JPH0461390A - Optical transmission device - Google Patents

Abstract

PURPOSE:To transmit signal light having strength stabilized for long time under simple device structure which is immune to the surrounding temperature by using the electric current which flows in proportion to optical power absorbed in an optical modulator and detecting the strength of optical power output from the optical modulator. CONSTITUTION:The voltage VP generated on both ends of a resistor 4 is proportional to optical power absorbed in a semiconductor optical modulator 2. The other output from a semiconductor laser 1 is input into an optical detector 3. The detector 3 is connected with a power source Vb by way of an electric current detection circuit 5. The output of the current detection circuit 5 is further input into a bias circuit 8 so that the output of the semiconductor laser 1 may be stabilized. The voltage VP is input into a comparator 7 on one side by way of an inductor 11 while the output of the current detector 5 is also input into a comparator 7 on the other side. It is possible to obtain the output voltage in proportion to the strength of signal light Oout from the output of the comparator 7 by adjusting the constant of each circuit. The bias voltage of a modulator drive circuit 6 is controlled by the application of an average circuit 20, thereby stabilizing the average strength of output light of an optical transmission device.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical transmitter, and more particularly to an optical transmitter used in high-speed digital optical communications.

[Conventional technology]

The external optical modulation method used in conventional optical transmission equipment is extremely effective as a method of suppressing the broadening of the wavelength spectrum, which causes dispersion degradation during optical fiber transmission. Since this is possible, its application to ultra-high-speed optical communication systems is being considered. A conventional example is shown in Fig. 5 ("2,4 Gb/s 5-100k optical fiber transmission experiment using InGaAsP absorption type optical modulation element", Preliminary Lectures for the 1989 Institute of Electronics, Information and Communication Engineers Spring National Conference) Conventional In an example, a semiconductor laser 1 emits light at a constant light intensity, the light is input to an optical modulator 2, and the absorption coefficient of the optical modulator 2 is changed to perform optical intensity modulation. The wavelength spectrum spread of the signal is approximately one-fifth that of the previously used method of directly modulating a semiconductor laser, making it possible to increase the transmission distance of information, which was limited by dispersion degradation, by approximately five times. It is.

[Problem to be solved by the invention]

In the external modulation method of the conventional optical transmitter described above,
In order to detect the output light from an optical modulator, a method has been adopted in which a portion of the output light is dispersed and its optical power is measured. Therefore, there is a drawback that the output light intensity from the optical transmitter is lost and the configuration of the device becomes complicated in order to control the intensity to always keep it stable.

[Means to solve the problem]

The optical transmitter of the present invention includes means for outputting a first detection signal according to the amount of light emitted from the semiconductor laser, and an electro-absorption type semiconductor optical modulator that modulates the intensity of the light emitted from the semiconductor laser and outputs it as signal light. and means for outputting a second detection signal corresponding to the light intensity of the signal light based on a comparison value with the first detection signal.

[Effect]

The transmitting device of the present invention utilizes the fact that when performing intensity modulation of light using an electro-absorption semiconductor optical modulator, a current flows that is proportional to the optical power absorbed by the optical modulator. The feature is that it detects the intensity of light output from. Fig. 3 shows its operation.

This will be explained using FIG.

One output of the semiconductor laser 1 is input to an optical modulator 2, and the other output is input to a photodetector 3. Here, by applying a driving voltage to the optical modulator 2, the intensity of light is modulated and the signal light Pout is output. FIG. 4(a) shows the modulator driving voltage, and since the absorption rate inside the optical modulator changes according to this driving voltage, the optical output intensity from the optical modulator changes as shown in FIG. 4(a). It is modulated as shown in b). At this time, a current is generated inside the optical modulator in proportion to the amount of light absorbed, so the current waveform becomes as shown in FIG. 4(c).

Here, if the optical power input to the modulator is Po, the absorption rate of the optical modulator when the optical modulator drive voltage is Vd is D, and the optical power of the output light from the optical modulator is P, then PI =
It can be expressed as Po (ID>).

At this time, if the 1f current generated in the optical modulator is 1, it is known that it is proportional to PoD.

On the other hand, a photodetector 3 is connected from the other emission end of the semiconductor laser.
If the optical power incident on the photodetector 3 is PO', since PO and Po' are in a proportional relationship, current flow occurs in the photodetector 3. is P.

is proportional to. Therefore, by determining each proportionality constant so that the intensity P of the output light from the optical modulator can be expressed as the difference between 1 and IO, the intensity of the output light from the optical modulator can be detected.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of a first embodiment of the present invention.

