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Blue laser light generation by intracavity frequency doubling of Cesium vapor laser
Optics Communications 282 (2009) 4585–4586
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Blue laser light generation by intracavity frequency doubling of Cesium vapor laser B.V. Zhdanov *, M.K. Shaffer, W. Holmes, R.J. Knize US Air Force Academy, Department of Physics, 2354 Fairchild Dr., Ste. 2A31, USAF Academy, CO 80840, USA
a r t i c l e
i n f o
Article history: Received 25 June 2009 Received in revised form 30 July 2009 Accepted 20 August 2009
a b s t r a c t Blue laser light with a wavelength 447 nm was generated by intracavity frequency doubling of a Cs laser in PPKTP crystal. A continuous wave power of 600 mW was obtained at an optical-to-optical efficiency of 4%. In the pulsed operation, when thermal effects were reduced, the efficiency obtained was 14% and the peak blue power was 2.1 W. Ó 2009 Elsevier B.V. All rights reserved.
PACS: 42.55.Lt 42.70.Hj Keywords: Alkali lasers Second harmonic generation
1. Introduction High power lasers operating in the blue spectral range are desirable for many important applications such as data recording, laser displays, underwater communication, etc. One possible way to generate blue laser radiation is to frequency double the output of high power near infrared lasers. Recently developed diode pumped alkali vapor lasers [1–3] are good candidates for frequency doubling because of their ability to generate high power beams with excellent beam quality and very narrow emission linewidth, which are important features to realize efficient frequency conversion. Previous experiments on second harmonic generation (SHG) of Rb laser [4] in BiBO crystal using an intracavity design demonstrated generation of 397 nm light with 250 mW power and an efficiency of about 1.5% with a fundamental power about 20 W. In our previous experiments on direct frequency doubling of Cs laser [5], we used a periodically poled KTP (PPKTP) crystal that generated 447 nm light with an efficiency of 10% using 3 W of fundamental power. In this paper, we present the results of our experiments on intracavity frequency doubling of Cs laser operating at 894 nm in continuous wave (CW) and pulsed mode.
2. Experiment and discussion The diagram of the experimental setup is presented in Fig. 1. The stable Cs laser cavity was created by two mirrors: a highly * Corresponding author. Tel.: +1 719 3332109; fax: +1 719 3337098. E-mail address: [email protected] (B.V. Zhdanov). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.08.032
reflective at 894 nm and 852 nm concave mirror with a radius of curvature 50 cm and a flat mirror with high reflection at 894 nm and 447 nm. The cavity length was about 45 cm long. The 2 cm long Cs vapor cell with 600 torr ethane buffer gas heated to 95 °C was positioned inside the laser cavity close to the concave mirror. At this position the cavity mode had maximum size (radius 650 lm at 1/e2 level) that allowed the pump beam size to match the cavity mode. The Cs vapor was longitudinally pumped by a narrowband laser diode array (LDA) operating at 852 nm with a linewidth less than 12 GHz that matches the pressure broadened absorption line of Cs vapor (12 GHz). The pump beam was introduced into the Cs cell through the polarizing beam splitting cube (PBS) that provided separation of the pump and lasing beams having orthogonal polarizations. The 3 cm long PPKTP crystal (Raicol Crystals Ltd., Israel) was positioned close to the flat cavity mirror, where the cavity mode had minimal size (radius 210 lm at 1/e2 level) and, thus, the intensity of the lasing beam was the highest. The temperature of the PPKTP crystal was kept with a precision of 0.02 °C at its phase matching value of 37.95 °C, which was determined experimentally (see Fig. 2). A dichroic mirror was positioned inside the laser cavity between the PPKTP crystal and PBS and set at an angle of 45° to the cavity axis with 99.8% reflectivity for the second harmonic (447 nm) and 96.3% transmission for Cs laser radiation (894 nm). This mirror was used as an output coupler for the second harmonic radiation. At the same time, the small portion of the lasing radiation reflected by the dichroic mirror was used to measure the lasing power inside the laser cavity. We studied the second harmonic conversion efficiency for two modes of Cs laser operation: CW and pulsed with 10 ls pulses at 1 kHz repetition rate. In the pulsed mode we could significantly
4586
B.V. Zhdanov et al. / Optics Communications 282 (2009) 4585–4586
Diode Laser Pump 852 nm
HR Mirror for 894 nm and 447 nm
HR Mirror for 894 nm And 852 nm
Dichroic Mirror
PPKTP
Cs Cell
PBS cube SH Output 447 nm Fig. 1. Experimental setup for intracavity SHG of Cs laser in PPKTP.
