DEVELOPMENT OF HIGH-POWER PICOSECOND FIBER-BASED ULTRAVIOLET SOURCE

MSc in Photonics Universitat Politècnica de Catalunya (UPC) Universitat Autònoma de Barcelona (UAB) Universitat de Barcelona (UB) Institut de Ciències Fotòniques (ICFO) PHOTONICSBCN http://www.photonicsbcn.eu Master in Photonics MASTER THESIS WORK DEVELOPMENT OF HIGH-POWER PICOSECOND FIBER-BASED ULTRAVIOLET SOURCE Josep Canals Casals Supervised by Dr. S. Chaitanya Kumar (ICFO) and Prof. M. Ebrahim-Zadeh (ICFO) Presented on date 8 th September 4 Registered at

Development of high-power picosecond fiber-based ultraviolet source Josep Canals Casals Optical parametric oscillators Group, ICFO - Institut de Ciències Fotòniques, Av. Carl Friedrich Gauss, 3, 886 Castelldefels, Barcelona, Spain. E-mail: jcanalscasals@gmail.com Abstract. We report a high-power, picosecond ultraviolet (UV) source at 66 nm in a compact design based on single-pass fourth harmonic generation (FHG) and mode-locked Yb-fiber laser. The configuration has two stages: second harmonic generation (SHG) of 64 nm to 53 nm based on crystal followed by SHG of 53 nm to 66 nm based on crystal. The obtained power is 55 mw at 8 MHz for a fundamental average power of 5 W at a green to UV conversion efficiency.5%. The long term UV power stability is recorded to be 4.65% rms in TEM spatial mode profile. Keywords: non-linear optics, frequency conversion, harmonic generation, non-linear materials, fiber lasers, visible lasers, UV lasers. Introduction Since the demonstration of the first working laser by Maiman in 96 [], major research in non-linear optics was initiated and several non-linear processes could be observed such as the second harmonic generation (SHG) which was demonstrated in 96 by Franken et al []. The coherent ultra-fast optical sources in the ultraviolet (UV) are of great interest for a variety of applications including quantum optics, optical data storage, atmospheric sensing, combustion diagnostics and bio-imaging [3]-[4]. The access to this spectral region has been achieved using bulky, complex and power-hungry gas lasers such as excimer lasers. Semiconductor lasers as diode laser can also achieve coherent light in this spectral region but with low quality beam and with low power. Hence, the development of high-repetition-rate ultra-fast sources in the UV at practical average power and efficiency, in simple, compact, cost-effective and practical architecture remains challenging. Non-linear frequency conversion techniques represent a potentially viable and effective approach to the direct generation of ultra-fast radiation in UV applying frequency doubling tripling or quadrupling of a pulsed laser with a suitable non-lineal crystal. Earlier reports demonstrate a 355 nm source generation at low repetition rate 5 Hz using a third harmonic generation (THG) of mode-locked and amplified picosecond Nd:YAG laser and a crystal [5]. Efficient generation of tunable pulses from 375

