JPH05235865A - Optical digital signal transmission system - Google Patents

Abstract

PURPOSE:To attain simple identification and reproduction by applying wavelength multiplex to an optical signal using codes whose code logic is inverted and outputting the result in a transmission system, taking the difference of power of optical signals of two waves after wave branching in a reception system and identifying the signal to be obtained. CONSTITUTION:Two wavelength lights outputted from laser diodes LD1, LD2 are modulated by digital signals Q, inverse of Q whose code logic is inverted by a drive circuit 1 and the result is multiplexed at a synthesizer 6 by polarized wave controllers 2, 3 and variable attenuators 4,5. The light of two wavelengths passes through a same bus in an optical talking device comprising an optical transmission line 7 and an optical switch network 8 and is subjected to same fluctuation. A differential signal D and its inversional signal D are obtained from the signal of two wavelengths branched at a branching device 9 on a reception side. The signals are received by a differential input light receiving device 10, the signals D and the inverse of D are obtained from the difference at an identification device 11. Thus, the fluctuation of an OFF level is canceled and the reference level of the identification is made constant, then the signal is identified.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical digital signal transmission system in an optical switching system, for example.

[0002]

SUMMARY OF THE INVENTION The present invention is applied between input and output of an optical communication path, and is characterized by performing differential transmission of signals using two wavelength-multiplexed lights. By using a differential signal that passes through the same path, bit reproduction is possible even if the level changes.

Furthermore, when a signal and a clock are passed through the same path by using wavelength division multiplexing, a clock having an optimum phase relationship with the signal is always obtained, so that bit reproduction is performed even if phase fluctuation occurs. Is possible.

[0004]

2. Description of the Related Art In the conventional optical digital signal transmission system,
The light level is the AGC (Automatic g
ain control (automatic gain control) circuit electrically corrects. For example, as the optical input changes, A
The signal output is kept constant by controlling the current multiplication factor of the PD, controlling the gain of the amplifier by an electric circuit, or using both of them. Such an AGC circuit cannot cope with high-speed level fluctuation and requires a complicated circuit.

A clock synchronized with the signal is required for bit reproduction of the received light. A PLL (Phase Locked Loo) is used to obtain this clock.
(p, phase synchronization) circuit, tank circuit to extract the clock from the transmitted signal, and a plurality of clocks with a phase shifted in advance (multi-phase clock), and select the optimal clock from them There is.

[0006]

The integrated optical switch needs to be provided with an optical amplifier for compensating for the loss. In that case, the spontaneous emission light is superimposed on the signal as noise. Loss of optical switch element, amplification of optical amplifier,
The levels of spontaneous emission are different. Therefore, each time the output is switched, the light path is different, and the signal level is different as shown in FIG. When the on and off light levels fluctuate, it is necessary to change the reference for identification, but since the optical switch is switched at high speed, it is necessary to change the reference at high speed according to the light level. Further, since the AGC circuit is a circuit intended to correct long-term fluctuations in terms of time, it cannot cope with high-speed fluctuations due to switching of optical switches. Furthermore, such a system intended for signal identification has been a major problem in signal identification and reproduction, which has never been seen before. An object of the present invention is to identify and reproduce such a signal whose level fluctuates at high speed in an optical digital signal transmission system.

Further, in the optical communication path, the output changes the optical path each time the optical switch is switched, so that the bit phase fluctuates as shown in FIG. 10 due to variations in the delay amount. In the conventional technology, a circuit for generating a clock synchronized with a signal required for bit reproduction becomes very complicated. Further, an optimum clock for signal identification is not obtained until the clock is generated or while the clock is selected, which is a problem in signal identification and reproduction. Another object of the present invention is to identify and reproduce such a signal whose bit phase varies.

[0008]

In order to achieve the above object, according to the present invention, regarding level fluctuation, light of two wavelengths modulated by signals each having a code logic inverted in a transmission system is multiplexed and output, and transmitted. To do. In the receiving system, the received light of two wavelengths is demultiplexed, the power difference between the two waves is calculated, and identification reproduction is performed.

