JPH08331100A - Optical communication system for optical frequency multiplex communication, optical transmitter and optical frequency control method - Google Patents

Abstract

PURPOSE: To conduct high density multiplexing efficiently on an optical frequency axis by providing an output of an optical frequency for optical frequency control in addition to a received optical frequency from plural optical transmitters and matching the optical frequency with a similar optical frequency for optical frequency control outputted from other optical transmitter. CONSTITUTION: The optical transmitter has a light source section 2-3 keeping at last three optical frequency lights at a prescribed interval on an optical transmission path and providing an output of them with control. Furthermore, an optical frequency control system 2-1 provided to the optical transmitter unifies its control such that a communication signal frequency is higher than an optical frequency of maximum optical frequency among three optical frequencies and two adjacent optical frequencies are matched in the optical frequency area for multiplex communication with the maximum optical frequency or a communication signal frequency is lower than an optical frequency of minimum optical frequency among three optical frequencies and two adjacent optical frequencies are matched in the optical frequency area for multiplex communication with the minimum optical frequency. Thus, interference at output start is avoided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system and an optical communication method for performing optical frequency multiplex communication.

[0002]

2. Description of the Related Art Optical frequency multiplexing communication can have a large number of independent channels in one transmission line. Since multiplexing on the time axis such as frame synchronization is not necessary, it is not necessary to match the transmission speed of each channel, and it is also suitable for multimedia communication where network flexibility is required.

As an example of the optical frequency multiplex communication system,
There is a system in which each terminal has a set of optical transmitters and optical receivers whose optical frequencies are variable. The transmitting terminal adjusts the optical frequency of the variable optical frequency light source of the optical transmitter to the optical frequency not used for communication (“channel” of optical frequency multiplex communication). On the other hand, the receiving terminal station matches the center optical frequency (hereinafter also referred to as the optical frequency of the optical filter) of the transmission spectrum of the optical bandpass filter (hereinafter referred to as the optical filter) of the optical receiver with the optical frequency of the reception. To receive. The optical frequency range available in the system is determined by the optical frequency variable range of the optical transmitter and the optical receiver. The optical frequency interval of channels (hereinafter referred to as channel interval) is determined by the width of the transmission spectrum of the optical filter of the optical receiver.

An optical frequency variable semiconductor laser (hereinafter, the semiconductor laser is referred to as an LD) can be used as the optical frequency variable light source. Research is underway to expand the variable range of the optical frequency, but the ones at the practical level at the present time are DBR (distributed Bragg refrector) type of multi-electrode and DFB (distributor).
Buted feedback) type, the optical frequency variable width is several nm. As an example, a publication of the Institute of Electronics, Information and Communication Engineers OQE89-
116, "Three-electrode long cavity λ / 4-shift MQW-DFB laser". On the other hand, as the variable optical frequency filter, a Fabry-Perot resonator type filter can be used. Optical frequency variable width is several tens nm, spectrum width is 0.1 nm
Something is at a practical level. As an example,
Publication ECOC '90 -605, “A field-worthy, high-perform
ance, tunable fiber Fabry-Perot filter ”.

[0005]

By narrowing the channel interval, many channels can be taken with the same optical frequency variable width. In order to keep the interval below the fluctuation range due to the drift of the optical frequency of the LD and the optical filter, the influence of the drift must be suppressed by control. In order to suppress the influence of drift, it is necessary to stabilize the optical frequency absolutely or relatively. However, the absolute standard of optical frequency is not simple. Further, in a communication system in which an optical transmitter is located at a remote place like a LAN, it is difficult to set an absolute reference in the communication system and stabilize it relative to it.

[0006]

In order to solve the above problems, the present invention provides an optical communication system, an optical transmitter and an optical frequency control method which do not require absolute stabilization of the optical frequency. Further, in the present invention, it is not necessary to set a reference optical frequency. The basic principle of the present invention is as follows.

In the optical transmitter, the optical frequency of the output light of the own optical transmitter is moved to the end of the optical frequency region where the optical frequency multiplex communication is performed.

[0008] In order to avoid interference caused by the optical frequency of the output light of the self-optical transmitter approaching the optical frequency of other light beyond a predetermined interval, the interval between adjacent optical frequencies of other light is specified. The control is performed so that the optical frequency interval is maintained.

As described above, on the optical frequency axis, the transmission optical frequencies of the respective stations are sequentially multiplexed from the end of the optical frequency region in the transmission start order. In the present invention, in order to keep the interval between the optical frequencies of other adjacent lights at a predetermined optical frequency interval, the light of at least three optical frequencies is output from the optical transmitter at a predetermined interval. Then, for example, when the optical frequency of the output light of the self-optical transmitter is approached to the one with the higher optical frequency, the light of the maximum optical frequency of the light of the three optical frequencies output by the self-optical transmitter is output. The frequency of light having a frequency higher than the adjacent frequency and the frequency of light adjacent to each other are matched with the optical frequency of light having the maximum optical frequency of the three optical frequencies output from the own optical transmitter. By maintaining this coincidence state, light having an intermediate optical frequency among lights having the three optical frequencies is prevented from interfering with other light. At this time, the light of the minimum optical frequency among the lights of the three optical frequencies is the maximum optical frequency of the lights of the three optical frequencies output by the other optical transmitter that starts transmission later. It is used to match the frequency of light.

In order to realize the above control, the present invention uses
The following optical communication system and optical frequency control method are provided. Further provided is an optical transmitter in an optical communication system.

1. First, the optical communication system is an optical communication system that performs optical frequency multiplex communication on an optical transmission line, and an optical transmitter in the optical communication system transmits light of at least three optical frequencies to each other. Maintain a predetermined optical frequency interval,
And a light source unit capable of controlling and outputting an optical frequency, an optical frequency of a light having a maximum optical frequency among lights of the three optical frequencies, and an optical frequency of the maximum light in an optical frequency region in which the optical frequency multiplex communication is performed. The optical frequency is higher than the optical frequency and the optical frequency of the optical frequency of the adjacent optical frequency is controlled to match, or the minimum optical frequency of the three optical frequencies is controlled. The optical frequency of the light of the frequency,
Control means for controlling to match two optical frequencies, which have an optical frequency smaller than that of the light of the minimum optical frequency in the optical frequency region in which the optical frequency multiplex communication is performed, and the optical frequency of light of an adjacent optical frequency. There is provided an optical communication system comprising: As a result, the optical frequency can be absolutely stabilized, the interval between adjacent optical frequencies can be maintained at a predetermined interval without using the reference optical frequency, and high-density optical frequency multiplexing can be realized.

