JP5385858B2 - Generation method and apparatus for optical orthogonal frequency division multiplexing optical signal - Google Patents

Description

  The present invention relates to a generation method and a generation apparatus for generating an optical orthogonal frequency division multiplexing optical signal for optical orthogonal frequency division multiplexing transmission realizing high frequency utilization efficiency in optical fiber communication.

  As a recent demand for optical fiber communication systems, research and development of transmission systems and devices based on wavelength division multiplexing optical transmission technology and capable of a total transmission capacity of several terabits / second to several tens of terabits / second has been actively conducted. It has been broken. In the case of the intensity modulation by binary transmission using only the intensity “0” and “1” as signals as used in the conventional optical fiber communication, and the direct detection method, the speed increases to 40 gigabit / second or 100 gigabit / second. As a result of the spread of the optical spectrum of the transmitted signal, not only the dispersion tolerance is abruptly deteriorated, but it is necessary to ensure a wide wavelength interval in order to avoid crosstalk between channels in wavelength division multiplexing. And the network flexibility. Therefore, as a method for increasing the bit rate without occupying the frequency band per channel, techniques such as multi-level modulation and multiplexing are important.

  Orthogonal Frequency Division Multiplexing (OFDM) is promising as a technique for improving dispersion tolerance and increasing frequency utilization efficiency. This method is a kind of multi-carrier transmission method, and it is separated at the receiving end against the overlap of modulation spectrum between subcarriers by multiplexing under the condition that each subcarrier is orthogonal. Since it can be demodulated, it has been put to practical use in wireless transmission including mobile radio communication. In orthogonal frequency division multiplexing, the bit rate per subcarrier can be reduced to 1 / N (N: the number of optical subcarriers) compared to single carrier transmission. By applying this, it is possible to suppress the spread of the spectrum of the transmission signal of each subcarrier and relax the limitation on the transmission capacity caused by the chromatic dispersion or polarization mode dispersion of the optical fiber.

  As means for applying the orthogonal frequency division multiplexing method to an optical fiber communication system, there is a method of driving an optical modulator using an OFDM baseband signal generated by electrical processing as shown in Non-Patent Document 1. This system can be expected to have an affinity with the conventional wireless system, but in an optical fiber communication system that requires high speed and large capacity, the transmission speed is limited by the band of the electrical processing system.

  On the other hand, as shown in Patent Literature 1 and Patent Literature 2 and the like, a method has been proposed in which optical orthogonal subcarriers are generated in the light region and each subcarrier is intensity-modulated or phase-modulated. For the purpose of transmission simulation, a single modulator can be used to modulate optical orthogonal subcarriers at once to obtain transmission capacity. However, in actual applications, each subcarrier modulates with a different data signal. Therefore, a plurality of modulators are essential.

JP 2009-017320 A JP 2009-124700 A

SL Jansen, I. Morita, N. Takeda, and H. Tanaka, "20-Gb / s OFDM Transmission over 4,160-km SSMF Enabled by RF-Pilot Tone Phase Noise Compensation", Proc. Of OFC / NFOEC2007, paper PDP15, 2007

  In the method of generating and separating optical orthogonal subcarriers as shown in Patent Literature 1 and Patent Literature 2 and individually performing optical modulation, the bandwidth of an electric circuit for driving the optical modulator is increased by increasing the number of subcarriers. Simultaneously with the relaxation, the frequency utilization efficiency can be improved. On the other hand, the increase in subcarriers increases not only the number of optical modulators and high-frequency electrical circuits for driving them, but also between optical modulators due to high-frequency electrical wiring for connecting electrical circuits and optical modulators. Electrical crosstalk becomes a problem. As a result, the number of practical optical subcarriers is limited. The setting of the number of optical subcarriers is an important parameter for obtaining the transmission characteristics of the optical OFDM signal such as obtaining good transmission characteristics according to the characteristics of the optical transmission path and improving the frequency utilization efficiency.

  The present invention has been made in view of the above problems, and an optical orthogonal frequency division multiplexing optical signal that generates an optical OFDM signal with high frequency utilization efficiency by a method different from the method of individually modulating optical subcarriers as described above. An object of the present invention is to provide a generation method and a generation apparatus.

A method for generating an optical orthogonal frequency division multiplexing optical signal according to a first invention for solving the above-described problem is as follows.
Inputting an input optical signal of a single wavelength;
Converting the input optical signal into an optical signal having two or more wavelengths, and demultiplexing the optical signal for each wavelength;
Generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
For each wavelength of the demultiplexed optical signal, superimposing the optical signal with the different converted light, and performing wavelength conversion to another optical signal having the converted light as a central wavelength;
For each wavelength of the other optical signal subjected to wavelength conversion, the bandwidth is limited so that the bandwidth of the frequency of the other optical signal corresponds to twice the bit rate. Passing the signal;
Combining and outputting the other optical signals transmitted through the number of wavelengths, and
An optical orthogonal frequency division multiplexed optical signal is generated from the input optical signal that has been input.

A method for generating an optical orthogonal frequency division multiplexing optical signal according to a second invention for solving the above-described problems is as follows.
In the method for generating an optical orthogonal frequency division multiplexing optical signal according to the first invention,
The step of demultiplexing comprises:
For each bit of the input optical signal, superimposing the input optical signal on a different wavelength, converting the wavelength into an optical signal having the wavelength as a central wavelength, and converting the optical signal into an optical signal having the number of wavelengths. ,
And a step of demultiplexing the converted optical signal for each wavelength.

A method for generating an optical orthogonal frequency division multiplexing optical signal according to a third aspect of the present invention for solving the above-described problem is as follows.
Inputting an input optical signal of a single wavelength;
Dividing the bit rate of the input optical signal by two or more wavelengths to obtain another bit rate, and generating converted light having the number of wavelengths having a frequency interval corresponding to the other bit rate;
For each bit of the input optical signal, superimposing the input optical signal with the different converted light, and performing wavelength conversion to an optical signal having the converted light as a central wavelength;
For each bit of the input optical signal, pass the optical signal by limiting the bandwidth so that the frequency bandwidth of the wavelength-converted optical signal is a frequency equivalent to twice the other bit rate. And a step of causing
An optical orthogonal frequency division multiplexed optical signal is generated from the input optical signal that has been input.

