JPH04123482A - Light transmitter - Google Patents

Abstract

PURPOSE:To reduce a rising jitter due to a zero-bias modulation by a simple addition element by forming a rise differentiated signal of a pulse signal to be input to a semiconductor laser, combining the signal with the pulse signal to be input to the laser, and pulse-signal-modulating the laser by the combined signal. CONSTITUTION:A differentiator 3 generates a differentiated signal of an input pulse signal. and is set to a time constant of a minimum pulse width or less by using a CR circuit, an LR circuit, etc. A half-wave rectifier 4 outputs part of the input pulse differentiated signal, and outputs a differentiated signal of a polarity corresponding to the rise of the input pulse. A driver 2 is a drive signal generator of a semiconductor laser, and converts the pulse signal to a signal of a driving level of the laser. When the drive signal responsive to the pulse signal is generated, the output of the rectifier 4 is simultaneously combined and produced. The differentiated signal is combined with the input pulse to drive the laser. Thus, the current width of the rise is increased, shortening of an oscillation delay time due to a zero-bias modulation is facilitated, and a rising jitter due to a pattern effect can be suppressed to a minimum.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a digital optical transmission device that achieves cost reduction.

(Prior Art) Optical transmission devices occupy an important position as high-speed, long-distance communication devices. In particular, when a semiconductor laser is used as a light source, it has become possible to carry out multi-gigabit transmission over distances of tens of kilometers or more, which would be difficult to achieve with electrical signal transmission.

In recent years, efforts have been made to apply optical transmission to short-distance signal transmission, focusing on the high speed of such optical transmission equipment and its resistance to electromagnetic interference, which is one of its original characteristics. . For example, there are attempts to introduce optical transmission between boards and frames of high-speed digital equipment, etc., and to transmit signals in the gigabit class in the main processing departments. In such signal transmission, the transmission distance is not very long, for example, in the case of inter-frame transmission, the maximum length is about 10 m to 100 m, which is a distance that hardly causes any problems in terms of the performance of the optical transmission equipment. Although such high-speed transmission is not impossible in electrical signal transmission, it is not a technology that can be easily put to practical use even when transmitting several gigabit signals over a distance of about 10m. This means that electrical signal transmission suffers from problems such as loss in the signal line, induced noise, and loop noise caused by connecting the ground wire, making it less expensive to transmit signals, and also because the electrical cables capable of gear bit class transmission become larger. This is due to the fact that restrictions tend to be imposed on the number of lines and the mounting form.

Under these circumstances, it is almost inevitable that optical transmission devices be introduced into high-speed digital equipment such as high-speed computer image information processing equipment. However, when attempting to introduce optical transmission equipment for this purpose, the cost of the equipment poses a major obstacle. Below, circumstances that make it difficult to reduce the price of conventional optical transmission equipment will be explained.

In general trunk systems and optical transmission equipment, semiconductor lasers are used as light sources for gigabit per second class signal transmission. Gigabit signal transmission uses light emitting diodes (Li
This is also possible with a light emitting diode (hereinafter referred to as LED), but in LEDs, the main controlling factor for response speed is the carrier lifetime, and operations such as adding high concentrations of impurities are used to shorten the carrier lifetime. There is a need to do. In this case, since the carrier lifetime becomes short, the luminous efficiency inevitably decreases. Furthermore, it is difficult to ensure reproducibility in the carrier life shortening operation, and practical systems using LEDs have a transmission speed of about several hundred megabits per second. On the other hand, when a semiconductor laser is used, response speed can be easily improved by stimulated emission, and a semiconductor laser is generally used as a light source in an optical transmission device of several gigabits per second. However, when a semiconductor laser is used as a light source, it is difficult to reduce the cost of the device, and it is difficult to apply it to high-speed digital equipment as described above. The main reason for this is the instability of semiconductor laser characteristics. 7th
The figure shows an example of the characteristics of a semiconductor laser. The characteristic bending point in the figure is called the threshold current, and is the boundary point at which the semiconductor laser starts operating.

