Widely tunable ultraviolet C generation using wavelength selective external high-q-cavity and a blue laser diode system C. Tangtrongbenchasil a and K. Nonaka b a Department of Electronic and Photonic Systems Engineering, b Frontier Engineering Course, Kochi University of Technology, Tosayamada, Kami City, Kochi Prefecture 782-8502, Japan ABSTRACT This paper presents a second harmonic generation scheme using a nonlinear optic crystal and a multimode Fabry-Perot blue laser diode that has potential to generate widely tunable coherent deep ultraviolet at approximately 225 nm. Using the Fabry-Perot multimode laser diode with the sum-frequency technique, a high second harmonic power is hardly observed due to low conversion efficiency. In this paper, an approximately 1 µw second harmonic ultraviolet power at around 225 nm ultraviolet wavelength and approximately 6 nm ultraviolet wavelength tunability can be obtained using a multi mode blue LD chip, a nonlinear optic crystal, and an external high-q-cavity setup. Keywords: second harmonic generation, nonlinear optic crystal, wide wavelength tunability, external high-q-cavity, second harmonic conversion efficiency improvement 1. INTRODUCTION Coherent deep ultraviolet C (UV-C) approximately 220 nm is very useful for nano-focus beam applications such as beam lithography for very large scale integrated circuit (VLSI) and molecular spectroscopy. Conventional coherent UV sources in these wavelengths are excimer lasers and sum frequency from solid state lasers that can generate high power 1) but these lasers have very large bodies, complex structures, fixed wavelength, high manufacturing costs, and high maintenance costs. Due to these disadvantages of these UV sources, compact, simple to fabricate, cost effective and coherent wavelength flexible UV sources are desired. External cavity diode lasers (ECDLs) and nonlinear optic crystals for second harmonic generation (SHG) researches have been reported for these solutions. However, the generation of a short wavelength around 220 nm was reported with a very low efficiency and a complex setup, due to insufficient laser diode (LD) power, oscillation quality, and crystal conversion efficiency 2-6). In, this paper, a simple and more efficient method to generate coherent deep UV around 225 nm with widely single mode wavelength tunability is presented. In addition, mathematical model of a SH conversion efficiency improvement is also discussed. 2. UV-C SECOND HARMONIC GENERATION DESIGN AND OPTIMIZATION Simple SHG mathematical estimations when uniform beam is employed were published in ref. 7 and 8. Boyd and Kleinman published a SHG mathematical model including phase mismatch factor, focal position factor, strength of focusing factor, birefringence factor, and absorption factor, that is suitable to estimate the SH power when focusing Gaussian beam is employed 9). In this paper, a BBO nonlinear optic crystal and a 450 nm fundamental wavelength multimode Fabry-Perot blue LD were implemented to generate widely tunable coherent deep UV-C. The shortest usable wavelength of the BBO crystal is 205 nm, due to phase matching angle limitation of fundamental and SH waves 10). Using Sellmeier s equations 8, 9), operating refractive index, phase matching angle, walk-off angle, and effective conversion coefficient can be theoretically obtained. The optimized SH output power ( P 2ω ), when phase mismatch = 0, focal position is at the center of the BBO crystal, and no absorption, can be calculated as 9), 096401e@gs.kochi-tech.ac.jp; Phone +81 887-57-2106; Fax +81 887-57-2299 International Workshop and Conference on Photonics and Nanotechnology 2007, edited by Preecha P. Yupapin, Wicharn Techitdheera, Proc. of SPIE Vol. 6793, 679306, (2008) 0277-786X/08/$18 doi: 10.1117/12.798952 2008 SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol. 6793 679306-1
