JP2017143338A - Light transmission and reception system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform high speed transmission with filter tolerance without using complicated demodulation processing in a light receiver.SOLUTION: A light transmitter includes: an encoding part which converts a quaternary data signal composed of a column of quaternary data information from 0 to 3 into a 7-valued data signal composed of a column of symbols in value adding to the data information value a symbol whose positive and negative reverse every time 0 appears; a signal generation part generating a 7-valued electric signal from the 7-valued data signal; and a light modulation part generating a transmission optical signal to which 7-valued optical amplitude modulation equivalent to quaternary light intensity has been applied by applying the 7-valued electric signal to laser beam outputted from a signal light source. The light receiver is provided with: a light receiving part which directly detects the optical signal transmitted by the light transmitter and converts intensity information of the optical signal into an electrical signal; and a demodulation part demodulating the quaternary data signal from the electrical signal converted by the light receiving part.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmission / reception system.

  With an increase in demand for data communication, an optical transmission network using an optical signal modulation technique and an optical signal multiplexing technique capable of transmitting a large volume of traffic is becoming widespread. In particular, in an ultrahigh-speed optical transmission / reception system having a transmission rate per wave of 100 Gb / s (gigabit per second) or more, a digital coherent technology combining a coherent detection and a digital signal processing technology has been widely used. On the other hand, large-capacity data communication using mobile terminals, as represented by LTE (Long Term Evolution), has become widespread. Due to this widespread use, it is required to realize ultra high speed optical transmission of 100 Gb / s class at a lower cost, that is, with a simpler transceiver configuration.

  As a method for realizing ultra-high-speed optical transmission of 100 Gb / s class with a simple configuration, a direct detection method that demodulates a data signal based on the intensity information of the optical signal has attracted attention. In particular, an ultrahigh-speed optical transmission system using a quaternary intensity modulation system (PAM4: 4-level Pulse Amplitude Modulation) is being studied. The quaternary intensity modulation method has higher frequency utilization efficiency than the NRZ (Non Return-to-Zero) method that is a binary intensity modulation signal.

  In 100 Gb / s class optical transmission using a digital coherent technique, generally, a polarization QPSK (Quadrature Phase Shift Keying) modulation method (PDM-QPSK) is used. The modulation speed of PDM-QPSK is about 25 Gbaud. On the other hand, the modulation rate when super-high-speed optical transmission of 100 Gb / s class is implemented using PAM4 is about 50 Gbaud. For this reason, the signal spectrum when PAM4 is used occupies a wider frequency than PDM-QPSK. This means that the 100 Gb / s class PAM4 system is greatly affected by the waveform deterioration due to filtering compared to the PDM-QPSK system. In order to realize 100 Gb / s class ultra high-speed optical transmission with a simpler configuration, it is important to apply a narrower-band electrical filter, and therefore techniques for improving the filter strength against transmission signals have been studied. A duobinary PAM4 method has been proposed as a technique for improving the filter strength for PAM4 signals (see, for example, Non-Patent Document 1). In the duobinary PAM4 system, the quaternary signal of the light intensity used in the PAM4 signal is duobinary encoded and converted into a quinary signal of the light intensity, thereby realizing an improvement in the filter strength and the strength against wavelength dispersion of the transmission line.

N. Stojanovic, et. Al., "Performance and DSP Complexity Evaluation of a 112-Gbit / s PAM-4 Transceiver Employing a 25-GHz TOSA and ROSA," ECOC 2015 Tu. 3.4.5

  The PAM4 system has been studied as a system for realizing 100 Gb / s class ultra-high-speed optical transmission with a simple configuration. By applying duobinary encoding to this PAM4 system, improvement in filter tolerance and chromatic dispersion tolerance is realized. A duobinary PAM4 scheme has been proposed. However, in this duobinary PAM4 system, the light intensity takes seven values, and the inter-symbol distance is shorter than that of the PAM4 signal, so that the tolerance to noise is reduced. In addition, since duobinary coding is applied in the optical transmitter, it is necessary to use maximum likelihood sequence estimation (MLSE) in the optical receiver during demodulation, which complicates the optical receiver function. There is a problem of inviting.

  In view of the above circumstances, an object of the present invention is to provide an optical transmission / reception system that realizes high-speed transmission with filter tolerance without using complicated demodulation processing in an optical receiver.

  One aspect of the present invention is an optical transmission / reception system having an optical transmitter and an optical receiver, wherein the optical transmitter receives a quaternary data signal composed of a sequence of quaternary data information from 0 to 3. An encoding unit that converts the value of the data information into a 7-value data signal that is a sequence of symbols each having a sign that is inverted every time 0 appears in the value of the data information; A signal generation unit that generates a seven-value electric signal, and an optical signal that is subjected to seven-value optical amplitude modulation corresponding to the four-value light intensity by applying the electric signal to the laser light output from the signal light source. An optical modulation unit, wherein the optical receiver directly detects the optical signal transmitted from the optical transmitter and converts intensity information of the optical signal into an electric signal; and the light receiving unit 4 value data signal is demodulated from electric signal converted by It includes a control unit.

  One embodiment of the present invention is the optical transmission / reception system described above, wherein the optical transmitter further includes a digital filter function unit that narrows the band of the seven-value data signal by digital filter processing, and the signal generation unit Generates a seven-value electrical signal from the digital-filtered seven-value data signal.

