JP4809270B2 - Optical transmission apparatus and method - Google Patents

Description

  The present invention relates to an optical transmission apparatus and method.

  With the recent increase in demand for data transmission, an optical fiber communication system exceeding 100 Gb / s is desired. As an optical modulation system that realizes such high-speed communication, a multi-level modulation technique has attracted attention. In particular, differential quadrature phase shift modulation, so-called DQPSK (Differential Quadrature Phase Shift-Keying) modulation, has high resilience due to a certain envelope characteristic with respect to nonlinearity of the optical fiber and high spectral efficiency. There are advantages such as few electronic devices to be used and a large tolerance for chromatic dispersion and polarization mode dispersion. DQPSK modulation is a promising modulation scheme for Dense Wavelength Division Multiplexing (DWDM) systems and high-speed optical networks with high spectral efficiency.

Non-Patent Document 1 describes a DQPSK direct detection method in an optical communication system. DQPSK direct detection is a quaternary phase modulation method, and 2 bits can be transmitted in one symbol. That is, the symbol rate is half the bit rate B.
RA Griffin and AC Carter, "Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission," in Optical Fiber Communications Conference (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 2002), Paper WX6, 2002.

  A high transmission rate can be realized by introducing a multilevel optical modulation technique such as DQPSK modulation. However, even in that case, the transmission rate is mainly limited by the response speed of the electric circuit.

  An object of the present invention is to provide an optical transmission apparatus and method capable of realizing a higher bit rate while using a relatively low-speed electric circuit.

The optical transmission apparatus according to the present invention precodes 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate of B / 2N (N is an integer equal to or greater than 2). Precoders for generating _{N first} modulation signals I _{1 to} I _N and N second modulation signals Q _{1 to} Q _N having a bit rate B / 2N, and the N first modulation signals I ₁ the first phase interleaver phase interleaving ~I _N in 2N / B period, a second phase inter phase interleaving the N second modulation signal _Q 1 to Q _N in 2N / B period A reverber, a coherent light source that generates coherent light, an optical demultiplexer that divides the output light of the coherent light source into two, first and second arms that respectively propagate divided light by the optical demultiplexer, and the first And the light from the second arm An optical IQ multiplexer including a wave-separating optical multiplexer and a phase shifter that shifts the phase of the light on the second arm by π / 2, and N serially connected units arranged on the first arm _{N first} phase modulators that phase-modulate input light according to the _{N first} modulation signals I _{1 to} I _N phase-interleaved by the first phase interleaver. A phase modulator and serially connected N second phase modulators disposed on the second arm, the N second phase modulators interleaved by the second phase interleaver And _N second phase modulators that phase-modulate the input light in accordance with the modulation signals Q _{1 to} Q _N.

The optical transmission method according to the present invention pre-codes 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate B / 2N (N is an integer equal to or greater than 2) to generate a bit rate B / 2N. N first modulated signals I _{1 to} I _N and N second modulated signals Q _{1 to} Q _N having a bit rate B / 2N are generated, and the N first modulated signals I ₁ are generated. to phase interleaving ~I _N in 2N / B period, the N first modulation signal _Q 1 to Q _N to phase interleaved into 2N / B period, the coherent light from the coherent light source in-phase ( I) The components are sequentially phase-modulated according to the _{N first} modulated signals I _{1 to} I _N that have been phase-interleaved to generate an I-component optical signal, and the orthogonal (Q) component of the coherent light For the N phase interleaved In accordance with the second modulation signals Q _{1 to} Q _N , sequentially phase-modulating to generate a Q component optical signal, and combining the I component optical signal and the Q component optical signal Features.

The optical transmission apparatus according to the present invention precodes 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate of B / 2N (N is an integer equal to or greater than 2). Precoders for generating _{N first} modulation signals I _{1 to} I _N and N second modulation signals Q _{1 to} Q _N having a bit rate B / 2N, and the N first modulation signals I ₁ the first phase interleaver phase interleaving ~I _N in 2N / B period, a second phase inter phase interleaving the N second modulation signal _Q 1 to Q _N in 2N / B period A Lever, a coherent light source that generates coherent light, an I component processing system that phase-modulates the output light of the coherent light source by (0, π), and an output light of the I component processing system (0, π / 2) And a Q component processing system for phase modulation at I component processing system, in accordance with the first of the first of the N number of phase has been interleaved by the phase interleaver modulation signal I ₁ ~I _N, are serially connected to the phase modulation at each input light (0, [pi) N first phase modulators, and the Q component processing system is phase-interleaved by the second phase interleaver (330). The N second modulated signals Q _{1 to} Q _N are interleaved by the second phase interleaver (330). And N serially connected second phase modulators each phase-modulating input light by (0, π / 2).

The optical transmission method according to the present invention precodes 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate B / 2N, and generates _N first modulated signals having a bit rate B / 2N. Generating _N second modulation signals Q _{1 to} Q _N having I _{1 to} I _N and a bit rate B / 2N, and the N first modulation signals I _{1 to} I _N within a 2N / B period Phase interleaving, phase interleaving the N first modulated signals Q _{1 to} Q _N within a period of 2N / B, and the N _first phase-interleaved coherent light from the coherent light source. Are sequentially phase-modulated by (0, π) in accordance with the modulation signals I _{1 to} I _N of the _N , and are further sequentially (0, π) in accordance with the _N second modulated signals Q _{1 to} Q _N that are phase-interleaved. / 2) phase modulation.

