40 Matching Results

Search Results

Advanced search parameters have been applied.

Injection-locked composite lasers for mm-wave modulation : LDRD 117819 final report.

Description: This report summarizes a 3-year LDRD program at Sandia National Laboratories exploring mutual injection locking of composite-cavity lasers for enhanced modulation responses. The program focused on developing a fundamental understanding of the frequency enhancement previously demonstrated for optically injection locked lasers. This was then applied to the development of a theoretical description of strongly coupled laser microsystems. This understanding was validated experimentally with a novel 'photonic lab bench on a chip'.
Date: September 1, 2010
Creator: Wendt, Joel Robert; Vawter, Gregory Allen; Raring, James; Tauke-Pedretti, Anna; Alford, Charles Fred (Sandia Staffing Alliance, LLC, Albuquerque, NM); Skogen, Erik J. et al.
Partner: UNT Libraries Government Documents Department

Two-Element Phased Array of Anti-Guided Vertical-Cavity Lasers

Description: We demonstrate for the first time anti-guided coupling of two adjacent vertical-cavity surface-emitting lasers (VCSEL's), obtaining a 1-by-2 phase-locked array at 869 nm. The lateral index modification required for anti-guiding is achieved by a patterned 3-rim etch performed between two epitaxial growths. In contrast with prior evanescently coupled VCSEL's, adjacent anti-guided VCSEL's can emit in-phase and produce a single on-axis lobe in the far field. Greater than 2 mW of in-phase output power is demonstrated with two VCSEL's separated by 8 {micro}m. Moreover, phase locking of two VCSEL's separated by 20 {micro}m is observed, indicating the possibility of a new class of optical circuits based upon VCSEL's that interact horizontally and emit vertically.
Date: September 27, 1999
Creator: ALLERMAN, ANDREW A.; CHOQUETTE, KENT D.; GEIB, KENT M.; HADLEY, G. RONALD & SERKLAND, DARWIN K.
Partner: UNT Libraries Government Documents Department

Fiber Laser Replacement for Short Pulse Ti:Sapphire Oscillators -- Scalable Mode Locking to Record Pulse Energies

Description: We have investigated fiber-based lasers that mode-lock via three nonlinear mechanisms: pulse evolution, bend loss, and tunneling. Experiments with nonlinear pulse evolution proved especially promising; we report here a fiber laser that produces 25 nJ, sub-200 fs pulses, an energy that is 60% higher than previous reports. Experiments with nonlinear bend loss were inconclusive; though bend-loss data show that the effect exits, we were not able to use the phenomenon to lock a laser. New models suggest that nonlinear tunneling could provide an alternate path.
Date: February 14, 2006
Creator: Dawson, J W; Messerly, M J & An, J
Partner: UNT Libraries Government Documents Department

Nonlinear tearing mode interactions and mode locking in reversed field pinches

Description: The nonlinear interaction of a set of tearing instabilities and plasma flow is studied in a cylindrical plasma. An analytic theory of mode locking is developed which includes the effects of the localized electromagnetic torques, plasma inertia and cross-field viscosity. The calculation is specialized for the case of mode locking on the Madison Symmetric Torus (MST) reversed field pinch. In MST plasmas, a set of m = 1 tearing instabilities become phase locked and form a toroidally localized, rotating magnetic disturbance. An evolution equation for the phase velocity of this magnetic disturbance is derived which accounts for two types of electromagnetic torques. The external torques describe the interaction of the tearing modes with static magnetic perturbations located outside the plasma region. The interior torques describe the nonlinear interaction of three tearing modes which satisfy a wave number resonance condition. For conditions typical of MST, the internal torques dominate the external torques, which suggest the nonlinear interaction of tearing instabilities play a prominent role in the momentum degradation and mode locking.
Date: June 1, 1996
Creator: Hegna, C.C.
Partner: UNT Libraries Government Documents Department

Physics of locked modes in ITER: Error field limits, rotation for obviation, and measurement of field errors

