Michael G. Wickham

Coherently coupled high-power fiber arrays

A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light energy in a single far-field spot. Each element consists of a single-mode fiber-amplifier chain. Phase control of each element is achieved with a Lithium-Niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channels signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channels phase modulator. This effort represents the results of a multi-year effort to achieve high power from a single element fiber amplifier and to understand the important issues involved in coherently combining many individual elements to obtain sufficient optical power for directed energy weapons. Northrop Grumman Corporation and the High Energy Laser Joint Technology Office jointly sponsored this work.

Diffractive-optics-based beam combination of a phase-locked fiber laser array

A diffractive optical element (DOE) is used as a beam combiner for an actively phase-locked array of fiber lasers. Use of a DOE eliminates the far-field sidelobes and the accompanying loss of beam quality typically observed in tiled coherent laser arrays. Using this technique, we demonstrated coherent combination of five fiber lasers with 91% efficiency and M2=1.04. Combination efficiency and phase locking is robust even with large amplitude and phase fluctuations on the input laser array elements. Calculations and power handling measurements suggest that this approach can scale to both high channel counts and high powers.

Brightness-Scaling Potential of Actively Phase-Locked Solid-State Laser Arrays

Recent progress in developing phased arrays of high-brightness solid-state lasers is summarized. We address the prospects for continued brightness-scaling via a model that extrapolates measured results to large numbers of array elements and provides a quantitative illustration of the features of coherent beam combination. This demonstrates that with present-day technology, both slab and fiber lasers have the capability to scale to unprecedented brightness levels.

Coherently coupled high power fiber arrays

A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light. The results of this experiment as well as comparisons to other fiber array approaches will be presented

Diode array pumped kilowatt laser

The diode array pumped kilowatt laser (DAPKL) has demonstrated more than an order of magnitude increase in brightness and average power for short pulse diode-pumped solid-state lasers since its inception in 1991. Significant advances in component technology have been demonstrated, including: development of a diffusion bonding process for producing large slabs of Nd:YAG laser material. Phase conjugation by stimulated Brillouin scattering (SBS) has been demonstrated with high reflectivity and fidelity in a simple focused geometry with input powers of 100 W. Pulse energies at 1.06 /spl mu/m of 10 J have been demonstrated with a beam quality of 1.5 times diffraction limited at the 500-W level. An average power of 875 W at 100 Hz has been obtained. Efficient frequency doubling with a record power of 165 W has been demonstrated with 5 J per pulse at 0.53 /spl mu/m. Work is ongoing to enclose the system in a compact brassboard with improved performance and long term stability.

8-W coherently phased 4-element fiber array

A four-element fiber array has been constructed to yield 8 watts of coherently phased, linearly polarized light energy in a single far field spot. Each element consists of a 2-W single-mode fiber-amplifier chain. Phase control of each element is achieved with a lithium-niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. Frequency broadening of the signal is necessary to avoid stimulated Brillouin scattering. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channels signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channels phase modulator. The stability of the optical train is an essential contributor to its success. A state-of-the-art interferometer was built with mountless optics. A lens array was constructed using nano-positioning tolerances, where each lens was individually aligned to its respective fiber to collimate its output and point it at a common far field spot. This system proved to be highly robust and handled any acoustic perturbations.

Loop-mirror filters based on saturable-gain or -absorber gratings

We present a novel all-fiber narrow-band filter based on pump-induced saturable-gain or-absorber gratings in a loop mirror. Our design provides built-in interferometric phase alignment of the signal to the grating for optimal filtering. Notch or bandpass functionality is determined by the choice of gain or absorption and the input ports selected for the pump and signal. The loop-mirror filter has potential bandwidths from the submegahertz to beyond the gigahertz regimes, and one can tune it optically by changing the wavelength of the pump light that establishes the grating. Such filters have potential applications to wavelength-division-multiplexed optical networks and optical rf signal processing.

Suppression of stimulated Brillouin scattering in single-frequency multi-kilowatt fiber amplifiers

Previous research has shown that temperature gradients along a fiber can broaden the Stimulated Brillouin Scattering (SBS) gain profile and thereby increase the SBS threshold. However, within practical temperature ranges this method has been limited to SBS thresholds of a few hundred Watts. It is also well known that strain gradients applied to a fiber can broaden the SBS resonance. To suppress the SBS threshold to kW levels in fiber amplifiers of length ~5 m requires broadening of the SBS resonance width to ~1 GHz, which can be achieved with a strain of 1 - 2%. Although tensile strain is generally limited by fiber failure to less than ~1%, compressive strain has been employed to the level of many percent in a number of applications in the tuning of fiber Bragg gratings. We demonstrate the effect of SBS gain broadening and suppression by strain gradients at high power (~ 190 W) for the first time to our knowledge, and explore scaling of this method to kW output levels.

Low-loss fiber optic time-delay element for phased-array antennas

We present a novel concept, the fiber optic Bragg grating true- time-delay (TTD) element, for implementing true time delay in the distribution network of an optically fed phased array antenna. the device utilizes narrowband optical Bragg reflection gratings written holographically into the core of a single-mode fiber at various positions along its length. An optical carrier is modulated by the RF signal of interest and launched into this delay-line fiber. The desired RF time delay may be realized by wavelength-selectable nature of the TTD device offers the possibility for simplified beamsteering control and channel multiplexing.

One Joule Per Pulse, 100 Watt, Diode-Pumped, Near Diffraction Limited, Phase Conjugated, Nd:YAG Master Oscillator Power Amplifier

We have assembled and tested a diode-pumped, phase conjugated Nd:YAG master oscillator power amplifier (PC MOPA) operating at an average power of 100 Watts. 1 J per pulse has been extracted at a repetition rate of 100 Hz with a beam quality (BQ) of 1.1 x diffraction limited (D.L.). This combination of average power and beam quality makes this the brightest short pulse solid-state laser reported to date. The optical efficiency of 22% and the overall efficiency of 9.4% also represent record performance for high energy short pulse lasers. Excellent spatial uniformity and a pulse length of 7 ns make this laser ideal for frequency doubling and parametric conversion.