JP2705291B2 - Optical transmitter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical transmission device used for optical communication and the like.

(Prior Art) In optical communication, an intensity-modulated signal light is obtained by changing an injection current to a semiconductor laser, and the intensity-modulated signal light is transmitted through an optical fiber that is a transmission line, and is subjected to photoelectric conversion such as a PIN diode. An intensity modulation-direct detection optical communication device that is received by an optical receiver using an element is mainly used. In this optical communication device, the wavelength band at which the loss of the optical fiber is the lowest is 1.5 μm.
It is known that in m-band transmission, if communication is performed at a transmission speed of gigabit or more, the dispersion of the optical fiber is affected, and the quality is significantly degraded after transmission (M. Shikada et a
l., “Long-distance Gigabit-Range Optical Fiber T
ransmission Experiments Employing DFB-LD's and I
nGaAs-APD's "IEEE, Journal of Lightwave Technolog
y, Vol. LT-5, No. 10, pp. 1488-1497).

In addition, in an optical heterodyne communication device in which information is placed on the frequency, phase, and amplitude of light and a receiving side obtains a beat with local oscillation light and obtains information from the beat, chirping that occurs when a semiconductor laser is directly modulated is used. Since there is no influence of the spectrum spread, the influence of the dispersion of the optical fiber is small as compared with the intensity modulation-direct detection optical communication device. However, it has been reported that degradation occurs in ultra-high speed and long distance transmission (N. Takachio et al., “Chromatic Dispersion E
qualization in an 8 Gb / s 202 km CPFSK Transmission
Experiment "17 th Conference on Itegrated Optics a
nd Optical Fiber Communication, Post-deadline Pape
rs 20 PDA-13).

On the other hand, in recent years, research on optical amplifiers has been carried out, and studies on direct amplification relay systems using optical amplifiers have been actively conducted (S. Yamada).
moto et al., “516 km 2.5 Gb / s Optical Fiber Transm
ission Experiment using 10 Semiconductor Laser Amp
lifiers and Measurement of Jitter Accumulation "17
th Conference on Integrated Optics and Optical Fib
er Communication, Post-deadline Papers 20 PDA-
9). In such a direct amplification relay system, the transmission distance can be extended by compensating for the loss, so that the possibility of ultra-long distance transmission is expected.

As described above, the signal light receives power loss in the optical fiber and generates waveform distortion due to the influence of dispersion. The transmittable distance of the optical communication is limited by the two effects of the power loss and the dispersion. In high-speed transmission of several gigabits or more, there is a large spectrum spread due to modulation in signal light, which is greatly affected by dispersion and is therefore limited by dispersion before being limited by loss of an optical fiber. In ultra-long-distance transmission in which an optical amplifier is used as an amplification repeater, loss limitation can be compensated by the optical amplifier, but the transmission distance is limited by the influence of dispersion.

The dispersion of an optical fiber is caused by the fact that the time required for propagation differs when the frequency of light input to the optical fiber differs. Therefore, if spectrum spread due to modulation exists in the signal light, the waveform is distorted after transmission due to the spectrum spread. For example, when transmitting light in the 1.5 μm band through a 1.3 μm zero-dispersion fiber, the short wavelength side component (high frequency signal component) in the signal light has a high propagation speed, and the long wavelength side component (low frequency signal component). ) Has a slow propagation speed. Therefore, after transmission, high-frequency signals are concentrated in front of the pulse, and low-frequency signals are concentrated in the rear of the pulse. as a result,
Waveform distortion occurs in the transmitted pulse, making it impossible to determine the sign of the mark or space.

