JP2705293B2 - 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 modulation signal of light is obtained by changing an injection current to a semiconductor laser, and the intensity modulation signal is transmitted through an optical fiber which is a transmission line, and is used for a PIN diode or the like. An intensity modulation-direct detection communication device that receives the intensity modulation signal with an optical receiver using a photoelectric conversion element is mainly used. In this communication device, when communication is performed at a transmission speed of gigabit or more in 1.5 μm band transmission, which is the wavelength band where the loss of the optical fiber is minimized, the quality of the transmission deteriorates after transmission due to the effect of dispersion of the optical fiber. (M. Shikada et al., “Long-distance Gi
gabit-Range Optical Fiber Transmission Experiment
s Employing DFB-LD's and In GaAs-APD's "IEEE, Jour
nal of Lightwave Technology, Vol. LT-5, No. 10, pp. 148
8-1497).

In addition, in an optical heterodyne communication device that obtains information by receiving information on the frequency, phase, and amplitude of light and receiving a beat with local light on the receiving side, the spectrum spread due to chirping that occurs when the semiconductor laser is directly modulated is reduced. Since there is no influence, the influence of the dispersion of the optical fiber is smaller than that of the intensity modulation-direct detection communication device. However, it has been reported that degradation occurs in ultra-high-speed and long-distance transmissions (N. Takach
io et al., “Chromatic Dispersion Equalization in a
n 8 Gb / s 202 km CPFSK Transmission Experiment "17 t
h Conference on Integrated Optics and Optical Fibe
r Communication, Post-deadline Papers 20 PDA-1
3).

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 also been actively conducted (S. Yamada).
moto et al., “516km 2.4 Gb / s Optical Fiber Transmi
ssion Experiment using 10 Semiconductor Laser Ampl
ifiers and Measurement of Jitter Accumulation "17 t
h Conferece on Integrated Optics and Optical Fiber
Communication, post−deadline Papers 20 PDA−
9). In such a direct amplification relay system, the possibility of extending the transmission distance by compensating for the loss can be expected, 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 optical communication is limited by the influence of the power loss and the dispersion. In high-speed transmission of several gigabits or more, the signal light has a large spectrum spread due to modulation, and the effect of dispersion increases, and the dispersion is limited before the loss of the optical fiber is limited. In ultra-long distance transmission using an optical amplifier as an amplifying repeater, the optical amplifier compensates for the loss, so that 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 varies depending on the frequency of light input to the optical fiber. 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 band zero-dispersion fiber, the propagation speed of the short wavelength side component (high frequency signal component) in the signal light is high, and the long wavelength side component (low frequency signal component) is transmitted. ) Is slow. 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,
In some cases, an appropriate frequency modulation is applied to 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 in front and a high frequency in the back. It has already been reported that the method reduces the waveform distortion and extends the transmittable distance (N. Henmi et al., “A Novel Dispersion Compensation”.
Technique for Multigigabit Transmission with Norm
al Optical Fiber at 1.5um Wavelength "Optical Fiber
Communication Conference '90, Post-deadline Paper
s PD-8). This dispersion compensation method is called a pre-chirp method. By this method, in a 10 Gb / s transmission system, the transmittable distance can be increased from 20 km to 50 km.

(Problems to be Solved by the Invention) In the prechirp method described above, if the pulse width of one pulse is expanded to one or more time slots and the optical frequency is appropriately changed within the pulse, the transmission distance theoretically increases. It can be further expanded. 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.

Therefore, the transmittable distance using the above-described prechirp method is theoretically limited, and is about 2.5 times the transmittable distance using a normal external modulation method.

