JPH11340926A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid the effect of a phase noise on a light source that is a problem in the case of generating a high frequency signal such as a millimeter wave in optical heterodyne. SOLUTION: One and the same light source 1 is branched into two, a data modulator 4 applies data modulation to one light and a phase modulator shifts an optical frequency of the other light by a prescribed phase to produce an optical spectral line. When an optical branching coupler 6 makes the both confluent, since the lights are originated from the same light source, phase noise is automatically cancelled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter for transmitting a high frequency signal such as a millimeter wave through an optical fiber.

[0002]

2. Description of the Related Art In wireless communication, a millimeter wave band having a large transmission capacity is expected. However, the disadvantage of the millimeter wave band is that the attenuation in air is large, which hinders the expansion of the communication area. The expansion of the communication area is an unavoidable issue from the viewpoint of increasing communication demand and providing precise communication services in the future.

[0003] Various technical developments have been attempted to expand the communication area in millimeter-wave band communication. One of them,
There has been proposed a system in which a signal in the millimeter wave band to be transmitted is converted into an optical signal and transmitted to a distant place via an optical fiber.

There are various methods for transmitting a high-frequency signal such as a millimeter wave through an optical fiber, one of which is optical heterodyne.

[0005] The optical heterodyne is a system as shown in FIG. 30 in which a signal light in which data is loaded at an optical frequency f0 by a signal of a baseband or a low frequency close to the baseband, and this signal light is an optical signal. In this system, unmodulated local light having a frequency fL slightly separated from the frequency is mixed with polarized light and transmitted, and the receiving side receives the light with a photodiode.

FIG. 31A shows an optical spectrum on the transmitting side in optical heterodyne, and FIG.
This is shown in FIG. Since the photodiode used for receiving the optical signal is an ideal square detector, the signal light and the beat of the local light are received. In the received electric signal, the data near the baseband on the signal light is frequency-converted to a frequency near the optical frequency difference between the signal light and the local light. At this time, if the frequency difference is a frequency in the millimeter wave band, it is converted to a frequency in the millimeter wave. In this way, it is possible to generate a millimeter wave at the receiver without directly modulating the light with the millimeter wave signal.

However, a semiconductor laser device generally used as a light source for optical communication has large phase noise, which is usually several [MHz] in oscillation line width, and several tens [kHz] even when an external resonator is used. There is a degree. The optical phase noise is down-converted in the millimeter wave obtained by optically heterodyneing these, and only a signal with large noise is obtained.

In order to avoid this, there has been proposed a method of applying a PLL (Phase Locked Loop Control) to the optical phases of the local light and the signal light. This is because, as shown in FIG. 32, the beat signal of the optical frequency difference between the local light from the local light source 49 and the signal light from the signal light source 50 is detected by the photodiode 53, and this is amplified by the amplifier 54 to be amplified. At 56, the reference signal from the reference signal generator 55 is mixed, the low-frequency component is extracted through the low-pass filter 57, and the output wavelength of the local light source 49 is adjusted. The PLL is applied to the optical phase of the local light or the signal light so that the phase noise of the beat signal is eliminated.

The light from the signal light source 50 is modulated by the data modulator 51 in accordance with the data, and this is combined with the local light from the local light source 49 by an optical branching / combining device 52 and mixed.
Transmit to optical fiber.

However, in the case of such a method in which the PLL is applied to the optical phase of the local light and the signal light, the beat signal is difficult to handle when the optical frequency difference is as high as a millimeter wave, and
Since high frequencies must be handled, amplifiers, mixers, filters, and the like must be used with good high frequency characteristics, resulting in increased costs.

[0011]

One of the methods for transmitting a high-frequency signal such as a millimeter wave through an optical fiber is an optical heterodyne. This is a signal light in which data is carried at the optical frequency f0 by a baseband signal or a low-frequency signal close to the baseband, and an unmodulated local light having a frequency fL, which is slightly apart from the optical frequency. In this method, the signals are mixed and transmitted, and the receiving side receives the signals by a photodiode. A low-cost semiconductor laser device is used as a light source for optical communication.

However, the semiconductor laser device has a large phase noise, and the optical phase noise is down-converted in a millimeter wave obtained by optically heterodyning the phase noise into a signal having a large noise, so that the S / N (signal / noise) is increased. Ratio) becomes worse.

To avoid this, a method of applying a PLL to the optical phase of the local light and the signal light has been proposed. However, when the optical frequency difference is as high as a millimeter wave, the beat signal is difficult to handle, and the cost of the device is low. Up was inevitable.

Therefore, in optical communications, even when a high frequency such as a millimeter wave is generated by optical heterodyne, the problem of optical phase noise of a semiconductor laser device as a light source can be avoided, and the cost can be reduced easily. The urgent development of technology is expected.

Therefore, an object of the present invention is to easily avoid the influence of optical phase noise of a semiconductor laser device as a light source even when a high frequency such as a millimeter wave is generated by optical heterodyne in optical communication. It is an object of the present invention to provide an optical transmitter which can perform optical communication at low cost and with good S / N.

[0016]

In order to achieve the above object, the present invention is configured as follows. That is, [1] In the first invention of the present application, a single longitudinal mode light source, a splitter for splitting light output from the light source, and a data modulator for modulating one of the split lights by a data signal. Generating means for processing another one of the branched lights to generate an optical spectrum line shifted from the optical spectrum of the light source to an optical frequency separated from the optical frequency of the light source by a predetermined frequency; and An optical transmitter comprising a coupler for coupling the output light of the device and the output light of the generating means.

In the optical transmitter having such a configuration, light of a single longitudinal mode is emitted from a light source, and this light is split into two systems by a splitter, and one of them is a data modulator and is relatively low corresponding to data. The modulation of the frequency is applied, and the other is converted into an optical spectrum line by shifting the optical spectrum to an optical frequency separated by a predetermined frequency by the optical spectrum line generating means.

Then, the output of the data modulator and the output light of the optical spectrum line generating means are coupled by a coupler, and sent out to an optical fiber which is an optical transmission line.

The optical spectrum generated by the optical spectrum line generating means is separated from the optical signal modulated and output by the data modulator by the desired optical frequency f1, and has the same optical phase noise as the light source. Also, the data modulated light has the same optical phase noise as the light source. Therefore, when these are combined and interfered by the coupler, the optical phase noise is automatically canceled, and the received signal is not affected.

As described above, in a mode in which the light source is shared,
Since the two branched lights inherit the optical phase noise of the common light source, the same optical phase noise is superimposed, and one of the two branched lights has an optical spectrum of only the desired frequency f1. After being shifted, the light is merged with the other of the two branched lights and caused to interfere with each other, whereby the influence of optical phase noise can be canceled.

Therefore, according to the present invention, it is possible to obtain a received signal free from the influence of optical phase noise while using a method equivalent to optical heterodyne.

[2] In the second invention of the present application, a single longitudinal mode light source, a splitter for splitting the light output from the light source, and a data modulator for modulating one of the split lights by a data signal A phase modulator that performs phase modulation on another one of the branched lights with a periodic signal, an optical filter that filters output light of the phase modulator, output light of the data modulator, and output of the optical filter. The frequency of the periodic signal is a frequency difference between an optical frequency of output light of the data modulator and an optical frequency of a specific sideband of output light of the phase modulator. Determined to be a value, the optical filter, the sideband, for a sideband at an optical frequency symmetrical with respect to the optical frequency of the light source of the sideband, to pass either one. Providing a characteristic optical transmitter That.

The light output from the single longitudinal mode light source is split into two, and one of the two modulates data at a low subcarrier frequency fs close to the baseband. FIG. 4A shows an optical spectrum after data modulation. The other light has a frequency f1
Is phase-modulated by the periodic signal. The phase-modulated light spectrum is as shown in FIG. That is, in FIG. 4, f0 is the optical frequency of the light source, and the frequency f
Every other sideband occurs. In the data modulated light, a sideband in which data is present at f0 ± fs is generated. In the present invention, f1 and fs are determined so that the frequency f1-fs becomes a desired frequency.

The optical transmitter shown here uses a common light source, and obtains phase-modulated light from the light output from the light source by modulating with a periodic signal, and obtains data-modulated light by modulating with a data signal. . The phase modulation is performed to obtain a harmonic component. Even if the phase modulation is used, if the phase modulation is performed by a periodic signal, the same sideband as when the intensity modulation is used, that is, the harmonic component is used. Is generated in the present invention. The phase modulator focuses on the fact that the cost of the device is lower than in the case of using intensity modulation.

The phase-modulated light and data-modulated light thus obtained interfere with each other by multiplexing. That is, when the data-modulated light (FIG. 4A) and the phase-modulated light (FIG. 4B) of the same light source interfere with each other, the respective spectral lines of the phase-modulated light and the data-modulated light are changed. Both the phase-modulated light and the data-modulated light use the same light from the same light source, and therefore both have the same phase noise. The influence is canceled and disappears, so that an optical signal without phase noise can be obtained by this configuration.

In the phase-modulated light, sidebands are formed symmetrically on both sides of the optical carrier f0. This is caused to interfere with the data-modulated light, and is detected by a photodetector having a square detection characteristic. In this state, interference waves from both sidebands are detected at the same reception frequency. The magnitude of the detected data changes depending on the phase relationship between the sidebands on both sides, and in some cases, no signal is detected. Therefore, it is necessary to adjust the sidebands, but the phase relationship between the sidebands changes due to the chromatic dispersion of the optical fiber during transmission and cannot be easily controlled.