A method for generating an optical orthogonal frequency division multiplexing optical signal according to a fourth invention for solving the above-described problems is as follows.
In the method for generating an optical orthogonal frequency division multiplexing optical signal according to the third invention,
The passing step includes:
For each bit of the input optical signal, the optical signal is allowed to pass by synchronizing the center wavelength of the bandwidth to be limited to the wavelength of the converted light.

A method for generating an optical orthogonal frequency division multiplexing optical signal according to a fifth invention for solving the above-described problems is as follows.
Inputting a wavelength division multiplexed optical signal composed of two or more wavelengths;
Demultiplexing the wavelength division multiplexed optical signal into optical signals for each wavelength;
Generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
For each wavelength of the demultiplexed optical signal, superimposing the optical signal with the different converted light, and performing wavelength conversion to an optical signal having the converted light as a central wavelength;
Combining and outputting the wavelength-converted optical signals of the number of wavelengths,
An optical orthogonal frequency division multiplexed optical signal is generated from the input wavelength division multiplexed optical signal.

An apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to a sixth aspect of the present invention for solving the above-described problem is provided.
A converter that converts an input optical signal having a single wavelength into an optical signal composed of two or more wavelengths, and demultiplexes the optical signal for each wavelength;
A converted light generator for generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
A wavelength converter that is provided for each wavelength of the demultiplexed optical signal, superimposes the optical signal with the different converted light for each wavelength, and converts the wavelength into another optical signal having the converted light as a central wavelength. When,
Provided for each wavelength of the demultiplexed optical signal, and for each wavelength of the other optical signal subjected to wavelength conversion, the frequency bandwidth of the other optical signal is a frequency corresponding to twice the bit rate. A filter that limits the bandwidth and passes the other optical signal;
An optical multiplexer that multiplexes and outputs the other optical signals transmitted through the number of wavelengths;
An optical orthogonal frequency division multiplexed optical signal is generated from the input optical signal that has been input.

An apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to a seventh aspect of the present invention for solving the above-described problem is provided.
In the apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to the sixth invention,
The converter is
For each bit of the input optical signal, the input optical signal is superimposed on a different wavelength, converted into an optical signal having the wavelength as a central wavelength, and converted into an optical signal having the number of wavelengths. A wavelength converter;
And an optical demultiplexer that demultiplexes the converted optical signal for each wavelength.

An apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to an eighth invention for solving the above-described problem is provided.
Dividing the bit rate of the input optical signal of a single wavelength by the number of two or more wavelengths to obtain another bit rate, and converting the converted number of wavelengths having a frequency interval corresponding to the other bit rate. A converted light generator to generate,
For each bit of the input optical signal, a wavelength converter that superimposes the input optical signal with the different converted light and converts the wavelength into an optical signal having the converted light as a center wavelength;
For each bit of the input optical signal, pass the optical signal by limiting the bandwidth so that the frequency bandwidth of the wavelength-converted optical signal is a frequency equivalent to twice the other bit rate. And a filter to be
An optical orthogonal frequency division multiplexed optical signal is generated from the input optical signal that has been input.

An apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to a ninth aspect that solves the above-described problems is provided by:
In the apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to the eighth invention,
The filter is
It is a synchronous wavelength tunable filter that allows the optical signal to pass by synchronizing the center wavelength of the limited bandwidth with the wavelength of the converted light for each bit of the input optical signal.

An apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to a tenth aspect of the present invention for solving the above-described problem is provided.
An optical demultiplexer for demultiplexing an input wavelength division multiplexed optical signal composed of two or more wavelengths into an optical signal for each wavelength;
A converted light generator for generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
A wavelength converter that is provided for each wavelength of the demultiplexed optical signal, superimposes the optical signal with the different converted light for each wavelength, and converts the wavelength into an optical signal having the converted light as a central wavelength;
An optical multiplexer that multiplexes and outputs the optical signal having undergone wavelength conversion of the number of wavelengths,
An optical orthogonal frequency division multiplexed optical signal is generated from the input wavelength division multiplexed optical signal.

  According to the present invention, an optical OFDM signal with high frequency utilization efficiency can be generated without being affected by electrical crosstalk.

It is a schematic block diagram which shows the production | generation apparatus of the conventional optical OFDM signal. It is a schematic block diagram which shows an example (Example 1) of the embodiment about the production | generation apparatus of the optical OFDM signal which concerns on this invention. It is a figure explaining the production | generation of an optical OFDM signal, (a) is an input signal spectrum, (b) is a spectrum of an optical subcarrier, (c) is a spectrum after wavelength conversion, (d) is the light after passing an optical filter. The spectrum, (e), is the optical spectrum after combining. FIG. 3 is a schematic configuration diagram illustrating an optical serial / parallel converter in the optical OFDM signal generation device illustrated in FIG. 2. It is a figure which shows the time response of the wavelength control light source in 1 to 4 optical serial parallel conversion. It is a figure which shows the time response waveform in 1 to 4 optical serial parallel conversion, (a) is the input optical signal, (b) is the output signal after a wavelength converter, (c) is after an optical demultiplexer Signal. It is a schematic block diagram which shows another example (Example 2) of the embodiment about the production | generation apparatus of the optical OFDM signal which concerns on this invention. It is a figure which shows the time response of a wavelength variable light source transmission frequency when the number of optical subcarriers N is 4, and a synchronous high-speed wavelength variable optical filter center wavelength. It is a graph which shows the relationship between the transmission distance and the bit rate which considered the loss limit and dispersion | distribution limit of the optical fiber in single carrier transmission. It is a schematic block diagram which shows another example (Example 3) of the embodiment about the production | generation apparatus of the optical OFDM signal which concerns on this invention. It is a figure which shows the spectrum of the multiplexing system by the conventional WDM system and an optical OFDM signal.

