Russell B. Wilcox

Scalable antiguided ribbon laser

A new scalable fiber laser approach to phase locking of multiple gain cores in an antiguided structure is described and modeled. In essence, the waveguide comprises a periodic sequence of gain-loaded and no-gain segments that has a uniform refractive index (referred to as the ribbon) encapsulated within a reduced-index cladding region. Our calculations reveal that the constant-index profile within the ribbon structure provides optimal mode discrimination; the refractive index must be constant within ±0.001 to ensure single-mode operation for a five-core design. One-dimensional and two-dimensional calculations are pursued to support the design criteria. Slight periodic variation in the refractive index of the ribbon laser leads to the emergence of a photonic bandgap, in analogy to so-called holey fibers. Our constant-index design, together with the periodic gain profile, may be described as a photonic metal.

Issue of FM to AM conversion on the National Ignition Facility

The National Ignition Facility (NIF) baseline configuration for inertial confinement fusion requires phase modulation for two purposes. First, approximately 12 angstrom of frequency modulation (FM) bandwidth at low modulation frequency is required to suppress buildup of Stimulated Brioullin scattering in the large aperture laser optics. Also, approximately 3 angstrom or more bandwidth at high modulation frequency is required for smoothing of the speckle pattern illuminating the target by the smoothing by spectral dispersion method. Ideally, imposition of bandwidth by pure phase modulation does not affect the beam intensity. Ideally, imposition of bandwidth by pure phase modulation does not affect the beam intensity. However, as a result of a large number of effects, the FM converts to amplitude modulation (AM). In general this adversely affects the laser performance, e.g. by reducing the margin against damage to the optics. In particular, very large conversion of FM to AM has been observed in the NIF all-fiber master oscillator and distribution systems. The various mechanisms leading to AM are analyzed and approaches to minimizing their effects are discussed.

High-channel-count fiber Bragg gratings fabricated by phase-only sampling

In conclusion, it has been demonstrated that phase-only sampling can generate high-channel count fibre Bragg gratings (FBGs) with the conventional technique of side writing through a phase mask. Additional phase-only sampling schemes and experiment results will be discussed.

Tunable dispersion slope compensation for 40-Gb/s WDM systems using broadband nonchannelized third-order chirped fiber Bragg gratings

We demonstrate tunable dispersion slope compensation using broadband nonchannelized third-order chirped fiber Bragg gratings (FBGs) for 40-Gb/s wavelength-division-multiplexed (WDM) systems. The slope is tuned by stretching as it has a third-order time delay variation with wavelength, and thus a second-order dispersion-wavelength curve. Calculation results include the examples of the slope tuning in addition to the interchannel dispersion differences and the slope tuning ranges with respect to the wavelength in a single grating. To verify the functionality of the third-order grating, we prepare a third-order grating with a dispersion variation of up to 400 ps/nm over the 3.5-nm usable bandwidth, and a dispersion slope tuning range of /spl sim/65 ps/nm/sup 2/ is achieved. For 40-Gb/s systems, two new gratings are designed and inversely cascaded to eliminate the inherent errors originating from the deleterious higher-order dispersion components in a single grating. The slope tuning range is -20 to +20 ps/nm/sup 2/ over the 5-nm usable bandwidth. A /spl sim/10-dB power penalty improvement is achieved and the BER floor is eliminated for the worst-case channel after transmission through a 170-km link using dispersion shifted fiber (DSF) with a 4/spl times/40-Gb/s RZ data stream.

