JPH0378335A - Stabilizing circuit for optical fsk frequency deviation - Google Patents

Abstract

PURPOSE:To stabilize frequency deviation by controlling the frequency deviation of respective FSK signal light beams in a frequency range corresponding to the half cycle of a transmissivity cycle in a periodical filter. CONSTITUTION:A selection means 24 extracts one FSK signal light beam from signal light beams (FSK signal light beams) 151-15n with a frequency corresponding to an injected current which is optical frequency-multiplexed. The periodical filter 25 and control means 261 and 26n make the frequency whose transmissivity is to a maximum value or a minimum value to coincide with the central frequency of the FSK signal light beams 151-15n. Then, an output signal 17 corresponding to the frequency deviated amount of the FSK signal light beams 151-15n of the transmissivity cycle is taken out, and the output signal 17 is fed back to corresponding light sources 221-22n through a switching means 28. Thus, the deviation of the frequency of FSK signal light beams can be stabilized.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention is an optical FSK frequency shifter that stabilizes the frequency shift of FSK signal light obtained by direct modulation of each laser diode in an optical frequency multiplexing transmission system. Regarding stabilization circuits.

[Prior Art] FIG. 9 is a diagram showing a schematic configuration of an optical FSK modulation device that directly modulates signal light output from a laser diode.

In the figure, a laser diode 91 is configured to output a signal light (FSK signal light) with a frequency corresponding to an injected current, and an adder 97 adds a bias current 93 and a digital signal 95, which is a transmission signal. By using the injection current, FSK signal light 99 whose frequency changes according to the digital signal 95 is output.

However, in response to changes in the temperature of the board on which the laser diode 91 is mounted and other laser diode driving conditions,
Since the frequency shift of FSK signal light fluctuates, a frequency shift stabilization circuit for stabilizing the frequency shift has been proposed (Japanese Patent Application No. 192337/1983).

This stabilization method uses a periodic filter whose transmittance changes periodically according to the frequency of the FSK signal light (hereinafter, the frequency interval at which the transmittance changes periodically is referred to as the "transmittance period").
, and extracts a signal detected according to the deviation from the transmittance period of the periodic filter, which is set corresponding to the frequency shift of the FSK signal light. That is, by utilizing the fact that the periodic filter can detect fluctuations in the frequency shift of the FSK signal light, the detection signal is used to apply feedback to the FSK signal light source, and the amplitude control of the input digital signal or multi-electrode laser diode is performed. This configuration stabilizes the frequency shift of the FSK signal light by controlling the frequency modulation efficiency.

Note that a configuration is also shown in which the FSK signal light is converted into an electrical signal and the frequency shift of the FSK signal light is stabilized at the transmittance period using a periodic filter at the electrical signal level.

[Problem to be solved by the invention]

By the way, the frequency shift stabilization circuit that has already been proposed is used in an optical FSK modulator that uses a single laser diode as the FSK signal light source, and is used in optical frequency multiplexing transmission using multiple FSK signal light sources. The optical FSK frequency shift stabilization circuit used in the system is not shown.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical FSK frequency shift stabilization circuit that can individually apply feedback control to a plurality of FSK signal light sources and suppress fluctuations in frequency shift of a plurality of FSK signal lights.

[Means to solve the problem]

FIG. 1 is a block diagram showing the principle configuration of the system of the present invention.

In the figure, a plurality of light sources emit a plurality of FSK signal lights whose frequencies are shifted according to a transmission signal and have different center frequencies, and an FSK signal light whose center frequency is within a predetermined frequency range from the plurality of FSK signal lights. a selection means for selectively extracting signal light; a periodic filter to which the selected FSK signal light is input and changes its transmittance periodically at predetermined frequency intervals; Frequency at which the rate is the maximum value or minimum value and the FSK
a control means for matching the center frequency of the signal light and detecting a shift in the frequency shift amount of the FSK signal light with respect to a half cycle of the frequency interval; and an output of the control means in conjunction with a selection operation of the selection means. and switching means for sending a signal to the injection current control section of the corresponding light source.

[For production]

The selection means extracts one FSK signal light from a plurality of optical frequency multiplexed FSK signal lights.

