Narrow-linewidth Continuous-wave Laser Research Articles

Laser-induced fluorescence of filament-produced plasmas

Self-guided ultrafast laser filaments are a promising method for laser beam delivery and plasma generation for standoff and remote detection of elements and isotopes via filament-induced breakdown spectroscopy (FIBS). Yet, there are several challenges associated with the practical application of FIBS, including delivery of sufficient laser energy at the target for generating plasma with a copious amount of emission signals for obtaining a high signal-to-noise ratio. Here, we use laser-induced fluorescence (LIF) to boost the emission signal and reduce self-reversal in the spectral profiles. Ultrafast laser filaments were used to produce plasmas from an Al 6061 alloy target at various standoff distances from 1 to 10 m. For LIF emission enhancement, a narrow linewidth continuous-wave laser was used in resonance with a 394.40 nm Al I resonant transition, and the emission signal was monitored from the directly coupled transition at 396.15 nm. Emission signal features of Al I are significantly enhanced by resonant excitation. In addition, LIF of filament ablation plumes reduces the self-reversal features seen in the thermally excited spectral profiles. Time-resolved two-dimensional fluorescence spectroscopy was performed for evaluating the optical saturation effects, which are found to be non-negligible due to high Al atomic densities in the filament-produced plasmas.

Spectrally-resolved Rayleigh scattering to measure velocity, temperature, density, and density fluctuations in high-speed flows

A new spectroscopic setup to analyze Rayleigh-scattered light from a high-speed jet was created at the NASA Ames Research Center for simultaneous measurement of velocity, temperature, and density. The point-measurement arrangement uses a narrow-linewidth continuous-wave laser and a stabilized, air-spaced, Fabry–Perot etalon to resolve the Rayleigh–Brillouin (RB) spectrum. Light scattered from a 0.4 mm-long and 0.15 mm-diameter probe volume was collected and 10% of the light was split to measure the scattering intensity via a photo-multiplier tube, which provided a measure of the gas density and density fluctuation spectra. The rest was directly imaged through the Fabry–Perot interferometer to an EMCCD camera. A new software program was developed in the Matlab® platform to model the fringes seen in the camera image. At first, a small part of the incident laser light was analyzed to find the instrument function. When Rayleigh-scattered light was passed, a change in the fringe diameter corresponding to the Doppler shift from the air velocity and a thickening of the fringe corresponding to the thermal broadening were observed. A least-squares fit utilizing Tenti’s S6 model of the RB scattering provided velocity and temperature at the probe volume. A high-speed, clean-air jet, operated in the Mach range of 0 ≤ M ≤ 1.2 provided validation of the technique, and demonstrated applicability in shock-containing flows. The compact, transportable, efficient, and affordable setup was fairly accurate: uncertainties were < 7 m/s in velocity, < 5 K in temperature, and < 1% in density. Spectra of density fluctuations were measured over a frequency range of 50 kHz. Principle of Rayleigh scattering technique, (a) schematic representation; (b) superimposed Fabry–Perot images from the present setup, U = 323 m/s, T = 249 K.

Low Noise Frequency Combs for Higher Order QAM Formats Through Cross-Phase Modulation of Modelocked Laser Pulses

An active, harmonically modelocked fiber laser with phase unstable output pulses from its long cavity is shown transformable into a low noise frequency comb suited for phase noise sensitive communications. Cross-phase modulation (XPM) of its output pulses with a continuous wave (CW) probe laser in a highly nonlinear fiber (HNLF) produces a frequency comb with noise at its minimum for low timing jitter pulses. Experiments with 2 ps pulses at 40 GHz repetition show that spectral lines obtained from within a bandwidth of 30 lines achieve low bit error rate (BER) after their individual modulation with 96 Gb/s dual polarization 64-QAM signals, reaching below the limit of hard-decision forward error correction (FEC), and close to that of a narrow linewidth CW laser. Its suitability as a seed for even broader comb generation is demonstrated by addition of a second stage HNLF, enabling a larger bandwidth of nearly 50-comb lines to reach a BER below the FEC limit for the same signal. The benefit of XPM for improving the phase noise of a modelocked semiconductor laser frequency comb is also shown, with limitation from larger pulse timing jitter. The results highlight the capability for enabling new applications of otherwise unusable pulse sources.

Reduced phase noise in an erbium frequency comb via intensity noise suppression.

