Appl. Phys. B 62, 333-338 (1996) Applied Physics B ,..s,s and Optics © Springer-Verlag 1996 Tunable frequency-controlled laser source in the near ultraviolet based on doubling of a semiconductor diode laser M. de Angelis 1, G. M. Tino 1, P. De Natale 2, C. Fort 2, G. Modugno 2, M. Prevedelli 2, C. Zimmermann 2'* ~Dipartimento di Scienze Fisiche dell'Universit/tdi Napoli "FedericoII', I.N.O. Sezionedi Napoli, Mostra d'Oltremare Pad. 20, 1-80125 Napoli, Italy (Fax: + 39-81/2394508, E-mail: MARELLA@axpnal.na.infn.it) 2European Laboratory for Nonlinear Spectroscopy(LENS), Largo E. Fermi 2, 1-50125 Firenze, Italy Received: 12 July 1995/Accepted: 6 September 1995 Abstract. Continuously tunable ultraviolet laser radi- ation at 397 nm was generated by doubling the output of a semiconductor diode laser. The fundamental radiation was provided by a 150 mW A1GaAs laser diode injected by a low-power A1GaAs laser diode which was frequency stabilized by optical feedback using a new scheme of a miniature external cavity. Second-harmonic generation was produced in a lithium-triborate crystal placed in a compact enhancement cavity. The fundamental radi- ation was used for sub-Doppler spectroscopy of the Ar I 4s3p°o-4plP1 transition at 795 nm; the second-har- monic radiation was used for spectroscopy of the Ca II 4 2S1/2-4 2P~/2 transition at 397 nm. PACS: 42.55.Px; 42.65.Ky Semiconductor diode lasers are attractive light sources for spectroscopic experiments in the visible and near-infrared region of the spectrum [1]. Because of their unique prop- erties of compactness, efficiency, low amplitude noise and fast frequency tunability, they not only allow a drastic simplification of the experimental setups, but also lead to an improvement of their performances. On the other hand, the low spectral purity of this kind of lasers may represent a serious drawback for their use in high-resolu- tion spectroscopy experiments. Different techniques have been demonstrated for the frequency stabilization of diode lasers. The use of diode lasers has been limited also be- cause they are available only at specific wavelength re- gions and at relatively low power when they operate in cw single-mode. Injection locking allows to use full power while having single-mode operation. This technique couples two oscillators so that their frequencies and phases are highly correlated. As applied to lasers, this * Permanent address: Sektion Physik der Universitf.t Miinchen, D-80799 Miinchen, Germany technique is more interesting when a low-power laser with desirable frequency properties (the master) is used to im- pose its frequency and mode structure onto a high-power laser (the slave) whose spectral properties would otherwise not be so good. This is accomplished by injecting the master laser output into the slave laser's cavity. Injection locking operation has been demonstrated to be an easy and valid way to convert on single mode the power of laser diodes which are multimode free-running lasers [2]. This technique, applied to laser diodes, offers the advant- age of single-frequency operation of a high-power laser without the use of external cavities that reduce the efficien- cy and output power of the oscillator. Generally, efficient frequency doubling of laser diodes is limited by the fundamental power available from single- mode lasers. By using an enhancement cavity for the fundamental radiation, good second-harmonic generation efficiency can be obtained [3, 4]. In [3] the laser diode was frequency stabilized by optical feedback fl'om the en- hancement external cavity. The high conversion efficiency demonstrated in these works was mainly due to the large nonlinear susceptibility of the crystal used (KNbO3) and to the possibility of using non-critical phase matching. However, generation of radiation at shorter wavelength in KNbO3 is not possible because the crystal has a low- wavelength cut-off at 419 nm. Generation of UV radiation at 397 nm by frequency doubling a laser diode was dem- onstrated [5] by using angle tuning phase matching of LiIO3 crystal in an enhancement cavity. Hayasaka et al. [5] used a laser diode frequency stabilized by optical feedback from the external enhancement cavity while the tuning of the wavelength was realized only by changing the temperature of the diode, which gives, as is well known, a discontinuous tuning. Furthermore, optical feedback from the doubling external cavity limits the continuous frequency scanning of the UV radiation. In case the laser diode is optically locked to the enhancement cavity, the frequency scanning range is never greater than 6 GHz in the UV region [5]. Frequency doubling of the laser diode at shorter wavelength has been realized by Tamm [6]. In his work, a low-power frequency-stabilized diode laser emitting at 740 nm was doubled by using an