The semiconductor laser 1 is caused to emit light by supplying a current from the bias circuit 8, and one of the output lights of the semiconductor laser 1 is input to the electro-absorption type semiconductor optical modulator 2. One terminal of the semiconductor optical modulator 2 is connected to the output of a modulator drive circuit 6, and the other terminal is connected to ground via a resistor 4.

The semiconductor optical modulator 2 generates an optical signal P which is modulated by changing the absorption rate in the waveguide section by the modulation voltage from the modulator drive circuit 6.
Output out. Here, the voltage Vp generated across the resistor 4 is proportional to the optical power absorbed by the semiconductor optical modulator 2. Further, a resistor 10 for impedance matching is connected between the output of the modulator drive circuit 6 and the ground. The other output from the semiconductor laser 1 is input to a photodetector 3, which is connected to a power supply vb via a current detector F#I5. Further, the output of the current detection circuit 5 is input to the bias circuit 8, and the output of the semiconductor laser 1 is stabilized. The pressure Vp is input to one side of the comparator 7 via the inductor 11. The output of the current detector 5 is also input to the other comparator 7, and by adjusting the constants of each circuit, an output voltage proportional to the intensity of the signal light Pout is obtained from the output of the comparator 7. An average north circuit n20 is connected to the output of this comparator 7, and the output of the averaging circuit 20 is used to control the bias voltage of the modulator drive circuit 6, thereby stabilizing the average intensity of the output light of the optical transmitter. conduct.

By using this embodiment, the output intensity fluctuation due to temperature changes of the semiconductor laser or electro-absorption type semiconductor optical modulator is reduced to about one-tenth of that of the conventional one, and the output intensity is 2.4 Gb/s.
In a 1100K transmission experiment, stable operation with no code errors was achieved for 10 hours.

Next, a second embodiment of the present invention will be explained with reference to FIG.
In this embodiment, a semiconductor laser 1, a semiconductor optical modulator 2
The device is characterized by the use of a monolithically integrated device including the photodetector 3 and the photodetector 3. The connections of the optical elements within this monolithic integrated element are similar to the first embodiment. However, since the electrodes on one side of these optical elements are common, the resistor 4 for detecting the current flowing through the electroabsorption type semiconductor optical modulator 2 is connected between the semiconductor optical modulator 2 and the modulator drive circuit 6. connected to. Both ends of this resistor 4 are connected to a differential amplifier 21 via inductors 11 and 12, and a photocurrent generated in the semiconductor optical modulator 2 is detected. The configuration and connections of the circuits other than these are the same as in the first embodiment.

By using this embodiment, the optical coupling loss occurring between the semiconductor laser 1 and the semiconductor optical modulator 2, which was a problem in the first embodiment, can be reduced by about 5 dB, and the
We achieved 10 hours of operation without code errors in 120 km transmission over 100 ms.

Although two examples have been described as embodiments of the optical transmitter of the present invention, various other embodiments can be realized.For example, the first current detector connected to the semiconductor optical modulator may be , is inserted between the semiconductor optical modulator and the modulator drive circuit in both the first and second embodiments, but this is inserted between the semiconductor optical modulator and the ground, etc. It can be inserted anywhere as long as an amount proportional to the optical power of This can be easily realized by Furthermore, the comparator in this description can be implemented using any electronic circuit that is capable of detecting a difference between two inputs.

〔Effect of the invention〕

As explained above, according to the present invention, by controlling the intensity of signal light output from an electro-absorption semiconductor modulator, stable intensity can be achieved over a long period of time without being affected by ambient temperature with a simple device configuration. This has the effect of being able to transmit signal light.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a first embodiment of the present invention;
The figure is a block diagram showing the second embodiment of the present invention, FIG. 3 is a diagram showing the structure of the light output detection method of this embodiment, and FIG. 4 is for explaining the light output detection method of this embodiment. FIG. 5 is a block diagram of an example of a conventional optical transmitter. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 2... Lightning absorption type semiconductor optical modulator, 3... Photodetector, 4, 10... Resistor, 6...
...Modulator drive circuit, 7...Comparator, 8...Bias circuit, 11.12...Inductor, 20...Averaging circuit, 21...Differential amplifier.

Claims (1)

[Scope of Claims] 1. Means for outputting a first detection signal according to the amount of light emitted from the semiconductor laser, and electro-absorption type semiconductor optical modulation that modulates the intensity of the light emitted from the semiconductor laser and outputs it as signal light. 1. An optical transmitter comprising: a detector; and means for outputting a second detection signal corresponding to the optical intensity of the signal light based on a comparison value with the first detection signal. 2. The optical transmitter according to claim 1, wherein the output of the semiconductor optical modulator is controlled by a comparison output of the first and second detection signals.