IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 15, AUGUST 1, 2013
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Self-Frequency Conversion Laser in Nd-Doped Calcium Barium Niobate Ferroelectric Crystal Wen Lan Gao, Guo Qiang Xie, Jie Ma, Peng Yuan, Lie Jia Qian, Ju Qing Di, Xiao Dong Xu, Jun Xu, and M. A. Swirkowicz
Abstract— We demonstrate an efficient diode-pumped Nd:CBN-28 ferroelectric crystal laser for the first time. The maximum output power of 0.536 W is achieved with a slope efficiency as high as 27.7%, which is higher than that in the Nd:SBN ferroelectric crystal laser. The self-frequency doubling and self-sum-frequency processes are obtained simultaneously under intracavity conditions in the diode pumped Nd:CBN-28 laser. These processes show the ability of the Nd:CBN-28 crystal as a nonlinear optical multi-frequency converter without any angle or temperature tuning. Index Terms— Ferroelectric crystal, diode-pumped lasers, solid state laser, self-frequency doubling (SFD), self-sum-frequency (SSF).
I. I NTRODUCTION
F
ERROELECTRIC crystals have been widely used in nonlinear optical devices such as frequency doublers, optical parametric oscillators and light modulators [1], [2]. The typical ferroelectrics Srx Ba1−x Nb2 O6 (SBN) crystal with the tetragonal tungsten bronze (TTB) structure has been extensively studied due to its outstanding nonlinear properties [3]. Second harmonic generation (SHG) and other frequency mixing processes have been demonstrated by taking advantage of the suitable ferroelectric domains structures [1], [3]–[6]. SBN crystal doped with Nd3+ has also shown to be quite suitable for laser applications [7]–[9]. The Nd: SBN crystal can be used to generate visible laser radiation at different wavelengths by means of self-frequency doubling (SFD) and self-sumfrequency (SSF) processes without any temperature and angle tuning [8], [10]. But the phase transition temperature (Curie
Manuscript received December 10, 2012; revised May 11, 2013; accepted May 24, 2013. Date of publication June 3, 2013; date of current version July 9, 2013. This work was supported in part by the National Natural Science Foundation of China under Grants 61008018, 60890202, 60725418, and 11121504, and in part by the National Basic Research Program of China under Grant 2013CBA01505. W. L. Gao, G. Q. Xie, J. Ma, P. Yuan, and L. J. Qian are with the Department of Physics, Key Laboratory for Laser Plasmas, State Key Lab of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). J. Q. Di is with the Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China (e-mail:
[email protected]). X. D. Xu and J. Xu are with the Key Laboratory of Transparent and OptoFunctional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China (e-mail:
[email protected];
[email protected]). M. A. Swirkowicz is with the Institute of Electronic Materials Technology, Warsaw 01-919, Poland. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2265273
temperature, TC ) of SBN crystal (around 79 °C, x = 0.61) is low [11], which is a main drawback for laser applications. At high pump power, the crystal will undergo the transition to the paraelectric state, correspondingly, the frequency conversion process will disappear [12]. Therefore, compared with SBN, calcium barium niobate (Ca0.28 Ba0.72 Nb2 O6 , CBN-28) crystal should be more suitable for practical optical devices operating at higher pump powers because of its higher phase transition temperature (around 265 °C)[13]. The CBN-28 crystal consists of dense random-sized 180° ferroelectric domains. So far, conical SHG with large angular deflection in CBN-28 crystal has been demonstrated [14]. The localizations of Nd:CBN ferroelectric crystal has also been studied [15]. However, the laser and self-frequency conversion performances of Nddoped CBN-28 crystal have not been investigated yet. In this letter, the continuous wave laser performance of Nd: CBN28 crystal has been studied for the first time. The SFD and SSF processes have also been obtained simultaneously under intracavity conditions in the diode pumped Nd: CBN-28 laser. II. E XPERIMENTS The schematic of the continuous-wave laser setup is shown in Fig. 1. A 10 W single-emitter AlGaAs laser diode at 803 nm (Nlight Laser, NL-CN-10-808-F-3) was used as the pump source. The pump light was focused into the Nd:CBN-28 crystal by two coupling convex lenses with the focal length of 100 and 150 mm, respectively. The input mirror M1 was coated with high reflectivity (> 99.7%) for 1030–1090 nm and high transmission (> 95%) for 780–810 nm. The M2 mirror was a plano-plano output coupler. The laser output performances were compared under four different output couplings of 0.6%, 1.5%, 2%, and 5%. A polished and coated Nd: CBN-28 crystal was used as laser gain medium. The Nd: CBN-28 crystal was grown by Czochralski technique with Nd3+ concentration of 0.5 at.%. The crystal had dimensions of 3 mm × 3 mm in cross section and 6 mm in length. To remove the generated heat in the laser crystal, the crystal was wrapped with indium foil and tightly mounted in a water-cooled copper block. In the experiment, the cooling water temperature was sustained at 18 °C. The laser output power was measured with a thermosensitive power meter (PM320E, THORLABS). III. R ESULTS AND D ISCUSSION Figure 2 shows the output powers dependence on the absorbed pump powers with four different couplers. The output powers of the Nd: CBN-28 laser shows linear dependence on pump powers. The laser thresholds are 0.46, 0.53, 0.67 and
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Fig. 1.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 15, AUGUST 1, 2013
Experimental setup of the Nd: CBN-28 laser.
