Abstract-Exact frequency and phase control of a terahertz laser

Reshma A. Mohandas, Lalitha Ponnampalam, Lianhe Li, Paul Dean, Alwyn J. Seeds, Edmund H. Linfield, A. Giles Davies, and Joshua R. Freeman

Schematic diagram of the experimental arrangement. EDFA, erbium doped fibre amplifier; Tx, photomixer emitter; Rx1 and Rx2, photomixer receivers; PLL, phase lock loop; Δ𝜑$\mathrm{\Delta }\phi$, variable delay line. Electrical connections are shown in black, optical fiber and IR connections in red, and terahertz connections in green.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-9-1143

The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of <1Hz$<1\mathrm{H}\mathrm{z}$, robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range ±0.3𝜋$\pm 0.3\pi$. These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.

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.

Abstract-Two-dimensional coherent spectroscopy of a THz quantum cascade laser: observation of multiple harmonics

Sergej Markmann, Hanond Nong, Shovon Pal, Tobias Fobbe, Negar Hekmat, Reshma A. Mohandas, Paul Dean, Lianhe Li, Edmund Linfield, A. G. Davies, Andreas D. Wieck, Nathan Jukam,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-18-21753&origin=search

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ωτ = 0, ωt = ω0), (ωτ = ω0, ωt = ω0), and (ωτ = 2ω0, ωt = ω0) where ω0 denotes the lasing frequency. The peak at ωτ = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery.

© 2017 Optical Society of America

Abstract- Injection locking of a terahertz quantum cascade laser to a telecommunications wavelength frequency comb

Joshua R. Freeman, Lalitha Ponnampalam, Haymen Shams, Reshma A. Mohandas, Cyril C. Renaud, Paul Dean, Lianhe Li, A. Giles Davies, Alwyn J. Seeds, and Edmund H. Linfield

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-9-1059

High-resolution spectroscopy not only can identify atoms and molecules but also can provide detailed information on their chemical and physical environment and relative motion. In the terahertz frequency region of the electromagnetic spectrum, where many molecules have fundamental vibrational modes, there is a lack of powerful sources with narrow linewidths that can be used for absorption measurements or as local oscillators in heterodyne detectors. The most promising solid-state source is the THz frequency quantum cascade laser (QCL), however, the linewidth of this compact semiconductor laser is typically too broad for many applications, and its frequency is not directly referenced to primary frequency standards. In this work, we injection lock a QCL operating at 2 THz to a compact fiber-based telecommunications wavelength frequency comb, where the comb line spacing is referenced to a microwave frequency reference. This results in the QCL frequency locking to an integer harmonic of the microwave reference, and the QCL linewidth reducing to the multiplied linewidth of the microwave reference, <100  Hz$<100\mathrm{Hz}$. Furthermore, we perform phase-resolved detection of the locked QCL and measure the phase noise of the locked system to be −75  dBc/Hz$-75\mathrm{dBc}/\mathrm{Hz}$ at 10 kHz offset from the 2 THz carrier.

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.

Abstract-Measurement of the emission spectrum of a semiconductor laser using laser-feedback interferometry

• James Keeley, 
• Joshua Freeman, 
• Karl Bertling, 
• Yah L. Lim, 
• Reshma A. Mohandas, 
• Thomas Taimre, 
• Lianhe H. Li, 
• Dragan Indjin, 
• Aleksandar D. Rakić, 
• Edmund H. Linfield, 
• A. Giles Davies & 
• Paul Dean

https://www.nature.com/articles/s41598-017-07432-0?WT.feed_name=subjects_chemistry

The effects of optical feedback (OF) in lasers have been observed since the early days of laser development. While OF can result in undesirable and unpredictable operation in laser systems, it can also cause measurable perturbations to the operating parameters, which can be harnessed for metrological purposes. In this work we exploit this ‘self-mixing’ effect to infer the emission spectrum of a semiconductor laser using a laser-feedback interferometer, in which the terminal voltage of the laser is used to coherently sample the reinjected field. We demonstrate this approach using a terahertz frequency quantum cascade laser operating in both single- and multiple-longitudinal mode regimes, and are able to resolve spectral features not reliably resolved using traditional Fourier transform spectroscopy. We also investigate quantitatively the frequency perturbation of individual laser modes under OF, and find excellent agreement with predictions of the excess phase equation central to the theory of lasers under OF.

Abstract-Terahertz generation mechanism in nano-grating electrode photomixers on Fe-doped InGaAsP

Reshma A. Mohandas, Joshua R. Freeman, Michele Natrella, Mark C. Rosamond, Lalitha Ponnampalam, Martyn J. Fice, Alwyn J. Seeds, Paul. J. Cannard, Michael. J. Robertson, David. G. Moodie, A. Giles Davies, Edmund H. Linfield, and Paul Dean

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-9-10177

We report the generation mechanism associated with nano-grating electrode photomixers fabricated on Fe-doped InGaAsP substrates. Two different emitter designs incorporating nano-gratings coupled to the same broadband antenna were characterized in a continuous-wave terahertz (THz) frequency system employing telecommunications wavelength lasers for generation and coherent detection. The current-voltage characteristics and THz emission bandwidth of the emitters is compared for different bias polarities and optical polarisations. The THz output from the emitters is also mapped as a function of the position of the laser excitation spot for both continuous-wave and pulsed excitation. This mapping, together with full-wave simulations of the structures, confirms the generation mechanism to be due to an enhanced optical electric field at the grating tips resulting in increased optical absorption, coinciding with a concentration of the electrostatic field.

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.

Abstract-Gain recovery time in a terahertz quantum cascade laser

David R. Bacon1, Joshua R. Freeman1,a), Reshma A. Mohandas1, Lianhe Li1, Edmund H. Linfield1, A. Giles Davies1 and Paul Dean1
 1 School of Electronic and Electrical Engineering, University of Leeds, Woodhouse Lane, Leeds LS9 2JT, United Kingdom
a) Electronic mail: j.r.freeman@leeds.ac.uk
Appl. Phys. Lett. 108, 081104 (2016); http://dx.doi.org/10.1063/1.4942452

The gain recovery time of a bound-to-continuum terahertz frequency quantum cascade laser, operating at 1.98 THz, has been measured using broadband terahertz-pump-terahertz-probe spectroscopy. The recovery time is found to reduce as a function of current density, attaining a value of 18 ps as the laser is brought close to threshold. We attribute this reduction to improved coupling efficiency between the injector state and the upper lasing level as the active region aligns.