Time-domain mode division multiplexing transmission method and time-domain mode division multiplexing transmission device

Abstract

[Problem] To provide a time-domain mode division multiplexing transmission method and a time-domain mode division multiplexing transmission device capable of mode division multiplexing in the time domain. [Solution] A coherent Hermite-Gaussian pulse with an amplitude given by the Hermite-Gaussian function is used as signal light and local oscillator light.

Description

Time domain mode multiplexing transmission method and time domain mode multiplexing transmission device

[0001] The present invention relates to a time domain mode multiplexing transmission method and a time domain mode multiplexing transmission device using a Hermite-Gaussian pulse whose amplitude is given by a Hermite-Gaussian function.

[0002] Over the past 40 years, optical communication infrastructure has dramatically increased its capacity by more than 10,000 times thanks to numerous technological innovations. This increase in capacity has been primarily achieved through time division multiplexing (TDM) and wavelength division multiplexing (WDM). Meanwhile, Internet traffic continues to grow exponentially at an annual rate of more than 40%, necessitating further capacity expansion in backbone optical transmission networks. Recently, three new high-capacity technologies (known as the "3M" or "Multi" technologies) have been actively researched and developed: multilevel transmission, space division multiplexing (SDM) using multicore fiber, and mode division multiplexing (MDM) using multimode fiber (see, for example, Non-Patent Document 1).

[0003] Mode multiplexing transmission is based on the LP mode, which is the fundamental propagation mode of optical fiber. 01 In addition to the mode, LP 11 , LP 21 , LP 02 This is a method of simultaneously transmitting information in higher-order modes such as 125 μm. Mode multiplexing is useful as a high-density spatial multiplexing technology because it can increase the transmission capacity by N times through N-mode transmission while maintaining the standard outer diameter of the fiber at 125 μm. However, crosstalk due to coupling between modes is unavoidable, so mode separation is required after transmission using signal processing called MIMO (Multi-Input Multi-Output), and realizing this method has been a challenge (see, for example, Non-Patent Document 2).

[0004] Thus, what is called a normal mode in optical communications is a spatial mode that propagates through an optical fiber, and until now, no transmission technology has existed that introduces the concept of a mode to optical pulses that operate in the time domain and realizes mode multiplexing in the time domain.

[0005] Optical pulses used in conventional optical communications typically have waveforms based on RZ (return-to-zero) pulses, i.e., Gaussian or Sech pulses. These pulses can be interleaved on the time axis to enable high-speed TDM (time division multiplexing). Recently, inventors have proposed coherent optical Nyquist pulses as a new optical pulse for TDM to replace RZ pulses (see, for example, Patent Document 1). Nyquist pulses have a Sinc function shape and are characterized by periodic zero intensity. When pulses are interleaved according to this period, adjacent pulses always have zero intensity at the signal decision point, enabling signal decision without interference even when adjacent pulses overlap. As a result, Nyquist pulses enable denser and more efficient TDM than conventional RZ pulses, and have achieved ultra-high-speed transmission of 15 Tbit / s at a single wavelength, which was previously difficult with conventional RZ pulses (see, for example, Non-Patent Document 3).

[0006] M. Nakazawa, M. Suzuki, Y. Awaji, and T. Morioka, “Space-Division Multiplexing in Optical Communication Systems: Extremely Advanced Optical Transmission with 3M Technologies”, Springer, 2022R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. C. Burrows, R.-J. Essiambre, P. J. Winzer, D. W. Peckham, A. H. McCurdy, and R. Lingle Jr, “Mode-Division Multiplexing Over 96 km of Few-Mode Fiber Using Coherent 6x6 MIMO Processing”, J. Lightwave Technol., February 2012, vol. 30, no. 4, p. 521-531M. Yoshida, K. Kimura, T. Iwaya, K. Kasai, T. Hirooka, and M. Nakazawa, “Single-channel 15.3 Tbit / s, 64 QAM coherent Nyquist pulse transmission over 150 km with a spectral efficiency of 8.3 bit / s / Hz”, Opt. Express, September 2019, vol. 27, no. 20, p. 28952-28967