The threshold current of semiconductor lasers tends to fluctuate depending on the temperature, and generally APC (^cto) is used to automatically control the optical output.
ma+ic Power "Control", hereinafter referred to as APC
) circuit is required, including a monitor photodiode,
Requires comparison/calibration circuit, etc. Further, semiconductor lasers tend to cause instability in operation when the light emitted by the semiconductor laser is reflected back. Due to such unstable characteristics, devices using semiconductor lasers require special additional components such as additional mechanisms such as APC circuits and anti-reflection treatment of the optical system.
Furthermore, since these assembly adjustments must be performed precisely, the costs for the peripheral parts and their assembly and adjustment become enormous compared to the price of the semiconductor laser element.

When the present invention is intended to be applied to high-speed digital equipment as described above, it is possible to reduce the price by taking into consideration the short transmission distance. In other words, in the case of short-distance transmission, the transmitted light output does not need to be large in the trunk optical transmission equipment, and therefore the optical system can be significantly simplified, and only a portion of the output light from the semiconductor laser can be transferred to the optical fiber. Since it is only necessary to introduce the semiconductor laser and the optical fiber, it becomes possible to directly couple the semiconductor laser and the optical fiber. Therefore, there is no need to use special lenses in the optical system, and the semiconductor laser and optical fiber can be directly coupled at a suitable distance, making it possible to significantly reduce the cost of optical components and their adjustment costs. Become. Further, since the output light from the semiconductor laser has a divergence angle of about 20° to 30°, a wide tolerance can be obtained for the alignment tolerance of the optical fiber. This eliminates the need for precise positioning and only requires rough adjustment during assembly, which also makes it possible to reduce assembly costs.

Although it is possible to reduce the cost of the optical system in a short-distance optical transmission device as described above, the characteristic fluctuation due to temperature still remains unsolved. Therefore, as a method of suppressing characteristic fluctuations due to temperature, attempts have been made to make the threshold current of the semiconductor laser very small and increase the drive current relatively to the threshold current. For example, the threshold current of a semiconductor laser can be reduced to 5 mA or less, and the drive current can be reduced to 5 mA or less.
If the value is 0 mA or more, the fluctuation of the threshold current due to temperature can be substantially ignored. In this method, the above-mentioned A
Additional parts such as PC circuits can be reduced;
This eliminates circuit-like adjustment work. If an optical transmission device using a semiconductor laser with a low threshold current and limited to short-distance transmission is put into practical use, it will be possible to achieve a significant cost reduction that is not possible with conventional optical transmission devices, and it will be possible to realize ultra-high-speed digital transmission. The device has become easier to implement and has made a huge contribution to industry.

However, the above-mentioned optical transmission device has the difficulty of causing problems depending on the practitioner. That is, if a semiconductor laser with a low threshold current is used and an additional circuit dedicated to the APC circuit is omitted, bias control of the semiconductor laser inevitably becomes impossible, and so-called zero-by-two-female modulation must be performed. . In this zero bias modulation, the second problem is the well-known oscillation delay time. FIG. 8 schematically shows the pulse modulated light 111 power of a semiconductor laser, and the waveform shown by the broken line is a general modulation waveform when a DC bias current is applied near or above the threshold current. The solid line is the modulation waveform when no DC bias current is applied.

As can be seen from the figure, in so-called zero bias modulation, l,
This causes an oscillation delay at the rising edge that is related to the threshold current value, the drive current value, and the drive current value. This oscillation delay can be considered as the time until carrier filling corresponding to the threshold current value is performed, and in the case of modulation by a single pulse, it has approximately the same value. In the case of actual signals, the arrangement of pulse trains must be regarded as random, and it is also known that the oscillation delay time varies depending on the pattern of the pulse train. The reason for this is that the level of residual carriers remaining in the semiconductor laser varies depending on whether the previous pulse is "0" or "1".

