1. Introduction

Tunable coherent ultrafast optical sources in the ultraviolet (UV) are of interest for a variety of applications including quantum optics [1], optical data storage [2], atmospheric sensing (both cw and ultrafast lasers are used), combustion diagnostics, and bio-imaging [3]. Traditionally, access to this spectral region has been achieved using bulky, complex and power-hungry gas lasers such as excimer lasers [4], offering limited or no wavelength tuning capability. Cerium-doped fluoride lasers, on the other hand, are promising candidates for tunable UV generation, but such lasers require deep-UV pump sources typically obtained by harmonic conversion of near-infrared solid-state lasers [5]. In addition, due to the short upper-state lifetime of the gain medium [6], such lasers are typically pumped synchronously by the deep-UV radiation.

Nonlinear frequency conversion techniques represent a potentially viable and effective approach to the direct generation of tunable ultrafast radiation in the UV. In particular, frequency doubling, tripling, or quadrupling of mode-locked solid-state lasers in suitable nonlinear materials can, in principle, allow access to the UV spectral regions, not attainable with alternative methods. Some examples of earlier reports on UV generation in the ultrafast femtosecond [7] and picosecond [8] time-scales include the second harmonic generation (SHG) of a Kerr-lens-mode-locked (KLM) Ti:sapphire laser at 76 MHz repetition rate in the birefringent nonlinear crystal of BiB₃O₆ (BIBO), providing tunable UV-blue radiation from 370 to 450 nm at >50% efficiency. Third harmonic generation of mode-locked and amplified picosecond Nd:YAG laser has also been demonstrated using BIBO, providing UV radiation at 355 nm at a low repetition rate 25 Hz [9]. More recently, fourth harmonic generation of a mode-locked EDFA at 1550 nm operating at 10 MHz resulted in the generation of UV radiation at 390 nm with ~12% conversion efficiency using the quasi-phase-matched (QPM) nonlinear material of periodically-poled KTiOPO₄ [10]. While SHG of the Ti:sapphire laser can provide tunable UV generation [7,8], the other techniques result in fixed output wavelengths [9,10]. Moreover, the generation of shorter UV wavelengths using higher harmonics of near-infrared solid-state or Ti:sapphire lasers results in reduced conversion efficiencies, requiring low-repetition-rate amplified systems to achieve higher pulse intensities, and thus practical output pulse energies and average powers in the UV. Given such prerequisites, the development of tunable high-repetition-rate ultrafast sources in the UV at practical average powers and efficiencies, in simple, compact, cost-effective, and practical architecture remains challenging, and alternative new approaches need to be devised to access this spectral range.

Optical parametric oscillators (OPOs) are now established as viable and practical sources of tunable coherent radiation from the UV to mid-IR. Using the KLM Ti:sapphire, crystalline solid-state, and fiber lasers, the potential of OPOs for the generation of tunable radiation in all time-scales from the continuous-wave (cw) to ultrafast picosecond and femtosecond domain has been demonstrated [11,12]. With the advent of QPM nonlinear materials such as MgO-doped periodically-poled stoichiometric LiTaO₃ (MgO:sPPLT) with high photorefractive damage threshold, green-pumped OPOs in cw [13,14] and picosecond [15] time-scales have been developed, providing tunable radiation in the visible range, including spectral regions covered by the Ti:sapphire laser. In cw operation, both diode-pumped solid-state green lasers [13] and frequency-doubled Yb-fiber lasers [14] have been deployed as the pump source, while in the ultrafast picosecond regime the second harmonic of mode-locked Yb-fiber laser at 532 nm has been successfully used for synchronously pumping of the OPO [15]. By exploiting additional internal frequency doubling of the resonant signal radiation in green-pumped cw OPOs, tunable generation in the blue has also been achieved at practical power levels [16], while using similar techniques in a picosecond OPO synchronously pumped by a mode-locked Yb-fiber laser at 1064 nm has enabled tunable pulse generation in the visible spectrum [17]. Internal SHG of femtosecond OPOs synchronously pumped by frequency-doubled output of the KLM Ti:sapphire laser in the blue has also enabled tunable ultrashort pulse generation in the UV [18]. Here, we deploy the same generic approach [16–18] to demonstrate a versatile tunable ultrafast source for the UV by using internal frequency doubling of a picosecond OPO based on MgO:sPPLT, synchronously pumped at 532 nm in the green. The OPO deploys a mode-locked picosecond Yb-fiber laser at 1064 nm as the primary pump source, resulting in a compact, robust, and practical system architecture, also offering the advantages of air cooling and potential power scaling. The green pump radiation is obtained by simple single-pass SHG of the Yb-fiber laser in BIBO crystal [19]. The internal frequency doubling of the OPO visible signal pulses into the UV is also achieved in a second BIBO crystal. Using a 30-mm-long MgO:sPPLT crystal for the OPO and a 5-mm-long BIBO crystal for internal SHG, we have generated tunable UV radiation across 316-339 nm with an average power of up to 30 mW at a pulse repetition rate of 240 MHz.