Therefore, in the present invention, one of the two sidebands providing a desired reception frequency is blocked by an optical filter.

In the present invention, a high-frequency system such as millimeter wave transmission is assumed, and the optical frequency interval between a desired sideband and a sideband (image component) symmetrically positioned with respect to the optical carrier is: Relatively large. When there are no restrictions on components other than the desired sideband and its image, the stability of the transmitted light frequency of the optical filter can be low, and the system can be simplified.

As described above, the system of the present invention is characterized by using an optical modulator. A general optical modulator is a lithium niobate modulator. In the case of the lithium niobate modulator, a phase modulation is performed on light using a refractive index change by an applied voltage. Various modulators can be configured by combining or modifying them.

Although the phase modulator itself is rarely used in the world of optical communication, the phase modulator has the simplest configuration, does not require bias voltage control, and is a low-cost device. Focusing on this point, in the present invention, the light from the same light source is split into two, one is data-modulated, and the other is phase-modulated and then multiplexed with each other to cancel the phase noise of the light source. I made it.

As described above, according to the second aspect of the present invention, the influence of the optical phase noise is eliminated by generating the beat from the same light source, and one of the interference waves is generated by phase modulation by the optical phase modulator. Accordingly, the cost of the modulator can be reduced.

[3] Further, in the third invention of the present application, in the configuration of the above item [2], the periodic signal is a sine wave,
The frequency is 1 / n (n is an integer of 2 or more) of the frequency difference between the sideband giving the desired value and the optical frequency of the light source.
It is characterized by the following.

In the invention described in the above item [2], the sideband closest to the optical carrier f0 among the sidebands generated by the phase modulation has been described.

However, in the present invention, a high frequency such as a millimeter wave is assumed, and in the example of FIG. 4, the phase modulation must be performed at the frequency of the millimeter wave, which increases the cost. In phase modulation, when the modulation degree is increased, the power of the sideband of the harmonic component of the modulation frequency increases.

The third invention of the present application proposes a method using such harmonic components.

In the example of FIG. 4, f0 + 2f1 and f0 + 3f
When an interference wave generated by interference between the data modulation light and the data 1, 2,... Is received, the frequency f1 of the phase modulation is determined so that the desired frequency is obtained.

As a result, f1 is reduced, and the cost around the phase modulator can be reduced.

[4] Next, according to the fourth aspect of the present invention, in the optical transmitter for controlling the optical frequency so that the signal light and the local light have a desired optical frequency difference, and multiplexing and transmitting the same, And a photodetector detects an interference wave between one of a plurality of sidebands generated by modulating with a periodic signal and the signal light, and the detected error signal is constant. An optical transmitter characterized by controlling the optical frequency and optical phase of the signal light or the local light so as to maintain a DC value.

Separately from the light source of the data modulation light (signal light source) and the light source of the interference light (local light source), P
In the case of the LL method, if the beat is a high frequency such as a millimeter wave, an element having good high frequency characteristics must be used, and the cost increases.

Therefore, in the fourth invention of the present application, PLL is not performed in a high frequency band, but is performed in a low frequency band. That is, the error signal for applying the PLL is detected at a low frequency near DC. As shown in FIG. 22, the light output from the signal light source 8 and the light output from the local light source 7 are respectively branched, and the local light is subjected to phase modulation with a repetitive signal such as a sine wave. The split light is multiplexed again, detected by the photodetector 10, and the local signal or the optical frequency of the signal light is adjusted so that the filter output becomes a constant DC value by the error signal passed through the low-pass filter 11. Control the phase. The local light is subjected to phase modulation to generate sidebands, and in addition to the sideband having a frequency corresponding to the cycle of the phase modulation signal, a number of sidebands having frequencies corresponding to harmonics are generated. The same optical phase noise as the optical phase noise of the light source is placed in the sideband.

Therefore, in the present invention, the signal light is locked by the PLL to the sideband corresponding to the harmonic. If the desired sideband of the local light is made to interfere with the signal light in such a manner that the signal light approximately overlaps in advance, the component near the DC of the signal detected by the photodetector becomes an error signal of the PLL, and synchronization becomes possible.

On the other hand, the signal light at the branch source and the local light are combined by another optical branching coupler 6 and transmitted. Since this local light is not subjected to phase modulation, only the component of the optical frequency fL of the local light is transmitted. The signal light is near f0, and the receiver of the transmitted signal detects the beat of the optical carrier frequency f0 of the signal light and the optical frequency fL of the local light. According to the fourth aspect of the present invention, since the error signal used in the PLL is detected at a low frequency near DC, a loop can be assembled with low-cost elements. Further, since the frequency of the phase modulation is only a fraction of the frequency of the desired millimeter wave band, the cost around the phase modulator can be reduced.

[5] Further, in the fifth invention of the present application, in the optical transmitter for multiplexing and transmitting the signal light and the local light by the optical branching coupler, the signal light is modulated by a subcarrier signal. The local light is modulated by a periodic signal, one of the plurality of outputs of the optical branching coupler is transmitted as the output of the optical transmitter, and another is detected by a photodetector, Controlling the optical frequency and optical phase of the signal light or the local light so that an error signal caused by interference between one of a plurality of sidebands generated in light emission and an optical carrier component of the signal light maintains a constant DC value. An optical transmitter is provided.

In the fourth invention of the present application, the local light and the signal light are branched from the main line (main stream) to generate an error signal. However, in the fifth invention of the present application, the local light and the main light of the local light are phase-modulated. Combines with the data modulated signal light. An optical branching coupler used for multiplexing has a plurality of output ports. In a directional coupler (coupler) often used as an optical branching coupler,
The basic form when two inputs are merged is two outputs. 1
Outputs can also be made, but this can be considered as a form of discarding one of the two outputs, and one output port suffers the same loss as two output ports.

Therefore, in the present invention, a 2 × 2 coupler is used, one of the outputs is sent to the receiver, and the other output is used to synchronize the optical phase of the local light with the optical phase of the signal light.
Apply LL. The signal light used here is obtained by subjecting an optical carrier to subcarrier modulation using a band signal. There is a sideband on which the optical carrier component and data are superimposed, and the optical frequency of the optical carrier and the sideband. Optical frequency is PLL
Away from the retraction range.

The fifth invention is to transmit the local light phase modulated and the signal light as they are. Although any one sideband of the local light is synchronized with the optical carrier component of the signal light, any sideband of the local light with which the optical carrier of the signal light is synchronized may be used. As in the case of the fourth aspect, it is not necessary to select a sideband in which the optical frequency fL of the local light and the optical carrier frequency f0 of the signal light are separated by a required value.
In this way, when received, an interference wave with any sideband falls to a desired frequency band. Two sidebands separated from the signal light by plus and minus by a required frequency difference simultaneously fall to a desired frequency. However, when phase modulation is applied to an optical carrier, an envelope as shown in FIG. 28 generally has a mountain-shaped spectrum, depending on the degree of modulation. Therefore, if the optical frequency fL of the local oscillation light and the optical carrier frequency f0 of the signal light are appropriately separated, there is a remarkable difference in the size of the sideband separated from the positive side and the sideband separated from the negative side, When the interference wave with the signal light is received, the interference wave with the larger sideband becomes dominant.

According to the fifth aspect of the present invention, the wavelengths of the signal light and the local light do not need to be strictly controlled, and the cost can be reduced. Also, since there are few branches in the system, loss is small and higher quality transmission is possible.

[0048]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Various embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a block diagram of an optical transmitter according to a first embodiment of the present invention. That is, in FIG. 1, 1 is a light source, 4 is a data modulator, 5
Numerals 6 and 6 denote an optical branching coupler, and 70 denotes an optical spectrum line generating means.

The light source 1 generates light in a single longitudinal mode, and is constituted by using, for example, a semiconductor laser device. The optical splitter / coupler 5 is for splitting the light output from the light source 1, and the data modulator 4 modulates one of the lights split by the optical splitter / coupler 5 by a data signal. It is for multiplying.

Further, the optical spectrum line generating means 70 processes another one of the lights branched by the optical branching coupler 5 to an optical frequency separated by a predetermined frequency from the optical frequency of the light source. A light spectrum line is generated by shifting the light spectrum of the light source. The optical branching coupler 6 combines the output light of the data modulator 4 and the output light of the optical spectrum line generating means 70, and the light coupled through the optical branching coupler 6 is finally converted. Output light.

In the optical transmitter having such a configuration, light of a single longitudinal mode is emitted from the light source 1. The light output from the light source 1 is split into two systems by an optical splitter / combiner 5. One of them undergoes a relatively low frequency modulation by the data modulator 4. The data modulation format in the data modulator 4 is, for example, AM modulation using a subcarrier signal. The other of the two branched by the optical branching coupler 5 is input to an optical spectrum line generating means 70, where the optical spectrum of the light source 1 is shifted to an optical frequency separated by a predetermined frequency. It becomes a line.

As the optical spectrum line generator 70 having such a spectrum shift function, for example, an optical frequency shifter, an optical phase modulator, an optical intensity modulator, and in some cases, a device for shaping the output optical spectrum thereof It can be realized by using.

The output of the data modulator 4 and the output light of the optical spectrum line generating means 70 are coupled by the optical branching coupler 6 and sent out to an optical fiber which is an optical transmission line. However, at the time of coupling by the optical branching coupler 6, the optical path length of both branches is adjusted so that the delay difference between the two is within the coherence length of the light source 1, and
It is preferable that the whole be composed of cables and devices that maintain the polarization so that they merge with the same polarization.

