GB2243713A - Injection laser modulation - Google Patents

Abstract

A method of amplitude modulating the light output of an injection laser D1 with a binary digital bit stream employs a monitor photodiode D2 for regulation of the bias and modulation current drives. Regulation of the modulation current drive is achieved by subtracting from the photocurrent during data 1 bit periods the desired photocurrent increment associated with data 1's, the resulting signal being multiplied with a signal representative of the integral of the applied bit stream, thereby producing a control signal employed in a feedback control loop A3. The difference signal is additionally used in a separate feedback control loop A1/A2 to regulate the bias current. <IMAGE>

Description

Inject ion Laser Modulation This invention relates to the amplitude modulation of injection lasers with binary digital information. Especially in the case of modulation at high bit rates, it is preferred for the current drive corresponding to the lower required value of light output to be a non-zero current at about the value corresponding to the lasing threshold current. This is because of the time delay and frequency chirp associatec with attempting to drive an injection laser rapidly hard on from well beneath threshold.

Accordingly an inject ion laser driver for binary modulation needs to provide a bias current of sufficient amplitude to drive the laser appropriately close to threshold, and also to provide a modulation current of appropriate amplitude to provide the requisite proportional difference of light output between data l's and data O's. Having regard to the fact that the lasing threshold of an injection laser varies with temperature and with the effects of ageing, and that similar properties are also observed in respect of slope efficiency (ratio of incremental laser light output to current drive), a conventional laser driver incorporates some form of monitor photodetector, typically a photodiode, positioned to receive a proportion of the emitted light.

If the response of the monitor photodiode were fast enough, it would in principle be possible to measure its output in data 0 bit periods and data l's to generate respective control signals for regulating the bias and modulation currents respectively. Particularly at high bit rates, this approach would be inconveniently expensive to implement in practice. One solution to this problem, when the binary modulation uses a balanced code, is to impress a ripple, a small amplitude low frequency current modulation, upon the data 0's, this low frequency modulation being within the pass band of the monitor photodiode detector circuitry. The slope efficiency of the laser changes relatively abruptly in the region of the lasing threshold from a low value below threshold to a significantly higher value above threshold.If the laser bias is significantly below threshold this ripple will produce a correspondingly small ripple of the light output, whereas it will be significantly larger if the bias is significantly above threshold. A feedback control loop can therefore be used to regulate the bias to the neighbourhood of the lasing threshold by selecting an intermediate control value for the ripple observed by the monitor photodiode. The modulation current can be independently controlled by a second feedback control loop deriving its control signal from a measure of the long-term mean current output of the monitor photodiode which, since the code is a balanced code, will correspond to the point midway between the light output of a data 1 and that of a data 0.

One of the disadvantages of this use of a ripple frequency to control the operation of the driver is that the binary data stream may from time to time include a spectral component at the ripple frequency.

This can give rise to spurious effects in the monitor detection system causing the proper stabilisation of the drive conditions to be disrupted.

The present invention is directed to a method of driving an inject ion laser with binary digital amplitude modulation in a manner using independent feedback control loops to regulate the laser bias and modulation in an independent manner without recourse to ripple techniques.

According to the present invention there is provided a method of amplitude modulating the light output of an injection laser with a binary digital bit stream, in which method a photocurrent representative of the light output is provided by a photodetector positioned to receive a proportion of said output, wherein the laser is supplied with bias and modulation current drives, wherein a current of amplitude equal to the desired photocurrent increment associated with bit periods for which the modulation provides the higher laser drive current is subtracted from the photocurrent during said higher laser drive current bit periods to produce a signal which is multiplied with a signal that represents the integral of the applied bit stream to provide a feedback control signal which is employed in a feedback control loop to regulate the amplitude of the modulation current drive.

The use of such a method permits the satisfactory operation of a laser driver driven with unbounded codes and affords a quick power up of the transmitter. Data can be applied at a very high bit rate.

The invention further provides an injection laser binary digital modulation drive circuitry that includes a data input terminal and a monitor photodetector positioned to receive a proportion of the laser light output, which drive circuitry incorporates first and second feedback loops to compensate for changes in laser threshold and slope efficiency characteristics respectively, wherein the threshold feedback loop has a control signal input connected to the output of a multiplier having first and second inputs, wherein the multiplier first input is a.c. coupled with the output of the monitor photodetector and with the output of a switched constant current generator connected to supply a current oppositely directed with respect to the monitor photodetector current, which switched constant current generator has a switch control input connected with the data input terminal such that it is adapted to a.c.

couple its current with the multiplier only during high laser drive current bit periods, and wherein the data input terminal is also connected to an integrator adapted to form the integral of data applied to the data input terminal, the output of the integrator being connected to the second input of the multiplier.

There follows a description of an injection laser driver, and its mode of operation, embodying the invention in a preferred form. The description refers to the accompanying drawing which depicts a block diagram of the laser driver.

Referring to the accompanying drawing, an injection laser diode D1 is supplied with current from two adjustable constant current generators I1 and I2.

Associated with the laser diode is a monitor photodiode D2 whose anode is connected to two further adjustable constant current generators I3 and I4. CD2 represents the stray capacitance associated with the monitor photodiode D2 and its terminal connections. Constant current generator I1 provides the bias current Il for the laser D1, and the magnitude of this bias current i1 is regulated by means of a first feedback loop incorporating amplifiers Al and A2. (A2 is a unity gain buffer with high input impedance). Constant current generator I2 provides the modulation current 12 for the laser, and the magnitude of this modulation current i2 is regulated by means of a second feedback loop incorporating amplifier A3.Data is applied to the driver by way of an input terminal T where it is employed to operate three switches SW1, SW2 and SW3.

