Coherent light generators – Particular component circuitry – Optical pumping
Patent
1995-03-05
1998-03-31
Healy, Brian
Coherent light generators
Particular component circuitry
Optical pumping
H01S 300
Patent
active
057346682
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a drive circuit for a laser diode.
2. Description of the Prior Art
The lasing output of a laser diode is controlled by the current through the diode.
JP-A-61 256 682 discloses a system for driving a laser diode irrespective of the polarity of the diode by separating the light output control circuit of the diode and the diode in a DC manner.
Previously, laser diodes have been driven by linear control circuits as shown in FIG. 1 of the instant application. Such a system is disclosed in e.g. Melles Griot Optics Guide 5, Chapter 20, page 29 (Power Supply Design (1990)).
In the prior art drive circuit of FIG. 1, an external control signal indicative of the desired lasing output of a diode 1 is applied, via an operational amplifier 3, to a transistor 2 which acts as a variable resistance connected in series with another resistor to vary the current through the diode 1 to produce the required coherent light emission output power. The output of the diode is detected by a photodetector and a signal commensurate therewith is fed back to the other input of the op-amp 3 where it is compared to the external control signal and the difference is used to adjust the current through the laser diode, 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 1 in accordance with the external control signal and the feedback signal, and the remaining voltage from the direct current supply bus 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 0.5 A. With, for example a 5V power supply, 3V would therefore be dropped across the transistor and resistor, i.e. a power loss of 1.5 W, Thus, of the 2.5 W used, only 1 W gets used by the laser diode which is very inefficient.
Further, since such a large amount of power is dissipated as heat in 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, WO-92/02844, describes a bank of e.g. sixteen laser diodes 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 be 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 volts will often result in a small amplifier output signal and this signal may be sufficient to cause the laser diode to 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.
SUMMARY OF THE INVENTION
It is, therefore, an object of the preferred embodiment of the present invention to provide a compact laser diode drive circuit whereby electrical power losses are minimised an
Lee Robin Michael
Raven Anthony
Diomed Limited
Healy Brian
Wise Robert E.
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