JPH0241043A - Optical fsk optical frequency shift stabilizing circuit - Google Patents

Abstract

PURPOSE:To make the optical frequency shift stable in the optical FSK modulation system by controlling a laser diode so that the frequency shift of the optical FSK signal is made coincident with the frequency of an electric or optical periodic filter. CONSTITUTION:An FSK signal light 21 obtained from a laser diode 15 is divided into two by a coupler 23. The one is extracted as an output signal light 25 and the other is subject to FM-AM conversion by a Mach Zender interferrometer 27 being an optical periodic filter. Then the two output lights from the interferrometer 27 are received by photodiodes 291, 292 and their difference signal is divided into two. The one is given to an optical path length control section 31 and the other is given to a mixer 35 through a DC interrupter 33. Then a mixer 35 multiplies a signal through the DC interrupter 33 with a digital electric signal 11, and the result is outputted, which is given to a programmable attenuator 37, where the amplitude of a digital electric signal 11 is controlled.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an optical FSK optical frequency shift stabilization circuit, and in particular to an optical FSK optical frequency shift stabilization circuit obtained by direct modulation of a laser diode.
Optical FS that stabilizes the optical frequency shift of the modulated signal
This invention relates to a K-optical frequency shift stabilization circuit.

[Conventional technology]

An example of the configuration of a conventional optical FSK modulation system using direct modulation of a laser diode is shown in FIG. In the figure, by superimposing a digital electrical signal 11 and a bias current 13 and applying it to a laser diode 15, an FSK signal whose optical frequency changes digitally is generated as an optical FSK modulation signal.
A signal light 17 can be obtained.

(Problem to be Solved by the Invention) By the way, in the conventional optical FSK modulation method described above, since feedback is not applied to the laser diode 15, the FSK signal light 17 may change depending on laser diode driving conditions such as changes in substrate temperature. There was a problem that the frequency deviation of

The present invention was created in view of these points, and provides an optical FSK optical frequency deviation stabilization circuit that suppresses fluctuations in optical frequency deviation in FSK signal light output by a laser diode. is intended to provide.

[Means to solve the problem]

FIG. 1 is a block diagram of an optical FSK optical frequency shift stabilizing circuit according to the present invention.

In the figure, the laser diode has an optical frequency shift state that changes depending on the injected current, and an optical FSK modulation signal corresponding to the optical frequency shift state is obtained.

The periodic filter variably controls the optical frequency shift of the optical FSK modulated signal.

The control means controls the frequency period of the periodic filter and the optical frequency shift of the optical FSK modulation signal to have a predetermined relationship.

Therefore, the optical frequency shift of the optical FSK modulated signal is maintained at a desired state.

(Function) In the present invention, the laser diode is directly modulated and outputs an optical FSK modulation signal. The optical FSK modulated signal is converted into an electrical signal after passing through a periodic filter that variably controls its optical frequency shift. Based on the converted electrical signal, the frequency shift of the optical FSK modulation signal and the frequency period of the periodic filter are controlled so as to have a predetermined relationship (eg, match).

This stabilizes the frequency shift of the optical FSK modulated signal, and suppresses fluctuations in the optical frequency shift of the optical FSK modulated signal caused by the laser diode.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 2 shows the configuration of an optical FSK optical frequency shift stabilizing circuit in a first embodiment of the present invention. FIG. 5 is a block diagram of a second embodiment of the present invention. FIG. 6 shows the configuration of an optical FSK optical frequency shift stabilizing circuit in a third embodiment of the present invention. FIG. 7 is a block diagram of a fourth embodiment of the present invention.

(2) Correspondence between the embodiment and FIG. 1 Here, the correspondence between the embodiment of the present invention and FIG. 1 is shown.

The laser diode corresponds to a 15° multi-electrode laser diode 61.

The periodic filter corresponds to the Matsuhatsu Ender interferometer 27° intermediate frequency delay detector 59.

