Continuous Sun-Pumped Room Temperature Glass Laser Operation G. R. Simpson

American Optical Company, Southbridge, Massachusetts. Received 16 February 1964.

Continuous laser oscillation at 1.06 μ has been attained at 30°C with 6.25 wt % Nd2O3-doped barium crown glass1 when pumped by the sun and when pumped by a carbon arc sun-simulator. The Nd:glass employed (A.O. Type 616W1) was the same type Young used in attaining continuous laser operation when pumped by a mercury lamp.2 This glass has a fluorescent lifetime of 400 μsec, a fluorescent line-width of 320 Å at 20°C, and an internal loss coefficient (absorption at 1.06 Μ) of 0 . 1 % cm - 1 . Flash threshold of the rods is on the order of 1 J when optically coupled to an EG&G FX-33 flashtube by placing rod and flashtube at unit magnification conjugates within a 23.5-cm diam reflecting sphere.2

Solar operation was attained through an air mass of 1.3 at Southbridge, Massachusetts, on a day when a high, thin, cirrus cloud coverage existed. Operation was approximately 15% above threshold. Laser oscillation 50% above threshold was attained using a carbon arc sun-simulator. The average radiance of the effective cross-section area of the carbon arc, when operated at 340 A and 80 V, was 70% of the exospheric solar radiance, and its spectral distribution was very similar to that of the sun. The 30-mm laser rod consisted of a 0.1-mm Nd:glass core clad by a 1.0-mm diam soda-lime silicate glass. One end of the laser rod was opaquely silvered, and the other end silvered to transmit 0.2% at 1.06 μ.

The solar pump optical system, illustrated in Fig. 1, is com­posed of a 61-cm diam ƒ / l . 5 aluminized parabolic mirror and two SF-4 glass aplanatic refractors.3 The resulting solar image is formed with a numerical aperture of 0.95 and has a diameter of 2.8 mm. Reflection and absorption losses of the optical system result in a transmission of 78%. The solar image is formed at the entrance aperture of a highly reflecting silvered cylindrical cavity 2.7 mm in diameter and 25 mm in length. The fiber laser is mounted within this cavity by cementing it to the end reflector through which it passes. Spectral filtering of unusable solar energy was not attempted. No provisions were made for cooling the laser cavity, so operation was limited to 0.5 sec by use of a hand-operated dowser situated just in front of the refracting elements. An exposure in excess of 0.5. sec under

Fig. 1. Schematic illustration of pumping and detection scheme for sun-pumped laser.

Fig. 2. Sun-pumped Nd:glass laser operation at 30°C. (a) Threshold operation; time scale: 5 msec/div. (b) Approxi­mately 10% above threshold; time scale: 5 msec/div. (c) Approximately 15% above threshold; time scale: 20 msec/div.

these conditions resulted in the silver on the exposed laser facet burning off.

The optical system for pumping with the carbon arc was similar to tha t employed in solar pumping. A 61-cm diam spherical mirror was used off-axis at unit magnification conjugates in place of the paraboloid. The same refracting elements were used, resulting in an arc image approximately 4.3 mm in diameter formed at a numerical aperture of 0.95. The reflecting cylindrical cavity containing the laser was 25 mm in length and 3.7 mm in diameter. A shutter composed of three conical elements ro­tating at different speeds so as to give one pumping pulse per second, provided a pulse with a 10-msec rise time, 8-msec full exposure, and 10-msec decay time. This alleviated the neces­sity of cooling the laser activity.

Laser output, detected by a narrow bandpass filtered DuMont K1430, S-1 photomultiplier, is depicted in Figs. 2 and 3 for solar operation and solar-simulator operation, respectively. Different laser rods were used for the illustrated solar and solar-simulator operation. I t will be noted tha t the laser output depicted in Fig. 2 is of a spiking nature, whereas tha t in Fig. 3 (from a dif­ferent rod) is a smooth continuum which closely follows the 360-cps rectified carbon arc power supply ripple and occasionally lapses into a spiking behavior. The differing types of outputs have been observed previously in clad Nd:glass laser rods with diameters ranging from 0.1 mm to 12 mm, and have been as­cribed to differences in the glass optical quality and the core/ cladding interface. The low-frequency oscillations shown in Fig. 2(c) are not clearly understood. One possible explanation is microphonism in the laser cavity caused by the mechanical shock of opening the hand-operated dowser.

The work described herein has been supported for the most part by the Avionics Laboratory, Research and Technology Division of the Air Force Systems Command, at Wright-Patter­son AFB, Ohio. The author gratefully acknowledges the help given by C. G. Young in testing and selecting the most promising

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Fig. 3. Nd:glass laser operation at 25°C when pumped by-carbon arc solar-simulator. Time scale: 2 msec/div. (a) Pump­ing light trace showing 360-cps ripple. Laser output; pumping

aperture opening: (b) ½, (c) 3/4, (d) ⅞, (e) full.

Nd:glass fibers for use in the sun-pumped laser experiment. In addition, the author is indebted to E. O. Dixon for many helpful discussions and valuable suggestions.

References 1. E. Snitzer, Phys. Rev. Letters 7, 444 (1961). 2. C. G. Young, Appl. Phys. Letters 2, 8 (1963). 3. G. R. Simpson, J. Opt. Soc. Am. 52, 595 (1962).

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