JPH04117036A - Optical transmitter - Google Patents

Abstract

PURPOSE:To attain high speed long range transmission while the effect of decentralization is reduced by dividing a transmission signal light into two series, extending one time slot of each series more than one time slot of the original transmission signal before the division, setting an optimum optical frequency change and adding the series. CONSTITUTION:The transmitter is provided with 1st and 2nd signal sources 13, 14, a semiconductor laser light source 1, a modulation signal source 8 outputting a modulation signal, an optical branching device 19 branching an output light of the light source 1, and an optical delay means 20 giving a time difference to the branched lights, and also with 1st and 2nd external modulators 17, 18 applying intensity modulation to the branched lights receiving a time difference by the means 20 with 1st and 2nd signals outputted from signal lines 13, 14 and a polarized optical multiplexer 21 applying polarized light multiplex to the output light and sending the result to the transmission line. In this case, when one transmission signal is divided into two series, since one time slot of each series is extended twice, the pulse width in the time slot is expanded and an optical frequency change is added and then polarized light multiplexing is implemented. Thus, the effect of decentralization in a long distance transmission is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical transmitter used for optical communications and the like.

(Prior art) In optical communication, intensity-modulated signal light is obtained by changing the current injected into a semiconductor laser, and the intensity-modulated signal light is transmitted through an optical fiber as a transmission path, and is then connected to a photoelectric converter such as a PIN diode. Intensity modulation-direct detection optical communication devices that receive signals with optical receivers using optical elements are in practical use. This optical communication device uses wavelengths of 1 and 5, which are the wavelength bands where the optical fiber has the lowest loss.
In μm band transmission, CM is known to be affected by optical fiber dispersion when communicating at transmission speeds of gigabit or higher, resulting in significant quality deterioration after transmission.

5hikada et al, -Long-dis
tance Gtgabft-Range Optica
l Fiber Transmssion Expe.
riments Eggp! oying DFB-LD
s and InGaAs-APD's"IEEE,
Journal of Lightvave Tech
nology, Vow, LT-5, No, 10pp
, 14811-1497).

In addition, in an optical heterodyne communication device that puts information on the frequency, phase, and amplitude of light and obtains the beat with local oscillation light on the receiving side and obtains information from the beat, chirping that occurs when directly modulating a semiconductor laser Since there is no effect of spectrum broadening, the effect of optical fiber dispersion is smaller than that of an intensity modulation/direct detection optical communication device. However, it has also been reported that deterioration occurs in ultra-high-speed, long-distance transmission (N, Takachio et at, Chro
iaLic Dispersion Equaliza
tjon in an 8 Gb/s 202 klI
CPFSK Transmission Experiment
llent-17th Conrerenee on
1tegrated 0ptfcs and 0pti
cal Fiber Counicatton, P
o5t-deadline Papers 20 PD
A-13).

On the other hand, research on optical amplifiers has been conducted in recent years, and studies on direct amplification repeating systems using optical amplifiers are also gaining momentum (S, Ya
iamoto et al, -516km 2.5
Gb/s 0ptlcal Fiber Trans
mission EXI）erillent usin
gt.

Sem1conductor La5er Aa+pl
iriers and MeasureIlant o
f JLtter Accumulatlon-17t
h ConcrincceOn Integrated
0ptics and 0ptical fiber
Comff1unication, Po5t-de
adlfne Papers 20 PDA-9).

In such a direct amplification repeater system, the possible transmission distance can be extended by compensating for loss, so it is expected to have the potential for ultra-long distance transmission.

In this way, the signal light suffers power loss in the optical fiber and also suffers waveform distortion due to the influence of dispersion. Both power loss and dispersion limit the transmission distance of optical communications. In high-speed transmission of several gigabit or more, the signal light has a large spectrum spread due to modulation, and is greatly influenced by dispersion, and is first limited by dispersion before being limited by optical fiber loss. Furthermore, in ultra-long distance transmission where an optical amplifier is used as an amplification repeater, the optical amplifier can compensate for loss limitations, but the transmission distance is limited by the effects of dispersion.

Dispersion in an optical fiber is caused by the fact that the time required for propagation differs depending on the frequency of light input into the optical fiber. Therefore, if the signal light has spectrum broadening due to modulation, the waveform will be distorted after transmission due to this spectrum broadening. For example, L, Sμ-band zero dispersion fiber with 1.5μ
When transmitting high frequency light, the short wavelength components (signal components with high frequency) in the signal light have a high propagation speed, and the long wavelength components (signal components with low frequency) have 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 pulse after transmission, making it impossible to distinguish between marks and spaces.