As a method of compensating for waveform distortion due to such dispersion,
Appropriate frequency modulation is performed on the output light of the semiconductor laser, and the external modulator modulates the intensity of the frequency-modulated light so that one pulse of the transmission signal has a low frequency and the rear has a high frequency. There is a report that distortion is reduced and transmission distance is extended (N. Henmi et al., “A Novel Dispersion Co.
mpensation Technique for Multigigabit Transmission
with Normal Optical Fiber at 1.5um Wevelength "Opt
ical Fiber Communication Conference '90, Poost-dead
line Papers PD-8). This dispersion compensation method is called the pre-charging method. By this method, in a 10 Gb / s transmission system, the transmission range was 20 km,
Can be expanded to 50km.

(Problem to be Solved by the Invention) The theoretically possible transmission distance in the optical transmission device using the pre-charging method is about 2.5 times as long as the transmission possible distance using the ordinary external modulation method. The dispersion was not sufficiently compensated for the distance.

Therefore, an object of the present invention is to provide an optical transmission device that can further increase the amount of dispersion degradation that can be compensated.

(Means for Solving the Problems) In order to solve the above problems, a first optical transmission device provided by the present invention includes a clock signal source for generating a clock signal, one semiconductor laser light source, and the semiconductor laser light source. A DC bias source for applying a bias to the injection current of the laser light source, an adder for adding the output of the DC bias source and the clock signal and applying the output to the injection current of the semiconductor laser, and input to the adder A variable attenuator and a variable delay for adjusting the amplitude and the phase of the clock signal, and first and second transmission signal sources respectively generating transmission signals independent of each other in synchronization with a clock signal output from the clock signal source; First and second pulse width converters for setting the pulse widths of the outputs of the first and second transmission signal sources; An optical splitter for splitting into a second split light, an optical delay means for providing a time difference between the first and second split lights split by the optical splitter, and the first and second pulse width converters A first and a second external modulator for respectively modulating the intensity of the first and second branch output light using the output signal of the first and second branch output light to generate first and second intensity modulated light; An optical multiplexer for multiplexing the output light of the external modulator and transmitting the multiplexed output light to a transmission line, wherein the optical delay means is provided between the optical splitter and the external modulator, and Means for providing a time difference between them, or means provided between the external modulator and the optical multiplexer to provide a time difference between the intensity-modulated lights, wherein the first and second intensity modulations are provided. So that the first half of each light pulse has a lower frequency and the second half has a higher frequency The adder uses the variable attenuator to adjust the phase of the clock signal input to the adder using the variable delay device and to adjust the optical frequency displacement of the output light of the semiconductor laser light source. The amplitude of the clock signal input to the input terminal is adjusted.

Further, a second optical transmitter provided by the present invention includes a clock signal source for generating a clock signal, first and second semiconductor laser light sources, and injection currents of the first and second semiconductor laser light sources. First and second DC bias sources for applying a bias, and the clock signal is added to the output of the first DC bias source, and the output is added to the first DC bias source.
A first adder applied to the injection current of the semiconductor laser of
A second adder for adding the clock signal to an output of the second DC bias source and applying the output to an injection current of the second semiconductor laser; and a clock signal input to the first adder. First to adjust the amplitude and phase of
A variable attenuator and a first variable delay, a second variable attenuator and a second variable delay for respectively adjusting an amplitude and a phase of a clock signal input to the second adder, and the clock First and second transmission signal sources respectively generating transmission signals independent of each other in synchronization with an output clock signal of the signal source, and a first for setting a pulse width of an output of each of the first and second transmission signal sources. And a second pulse width converter, and using the output signals of the first and second pulse width converters to intensity-modulate the first and second semiconductor laser output lights, respectively, to obtain first and second intensities. A first and a second external modulator for generating modulated light; and an optical multiplexer for multiplexing output light of the first and second external modulators and transmitting the multiplexed light to a transmission path, wherein the first and second external modulators are provided. 2 in the first half of each pulse of the externally modulated light The first and second variable delays are used to lower the frequency and increase the latter half, and to make a difference between the generation timings of the first and second semiconductor laser light sources. The first and second variable attenuations are used to adjust the phase of the clock signal input to the second adder and to adjust the optical frequency displacement of the output light of the first and second semiconductor laser light sources. The amplitudes of the clock signals input to the first and second adders are adjusted by using respective adders.

