JP3425027B2 - Optical receiver - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to increasing the sensitivity of an optical receiver used for optical communication.

[0002]

2. Description of the Related Art Conventionally, an optical receiver used for optical communication is
The input optical signal is directly input to a photodetector such as a photodiode (PD) or an avalanche photodiode (APD). In this case, the sensitivity of the optical receiver is limited by circuit noise such as noise generated by the light receiver, noise of the amplifier, and shot noise.

For example, the first of the conventional examples is as follows. In recent years, excellent optical amplifiers such as erbium-doped fiber amplifiers have been developed, and it has become possible to improve the sensitivity of the receiver by installing the optical amplifier in the front stage of the optical receiver. FIG. Sano et al. 、 ”
10 Gbit / s, 300 km repeaterl
ess transmission with SBS
suppression by the use o
f theRZ format ", Electron.
Lett. , Pp. The configuration of the optical receiver shown in 1694-1695 (1994) is rewritten for easy understanding. In the figure, 1 is an optical receiver input end, 2 is an optical amplifier, 5 is an optical filter, 6 is a light receiver, 7 is a receiver, 8 is a clock output end, and 9 is a data output end. By installing the optical amplifier 2 in the front stage of the light receiver 7, the optical signal input to the light receiver 6 is amplified, so that the sensitivity of the receiver is significantly improved. In an optical receiver of this kind of configuration, the beat component between the noise light and the signal light of the optical amplifier, which is called spontaneous emission light (hereinafter referred to as signal-spontaneous emission light beat noise), and the beat component between spontaneous emission lights (hereinafter referred to as natural emission light). Emitted light-spontaneous emitted light beat noise) is the main cause of noise. The spontaneous emission light intensity Pase output from the optical amplifier is given by Pase = nsp (G-1) hv Bo (1). Here, nsp is the population inversion coefficient of the optical amplifier, G is the gain, h is the Planck's constant, v is the optical frequency, and Bo is
Is the band of the optical filter. When the signal current is output from the photodetector and the spontaneous emission photocurrent are isp, is = s GP (2) isp = s Pase (3). Here, P is the optical receiver input signal strength, s is the light-current conversion efficiency by the PD, and s = ηe / hv.

Signal-Spontaneous emission light beat noise ss-sp
2. Spontaneous emission light-Spontaneous emission light Beat noise ssp-sp
2 is It can be expressed as. Signal-Spontaneous emission beat noise s
s-sp2 is coherent interference between the signal light and spontaneous emission light existing in the same frequency band, and is noise depending on the intensity of spontaneous emission light existing in the same frequency band regardless of the band of the optical filter. Since spontaneous emission light-spontaneous emission light beat noise ssp-sp2 is coherent interference between spontaneous emission lights, noise intensity is proportional to the bandwidth of the optical filter. S / N of receiver is Given in. Since isp is proportional to the optical band Bo, the signal-spontaneous emission light beat noise ss-sp2 does not depend on the optical band, whereas the spontaneous emission light-spontaneous emission beat noise ssp-sp2 is proportional to the optical band Bo. The signal-spontaneous emission light beat noise ss-sp2 is proportional to the signal intensity, while the spontaneous-emission light-spontaneous emission beat noise ssp
Since -sp2 has a constant value regardless of the signal intensity, it is possible to reduce the spontaneous emission light-spontaneous emission beat noise ssp-sp2 in a region where the signal intensity is small.
It is effective in improving the ratio. Therefore, the optical filter 5 is inserted in FIG. Although spontaneous emission light can be reduced by this optical filter, the band of the optical filter cannot be narrowed too much considering the spectral line width of the signal and the wavelength fluctuation of the signal.

The second conventional example is as follows. Spontaneous emission light is removed even in the time domain, and the signal / noise ratio (S
There is an attempt to improve the / N ratio). FIG. 24 shows M. N
akazawa, K .; Suzuki, H .; Kubot
a, E. Yamada and Y. Kimur
a: 'Straight-line soliton
data transmission data 20
Gbit / sbeyond Gordon-Haous
limit ', Electron. Lett. , 3
0, pp. It is a rewrite of the optical repeater shown in 1331-1332 (1994). A similar technique is also introduced in JP-A-6-112908. The configuration shown in the above figure was devised to suppress the jitter caused by the accumulation of noise in the optical amplifier in the optical soliton transmission system. In the figure, 30 is an input end, 2 is an optical amplifier, 26 is a coupler, 3 is an optical gate, 29 is a clock extraction circuit, 10
Is a phase shifter, and 11 is an optical gate drive circuit. The operation of FIG. 24 will be described with reference to FIG. FIG. 25A shows the input terminal 3
An optical signal waveform input to 0 is shown. The optical signal input to the input terminal 30 is amplified by the optical amplifier 2, then branched by the coupler 26, and input to the optical gate 3 and the clock extraction circuit 29. The phase of the clock signal extracted by the clock extraction circuit is adjusted by the phase shifter 10 and input to the optical gate drive circuit 11. FIG. 25B shows the output signal of the optical gate drive circuit 11. FIG. 2 shows the optical signal of FIG.
FIG. 25C shows an optical signal waveform output to the output end when input to the optical gate 3 driven by 5b. The spontaneous emission light in the time domain where there is no signal is removed by an optical gate to suppress the accumulation of the spontaneous emission light and further suppress the jitter.

