GB2245756A - Digital driving of injection lasers - Google Patents

Abstract

In a laser driver a clock pulse 11 is added to the data waveform 10 of the NRZ laser drive current to form a waveform 13 which drives the laser above lasing threshold before the end of every data 0 bit period. This avoids in data 1 bit periods, the turn-on and chirp problems associated with driving the laser hard on from beneath lasing threshold. Optionally the clock pulse addition is inhibited in data 1 bit periods to provide the modified drive waveform 14. The driver circuit is detailed (Fig. 3). <IMAGE>

Description

Digital Driving of Injection Lasers This invention relates to the digital driving of injection lasers. Automatic level control in an injection laser driver is generally required because injection lasers are threshold devices with light output/current drive transfer characteristics that are temperature sensitive andrlifetime dependent. Such automatic level control can conveniently be achieved by monitoring the slope of the transfer characteristic, which changes significantly just beneath the lasing threshold. A particular example of this is described in United Kingdom patent GB 2 025 121 B to which attention is directed. This form of automatic level control provides a bias point just around the lasing threshold because this is the point at which the slope changes most rapidly with increasing drive current.On the other hand an operating point just above lasing threshold is highly desirable for high speed narrow bandwidth transmission because driving from beneath threshold produces excessive chirp and delay associated with the turning on of the laser.

A method of addressing this problem is disclosed in EP 0 319 852 A, to which attention is directed, this method involving the use of special pulse shaping techniques to predistort each electrical pulse in such a way as to compensate for the distortion introduced by the laser. Relatively complicated circuitry involving the use of a field effect transistor and a transmission line is employed to produce this predistortion. Somewhat more simple to implement circuitry is employed in the method of addressing this chirp and turn-on delay problem that is described by L. Bickers and L.D. Westbrook in the OFC 88 post-deadline paper entitled 'Chirp Reduction in 1.5 pm Distributed Feedback Lasers by Modulation Pulse Shaping' (PD 12-1 to PD 12-4), to which attention is also directed.In this instance a prepulse is derived from the main pulse by splitting the data signal, and then delaying the main signal before recombining it with the (undelayed) split-off signal. The combining of these two signals provides a pulse waveform with stepped rising and falling edges. The step on the rising edge is used to take the laser slightly above threshold just before it is driven hard. On the other hand the step in the falling edge is not helpful in turning the laser rapidly off.

According to the present invention there is provided a method of electrically driving an injection laser to provide a digitally modulated output using non-return-to-zero (NRZ) amplitude modulation of the injection laser drive current, wherein the d.c. bias of the NRZ amplitude modulation is regulated by electro-optic feedback, and wherein at least in each data period of the digital modulation with the lowest level of laser drive current the laser drive is augmented by a clock-derived auxiliary current drive which raises the level of the laser drive current at the final portion of that data period relative to the drive current at the initial portion of that data period.

Conventionally information that is impressed upon an optical carrier is in binary form, in which case with amplitude modulation there are just two levels of laser drive current corresponding respectively to a data '0' and to a data '1'. It will be evident however that the principles of the present invention are applicable not only to binary level amplitude modulation systems, but also to higher, multi-level amplitude modulation systems.

The invention also resides in a non-return-to-zero (NRZ) amplitude modulation drive circuitry for an injection laser, said circuitry including first driver means adapted to provide an NRZ current drive output in response to the application of a digital data input signal to an input of said first driver means, electro-optic feedback means adapted to regulate the output of the first driver means, and an auxiliary driver adapted to generate from a clock input signal an auxiliary signal which is added to the output of the first drive means to provide a combined output current, which auxiliary signal, at least in each data period of the digital modulation for which the first driver means provides its lowest level of current drive output, raises the level of the combined output current at the final portion of that data period relative to the level of the combined output current at the initial portion of that data period.

There follows a description of the driving of an injection laser by methods embodying the invention in preferred forms, and of drivers for performing these methods. The description refers to the accompanying drawings in which: Figure 1 depicts data, clock, and drive waveforms, Figure 2 depicts a typical transfer characteristic of an injection laser, and Figure 3 is a block diagram of an injection laser driver.

The injection laser driving method that is now to be described may be characterised as a slope control method with clock addition. In Figure 1 a representative binary data stream, (0,1,0,0,1,1,01) is depicted at 10, and a clock pulse stream at 11. The addition of the clock pulse stream to the data stream produces the combined stream 13. If however the clock pulse stream is gated so that it is added only to data 0's of the data stream 10, the result is the gated combined stream 14.

