Detection sensitivity of laser feedback interferometry using a terahertz quantum cascade laser.

We report on the high detection sensitivity of a laser feedback interferometry scheme based on a terahertz frequency quantum cascade laser (QCL). We show that variations on the laser voltage induced by optical feedback to the laser can be resolved with the reinjection of powers as low as ∼-125 dB of the emitted power. Our measurements demonstrate a noise equivalent power of ∼1.4 pW/√Hz, although, after accounting for the reinjection losses, we estimate that this corresponds to only ∼1 fW/√Hz being coupled to the QCL active region.

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

We demonstrate an active phase-nulling scheme for terahertz (THz) frequency quantum cascade lasers (QCLs) under optical feedback, by active electronic feedback control of the emission frequency. Using this scheme, the frequency tuning rate of a THz QCL is characterized, with significantly reduced experimental complexity compared to alternative approaches. Furthermore, we demonstrate real-time displacement sensing of targets, overcoming the resolution limits imposed by quantization in previously implemented fringe-counting methods. Our approach is readily applicable to high-frequency vibrometry and surface profiling of targets, as well as frequency-stabilization schemes for THz QCLs.

Three-dimensional terahertz imaging using swept-frequency feedback interferometry with a quantum cascade laser.

We demonstrate coherent three-dimensional terahertz imaging by frequency modulation of a quantum cascade laser in a compact and experimentally simple self-mixing scheme. Through this approach, we can realize significantly faster acquisition rates compared to previous schemes employing longitudinal mechanical scanning of a sample. We achieve a depth resolution of better than 0.1 µm with a power noise spectral density below -50 dB/Hz, for a sampling time of 10 ms/pixel.

Terahertz inverse synthetic aperture radar imaging using self-mixing interferometry with a quantum cascade laser.

We propose a terahertz (THz)-frequency synthetic aperture radar imaging technique based on self-mixing (SM) interferometry, using a quantum cascade laser. A signal processing method is employed which extracts and exploits the radar-related information contained in the SM signals, enabling the creation of THz images with improved spatial resolution. We demonstrate this by imaging a standard resolution test target, achieving resolution beyond the diffraction limit.

Terahertz in vivo imaging of human skin: Toward detection of abnormal skin pathologies.

Terahertz (THz) imaging has long held promise for skin cancer detection but has been hampered by the lack of practical technological implementation. In this article, we introduce a technique for discriminating several skin pathologies using a coherent THz confocal system based on a THz quantum cascade laser. High resolution in vivo THz images (with diffraction limited to the order of 100 µm) of several different lesion types were acquired and compared against one another using the amplitude and phase values. Our system successfully separated pathologies using a combination of phase and amplitude information and their respective surface textures. The large scan field (50 × 40 mm) of the system allows macroscopic visualization of several skin lesions in a single frame. Utilizing THz imaging for dermatological assessment of skin lesions offers substantial additional diagnostic value for clinicians. THz images contain information complementary to the information contained in the conventional digital images.

Spectral broadening caused by dynamic speckle in self-mixing velocimetry sensors.

Self-mixing laser sensors require few components and can be used to measure velocity. The self-mixing laser sensor consists of a laser emitting a beam focused onto a rough target that scatters the beam with some of the emission re-entering the laser cavity. This 'self-mixing' causes measurable interferometric modulation of the laser output power that leads to a periodic Doppler signal spectrum with a peak at a frequency proportional to the velocity of the target. Scattering of the laser emission from a rough surface also leads to a speckle effect that modulates the Doppler signal causing broadening of the signal spectrum adding uncertainty to the velocity measurement. This article analyzes the speckle effect to provide an analytic equation to predict the spectral broadening of an acquired self-mixing signal and compares the predicted broadening to experimental results. To the best of our knowledge, the model proposed in this article is the first model that has successfully predicted speckle broadening in a self-mixing velocimetry sensor in a quantitative manner. It was found that the beam spot size on the target and the target speed affect the resulting spectral broadening caused by speckle. It was also found that the broadening is only weakly dependent on target angle. The experimental broadening was consistently greater than the theoretical speckle broadening due to other effects that also contribute to the total broadening.

Adaptive self-mixing vibrometer based on a liquid lens.