Regarding the phase fluctuation, a signal and a clock for the signal are wavelength-multiplexed in a transmission system, output, and transmitted through the same path. In the receiving system, bit reproduction is performed using a signal and a clock obtained by converting the demultiplexed light into light.

[0010]

The pair of wavelength-multiplexed optical signals and the clock-modulated optical signal pass through the same path, undergo the same fluctuation, and have the same delay amount. Since the differential signals in the optical signals received by the receiving system serve as references for identifying each other, it is possible to identify even if the level changes. Moreover, since the optimum clock is always obtained, bit reproduction is possible.

[0011]

FIG. 1 shows an embodiment of the present invention. In the transmission system, the light of two wavelengths (λ ₁ , λ ₂ ) output from the laser diodes LD1 and LD2 is modulated by the drive circuit 1 by the digital signal Q and the inverted Q, respectively, which are sign-logically inverted, Polarization controller 2,
3 and variable attenuators 4 and 5 for multiplexing by a multiplexer 6. The light of two wavelengths passes through the same path in the optical communication path consisting of the optical transmission path 7 and the optical switch network 8 and is similarly subject to fluctuations. In the receiving system, the differential signal D and the inversion D as shown in FIG. 8A are obtained from the two wavelengths demultiplexed by the demultiplexer 9. When these two signals D and inverted D are received by the differential input photodetector 10 and the difference between them is taken by the discriminator 11, the signal (D, inverted D) of FIG. 8B is obtained. Since the off-level fluctuation is canceled and the reference level for identification can be made constant, the signal can be identified. Although a pair of wavelength pairs (λ ₁ , λ ₂ ) is used in this figure, it is also possible to use a plurality of wavelength pairs in order to increase the transmission capacity.

A specific example of the transmission system is shown in FIG. The wavelength from the laser diode (DFB laser) LD1, LD2 is λ
The light of ₁ and λ ₂ are variable attenuators 4 and 5 and wave plates 12 and 1.
After passing through 3, the light is multiplexed by the half mirror 14, passes through the polarizer 15, and is output to the optical transmission line 16. A multiplexer may be used instead of the half mirror 14. 17,1
Reference numerals 8 and 19 are lenses. In such a configuration,
The variable attenuators 4 and 5 adjust the light intensity of two wavelengths.
In addition, the wavelength plates 12 and 13 and the polarizer 15 align the polarized lights of two wavelengths of light. In the case of an optical communication path having no polarization dependence, it is not necessary to align the polarized light.

A concrete example of the receiving system is shown in FIG. Optical transmission line 2
The two wavelengths of light from 0 are generated by the coupler 21 and the wavelength filter 2
The signals are demultiplexed by 2 and 23, converted into an electric signal in the differential input photodetector 10, and bit-reproduced in the discriminator 11 based on the clock input. A demultiplexer can be used instead of the coupler and the wavelength filter.

Specific examples after the differential input photodetector are shown in FIGS. 5 (a) and 5 (b). In the circuit shown in FIG. 5A, the two-wavelength optical signals D and the inverted D are converted into two photodiodes PD1 and P
D2 separately performs photoelectric conversion, and the differential amplifier 24 amplifies to a level that can be identified by the differential input identifier 25. Figure 5
In the circuit (b), the two photodiodes D1 and D2 connected in series convert the difference signal of the optical signals D of two wavelengths and the inversion D into an electric signal, which is taken out, amplified by the single-phase amplifier 26, and outputted to the discriminator 11. input.

