Generation of 1.2 W green light using a resonant cavity-enhanced second-harmonic process with a periodically poled KTiOPO4

Optics Communications 294 (2013) 271–275

Contents lists available at SciVerse ScienceDirect

Optics Communications journal homepage: www.elsevier.com/locate/optcom

Generation of 1.2 W green light using a resonant cavity-enhanced second-harmonic process with a periodically poled KTiOPO4 Yun Zhang a,n, Nobuyuki Hayashi a, Hiroshi Matsumori a, Ryohei Mitazaki a, Yinghong Xue a, Yoshiko Okada-Shudo a, Masayoshi Watanabe a, Katsuyuki Kasai b a b

Department of Engineering Science, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan Advanced ICT Research Institute, National Institute of Information and Communications Technology, 588-2 Iwaoka, Nishi-ku, Kobe, Hyogo 651-2492, Japan

a r t i c l e i n f o

abstract

Article history: Received 2 August 2012 Received in revised form 22 November 2012 Accepted 23 November 2012 Available online 2 January 2013

We report on frequency doubling of a single frequency cw Nd:YAG laser at wavelength of 1064 nm using a periodically poled KTiOPO4 crystal (PPKTP) placed in a single resonant ring enhanced cavity. A maximum output power of 1.23 W, corresponding to 72% of conversion efficiency with 1.7 W of fundamental power, was achieved. This is, to the best of our knowledge, the highest power obtained at this wavelength with PPKTP crystal in an external enhanced cavity. The output power is continuously stable with a peak to peak fluctuation of less than 1% over 4 h owing to optimum design of an enhanced cavity; moreover, the setup is inherently simple and robust. Therefore, it is suitable to be a light source for quantum optics experiments. & 2012 Elsevier B.V. All rights reserved.

Keywords: Second harmonic generation Periodically poled KTP High efficiency resonant doubling

1. Introduction Second harmonic generation (SHG) is an important technique for producing radiations at wavelengths for which lasers are not available [1]. Two different configurations have been used to achieve frequency doubling: single-pass and resonant cavity systems. While single-pass design has advantages of simplicity and compactness, resonant-cavity SHG represents a powerful technique for the efficient frequency doubling of low- (below 1 W) and medium-power (several Watts) continuous wave (cw) laser sources. On the other hand, in past decade, quasi-phasematching (QPM) periodically poled ferroelectrics crystals, such as periodically poled LiNbO3 (PPLN), periodically poled KTiOPO4 (PPKTP) and periodically poled stoichiometric LiTaO3 (PPLT), have become a well assessed and versatile technique for efficient nonlinear generation and engineering of new optical devices [2]. To date, different materials have been used for SHG in QPM crystals, for wavelength from IR to UV range [3–5]. Doubling efficiencies as high as 95% were achieved with resonant cavity enhanced doubling using PPKTP [6]. In quantum optics, lots of experiments have reported to generate cw nonclassical state by SHG or optical parametric oscillator (OPO) by employing QPM crystals as nonlinear crystal [7,8]. For the generation of green light, different materials and different configurations have also been demonstrated. For example, whereas PPLN or PPKTP was

n

Corresponding author. E-mail address: [email protected] (Y. Zhang).

0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.11.097

early used for efficient frequency doubling with the resonant cavity system at low power [9,10] of fundamental light, MgOdoped PPLN and PPSLT were recently employed with single-pass, multi-pass or multi-crystal pass configuration [11–13] at high power (more than 5 W) of fundamental light because high-quality crystals are available. Especially, the single pass SHG of a fiber laser with MgO-doped PPSLT gives alternative approaches for development of high power green source [13–15]. The search for stable, efficient, and robust cw light sources at medium-power is essential for more fundamental studies such as spectroscopy, atom cooling and quantum optics [4,16]. To generate the squeezed state, one usually adopts an experimental configuration that consists of an OPO pumped by a frequency doubled laser source [17]. Although commercial laser sources, in which LiNbO3 or LBO is employed as a nonlinear crystal, are available; power instabilities and thermal lens effects could not be completely suppressed because phase matching at a high temperature was used for these crystals. Further improvements of this kind of lasers are desired. The PPKTP crystal is attractive for frequency conversion at the medium power level. One advantage of PPKTP is that it offers increased resistance to photorefractive damage and relatively high effective nonlinearity. Advances in material growth and fabrication have also led to the availability of such crystals with high optical quality. Comparing with the PPLN and PPSLT, PPKTP has a good power-handing capability in the visible spectra. For green light generation, PPKTP is preferred to PPLN and PPSLT [18–22]. Unfortunately, PPKTP was considered less as a doubling crystal for cw lasers, except for a few reports of intracavity