The control means includes mixer 35. Programmable attenuator 37. Intermediate frequency discriminator 51. Laser diode 57.
It corresponds to the adder 67.

Examples of the present invention will be described below assuming that the correspondence relationship as described above exists.

(2) First Embodiment The first embodiment shown in FIG. 2 uses a Matsuhatsu Enda interferometer as a periodic optical filter. In this embodiment, frequency shift is stabilized by adjusting the amplitude of a digital electrical signal so that it matches the frequency period of an optical periodic filter.

In FIG. 2, a digital electric signal 11 and a bias current 13 are superimposed and injected into a laser diode 15, and the resulting FSK signal light 21 is transferred to a coupler 2.
It is divided into two by 3.

One of the two halves of the light is taken out as the output signal light 25, and the other is FM-AM converted by the Matsuhatsu Ender interferometer 27, which is a periodic optical filter, and then output from the two output ports of the Matsu Hatsu Ender interferometer 27. The output light is received by two photodiodes 29 and 29□, and the difference signal therebetween is divided into two. One is input to the optical path length controller 31, and the other is inputted to the mixer 35 along with the digital electric signal 11 through the DC breaker 33, and the mixer 35 multiplies the signal passing through the DC breaker 33 and the digital electric signal 11. The calculated signal is output. This output signal is input to the programmable attenuator 37 to control the amplitude of the digital electrical signal 11.

FIG. 3 shows the relationship between the optical frequency dependence of the transmittance of the Matsuhatsu Ender interferometer 27 and the frequency arrangement of the FSK signal light beam 2 in the first embodiment described above. The transmittance of the Matsuhatsu Ender interferometer 27 at the center frequency of the FSK signal light 21 is 50%.
By adjusting the Matsuhatsu Ender interferometer 27 so that The solid line shows the case where the frequency shift of the FSK signal light 21 matches the frequency period of the Matsuhatsu Ender interferometer 27, and shows that the output after FM-AM conversion becomes DC. The broken line and the dotted line are cases in which the frequency shift of the FSK signal light 210 is longer and shorter than the frequency period of the Matsuhatsu Ender interferometer 27, respectively, and in the waveform after FM-AM conversion, the amplitude is the frequency shift of the FSK signal light 21 and the frequency shift of the This shows that the pulse signal is proportional to the difference from the frequency period of the interferometer 27, and the phase is opposite when the frequency deviation of the FSK signal light 210 is longer than or shorter than the frequency period of the Matsuhatsu Ender interferometer 27. .

FIG. 4 shows the mixer 35 in the first embodiment shown in FIG.
The relationship between the input signal to the mixer 35 and the output signal from the mixer 35 is shown, and the broken line indicates the case where the frequency shift of the FSK signal light 21 is longer than the frequency period of the Matsuhatsu Ender interferometer 27.
The dotted lines each indicate the case where the frequency shift of the FSK signal light 21 is shorter than the frequency period of the Matsuhatsu Ender interferometer 27.

Since the DC component is removed by the DC breaker 33, when the frequency shift of the FSK signal light 21 matches the frequency period of the Matsuhatsu Ender interferometer 27, the output from the mixer 35 is 0.
(2), and the pulse signal obtained when the frequency shift of the FSK signal light 210 is different from the frequency period of the Matsuhatsu Ender interferometer 27 will also have equal values above and below centering on O2.

When this pulse signal and the digital electric signal 11 are synchronized, the output signal of the mixer 35 obtained as the product of these becomes a direct current, but the output voltage value is the amplitude of the input pulse, that is, the frequency of the FSK signal light 21. The polarity is proportional to the difference between the deviation and the frequency period of the Matsuhatsu Enda interferometer 27, and its polarity is The opposite is true when the frequency period is shorter than the frequency period of the interferometer 27.

