Cascade Laser Operating Research Articles

Optimization of quantum cascade laser operation by geometric design of cascade active band in open and closed models

Using the effective mass and rectangular potential approximations, the theory of electron dynamic conductivity is developed for the plane multilayer resonance tunnel structure placed into a constant electric field within the model of open nanosystem, and oscillator forces of quantum transitions within the model of closed nanosystem. For the experimentally produced quantum cascade laser with four-barrier active band of separate cascade, it is proven that just the theory of dynamic conductivity in the model of open cascade most adequately describes the radiation of high frequency electromagnetic field while the electrons transport through the resonance tunnel structure driven by a constant electric field.

On the active conductivity of a three-barrier resonant-tunneling structure and optimization of quantum cascade laser operation

Within the context of the model of effective masses and rectangular potentials, the theory of active electronic conductivity is proposed for a three-barrier resonant-tunneling structure in a dc electric field, as the active element of a quantum cascade laser. It is shown that the chosen geometrical parameters of the active regions in the first experimentally fabricated quantum cascade lasers are most optimal and the experimental and theoretical values of the radiation energies correlate with each other with an accuracy of up to a few percent.

A CO 2 sensor based upon a continuous-wave thermoelectrically-cooled quantum cascade laser

A CO 2 sensor based upon a continuous-wave thermoelectrically-cooled distributed feedback quantum cascade laser operating between 2305 and 2310 cm −1 and a 54.2 cm long optical cell has been developed. Two approaches for direct absorption spectroscopy have been evaluated and applied for monitoring of the CO 2 concentration in gas lines and ambient laboratory air. In the first approach optical transmittance was derived from the single channel laser intensity, whilst in the second approach a ratio of signal and reference laser intensities (balanced detection) was used. The optimum residual absorption standard deviation was estimated to be 1.9 × 10 −4 for 100 averages of 1 ms duration and 0.1 cm −1 scans over the P(46) CO 2 absorption line of the ν 3 vibrational band at 2306.926 cm −1. A CO 2 detection limit (1 standard deviation) of 36 ppb was estimated for 0.1 s average and balanced detection.

Extremely sensitive detection of NO_2 employing off-axis integrated cavity output spectroscopy coupled with multiple-line integrated absorption spectroscopy

We report on the development of a new sensor for NO₂ with ultrahigh sensitivity of detection. This has been accomplished by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several kilometers in a small volume cell) with multiple-line integrated absorption spectroscopy (MLIAS) (where we integrate the absorption spectra over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity quantum cascade laser operating in the 1601-1670 cm⁻¹ range and a high-finesse optical cavity, the absorption spectra of NO₂ over 100 transitions in the R band have been recorded. From the observed linear relationship between the integrated absorption versus concentration of NO₂ and the standard deviation of the integrated absorption signal, we report an effective sensitivity of detection of approximately 28 ppt (parts in 10¹²) for NO₂ To the best of our knowledge, this is among the most sensitive levels of detection of NO₂ to date.

Coherent sampling of active mode-locked terahertz quantum cascade lasers and frequency synthesis

Researchers demonstrate a terahertz quantum cascade laser operating in a regime of active mode-locking by modulating its bias current with a radiofrequency synthesizer. This technique allows coherent sampling of the terahertz electric field as well as control over the laser's carrier–envelope phase shift.

Diode Pumped Erbium Cascade Fiber Lasers

Cascading the 4I11/2→4I13/2 transition at 2.8 μm and 4I13/2→4I15/2 transition at 1.6 μm offers a solution to the thermal management of high power Er3+-doped fluoride fiber lasers. We demonstrate an output power of 8.2 W at 2.8 μm from an Er3+-doped fluorozirconate fiber laser with 56 W of launched pump power at 975 nm by cascade laser operation with the 1.6-μm ground-state transition. By careful selection of the Er3+ concentration that prevents significant interaction between Er3+ ions, it is shown that cascade lasing can occur from simple resonator arrangements including Fresnel reflection from the ends of the fiber only. The unsaturated output power suggests that no competing transitions are oscillating which advocates that radiative quenching of the metastable 4I13/2 level provides an elegant way of reducing undesired heat generation. We show that core temperature increases at the tip of the pumped end of the fiber can be an order of magnitude lower than fiber lasers employing single transition oscillation. The potential of further power scaling is demonstrated and the experimental results are verified with the use of a numerical model.

Modelling of optical Kerr effects on the static and dynamic behaviors of quantum cascade laser

The effect of optical Kerr nonlinearity on the static and dynamic behaviors of quantum cascade laser operating in the mid-infrared is theoretically investigated. Our model is based on three-level rate equations including the dependence of the loss on photon number in the cavity. The optical stability domain that allows for the determination of current injection is derived within the premises of our model. The analytical solutions of eigenvalue equation allowing the investigation of stability analysis are obtained. Furthermore, nonlinear effects influence significantly the dynamics of photons in cavity and electrons in the upper laser level.