That is, the optical path length from the optical splitter / coupler 5 to the optical splitter / coupler 6 via the data modulator 4 and the optical splitter / coupler from the optical splitter / coupler 5 via the optical spectrum line generating means 70. The optical path length up to the device 6 is adjusted so that the delay difference between the two is within the coherence length of the light source 1.

The optical spectrum generated by the optical spectrum line generating means 70 is separated from the optical signal output by data modulation by the data modulator 4 by a desired optical frequency, and
It has the same optical phase noise as light source 1.

Since the data-modulated light also has the same optical phase noise as the light source 1, if these are combined and interfered by the optical splitter / coupler 6, the optical phase noise is automatically canceled and received. No effect on the signal.

The operation will be specifically described with reference to FIGS. As shown in FIG. 2A, the light source 1 emits light having an optical frequency f0 as its output light. This f
Light having a frequency of 0 is split into two. The data modulator 4 applies data modulation to one of the two split lights. The optical spectrum that has undergone this data modulation is, for example,
The result is as shown in FIG.

The other of the two branched lights is given to the optical spectrum line generating means 70. Then, the optical spectrum line generating means 70 shifts the input light by a predetermined frequency f1. As a result, as shown in FIG. 2C, an optical spectrum in which the optical spectrum of the light source 1 is shifted to a frequency position separated by f1 from the f0 frequency is generated. When the output light of the data modulator 4 and the output light of the optical spectrum line generating means 70 are combined and transmitted by the optical branching coupler 6, the data is modulated to f1 by the interference at the receiver as shown in FIG. A signal is received.

As described above, in a mode in which the light source is shared,
Since the two branched lights inherit the optical phase noise of the common light source 1, the same optical phase noise is present, and the light spectrum of one of the two branched lights is shifted by the desired frequency f1. If they are merged and interfered in a form, the influence of optical phase noise can be canceled.

Therefore, according to the present invention, it is possible to obtain a received signal free from the influence of optical phase noise while using a method equivalent to optical heterodyne.

As described above, according to the optical transmitter of the first embodiment, a single longitudinal mode light source, ie, a laser element,
A splitter for splitting the light output from the light source, a data modulator for modulating one of the split lights by a data signal, and processing another one of the split lights from the optical frequency of the light source. A light source that generates an optical spectrum line shifted from the light spectrum of the light source to an optical frequency separated by a predetermined frequency, and a coupler that combines the output light of the data modulator and the output light of the generator. The light from one light source is split into two, and one is supplied to an optical spectrum line generating means and the other is supplied to a data modulator. Generates an optical spectrum line that is obtained by shifting the optical spectrum of the light source to an optical frequency that is separated from the frequency by a predetermined frequency, and the other light that is bifurcated is subjected to data modulation by a data modulator. Are output, these data modulated optical signal and the optical spectral line is obtained as a final output is multiplexed by the multiplexer.

As described above, the optical spectrum generated by the optical spectrum line generating means is separated from the optical signal modulated and output by the data modulator by a desired optical frequency, and has the same optical frequency as the light source. Has phase noise.

Since the data-modulated light also has the same optical phase noise as the light source, if these are combined and interfered by a coupler, the optical phase noise is automatically canceled and the received signal is affected. It does not appear.

Therefore, according to the optical transmitter of this embodiment,
In an optical heterodyne, it is possible to provide an inexpensive optical communication device that has a simple configuration using an optical spectrum line generating means on the transmission side and is not affected by the optical phase noise of the light source and therefore has a good S / N.

Next, a specific example in which the beat is generated from the same light source to eliminate the influence of optical phase noise, and one of the interference waves is generated by phase modulation to reduce the cost of the modulator. A description will be given as a second embodiment.

(Second Embodiment) FIG. 3 is a block diagram as a second embodiment corresponding to the second invention of the present application.
In the figure, 1 is a light source, 2 is a phase modulator, 3 is an optical filter, 4 is a data modulator, and 5 and 6 are optical branching / combining devices.

The light source 1 generates light in a single longitudinal mode, and is composed of, for example, a semiconductor laser device. The optical splitter / coupler 5 is for splitting the light output from the light source 1 into two, and the data modulator 4 is for modulating one of the split light by a data signal. Things.

The phase modulator 2 is for applying phase modulation to another one of the two branched lights by a periodic signal, and the optical filter 3 converts the output light from the phase modulator 2 into a predetermined light. Is filtered to extract only the components of the band.

The optical splitter / coupler 6 combines the output light from the data modulator 4 and the output light from the phase modulator 2 obtained through the optical filter 3 and multiplexes them. It is for outputting to a certain optical fiber.

The frequency of the periodic signal is such that the frequency difference between the optical frequency of the output light of the data modulator 4 and the optical frequency of a specific sideband of the output light of the phase modulator 2 has a desired value. And the optical filter 3 includes: the sideband;
With respect to the sideband having an optical frequency symmetric with respect to the optical frequency of the light source in the sideband, one of the sidebands is set to pass.

In the optical transmitter according to the second embodiment having such a configuration, the single longitudinal mode light output from the light source 1 is split into two by the optical splitter / coupler 5, and one of the split light is split into two. The data modulator 4 undergoes data modulation corresponding to the data signal, and the other of the two branched lights undergoes phase modulation in the phase modulator 2 corresponding to the phase modulation control signal.

The data signal input to the data modulator 4 is a signal of data to be transmitted, and is a signal of a relatively low frequency such as a baseband or an intermediate frequency. Also,
The phase modulation control signal input to the phase modulator 2 is a periodic signal, and if the frequency band required on the receiving side is a millimeter wave, for example, a sine wave signal near 60 [GHz].

The light source 1 is a single longitudinal mode light source. More specifically, a semiconductor laser device such as a DFB laser device or a DBR laser device, or Nd: YA
A solid-state laser device such as a G laser can be used. As the optical modulator, a modulator using the electro-optic effect of lithium niobate is often used with relatively low loss.

In the embodiment of the present invention, various components such as phase modulation of light and passage of light through an optical filter are included, so that the light loss tends to increase. At the time of transmission, a signal-to-noise ratio as good as possible is required, so that the loss should be as small as possible. Therefore, a lithium niobate modulator is suitable for constituting the device of the present invention.

The data modulation method in the data modulator 4 is A based on a signal obtained by adding a data signal to the subcarrier frequency fs.
In the case of the M modulation (amplitude modulation) method, the light subjected to the data modulation is as shown in FIG.

That is, when AM modulation is performed at the subcarrier frequency fs, sidebands with data components are generated around f0−fs, f0 + fs on both sides of the optical carrier having the frequency f0. Here, the component whose frequency is f0-fs is an image component of the component of f0 + fs.

At this time, in order to down-convert the sideband at f0 + fs to a desired frequency fr on the receiving side, the subcarrier frequency fs or the frequency f1 of the sine wave signal input to the phase modulator is changed. , F1-fs =
It may be determined to be fr.

The light phase-modulated by the sine wave signal of the frequency f1 in the phase modulator 2 is a light having a sideband standing at every frequency f1 on both sides around f0 as shown in FIG. 4B. It becomes a spectrum. These sidebands have exactly the same phase noise as the light source 1 has.

As described above, the subcarrier frequency fs
The light output from the data modulator 4 after data modulation has a sideband in which data is present at f0 ± fs. In the system of the present invention, f1 and fs are determined so that the frequency f1-fs becomes a desired frequency.

One of the side bands “f0 + f1” and “f0−f1” is removed by the optical filter 3. The components other than “f0 + f1” and “f0−f1” may or may not pass through the filter.

As a filter that can be used, any of a band pass filter, a band rejection filter, a low pass filter, and a high pass filter may be used.

In general, a periodic transmission type filter and a period rejection type filter are often used as an optical filter. The periodic type filter is also described above depending on the transmission range (rejection range) other than the transmission range (or rejection range) to be used. "" F0 + f1 "
Alternatively, any one of “f0−f1” can be transmitted and the other is blocked ”, as long as the function is not impaired.

FIG. 5B shows the spectrum of the phase-modulated light that has passed through the optical filter. This is an example in which a band-pass filter is used, and the end of the pass band of the filter lies between “f0−f1” and “f0 + f1”, and passes the “f0 + f1” component.

The light shown in FIG. 5B is multiplexed with the signal light shown in FIG. 5A by using a coupler or the like so that the polarizations are matched so that the interference is maximized. For this purpose, a polarization controller may be used, or the system may be composed of components that maintain polarization in the entire system.

At the time of merging, the branch for phase modulation (the path on the phase modulation system side) and the branch for data modulation (the path for the data modulation system side) are performed from the branch at the optical branching coupler 5 until the branching at the optical branching coupler 6. Path) is sufficiently reduced.

This is because, in order to cancel the influence of phase noise when interference occurs, it is necessary to multiplex signals within a range in which the delay difference maintains coherence.

The coherence length is determined by the phase noise of the light source. If a single longitudinal mode semiconductor laser is used as the light source 1, the coherence length is about several meters. If the delay difference between the two paths (paths) falls within this range, there is almost no problem. However, in order to minimize the effect of phase noise, the delay is within about 1/10 of the coherence length. Is desirable.

In most cases, it is sufficient to adjust the length of the optical fiber cord connecting the components. However, when the optical fiber amplifier 24 is inserted into one of the branches as in the configuration shown in FIG. 6, the optical fiber amplifier 24 significantly increases the delay. In such a case, it is advisable to insert a delay adjusting fiber 23 or the like into the other branch as shown in FIG.