(Operation) In the precharge method, if the pulse width of one pulse is expanded to one time slot or more and the optical frequency is appropriately changed within the pulse, the transmission distance can be theoretically further increased. However, if the pulse width of one pulse is set to one time slot or more, it is not possible to appropriately set a change in the optical frequency in a portion overlapping with an adjacent pulse. The present invention has the following features. One signal light set to an optimum optical frequency change state within one time slot is output from two divided signal lights or two transmission light sources set to an optimum optical frequency change state within one time slot. Each signal light is intensity-modulated with a pulse width within a range of one time slot of each time series. At this time, for example, if the transmission line is an anomalous dispersion optical fiber, the optical frequency is set to be low in the first half of the pulse and to be high in the second half. Then, an appropriate delay difference is given to each time-series signal light, and they are added. The signal light obtained by adding the two time-series signal lights is subjected to pulse width compression by propagating through a transmission path, and is multiplexed time slot after transmission (1/2 of one time slot before multiplexing). , A sufficient eye opening can be obtained. Furthermore, since polarization multiplexing is performed in the transmission section, optical multiplexing without loss is theoretically possible, and a high-power optical transmission output can be realized.

Next, the present invention will be described with reference to examples.

FIG. 1 is a configuration diagram of one embodiment of the first optical transmission device of the present invention. In FIG. 1, the configuration of the optical transmitter 31 will be described first. A clock signal 201 output from a clock generator 8 that generates a sine wave at a clock frequency of 5 GHz passes through a variable attenuator 9 and a variable delay unit 11 to a semiconductor laser light source 1 that oscillates in a single longitudinal mode in a 1.5 μm band. The resulting frequency modulation signal 202 and the DC bias current are added by the adder 6 and applied. This bias current is supplied from a DC bias source 4. The output light 101 frequency-modulated by the frequency modulation signal 202 is output from the semiconductor laser light source 1. At this time, the output light 101 is simultaneously subjected to intensity modulation and frequency modulation to modulate the injection current of the semiconductor laser light source 1. In the present invention, since the frequency modulation effect is used, the modulation degree of the injection current when obtaining the desired frequency modulation is small, and as a result, the intensity modulation degree is small. Further, the intensity modulation is performed by the intensity modulation performed by the external modulators 17 and 18 and the optical delay means is provided between the optical splitter and the external modulator, and a time difference is generated between the split output lights. Or at least one of means for providing a time difference between the intensity-modulated lights provided between the external modulator and the optical multiplexer, and each of the pulses of the first and second externally-modulated lights. The phase of the clock signal input to the adder by using the variable delay device so that the optical frequency of the first half is lower and the latter is higher, or the optical frequency of the first half of the pulse is higher and the latter is lower. And adjusting the amplitude of a clock signal input to the adder using the variable attenuator to adjust the optical frequency displacement of the output light of the semiconductor laser light source. apparatus. Synchronization is not a problem. This frequency-modulated output light 10
1 is split into two branched lights 103 and 104 by an optical coupler 19.
The signal sources 13 and 14 synchronize with the clock generator 8 and
Gb / s RZ signals 204 and 205 are generated. One of the two split beams 103 is intensity-modulated by the external modulator 17 of LiNbO ₃ by the intensity modulation signal 206 generated by the pulse width variable circuit 15 receiving the RZ signal 204. The other split light 104 is
The optical delay device 20 time-delays the branched light 103, and the delayed light 1
It becomes 05. The delayed light 105 is supplied to the pulse width
The signal 205 receives the intensity modulated signal 207 generated by the
The intensity is modulated by the O ₃ external modulator 18. As a result, intensity-modulated signal lights 106 and 107 are obtained. The intensity-modulated signal light 106 and the intensity-modulated signal light 107 are polarization-multiplexed using the polarization multiplexer 21, and a polarization-multiplexed transmission signal light 108 is obtained. The polarization multiplexed transmission signal light 108 is transmitted through the optical fiber 3 having a zero-dispersion wavelength in the 1.3 μm band, then becomes a reception signal light 109 and is detected by the optical receiver 32. In the optical receiver 32, the avalanche photodiode 22 is used as a photoelectric conversion element. The received signal light 109 is converted into an electric signal and then amplified by the electric amplifier 23 to obtain an information signal.

