Massachusetts Institute of Technology

Andrew Benedick

John J Zayhowski

Jonathan C Twichell

Jason Edward Langseth

Dale H Martz

Adam Alexander Libson

Seth Trotz

Stephen Conrad

Kevin Creedon

Eli Doris

Binary-phase-shift-key, phase-modulated waveforms with gigahertz bandwidths, suitable for kilowatt-class fiber amplifiers, can be narrowed back to the source laser's linewidth via second-harmonic, sum-frequency, or difference-frequency generation in a second-order nonlinear crystal. The spectrum of an optical signal phase-modulated with a pseudo-random bit sequence (PRBS) waveform recovers its original optical spectrum when frequency-doubled using second-harmonic generation (SHG). Conceptually, the PRBS waveform is cancelled by the SHG process, and the underlying laser spectrum is converted to the second-harmonic wavelength as though the PRBS modulation were not present. The same cancellation is possible with sum-frequency generation (SFG) and difference frequency generation (DFG), making it possible to construct high-power, narrow-linewidth lasers at wavelengths from the visible to the long-wave infrared. Using ytterbium-, erbium-, thulium-, and neodymium-doped fibers with SHG, SFG and DFG processes allows generation of high-power beams with very narrowband optical spectra and wavelengths from below 400 nm to beyond 5 μm.

Manipulating the optical phase of a laser beam

In coherent beam combining, the beams can be phase-modulated with a pseudo-random bit sequence (PRBS) to prevent stimulated Brillouin scattering (SBS) downstream. To coherently combine the phase-modulated beams, however, the PRBS waveforms should be true-time-delayed to within a small fraction of the bit duration. Traditionally, this true time delay is achieved by cutting optical fibers to length or with optical trombones. But trimming fibers is hard to do precisely, and optical trombones have large insertion loss. In addition, the path length mismatch varies as the fibers heat up and/or vibrate. Here, the beams are generated from a kilohertz linewidth seed split among N&#x3e;1 (e.g., N=100) arms. Each arm is phase-modulated with a separate copy of the unique PRBS pattern. The relative phase of the PRBS patterns is stabilized by phase-locking the master oscillators used to read out the PRBS patterns. The PRBS patterns can be phase shifted with respect to one another to compensate for physical path length mismatches of the optical fibers. Scanning the relative phase of the PRBS pattern used to modulate different arms yields a cross-correlation peak in combined power when the phases are matched at the combination plane.

SBS suppression and electronic path length matching in coherent beam combining

A High Bandwidth Individual Channel Control via Optical Reference Interferometry (HICCORI) system actively controls the phase and/or polarization of the optical emission of each element in a tiled optical array. It can also actively align any high-frequency broadening waveform applied to the array beams for spectral broadening or data transmission. By maintaining consistent polarization and manipulating the phase relationships of the beams emitted by the array elements, the HICCORI system can manipulate the spatial pattern of constructive and destructive interference formed as the individual emissions coherently combine. Active feedback control allows the desired phase, polarization, and/or spectral broadening alignment to be maintained in the presence of external disturbances.

High bandwidth individual channel control via optical reference interferometry control system architecture

Multi-stage fiber amplifiers can amplify signals from a few Watts to several kilowatts. These amplifiers are limited in power by intensity instabilities resulting from a sequence of nonlinear optical effects. These nonlinear optical effects include stimulated Brillouin scattering (SBS), with produces a high-intensity pulse close to the signal wavelength that propagates backward up the amplifier chain, causing permanent damage to the upstream components. This SBS pulse can be blocked by an optical isolator that blocks backward-propagating light at or near the signal wavelength. At high enough power levels, the SBS pulse can also induce backward-propagating light at wavelengths tens to hundreds of nanometers away from the signal wavelength. This SBS-Pulse Induced Non-linear Spectrum light is outside the isolator's reject band, so it can propagate upstream and de-stabilize the upstream amplifier stages. It can be suppressed using a filter with a broad reject band and a suppression ratio of &#x2265;30 dB, enabling higher power operation.

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Stow, MA, United States of America

Andrew Benedick