As a method of compensating for waveform distortion due to such dispersion, the output light of the semiconductor laser is subjected to appropriate frequency modulation, and the frequency modulated light is intensity-modulated using an external modulator, so that one part of the transmitted signal is
There is a report that the front part of the pulse has a low frequency and the rear part has a high frequency to reduce waveform distortion and extend the transmission distance (N.

Henm1 et al, -A Novel Di
spersion Compensation Tec
hnique for Multigigabit T
ransIIission with Norsal
0ptical fiber at 1.5
um Wevelength-0ptical Fi
ber Co5g+unication Conference
ence '90Poost-deacfline P
apers PD-8) o This dispersion compensation method is called the Blichave method, and with this method, 10Gb
/s transmission system, the possible transmission distance is 201C
What used to be 11 km can be expanded to 50 km.

(Problems to be Solved by the Invention) The possible transmission distance in the optical transmission device using the above-mentioned Prichab method is theoretically about 2.5 times the transmission distance using a normal external modulation method. Dispersion was not compensated for sufficiently to obtain long transmission distances.

Therefore, an object of the present invention is to provide an optical transmitter that can further expand the amount of dispersion degradation that can be compensated for.

(Means for Solving the Problems) In order to solve the above-mentioned problems, a first optical transmitting device provided by the present invention includes first and second signal sources, a semiconductor laser light source, and the first and second signal sources. a modulation signal source that outputs a modulation signal that modulates the injection current of the semiconductor laser light source in synchronization with the second signal source;
and an optical splitter that branches into a second branched light, an optical delay means for providing a time difference between the first and second branched lights branched by the optical splitter, and a time difference provided by the optical delay means. first and second external modulators that intensity-modulate the first and second branch destinations with first and second signals output from the first and second signal sources, respectively; It is characterized by comprising a polarization multiplexer that polarization-multiplexes the output light of the second external modulator and sends the polarization multiplexed light to the transmission path.

Further, a second optical transmitting device provided by the present invention includes first and second signal sources, first and second semiconductor laser light sources, and the second optical transmitter in synchronization with the first and second signal sources. first and second modulation signal sources that respectively output modulation signals that modulate the injection currents of the first and second semiconductor laser light sources; and first and second external modulators that modulate the intensity with the first and second signals output from the second signal source, respectively, and provide a time difference between the output lights of the first and second external modulators. and a polarization multiplexer for polarization multiplexing and transmitting the polarized light to a transmission path.

(Function) In the Blichave method, if the pulse width of one pulse is expanded to one time slot or more and the optical frequency is appropriately changed within that pulse, the transmission distance can theoretically be further extended. However, if the pulse width of one pulse is made greater than one time slot, it is not possible to appropriately set the change in the optical frequency in the portion where the pulse overlaps with the adjacent pulse. The present invention divides the transmitted fJ signal light into two streams, sets one time slot of each series to an optimal optical frequency change state with a length longer than the one time slot of the original transmitted signal before division, After that, each series is added together.For example,
If one transmission signal is divided into two sequences, one time slot of each sequence can be up to twice as long, but within that twice the time slot, the pulse width is widened, the optical frequency is changed, and then the pulse is added again. Just match it. Furthermore, since polarization multiplexing is performed in the transmitter, lossless optical multiplexing is theoretically possible, and a high optical transmission output can be achieved.

(Example) Next, the present invention will be explained by giving examples.

FIG. 1 is a block diagram of an embodiment of a first optical transmitter according to the present invention. In FIG. 1, the configuration of the optical transmitter 31 will be explained first. A semiconductor laser light source 1 that oscillates in a single longitudinal mode in the 1.5 μm band receives a clock signal 201 output from a clock generator 8 that generates a sine wave at a clock frequency of 5 GHz through a variable attenuator 9 and a variable delay device 11. The frequency modulation signal 202 and the DC bias current generated by passing through the adder 6 are added together and applied. This bias current is supplied from a DC bias source 4. The semiconductor laser light source 1 outputs an output light 101 that is frequency-modulated using a frequency modulation signal 202 . This frequency modulated output light 1
01 is split into two branched lights 103 and 104 by an optical coupler 19. The signal sources 13, 14 provide 5 Gb/s RZ signals 204, 20, respectively, in synchronization with the clock generator 8.
Generates 5.

