Gary L. Herrit

Infrared optical characteristics of type 2A diamonds.

The absorption coefficient of two samples of type 2A diamond was measured by laser calorimetry at 10.6 microm and found to be 0.033 and 0.0415 cm(-1). Transmission properties from 2.5 to 20 microm are also presented.

Laser beam analysis in direct metal deposition process

The quality of a laser beam profile and its stability play fundamental roles in laser material processing. Beam reflections may dramatically affect the beam quality under certain material processing conditions. In this study, experiments were carried out to examine the variations of the continuous wave CO2 laser beam profile during the direct metal deposition (DMD) process. A pyroelectric-camera-based new industrial laser beam analysis device was installed in the optical line near the laser output coupler. Experimental results are presented with the comparison of beam profiles under different cladding processing conditions, such as the coaxial powder flow cladding, preplaced powder cladding, the remelting process, and the dumping of the raw beam. It was observed that cladding using preplaced powder gives off more reflective energy from the cladding zone which deforms the beam profile and causes great power fluctuation. Laser beam profiles and stability during DMD processes were monitored and analyzed with...

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...

Comparison of 1 micron transmissive optical materials for high power lasers

With the advent of very high-power direct-diode, fiber and disk lasers, choosing the right optical material for a given application is vitally important. What will determine the best optic for the application are a combination of factors about the application itself and the material properties of the optics. Application factors of importance include the laser power, laser type, and the users’ application. Important material properties that the designer must consider in the selection of optical materials are the fundamental optical, thermal, and mechanical properties as well as the cost of purchasing the optics. This paper will discuss in general the laser and application factors, however, the main focus will be on various properties of the available optical materials and the relative cost of finished optics. During the discussion of the material properties we will present a figure of merit calculation for optical materials that can be used to determine the suitability of an optic to an application. We will discuss the use of Fused Silica, Zinc Sulfide MultiSpectral (ZnS MS), and Zinc Selenide (ZnSe) for lenses and windows. Finally we will present some novel optic configurations that can be diamond turned for special beam shaping applications such as heat treating, cladding, and welding.With the advent of very high-power direct-diode, fiber and disk lasers, choosing the right optical material for a given application is vitally important. What will determine the best optic for the application are a combination of factors about the application itself and the material properties of the optics. Application factors of importance include the laser power, laser type, and the users’ application. Important material properties that the designer must consider in the selection of optical materials are the fundamental optical, thermal, and mechanical properties as well as the cost of purchasing the optics. This paper will discuss in general the laser and application factors, however, the main focus will be on various properties of the available optical materials and the relative cost of finished optics. During the discussion of the material properties we will present a figure of merit calculation for optical materials that can be used to determine the suitability of an optic to an application. We wil...

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...

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.

Thermal management to reduce focus shift using ZnS MS optics

Focus shift or thermal lensing is one of the key factors when determining the quality of a cutting or welding head. High power CO2 laser industry cutting heads evolved over time to produce focused beams with a minimum of thermal lensing. One of the key lens parameters that determine thermal lensing produced by a focusing lens is the lens materials thermal conductivity. CO2 OEM system builders have found that the best optical material for focusing lenses in cutting heads is ZnSe; this is largely due to its relatively high thermal conductivity and low absorptivity. It is expected that the 1 µm cutting heads will evolve in a similar manner with regards to managing the thermal loads in the cutting head including the need for a high thermal conductivity lens material and relatively low absorptivity. In this paper we will present test results for focus shift utilizing direct water cooled mounts with ZnS MS lenses. We will also discuss the effects of beam diameter changes on the focus shift when changing to different magnifications.Focus shift or thermal lensing is one of the key factors when determining the quality of a cutting or welding head. High power CO2 laser industry cutting heads evolved over time to produce focused beams with a minimum of thermal lensing. One of the key lens parameters that determine thermal lensing produced by a focusing lens is the lens materials thermal conductivity. CO2 OEM system builders have found that the best optical material for focusing lenses in cutting heads is ZnSe; this is largely due to its relatively high thermal conductivity and low absorptivity. It is expected that the 1 µm cutting heads will evolve in a similar manner with regards to managing the thermal loads in the cutting head including the need for a high thermal conductivity lens material and relatively low absorptivity. In this paper we will present test results for focus shift utilizing direct water cooled mounts with ZnS MS lenses. We will also discuss the effects of beam diameter changes on the focus shift when changing to diff...