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Results for “MODE LOCKING”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Control over pulsed operation modes of an erbium-doped fibre laser passively mode-locked via nonlinear polarisation rotation

We have experimentally demonstrated the possibility of controlling pulse generation modes of an erbium-doped fibre ring laser operating in the anomalous dispersion regime and passively mode-locked via nonlinear polarisation rotation. It has been shown that individual adjustment of the pump power and intracavity polarisation controllers leads to changes in pulse generation mode, without changing the cavity. We have evaluated the key spectral, temporal and energetic characteristics of the observed pulse generation modes. (control of laser radiation parameters. active media)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

First observation of unlocking the locked mode by electrode biasing on J-TEXT tokamak

Major disruptions, often caused by locked m/n = 2/1 modes (m/n is the poloidal/toroidal mode number), are great threat to a tokamak fusion reactor and should be mitigated or avoided. The locked 2/1 modes have been unlocked by electrode biasing (EB) for the first time in J-TEXT tokamak experiments. Furthermore, the application of a sufficiently negative EB voltage in plasma edge region quickly changes the phase and amplitude of the locked 2/1 mode and drives the mode to rotate in about 10–30 ms. For a larger EB current or a smaller locked mode amplitude, the mode is more easily to be unlocked, revealing a new method for mode unlocking and avoiding plasma disruptions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1976

Mode-locked frequency doubled Nd:YAG laser

The design, fabrication, test, and delivery of two mode-locked, frequency doubled Nd:YAG laser systems are described. Each system was comprised of two units, the laser head and optics on an Invar plate and the electronics control unit in a relay rack chassis panel. Laser number one operated at a repetition rate of 400 MHz and was designed for use in an optical communication system. Laser number two operated at 200 MHz repetition rate and was designed for optical ranging and target signature experiments. Both lasers had a pulse width of 200 ps at the 10% amplitude points at 1.064 micrometer wavelength (150 ps at 0.532 micrometers) with an amplitude stability of + or - 4%. Output power exceeded the design goals.

Brookman, J. S.↗
NASA NTRS1991

High-power pulsed and CW diode-pumped mode-locked Nd:YAG lasers

The operation of both pulsed and CW diode-pumped mode-locked Nd:YAG lasers are presented. The pulsed laser produced 1.0 mJ with pulsewidths of 90 psec at 20 Hz. The CW pumped laser produced 6 W output at 1.064 microns and 3 W output at 532 nm.

Marshall, Larry R.↗
DOE OSTI2024

Probabilistic locked mode predictor in the presence of a resistive wall and finite island saturation in tokamaks

We present a framework for estimating the probability of locking to an error field in a rotating tokamak plasma. This leverages machine learning methods trained on data from a mode-locking model, including an error field, resistive magnetohydrodynamics modeling of the plasma, a resistive wall, and an external vacuum region, leading to a fifth-order ordinary differential equation (ODE) system. It is an extension of the model without a resistive wall introduced by Akçay et al. [Phys. Plasmas 28, 082106 (2021)]. Tearing mode saturation by a finite island width is also modeled. We vary three pairs of control parameters in our studies: the momentum source plus either the error field, the tearing stability index, or the island saturation term. The order parameters are the time-asymptotic values of the five ODE variables. Normalization of them reduces the system to 2D and facilitates the classification into locked (L) or unlocked (U) states, as illustrated by Akçay et al., [Phys. Plasmas 28, 082106 (2021)]. This classification splits the control space into three regions: L̂, with only L states; Û, with only U states; and a hysteresis (hysteretic) region Ĥ, with both L and U states. In regions L̂ and Û, the cubic equation of torque balance yields one real root. Region Ĥ has three roots, allowing bifurcations between the L and U states. The classification of the ODE solutions into L/U is used to estimate the locking probability, conditional on the pair of the control parameters, using a neural network. We also explore estimating the locking probability for a sparse dataset, using a transfer learning method based on a dense model dataset.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1980

Analysis of mode-locked and intracavity frequency-doubled Nd:YAG laser

The paper presents analytical and computer studies of the CW mode-locked and intracavity frequency-doubled Nd:YAG laser which provide new insight into the operation, including the detuning behavior, of this type of laser. Computer solutions show that the steady-state pulse shape for this laser is much closer to a truncated cosine than to a Gaussian; there is little spectral broadening for on-resonance operation; and the chirp is negligible. This leads to a simplified analytical model carried out entirely in the time domain, with atomic linewidth effects ignored. Simple analytical results for on-resonance pulse shape, pulse width, signal intensity, and harmonic conversion efficiency in terms of basic laser parameters are derived from this model. A simplified physical description of the detuning behavior is also developed. Agreement is found with experimental studies showing that the pulsewidth decreases as the modulation frequency is detuned off resonance; the harmonic power output initially increases and then decreases; and the pulse shape develops a sharp-edged asymmetry of opposite sense for opposite signs of detuning.

