IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 4, NO. 5, SEPTEMBER 2014
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THz Letters Continuous-Wave Reflection Imaging Using Optical Feedback Interferometry in Terahertz and Mid-Infrared Quantum Cascade Lasers F. P. Mezzapesa, M. Petruzzella, M. Dabbicco, H. E. Beere, D. A. Ritchie, M. S. Vitiello, and G. Scamarcio
Abstract—In this paper, we investigate a coherent imaging system based on Terahertz and mid-infrared (MIR) quantum cascade lasers (QCLs) in terms of phase sensitivity against optical feedback. In the self-mixing scheme, a single QCL acting as oscillator, mixer and detector of infrared radiation, is used for continuous-wave reflection imaging as well as phase profiling. We study the dependence of the phase signature on the optical feedback level and assess the limit for phase signal detection. Index Terms—Quantum cascade laser (QCL), THz imaging, optical feedback interferometry, optical sensing, laser sensors.
I. INTRODUCTION
O
PTICAL feedback interferometry in quantum cascade lasers (QCLs) is well suited for contactless sensing where the wavelength agility and spectral purity associated with QCLs are required. QCLs are characterized by intrinsic linewidth of tens of hertz (Hz) [1], [2], potentially limited to the quantum noise, and continuous-wave CW output power on the order of several watts in the mid-infrared (MIR) [3] and few milliwatts (mW) in the terahertz (THz) range [4]. Moreover, QCLs are intrinsically stable against optical feedback [5], tolerating strong feedback level without exhibiting dynamical instabilities typical of bipolar semiconductor lasers, such as mode-hopping, intensity pulsation or coherence collapse [6]. This unique behavior of QCLs can be ascribed to two main factors: 1) the small linewidth enhancement factor ( in THz-QCLs [7]) with respect to conventional diode lasers, that reduces the number of external cavity modes possibly concurring in destabilizing the CW emission and 2) the absence Manuscript received March 05, 2014; revised April 24, 2014; accepted May 30, 2014. Date of publication June 18, 2014; date of current version August 22, 2014. This work was supported in part by MIUR—PON02-0576 INNOVHEAD and by MASSIME. The work of M. S. Vitiello was supported by the Italian Ministry of Education, University Research (MIUR) through the program “FIRB —Futuro in Ricerca 2010” RBFR10LULP “Fundamental research on Terahertz photonic devices”. F. P. Mezzapesa, M. Petruzzella, M. Dabbicco, and G. Scamarcio are with the Department of Physics, University of Bari “Aldo Moro”, I-70126 Bari, Italy, and also with CNR—Istituto di Fotonica e Nanotecnologie UOS Bari, I-70126 Bari, Italy (e-mail:
[email protected],
[email protected],
[email protected],
[email protected]). H. E. Beere and D. A. Ritchie are with Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK (e-mail:
[email protected],
[email protected]). M. S. Vitiello is with the NEST, CNR—Istituto Nanoscienze and Scuola Normale Superiore, I-56127 Pisa, Italy (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TTHZ.2014.2329312
of relaxation oscillations (class-A laser) owing to high values of photon-to-carrier lifetime ratio (i.e., ultrafast intersubband relaxation time) [5]. Also, the inherent sensitivity of the compliance voltage to the optical feedback is ideal for compact and monolithic sensors using the self-mixing approach [8], since the power modulation can be detected directly as voltage drop across the QCL active region with no need of external detectors. Indeed, the optical feedback interferometry in QCLs does incorporate the detector functionality within the laser cavity itself, and has been extensively exploited for a variety of relevant applications including, but not limited to, phase spectroscopy [9] in-line laser ablation monitoring [10], metrology [11], [12]. THz imaging, though self-mixing effect in a QCL, has been recently demonstrated [13]–[15]. A detector-less coherent system to monitor in reflection mode the density of photo-generated charge carriers on a semiconductor surface was also reported [16]. In the following, we study the effect of optical feedback levels on the phase sensitivity in a raster-scanned imaging system based on continuous-wave MIR and THz QCLs, showing the existence of feedback thresholds for accurate phase-detection. Its potential applicability in active 3D profiling may boost breakthrough innovation for fast and reliable QCL-based reflection imaging systems in the THz-gap of the electromagnetic spectrum. II. EXPERIMENTAL RESULT Fig. 1 shows the homodyne (coherent) imaging system consisting of a single-arm optical feedback interferometer based on the self-mixing effect in quantum cascade lasers. Self-mixing interferometry (SMI) occurs when a small fractions of emitted radiation is fed back from an external target into the laser cavity. The reflected light coherently interferes with the optical field inside the active cavity, producing variations in the threshold gain, emitted power, lasing spectrum and voltage drop across the active region. Thus, a single QCL subjected to optical feedback can both generate and sense the THz radiation, with no need of an external detector. In the MIR domain, a tunable QCL emitting at m and temperature stabilized at 283 K, was driven in continuous-wave operating mode at a constant current . The probe beam was collimated and tightly focused on the specimen by a couple of gold-plated off-axis parabolic reflectors. Poly-propylene sheets (83% transmittance at 6.2 m) were used as attenuator to adjust the effective optical feedback reflected off the
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IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 4, NO. 5, SEPTEMBER 2014
Fig. 3. Experimental results: SMI signal amplitude at various feedback strength level in MIR QCL, disclosing the decrease of fringe contrast for high feedback. Inset: representative waveforms of the interferometric signal in region III showing different fringe contrast. Fig. 1. Schematic layout of the experimental setup. The QCL beam m) was focused onto the sample surface by a couple of (optical path length mm 90 gold-plated off-axis parabolic reflectors. 2-inch diameter, The chopper frequency was set at 1 kHz.
