1. Introduction

Tunable continuous-wave (cw) ultraviolet (UV) sources are of importance for a variety of applications. Typically, the 330-345 nm wavelength range is of interest for spectroscopy, air pollution monitoring and biochemical analysis. For many years, access to the UV spectral region has been achieved using gas lasers [1] and diode lasers [2,3]. While gas lasers are very bulky and power- hungry, diode lasers suffer from poor beam quality. As such, the generation of tunable cw UV radiation below 345 nm, with high beam quality, spectral purity, and in compact solid-state design, still remains a challenge.

Nonlinear frequency conversion processes can offer a viable alternative to cover spectral regions that are inaccessible to conventional laser sources [4]. However, access to the UV spectral range in the cw regime is constrained by the low gain of the available nonlinear materials. The most established birefringent crystals for UV generation are lithium triborate, LiB₃O₅ (LBO) and beta-barium borate, β-BaB₂O₄ (BBO). By exploiting LBO and BBO crystals, with effective nonlinearity of d_eff~0.7 and 2.0 pm/V, respectively, cw UV output has been achieved by third-harmonic-generation of 1.064-μm solid-state lasers and optically-pumped semiconductor lasers [5], providing output at 355 nm, or by cascaded second-harmonic-generation (SHG) in external resonant enhancement cavities [6], generating radiation at 266 nm. Using CsLiB₆O₁₀ (CLBO), UV output at a fixed wavelength of 244 nm has also been achieved by fourth-harmonic generation of an optically-pumped semiconductor laser [7]. In addition, sum-frequency-generation (SFG) of UV radiation using 3rd-order quasi-phase-matched (QPM) interaction under temperature phase-matching has been demonstrated, providing UV output at 355 nm [8,9]. Based on a periodically-poled material, intracavity non-phase-matched SFG has been reported to generate tunable UV radiation in a pulsed optical parametric oscillator (OPO) [10]. The UV output power achieved using this technique is low, and in cw regime this approach results in further reduction in UV power. To exploit the highest nonlinear gain in QPM materials, 1st-order quasi-phase-matching is required [11–13]. For SFG into the UV, this demands a grating period of Λ~2 µm, which is still challenging to fabricate. Moreover, the material dispersion of such QPM crystals results in very narrow wavelength range of ~3-4 nm/grating period in the UV under temperature phase-matching, necessitating multiple grating periods to achieve significant UV tuning range, thus making the use of QPM crystals even more demanding. As such, it is imperative to consider alternative nonlinear materials, as well as new techniques, for the generation of coherent radiation with significant tuning coverage in the UV. In this context, the birefringent nonlinear crystal of bismuth triborate, BiB₃O₆ (BIBO) is a promising candidate, offering UV transparency down to 280 nm combined with a relatively high nonlinear coefficient (d_eff~3.8 pm/V) [14]. It can also be phase-matched for UV generation using SHG/SFG under type-I (ee→o) interaction at room temperature. In the cw regime, where low pumping intensities are available, an attractive approach to further enhance the nonlinear gain is to deploy intracavity frequency conversion schemes, thereby exploiting the high circulating power inside the resonator. In pulsed OPOs, under high pumping intensity, this approach has been demonstrated to generate different wavelengths of interest in the UV [15,16]

The cw OPO in singly-resonant oscillator (SRO) configuration is now recognized as a viable source of widely tunable coherent radiation, providing multi-watt cw output powers in the near- to mid-infrared, while affording significant signal output coupling loss [17,18]. In addition, the high circulating intensities in cw SROs based on QPM materials has enabled internal SHG/SFG over extended spectral regions in the visible and near-infrared (near-IR) at high output powers [4,19,20]. Hence, an attractive approach to the generation of tunable radiation in the UV could be intracavity SHG of the resonant signal, or SFG between the pump and signal, in cw SROs pumped in the green. Such cw SROs, based on the QPM material such as MgO:sPPLT, can provide a wide signal tuning from the visible to near-IR, and high circulating powers. Thus, internal SHG/SFG in green-pumped SROs can lead to the realization of compact and tunable cw sources in the UV and, moreover, allows simultaneous generation of high-power infrared idler wavelengths, providing increased flexibility for many applications. Here we report what we believe to be the first demonstration of a cw UV source tunable across 333-345 nm, using BIBO as the nonlinear gain medium for intracavity SFG of the resonant signal and pump in a cw OPO. The SRO, pumped at 532 nm, is based on a single-grating MgO:sPPLT crystal. The device provides up to 21.6 mW of output power at 339.7 nm, together with >1 W of idler power over 65% of the tuning range of 1158-1312 nm for 10 W of pump power.