2. Experimental configuration

The schematic of the intra-cavity frequency-doubled picosecond OPO for tunable UV generation is shown in Fig. 1. The primary pump source is an Yb-fiber laser providing up to 20 W of average power at 1064 nm in pulses of ~21 ps duration at 80 MHz repetition rate [15]. The green pump radiation for the OPO is obtained by external single-pass SHG of the laser in a BIBO crystal [19]. We choose BIBO because of its versatile nonlinear optical properties [20,21], including a relatively high nonlinear gain coefficient (d_eff~3.2 pm/V), wide spectral and angular acceptance bandwidths, and lower spatial walk-off (ρ~25.6 mrad) as compared to other crystals in the borate family such as uniaxial β-BaB₂O₃ (ρ~55 mrad) for SHG into the green under type I (ee→o) phase-matching in the optical yz-plane. Since the performance of the Yb-fiber laser with regard to pulse duration and spectral stability is optimum at the highest power, we operate the laser at the maximum output power and used a combination of a half-wave plate and a polarizing beam-splitter as an attenuator. A second half-wave plate is used to control the beam polarization for optimum phase-matched SHG in the BIBO crystal. Using a 10-mm-long BIBO crystal, we generate as much as 5.4 W of picosecond green radiation at 80 MHz for an average Yb-fiber fundamental power of 14.2 W at single-pass SHG conversion efficiency of 38% [19].

 

Fig. 1 Experimental design of the intra-cavity frequency-doubled green-fiber-laser-pumped picosecond OPO tunable ultraviolet generation. FI: Faraday isolator, λ/2: Half-wave plate, PBS: Polarizing beam-splitter; L: Lens; M: Mirror.

