A swept-source optical coherence tomography (OCT) system is demonstrated in the mid-infrared region. A Michelson interferometric setup is illuminated by an external cavity quantum cascade laser (QCL), with a scanning frequency of 1 Hz. A-scans were collected using three different samples: a mirror, CaF2 coated with germanium on both of its surfaces, and CaF2 coated with germanium on the back side of the sample. These depth-profiles were used to mimic a tissue sample with multiple reflective boundaries. Fourier transformation of these interference fringes clearly showed the expected depths of reflection, allowing for the signal to noise ratio of the system to be determined.
Infrared spectroscopy is a highly attractive read-out technology for compositional analysis of biomedical specimens because of its unique combination of high molecular sensitivity without the need for exogenous labels. Traditional techniques such as FTIR and Raman have suffered from comparatively low speed and sensitivity however recent innovations are challenging this situation. Direct mid-IR spectroscopy is being speeded up by innovations such as MEMS-based FTIR instruments with very high mirror speeds and supercontinuum sources producing very high sample irradiation levels. Here we explore another possible method – external cavity quantum cascade lasers (EC-QCL’s) with high cavity tuning speeds (mid-IR swept lasers).
Swept lasers have been heavily developed in the near-infrared where they are used for non-destructive low-coherence imaging (OCT). We adapt these concepts in two ways. Firstly by combining mid-IR quantum cascade gain chips with external cavity designs adapted from OCT we achieve spectral acquisition rates approaching 1 kHz and demonstrate potential to reach 100 kHz. Secondly we show that mid-IR swept lasers share a fundamental sensitivity advantage with near-IR OCT swept lasers. This makes them potentially able to achieve the same spectral SNR as an FTIR instrument in a time x N shorter (N being the number of spectral points) under otherwise matched conditions. This effect is demonstrated using measurements of a PDMS sample.
The combination of potentially very high spectral acquisition rates, fundamental SNR advantage and the use of low-cost detector systems could make mid-IR swept lasers a powerful technology for high-throughput biomedical spectroscopy.
FTIR spectroscopy using a thermal light source has been the dominant method for obtaining infrared spectra since the 1950’s. Unfortunately the limited surface brightness and low spatial coherence of black-body radiators limits the spectral SNR in microspectroscopy and stand-off detection. Two recent innovations are addressing this problem a) FTIR instruments illuminated by high-spatial coherence broad-band supercontinuum sources and b) high spatial coherence narrow-band EC-QCL’s.
Here we ask whether these two approaches offer equivalent sensitivity. By noting an analogy with near-infrared optical coherence tomography we rigorously show that the high temporal coherence of the EC-QCL brings an additional, very large SNR advantage over an FTIR instrument illuminated by a supercontinuum source under otherwise matched conditions. Specifically if a spectrum containing N points is recorded by both instruments using the same illumination intensity and the same detector noise level, then the EC-QCL can deliver a given spectral SNR in a time xN shorter than the FTIR instrument. This factor can reach x100, potentially even x1000, in realistic applications.
We exploit the analogy with OCT further by developing a mid-infrared “swept laser”, using commercially available components, in which the tuning rate is much higher than in commercial EC-QCL devices. We use this swept laser to demonstrate the SNR advantage experimentally, using a custom-made EC-QCL spectrometer and PDMS polymer samples. We explore the potential upper limits on spectral acquisition rates, both from the fundamental kinetics of gain build-up in the external cavity and from likely mechanical limits on cavity tuning rates.
Near-infrared external cavity lasers with high tuning rates (“swept lasers”) have come to dominate the field of nearinfrared
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 ×<i>N</i> (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 ×<i>N</i> (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<sup>-1</sup> with 2 cm-<sup>1</sup> 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.
We review the development of high performance, short wavelength (3 μm < λ < 3.8 μm) quantum cascade lasers (QCLs)
based on the deep quantum well InGaAs/AlAsSb/InP materials system. Use of this system has enabled us to demonstrate
room temperature operation at λ ~ 3.1 μm, the shortest room temperature lasing wavelength yet observed for InP-based
QCLs. We demonstrate that significant performance improvements can be made by using strain compensated material
with selective incorporation of AlAs barriers in the QCL active region. This approach provides reduction in threshold
current density and increases the maximum optical power. In such devices, room-temperature peak output powers of up
to 20 W can be achieved at λ ~ 3.6 μm, with high peak powers of around 4 W still achievable as wavelength decreases to
We report the first realization of short wavelength (λ ~ 3.05 - 3.6 μm) lattice matched In<sub>0.53</sub>Ga<sub>0.47</sub>As/AlAs<sub>0.56</sub>Sb<sub>0.44</sub>/InP
quantum cascade lasers (QCLs). The highest-performance device (λ ~ 3.6μm) displays pulsed laser action for
temperatures up to 300 K. The shortest wavelength QCL (λ ≈ 3.05 μm) operates in pulsed mode at temperatures only up
to 110 K. The first feasibility study of the strain compensated InGaAs/AlAsSb/InP QCLs (λ ~ 4.1 μm) proves that the
lasers with increased indium fractions in the InGaAs quantum wells of 60 and 70% display no degradation compared
with the lattice matched devices having identical design. This strain compensated system, being of particular interest for
QCLs at λ <~ 3.5μm, provides increased energy separation between the Γ and X conduction band minima in the quantum
wells, thus decreasing possible carrier leakage from the upper laser levels by intervalley scattering. We also demonstrate
that the performance of strain compensated InGaAs/AlAsSb QCLs can be improved if AlAsSb barriers in the QCL
active region are replaced by AlAs layers. The introduction of AlAs is intended to help suppress compositional
fluctuations due to inter diffusion at the quantum well/barrier interfaces.
