Spatio-temporal focal spot characterization and modeling of the NIF ARC kilojoule picosecond laser.

The advanced radiographic capability (ARC) laser system, part of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, is a short-pulse laser capability integrated into the NIF. The ARC is designed to provide adjustable pulse lengths of ∼1-38ps in four independent beamlets, each with energies up to 1 kJ (depending on pulse duration). A detailed model of the ARC lasers has been developed that predicts the time- and space-resolved focal spots on target for each shot. Measurements made to characterize static and dynamic wavefront characteristics of the ARC are important inputs to the code. Modeling has been validated with measurements of the time-integrated focal spot at the target chamber center (TCC) at low power, and the space-integrated pulse duration at high power, using currently available diagnostics. These simulations indicate that each of the four ARC beamlets achieves a peak intensity on target of up to a few 1018W/cm2.

Injection laser system for Advanced Radiographic Capability using chirped pulse amplification on the National Ignition Facility.

We report on the design, performance, and qualification of the injection laser system designed to deliver joule-level chirped pulse beamlets arranged in dual rectangular beam formats into two main laser amplifier beamlines of the National Ignition Facility. The system is designed to meet the requirements of the Advanced Radiographic Capability upgrade with features that deliver performance, adjustability, and long-term reliability.

Diode-based additive manufacturing of metals using an optically-addressable light valve.

Selective Laser Melting (SLM) of metal powder bed layers, whereby 3D metal objects can be printed from a digital file with unprecedented design flexibility, is spurring manufacturing innovations in medical, automotive, aerospace and textile industries. Because SLM is based on raster-scanning a laser beam over each layer, the process is relatively slow compared to most traditional manufacturing methods (hours to days), thus limiting wider spread use. Here we demonstrate the use of a large area, photolithographic method for 3D metal printing, using an optically-addressable light valve (OALV) as the photomask, to print entire layers of metal powder at once. An optical sheet of multiplexed ~5 kW, 20 ms laser diode and ~1 MW, 7 ns Q-switched laser pulses are used to selectively melt each layer. The patterning of near infrared light is accomplished by imaging 470 nm light onto the transmissive OALV, which consists of polarization-selective nematic liquid crystal sandwiched between a photoconductor and transparent conductor for switching.

Field-flattened, ring-like propagation modes.

We present a method for designing optical fibers that support field-flattened, ring-like higher order modes, and show that the effective and group indices of its modes can be tuned by adjusting the widths of the guide's field-flattened layers or the average index of certain groups of layers. The approach provides a path to fibers that have simultaneously large mode areas and large separations between the propagation constants of their modes.

Mode-converters for rectangular-core fiber amplifiers to achieve diffraction-limited power scaling.

A rectangular-core (ribbon) fiber that guides and amplifies a single higher-order-mode (HOM) can potentially scale to much higher average powers than what is possible in traditional circular-core large-mode-area fibers. Such an amplifier would require mode-conversion at the input to enable interfacing with seed sources that typically output TEM(00) mode radiation and at the output to generate diffraction-limited radiation for end-user applications. We present the first simulation and experimental results of a mode conversion technique that uses two diffractive-optic-elements in conjugate Fourier planes to convert a diffraction limited TEM(00) mode to the HOM of a ribbon fiber. Mode-conversion-efficiency is approximately 84% and can theoretically approach 100%. We also demonstrate a mode-converter system that converts a single HOM of a ribbon fiber back to a diffraction-limited TEM(00) mode. Conversion efficiency is a record 80.5%.

Mode conversion in rectangular-core optical fibers.

Mode conversion from the fundamental to a higher-order mode in a rectangular-core optical fiber is accomplished by applying pressure with the edge of a flat plate. Modal analysis of the near and far field images of the fiber's transmitted beam determines the purity of the converted mode. Mode conversion reaching 75% of the targeted higher-order mode is achieved using this technique. Conversion from a higher-order mode back to the fundamental mode is also demonstrated with comparable efficiency. Propagation of a higher-order mode in a rectangular-core fiber allows for better thermal management and bend-loss immunity than conventional circular-core fibers, extending the power-handling capabilities of optical fibers.

Chipscale, single-shot gated ultrafast optical recorder.

We introduce a novel, chipscale device capable of single-shot ultrafast recording with picosecond-scale resolution over hundreds of picoseconds of record length. The device consists of two vertically-stacked III-V planar waveguides forming a Mach-Zehnder interferometer, and makes use of a transient, optically-induced phase difference to sample a temporal waveform injected into the waveguides. The pump beam is incident on the chip from above in the form of a diagonally-oriented stripe focused by a cylindrical lens. Due to time-of-flight, this diagonal orientation enables the sampling window to be shifted linearly in time as a function of position across the lateral axis of the waveguides. This time-to-space mapping allows an ordinary camera to record the ultrafast waveform with high fidelity. We investigate the theoretical limits of this technique, present a simulation of device operation, and report a proof-of-concept experiment in GaAs, demonstrating picosecond-scale resolution over 140 ps of record length.

High brightness, quantum-defect-limited conversion efficiency in cladding-pumped Raman fiber amplifiers and oscillators.

