JPS6248823A - Optical communication method and optical transmission equipment - Google Patents

Abstract

PURPOSE:To attain the transmission of a signal distorted to be a signal without distortion at the receiving end by using a bit pattern comprising a code sent already, a code to be sent at present and a code sent later so as to apply intensity and phase modulation of light. CONSTITUTION:A data to be sent is inputted to a shift register 201 to sample the data at the leading of a clock. An output S1 of an intermediate register holds a bit going to be sent at present, an output S2 corresponds to a bit sent precedingly and an output S0 corresponds to a bit sent next. The outputs S0-S2 are represented in binary number and they are converted into a decimal number by a binary/decimal converter 202. Outputs C0-C7 are connected respectively to different waveform generators 230-237 and signals W0-W7 generated alternately are arranged into one signal line at an analog OR circuit 204. A DC bias is applied to an output of the circuit 204 by a level shift 205 and sent to an intensity modulator via an amplifier 206. An amplifier 207 driving the phase modulator is controlled directly by a data.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical communication method and an optical transmitter that implements the method.

(Prior Art) Optical transmission lines such as single mode optical fibers inherently have group velocity dispersion and optical nonlinearity. Therefore, in an optical communication system using an optical transmission line, the light transmitted from the transmitting end is
Dispersion and nonlinearity of the optical transmission path are affected during propagation through the optical transmission path, and the way the intensity and phase of the light obtained at the receiving end changes is generally different from the state at the transmitting end. However, conventional optical communication systems transmit light suitable for code identification, that is, angle-modulated light that is close to the ideal and avoids changes in light intensity as much as possible.

(Problem to be solved by the invention) When transmitting light suitable for identification at the receiving end in this way,
As mentioned above, as the light propagates through the optical transmission path, the intensity and phase of the light change in different ways due to the effects of dispersion and nonlinearity.As a result, the light after long-distance propagation is unsuitable for code identification. This controlled the achievable transmission path distance and transmission speed (bit rate).

An object of the present invention is to provide a method that enables increasing the transmission path distance and/or increasing the transmission speed (bit rate) of an optical transmission system using an optical transmission path having group velocity dispersion and optical nonlinearity. That's true. Another object of the present invention is to provide an optical transmitter that enables an optical transmission system using an optical transmission line with dispersion and nonlinearity to have a longer transmission line length and a higher transmission speed. .

(Means for Solving the Problems) The present invention is an optical communication method characterized by transmitting a distorted signal in consideration of the characteristics of an optical transmission path so that the signal is distortion-free at the receiving end. ing. The present invention also includes a light source, a modulating means for modulating the angle (phase or frequency) and intensity of the output light from the light source, data to be transmitted, data already transmitted, and data to be transmitted thereafter as a device for realizing this method. and control means for controlling the modulation means by determining the modulation state of the data to be currently transmitted based on the data to be transmitted.

(Function) The optical propagation characteristics of a single mode optical fiber are well expressed by a wave equation (for example, B. P. Nelson et al., Optics Communications, Vol. 48, No. 4, 2).
92 pages, 1983). Here, the electric field strength of light is t,
(t, z) = −E (t, z) eX p[i(
wt-hz)l+multi-acting conjugate term plane wave. 2 is a coordinate representing the traveling direction of light, t is time, W is optical frequency, k
, kl, and k2 are the phase velocity, group velocity, and group velocity dispersion of light, respectively. Also, Y is the attenuation rate of light, - and n2
are the linear refractive index and the nonlinear refractive index, respectively. The unit is c
gs is used.

As mentioned above, the optical propagation characteristics of a single mode optical fiber are expressed by equation (1), so for example, if a certain type of optical pulse is transmitted from the input end of the optical fiber, the shape of the optical pulse will change depending on the propagation characteristic of the optical fiber. It is possible to predict how the material will deform as a result of this, and this has been done for a long time.

