GB2150383A

GB2150383A - Optical transmitter distortion compensation circuit

Info

Publication number: GB2150383A
Application number: GB8429061A
Authority: GB
Inventors: Norbert Kaiser; Friedemann Mohr
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1983-11-22
Filing date: 1984-11-16
Publication date: 1985-06-26
Also published as: ES537859A0; DE3342001A1; ES8604377A1; GB8429061D0; GB2150383B

Abstract

The circuit for compensating third-order distortion caused by a laser (3) in a signal to be transmitted contains a pre-equaliser (2) ahead of the laser (3) and an evaluating circuit (4) which senses the distortion in the signal at the output of the laser (3) and controls the pre-equaliser (2). A test signal generator (5) feeds a first out of band test signal (fo) into the pre-equaliser (2), and the third-order distortion of this signal at the output of the laser (3) is compared in the evaluating circuit (4) with a second test signal (3fo) having three times the frequency of the first test signal (fo). The comparison gives a manipulated variable (I) for the pre-equaliser (2). <IMAGE>

Description

SPECIFICATION Distortion compensation circuit The present invention relates to a circuit for compensating third-order distortion caused by a transmitter in a signal to be transmitted.

A circuit of this kind is disclosed in DE-OS 30 20 995. In the equalising system described there, the signal to be transmitted is sent over two parallel paths in the pre-equaliser. The first path contains a delay line, and the second path a series combination of a harmonic generator, a phase shifter, and an attenuator. The harmonic generator generates thirdorder distortions. The pre-equaliser is followed by a travelling-wave tube amplifier acting as the transmitter. The evaluating circuit consists of a distortion detector in which the signal sensed at the output of the travelling-wave tube amplifier is converted down to a lower frequency twice and then rectified, and a distortion controller which acts on the phase shifter and the attenuator in the pre-equaliser.

The invention seeks to provide a simple distortion compensation circuit which can be used for an optical transmitter.

According to the invention there is provided a distortion compensation for compensating thirdorder distortion caused by a transmitter in a signal to be transmitted, comprising a pre-equaliser ahead of the transmitter, and an evaluating circuit sensing the distortion in the signal at the output of the transmitter and controlling the pre-equaliser, characterised in that the transmitter is a laser, that a test signal generator is present which provides a first test signal of constant frequency and amplitude and a second test signal of three times the frequency of the first test signal, that the pre-equaliser is preceded by an adder in which the signal to be transmitted and the first test signal are combined, that the evaluating circuit is supplied with the second test signal from the test signal generator, and that the evaluating circuit derives from the output signal of the laser and the second test signal a manipulated variable which controls the pre-equaliser so that the signal at the output of the laser is equalised.

The circuit permits the use of optical transmitter components which do not meet stringent linearity requirements.

In order that the invention and its various other preferred features may be understood more easily, an embodiment thereof will now be described, by way of example only, with reference to the drawings, in which: Figure lisa block diagram of a transmitter with a distortion compensation circuit constructed in accordance with the invention, Figure 2 is a block diagram showing in more detail the pre-equaliser in Figure 1, and Figures 3a and 3b are two block diagrams of the test signal generator of Figure 1.

Referring to Figure 1, the circuit for compensating third-order distortions caused by a transmitter contains an adder 1, a pre-equaliser 2 following the adder 1, an evaluating circuit 4 controlling the pre-equaliser, and a test signal generator 5. Connected to the pre-equaliser 2 is the transmitter device, in this case a laser 3. The signal SA provided by the laser, i.e., the laser light, is transmitted to its destination over an optical waveguide 7. A voltage UD corresponding to the signal SA iS applied to the evaluating circuit 4 from a photodiode 6, which receives a portion of the laser light. The test signal generator 5 applies a first test signal fo to the adder land a second test signal 3fo to the evaluating circuit 4.The signal to be transmitted Sr, iS applied to the adder 1 via an input terminal E.

