multi-value optical transmitter 500 of a general division | segmentation electrode structure. FIG. 6 is a diagram schematically showing a configuration of an optical multiplexer / demultiplexer 513. FIG. 3 is a diagram schematically showing a configuration of an optical multiplexer / demultiplexer 514. 5 is an operation table showing the operation of the optical transmitter 500. FIG. 6 is a diagram schematically illustrating a light propagation mode in the optical transmitter 500. FIG. 5 is a constellation diagram showing lights L1 and L2 when phase modulation is not performed by the phase modulation areas PM51_1 to 51_4 and the phase modulation areas PM52_1 to 52_4. FIG. 11 is a constellation diagram showing light L1 and light L2 when a binary code of an input digital signal is “0000” in the optical transmitter 500. FIG. 4 is a constellation diagram showing lights L1 and L2 in the optical transmitter 500. FIG. 11 is a constellation diagram showing the light intensity of output light OUT due to light L1 and L2 being combined in the optical transmitter 500. 1 is a block diagram schematically showing a configuration of an optical transmitter 100 according to a first embodiment. FIG. 4 is a diagram schematically showing the number of gradation levels of D / A converters DAC1 to DAC4. 3 is an operation table illustrating an operation of the optical transmitter 100 according to the first embodiment. FIG. 3 is a constellation diagram illustrating a modulation operation of the optical transmitter 100 according to the first embodiment. FIG. 3 is a constellation diagram illustrating a modulation operation of the optical transmitter 100 according to the first embodiment. FIG. 4 is a block diagram schematically showing a configuration of an optical transmitter 200 according to a second embodiment. FIG. 6 is a block diagram schematically showing a configuration of an optical transmitter 300 according to a third embodiment. FIG. 6 is a block diagram schematically showing a configuration of an optical transmission / reception system 400 according to a fourth exemplary embodiment. It is a figure which shows the waveform and appearance probability of a pre-equalized optical signal. It is a graph which shows the gradation change of 4-bit output light which has a general linear characteristic. It is a graph which shows the gradation change of 4-bit output light which has the nonlinear characteristic in which the gradation width | variety near "1000" becomes narrow.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted as necessary.

As a premise for understanding the configuration and operation of an optical transmitter according to the following embodiment, a general multi-value optical transmitter 500 having a divided electrode structure will be described. The optical transmitter 500 is a multi-level modulation optical transmitter, but here, for the sake of simplicity of description, the optical transmitter 500 will be described as a 4-bit optical transmitter. FIG. 1 is a block diagram schematically showing a configuration of a multi-value optical transmitter 500 having a general divided electrode structure. The optical transmitter 500 includes an optical modulator 51, a decoder 52, and a drive circuit 53.

The optical modulator 51 outputs an output light OUT obtained by modulating the input light IN. The optical modulator 51 includes

optical waveguides

511 and 512, optical multiplexers /

demultiplexers

513 and 514, and phase modulation regions PM51_1 to PM51_4, PM52_1 to PM52_4. The

optical waveguides

511 and 512 are arranged in parallel.

An optical multiplexer / demultiplexer 513 is inserted on the optical signal input (input light IN) side of the

optical waveguides

511 and 512. On the input side of the optical multiplexer / demultiplexer 513, the input light IN is input to the input port P1, and the input port P2 is not input. On the output side of the optical multiplexer / demultiplexer 513, the optical waveguide 511 is connected to the output port P3, and the optical waveguide 512 is connected to the output port P4.

FIG. 2A is a diagram schematically showing the configuration of the optical multiplexer / demultiplexer 513. In the optical multiplexer / demultiplexer 513, the light incident on the input port P1 propagates to the output ports P3 and P4. However, the phase of light propagating from the input port P1 to the output port P4 is delayed by 90 ° compared to the light propagating from the input port P1 to the output port P3. Further, the light incident on the input port P2 propagates to the output ports P3 and P4. However, the phase of light propagating from the input port P2 to the output port P3 is delayed by 90 ° compared to the light propagating from the input port P2 to the output port P4.

An optical multiplexer / demultiplexer 514 is inserted on the optical signal output (output light OUT) side of the

optical waveguides

511 and 512. On the input side of the optical multiplexer / demultiplexer 514, the optical waveguide 511 is connected to the input port P5, and the optical waveguide 512 is connected to the input port P6. On the output side of the optical multiplexer / demultiplexer 514, the output light OUT is output from the output port P7.

FIG. 2B is a diagram schematically showing the configuration of the optical multiplexer / demultiplexer 514. The optical multiplexer / demultiplexer 514 has the same configuration as the optical multiplexer / demultiplexer 513. The input ports P5 and P6 correspond to the input ports P1 and P2 of the optical multiplexer / demultiplexer 513, respectively. The output ports P7 and P8 correspond to the output ports P3 and P4 of the optical multiplexer / demultiplexer 513, respectively. The light incident on the input port P5 propagates to the output ports P7 and P8. However, the phase of light propagating from the input port P5 to the output port P8 is delayed by 90 ° compared to the light propagating from the input port P5 to the output port P7. Further, the light incident on the input port P6 propagates to the output ports P7 and P8. However, the phase of light propagating from the input port P6 to the output port P7 is delayed by 90 ° compared to the light propagating from the input port P6 to the output port P8.

