Continuous wave room temperature external ring cavity quantum cascade laser

An external ring cavity quantum cascade laser operating at ∼5.2 μm wavelength in a continuous-wave regime at the temperature of 15 °C is demonstrated. Out-coupled continuous-wave optical powers of up to 23 mW are observed for light of one propagation direction with an estimated total intra-cavity optical power flux in excess of 340 mW. The uni-directional regime characterized by the intensity ratio of more than 60 for the light propagating in the opposite directions was achieved. A single emission peak wavelength tuning range of 90 cm-1 is realized by the incorporation of a diffraction grating into the cavity

Visible-light optical coherence tomography platform for the characterization of the skin barrier

We demonstrate a free-space, trolley-mountable Fourier domain visible-light optical coherence tomography (OCT) system for studying the stratum corneum in non-palmar human skin. An axial resolution of 1 µm in tissue and at least −75 dB sensitivity have been achieved. High-quality B-scans, containing 1600 A-scans, are acquired at a rate of 39 Hz. Images from the dorsal hand, ventral wrist and ventral forearm areas are obtained, with a clearly resolved stratum corneum layer (typically 5–15 µm thick) presenting as a hypoechogenic dark layer below the bright entrance signal, similar to that found in palmar skin with traditional OCT systems. We find that the appearance of the stratum corneum layer strongly depends on its water content, becoming brighter after occlusive hydration

Sensitivity Advantage of QCL Tunable-Laser Mid-Infrared Spectroscopy over FTIR Spectroscopy

Interest in mid-infrared spectroscopy instrumentation beyond classical FTIR using a thermal light source has increased dramatically in recent years. Synchrotron, supercontinuum, and external-cavity quantum cascade laser light sources are emerging as viable alternatives to the traditional thermal black-body emitter (Globar), especially for remote interrogation of samples ("stand-off" detection) and for hyperspectral imaging at diffraction-limited spatial resolution ("microspectroscopy"). It is thus timely to rigorously consider the relative merits of these different light sources for such applications. We study the theoretical maximum achievable signal-to-noise ratio (SNR) of FTIR using synchrotron or supercontinuum light vs. that of a tunable quantum cascade laser, by reinterpreting an important result that is well known in near-infrared optical coherence tomography imaging. We rigorously show that mid-infrared spectra can be acquired up to 1000 times faster - using the same detected light intensity, the same detector noise level, and without loss of SNR - using the tunable quantum cascade laser as compared with the FTIR approach using synchrotron or supercontinuum light. We experimentally demonstrate the effect using a novel, rapidly tunable quantum cascade laser that acquires spectra at rates of up to 400 per second. We also estimate the maximum potential spectral acquisition rate of our prototype system to be 100,000 per second

Metalorganic vapour phase epitaxy of InGaAs/InAlAs and GaAs/AlGaAs quantum cascade laser structures

Metalorganic vapour phase epitaxy (MOVPE) has been successfully introduced by our group as an alternative growth technology of mid-IR InGaAs/InAlAs/InP quantum cascade lasers (QCLs) [1, 2]. Later on, we have transferred this technology to a production type multi wafer MOVPE reactor [3]. Many research groups and industrial companies have since followed our technological approach. The crystalline quality of the MOVPE grown material meets the stringent requirements imposed by the QCLs designs for operation in a wide spectral range of ~5-16 µm. However, developing an epitaxial process of highly strain-compensated QCL structures for operation at shorter wavelengths of ~3-5 µm appeared to be extremely challenging. Careful tuning the growth temperature regime was used to produce 30-period In0.7Ga0.3As/In0.34Al0.66As structures with ~1.2% mismatch from InP in the individual constituent layers. Fig. 1 shows an STEM image of a part of the QCL core. The measured length of one cascaded period of 51.5 nm is identical to the intended value. The same period length was derived from the X-ray diffraction data. 10 µm wide and 3 mm long devices with as-cleaved facets operate at λ ≈ 4 µm and deliver more than 2.4 W of peak optical power from both facets at 300 K with threshold current density of 2.5 kA/cm2 (Fig. 2). The lasers operate up to at least 400 K with characteristic temperature of 153 K. The developed epitaxial process represents a solid platform for engineering straincompensated QCLs structures for shorter emission wavelengths around 3.5 µm. Another direction of our recent research efforts was revisiting GaAs-based QCLs to develop a robust and costeffective growth technology of devices operating around 9 µm. InGaP and InAlP waveguides were used to improve optical confinement and reduce waveguide losses. STEM confirmed the intended thickness of individual GaAs and Al0.45Ga0.55 layers in the laser core. The amplitude of the interface roughness is less than 0.5 nm (the nominal thickness of the thinnest layer in the active region is 0.9 nm). QCLs with In0.47Al0.53P waveguides demonstrate record low threshold current densities for the GaAs/AlxGa1-xAs materials system. Under pulsed operation, threshold current densities of 2.2 and 4.4 kA/cm2 were observed at 240 and 300 K respectively, and laser emission was maintained up to temperatures of at least 330 K. The laser emitted peak optical powers of 0.57 W at 240 K and 0.16 W at 300 K. The presented laser performance should greatly increase the prospects of mid-IR GaAs-based QCLs for technological applications

