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Oct 6, 2017 - THOMAS G. FOLLAND, ... P. Dean, A. Valavanis, J. Keeley, K. Bertling, Y. L. Lim, R. Alhathlool, A. D. Burnett, L. H. Li, S. P. Khanna, ... Pepper, G. Aeppli, A. G. Davies, P. Dean, E. Linfield, and C. C. Renaud, “Coherent ...

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Coherent detection of THz laser signals in optical fiber systems THOMAS G. FOLLAND,1,3 OWEN P. MARSHALL,1 HARVEY E. BEERE,2 DAVID A. RITCHIE,2 AND SUBHASISH CHAKRABORTY1,4 1

School of Electrical and Electronic Engineering, University of Manchester, UK Semiconductor Physics Group, Cavendish Laboratory, University of Cambridge, UK 3 th[email protected] 4 [email protected] 2

Abstract: Terahertz (THz) coherent detectors are crucial for the stabilization and measurement of the properties of quantum cascade lasers (QCLs). This paper describes the exploitation of intra-cavity sum frequency generation to up-convert the emission of a THz QCL to the near infrared for detection with fiber optic coupled components alone. Specifically, a low cost infrared photodiode is used to detect a radio frequency (RF) signal with a signal-to-noise ratio of approximately 20dB, generated by beating the up-converted THz wave and a near infrared local oscillator. This RF beat note allows direct analysis of the THz QCL emission in time and frequency domains. The application of this technique for QCL characterization is demonstrated by analyzing the continuous tuning of the RF signal over 2 GHz, which arises from mode tuning across the QCL’s operational current range. 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. OCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (040.2235) Far infrared or terahertz; (060.0060) Fiber optics and optical communications.

References and links 1.

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#300724 Journal © 2017 Received 23 Jun 2017; revised 14 Aug 2017; accepted 17 Aug 2017; published 6 Oct 2017

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12. M. Ravaro, V. Jagtap, C. Manquest, P. Gellie, G. Santarelli, C. Sirtori, S. P. Khanna, E. H. Linfield, and S. Barbieri, “Spectral Properties of THz Quantum-Cascade Lasers: Frequency Noise, Phase-Locking and Absolute Frequency Measurement,” J. Infrared Millim. Terahertz Waves 34(5-6), 342–356 (2013). 13. S. Barbieri, M. Ravaro, P. Gellie, G. Santarelli, C. Manquest, C. Sirtori, S. P. Khanna, E. H. Linfield, and A. G. Davies, “Coherent sampling of active mode-locked terahertz quantum cascade lasers and frequency synthesis,” Nat. Photonics 5(5), 306–313 (2011). 14. V. Berger and C. Sirtori, “Nonlinear phase matching in THz semiconductor waveguides,” Semicond. Sci. Technol. 19(8), 964–970 (2004). 15. S. S. Dhillon, C. Sirtori, S. Barbieri, A. de Rossi, M. Calligaro, H. E. Beere, and D. A. Ritchie, “THz sideband generation at telecom wavelengths in a GaAs-based quantum cascade laser,” Appl. Phys. Lett. 87(7), 071101 (2005). 16. S. S. Dhillon, C. Sirtori, J. Alton, S. Barbieri, A. de Rossi, H. E. Beere, and D. A. Ritchie, “Terahertz transfer onto a telecom optical carrier,” Nat. Photonics 1(7), 411–415 (2007). 17. S. Chakraborty, “Terahertz Mixer and Optical Fiber Coupled Terahertz Mixer,” U.S. patent US20150248047 A1 (2015). 18. M. Khairuzzaman, “Digitally Selected Electronically Switchable Terahertz-over-Fibre,” PhD Thesis, The University of Manchester (2014). 19. J. Madéo, P. Cavalié, J. R. Freeman, N. Jukam, J. Maysonnave, K. Maussang, H. E. Beere, D. A. Ritchie, C. Sirtori, J. Tignon, and S. S. Dhillon, “All-optical wavelength shifting in a semiconductor laser using resonant nonlinearities,” Nat. Photonics 6(8), 519–524 (2012). 20. W. Maineult, L. Ding, P. Gellie, P. Filloux, C. Sirtori, S. Barbieri, T. Akalin, J.-F. Lampin, I. Sagnes, H. E. Beere, and D. A. Ritchie, “Microwave modulation of terahertz quantum cascade lasers: a transmission-line approach,” Appl. Phys. Lett. 96(2), 021108 (2010). 21. E. Ip, A. P. T. Lau, D. J. F. Barros, and J. M. Kahn, “Coherent detection in optical fiber systems,” Opt. Express 16(2), 753–791 (2008). 22. T. Folland, A. Ramos - Pulido, O. Marshall, H. Beere, D. Ritchie, and S. Chakraborty, “High-accuracy heterodyne detection of THz radiation exploiting telecommunication technologies,” in CLEO: 2015 (OSA, 2015), paper STu4H.3. 23. T. G. Folland, A. Ramos-Pulido, O. P. Marshall, H. E. Beere, D. A. Ritchie, and S. Chakraborty, “Time-resolved THz Laser spectra using a Fibre-interfaced Optical Heterodyne system,” in CLEO: 2015 (OSA, 2015), paper STu4H.3. 24. A. Scheuring, P. Dean, A. Valavanis, A. Stockhausen, P. Thoma, M. Salih, S. P. Khanna, S. Chowdhury, J. D. Cooper, A. Grier, S. Wuensch, K. Il’in, E. H. Linfield, A. G. Davies, and M. Siegel, “Transient Analysis of THzQCL Pulses Using NbN and YBCO Superconducting Detectors,” IEEE Trans. Terahertz Sci. Technol. 3(2), 172–179 (2013). 25. A. Yariv and P. Yeh, Photonics: Optical Electronics in Modern Communications (Oxford University, 2007). 26. O. P. Marshall, M. Khairuzzaman, H. E. Beere, D. A. Ritchie, and S. Chakraborty, “Broadband photonic control for dual-mode terahertz laser emission,” Appl. Phys. Lett. 102(18), 181106 (2013). 27. G. Fasching, V. Tamošiunas, A. Benz, A. M. Andrews, K. Unterrainer, R. Zobl, T. Roch, W. Schrenk, and G. Strasser, “Subwavelength microdisk and microring terahertz quantum-cascade lasers,” IEEE J. Quantum Electron. 43(8), 687–697 (2007). 28. B. Meng, J. Tao, X. Hui Li, Y. Quan Zeng, S. Wu, and Q. Jie Wang, “Tunable single-mode slot waveguide quantum cascade lasers,” Appl. Phys. Lett. 104(20), 201106 (2014). 29. M. S. Vitiello, G. Scamarcio, and V. Spagnolo, “Time-resolved measurement of the local lattice temperature in terahertz quantum cascade lasers,” Appl. Phys. Lett. 92(10), 101116 (2008). 30. R. P. Green, J.-H. Xu, L. Mahler, A. Tredicucci, F. Beltram, G. Giuliani, H. E. Beere, and D. A. Ritchie, “Linewidth enhancement factor of terahertz quantum cascade lasers,” Appl. Phys. Lett. 92(7), 071106 (2008).