Phase modulation regions PM51_1 to PM51_4 are arranged in the optical waveguide 511 between the optical multiplexer / demultiplexer 513 and the optical multiplexer / demultiplexer 514. Phase modulation regions PM52_1 to PM52_4 are disposed in the optical waveguide 512 between the optical multiplexer / demultiplexer 513 and the optical multiplexer / demultiplexer 514.

Here, the phase modulation region is a region having electrodes formed on the optical waveguide. When an electric signal, for example, a voltage signal is applied to the electrode, the effective refractive index of the optical waveguide under the electrode changes. As a result, the substantial optical path length of the optical waveguide in the phase modulation region can be changed. Thereby, the phase modulation region can change the phase of the optical signal propagating through the optical waveguide. The optical signal can be modulated by giving a phase difference between the optical signals propagating between the two

optical waveguides

511 and 512. That is, the optical modulator 51 constitutes a multi-value Mach-Zehnder optical modulator having two arms and an electrode division structure.

The decoder 52 decodes the 4-bit input digital signal D [3: 0] and outputs, for example, multi-bit signals D 1 to D 4 to the drive circuit 53.

The drive circuit 53 includes five-value D / A converters DAC51 to DAC54. Signals D1 to D4 are supplied to the D / A converters DAC51 to DAC54, respectively. The D / A converters DAC51 to DAC54 output a pair of differential output signals according to the signals D1 to D4. At this time, the positive-phase output signals of the differential output signals output from the D / A converters DAC51 to DAC54 are output to the phase modulation regions PM51_1 to 51_4. Respective negative-phase output signals of the differential output signals output from the D / A converters DAC51 to DAC54 are output to the phase modulation areas PM52_1 to 52_4.

Here, the differential output signals output from the D / A converters DAC51 to DAC54 will be described. As described above, the D / A converter DAC 51 is a D / A converter with five values (0, 1, 2, 3, 4). That is, the DAC 51 increases the value of the positive phase output signal in the order of “0” → “1” → “2” → “3” → “4” in accordance with the increase in the value of the signal D1.

On the other hand, the DAC 51 outputs a signal obtained by inverting the normal phase output signal as a negative phase output signal. That is, the DAC 51 increases the value of the reverse phase output signal in the order of “4” → “3” → “2” → “1” → “0” in accordance with the increase in the value of the signal D1. It can also be understood that the value of the negative phase output signal is determined such that the sum of the values of the positive phase output signal and the negative phase output signal is equal to the maximum value “4” of the quinary output. .

FIG. 3 is an operation table showing the operation of the optical transmitter 500. The D / A converter DAC 51 increases the value of the positive phase output signal to “0” as the input digital signal D [3: 0] increases from “0000” → “0001” → “0010” → “0011” → “0100”. ”→“ 1 ”→“ 2 ”→“ 3 ”→“ 4 ”and increase the value of the negative phase output signal from“ 4 ”→“ 3 ”→“ 2 ”→“ 1 ”→“ 0 ” Decrease in order. However, when the input digital signal D [3: 0] is “0101” or more, the value of the positive phase output signal of the D / A converter DAC 51 is “4” and the value of the negative phase output signal is “0”. .

The D / A converter DAC 52 increases the value of the positive phase output signal to “0” as the input digital signal D [3: 0] increases from “0100” → “0101” → “0110” → “0111” → “1000”. ”→“ 1 ”→“ 2 ”→“ 3 ”→“ 4 ”and increase the value of the negative phase output signal from“ 4 ”→“ 3 ”→“ 2 ”→“ 1 ”→“ 0 ” Decrease in order. However, when the input digital signal D [3: 0] is “0011” or less, the value of the positive phase output signal of the D / A converter DAC 52 is “0” and the value of the negative phase output signal is “4”. . When the input digital signal D [3: 0] is “1001” or more, the value of the positive phase output signal of the D / A converter DAC 52 is “4” and the value of the negative phase output signal is “0”. .

The D / A converter DAC 53 increases the value of the positive phase output signal to “0” as the input digital signal D [3: 0] increases from “1000” → “1001” → “1010” → “1011” → “1100”. ”→“ 1 ”→“ 2 ”→“ 3 ”→“ 4 ”and increase the value of the negative phase output signal from“ 4 ”→“ 3 ”→“ 2 ”→“ 1 ”→“ 0 ” Decrease in order. However, when the input digital signal D [3: 0] is “0111” or less, the value of the positive phase output signal of the D / A converter DAC 53 is “0” and the value of the negative phase output signal is “4”. . When the input digital signal D [3: 0] is “1101” or more, the value of the positive phase output signal of the D / A converter DAC 53 is “4”, and the value of the negative phase output signal is “0”. .

