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We report a compact source of high power, tunable, ultrafast yellow radiation using fourth-harmonic generation of a mid-IR laser in two-stage frequency-doubling processes. Using C r 2+ : Z n S laser at 2360 nm frequency-doubled in a multi-grating MgO:PPLN crystal, we have generated near-IR radiation tunable across 1137-1200 nm with average output power as high as 2.4 W and pulse width of ∼60 f s . Subsequently, the near-IR radiation is frequency-doubled using a bismuth triborate (BIBO) crystal to produce coherent yellow radiation tunable across 570-596 nm with a maximum average power of ∼1 W . The source has a maximum mid-IR to yellow (near-IR to yellow) single-pass conversion efficiency as high as ∼29.4 % (∼47 % ). Without any pulse compression, the yellow source has output pulses at a repetition rate of 80 MHz with a pulse width of ∼130 f s in Gaussian-shaped and a spectral width of ∼4 n m corresponding to a time-bandwidth product of 0.45. The generated output beam has a Gaussian transverse beam profile with measured M 2 values of M x 2∼1.07 and M y 2∼1.01.

We report a continuous-wave (cw) optical parametric oscillator (OPO) generating optical vortices tunable in the ultraviolet (UV). Based on MgO:sPPLT as the nonlinear crystal, the singly resonant OPO is pumped by a cw vortex beam in the green, and deploying intracavity sum-frequency generation (SFG) between the undepleted pump and the Gaussian resonant signal in the crystal of BiB 3 O 6 , it can generate optical vortices of order, l uv =1 and 2, tunable across 332-344 nm in the UV with a maximum power of 12 mW. Due to conservation of orbital angular momentum in the parametric process, the OPO also produces a non-resonant idler output beam in a vortex spatial profile of order l i =1 and 2, identical to the pump vortex, with the signal beam in Gaussian distribution. The idler vortex is tunable across 1172-1338 nm with maximum output power of 1.3 W.

We report on a compact and simple ultrafast source producing tunable radiation in the near-IR wavelength range. Based on single-pass frequency doubling of an ultrafast Cr 2+ :ZnS laser at 2360 nm with pulse width of 43 fs at a repetition rate of 80 MHz in MgO:PPLN crystal, the source produces maximum average output power of ∼2.43  W tunable across 1137-1200 nm with a maximum single-pass conversion efficiency as high as 65%. Without use of any pulse compression technique, the source produces output pulses in Gaussian shape with measured pulse width of ∼60  fs and spectral width of 39 nm centered at 1180 nm corresponding to a time-bandwidth product of 0.5. The output beam has a Gaussian spatial profile with measured M 2 <1.32 and a peak-to-peak power fluctuation of 3% over 2 h. Using MgO:PPLN crystal of two different lengths, 1 mm and 2 mm, we have observed that the optimum second-harmonic generation efficiency of an ultrafast pulse laser, even in the presence of temporal walk-off, appears in the low pump focusing condition.

We report on direct generation of optical vortices from a continuous-wave (cw), Gaussian beam pumped doubly resonating optical parametric oscillator (DRO). Using a 30-mm long MgO doped periodically poled lithium tantalate (MgO:sPPLT) crystal based DRO, pumped in the green by a frequency-doubled Yb-fiber laser in Gaussian spatial profile we have generated signal and idler beams in vortex mode of order, l = 1, tunable across 970-1178 nm. Controlling the overlap between the Gaussian pump beam with the fundamental cavity mode of the resonant signal and idler beams of the DRO through the tilt of the pump beam and/or the cavity mirror in transverse plane, we have generated both signal and idler beams in vortex and vortex dipole spatial profiles. Using the theoretical formalism for the vortex beam generation through the superposition of two Gaussian beams we have numerically calculated the spatial profile of the generated beam in close agreement with our experiment results. The generic experimental scheme can be used to generate optical vortex across the electromagnetic spectrum and in all time scales (cw to ultrafast) using suitable OPO.