Composite Nano-Antenna Integrated With Quantum Cascade Laser

Exploiting optical nano-antennas to boost the near-field confinement within a small volume can increase the limit of molecular detection by an order of magnitude. We present a novel antenna design based on Au-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -Au single nanorod integrated on the facet of a quantum cascade laser operating in the midinfrared region of the optical spectrum. Finite-difference time-domain simulations showed that for sandwiched dielectric thicknesses within the range of 20-30 nm, peak optical intensity at the top of the antenna ends is 500 times greater than the incident field intensity. The device was fabricated using focused ion beam milling. Apertureless midinfrared near-field scanning optical microscopy showed that the device can generate a spatially confined spot within a nanometric size about 12 times smaller than the operating wavelength. Such high intensity, hot spot locations can be used in increasing photon interaction with bio-molecules for sensing applications.

Loss-reduction in midinfrared photonic crystal quantum cascade lasers using metallic waveguides

We describe a mechanism for plasmonic loss reduction in midinfrared metallic photonic crystals and apply it to surface-plasmon quantum cascade lasers. We obtain pulsed, room-temperature operation of surface-emitting photonic crystal quantum cascade lasers operating at λ≈7.4 μm. The photonic crystal resonator is patterned in the de- vice top metallization, and laser operation is obtained on a band-edge mode of the photonic band structure. The emission is spectrally single mode, with a side-mode suppression ratio of 20 dB, and on-chip tun- ability is obtained over a wavelength range of ≈0.52 μm. Simulations based on a finite elements approach and on the finite-difference time- domain method allow us to study the photonic-band structure, the elec- tromagnetic field distributions, and especially, the influence of the device parameters on the losses. The comparison between the measured and simulated far-field emission patterns and polarization proves the lasers operate on a monopolar-symmetry mode C 2010 Society of Photo-Optical Instru-

Rapid passage signals induced by chirped quantum cascade laser radiation: K state dependent-delay effects in the ν_2 band of NH_3

In this Letter, a 10 microm quantum cascade laser operating in the intrapulse mode is used observe rapid passage (RP) effects within a 40 cm single-pass gas cell containing low pressures of NH(3). The laser tuning range allows the rotational states J=2 with K=0, 1, and 2 to be probed. We show that the RP structures change as a function of optical density and that the magnitude of the delay in the switch from absorption to emission as a function of increased gas pressure is dependent upon the initial value of K. These measurements are qualitatively well modeled using the Maxwell-Bloch equations.

Real-time modeling of quantum cascade laser operation using linear combinations of intrawell properties

Description of multi-quantum well heterostructures based on localized properties of the constituent single well eigenstates permits invaluable insight into the relationships between structure design and device operation. Analytical descriptions of single well electronic states permits analytical evaluation of tight-binding parameters used to describe coupled well interactions. An expansion of Fermi’s Golden Rule in the single well basis is presented under the assumption that transitions between delocalized states can be written as linear combinations of intrawell mean scattering rates. This assumption of localized transitions permits the construction of predetermined sets for the limited single well scattering events. Using a simplified density matrix description of transport, where incoherent scattering and coherent tunneling is restricted to intrawell and interwell transitions, respectively, allows for an easily accessible model of transport. The combination of these computational techniques produces a simulation tool requiring near zero computational time. Application of these techniques to a four-well and a three-well resonant phonon depopulation quantum cascade laser demonstrates this method as a valuable tool for fast approximation of device operation.

Direct and wavelength modulation spectroscopy using a cw external cavity quantum cascade laser

A continuous wave external cavity quantum cascade laser (EC-QCL) operating between 1872 and 1958 cm−1 has been used to make rotationally resolved measurements in the fundamental band of nitric oxide at 140 mTorr, and the ν2 band of water at atmospheric pressure. These measurements demonstrate the advantages of wide tunability and high resolution of the EC-QCL system. From direct absorption spectroscopy on nitric oxide a laser bandwidth of 20 MHz has been deduced and a sensitivity of 8.4×10−4 cm−1 Hz−1/2 was achieved. Wavelength modulation spectroscopy using current modulation enhances the sensitivity by a factor of 23 to 3.7×10−5 cm−1 Hz−1/2.

Quantum Cascade Laser-Based Photoacoustic Sensor for Trace Detection of Formaldehyde Gas

We report on the development of a photoacoustic sensor for the detection of formaldehyde (CH2O) using a thermoelectrically cooled distributed-feedback quantum cascade laser operating in pulsed mode at 5.6 μm. A resonant photoacoustic cell, equipped with four electret microphones, is excited in its first longitudinal mode at 1,380 Hz. The absorption line at 1,778.9 cm−1 is selected for CH2O detection. A detection limit of 150 parts per billion in volume in nitrogen is achieved using a 10 seconds time constant and 4 mW laser power. Measurements in ambient air will require water vapour filters.

