JP3450668B2 - WDM optical signal transmitter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing optical signal transmitter which uses a plurality of lights having different wavelengths.

[0002]

2. Description of the Related Art FIG. 1 shows an example of a conventional wavelength division multiplexing optical signal transmitting apparatus, in which individual optical signals are transmitted from a plurality of optical frequency variable optical signal transmitters 2 whose optical frequencies are precisely controlled by a wavelength reference 1. The modulated optical signals having the frequency carriers ν1 to νn are output, and these are multiplexed by the multiplexer 3. The multiplexer 3 may be a 1 × N coupler, but an arrayed waveguide grating filter (hereinafter referred to as AWG) is preferable.

In such a wavelength division multiplexing optical signal transmitter capable of individually controlling the wavelength (optical frequency), the wavelength intervals can be made unequal or even, so that the zero-dispersion wavelength of the fiber and the wavelength band can be set. Can be arbitrarily set to construct a transmission system.

However, since each transmitter 2 requires a wavelength control mechanism individually, it is difficult to miniaturize or integrate them. As a result, not only the entire apparatus becomes large-scale, but also maintenance is required. The cost also becomes enormous, and there is a problem that such a hardware configuration imposes a heavy burden in constructing a wavelength division multiplexing (WDM) transmission system having a large number of channels with narrower wavelength intervals.

FIG. 2 shows another example of a conventional wavelength division multiplexing optical signal transmitter, here, a single element produces a large amount of continuous light (CW light).
An example using a mode-locked semiconductor laser that can utilize In the figure, 11 is a reference light source, 12 is a mode-locked semiconductor laser, 13 is an electric oscillator, 14 is an isolator, 15 is an AWG, 16 is a plurality of fiber loops, 17 is a plurality of pattern generators, and 18 is a plurality of modulators. .

The mode-locked semiconductor laser 12 generates a large number of continuous monochromatic lights (multi-wavelength light) corresponding to each oscillation mode and arranged at equal intervals at wavelength intervals where each wavelength is equal to the repetition frequency. These multi-wavelength lights fluctuate in free running, but due to the injection locking using the reference light from the reference light source 1 and the forced mode-locking operation using the modulation signal current from the electric oscillator 13, the absolute wavelengths of all modes are changed. It is possible to lock (H. Yasaka et al., “Mu
ltiwavelength light source with precise frequency
spacing using mode-locked semiconductor laser and
arrayed waveguide grating filter ”OFC '96 F
See B2).

The multi-wavelength light arranged at equal intervals in this way is demultiplexed by the AWG 15 for each optical frequency (wavelength),
After being individually data coded by each fiber loop 16, pattern generator 17 and modulator 18,
It is again coupled to the AWG filter 15 and multiplexed.

The WDM inter-channel spacing is selected so that crosstalk does not occur depending on the data band.
Normally, 100 GHz intervals are recommended. At present, the wavelength spacing that can be stabilized is about 50 GHz, but in the future, due to the development of ultra-high-speed devices, sub-THz (terahertz)
Is expected to be extended, and it will be easy to satisfy this recommendation in the future.

[0009]

However, the multi-wavelength light arranged at equal intervals on the optical frequency axis has a four-wave mixing (FW) when the wavelength band is in the vicinity of the zero dispersion wavelength of the transmission fiber.
However, there was a problem of causing crosstalk due to M (FWM: ν1 and ν due to the nonlinear effect of the transmission fiber).
A phenomenon in which 2 interacts to generate new light for each ν1-ν2. It appears remarkably in the zero-dispersion wavelength region where there is little phase shift when propagating in the fiber. In WDM transmission, if optical frequency carriers are set at equal intervals, crosstalk occurs due to this FWM, and transmission quality is significantly deteriorated. ).

Since the multi-wavelength light generated by the mode-locked semiconductor laser has the same phase, the FWM is particularly likely to be generated as compared with the case of the individual light source. For this reason, when trying to realize a long span, large capacity optical transmission system by wavelength multiplexing in a wide band including the zero dispersion wavelength, it is difficult to use the above mode-locked semiconductor laser as it is as a multi-wavelength light source. It was

An object of the present invention is to provide a wavelength division multiplexing optical signal transmitter which can utilize a mode-locked semiconductor laser as a multi-wavelength light source.

[0012]

In order to solve the above problems, the present invention is characterized in that means for individually shifting the optical phase of continuous monochromatic light of each oscillation mode of a mode-locked semiconductor laser is provided.

The FWM generation efficiency in the fiber increases rapidly with increasing light intensity due to the nonlinear Schrodinger equation. Therefore, in order to suppress the FWM, it is necessary to collapse the mode-locking pulse on the time axis.