The periodic filter and the control means match the center frequency of the FSK signal light to the frequency at which the transmittance takes a maximum value or a minimum value, and calculates the FS for a half period of the transmittance period.
It is possible to stabilize the frequency deviation of the FSK signal light by extracting an output signal corresponding to the deviation in the frequency deviation of the K signal light and feeding the output signal back to the corresponding light source via the switching means. can.

That is, the transmittance of the periodic filter is controlled so as to have a maximum value or a minimum value at the center frequency of the FSK signal light,
By using a periodic filter and control means that stabilize the frequency shift of the FSK signal light to a half period of its transmittance period, by simply adding selection means and switching means,
Multiple FS with one periodic filter and control means
It becomes possible to stabilize each frequency shift of the K signal light.

In addition, when each center frequency interval of the FSK signal light to be optically frequency multiplexed is an integral multiple of the periodic filter's transmittance period, the frequency characteristics of the periodic filter's transmittance for each FSK signal light are as follows. They are the same, and the injection current control sections of each light source can also be configured in the same way.

In addition, if each center frequency interval of the FSK signal light to be optically frequency multiplexed is a half-integer multiple of the transmittance period of the periodic filter (an integer multiple of the half period), the periodic filter for each FSK signal light is The frequency characteristics of the transmittance of are sequentially reversed,
Since the sign of the output signal of the control means is reversed, it is necessary to sequentially set the control characteristics of the injection current control section of each light source to reverse characteristics accordingly.

〔Example〕

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

FIG. 2 is a block diagram showing the configuration of the first embodiment of the optical FSK frequency shift stabilizing circuit of the present invention.

Note that this embodiment shows a configuration using a waveguide type Matsuhatsu Ender interferometer as a periodic filter.

The transmittance period of this waveguide type Matsuhatsu Enda interferometer is 4
(GHz) and each FSK is optically frequency multiplexed.
The center frequency interval of the signal light is set to 10 by a predetermined control means.
(GHz), each FSK
The frequency deviation of the signal light is half the transmittance period 2 (
GHz).

In the figure, N digital signals 1 are multiplexed and transmitted.
1. ~11N are connected to adders 21. to 11N via variable attenuators 20+ to 2ON. ~21. N bias currents 13
+ to 13N respectively to form N laser diodes 22. Injected to ~22N. Each laser diode 2
N FSK signal lights 151 emitted from 21 to 22N
158 are optically frequency multiplexed into the FSK signal light 15 via the optical multiplexer 23.

From this optical frequency multiplexed FSK signal light 15, one wavelength of the FSK signal light 15i (1≦M≦N) is selected via an optical frequency selection switch 24, and the FSK signal light 15i (1≦M≦N) is sent to a waveguide type Matsuhatsu interferometer 25. It is incident. The light emitted from the two output ports (a) and (b) of the waveguide type Matsuhatsu Ender interferometer 25 is transmitted to a photodiode 26.b connected in series. ．． 2
The light is received in 6□ respectively. Photodiode 26. ．．
The control signal 17 taken out as a differential signal of each photodiode output from the connection point 26□ is input to the optical path length control section 27 of the waveguide type Matsuhatsu Ender interferometer 25 and the changeover switch 28 that performs 1-to-N switching. . Each output of the changeover switch 28 is connected to a variable attenuator 20. ~2
Connected to the ON attenuation amount control terminal. The control circuit 29 is
The selection operation of the optical frequency selection switch 24 and the switching operation of the changeover switch 28 are controlled in a corresponding manner.

Note that if the maximum or minimum point of the transmittance of the waveguide type Matsuhatsu Ender interferometer 25 does not match the center frequency of the FSK signal light 15.4, an alternating current component is generated in the control signal 17. Therefore, this control signal 17 is sent to the optical path length control section 27 of the waveguide type Matsuhatsu Ender interferometer 25,
By controlling the AC component to zero using the optical path length control unit 27, the FSK is adjusted to the frequency at which the transmittance of the waveguide type Matsuhatsu Ender interferometer 25 reaches its maximum or minimum point.
The center frequencies of the signal lights 15M can be matched.

