JP3099352B2 - 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 light with an optical receiver using a photoelectric conversion element is mainly used. In this communication device, if 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 optical fiber is minimized, dispersion of the optical fiber causes the quality degradation after transmission. Is known (M.
Shikada et al., “Long-distance Gigabit-Range Opt
ical Fiber Transmission Experiments Employing DFB
−LD's and InGaAs−APD's "IEEE, Journal of Lightwave
Technology, Vol. LT-5, No. 10, pp. 1488-1497).

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 8Gb / s 202km CPFSK Transmission Experiment "17th C
onference on Integrated Optics and Optical Fiber C
ommunication, Post-deadline Papers 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 also been actively conducted (S. Yamada).
moto et al., “516km 2.4Gb / s Optical Fiber Transmis
son Experiment using 10 Semiconductor Laser Amplif
iers and Measurement of Jitter Accumulatiom "17th C
onference on Integrated Optics and Optical Fiber C
ommunication, 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 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, and the influence of dispersion increases, and dispersion is restricted before loss of an optical fiber is restricted. 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 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,
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 1.5μm Wavelength "Optical Fiber
r Communication Conference '90, Post-deadline Pape
rs PD-8). This dispersion compensation method is called a pre-charging method, and 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 above-described pre-charging method, if the pulse width of one pulse is extended to one time slot or more and the optical frequency is appropriately changed within the pulse, the transmission distance theoretically becomes longer. 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-mentioned pre-charging 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 transmission device according to the present invention generates n (n is a positive integer) semiconductor laser light sources and clock signals having a predetermined phase relationship with each other, and generates each clock signal. N clock signal sources for modulating injection currents of the n semiconductor laser light sources corresponding to the above, n transmission signal sources for respectively generating transmission signals synchronized with the clock signal, and the n semiconductor lasers N external modulators that generate intensity-modulated light by intensity-modulating the output light of the light source with corresponding transmission signals, and multiplex the n intensity-modulated light output from each of the n external modulators And an optical multiplexer for transmitting to the transmission line, wherein the n clock signals generated by the n clock signal sources have different phases from each other.

(Operation) In the present invention, the transmission signal is divided into a plurality of streams, and an optimal optical frequency change state is set in a state where one time slot of each stream is longer than one time slot of the original transmission signal. And add. For example, if one transmission signal is divided into two sequences, one time slot of each sequence will be up to twice as long, so after expanding the pulse width in the double time slot and adding an optical frequency change, Just add them again.

(Embodiment) FIG. 1 is a configuration diagram showing an embodiment of the optical transmission apparatus of the present invention. This embodiment shows a case where the number of signal sources is two and the transmission signal light is two-multiplexed.

First, the configuration of the optical transmission device 31 will be described with reference to FIG.
The two semiconductor laser light sources 1 and 2 oscillate in a single longitudinal mode in the 1.5 μm band. A frequency modulation signal is applied to the semiconductor laser light source 1.
202 and the DC bias current supplied from the DC bias source 4 are added by the adder 6 and applied. Like the semiconductor laser light source 1, the frequency modulation signal 203 and the DC bias current from the DC bias source 5 are added to the semiconductor laser light source 2 by the adder 7 and applied. The frequency modulation signal 202 is obtained by converting the clock signal 201 from the clock generator 8 which generates a sine wave at a clock frequency of 5 GHz into a variable attenuator 9.
And a variable delay unit 10 for processing.
The frequency modulation signal 203 converts the clock signal 201 into a variable attenuator 11
And processing by the variable delay unit 12.
The semiconductor laser light source 1 outputs light 101 frequency-modulated by the frequency modulation signal 202, and the semiconductor laser light source 2 outputs light 102 frequency-modulated by the frequency modulation signal 203.

The two signal sources 13 and 14 are synchronized with the clock generator 8 and 5 Gb
/ s RZ signals 204 and 205 are generated, respectively. The RZ signal 204 passes through the pulse width variable circuit 15 and becomes LiNbO ₃
Are provided to an external modulator 17. The RZ signal 205 is the RZ signal 204
Similarly, the intensity modulated signal 207
To the LiNbO ₃ external modulator 18 as

The external modulator 17 intensity-modulates the output light 101 from the semiconductor laser light source 1 using the intensity modulation signal 206, and outputs the output light 101 as a transmission signal light 103. The external modulator 18 uses the intensity modulation signal 207 to output light 102 from the semiconductor laser light source 2.
Is intensity-modulated, and the output light 102 is output as a transmission signal light 104.

The transmission signal light 103 and the transmission signal light 104 are time-multiplexed by the optical coupler 19 to obtain a combined transmission signal light 105. The multiplexed transmission signal light 105 becomes a reception signal light 106 after being transmitted through the optical fiber 3 having a zero dispersion wavelength in the 1.3 μm band. The received signal light 106 is detected by the optical receiver 32. In the optical receiver 32, the avalanche photodiode 20 is used as a photoelectric conversion element, and the electric signal converted from the received signal light 106 is amplified by the electric amplifier 21 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 204 and the intensity modulation signal 205 is 1/2
The pulse width of the pulse is set to twice the original pulse width using the pulse width variable circuits 15 and 16. By using the variable delay devices 10 and 12 to adjust the phase difference between the frequency modulation signal 202 and the intensity modulation signal 206 and the phase difference between the frequency modulation signal 203 and the intensity modulation signal 207, respectively, each transmission signal light 103 and 104 is The optical frequency is set so that the optical frequency becomes lower at the rising portion of the mark signal and becomes higher at the falling portion.

As a result, when using the conventional bleaching method, 10 Gb
At / s, the transmittable distance was about 50 km, but when transmitted with this device, a received waveform with small waveform distortion was obtained even after 100 km transmission, and good transmission without code errors was realized. Was completed.

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. Li as external modulator
A semiconductor external modulator may be used instead of the NbO ₃ modulator. The bit rate is not limited to 5 Gb / s, but can be 2 Gb / s or 10 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 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) 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. 1,2 ... semiconductor laser light source, 3 ... optical fiber, 4,5 ...
DC bias source, 6,7 Adder, 8 Clock generator, 9,11 Variable attenuator, 10,12 Variable delayer 13,14
………………………………………………………………………………………………………………………………………………………………………………………………………………. Device, 32 ... optical receiver.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04B 10/142 H04J 14/08 (56) References JP-A-2-107704 (JP, A) JP-A-2-167524 (JP) JP-A-63-248234 (JP, A) JP-A-64-51834 (JP, A) JP-A-1-195428 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) H04B 10/00 G02F 1/01 H01S 5/06 H04J 14/08

Claims (1)

(57) [Claims] 1. A method of generating n (n is a positive integer) semiconductor laser light sources and clock signals having a predetermined phase relationship with each other, and injecting the n semiconductor laser light sources corresponding to the respective clock signals. N clock signal sources for modulating current, n transmission signal sources for generating transmission signals synchronized with the clock signal, and output light of the n semiconductor laser light sources with corresponding transmission signals for respective intensities. And n optical modulators for modulating to generate intensity-modulated light, and an optical multiplexer for multiplexing n intensity-modulated lights output from the n external modulators and sending the multiplexed light to a transmission path. An optical transmitter, wherein the n clock signals generated by the n clock signal sources have different phases.