Download Full Size | PDF

The green output power is naturally stable with a fluctuation of 0.24% rms over 15 hours and has a TEM₀₀ spatial mode (M²<1.93) with elliptic beam profile of circularity >0.4. Such low circularity in the green beam can be attributed to the spatial walk-off between the fundamental and SH wavelengths in BIBO resulting from the tight focusing of the pump beam (w_p~45 µm) and long crystal length (10 mm) used to achieve higher SHG efficiency. Detailed characterization of the green beam can be found in Ref [19]. The green beam is readily circularized using two cylindrical lenses (not shown in Fig. 1) with focal lengths, f = 75, 150 mm, separated by 245 mm, resulting in a circularity of >0.7. But, due to unavailability of the suitable cylindrical lenses, we were not able to completely circularize the beam. The circularity of the green beam can also be further improved by increasing the beam waist radius (loose focusing) of the fundamental beam in the BIBO crystal, but at the cost of reduced efficiency. An effective alternative approach to improve the circularity, yet generating high SH power and efficiency, is to deploy walk-off compensation schemes. The single-pass SHG scheme in combination with the Yb-fiber laser makes this picosecond green source a simple, compact and practical source for many applications, including synchronous pumping of OPOs. The OPO is configured in a folded ring cavity comprising four plano-concave mirrors (M₁−M₄) of radius of curvature, r = 10 cm, providing two intra-cavity foci, a plane mirror (M₅), and an output coupler (OC). All cavity mirrors are highly transmitting for the pump (T>94% at 532 nm) and idler (T>80% over 1.5–3.5 μm), while highly reflecting for the signal (R >99.5% over 625–850 nm), ensuring singly-resonant signal oscillation. In addition, all mirrors have moderate transmission (T>75%) over 315-425 nm, allowing extraction of the generated UV radiation. A conventional plane OC with partial transmission (T~14 ± 5%) over 500-700 nm is used to extract signal power from the OPO and a dichroic mirror, M, separates the generated idler from the unconverted pump. The nonlinear crystal for the OPO is a 30-mm-long, multi-grating MgO:sPPLT crystal, with three grating periods, Λ = 10.5, 11.0, 11.5 µm, and is housed in an oven adjustable from room temperature to 200 °C with a stability of ± 0.1 °C. This new multi-grating design for the MgO:sPPLT allows us to generate substantially shorter signal wavelengths in the visible than our earlier work [15], hence enabling us to reach the UV spectral range with internal frequency doubling. The green beam is focused to an elliptic waist radius of w₀~46 × 69 µm at the centre of the MgO:sPPLT crystal placed at the primary focus of the cavity between M₁ and M₂. A 5-mm-long BIBO crystal with 8 × 4 mm² aperture cut at θ = 135° (ϕ = 90°) for type-I (ee→o) phase-matching in the optical yz-plane is placed at the secondary focus of the cavity between mirrors M₃ and M₄ for internal SHG of the resonant signal pulses into the UV. Both crystals have antireflection (AR) coatings for the resonant signal (R<0.5% over 625–700 nm), while the BIBO has additional AR coating for the UV (R<0.5% over 310-350 nm). The total optical length of the OPO cavity is ~1.25 m, corresponding to a repetition rate of 240 MHz, ensuring synchronization with the third harmonic of the pump laser repetition rate, making the system a compact, high-repetition-rate UV source, as shown in the laboratory experimental setup depicted in Fig. 2.

 

Fig. 2 Photograph of laboratory setup of the compact, high-repetetion-rate, picoscond OPO showing the resonant red signal with internal frequency-doubling for tunable UV generation.

Download Full Size | PDF

3. Results and discussions

3.1 Wavelength tuning range

In order to characterize the intra-cavity frequency-doubled picosecond OPO, we performed measurements of wavelength tuning without the SHG crystal in the cavity. Wavelength tuning was achieved by varying the temperature as well as the grating period of the MgO:sPPLT crystal. Using the three different grating periods, Λ = 10.5, 11.0, 11.5 µm, and by changing the temperature of the crystal from 30 °C to 200 °C, we were able to tune the OPO over 634-681 nm in the red (signal), together with the mid-IR (idler) across 2429-3298 nm, resulting in a total signal (47 nm) plus idler (869 nm) tuning of 916 nm, as shown in Fig. 3.The signal wavelength was measured using a spectrometer with a resolution of 0.27 nm (Ocean optics HR 4000), while the idler wavelength was calculated from energy conservation. The solid lines in Fig. 3 correspond to the calculated tuning range using the relevant Sellmeier equations for stoichiometric LiTaO₃ [22], where good agreement between the experimental data and theory is confirmed.

 

Fig. 3 Wavelength tuning range of the picosecond OPO in the red (signal) and the mid-IR (idler) as a function of temperature for three different grating periods of the MgO:sPPLT crystal, Λ = 10.5, 11.0 and 11.5 µm.

