JP3831393B2 - Optical transmitter - Google Patents

Description

  The present invention relates to an optical transmitter that transmits a high-frequency signal such as a millimeter wave through an optical fiber.

  In wireless communication, a millimeter wave band with a large transmission capacity is expected. However, the millimeter-wave band has a drawback that the attenuation in the air is large, which hinders the expansion of the communication area. Expansion of the communication area is an unavoidable issue from the viewpoint of increasing future communication demand and providing precise communication services.

  Accordingly, various attempts have been made to develop technology for expanding the communication area in millimeter wave band communication. For example, a system has been proposed in which a millimeter-wave band signal to be transmitted is converted into an optical signal and transmitted far away by an optical fiber.

  There are various methods for transmitting high-frequency signals such as millimeter waves through optical fibers, one of which is optical heterodyne.

  This optical heterodyne is a system as shown in FIG. 30. In the system shown in FIG. 30, the signal light in which data is placed on the optical frequency f0 by a low-frequency signal close to the baseband, and this signal light has a slight optical frequency. This is a system in which unmodulated local light having a frequency of fL is mixed and transmitted after being combined with polarized light, and this is received by a photodiode on the receiving side.

  The optical spectrum on the transmission side in the optical heterodyne is shown in FIG. 31 (a), and the spectrum of the received light is shown in FIG. 31 (b). Since the photodiode used for receiving the optical signal is an ideal square detector, the signal light and the local light beat are received. In the received electrical signal, data near the baseband that was on the signal light is frequency-converted to the vicinity of the frequency of the optical frequency difference between the signal light and the local light. At this time, if the frequency difference is a millimeter-wave band frequency, it is converted to a millimeter-wave frequency. In this way, a millimeter wave can be generated at the receiver without directly modulating the light with the millimeter wave signal.

  However, a semiconductor laser element generally used as a light source for optical communication has a large phase noise, which is usually several [MHz] in terms of oscillation line width, and several tens [kHz] even if an external resonator is used. is there. Then, in the millimeter wave obtained by optically heterodyning these, the optical phase noise is down-converted, and only a signal with a large noise can be obtained.

  In order to avoid this, a method of applying PLL (phase-locked loop control) to the optical phases of the local light and the signal light has been proposed. 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 a photodiode 53, and this is amplified by an amplifier 54 and mixed. 56, the reference signal from the reference signal generator 55 is mixed, the low-pass 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 signal light so that the phase noise of the beat signal is eliminated.

  Then, the light from the signal light source 50 is modulated in correspondence with the data by the data modulator 51, and this is combined with the local light from the local light source 49 by the optical branching coupler 52, mixed, and transmitted to the optical fiber. To.

  However, in the case of a method in which a PLL is applied to the optical phase of local light and signal light in this way, the beat signal is difficult to handle at a high frequency such as a millimeter wave and the amplifier must be handled at a high frequency. In addition, a mixer, a filter, etc. that have good high frequency characteristics must be used, resulting in an increase in cost.

  One method for transmitting high-frequency signals such as millimeter waves through optical fibers is optical heterodyne. This is a signal light in which data is placed at the optical frequency f0 by a signal having a low frequency close to the baseband or the baseband, and this signal light is polarized with unmodulated local light having a frequency fL slightly separated from the optical frequency. In this method, the signals are mixed and transmitted, and the receiving side receives them with a photodiode. A low-cost semiconductor laser element is used as a light source for optical communication.

  However, the semiconductor laser element has a large phase noise, and the optical phase noise is down-converted in the millimeter wave obtained by optical heterodyne, resulting in a noisy signal and an S / N (signal / noise ratio). Deteriorate.

  In order to avoid this, a method of applying a PLL to the optical phase of local light and signal light has been proposed, but beat signals are difficult to handle at high frequencies such as millimeter waves, avoiding an increase in device cost. I couldn't.

  Therefore, in optical communication, even when high frequencies such as millimeter waves are generated by optical heterodyne, the problem of optical phase noise of a semiconductor laser element as a light source can be avoided, and a simple technique that enables cost reduction is urgently required. Development is envied.

  Accordingly, an object of the present invention is to easily avoid the influence of the optical phase noise of the semiconductor laser element as the light source even in the case where a high frequency such as millimeter wave is generated by optical heterodyne in optical communication. It is an object of the present invention to provide an optical transmitter that enables inexpensive and good S / N optical communication.

  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 branching device for branching the light output from the light source, a data modulator for modulating one of the branched lights with a data signal, Generating means for processing the other of the branched light to generate an optical spectrum line obtained by shifting 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 the data modulation An optical transmitter comprising a coupler for combining the output light of the transmitter and the output light of the generating means is provided.

  The optical transmitter having such a configuration emits light in a single longitudinal mode from a light source, and this light is split into two systems by a branching device, and one of them is a data modulator and a modulation of a relatively low frequency corresponding to data. The other is made into an optical spectrum line in the form of an optical spectrum shifted 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 path.

  The optical spectrum generated by the optical spectrum line generating means is separated from the optical signal that is data-modulated by the data modulator and output by a desired optical frequency f1, and has the same optical phase noise as the light source, and Data modulated light also has the same optical phase noise as the light source. For this reason, when they are combined and interfered by the coupler, the optical phase noise is automatically canceled and the received signal is not affected.

  In this way, in the form in which the light source is made common, both of the two branched lights inherit the optical phase noise of the common light source, so the same optical phase noise is on, and among the two branched lights, One light can cancel the influence of the optical phase noise when the light spectrum is shifted by the desired frequency f1 and then merged with the other light of the two branched lights.

  Therefore, according to the present invention, it is possible to obtain a received signal that is not affected by optical phase noise at all while using a method equivalent to optical heterodyne.

  [2] In the second invention of the present application, a single longitudinal mode light source, a branching device that branches the light output from the light source, a data modulator that modulates one of the branched lights with a data signal, and a branch A phase modulator that modulates the phase of the other modulated light by a periodic signal, an optical filter that filters the output light of the phase modulator, and the output light of the data modulator and the output light of the optical filter The frequency of the periodic signal has a desired frequency difference between the optical frequency of the output light of the data modulator and the optical frequency of a specific sideband of the output light of the phase modulator. The optical filter is configured to pass either one of the sidebands and sidebands having optical frequencies that are symmetrical with respect to the optical frequency of the light source of the sidebands. An optical transmitter is provided.

  The light output from the single longitudinal mode light source is bifurcated, and one of the light is modulated at a subcarrier frequency fs close to the baseband. The data-modulated light spectrum is as shown in FIG. The other light is phase-modulated by a periodic signal having a frequency f1. The phase-modulated optical spectrum is as shown in 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. Further, sidebands in which data is carried at f0 ± fs are generated in the data-modulated light. 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, obtains phase-modulated light from light output from the light source by modulation with a periodic signal, and obtains data-modulated light by modulation with a data signal. Phase modulation is performed to obtain harmonic components, and even if phase modulation is used, if it is phase modulation using a periodic signal, the same sidebands as when intensity modulation is used, that is, harmonic components Is used in the present invention. The phase modulator pays attention to the fact that the cost of the apparatus is lower than that in the case of using intensity modulation.

