Browsing by Author "Sirtori, C."

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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.
Open Access
Time resolved Fabry-Perot measurements of cavity temperature in pulsed QCLs
(OSA - The Optical Society, 2018) Gündoğdu, Sinan; Pisheh, ‪Hadi Sedaghat; Demir, Abdullah; Günöven, M.; Aydınlı, Atilla; Sirtori, C.
Temperature rise during operation is a central concern of semiconductor lasers and especially difficult to measure during a pulsed operation. We present a technique for in situ time-resolved temperature measurement of quantum cascade lasers operating in a pulsed mode at ~9.25 μm emission wavelength. Using a step-scan approach with 5 ns resolution, we measure the temporal evolution of the spectral density, observing longitudinal Fabry-Perot modes that correspond to different transverse modes. Considering the multiple thin layers that make up the active layer and the associated Kapitza resistance, thermal properties of QCLs are significantly different than bulk-like laser diodes where this approach was successfully used. Compounded by the lattice expansion and refractive index changes due to time-dependent temperature rise, Fabry-Perot modes were observed and analyzed from the time-resolved emission spectra of quantum cascade lasers to deduce the cavity temperature. Temperature rise of a QCL in a pulsed mode operation between -160 °C to -80 °C was measured as a function of time. Using the temporal temperature variations, a thermal model was constructed that led to the extraction of cavity thermal conductivity in agreement with previous results. Critical in maximizing pulsed output power, the effect of the duty cycle on the evolution of laser heating was studied in situ, leading to a heat map to guide the operation of pulsed lasers.