Search

We report a generic method for transforming the non-collinear down-converted photons in a “perfect” annular ring spatial distribution insensitive to the change in phase matching conditions as required for the stable single photon and entangled photon sources.

We report on an imaging-inspired measurement of orbital angular momentum (OAM) using only a simple tilted lens and an Intensified Charged Coupled Device (ICCD) camera, allowing us to monitor the propagation of OAM structured photons over distance, crucial for free-space quantum communication networks. We demonstrate measurement of OAM orders as high as 14 in a heralded single-photon source (HSPS) and show, for the first time, the imaged self-interference of photons carrying OAM in a modified Mach-Zehnder Interferometer (MZI). The described methods reveal both the charge and order of a photons OAM, and provide a proof of concept for the interference of a single OAM photon with itself. Using these tools, we are able to study the propagation characteristics of OAM photons over distance, important for estimating transport in free-space quantum links. By translating these classical tools into the quantum domain, we offer a robust and direct approach for the complete characterization of a twisted single-photon source, an important building block of a quantum network.

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 > 1 h . The signal pulses have a Gaussian duration of < 19 p s with a time–bandwidth product of Δ τ Δ ν < 3.6 across the tuning range.

The optical vortex beams have attracted a great deal of interest in recent times due to their wide range of applications in classical and quantum optics [1] . Due to the presence of the orbital angular momentum (OAM) per photon and the doughnut-shaped intensity distribution, the optical vortices have been used for a variety of studies with free space propagation [2] . While the propagation dynamics of the classical optical vortices have been studied in the literature, there is no such study for the single photons carrying OAM. Here we report on the experimental measurement of divergence of single-photon carrying OAM.

Two-photon interference, commonly known as Hong-Ou-Mandel (HOM) interference [1] , has attracted a great deal of interest in quantum sensing, including the precision timing measurements [2] and quantum optical coherence tomography [3] . While the dip of the HOM interference is the direct measure of the purity of the single-photon, the width of the HOM dip relates to the coherence time of the interfering photons. The broad consensus for using the HOM interferometer for sensing application remained the HOM dip width as the limiting factor for the precision measurements. As a consequence, the ultra-broadband photon sources have long been hailed as a vital prerequisite for ultra-precise HOM interferometry. Here, we have studied the variation in the width of the HOM dip with the crystal length for the continuous-wave pump laser and measured the HOM dip as low as 8.2 ± 0.2 µm corresponding to the spectral width of the single-photon source of 79.6±1.9 nm useful for sensitive sensing applications.

The recent advances in the field of spatial engineering of coherent sources have opened up wide interest for utility in new application areas. Among various structured beams, optical vortices [1] and Airy beams [2] have proved useful in a variety of applications including quantum information and communication, material lithography, micro particle manipulation and light sheet microscopy. Typically, such beams are generated through spatial modulation of Gaussian laser beams. Since the generation of vortex and Airy beams requires different spatial phase modulation techniques, such beams are generated in separate experimental schemes. Here, we report a novel experimental scheme for simultaneous generation of various types of structured beams including Gaussian, vortex, Airy, and vortex Airy beams, which are also tunable across a broad wavelength range by using an optical parametric oscillator (OPO) in singly resonant oscillator (SRO) configuration.

Here, we report on a simple experimental scheme to demonstrate the second harmonic (SH) Talbot effect. Using a dielectric Microlens array (MLA) as an array illuminator at 1064 nm on to a 1.2 mm thick BiBO crystal, we have observed the SH Talbot effect in green at 532 nm. Furthermore, we have shown a simple control scheme to tune the Talbot lengths of the self-images in both pump and SH. This generic experimental scheme can be used to generate long range self-images of periodic structures, and can be used to program desired Talbot planes, at predetermined positions.

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

We report on efficient, two stage single-pass second harmonic generation of ultrafast Cr2+:ZnS laser with spectral bandwidth of 138 nm centered at ~2360 nm and pulse width of ~43 fs at a repetition rate of 80 MHz into tunable yellow radiation across 577 - 589 nm in multi-grating MgO:PPLN crystals. A maximum average output power ~940 mW at 589 nm wavelength with a single-pass conversion efficiency as high as 41% was achieved. The yellow radiation has a spectral bandwidth of 2 nm and pulse-width of ~913 fs in absence of any pulse compression with a time-bandwidth product of 1.58.

