JPH03222381A - Light frequency stabilized device - Google Patents

Abstract

PURPOSE:To make electrical spectrums coincident in density and to obtain a light frequency stabilized device high in stability of frequency by a method wherein an original signal and a modulated signal are electrically filtered, and a relative error signal is detected. CONSTITUTION:Lasers 1 and 2 contained in a feedback systems 3 and 4 respectively are directly modulated by an FSK modulating signal, the optical signals converted in intensity fluctuation through a Fabry-Perot resonator 10 are detected by a photodetector 11, and the detected signals are added to the either input of mixers 5 and 6 after they pass through a low-pass filter 12. The signals obtained by making the FSK modulating signals which modulate the lasers 1 and 2 pass through a low-pass filters 7 and 8 possessed of nearly the same frequency characteristic with the filter 12 are added to the other input of the mixers 5 and 6. The mixers 5 and 6 have a function as a correlator and generate error signals correspondent to correlative frequency deviation from the resonant frequency of a resonator 10 of the light frequency of the lasers 1 and 2. When the error signals concerned are fed back to the drive current or the temperature of the lasers 1 and 2, the optical frequency of the lasers 1 and 2 is locked to the resonant frequency of the resonator 10.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention provides a coherent optical frequency multiplexing transmission system that transmits multiple optical frequencies through a single optical fiber, which has a low error rate, low cost, and high reliability. The present invention relates to an optical frequency stabilization method suitable for realizing the device and its device.

[Conventional technology]

In a coherent optical frequency multiplexing transmission system that transmits signals at multiple optical frequencies through a single optical fiber, as the channel spacing becomes narrower, the signal error rate increases due to crosstalk between channels.

In order to prevent this increase in error rate, it is essential to stabilize the channel spacing. Regarding channel spacing control in conventional optical frequency division multiplexing transmission systems, Japanese Patent Application Laid-Open No. 64-60033
There is an example in the publication. A specific example is shown in FIG. 2
The modulated optical signals from the lasers 101 and 102 are frequency multiplexed by the optical coupler 107, filtered by the Fabry-Perot resonator 108, and then transmitted to the photodetector 1.
Detected in 09. The optical frequency of each signal is controlled by feedback systems 103 and 104 so as to match the comb-shaped and equally spaced resonant frequencies of the Fabry-Perot resonator 108. In this method, frequency mixer 105 is used to obtain optical frequency control signals for lasers 101 and 102.
, 106, a correlation is taken between the modulation component of the optical frequency multiplexed signal that has passed through the Fabry-Perot resonator 108 and the original signal used to modulate each laser. As a result,
Information on each channel can be separated and identified from the modulated components of two channels output by one photodetector 109.

[Problem to be solved by the invention]

In the above conventional technology, it is difficult to increase the stability of the channel spacing due to noise caused by the mismatch in the electric flux vector density between the original signal that becomes the 1° reference correlation signal and the correlated signal obtained by photoelectric conversion from the FSK signal. was difficult.

2. Since a binary code is used for the original signal, it is not possible to determine the direction of fluctuation in the total light intensity when the optical frequency fluctuates, making it difficult to detect the locked or unlocked state of the light source. was there.

A first object of the present invention is to provide an optical frequency stabilizing device that achieves high frequency stability by matching the electric charge vector densities of a reference correlation signal and a correlated signal.

A second object of the present invention is to provide an optical frequency stabilizing device that can reliably discriminate between a locked state and an unlocked state by using a ternary code as a modulation signal.

[Means to solve the problem]

The second object of the present invention is achieved by correlating the original signal and the signal obtained by photoelectrically converting the FSX signal after matching the electric scum vector densities using electric signal filters. The second object of the present invention is to use a ternary code that is a DC balanced code, for example, AM I (Al
ternate MarkInversion) code,
This is achieved by using as a modulation signal for each light source.

[Effect]

FIG. 3 shows the principle by which the average optical frequency of the laser is locked to the resonant frequency of the Fabry-Perot resonator when the frequency of the laser is modulated with a binary code. Figure 3 (a) shows the signal obtained from the photodetector when the average optical frequency of the laser is lower than the resonant frequency of the Fabry-Perot cavity, and (b) shows the signal obtained from the photodetector when the average optical frequency of the laser matches the resonant frequency of the Fabry-Perot cavity. (c) is the signal obtained from the photodetector when the frequency of the light source is shifted to the other side of the resonant frequency; (d) is the original signal of modulation. It shows. Figures 3(d) and (a).