  According to the present invention, in order to generate an optical signal with a bit rate B, it is only necessary to operate the electrical stage at B / 2N, and an optical signal with a high bit rate can be generated with a low-speed electric circuit.

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

  FIG. 1 shows a schematic block diagram of a data transmission system according to an embodiment of the present invention applied to an NRZ-DQPSK system.

  The optical transmission device 10 generates a DQPSK signal So having a bit rate B from four data signals U1, U2, V1, and V2 having a bit rate B / 4, and outputs the DQPSK signal So to the optical transmission line 12. The optical receiver 14 receives the optical signal So input from the optical transmission line 12, and restores the data signals U1, U2, V1, and V2. In this embodiment, the bit rate of each signal is indicated after the notation so that the rate of each signal can be easily understood. For example, in this embodiment, the bit rate of the data signals U 1, U 2, V 1, V 2 is ¼ with respect to the bit rate B on the optical transmission line 12.

  In the optical transmitter 10, the laser diode 20 generates coherent laser light that is carrier light for carrying data. The output laser light from the laser diode 20 is input to the optical IQ multiplexer 22 having a Mach-Zehnder interferometer structure.

  The optical IQ multiplexer 22 includes an optical demultiplexer 24, arms 26 a and 26 b, and an optical multiplexer 28. Two phase modulators 30 and 32 are serially arranged on the arm 26a. On the other arm 26 b, two phase modulators 34 and 36 and a phase shifter 38 that shifts the optical phase by π / 2 with respect to the laser wavelength of the laser diode 20 are serially arranged. If the optical carrier on the arm 26a is, for example, an in-phase component, so-called I component, by the phase shifter 38, the optical carrier on the arm 26b becomes a quadrature component, so-called Q component.

Each phase modulator 30, 32, 34, 36 modulates the phase of the input light by (0, π) according to the binary value of the applied modulation signal. The phase modulators 30, 32, 34, and 36 preferably have a Mach-Zehnder interferometer structure in terms of phase modulation characteristics. Further, by employing a phase modulator having a Mach-Zehnder interferometer structure, there is an advantage that the optical IQ multiplexer 22 can be manufactured integrally. For the MZI phase modulator, the phase modulators 30, 32, 34, 36 are biased at null bias point by adjusting the corresponding DC bias, is driven by the peak-to-peak driving voltage of 2 × V _[pi.

The precoder 40 precodes the data signals U1, U2, V1, and V2 having the bit rate B / 4 for DQPSK in the phase modulators 30, 32, 34, and 36, and outputs the phase modulators 30, 32, 34, and 36. Modulation signals (driving signals) I ₁ , I ₂ , Q ₁ , and Q ₂ with a bit rate B / 4 are generated.

In this embodiment, modulated signals or drive signals I ₁ and I ₂ are applied to the phase modulators 30 and 32 that are serially connected to the I component, with the phase shifted. That is, phase interleaving is applied. Similarly, the modulation signals (drive signals) Q ₁ and Q ₂ are also applied to the phase modulators 34 and 36 that are serially connected to the Q component, with the phase shifted. For phase interleaving, the modulation signal I ₂ is delayed by ΔT by the delay circuit 42 and applied to the phase modulator 32, and the modulation signal Q ₂ is delayed by ΔT by the delay circuit 44 to the phase modulator 36. Applied. The delay time ΔT of the delay circuits 42 and 44 is equal to 2 / B (seconds).

The delay circuit 42 functions as a phase interleaver that distributes the modulation signals I ₁ and I ₂ in a 4 / B period. The delay circuit 44 functions as a phase interleaver that distributes the modulation signals Q ₁ and Q ₂ in a 4 / B period.

  The optical multiplexer 28 combines the output light from the phase modulator 32 and the output light from the phase shifter 38. The multiplexed light So is DQPSK signal light having a bit rate B, and is output to the optical transmission line 12. The phase modulators 30, 32, 34, and 36 need only operate at a rate that is ¼ of the final bit rate B, and thus the cost of the optical transmission device 10 can be greatly reduced. For example, when the final transmission rate B is 20 Gbps, each phase modulator 30, 32, 34, 36 only needs to be operable at 5 Gbps.

  2A to 2F show timing charts of the phase modulators 30, 32, 34, and 36. FIG. Here, for easy understanding, each phase modulator 30, 32, 34, 46 is input when the applied modulation signal or drive signal is high (binary value “1”). A device that relatively shifts the optical phase of the optical signal of light by π, and outputs the optical phase of the optical signal of the input light as it is when it is low (binary value “0”). To do.

2 (A) shows an example of the waveform of the modulation signal _{I 1.} Here, the modulation signal I ₁ is “000101” as a binary value, and one bit period is 4 / B (seconds). The relative optical phase change in the phase modulator 30 is 0, 0, 0, π, 0, π. Figure 2 (B) shows an example of the waveform of the modulation signal _{I 2.} Here, the modulation signal I ₂ is “010001” as a binary value, and the 1-bit period is 4 / B (seconds), and is delayed by 2 / B with respect to the drive signal I ₁ by the delay circuit 42. The relative optical phase change in the phase modulator 32 is 0, π, 0, 0, 0, π. 2C shows an example of a phase waveform of light that is phase-modulated by the phase modulator 30 as shown in FIG. 2A and phase-modulated by the phase modulator 32 as shown in FIG. Indicates. When both phase modulators 30 and 32 receive a phase shift of π, the total becomes 2π and returns to 0. The 1-bit period of the output signal light of the phase modulator 32 becomes 2 / B due to phase interleaving, and becomes the bit rate B / 2 of the output signal light of the phase modulator 32.

