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

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.

High-order femtosecond vortices up to the 30th order generated from a powerful mode-locked Hermite-Gaussian laser

Femtosecond vortex beams are of great scientific and practical interest because of their unique phase properties in both the longitudinal and transverse modes, enabling multi-dimensional quantum control of light fields. Until now, generating femtosecond vortex beams for applications that simultaneously require ultrashort pulse duration, high power, high vortex order, and a low cost and compact laser source has been very challenging due to the limitations of available generation methods. Here, we present a compact apparatus that generates powerful high-order femtosecond vortex pulses via astigmatic mode conversion from a mode-locked Hermite-Gaussian Yb:KGW laser oscillator in a hybrid scheme using both the translation-based off-axis pumping and the angle-based non-collinear pumping techniques. This hybrid scheme enables the generation of femtosecond vortices with a continuously tunable vortex order from the 1st up to the 30th order, which is the highest order obtained from any femtosecond vortex laser source based on a mode-locked oscillator. The average powers and pulse durations of all resulting vortex pulses are several hundred milliwatts and <650 fs, respectively. In particular, 424-fs 11th-order vortex pulses have been achieved with an average power of 1.6 W, several times more powerful than state-of-the-art oscillator-based femtosecond vortex sources.

Intense vortical-field generation using coherent superposition of multiple vortex beams

Coherent beam combining technology applied to multiple vortex beams is a promising method to generate high-power vortex beams. We utilize the coherent combination of multiple Laguerre-Gaussian beams at the waist plane and propose theoretically a practical generation system for a high-power beam carrying orbital angular momentum by considering oblique incidence. The results demonstrate that the orbital angular momentum distribution of the combined field is similar to that of a single Laguerre-Gaussian beam within the Rayleigh length. Moreover, the combined field has relativistic intensity local spots that exhibit stable spatial propagation. The proposed system may potentially be applied to intense vortical fields, large scale nuclear fusion device, such as suppressing stimulated Raman scattering and filamentation when a laser beam propagates in plasma.

Third-order orbital angular momentum pulse generation from a passively Q-switched fiber laser

We propose and demonstrate an all-fiber passively Q-switched laser generating a third-order orbital angular momentum (OAM) pulse by introducing a few-mode long-period fiber grating (LPFG) into the laser cavity. The LPFG with asymmetric cross structure and strong refractive index modulation overcomes the coupling issue between the fundamental and the third-azimuthal-order (LP₃₁ or OAM₃) modes and realizes their direct conversion. A homemade graphene-based saturable absorber is used to realize Q-switched operation. The laser operates at a center wavelength of 1548.2nm, with a 3 dB spectral bandwidth of 0.4nm, and the OAM₊₃ and OAM_-3 beams can achieve the purity of 90.0% and 90.2%, respectively. This all-fiber Q-switched laser has simple and compact structure and high purity of OAM_±3 beams, which has potential applications in the fields of optical tweezers and material processing.

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, l_p= 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, l_i= 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 l_p= 5 and generating idler vortices of high order in the mid-IR.

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 l_i=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.

Controlled generation of array beams of higher order orbital angular momentum and study of their frequency-doubling characteristics

We report on a simple and compact experimental scheme to generate high-power, ultrafast, higher-order vortex-array beams. Simply by using a dielectric microlens-array (MLA) and a plano-convex lens, we have generated array-beams carrying the spatial property of the input beam. Considering the MLA as a 2D sinusoidal phase-grating, we have numerically calculated the intensity pattern of the array-beams in close agreement with the experimental results. Using vortex beams of order as high as l = 6, we have generated vortex array-beam with individual vortices of orders up to l = 6. We have also theoretically derived the parameters controlling the intensity pattern, size, and the array-pitch and verified experimentally. The single-pass frequency-doubling of vortex-array at 1064 nm in a 1.2 mm long BiBO crystal produced green vortex-array of order, l_sh = 12, twice the order of pump beam. Using lenses of different focal lengths, we have observed the vortex-arrays of all orders to follow a focusing dependent conversion similar to the Gaussian beam. The maximum power of the green vortex-array is measured to be 138 mW at a single-pass efficiency as high as ~3.65%. This generic experimental scheme can be used to generate the array beams of desired spatial intensity profile across a wide wavelength range by simply changing the spatial profile of the input beam.