  One embodiment of the present invention is the above-described optical transmission / reception system, wherein the signal generation unit outputs light of an output optical signal when a symbol value in a 7-value data signal is i (i = 1, 2, 3). When the intensity is equal to the optical intensity of the output optical signal when the symbol value is -i, the optical phase of the output optical signal is 0 when the symbol value is i, and the output light when the symbol value is -i When the optical phase of the signal is π and the symbol value is 0, an electrical signal for applying a modulation voltage to the laser beam is generated so that the optical intensity of the output optical signal is minimized.

One embodiment of the present invention is the above-described optical transmission / reception system, in which the signal generation unit has a voltage necessary for V _π to change the output optical power from the optical modulation unit from a minimum value to a maximum value. When the value V _bias is a voltage value at which the output optical power from the optical modulation unit is a minimum value, the seven symbol values from -3 to 3 in the seven-value data signal are respectively expressed as V _bias −V _π to V An electric signal for driving the optical modulation unit is generated by an applied voltage corresponding to each of the seven voltage values dividing _bias + _Vπ at equal intervals.

  One aspect of the present invention is an optical transmission / reception system including an optical transmitter and an optical receiver, wherein the optical transmitter is configured to store N-value data information from 0 to N−1 (N is an integer of 1 or more). An N-value data signal composed of a column is converted into a 2N-1 value data signal composed of a symbol column having a value to which the sign is inverted every time 0 appears in the data information value. An encoding unit, a signal generation unit that generates a 2N-1 value electrical signal from the 2N-1 value data signal, and an N-value light intensity by applying the electrical signal to a laser beam output from a signal light source. An optical modulation unit that generates an optical signal subjected to optical amplitude modulation of 2N-1 value corresponding to the optical signal, and the optical receiver directly detects the optical signal transmitted from the optical transmitter, and A light receiving unit that converts intensity information of an optical signal into an electrical signal, and the light receiving unit And a demodulator for demodulating the data signals of N values from the conversion electrical signals.

  One embodiment of the present invention is the above-described optical transmission / reception system, wherein the optical transmitter further includes a digital filter function unit that narrows the band of the 2N-1 value data signal by digital filter processing. The generation unit generates a 2N-1 value electric signal from the 2N-1 value data signal subjected to the digital filter processing.

  One embodiment of the present invention is the above-described optical transmission / reception system, wherein the signal generation unit is configured such that a symbol value in a 2N−1 value data signal is i (i is an integer of 1 to N−1). When the optical intensity of the output optical signal is equal to the optical intensity of the output optical signal when the symbol value is -i, and the symbol value is i, the optical phase of the output optical signal is 0 and the symbol value is -i In this case, an electrical signal for applying a modulation voltage to the laser beam is generated so that the optical phase of the output optical signal is π and the optical intensity of the output optical signal is minimized when the symbol value is 0.

One embodiment of the present invention is the above-described optical transmission / reception system, in which the signal generation unit has a voltage necessary for V _π to change the output optical power from the optical modulation unit from a minimum value to a maximum value. When the value, V _bias is a voltage value at which the output optical power from the optical modulation unit is a minimum value, 2N−1 symbol values from −N + 1 to N−1 in the 2N−1 value data signal are respectively the V _bias -V applied voltage corresponding to _{V bias} + _{V π} are divided equally 2N-1 pieces of voltage values from each _[pi generates an electrical signal for driving the light modulation unit.

  According to the present invention, high-speed transmission with filter tolerance can be realized without using complicated demodulation processing in an optical receiver.

It is a block diagram which shows the structure of the optical transmitter by embodiment of this invention. It is a figure which shows the relationship between the light intensity of the 7 value electric signal and transmission optical signal by the embodiment. It is a figure which shows the relationship between the quaternary electric signal in the conventional PAM4 system, and the optical intensity of a transmission optical signal. It is a figure which shows the relationship between the 7 value electric signal in the conventional duobinary PAM4 system, and the optical intensity of a transmission optical signal. It is a block diagram which shows the structure of the optical receiver by the embodiment. It is a figure which shows the relationship between the quaternary data signal before an encoding, and the optical signal waveform after an encoding in each of the 4 value / 7 value encoding PAM4 system of the same embodiment, and the conventional PAM4 system. It is a figure which shows the transmission optical signal spectrum in each of the 4-value / 7 value encoding PAM4 system of the same embodiment, and the conventional PAM4 system. It is a figure which shows the result of OSNR (Optical Signal-to-Noise Ratio) characteristic evaluation with respect to each of the 4-value / 7 value encoding PAM4 system of the same embodiment, and the conventional PAM4 system. It is a figure which shows how each waveform of the 4-value / 7 value encoding PAM4 system of the same embodiment and the conventional PAM4 system deteriorates by wavelength dispersion. It is a figure which shows the evaluation result of the chromatic dispersion tolerance with respect to each of the 4 value / 7 value encoding PAM4 system of the same embodiment, and the conventional PAM4 system. It is a block diagram which shows the structure of the optical transmitter in 1st Embodiment. It is a figure which shows the relationship between the light intensity of the 7-value electrical signal and transmission optical signal in 1st Embodiment. It is a block diagram which shows the structure of the optical receiver in 1st Embodiment. It is a block diagram which shows the structure of the optical transmitter in 2nd Embodiment. It is a block diagram which shows the structure of the optical transmitter in 3rd Embodiment. It is a block diagram which shows the structure of the optical receiver in 3rd Embodiment. It is a block diagram which shows the structure of the optical transmitter in 4th Embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In the conventional PAM4 system (4-level intensity modulation system), 2-bit data information is allocated to 4-level light intensity of 0, 1, 2, 3 to transmit 2 bits / symbol (2 bits per symbol). Realize. In the duobinary PAM4 method, 2-bit data information is assigned to four values of light intensity, and then the four values of light intensity are assigned to seven values of light intensity of 0, 1, 2, 3, 4, 5, 6. Thus, 2-bit / symbol transmission having high filter tolerance and high wavelength dispersion tolerance is realized. However, since the optical receiver must identify the 7-value light intensity at the time of demodulation, the tolerance to noise is reduced as compared with the conventional PAM4 system. Further, in order to restore the transmission data information, duobinary decoding using MLSE (maximum likelihood sequence estimation) or the like must be performed at the time of demodulation, so that a complicated digital signal is transmitted to the optical receiver. A processing function is required.

