Herman E. Reedy

Advanced Figure Of Merit Evaluation For CO2 Laser Optics Using Finite Element Analysis

Thermal lensing of optics used in high power CO2 laser cavities and beam delivery systems is a constant problem facing the designer and the end user. CO2 laser mirrors, used as total reflectors in the laser cavity and beam turning optics in the delivery system, play a key role in the performance of the laser system. Silicon and copper are the two most popular substrate materials used for high power CO2 laser mirrors today. The amount of thermal lensing in these mirrors depends on the amount of absorption in the mirror coating and the ability of the mirror substrate to dissipate the heat energy absorbed by the coating. Since the coating applied to silicon and copper mirrors is the same, the decision as to which substrate material to use can be based on the mechanical and thermal characteristics of the respective materials. This paper will present current data on mirror coating absorptivities, a comparison of the mechanical and thermal properties of the two substrate materials, and finally, a figure of merit analysis based on data from a finite element analysis program.

Methods for reducing stress birefringence in cadmium telluride electro‐optic modulators

By studying the static birefringence pattern of cadmium telluride electro‐optic modulator crystals after fabrication, it has been determined that standard fabrication processes induce excessive stress birefringence. This stress birefringence is produced during the grinding stage of fabrication. Two methods for removing this birefringence have been explored: (1) mechanical polishing of the sides of the crystal to an optical finish, and (2) chemical etching of the sides of the crystal with a 5% bromine solution. Both methods reduced the static birefringence of the crystal. The stress birefringence pattern can be recreated by grinding the sides of the crystal with 9 μm polishing grit.

Measuring the specific heat of solids using a CO2 laser calorimeter

A method has been devised to measure the specific heat of solid materials, especially those used as optical components for the infrared part of the spectrum, using a CO2 laser vacuum calorimeter. The CO2 laser vacuum calorimeter is a device that can precisely determine the absorptivity, at 10.6 μm, of an optic that has been coated to reflect the laser beam. A sample to be tested is fabricated in the shape of a simple optical element, and a high‐reflecting thin‐film coating is applied. Witness samples of known specific heat are first tested to determine the percent absorption of the coating. The sample of unknown specific heat is then tested by irradiation by the CO2 laser in vacuum to find the specific heat of the substrate. The method was tested using samples fabricated of ZnSe, GaAs, Ge, and CdTe. Specific heat values calculated using this method were within 10% of commonly reported values. The accuracy of this method is ±15%. Because the mirror coating is highly reflective, substrate materials that are...

Novel focusing optics for IR lasers

Traditional focusing optics for IR lasers are refractive lenses and off-axis reflective mirrors. ZnSe meniscus and plano-convex lenses dominate the refractive lens types in use today for CO2 laser cutting, welding, marking, and engraving systems, to name a few of the more common applications.Silicon and copper spherical mirrors that are used near normal incidence and off-axis copper parabolic mirrors dominate the type of reflective mirrors used in very-high-power CO2 lasers for welding and heat treating. These types of mirrors are made with common polishing and diamond-turning techniques that are well known.In the last three years, advances in fabrication techniques have allowed optic manufacturers to produce more advanced optical surfaces on metal and refractive materials. This paper will provide details of some of these novel focusing optics for IR laser applications. The paper will include design details of ring-focus off-axis parabolas, focused flat-top generators, toroidal lenses and mirrors, faceted integrators, and long-radius off-axis parabolas. Both design and theoretical performance will be given for the optics. In some cases, laser test data will also be provided.Traditional focusing optics for IR lasers are refractive lenses and off-axis reflective mirrors. ZnSe meniscus and plano-convex lenses dominate the refractive lens types in use today for CO2 laser cutting, welding, marking, and engraving systems, to name a few of the more common applications.Silicon and copper spherical mirrors that are used near normal incidence and off-axis copper parabolic mirrors dominate the type of reflective mirrors used in very-high-power CO2 lasers for welding and heat treating. These types of mirrors are made with common polishing and diamond-turning techniques that are well known.In the last three years, advances in fabrication techniques have allowed optic manufacturers to produce more advanced optical surfaces on metal and refractive materials. This paper will provide details of some of these novel focusing optics for IR laser applications. The paper will include design details of ring-focus off-axis parabolas, focused flat-top generators, toroidal lenses and mirrors, faceted...