The spontaneous emission light having a wide spectrum component is averaged on the time axis by the group velocity dispersion of the optical fiber connected after the optical repeater shown in FIG.
A signal that is an optical soliton propagates while maintaining its waveform on the time axis. As a result, after propagation through the optical fiber, the signal / noise ratio (S / N ratio) is improved as shown in FIG. 25D as compared with FIG. 25A. Since this repeater utilizes the fact that an optical pulse scraped by an optical gate propagates as a soliton in a transmission fiber, the length of the transmission line is the length required for the soliton to exhibit the properties of soliton (soliton). Length) or more. Further, the pulse width and intensity incident on the transmission line in order to propagate as solitons must satisfy the condition known as soliton condition.

[0007]

In the optical receiver using the optical amplifier as described above, the spontaneous emission light of the optical amplifier causes noise. In the optical receiver of Conventional Example 1, it is necessary to narrow the band of the optical filter that removes spontaneous emission light in order to obtain a high S / N ratio, but the optical filter band is the spectral line width of the optical signal and the signal light. However, there is a problem in that a sufficiently high S / N ratio cannot be achieved because the wavelength fluctuation width is limited. The idea of removing spontaneous emission light in the time domain, which is applied to the optical repeater of the second idea of the conventional example, utilizes the difference in the behavior of soliton and noise light in the optical fiber to improve the S / N ratio. Since the improvement has been realized, there is a problem that it cannot be applied to an optical receiver without a long-distance optical fiber. In addition, a new clock extraction circuit must be provided to remove spontaneous emission in the time domain, and the clock signal for driving the optical gate and the optical signal input to the optical gate must be in phase. Therefore, there is a problem that the phase shifter must be adjusted, the circuit configuration becomes complicated, and cold start is not guaranteed.

An object of the present invention is to obtain an optical receiver having an S / N ratio higher than the S / N ratio limited by the band of the optical filter. Another object is to facilitate the phase control required to maximize the light intensity and S / N output from the optical gate and prevent deadlock. Another object is to demultiplex the received optical signal by driving the periodic short pulse light source at a frequency that is an integer fraction of the modulation frequency of the received optical signal.

[0009]

An optical receiver according to the present invention is provided with an optical gate provided on the signal input end side, a high dispersion medium connected to the optical gate output side, and a high dispersion medium output side. A receiver for recovering data and a clock from a light receiver connected to the receiver and a phaser for controlling the gate time of the optical gate from the clock of the output of the receiver are provided.

Furthermore, a waveguide grating filter is used as the high dispersion medium.

Further, peak detecting means for detecting the peak value of the clock of the output of the receiver is added to the basic structure, and the time of the optical gate is controlled so as to maximize the peak value of this clock.

Further, voltage detection means for detecting the voltage value of the output voltage of the photodetector is added to the basic structure, and the time of the optical gate is controlled so as to maximize the voltage value of this output voltage.

Further, a clock break detecting means for detecting a break in the output clock of the receiver is added, and the drive width of the optical gate is reduced by detecting the clock break.

In addition to the basic structure, a supersaturated absorber is added between the optical gate output side and the photodetector input side.

Furthermore, an optical amplifier and an optical fiber are used as the high dispersion medium, and the optical signal strength in the optical fiber is set to a level higher than the optical soliton power.

In the basic configuration, instead of a high dispersion medium,
A first optical filter, an optical fiber, and a second optical filter having a passing wavelength different from that of the first optical filter are cascade-connected between the optical gate output side and the optical receiver input side, and the optical input level is increased. It is set to a predetermined value or more.

Further, a multiplexer for the optical fiber is provided on the output side of the first optical filter, and a periodic short pulse light source is provided as another input to this multiplexer, and a signal of the output of the first optical filter is provided. The received signal is demultiplexed so that the sum level of the short cycle pulse output is a predetermined value or more.

Furthermore, a multiplexer for the optical fiber on the output side of the first optical filter, a short-period pulse light source as another input to this multiplexer, and a clock provided on the output side of the first optical filter. A prescaler for dividing the phase signal from the clock output from the reproduction circuit to be a drive signal to the short cycle pulse light source is provided, and the level of the sum of the signal of the first optical filter output and the short cycle pulse output is set. The received signal is demultiplexed at a level higher than the optical soliton power.