The D / A converter DAC 54 increases the value of the positive phase output signal from “0” to “1” as the input digital signal D [3: 0] increases from “1100” → “1101” → “1110” → “1111”. ”→“ 2 ”→“ 3 ”, and the value of the negative phase output signal is decreased in the order of“ 4 ”→“ 3 ”→“ 2 ”→“ 1 ”→. However, when the input digital signal D [3: 0] is “1011” or less, the value of the positive phase output signal of the D / A converter DAC 51 is “0”, and the value of the negative phase output signal is “4”. .

Here, the phase modulation operation of the optical transmitter 500 will be described. FIG. 4 is a diagram schematically illustrating a light propagation mode in the optical transmitter 500. In this example, as shown in FIG. 1, the input light IN is input to the input port P1 of the optical multiplexer / demultiplexer 513. Therefore, the phase of the light output from the output port P4 is delayed by 90 ° compared to the light output from the output port P3. Thereafter, the light output from the output port P3 passes through the phase modulation regions PM51_1 to 51_4 and reaches the input port P5 of the optical multiplexer / demultiplexer 514. The light that reaches the input port P5 reaches the output port P7 as it is. On the other hand, the light output from the output port P4 passes through the phase modulation regions PM52_1 to 52_4 and reaches the input port P6 of the optical multiplexer / demultiplexer 514. The light reaching the input port P6 reaches the output port P7 with a phase delay of 90 °.

That is, even when the phase modulation regions PM51_1 to 51_4 and the phase modulation regions PM52_1 to 52_4 are not subjected to phase modulation, the light L2 reaching the output port P7 from the input port P6 is the light reaching the output port P7 from the input port P5. Compared to L1, the phase is delayed by 180 °.

FIG. 5A is a constellation diagram showing the lights L1 and L2 when the phase modulation regions PM51_1 to 51_4 and the phase modulation regions PM52_1 to 52_4 are not subjected to phase modulation. As described above, the phase of the light L2 reaching the output port P7 from the input port P6 is delayed by 180 ° compared to the light L1 reaching the output port P7 from the input port P5.

In contrast, in the optical transmitter 500, the normal phase output signal is input to the phase modulation regions PM51_1 to 51_4, and the negative phase output signal is input to the phase modulation regions PM52_1 to 52_4. Thereby, the phase delay of the light L2 reaching the output port P7 from the input port P6 is compensated. FIG. 5B is a constellation diagram showing the lights L1 and L2 when the binary code of the input digital signal D [3: 0] is “0000” in the optical transmitter 500. For example, if the binary code of the input digital signal D [3: 0] is “0000”, “0” that is a positive phase output signal is input to the phase modulation regions PM51_1 to 51_4. On the other hand, “4” that is an anti-phase output signal is input to the phase modulation regions PM52_1 to 52_4. Thereby, the phase of the light passing through the phase modulation regions PM52_1 to 52_4 is further delayed by 180 °.

That is, the light L2 reaching the output port P7 from the input port P6 is added with 180 °, which is the phase delay due to the phase modulation regions PM52_1 to 52_4, in addition to the original 180 ° phase delay. As a result, a 360 ° phase lag occurs in the light L2 reaching the output port P7 from the input port P6, so that the phase lag with respect to the light L1 reaching the output port P7 from the input port P5 is substantially eliminated. Further, every time the binary code of the input digital signal D [3: 0] increases and the normal phase output signal output from the DACs 51 to 54 increases, the negative phase output signal decreases.

FIG. 5C is a constellation diagram showing the lights L1 and L2 in the optical transmitter 500. As shown in FIG. 5C, by using the differential output signal, the light reaching the output port P4 from the input port P1 and the output port P7 from the input port P6 according to the change of the input digital signal D [3: 0]. While compensating for the phase delay of L2, each of L1 / L2 changes the phase of the light in contrast to the Re axis, and optical D / A conversion in the optical transmitter becomes possible. Thereby, as shown in the operation table of FIG. 3, the phase modulation amount of the light L1 is 0 to 15Δθ and the phase modulation amount of the light L2 is 0 to −15Δθ according to the value of the input digital signal D [3: 0]. It can be changed in 16 stages.

In FIGS. 5B and 5C, when the binary code of the input digital signal D [3: 0] is “0000” or “1111”, the positions of the lights L1 and L2 do not match. It is only for reviewing the drawing. That is, when the binary code of the input digital signal D [3: 0] is “0000” or “1111”, the positions of the lights L1 and L2 may match. In addition, here, a case has been described in which the amount of phase change modulated in the phase modulation region changes from 0 to 180 degrees in accordance with the input digital signal. However, the present invention is not limited to this.

The optical transmitter 500 functions as a 4-bit optical transmitter with the above configuration. However, when the gray levels of the phases of L1 and L2 phase-modulated by the drive circuit 53 are equally spaced, the following problem occurs. FIG. 5D is a constellation diagram illustrating the light intensity of the output light OUT due to the light L1 and L2 being combined in the optical transmitter 500. As shown in FIG. 5D, when the phase of the optical signal is shifted at equal intervals, the gradation interval of the light intensity of the output light becomes non-uniform, and the linearity of the signal intensity of the output light with respect to the input digital signal is reduced. It cannot be secured.

