Abstract

The output spectrum of a multi-mode laser is extremely sensitive to frequency selective loss in the resonator, and this loss can be measured from recorded spectra (ICAS^[1]). Application of diode lasers to this technique requires the elimination of the 30% reflection of at least one of the diode’s facets, and setting up an external resonator. A double heterostructure AlGaAs laser diode (Hitachi) with 780 nm emission wavelength has been vacuum-coated by HfO₂ such that the residual reflectivity was R ≤ 10^-2. A 70 cm-long external cavity provided 30% feedback. A Rb heatpipe with density 10⁶ cm^-3 < N_Rb, < 3·10¹¹ cm^-3 and 3 cm absorption length was placed in the resonator; it shows line absorption at 780.027 nm, pressure broadened by 1bar Ar to 25GHz. Output spectra have been recorded with a spectrograph, diode array, and digital scope (i) at N_Rb = 5·10¹⁰ cm^-3 and various values of the diode current (Fig. 1), and (ii) with varied absorption at threshold current.

© 1992 IQEC