One of the two branched lights 103 is converted into LiNb0. The intensity is modulated by an external modulator 17. Further, the other branched light 104 is connected to the optical delay device 2
0, the branched light 103 is time-delayed, and the delayed light 105
becomes. This delayed light 105 is an intensity modulated signal 207 generated by the pulse width variable circuit 16 manually from the RZ signal 205.
The intensity is modulated by the external modulator 18 of L i N b OM. 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 a polarization multiplexer 21, and a polarization-multiplexed transmission signal light 108 is obtained. After 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, it becomes a received signal light 109 and is detected by the optical receiver 32 . The optical receiver 32 uses an avalanche photodiode 22 as a photoelectric conversion element, converts the received signal light 109 into an electrical signal, and then amplifies it with an electrical amplifier 23 to obtain an information signal.

Next, the operation of the main parts of this optical transmission 31 will be explained. FIG. 2 shows a timing chart of the phase relationship between the clock signal and the signal light. The intensity modulated signals 206 and 207 are each controlled by variable pulse width circuits 15 and 16 so that the phase difference between them becomes 1/2 time slot and the pulse width of the pulse becomes twice the original pulse width. Intensity modulated signal light 106 which is the output light of external modulators 17 and 18
At ゜107, the optical frequency is low at the rising edge of the mark signal and high at the falling edge (so that
The variable delay device 11 was used to adjust the phase difference between the frequency modulation signal 202 and the intensity modulation signals 206 and 207 to the semiconductor laser light source 1. Further, since lossless optical multiplexing is possible by polarization multiplexing, an optical transmission power of +2 dB■ can be achieved. In transmission experiments, when using the conventional Blichave method, the possible transmission distance is approximately 50 km at 10 Gb/s.
m, but in the case of transmission in this embodiment, it is 110
Even after 0k transmission, a received waveform with small waveform distortion can be obtained, and good long-distance transmission without code errors can be realized.

There are various other variations of the first optical transmitter of the present invention. The light source is not limited to a light source in the 1°5μ band, but may also be in the 1.3μ band or other wavelengths. An optical amplifier may be used to compensate for the loss of optical intensity due to branching of the output light of the light source. As an external modulator,
A semiconductor external modulator may be used instead of the L i N b O3 modulator. The optical delay may be performed between the external modulator and the multiplexer instead of between the splitter and the external modulator, or between both. Also, the bit rate is 5
Not limited to Gb/s, 2 Gb/s or 10 Gb
/s can also be used. 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 twirl wave or a triangular wave. The transmission line may include an optical amplifier as an amplification repeater in the middle. The zero-dispersion wavelength of the optical fiber of the transmission line is not limited to the 1.3 μι band either. Further, the configuration of the receiver is not limited to direct detection, and heterodyne detection can also be used.

FIG. 3 is a configuration diagram of an embodiment of the second optical transmitter of the present invention. In FIG. 3, the configuration of the optical transmitter 31 will be explained. The two semiconductor laser light sources 1 and 2 that oscillate in a single longitudinal mode in the 1.5 μι band receive a clock signal 201 outputted from a clock generator 8 that generates a sine wave at a clock frequency of 5 GHz through variable attenuation. unit 9 and variable delay unit 1
1, the frequency modulated signals 202 and 203 generated by passing through the variable attenuator 10 and the variable delay device 12, respectively, and the DC bias current are added by adders 6 and 7 and applied. This bias current is supplied from DC bias sources 4 and 5, respectively. The semiconductor laser light source 1.2 outputs output lights 101 and 102 that are frequency modulated using frequency modulation signals 202 and 203, respectively. Signal source 13
．． 14 are each 5 Gb in synchronization with the clock generator 8.
/s RZ signals 204 and 205 are generated. The frequency-modulated output light 101, 102 is transmitted to a variable pulse width circuit 15.
16 are intensity-modulated by LiNbO3 external modulators 17 and 18 using intensity modulation signals 206 and 207 generated by inputting RZ signals 204 and 205, respectively. the result,
Intensity modulated signal lights 106 and 107 are obtained. The intensity modulated signal lights 106 and 107 are time-division polarization multiplexed using a polarization multiplexer 21 to obtain a polarization multiplexed transmission signal light 108. After the polarization multiplexed transmission signal light 108 is transmitted through the optical fiber 3 having a zero dispersion wavelength in the 1.3 μι band, it becomes a received signal light 109 and is detected by the optical receiver 32 . Optical receiver 3
In No. 2, an avalanche photodiode 22 is used as a photoelectric conversion element, and the received signal light 1(')9 is converted into an electrical signal and then amplified by an electrical amplifier 23 to obtain an information signal.