Similarly, FIG. 2 (D) shows a waveform example of the modulation signal _{Q 1.} Here, the modulation signal Q ₁ is “001011” as a binary value, and one bit period is 4 / B (seconds). The relative optical phase change in the phase modulator 34 is 0, 0, π, 0, π, π. Figure 2 (E) shows a waveform example of the modulation signal _{Q 2.} Here, the modulation signal Q ₂ is “101010” as a binary value, and a 1-bit period is 4 / B (seconds), and is delayed by 2 / B with respect to the drive signal Q ₁ by the delay circuit 44. The relative optical phase change in the phase modulator 36 is π, 0, π, 0, π, 0. FIG. 2F shows an example of a phase waveform of light that is phase-modulated by the phase modulator 34 as shown in FIG. 2D and phase-modulated by the phase modulator 36 as shown in FIG. Indicates. When both of the phase modulators 34 and 36 receive a phase shift of π, the total becomes 2π and returns to 0. The 1-bit period of the output signal light from the phase modulator 36 becomes 2 / B due to phase interleaving, similarly to the output signal light from the phase modulator 32, or the bit rate B / 2 of the output signal light from the phase modulator 36. become.

In this embodiment, for each of the I component and Q component, the phase modulators 30, 32; 34, 36 are serially connected, so that the signal to be transmitted is phase-interleaved or bit-interleaved. In order to electrically generate a modulation signal of a short bit period after phase interleaving, a high-speed electric circuit is required. In this embodiment, a plurality of phase modulators are connected in series to achieve the same effect. Realized and does not require a high-speed electrical circuit. However, the precoder 40 needs to generate the modulation signals I ₂ and Q ₂ in consideration of the influence of the phase modulation by the phase modulator 30. In general, when N-stage phase modulators are serially connected, the modulation signal to the i-th phase modulator is the phase from the first stage to the (i-1) -th stage. It is determined in consideration of the modulation signal to the modulator.

  The phase shifter 38 shifts the optical phase of the output signal light from the phase modulator 36 by π / 2. Thereby, the output signal light of the phase modulator 32 and the output signal light of the phase shifter 38 are orthogonal to each other.

  The optical multiplexer 28 combines the output signal light of the phase modulator 32 and the output signal light of the phase shifter 38 in a quadrature phase state. The bit rate of the output light So from the optical multiplexer 28 is rate B.

  The configuration and operation of the optical receiver 14 will be described. The signal light So may be obtained by separating the DQPSK signal transmitted by the I component and the DQPSK signal transmitted by the Q component, and separating each DQPSK signal on the time axis.

  The optical demultiplexer 50 divides the optical signal from the optical transmission path 12 into two powers, supplies one to an MZ-Zehnder Delay Interferometer (MZDI) 52, and supplies the other to an MZ delay interferometer 54. To supply. The MZ delay interferometer 52 includes a delay circuit 52a of one bit period Δτ (= 2 / B) on one arm and a + π / 4 phase shifter 52b on the other arm. Similarly, the MZ delay interferometer 54 includes a delay circuit 54a of one bit period Δτ (= 2 / B) on one arm and a −π / 4 phase shifter 54b on the other arm. The MZ delay interferometer 52 functions as means for converting an I-component DQPSK optical signal into an OOK (On-Off-Keying) optical signal, and the MZ delay interferometer 54 converts a Q-component DQPSK optical signal into an OOK optical signal. It functions as a means for conversion.

  The MZ delay interferometer 52 includes a constructive port and a destructive port as output ports. A balanced receiver 56 comprising two light receiving elements connected in series receives the balanced output light from the constructive port of the MZ delay interferometer 52 and the output light from the destructive port, and receives a data signal U of bit rate B / 2. To restore. The data signal U is an electrical signal that carries the binary value shown in FIG.

  Similarly, the MZ delay interferometer 54 includes a constructive port and a destructive port as output ports. A balanced receiver 58 composed of two light receiving elements connected in series receives the balanced output light from the constructive port and the output light from the destructive port of the MZ delay interferometer 54, and receives the data signal V of bit rate B / 2. To restore. The data signal V is an electric signal that carries the binary value shown in FIG.

  The electric separation device 60 separates the output electric signal of the balanced receiver 56 into two signals U1 and U2 for each bit, and outputs the separated signals U1 and U2 at the bit rate B / 4. The electrical separator 62 separates the output electrical signal of the balanced receiver 58 into two signals V1 and V2 for each bit, and outputs the separated signals V1 and V2 at the bit rate B / 4.

  In this way, the optical receiver 14 can restore the signals U1, U2, V1, and V2 that are input to the precoder 40 of the optical transmitter 10. The precoder so that the I component DQPSK optical signal of the output optical signal of the optical transmitter 10 carries U1 and U2 by bit interleaving and the Q component DQPSK optical signal carries signals V1 and V2 by bit interleaving. By designing 40, the configuration of the optical receiver 14 is simplified.

