LASER DIODE SUPPLY CIRCUIT

Abstract: A laser diode supply circuit comprises a laser diode (LD1) connected in series with a junction transistor (TR1). The base of the transistor is controlled through a second transistor (TR2) receiving at its base a control voltage developed by the series connection of a photo-diode (LD2) and a variable resistor (VR1). The photo-diode is optically coupled with the laser diode, such that as the power output of the laser diode increases beyond a desired value the control voltage exceeds the threshold value of the second transistor (TR2).

Description: LASER DIODE SUPPLY CIRCUIT This invention relates to a supply circuit for a laser diode and is particularly concerned with such a circuit in which the optical power output of the laser diode may be stabilised. Laser diodes are available at wavelengths over a broad range both in the visible and non-visible portion of the electromagnetic spectrum. For convenience, terms such as "light" are used herein to describe the invention, but the invention should not be regarded as restricted to the visible portion of the spectrum. It is an object of the present invention to provide an improved laser diode supply circuit which is capable of stabilising the optical power output of the diode notwithstanding small variations in, for example, the supply voltage or temperature. Accordingly, the present invention consists in one aspect in a laser diode supply circuit comprising a laser diode connected with a voltage source and with control means serving to control current through the laser diode, wherein said control means comprises semiconductor switch means switched into conduction on the application of a threshold voltage value thereto and wherein light sensing means are provided optically coupled with the laser diode and adapted to apply to said semiconductor switch means a control voltage related to the optical power output of the laser diode, such that as said control voltage passes said threshold voltage value, the semiconductor switch means is switched into conduction to control current through the laser diode. The semiconductor switch means may take the form of a junction or other transistor. The threshold voltage value - for example Vbe - therefore serves in this invention as a reference against which the control voltage is compared. The control voltage may be provided by a laser diode connected in series with a variable resistor, this variable resistor enabling presetting of the controlled optical power output of the laser diode. By making use of Vbe, or another semiconductor switch means threshold voltage value, this invention ingeniously avoids the need for a separate reference voltage. The effects of variation in supply voltage or temperature of the laser diode are largely nulled. The invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a simplified circuit diagram illustrating the present invention, and Figure 2 is a detailed circuit diagram of one embodiment of the present invention Referring initially to Figure 1, a laser diode LD1 is connected in series with a junction transistor TR1 between a voltage source +V and ground. The base of transistor TR1 is connected to the common point of resistor Rl and a second transistor TR2, these being connected in series between the supply voltage and ground. The base of TR1 is further connected to ground through resistor R2. The base of TR2 is connected to the common point of a photo-diode LD2 and variable resistor VR2, these components again being connected in series between the supply voltage and ground. The photo-diode LD2 is optically coupled with the laser diode LD1 to provide a reverse current which is proportional to the optical power output of the laser diode. In operation, the current in laser diode LD1 is controlled by transistor TR1, the operating point of which is determined by resistors Rl and R2. The value of variable resistor VRl is set so that the voltage applied to the base of transistor TR2 is beneath the voltage threshold determined by the base-emitter voltage. In the circumstance where the optical power output of the laser diode begins to exceed the desired value, the reverse current through photo-diode LD2 increases so increasing in turn, the voltage at the base of transistor TR2. This transistor is switched into conduction with the effect that current is diverted from the base of transistor TR2. This in turn reduces the collector current of transistor TR2 so stabilising the current in the laser-diode LD1. In addition to the stabilisation described above, the optical power output of the laser diode can be modulated using a variety of techniques. Thus, for example, the supply voltage +V could be pulsed; separate means could be provided for modulating the base current of transistor TR1; or, current could be directly injected at the collector of TR1. It will be understood that the photo-diode LD2 and variable resistor VRl could be replaced by other means - such as an appropriately biased phototransistor or resistance network including a light sensitive resistor - for providing a control voltage related to the optical power output of the laser diode. It will further be understood by the skilled man that the transistors TR1 and TR2 need not be junction transistors and that FET's or a wide range of further semiconductor control devices could be employed with modifications to the circuitry that would be obvious to the skilled man. A further embodiment of the present invention will now be described in more detail with reference to Figure 2. A laser diode LD1 (NDL 3200) is provided in a common cathode arrangement with a photo-diode LD2. The laser diode LD1 is connected between the voltage rail Vcc and the collector of a junction transistor TR1 having its emitter connected to ground. A diode DI and a capacitor Cl are both connected in parallel with the laser diode, the diode DI serving to prevent reverse bias damage and the capacitor Cl serving to prevent damage from power surges. The photo-diode LD2 is connected between the voltage rail Vcc and the base of junction transistor TR2. This transistor is connected between the base of transistor TR1 (through rail 10) and ground. Resistors Rl and R2 are connected in series between the voltage rail Vcc and the rail 10; resistor R3 is connected between the rail 10 and ground. Capacitor C4 is similarly connected between the rail 10 and ground and serves to restrict the turn-on rate of transistor TR1 and also ensures stability. The turn-off rate is determined by the value of resistor R3 which discharges capacitor C4. The common point of resistors Rl and R2 is connected to ground through Zener diode ZDl which limits the drive should the supply voltage increase beyond a limit. The operation of the circuit, as so far described, is analogous with that of Figure 1. The optical power output of the laser diode LD1 is detected by the photo-diode LD2 and a reverse current, which is proportional to the optical power output of the laser diode, develops a voltage across the variable resistor VRl. Pre-setting of the variable resistor provides compensation for variations in sensitivity of photo-diodes. With increasing power output from the laser diode, the voltage across variable resistor VRl rises in proportion to the photo-diode reverse current until a voltage threshold is reached which is set by the base-emitter voltage of transistor TR2. As the voltage across variable VRl increases further, current is fed into the base of transistor TR2 allowing its collector-emitter to turn on and divert current from resistor Rl and R2 which would otherwise drive the base of transistor TRl. This in turn reduces the collector current of transistor TR2 stabilising the current in the laser diode. Variations in supply voltage or temperature of the laser diode have no effect on the control of the transistor TR2. Returning to a detailed consideration of Figure 2, a multivibrator is provided, as shown generally at 12, comprising transistors TR3 and TR4, capacitors C2 and C3, and resistors R4 to R7. The circuit of the multivibrator is of conventional form and detailed description is felt unnecessary. An output of the multivibrator is taken through a resistor RIO to the base of a transistor TR5 connected between the rail 10 and ground. It will be understood that the effect of the multivibrator is to pulse conduction of the transistor TRl thus providing modulation of the laser diode output to a level determined by the parameters of the multivibrator circuit, resistor R4 and transistor TR5- It should be understood that still further modifications beyond those specifically described herein can be made without departing from the scope of the invention as set forth in the appended claims.