Development of high-power picosecond fiber-based UV source to 435 nm are also shown using a mode locked Ti:sapphire laser at 76 MHz and a crystal in femptosecond regime and in picosecond regime [6]-[7]. Here we report a picosecond UV source at 66 nm, with 55 mw output power in a compact design based in two stages. Firstly, SHG of Yb-fiber laser using a mm length crystal is obtained as in [8]. Secondly, fourth harmonic generation (FHG) of the fundamental fiber laser beam is obtained with a SHG process from the generated green at 53 nm to ultraviolet at 66 nm using a 5 mm length crystal. Single-pass configuration is used recording long term stability of 4.65% rms over 7 minutes and a TEM spatial profile.. Basics of non-linear optics Non-linear optics is the study of the phenomena derived from the modification of the optical properties of a material system by the presence of light. When a electromagnetic radiation Ẽ(t) is applied to the material, the electrons which are inside are displaced with respect to the nuclei and the system is thus polarized. This polarization P (t) is a function of the field P = P (Ẽ), and can be expanded in a power series in terms of it, following () from [9]. ] P (t) = ɛ [χ () Ẽ(t) + χ () Ẽ (t) + χ (3) Ẽ 3 (t) +... P (t) () + P (t) () + P (t) (3) +... () The constant of proportionality χ (n) is known as the susceptibility and ɛ is the permittivity of free space. In general χ (n) is a tensor and it is independent on frequency ω of the optical radiation. In the linear optics case, the induced polarization depends linearly on the electric field strength and only the first term P (t) () in () is considered. In nonlinear optics case, the electrical field strength is comparable to intra-atomic electric field and the optical response of the matter needs to consider all terms in (). In this work we use SHG, which only involves second-order non-linear susceptibilities χ () and two input beams with ω = ω = ω in (). Ẽ(t) = Ẽ(t)e iω t + Ẽ(t)e iω t With this input beam, the second order polarization can be expressed as (3) where a term at frequency ω called second harmonic or SH beam and other independent of ω called optical rectification can be observed. P (t) () = ɛ χ () Ẽ(t) = ɛ χ () E e iωt + ɛ χ () E E (3) (). Second harmonic generation (SHG) and fourth harmonic generation (FHG) SHG is a process in which a beam at frequency ω interacts with matter to generate an output beam at ω. If this beam at frequency ω interacts with other non-linear crystal creating other SHG process, the output beam has a frequency at 4ω. Thus, in figure the block diagram of FHG process using two stages of SHG interactions is represented. The efficiency of this process depends on several parameters such as the non-linear optical coefficient of the crystal, the phase-matching angle in a certain optical plane, the

Development of high-power picosecond fiber-based UV source 3 4 χ () χ () SHG SHG FHG Figure : Block diagram of FHG using two SHG processes. length of the non-linear crystal, the spectral and angular acceptance bandwidth or the spatial walk-off... Phase-matching angle and effective non-linear coefficients The phase-matching direction for a yz plane and its corresponding effective nonlinear coefficient d eff to have SHG and FHG are calculated using the Sellmeier equations in []-[] for and respectively and the refractive index in each axis are represented as a function of the wavelength in figure (a) for and figure (b) for. 64 nm 53 nm 53 nm 66 nm Refractive Index..9.8 (a) Transparency range.9 μm -.5 μm n x n y n z Refractive Index 3.8.6.4 (b) Transparency range.96 μm -. μm n o n e.7 3 4 Wavelegth ( m). 3 Fundamental 3 (c).64 68.9.5.5 4 6 8 SHG FH. (d).53 47.4 4 6 8.5.5 SH Figure : Refractive index as a function of the wavelength for (a) and (b) and phase-matching angle as a function of the input and output wavelength in (c) and (d). The shadowed areas show the transparency range of (a) and (b).

Development of high-power picosecond fiber-based UV source 4 The θ angle represents the polar angle relative to the optical z-axis and φ represents the azimuthal angle measured from x-axis. The polarization direction normal to the plane is called ordinary o and the other allowed direction parallel to the plane is called extraordinary e. is a positive biaxial crystal with type I (eeo) interaction and is a negative uniaxial crystal with type I (ooe) interaction. In yz plane, φ = 9 and angle θ is that satisfy the phase-matching condition in (4), k ω + k ω = k ω πn ω(θ) λ ω + πn ω(θ) λ ω = πn ω λ ω n ω (θ) = n ω (4) where n ω and n ω are the refractive index at ω and ω, λ ω and λ ω are the fundamental and second harmonic wavelength and k ω and k ω are the wave vectors at ω and ω respectively. The refractive index of the e polarized wave depends on the angle θ whereas the refractive index of the o does not. The refractive index expression of the e polarized wave in yz plane in type I (eeo) interaction as function of θ is (5). n ω(θ) = cos (θ) + sin (θ) n ω,y n ω,z The phase angle θ pm as a function of the wavelength is represented for and crystals in figure (c) and figure (d). In figure 3(a) and 3(b) is graphically solved the phase-matching condition in (4) for and crystals. (5) 64 nm 53 nm 53 nm 66 nm n( ).95 (a) n e = n(θ, n,y, n,z ).9.85 λ=64 nm.9º 69º.8 n o = n,x n( ).8.75.7.65 (b) λ=53 nm n e = n(θ, n,x, n,z ) n o = n,x 47º 3º.75 5 5.6 5 5 d eff (pm/v) 4 3 (c) Φ=9º, 8º-θ Φ=º, θ.64 3.36..65.9.. d eff (pm/v).6.4. (d).4.53 Figure 3: Phase-matching condition solution for (a) and (b) and effective non-linear coefficient in (c) and (d).