2. The optical transmitter may have a means for modulating light of at least an intermediate optical frequency among lights of the three optical frequencies.

3. The optical transmitter has an optical bandpass filter capable of controlling a transmission center optical frequency, and the control means controls the light source unit and the optical bandpass filter to provide an optical bandpass filter. In the optical communication system according to any one of 1 and 2, the pass filter controls the light of the two optical frequencies to be matched so as to pass through the optical band pass filter at the same time. It is possible to realize simplification of the configuration and ease of control by using it. Further, the optical bandpass filter only needs to have one for each optical transmitter.

4. In all the optical transmitters in the optical communication system, the control means controls the optical frequency of the light of the maximum optical frequency among the lights of the three optical frequencies and the optical frequency multiplex communication. In the optical frequency range to be performed, the optical frequency is larger than the optical frequency of the maximum optical frequency, and the optical frequency of the light of the adjacent optical frequency is controlled to match the two optical frequencies, or the three optical frequencies are controlled. Among the light of the frequency, the light of the light of the minimum optical frequency and the light of the adjacent optical frequency that is smaller than the light of the minimum optical frequency in the optical frequency region for performing the optical frequency multiplex communication. In the optical communication system according to any one of 1 to 3, which is unified to control to match two optical frequencies with the optical frequency Since the wave number is multiplexed, when an optical transmitter that has a transmission request starts output on the optical transmission line, by starting output at the optical frequency on the opposite side that is being multiplexed, interference at the start of output Can be avoided.

5. The optical communication system according to any one of 1 to 4, wherein a part of the light output from one of the optical transmitters to the optical transmission line is also input to the optical transmitter. In the optical communication system, each optical transmitter does not require a special configuration for inputting the self-emitted light to the detecting means in the self-optical transmitter.

6. On the other hand, the optical transmitter further comprises means for directly inputting a part of the output from the light source section to the detecting means in the self-optical transmitter, so that the self-luminous light once outputted on the optical transmission line is provided. It is not necessary to have a configuration in which light can be input to the self-light transmitter. Further, it is possible to directly input the light of a level sufficiently higher than the light input to the own optical transmitter via the optical transmission line of the optical communication system to the detection means. Further, by this, even when the optical frequency output by the optical transmitter is finely modulated to generate an error signal and feedback control is performed, it is necessary to make the modulation frequency of the fine modulation different in each optical transmitter in the optical communication system. Disappears.

7. Further, as a means for obtaining the three optical frequencies, there is a configuration for performing optical modulation with a ternary FSK signal. Specifically, there is a configuration in which a light source such as an LD is directly modulated with a ternary signal, or light from the light source is passed through a modulator and modulated with a ternary signal. In this configuration, the optical transmitter need only have one light source.

8. The optical frequency control method is an optical frequency control method in an optical transmitter used in an optical communication system for performing optical frequency multiplex communication on an optical transmission line, and at least three light source units from the light source section in the optical transmitter are used. Lights having an optical frequency are output at a predetermined interval from each other, and among the lights having the three optical frequencies, the optical frequency of the light having the maximum optical frequency and the optical frequency range in which the optical frequency multiplex communication is performed are performed. The light frequency is larger than the light of the maximum light frequency, and is controlled to match the two light frequencies with the light frequencies of the light of the adjacent light frequencies, or among the light of the three light frequencies, An optical frequency of light having a minimum optical frequency and an optical frequency of light having an optical frequency smaller than that of the optical frequency of the minimum optical frequency in the optical frequency region in which the optical frequency multiplex communication is performed and having an adjacent optical frequency. Provided is an optical frequency control method characterized by controlling two optical frequencies to match each other. As a result, the optical frequency can be absolutely stabilized, the interval between adjacent optical frequencies can be maintained at a predetermined interval without using the reference optical frequency, and high-density optical frequency multiplexing can be realized.

9. In the optical frequency control method, the optical transmitter has an optical bandpass filter capable of controlling a transmission center optical frequency, and the optical source is controlled by controlling the light source unit and the optical bandpass filter. The two optical frequencies to be matched can be easily matched by controlling the bandpass filter so that the lights of the two optical frequencies to be matched simultaneously pass through the optical bandpass filter.

10. As a concrete method thereof, the control for causing the lights of the two optical frequencies to be matched to be in a state of being simultaneously transmitted through the optical bandpass filter is performed by the control of the two frequencies to be matched. The first step of matching any one of them with the transmission center optical frequency of the optical band pass filter, and the predetermined interval was maintained while maintaining the matched state in the first step. Three
There is an optical frequency control method having a second step of changing two frequencies while keeping the interval and controlling so that lights of the two frequencies to be matched are simultaneously transmitted through the optical bandpass filter.

11. In the optical frequency control method, the optical transmitter finely modulates at least the optical frequency of the three optical frequencies to be transmitted through the optical bandpass filter to perform feedback control. By generating an error signal and pulling in and stabilizing the optical frequency to the transmission wavelength of the optical bandpass filter, it is possible to easily pull in and stabilize.

12. At that time, in each optical transmitter in the optical communication system, by making the modulation frequency of the minute modulation different, even when the two optical frequencies to be matched are respectively slightly modulated, The control error can be reduced.

[0023]

【Example】

(Example 1) In order to solve the above-mentioned problems
The three optical frequencies output by one optical transmitter do not have to keep emitting the respective optical frequencies. That is, the optical frequency of the output light of one light source is set to three optical frequencies f at predetermined intervals.
By switching and outputting with s, fm, and fg, it is possible to obtain the same operation as that of providing three light sources in the light source unit and outputting with fs, fm, and fg, respectively. At this time, as described above, when controlling and changing the output optical frequency, for example, when shifting to the larger optical frequency,
The same optical frequency change is given to the three optical frequencies.