[0019]

DETAILED DESCRIPTION OF THE INVENTION

Embodiment 1. FIG. 1 shows the configuration of an optical receiver according to the first embodiment of the present invention. In FIG. 1, 1
Is an input end, 2 is an optical amplifier, 3 is an optical gate, 4 is a high-dispersion medium, and 5 is an optical filter. Although this optical filter is not essential, the stability becomes higher. The transmission band may be wider than that of the conventional example. 6 is a light receiver, 7 is a receiver, 8 is a clock output terminal, 9 is a data output terminal, 10
Is a phase shifter, and 11 is an optical gate drive circuit. Optical amplifier 2
Erbium-doped fiber amplifier (EDF
A) or a semiconductor optical amplifier can be used. As the optical gate 3, a semiconductor modulator, a dielectric modulator or the like is used. Further, as the high dispersion medium, a dispersion compensating optical fiber, a diffraction grating, a Fabry-Perot interferometer or the like can be used.

Next, the operation will be described with reference to FIG. The signal waveform output from the optical amplifier 2 is shown in FIG. Here, although the pulse train of the RZ code (Return to Zero code) is shown, the NRZ code (Non Retu) is shown.
It is also possible to use a pulse train of rn to Zero code). In other words, even for NRZ signals, R
Since the Z code is converted in the optical region, inter-code interference can be suppressed and the S / N ratio is improved. The optical gate 3 is shown in FIG.
When such an electric signal is applied, the output waveform of the optical gate 3 becomes as shown in FIG. 2C because the optical gate 3 plays a role of the gate. Since the time of propagation in the high-dispersion medium 4 varies depending on the wavelength, the spontaneous emission light distributed over a wide wavelength range is expanded on the time axis, whereas the signal has a relatively narrow spectral line width, and thus the signal on the time axis is expanded. It does not spread. Since the spread on the time axis of the spontaneous emission light in the same band as the signal spectrum is the same as the signal, the signal-spontaneous emission beat noise does not decrease, but the spontaneous emission-spontaneous emission beat noise can be reduced. it can. In particular, noise at the time of space (signal “0”) can be reduced. Therefore, the waveform after propagating the high-dispersion medium 4 has a high S / N ratio as shown in FIG. 2D as compared with FIG.

For example, the signal transmission rate is 10 Gbit /
s, the transmission code is an RZ code with a duty of 25%, and the band of the optical filter is 1 nm. Both the signal and the spontaneous emission light are cut out to 25 ps by the optical gate 3. In this case, the signal spectral line width is about 0.1 nm and the spontaneous emission light spectral width is 1 nm. When 100 ps / nm is used as the dispersion value of the high dispersion medium 4, the pulse spread of the signal can be ignored, while the spontaneous emission Since the light spreads by 100 ps, the spontaneous emission light component that overlaps the signal on the time axis becomes about 1/4 before being input to the high dispersion medium 4, and the S / N ratio is improved. Since the spontaneous emission light-spontaneous emission light beat noise can be reduced by the present invention, it is possible to effectively improve the S / N ratio of the receiver especially in a region where the signal intensity is small, and the minimum receiver The light receiving sensitivity can be reduced.

The optical gate signal shown in FIG. 2B is generated from the clock signal output from the clock output terminal 8 of the photodetector 6. The phases of the optical amplifier output signal and the optical gate signal shown in FIG. 2A are adjusted by the phase shifter 10, and the gate width of the optical gate signal corresponds to the signal pulse width input to the optical receiver by the optical gate drive circuit 11. Will be adjusted accordingly. In the present embodiment, the connection position of the optical filter 5 may be before the optical gate 3 or before the high dispersion medium 4. The S / N ratio of the optical signal is hardly deteriorated by the loss after the optical amplifier 2. This is because the spontaneous emission light that causes a noise is generated in the optical amplifier 2, and the loss thereafter does not deteriorate the S / N ratio because the signal light and the noise light are attenuated equally. Further, even if a second optical amplifier (not shown) is connected in cascade, when the gain of the second optical amplifier is smaller than the gain of the previous optical amplifier 2, the noise generated in the second optical amplifier is small and is ignored. it can. Therefore, the second optical amplifier can be provided in front of the light receiver 6 if necessary. In this embodiment, the output pulse width and intensity of the optical amplifier are arbitrary. Further, according to the present invention, the noise of the optical amplifier is removed even on the time axis, so that the S / N ratio can be increased without narrowing the band of the optical filter 5. Therefore, a control device for matching the center frequency of the narrow band filter and the transmission band with the signal light frequency is unnecessary, and the device configuration is simplified.

Embodiment 2. FIG. 3 shows the configuration of the optical receiver according to the second embodiment of the present invention. Figure 1
The difference is that a high-dispersion medium and a waveguide grating filter 39 are used as an optical filter. The operation principle of the entire optical receiver is similar to that of the first embodiment. The waveguide grating filter 39 is described, for example, in K.K. O. Hi
ll, "Photosensitive and and
its application tooptica
l fiber communications ”, T
Utmostal Section, Optical F
iber Communications Conf
Rence, pp. 146-193 (1995).