Embodiment 1
Next, the optical transmitter 100 according to the first embodiment of the present invention will be described. The optical transmitter 100 is configured as an optical transmitter having a function of adjusting the linearity of output light in order to solve the problem in the optical transmitter 500 described above. Further, although the optical transmitter 100 is a multi-level modulation optical transmitter, here, the optical transmitter 100 is described as a 4-bit optical transmitter for simplification of description. FIG. 6 is a block diagram schematically illustrating the configuration of the optical transmitter 100 according to the first embodiment. The optical transmitter 100 includes an optical modulator 11, a decoder 12, a drive circuit 13, and a control circuit 14.

The optical modulator 11 outputs an output light OUT obtained by modulating the input light IN. The optical modulator 11 includes

optical waveguides

111 and 112, optical multiplexers /

demultiplexers

113 and 114, and phase modulation regions PM11_1 to PM11_4 and PM12_1 to 12_4. The

optical waveguides

111 and 112 are arranged in parallel.

An optical multiplexer / demultiplexer 113 is inserted on the optical signal input (input light IN) side of the

optical waveguides

111 and 112. The optical multiplexer / demultiplexer 113 has the same configuration as the optical multiplexer / demultiplexer 513 described above. On the input side of the optical multiplexer / demultiplexer 113, the input light IN is input to the input port P1, and the input port P2 is not input. On the output side of the optical multiplexer / demultiplexer 113, the optical waveguide 111 is connected to the output port P3, and the optical waveguide 112 is connected to the output port P4.

An optical multiplexer / demultiplexer 114 is inserted on the optical signal output (output light OUT) side of the

optical waveguides

111 and 112. The optical multiplexer / demultiplexer 114 has the same configuration as the optical multiplexer / demultiplexer 514 described above. On the input side of the optical multiplexer / demultiplexer 114, the optical waveguide 111 is connected to the input port P5, and the optical waveguide 112 is connected to the input port P6. On the output side of the optical multiplexer / demultiplexer 114, the output light OUT is output from the output port P7.

In the optical waveguide 111 between the optical multiplexer / demultiplexer 113 and the optical multiplexer / demultiplexer 114, phase modulation regions PM11_1 to PM11_4 are arranged. Phase modulation regions PM12_1 to PM12_4 are arranged in the optical waveguide 112 between the optical multiplexer / demultiplexer 113 and the optical multiplexer / demultiplexer 114.

Here, similarly to the optical transmitter 500, the phase modulation region has electrodes formed on the optical waveguide. When an electric signal, for example, a voltage signal is applied to the electrode, the effective refractive index of the optical waveguide under the electrode changes. As a result, the substantial optical path length of the optical waveguide in the phase modulation region can be changed. Thereby, the phase modulation region can change the phase of the optical signal propagating through the optical waveguide. The optical signal can be modulated by providing a phase difference between the optical signals propagating between the two

optical waveguides

111 and 112. That is, the optical modulator 11 constitutes a multi-value Mach-Zehnder optical modulator having two arms and an electrode division structure.

The drive circuit 13 includes D / A converters DAC1 to DAC4. The D / A converters DAC1 to DAC4 have the same full-scale amplitude FSA. However, the D / A converters DAC1 to DAC4 have different gradation numbers. In this example, it is assumed that the number of gradations of the D / A converters DAC1 and DAC4 is smaller than the number of gradations of the D / A converters DAC2 and DAC3. FIG. 7 is a diagram schematically showing the number of gradations of the D / A converters DAC1 to DAC4. Here, the number of gradations of the D / A converters DAC1 and DAC4 is 3, and the number of gradations of the D / A converters DAC2 and DAC3 is 5.

The D / A converters DAC1 to DAC4 output a pair of differential output signals according to the signals D1 to D4. At this time, the positive phase output signals of the differential output signals output from the D / A converters DAC1 to DAC4 are output to the phase modulation regions PM11_1 to 11_4. Respective negative-phase output signals of the differential output signals output from the D / A converters DAC1 to DAC4 are output to the phase modulation areas PM12_1 to 12_4.

