1. Introduction

Tunable femtosecond laser sources in the ultraviolet (UV) are practical and valuable for a variety of applications such as spectroscopy [1], optical data storage [2] and biophotonics [3]. Generally, lasers covering the UV spectral region are commonly realized with gas lasers such as excimer lasers [4] and diode lasers [5], which are constricted by their wavelength tuning capability. On the other hand, solid-state UV laser based on Cerium or other rare-earth doped materials [6], which can provide an alternative tunable UV generation. However, such lasers require pump sources in the deep-UV wavelength, leading to an increasing system complexity and cost. Nonlinear frequency conversion techniques can offer the possibility to realize effectively tunable ultrafast laser in the UV spectral region. Typically, frequency doubling, tripling or quadrupling of lasers in the visible and near-IR wavelength provide the most direct route to the generation of UV pulses, such as second harmonic generation (SHG) of mode-locked Ti:sapphire laser [7], third harmonic generation of Nd:YAG laser and fourth harmonic generation of EDFA [8]. Nevertheless, except for the Ti:sapphire setup, all described techniques result in a fixed output wavelength from the fundamental input lasers. Therefore, a compact and cost-efficiently tunable UV source with high efficiency as well as high average output power needs to be devised.

For many years, optical parametric oscillators (OPOs) have been established as tunable and practical sources of coherent radiation and broad spectrum. The generation of tunable ultrashort pulse in the UV region based on internal SHG of femtosecond OPOs pumped by frequency-doubled of the Ti:sapphire laser at 415 nm has been reported [9]. By using a β-BaB₂O₄ internal to the OPO cavity, M.Ghotbi provided tunable femtosecond pulses across 250-355 nm in the UV at up to 225 mW average power, while the relatively high complexity, large size and high cost, partly due to the need for water-cooling and bulk solid-state pump lasers, impose critical limitations [10]. Therefore, the combination of deploying frequency-doubled Yb-fiber lasers as the pump sources [11,12], and exploiting additional internal frequency doubling of resonant signal radiation has been developed to obtain compact, robust and practical sources in the UV spectral region. Recently, G.K.Samanta reported a tunable picosecond source for the UV based on intra-cavity frequency doubling of a MgO:sPPLT signal-resonant oscillator (SRO), synchronously pumped by the second harmonic of a mode-locked Yb-fiber laser with average power up to 30 mW [13]. Thanks to the high power and efficiency, the frequency-doubled Yb-fiber laser pumped OPOs have been natural suitable laser sources to generate high power tunable UV pulses, which could provide increased flexibility for many applications. To obtain the shorter wavelength in UV regime is one of the hot topics in this field.

Previously, we demonstrated a high-power tunable femtosecond lithium tribotate (LBO) optical parametric oscillator pumped by second harmonic of a femtosecond Yb-fiber laser [14], operating in visible regime with dual-wavelength. More recently, we reported a high average power tunable UV source with the wavelength ranging from 385 to 400 nm, based on a noncollinearity intra-cavity sum frequency mixing PPLN-OPO [15]. In this letter, we extend this approach to 330 nm by using a β-barium borate (BBO) crystal as the nonlinear gain medium for internal frequency doubling of the signal pulses. A more compact setup and a more economical crystal are applied to generate 330 nm femtosecond laser with wider wavelength tunable range, compared with [15]. The signal pulses are generated from a femtosecond LBO-OPO, which is synchronously pumped at 520 nm in the green. Compared with the approach in [9], signal wavelength tuning is achieved by varying the temperature of crystal. This is a popular way owing to the fact that no further angle tuning of the OPO crystal is needed, which makes the whole system easier to align. The green pump source is obtained by SHG of the Yb-fiber laser at 1040 nm as the primary pump source in a LBO crystal. To our best knowledge, this is the first example of tunable femtosecond UV source based on internal SHG from a fiber laser pumped LBO-OPO, resulting in a compact, robust and practical system. Pumped at 3 W in the green, we have generated a tunable UV source across 330~442 nm with an average power of up to 364 mW at 402 nm, corresponding to 12.3% green to ultraviolet conversion efficiency.

