1. Introduction

Yellow-orange (570-620 nm) laser sources are in particular useful in applications such as bio-medicine, Lidar, astronomy, and military. Narrow-line operation of these laser sources is of further interest in spectroscopy and remote-sensing applications [1]. However, no efficient laser sources currently available in this spectral region nor applicable fundamental infrared lasers for performing efficient frequency doubling to the region. Recent development of frequency doubling of a Raman-shifted Nd³⁺ laser to the region has thus become one of the solutions [2,3]. Besides, sum frequency generation (SFG) of two emission lines from transitions ⁴ F_3/2 – ⁴ I_11/2 (λ₁~1.06 μm) and ⁴ F_3/2 - ⁴ F_13/2 (λ₂~1.3 μm) of Nd-lasers has long become an important approach of generating coherent radiation in the yellow-orange spectral region. While this SFG technique has been intensively studied for the generation of pulsed 589-nm light sources in Q-switched Nd:YAG laser systems [4,5], the first cw 589-nm generation was reported by Moosmüller in 1997 [1]. However, Moosmüller’s scheme and consequent developments [6,7] were all based on the SFG (or cavity-enhanced SFG) of two independent Nd³⁺ lasers oscillating at the two infrared frequencies. Chen firstly reported a cw dual-wavelength (λ₁ = 1064 nm, λ₂ = 1342 nm) Nd:YVO₄ laser system using a three-mirror cavity scheme [8] and demonstrated afterwards an extracavity SFG for 593-nm generation with such a scheme [9]. Yet such a compact and efficient 593-nm generation system operated at numerous longitudinal modes with a total spectral width of ~0.1 nm (mainly determined by the linewidths of the two infrared waves) [9]. It is thus essential to reduce the spectral linewidths of both the dual wavelengths operated in such a system so as to achieve a narrow-line or even single-longitudinal-mode (SLM) operation of the SFG light with it. Lin et al. has constructed a cw, SLM dual-wavelength Nd:YVO₄ laser by using a grazing-incident grating and coupled cavities [10]. Nevertheless, such a system uses a more complex geometry and generally suffers from high losses due to the usually low grating diffraction efficiency, which might prevent it from being a favorable solution for the further construction of an efficient narrow-line SFG system.

Photo-thermal reflective (PTR) glass volume Bragg grating (VBG) has caught a lot of attention on laser science and applications mainly because of its versatile and unique optical properties such as narrow diffraction spectrum, good angular selectivity, flexible diffraction efficiency design, good thermal stability, large transparent range, and high laser damage threshold (typically > 40 J/cm²) [11,12]. Using VBG as laser mirrors can drastically reduce laser output spectral width [13,14] and even can facilitate the lasers operating in single longitudinal mode [15,16].

Stable operation of a cw dual-wavelength Nd:YVO₄ laser in a three-mirror cavity scheme has been demonstrated and studied with respect to the gain competition between the two emission lines [8]. In addition, chaotic power fluctuation of a diode-pumped intracavity-doubled Nd-laser has been reported and attributed to the cross coupling of the fundamental longitudinal modes via the SFG process [17]. Recently, this output amplitude noise (so called “green noise”) has been successfully suppressed in an intracavity-doubled, SLM Nd-laser achieved using VBG [18]. In this paper, we report a simple laser scheme in generating a narrow-line even SLM 593-nm light via intracavity SFG (ISFG) of a cw dual-wavelength Nd:YVO₄ laser using two VBG reflectors in three-mirror cavity configuration. We believe this could be the first scheme demonstrated in producing highly spectral-narrowed ISFG orange light in a single Nd-laser system.

