Effect of pulse to pulse variation of divergence, pointing and amplitude of copper vapor laser radiations on their second harmonic and sum frequency conversion Om Prakash n , Ramakanta Mahakud, Shankar V. Nakhe, Sudhir K. Dixit Raja Ramanna Centre for Advanced Technology, R&D C-3 Block, Laser System Engineering Section, Indore 452013, M.P., INDIA article info Article history: Received 16 November 2012 Received in revised form 20 January 2013 Accepted 29 January 2013 Available online 16 March 2013 Keywords: Sum frequency mixing Second harmonic of green and yellow radiation Divergence and beam pointing stability abstract This paper presents the effect of single pulse stability of divergence angle, beam pointing angle and amplitude of green and yellow radiation pulses of an unstable resonator copper vapor laser (CVL) oscillator in the sum frequency (SF) mixing and second harmonic (SH). The conversion efficiency of sum frequency generation was lower compared to second harmonic processes despite larger fundamental power being used in sum frequency experiments. However the net UV power obtained at the sum frequency was higher than both of the second harmonic UV frequencies. Lower sum frequency generation (SFG) conversion efficiency compared to second harmonic generation (SHG) of individual CVL radiation is attributed to difference in single pulse stability of beam pointing, divergence and amplitude fluctuation of both CVL radiations in addition to commonly known fact of spatio-temporal mis-match. At the same fundamental input power, higher SH conversion efficiency of yellow compared to green is attributed to its better single pulse stability of beam pointing and divergence. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction The high repetition rate, narrow line-width, high average power UV radiations (255 nm, 271 nm, and 289 nm) generated from the second harmonic [1–3] or the sum frequency mixing [2,4] of copper vapor laser (CVL) visible radiations (510 nm and 578 nm) are the potential sources for high speed manufacturing of photonics components such as Fiber Bragg Grating (FBG) [5], precision micro-machining, cutting and etching of semiconduc- tors [6] and for optical pumping of various cerium-doped laser [7] to obtain tunable UV radiations (280 nm–315 nm). In reported literature, the CVL to UV frequency conversion efficiencies were compared for all the three UV wavelengths [1–4]. The relative sum and second harmonic efficiencies depended on the green and yellow power ratio, their temporal mismatch, details of focusing geometry and the choice of non-linear crystal. In particular, in a spherical focusing geometry with beta barium borate (BBO) crystal, the UV conversion efficiencies were 9.6%, 5.5% and 6.4% for the second harmonic generation (SHG) of green, sum fre- quency generation (SFG) of green and yellow and SHG of yellow at the average power of 4.8 W, 8.4 W and 3.6 W respectively [8]. The UV output power level scaled to more than 1.0 W from a single CVL oscillator using the cylindrical focusing geometry in BBO, as it is a good candidate for high power operation in view of its high damage threshold and large acceptance angle along the azimuthal direction [2]. The conversion efficiency of 34%, 19% and 17% were obtained at the SH of green, SF of green and yellow and SH of amplified yellow [2]. Recently in the BBO crystal, the conversion efficiencies of 28%, 23%, and 17.5% at 255 nm, 271 nm and 289 nm respectively, are reported from a single CVL system using tunable acoustic optical filter [9]. In this set-up, the different CVL radia- tions and amplitudes are controlled selectively using electroni- cally tunable acousto-optical filter. From the reported literature, it is seen that the SFG conversion efficiency was lower as compared to that of second harmonic (SH) of green CVL radiation [10]. It is also observed that from a single CVL oscillator, the conversion efficiency of SH of yellow is lower than the SH of green [2,9]. However, the average power of green and yellow were not the same and the conversion efficiency for both the radiations is not compared at the same input power. Also a little attention is paid to understand the reason of the low conversion efficiency in SFG as compared to SHG of CVL radiations except some of the commonly known facts of spatio-temporal mismatch between the green and yellow radiations and green/yellow average power ratio. It is well known that the non-linear crystals have limited angular acceptance. Hence any pulse to pulse variation of CVL beam divergence pointing that exceeds the acceptance angle of crystal will affect the frequency conversion process by deterior- ating the phase matching conditions. In a study on beam pointing stability on CVL, it was demonstrated that the beam pointing Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology 0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.01.026 n Corresponding author. Tel.: þ91 7312442472; fax: þ91 07312442400. E-mail address: oprakash@rrcat.gov.in (O. Prakash). Optics & Laser Technology 50 (2013) 43–50