LASER DIODE DRIVE CIRCUIT

Abstract: The lasing output of a laser diode (1) is controlled by the current through the diode (1). The laser diode (1) is driven by a switched mode drive circuit in which a pulsed signal is applied to a smoothing circuit, typically including a capacitor C and an inductor L, arranged to provide a smoothed output voltage whose magnitude depends on the pulse width of the drive signal.

Description: LASER DIODE DRIVE CIRCUIT The present invention relates to a drive circuit for a laser diode. The lasing output of a laser diode is controlled by the current through the diode. Previously, laser diodes have been driven by linear control circuits as shown in Fig.l. An external control signal indicative of the desired lasing output of the diode 1 is applied, via an op-amp 3, to a transistor 2 which acts as a series element together with the resistor to vary the current through the diode 1 to produce the required output power. The output of the diode is detected by a photodetector and fed back to the other input of the op- amp where it is compared to the external control signal to adjust the output so as to minimise the error between the actual and desired output values. Thus, in such drive circuits the laser diode current and voltage are controlled by the transistor in accordance with the external control signal and the feedback signal, and the remaining voltage from the supply rail is dropped across the series elements (the transistor and the resistor) . We have found that these known drive circuits for laser diodes are not, however, very efficient. Typically, at full power, the laser diode has approximately 2V across it and a current of 0.5A. With, for example a 5V power supply, 3V would therefore be dropped across the transistor and resistor, i.e. a power loss of 1.5W. Thus, of the 2.5W used, only 1W gets used by the laser diode which is very inefficient. Further, since such a large amount of power is dissipated as heat through the series elements, large heat sinks are required. This results in a bulky circuit. The above problems are most significant where a light source is provided with a plurality of laser diodes whose light beams are combined to provide a power output substantially higher than that available from a single diode. For example, as described in WO-92/02844, a bank of e.g. sixteen laser diodes are arranged in a portable light source for an optical fibre such as a body implantable probe or retina laser treatment fibre. In such a device sixteen drive circuits must be provided in a compact unit and it is, therefore, important that the circuits are as efficient and compact as possible. Another problem with known linear laser diode drive circuits arises when it is necessary to switch the diode off. Since most op-amps have slight offsets and tolerance mismatches, an external control signal of zero will often result in a small signal being output and so the laser diode would still be activated to provide a very low power output. In practice, in order to overcome this problem, a negative external control signal is used when zero output is required. However, this is not ideal and leads to difficulties when a very low output or precise control is required. Thus, known laser drive circuits as shown in Fig. 1 are typically only 30 to 40% efficient or less, and therefore require a higher power input and, due to the power loss through the transistor and resistor, large heat sinks are required resulting in a large bulky circuit . Such circuits are particularly unsuitable for use in applications such as disclosed in WO-92/02844 where the size of the elements is an important consideration. It is, therefore, an object of the present invention to provide a compact laser diode drive circuit whereby electrical power losses are minimised and precise control of the laser, even at very " low or zero output, is possible. In order to achieve this object the present invention uses a switched mode drive circuit to drive a laser diode. Accordingly, viewed from a first aspect, the invention provides a laser diode electrically connected to a switched mode drive circuit. Viewed from a further aspect, the invention provides a method of driving a laser diode comprising the use of a switched mode drive circuit. The invention extends to a light source unit for use in conjunction with an optical fibre, comprising a plurality of laser diodes, optical means for combining the light beams produced by the diodes and for directing the combined beam into an optical fibre, wherein each laser diode is driven by a switched mode drive circuit contained in said unit. Various types of switched mode drive circuits are known but these have, hitherto, never been used to drive laser diodes in connection with which they represent a new departure with special advantages. A switched mode drive circuit as referred to herein is one in which a pulsed signal is applied to a smoothing circuit, typically including a capacitor and an inductor, arranged to provide a smoothed output voltage whose magnitude depends on the pulse width of the drive signal. Switched mode drive circuits have a much greater thermal efficiency and, since excess voltage is not dropped across an element in series with the diode resulting in power being dissipated as heat, no bulky cooling elements are required allowing the circuit to be much smaller. A further advantage of a switched mode drive circuit is that the laser diode can be switched off precisely in response to a zero power external control signal i.e. with no drive pulses there can ~ be no current flowing in the laser diode. It is not affected by voltage offsets. The use of high