2. Experimental setup

The schematic of the experimental setup is shown in Fig. 1. The OPO is synchronously pumped by the second harmonic of an Yb-fiber laser-amplifier system at 520 nm in the green. The primary pump source centered at 1040 nm with a repetition rate of 57 MHz provides up to 7.4 W of average power, and pulse duration of 180 fs. To maintain stable output characteristics, the laser system is operating at maximum power, a combination of a half-wave plate and a polarizing beam splitter cube (PBS) are used to control the pump power to the OPO. L1 (f = 70 mm) and L2 (f = 110 mm), are two focusing and collimating lenses used for second harmonic generation (SHG), respectively. Additionally, the latter lens L2 (f = 110 mm) expands the SHG beam to realize the optimum mode matching with cavity mode. The OPO cavity is a bifocal ring, comprising of four concaves (r = 150 mm) and two plane mirrors, making sure the green beam be focused to an circle waist radius of ~50 um at the center of the OPO crystal between M1 and M2. Mirror M1 is highly reflective both for the signal (R = 99.8%, over 650~1100 nm) and the pump (R>90%, at 520 nm). Mirrors M2-6 are highly reflective for the signal (R = 99.8%, over 650~1100 nm), while transmitting the idler (T>80%, over 1200~2500 nm) and the pump. This design ensures the singly-resonant oscillation for the signal. M5 is the UV output coupling mirror which has a transmission of 90% over the range from 310 nm to 450 nm. The total optical length of the OPO cavity is ~2.63 m, corresponding to a repetition rate of 114 MHz, which is set to be twice of the pump oscillator’s, ensuring synchronization with Yb-laser system. Furthermore, the stability and the output power of the OPO highly depend on the distance between concave mirrors. Therefore, the mirrors M1, M2, M5 and M6 are mounted on the precision translation stages to provide the required adjustment of the distances.

 

Fig. 1 Experimental design of the tunable UV generation setup; HWP: half- wave plate; PBS: polarizing beam-splitter; L: lens; DM: dichroic mirror; M: mirror

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The green pump source for the OPO is obtained by SHG of the Yb-laser in a 2 mm thick LBO crystal, whose cutting is $\phi \text{=}{12.9}^{\circ }$, $\theta \text{=}{90}^{\circ }$ for type-I (e→o + o) interaction. In this experimental setup, we choose the LBO crystal as SHG crystal because of its wide angular acceptance, which enables a relatively symmetric TEM00 spatial mode for the OPO operation. We have generated as much as 3 W of the SHG green source, corresponding to 40.5% conversion of the primary pump source, with pulse duration about 250 fs. The nonlinear crystal for the OPO is the same as the one in the earlier work [14], which is a 4 mm long noncritical xy-plane phase-matched LBO ($\phi \text{=}{0}^{\circ }$, $\theta \text{=9}{0}^{\circ }$) housed in an oven adjustable from room temperature to 200 °C with a stability of ± 0.1 °C. Considering the broadband UV transparency (down to about 180 nm) and high nonlinear efficiency for UV generation, we employ BBO crystal for internal frequency doubling due to its perfect SHG property in UV. The BBO crystal is 3 mm long, 5 mm × 5 mm cross section cut at $\theta \text{=}{29.2}^{\circ }$($\phi \text{=}{0}^{\circ }$) for type- I (e→o + o) phase-matching interaction. Both crystals are antireflection (AR) coated for the signal, while the BBO has an additional AR coating in the UV wavelength.

3. Results and discussion

In order to configure the intra-cavity frequency-doubled OPO system, we performed measurement of signal wavelength tuning without BBO crystal deposited between mirrors M5 and M6 in the cavity. Wavelength tuning from red to near IR was achieved by varying the temperature of crystal due to the temperature-tuned noncritical phase-matching capability of LBO. Temperature tuning is a popular way owing to the fact that no further alignment of an OPO cavity is needed. The temperature-tuning property of the LBO-OPO was discussed in our previous work [14], in which we obtained dual-wavelength operation of an OPO. As we had mentioned before, the dual-wavelength resonance could be acquired when two wavelengths had identical cavity optical length, and the group velocities mismatch (GVM) brought by the nonlinear crystal and the broadband dielectric mirrors was compensated in the femtosecond laser case. However, in this letter, a single wavelength operation OPO is needed for high power UV generation and thus we use mirrors with really high negative group-velocity dispersion to get an approximate linear GVD of OPO system, which make only single wavelength can be oscillated for the signal.

