Fiber-optic delivery of high-peak-power Q-switched laser pulses for in-cylinder flow measurement

A bundle of optical fibers was constructed to deliver Q-switched frequency-doubled Nd:YAG laser pulses for the purpose of particle image velocimetry. Data loss that is due to fiber speckle was reduced by ensuring that each fiber was different in length by more than the coherence length of the laser being delivered. Hence, their speckle patterns will overlap but not interfere, producing more even illumination that is shown to reduce data loss. A custom-made diffractive optical element and careful endface preparation help to reduce damage to the fibers by the required high peak powers. With this method, pulse energies in excess of 25 mJ were delivered for a series of experimental trials within the cylinder head of an optically accessed internal combustion engine. Results from these trials are presented along with a comparison of measurements generated by conventionally delivered beams.     

Bundled hollow optical fibers for transmission of high-peak-power Q-switched Nd:YAG laser pulses

A hollow-fiber bundle was designed and used to deliver high-peak-power pulses from a Q-switched Nd:YAG laser. An 80 cm long bundle with a total diameter of 5.5 mm was composed of 37 glass capillaries with bore diameters of 0.7 mm. Beam-resizing optics with two lenses were used to couple the laser beam into the bundle. The measured coupling loss due to the limited aperture ratio of the bundle was 2.3 dB, and the transmission loss at wavelengths of 1064 and 532 nm was 0.3 dB. When an inert gas flowed through the bores of the capillaries, the maximum output pulse energy was 200 mJ, which was the limit of the laser used in the experiment. Hollow-fiber bundles withstand irradiation better than single hollow fibers and silica-glass optical fibers do. They are suitable for many dermatological applications because they can be used to irradiate a large area.     

Analysis of optical damage mechanisms in hollow-core waveguides delivering nanosecond pulses from a Q-switched Nd:YAG laser

Hollow-core waveguides consisting of a glass capillary tube with an internal reflective coating are capable of delivering pulse energies of tens of millijoules with improved focusability compared to step index fibers of similar core diameter. We demonstrate the capability of these fibers to deliver high-power Q-switched pulses at the fundamental (1064 nm), second (532 nm), and third (355 nm) harmonics of a Nd:YAG laser, both in terms of peak power and beam quality delivered. In terms of peak power delivery, the primary limitation is the occurrence of bend-induced optical damage to the reflective coating. The damage mechanism and the influential factors are analyzed, in particular, the dependence upon the number of guided modes, core diameter, coating thicknesses, and input polarization alignment.     

Cost-effective low timing jitter passively Q-switched diode-pumped solid-state laser with composite pumping pulses

A novel scheme that combines gain switching with passive Q switching of a miniature diode-pumped solid-state laser is proposed and implemented. A composite pumping pulse, consisting of a long, low-intensity pulse and a following short, high-intensity pulse, is used to reduce the timing jitter. A greater-than-tenfold reduction in timing jitter is demonstrated.     

Divergence temporal dynamics of a Q-switched laser

I address the influence of the direct coupling of the average lattice strains to the active ions on the pulse shape and the far-field beam spreading of a Q-switched laser. Calculations are made for the Cr(3+): sapphire system. The dynamic behavior of laser divergence as the pulse develops is studied without and with an internal aperture. The results can be used to search for similar effects in lasers involving other ions, such as Ti(3+) and Cr(4+).     

Increasing output energy from a passively Q-switched Er:glass laser

A method for increasing the output energy from newly developed passively Q-switched Er:glass eyesafe lasers is presented. The increase of energy is achieved by incorporating binary phase elements inside the laser cavities. Experimental results reveal that the output energies can be increased by more than a factor of two. Moreover, by manipulating the output phase with the binary phase elements, the peak energy density in the far field is increased by more than a factor of 4.5.     

Tungsten tri-oxide (WO3) film absorber for generating Q-switched pulses in erbium laser

ABSTRACT

This paper reports a new type of passive saturable absorber (SA) made of transition metal oxide (TMO) embedded in polyvinyl alcohol (PVA). The Tungsten trioxide (WO₃)-PVA SA is placed in an erbium-doped fibre laser cavity to produce Q-switched pulses operating at 1562.82?nm. The pulse laser starts to manifest at the threshold pump power of 40?mW and continues to exist until the maximum pump power of 195?mW. Within that pump power range, its pulse energy, repetition rate and pulse width vary from 98 to 142.85?nJ, 29.86 to 56.7?kHz and 5.032 to 1.85?µs, respectively. The pulse train is stable with a signal to noise ratio of 70?dB. This is the first demonstration of a Q-switched laser using such a SA.     

Single attosecond light pulses from multi-cycle laser sources

High harmonic generation provides a means of producing attosecond pulses of light which are the shortest, controllable probes available to science for time-resolving ultrafast dynamics. We review techniques based on high harmonic generation for generating single attosecond pulses using high-power, multi-cycle laser sources, including optical-, polarisation-, and ionisation-gating schemes as well as techniques based on field synthesis. By significantly reducing the technical demands placed on the driving laser, these techniques have the potential to greatly broaden the application base for attosecond pulses.     