Fig. 3.
Fig. 2.
Infrared laser spectrum of Nd:CBN-28 laser.
Output power versus absorbed pump power.
0.91 W for coupler transmission of 0.6%, 1.5%, 2%, and 5%, respectively. From Fig. 2, we can see that more efficient operation was achieved by using the coupler of 5% in the experiment. The maximum output power and the corresponding slope efficiency are 0.536 W and 27.7%. In the case of 0.6%, 1.5% and 2% couplers, the maximum output powers were a bit lower and the corresponding slope efficiencies were 10.9%, 19.7% and 24.7%, respectively. The maximum output power and efficiency reported here is significantly higher than that of Nd: SBN laser [9], [10], [12], [16]. From Fig. 3, it can be clearly observed that two laser lines oscillate simultaneously at 1064 and 1081 nm. This multiline oscillation is attributed to the inhomogeneous spectrum broadening of the emission bands induced by the Nd3+ multicenter distribution in CBN crystals. When infrared laser oscillation occurs, SFD and SSF processes with diffuse green light are also observed by the naked eyes. The picture of the generated green light was obtained by means of a digital camera. Infrared radiations were removed by using the filters. Figure 4 shows the circular ring obtained by SFD and SSF process, while the conical green light emerging from the crystal was projected onto a screen. The polarization state of the conical converted radiation was found to be radial according to the configuration of our noncollinear scheme and the symmetry of the SHG tensor in CBN, in agreement with SBN works [9]. With a lens, the partial ring of the green light was focused into a spectrum analyzer outside the cavity. Fig. 5 shows the self-converted green light spectra of Nd: CBN-28 laser. There are three wavelength components in the green light, which are located at 532, 536, 540 nm, respectively. Two of them (532 and 540 nm) are generated by SFD of the two laser lines at
Fig. 4. The generated circular ring of second-harmonic generation in Nd:CBN-28 laser.
1064 and 1080 nm. The third one (536 nm) originates from SSF process of the 1064 and 1080 nm laser lines. Like SBN crystal, the cone-shaped SHG light emission is generated when an intense laser beam propagates parallel to the polar axis of the crystal [3]. The phase matching condition for SHG can be written as K2ω − 2Kω − G = 0.
(1)
For the momentum conservation, where G is a vector of the reciprocal lattice for the ferroelectric domain structure, Kω represents the wave vector of the fundamental beam, and K2ω is the wave vector of the SHG beam. The direction of the generated SH emission is dependent on the fundamental beam propagation direction with respect to the c axis. In our work, the fundamental beam propagates parallel to the c axis. The scheme of momentum conservation for the quasi-phase matching is shown in Fig. 4. In this case, a cone ring of harmonic lights around the fundamental beam is generated. The conical angle of the SH ring is defined by the refractive index of CBN-28 crystal [17] for fundamental and harmonic wavelengths as follows: n o (ω) kω + kω . (2) = k 2ω n e (θ, 2ω) The internal angles of 532 and 540 nm SHG components were calculated to be 16.7° and 16.4°, corresponding to the external angle of 42.6° and 41.7°, respectively. For SSF process, the internal angel is defined as follows: cos θ =
cos θ =
kω + kω . kSSF
(3)
GAO et al.: SELF-FREQUENCY CONVERSION LASER
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R EFERENCES
Fig. 5.
Self-converted green light spectrum of Nd: CBN-28 laser.
Where kω , kω , and kSSF are the wave vector of 1064 nm, 1081 nm laser in the cavity and self-sum frequency wave, respectively. The internal angle of SSF generation of 536 nm was calculated to be 16.6°, corresponding to the external angle of 42.1°. The wavelengths of SFD and SSF are 532, 540, and 536 nm, respectively, and the corresponding external angles are 42.6°, 41.7°, and 42.1° respectively. These results demonstrated the Nd: CBN-28 crystal can serve as a self-nonlinear prism. IV. C ONCLUSION We have demonstrated an efficient diode-pumped Nd: CBN-28 ferroelectric crystal laser for the first time. The maximum output power of 0.536 W was obtained with a slope efficiency as high as 27.7%. The maximum output power and highest slope efficiency reported here are significantly higher than that of Nd: SBN ferroelectric crystal lasers. The SFD and SSF processes can be obtained simultaneously under intracavity conditions in the diode pumped Nd: CBN-28 laser. Their radiation wavelengths are 532, 540, and 536 nm, respectively, and the corresponding external angles are 42.6°, 41.7°, and 42.1° respectively. These processes show the ability of Nd: CBN-28 crystal as a nonlinear optical multi-frequency converter without any angle or temperature tuning. The noncollinear conical SFD and SSF radiations also make this laser attractive as a light source for color displays, optical memories, and multi-wavelength scanners.
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