frequency switching is preferred and allows the inductor in the smoothing circuit to be small both in size and value thus having only a small energy storage capability. Once the input power to the drive circuit is switched off, the laser's ability to continue lasing depends on the energy storage of the inductor. Therefore, the smaller the inductance the quicker the laser will stop lasing in response to a zero command signal input. This is particularly advantageous in medical apparatus since it is a standard requirement of all electrical medical equipment that if a failure is detected the apparatus must react quickly to being switched off. It is also a requirement that medical products comply with an international standard (IEC 601) which requires that the system must not become hazardous by reason of any one component failing, and so additional safety measures must be included to prevent this. In the linear supply shown in Fig.l, if the transistor fails, a short circuit between the diode and the resistor would result causing the diode to be run at higher power. Unless this destroys the diode (which is also undesirable) the high power output is potentially hazardous to the patient. Until now, in order to comply with the regulation, additional safety monitoring elements have been added to the drive circuit to avoid this problem but this all adds to the overall size, complexity and cost of the package. Thus, in a particularly preferred embodiment the present invention comprises an isolating transformer, preferably a step down transformer isolating the driving end of the circuit from the smoothing circuit and laser diode. A suitable circuit is a Buck step-down isolated, push-pull converter. An isolating transformer enables the apparatus to be made intrinsically safe against single component failure without the need for additional safety measures. The use of a step-down transformer also increases the efficiency of the circuit and minimises power losses. Previously, switched mode voltage supplies have been used to provide standard 5 or 12V supply rails. However, in driving a laser diode the voltage requirement is much less than this, usually around 2V. It has been found, however, that efficiency in these circuits decreases dramatically for a given power output as the output voltage decreases. Therefore, in a preferred embodiment the diodes, traditionally provided on the output side of the transformer in a transformer isolated push-pull converter, which act as self- commutating switches, are replaced by active switches, preferably FETs driven by tertiary windings on the transformer. The current required to drive the FETs is very low and this results in very low loss. Certain embodiments of the invention will now be described by way of example only with reference to the accompanying drawings wherein. Fig. 1 shows a known linear laser diode drive circuit. Fig. 2 shows a Buck push-pull voltage converter with an isolating transformer as used in one embodiment of the present invention. Fig. 3 shows an adapted form of the circuit shown in fig. 2 to drive a laser diode. In general, in a switched mode drive circuit, drive pulses are applied to a driven high frequency switch Q,,, Q 2 which opens and closes to produce a pulsed output signal. This output signal is smoothed by an LC circuit to provide an output signal whose amplitude is determined by the width of the input pulses. This output signal is applied across the laser diode 1. The width of the input pulses is controlled in dependence on an external input command signal indicative of the desired output power and a feedback signal from a sensor which detects the actual output of the laser diode. In linear systems as shown in fig. 1 all of the supply voltage is dropped across the diode and series elements such as a transistor and a resistor. Thus if, for example, the power supply voltage was 12V and, at full power, the diode voltage was 2V and the operating current was 3A, the overall power would be 36W, with 30W of that being dissipated as heat in the series element and only 6W being useful power output from the diode. This is clearly very inefficient and also results in the series components becoming very hot and so additional cooling elements are required. Using the present invention, almost all of the generated power is from the laser and the system is, therefore, around 90% efficient. The use of a step-down transformer considerably increases the efficiency of the system. In a typical circuit, the ratio of primary turns to secondary turns is 47:3 turns, for a 2V, 3A output using a 50V mains supply on the primary, the primary current is only about 140mA and thus the current related losses are greatly reduced. In practice a bridge rectifier (not shown) will usually be connected between the mains transformer and the switching circuit. Without the step-down transformer the diode operating current, typically 3A, would need to be supplied from the mains transformer and through the bridge rectifier diodes. Since each diode in the bridge rectifier has a 0.7V drop across it this would result in a high power loss across the rectifier. Thus, since the step-down transformer results in a very low current through the rectifier, these losses are minimised. Due to the very low current, Joule losses on all the cables are also minimised. The use of an isolating transformer is also a safety feature which, in accordance with medical instrumentation regulations, protects against failure of any single component being hazardous. Only A.C. power can get across the transformer. Therefore, if either of the driven switches, Q.,, Q 2 , fails resulting in a short circuit