Development of high-power picosecond fiber-based UV source 5 Comparing the results, can be observed that both satisfy the same phase-matching angle for the given wavelength. Since there are two possible solutions, we take the phasematching angle in which its effective non-linear coefficient is larger for the same crystal length which is θ = 68.9 for and θ = 47 for. The effective non-linear coefficient for these phase-matching angles in the working wavelength are 3.36 pm/v in and.4 pm/v for using the relevant formulae in []-[]. They are represented as a function of the wavelength in figure 3(c) and figure 3(d) for and respectively... Angular and spectral acceptance bandwidths Angular and spectral acceptance bandwidth are defined as the tolerance of phasematching to the spatial and spectral spread of the input beam. They have a huge effect in the conversion efficiency and output power of the crystal. They are calculated representing the normalized efficiency curve as function of the angle θ and the input wavelength respectively and calculating the full width at half maximum (FWHM) of the resulting sinc function. Hence, the angular acceptance bandwidth for SHG is.3 mrad and.7 mrad for FHG both represented in figure 4(a) and figure 4(b) respectively. On the other hand, the angular acceptance bandwidth for SHG is.97 nm and.74 nm for FHG and they are represented in figure 4(c) and figure 4(d). All these calculations are done considering a and crystal length of mm...3 Spatial walk-off The spatial walk-off causes angular separation between the orthogonally polarised o and e waves within the non-linear crystal reducing the efficiency of the process. To characterize this effect the angular walk-off is represented as a function of the input wavelength for SHG and FHG in figure 4(e) and figure 4(f) respectively. 3. Experimental set-up The schematic of the high power picosecond fiber-based UV source is shown in figure 5. The primary pump source is an Yb-fiber laser providing up to W of average power at 64 nm in pulses of ps duration at 8 MHz repetition rate. 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 a combination of a half-wave plate and a polarising beam-splitter as an attenuator are used. The other half-wave plate controls the polarization of the beam to achieve the optimus phase matching in the crystal. The crystal is 4 mm 4 mm mmlong, cut at θ = 68.9 and φ = 9 for type-i (eeo) phase-matching in the yz optical plane. The fundamental beam is focused with L at the center of the beam waist radius of ω 4 µm, corresponding to a focusing parameter of ξ 7. The dichroic mirrors M (R > 99% at 53 nm; T > 99% at 64 nm) separate and deliver the generated green from the fundamental IR. Two cylindrical lenses L and L3 with focal length f = 75 mm and f = 5 mm are used to circularize the green beam obtaining a ellipticity of.9. The second part of the experiment is composed for a 4 mm 4 mm 5 mm-long crystal, cut at θ = 47 and φ = 9 for type I (ooe) phase-matching in the yz optical plane.