We present a detailed theoretical investigation of cladding-pumped Raman fiber amplification in an unexplored parameter space of high conversion efficiency (> 60%) and high brightness enhancement (> 1000). Fibers with large clad-to-core diameter ratios can provide a promising means for Raman-based brightness enhancement of diode pump sources. Unfortunately, the diameter ratio cannot be extended indefinitely since the intensity generated in the core can greatly exceed that in the cladding long before the pump is fully depleted. If left uncontrolled, this leads to the generation of parasitic second-order Stokes wavelengths in the core, limiting the conversion efficiency and as we will show, clamping the achievable brightness enhancement. Using a coupled-wave formalism, we present the upper limit on brightness enhancement as a function of diameter ratio for conventionally guided fibers. We further present strategies for overcoming this limit based upon depressed well core designs. We consider two configurations: 1) pulsed cladding-pumped Raman fiber amplifier (CPRFA) and 2) cw cladding-pumped Raman fiber laser (CPRFL).

Solid-state ultrafast all-optical streak camera enabling high-dynamic-range picosecond recording.

We demonstrate an ultrafast optical recording system based on a novel optical beam deflection technique. An optical pump temporarily creates an array of prisms that deflect an optical signal beam within a GaAs/AlGaAs planar waveguide. The fabricated device yielded, to our knowledge, the fastest sustained optical deflection reported to date and was used to create spatial representations of ultrafast temporal waveforms. A conventional camera was then used to record single-shot waveforms with a 2.5 ps resolution over a 50 ps record with a dynamic range in excess of 3000:1. Through further development, this all-optical streak camera could provide insight into previously unmeasurable phenomena in many fields.

Brightness enhancement in a high-peak-power cladding-pumped Raman fiber amplifier.

We demonstrate a cladding-pumped Raman fiber amplifier (CPRFA) whose brightness-enhancement factor depends on the cladding-to-core diameter ratio. The pump and the signal are coupled independently into different input arms of a pump-signal combiner, and the output is spliced to the Raman amplifier fiber. The CPRFA generates 20 microJ, 7 ns pulses at 1100 nm at a 2.2 kHz repetition rate with 300 microJ (25.1 kW peak power) of input pump energy. The amplified signal's peak power is 2.77 kW, and the brightness-enhancement factor is 192--the highest peak power and brightness enhancement achieved in a CPRFA at any wavelength, to our knowledge.

Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power.

We analyze the scalability of diffraction-limited fiber lasers considering thermal, non-linear, damage and pump coupling limits as well as fiber mode field diameter (MFD) restrictions. We derive new general relationships based upon practical considerations. Our analysis shows that if the fiber's MFD could be increased arbitrarily, 36 kW of power could be obtained with diffraction-limited quality from a fiber laser or amplifier. This power limit is determined by thermal and non-linear limits that combine to prevent further power scaling, irrespective of increases in mode size. However, limits to the scaling of the MFD may restrict fiber lasers to lower output powers.

Generalized formulation for performance degradations due to bending and edge scattering loss in microdisk resonators.

We present a generalized formulation for the treatment of both bending (whispering gallery) loss and scattering loss due to edge roughness in microdisk resonators. The results are applicable to microrings and related geometries. For thin disks with radii greater than the bend-loss limit, we find that the finesse limited by the scattering losses induced by edge roughness is independent of radii. While a strong lateral refractive index contrast is necessary to prevent bending losses, unless the radii are of the order of a few microns, lateral air-cladding is detrimental and only enhances scattering losses. The generalized formulation provides a framework for selecting the refractive index contrast that optimizes the finesse at a given radius.

Transverse instabilities and pattern formation in two-beam-excited nonlinear optical interactions in liquids.

We describe observations of various transverse instabilities that occur when two laser beams intersect in nonlinear optical liquids. Patterns that we observe include two types of conical emission and the generation of a linear array of spots. These results can be understood in terms of the physical processes of self-diffraction, two-beam-excited conical emission, and seeded modulational instability.

Enhanced linear and nonlinear optical phase response of AlGaAs microring resonators.

We have constructed and characterized several optical microring resonators with scale sizes of the order of 10 microm. These devices are intended to serve as building blocks for engineerable linear and nonlinear photonic media. Light is guided vertically by an epitaxially grown structure and transversely by deeply etched air-clad sidewalls. We report on the spectral phase transfer characteristics of such resonators. We also report the observation of a pi-rad Kerr nonlinear phase shift accumulated in a single compact ring resonator evidenced by all-optical switching between output ports of a resonator-enhanced Mach-Zehnder interferometer.

Beyond the absorption-limited nonlinear phase shift with microring resonators.

We show that the nonlinear phase shift produced by a ring resonator constructed from a given nonlinear optical material can be greater than the phase shift produced by a single pass through an infinite length of the same material when linear and nonlinear absorption are taken into consideration. The figure of merit (defined by the phase shift times the throughput) also improves for the ring resonator over that of the native nonlinear absorbing material. We finally show that these benefits of using the ring resonator as a nonlinear phase-shifting element can enhance the switching characteristics of a Mach-Zehnder interferometer.

Gap solitons in a two-channel microresonator structure.

We show that, when two channel waveguides are coupled by a sequence of periodically spaced microresonators, the group-velocity dispersion is low in the vicinity of the gap associated with the resonant frequency of the resonators. This low dispersion permits the excitation of a gap soliton with much lower energy than in a gap of similar width caused by Bragg reflection.