Fig. 2. Temperature tuning curve for SHG in PPKTP showing optimal temperature of 37.95 °C and FWHM 0.55 °C.
sion efficiency for the CW pump is approximately the same as for the pulsed pump if the pump power is below 3 W. For the higher pump power, the SH power grows quadratically for the pulsed case (as predicted by theory; see, for example, [6]), while for the CW pump the SH power deviates from the quadratic law and grows approximately linearly. The resulting optical-to-optical efficiency for the pulsed excitation was about 14% and for CW pump was about 4% at the fundamental power of 15 W. The maximum blue light CW power demonstrated was 0.6 W and the pulsed peak power was 2.1 W. The reduction of the SHG efficiency for the CW pump is due to the thermal effects, creating a lens in the crystal and shifting its temperature from the phase matching value. Selecting other nonlinear crystals that have lower SH absorption should reduce the SH power degradation due to the thermal effects and increase SHG efficiency. 3. Conclusion We have demonstrated efficient blue laser light generation by intracavity second harmonic generation of diode pumped Cs vapor laser in PPKTP crystal. Thermal effects reducing the efficiency of second harmonic generation were observed for the fundamental power higher than 3 W. The contribution of these thermal effects can be reduced by using another nonlinear crystal with lower absorption. Acknowledgements This work was supported by the High Energy Laser Joint Technology Office (JTO), Air Force Office of Scientific Research (AFOSR), Air Force Research Laboratories (AFRL), and National Science Foundation (NSF).
Fig. 3. Second harmonic power dependence on fundamental power for CW and pulsed fundamental radiation.
reduce the contribution of thermal effects observed in our previous experiments [2], which reduced the SHG efficiency. The dependence of the second harmonic power on the fundamental power for these two cases is presented in Fig. 3 (the peak power values provided there for the pulsed mode). As shown, the SHG conver-
References [1] R.J. Beach, W.F. Krupke, V.K. Kanz, S.A. Payne, J. Opt. Soc. Am. B 21 (12) (2004) 2151. [2] B. Zhdanov, R.J. Knize, Opt. Lett. 32 (15) (2007) 2167. [3] B.V. Zhdanov, R.J. Knize, Proc. SPIE 6874 (2008) 68740F-1. [4] A. Petersen, R. Lane, Proc. SPIE 7005 (2008) 700529-1. [5] B.V. Zhdanov, Yalin Lu, M.K. Shaffer, W. Miller, D. Wright, R.J. Knize, Opt. Exp. 16 (22) (2008) 17585. [6] RW. Boyd, Nonlinear Optics, second ed., Academic Press, 2003.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Blue laser light generation by intracavity frequency doubling of Cesium vapor laser B.V. Zhdanov *, M.K. Shaffer, W. Holmes, R.J. Knize US Air Force Academy, Department of Physics, 2354 Fairchild Dr., Ste. 2A31, USAF Academy, CO 80840, USA
a r t i c l e
i n f o
Article history: Received 25 June 2009 Received in revised form 30 July 2009 Accepted 20 August 2009
a b s t r a c t Blue laser light with a wavelength 447 nm was generated by intracavity frequency doubling of a Cs laser in PPKTP crystal. A continuous wave power of 600 mW was obtained at an optical-to-optical efficiency of 4%. In the pulsed operation, when thermal effects were reduced, the efficiency obtained was 14% and the peak blue power was 2.1 W. Ó 2009 Elsevier B.V. All rights reserved.
PACS: 42.55.Lt 42.70.Hj Keywords: Alkali lasers Second harmonic generation
1. Introduction High power lasers operating in the blue spectral range are desirable for many important applications such as data recording, laser displays, underwater communication, etc. One possible way to generate blue laser radiation is to frequency double the output of high power near infrared lasers. Recently developed diode pumped alkali vapor lasers [1–3] are good candidates for frequency doubling because of their ability to generate high power beams with excellent beam quality and very narrow emission linewidth, which are important features to realize efficient frequency conversion. Previous experiments on second harmonic generation (SHG) of Rb laser [4] in BiBO crystal using an intracavity design demonstrated generation of 397 nm light with 250 mW power and an efficiency of about 1.5% with a fundamental power about 20 W. In our previous experiments on direct frequency doubling of Cs laser [5], we used a periodically poled KTP (PPKTP) crystal that generated 447 nm light with an efficiency of 10% using 3 W of fundamental power. In this paper, we present the results of our experiments on intracavity frequency doubling of Cs laser operating at 894 nm in continuous wave (CW) and pulsed mode.