  The phase-modulated light and the data-modulated light thus obtained are combined to interfere with each other. That is, when data-modulated light (FIG. 4 (a)) and phase-modulated light (FIG. 4 (b)) of the same light source interfere with each other, spectral lines of the phase-modulated light and data-modulated light are generated. Since both phase-modulated light and data-modulated light use light from the same light source, and both have the same phase noise, interference between the two causes phase noise. Since the influence is canceled and disappears, an optical signal having no 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 interfered with the data-modulated light and detected by a photodetector having a square detection characteristic, but interference waves from the sidebands on both sides are detected at the same reception frequency as it is. The size 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 giving 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) at a symmetrical position across the optical carrier is relatively wide. . If there are no restrictions on components other than the desired sideband and its image, the stability accuracy of the transmitted light frequency of the optical filter may be low, and the system can be simplified.

  Thus, the system of the present invention is characterized by using an optical modulator. Then, a general light modulator is a lithium niobate modulator, and in the case of the lithium niobate modulator, based on a form in which phase modulation is applied to light using a change in refractive index due to an applied voltage, Various modulators can be configured by combinations and modifications thereof.

  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. Then, paying attention to this point, in the present invention, the light from the same light source is bifurcated, one is data-modulated, the other is phase-modulated and then combined to cancel the phase noise of the light source. I made it.

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

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

  In the invention shown in the above item [2], the example in which the sideband closest to the optical carrier f0 is used 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, phase modulation must be performed at a millimeter wave frequency, which increases costs. In phase modulation, the power of the sideband of the harmonic component of the modulation frequency increases as the modulation depth is increased.

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

  In the example of FIG. 4, the phase modulation frequency f1 is determined so that the desired frequency is obtained when an interference wave generated by interference between f0 + 2f1, f0 + 3f1,.

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

  [4] Next, in the fourth invention of the present application, 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 combining and transmitting, the local light is branched. An interference wave between one of a plurality of sidebands modulated and generated by a periodic signal and the signal light branched is detected by a photodetector, and the detected error signal has a constant DC value. An optical transmitter characterized by controlling an optical frequency and an optical phase of the signal light or the local light so as to keep the signal light.

  In the case of a system in which a data modulation light source (signal light source) and an interference light source (local light source) are separated and a PLL is applied to those beats, if the beat is a high frequency such as a millimeter wave, the high frequency characteristics A good element must be used, which increases the cost.

  Therefore, in the fourth invention of the present application, the PLL is not performed in the high frequency band, but is performed in the low frequency band. That is, an error signal for applying the PLL is detected at a low frequency near the direct current. As shown in FIG. 22, the lights output from the signal light source 8 and the local light source 7 are branched, and the local light is phase-modulated with a repetitive signal such as a sine wave. The branched light is recombined and detected by the photodetector 10, and the optical frequency and light of the local light or signal light are adjusted so that the filter output becomes a constant DC value by the error signal that has 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 sidebands having the frequency corresponding to the period of the phase modulation signal, many sidebands having the frequencies corresponding to the harmonics are generated. The sideband has the same optical phase noise as the optical phase noise of the light source.

  Therefore, in the present invention, the signal light is locked to the sideband corresponding to the harmonic by the PLL. If interference is performed in a state where the desired sideband of the local light and the signal light are approximately overlapped in advance, the component near the direct current 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 of the branch source and the local light are combined and transmitted by another optical branching coupler 6. 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 in the vicinity of f0, and the beat of the optical carrier frequency f0 of the signal light and the optical frequency fL of the local light is detected in the transmitted receiver. According to the fourth aspect of the present invention, since the error signal used in the PLL is detected at a low frequency near the direct current, a loop can be assembled with low-cost elements. In addition, since the phase modulation frequency is about a fraction of the desired millimeter wave band frequency, the cost around the phase modulator can be reduced.

  [5] Further, in the fifth invention of the present application, in the optical transmitter for combining the signal light and the local light with an optical branching coupler and transmitting the optical signal, the signal light is modulated with a subcarrier signal, The light emission is modulated by a periodic signal, one of a plurality of outputs of the optical branching coupler is transmitted as the output of the optical transmitter, and the other one is detected by a photodetector and generated in the local light. Controlling the optical frequency and optical phase of the signal light or the local light so that an error signal due to interference between one of the plurality of sidebands and the 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, but in the fifth invention of the present application, it is not branched, and the main line of the local light is subjected to phase modulation and data modulation is performed. Combines with 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. One output can also be produced, but this can be regarded as a form in which one of the two outputs is discarded, and even with one output port, it suffers the same loss as when there are 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 apply a PLL that synchronizes the local light and the optical phase of the signal light. The signal light used here is an optical carrier that has been subjected to subcarrier modulation by a band signal, and has optical carrier components and sidebands carrying data, and the optical frequency and sidebands of the optical carrier. The optical frequency is set to be more than the PLL pull-in range.

  According to a fifth aspect of the invention, the local light phase-modulated light and the signal light are transmitted as they are. Any one sideband of the local light and the optical carrier component of the signal light are synchronized, but any local band of the local light to which the optical carrier of the signal light is synchronized may be used. As in the case of the fourth invention, it is not necessary to select a sideband in which the optical frequency fL of local light and the optical carrier frequency f0 of signal light are separated by a required value. If it does in this way, when it receives, the interference wave with a certain sideband will fall in a desired frequency band. Centering on the signal light, two sidebands separated by plus and minus by the required frequency difference simultaneously drop to the desired frequency. However, when phase modulation is performed on an optical carrier, an envelope as shown in FIG. 28 generally forms a mountain-shaped spectrum, depending on the degree of modulation. Therefore, if the local light frequency fL and the optical carrier frequency f0 of the signal light are appropriately separated, there is a significant difference in the size of the sidebands that are separated positively and the sidebands that are separated negatively, When an interference wave with the signal light is received, the interference wave with the larger sideband becomes dominant.

  In the fifth invention of the present application, it is not necessary to strictly control the wavelength of the signal light and the local light, and the cost can be reduced. Further, since there are few branches in the system, there is little loss and higher quality transmission is possible.

  The present invention has proposed a method for avoiding the influence of the phase noise of the light source, which is a problem, at low cost when generating a high-frequency signal such as a millimeter wave by optical heterodyne.

  In the first invention, the same light source is bifurcated, data modulation is applied to one, and the other generates an optical spectral line where the optical frequency is shifted by a predetermined value. When both are merged, the phase noise is automatically canceled because the source is the same light source.

  In the second and third inventions, the phase noise is always synchronized to generate the data modulation light from the same light source and the phase modulation sideband for generating the beat, and the influence of the phase noise is automatically Canceled by 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 prepared separately, but the local light is phase-modulated at a frequency that is a fraction of the desired reception frequency, and the signal light is locked to the generated sideband. To do. Since the phase modulation frequency can be low, the cost around the phase modulator can be reduced. Further, although a PLL is required, an error signal for that purpose is received at a low frequency close to direct current, so that the cost of the system can be reduced.

  Hereinafter, various embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram of an optical transmitter showing an embodiment of the first invention of the present application. That is, in FIG. 1, 1 is a light source, 4 is a data modulator, 5 and 6 are optical branching couplers, and 70 is an optical spectral line generating means.

  The light source 1 generates light in a single longitudinal mode, and is configured using, for example, a semiconductor laser element. The optical branching coupler 5 is for branching the light output from the light source 1, and the data modulator 4 modulates one of the lights branched by the optical branching coupler 5 with a data signal. It is meant to be applied.