  Hereinafter, some embodiments of a method and an apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to the present invention will be described with reference to FIGS.

Example 1
First, before describing the present embodiment, the basic principle of orthogonal frequency division multiplexing (hereinafter referred to as OFDM) will be described. OFDM is a type of multi-carrier modulation, and at the same time it reduces the speed per subcarrier by parallel transmission of high-speed signal sequences, and at the same time, suppresses the occupied bandwidth to be transmitted by using the orthogonal relationship between subcarriers. be able to. Here, the orthogonal relationship in OFDM is a transmission spectrum for each subcarrier when a relationship of Δf = 1 / T is established between the symbol rate 1 / T for modulating each subcarrier and the optical subcarrier frequency interval Δf. Even if they overlap, it is possible to separate and identify each subcarrier at the receiving end.

  When applying the above basic principle of OFDM to an optical fiber communication band, optical carrier frequencies (= wavelengths) are arranged at frequency intervals (or wavelength intervals) satisfying an orthogonal relationship, and the frequency intervals are set as optical subcarriers. It is implemented by modulating at a symbol rate that matches

  Here, a conventional generation apparatus for generating an optical OFDM signal is shown in FIG. 1, and an OFDM generation method will be described using this.

  In a conventional optical OFDM signal generator, an optical subcarrier generator 11 uses an output light from a light source 12 such as a semiconductor laser as a seed light, and uses an input optical subcarrier (converted light) with a constant optical subcarrier frequency interval. Generate. The generated input optical subcarrier is demultiplexed for each input optical subcarrier via the optical demultiplexer 13 and input to the optical modulators M1 to M4. In the optical modulator driving high-frequency electric circuits E1 to E4, the electric baseband signal for driving the optical modulators M1 to M4 is based on the data signals 1 to 4 and has a symbol rate that matches the optical subcarrier frequency interval. Thus, the orthogonal relationship of OFDM is satisfied. Optical OFDM signals are generated by collectively multiplexing the modulated optical subcarriers output from each of the optical modulators M1 to M4 using the optical multiplexer 14. At this time, by increasing the number of optical subcarriers, the total transmission capacity is made constant, and the occupied bandwidth of each optical subcarrier is suppressed, so that it is possible to improve dispersion tolerance and relax the bandwidth of the electric processing system.

  However, as described above, the increase in the number of optical subcarriers causes an increase in the number of optical modulator driving high-frequency electric circuits and the number of electrical wirings required for the connection with the increase in the number of optical modulators, and between the wiring channels This is a problem of crosstalk. In particular, when modularization is assumed, it can be said that the upper limit of the number of optical subcarriers is determined by a realistic configuration. Although the increase in the number of optical subcarriers is important for improving the frequency utilization efficiency, the upper limit is limited by electric crosstalk.

  Therefore, in the present embodiment, the generation apparatus that generates the optical OFDM signal is configured as shown in FIG.

  The generating apparatus of this embodiment includes an optical subcarrier generator 11 (converted light generator), a light source 12, an optical demultiplexer 13, an optical multiplexer 14, an optical serial / parallel converter 20 (converter), and a wavelength converter. C1-C4 and band-pass filters (hereinafter referred to as filters) T1-T4.

  The optical serial / parallel converter 20 receives an input optical signal having a single wavelength. Here, the input optical signal is a light having two or more wavelengths (= the number of optical subcarriers). While converting into a signal, the converted optical signal is demultiplexed for every wavelength. The optical serial / parallel converter 20 will be further described in detail with reference to FIG. In the optical subcarrier generator 11 and the light source 12, a constant frequency corresponding to the bit rate B of the optical signal demultiplexed by the optical serial / parallel converter 20 using the output light from the light source 12 such as a semiconductor laser as seed light. Input optical subcarriers (converted light) corresponding to the number of wavelengths having intervals are generated. The optical demultiplexer 13 demultiplexes the input optical subcarriers corresponding to the number of wavelengths generated by the optical subcarrier generator 11 for each input optical subcarrier.

  In this embodiment, as an example, the number of wavelengths (= the number of optical subcarriers) is set to “4”, and wavelength converters C1 to C4 and filters T1 to T4 corresponding to the number of wavelengths are provided. The optical signals demultiplexed by the optical serial / parallel converter 20 are respectively input to the wavelength converters C1 to C4, are generated by the optical subcarrier generator 11, and are demultiplexed by the optical demultiplexer 13. Each input optical subcarrier is input. Each wavelength converter C1 to C4 superimposes an optical signal with a different input optical subcarrier for each wavelength, and converts the wavelength into an optical signal (optical subcarrier) having the input optical subcarrier as a central wavelength. Yes. In the filters T1 to T4, for each wavelength, the frequency bandwidth of the wavelength-converted optical subcarrier is set to a frequency corresponding to twice the bit rate B (that is, a band that is an orthogonal condition for optical OFDM). The optical subcarriers are allowed to pass with limited bandwidth. The optical multiplexer 14 combines and outputs optical subcarriers for the number of wavelengths that have passed through the filters T1 to T4, and as a result, generates an optical OFDM signal.

  As described above, in this embodiment, instead of the optical modulator driving high-frequency electric circuits E1 to E4 and the optical modulators M1 to M4 shown in FIG. 1, optical serial / parallel conversion for converting an optical signal from serial to parallel is performed. The optical OFDM signal is generated using the optical device 20, the wavelength converters C1 to C4, and the filters T1 to T4. That is, instead of driving the optical modulators M1 to M4 with the high frequency signals sent from the high frequency electric circuits E1 to E4 for driving the optical modulators through the electrical wiring, the optical transmission sent from the optical serial / parallel converter 20 is used. An optical OFDM signal is generated by wavelength converters C1 to C4 using an optical signal passing through the road.