2ω d k 1 e P = P L dτ dτ, (1) 2 2 ξ ξ 2 2 [ β ( τ τ ) ] 2 eff F 2ω ω 2 3 i( τ τ)] πε onofneuvc 4ξ ξ ξ e where P ω is fundamental power [W], L is crystal length [m], eff 1.022 10 12 m V at 450 nm 12 As fundamental wavelength, k F is fundamental propagation constant, ε 0 is Planck s constant = 8.854 10 Vm, c is light speed in free space = 3 10 8 [ m s ], n of is 445 nm ordinary fundamental wave refractive index = 1.683, n euv is 225 L nm extraordinary SH wave refractive index = 1.653, b is confocal parameter, ξ is strength of focus =, β is b 1 ρ birefringence parameter =, and ρ is walk off angle [radian]. ξ 2 LkF d is conversion efficiency [ ] β = 0 if and only if ρ = 0 that is invalid at 445 nm fundamental wavelength which has ρ = 0.073 radian, so β = 14.05 radian at 445 nm fundamental wavelength. Moreover, effective focal length, which is a very important factor to optimized the SH conversion efficiency, is required to estimate but it is not included in (1). The effective focal length can be estimated by 10), L eff πb 2π nof f = =, (2) 2 w c where f is operating focal length and w c is beam radius of the collimated input beam. The effective focal length is directly proportional to the confocal parameter. Equation 2 implies that there is an optimum effective focal length of any arbitrary confocal parameter depending on the focal length of focusing lens. By the symmetry of focusing and defocusing with the identical focal length of focusing lenses, the optimized crystal length is consequently equal to 2L eff. In addition, the confocal parameter is also directly proportional to the operated focal length, so longer focal length requires longer crystal length to optimize the conversion efficiency. Narrow wavelength tolerance or single longitudinal mode oscillation, on the other hand, is one of requirements to realize theoretical SHG efficiency. Thus the wavelength tolerance must be control as narrow as possible. The UV wavelength tunablility depends on the angle and position of feedback fundamental light through LD. To stabilize fundamental wavelength as single mode oscillation, transmission grating and flat mirror are implemented as fundamental wavelength selection and enhanced high-q ECDL, respectively. The feedback angle of fundamental light must be set as close as possible to the polarization plane of LD, so that narrow single mode fundamental wavelength can be realized. To tune fundamental wavelength, the position shift with respect to the orthogonal of polarization plane must be tuned. However, narrowing wavelength tolerance can cause phase mismatch, adjusting the nonlinear optic crystal can overcome this problem. The difference position and angle of feedback fundamental light can cause mode suppression of fundamental light because of the employed LD in this paper is multimode Fabry-Perot LD. Consequently, fundamental power of nearby wavelength is decreased, due to mode suppression. In addition, the limitation of wavelength tuning range is profile of gain of LD waveguide. 3. EXPERIMENTAL SETUP Fig.1 shows a setup of widely tunable 225 nm UV-C generation using SHG scheme with multimode Fabry-Perot blue LD. In this paper, the approximate 225 nm SH wavelengths were generated using a multimode Fabry-Perot 450 nm blue LD that operated in continuous wave (CW) oscillation with Gaussian beam profile. The blue LD launched the collimating beam profile having the effective parallel and perpendicular beam axes are 3.5 mm and 1.5 mm, respectively. The beam was focused at the center of 10 mm length BBO crystal by a 100 mm plano-convex lens. The output radiation from BBO which consisted of 450 nm fundamental wavelength and 225 nm SH wavelength were, consequently, collimated by another 100 mm plano-convex lens to obtain the similar collimating beam profile as launching from multimode blue LD. Then, the 450 nm fundamental wavelength continuously propagated to a transmission grating setting at 450 nm fundamental wavelength Brewster angle of 60º with respect to incident wave and reflected back by a 100% reflection for 440 nm wavelength flat mirror for wavelength selection, wavelength stabilizer, and to enhance an external high-q ECDL. The 450 nm fundamental wavelength passing through the transmission grating was split into 0 nd order and 1 st order by 5% and 95% power of incident wave, respectively. The 0 nd order wave was for monitor the stability of the 450 nm fundamental wavelength. If the 450 nm fundamental wavelength was single mode and stable, the 225 nm wavelength was consequently single mode and stable. In order to complete the external Proc. of SPIE Vol. 6793 679306-2
