Question

For a typical laser, the beam divergence is less than

1 milliradian

5 milliradian

10 milliradian

15 milliradian​

Answer 1

Beam divergence measurements were made by reducing the aerolens module temperature to—84°C and propagating a helium neon laser beam through the aerolens channel to a detector or screen 244 in. (620 cm) away for observation. The room temperature (15°C) nitrogen flow velocity was optimized to obtain simultaneously maximum beam divergence and minimum profile distortion for this operating gas to wall temperature differential [15−(−84)= 99°C]. Measurement of the beam spread of the elliptical pattern was made visually and by detector scanning. The visual observations of the beam spread were about 36% greater than the spread determined by the 1/e2 points obtained by scanning. As the aerolens returned to room temperature, measurement of the diverged beam spread was made for selected temperature differentials (typically every 70 degrees). Transverse flow gas velocity tuning was performed for each measurement condition. In Fig. 7, a photograph of the elliptical beam observed on the screen for a temperature differential of 75°C is (above the scale), is compared with a beam observed with the aerolens flow momentarily turned off (below the scale). Note that the beam size in the direction affected by the aerolens has increased by a factor ∼16 while the beam size in the orthogonal direction has remained constant. A plot of the beam spread observed at a distance 244 in. (620 cm) from the aerolens for differential temperatures ranging from zero to 99°C is presented in Fig. 8. The experimental data points represent the beam spread for a beam size determined by the 1/e2 points (13.5% of peak intensity level) of a Gaussian beam profile. The theoretical curve was calculated [Eqs. (1–6)] with ordinate and divergence values at the entrance of the gas of0= 1.0 mm and0= 0.5 mrad, respectively. The laser used in these measurements was a Spectra Physics Model 155 with an output beam diameter of 0.9 mm and full angle divergence of 1.0 mrad. It is seen that the correlation between experiment and theory is excellent. Note also the concave upward character of the data indicating the nonlinear influence of aerolens temperature differential in this index gradient lens. Maximum beam divergence for a 99°C temperature differential was observed to be 22.6 mrad which is a factor 22.6 increase in beam divergence over the 1 mrad divergence incident beam.

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