Distributed Feedback Quantum Cascade Research Articles

Methane-oxygen rotating detonation exhaust thermodynamics with variable mixing, equivalence ratio, and mass flux

Time-resolved in situ measurements of thermodynamic properties (pressure, temperature and species density) were performed in the annular exhaust of a methane-oxygen rotating detonation rocket engine (RDRE) using high-speed laser absorption spectroscopy. Bias-tee circuitry was coupled with a distributed feedback quantum cascade near 5 μm to spectrally-resolve a cluster of rovibrational transitions in the fundamental vibrational band of CO at MHz measurement rates, from which temperature and density were inferred from the line areas and pressure from line-width. The laser source was fiber-coupled for remote light delivery to the exhaust plane of the RDRE combustor. A single-ended optical interface with retro-reflection off of the annulus center body was utilized for stand-off in situ detection. The test article consisted of a 76.2 mm diameter annulus with a annular width of 0.5 cm and doublet impingement injection. Time-resolved CO absorption data was analyzed to examine the evolution of gas properties during engine test firings of 0.5–1.0 s in duration. Start-up transients were examined and intra-cycle profiles of gas properties were compared over a range of equivalence ratios and mass fluxes comprising approximately 25 test conditions. The effects of variable mixing were also examined through a staggered doublet injector configuration. The novel thermodynamic dataset was compared with thrust measurements, wave speed visualizations and first-order detonation models, revealing non-ideal behavior such as parasitic deflagration and modulating oxidizer-to-fuel ratio related to post-detonation injector recovery.

MHz laser absorption spectroscopy via diplexed RF modulation for pressure, temperature, and species in rotating detonation rocket flows

A mid-infrared laser absorption sensing method has been developed to quantify gas properties (temperature, pressure, and species density) at MHz measurement rates, with application to annular rotating detonation rocket flows. Bias-tee circuitry is integrated with distributed feedback quantum cascade and interband cascade lasers in the $$4{-}5~\mu \hbox {m}$$ range enabling diplexed radio frequency (RF) wavelength modulation on the order of several MHz while yielding sufficient scan depth to capture multiple rovibrational transitions in the fundamental vibrational bands of $${\text {CO}}$$ and $${\text {CO}}_{2}$$ . Sub-microsecond spectrally-resolved $${\text {CO}}$$ absorption lineshapes provide for inference of temperature and species from a two-line area ratio and pressure from collision line-width. $${\text {CO}}_{2}$$ column density is inferred from peak-to-valley differential absorption at the bandhead near $$4.19~\mu \hbox {m}$$ . A field demonstration on a methane-oxygen rotating detonation rocket engine was performed utilizing an in situ single-ended retro-reflection optical configuration aligned at the exhaust plane. The target gas properties are temporally-resolved at up to 3 MHz across rotating detonations with up to 20 kHz cycle frequency.

Quantum cascade laser intensity noise under external feedback conditions estimated from self‐mixing method

Measured is the relative intensity noise (RIN) of a mid-infrared Peltier-cooled distributed-feedback (DFB) quantum cascade laser (QCL) with a feedback loop, where optical feedback parameters affecting the RIN were estimated by using the self-mixing method. To that end, a reflecting mirror forming the external cavity was placed on a voice coil and was moved back and forth after the RIN measurements, by which characteristic waveforms appeared. The amount of external feedback could thereby be deduced. There was no appreciable change in the RIN of the DFB-QCL, even with an optical feedback being in the coherence collapse regime in the case of conventional DFB-laser diodes.

Single-mode, tunable distributed-feedback and multiple-wavelength quantum cascade lasers

Single-mode and tunable quantum cascade distributed feedback (QC-DFB) lasers in the wavelength range from 4.5 to 16.5 /spl mu/m are reviewed. In the case of QC lasers with dielectric waveguides, DFB; lasers are fabricated either with a top-grating approach, which is simpler to manufacture, or a buried grating with epitaxial regrowth, which generally has a higher single-mode yield as a result of a larger coupling factor. Long-wavelength QC-DFB lasers based on surface plasmon waveguides use bi-metal gratings for Bragg reflection. Single-mode emission with a side-mode suppression ratio of 30 dB and a tunability (depending on wavelength) of 0.3-1.0 nm/K heat sink temperature or of 2040 nm/A CW current are customarily achieved. These features together with the potential for high optical power, room-temperature operation, and narrow intrinsic linewidth make QC-DFB lasers prime choices as narrow-band light sources in mid-infrared trace gas sensors. As a result of their unipolar nature and the possibility to serially stack, or cascade, many active regions, QC lasers also have an intrinsic potential for multi pie-wavelength operation in a wide variety of device concepts. Multiple different optical transitions in single active regions stacked in a homogeneous cascade as well as multiple single-wavelength active regions cascaded in various schemes of heterogeneous cascades have. been demonstrated. Based upon multiple-wavelength QC lasers, multiple single-mode QC-DFB lasers have been fabricated using sectioned laser cavities with multiple gratings. Adjusting the length of each Bragg-grating section as well the mode-overlap factor by tailoring the heterogeneity of the cascade has lead to a doubly single-mode QC-DFB laser with simultaneous single-mode emission around 5.0 and 7.5 /spl mu/m and a tunability at each wavelength as expected from equivalent single-wavelength single-mode lasers. Finally, the concept of multiple-wavelength emission was extended to ultrabroad-band emission, with a QC laser that exhibited gain from 5 to 8 /spl mu/m and simultaneous laser action from 6 to 8 /spl mu/m.

Chemical sensors based on quantum cascade lasers

There is an increasing need in many chemical sensing applications ranging from industrial process control to environmental science and medical diagnostics for fast, sensitive, and selective gas detection based on laser spectroscopy. The recent availability of novel pulsed and CW quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared spectroscopic sources address this need. A number of spectroscopic techniques have been demonstrated worldwide by several groups. For example, the authors have employed QC-DFB lasers for the monitoring and quantification of several trace gases and isotopic species in ambient air at ppmv and ppbv levels by means of direct absorption, wavelength modulation, and cavity enhanced and cavity ringdown spectroscopy.

Simultaneously at two wavelengths (5.0 and 7.5 [micro sign]m) singlemode and tunable quantum cascade distributed feedback lasers

Quantum cascade distributed feedback (QC-DFB) lasers based on a heterogeneous-cascade two-wavelength active waveguide core and a multisectioned cavity featuring two different Bragg gratings are demonstrated. Optimised lasers display singlemode emission at λ∼5.0 and 7.5 µm simultaneously and a tunability on both modes equal to single-wavelength QC-DFB lasers.

Single-mode tunable quantum cascade lasers in the spectral range of the CO2 laser at /spl lambda/=9.5-10.5 μm

Widely tunable, single-mode quantum cascade distributed feedback (QC-DFB) lasers based on a complex coupling scheme and operating in the wavelength range of the CO/sub 2/ laser (/spl lambda//spl ap/9.5-10.5 /spl mu/m) are reported. Dynamic single-mode emission up to high current levels is obtained. The continuous single-mode tuning range is 150 nm, while the tuning range of the equivalent Fabry-Perot laser is /spl sim/400 nm. By homogeneously reducing all layer thicknesses by 10%, the wavelength coverage of a single QC-laser design can be extended to cover one entire regular band of the fundamental CO/sub 2/ laser spectrum.

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.

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.