Development of high-power picosecond fiber-based UV source 6 64 nm 53 nm 53 nm 66 nm SH (a).3 mrad L= mm SH (b).7 mrad L= mm.. 68.6 68.7 68.8 68.9 47.35 47.4 47.45 47.5 SH (c) L= mm.97 nm SH (d) L= mm.74 nm.. Walk-off angle (mrad) 6 5 4 3.6.64.66 (e) 5.64.64.9.. Walk-off angle (mrad).536.538.53.53 9.7 9 8 7 6.53 (f) 5.5.7 Figure 4: Theoretical calculations of angular acceptance bandwidth in (a) and (b), spectral acceptance bandwidth in (c) and (d) and walk-off angle in (e) and (f). The output beam is focused with L4 at the center of the crystal with a beam waist radius of of ω 3 µm, corresponding to a focusing parameter of ξ.5. The dichroic mirrors M (R > 99% at 66 nm; T > 99% at 53 nm) separate and deliver the generated UV from the green. A UV filter (FGUV- 5 mm) with high transmission in wavelength from 75 nm to 375 nm is finally used to separate FH beam from any residual SH beam.

Development of high-power picosecond fiber-based UV source 7 Figure 5: Experimental design of high-power picosecond fiber-based UV source. FI: Faraday isolator, λ/: Half-wave plate, PBS: Polarizing beam-splitter, L: Lens, M: Mirrors, F:UV filter 4. Results 4. SHG characterization 4.. Power scaling The SHG efficiency and the power scaling measurements for the SHG as a function of the fundamental power are represented in figure 6(a). As the input power is increased, the output power increases quadratically reaching a maximum SH power of 5.77 W for a fundamental power of 5 W. Hence, a single-pass SHG efficiency conversion of 38.% is obtained. Figure 6(b) shows the variation of the SH power as function of the square of fundamental power which is expected to be linear. 4.. Spectrum, stability and beam quality The SHG spectral measurements carried out in fundamental input power of 5 W is represented in figure 6(c). The long-term stability of the SH beam at an average power of 5.77 W at 53 nm is represented in figure 6(d). The SH generated has excellent passive stability of.7% rms over 8 minutes. The SH beam profile is measured at a distance 55 cm from the crystal with an output power of 5.4 W together with the intensity profile and represented in figure 6(e). The green beam presents an ellipticity of due to the spatial walk-off between the two beams with different wavelengths interacting in the crystal seen previously in figure 4(e). Using two cylindrical lens with f = 75 mm and f = 5 mm of focal length (L and L3 in figure 5) the green beam is circularized up to ellipticity of.9 as figure 6(f) to improve the spatial behaviour of the generated UV beam. 4. FHG characterization 4.. Power scaling The FH efficiency and the power scaling measurements for the FHG as a function of the SH power are represented in figure 7(a). As the input power is increased, the output power increases quadratically reaching a maximum FH power of 55 mw for a SH power of 5 W. Hence, a single-pass from green to UV efficiency conversion of.5%

Development of high-power picosecond fiber-based UV source 8 SH Power (W) 5 (a) 4 SHG Efficiency (%) SH Power (W) 8 6 4 (b) 5 5 Fundamental Power (W) 3 [Fundamental Power (W)] Intensity (a.u.). (c) SH Power (W) 6 4 (d) SH power stability:.7% rms 58 53 53 534 536 Wavelength (nm) 4 6 8 Time (min) (e) Ellipticity= Power=5.4 W (f) Ellipticity=.9 Figure 6: Experimental measurements of SHG: (a) power scaling and efficiency curve, (b) SH power variation as function of the square of the fundamental, (c) spectrum of the generated green beam, (d) long term stability over 8 minutes, spatial profile of the generated TEM beam, (e) without and (f) with circularization with cylindrical lenses. is reported. Figure 7(b) shows the variation of the FH power as function of the square of SH power which is expected to be linear. 4.. Spectrum, stability and beam quality The FHG spectral measurements carried out in fundamental input power of 5 W is represented in figure 7(c).The long-term stability of the FHG beam at average power of 55 mw at 66 nm is represented in figure 7(d) and is recorded to have a stability of 4.65% rms over 7 minutes. The FHG beam profile is measured at a distance 6 cm from the crystal with