[0007] Japanese Patent Publication No. 5634296

[0008] Because the optical pulses used in TDM can only transmit one pulse at a time in one symbol time slot, achieving ultra-high speeds through TDM requires an increase in the symbol rate, i.e., a narrower time slot width, and therefore ultrashort pulses. However, because ultrashort pulses generally have a wide spectral width, the number of wavelengths that can be multiplexed by WDM within a limited frequency bandwidth is reduced. In other words, there is a trade-off between the increase in speed achieved by TDM and the multi-channelization achieved by WDM, and there has been a problem in that there is a limit to the expansion of transmission capacity using TDM and WDM alone.

[0009] In recent digital coherent transmission, a multilevel transmission technology called QAM (Quadrature Amplitude Modulation) has been used to increase the transmission capacity per unit frequency (spectral efficiency). However, there is a theoretical limit to spectral efficiency called the Shannon limit, and it is difficult to overcome this limit by simply increasing the QAM level. Therefore, as mentioned above, SDM transmission technology using multicore fiber and spatial modes has been actively researched, but it has been difficult to expand the transmission capacity of a single core and single spatial mode by methods other than TDM, WDM, and multilevel modulation.

[0010] The present invention has been made to solve the above problems, and has as its object to provide a time domain mode multiplexing transmission method and a time domain mode multiplexing transmission device that enable mode multiplexing in the time domain.

[0011] In order to achieve the above object, a time domain mode multiplexing transmission method according to the present invention is characterized in that coherent Hermite-Gaussian pulses whose amplitudes are given by Hermite-Gaussian functions are used as signal light and local light.

[0012] A time-domain mode multiplexing transmission apparatus according to the present invention is characterized by comprising: coherent Hermite-Gaussian pulse generating means for generating a coherent Hermite-Gaussian pulse whose amplitude is given by a Hermite-Gaussian function; signal light generating means for using the coherent Hermite-Gaussian pulse as signal light; local light generating means for using the coherent Hermite-Gaussian pulse as local light; and detection / separation / demodulation means for detecting, separating, and demodulating the signal light and the local light.

[0013] The time-domain mode multiplexing transmission method according to the present invention can be suitably implemented by the time-domain mode multiplexing transmission device according to the present invention. The time-domain mode multiplexing transmission method and time-domain mode multiplexing transmission device according to the present invention can simultaneously transmit N pulses in one symbol time slot, thereby transmitting N times the amount of information compared to conventional methods. Furthermore, since there is no need to increase the symbol rate by TDM, a high-speed optical transmission system can be provided using low-speed optical / electronic devices. Furthermore, a highly efficient, large-capacity optical transmission system can be provided using a conventional single-core, single-spatial-mode optical fiber.

[0014] In the time-domain mode multiplexing transmission device according to the present invention, unlike conventional TDM methods, the coherent Hermite-Gaussian pulse generating means is preferably configured to generate and simultaneously transmit N coherent Hermite-Gaussian pulses of orders 0 to N-1 in a one-symbol time slot. In the time-domain mode multiplexing transmission method according to the present invention, the coherent Hermite-Gaussian pulses are preferably generated and simultaneously transmitted N coherent Hermite-Gaussian pulses of orders 0 to N-1 in a one-symbol time slot. In this case, N-fold increased transmission capacity can be achieved. Furthermore, the N coherent Hermite-Gaussian pulses may be synchronized with each other in the time domain.