  In the present embodiment, the basic operation is to convert the wavelength, which is the carrier frequency of the input optical signal, into optical subcarriers having a frequency interval that is an orthogonal condition of optical OFDM by the wavelength converters C1 to C4. Here, the wavelength converters C1 to C4 mean to operate as optical carrier frequency converters.

  The input optical signal is converted from a serial signal sequence to a parallel signal by the optical serial / parallel converter 20, and the number of parallel output ports at this time corresponds to the number of optical subcarriers. The parallel signal output is used as an optical signal of the wavelength converters C1 to C4, and the output from the optical subcarrier generator 11 is demultiplexed by the optical demultiplexer 13 as CW light (converted light). The outputs from the wavelength converters C1 to C4 are optical subcarriers with frequency intervals that are orthogonal conditions of optical OFDM, and after being converted into such optical subcarriers, the spectrums satisfy the orthogonal conditions of optical OFDM. An optical OFDM signal is generated by being input to the optical multiplexer 14 via the filters T1 to T4 to be shaped and multiplexing using the optical multiplexer 14.

  Here, in the case of binary intensity modulation in which k = 1 using the relationship B = k / T between the symbol rate 1 / T and the bit rate B in the 2 ^ k value modulation method, A method for generating an optical OFDM signal in the apparatus will be described with reference to conceptual diagrams of optical spectra shown in FIGS. In FIG. 3, as an example, the number of optical subcarriers N = 4.

The optical signals input from the optical serial / parallel converter 20 to the wavelength converters C1 to C4 are converted from serial to parallel, and as shown in FIG. The optical spectrum of Here, since the number of optical subcarriers N = 4, the occupied bandwidth = 8B. The optical signals input to the wavelength converters C1 to C4 may have any wavelength as long as the frequencies _fsp are different from each other. On the other hand, as shown in FIG. 3B, the output from the optical subcarrier generator 11 has optical frequencies f1, f2, f3, and f4 with an optical subcarrier frequency interval Δf, and the optical frequencies f1, f2, f3, and f4 are demultiplexed by the optical demultiplexer 13 and input to the wavelength converters C1 to C4, respectively, and used as converted light.

  As an optical signal to be input to the wavelength converters C1 to C4, a parallel signal having an occupied bandwidth N × 2B shown in FIG. 3A, which is an optical output from the optical serial / parallel converter 20, is used. Wavelength conversion is performed using each of the optical frequencies f1, f2, f3, and f4 shown in FIG. 3B, which are the outputs from the optical demultiplexer 13, as the converted lights input to .about.C4. As shown in FIG. 3C, the spectrum of the optical subcarriers converted by the wavelength converters C1 to C4 is shifted to the frequency bands f1, f2, f3, and f4 of the converted light to which the center frequency is input. It can be seen that the shape has an optical spectrum with the same occupied bandwidth N × 2B as the input optical signal.

  In order to satisfy the orthogonal condition B = Δf of the optical OFDM, each spectrum is shaped by the filters T1 to T4. As shown in FIG. 3 (d), the optical spectrum after passing through the filters T1 to T4 has an optical subcarrier frequency interval of Δf, a center frequency of frequency bands f1, f2, f3, and f4, and its shape is occupied. The optical spectrum has a bandwidth 2B, and the orthogonal condition B = Δf is satisfied.

  Then, the optical subcarriers after passing through the filters T1 to T4 are multiplexed by the optical multiplexer 14, and the optical spectrum after the multiplexing by the optical multiplexer 14 is as shown in FIG. 3 (e). The occupied bandwidth (N + 1) Δf is obtained by the number N and the optical subcarrier frequency interval Δf.

  As an example, while the occupied bandwidth of a binary modulation signal of 40 Gbit / s is 80 GHz, the optical subcarrier frequency interval Δf = 10 GHz and the number of optical subcarriers N = 4 optical subcarriers is used. For an optical OFDM signal, it is clear that this results in an occupied bandwidth (N + 1) Δf = (4 + 1) × 10 GHz = 50 GHz. The frequency utilization efficiency defined by dividing the bit rate by the occupied bandwidth is 0.5 bit / s / Hz and 0.8 bit / s / Hz, respectively, and the frequency utilization efficiency is greatly improved by the optical OFDM signal. I understand. Furthermore, by using optical subcarriers with an optical subcarrier frequency interval Δf = 2.5 GHz and the number of optical subcarriers N = 16, the frequency utilization efficiency becomes 0.94 bits / s / Hz, further improving the frequency utilization efficiency. Obviously you can.

  In the present invention, including this embodiment, as a wavelength converter, cross-phase modulation using a semiconductor optical amplifier, a nonlinear optical medium, or the like, cross-gain modulation, or cross-absorption modulation of an electroabsorption modulator is used. Therefore, unlike the conventional method, the optical modulator can be driven by only a bias such as a DC power supply without using a high-frequency electric signal.

  This is a means that can fundamentally solve the problem due to electrical crosstalk as compared with the conventional method in which the number of modulators is increased when the number of optical subcarriers is increased. As the optical multiplexer / demultiplexer, a semiconductor and quartz-based array diffraction grating waveguide, a multimode interference waveguide, or the like can be used. Devices having these waveguides as basic structures can be monolithically or hybridly integrated with semiconductor light sources, optical modulators, optical amplifiers, and the like.

  FIG. 4 is a diagram showing a schematic configuration of the optical serial / parallel converter 20 in the present embodiment. The optical serial / parallel converter 20 uses a wavelength converter 22 (another wavelength converter) and a high-speed wavelength control light source 24 to superimpose the input optical signal on a different wavelength for each bit of the input optical signal. The wavelength is converted into an optical signal having the wavelength as the center wavelength, and thereby converted into an optical signal having wavelengths corresponding to the number of optical subcarriers. Thereafter, the optical demultiplexer 25 is used to demultiplex the converted optical signal for each wavelength and convert it into a parallel signal. At this time, the input optical signal is branched into two using the optical shunt 21, one being input to the wavelength converter 22 and the other being input to the timing transmitter 23. A timing electric signal (clock signal) synchronized with the optical signal is generated, and the high-speed wavelength control light source 24 is driven based on the timing electric signal to generate converted light having different wavelengths to be input to the wavelength converter 22.

