Résultat de la recherche
Interferometric measurements beyond the coherence length of the laser source
Interferometric measurements beyond the coherence length of the laser are investigated theoretically and experimentally in this paper. Thanks to a high-bandwidth detection, high-speed digitizers and a fast digital signal processing, we have demonstrated that the limit of the coherence length can be overcome. Theoretically, the maximal measurable displacement is infinite provided that the sampling rate is sufficiently short to prevent any phase unwrapping error. We could verify experimentally this concept using a miniature interferometer prototype, based on a frequency stabilized vertical cavity surface emitting laser. Displacement measurements at optical path differences up to 36 m could be realized with a relative stability better than 0.1 ppm, although the coherence length estimated from the linewidth and frequency noise measurements do not exceed 6.6 m.
Frequency noise of free-running 4.6 um distributed feedback quantum cascade lasers near room temperature
The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 um range and operated in cw mode near room temperature (277K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz2/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.
Intersubband spectroscopy probing higher order interminiband transitions in AlN-GaN-based superlattices
We investigate midinfrared intersubband photodetectors based on short-period AlN/GaN superlattices with different quantum well thicknesses. Band structure calculations, as well as optical transmission and photovoltage measurements, underline the importance of higher order interminiband transitions. In particular, it was found that optical transitions between the second and third minibands benefit from much larger electron displacements and oscillator strengths than those between the first and second minibands. Our results suggest that optical rectification is therefore much more efficient for devices based on a higher order interminiband transition.
Real-time Amyloid Aggregation Monitoring with a Photonic Crystal-based Approach
We propose the application of a new label-free optical technique based on photonic nanostructures to real-time monitor the amyloid-beta 1-42 (A?(1-42)) fibrillization, including the early stages of the aggregation process, which are related to the onset of the Alzheimer's Disease (AD). The aggregation of A? peptides into amyloid fibrils has commonly been assocd. with neuronal death, which culminates in the clin. features of the incurable degenerative AD. Recent studies revealed that cell toxicity is detd. by the formation of sol. oligomeric forms of A? peptides in the early stages of aggregation. At this phase, classical amyloid detection techniques lack in sensitivity. Upon a chem. passivation of the sensing surface by means of polyethylene glycol, the proposed approach allows an accurate, real-time monitoring of the refractive index variation of the soln., wherein A?(1-42) peptides are aggregating. This measurement is directly related to the aggregation state of the peptide throughout oligomerization and subsequent fibrillization. Our findings open new perspectives in the understanding of the dynamics of amyloid formation, and validate this approach as a new and powerful method to screen aggregation at early stages. [on SciFinder(R)]
Si-Interdiffusion in heavily doped AIN-GaN-based quantum well intersubband photodetectors
We demonstrate the effect of rapid thermal annealing on heavily Si-doped AlN/GaN quantum wells. After 1000 °C annealing during 5, 10, and 20 min, the dominant effect was interdiffusion of Si rather than intermixing between the Al and Ga atoms. Both their original value and the magnitude of the changes after annealing reveal that intersubband absorption and photovoltage are related to two different optical transitions as follows: absorption occurs in the 1 to 2 intersubband transition, whereas photovoltage is due to a subsequent process from the 1 to 2 and the manifold of 2 to higher order transitions. © 2011 American Institute of Physics.
Performance Improvement of AIN-GaN-Based Intersubband Detectors by Using Qantum Dots
Linewidth of a quantum cascade laser assessed from its frequency noise spectrum and impact of the current driver
We report on the measurement of the frequency noise properties of a 4.6-μm distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6 cm^−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below ≈1 nA/√Hz allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of ≈1.6 MHz/(nA/√Hz) for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several nA/√Hz , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only 350 pA/√Hz current noise, which is suitable to observe the ≈550 kHz linewidth of our QCL.
CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade detector and a Fourier spectrometer
Abstract: We report a prototype CO2 gas sensor based on a simple blackbody infrared source and a spectrally narrow quantum cascade detector (QCD). The detector absorption spectrum is centered at 2260 cm(-1) (4.4 um) and has a full width at half maximum of 200 cm(-1)(25 meV). It covers strong absorption bands of two spectrally overlapping CO2 isotopomers, namely the P-branch of 12CO2 and the R-branch of 13CO2. Acquisition of the spectral information and data treatment were performed in a Fourier transform infrared (FTIR) spectrometer. By flushing its sample compartment either with nitrogen, dry fresh air, ambient air, or human breath, we were able to determine CO2 concentrations corresponding to the different gas mixtures. A detection limit of 500 ppb was obtained in these experiments.
Interferometric measurements beyond the coherence length of the laser source
Interferometric measurements beyond the coherence length of the laser are investigated theoretically and experimentally in this paper. Thanks to a high-bandwidth detection, high-speed digitizers and a fast digital signal processing, we have demonstrated that the limit of the coherence length can be overcome. Theoretically, the maximal measurable displacement is infinite provided that the sampling rate is sufficiently short to prevent any phase unwrapping error. We could verify experimentally this concept using a miniature interferometer prototype, based on a frequency stabilized vertical cavity surface emitting laser. Displacement measurements at optical path differences up to 36 m could be realized with a relative stability better than 0.1 ppm, although the coherence length estimated from the linewidth and frequency noise measurements do not exceed 6.6 m.