We present a coherent erbium fiber frequency comb that achieves low phase noise operation through the active suppression of amplitude fluctuations within the laser oscillator. The amplitude noise servo has a bandwidth of 550 kHz and is achieved by current actuation of the laser pump diode. This servo reduces the integrated phase noise of the carrier envelope offset frequency of the comb, fceo, due to the strong coupling of amplitude and phase noise in the laser oscillator. Additionally, we use a composite error signal that utilizes information from both the amplitude noise and the fceo error signal to actuate the pump diode current, which further increases the coherence of the comb. With this locking scheme, the integrated phase noise on fceo is measured to be 270 mrad from 10 Hz to 1.5 MHz, indicating 93% of the optical carrier power is in the coherent signal. A simultaneous phase lock to a narrow-linewidth continuous-wave laser is achieved by actuating on the cavity length, and shows an integrated phase noise of 44 mrad.

Tunable, stable source of femtosecond pulses near 2 μm via supercontinuum of an Erbium mode-locked laser.

A source of ultrashort pulses of light in the 2 μm region was constructed using supercontinuum broadening from an erbium mode-locked laser. The output spectrum spanned 1000 nm to 2200 nm with an average power of 250 mW. A pulse width of 39 fs for part of the spectrum in the 2000 nm region, corresponding to less than six optical cycles, was achieved. A heterodyne measurement of the free-running mode-locked laser with a narrow-linewidth continuous wave laser resulted in a near shot noise-limited beat note with a signal-to-noise ratio of 45 dB in a 10 kHz resolution bandwidth. The relative intensity noise of the broadband system was investigated over the entire supercontinuum, and the integrated relative intensity noise of the 2000 nm portion of the spectrum was 1.7 × 10(-3). The long-term stability of the system was characterized, and intensity fluctuations in the spectrum were found to be highly correlated throughout the supercontinuum. Spectroscopic limitations due to the laser noise characteristics are discussed.

Endoscopic filtered Rayleigh scattering for the analysis of ducted gas flows

Laser-induced Rayleigh scattering makes use of the fact that elastically scattered laser light from atoms or molecules hold information on pressure, temperature and velocity inside the observed region of interest. Being several orders of magnitude weaker than other elastic light scattering effects such as Mie scattering from larger particles or laser flare from surfaces, the accuracy of Rayleigh scattering measurements strongly depends on the attenuation of these unwanted but omnipresent noise effects. The image-based technique of filtered Rayleigh scattering makes use of the absorption bands of atomic or molecular gases to remove strong elastic scattering effects from the measured signal. The concept is extended by frequency tuning a narrow line-width continuous wave laser light source along the absorption filter’s transmission profile. In acquiring images at several known frequencies, this results in spectra for each camera pixel from which time-averaged temperature, pressure and velocity fields can be deduced simultaneously. In order to qualify the frequency scanning filtered Rayleigh scattering technique for applications with strongly limited accessibility, it is applied to characterize the flow inside a bell-mouthed circular duct. The image data are acquired endoscopically via a fiberscope that is placed downstream from the measurement plane.

Resonant laser power build-up in ALPS—A “light shining through a wall” experiment

The ALPS Collaboration runs a “light shining through a wall” (LSW) experiment to search for photon oscillations into “weakly interacting sub-eV particles” (WISPs) inside of a superconducting HERA dipole magnet at the site of DESY. In this paper we report on the first successful integration of a large-scale optical resonant cavity to boost the available power for WISP production in this type of experiments. The key elements are a frequency tunable narrow line-width continuous wave laser acting as the primary light source and an electronic feed-back control loop to stabilize the power build-up. We describe and characterize our apparatus and demonstrate the data analysis procedures on the basis of a brief exemplary run.

Injection power and wavelength dependence of an external-seeded gain-switched Fabry–Perot laser

We experimentally investigate a gain-switched Fabry–Perot semiconductor laser externally injection seeded with a narrow linewidth continuous wave laser. We determine the dependence of pulse width, side-mode suppression ratio, and timing jitter on seeding power and seeding wavelength. As external seeding power is increased up to a certain value, we observe negligible change in pulse width, a linear increase in side-mode suppression ratio, and a linear decrease in timing jitter. Exceeding a certain value leads to substantial pulse broadening. By tuning the seeding wavelength across a chirped mode of the laser, we observe the lowest timing jitter and the largest side-mode suppression when the seed wavelength is aligned with the center of the chirped spectrum of a desired mode.