  Further, the optical spectral line generating means 70 processes the other one of the lights branched by the optical branching coupler 5 and changes the optical frequency of the light source to an optical frequency separated from the optical frequency of the light source by a predetermined frequency. An optical spectrum line with a shifted optical spectrum is generated. The optical branching coupler 6 combines the output light of the data modulator 4 and the output light of the optical spectral line generating means 70. The light coupled through the optical branching coupler 6 is finally obtained. Output light.

  The optical transmitter configured as described above emits light in a single longitudinal mode from the light source 1. The light output from the light source 1 is branched into two systems by the optical branching coupler 5. One is subjected to modulation at a relatively low frequency by the data modulator 4. The data modulation format in the data modulator 4 is AM modulation using a subcarrier signal. The other of the two branches branched by the optical branching coupler 5 is input to the optical spectral line generating means 70, where the optical spectrum of the light source 1 is shifted to the optical frequency separated by a predetermined frequency. Become 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 that shapes the output optical spectrum thereof is used. Can be realized.

  The output of the data modulator 4 and the output light of the optical spectrum line generating means 70 are combined by the optical branching coupler 6 and sent out to an optical fiber which is an optical transmission line. However, when coupling is performed by the optical branching coupler 6, the optical path lengths of both branches are adjusted so that the delay difference between them is within the coherence length of the light source 1, and the polarization is combined so as to be merged with the same polarization. It is better to compose the whole with cables and devices that hold waves.

  That is, the optical path length from the optical branching coupler 5 to the optical branching coupler 6 via the data modulator 4 and the optical branching coupler 5 to the optical branching coupler 6 via the optical spectral line generating means 70. The optical path length 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 spectral line generating means 70 is separated from the optical signal that is data-modulated by the data modulator 4 and output by a desired optical frequency, and has the same optical phase noise as the light source 1. .

  Since the data-modulated light also has the same optical phase noise as that of the light source 1, if the optical branching coupler 6 joins and interferes the optical phase noise, the optical phase noise is automatically canceled and affects the received signal. Does not appear.

  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. The light having a frequency of f0 is bifurcated. Then, data modulation is performed by the data modulator 4 on one of the two branched lights. The optical spectrum subjected to this data modulation is, for example, as shown in FIG.

  The other branched light is applied to the optical spectral 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 from the f0 frequency by f1 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 by the optical branching coupler 6 and transmitted, the receiver modulates the data to f1 by these interferences as shown in FIG. A signal is received.

  As described above, in the form in which the light source is shared, both of the two branched lights inherit the optical phase noise of the common light source 1, so the same optical phase noise is on the one of the two branched lights. Can be made to interfere with each other by causing the optical spectrums to be shifted by the desired frequency f1, thereby canceling the influence of the optical phase noise.

  Therefore, according to the present invention, it is possible to obtain a received signal that is not affected by optical phase noise at all 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, that is, a laser element, a branching device that branches the light output from the light source, and data in one of the branched lights. A data modulator that modulates according to a signal, and an optical spectrum line obtained by shifting the optical spectrum of the light source to an optical frequency that is processed by processing one of the branched lights and is separated from the optical frequency of the light source by a predetermined frequency. Generating means, and a coupler that combines the output light of the data modulator and the output light of the generating means, and splits the light from one light source to generate one optical spectrum line The optical spectrum line generating means shifts the light spectrum of the light source to an optical frequency separated by a predetermined frequency from the optical frequency of the light source. And the other branched light is data-modulated by a data modulator and output, and the data-modulated optical signal and the optical spectrum line are combined by a multiplexer. Is the final output.

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

  Since the data-modulated light also has the same optical phase noise as the light source, the optical phase noise is automatically canceled when they are combined and interfered by the coupler, and the received signal is not affected. Therefore, according to the optical transmitter of this embodiment, in the optical heterodyne, the optical spectral line generating means is used on the transmission side, and it is not affected by the optical phase noise of the light source. Therefore, the S / N is good and inexpensive. An optical communication device can be provided.

  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, thereby reducing the cost of the modulator. This will be described as an 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 couplers.

  The light source 1 generates light in a single longitudinal mode, and is composed of, for example, a semiconductor laser element. The optical branching / coupling device 5 is for bifurcating the light output from the light source 1, and the data modulator 4 is for modulating one of the bifurcated light by a data signal. Is.

  The phase modulator 2 is for applying phase modulation to another one of the branched light by a periodic signal, and the optical filter 3 converts the output light from the phase modulator 2 into a predetermined band. Only the components are filtered to be extracted.

  The optical branching coupler 6 couples and combines the output light from the data modulator 4 and the output light from the phase modulator 2 obtained via the optical filter 3, and an optical fiber as a transmission path. It is for output to.

  The frequency of the periodic signal is determined so 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 becomes a desired value. The optical filter 3 is set so as to pass either the sideband or the sideband that is at a symmetric optical frequency with respect to the optical frequency of the light source in the sideband.

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

  The data signal input to the data modulator 4 is a data signal to be transmitted, and is a signal having a relatively low frequency such as a baseband or an intermediate frequency. Further, 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, it is a sine wave signal around 60 [GHz]. .

  What is used for the light source 1 is a single longitudinal mode light source. Specifically, a semiconductor laser element such as a DFB laser element and a DBR laser element, or a solid-state laser element such as an Nd: YAG 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 are included such as applying phase modulation to light or passing through an optical filter, so that there is a considerable tendency to increase optical loss. Since a good signal-to-noise ratio is required at the time of transmission, the loss should be as small as possible. Therefore, a lithium niobate modulator is suitable for constructing the device of the present invention.

  When the data modulation method in the data modulator 4 is an AM modulation (amplitude modulation) method using a signal obtained by placing a data signal on the subcarrier frequency fs, the light subjected to the data modulation is as shown in FIG. That is, when AM modulation is applied with the subcarrier frequency fs, sidebands with data components are generated near f0−fs and 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 downconvert the sideband at f0 + fs to the desired frequency fr at the receiving side, the subcarrier frequency fs or the frequency f1 of the sine wave signal input to the phase modulator is changed to f1− What is necessary is just to determine so that it may become fs = fr.

  In the phase modulator 2, the light phase-modulated by the sine wave signal having the frequency f1 has an optical spectrum in which sidebands appear on both sides of the frequency f1 around f0 as shown in FIG. 4B. . These sidebands have exactly the same phase noise as that of the light source 1.

As described above, the sideband in which data is carried at f0 ± fs is generated in the light that is data-modulated at the subcarrier frequency fs and output from the data modulator 4. In the system of the present invention, f1 and fs are determined so that the frequency f1-fs is a desired frequency.
One of the sidebands “f0 + f1” and “f0−f1” is removed by the optical filter 3. 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 periodic blocking type filter are often used as optical filters. However, the periodic type filter also has the above-described ““ f0 + f1 ”depending on a transmission band (blocking band) other than the transmission band (or blocking band) to be used. It can be used as long as the function of “transmitting one of“ f0-f1 ”and blocking the other” is not impaired.

