tutorial modifying laser beams -...
TRANSCRIPT
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
Tutorial: Modifying Laser Beams – No Way
Around It, So Here’s How
By John McCauley, Product Specialist, Ophir Photonics
There are many applications for lasers in the world today with even more on
the horizon. In order to apply a laser to an industrial process, medical
therapy or communications technology it is usually necessary to modify the
laser beam to achieve the desired results. Laser beam measurement tools
make this possible. In this article we will discuss how to match an
appropriate laser beam profiling technique to a laser application. The topics
will include building an aligned optical system, collimation and focusing, high
power laser quality measurements and proper attenuation techniques for
making accurate laser measurements.
Building an Aligned Optical System
Many laser applications are based on a system that includes a laser with
associated optics to deliver a laser beam of known size and position to do
something. Examples of these are as diverse as laser printers and marking
machines, laser range finder, LIDAR or military targeting systems, and laser
projection systems for movie theatres. What all of these have in common is
the need to put a beam of a certain size at a certain place in space. Using a
beam profiler allows this to be done quickly and efficiently — simply place
the profiler where the beam is to be aimed and measure the size and position.
These parameters can be adjusted with direct feedback from the profiler.
Beam profilers to do this can be either based on CCD or CMOS array or
scanning slit technology. Each has its advantages. Arrays will provide a true
two dimensional image of the beam and show any irregularities in power
distribution; scanning slits will provide an XY profile, but can measure much
smaller beams, as small as 10µm, and will require little or no attenuation or
adjustment to the profiler itself. In addition, the scan plane of the slit
profiler is very well known, allowing the z-location of a beam waist to be
located very accurately. Both camera and slit profilers can determine the x-y
position of the beam in space to less than a µm.
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
Fig 1. Scanning slit and array profilers.
For applications where pointing accuracy is very important it is necessary to
isolate linear and angular pointing error and also to eliminate external
influences on the measurement. Pointing accuracy is dependent on the
profiler, the fixturing of the laser system and external influences. Localized
heating, air currents, vibration all contribute to uncertainty in pointing
measurements. One way to isolate external influences is to shrink the
measurement to as short a distance as possible. Rather than measuring
pointing over a distance of 10s of meters, using a very accurate profiler on the
bench top can deliver better results. By using scan averaging with a
NanoScan slit scanning profiler it is possible to measure pointing accuracy to
submicron precision, allowing a range finder to be aligned in a very small test
fixture.
Isolating linear from angular pointing error can be done with a two profiler
method. By splitting the beam and looking at the near-field and far-field
through an f-theta lens these motions can be separated. Angular motion will
show up as a shift in the near-field measurement, but will not be seen in the
far-field due to the f-theta correction. Linear motion, on the other hand will
be seen in the far-field as a shift in position on the far-field profiler.
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
Fig 2. Angular vs linear pointing measurement set up.
This may be more than most applications need, and simply placing the
profiler where the beam is to be delivered is usually enough.
Collimation and Focusing
Other applications often require that a laser either be focused to a particular
beam waist or collimated to a minimally diverging or converging beam.
Focusing can be a fairly simple process for low power beams, such as those
used in fiber optic telecommunication or point-of-sale scanners. It is much
more complicated for higher power applications like ophthalmic surgery,
medical therapeutics or precision welding. For a low power application, one
can simply place the profiler at the focus point and adjust the optics until the
desired beam size if obtained. This is routinely done with passive optical
components for free space coupling of fiber.
Fig 3. Focusing fiber optic components.
Measuring a higher power focused beam can be much more difficult because
the power density of the laser increases geometrically as beam size shrinks.
Although the slit scanner can often handle direct measurement of fairly high
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
power beams, often the power density can exceed the damage threshold for
the slit material. Camera measurement of focused beams is always a bit
more complicated, because arrays always need attenuation to measure lasers
without saturating the signal. With a focused beam it is usually impossible to
get the focused beam to the array through attenuators without distortion.
Fig 4. Focused beam has different path lengths through attenuator.
The attenuation in Fig. 4 is much simpler than is normally used and it would
still cause distortion. The solution to this problem is to image the beam waist
with a lens, with the attenuation added to the image.
Fig 5. Focused spot profiler.
Ophir Photonics Group
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Tel: 435-753-3729
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This method can also be used with a scanning slit profiler to reduce the
power density of the beam, again geometrically. For example if one has a
15W laser beam that is focused to 10µm, the power density would exceed the
damage threshold of the slit material. However, by expanding the beam 10X,
the power density will be reduced by 100-fold, bringing it back to the safe
level.