FIG. 3 is a diagram showing the frequency characteristics of the transmittance of the waveguide type Matsuhatsu Ender interferometer 25.

In the figure, the horizontal axis shows the frequency of incident light, and the vertical axis shows the transmittance in percent. In addition, the solid line and the dashed-dotted line are
The frequency dependence of the light emitted from the two output ports a and b is shown, and the phases differ by 180'' from each other.

Here, the center frequency of the FSK signal light is determined by the periodic filter (
The transmittance of the waveguide type Matsuhatsu Enda interferometer 25) is maximum (
When the frequency deviation is set to half the transmittance period, the frequencies fll and f at which the transmittance becomes 50% are controlled as indicated by the large solid line arrows in Fig. 3.
Shifts by 1□. In addition, when the frequency shift of the FSK signal light fluctuates and becomes longer than half the transmittance period (thin solid line arrow) or becomes shorter (dotted line arrow), the FSK signal light changes to the frequency fil, f2□ or the frequency f
'll, shift at f32.

FIG. 4 is a diagram showing, on the time axis, the detection outputs of each photodiode 265, 26□ corresponding to the frequency shift and fluctuation of the FSK signal light, and the control signal 17 obtained by taking the difference thereof.

FIG. 4(a) is a diagram showing the state of frequency shift of FSK signal light.

FIG. 4 Q: 1) and (C) show that FSK signal light shifted at frequencies fil and f12 is transmitted to each photodiode 26 . ．． 26□ shows the detection output i1, i, and FIG. 4(d) shows the control signal 17 which is the difference (1m!b).
shows.

In other words, there is no fluctuation in the frequency shift of the FSK signal light,
Predetermined deviation amount (frequency deviation is half the transmittance period (2G)
Iz)), the level (α) of the light emitted from the two output ports a and b of the waveguide type Matsuhatsu interferometer 25 is the same, and the control signal 17 is not output (output O).

Fig. 4(e) and (f) show that the FSK signal light shifted at frequencies rz+ and f22 is detected by the detection outputs (solid lines )
1 and 4 (the difference is the control signal 1
It is 7.

That is, if there is a fluctuation in the frequency shift of the FSK signal light that exceeds a predetermined shift amount, the level difference (α+Δi/2, α−Δi/2) causes a forward current +Δi, and the control signal 17 becomes a positive voltage DC signal.

4(c) and (i) show that FSK signal light shifted at frequencies fall and fzz is transmitted to each photodiode 26. ．． When the detection output of 26□ (cotton solid line) is shifted at frequencies f II, f, □ (large solid line)
FIG. 4(j) shows the control signal 17 which is the difference.

That is, if there is a fluctuation in the frequency shift of the FSK signal light that is less than a predetermined amount of shift, the reverse current -
Δj occurs, and the control signal 17 becomes a negative voltage DC signal.

In this way, the control signal 17 is obtained as a DC signal according to the fluctuation of the frequency shift of the FSK signal light, and by feeding back this signal as a control signal of the laser diode 21, the frequency shift of the FSK signal light is controlled. It can be performed.

In this embodiment, this control signal 17 is taken into the corresponding variable attenuator 20 via the changeover switch 2 linked to the optical frequency selection switch 24, and the digital signal 1
Although the frequency shift is controlled by controlling the amplitude of 1.4, the FSK signal light 15.4 is optically frequency multiplexed. -15° center frequency interval (10GHz) is
Transmittance period of waveguide type Matsuhatsu Ender interferometer 25 (4G
Each FSK signal light 15. ~15N
The deviation direction of the frequency deviation and the sign of the control signal 17 are determined for each FS.
It is inverted every K signal light. Therefore, variable attenuator 20
．． ~20. It is necessary to sequentially reverse the attenuation characteristics for the control signal 17 of l.

In addition, each FSK signal light 15. By matching the center frequency interval of ~15N to an integral multiple of the transmittance period of the waveguide-type Matsuhatsu Interferometer 25, it is possible to make all variable attenuators have exactly the same attenuation characteristics for the input signal. be.