Download Full Size | PDF

We also recorded the output power of the picosecond OPO in the visible red signal and mid-IR idler, and the corresponding pump depletion across the tuning range, with the results shown in Fig. 4.The data were obtained while pumping the OPO with a constant average green power of ~2 W. In these measurements, a conventional OC with a transmission of T~14 ± 5% over 500-700 nm was used to extract the visible signal power from the cavity. As evident from Fig. 4(a), the out-coupled signal power in the red spectral region varies from 765 mW at 634 nm to 560 mW at 681 nm, with a maximum of ~800 mW at 642 nm, corresponding to a signal extraction efficiency of ~40%. A signal power >500 mW is extracted from the OPO over the entire tuning range. Further enhancement in the extracted signal power could be achieved by optimizing the output coupling transmission using conventional [23,24] or interferometric [25] techniques. The non-resonant forward-pass idler power was separated from the depleted pump using a germanium filter. As shown in Fig. 4(b), the idler output power varies from 130 mW at 2429 nm to 51 mW at 3298 nm, with a maximum of 147 mW at 2534 nm. Over the entire idler wavelength range, the OPO provides a mid-IR average power >50 mW, sufficient for many applications.

 

Fig. 4 Output power and pump depletion across the tuning range of the picosecond OPO in the red (signal) and mid-IR (idler) spectral range. Solid lines are guide to the eye.

Download Full Size | PDF

The corresponding pump depletion is recorded to be ~60% over the entire tuning range with a maximum of ~73%, as shown in Fig. 4(c). With the maximum available green power of 5.4 W, we expect the operation of the OPO at higher pumping levels beyond 2 W, together with optimized signal output coupling, will result in increased overall extraction efficiency and output power. However, we restricted OPO operation to lower green pump powers to avoid any possible damage to the MgO:sPPLT crystal.

3.2 Power scaling

We studied power scaling of the picosecond OPO by recording the single-pass idler and out-coupled signal power through the OC (T~14 ± 5%) as a function of average input pump power for two different pairs of signal-idler wavelengths within the tuning range of the OPO, with the results shown in Fig. 5.Using the grating period of Λ = 10.5 µm and operating at a MgO:sPPLT crystal temperature of 105 °C, we were able to extract up to 517 mW of signal at 647 nm together with 120 mW of idler at 2525 nm for an average pump power of 2 W, as shown in Fig. 5(a). Slope efficiencies of 32.9% and 7.2% are estimated from the linear fits to the power scaling data at the signal and idler wavelengths, respectively. The threshold of the OPO under these operating conditions is recorded to be 460 mW. Figure 5(b) represents the simultaneous power scaling measurement of the signal and idler as a function of pump power, using Λ = 11.5 µm, at a crystal temperature of 200 °C. This grating period and temperature correspond to a red signal wavelength of 634 nm and an idler wavelength of 3306 nm. Under these conditions, the average pump power threshold of the OPO is 360 mW, while a signal power up to 826 mW and idler power of 56 mW is generated. The signal and idler powers grow at an estimated slope efficiency of 50.6% and 3.1%, respectively. As evident from Fig. 5(a) and 5(b), no saturation in output power is observed while pumping up to 2 W at 532 nm.

 

Fig. 5 Variation of idler power and out-coupled red signal power as a function of input pump power at two different pairs of signal and idler wavelengths. Solid lines are best fit to the experimental data.

Download Full Size | PDF

3.3 Spectral and temporal characteristics

For temporal characterization of the OPO, we performed autocorrelation measurements of the output signal pulse duration and repetition rate, while at the same time recording the output spectrum. Representative results with the OPO operating at a crystal temperature of 200 °C and a grating period of Λ = 10.5 µm, generating a signal wavelength at 665 nm, are shown in Fig. 6.Using a Michelson interferometer for intensity autocorrelation based on non-collinear SHG of the signal radiation in a 5-mm long BIBO crystal and a sensitive power meter, we measured the temporal width of the signal pulses, as shown in Fig. 6(a). The measurement indicates that the signal pulses have duration of 12 ps with a Gaussian temporal profile. The corresponding signal spectrum centred at 665.5 nm is shown in the inset of Fig. 6(a). As evident, the spectrum has a smooth profile with a full-width at half-maximum (FWHM) bandwidth of 2.1 nm, resulting in a time-bandwidth product of ΔτΔν~17. This is clearly many times above the transform limit for a Gaussian pulse (ΔτΔν~0.44), and is due to the lack of dispersion control within the OPO cavity and the group-delay dispersion (GDD) of the cavity mirrors, which is unknown. The measured FWHM spectral bandwidth of 2.1 nm at 665 nm can potentially support signal pulses of ~0.3 ps duration, therefore the time-bandwidth product can be significantly improved using dispersion compensation within the OPO cavity in combination with GDD-controlled mirrors to reduce the signal pulse duration. In addition, bandwidth reduction techniques using, for example, a diffraction grating instead of the OC as a feedback mirror, or intra-cavity wavelength selection elements such as a birefringent filter or an etalon can be deployed to reduce the signal spectrum, thereby improving the time-bandwidth product. We also measured the repetition rate of the output signal pulses using an InGaAs photo-detector (20 GHz, 18.5 ps) and a fast oscilloscope (3.5GHz, 40GS/s), with the results shown in Fig. 6(b), where a repetition rate of 240 MHz, synchronized to the third harmonic of the pump laser repetition frequency of 80 MHz, is confirmed.