Therefore, when there are many residual carriers, the oscillation delay time becomes close to that equivalent to applying a DC bias, and the oscillation delay time becomes short, whereas when there are few residual carriers, the oscillation delay time becomes close to pure zero bias modulation, and the oscillation delay time becomes short. becomes longer. This phenomenon is called pattern effect and is known as rising jitter in zero bias modulation. This rise jitter causes identification errors in transmission signals, which is a major problem that lowers the reliability of optical transmission equipment.

Conventionally, in order to prevent such rising jitter, methods have been used to add a short pulse to the rising edge of the pulse modulated signal, or to add an undershoot to the falling edge of the pulse modulated signal to forcibly draw out the residual carriers. Ta. However, conventional methods have had the adverse effect of complicating the semiconductor laser drive circuit and increasing the price of the optical transmission device again.

(Problems to be Solved by the Invention) The present invention has been made in consideration of the problems of the prior art,
The aim is to reduce the rising jitter caused by zero-bias modulation using very simple additional elements, and to provide an extremely low-cost optical transmission device that suppresses the effects of rising jitter on the receiving circuit, enabling high-quality signal transmission. It is said that

[Structure of the Invention] (Means for Solving the Problems) In the present invention, when performing zero bias modulation of a semiconductor 1 noser,
Pulse modulation is performed by combining the rising differential signal of the input pulse to the semiconductor laser into a pulse. Further, in the optical reception, a falling differential signal is formed at the falling edge of the optical pulse, and the C reception state is set using the falling differential signal.

(Function) According to the present invention, a high-performance optical transmission device can be realized while suppressing a rise in device cost because the rise jitter in semiconductor laser zero-bias modulation is suppressed using very simple additional elements. Since the reception state is set using the differential signal of the pulse trailing edge, deterioration in transmission quality due to semiconductor laser zero bias modulation can be suppressed while keeping the device price approximately the same.

(Example) Examples of the present invention will be described below with reference to the drawings.

FIG. 1 is a configuration block diagram showing an optical transmission section of an optical transmission apparatus according to a first aspect of the present invention. 1 is a semiconductor laser, 2 is a drive circuit, 3 is a differential circuit, 4 is a half-wave rectifier circuit, 5 is an input buffer circuit, and 6 is a signal input terminal. The buffer circuit 5 performs input impedance matching for the input signal line or signal generation circuit, and prevents the influence of the drive circuit 2 and the differentiation circuit 3 from flowing backward. For example, the buffer circuit 5 is used for an impedance conversion circuit or Use a buffer amplifier. The differentiating circuit 3 generates a differentiated signal of the input pulse signal, and uses a CR circuit, LR circuit, etc. to set the time constant to be less than the minimum pulse width, for example, less than 1/4 of the minimum pulse width. The half-wave rectifier circuit 4 is a circuit that takes out one side of the input pulse differential signal, and uses the base input characteristics of the diode or transistor to take out the differential signal of the polarity corresponding to the rising edge of the semiconductor laser input pulse. The drive circuit 2 is a drive signal generator for a semiconductor laser, and converts an input pulse signal into a signal at a drive level for the semiconductor laser. Further, the drive circuit 2 is configured to simultaneously generate a drive signal corresponding to the input callus signal and simultaneously synthesize and output the output of the half-wave rectifier circuit 4. For example, the input pulse signal is input to one input terminal of the differential amplifier circuit. and connect the half-wave rectifier circuit output to the opposite phase input terminal or constant current generator. FIG. 2 schematically shows the main signal waveforms in the configuration of FIG. 1. Figure 2 (a) shows the input pulse waveform or buffer circuit output waveform, Figure 2 (b) shows the output waveform of the half-wave rectifier circuit 4, and the broken line shows the waveform removed by the half-wave rectifier circuit from the output waveform of the differentiating circuit 3. , (c) in the same figure shows the waveform output from the drive circuit 2, and (d) in the same figure shows the optical output waveform from the semiconductor laser 1.