FIG. 5B shows the spectrum of the phase-modulated light that has passed through the optical filter. This is an example using a band pass filter, and there is an end of the pass band of the filter between “f0−f1” and “f0 + f1”, and the “f0 + f1” component is passed.
The light shown in FIG. 5B is combined by using the signal light as shown in FIG. 5A and a coupler so that the polarizations are matched so that the interference is maximized. For this purpose, a polarization controller may be used, or it may be configured with components that maintain polarization in the entire system.

  When merging, a branch for phase modulation (path on the phase modulation system side) and a branch for data modulation (path on the data modulation system side) from branching at the optical branching coupler 5 to joining at the optical branching coupler 6 The delay difference is made sufficiently small. This is because, in order to cancel the influence of phase noise when interference occurs, it is necessary to multiplex 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 (routes) is within this range, there is almost no problem, but in order to reduce the influence of phase noise as much as possible, it is within the range of about 1/10 or less of the coherence length. Is desirable.

  In most cases, it is only necessary 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, as shown in FIG. 6, it may be adjusted by inserting a delay adjusting fiber 23 or the like in the other branch.

When the combined 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 data components are received around f1, 2f1, and 3f1 due to interference between the data-modulated light and the respective components of the phase-modulated light that has passed through the optical filter. In order to extract the signal at f1-fs on the receiving side, it is only necessary to filter the received signal using a bandpass filter 14 for electric signals 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, when it interferes with the sidebands of the phase modulated light having the same phase noise, the phase noises cancel each other, and the phase noise is It will be cancelled. As a result, phase noise due to the line width of the light source is not detected in the received electrical signal, and good reception is possible.

  The main points and reasons of this embodiment are summarized as follows. That is, in this 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 bifurcated. 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 as shown in FIG. 4 (a), and the phase-modulated optical spectrum is as shown in FIG. 4 (b). 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. Further, sidebands in which data is carried at f0 ± fs are generated in the data-modulated light. In the present invention, f1 and fs are determined so that the frequency f1-fs becomes a desired frequency.

Here, in this embodiment, the light source is shared, 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 by modulation with a data signal. For the following reason.
In other words, phase modulation is performed in order to obtain harmonic components, but even if phase modulation is used, if the phase modulation is based on a periodic signal, sidebands similar to those in the case of using intensity modulation, that is, harmonic components are generated. Is being used. The phase modulator pays attention to the fact that the cost of the apparatus is lower than that in the case of using intensity modulation.

  The phase-modulated light and the data-modulated light thus obtained are combined to interfere with each other, thereby canceling phase noise of the light source. That is, as shown in FIG. 4, when the data-modulated light (FIG. 4 (a)) and the phase-modulated light (FIG. 4 (b)) of the same light source interfere with each other, Data modulated light interferes. Both the phase-modulated light and the data-modulated light use the light of the same light source, and therefore the same phase noise is present, so if they interfere with each other, the influence of the phase noise is cancelled. Disappear. Therefore, with this configuration, a received signal without phase noise can be obtained.

  Here, side-bands are formed symmetrically on both sides of the optical carrier f0 in the phase-modulated light, and this is a problem if it remains as it is. That is, if phase modulated light having sidebands on both sides of the optical carrier f0 is interfered with the data modulated light and detected by a photodetector having a square detection characteristic, interference from both sidebands at the same reception frequency occurs. A wave is detected. The size 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 giving a desired reception frequency is blocked by the optical filter. For example, as shown in FIG. 4, the sideband wave has two symmetrical components f0−f1 and f0 + f1 centered on the optical carrier having the frequency of f0, but only one of these two sideband waves f0 ± f1. One sideband wave is allowed to pass through using an optical filter having a filtering characteristic that passes the light. Components other than these two sidebands, such as sidebands in the optical carriers f0, f0 ± 2f1, f0 ± 3f1,..., Are either blocked by the optical filter or allowed to pass through. But it ’s okay. When such a component remains, for example, if a component of f0 remains, in this case, the component of f0 interferes with f0 + f1, and a beat is generated in a desired band when received. However, there is no problem if the data does not contain 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 change include phase fluctuations between components causing interference, power fluctuations of components, and the like, which are caused by changes in optical fiber length, fluctuations in wavelength characteristics of optical filters, and the like. However, the rate of change at that time is very slow, such as daily or monthly. In general, pure DC in baseband signals and pure carrier components in band signals rarely have information, and even if the level or phase of the carrier itself changes from day to day or from month to month, information is transmitted. Does not affect. Therefore, there is no problem even if components other than the desired sideband to be obtained, such as f0, 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) at a symmetrical position across the optical carrier is relatively wide. . If there are no restrictions on components other than the desired sideband and its image, the stability accuracy of the transmitted light frequency of the optical filter may 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. Then, a general light modulator is a lithium niobate modulator, and in the case of the lithium niobate modulator, based on a form in which phase modulation is applied to light using a change in refractive index due to an applied voltage, Various modulators are configured by combinations and modifications thereof.

  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. Then, paying attention to this point, in the present invention, the light from the same light source is bifurcated, one is data-modulated, the other is phase-modulated and then combined to cancel the phase noise of the light source. I made it.

  Thus, in the optical transmitter shown in the second embodiment, the influence of the optical phase noise is eliminated by creating a beat from the same light source, and one of the interference waves is eliminated by the optical phase modulator. By generating by phase modulation, the cost of the modulator can be reduced.

  Note that 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 in the optical transmitter of the present invention, but such a component is not shown.

  An object of the present invention is to make it possible to easily obtain a high-quality high-frequency signal on the receiving side with an inexpensive configuration. In the above-described embodiment, the phase modulator is phase-modulated with a sine wave having the frequency f1, and the desired frequency on the reception side is centered on f1-fs. This method can achieve the purpose of avoiding phase noise, but when f1 is a high frequency such as a millimeter wave, it is not easy to modulate even on the transmission side.

  As can be seen from FIG. 7, depending on how the sidebands are cut out by the optical filter, signals are also detected around frequencies corresponding to harmonics of f1, such as 2f1 and 3f1. Therefore, in the present invention, when n is an arbitrary integer, the modulation frequency f1 of the phase modulation is determined so that “n × f1−fs” becomes the desired frequency on the receiving side, and “f0−n × f1” in the optical filter. And the transmitted light wavelength is adjusted so that either one of “f0 + n × f1” passes.

  FIG. 10B shows a spectrum when n = 3. The optical filter transmits the “f0 + 3 × f1” component and blocks the “f0-3 × f1” component. Then, when the transmitted phase-modulated light of the component “f0 + 3 × f1” and the data-modulated light as shown in FIG. 10A are interfered and transmitted, this is square-detected like a photodiode on the receiving side. If detection is performed by a photodetector having characteristics, the spectrum of the received electric signal obtained is as shown in FIG.

  That is, as shown in FIG. 11, interference waves of each sideband and data modulated light are 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 allowed to pass through. The band pass for electrical signals having the band pass characteristics as shown by the dotted line in FIG. Only that component needs to be extracted through the filter. At the time of phase modulation, it is preferable to modulate with a modulation degree that sufficiently generates harmonic components. By doing so, the frequency of the phase modulation can be lowered, and the configuration around the phase modulator can be reduced.

[Optical filter used in optical transmitter of this embodiment]
In the present invention, the transmitted light frequency of the optical filter 3 is adjusted and controlled so that only one of the two sidebands giving a desired reception frequency remains. Therefore, it is preferable to use an optical filter having a variable transmitted light frequency.