  In the optical receiver 14, the bit interleaved signals U1, U2; V1, V2 are separated by the electrical stage, but may be separated by the optical stage. FIG. 3 shows a schematic block diagram of an optical receiver 14a that separates the signals U1, U2; V1, V2 at the optical stage.

  The optical demultiplexer 70 power-divides the optical signal from the optical transmission line 12 into two, supplies one to the MZ delay interferometer 72, and supplies the other to the MZ delay interferometer 74. The MZ delay interferometer 72 includes a delay circuit 72a of one bit period Δτ (= 2 / B) on one arm, and a + π / 4 phase shifter 72b on the other arm. Similarly, the MZ delay interferometer 74 includes a delay circuit 74a of one bit period Δτ (= 2 / B) on one arm and a −π / 4 phase shifter 74b on the other arm. The MZ delay interferometer 72 causes the output light of the delay circuit 72a and the output light of the phase shifter 72b to interfere with each other, thereby converting the I component DQPSK optical signal into an OOK optical signal. The MZ delay interferometer 74 causes the output light of the delay circuit 74a and the output light of the phase shifter 74b to interfere with each other, thereby converting the Q component DQPSK optical signal into an OOK optical signal.

  The optical separation device 76 separates the optical signal carrying the signal U1 and the optical signal carrying the signal U2 from the output optical signal of the MZ delay interferometer 72 in a 2 / B cycle on the time axis. Similarly, the optical separation device 78 separates the optical signal carrying the signal V1 and the optical signal carrying the signal V2 from the output optical signal of the MZ delay interferometer 74 in a 2 / B cycle on the time axis.

  In the optical separation device 76, the optical demultiplexer 80 divides the output light of the MZ delay interferometer 72 into two, and supplies one to the electroabsorption optical modulator 82 and the other to the electroabsorption optical modulator 84. The clock generator 86 generates a clock having a frequency B / 4 and a duty ratio of 50%, and applies it to the electroabsorption optical modulator 82 as a drive signal. The inverting circuit 88 inverts the output clock of the clock generator 86 and applies it to the electroabsorption optical modulator 84 as a drive signal. The electroabsorption optical modulators 82 and 84 transmit the input light during a period in which a pulse is applied as a drive signal. With this configuration, two bit-interleaved signals can be separated.

  The light separation device 78 has the same configuration as the light separation device 76, and separates the optical signal carrying the signal V1 and the optical signal carrying the signal V2 from the output optical signal of the MZ delay interferometer 74.

  The light receiver 90 converts one output optical signal of the light separating device 76 into an electrical signal. The light receiver 92 converts one output optical signal of the light separating device 76 into an electrical signal. The output of the light receiver 90 becomes the signal U1 of the bit rate B / 4, and the output of the light receiver 92 becomes the signal U2 of the bit rate B / 4. Since the RZ waveform is maintained as it is, an RZ / NRZ conversion circuit, a latch circuit, or the like may be disposed after the light receivers 90 and 92. By performing feedback control of the frequency or phase of the clock generator 86 according to the amplitude or error rate of the output signal of the light receiver 90 or 92, the operation of the light separating device 76 is stabilized.

  The light receiver 94 converts one output optical signal of the light separating device 78 into an electric signal. The light receiver 92 converts one output optical signal of the light separating device 78 into an electric signal. The output of the light receiver 94 becomes the signal V1 of the bit rate B / 4, and the output of the light receiver 96 becomes the signal V2 of the bit rate B / 4. Since the RZ waveform is maintained as it is, an RZ / NRZ conversion circuit or a latch circuit may be disposed after the light receivers 94 and 96. The operation of the light separating device 78 is stabilized by feedback controlling the frequency or phase of the clock generator 86 of the light separating device 78 in accordance with the amplitude or error rate of the output signal of the light receiver 94 or 96.

  Obviously, a balanced receiver configuration may be used as the light receivers 90, 92, 94, 96.

4, FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9 and FIG. The waveform was measured with the NRZ-DPSK signal for the convenience of the measuring instrument. FIGS. 4A and 4B show examples of waveforms of modulated signals I ₁ and I ₂ of 5.0 Gbps, respectively. FIGS. 5A and 5B show examples of waveforms of modulated signals Q ₁ and Q ₂ of 5.0 Gbps, respectively. 4 and 5, the horizontal axis indicates time, and the vertical axis indicates amplitude.

FIG. 6 shows an example of the amplitude and phase of the DQPSK signal light generated by the modulation signals I ₁ , I ₂ , Q ₁ , and Q ₂ shown in FIGS. 4 and 5, that is, the output signal light So. FIG. 6A shows changes in light intensity. In FIG. 6A, the horizontal axis indicates time, and the vertical axis indicates light intensity. FIG. 6B shows a change in the optical phase. In FIG. 6B, the horizontal axis represents time, and the vertical axis represents the optical phase (radian).

In the example of the data string of the modulation signals I ₁ , I ₂ , Q ₁ , and Q ₂ shown in FIG. 2, the final I component and Q component phase patterns are shown in FIGS. 2 (C) and (F), respectively. As shown, the bit rates B / 2 are “00110110010” and “11010101001”. After combining the I component and the Q component, the combinations of the I component and the Q component are (0, 1), (0, 1), (1, 0), (1, 1), (0, 0). , (1,1), (1,0), (0,1), (0,0) and (1,0) are transmitted. FIG. 6B shows the combination of the I component and Q component transport data. The drop in light intensity at the phase transition boundary is due to the use of a dual drive MZ modulator.