Here, the differential output signals output from the D / A converters DAC1 to DAC4 will be described. As described above, the D / A converters DAC1 and DAC4 are four-value (3-gradation) output D / A converters. At this time, the positive phase output signals of the D / A converters DAC1 and DAC4 are “0 (0 × FSA / 3)” → “1 × FSA / 3” → “2 × FSA / 3” → “3 × FSA / 3”. The negative phase output signal decreases in the order of “3 × FSA / 3” → “2 × FSA / 3” → “1 × FSA / 3” → “0 (0 × FSA / 3)”. . As described above, the D / A converters DAC2 and DAC3 are 6-value (5-gradation) output D / A converters. At this time, the positive phase output signals of the D / A converters DAC2 and DAC3 are “0 (0 × FSA / 5)” → “1 × FSA / 5” → “2 × FSA / 5” → “3 × FSA / 5”. → "4xFSA / 5" → "5xFSA / 5" increases in this order, and the reverse phase output signal is "5xFSA / 5" → "4xFSA / 5" → "3xFSA / 3" → “2 × FSA / 3” → “1 × FSA / 3” → “0 (0 × FSA / 3)”. It can also be understood that the value of the negative phase output signal is determined such that the sum of the values of the positive phase output signal and the negative phase output signal is equal to the maximum value of the outputs of DAC1 to DAC4.

The decoder 12 decodes the 4-bit input digital signal D [3: 0] and outputs the signals D 1 to D 4 to the drive circuit 13.

The control circuit 14 controls the decoder 12 to output the signals D1 to D4 to any of the D / A converters DAC1 to DAC4. In this example, under the control of the control circuit 14, the decode 12 outputs signals D1 to D4 to the D / A converters DAC1 to DAC4, respectively. That is, the control circuit 14 specifies the correspondence between the signals D1 to D4 and the D / A converters DAC1 to DAC4. In other words, the control circuit 14 specifies the correspondence relationship between the decode value and the D / A converters DAC1 to DAC4, that is, the operation range of the DAC1 to DAC4 with respect to the decode value.

FIG. 8 is an operation table showing the operation of the optical transmitter 100 according to the first embodiment. As the input digital signal D [3: 0] increases in the order of “0000” → “0001” → “0010” → “0011”, the D / A converter DAC1 sets the positive phase output signal to “0 (0 × FSA / 3). ) ”→“ 1 × FSA / 3 ”→“ 2 × FSA / 3 ”→“ 3 × FSA / 3 ”. On the other hand, the D / A converter DAC1 outputs the reverse phase output signal in the order of “3 × FSA / 3” → “2 × FSA / 3” → “1 × FSA / 3” → “0 (0 × FSA / 3)”. Decrease with. However, when the input digital signal D [3: 0] is “0100” or more, the positive phase output signal of the D / A converter DAC1 is “3 × FSA / 3”, and the negative phase output signal is “0 (0 × FSA / 3) ”.

The D / A converter DAC2 outputs the positive phase output signal as the input digital signal D [3: 0] increases from “0011” → “0100” → “0101” → “0110” → “0111” → “1000”. “0 (0 × FSA / 5)” → “1 × FSA / 5” → “2 × FSA / 5” → “3 × FSA / 5” → “4 × FSA / 5” → “5 × FSA / 5” Increase in the order. On the other hand, the D / A converter DAC2 converts the reverse phase output signal from “5 × FSA / 5” → “4 × FSA / 5” → “3 × FSA / 5” → “2 × FSA / 5” → “1 × FSA”. / 5 ”→“ 0 (0 × FSA / 5) ”. However, when the input digital signal D [3: 0] is “0010” or less, the positive phase output signal of the D / A converter DAC2 is “0 (0 × FSA / 5)”, and the negative phase output signal is “ 5 × FSA / 5 ”. When the input digital signal D [3: 0] is “1001” or more, the positive phase output signal of the D / A converter DAC2 is “5 × FSA / 5”, and the negative phase output signal is “0 (0 × FSA). / 5) ".

The D / A converter DAC3 outputs the positive phase output signal as the input digital signal D [3: 0] increases from “1000” → “1001” → “1010” → “1011” → “1100” → “1101”. “0 (0 × FSA / 5)” → “1 × FSA / 5” → “2 × FSA / 5” → “3 × FSA / 5” → “4 × FSA / 5” → “5 × FSA / 5” Increase in the order. On the other hand, the D / A converter DAC3 converts the reverse phase output signal from “5 × FSA / 5” → “4 × FSA / 5” → “3 × FSA / 5” → “2 × FSA / 5” → “1 × FSA”. / 5 ”→“ 0 (0 × FSA / 5) ”. However, when the input digital signal D [3: 0] is “0111” or less, the positive phase output signal of the D / A converter DAC3 is “0 (0 × FSA / 5)”, and the negative phase output signal is “ 5 × FSA / 5 ”. When the input digital signal D [3: 0] is “1110” or more, the positive phase output signal of the D / A converter DAC3 is “5 × FSA / 5”, and the negative phase output signal is “0 (0 × FSA). / 5) ".

As the input digital signal D [3: 0] increases from “1101” → “1110” → “1111”, the D / A converter DAC4 changes the positive phase output signal from “0 (0 × FSA / 3)” → “ Increase in the order of “1 × FSA / 3” → “2 × FSA / 3”. On the other hand, the D / A converter DAC4 decreases the reverse phase output signal in the order of “3 × FSA / 3” → “2 × FSA / 3” → “1 × FSA / 3”. However, when the input digital signal D [3: 0] is “1100” or less, the positive phase output signal of the D / A converter DAC 4 is “0 (0 × FSA / 3)”, and the negative phase output signal is “ 3 × FSA / 3 ”.