As is clear from these figures, in the present invention, the semiconductor laser is driven by combining the rising differential signal of the input pulse. Of course, at this time, no DC bias current is applied to the semiconductor laser 1, and bias-free driving is performed. As a result, in the optical transmission device according to the embodiment of the present invention, the semiconductor laser 1 is driven without using bias APC, and a short pulse can be added to the rising edge of the semiconductor laser drive signal by adding an extremely simple circuit. Therefore, the current amplification at the rising portion of the semiconductor laser becomes large, the oscillation delay time can be easily shortened by zero bias modulation, and the rising jitter due to pattern effects can be suppressed to a minimum. Also, since the short pulse added to the rising edge of the signal is a differential signal,
It is possible to set the differential signal time constant so as to cancel out the rise time constant of the internal carrier density caused by the carrier life of the semiconductor laser. In this case, the substantial semiconductor laser internal carrier density rise time can be made equal to that when a DC bias current is applied. Here, it will be specifically shown that the embodiment of the present invention shown in FIG. 1 can be implemented by adding an extremely simple circuit. FIG. 3 is a circuit configuration diagram embodying the differentiator circuit 3 and half-wave rectifier circuit 4 of the embodiment shown in FIG. 3. The differentiating circuit shown in FIG. 3 can be constructed with a CR differentiator, and the half-wave rectifier circuit 4 can be constructed with a diode. These circuit elements do not particularly need to be externally attached, and since they are easy to integrate elements, they can be provided within one IC chip as a driving IC. In addition, the buffer circuit 5 may be an input buffer switch circuit in a normal drive circuit, and is not newly provided for the purpose of the present invention, but may be a part of a normal semiconductor laser drive circuit that is practically used. be.

Therefore, in order to implement the present invention, it is only necessary to add a few passive elements during circuit design, and there is no need for major circuit changes.

Next, an example will be shown regarding the second aspect of the present invention. Fourth
The figure is a configuration block diagram showing an optical receiving section of the optical transmission device of the present invention. 7 is a light receiving element such as a PIN photodiode; 8 is a preamplifier such as a transimpedance amplifier; 9 is a main amplifier; 1G is an identification circuit for reproducing digital signals; 11 is a differentiation circuit; 12 is a half-wave rectifier circuit;
3 is a reception level detection circuit which has a function of controlling the threshold signal level of the identification circuit IO. Further, I4 is a data output terminal.

In this embodiment, 13 level detection circuits and 10 discrimination circuits are used to automatically control the discrimination level.
ol, hereinafter referred to as ATC) circuit.

At this time, the differentiator circuit 11 and the half-wave rectifier circuit 12 generate the level detection timing of the level detection circuit.
A differential signal of the input pulse is extracted at the falling edge of the received optical pulse. The level detection circuit 13 performs level detection in synchronization with the output of the half-wave rectification circuit 12. At this time, the half-wave rectifier circuit output occurs when the optical signal changes from the "ON" state to the "OFF" state, so if it is left as it is, it will remain "ON".
Condition level cannot be detected. Therefore, level detection circuit 1
3. For example, peak hold is performed at a sampling period shorter than the small signal pulse period (for example, 1/4 of the minimum signal pulse period), and the discrimination threshold is determined by the hold value at the time of the output from the half-wave rectifier circuit I2. All you have to do is decide and output it. By configuring and functioning in this way, the discrimination threshold value is determined by the value immediately before the fall of the optical signal, and the following advantages appear. That is, as shown by the solid line in FIG. 8, when a semiconductor laser is subjected to zero-bias modulation, in addition to the oscillation delay time described above, an oscillation phenomenon in the direction of light output at the rising edge of oscillation tends to occur. This is the so-called Q inside the semiconductor laser.
This is because switching occurs, causing rapid fluctuations in carrier density and photon density, and so-called relaxation oscillations appear due to the interaction between carrier density and photon density. This relaxation oscillation converges and subsides in a time approximately equal to the carrier life, but during this time the initial output power often increases in a spike-like manner. Therefore, detecting the reception level during the time when this relaxation oscillation is occurring often results in incorrect reception level detection.
One of the major problems in zero-bias modulation of semiconductor lasers
It has become. In the embodiment of the present invention shown in FIG. 4, the reception level is detected using the falling part of the optical pulse as a trigger while avoiding the rising part of the optical pulse. Therefore, the optical transmission device of the present invention has the feature of suppressing reception errors due to zero-bias modulation of the semiconductor laser and ensuring transmission quality equivalent to a modulation method using a normal bias. FIG. 5 schematically shows the signal waveform according to the embodiment shown in FIG. 4, and is shown as a so-called eye pattern in which random patterns are superimposed.

