GB2291511A

GB2291511A - A method of stabilising the output intensity and or output frequency of an optical light source using an electronic counting technique

Info

Publication number: GB2291511A
Application number: GB9414465A
Authority: GB
Inventors: Martin Lawrence
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-07-18
Filing date: 1994-07-18
Publication date: 1996-01-24
Also published as: GB9414465D0

Abstract

The intensity or frequency of the light from a laser is stabilised using an electrical up/down counter and digital to analogue conversion techniques to generate ramp voltages of particular magnitude and sign, optical characteristics are controlled by monitoring the optical output and controlling the direction of the counter such that the electronic feedback is always driving towards some desired point in the optical characteristic.

Description

A method of stabilising or locking to a reference the output intensity and or output frequency of an optical light source using an electronic counting technique.

This invention relates to a method for stabilising or locking the output of an optical radiation source such as a laser. Specifically the optical intensity or the frequency of the output are the parameters which are stabilised by this method. The need to maintain stable operation of intensity and or frequency arises from applications such as Spectroscopy but is by no means limited to such areas.

This invention uses as a working mechanism a feedback loop in which a portion of the output light from the radiation source is analysed in terms of its amplitude or frequency or both and such a derived signal is then used to control the parameters of interest. For analysis involving amplitude then the output from a sensor such as a photodiode might be used in production of the feedback signal. For analysis involving optical frequency, then the transmission of an optical filter, fabry perot etalon or selective absorbing medium might be observed by a sensor such as a photodiode from which the feedback signal would be initiated but these mechanisms are only given by way of example and this list is not exhaustive.

This invention uses a device such as a photodiode to produce an electrical signal that carries infonnation about the parameters of the optical source that it is desired to control or modify.

It is the detail of the operation of the feedback loop that is the specific subject of this application. Having recovered an electrical signal, the signal is conditioned and an electrical measuring circuit such as a peak detector or comparator produces a pulse at some specific change in the optical characteristic such as a transmission peak in a filter response or an amplitude peak or a particular amplitude level. This pulse controls the direction of counting of a digital counting circuit, meaning that the circuit can be directed to count either up or doll, and the output of the counting circuit is passed through a digital to analogue converter to produce an electrical ramp which forms part or all of the electrical drive to the optical device Suppose that at the beginning of the operation the counter electronics is initiated to count up from zero.This results in a positive - voltage increasing ramp from the digital to analogue converter that is driving the optical device. After some lime the optical characteristic changes or reaches a level and the peak or level is detected and the output from the peak or level detector then switches the counting direction of the counter to count down, this then causing the output voltage produced by the digital to analogue converter now to ramp downy so driving the optical output back in the direction of the peak or level just detected. As the optical peak or level is passed again the peak detector circuitry again emits an electrical pulse which again changes the direction of the counter to up. Thus, the essential mechanism of this invention is the control and operation of the counter circuitry, feedback input to and electrical output from which is derived voltage levels that control optical emissions in a sense that will always drive and hold such emissions at or to selected reference points in their amplitude and or frequency characteristics.

Claims

Claims.

1. An apparatus consisting of a pulse generator, up/dossn counter and digital to analogue converter the output from which is an electrical signal which ramps up or down depending on whether the counter is counting up or down, the action of this voltage ramp being to control the amplitude and or frequency characteristic of an optical source such as a laser through suitable control interface circuitry. The direction of counting of the counter device is controlled by circuitry designed to detect specific levels or peaks in the optical characteristics that are monitored by sensors such as photodiodes operating either directly or in conjunction with optical elements such as filters or specific absorbing media an example of which could be a cell containing a particular chemical knout to absorb optical radiation in the waveband in question.

2. An apparatus as in claim 1 where several subsidiary circuits might comprise the pulse generator, counter, digital to analogue functions or circuitry for level or peak detection.

3. An apparatus as in claim 1 where the apparatus might not lie in proximity and where the sensor might sense the optical characteristics through free space or a medium such as a fibre optic cable.

4. An apparatus as in claim 1 where the optical radiation source is a laser or a light emitting diode (LED) or an incandescent source.

5. An apparatus as in claim 1 where the reference level or peak which is detected and used to switch the direction of the counter might be tuned or varied to accomodate tuning of some characteristic of the optical source.

6. An apparatus as substantially described herein.