However, if we know the shape of the optical pulse during propagation through the fiber (for example, the shape of the optical pulse at the light output end of the fiber), we can use equation (1) to calculate the shape of the optical pulse in the past (for example, the shape of the optical pulse at the input end of the fiber). Light pulse shape) can be understood. This is t in equation (1)
It is clear that if we replace -t with -t (reverse the time), we get: In other words, equations (1) and (2
), the left side represents the propagation of light itself, and the right side represents the change accompanying the propagation of light.
), it can be seen that the light is traveling in the opposite direction since 1 becomes -1 by comparing the left side. Also, the signs of the terms on the right side are all reversed, which means that the light undergoes completely opposite changes during propagation.The above argument holds regardless of time and position z, so
It is possible to determine the past state of a certain light from equation (1). (This is said to mean that Equation (1) is invariant with respect to time reversal. If a phenomenon expressed by the third order differential of time is acting on light on the right side of Equation (1), the above invariance holds true. ) Therefore, in an optical communication system using an optical fiber, the optical waveform at the transmitting end can be determined from the desired optical waveform at the receiving end. - Consider as an example a phase-shift keying (PSK) coherent optical communication system. In PSK systems, information is encoded as phase changes in light. For example, the binary number “°1”
'', the phase of the light is changed from the reference value to n/2,
It is also possible to represent °'0'' by a phase shifted by -n/2 from the same standard. This state is represented by (a
). In the same figure, °'1101000111'
” information is encoded as a phase change.In this way, in the PSK system, information is encoded as a phase change, so it is desirable that the light intensity is constant.If the light intensity also changes, this change becomes noise when identifying codes at the receiving end, degrading the performance of optical communication systems.For this reason, conventional optical transmitters for PSK coherent optical communication systems generate light with constant optical intensity and phase modulation. However, as such light propagates through the optical fiber, it is strongly affected by dispersion. This tendency is especially noticeable as the attenuation rate increases. As a result, the intensity of the light that reaches the receiving end is The signal had changed significantly compared to when it was transmitted, which caused a deterioration in the performance of the system. (The phase also changes to some extent, but it is negligible when the optical power is less than a few mW. This was discovered theoretically by the author. However, according to the optical communication method of the present invention,
When the phase change due to encoding is as shown in FIG. 1(a), the above problem can be avoided by simultaneously modulating the light intensity as shown in FIG. 1(a). The intensity waveform in Fig. 1 is calculated from Equation (1) at the receiving end using the ideal electric field waveform at the receiving end, that is, the electric field waveform of constant intensity light whose phase changes as shown in the phase waveform in Fig. 1(a) as a boundary condition. Therefore, l is obtained as a waveform at the time of return, that is, approximately l in the past in terms of time. The validity of such a procedure has already been explained as the invariance of equation (1) with respect to time reversal. Therefore, if light with the phase and light intensity changes shown in Figure 1(a) is transmitted, this light will change during propagation through the optical fiber, resulting in ideal PSK signal light at the receiving end, which was impossible with conventional methods. Long distance transmission and high speed transmission are possible.

When transmitting a certain code, an optical transmitter that implements such an optical transmission method must take into consideration the codes that have already been transmitted and the codes that will be transmitted thereafter. This is clear from the light intensity waveform in FIG. 1(a). In other words, when transmitting the code ``0'', the required intensity modulation method differs depending on whether the previous code and subsequent code are 0'0'' or flits. For this reason, an optical transmitter that implements the above-mentioned optical transmission method uses a means for modulating the intensity and phase of light, and a bit pattern consisting of a code that has already been transmitted, a code that is currently to be transmitted, and a code that will be transmitted thereafter. Control means for intensity and phase modulation.

(Example) An example of the angle modulation type optical communication method and optical transmitter of the present invention will be described using FIG. The laser diode 2 in FIG. 1(b) is made of GaInAsP/InP DFB DCPB
The oscillation wavelength of the H laser is 1.55 pm. This laser diode is continuously oscillated by a laser drive circuit 1 with an output of 45 mW. The output light from the laser diode 2 is modulated by a phase modulator 3 and an intensity modulator 4, and then focused by a lens 5 and injected into a single mode optical fiber 6. The optical loss of the phase modulator 3 and the intensity modulator 4 and the optical fiber coupling loss as mentioned above were 10 dB in total, and the fiber input optical power was 4.5 mW. On the other hand, the transmitted waveform is a PSK signal shown in Figure 1(a), and the bit rate is 2Gb/5 (
NRZ). In the normal PSK system, Figure 1 (
Transmit angle-modulated light as shown in a),
Care is taken to keep the light intensity constant. However, in this transmitter, according to the optical transmission method of the present invention, the optical intensity is also as shown in FIG. 1(a).
The light intensity is also modulated as shown in . This light intensity waveform is a complete PSK signal with a light intensity of 143 dBm.
In other words, the phase was determined from equation (1) as described above using the constant intensity light shown in FIG. 1(a) as a boundary condition. However, as the parameters of equation (1), the constant of the single mode optical fiber 6, n, = 1.45°' n2 = 1. I
X1 leaf 13esu, effective radius = 10pm, 10p 1.
55pm.

