Transverse-mode selective laser operation by unicursal fast-scanning pumping

Single-frequency orbital angular momentum switchable modes from a microchip laser

We demonstrate the direct generation of single-frequency switchable orbital angular momentum (OAM) modes in a 1 µm wavelength range using a Nd:YVO4 microchip laser. The 808 nm laser diode pump beam is shaped into annular through an axicon associated with a lens. By adjusting the diameter and power of the annular pump beam, various OAM modes with different mode volumes can oscillate inside the Nd:YVO4 microchip. Moreover, a single-frequency output is also available due to the short cavity of the microchip. In the proof-of-principle experiment, single-frequency twofold multiplexed OAM modes | ± 1> and | ± 2> are generated, with experimentally measured fidelity higher than 96%. This work presents a compact and versatile single-frequency OAM source and will inspire multiple advanced scenarios ranging from classical to quantum photonics.

Stable pulsed operation of Lissajous structured beams by Nd:YVO4/Cr4+:YAG laser in a concave-convex resonator

A Nd:YVO₄/Cr⁴⁺:YAG laser with a symmetric concave-convex cavity ensuring strong intracavity beam focusing on the absorber is designed for stable pulsed operation of Lissajous structured modes with transverse patterns as Lissajous figures. Setting the cavity length to fulfill the criterion for efficient passive Q switching (PQS), as well as to meet the accidental degenerate conditions, Lissajous pulsed beams with well-defined structures and good temporal stability are created under two-dimensional off-axis pumping. Although the multi-transverse-mode oscillation inevitably induces asynchronous pulsation and leads the short-term pulse profiles to reveal parasitic effects, the overall long-term behavior of Lissajous pulses can be kept regular with amplitude fluctuations ≤15% and pulse-to-pulse timing jitter ≤5%. With the maximum peak power exceeding 500 W at a pump power of 4.5 W, the PQS Lissajous modes are further transformed into trochoidal pulsed beams to realize high-order and high-peak power structured vortex fields.

1645-nm single-frequency vortex laser from an Er:YAG nonplanar ring oscillator

A 1645-nm single-frequency vortex beam with narrow linewidth from an Er:YAG nonplanar ring oscillator (NPRO) using an annular pump beam is demonstrated. The pump beam from a 1532-nm fiber laser is shaped to an annular beam by an axicon. The Er:YAG NPRO generates a 1.96-W single-frequency vortex beam under a pump power of 13 W. The linewidth of the 1645-nm vortex laser is measured as 6 kHz. This work provides a convenient way of single-frequency vortex beam generation.

Control of transverse mode in a He-Ne laser using an astigmatic resonator

The realization of output of the controllable transverse mode in a laser resonator has always been the key problem in applications of lasers. At present, the theory of optical resonators for passive resonators is relatively mature, but the non-uniformity of gain media greatly affects the output of the laser transverse mode for astigmatic resonators in operation; especially for gas lasers, controllable high-order modes have not been studied. To realize the theory of an astigmatic passive resonator of a gas laser as a good approximation of an active resonator, this paper develops the theory of selecting the laser eigenmode through an astigmatic resonator, and verifies that the two-dimensional tilt of the cavity mirror can break the axial symmetry. Controllable output of the laser mode is realized in real time and conveniently for the first time, to the best of our knowledge. This scheme is not only common to all kinds of lasers, but also has important research significance for the high-order modes required for real-time and rapid regulation of gas lasers under operating conditions.

Self-mixing thin-slice solid-state laser Doppler velocimetry with much less than one feedback photon per Doppler cycle

The drastic shortening of a photon lifetime as compared with normal TEM00 operations has been shown to be associated with the formation of annular mode operations in a thin-slice Nd:GdVO4 laser with tilted laser diode end pumping. The 15 dB enhancement of the signal-to-noise ratio, owing to the shortened photon lifetime, has been demonstrated in the self-mixing laser Doppler velocimetry experiment in comparison with the TEM00 operations, where the minimum intensity feedback rate from a target to the laser for successful measurements was estimated to be -123  dB, which corresponds to 0.007 photon per Doppler cycle.