Omid Kokabee

We report a high-power picosecond optical parametric oscillator (OPO) synchronously pumped by a Y... more We report a high-power picosecond optical parametric oscillator (OPO) synchronously pumped by a Yb fiber laser at 1.064μm, providing 11.7W of total average power in the near to mid-IR at 73% extraction efficiency. The OPO, based on a 50mm MgO:PPLN crystal, is pumped by 20.8ps pulses at 81.1MHz and can simultaneously deliver 7.1W of signal at 1.56μm and 4.6W of idler at 3.33μm for 16W of pump power. The oscillator has a threshold of 740mW, with maximum signal power of 7.4W at 1.47μm and idler power of 4.9W at 3.08μm at slope efficiencies of 51% and 31%, respectively. Wavelength coverage across 1.43–1.63μm (signal) and 4.16–3.06μm (idler) is obtained, with a total power of ~11 W and an extraction efficiency of ~68%, with pump depletion of ~78% maintained over most of the tuning range. The signal and idler output have a single-mode spatial profile and a peak-to-peak power stability of ±1.8% and ±2.9% over 1h at the highest power, respectively. A signal pulse duration of 17.3ps with a clean single-peak spectrum results in a time-bandwidth product of ~1.72, more than four times below the input pump pulses.

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Optimum output coupling in optical oscillators using an antiresonant ring interferometer

We describe a simple and universal method for absolute optimization of output power from optical ... more We describe a simple and universal method for absolute optimization of output power from optical oscillators using interferometry. By incorporating an antiresonant ring interferometer in one arm of the oscillator cavity, simple adjustments to the interferometer provide continuously variable output coupling over a broad spectral range and under any operating conditions. We demonstrate the technique using a femtosecond optical parametric oscillator (OPO), where we show continuously adjustable output coupling from 1% to 60%. By operating the OPO under an optimized output coupling of ~30%, we obtain ~200 mW of extracted power, more than twice that with an ~4% conventional output coupler, across the full tuning range. We also show that the technique has no detrimental effect on the spatiotemporal characteristics of the output, with the extracted signal exhibiting a Gaussian beam profile and near-transform-limited pulse durations.

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Extended-cavity, tunable, GHz-repetition-rate femtosecond optical parametric oscillator pumped at 76 MHz

We report the generation of tunable GHz-repetition-rate femtosecond pulses in a synchronously-pum... more We report the generation of tunable GHz-repetition-rate femtosecond pulses in a synchronously-pumped optical parametric oscillator (SPOPO) with extended cavity length pumped by a 76 MHz Kerr-lens mode-locked Ti:sapphire laser. In a SPOPO based on periodically-poled LiNbO3, insertion of a prism pair for dispersion compensation internal to the linear cavity provides stable output pulse trains of up to 14th harmonic of pump repetition-rate (1064 MHz) with 70 mW of average power for 1.45 W of pump. Near-transform-limited pulses down to 216 fs are achieved with wide tunability across 1500-1540 nm by continuous detuning of the SPOPO cavity delay over 8 microm.

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Efficient, high-repetition-rate, femtosecond optical parametric oscillator tunable in the red

We report efficient generation of tunable femtosecond pulses in the red by internal frequency dou... more We report efficient generation of tunable femtosecond pulses in the red by internal frequency doubling of an optical parametric oscillator (OPO) based on periodically poled LiNbO3 (PPLN). The OPO, based on a 1-mm-thick PPLN crystal, is synchronously pumped by a femtosecond Ti:sapphire laser at 810 nm, providing signal pulses across 1.33-1.57 microm at a 76 MHz repetition rate. Using a 1-mm-thick crystal of BiB3O6 (BIBO) internal to the OPO cavity, we achieve frequency doubling of signal pulses across 665-785 nm with up to 260 mW of average power for 1.51 W of pump. The high nonlinear gain and phase-matching acceptance in PPLN and BIBO permit convenient tuning across the full range by simple detuning of OPO cavity delay. Intracavity dispersion compensation results in near-transform-limited red pulses with durations down to 140 fs for 185 fs input pump pulses.