  In this embodiment, after assigning 2-bit data information to four-level light intensity, the four-value light intensity is set to seven-value light amplitudes of -3, -2, -1, 0, 1, 2, and 3. Assign to. This corresponds to 4-level / 7-level encoding. The quaternary signal before encoding is S (S = 0, 1, 2, 3), and the quinary signal after encoding is C (C = -3, -2, -1, 0, 1, 2, 3). When the management code is P (P = -1, 1), the encoding is executed according to the following (Expression 1) and (Expression 2).

  However, when S = 0, the sign of P is inverted. At this time, the initial value of P may be either 1 or -1. When quaternary / 7-value encoding is performed according to the rules shown in (Equation 1) and (Equation 2), for example, the quaternary signal S = [1 0 3 2 1 0 2 0 0 3 2 0 2] A 7-value signal sequence of C = [1 0 -3 -2 -1 0 2 0 0 3 2 0 -2] is assigned to the sequence. Here, the initial value of the management code P is set to 1, and the management code sequence is P = [1 −1 −1 −1 −1 1 1 −1 1 1 1 −1 −1].

  The optical transmission / reception system according to the present embodiment includes the optical transmitter 1 shown in FIG. 1 and the optical receiver 3 shown in FIG. The optical transmitter 1 and the optical receiver 3 are connected via an optical fiber transmission line. The optical fiber transmission line transmits signal light between the optical transmitter 1 and the optical receiver 3. One or both of the transmission side and the reception side may be an optical transceiver including the optical transmitter 1 and the optical receiver 3.

  FIG. 1 is a block diagram showing the configuration of the optical transmitter 1. The optical transmitter 1 includes a quaternary / 7-level encoding unit 11, a signal generation unit 12, a signal light source 13, and a Mach-Zehnder optical intensity modulator (hereinafter referred to as “MZ modulator”) 14. Is provided. The 4-level / 7-level encoding unit 11 converts a 4-level data signal (S), which is a 4-level data signal to be transmitted, into a 7-level data signal according to the rules shown in (Equation 1) and (Equation 2). It is converted into a 7-value data signal (C). The signal generator 12 converts the seven-value data signal (C) into a seven-value electric signal that is a seven-value electric signal. The signal generation unit 12 applies the seven-value electric signal to the MZ modulator 14 as an applied voltage. As a result, the MZ modulator 14 generates a transmission optical signal (7-value amplitude modulated optical signal) obtained by subjecting the laser light output from the signal light source 13 to 7-value optical amplitude modulation by the applied voltage. The optical transmitter 1 outputs the transmission optical signal generated by the MZ modulator 14 to the optical fiber transmission line. The transmission optical signal is transmitted to the optical receiver 3 through the optical fiber transmission line.

FIG. 2 is a diagram showing the relationship between the 7-value electrical signal and the light intensity of the transmission optical signal according to this embodiment.
The optical transmitter 1 sets the bias voltage V _bias to be applied to the optical modulation unit (for example, the MZ modulator 14) to the null point of the light intensity, and the maximum amplitude (that is, −3 and 3) of the 7-value electric signal is set. to be equal to twice the value of the wavelength voltage V _[pi light modulation portion (i.e. 2V _[pi), adjusts the amplitude. Since the optical phase is inverted at the null point of the optical signal, the modulated optical signal becomes an optical signal subjected to seven-value optical amplitude modulation. At this time, the applied voltage is adjusted so that the light intensities in the 7-value data “3 and -3”, “2 and −2”, and “1 and −1” are equal to each other. As a result, it is possible to generate an optical signal having 7 values in terms of light amplitude and 4 values in terms of light intensity.

FIG. 3 is a diagram showing the relationship between the quaternary electric signal and the optical intensity of the transmission optical signal in the conventional PAM4 system, and FIG. 4 shows the optical intensity of the quinary electric signal and the transmission optical signal in the conventional duobinary PAM4 system. It is a figure which shows the relationship.
In the conventional PAM4 system shown in FIG. 3 and the conventional duobinary PAM4 system shown in FIG. 4, the data signal is superimposed in the form of light intensity. On the other hand, in the quaternary / 7-value encoding PAM4 system of this embodiment, as shown in FIG. 3, the data signal is superimposed in the form of optical amplitude.

