JPS63228829A - Optical frequency draw-in device - Google Patents

Abstract

PURPOSE:To draw the oscillation frequency of an oscillating light source into a certain frequency range, e.g., a lock-in range of a stabilizing loop for frequency difference against reference signal light, by detecting and correcting the variance of frequency by means of the sweep signal of a frequency sweeping part and an output signal received from a power monitor part. CONSTITUTION:The positional relation between the signal light 1 and local oscillation light 3 is detected by sweeping the oscillation frequency of the light 3. The difference of frequency between the light 1 and the light 3 is set within the band of a light receiver 5 at a certain point in a sweeping. The presence or absence of an intermediate frequency signal is monitored at a power monitor part 10 consisting of an envelope detecting circuit. The pulse output of the part 10 is multiplied by the output of a sawtooth wave generating circuit 9 via a multiplier 12 for production of a signal 13 whose polarity corresponds to the frequency disposition. Based on the signal 13, a light source control part 14 changes the bias level of a local oscillating light source 2 in response to the polarity and the amplitude of the signal 13. In such a way, the oscillation frequency of the light source 2 is drawn in.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical frequency pulling device, and more particularly to an optical frequency pulling device suitable for use in an optical communication system using optical heterodyne detection.

[Conventional technology]

In general, optical communication systems that use optical heterogain detection have a
Since it is possible to achieve optical reception sensitivity that is ~100 times higher, optical communication systems using optical heterogain detection are expected to be effective optical communication systems for long-distance optical communication trunk systems and the like.

In addition, in recent years, the performance of semiconductor lasers has improved, and optical heterodyne detection systems using semiconductor lasers as light sources have been developed.

By the way, optical heterodyne detection systems use optical phase and frequency information, so the frequency stability of the light source becomes an issue.When a semiconductor laser is used as a light source, its oscillation frequency changes depending on its injected current and ambient temperature. Therefore, its stability is a particular issue. That is, in the optical heterodyne detection system, it is necessary to keep the relative frequency difference between the transmitting light source and the local oscillation light source constant, and for this reason, various studies have been made to stabilize the system. For example, the paper "Study on Offcent Lock of Semiconductor Laser" by Kuboki and Otsu in IEICE Technical Inn, MW85-29 is an example. Through such research, it can be said that a technique for stabilizing the frequency difference between two semiconductor lasers having a relatively close oscillation frequency difference is already being established.

[Problem that the invention seeks to solve]

By the way, as mentioned above, the oscillation frequency of a semiconductor laser increases (changes) depending on the injected current and ambient temperature, so when the system is started up, the oscillation frequency difference between the transmitting light source and the local oscillation light source increases due to the frequency stabilization loop described above. Lock-in range (range where pull-in is performed to stabilize the frequency difference)
It doesn't necessarily mean it's contained within. There have been no reports on the frequency pull-in method when the oscillation frequency difference between the transmitting light source and the local oscillation light source is outside the lock-in range, and this poses a problem when putting an optical heterogain detection system into practical use. It had become.

An object of the present invention is to provide an optical frequency pulling device that can pull the oscillation frequency of an oscillating light source into a certain frequency range, for example, within the lock-in range of a frequency difference stabilization loop, with respect to a reference signal light. be.

[Means for solving problems]

The present invention is an optical frequency pulling device that pulls the frequency of a light source into a predetermined frequency range with respect to a reference signal light, which comprises a frequency sweep section that sweeps the frequency of the light source, and a monitor that monitors the signal power appearing within the signal band. a power monitor unit that detects the frequency deviation of the light source with respect to the signal light based on the sweep signal of the frequency sweep unit and the output ε from the power monitor unit; and a light source control section that corrects the frequency shift based on a signal containing information about the frequency shift.