By inputting this signal to the programmable attenuator 37 whose attenuation increases in proportion to the input voltage, the amplitude of the digital electrical signal 11 can be controlled and the optical frequency shift of the FSK signal light 21 can be controlled. The role of the programmable attenuator 37 is to control the amplitude of the digital electrical signal 11 according to the output voltage from the mixer 35, so it can be used with other devices with similar functions, such as an automatic gain controller. Get results.

If the FM signal amplitude after FM-AM conversion is sufficiently small compared to the optical intensity, or if the modulation speed is sufficiently fast, the optical path length controller 3
Consider the case where 1 cannot detect the modulated signal. in this case,
Of the difference signals between the two photodiodes 29 and 29□, the signal that controls the optical path length controller 31 is a difference signal between the optical intensities output from the two ports of the Matsuhatsu Ender interferometer 27, and this signal It becomes 0 when the transmittance of No. 27 is 50%. Therefore, the optical path length controller 31 controls the optical path length so that the absolute value of the optical intensity difference signal is minimized, so that the transmittance of the Matsuhatsu Ender interferometer 27 at the center frequency of the FSK signal beam 21 is 50.
Adjust so that it is %.

In the above-described embodiment, an example using the Matsuhatsu Ender interferometer 27 was shown, but it can also be realized using an interference needle other than the Matsuhatsu Ender interferometer 27.

FIG. 5 shows the structure of a second embodiment of the present invention. In this embodiment, an intermediate frequency delay detector is used as the electrical periodic filter, and frequency shift is stabilized by adjusting the amplitude of the digital electrical signal so that it matches the frequency period of the electrical periodic filter.

In FIG. 5, a digital electric signal 11 and a bias current 13 are superimposed and injected into a laser diode 15 to obtain FSK signal light 21. This is divided into two by coupler 23. One is taken out as the output signal light 41, and the other is received by the photodiode 47 together with the local light 45 through the coupler 43, and the intermediate frequency signal 49 generated by the interference between the FSK signal light 21 and the local light 45 is generated.
get. This signal is divided into two parts, one of which is input to an intermediate frequency discriminator 51 to generate a frequency difference signal 5 representing the difference between the center frequency of the FSK signal light 21 and the center frequency of the local light 45.
3 is superimposed on another bias current 55 and injected into another laser diode 57. intermediate frequency signal 4
The other half of 9 is subjected to FM-AM conversion by an intermediate frequency delay detector 59, which is an electric periodic filter, and then inputted to a mixer 35 together with the digital electric signal 11 through a DC breaker 33, and the multiplication is the calculated output. The signal is supplied to a programmable attenuator 37.

Frequency difference signal 53 obtained from intermediate frequency discriminator 51
By injecting this into the laser diode 57, the laser diode 57 is controlled so that the difference between the optical frequency of the local light 45 generated by the laser diode 57 and the center frequency of the FSK signal light 21 becomes constant.

The intermediate frequency delay detector 59 has the functions shown in FIGS. 3 and 4 in the electrical stage, similar to the Matsuhatsu Ender interferometer 27 in the first embodiment shown in FIG. Deviation and intermediate frequency delay detector 5
A pulse waveform having an amplitude and a phase corresponding to the difference from the frequency period of 9 is output. By passing the pulse signal obtained in this way through a programmable attenuator 37, the first
The frequency shift of the FSK signal light 21 is stabilized to the frequency period of the intermediate frequency delay detector 59 using the same principle as in the embodiment.

This second embodiment uses heterodyne detection to convert an optical signal into an electrical signal in an intermediate frequency band.
The feature lies in that the frequency shift of the signal light 21 is stabilized to the frequency period of the electrical periodic filter. Another feature is that the frequency shift of the FSK signal light 21 is stabilized to the frequency period of the electrical periodic filter by converting the optical signal into an electrical signal in the baseband frequency band using homodyne detection.

(2) Third Embodiment FIG. 6 shows the configuration of a third embodiment of the present invention. In this embodiment, the frequency shift is stabilized by adjusting the bias current so as to match the frequency period of the optical periodic filter.

