Highly efficient generation of blue-orange femtosecond pulses from intracavity-frequency-mixed optical parametric oscillator

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15 January 1996

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OPTICS COMMUNICATIONS Optics Communications123 (1996) 121-128

Highly efficient generation of blue-orange femtosecond pulses from intracavity-frequency-mixed optical parametric oscillator A. Shirakawa, H.W. Mao ‘, T. Kobayashi Department of Physics, University of Tokyo, Hongo, Bunkyo, I13 Tokyo, Japan

Received 15 June 1995

Abstract The second harmonic generation of the signal or sum frequency generation of the signal and pump in a P-BaB,O, crystal in the cavity of an optical parametric oscillator pumped synchronously by a mode-locked Ti:sapphire laser is reported. Visible femtosecond pulses with an average power of 330 mW are generated with a pumping power of 1.3 W. Broad tunability is achieved from 426 to 483 nm and 520 to 585 nm using a single mirror set.

1. Introduction

Recently Kerr-lens mode-locking has been applied to several solid-state lasers and the tuning range of femtosecond pulses from lasers has been extended substantially. However, the output wavelength is almost limited to red to near-infrared region and even with the second harmonics (SH) the whole visible region cannot be covered. This has been a severe problem for femtosecond time-resolved spectroscopy of a material with an absorption maximum outside this region. To solve this problem, several optical parametric oscillators ( OPO’s) synchronously pumped by mode-locked Ti:sapphire lasers were reported and near-infrared [l31 and visible [4,5] femtosecond pulses have been obtained. There are two methods to obtain visible femtosecond pulses from a Ti:sapphire-pumped OPO. One is using the SH of a Ti:sapphire laser as a pump. Driscoll et al. reported /3-BaB204 (BBO) -based OPO pumped with ’ Permanentaddress,Fujian Institute of Researchon the Structureof Matter, P.O. Box 143,Fuzhou, Fujian, 350002,P. R. China. 0030-4018/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDIOO30-4018(95)00469-6

1 W SH pulses at 400 nm generated from a 2 W modelocked Ti:sapphire laser and obtained up to 150 mW average output power with pulse width of as short as 17 fs tunable from 590 to 666 nm [ 41. In this case the net conversion efficiency from the Ti:sapphire laser to the OPO output is limited to 7.5% by the second harmonic generation (SHG) of the Ti:sapphire laser. The other method is a nonlinear frequency mixing. Ellingson et al. constructed a femtosecond OPO pumped by a 2.1 W Ti:sapphire laser based on a KTiOPO, (KTP) crystal and by the intracavity SHG of the near-infrared signal in a BBO crystal 115 fs pulses with the average power of 240 mW were generated tunable from 580 to 657 nm [ 51. The relatively high efficiency of 11.4% was achieved with intracavity doubling geometry. Even though high power visible femtosecond pulses can be efficiently obtained from an OPO, the tuning range is limited by the absorption of the idler in the parametric crystal and in KTP the cut off wavelength of the idler is about 3.4 Frn, although the crystal is almost transparent up to 4.5 Fm. Because of this cut off the shortest wavelength of the signal SH is about 520 nm. Even though KTiOAsO, (KTA),

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CsTiOAsO, (CTA) , and RbTiOAsO, (RTA) [ 6,7], which are transparent to about 5 pm, have been used for the gain crystals to extend the tunability to shorter wavelength, the idler cut-off is not much improved and output power is substantially reduced beyond 3.3 pm in an RTA OPO [ 61. Powers et al. explained this reduction in terms of the broad absorption at longer wavelength [ 61. To extend the tunability in the visible it is convenient to perform sum frequency generation (SFG) of pump and signal. In spite of the absence of the phase-matching condition the pulses of the sum frequency (SF) are directly generated from a parametric crystal because of the large nonlinearity of the crystal and high intracavity power, but with the disadvantage of small output coupling [l-3,6,7]. By using the SF of pump and intracavity signal under a phase-matching condition the tunability of the high power femtosecond pulses can be significantly extended to shorter wavelength. To the best of our knowledge no experimental work, except Ref. [8], has been reported on the intracavity SFG configuration. In that work a doubly resonant OPO using a LiIO, (LIO) crystal was pumped by a Qswitched ruby laser and the SF of the pump and signal was generated from another intracavity LIO with nanosecond pulse width and 4% conversion efficiency [ 81. Intracavity SFG of singly resonant synchronously pumped OPO increases the efficiency owing to high peak power of interacting pulses. In the present paper we demonstrate efficient intracavity frequency mixing in a KTP-based OPO synchronously pumped by a mode-locked Ti:sapphire laser. The signal SH and SF of the pump and signal generated with an intracavity BBO crystal cover from 520 to 585 nm and 426 to 483 nm with high power of 330 mW and 210 mW, respectively, with sub-100 fs width. This broad tunability is obtained without replacing the mirror set and the high conversion efficiency (44% for SHG OPO and 3 1% for SFG OPO including idler) is achieved by an output coupling mixing crystal BBO. This offers a useful femtosecond light source for time-resolved femtosecond spectroscopy in the blueorange region.. 2. Experimental setup Fig. 1 shows the schematic of the femtosecond OPO. The pump laser is a regeneratively mode-locked

