Visible lasers have a wide range of applications in imaging, spectroscopy and displays. Unfortunately, they suffer from coherent artifacts such as speckle. Various compounding techniques have been developed to remove speckle, but these methods usually involve mechanically moving parts and require long acquisition times. A different approach to prevent speckle formation is developing lasers with low spatial coherence. A careful design of the laser cavity can facilitate lasing in many spatial modes with distinct emission pattern. The total emission from those mutually incoherent lasing modes has low spatial coherence. To date, several types of such lasers have been developed, but most of them have emission beyond the visible spectrum, making them unsuitable for imaging or display applications that require visible light.
An alternative way of making visible sources, especially of green color, is frequency doubling of infrared (IR) lasers. We develop a green light source with low spatial coherence via intracavity frequency doubling of a solid-state degenerate laser. The second harmonic emission is distributed over a few thousands independent transverse modes, and exhibits low spatial coherence. A strong suppression of speckle formation is demonstrated for both fundamental and second harmonic beams. Using the green emission for fluorescence excitation, we show the coherent artifacts are removed from the full-field fluorescence images. The achievable high power, low spatial coherence, and good directionality make the green degenerate laser an attractive illumination source for parallel imaging and projection display.
Semiconductors such as Si and GaAs are transparent to infrared laser radiation with wavelengths >1.2 μm. Focusing
laser light at the back surface of a semiconductor wafer enables a novel processing regime that utilizes this transparency.
However, in previous experiments with ultrashort laser pulses we have found that nonlinear absorption makes it
impossible to achieve sufficient optical intensity to induce material modification far below the front surface. Using a
recently developed Tm:fiber laser system producing pulses as short as 7 ns with peak powers exceeding 100 kW, we
have demonstrated it is possible to ablate the “backside” surface of 500-600 μm thick Si and GaAs wafers. We studied
laser-induced morphology changes at front and back surfaces of wafers and obtained modification thresholds for multipulse
irradiation and surface processing in trenches. A significantly higher back surface modification threshold in Si
compared to front surface is possibly attributed to nonlinear absorption and light propagation effects. This unique
processing regime has the potential to enable novel applications such as semiconductor welding for microelectronics,
photovoltaic, and consumer electronics.
Utilizing the transparency of silicon at 2 μm, we are able to ablate the backside of 500-μm thick
silicon wafers without causing any damage to the front surface using a novel nanosecond
Tm:fiber laser system. We report on our high energy/high peak power nanosecond Tm:fiber
laser and provide an initial description of the effects of laser parameters such as pulse duration
and energy density on the ablation, and compare thresholds for front and backside machining.
The ability to selectively machine the backside of silicon wafers without disturbing the front
surface may lead to new processing techniques for advanced manufacturing in solar cell and
Femtosecond laser direct writing (FLDW) has been widely employed to create volumetric structures in transparent
materials that are applicable as various photonic devices such as active and passive waveguides, couplers, gratings,
and diffractive optical elements (DOEs). The advantages of fabrication of volumetric DOEs using FLDW include
not only the ability to produce embedded 3D structures but also a simple fabrication scheme, ease of customization,
and a clean process. DOE fabrication techniques using FLDW are presented as well as the characterization of laserwritten
DOEs by various methods such as diffraction efficiency measurement. Fresnel zone plates were fabricated in
oxide glasses using various femtosecond laser systems in high and low repetition rate regimes. The diffraction
efficiency as functions of fabrication parameters was measured to investigate the dependence on the different
fabrication parameters such as repetition rate and laser dose. Furthermore, several integration schemes of DOE with
other photonic structures are demonstrated for compact photonic device fabrication.
In recent years, a major interest in surface as well as bulk property modification of semiconductors using laser irradiation
has developed. A.Kar et al.  and E.Mazur et al.  have shown introduction and control of dopants by long-pulse
laser irradiation and increased absorption due to femtosecond irradiation respectively. With the development of mid-IR
sources, a new avenue of irradiation can be established in a spectral region where the semiconductor material is highly
transparent to the laser radiation. The characterization of the light-matter-interaction in this regime is of major interest.
We will present a study on GaAs and its property changes due to pulsed laser irradiation ranging from the visible to the
mid-IR region of the spectrum. Long-pulse as well as ultra-short pulse radiation is used to modify the material.
Parameters such as ablation threshold, radiation penetration depth and thermal diffusion will be discussed.
The ability to integrate micro-channels for fluid transport with optical elements is attractive for the development of
compact and portable chip-based sensors. Femtosecond Laser Direct Writing (FLDW) in transparent materials is a
powerful tool for the fabrication of such integrated devices. We demonstrate the use of FLDW to fabricate coupled
micro-fluidic channels and optical waveguides towards an integrated sensing device for molecular detection.
Waveguides were directly written into the host material and channels were formed by modifying the molecular structure
through FLDW followed by wet chemical etching. Multiple host materials including chalcogenide glasses for IR
detection are discussed.