When the multiplexed signal is transmitted through an optical fiber and detected using a photodetector having a square detection characteristic such as a photodiode, the spectrum of the received electric signal is as shown in FIG. .

That is, the components of the data modulated light and the components of the phase modulated light that have passed through the optical filter interfere with each other, so that the data components are received around f1, f1, and 3f1. Then, on the receiving side, to extract the signal at f1-fs, the received signal may be filtered using the bandpass filter 14 for the electric signal that passes only the required frequency component as shown in FIG. Since the phase noise of the data modulated light is exactly the same as the phase noise of the light source, if it interferes with the sideband of the phase modulated light having the same phase noise, the phase noises will cancel each other out, and the phase noise will be It will be canceled.

As a result, phase noise due to the line width of the light source is not detected in the received electric signal, and good reception is possible.

The main points and reasons of the present embodiment are summarized as follows. That is, in the present embodiment, one light source is shared for data modulation and phase modulation, and the light output from the single longitudinal mode light source 1, which is the shared light source, is branched into two. One is data-modulated at a low subcarrier frequency fs close to the baseband, and the other light is phase-modulated by a periodic signal (sine wave signal) having a frequency f1.

The data modulated optical spectrum is shown in FIG.
FIG. 4A shows the optical spectrum subjected to the phase modulation, and FIG. That is, in FIG. 4, f0 is the optical frequency of the light source, and sidebands are generated on both sides at every frequency f1. In the data modulated light, a sideband in which data is present at f0 ± fs is generated. In the present invention, f1 and fs are determined so that the frequency f1-fs becomes a desired frequency.

In this embodiment, a common light source is used, and phase-modulated light is obtained from light output from the light source by modulation with a periodic signal, and data-modulated light is obtained from modulation with a data signal. But it is
For the following reasons.

That is, the phase modulation is performed to obtain a harmonic component. Even if the phase modulation is used, if the phase modulation is performed by a periodic signal, the same sideband as that in the case of using the intensity modulation, that is, the harmonic is used. It utilizes the fact that components are generated. The phase modulator focuses on the fact that the cost of the device is lower than in the case of using intensity modulation.

The phase-modulated light and the data-modulated light obtained in this way are combined to cause interference, thereby canceling the phase noise of the light source.

That is, as shown in FIG. 4, when the data modulated light (FIG. 4A) and the phase modulated light (FIG. 4B) of the same light source interfere with each other, each of the phase modulated light Spectral lines and data modulated light interfere. Both the phase-modulated light and the data-modulated light use light from the same light source, and therefore have the same phase noise.If they interfere with each other, the influence of the phase noise is canceled. Disappear. Therefore, with this configuration, a received signal without phase noise can be obtained.

Here, the optical carrier f
Sidebands are formed symmetrically on both sides of 0, which is a problem if left as is. That is, the optical carrier f
When phase modulated light having sidebands on both sides of 0 is interfered with data modulated light and detected by a photodetector with square detection characteristics, interference waves from both sidebands are detected at the same reception frequency. You. The magnitude of the detected data changes depending on the phase relationship between the sidebands on both sides, and in some cases, no signal is detected. Therefore, it is necessary to adjust the sidebands, but the phase relationship between the sidebands changes due to the chromatic dispersion of the optical fiber during transmission and cannot be easily controlled.

Therefore, in the present invention, one of the two sidebands providing a desired reception frequency is blocked by an optical filter. For example, as shown in FIG. 4, the sideband waves are f0−f1 and f0 + around the optical carrier having the frequency f0.
Although two symmetrical components f1 appear, one of the two sidebands f0 ± f1 is passed by using an optical filter having a filtering characteristic of passing only one of them. Components other than the two sidebands, for example, optical carriers f0, f0 ± 2f1, f0 ± 3f1,...
May be blocked by an optical filter or passed.
When such a component remains, for example, if a component of f0 remains, in this case, the component of f0 and f0 + f1 interfere with each other, and a beat is generated in a desired band upon reception. However, there is no problem if the data does not include a DC component as shown in FIG.

On the other hand, when the data has a component near DC as shown in FIG. 9, the received carrier component of the desired frequency changes in an interference state. Factors of the change are a phase change between components causing interference, a power change of the components, and the like, and are caused by a change in an optical fiber length, a change in a wavelength characteristic of an optical filter, and the like. However, the speed of change at that time is very slow, such as daily or monthly. And
Normally, pure DC in baseband signals and pure carrier components in band signals hardly have information.Even if the level or phase of the carrier itself changes on a daily or monthly basis, information transmission is not possible. It does not affect.

Therefore, there is no problem even if components other than the desired desired sideband, such as f0 and f0 ± 2f1, are transmitted through the optical filter.

In the present invention, a high-frequency system such as millimeter-wave transmission is assumed, and the optical frequency interval between a desired sideband and a sideband (image component) symmetrically positioned across the optical carrier is: Relatively large. When there are no restrictions on components other than the desired sideband and its image, the stability of the transmitted light frequency of the optical filter can be low, and the system can be simplified.

As described above, the system of the present invention is characterized by using an optical modulator for cost reduction. A general optical modulator is a lithium niobate modulator. In the case of the lithium niobate modulator, a phase modulation is performed on light using a refractive index change by an applied voltage. Various modulators are configured by these combinations and modifications.

Although the phase modulator itself is rarely used in the world of optical communication, the phase modulator has the simplest configuration, does not require bias voltage control, and is a low-cost device. Focusing on this point, in the present invention, the light from the same light source is split into two, one is data-modulated, and the other is phase-modulated and then multiplexed with each other to cancel the phase noise of the light source. I made it.

As described above, in the optical transmitter shown in the second embodiment, the influence of the optical phase noise is eliminated by generating a beat from the same light source, and one of the interference waves is converted to the optical phase. Since the modulator is generated by phase modulation, the cost of the modulator can be reduced.

The description of the embodiment shows only elements that are essentially related to the present invention. For example, in practice, an optical amplifier may be appropriately inserted into the optical transmitter of the present invention, but such components are not shown.

An object of the present invention is to provide an inexpensive configuration, and
An object is to make it possible to easily obtain high-quality high-frequency waves on the receiving side. In the above-described embodiment, the phase modulator modulates the phase with the sine wave having the frequency f1, and the desired frequency on the receiving side is centered on f1-fs. With this method, the purpose of avoiding phase noise can be achieved, but when f1 is a high frequency such as a millimeter wave, it is not easy to modulate even on the transmitting side.

As can be seen from FIG. 7, depending on how the side filter is cut out by the optical filter, a signal is also detected around a frequency corresponding to a harmonic of f1, such as 2f1, 3f1.

Therefore, in the present invention, if n is an arbitrary integer, the modulation frequency f1 of the phase modulation is determined so that "nxf1-fs" becomes the desired frequency on the receiving side. The wavelength of the transmitted light is adjusted so that one of “× f1” and “f0 + n × f1” passes.

FIG. 10B shows a spectrum when n = 3. The component “f0 + 3 × f1” is transmitted through the optical filter, and the component “f0−3 × f1” is blocked.

Then, the transmitted “f0 + 3 × f
If the phase-modulated light of the 1 ″ component and the data-modulated light as shown in FIG. 10A are transmitted while interfering with each other, if this is detected by a photodetector having a square detection characteristic such as a photodiode on the receiving side, The spectrum of the obtained received electric signal is as shown in FIG.

That is, as shown in FIG. 11, an interference wave of each sideband and data modulated light is received. If the frequency desired by the receiving side is “3 × f1-fs”, the frequency band is included as a pass band, and the others are not passed. The band pass for an electric signal having a band pass characteristic as shown by a dotted line in FIG. Only the component needs to be extracted through the filter.

At the time of phase modulation, it is preferable to perform modulation at a modulation factor at which a harmonic component is sufficiently generated. By doing so, the frequency of the phase modulation can be reduced, and the configuration around the phase modulator can be reduced in cost.

[Optical Filter Used in the Optical Transmitter of the Present Embodiment] In the present invention, the transmission of the optical filter 3 is performed so that only one of the two sidebands providing a desired reception frequency is left. Adjust and control the optical frequency. Therefore, it is preferable to use an optical filter whose transmission light frequency is variable.

As a control method of the optical filter, a method will be described in which the end of the transmission band 13 of the optical filter is locked to a position where the carrier component f0 of the phase-modulated light is transmitted about half as shown in FIG. In this case, the optical filter 3 has a sufficient bandwidth and a sufficiently sharp rising edge to transmit one of the desired sidebands and block the other while the optical carrier f0 is transmitted through about half. Use

For example, when the rise of the filter is “2 × f
1 ”and the full width at half maximum of the filter band is“ 2 × f
1 ”or a band-pass filter (or a periodic transmission type filter) whose rising edge is smaller than“ 2 × f1 ”and the full width at half maximum of the stopband is“ 2 × f1 ”. Filter) or a high-pass filter or a low-pass filter whose rising edge is smaller than “2 × f1”.

With respect to the band pass filter and the band rejection filter, the shape of the pass band characteristic or the stop band characteristic is as follows.
Instead of a rounded top like a Gaussian type, it is better to have a flat top such as a Chebyshev type or a Butterworth type. It is good to be in the band where it is applied.

