Harnessing nonlinear frequency upconversion of Talbot effect with flexible Talbot lengths.

We report on a simple experimental scheme demonstrating nonlinear frequency upconversion of the Talbot effect with controllable Talbot lengths at high conversion efficiency. Using a microlens array (MLA) as an array illuminator at 1064 nm onto a 1.2-mm-thick BiBO crystal, we have observed the second harmonic Talbot effect in green at 532 nm with a Talbot length twice that of the pump Talbot length. However, the Talbot length is constant for fixed parameters of the periodic object and the laser wavelength. With the formulation of a suitable theoretical framework, we have implemented a generic experimental scheme based on the Fourier transformation technique to independently control the Talbot lengths of the MLA in both the pump and the second harmonic, overcoming the stringent dependence of MLA parameters on the self-images. Deploying the current technique, we have been able to tune the Talbot lengths from zT = 26 cm to zT = 62.4 cm in the pump and zT = 12.4 cm to zT = 30.8 cm in the second harmonic, respectively. The single pass conversion efficiency of the Talbot images is 2.91% W-1, an enhancement of a factor of 106 as compared to the previous reports. This generic experimental scheme can be used to generate long-range self-images of periodic structures and also to program desired Talbot planes at required positions at both pump and upconverted frequency to avoid any mechanical constraints of experiments.

Tunable, high-power, high-order optical vortex beam generation in the mid-infrared.

We report the generation of tunable high-order optical vortices in the mid-infrared (mid-IR) using a picosecond optical parametric oscillator (OPO). The OPO is based on MgO:PPLN as the nonlinear gain medium and synchronously pumped by a mode-locked Yb-fiber laser at 1064 nm. Using a singly-resonant oscillator configuration for the OPO, we have achieved direct transfer of pump optical vortices to the non-resonant idler beam, with the resonant signal in the Gaussian cavity mode. We demonstrate the successful transfer of pump optical vortices of order, lp = 1 to 5, to the idler beam of the same order across the mid-IR, with an output power of 630 mW to 130 mW across 2538 nm to 4035 nm for the highest idler vortex order, li = 5. To the best of our knowledge, this is the first report of an OPO pumped by a vortex beam of order as high as lp = 5 and generating idler vortices of high order in the mid-IR.

Nonlinear frequency conversion of 3D optical bottle beams generated using a single axicon.

We demonstrate a novel experimental scheme to generate and study the nonlinear frequency conversion of a three-dimensional (3D) optical Bessel bottle beam (BBB). Using a single axicon and standard optical components and controlling the spot size and divergence of the input Gaussian beam to the axicon, we have generated stable micron-size, high-power optical BBB with tunable spatial characteristics. The BBB has a series of low-intensity regions surrounded by high intensity with diameters of ∼30µm and 17 µm, respectively, at a variable period of 2.3 to 6.4 mm along with the beam propagation. Using the single-pass second harmonic generation (SHG) of femtosecond BBB at 1064 nm in a bismuth triborate nonlinear crystal, we have generated BBB at 532 nm with output power as high as 75 mW and single-pass SHG efficiency of 1.9%. We also observed the self-healing of the BBB at both pump and SHG wavelengths. It is interesting to note that the pump beam truncation shows self-healing in the SHG beam. Such observation proves the direct transfer of the pump's spatial characteristics to the SHG beam in the nonlinear process, potentially useful for imaging even in the turbid medium in biology. This generic scheme can be used at different wavelengths and timescales (continuous-wave to ultrafast).

Tunable vortex beam generation using an optical parametric oscillator with an antiresonant-ring interferometer.

We report a high-average-power picosecond optical vortex source tunable in the near-infrared, using an antiresonant-ring (ARR) interferometer internal to an optical parametric oscillator (OPO) in combination with an external cylindrical lens for astigmatic mode conversion. The ARR OPO is tunable in the signal across 1457-1647 nm with a vortex intensity profile and up to 1 W of average power at 1602 nm. The corresponding idler is tunable over 3006-3945 nm in a Gaussian intensity profile with as much as 1.6 W at 3168 nm. The vortex signal and the Gaussian idler exhibit passive power stability better than 1.7% rms and 1.3% rms, respectively, over >1h. The signal pulses have a Gaussian duration of <19ps with a time-bandwidth product of ΔτΔν<3.6 across the tuning range.

Multi-structured-beam optical parametric oscillator.

Structured beams, conventionally generated through the spatial mode conversion of the Gaussian laser beams, have attracted great interest in recent years. Optical parametric oscillators (OPOs) have demonstrated the potential for the generation of tunable structured beams directly from an input pump source. However, to date, a particular OPO design has been shown to produce such beams only in a specific configuration and different spatial structured beams require different system architectures. Here, we report the generation of multiple-structured beams from a single OPO device. Using a vortex-beam-pumped ultrafast OPO in singly-resonant oscillator design and through the control of the mode structure of the resonant beam using a simple intracavity aperture, we generate vortex, Airy, vortex Airy, and Gaussian signal beams over a tunable wavelength range across 1457-1680 nm, simultaneous with vortex beam in the non-resonant idler across 2902-3945 nm, from different ports of the device. The signal and idler vortices have output power in excess of 1 W and maximum vortex order of li=2, while the Airy beam and vortex Airy beam have output power of more than 200 mW. The generic experimental design can be used to provide multi-structured spatial beams with broad tunability across different spectral regions by proper selection of pump laser and nonlinear material and in all times-scales from continuous-wave to ultrafast femtosecond domain.