FIGS. 9A and 9B are constellation diagrams showing the modulation operation of the optical transmitter 100. FIG. As shown in FIG. 9A, the D / A converters DAC1 and DAC4 can change the phase modulation amount of the light L1 by FSA / 3 × Δθ and the phase modulation amount of the light L2 by −FSA / 3 × Δθ. The D / A converters DAC2 and DAC3 can change the phase modulation amount of the light L1 by FSA / 5 × Δθ and the phase modulation amount of the light L2 by −FSA / 5 × Δθ.

In this example, the D / A converters DAC1 and DAC4 have fewer gradations than the D / A converters DAC2 and DAC3. Therefore, the gradation intervals of the D / A converters DAC1 and DAC4 are wider than those of the D / A converters DAC2 and DAC3. As a result, the phase change of the optical signal by the D / A converters DAC1 and DAC4 is larger than the phase change of the optical signal by the D / A converters DAC2 and DAC3.

That is, a D / A converter having a large number of gradations is assigned to a D / A converter that handles gradations close to the center gradation of output light, and a D / A converter that handles gradations away from the center gradation of output light is assigned to the D / A converter. By assigning a D / A converter with a small number of exponents, as shown in FIG. 9B, the gradation intervals of the signal intensity of the output light can be made uniform.

As a result, compared to the case where the phase is changed at equal intervals as in the optical transmitter 500, the optical transmitter 100 can make the gradation intervals of the output light uniform. As described above, according to this configuration, it is possible to provide an optical transmitter capable of adjusting the linearity of the signal intensity of output light.

Embodiment 2
Next, an optical transmitter 200 according to the second embodiment of the present invention will be described. An optical transmitter 200 is a modification of the optical transmitter 100 according to the first embodiment. FIG. 10 is a block diagram schematically illustrating a configuration of the optical transmitter 200 according to the second embodiment. The optical transmitter 200 has a configuration in which a storage device 15 is added to the optical transmitter 100 according to the first embodiment.

The storage device 15 has a DAC selection table 16. The control circuit 14 reads the DAC selection table 16 stored in the storage device 15 and outputs the signals D1 to D4 to any of the D / A converters DAC1 to DAC4 based on the DAC selection information in the DAC selection table 16. .

Note that the DAC selection table 16 may be a fixed value stored in advance in the storage device 15. Further, the DAC selection table 16 may be input from the outside to the storage device 15 as initial setting information when the optical transmitter 200 is incorporated into the optical transmission / reception system. Furthermore, it is possible to update the DAC selection table 16 of the storage device 15 from the outside while the optical transmitter 200 is transmitting an optical signal.

According to this configuration, the control circuit 14 can refer to the DAC selection table 16 and assign the respective operation ranges of the D / A converters DAC1 to DAC4 having different gradation numbers to suitable decode values. Become.

Embodiment 3
Next, an optical transmitter 300 according to the third embodiment of the present invention will be described. An optical transmitter 300 is a modification of the optical transmitter 100 according to the first embodiment. FIG. 11 is a block diagram schematically illustrating a configuration of the optical transmitter 300 according to the third embodiment. The optical transmitter 300 has a configuration in which an optical monitor circuit 17 and an arithmetic device 18 are added to the optical transmitter 100 according to the first embodiment.

The optical monitor circuit 17 monitors the output light OUT of the optical modulator 11 and detects the light intensity of the output light OUT. Then, the light monitor circuit 17 outputs a detection signal S _d corresponding to the detected light intensity to the arithmetic device 18.

The computing device 18 calculates the difference between the light intensity of the output light OUT obtained from the detection signal Sd and the expected value of the light intensity corresponding to the value of the input digital signal D [3: 0]. Then, an adjustment instruction signal So corresponding to the calculated difference is output to the control circuit 14. The adjustment instruction signal So includes DAC selection information.

The control circuit 14 outputs signals D1 to D4 to the DAC specified by the adjustment instruction signal So.

According to this configuration, it is possible to appropriately and automatically select the D / A converters DAC1 to DAC4 while monitoring the light intensity of the actual output light OUT. Therefore, it is possible to provide an optical transmitter capable of precisely adjusting the linearity with respect to the signal intensity of the output light OUT.

Embodiment 4
Next, an optical transmission / reception system 400 according to Embodiment 4 of the present invention will be described. The optical transmission / reception system 400 is an optical transmission / reception system using any one of the

optical transmitters

100, 200, and 300 described above. Here, an example in which the optical transmission / reception system 400 includes the optical transmitter 100 will be described. FIG. 12 is a block diagram schematically illustrating a configuration of the optical transmission / reception system 400 according to the fourth embodiment.

The optical transmission / reception system 400 includes an optical transmitter 100, an optical receiver 401, a transmission path 402, and an optical amplifier 403.

The optical transmitter 100 outputs, as an optical signal, a QPSK optical signal that has been subjected to, for example, quadrature phase shift keying (hereinafter, referred to as QPSK).