[0015] Furthermore, the time-domain mode multiplexing transmission method and time-domain mode multiplexing transmission apparatus according to the present invention may achieve demultiplexing by utilizing the orthogonality of coherent Hermite-Gaussian pulses. That is, to demultiplex an m-th order (0≦m≦N−1) coherent Hermite-Gaussian pulse from N coherent Hermite-Gaussian pulses, the m-th order coherent Hermite-Gaussian pulse may be used as a local pulse and homodyne coherent detection may be performed. More specifically, in the time-domain mode multiplexing transmission apparatus according to the present invention, the signal light generating means may generate a signal in which N coherent Hermite-Gaussian pulses are multiplexed as the signal light, the local light generating means may generate the m-th order coherent Hermite-Gaussian pulse as the local light, and the detection / separation / demodulation means may perform homodyne coherent detection of the signal light and the local light and integrate the detected signal over a time slot corresponding to one symbol to demultiplex the m-th order Hermite-Gaussian pulse from the N Hermite-Gaussian pulses. Furthermore, in the time-domain mode multiplexing transmission method according to the present invention, an m-th order coherent Hermite-Gaussian pulse may be used as a local light, and signal light in which N coherent Hermite-Gaussian pulses are multiplexed may be subjected to homodyne coherent detection, and the detected signal after detection may be integrated over a time slot for one symbol, thereby demultiplexing the m-th order coherent Hermite-Gaussian pulse from the N coherent Hermite-Gaussian pulses.

[0016] According to the present invention, it is possible to provide a time domain mode multiplexing transmission method and a time domain mode multiplexing transmission device that are capable of mode multiplexing in the time domain.

[0017] The time waveform a of the Hermite-Gaussian pulse of m=0 to 2 for the time domain mode multiplexing transmission device of the embodiment of the present invention HGm (t) and frequency spectrum A HGm (ω). The time waveform a of the Hermite-Gaussian pulse of m=3 to 5 degrees in the time domain mode multiplexing transmission device according to the embodiment of the present invention. HGm (t) and frequency spectrum A HGm4 is a block diagram showing the configuration of a time-domain mode multiplexing transmission device according to an embodiment of the present invention. It is a block diagram showing the configuration of a time-domain mode multiplexing optical transmission system used in an experiment for the time-domain mode multiplexing transmission device according to an embodiment of the present invention. It shows (a-1) the waveform and (a-2) optical spectrum of a zeroth-order coherent Hermite-Gaussian pulse, (b-1) the waveform and (b-2) optical spectrum of a first-order coherent Hermite-Gaussian pulse, (c-1) the waveform and (c-2) optical spectrum of a second-order coherent Hermite-Gaussian pulse, and (d-1) the waveform and (d-2) optical spectrum of a third-order coherent Hermite-Gaussian pulse, all of which were generated using the experimental system shown in FIG. 1 shows (a-1) back-to-back experimental results (results when a transmitter and a receiver are directly connected without a transmission line) and (a-2) a constellation of a 16-QAM signal after 450 km transmission, showing experimental results using a zero-order Hermite-Gaussian pulse, for a time-domain mode multiplexing transmission device according to the first embodiment of the present invention; (b-1) back-to-back experimental results using a first-order Hermite-Gaussian pulse and (b-2) a constellation of a 16-QAM signal after 450 km transmission; (c-1) back-to-back experimental results using a second-order Hermite-Gaussian pulse and (c-2) a constellation of a 16-QAM signal after 450 km transmission; and (d-1) back-to-back experimental results using a third-order Hermite-Gaussian pulse and (d-2) a constellation of a 16-QAM signal after 450 km transmission.1 shows a constellation of a 16-QAM signal after back-to-back and (a-2) 450 km transmission showing experimental results using a zero-order Hermite-Gaussian pulse, (b-1) a constellation of a 16-QAM signal after back-to-back and (b-2) 450 km transmission showing experimental results using a first-order Hermite-Gaussian pulse, (c-1) a constellation of a 16-QAM signal after back-to-back and (c-2) 450 km transmission showing experimental results using a second-order Hermite-Gaussian pulse, and (d-1) a constellation of a 16-QAM signal after back-to-back and (d-2) 450 km transmission showing experimental results using a third-order Hermite-Gaussian pulse, for a time-domain mode multiplexing transmission device according to a second embodiment of the present invention.

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings etc. As mentioned above, the most common RZ pulse used in optical communications is a Gaussian pulse. The Gaussian function is given by the following equation (1).

[0019] Here, A represents the amplitude and T represents the pulse width (1 / e width).

[0020] The Gaussian function is known as the function with the lowest degree among functions called Hermite-Gaussian functions. In general, the m-th degree Hermite-Gaussian function is given by the following equation (2).