 

Fig. 6 (a) Intensity autocorrelation, and Inset: corresponding spectrum, of red signal pulses generated by the green-pumped picosecond OPO. (b) Output signal pulse train at 240 MHz.

Download Full Size | PDF

3.4 Power stability and the spatial beam quality

In order to study the stability of the out-coupled signal power over time, we operated the OPO in the middle of the tuning range at ~665 nm using a grating period of Λ = 10.5 µm and a crystal temperature of 200 °C. For the measurement, we pumped the OPO with an average green power of 2.2 W, producing a signal power of 610 mW with ~8% output coupling. The results are shown in Fig. 7(a).As evident, the output signal exhibits good power stability with a fluctuation better than 1.4% rms over 14 hours under passive conditions and in the absence of thermal and mechanical isolation. However, the signal power still exhibits six times larger fluctuation compared to that of the green pump source (0.24% rms over 15 hous) [19]. This reduced power stability in the output signal can be attributed mainly to temperature fluctuations of the oven while operating the OPO at elevated temperatures (~200 °C) well above the laboratory temperature (~30 °C). Other factors include mode-matching instability arising from thermal effects in the MgO:sPPLT crystal due to the high intra-cavity signal power as well as the green pump power [23]. Therefore, further improvement in the output power stability is expected by operating the OPO at lower crystal temperatures and with higher output coupling using suitable grating periods. To determine the spatial profile of OPO output, we recoded the far-field energy distribution of the out-coupled signal beam at the same wavelength of ~665 nm and 610 mW of power. The results are shown in Fig. 7(b), where the energy distribution together with the intensity profiles and Gaussian fits along the two orthogonal axes confirm TEM₀₀ spatial mode with a circularity of 0.88.

 

Fig. 7 (a) Power stability of the out-coupled signal for 14 hours. (b) Spatial beam profile of the out-coupled signal at 665.5 nm together with the intensity profiles (pink) and Gaussian fits (yellow) measured in the far-field.

Download Full Size | PDF

3.5 Ultraviolet generation

Following the characterization of the green-pumped picosecond OPO with regard to the visible signal and mid-IR idler output, as described in Sections 3.1 to 3.4, we inserted the 5-mm-long BIBO crystal between mirrors M₃ and M₄ (Fig. 1) for intra-cavity frequency doubling of the resonant signal pulses into the UV. In order to increase the circulating signal power for enhanced SHG efficiency, we reduced the output coupling of the OPO. In principle, one can increase the intra-cavity signal power by reducing the output coupling to zero. However, we have found that higher intra-cavity powers lead to a degradation of the intra-cavity signal mode, and hence the generated UV power. On the other hand, increasing the pump power can produce higher intra-cavity signal power, but at the expense of increased thermal loading of the MgO:sPPLT crystal. Both these limitations can be potentially overcome by using a more optimized cavity design to provide a larger signal mode volume in the MgO:sPPLT crystal. However, with the present cavity design, we operated the OPO with an output coupling of ~8% and pump power of 2.7 W.

