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Abstract
We present the first frequency-quadrupled linearly-polarized Q-switched neodymium-doped fiber laser generating > 500 mW average power at 226 nm. For this purpose, an amplified Q-switched oscillator using novel large-mode-area (LMA) fibers and generating up to 24 W average power (15 kW peak power) at 905 nm was developed. Two nonlinear frequency conversion stages using a LBO crystal for SHG and a BBO crystal for FHG generate respectively up to 4.9 W average power in the deep blue at 452 nm and a maximum of 510 mW average power in the deep ultra-violet (DUV) at 226 nm. Performance limitations and further improvements are discussed.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The development of high-energy pulsed laser sources emitting in the DUV spectral range at wavelengths < 250 nm is attractive for many scientific and industrial applications such as skin treatment [1], material processing [2] or laser-induced spectroscopy (LIBS or LIF) [3,4]. Until now, practical realizations of compact laser sources with nearly diffraction-limited beam in the DUV have mainly relied on cascaded non-linear frequency conversions of infrared (IR) emission from solid-state lasers. While these techniques have been proven very effective at longer UV wavelengths near 258 nm [5] or 266 nm [6], the generation of short DUV wavelengths via fourth harmonic generation (FHG) requires a powerful laser source emitting at a fundamental wavelength below 1 µm combined with efficient non-linear conversion stages. As a matter of fact, watt-level and diffraction-limited DUV laser sources have been little explored until now. Average power of 600 mW at 236.5 nm [7], 1.37 W at 213 nm [8] and 2.5 W at 206 nm [9] have been recently reported but these laser sources are based on crystalline solid-state lasers, hence providing only limited fixed wavelengths. Fiber laser technology is undoubtedly a good option for further developments of laser sources in the DUV spectral domain, especially in terms of compactness, power scalability, beam quality and wavelength tunability. Furthermore, pulsed Yb-doped fiber lasers have already proven to be effective for generating high average power in blue/green [10,11] and UV regions [12] through frequency conversion. Continuous Wave (CW) laser emission in the DUV is also of great interest, especially for atomic, molecular and optical physics (AMO). CW single-frequency emission in the DUV allows for example laser cooling of Aluminum Monofluoride [13] or Cadmium [14]. Yb doped fiber laser has already been used for single-frequency CW laser emission in the DUV [15]. However, efficient frequency conversion in CW regime is challenging as it requires extremely narrow linewidth infrared laser sources and cascaded resonant SHG cavities with active stabilization [15,16]. In order to reach DUV wavelengths by using a simple frequency-quadrupling scheme, Neodymium-doped fiber (NDF) laser based on the three-level 4F3/2→4I9/2 transition has a great potential with the possibility of tunable laser emission in the 880-930 nm spectral range [17]. This is by far the shortest wavelengths directly available from rare-earth doped silica fiber laser. The main challenge to obtain high power laser operation around 900 nm in neodymium-doped laser materials is the competition with the 4-level transition near 1060 nm. Indeed, in addition to signal reabsorption from the ground-state level, the emission cross-section is more than two times lower for the 3-level transition compared to the 4-level transition. Nd-doped materials may also present ions clustering effect for high doping concentration level, which is more detrimental to the transition near 900 nm [18]. In fact, the pump fluence is a determining parameter and must be high enough to balance the gains of the two transitions. In a classical cladding-pumped single-mode fiber laser, a high pump fluence is difficult to achieve. Nevertheless, the parasitic lasing near 1060 nm can be suppressed by using a special W-type core index profile, which acts as a short-pass filter and therefore introduces very high propagation losses for longer wavelengths including the parasitic emission near 1060 nm [19]. Following this approach, a few watts of output power