In this embodiment, as described above, the output light frequency of one light source is switched to obtain three light frequencies.
In connection with this, the optical FSK system is conventionally known as an optical modulation system. This method assigns binary transmission signals to different optical frequencies, that is, when the transmission signal is "0", the space frequency fs is transmitted, and when the transmission signal is "1", the optical frequency is the mark frequency fm, and the receiving side transmits it. In this method, at least one of fs and fm is received, and demodulation is performed according to the intensity change. Among them, in the direct FSK modulation, a laser or the like is used as a light source and an FSK modulation output can be obtained by superimposing a modulation current on a drive current, which is excellent in simplicity. As an example of the receiving method, one optical frequency is extracted and demodulated by an optical bandpass filter on the receiving side. Regarding the optical frequency multiplex communication system using optical FSK modulation, the publication Electronics Letters, Vol. 25, 23
Issue 1600-1602, "FDMA-FSK noncoherent star netwo
rkoperated at 600Mbit / s using two-electrode DFB l
An example is given in "asers and fiber optical filter demultiplexer".

In this embodiment, this optical FSK modulation is applied,
A semiconductor laser is directly modulated by using a ternary modulation current to obtain three optical frequency outputs. The above-mentioned publication OQE89-116 of the Institute of Electronics, Information and Communication Engineers, "Three-electrode long resonator λ / 4 shift MQW-D
When using an element with the configuration described in "FB laser", 3
Of the two electrodes, the optical frequency of the output light can be switched by superposing the modulation current on the injection electrode of the center electrode. At this time, by superimposing the ternary modulation current,
It can be switched at three optical frequencies. Further, the optical frequency of the output light can be swept by adjusting the injection currents of the electrodes on both sides (side electrodes) and the electrode on the center (center electrode). At this time, it is possible to sweep while maintaining the frequency interval of the three optical frequencies by the above-mentioned ternary modulation current at a predetermined interval. The sweep range is 1547.3 to 1549.0 nm, which is about 1.7 nm, and when converted to optical frequency, about 210 GHz can be swept. In the present embodiment, following the optical FSK method, the three optical frequencies are described as a mark frequency fm and a space frequency fs, and in the present invention, a guard frequency fg that is a third optical frequency by using the three optical frequencies.

The first embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a schematic diagram of the arrangement of optical frequencies according to the present invention. In an optical communication system to which the optical frequency control method of the present invention is applied, first, second, and third transmission optical nodes are set in order of earliest transmission start time among transmissions performed at a certain time point, and The positional relationship between the optical frequency of the transmitted light and the optical frequency of the optical filter is shown. In the figure, fmn (n = 1 to 3)
Is the mark light frequency, fsn (n = 1 to 3) is the space light frequency, fgn (n = 1 to 3) is the guard light frequency, and ffn (n = 1 to 3) is the light frequency of the optical filter.

FIG. 2 is a block diagram of an optical transmitter using the optical frequency control system of the present invention. As shown in the figure, the optical frequency control system 2-1, the guard signal adding circuit 2-2, the LD 2-3, the optical filter 2
-4, a light receiving element 2-5, LD drive circuits 2-6 and 7, an amplifier 2-8, and an optical filter drive circuit 2-9.

LD2-3 is a publication of the Institute of Electronics, Information and Communication Engineers OQE89
-116, "Three-electrode long cavity λ / 4 shift MQW-DFB laser"
The described elements can be used. The optical filters 2-4 are
Publication ECOC '90 -605, “A field-worthy, high-perform
ance, tunable fiber Fabry-Perot filter ”can be used. It operates as a voltage-controlled wavelength (optical frequency) tunable filter in the range of the longitudinal mode interval or less. In this embodiment, the full width at half maximum of the transmission spectrum is: 1 GHz, vertical mode interval: 100 GHz The guard signal adding circuit 2-2 converts the mark / space binary signal into a mark / space / guard ternary signal. It forms the core of the invention and its detailed configuration will be described later.

This optical transmitter is a part of an optical node that connects the terminal station and the transmission line. Transmit signal and tuning from terminal
ON / OFF signal is input. The transmission signal is a binary signal of mark and space. The guard signal addition circuit 2-2 converts this signal into a ternary signal and outputs it to the LD drive circuit 2-6. The LD drive circuit 2-6 receives a low frequency (several tens of kHz or less) modulation signal for tracking and a bias signal for optical frequency control from the optical frequency control system 2-1 from the guard signal adding circuit 2-2. Input the modulation signal (several MHz or more) for FSK modulation.
A bias signal for controlling the optical frequency is input to the LD drive circuit 2-7. The LD drive circuit 2-6 injects a current corresponding to the input voltage into the center electrode of the LD 2-3, and the LD drive circuit 2-7 injects a current corresponding to the input voltage into the side electrode. A low-frequency (several tens of kHz or less) modulation signal for tracking and a bias signal for optical frequency control are input from the optical frequency control system 2-1 to the optical filter control circuit 2-9. Optical filter control circuit 2-
9 applies a voltage corresponding to the input voltage to the optical filter 2-4.
The optical filter 2-4 transmits only the light corresponding to the optical frequency of the peak of the transmission spectrum among the lights incident from the transmission path. The light is converted into an electric signal by the light receiving element 2-5, amplified by the amplifier 2-8, input to the optical frequency control system 2-1, and LD2-
3 and the optical filter 2-4 are used to control the optical frequency.

FIG. 3 is a block diagram of the optical frequency control system. As shown in the figure, the ramp signal generator 3-1, 2, LD control circuit 3-
3, optical filter control circuit 3-4, adder 3-5,6,7,8, sine wave oscillator 3-9,10, multiplier 3-11,12, low-pass filter (hereinafter referred to as LPF) 3- 13,14,15,16, feedback control circuit 3-17,1
8, switch 3-19, 20, reference voltage source 3-21, 22, comparator 3-23,
Consists of 24.

