JP6074823B2 - Optical time division multiplexing transmission system - Google Patents

Description

  The present invention relates to an optical time division multiplex transmission system for increasing a transmission rate of information transmission.

With the rapid development of the Internet, the transmission rate required for optical communication is increasing year by year, but the operation speed of electronic devices is reaching its limit.
In general, the number of codes (symbols) that can be transmitted per unit time is called a symbol rate [baud / second]. To transmit a signal at a symbol rate exceeding the upper limit of an electronic device, the signal is multiplexed in the optical domain. Optical time division multiplexing (OTDM) is required.
In OTDM transmission, a plurality of optical pulse signals (called RZ signals) modulated in intensity or phase are prepared, and the signals are multiplexed by switching them at high speed and converting them into serial signals.

FIG. 1 shows the principle of OTDM transmission.
For example, as the multi-channel signal transmitter 1, N (Ch.1 to Ch.N) optical transmitters 2 that output RZ signals having a symbol rate (clock frequency) of f ₀ [baud / sec] are used. Optical signal pulses to be output are arranged at equal intervals by a multiplexer (Mux) 3 and converted into serial signals. However, f ₀ [baud / sec] is set to a value equal to or less than the upper limit that can be handled by the electrical phase synchronization circuit (PLL circuit).
As a result, the multiplexer (Mux) 3 generates an OTDM signal having a symbol rate of f ′ = N × f ₀ [baud / sec].

On the other hand, the receiver includes a demultiplexer (Demux) 4 on the optical input side, divides this OTDM signal into N parallel signals (Demux), and performs signal processing for each channel.
In order to perform this Demux operation, a reference signal (clock) having a frequency equal to the symbol rate (= f ₀ ) of the original transmitter is required. Therefore, the receiver is required to have a so-called optical clock extraction function for generating this clock signal by the optical clock extraction unit 5 from the received OTDM optical signal.
Since the fundamental frequency of the OTDM signal exceeds the limit of the operation speed of the electronic device, an ultrafast optical device using a nonlinear optical effect is usually required to extract a clock signal from the OTDM signal.

  As described above, in the OTDM system, it is necessary to execute signal processing such as multiplexing / division at a speed exceeding the operation limit of the electronic device. For this, an optical device using a nonlinear optical effect is used. A general optical device has an input / output port for signal light and an input port for control light, and controls the intensity and phase of the signal light by turning on / off the control light.

  As a typical ultrahigh-speed optical device, there is one using four-wave mixing generated in a nonlinear fiber or cross-phase modulation indicated by semiconductor intersubband transition (ISBT). As shown in Non-Patent Document 1 below, ISBT OTDM with a symbol rate of 172 G [baud / sec] is realized using an optical gate.

Several clock extraction methods using ultrafast optical devices have been devised so far. As shown in the upper part of FIG. 2, a multiplexed OTDM signal (f ′ ₀ [baud / second]) is usually used. Since the information about the channel is not included, the timing of the extracted clock extraction signal has N options as shown in the lower part of FIG. 2 (see Non-Patent Document 2 below).
Therefore, in order to identify the channel of the demultiplexed signal in the receiving apparatus, it is required that the clock signal is always extracted at a constant timing with respect to the OTDM signal. For this reason, as shown in Non-Patent Document 3 below, it is necessary to transmit data with channel identification information added thereto.

  As shown in FIG. 3, the channel identification information is transmitted using a wavelength in a band (Outband) different from that of data by using a signal in which a clock pulse is superimposed on an optical time division multiplexed signal as a transmission signal. You can also. However, in that case, an extra band or power is required as the occupied band of the wavelength, and further, there is a problem that the timing of the extraction clock is likely to fluctuate due to the influence of the dispersion of the transmission path. It is desirable to transmit the channel identification information by Inband).

A method of transmitting channel identification information within a data band has been devised for OTDM transmission of OOK (On-off keying), which is a method of transmitting information using the intensity of an optical pulse, It is difficult to adopt such a method for a signal of the PSK method (Phase-shift keying), which is a method for transmitting information using the phase of an optical pulse.
In recent years, with the spread of digital coherent communication, the importance of the PSK system has increased, and the development of a clock extraction technique that enables channel identification in an OTDM signal of the PSK system has become an important issue.