  FIG. 5 is a block diagram showing the configuration of the optical receiver 3 according to the present embodiment. The optical receiver 3 includes a light receiving unit 31. The light receiving unit 31 receives the seven-value amplitude modulated optical signal output from the optical transmitter 1 and transmitted through the optical fiber transmission line by direct detection, and converts the intensity information of the received optical signal into an electrical signal. Here, the optical signal subjected to the seven-value optical amplitude modulation as the transmission optical signal is converted into a four-value electric signal which is an electric signal having a four-value amplitude. Specifically, the amplitude value of the electrical signal corresponding to the optical signal assigned with -3 or 3 as the optical amplitude is 3, and the amplitude value of the electrical signal corresponding to the optical signal assigned with -2 or 2 as the optical amplitude. 2, the amplitude value of the electrical signal corresponding to the optical signal assigned with −1 or 1 as the optical amplitude is 1, and the amplitude value of the electrical signal corresponding to the optical signal assigned with 0 as the optical amplitude is 0. Assuming that the quaternary electric signal after direct detection by the light receiving unit 31 is R (R = 0, 1, 2, 3), this quaternary electric signal (R) is exactly the quaternary data signal (S) before encoding. It can be expressed by the following (formula 3).

  Thus, in this embodiment, it is possible to demodulate transmission data without using a decoding process using MLSE or the like such as the duobinary PAM4 system. Therefore, it is not necessary to provide a complicated digital signal processing function in the optical receiver. This means that the signal can be demodulated with a simple optical receiver configuration.

  FIG. 6 is a diagram showing the relationship between the quaternary data signal (S) before encoding and the optical signal waveform after encoding in each of the quaternary / 7-value encoding PAM4 system and the conventional PAM4 system of this embodiment. It is. FIG. 6A shows the 4-value / 7-value encoding PAM4 system of this embodiment, and FIG. 6B shows the conventional PAM4 system.

  The quaternary data signal (S) before encoding is [3 0 3 2 0 1 0 3] in both the 4-value / 7-value encoding PAM4 system and the conventional PAM4 system in this embodiment. As shown in FIG. 6B, in the conventional PAM4 system, in a data sequence such as [3 0 3], a large change in optical amplitude occurs at a high frequency corresponding to the modulation frequency. On the other hand, as shown in FIG. 6A, in the quaternary / 7-value encoding PAM4 system of this embodiment, a data sequence such as [3 0 3] is [3 0 − 3], the change in the optical amplitude only occurs at a frequency as low as about half of the modulation frequency. This means that in the four-value / seven-value encoded PAM4 system of the present embodiment, a large change in optical amplitude does not occur at a high frequency.

  FIG. 7 is a diagram showing the transmission optical signal spectrum in each of the 4-value / 7-value encoding PAM4 system and the conventional PAM4 system of this embodiment. FIG. 7A shows the 4-value / 7-value encoding PAM4 system of this embodiment, and FIG. 7B shows the conventional PAM4 system. Here, in both cases, the modulation speed is set to 56 Gbaud.

  As shown in FIG. 7A, in the four-value / seven-value encoded PAM4 system of the present embodiment, the signal power is concentrated in the low frequency region, compared with the conventional PAM4 system shown in FIG. 7B. It can be confirmed that the power in the high frequency region is small. This means that high-frequency fluctuations are suppressed in the quaternary / 7-value encoded PAM4 system.

  FIG. 8 shows the results of OSNR (Optical Signal-to-Noise Ratio) characteristic evaluation for each of the quaternary / 7-value coded PAM4 system of the present embodiment and the conventional PAM4 system by transmission simulation. FIG. Here, the modulation speed of each signal is set to 56 Gbaud, and the OSNR and BER (bits) are set for each of the cases where the electrical band of the optical transmitter is limited to 26 GHz, limited to 28 GHz, and when no limitation is given. (Error rate) relationship.

  It can be confirmed that the OSNR characteristic of the four-value / seven-value encoded PAM4 system when the band limitation is not given is almost equal to the OSNR characteristic of the conventional PAM4 system. On the other hand, it can be confirmed that the OSNR characteristic of the four-value / seven-value encoded PAM4 system of the present embodiment when band limitation is given shows better characteristics than the conventional PAM4. In particular, when focusing on the case where the electrical band of the optical transmitter is limited to 26 GHz, the OSNR required for the 4-level / 7-level encoding PAM4 system of this embodiment to satisfy the BER of 1E-3 is approximately 33 dB. On the other hand, the OSNR required for the conventional PAM4 system to satisfy the BER of 1E-3 is approximately 35 dB. This means that by applying the four-value / seven-value encoded PAM4 system of the present embodiment in a band limited environment, a characteristic improvement effect of about 2 dB can be obtained in terms of OSNR.

  FIG. 9 is a diagram showing how the waveforms of the quaternary / 7-value encoding PAM4 system and the conventional PAM4 system of this embodiment are degraded by chromatic dispersion. FIG. 9A is a diagram showing waveform deterioration of the 4-value / 7-value encoding PAM4 system of this embodiment, and FIG. 9B is a diagram showing waveform degradation of the conventional PAM4 system. Here, the quaternary data signal (S) before encoding is [3 0 3]. As shown in FIG. 9B, in the conventional PAM4 system, an optical signal corresponding to “3” has a waveform spread due to chromatic dispersion, and intersymbol interference (ISI) between adjacent symbols corresponding to “0”. : Inter-Symbol Interference) occurs. On the other hand, as shown in FIG. 9A, in the quaternary / 7-value encoded PAM4 system of the present embodiment, a data sequence such as [3 0 3] is [3 0 − 3]. Therefore, the ISI between the symbol corresponding to “3” and the adjacent symbol corresponding to “0” is the ISI between the symbol corresponding to “−3” and the symbol corresponding to “0” next to the symbol. Negate each other. Therefore, ISI does not occur between “3”, “0”, and “−3”. This means that the four-value / seven-value encoded PAM4 system of the present embodiment is less influenced by signal quality degradation due to chromatic dispersion than the conventional PAM4 system, that is, has a high chromatic dispersion tolerance. doing.

