In this paper, the spontaneous Raman spectra and second harmonic generation (SHG) properties at 589 nm of a novel Raman crystal BaTeMo2O9 (BTM) were investigated. The BTM crystal was cut along the type-II SHG phase-matching direction for the first-order Raman shift at 1178 nm to realize the SRS and SHG simultaneously. Pumped by a nanosecond 1064 nm laser source, a self-frequency-doubled BTM Raman laser operating at 589 nm has been demonstrated for the first time. At the pump pulse energy of 48 mJ, the maximum yellow laser output pulse energy of 5.6 mJ was obtained with an optical-to-optical conversion efficiency of 11.7%. Our results show that BTM crystal is one of the promising candidate Raman materials to generate yellow laser radiation.
©2013 Optical Society of America
Yellow-orange lasers are of increasing interest for a number of applications, including laser medicine, guide-stars and coastal bathymetry. They are usually obtained by combining stimulated Raman scattering (SRS) and second harmonic generation (SHG) processes [1–5]. A wide variety of Raman and self-Raman crystals can be used to generate Raman lasers at 1100~1200 nm, including Ba(NO3)2, BaWO4, SrWO4, KGd(WO4)2, PbWO4, Nd:YVO4 and Nd:GdVO4 [3,6,7]. While suitable doubling crystals include KTiOPO4 (KTP) and LiB3O5 (LBO). In this way, the SRS and frequency-doubling processes are usually completed in different crystals. Therefore, the two optical processes can be guaranteed by SRS and SHG crystals, and high efficient yellow laser operations can be ensured. However, some nonlinear optical crystals possess excellent properties of both SRS and SHG, and they provide a new method for realizing compact and highly efficient self-frequency-doubled Raman laser outputs. Among this kind of crystals, 20-mm-long KTP and 30-mm-long KTiOAsO4 (KTA) have been realized self-frequency-doubled Raman laser output by Chen and Liu, respectively [8,9]. The optical-to-optical conversion efficiencies are 2.5% and 7.5% for KTP and KTA.
Crystals with better SRS and SHG properties than that of KTP and KTA are respected to achieve higher conversion efficiencies for self-frequency-doubled Raman laser. Quaternary molybdates BaTeMo2O9 (BTM), a novel nonlinear crystal, was synthesized and single crystals with large size of 34 × 37 × 45 mm3 were grown in our group [10,11]. It crystallized in monoclinic system with space group P21, and the crystal structure analysis showed that Te-O4 terahedron and Mo-O6 octahedrion were formed in the crystal . Our previous studies demonstrated that BTM exhibits wide transmission band (0.4-5 μm), large nonlinear coefficients, flexible phase-matching, high optical damage tolerance, chemical, and mechanical stabilities [13,14]. The strong Raman shift at 921 cm−1 has been used to realize a high efficiency Raman laser at 1178 nm . Its excellent nonlinear optical and SRS properties make the crystal be a candidate for high efficiency self-frequency-doubled Raman lasers in yellow region.
In this paper, the spontaneous Raman spectra in the quasi noncritical phase-matching (QNCPM) direction and second order nonlinear optical properties at 1178 nm (first-order Stokes wavelength) have been investigated. A 10-mm-long BTM crystal cut along QNCPM direction has been used to realize a self-frequency-doubled Raman laser at 589 nm successfully. The maximum output energy was 5.6 mJ with a pulse width of 3.5 ns. And the conversion efficiency from the pump energy to the yellow laser output energy is 11.7%.
2. Spontaneous Raman spectra and SHG properties
Large Raman gains determines high efficient 1178 nm laser generation, and good SHG properties can provide high efficient 589 nm laser output. For BTM self-frequency-doubled Raman laser, high efficient SRS and SHG processes should be realized in a BTM crystal simultaneously. Therefore, both the SHG properties of BTM at 1178 nm and spontaneous Raman spectra under the SHG configuration should be studied in detail. As our previous studies shown, the first-order Stokes wavelength is 1178 nm with the pump wavelength of 1064 nm propagating along Z-axis . According to the Sellmeir equations of BTM crystal, we calculated the phase-matching curves in XY-plane for SHG at 1178 nm. As shown in Fig. 1, BTM can support QNCPM in the direction of (θ, φ) = (90°, 52.7°) for type-II phase-matching condition (o + e→o). In this direction, the SHG properties were calculated as follows.
2.1 Effective nonlinear coefficients
For biaxial crystals, the effective nonlinear coefficients (deff) cannot be easily given in simple equation expressions as that of uniaxial crystals. The expressions of deff for biaxial crystals can be calculated as follows 11]. The result calculated according to the above expressions is 8.44 pm/V.
2.2 Acceptance angle / bandwidth and walk-off angle
The incidence light which deviates phase-matching angle would cause phase-mismatch (Δk = kp - ks - ki ≠ 0), where kp, ks and ki are propagation constants for the pump, signal and idler wavelengths, respectively). The efficient frequency conversion can generate when Δk ≤ π/l, where l is the length of the crystal. In order to illustrate the extended phase-matching, the phase-mismatch factor Δk around the phase-matching angle (θ0) can be expanded as follows:
When the incident light propagates along (θ, φ) = (90°, 52.7°), both the first and second terms in the expansion of Δk are zero. Hence, the acceptance angle Δθ is essentially determined by the second order derivative term, asEq. (2) and (3), respectively. Then the acceptance angle and the acceptance bandwidth are calculated of Δθ = 124 mrad· cm and Δλ = 0.65 nm.
The walk-off angles of biaxial crystals require numerical method. According to the reference , the walk-off angles of BTM at 1178 nm in the QNCPM direction can be calculated as αe = 100 mrad.