2. Laser design and construction

Figure 1 shows the schematic arrangement of the laser system constructed in this work. The pump source of the system is a fiber coupled diode laser at 809-nm wavelength. The gain medium, having a 3-mm × 3-mm laser aperture and 7-mm length, is an a-cut Nd:YVO₄ crystal with a 0.5-at. % Nd³⁺ doping concentration. The laser oscillated at both 1064-nm and 1342-nm wavelengths achieved in a three-mirror cavity scheme formed by a dielectric mirror M1 at the pumping end and two VBG reflectors at the output ends. M1 is a 150-mm radius-of-curvature meniscus (zero-power) dielectric mirror having >99.8% reflectance at both 1064 nm and 1342 nm and ~95% transmittance at 809 nm. The two VBGs, both having a clear aperture of ~5 mm × 5 mm and a grating thickness of ~4.5 mm, were designed for Bragg wavelengths at 1342 nm and 1064 nm, respectively. The VBGs were mounted on temperature controlled ovens. The diffraction efficiency of both VBG reflectors is higher than 99% at their respective central diffraction wavelengths ~1342.2 nm and ~1064.2 nm at room temperature. Thereinafter we will refer to the two VBG reflectors as 1342-nm VBG and 1064-nm VBG, respectively, for short. The reflection spectral bandwidths (FWHM) of the 1342-nm and 1064-nm VBGs are 0.34 and 0.3 nm, respectively. One of the 1342-nm VBG surfaces (S1) was polished to have a 10-deg angle with respect to the Bragg grating surface planes and applied with a coating which is highly reflective at 1064 nm and anti-reflective at both 1342 nm and 593 nm. This is to form a V-shape cavity for the 1064-nm laser. The S2 surface of the 1342-nm VBG was coated with an anti-reflection (AR) dielectric layer for both 1342 nm and 593 nm. We set up the three-mirror cavity system in a folded geometry rather than in a linear fashion as usually did in a system with dielectric cavity mirrors [8] to prevent the oscillating 1064-nm wave from traversing the 1342-nm VBG since a high diffraction efficiency VBG has inevitable holographic scattering [19] which can cause noticeable scattering loss for incident light different from the Bragg resonance wavelength. Scattering and absorption losses in PTR glass VBG can be technologically controlled on the level of 10⁻² and 10⁻³ cm⁻¹ in the near IR region, respectively [20], which is comparable to what we have characterized for the presently used VBGs (~0.02 cm⁻¹). The 1064-nm VBG reflector was placed 26 mm from the 1342-nm one. Both end surfaces of the 1064-nm VBG reflector were AR coated at 1064 nm and had a high transmittance (>93%) at 1342 nm. A BIBO crystal (Castech Inc.), having a 3 × 3 mm² cross section and 8 mm length, was placed 6 mm upstream from the 1342-nm VBG and 37 mm downstream from the Nd:YVO₄ crystal for performing the intracavity type-I (ooe) phase-matched SFG process of the 1064-nm and 1342-nm lasers.

 

Fig. 1 Schematic arrangement of the cw 593-nm intracavity SFG laser system in a diode-pumped, dual-wavelength Nd:YVO₄ laser using two VBG reflectors.

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3. Laser performance characterization and discussion

If both 1064-nm and 1342-nm outputs radiate approximately in single longitudinal mode, one can expect the 593-nm SFG output a SLM operation. Figures 2(a) and 2(b) show the measured output spectra of the 1064-nm and 1342-nm lasers (solid red lines) from the two-VBG laser system when pumped at 4.2-W diode power, respectively. The two VBGs were temperature stabilized at 25°C. These spectra were recorded by an optical spectrum analyzer (OSA; ADVANTEST-Q8384) operated at the high-resolution mode. The spectral linewidths of the two laser lines are 10 pm (~2.6 GHz) and 8 pm (~1.3 GHz), respectively. These linewidths have reached about the resolution limit of the OSA (10 pm). In this case, the spectral bandwidth of the SFG signal can be estimated as $\Delta {\nu }_s=\sqrt{{(\Delta {\nu }_1)}^2+{(\Delta {\nu }_2)}^2}\approx 3\text{\hspace{0.17em} GHz}$, rested on the convolution theorem for Gaussian spectral profiles, where Δν₁ and Δν₂ are the spectral bandwidths of the two incident lasers. The measured output spectra of the 1064-nm and 1342-nm lasers (solid grey lines) from the same laser system but with the VBG reflectors replaced by typical dielectric-coated high-reflectivity (HR) laser mirrors (>99.6% reflectance over 1020-1160 nm and 1280-1480 nm, respectively) are also plotted in Figs. 2(a) and 2(b) for comparisons. In this measurement, a 10-deg tilted dichroic mirror highly reflective to the 1064-nm wave and highly transmissive to the 1342 and 593-nm waves has been inserted in between the BIBO crystal and the 1342-nm HR laser mirror so as to form a V-shape cavity as arranged for the system using VBG mirrors (see Fig. 1). From Fig. 2 it clearly shows a remarkable reduction of lasers linewidths has been achieved with the system using VBGs. The linewidth reduction was more than 17 folds for both laser lines.