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) An optical transmitter according to the present invention comprises a clock signal source for generating a clock signal, one semiconductor laser light source, and a DC bias source for applying a bias to an injection current of the semiconductor laser light source. An adder that adds the output of the DC bias source and the clock signal and applies the output to the injection current of the semiconductor laser; and a variable attenuator that adjusts the amplitude and phase of the clock signal input to the adder. A variable delay unit, n (n is a positive integer of 3 or more) transmission signal sources that generate transmission signals independent of each other in synchronization with a clock signal output from the clock signal source, and the n transmissions N pulse width modulators each for setting a pulse width of an output of a signal source, an optical splitter for splitting output light of the semiconductor laser light source into n paths, and the n pulses N external modulators for respectively generating intensity-modulated light obtained by intensity-modulating the n-piece branched output light using an output signal of the width modulator, and optical delay means for giving a time difference between the n-pieces of intensity-modulated light When,
An optical multiplexer for multiplexing n intensity-modulated lights output from the n external modulators and transmitting the multiplexed light to a transmission path,
The optical delay means is provided between the optical splitter and the external modulator to provide a time difference between the n split output lights, or the optical delay means and the optical multiplexer At least one of means for providing a time difference between the n intensity-modulated lights provided between the n intensity-modulated lights so that the optical frequency of the first half in each pulse of the n intensity-modulated lights is low and the second half is high. The variable delay device is used to adjust the phase of the clock signal input to the adder, and the variable attenuator is used 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 device is adjusted.

(Operation) The present invention has the following features. The transmission signal light set to the optimum optical frequency change state within one time slot is divided into a plurality (n) of systems, and the intensity is modulated with a pulse width within a range of one time slot in 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 to which the n number of time-series signals are added is subjected to pulse width compression by propagating through a transmission path, and is multiplexed time slot after transmission (1 / n of one time slot before multiplexing). , A sufficient eye opening can be obtained.

(Embodiment) FIG. 1 is a configuration diagram showing an embodiment of the optical transmission apparatus of the present invention. In the present embodiment, there is shown a case where there are two signal sources independent of each other and synchronized with each other and the transmission signal light is two-multiplexed.

First, the configuration of the optical transmission device 31 will be described with reference to FIG.
Semiconductor laser light source 1 that oscillates in single longitudinal mode in 1.5μm band
, The frequency modulation signal 202 and the DC bias current are added by the adder 4 and applied. Frequency modulated signal 202
Is obtained by processing the clock signal 201 from the clock generator 5 that generates a sine wave at a clock frequency of 5 GHz by the variable attenuator 6 and the variable delay unit 7. On the other hand, the DC bias current is supplied from the DC bias source 3. The semiconductor laser light source 1 outputs the light 101 frequency-modulated by the frequency modulation signal 202. At this time, the output light 101
In order to modulate the injection current of the semiconductor laser light source 1, intensity modulation and frequency modulation are performed simultaneously. 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, since the intensity modulation is synchronized with the intensity modulation performed by the external modulators 12, 13, there is no problem. The output light 101 is split by the optical coupler 19 into two split lights 102 and 103. The two signal sources 8 and 9 are synchronized with the clock generator 5 and 5 Gb / s
RZ signals 203 and 204 are generated independently of each other. RZ signal 203
Is LiN as the intensity modulated signal 205 through the pulse width variable circuit 10.
bO ₃ is provided to an external modulator 12. RZ signal 204 is the RZ signal
Similarly to 203, the intensity modulated signal 2
06 is given to the external modulator 13 of LiNbO ₃ .

One branch light 102 is intensity-modulated by the external modulator 12 using the intensity modulation signal 205. The other split light 103 is
The optical delay circuit 21 delays the branched light 102 by a predetermined time to become a delayed light 104. The delay light 104 is intensity-modulated by the external modulator 13 of LiNbO ₃ using the intensity modulation signal 206. As a result, transmission signal lights 105 and 106 are obtained. The transmission signal light 105 and the transmission signal light 106 are time-multiplexed by the optical coupler 20, and a combined transmission signal light 107 is obtained. Combined transmission signal light 1
Reference numeral 07 denotes a received signal light 108 after being transmitted through the optical fiber 2 having a zero-dispersion wavelength in the 1.3 μm band. The received signal light 108 is detected by the optical receiver 32. In the optical receiver 32, an avalanche photodiode 22 is used as a photoelectric conversion element, and an electric signal converted from the received signal light 108 is amplified by an electric amplifier 23 to obtain a signal.