We first placed a 3% output coupler (OC) in the position of M3 and optimized the cavity to ensure OPO system operating at the maximum signal output power. Figure 2(a) shows recorded spectrum of the signal by changing the temperature of the crystal from 140 °C to 200 °C and cavity length adjustment. In this way, we are able to obtain the signal wavelength from 660 nm to 884 nm, together with the corresponding mid-IR idler wavelength coving the range from 2451 nm to 1263 nm. The 224 nm bandwidth of the signal tuning, is in good agreement with the calculated data in the earlier work [14]. The signal wavelength was measured using an Ocean optics SD 2000 spectrometer, while the idler wavelength was calculated from energy conservation. Figure 2(b) shows the pulse duration of the signal and the inset indicates a typical autocorrelation of signal pulses at 804 nm, with the pulse duration of 230 fs. On account of the temporal walk off effects due to the group velocity mismatch(GVM) between the signal pulses and the pump pulse in the cavity, the pulse durations of signal pulses varies from 284 fs to 154 fs are measured, when signal wavelength is tuned from 660 nm to 884 nm. Assuming a Gaussian pulse shape, the Fourier-limited pulse duration of the pulse is 69 fs at 660 nm and 114 fs at 884 nm, which means that the pulses are broadened to be between 4.1 and 1.4 times the Fourier transform limit. We also studied the output power characteristics of the signal, as the filled black square indicated in the Fig. 2(a). All the power data shown in the Fig. 2(a) were measured while the pump green source at a constant average power of 3 W. As evident from the Fig. 2(a), the maximum output power of 492 mW at 740 nm was generated, corresponding to a signal extraction efficiency of ~16.4%.

 

Fig. 2 (a) Wavelength tuning range from 660 to 884 nm and corresponding power across the tuning range of the signal; (b) Pulse duration of the OPO as a function of signal wavelength, the inset shows a typical autocorrelation of signal.

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In order to accomplish the phase-matching of each nonlinear crystal, a suitable type of wavelength tuning method is essential. In this letter we choose two methods, temperature or angle based, taking into account the required range of wavelength and the angular and temperature of the chosen crystal [16]. Hence, we calculated the temperature-tuning property of the LBO-OPO, and the phase-matching property of the inter-crystal BBO. Figure 3(a) shows the calculations of the typical dual-wavelength phase matching property of the LBO-OPO. The OPO which is pumped by green at 520 nm allowed a continuous tuning range from 660 nm to 1040 nm for the signal and 2500 nm to 1040 nm for the corresponding idler while the temperature adjusts from 120°C to 180°C (20°C left shift between the experiment and calculation). The area between two white dashes represents the signal wavelength region in our experiment. The best performance of a BBO crystal is realized through a type-I (e→o + o) angle-tuned interaction for its largest effective second-order nonlinearity within this direction [16]. The calculated wavelength tuning range of intra-cavity doubled of signal wavelength is indicated in the Fig. 3(b). By changing the angle of BBO crystal from 36 ° to 26.5 °, UV spectral tuning across 330~442 nm is achieved for a resonant signal wavelength range of 660~884 nm. We can observe that, the calculated bandwidth of UV range is narrower than the signal region as a consequence of the acceptance of crystal phase-matching angle.

 

Fig. 3 (a) Calculated signal, idler wavelength tuning range of OPO system as a function of the LBO crystal temperature, (b) Calculated intracavity frequency-doubled UV wavelength with the angle of BBO crystal from 36 ° to 26.5 °

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Following the characteristics of the signal wavelength tuning and power scaling, we placed the 3 mm long BBO crystal at the focus of concave mirror M5 and M6 to realize a high UV output power, although sacrificed part of the bandwidth of signal pulses. The output coupler was replaced by AR mirror M3 to enhance the intra-cavity power for UV generation. In addition, the cavity length was slightly shortened by moving M6 to restart the oscillation. Then the wavelength tuning in the UV region was achieved by simultaneously varying temperature of the LBO crystal, and angular of the BBO frequency doubling crystal for each signal wavelength to access optimum phase-matching. In contrast to sum frequency technique in [15], the signal pulses didn’t need to synchronize with another pump beam, so we got a more practical way to generate UV pulses. BBO has been shown to be a practical crystal for SHG conversion process in the near-IR due to the weak dependence of the phase-matching angle on the wavelength, as well as its small GVM [17]. As the crystal temperature changed from 140 °C to 200 °C, we obtained signal spectrum tuning. While varying the internal angle of BBO crystal from 36 ° to 26.5 °, in good agreement with the calculated curve, the UV wavelength could be extended to 330 nm with a continuous wavelength tunability of 112 nm as shown in Fig. 4(a). With the present cavity design, we operated the OPO with ~3W of green pump power, as shown in Fig. 4(b), when the extracted UV power varied from 102 mW at 331 nm to 181mW at 441nm with a maximum UV power of 364 mW at 402 nm corresponding to a green to UV conversion efficiency as much as 12. 3%.