Round-robin test on optical absorption at 10.6 μm

The approved Draft International Standard 11551 on test methods for absorptance of optical laser components recently passed the international voting procedure. The utility of this standard practice document is the subject of the present round robin test. In order to cover a broad range of CO2-laser optical components, different types of metal mirrors and transmissive ZnSe-optics were included in the master sample set. After an initial inspection, this set passed through a series of optical laboratories in the United States, the United Kingdom, and Germany. The absorption of the samples was measured by calorimetric methods according to ISO DIS 11551, and the measurement results were compiled by the coordinating institute. The evaluation of the experiment was not started until all tests were completed. The results of the round robin test are discussed, compared and evaluated with respect to ageing mechanisms in optical coatings for CO2-laser systems. Although a great variety of different test facilities was employed by the round robin partners, a good agrement of the absorption values was observed for the wavelength of 10.6 micrometers . This demonstrates the versatility of the approved Draft International Standard 11551 for the calorimetric measurement of optical absorption in CO2-laser components.

Measurement of the birefringence in cadmium telluride electro-optic modulators

Cadmium telluride is the primary choice for electro-optic modulator applications in the mid infrared region - particularly at 10.6 micron. In principle, single crystal CdTe in the rest state is only weakly birefringent along the {1 1O} planes. Some internal birefringence does exist, however, due to fabricated-in stress birefringence and birefringence associated with slippage of the crystal along the {1 1 1} planes. When a voltage is applied to a CdTe crystal, the total phase shift introduced by the crystal is a result of the combination of the electro-optic effect and the residual birefringence. This paper will present a method of measuring the phase shift produced by residual birefringence in CdTe modulators at 10.6 micron. The test method is a modification of the crossed polarizer technique. Test results will be presented for CdTe modulators with voltage and without voltage.

Comparison Of GaAs And ZnSe For High Power CO 2 Laser Optics

A critical factor in the performance of high power industrial CO2 lasers is the substrate material used for optical components in the laser and beam delivery system. The choices for transmissive optics such as lenses and output couplers can be narrowed down to GaAs or ZnSe with low absorbing coatings, however, determining which one of these two materials will work best in a particular application can be difficult and in some cases ambiguous. This paper compares ZnSe and GaAs as used in high power laser applications by use of thermal finite element analysis, laser calorimetry data, and a relative figure of merit function.

Novel method for imaging high-power CO 2 laser beams

Continuous monitoring of high power (above 2 kW) CO2 laser beams with camera based systems has not been effective because beam sampling optics have not been available. Camera based systems allow real-time imaging of the entire beam profile which in turn enables real-time tuning and alignment of the laser, as well as enabling instantaneous recognition of beam misalignment in the optical delivery train. Spiricon and II-VI have jointly developed a new method for in-line, passive sampling and beam profiling of high power, multi-kilowatt CO2 lasers. The system uses conventional optics in a novel sampling arrangement, coupled to a Spiricon Pyroelectric IR Camera and Laser Beam Analysis software.

Residual stress birefringence in ZnSe and multispectral ZnS

Birefringence is an important factor in determining the imaging quality of visible and infrared optical systems. This paper presents residual birefringence data obtained at 0.6328 micrometers and 10.591 micrometers from several ZnSe and multispectral grade ZnS windows. Refractive index inhomogeneity tests were also performed on the samples at 0.6328 micrometers and their results are given. Residual birefringence data at 0.6328 micrometers is compared to data at 10.591 micrometers .

Laser damage studies of metal mirrors and ZnSe optics by long-pulse and TEA-CO2 lasers at 10.6 um

Single shot laser damage studies have been performed using an RF-excited long pulse laser and a TEA-CO2-laser with pulse durations of 1.2 ms and 100 ns, respectively. Besides bare diamond turned copper mirrors with different metal and dielectric coatings, ZnSe-optics with selected coating types were tested. The temporal damage behavior in the long pulse regime was investigated on the basis of a damage detection system with a time resolution of 10 microsecond(s) . The dependence of the damage threshold on the intensity is discussed in consideration of the integral absorptance of the coatings. The measured damage thresholds of this detection system are compared to those obtained by Nomarski/darkfield microscopy. The local variation of the laser induced damage threshold is correlated to the corresponding photothermal deflection signal, reflectance, and defect density of the coated surfaces.