2. Experimental setup

The schematic of the experimental setup is shown in Fig. 1. The pump source is frequency-doubled, diode-pumped cw Nd:YVO₄ laser delivering up to 10 W of output power at 532 nmin a single-frequency, linearly polarized beam with M²<1.1. To maintain stable output characteristics, the laser is operated at maximum power and a combination of a half-wave plate (HWP) and a polarizing beam-splitter is used for power attenuation. A second HWP is used to adjust the pump polarization for phase-matching in the nonlinear crystal. The nonlinear crystal for the OPO is a 30-mm-long, 2.14-mm-wide and 1-mm-thick, 1% bulk MgO:sPPLT. It is housed in an oven with a stability of ± 0.1 °C, enabling temperature tuning from 25 °C to 200 °C. The crystal contains a single grating period (Λ = 7.97 μm) and its end-faces are antireflection (AR)-coated (R <1%) over 800–1100 nm. The AR coating has a residual reflectivity of 1% to 15% for idler wavelengths over 1100–1400 nm. For intracavity SFG, we use a 5-mm-long, 8-mm-wide, 4-mm-thick BIBO crystal cut at θ = 136° (ϕ = 90°) for type-I (ee→o) interaction and AR-coated for 532 nm, 850-970 nm, and 325-350 nm. The OPO is configured in a ring cavity comprising two concave mirrors, M_1,2 (r = 50 mm) and two plane mirrors, M_3,4. Mirror M₁ is highly reflecting for the signal (R>99% over 840-1000 nm), and transmitting for the pump (T>95% at 532 nm) and idler (T>85% over 1100-1500 nm). Mirrors M_2-4 are highly reflecting for the signal (R>99% over 877-985 nm) and the pump (R>97% at 532 nm), while transmitting for the idler (T>80% over 1100-1400 nm) and the UV (T>80% over 310-390 nm). Thus, the OPO cavity ensures singly-resonant oscillation for the signal with a single-pass pump. A lens of focal length, f = 100 mm, is used to focus the pump beam at the center of the MgO:sPPLT crystal, to a waist radius of w_p1~31 μm, corresponding to a confocal focusing parameter of ξ_p1 = l/b_p1~1.2, where l is the length of the nonlinear crystal and b_p1 = kw_p1² is the confocal parameter of the pump beam. The design of the SRO cavity results in a primary signal waist radius of w_s1~46 μm at the center of the MgO:sPPLT crystal, leading to b_p1~b_s1, where b_s1 = kw_s1² is the confocal parameter of the signal beam, between mirrors M₁ and M₂, and secondary pump and signal waist radii of w_p2~95 μm (ξ_p2~0.025) and w_s2~125 μm (ξ_s2~0.026), respectively, at the center of the BIBO crystal, between mirrors M₃ and M₄. To filter out any residual signal and green power, while measuring the UV output, we use a Schott glass (FGUV11) with high transmission (T = 81%) over 332-344 nm.

 

Fig. 1 Schematic of the tunable, all-solid-state, cw UV source. λ/2, half-wave plate; PBS, polarizing beam-splitter; L, lens; M_1-4, mirrors; F, Filter.

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3. Results and discussion

We first studied the angular acceptance bandwidth of the BIBO crystal for SFG by measuring the variation of the UV output at 339.7 nm as a function of the crystal angle, θ, at a signal wavelength of λ_signal = 939.8 nm, while keeping MgO:sPPLT temperature fixed at T_QPM = 116 °C, and at the maximum input power of 10 W. The angular acceptance profile obtained is shown in Fig. 2(a). The sinc² fit to the experimental data has a full-width at half-maximum (FWHM) bandwidth of Δθ = 0.02°. Figure 2(b) shows the theoretical angular acceptance curve calculated using the relevant Sellmeier equations for a 5-mm-long BIBO [21], where a FWHM bandwidth of Δθ = 0.025°, at a phase-matching angle of θ = 141.54°, corresponding to λ_UV = 339.7 nm, is obtained, in good agreement with the measured value.