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.

We report on the direct measurement of topological charge of single photon generated through the down-conversion of vortex pump beam using a tilted lens and coincidence detection based on intensified charge coupled device.

We report on the theoretical and experimental results on single-pass, second harmonic generation (SHG) of ultrafast, non-collinear Poincaré into blue. The lemon and star topologies of the SHG are twice that of the pump beam.

We report on generation of micro-size high peak power optical Bessel bottle beams at 1064 nm wavelength using a single axicon and optical feedback consists of linear optical elements and studied the frequency doubling characteristics.

We experimentally demonstrate the generation of vector beams through the spin-orbit interaction of circularly-polarized light in indium-tin-oxide film at wavelengths close to the zero-permittivity spectral region.

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

The ultrafast optical radiation in the yellow spectral region is of great interest for variety of scientific and technological applications including astronomy, spectroscopy and medicine, with potential to revolutionize eye surgeries through exquisite accuracy [1, 2]. However, it is relatively difficult to access the yellow region, especially when good beam quality, high power and high efficiency are required. Till date, various types of yellow lasers have been reported [3, 4] mostly in CW regime and nanosecond timescales. Here, we report on a compact, high-power, high repetition-rate, ultrafast source of yellow radiation. Using an ultrafast Cr2+:ZnS laser of average power of 4.5 W with spectral width of Δλ=138 nm centered at 2.36 μm, producing output pulses of width ∼40 fs at a repetition rate of 80 MHz, and two stage single-pass second harmonic generation (SHG), we have generated ultrafast radiation of power 1 W tunable across 577–589 nm. The schematic of the experimental setup is shown in Fig. 1(a). The single-pass SHG of Cr2+:ZnS laser in a 2 mm long 5% mol doped MgO:PPLN multi-grating crystal (Λ=33.15–35.25 μm), giving second harmonic (SH) pulses of 90 fs at 1.18 μm, with a maximum average power of 2.3 W at a single-pass efficiency as high as 65%. Further the 1.18 μm radiation is frequency-doubled in a 1 mm long and 0.5∗1 mm2 in aperture, MgO:PPLN crystal (Λ=8.9 −9.45 μm) producing yellow radiation. The acceptance bandwidth (FWHM) and temporal walk-off length (Lt) of the second crystal are calculated to be 3 nm and 288 μm, respectively. In order to optimize the focusing condition for maximum fourth harmonic generation (FHG) efficiency, we pumped the crystal at constant power of 1.3 W at 1.18 μm and recorded the fourth harmonic (FH) power as a function of focusing parameter, ξ =L/b, where, L is the length of the crystal and b=2πnw 0 2/λ, w 0 is the beam waist radius, n is the refractive index of the crystal and λ is the input wavelength.

Optical vortices are beams with screw like (helical) phase variation along the propagation direction. This phase variation is given by exp(imθ) in the azimuthal plane, where m is the charge of optical vortices. Due to the doughnut shape intensity distribution and orbital angular momentum (OAM) content, optical vortices have found applications in various fields of science and technology. Commercially available mode converters transforming Gaussian beam into vortices suffer from the common drawback of low power handling capabilities and/or narrow wavelength coverage. To avoid such limitations, here we present a new technique of generating high power optical vortices over a wide wavelength range by using a Gaussian beam pumped anti-resonant-ring (ARR) optical parametric oscillator (OPO) [1]. By controlling the superposition of the Gaussian beams of an intracavity ARR, we have generated first-order HG modes [2] and further transformed into optical vortices using cylindrical lens [3], providing tunability across1457 nm to 1647 nm with an output power as high as 539 mW.