  FIG. 10 is a diagram showing evaluation results of chromatic dispersion tolerance for each of the four-value / seven-value encoded PAM4 system of the present embodiment and the conventional PAM4 system based on transmission simulation. Here, the modulation rate of each signal is set to 56 Gbaud, and the relationship between transmission line chromatic dispersion and BER when the OSNR is 34 dB is shown. Focusing on the chromatic dispersion value satisfying the BER of 1E-3, in the conventional PAM4 system, the BER becomes 1E-3 or more when exceeding ± 17.5 ps / nm, whereas the four values of the present embodiment. In the 7-value encoding PAM4 system, it can be confirmed that the BER does not exceed 1E-3 as long as it is up to ± 27.5 ps / nm. This means that the quaternary / 7-value encoded PAM4 system of this embodiment has a chromatic dispersion tolerance of about 1.4 times that of the conventional PAM4 system.

  The 4-level / 7-level encoding PAM4 system of the present embodiment can be extended as an encoding system for an arbitrary N-value intensity modulation signal (PAM-N system). Specifically, an N-value data signal is encoded by the method shown in (Equation 1) and (Equation 2) to be converted into a 2N-1 value data signal, and N-value / 2N-1 value encoding is performed. PAM-N method is realized. Here, S is an N-value data signal (S = 0, 1,..., N−1) before encoding, and C is a 2N−1 value data signal (C = −N + 1,. 1, 0, 1,..., N−1) and P are management codes (P = −1, 1).

Subsequently, first to fourth embodiments to which the above-described embodiment is applied will be described.
(First embodiment)
FIG. 11 is a block diagram showing the configuration of the optical transmitter 110 in the first embodiment. The optical transmitter 110 includes a quaternary / seven-value encoding unit 111, a signal generation unit 112, an electric amplifier 113, a signal light source 114, and an MZ modulator 115. The 4-level / 7-level encoding unit 111 converts the 4-level data signal (S) to be transmitted into a 7-level data signal (C) according to the rules shown in (Equation 1) and (Equation 2). The data is output to the generation unit 112. The signal generation unit 112 converts the seven-value data signal (C) input from the four-value / seven-value encoding unit 111 into a seven-value electric signal and outputs it to the electric amplifier 113. The electric amplifier 113 amplifies the seven-value electric signal input from the signal generation unit 112 and applies it to the MZ modulator 115 as a voltage. The MZ modulator 115 generates a transmission optical signal (seven-value amplitude modulated optical signal) obtained by subjecting the laser light output from the signal light source 114 to seven-value optical amplitude modulation by an applied voltage. The optical transmitter 110 outputs the transmission optical signal generated by the MZ modulator 115 to the optical fiber transmission line. The transmission optical signal is transmitted to the optical receiver 130 shown in FIG. 13 through the optical fiber transmission line.

The bias voltage applied to the MZ modulator 115 is set to a value (null point) at which the power of the output light from the MZ modulator 115 is minimized. _Assuming that the bias voltage is V _bias , the applied voltage values corresponding to the symbols -3, -2, -1, 0, 1, 2, and 3 are as shown in the following (Equation 4) to (Equation 10), respectively. . The symbol i is a symbol having a value i (i is an integer of −3 to 3).

FIG. 12 is a diagram showing the relationship between the 7-value electrical signal and the light intensity of the transmission optical signal. The V _[pi in the above formula, is a voltage value at which street the maximum output optical power from the MZ modulator 115 is the minimum shown in Figure 12. By setting the applied voltage in this way, it is possible to generate a seven-value optical amplitude modulation signal in which the output optical power from the MZ modulator 115 is arranged at equal intervals between symbols.

  FIG. 13 is a block diagram illustrating a configuration of the optical receiver 130 according to the first embodiment. It is connected to the optical transmitter 110 via an optical fiber transmission line. The optical receiver 130 includes a light receiver (PD) 131 and an identification circuit 132. The light receiver 131 receives the seven-value amplitude modulated optical signal output from the optical transmitter 110 and transmitted through the optical fiber transmission line, and receives the received optical signal by direct detection. The light receiver 131 converts the intensity information of the received optical signal into a quaternary electrical signal (R) that is a quaternary electrical signal, and outputs the quaternary electrical signal (R) to the identification circuit 132. The identification circuit 132 extracts the clock of the signal and identifies 0, 1, 2, 3 according to the intensity value represented by the quaternary electric signal (R) corresponding to the intensity information of the optical signal, thereby transmitting the signal. Demodulate the data.

(Second Embodiment)
This embodiment generates a transmission optical signal having a narrower band than that of the first embodiment. The configuration of the optical receiver of this embodiment is the same as that of the optical receiver 130 of the first embodiment.

  FIG. 14 is a block diagram illustrating a configuration of the optical transmitter 210 according to the second embodiment. The optical transmitter 210 includes a four-value / seven-value encoding unit 211, a digital filter function unit 212, a signal generation unit 213, an electric amplifier 214, a signal light source 215, and an MZ modulator 216. As described above, the optical transmitter 210 has a configuration in which the digital filter function unit is further added to the optical transmitter 110 of the first embodiment.