  FIG. 7 shows an eye pattern of the output signal So. The horizontal axis indicates time, and the vertical axis indicates optical power (arbitrary scale). You can see that the eyes are open enough.

  8A shows an example of the waveform of the reception signal U of the balanced receiver 56, and FIG. 8B shows an example of the waveform of the reception signal V of the balanced receiver 58. The horizontal axis indicates time, and the vertical axis indicates amplitude. FIG. 9A shows the eye pattern of the received signal U, and FIG. 9B shows the eye pattern of the received signal V. FIG. 10 shows an example of an optical spectrum of a 20 Gbps DQPSK optical signal generated by the optical transmitter 10. The spectral band of 20 dB was 15 GHz.

  In the optical transmission device 10 shown in FIG. 1, the I component processing system and the Q component are arranged in parallel, but they may be arranged serially. FIG. 11 shows a schematic block diagram of an optical transmission apparatus 110 having a configuration in which an I component processing system and a Q component are serially connected.

  In the optical transmission device 110, (0, π) phase modulation is used for the I component processing system 122, and (0, π / 2) phase modulation is used for the Q component processing system 124.

The laser diode 120 generates coherent laser light serving as carrier light for carrying data. The I component processing system 122 is composed of two phase modulators 122 ₁ and 122 _{2 of} (0, π) phase modulation connected in series. The Q component processing system 124 is composed of two phase modulators 124 ₁ and 124 _{2 of} (0, π / 2) phase modulation connected in series.

The output laser beam of the laser diode 120 is inputted to the first phase modulator 122 the _first processing system 122 of the I component. The output light of the phase modulator 122 ₁ inputs the second phase modulator 122 _2. The output light of the phase modulator 122 _2, and inputs to the first phase modulator 124 the _first Q component of treatment system 124. The output light of the phase modulator 124 ₁ inputs the second phase modulator 124 _2. Of the output light phase modulator 124 _2, the final output optical signal So, and the output to the optical transmission line.

The phase modulators 122 ₁ and 122 ₂ modulate the phase of the input light by (0, π) according to the binary values of the applied modulation signals I ₁ and I ₂ . The phase modulators 124 ₁ and 124 ₂ modulate the phase of the input light by (0, π / 2) according to the binary values of the applied modulation signals Q ₁ and Q ₂ . As the phase modulators 122 ₁ and 122 ₂ , a phase modulator having a Mach-Zehnder interferometer structure can be used.

The precoder 126 precodes the data signals U1, U2, V1, and V2 of the bit rate B / 4 for DQPSK in the phase modulators 122 ₁ , 122 ₂ , 124 ₁ , and 124 ₂ , and the phase modulators 122 ₁ , 122 _{2, 124} 1, 124 modulated signals for _two (drive _current) to generate a I _1, I _2, Q 1, _{Q 2.}

For the same phase interleaving as in the embodiment shown in FIG. 1, the delay circuit 128 having a delay time ΔT (= 2 / B) delays the modulation signal I ₂ from the precoder 126 by ΔT to the phase modulator 122 ₂ . Apply. The delay circuit 128 functions as a phase interleaver that distributes the modulation signals I ₁ and I ₂ in a 4 / B period. Further, the delay circuit 130 having the delay time ΔT (= 2 / B) delays the modulation signal Q ₂ from the precoder 126 by ΔT and applies it to the phase modulator 124 ₂ . The delay circuit 130 functions as a phase interleaver that distributes the modulation signals Q ₁ and Q ₂ in a 4 / B period.

  The optical signal So output from the optical transmitter 110 can be received by any of the configurations of the optical receiver 14 shown in FIG. 1 and the optical receiver 14a shown in FIG.

  Implementation of a general configuration in which an I-component processing system and a Q-component processing system are connected in parallel, and N (N is an integer of 3 or more) phase modulators are serially connected in each processing system. Example 3 will be described. FIG. 12 shows a schematic block diagram of the third embodiment.

12 multiplexes 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate B / 2N to generate a DQPSK signal So having a bit rate B, and performs optical transmission. Output to the road.

  In the optical transmitter 210, the laser diode 220 generates coherent laser light serving as carrier light for carrying data. The laser beam output from the laser diode 220 is input to the optical IQ multiplexer 222 having a Mach-Zehnder interferometer structure.

The optical IQ multiplexer 222 includes an optical demultiplexer 224, arms 226a and 226b, and an optical multiplexer 228. N phase modulators 230 _{1 to} 230 _N are serially arranged on the arm 226a. N phase modulators 232 _{1 to} 232 _N are serially arranged on the other arm 226b. A phase shifter 234 that shifts the optical phase by π / 2 with respect to the laser wavelength of the laser diode 220 is further disposed on the arm 226b. If the optical carrier on the arm 226a is, for example, an in-phase component, so-called I component, by the phase shifter 234, the optical carrier on the arm 226b becomes a quadrature component, so-called Q component.