Description: The existing theoretical and experimental basis for predicting the levels of resonant static error field at different components m,n that stop plasma rotation and produce a locked mode is reviewed. For ITER ohmic discharges, the slow rotation of the very large plasma is predicted to incur a locked mode (and subsequent disastrous large magnetic islands) at a simultaneous weighted error field ({Sigma}{sub 1}{sup 3}w{sub m1}B{sup 2}{sub rm1}){sup {1/2}}/B{sub T} {ge} 1.9 x 10{sup -5}. Here the weights w{sub m1} are empirically determined from measurements on DIII-D to be w{sub 11} = 0. 2, w{sub 21} = 1.0, and w{sub 31} = 0. 8 and point out the relative importance of different error field components. This could be greatly obviated by application of counter injected neutral beams (which adds fluid flow to the natural ohmic electron drift). The addition of 5 MW of 1 MeV beams at 45{degrees} injection would increase the error field limit by a factor of 5; 13 MW would produce a factor of 10 improvement. Co-injection beams would also be effective but not as much as counter-injection as the co direction opposes the intrinsic rotation while the counter direction adds to it. A means for measuring individual PF and TF coil total axisymmetric field error to less than 1 in 10,000 is described. This would allow alignment of coils to mm accuracy and with correction coils make possible the very low levels of error field needed.
Date: February 1, 1997
Creator: La Haye, R.J.
Partner: UNT Libraries Government Documents Department

Femtosecond, Cr{sup 4+}:YAG laser

Description: Results from both a regeneratively-initiated and self-initiated, mode-locked CR.YAG laser which is tunable from 1.51 to 1.53 {mu}m are reported. One hundred and twenty femtsosecond, nearly transform-limited pulses have been generated with peak output powers of 45 kW. The stable, high peak power pulses and room temperature operation of this laser make it a very suitable alternative to the cumbersome, cryogenic mode-locked NaCl laser commonly used in both narrow bandgap semiconductor and optical communications research.
Date: August 1, 1994
Creator: Nathel, H.; Sennaroglu, A. & Pollock, C.R.
Partner: UNT Libraries Government Documents Department

Self-seeded injection-locked FEL amplifier

Description: A self-seeded free electron laser (FEL) provides a high gain and extraction efficiency for the emitted light. An accelerator outputs a beam of electron pulses to a permanent magnet wiggler having an input end for receiving the electron pulses and an output end for outputting light and the electron pulses. An optical feedback loop collects low power light in a small signal gain regime at the output end of said wiggler and returns the low power light to the input end of the wiggler while outputting high power light in a high signal gain regime.
Date: December 1, 1998
Creator: Sheffield, Richard L.
Partner: UNT Libraries Government Documents Department

Techniques for increasing output power from mode-locked semiconductor lasers

Description: Mode-locked semiconductor lasers have drawn considerable attention as compact, reliable, and relatively inexpensive sources of short optical pulses. Advances in the design of such lasers have resulted in vast improvements in pulsewidth and noise performance, at a very wide range of repetition rates. An attractive application for these lasers would be to serve as alternatives for large benchtop laser systems such as dye lasers and solid-state lasers. However, mode-locked semiconductor lasers have not yet approached the performance of such systems in terms of output power. Different techniques for overcoming the problem of low output power from mode-locked semiconductor lasers will be discussed. Flared and arrayed lasers have been used successfully to increase the pulse saturation energy limit by increasing the gain cross section. Further improvements have been achieved by use of the MOPA configuration, which utilizes a flared semiconductor amplifier s amplify pulses to energies of 120 pJ and peak powers of nearly 30W.
Date: February 1, 1996
Creator: Mar, A. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