We report on a simple experimental scheme based on a pair of cylindrical lenses (convex and concave) of the same focal length and common optical elements, producing high power optical beams in 1D and/or 2D Airy intensity profiles with laser polarization as the control parameter. Using an ultrafast Yb-fiber laser at 1064 nm of average power of 5 W in a Gaussian spatial profile and pulse width of ∼180  fs, we have generated 1D and 2D Airy beams at an efficiency of 80% and 70%, respectively, and a pulse width of ∼188 and ∼190  fs, respectively. We have measured the transverse deflection rate of 1D and 2D beams to be ∼5.0×10 -5 1/mm and ∼2.0×10 -5 1/mm, respectively. Simply rotating the polarization state of the 1D cubic phase modulated beam in the experiment, we can produce 1D and 2D Airy beams on demand. Using a 5 mm long bismuth borate (BiB 3 O 6 ), we have also studied frequency-doubling characteristics of both 1D and 2D Airy beams. Like the 2D Airy beam, the 1D Airy beam also produces a frequency-doubled 1D Airy and an additional 1D spatial cubic structure. Like the Gaussian beams, we have observed the focusing dependence of conversion efficiency for both 1D and 2D Airy beams, producing green 1D and 2D Airy beams of output powers in excess of 110 and 150 mW for 3.4 and 2.8 W of fundamental power, respectively.

We report on the orbital angular momentum (OAM) exchange among the interacting beams in an ultrafast optical parametric oscillator (OPO). The singly-resonant OPO is synchronously pumped by a picosecond vortex beam from a frequency-doubled Yb-fiber laser at 532 nm in the green. We demonstrate successful transfer of the pump OAM mode to the non-resonant idler beam tunable across 1109-1209 nm, with OAM as high as l p =3. Controlling the cavity loss and spatial overlap between the resonant signal and the pump beam in the nonlinear crystal, we have generated signal and idler OAM mode combinations, (l s ,l i ) of (0,2) and (1,1), and (0,3) and (1,2) for pump OAM mode l p =2 and 3, respectively. Using a pump power of 1 W, we have generated idler OAM mode of orders, l i =1, 2, and 3, with maximum output powers of 202, 113, and 57 mW, respectively. To the best of our knowledge, this is the first report on controlled generation of OAM modes from an ultrafast OPO.

We propose and experimentally demonstrate a novel experimental scheme to generate high on-axis peak intensity, segmented, smooth, zero-order quasi-Bessel beams with tunable ranges. Illuminating the axicon with hollow Gaussian beams (HGBs) of different orders, we have generated Bessel beams of varying ranges at different positions away from the axicon. The presence of a dark core at the center of the HGBs removes the effect of imperfection in the axicon tip. As a result, the entire power of the input beam is transformed into a zero-order Bessel beam without any on-axis intensity modulation. We observe the decrease in range and increase in on-axis peak intensity of the zero-order Bessel beam with the order of HGBs. Controlling the superposition of the HGBs of different orders to the axicon, we have demonstrated the increase in the range of the Bessel beam. The current technique can also produce Bessel beams of different intensity distributions, including single-peak or multiple-peak Bessel beams. Using single-pass second-harmonic generation in nonlinear crystals of different lengths, we have further verified the increase of on-axis peak intensity of the Bessel beam with the order of the HGBs and the increase in the range of the Bessel beam due to the superposed HGBs.

We report on a novel experimental scheme to generate continuous-wave (cw), high-power, and higher-order optical vortices tunable across a mid-IR wavelength range. Using a cw, two-crystal, singly resonant optical parametric oscillator (T-SRO) and pumping one of the crystals with a Gaussian beam and the other crystal with optical vortices of orders l p =1-6, we have directly transferred the vortices at near-IR to the mid-IR wavelength range. The idler vortices of orders l i =1-6 are tunable across 2276-3576 nm with a maximum output power of 6.8 W at an order of l i =1 for the pump power of 25 W, corresponding to a near-IR vortex to mid-IR vortex conversion efficiency as high as 27.2%. Unlike the SROs generating optical vortices restricted to lower orders (≤2) due to the elevated operation threshold of SROs with higher-order pump vortices, here the coherent energy coupling between the resonant signals of two crystals of T-SRO facilitates the transfer of pump vortex of any order to the idler wavelength without a stringent operation threshold condition. The generic experimental scheme can be used in any wavelength range across the electromagnetic spectrum and in all timescales, from cw to ultrafast regimes.