The optical transmitter 100 and the optical receiver 401 are optically connected by an optical transmission path 402, and a QPSK optical signal propagates. An optical amplifier 403 is inserted into the transmission line 402 and amplifies a QPSK optical signal propagating through the transmission line 403. The optical receiver 401 demodulates the QPSK optical signal into an electrical signal.

The optical transmission / reception system 400 can transmit an optical signal using the optical transmitter 100 with the above configuration. Of course, the optical transmitter 100 can be appropriately replaced with the

optical transmitter

100 or 200.

Embodiment 5
Next, a fifth embodiment of the present invention will be described. In the first to third embodiments described above, the example of improving the linearity of the output light has been described, but the method for adjusting the linearity of the output light is not limited to this. In the present embodiment, another method of adjusting the linearity of output light using the

optical transmitters

100, 200, and 300 according to the first to third embodiments will be described.

For example, in an optical transmission / reception system or the like, pre-equalization processing may be performed on an optical signal transmitted from an optical transmitter when performing long-distance transmission. In this case, the optical signal subjected to the pre-equalization process has a high probability that a component having a medium amplitude appears. In such a case, it is possible to perform signal processing with a small number of bits by processing a component with a high appearance probability with high accuracy and a component with a low appearance probability with low accuracy.

FIG. 13 is a diagram showing the waveform and appearance probability of the pre-equalized optical signal. When the pre-equalization signal as shown in FIG. 13 is subjected to optical signal processing with, for example, 4-bit gradation, the appearance probability near the center of the signal, that is, near “1000” increases. On the other hand, the appearance probabilities near both ends of the signal, that is, near “0000” and “1111” are reduced.

FIG. 14A is a graph showing a gradation change of 4-bit output light having general linear characteristics. FIG. 14B is a graph showing a gradation change of 4-bit output light having nonlinear characteristics in which the gradation width near “1000” becomes narrow. As shown in FIG. 14B, the gradation change of the output light is not quantized with the linear state characteristic shown in FIG. 14A, but nonlinear characteristics such that the gradation width near “1000” shown in FIG. By performing the quantization, the processing near “1000” can be performed with high accuracy. As a result, when a signal as shown in FIG. 13 is handled, it is possible to perform highly accurate signal processing by giving a nonlinear characteristic as shown in FIG. 14B even with the same 4-bit gradation.

As described above, the linearity with respect to the signal intensity of the output light can be adjusted by using the adjustment method of the present invention even for the purpose of giving the signal nonlinearity. Therefore, the

optical transmitters

100, 200, and 300 according to the first to third embodiments give the output light OUT non-linearity as shown in FIG. 13 to reduce processing bits depending on the communication method. And high-precision processing can be realized.

Other Embodiments The present invention is not limited to the above-described embodiments, and can be appropriately changed without departing from the spirit of the present invention. For example, since the phase change of light can be added regardless of the order of the change, the selection pattern of the D / A converter is not limited to the above example. In other words, the order of the D / A converters DAC1 to DAC4 can be arbitrarily changed.

In the above-described embodiment, the

optical transmitters

100, 200, and 300 have been described as 4-bit optical transmitters, but this is merely an example. That is, it goes without saying that an optical transmitter capable of higher-order multilevel modulation can be configured by increasing the number of phase modulation regions (divided electrodes), the number of D / A converters, and the number of gradations.

In the third embodiment, the example in which the light intensity of the output light is monitored and the signal to be supplied to the D / A converter is selected, but this is only an example. That is, the light intensity of the output light may be monitored by the optical receiver, and the light intensity information may be fed back from the optical receiver to the optical transmitter. Further, the arithmetic device 18 may be incorporated in the optical transmitter.

The present invention has been described above with reference to the embodiment, but the present invention is not limited to the above. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the invention.

This application claims priority based on Japanese Patent Application No. 2012-064769 filed on Mar. 22, 2012, the entire disclosure of which is incorporated herein.

11, 51

Optical modulator

12, 52

Decoder

13, 53 Drive circuit 14 Control circuit 15 Storage device 16 DAC selection table 17 Optical monitor circuit 18

Arithmetic device

100, 200, 300, 500

Optical transmitters

111, 112, 511, 512

Optical Waveguide

113, 114, 513, 514 Optical multiplexer / demultiplexer 400 Optical transceiver system 401 Optical receiver 402 Transmission path 403 Optical amplifiers DAC1 to DAC4, DAC51 to DAC54 D / A converter OUT Output light PM11_1 to PM11_4, PM12_1 to PM12_4, PM51_1 to PM51_4, PM52_1 ~ PM52_4 the phase modulation region _{S d} detection signal So adjustment instruction signal

