James N. Baillargeon

High power mid‐infrared (λ∼5 μm) quantum cascade lasers operating above room temperature

The high power operation of mid‐infrared quantum cascade lasers at temperatures up to T=320 K is reported. Gain at high temperature is optimized by a design combining low doping, a funnel injector, and a three‐well vertical transition active region. A molecular beam epitaxy grown InP top cladding layer is also used to optimize heat dissipation. A peak pulsed optical power of 200 mW and an average power of 6 mW are obtained at 300 K and at a wavelength λ=5.2 μm. The devices also operate in continuous wave up to 140 K.

Distributed feedback quantum cascade lasers

Pulsed single mode operation of distributed feedback quantum cascade lasers is reported above room temperature at both 5.4 and 8 μm wavelengths. Peak optical powers up to 60 mW at 300 K are obtained with a tuning range of ∼60 nm from 100 to ∼320 K. The linewidth is limited by thermal drift during the pulse with a typical value of 0.3 cm−1 for a 10 ns long pulse at 300 K.

New frontiers in quantum cascade lasers and applications

Recent advances and new directions in quantum cascade (QC) lasers are discussed. Invented in 1994 following many years of research on band-structure engineered semiconductors and devices grown by molecular beam epitaxy, this fundamentally new laser has rapidly advanced to a leading position among midinfrared semiconductor lasers in terms of wavelength agility as well as power and temperature performance. Because of the cascaded structure, QC lasers have a slope efficiency proportional to the number of stages. Devices with 100 stages having a record peak power of 0.6 W at room temperature are reported. QC lasers in the AlInAs-GaInAs lattice matched to InP material system can now be designed to emit in the whole midinfrared range from 4 to 20 /spl mu/m by appropriately choosing the thickness of the quantum wells in the active region. Using strained AlInAs-GaInAs, wavelengths as short as 3.4 /spl mu/m have been produced. New results on QC lasers emitting at 19 /spl mu/m, the longest ever realized in a III-V semiconductor laser, are reported. These devices use innovative plasmon waveguides to greatly enhance the mode confinement factor, thereby reducing the thickness of the epitaxial material. By use of a distributed feedback (DFB) geometry, QC lasers show single-mode emission with a 30-dB side-mode suppression ratio. Broad continuous single-mode tuning by either temperature or current has been demonstrated in these DFB QC lasers at wavelengths in two atmospheric windows (3-5 and 8-13 /spl mu/m), with continuous-wave linewidths <1 MHz when free running and /spl sim/10 KHz with suitable locking to the side of a molecular transition. These devices have been used in a number of chemical sensing and spectroscopic applications, demonstrating the capability of detecting parts per billion in volume of several trace gases. Sophisticated band-structure engineering has allowed the design and demonstration of bidirectional lasers. These devices emit different wavelengths for opposite bias polarities. The last section of the paper deals with the high-speed operation of QC lasers. Gain switching with pulse widths /spl sim/50 ps and active modelocking with a few picosecond-long pulses have been demonstrated. Finally, a new type of passive modelocking has been demonstrated in QC lasers, which relies on the giant and ultrafast optical Kerr effect of intersubband transitions.

Cavity ringdown spectroscopic detection of nitric oxide with a continuous-wave quantum-cascade laser

A spectroscopic gas sensor for nitric oxide (NO) detection based on a cavity ringdown technique was designed and evaluated. A cw quantum-cascade distributed-feedback laser operating at 5.2 mum was used as a tunable single-frequency light source. Both laser-frequency tuning and abrupt interruptions of the laser radiation were performed through manipulation of the laser current. A single ringdown event sensitivity to absorption of 2.2 x 10(-8) cm(-1) was achieved. Measurements of parts per billion (ppb) NO concentrations in N(2) with a 0.7-ppb standard error for a data collection time of 8 s have been performed. Future improvements are discussed that would allow quantification of NO in human breath.

Photoacoustic spectroscopy using quantum-cascade lasers

Photoacoustic spectra of ammonia and water vapor were recorded by use of a continuous-wave quantum-cascade distributed-feedback (QC-DFB) laser at 8.5 mum with a 16-mW power output. The gases were flowed through a cell that was resonant at 1.6 kHz, and the QC-DFB source was temperature tuned over 35 nm for generation of spectra or was temperature stabilized on an absorption feature peak to permit real-time concentration measurements. A detection limit of 100 parts in 10(9) by volume ammonia at standard temperature and pressure was obtained for a 1-Hz bandwidth in a measurement time of 10 min.

Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser

A quantum-cascade laser operating at a wavelength of 8.1 micrometers was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm-1, and absorption spectra of the CH4 nu 4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) ( ppm) in 50 Torr of air was measured in a 43-cm path length with +/- 0.5-ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of 13CH4 and CH3D species in a CH4 sample was determined.

Continuous-wave and high-power pulsed operation of index-coupled distributed feedback quantum cascade laser at λ≈8.5 μm

High performance index-coupled quantum cascade distributed feedback (QC-DFB) lasers operating at λ≈8.5 μm are reported. Reliable dynamic single-mode emission with a side mode suppression ratio ⩾30 dB is obtained. The continuous single-mode tuning range is 140 nm. In pulsed operation a record high peak output power of 60 mW at 300 K is achieved. We further report on the first continuous-wave QC-DFB lasers. These devices display an output power of 10 mW at 120 K.

High-power λ≈8 μm quantum cascade lasers with near optimum performance

Quantum cascade (QC) lasers emitting at λ≈8 μm with a power performance equal to short-wavelength (λ≈5 μm) QC lasers are reported. The device improvement is mainly achieved by a design of the injector/relaxation region, which at laser threshold allows resonant carrier injection between the ground state of the preceding and the upper laser level of the subsequent active region. In pulsed operation a peak output power of 1.3 W per facet has been measured at 100 K. At room temperature a record peak power of 325 mW and a record slope efficiency of 180 mW/A have been measured. In continuous-wave operation the maximum power at 30 K was 510 mW per facet and still 200 mW per facet at 80 K. The high values of the output power and slope efficiency demonstrate the validity of the cascading scheme, in which electrons above threshold generate one photon per each active region they successively traverse.

Kilohertz linewidth from frequency-stabilized mid-infrared quantum cascade lasers

Frequency stabilization of mid-IR quantum cascade (QC) lasers to the kilohertz level has been accomplished by use of electronic servo techniques. With this active feedback, an 8.5-microm QC distributed-feedback laser is locked to the side of a rovibrational resonance of nitrous oxide (N(2) O) at 1176.61cm (-1) . A stabilized frequency-noise spectral density of 42Hz/ radicalHz has been measured at 100 kHz; the calculated laser linewidth is 12 kHz.

Cavity ringdown spectroscopy using mid-infrared quantum-cascade lasers.

Cavity ringdown spectra of ammonia at 10 parts in 10(9) by volume (ppbv) and higher concentrations were recorded by use of a 16-mW continuous-wave quantum-casacde distributed-feedback laser at 8.5 mum whose wavelength was continuously temperature tuned over 15 nm. A sensitivity (noise-equivalent absorbance) of 3.4x10(-9) cm(-1) Hz(-1/2) was achieved for ammonia in nitrogen at standard temperature and pressure, which corresponds to a detection limit of 0.25 ppbv.