We review our previous results on efficient, high-power, and compact terahertz (THz) sources based on parametric processes. By reversely stacking GaP plates, we have improved photon conversion efficiency from 25% to 40%, which corresponds to the maximum value. As the number of the plates is increased from 4 to 5, the output power is decreased due to back conversion. The corresponding peak power of the THz output reaches 4 kW. We have investigated THz generation by mixing two frequencies generated by a single Nd:YLF solid-state laser. The average output power reaches 1 μW. By introducing two Nd:YLF crystals, we significantly improved the output power to 4.5 μW. Such a configuration allows us to generate different output frequencies. We review our progress for making our THz source further compact by taking advantage of passively Q-switched laser pulses. We have demonstrated that an intracavity can be utilized to enhance the THz output powers by two orders of magnitude.

Progress in terahertz sources based on difference-frequency generation [Invited]

This review paper covers the forensic-relevant literature for the analysis and detection of explosives and explosives residues from 2016-2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/Resources/Documents#Publications.

Interpol review of detection and characterization of explosives and explosives residues 2016-2019

When two Nd:YLF crystals share a Cr:YAG crystal functioning as a single passive Q switch, the timing jitter between each pair of dual-frequency pulses generated by the two crystals has been reduced by a factor of 20. Such a reduction in the timing jitter allows us to generate terahertz pulses by focusing such a passively Q-switched laser beam onto a nonlinear crystal. Such a result represents the first step for us to eventually implement a compact terahertz source based on ultracompact microchip lasers.

Investigation of terahertz generation from passively Q-switched dual-frequency laser pulses.

Optical fiber lasers and components for optical fiber laser. An optical fiber laser can comprise a fiber laser cavity having a wavelength of operation at which the cavity provides output light, the cavity including optical fiber that guides light having the wavelength of operation, the fiber having first and second lengths, the first length having a core having a V-number at the wavelength of operation and a numerical aperture, the second length having a core that is multimode at the wavelength of operation and that has a V-number that is greater than the V-number of the core of the first length optical fiber at the wavelength of operation and a numerical aperture that is less than the numerical aperture of the core of the first length of optical fiber. At least one of the lengths comprises an active material that can provide light having the wavelength of operation via stimulated emission responsive to the optical fiber receiving pump light. Components include a mode field adapter and optical fiber interconnection apparatus, which can be used to couple the first and second lengths of optical fiber, or can couple the fiber laser to an optical fiber power amplifier, which can be a multimode or single mode amplifier.

Optical fiber laser, and components for an optical fiber laser, having reduced susceptibility to catastrophic failure under high power operation

An optical waveguide having means for performing optical coupling with high efficiency at a predetermined position in an optical circuit substrate and which optionally includes an optical-electrical circuit board. Also provided are an optical waveguide, an optical path thereof being changed in an optical circuit at a steep angle and the optical waveguide for performing coupling and splitting of light being decreased in size in the optical circuit. The optical waveguide has a core and a cladding layer, and a wall surface, which is formed by cutting out at least a part of the core in a thickness direction of the core through irradiation of a laser beam and crosses at least a part of the core, which is a specular surface.

Optical waveguide having specular surface formed by laser beam machining

We have explored using 808nm Vertical Cavity Surface emitting laser (VCSEL) arrays for end-pumping of Nd:YAG
lasers. A variety of laser designs were explored including a compact passively Q-switched lasers that produced a 22mJ
pulse having a pulse width of <1.5ns, and an actively Q-switched laser that produced a 40mJ pulse having a 7 ns pulse
width. The VCSEL pumped actively Q-switched laser was used as a source for sum frequency generation. Using a
2mm type II KTP and 3mm type I LBO, we generated greater than 5mJ at 355nm with a 21% THG conversion
efficiency.

Compact VCSEL pumped Q-switched Nd:YAG lasers

A method and a corresponding apparatus for side-coupling light into an optical waveguide, such as a double-cladding fiber, are described. The method includes emitting light from a light source and coupling the emitted light into a side of a waveguide that defines a slit disposed on the side of the waveguide at an angle relative to an axis of the waveguide. The emitted light is incident on a surface of the slit and the slit surface reflects the incident light into the waveguide.

Method and apparatus for coupling light into a waveguide using a slit

A novel eye-safe, long range laser pointer system for use in day or night conditions is described. The system uses a short pulse laser and a gated camera to detect the laser spot at long ranges in the presence of a strong solar background. The camera gate is synchronized with incident laser pulses using a separate large area, fast photodiode to detect the high peak power pulses. Alternately, gate synchronization using a GPS-disciplined clock can be used. Eye-safe systems operating in the near-UV or SWIR band are described.

Laser pointer system for day and night use

We present a compact, side pumped passively Q-switched Yb:YAG laser that was operated in a burst mode with pump durations of 2-4 ms at low duty cycles. Intra-pump pulse Q-switched pulse repetition frequencies varied from 5-20 kHz depending on the transmission of the Cr:YAG saturable absorber, which was varied from 70% to 94%. Pump duration, pulse repetition frequency and output coupler reflectivity were optimized to yield maximum Yb:YAG laser average power and laser efficiency, while providing sufficient peak intensity, typically 0.3-1 MW, to enable efficient forth harmonic generation (FHG). Pulse energies and durations were in ranges of 0.3-1.8 mJ and 1.5-7ns, respectively, dependent on the unbleached transmission of the Cr:YAG saturable absorber. We achieved an optical efficiency of greater than 15% for the Yb:YAG laser. Extra-cavity 515 nm second harmonic generation (SHG) was achieved using a 5mm long KTP crystal. The 515 nm light was then frequency doubled by focusing it into a 7mm long BBO crystal, resulting in a 15% conversion efficiency from 1030nm to 257.5 nm, with an average UV power greater than 100 mW.

UV by the fourth harmonic generation of compact side-pumped Yb:YAG laser emission

We have fabricated prototype frequency tripled Nd:YAG lasers using 808nm Vertical Cavity Surface emitting laser (VCSEL) arrays for end-pumping. The passively Q-switched Nd:YAG laser generated 15mJ pulses with a duration of 2-4 ns. Used as a source for third harmonic generation, the laser produced in excess of 2mJ at 355nm. Of particular concern was the impact of temperature variation on conversion efficiency, which included effects for both the source laser and non-linear crystals. Various solutions to the temperature effects were explored to enable operation of the frequency tripled laser over a wide temperature range.

Wide temperature operation of a VCSEL pumped 355nm frequency tripled Nd:YAG laser

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