If we take the correlation signals with (b) and (c), they are both positive.

O9 becomes negative. Therefore, when this correlation signal is fed back to the laser beam as an error signal, the average optical frequency of the light source is locked to the resonant frequency of the Fabry-Perot resonator. The original signal (d) and the signals (a) and (b) obtained from the photodetector when taking the correlation in Fig. 3.

If there is a mismatch in the electric scum vector density in (c), noise will be generated in the correlation error signal, and the frequency stability will not be high.

[Example] An example of the present invention is shown in FIG. This embodiment includes two independent feedback systems 3 and 4, an optical coupler 9, a Fabry-Perot resonator 10, a photodetector 11, and a low-pass filter 1.
It is composed of 2.

Each feedback system is a laser and low-pass filter.

It consists of a frequency mixer. The figure shows the case where the number of transmission channels is 2, but if the number of channels is any number n, n channels can be stabilized simultaneously by preparing n light sources and n independent feedback systems. It becomes possible to do so.

First, the configuration and operating principle of this embodiment will be explained. In FIG. 1, 1 and 2 are lasers, each of which has a feedback system 3.
, 4. Each laser is directly modulated by the FSX modulation signal. This frequency-modulated optical signal is multiplexed by an optical coupler 9 and then transmitted through a Fabry-Perot resonator 10. At this time, the optical frequency of each laser corresponds to two different resonances of the Fabry-Perot cavity. The optical signal converted into intensity fluctuation by the Fabry-Perot resonator is detected by the photodetector 11. The signal obtained here is transmitted through a low-pass filter 12 and then applied to two feedback systems 3 and 4. This signal is applied to one input of a frequency mixer 5.6 in each feedback system. To the other input of the frequency mixer 5.6, a signal obtained by transmitting the FSX modulation signal for modulating each laser through a low-pass filter 7.8 having a frequency characteristic approximately equal to 12 is applied.

Each frequency mixer functions as a correlator and generates an error signal corresponding to the relative frequency variation of each laser's optical frequency from the resonant frequency of the Fabry-Perot cavity.

By feeding back this error signal to the drive current or temperature of each laser, the optical frequency of each laser is locked to the resonant frequency of the Fabry-Perot resonator.

FIG. 4 is a diagram showing the effect of the low-pass filter in the embodiment of FIG. 1. This is a feedback loop that locks a single channel laser FSK modulated at 400 MHz to the resonant frequency of a Fabry-Perot resonator, and when the loop is opened, the optical frequency of the laser is changed and the error signal from the mixer is measured. FIG. 4 shows the results when a 200 MHz low-pass filter was used in the feedback loop and when it was not used. The horizontal axis of FIG. 4 is the relative frequency deviation of the laser light frequency from the resonance frequency of the Fabry-Perot resonator, and the vertical axis is the error signal from the frequency mixer. From the same figure, if we calculate the detection sensitivity of relative frequency fluctuations, that is, the error signal output per I MHz deviation of the laser light frequency from the resonance frequency of the Fabry-Perot cavity, it is 0 when there is no low-pass filter.
．． 65 mV/MHz, while with the low-pass filter it was 1.17 mV/MHz. This shows that higher frequency stability can be obtained with a low-pass filter, confirming the effectiveness of the filter. The reason why the low-pass filter increases the detection sensitivity of optical frequency fluctuations and also increases the frequency stability is that the filter smoothes the rise and fall of the signal, and a strong correlation appears in the frequency mixer.

FIG. 5 shows the relative variation of one laser frequency from the resonance frequency of the Fabry-Perot resonator using an error signal in the embodiment of FIG.

The broken line represents the resonant frequency of the Fabry-Perot resonator, which serves as a frequency reference. The frequency fluctuation, which was about 300 MHz when not controlled, was reduced to about 5 MHz by the method shown in FIG.

This shows that the embodiment shown in FIG. 1 is very effective in stabilizing the optical frequency of the laser.