Widely tunable high-power external cavity quantum cascade laser operating in continuous-wave at room temperature

A grating-coupled external cavity quantum cascade laser operating in continuous-wave at room temperature is reported. Single-frequency operation tunable over more than 160 cm−1 around the centre wavelength of 4.6 µm has been observed at a chip temperature of 300 K. The maximum optical power at the gain peak was 300 mW, corresponding to a wall-plug efficiency of 6%. Observed power output at the gain bandwidth edges was in excess of 125 mW.

Development of a spectrometer using a continuous wave distributed feedback quantum cascade laser operating at room temperature for the simultaneous analysis of N_2O and CH_4 in the Earth's atmosphere

We report on the development and performance of a gas sensor based on a distributed feedback quantum cascade laser operating in continuous wave at room temperature for simultaneous measurement of nitrous oxide (N(2)O) and methane (CH(4)) concentrations at ground level. The concentrations of the gases are determined by a long path infrared diode laser absorption spectroscopy. The long-term stability of the instrument is evaluated using the Allan variance technique. A preliminary evaluation of the instrument performance is realized by in situ measurements of N(2)O and CH(4) concentrations at ground level during 1 day. The sensor has also been applied to study the time response of N(2)O concentrations to a fertilizer addition in a soil sample and for the comparison between various types of soils.

Sub-parts-per-billion level detection of NO 2 using room-temperature quantum cascade lasers

We report the sub-parts-per-billion-level detection of NO2 using tunable laser-based photoacoustic spectroscopy where the laser radiation is obtained from a room-temperature continuous-wave high-power quantum cascade laser operating in an external grating cavity configuration. The continuously tunable external grating cavity quantum cascade laser produces maximum single-frequency output of approximately 300 mW tunable over approximately 350 nm centered at 6.25 microm. We demonstrate minimum detection level of approximately 0.5 parts per billion of NO2 in the presence of humidified air.

Analysis of limitations to wallplug efficiency and output power for quantum cascade lasers

We have carried out detailed optical and thermal simulations of quantum cascade laser (QCL) operation as a function of material and device parameters. The main objective of the study is to evaluate the potential for improvements in the cw wallplug efficiencies and output powers beyond the current state of the art, when such parameters as the internal loss, series resistance, doping level in the active region, number of QCL stages, ridge width, cavity length, current density, and operating temperature are varied within reasonable ranges. For the test case of a narrow-ridge λ=4.8μm QCL mounted epitaxial side down, we project that a maximum wallplug efficiency of &amp;gt;10% and cw output power of &amp;gt;1W may be feasible at room temperature, if the net internal loss can be reduced by 30% from its best current value.

Sub-part-per-billion monitoring of nitric oxide by use of wavelength modulation spectroscopy in combination with a thermoelectrically cooled, continuous-wave quantum cascade laser

We used a thermoelectrically cooled, continuous-wave, quantum cascade laser operating between 1847 and 1854 cm(-1) in combination with wavelength modulation spectroscopy for the detection of nitric oxide (NO) at the sub-part-per-billion by volume (ppbv) level. The laser emission overlaps the P7.5 doublet of NO centered around 1850.18 cm(-1). Using an astigmatic multiple-pass absorption cell with an optical path length of 76 m, we achieved a detection limit of 0.2 ppbv at 10 kPa, with a total acquisition time of 30 s. The corresponding minimal detectable absorption is 8.8 x 10(-9) cm(-1) Hz(-1/2).

Quantum cascade laser-based carbon monoxide detection on a second time scale from human breath

We present three different detection schemes for measuring carbon monoxide (CO) in direct absorption using a thermoelectrically cooled, distributed-feedback pulsed quantum cascade (qc) laser operating between 2176 and 2183 cm-1. The laser emission has overlap with the strong R(8)1 ro-vibrational transition in CO at 2176.2835 cm-1. Firstly, by utilizing the frequency chirp of the qc-laser with long laser pulses, a minimal detectable absorption of 1.2×10-5 cm-1 is achieved at an acquisition rate of 3 Hz. Additionally, with short laser pulses and slow frequency scanning a minimal detectable absorption 8.2×10-7 cm-1 is reported, with an acquisition time of 60 s. Finally, a novel amplitude modulation technique is developed to facilitate real-time measurement of CO in exhaled air. The application of this detector to detection of CO in a single breath as a potential non-invasive diagnostic tool is shown.

2.9 THz quantum cascade lasers operating up to 70K in continuous wave

We report the operation of a quantum cascade laser emitting at a 103μm wavelength (2.9THz). The active region is based on a bound-to-continuum design allowing a low parasitic leakage current, and a high upper-to-lower-state lifetime ratio. The latter is demonstrated by a pronounced decrease of the differential resistance at threshold, which is visible up to high temperatures, and by a weak temperature dependence of the slope efficiency. At 4K, we report a threshold current density of only 105A∕cm2 both in pulsed and continuous-wave operation, and an emitted peak power of 15mW independent of the duty cycle. Maximum operating temperatures of 95K and 70K are observed in pulsed and continuous wave modes, respectively.