Coherent light generators – Particular component circuitry – Optical pumping
Patent
1991-07-22
1993-03-30
Scott, Jr., Leon
Coherent light generators
Particular component circuitry
Optical pumping
H01S 300
Patent
active
051990393
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
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.
SUMMARY
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.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference to the accompanying drawings in which:
FIG. 1 is a simplified circuit diagram illustrating the present invention, and
FIG. 2 is a detailed circuit diagram of one embodiment of the present invention
DETAILED DESCRIPTION OF THE INVENTION
Referring initially to FIG. 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 R1 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 R1 and R2. The value of variable resistor VR1 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
REFERENCES:
patent: 3898583 (1975-08-01), Shuey et al.
patent: 4109217 (1978-08-01), Brackett et al.
patent: 4594717 (1986-06-01), Bracht et al.
patent: 4674093 (1987-06-01), Angerstein et al.
patent: 4709370 (1987-11-01), Bednarz et al.
Imatronic Limited
Jr. Leon Scott
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