Short pulses from a plasma-controlled HCN gas laser

PHYSICS LETTERS

Volume 46A, number 4

SHORT PULSES FROM

31 December 1973

A PLASMA-CONTROLLED

HCN GAS LASER

G.D. TAIT, L.B. WHITBOURN and L.C. ROBINSON School of Physics,

University

of Sydney,

Sydney,

Australia

Received 20 August 1973 A pulsed plasma source inserted of the laser by varying

the effective

in the cavity of a continuous length of the cavity.

In a previous paper [l] the authors studied the generation of multiple short pulses of radiation at a wavelength of 337 pm by a pulsed hydrogen cyanide laser. These pulses were shown to arise when the time-varying plasma refractive index of the gas discharge swept the effective length of the laser through the narrow gainlengths of successive modes of its Fabry-Perot resonator, In the pulsed laser, discharge plasma effects and laser effects (e.g. the production and excitation of HCN molecules) are interconnected as they both depend on the same discharge. To investigate the mode sweeping process it is desirable to study the influence of the plasma alone. To this end we have built a continuous laser and have located within the laser resonator a quite independent pulsed hydrogen plasma. We wish now to describe this plasma-laser experiment and to show our first observations of short pulses produced by the plasma-controlled laser. We illustrate the experimental system in fig. 1. The Fabry-Perot resonator consists of a plane mirror separated by 2.77 m from a 3.7 m radius of curvature concave mirror having a small central output coupling hole 1.5 mm in diameter. The concave mirror is movable,

DC power

supply

with output

hole

Fig. 1. D.C. laser with intracavity pulsed plasma source. The mylar diaphragm

is tilted to avoid spurious

effects from reflections.

HCN gas laser has been used to control

the output

Fig. 2. Laser output pulses. The pulsed discharge is fired 100 ps before the start of the photograph. permitting alignment of the resonator and adjustment of its length by micrometers. The laser discharge takes section of the 7.5 cm diameter glass laser vessel, through which a 2: 1 mixture of methane and nitrogen flows at 0.5 R torr/s and a total pressure of 1 torr, excited by a 1 A continuous discharge current. Next to the laser discharge, but separated from it by a mylar diaphragm 5 pm thick is another glass vessel 33 cm long and 25 cm in diameter. A steady flow of hydrogen gives a pressure of 20 mtorr in this vessel. The discharge of 1 /JF capacitor charged to 20 kV ionizes the hydrogen. Suppose we tune the continuous laser to give output when the hydrogen discharge is inoperative. Then, when the discharge is fired, we observe firstly a period when the laser power drops to zero (fig. 2). As the hydrogen plasma density falls, two pulses are generated as the plasma refractive index tunes the laser length through successive resonances. Finally, as the electron density falls towards zero, the laser switches on again and the output settles to its quiescent level. We have recorded the response of the laser to pulsing the hydrogen plasma for many positions of the moving mirror separated by steps of 20 pm. In general we have observed 1 to 3 pulses (depending on mirror spacing) in place in a 1.5 m long water-cooled

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number

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PHYSICS

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I

I

200

300 (pm)

in cavity

length

I

I

400

Fig. 3. The dots show the measured onset times of laser radiation pulses as a function of mirror separation. The curves are calculated from electron density measurements,

the afterglow of the plasma. From a portion of this data we plot fig. 3. The proposed explanation of these results is as follows. The sets of points designated “A” in fig. 3 are repeated at intervals of mirror displacement of about 168 pm and accordingly represent output at 337 pm, the usual dominant HCN laser line. Within a given set of these points it is seen that as the mirror separation increases, the output pulse appears progressively earlier in time such that the decreased refractive index of the plasma compensates for the increased physical length of the resonator. The fact that no output is observed earlier than about 250 ps after preparation of the plasma we attribute to strong refractive effects associated with the radial electron density gradient in the plasma, which presumably make the resonator unstable at early times [2] . The other groups of points designated “B” in fig. 3, repeated with a slightly different interval in mirror spacing of about 156 pm, are due to laser action at 3 11 pm. Oscillation at either wavelength prob-

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LETTERS

31 December

1973

ably occurs in the TEMOO transverse mode, no other modes having been observed. We have examined the quantitative relationship between the pulse delay versus mirror position observations and the plasma decay. The electron density II of the plasma was measured by 35 CHz microwave interferometry. The reduction in the optical length of the laser resonator caused by the plasma is [I] Lrr/2N where L is the length of the plasma and N is the appropriate cut-off density for the laser wavelength (337 or 3 11 pm). Using this formula we have plotted the curves in fig 3. The curves are positioned to match the observed points late in time when the electron density tuning effect is negligible, with adjacent curves corresponding to a given laser line spaced half a wavelength apart. Good agreement is obtained with the experimental points. The authors wish to acknowledge financial support from the A.R.G.C. and A.I.N.S.E. and Professor H. Messel and the Science Foundation for Physics for providing the facilities that enables this work to be carried out.

References [l]

L.B. Whitbourn, L.C. Robinson and G.D. Tait, Phys. Lett. 38A (1972) 315. [ 21 B.W. McCaul, Appl. Opt. 9 (1970) 65 3. L.B.~Whitbourn, Ph.D. thesis (unpublished) University of Sydney, Sydney, Australia (1973).