Watt-level, ultrafast, tunable yellow source based on single-pass, fourth-harmonic generation of Cr2+:ZnS laser at 2360 nm.

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 Cr2+:ZnS 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 ∼60fs. 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 ∼1W. 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 ∼130fs in Gaussian-shaped and a spectral width of ∼4nm corresponding to a time-bandwidth product of 0.45. The generated output beam has a Gaussian transverse beam profile with measured M2 values of Mx2∼1.07 andMy2∼1.01.

Controlled generation of vortex and vortex dipole from a Gaussian pumped optical parametric oscillator.

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.

Single-pass, second-harmonic generation of high-power, ultrafast mid-IR Cr2+:ZnS laser at 2360 nm.

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 Cr2+: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 M2<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.

Tunable ultraviolet vortex source based on a continuous-wave optical parametric oscillator.

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 BiB3O6, it can generate optical vortices of order, luv=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 li=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.

On-axis intensity modulation-free, segmented, zero-order Bessel beams with tunable ranges.

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.

High-power, continuous-wave, tunable mid-IR, higher-order vortex beam optical parametric oscillator.

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 lp=1-6, we have directly transferred the vortices at near-IR to the mid-IR wavelength range. The idler vortices of orders li=1-6 are tunable across 2276-3576 nm with a maximum output power of 6.8 W at an order of li=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.

Orbital angular momentum exchange in a picosecond optical parametric oscillator.

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 lp=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, (ls,li) of (0,2) and (1,1), and (0,3) and (1,2) for pump OAM mode lp=2 and 3, respectively. Using a pump power of 1 W, we have generated idler OAM mode of orders, li=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.

Simultaneous generation of high-power, ultrafast 1D and 2D Airy beams and their frequency-doubling characteristics.

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 (BiB3O6), 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.

High power, high repetition rate, tunable broadband mid-IR source based on single-pass optical parametric generation of a femtosecond laser.

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 LiNbO3 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%.

Continuous-wave, singly resonant parametric oscillator-based mid-infrared optical vortex source.

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 lp=1 at 1064 nm, the SRO, configured in a four curved mirror-based ring cavity with a 50 mm long MgO-doped periodically poled LiNbO3 crystal, produces an idler beam with an output power in excess of 2 W in a vortex spatial profile with the order li=1, tunable across 2217-3574 nm and corresponding signal beam in Gaussian intensity distribution across 1515-2046 nm. For pump vortices of the order lp=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.

Efficient nonlinear generation of high power, higher order, ultrafast "perfect" vortices in green.

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 LiNbO3 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.

Ultrafast optical vortex beam generation in the ultraviolet.

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 ß-BaB2O4 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.

High-power, high-repetition-rate, Yb-fiber laser based femtosecond source at 355 nm.

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.

Frequency-doubling characteristics of high-power, ultrafast vortex beams.

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.

All-periodically poled, high-power, continuous-wave, single-frequency tunable UV source.

We report on experimental demonstration of an all-periodically poled, continuous-wave (CW), high-power, single-frequency, ultra-violet (UV) source. Based on internal second-harmonic-generation (SHG) of a CW singly resonant optical parametric oscillator (OPO) pumped in the green, the UV source provides tunable radiation across 398.94-417.08 nm. The compact source comprising of a 25-mm-long MgO-doped periodically poled stoichiometric lithium tantalate (MgO:sPPLT) crystal of period Λ(SLT)=8.5 µm for OPO and a 5-mm-long, multi-grating (Λ(KTP)=3.3, 3.4, 3.6 and 3.8 µm), periodically poled potassium titanium phosphate (PPKTP) for intra-cavity SHG, provides as much as 336 mW of UV power at 398.94 nm, corresponding to a green-to-UV conversion efficiency of ∼6.7%. In addition, the singly resonant OPO (SRO) provides 840 mW of idler at 1541.61 nm and substantial signal power of 108 mW at 812.33 nm transmitted through the high reflective cavity mirrors. UV source provides single-frequency radiation with instantaneous line-width of ∼18.3 MHz and power >100 mW in Gaussian beam profile (ellipticity >92%) across the entire tuning range. Access to lower UV wavelengths requires smaller grating periods to compensate high phase-mismatch resulting from high material dispersion in the UV wavelength range. Additionally, we have measured the normalized temperature and spectral acceptance bandwidth of PPKTP crystal in the UV wavelength range to be ∼2.25°C·cm and ∼0.15 nm·cm, respectively.