1. Introduction Compact, tunable [1,2] and high power [3] THz quantum cascade lasers (QCLs) could significantly improve the applicability of commercial THz technologies. Realizing this potential in both imaging [4] and communications [5] requires the capacity to measure and control the phase and amplitude of the THz wave. Coherent THz-to-microwave links, exploiting homodyne or heterodyne detection, provide a means to measure the amplitude and phase of laser emission electrically. However, existing methods for coherent detection of QCL radiation from THz sources [6–9] or near-infrared (NIR) mode-locked lasers [10–13] use bulky and expensive free space optics and detectors, making them commercially unappealing. This work demonstrates coherent detection of QCL emission with off-the-shelf fiber-optic components (plus cryostat) by exploiting the optical nonlinearity of the active region to up-convert the THz wave to the NIR [14–18]. Although the up-conversion process significantly reduces the power of the THz signal, we are able to demonstrate a signal-to-

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noise ratio of ~20dB without optical amplification or servo-loop control, comparable to earlier work [9,10]. The use of cheap fiber-optic components improves the practical viability of QCLs for incorporation into compact and cost effective THz systems [17]. It has been shown that the GaAs-based active region of a typical THz QCL can support a phase matched interaction between NIR and THz waves by exploiting the anomalous dispersion of the Reststrahlen band [14–17]. This nonlinear process results in up-conversion of the THz wave into sum and difference frequency sidebands on an optical carrier; these sidebands can be transmitted and manipulated by traditional fiber-optics [15]. The process can be engineered [16,17,19] and used to directly detect the emission spectrum of modulated QCLs [20]. In this work, for the first time, we extract the time-varying amplitude and phase information from the up-converted sideband signal, and by exploiting optical heterodyne detection, we can down-convert the THz signal to microwave frequencies for electrical measurement. The result constitutes a fully tunable THz-to-microwave link enabled by telecoms technology, allowing the use of high performance fiber-optic components (fiber amplifiers, ultrafast diodes) and associated techniques (such as auto-correlators) to analyze and stabilize the properties of THz QCLs.