[0021] Here, T is a parameter that represents the pulse width. m (t) is a function called a Hermite function, and is defined by the following formula (3) when m is an even order, and by the following formula (4) when m is an odd order.

[0022]

[0023] Specifically, when m=0 to 6, it is defined by the following formulas (5-1) to (5-7).

[0024] The Fourier transform of the Hermite-Gaussian function is given by the following equation (6).

[0025] Specifically, when m=0 to 6, the following equations (7-1) to (7-7) are obtained.

[0026] Figure 1 shows the case of m = 0 to 2, and Figure 2 shows the case of m = 3 to 5. HGm (t) and A HGm The shapes of (ω) are shown. As shown in these figures, the time waveform is an odd function when m is odd, and an even function when m is even. The spectrum has the same shape as the time waveform, with the sign inverted as the order increases, and the real and imaginary parts swapped. Specifically, when m is odd, it is an odd function with only an imaginary part, and when m is even, it is an even function with only a real part.

[0027] The Hermite-Gaussian function is a function widely known in the fields of science and engineering, such as a high-order eigensolution of the Schrodinger equation for a harmonic oscillator in quantum mechanics, an eigensolution of a high-order beam in a laser resonator (Hermite-Gaussian beam), etc. Furthermore, it has properties such as its shape not changing even when subjected to Fourier transform as shown in equation (6), and an orthogonal relationship between Hermite-Gaussian functions of different orders, i.e., it satisfies the following equation (8).

[0028]

[0029] A method for generating an optical pulse having such a Hermite-Gaussian function as its pulse shape (hereinafter referred to as a Hermite-Gaussian pulse) is disclosed by the inventor in Japanese Patent Application No. 2023-037273. That is, by inserting an optical filter with variable amplitude and phase characteristics into the resonator of an FM mode-locked laser and setting the amplitude and phase characteristics of the optical filter according to the Hermite-Gaussian function of the order to be output, a Hermite-Gaussian pulse of any order can be generated. The specific shapes of the amplitude and phase characteristics of the optical filter are disclosed in Japanese Patent Application No. 2023-037273.

[0030] The configuration of a time domain mode multiplexing transmission device according to an embodiment of the present invention is shown in FIG. 3. The time domain mode multiplexing transmission device according to an embodiment of the present invention can suitably implement the time domain mode multiplexing transmission method according to an embodiment of the present invention. The time domain mode multiplexing transmission device according to an embodiment of the present invention includes N pulse light sources (HG0 to HG N-1 ) 1, N optical modulators 2 connected to the pulse light source 1, N polarization multiplexing circuits 3 connected to the optical modulators 2, an optical fiber transmission line 4 through which the outputs from the N polarization multiplexing circuits 3 are multiplexed and transmitted, and a local pulse light source (HG m ) 5, a 90-degree optical hybrid circuit 6 to which the output from the optical fiber transmission line 4 and the output from the local pulse light source 5 are input, a balanced photodetector 7 connected to the 90-degree optical hybrid circuit 6, and a digital signal processing circuit 8 connected to the balanced photodetector 7.

[0031] In FIG. 3, the pulse light source 1 corresponds to a coherent Hermite-Gaussian pulse generating means that generates a coherent Hermite-Gaussian pulse, but the coherent Hermite-Gaussian pulse generating means is not limited to a pulse light source, and may be any means that can generate a coherent Hermite-Gaussian pulse by shaping light that is not a pulse light source.

[0032] The optical modulator 2 corresponds to a signal light generating means that generates signal light using a coherent Hermite-Gaussian pulse. The local pulse light source 5 corresponds to a local light generating means that generates local light using a coherent Hermite-Gaussian pulse. The 90-degree optical hybrid circuit 6, balanced photodetector 7, and digital signal processing circuit 8 correspond to a detection / separation / demodulation means that detects, separates, and demodulates the signal light and local light using the signal light and local light.