The ramp signal generator 3-1 produces a ramp signal for sweeping the optical frequency of the LD 2-3. The control of the output ramp signal is controlled by two signals, a trigger signal and a hold signal. [Trigger signal: L, hold signal: L. (H is digital signal 1, L is digital signal 0)] initial value, [Trigger signal: H, hold signal: L] sweeps, [Trigger signal: H, hold signal: H] hold signal Holds the output when changes from L to H.

The ramp signal generator 3-2 produces a ramp signal for sweeping the optical frequency of the optical filter 2-4. The control of the output ramp signal is controlled by two signals, a trigger signal and a hold signal. [Trigger signal: L, hold signal: L] initial value, [Trigger signal: L
H, hold signal: L] sweeps from the output when the signal changes, and [trigger signal: H, hold signal: H] holds the output when the hold signal changes from L to H.

The LD control circuit 3-3 responds to the input signal LD2.
The LD drive circuits 2-6 and 7 are controlled so that the optical frequency of -3 changes. Similarly, the optical filter control circuit 3-4 controls the optical filter drive circuit 2-9 so that the optical frequency of the optical filter 2-4 changes according to the input signal.

The feedback control circuits 3-17 and 18 generate an operation signal from the error signal. Well-known PID (proportional integral d
ifferential) can be used. Switch 3-19, 20
Is an ON / OFF signal: H, connect the output to the input, ON / OFF signal: L
To set the output to 0. Comparator 3-23,24 is H for [-input terminal voltage] ≤ [+ input terminal voltage], [-input terminal voltage]>
Output L at [+ input terminal voltage].

The trigger signal of the ramp signal generator 3-1 is the tuning ON / OFF signal from the terminal station, and the hold signal is the output of the comparator 3-23. The output is added to the output of the switch 3-19 by the adder 3-5 and input to the LD control circuit 3-3. LD control circuit 3-3 has two outputs, one of which is LD
The other is input to the drive circuit 2-6 and the other to the LD drive circuit 2-7.
The output of the comparator 3-23 is input to the trigger signal of the ramp signal generator 3-2, and the output signal of the comparator 3-24 is input to the hold signal. The output is added to the output of the switch 3-20 by the adder 3-6 and input to the optical filter control circuit 3-4. The output of the filter control circuit 3-4 is input to the optical filter drive circuit 2-9.

The output signal of the amplifier 2-8 and the modulation signal from the sine wave oscillator 3-9 are input to the multiplier 3-11. The low frequency component of the output signal is extracted by LPF3-13, and the feedback control circuit 3-1
Type in 7. The operation signal generated by the feedback control circuit 3-17 is added to the modulation signal from the sine wave oscillator 3-9 by the adder 3-7 and input to the switch 3-19. The same applies to the multiplier 3-12, the sine wave oscillator 3-10, the LPF 3-13, the feedback control circuit 3-18, the adder 3-8, and the switch 3-20.

The LPFs 3-15 and 16 extract low frequency components of the output of the amplifier 2-8. The output of LPF3-15 is input to the + input terminal of comparator 3-23, and the output of reference voltage source 3-21 is input to the-input terminal. The output of LPF3-16 is input to the + input terminal of comparator 3-24, and the output of reference voltage source 3-22 is input to the-input terminal. Comparator 3-
23 and 3-24 output H when the input from the + terminal exceeds the input from the-terminal, and output L when the input does not exceed the input.

FIG. 4 is a block diagram of an example of an optical communication system using the optical transmitter of the present invention. It is a star-type network with n terminal stations. As shown in the figure, terminal stations 4-1-1 to n, optical nodes 4-2-1 to n, n × n star coupler 4-3, optical fiber 4
It consists of -4-1 to n and 4-5-1 to n. Also, optical node 4-2-1 ~
n is composed of an optical transmitter 4-6, an optical receiver 4-7, and an optical branching device 4-8.

The terminal stations 4-1-1 to 4-n are connected to the network by optical nodes 4-2-1 to 4-n, respectively. Each optical node 4-2-1 to n has a transmitting optical fiber 4-4-1 to n and a receiving optical fiber 4-5-1 to n.
Connect with n × n star coupler 4-3. The transmission light from the optical transmitter is sent to the n × n star coupler 4-3 through the transmission optical fibers 4-4-1 to 4-n. The n × n star coupler 4-3 evenly distributes the transmitted light to the respective receiving optical fibers 4-5-1 to 4-n and sends it to the respective optical nodes 4-2-1 to 4-n. Incident light from the receiving optical fibers 4-5-1 to 4-n is divided into two by the optical branching device 4-8 and input to the optical receiver 4-7 and the optical transmitter 4-6.

In this configuration, the amount of light incident on the optical filters of the optical receiver and the optical transmitter is almost equal in the output light of each optical transmitter.

FIG. 5 is an explanatory diagram of the operation of the optical frequency control system. The LD tuning operation performed by an optical node in the optical communication system prior to transmission is described in six stages. In the figure, fg, fm, and fs are the guard optical frequency of the LD of the optical transmitter of the optical node, the mark optical frequency, the space optical frequency, and ff is the optical frequency of the optical filter. fgo, fmo, fs
o is the guard optical frequency, mark optical frequency, and space optical frequency of the optical transmitter of the optical node that started transmission before the optical node started transmission. The arrow indicates that the sweep direction and the spectrum (LD indicates a line, and the optical filter indicates a peak) are thick, which indicates that fine modulation is applied for tracking.

"Operation 0" is before the start of tuning, "Operation 1" is "at the start of tuning (fs sweep)", "Operation 2" is "pulling fs into ff, sweeping ff", and "action 3". Is [sweep of ff], "Operation 4" is [pull of ff into fgo], "
The operation 5 "indicates a steady state of tuning (steady state at the time of transmission).

FIG. 6 is a schematic diagram of a guarded FSK signal. In the present embodiment, a 1-bit guard signal is added to the FSK signal of 4 bits to form a ternary signal. Specifically, the guard signal adding circuit 2-2 adds a guard signal for every 4 bits of the binary signal from the terminal station to form a ternary signal. The LD drive circuit 2-6 outputs currents corresponding to the guard signal, mark signal, and space signal. The optical frequency allocation is fg>fm> fs, and the interval is 2 GHz. As a result, the output light of LD2-3 becomes a ternary FSK signal as shown in FIG.