272

Y. Zhang et al. / Optics Communications 294 (2013) 271–275

configuration [23] at the medium power of fundamental radiations. One reason may be that the quality of PPKTP was poor comparing with LiNbO3 and LBO crystals. In recent years, the SHG using PKTP re-bloomed and the research focus on the further improvement of the output power and conversion efficiency by employing an enhanced cavity since high quality crystals are available. The output power was improved to 6.2 W for the fundamental power of near 30 W at conversion efficiency of 20.8% by employing a 17-mm long PPKTP crystal with singlepass configuration [11]. At the telecommunication wavelength of 1550 nm, the conversion efficiency of 95% was also yielded using a semi-monolithic enhanced cavity configuration [6]. In this paper, we describe the generation of 1.2 W green light based on frequency doubling of a single-frequency Nd:YAG laser output with an external enhancement cavity containing a PPKTP crystal. Furthermore, the stability and beam quality of the output light were investigated. This simple and robust system is suitable to be a light source for quantum optics.

between the curved mirrors, where the smallest waist of 37 mm occurs. The temperature of PPKTP was controlled to the optimal phase-matching temperature within an accuracy of 0.1 1C. The green light output was separated from the infrared light by a filter and measured using a power meter. In order to improve the stability of the enhancement cavity, we build SHG cavity from a single aluminum block by digging out the inner part. The PPKTP along with the oven, the thermal electric cooler and cavity mirror mounts are fixed firmly inside. The cavity is locked according to the dither scheme, that was already successfully used for SHG and OPO in many experiments [24]. To this purpose, the PZT of the cavity was dithered by a 30 kHz sinusoidal signal with a amplitude of about 500 mV and the light leaking from the cavity was monitored by a photodiode (locking PD) and then demodulated by a lock-in amplifier to provide an error signal for the length control of the cavity, then the error signal was fed back to PZT via a piezo driver.

2. Experimental setup

3. Experimental results and discussion

A schematic of the experimental setup is shown in Fig. 1. All the optical parts of the SHG system were arranged on a 40 cm  50 cm breadboard. The fundamental laser source is a monolithic Nd:YAG laser (Innolight: Mephisto) emitting up to 2 W of cw radiation at 1064 nm, with a linewidth of less than 1 kHz. To maintain stable output characteristics, we operated the pump laser at the maximum power and used an attenuator comprising a half-wave plate (HWP1) and a polarizing beam splitter to vary the input fundamental power of enhancement cavity. The second HWP(HWP2) was used to yield the correct pump polarization for phase matching. The cavity is made by four mirrors in a bow-tie configuration, using two plane mirrors (M1, M2) and two curved mirrors (M3, M4) having 50 mm radius of curvature. The total length of the cavity is about 400 mm, and the free spectral range is about 367 MHz. We use three kinds of input coupling mirrors (M1), with different reflectivity at the fundamental wavelength, namely 90%, 95% and 98%. The output mirror (M4) is coated dichromatically so that it transmits 98% of the power of 532 nm light and reflects more than 99.9% of 1064 nm light. The other two mirrors (M2, M3) are high reflectivity mirrors at fundamental wave. A plane mirror (M2) is mounted on a PZT actuator which is used to actively control the cavity length and keep the cavity resonant with the laser frequency. A PPKTP crystal has a 9:00 mm grating period and both end facets antireflection coated for both 1064 nm and 532 nm. The PPKTP, with dimensions of 1 mm 1 mm  10 mm, is housed in a home-made oven and placed

3.1. Single-pass configuration SHG

1064 nm

In order to characterize the PPKTP crystal, the single-pass SHG performance of the crystal was investigated. It was performed by removing the input-coupling mirror (M1). Phase matching is the most restrictive requirement in nonlinear frequency conversion. For a particular periodically poled material, phase matching strongly depends on temperature and linewidth of the incident laser frequency. Usually, the tolerance frequency bandwidth of phase matching is several tens of GHz. Here, we employed a laser with linewidth of only several kHz, which is much smaller than the tolerance frequency bandwidth of phase matching; so the linewidth of incident laser frequency is not a limiting factor in the attainment of efficient SHG. There is an optimal phase matching temperature and the conversion efficiency of the crystal decreases rapidly away from this point. We measured the SHG output power as a function of temperature. The normalized output power as a function of temperature is shown in Fig. 2. We can see that the optimal phase matching temperature is about 32.5 1C and the full width at half maximum (FWHM) of the temperature tolerance is about 5.5 1C. This value is wider than the calculated value of 2.6 1C using the relevant Sellmeier equations [25]. The solid curve is a sinc2 fit to the data, confirming the expected temperature dependence of SHG. We find it can be reduced by changing the beam waist sizes in the crystal. However the

HWP1

2W

PBS 1.7 W

HWP2

Lens

Aluminum Box M2

M1 Locking PD

M3

M4 PPKTP

PZT

Filter

Fig. 1. Schematic of experimental setup for frequency doubling.