[0033] On the transmitting side, N pulse light sources 1 are used to generate time-synchronized coherent Hermite-Gaussian pulses of orders 0 to N-1 at a repetition frequency R. Each order of Hermite-Gaussian pulse is independently data-modulated at a symbol rate R (the value of the coherent Hermite-Gaussian pulse is always equal to the repetition frequency) using N optical modulators 2, and then polarization-multiplexed using a polarization multiplexing circuit 3 as needed. The generated Hermite-Gaussian pulse trains of orders 0 to N-1 are multiplexed and then transmitted over an optical fiber transmission line 4. In the optical fiber transmission line 4, loss is compensated for using an erbium-doped fiber amplifier (EDFA) or a Raman amplifier as needed.

[0034] On the receiving side, the data signal light after transmission through the optical fiber transmission line 4 is multiplexed with a Hermite-Gaussian pulse of order m (0≦m≦N−1) with a repetition frequency R output from a local pulse light source 5, and homodyne coherent detection is performed using a 90-degree optical hybrid circuit 6 and a balanced photodetector 7. Here, the local pulse output from the light source 5 is synchronized in phase and clock with the transmitted data signal light. An optical phase-locked loop (OPLL) circuit or an optical injection locking circuit is used for phase synchronization. The homodyne coherently detected data signal undergoes mode demultiplexing and demodulation processing as described below using a digital signal processing circuit 8.

[0035] N-mode multiplexed Hermite-Gaussian pulses of orders 0 to (N-1) are expressed by the following equation (9).

[0036] This is the local light a given by the m-th order Hermite-Gaussian pulse. HGm When homodyne detection is performed at (t), the detected signal is a(t) and a HGm (t), that is, given by the following equation (10).

[0037] After this signal is A / D converted, the digital signal processing circuit 8 calculates the integral of the following equation (11). Here, T is the symbol interval 1 / R given by the reciprocal of the symbol rate R, and equation (11) calculates the integral over a time slot (span) for one symbol.

[0038] At this time, due to the orthogonal relationship (Equation (8)) of the Hermite-Gaussian pulse, only the term in the following Equation (12) remains in the integral of Equation (11), and the rest are zero. As a result, it is possible to demultiplex the m-th order Hermite-Gaussian pulse from the N modes.

[0039] The integration range of equation (11) is a time slot for one symbol, and strictly speaking, is not −∞< t<∞ shown in equation (8). However, if the base of each order of the Hermite-Gaussian pulse is sufficiently attenuated within a time slot for one symbol, the integration range of equation (11) can be regarded as −∞< t<∞.

[0040] The following describes in detail the time-domain mode multiplexing transmission method and time-domain mode multiplexing transmission device according to an embodiment of the present invention, using specific experimental examples. The experimental setup is shown in Figure 4. In this experiment, four-mode multiplexing transmission was performed using N=4, i.e., 0th- to 3rd-order Hermite-Gaussian pulses. Instead of using four Hermite-Gaussian pulse light sources as coherent Hermite-Gaussian pulse generators, for simplicity, a CW (Continuous Wave) light source (ECLD: External Cavity Laser Diode) oscillating at a narrow linewidth and single frequency at a wavelength of 1.55 μm and an optical comb generator was used to generate the coherent Hermite-Gaussian pulses. This light source is commonly used in coherent transmission.

[0041] Next, an optical comb generator generates optical pulses with a repetition rate of 10 GHz, which are then split into five branches. Four of these branches are passed through a waveform shaping filter called a Waveshaper (WS) to generate Hermite-Gaussian pulses of order 0 to 3. The remaining branch is frequency-downshifted by 90 GHz from the center and extracted using an optical filter to serve as a pilot tone for phase synchronization. The generated Hermite-Gaussian pulses of order 0 to 3 are then modulated by a 10 Gbaud QAM signal generated by an arbitrary waveform generator (AWG). Specifically, each pulse is QAM-modulated by an IQ modulator (IQM) driven by the QAM signal. After adjusting the timing of each pulse using an optical delay line (Δτ), the four modes are combined and input to a polarization multiplexing circuit (Pol-MUX) for simultaneous polarization multiplexing. A 10 GHz sinusoidal optical signal for distributing a 10 GHz clock signal is generated using a semiconductor laser and an optical intensity modulator, and after being multiplexed with a signal optical pulse and a pilot tone, it is input into the optical fiber transmission line.