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Fig. 1. Schematic of the intracavity-frequency-mixed OPO. The pump beam for SFG is indicated by the dotted line. Ml-M& highly reflective cavity mirrors, Pl, P2: SF2 Brewster prisms, PZT: piezoelectric transducer, Ll: pump focusing lens, L2-L4: lenses for recollimating and mode-matching the pump beam for SFG, D: delay line.

Ti:sapphire laser (Spectra-Physics Tsunami) and generates 81 MHz pulses with 1.1 W average power and 90 fs pulse width (assumed as a Gaussian shape) at 790 nm. The polarization is changed from vertical to horizontal using two mirrors and then it synchronously pumps the OPO through a concave cavity mirror with anf= 50 mm lens (the effective focal length is 62 mm). The OPO is composed of six broadband dielectric mirrors with R > 99.7% between 1.O and 1.2 pm. Two of the mirrors are planar and the other four are concave with radius r= 100 mm. One plane mirror mounts on a translation stage with a piezoelectric transducer for the synchronization of the cavity with the pump laser. The KTP crystal is cut at noncritically phase-matched angle (0=90”, $=O’, o+o+e type II) with 1 mm thickness and anti-reflection (AR) coated for 1.Oto 1.2 p,m. Including the total loss (about 9%) at M2 and the AR-coating at 790 nm the net pump power injected to the KTP is 1.0 W. The group velocity mismatches (GVM’s) between pump (790 nm) and signal ( 1.13 p,m) and pump and idler (2.60 p,m) are calculated as 107 fs and - 178 fs, respectively. The cavity is aligned by using the non-phase-matched SH of the pump from the KTP. A group-velocity-dispersion (GVD) -compensating SF2 prism pair with the slant length of 27 cm is used to reduce the third order dispersion of the cavity. The signal SH and SF of the signal and pump are generated with aBB0 crystal in the OPO. BBO is suited to SHG in the near-infrared region because of the small GVM and the weak dependence of the phase matching

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angle on the wavelength [ 51. The 0.3 mm thick BBO is cut at 0 = 22.4” and 4 = 0” with an AR coating centered at 1.06 pm. The GVM between the fundamental ( 1.1 km) and its SH is 21 fs. The signal SH is obtained simply by inserting the BBO crystal in the cavity. The interaction is of type I (o + o -+ e) and the vertically polarized green pulses are generated to the dual directions through the folding mirrors. The reflection loss of the SH on the coating of the BBO is less than 1% in the spectral range between 505 and 575 nm and the losses at the mirrors are 5-lo%, resulting in the efficient extraction of the SH. The oscillation is maintained with the acceptance of the cavity length mismatch of 3 to 8 pm, depending on the wavelength. The second mixing is SFG between the pump and signal. The Ti:sapphire beam after pumping the KTP is recollimated and again introduced in the cavity with an f=50 mm lens, as shown in Fig. 1 with a dashed line. After passing the non-coated lenses, M3, M5, and the AR-coatings of the crystals, the residual pump power of about 300 mW is estimated to be injected to the BBO at 790 nm. The reinjected pump beam is mode-matched with the oscillating signal beam by adjusting the positions of L3 cf= 500 mm) and L4. In the time domain the pump pulses are synchronized with the oscillating signal pulses in the crystal by adjusting the delay line. The allowance of the delay variation is about 100 pm. Routing the pump beam outside the cavity is needed to compensate the temporal walk-off between the pump and signal caused by the GVM in the KTP to increase the SFG efficiency. The interaction is also of type I and vertically polarized blue femtosecond pulses are generated in a single direction. Fig. 2 shows the tuning curves calculated using the Sellmier equations for KTP [ 91 and BBO [ lo]. The alignment of the SFG is performed from the SHG configuration simply by rotating the BBO only by a few degrees. The reinjected Ti:sapphire beam is adjusted to coincide with the SH beam line at the angle of the SHG, and by scanning only the delay line the SF output is easily observed. At the optimum angle of SFG a few mW of the SH output remains. The GVM’s in the BBO between the blue output and pump and between blue and signal are 30 fs and 42 fs, respectively with pumping at 790 nm, and are increased to 39 fs and 54 fs, respectively at 720 nm.