Fusion laser oscillator and pulse-forming system using integrated optics

In order to demonstrate new technology for the proposed National Ignition Facility (NIF), we are currently building a 5-kilojoule laser called Beamlet. The oscillator and pulse shaping system for Beamlet represents a major technological improvement over previous designs. Using integrated optics, fiber optics, and diode-pumped lasers instead of bulk optics and flashlamp-pumped lasers, this new master oscillator takes advantage of current technology to make a system with numerous advantages. The requirements for a NIF for greater flexibility and reliability necessitate this new approach; the pulse-forming system for the Beamlet demonstrates a subset of the capabilities required for a NIF. For the Beamlet, we must produce a single 1 - 10 ns, shaped- and frequency-modulated pulse. The Beamlet needs only to generate square output pulses for technology demonstration purposes, but the input pulses must be shaped to compensate for gain saturation in the power amplifier. To prevent stimulated Brillouin scattering (SBS) from damaging the output optics, the output pulse must have some bandwidth, and thus the pulse-forming system phase modulates the input pulse. These requirements are very similar to those for the Nova master oscillator system, but Nova technology is not the best choice for the Beamlet. In developing an oscillator design for a fusion laser system, the system requirements are defined by the oscillators place in the overall laser architecture. Both Nova and Beamlet use a master oscillator-power amplifier (MOPA) architecture. In a MOPA-laser architecture, a low-power oscillator is followed by a high-gain, high-power amplifier. If the output signal is to have a high signal-to-noise ratio (SNR), the oscillator-signal power must be high above the amplifier noise power.

Temporal shaping of third-harmonic pulses on the Nova laser system

We demonstrate temporal shaping of 0.35-microm-wavelength pulses produced by a third-harmonic conversion of the output from the Nova Nd:phosphate glass-laser amplifier system for use in inertial confinement fusion experiments. We describe the computer models used to calculate the pulse shape that is required as the input to the amplifier system, the experimental apparatus used to produce these pulses, and the high-power 0.35-microm shaped pulses produced in recent experiments.

Tunable interchannel broad-band dispersion-slope compensation for 10-Gb/s WDM systems using a nonchannelized third-order chirped FBG

We demonstrate tunable dispersion-slope compensation using a single broad-band nonchannelized fiber Bragg grating. Tunability of the dispersion slope is achieved by stretching the grating, which has a third-order time-delay variation with wavelength, thereby causing a second-order variation in dispersion. Although this nonlinear dispersion characteristic causes some inherent mismatch between the grating and real fiber links, the induced errors are negligible in 10-Gb/s systems. Dispersion-slope tunability of /spl sim/65 ps/nm/sup 2/ is achieved using a grating with dispersion variation of up to 400 ps/nm over a 3.5-nm usable bandwidth. Using this grating, we show over 10-dB improvement in the worst-case channel in a 10-Gb/s return-to-zero system. Two gratings are cascaded in order to double the dispersion values that can be compensated by the grating.

Amplitude and phase modulation with waveguide optics

We have developed amplitude and phase modulation systems for glass lasers using integrated electro-optic modulators and solid state high-speed electronics. The present and future generation of lasers for Inertial Confinement Fusion require laser beams with complex temporal and phase shaping to compensate for laser gain saturation, mitigate parametric processes such as transverse stimulated Brillouin scattering in optics, and to provide specialized drive to the fusion targets. These functions can be performed using bulk optoelectronic modulators, however using high-speed electronics to drive low voltage integrated optical modulators has many practical advantages. In particular, we utilize microwave GaAs transistors to perform precision, 250 ps resolution temporal shaping. Optical bandwidth is generated using a microwave oscillator at 3 GHz amplified by a solid state amplifier. This drives an integrated electrooptic modulator to achieve laser bandwidths exceeding 30 GHz.

Tunable dispersion slope compensation for WDM systems using a single non-channelized third-order-chirped FBG

We demonstrate a novel technique for tunable dispersion slope compensation that relies on a single nonchannelized chirped fiber Bragg grating (FBG). This grating is unique in that it is written such that the time delay as a function of wavelength is a third-order profile that covers several WDM channels over a continuous bandwidth of many nanometers. The dispersion range provided by the grating for the WDM channels can be up to 400 ps/nm, enabling dispersion slope compensation.

Stimulated Brillouin scattering thresholds for square pulses in polarizing fiber

Summary form only given. The stimulated Brillouin scattering (SBS) threshold gives an upper limit for the optical power that can be efficiently launched into an optical fiber. Recently, front-end lasers for large-scale laser systems were reported based on fiber optics using single-frequency square pulses propagating in polarizing fibers. Exceeding the SBS threshold could cause excess noise at low levels, and possibly damage to components at higher levels in the front end and/or following amplifier systems. While the general features of SBS are qualitatively well understood, the work reported treats SBS in polarizing fibers using square pump pulses at a wavelength of 1064 nm. Thresholds are given in terms of launched peak fluence and intensity. Results were compared with non-polarizing fibers.