were achieved in CW [20] or pulsed regime [21]. However, this W-type index profile is only applicable to small diameter core (< 6 µm) which implies a low threshold for non-linear effects. In order to both mitigate the non-linear effects and promote the gain at 900 nm, specifically designed Nd-doped large-mode-area fibers with reduced clad-to-core ratio and low numerical aperture (NA) were first developed by Dawson et al. Up to 15 W of CW power at 938 nm was achieved using this design [22]. Reduced clad-to-core ratio helps increasing the pump absorption, enabling lower neodymium concentration, which also reduces the gain difference between 900 nm and 1060 nm optical transitions. More than 20 W CW power at 910 nm were obtained in 2013 by our group using a 20 µm/80 µm (core/clad diameters) Nd-doped LMA fiber laser [23]. Later on, 27 W of output power was demonstrated at 925 nm in a dual wavelength pumped Nd fiber laser using photonic crystal fiber designed with a spectral filtering to suppress the amplified spontaneous emission (ASE) from the four-level transition [24]. Despite these recent developments, the non-linear frequency conversion of a pulsed Nd-doped fiber laser system has been little explored so far. Bartolacci et al. reported 300 mW of blue power from a frequency-doubled MOPA laser system operating in sub-nanosecond pulse regime [21]. However, because of the low pulse peak power, SHG was realized in a PPLN crystal, which strongly limits further power scaling in the blue. In addition, non-linear crystals currently available for efficient frequency conversion in the DUV are limited to a few bulk crystals (BBO, CLBO). As a consequence, the development of NDF laser source with high peak-power (> 10 kW) is required first.
In this paper, we report a nanosecond pulsed Master-Oscillator Power-Amplifier (MOPA) fiber laser system emitting at 905 nm, based on alumino-phospho-silicate LMA Nd-doped fibers, allowing the generation of 510 mW of average power at 226 nm after frequency quadrupling. An actively Q-switched master-oscillator delivering 45 ns pulses at a repetition rate adjustable between 10 and 60 kHz was amplified in a low-NA 30/130 µm PM NDF at an average power of 24 W (0.6 mJ energy per pulse at 40 kHz repetition rate). Two nonlinear stages based on LBO and BBO crystals were used respectively for cascaded second-harmonic generation (SHG) and FHG, giving conversion efficiencies of 20% and 10%. To the best of our knowledge, this is the shortest DUV wavelength obtained using the 4th harmonic generation of a fiber laser.
2. Fiber design and CW laser performance
The low-NA alumino-phospho-silicate fiber preforms were synthetized using a homemade Surface Plasma Chemical Vapor Deposition (SPCVD) process. This deposition technique is based on a low-pressure microwave plasma assisted oxidation reactions on the inner surface of the synthetic silica tube [25]. After the fabrication of this preliminary preform, the core/cladding ratio was adjusted by adding or removing silica and a pair of stress rods were inserted in the preform in order to obtain a polarization-maintaining (PM) fiber. From this process, various size of PM-LMA Nd-doped fibers, with a core composition of SiO2-Al2O3-P2O5-Nd2O3 and a clad-to-core ratio ∼ 4 were fabricated. The aluminum-phosphorus co-doping of the core is necessary to increase neodymium concentration while reducing clustering effects [26]. Also this core composition helps to maintain a low refractive index difference between the core and the cladding. The possibility of producing our own tailored fibers, with custom geometries and compositions, is greatly helpful to reduce parasitic emission of neodymium while maintaining good beam quality. This type of design would also be beneficial for studies of Yb-fiber lasers at 976 nm [27]. In the frame of this work, two PM-LMA Nd doped fibers were incorporated in the pulsed laser source. The Q-switched Master Oscillator was based on a 19 µm core (NA 0.075), 80 µm cladding (NA 0.46) with a pump absorption coefficient of 2.75 dB/m at 808 nm. The power amplifier relies on a 30 µm core (NA 0.05), 130 µm cladding (NA 0.46) with a pump absorption coefficient of 0.92 dB/m at 808 nm. The two mode field diameters (MFD) were respectively estimated to 17 µm and 26 µm at 905 nm.