In FIG. 6, a digital electrical signal 11 is connected to a bias current 13. FSK signal light 63 is obtained by injecting it into one electrode of the multi-electrode laser diode 6e and injecting another bias current 13□ into the other electrode of the multi-electrode laser diode 61. .

This FSK signal light 63 is divided into two by a coupler 23. One is taken out as the output signal light 65, and the other is FM-AM converted by the Matsuhatsu Ender interferometer 27, and then the output lights from the two output boats are received by two photodiodes 291 and 29□, and their difference signal is divided into two. do. One side is input to the optical path length controller 31 of the Matsuhatsu Ender interferometer 27, and the other side is input to the mixer 35 together with the digital electric signal 11 after passing through the DC breaker 33. The bias current is superimposed with the bias current 13□ and injected into the multi-electrode laser diode 61 through 67.

The electrode that injects the bias current 13□ adjusts the bias current flowing through each electrode of the multi-electrode laser diode 61, so that the oscillation center frequency of the multi-electrode laser diode 61 does not change due to changes in the injection current, but the frequency modulation efficiency changes. Set it to change. As described above in the first embodiment, the output of the mixer 35 is a DC signal proportional to the difference between the optical frequency shift of the FSK signal light 63 and the frequency period of the Matsuhatsu Ender interferometer 27.
Multi-electrode laser diode 6 can be created by overlapping □.
The frequency modulation efficiency of 1 is controlled to stabilize the frequency shift of the signal light 63 in the FS. At this time, the transmittance of the Matsuhatsu Ender interferometer 27 at the center frequency of the FSK signal light 63 is 50%.
The Matsuhatsu Ender interferometer 27 is adjusted eight times by the optical path length controller 31 so that .

This embodiment is characterized in that the frequency of the FSK signal light 63 is shifted by controlling the bias current. It is possible to control the frequency shift of the FSK signal light by controlling the bias current using a single electrode laser diode, but with a single electrode laser diode, the center frequency of the FSK signal light changes due to changes in the bias current. It is impossible to suppress. The configurations of the control system and demodulation system become complicated, as the center frequency of the Matsuhatsu Enda interferometer must be changed rapidly in accordance with the change in the center frequency of the FSK signal light.

(2) Fourth Embodiment FIG. 7 shows a fourth embodiment of the present invention. In this embodiment, the frequency shift is stabilized by adjusting the bias current so as to match the frequency period of the electric periodic filter.

In FIG. 7, a digital electrical signal 11 and a bias current 13. A multi-electrode laser diode 61 is formed by overlapping the
FSK signal light 71 is obtained by injecting another bias current 13□ into one electrode of the multi-electrode laser diode 61.

This FSK signal light 71 is divided into two by a coupler 23. One is taken out as the output signal light 73, and the other is taken out from the coupler 43.
The light is received together with the local light 45 by the photodiode 29, and the intermediate frequency signal 49 obtained by interference between the FSK signal light 71 and the local light 45 is divided into two. One is input to the intermediate frequency discriminator 51 to obtain a frequency difference signal 53 indicating the difference between the center frequency of the FSK signal light 71 and the oscillation frequency of the local light 45, which is superimposed with the bias current 55 and injected into the laser diode 57. . The other one is subjected to AM-FM conversion by an intermediate frequency delay detector 59 and then passed through a DC breaker 33 and sent to a mixer 35 together with the digital electric signal 11.
Enter. Here, the multiplied output of the mixer 35 is superimposed on the bias current 13□ through an adder 67, and is injected into the multi-electrode laser diode 61.

Frequency difference signal 53 obtained from intermediate frequency discriminator 51
By injecting this into the laser diode 57, the laser diode 57 is controlled so that the difference between the optical frequency of the local light 45 generated by the laser diode 57 and the center frequency of the FSK signal light 21 becomes constant.

By adjusting the bias current injected into each electrode of the multi-electrode laser diode 61, changes in the injection current do not affect the oscillation center frequency of the multi-electrode laser diode 61. The frequency modulation efficiency is set to vary.

