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High-power, continuous-wave, second-harmonic


December 15, 2008 / Vol. 33, No. 24 / OPTICS LETTERS

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High-power, continuous-wave, second-harmonic generation at 532 nm in periodically poled KTiOPO4
G. K. Samanta,1,* S. Chaitanya Kumar,1 M. Mathew,1 C. Canalias,2 V. Pasiskevicius,2 F. Laurell,2 and M. Ebrahim-Zadeh1,3
1

ICFO–Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain 2 Royal Institute of Technology, Roslagstullsbacken 21 10691 Stockholm, Sweden 3 Institucio Catalana de Recerca i Estudis Avancats, Passeig Lluis ompanys 23, Barcelona 08010, Spain *Corresponding author: goutam.samanta@icfo.es

Received October 7, 2008; accepted October 15, 2008; posted November 3, 2008 (Doc. ID 102494); published December 5, 2008 We report ef?cient generation of high-power, cw, single-frequency radiation in the green in a simple, compact con?guration based on single-pass, second-harmonic generation of a cw ytterbium ?ber laser at 1064 nm in periodically poled KTiOPO4. Using a crystal containing a 17 mm single grating with period of 9.01 m, we generate 6.2 W of cw radiation at 532 nm for a fundamental power of 29.75 W at a single-pass conversion ef?ciency of 20.8%. Over the entire range of pump powers, the generated green output is single frequency with a linewidth of 8.5 MHz and has a TEM00 spatial pro?le with M2 1.34. The demonstrated green power can be further improved by proper thermal management of crystal heating effects at higher pump powers and also by optimized design of the grating period to include thermal issues. ? 2008 Optical Society of America OCIS codes: 140.7300, 140.3510, 190.2620, 190.4400.

Quasi-phase-matched (QPM) nonlinear materials have had a major impact on optical frequencyconversion technology. In particular, the advent of periodically poled LiNbO3 (PPLN) has enabled the development of a new generation of devices with unprecedented performance capabilities. At the same time, the onset of photorefractive damage under exposure to visible radiation has con?ned the utility of PPLN mainly to the IR. Doping with MgO can partially alleviate this problem, permitting the use of MgO:PPLN at higher powers and lower temperatures [1,2]. However, residual effects of photorefractive damage continue to restrict operation of devices based on MgO:PPLN mainly to the near- and mid-IR. For frequency conversion in the visible, QPM materials such as MgO-doped periodically poled stoichiometric LiTaO3 (MgO:sPPLT) [2,3] and periodically poled KTiOPO4 (PPKTP) [2,4–7] are attractive alternatives, offering increased resistance to photorefractive damage and relatively high effective nonlinearities deff 10 pm/ V . Advances in material growth and fabrication have also led to the availability of such QPM materials with high optical quality, shorter grating periods, and long interaction lengths (up to 30 mm), enabling the development of frequency-conversion devices for the visible in the cw [2,4–6] and low-intensity pulsed regimes [7]. In this Letter, we report ef?cient generation of high-power cw radiation at 532 nm in a compact and practical design based on simple single-pass, secondharmonic-generation (SHG) of a cw ytterbium (Yb) ?ber laser in PPKTP. Earlier reports of cw SHG in the visible based on PPKTP include intracavity [4], external enhancement [5], and single-pass con?gurations [6], but the highest cw SH power achieved so far has been 2.3 W in multimode output [4]. As such, power scaling of SHG and performance of the material at in0146-9592/08/242955-3/$15.00

creased powers has not been reported, to our knowledge. Here, we demonstrate the generation of 6.2 W of cw, single-frequency radiation at 532 nm with a single-pass conversion ef?ciency as high as 20.8%. We also investigate thermal issues relating to power scaling of SHG to fundamental powers up to 30 W and discuss strategies for increased output power and ef?ciency at higher pump powers. A schematic of the experimental setup is shown in Fig. 1. The fundamental pump source is a cw Yb ?ber laser (IPG Photonics, YLR-30-1064-LP-SF). The laser delivers up to 30 W of single-frequency radiation at 1064 nm in a linearly polarized beam with a diameter of 3 mm, M2 factor 1.01, and a measured instantaneous linewidth of 12.5 MHz. To maintain stable output characteristics, we operated the pump laser at the maximum power and used an attenuator comprising a half-wave plate (HWP) and a polarizing beam splitter to vary the input fundamental power. A second HWP was used to yield the correct pump polarization for phase matching. The fundamental beam was focused to a measured beam waist radius of 37 m (1 / e2-intensity) at the center of the crystal = 1.28 . The bulk PPKTP crystal is fabricated inhouse from a c-cut 19-mm-long, 1-mm-thick, and 6-mm-wide ?ux-grown KTP sample. The c? face of the crystal was patterned with a photoresist grating