[Effect]

In order to pull the frequency of the light source into a predetermined frequency range with respect to the reference signal light, the present invention sweeps the oscillation frequency of the light source by a frequency sweep section that sweeps the frequency of the light source, and also uses the frequency of the light source with respect to the signal light. The amount of deviation in frequency is detected using the sweep signal of the frequency sweep section and the output signal from the power monitor section. The detected signal includes information about the frequency deviation of the light source with respect to the signal light, and the light source control unit changes the oscillation frequency of the light source to correct the frequency deviation based on this signal. The oscillation frequency of the light source is pulled in.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

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

This embodiment is applied to the case where the oscillation frequency of the local oscillation light source of an optical receiver using optical heterogain detection is brought into a predetermined frequency range for the signal light sent from the transmitting side. As shown in the figure, a local oscillation light source 2,
An optical multiplexer 4 that multiplexes the signal light 1 and the local oscillation light 3 from the local oscillation light source 2; an optical receiver 5 that receives the light multiplexed by the optical multiplexer 4; It also includes a frequency sweep circuit 8, a sawtooth wave generation circuit 9 for obtaining a sweep signal, a power monitor section 10, and a comparator 11.
, a multiplier 12 , a light source control section 14 , and a sweep control section 15
is provided.

The frequency sweep circuit 8 is used to sweep the oscillation frequency of the local oscillation light source 2, and this sweep is performed based on the output of the sawtooth wave generation circuit 9. Furthermore, as will be described later, an error signal 7 is applied to the frequency sweep circuit 8 when a demodulated output cannot be obtained, and the sweep width is controlled by a sweep control section 15. ing.

The power monitor unit 10 is supplied with an output from an optical receiver 5 that performs heterogain detection. As will be described later, the power monitor unit 10 monitors the signal power appearing within the signal band, and can be configured with an envelope detection circuit.

The output of the power monitor unit 10 is supplied as one input to the multiplier 12 via the comparator 11, and
2 is applied with the output of the sawtooth wave generating circuit 9 as the other input. In this embodiment, with such a configuration,
A means for detecting a frequency deviation of the local oscillation light source 2 with respect to the signal light 1 is constituted by the sweep signal and the output signal from the power monitor section 10.

The signal 13 obtained from the multiplier 12 is a signal containing information about the above-mentioned frequency shift, and this is the signal that is transmitted to the light source control section 14.
and the sweep control section 15.

This light source control unit 14 is a control unit that controls the local oscillation light source 2 to correct the frequency deviation based on the signal 13 containing information about the frequency deviation, and such control changes the bias level of the local oscillation light source 2. This is done by On the other hand, the sweep control section 15 is configured to control the sweep width based on the signal 13 corresponding to the frequency shift.

Further, a detailed explanation will be given with reference to FIG. 2 as well.

First, in optical heterogain detection, a signal is demodulated from an intermediate frequency electrical signal corresponding to the difference frequency between the signal light and the local oscillation light. However, there are limits to the frequency response of the light receiving element and the electrical circuit, and the possible intermediate frequency band is currently 10 GHz. The lock-in range of the frequency difference stabilization loop is also limited in this intermediate frequency band.

In particular, when a semiconductor laser is used as a light source, its oscillation frequency changes by 10 GHz or more with a change in ambient temperature of 1'C, and by several GHz with a change in injection current of 1 mA. Therefore, the difference frequency between the signal light and the local oscillation light easily becomes a frequency outside the intermediate frequency band. Therefore, in this case, in order to bring the signal back into the intermediate frequency band, as shown in Figure 1, the signal light
In contrast, means for detecting which side and how much the local oscillation light 3 is shifted, and means for sweeping the frequency of the local oscillation light 3 within the intermediate frequency band in response to the detection signal are used. .

In this embodiment, the relative positional relationship between the signal light 1 and the locally oscillated light 3 is detected by sweeping the oscillation frequency of the locally oscillated light 3.