In order to prevent about half of the power of the optical carrier f0 from being blocked at the edge of the optical filter 3, the following is performed. The rising and falling characteristics of the optical filter 3 often have a gentle S-shaped slope as shown in FIG. That is, it rises slowly at the beginning, rises steeply in the middle, and rises slowly again at the end. When this is represented by a differential coefficient, FIG.
(B). As can be seen from FIG.
The slope is the largest during the rise. In an ordinary optical filter, the portion where the inclination is the largest is often near a point where the light transmittance is about half of the maximum value (half-value point).

In the present invention, the transmitted light frequency of the optical filter is slightly oscillated right and left by a low frequency signal of a certain specific frequency F1. Then, the intensity of the light of the frequency component applied to the end of the transmission band of the optical filter changes at the frequency of F1.

The spectrum when the interference wave of the phase-modulated light and the data-modulated light that have passed through the optical filter is received by the photodetector is as shown in FIG. 14 when the optical carrier f0 is blocked by about half the optical filter. . That is,
The data component 15 of the frequency fs is obtained by directly detecting the data modulated light, and combining the optical carrier component f0 of the phase modulated light and the interference wave of the data modulated light.

When the magnitude of the f0 component of the phase-modulated light changes at the frequency F1, the power of the data component 15 becomes F1.
It changes at the frequency. Only the data component 15 is extracted from the signal that receives the interference wave of the phase modulation light and the data modulation light with a filter or the like, and the power is detected. Its power is F1
But the optical carrier f of the phase modulated light
When 0 is at the point where the differential coefficient is maximum in FIG. 13B, the rate of fluctuation of the power of the detected data component 15 is maximum.

Therefore, the ratio may be detected, and the optical filter may be locked at the maximum point. To find the maximum point, it is better to use a hill-climbing method or the like.

It is to be noted that a point where the differential coefficient is locally large may exist in the stop band of the optical filter. There is a possibility of locking at such a point.

To prevent this, the absolute value of the amplitude of the component oscillating at F1 when the transmitter is ideally locked is stored at the time of shipping adjustment of the transmitter, and the absolute value of the data component 15 at the time of startup is stored. In locking the optical filter at the point where the rate of power fluctuation is maximum, the value of the point at which the rate of power fluctuation of the data component is recognized as the maximum is compared with the stored value, and as a result, If it is too small, it is preferable to determine that it is not locked to the correct optical frequency and search for another point.

However, when the above-described optical filter is not used, for example, a band-pass filter having only a band width that passes only one sideband, or a stopband that blocks only one sideband is provided. If no bandstop filter is used, the above method cannot be used. This is because the band is too narrow to allow the desired sideband to pass sufficiently or to block the sideband to be blocked. The following may be applied to such a filter.

That is, in the case of a band-pass filter having a band that is too narrow, the light combined with the data modulated light is branched and received, and the optical filter is controlled so that the power of the data signal of the desired frequency is maximized. In the case of a band rejection filter having a narrow stop band, an optical filter is configured to reflect the rejected light, and a circulator is inserted in front of the optical filter to separate the reflected light as shown in FIG. ,
It combines with the branched data modulated light. This may be received by a photodetector and the stop frequency of the optical filter may be controlled so that the data signal of the desired frequency is maximized.

[Example of Making Optical Integrated Circuit Element] By the way, when the configuration as shown in FIG. 3 is adopted as the optical transmitter,
Except for the light source, other elements can be combined into one optical integrated circuit. An example is shown in FIG. FIG. 16 shows an example in which the phase modulator 2, the optical filter 3, the data modulator 4, the optical branching couplers 5, 6, the optical paths, and the like, which are the components in FIG. 3, are integrated on a lithium niobate substrate.

In FIG. 16, reference numeral 29 denotes a phase modulator,
Denotes an AM modulation unit, and 31 denotes an optical filter unit.
Phase modulator 2, data modulator 4, optical filter 3 in FIG.
Corresponds to. 26 is an optical waveguide, 34, 35, 36
Denotes a Y-branch (optical branch coupler) of the optical waveguide, and the Y-branches 34, 35, and 36 constitute an optical branch coupler.

In the case of the structure shown in FIG. 16, an optical waveguide 26 serving as an optical path is formed on a lithium niobate substrate by titanium diffusion or the like, and electrodes are provided thereon. The electrodes are traveling wave electrodes. (However, in the figure, only the signal electrode is shown and the ground electrode is omitted.) The optical filter 31 is formed by forming a Bragg grating 33 in a part of the optical waveguide 26, and A Bragg grating 33 is formed in the portion, and an electrode 32 is attached to the Bragg grating 33 so that the stop band is variable.

Further, there are an input section for guiding light from a light source outside the optical waveguide 26 and an output section for extracting light after interference, and the optical path is bifurcated via a Y-branch 34 between these, and bifurcated. The optical paths are combined into one by a Y-branch 35 to reach an output unit.

A phase modulator 29 and an optical filter 31 are formed on one side of the optical waveguide 26 branched from the input side, and the other side of the optical waveguide 26 branched off from the input side via a Y branch 36. Further, after splitting the optical path into two, an electrode 28 is formed on each optical path so that amplitude modulation can be applied to light in the optical path by a data signal, and an AM modulator 30 is formed. The two-branched optical path of the AM modulator 30 has an optical path configuration in which the optical paths are combined again by another Y-branch 36 and then reach the Y-branch 35.

The phase modulator 29 has a structure in which an electrode 27 is formed on a part of the optical waveguide 26 branched by the Y-branch 34 and a phase modulation signal is applied thereto to perform phase modulation on light in the optical waveguide. It is.

By integrating each element on the lithium niobate substrate in this way, connection between elements becomes unnecessary, and low loss and miniaturization of the optical transmitter become possible.

[Modification (1) of Second Embodiment] FIG. 17 shows a modification of the optical transmitter according to the present invention shown in FIG. In FIG. 17, 1 is a light source, 2 is a phase modulator, 4 is a data modulator, 5 and 6 are Y-branches (optical branch couplers), and 25 is a semiconductor laser device.

Here, the data modulator 4 and the phase modulator 2 have the same configuration as that of FIG. 3, but the semiconductor laser element 25 is inserted in the configuration of FIG. 17 instead of the optical filter 3 in the configuration of FIG. Have been. Semiconductor laser device 2
Reference numeral 5 denotes a device which oscillates in a single longitudinal mode, such as a λ (wavelength) / 4 shift DFB laser or a DBR laser, and whose reflection structure can be used as an optical filter transmitting the oscillation wavelength. .

The drive current of the semiconductor laser element 25 is adjusted in advance so that light having a wavelength close to the wavelength of a desired sideband to be transmitted oscillates. The semiconductor laser device has a characteristic that when light near the oscillation wavelength when oscillating alone (free-run) is injected from the outside, the semiconductor laser device oscillates at a wavelength synchronized with the wavelength of the injected light.

At this time, the oscillation light has the same phase noise as that of the injected light. Further, the structure for oscillating in a single longitudinal mode has wavelength selectivity.

Therefore, if the phase-modulated light is injected as shown in FIG. 4B and the free-running oscillation wavelength of the semiconductor laser 25 is adjusted to around f0 + f1, the light of f0 + f1 is selected by the wavelength selectivity. Oscillate. In order to keep the oscillation wavelength close to the wavelength of the desired sideband, the output multiplexed by the optical branching coupler 6 is partially branched and received so that the interference wave with the data modulated light has a desired frequency. Then, the free-run wavelength of the semiconductor laser element 25 may be adjusted.

In such an embodiment, light having a strong power synchronized with a desired sideband can be easily obtained.

The optical transmitter of the present invention can be used by wavelength multiplexing. In that case, as shown in FIG. 18, a plurality of optical transmitters of the present invention (for example, the optical transmitter having the configuration of FIG. 3) are prepared, and the optical transmitters 37a, 37b, to 37n do not overlap with each other. Prepare a light source of the wavelength. 38
a, .about.38n-1 are optical branching couplers, and 39 is a light for receiving optical signals from the optical transmitters 37a, 37b, .about.37n coupled by the optical branching couplers 38a, 38n-1. It is a receiver.

In such a configuration, on each transmitting side,
As shown in FIG. 19A, each of the optical transmitters 37a, 37b,.
AM using subcarrier signals of subcarrier frequencies fs1, fs2,.
Data modulation by modulation. Thus, each subcarrier is changed for each transmitter. As shown in FIG. 18, these are coupled by optical branching couplers 38a, 38b, to 38n-1, transmitted, and received by the receiver 39.

The receiver 39 does not separate the wavelengths and collectively receives them with one photodetector. Then, as shown in FIG. 19B, it is possible to receive data signals in the same band and not overlapping.

[Modification (2) of Second Embodiment] FIG. 20 shows another modification. In FIG. 20, 40 is a comb-like wave generator, 41-1, 41-2,..., 41-n are data transmitters, 42 is a receiver, and 67 is a trunk fiber (optical fiber).
It is.

The configuration shown in FIG. 20 is different from each data transmitter 41-1,
41-2,..., 41-n have no light source, and a comb-shaped wave generator 40 composed of a light source and a phase modulator is provided at the end of an optical fiber 67 serving as a trunk. The light source is a single longitudinal mode light source, and a plurality of sidebands f0, f0 ± f1,
f0 ± 2f1, f0 ± 3f1, f0 ± 4f1,..., f0
± nf1 is generated. The comb-wave generator 40 has a plurality of sidebands f0, f0 ± f as shown in FIG. 21 by increasing the degree of phase modulation of the single longitudinal mode light from the light source.
1, f0 ± 2f1, f0 ± 3f1, f0 ± 4f1,...,
The optical power of f0 ± nf1 is made relatively uniform.