The structure and operation of the waveguide grating filter 39 will be described with reference to FIG. In FIG. 4, 32 is a waveguide grating filter input terminal, 34 is a circulator, 33 is a waveguide grating filter output terminal, and 35 is a chirped reflection grating. The chirped reflection grating 35 is provided with a diffraction grating having a different pitch depending on the position, and the reflected position differs depending on the wavelength. Therefore, as shown in FIG. 5, the time until the optical signal input to the waveguide grating filter input terminal 32 is output to the waveguide grating filter output terminal 33 is a function of wavelength. In addition, light having a wavelength other than the Bragg wavelength of the diffraction grating is not reflected by the chirped reflection grating 35, and thus is not output to the waveguide grating filter output terminal 33. Therefore, the waveguide grating filter 39 serves both as a high dispersion medium and an optical filter. Since the waveguide grating can realize arbitrary dispersion characteristics, it is possible to obtain optimum dispersion characteristics. In this embodiment, the high dispersion medium 4 may be further cascaded in combination with the first embodiment.

Embodiment 3. FIG. 6 shows the configuration of the optical receiver according to the third embodiment of the present invention. Figure 1
The difference is that the peak detection circuit 12 and the phase shifter control means 13 are added. The optical gate signal shown in FIG. 2B needs to be in phase with the optical amplifier output signal shown in FIG. In this embodiment, the peak value of the clock signal output from the receiver 7 is detected by the peak detection circuit 12, and the delay amount of the phase shifter 10 is adjusted so that the peak value becomes the largest.

An example of implementation of the phase shifter control means 13 is shown in FIG. In FIG. 7, 16 is a mixer, 17 is a low-pass filter, 20 is an oscillator, 18 is an adder, and 19 is a phaser control means. The phaser control means input signal 21 and the oscillator output signal 22 are input to the mixer 16, and the mixer output signal 23 is obtained. The error signal obtained by inputting the mixer output signal 23 to the low pass filter and the oscillator output signal 22 are added by the adder 18 to obtain a phase shifter drive signal. The phaser generates a delay amount proportional to the applied voltage.

Next, the operation will be described with reference to FIG. When the voltage applied to the phase shifter is correct (FIG. 8B) and the phases of the optical gate signal and the optical amplifier output signal are aligned, the phaser control means input signal 21 becomes FIG. 8D and the oscillator output The mixer output signal 16 becomes as shown in FIG. 8 (h) by being input to the mixer 16 together with the signal 22 (FIG. 8 (g)). As for the mixer output signal 16, the output of the low-pass filter 17 for inputting the low-pass filter shown in FIG. When the voltage applied to the phase shifter is small (FIG. 8 (a)), the phase shifter control means input signal 21 is shown in FIG. 8 (f), and the mixer output signal 16 is shown in FIG. 8 (j).
The output of the low pass filter 17 becomes positive and a positive error signal is generated. On the other hand, when the voltage applied to the phase shifter is large (FIG. 8 (c)), the phase shifter control means input signal 21
8 (e), the mixer output signal 16 is shown in FIG. 8 (i), the output of the low-pass filter 17 is negative, and a negative error signal is generated. In this way, the bias voltage applied to the phaser is optimized. The output signal frequency of the oscillator 20 is sufficiently lower than the clock frequency and is equal to or lower than the response frequency of the peak detection circuit 12. The adder 18 is realized by an analog adder using, for example, an operational amplifier, and the phase shifter control means 19 is realized by an appropriate amplifier. It goes without saying that the phaser control means 13 can also be realized by a CPU and a simple control program.

Fourth Embodiment FIG. 9 shows the configuration of the optical receiver according to the fourth embodiment of the present invention. Figure 1
The difference is that the voltage detection circuit 15 and the phase shifter control means 13 are added. The configuration and operation of the phase shifter control means 13 are the same as those of the third embodiment, the output voltage of the photodetector 6 is detected by the voltage detection circuit 15, and the delay amount of the phase shifter 10 is set so that the voltage becomes the maximum. adjust. Voltage detection circuit 1
The response speed of 5 is sufficiently slower than the clock frequency and faster than the oscillator output signal frequency included in the phase shifter control means 13. The difference in operation based on the difference in configuration from the third embodiment is that the bias of the phase shifter can be controlled even when the receiver 7 is provided with a gain control mechanism because the receiver output signal is not used. is there.