Takayuki Kurosu and others, "Transmission of Super Hi-Vision video using optical time division multiplexing", IEICE Technical Report, IEICE Technical Report, IA2009-91, p.53-p.58, (2010) Keiyuki Ome, Kyoichi Onoji, Ikuo Yamashita, Katsuji Higuchi, "Examination of 10GHz clock signal extraction from 160Gbit / s optical RZ signal", IEEJ Trans. EIS, vol.125, No.10, p.1608-p .1613, (2005) Kozo Fujii, “Development of Ultra-High-Speed Optical Signal Processing Technology”, Oki Technical Review, No. 204, Vol.72, No.4, p.70-p.75, October 2005 Takayuki Kurosu et al., “Optical Time Division "Transmission of Super Hi-Vision Video Using Multiplex", IEICE Technical Report, IEICE Technical Report, IA2009-91, p.53-p.58, (2010)

For the OOK type OTDM signal, two methods have been proposed in which the phase of the electric field of the optical pulse signal is controlled to add channel identification information to the data.
In the first example, as shown in FIG. 4, when an RZ optical signal having an N-channel symbol rate f ₀ [baud / sec] generated by applying intensity modulation to the output of the short pulse light source 6 is multiplexed. In addition, the optical path length of each channel is adjusted to give phase shifts φ1 to φN fixed to the photoelectric field between adjacent pulses. By changing the magnitude of these phase shifts for each channel, a clock frequency component is added to the light intensity spectrum of the OTDM signal and output to the optical clock extraction unit 8.
(See Non-Patent Document 3 above).
In this example, it is necessary to precisely control the electric field phase of all the optical pulses, and high fabrication accuracy is required for the optical signal multiplexing (MUX) unit.
Further, it is necessary to perform feedback control on the optical path length of each channel so that the photoelectric field phase does not change with respect to environmental changes such as temperature, and the system becomes more complex as the multiplicity N increases.

The second example is proposed by the present inventors in a previous application (Japanese Patent Application No. 2011-000122). As shown in FIG. 5, the multi-channel signal generation unit uses a symbol rate f ₀ [baud / Second] OOK-RZ optical signal is generated, and the MUX / channel identification information adding unit 10 applies phase modulation only to a specific channel (reference channel) to perform time division multiplexing.
This makes it possible to distinguish the reference channel shown in black from other channels (normal channels without phase modulation). In the figure, 11 is a phase modulation detector, 12 is a clock extractor, and 13 is an OTDM signal divider.
This method does not require precise control of the photoelectric field phase over all pulses, but the transmitter requires an intensity modulator for data transmission and a phase modulator for channel information transmission, and the receiver also performs phase modulation. The system may be complicated, such as requiring a detector.
Both of these methods are effective for the OOK type OTDM because they use the photoelectric field phase, but cannot be applied to the PSK type OTDM.

  The present invention has been made to solve the above-described problems of the prior art, and an object of the present invention is to obtain an optical time division multiplex transmission system having a simple system configuration without lowering spectrum use efficiency. That is.

In order to solve this problem, the present invention modulates the pulse interval in OTDM transmission and adds a clock frequency component to the light intensity spectrum, so that the receiver always outputs a clock signal to the data at a constant timing. Makes it possible to extract.
More specifically, in the optical time division multiplexing transmission system of the present invention, the following technical means were taken. That is, by using N optical transmitters that output optical pulse signals (RZ signals) having a symbol rate of f ₀ [baud / sec], the N parallel optical signals are overlapped with a multiplexing device shifted in time. An optical signal output from at least one of the optical transmitters in an optical time division multiplex transmission system that converts the signal into a serial signal, transmits it, separates the received signal into parallel signals at the receiver, and performs signal processing for each channel. The wavelength of the pulse is made different from the wavelength of the optical signal pulse output from the other optical transmitter, and the symbol rate f _{0 is} determined based on the optical signal pulse from the optical signal pulse having the different wavelength by the receiver. Are extracted and signal processing is performed for each channel.

According to the present invention, by inserting optical signal pulses having different wavelengths between data, the intensity spectrum of the OTDM signal is equal to the symbol rate f ₀ or an integral multiple thereof without manipulating the phase or intensity of the optical signal pulse. A frequency component can be added. Since the clock frequency f ₀ [Hz] can be extracted on the basis of the added frequency component, PSK that transmits information using the phase of an optical pulse, which has recently become increasingly important with the spread of digital coherent communication. Also in the system, channel identification information can be put on the OTDM signal with the same system configuration as that of normal OTDM transmission. Moreover, the clock can be extracted by a normal electronic circuit on the receiving side without using an ultrahigh-speed optical device or the like.
Therefore, the apparatus is simple and can transmit a signal with high quality as compared with a method of adding channel identification information to data using phase or intensity modulation. In addition, this method can be applied to both OOK and PSK transmission formats.