The field component of the mode-locked pulse is generally

[Equation 1] It can be expressed as. Here, F is a repetition frequency and ψ is a constant. ρ is a coefficient representing the mode distribution and depends on the pulse waveform.

The simplest method of collapsing the pulse is to make the carrier frequencies of light unequal intervals.

In the Mach-Zehnder type phase modulator, the optical frequency can be easily converted (shifted) by simultaneously modulating both arms with the phase difference π in opposite directions. That is, the modulated light from one arm

[Equation 2] And the other

[Equation 3] Ignoring the higher order terms as, the light field component after passing through the modulator is mainly

[Equation 4] Becomes This means that only the modulation frequency is converted into the optical frequency with the accuracy of the synthesizer.

By performing this conversion individually for all oscillation modes, it is possible to set the optical frequencies strictly at unequal intervals. In wavelength division multiplex transmission using light arranged in this way, it is possible to significantly improve the power penalty in the vicinity of the zero-dispersion wavelength, as compared with those arranged at equal intervals.

When there is a margin in the modulation power of the modulator, the modulation amplitude J ₀ (δ) which is the first zero point of the Bessel.
It is possible to eliminate the carrier component by phase modulation at = 0. In this case, since sidebands are generated on both sides over a wide band at modulation amplitude intervals, the modulation frequency is limited depending on the bandwidth of the transmission channel.

Further, even if the carrier component exists, it is possible to collapse the pulse by making the modes completely uncorrelated, and it can be realized by chaotically performing the phase shift. . Fig. 3 (a) shows the pulses synchronized in each mode, while Fig. 3 (b) shows the time waveform when the DC component of the phase is set completely randomly.
It can be seen that the pulse waveform is largely collapsed. Here, these graphs are calculated assuming that the repetition frequency is 100 GHz and the number of modes is 5.

[0020]

DETAILED DESCRIPTION OF THE INVENTION

[0021]

First Embodiment FIG. 4 shows a first embodiment of the present invention corresponding to claims 1 and 2, in which a Mach-Zehnder type optical frequency shifter is used, and is the same as FIG. 2 in the figure. The components are represented by the same symbols. That is, 11 is a reference light source, 12 is a mode-locked semiconductor laser, 13 is an electric oscillator, 14 and 21 are isolators, 15 and 22 are AWGs,
Reference numeral 17 is a plurality of pattern generators, 18 is a plurality of modulators, 23 is an optical amplifier, 24 is an optical filter, 25 is an optical modulator, and 26 is a plurality of optical frequency shifters.

In the above structure, the reference light from the reference light source 11 is amplified by the optical amplifier 23, and the spontaneous emission light (ASE) in the optical amplifier 23 is removed by the optical filter 24.
After being modulated by the modulator 25, the mode-locked semiconductor laser 1
Injection synchronized to 2. The modulator 25 is an EA modulator or L
It is composed of an N (lithium niobate) intensity modulator, and the modulation frequency is equal to the repetition frequency of the mode-locking pulse or an integral multiple thereof. By such light injection, the absolute wavelength and the repetition frequency are simultaneously stabilized. It is desirable to insert a polarization controller between the modulator 25 and the mode-locked semiconductor laser 12 to improve the injection efficiency.

The pulse composed of multi-wavelength light output from the mode-locked semiconductor laser 12 is demultiplexed by the AWG 15 for each mode (optical frequency), and individually converted by the optical frequency shifters 26 so that the optical frequencies are converted at different intervals. To be done.

FIG. 5 shows the details of the individual optical frequency shifters 26. The Mach-Zehnder type phase modulator 261, the (electrical) sign inverter 262, the (electrical) 180-degree phase shifter 263, and the optical frequency converter are used. It consists of an electric oscillator 264. By precisely adjusting the branching ratio 1: 1 of the Mach-Zehnder type phase modulator 31 and the phase difference π of the 180-degree phase shifter 263, it is possible to completely remove the carrier frequency and convert the optical frequency. (In the current experiment, the carrier extinction ratio is about 30 dB). Here, this optical frequency shift amount is within the band of each WDM channel.

The optical signal whose optical frequency has been shifted is data coded by each pattern generator 17 and modulator 18, and then multiplexed by another AWG 22 and introduced into one optical fiber.

[0026]

[Embodiment 2] FIG. 6 shows a second embodiment of the present invention corresponding to claims 1 and 3, in which a carrier component of light is removed by phase modulation. ,
4 are designated by the same reference numerals. That is, 11
Is a reference light source, 12 is a mode-locked semiconductor laser, 13 is an electric oscillator, 14 and 21 are isolators, 15 is an AWG,
16 is a plurality of fiber loops, 17 is a plurality of pattern generators, 18 is a plurality of modulators, 23 is an optical amplifier, 2
Reference numeral 4 is an optical filter, 31 is a plurality of electric oscillators, and 32 is a plurality of phase modulators.