Power meter Fig. 2. Temperature tuning bandwidth for frequency doubling in PPKTP.

Y. Zhang et al. / Optics Communications 294 (2013) 271–275

mðTLa Lp Þ2 Pr ¼ ð1TÞð1mÞ þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 , Pin ð12 ð1TÞð1La Lp ÞÞ

Fig. 3. SH output power by a single-pass frequency doubling configuration. The filled circles are experimental data and curve is best fitting with P 2 ¼ ENL P2in , giving a ENL ¼ 0:0097 7 0:0003 W1 .

thermal-induced instabilities in the doubling cavity will be caused, i.e. wide phase matching range ensures very stable and reliable operating in an enhanced cavity configuration. Temperature curves for different fundamental power were also measured. A significant difference was not observed, therefore thermal effects does not occur when light is less than 2 W in our system. Figure 3 shows the SH power as a function of fundamental laser power for single-pass SHG configuration. Also shown in Fig. 3 are theoretical fitting curve with equation of P 2 ¼ ENL P 2in . We find the single-pass conversion coefficient of ENL ¼ 0:97% 7 0:03% W1 (conversion coefficient ENL is the only fitting parameter). The quadratic increase in the SH power is maintained at the full fundamental power. This also indicates that thermal dephasing has not been caused when the input fundamental light power was less than 2 W. Comparing with the former reports, the conversion coefficient is a little smaller [8–10]. Once again, it can be further improved with a tighter focusing inside the PPKTP crystal, but beam waist size had to be optimized to eliminate thermal-induced instabilities in the doubling cavity.

273

ð3Þ

where T is the transmission of the input coupler (M1) for fundamental wave. Total intracavity losses include Lp represents all passive losses in the cavity including absorption and scattering of the mirrors and crystal; La ¼ ENL Pc is active loss, which represents the conversion of the fundamental wave to second harmonic wave. We have tested three different cavities with different input coupler (T¼ 2%, 5%, and 10%). The passive loss is determined to be Lp ¼ 1:5 7 0:2% by measuring the finesse of different cavities. The choice of the input coupler is critical for achieving a high conversion efficiency. Optimal coupling of the laser power into the cavity depends on the total round-trip losses (including passive and active loss). Using the above formulas and parameters, the intracavity power depending on the transmission of input coupler and mode matching factor is calculated at the fundamental power of 1.7 W and shown in Fig. 4. The maximum intracavity power can be obtained at the condition of impedance matching, which is achieving by optimum input coupler transmission T opt ¼ La þ ENL Pc . In our case, the impedance matching is almost achieved with the current parameter by setting T¼10% for mode-matching factor of 0.8–0.9. Fig. 5 gives the input–output characteristics and conversion efficiency of the external resonant doubler at input coupler of T¼10% when the external cavity was locked to an incident fundamental resonance. The pump power in front of the enhancement cavity was measured by an infrared power meter, whereas the second harmonic power at the cavity output was measured by another power meter. The only correction that we made to the measured data was to dived the measured green power by the transmission of the cavity output coupler and the filter. The dots represent raw values measured with two power meters. For 1.7 W input, 1.23 W of green light was emitted out from the cavity with a 90%-reflectivity input mirror. This corresponds to a conversion efficiency of 72%. On the other hand, 330 mW and 700 mw of SH power were obtained when 98%-reflectivity mirror and 95%reflectivity mirror are employed as input coupler mirror. Taking these results and above-mentioned parameters into Eqs. (1)–(3), we obtained mode matching factor of 0:85 7 0:02. So, our conversion efficiency could be 85% when mode-matching factor be taken account into. From the measured values for ENL, passive loss Lp and other parameters, the theoretical prediction for the

3.2. SHG with enhancement cavity The SHG power generated in a enhanced cavity is related to the intracavity fundamental power Pc entering the crystal P 2 ¼ ENL Pc ¼ ENL eP in