Next, the operation of the main part of the optical transmission device 31 will be described in more detail. FIG. 2 is a timing chart showing the phase relationship between the clock signal and the signal light.

The phase difference between the intensity modulation signal 205 and the intensity modulation signal 206 is 1/2.
The time width is set as a time slot, and the pulse width of the pulse is arbitrarily set within the range of one time slot using the pulse width variable circuits 10 and 11. In this case, the pulse width is expanded as much as possible within one time slot. By adjusting the phase difference between the frequency modulation signal 202 and the intensity modulation signals 205 and 206 by using the variable delay device 7, the optical frequency of each of the transmission signals 105 and 106 becomes lower at the rising portion of the mark signal and becomes lower at the falling portion of the mark signal. Set to be higher. As a result, when using the conventional prechirp method, the transmittable distance was about 50 km at 10 Gb / s, but when transmitted with this device, a received waveform with small waveform distortion was obtained even after 100 km transmission. As a result, good transmission without any code error could be realized.

The present invention has various other modifications. 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. The number of time multiplexing is not limited to two but may be three or more. An optical amplifier may be used to compensate for the loss of light intensity caused by splitting the output light of the light source. As the external modulator, a semiconductor external modulator may be used instead of the LiNbO ₃ modulator. The place where the optical delay is performed may be between the external modulator and the multiplexer or both, instead of between the splitter and the external modulator. Further, the bit rate is not limited to 5 Gb / s, but can be 10 Gb / s even at 2 Gb / s. The waveform for frequency-modulating the semiconductor laser 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 line is also 1.3
It is not limited to the μm band. The configuration of the receiver is not limited to direct detection, and heterodyne detection can be used.

(Effects of the Invention) According to the present invention, high-speed and
Even in long-distance transmission, it is possible to obtain an optical transmission device that enables transmission in which the influence of dispersion is reduced or compensated.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of an optical transmission device of the present invention,
FIG. 2 is a timing chart showing the phase relationship between each signal and each signal light during operation. 1 ... Semiconductor laser light source, 2 ... Optical fiber, 3 ... DC bias source, 4 ... Adder, 5 ... Clock generator,
6 ... variable attenuator, 7 ... variable delay unit, 8,9 ... signal source, 10,11 ... pulse width variable circuit, 12,13 ... external modulator, 19,20 ... optical coupler, 21 …… Optical delay device, 22 …… Avalanche photodiode, 23 …… Electrical amplifier, 31
... Optical transmitter 32, optical receiver.

──────────────────────────────────────────────────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 (1)

(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. N (n is a positive integer of 3 or more) transmission signal sources that respectively generate transmission signals independent of each other in synchronization with the clock signal of n, and n that sets the pulse width of the output of the n transmission signal sources. Pulse width converters, an optical splitter that splits the output light of the semiconductor laser light source into n paths, and the n number of output signals from the n pulse width converters. N external modulators for respectively generating intensity-modulated light obtained by intensity-modulating the output light, optical delay means for providing a time difference between the n intensity-modulated lights, and output from the n external modulators an optical multiplexer for multiplexing n number of intensity-modulated lights and transmitting the multiplexed light to a transmission line, wherein the optical delay means is provided between the optical splitter and the external modulator, and the n number of branch outputs are provided. Means for providing a time difference between the lights or at least one of means for providing a time difference between the n intensity-modulated lights provided between the external modulator and the optical multiplexer, wherein the n Adjusting the phase of the clock signal input to the adder by using the variable delay unit so that the first half of the pulse of the intensity-modulated light has a lower optical frequency and the latter half has a higher optical frequency; To adjust the optical frequency displacement of the output light Optical transmission apparatus characterized by adjusting the amplitude of the clock signals input to the adder using the variable attenuator.