Figure 2 depicts at 20 the transfer characteristic of a typical injection laser in which light output is plotted as a function of laser drive current. This characteristic has, beneath laser threshold, a region 20a over which the slope of the characteristic is relatively constant and of a relatively low value. Just beneath the lasing threshold the characteristic exhibits a 'knee' region 20b where the slope is increasing relatively rapidly with inversing threshold. Then, above the lasing threshold, there is a further region 20c where once again the slope is relatively constant, but of a higher value than that existing in region 20a. Because of problems of turn-on time and increased chirp when driving a laser from below lasing threshold, one would like to drive the laser between a low current value near the bottom end of region 20c and some higher current value well up the region 20c.A difficulty in attaining this objective is associated with the fact that the position of the 'knee' region 20b is a function of temperature, and is also liable to change with the effects of ageing. The slope control feedback system to which previous reference has been made imparts a low frequency ripple of known amplitude upon the drive current. This in turn imparts a corresponding ripple upon the light output. The magnitude of this ripple depends upon the slope of the characteristic at the operating drive current point. A feedback loop can be set to regulate the operating point to a value where this slope is changing relatively rapidly. Thus the slope control feedback technique sets the operating point of the data 0 level to some point on the 'knee' 20b of the characteristic.This is a point (ISL' BSL) just below the lasing threshold ITCH' TH), rather than just above it. However, by adding to the data stream 10 a gated or ungated clock pulse stream to produce respectively the waveform 14 or the waveform 13, thereby providing the light output waveform 24 or 23, it is seen from Figure 2 that in a data 0 bit period the current is taken from a value below the lasing threshold current ITH in the first half of that period to a value just above this threshold in the second half. If this data 0 bit period is then immediately followed with a data 1, the light output during this data 1 does not suffer from the time delay and chirp problems associated with driving the laser from below lasing threshold because the laser is already just above threshold at the commencement of this data 1 bit period.

The laser driver of Figure 3 has its laser 30 fed via a modulator 31 with conventional binary data applied to an input line 32. A monitor photodiode 33, positioned to receive a proportion of the light emitted by the laser 30, provides an output employed in two feedback paths which regulate the laser drive current.

The first of these feedback paths includes a differential amplifier 34 with a time constant relatively long compared with the longest period of consecutive l's or 0's in order to provide effectively a d.c. output from this amplifier 34. This d.c. output is set to a value to provide the required mean level of drive current applied to the laser having regard to the particular mark density of the binary data being supplied on input line 32.

The second feedback path is the slope control feedback path, and provides a gain control signal on line 35 which regulates the depth of modulation provided by modulator 31 to a value that provides a mean drive current in data 0 bit periods of ISL. For this purpose the modulator 31 impresses a fixed amplitude ripple signal of frequency fr upon the current drive supplied to laser 30. This ripple produces a corresponding ripple in the output of the monitor photodiode 33, and its amplitude is determined by means of a filter 36 tuned to f and a detector 37 connected in this second feedback path.Exact correspondence between the ripple frequency impressed upon the laser drive current and the frequency of the filter 36 is conveniently obtained by feeding part of the output of filter 36 upon a line 38 connected to a saturating amplifier 39 forming part of the modulator 31. The data input on line 32 is liable to have a frequency spectrum including a component at the ripple frequency, and so the component at the ripple frequency appearing in the monitor photodiode output will derive in part from the saturating amplifier 39, and in part from the data input on line 32. Optionally this second part, the part derived from the data input, can be cancelled by tapping-off a fraction of the input data signal, and using it on line 40 to control a current source 41 connected in series with the monitor photodiode 33.

To this circuitry which regulates both the mean current drive level and the depth of modulation is added signal processing means 42 which performs the clock addition function of the circuitry by adding its output delivered on line 43 to that of the modulator 31 and of the amplifier 34 in the driving of the laser 30. The signal processing means 42 is fed with a clock input signal on line 44, and in response thereto, provides a clock output signal on line 43 which has a rising edge at the midpoint of each bit period and is of the required amplitude to take the laser drive current above lasing threshold value, ITH, in the second half of each data 0 bit period. Optionally the signal processing means 42 includes a gate (not shown) which inhibits the clock output signal in the presence of data 1 bits applied to the signal processing means over line 45.

Claims (7)

CLAIMS.

1. A method of electrically driving an injection laser to provide a digitally modulated output using non-return-to-zero (NRZ) amplitude modulation of the injection laser drive current, wherein the d.c. bias of the NRZ amplitude modulation is regulated by electro-optic feedback, and wherein at least in each data period of the digital modulation with the lowest level of laser drive current the laser drive is augmented by a clock-derived auxiliary current drive which raises the level of the laser drive current at the final portion of that data period relative to the drive current at the initial portion of that data period.

2. A method as claimed in claim 1, wherein the laser drive is augmented by a clock-derived auxiliary current drive in every data period.

3. A method as claimed in claim 1 or 2, wherein the data is binary data.

4. A method of electrically driving an injection laser to provide a digitally modulated output using NRZ amplitude modulation of the injection laser drive current, which method is substantially as hereinbefore described with reference to the accompanying drawings.

5. A non-return-to-zero (NRZ) amplitude modulation drive circuitry for an injection laser, said circuitry including first driver means adapted to provide an NRZ current drive output in response to the application of a digital data input signal to an input of said first driver means, electro-optic feedback means adapted to regulate the output of the first driver means, and an auxiliary driver adapted to generate from a clock input signal an auxiliary signal which is added to the output of the first drive means to provide a combined output current, which auxiliary signal, at least in each data period of the digital modulation for which the first driver means provides its lowest level of current drive output, raises the level of the combined output current at the final portion of that data period relative to the level of the combined output current at the initial portion of that data period.

6. Drive circuitry as claimed in claim 5 wherein the auxiliary driver is adapted to generate the auxiliary signal for each data period.

7. Non-return-to-zero amplitude modulation drive circuitry for an injection laser, which circuitry is substantially as hereinbefore described with reference to the accompanying drawings.