Alhathlool, R

Bertling, K

Davies, AG

Dean, P

Indjin, D

Keeley, J

Li, LH

Lim, YL

Linfield, EH

Rakic, AD

Taimre, T

Valavanis, A

White Rose Research Online

This is a repository copy of Active phase-nulling of the self-mixing phase in a terahertz frequency quantum cascade laser.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/89957/Version: Accepted VersionConference or Workshop Item:Dean, P, Keeley, J, Valavanis, A et al. (9 more authors) Active phase-nulling of the self-mixing phase in a terahertz frequency quantum cascade laser. In: Intersubband Transitions in Quantum Wells (ITQW) 2015, 6 - 11 September 2015, Vienna. (Unpublished) eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Active phase-nulling of the self-mixing phase in a terahertz frequency quantum cascade laser P. Dean1,*, J. Keeley1, A. Valavanis1, K. Bertling2, Y. L. Lim2, T. Taimre2, R. Alhathool1, L. H. Li1, D. Indjin1, A. D. Rakic2, E. H. Linfield1, and A. G. Davies1 1 School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK  2 School of Information Technology and Electrical Engineering, University of Queensland, Brisbane, QLD 4072, Australia *Contact Email: p.dean@leeds.ac.uk Introduction Self-mixing (SM) interferometry  using quantum cascade lasers (QCLs) has recently emerged as a powerful sensing technique in the terahertz (THz) frequency range [1]. In this work, we develop an active phase-nulling scheme for THz QCLs under optical feedback (OF), through electronic feedback control of the emission frequency. We first apply this method to measurement of the frequency tuning of a laser as a function of drive current. Such characterization is essential for many applications, including gas spectroscopy, and frequency-modulated approaches to imaging and materials analysis using THz QCLs. In addition to offering real-time capability, our measurement approach provides a greatly reduced experimental complexity compared to previously reported schemes for characterizing frequency tuning in QCLs—these typically require detection of the high-frequency beat note generated by mixing the QCL field with that derived from a local oscillator source (e.g. either a second QCL or an optically pumped gas laser) [2]. In contrast to these approaches, which are restricted to the operating frequency range of the mixer, our technique can be applied across the full 1.2–5.2 THz spectral coverage range of THz QCLs. In addition, we demonstrate a simple SM scheme for determination of the absolute emission spectrum of the free-running laser. We also demonstrate that our phase-nulling scheme can be applied to real-time displacement sensing of targets. This scheme achieves all of the advantages of previous SM-based displacement sensors whilst overcoming the resolution limits imposed by the quantization associated with fringe counting methods and simultaneously avoiding the need for parametric fitting to an OF model in post-processing.              Fig. 1: Schematic diagram of the experimental system. The dashed box indicates the compensation loop used to cancel voltage modulation of the laser. LPF—low pass filter. Measurement of frequency tuning and absolute emission frequency The THz QCL consisted of a bound-to-continuum active region emitting at a frequency ߥ଴̱2.6 THz, which was processed into a semi-insulating surface-plasmon ridge waveguide. The phase change in the SM voltage signal arising from modulation of the external cavity length οܮ was compensated by an electronic feedback loop acting on the QCL emission frequency through dynamic control of the driving current. An adjustable inverting amplifier stage acting on the filter output Vout was used to subtract the unwanted direct modulation of the QCL terminal voltage from the input to the loop filter, as shown by the dashed box in Fig. 1. The filter output Vout was recorded in response to a ο25׽ܮ鳥鳥たm modulation of the external cavity length (approximately half a fringe). The resulting time-dependent frequency excursion of the laser οߥ can be deduced from the recorded mirror trajectory through the relation οߥ ଴ߥ ൌ െΤ ȟܮ ଴Τܮ . By correlating this with the measured control signal Vout, the frequency tuning can be mapped as a function of current perturbation ο݅, enabling the tuning rate k to be determined. Figure 2 shows the relationship between ο݅ and οߥ obtained in this, which yields a value k = −8.2鳥鳥MHz/mA. We have also applied a simple SM scheme for determination of the absolute emission frequency of the free-running laser. In this case, the SM voltage signal was recorded as the cavity mirror was displaced through a distance 20 cm. By Fourier analysis of the interferometric data, the emission spectrum was acquired with a resolution of 750 MHz, as shown in the inset of Fig. 2.      Real-time displacement sensing As a second demonstration of our scheme, we have applied it to the real-time displacement sensing of targets. In this case, the filter output Vout generated in response to a moving target represents a replica of the target displacement, scaled by a responsivity factor. Figure 3 shows the displacement measured over two oscillation cycles of a mirror vibrating at 2 Hz with a peak-to-peak displacement, D=22.5鳥鳥たm. From Fourier analysis of the filter output signal, the noise equivalent displacement was found to be 0.2׽鳥鳥たm for a 1-Hz measurement bandwidth centered around the oscillation frequency. Furthermore, the operating range of the electronic feedback loop in our system is expected to be 3.6׽鳥鳥cm, suggesting much greater displacement amplitudes could, in principle, be measured using our technique.  [1] P. Dean et al., Opt. Lett. 36, 2587-2589 (2011) [2] A. Barkan, et al., Opt. Lett. 29, 575 (2004). Fig. 2: Frequency tuning 〉Ȟ of the QCL arising from a current perturbation 〉i, as determined from the feedback control signal Vout measured in response to a modulation of the external cavity length. The dashed line shows a linear fit to the data. Inset: Emission spectrum determined by SM interferometry. Fig. 3: (a) Displacement (solid line) determined from the feedback control signal Vout, in response to a vibrating target (actual displacement shown by the dashed line). (b) Error determined from the difference between the measured and actual displacements. 

Active phase-nulling of the self-mixing phase in a terahertz frequency quantum cascade laser

https://eprints.whiterose.ac.uk/89957/1/ITQW2015_SM.pdf

Active phase-nulling of the self-mixing phase in a terahertz frequency quantum cascade laser

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