JP3789783B2 - Wavelength division multiplexing optical transmission system and transmission method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength division multiplexing optical transmission system and transmission method in the field of optical communication.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in order to increase the transmission capacity of optical communication, a wavelength division multiplexing optical transmission system that transmits signals using a large number of wavelengths has been used.
[0003]
FIG. 6 is a block diagram of a conventional wavelength division multiplexing optical transmission system. In the transmission unit, signals from a plurality of optical transmitters 1-1 having different wavelengths (= optical frequencies) are multiplexed by the wavelength multiplexer 1-3. A number of combined optical signals are transmitted by sharing one optical transmission medium 1-5. In the receiving unit, the wavelength demultiplexer 1-8 demultiplexes each wavelength, and the optical signal of each wavelength is received by the optical receiver 1-10. The wavelength division multiplexing optical transmission system has been realized with the above configuration.
[0004]
From the viewpoint of effective use of the wavelength region of light, high-density wavelength division multiplex transmission is desired in which the wavelength intervals of light are close. When the optical frequency interval is Δf [Hz] and the transmission speed is B [bit / s], B / Δf [bit / s / Hz] is referred to as band efficiency (Spectral Efficiency). The theoretical limit of the band use efficiency of the ON / OFF modulation method of the double sideband is 1 [bit / s / Hz].
[0005]
In the conventional wavelength division multiplex transmission system, a desired signal light is extracted from a receiving unit by a wavelength selection filter using a wavelength demultiplexer such as an arrayed waveguide type diffraction grating. However, when a large number of signals are transmitted at a high density, they overlap with signals of adjacent wavelengths, so that there is a problem that a desired signal cannot be detected by the wavelength selective filter. If the wavelength interval is widened to prevent interference with adjacent frequencies, the band utilization efficiency decreases. In the conventional wavelength division multiplexing transmission system, the band utilization efficiency is usually about 0.4 [bit / s / Hz] or less.
[0006]
[Problems to be solved by the invention]
When a large number of signals are transmitted at a high density, signals of adjacent wavelengths are overlapped with each other, so that there is a problem that the wavelength selection filter used in the conventional wavelength multiplexing transmission system cannot separate the signals. On the other hand, when the wavelength interval is widened, the conventional wavelength multiplexing transmission system has a problem that the band use efficiency is poor. An object of the present invention is to provide a wavelength division multiplexing optical transmission system and transmission method with good band utilization efficiency.
[0007]
[Means for Solving the Problems]
The wavelength division multiplexing transmission system of the present invention uses an N wave (B / Δf ≦ 1 [bit / s / Hz]) modulated by an optical element at an optical frequency interval Δf [Hz] and modulation speed B [bit / s] (where B / Δf ≦ 1 [bit / s / Hz]). N is an integer greater than or equal to 2) an optical transmitter for generating an optical signal, an optical transmitter comprising means for combining the optical signals, and a transmission signal from the optical transmitter, the length difference is _c / (2n c Δf) (where, c is the speed of light, n _c is the effective refractive index of the waveguide) and an asymmetric Mach-Zehnder interference element of the desired wavelength from the output signal of the asymmetric Mach-Zehnder interference element Selectively transmitting wavelength filter means , connected after the wavelength filter means, from 1 / (2Δf) [s] to T [s] (where T is a time of 1 bit) in units of bits from the output of the wavelength filter Time gate means to extract the time signal between An optical receiver comprising, electrically connected to the modulation device, or, provided between said multiplexing means and said optical transmitter, all the wavelength signals at the input of the asymmetric Mach-Zehnder interference element And a bit phase adjusting means for matching the bit phases.
[0008]
According to a second aspect of the present invention, the bit phase adjusting means is electrically connected to an optical transmitter and controls a bit phase of an electrical modulation signal to a modulation element in the optical transmission unit. It is characterized by. The bit phase adjusting means is provided in an optical transmission path disposed between the optical transmitter and the multiplexing means , and adjusts the optical bit phase by controlling the optical path length of the optical signal. The time gate means is an optical gate switch. The time gate means is an electric gate circuit in an optical receiver.