Each of the phase modulators 230 _{1 to} 230 _N and 232 _{1 to} 232 _N is input light at (0, π) according to the binary value of the applied modulation signal, similarly to the phase modulators 30, 32, 34, and 36. Modulate the phase of. The phase modulators 230 _{1 to} 230 _N and 232 _{1 to} 232 _N preferably have a Mach-Zehnder interferometer structure in view of phase modulation characteristics. Further, by employing a phase modulator having a Mach-Zehnder interferometer structure, there is an advantage that the optical IQ multiplexer 222 can be manufactured integrally. For the MZI phase modulator, the phase modulators _{230 1 ~230 N, 232 1 ~232} N is biased at null bias point by adjusting the corresponding DC bias, a peak-to-peak driving voltage of 2 × V _[pi Driven. The phase modulators 230 _{1 to} 230 _N and 232 _{1 to} 232 _N may be operated at the bit rate B / 2N.

Precoder 236, precoded for DQPSK at bit rate B / 2N of the data signal _U 1 _{~U N,} the phase modulator ₂₃₀ to _{V 1 ~V N 1 ~230 N,} 232 1 ~232 N, phase modulation vessel _{230 1 ~230 N, 232 1 ~232} N bit rate B / 2N modulated signals for (drive _signal) I ₁ ~I _N, to generate the Q 1 to Q _N.

In this embodiment, the modulation signals I _{1 to} I _N from the precoder 236 are phase-interleaved by the delay circuit 238 (238 _{1 to} 238 _N ) and applied to the phase modulators 230 _{1 to} 230 _N on the arm 226a. The Similarly, the modulation signals Q _{1 to} Q _N from the precoder 236 are phase-interleaved by the delay circuit 240 (240 ₁ to 240 _N ) and applied to the phase modulators 232 _{1 to} 232 _N on the arm 226b. The delay time ΔT _i of the delay circuits 238 _i , 240 _i for delaying the modulation signals I _i , Q _i , where i = 1 to N, is 2 (i−1) / B. Although the delay circuits 238 ₁ and 240 ₁ are actually omitted, they are shown for logical consistency. The DQPSK optical signals generated by the phase modulators 230 _{1 to} 230 _N carry _N data signals indicated by the modulation signals I _{1 to} I _N by time division multiplexing. Similarly, DPQSK optical signals generated by the phase modulators 232 _{1 to} 232 _N carry _N data signals indicated by the modulation signals Q _{1 to} Q _N by time division multiplexing.

The delay circuit 238 (238 _{1 to} 238 _N ) functions as a phase interleaver that distributes the modulation signals I _{1 to} I _N at 2 / B intervals in a 2N / B period. Similarly, the delay circuit 240 (240 ₁ to 240 _N ) functions as a phase interleaver that disperses and arranges the modulation signals Q _{1 to} Q _N at 2 / B intervals in 2N / B periods.

The optical multiplexer 228 and an output light of the phase modulator 230 _N, the output light of the phase shifter 234 multiplexing. The multiplexed light So is DQPSK signal light having a bit rate B, and is output to the optical transmission line.

  The separation characteristics of the electrical separation devices 60 and 62 of the optical receiver 14 shown in FIG. 1 are changed to a configuration expanded from 1: 2 to 1: N, or the optical separation device 76 of the optical receiver 14a shown in FIG. , 78 is expanded from 1: 2 to 1: N, the optical signal So output from the optical transmitter 210 can be received.

  An optical transmission apparatus that serially connects an I-component processing system and a Q-component processing system, and a general configuration in which N (N is an integer of 3 or more) phase modulators are serially connected in each processing system. Example 4 will be described. FIG. 13 shows a schematic block diagram of the fourth embodiment.

13 multiplexes 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate B / 2N, generates a DQPSK signal So having a bit rate B, and performs optical transmission. Output to the road.

The laser diode 320 generates coherent laser light that is carrier light for carrying data. Processing system 322 of the I component consists connected to the serial (0, [pi) phase modulation of the N phase modulators ₃₂₂ 1 _{~322 N.} The processing system 324 of the Q component consists connected to the serial (0, π / 2) phase modulation of the N phase modulators ₃₂₄ 1 _{~324 N.}

The output laser beam of the laser diode 320 sequentially to the phase modulator ₃₂₂ 1 _{~322 N} processing system 322 of the I component, type, is phase modulated by the phase modulators ₃₂₂ 1 _{~322 N.} The output light of the phase modulator 322 _N sequentially to the phase modulator ₃₂₄ 1 _{~324 N} of Q component of the processing system 324, and the input is phase modulated by the phase modulators ₃₂₄ 1 _{~324 N.} The output light of the phase modulator 324 _N is, the final output optical signal So, and the output to the optical transmission line.

The phase modulators 322 _{1 to} 322 _N modulate the phase of the input light by (0, π) according to the binary values of the applied modulation signals I _{1 to} I _N. The phase modulators 324 _{1 to} 324 _N modulate the phase of the input light by (0, π / 2) according to the binary values of the applied modulation signals Q _{1 to} Q _N. As the phase modulators 322 _{1 to} 322 _N , a phase modulator having a Mach-Zehnder interferometer structure can be used.

Precoder 326, precoded for DQPSK at bit rate B / 2N of the data signal _{U 1 ~U N, V 1 ~V} N phase modulators _{322 1 ~322 N, 324 1 ~324} N, phase modulation vessel ₃₂₂ 1 _{~322 N, 324} 1 _~324 modulation signal (drive _signal) of the bit rate B / 2N for _N I ₁ ~I _N, to generate the Q 1 to Q _N.