Fig. 1. The all-fiber coupled THz to RF link. (a) Schematic of the optical process. THz radiation is converted to a NIR intermediate frequency (IF) before optical heterodyne detection in a photodiode. (b) The experimental setup, including a schematic of the waveguide integrated QCL (PC – polarization controller, LNA – low noise amplifier, LO – local oscillator, ESA – electrical spectrum analyser, RTO – real time oscilloscope).

The proposed technique for coherent detection of the THz signals by a two-stage mixing process is illustrated in Fig. 1(a). Firstly the THz signal (fTHz) is mixed with a NIR carrier wave (f1) within the QCL, up-converting it to a sum (and/or a difference) frequency sideband. Subsequently, the sideband is mixed with a NIR local oscillator (LO) signal (f2) in a fast photodiode, generating a radio frequency (RF) beat note (Δ) at the detector output. This beat note contains information about the amplitude, relative phase and relative frequency between all three lasers. The process is essentially equivalent to mixing between the QCL output and the THz difference signal between the NIR lasers (Fig. 1(a) bottom). This approach is particularly flexible for both measuring and stabilizing laser emission. First, the generated signal can be easily fine-tuned by controlling the frequency of either the QCL or NIR laser sources. Whilst the technique relys on QCLs with an integrated NIR waveguide, it is possible to integrate such guides into essentially any QCL active region. It should therefore be straightforward to implement either heterodyne or homodyne coherent detection [21] for measuring the high speed modulation [20] or mode locking [13] properties of THz QCLs emitting at arbitrary frequencies. Second, if noise in the two NIR signals can be correlated, as in dual frequency or mode locked laser sources, then the output signal could be used to stabilize the QCL for meteorological or communication applications. As an application of this

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technique, pulsed QCL emission is subsequently characterized in both the frequency [22] and time domains [23]. 2. Detection scheme Two QCL ridges on the same chip were fabricated from a GaAs/Al0.15Ga0.85As wafer with a 12.8 µm high bound-to-continuum active region and an integrated NIR guiding layer [15,18]. The two 200 µm-wide laser ridges exploit semi-insulating surface plasmon waveguides, and were cleaved into cavities ~4.6 mm long. Devices were operated in pulsed mode (repetition rate frep, peak current IQCL), well above threshold and at a heatsink temperature of 15K. Current pulses were provided by an Agilent 8144A pulse generator, amplified by an AVtech AVX-MRB2 pulse amplifier. Both devices showed similar emission properties. These devices were characterized in the THz with a Bruker Vertex 80 Fourier transform infrared (FTIR) spectrometer and a calibrated thermopile (placed directly in front of a laser facet) while mounted in a Janis ST100 cryostat. The fiber optic experimental setup used to implement the coherent detection system is illustrated in Fig. 1(b) [22]. NIR light generated by a tunable external cavity laser (Yenista Tunics T100, optimum power 13dBm) is injected into the NIR waveguide embedded within the QCL [15] via single mode optical fiber (SMF) butt coupled to a QCL facet in a Janis ST500 cryostat. The total injection/extraction losses in the coupling process were approximately 21dB. THz light is generated and mixed with the NIR carrier wave inside the electrically driven QCL (Optimum peak power 35mW), generating sum and difference frequency sidebands. To optimize the sideband intensity, the frequency of the carrier is chosen to lie at the phase matching point of the up-conversion process (~1.3μm) and is TE polarized. The NIR sidebands were collected alongside the NIR carrier with a second butt coupled SMF. Light from a second external cavity laser (Yenista Osics T100) was then coupled into the fiber, after the QCL, as a local oscillator. The NIR signals on the fiber could be measured with both a fast photodiode (Thorlabs DET08CFC/M) and an optical spectrum analyzer (Yokogawa AQ6370Z, 18GHz spectral resolution). The RF signal was amplified (Minicircuits ZX60-3018G-S + ) and measured either by an electrical spectrum analyzer (ESA - Keysight HSA N9344C) or digital real-time oscilloscope (Keysight Infinium MSO9104A).

Fig. 2. Heterodyne detection of a THz sideband by NIR lasers. (a) Optical spectrum showing both the up-conversion of the THz emission to a sideband (black), and spectrum during heterodyne detection (red). Insets show the THz pulsed LIV curve and THz spectra of the QCL at the indicated operated currents. (b) RF beat frequency measurement of the THz QCL emission. The inset is a reference measurement of the beat signal generated between the NIR lasers used in the experiment.

