Broadband Optical Sources Based on Highly Nonlinear Quasi-Phasematched Interactions


Quasi-phasematching (QPM) is a powerful and versatile technique for manipulating coherent radiation. With current QPM media, we can perform a number of functionalities, including generating, amplifying, or spectrally broadening light across a wide range of frequencies. An advantage of QPM is that it allows us to engineer very strong nonlinear interactions with available commercial lasers. However, devices involving such strong nonlinearities can exhibit a number of subtle effects, necessitating careful numerical and theoretical modeling. In this dissertation, we consider a number of different applications of QPM, all of which share this common theme.

First, we consider supercontinuum (SC) generation in quasi-phasematching waveguides, an approach which has several potential advantages including relatively low energy requirements (compared to those required in bulk), automatic carrier envelope offset frequency detection, the potential to engineer the QPM grating profile to produce a tailored output spectrum, and the use of mid-IR QPM media. We first study SC generation in reverse-proton-exchanged (RPE) lithium niobate waveguides numerically, showing good agreement between simulations and existing experimental results. Our model reveals a strong competition between second- and third-order nonlinear effects, significantly greater than previously assumed for lithium niobate, which led to almost an order-of-magnitude increase in SC generation energy requirements and in some cases degraded coherence properties.

We performed SC generation experiments in QPM RPE waveguides using a Tm-doped fiber laser system. These experiments yielded a multiple-octave-spanning spectrum, and facilitated self-referencing of the laser directly from the waveguide. We modeled the experiments numerically, showing good agreement between simulated and measured spectra. Next, in order to resolve the competition between second- and third-order nonlinear effects and several other limitations of RPE waveguides, we studied dispersion-engineered lithium niobate ridge waveguides for the purpose of obtaining group velocity matching between the first and second harmonic pulses. We showed numerically that when properly designed, these ridge waveguides can facilitate SC generation with pump energies of around 50 pJ, around 50 times lower than in RPE waveguides.

There is also significant interest in generating tunable, high-spectral-density frequency combs in the mid-IR for spectroscopic applications. To accomplish this goal, we demonstrated tunable mid-IR generation based on difference frequency generation (DFG) in orientation patterned gallium arsenide (OP-GaAs). The OP-GaAs sample had a fan-out QPM design, facilitating smooth tuning across the mid-IR from 6.7-12.7 microns. The DFG system was based on a Tm-doped fiber laser producing 150-fs pulses at 1950-nm, and utilized the Raman soliton self frequency shift in a fluoride fiber to obtain a power-tunable 2500-nm seed for the DFG process. Average powers of up to 1.3 mW were obtained. We investigated the limitations of this approach theoretically and numerically, and showed that with realistic upgrades to the pump laser, high-gain optical parametric amplification (OPA) and high overall conversion efficiency can be obtained. This would allow for the generation of powers exceeding of as much as 100 mW, with a power spectral density sufficient for spectroscopic applications.

The last topic discussed in this dissertation is optical parametric chirped pulse amplification (OPCPA). There is significant interest in generating high-intensity, few-cycle pulses in the mid-IR for high-harmonic generation. However, there are few laser systems with the required properties beyond 3000 nm, which motivates the use of existing near-IR laser sources and nonlinear optics to generate and amplify light in this spectral region. Due to dispersion, amplification of few-cycle pulses is challenging unless one of several approaches is taken in order to facilitate broadband gain. Our approach is to utilize chirped (aperiodic) QPM gratings, in which the grating k-vector is varied monotonically through the length of the device. With these devices, the grating bandwidth can exceed the phasematching bandwidth of the input pulses, thereby allowing for ultra-broadband amplification. We first study the properties of this type of OPA device, showing a number of interesting physical properties including adiabatic following solutions to the nonlinear three-wave mixing equations. We then give an overview of our OPCPA experimental results, demonstrating 7 microjoule, 75-fs pulses centered at 3400-nm, with a repetition rate of 100 kHz. With chirped QPM OPA devices, high-gain, broad-bandwidth amplification, and tailored spectral gain and phase profiles can be achieved. Furthermore, these devices enable a number of system-level improvements. For example, we achieve highly efficient pulse compression by sending the amplified mid-IR pulses through low-loss, anti-reflection-coated bulk silicon. Spectral phase not corrected by the silicon can be compensated by pre-distorting the 1560-nm seed laser with a pulse shaper. This approach is possible because the seed phase is parametrically transferred to the mid-IR pulses by the broadband, collinear pre-amplifier. Finally, we discuss in detail the design opportunities and constraints which arise when deploying chirped QPM gratings in OPCPA systems. The main system constraints are on the duration and peak power of the pump pulses. We show that recent upgrades to the pump laser allowed these constraints to be met. The compressed idler pulses were able to ionize xenon, indicating the system is now adequate for use in high-field experiments.


June, 2012