  The four-value / seven-value encoding unit 211 converts the four-value data signal (S) to be transmitted into a seven-value data signal (C) and outputs it to the digital filter function unit 212. The digital filter function unit 212 performs digital filter processing on the seven-value data signal (C) input from the four-value / seven-value encoding unit 211 and outputs the result to the signal generation unit 213. The signal generation unit 213 converts the de 7-value data signal that has been subjected to the digital filter processing into a 7-value electric signal and outputs it to the electric amplifier 214. As a result, it is possible to narrow the band of the seven-value electric signal input to the electric amplifier 214. In particular, as a digital filter process, by outputting a signal subjected to a raised cosine filter (RCF) process whose transfer function is represented by the following (Expression 11), (Expression 12), and (Expression 13) from the signal generation unit 213: Thus, it is possible to generate a transmission optical signal having a narrower signal band. The electric amplifier 214 amplifies the seven-value electric signal input from the signal generation unit 213 and applies it to the MZ modulator 216 as a voltage. The MZ modulator 216 generates a transmission optical signal obtained by subjecting the laser light output from the signal light source 215 to seven-value optical amplitude modulation.

  Here, W is a modulation speed, α is a roll-off coefficient, and 0 ≦ α ≦ 1.

  In the first embodiment, the signal generator 112 only needs to have a digital-to-analog converter (DAC) function having a resolution necessary for generating a seven-value electric signal. In the second embodiment, a DAC function having a higher resolution is required for the signal generation unit 213 in accordance with the RCF processing, while having the characteristics of the quaternary / 7-value encoding PAM4 system, and the first embodiment. It is possible to generate a transmission optical signal having a narrower signal band than that of.

(Third embodiment)
This embodiment corresponds to a case where the first embodiment is extended to an intensity modulation signal having N values (N is an integer of 1 or more).
FIG. 15 is a block diagram showing a configuration of the optical transmitter 310 in the third embodiment. The optical transmitter 310 includes an N-value / 2N-1 value encoding unit 311, a signal generation unit 312, an electric amplifier 313, a signal light source 314, and an MZ modulator 315. The N-value / 2N-1 value encoding unit 311 converts the N-value data signal (S), which is an N-value data signal to be transmitted, into 2N-1 according to the rules shown in (Expression 1) and (Expression 2). It is converted into a 2N-1 data signal (C) which is a value data signal, and is output to the signal generator 312. The signal generation unit 312 converts the 2N-1 value data signal (C) input from the N value / 2N-1 value encoding unit 311 into a 2N-1 value electric signal that is a 2N-1 value electric signal, Output to the electric amplifier 313. The electric amplifier 313 amplifies the 2N−1 value electric signal input from the signal generation unit 312 and applies it to the MZ modulator 315 as a voltage. The MZ modulator 315 generates a transmission optical signal (2N−1 value amplitude modulated optical signal) obtained by applying 2N−1 value optical amplitude modulation to the laser light output from the signal light source 314 by an applied voltage.

The bias voltage applied to the MZ modulator 315 is set to a value (null point) at which the power of the output light from the MZ modulator 315 is minimized. _Assuming that the bias voltage is V _bias , the applied voltage values corresponding to the respective symbols -N + 1,..., -1, 0, 1,. . The symbol j is a symbol having a value j (j is an integer from −N + 1 to N−1).

Here, _Vπ is a voltage value at which the output optical power from the MZ modulator 315 becomes minimum to maximum as shown in FIG. By setting the applied voltage in this way, it is possible to generate an N−1 value optical amplitude modulation signal in which the output optical power from the MZ modulator 315 is arranged at equal intervals between each symbol.

  FIG. 16 is a block diagram showing a configuration of the optical receiver 330 in the third embodiment. The optical receiver 330 is connected to the optical transmitter 310 via an optical fiber transmission line. The optical receiver 330 includes a light receiver 331 and an identification circuit 332. The light receiver 331 receives the 2N-1 value amplitude modulated optical signal output from the optical transmitter 310 and transmitted through the optical fiber transmission line by direct detection. The light receiver 331 converts the intensity information of the received optical signal into an N-value electrical signal (R) that is an N-value electrical signal, and outputs the N-value electrical signal (R) to the identification circuit 332. The identification circuit 332 extracts the clock of the signal and identifies 0, 1,..., N−1 according to the intensity value represented by the N-value electric signal (R) corresponding to the intensity information of the optical signal. The transmission data is demodulated.

(Fourth embodiment)
This embodiment generates a transmission optical signal having a narrower band than that of the third embodiment. The configuration of the optical receiver according to the present embodiment is the same as that of the optical receiver 330 according to the third embodiment.

  FIG. 17 is a block diagram illustrating a configuration of the optical transmitter 410 according to the fourth embodiment. The optical transmitter 410 includes an N-value / 2N-1 value encoding unit 411, a digital filter function unit 412, a signal generation unit 413, an electric amplifier 414, a signal light source 415, and an MZ modulator 416. Thus, the optical transmitter 410 has a configuration in which the digital filter function unit is further added to the optical transmitter 310 of the third embodiment.