2.3 Spontaneous Raman spectra
To guarantee a highly efficient type -II phase-matching SHG output, the polarization of Raman scattered light parallel and perpendicular to Z-axis in the incidence cross section must be strong and comparative. A BTM crystal cut along QNCPM direction was used to determine the spontaneous Raman spectra by using an NXR FT-Raman spectrometer (Horiba JY HR800). As shown in Fig. 2, it can be seen that the strongest Raman shift are at 915.2 cm−1 for the two Raman configurations, and both the intensity values (14000, 11000 au) and line-widths (4.2, 4.4 cm−1) are nearly equal. Our experiments results show that the Raman shifts have little changes with the azimuthal φ around φ = 52.7 o in the XY plane. The spectral acceptance bandwidth of the phase-matching for BTM is wide enough to cover the first-order Raman Stokes wavelength.
From the above analysis, it can be seen that the BTM crystal cut along (θ, φ) = (90°, 52.7°) exhibits excellent SRS and SHG properties simultaneously, and the self-frequency-doubled Raman laser is expected. Therefore, a sample oriented along QNCPM direction with dimensions of 4 × 4 × 10 mm3 was fabricated, polished, and coated with anti-reflective (AR) films at 589, 1064, and 1178 nm.
3. Experimental setup
The experimental setup of the self-frequency-doubled BTM Raman laser is shown in Fig. 3. A simple external resonator setup was employed. The pump source was a lamp-pumped electric-optics Q-switched Nd:YAG laser amplifier system operating at 1064 nm with a pulse width of 10 ns, repetition rate of 1Hz and TEM00 mode. And the output laser was polarized in horizontal direction. The divergence angle of the laser beam was less than 3 mrad and the spot diameter was about 2 mm. The Raman cavity consisted of two plane mirrors M1 and M2 with separation distance of 12 mm. The mirror M1 was AR coated at 1064 nm and high reflectivity (HR, R>99.8%) coated at 579-590 nm and 1140-1185 nm. Output mirror M2 was HR coated at 1064 & 1140-1185 nm (to doubly pass the unconverted fundamental wave), and AR coated at 570-590 nm. The output pulse energy was measured by an energy meter, and the pulse characteristics were recorded by a digital oscilloscope (Tektronix, DPO7104) and a photodetector (New Focus, 1611). An optical spectrum analyzer with resolution of 0.5 ns (AvaSpec-3648-NIR-256-2.2) was used to measure the spectral information.
4. The results and discussions
The output energies of the self-frequency-doubled Raman laser are shown in Fig. 4. The threshold pump intensity was observed as about 110 MW/cm2 at 1064 nm. The output pulse energies increased with the increase of the pump pulse energies. The maximum output pulse energy from the 589 nm laser was 5.6 mJ at the pump pulse energy of 48 mJ, with an optical-to-optical conversion efficiency of 11.7% and a slope efficiency of 15%, respectively.
With a spectrum analyzer, a 589 nm spectral line was recorded as shown in Fig. 5. The output pulse widths decreased with the increase of the pump pulse energies, and the achieved shortest pulse width was 3.5 ns. A typical oscilloscope trace of the 589 nm pulse is shown in Fig. 6.
To understand the self-frequency-doubled Raman laser properties, a similar work based on KTA is cited here as a comparison . In their experiment, with a 30 mm long x-cut KTA crystal used, an optical-to-optical conversion efficiency of 7.5% was obtained, while 11.7% optical-to-optical conversion efficiency was realized only by using a 10 mm BTM crystal in our experiment. It is well known that the optical-to-optical conversion efficiency is determined by both SRS and SHG processes. For BTM crystal, the strong covalent bondings for molybdate molecular groups make the crystal exhibit strong spontaneous Raman shifts at 915.2 cm−1 for the two Raman configurations and a low threshold as well as a high efficiency SRS process . Moreover, the BTM can support QNCPM at the first-order Raman Stokes wavelength. In addition, the effective nonlinear coefficient of BTM (8.44 pm/V) is much larger than that of KTA (−3.56 pm/V), which guarantees a high SHG efficiency. Consequently, all the better second-order nonlinear and SRS properties than those of KTA make the BTM higher optical-to-optical conversion efficiency, although the length of BTM used in the experiment was much shorter than that of KTA. As the transmittance spectra shown, the transmittances of the crystal in our experiment at 1064, 1178 and 589 nm were measured to be 98.7%, 98.5% and 75%, respectively. Our studies on the crystal quality showed that the ultraviolet absorption edge was at 400 nm, and the transmittance without any AR films was about 70% at 589 nm with a crystal sample of 2 mm long. The scattering particles in the microscope revealed that the transmittance at 589 nm could become higher by improving the crystal optical quality. So it is expected that the optical-to-optical conversion efficiency could be improved by using a higher quality crystal, which is under the way in our lab.
In conclusion, a self-frequency-doubling BTM Raman laser in an external resonator was demonstrated. An 1178 nm Raman laser was generated for 915.2 cm−1 Raman shift from a 10 mm (90°, 52.7°)-cut crystal. The self-frequency doubling of the Raman laser was accomplished in the same crystal, and a 589 nm yellow laser was obtained. The maximum output pulse energy is 5.6 mJ for the yellow laser with an optical-to-optical conversion efficiency of 11.7%. Higher optical-to-optical conversion efficiencies are expected by using longer and higher optical quality crystals in the near future.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51021062, 50990061 and 50802054), the 973 program of the People’s Republic of China (Grant No. 2010CB630702), China Postdoctoral Science Foundation (Grant No. 20110491600), Graduate Independent Innovation Foundation of Shandong University (Grant No. yzc12115) and the Program of Introducing Talents of Disciplines to Universities in China (111 program no. b06017).
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