FIG. 5 shows the time response of the high-speed wavelength control light source 24 when one-to-four optical serial / parallel conversion is taken as an example. The oscillation frequency and oscillation intensity of the high-speed wavelength control light source 24 are driven so as to change in synchronization with the clock signal, and these optical outputs are used as converted light to the wavelength converter 22. As a result, a signal synchronized with the oscillation frequencies f _sp1 , f _sp2 , f _sp3 , and f _sp4 of the high-speed wavelength control light source 24 is obtained as the output of the wavelength converter 22 in units of one bit time of the input optical signal.

FIG. 6 shows a conceptual diagram of time response waveforms in the wavelength converter 22 and the optical demultiplexer 25 when one-to-four optical serial / parallel conversion is taken as an example, as in FIG. The input optical signal input to the wavelength converter 22 is a serial signal as shown in FIG. Then, as shown in FIG. 6B, the output optical signal of the wavelength converter 22 has the transmission frequencies f _sp1 , f _sp2 , f _sp3 , and f _sp4 of the high-speed wavelength control light source 24 for each bit on the time axis. It becomes a transcribed waveform. By demultiplexing this signal for each wavelength (frequency) by the optical demultiplexer 25, a parallel signal is obtained as shown in FIG. As described above, each parallel signal is input as an optical signal to the wavelength converters C1 to C4, and each of the input optical subcarriers (converted light) from the optical demultiplexer 13 is used to orthogonalize the optical OFDM. Wavelength conversion is performed on optical subcarriers having a frequency interval as a condition. After that, the filter is shaped into a spectrum that satisfies the orthogonal condition of the optical OFDM by the filters T1 to T4, and further multiplexed by the optical multiplexer 14, thereby generating an optical OFDM signal.

  In this embodiment, the number N of optical subcarriers is determined by the number of wavelengths of converted light used for wavelength conversion and the number of output ports of the optical demultiplexer 25. Therefore, by controlling the driving method of the high-speed wavelength control light source 24, the number of wavelengths of the converted light to be output is increased, and by using the optical demultiplexer 25 having the corresponding number of output ports, it can be easily controlled. is there. Here, by using an electron injection control type semiconductor wavelength control light source using a multi-electrode structure, a high-speed frequency modulation light source, or the like as the high-speed wavelength control light source 24, high-speed wavelength tuning becomes possible.

(Example 2)
FIG. 7 is a schematic configuration diagram illustrating an optical OFDM signal generation apparatus according to the present embodiment. The basic principle relating to the generation of the optical OFDM signal is the same as that in the first embodiment described above and is as described in FIG. 3, but this embodiment has a simpler configuration.

  In the first embodiment, a wavelength converter 22 for serial / parallel conversion of an input optical signal and wavelength converters C1 to C4 for generating an optical OFDM signal are necessary for each of the optical signals. A high-speed wavelength control light source 24 for serial / parallel conversion and an optical subcarrier generator 11 for generating an optical OFDM signal are necessary, and separate light sources are necessary. Furthermore, it is essential to use the wavelength converters C1 to C4 and the filters T1 to T4 by the number of optical subcarriers to be used.

  Therefore, in the present embodiment, the generation apparatus that generates the optical OFDM signal is configured as shown in FIG.

  The generator of this embodiment includes an optical shunt 31, a wavelength converter 32, a timing transmitter 33, a control circuit 34, a high-speed wavelength control light source 35 (converted light generator), a synchronous high-speed wavelength tunable filter (hereinafter referred to as a variable filter). 36).

  In the optical shunt 31, an input optical signal having a single wavelength is branched into two, one of the two branched optical signals is input to the wavelength converter 32, and the other is input to the timing transmitter 33. Is done. The timing transmitter 33 generates a timing electric signal (clock signal) synchronized with one bit of the input optical signal, and this clock signal is input to the control circuit 34. The control circuit 34 generates an electrical control synchronization signal based on the clock signal, and this electrical control synchronization signal is input to the variable filter 36 together with the high-speed wavelength control light source 35. The high-speed wavelength control light source 35 is driven based on an electrical control synchronization signal. Here, the input optical subcarrier (converted light) input to the wavelength converter 32 is generated for each bit. The generated input optical subcarriers have a certain frequency interval. This frequency interval is obtained by dividing the bit rate of the input optical signal by the desired number of wavelengths (number of subcarriers) (other bit rates). This corresponds to B.

  Therefore, one of the input optical signals branched into two by the optical shunt 31 is input to the wavelength converter 32 and the input optical subcarrier generated by the high-speed wavelength control light source 35 is input to the wavelength converter 32. The wavelength converter 32 superimposes the input optical signal with a different input optical subcarrier for each bit of the input optical signal, and converts the wavelength into an optical signal (optical subcarrier) having the input optical subcarrier as the center wavelength. It has been converted. That is, the wavelength, which is the carrier frequency of the input optical signal, is wavelength-converted into optical subcarriers having frequency intervals that satisfy the orthogonal condition of optical OFDM.

  The variable filter 36 is also driven based on the electrical control synchronization signal. Here, for each bit of the input optical signal, the frequency bandwidth of the wavelength-converted optical subcarrier is twice that of the other bit rate B. The bandwidth is limited so that the optical subcarrier is allowed to pass so that the frequency is equivalent to that of the optical OFDM (that is, the optical OFDM orthogonal condition is satisfied). As a result, an optical OFDM signal is generated. ing.

  As described above, in this embodiment, the wavelength converter 32 performs wavelength conversion for satisfying the orthogonal condition of the optical OFDM simultaneously with the optical serial / parallel conversion by the wavelength converter 32, and further, variable in synchronization with the wavelength conversion. It is characterized in that the spectrum is shaped by the filter 36.