FIG. 6 shows a second embodiment of the invention. In this embodiment, there are three feedback systems 204, 205,
206, optical coupler 213, Fabry-Perot resonator 214,
It is composed of a photodetector 215 and a bandpass filter 216. Each feedback system includes a laser, frequency mixer,
It consists of a bandpass filter. This embodiment stabilizes each channel spacing and at the same time stabilizes their absolute optical frequencies. A laser 203 whose absolute optical frequency is stabilized to stabilize the absolute optical frequency of each channel, and a Fabry-Perot resonator resonance frequency control device 21 which can control the temperature or optical path length of the Fabry-Perot resonator.
Prepare 7. In the feedback system 206, the first
The correlation error signal obtained in the same manner as in the illustrated embodiment is fed back to the Fabry-Perot resonator resonance frequency control device 217. Due to this feedback, the resonance frequency of the Fabry-Perot cavity 213 is equal to the optical frequency of the laser 203.
The frequency above and below is determined to be an integral multiple of the resonance frequency interval. Further, the optical frequencies of the lasers 201 and 202 are locked to different resonance frequencies of the Fabry-Perot resonator 213, similar to the embodiment shown in FIG. Therefore, the absolute optical frequency of each laser is determined to be an integral multiple of the resonant frequency interval of the Fabry-Perot resonator 213 from the absolute optical frequency of the laser 203, and the frequency interval is also stabilized.

FIG. 7 shows that in the embodiment of FIG. 1 or FIG.
This figure shows the positional relationship between the resonance curve of the Fabry-Perot cavity and the optical spectrum of the laser on the optical frequency axis when the laser is FSX-modulated using the M I code. A.M.I.
When a laser is FSK modulated using a code, side bands of optical frequencies corresponding to positive marks and negative marks appear at equal intervals on the left and right sides of the optical frequency corresponding to the space. 8th
The figure shows a signal detected by a photodetector after the modulated optical signal of the laser passes through the Fabry-Perot resonator. Figure 8a
is the time series of the AMI code modulated by the laser, and the figure is the time series of the signal detected by the photodetector when the optical frequency corresponding to the space matches the resonant frequency of the Fabry-Perot resonator.

In addition, C in Fig. 8 is a time series of the signal detected by the photodetector when the optical frequency corresponding to a positive mark matches the resonant frequency of the Fabry-Perot resonator, and d in Fig. 8 is a time series of the signal detected by the optical detector corresponding to a negative mark. This is a time series of signals detected when the frequency matches the resonant frequency of the Fabry-Perot resonator. Figure 8a
When the correlation between the signal 1 and the signals s, c, and d in the same figure is taken, O2 becomes positive and negative, respectively. In the feedback system of the embodiment shown in FIG. 1 or 6, the optical frequency of the laser or the resonant frequency of the Fabry-Perot resonator is controlled so that the correlation becomes O. Therefore, the laser is FSK by AMI code.
When modulated and feedback as shown in FIG. 1 or 6 is performed, the optical frequency corresponding to the space of the transmission signal coincides with the resonance peak of the Fabry-Perot resonator as shown in FIG. 7. That is, when the AMI code is used to lock to the resonant frequency of the Fabry-Perot resonator, the intensity of light transmitted through the Fabry-Perot resonator becomes maximum when all channels are locked. When either channel goes out of lock, the light intensity decreases. Therefore, by monitoring the DC level of the output of the photodetector, it is possible to detect that the lock has been released.

Although the above method uses an optical signal transmitted through a Fabry-Perot resonator, the channel spacing can be stabilized in a similar manner by using an optical signal reflected from the Fabry-Perot resonator.

In addition, although the laser is FSK modulated in the embodiments shown in Figs. 1 and 6, it can also be applied to ASK modulation by adding a DC level monitoring function of the photodetector, thereby setting a limit on the direction in which the phase of the light source changes. Therefore, it can also be applied to PSK modulation.

Furthermore, by using the optical frequency stabilizing devices of the embodiments shown in FIGS. 1 and 6 in the transmitting device of an optical frequency multiplexing transmission system, a system with stable channel spacing and a system with stable absolute optical frequency can be realized. Can be built. This is the 9th
As shown in the figure. As shown in the figure, an optical frequency multiplexing transmission bag using an optical frequency stabilizing device 13 [1301 is an optical fiber 30
3 to the optical frequency multiplex receiver 302.

