For CO2 and Other Molecular Lasers

Displays IR Laser Beams

No longer are the firebricks and smoldering paper "tools of the trade" for researchers working with CO2 lasers. Now it's possible to see IR laser beams in real time and with high resolution using a Thermal Image Plate from Macken Instruments. The characteristics of this instrument enable it to solve a wide range of problems. For example:

• The fast response time permits it to be used for viewing rapidly changing laser mode patterns when adjusting resonator mirrors.
• The convenient shape permits it to be placed next to optical elements to locate and center beams.
• The high resolution permits it to be used as a diagnostic tool to examine imperfections in IR optical elements or as an aide in optical alignment.
• Even the critical adjustment of a laser interferometer is greatly simplified when the interference fringes of two misaligned beams become clearly visible.

Theory of Operation

The Thermal Image Plate displays IR laser beam through the use of thermal-sensitive phosphors. When illuminated these phosphors fluoresce by a long wavelength ultraviolet light (360nm). The intensity of the fluorescence decreases with increasing temperature.

When an IR laser beam strikes the thermal-sensitive surface, the absorbed energy raises the surface temperature and produces a corresponding thermal image. The produced pattern appears as a dark image on a bright fluorescent background when the surface is illuminated by an ultraviolet light. Different sensitivity ranges are obtained by using different phosphors and by varying the amount of thermal insulation between the phosphors and the anodized aluminum heat sink.

Any long wavelength, ultraviolet light can be used to illuminate the surface, but Macken Instruments' Lamp Model 22-UV is the most satisfactory. Its small size and high-illumination level permit it to be conveniently positioned out of the working area.

Description

The power density of CO2 laser beams ranges from those which can rapidly burn holes in firebrick to those that can't be felt on the back of the hand. Eight different surfaces with overlapping sensitivities are used to smoothly span the power density range from 200 watts/cm2 to 0.01 watts/cm2. (The minimum detectable power for a focused beam is less than 0.001 watts.)

Thermal Image Plate 22-A contains four surfaces which cover the power densities commonly obtained from CO2 laser beams as they emerge from the laser. The lower power densities usually found in optical systems and divergent beams are covered by Thermal Plate 22-B.

The physical dimensions of each Thermal Plate (22-A and 22-B) are 6" x 3" x 0.5". Each side of these plates contains two sensitive surfaces which are 3" x 3". The ultraviolet lamp (22-UV) measures 6" x 2" x 2".

The following table gives the specifications of the eight surfaces measured at 8 to 12 microns. For use at 4 to 8 microns, multiply columns a, b and c by 2.

For use at 2 to 4 microns, multiply columns a, b and c by 2.5.

1. The normal sensitivity range is the spread of power densities which can be easily viewed with no reduction in background illumination. The upper power density occurs when the surface becomes saturated, turning the area completely black and making it incapable of displaying any further detail within the beam.
2. The minimum-detectable power densities are the lowest power densities observable under the most favorable illumination levels. For this sensitivity the room lights must be dimmed and the level of ultraviolet illumination recreated to produce a dim fluorescence.
3. Damage threshold is the power density which produces a permanent change in the thermal- sensitive surface. This occurs at power factors at least four times greater than saturation. Therefore, when saturation occurs, a lower sensitivity surface should be used to display the beam.
4. The response time is the length of time it takes a change in the beam to be displayed as a change in the thermal image.
5. The resolution of a surface is the maximum number of dark and light line pairs which can be displayed with good contrast. For comparison, the resolution of a newspaper photograph is approximately 70 lines/inch. Two C)2 laser beams intersecting at an angle of two degrees produce an 80 line/inch interference pattern.

Power Dissipation A Thermal Image Plate device is capable of displaying beams up to 200 watts. However, it must be removed from the beam and allowed to cool when there is a marked decrease in overall fluorescence. This occurs when the plate has absorbed 15,000 watt seconds of power. Powers less than 30 watts can be dissipated indefinitely. Safety Precautions have been taken to eliminate specular reflections from the Thermal Image Plate. Both the thermal-sensitive surfaces and the anodized aluminum have matte finishes and show little surface reflection at 10.6 microns. However, standard safety precautions such as wearing protective glasses should always be observed when working with IR lasers. Price The Thermal Image Plates 22-A and 22-B as well as the ultraviolet lamp 22-UV can be purchased separately or together in the form of a Thermal Image Kit. When the kit is purchased, a hard plastic case is also included. Considering the wide range of power densities which can be obtained from non-uniform beams, we recommend an initial purchase of the Thermal Image Kit.

Ordering Information Price List Thermal Image Plates: Thermal Image Plate, Model 22-A Thermal Image Plate, Model 22-B Ultraviolet Lamp, Model 22-UV Thermal Image Kit, Model 22-K -includes Model 22-A, 22-B, 22-UV, in a hard plastic case. Prices on custom thermal image surfaces are available upon request. These include larger sizes, models or coatings applied to supplied parts.