We report on the experimental study of the structural, electronic and thermal properties of state-of-art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 µm. This information has been obtained by investigating the active region band-to-band photoluminescence signals, detected by means of an GaInAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in QCLs based on Sb-ternary barriers may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and the electron-lattice coupling in Sb-based QCLs based on a quaternary-alloy. We extracted the thermal resistance (RL = 8.9 K/W) and the electrical power dependence of the electronic temperature (Re = 11.7 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx structures operating at 4.9 µm, in the lattice temperature range 50 K - 80 K. The corresponding electron-lattice coupling constant ( = 10.8 Kcm2/kA) reflects the reduction of the electron-leakage channels associated with the use of a high conduction band-offset.
We report on the experimental study of the electronic and thermal properties in state of art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 &mgr;m. This information has been obtained by investigating the band-to-band photoluminescence signals, detected by means of an InGaAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in these structures may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and of the electron-lattice coupling in Sb-based QCLs based on a
quaternary-alloy. We extracted the thermal resistance (R<sub>L</sub> = 9.6 K/W) and the electrical power dependence of the
electronic temperature (R<sub>e</sub> = 12.5 K/W) of Ga<sub>0.47</sub>In<sub>0.53</sub>As/Al<sub>0.62</sub>Ga<sub>0.38</sub>As<sub>1-x</sub>Sb<sub>x</sub> structures operating at 4.9 &mgr;m, in the lattice temperature range 60 K - 90 K. The corresponding electron-lattice coupling &agr;= 9.5 Kcm<sup>2</sup>/kA) reflects the efficient electronic cooling via optical phonon emission. The experimental normalized thermal resistance R<sub>L</sub><sup>*</sup> = 3.9 Kxcm/W
demonstrates the beneficial use of quaternary thicker barriers in terms of device thermal management.
We report the realisation of spectroscopic broadband transmission experiments on quantum cascade lasers (QCLs)
under continuous wave operating conditions for drive currents up to laser threshold. This technique allows, for the first
time, spectroscopic study of light transmission through the waveguide of QCLs in a very broad spectral range (λ~1.5-12
μm), limited only by the detector response and by interband absorption in the materials used in the QCL cladding
regions. Waveguide transmittance spectra have been studied for both TE and TM polarization, for InGaAs/InAlAs/InP
QCLs with different active region designs emitting at 7.4 and 10μm. The transmission measurements clearly show the
depopulation of the lower laser levels as bias is increased, the onset and growth of optical amplification at the energy
corresponding to the laser transitions as current is increased towards threshold, and the thermal filling of the second
laser level and decrease of material gain at high temperatures. This technique also allows direct determination of key
parameters such as the exact temperature of the laser core region under operating conditions, as well as the modal gain
and waveguide loss coefficients.
The quantum cascade laser is a semiconductor light source based on resonant tunnelling and optical transitions between quantised conduction band states. In these devices the principles of operation are not based on the physical properties of the constituent materials, but arise from the layer sequence forming the heterostructure. The quantum design and the control of the layer thickness, down to an atomic mono-layer, allows one to ascribe into a semiconductor crystal, artificial potentials with the desired electronic energy levels and wavefunctions. In recent years the performance of quantum cascade lasers has improved markedly and this semiconductor technology is now an attractive choice for the fabrication of mid-far infrared lasers in a very wide spectral range (3.5-160 μm). At present, the best performances are reached at wavelength between 5-10 μm, but recent results on new material systems with deeper quantum wells are indicating that this technology will be soon available also in the 3-5 μm spectral region.
We report MOVPE-grown quantum cascade lasers with operating wavelengths between λ~7.5-9.5μm with threshold current densities as low as 2.4kA/cm2 at room temperature. Seven wafers grown for operation at ~9μm show a variation of just 3% in the superlattice periods obtained from X-ray analysis, and laser emission is observed from all wafers with a ~5meV spread of emission energies. Multimode Fabry-Perot and singlemode distributed feedback lasers have been fabricated, operating at λ~7.8μm at room temperature, corresponding with absorption lines in the infrared spectra of methane. In addition, we have produced a strain compensated MOVPE-grown quantum cascade laser operating at λ~4.5μm.