These are some Frequently Asked Questions about Star Tech
ultraviolet beam profilers. Feel free to ask more if you have any.
How do they work?
All Star Tech Instruments UV beam profilers work by the process of downconversion. Ultraviolet light is converted to the visible or near-IR by one of our proprietary conversion crystals. By this process the energy distribution of the input UV beam is reproduced in a real object, which emits non-coherent, non-collimated light in a band that is much easier to work with. The light is collected using a high-quality imaging system, and reimaged onto a silicon CCD camera.
Isn't the image blurred within the crystal? What's the resolution?
The downconverting crystals used in our beam profilers are very thin. The stock items have 0.5 mm thick crystals, but much thinner ones (less than 0.1 mm) are available. This fact keeps the image loss generated within the crystal to something much less than the resolution of the CCD camera.
The ultimate limit on the resolution of the system due to the crystal is approximately equal to the full-angle divergence of the input beam times the thickness of the crystal. So an excimer beam with a half-angle divergence of 1 mrad going into a standard 0.5 mm f-plate would have a limit on resolution of about 1 micron. To actually achieve 1 micron resolution requires great care in mounting the beam profiler and in choice of f-plates and CCD cameras, although such high resolution has been observed by our customers. Under normal excimer laboratory conditions using our standard equipment, the highest resolution achieved is around 5 to 10 microns.
What about the imaging optics?
The imaging system is made of high quality, diffraction-limited achromats. They do not introduce any measurable beam distortion.
Among the most unique and powerful features of Star Tech Instruments beam profilers is the ability to control the magnification and intensity of your image optically. The BIP-5100 beam profiler comes with a parfocal 6:1 zoom feature, which allows you to adjust the field of view to fit the beam, or to zoom in on features you wish to observe more closely.
What is the dynamic range?
All of our beam profilers come with an adjustible iris, which allows the user to control the intensity of the image. Used with our standard CCD camera, this results in a dynamic range of appoximately 10000.
How about the dynamic range of the crystal? Isn't downconversion a nonlinear process?
Of course it is! All natural processes are nonlinear. However, the range over which the input-output relationship approximates a straight line is again around 4 to 5 orders of magnitude, depending on wavelength and choice of material.
What is the damage threshold? Don't the components degrade?
There are three basic types of radiation damage associated with excimer laser beam profilers-
Ablation Damage- A focused excimer beam will vaporize any known material, and our instruments are no exception. Ablation damage will occur when the energy density of a pulse exceeds the single-shot damage threshold. This is normally around 500 mJ/cm2 for most of our f-plates, with versions available having damage thresholds as high as 5 J/cm2
Thermal Damage- Thermal damage occurs when the total optical power of the incident beam exceeds the amount that can be dissipated by the beam profiler, and the resulting temperature rise damages the absorbing elements inside the profiler. The maximum input power for a Star Tech Instruments beam profiler ranges from 5 watts to over 1 kilowatt, depending on model, wavelength, and configuration.
Long-Term Degradation- After hundreds of units and billions of pulses, in-house and field tests of our downconverter plates show that they never deteriorate, when used within their specified energy and power ranges. The downconverter will never have to be replaced because it is "worn out". The visible optics and CCD camera are protected by a proprietary visible-transparent absorber, so no ultraviolet radiation at all reaches the CCD array, protecting it from degradation also.
What is the usable wavelength range?
Star Tech Instruments beam profilers have been used at all excimer wavelengths from 157 nm to 351 nm, and with other pulsed and CW lasers with wavelengths as long as 400 nm. On the other end of the spectrum, our beam profilers have been used with soft x-ray sources, such as the kind used for x-ray lithography, as short as 1 nm. With such short wavelengths the downconverter needs to be mounted in a vacuum-tight fixture, but being the radiation is then converted to visible light no vacuum is necessary after that point.
What is the maximum repetition rate of the input beam?
There is really no maximum or minimum repetition rate. In most cases, the maximum rate at which you can acquire data is going to be the frame rate of the camera- for a standard CCD camera this is 30 Hz. Star Tech Instruments beam profilers have been used at repetition rates over a kilohertz. The only concern is keeping the total input power less than the power dissipation of the beam profiler.
How large of a beam can I image?
Thanks to our high-quality variable magnification imaging system, the input aperture of the profiler is independent of the size of the sensing CCD array. Stock items have input aperturesfrom 10 mm to 100 mm, and apertures 150 mm or larger are available on special order.
What makes your beam profilers so much better than everyone else's?
Our downconversion process is patented and proprietary. Other kinds of UV beam profilers rely on direct irradiation of pyroelectric arrays or silicon CCD's. Pyroelectric arrays are fine for CO2 lasers, but their slow response times and low resolution make them less than ideal for ultraviolet laser use. Other brands of beam profilers rely on merely irradiating a CCD camera with an attenuator in front of it. Not only does the attenuator degrade over time due to the effects of UV radiation, but the silicon camera does also. Neither technique affords the large dynamic range, large input apertures, freedom from degradation,and optical control over magnification and image intensity that you get with a Star Tech Instruments beam profiler.
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