The phase-modulated light is converted to a comb-wave generator 40.
Is generated and flows through the trunk fiber 67. The data transmitters 41-1, 41-2,..., 41-n have an input side and an output side of an optical signal, and the data transmitters 41-1, 41-2,.
They are connected in the line of the trunk fiber 67 so as to be connected in series in the order of 1-n.

Each of the data transmitters 41-1, 41-2,..., 41
In the case of -n, one of different sidebands is extracted from a plurality of sidebands whose optical power is relatively uniform as shown in FIG. , And terminates the adjacent sideband. For example, the first-stage data transmitter 41-1 has f0-f
1 is data-modulated, f0-2f1 is transmitted, f0 is terminated, and the second-stage data transmitter 41-2 data-modulates the sideband f0 + 2f1, f
In other words, 0 + f1 is transmitted, and f0 + 3f1 is terminated.

Each of the data transmitters 41-1, 41-2,..., 41
At -n, the light subjected to AM modulation is added to the trunk fiber 67 again.

As described above, the sideband to be modulated and the three adjacent sidebands on both sides thereof are set as one set, and different data transmitters perform the same operation for different sets of sidebands. I do. The subcarrier frequency of the AM modulation is changed for each data transmitter.

When the receiver 42 of FIG. 20 receives these signals without separating them, a plurality of subcarrier-multiplexed data transmissions are performed around the frequency f1 which is the interval between the sidebands as shown in FIG. A signal from the vessel is received.

In this embodiment, a light source need not be provided for each data transmitter, and the wavelength control of the light source between the data transmitters is not required, so that the data transmitter can be simplified.

In the first and second embodiments described above, the optical carrier f0 out of the sidebands generated by the phase modulation is used.
In the example described above, the sideband closest to is used.

However, in the present invention, a high frequency such as a millimeter wave is assumed, and in the example of FIG. 4, the phase modulation must be performed at the frequency of the millimeter wave, which increases the cost. In phase modulation, when the modulation degree is increased, the power of the sideband of the harmonic component of the modulation frequency increases.

A method using such a harmonic component will now be described as a third embodiment.

(Third Embodiment) The periodic signal used for phase modulation in the phase modulator in the configuration of the second embodiment is a sine wave in the third embodiment, and the frequency is the desired value. The frequency difference between the given sideband and the optical frequency of the light source is 1 / n (n is an integer of 2 or more).

A specific example of an optical transmitter using a sideband as such a harmonic component will be described. Here, the phase modulator 2 in FIGS. 3, 6, 15, 17, and 22, and the phase modulator of the comb-shaped wave generator 40 in FIG. The frequency is set to 1 / n (n is an integer of 2 or more) of the frequency difference between the sideband giving a desired value and the optical frequency of the light source.

In the example of FIG. 4, f0 + 2f1 and f0 + 3f
When the photodetector receives an interference wave generated by the interference between the data modulated light and
The frequency f of the phase modulation so that the desired frequency is obtained
1 is determined.

As a result, f1 becomes smaller, and hardware around the phase modulator can be reduced in cost.

(Fourth Embodiment) Next, a fourth embodiment of the present invention will be described as a fourth embodiment.

FIG. 23 is a block diagram thereof. In the figure, 2
Is a phase modulator, 4 is a data modulator, 7 is a local light source, 8 is a signal light source, 9a and 9b are optical branching couplers, 10 and 44.
Is a photodiode, 11 is a low-pass filter, 12 and 43 are optical branching / coupling devices, 45 is a band-pass filter,
6 is a control device, 68 is a PLL loop, and 69 is a frequency control loop.

The signal light source 8 is an original light source to be subjected to data modulation, and is composed of, for example, a single longitudinal mode semiconductor laser device.

The local light source 7 is an original light source to be subjected to phase modulation, and is also formed of, for example, a single longitudinal mode semiconductor laser device like the signal light source. The optical branching coupler 9a is a coupler having a one-input-port two-output-port configuration for branching the output light of the local light source 7 into two,
The optical branching coupler 9b is a coupler having a one-input-port and two-output-port configuration for splitting the output light from the signal light source 8 into two.

The phase modulator 2 modulates the phase of the light from the local light source 7 guided through the optical splitter / coupler 9a and outputs the light. In addition, the data modulator 4 outputs A to the output light from the signal light source 8 guided via the optical branching / combining device 9b by using, for example, a subcarrier signal fs corresponding to data.
The signal is subjected to M modulation and output as data modulated light.

The optical branching / combining device 43 has two input ports and two outputs for coupling one of the output lights of the local light source 7 bifurcated by the optical branching / combining device 9a and the data modulated light output from the data modulator 4. The coupler has a port configuration. One of the two output ports is an output port to the photodiode 44, and the other is an output port to the transmission line.

The optical splitter / coupler 12 combines the output of the phase modulator 2 and the output light of the signal light source 8 and multiplexes them. The photodiode 10 is a photodetector for detecting the multiplexed output. The low-pass filter 11 is for extracting a low-frequency component from the detection output of the photodiode 10, and the control device 46 controls the frequency and the frequency of the output light of the local light source 7 corresponding to the low-frequency component. In order to shift the optical phase, the local light source 7 is controlled.

A PLL control system for controlling the optical frequency and optical phase of the local light source 7 with reference to the output optically coupled by the optical branching / coupling unit 12 by the photodiode 10, the low-pass filter 11 and the control unit 46. ing.

The photodiode 44 is connected to the optical branching coupler 43.
The bandpass filter 45 is for extracting a predetermined band component from the detection output of the photodiode 44, and the control device 46 It also has a function of controlling the local light source 7 to shift the frequency of the output light from the local light source 7 in accordance with the extracted frequency component.

A frequency control system for controlling the optical frequency of the local light source 7 with reference to the transmission light output by the photodiode 44, the band-pass filter 45, and the control device 46.

The optical transmitter described here controls the optical frequency so that the signal light and the local light have a desired optical frequency difference, and multiplexes and transmits the local light to form a periodic signal. One of a plurality of sidebands generated by the modulation and the interference wave of the signal light is detected by a photodetector, so that the detected error signal maintains a constant DC value, The optical phase of the signal light or the local light is controlled.

In the case of the configuration shown in FIG.
And the local light source 7 are separately provided. The light from the signal light source 8 is modulated by the data modulator 4 to become data-modulated light, and is sent out toward the optical branching / combining device 43. The light from the local light source 7 is also directed toward the optical branching / combining device 43. After being multiplexed here, they are sent out to the transmission line as transmission output (transmission light).

As described above, the signal light and the local light are multiplexed by the optical splitter / coupler 43 with the polarization and the delay matched, and are transmitted. The adjustment of the polarization and the delay and the form of the data modulation are the same as those of the second and third embodiments described above.

In the example of FIG. 23, the local light and the signal before being subjected to modulation are branched. Phase modulator 2 for local light
, And the combined signal light, the polarization and the delay are combined and received by the photodiode 10.
Optical carrier frequency f0 of signal light and optical frequency fL of local light
When it is desired to make the difference fr, the frequency for modulating the branched local light is, for example, fr / n (where n is an integer of 2 or more).

The PLL is applied so that the n-th sideband (or -nth sideband) generated by the phase modulation and the optical frequency and optical phase of the signal light match. “2 × 2”
One of the outputs of the optical splitter / coupler 43 is transmitted as transmission light, while the other output is received by the photodiode 44, and the absolute value of the difference fc = fr−fd between the difference fd between f0 and fL and fr is calculated. , The pull-in range frequency fi of the PLL loop 68
The optical frequency of the local light or the signal light (local light in the figure) is controlled so as to be inside.

For example, a method of arranging a band-pass filter having a center frequency fr and a band of 2fi behind the photodiode 44 and controlling the optical frequency so that the power of the received signal passing through the filter becomes maximum is adopted. Used.

The purpose here is to prevent the signal light from being locked to a sideband other than the desired sideband, so that the following system may be used. First, the frequency fd of the interference wave signal detected by the photodiode 44 is equal to the frequency fr-fr / 2n.
<Fd <fr + fr / 2n (where n is an arbitrary integer).

For example, the optical frequency of the local light (or signal light) is controlled such that the band of the band-pass filter 45 is maximized so that the power of the transmitted electric signal is maximized.

The optical frequency of the local light (or signal light) is controlled so that the signal detected by the photodiode 10 falls within the pull-in frequency of the PLL while keeping the signal within the band of the filter 45. . For example, the optical frequency may be changed while the signal is passing through the band of the filter 45. When a signal within the pull-in range of the PLL is detected, the control of the optical frequency by the loop 69 is stopped,
Turn on the PLL loop so that it is controlled automatically.

A semiconductor laser used as a light source has a property that an oscillation wavelength changes according to an injection current. When a very small pulse-like current is passed in addition to the DC drive current, the optical frequency changes for a moment and returns to its original state, and the optical phase slightly changes. Thus, the optical phase of the semiconductor laser can be controlled. When a solid-state laser element is used as a light source, an external infinite phase modulator is preferably used.

In the branch for modulating the signal light, the data modulator is arranged after the branch. However, if the optical spectrum of the signal light obtained by the data modulation is composed of an optical carrier component and a subcarrier data component having a frequency slightly away from the optical carrier (outside the PLL pull-in range),
Since the PLL can be applied using the optical carrier component, the data modulator may be before the branch.