That is, in FIG. 1, signal light 1 is transmitted from local oscillation light source 2.
Local oscillation light 3, which is the output of 7
is applied to the frequency sweep circuit 8 of the local oscillation light source 2. Thereby, the frequency sweep circuit 8 sweeps the oscillation frequency of the local oscillation light source 2 based on the output of the sawtooth wave generation circuit 9. Somewhere during this sweep, the frequency difference between the signal light 1 and the local oscillation light 3 falls within the band of the optical receiver 5. The presence or absence of this intermediate frequency signal is monitored by a power monitor section 10 consisting of an envelope detection circuit. By passing the output of the power monitor section 10 through a comparator 11, a pulse output is obtained at the moment the signal enters the optical receiver 5. By multiplying this pulse output by the output of the sawtooth wave generation circuit 9 in the multiplier 12, the amplitude is proportional to the frequency difference between the signal light 1 and the local oscillation light 3, and the polarity is changed between the signal light 1 and the local oscillation light 3. A signal 13 corresponding to the frequency arrangement is obtained.

FIG. 2 is a diagram showing the waveforms of various parts when the oscillation frequency of the local oscillation light 3 is swept as described above.

For example, if we consider the case where the current injected into the local oscillation light source 2 is modulated by a sawtooth wave as shown in Fig. 2(a) and the frequency is swept, in this case, as the local oscillation frequency is swept, at a certain moment, as shown in Fig. 2(a), As shown in (b), a signal is generated within the intermediate frequency band. Here, Fig. 2 (a), (b) 2
When the two signals are multiplied, a signal 13 having an amplitude corresponding to the difference between the signal light 1 and the local oscillation light 3 is obtained as shown in FIG. 2(C). Here, the amplitude of the signal 13 obtained also changes depending on the positional relationship between the signal light 1 and the local oscillation light 3. Therefore, by changing the bias level of the local oscillation light source 2 using this signal 13, it is possible to bring the difference frequency between the signal light 1 and the local oscillation light 3 into the intermediate frequency band.

In this manner, the light source control section 14 changes the bias level of the local oscillation light source 2 based on the signal 13 in accordance with the scratchiness and amplitude of the signal 13. As a result, the oscillation frequency of the local oscillation light source 2 is pulled in. Further, the sweep control section 15 changes the amplitude of the output signal of the frequency sweep circuit 8 in proportion to the amplitude of the signal generator 3. As a result, when the oscillation frequency of the local oscillation light source 2 has been pulled in, the frequency sweep circuit 8 does not perform frequency sweep.

This is done in order to solve the following problems.

That is, when stabilizing the difference frequency within the intermediate frequency band, if the local oscillation light 3 continues to be swept, this sweeping will interfere with stabilization. Therefore, in order to solve this problem, as mentioned above, the amplitude of the signal 13 obtained by the product of the sweep signal and the signal from the intermediate frequency band is
Since it is proportional to the difference frequency between the signal 13 and the local oscillation light 3, the sweep width of the local oscillation light 3 is changed in proportion to the amplitude of this signal 13. By controlling in this manner, the sweep amplitude of the local oscillation light 3 can be made zero when a signal is drawn into the intermediate frequency band, and it is possible to prevent the sweep from interfering with stabilization.

In this example, a distributed feedback semiconductor laser with a wavelength of 1 and 55 μm was used as the local oscillation light source 2. The signal light 1 is an ASK modulated signal at 400 Mb/s, and is combined with the local oscillation light 3 to obtain the center frequency IG.
Hz, and the optical receiver 5 in the band IGHz performed hetero gain detection. The frequency sweep circuit 8 varied the injection current of the local oscillation light source 2 by a maximum of ±20 mA. This results in approximately 100GH
It was possible to change the oscillation frequency of the z local oscillation light source 2. Actually, before the frequency pull-in device of FIG. 1 according to the present invention operated, the oscillation frequencies of the signal light 1 and the local oscillation light 3 were about 300H2 apart, but by operating the frequency pull-in device of FIG. The frequency of the oscillation light 3 has changed, and the beat signals of the signal light 1 and the local oscillation light 3 can now be obtained within the band of the optical receiver 5. As a result, a correct demodulated output was obtained from the demodulating circuit 6.

In this way, according to this embodiment, even if the oscillation frequency of the local oscillation light source 2 is considerably deviated from the frequency of the reference signal light 1 in the receiving section, the signal light 1 and the local oscillation light 3 are
It was possible to automatically sweep the oscillation frequency of the local oscillation light source 2 and pull in the frequency until the beat frequency of the local oscillation light source 2 was within the lock-in range of the electrical frequency stabilization loop.