Vector-vortex beams, having both phase and polarization singularities, are of great interest for a variety of applications. Generally, such beams are produced through systematic control of phase and polarization of the laser beam, typically external to the source. However, efforts have been made to generate vector-vortex beams directly from the laser source. Given the operation of the laser at discrete wavelengths, vector-vortices are generated with limited or no wavelength tunability. Here, we report an experimental scheme for the direct generation of vector-vortex beams. Exploiting the orbital angular momentum conservation and the broad wavelength versatility of an optical parametric oscillator, we systematically control the polarization of the resonant beam using a pair of intracavity quarter-wave plates to generate coherent vector-vortex beam tunable across 964–990 nm, with output states represented on the higher-order Poincaré sphere. The generic experimental scheme paves the way for new sources of structured beams in any wavelength range across the optical spectrum and in all time-scales from continuous-wave to ultrafast regime.

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 ∼ 30 µ m 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).

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 .

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.

We report on the generation of high power ultrafast optical vortex arrays of controlled intensity distribution. Using microlens array we have generated vortex arrays at 1064 nm and frequency-doubled into vortex arrays at new wavelength.

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 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.

We demonstrate the direct transfer of pump vortex mode to the idler beam in a singly- resonant optical parametric oscillator, generating ultrafast vortex beam of order as high as l=3, tunable across 1109-1209 nm.

We report on a novel method to generate high peak intensity, segmented, smooth, zero-order Bessel beams with tunable range peak position using axicon with hollow Gaussian beam (HGB) as pump mode.

We report on controlling the bi-photon orbital angular momentum (OAM) eigenmodes in the spontaneous parametric down conversion process by simply adjusting the asymmetry of the pump vortex beam. Adjusting the optic axis of the spiral phase plate (SPP) of phase winding corresponding to OAM mode, l, with respect to the beam propagation axis, we have transformed a Gaussian beam into an asymmetric vortex beam with OAM modes, l, l-1, l-2, 0 with different weightages. Pumping the nonlinear crystal with such asymmetric vortices and controlling their asymmetry we have tailored the spiral spectrum of the biphoton OAM eigenmodes. Calculating the Schmidt number of the biphotons we observe the increase in the spiral bandwidth of the OAM eigenmodes and hence the dimensionality of the system. Although we have restricted our study to show the increase in spiral bandwidth of the biphotons by simply controlling the asymmetry of the pump vortices, we can, in principle, further enhance the dimensionality of the entangled states by manipulating the pump beam size and crystal length.

We demonstrate a novel experimental scheme to generate tunable vector vortex beam directly from an optical parametric oscillator (OPO). Using a nanosecond doubly-resonant OPO, we produce vector vortex beam tunable across 964-990 nm.

We report on simultaneous generation of high power, 1D and 2D Airy beam using a pair of concave and convex cylindrical lenses in a novel experimental scheme and studied their frequency doubling characteristics.

We report on direct transfer of vortices at near-IR to the mid-IR wavelengths using a dual-crystal singly-resonant optical parametric oscillator producing vortices of order up to l=6 and power of 6 W across 2270-3560 nm.

We demonstrate a novel experimental scheme to generate tunable vector vortex beam directly from an optical parametric oscillator (OPO). Using a nanosecond doubly-resonant OPO, we produce vector vortex beam tunable across 964–990 nm.

We report a continuous-wave source of ultraviolet radiation in vortex spatial intensity profile tunable across 332-344 nm based on a green-pumped optical parametric oscillator.

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%.

Optical vortices, having doughnut-shaped spatial profile with zero intensity around the phase singularity, are of great interest due to its wide range of applications in variety of fields in science and technology. Two spatially separated entwined vortices with opposite topological charge forms a vortex dipoles. Due to their entwining nature, the vortex dipoles are created and annihilated in pairs of opposite charge, and carry a very stable topological structures. In recent times, vortex dipoles have created a lot of interest in studying in propagation dynamics [1] and optical vortex metrology [2]. Typically, optical vortex dipoles are created using holographic techniques. Recently, a new technique has been demonstrated to generate vortex dipole through coupling between the amplitude and phase differences of the superposing beams [3]. Here we report, for the first time to the best of our knowledge, on the generation of tunable vortex dipole directly from an optical parametric oscillator (OPO). Pumping a continuous wave (cw), doubly resonant OPO with Gaussian beam and carefully adjustment of the cavity mirrors, we have generated vortex dipole of power as much as 84 mW tunable across 968 nm-1181 nm. The generic scheme can used to generate vortex dipole at any wavelength and at all time scales.