Loss = 0.2dB/1cm, D = -15ps/(
nm-km) was used. The transmission distance is 230 km.

Now, the intensity modulator 4 of FIG. 1(b) used to obtain this waveform is a Pockels cell and has a half-wavelength voltage of 5V. Further, the phase modulator 3 is also a substantially similar Pockels cell and has the same half-wavelength voltage of <5V. These modulators are driven by a phase and intensity modulator drive circuit 7 (hereinafter abbreviated as modulator drive circuit) so as to obtain the waveform shown in FIG. 1(a). The configuration of the modulator drive circuit 7 is shown in FIG. 2, and the waveforms of each part in FIG. 2 are shown in FIGS. 3(a) to (1). Figure 2
Most of the elements inside are comprised of high-speed emitter-powered pulled logic circuits. The delay time per gate of this circuit is 0°2 ns, and it operates with a clock of 5 Gb/s or more.

In FIG. 2, the data to be transmitted is first sent to the shift register 20.
1 is input. The shift register 201 samples the data at the rising edge of the clock. Signal lines SO to S2 in FIG. 2 represent instantaneous values of each register. Therefore, the relationship between input data, clock, and SO to S2 is shown in Figure 3.
(a) to (e). Here, the output S1 of the intermediate register holds the bit currently being sent, and S2
corresponds to the bit that was transmitted last time, and SO corresponds to the bit that will be transmitted next time. In other words, there are eight ways to combine 3 bits, and SO to S2 are expressed as binary numbers. Therefore, this binary number is converted into a binary l-decimal converter 202.
Convert to decimal number with 1, where the bit pattern is 111
If this is the case, the C7 output will be turned on, if it is 000, the CO high output will be turned on, and if it is 101, the C5 output will be turned on. This situation is shown in FIGS. 3(f) to (h). These CO-C7 outputs are connected to waveform generators 230-237 that generate different waveforms, respectively. Therefore, different waveforms can be generated depending on the bit pattern. The waveform that should be generated here is shown in Figure 1 (a
), but this waveform generator 230
237 generates a waveform obtained by removing the bias component (DC component) from the light intensity waveform shown in FIG. 1(a). For example, the bit that has already been sent is '0', and the bit that will be sent in the future is '0'.
1", C1 is turned on and the waveform generator 231 is triggered.Then, the waveform generator 231 generates the waveform shown in FIG. 3(j). That is, the waveform generators 230 to 237
One of them is selected according to a bit pattern, and each generates an appropriate waveform according to the bit pattern. Here, in Figure 3, the lines of CO and 07, (r) and (
The reason why there is no output in h) is that the light intensity may be constant in the corresponding bit patterns of these outputs. This means that the bit pattern is 000 and 1 in Figure 1.
This can be seen from the light intensity waveform at 11. Therefore, waveform generators 230 and 237 may be omitted. Next, waveform generator 230
~237, the signals WO~W7 (Fig. 3(i)
) to (k)) are combined into one signal line by the analog OR circuit 204. Therefore, the output D of the analog OR circuit 204 becomes as shown in (1) of FIG. A DC bias is applied to this signal by the level shifter 205, and the resulting waveform is similar to the intensity waveform in FIG. 1(a). By converting this into a voltage suitable for driving the intensity modulator 4 of FIG. 1(b) using the amplifier 206,
The desired light intensity modulation shown in FIG. 1(a) is possible.