Fig. 1. Schematic of the experimental design for singlepass SHG of cw Yb ?ber laser in PPKTP. / 2, half-wave plate; PBS, polarizing beam splitter; L, lens f = 20 cm ; M, dichroic mirror. ? 2008 Optical Society of America

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of 9.01 m period and was covered with an Al ?lm over 17 mm length at the center, keeping a 1 mm free zone on each side of the grating. The actual length of the grating is thus 17 mm. The crystal was poled by applying 5-ms-long electric ?eld pulses of 2.5 kV/ mm in magnitude [8]. The PPKTP is housed in an oven with a temperature stability of ±0.1° C. The crystal faces are antire?ection coated R 1 % at 1064 nm and 532 nm. A dichroic mirror, M, coated for high re?ectivity R 99% at 1064 nm and high transmission T 99% at 532 nm, extracted the generated green output from the fundamental. To characterize the PPKTP sample, we ?rst determined the dependence of SHG power on crystal temperature. To avoid unwanted contributions from thermal effects, we performed the measurements at a low fundamental power of 1 W, resulting in 8.3 mW of green output at a phase-matching temperature of 30.8° C. The result is shown in Fig. 2. The solid curve is a sinc2 ?t to the data, con?rming the expected temperature dependence of SHG. The FWHM of the curve is T = 3 ° C, which is slightly wider than the calculated value of 2.62° C from the Sellemier equations [9]. The effective interaction length can be de?ned as Leff = L 2.62/ 3 = 14.84 mm, where L = 17 mm is the physical length of the grating. The discrepancy between the physical length and measured effective length is attributed to imperfections in the grating structure and nonideal focusing. We performed measurements of SHG ef?ciency and power up to the maximum ?ber laser power of 30 W, with the results shown in Fig. 3. The fundamental power was measured at the input to the crystal, while the generated green power was corrected for the 6% transmission loss of mirror M. We obtained 6.2 W of green power at the full input power of 29.75 W, representing a single-pass conversion ef?ciency of 20.8%. As evident in Fig. 3, the quadratic increase in SH power and corresponding linear variation in ef?ciency are maintained up to 15 W of input power, after which saturation sets in. Beyond this value, SH power increases linearly with fundamental power. The saturation effect is attributed to depletion of the fundamental [2] and thermal phase mismatch in the PPKTP crystal, owing to a number of factors. These include green-induced IR absorption (GIIRA) of fundamental, two-photon absorption (TPA) of fun-

Fig. 3. Dependence of the measured SH power and the corresponding conversion ef?ciency on the incident fundamental power.

Fig. 2. Temperature dependence of SH power (?lled circles) and the sinc2 ?t (solid curve). The resulting temperature bandwidth is 3 ° C.

damental and generated green, and linear absorption at both wavelengths. To verify any contribution from GIIRA to thermal dephasing, we focused 6 W of green radiation from a cw laser at 532 nm (Coherent, Verdi-10) to a beam radius of 34 m inside the crystal to obtain similar maximum green power density 0.33 MW/ cm2 as in the SHG experiments. The arrangement resulted in the simultaneous focusing of the fundamental ?ber laser IR beam to a waist radius of 28 m inside the crystal, close to 37 m used in the SHG experiments. We then measured the variation in transmitted IR power at different green power levels over 10 min periods. However, we observed no difference in the transmitted IR power with and without green radiation, indicating the absence of GIIRA up to the maximum green power of 6 W. We also investigated the possible role of TPA by recording the crystal transmission at both fundamental and green wavelengths at power levels up to 30 W and 6 W, respectively. We obtained a linear increase in transmitted power with input power at both wavelengths, from which linear absorption losses of 0.3% / cm at 1064 nm and 4.5% / cm at 532 nm were deduced, in agreement with previous results [10]. These measurements thus con?rmed that thermal dephasing in the PPKTP crystal was also not due to TPA, but a result of intrinsic linear absorption of IR and green, leading to increased heating at higher powers. We determined a normalized conversion ef?ciency of 1.2% / W below 15 W of fundamental power, monotonically decreasing to 0.7% / W with the rise in input power to 29.75 W, con?rming increased thermal dephasing due to linear absorption at higher powers. The normalized ef?ciency values do not account for absorption losses in the crystal. From the normalized ef?ciency of 1.2% / W and the focusing parameter of = 1.28 h 0.85 , the effective nonlinearity is calculated as deff 7.63 pm/ V for the 17 mm grating, corresponding to d33 12 pm/ V. This value is 77.9% of previous results [11], which we attribute to crystal absorption of the green and IR, laser linewidth, and imperfections in the grating structure such as missing periods and duty factor variations. Considering the absorption losses of 0.3% / cm at 1064 nm and 4.5% / cm at 532 nm, we determined a normalized ef?ciency of 1.33% / W below 15 W of fundamental power, resulting in deff 8.6 pm/ V, corresponding to