ð1Þ

where e is the enhancement factor. There is several ways to get enhancement factor. One method is that we can obtain by detecting the leakage light powers(P wi ,Pwo ) from one mirror (such as M3) with and without the cavity enhancement of the fundamental light, then e ¼ P w =Pwo . We can also obtain the detail information by employing the standard model for frequency doubling in a cavity [24,26]. Because mode matching is usually not perfect in experiment, we have extended the method to account for the mode matching factor mð0 om o 1Þ, which includes the impedance matching of the enhancement cavity and mode matching between the laser mode and cavity mode. It is straightforward to derive formulas for intra-cavity fundamental power Pc, reflected fundamental power Pr, as a function of input power Pin. They are given by Pc Tm ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi , P in ð12 ð1TÞð1La Lp ÞÞ2

ð2Þ

Fig. 4. Intracavity power (Pc) as a function of the transmission of input coupler (T) and mode matching factor (m) at the fundamental power of 1.7 W.

274

Y. Zhang et al. / Optics Communications 294 (2013) 271–275

Fig. 5. Plots of the measured second harmonic output power from the cavity and the conversion efficiency as a function of the incident fundamental power.

harmonic output from Eq. (1) and conversion efficiency are also shown in Fig. 5 as the fundamental power Pin . It shows good agreement between the experiment and theory, resulting that the estimation of intracavity loss and mode matching factor were reasonable. Hence it is reasonable to say that the impedance matching was nearly optimized for this experiment. At present, the conversion efficiency, we believe, was only limited by intracavity passive loss. This can be reduced by employing a low loss mirror and monolithic cavity or semimonolithic cavity configuration. We note that doubling efficiencies as high as 95% were achieved with resonant cavity enhanced doubling using PPKTP owing to lower cavity losses of 0.3% and perfect mode matching factor of 99% [6]. 3.3. Long-term stability Thermal effects are critical issues for stability at high power operation. Material absorption is expected to become important with increasing fundamental power, leading to heating of the nonlinear crystal. As a result, the oven temperature has to be lowered to compensate for the rise in crystal temperature, thus resulting in a shift of the phase-matching peak towards lower temperatures. This is confirmed by measurements and the normalized output power vs temperatures is also shown in Fig. 2. The phase-matching temperature decreases about 1.5 1C. The experimental results show the temperature tolerance was about 6.5 1C for the cavity configuration, which is almost as the same as the temperature tolerance for the single-pass configuration. Obviously, these indicate that inhomogeneous temperature distribution in the nonlinear crystal does not occur thank to a big waist size was selected for the enhancement cavity [21,22]. In other words, we chose an optimal intra-cavity fundamental power Pc to avoid the inhomogeneous temperature distribution in nonlinear crystal. Also the sinc2 phase-matching curve loses its typical symmetric shape, a sudden drop of the green power after the optimal phase-matching temperature was observed. The long-term power stability near the maximum output power is illustrated in Fig. 6. Each point was recorded every ten seconds by averaging the power meter readout. In the specific data set in Fig. 6, the power stayed within 0.5% peak-to-peak for 4 h with the electronic servo on. Typical fluctuation was smaller than 1% per hour. Although we used a discrete enhancement cavity whose mechanical stability was lower than a monolithic cavity, a dynamic range of the correction signal, corresponding to about two free spectral ranges, is sufficient to keep the cavity to locked for several hours. Unlocks of the cavity are usually caused

Fig. 6. Long-term measurement of the generated second harmonic power at a incident power of 1.7 W and TEM00 mode intensity profiles of the generated light. The same data are also plotted with a vertical scale that is enlarged by a factor of 6.

by environmental perturbations or by long term mechanical deformation due to temperature drifts. We also measured the beam quality factor M2 for the generated SH out of the cavity with a beam profiler, and found M 2x  1:09 70:02 and M2y  1:10 7 0:02 with a waist size asymmetry between two perpendicular radii less than 3%. The intensity distribution of green beam was also shown in the inset of Fig. 6. So the generated SH beam is near to a symmetrical Gaussian mode and is suitable for pumping the OPO. Other issues are the reproducibility of the system and dayto-day operation. In the matter of fact, this system has been operated for more than 500 h (over half a year) with the same PPKTP crystal in our lab. It is very easy to keep the output power of more than 900 mW. By slightly adjusting the enhancement cavity, the output power can be restored to more than 1 W. Significant power drops on days operation were not observed. These performance ensure potential for commercial product in future.

