This work presents a review on the effect of transverse mode instability in high-power fiber laser systems and the corresponding investigations led worldwide over the past decade. This paper includes a description of the experimental observations and the physical origin of this effect, as well as some of the proposed mitigation strategies.

Transverse mode instability

This work presents a review on the effect of transverse mode instability in high-power fiber laser systems and the corresponding investigations led worldwide over the last decade. This manuscript includes the description of the experimental observations, the physical origin of this effect, as well as some of the proposed mitigation strategies.

Transverse Mode Instability

Solid-state optical refrigeration uses anti-Stokes fluorescence to cool macroscopic objects to cryogenic temperatures without vibrations. Crystals such as Yb3+-doped YLiF4 (YLF:Yb) have previously been laser-cooled to 91 K. In this study, we show for the first time laser cooling of a payload connected to a cooling crystal. A YLF:Yb crystal was placed inside a Herriott cell and pumped with a 1020-nm laser (47 W) to cool a HgCdTe sensor that is part of a working Fourier Transform Infrared (FTIR) spectrometer to 135 K. This first demonstration of an all-solid-state optical cryocooler was enabled by careful control of the various desired and undesired heat flows. Fluorescence heating of the payload was minimized by using a single-kink YLF thermal link between the YLF:Yb cooling crystal and the copper coldfinger that held the HgCdTe sensor. The adhesive-free bond between YLF and YLF:Yb showed excellent thermal reliability. This laser-cooled assembly was then supported by silica aerogel cylinders inside a vacuum clamshell to minimize undesired conductive and radiative heat loads from the warm surroundings. Our structure can serve as a baseline for future optical cryocooler devices.

/pdf/first-demonstration-of-an-all-solid-state-optical-cryocooler-42fx5f3kdn.pdf

First demonstration of an all-solid-state optical cryocooler.

Resonant excitation of terahertz (THz) radiation based on beating of two spatial-Gaussian lasers having different frequencies and wave numbers but the same electric field amplitudes is proposed in a spatially periodic density plasma in the presence of a static magnetic field applied perpendicular to the direction of propagation of the lasers. In this process, the ponderomotive force is developed with its components parallel and perpendicular to the direction of propagation of the lasers. This leads to a nonlinear oscillatory current that resonantly excites the THz radiation with the frequency of the order of the upper hybrid frequency. The contribution of magnetic field, laser beamwidth, and amplitude and periodicity of the density ripples is discussed for the efficient THz radiation generation. With the optimization of these parameters, the efficiency on the order of 10${}^{\ensuremath{-}3}$ or larger can be achieved in the present scheme.

Terahertz radiation generation by beating of two spatial-Gaussian lasers in the presence of a static magnetic field.

Nondegenerate four-wave mixing is demonstrated in Ti in-diffused lithium niobate waveguides. The second-order nonlinear interaction between two oppositely propagating guided modes was observed to generate a radiation field at the sum frequency into a direction perpendicular to the surface during the overlap of the incident waves.

Nondegenerate four wave mixing in integrated optics (A)

By stacking alternatively rotated gallium phosphide (GaP) plates, the maximum photon conversion efficiency of 40% for the terahertz (THz) generation based on difference-frequency generation has been achieved. The corresponding peak power generated inside the four GaP plates approaches 4 kW. As the number of plates is increased from four to five, the THz output power is significantly decreased, due to back parametric conversion.

Terahertz generation based on parametric conversion: from saturation of conversion efficiency to back conversion

We have implemented a single-photon detection system in the 1.55-μm region, based on frequency up-conversion in MgO-doped periodically poled LiNbO3 waveguide. We have subsequently reached a record-low dark count rate of 45 counts per second. The detectable signal photon rate, i.e., the rate for counting the up-converted photons corrected by the dark counts, noises, and losses, reaches a record-low value of 81 photons per second. Through free-space coupling, we have eliminated the dark counts induced by parametric fluorescence.

Approaching single-photon detection in near-infrared region

We have demonstrated the recovery of the distorted images by a phase plate to nearly their original qualities under a refection configuration. This is made possible by utilizing difference-frequency generation in a bulk KTiOPO4 (KTP) crystal and an AFB–KTP crystal composite. In such a geometry, both input and phase-conjugated waves propagate through the distorted phase plate for a round trip. They are reflected by the object, which can be far away from the imaging system, and captured by the cameras. Therefore, such a configuration is much more practical for eventually deploying our imaging system for realizing practical applications. We have demonstrated the advantages of the imaging system such as polarization insensitivity and broadband capability.

Image restoration based on phase conjugation in second-order nonlinear materials under reflection configuration.

The progress achieved on power scaling and compact and portable THz sources
 is reviewed. By reversely stacking the GaP plates, the photon conversion efficiency
 is improved from 25% to 40% which corresponds to the maximum value. When the number
 of the plates is increased from four to five, the output power decreases because of
 back conversion. The THz generation is also investigated by mixing the two
 frequencies generated by a single Nd:YLF solid-state laser. The average output power
 reaches 1 \mu W. The introduction of two Nd:YLF crystals significantly improves the
 output power to 4.5 µW. This configuration facilitates the generation of different
 output frequencies.

Recent progress on terahertz generation based on difference frequency generation: from power scaling to compact and portable sources (Invited Paper)

The bonds established with Onyx’s adhesive-free bonding (AFB) technique between optical materials, such as trivalent rare-earth ion doped YAG, un-doped YAG, or sapphire, rely on Van der Waals forces. The AFB technique is being applied to fabricate true crystalline fiber waveguides for high power, high beam quality laser emission, and walk-off compensated nonlinear crystal stacks for high efficiency, high beam quality optical nonlinear conversion.

Adhesive-free bond (AFB) true crystalline fiber waveguides and walk-off compensated nonlinear crystal stacks: optical components for high performance lasing and frequency conversion

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