Development of high-power picosecond fiber-based UV source 9 FHG Efficiency (%).5.5 (a) 6 4 FH Power (mw) FH Power (mw) 8 6 4 (b) 4 6 SH Power (W) 3 [SH Power (W)] 6 Intensity (a.u). (c) FH Power (W) 4 (d) FH power stability: 4.65% rms 64 65 66 67 68 69 Wavelength (nm) 4 6 Time (min) (e) Ellipticity=6 Power=7 mw (f) Figure 7: Experimental measurements of FHG: (a) power scaling and efficiency curve, (b) FH power variation as function of the square of the SH power, (c) spectrum of the generated beam, (d) long term stability over 7 minutes, (e) spatial profile of the generated TEM beam and (f) picture of experimental set-up in the laboratory where in the bottom-right corner of the picture infra-red beam crossing L and going inside the. Then the green beam can be observed in the middle of the figure crossing the generating the UV beam in the top-left corner of the image. an output power of 7 mw together with the intensity profile and represented in figure 7(e). The UV also presents an ellipticity of 6 due to the circularity of the green beam is not and there is also spatial walk-off in between the two beams with different wavelengths interacting in the crystals presented in figure 4(f). Finally, figure 7(f) shows a picture of the complete set-up in the laboratory showing the infra-red, green and UV beams interacting with and crystals.

Development of high-power picosecond fiber-based UV source 5. Conclusion In conclusion, we have demonstrated a high-power, picosecond UV source based on single-pass FHG of Yb-fiber laser. The set-up consists in a first stage based on mm length crystal from 64 nm to 53 nm followed with a second stage based on 5 mm length crystal from 53 nm to 66 nm. The generated average power is 55 mw at 66 nm at 8 MHz for a fundamental power of 5 W. The green to UV conversion efficiency is.5% and the long term UV power stability is recorded to be 4.65% rms over 7 minutes. This results could be improved by optimizing spatial mode-matching in the non-linear crystals, using a UV filter with maximum transmission in central wavelength of the spectrum and optimizing the transparency of the mirrors in green and UV wavelength. This technique represents a single, compact and practical approach to the development of ultra-fast source in the UV. [] T. M. Maiman, "Simulated optical radiation in ruby," Nature 87, 493-494, (96). [] P. A. Franken et al., "Generation of optical harmonics," Physical Review Letters 7 (4), 8-9, (96). [3] S. Khripunov et al., "Variable-wavelength second harmonic generation of CW Yb-fiber laser in partially coupled enhancement cavity," Opt. Express (6), 746-75, (4). [4] A. K. Jayasinghe et al., "Holographic UV laser microsurgery," Biomedical Opt. Express (9), 59-599, (). [5] M. Ghotbi et al., "Efficient third harmonic generation of microjoule picosecond pulses at 355 nm in BiB 3 O 6," Applied Physics Letters 89 (7), 734-734, (6). [6] M. Ghotbi et al., "High-average-power femptosecond pulse generation in the blue using BiB 3 O 6," Opt. Lett. 9 (), 53-53, (4). [7] M. Ghotbi and M. Ebrahim-Zadeh, "99 mw average power, 5% efficient, high-repetition-rate picosecond-pulse generation in the blue with BiB 3 O 6," Opt. Lett. 3 (4), 3395-3397, (5). [8] S. Chaitanya Kumar and M. Ebrahim-Zadeh, "High-power, fiber-pumped, picosecond green source based on BiB 3 O 6," Laser Physics 4 (), 54-54, (4). [9] S. Chaitanya Kumar and M. Ebrahim-Zadeh, "High-power, fiber-laser-pumped, picosecond optical parametric oscillator based on MgO: spplt," Opt. Express 9 (7), 666-6665, (). [] K. Miyata et al., "Phase-matched pure χ (3) third-harmonic generation in noncentrosymmetric BiB 3 O 6," Opt. Lett. 34 (4), 5-5, (9). [] D. N. Nikigosyan, "Beta barium borate ()," Applied Physics 5 (6), 359-368, (99). [] M. Ghotbi and M. Ebrahim-Zadeh, "Optical second harmonic generation properties of BiB 3 O 6," Opt. Express (4), 6-69, (4).