Claims: CLAIMS : 1. A laser diode supply circuit comprising a laser diode connected with a voltage source and with control means serving to control current through the laser diode, wherein said control means comprises semiconductor switch means (TR2) switched into conduction on the application of a threshold voltage value thereto and wherein light sensing means (VRl, LD2) are provided optically coupled with the laser diode and adapted to apply to said semiconductor switch means a control voltage related to the optical power output of the laser diode (LD1), such that as said control voltage passes said threshold voltage value, the semiconductor switch means is switched into conduction to control current through the laser diode. 2. A circuit according to Claim 1, wherein said switch means comprises a first transistor (TR2) and wherein said control means comprises a second transistor (TRl) connected in series with the laser diode, conduction of said second transistor being controlled through an output of said first transistor. 3. A circuit according to Claim 2, wherein said first and second transistors comprise junction transistors with said control voltage being applied to the base of the first transistor (TR2) and with said first transistor being connected in the base circuit of said second transistor (TR2) . 4. A circuit according to any one of the preceding claims, wherein said light sensitive means comprises a photo-diode (LD2) connected in series with a variable resistance (VRl). 5. A circuit according to Claim 4, wherein said photo-diode (LD2) is formed integrally with said laser diode (LD1) . 6. A circuit according to any one of the preceding claims, further comprising means (12) for modulating the optical power output of said laser diode. 7- A circuit according to Claim 6, wherein said modulating means comprises means (12) for modulating the output of said voltage source.

Inventors: RAPER KEITH, RAPER, KEITH
Applicants: IMATRONIC LTD, IMATRONIC LIMITED
Application year: 1990
Main classification: H01S3
IPC codes: H01L33, H01S5