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d33 13.5 pm/ V for a 14.8 mm PPKTP crystal with 50% duty factor. As the missing periods are included in the effective length, the deviation in the calculated d33 from earlier value [11] can be due to the grating duty factor and the laser linewidth. However, using the Sellmeier equations [9], the calculated FWHM spectral acceptance of the 17 mm crystal 0.157 nm is much wider than the laser linewidth 12.5 MHz , con?rming the discrepancy due to duty factor of the grating. To gain further insight into thermal effects in the PPKTP crystal, we recorded the SHG phasematching temperature at different fundamental powers. Although no signi?cant shift in the phasematching temperature was observed at the lower input powers, a monotonous decrease in phasematching temperature, at a rate of 0.19° C / W, was measured above 15 W, further con?rming crystal heating at higher input powers. At lower powers, linear absorption does not signi?cantly affect the phasematching temperature, whereas at higher powers it heats the crystal, reducing the optimum value for phase matching and thus requiring a reduction in crystal temperature for maximum green generation. Since phase matching for the present PPKTP sample is near room temperature, further optimization of the crystal temperature using Peltier cooling or the use of a grating period with a higher phase-matching temperature could potentially lead to increased green power and ef?ciency at the higher input powers up to 30 W and above, with the possibility of power scaling of green beyond 6 W. Proper management of the generated heat or inclusion of the crystal heating effects in the grating design are other possible strategies to achieve the highest SH power and ef?ciency. We analyzed the output spectrum at 532 nm using a scanning confocal interferometer (free spectral range, 1 GHz; ?nesse, 400). A typical fringe pattern at 25 W of fundamental power (5 W of green), is shown in Fig. 4, verifying single-frequency operation. The instantaneous linewidth, measured directly without accounting for the interferometer resolution, is 8.5 MHz. The same behavior was observed throughout the pumping range with similar linewidth, con?rming robust single-mode operation at all pump powers.

Fig. 5. (Color online) TEM00 energy distribution, beam pro?les, and Gaussian ?ts (solid curves) of the generated green beam in the far-?eld.

The far-?eld energy distribution of the green output beam at 5 W together with the intensity pro?le and the Gaussian ?ts, recorded at 25 W of input power, are shown in Fig. 5. Using a 25 cm focal length lens and scanning beam pro?ler, we measured M2 values of the beam as M2 1.29 and M2 1.31, x y con?rming TEM00 spatial mode. Similar M2 values were measured at different input power levels, showing a small variation in M2 from 1.25 to 1.29 and M2 x y from 1.2 to 1.34. We have thus generated 6.2 W of cw, singlefrequency radiation at 532 nm at 20.8% ef?ciency in a highly compact and practical design using simple single-pass SHG of a cw Yb ?ber laser in PPKTP near room temperature. With proper thermal management and optimized grating design, further improvements in green power and ef?ciency as well as power scaling are feasible. This work was supported by the Ministry of Education and Science, Spain, under grant TEC200612360. References
1. H. Furuya, A. Morikawa, K. Mizuuchi, and K. Yamamoto, Jpn. J. Appl. Phys. 45, 6704 (2006). 2. F. J. Kontur, I. Dajani, Y. Lu, and R. J. Knize, Opt. Express 15, 12882 (2007). 3. S. V. Tovstonog, S. Kurimura, I. Suzuki, K. Takeno, S. Moriwaki, N. Ohmae, N. Mio, and T. Katagai, Opt. Express 16, 11294 (2008). 4. S. Greenstein and M. Rosenbluh, Opt. Commun. 238, 319 (2004). 5. R. Le Targat, J. J. Zondy, and P. Lemonde, Opt. Commun. 247, 471 (2005). 6. Z. Sun, G. K. Samanta, G. R. Fayaz, M. EbrahimZadeh, C. Canalias, V. Pasiskevicius, and F. Laurell, in Proceedings of the Conference on Lasers and ElectroOptics (Optical Society of America, 2007), paper CTuK1. 7. G. R. Fayaz, M. Ghotbi, and M. Ebrahim-Zadeh, Appl. Phys. Lett. 86, 061110 (2005). 8. C. Canalias, S. Wang, V. Pasiskevicius, and F. Laurell, Appl. Phys. Lett. 88, 032905 (2006). 9. K. Kato and E. Takaoka, Appl. Opt. 41, 5040 (2002). 10. G. Hansson, H. Karlsson, S. Wang, and F. Laurell, Appl. Opt. 39, 5058 (2000). 11. M. V. Pack, D. J. Armstrong, and A. V. Smith, Appl. Opt. 43, 3319 (2004).

Fig. 4. Single-frequency spectrum of the green output recorded by a scanning Fabry–Perot interferometer.


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