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Fig. 2. Calculated tuning curves and phase-matching curves of the SHG OPO (solid line) and SFG OPO (dashed line). The experimental tuning curves are also shown for the SHG output (full circle) and the SFG output (open circle) when the pump is tuned between 720 and 820 nm.

3. Results and discussion The tunability of the OPO using the KTP crystal cut for noncritically phase-matching is obtained by tuning the pump Ti:sapphire laser from 720 to 820 nm. The average output power and pulse width depend on the wavelength and typically changed between 0.85 and 1.3 W and 90 and 170 fs, respectively. The near-infrared signal wavelength is from 1.06 to 1.17 p,rn by this pump wavelength tuning, with the corresponding tuning range of the idler from 2.24 to 2.74 p,m. Under pumping at 790 nm ( 1.1 W, 90 fs) , the wavelength of the signal is 1.13 p,m and the output powers are about 80 mW and 150 mW with and without a prism pair, respectively, which are measured with a high reflector Ml replaced with a 3% output coupler. The pulse width is 50 fs with the time-bandwidth product of 0.34 when a sech’ shape is assumed. M 1 is returned to a high reflector and the BBO crystal is introduced. The SH output is obtained only by the adjustment of the crystal angle and cavity length. The tuning range of the SH is from 529 to 585 nm, as shown in Fig. 2. The background-free SHG autocorrelation trace and spectrum centered at 565 nm with pumping at 790 nm are shown in Fig. 3. For the autocorrelation measurement a 0.1 mm thick BBO crystal ( 0 = 45.2”, 4 = 0”) is used. The Gaussian fit pulse width is 73 fs and the time-bandwidth product is 0.55, indicating a chirp. The pulse width of the output through M5 (M5-

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Fig. 3. (a) Autocorrelation trace and (b) spectrum of the output pulses from the intracavity SHG OPO pumped at 790 nm. The Gaussian fit (solid line) pulse width is 73 fs (fwhm) and the bandwidth is 7.9 nm ( fwhm) , indicating the time-bandwidth product is 0.55.

output) is longer than that through M4 (M4-output) by about 5% because the latter pulses are generated just after the compression of the fundamental signal by the prism pair. The wavelength dependence of the available output power is shown in Fig. 4. Even though the BBO crystal with two surfaces is added, the output power of the SH with all high reflectors for the fundamental is more than twice as large as that of the near-infrared signal extracted with a 3% output coupler. With pumping at 790 nm, the output power of the SHG OPO is 220 mW at 565 nm even though that of the fundamental OPO is only 80 mW, indicating the effective conversion efficiency is 270% from the signal to its SH. The maximum output power is 330 mW with 100 fs pulse width at 551 nm when the pumping power is 1.3 W with 120 fs width at 769 nm. If the losses of the pump and SH at

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the cavity mirrors and crystal surfaces are corrected, the pump and output power are estimated as 1.2 W and 370 mW, respectively, and the net conversion efficiency is 3 1% of SH and the total efficiency including the idler is 44%, which is, to the best of our knowledge, the highest among the intracavity dispersion-compensated OPO’s ever reported. This high efficiency is mainly due to the almost optimized output coupling of this OPO. The output coupler is the nonlinear loss caused by the SHG in the BBO and the coupling factor can be adjusted by scanning the position along the beam line. Fig. 5 shows the dependence of the output powers of the two directions and the sum of them on the position of the BBO crystal at 55 1 nm in the following. cases: the OPO is adjusted for maximum output power (a) at the focus and (b) at the edge (around -7 or +7 nm). In (a) the maximum total output power is 330 mW at the focus and decreases rapidly away from this point. On the other hand, in (b) the curve of the total output power has a minimum at the focus and maxima on both sides (around - 3 mm or + 3 mm). The maximum power is 60 mW smaller than in the case of (a) but at the edge the power is about 1.5 times larger. The curve of the M4-output has two peaks but they are not clearly observed in that of the MS-output. The BBO position also affects the total GVD in the OPO dominated by the self phase modulation (SPM) in the crystals [ l-31. Its amount is estimated from the signal wavelength dependence on the cavity length [ 2-

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Fig. 4. Wavelength dependence of the available average power of dual outputs from the intracavity SHG OPO with the noncritical KTP (full circle) or the critical KTP (open circle), without correcting the reflection losses in the cavity.