Embodiment 5. FIG. 10 shows the configuration of the optical receiver according to the fifth embodiment of the present invention. The difference from FIG. 1 is that a clock loss detection circuit 14 is added and a function of stopping or reducing the output is added to the optical gate drive circuit 11a. Clock loss detection circuit 14
Monitors the clock signal output from the receiver 7, and outputs a clock loss detection signal when the clock signal amplitude becomes equal to or less than a certain threshold. The optical gate drive circuit 11a reduces the pulse width drive amplitude to the optical gate when the clock loss detection signal is input. The clock loss detection circuit 14 can be configured by using a low pass filter having an appropriate time constant and a comparator. In addition, the optical gate drive circuit 11a
Can be easily realized by adding an analog switch or using a gain control amplifier.

In FIG. 2, when the optical gate signal FIG. 2 (b) and the optical amplifier output signal FIG. 2 (a) are out of phase with each other by 180 degrees, the optical gate output FIG. 2 (c) becomes zero. The bias control function of the phase shifter shown in the forms 3 and 4 of the present invention does not operate (deadlock). On the other hand, in the present embodiment, when the optical gate output FIG. 2C becomes zero and the clock output of the receiver 7 is cut off, the drive amplitude of the optical gate 3 is reduced. Since the optical gate output FIG. 2 (c) does not become zero only when the drive amplitude of the pulse width of the optical gate 3 becomes small, the control function of the phase shifter shown in the third and fourth embodiments operates and an appropriate bias is obtained. Applied to the phaser. As a result, the amplitude output from the optical gate is increased, a clock having a normal amplitude is output from the photodetector 7, and the optical gate drive circuit 11a is restored to the normal operation.
In this way, the phase shifter can be controlled without deadlocking even when the optical gate signal and the optical amplifier output signal are out of phase by 180 degrees. This embodiment operates effectively when used in combination with the third or fourth embodiment.

Sixth Embodiment FIG. 11 shows the configuration of the optical receiver according to the sixth embodiment of the present invention. The difference from FIG. 1 is that a supersaturated absorber 24 is added.
Next, the operation will be described with reference to FIG. In FIG. 12, (a) is an optical amplifier output signal, (b) is an optical gate signal,
(C) is an optical gate output signal and (d) is a high dispersion medium output signal. Since the supersaturated absorber 24 is transparent only to an input having a certain light intensity or more and has a function of blocking an optical signal having a certain light intensity or less, when an optical signal such as (d) is input to the saturable absorber (e) ) Output is obtained. As a result, noise on the space side (“0” level) is effectively removed, and a higher S / N ratio can be obtained. Since only the noise is spread on the time axis by the optical gate and the high-dispersion medium as described in the description of the operation of the first embodiment, a high S / N ratio improving effect by the saturable absorber can be expected.

As the saturable absorber 24, for example, a nonlinear loop mirror (Nonlinear op) as shown in FIG. 13 is used.
The optical loop mirror (NOLM) can be used. In FIG. 13, 27 is a saturable absorber input terminal, 28 is a saturable absorber output terminal, 2c is an optical amplifier, 26 is an optical coupler, 25 is an optical fiber, and 40 is an optical attenuator. Next, the operation will be described. The optical signal input from the saturable absorber input terminal 27 is amplified by the optical amplifier 2c and then input to the optical coupler 26. The light demultiplexed by the optical coupler returns to the optical coupler 26 after propagating through the two paths a and b, respectively. At this time, if the optical attenuator 40 is inserted so that the light passing through the paths a and b has different light intensities when propagating through the optical fiber 25, the self-phase modulation that the light passing through the two paths receives in the optical fiber 25 is performed. Since the magnitude of the effect is different, the phase of the light returning to the coupler 26 is different. When the phases input to the optical coupler 26 are different, they are output to the saturable absorber output 28. As the intensity of the input light increases, the phase difference of the light transmitted through the two paths a and b increases, so that FIG. 13 operates as a saturable absorber. The optical attenuator 40 is a,
b Since the light passing through the two paths is used to change the intensity when passing through the optical fiber, it may have a negative loss or gain.

Embodiment 7. FIG. 14 shows the configuration of the optical receiver according to the seventh embodiment of the present invention. The difference from FIG. 1 is that the optical fiber 25 is provided as the high-dispersion medium 4, and the optical amplifier 2c is provided so that the optical signal intensity input to the optical fiber 25 is equal to or higher than the optical soliton power to cause pulse compression. Is. Next, the operation will be described with reference to FIG. (A) Optical amplifier output signal, (b)
The optical gate signal and (c) optical gate output signal are the same as in FIG. The optical signal input to the optical fiber 25 is the optical amplifier 2
amplified by b. The dispersion value of the optical fiber 25 is anomalous dispersion, and the optical signal intensity input to the optical fiber 25 is set to be equal to or higher than the soliton power intensity.
In 5, the signal pulse is compressed. This is an optical fiber 25
This is due to the interaction between the self-phase modulation effect and the anomalous dispersion of the optical fiber. Due to this effect, the signal pulse is compressed, while the noise light is spread on the time axis by the dispersion of the optical fiber 25, so that a high S / N ratio can be obtained.

