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Full text of "DTIC ADA244958: CO2 Laser Frequency Multiplication Report for December 1991"

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AD-A244 958 

JAN 2 3 1992 

CONTRACT NO. 00014-91-0-2279 

A Lumonics 203 TEA discharge laser was refurbished for use as an 
amplifier. After conditioning, only two out of the three 40 cm gain modules 
would operate with greater than 90% reliability. The third module would 
arc prematurely on 50% of shots, so it was not used in our initial 
amplification experiments. Using an input mode-locked pulse train 
(TEMqo. 9-55 ijm, 3 |j.sec duration, 2 nsec pulses spaced by 40 nsec) a gain of 

4 was obtained at an input energy of 40 mJ. The input Gaussian spot 
radius parameter was 0.5 cm, so that an output energy density of 0.4 J cm'^ 
was achieved in the unfocussed exit beam. Further increase in energy 
density could be achieved in principle by beam expansion before 
amplification, followed by reduction to the 1 cm aperture of the AgGaSe 2 
doubling crystal. 

By appropriate timing of the oscillator and amplifier pulses, the 
envelope of the mode-locked pulse train could be improved. Without 
amplification, the "spike" to "tail" ratio ranged between 4 and 8, depending 
on the intensity of line injection, but after amplification of only the tail 
portion of the pulse, with a gain of 4, the "spike" to "tail" ratio was 
approximately in the range of 2:1. This improvement will allow a 
meaningful study of frequency doubling with trains of 100 pulses in 
duration, contrasted with the 10 pulse duration of a typical spike. 

The reproducibility of the envelope of the mode-locked pulse train was 
improved greatly through the introduction of an intra-cavity monitor of the 
injected intensity. This intensity peaks when the cavity length is an exact 
multiple of X/2, where X is the injected wavelength. We tuned the cavity 
length manually, if needed, prior to a shot and thereby achieved 100% 
successful line injection with a stable envelope. Prior to this development, 
as a back-up approach to line selection, a 9.5 p-m blazed grating had been 
ordered which is due for delivery in the near future and will be used to 
obtain envelopes in which the "spike” dominates the "tail" by a ratio of 10:1 
or greater. 

A 35 mm long x 10 mm x 10 mm type I AgGaSe 2 crystal was received 
from Cleveland Crystals in mid December. It has been mounted on a 


1 in 089 

temperature-controlled stage with rotation and translation. A MgF 2 
window was bought to separate 9.55 [im from 4.775 pm by differential 
absorption. Preliminary frequency doubling at relatively low intensities 
(< 1 MW cm-2) has shown that the crystal is optically perfect throughout its 
length, as evidenced by the clearly defined (sin 0/0)2 pattern of second 
harmonic power versus crystal angle, with a half-width to the first zero of 
only 0.2° (internal). The same curve taken at two temperatures (30°C and 
58°C) shows almost no change at all in the phase-matching angle (< 0.01° 
internal) indicating that the temperature coefficient of the index difference 
is less than 3 x 10-7 per °C. This is even smaller than the low value of 1.1 x 
10-6 obtained for frequency doubling of 10.6 pm by Barnes et al in 1979, 
obtained by comparing between 100°K and 298°K 

In the coming month we plan to study second harmonic generation 
with short and long pulses trains at increasing fluences. We shall look at 
the optical quality of the second harmonic beam. A second type I crystal 
will be purchased in order to generate the third harmonic of 9.55 pm at 
3.18 pm, by mixing with 4.77 pm. (Type II material is still unavailable). 

Defense Systems 

Textron Defense Systems/ 
Subsidiary of Textron Inc. 

2385 Revere Beach ParKway 
Everett. MA 02149 

6 January 1992 

Naval Research Laboratory 
Attn: Mr. Barry Feldman, Code: 6540 
4555 Overlook Avenue, SW 
Washington, DC 20375-oGCl 

Subject: Contract No. N00014-91-C-2279 

Monthly Progress Report - December 1991 

Dear Mr. Feldman: 

In accordance with the 91-C-2279 contract, Textron Defense Systems (TDS) 
hereby forwards one copy of the subject report. Additional distribution has 
been made to the offices set forth below, per Enclosure 1 of the DD Form 1423. 

Very truly yours, 
Textron Defense Systems 

C. Thomas Christiano 
Manager of Contracts 




cc: Director 

Naval Research Laboratory 
ATTN: Code 4827 

Washington, DC 20375-5000, (1 copy) 

^Defense Technical Information Center 
Bldg. 5, Cameron Station 
Alexandria, VA 22314, (4 copies) 

Cefense Contract Management Area Operations 
495 Summer Street, Boston, MA 02210-2184 
Attn: GFACB-Z3/Mr. David Noyes 

Administrative Contracting Officer, (1 copy)