Fig. 2. Effect of optical feedback levels on the phase sensitivity in a QCL-based imaging. The intensity of SM signal is in millivolts. (a) Reflectivity image at the highest level of optical feedback. (b) Phase information (SM fringes) are retrieved only by reducing the feedback level. (c) Representative waveform obmm/s. The specimen surface was tained in response to raster-scan at to detect the purposely tilted with respect to the optical axis phase signature of the reflected field (i.e., surface morphology).
target surface and refocused upon the front facet of the QCL. Coherent mixing of the reflected beam with the intracavity field produced voltage modulation on the laser terminals, i.e., interferometric fringes associated with the spatial phase of the reflected field. The voltage offset measured across the device (i.e., the incoherent amount of back-scattered radiation) was subtracted by ac-coupling to a lock-in amplifier with time constant of 20 ms. The laser beam was modulated at the frequency of 1 kHz via a mechanical chopper set along the optical axis. Fig. 2 shows representative images in reflection mode of an aluminum label deposited on a diffusive substrate of polycarbonate. SMI signal acquisitions at various feedback levels were
made by raster-scanning the specimen with a linear-motor stage ( m/s) at step-size of m. The sample was purposely tilted ( ) to acquire the phase profile. At the highest feedback level [see Fig. 2(a)], fringes associated with the spatial phase information were not visible on the waveform of the self-mixing signal, nearly buried in the DC signal arising from the effective sample reflectivity. Thus, the imaging system failed to detect changes in phase of the field relative to optical path modifications experienced by photons. However, conventional asymmetric modulations on the SMI signal were disclosed at higher attenuation, i.e., reducing the optical feedback strength, as shown in Fig. 2(b). Indeed, Fig. 2(c) shows a representative waveform with contiguous sawtooth like fringes superimposed to the DC voltage and corresponding to multiple displacements of half the laser wavelength in the external cavity. Fast-switching transitions were exhibited each time the interferometric phase was varied by , giving access to the information relative to the surface tilting of the specimen with respect to the optical axis. Furthermore, the amplitude of the fringes was dependent on the feedback attenuation. In order to investigate the SMI signal amplitude dependence on the feedback strength, the laser was focused onto the Al surface of the sample at the position marked by the arrow in Fig. 2, and the sample was moved longitudinally along the optical axis. Fig. 3 shows the peak-to-peak self-mixing voltage - recorded as a function of the feedback strength parameter (k). The latter was carefully measured under various coupling conditions, as described in [5]. In the region I, we verified that the fringe amplitude - monotonically increased when the -value was increased, as predicted by the theory [6]. A further reduction of the attenuation yield to an interval of k-values in which the fringe magnitude saturates (see plateau for in region II), as expected in conventional laser diodes subjected to moderate feedback [17]. Finally, the SMI signal switched to decrease in amplitude as shown in the region III when the feedback was further increased, i.e., for the highest feedback levels accessible in the experiment. Notably, neither multistable regime nor coherent collapse typically occurring in bipolar laser was ever observed with QCLs regardless of the optical feedback level, owing to their intrinsic stability [5]. Nonetheless, the fringe contrast of SMI waveforms in region III followed an evolution representatively depicted in the inset of Fig. 3, where fringes gradually disappeared at extreme feedback levels. In particular, the signal merged with the noise floor at and zero crossing with steep slope barely
MEZZAPESA et al.: CONTINUOUS-WAVE REFLECTION IMAGING IN THz AND MIR QCLs
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terahertz and mid-infrared QCLs. The capability to retrieve information about the phase of the field was demonstrated and the threshold for phase detection was accurately measured, paving the way towards high resolution 3D imaging. REFERENCES
Fig. 4. Imaging via optical feedback interferometry in quantum cascade lasers: (a) MIR image of a “one-cent-euro” coin, and (b) THz image of a dime (ten-cent US) coin, respectively. The gray color scale of the picture codifies the value of the self-mixing signal.
occurred. Hence, complex algorithm may be required to handle the phase signature and retrieve information from the phase profile with accuracy. III. COHERENT REFLECTION IMAGING The extension of this study in the THz domain yield to comparable results. A resonant phonon single longitudinal mode THz QCL emitting at the wavelength of m (3.93 THz) and embedded in a surface plasmon waveguide [18] was used in the experiment. The device was mounted on the cold finger of a continuous-flow cryostat to keep the heat sink temperature fixed at 17 K. To maximize the sensitivity to optical feedback, the QCL was driven near threshold (i.e., mA for mA for the solitary QCL) at a constant current CW mode operation. Polymethylpentene foils (TPX) were inserted in the optical path as attenuator in the THz region (82% of transmission) so that various feedback levels could be obtained. Fig. 4 shows high-resolution imaging via optical feedback interferometry in quantum cascade lasers: a) MIR image of onecent–euro coin; and b) THz image of a dime (ten-cent US) coin, respectively. The range of working operation was investigated in terms of suitable attenuation to resolve the sample morphology. In Fig. 4(a), the main contribute to the image formation arises from the DC term of the signal, i.e., no feedback attenuation was used. The sensitivity to the phase was achieved in the THz image shown in Fig. 4(b), by setting the optical feedback strength at . SMI adjacent fringes were generated at the flat zone of the coin (i.e., roughness smaller than ) and correspond to the sample tilt in the y-direction. The contrast of the coin edges can be attributed to the scattering mechanism of radiation at interfaces. The spatial resolution of the imaging scheme is in the sub-millimeter range, with a signal to noise ratio larger than dB). 35 dB and a relatively high dynamic range ( IV. CONCLUSION We have studied the effect of optical feedback strength on the phase sensitivity in CW reflection imaging based on SMI with
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