  The N value / 2N−1 value encoding unit 411 converts the N value data signal (S) to be transmitted into a 2N−1 value data signal (C), and outputs the converted signal to the digital filter function unit 412. The digital filter function unit 412 performs digital filter processing on the 2N−1 value data signal (C) input from the N value / 2N−1 value encoding unit 411 and outputs the result to the signal generation unit 413. The signal generation unit 413 converts the 2N−1 value data signal that has been subjected to the digital filter processing into a 2N−1 value electric signal, and outputs the 2N−1 value electric signal to the electric amplifier 414. As a result, it is possible to narrow the band of the 2N-1 value electric signal input to the electric amplifier 414. In particular, as a digital filter process, a signal having a RCF process whose transfer function is represented by (Equation 11), (Equation 12), and (Equation 13) is output from the signal generation unit 413, thereby transmitting a signal having a narrower signal band. An optical signal can be generated. The electric amplifier 414 amplifies the 2N−1 value electric signal input from the signal generation unit 413 and applies it to the MZ modulator 416 as a voltage. The MZ modulator 416 generates a transmission optical signal in which the laser light output from the signal light source 415 is subjected to 2N-1 value optical amplitude modulation by an applied voltage.

  In the third embodiment, the signal generator 312 only needs to have a digital-to-analog converter (DAC) function having a resolution necessary for generating a 2N-1 value electric signal. In the fourth embodiment, a DAC function having a higher resolution is required for the signal generation unit 413 along with the RCF processing, while the N-value / 2N-1 value encoding PAM4 scheme is provided, and the third It is possible to generate a transmission optical signal having a narrower signal band than in the embodiment.

According to the embodiment described above, the optical transmission / reception system includes an optical transmitter and an optical receiver. The optical transmitter includes an encoding unit, a signal generation unit, a signal light source, and an optical modulation unit (for example, an MZ modulator), and the optical receiver includes a light receiving unit and a demodulation unit (for example, an identification circuit). With.
In the optical transmitter, an encoding unit (for example, a quaternary / 7-level encoding unit) inputs a quaternary data signal composed of a sequence of quaternary data information from 0 to 3. The encoding unit converts the quaternary data signal into a ternary data signal composed of a sequence of symbols having a value added with a sign that reverses the sign every time data information with a value of 0 appears. . The signal generator generates a seven-value electric signal from the seven-value data signal. The light modulation unit applies a seven-value electrical signal to the laser light output from the signal light source to generate an optical signal that has been subjected to seven-value light amplitude modulation corresponding to the four-value light intensity. In the optical receiver, the light receiving unit directly detects the optical signal transmitted from the optical transmitter, and converts the intensity information of the optical signal into an electric signal. The demodulating unit demodulates the quaternary data signal from the electrical signal converted by the light receiving unit.
The optical transmitter may further include a digital filter function unit that narrows the 7-value data signal by digital filter processing. The signal generation unit generates a seven-value electric signal from the seven-value data signal subjected to the digital filter processing.
In addition, the signal generation unit includes the light intensity of the output optical signal when the symbol value in the 7-value data signal is i (i = 1, 2, 3) and the light intensity of the output optical signal when the symbol value is −i. Are equal, and the optical phase of the output optical signal is 0 when the symbol value is i, the optical phase of the output optical signal is π when the symbol value is −i, and the output is when the symbol value is 0 An electric signal for applying a modulation voltage to the laser light is generated so that the light intensity of the optical signal is minimized.
In the signal generation unit, V _π is a voltage value necessary for changing the output optical power from the optical modulation unit from the minimum value to the maximum value, and V _bias is the output optical power from the optical modulation unit. In the case of the voltage value, the applied voltage corresponding to each of the seven symbol values from −3 to 3 in the 7-value data signal and each of the seven voltage values dividing V _bias −V _π to V _bias + V _π at equal intervals. To generate an electric signal for driving the light modulation unit.

Alternatively, in the optical transmitter, the encoding unit (for example, N value / 2N-1 value encoding unit) is composed of a string of N-value data information from 0 to N-1 (N is an integer of 1 or more). An N-value data signal is input. The encoding unit converts the N-value data signal into a 2N-1 value data signal composed of a sequence of symbols having a value in which the sign of the sign is inverted every time data information having a value of 0 appears. Convert. The signal generator generates a 2N-1 value electric signal from the 2N-1 value data signal. The optical modulator generates an optical signal by applying an electrical signal to the laser light output from the signal light source and performing 2N-1 value optical amplitude modulation corresponding to the N-level light intensity. In the optical receiver, the light receiving unit directly detects the optical signal transmitted from the optical transmitter, and converts the intensity information of the optical signal into an electric signal. The demodulator demodulates the N-value data signal from the electrical signal converted by the light receiver.
The optical transmitter may further include a digital filter function unit that narrows the band of the 2N-1 value data signal by digital filter processing. The signal generation unit generates a 2N-1 value electric signal from the 2N-1 value data signal subjected to the digital filter processing.
In addition, the signal generation unit outputs the light intensity of the output optical signal when the symbol value in the 2N−1 value data signal is i (i is an integer of 1 to N−1) and the output light when the symbol value is −i. When the signal optical intensity is equal and the symbol value is i, the optical phase of the output optical signal is 0, and when the symbol value is −i, the optical phase of the output optical signal is π, and the symbol value An electric signal for applying a modulation voltage to the laser beam is generated so that the light intensity of the output optical signal is minimized when is.
Further, the signal generator, the 2N-1 symbol values of N-1 respectively from -N + 1 in 2N-1 level data _signal, 2N-1 or to separate from the _{V bias} -V _[pi at equal intervals _{V bias} + _{V π} An electric signal for driving the optical modulation unit is generated by an applied voltage corresponding to each of the voltage values.