[0042] The transmission path is a 75-km span of dispersion-managed fiber (50 km SLAF + 25 km IDF). At the receiving end, a pilot tone is extracted from the received signal light using an optical filter, its frequency is upshifted using an optical frequency shifter (OFS), and then injected into a distributed feed-back laser diode (DFB-LD) used as a local oscillator. As a result, local oscillations phase-locked to the signal light via the pilot tone are generated from the DFB-LD by injection locking. In parallel, a 10-GHz clock signal is extracted using another optical filter and used to drive an optical comb generator. This optical comb generator generates m-th order (m = 0 to 3) Hermite-Gaussian local oscillation pulses whose repetition rate is synchronized with the signal light. The signal light is then homodyne coherently detected using a 90-degree optical hybrid circuit and a balanced photodiode (B-PD). The signal then passes through a low-pass filter (LPF), undergoes A / D conversion, and is then subjected to mode demultiplexing and demodulation in a digital signal processing circuit (DSP).

[0043] The waveforms of the generated 0th to 3rd order Hermite-Gaussian pulses are shown in Figures 5(a-1) to 5(d-1), with time on the horizontal axis and intensity on the vertical axis, and the optical spectra corresponding to these waveforms are shown in Figures 5(a-2) to 5(d-2), with time on the horizontal axis and power on the vertical axis. The solid lines show the measured values, and the dashed lines show the theoretical values. Figure 5 shows that the two are in good agreement.

[0044] [First Embodiment] In the first embodiment of the present invention, coherent Hermite-Gaussian pulses of each order are mode-multiplexed and transmitted with the same average power. Figure 6 shows the results of 450-km transmission of a 10-Gbaud, 16-QAM signal, four-mode multiplexed using the first embodiment. The average power of each order is set to -4.0 dBm. Figures 6(a-1) to 6(d-1) show back-to-back results (results when the transmitter and receiver are directly connected without a transmission path), and Figures 6(a-2) to 6(d-2) show the constellations of the 16-QAM signal after 450-km transmission. In each figure, BER (Bit Error Rate) is an index representing the bit error rate, and EVM (Error Vector Magnitude) is an index representing the signal point dispersion (degree of deviation from the ideal value) on the constellation.

[0045] As shown in Figure 6, the error rate after transmission is 0th order: 1.3 × 10 -3 , 1st order: 2.7×10 -3 , quadratic: 1.5×10 -3 , cubic: 1.6×10 -3 It is known that these error rate values ​​can be achieved by using forward error correction (FEC) technology. This demonstrates that time-domain multiplexing technology using coherent Hermite-Gaussian pulses can be realized.

[0046] [Second Embodiment] In the second embodiment of the present invention, coherent Hermite-Gaussian pulses of each order are mode-multiplexed and transmitted with the same peak power. Figure 7 shows the results of 450-km transmission of a 10-Gbaud, 16-QAM signal, four-mode multiplexed using the second embodiment. The peak power of each order is set to the same value of 3.4 dBm. The average power of each order is: 0th order: -5.1 dBm, 1st order: -3.8 dBm, 2nd order: -3.3 dBm, and 3rd order: -3.0 dBm. When comparing Hermite-Gaussian pulse waveforms at the same average power, the lower the order, the higher the peak power. Therefore, when comparing at the same peak power, the lower the order, the lower the average power. Figures 7(a-1) to (d-1) show back-to-back constellations, and Figures 7(a-2) to (d-2) show the 16-QAM signal constellations after 450-km transmission.

[0047] As shown in Figure 7, the error rate after transmission is 0th order: 1.7 × 10 -3 , 1st order: 1.6×10 -3 , quadratic: 1.4×10 -3 , cubic: 1.5×10 -3 It can be seen that better transmission characteristics are obtained compared to Figure 6. Therefore, just like Figure 6 (constant average power), it has been demonstrated that error-free transmission is possible with the method in Figure 7 by employing FEC. Note that in Figure 6, the performance deteriorates slightly when transmitted at the same average power because the lower the order pulse, the higher the peak power, making it more susceptible to the effects of nonlinear optical effects in the fiber.