 

Fig. 2 (a) Variation of the cw UV power as a function of angle, θ, of the BIBO at λ_UV = 339.7 nm. The solid line is the sinc² fit to the experimental data. (b) Theoretical angular acceptance bandwidth of 5-mm-long BIBO crystal.

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In order to characterize the spectral coverage of the device, we varied the temperature of the MgO:sPPLT crystal, T_QPM, while simultaneously changing the BIBO phase-matching angle. By varying T_QPM from 76 °C to 176 °C, and θ over 139.1°-143.2°, UV spectral tuning across 333-345 nm was achieved for a resonant signal wavelength range of 895-984 nm, with a corresponding idler tuning of 154 nm over 1158-1312 nm. Figure 3(a) shows the measured UV power across the tuning range for a fixed pump power of 10 W. The generated UV was continuously tunable across the full 333-345 nm range, providing >15 mW over 64% of the tuning range, with a maximum of 21.6 mW at 339.7 nm. The simultaneously measured poweracross the idler tuning range is shown in Fig. 3(b). The device can deliver >1 W of idler power over 65% of the tuning range, with a maximum of 1.7 W at 1247 nm. The drop in idler power beyond ~1290 nm is due to the crystal coating losses and parametric gain reduction away from degeneracy. The short-wavelength idler cut-off at 1158 nm is due to the reflectivity fall-off of mirrors M_2-4 above the signal wavelength of 985 nm. We also measured the signal power leaked-out through mirror M₃, and obtained up to 257 mW at 908 nm, with >65 mW over the full signal tuning range, as shown in Fig. 3(c). The drop in signal output power across 915-944 nm is due to the high reflectivity (R>99.8%) of M₃ over this wavelength range.

 

Fig. 3 Variation of (a) UV, (b) idler, and (c) signal power across the respective tuning ranges, at the maximum pump power of 10 W at 532 nm.

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Keeping T_QPM = 126 °C, and generating UV in the middle of the tuning range at λ_UV = 338.2 nm, we studied the simultaneous variation of UV power and the idler output power at λ_idler = 1247 nm up to the maximum input pump power. As can be seen in Fig. 4, we obtained up to 18.6 mW of UV, together with 1.7 W of idler, at the pump power of 10 W. The variation of the corresponding leaked-out signal power as a function of pump power is shown in inset (a) of Fig. 4. Using the measured output signal power and transmission of mirror M₃ at 928 nm, we calculated the intracavity signal power at maximum pump power of 10 W to be 64.5 W. As the confocal focusing parameters for the pump and signal in BIBO are <<1, by considering plane-wave undepleted pump approximation and an effective length of 3.2 mm due to the spatial walk-off for SFG in BIBO, we calculated the expected UV power as a function of intracavity signal power and the undepleted green pump power [22]. The inset (b) of Fig. 4 shows the theoretical UV power scaling curve, where good agreement with the experimental data is confirmed. At λ_UV = 339.7 nm, with slight misalignment of the cavity, we have also observed UV power up to ~28 mW, together with a simultaneous decrease in the OPO pump depletion, indicating an increase in the ratio of intracavity undepleted green to signal power. However, as a consequence of this misalignment, the UV power was not stable. To understand this increase in UV power, further studies are necessary.

 

Fig. 4 Variation of UV power and idler output power with input pump power. The solid lines are the quadratic and linear fit to the UV and idler experimental data, respectively. Inset: (a) Signal power scaling. The solid line is the linear fit to the experimental data. (b) Theoretical UV power scaling.

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As the two input beams to the BIBO crystal, the signal and the green, are extraordinary, they experience a spatial walk-off angle of 67.79-70.95 mrad and 71.03-74.05 mrad, respectively, over the full tuning range. This also results in a small relative walk-off angle of Δρ~3.10-3.24 mrad. However, as evident from Fig. 5, the far-field energy distribution of the UV output at 338.2 nm, together with the orthogonal intensity profiles, at maximum power show good circularity of >75%, which could be due to the long Rayleigh range (>50 mm) for both input beams, resulting in a reduction in the ellipticity of the generated UV beam. For different pumping levels, and across the tuning range, we have observed similar behavior.