  A series of operations will be described. An input optical signal is branched into two by an optical shunt 31, and is sent to a wavelength converter 32 and a timing transmitter 33 in the same manner as the optical serial / parallel converter 20 described in the first embodiment. Each is entered. The timing transmitter 33 generates a clock signal synchronized with the input optical signal and inputs the clock signal to the control circuit 34 for driving the high-speed wavelength control light source 35 and the variable filter 36.

  The control circuit 34 performs drive control as shown in FIG. 8 on the high-speed wavelength control light source 35 and the variable filter 36 based on the clock signal. Here, FIG. 8 is an example when the number of optical subcarriers N is four. The input optical subcarriers of the oscillation frequencies f1, f2, f3, and f4 of the high-speed wavelength control light source 35 are operated at a frequency interval Δf that satisfies the orthogonal condition of the optical OFDM, and input to the wavelength converter 32 as converted light. The output optical signal of the wavelength converter 32 is output in synchronization with the oscillation frequency of the high-speed wavelength control light source 35, but its optical spectrum shape is equivalent to the input optical signal. Therefore, the optical OFDM orthogonal condition is not satisfied by simply multiplexing the signals.

  Therefore, as shown in FIG. 8, the time response of the variable filter 36 is such that the occupied bandwidth is fixed to 2B (= 2Δf) with respect to the oscillation frequency (oscillation wavelength) of the input optical subcarrier from the high-speed wavelength control light source 35. At the same time, for each bit of the input optical signal, the center wavelength of the limited bandwidth, that is, the center frequency of the variable filter 36 is synchronized with the oscillation frequency (oscillation wavelength) of the input optical subcarrier of the high-speed wavelength control light source 35. It is variable. Thus, the spectrum is shaped by the variable filter 36 simultaneously with the operation of the optical serial / parallel conversion, and an optical OFDM signal is generated.

In the first embodiment, as shown in FIGS. 3 and 5, the optical frequencies f1, f2, f3, and f4 of the optical subcarrier generator 11 for generating the optical OFDM signal and the high speed for optical serial / parallel conversion are used. The optical frequencies f _sp1 , f _sp2 , f _sp3 , and f _{sp4 of the} wavelength control light source 24 are different optical frequencies, but in this embodiment, the oscillation frequency of the high-speed wavelength control light source 35 and the variable filter synchronized therewith. By using 36, optical serial / parallel conversion and wavelength conversion for generating an optical OFDM signal can be simultaneously performed by a single wavelength converter 32 using the same optical frequency.

  As a synchronous optical filter, an occupied bandwidth that can be an orthogonal condition is designed in advance, or a variable bandwidth optical filter that can be controlled by a DC electric signal is used. By controlling so as to match the optical frequency, an optical OFDM signal can be generated. The basic structure of the optical filter is one that uses a Bragg diffraction grating or one that uses a Fabry-Perot resonator, and can control the center frequency and occupied bandwidth by electron injection control, similar to a high-speed wavelength control light source. Is used.

  In this embodiment, since each of the high-speed wavelength control light source 35 and the variable filter 36 is controlled using a clock signal from the timing transmitter 33, a portion requiring high frequency band electrical processing is substantially controlled. Only the electric wiring for driving the circuit 34, the high-speed wavelength control light source 35, and the variable filter 36 is provided. Therefore, in order to increase the number of optical subcarriers, the control circuit 34 sets the operating point of the oscillation frequency of the high-speed wavelength control light source 35, the center frequency of the variable filter 35 synchronized therewith, and the occupied bandwidth for satisfying the orthogonal condition. This is possible. Therefore, it is possible to solve the problem of high-frequency electrical wiring accompanying the increase in the number of optical subcarriers. In addition, it is possible to flexibly cope with changes in the number of optical subcarriers.

  In this embodiment, the number of wavelength converters 32 is also one, and a reduction in power consumption when the number of wavelength converters 32 increases as the number of optical subcarriers increases can be expected. The upper limit of the number of optical subcarriers N is a variable filter having an occupied bandwidth of 2ΔF / N, assuming that the entire bandwidth is used as an optical subcarrier with respect to the variable bandwidth ΔF of the high-speed wavelength control light source 35 and the variable filter 36. It is determined depending on whether 36 can be configured realistically. When an optical OFDM signal is generated using 10 subcarriers for a 50 GHz variable band, the required occupied bandwidth of the variable filter 36 is 10 GHz.

  When an optical OFDM signal is generated in the 1550 nm band that is a typical optical fiber communication band wavelength, an optical subcarrier is generated in the vicinity of 193.5483 THz in terms of optical frequency. As an example, when generating an optical OFDM signal from a 40 Gbit / s serial signal using an optical subcarrier frequency interval of 2.5 GHz and 16 optical subcarriers, the frequency band of the optical subcarrier is 193.5283 The frequency range is 193.5683 THz, and the wavelength range is 0.32 nm. The occupied bandwidth of the variable filter 36 is 5 GHz due to the orthogonal condition of the optical OFDM. By using an optical OFDM signal, the occupied bandwidth and the frequency utilization efficiency are 42.5 GHz and 0.94 bit / s / Hz, respectively.

  FIG. 9 shows the relationship between the transmission distance and the bit rate in consideration of the loss limit and dispersion limit of the optical fiber in single carrier transmission. Here, it was assumed that the dispersion constant D = 17 ps / km-nm and the wavelength 1550 nm.

  As can be seen from FIG. 9, the transmittable distance is limited in inverse proportion to the bit rate. The limit of the transmission distance of 40 Gbit / s by a single carrier is about 2 to 3 km. However, in the optical OFDM signal, 16 bits modulated at a bit rate of 2.5 Gbit / s due to the orthogonal relationship of each optical subcarrier. The optical subcarriers can be transmitted independently without interfering with each other. This means that the theoretical limit of the transmission distance can be expanded to 600 km despite the transmission capacity of 16 × 2.5 Gbit / s = 40 Gbit / s. Similarly, when a 40 Gbit / s optical signal is converted into an optical OFDM signal with the number of optical subcarriers of 4 and a frequency interval of 10 GHz, the transmission distance can be about 40 km with a frequency utilization efficiency of 0.8 bit / s / Hz.