cavity of the system, the 1 st order wave from transmission grating must be reflected by the 100% reflection for 440 nm flat mirror back to the blue LD for enhancement of the 450 nm fundamental power. The flat mirror is not only for constructing the external high-q-cavity but also tuning and stabilizing oscillation wavelength. Adjusting the angle of the 100% reflection for 440 nm flat mirror can stabilize and tune fundamental and SH wavelengths. When position and reflected angle of 100% reflection for 440 nm flat mirror were properly set, the resonant condition was enhanced. Consequently, the fundamental power was increased, due to cavity enhancement. Oscillation wavelength of ECDL using feedback mirror was determined by optical cavity length of an external cavity, a LD cavity length, and a gain band profile. Thus, external cavity can be constructed using temperature controller to control gain spectrum of LD and wavelength selecting filter (transmission grating) to select and to stabilize the feedback wavelength 11-16). However, optical phase fluctuation in the external cavity by the effect of airflow and thermal fluctuation cause slight oscillation wavelength shift. UV Generation Power Meter - -L4- - Movement Direction for wavelength tuning FM 1st de DM LENS BBO l-2 sof445nm TEC Controller LDDriver 0th order,,, TG UV uv of445nm LENS: 100-mm focusing length DM: 220 nm dichroic mirror TG: transmission grating FM: 440 nm flat mirror OSA: optical spectrum analyzer PMT: photomultiplier tube P: prism TEC: thermo-electric cooler Fig. 1. A bi-directional detection setup of widely tunable 225 nm UV-C generation using SHG scheme with multimode blue LD. The polarizations of fundamental wavelength and SH wavelength differ by 90º. To separate the 450 nm fundamental wavelength and 225 nm SH wavelength, two dichroic mirrors were placed before and after plano-convex lenses (see Fig.1). The dichoic mirror reflected the 225 nm SH wavelength while the dichoic mirror was transparent for 450 nm fundamental wavelength. In practice, the reflected the 225 nm wavelength from the dichroic mirror always contains a few percent of the 450 nm wavelength even the dichroic mirror is exactly placed at the Brewster angle, so the reflected wavelength can be perfectly separated by prism that was set to Brewster angle for the 225 nm SH wavelength transparency obtain purely 225 nm coherent deep UV-C. On the other hand, beam waists at the center of BBO crystal were equal to 37.35 µm and 16 µm for parallel and perpendicular axis, respectively. The maximum average power of the fundamental wavelength inside the cavity was 103 mw, consequently, a 17.24 kw/cm 2 excitation is expected at around focus region. 5.0 db/d 443 nm 449mn 1.5OnnVD 455nm Fig. 2. Single mode fundamental spectrums of multimode blue LD with an external high-q-cavity. Proc. of SPIE Vol. 6793 679306-3
4. EXPERIMENTAL RESULTS When the high-q-cavity was enhanced, the fundamental wavelength was stabilized to single mode wavelength, the fundamental was also increased and spectrum width ( λ) was approximately less than 0.08 nm as shown in Fig.2. Consequently, single mode coherent deep UV-C lights were also observed. Fig.2 shows single mode spectrums of multimode blue LD with a high-q ECDL. The central oscillation fundamental wavelength of this system was approximately 448.9 nm with the spectrum width was 0.08 nm (see Fig.2). The maximum fundamental power inside the cavity at 448.9 nm was measured by optical power meter to be equal to 103.30 mw. The shortest and the longest fundamental wavelengths, which were 443.9 nm and 455.4 nm, had the spectrum widths that were equal to 0.04 nm and 0.05 nm, respectively. The maximum fundamental powers at the shortest and longest wavelengths were reduced; even the