Abstract-Gain measurement of terahertz quantum cascade laser via a master-oscillator power-amplifier configuration

Chenren Yu, Huan Zhu, Fangfang, Wang, Gaolei Chang, Haiqing Zhu, Jianxin Chen, Gangyi Xu, Li He,



https://www.sciencedirect.com/science/article/pii/S0022024819301551

We report a method to measure the net loss and gain of a terahertz quantum cascade laser (THz-QCL), which is based on a master-oscillator power-amplifier (MOPA) configuration. In the measurement, the master-oscillator (MO) section and power-amplifier (PA) section are separately biased. With the fixed seed power from the MO section, the output power of the device is measured at various bias voltage applied on the PA section, from which the net loss and gain is deduced as a function of bias. We demonstrate that gain clamping and gain saturation can be avoided in our measurement, which allow a complete evolution of the loss and gain characteristics. For a THz-QCL with the bound-to-continuum active region and the metal-metal waveguide, the measured maximal net gain is about 13.0-16.5 cm-1 at 2.58 THz at a 20 K, which is in qualitative agreement with the theoretical analysis.

Abstract-Sideband generation of coupled-cavity terahertz semiconductor lasers under active radio frequency modulation

Ziping Li, Hua Li, Wenjian Wan, Kang Zhou, Juncheng Cao, Gaolei Chang, and Gangyi Xu

Fig. 1 (a) Schematic of the coupled-cavity terahertz QCL. The inset is a scanning electron microscope image of the fabricated short cavity and air gap of the laser. (b) Calculated total propagation losses as a function of eigen mode frequency of the coupled-cavity with material loss (green) and without material loss (black). The scatters are the calculated results and the solid lines are for the guide of eyes. The red stars are the measured lasing frequencies obtained from (d). (c) Measured light-current-voltage (L-I-V) characteristic of the coupled-cavity laser in continuous wave (cw) mode at 20 K. (d) Normalized terahertz emission spectra as a function of drive current without RF modulation at 20 K. Each spectrum is shifted vertically for clarity. The insets of (d) are magnified spectra to clear show the spectral degeneracy.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-25-32675

The radio frequency (RF) modulation is a powerful tool, which is used for generating sidebands in semiconductor lasers for active mode-locking. The two-section coupled-cavity laser geometry shows advantages over traditional Fabry-Pérot cavities in the RF modulation efficiency, because of its reduced device capacitance of short section cavity. Further, it has been widely used for active/passive mode-locking of semiconductor diode lasers. For semiconductor-based quantum cascade lasers (QCLs) emitting in the far-infrared or terahertz frequency bands, the two-section coupled-cavity configuration can strongly prevent the laser from multimode emissions. This is because of its strong mode selection (loss modulation), which the cavity geometry introduces. Here, we experimentally demonstrate that the coupled-cavity terahertz QCL can be actively modulated to generate sidebands. The RF modulation is efficient at the frequency that equals the difference frequency between the fundamental and higher order transverse modes of the laser, and its harmonics. We show for the first time that, when the laser is modulated at the second harmonic of the difference frequency, the sideband generation in coupled-cavity terahertz QCLs and the generated sidebands are equally spaced by the injected microwave frequency. Our results, which are presented here, provide a novel approach for modulating terahertz coupled-cavity lasers for active mode-locking. The coupled-cavity geometry shows advantages in generating dense modes with short cavities for potential high-resolution spectroscopy. Furthermore, the short coupled-cavity laser consumes less electrical power than Fabry-Pérot lasers that generate a similar mode spacing.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz master-oscillator power-amplifier quantum cascade laser with a grating coupler of extremely low reflectivity

Huan Zhu, Haiqing Zhu, Fangfang Wang, Gaolei Chang, Chenren Yu, Quan Yan, Jianxin Chen, Lianhe Li, A. Giles Davies, Edmund H. Linfield, Zhou Tang, Pingping Chen, Wei Lu, Gangyi Xu, and Li He

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-2-1942

A terahertz master-oscillation power-amplifier quantum cascade laser (THz-MOPA-QCL) is demonstrated where a grating coupler is employed to efficiently extract the THz radiation. By maximizing the group velocity and eliminating the scattering of THz wave in the grating coupler, the residue reflectivity is reduced down to the order of 10−3. A buried DFB grating and a tapered preamplifier are proposed to improve the seed power and to reduce the gain saturation, respectively. The THz-MOPA-QCL exhibits single-mode emission, a single-lobed beam with a narrow divergence angle of 18° × 16°, and a pulsed output power of 136 mW at 20 K, which is 36 times that of a second-order DFB laser from the same material.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.