Next, the operation of the main part of the optical transmission 31 will be described. FIG. 2 is a timing chart showing the phase relationship between the clock signal and the signal light. The phase difference between the intensity-modulated signals 206 and 207 is タ イ ム time slot, and the pulse width is arbitrarily set within the range of one time slot using the pulse width variable circuits 15 and 16. In this case, the pulse width is expanded as much as possible within one time slot. External modulator 1
In the intensity modulated signal light 106, 107 which is the output light of 7,18,
The phase difference between the frequency modulation signal 202 and the intensity modulation signals 206 and 207 to the semiconductor laser light source 1 is changed using the variable delay device 11 so that the optical frequency is low at the rising portion of the mark signal and high at the falling portion. It was adjusted. Also, since optical multiplexing with small loss is possible by polarization multiplexing, optical transmission power of +2 dBm is realized. In the transmission experiment, when the conventional pre-charging method was used, the transmittable distance was about 50 km at 10 Gb / s, but in the case of transmission in the present embodiment, the received waveform with a small waveform distortion even after 100 km transmission was obtained. As a result, good long-distance transmission without code errors can be realized.

There are various other modified examples of the first optical transmission device of the present invention. The light source is not limited to the 1.5 μm band light source, but may be a 1.3 μm band or another wavelength. An optical amplifier may be used to compensate for a loss in light intensity caused by splitting the output light of the light source. As an external modulator, Li
A semiconductor external modulator may be used instead of the NbO ₃ modulator. The place where the optical delay is performed may be between the external modulator and the multiplexer instead of between the splitter and the external modulator, or both. Further, the bit rate is not limited to 5 Gb / s, but may be 2 Gb / s or 10 Gb / s. The waveform for frequency-modulating the output light of the semiconductor laser light source is not limited to a sine wave but may be a sawtooth wave or a triangular wave. The transmission path may be a transmission path including an optical amplifier as an amplification repeater in the middle.
The zero-dispersion wavelength of the optical fiber in the transmission path is not limited to the 1.3 μm band. The configuration of the receiver is not limited to direct detection, and heterodyne detection can be used.

FIG. 3 is a configuration diagram of one embodiment of the second optical transmission device of the present invention. 3, the configuration of the optical transmitter 31 will be described. A clock signal 201 output from a clock generator 8 that generates a sine wave at a clock frequency of 5 GHz is applied to a variable attenuator 9 by two semiconductor laser light sources 1 and 2 that oscillate in a single longitudinal mode in a 1.5 μm band. The frequency modulation signals 202 and 203 generated by passing through the variable delay unit 11 and the variable attenuator 10 and the variable delay unit 12, respectively, and the DC bias current are added by the adders 6 and 7, respectively, and applied. This bias current is supplied from DC bias sources 4 and 5, respectively. The semiconductor laser light sources 1 and 2 output output lights 101 and 102 frequency-modulated by frequency modulation signals 202 and 203, respectively. At this time, the output lights 101 and 102 are simultaneously subjected to intensity modulation and frequency modulation in order to modulate the injection current of the semiconductor laser light source 1. Since the frequency modulation effect is used similarly to the first embodiment, the modulation degree of the injection current when obtaining the desired frequency modulation is small, and as a result, the intensity modulation degree is small. Further, since the intensity modulation is synchronized with the intensity modulation performed by the external modulators 17 and 18, there is no problem. Signal source
13 and 14 are 5Gb / s RZ in synchronization with clock generator 8
Generate signals 204 and 205. Frequency-modulated output light 101,1
02 is subjected to intensity modulation by external modulators 17 and 18 of LiNbO ₃ by intensity modulation signals 206 and 207 generated by the pulse width variable circuits 15 and 16 receiving the RZ signals 204 and 205, respectively. As a result, intensity-modulated signal lights 106 and 107 are obtained. This intensity modulated signal light 10
6,107 are time division polarization multiplexed using the polarization multiplexer 21,
A polarization multiplex transmission signal light 108 is obtained. The polarization multiplexed transmission signal light 108 is an optical fiber 3 having a zero-dispersion wavelength in the 1.3 μm band.
Is transmitted, and becomes a received signal light 109, which is detected by the optical receiver 32. In the optical receiver 32, the avalanche photodiode 22 is used as a photoelectric conversion element.
After converting 109 into an electric signal, it is amplified by an electric amplifier 23 to obtain an information signal.