external high-q-cavity cavity was enhanced, due to the mode suppression inside LD waveguide. At the shortest and longest wavelengths, the maximum fundamental powers were reduced to approximately 62.5 mw. The setup as shown in Fig.1 is able to detect SH power for 2 directions, which were named as forward detection and backward detection. The forward detection was located between plano-convex lens and transmission grating. The backward detection was located in front of LD mount. By this setup, when the angle and position of feedback light from the 440 nm flat mirror were properly set, the SH power is able to enhanced without changing the nonlinear optical crystal angle which is for optimized SHG scheme. The maximum fundamental power inside the cavity at 448.9 nm was equal to 103.30 mw resulting 0.67 µw and 0.34 µw of the maximum forward detection and the maximum backward detection of SH power, respectively. So, the maximum total detection of SH power was equal to 1.01 µw. Using bidirectional detection technique, an approximately 50% of SH power was obtained. Consequently, the total SH conversion efficiency of this setup was equal to 0.98 10 3 % while the SH conversion efficiency of single directional forward detection was only 0.65 10 3 %. Fig.3 shows an experimental result of SHG at 448.9 nm and the variation of fundamental power vs. fundamental wavelength. The simulation of SH power forward detection was also shown in Fig.3 that implied our compact setup satisfied following the Boyd and Kleinmann estimation model as shown in eq.(1). However, there are some losses; scattering loss, absorption loss, etc. The total SH powers of shortest and longest wavelengths in this system were obtained as 0.101 µw and 0.104 µw, respectively. Fig. 3. An experimental result of SHG at 448.9 nm and the variation of fundamental power vs. fundamental wavelength. On the other hand, the actual nonlinear conversion is slightly different from ideal. In practice, the conversion efficiency is governed by the phase mismatch factor. Phase mismatch factor is a function of crystal temperature, frequencies of the interaction waves, and deviation from phase matching angle. To optimize the conversion efficiency, the total phase mismatch factor must be minimized; 1) directly applying the temperature controller to the BBO crystal to suppress the crystal temperature mismatch, 2) adjusting the proper angle of the BBO crystal for a certain operating wavelength, and using the appropriate focal length when the focusing beam is employed to reduce the crystal angle mismatch, 3) narrowing and tuning the spectrum of input LD oscillation using a wavelength selective and an external high-q-cavity system to suppress the frequencies mismatch. Consequently, the fundamental input power was enhanced by the external high-q-cavity system and the operating wavelengths were single mode and stable. In addition, the crystal temperature mismatch and the crystal angle mismatch can be completely controlled as mentioned above but the frequencies Proc. of SPIE Vol. 6793 679306-4
mismatch cannot be completely controlled to get the ideal single mode wavelength due to the slightly spread of wavelength tuning. 5. DISCUSSIONS AND CONCLUSIONS The maximum SH power obtaining from this setup was equal to 1.01 µw at 448.9 nm fundamental wavelength or 224.45 nm SH wavelength. The 3-dB wavelength band, which was between 445 nm to 453 nm, had the SH power approximately to be equal to 0.7 µw. This system improved the SH wavelength tunabilty to be approximately 6 nm from 221.5 nm to 227.5 nm for UV-C and 443 nm to 453 nm for fundamental oscillation. However, the SH power decrease to an approximately 0.1 µw due to mode suppression causing fundamental power reduction. In order to improve the SH output power, the increasing of crystal length or longer interaction length with higher confocal parameter are required. For example, if the 100 mm focal length lens is used to focus the fundamental beam to the center of BBO nonlinear optic crystal, the most effective crystal length would be 105.5 mm or 2 time of effective focal length (see eq.