The same effect can be obtained by replacing the optical fiber 25 with the Kerr effect device such as a semiconductor waveguide and the high dispersion medium 4. Further, since the dispersibility of the optical fiber 25 can be utilized, the high dispersion medium 4 may be omitted.

Embodiment 8. FIG. 16 shows the configuration of the optical receiver according to the eighth embodiment of the present invention. In FIG. 16, reference numeral 1 is an input end, and 2 is an optical amplifier, which amplifies the input signal light to a certain level or more to cause spread spectrum. 5a is a first optical filter, 5b
Is a second optical filter, 25 is an optical fiber, 6 is a light receiver,
Reference numeral 7 is a receiver, 8 is a clock output terminal, and 9 is a data output terminal. The important thing is that the first optical filter 5a and the second optical filter 5a
The central wavelength of the transmission band of the optical filter 5b is set to a different value.

Next, the operation will be described with reference to FIGS. In the present embodiment, the effect of improving the S / N ratio is obtained by separating the signal and noise by the self-phase modulation effect. In FIG. 17, (a) shows the transmission characteristics of the first optical filter 5a. (B) is an optical spectrum output from the first optical filter 5a. It is preferable that the center wavelength of the signal spectrum and the center wavelength of the transmission band of the first optical filter match. The optical signal, which has been sufficiently raised in level and is input to the optical fiber 25, has its spectrum spread as shown in (c) by the self-phase modulation effect in the optical fiber. Second optical filter 5b having transmission characteristic (d)
When the signal spectrum spread in the optical fiber is input to, an output spectrum as shown in (e) is obtained. Here, by making the transmission band of the first optical filter 5a different from the transmission band of the second optical filter 5b, noise in the time domain where no signal pulse exists can be removed. This is because only light having a light intensity of a certain value or more spreads the spectrum due to the self-phase modulation effect. Therefore, in the time domain, the optical amplifier output pulse shape of FIG. 18A and the second optical filter output pulse shape of FIG. 18B are obtained.

For example, an input pulse to the optical fiber 25 has a pulse width of 25 ps, a light intensity of +22 dBm, an optical fiber 2
When the dispersion value of 5 is −0.5 ps / nm / km and the length is 20 km, the spectrum of the optical signal is expanded to about 3 nm. The pulse width obtained by setting the band of the first and second optical filters to 1 nm and shifting the center wavelength by 1 nm is about 20 ps.
Thus, a pulse waveform with an improved S / N ratio as shown in FIG. 18B is obtained. If the dispersion of the optical fiber 25 is anomalous dispersion or zero dispersion, the light intensity required to broaden the spectrum can be reduced by the self-phase modulation effect, but the modulation instability and the nonlinear effect known as the four-wave mixing effect are large. Since the waveform change is also significant, the margin for the optical fiber incident intensity is reduced. Therefore, the optical fiber 25 is preferably used in the normal dispersion region.

Ninth Embodiment FIG. 19 shows the configuration of the optical receiver according to the ninth embodiment of the present invention. This embodiment is a combination of the first embodiment and the eighth embodiment. As for the operation, the S / N ratio improving effect obtained by only the noise being spread on the time axis by the optical gate and the high dispersion medium as shown in FIG. 2 and the self phase modulation effect as shown in FIG. Both the S / N ratio improving effect due to the separation of the used signal and the noise can be obtained.

Embodiment 10. FIG. 20 shows the configuration of the optical receiver according to the tenth embodiment of the present invention. This embodiment also sets the optical signal level to a certain value or more,
Although it utilizes the spread of the spectrum,
The difference from 6 is that a periodic short pulse light source 36 is provided as an addition optical signal. As the short pulse light source 36, a semiconductor absorption modulator, a mode-locking operation of a semiconductor laser, a gain switching operation, a ring resonator laser, or the like can be used. It is desirable that the wavelength of the short pulse light source 36 be different from the signal wavelength, and also be different from the first optical filter 5a and the second optical filter 5b.

By matching the cycle of the short period pulse with the cycle of the input signal to be separated, it is possible to perform the operation of demultiplexing only the input to be separated from the multiplexed inputs. Next, the operation will be described with reference to FIG. Figure 2
In FIG. 1, (a) is a first optical filter output, and (b) is a short pulse light source output. When the output of the first optical filter and the output of the short pulse light source overlap on the time axis, the optical fiber 2
Since the cross phase modulation effect occurs at 5, the signal spectrum is broadened. By shifting the transmission bands of the first optical filter 5a and the second optical filter 5b as in the eighth and ninth embodiments, the signal light passes through the second optical filter 5b only when the mutual phase modulation effect occurs. Set to do. As a result, the output waveform of the second optical filter 5b becomes as shown in FIG. 21C, and not only a high S / N ratio can be obtained by the same principle as that of the eighth embodiment, but also only a predetermined pulse can be extracted. it can. For example, as shown in FIG. 21, if the repetition period of the short pulse light source is set to 1/2 of the clock frequency,
1: 2 optical DEMUX (Demultiplexin)
g) can be realized. Since the optical DEMUX can narrow the electronic circuit band required for the light receiver 6 and the receiver 7, not only the circuit can be easily realized but also noise can be reduced. this is,
Even when the optical input is RZ, it can be applied without any processing, and the configuration is simple.