Hereinafter, the coincidence of optical frequencies (fs and ff, ff and fg
Method o), switching of tuning control, and overall tuning operation will be explained.

In the present invention, one optical filter (optical frequency
The signal from ff) is used to keep the optical frequency interval between adjacent channels constant. Therefore, the guard optical frequency is added to the conventional binary FSK signal of the mark and space, and the optical frequency of each channel is changed to the guard optical frequency fg, the mark optical frequency fm,
Arrange in order of space optical frequency fs. In the present invention, in a certain node, of the three optical frequencies (fg, fm, fs) output by the node, the optical frequency closer to the adjacent channel to be kept at a predetermined interval and the output of the adjacent channel are output. Of the optical frequencies, the optical frequency closer to the local node is matched. (However, in the present application, the two optical frequencies coincide with each other when the two optical frequencies are close to each other within a predetermined interval (in the present embodiment, the two optical frequencies are input to the optical filter). The state in which a transmission output that exceeds a predetermined threshold value is obtained by the above) is included.)
In this embodiment, fs is matched with fgo, which is the closest frequency of the optical frequencies of the adjacent channels, to keep the optical frequency interval with the adjacent channels at a predetermined interval.
For that, make fs match ff, keep that state,
Match fg of adjacent channels, that is, fgo. As a result, fs and fgo match. The optical frequency arrangement of each channel is as shown in FIG.

In the present embodiment, the two optical frequencies match (fs
And ff, ff and fgo), sweeping, and pulling in / stabilizing. That is, [matching fs and ff] means sweeping fs and
It means approaching to and further pulling in and stabilizing to ff,
[Match of ff and fgo] means sweeping the sweep of ff to bring it closer to fgo, and further pulling in and stabilizing fgo. The sweep will be described later as a general tuning operation, and the pull-in / stabilization will be described here.

[Induction / stabilization of fs to ff], [fgo of ff
Both the optical frequency of LD, which is a line spectrum, and the optical frequency of the peak of the transmission spectrum of an optical filter with a half-value width of 1 GHz are the targets. The transmitted light intensity of the optical filter 2-4 becomes maximum when the optical frequency of the LD and the peak optical frequency of the optical filter match, and becomes smaller as the distance increases. In order to perform the pull-in, it is necessary to detect and control the magnitude relationship between the optical frequency of the LD and the peak optical frequency of the optical filter (whether the high optical frequency side or the low optical frequency side). As a method of retracting / stabilizing, Japanese Patent Laid-Open No. Hei 1-17
The one described in the conventional example of 7518, "Wavelength filter automatic tuning control system" can be used. The outline is
The control amount (optical frequency) is modulated with a small amplitude, the modulation signal and the detection signal (the amount of transmitted light of the optical filter 2-4) are multiplied, and feedback control is performed using the low frequency component as an error signal.

[Including / stabilizing fs to ff] is performed by the adder 3-
7. Sine wave oscillator 3-9, multiplier 3-11, LPF3-13, feedback control circuit 3-17. The amplitude of the fs minute modulation is set in a range where the periodic fluctuation of the optical frequency due to the modulation does not affect the communication. On the other hand, the frequency of fs minute modulation is set to be sufficiently lower than the transmission signal band (> 1 MHz). In this embodiment, the amplitude is set to 50 MHz and the frequency is set to be different for each transmitter in the optical communication system within the range of 1 kHz to 10 kHz. This is to prevent an increase in the pull-in control error that occurs when tuning ends and enters the steady state (modulating fs also modulates fm and fg. Steady state (fs and fgo match) Fs and fgo for the transmitted light of the optical filter 2-4.
And the transmitted light output of the optical filter exceeds a predetermined threshold value. Fine modulation frequency of fs and fg
If the minute modulation frequency of o is the same, the fgo component contributes to the signal obtained by multiplication with the modulation signal, which increases the control error, which is not preferable). LPF3-13
The cutoff frequency of is set so as to sufficiently attenuate the modulation component of the output signal of the amplifier 2-8.

[Addition / stabilization of ff to fgo] is performed by the adder 3
-8, sine wave oscillator 3-10, multiplier 3-12, LPF 3-14, feedback control circuit 3-18. The amplitude of the minute modulation of ff is fs
It is about the same as that of 50MHz. The frequency is set to 20kHz (modulation of the optical filter does not affect the pull-in / stabilization of other optical transmitters in the optical communication system, so there is no need to change it for each optical transmitter). The cutoff frequency of the LPF3-14 is set so as to sufficiently attenuate the modulation component of the output signal of the amplifier 2-8.

In the tuning operation of this embodiment, first, fs is swept up to the vicinity of ff, then fs is pulled to ff and stabilized, and while keeping that state, or fs is pulled to ff and stabilized. Swapping ff to the vicinity of fgo while overlapping with the operation, then pulling ff into fgo and stabilizing it. Tuning starts with a tuning ON / OFF signal from the terminal station, and subsequent switching of operations (sweep, pull-in / stabilization) is performed by monitoring the amount of transmitted light of the optical filter 2-4.

The amount of transmitted light is monitored by extracting the low frequency component of the output signal of the amplifier 2-8 with an LPF and comparing it with a reference voltage by a comparator. The switching of control is performed in "
It is operation 2 "and" operation 4 ". Switching in" operation 2 "is performed by LPF3-15, reference voltage source 3-21, and comparator 3-23, and switching in" operation 4 "is performed by LPF3-16. , The reference voltage source 3-22 and the comparator 3-24. The switching point is set between the time when fs and fgo fall on the tail of the transmission spectrum of the optical filter 2-4 and the time when the peak is reached. In this example, fs, f
Switch when 50% of the go light amount is transmitted through the optical filter 2-4.