Using all three grating periods of the MgO:sPPLT crystal, Λ = 10.5 µm, 11.0 µm and 11.5 µm, and by varying crystal temperature from 30 °C to 200 °C, we were able to tune the OPO signal from ~634 to ~681 nm, enabling intra-cavity SHG over a tunable range from ~317 nm to ~340.5 nm, resulting in a gap-free wavelength coverage of 23.5 nm in the UV. The full tuning range in the UV was achieved by rotation of the BIBO crystal from an internal phase-matching angle of θ = 130.7° to θ = 138.1°. We recorded the UV power across the tuning range, while maintaining the green pump power fixed at 2.7 W. The results are shown in Fig. 8, where the extracted UV power is observed to vary from 13 mW at 317 nm to 18 mW at 340.5 nm, with as much as 30 mW available at 334.5 nm. These values do not account for the UV transmission loss of ~25% through mirror M₃, so that a maximum UV power of ~38 mW at 334.5 nm is generated at the output of the BIBO crystal.

 

Fig. 8 Extracted output power from the intra-cavity frequency-doubled picosecond OPO across the UV tuning range. Solid lines are guide to the eye.

Download Full Size | PDF

3.6 Power scaling in the UV

To characterize the power scaling of the intra-cavity frequency-doubled picosecond OPO in the UV, we performed simultaneous measurements of the signal, idler and SHG output power as a function of input pump power, with the results shown in Fig. 9.The results correspond to measured extracted UV power, without correction for the ~25% coating transmission loss of mirror M₃ in the UV. We operated the OPO at 664.7 nm and adjusted the angle of the BIBO crystal to access optimum phase matching to generate maximum UV power for the available pump power. As evident from the Fig. 9(a), the UV power increases from 0.1 mW for a pump power of 390 mW to a maximum of 23 mW for 2.7 W of green power. We also measured the spectrum of the generated UV radiation using a spectrometer of resolution 0.27 nm. The result is shown in the inset of Fig. 9(a), where the generated UV radiation is recorded to have a bandwidth of 1.7 nm (FWHM) centered at 332.37 nm. The corresponding signal and idler power scaling is shown in Fig. 9(b). We were able to generate a maximum of 540 mW of signal at 664.74 nm and 69 mW of idler at 2664 nm for 2.7 W of pump power. The slope efficiencies of the signal and idler power are estimated to be 22.6% and 2.6%, respectively. The threshold of the OPO was recorded to be as low as 390 mW in the presence of the BIBO intra-cavity doubling crystal.

 

Fig. 9 Variation of (a) UV power, and (b) corresponding signal and idler power as a function of green pump power to the internally frequency-doubled picosecond OPO. Inset: Spectrum of the generated UV radiation. Solid lines are guide to the eye.

Download Full Size | PDF

From the out-coupled signal power (~600 mW) at 669 nm and the transmission (T~8%) of the output coupler, we estimated an average intra-cavity signal power of ~7.5 W. With the estimated signal power and the 30 mW of UV output power at 334.5 nm, and the coating transmission loss of ~25% through mirror M₃, we calculate a single-pass SHG efficiency of ~0.5%, which is very small considering the 5-mm length of the BIBO crystal and the peak intra-cavity signal power of 2.6 kW. However, from the cavity design, we estimate the signal beam waist at the center of the BIBO crystal between M₃ and M₄ (Fig. 1) to be w_0s~23 µm, because of the relatively short radius of curvature of the mirrors (r = 10 mm) in the present resonator configuration. Such a tight focusing limits the effective usable length of the BIBO crystal to <1 mm, due to the effect of spatial walk-off. Therefore, we expect that it will be possible to achieve higher conversion efficiency by optimizing the signal beam waist inside the BIBO crystal using more suitable focusing mirrors with larger radius of curvature to exploit the full 5-mm interaction length for UV generation. Another important factor limiting UV efficiency in the present experiment is the 2.1-nm bandwidth (FWHM) of the fundamental signal pulses. Given that the spectral acceptance bandwidth of BIBO for phase-matched SHG into the UV under type I (ee→o) interaction in the optical yz-plane is ~0.2 nm.cm [21], only a small fraction of signal spectrum (0.4 nm) currently contributes to efficient frequency doubling into the UV. As such, substantial improvements in SHG efficiency and UV output power can be brought about by implementing techniques to reduce the spectral bandwidth of signal pulses as mentioned in Section 3.3. Such techniques should result in significant bandwidth reduction, given the large time-bandwidth product of ΔτΔν~17 measured for the signal pulses.