Similarly to the third embodiment, the modulated signals I _{1 to} I _N from the precoder 326 are phase-interleaved by the delay circuits 328 (328 _{1 to} 328 _N ), respectively, and the phase modulators 322 ₁ to 322 ₁ to I of the I component processing system 322 are processed. It applied to 322 _N. Similarly, the modulation signals Q _{1 to} Q _N from the precoder 326 are phase interleaved by the delay circuit 330 (330 _{1 to} 330 _N ) and applied to the phase modulators 324 _{1 to} 324 _N of the Q component processing system 324. The Assuming i = 1 to N, the delay time ΔT _i of the delay circuits 328 _i and 330 _i for delaying the modulation signals I _i and Q _i is 2 (i−1) / B, similarly to the delay circuits 238 _i and 240 _i. It is. The delay circuits 328 ₁ and 330 ₁ can be omitted, but are illustrated for logical consistency.

The delay circuit 328 (328 _{1 to} 328 _N ) functions as a phase interleaver that disperses and arranges the modulation signals I _{1 to} I _N at 2 / B intervals in a 2N / B period. Similarly, the delay circuits 330 (330 _{1 to} 330 _N ) function as phase interleavers that disperse and arrange the modulation signals Q _{1 to} Q _N at 2 / B intervals in 2N / B periods.

In the present embodiment, the phase modulators 322 _{1 to} 322 _N mount _N data signals indicated by the modulation signals I _{1 to} I _{N on} the I component of the optical carrier from the laser diode 320 by time division multiplexing, and perform phase modulation. The units 324 _{1 to} 324 _N load _N data signals indicated by the optical modulation signals Q _{1 to} Q _{N on} the Q component of the same optical carrier by time division multiplexing.

  The separation characteristics of the electrical separation devices 60 and 62 of the optical receiver 14 shown in FIG. 1 are changed to a configuration expanded from 1: 2 to 1: N, or the optical separation device 76 of the optical receiver 14a shown in FIG. , 78 is expanded from 1: 2 to 1: N, the optical signal So output from the optical transmitter 210 can be received.

  Although an example of DQPSK modulation has been described, QPSK modulation can also be applied.

  The present invention is applicable to homodyne QPSK systems. The present invention is also applicable to RZ-DQPSK or RZ-QPSK systems, and moreover, differential multi-phase phase shift such as Differential 8-Phase Shift-Keying (D8PSK). The present invention is also applicable to modulation (Differential Multiple-Phase Shift Keying: DMPSK).

  Although the invention has been described with reference to specific illustrative embodiments, various modifications and alterations may be made to the above-described embodiments without departing from the scope of the invention as defined in the claims. This is obvious to an engineer in the field to which the present invention belongs, and such changes and modifications are also included in the technical scope of the present invention.

1 shows a schematic block diagram of a data transmission system according to an embodiment of the present invention applied to an NRZ-DQPSK system. FIG. The timing chart of the phase modulators 30, 32, 34, and 36 is shown. It is a schematic block diagram of another structural example of an optical receiver. Waveform examples (A) and (B) of modulated signals I ₁ and I ₂ of 5.0 Gbps are shown, respectively. Waveform examples (A) and (B) of modulated signals Q ₁ and Q ₂ of 5.0 Gbps are shown, respectively. An example of the DQPSK signal light generated by the modulation signals I ₁ , I ₂ , Q ₁ , and Q ₂ shown in FIGS. 4 and 5, that is, the amplitude (A) and optical phase (B) of the output signal light So is shown. An example of the eye pattern of the output signal So is shown. An example of waveforms of the reception signals U and V of the balanced receivers 56 and 58 is shown. An example of the eye pattern of the received signals U and V is shown. The optical spectrum example of the produced | generated 20 Gbps DQPSK optical signal is shown. It is a schematic block diagram of another structure of the optical transmitter of this invention. It is a schematic block diagram of still another configuration of the optical transmission apparatus of the present invention. It is a schematic block diagram of still another configuration of the optical transmission apparatus of the present invention.

Explanation of symbols

10: Optical transmitter 12: Optical transmission line 14, 14a: Optical receiver 20: Laser diode 22: Optical IQ multiplexer 24: Optical demultiplexer 26a, 26b: Arm 28: Optical multiplexer 30, 32, 34, 36 : Phase modulator 38: phase shifter 40: precoder 42, 44: delay circuit 50: optical demultiplexer 52, 54: MZ delay interferometer 52a, 54a: delay circuit 52b, 54b: phase shifter 56, 58: balanced reception Units 60 and 62: Electric separators 70: Optical demultiplexers 72 and 74: MZ delay interferometers 72a and 74a: Delay circuits 72b and 74b: Phase shifters 76 and 78: Optical separators 80: Optical demultiplexers 82 and 84 : Electroabsorption optical modulator 86: clock generator 88: inversion circuits 90, 92, 94, 96: light receiver 110: optical transmitter 120: laser diode 122: I component processing system 122 ₁ , 122 ₂ : Phase modulator 124: Q component processing system 124 ₁ , 124 ₂ : Phase modulator 126: Precoder 128 and 130: Delay circuit 210: Optical transmitter 220: Laser diode 222: Optical IQ multiplexer 224: Optical demultiplexers 226a and 226b: Arm 228: Optical multiplexers 230 _{1 to} 230 _N : Phase modulators 232 _{1 to} 232 _N : Phase modulator 234: Phase shifter 236: Precoder 238 (238 _{1 to} 238 _N ): Delay circuit 240 (240 ₁ to 240 _N ): delay circuit 310: optical transmitter 320: laser diode 322: I component processing system 322 _{1 to} 322 _N : phase modulator 324: Q component processing system 324 _{1 to} 324 _N : phase modulator 326: precoder _{328 (328} 1 _{~328 N):} the delay circuit _{330 (330} 1 _{~330 N} : Delay circuit