The setting of the output voltage of the reference voltage sources 3-21 and 22 for switching with the above light quantity will be described. fg, f
Each light quantity of m and fs corresponds to the number of bits of each optical frequency. In this embodiment, 1-bit fg is added to 4-bit data composed of fm and fs. Furthermore, the time average of fm and fs (on the order of several microseconds)
Are equal. As a result, the light quantity of fg: the light quantity of fm: the light quantity of fs = 1: 2: 2. Further, the light amount ratio of the two optical frequencies fs and fgo to be matched is determined by the system configuration. In this embodiment, as described in the description of FIG. 4, the light quantity of the light source of the own station and the light quantity of the light source of the other station which are incident on the optical filter 2-4 are substantially equal. That is, the light quantity of fs: the light quantity of fgo = 2: 1. Assuming that the light intensity when fs and ff completely match is P, the threshold for pulling in / stabilizing fs to ff and shifting to ff sweep in "Operation 2" is 50% of the light intensity of fs.
0.5P, the threshold of the shift to the stabilization of ff to fgo in "Operation 4" and stabilization is 0.25P, which is 50% of the light intensity P of fgo and the light intensity of fgo due to the perfect match of fs and ff. So 1.
It becomes 25P. Therefore, set the output voltage of the reference voltage source 3-21 to V
When t, the output voltage of the reference voltage source 3-22 is 2.5 times Vt.

The overall tuning operation will be described below. In the present embodiment, usually, tuning is performed from "operation 0" to "operation 5" as shown in FIG. Hereinafter, each operation will be described.

"Operation 0" This operation represents the state before tuning is started (that is, when transmission is not performed). The tuning ON / OFF signal from the terminal station is L, and the output signals of the comparators 3-23 and 24 are both
It is L. As a result, the ramp signal generators 3-1, 2 are held in the initial value (minimum value), and the switches 3-19, 20 are turned off. ff is to some extent from the side of the highest optical frequency used in this optical communication system (about 10 GHz; this value is used to reliably detect the emission frequency of the own station that is swept from the side of high optical frequency to the side of low optical frequency. It can be determined by the control accuracy, etc.) It is in a low position.

"Operation 1" This operation is an operation at the time of starting tuning (sweeping of fs) when a transmission request is generated. The tuning ON / OFF signal from the terminal station is H, and the output signals of the comparators 3-23 and 24 are LPF3-1
Since both inputs from 5,3-16 do not exceed the threshold, both
Is. As a result, the ramp signal generator 3-1 is in the sweep state, the ramp signal generator 3-2 is in the hold state at the initial value (minimum value), and the switches 3-19 and 20 are in the OFF state. The optical frequency of the LD is swept from the highest side to the lowest side of the optical frequency used in this optical communication system. During tuning, an idling signal is being input from the terminal station, fg, fm,
The fs are arranged at regular intervals in order from the high optical frequency side, and are swept to the low optical frequency side.

"Operation 2" In this operation, fs is pulled into ff and ff is swept. When fs is applied to the tail of the transmission spectrum of the optical filter 2-4 and exceeds the threshold value, the output signal of the comparator 3-23 changes from L to H (the tuning ON / OFF signal from the terminal station is H, the comparator 3 -24 output signal remains L). The switch 3-20 remains in the OFF state, the switch 3-19 is in the ON state, the ramp signal generator 3-1 is in the hold state, and the ramp signal generator 3-2 is in the sweep state. The optical frequency of the LD (fg, f
(m, fs) is modulated with a small amplitude, and fs is pulled into ff and stabilized by feedback control. Since the ramp signal generator 3-2 is in the sweep state, ff starts to be swept toward the side where the optical frequency is low.

"Operation 3" In this operation, ff is continuously swept. From "action 2"
The ff sweep state is maintained, and the LD optical frequency (fg, fm, fs),
The whole ff moves to the lower frequency side.

"Operation 4" In this operation, ff is pulled into fgo and stabilized. When fgo is applied to the tail of the transmission spectrum of the optical filter 2-4 and exceeds the threshold value, the output signal of the comparator 3-24 changes from L to H. Switch 3-19 is in the ON state, ramp signal generator 3-1 is in the hold state, switch 3-20 is in the ON state, ramp signal generator
3-2 goes into hold status. When the switch 3-20 is turned on, the optical frequency of the optical filter 2-4 is modulated with a small amplitude, and ff is pulled into fgo.

"Operation 5" This operation shows a steady state of tuning. The state of "operation 4" is maintained and fs, ff, fgo match (fs is pulled into ff and stabilized, and ff is pulled into fgo and stabilized).

When the optical transmitter at an optical frequency lower than that of the own station finishes transmission, there is a space, and the transmission optical frequency of the own station can be further shifted to the lower frequency side. Specifically, when there are no adjacent channels, fgo is not input to the optical filter, so the output signal of the comparator 3-24 changes from H to L. As a result, the state of "operation 5" is switched to the state of "operation 3", and the ramp signal generator 3-2 performs sweeping again from the value in the hold state. Further, the switch 3-20 is turned off, and the minute modulation of the optical filter 2-4 is stopped until the state of "operation 4" is reached. Also, when the adjacent channel shifts to the low frequency side, the optical filter is controlled to follow it, so it changes from "operation 5" to "operation 4".
While maintaining the same state, it moves to the lower frequency side with the adjacent channel.

If there is no channel already transmitting on the lower optical frequency side of the local station, the ramp signal generator
The sweep is held at the maximum output value preset in 3-1 and the sweep of the LD optical frequency (fg, fm, fs) stops at a certain optical frequency.

The LD tuning of the optical transmitter, which is a feature of the present invention, has been described above. Next, an example of transmission and reception when this method is used will be described.

In this embodiment, communication data is transmitted as shown in FIG.
After "Operation 5", the operation is started after waiting for a certain time. Until then, the idling signal is transmitted. This is to wait for the time required for matching the optical frequency of the optical filter in the optical receiver on the receiving side to this channel and for identifying the destination address. A destination address is attached to the idling signal,
It is used to identify the receiving channel on the receiving side.

