Yamac Dikmelik

Fiber-coupling efficiency for free-space optical communication through atmospheric turbulence

High-speed free-space optical communication systems have recently used fiber-optic components. The received laser beam in such a system must be coupled into a single-mode fiber at the input of the receiver module. However, propagation through atmospheric turbulence degrades the spatial coherence of a laser beam and limits the fiber-coupling efficiency. We numerically evaluate the fiber-coupling efficiency for laser light distorted by atmospheric turbulence. We also investigate the use of a coherent fiber array as a receiver structure and find that a coherent fiber array that consists of seven subapertures would significantly increase the fiber-coupling efficiency.

Femtosecond and nanosecond laser-induced breakdown spectroscopy of trinitrotoluene

Femtosecond and nanosecond laser-induced breakdown spectroscopy (LIBS) were used to study trinitrotoluene (TNT) deposited on aluminum substrates. Over the detection wavelength range of 200-785 nm, we have observed emission from CN and C(2) molecules as the marker for the explosive with femtosecond LIBS. In contrast, the signal for nanosecond LIBS of TNT is dominated by emission from the elemental constituents of the explosive. Aluminum emission lines from the substrate are also observed with both femtosecond and nanosecond excitation and indicate the role played by the substrate in the interaction.

Coherent frequency combs produced by self frequency modulation in quantum cascade lasers

One salient characteristic of Quantum Cascade Laser (QCL) is its very short τ ∼ 1 ps gain recovery time that so far thwarted the attempts to achieve self-mode locking of the device into a train of single pulses. We show theoretically that four wave mixing, combined with the short gain recovery time causes QCL to operate in the self-frequency-modulated regime characterized by a constant power in time domain and stable coherent comb in the frequency domain. Coherent frequency comb may enable many potential applications of QCLs in sensing and measurement.

Ultrafast laser-based spectroscopy and sensing: applications in LIBS, CARS, and THz spectroscopy.

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating. While some of these techniques are currently confined to sensing at close ranges, others can be implemented for remote spectroscopic sensing owing principally to the laser pulse duration. We present a review of ultrafast laser-based spectroscopy techniques and discuss the use of these techniques to current and potential chemical and environmental sensing applications.

Importance of interface roughness induced intersubband scattering in mid-infrared quantum cascade lasers

The electron transit time of many different quantum cascade lasers has been measured and compared to the calculated upper laser level lifetimes with and without taking into account interface roughness induced intersubband scattering. A significantly better correlation is found between the experimental results and the calculation when including the contribution from the interface roughness (corr. coeff.: 0.79 vs. 0.43 with and without the consideration of interface roughness, respectively). This suggests that in addition to longitudinal optical phonons, interface roughness is also crucial in determining the intersubband lifetimes in mid-infrared quantum cascade laser and should routinely be included in design.

Effects of surface roughness on reflection spectra obtained by terahertz time-domain spectroscopy

We present an analytical model that shows that reflection from a rough surface causes a Gaussian frequency roll-off for the spectral magnitude of a terahertz wave and reduces the signal-to-noise ratio of terahertz time-domain spectroscopy. The parameter that determines the width of the frequency roll-off is the standard deviation of the surface height distribution. Measurements of terahertz waves reflected from copper powder samples provide experimental evidence for this effect.

Femtosecond-laser-induced breakdown spectroscopy of explosives

We use femtosecond laser-induced breakdown spectroscopy (LIBS) to detect trace amounts of TNT and RDX. A high-power pulsed laser is used in LIBS to form a plasma on the material surface and the optical emission from the plasma is spectrally analyzed to determine the material composition. Femtosecond LIBS results for TNT and RDX on aluminum substrates and glass slides are reported. Results are examined in terms of the optical properties of the substrate and the strong linear absorption for aluminum is contrasted with the weaker multiphoton absorption for glass. Optical microscope images of the ablated explosives are shown for femtosecond and nanosecond laser excitation. Fragmentation studies by femtosecond laser mass spectrometry are used to interpret LIBS results.

Transport and gain in a quantum cascade laser: model and equivalent circuit

A simple rate-equation-based model for transport and gain in quantum cascade lasers is developed. According to the model, the IV characteristics of a quantum cascade laser can be well described by equivalent circuit containing a serial connection of a Schottky diode, a tunnel diode, and a field-effect transistor. As a consequence, a quantum cascade laser can be described in terms accessible to circuit designers.

Short Injector Quantum Cascade Lasers

We report our study on the effects of shortened quantum cascade (QC) laser injector regions. While conventional short-wavelength QC lasers typically have around seven or more injector region quantum wells, we investigate QC structures with three and two injector wells. Improvements in threshold currents, output powers, and wall-plug efficiencies are expected for fundamental reasons. At heat sink temperatures near 80 K, we observe threshold current densities less than 0.5 kA/cm2, nearly 4 W peak output power, and wall-plug efficiencies in excess of 20%. At room temperature, we see threshold current densities around 2.3 kA/cm2, output powers in excess of 1 W, and wall-plug efficiencies around 7.6%. We also observe new effects in midinfrared QC lasers, such as a pronounced negative differential resistance, pulse instabilities, and multiple and varied turn-off mechanisms. These effects result from the greatly abbreviated injector regions with highly discrete states.

Femtosecond laser-induced breakdown spectroscopy of explosives and explosive-related compounds

In this work, we describe femtosecond laser-induced breakdown spectroscopy (LIBS) to detect trace amounts of explosive-related compounds (ERCs). A high-power pulsed laser is used in LIBS to form a plasma on the material surface and the optical radiation from the plasma is spectrally analyzed to determine the material composition. LIBS is minimally destructive because only a minute amount of material is consumed in the process. LIBS also enables remote analysis because only optical access to the material is needed. Femtosecond LIBS results for TNT on brass and molybdenum substrates, and RDX on molybdenum substrates are reported. We will also show the effect of detection gate delay and gate width on the enhancement of spectral information provided by LIBS.