In this case, the cross phase modulation effect causes a change in the refractive index in the optical fiber 25 according to the light intensity of the wavelength emitted from the periodic short pulse light source 36, and the change in the refractive index causes the optical receiver input end 1 to change. This is the effect that the signal light input from is subjected to phase modulation and the spectrum is broadened. In order to perform the optical DEMUX by the periodic short pulse light source 36, it is preferable that a large mutual phase modulation effect is generated as compared with the self phase modulation effect. Therefore, the optical intensity of the wavelength emitted from the periodic short pulse light source 36 in the optical fiber 25 is preferable. The optical filter 5
It is better to set a larger than the output signal light intensity. Further, if the optical fiber 25 is used in the anomalous dispersion region or the zero dispersion region, non-linear effects such as four-wave mixing effect and modulation instability increase, so that it is easier to use in the normal dispersion region. Further, in the present embodiment, when the signal input to the optical receiver input terminal is the NRZ code, it is possible to receive the RZ code by driving the short pulse light source at the clock frequency of the signal. By converting the NRZ code into the RZ code in the optical domain, inter-code interference can be suppressed and the receiving sensitivity of the receiver can be increased.

Eleventh Embodiment 22 shows the structure of an optical receiver according to an eleventh embodiment of the present invention. The difference from FIG. 20 is that the phase shifter 10, the clock recovery circuit 41, the prescaler 38, and the periodic short pulse light source drive circuit 3 are provided.
7 is added. An appropriate delay is given to the output signal of the clock recovery circuit 41 by the phase shifter 10, and then the frequency is divided by the prescaler 38 into a frequency of an integer fraction. An optical DEMUX can be realized by inputting the divided signal to the short pulse light source drive circuit 37 and driving the short pulse light source. It goes without saying that it is useful to use the third and fourth embodiments in combination with the present embodiment to automatically control the bias of the phase shifter 10.

The optical receivers of Embodiments 1 to 11 above are
Both can be applied as a receiver in optical communication such as optical fiber communication and satellite communication. Further, an optical amplifier can be connected immediately before the light receiver 6.

[0044]

The optical receiver of the present invention is configured as described above, and since a part of the spontaneous emission light of the optical amplifier is removed by the optical gate to diffuse noise, it is limited in the band of the optical filter. The S / N ratio is higher than the S / N ratio.

Since the waveguide grating is used as the high-dispersion medium, there is an effect that the structure is simplified.

Since the configuration is controlled so that the phase of the clock signal for driving the optical gate and the phase of the optical signal input to the optical gate match, the intensity of the light output from the optical gate is increased and a high S / N ratio is obtained. There is an effect to be obtained.

Further, even if the receiver detects a clock break, the drive signal amplitude of the optical gate is narrowed, so that deadlock does not occur and cold start is possible.

Further, since the spontaneous emission light of the time slot having no optical signal is removed by using the supersaturated absorber together, there is an effect that a higher S / N ratio can be obtained.

Further, since the signal is separated from noise as a non-linear pulse by the optical fiber and high optical level driving, there is an effect that a higher S / N ratio can be obtained.

Further, since the periodic short pulse light source is driven at a frequency which is an integer fraction of the modulation frequency of the received optical signal, there is an effect that the received optical signal can be demultiplexed simultaneously with a good S / N ratio.

[Brief description of drawings]

FIG. 1 is a configuration block diagram of an optical receiver according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an operation of the optical receiver of FIG.

FIG. 3 is a configuration block diagram of an optical receiver according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing details of the waveguide grating filter of FIG.

5 is a diagram illustrating the operation of the filter of FIG.

FIG. 6 is a configuration block diagram of an optical receiver according to a third embodiment of the present invention.

7 is a configuration diagram showing details of the phase shifter control means in FIG.

FIG. 8 is a diagram illustrating the operation of the apparatus of FIG.

FIG. 9 is a configuration block diagram of an optical receiver according to a fourth embodiment of the present invention.

FIG. 10 is a configuration block diagram of an optical receiver according to a fifth embodiment of the present invention.

FIG. 11 is a configuration block diagram of an optical receiver according to a sixth embodiment of the present invention.

FIG. 12 is a diagram illustrating an operation of the optical receiver of FIG.

13 is a configuration diagram showing details of the supersaturated absorber of FIG. 11. FIG.

FIG. 14 is a configuration block diagram of an optical receiver according to a seventh embodiment of the present invention.