We report on a high-power, continuous-wave source of optical vortices tunable in the mid-infrared (mid-IR) wavelength range. Using the orbital angular momentum (OAM) conservation of the parametric processes and the threshold conditions of the cavity modes of the singly resonant optical parametric oscillator (SRO), we have transferred the OAM of the pump beam at the near-infrared wavelength to the idler beam tunable in the mid-IR. Pumped with a vortex beam of order l p =1 at 1064 nm, the SRO, configured in a four curved mirror-based ring cavity with a 50 mm long MgO-doped periodically poled LiNbO 3 crystal, produces an idler beam with an output power in excess of 2 W in a vortex spatial profile with the order l i =1, tunable across 2217-3574 nm and corresponding signal beam in Gaussian intensity distribution across 1515-2046 nm. For pump vortices of the order l p =1 and 2, and a power of 22 W, the SRO produces idler vortices of the same order as that of the pump beam with a maximum power of 5.23 and 2.3 W, corresponding to near-IR to mid-IR vortex conversion efficiency of 23.8% and 10.4%, respectively. The idler vortex beam has a spectral width, and a passive rms power stability of 101 MHz and 4.9% over 2 h, respectively.

We report on single-pass optical parametric generation for high power, high repetition rate (RR), ultrafast broadband optical radiation in the mid-IR. Taking advantage of broad phase-matching bandwidth (BW) of the crystals for the interacting waves having zero group velocity mismatch, we have used a 50 mm long MgO-doped periodically poled LiNbO 3 crystal to develop a single-pass, parametric source producing femtosecond output pulses at a RR of 78 MHz. Pumping with a femtosecond Yb-fiber laser at 1064 nm, the source produces signal and idler radiation tunable across 1422-1561 nm and 4229-3342 nm, respectively. The signal radiation has a pulse and spectral BW of 296 fs and 9.2 nm centered at 1492 nm, respectively, with a time-BW product ∼0.37, close to the transform limit. The idler radiation has spectral BW as high as 123 nm centered at 3709 nm. The source produces a signal (idler) beam of power of 2.07 W (0.54 W) at 1492 nm (3709 nm) in a Gaussian spatial profile with peak-to-peak passive power fluctuation better than 5% (4%) over 4 h at a single-pass conversion efficiency as high as ∼55%.

We report on the generation of ultrafast vortex beams in the deep ultraviolet (DUV) wavelength range at 266 nm, for the first time to our knowledge. Using a Yb-fiber-based green source in combination with two spiral phase plates of orders 1 and 2, we were able to generate picosecond Laguerre-Gaussian (LG) beams at 532 nm. Subsequently, these LG beams were frequency doubled by single-pass, second-harmonic generation in a 10 mm-long β-BaB 2 O 4 crystal to generate ultrafast vortex beams at 266 nm with a vortex order as high as 12, providing up to 383 mW of DUV power at a single-pass, green-to-DUV conversion efficiency of 5.2%. The generated picosecond UV vortex beam has a spectral width of 1.02 nm with a passive power stability better than 1.2% rms over >1.5  h.

We report on efficient nonlinear generation of ultrafast, higher order "perfect" vortices at the green wavelength. Based on Fourier transformation of the higher order Bessel-Gauss (BG) beam generated through the combination of the spiral phase plate and axicon, we have transformed the Gaussian beam of the ultrafast Yb-fiber laser at 1060 nm into perfect vortices of power 4.4 W and order up to 6. Using single-pass second-harmonic generation (SHG) of such vortices in 5 mm long chirped MgO-doped, periodically poled congruent LiNbO 3 crystal, we have generated perfect vortices at green wavelength (530 nm) with output power of 1.2 W and vortex order up to 12 at a single-pass conversion efficiency of 27%, independent of the orders. This is the highest single-pass SHG efficiency of any optical beams other than Gaussian beams. Unlike the disintegration of higher order vortices due to spatial walk-off effect in birefringent crystals, here, the use of the quasi-phase-matching process enables generation of high-quality vortices, even at higher orders. The green perfect vortices of all orders have temporal and spectral widths of 507 fs and 1.9 nm, respectively, corresponding to a time-bandwidth product of 1.02.