Claims (5)

2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate B / 2N (N is an integer equal to or greater than 2) are precoded, and N first modulated signals I having a bit rate B / 2N are encoded. A precoder (236) for generating _N second modulated signals Q ₁ -Q _{N of 1} -I _N and a bit rate B / 2N;
A first phase interleaver (238) for phase interleaving the _{N first} modulated signals I _{1 to} I _N within a 2N / B period;
A second phase interleaver (240) for phase interleaving the _N second modulated signals Q _{1 to} Q _N within a 2N / B period;
A coherent light source (220) for generating coherent light;
The optical demultiplexer (224) that divides the output light of the coherent light source into two, the first and second arms (226a, 226b) that propagate the divided light by the optical demultiplexer, respectively, the first and second An optical IQ multiplexer (222) comprising an optical multiplexer (228) for multiplexing light from the arm and a phase shifter (234) for shifting the phase of the light on the second arm (226b) by π / 2. )When,
N serially connected N first phase modulators disposed on the first arm, the N first modulation signals phase-interleaved by the first phase interleaver (238) _N first phase modulators (230 _{1 to} 230 _N ) that phase-modulate input light according to I _{1 to} I _N ;
Serially connected N second phase modulators arranged on the second arm, the N second modulated signals phase-interleaved by the second phase interleaver (240) _N second phase modulators (232 _{1 to} 232 _N ) that phase-modulate the input light according to Q _{1 to} Q _N
An optical transmitter characterized by comprising:   2. The optical transmission device according to claim 1, wherein the first and second phase modulators are Mach-Zehnder type phase modulators. 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate of B / 2N (N is an integer equal to or greater than 2) are precoded to generate _N first modulated signals having a bit rate of B / 2N. Generating _N second modulated signals Q _{1 to} Q _N with I _{1 to} I _N and bit rate B / 2N;
Phase interleaving the _{N first} modulated signals I _{1 to} I _N within a 2N / B period,
Phase interleaving the _N second modulated signals Q _{1 to} Q _N within a 2N / B period,
The in-phase (I) component of the coherent light from the coherent light source is sequentially phase-modulated according to the _{N first} modulation signals I _{1 to} I _N that are phase-interleaved to generate an I-component optical signal,
The quadrature (Q) component of the coherent light is sequentially phase-modulated according to the _N second modulated signals Q _{1 to} Q _N that are phase-interleaved to generate a Q-component optical signal,
An optical transmission method comprising combining the I component optical signal and the Q component optical signal. 2N data signals U _{1 to} U _N and V _{1 to} V _N having a bit rate B / 2N (N is an integer equal to or greater than 2) are precoded, and N first modulated signals I having a bit rate B / 2N are encoded. a precoder (326) for generating a ₁ ~I _N and the bit rate B / 2N of N second modulation signal _Q 1 to Q _N,
A first phase interleaver (328) for phase interleaving the _{N first} modulated signals I _{1 to} I _N within a 2N / B period;
A second phase interleaver (330) for phase interleaving the _N second modulated signals Q _{1 to} Q _N within a 2N / B period;
A coherent light source (320) for generating coherent light;
An I component processing system (322) for phase-modulating the output light of the coherent light source by (0, π);
Q component processing system (324) for phase-modulating the output light of the I component processing system by (0, π / 2)
And
The I component processing system phase-modulates the input light by (0, π) according to the _{N first} modulation signals I _{1 to} I _N phase-interleaved by the first phase interleaver (328), respectively. Serially connected N first phase modulators (322 _{1 to} 322 _N )
Comprising
In accordance with the _N second modulated signals Q _{1 to} Q _N phase-interleaved by the second phase interleaver (330), the Q component processing system converts the input light to (0, π / 2), respectively. N serially connected second phase modulators (324 _{1 to} 324 _N ) for phase modulation
An optical transmission device comprising: 2N data signals U _{1 to} U _N , V _{1 to} V _N having a bit rate B / 2N are precoded, and N first modulation signals I _{1 to} I _N and a bit rate B having a bit rate B / 2N are pre-coded. N second modulated signals Q _{1 to} Q _N of / 2N are generated,
Phase interleaving the _{N first} modulated signals I _{1 to} I _N within a 2N / B period,
Phase interleaving the _N second modulated signals Q _{1 to} Q _N within a 2N / B period,
The coherent light from the coherent light source is sequentially phase-modulated with (0, π) in accordance with the _{N first} modulation signals I _{1 to} I _N that have been phase interleaved, and further, the N number of phase interleaved N An optical transmission method characterized in that phase modulation is sequentially performed at (0, π / 2) in accordance with the second modulation signals Q _{1 to} Q _N.

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