3. Detected RF signal Measurements of the QCL emission at THz frequencies are presented in the inset of Fig. 2(a), which shows multi-mode operation across the full operational current range, with a mode

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spacing of ~8.4 GHz. This behavior is typical of a pulsed bound-to-continuum QCL. The NIR spectrum from the mixing experiment presented in Fig. 2(a) shows the carrier wave, upconverted THz emission (IQCL = 1950 mA, frep = 10 kHz, 17.5% duty cycle), and NIR local oscillator laser. The sideband shows the up-converted multi-mode emission of the QCL, with a peak conversion efficiency of ~4 × 10−6, comparable with earlier experiments [15]. The spectrum of the beat note measured by the coherent detection system is shown in Fig. 2(b) (100 kHz RBW, 30 s sweep time, IQCL = 1650 mA), showing approximately 20 dB of dynamic range. The signal has a linewidth of around 240 MHz, which can be attributed to instability in each of the three lasers. Further insight into the source of this instability can be gained by measuring the microwave signal generated when the two NIR lasers are mixed directly on the photodiode (inset Fig. 2(b)). The resulting 30MHz linewidth suggests that the QCL is the most significant source of instability, which we will show is a consequence of pulsed emission.

Fig. 3. Time domain waveforms (a) Illustration of the current pulse applied to the QCL, showing the period Trep and pulse length (b) RF waveforms recorded when a current pulse 2µs long is applied to the QCL. (i) Complete RF pulse detected by the oscilloscope (ii)/(iii) Time slices of the waveform showing the sinusoidal RF signal (iv)/(v) the Fourier transform of the two different time slices, revealing a small variation in beat note frequency across the waveform. (c) Transient analysis of the QCL waveform.

Subsequently we measured the time varying power of the beat signal with a high speed oscilloscope. Figure 3(a) shows a schematic of the current pulse train applied to the QCL, which turns the QCL on and off to produce a gated microwave signal. Figure 3(b)(i) shows a waveform of the RF signal detected by the photodiode from a single 2µs pulse of QCL emission (IQCL = 1650 mA, frep = 100kHz, 20% duty cycle). Individual oscillations of the RF signal are resolvable for different time points in the waveform, shown in Figs. 3(b)(ii) and 3(b)(iii). This oscilloscope waveform contains time varying frequency information about the QCL emission, which can be extracted with the aid of basic signal processing techniques [9,23,24]. For example, taking the Fourier transform (FT) of the oscillations in Figs. 3(b)(ii) and 3(b)(iii) results in the spectra shown in Figs. 3(b)(iv) and 3(b)(v). These show a small variation in the QCL emission frequency between the two time slices, indicative of intra pulse tuning. This analysis approach can be extended to the full waveform by breaking it down into a series of frames, as shown in Fig. 3(c). A FT can then be performed frame by frame to analyze transient frequency variations in the microwave beat note (shown in Fig. 3(c)). In

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such an analysis, the sampling window needs to be chosen to maximize temporal resolution (50ns), while maintaining sufficient frequency resolution (40 MHz) to resolve changes in THz emission. Note that the observed inter-pulse tuning range observed in Fig. 3(c) differs from Fig. 2(b), a consequence of two effects. First; the QCL power changes across the tuning pulse, resulting in the stable peak dominating the averaged spectrum produced by the analyzer. Secondly, Fig. 2(c) shows the spectrum of longer pulses, where the current varies across the pulse – introducing additional instability. The origin of this instability will be discussed later in the paper. We next illustrate the power of this coherent detection scheme by analyzing the properties of QCL emission. 4. Analysis of QCL emission As an application of this coherent detection system, we study the current-induced fine tuning of the QCL. Such tuning is generally a consequence of two mechanisms: temperature tuning (arising from joule heating) and gain-induced mode-pulling [25] (for example arising from Stark shifting of the gain curve [26,27]). Whereas the first is an outcome of the temperature dependence of the refractive index in GaAs, which redshifts laser modes [26], in the second mechanism a spectrally varying material gain induces a change in the refractive index of a material, pulling laser mode solutions towards the gain center frequency [26]. When the QCL operating current is varied the gain center frequency can change, producing a consequent change in the lasing mode frequencies tuning the laser mode frequencies in the same direction. In this case the laser emission spectra in the inset of Fig. 2(a) indicate that the gain curve redshifts and broadens as operating current is increased, which would induce a redshift in the laser modes. Unlike in most laser systems, where temperature effects always dominate, the small thermal coefficient of the QCL active region [6] means that both effects influence tuning. Understanding these two effects is critical for optimizing laser tuning ranges, requiring precise measurement of the QCL tuning. First, we measure QCL frequency tuning accurately using an electrical spectrum analyzer.

