Monica Minden

Power-scalable phase-compensating fiber-array transceiver for laser communications through the atmosphere

We report laboratory experiments demonstrating a phase-compensating 70-mm-diameter aperture transceiver that comprises a hexagonally close-packed array of seven 23-mm-diameter fiber collimator subapertures. Other than the collimators, the transceiver uses only fiber optics, connected as a master oscillator-multiple amplifier. The master oscillator is a fiber-coupled 1.5-µm diode laser, which is split and fed to 1-W fiber amplifiers before it exits the collimators. To obtain a phase-coherent far field we control each subapertures phase by adjusting the current to its amplifiers pump diodes in a multidither arrangement, maximizing the signal at the receiver. We achieve a diffraction-limited coherent beam combination in the far field that produces 1.4 W of power in the main lobe, in agreement with theory.

All-fiber 50 W coherently combined passive laser array

We experimentally demonstrate 50 W of spontaneously phase-locked two-laser array in an all-fiber and all-passive configuration using large-mode-area (LMA) polarization-maintaining fiber laser cavities and an LMA fiber coupler. We show that both laser cavity length difference and fiber nonlinearity play an important role in achieving efficient and stable coherent beam combining. In addition, we compare the difference in coherent combining efficiency by using fibers with different mode-field diameters and discuss the underlying phase-locking mechanism and its power scalability.

200 W self-organized coherent fiber arrays

We report producing 200 W coherent fiber laser arrays without active control. This outcome is obtained via self-organization using a non-fiber coupler for two- to ten-laser arrays.

Self-organized coherence in fiber laser arrays

Self-organized coherence between fiber lasers has been reported both via all-fiber 2x2 directional coupler trees and in spatially multi-core fibers. We have taken this a major step forward, coupling together a number of independent fiber lasers to obtain a spatially and spectrally coherent far field, with no active length, polarization, or amplitude control. The near field output comes from a spatial array rather than from a single fiber, making this approach scalable to extremely high power.

Detection of 532-nm frequency-doubled Nd:YAG radiation in an active rubidium atomic resonance filter.

We report the detection of frequéncy-doubled Q-switched Nd:YAG laser radiation at 532.24 nm by an active rubidium atomic resonance filter. The Rb 5p level was populated by using a diode laser operating at 795 nm. The 532-nm laser excited atoms to the 10s level; these decay through p levels to the ground state. Signals were observed at 420/422 nm and 359 nm, corresponding to fluorescence from states 6p and 7p. Fluorescence noise was observed only on the 420/422-nm transition; the 359-nm signal had no detectable noise.

A range-resolved Doppler imaging sensor based on fiber lasers

We have developed a unique eyesafe (1.5 /spl mu/m) laser with a pseudorandom pulse output. The laser is based on a mode-locked pulse format; the pulse pattern repeats over a time consistent with the cavity round-trip period but appears random within that period. The pseudorandom pulse format occurs when we overdrive a passive mode-locking mirror in a long fiber laser. This laser is used for generating range-resolved Doppler images.

Mode splitting and the coherent instability in high-gain lasers

A high-gain continuous-wave laser may exhibit an output instability in the form of a periodic pulse train or chaotic signal. In the time domain, the instability is interpreted as resulting from the inability of the medium’s polarization to respond quickly enough to perturbations in the optical field of the cavity. In the frequency domain, it can be interpreted as arising from the splitting of a single longitudinal mode into several oscillating lines, each of which satisfies the same cavity-resonance condition as the original mode. We show that the mode-splitting interpretation of the instability yields a new understanding of the complex behavior reported in xenon, helium–xenon, and helium–neon lasers. The fundamental frequencies of the instability and the possibility of chaotic output are shown to be consistent with unequally spaced resonant modes that arise close to the laser threshold. With saturation, nonresonant harmonics of the fundamental frequencies appear in the laser output. These harmonics have fixed phases relative to the resonant modes and lead to the complex asymmetries of the pulse shapes. Interaction between the resonant and nonresonant modes can trigger period doubling in the pulse train.

Coherent combining of fiber lasers

It seems straightforward to combine the latest high power fiber lasers together to scale up power. Yet every approach, including HRLs self-organization architecture, has proved more challenging than expected at high power. In practice power scaling elicits several undesirable phenomena whose consequences inhibit coherent combining. Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), for example, can together turn the relatively benign power fluctuations associated with mode beating into fiber-damaging pulses. Coherence and polarization integrity are inevitably impaired by glass-related nonlinear index changes (n2). HRL has demonstrated two to four lasers coherently combined to the 200 Watt level, using free space coupling with beamsplitters. In addition, nine independent lasers were locked in an all-fiber coupler

A coherent fiber-array-based laser link for atmospheric aberration mitigation and power scaling

We report a laser link that can correct atmospheric aberrations. We use a fiber collimator array, fed by a master oscillator with multiple fiber amplifiers (MOMA), and accomplish phase adjustment via pump diode current control. Each of seven channels is tagged by a different 1-20 kHz diode current dither. At the receiver, each channels phase information is extracted from the <50 kHz signal. Our measurements show 5 kHz phase adjustment capability, so even turbulence-induced aberrations, as well as typical atmospheric aberrations (< 200 Hz) can be corrected. Only in >~100 km-range scenarios is the correction bandwidth limited by lights travel time. The low dither frequencies and amplitudes do not interfere with the typically GHz laser communications signal. Importantly, our system reduces transmitter power requirements by correcting small pointing errors and atmospheric-path aberrations. Of course the multiple-fiber amplifier array also enables power scaling. We describe our near- and far-field beam measurements in the laboratory.

Ion-Etched Gratings For Laser Applications

Ion beam sputter etching has proven to be a superior technique for producing grating sampling mirrors for large optical systems. The patterns to be etched are defined by a photoresist masking film on the mirror surface. Grating patterns have been produced on laser mirrors by replication of diamond-scribed master patterns, while holographic construction has been used to produce linear and nonlinear gratings. The microscopic details of ion etched grating profiles show that the process is capable of high resolution pattern delineation and large area device fabrication.