These three switches are shown in the positions they are caused to assume in a data 1 (high laser drive, high light output) bit period. In such data 1 bit periods switch SW1 directs the modulation current 12 through the laser diode together with the bias current i il, and similarly switch SW2 connects the current i3 flowing through current generator I3 to the anode connection of the monitor photodiode. In data 0 bit periods the switches SW1 and SW2 shunt currents i2 and i3 to ground. Hence, in data 0 bit periods, only the bias current il 1 is caused to flow through the laser diode.

In the absence of current generator I3, the potential of the monitor photodiode anode will fluctuate with the applied data stream in consequence of the photodiode receiving more light, and hence passing more current, in data 1 bit periods than in data bit 0 periods. However, because the time constant of the monitor photodiode is not short compared with a single bit period, the fluctuation resulting from an isolated data 0 or data 1 bit period will not be as great as occurs in the presence of a consecutive sequence of data 0 or data 1 bit periods. Therefore simply to use a peak detector to measure the amplitude of these fluctuations does not provide an adequate feedback control signal for regulating the second feedback control loop.

In order to overcome this problem an integrator S is fed from two voltage sources under the control of switch SW3 in order to produce a voltage waveform which fluctuates wither in the same way as, or in antiphase with, the voltage fluctuations produced at the monitor photodiode anode by virtue of the changes in photocurrent that it passes. Additionally constant current generator I3 is introduced to apply, under the control of switch SW2, a current to the monitor photodiode anode during data 1 bit periods which is designed to exactly compensate the increased photocurrent occurring during those bit periods. The monitor photodiode anode is connected via a dc blocking capacitor C1 to one input, the first input, of a multipler M whose other input is taken from the output of the integrator S.The output of the multiplier forms the feedback control signal for the second feedback control loop, and hence is applied to amplifier A3.

If the current i3 provided by current generator I3 is set to a specific value, and the modulation of the light output provided by the laser changes so that the photocurrent increment induced in the monitor photodiode in data 1 bit periods changes from being undercompensated by 13 to being overcompensated by i3, then the phase of the voltage fluctuations applied to one input of the multiplier is 0 changed by 180 with respect to the voltage fluctuations applied to the other input, and in consequence the multipler output voltage changes sign.

Thus the second feedback control loop operates to control the laser modulation to produce a specific increment of light output in data 1 bit periods, this specific increment being determined by the chosen value of 13 provided by current generator I3.

When the second feedback loop is functioning, the photocurrent increment present during data 1 bit periods is exactly matched by i3, and so the nett current at the photodiode anode is the same in data 1 bit periods as it is in data 0 bit periods. This is arranged to be balanced by the current i4 by taking the control signal for the first feedback control loop from the monitor photodiode anode and using it as the input to amplifier A2.

Claims (5)

CLAIMS.

1. A method of amplitude modulating the light output of an injection laser with a binary digital bit stream, in which method a photocurrent representative of the light output is provided by a photodetector positioned to receive a proportion of said output, wherein the laser is supplied with bias and modulation current drives, wherein a current of amplitude equal to the desired photocurrent increment associated with bit periods for which the modulation provides the higher laser drive current is subtracted from the photocurrent during said higher laser drive current bit periods to produce a signal which is multiplied with a signal that represents the integral of the applied bit stream to provide a feedback control signal which is employed in a feedback control loop to regulate the amplitude of the modulation current drive.

2. A method of amplitude modulating the light output of an injection laser with a binary digital bit stream, in which method a photocurrent representative of the light output is provided by positioning a photodetector to receive a proportion of said output, wherein the laser is supplied with bias and modulation current drives, wherein the magnitude of the bias current is regulated by means of a first feedback control loop employing a bias amplitude control signal representative of the photocurrent of the photodetector during bit periods for which the modulation provides the lower laser drive current, wherein the magnitude of the modulation current is regulated by means of a second feedback control loop employing a modulation amplitude control signal which is derived from multiplying a first signal with a second signal, said first signal being representative of the integral of the bit stream and said second signal being representative of the difference between the photocurrent of the photodetector and a fixed amplitude current present only during bit periods for which the modulation provides the higher laser drive current.

3. A method substantially as hereinbefore described with reference to the accompanying drawing of amplitude modulating the light output of an injection laser with a binary digital bit stream.

4. An injection laser binary digital modulation drive circuitry that includes a data input terminal and a monitor photodetector positioned to receive a proportion of the laser light output, which drive circuitry incorporates first and second feedback loops to compensate for changes in laser threshold and slope efficiency characteristics respectively, wherein the threshold feedback loop has a control signal input connected to the output of a multiplier having first and second inputs, wherein the multiplier first input is a.c. coupled with the output of the monitor photodetector and with the output of a switched constant current generator connected to supply a current oppositely directed with respect to the monitor photodetector current, which switched constant current generator has a switch control input connected with the data input terminal such that it is adapted to a.c.

couple its current with the multiplier only during high laser drive current bit periods, and wherein the data input terminal is also connected to an integrator adapted to form the integral of data applied to the data input terminal, the output of the integrator being connected to the second input of the multiplier.

5. Injection laser binary digital modulation drive circuitry substantially as hereinbefore described with reference to the accompanying drawing.