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High-harmonic-repetition-rate, 1 GHz femtosecond optical parametric oscillator pumped by a 76 MHz Ti:sapphire laser

We demonstrate the generation of femtosecond pulses in a synchronously pumped optical parametric ... more We demonstrate the generation of femtosecond pulses in a synchronously pumped optical parametric oscillator (SPOPO) at the harmonics of pump repetition rate using a SPOPO cavity longer than the fundamental synchronous length. The SPOPO is based on a 1 mm crystal of periodically poled LiNbO(3) and pumped by a Kerr-lens mode-locked Ti:sapphire laser at 76 MHz. By increasing the SPOPO synchronous cavity length we have generated output signal pulses at successive harmonics of the pump repetition rate up to the 13th harmonic at 988 MHz, where average signal powers of 30 mW are still available for 1.45 W of pump power. The generated signal pulses at 988 MHz are near transform limited with average durations of 227 fs and a time-bandwidth product of 0.41 for 185 fs input pump pulses.

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Femtosecond nonlinear frequency conversion based on Bismuth triborate

We review the physical properties, linear and nonlinear optical characteristics, and phase-matchi... more We review the physical properties, linear and nonlinear optical characteristics, and phase-matching configurations of BiB3O6 (BIBO), the first low-symmetry (monoclinic) inorganic nonlinear crystal that has found broad applications for frequency conversion of laser sources from the UV, across the visible, to the near-IR based on three-wave interactions. We describe in detail the most relevant optical properties that make this material an attractive candidate for nonlinear frequency conversion of laser light in general, and ultrafast femtosecond laser sources in particular. With special focus on ultrafast frequency conversion, characteristics such as group-velocity mismatch and spectral acceptance, parametric gain bandwidth, group-velocity dispersion, as well as angular acceptance and spatial walk-off are evaluated and optimum configurations for the attainment of maximum conversion efficiency, minimum pulse duration, and highest spatial beam quality are identified and compared with the most widely established alternative borate crystal, -BaB2O4. Experimental results are presented on both parametric up- and down-conversion of femtosecond pulses, from the high-energy, low-repetition-rate (1 kHz) to the low-energy, high-repetition-rate (56-76 MHz) regime, demonstrating the unique versatility of BIBO for efficient frequency conversion of femtosecond pulses with broad tunability from 250 nm in the UV, throughout the visible, up to 3000 nm in the IR.

$Table 2 Sellmeier coefficients of BIBO: n? = A + B/(X? - C)- D)? valid in the 0.32-3.09 um range (from [63]). Other refractive index measurements of BIBO appeared in [2] and other measurements of the thermo-optic coeffi- cients can be found in [79]. In [80] the refractive indices were measured at four temperatures for four wavelengths and four sets of Sellmeier equations were fitted for different temperatures. However, the obtained thermo-optic coeffi- cients are very different from those in [63], see Table 3. A possible explanation for this discrepancy is that in [80] the effect of the thermal expansion on the index values had not There is no certainty that all possible polarization con- figurations with nonvanishing d.g in Table 1 are phase matchable. The latter depends on the dispersive properties. The first refractive index measurements for BIBO at 1079.5 and 539.75 nm appeared in [68], and Sellmeier equations containing one UV pole and a quadratic IR correction term were first derived in [1] by fitting measurements extending from 365 to 2325 nm. Note that the difference between n,, and n, in BIBO is much smaller compared with the differ- ence with n,, which is reflected in the fact that Q << 90° (quasi-uniaxiality). The refinement in [45,71] was based on OPO data and had validity from 474 to 3083 nm. More recently, the same authors published a further refinement of the Sellmeier equations based on direct THG (four-wave process) for propagation along the x principal optical axis, where the second-order nonlinearity deg (see Table 1) van- ishes [63]. These equations, reproduced in Table 2, which are used in the present work, are valid between 326.3 and 3083 nm. It should be noted that OPO, SHG and SFG pro- cesses have also been used to derive thermo-optic disper- sion formulae for BIBO [45, 63,71, 72], important for tem- perature dependent phase matching in the 20-120 °C range.$

$“The Sellmeier and nonlinear coefficient data used for the table, with the exception of BIBO, is from [88]: For KH2PO4 (KDP), LBO, CsLiBeO10 (CLBO), BBO, PPKTP, LiIO3, and KNbO3, the so-called “best dispersion relations” were used; for LINbO3 and PPLN, the data chosen was for congruent material, and in the case PPSLT the data used was for stoichiometric lithium tantalate. The validity of the Sellmeier expansion does not always cover the full A ranges. Table 5 Parameters of several crystals that can be used in ultrabroadband OPA/OPG schemes pumped below 1 um: \p is the “magic” pump wavelength for which the signal/idler GVD vanishes near degeneracy, AF is the fundamental wavelength for broadband SHG, AA/Av correspond to the wavelength/frequency range for maximum gain bandwidth calculated directly from Eq. (5) at the 1/2 level (crystal length 5 mm, pump intensity 50 GW/cm’), 6 /p and A are the phase-matching angle and the period for degenerate operation at the given p in birefringent and quasi-phase-matching, respectively, and the GVM parameter 1/vp — 1/vs,r is calculated for degenerate operation at Ap.”$

