Rainer Martini

Quantum cascade lasers: ultrahigh-speed operation, optical wireless communication, narrow linewidth, and far-infrared emission

Following an introduction to the history of the invention of the quantum cascade (QC) laser and of the band-structure engineering advances that have led to laser action over most of the mid-infrared (IR) and part of the far-IR spectrum, the paper provides a comprehensive review of recent developments that will likely enable important advances in areas such as optical communications, ultrahigh resolution spectroscopy and applications to ultrahigh sensitivity gas-sensing systems. We discuss the experimental observation of the remarkably different frequency response of QC lasers compared to diode lasers, i.e., the absence of relaxation oscillations, their high-speed digital modulation, and results on mid-IR optical wireless communication links, which demonstrate the possibility of reliably transmitting complex multimedia data streams. Ultrashort pulse generation by gain switching and active and passive modelocking is subsequently discussed. Recent data on the linewidth of free-running QC lasers (/spl sim/150 kHz) and their frequency stabilization down to 10 kHz are presented. Experiments on the relative frequency stability (/spl sim/5 Hz) of two QC lasers locked to optical cavities are discussed. Finally, developments in metallic waveguides with surface plasmon modes, which have enabled extension of the operating wavelength to the far IR are reported.

High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers

Quantum cascade (QC) lasers, based on intersubband transitions in semiconductor quantum wells, are characterized by ultrafast (picosecond) carrier lifetimes. An important consequence of this unique property is the expected absence of relaxation oscillations in the transient response of these devices. Here, we discuss and experimentally verify this prediction by measuring the modulation response of several 8-μm-QC lasers, properly processed and packaged for high-speed operation, up to 10 GHz.

Modulation of pro- and anti-inflammatory cytokine production in very preterm infants.

BACKGROUND In premature infants, outcome of infection-associated complications is heterogeneous despite advances in antibiotic treatment and diagnosis. Information on the immune response in preterm infants is limited. Immune modulatory strategies require detailed analysis of mediators and their kinetics. OBJECTIVE To determine the kinetics of IL-1beta, TNFalpha, IL-6, IL-8, IL-10, gammaINF and G-CSF in preterm and term infants in an ex vivo cord blood culture (CBC) endotoxin model. DESIGN AND METHODS Cord blood of 25 infants was obtained immediately after birth from the fetal side of the placenta and incubated in culture medium (RPMI 1640) in the presence or absence of 500 pg/ml lipopolysaccharide (LPS) for 48h. TNFalpha, IL-1beta, IL-6 and IL-8 were measured by sequential immunometric assay (IMMULITE, DPC Biermann, Germany); IL-10 (Milenia Biotec, Bad Nauheim, Germany), gammaINF (Diaclone, Besancon, France) and G-CSF (R & D Systems, Wiesbaden, Germany) were determined by ELISA in supernatants at 0, 4, 8, 12, 24 and 48h. Infants were stratified into three gestational age groups (< or =32 weeks, 33-36 weeks, > or =37 weeks). Variations between the groups were first analyzed for significance by Kruskal-Wallis test and pairs were compared by Mann-Whitney-U test. Effects of gestational age, leucocyte count, hematocrit and frequency of antenatal steroid exposure were tested by linear regression analysis. To correct a possible impact of variable, WBC count, cytokine levels were adjusted according to individual leucocyte numbers. RESULTS LPS-stimulated maximum levels of IL-6, IL-1beta,TNFalpha and G-CSF in CBC were significantly lower in very preterm infants compared to more advanced gestational age groups. After adjusting the cytokine levels for 10(5) leucocytes, a significant effect of gestational age on IL-6 and G-CSF production (p<0.05) was detected. A non-significant trend towards reduced cytokine levels was observed following multiple antenatal steroid exposures. IL-10:TNFalpha ratio increased in very preterm neonates when compared with the advanced gestational age, although the increase was not significant. CONCLUSIONS Pro-inflammatory cytokine activity in CBC correlates with gestational age, whereas IL-10 does not. Although ex vivo synthesis of IL-1beta, TNFalpha, IL-6, G-CSF in CBC depends in part on leucocyte numbers, IL-6 and G-CSF synthesis appeared to be related to immaturity. Non-significant effects of multiple antenatal steroid exposure and increased IL-10:TNFalpha ratio in preterm neonates, observed in a small sample size, warrant further investigation.

Quantum cascade lasers and the Kruse model in free space optical communication.