In the adder 1, the signal to be transmitted, S, iS combined with the first test signal, which has a constant frequency and amplitude. This combined signal passes through the pre-equaliser 2 and is applied as modulating current to the laser 3. A regulated bias current can be applied to the laser 3 in the usual manner. The photodiode 6 senses a portion of the light emitted by the laser, and forms the voltage U0 therefrorn. In the evaluating circuit 4, a multiplier 8 multiplies this voltage UD by a second test signal 3fo. The resulting voltage U3 corresponds to the third-order distortion occuring in the laser 3. It is compared with a desired voltage U35 in a differential amplifier 9.In an integral-action controller 10 following the differential amplifier 9, the output voltage of the differential amplifier is converted into the manipulated variable I, which controls the preequaliser 2. By adjusting the desired voltage U3s, third-order distortions in the signal SA at the output of the laser 3 can be minimised.

Figure 2 shows a block diagram of the preequaliser 2. The signal to be transmitted is fed from the adder 1 to a variable-gain amplifier 13. The output of this amplifier 13 is connected to the noninverting input + of a first differential amplifier 11. The signal from the adder 1 is also applied to the noninverting input + of a second differential amplifier 12. The inverting input of the latter is grounded, and the output is connected to the inverting input - of the first differential amplifier 11. The second differential amplifier 12 is also presented with the manipulated variable I from the evaluating circuit 4, which varies the operating point of the differential amplifier.Thus, the nonlinearity of the large-signal transfer characteristic of the pre-equaliser 2 is controlled, so that the output of the first differential amplifier 11 provides a modulating current which is so distorted that the signal SA at the output of the laser 3 is free from third-order distortion.

Figures 3a and 3b show block diagrams of preferred embodiments of the test signal generator 5. In Figure 3a, a generator 14 delivers a sinusoidal sigal of frequency 3fo. This signal is used directly as the second test signal. By a divider 15 and a following band-pass filter 16, the first test signal fo is derived from the signal of the generator 14.

In Figure 3b, a generator 17 provides a sinusoidal signal of frequency fo, which is used directly as the first test signal. The second test signal 3fo is derived from the sinusoidal signal of the generator 17 by multiplication using a phase-locked loop 18 followed by a band-pass filter 19.

Claims

1. A distortion compensation circuit for compensating third-order distortion caused by a transmitter in a signal to be transmitted, comprising a pre equaliserahead of the transmitter, and an evaluating circuit sensing the distortion in the signal at the output of the transmitter and controlling the preequaliser, characterised in that the transmitter is a laser, that a test signal generator is present which provides a first test signal of constant frequency and amplitude and a second test signal of three times the frequency of the first test signal, that the preequaliser is preceded by an adder in which the signal to be transmitted and the first test signal are combined, that the evaluating circuit is supplied with the second test signal from the test signal generator, and that the evaluating circuit derives from the output signal of the laser and the second test signal a manipulated variable which controls the preequaliser so that the signal at the output of the laser is equalised.

2. A circuit as claimed in claim 1, characterised in that the first test signal and the second test signal lie outside the frequency band of the signal to be transmitted.

3. A circuit as claimed in claim 1 or 2, characterised in that a photodiode receiving light from the laser is present between the evaluating circuit and the laser.

4. A circuit as claimed in claim 3, characterised in that the evaluating circuit contains a series combination of a multiplier for combining the voltage coming from the photodiode with the second test signal a differential amplifier to whose second input a desired voltage is applied, and an integral-action controller.

5. A circuit as claimed in any one of the preceding claims, characterised in that the pre-equaliser contains a first differential amplifier, a second differential amplifier, and a variable-gain amplifer, that the signal to be transmitted is applied to the noninverting input (+) ofthe first differential amplifierthrough the variable-gain amplifier, and to the inverting input through the second differential amplifier, and that the manipulated variable varies the operating point of the second differential amplifier.

6. A circuit as claimed in any one of the preceding claims, characterised in that the test signal generator generates the first test signal directly and derives the second test signal from the first test signal by multiplication using a phase-locked loop.

7. A circuit as claimed in any one of claims 1 to 5, characterised in that the test signal generator generates the second test signal directly and dervies the first test signal from the second test signal by division.

8. A distortion compensation circuit substantially as described herein with reference to the drawings.