  As a method of controlling the optical filter, a method of locking the end of the transmission band 13 of the optical filter at a position where the end of the carrier component f0 of the phase modulated light is transmitted by about half as shown in FIG. In this case, the optical filter 3 has a sufficient bandwidth and a sufficiently sharp rise so that one of the desired sidebands can be transmitted while the other half of the optical carrier f0 is transmitted. Is used.

  For example, the rise of the filter is smaller than “2 × f1” and the full width at half maximum of the filter band is about “2 × f1”, or the rise of the filter is higher than “2 × f1”. A band-stop filter (or a period-stop filter) having a small full width at half maximum of the stop band of about “2 × f1”, or a high-pass filter or a low-pass filter whose rise of the filter is smaller than “2 × f1”. Use it.

  Regarding the band-pass filter and band-stop filter, the shape of the pass-band characteristic or stop-band characteristic is not a round shape like a Gaussian shape, but is preferably a Chebyshev type or a Butterworth type that has a flat top shape. It is preferable that the sideband to be transmitted or the sideband to be blocked is in a band that lies on a flat portion of the top.

  In order to stop about half of the power of the optical carrier f0 at the edge of the optical filter 3, the following is performed. The shape of the rising and falling characteristics of the optical filter 3 is often a gentle S-shaped slope as shown in FIG. That is, at the beginning, it rises gently, rises abruptly, and rises slowly at the end. This is expressed as a differential coefficient as shown in FIG. As can be seen from FIG. 13B, the slope is the largest during the rise. In a normal optical filter, the portion with the largest inclination 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 vibrated slightly to the left and right with a low frequency signal having a specific frequency F1. Then, the intensity of the frequency component light applied to the end of the transmission band of the optical filter changes at the frequency of F1.

  The spectrum when the photodetector receives the interference wave of the phase-modulated light and the data-modulated light that has passed through the optical filter is as shown in FIG. 14 in a state where the optical carrier f0 is blocked about half by the optical filter. That is, the data component 15 of the frequency fs is obtained by directly detecting the data modulated light, and is obtained by 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 changes at the frequency of F1. Only the data component 15 is extracted from the signal that received the interference wave of the phase-modulated light and the data-modulated light by a filter or the like, and its power is detected. The power fluctuates at the frequency of F1, but when the optical carrier f0 of the phase-modulated light is at the point where the differential coefficient of FIG. 13B is maximized, the power fluctuation of the detected data component 15 is fluctuated. The ratio of becomes the maximum. Therefore, it is only necessary to detect the ratio and lock the optical filter at the point where it becomes the maximum. To find the maximum point, you can use the hill climbing method.

  Note that there may be a point where the differential coefficient is locally increased in the stop band of the optical filter. In a transition period such as when the optical transmitter is started up, such a case may occur. There is also the possibility of locking to a point. Therefore, to prevent this, the absolute value of the amplitude of the component that vibrates at F1 when ideally locked is stored at the time of shipment adjustment of the transmitter, and the power fluctuation of the data component 15 at startup When the optical filter is locked to the point where the ratio of the maximum is the maximum, the value at the point where the ratio of the power fluctuation of the data component is recognized as the maximum is compared with the stored value, and as a result, it is likely to be much smaller than the stored value. If so, it may be configured to search for another point by determining that the optical frequency is not locked.

  However, when the optical filter as described above is not used, for example, a bandpass filter having only a bandwidth for passing one sideband or a bandstop having only a stopband for blocking one sideband. If a filter is used, the above method cannot be used. This is because the band is too narrow for the desired sidebands to pass sufficiently or to block the sidebands to be blocked sufficiently. For such a filter, the following is preferable.

  That is, in the case of a band pass filter whose band is too narrow, the optical filter is controlled so that the light combined with the data modulated light is branched and received, and the power of the data signal of the desired frequency is maximized. In the case of a band stop filter with a narrow stop band, an optical filter is configured to reflect the blocked light, and a circulator is inserted in front of the optical filter to separate the reflected light as shown in FIG. And multiplexed 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 optical integrated circuit device]
By the way, when the configuration as shown in FIG. 3 is adopted as the optical transmitter, other elements can be combined into one optical integrated circuit except for the light source. 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 and 6, the optical path, etc., which are the components in FIG. 3, are integrated on the lithium niobate substrate.

  In FIG. 16, reference numeral 29 denotes a phase modulation unit, 30 denotes an AM modulation unit, and 31 denotes an optical filter unit, which correspond to the phase modulator 2, the data modulator 4, and the optical filter 3 in FIG. Reference numeral 26 denotes an optical waveguide, and reference numerals 34, 35, and 36 denote Y branches (optical branch couplers) of the optical waveguide. The Y branches 34, 35, and 36 constitute an optical branch coupler.

  In the case of the configuration of 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 an electrode is attached thereon. The electrode is a traveling wave electrode. (However, in the figure, only the signal electrode is shown and the ground electrode is omitted.) The optical filter 31 is configured by forming a Bragg grating 33 in a part of the optical waveguide 26, and one of the optical waveguides 26. 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.

  In addition, there is an input unit for guiding light from a light source outside the optical waveguide 26 and an output unit for deriving light after interference. Between these, the optical path is bifurcated via the Y branch 34, and each of the bifurcated optical paths is The Y branch 35 is combined into one to reach the output unit.

  A phase modulator 29 and an optical filter 31 are formed on one of the optical waveguides 26 branched from the input unit side, and an optical path is further provided to the other of the optical waveguides 26 branched from the input unit side via the Y branch 36. After bifurcating, an electrode 28 is formed in 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 optical path that is bifurcated by the AM modulator 30 is configured such that the optical path is brought together by another Y branch 36 to reach the Y branch 35 again.

  The phase modulator 29 is configured to apply phase modulation to light in the optical waveguide by forming an electrode 27 on a part of the optical waveguide 26 that is bifurcated by the Y branch 34 and giving a phase modulation signal thereto. By integrating each element on the lithium niobate substrate in this manner, connection between elements becomes unnecessary, and the optical transmitter can be reduced in loss and reduced in size.

[Modification (2) of the 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 branching couplers), and 25 is a semiconductor laser element.

  Here, the data modulator 4 and the phase modulator 2 are the same as in the configuration of FIG. 3, but instead of the optical filter 3 in the configuration of FIG. 3, a semiconductor laser element 25 is inserted in the configuration of FIG. . The semiconductor laser element 25 oscillates in a single longitudinal mode, such as a λ (wavelength) / 4 shift DFB laser or a DBR laser, and its reflection structure can be used as an optical filter that transmits the oscillation wavelength. Is used.

  In the semiconductor laser element 25, the drive current 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 element has a feature that when light close to the oscillation wavelength when oscillating alone (free run) is injected from the outside, it 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. The structure for oscillating in a single longitudinal mode has wavelength selectivity.

  Therefore, if light subjected to phase modulation is injected as shown in FIG. 4B and the free-run oscillation wavelength of the semiconductor laser 25 is adjusted in the vicinity of f0 + f1, light of f0 + f1 is oscillated by wavelength selectivity. It becomes like this. In order to make the oscillation wavelength close to a desired sideband wavelength, the output combined 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. In addition, the free run wavelength of the semiconductor laser element 25 may be adjusted. In such a form, light with 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,. Prepare a light source of wavelength. Reference numerals 38a and 38n-1 denote optical branching couplers, and 39 receives optical signals from the optical transmitters 37a, 37b and 37n coupled by the optical branching couplers 38a and 38n-1. It is an optical receiver.