2. Experiment and discussion The diagram of the experimental setup is presented in Fig. 1. The stable Cs laser cavity was created by two mirrors: a highly * Corresponding author. Tel.: +1 719 3332109; fax: +1 719 3337098. E-mail address: [email protected] (B.V. Zhdanov). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.08.032
reflective at 894 nm and 852 nm concave mirror with a radius of curvature 50 cm and a flat mirror with high reflection at 894 nm and 447 nm. The cavity length was about 45 cm long. The 2 cm long Cs vapor cell with 600 torr ethane buffer gas heated to 95 °C was positioned inside the laser cavity close to the concave mirror. At this position the cavity mode had maximum size (radius 650 lm at 1/e2 level) that allowed the pump beam size to match the cavity mode. The Cs vapor was longitudinally pumped by a narrowband laser diode array (LDA) operating at 852 nm with a linewidth less than 12 GHz that matches the pressure broadened absorption line of Cs vapor (12 GHz). The pump beam was introduced into the Cs cell through the polarizing beam splitting cube (PBS) that provided separation of the pump and lasing beams having orthogonal polarizations. The 3 cm long PPKTP crystal (Raicol Crystals Ltd., Israel) was positioned close to the flat cavity mirror, where the cavity mode had minimal size (radius 210 lm at 1/e2 level) and, thus, the intensity of the lasing beam was the highest. The temperature of the PPKTP crystal was kept with a precision of 0.02 °C at its phase matching value of 37.95 °C, which was determined experimentally (see Fig. 2). A dichroic mirror was positioned inside the laser cavity between the PPKTP crystal and PBS and set at an angle of 45° to the cavity axis with 99.8% reflectivity for the second harmonic (447 nm) and 96.3% transmission for Cs laser radiation (894 nm). This mirror was used as an output coupler for the second harmonic radiation. At the same time, the small portion of the lasing radiation reflected by the dichroic mirror was used to measure the lasing power inside the laser cavity. We studied the second harmonic conversion efficiency for two modes of Cs laser operation: CW and pulsed with 10 ls pulses at 1 kHz repetition rate. In the pulsed mode we could significantly
4586
B.V. Zhdanov et al. / Optics Communications 282 (2009) 4585–4586
Diode Laser Pump 852 nm
HR Mirror for 894 nm and 447 nm
HR Mirror for 894 nm And 852 nm
Dichroic Mirror
PPKTP
Cs Cell
PBS cube SH Output 447 nm Fig. 1. Experimental setup for intracavity SHG of Cs laser in PPKTP.
Fig. 2. Temperature tuning curve for SHG in PPKTP showing optimal temperature of 37.95 °C and FWHM 0.55 °C.
sion efficiency for the CW pump is approximately the same as for the pulsed pump if the pump power is below 3 W. For the higher pump power, the SH power grows quadratically for the pulsed case (as predicted by theory; see, for example, [6]), while for the CW pump the SH power deviates from the quadratic law and grows approximately linearly. The resulting optical-to-optical efficiency for the pulsed excitation was about 14% and for CW pump was about 4% at the fundamental power of 15 W. The maximum blue light CW power demonstrated was 0.6 W and the pulsed peak power was 2.1 W. The reduction of the SHG efficiency for the CW pump is due to the thermal effects, creating a lens in the crystal and shifting its temperature from the phase matching value. Selecting other nonlinear crystals that have lower SH absorption should reduce the SH power degradation due to the thermal effects and increase SHG efficiency. 3. Conclusion We have demonstrated efficient blue laser light generation by intracavity second harmonic generation of diode pumped Cs vapor laser in PPKTP crystal. Thermal effects reducing the efficiency of second harmonic generation were observed for the fundamental power higher than 3 W. The contribution of these thermal effects can be reduced by using another nonlinear crystal with lower absorption. Acknowledgements This work was supported by the High Energy Laser Joint Technology Office (JTO), Air Force Office of Scientific Research (AFOSR), Air Force Research Laboratories (AFRL), and National Science Foundation (NSF).
Fig. 3. Second harmonic power dependence on fundamental power for CW and pulsed fundamental radiation.
reduce the contribution of thermal effects observed in our previous experiments [2], which reduced the SHG efficiency. The dependence of the second harmonic power on the fundamental power for these two cases is presented in Fig. 3 (the peak power values provided there for the pulsed mode). As shown, the SHG conver-
References [1] R.J. Beach, W.F. Krupke, V.K. Kanz, S.A. Payne, J. Opt. Soc. Am. B 21 (12) (2004) 2151. [2] B. Zhdanov, R.J. Knize, Opt. Lett. 32 (15) (2007) 2167. [3] B.V. Zhdanov, R.J. Knize, Proc. SPIE 6874 (2008) 68740F-1. [4] A. Petersen, R. Lane, Proc. SPIE 7005 (2008) 700529-1. [5] B.V. Zhdanov, Yalin Lu, M.K. Shaffer, W. Miller, D. Wright, R.J. Knize, Opt. Exp. 16 (22) (2008) 17585. [6] RW. Boyd, Nonlinear Optics, second ed., Academic Press, 2003.