ISSN 1068-3356, Bulletin of the Lebedev Physics Institute, 2018, Vol. 45, No. 3, pp. 83–86. c  Allerton Press, Inc., 2018. Original Russian Text c  A.K. Chernyshov, P.A. Mikheyev, N.N. Lunev, 2018, published in Kratkie Soobshcheniya po Fizike, 2018, Vol. 45, No. 3, pp. 28–33. Diode Laser with External Double Reﬂector for Gas Analysis A. K. Chernyshov a,b , P. A. Mikheyev a,b , and N. N. Lunev a,b a Samara Branch of the Lebedev Physical Institute, Russian Academy of Sciences, ul. Novo-Sadovaya 221, Samara, 443011 Russia b Samara National Research University, Moskovskoe sh. 34, Samara, 443086 Russia; e-mail: chak@ﬁan.smr.ru Received January 29, 2018 Abstract—A diode laser conﬁguration with a short external cavity in which an additional reﬂector is formed by a packet of two thin cover glasses is proposed. The use of such external cavities in combination with diode lasers based on quantum-well heterostructures is promising for Doppler broadening spectroscopy and the cases where a wide range of continuous tuning at moderate requirements for the laser linewidth is important. DOI: 10.3103/S1068335618030053 Keywords: diode lasers, multiple-quantum-well heterostructure, short external cavity, spectral charac- teristics, gas analysis. To span a maximum spectral range using the same emitter in spectroscopy and optical gas analyzers, diode lasers (DLs) based on multiple-quantum-well (MQW) heterostructures with optical gain band- widths reaching hundreds of nanometers are promising [1]. At the same time, the wide gain bandwidth promotes simultaneous generation of several longitudinal modes by the emitter, which distorts measured spectra. One of the methods for suppressing multimode DL generation is placing a thin cover glass near the output laser face [2]. However, the wider the laser optical gain band, the shorter external cavity is required for guaranteed single-mode lasing. Therefore, in the case of MQW diode lasers, the application of this approach is limited, since the cover glass should be brought almost right up to the semiconductor crystal. To remove this constraint, the “nonius” eﬀect can be used, which consists in rarefying the resonance pattern during sequential passage of light through a pair of interferometers with approximately equal lengths [3, 4]. As a result, the “thick” tandem interferometer appears equivalent to the “thin” etalon. In this study, it was demonstrated that an addition of an external reﬂector formed by a pair of cover glasses to the MQW diode laser makes it possible (due to the “nonius” eﬀect) to easily achieve frequency-tunable single-mode lasing. The results on observations of water and argon absorption lines, obtained using MQW diode lasers equipped with double external reﬂectors are presented. The conﬁguration of the diode laser with short external cavity (DLEC) based on a double reﬂector is shown in Fig. 1 (left). Preliminarily, to provide access to the semiconductor crystal, a protective cover with output window was cut oﬀ from the commercial laser assembly. Then the open assembly was placed into a holder whose temperature was stabilized by a Peltier element. Laser radiation was collimated into a parallel beam using a selected microobjective (F =6 mm, NA =0.5) with a ﬂange focal distance of ∼1.7 mm. This is necessary to accommodate two microscope cover glasses forming an external reﬂector in the gap between the microobjective and semiconductor crystal face. Cover glasses ∼100 μm thick were stacked and glued to a corner frame using which ﬁne tuning of their spatial position was provided. The spectral characteristics of the diode laser with a short external cavity was studied using the setup shown in Fig. 1 (right). To operate the emitter in continuous mode, an ITC4001 (Thorlabs) dual current/temperature controller was used. The linearly polarized DLEC output beam initially passed through a λ/4 phase plate P based on a liquid-crystal cell [5] and became circularly polarized. Such polarization conversion is necessary to suppress possible parasitic optical feedback. As is known, the rotational direction of the circular light polarization changes after reﬂecting from optical elements. Therefore, after passing through the λ/4 plate, back-reﬂected light gains an additional phase increment 83