In such a configuration, on each transmitting side, as shown in FIG. 19 (a), subcarriers of subcarrier frequencies fs1, fs2,..., Fsn having different frequencies in the optical transmitters 37a, 37b,. Data modulation is performed by AM modulation using a signal. In this way, each subcarrier is changed for each transmitter. This is coupled by optical branching couplers 38a, 38b, .about.38n-1 as shown in FIG. 18, transmitted, and received by receiver 39.
The receiver 39 does not separate the wavelengths but collects the signals as they are and receives them with a single photodetector. Then, as shown in FIG. 19B, it is possible to receive data signals that are in the same band and do not overlap.

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

  In the configuration of FIG. 20, each of the data transmitters 41-1, 41-2,..., 41-n does not have a light source, and a comb wave generator including a light source and a phase modulator at the end of an optical fiber 67 serving as a trunk line. There are 40. 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 are generated by phase modulation. Comb wave generator 40 has a plurality of sidebands f0, f0 ± f1, f0 ± 2f1 as shown in FIG. 21 by deepening the degree of modulation of the single longitudinal mode light from the light source. , F0 ± 3f1, f0 ± 4f1,..., F0 ± nf1 are made relatively uniform.

  Comb wave generator 40 generates such phase-modulated light and passes it through trunk fiber 67. The data transmitters 41-1, 41-2,..., 41-n have an optical signal input side and an output side, and the data transmitters 41-1, 41-2,. They are connected in the main fiber 67 so as to be connected in series.

  In each of the data transmitters 41-1, 41-2,..., 41-n, among the plurality of sidebands having relatively uniform optical power as shown in FIG. One different sideband is extracted and AM modulation is performed with a specific subcarrier frequency, and the adjacent sideband is terminated. For example, the first-stage data transmitter 41-1 modulates the sideband f0-f1, transmits f0-2f1, transmits f0, and terminates the second-stage data transmitter 41-2 by f0 + 2f1. The sideband is data modulated, f0 + f1 is transmitted, and f0 + 3f1 is terminated.

  In each of the data transmitters 41-1, 41-2,..., 41-n, AM-modulated light is again applied to the trunk fiber 67. In this way, the sideband to be modulated and the adjacent three sidebands on both sides are set as one set, and different data transmitters perform the same operation on different sets of sidebands. Further, the AM modulation subcarrier frequency is changed for each data transmitter.

  When the receiver 42 in FIG. 20 receives all of them without separating them, the frequency from a plurality of subcarrier-multiplexed data transmitters as shown in FIG. A signal is received. In this embodiment, it is not necessary to place a light source on each data transmitter, and it is not necessary to control the wavelength of the light source between the data transmitters, thereby simplifying the data transmitter.

  In the first and second embodiments described above, the example in which the sideband closest to the optical carrier f0 is used 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, phase modulation must be performed at a millimeter wave frequency, which increases costs. In phase modulation, the power of the sideband of the harmonic component of the modulation frequency increases as the modulation depth is increased. Next, a method using such harmonic components will 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 a sideband wave that gives the desired value and the optical frequency of the light source. 1 / n (n is an integer of 2 or more).

  A specific example of an optical transmitter that uses such sideband waves as harmonic components will be described. Here, the phase modulator 2 in FIG. 3, FIG. 6, FIG. 15, FIG. 17, and FIG. 22, and the phase modulator of the comb wave generator 40 in FIG. The frequency is 1 / n (n is an integer of 2 or more) of the frequency difference between the sideband wave that gives a desired value and the optical frequency of the light source.

  In the example of FIG. 4, the phase modulation frequency is set such that the interference wave generated by the interference between f0 + 2f1, f0 + 3f1,... And the data modulated light is a desired frequency when received by the photodetector on the receiver side. Determine f1. As a result, f1 becomes small, and the hardware around the phase modulator can be reduced in cost.

(Fourth embodiment)
Next, the 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 are photodiodes, 11 is a low-pass filter, 12 And 43 are optical branching couplers, 45 is a band pass filter, 46 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 element.
The local light source 7 is an original light source to be subjected to phase modulation, and is also composed of, for example, a single longitudinal mode semiconductor laser element, like the signal light source. The optical branching coupler 9a is a coupler having a 1 input port 2 output port configuration for bifurcating the output light of the local light source 7, and the optical branching coupler 9b 1 is for bifurcating the output light from the signal light source 8. It is a coupler having an input port 2 output port configuration.

  The phase modulator 2 phase-modulates and outputs the light from the local light source 7 guided through the optical branching coupler 9a. Further, the data modulator 4 performs AM modulation on the output light from the signal light source 8 guided through the optical branching coupler 9b, for example, by a data-corresponding subcarrier signal fs, and outputs it as data modulated light. To do.

  The optical branch coupler 43 has a 2-input port 2-output port configuration for coupling one of the output lights of the local light source 7 branched into two by the optical branch coupler 9 a and the data modulated light output from the data modulator 4. Of the two output ports, one is an output port to the photodiode 44, and the other is an output port to the transmission line.

  The optical branching coupler 12 combines the output of the phase modulator 2 and the output light of the signal light source 8 and combines them. The photodiode 10 is a photodetector for detecting this combined output, The band-pass filter 11 is for extracting a low frequency component from the detection output of the photodiode 10, and the control device 46 sets the frequency and optical phase of the output light of the local light source 7 in correspondence with the low frequency component. In order to shift, the local light source 7 is controlled.

  A PLL control system that controls the optical frequency and optical phase of the local light source 7 with reference to the optically coupled output from the optical branching coupler 12 by the photodiode 10, the low-pass filter 11, and the control device 46 is configured.

  The photodiode 44 is a photodetector for detecting the transmission light bifurcated by the optical branching coupler 43, and the band pass filter 45 is for extracting a predetermined band component from the detection output of the photodiode 44. The control device 46 also has a function of controlling the local light source 7 so as to shift the frequency of the output light of the local light source 7 in correspondence with the extracted frequency component.

  The photodiode 44, the band pass filter 45, and the control device 46 constitute a frequency control system that controls the optical frequency of the local light source 7 with reference to the transmitted light output.

  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 when the light is combined and transmitted, the local light is branched and modulated by a periodic signal. An interference wave between one of a plurality of generated sidebands and a branch of the signal light is detected by a photodetector, and the signal light is maintained so that the detected error signal maintains a constant DC value. Alternatively, the optical phase of the local light is controlled.

In the case of the configuration shown in FIG. 23, the signal light source 8 and the local light source 7 are provided separately. Then, the light from the signal light source 8 is modulated by the data modulator 4 to become data-modulated light, which is sent out toward the optical branching coupler 43, and the light from the local light source 7 is also directed toward the optical branching coupler 43. After being combined here, both are sent to the transmission line as transmission output (transmission light).
In this way, the signal light and the local light are combined by the optical branching / combining device 43 with the polarization and the delay combined, and sent out. The forms of polarization and delay adjustment and data modulation are the same as those in the second and third embodiments described above.