The laser efficiency of each fiber has been first measured in CW regime. For these measurements, we used a linear laser cavity formed with two bulk mirrors, respectively a high reflective dichroic mirror at 900 nm (R = 100%) and a broadband output coupler (R=8%). The fiber was free-space pumped with a fiber-coupled laser diode at 808 nm. The PM-LMA Nd-doped fiber length was adjusted to obtain a pump absorption of ∼80% at the laser threshold. Both fiber end-facets were angle cleaved and ASE at 1060 nm was filtered out on each side of the cavity to avoid parasitic oscillation on the 4-level optical transition. Figure 1 shows the output power versus injected pump power obtained in CW regime for each fiber. The 20/80 µm fiber shows a 44% slope efficiency with respect to the injected pump power and a threshold of 2.5 W. The M2 factor of the output optical beam was measured equal to 1.48 and 1.35 on each axis.
Fig. 1. CW laser output powers at 905 nm versus injected pump power for the two PM-LMA Nd-doped fibers.
The 30/130 µm fiber is characterized by the same slope efficiency of 44% but with a higher threshold of 6 W. However, a maximum output power of 22.8 W could be obtained for 56 W of injected pump power. One can note that the output power is only limited by the available pump power as no saturation or thermal effects can be observed. Despite the large core diameter of 30 µm, the output beam was diffraction-limited with M2 of 1.01 and 1.04. This good result is attributed to the low core NA of this fiber thanks to a precise control of the refractive index using SPCVD fabrication process.
3. Experimental set-up and results in pulsed regime
A pulsed MOPA laser system based on the two PM-LMA Nd-doped fibers was developed following the experimental set-up illustrated on Fig. 2. The Q-switched Master Oscillator was composed of a high-reflection broadband mirror (R = 100% at 900-920 nm), a 3.5 m long 20/80 µm NDF with a coil diameter of 9 cm and a partially reflecting fiber Bragg Grating (FBG) as an output coupler. The FBG was photo-inscribed in a PM 20/80 passive fiber (0.08 NA) and has a reflectivity of 10% at 905 nm with 3 dB bandwidth of 0.55 nm. The NDF was free-space pumped with a 35 W fiber-coupled (100 µm NA=0.22) laser diode. The Q-switched regime was obtained by inserting in the cavity an acousto-optic modulator (Isomet M1080-T80L-1.5) aligned on the first order-diffracted beam. The switching gate time was set to 170 ns and its rise time to 85 ns. Special care was taken to avoid spurious lasing at 1060 nm by suppressing any broadband optical feedback in the NDF. The fiber end-facets were angle cleaved and a dichroic mirror DM1 filtered out the ASE at 1060 nm. The pulse repetition frequency (PRF) of the oscillator could be tuned from 10 kHz up to 60 kHz while maintaining a stable output pulses train with 45 ns full width at half maximum (FWHM) pulse duration. For 20 W of injected pump power at 40 kHz PRF, we obtained 3.3 W average output power measured after the dichroic mirror DM2, corresponding to an estimated peak power of 2 kW.
Fig. 2. Schematic of the pulsed MOPA laser system operating at 905 nm (HR: High Reflective; TFP: Thin Film Polarizer; DMi: Dichroic Mirrors; LD: Laser Diode; AOM: Acousto-Optic Modulator; PM: Polarization Maintaining; FBG: Fiber Bragg Grating; HWP: Half-Wave Plate).
To further increase the pulse peak power, the output beam from the oscillator was injected in a Power Amplifier made from 10 m of 30/130 µm LMA NDF. Mode-matching between the FBG output fiber and the 30/130 NDF was achieved through two lenses L1 and L2 with respective focal lengths f1=11 mm and f2=15 mm. The coil diameter of the LMA fiber was adjusted around 25 cm to avoid coupling on higher order modes. After the isolator, a half-wave plate was used to align the beam polarization on one eigenaxis of the PM NDF. The fiber amplifier was free-space pumped using a 60 W fiber-coupled (100 µm NA=0.22) laser diode. After reflection on the two dichroic mirrors DM5 and DM6, 55 W of pump power could be launched into the 130 µm cladding. The unsaturated pump absorption was ∼82%. The power injected in the 30 µm core was around 2.5 W at a repetition rate of 40 kHz. The output power was amplified up to 24 W at maximum pump power with a conversion efficiency of 44%, with respect to the launched pump power. For lower repetition rates, the peak power could be increased but with the disadvantage of lower average power and reduced saturation of the amplifier stage. Hence, the peak power reaches 15 kW, which is usually sufficient to achieve efficient single-pass frequency conversion in non-linear crystals.