4. Conclusion In summary, we demonstrated stable operation of a cavity enhanced SHG using a PPKTP as nonlinear medium, generating up to 1.23 W of green power on a pure TEM00 mode with 1.7 W of fundamental power, corresponding to a conversion efficiency of 72%. To our knowledge, this is the highest cw SH power generated in external cavities with PPKTP. The output power is stable with a peak to peak fluctuation of less than 1% over 4 h benefiting from optimum design of an enhancement cavity. With furtherer reduction of passive loss of the enhancement cavity and improvement of mode-matching factor, improvements in output power and conversion efficiency are feasible.

Acknowledgments This work is supported by the Matsuo Foundation and a Grantin-Aid for Scientific Research (Nos. 23560051 and 24246022) from the Ministry of Education, Culture, Sports, Science and Technology. References [1] R.W. Boyed, Nonlinear Optics, 3rd ed., Academic, New York, 2008. [2] G. Rosenman, A. Skliar, I. Lareah, N. Angert, M. Tseitlin, M. Roth, Physical Review B 54 (1996) 54.

Y. Zhang et al. / Optics Communications 294 (2013) 271–275

[3] F.Y. Wang, B.S. Shi, Q.F. Chen, C. Zhai, G.C. Guo, Optics Communications 281 (2008) 4114. [4] F. Villa, A. Chiummo, E. Giacobino, A. Bramati, Journal of the Optical Society of America B 24 (2007) 576. [5] I. Ricciardi, M. De Rosa, A. Rocco, P. Ferraro, P. De Natale, Optics Express 18 (2010) 10985. [6] S. Ast, R.M. Nia, A. Schonbeck, N. Lastzka, J. Steinlechner, T. Eberle, M. Mehmet, S. Steinlechner, R. Schnabel, Optics Letters 36 (2011) 3467. [7] M. Stefszky, C. Mow-Lowry, K. McKenzie, S. Chua, B.C. Buchler, T. Symul, D. McClelland, P.K. Lam, Journal of Physics B: Atomic, Molecular and Optical Physics 44 (2011) 015502. [8] Y.M. Li, S.R. Yang, S.J. Zhang, K.S. Zhang, Optics Communications 265 (2006) 576. [9] A. Arie, G. Rosenma, A. Korenfeld, A. Skliar, M. Oron, M. Katz, D. Eger, Optics Letters 23 (1998) 28. [10] I. Juwiler, A. Arie, A. Skliar, G. Rosenman, Optics Letters 24 (1999) 1236. [11] G.K. Samanta, S. Chaitanya Kumar, M. Mathew, C. Canalias, V. Pasiskevicius, F. Laurell, M. Ebrahim-Zadeh, Optics Letters 33 (2008) 2955. [12] T. Mizushima, H. Furuya, S. Shikii, K. Kusukame, K. Mizuuchi, K. Yamamoto, Applied Physics Express 1 (2008) 032003. [13] G.K. Samanta, S.C. Kumar, K. Devi, M. Ebrahim-Zadeh, Optics Letters 35 (2010) 3513.

275

[14] G.K. Samanta, S.C. Kumar, M. Ebrahim-Zadeh, Optics Letters 34 (2009) 1561. [15] S.V. Tovstonog, S. Kurimura, K. Kitamura, Applied Physics Letters 90 (2007) 051115. [16] K. Schneider, S. Schiller, J. Mlynek, M. Bode, I. Freitag, Optics Letters 21 (1996) 1999. [17] Y. Zhang, Frontiers of Physics in China 3 (2008) 126. [18] M.G. Pullen, J.J. Chapman, D. Kielpinski, Applied Optics 47 (2008) 1397. [19] H. Furuya, A. Morikawa, K. Mizuuchi, K. Yamamoto, Japanese Journal of Applied Physics 45 (2006) 6704. [20] N. Pavel, I. Shoji, T. Taira, K. Mizuuchi, A. Morikawa, T. Sugita, K. Yamamoto, Optics Letters 29 (2004) 830. [21] F.J. Kontur, I. Dajani, Y. Lu, R.J. Knize, Optics Express 15 (2007) 12882. [22] S.C. Kumar, G.K. Samanta, M. Ebrahim-Zadeh, Optics Express 17 (2009) 13711. [23] S. Greenstein, M. Rosenbluh, Optics Communications 238 (2004) 319. [24] K. Hayasaka, Y. Zhang, K. Kasai, Optics Express 12 (2004) 3567. [25] M.M. Fejer, G.A. Magel, D.H. Jundt, R.L. Byer, IEEE Journal of Quantum Electronics 28 (1992) 2631. [26] W.J. Kozlovsky, C.D. Nabors, R.L. Byer, IEEE Journal of Quantum Electronics 28 (1988) 913.