We report on the development of a high-power, high-repetition-rate, fiber laser based source of ultrafast ultraviolet (UV) radiation. Using single-pass second-harmonic generation and subsequent sum-frequency generation (SFG) of an ultrafast ytterbium fiber at 1064 nm in 1.2 and 5 mm long bismuth triborate (BIBO) crystals, respectively, we have generated UV output power as high as 1.06 W at 355 nm with single-pass near-infrared-to-UV conversion efficiency of ∼22%. The source has output pulses of temporal and spectral widths of ∼576  fs and 1.6 nm, respectively, at 78 MHz repetition rate. For given crystals and laser parameters, we have experimentally verified that the optimum conversion efficiency of the SFG process requires interacting pump beams to have the same confocal parameters. We also present a systematic study on the power ratio of pump beams influencing the overall conversion of the UV radiation. The UV source has a peak-to-peak short-term power fluctuation of <2.2%, with a power drift of 0.76%/h associated to different loss mechanisms of the BIBO crystal at UV wavelengths. At tight focusing, the BIBO crystal has a broad angular acceptance bandwidth of (∼2  mrad·cm) for SFG of the femtosecond laser.

We report on frequency-doubling characteristics of high-power, ultrafast optical vortex beams in a nonlinear medium. Based on single-pass second-harmonic generation (SHG) of optical vortices in 1.2 mm long bismuth triborate (BIBO) crystal, we studied the effect of different parameters influencing the SHG process in generating high-power and higher-order vortices. We observed a decrease in SHG efficiency with the order, which can be attributed to the increase of the vortex beam area with order. Like a Gaussian beam, optical vortices show focusing-dependent conversion efficiency. However, under similar experimental conditions, the optimum focusing condition for optical vortices is reached at tighter focusing with orders. We observed higher angular acceptance bandwidth in the case of optical vortices than that of a Gaussian beam; however, there is no substantial change in angular acceptance bandwidth with vortex order. We also observed that in the frequency-doubling process, the topological charge has negligible or no effect in temporal and spectral properties of the beams. We have generated ultrafast vortices at 532 nm with power as much as 900 mW and order as high as 12. In addition, we have devised a novel scheme based on linear optical elements to double the order of any optical vortex at the same wavelength.

We report on experimental realization of optimum focusing condition for type-I second-harmonic generation (SHG) of high-power, ultrafast laser in "thick" nonlinear crystal. Using single-pass, frequency doubling of a 5 W Yb-fiber laser of pulse width ~260 fs at repetition rate of 78 MHz in a 5-mm-long bismuth triborate (BIBO) crystal we observed that the optimum focusing condition is more dependent on the birefringence of the crystal than its group-velocity mismatch (GVM). A theoretical fit to our experimental results reveals that even in the presence of GVM, the optimum focusing condition matches the theoretical model of Boyd and Kleinman, predicted for continuous-wave and long-pulse SHG. Using a focusing factor of ξ=1.16 close to the estimated optimum value of ξ=1.72 for our experimental conditions, we generated 2.25 W of green radiation of pulse width 176 fs with single-pass conversion efficiency as high as 46.5%. Our study also verifies the effect of pulse narrowing and broadening of angular phase-matching bandwidth of SHG at tighter focusing. This study signifies the advantage of SHG in "thick" crystal in controlling SH-pulse width by changing the focusing lens while accessing high conversion efficiency and broad angular phase-matching bandwidth.

We report a compact tunable 240-MHz picosecond source for the ultraviolet based on intra-cavity frequency doubling of a signal-resonant MgO:sPPLT optical parametric oscillator (OPO), synchronously pumped at 532 nm in the green by the second harmonic of a mode-locked Yb-fiber laser at 80-MHz repetition rate. By deploying a 30-mm-long multi-grating MgO:sPPLT crystal for the OPO and a 5-mm-long BiB(3)O(6) crystal for internal doubling, we have generated tunable UV radiation across 317-340.5 nm, with up to 30 mW at 334.5 nm. The OPO also provides tunable visible signal in the red, across 634-681 nm, and mid-infrared idler radiation over 2429-3298 nm, with as maximum signal power of 800 mW at 642 nm. The signal pulses have a temporal duration of 12 ps at 665 nm and exhibit high spatial beam quality with Gaussian profile. The signal power is recorded to be naturally stable with a fluctuation of 1.4% rms over 14 hours, while UV power degradation has been observed and studied.