[0048] As described above in detail, the present invention can realize mode-multiplexed transmission in the time domain by utilizing the orthogonality of coherent Hermite-Gaussian pulses. As a result, it is possible to expand the transmission capacity per single wavelength without using TDM. Furthermore, by combining the present invention with TDM, WDM, and SDM technology using multicore fiber and multimode fiber, it is possible to provide an even larger-capacity transmission system.

[0049] 1 Pulse light source 2 Optical modulator 3 Polarization multiplexing circuit 4 Optical fiber transmission line 5 Local pulse light source 6 90-degree optical hybrid circuit 7 Balanced photodetector 8 Digital signal processing circuit

Claims

1. A time-domain mode multiplexing transmission method characterized in that coherent Hermite-Gaussian pulses whose amplitudes are given by Hermite-Gaussian functions are used as signal light and local light.

2. A time domain mode multiplexing transmission method according to claim 1, characterized in that N coherent Hermite-Gaussian pulses of orders 0 to N-1 are generated in a time slot for one symbol as the coherent Hermite-Gaussian pulses and transmitted simultaneously.

3. The time domain mode multiplexing transmission method according to claim 2, wherein said N coherent Hermite-Gaussian pulses are synchronized with each other in the time domain.

4. A time domain mode multiplexing transmission method according to claim 2, characterized in that an m-th order coherent Hermitian Gaussian pulse out of said N coherent Hermitian Gaussian pulses is used as said local light, signal light in which said N coherent Hermitian Gaussian pulses are multiplexed is subjected to homodyne coherent detection, and the detected signal after detection is integrated over a time slot for one symbol, thereby demultiplexing said m-th order coherent Hermitian Gaussian pulse from said N coherent Hermitian Gaussian pulses.

5. A time domain mode multiplexing transmission method according to claim 2, characterized in that said N coherent Hermite-Gaussian pulses are multiplexed and transmitted with the same average power.

6. A time domain mode multiplexing transmission method according to claim 2, wherein said N coherent Hermite-Gaussian pulses are multiplexed and transmitted with the same peak power.

7. A time domain mode multiplexing transmission device comprising: coherent Hermite Gaussian pulse generating means for generating a coherent Hermite Gaussian pulse whose amplitude is given by a Hermite Gaussian function; signal light generating means for using said coherent Hermite Gaussian pulse as signal light; local light generating means for using said coherent Hermite Gaussian pulse as local light; and detection / separation / demodulation means for detecting, separating and demodulating the signal light and the local light.

8. A time domain mode multiplex transmission device according to claim 7, characterized in that said coherent Hermite Gaussian pulse generating means is configured to generate N coherent Hermite Gaussian pulses of orders 0 to N-1 in a time slot for one symbol and transmit them simultaneously.

9. A time domain mode multiplex transmission apparatus according to claim 8, wherein said coherent Hermite-Gaussian pulse generating means is configured to synchronize said N coherent Hermite-Gaussian pulses with each other in the time domain.

10. A time domain mode multiplexing transmission device according to claim 8, characterized in that: said signal light generating means generates a signal in which said N coherent Hermitian Gaussian pulses are multiplexed as said signal light; said local light generating means generates an m-th order coherent Hermitian Gaussian pulse from said N coherent Hermitian Gaussian pulses as said local light; and said detection separation demodulation means is configured to perform homodyne coherent detection of said signal light and said local light and to integrate the detection signal after detection over a time slot for one symbol, thereby demultiplexing said m-th order coherent Hermitian Gaussian pulse from said N coherent Hermitian Gaussian pulses.

11. A time domain mode multiplex transmission apparatus according to claim 8, wherein said coherent Hermite-Gaussian pulse generating means is configured to multiplex and transmit said N coherent Hermite-Gaussian pulses with the same average power.

12. A time domain mode multiplex transmission apparatus according to claim 8, wherein said coherent Hermite-Gaussian pulse generating means is configured to multiplex and transmit said N coherent Hermite-Gaussian pulses with the same peak power.

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