A self-mixing laser diode vibrometer including an adaptive optical element in the form of a liquid lens (LL) has been implemented and its benefits demonstrated. The LL arrangement is able to control the feedback level of the self-mixing phenomenon, keeping it in the moderate feedback regime, particularly suitable for displacement measurements. This control capability has enabled a remarkable increase in the sensor-to-target distance range where measurements are feasible. Target vibration signal reconstructions present a maximum error of lambda radical16 as compared with a commercial sensor, thus providing an improved working range of 6.5 cm to 265 cm.

Lasers-an effective artificial source of radiation for the cultivation of anoxygenic photosynthetic bacteria.

The laser diode (LD) is a unique light source that can efficiently produce all radiant energy within the narrow wavelength range used most effectively by a photosynthetic microorganism. We have investigated the use of a single type of LD for the cultivation of the well-studied anoxygenic photosynthetic bacterium, Rhodobacter capsulatus (Rb. capsulatus). An array of vertical-cavity surface-emitting lasers (VCSELs) was driven with a current of 25 mA, and delivered radiation at 860 nm with 0.4 nm linewidth. The emitted light was found to be a suitable source of radiant energy for the cultivation of Rb. capsulatus. The dependence of growth rate on incident irradiance was quantified. Despite the unusual nearly monochromatic light source used in these experiments, no significant changes in the pigment composition and in the distribution of bacteriochlorophyll between LHII and LHI-RC were detected in bacterial cells transferred from incandescent light to laser light. We were also able to show that to achieve a given growth rate in a light-limited culture, the VCSEL required only 30% of the electricity needed by an incandescent bulb, which is of great significance for the potential use of laser-devices in biotechnological applications and photobioreactor construction.

Algorithm for the determination of intrinsic optical constants of metal films: application to aluminum.

Optical and electron-energy-loss data for evaporated-aluminum films have been critically analyzed and used in an iterative, self-consistent algorithm that represents a combination of the Kramers-Kronig analysis and the semiquantum-model application. The novel values of the intrinsic optical functions of aluminum have been determined in a wide spectral range from 200 µm (6.2 meV) to 0.12 nm (10 keV). These functions are in accordance with recent calculations by Lee and Chang [Phys. Rev. B 49, 2362 (1994)], with dc conductivity measurements, and are in good agreement with both peak positions and line widths obtained from electron-energy-loss experiments. The results are examined for internal consistency by inertial and f-sum rules.

Determination of the reflection coefficients of laser light of wavelengths lambda(0.22 microm,200 microm) from the surface of aluminum using the Lorentz-Drude model.

Using four different fitting methods, the applicability of the Lorentz-Drude (LD) model has been investigated for calculating the reflection coefficients of the laser light of wavelengths of lambda(0.22 microm,200 microm) from the surface of aluminum. The applicability of the LD model has been established not only in the IR region but also in the visible and UV regions. The values of plasma frequency omega(rho) and damping frequency I have been determined. The values of the reflection coefficients calculated by the LD model have been compared with corresponding experimental values. It has been established that in both the far and medium IR region as well as in both the visible and UV, discrepancies are smaller than 3%. The maximum discrepancy is smaller than 7% and occurs in a narrow region (~lambda = 0.825 microm), where aluminum strongly absorbs caused by band-band transitions.

Optical properties of metallic films for vertical-cavity optoelectronic devices.

We present models for the optical functions of 11 metals used as mirrors and contacts in optoelectronic and optical devices: noble metals (Ag, Au, Cu), aluminum, beryllium, and transition metals (Cr, Ni, Pd, Pt, Ti, W). We used two simple phenomenological models, the Lorentz-Drude (LD) and the Brendel-Bormann (BB), to interpret both the free-electron and the interband parts of the dielectric response of metals in a wide spectral range from 0.1 to 6 eV. Our results show that the BB model was needed to describe appropriately the interband absorption in noble metals, while for Al, Be, and the transition metals both models exhibit good agreement with the experimental data. A comparison with measurements on surface normal structures confirmed that the reflectance and the phase change on reflection from semiconductor-metal interfaces (including the case of metallic multilayers) can be accurately described by use of the proposed models for the optical functions of metallic films and the matrix method for multilayer calculations.