FIG. 3 is a block diagram for explaining a second embodiment of the present invention. In this embodiment, an external resonator type LD was used as the local oscillation light source 2, and the frequency of the local oscillation light source 2 was swept by rotating a grating constituting the external resonator using a signal from the frequency sweep circuit 8. In addition, a signal 13 to the light source control section 14 and the sweep M control section 15 is transmitted.
was extracted as follows. First, comparator 11
and the outputs of the sawtooth wave generating circuit 9 are added by an adder 20.

The output of this adder 20 is passed through a clip circuit 21 to take out the output of the portion above a certain clip level.

The signal 13 is obtained by inputting this output to the differential amplifier 23 and comparing it with the reference signal 22. FIG. 4 is a diagram showing the waveforms of each part at that time.

The rest of the structure is the same as that of the first embodiment, and the same reference numerals as in FIG. 1 are used.

In this example, an external resonator type LD is used as the light source, so the frequency range that can be swept is as large as approximately 1000 GHz, and the oscillation frequency of the signal light 1 and local oscillation light 3 is 100 GHz.
Even when the distance is above, it was possible to draw the beat signal into the band of the optical receiver 5 as in the first embodiment.

It should be noted that the present invention may include various modifications other than the above-described embodiments. For example, instead of the sawtooth wave generation circuit 9, a triangular wave generation circuit or a sine wave generation circuit may be used. Further, the sweep control section 15 may perform a switch operation to stop the sweep when the signal 13 becomes below a certain amplitude level. Further, the light source control unit 14 can also control the local oscillation light source 2 by applying a constant level bias change to the local oscillation light source 2 only in response to the polarity of the signal 13.

It is also possible to feed back the signal from the light source control section 14 to the ambient temperature of the local oscillation light source 2. Furthermore, by adding an electrical frequency offset loop to the above embodiment, the beat frequency can be stabilized with high precision.

〔Effect of the invention〕

As explained above, according to the present invention, even when the oscillation frequency of the oscillation light source deviates considerably from the frequency of the reference signal light, the oscillation light source oscillation frequency is automatically swept. Therefore, the optical frequency pull-in device of the present invention is suitable for pulling into the lock-in range of a frequency difference stabilization loop in a receiving section using optical heterogain detection, for example, and is suitable for automatically starting up the system and restoring a momentary system interruption. effective.

[Brief explanation of drawings]

Fig. 1 is a block diagram for explaining the first embodiment of the present invention, Fig. 2 is a diagram showing waveforms of various parts when the oscillation frequency of local oscillation light is swept, and Fig. 3 is a block diagram for explaining the first embodiment of the present invention. FIG. 4 is a block diagram for explaining the second embodiment. FIG. 4 is a diagram showing waveforms of various parts in the second embodiment. 1... Signal light 2... Local oscillation light source 3... Local oscillation light 4... Optical multiplexer 5... Optical receiver 6... Demodulation circuit 7... Error signal 8... Frequency sweep circuit 9...Sawtooth wave generation circuit 10...Power monitor section 11...Comparator 12...Multiplier 13...Signal 14...Light source control section 15...Sweep control section 20 ... Adder 21 ... Clip circuit 22 ... Reference signal 23 ... Differential amplifier Patent attorney Yoshiyuki Iwasa Figure 1 Figure 2 Figure 3

Claims (2)

[Claims] (1) An optical frequency pulling device that pulls the frequency of a light source into a predetermined frequency range with respect to a reference signal light, which includes a frequency sweep section that sweeps the frequency of the light source and monitors the signal power appearing within the signal band. a power monitor section; a detection means for detecting a frequency shift of the light source with respect to the signal light based on a sweep signal of the frequency sweep section and an output signal from the power monitor section; and information about the frequency shift obtained by the detection means. and a light source control unit that corrects the frequency shift based on a signal containing the frequency difference. (2) In the optical frequency pull-in device according to claim 1, the sweep width of the frequency sweep section is controlled by a sweep control section based on a signal corresponding to the frequency shift. Optical frequency pull-in device.