 

Fig. 4 (a) Wavelength tuning property of UV generation from 330 to 442nm; (b) Extracted output power from the intra-cavity frequency-doubled OPO across the UV tuning range

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To characterize the power scaling of the intra-cavity frequency-doubled OPO in the UV, we kept T = 150ºC and operated the OPO at 402 nm, where the OPO was the most efficient. Figure 5(a) indicates that the UV power increased from 12 mW at a green pump power of 1.2 W to a maximum of 364 mW at pump power of 3W, representing a conversion efficiency of 12.3%. The threshold of the OPO was recorded to be 986 mW in the presence of the intra-cavity doubling crystal BBO and the slope efficiency of the UV source was estimated to be 19.6%. Considering the linear increasing and no evidence of saturation of UV power, more UV output power would be acquired with a higher pump green power. The short length BBO crystal enables minimum beam distortion arising from spatial walkoff, tight focusing, or cavity astigmatism, so that the UV output beam have a shape close to fundamental mode. The inset shows the brilliant beam profile of the UV traverse mode at 402 nm, and almost maintains the same with the wavelength tuning. As shown in Fig. 5(b), we characterized the spectral density of the relative intensity noise (RIN) and calculated the integrated RIN of the UV output at 402 nm. For comparison, the primary pump source was also recorded and shown in Fig. 5(b). Both of them were detected by a high-sensitivity photo-detector (Thorlabs, PDA36A-EC), and characterized by a fast Fourier transform (FFT) analyzer (Stanford research systems, SR770), as well as an RF analyzer (Agilent, 8560EC). The root-mean-square (RMS) RIN was 0.3% and 0.5% integrated from 10 Hz to 5 MHz for the laser pump source and UV output, respectively. The fluctuation of the UV output was 1.67 times of the pump source. According to the approach in [18], a closed-loop wavelength stabilization system will optimize the power fluctuation behavior of the OPO system.

 

Fig. 5 (a) Variation of the generated UV average power and conversion efficiency at 402 nm as a function of the input green pump power at 520nm and the inset shows the beam profile; (b) Measured RIN spectra (top) and integrated RIN (bottom) at the pump laser and UV output.

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By keeping the LBO temperature at 150 °C, and carefully adjusting cavity length and the angle of internal BBO crystal, we obtained the influence of a cavity length mismatch on the generated signal and UV source. This influence is attributed to the broad-signal phase-matching bandwidth brought by wide spectrum femtosecond pump pulses. The reason for the movement of signal spectrum peaks with changing the cavity length is that different wavelengths of the parametric gain bandwidth can be achieved in nonlinear crystal, and synchronous pumping condition can be fulfilled in different wavelength ranges by slightly changing the cavity length according to the gain bandwidth [19]. As shown in Fig. 6, the signal wavelength varies across 695~806 nm, enabling intra-cavity SHG over a tunable spectrum range from 347.5 nm to 403 nm and the spectral bandwidth of the signal pulse vary from 5 to 7 nm. Moreover, the generated UV spectrum exhibits consistent behavior, with bandwidths ranging from ~1nm to ~2 nm. We believe that the spectral acceptance in BBO leading to gain narrowing and thus constraining the SHG conversion bandwidth for fundamental pulses, reducing the bandwidth from the visible to the UV. The pulse duration at 403 nm is calculated to be 270 fs, taking into account the temporal walkoff between signal and second harmonic pulses in the nonlinear crystal.

 

Fig. 6 Spectra of signal across the OPO tuning range and corresponding generated intracavity frequency-doubled UV at LBO temperature of 150 °C

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4. Conclusion

In conclusion, we have demonstrated a femtosecond UV source with wide tunability and practical output power. This femtosecond UV source is based on a temperature-tuned LBO OPO pumped at 520nm generated by the second harmonic of an Yb-fiber laser. And the UV region has been reached through intra-cavity frequency doubling of the signal pulses in BBO crystal. We have extended tunable femtosecond UV pulse wavelength ranging from 330 nm to 442nm, with output power up to 364 mW at 402 nm, corresponding to 12.3% green to ultraviolet conversion efficiency. With improvements in mirror coating, strategies to reduce the duration of signal pulses and adding a feedback setup to the cavity, further increasing in extracted UV output power and long-term power stability can be expected. This device with practical output powers and compact system will offer a novel approach to the development of tunable femtosecond UV generation. Therefore, such an approach provides a convenient way in many applications, such as time-domain and photoelectron spectroscopy, bio imaging, atmosphere sensing and nanotechnology.