The output of the mixer 35 is FSK as in the second embodiment described above.
A DC signal is generated that is proportional to the difference between the frequency shift of the signal light 71 and the frequency period of the intermediate frequency delay detector 59. By superimposing this on the bias current 13□, the frequency modulation efficiency of the multi-electrode laser diode 61 can be increased. control, F
The frequency shift of the SK signal light 71 is stabilized.

This embodiment is characterized in that the frequency shift of the FSK signal light 71 is stabilized to the frequency period of the electrical periodic filter by controlling the bias current injected into the multi-electrode laser diode 61. Also, like the second embodiment described above, this second embodiment also converts an optical signal into an electrical signal in an intermediate frequency band by heterodyne detection, or converts an optical signal into an electrical signal in the baseband frequency band by homodyne detection. By converting it into an electrical signal, FSK signal light 2
It is characterized in that the frequency deviation of 1 is stabilized to the frequency period of the electrical periodic filter.

By the way, when using a single electrode laser diode,
It is impossible to suppress changes in the center frequency of FSK signal light due to changes in bias current. As a result, the configurations of the control system and demodulation system become complicated, such as the need to change the center frequency of the Matsuhatsu Enda interferometer at high speed in accordance with changes in the center frequency of the FSK signal.

■0 Modified aspects of the invention Furthermore, in ``correspondence between the embodiment examples and FIG. 1'',
Although the correspondence between the present invention and the embodiments has been explained, the present invention is not limited thereto (although those skilled in the art can easily imagine that there are various modifications).

〔Effect of the invention〕

As described above, according to the present invention, by controlling the laser diode so that the frequency shift of the optical FSK signal matches the frequency period of the electrical or optical periodic filter, the optical frequency can be adjusted in the optical FSK modulation method. It becomes possible to stabilize the deviation.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an optical FSK optical frequency shift stabilization circuit according to the present invention, Fig. 2 is a block diagram of an optical FSK optical frequency shift stabilization circuit according to the first embodiment of the present invention, and Fig. 3 is a periodic diagram. 4 is a conceptual diagram showing mixing in a mixer, and FIG. 5 is a configuration diagram of an optical FSK optical frequency shift stabilization circuit according to a second embodiment of the present invention. , FIG. 6 is a block diagram of an optical FSK optical frequency shift stabilizing circuit according to a third embodiment of the present invention, and FIG. 7 is a block diagram of an optical FSK optical frequency shift stabilizing circuit according to a fourth embodiment of the present invention. , FIG. 8 is a block diagram showing the basic configuration of conventional optical FSK modulation method 2. In the figure, 11 is a digital electric signal, 13.131, 132.55 are bias currents, and 15.5
7 is a laser diode, 17.21 and 63.71 are FSK signal lights, 23.43 is a coupler, 25.41 and 65.73 are output signal lights, 27 is a Mannha-Zehnder interferometer, 29.29. ．． 29□, 47 is a photodiode, 31 is an optical path length controller, 33 is a DC breaker, 35 is a mixer, 37 is a programmable attenuator, 45 is a local light, 49 is an intermediate frequency signal, 51 is an intermediate frequency discriminator, 53 is a frequency 59 is an intermediate frequency delay detector, 61 is a multi-electrode laser diode, and 67 is an adder. Fig. 1 Fig. Fig. Fig. Fig. Fig. 771 Placement I (output value of the members + r)

Claims (1)

[Claims] (1) A laser diode whose optical frequency shift state changes according to the injected current and an optical FSK modulation signal corresponding to the optical frequency shift state is obtained, and the optical frequency shift of the optical FSK modulation signal is variably controlled. a periodic filter that controls the optical frequency of the optical FSK modulated signal; and a control means that controls the frequency period of the periodic filter and the optical frequency shift of the optical FSK modulated signal to have a predetermined relationship. An optical FS characterized by being configured such that the deviation is maintained at a desired state.
K optical frequency shift stabilization circuit.