FIG. 15 is a diagram illustrating an operation of the optical receiver of FIG.

FIG. 16 is a configuration block diagram of an optical receiver according to an eighth embodiment of the present invention.

17 is a diagram for explaining the operation of the optical receiver of FIG.

FIG. 18 is a diagram for explaining the operation of the optical receiver of FIG.

FIG. 19 is a configuration block diagram of an optical receiver according to a ninth embodiment of the present invention.

FIG. 20 is a configuration block diagram of an optical receiver according to a tenth embodiment of the present invention.

21 is a diagram for explaining the operation of the optical receiver of FIG.

FIG. 22 is a configuration block diagram of an optical receiver according to an eleventh embodiment of the present invention.

FIG. 23 is a configuration block diagram of a first optical receiver of a conventional example.

FIG. 24 is a configuration block diagram of a second optical receiver of a conventional example.

FIG. 25 is a diagram for explaining the operation of the second conventional optical receiver.

[Explanation of symbols]

1 optical receiver input end, 22a, 2b, 2c optical amplifier, 3 optical gate, 4 highly dispersive medium, 5 optical filter,
6 receiver, 7 receiver, 8 clock output terminal, 9
Data output terminal, 10 phaser, 11 11a optical gate drive circuit, 12 peak detection circuit, 13 phaser control means, 14 clock loss detection circuit, 15 voltage detection circuit, 16 mixer, 17 low pass filter, 18 adder, 19 phase Drive circuit, 20 oscillators, 21
Phaser control means input signal, 22 oscillator output signal,
23 mixer output signal, 24 oversaturation absorber, 25 optical fiber, 26 optical coupler, 27 oversaturation absorber input terminal, 28 oversaturation absorber output terminal, 29 clock extraction circuit, 30 optical repeater input terminal, 31 optical repeater output terminal , 32 waveguide grating input terminal, 33 waveguide grating output terminal, 34 circulator, 3
5 Chirped reflection grating, 3
6 short pulse light source, 37 short pulse light source drive circuit, 38
Prescaler, 39 waveguide grating filter, 40 optical attenuator, 41 clock recovery circuit.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI H04B 10/14 10/18 10/26 10/28 (56) Reference JP-A-6-112908 (JP, A) JP-A 9-23188 (JP, A) JP-A-3-278627 (JP, A) JP-A-7-221706 (JP, A) JP-A-2-230221 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02B 6/00 306 G02F 1/35 501 JISST file (JOIS)

Claims (10)

(57) [Claims] 1. An optical gate provided on a signal input end side, a high-dispersion medium connected to the optical gate output side, and a light receiver connected to the high-dispersion medium output side to reproduce data and a clock. An optical receiver comprising a receiver and a phaser for controlling a gate time of the optical gate from a clock output from the receiver. 2. The optical receiver according to claim 1, wherein a waveguide grating filter is used as the high dispersion medium. 3. A peak detecting means for detecting the peak value of the clock of the output of the receiver is added, and the time of the optical gate is controlled so as to maximize the peak value of the clock. The optical receiver according to item 1. 4. A voltage detecting means for detecting the voltage value of the output voltage of the photodetector is added, and the time of the optical gate is controlled so as to maximize the voltage value of the output voltage. The optical receiver according to claim 1. 5. The optical gate driving width is narrowed by detecting a clock break detecting means for detecting a break in the output clock of the receiver output, and the driving width of the optical gate is reduced. Alternatively, the optical receiver according to claim 4. 6. A saturable absorber is added between the output side of the optical gate and the input side of the photodetector.
Optical receiver as described. 7. The optical receiver according to claim 1, wherein an optical amplifier and an optical fiber are used as the high dispersion medium, and the optical signal strength in the optical fiber is set to a level higher than the optical soliton power. 8. A first optical filter, an optical fiber, and a second optical fiber having a different passing wavelength from the optical gate output side to the optical receiver input side instead of the high-dispersion medium. 2. The optical receiver according to claim 1, wherein the optical receivers are connected in cascade and the optical input level is set to a predetermined value or more. 9. A multiplexer for the optical fiber on the output side of the first optical filter, and a periodic short pulse light source as another input to the multiplexer, and a signal of the output of the first optical filter. 9. The optical receiver according to claim 8, wherein the received signal is demultiplexed so that the level of the sum of the output of the short cycle pulse is a predetermined value or more. 10. A multiplexer for the optical fiber on the output side of the first optical filter, a periodic short pulse light source as another input to the multiplexer, and a clock regeneration provided on the output side of the first optical filter. A prescaler that divides the phase signal from the clock output from the circuit to be a drive signal to the periodic short pulse light source is provided, and the level of the sum of the signal of the first optical filter output and the periodic short pulse output. 9. The optical receiver according to claim 8, wherein the received signal is demultiplexed so that the signal level is equal to or higher than a predetermined value.