Claims

An optical modulator having an optical transmission path through which an optical signal propagates, in which a plurality of phase modulation regions are formed;
A decoder that decodes the input digital signal and outputs a signal corresponding to the decoded value;
A drive circuit that outputs a drive signal of three or more gradations including a different number of gradations to each of the plurality of phase modulation regions based on a signal according to the decode value;
A control circuit that specifies a correspondence relationship of the plurality of drive signals with respect to the decode value by controlling the drive circuit;
Optical transmitter.
Among the plurality of drive signals output from the drive circuit, the control circuit is output to a phase modulation region that performs modulation at a gradation close to the center of the modulation gradation number represented by the input digital signal. The drive circuit is controlled to increase the number of gradations,
The optical transmitter according to claim 1.
The control circuit is output to a phase modulation region that performs modulation at a gradation having a high appearance probability among the modulation gradation numbers represented by the input digital signal among the plurality of drive signals output from the drive circuit. The drive circuit is controlled so that the number of gradations increases as
The optical transmitter according to claim 1.
The drive circuit is
A plurality of multi-level DACs that output the drive signal to each of the plurality of phase modulation regions, including ones having different numbers of gradations,
The control circuit specifies an operation range of the plurality of multi-level DACs with respect to the decoded value,
The optical transmitter according to any one of claims 1 to 3.
A storage device storing operation range specification information for each of the decode values of the plurality of DACs;
The control circuit designates an operation range for the decode values of the plurality of DACs based on the designation information.
The optical transmitter according to claim 4.
An optical monitor circuit for detecting the light intensity of the output light from the optical modulator;
An arithmetic unit that outputs an adjustment instruction signal based on the difference between the light intensity detected by the light monitor circuit and the expected value of the light intensity obtained from the input digital signal;
The control circuit designates an operation range for the decoded values of the plurality of DACs based on the adjustment instruction signal.
The optical transmitter according to claim 4.
An optical transmitter for transmitting an optical signal;
A transmission path through which the optical signal propagates;
An optical receiver for receiving the optical signal via the transmission path,
The optical transmitter is
An optical modulator having an optical transmission path through which an optical signal propagates, in which a plurality of phase modulation regions are formed;
A decoder that decodes the input digital signal and outputs a signal corresponding to the decoded value;
A drive circuit that outputs a drive signal of three or more gradations including a different number of gradations to each of the plurality of phase modulation regions based on a signal according to the decode value;
A control circuit that specifies a correspondence relationship of the plurality of drive signals with respect to the decode value by controlling the drive circuit;
Optical transmission / reception system.
Among the plurality of drive signals output from the drive circuit, the control circuit is output to a phase modulation region that performs modulation at a gradation close to the center of the modulation gradation number represented by the input digital signal. The drive circuit is controlled to increase the number of gradations,
The optical transmission / reception system according to claim 7.
The control circuit is output to a phase modulation region that performs modulation at a gradation having a high appearance probability among the modulation gradation numbers represented by the input digital signal among the plurality of drive signals output from the drive circuit. The drive circuit is controlled so that the number of gradations increases as
The optical transmission / reception system according to claim 7.
The drive circuit is
A plurality of multi-level DACs that output the drive signal to each of the plurality of phase modulation regions, including ones having different numbers of gradations,
The control circuit specifies an operation range of the plurality of multi-level DACs with respect to the decoded value,
The optical transmission / reception system according to any one of claims 7 to 9.
A storage device storing operation range specification information for each of the decode values of the plurality of DACs;
The control circuit designates an operation range for the decode values of the plurality of DACs based on the designation information.
The optical transmission / reception system according to claim 10.
An optical monitor circuit for detecting the light intensity of the output light from the optical modulator;
An arithmetic unit that outputs an adjustment instruction signal based on the difference between the light intensity detected by the light monitor circuit and the expected value of the light intensity obtained from the input digital signal;
The control circuit designates an operation range for the decoded values of the plurality of DACs based on the adjustment instruction signal.
The optical transmission / reception system according to claim 10.
Based on a signal corresponding to the decoded value of the input digital signal, a drive signal of three or more gradations including a plurality of gradations in each of the plurality of phase modulation regions formed in the optical waveguide provided in the optical modulator A plurality of DACs that can specify an operation range for the decoded value by an external control circuit,
Driving circuit.
Among the plurality of DACs, the number of gradations increases as the drive signal is output to a phase modulation region that performs modulation at a gradation close to the center of the modulation gradation number represented by the input digital signal. To
The drive circuit according to claim 13.
Of the plurality of DACs, the number of gradations increases as the drive signal is output to the phase modulation region that performs modulation at a gradation having a high appearance probability among the modulation gradations represented by the input digital signal. Characterized by the
The drive circuit according to claim 13.
An operation range for each of the decode values of the plurality of DACs is designated based on designation information for the decode values of the plurality of DACs stored in an external storage device.
The drive circuit according to any one of claims 13 to 15.
Based on the adjustment instruction signal output from the arithmetic unit based on the difference between the light intensity of the output light from the optical modulator detected by the optical monitor circuit and the expected value of the light intensity obtained from the input digital signal, An operation range for each of the decode values of a plurality of DACs is designated.
The drive circuit according to any one of claims 13 to 15.