Wavelength- and OAM-tunable vortex laser with a reflective volume Bragg grating

Wavelength-tunable Tm:Y2O3 ceramic vortex laser with a changeable orbital angular momentum state

Wavelength-tunable orbital angular momentum (OAM) lasers with controllable topological charges have the potential for serving as light sources for large-capacity optical communication by combining conventional wavelength division multiplexing (WDM) with OAM mode-division multiplexing (OAM-MDM). In this study, we demonstrate a wavelength-tunable Tm-bulk laser that can control OAM states in the 2-µm spectral range. The excitation conditions for different Laguerre-Gaussian (L G 0,l ) modes in a bulk laser cavity are theoretically determined by measuring the spatial propagation dynamics of the annular pump beam. As a proof-of-principle study, we experimentally generate OAM states of |ℏ| and |2ℏ| from a T m:Y 2 O 3 ceramic laser with a tunable emission wavelength using a Lyot filter (LF). The spatial properties of the scalar optical vortices are well conserved during wavelength tuning, indicating the feasibility of our approach for producing wavelength-tunable structured light. These OAM laser sources, which are characterized by their robustness and compactness, have potential applications in various areas such as optical communications, quantum optics, super-resolution microscopes, and more.

Flexible generation of broadly wavelength- and OAM-tunable Laguerre-Gaussian (LG) modes from a random fiber laser

Broadband wavelength tunable Laguerre-Gaussian (LG) mode with flexibly manipulated topological charge is greatly desired for large-capacity optical communication. However, the operating wavelengths achieved for the current LG modes are significantly restricted either by the emission spectrum of the intracavity gain medium or by the operation wavelengths of mode-conversion or modulation components. Here, broadband wavelength-tunable LG modes with a controllable topological charge are generated based on a random fiber laser (RFL) and a digital micromirror device (DMD). The RFL can produce broadly wavelength-tunable laser emissions spanning from 1044 to 1403 nm with a high spectral purity and an excellent beam quality, benefiting from the cascaded random Raman gain starting from a ytterbium fiber based active gain. A commercially available broadband DMD is then utilized to excite the LG modes with a flexibly tunable topological charge of up to 100 order through the super-pixel wavefront shaping technique. The combination of the RFL and the DMD greatly broadens the operating wavelength region of the LG modes to be achieved, which facilitates the capacity scaling-up in the orbital angular momentum multiplexed optical communication application.

Theoretical and Experimental Research on the Mode Modulation Regulation for the Mode-Tunable Vortex Laser Based on Mode Conversion and Intra-Cavity Modulation

The vortex laser beam has been widely applied in many fields for its unique properties. However, researchers have to conduct extensive and recurring experiments to find the modulation abilities of the vortex beam modes for a given resonant cavity. In this paper, a mode modulation regulation acquisition (MORA) method, investigating the relationship between the modes of the vortex beam and modulation parameters, is proposed and verified. A typical mode-tunable vortex laser, consisting of a classic plano-concave straight cavity, a vortex beam generation beamline, and a reference beam acquisition beamline, is used as the analysis and experiment object. The principle and working process of the MORA method is analyzed in the simulation, and its validity is verified in the experiment. Based on the obtained theoretical relationship between the modes of the vortex beam and modulation parameters, the MORA method could be used to help researchers in designing the practical vortex laser with target vortex beams output by optimizing the structure of the vortex laser, selecting the suitable intra-cavity modulation elements (IMEs), and pre-positioning the location of the IMEs.

Advances on Solid-State Vortex Laser

Vortex beams (VBs) are structured beams with helical wavefronts carrying orbital angular momentum (OAM) and they have been widely used in lots of domains, such as optical data-transmission, optical tweezer, quantum entanglement, and super-resolution imaging. The ability to generate vortex beams with favorable performance is of great significance for these advanced applications. Compared with extra-cavity schemes, such as spatial light modulation, mode conversion, and others which transform other modes into vortex modes, solid-state vortex lasers can output vortex beams directly and show advantages including a compact structure, high robustness, easy to integrate, and low cost. In this review, we summarize intra-cavity generation approaches to vortex beams in solid-state lasers. Our work on 1.6μm eye-safe vector vortex lasers is also introduced.