(2)) that would produce SH power approximately 7.09 µw for forward detection. Consequently, the SH power of backward detection would approximately be 3.5 µw. In summary, using the multimode blue LD with widely wavelength tunable and single mode oscillation external high-qcavity with BBO crystal encouraging maximum SH power at 1.01 µw coherent deep UV-C at 224.45 nm SH wavelength was successfully observed. The SH wavelength tunabiltiy was improved to be wider as 6 nm in this system. The conversion efficiency was improved to be 0.98 10 3 % at 224.45 nm fundamental wavelength, when fundamental input power is 103.30 mw, due to bi-direction detection technique. The experimental results of this region were well matched to the Boyd and Kleinmann model estimation. To improve the SH conversion efficiency, the increasing of confocal parameter and the crystal length are extremely required. However, there is a trade-off between system size and conversion efficiency. If the high conversion efficiency is required, the optical system size must be increased. In contrast, if the compactness is required, the conversion efficiency is low. On the other hand, the main parameters; phase matching angle, walk-off angle, and effective coefficient must be carefully controlled due to very slightly change of these parameters cause suddenly decreasing of SH efficiency. This paper showed the sufficient of wavelength tunability and compactness comparing with the conventional excimer and YAG laser. Moreover, this system has potential to focus and achieve the higher power density than bulk laser at the selected area. REFERENCES [1] W. L. Zhou, Y. Mori, T. Sasaki, and S. Nakai, Optics Communications 123, pp. 583-586, 1996. [2] NICHIA Corp., Ultra Violet Laser Diode, NDHU110APAE2. [3] K. Ohara, M. Sako, and K. Nonaka, 210 nm ultraviolet generation using blueviolet laser diode and BBO SHG crystal, CLEO Pacific RIM conference, Taipei, 2003. [4] K. Ohara K. Nonaka, and P. Vesarach, 0.2 µm Deep UV Generation using 0.4 µm Blue Laser Diode with Wavelength Tunable Cavity, CLEO Pacific RIM conference, Tokyo, 2005. [5] C. Tangtrongbenchasil, K. Ohara, T. Itagaki, P. Vesarach, and K. Nonaka, 219-nm Ultra Violet Generation Using Blue Laser Diode and External Cavity, Japanese Journal of Applied Physics, Vol. 45, No. 8A, pp. 6315 6316, 2006. [6] C. Tangtrongbenchasil, K. Nonaka, and K. Ohara, 220-nm Ultra Violet Generation Using an External Cavity Laser Diode with Transmission Grating, MOC 2006, Sep. 2006, Seoul, Korea, Vol. 2, pp. 5 8. [7] D.L. Mills, Nonlinear Optics: Basic Concepts, 2 nd edition, ed. (Springer, New York, 1998). [8] V. G. Dmitriev, G. G. Gurzadyan, and D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer Series in Optical Sciences Volume 64. [9] G. D. Boyd and D.A. Kleiman, Parametric Interaction of Focused Gaussian Light Beam, Journal of Applied Physics, Vol. 39, No.8, pp 3597 3639, 1968. [10] CASIX Co., Ltd., Product Catalog 2004. [11] Mark W. F. and Adam M., Spectral Characteristics of External-Cavity Controlled Semiconductor Lasers, IEEE J. Quantum Electron, Vol. QE-17, No. 1, pp. 44-59, 1981. [12] T. Laurila, T. Joutsenoja, R. Hernberg, and M. Kuittinen, Tunable external-cavity diode laser at 650 nm based on a transmission diffraction grating, Applied Op., Vol. 41, No. 27, pp. 5632-5637, 2002. Proc. of SPIE Vol. 6793 679306-5
[13] H. Patrick and C.E. Wieman, Frequency stabilization of a diode laser using simultaneous optical feedback from a diffraction grating and narrowband Fabry-Perot cavity, Rev. Sci. Instrum., Vol. 62, No. 11, pp. 2593-2595, 1991. [14] A. Wicht, M. Rudolf, P. Huke, R. Rinjkeff, and K. Danzmann, Grating enhanced external cavity diode laser, Appl. Phys. B, 2003. [15] M. W. Flemming and A. Mooridian, Spectral Characteristics of External-Cavity Controlled Semiconductor Lasers, IEEE J. Quantum Electron, Vol. QE-17, No. 1, pp. 44-59, 1981. [16]K. Hayasaka, Frequency stabilization of an extended-cavity violet diode laser by resonant optical feedback, Optics Comm. 206, pp. 401-409, 2002. Proc. of SPIE Vol. 6793 679306-6