Next, the operation of the main part of the optical transmission device 31 will be described. FIG. 4 is a timing chart showing the phase relationship between the clock signal and the signal light. The intensity modulated signals 206 and 207 are
The phase difference becomes 1/2 time slot, and the pulse width is arbitrarily set within the range of one time slot using the pulse width variable circuits 15 and 16. In this case, the pulse width is expanded as much as possible within one time slot. In the intensity-modulated signal lights 106 and 107, which are the output lights of the external modulators 17 and 18, the variable delay units 11 and 1 are configured so that the optical frequency is low at the rising portion of the mark signal and high at the falling portion.
2, the frequency modulation signal 202 to the semiconductor laser light sources 1 and 2
The phase difference between 203 and the intensity modulation signals 206 and 207 was adjusted respectively. Also, since optical multiplexing with small loss is possible by polarization multiplexing, optical transmission power of +2 dBm is realized. In the transmission experiment, when using the conventional precharge method, 10 Gb
In the case of / s, the transmittable distance was about 50 km, but in the case of transmission in this embodiment, a received waveform with small waveform distortion was obtained even after 100 km transmission, and good long-distance transmission without code error Can be realized.

There are various other modified examples of the second optical transmission device of the present invention. The light source is not limited to the 1.5 μm band light source, but may be a 1.3 μm band or another wavelength. As the external modulator, a semiconductor external modulator may be used instead of the LiNbO ₃ modulator. Further, the bit rate is not limited to 5 Gb / s, but may be 2 Gb / s or 10 Gb / s.
The waveform for frequency-modulating the output light of the semiconductor laser light source is not limited to a sine wave but may be a sawtooth wave or a triangular wave. The transmission path may be a transmission path including an optical amplifier as an amplification repeater in the middle. The zero-dispersion wavelength of the optical fiber in the transmission path is not limited to the 1.3 μm band. The configuration of the receiver is not limited to direct detection, and heterodyne detection can be used.

(Effects of the Invention) As described above, according to the present invention, even in high-speed and long-distance transmission in which the influence of dispersion in a transmission line is large, an optical transmission apparatus capable of reducing or compensating for the influence of dispersion is provided. Can be obtained. Further, the loss in the transmission unit can be reduced by polarization multiplexing.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of a first optical transmission apparatus of the present invention, FIG. 2 is a timing chart showing the state of each part during operation of the embodiment of FIG. 1, and FIG. FIG. 4 is a configuration diagram of one embodiment of the second optical transmission device of the present invention, and FIG. 4 is a timing chart showing the state of each part during operation of the embodiment of FIG. 1,2 ... semiconductor laser light source, 3 ... optical fiber, 4,5 ...
DC bias source, 6,7 Adder, 8 Clock generator, 9,10 Variable attenuator, 11,12 Variable delayer, 13,14
…………………………………………………………………………………………………………………………………………………………………………………………………………………. ·diode,
23 ... electric amplifier, 31 ... optical transmitter, 32 ... optical receiver,
101, 102 ... output light, 103, 104 ... branch light, 105 ... delay light, 106, 107 ... intensity modulated signal light, 108 ... polarization multiplexed transmission signal light, 109 ... received signal light, 201 ... clock signal, 20
2,203: frequency modulation signal, 204,205 ... RZ signal, 206,20
7 ... Intensity modulated signal.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/142 10/18 H04J 14/08