Siegman, A. E.↗
NASA NTRS1972

Effect of saturated frequency chirping on mode-locked laser pulses.

A dye-induced nonlinear frequency chirping is analytically shown to have significant modifications on the nature of the output mode-locked lasers. It is shown that the saturated frequency sweeping is responsible for substantial pulse broadening as well as substructures.

Kim, D. M.↗
NASA NTRS2010

Time-Gating Processes in Intra-Cavity Mode-Locking Devices Like Saturable Absorbers and Kerr Cells

Photons are non-interacting entities. Light beams do not interfere by themselves. Light beams constituting different laser modes (frequencies) are not capable of re-arranging their energies from extended time-domain to ultra-short time-domain by themselves without the aid of light-matter interactions with suitable intra-cavity devices. In this paper we will discuss the time-gating properties of intra-cavity "mode-locking" devices that actually help generate a regular train of high energy wave packets.

Prasad, Narasimha↗
NASA NTRS2019

Passively mode-locked interband cascade lasers

Optical frequency combs have revolutionized the field of high resolution real-time molecular spectroscopy. Here, we demonstrate an electrically-driven optical frequency comb whose sub-picosecond pulses span more than 1 THz of spectral bandwidth centered near 3.3 mm. This is achieved by passively mode locking an interband cascade laser in a multi-contact architecture with gain and saturable absorber sections monolithically integrated on the same chip.

Wysocki, Gerard↗
DOE OSTI2022

Colliding Pulse Mode Locking and Nonequilibrium Carrier Dynamics within Vertical External-Cavity Surface-Emitting Lasers

The complex nonlinear dynamics of colliding counterpropagating ultrashort laser pulses circulating in a vertical external-cavity surface-emitting ring laser cavity are captured by a first-principles quantum-mechanical microscopic model coupled to an intracavity beam propagator. Furthermore, our numerical simulations establish that overall mode locking stability is sensitive to relative phases and delays of the pulses impinging on the semiconductor saturable absorber mirror. Delay offsets of the counterpropagating pulses lead to asymmetric carrier recovery and instability, often leading in turn to single unidirectional circulating pulses. Although computationally challenging, we also include transverse effects and show that the influence of interference patterns from spatial gratings written by the counterpropagating pulses is significant in stabilizing the modelocked pulses.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Mode locking in an optomechanical cavity

Here, we experimentally study a fiber-based optical ring cavity integrated with a mechanical resonator mirror and an optical amplifier. The device exhibits a variety of intriguing nonlinear effects including synchronization and self-excited oscillation. Passively generated optical pulses are observed when the frequency of the optical ring cavity is tuned very close to the mechanical frequency of the suspended mirror. The optical power at the threshold of this process of mechanical mode locking is found to be related to quantum noise of the optical amplifier.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Kerr-lens mode-locked Cr:ZnS oscillator reaches the spectral span of an optical octave

We report, to the best of our knowledge, the first super-octave femtosecond polycrystalline Cr:ZnS laser at the central wavelength 2.4 µm. The laser is based on a non-polarizing astigmatic X-folded resonator with normal incidence mounting of the gain element. The chromatic dispersion of the resonator is controlled with a set of dispersive mirrors within one third of an optical octave over 2.05–2.6 µm range. The resonator’s optics is highly reflective in the range 1.8–2.9 µm. The components of the oscillator’s output spectrum at the wavelengths 1.6 µm and 3.2 µm are detected at –60 dB with respect to the main peak. Average power of few-cycle Kerr-lens mode-locked laser is 1.4 W at the pulse repetition frequency 79 MHz. That corresponds to 22% conversion of cw radiation of Er-doped fiber laser, which we used for optical pumping of the Cr:ZnS oscillator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1992

An analysis of the Hubble Space Telescope fine guidance sensor Fine Lock mode

There are two guiding modes of the Hubble Space Telescope used for the acquisition of astronomical data by one of its six scientific instruments. The more precise one is called Fine Lock. Command and control problems in the on-board electronics, compounded by the aberrations in the main optics, has limited routinely successful Fine Lock to brighter stars, V less than 13.0 mag, instead of fulfilling its prelaunch goal of V = 14.5 mag. Consequently, the less precise guiding mode of Coarse Track (about 25 milliarcsecond in practice) has to be used fairly frequently. Indeed, almost half of the celestial hemisphere has stars too faint to support guidance in the Fine Lock mode. Hence, some of the scientific observations to have been made with the Hubble Space Telescope will be compromised. In this paper a report on the only realistic or extensive simulations of the Fine Lock guidance mode is presented. The theoretical analysis underlying the Monte Carlo experiments and the numerical computations show that the control electronics have significant limitations and how to adjust the various control parameters in an attempt to extend Fine Lock guiding performance back to V - 14.0 mag.

Taff, L. G.↗