Continuous-wave Distributed-feedback Laser Research Articles

Ppb-level methane detection sensitivity based on a homemade Raman fiber amplifier and differential photoacoustic technology.

A high-power near-infrared wavelength-modulated differential photoacoustic spectroscopy sensor for parts-per-billion (ppb) level methane detection is reported by using a homemade Raman fiber optical amplifier. A commercial 1653.7nm continuous wave distributed feedback laser is employed as a seed source to excite a high light power of ∼550m W, which greatly improves sensor performance. Wavelength modulation spectroscopy and differential techniques are applied to further improve the signal-to-noise ratio of the photoacoustic signal. A 1σ minimum detection limit of ∼10p p b for methane detection is achieved with an integration time of 10s.

Pressure-dependent sensitivity of a single-pass methane detection system using a continuous-wave distributed feedback laser at 3270 nm.

A single-pass, compact, and low-power-consumption methane sensing system is presented, and its sensitivity is examined for a set of reference cells with pressures of 7.4, 74, and 740Torr and the same methane concentration at laboratory temperature of 23°C. A GaSb-based continuous-wave distributed feedback tunable diode laser at 3270nm and wavelength modulation spectroscopy were employed to collect 2f spectra in the ν3, R3 band of CH412. The collected 2f spectra were fitted to a modulated Voigt line profile model. An Allan-Werle variance analysis shows that the best detectivity, 4ppbm, is obtained for the highest pressure cell. At pressures of 74 and 7.4Torr the detectivities are 16 and 28ppbm, respectively. For these low-pressure cells, further sensitivity improvements were achieved using a large modulation depth (m>2.2) at the expense of spectral resolution.

Fiber-integrated frequency-doubling of a picosecond Raman laser to 560 nm.

We report the development of a fiber-integrated picosecond source at 560 nm by second harmonic generation of a Raman fiber laser. A picosecond ytterbium master oscillator power fiber amplifier is used to pulse-pump a Raman amplifier, which is seeded by a continuous wave distributed feedback laser diode operating at 1120 nm. The pulse train generated at 1120 nm is frequency-doubled in a fiber-coupled periodically-poled lithium niobate crystal module, producing 450 mW of average power at 560 nm with a pulse duration of 150 ps at a repetition rate of 47.5 MHz. The near diffraction-limited (M(2) = 1.02) collimated output beam is ideal for super-resolution microscopy applications.

Measurements of Extinction by Aerosol Particles in the Near-Infrared Using Continuous Wave Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy using a fiber-coupled continuous wave distributed feedback laser at a wavelength of 1520 nm has been used to measure extinction of light by samples of nearly monodisperse aerosol particles <1 μm in diameter. A model is tested for the analysis of the sample extinction that is based on the Poisson statistics of the number of particles within the intracavity laser beam: variances of measured extinction are used to derive values of the scattering cross section for size-selected aerosol particles, without need for knowledge of the particle number density or sample length. Experimental parameters that influence the performance of the CRD system and the application and limitations of the statistical model are examined in detail. Determinations are reported of the scattering cross sections for polystyrene spheres (PSSs), sodium chloride, and ammonium sulfate, and, for particles greater than 500 nm in diameter, are shown to be in agreement with the corresponding values calculated using Mie theory or Discrete Dipole Approximation methods. For smaller particles, the experimentally derived values of the scattering cross section are larger than the theoretical predictions, and transmission of a small fraction of larger particles into the cavity is argued to be responsible for this discrepancy. The effects of cubic structure on the determination of optical extinction efficiencies of sodium chloride aerosol particles are examined. Values are reported for the real components of the refractive indices at 1520 nm of PSS, sodium chloride, and ammonium sulfate aerosol particles.

Experimental investigation of high-frequency noise and optical feedback effects using a 97 μm continuous-wave distributed-feedback quantum-cascade laser

An experimental investigation of high-frequency noise, i.e., up to 3 GHz, exhibited by a 9.7 microm quantum cascade laser, is described. Noise characteristics and measurements of a liquid-nitrogen-cooled continuous-wave distributed-feedback laser are presented. Well defined sets of narrow and intense resonance peaks have been observed in the 10-300 MHz range. Measurements of relative intensity noise have been performed. It is also shown that quantum-cascade lasers are sensitive to optical feedback. The excess noise generated by the feedback has been investigated under well defined conditions. A description of the experimental phenomenon is presented along with methods of minimizing optical feedback.