17 research outputs found
Continuous wave room temperature external ring cavity quantum cascade laser
An external ring cavity quantum cascade laser operating at âŒ5.2 ÎŒm 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 ”m in tissue and at least â75â
dB sensitivity have been achieved. High-quality B-scans, containing 1600 A-scans, are acquired at a rate of 39â
Hz. Images from the dorsal hand, ventral wrist and ventral forearm areas are obtained, with a clearly resolved stratum corneum layer (typically 5â15 ”m 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 ”m.
However, developing an epitaxial process of highly strain-compensated QCL structures for operation at shorter
wavelengths of ~3-5 ”m 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 ”m wide and 3
mm long devices with as-cleaved facets operate at λ â 4 ”m 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 ”m.
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 ”m. 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 ĂN (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 ĂN (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 3.4âÎŒm at 200 K. A unidirectional operation is achieved by incorporating an âS-shapedâ 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 ⌠3.35Ό 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 ÎŒm 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 (â11.3â [micro sign]m) MOVPE-grown quantum cascade lasers up to 350â K
International audienc
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