In this embodiment, for this purpose, the output voltage of the amplifier 206 is as shown in FIG.
The voltage was adjusted so that the voltage was 3.25V at the portion corresponding to the DC level in (a), and 5V at the portion corresponding to the maximum value. On the other hand, phase modulation can be performed using the same method as before. That is, the amplifier 207 driving the phase modulator in FIG. 2 is directly controlled by data. Waveform generators 230-237
consists of a combination of a delay line, a monostable multivibrator circuit, and an OR circuit. Also, the analog O in the same figure
The R circuit 204 is a multi-input amplifier circuit and also has the function of a level shifter 205. Also, due to the delay time of these circuits, a propagation delay of 6.2 ns occurred on the path leading to the amplifier 206 compared to the path leading to the amplifier 207. Although it is not shown in FIG. 2, there is a delay before the amplifier 207. I fixed this by inserting a circuit. Further, since the amplifiers 206 and 207 require an output voltage of 5 cm to drive the phase modulator 3 and the intensity modulator 4 of FIG. 1(b), they were constructed from high-speed GaAs FETs for microwave use.

When the phase modulator 3 and intensity modulator 4 shown in FIG. 1(b) were driven using the above circuit, modulation substantially close to the waveform shown in FIG. 1(a) could be performed. After this light propagates for 230 km through the single mode optical fiber 6 shown in FIG.
The PSK heterodyne receiver, which has a sensitivity of -43 dBm (NRZ), was able to secure the required error rate. Of course, some intensity variations were visible in the received light, but these consisted of frequency components much higher than the data rate and could be easily removed with a bandpass filter. As a test, when the intensity modulator 4 was removed at the transmitting end, as expected, a large change in light intensity occurred at the receiving end, making code identification completely impossible.

Although the angle modulation optical communication method and optical transmitter of the present invention have been described above using one embodiment, the present invention is not limited to this embodiment. In this embodiment, an optical communication system affected by dispersion of an optical fiber is taken as an example, but the optical transmission method of the present invention can be similarly applied even when nonlinearity of an optical fiber is a problem. Furthermore, although a PSK coherent optical communication system has been described here, the present invention can also be applied to a frequency modulation (FSK) system as is.

Furthermore, as the bit rate increases, it may be necessary to modulate not only the light intensity but also the phase of the light differently depending on the front and rear bit patterns. This is possible by driving not only a phase modulator but also a phase modulator.

Furthermore, in this embodiment, for simplicity, only the code to be transmitted and the codes before and after it were considered; however, especially when the bit rate is high, not only one code before and after but also two or more codes before and after (
Therefore, it is necessary to take into consideration a pattern consisting of 5 bits or more, but this is also a simple extension of the present embodiment. In addition, in this example, the diode laser was continuously oscillated and the intensity was modulated using an external intensity modulator, but similar results can be obtained by directly modulating the laser diode, which has almost no change in oscillation frequency even during high-speed modulation. . A similar result can also be obtained by appropriately controlling the light emitting region of a laser diode having a chirp control region and the chirp control region drive current. Further, the phase and intensity modulator drive circuit in the optical transmitter of the present invention does not require any active element faster than the data clock. Therefore, it can be realized using the same technology as the parity bit and CRC (cyclic redundancy code) generation circuit in conventional devices.

(Effects of the Invention) The present invention makes it possible to increase the transmission distance and increase the transmission speed (bit rate) of an optical transmission system in which the dispersion characteristics and nonlinearity of the optical transmission line determine the transmission limit. be.

202 is a diagram showing one embodiment of the device; in the same figure, 1 is a laser drive circuit; 2 is a laser diode; 3 is a phase modulator; 4 is an intensity modulator; 5 is a lens; 6 is a single mode light source;
is a binary l decimal converter, 230 to 237 are wave 80
-82, (D to (h) are CO to C7, (i) to (k) are WO-W7, and l indicates the waveform at D.

Claims (1)

[Claims] 1. An optical communication method characterized by transmitting a distorted signal in consideration of the characteristics of an optical transmission path so that the signal is distortion-free at the receiving end. 2. A light source, a modulating means for modulating the angle (phase or frequency) and intensity of the output light from this light source, and data to be transmitted, data to be currently transmitted based on data already transmitted and data to be transmitted thereafter. an optical transmitter comprising at least a control means for determining a modulation state of the modulation means and controlling the modulation means.