Incidentally, in this embodiment, a configuration example using the waveguide-type Matsuhatsu-Enda interferometer 25 as the periodic filter has been shown, but it is also possible to use, for example, a Michelson interferometer or the like. Also, variable attenuators 20, -20. 1 controls the amplitudes of the digital signals 111 to LIH in accordance with the control signal 17, so other equipment having a similar function, such as an automatic gain controller, can also be used.

In addition, although a configuration is shown in which the optical frequency selection switch 24 is used to select the FSK signal light 15M to be subjected to stabilization control at the downstream stage of the optical multiplexer 23, one FSK signal light before multiplexing is used at the stage before the optical multiplexer 23. It is also possible to use a switch that selectively branches.

FIG. 5 is a block diagram showing the configuration of a second embodiment of the optical FSK frequency shift stabilizing circuit of the present invention.

Note that this embodiment is characterized by a configuration in which the periodic filter is processed at the electrical signal level. That is, two intermediate frequency delay detectors are used, and their transmittance period is 4 (GHz
), and the center frequency interval of each optical frequency multiplexed FSK signal light is stabilized at 10 (GHz), the frequency deviation of each FSK signal light is set to 2, which is half the transmittance period. (GHz).

In FIG. 5, variable attenuator 20. ~20,4, adder 21. 21N, laser diodes 221 to 22N, optical multiplexer 23, optical frequency selection switch 24, changeover switch 28, and control circuit 29 are the same as in the first embodiment.

In this embodiment, FSK signal light 15.4 (1≦M≦N) of one wavelength selected by the optical frequency selection switch 24 is input to the coupler 51 and combined with the local light 31 emitted from the laser diode 52. Wave. The combined light emitted from the coupler 51 is transmitted to each photodiode 53+, 53
Intermediate frequency signal 3 received by □ and obtained as each output
3I and 338 are respectively intermediate frequency delay detectors 54+
, 54g to the differential amplifier 55, and its output is sent as the control signal 17 to the corresponding variable attenuator 20M via the changeover switch 28 that performs 1:N switching.

Further, the intermediate frequency signal 33□ output from the photodiode 53□ is branched and input to the intermediate frequency discriminator 56, and the output frequency difference signal 35 is applied to the bias current 37 of the laser diode 52 for local light. This is an additive configuration.

Here, one intermediate frequency delay detector 54. By using a divider or other branching element with no phase shift, the transmittance can have the frequency characteristic shown by the solid line a in FIG. 3, and the other intermediate frequency delay detector 54□
By using a 180° hybrid branching element, the transmittance can have the frequency characteristic shown by the dashed line in FIG. 3. Therefore, each intermediate frequency delay detector 54. ．． By inputting the output of 54□ to the differential amplifier 55, the same control signal 17 as in the first embodiment can be obtained based on the principle explained using FIG.

Note that in this embodiment, each optical frequency multiplexed F
The center frequency interval of the SK signal light is determined by each intermediate frequency delay detector 54. ．． Since the transmittance period of 54□ is set to several times the transmittance period as in the first embodiment, the variable attenuator 20. ~2
It is necessary to sequentially reverse the attenuation characteristics for the control signal 17 of 08.

Further, the output of the intermediate frequency discriminator 56 includes the selected F.
The difference between the center frequency of the SK signal light 15 and the center frequency of the local light 31 is obtained. Therefore, by controlling the laser diode 52 using the frequency difference signal 35, the intermediate frequency signals 33, . The center frequencies of 33□ can be matched.

In this embodiment, a configuration is shown in which the frequency of the local light 31 is controlled by the frequency difference signal 35 extracted from the intermediate frequency signal 33□, but the other intermediate frequency signal 338
, or a signal extracted from both.

In this way, in this embodiment, the FSK signal light 15□ is converted into an intermediate frequency band electrical signal by heterodyne detection,
The frequency shift is detected by intermediate frequency delay detectors 541, 54□
It is characterized by a configuration that stabilizes the transmittance according to the transmittance period of the periodic filter configured by.

