Discrete Vernier tuning in terahertz quantum cascade lasers using coupled cavities

Abstract

Discrete Vernier frequency tuning of terahertz quantum cascade lasers is demonstrated using a device comprising a two-section coupledcavity. The two sections are separated by a narrow air gap, which is milled after device packaging using a focused ion beam. One section of the device (the lasing section) is electrically biased above threshold using a short current pulse, while the other section (the tuning section) is biased below threshold with a wider current pulse to achieve controlled localized electrical heating. The resulting thermally-induced shift in the longitudinal cavity modes of the tuning section is engineered to produce either a controllable blue shift or red shift of the emission frequency. This discrete Vernier frequency tuning far exceeds the tuning achievable from standard ridge lasers, and does not lead to any corresponding change in emitted power. Discrete tuning was observed over bandwidths of 50 and 85 GHz in a pair of devices, each using different design schemes. Interchanging the lasing and tuning sections of the same devices yielded red shifts of 20 and 30 GHz, respectively. 659-670 (1984

Model for a pulsed terahertz quantum cascade laser under optical feedback

“…For our exemplar modeling demonstration we used a laboratory-determined emission frequency coefficient of −12 MHz/mA for the driving current range of interest, 420 mA -510 mA. Thermal frequency modulation is due to thermal expansion of the cavity [67] as well as change in refractive index with AR temperature [34]. The two effects together can produce a complex emission frequency vs. temperature characteristic which varies from laser to laser.…”

“…High-powered THz QCLs can require drive currents in the region of amperes, producing several watts of Joule heating (self-heating) power [31]. The thermal transients and accompanying effects brought about by self-heating are far more prominent in these devices than in other types of laser [32,33], to the extent that they may be used as a tool for tuning QCLs [34].…”

Photonic Engineering Technology for the Development of Terahertz Quantum Cascade Lasers

“…In general, frequency tuning in this way is inefficient for THz QCL due to the small refractive index change as a function of temperature and its long wavelength. To enhance the frequency tuning range, complicated optical cavities, such as two-section coupled-cavity structures, [87] aperiodic gratings within a standard FP cavity, [88] were utilized to expand the tuning capability based on the so-called Vernier effect. [86] Therefore, it allows very precise frequency control.…”

Power Amplification and Coherent Combination Techniques for Terahertz Quantum Cascade Lasers

“…Compared with surface emission, although the output beam of the facet-emitting devices suffers higher divergence originated from the diffraction of the sub-wavelength size of the aperture, facet-emitting devices are expected to give higher output power due to the longer length of the waveguide along the propagating direction. In 2014, Kundu et al demonstrated frequency-tunable THz QCL using coupled cavities [42]. In their device, one section of the device (the lasing section) was electrically biased above threshold using a short current pulse, while the other section (the tuning section) was biased below threshold with a wider current pulse to achieve controlled localized electrical heating.…”