FIG. 2 shows another embodiment of the present invention. In this embodiment, light of wavelength λ ₃ is used for clock transmission,
The other structure is the same as that of the embodiment (FIG. 1). That is, the drive circuit 27 causes the digital signal Q and the inverted Q
Light of two wavelengths (λ ₁ , λ ₂ ) modulated by is extracted from the laser diodes LD1 and LD2, and further, light modulated by the clock C is extracted from the laser diode LD3. Light of two wavelengths from the laser diodes LD1 and LD2 passes through the polarization controllers 2 and 3 and the variable attenuators 4 and 5, and light from the laser diode LD3 passes through only the polarization controller 28 and is multiplexed by the multiplexer 6. Three wavelengths (λ ₁ , λ ₂ ,
The light of λ ₃ ) passes through the same path in the optical communication path, is delayed by the same time, and reaches the receiving system. In the receiving system, the lights of three wavelengths are demultiplexed by the demultiplexer 9 to obtain the differential signals D, inversion D and the clock C, and the differential signals are input to the differential input photodetector 10 and photoelectrically converted. , The clock is input to the optical receiver 29 and photoelectrically converted, and the differential input optical receiver 1
Bits are reproduced from the electric signal from 0 based on the clock from the light receiver 29. Note that the differential transmission method having the configuration of FIG. 1 can also be applied to the clock to be transmitted.

FIG. 6 shows a concrete example of the receiving system when the clocks are wavelength-multiplexed. Light of three wavelengths is demultiplexed using the demultiplexer 9, and photoelectric conversion is separately performed in the photodetectors 10 and 29, and amplified to required levels by the amplifiers 30 and 31, respectively. The clock is a BPF (bandpass filter,
Only the clock frequency is extracted using a band pass filter) and input to the discriminator 11. The delay circuit 32 delays the signal from the amplifier 30 so that the phase of the signal from the amplifier 30 and the phase of the clock from the BPF 33 match the bit reproduction in the discriminator 11, and inputs the signal to the discriminator 11. This delay circuit 32
The position of may be the signal path on the clock side or the transmission system side. Further, the delay circuit may be either an optical delay circuit or an electric delay circuit.

FIG. 9 shows an example of the transmitted signal train and clock. If the discriminator 11 reproduces bits at the rising edge of the clock, the optimum phase relationship is that the rising edge of the clock comes to the center of the signal.

[0018]

According to the present invention, a signal whose optical level fluctuates at high speed can be easily identified and reproduced without a complicated control circuit. Further, even when the phase of the signal changes, the signal can be easily identified and reproduced without a complicated control circuit.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention.

FIG. 2 is a block diagram of a second embodiment of the present invention.

FIG. 3 is a diagram showing a specific example of a transmission system.

FIG. 4 is a diagram showing a specific example of a receiving system.

FIG. 5 is a diagram showing a specific example of the configuration after the differential input photodetector.

FIG. 6 is a diagram showing a specific example of a receiving system when clocks are wavelength-multiplexed.

FIG. 7 is a diagram showing a signal string whose level changes at high speed.

FIG. 8 is a diagram showing a signal train in a differential input light receiver.

FIG. 9 is a diagram showing a phase relationship among a transmitted signal train, a clock, and a reproduced signal.

FIG. 10 is a diagram showing a signal sequence having a bit phase shift.

[Explanation of symbols]

Q, inverted Q Digital signal transmitted from transmission system and its sign inverted digital signal D, inverted D Digital signal obtained from multiplexed light with level fluctuation in receiving system C clock BPF bandpass filter

Claims (4)

[Claims] 1. In an optical digital signal transmission system for transmitting an optical digital signal between input / output ports connected by an optical communication path using an optical switch, an optical signal using codes whose code logics are mutually inverted in a transmission system. The signals are wavelength-multiplexed and output, the output optical signals are transmitted through the same path in the optical communication path, and then demultiplexed by the receiving system, and the power difference between the two demultiplexed optical signals is calculated. An optical digital signal transmission system characterized by identifying the obtained signal. 2. The optical digital signal transmission system according to claim 1, wherein the transmission system has a function of aligning the polarization and the optical intensity of an optical signal to be output. 3. The optical digital signal transmission system according to claim 1 or 2, wherein a clock signal used for bit reproduction of an optical signal is wavelength-multiplexed with the optical signal and transmitted through the same path. 4. The optical digital signal transmission system according to claim 1, wherein a plurality of pairs of differential optical signals are wavelength-multiplexed using different wavelengths.