 

Fig. 2 Measured output spectra of the (a) 1064-nm and (b) 1342-nm lasers from the two-VBG laser system at 4.2-W diode pump power when the VBGs were worked at 25°C (solid red lines) or tuned to 35°C (dashed green lines). The spectra measured from the same laser system but with typical dielectric-coated laser mirrors are also plotted for comparisons (solid grey lines).

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A scanning Fabry-Perot interferometer (Thorlabs SA200-5A; working spectral range: 525-650 nm) having a spectral resolution of 7.5 MHz and finesse of >200 was used to analyze the spectral mode operation of the SFG output from the system using VBGs. Figure 3 shows the measured Fabry-Perot scanning trace of the 593-nm output from the intracavity SFG system shown in Fig. 1 when pumped at 3.6-W diode power. The grey line indicates the ramp voltage in driving the scanning Fabry-Perot and the red line is the corresponding Fabry-Perot signal. Two obvious signal spikes were emerged and spaced by ~1.5 GHz, corresponding to the free spectral range (FSR) of the scanning Fabry-Perot cavity. Without observing other signal in the free spectral range, the signal trace shows that the 593-nm output has been operated in SLM condition. We expanded one of the Fabry-Perot signals and estimated the linewidth (full width at half maximum) of the SLM 593-nm light to be as narrow as ~15 MHz, as shown in the inset of Fig. 3. From the inset, one accompanying peak aside the main signal is also observed. This is believed to arise from the transverse mode of an oscillating beam participating in the SFG process. The measured beam quality factor M² for this 593-nm output was >~1.3. The maximum output power we measured for the 593-nm SFG orange laser operated in the SLM condition was around 23 mW. At this power level, the output stability of this SLM ISFG system was found to be better than 2% over a 30-min observation time, while it has deteriorated to worse than 9% with the system using the dielectric-coated laser mirrors over the same observation time. Figure 4 plots the power fluctuations of the 593-nm outputs recorded from the ISFG system constructed with the VBG mirrors (red trace) and with the dielectric-coated mirrors (black trace). An outstanding reduction of the output power fluctuation with the VBG system is shown as expected as a result of its SLM operation that eliminates the aforementioned mode cross coupling problem. Both the VBG and dielectric-mirror ISFG systems have similar dual-wavelength laser thresholds of ~0.85-W diode power.

 

Fig. 3 Fabry-Perot scanning trace of the 593-nm output from the intracavity SFG system at 3.6-W diode pump power. The grey trace is the Fabry-Perot ramp voltage. The red trace is the Fabry-Perot signal, indicating the SLM operation of the 593-nm output. The inset shows an expanded Fabry-Perot signal from which the linewidth of the SLM 593-nm light is estimated (~15 MHz).

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Fig. 4 Recorded power fluctuations of the 593-nm lights output from the ISFG system constructed with the VBG mirrors (red trace) and with the dielectric-coated mirrors (black trace).