Optical vortex switch based on multiplexed volume gratings with high diffraction efficiency

Systems of controllable orbital angular momentum (OAM) require more compact, higher conversion efficiency and more tolerable wavelength or polarization. We introduce an optical vortex switch based on a multiplexed volume grating (MVG). The MVG recorded in a piece of photo-thermo-refractive (PTR) glass exhibits high diffraction efficiency (DE, also known as conversion efficiency in transporting), sensitive angular selectivity, and polarization-insensitivity. The effects of the incident divergence angle and polarization on the DE and the far-field diffraction profiles are demonstrated and investigated. It turns out that the divergence angle of the probe beam can greatly affect the DE. The fluctuation of the DE caused by polarization variation is less than 1.59%. This switch can be potentially applied in vortex tweezers, optical communication, and high power systems.

Generation of wavelength-tunable optical vortices using an off-axis spiral phase mirror

Wavelength-tunable optical vortices with a topological charge equal to l=1 of orbital angular momentum (OAM) were experimentally realized using a single off-axis spiral phase mirror (OSPM) with lasers of various visible-light wavelengths. Using an OSPM designed for 561 nm and an incidence angle of 45°, circular doughnut-shaped l=1 optical vortices were obtained at 561, 473, and 660 nm by rotating the OSPM to modify the laser incidence angle. Wavelength-tunable l=1 optical vortices were obtained at the respective incidence angles of 45°, 53.4°, and 33.7°, because the effective geometrical thickness of the OSPM, which determines the order of OAM, was identical at each wavelength. This flexible OSPM which operates over a wide wavelength range will provide continuously wavelength-tunable optical vortices for applications in the fields of advanced optics and photonics in which optical vortices with wide wavelength tunability are in demand.

Resonantly pumped Er:YAG vector laser with selective polarization states at 1.6 µm

A resonantly pumped Er:YAG vector laser emitting at 1645 nm with selective polarization states is demonstrated. A compact five-mirror resonator incorporated a pair of quarter-wave plates (QWPs), and a pair of q-plates (QPs) is employed. Cylindrical vector beams of all states on a single high-order Poincaré sphere could be obtained by rotating the QWPs and QPs relatively.

Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

Direct generation of vortex beams from a double-end polarized pumped Yb:KYW laser

Double-end polarized pumping scheme combined with off-axis pumping technique has been first introduced to generate vortex beams in a z-type cavity. By employing double-end pumping, two different transverse modes can be excited simultaneously. The phase delay between these two modes can be finely tuned by manipulating the cavity structure. Direct emission of a chirality controllable Laguerre Gaussian LG₀₁ vortex beam with slope efficiency of more than 40% has been realized by a double-end polarized pumped Yb:KYW laser. Other modes, such as dual-LG₀₁ mode, cross-shaped mode, and LG₁₀ mode, have also been demonstrated from our laser setup.

Generation of wavelength- and OAM-tunable vortex beam at low threshold

Wavelength- and OAM- tunable laser with large tunable range is the key source for the application in large capacity optical communications. In this paper, we demonstrate a wavelength- and OAM-tunable vortex laser in a 1.2 W single mode fiber coupled LD pumped Yb:phosphate laser. A z-type cavity has been used to precisely control the laser mode diameter. A thin film polarizer (TFP) is inserted to finely control the intra-cavity loss and tune the wavelength. Corresponding laser fundamental mode to pump beam ratio has been optimized to decrease the pump threshold for high order HG mode running. A pair of cylindrical lenses has been used to convert the HG mode to vortex output. The vortex beam with OAM-tunable range from 1ħ to 14 ħ with wavelength tuning range of ~36.2 nm for LG_0,1 vortex beam, and ~14.5 nm for LG_0,14 vortex beam at pump power of only 1.2 W have been realized, which is the largest tuning range of both OAM and wavelength at ~1 W pump power range to the best of our knowledge.

Wavelength-tunable Hermite-Gaussian modes and an orbital-angular-momentum-tunable vortex beam in a dual-off-axis pumped Yb:CALGO laser

A dual-off-axis pumping scheme is presented to generate wavelength-tunable high-order Hermite-Gaussian (HG) modes in Yb:CaGdAlO₄ lasers. The mode and wavelength can be actively controlled by the off-axis displacements and pump power. The purities of the output HG modes are quantified by intensity distributions and the measured M² values. The highest order reaches m=15 for stable HG_m,0 mode, and wavelength-tunable width is about 10 nm. Moreover, through externally converting the HG_m,0 modes, the vortex beams carrying orbital angular momentum (OAM) with a large OAM-tunable range from ±1ℏ to ±15ℏ are produced. This work is effective for largely scaling the spectral and OAM tunable ranges of optical vortex beams.