  In this embodiment, by determining the transmission capacity and transmission distance to be transmitted, considering the specifications of each component such as a wavelength converter, an optical filter, and an optical multiplexer / demultiplexer for generating an optical OFDM signal, The number of optical subcarriers is determined. Therefore, as compared with the conventional method in which a plurality of modulators are driven, this embodiment can flexibly cope with a change in the number of optical subcarriers, and is extremely effective in designing an optical communication system.

(Example 3)
FIG. 10 is a schematic configuration diagram illustrating an optical OFDM signal generation apparatus according to the present embodiment. In this embodiment, an optical signal according to a conventional WDM system is converted into an optical OFDM signal. In the present embodiment, the same reference numerals are given to the same components as those of the generation apparatus shown in the first embodiment.

  The generation apparatus of the present embodiment includes an optical subcarrier generator 11 (converted light generator), a light source 12, an optical demultiplexer 13, an optical multiplexer 14, an optical demultiplexer 42, and wavelength converters C1 to C4. The wavelength converters C <b> 1 to C <b> 4 are provided for each wavelength of the optical signal demultiplexed by the optical demultiplexer 42.

  The wavelength division multiplexed optical signal demultiplexed by the optical demultiplexer 42 is multiplexed (multiplexed) in advance by a plurality of channels, that is, optical signals having two or more wavelengths. Such a wavelength division multiplexed optical signal is input to the optical demultiplexer 42 and demultiplexed into optical signals for each wavelength by the optical demultiplexer 42. In the optical subcarrier generator 11 and the light source 12, a constant frequency interval corresponding to the bit rate B of the optical signal demultiplexed by the optical demultiplexer 42 with the output light from the light source 12 made of a semiconductor laser or the like as seed light. The number of input optical subcarriers (converted light) corresponding to the number of wavelengths is generated. The optical demultiplexer 13 demultiplexes the input optical subcarriers corresponding to the number of wavelengths generated by the optical subcarrier generator 11 for each input optical subcarrier.

  In the present embodiment, as an example, the wavelength division multiplexed optical signal has four channels ch1 to ch4, the number of wavelengths (= the number of optical subcarriers) is “4”, and the wavelength converters C1 to C4 for this number of wavelengths are provided. Is provided. The optical signals demultiplexed by the optical demultiplexer 42 are input to the wavelength converters C1 to C4, respectively, generated by the optical subcarrier generator 11, and demultiplexed by the optical demultiplexer 13. Each input optical subcarrier is input. Each wavelength converter C1 to C4 superimposes an optical signal with a different input optical subcarrier for each wavelength, and converts the wavelength into an optical signal (optical subcarrier) having the input optical subcarrier as a central wavelength. Thus, the optical subcarrier is converted into an optical subcarrier that satisfies the orthogonal condition of optical OFDM. The optical multiplexer 14 combines and outputs the optical subcarriers wavelength-converted by the wavelength converters C1 to C4, that is, the optical subcarriers corresponding to the number of wavelengths, and generates an optical OFDM signal as a result. doing.

  As described above, this embodiment is characterized in that a conventional multiplexing method based on the WDM system is converted into an optical OFDM signal having high frequency utilization efficiency.

  FIG. 11 shows a conceptual diagram of a spectrum multiplexed by a conventional WDM system and a spectrum multiplexed by an optical OFDM signal. In the intensity modulation by the conventional WDM method, when one optical signal is assigned to one frequency grid (= channel) determined by the standard, the frequency utilization efficiency is limited to 0.5 bit / s / Hz at the maximum. The When the interval of the frequency grid is strictly fixed, it becomes smaller than this value. On the other hand, in this embodiment, an optical signal according to a conventional WDM system is converted into an optical OFDM signal and aggregated in one grid, thereby increasing the frequency utilization efficiency.

  As an example, when a 10 Gbit / s optical signal is multiplexed in the WDM system at four wavelengths of a frequency grid of 100 GHz, the frequency utilization efficiency is 0.1 bit / s / Hz. By converting it to a signal, the signal becomes 0.4 bit / s / Hz, which is four times.

  In this embodiment, since a WDM optical signal transmitted at a constant bit rate in an optical fiber is used as an object to be converted into an optical OFDM signal, the optical demultiplexer 42 demultiplexes the WDM optical signal for each wavelength, Are input as optical signals to the wavelength converters C1 to C4. As the converted light, the optical output of the optical subcarrier generator 11 is used as in the first embodiment. At this time, wavelength conversion is performed on the optical signal bit rate B input to the wavelength converters C1 to C4 using input optical subcarriers (converted light) at optical frequency intervals that can be an orthogonal condition of optical OFDM. .

  In this embodiment, optical subcarriers are generated at a frequency interval corresponding to a predetermined bit rate B optical signal, so that the optical OFDM orthogonal condition can be satisfied only by wavelength conversion operation. As a result, spectrum shaping by optical filtering becomes unnecessary. As a result, unlike the first and second embodiments, the optical multiplexer 14 can multiplex the optical OFDM signal without using an optical filter for spectrum shaping. Also, in the case of conversion from a conventional WDM optical signal to an optical OFDM signal, an optical OFDM signal can also be generated in this embodiment without using high-frequency electrical processing.

  The method and apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to the present invention is suitable for optical fiber communication realizing high frequency utilization efficiency.