  According to the embodiment described above, the optical transmitter generates a transmission optical signal by converting the optical amplitude into seven values or 2N−1 values, instead of converting the light intensity into 7 values or 2N−1 values. . Thereby, this embodiment can implement | achieve the filter tolerance improvement and chromatic dispersion tolerance improvement of a PAM4 signal compared with the conventional PAM4 system. In addition, this embodiment has higher noise immunity than the conventional duobinary PAM4, and it is not necessary to use complicated demodulation processing such as MLSE in the optical receiver.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

  By directly receiving a transmitted optical signal, it can be applied to an optical transmission / reception system using a direct detection method in which a data signal is demodulated based on intensity information of the optical signal.

1, 110, 210, 310, 410 ... optical transmitters 3, 130, 330 ... optical receivers 11, 111, 211 ... four-value / seven-value encoding units 12, 112, 213, 312, 413 ... signal generation unit 13 , 114, 215, 314, 415... Signal light sources 14, 115, 216, 315, 416... MZ modulator 31, light receivers 113, 214, 313, 414, electric amplifiers 131, 331. 212, 412 ... digital filter function units 311, 411 ... N value / 2N-1 value encoding unit

Claims (8)

An optical transmission / reception system having an optical transmitter and an optical receiver,
The optical transmitter is
A quaternary data signal consisting of a sequence of quaternary data information from 0 to 3 is added to the value of the symbol of a value obtained by adding a sign that reverses the sign every time 0 appears in the value of the data information. An encoding unit for converting into a 7-value data signal consisting of a sequence;
A signal generator for generating a seven-value electrical signal from the seven-value data signal;
An optical modulation unit that generates an optical signal obtained by applying the electrical signal to the laser light output from the signal light source and performing seven-level optical amplitude modulation corresponding to the four-level optical intensity;
With
The optical receiver is:
A light receiving unit that directly detects the optical signal transmitted from the optical transmitter and converts intensity information of the optical signal into an electrical signal;
A demodulator that demodulates a four-value data signal from the electrical signal converted by the light receiver;
Comprising
An optical transmission / reception system. The optical transmitter further includes a digital filter function unit that narrows the 7-value data signal by digital filter processing,
The signal generation unit generates a seven-value electrical signal from the digital-filter-processed seven-value data signal.
The optical transmission / reception system according to claim 1. The signal generation unit determines the light intensity of the output optical signal when the symbol value in the 7-value data signal is i (i = 1, 2, 3) and the light intensity of the output optical signal when the symbol value is −i. The optical phase of the output optical signal is 0 when they are equal and the symbol value is i, the optical phase of the output optical signal is π when the symbol value is −i, and the output light when the symbol value is 0 Generating an electrical signal for applying a modulation voltage to the laser beam so that the light intensity of the signal is minimized;
The optical transmission / reception system according to claim 1 or 2, characterized by the above. In the signal generation unit, V _π is a voltage value necessary for changing the output optical power from the optical modulation unit from the minimum value to the maximum value, and V _bias is the output optical power from the optical modulation unit is the minimum value. If a voltage value which is the -3 seven symbol values of 3 from each of the 7 value data _signal is made to correspond to the seven voltage values respectively to separate from the _{V bias} -V _[pi at equal intervals _{V bias} + _{V π} applied Generating an electric signal for driving the light modulator by voltage;
The optical transmission / reception system according to claim 1, wherein the optical transmission / reception system is an optical transmission / reception system. An optical transmission / reception system having an optical transmitter and an optical receiver,
The optical transmitter is
An N-value data signal consisting of a sequence of N-value data information from 0 to N-1 (N is an integer equal to or greater than 1) is positive or negative each time 0 appears in the data information value. An encoding unit for converting into a 2N-1 value data signal composed of a sequence of symbols of values to which a sign to be inverted is assigned;
A signal generator for generating a 2N-1 value electrical signal from the 2N-1 value data signal;
An optical modulation unit that generates an optical signal by applying the 2N-1 value optical amplitude modulation corresponding to the N value optical intensity by applying the electrical signal to the laser light output from the signal light source;
With
The optical receiver is:
A light receiving unit that directly detects the optical signal transmitted from the optical transmitter and converts intensity information of the optical signal into an electrical signal;
A demodulator that demodulates an N-value data signal from the electrical signal converted by the light receiver;
Comprising
An optical transmission / reception system. The optical transmitter further includes a digital filter function unit that narrows the band of the 2N-1 value data signal by digital filter processing,
The signal generation unit generates a 2N-1 value electric signal from the digital filter processed 2N-1 value data signal.
The optical transmission / reception system according to claim 5. The signal generation unit outputs the optical intensity of the output optical signal when the symbol value in the 2N-1 value data signal is i (i is an integer between 1 and N-1) and the output optical signal when the symbol value is -i. When the symbol value is i, the optical phase of the output optical signal is 0, and when the symbol value is −i, the optical phase of the output optical signal is π, and the symbol value is 0. Generating an electrical signal for applying a modulation voltage to the laser beam so that the light intensity of the output optical signal is minimized at the time of
The optical transmission / reception system according to claim 5 or 6, In the signal generation unit, V _π is a voltage value necessary for changing the output optical power from the optical modulation unit from the minimum value to the maximum value, and V _bias is the output optical power from the optical modulation unit is the minimum value. If a voltage value which is, 2N-1 to separate from -N + 1 in 2N-1 level data signal 2N-1 one symbol values each of _N-1, at equal intervals _{V bias} + _{V π} from _{V bias} -V _[pi Generating an electric signal for driving the light modulator by an applied voltage corresponding to each of the voltage values;
The optical transmission / reception system according to any one of claims 5 to 7, wherein

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