Mid-infrared (MIR) free space optical communication has seen renewed interest in recent years due to advances in quantum cascade lasers. We present data from a multi-wavelength test-bed operated in the New York metropolitan area under realistic weather conditions. We show that a mid-infrared source (8.1 microm) provides enhanced link stability with 2x to 3x greater transmission over near infrared wavelengths (1.3 microm & 1.5 microm) during fog formation and up to 10x after a short scavenging rain event where fog developed and visibility reduced to approximately 1 km. We attribute the improvement to less Mie scattering at longer wavelengths. We confirm that this result is generally consistent with the empirical benchmark Kruse model at visibilities above 2.5 km, but towards the 1 km eye-seeing limit we measured the equivalent MIR visibility to be > 10 km.

Forward-propagating surface-enhanced Raman scattering and intensity distribution in photonic crystal fiber with immobilized Ag nanoparticles

A 30 cm long solid-core photonic crystal fiber (PCF) with immobilized and discrete Ag nanoparticles was used to obtain forward-propagating surface-enhanced Raman scattering (SERS) of 2 microM Rhodamine 6G (R6G) aqueous solution filled in the cladding air channels. The intensity distributions of characteristic Raman vibrational bands of silica and R6G in PCF were mapped for the first time to our knowledge by hyperspectral Raman imaging. We show that the measured SERS intensity arises exclusively from the forward-propagating core mode as a result of evanescent-field interaction with R6G in the innermost ring of the cladding air channels.

Optically induced fast wavelength modulation in a quantum cascade laser

An optically induced fast wavelength shift is demonstrated in a standard middle infrared (MIR) quantum cascade laser (QCL) by illuminating the front facet with a femtosecond (fs) near infrared (NIR) laser, allowing fast optical frequency modulation (FM) for free space optical communication (FSOC) and FM spectroscopy. Using an etalon as a narrow band-pass wavelength filter, the wavelength modulation (WM) was clearly observed at frequencies up to 1.67 GHz. This approach can also be used for wavelength conversion and might be extended to QCLs operating in different wavelength regions.

High-speed all-optical modulation of a standard quantum cascade laser by front facet illumination

A simple experimental scheme to control intersubband lasing transition by optically induced nonresonant interband transition is presented, allowing for high-speed all-optical modulation of mid-infrared (MIR) quantum cascade laser (QCL). Illuminating the QCL front facet with 100 fs Ti:sapphire laser pulse, a fast modulation of a cw-MIR emission is observed with an estimated transient time of less than 81 ps, limited by instrument bandwidth, and a modulation depth of 18%. It has a great potential for high-speed modulation of high power QCL at room temperature.

Comparison of subspace and ARX models of a waveguide's terahertz transient response after optimal wavelet filtering

A quasi-optical deembedding technique for characterizing waveguides is demonstrated using wide-band time-resolved terahertz spectroscopy. A transfer function representation is adopted for the description of the signal in the input and output port of the waveguides. The time-domain responses were discretized and the waveguide transfer function was obtained through a parametric approach in the z-domain after describing the system with an AutoRegressive with eXogenous input (ARX), as well as with a state-space model. Prior to the identification procedure, filtering was performed in the wavelet domain to minimize both signal distortion, as well as the noise propagating in the ARX and subspace models. The optimal filtering procedure used in the wavelet domain for the recorded time-domain signatures is described in detail. The effect of filtering prior to the identification procedures is elucidated with the aid of pole-zero diagrams. Models derived from measurements of terahertz transients in a precision WR-8 waveguide adjustable short are presented.

Measurement of propagation constant in waveguides with wideband coherent terahertz spectroscopy

A quasi-optical technique for characterizing micromachined waveguides is demonstrated with wideband time-resolved terahertz spectroscopy. A transfer-function representation is adopted for the description of the relation between the signals in the input and output port of the waveguides. The time-domain responses were discretized, and the waveguide transfer function was obtained through a parametric approach in the z domain after describing the system with an autoregressive with exogenous input model. The a priori assumption of the number of modes propagating in the structure was inferred from comparisons of the theoretical with the measured characteristic impedance as well as with parsimony arguments. Measurements for a precision WR-8 waveguide-adjustable short as well as for G-band reduced-height micromachined waveguides are presented.

High duty cycle operation of quantum cascade lasers based on graded superlattice active regions

High duty cycle operation of quantum cascade superlattice lasers with graded superlattice active regions is investigated with the goal of achieving high average optical power. The optical output power increases with pulse width and decreases with heat sink temperature. This behavior is explained on the basis of the laser core temperature oscillations during the pulsed, high duty cycle operation. Between 175 and 325 K heat sink temperature, optimum duty cycles vary from 10% to 1% and average power levels vary from 50 to 1 mW for various lasers used in this study.