Fig 6. Beam expander with slit profiler.
Collimation
Collimation is the opposite of focusing, that is, it is the process of making the
laser have the same diameter for as long a distance as possible. Laser
applications that need this type of adjustment include laser point-of-sale
scanners, LIDAR and rangefinders, guide stars and free-space optical
communications; basically anything that needs a beam that maintains a
uniform beam size over a long distance. Another application is the
collimation of a large beam that will then be focused to a small diameter.
There are actually two types of collimation; one can be described as the
minimum divergence, and the other as the maximum distance to the waist.
When collimating for minimum divergence, the following formula defines the
divergence angle, θ = Dƒ / ƒ, using a lens of known focal length. By placing a
beam profiler at the geometric focus of the lens, adjusting the collimation to
produce the minimum spot size at this point achieves the best collimation.
This type of collimation is important for situations where the mathematically
accurate value for θ is important, such as when creating a collimated beam to
be refocused to a specific size and Rayleigh range. In many cases, simply
adjusting for the maximum distance to the waist, or focusing to infinity, is
sufficient to provide the performance required.
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
Fig. 7. Lens collimation method.
It is also possible to use the above divergence measurement technique to
achieve the divergence that will provide the proper distance-to-the-waist
desired. There is a more thorough discussion of collimation of Gaussian
beams in Donald O’Shea’s Elements of Modern Optical Design.1
High Power Beam Quality Measurements
High power lasers are used for many industrial applications, particularly
high precision welding, brazing and cutting operations. The laser is well
suited to these operations and can be a rapid and efficient method, but only if
the laser is operation properly and delivering the correct beam shape and
power to the work piece. In order to guarantee this, there are several things
that can and should be measured periodically.
• Output power or energy
• Spatial or Beam Profile
• Waist position
• Temporal profile (for pulsed beams)
All of these beam quality parameters can affect the performance of the laser
in accomplishing its assigned task. Measurements should be made in order to
establish the criteria of a laser process to ensure stable, reliable and
consistent performance and to monitor this performance over time. Lasers
and optical delivery systems degrade over time, and monitoring the laser
criteria can help to prevent waste and rework if the laser performance no
longer meets the requirements of the process. Measurements should be made
whenever one is moving or expanding a process to another system, for batch
validation and for laser maintenance and troubleshooting. Many
1 O’Shea, Donald C., Elements of Modern Optical Design, John Wiley and Sons, 1985, pp. 241-5
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
organizations operating under ISO 9001, GMP or GLP protocols will mandate
routine periodic measurement to ensure consistent and traceable results.
The technologies available for laser measurement include power and energy
meters, which indicate the total power or energy being delivered by the laser
system; beam profilers, which show the shape and size of the beam; and fast
photodiodes to measure the pulse shape of pulsed laser beams. Power and
energy meters are available in many varieties, depending on the nature of
the laser to be measured. Power meters are used for continuous wave (CW)
lasers and energy meters are used for pulsed beams. For most industrial
strength lasers these are either thermal sensors or pyroelectric sensors.
Thermal sensors, which detect the temperature rise caused by the laser and
convert it to an electrical signal, can be used to measure average power or
single pulse energy up to kWs or 100s of Joules. Depending on the power or
energy levels that they are designed to handle, they will have convection,
fans or water for cooling. For measuring repetitive pulse energies,
pyroelectric sensors are used. These comprise a crystal lattice that expands
and contracts in response to the laser pulses, creating a proportional
electrical signal and can be used up to 20kHz repetition rates and 10s of
Joules of energy. These are typically cooled by convection.
Fig 8. Various power and energy sensors.
Further information about the laser’s performance is provided by beam
profiling, as discussed above. However with high power lasers, beam
profiling becomes more complicated due to the potential for damage to the
profiler. Using attenuation devices, lasers can be sampled by camera based
array profilers, which will provide measurement of beam size, shape, spatial
energy distribution and beam wandering. Both array-based and scanning slit
based profilers can be used to measure focused spots to ensure that the laser
beam being delivered to the work piece is of the proper size to do the required
job. Other types of profilers such as a rotating pinhole or a spinning wand
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
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are able to sample a small portion of the beam and construct an image of the
profile, beam caustic and energy distribution of the beam.
