BiB 3 O 6 has been used for second-harmonic generation of a femtosecond Er-fiber laseramplifier at 56 MHz. An internal conversion efficiency of 23% was obtained for second-harmonic pulses with a duration of 64 fs at 782 nm. Mode-locked Er-fiber lasers provide compact and stable ultrashort light sources near 1600 nm at high repetition rate. Frequency doubling to ~800 nm is useful for injection seeding of Ti:sapphire or alexandrite regenerative amplifiers [1] . Output power of Er-fiber lasers,

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... ever, is rather low in the sub-100-fs regime, even amplified in an Er-fiber amplifier, and this makes the second-harmonic generation (SHG) inefficient. Thus, using a 1-cm-long E-BaB 2 O 4 (BBO) crystal, a conversion efficiency of 10 was achieved for 86-fs pulses at the second harmonic [2], corresponding to an energy of 270 pJ at 772 nm for a repetition rate of 31.8 MHz. Higher conversion efficiency of 25 was achieved with a 1-mm-long, periodically poled LiNbO 3 (PPLN) crystal [3], generating 90-pJ, 190-fs pulses at 777 nm. To preserve the short pulse duration of the fundamental, broad spectral acceptance bandwidth of nonlinear crystal is required. The type-0 (ee-e) PPLN crystal possesses a very narrow bandwidth of 'OƐ ~ 1 nmcm (FWHM). Therefore, the interaction length needs to be extremely short despite the large effective nonlinear constant (d eff = 16.5 pm/V [3]), which requires tight focusing inside the SHG crystal to achieve high conversion efficiency (e.g. w o = 10 µm for Ɛ = 1 mm [3] ). This indicates the limits of this material for an ultimate short pulse (W < 100 fs), having in mind also the damage-threshold. In contrast, the spectral bandwidth of type-1 BBO is very large, but the small angular acceptance ('T ext Ɛ ~ 1.4 mradcm) and large walk-off angle (U ~ 2.8q) limit the achievable conversion efficiency and lead to spatially poor beam quality [2]. Here we report efficient SHG of a low-power femtosecond Erfiber laser-amplifier at 1564 nm by the use of a BiB 3 O 6 (BIBO) frequency-doubler. Transform-limited 64-fs SHG pulses with an internal conversion efficiency as high as K = 23 have been obtained with a 5-mm-long sample. Also, a maximum conversion efficiency of K = 27 with a slightly longer SHG pulse of W = 73 fs has been obtained with a 6-mm-long sample. An experimental comparison with a 6-mm BBO crystal is presented in detail. BIBO is a positive biaxial crystal belonging to the monoclinic system with point symmetry 2. Since this material simultaneously fulfills the phase-and group-velocity matching (GVM) between fundamental and secondharmonic beams at O ~ 1.637 nm [4], the spectral acceptance bandwidth for phase-matched SHG of an Er-fiber laser is expected to be large. The recently published Sellmeier equations [4] predict 'OƐ = 24.9 nmcm at O = 1564 nm in the x-z plane (T pm = 10.9q) for oo-e type-1 interaction, which is much broader than those of other existing nonlinear crystals, except BBO ('OƐ > 100 nmcm [5]). The very broad bandwidth of BBO is attributed to the fact that its phase-matching condition for GVM is located at O ~ 1547 nm. However, BIBO exhibits larger angular acceptance ('T ext Ɛ = 2.28 mradcm), smaller walk-off angle (U = 1.75q), and ~1.5 times larger effective nonlinear constant (d eff = 3.1 pm/V). Given the spectral bandwidth of a transform-limited ~60-fs-long sech 2 -shaped pulse, the crystal length to preserve the pulse-duration of our system is between 5 and 6 mm. Therefore, we compared two BIBO crystals, 5and 6-mm-long, cut at T = 11.4q in the x-z plane, with a 6-mm-long type-1 BBO crystal. All samples were uncoated. The SHG experiments were carried out on a diode-pumped, mode-locked (by nonlinear polarization rotation) Er-fiber laser-amplifier. This system provided a linearly polarized, diffraction-limited (M 2 # 1.0) beam with an average power of 65 mW at a repetition rate of 56 MHz, corresponding to an energy of 1.16 nJ at 1564 nm. Intensity autocorrelation measurement with a 2-mm BBO crystal gave a fundamental pulse duration of 59 fs assuming a sech 2 -pulse shape. Several different lenses were tested for focusing the output beam into the SHG crystals and f = 75 mm was chosen to achieve maximum SHG powers. The beam spot radius at the focus was measured to be w o (1/e 2 ) # 25 µm. A KDP crystal was placed after the SHG crystals to block the fundamental beam in the power measurements but this filter was removed during the spectral and autocorrelation measurements. a1271_1.pdf CTuR6.pdf