According to the fourth embodiment of the present invention, the error signal used for the PLL for synchronizing the phase while keeping the optical frequency interval between the signal light and the local light at a desired value is a signal near DC. Since the detection is performed at a low frequency, the cost of the system can be reduced. Further, since the frequency of the phase modulation is small, the configuration around the phase modulator becomes easy, and the cost can be reduced.

As described above, the optical transmitter of this embodiment is
In an optical transmitter that controls the optical frequency so that the signal light and the local light have a desired optical frequency difference, and multiplexes and transmits the signals, a plurality of sides generated by branching the local light and modulating with a periodic signal. An interference wave between one of the wavebands and the one obtained by splitting the signal light is detected by a photodetector, and the signal light or the light of the local light is emitted so that the detected error signal maintains a constant DC value. It is characterized by controlling the phase.

Separately from the light source of the data modulation light (signal light source) and the light source of the interference light (local light source), P
When LL is applied, when the beat is a high frequency such as a millimeter wave, an element having good high frequency characteristics must be used, and the cost becomes extremely high.

Therefore, here, the PLL is not performed in the high frequency band, but is performed in the low frequency band. That is, PLL
The error signal for multiplying is detected at a low frequency near DC.

As shown in FIG. 22, the light output from the signal light source 8 and the light output from the local light source 7 are respectively branched, and the local light is subjected to phase modulation with a repetitive signal such as a sine wave. The split light is combined again, detected by the photodetector 10, and the local light or the phase of the signal light is controlled by the error signal passed through the low-pass filter 11 so that the filter output becomes a constant DC value.

The signal light after branching to FIG.
FIG. 4B shows an optical spectrum of the local light after branching. The local light has undergone phase modulation to generate sidebands. In addition to the sideband of the frequency corresponding to the period of the phase modulation signal, a number of sidebands of the frequency corresponding to the harmonics are generated. The same optical phase noise as the optical phase noise of the light source is placed in the sideband.

Therefore, here, the signal light is locked by the PLL to the sideband corresponding to the harmonic.

If the desired sideband of the local light is caused to interfere with the signal light in such a manner that the signal light substantially overlaps, a component near the direct current of the signal detected by the photodetector becomes PL.
It becomes an L error signal, and synchronization becomes possible.

On the other hand, the signal light at the branch source and the local light are combined by another optical branching coupler 6 and transmitted. Since this local light is not subjected to phase modulation, only the fL component is transmitted.
The signal light is in the vicinity of f0, and the transmitted receiver detects beats of f0 and fL.

Therefore, according to the fourth embodiment of the present invention shown here as the fourth embodiment, since the error signal used in the PLL is detected at a low frequency near DC, a loop can be assembled with low-cost elements. Can be. Further, since the frequency of the phase modulation is only a fraction of the frequency of the desired millimeter wave band, the cost around the phase modulator can be reduced.

(Fifth Embodiment) Next, a fifth embodiment of the present invention will be described as a fifth embodiment.

FIG. 25 is a block diagram thereof. In the figure, 5
8 is a local light source, 59 is a signal light source, 60 is a data modulator,
61 is a phase modulator, 62 is an optical splitter / coupler, 63 is a photodiode, 64 is a low-pass filter, and 65 is a control device.

The signal light source 59 is an original light source to be subjected to data modulation, and is composed of, for example, a single longitudinal mode semiconductor laser device. Data modulator 6
0 is, for example, the output light from the signal light source 59,
The data is subjected to AM modulation by the data-compatible subcarrier signal fs and output as data-modulated light.

The local light source 58 is an original light source to be subjected to phase modulation, and is also formed of, for example, a single longitudinal mode semiconductor laser device like the signal light source. The phase modulator 61 is for phase-modulating the light from the local light source 58 and outputting the light.

The optical branching / combining unit 62 combines the output of the phase modulator 61 and the output of the data modulator 60 and multiplexes them. The photodiode 63 is a photodetector for detecting the multiplexed output. , Low-pass filter 64
Is for extracting a low frequency component from the detection output of the photodiode 63, and the control device 65 controls the local light source to shift the frequency of the output light of the local light source 58 corresponding to the low frequency component. 58 is controlled.
A PLL that controls the optical frequency of the local light source 58 with reference to the output optically coupled by the optical branching coupler 62 by the photodiode 63, the low-pass filter 64, and the control device 65.
Is composed.

In such a configuration, the signal light source 59,
For example, light output from a single longitudinal mode semiconductor laser device enters the data modulator 60, where it undergoes, for example, AM modulation by a subcarrier signal fs. The subcarrier frequency fs is much lower than the frequency fp when the optical carrier is down-converted into the desired reception band, for example, a frequency of about fp / 100.

On the other hand, the light output from the local light source 58 is subjected to optical phase modulation by a phase modulator 61 by a predetermined sine wave signal. The frequency of the phase modulation is specifically, for example, fp / n
It is. Note that n is an integer of 2 or more, for example, “4”,
“6”, “10”, etc.

In this system, similarly to the signal light source 59, the local light source 58 is constituted by a single longitudinal mode semiconductor laser element, and the semiconductor laser element has a characteristic that the oscillation light frequency changes depending on current and temperature. ing. Therefore, in this system, the frequency of the output light of the light source is controlled using this fact. That is, the local light source 58 and the signal light source 59 measure the characteristics of the current and the temperature versus the oscillation light frequency in advance, grasp the characteristics, and correspond to the characteristics.
Has a function of controlling the optical frequency of the local light source 58 based on the combined output so that the optical carrier of the signal light is locked to the sideband (not the optical carrier) of the local light. It is assumed that

Accordingly, the PLL controls the optical frequency of the local light source 58 corresponding to the multiplexed output so that the optical carrier of the signal light is locked to the sideband (not the optical carrier) of the local light. This constitutes a loop.

The object controlled by the PLL control loop may be a signal light source. This PLL control loop controls the optical frequency and the optical phase of the local light source or the signal light source, and the optical carrier of the signal light is transmitted to the local light source. It is responsible for locking to the sideband of the emission (not to the optical carrier).

That is, the current control device 65 first varies the optical frequency of the local light source 58 (or the signal light source 59), and the optical frequency difference between any one of the sidebands of the local light and the signal light becomes P.
Search for a point that falls within the LL pull-in range.

When the PLL enters the pull-in range,
The current control device 65 activates the PLL to lock the signal light to one of the sidebands of the local light.

If the optical carrier frequency of the signal light to be locked by the control by the PLL is likely to coincide with the optical carrier frequency of the local light of the signal light, the control device 65 forcibly sets the PLL. And the injection current to the light source is controlled so that the optical frequency is separated enough to lock to the sideband.

By controlling the light source by such a PLL, the influence of the phase noise of the light source can be avoided.

With respect to the local light obtained by controlling the light source in this way, the local light after phase modulation by the phase modulator 61 and the signal light from the signal light source 59 data-modulated by the data modulator 60 are light. The signals are multiplexed in the branch coupler 62. Then, a part of the multiplexed optical signal is transmitted to the transmission line to be a transmission output, and a part is transmitted to the photodiode 63 and used for the light source control by the PLL.

The optical branching coupler 62 is a 2 × 2 directional coupler, that is, a directional coupler having two input ports and two output ports, and the output light from one output port is an optical transmission line. And the output light from the other output port is detected by the photodetector 63.

The current control device 65 first sets the local light source 58
The optical frequency of the signal light source 59 (or the signal light source 59) is shaken to find a point at which the optical frequency difference between one of the sidebands of the local light and the signal light falls within the pull-in range of the PLL. For example, the current control device 65 can avoid the influence of the phase noise of the light source by controlling the PLL to lock the signal light to one of the sidebands of the local light by operating the PLL. Since the error signal to be used is a low-frequency signal in the vicinity of DC, it can be realized by control system hardware using an element having a low operation speed, so that the cost of the control system hardware can be reduced.

Further, according to this embodiment, any of the sidebands of the data component and the local light on the signal light always falls to a desired frequency band on the receiving side. As a result, there is no need to strictly set the frequency interval between the signal light and the local light to a desired frequency interval, and there is an advantage that the control is simplified.

In the embodiments described with reference to FIGS. 23 and 25, the description has been made such that a “2 × 2” optical directional coupler (coupler) is used as the optical branching coupler.
As shown in Fig. 6, once a "2 x 1" coupler (2 input ports 1
After merging by a coupler having an output port configuration), the signal may be branched again by a “1 × 2” coupler (a coupler having a one-input-port two-output-port configuration).

As described above, in the fifth embodiment of the fifth invention, in the optical transmitter for multiplexing the signal light and the local light by the optical branching coupler and transmitting the signal light, the signal light is modulated by the subcarrier signal. Wherein the local oscillation light is modulated with a periodic signal, one of the plurality of outputs of the optical branching coupler is transmitted as the output of the optical transmitter, and another one is detected by a photodetector, A configuration for controlling the optical phase of the signal light or the local light so that an error signal due to interference between one of a plurality of sidebands generated in the local light and an optical carrier component of the signal light maintains a constant DC value. It is characterized by having done.

That is, in the fourth invention of the present application described in the fourth embodiment, the local light and the signal light are branched off from the main line to generate an error signal, but in the fifth invention of the present application, the branching is not performed. The main line of the local light is phase-modulated so as to be multiplexed with the signal light that has been subjected to data modulation.

An optical branching coupler used for multiplexing has a plurality of output ports. This is for the following reason. That is, in a directional coupler (coupler) often used as an optical branching coupler, the basic form when two inputs are merged is two outputs. Of course, a one-output structure can also be manufactured, but this can be regarded as a form in which one of the two outputs is discarded. Receive.