Reception is an optical frequency fm which does not overlap in the optical frequency arrangement among the optical frequencies output by each node.
Is transmitted by an optical filter in the optical receiver.
The following operation is performed in order to match the peak of the transmission spectrum of the optical filter with fm. When not receiving, the optical receiver of the optical node in the optical communication system adjusts the optical filter to the light with the highest optical frequency of the incident light (fg of the channel with the highest optical frequency). When the channel that newly starts transmission is squeezed and the fs of the channel that newly starts transmission is drawn into fg of the original channel, the amount of light transmitted through the optical filter increases. At this time, the optical frequency of the optical filter is swept to the higher optical frequency side, and it is pulled in and stabilized at fm of this channel. The destination address is written in the transmission signal, and when the address is the local station, the state is maintained and the signal is received. If it is not the address of the own station, ff is again swept to the higher optical frequency side, and it is pulled in and stabilized at fg of that channel. As the sweep, pull-in, and stabilization methods, those used in the control of the optical filter of the optical transmitter described above are applied.

(Second Embodiment) A second embodiment of the present invention will be described below with reference to the drawings. In this embodiment, the configuration of the optical transmitter is different from that of the first embodiment. FIG. 7 shows the configuration diagram.

The difference from FIG. 2 is that, of the two end faces of LD2-3, the output light from the end face opposite to the output end face to the transmission line is used as the incident light from the transmission line by the optical combiner 7-1. Merge, optical filter 2
-4 is to be incident. The light fg, fm, fs of the own station directly enters the optical filter 2-4 without going through the network, so the amount of fs entering the optical filter 2-4 is n times fgo (n is the number of branches of the star coupler). That's all. As a result, even if all the optical transmitters have the same fine modulation frequency for pulling in and stabilizing, the error due to fgo in pulling in and stabilizing fs to ff is reduced. That is, the same optical frequency control system can be used in all the optical transmitters in the optical communication system.

In this embodiment, fs incident on the optical filter 2-4
The output voltage of the reference voltage sources 3-21 and 22 is different from that of the first embodiment because the amount of light is increased. The output voltage of the reference voltage source 3-21 is the value of the output voltage of the amplifier 2-8 when 50% of the transmitted light amount of ff when fs and ff completely match is incident on the light receiving element 2-5.
Set to Vt2. The output voltage of the reference voltage source 3-22 is fgo and ff
50% of the transmitted light amount of fgo when the
100% of the transmitted light amount of ff when fs and ff are completely matched with the output voltage value of amplifier 2-8 when incident on 5
It is set to the sum of the output voltage values of the amplifier 2-8 when incident on.

The rest of the configuration of the optical frequency control system and the tuning operation are the same as in the first embodiment, and therefore their explanations are omitted here.

(Other Embodiments) Each component is not limited to the one described in the embodiment as long as it has a similar function (the same applies to a system consisting of several components). Further, the numerical values are not limited to the described values as long as they are within the allowable range of operation.

Specifically, the LD is a three-electrode λ / 4 shift.
DFB-LD was used, but multi-electrode DBR (distributed Bragg ref
rector) -LD can also be used. A fiber / Fabry / Perot type was used as the optical filter.
A semiconductor type optical filter such as a B-LD filter can also be used. As the pull-in / stabilization control system, a system using minute modulation is used, but it is also possible to use another control system.

The operation of the control system may be any other if fgo and fs match in the steady state of tuning (in this case, the configuration of the control system is changed according to the operation).

The optical frequency of the light source is set to the sweep direction from the high optical frequency side to the low optical frequency side, but the sweep direction may be from the low optical frequency side to the high optical frequency side.

In the above embodiment, after matching fs and ff, fs and fgo are controlled to match ff and fgo while maintaining that state (or overlapping with the matching operation of fs and ff). Matched, but after matching fgo and ff, keeping that state (or overlapping with the matching operation of fgo and ff), fs
And fff may be controlled so that fs and fgo match.

Arrangement of fg, fm, fs, and which of them is ff
Whether or not to match with can be changed within a range where there is no problem in the operation.

As the optical transmission line of the communication system, not only the optical fiber used in the above-mentioned embodiment but various media such as space can be used as long as it can transmit light. The light referred to in the present application is not limited to visible light,
It includes all frequency regions to which the present application can be applied.

[0077]

As described above, according to the present invention, high-density optical frequency multiplexing can be performed without absolutely stabilizing the optical frequency and without providing the reference optical frequency. 1 optical filter for tuning the optical transmitter
Configuration is easy because all that is needed. In addition, the control operation of the optical frequency control system is simplified, and MPU (micro processing un
It does not require a sophisticated control system such as it).

[Brief description of drawings]

FIG. 1 is a schematic diagram of an optical frequency arrangement of an optical frequency control method of the present invention.

FIG. 2 is a configuration diagram of a first embodiment of an optical transmitter using the optical frequency control method of the present invention.

FIG. 3 is a configuration diagram of an optical frequency control system.

FIG. 4 is a block diagram of an optical communication system.

FIG. 5 is an explanatory diagram of the operation of the optical frequency control system.

FIG. 6 is a schematic diagram of a guarded FSK signal.

FIG. 7 is a configuration diagram of a second embodiment of an optical transmitter using the optical frequency control method of the present invention.

[Explanation of symbols]

 2-1 Optical frequency control system 2-2 Guard signal addition circuit 2-3 LD 2-4 Optical filter 2-5 Light receiving element 2-6,7 LD drive circuit 2-8 Amplifier 2-9 Optical filter drive circuit 3-1 , 2 Ramp signal generator 3-3 LD drive circuit 3-4 Optical filter drive circuit 3-5-8 Adder 3-9,10 Sine wave oscillator 3-11,12 Multiplier 3-13-16 LPF 3-17 , 18 Feedback control circuit 3-19, 20 Switch 3-21, 22 Reference voltage source 3-23, 24 Comparator 4-1 Terminal station 4-2 Optical node 4-3 n × n Star coupler 44,5 Optical Fiber 4-6 Optical transmitter 4-7 Optical receiver 4-8 Optical splitter 7-1 Optical splitter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H04B 10/26 10/14 10/04 10/06

Claims (17)