In the above structure, the reference light from the reference light source 11 is amplified by the optical amplifier 23, and after the spontaneous emission light (ASE) in the optical amplifier 23 is removed by the optical filter 24, it is injected into the mode-locked semiconductor laser 12. As a result, the absolute wavelength is stabilized, and the repetition frequency is stabilized by the forced mode-locking operation based on the modulation signal current from the electric oscillator 13.

The pulse composed of multi-wavelength light output from the mode-locked semiconductor laser 12 is demultiplexed by the AWG 15 for each mode (optical frequency), and each pattern generator 1
After the data is coded by 7 and the modulator 18, the phase is modulated by the electric oscillator 31 and the phase modulator 32 so that the carrier component disappears. Here, a resonant LN oscillator capable of large amplitude modulation can be used as the phase modulator 32.

The optical signal from which the optical carrier component has been removed is looped back by the AWG 15 and multiplexed to be introduced into one optical fiber.

[0030]

[Third Embodiment] FIG. 7 shows a third embodiment of the present invention corresponding to claims 1 and 4, in which the pulse waveform is disrupted by chaotically disturbing the bias component of each phase of the mode-locking pulse. In the figure, the same components as those in FIG. 6 are represented by the same reference numerals. That is, 11 is a reference light source, 12 is a mode-locked semiconductor laser, 13 is an electric oscillator, 14 and 21 are isolators, 15 is an AWG, 16
Is a plurality of fiber loops, 17 is a plurality of pattern generators, 18 is a plurality of modulators, 23 is an optical amplifier, 24 is an optical filter, 41 is a plurality of chaos generation sources, and 42 is a plurality of optical phase shifters.

In the above structure, generation of multi-wavelength light in the mode-locked semiconductor laser 12, data coding, and loopback topology are the same as those in the second embodiment. Here, instead of an electric oscillator for phase modulation, each independent chaos generation source 4 with no temporal correlation is used.
By determining the amount of phase shift of each optical phase shifter 42 by 1, the pulse waveform is destroyed. Such an optical phase shifter 42
For this, an LN phase modulator can be used.

Chaos can be generated in a software manner by a computer or the like, and can be used by converting it into an electric signal. In addition, it is also possible to use a noise source in which a time correlation component does not remain as a chaos generation source.

[0033]

As described above, according to the present invention,
A mode-locked semiconductor laser can be used as a multi-wavelength light source, a simple structure and a compact size can be realized, and a wavelength-multiplexed optical signal transmitter that requires less maintenance and management can be realized. It is possible to reduce costs.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an example of a conventional wavelength division multiplexing optical signal transmitter.

FIG. 2 is a block diagram showing another example of a conventional wavelength division multiplexing optical signal transmitter.

FIG. 3 is a diagram showing how a pulse waveform is collapsed by a bias of random (chaotic) phase.

FIG. 4 is a configuration diagram showing a first embodiment of a wavelength division multiplexing optical signal transmission device of the present invention.

5 is a configuration diagram showing details of an optical frequency shifter in FIG.

FIG. 6 is a configuration diagram showing a second embodiment of a wavelength division multiplexing optical signal transmission device of the present invention.

FIG. 7 is a configuration diagram showing a third embodiment of a wavelength division multiplexing optical signal transmission device of the present invention.

[Explanation of symbols]

11 ... Reference light source, 12 ... Mode-locked semiconductor laser, 13
... Electric oscillators, 14, 21 ... Isolators, 15, 22
... AWG, 16 ... plural fiber loops, 17 ... plural pattern generators, 18 ... plural modulators, 23 ... optical amplifier, 24 ... optical filter, 25 ... optical modulator, 26 ... plural optical frequency shifters, 31 ... Multiple electric oscillators, 32 ...
Multiple phase modulators, 41 ... Multiple chaotic sources, 42 ... Multiple optical phase shifters.

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Claims (4)

(57) [Claims] 1. A wavelength-division-multiplexed optical signal transmitter for demultiplexing continuous monochromatic light of each oscillation mode from a mode-locked semiconductor laser, performing data coding by an external modulator, and then re-combining the monochromatic light of each oscillation mode. A wavelength division multiplexing optical signal transmission device, characterized in that an optical phase shift means for individually shifting the optical phase of light is provided before or after an external modulator. 2. The wavelength-division-multiplexed optical signal transmitter according to claim 1, wherein an optical phase shift means for shifting to an optical frequency having no carrier component is used. 3. An optical phase shifter for phase-modulating so as to eliminate a carrier component is used.
The wavelength-division-multiplexed optical signal transmitter described. 4. The wavelength-division-multiplexed optical signal transmitter according to claim 1, wherein an optical phase shifter for performing chaotic phase modulation is used.