High-power vortex beam generation enabled by a phased beam array fed at the nonfocal-plane

High-power vortex beams have extensive applications in optical communication, nonlinear frequency conversion, and laser processing. To overcome a single beam's power limitation, generating vortex beams, based on a phased beam array, is an intuitive idea that requires locking each beamlet's phase to a specific different value. Conventionally, the intensity profiles of the focal plane (far field) are used for extracting the cost functions in active phase control systems. However, as for generating vortex beams, the cost function extraction method at the focal plane suffers because the same intensity profile of the beam array could correspond to different phase distributions in near field. Thus, the accurate phase control signals are difficult to obtain. In this paper, a new concept of extracting cost functions at the non-focal-plane is firstly presented and analyzed in detail by numerical simulation. This cost function extraction method is an efficient way of generating vortex beams with different topological charges, including second-order Bessel-Gaussian beams. The new concept could provide a valuable reference and contribute to the practical implementation of generating vortex beams by coherent beam combining technology.

Direct generation of a narrow-linewidth Laguerre-Gaussian vortex laser in a monolithic nonplanar oscillator

Vortex laser beams carrying orbital angular momentum have been attracting a lot of interest in recent years. Here we demonstrate the direct generation of a vortex laser in a monolithic nonplanar ring cavity. The unidirectional and single-frequency operation of Laguerre-Gaussian modes is observed and characterized. Fork interferograms have been obtained using a simple interferometer based on a plano-concave lens, and the topological charge of vortex beam is determined. A spectral linewidth as narrow as 2.3 kHz is measured by beating with a reference laser. We believe that such a high coherent vortex laser can be beneficial for numerous applications, including precision measurements and optical communications.

Direct transfer of classical non-separable states into hybrid entangled two photon states

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.

Ultrashort vortex from a Gaussian pulse - An achromatic-interferometric approach

The more than a century old Sagnac interferometer is put to first of its kind use to generate an achromatic single-charge vortex equivalent to a Laguerre-Gaussian beam possessing orbital angular momentum (OAM). The interference of counter-propagating polychromatic Gaussian beams of beam waist ω_λ with correlated linear phase (ϕ₀ ≥ 0.025 λ) and lateral shear (y₀ ≥ 0.05 ω_λ) in orthogonal directions is shown to create a vortex phase distribution around the null interference. Using a wavelength-tunable continuous-wave laser the entire range of visible wavelengths is shown to satisfy the condition for vortex generation to achieve a highly stable white-light vortex with excellent propagation integrity. The application capablitiy of the proposed scheme is demonstrated by generating ultrashort optical vortex pulses, its nonlinear frequency conversion and transforming them to vector pulses. We believe that our scheme for generating robust achromatic vortex (implemented with only mirrors and a beam-splitter) pulses in the femtosecond regime, with no conceivable spectral-temporal range and peak-power limitations, can have significant advantages for a variety of applications.

Hollow Gaussian beam generation through nonlinear interaction of photons with orbital angular momentum

Hollow Gaussian beams (HGB) are a special class of doughnut shaped beams that do not carry orbital angular momentum (OAM). Such beams have a wide range of applications in many fields including atomic optics, bio-photonics, atmospheric science, and plasma physics. Till date, these beams have been generated using linear optical elements. Here, we show a new way of generating HGBs by three-wave mixing in a nonlinear crystal. Based on nonlinear interaction of photons having OAM and conservation of OAM in nonlinear processes, we experimentally generated ultrafast HGBs of order as high as 6 and power >180 mW at 355 nm. This generic concept can be extended to any wavelength, timescales (continuous-wave and ultrafast) and any orders. We show that the removal of azimuthal phase of vortices does not produce Gaussian beam. We also propose a new and only method to characterize the order of the HGBs.

Generation of "perfect" vortex of variable size and its effect in angular spectrum of the down-converted photons

The "perfect" vortex is a new class of optical vortex beam having ring radius independent of its topological charge (order). One of the simplest techniques to generate such beams is the Fourier transformation of the Bessel-Gauss beams. The variation in ring radius of such vortices require Fourier lenses of different focal lengths and or complicated imaging setup. Here we report a novel experimental scheme to generate perfect vortex of any ring radius using a convex lens and an axicon. As a proof of principle, using a lens of focal length f = 200 mm, we have varied the radius of the vortex beam across 0.3-1.18 mm simply by adjusting the separation between the lens and axicon. This is also a simple scheme to measure the apex angle of an axicon with ease. Using such vortices we have studied non-collinear interaction of photons having orbital angular momentum (OAM) in spontaneous parametric down-conversion (SPDC) process and observed that the angular spectrum of the SPDC photons are independent of OAM of the pump photons rather depends on spatial profile of the pump beam. In the presence of spatial walk-off effect in nonlinear crystals, the SPDC photons have asymmetric angular spectrum with reducing asymmetry at increasing vortex radius.

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.