FIG. 1 is a diagram illustrating the principle of OTDM transmission. FIG. 2 is a diagram for explaining the necessity of extracting a clock signal. FIG. 3 is a diagram illustrating an example of a conventional technique. FIG. 4 is a diagram illustrating another example of the prior art. FIG. 5 is a diagram showing still another example of the prior art. FIG. 6 is a diagram showing a conventional pulse train in which optical signal pulses are arranged at the same time interval in order to explain the basic principle of the present invention. FIG. 7 is a diagram showing a pulse train in which a time interval of ΔT is inserted for each N channel signal. FIG. 8 is a diagram illustrating the transmission side. FIG. 9 is a diagram illustrating the reception side. FIG. 10 is a diagram illustrating the transmission side in the modified example. FIG. 11 is a diagram illustrating a reception side in the modified example. FIG. 12 is a diagram illustrating a transmission side in another modified example. FIG. 13 is a diagram illustrating a receiving side in another modified example. FIG. 14 is a diagram showing the transmission side in the embodiment of the present invention. FIG. 15 is a diagram showing the receiving side in the embodiment of the present invention.

6 and 7 show the principle of the present invention.
In OTDM, as shown in FIG. 6, optical signal pulses (RZ optical signals) a of Ch _{1 to} Ch _N are usually arranged at the same time interval T, where the horizontal axis is time.
Here, if the clock frequency of the transmitter is f ₀ [Hz] and the signal multiplicity is N, the fundamental frequency of the spectrum of the light intensity waveform of this OTDM signal is f ′ = N × f ₀ [Hz], and f The frequency component of ₀ [Hz] is not included.
Therefore, in the present invention, a new frequency component is added to the light intensity spectrum of the OTDM signal by arranging the pulse intervals so that they are periodically changed when multiplexing optical signals.

That is, in the example shown in FIG. 1, when the clock frequency of the optical signal pulse to be output from each optical transmitter on the transmission side is f ₀ [Hz] and the signal multiplicity is N, in the multiplexer (Mux) 3, A pulse arrangement as shown in FIG. 7 is inserted by inserting a time interval of ΔT between the optical signal pulse from the last channel in the series of N optical signal pulses and the first channel starting next. Can be obtained.
A clock frequency component having a sufficient strength to be extracted using a PLL circuit is added to the light intensity spectrum of the OTDM signal thus generated. Further, in the dividing device, it is possible to identify the channel based on the presence of ΔT.

8 and 9 show typical examples of pulse arrangements that enable channel identification. 8 shows the transmitting side, and FIG. 9 shows the receiving side.
On the transmission side, in synchronization with a clock signal (electric signal) with a clock frequency f ₀ [Hz], the signal generator 14 generates N channels of electric signals with a transmission rate f ₀ [baud / sec]. The signal is converted into an RZ optical signal by an electric / optical converter 15 including an electro-optic modulator. These N-channel parallel RZ optical signals are converted by the optical time division multiplexing unit 16 into serial signals having a transmission rate of f ′ = N × f ₀ [baud / sec].
Then, by changing the optical path length connecting the electrical / optical conversion unit 15 and the optical time division multiplexing unit 16 by a certain length for each channel, the optical time division multiplexing unit 16 receives the channel ( Optical signal pulses arranged at uniform time intervals are output from Ch ₁ ) to channel (Ch _N ), and thereafter the optical time division multiplexing unit 16 inserts a gap corresponding to the above-described ΔT, and this cycle is repeated. .
Thus, as shown in FIG. 7A, after the optical signal pulse train of equal intervals from the channel (Ch ₁ ) to the channel (Ch _N ) from the optical time division multiplexing unit 16, a pulse interval longer than this uniform time interval by ΔT. The OTDM signal (transmission rate N × f ₀ [baud / sec]) is output.