Tunable picosecond blue and ultraviolet pulses from a diode-pumped laser system seeded by a gain-switched laser diode

Picosecond pulses emitted from a gain-switched laser diode have been amplified in a Ti:sapphire regenerative amplifier indirectly pumped by a 4-W laser diode. This all-solid-state system produced microjoule pulses tunable from 803 to 840 nm at repetition rates up to 25 kHz with durations of 70-100 ps. By frequency doubling and tripling the output, we generated blue and UV pulses tunable from 401 to 420 nm and from 268 to 280 nm, respectively. Average powers larger than 4 mW were reached in these two wavelength regions.     

Heat source density and temperature distribution in a laser ruby crystal

The heat source density in a laser ruby is calculated by numerical integration of the pumping and absorption spectra. The heat source density can be represented approximately by the formula q(r) = =q₁ + q₂ I₀ (r/r₀), which can be used to determine the temperature distribution in the crystal for typical pumping and cooling conditions.     

Design of a high-power Nd:YAG Q-switched laser cavity

An electro-optically Q-switched Nd:YAG laser resonator that uses two end prisms placed orthogonally perpendicular to each other has been designed. This configuration improves the stability of the resonator and does not alter the characteristics of the electro-optical Q switch. The outcoupling ratio of the cavity is optimized by a change in the azimuthal angle of a phase-matched Porro prism placed at one end of the cavity. The prism placed at the other end of the cavity is designed so that it introduces a phase change of II, regardless of its orientation and index of refraction, resulting in a more efficient and stable cavity.     

Fiber-optic detector using a mid-infrared diode laser and an acoustooptic modulator

Data are given for an infrared detector based on a semiconductor injection laser, which operates at a wavelength of 3.3 μm and is coupled by a chalcogenide optical fiber to an acoustooptic modulator made from an amorphous Si-Te alloy. The beam modulation coefficient reaches 70% for a pulse duration ≳0.3 μs, making the detector well suited to gas-analysis applications. Pis’ma Zh. Tekh. Fiz. 23, 14–18 (October 26, 1997)     

Polarization control of a Q-switched, diode-pumped Nd:YAG laser

We describe control of the polarization state of a diode-pumped Nd:YAG laser that is Q switched with an acousto-optic modulator (AOM). One of two orthogonal linear polarization states can be made dominant, depending on the amount of loss introduced by the AOM. Heterodyne beating indicates that the two polarization states are of slightly different frequencies.     

Speckle contrast reduction in a large-core fiber delivering Q-switched pulses for fluid flow measurements

Due to their capability for supporting high-peak powers, large-core hollow optical fibers may be used to deliver high-peak-power nanosecond pulses for the fluid flow measurement technique of particle image velocimetry [Meas. Sci. Technol. 16, 1119 (2005)]. One drawback of using such fibers for fluid flow measurements is that the output suffers from a speckled interference pattern due to the fiber's multimode nature, which can lead to a loss of spatial information and reduced data quality. Presented here is a technique to reduce the speckle contrast from these fibers when delivering nanosecond pulses. Significant smoothing of the output intensity distribution is demonstrated, giving an improved source of illumination for fluid flow measurements and other imaging techniques requiring pulsed laser illumination.     

Direct-write fabrication of functional protein matrixes using a low-cost Q-switched laser

We report the use of an inexpensive, small, and "turn-key" Q-switched 532-nm Nd:YAG laser as a source for nonlinear, direct-write protein microfabrication. In this approach, microJoule pulses (pulse widths, approximately 600 ps) are focused using high numerical aperture optics to submicrometer focal spots, creating instantaneous intensities great enough to promote multiphoton excitation of a photosensitizer and subsequent intermolecular cross-linking of protein molecules. By scanning the femtoliter focal volume through reagent solution, extended protein-based structures can be fabricated with precise, three-dimensional topographies. As with earlier studies using a femtosecond titanium:sapphire laser costing more than 100K, physically robust and chemically responsive microstructures can be fashioned rapidly with feature sizes smaller than 0.5 microm, and cross-linking can be achieved using both biologically benign sensitizers (e.g., flavins) and by using the proteins themselves to sensitize cross-linking. We demonstrate in situ fabrication to corral neurite outgrowth and show the ability to functionalize avidin structures with biotinylated reagents, an approach that enables chemical sensing to be performed in specified microenvironments. Characterization of this inexpensive, low-power source will greatly broaden access to direct-write protein microfabrication.     

Surface hardening of oil-hardened nonshrinking steel using ruby laser

Surface modification of metals and alloys using lasers has acquired considerable importance because the changes achieved are highly localized and superficial. In this process the mechanical and structural properties of the bulk are retained as such because of the high temperature gradient and high rate of change of temperature that are unattainable by conventional methods. The present report is on surface hardening of oil-hardened nonshrinking (OHNS) steel using Ruby laser in normal mode. It is found that surface hardening increases thrice the size of base material and the microstructure of the laser-heated zone changes from pearlite phase to martensite phase.