  In the example of FIG. 23, the local light and the signal before being modulated are branched. The local light is phase-modulated by the phase modulator 2, and the branched signal light, polarization and delay are combined and received by the photodiode 10. If the difference between the optical carrier frequency f0 of the signal light and the optical frequency fL of the local light is to be 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 nth sideband (or -nth sideband) generated by the phase modulation matches the optical frequency and optical phase of the signal light. One of the outputs of the “2 × 2” optical splitter / coupler 43 is transmitted as transmitted light, but the other output is received by the photodiode 44, and the difference between f0 and fL, the difference between fd and fr fc = fr− The optical frequency of the local light or signal light (local light in the figure) is controlled so that the absolute value of fd is within the pull-in range frequency fi of the PLL loop 68. For example, a method is used in which a band-pass filter having a center frequency fr and a band 2 fi is disposed downstream of the photodiode 44 and the optical frequency is controlled so that the power of the received signal passing through the filter is maximized.

  The purpose here is not to lock the signal light to a sideband other than the desired sideband, so the following system may be used. First, the frequency fd of the interference wave signal detected by the photodiode 44 is set within the range of 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 so that the power of the transmitted electric signal is maximized in the band of the band pass filter 45.

  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 range frequency of the PLL while keeping the signal in the band of the filter 45. For example, the optical frequency may be varied while a signal is passing through the band of the filter 45. When a signal within the PLL pull-in range is detected, the control of the optical frequency by the loop 69 is stopped, and the PLL loop is turned on so that it is automatically controlled.

  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 current is supplied in addition to the direct current drive current, the optical frequency changes momentarily and returns to the original state, and the optical phase changes slightly. In this way, the optical phase of the semiconductor laser can be controlled. When a solid-state laser element is used as a light source, an external optical infinite phase modulator may be used.

  In the branch that modulates the signal light, the data modulator is arranged behind the branch. However, if the optical spectrum of the signal light obtained by data modulation is composed of the optical carrier component and the subcarrier data component having a frequency slightly separated from the optical carrier (away from the PLL pull-in range), the optical carrier component is used. The data modulator may be in front of the branch.

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

  As described above, the optical transmitter according to this embodiment controls the optical frequency so that the signal light and the local light have a desired optical frequency difference, and branches the local light in the optical transmitter that transmits the combined light. An interference wave between one of a plurality of sidebands modulated and generated by a periodic signal and the signal light branched is detected by a photodetector, and the detected error signal has a constant DC value. It is characterized in that the optical phase of the signal light or the local light is controlled so as to maintain.

  Separately applying a data modulated light source (signal light source) and an interference light source (local light source) and applying a PLL to those beats has good high frequency characteristics when the beat has a high frequency such as a millimeter wave. The element must be used, which is very expensive. Therefore, here, PLL is not performed in the high frequency band, but is performed in the low frequency band. In other words, the error signal for applying the PLL is detected at a low frequency around the direct current.

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

FIG. 24A shows the signal light after branching, and FIG. 24B shows the optical spectrum of the local light after branching. The local light is subjected to phase modulation and sidebands are generated. In addition to the sidebands having a frequency corresponding to the period of the phase modulation signal, many sidebands having a frequency corresponding to the harmonics are generated. The sideband carries the same optical phase noise as that of the light source. Therefore, here, the signal light is locked by the PLL with respect to the sideband corresponding to the harmonic.
If interference is performed in a state where the desired sideband of the local light and the signal light are approximately overlapped in advance, the component near the direct current 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 of the branch source and the local light are combined and transmitted by another optical branching coupler 6. 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 beats of f0 and fL are detected in the transmitted receiver. Therefore, according to the fourth aspect 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 the direct current, a loop can be assembled with low-cost elements. Further, since the phase modulation frequency is about a fraction of the desired millimeter-wave band frequency, the cost around the phase modulator can be reduced.

(Fifth embodiment)
Next, an embodiment of the fifth invention of the present application will be described as a fifth embodiment.
FIG. 25 is a block diagram thereof. In the figure, 58 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 branching coupler, 63 is a photodiode, 64 is a low-pass filter, and 65 is a control device. is there.

  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 element. The data modulator 60 subjects the output light from the signal light source 59 to AM modulation by, for example, a data-corresponding subcarrier signal fs, and outputs the result as data modulated light.

  The local light source 58 is an original light source to be subjected to phase modulation, and is also composed of, for example, a single longitudinal mode semiconductor laser element like the signal light source. The phase modulator 61 phase-modulates the light from the local light source 58 and outputs it.

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

  In such a configuration, light output from the signal light source 59, for example, a single longitudinal mode semiconductor laser element, is incident on the data modulator 60, where it is subjected to AM modulation by, for example, the subcarrier signal fs. The subcarrier frequency fs is a frequency that is much lower than the frequency fp when the optical carrier is down-converted within the desired reception band, for example, about fp / 100.

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

  In this system, similarly to the signal light source 59, the local light source 58 is formed of 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. Therefore, this system uses this fact to control the frequency of the output light of the light source. That is, the local light source 58 and the signal light source 59 measure the characteristics of current and temperature versus the oscillation optical frequency in advance and grasp the characteristics, and the optical carrier of the signal light is transmitted to the control device 65 in response to the characteristics. It is assumed that the optical frequency control of the local light source 58 is applied based on the combined output so as to lock to the sideband (not to the optical carrier).

  Therefore, the PLL constitutes a control loop for performing the optical frequency control of the local light source 58 corresponding to the combined output so that the optical carrier of the signal light is locked to the sideband of the local light (not the optical carrier). Will be.

  The signal to be controlled by the PLL control loop may be a signal light source. This PLL control loop applies optical frequency and optical phase control to the local light source or signal light source, and the optical carrier of the signal light is local light ( It plays the role of locking to the sidebands (not to the optical carrier). That is, the current control device 65 first oscillates the optical frequency of the local light source 58 (or the signal light source 59), and the optical frequency difference between one of the local light sidebands and the signal light is within the pull-in range of the PLL. Find a point to come.

If the PLL is within the pull-in range, the current control device 65 activates the PLL to lock the signal light to one of the local light sidebands.
Further, if the optical carrier frequency of the signal light 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 turns off the PLL, The injection current to the light source is controlled so that the optical frequency is separated to such an extent that it can be locked to the sideband.

By controlling the light source using 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 divided into optical branching couplers. Combine at 62. A part of the combined optical signal is sent out to the transmission path to become a transmission output, and a part is sent to the photodiode 63 to be used for 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 output light from one output port is light that is an optical transmission line. The light is sent to the fiber, and the output light from the other output port is detected by the photodetector 63.

  First, the current controller 65 oscillates the optical frequency of the local light source 58 (or the signal light source 59), and the optical frequency difference between one of the local light sidebands and the signal light comes within the pull-in range of the PLL. If the current control device 65 enters the PLL pull-in range, the current control device 65 operates the PLL and locks the signal light to one of the sidebands of the local light. The error signal used in the PLL is a low-frequency signal in the vicinity of DC, so that it can be realized by control system hardware using an element having a low operating speed. The cost of hardware can be reduced.