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According to the measured output spectra of the dual-wavelength lasers using dielectric-coated mirrors shown in Fig. 2 (solid grey lines), the laser output linewidth can be wider than 200 pm. This output linewidth is considerably smaller than the spectral acceptance bandwidth (Δλ) of the current SFG process using an 8-mm long, type-I BIBO crystal (Δλ~6.7 and 7.9 nm for the 1064 and 1342-nm waves, respectively, estimated from Ref [21].). In the present narrow-line laser system, it is possible to achieve the wavelength tuning over the whole laser emission linewidth simply by tuning the temperature of the VBGs utilized [22]. The dashed green curves shown in Figs. 2(a) and 2(b) were the measured output spectra of the 1064-nm and 1342-nm lasers from the present two-VBG laser system when the temperatures of the two VBG reflectors were both tuned to 35°C. Upper plot of Fig. 5 shows the measured two infrared laser wavelengths as a function of the temperature of the two VBGs. Temperature tuning rates of ~7 and ~11 pm/°C for the two lasers were achieved with the 1064-nm and 1342-nm VBGs in this system, respectively, corresponding to an effective tuning rate of ~5 pm/°C for the SFG orange light, as the measured data shown in the lower plot of Fig. 5.

 

Fig. 5 Measured two infrared laser wavelengths (upper plot) and SFG orange wavelength (lower plot) as a function of the temperature of the two VBGs. Temperature tuning rates of ~7, ~11, and ~5 pm/°C for the two infrared and the orange lasers were achieved, respectively.

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Figure 6 shows the measured 593-nm SFG output power as a function of the diode pump power (solid red triangles) for the system shown in Fig. 1. The dashed blue line is the quadratic fitting-curve of the 593 nm output, which clearly shows the signature of the nonlinear gain process of a SFG process. At the diode pump power higher than 3.6 W, the system starts to operate at more than one longitudinal modes. At the diode pump power of 4.2 W, we identified two longitudinal modes from the output interference fringe of the 593-nm wave with a FSR~21-GHz Fabry-Perot interferometer, from which a spectral linewidth of <8.5 GHz (10 pm) was estimated for the 593-nm output. Nevertheless, such a spectral linewidth has been still >10 times narrower than that observed from present system with the dielectric-coated mirrors replacing the VBG reflectors, as revealed in Fig. 2. Typically the reflection spectral width of a high reflectivity VBG can be as narrow as 0.2-0.4 nm depending on its design. However, such a VBG is generally characterized by a flat-top like reflection spectrum and thus provides little discrimination for laser modes around the highest reflectivity wavelength. Therefore, the emission of other laser longitudinal modes near around the central mode can be observed in this laser system when operated at a higher pump power. The output 593-nm power was >40 mW at this pump power level (4.2 W), corresponding to an optical to optical conversion efficiency of ~1%. An appreciable enhancement of conversion efficiency can be expected if the BIBO (effective nonlinear coefficient d_eff~2.2 pm/V) SFG crystal can be replaced by a periodically poled (PP) crystal like a PPKTP (d_eff ~11 pm/V) [7] or an MgO:PPLiNbO₃ (d_eff ~17 pm/V).

 

Fig. 6 Measured 593-nm SFG output power (solid red triangles) as a function of the diode pump power. The dashed blue line is the corresponding quadratic fitting curve.

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4. Conclusion

We have successfully demonstrated a highly spectral-narrowed cw 593-nm laser source based on the intracavity SFG of a diode-pumped, dual-wavelength Nd:YVO₄ laser using two VBG reflectors with central diffraction wavelengths at 1064 nm and 1342 nm, respectively. The intracavity SFG process was performed in a type-I BIBO crystal. More than 23-mW orange 593-nm generation radiating at single longitudinal mode of ~15 MHz spectral linewidth can be obtained from this compact laser system at diode pump power of 3.6 W. At a higher pump power of 4.2 W, the SFG output reached a power of >40 mW and radiated at two longitudinal modes (corresponding to a spectral linewidth of <10 pm), which is still >10 times narrower than that observed from SFG of a conventional dual-wavelength Nd³⁺ laser system. We achieved also in this system an effective tuning rate of ~5 pm/°C for the SFG output via the temperature tuning of the two VBG mirrors utilized.