A Theoretical Investigation of Mode-Locking Phenomena in Reversed Field Pinches

Description: OAK-B135 This paper investigates the formation and breakup of the ''slinky mode'' in an RFP using analytic techniques previously employed to examine mode locking phenomena in tokamaks. The slinky mode is a toroidally localized, coherent interference pattern in the magnetic field which co-rotates with the plasma at the reversal surface. This mode forms, as a result of the nonlinear coupling of multiple m = 1 core tearing modes, via a bifurcation which is similar to that by which toroidally coupled tearing modes lock together in a tokamak. The slinky mode breaks up via a second bifurcation which is similar to that by which toroidally coupled tearing modes in a tokamak unlock. However, the typical m = 1 mode amplitude below which slinky breakup is triggered is much smaller than that above which slinky formation occurs. Analytic expressions for the slinky formation and breakup thresholds are obtained in all regimes of physical interest. The locking of the slinky mode to a static error-field is also investigated analytically. Either the error-field arrests the rotation of the plasma at the reversal surface before the formation of the slinky mode, so that the mode subsequently forms as a non-rotating mode, or the slinky mode forms as a rotating mode and subsequently locks to the error-field. Analytic expressions for the locking and unlocking thresholds are obtained in all regimes of physical interest. The problems associated with a locked slinky mode can be alleviated by canceling out the accidentally produced error-field responsible for locking the slinky mode, using a deliberately created ''control'' error-field. Alternatively, the locking angle of the slinky mode can be swept toroidally by rotating the control field.
Date: April 7, 2004
Creator: Fitzpatrick, Richard
Partner: UNT Libraries Government Documents Department

A theoretical investigation of mode-locking phenomena in reversed field pinches

Description: OAK-B135 This paper investigates the formation and breakup of the ''slinky mode'' in an RFP using analytic techniques previously employed to examine mode locking phenomena in tokamaks. The slinky mode is a toroidally localized, coherent interference pattern in the magnetic field which co-rotates with the plasma at the reversal surface. This mode forms, as a result of the nonlinear coupling of multiple m = 1 core tearing modes, via a bifurcation which is similar to that by which toroidally coupled tearing modes lock together in a tokamak. The slinky mode breaks up via a second bifurcation which is similar to that by which toroidally coupled tearing modes in a tokamak unlock. However, the typical m = 1 mode amplitude below which slinky breakup is triggered is much smaller than that above which slinky formation occurs. Analytic expressions for the slinky formation and breakup thresholds are obtained in all regimes of physical interest. The locking of the slinky mode to a static error-field is also investigated analytically. Either the error-field arrests the rotation of the plasma at the reversal surface before the formation of the slinky mode, so that the mode subsequently forms as a non-rotating mode, or the slinky mode forms as a rotating mode and subsequently locks to the error-field. Analytic expressions for the locking and unlocking thresholds are obtained in all regimes of physical interest. The problems associated with a locked slinky mode can be alleviated by canceling out the accidentally produced error-field responsible for locking the slinky mode, using a deliberately created ''control'' error-field. Alternatively, the locking angle of the slinky mode can be swept toroidally by rotating the control field.
Date: March 17, 2004
Creator: Fitzpatrick, Richard
Partner: UNT Libraries Government Documents Department

Coupled Resonator Vertical Cavity Laser Diodes

Description: For many applications, the device performance of edge emitting semiconductor lasers can be significantly improved through the use of multiple section devices. For example, cleaved coupled cavity (C3) lasers have been shown to provide single mode operation, wavelength tuning, high speed switching, as well as the generation of short pulses via mode-locking and Q-switching [1]. Using composite resonators within a vertical cavity laser opens up new possibilities due to the unique ability to tailor the coupling between the monolithic cavities, incorporate passive or active resonators which are spectrally degenerate or detuned, and to fabricate these devices in 2-dimensional arrays. Composite resonator vertical cavity lasers (CRVCL) have been examined using optical pumping and electrical injection [2-5]. We report on CRVCL diodes and show that efficient modulation of the laser emission can be achieved by either forward or reverse biasing the passive cavity within a CRVCL.
Date: July 22, 1999
Creator: Choquette, K.D.; Chow, W.W.; Fischer, A.J.; Allerman, A.A.; Hou, H.Q. & Geib, K.M.
Partner: UNT Libraries Government Documents Department

A wide bandwidth free-electron laser with mode locking using current modulation.