Note that in a configuration in which homodyne detection is used to convert the signal into an electrical signal in the baseband frequency band, the intermediate frequency delay detector 54
．． ．． By simply adjusting the delay time constant of 54□, the control signal 17 that stabilizes the frequency shift can be obtained using a similar circuit configuration.

FIG. 6 is a block diagram showing the configuration of a third embodiment of the optical FSK frequency shift stabilizing circuit of the present invention.

Note that this example shows a configuration using a waveguide-type Matsuhatsu-Zender interferometer as a periodic filter, as in the first example, and the transmittance period of the waveguide-type Matsuh-Zehnder interferometer,
The same applies to the center frequency interval of each optical frequency multiplexed FSK signal light and the value of the stabilized frequency shift of each FSK signal light.

Here, the feature of this embodiment is that each of the FSK signal lights 15+ to 15. The frequency deviation of digital signal 11.
It is not a configuration that is stabilized by ~11N amplitude control,
Using a multi-electrode laser diode for each laser diode,
This is done by controlling the bias current. That is, the present invention is characterized by a configuration in which the bias current of one of the multi-electrode laser diodes is controlled to change its frequency modulation efficiency and thereby stabilize the frequency shift.

That is, in FIG. 6, the multi-electrode laser diode 6
One electrode B of 1+ to 61N receives a digital signal 11.
．． ~11N and the bias current 41°~41N are added to the adder 6.
3. ~63M, and the other electrode A receives a control signal 17 and a bias current 43. ~43s and adder 65. ~65.4 is added and then injected. Therefore, the variable attenuator 2 of the first embodiment shown in FIG.
0°~20.4 is not required, the optical multiplexer 23, optical frequency selection switch 24, waveguide type Matsuhatsu Ender interferometer 25, photodiode 26. ．． 26□, the changeover switch 28, and the control circuit 29 have the same configuration.

Here, an example of controlling the frequency modulation efficiency in a distributed feedback (DFB) type laser diode having three electrodes will be described with reference to FIG.

FIG. 7(a) shows electrode A, electrode B,
This figure shows a distributed feedback laser diode having an electrode C, in which a transmission signal (digital signal) is added to the bias current and injected into the electrode C.

7, etc.), 40 (m
A) bias current of 10 (mA) is injected into electrode A.
Alternatively, the frequency shift of the light emitted when a bias current of 20 (mA) is injected is shown by a solid line and a dashed-dotted line, respectively. The horizontal axis is the transmitted signal (digital signal)
The frequency (MHz) of , the vertical axis shows the frequency deviation (one scale is 2 dB).

In this way, when the current amplitude of the transmitted signal is equal, the frequency deviation changes according to the change in the bias current injected into electrode A, so the frequency modulation efficiency of the laser diode changes. It can be said.

FIG. 8 is a diagram showing the change in the oscillation wavelength when the injection current value of the electrode A closest to the light emitting end of this three-electrode distributed feedback laser diode is changed.

As shown in the figure, even if the current injected into electrode A changes from 10 (mA) to 20 (mA), the change in the oscillation wavelength is small.

Regarding changes in frequency modulation efficiency and oscillation wavelength according to changes in bias current in multi-electrode distributed feedback laser diodes, for example, see the literature "Yoshikun
I and G, MOTOSU Gl, “Multiel
ectrodeDistributed Feedba
ck La5er for Pure Frequen
cyModulation and Chirping
5uppressed A+oplitutedMod
ulation, J., Lightwave tec.
hnol, Vol. LT-5+Sou 4, 1987. p
p, 516-522 J.

In this way, in this embodiment, by adding the control signal 17 to the bias current 43.4 and injecting it into the electrode A of the multi-electrode laser diode 61M, the FS
The frequency shift of the K signal light 15.4 can be controlled (stabilized).

Note that in this embodiment as well, each optical frequency multiplexed F
SK signal light 15. The center frequency interval of ~15N is set to be several times as large as the transmittance period of the waveguide type Matsuhatsu interferometer 25, as in the first embodiment. Therefore, each adder 65. ~65° is the control characteristic of the frequency shift with respect to the control signal 17, that is, the multi-electrode laser diode 61
．． It is necessary to sequentially reverse the change characteristics of the bias current injected into the electrode A of ~61N. In addition, each FSK signal light 15. By matching the center frequency spacing of ˜15N to an integral multiple of the transmittance period of the waveguide-type Matsuzender interferometer 25, it is possible to use an adder that gives the same bias current change to the control signal 17.