The output spectrum and the M2 beam quality factor are shown respectively in Fig. 3(a) and 3(b). The amplified spectrum is significantly broadened and presents a total spectral linewidth of more than 10 nm and a FWHM equal to 2 nm. Denisov et al. already reported similar spectral broadening in a fiber amplifier seeded by a Q-switched fiber laser [28]. This effect was also explained by a self-phase-modulation based broadening model taking into account the presence of short stochastic pulses inside the incoherent nanosecond pulse generated by a Q-switched fiber laser [29]. Spectral broadening could be reduced by using a shorter fiber length for the high-power amplifier, which would require increasing the neodymium doping concentration. The temporal shape of the pulse envelope shown in Fig. 3(a) is nearly Gaussian with a FWHM pulse duration equal to 45 ns. The beam quality was measured with a camera beam profiler (Dataray WinCamD-LCM). The M2 beam quality was found to be 1.43 and 1.36 on each axis. The degradation of the beam quality, compared to the results in CW regime, is mainly attributed to the imperfect free-space signal injection. Nevertheless, free-space coupling could be avoided using a mode-field adapter (MFA) and other PM fiber components to replace the bulk isolator and the dichroic mirror.
Fig. 3. (a) Output spectrum of the oscillator and the amplifier laser emission, inset: temporal pulse shape; (b) M2 measurement, inset: beam image.
4. Frequency conversion
The infrared emission at 905 nm from the MOPA system was frequency converted successively in the blue at 452.5 nm (SHG) and in the DUV at 226.2 nm (FHG) using two single-pass cascaded nonlinear stages (Fig. 4.). Considering the high peak power of the incident IR pulse, periodically poled crystals were excluded because of the low threshold for photorefractive damage, despite the very large nonlinear coefficient (5-10 pm/V). Therefore, only two nonlinear crystals have been finally considered for SHG: bismuth borate (BiBO) and lithium triborate (LBO). BiBO has a large nonlinear coefficient (3.34 pm/V) but low angular acceptance and large walk-off angle. LBO has a lower nonlinear coefficient (0.8 pm/V) compared to BiBO and pp-KTP but presents higher damage threshold, large angular acceptance and low walk-off angle. The properties of the non-linear crystals available for IR-to-blue conversion are summarized in Table 1. LBO crystal was preferably selected because of its large spectral acceptance, which is a significant advantage considering the broad spectrum of the fiber laser source. In addition, the low walk-off angle can limit the beam degradation and subsequently improve the FHG efficiency. The choice for the FHG crystal is simplified by the fact that BBO is the only commercially available crystal allowing phase matching conditions from 452 nm to 226 nm conversion at room temperature. CLBO was not considered because of its high hygroscopicity and the need to cool it down to -15°C for FHG at this wavelength.
Table 1. Properties of nonlinear crystals for SHG from 905 nm to 452 nm
The LBO is a 5 × 5x20 mm crystal cut for type I SHG at θ = 90° and φ = 22.2°, with AR coatings at 910 nm and 455 nm. The output beam from the 30 µm fiber core (MFD ∼26 µm) was collimated using an aspheric lens with a 15 mm focal length. A half-wave plate was used to adjust the angular orientation of the linear polarization with respect to the optical axes of the LBO crystal for phase-matching condition. The collimated beam was then focused with a spherical lens (40 mm focal length) into the LBO crystal. The focused beam sizes in the LBO, estimated to 2w0x=74 µm and 2w0y=60 µm at the waist, were optimized during experiments in order to generate a maximum blue power. A 100 mm focal lens was used to collimate the beam after the LBO and the unconverted IR beam was filtered out with a dichroic mirror. From the SHG process, we measured a maximum average power of 4.9 W at 452 nm, corresponding to a conversion efficiency of 20% (Fig. 5). The measured spectral linewidth for the blue emission is 1.5 nm FWHM, mainly limited by the spectral acceptance of the 20 mm long LBO crystal. It also shows that the SHG conversion efficiency is strongly reduced by the broad emission spectrum of the fundamental IR beam. The blue output power is quadratic versus the incident power at 905 nm. A degradation of the beam quality was observed after the SHG, with an M2 factor of the blue beam measured equal to Mx2 = 2.7 and My2=3.6. We attribute the beam degradation to the walk-off inside the LBO crystal combined to the tight focusing necessary to compensate the broad spectrum of the fundamental IR beam.