Description: A new scheme for mode locking a free-electron laser amplifier is proposed based on electron beam current modulation. It is found that certain properties of the original concept, based on the energy modulation of electrons, are improved including the spectral brightness of the source and the purity of the series of short pulses. Numerical comparisons are made between the new and old schemes and between a mode-locked free-electron laser and self-amplified spontaneous emission free-electron laser. Illustrative examples using a hypothetical mode-locked free-electron laser amplifier are provided. The ability to generate intense coherent radiation with a large bandwidth is demonstrated.
Date: January 20, 2011
Creator: Kur, E.; Dunning, D. J.; McNeil, B. W. J.; Wurtele, J. & Zholents, A. A.
Partner: UNT Libraries Government Documents Department

Millimeter-wave signal generation using an integrated mode-locked semiconductor laser and photodiode

Description: A compact optoelectronic integrated circuit for generation of mm-wave frequencies is demonstrated. A monolithically integrated semiconductor ring laser, optical amplifier and waveguide photodiode are used to generate electrical signals up to 85.2 GHz.
Date: February 1, 1997
Creator: Vawter, G.A.; Mar, A.; Hietala, V. & Zolper, J.
Partner: UNT Libraries Government Documents Department

An injection modelocked Ti-sapphire laser for synchronous photoinjection

Description: The CEBAF 4 GeV accelerator has recently begun delivering spin-polarized electrons for nuclear physics experiments. Spin-polarized electrons are emitted from a GaAs photocathode that is illuminated with pulsed laser light from a diode laser system synchronized to the injector chopping frequency (499 MHz). The present diode laser system is compact, reliable and relatively maintenance-free; however, output power is limited to less than 500 mW. In an effort to obtain higher average power and thereby prolong the effective operating lifetime of the source, they have constructed an injection modelocked Ti-sapphire laser with picosecond pulsewidths and gigahertz repetition rates. Modelocked operation is obtained through gain modulation within the Ti-sapphire crystal as a result of injection seeding with a gain-switched diode laser. Unlike conventional modelocked lasers, the pulse repetition rate of this laser can be discretely varied by setting the seed laser repetition rate equal to multiples of the Ti-sapphire laser cavity fundamental frequency. They observe pulse repetition rates from 223 MHz (fundamental) to 1,560 MHz (seventh harmonic) with average output power of 700 mW for all repetition rates. Pulsewidths ranged from 21 to 39 ps (FWHM) under various pump laser conditions.
Date: July 1, 1997
Creator: Hovater, C. & Poelker, M.
Partner: UNT Libraries Government Documents Department

Use of an m=2, n=1 static error field correction coil, {open_quotes}The C-coil,{close_quotes} on DIII-D to avoid disruptive locked modes

Description: Minimizing resonant, static n = 1 error field with a phase steerable correction coil the C-coil, in DIII-D allows avoidance of disruptive locked modes. Alternatiely, increasing n = 1 error field in rapidly rotating plasmas can induce magnetic braking of rotation without locking for the study of the role of rotation on stability. Small toroidally asymmetric m = 2, n = 1 static field errors are of concern for the design of next-generation devices and for the operation of existing tokamaks. In low density ohmic plasmas for example, the torque of a small resonant error at the q = 2 surface can overcome the plasma inertial and/or viscous forces, stop the rotation and produce a large island which can cause disruption.
Date: May 1, 1995
Creator: La Haye, R.J. & Scoville, J.T.
Partner: UNT Libraries Government Documents Department

