Download LBA - Laser Beam Analyzer Working Principle

LBA - Laser Beam Analyzer
In industrial and laboratory applications of CO2 and YAG lasers it is
important to know the exact instantaneous distribution of the intensity
across the laser beam, i.e. the laser's mode. All the beam analyzing
methods known to date are cumbersome and have one serious
drawback: they do not allow measurement and continuous monitoring
during an experiment or during material processing. The Laser Beam
Analyzer - LBA - makes it possible to control the intensity distribution
across the laser beam while a work piece is being processed, without
noticeable loss of beam intensity. Indeed in CW mode, the intensity is
reduced by mere 0.8 %.
Working Principle
Driven by a synchronous motor, a needle with a high reflective
surface rotates in a plane perpendicular to the beam. While
passing across the beam it reflects a tiny fraction of the beam onto
two pyroelectric detectors. The needle moving at constant speed,
the power distribution across the beam section is represented by
the amplitude of the amplified signals from the detectors. The LBA
is designed for the simultaneous displaying of the intensity profiles
in the x and y axes (see picture) on a dual trace oscilloscope.
APPLICATIONS
1. Adjustment of CW-Lasers
The LBA is mounted and centered on the beam in a convenient position while the
oscilloscope is placed beside the mirror to be adjusted. During adjustment the variation of the
intensity profile can be directly observed and the laser can easily be adjusted to get an
optimum mode.
2. Intensity Profile Control in Material Processing
The LBA is permanently installed in an easy accessible place in the beam path between the
laser and the work station, the oscilloscope being installed at the work station. This
arrangement allows control of the instantaneous mode and power during the working process.
The beam intensity loss due to the LBA is 0.8%. Permanent installation in the beam path
does not impair production quality. When a laser is used in production - e.g. for steel cutting a "good mode" is a prerequisite for a clean cut. The Laser Beam Analyzer offers permanent
control of the intensity profile. Further development toward fully automatic operation is
feasible. The signal can be processed electronically and be used for automatic production
control by comparison of actual to optimum parameters. There are some interesting aspects
to lab application. It is generally known that in metals processing a considerable portion of the
radiation is reflected from the metal surface and re-enters the laser via the focusing lens in
the form of a parallel beam. This retro reflection increases the power density inside the laser
and can lead to a substantial variation of the laser's mode. Direct observation of this
phenomenon by means of the LBA allows the optimization of the process parameters.
3. Measurement of Beam Divergence
As the rotating needle passes across the beam at constant speed there is a fixed relationship
between passage time and beam width. Taking model LBA1 for an example, 400us shown on
the CRT screen are equivalent to a beam width of 10mm. By successively installing the Beam
Analyzer at different distances from the laser the beam diameter (1/e) can be measured and
divergence can easily be calculated from these values.
4. Adjustment of Pulsed-Lasers
For pulsed lasers it is suggest able (even not indispensable) to employ a memory
oscilloscope. When a laser pulse meets the needle, a very narrow "peak" will appear on the
screen in a position corresponding to that of the needle at the pulse instant. As the time goes
on, the screen will be filled with many such "peaks", whose enveloping curve represents the
laser's mode. To avoid long waiting time with low repetition rate lasers, a synchronization
signal is supplied by the LBA to drive a laser pulse when, after every revolution the needle is
in a slightly different position. A reset signal is also provided to erase the screen after every
scanning. Furthermore the facility of locking the needle across the beam enables the
observation of the pulse's time-shape, point-to-point in the beam cross-section.
5. Measurements of Focus
The needle's high reflective surface and its moving speed make the LBA suitable for
measurement of the focus diameter. The unit is displaced along the beam until the minimum
beam diameter has been found. This measurement allows to determine the power density at
the focus, as well as the influence of lenses of different focal lengths and shapes. Distorted
lenses or mirrors can the be spotted by comparative measurement.
SPECIFICATIONS
The LBA is available in three different sizes:
LBA1 for beam diameter till 25mm; 50(60) Hz scanning rate
LBA2 for beam diameter till 40mm; 50(60) Hz scanning rate
LBA3 for beam diameter till 60mm; 50(60) Hz scanning rate
Detectors window is available either CO2 or YAG laser as well as for both
Detectors amplifier is switchable in 4 gains ranges (0.02-1-3-10). The spatial resolution
and the sensitivity depend linearly on the chosen gain.
o With gain 0.02 the following values are reached:
spatial resolution: 4um
sensitivity: 0.3mV/sq. mm
increasing the gain increase the sensitivity and reduce the resolution of the same
factor.
Output noise signal is always less than 5mV (rms)
Please specify in your order required version and model, detector's window and
supply voltage (220 or 110V) and frequency (50 or 60Hz).
o Dimension (mm)
LBA1 187x89x185
LBA2 293x130x287
LBA3 360x150x355
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