Fig. 5. SHG power at 452 nm versus incident power at 905 nm (a) and M2 measurement of the blue beam (b). Insets: blue beam profiles near the waist and in far field.
A half-wave plate was added between the two conversion stages to rotate by 90° the polarization state in order to partially compensate the walk-off in the BBO crystal [7]. The blue beam was focused into a BBO crystal with a 40 mm focal lens. The BBO crystal is a 4 × 4x8 mm type I, cut at θ = 61.9° and φ = 90° with AR coatings at 455 nm and 227 nm. The residual blue and UV beams were separated using a Pellin-Broca prism made with UV grade fused silica. The FHG generated up to 510 mW of DUV power for the maximum incident blue power of 4.9 W, representing a 10% conversion efficiency.
The conversion curve of the FHG stage and the spectrum of the DUV beam are given in Fig. 6. The linewidth of the DUV laser emission is close to 0.5 nm, hence slightly reduced due to the low spectral acceptance of the BBO crystal in the DUV. The pulse duration was shortened to 37 ns, which gives a peak power of 370 W. The power stability at 226 nm was monitored for 90 minutes and the result is presented in Fig. 7. The RMS value was 0.48 W with +/-6% fluctuations. After adjusting the position of the BBO for optimal DUV output power, a slow decrease is observed for 30 min, which is mainly attributed to thermal stabilization rather than photo-degradation of the BBO crystal. Indeed, we did not observed irreversible or long-term degradations of the BBO crystal during experiments. In addition, a simple adjustment of the angular phase matching of the BBO crystal allows recovering the initial DUV power.
Fig. 6. FHG power at 226 nm versus incident blue power at 452 nm (a) and spectrum of the DUV output (b). Inset: DUV far field beam profile.
5. Conclusion
In conclusion, we report a pulsed DUV laser source at 226 nm using FHG from a MOPA laser system based on PM-LMA Nd-doped fibers. This is to the best of our knowledge the first demonstration of a frequency-quadrupled Nd-doped fiber laser emitting > 0.5 W in the deep-UV. The MOPA laser system at 905 nm consisted of an actively Q-switched fiber oscillator followed by a power amplifier based on a novel low-NA 30 µm core PM Nd-doped fiber. Up to 24 W of average power was obtained at a repetition rate of 40 kHz, with pulse duration of 45 ns. The highest average power achieved after SHG in a LBO crystal was 4.9 W at 452 nm. Moreover, FHG in a BBO crystal permitted to obtain a stable average power of 0.48 W (with a maximum of up to 0.51 W) at 226 nm. With appropriate passive fiber components, this DUV source has the potential of being compact and robust. In addition, DUV wavelengths between 220 nm and 235 nm could be easily accessed by tuning the Nd-doped fiber laser. Future work will focus on a Nd doped fiber laser emitting around 900 nm in CW single-frequency regime for FHG in the DUV.
Funding
Agence Nationale de la Recherche (ANR-10-LABX-09-01, ANR-19-CE24-0029).
Acknowledgments
The authors acknowledge financial support from the French National Research Agency (ANR) in the frame of “the investments for the future” (ANR-10-LABX-09-01) and NEODUV (ANR-19-CE24-0029).
Disclosures
The authors declare no conflicts of interest.
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