John A. Detrio

Thermo-optic coefficient of barium gallogermanate glass

Barium gallogermanate (BGG) glasses are currently being explored as a viable low cost material for numerous U.S. defense and commercial visible-infrared window applications. These glasses are transparent from 0.4 mum to beyond 5.0 mum and can be easily made in large optics and complex shapes with high index homogeneity. For high-energy laser (HEL) applications, knowledge of the thermo-optic coefficient (dn/dT) of the window material is important in determining the optical path distortion. The dn/dT measurements were made on BGG glass at 633 and 3390 nm and compared with the values for multispectral ZnS. The dn/dT for BGG glass was approximately 1/5 the value for multispectral ZnS, giving BGG glass a clear advantage for HEL applications.

Light Scattering Surface Roughness Characterization

Light scattering is a sensitive method for characterizing the topography of a smooth, reflecting surface. Particulate contaminants which may influence the yield of semiconductor devices are easily detected in scattered light. Surface irregularities which may influence the lifetime or mobility may be quantitatively evaluated by scattering. The total integrated scatter (TIS) fromoa silicon surface can be related to the rms surface roughness with a sensitivity of 10 Å. Some applications of light scattering to the char-acterization of silicon will be summarized. The TIS method for measuring the surface roughness will be presented in detail.

Development of low-OPD windows for airborne laser

We begin with a brief review of prior work relating to optical windows for use with high power laser beams. A typical window must provide pressure separation between system segments, ultra-low loss, and small wavefront distortion of the many outgoing laser beams and signal returns despite heating by the high energy laser beam. Historically, two approaches have been examined to improve such windows.

ASTM And OSA Efforts To Develop An Optical Surface Roughness Measurement Standard

The development of a formal standard test method for the surface microroughness characterization of optical components is outlined. The results of an interlaboratory comparison of the test method are presented to illustrate the advantages and limitations of this approach to measure effective rms surface roughness. The lab-to-lab variations in measured total integrated scatter (TIS), effective roughness, were found to be about +15%. The within-lab variations were about ±5%.

Rain impact testing of durable coatings on water-clear ZnS

Quantitative measurements of the influence of high-speed rain impact damage on uncoated water clear ZnS as a function of velocity and impact exposure were made using Nomarski microscopy, total integrated scattering, integrated spectral transmission and equibiaxial flexural testing. The rotating arm tests assessed the response of the material to nominal 2 mm diameter drops at normal incidence. The measure of drop exposure selected for these studies was the density of drop impacts expected to strike the specimen for the test conditions (rain rate, velocity, run time, and spatial distribution of drops). The expected exposure is compared to measurements on PMMA calibration specimens. The influence of high durability coatings on the water clear ZnS was assessed using the same characterization methods. The limitations imposed by statistical variability illustrate the difficulty of comparisons made without adequate control of the parameters.

Photoconductivity of Ga implanted Si

Abstract High resolution spectral photoconductivity measurements are reported for 360 keV, 10 13 cm −2 Ga ion implanted Si. The 32 K spectrum shows excited state features and BWF transitions m addition to the impurity-to-band continuum. High resistivity n -type substrates were used without multiple implantation to adjust the compensation. Careful electrical contact to the implanted layers and reverse bias isolation of the shallow implant were necessary to produce the Ga spectra without observing contributions from the substrate.

Scattering In Znse Laser Windows

The scattering of laser radiation in window materials gives rise to several effects which have considerable practical importance. The angular distribution of the radiation scattered within the window can be interpreted to aid in understanding the homogeneity and microstructure of the material. The distribution of scattered radiation which escapes from the window may have important consequences for applications of the material in a laser system. The absorption of scattered radiation within the window will confound the measurement of the linear absorption coefficient when a calorimetric absorption technique is employed. Experiments designed to measure the internal scattering distribution, the external scattering distribution and the influence of absorbed scattering on calorimetric absorption are described. Data for CVD ZnSe are presented.

Effects of Deuterium Treatments on the Optical Properties of Fused Silica

Flame produced fused silica, the process used by all American manufacturers, contains a large amount of OH - . The fundamental OH - vibration in SiO 2 is at 2.75 jim and its first overtone is at 1.38 μm. Much of the optical absorption at 1.3 μm is due to the wing of the 1.38 μm absorption band. Stone and Burrus [ 1 ] l described a deuterium treatment given optical fibers to transform the OH - absorption to OD - whose fundamental is at 3.72 μm and first overtone at 1.85 μm. Here we describe some related treatments given to Suprasil II window samples. Spectral traces show that the 1.38 μm absorption peak is diminished while a new absorption peak appears at 1.85 μm. Similar changes appear in the spectrum between 2 and 4 μm. 1.3 μm laser rate calorimetry performed before and after treatment on the sample showing the most complete HAD exchange exhibited a drop in absorption value to about 1/5. Thus we conclude that over 80% of the optical absorption in Suprasil II at 1.3 μm is due to the OH - molecule. The method offers a way of improving the performance of flame produced fused silica windows used at 1.3 μm.

Optical Distortion And Far Field Measurements For Laser Window Materials

Laser-induced optical distortion in laser window materials may seriously impair the performance of a high power optical system. The distortions result from refractive index changes produced by laser-induced thermal gradients. The thermo-optic effect (dn/dT), thermal expansion, and stress-optic effect all contribute to the loss of far field intensity. Experimental measurements of thermal lensing in ZnSe and experiments to monitor the distortion of laser-irradiated fluoride materials are presented. For a well behaved Gaussian beam profile the window distortion produces a thermal lensing effect in ZnSe which changes the focus but produces only slight distortion; refocusing of the optical system can restore the on-target intensity in this situation. In general, the laser-induced distortions will follow the beam profile variations. Irregular beam intensity profiles introduce greater distortion than uniform beam profiles.

Synchronous Detection Technique For Determining Brewster's Angle

A direct derivative measurement technique for determining Brewsters angle has been developed using a lock-in amplifier which synchronously detects the reflected intensity of a directionally modulated or dithered laser beam. In our apparatus the angle of incidence of a polarized laser beam is slowly varied while it is dithered at a high frequency by a galvanometer-mounted mirror. The modulated signal, which is proportional to the derivative of the reflectance versus angle curve, is detected with a lock-in amplifier tuned to the dither frequency. The angle at which the signal crosses zero is Brewsters angle. The angle is more precisely defined using this method as opposed to the more conventional method of determining the angle of minimum reflectance because it is easier to locate a zero crossing than a minimum in a curve. The technique has been tested at visible and infrared HeNe laser wavelengths. The precision of the measurement is typically better than five parts per thousand for materials with refractive index near 1.5 and ten parts per thousand for materials with refractive index near 2.5. Data have been gathered for a number of visible and infrared transmitting materials.