John R. Marciante

Nonlinear mechanisms of filamentation in broad-area semiconductor lasers

There are three nonlinear mechanisms that can lead to filamentation in broad-area semiconductor lasers: gain-saturation-induced changes in the refractive index through the linewidth-enhancement factor, self-focusing due to heat-induced index changes, and self-defocusing through intensity-dependent index changes in the cladding layer. We present a theoretical model to analyze these mechanisms and their relative roles in destabilizing the laser output. We find that there exists a critical value for the linewidth-enhancement factor below which broad area lasers are stable for wide stripe widths (as wide as 250 /spl mu/m) and high pumping levels (as high as 20 times threshold). We also find that broad-area lasers are less susceptible to filamentation through self-defocusing and show how an intensity-dependent index in the cladding layer may be used to suppress filamentation caused by the linewidth-enhancement factor.

All-fiber optical magnetic-field sensor based on Faraday rotation in highly terbium-doped fiber

An all-fiber optical magnetic field sensor with a sensitivity of 0.49 rad/T is demonstrated. It consists of a fiber Faraday rotator (56-wt.%-terbium-doped silica fiber) and a fiber polarizer (Corning SP1060 fiber).

Distributed Waveform Generator: A New Circuit Technique for Ultra-Wideband Pulse Generation, Shaping and Modulation

A new circuit technique, the distributed waveform generator (DWG), is proposed for low-power ultra-wideband pulse generation, shaping and modulation. It time-interleaves multiple impulse generators, and uses distributed circuit techniques to combine generated wideband impulses. Built-in pulse shaping can be realized by programming the delay and amplitude of each impulse similar to an FIR filter. Pulse modulation schemes such as on-off keying (OOK) and pulse position modulation (PPM) can be easily applied in this architecture. Two DWG circuit prototypes were implemented in a standard 0.18 mum digital CMOS technology to demonstrate its advantages. A 10-tap, 10 GSample/s, single-polarity DWG prototype achieves a pulse rate of 1 GHz while consuming 50 mW, and demonstrates OOK modulation using 16 Mb/s PRBS data. A 10-tap, 10 GSample/s, dual-polarity DWG prototype was developed to generate UWB pulses compliant with the transmit power emission mask. Based on the latter DWG design, a reconfigurable impulse radio UWB (IR-UWB) transmitter prototype was implemented. The transmitters pulse rate can be varied from 16 MHz range up to 2.5 GHz. The bandwidth of generated UWB pulses is also variable, and was measured up to 6 GHz (- 10 dB bandwidth). Both OOK and PPM modulation schemes are successfully demonstrated using 32 Mb/s PRBS data. The IR-UWB transmitter achieves a measured energy efficiency of 45 pJ/pulse, independent of pulse rate.

Spectral properties of angled-grating high-power semiconductor lasers

We study the spectral properties of angled-grating high-power semiconductor lasers, also known as /spl alpha/ distributed feedback (DFB) lasers. We have derived a closedform expression to describe the cavity resonance. The results of this model are shown to compare favorably with experimental data. Intrinsic device parameters such as coupling coefficient and grating period are shown to be correlated to spectral and nearfield characteristics. The formulations and insights developed in this paper allow one to calculate these critical design parameters for optimum performance.

Spatio-temporal characteristics of filamentation in broad-area semiconductor lasers

Using the time dependent rate equations appropriate for semiconductor lasers, we perform a linear stability analysis to derive an analytic expression for the filament gain as a function of the filament spacing, oscillation frequency, linewidth-enhancement factor, pumping level, and nonlinear refractive index. The spatio-temporal characteristics of filamentation are obtained for a given set of operating parameters by maximizing the filament gain. Our theory predicts that there is a critical value of the linewidth-enhancement factor below which broad-area lasers remain stable even at high pumping levels. Moreover, under certain conditions, the self-defocusing Kerr-type nonlinearity can be used to suppress filamentation even when the linewidth-enhancement factor exceeds this critical value.

Gain Filtering for Single-Spatial-Mode Operation of Large-Mode-Area Fiber Amplifiers

Gain filtering of higher-order modes in large-mode-area fibers is an extremely robust method for providing diffraction-limited performance regardless of core diameter or input-beam quality. These predictions extend to 100- mum cores to the 10-kW level.

Compact all-fiber optical Faraday components using 65-wt%-terbium–doped fiber with a record Verdet constant of −32 rad/(Tm)

A compact all-fiber Faraday isolator and a Faraday mirror are demonstrated. At the core of each of these components is an all-fiber Faraday rotator made of a 4-cm-long, 65-wt%-terbium-doped silicate fiber. The effective Verdet constant of the terbium-doped fiber is measured to be -32 rad/(Tm), which is 27 x larger than that of silica fiber. This effective Verdet constant is the largest value measured to date in any fiber and is 83% of the Verdet constant of commercially available crystal used in bulk optics-based isolators. Combining the all-fiber Faraday rotator with fiber polarizers results in a fully fusion spliced all-fiber isolator whose isolation is measured to be 19 dB. Combining the all-fiber Faraday rotator with a fiber Bragg grating results in an all-fiber Faraday mirror that rotates the polarization state of the reflected light by 88 +/- 4 degrees .

OMEGA EP high-energy petawatt laser: progress and prospects

OMEGA EP (extended performance) is a petawatt-class addition to the existing 30-kJ, 60-beam OMEGA Laser Facility at the University of Rochester. It will enable high-energy picosecond backlighting of high-energy-density experiments and inertial confinement fusion implosions, the investigation of advanced-ignition experiments such as fast ignition, and the exploration of high-energy-density phenomena. The OMEGA EP short-pulse beams have the flexibility to be directed to either the existing OMEGA target chamber, or the new, auxiliary OMEGA EP target chamber for independent experiments. This paper will detail progress made towards activation, which is on schedule for completion in April 2008.

High-efficiency, high-dispersion diffraction gratings based on total internal reflection

We report a new class of high-dispersion immersed diffraction gratings for which the reflective nature of the diffraction is provided by the phenomenon of total internal reflection (TIR) regardless of grating tooth shape. Thus, the component can be fabricated from a single dielectric material and requires no metallic or dielectric film layers for high reflection diffraction efficiency. With the absence of metallic absorption, diffraction efficiencies of these TIR gratings can reach more than 99% for 15-20-nm spectral bandwidths, making them suitable for many laser-based technologies.

Dual-Frequency Operation in a Short-Cavity Ytterbium-Doped Fiber Laser

A dual-frequency 2-cm silica fiber laser with a wavelength spacing of 0.3 nm has been demonstrated using a polarization-maintaining (PM) fiber-Bragg-grating (FBG) reflector. The birefringence of the PM FBG was used to generate the two single-mode (SM) lasing frequencies of orthogonal polarizations. The SM operation in each wavelength has been verified. The output power reaches 43 mW with the optical signal-to-noise ratio of greater than 60 dB. The fiber laser shows stable dual-frequency output under pump variations