[Claims] 1. An optical communication system for performing optical frequency multiplex communication on an optical transmission line, wherein an optical transmitter in the optical communication system keeps lights of at least three optical frequencies at predetermined optical frequency intervals with each other. And a light source unit capable of controlling and outputting an optical frequency, an optical frequency of light having a maximum optical frequency among lights of the three optical frequencies, and an optical frequency of the maximum in an optical frequency region for performing the optical frequency multiplex communication. The optical frequency is larger than the optical frequency and the optical frequency of the adjacent optical frequency is controlled so as to match the two optical frequencies.
Of the two optical frequencies, the optical frequency of the light having the minimum optical frequency and the optical frequency that is smaller than the optical frequency of the minimum optical frequency in the optical frequency region in which the optical frequency multiplex communication is performed, and adjacent optical frequencies An optical communication system comprising: a control unit that controls to match two optical frequencies with the optical frequency of the light. 2. The optical communication system according to claim 1, wherein the optical transmitter further comprises means for modulating light of at least an intermediate optical frequency among lights of the three optical frequencies. 3. The optical transmitter has an optical bandpass filter capable of controlling a transmission center optical frequency, and the control means controls the light source unit and the optical bandpass filter to provide an optical bandpass filter. The optical communication system according to any one of claims 1 and 2, wherein the pass filter controls so that lights of two optical frequencies to be matched are simultaneously transmitted through the optical band pass filter. 4. In all the optical transmitters in the optical communication system, the control means controls the optical frequency of the light of the maximum optical frequency among the lights of the three optical frequencies and the optical frequency multiplex communication. In the optical frequency range to be performed, the optical frequency is larger than the optical frequency of the maximum optical frequency, and the optical frequency of the light of the adjacent optical frequency is controlled to match the two optical frequencies, or the three optical frequencies are controlled. Among the light of the frequency, the light of the light of the minimum optical frequency and the light of the adjacent optical frequency that is smaller than the light of the minimum optical frequency in the optical frequency region for performing the optical frequency multiplex communication. The optical communication system according to any one of claims 1 to 3, wherein the optical communication system and the optical system according to any one of claims 1 to 3 are unified so as to match the two optical frequencies with each other. 5. The optical communication system according to claim 1, wherein a part of the light output from one of the optical transmitters to the optical transmission line is also input to the optical transmitter. The optical communication system described. 6. The optical communication system according to claim 1, wherein the optical transmitter generates the three optical frequencies by performing optical modulation with a ternary FSK signal. 7. An optical transmitter for use in the optical communication system according to claim 1, wherein light of at least three optical frequencies can be output while maintaining a predetermined optical frequency interval with each other and controlling the optical frequencies. And a light frequency of light of the maximum light frequency among lights of the three light frequencies, and a light frequency of light higher than the light of the highest light frequency in a light frequency region for performing the light frequency multiplex communication, And controlling so that the two optical frequencies of the adjacent optical frequencies and the optical frequency of the light coincide with each other, or
Of the two optical frequencies, the optical frequency of the light having the minimum optical frequency and the optical frequency that is smaller than the optical frequency of the minimum optical frequency in the optical frequency region in which the optical frequency multiplex communication is performed, and adjacent optical frequencies And a control means for controlling to match two optical frequencies with the optical frequency of the light. 8. The optical transmitter according to claim 7, further comprising means for modulating light of at least an intermediate optical frequency among lights of the three optical frequencies. 9. An optical bandpass filter capable of controlling a transmission center optical frequency is further provided, and the control means controls the light source unit and the optical bandpass filter so that the optical bandpass filter is the same. 9. The optical transmitter according to claim 7, wherein light of two optical frequencies to be controlled is controlled so as to simultaneously pass through the optical bandpass filter. 10. The optical transmitter according to claim 9, further comprising means for directly inputting a part of the output from the light source section to the optical bandpass filter. 11. The three optical frequencies are generated by performing optical modulation with a ternary FSK signal.
The optical transmitter according to any one of the above. 12. A method for controlling an optical frequency in an optical transmitter used in an optical communication system for performing optical frequency multiplex communication on an optical transmission line, wherein light having at least three optical frequencies is mutually emitted from a light source unit in the optical transmitter. Out of the light of the three optical frequencies, the optical frequency of the light of the maximum optical frequency and the light of the maximum optical frequency in the optical frequency region in which the optical frequency multiplex communication is performed are output at a predetermined interval. Also has a large optical frequency, and is controlled so that the two optical frequencies of the adjacent optical frequencies and the optical frequency of the adjacent optical frequency are matched, or
Of the two optical frequencies, the optical frequency of the light with the minimum optical frequency and the optical frequency that is smaller than the light with the minimum optical frequency in the optical frequency region in which the optical frequency multiplex communication is performed, and the adjacent optical frequency The optical frequency control method is characterized by controlling the two optical frequencies to match the optical frequency of the light. 13. The optical transmitter has an optical bandpass filter capable of controlling a transmission center optical frequency, and controls the light source unit and the optical bandpass filter so that the optical bandpass filter makes the coincidence. 13. The optical frequency control method according to claim 12, wherein light having two optical frequencies to be controlled is controlled so as to be simultaneously transmitted through the optical bandpass filter. 14. The control for causing the light of the two optical frequencies to be matched to simultaneously pass through the optical bandpass filter is one of the two frequencies to be matched, and The first step of matching the transmission center optical frequency of the optical bandpass filter and the three frequencies having the predetermined intervals are kept at the intervals while keeping the matched state at the first step. 14. The optical frequency control method according to claim 13, further comprising a second step of controlling the light having two frequencies to be matched so as to be simultaneously transmitted through the optical bandpass filter while being changed. 15. The optical transmitter generates an error signal for feedback control by finely modulating at least the optical frequency that is transmitted through the optical bandpass filter among the three optical frequencies to be output, The optical frequency control method according to any one of claims 13 and 14, wherein an optical frequency is drawn into and stabilized at a transmission wavelength of the optical bandpass filter. 16. The modulation frequency of the minute modulation is different in each optical transmitter in the optical communication system.
The optical frequency control method described. 17. The optical frequency control method according to claim 12, wherein the optical transmitter generates the three optical frequencies by performing optical modulation with a ternary FSK signal.