[0009]
Further, in the invention of claim 6, the means for multiplexing in the optical transmission section is a first wavelength multiplexing for multiplexing odd-numbered optical signals with respect to N-wave optical signals with an optical frequency interval Δf. And second wavelength multiplexing means for multiplexing even-numbered optical signals. The invention according to claim 7 is characterized in that the asymmetric Mach-Zehnder type element has a built-in phase shifter. Furthermore, the phase shifter adjusts the phase shift so that either the odd-numbered or even-numbered signal light wavelength corresponds to the peak of the transmission spectrum of the asymmetric Mach-Zehnder interference element.
[0010]
In the wavelength division multiplexing optical transmission method of the present invention, modulation is performed by the modulation element at an optical frequency interval Δf [Hz] and a modulation speed B [bit / s] (B / Δf ≦ 1 [bit / s / Hz]). An optical signal of N waves (N is an integer of 2 or more) is generated by an optical transmitter, and the optical signals are combined and transmitted, and electrically connected to the optical transmitter, or combined with the optical transmitter. The bit phase adjustment means provided between the means adjusts the bit phase so that the bit phases of the signals of all wavelengths match, and the transmitted signal has an optical path length difference of c / (2n _c Δf ) (where, c is the speed of light, n _c is input to the asymmetric Mach-Zehnder interference element of the equivalent refractive index of the waveguide), using the wavelength filter means from an output of the asymmetric Mach-Zehnder interferometer device selects the desired wavelength after, from the selected signal, in bits 1 / (2Δf) [s] from T [s] (where, T is 1-bit time) and outputs removed time of the signal until.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention.
[0012]
In the transmission unit, N pieces of light (N is an integer of 2 or more, where N is an even number in this embodiment in order to simplify the way of attaching reference numerals) arranged at an optical frequency interval Δf [Hz]. Transmitters 1-1 _{1 to} 1-1 _N modulate optical signals having wavelengths λ _{1 to} λ _{N at} a modulation speed B [bit / s], respectively, and use efficiency B / Δf [bit / s / Hz]. ] Is set to a value equal to or less than 1 and close to 1. The modulation method is assumed to be an ON / OFF modulation method.
[0013]
Optical transmitter 1-1 ₁ ~1-1 _N electrical modulation by electrical bit phase adjusting means 1-2 ₁ ~1-2 _N to modulating element in the optical transmitter 1-1 ₁ ~1-1 _N The bit phase of the signal can be controlled, and the bit phase of the signal of all wavelengths is at the input end of the Mach-Zehnder type interference element 1-6 of the receiving unit so that signal processing can be performed while paying attention to one bit. The bit phase of the electrical signal is adjusted so as to match. Alternatively, as the bit phase adjusting means, an optical bit phase adjusting means such as a variable delay waveguide is provided between the optical transmitters 1-1 _{1 to} 1-1 _N and the wavelength multiplexers 1-3a and 1-3b. Alternatively, the optical path length may be adjusted so that the bit phases of the signals of all wavelengths at the input end of the Mach-Zehnder type interference element 1-6 of the receiving unit are matched.
[0014]
In the present invention, since the band use efficiency B / Δf [bit / s / Hz] is set to be 1 or less and close to 1, the wavelength interval becomes as narrow as the signal band. For this reason, if the transmitter unit multiplexes using a wavelength combiner as in the conventional example, the signal component is cut off during the wavelength combination. Therefore, in order to avoid this, in the present embodiment, when multiplexing is performed using a wavelength multiplexer, a plurality of optical signals (optical signal wavelengths λ ₁ , λ) of odd channels arranged at the optical frequency interval 2Δf are used. ₃ , λ ₅ ,..., Λ _N−1 ) are combined by a wavelength multiplexer 1-3 a with an optical frequency interval of 2Δf, and a plurality of even-channel optical signals (optical signal wavelengths λ ₂ ,. λ ₄ , λ ₆ ,..., λ _N ) are combined by another wavelength multiplexer 1-3b with an optical frequency interval of 2Δf, and the outputs of the two wavelength multiplexers are 1: 1 couplers 1. -4 etc. to combine. Since the optical frequency interval 2Δf of the wavelength multiplexers 1-3a and 1-3b is wider than the signal band, the signal component is not deleted and information is not lost. Instead of using the wavelength multiplexers 1-3a and 1-3b, multiplexing may be performed using an N × 1 multiplexing element such as a multimode interference optical element or a multistage connection of a 1: 1 coupler.
