Arun K. Sridharan

Power scaling analysis of fiber lasers and amplifiers based on non-silica materials

A developed formalism1 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.

Mode conversion in rectangular-core optical fibers

Mode conversion from the fundamental to a higher-order mode in a rectangular-core optical fiber is accomplished by applying pressure with the edge of a flat plate. Modal analysis of the near and far field images of the fibers transmitted beam determines the purity of the converted mode. Mode conversion reaching 75% of the targeted higher-order mode is achieved using this technique. Conversion from a higher-order mode back to the fundamental mode is also demonstrated with comparable efficiency. Propagation of a higher-order mode in a rectangular-core fiber allows for better thermal management and bend-loss immunity than conventional circular-core fibers, extending the power-handling capabilities of optical fibers.

High brightness, quantum-defect-limited conversion efficiency in cladding-pumped Raman fiber amplifiers and oscillators

We present a detailed theoretical investigation of cladding-pumped Raman fiber amplification in an unexplored parameter space of high conversion efficiency (> 60%) and high brightness enhancement (> 1000). Fibers with large clad-to-core diameter ratios can provide a promising means for Raman-based brightness enhancement of diode pump sources. Unfortunately, the diameter ratio cannot be extended indefinitely since the intensity generated in the core can greatly exceed that in the cladding long before the pump is fully depleted. If left uncontrolled, this leads to the generation of parasitic second-order Stokes wavelengths in the core, limiting the conversion efficiency and as we will show, clamping the achievable brightness enhancement. Using a coupled-wave formalism, we present the upper limit on brightness enhancement as a function of diameter ratio for conventionally guided fibers. We further present strategies for overcoming this limit based upon depressed well core designs. We consider two configurations: 1) pulsed cladding-pumped Raman fiber amplifier (CPRFA) and 2) cw cladding-pumped Raman fiber laser (CPRFL).

First selective mode excitation and amplification in a ribbon core optical fiber

We propose and demonstrate amplification of a single high-order mode in an optical fiber having an elongated, ribbon-like core having an effective mode area of area of 600 µm(2) and an aspect ratio of 13:1. When operated as an amplifier, the double-clad, ytterbium doped, photonic crystal fiber produced 50% slope efficiency and a seed-limited power of 10.5 W, corresponding to a gain of 24 dB. The high order mode remained pure through 20 dB of gain without intervention or realignment.

First multi-watt ribbon fiber oscillator in a high order mode

Optical fibers in the ribbon geometry have the potential to reach powers well above the maximum anticipated power of a circular core fiber. In this paper we report the first doped silica high order mode ribbon fiber oscillator, with multimode power above 40 W with 71% slope efficiency and power in a single high order mode above 5 W with 44% slope efficiency.

High-gain photonic crystal fiber regenerative amplifier

We have demonstrated a photonic crystal fiber-based regenerative amplifier at 1.078 microm. The input signal pulse energy is 20 pJ in a 12 ns pulse at a 3 kHz repetition rate. At 8.6 W of input pump power, the amplified output pulse energy is 157 microJ, yielding a gain of 69 dB. To our knowledge, this is the highest gain achieved in a fiber-based regenerative amplifier to date at any wavelength.

Brightness enhancement in a high-peak-power cladding-pumped Raman fiber amplifier

We demonstrate a cladding-pumped Raman fiber amplifier (CPRFA) whose brightness-enhancement factor depends on the cladding-to-core diameter ratio. The pump and the signal are coupled independently into different input arms of a pump-signal combiner, and the output is spliced to the Raman amplifier fiber. The CPRFA generates 20 microJ, 7 ns pulses at 1100 nm at a 2.2 kHz repetition rate with 300 microJ (25.1 kW peak power) of input pump energy. The amplified signals peak power is 2.77 kW, and the brightness-enhancement factor is 192--the highest peak power and brightness enhancement achieved in a CPRFA at any wavelength, to our knowledge.

Mode-converters for rectangular-core fiber amplifiers to achieve diffraction-limited power scaling

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 TEM00 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 TEM00 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%.

Ultimate power limits of optical fibers

The fundamental power scaling limits for diffraction limited fiber lasers are reviewed. Relationships between the limits show there is an upper bound for single aperture power of conventional fiber lasers of 10-40 kW.

Yb3+ Doped Ribbon Fiber for High Average Power Lasers and Amplifiers

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.