On the other hand, on the receiving side, as shown in FIG. 9, a part of the OTDM signal is branched by the branching filter 17, and in the clock extraction unit 18, by the optical receiver 19 made of a photodiode or the like, By extracting the gap of ΔT, the clock frequency f ₀ [Hz] can be extracted via the PLL circuit 20.
In this example, _N optical signal pulses with a symbol rate (frequency) of f ₀ [baud / sec] are output from the channels (Ch _{1 to} Ch _N ) at equal intervals. 16, the optical signal from the (N + 1) channel is processed, and multiplexing is performed assuming that the (N + 1) th optical signal pulse is present after the optical signal pulse from Ch _N. However, since there is no (N + 1) th optical signal pulse, the optical signal pulse of the channel (Ch _N ) and the optical signal of the channel (Ch ₁ ) of the next cycle are equivalent to one optical signal pulse and the normal time interval. A time gap will be formed between the pulses.

Since there is no (Ch _{N + 1} ) optical transmitter, in the multiplexed optical signal pulse, the optical signal pulses from channel 1 to channel N are arranged at equal intervals, and then an empty pulse is inserted. Then, an optical signal pulse of equal intervals from the pulse from channel 1 to channel N continues again, and this is repeated.

In the receiver, the clock extraction unit 18 detects that the time interval of the optical signal pulse has increased due to this empty pulse, so that light having a symbol rate (frequency) of f ₀ [baud / sec] is transmitted from the transmitter side. It can be recognized that the signal is transmitted with N-channel multiplexing. In addition, since the channel 1 to channel N signals are sequentially transmitted after the empty pulse, the data channel can be identified based on the empty pulse, and the OTDM signal dividing unit 21 can separate the OTDM signal.

As shown in FIG. 10 (reception side) and FIG. 11 (transmission side), the signal generation unit 14 may be provided with a channel (Ch _{N + 1} ) that does not emit an optical signal pulse.
Also, in this example, the time interval between the optical signal pulses is different from the normal one, provided between the optical signal pulse from Ch _N and the optical signal pulse from Ch _{1 of the} next cycle. It may be provided between any of Ch _{1 to} Ch _N-1 , and the time interval between optical signal pulses is one empty pulse, but it may be a plurality.

In short, if at least one time interval different from the time interval set at equal intervals is formed among the optical signal pulses of the channel (Ch ₁ ) to the channel (Ch _N ), the PLL circuit 20 converts it. By specifying the clock frequency f ₀ , it is possible to extract the clock frequency f ₀ , so that the time interval may be shorter than the time interval set at equal intervals.

In this example, among the optical signal pulses of the channel (Ch ₁ ) to the channel (Ch _N ), the time interval between specific optical signal pulses is set to a time interval different from the time interval set to be equal. Thus, the clock frequency f ₀ is extracted. However, if at least one unique time interval different from the others among the optical signal pulses of the channel (Ch ₁ ) to the channel (Ch _N ) is formed, the optical frequency spectrum of the OTDM signal always has a clock frequency component. Therefore, the clock frequency f ₀ can be extracted by the PLL circuit 20 if a different time interval is set between arbitrary optical signal pulses as well as between specific optical signal pulses. It becomes.

In the above, by setting the time interval between the specific optical signal pulses among the optical signal pulses of the channel (Ch ₁ ) to the channel (Ch _N ) to be different from the time interval set at equal intervals. Although the clock frequency is extracted, the time interval of the optical signal pulses of the channels (Ch ₁ ) to (Ch _N ) may be changed at a predetermined period, and the clock frequency may be extracted at this period.
That is, as shown in FIG. 12 (transmission side) and FIG. 13 (reception side), N optical signal pulses from channel (Ch ₁ ) to channel (Ch _N ) are represented by M (M is a divisor of N). ) Then, the optical path length between the electrical / optical conversion unit 15 and the optical time division multiplexing unit 16 is changed so that the time interval between adjacent optical signal pulses for each M is T _{1 to} T _M, and N optical signals are obtained. N / M optical signal pulse groups whose time intervals change periodically are formed in the pulse.

A new component is added to the frequency (N / M) f ₀ [Hz] in the light intensity spectrum of the OTDM signal thus formed. This OTDM signal is input to the PLL circuit 20 via the optical receiver 19 in the clock extraction unit 18 to detect a frequency component of (N / M) f ₀ [Hz], and this is detected by the frequency divider 22 as M By multiplying by / N, the clock frequency f ₀ can be extracted and output to the OTDM signal divider 21.