3.7 Damage issues in the UV

Another important factor contributing to reduced output power in the UV can be material damage. Although we were able to generate 30 mW of UV power at the maximum available green power, we observed a drop in the UV power with time, which was identified as surface damage on the exit face of the BIBO crystal. In order to further understand the origin of this damage, we simultaneously recorded the signal as well as the UV output power over a period of 2 minutes, as shown in Fig. 10.The OPO was set to operate at a signal wavelength of 664.7 nm with a fixed pump power of 2.7 W. The BIBO crystal was adjusted to its optimum phase-matching angle to achieve highest UV power at 332.37 nm. As evident from the Fig. 10, the signal power remains almost constant over the measurement period, whereas the UV power drops from its maximum power of 30 mW to a stable 10 mW power level in ~45 seconds. It is to be noted that the ordinate in Fig. 10 corresponding to the UV power is plotted from a baseline of 10 mW.

 

Fig. 10 Output power stability of the out-coupled signal and corresponding frequency doubled UV power.

Download Full Size | PDF

By translating the BIBO crystal to a new position, we were able to recover the maximum UV power. However, returning to the same spot did not make any change in the UV power clearly indicating the permanent damage. Careful inspection also showed such damage marks on the crystal surface. This damage can be attributed to the damage of the AR coating on the crystal face at UV wavelength range and also the bulk surface damage. We also observed that the surface damage in the BIBO crystal resulted in the deformation of the output signal beam from its initial Gaussian spatial profile. We believe that by improving the surrounding environment of the BIBO crystal, for example by purging with nitrogen or enclosure of the crystal in vacuum, the crystal and coating damage issues can be mitigated, enabling stable power generation in the UV.

4. Conclusions

In conclusion, we have demonstrated a high-repetition-rate ultrafast UV source at 240 MHz based on a MgO:sPPLT picosecond OPO pumped at 532 nm in the green by the second harmonic of a mode-locked Yb-fiber laser, with internal frequency doubling of the visible signal pulses into the UV in BIBO. The device is tunable across 317-340.5 nm with a single set of mirrors, with the tuning range currently limited by the available grating periods in the MgO:sPPLT crystal and cut angle of the BIBO crystal. As such, an expansion of the tuning range beyond the current limits is readily feasible by using alternative grating periods in MgO:sPPLT and other suitable cut angles in BIBO for SHG. The spectral bandwidth of the UV radiation has been measured to be 1.7 nm (FWHM), while the corresponding bandwidth for the signal is 2.1 nm. We have extracted a maximum UV average power of 30 mW at 334.5 nm, with further scope for increased output power by optimizing spatial mode-matching of signal in the BIBO crystal, bandwidth reduction of signal pulses, and higher UV transmission of OPO mirrors. Improved circularization of the green pump beam, together with optimized mode-matching of pump and signal in the MgO:sPPLT crystal, should also enable the generation of higher UV output powers. In addition to UV generation, the OPO provides tunable signal pulses across 634-681 nm in the visible with a maximum average power of ~800 mW at 642 nm, and corresponding mid-IR idler radiation over 2429-3298 nm with up to 147 mW of output power. We have also observed that while the signal radiation has a power stability of 1.4% rms over 14 hours, the UV power drops from its maximum power of 30 mW to a stable 10 mW power in almost 45 seconds due to coating and surface damage on the BIBO sample, which we believe can be alleviated by environmental protection of the crystal. With the proposed improvements in OPO cavity design and mirror coatings, and strategies to reduce signal bandwidth and mitigate damage risk to the BIBO crystal surface and coating, further improvements in extracted UV output power, tuning coverage, and long-term power stability can be expected. The demonstrated technique represents a novel approach to the development of tunable ultrafast sources in the UV.