Therefore, in the present invention, a 2 × 2 coupler is used, one of the outputs is sent to a receiver, and the other output is used to form a PLL for synchronizing the optical phases of the local light and the signal light. Multiply. The signal light used here is obtained by subjecting an optical carrier to subcarrier modulation using a band signal as shown in FIG. There is a sideband on which an optical carrier component and data are superimposed, and the optical frequency of the optical carrier and the optical frequency of the sideband are separated from each other by more than the PLL pull-in range.

According to the fifth aspect, (a) and (b) of FIG. 24 are transmitted as they are. Although any one sideband of the local light is synchronized with the optical carrier component of the signal light, any sideband of the local light with which the optical carrier of the signal light is synchronized may be used. There is no need to select sidebands such that fL and f0 are separated by the required value as in the case of the fourth invention. In this way, when received, an interference wave with any sideband falls to a desired frequency band. Two sidebands separated from the signal light by plus and minus by a required frequency difference simultaneously fall to a desired frequency.

However, when phase modulation is applied to the optical carrier,
Although it depends on the degree of modulation, generally, the envelope as shown in FIG. 28 has a mountain-shaped spectrum. Therefore, if fL and f0 are appropriately separated from each other, there is a significant difference in the size of the sideband separated from the positive side and the sideband separated from the negative side, and when an interference wave with the signal light is received, Interference with the larger sideband becomes dominant. The level of the interference wave slightly changes due to the change in the phase difference between the plus sideband and the minus sideband,
If the level difference between the plus sideband and the minus sideband is large, the amount of change is small and does not matter.

According to the fifth aspect of the present invention, it is not necessary to strictly control the wavelengths of the signal light and the local light, and the cost can be reduced. In addition, since the number of branches in the system is small, the loss is small, and thus a system capable of higher quality transmission is provided.

[0216]

As described above in detail, according to the present invention, there is provided a method for producing a high frequency signal such as a millimeter wave by optical heterodyne at a low cost so as to avoid the influence of the phase noise of the light source, which is a problem. Proposed.

In the first invention, the same light source is branched into two, one of which is subjected to data modulation, and the other generates an optical spectrum line when the optical frequency is shifted by a predetermined value. When the two are merged, the phase noise is automatically canceled because the light sources are the same.

In the second and third inventions, the phase noise is always synchronized because the data modulated light and the phase modulation sideband for generating the beat are generated from the same light source. Is automatically canceled. Further, since phase modulation is used, the modulation system is simplified.

In the fourth and fifth inventions, the signal light source and the local light source are separately prepared, but the local light is phase-modulated at a frequency that is an integer fraction of the desired reception frequency, and the signal is generated in the generated sideband. Lock the light. Since the frequency of the phase modulation is low, the cost around the phase modulator can be reduced. Also, PL
Although L is required, an error signal for that is received at a low frequency close to DC, so that the cost of the system can be reduced.

[Brief description of the drawings]

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

FIG. 2 is a diagram for explaining the present invention, and is a waveform diagram for explaining the first embodiment of the present invention.

FIG. 3 is a diagram for explaining the present invention, and is a block diagram for explaining a second embodiment of the present invention.

FIG. 4 is a diagram for explaining the operation of the present invention.

FIG. 5 is a diagram for explaining the operation of the present invention.

FIG. 6 is a diagram for explaining the present invention, and is a block diagram for explaining a modification of the second embodiment of the present invention.

FIG. 7 is a diagram for explaining the operation of the present invention.

FIG. 8 is a diagram for explaining the operation of the present invention.

FIG. 9 is a diagram for explaining the operation of the present invention.

FIG. 10 is a diagram for explaining the operation of the present invention.

FIG. 11 is a diagram for explaining the operation of the present invention.

FIG. 12 is a diagram for explaining the operation of the present invention.

FIG. 13 is a diagram for explaining the operation of the present invention.

FIG. 14 is a diagram for explaining the operation of the present invention.

FIG. 15 is a diagram for explaining the present invention, and is a block diagram for explaining a modification of the second embodiment of the present invention.

FIG. 16 is a diagram for explaining the present invention, and is a diagram for explaining a configuration example in a case where the device configuration in the second embodiment of the present invention is integrated.

FIG. 17 is a diagram for explaining the present invention, and is a block diagram for explaining a modification of the second embodiment of the present invention.

FIG. 18 is a diagram for explaining the present invention, and is a block diagram for explaining a modification of the second embodiment of the present invention.

FIG. 19 is a diagram for explaining the operation of the present invention.

FIG. 20 is a diagram showing one embodiment of the present invention.

FIG. 21 is a diagram for explaining the operation of the present invention.

FIG. 22 is a diagram for explaining the present invention, and is a diagram for explaining a device configuration example according to the fourth embodiment of the present invention.

FIG. 23 is a diagram for explaining the present invention, and is a diagram for explaining an example of a device configuration according to a fourth embodiment of the present invention.

FIG. 24 is a diagram for explaining the operation of the present invention.

FIG. 25 is a diagram for explaining the present invention, and is a diagram for explaining an example of a device configuration according to a fifth embodiment of the present invention.

FIG. 26 is a diagram for explaining the present invention, and is a diagram showing a configuration example of some elements of the device according to the fourth embodiment of the present invention.

FIG. 27 is a diagram for explaining the operation of the present invention.

FIG. 28 is a diagram for explaining the operation of the present invention.

FIG. 29 is a view for explaining the operation of the present invention,
FIG. 4 is a waveform diagram illustrating an example of a data-modulated reception signal.

FIG. 30 is a diagram for explaining a conventional example.

FIG. 31 is a diagram for explaining a conventional example.

FIG. 32 is a diagram for explaining a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Phase modulator 3, 18, 31 ... Optical filter 4 ... Data modulator 5, 9a, 9b, 12, 21, 22, 38, 43, 4
7, 48, 52, 58, 62 optical branching coupler 7 local light source 8 signal light source 10, 20 photodiode 11 low-pass filter 13 optical filter transmission characteristics 14, 16 filter transmission characteristics 15 Components detected for control 17 Optical circulator 19, 46, 65 Controller 23 Optical delay line 24 Optical fiber amplifier 25 Semiconductor laser 26 Optical waveguide 27, 28, 33 Electrode 29, 61 Phase modulation Apparatus 30 AM modulator 33 Grating 34, 35, 36 Y branch 37 Optical transmitter 39 Optical receiver 40 Comb generator 41 Data transmitter 42 Receiver 44, 53, 63, 66 ... Photodiode 45 ... Bandpass filter 49 ... Local light source 50,59 ... Signal light source 51,60 ... Data modulator 54 ... Amplifier 55 ... Reference signal generator 56 ... Mixers 57,64 ... Low-pass filter 67 ... Main fiber 68 ... PLL loop 69 ... Frequency control loop

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/02 10/18 10/152 10/142

Claims (5)

[Claims] A single longitudinal mode light source; branching means for branching light output from the light source; data modulation means for modulating one of the lights branched by the branching means with a data signal; Processing the other light branched by the branching means, to an optical frequency separated by a predetermined frequency from the optical frequency of the light source,
An optical spectrum generating means for generating an optical spectrum line shifted from an optical spectrum of the light source, and a coupler for combining and outputting output light of the data modulating means and output light of the optical spectrum generating means. An optical transmitter. 2. A single longitudinal mode light source; branching means for branching light output from the light source; data modulation means for modulating one of the lights branched by the branching means with a data signal; Phase modulation means for phase-modulating the other light branched by the branching means with a periodic signal, an optical filter for filtering output light from the phase modulation means, output light from the data modulation means, and output from the optical filter Coupling means for coupling and outputting light, wherein the frequency of the periodic signal is a frequency between an optical frequency of the output light of the data modulation means and an optical frequency of a specific sideband in the output light of the phase modulation means. The difference is determined to be a desired value, the optical filter, the sideband, for a sideband at an optical frequency symmetrical with respect to the optical frequency of the light source of the sideband, one of the Through An optical transmitter characterized by having a characteristic of passing through. 3. The periodic signal is a sine wave, and its frequency is 1 / n (n is an integer of 2 or more) of the frequency difference between the sideband giving the desired value and the optical frequency of the light source. The optical transmitter according to claim 2, wherein: 4. An optical transmitter which controls an optical frequency so that a signal light and a local light have a desired optical frequency difference, and transmits the signal light and the local light in a multiplexed manner. And a photodetector detects one of a plurality of sidebands generated by modulating this with a periodic signal and an interference wave obtained by the signal light, and detects the detected error signal. An optical transmitter comprising control means for controlling an optical frequency and an optical phase of the signal light or the local oscillation light so that a constant DC value is maintained. 5. A light source having a signal light source and a local light source, wherein light from the signal light source is modulated by a modulating means into signal light, and multiplexed with light from the local light source by an optical branching coupler. In the optical transmitter, the optical branching coupler has a plurality of output ports, and the modulating means modulates light from a signal light source with a subcarrier signal and outputs the modulated light. The light from the local light source is phase-modulated by a periodic signal and provided to the optical branching coupler, and one of a plurality of outputs of the optical branching coupler is detected by a photodetector. Using the detection output, the signal light or the light of the local oscillation light such that an error signal due to interference between one of the plurality of sidebands generated in the local oscillation light and the optical carrier component of the signal light maintains a constant DC value. Control means for controlling frequency and optical phase An optical transmitter, comprising: