Phase locking of a 1.5 Terahertz quantum cascade ... - OSA Publishing

Phase locking of a 1.5 Terahertz quantum cascade laser and use as a local oscillator in a heterodyne HEB receiver D. Rabanus1 , U. U. Graf2 , M. Philipp2 , O. Ricken2 , J. Stutzki2 , B. Vowinkel2 , M. C. Wiedner2 , C. Walther3 , M. Fischer3 , J. Faist3 1 European Southern Observatory Alonso de Cordova 3107, Vitacura, Santiago, Chile (formerly KOSMA) [email protected] 2 KOSMA, Universit¨ at zu Köln Zülpicher Str. 77, 50937 Köln, Germany 3 Institut f¨ ur Quantenelektronik, ETH Zürich Wolfgang-Pauli-Str. 16, 8093 Zürich, Switzerland

Abstract: We demonstrate for the first time the closure of an electronic phase lock loop for a continuous–wave quantum cascade laser (QCL) at 1.5 THz. The QCL is operated in a closed cycle cryo cooler. We achieved a frequency stability of better than 100 Hz, limited by the resolution bandwidth of the spectrum analyser. The PLL electronics make use of the intermediate frequency (IF) obtained from a hot electron bolometer (HEB) which is downconverted to a PLL IF of 125 MHz. The coarse selection of the longitudinal mode and the fine tuning is achieved via the bias voltage of the QCL. Within a QCL cavity mode, the free-running QCL shows frequency fluctuations of about 5 MHz, which the PLL circuit is able to control via the Stark–shift of the QCL gain material. Temperature dependent tuning is shown to be nonlinear, and of the order of -16 MHz/K. Additionally we have used the QCL as local oscillator (LO) to pump an HEB and perform, again for the first time at 1.5 THz, a heterodyne experiment, and obtain a receiver noise temperature of 1741 K. © 2009 Optical Society of America OCIS codes: (250.5590) Quantum-well devices; (300.6310) heterodyne spectroscopy; (300.6495) Terahertz spectroscopy; (350.1270) Astronomy and astrophysics; (040.2235) Far infrared or terahertz detectors

References and links 1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum Cascade Laser,” Science 264, 553 (1994). 2. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156 (2002). (c) 2002: Nature. 3. G. Sonnabend, D. Wirtz, F. Schmulling, and R. Schieder, “Tuneable Heterodyne Infrared Spectrometer for atmospheric and astronomical studies,” Appl. Opt. 41, 2978 (2002). 4. M. C. Wiedner, G. Wieching, F. Bielau, K. Rettenbacher, N. H. Volgenau, M. Emprechtinger, U. U. Graf, C. E. ˚ Nyman, R. Güsten, S. Philipp, D. Rabanus, J. Stutzki, Honingh, K. Jacobs, B. Vowinkel, K. M. Menten, L.-A. and F. Wyrowski, “First observations with CONDOR, a 1.5 THz heterodyne receiver,” Astron. Astrophys. 454, L33 (2006).

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Received 28 Jul 2008; revised 29 Dec 2008; accepted 12 Jan 2009; published 16 Jan 2009

2 February 2009 / Vol. 17, No. 3 / OPTICS EXPRESS 1159

5. T. de Graauw, E. Caux, R. Guesten, F. Helmich, J. Pearson, T. G. Phillips, R. Schieder, X. Tielens, P. Saraceno, J. Stutzki, C. K. Wafelbakker, and N. D. Whyborn, “The Herschel-Heterodyne Instrument for the Far-Infrared (HIFI),” vol. 37, pp. 1219–+ (2005). 6. H.-W. Hübers, A. Semenov, H. Richter, M. Schwarz, B. Günther, K. Smirnov, G. Gol’tsman, and B. Voronov, “Heterodyne receiver for 3-5 THz with hot-electron bolometer mixer,” Millimeter and Submillimeter Detectors for Astronomy II. Edited by Jonas Zmuidzinas 5498, 579 (2004). 7. A. L. Betz, R. T. Boreiko, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Frequency and phase-lock control of a 3 THz quantum cascade laser,” Optics Letters 30, 1837 (2005). (c) 2005: American Institute of Physics. 8. M. Philipp, U. U. Graf, A. Wagner-Gentner, D. Rabanus, and F. Lewen, “Compact 1.9 THz BWO local-oscillator for the GREAT heterodyne receiver,” Infrared Physics and Technology 51, 54 (2007). Elsevier B.V. 9. E. E. Becklin, A. G. G. M. Tielens, R. D. Gehrz, and H. H. S. Callis, “Stratospheric Observatory for Infrared Astronomy (SOFIA),” Infrared Spaceborne Remote Sensing and Instrumentation XV. Edited by Strojnik-Scholl 6678, 8 (2007). 10. J. Gao, M. Hajenius, Z. Yang, J. Baselmans, P. Khosropanah, R. Barends, and T. Klapwijk, “Terahertz Superconducting Hot Electron Bolometer Heterodyne Receivers,” IEEE Trans. Appl. Supercond. 17(2, Part 1), 252 – 258 (2007). 11. U. U. Graf, S. Heyminck, E. A. Michael, S. Stanko, C. E. Honingh, K. Jacobs, R. T. Schieder, J. Stutzki, and B. Vowinkel, “SMART: The KOSMA Sub-Millimeter Array Receiver for Two frequencies,” vol. 4855, pp. 322– 329 (2003). 12. C. Kasemann, R. Güsten, S. Heyminck, B. Klein, T. Klein, S. D. Philipp, A. Korn, G. Schneider, A. Henseler, A. Baryshev, and T. M. Klapwijk, “CHAMP+: a powerful array receiver for APEX,” vol. 6275 (2006). 13. H.-W. Hübers, S. G. Pavlov, A. D. Semenov, R. Köhler, L. Mahler, A. Tredicucci, H. E. Beere, D. A. Ritchie, and E. H. Linfield, “Terahertz quantum cascade laser as local oscillator in a heterodyne receiver,” Opt. Express 13, 5890 (2005). 14. J. R. Gao, J. N. Hovenier, Z. Q. Yang, J. J. A. Baselmans, A. Baryshev, M. Hajenius, T. M. Klapwijk, A. J. L. Adam, T. O. Klaassen, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer,” Appl. Phys. Lett. 86, 4104 (2005). (c) 2005: American Institute of Physics. ˚ Nyman, P. Schilke, K. Menten, C. Cesarsky, and R. Booth, “The Atacama Pathfinder EXperiment 15. R. Güsten, L. A. (APEX) - a new submillimeter facility for southern skies -,” Astro. Astrophy. 454, L13 (2006). 16. C. Walther, M. Fischer, G. Scalari, R. Terazzi, N. Hoyler, and J. Faist, “Quantum cascade lasers operating from 1.2 to 1.6 THz,” Appl. Phys. Lett. 91, 1122 (2007). (c) 2007: American Institute of Physics. 17. C. Walther, G. Scalari, J. Faist, H. Beere, and D. Ritchie, “Low frequency terahertz quantum cascade laser operating from 1.6 to 1.8 THz,” Appl. Phys. Lett. 89, 1121 (2006). (c) 2006: American Institute of Physics. 18. B. S. Williams, S. Kumar, H. Callebaut, Q. Hu, and J. L. Reno, “Terahertz quantum-cascade laser operating up to 137 K,” Appl. Phys. Lett. 83, 5142 (2003). (c) 2003: American Institute of Physics. 19. R. Schieder, J. M. Horn, O. Siebertz, C. Moeckel, F. Schloeder, C. Macke, and F. Schmuelling, “Design of large-bandwidth acousto-optical spectrometers,” Proc. SPIE Vol. 3357 3357, 359 (1998). 20. A. J. L. Adam, I. Kaˇsalynas, J. N. Hovenier, T. O. Klaassen, J. R. Gao, E. E. Orlova, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions,” Appl. Phys. Lett. 88, 1105 (2006). (c) 2006: American Institute of Physics. 21. A. Yariv, “Quantum Electronics, 4th edition,” Chapter 6 p. 478 (1991). 22. J. Xu, J. M. Hensley, D. B. Fenner, R. P. Green, L. Mahler, A. Tredicucci, M. G. Allen, F. Beltram, H. E. Beere, and D. A. Ritchie, “Tunable terahertz quantum cascade lasers with an external cavity,” Appl. Phys. Lett. 91, 1104 (2007). (c) 2007: American Institute of Physics.

1.

Introduction

Quantum cascade lasers [1] (QCL) have been demonstrated for more than a decade in the IRregime and the push for longer wavelengths has given rise of laser sources which even cover the Terahertz frequency range [2]. It has been proposed for several years to use QCLs as local oscillators for heterodyne receivers. Recently, the Tunable Heterodyne Infrared Spectrometer (THIS) has successfully demonstrated astronomical observations at 10 μ m wavelength using a QCL local oscillator[3]. And only now, since continuous wave operation of THz-QCLs can be achieved with tolerable cooling effort, one might consider THz-QCLs as a practical, sufficiently controllable radiation source for heterodyne receivers. Heterodyne receivers up to about 1.5 THz [4] or 2 THz[5] have demonstrated the use of solid state multiplier chains as local oscillators. At higher frequencies, only optically pumped gas lasers[6, 7], limited to selected, fixed frequency operation, or multiplied Backward-Wave#99346 - $15.00 USD

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Oscillator sources[8], limited to a narrow tuning range, have demonstrated successful operation. From the astrophysics side, there is a strong demand for high spectral resolution observations (ν /Δν > 106 ) throughout the far-infrared and mid-infrared spectral regime, which houses, among other important spectral signatures, the brightest cooling lines of the dense interstellar medium ([CII] 158 μ m, [OI] 63 and 145 μ m) and the ground rotational lines of molecular hydrogen at 17 and 28 μ m. The Stratospheric Observatory for Infrared Astronomy [9] (SOFIA), carrying a 3m-class telescope into the stratosphere at 12 to 14 km observing altitude, will open the sky throughout this wavelength regime. Rapid progress in the detector technology provides sensitive THzmixers [6, 10] and advanced opto-mechanical designs nowadays allow the realization of moderate size heterodyne arrays for astronomical applications[11, 12]. Given the recent advances in the field, QCLs thus bear the potential to satisfy the strong demand for broadband tunable, high-power, frequency stable radiation sources as local oscillators, necessary to design and implement THz-heterodyne receivers for astronomy. Previously, experiments with QCLs have been either demonstrations of phase–locking [7] or usage as local–oscillator source [13, 14], but not both at the same time. In this paper, we report on an important step towards realizing this goal, namely the demonstration of a phase locked QCL operating at 1.5 THz and its use a the local oscillator source in a heterodyne receiver. The structure of this paper is as follows: In section 2 we describe the experimental setup of the heterodyne receiver demonstration. Section 3 contains the measurements and their interpretations, and finally section 4 we conclude the findings and give an outlook on what we assume to be the next steps towards a practical receiver for submillimeter astronomy. 2.

Experimental setup

Our experimental setup makes use of the already existing astrophysical heterodyne receiver CONDOR [4] which has been developed and built at KOSMA as a principal investigator instrument for highest frequency ground based observations at the Atacama Pathfinder EXperiment [15] (APEX) telescope. Receiver Cryostat

Window Martin−Puplett Interferometer

HEB

QCL Cryostat

QCL

Input

Teflon Lens

Port 1

Polarizer

Wire Grid

Port 2

Fig. 1. Experimental setup. For thermal reasons, the QCL and HEB are located in separate cryostats. For more details see the text.

The CONDOR receiver [4], a HEB mixer in a cryostat with a pulse tube cooler, comprises a combined cryogenic and warm quasi–optical path. The cryogenic optics consists of a corrugated feed horn that forms a Gaussian beam towards a collimating ellipsoidal mirror, and an infrared #99346 - $15.00 USD

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and optical blocker. The warm optics are the cryostat window, a Martin–Puplett diplexer for a low–loss overlay of the LO radiation on the RF signal (astronomical signal). The standard CONDOR LO, a computer controlled solid state multiplier chain by Virginia Diodes Inc. (VDI, hereafter VDI–LO), is used to pump the HEB. As a basis oscillator to drive the VDI multiplier chain, we used a YIG oscillator built in–house. On one hand, we have used the CONDOR receiver in its standard configuration to observe and characterize the phase-locked QCL. In a second experiment, we have replaced the standard LO of CONDOR by the QCL and have verified unchanged sensitivity of the heterodyne experiment with this new LO source. The QCL in this experiment is one device of a series produced at University of Neuchâtel [16, 17], Switzerland, which demonstrated the successful operation of low frequency terahertz quantum lasers covering the range of 1.2–1.8 THz. This particular device has been integrated in a cryostat which is cooled down to about 4 K by a Balzers closed–cycle cryo cooler. It is a metal–metal waveguide [18] device with cleaved facets on both sides of the active region, with no additional treatments of the radiating surfaces applied, as described by Walther [16]. The setup as depicted in Fig. 1 shows a top–view sketch of the optical bench with the beam path indicated. The QCL’s radiation was coupled in through the sky signal port (Port 1) while maintaining the CONDOR local oscillator (LO) in place (Port 2) for reference. To modulate the QCL’s intensity, a rotatable polarizer grid was introduced in the beam path. The experiments are split in three stages: 2.1.

Observation of the QCL emission as sky signal

Here the QCL emission line is observed with CONDOR in its standard receiver configuration. In this case the VDI–LO was used on port 2 in Fig. 1, and the intermediate frequency (IF) output of the receiver was analyzed with either a spectrum analyzer, for longer term integration, or an acousto–optical spectrometer (AOS [19]) for data dumps on short timescales of the order of 10ms. We observe the QCL laser emission at three longitudinal modes, and verify the tuning capability within such a single, longitudinal mode. 2.2.

Temperature–dependent tuning

QCLs change their emission frequency depending on the resonance condition of the modes in the device. Mainly driven by the longitudinal mode structure, the frequency depends on the cavity length and the index of refraction of the lasing material. At cryogenic temperatures of few tens of Kelvin (here TQCL ≈ 20 − 30K) we do not expect any significant length variation of the geometrical cavity length. Therefore, temperature–dependent variation of the index of refraction is the most prominent effect. An additional resistive heater was mounted on the cold head of the QCL cryostat, and the temperature could be raised from 21.9 K with the dissipation of only the QCL, to 29.1 K. 2.3.

Phase–locking the QCL with the down–converted CONDOR IF

Phase locking a monochromatic radiation source requires both measuring and adjusting the phase of the source. In our experiment, we are using the IF of the CONDOR receiver (see section 2.1 and Fig. 2) with a frequency down–converter. That allows us to drive a PLL circuit to control the bias voltage of the QCL. In this experiment, we down–convert the IF signal to 200 MHz, feed it into a PLL circuit, and herewith lock the phase of the QCL. Fig. 2 shows the details. The box labeled “PLL” in Fig. 2 is a circuit with a standard phase comparator chip which derives the anaolg control voltage from the phase difference to the reference oscillator.

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HEB VDI LO

1499.3 GHz

Mixer 1.2 GHz IF

REF OSC 200 MHz IF

20dB

PLL

1500.5 GHz QCL YIG OSC

1.0 GHz LO QCL Bias

Fig. 2. Setup of the locking scheme. The QCL and VDI–LO are mixed on the HEB, it’s IF is downconverted to about 200 MHz to drive a PLL cicuit which, in return, controls the bias voltage of the QCL.

2.4.

QCL as LO for CONDOR

In this stage we configured port 2 in Fig. 1 to be the input for the sky signal. This, together with a free–running QCL on port 1 –serving as LO– is an alternative receiver configuration, and allows us to obtain the figures of merit, e.g. noise temperature of the receiver system, via the Y–factor method, when pumped by the QCL as local oscillator. This method assumes separability of the whole system’s noise into receiver noise and the noise received by the antenna: Tsys = Trec + Ta . With two temperature baths at Thot and Tcold at the antenna input, we isolate and determine Trec . Note that at a frequency of 1.5 THz, i.e. hν /k = 72K, we are already far off the validity regime of the Rayleigh-Jeans approximation (hν