In such an optical frequency division multiplexing transmission system with stable channel spacing, the channel spacing can be narrowed to the minimum level that does not cause crosstalk, so that multiple channels can be frequency multiplexed and transmitted with high density. Further, an optical frequency division multiplexing transmission system in which both the channel spacing and the absolute frequency are stabilized can also form a network with other optical frequency division multiplexing transmission systems.

〔Effect of the invention〕

According to the present invention, an optical frequency stabilization method and device having the following effects are obtained.

1. Conventionally, a photodetector, an amplifier, and a two-frequency mixer having a frequency band equivalent to the transmission signal band were required, but in the present invention, a photodetector, an amplifier, and two frequency mixers with a frequency band that is narrower than the transmission signal band are required.
Since it can be realized with a frequency mixer, costs can be reduced.

2. By using a band-pass filter or a low-pass filter, the detection sensitivity of optical frequency fluctuations becomes higher than when no band-pass filter or low-pass filter is used, and frequency stability can be improved.

3. The frequency band required for frequency mixing to take the correlation between the original signal and the modulated signal can be made narrower than before.
Adjustment can be simplified.

4. The reliability of the device is improved because it is possible to determine whether the optical frequency of the light source is locked or not.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing an embodiment of the conventional technology, and Fig. 3 shows that the average frequency of the light source is locked to the resonant frequency of the Fabry-Perot resonator. A diagram showing the principle, Figure 4 is a diagram showing that the detection sensitivity of the frequency fluctuation of the light source has been improved by adding a frequency filter to the embodiment shown in Figure 1, and Figure 5 is an example of the embodiment shown in Figure 1. Fig. 6 shows another embodiment of the present invention, and Fig. 7 shows variations in the frequency of each channel when frequency stabilization is not performed and when it is performed. A diagram schematically showing how the center frequency of a modulated optical signal is locked to the resonance peak of a Fabry-Perot cavity,
Figure 8 is a diagram showing the principle that the center frequency of the optical signal modulated by the AMI code is locked to the resonance peak of the Fabry-Perot resonator, and Figure 9 is a diagram using the embodiment of Figure Y or Figure 6. It is an optical frequency multiplexing transmission system. 1.2...Light source, 3,4...Feedback system, 5
゜6...Frequency mixer, 7,8...Low pass filter, 9...Optical coupler, 10...Fabry-Perot resonator, 11...Photodetector, 12...Low pass filter,
13... Optical frequency stabilizer, 101, 102...
Light source, 103° 104... Feedback system, 105
．． 106... Frequency mixer, 107... Optical coupler,
108... Fabry-Perot resonator, 109... Photodetector, 201, 202... Light source, 203... Absolute frequency stabilization light source, 204°205.206... Feedback system, 207°208.209.・Frequency mixer,
210, 211° 212... Band pass filter, 2
DESCRIPTION OF SYMBOLS 13... Optical coupler, 214... Fabry-Perot resonator, 215... Photodetector, 216... Bandpass filter, 217... Fabry-Perot resonant resonance frequency control device, 301... Optical frequency of 13 Optical frequency multiplex transmitter using a stabilizing device, 302. Optical frequency multiplex receiver, 303. 6 Figure No. 6

Claims (1)

[Scope of Claims] 1. In an optical transmitter that stabilizes the optical frequency of the light source by a correlation error signal between an original signal modulating a light source and a signal modulated by the original signal, the original signal An optical frequency stabilizing device, which detects a correlated error signal after electrically filtering the modulated signal and the modulated signal. 2. In the optical frequency stabilization device according to claim 1, after frequency-multiplexing optical signals emitted from a plurality of transmitting light sources using an optical coupler, an optical filter, a photodetector, an electric filter, a correlator, and a plurality of independently controllable feedback systems to stabilize the optical frequency of the transmitting light source. 3. An optical frequency stabilizing device according to any one of claims 1 to 2, characterized in that a ternary code is used as a signal used for modulation. 4. In the optical frequency stabilization device according to any one of claims 1 to 3, one optical frequency is selected and transmitted from a plurality of optical frequencies corresponding to the original signal as an optical transmission signal. An optical frequency stabilizing device characterized by: 5. An optical frequency multiplexing transmitter characterized by using the optical frequency stabilizing device according to any one of claims 1 to 4. 6. An optical frequency multiplex transmission system characterized by using the optical frequency multiplex transmitter according to claim 5.