Spontaneous parametric down-conversion (SPDC), one of the most important nonlinear processes, is of paramount interest especially in the field of quantum optics due to its intrinsic capability in generating entangled photon pairs in different degree of freedoms (DoFs). The spatial distribution of the paired photons generated through SPDC process are highly influenced by different crystal parameters including birefringence and length, and the spatial structure of the pump beam [1]. Recent studies have shown the transfer of pump properties such as phase profile and doughnut intensity distribution [2] into the transverse amplitude of heralded single photon. Here we report, first time to best of our knowledge, the direct transfer of the non-separable state in OAM and polarization DoFs of classical pump beam to generate two photons state with hybrid entanglement in both OAM and polarization. Using same scheme one can produce macroscopic entanglement with very high quanta of OAM.

Hybrid entangled states, having entanglement between different degrees-of-freedom (DoF) of a particle pair, are of great interest for quantum information science and communication protocols. Among different DoFs, the hybrid entangled states encoded with polarization and orbital angular momentum (OAM) allow the generation of qubit-qudit entangled states, macroscopic entanglement with very high quanta of OAM and improvement in angular resolution in remote sensing. Till date, such hybrid entangled states are generated by using a high-fidelity polarization entangled states and subsequent imprinting of chosen amount of OAM using suitable mode converters such as spatial light modulator in complicated experimental schemes. Given that the entangled sources have feeble number of photons, loss of photons during imprinting of OAM using diffractive optical elements limits the use of such hybrid states for practical applications. Here we report, on a simple generic experimental scheme to generate hybrid entangled states in polarization and OAM through direct transfer of classical non-separable states of the pump beam in parametric down conversion process. As a proof of principle, using local non-separable pump states of OAM mode l = 3, we have produced quantum hybrid entangled states with entanglement witness parameter of ~1.25 ± 0.03 violating by 8 standard deviation.

Controlled switching of orbital angular momentum (OAM) of light at practical powers over arbitrary wavelength regions can have important implications for future quantum and classical systems. Here we report on a single source of OAM beams based on an optical parametric oscillator (OPO) that can provide all such capabilities. We demonstrate active transfer of OAM modes of any order, jl pj, of pump to the signal and idler in an OPO, to produce jl pj1 different OAM states by controlling the relative cavity losses of the resonated beams. As a proof-of-principle, we show that when pumping with the OAM states jl pj 1 and jl pj 2 for different relative losses of signal and idler, the OPO has two (j1;0i and j0;1i) and three (j2;0i, j1;1i, and j0;2i) output states, respectively. Our findings show that using a suitable loss modulator, one can achieve rapid switching of the OAM mode in OPO output beams in time. ©

We report a high power ultrafast Airy beam source producing femtosecond pulses at multi-gigahertz (GHz) repetition rate (RR). Based on intra-cavity cubic phase modulation of an optical parametric oscillator (OPO) designed in high harmonic cavity configuration synchronous to a femtosecond Yb-fiber laser operating at 78 MHz, we have produced ultrafast 2D Airy beam at multi-GHz repetition rate through the fractional increment in the cavity length. While small (3 (MgO:PPLN) crystal for efficient generation of ultrafast Airy beam and broadband mid-IR radiation. Pumping the MgO:PPLN crystal of grating period, Λ = 30 μm and crystal temperature, T = 100 °C using a 5-W femtosecond laser centred at 1064 nm, we have produced Airy beam radiation of 684 mW in ~639 fs (transform limited) pulses at 1525 nm at a RR of ~2.5 GHz. Additionally, the source produces broadband idler radiation with maximum power of 510 mW and 94 nm bandwidth at 3548 nm in Gaussian beam profile. Using an indirect method (change in cavity length) we estimate maximum RR of the Airy beam source to be ~100 GHz.