Nonlinear dynamics of additive pulse modelocked lasers

Description: Nonlinear dynamics have been studied in a number of modelocked laser systems, primarily in actively modelocked systems. However, less attention has been paid to the dynamics of passively modelocked laser systems. With the recent revolutionary advances in femtosecond modelocked laser technology, the understanding of instabilities and dynamics in passively modelocked lasers is an important issue. Here, the authors present experimental and numerical studies of the dynamics of an additive-pulse modelocked (APM) color-center laser.
Date: April 1, 1995
Creator: Sucha, G.; Bolton, S.R. & Chemla, D.S.
Partner: UNT Libraries Government Documents Department

Plasma rotation, dynamo, and nonlinear coupling in the reversed field pinch

Description: Two important effects of MHD fluctuations in the RFP and tokamak are current generation (the dynamo effect) and mode locking. In the T1 and MST RFP experiments new results reveal the mode dynamics underlying these phenomena. In T1 the effect of specific magnetic Fourier modes on the current density profile is evident. In MST, the MHD dynamo term ({delta}v x {delta}B) is measured in the plasma edge, and found to account for the time dependence of the edge current throughout a sawtooth cycle. As edge resistivity is increased in T1 the fluctuation amplitude increases to maintain the dynamo-driven current, as expected from MHD computation. The modes responsible for the dynamo often lock to the local magnetic field error at the vertical cut in MST. The plasma rotation velocity has been measured with a fast Doppler spectrometer to a time resolution of 1 {mu}s. The plasma rotation and mode phase velocity are remarkably well-correlated, with both slowing, in the presence of an impulsive field error, in a 100 {mu}s timescale.
Date: July 1, 1995
Creator: Prager, S.C.; Almagri, A.F. & Cekic, M.
Partner: UNT Libraries Government Documents Department

Generation of Attosecond X-ray Pulses Beyond the Atomic Unit of Time Using Laser Induced Microbunching in Electron Beams

Description: Ever since the discovery of mode-locking, efforts have been devoted to reducing the duration of laser pulses since the ultrashort pulses are critical to explore the dynamics occurred on a ever-shorter timescale. In this paper we describe a scheme that's capable of generating intense attosecond x-ray pulses with duration beyond the atomic unit of time ({approx}24 attoseconds). The scheme combines the echo-enabled harmonic generation technique with the bunch compression which allows one to generate harmonic numbers of a few hundred in a microbunched beam through up-conversion of the frequency of a UV seed laser. A few-cycle intense IR laser is used to generate the required energy chirp in the beam for bunch compression and for selection of an attosecond x-ray pulse. Using a representative realistic set of parameters, we show that 1 nm x-ray pulse with peak power of a few hundred MW and duration as short as 20 attoseconds (FWHM) can be generated from a 200 nm UV seed laser. The proposed scheme may enable the study of electronic dynamics with a resolution beyond the atomic unit of time and may open a new regime of ultrafast sciences.
Date: December 11, 2009
Creator: Xiang, D.; Huang, Z.; Stupakov, G. & /SLAC
Partner: UNT Libraries Government Documents Department

Evolution of toroidal flow during, after mode locking

Description: The response of the toroidal flow velocity to the abrupt locking of it in the vicinity of a magnetohydrodynamic (MHD) tearing-mode- induced magnetic island is examined analytically and numerically using a diffusive transport model in a cylindrical plasma model. The resultant toroidal momentum confinement is shown to degrade significantly after mode locking, as is often observed on many tokamaks and reversed field pinches (RFPs). The degraded toroidal momentum confinement time in the core and edge regions of the plasma are evaluated and compared to experimental observations, The mode locking time scale itself is also investigated by using a model toroidal torque balance equation. The decrease of mode frequency during mode locking seems to be governed mainly by the electromagnetic torque exerted on the resistive layer. For tokamak plasmas, the mode locking time scale is found to be much shorter than the diffusion time scale, which is in general agreement with experimental observations.
Date: November 1, 1995
Creator: Yokoyama, M.; Callen, J.D. & Hegna, C.C.
Partner: UNT Libraries Government Documents Department