We present theoretical and experimental study of a continuous-wave, two-crystal, singly-resonant optical parametric oscillator (T-SRO) comprising two identical 30-mm-long crystals of MgO:sPPLT in a four- mirror ring cavity and pumped with two separate pump beams in the green. The idler beam after each crystal is completely out-coupled, while the signal radiation is resonant inside the cavity. Solving the coupled amplitude equations under undepleted pump approximation, we calculate the maximum threshold reduction, parametric gain acceptance bandwidth and closest possible attainable wavelength separation in arbitrary dual-wavelength generation and compare with the experimental results. Although the T-SRO has two identical crystals, the acceptance bandwidth of the device is equal to that of a single-crystal SRO. Due to the division of pump power in two crystals, the T-SRO can handle higher total pump power while lowering crystal damage risk and thermal effects. We also experimentally verify the high power performance of such scheme, providing a total output power of 6.5 W for 16.2 W of green power at 532 nm. We verified coherent energy coupling between the intra-cavity resonant signal waves resulting Raman spectral lines. Based on the T-SRO scheme, we also report a new technique to measure the temperature acceptance bandwidth of the single-pass parametric amplifier across the OPO tuning range.

We describe the critical design parameters and present detailed experimental and theoretical studies for efficient, continuous-wave (cw), single-pass second harmonic generation (SHG) based on novel cascaded multicrystal scheme, providing >55% conversion efficiency and multiwatt output powers at 532 nm for a wide range of input fundamental powers at 1064 nm. Systematic characterization of the technique in single-crystal, double-crystal and multicrystal schemes has been performed and the results are compared. Optimization of vital parameters including focusing and phase-matching temperature at the output of each stage is investigated and strategies to achieve optimum SHG efficiency and power are discussed. Relevant theoretical calculations to estimate the effect of dispersion between the fundamental and the SH beam in air are also presented. The contributions of thermal effects on SHG efficiency roll-off have been studied from quasi-cw measurements. Using this multicrystal scheme, stable SH power with a peak-to-peak fluctuation better than 6.5% over more than 2 hours is achieved in high spatial beam quality with M2<1.6.

Characteristics of high-power, narrow-linewidth, continuous-wave (cw) green radiation obtained by simple single-pass second-harmonic-generation (SHG) of a cw ytterbium fiber laser at 1064 nm in the nonlinear crystals of PPKTP and MgO:sPPLT are studied and compared. Temperature tuning and SHG power scaling up to nearly 10 W for input fundamental power levels up to 30 W are performed. Various contributions to thermal effects in both crystals, limiting the SHG conversion efficiency, are studied. Optimal focusing conditions and thermal management schemes are investigated to maximize SHG performance in MgO:sPPLT. Stable green output power and high spatial beam quality with M(2)<1.33 and M(2)<1.34 is achieved in MgO:sPPLT and PPKTP, respectively.

We report efficient generation of high-power, cw, single-frequency radiation in the green in a simple, compact configuration based on single-pass, second-harmonic generation of a cw ytterbium fiber laser at 1064 nm in periodically poled KTiOPO(4). Using a crystal containing a 17 mm single grating with period of 9.01 microm, we generate 6.2 W of cw radiation at 532 nm for a fundamental power of 29.75 W at a single-pass conversion efficiency 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 TEM(00) spatial profile with M(2)<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.

We report a new source of cw, single-frequency radiation in the blue, offering extended tunability and practical powers in a compact, all-solid-state design. The device is based on a green-pumped, cw, singly resonant optical parametric oscillator using MgO-doped stoichiometric lithium tantalate (MgO:sPPLT) as the nonlinear material. By internal second-harmonic generation of the resonant near-infrared signal radiation in a 5 mm BiB(3)O(6) crystal, we generate nearly 450 mW of cw, single-frequency blue power over a tunable range of 425-489 nm with a linewidth of 8.5 MHz and a Gaussian spatial beam profile. The demonstrated wavelength coverage can be further extended by using alternative gratings for the MgO:sPPLT crystal.

We report major enhancements in the overall performance of continuous-wave singly-resonant optical parametric oscillators (cw SROs) through finite output coupling of the resonant wave. Using a cw SRO based on MgO:sPPLT pumped at 532 nm, we demonstrate improvements of 1.08 W in total output power, 10% in total extraction efficiency, and a 130-nm extension in the useful tuning range, while maintaining pump depletions of 70%, idler output powers of 2.59 W, and a minimal increase in oscillation threshold of 24%. The output-coupled cw SRO can deliver a total power of up to 3.6 W at 40% extraction efficiency across 848-1427 nm. The single-frequency resonant wave also exhibits a higher spectral purity than the non-resonant output.