Laser Engineering Flashcards
What is vibrational sum-frequency generation spectroscopy?
It is a method that characterizes surfaces and interfaces (structure, reactivity, composition, etc). It involves a second order nonlinear process. It mixes a pulsed tunable IR beam with a visible beam to produce an output with frequency the sum of the two. Detects resonances.
What’s XPS?
X-ray photoelectron spectroscopy. It’s a surface-sensitive effect based on the photoelectric effect, and can identify specific elements, chemical states, and electronic structure. Requires a vacuum. Measures electron count as a function of binding energy; certain peaks respond to certain electrons.
How does FTIR work?
Uses a Michelson interferometer and broadband light source (silicon carbide heated to high temperature, black body radiation like) to resolve spectral data over a wide spectral range. Very similar to OCT. Can be used to measure vibrational mode absorption or other resonant features.
How does Raman spectroscopy work?
It relies on inelastic scattering of photons (Raman scattering). A monochromatic light source (UV to IR but can also use X-rays), interacts with the molecular vibrations, phonons, or other excitations in a system, resulting in shift of the photon energy.
What does XRD measure?
atomic and molecular structure of crystals; measures X-ray counts as a function of angle. Diffraction angle gives information of electron density, chemical bonds, etc.
How does an SEM work?
Scanning Electron Microscope is an imaging technique that scans the surface with a focused beam of electrons, allowing for topography and composition. “Secondary” electrons are measured.
Describe what happens with harmonic generation?
n photons with the same frequency interact with a nonlinear material, are “combined”, and then generate a new photon with n times the energy (and wavelength divided by n). This happens because with sufficiently high electric field, polarizability starts to saturate (and behave nonlinearly). [related to electric susceptbility]
What is needed for efficient harmonic generation?
Lots of photons (low probability event); probability decreases with order of harmonic generation
Nonlinear medium must have an optimized symmetry (phase matching - birefringence)
Light source must be very intense and highly spatially and temporally coherent for efficient harmonic generation.
How is 266, 355 nm and 532 nm light typically generated?
532 nm light: frequency doubling of Nd:YAG crystal (1064 nm) light using lithium triborate (LBO) crystal
355 nm light: frequency tripling of Nd:YAG crystal via frequency doubling to 532 followed by sum frequency generation of the 532 nm light with the original 1064 nm light (1064 +532 = 355)
266 nm light: 1064 nm light frequency doubled to 532 (LBO) then again using another frequency doubling crystal (CLBO or cesium LBO).
What is an excimer laser?
optical amplification that occurs in a plasma with some noble gas and a halogen, pumped with nanosecond pulses to generate excimers; UV light is output typically
What are some issues with fiber coupling UV light?
Using (even) fused silica fibers results in solarization/degradation of the light. Hollow core fibers can be used
What is a fiber laser and what are some unique advantages?
Fiber laser: gain medium is a rare-earth-doped optical fiber. Typically they are diode-pumped. Typical to use fiber Bragg gratings (made within the doped fiber) as the resonator/reflector.
Generally they do NOT generate polarized output (sort of, it’s unstable). Most others generate linearly polarized light.
Advantages: good for high power (guiding effect avoids thermo-optical problems) in laser welding and cutting of metals.
good for upconversion lasers because you can maintain high pump intensities required for upconversion.
good for narrow linewidth/single longitudinal mode, can tune fiber bragg gratings very well
What’s Q switching?
A way of generating short (nanosecond, not pico or femto) by modulating the intracavity losses (Q factor). Keep losses high and feeding energy into the gain medium such that it accumulates, then abruptly reduce losses (this is all done using an acousto- or electro-optic modulator). Then the power builds up quickly in the laser resonator, then the gain gets saturated, and a pulse is extracted.
Unique properties and applications of laser diodes?
Highly tunable wavelength based on band gap flexibility
Very sensitive to current, requires fixed current (which is why a driver is typically used)
Generally cw (but can also generate very fast pulses)
Trade-off between beam quality and power
Very reliable
Applications: telecom, barcodes, DVDs, FaceID, pump source for other lasers and amplifiers.
