Diffraction-limited high power lasers in the region of 10s of kW to greater than 100 kW are needed for defense, manufacturing and future science applications. A balance of thermal lensing and Stimulated Brillouin Scattering (SBS) for narrowband amplifiers and Stimulated Raman Scattering (SRS) for broadband amplifiers is likely to limit the average power of circular core fiber amplifiers to 2 kW (narrowband) or 36 kW (broadband). A ribbon fiber, which has a rectangular core, operating in a high order mode can overcome these obstacles by increasing mode area without becoming thermal lens limited and without the on-axis intensity peak associated with circular high order modes. High order ribbon fiber modes can also be converted to a fundamental Gaussian mode with high efficiency for applications in which this is necessary. We present an Yb-doped, air clad, optical fiber having an elongated, ribbon-like core having an effective mode area of area of 600 μm² and an aspect ratio of 13:1. As an amplifier, the fiber produced 50% slope efficiency and a seed-limited power of 10.5 W, a gain of 24 dB. As an oscillator, the fiber produced multimode power above 40 W with 71% slope efficiency and single mode power above 5 W with 44% slope efficiency. The multimode M2 beam quality factor of the fiber was 1.6 in the narrow dimension and 15 in the wide dimension.
We present a method to repair damaged optics using laser-based chemical vapor deposition (L-CVD). A CO<sub>2</sub> laser
is used to heat damaged silica regions and polymerize a gas precursor to form SiO<sub>2</sub>. Measured deposition rates and
morphologies agree well with finite element modeling of a two-phase reaction. Along with optimizing deposition
rates and morphology, we also show that the deposited silica is structurally identical to high-grade silica substrate
and possesses high UV laser damage thresholds. Successful application of such a method could reduce processing
costs, extend optic lifetime, and lead to more damage resistant laser optics used in high power applications.
Diffraction limited fiber amplifiers in a circular geometry are likely to be limited by nonlinearities to 2 kW for narrowband and 10-36 kW for broadband lasers. We have proposed a ribbon fiber geometry to allow scaling fiber lasers above these limits in which a high order ribbon mode is amplified and converted back to the fundamental mode in free space. Novel methods of illuminating a high order ribbon fiber mode are discussed and compared with modeling and experimental results showing high purity illumination, > 90%. A 10 kW single frequency ribbon fiber amplifier design is presented and BPM simulation results verify the approach.
A rectangular-core fiber that guides and amplifies a
higher-order-mode can potentially scale to much higher
average powers than what is possible in traditional circular core large-mode-area fibers. Such an amplifier
would require mode-conversion at the input and output to enable interfacing with TEM<sub>00</sub> mode seed sources
and generate diffraction-limited radiation for various applications. We discuss the simulation and experimental
results of a mode conversion technique that uses two
diffractive-optic-elements in conjugate Fourier planes to
convert a diffraction limited TEM<sub>00</sub> mode to the higher-order-mode of a ribbon core fiber. Our experiments
show that the mode-conversion-efficiency exceeds 84% and can theoretically approach 100%.
A developed formalism<sup>1</sup> for analyzing the power scaling of diffraction limited fiber lasers and amplifiers is applied to a
wider range of materials. Limits considered include thermal rupture, thermal lensing, melting of the core, stimulated
Raman scattering, stimulated Brillouin scattering, optical damage, bend induced limits on core diameter and limits to
coupling of pump diode light into the fiber. For conventional fiber lasers based upon silica, the single aperture,
diffraction limited power limit was found to be 36.6kW. This is a hard upper limit that results from an interaction of the
stimulated Raman scattering with thermal lensing. This result is dependent only upon physical constants of the material
and is independent of the core diameter or fiber length. Other materials will have different results both in terms of
ultimate power out and which of the many limits is the determining factor in the results. Materials considered include
silica doped with Tm and Er, YAG and YAG based ceramics and Yb doped phosphate glass. Pros and cons of the
various materials and their current state of development will be assessed. In particular the impact of excess background
loss on laser efficiency is discussed.