DESCRIPTION OF SYMBOLS 11 Optical subcarrier generator 14 Optical multiplexer 20 Optical serial / parallel converter 22 Wavelength converter 24 High-speed wavelength control light source 25 Optical demultiplexer 32 Wavelength converter 35 High-speed wavelength control light source 36 Synchronous high-speed wavelength variable filter 42 Optical component Wavelength C1-C4 Wavelength converter T1-T4 Bandpass optical filter

Claims (10)

Inputting an input optical signal of a single wavelength;
Converting the input optical signal into an optical signal having two or more wavelengths, and demultiplexing the optical signal for each wavelength;
Generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
For each wavelength of the demultiplexed optical signal, superimposing the optical signal with the different converted light, and performing wavelength conversion to another optical signal having the converted light as a central wavelength;
For each wavelength of the other optical signal subjected to wavelength conversion, the bandwidth is limited so that the bandwidth of the frequency of the other optical signal corresponds to twice the bit rate. Passing the signal;
Combining and outputting the other optical signals transmitted through the number of wavelengths, and
An optical orthogonal frequency division multiplexing optical signal generation method, comprising: generating an optical orthogonal frequency division multiplexing optical signal from the input optical signal that has been input. The method of generating an optical orthogonal frequency division multiplexed optical signal according to claim 1,
The step of demultiplexing comprises:
For each bit of the input optical signal, superimposing the input optical signal on a different wavelength, converting the wavelength into an optical signal having the wavelength as a central wavelength, and converting the optical signal into an optical signal having the number of wavelengths. ,
And a step of demultiplexing the converted optical signal for each wavelength. A method of generating an optical orthogonal frequency division multiplexing optical signal. Inputting an input optical signal of a single wavelength;
Dividing the bit rate of the input optical signal by two or more wavelengths to obtain another bit rate, and generating converted light having the number of wavelengths having a frequency interval corresponding to the other bit rate;
For each bit of the input optical signal, superimposing the input optical signal with the different converted light, and performing wavelength conversion to an optical signal having the converted light as a central wavelength;
For each bit of the input optical signal, pass the optical signal by limiting the bandwidth so that the frequency bandwidth of the wavelength-converted optical signal is a frequency equivalent to twice the other bit rate. And a step of causing
An optical orthogonal frequency division multiplexing optical signal generation method, comprising: generating an optical orthogonal frequency division multiplexing optical signal from the input optical signal that has been input. The method for generating an optical orthogonal frequency division multiplexing optical signal according to claim 3,
The passing step includes:
A method for generating an optical orthogonal frequency division multiplexing optical signal, wherein the optical signal is passed in synchronization with the wavelength of the converted light for each bit of the input optical signal in synchronization with a center wavelength of the bandwidth to be limited. Inputting a wavelength division multiplexed optical signal composed of two or more wavelengths;
Demultiplexing the wavelength division multiplexed optical signal into optical signals for each wavelength;
Generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
For each wavelength of the demultiplexed optical signal, superimposing the optical signal with the different converted light, and performing wavelength conversion to an optical signal having the converted light as a central wavelength;
Combining and outputting the wavelength-converted optical signals of the number of wavelengths,
An optical orthogonal frequency division multiplexed optical signal generation method, comprising: generating an optical orthogonal frequency division multiplexed optical signal from the input wavelength division multiplexed optical signal. A converter that converts an input optical signal having a single wavelength into an optical signal composed of two or more wavelengths, and demultiplexes the optical signal for each wavelength;
A converted light generator for generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
A wavelength converter that is provided for each wavelength of the demultiplexed optical signal, superimposes the optical signal with the different converted light for each wavelength, and converts the wavelength into another optical signal having the converted light as a central wavelength. When,
Provided for each wavelength of the demultiplexed optical signal, and for each wavelength of the other optical signal subjected to wavelength conversion, the frequency bandwidth of the other optical signal is a frequency corresponding to twice the bit rate. A filter that limits the bandwidth and passes the other optical signal;
An optical multiplexer that multiplexes and outputs the other optical signals transmitted through the number of wavelengths;
An apparatus for generating an optical orthogonal frequency division multiplexed optical signal, wherein an optical orthogonal frequency division multiplexed optical signal is generated from the input optical signal inputted. The apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to claim 6,
The converter is
For each bit of the input optical signal, the input optical signal is superimposed on a different wavelength, converted into an optical signal having the wavelength as a central wavelength, and converted into an optical signal having the number of wavelengths. A wavelength converter;
An apparatus for generating an optical orthogonal frequency division multiplexed optical signal, comprising: an optical demultiplexer that demultiplexes the converted optical signal for each wavelength. Dividing the bit rate of the input optical signal of a single wavelength by the number of two or more wavelengths to obtain another bit rate, and converting the converted number of wavelengths having a frequency interval corresponding to the other bit rate. A converted light generator to generate,
For each bit of the input optical signal, a wavelength converter that superimposes the input optical signal with the different converted light and converts the wavelength into an optical signal having the converted light as a center wavelength;
For each bit of the input optical signal, pass the optical signal by limiting the bandwidth so that the frequency bandwidth of the wavelength-converted optical signal is a frequency equivalent to twice the other bit rate. And a filter to be
An apparatus for generating an optical orthogonal frequency division multiplexed optical signal, wherein an optical orthogonal frequency division multiplexed optical signal is generated from the input optical signal inputted. The apparatus for generating an optical orthogonal frequency division multiplexing optical signal according to claim 8,
The filter is
Optical quadrature frequency division, wherein the optical wavelength division filter is a synchronous wavelength tunable filter that passes the optical signal by synchronizing the center wavelength of the bandwidth to be limited to the wavelength of the converted light for each bit of the input optical signal Multiple optical signal generator. An optical demultiplexer for demultiplexing an input wavelength division multiplexed optical signal composed of two or more wavelengths into an optical signal for each wavelength;
A converted light generator for generating converted light of the number of wavelengths having a frequency interval corresponding to the bit rate of the demultiplexed optical signal;
A wavelength converter that is provided for each wavelength of the demultiplexed optical signal, superimposes the optical signal with the different converted light for each wavelength, and converts the wavelength into an optical signal having the converted light as a central wavelength;
An optical multiplexer that multiplexes and outputs the optical signal having undergone wavelength conversion of the number of wavelengths,
An apparatus for generating an optical orthogonal frequency division multiplexed optical signal, which generates an optical orthogonal frequency division multiplexed optical signal from the input wavelength division multiplexed optical signal.

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