or open circuit no power is transferred across the transformer to the diode. Further, the ground reference point of the laser diode can be arbitrarily selected such that if the transformer fails, e.g. a short circuit between windings is produced, the laser diode will be reverse biased and thereby switched off. In the embodiment shown in Figs. 2 and 3, the anode of the diode is connected to ground. If the transformer short circuits, all the voltages on the primary side are positive resulting in a positive potential being applied to the cathode of the diode which reverse-drives the laser diode. The arbitrary choice of ground potential also enables many laser diodes to be put in thermal and electrical contact with their anodes joined. If any of the components on the secondary side fail the lasing power will be reduced and no hazard arises. All of these features are particularly advantageous in medical instrumentation and, since additional safety elements are not needed, the resulting package is more compact. This is an especially important consideration for drive circuits for use in the system disclosed in WO-A-92/02844 where sixteen laser diodes are mounted in a compact unit. Although switched mode power supply topologies are known in other applications, e.g. to drive loudspeakers, they have not previously been used to drive laser diodes. One reason is that these circuits generally provide moderate or high (usually 5 or 12V) voltage rails, and in any case not less than 5V whereas laser diodes require a much lower voltage, typically 2V. It has been found that the losses in such circuits increase dramatically as the voltage falls below 5V. In general, switched mode power supplies such as that shown in Fig. 2 comprise two switches, or two switch pairs, as a driven switch, for which transistors Q. | , Q 2 are used, and a secondary, self-commutating switch, for which diodes S 3 , S 4 are used. Switching diodes do however result in some power loss, and this can be particularly serious in the case of a laser diode which constitutes a high current low voltage load. For example, a typical switching diode has a 0.7V drop across it. For a load which requires 10W at 10V the current is 1A, and the power loss across the switching diode is a relatively insignificant 0.7W. However, for a load which requires 10W at IV (such as a laser diode) the current is 10A and the power loss 7W. Thus, in order to minimise these losses, the drive circuit of Fig. 2 is adapted, in a preferred embodiment, by replacing the diodes S 3 , S , with active synchronous switches, e.g. in the form of FETs Q 3 , Q which are driven by tertiary coils on the transformer as pulses are applied. We have found that this arrangement dramatically reduces power loss in a laser diode drive circuit . It is preferable to minimise switching losses as much as possible by using a low-leakage inductance transformer. In the preferred system the transformer T 1 is formed by winding a first primary winding of bifilar wire very tightly and neatly around the core. A secondary winding is then wound over this, followed by a second primary winding and then a tertiary winding. The primary is split into two halves. Low leakage inductance is achieved by getting good coupling between the primary and secondary. The secondary should be physically close to the primary and covering it entirely. A suitable number of turns of each is specified by way of example in Figure 3. Although switched mode power supplies are known, their use in driving laser diodes, particularly in medical applications, requires special considerations as regards efficiency, size and safety as discussed above. It will be appreciated that the illustrated embodiments are given only by way of example. Other suitable forms of switched mode power supply will be known to those of relevant skill and could be used in the invention.

Claims: Claims 1. A laser diode electrically connected to a switched mode drive circuit. 2. A laser diode as claimed in claim 1 wherein said switched mode drive circuit comprises a smoothing circuit, to which a pulsed drive signal is applied, arranged to provide a smoothed output voltage whose magnitude depends on the pulse width of the drive signal. 3. A laser diode as claimed in claim 2 wherein said smoothing circuit comprises a capacitor and an inductor. 4. A laser diode as claimed in claim 2 or 3 comprising an isolating transformer isolating the drive end of the circuit from the smoothing circuit and laser diode. 5. A laser diode as claimed in claim 4 wherein said isolating transformer is a step down transformer. 6. A laser diode as claimed in any preceding claim wherein said switched mode drive circuit comprises a transformer isolated push-pull converter. 7. A laser diode as claimed in claim 6 wherein the self-commutating switches at the output of the push-pull converter are active switches driven by tertiary windings of the transformer. 8. A laser diode as claimed in claim 7 wherein said active switches are field effect transistors (FETs) . 9. A method of driving a laser diode comprising the use of a switched mode drive circuit. 10. A light source unit for use in conjunction with an optical fibre, comprising a plurality of laser diodes, optical means for combining the light beams produced by the diodes and for directing the combined beam into an optical fibre, wherein each laser diode is driven by a switched mode drive circuit contained in said unit.

Inventors: LEE ROBIN MICHAEL, RAVEN ANTHONY, LEE, ROBIN, MICHAEL, RAVEN, ANTHONY
Applicants: BIOMED LTD, LEE ROBIN MICHAEL, RAVEN ANTHONY, DIOMED LTD
Application year: 1993
Main classification: H01S3
IPC codes: H01S5