Browsing by Subject "Quantum Cascade Lasers"

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Open Access
Optical and thermal dynamics of long wave quantum cascade lasers
(2018-09) Gündoğdu, Sinan
Quantum Cascade Lasers (QCLs) are coherent light sources that make use of intraband transitions of wavefunction engineered semiconductor quantum wells. They have been designed to emit light in a wide spectral range; from mid-wave infrared to terahertz. Long wave QCLs are a subject of interest for some applications such as remote detection of harmful chemicals. These applications demand higher optical powers at room temperature. In this thesis we demonstrate simulation, design, fabrication and characterization of long-wave QCLs that emit light around 9.2 m. To increase optical power and enhance thermal performance, we explore the optical and thermal properties of QCLs. Thermal characteristics of QCLs are analyzed by nite element methods. We developed a spectral technique that relies on analysis of Fabry-Perot modes to measure cavity temperatures experimentally. By combining the simulations and experimental results we scrutinized the thermal properties of QCLs, and estimated the active region thermal conductivity. To increase the optical power, we conducted optical calculations and investigated the sources of loss. As a result of a search for alternative electrical passivation materials, we fabricated HfO2 passivated lasers and demonstrated about to two-fold reduction in optical loss and increase in optical power.
Open Access
Thermal characterisation of quantum cascade lasers with Fabry Perot modes
(SPIE, 2018) Gündoğdu, Sinan; Pisheh, Hadi Sedaghat; Demir, Abdullah; Günoven, M.; Aydınlı, Atilla; Sirtori, C.
Quantum cascade lasers are coherent light sources that rely on intrersubband transition in periodic semiconductor quantum well structures. They operate at frequencies from mid-infrared to terahertz. In cases of long wavelength and typical low thermal conductivity of the active region, temperature rise in the active region during operation is a major concern. Thermal conductivity of QCL epi-layers differ significantly from the values of bulk semiconductors and measurement of the thermal conductivity of epi-layers is critical for design. It is well known that Fabry-Perot spectra of QCL cavities exhibit fine amplitude oscillations with frequency and can be used for real time in-situ temperature measurement. Phase of the modulation depends on the group refractive index of the cavity, which depends on the cavity temperature. We fabricated QCL devices with from 12, to 24 um mesa widths and 2mm cavity length and measured high resolution, high speed time resolved spectra using a FTIR spectrometer in step scan mode in a liquid nitrogen cooled, temperature controlled dewar. We used the time resolved spectra of QCLs to measure average temperature of the active region of the laser as a function of time. We examined the effect of pulse width and duty cycle on laser heating. We measured the temperature derivative of group refractive index of the cavity. Building a numerical model, we estimated the thermal conductivity of active region and calculated the heating of the QCL active region in pulsed mode for various waveguide widths.