$where Aw = Awsg denotes the frequency change of the signal wave and, by energy conservation, the idler fre- quency change will be Aw; = — Aw. The phase-matching condition means that Aky = 0. The individual terms in Eq. (7) are used to evaluate the points, in terms of fre- quency, where the gain function G from Eq. (6) drops to 50% of its maximum value, which corresponds to the wave vector mismatch Aky/2 + +2(In 2)'/2(1/L)\/?. The re- sults, when using the first derivative (GVM approxima- tion) and the second derivative (GVD approximation), are well known [109, 110] and read, in terms of FWHM, for Av = Aw/27r In order to obtain analytical expressions for parametric gain bandwidth, the wave mismatch is usually expanded in a series as a function of frequency, assuming in a first approximation that the pump is monochromatic,$

$Figure 15 (online color at: www.lpr-journal.org) Normalized parametric gain in BIBO (negative type-I ooe interaction in the x—z plane) for different pump wavelengths (indicated) at exact phase-matching angles, for a crystal length of 5mm and pump intensity of 50 GW/cm”. Figure 16 Gain bandwidth (FWHM) as estimated from Fig. 15. The absolute gain G decreases from 1.41 x 108 at Ap = 720nm to 2.91 x 10’ at Ap = 780 nm and to 5.56 x 10° at \p = 840 nm.$

$Figure 17 (online color at: www.lpr-journal.org) Refractive index of BIBO for type-I SHG in the x—z plane. Red curves correspond to ooe and blue curves to eeo phase matching. The squares indicate the points where the first derivatives of the re- fractive index for the fundamental and second harmonic are equal (broadband SHG phase matching corresponding to Ar in Table 5) and the circles indicate the points of vanishing second derivative (corresponding to twice the “magic” pump wavelength or 2p in Table 5).$

$V. Petrov, M. Ghotbi, et al.: Femtosecond nonlinear frequency conversion based on BiB306 Figure 51 (online color at: www.Ipr-journal.org) Spectra of the WLC amplified in 3-mm thick (a) and 5-mm thick (b) BIBO: measured by the InGaAs spectrometer (black curves), reconstructed from the time-integrated XFROG trace (thin red curves) and computed from the Manley-Rowe relation (blue curves). The inset in (b) is a comparison of the spectra of the WLC seed generated in YAG (green line), and the amplified WLC in the 3-mm thick (purple line) and 5-mm thick (cyan line) BIBO crystals. shown in Figs. 51a and b, respectively. The partial spectrum up to 1600 nm (thick black curves) was directly recorded by an InGaAs-based spectrometer, while the complete spec- trum shown by the red curves was reconstructed from the XFROG trace recorded with a spectrometer and Si-detector array, see Fig. 53. Finally, the thick blue curves in Fig. 51 show the portion of the amplified spectrum (> 1600 nm) which was calculated from that directly measured below 1600 nm, using the Manley-Rowe relation. The obtained amplification bandwidth was ~ 80 THz for the 3-mm thick BIBO, and it increased to ~ 100 THz in the case of the 5-mm thick BIBO. InGaAs spectrometer and a power meter. Finally, using a fraction of the fresh pump beam as a probe (gate) pulse, XFROG (cross-correlation FROG based on SFG in a 10- um thick, type-I BBO crystal) measurements of the parametri- cally generated radiation were performed. The latter were used to reconstruct the entire spectrum and the integral pulse duration for the amplified WLC, as well as to obtain rough information about the phase-modulation. Care was taken to ensure collinear amplification in the BIBO crystals,$

$Figure 57 (online color at: www.lpr-journal.org) Parametric gain of BIBO for collinear-type ooe interaction at \p = 800 nm, calculatec for several fixed phase-matching angles close to degeneracy (0 ~ 11°). The crystal length, 3mm (a) and 5mm (b), and the pumy intensity, 100 GW/cm? (a) and 60 GW/cm? (b), correspond to the experimental conditions in Fig. 56. Figure 56 (online color at: www.lpr-journal.org) OPG spectra obtained with the 3-mm thick (a) and 5-mm thick (b) BIBO crystals, recorded with an InGaAs spectrometer at five different internal angles of the pump beam. The pump intensity is 100 GW/cm” (a) and 60 GW/cm*” (b).$