[0015]
As described above, the wavelength multiplexed signal combined in the transmission unit is transmitted by sharing one optical transmission medium 1-5.
[0016]
After the wavelength multiplexed signal is transmitted through the optical transmission medium 1-5, the optical path length difference is c / (2n _c Δf) (where c is the speed of light and n _c is the equivalent refractive index of the waveguide) at the receiver. Of the asymmetric Mach-Zehnder type interference element 1-6. Here, each signal is delayed by time 1 / (2Δf) in one optical path, and is combined with the other non-delayed signal and branched into an odd channel signal and an even channel signal. . At this time, the phase shifter 1-7 is adjusted so that the signal light wavelength of either the odd channel or the even channel matches the peak of the transmission spectrum of the Mach-Zehnder type interference element 1-6. At this time, if the wavelength dependence of the equivalent refractive index of the waveguide is small, the signal light wavelengths of all odd channels and the peak of the transmission spectrum of the Mach-Zehnder type interference element 1-6 are present at the signal output terminals for odd channels. In addition, at the signal output terminals for even channels, all even channel signal light wavelengths coincide with the peaks of the transmission spectrums 1-6 of the Mach-Zehnder type interference elements. Further, when the asymmetric Mach-Zehnder type interference element 1-6 is composed of an optical waveguide such as a semiconductor, glass, dielectric, etc., the optical path length difference of the asymmetric Mach-Zehnder type interference element 1-6 may be temperature dependent. Are known. When the optical path length difference of the asymmetric Mach-Zehnder type interference element 1-6 is temperature-dependent, the signal light wavelength of either the odd channel or the even channel matches the peak of the transmission spectrum of the Mach-Zehnder type interference element 1-6. In this way, the temperature of the Mach-Zehnder type interference element 1-6 may be controlled. In this case, the phase shifter 1-7 is not necessary. Further, when the manufacturing accuracy of the dimensions of the Mach-Zehnder type interference element 1-6 is sufficient, the signal light wavelength of either the odd channel or the even channel matches the peak of the transmission spectrum of the Mach-Zehnder type interference element 1-6. In this way, the phase shifter 1-7 may be unnecessary by precisely controlling the optical path length difference.
[0017]
The signal for the odd channel is separated into each wavelength by the wavelength demultiplexer 1-8a for the odd channel having the optical frequency interval 2Δf. The wavelength demultiplexer 1-8a (or wavelength demultiplexer 1-8b) includes a wavelength filter that selectively transmits the wavelength of a desired odd channel (or even channel) from the output signal of the asymmetric Mach-Zehnder type interference element 1-6. Has been. Considering only odd-numbered channels, the optical frequency interval 2Δf of the wavelength demultiplexer 1-8a is wider than the signal band, so that a desired signal component is not deleted and information is not lost. Due to the characteristics of 1-8a, signals of odd-numbered channels other than desired are removed.
[0018]
Further, from the output terminals, the time gates 1-9 _{1 to} 1-9 _N using an optical time gate switch are used to set 1 / (2Δf) to T (where, The signal at time T) is taken out.
[0019]
The asymmetric Mach-Zehnder interferometer 1-6 and the time gates 1-9 ₁ , 1-9 ₃ ,..., 1-9 _N-1 cancel all the even channel signal components at the odd channel output (details of this operation). Will be explained later.) For this reason, only the signal of only one wavelength of the odd-numbered channel is extracted by the combination with the wavelength demultiplexer 1-8a.