Figure 5 (a) shows the output waveform from the main amplifier, Figure 5 (b) shows the output from the half-wave rectifier, the solid line shows the output waveform, and the broken line shows the removed rising edge differential waveform. This is the output waveform. As shown in FIG. 5(a), when a semiconductor laser output subjected to zero bias modulation is received, signal spikes due to relaxation oscillation and rising jitter due to the pattern effect of the oscillation delay time appear at the rising edge of the optical pulse.

It will be understood that appropriate level detection can be performed by detecting the level while avoiding the rising portion of the optical pulse. In the embodiment shown in FIG. 4, level detection was performed using the output of the half-wave rectifier circuit 12 as a trigger;
The output may be averaged. Further, as can be seen from FIG. 5(b), the output of the half-waveform rectifier circuit 12 has stable periodicity, and can be used for extracting reception timing, clock, etc.

FIG. 6 is a block diagram showing another example of the embodiment of the present invention, in which the gain control of the main amplifier 9' is performed on the same principle as the embodiment of FIG. 4. Further, in this embodiment, the output of the differentiating circuit 11 is converted into half-wave rectifying circuits 12 and 15.
An example is added in which the output of the full-wave rectifier circuit 15 is input to the timing extraction circuit 16 to extract the identification timing.

With such a configuration, it is easy to reproduce the clock, and it is also possible to provide the clock output terminal 17.

As explained above, in the present invention, even when performing zero-bias modulation of a semiconductor laser, it is possible to ensure transmission quality equivalent to that of normal bias modulation, and the optical system described above can be used as a short-distance optical digital transmission device. In addition to the simplification of the system, the cost of parts, assembly, and adjustment can be significantly reduced, making it possible to realize an extremely low-cost high-speed optical transmission device.

Note that the present invention is not limited to the above embodiments,
For example, it goes without saying that the first and second inventions can be used in combination at the same time, and that various applications are possible for detailed circuits, specification parts, etc. of each.

[Effects of the Invention] The present invention is effective in significantly reducing the cost of high-speed optical transmission equipment that uses a semiconductor laser as a light source, and has a high degree of industrial contribution as it facilitates the construction of ultra-high-speed digital equipment, etc. It has this effect.

[Brief explanation of drawings]

Figures 1 to 6 are diagrams relating to embodiments of the present invention; Figure 7;
FIG. 8 is a schematic diagram for explaining the prior art and the general characteristics of a semiconductor laser. 1... Semiconductor laser, 2... Drive circuit, 3.11.
...Differentiating circuit, 4.12... Half-wave rectifier circuit 5, input buffer circuit, 7... Photodiode, 8, Preamplifier, 9. Main amplifier, 10. ...Identification circuit 13, ...
・Level detector

Claims (2)

[Claims] (1) In an optical transmission device that uses a semiconductor laser as a light source and performs pulse signal modulation without applying a DC bias current to the semiconductor laser, a rising differential signal of a pulse signal input to the semiconductor laser is formed, and the rising differential signal is An optical transmission device characterized in that a signal and a pulse signal input to the semiconductor laser are combined, and the combined signal performs pulse signal modulation of the semiconductor laser. (2) In an optical transmission device that uses a semiconductor laser as a light source and performs pulse signal modulation without applying a DC bias current to the semiconductor laser, the optical receiver forms a falling differential signal corresponding to the rising edge of the optical pulse, and An optical transmission device characterized in that reception conditions are set using the falling differential signal.