  Further, according to this embodiment, one of the sidebands of the data component on the signal light and the local light always falls in a desired frequency band on the receiving side. As a result, there is an advantage that the control can be simplified because the frequency interval between the signal light and the local light does not need to be strictly set to a desired frequency interval.

  In the embodiment described with reference to FIGS. 23 and 25, it is described that a “2 × 2” optical directional coupler (coupler) is used as the optical branching coupler. However, for example, as shown in FIG. After merging with a × 1 ”coupler (coupler having a configuration of two input ports and one output port), branching may be performed again with a coupler of“ 1 × 2 ”(coupler having a configuration of one input port and two output ports).

  As described above, in the fifth embodiment of the fifth invention, in the optical transmitter that combines and transmits the signal light and the local light with the optical branching coupler, the signal light is modulated with the subcarrier signal, The local light is modulated by a periodic signal, one of a plurality of outputs of the optical branching coupler is transmitted as an output of the optical transmitter, and another one is detected by a photodetector, and the local light is detected. The optical phase of the signal light or the local light is controlled so that an error signal due to interference between one of the plurality of sidebands generated in the optical signal and the optical carrier component of the signal light has a constant DC value. Features.

  That is, in the fourth invention of the present application described in the fourth embodiment, the local light and the signal light are branched from the main line to generate an error signal, but in the fifth invention of the present application, the branch light is not branched and the local light is emitted. The main line was phase-modulated and multiplexed with the data-modulated signal light.

  An optical branching coupler used for multiplexing has a plurality of output ports. This is due to 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, and even in the case of one output port, the loss is the same as in the case of having two output ports. Receive.

  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 apply a PLL that synchronizes the local light and the optical phase of the signal light. The signal light used here is obtained by subjecting an optical carrier to subcarrier modulation by a band signal as shown in FIG. There are sidebands carrying the optical carrier component and data, and the optical frequency of the optical carrier and the optical frequency of the sideband are more than the pull-in range of the PLL.

  The fifth invention transmits (a) and (b) of FIG. 24 as they are. Any one sideband of the local light and the optical carrier component of the signal light are synchronized, but any local band of the local light to 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 values as in the case of the fourth invention. If it does in this way, when it receives, the interference wave with a certain sideband will fall in a desired frequency band. Centering on the signal light, two sidebands separated by plus and minus by the required frequency difference simultaneously drop to the desired frequency.

  However, when phase modulation is performed on an optical carrier, an envelope as shown in FIG. 28 generally forms a mountain-shaped spectrum, depending on the degree of modulation. Therefore, if fL and f0 are appropriately separated, there is a significant difference between the sidebands that are separated positively and the sidebands that are separated negatively, and when an interference wave with the signal light is received, The interference wave with the larger sideband becomes dominant. The level of the interference wave changes slightly due to the change in the phase difference between the plus sideband and the minus sideband. do not become.

  In the fifth invention of the present application, it is not necessary to strictly control the wavelength of the signal light and the local light, and the cost can be reduced. In addition, since there are few branches in the system, there is little loss, so that a system capable of higher quality transmission is obtained.

It is a figure for demonstrating this invention, Comprising: It is a block diagram for demonstrating the 1st Embodiment of this invention. It is a figure for demonstrating this invention, Comprising: It is a wave form diagram for demonstrating the 1st Embodiment of this invention. It is a figure for demonstrating this invention, Comprising: It is a block diagram for demonstrating the 2nd Embodiment of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating this invention, Comprising: It is a block diagram for demonstrating the modification in the 2nd Embodiment of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating this invention, Comprising: It is a block diagram for demonstrating the modification in the 2nd Embodiment of this invention. It is a figure for demonstrating this invention, Comprising: It is a figure for demonstrating the structural example at the time of integrating the apparatus structure in the 2nd Embodiment of this invention into an integrated circuit. It is a figure for demonstrating this invention, Comprising: It is a block diagram for demonstrating the modification in the 2nd Embodiment of this invention. It is a figure for demonstrating this invention, Comprising: It is a block diagram for demonstrating the modification in the 2nd Embodiment of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure which shows one of the embodiment of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating this invention, Comprising: It is a figure for demonstrating the example of an apparatus structure in the 4th Embodiment of this invention. It is a figure for demonstrating this invention, Comprising: It is a figure for demonstrating the example of an apparatus structure in the 4th Embodiment of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating this invention, Comprising: It is a figure for demonstrating the example of an apparatus structure in the 5th Embodiment of this invention. It is a figure for demonstrating this invention, Comprising: It is a figure which shows the structural example of the one part element of the apparatus in the 4th Embodiment of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating operation | movement of this invention. It is a figure for demonstrating the effect | action of this invention, Comprising: It is a wave form diagram which shows the example of a received signal modulated by data. It is a figure for demonstrating a prior art example. It is a figure for demonstrating a prior art example. It is a figure for demonstrating a prior art 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, 47, 48, 52, 58, 62 DESCRIPTION OF SYMBOLS ... Optical branch coupler, 7 ... Local light source, 8 ... Signal light source, 10, 20 ... Photodiode, 11 ... Low pass filter, 13 ... Optical filter transmission characteristic, 14, 16 ... Filter transmission characteristic, 15 ... For control , 17 ... optical circulator, 19, 46, 65 ... control device, 23 ... optical delay line, 24 ... optical fiber amplifier, 25 ... semiconductor laser, 26 ... optical waveguide, 27, 28, 33 ... electrode, 29, 61 ... Phase modulator, 30 ... AM modulator, 33 ... Grating, 34, 35, 36 ... Y branch, 37 ... Optical transmitter, 39 ... Optical receiver, 40 ... Comb wave generator, 41 ... Data Transmitter, 42 ... Receiver, 44, 5 , 63, 66 ... photodiode, 45 ... band pass filter, 49 ... local light source, 50, 59 ... signal light source, 51,60 ... data modulator, 54 ... amplifier, 55 ... reference signal generator, 56 ... mixer, 57, 64 ... low-pass filter, 67 ... trunk fiber, 68 ... PLL loop, 69 ... frequency control loop

Claims (2)

In each of the optical transmitters that control the optical frequency so that the signal light and the local light using the semiconductor laser as the light source have a desired optical frequency difference, and the signal light and the local light are combined and transmitted, An interference wave obtained from one of a plurality of sidebands generated by capturing a part of local light and modulating it with a periodic signal and the signal light is detected by a photodetector, and this detection is performed. The optical frequency and optical phase of the signal light or local light are changed by changing the temperature or current of the signal light or the semiconductor laser that is the light source of local light, or both, so that the error signal maintains a constant DC value. An optical transmitter comprising phase-locked loop control means for controlling. Having a signal light source and the local light source according to the semiconductor laser, with an optical signal by modulating light from said signal light source at the modulating means, multiplexes in light and optical branching coupler from the local light source in the optical transmitter for transmitting Te, the optical branching coupler has a plurality of output ports, the modulating means is one and outputs the modulated light from said signal light source in the sub-carrier signal, the phase modulation The light from the local light source is phase-modulated by a periodic signal to be supplied to the optical branching coupler, and one of a plurality of outputs of the optical branching coupler is detected by a photodetector. The signal light source or the station is used 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. Semiconductor laser as light source Optical transmitter, characterized by a structure comprising a phase-locked loop control means for controlling the signal light or the local light optical frequency and optical phase by changing the temperature or current or both.

Images