231 Laser Physics, Vol. 6, No. 2, 1996, pp. 231–236. Original Text Copyright © 1996 by Astro, Ltd. Copyright © 1996 by åÄàä ç‡Û͇ /Interperiodica Publishing (Russia). 1. INTRODUCTION The use of semiconductor lasers for spectroscopy is very attractive, especially when long-term, reliable operation of the laser source is a primary requirement. This is the case for metrological applications. Low-cost single-mode diode lasers, which can be easily tuned in frequency, are commercially available in the red and at wavelength longer than 750 nm. Linewidths narrower than 1 MHz are easily obtained and a single laser can be tuned over a range larger than 20 nm. The technique of second harmonic generation (SHG) in a nonlinear crystal allows the extension of the use of semiconduc- tor lasers to the blue and near-UV spectral regions. The use of semiconductor laser for producing atomic laser cooling and trapping has yet been demon- strated for Cs and Rb and metastable noble gases, whose required wavelength is accessible to the com- mercial diode lasers. For metrological application to Ca and Mg atomic frequency standards (AFS), we are interested in three wavelengths. First of all, 654 nm, correspondent to the Ca intercombination line, in the range of the red laser diode. Then 422.7 nm, corre- sponding to the 1 S 0 – 1 P 1 resonance transition of Ca, and 383 nm corresponding to Mg transitions from the trip- let metastable levels to the 3 D(3s3d) multiplet. The last two wavelengths can be produced by frequency dou- bling commercial semiconductor laser in a nonlinear crystal. Here, we describe the duplication frequency laser system that we have developed in our laboratory and its application to the AFS, based on the 3 P 0 – 3 P 1 transition of Mg at 601 GHz. 2. FREQUENCY DOUBLED DIODE LASERS Second-order harmonic generation can be obtained by focusing a monochromatic laser beam inside a non- linear medium that presents a large second-order sus- ceptibility. In order to obtain relevant power conver- sion, the SHG waves produced along the laser path inside the nonlinear medium must do positive interfer- ence. The most convenient way is to accomplish this phase-matching condition between the ordinary and the extraordinary refraction index, by rotating the optical axis of a birefringent crystal with respect to the direc- tion of the propagation and of the polarization axis of the laser beam (type I, angle-tuned SHG). In these condi- tions, the SHG power P 2ω generated by a Gaussian laser beam focused into the center of the crystal can be evalu- ated, as a function of the incident power P ω and in the hypothesis of small conversion efficiency (P 2ω  P ω ), as , (1) where d eff is the nonlinear effective optical coefficient of the crystal, n is its refraction index, l is the length, α' = α ω + 1/2α 2ω is the absorption coefficient, and k ω is the wave number at the fundamental frequency, h m (B, ξ) is a function of the focusing parameter of the Gaussian beam ξ and of the crystal parameter B = ρ(lk ω ) 1/2 /2, proportional to the double refraction angle ρ, whose value was given by Boyd and Kleinman [1]. In the conditions of optimum focusing, h mm (B = 0) ≈ 1.068, while h mm (B) ≈ 0.714/B for B > 2. The efficiency of direct SHG of cw diode laser is lim- ited by the low power of the available lasers. The tech- nique of SHG in an external resonant cavity can be used to overcome this problem. Very high conversion effi- ciency in the blue [2] has been obtained with this tech- nique with KNbO 3 crystal, where type I phase-matching condition can be achieved by orienting the polariza- tion axis of the fundamental and of the SHG radiation along two principal optical axis (noncritical phase matching). The different dispersion temperature coeffi- cients for the three principal optical axes allow a large interval of wavelengths, in which the noncritical phase- matching condition is satisfied by tuning the tempera- ture. SHG resonant with the 422.7-nm Ca transition is obtained at about –13°C. The short-wavelength limit of KNbO 3 is around 420 nm. In the near UV, at 383 nm, the choice is between two less efficient angle-tuned crystals, LiIO 3 (LIO) and P 2 ω 2 ω 2 π n 3 ε 0 c 3 ------------------ d eff 2 P ω 2 lk ω α' l – ( ) exp h m B ξ , ( ) = HIGH-RESOLUTION LASER SPECTROSCOPY, FUNDAMENTAL MEASUREMENTS Frequency Doubled Laser Diodes: Application to Mg and Ca Atomic Frequency Standard N. Beverini*, E. Maccioni*, F. Strumia*, A. Godone**, and C. Novero** * Dipartimento di Fisica, Universita di Pisa, and I.N.F.M., piazza Torricelli 2, Pisa, I 56126 Italy ** Istituto Elettrotecnico Nazionale “Galileo Ferraris,” Strada delle Cacce, Torino, Italy Received November 16, 1995 Abstract—The technique of second harmonic generation by using nonlinear crystals extends the possible application of semiconductor lasers to the blue and the near-UV spectral region. Frequency doubled laser diode systems that fulfill the requirement of long-term reliable operation were developed at 383 nm and 422.7 nm and applied to the developing of Ca and Mg atomic frequency standards.