Furthermore, 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 output light (FSK signal The center frequency of light) also changes at the same time. Therefore, in this case, it is necessary to make the optical path length control section 27 of the waveguide type Matsuhatsu-Enda interferometer 25 follow the change in the center frequency at high speed, and a complicated control system and demodulation system are required.

Note that in a multi-electrode laser diode, as described above, by appropriately controlling the bias current injected into each electrode, only the frequency deviation can be changed without changing the center frequency of the FSK signal light.

By the way, the second embodiment is characterized by using a periodic filter (intermediate frequency delay detector 541.54□) that processes at the electrical signal level in contrast to the configuration of the first embodiment. In the configuration of the third embodiment, the same configuration as the second embodiment is possible for the periodic filter.

〔Effect of the invention〕

As described above, the present invention makes the center frequency interval of a plurality of FSK signal lights coincide with an integral multiple or a multiple of the transmittance period of a periodic filter, and corresponds to a half period of the transmittance period of the periodic filter. By controlling the frequency shift of each FSK signal light within the frequency range that
It is possible to stabilize the frequency shift of each signal light using one periodic filter and control means.

That is, a frequency shift stabilizing circuit for a plurality of FSK signal lights can be realized with a simple configuration, and a highly reliable optical frequency multiplexing transmission system can be easily installed horizontally.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the principle configuration of the present invention. FIG. 2 is a block diagram showing the configuration of a first embodiment of the present invention. FIG. 3 is a diagram showing the frequency characteristics of transmittance of a waveguide type Matsuhatsu Enda interferometer. FIG. 4 is a diagram illustrating the principle of detecting fluctuations in frequency shift of FSK signal light. FIG. 5 is a block diagram showing the configuration of a second embodiment of the present invention. FIG. 6 is a block diagram showing the configuration of a third embodiment of the present invention. FIG. 7 is a diagram illustrating an example of controlling the frequency modulation efficiency of a three-electrode distributed feedback laser diode. FIG. 8 is a diagram showing changes in the oscillation wavelength of a three-electrode distributed feedback laser diode. FIG. 9 is a diagram showing a schematic configuration of an optical FSK modulation device. 11... Digital signal, 13... Bias current,
15...FSK signal light, 17...control signal, 20...
... Variable attenuator, 21... Adder, 22... Laser diode, 23... Optical multiplexer, 24... Optical frequency selection switch, 25... Waveguide type Matsuhatsu Ender interferometer, 26... ... Photodiode, 27 ... Optical path length control section, 28 ... Changeover switch, 29 ... Control circuit,
31...Local light, 33...Intermediate frequency signal, 3
5... Frequency difference signal, 37... Bias current, 4
1.43...Bias current, 51...Coupler, 5
2...Laser diode, 53...Photodiode, 54...Intermediate frequency delay detector, 55...Differential amplifier, 56...Intermediate frequency discriminator, 6E...Multi-electrode laser diode , 63.65...adder, 91.
...Laser diode, 93...Bias current, 95
...Digital signal, 97...Adder, 99...
FSK signal light. Figure 1 Figure

Claims (1)

[Claims] (1) A plurality of light sources that emit a plurality of FSK signal lights whose frequencies are shifted according to a transmission signal and whose center frequencies are different; and an FSK whose center frequency is within a predetermined frequency range from the plurality of FSK signal lights. a selection means for selectively extracting signal light; a periodic filter to which the selected FSK signal light is input and whose transmittance is changed periodically at predetermined frequency intervals; a control means for matching a frequency at which the rate is a maximum value or a minimum value with a center frequency of the FSK signal light, and detecting a deviation in the frequency shift amount of the FSK signal light with respect to a half cycle of the frequency interval; 1. An optical FSK frequency shift stabilizing circuit comprising: switching means for transmitting an output signal of the control means to an injection current control section of a corresponding light source in conjunction with a selection operation of the control means.