Fig.9. High power monitoring setup.
For pulsed beam measurement the temporal profile of the pulses can provide
additional important information about the performance of the laser for a
particular operation. Leading edge spikes can cause a laser to create uneven
welds or ragged cuts. A fast photodiode can provide a good picture of the
temporal profile to see if poor performance of the laser system is the result of
irregular pulses, or some other cause.
Proper Attenuation Design for Laser Measurement
There are two basic types of attenuators used when making laser
measurements, reflective and absorptive. Within these two categories there
are different types and materials, but they effectively work in the same
manner. Absorptive attenuators are the colored or gray-glass filters that
work by absorbing some portion of the light, and they may be made of glass,
plastic or Inconel. Because they absorb light, they are subject to heating,
which then changes the optical properties of the material in a process called
“thermal lensing.” The localized heating in the filter material forms a lens
that can then modify the beam, causing erroneous profile information.
Reflective attenuators, such as laser end mirrors, quartz wedges or prisms
are not as susceptible to this phenomenon, and therefore should usually be
used for the first stages of attenuation of any lasers with powers greater than
100mW per millimeter diameter, or 12.7W/cm2 . This happens fast, and may
give the appearance of a stable laser profile. The following experiment shows
this dramatically:
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Experimental Design
An unfocussed CW beam from a Coherent DPSS 532 Laser (532nm
wavelength) with a nominal power of 500mW was measured using three
methods.
Method 1
Using the camera array beam profiler with all the attenuation provided by
absorptive filters, the beam was measured at a point 30cm from the exit
aperture of the laser. The attenuation comprised a 2.8OD gray-glass fixed
filter and the Photon ATP-K continuously variable wedge attenuator (1.7—
4.6OD).
Method 2
The first stage of attenuation was provided by using a front surface reflection
with additional attenuation provided by the ATP-K. The rest of the
measurement set up remained identical to that in Method 1.
Method 3
Measurements were then taken at the same measurement plane with the
Photon NanoScan scanning slit profiler, which needed no attenuation for this
laser power.
Results and Discussion
Because we expected to see thermal effects in the initial experimental design
(Method 1), we used time charting to observe the measurement of the laser.
In order to see any effects and eliminate any contribution from the laser
itself, we started the camera and then uncovered the laser beam. There was an immediate spike of the beam size to around 700µm. It settled to a value
around 500µm (1/e2) within a few milliseconds. With each measurement we
saw a similar pattern; however the value of the initial spike was somewhat
variable.
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
Fig 10. Time chart of beam measurement made with absorptive attenuation only, showing
initial spike of beam size with stabilization to lower value.
Had time charting not been employed, the measurement would have appeared to be stable at the 400-500µm beam size.
Fig 11. Data display of beam size measurements made with absorptive attenuation only.
When this same experiment was repeated using the front surface reflection
attenuator, the spiking effect was no longer visible and the beam measurement was ~850µm in one axis and 914µm in the other.
Fig 12. Data display of camera beam size measurements with 4% front surface reflector
and ATP-K.
Measurement with the NanoScan showed a beam size measurement of ~850µm
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
Fig 13. Data display of beam size measurements made with NanoScan profiler.
The measurements of the properly attenuated beam in the camera and the
NanoScan were within 2% agreement, which is the specification for accuracy
of these measurements. The beam measurements subject to thermal lensing
were 30-40% low, indicating that a very rapid lensing effect was taking place
in the first attenuator. Because this effect is so rapid, it is quite possible for
such errors to go unnoticed. The measurements did not exhibit the
“blooming” effect that might we expected from slower heating. Instead they
achieved equilibrium within a matter of milliseconds.
The power density of this laser beam was ~80W/cm2. The power going into
the ATP-K attenuator after the front surface reflector was 3.2W/cm2, which is
well below the theoretical thermal lensing limit of 12.7 W/ cm2. The
comparable results from this configuration and the NanoScan confirm that
thermal lensing effects do not occur at when the beam power incident on the
absorptive attenuator properly adjusted.
Conclusion
Matching your profiling technique to your laser application will allow you to
monitor and adjust you laser system to meet the requirements of the job at
hand. In many cases a laser profiler will make the process of building a laser
system much more efficient and accurate.
Ophir Photonics Group
3050 North 300 West
North Logan, UT 84341
Tel: 435-753-3729
www.ophiropt.com/photonics
Ophir Photonics Group
http://www.ophiropt.com/photonics
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