Photonic integrated circuit for all-optical millimeter-wave signal generation

Description: Generation of millimeter-wave electronic signals and power is required for high-frequency communication links, RADAR, remote sensing and other applications. However, in the 30 to 300 GHz mm-wave regime, signal sources are bulky and inefficient. All-optical generation of mm-wave signals promises to improve efficiency to as much as 30 to 50 percent with output power as high as 100 mW. All of this may be achieved while taking advantage of the benefits of monolithic integration to reduce the overall size to that of a single semiconductor chip only a fraction of a square centimeter in size. This report summarizes the development of the first monolithically integrated all-optical mm-wave signal generator ever built. The design integrates a mode-locked semiconductor ring diode laser with an optical amplifier and high-speed photodetector into a single optical integrated circuit. Frequency generation is demonstrated at 30, 60 and 90 Ghz.
Date: March 1, 1997
Creator: Vawter, G.A.; Mar, A.; Zolper, J. & Hietala, V.
Partner: UNT Libraries Government Documents Department

Operational Performance of the Two-Channel 10 Megawatt Feedback Amplifier System for MHD Control on the Columbia University HBT-EP Tokamak

Description: The operational characteristics and performance of the two channel 10 Megawatt MHD feedback control system as installed by Los Alamos National Laboratory on the Columbia University HBT-EP tokamak are described. In the present configuration, driving independent 300 {micro}H saddle coil sets, each channel can deliver 1100 Amperes and 16 kV peak to peak. Full power bandwidth is about 12 kHz, with capabilities at reduced power to 30 kHz. The present system topology is designed to suppress magnetohydrodynamic activity with m=2, n=1 symmetry. Application of either static (single phase) or rotating (twin phased) magnetic perturbations shows the ability to spin up or slow down the plasma, and also prevent (or cause) so-called ''mode-locking''. Open loop and active feedback experiments using a digital signal processor (DSP) have been performed on the HBT-EP tokamak and initial results show the ability to manipulate the plasma MHD mode frequency.
Date: October 6, 1997
Creator: Reass, W.A. & Wurden, G.A.
Partner: UNT Libraries Government Documents Department

Pulse propagation in inhomogeneous optical waveguides. Final report, September 15, 1992--March 14, 1996

Description: Accomplishments include two Ph.D. dissertations, twenty-six archival journal publications that have appeared in print, six articles that have appeared in conference or summer school proceedings, sixteen regular conference presentations, and eleven invited conference presentations. A complete record of the publications and presentations may be found in Sec. II.E. The areas in which the author has been working--randomly varying optical fiber birefringence, passively modelocked lasers, and quasi-phase matched second harmonic generation--are all still of great current interest. Recent progress in soliton transmission has been nothing short of outstanding with the recent achievement of single channel 15 Gbit/sec, nearly error-free transmission over 35,000 km. At the same time, remarkable progress with the presently used NRZ (non-return-to-zero) transmission mode makes it less clear that solitons will ultimately be used. The author has contributed in important respects to all these areas. In long-distance transmission systems, the length scale on which the birefringence varies randomly (30--100 m) is short compared to the nonlinear and dispersive scale lengths (100--1,000 km). Consequently, it is crucial to understand and characterize this randomly varying birefringence when studying long-distance evolution in optical fibers. That has been done in a series of studies that has also led to the proposal of a numerical scheme for modeling these systems that should be orders of magnitude faster than the schemes presently being used. In the studies of the fiber ring and figure-8 lasers, the author proposed that nonlinear polarization rotation is the mechanism responsible for fast saturable absorption in the fiber ring lasers--a result that was later verified experimentally. He also explored a new approach to determining the conditions for modelocking and self-starting in these lasers that uses the computer to determine the linear stability of both the pulsed and cw solutions. In all this work, the author has worked closely with leading ...
Date: August 17, 1998
Creator: Menyuk, C.R.
Partner: UNT Libraries Government Documents Department