Single Mode Optical Fiber Based Devices and Systems for Mid-infrared Light Generation, Communication and Metrology.

Abstract

Fiber-optic systems and devices for broadband mid-infrared light generation, communication and optical metrology are developed in this thesis.

Using the nonlinear properties of low mid-infrared loss ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber, a mid-infrared supercontinuum (SC) laser based on a thulium-doped fiber amplifier (TDFA) with spectrum extending from ~1.9-4.5 µm is demonstrated. A higher efficiency, power-scalable, all-fiber integrated mid-infrared light source is realized capable of generating ~0.7 W time-average power in wavelengths beyond 3.8 µm.

The novelty of the laser lies in its two-step spectral shifting architecture. First, amplified laser diode pulses at 1.55 µm are used to generate a SC extending beyond 2 µm in standard SMF using modulation-instability initiated pulse break-up. A TDFA stage is then used to amplify the ~2 µm components in the standard SMF continuum. By subsequently coupling the amplified ~2 µm pulses in to a ZBLAN fiber, an SC with up to ~2.6 W average power, and ~9% optical conversion efficiency from the power-amp pump to mid-IR output is demonstrated. The two-step methodology leads to extension in the long wavelength edge of the SC from 4.2 µm to ~4.5 µm, compared to previously demonstrated systems and ~2.5 times higher optical efficiency in generating wavelengths beyond 3.8 µm. Numerical simulations are also presented based on solving the generalized non-linear Schrödinger equation to verify and extend experimental results.

A broadband surface-normal optical modulator for communication applications with operation demonstrated over 1200-2400 nm is also presented. The modulator uses free-carrier effect in GaAs and mode selectivity of SMF to generate up to ~43% modulation depth with a maximum operating speed of ~270 MHz. The broad wavelength range of operation of the modulator can potentially enable higher throughput wavelength-division multiplexed optical network architectures based on broadband light sources.

Finally, an optical probe for detection of porosity defects in automotive parts is presented. The probe relies on the spatial coherence properties of SMF output to detect defects as small as ~50 µm lateral dimensions in bores down to 5 mm diameter. The probe uses a novel two-directional scattering-based non-contact approach to detect and classify defects on surfaces, where human inspection is labor-intensive.