FIGS. 14 and 15 show the configurations of the transmission side and the reception side according to the embodiment of the present invention, respectively.
In this example. Instead of changing the time interval between optical signal pulses, every N optical signal pulses from channel (Ch ₁ ) to channel (Ch _N ), an optical pulse having a wavelength different from that of other optical signal pulses Insert at least one.
That is, as shown in FIG. 14, in the signal generation unit 14, in addition to the channels (Ch ₁ ) to (Ch _N ) for transmitting original information, an electrical signal of the channel (Ch _{N + 1} ) is formed, Of the electrical / optical converter 15 corresponding to the channel (Ch _{N + 1} ), the optical transmitter corresponding to the channel (Ch ₁ ) to the channel (Ch _N ) outputs, for example, the wavelength λ 1 and the channel (Ch _{N + The} optical transmitter corresponding to ₁ ) outputs a pulse train of wavelength λ2. That is, from the optical time division multiplexing unit 16, a signal in which pulsed light of another wavelength is present is output as an OTDM signal for each N-channel data (black waveform).
The OTDM signal thus formed includes a data pulse and a clock pulse. As shown in FIG. 15, on the receiving side, the OTDM signal is input to a filter 23 that passes through a duplexer 17 and λ2, and has a wavelength λ2 Only the optical signal pulse is extracted, and the optical receiver 19 and the PLL circuit 20 can extract the clock frequency f ₀ [Hz].

On the other hand, the optical signal pulse of wavelength λ1 extracted from the demultiplexer 17 through the filter 24 that transmits λ1 is the optical signal pulse from the channel (Ch ₁ ) to the channel (Ch _N ). Is output to the OTDM signal dividing unit 21 and processed as signal data on the receiving side.
As shown in FIG. 15, on the receiving side, the filter 24 removes the optical signal pulse having the wavelength λ2 from the channel (Ch _{N + 1} ), thereby demultiplexing the OTDM signal in which the empty pulse is formed. The symbol rate f ₀ may be extracted by the PLL circuit 20 by sending it to the optical receiver 19 through 25 and making the time interval different from that of other optical signal pulses by the empty pulse.

As described above, according to the present invention, a clock frequency component can be added to the intensity spectrum of an OTDM signal by inserting optical signal pulses having different wavelengths into optical signal pulses from N channels. As a result, even in the PSK system in which information is transmitted using the phase of an optical pulse, channel identification information can be included in an OTDM signal with the same system configuration as that of normal OTDM transmission. Moreover, the clock can be extracted by a normal electronic circuit on the receiving side without using an ultrahigh-speed optical device or the like.
For this reason, for example, it can be expected to be widely used in communication systems for transmitting video signals for Super Hi-Vision that have been researched in recent years.

DESCRIPTION OF SYMBOLS 1 Multichannel signal transmitter 2 Optical transmitter 3 Multiplexer 4 Divider 5 Optical clock extractor 6 Short pulse light source 7 OTDM signal generator 8 Optical clock extractor 9 Multichannel signal generator 10 MUX / channel identification information adding unit DESCRIPTION OF SYMBOLS 11 Phase modulation detection part 12 Clock extraction part 13 OTDM signal division part 14 Signal generation part 15 Electricity / optical conversion part 16 Optical time division multiplexing part 17 Splitter 18 Clock extraction part 19 Optical receiver 20 PLL circuit 21 OTDM signal division Part 22 Frequency divider 23 Filter (transmits λ1)
24 filter (transmits λ2)
25 duplexer

Claims (2)

Using N optical transmitters that output optical pulse signals (RZ signals) with a symbol rate of f ₀ [baud / sec], the N parallel optical signals are overlapped with a time shift by a multiplexing device to obtain a serial signal. In an optical time division multiplex transmission system in which the received signal is separated into parallel signals in the receiver and signal processing for each channel is performed in the receiver,
The wavelength of an optical signal pulse output from at least one of the optical transmitters is different from the wavelength of an optical signal pulse output from another optical transmitter, and the optical signal pulse of which the receiver has a different wavelength is used. An optical time division multiplex transmission system characterized in that the symbol rate f ₀ is extracted based on the optical signal pulse from and the signal processing is performed for each channel. In the receiver, the optical signal pulse having the different wavelength is removed by a filter to form an empty pulse, which is different from the time interval between the other optical signal pulses, thereby reducing the symbol rate f ₀ . 2. The optical time division multiplex transmission system according to claim 1, wherein the optical time division multiplex transmission system is extracted.

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