[0020]
The signal input to the even-channel optical demultiplexer 1-8b is also wavelength-separated in the same manner as the odd-numbered channel.
[0021]
Signal separated into each wavelength is received by the wavelength lambda ₁ to [lambda] _N optical receiver 1-10 ₁ ~1-10 _N to light signals, thus high-density wavelength division multiplexing optical transmission is realized.
[0022]
In FIG. 1, an optical time gate switch is used as a time gate. Specifically, as an optical time gate switch, an electroabsorption modulator or a Mach-Zehnder intensity modulator is used as a switch, or a nonlinear optical effect is used. The optical switch used is used. Alternatively, there is an electrical time gate circuit, such as a method that detects the voltage at a specific time as the time gate by adjusting the identification timing of the identification circuit after it is converted into an electrical signal by the optical receiver circuit. sell.
[0023]
The operation of the asymmetric Mach-Zehnder type interference element 1-6 and the time gates 1-9 _{1 to} 1-9 _N will be described below with reference to FIG. When attention is paid to a signal of one bit, as shown in the figure, the signal (a) without delay by the asymmetric Mach-Zehnder type interference element and the signal (b) delayed by 1 / (2Δf) by the delay waveguide are combined. The output signal (c) of the asymmetric Mach-Zehnder type interference element is generated. From the figure, it can be seen that only the time portion of 1 / (2Δf) to T is an interference signal of the signal of the bit. Therefore, by extracting a signal having a time of 1 / (2Δf) to T by a time gate switch having a width T−1 / (2Δf), only an interference signal component of the signal of the bit can be extracted.
[0024]
If the phase shifter 1-7 is adjusted so that the odd channel signal light wavelength and the peak of the transmission spectrum of the Mach-Zehnder interference element 1-6 match, the phase of the signal light wavelength of the odd channel becomes stronger. Interfering like that. On the other hand, the signal light wavelengths of even-numbered channels separated by Δf are shifted in phase by π due to the optical path length difference c / (2n _c Δf), and thus interfere with each other so as to cancel each other. Therefore, the interference signal component cancels out and the even channel component does not appear.
[0025]
3 to 5 show simulation results using the configuration of the embodiment of the present invention. The wavelength interval was 5 GHz, the bit rate was 5 Gb / s, and the number of channels was N = 4. The bandwidth utilization efficiency is 1. Before the asymmetric Mach-Zehnder interference element 1-6 (FIG. 3), any one of the preceding optical gate switches 1-9 ₁ ～1-9 ₄ (FIG. 4), and an optical gate switch 1-9 ₁ ～1-9 The eye diagram after any one of ₄ (FIG. 5) is shown. It can be seen that sufficiently eye diagram is open even after the optical gate switches 1-9 ₁ ～1-9 _4.
[0026]
Therefore, according to the present invention, it is possible to realize a wavelength division multiplexing optical transmission system with good band utilization efficiency.
[0027]
【The invention's effect】
As described above, according to the present invention, it is possible to realize a wavelength division multiplexing optical transmission system with good band utilization efficiency. As a result, it is possible to realize a wavelength division multiplexing transmission system having a larger number of wavelengths with the same wavelength bandwidth as that of the prior art, thereby contributing to an increase in the transmission capacity of optical communication.
[0028]
In addition, since an optical transmission line with a narrow band can be constructed at a lower cost than an optical transmission line with a wide band, in the optical transmission system in which the construction cost of the transmission path occupies much of the cost of the optical transmission system, the present invention Cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram showing the configuration of an embodiment of a wavelength division multiplexing optical transmission system according to the present invention. FIG. 2 is a waveform diagram for explaining the operation of the configuration of FIG. Eye diagram showing results (before asymmetric Mach-Zehnder type interference element 1-6)
4 is an eye diagram showing a simulation result according to the configuration of FIG. 1 (before any _{one of the} optical gate switches 1-9 _{1 to} 1-9 ₄ ).