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BBO Position (mm) Fig. 5. Dependence of the SH output power on the position of the BBO along the beam line. The OPO is adjusted for maximum output power (a) at the focus and (b) at the sides (around - 7 or + 7 mm). Full circle: M4-output, open circle: MS-output, full square: total output power. 4.1. In the case of Fig. 5b the prism insertion amount is fixed for the shortest pulse of 90 fs at the positions of the two peaks with the total GVD of appro~mately - 400 fs*/m. When the BBO is located at the focus the increased negative GVD streches the width to 140 fs, while the weak wavelength dependence on the cavity length around 7 mm away from the focus indicates the nearly zero GVD with the noisy pulse train. The intracavity power increases as the BBO is moved away from the focus due to the smaller output coupling factor. At the focus the SHG efficiency is m~irniz~ while the intracavity signal power is overextracted, resulting in the reduction of the output power and the generation of the double-peaked structure. Fig. 5b shows the optimum coupling factor of the cavity is obtained around -3mmor +3mm.

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A similar dependence of the GVD on the crystal position is observed also in the single peak case (peaked when the BBO is at the focus, Fig. 5a) but the extra negative GVD region is not observed. The single peak means the coupling factor cannot exceed the optimum value even at the focus, probably because the linear loss of the cavity is smaller than in the case of (b) and the optimum coupling factor becomes larger than the maximum SHG efficiency. However, the large extracted power suggests that the OPO is almost optimally coupled. The conversion efficiency of SHG at the focus is estimated from the reflection at the Brewster prism to evaluate the intracavity power. The efficiency is about 4.5% to a single direction, hence the nonlinear output coupling factor is about 9% altogether in the two directions. This is measured at 541 nm, where the output power is 280 mW with 1.2 W pump power at 750 nm with the power dependence similar to that in Fig. 5a. Using the above coupling factor for the fund~en~ OPO, efficient operation in the near-infrared region may be also possible. In order to extend the tuning range of the OPO, another KTP ( 0 = 43.9’, 4 = O”, 1S mm) is also used and by tuning the angle with pumping at 790 nm the signal and idler are tunable in 1.03-l 25 pm and 2.153.39 pm, respectively. The tuning range of the SH is limited to a 520-577 nm range because of the AR coating of the BBO centered at 1.06 pm. The shorter edge is determined by the idler absorption in KTP. A maximum output power of 80 mW is obtained with weak wavelength dependence. The power is much lower than in the noncritical case because of the walkoff {about 3”) in the KTP. The SF of the pump and signal is obtained by reinjetting the pump into the BBO. In this case only the noncritical KTP is used for gain. Either SHG or SFG is selected by slight change of the angle of the BBO. In the case of an intracavity SFG OPO the pump is depleted in both parametric and SFG crystals. The output behaviour of OPO becomes complicated because of its dependence on the degree of pump depletion in both crystals [ 111. When the OPO is not optimally aligned, the oscillation stops just after the reinjection of the pump because of the signal depletion due to SFG. Under the proper alignment, however, a stable steady state can obtained.

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Fig. 6. (a) Cross correlation trace and (b) spectrum of the output pulses from the intracavity SFG OPO pumped at 790 nm. The cross correlation measurement is performed with the pump Ti:sapphire pulses and if they are assumed to be 90 fs Gaussian pulses, the trace is well fitted also with the Gaussian shape (solid line) from which the SFG puke width is estimated as 80 fs ( fwhm) . The bandwidth is 5.6 nm ( fwhm) , sothe time-bandwidth product is 0.62.

In Fig. 6 the cross correlation trace with the pump and spectrum centered at 467 nm with pumping at 790 nm are shown and the deconvoluted width of the SF pulses is 80 fs. The time-bandwidth product of 0.62 indicates the larger chirp than the SH pulses. This is partially due to the larger GVD of the M4 substrate and components in the autocorrelator at the SF wavelength, but the main reason is that the pulse width of the reinjetted pump is increased by about 20% after passing through the KTP, cavity mirrors and lenses. The compensation of the pump GVD before the reinjection will improve the time-bandwidth product. The wavelength dependence of the output power of the SF from 426 to 4X3 nm is shown in Fig. 7. Highest