The mid-infrared swept laser:Life beyond OCT?

Near-infrared external cavity lasers with high tuning rates ("swept lasers") have come to dominate the field of near-infrared low-coherence imaging of biological tissues. Compared with time-domain OCT, swept-source OCT a) replaces slow mechanical scanning of a bulky reference mirror with much faster tuning of a laser cavity filter element and b) provides a ×N (N being the number of axial pixels per A-scan) speed advantage with no loss of SNR. We will argue that this striking speed advantage has not yet been fully exploited within biophotonics but will next make its effects felt in the mid-infrared. This transformation is likely to be driven by recent advances in external cavity quantum cascade lasers, which are the mid-IR counterpart to the OCT swept-source. These mid-IR sources are rapidly emerging in the area of infrared spectroscopy. By noting a direct analogy between time-domain OCT and Fourier Transform Infrared (FTIR) spectroscopy we show analytically and via simulations that the mid-IR swept laser can acquire an infrared spectrum ×N (N being the number of spectral data points) faster than an FTIR instrument, using identical detected flux levels and identical receiver noise. A prototype external cavity mid-IR swept laser is demonstrated, offering a comparatively low sweep rate of 400 Hz over 60 cm-1 with 2 cm-1 linewidth, but which provides evidence that sweep rates of over a 100 kHz should be readily achievable simply by speeding up the cavity tuning element. Translating the knowledge and experience gained in near-IR OCT into mid-IR source development may result in sources offering significant benefits in certain spectroscopic applications. © 2015 SPIE

A unidirectional quantum cascade ring laser

We report on the design, fabrication, and characterization of a unidirectional quantum cascade ring laser operating at a wavelength of around 3.4 μm at 200 K. A unidirectional operation is achieved by incorporating an “S-shaped” crossover waveguide in a manner that it couples light from the counterclockwise direction to the preferred clockwise direction. The ring laser unidirectionality is confirmed by measuring the counterpropagating wave suppression ratio (CWSR) as a function of injection current. At 1.5 times the threshold current, the CWSR is 9 that is 90% of the light is emitted in the favored (clockwise) direction

λ ∼ 3.35μ m distributed-feedback quantum-cascade lasers with high-aspect-ratio lateral grating

We report the development of room-temperature distributed-feedback quantum-cascade lasers operating in a single mode in the 3.34 to 3.35 μm wavelength range. First-order lateral gratings with high aspect ratio (the ratio between the grating etch depth and its period) were formed using inductively coupled plasma etching. The as-cleaved lasers emit in pulsed regime with a sidemode suppression ratio of up to 24 dB and a peak single-mode output power of 130 mW from a single facet

Pulsed operation of long-wavelength (≃11.3 [micro sign]m) MOVPE-grown quantum cascade lasers up to 350 K

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InGaAs-AlAsSb-InP quantum cascade lasers : performance and prospects

We report the first demonstration of InGaAs/AlAsSb/InP quantum cascade lasers. Laser characteristics and structural investigations demonstrate the significant potential of this materials system for extending the short wavelength operating limit of quantum cascade lasers