[5] (after one of the optical gate switches 1-9 ₁ ～1-9 ₄₎ eye diagram showing the simulation result of the configuration of FIG. 1
FIG. 6 is a block diagram showing a conventional configuration example.
1-1 _{1 to} 1-1 _N optical transmitter 1-2 _{1 to} 1-2 _N- bit phase adjusting means 1-3a, 1-3b Waveform multiplexer 1-4 1: 1 coupler 1-5 optical transmission medium 1 -6 Asymmetric Mach-Zehnder type interference element 1-7 Phase shifter 1-8 Wavelength demultiplexer 1-9 _{1 to} 1-9 _N optical time gate switch 1-10 _{1 to} 1-10 _N optical receiver

Claims (9)

Light of N waves (N is an integer of 2 or more) modulated by a modulation element at an optical frequency interval Δf [Hz] and modulation speed B [bit / s] (B / Δf ≦ 1 [bit / s / Hz]) An optical transmitter for generating a signal, and an optical transmitter comprising means for multiplexing the optical signals;
An asymmetric Mach-Zehnder interference having an optical path length difference of c / (2n _c Δf) (where c is the speed of light and n _c is an equivalent refractive index of the waveguide). Elements,
Wavelength filter means for selectively transmitting a desired wavelength from the output signal of the asymmetric Mach-Zehnder interference element;
Time to connect after the wavelength filter means and extract a signal of time from 1 / (2Δf) [s] to T [s] (where T is a time of 1 bit) in units of bits from the output of the wavelength filter An optical receiver comprising gate means,
To match the bit phases of all wavelength signals at the input of the asymmetric Mach-Zehnder type interference element that is electrically connected to the modulation element or provided between the optical transmitter and the multiplexing means A wavelength division multiplexing optical transmission system characterized by comprising: The bit phase adjusting means is electrically connected to an optical transmitter and controls a bit phase of an electrical modulation signal to a modulation element in the optical transmission unit. Wavelength multiplexing optical transmission system. The bit phase adjustment means is provided in an optical transmission path disposed between the optical transmission unit and the multiplexing means , and adjusts the optical bit phase by controlling the optical path length of the optical signal. The wavelength division multiplexing optical transmission system according to claim 1. The wavelength multiplexing optical transmission system according to claim 1, wherein the time gate means is an optical gate switch. The wavelength multiplexing optical transmission system according to claim 1, wherein the time gate means is an electric gate circuit in an optical receiver. The means for multiplexing in the optical transmission unit combines first wavelength multiplexing means for multiplexing odd-numbered optical signals and even-numbered optical signals for N-wave optical signals with an optical frequency interval Δf. The wavelength division multiplexing optical transmission system according to any one of claims 1 to 5, further comprising: a second wavelength multiplexing unit that waves. The wavelength division multiplexing optical transmission system according to claim 1, wherein the asymmetric Mach-Zehnder element includes a phase shifter. The phase shifter adjusts the phase shift so that either the odd-numbered or even-numbered signal light wavelength corresponds to the peak of the transmission spectrum of the asymmetric Mach-Zehnder type interference element. WDM optical transmission system. Light of N waves (N is an integer of 2 or more) modulated by a modulation element at an optical frequency interval Δf [Hz] and modulation speed B [bit / s] (B / Δf ≦ 1 [bit / s / Hz]) signal generated by the optical transmitter transmits optical signals multiplexed by,
The bit phase is adjusted so that the bit phases of the signals of all wavelengths coincide with each other by the bit phase adjusting means electrically connected to the optical transmitter or provided between the optical transmitter and the multiplexing means. , the transmitted signal, the optical path length difference _c / (2n c Δf) (where, c is the speed of light, n _c is the effective refractive index of the waveguide) input to the asymmetric Mach-Zehnder interference element,
After selecting a desired wavelength using the wavelength filter means from the output of the asymmetric Mach-Zehnder type interference element,
From the selected signal, a signal in a time period from 1 / (2Δf) [s] to T [s] (where T is a time of 1 bit) is extracted and output in bits. Wavelength multiplexing optical transmission method.

Images