Claims (2)

(57) [Claims] 1. A clock signal source for generating a clock signal, one semiconductor laser light source, a DC bias source for applying a bias to an injection current of the semiconductor laser light source, and an output of the DC bias source and the clock signal. An adder for adding the output to the injection current of the semiconductor laser, a variable attenuator and a variable delay for respectively adjusting an amplitude and a phase of a clock signal input to the adder, and an output of the clock signal source. First and second transmission signal sources that respectively generate transmission signals independent of each other in synchronization with the clock signal of the first and second transmission signal sources, and first and second transmission signal sources that set the pulse widths of the outputs of the first and second transmission signal sources. A pulse width converter, an optical splitter for splitting the output light of the semiconductor laser light source into first and second split lights, and first and second splitters split by the optical splitter An optical delay means for providing a time difference between the first and second pulse-width converters, and intensity-modulating the first and second branch output lights using the output signals of the first and second pulse-width converters, respectively, to perform first and second intensity modulation. First and second external modulators for generating light; and an optical multiplexer for multiplexing output lights of the first and second external modulators and transmitting the multiplexed light to a transmission line, wherein the optical delay unit includes: Means provided between the optical splitter and the external modulator to provide a time difference between the split output lights, or between the external modulator and the optical multiplexer and between the intensity-modulated lights. At least one of means for giving a time difference to the first and second intensity-modulated lights, and the variable delay device uses the variable delay device so that the optical frequency of the first half in each pulse of the first and second intensity-modulated lights is low and the second half is high. Adjusts the phase of the clock signal input to the Optical transmission apparatus characterized by adjusting the amplitude of the variable attenuator clock signals input to the adder using to adjust the optical frequency shift amount of the output light of the semiconductor laser light source. 2. A clock signal source for generating a clock signal, first and second semiconductor laser light sources, and first and second biases respectively applied to injection currents of the first and second semiconductor laser light sources. A DC bias source; a first adder for adding the clock signal to an output of the first DC bias source and applying the output to an injection current of the first semiconductor laser; and the second DC bias source A second adder for adding the clock signal to the output of the second semiconductor laser and applying the output to the injection current of the second semiconductor laser, and adjusting the amplitude and phase of the clock signal input to the first adder, respectively. A first variable attenuator and a first variable delay, and a second variable attenuator and a second variable delay that respectively adjust the amplitude and the phase of the clock signal input to the second adder. , The first and second transmission signal sources respectively generating transmission signals independent of each other in synchronization with the output clock signal of the clock signal source, and the pulse widths of the outputs of the first and second transmission signal sources are set. First and second pulse width converters, and output signals of the first and second semiconductor lasers are respectively subjected to intensity modulation by using output signals of the first and second pulse width converters, thereby obtaining first and second pulse widths. A first and a second external modulator for generating intensity-modulated light, and an optical multiplexer for multiplexing output lights of the first and second external modulators and transmitting the multiplexed light to a transmission path, And the first half of each pulse of the second externally modulated light has a lower optical frequency and the latter has a higher optical frequency, and the first and second semiconductor laser light sources have different generation timings. 1st and 2nd variable delay To adjust the phases of the clock signals input to the first and second adders, respectively, and to adjust the optical frequency displacement of the output light of the first and second semiconductor laser light sources. An optical transmission device, wherein the amplitude of a clock signal input to each of the first and second adders is adjusted using first and second variable attenuators.