Fig. 4. Continuous tuning of the THz QCL emission measured by (a) coherent detection and (b) FTIR spectrometry. FTIR spectra are normalized as power information was not available.

The generated RF signal can be tuned by changing the driving current of the QCL, providing a precise measurement of tuning. Figure 4(a) shows a series of microwave spectra taken as the driving current of the QCL is continuously varied from 1150 mA to 1700 mA. The tuning range of 2.15 GHz, observed in this experiment, is limited by the bandwidth of the microwave amplifier. To ensure the correct tuning direction is presented the LO laser was tuned to higher frequency – the resultant shift in the RF signal determined if the sideband was above or below the LO frequency. This tuning can be compared directly against current tuning as measured with the FTIR, shown in Fig. 4(b). Although the low resolution of the FTIR (~2.1 GHz) limits the accuracy of the characterization, by fitting the mode position we can measure a comparable tuning range of ~2 GHz. This shows that the RF signal can be used to characterize emission frequency tuning at least as accurately as direct detection techniques such as FTIR spectrometry. For example, this technique has sufficient accuracy to measure

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tuning induced by directly changing the lasers operational temperature. We note that the range of tuning observed here is typical for such bound to continuum QCLs, however by appropriate cavity design it is possible to extend this range [28]. By improving the bandwidth of the detection electronics our system could measure tuning in such broadly tunable systems. Finally, we show that the transient tuning of the QCL emission within each pulse suggests that gain induced tuning plays a crucial part in frequency tuning.

Fig. 5. Intra-pulse tuning of QCL emission. (a) The current pulse applied to the QCL, measured with an inductive probe with a 2 ns response time. (b) Beat note frequency across a 3 µs pulse of QCL emission. Inset shows the Fourier transform of the indicated time period, the final 1 µs of the pulse.

First, note that Fig. 3 indicates significant intra pulse tuning. By measuring the 2µs current pulse applied to the QCL in Fig. 3(c) (as shown in Fig. 5(a)), we see that this is a consequence of the finite rise time provided by the driving circuit. However, this rise time can also reveal the transient amplitude and frequency tuning within pulses of QCL emission over submicrosecond timescales. Frame by frame time domain analysis was performed for a pulse 3 µs long, the maximum period over which our driving circuit can form and maintain flat current pulses. Figure 5(b) shows that there are significant frequency and power changes at the start of each pulse, > 2 GHz of tuning as the frequency goes from ~1.1 GHz through 0 Hz then back out to ~1.1 GHz. This tuning stabilizes after approximately 1.5 µs, after which the frequency remains constant. Note that this is essentially the full current driving tuning range, as can be seen in Fig. 4. The fact that the frequency stabilizes within the pulse suggests that temperature tuning is a minor effect; prior work indicates that the active region temperature will not equilibrate within pulses a few microseconds long [29], suggesting temperature tuning is minimal. This in turn suggests that the observed tuning is a consequence of gaininduced mode pulling effects, most likely Stark shifting in the bound-to-continuum active region [1,27]. We note that transient chirping can be neglected in this analysis, due to the inherently low alpha factor for THz QCLs [30]. At this point it is relevant to assess the ultimate stability of the QCL pulses for precision spectroscopy. To this end, we performed a Fourier transform on the final part of the pulse (inset of Fig. 5(b)) and found that 3 µs long pulses, without a servo-loop control, stabilize to a linewidth of 1.9 MHz. The remarkable ultimate stability of the QCL emission suggests that even pulsed QCLs may be appropriate for high resolution measurement of THz spectral lines. 5. Conclusions In conclusion, we have proposed and demonstrated a technique for the coherent detection of THz radiation with conventional off-the-shelf fiber optic components. This system is inherently tunable, and can detect any frequency of emission from a THz QCL with high temporal resolution. The detection bandwidth, modulation speed and signal strength are largely limited by the electronic components and coupling losses in our setup, and with appropriate RF design could be optimized for data transmission or spectroscopy experiments using QCLs. To illustrate its flexibility this technique was used to characterize the fine tuning properties of the THz emission from the laser itself. Specifically, we detected over 2GHz of

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tuning in the detected RF signal across the QCLs operational range, attributed to gain induced mode pulling phenomena. Funding EPSRC (G064504/1, EP/G03737X/1). Acknowledgments T. G. Folland acknowledges EPSRC North-West Nanoscience Doctoral Training Centre. The authors thank David Heard of Yenista Optics for loan of a 1.3µm tunable laser, Alwyn Seeds and Joshua Freeman for discussions on coherent detection and Md. Khairuzzaman for setup of the fiber coupling system.

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