Next, the operation of the main parts of this optical transmitter 31 will be explained. FIG. 4 shows a timing chart of the phase relationship between the clock signal and the signal light. Intensity modulation signals 206, 20
7 is their phase difference or 1/2 time slot,
Each pulse width is controlled by variable pulse width circuits 15 and 16 so that the pulse width of the pulse is twice the original pulse width.

Intensity modulated signal light 1 which is the output light of external modulators 17 and 18
At 06.107, the semiconductor laser light source 1 is controlled using the variable delay devices 11 and 12 so that the optical frequency is low at the rising edge of the mark signal, and the optical frequency is low at the falling edge.
, 2, the phase differences between the frequency modulated signals 202, 203 and the intensity modulated signals 206, 207 were adjusted, respectively. Also,
Since optical multiplexing without loss is possible by polarization multiplexing, +
An optical transmission power of 2 dBm is achieved. In transmission experiments, when using the conventional Blichave method, 10Gb/s
The transmission distance was approximately 50+o,
In the case of transmission in this embodiment, a received waveform with small waveform distortion can be obtained even after 100) oI transmission, and good long-distance transmission without code errors can be realized.

There are various other variations of the second optical transmitter of the present invention. The light source is not limited to a 1.5 .mu.-band light source, but may also be a 1.3 .mu.-band or other wavelength light source. As an external modulator, L i N b O! A semiconductor external modulator may be used instead of the 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 side wave or a triangular wave. The transmission line may include an optical amplifier as an amplification repeater in the middle. The zero dispersion wavelength of the fiber at the end of the transmission line is also 1.3 μl.
It is not limited to the obi. Further, the configuration of the receiver is not limited to direct detection, and heterodyne detection can also be used.

(Effects of the Invention) As explained above, according to the present invention, an optical transmitter can perform transmission that reduces or compensates for the influence of dispersion even in high-speed and long-distance transmission where the influence of dispersion on the transmission path is large. can be obtained. Furthermore, by polarization multiplexing, loss at the transmitter can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of an embodiment of the first optical transmitter 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. 3 is a diagram of the present invention. FIG. 4 is a block diagram of an embodiment of the second optical transmitter of the 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 delay device, 13.14...Signal source, 1
5゜16... Pulse width variable circuit, 17.18... External modulator, 19... Optical coupler, 20... Optical delay device,
21... Polarization multiplexer, 22... Avalanche photo diode, 23... Electrical amplifier, 31... Optical transmitter, 32... First receiver, 101, 102... Output light, 103゜104... Branched light, 105... Delayed light, 106, 107... Intensity modulated signal light, 108...
・Polarization multiplexed transmission signal light, 109... Reception signal light, 20
1... Clock signal, 202.203... Frequency modulation signal, 204, 205... RZ double signal 206, 20
7...Intensity modulation signal. Agent Patent Attorney Shinsuke Honjo 101, Output light 103 Branched light 105: Delayed light 106, 107 Intensity variable signal light 108 Confucian light multiplexing Confucian signal light 109 Received signal light 201 Clock 1 202, frequency variable m 101, 102: Output light 106, 107: Intensity modulated signal light 108 Polarization multiplexed transmission signal light 109゛Reception signal light 202. 203 204.205 206.207 Chronok signal frequency modulation signal output signal strength modulation signal of signal source

Claims (2)

[Claims] (1) first and second signal sources, a semiconductor laser light source, and a modulation signal source that outputs a modulation signal that modulates the injection current of the semiconductor laser light source in synchronization with the first and second signal sources; , an optical splitter that branches the output light of the semiconductor laser light source into first and second branched lights, and an optical delay means that provides a time difference between the first and second branched lights branched by the optical splitter. , the first signal is given a time difference by the optical delay means.
and first and second external modulators that intensity-modulate the second branched light with first and second signals output from the first and second signal sources, respectively; An optical transmitter comprising: a polarization multiplexer that polarizes output light from an external modulator and sends the polarization multiplexed light to a transmission path. (2) first and second signal sources, first and second semiconductor laser light sources, and injection currents of the first and second semiconductor laser light sources in synchronization with the first and second signal sources; a first and a second one each outputting a modulation signal modulating the
a modulated signal source, and first and second semiconductor laser light sources that intensity-modulate the respective output lights of the first and second semiconductor laser light sources with first and second signals output from the first and second signal sources, respectively. a second external modulator;
and a polarization multiplexer that imparts a time difference to the output light of the external modulator, polarization multiplexes the polarization multiplexed light, and sends the polarization multiplexed light to a transmission path.