 

Fig. 5 Far-field energy distribution of generated UV at 338.2 nm for pump power of 10 W.

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We also recorded the passive power stability of the generated UV at 338.2 nm, and the corresponding signal at 928 nm, at the maximum pump power of 10 W, under free-running conditions. The results are shown in Figs. 6(a) and 6(b), respectively, where the UV power is recorded to exhibit passive rms stability better than 3.4%, while the signal power rms stability is recorded to be better than 2.6% over more than 1 minute. The instability in power is attributed to the mechanical vibrations and air currents in the laboratory and the possible mode-hopping in the OPO in the absence of active stabilization. However, with mechanical and thermal isolation of the system and electronic locking, we expect substantial improvements in the power stability of the device. We also investigated stronger focusing with smaller beam waist radii (w_p2~31 μm, w_s2~42 μm) at the center of the BIBO crystal, by designing the OPO cavity using all-concave mirrors (r = 100 mm). However, this resulted in lower UV output power and reduced stability. The low output power could be due to the reduced effective interaction length of green and signal waves in the BIBO crystal. With the all-concave mirrors, the effective lengths for the pump and signal are reduced by ~3 times to ~1 mm compared to when plane mirrors are used for M₃ and M₄ (3.2 mm). The reduced power stability in the all-concave cavity configuration as compared to that when plane mirrors (M₃, M₄) are used is attributed to the increased sensitivity of mode-matching to perturbations such as mechanical vibrations and air currents in the laboratory.

 

Fig. 6 Passive power stability of (a) UV and (b) signal output from the SRO, over >1 min.

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We further studied the spectral characteristics of the UV source. Figure 7(a) shows the spectrum of the generated UV output, measured simultaneously with green pump and the signal radiation, at T_QPM = 145 °C, using a spectrometer with a resolution of 0.27 nm (OceanOptics, HR4000), showing central wavelength of 336 nm, 532 nm and 912 nm, respectively. We also measured the single-frequency spectrum of the signal (λ_signal = 928 nm) and the undepleted pump using a confocal Fabry-Perot interferometer (FSR = 1 GHz, finesse = 400), and found an instantaneous linewidth of 12.9 MHz and 6.5 MHz, respectively. Similar behavior was observed across the full tuning range. As such, it is expected that the UV output is also single-frequency. We were not able to determine the linewidth of the UV radiation due to the lack of suitable optics for our interferometer. However, we were able to record the frequency stability of the generated UV at maximum power as a function of time using a wavemeter (High finesse, WS/U-30), with a resolution of 5 MHz. The result is shown in Fig. 7(b). Under free-running conditions and in the absence of thermal isolation, the UV output exhibited a peak-to-peak frequency deviation of 193 GHz over >1 minute, measured at a central wavelength of 337.9912 nm, thus further confirming the narrow linewidth of the generated UV output. With active stabilization of OPO cavity, we expect further improvements in the frequency stability of the output.

 

Fig. 7 (a) Spectrum of the waves involved in SFG at T_QPM = 145 °C, and (b) frequency stability of the generated UV radiation at 337.99122 nm over >1 min.

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4. Conclusions

In conclusion, we have demonstrated a tunable cw single-frequency solid-state UV source for the 333-345 nm spectral range, which exploits BIBO as the nonlinear gain medium, for the first time. The approach is based on SFG of the input pump and resonant signal radiation internal to a cw SRO based on MgO:sPPLT and pumped at 532 nm. By deploying BIBO under type-I (ee→o) interaction for SFG, we have generated up to 21.6 mW of UV power at 339.7 nm in high beam quality with good passive power and frequency stability. Together with the UV, an idler tuning across 1158-1312 nm has been achieved, providing up to 1.7 W of infrared output power. The tuning range of the UV source is currently limited by the mirror and crystal coatings. By using the existing BIBO crystal cut angle together with suitable grating periods for MgO:sPPLT, the UV coverage can be further extended to shorter wavelengths. With improved thermal and mechanical isolation, the power and frequency stability of the device can be further improved. These features, together with a compact, practical, solid-state design make the device a promising source of tunable cw UV radiation for a variety of applications.