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power of 210 mW is obtained when pumped at 790 nm ( 1.1 W) . The total reflection loss of SF pulses at the AR-coating and M4 is 17%. The power decreases at shorter wavelength because of the higher loss at lvI4 (up to 42%) and the larger deviation of the phasematching angle of the BBO from the cutting angle (up to 7 degree externally, see Fig. 2). The net conversion efficiency of SF output at 467 nm is 25%, and that of the sum of the SF and idler is 30%. These are relatively large, but still smaller than the SHG OPO. The extracted signal power is estimated as only 100 mW from the same photon number of the SF and signal depleted in the BBO. This is less than a half of the output of the SHG OPO. When the reinjected pump power is decreased the SFG power is also decreased, indicating the pump intensity is insufficient to obtain the optimum coupling factor by the SFG process. The reduction of the reflection losses of the pump (about 40%) during the reinjection routing is effective in increasing the SFG efficiency. The efficiency is also limited by the inhomogeneous intensity distribution over cross section after passing the KTP, which is induced by weak higher-order transverse modes due to cross phase modulation in the KTP with the large nonlinear refractive index. In the case of the SHG OPG, no damage is observed in the crystals even with the high in~acavity peak intensity of about 30 GWlcm’ at the focus. In the SFG OPO, in contrast, surface damage could be observed on the BBO after several minutes operation. When the cavity is blocked with pump being injected, the damage is not

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Fig. 7. Wavelength dependence of available average output power of the intracavity SFG OPO. This case the gain crystal is only the noncritical KTP.

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with the pump Ti:sapphire laser. The main fluctuation of the pump intensity of 150 Hz is caused by the Ar laser, and slightly enhanced in the OPO. This stable pulse train is rn~n~~~ for about two hours, and the oscillation for about four hours, without any cavity length readjustment. The normal feedback using the wavelength dependence on the cavity length is effective in improving the long term stability, which is henceforth limited by the beam walking of the Ar laser.

4. Conclusions In this paper we presented the intracavity SHG of signal and SFG of pump and signal in a synchronously pumped femtosecond OPO. Stable visible femtosecond pulses are generated with an average power as high as 330 mW tunable from 426 to 483 nm (SFG) and 520 to 585 nm (SHG) with a single mirror set. The nonlinear output coupler almost optimizes the conversion efficiency of the OPO up to 44%, which is the highest efficiency among the intracavity GVD compensated OPO’s ever reported.

Acknowledgements

Fig. 8. Oscillograms of the pump Tixapphire laser (upper) and the SFG output of the OPO (lower) from the intracavity SFG OPO on time scales of (a) 20 &division and (b) 5 ms/division.

observed, indicating the damage is caused by the blue pulses, of which peak intensity in the BBO is at most 3 GWlcm’. It was reported that in BBO crystal the bulk damage threshold becomes lower at the shorter wavelength and the surface damage threshold is an order of magnitude lower than the bulk [ 121. The heat deposition causes not only the surface damages but also destroys phase-matching condition by the change of the refractive index because of the small angle acceptance of BBO (5.4 mrad mm at 1.1 pm). Cooling the crystal is needed to avoid this problem. The output pulses from the intracavity SFG OPO are stable, less than 5% amplitude fluctuation. A typical pulse train of the SF output is shown in Fig. 8 together

We are grateful to Drs. K. Misawa and S. Takeuchi for their useful suggestions and advices during the study. The technical support by Messrs. A. Ueki and S. Otsuka and the careful reading by Dr. R.J. York are also appreciated. This work is supported by the Grantin-Aid for Specially Promoted Research from the Ministry of Education, Science, and Culture (No. 05102~2).

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[6] P.E. Powers,C.L. TaugandL.K. Cheng, OpticsLett. 19 (1994) 1439 and references therein. [7] D.T. Reid, M. Ebrahimzadeh and W. Sibbet, Optics Lett. 20 (1995) 55. [S] A.J. Campillo, IBBE J. Qu~tumEl~~on. QE-8 (1972) 914. 191 K. Kato, LBEE J. Quantum Electron. QE-24 ( 1988) 3.

[ lo] D. Eimerl, L. Davis, S. Velsko, E.K. Graham and A. Zalkin, J. Appl. Phys. 62 (1987) 1968. [ I I] G.T. Moore and K. Koch, JBEE J. Quantum Electron. QE-29 (1993) 961. [ 121 H. Nakatani, W.R. Bose&erg, L.K. Cheng and CL. Tang, Appl. Phys. Lett. 53 (1988) 2587.