THE QUALITY CHECK QUERY
Dr Ephraim Greenfield argues that laser beam profiling can help diagnose many laser quality issues. Laser beam profiling is becoming more accepted as a necessary tool for diagnostics and quality assurance in the laser field. The need to know the laser beam intensity profile, as well as the power and energy, is becoming more accepted. However, until recently, beam profiling of pulsed lasers was complicated and difficult and often did not give satisfactory measurements.
Laser beam profilers are divided into two
principle types: rotating aperture types and array camera types.
In the rotating aperture type, an aperture, usually a thin slit, is rotated
past the laser beam, thereby creating a cross section of the laser beam.
This type is not suited for monitoring a pulsing laser, since it requires
a significant period of time to make the cross-section, and cannot be
synchronised with the laser pulses.
The array camera type is based on an array
camera, usually a silicon-based CCD or CMOS, covering the spectral range
190-1100nm, but also other types for longer wavelengths as well. In its
simplest form, it is an inexpensive commercial analogue camera without
the lens, where the protective cover glass has been removed to eliminate
optical interference effects. The profile is displayed as a contour map
of the beam intensity, a 3D picture, or profile.
CCD cameras operate as follows: The camera is exposed to the beam by electronically
turning on for 1/10,000th of a second to 1/50th
of a second. The camera does this repeatedly at a given frame rate. In
simple cameras, the frame rate is fixed to 25Hz or 30Hz, the standard
television rate. In the more expensive types, the frame rate can be synchronised
with an outside trigger signal. After the exposure, the electronics reads
out the information of the frame before the next frame. The proper exposure
is obtained by electronically varying the exposure time and the gain of
the camera. The simple camera types have several fixed exposure and gain
settings and the more expensive types have continuously variable settings.
When measuring a pulsed laser beam, varying the camera exposure is of not of much use, since the laser pulse width is fixed and is usually shorter than the shortest exposure time. In order to adjust the exposure to a pulsed laser, the laser beam is passed through suitable glass filters to reduce the intensity suitably, before impinging on the CCD. Continuously variable filters are available to make this adjustment easier. The gain adjustment can also be varied to help adjust the exposure.
The simple types of camera can operate with
pulsed lasers, but are seriously limited as follows:
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The camera runs at a fixed rate and cannot be synchronised with the laser pulses. All that can be done is to set the shutter exposure to the maximum, and display only those pulses that fall into the window of exposure (maybe 40 per cent of the pulses).
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With the limited gain settings, the only way to fine-adjust the exposure is to use expensive, manually adjusted variable filters.
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With infrared lasers, such as NdYAG at 1,064nm, ghost images will occur if the laser is not synchronised with the laser pulses; some of the pulse energy passes through the detector and hits the readout electronics on the rear, creating a spurious image.
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The set-up is complicated, must be done manually and must be adjusted each time the beam intensity changes.The more expensive cameras that can synchronise with the laser pulse, and have continuously variable gain, are better and eliminate objections 1, 2 and 3 above. However, they still have the following problems:
a) The adjustment must be done manually and is complicated;
b) The camera must be connected to a trigger output of the laser (if it has one);
c) For laser pulses shorter than ~10ms, the trigger output must come before the pulse so the camera has time to open before the pulse. Most lasers do not have this pre-trigger output.
The latest systems available have now eliminated
the last three problems:
1. Some systems now have automatic gain and exposure control to eliminate
problem a).
2. A method has been developed to eliminate problems b) and c), whereby
pulses can be synchronised by means of a sensitive built-in photodiode
trigger that senses the scattered laser light, thus eliminating the need
for connection to the laser. In addition, the camera circuitry measures
the timing and spacing of the pulses, and predicts when the next pulse
will arrive, automatically starting the exposure before the pulse occurs.
Thus, laser beam profilers for pulsed lasers have finally reached the
stage of 'plug and play'.
Dr. Ephraim Greenfield is research and development manager for the laser measurement group of Ophir Optronics.
Electro Optics Magazine
Volume number 33 Issue 168 February/March 2004
This article can also be read at http://195.40.133.66/cgi-bin/article.cgi?ID=%202089&vol=33&issue=168