Modellierung neuartiger modengekoppelter und Fourier-Domänen-modengekoppelter Faserringresonator-Laser
Final Report Abstract
The goal of this project has been to develop theoretical models of novel mode locked and Fourier domain mode locked (FDML) fiber ring cavity lasers. In the framework of this project, we have developed the first theoretical model for FDML operation, thus providing a basis for a theoretical understanding and numerical simulation of FDML lasers and a systematic design optimization. We have validated this model by extensive comparison to experimental results from Dr. Robert Huber’s group (LMU). We have used our model to study the instantaneous linewidth, which is related to the coherence length and thus the achievable imaging depth in optical coherence tomography applications. The effects governing the linewidth were identified, which is crucial for a further reduction of the linewidth. Furthermore, we have utilized our theoretical model for a careful study of the FDML dynamics, investigating how the interplay of the various effects leads to the formation of a steady state light field. We have also developed a semi-analytical description of the pulse propagation in strongly nonlinear optical fiber systems, significantly reducing the numerical complexity for the simulation of the optical dynamics, and providing an improved intuitive understanding for the pulse evolution in such systems. The key has been the introduction of a novel model test function, which can describe different pulse shapes by tuning a single parameter. In collaboration with Prof. F. Ömer Ilday, we have applied this description to similariton amplifiers and lasers. Specifically, this approach constitutes the first semi-analytical model for the soliton-similariton laser. An important subject of this project was the modeling of the instantaneous linewidth in FDML lasers, which is a measure of the coherence length of the optical output. In this context, several potential applications of FDML lasers are highly interesting, taking advantage of its unique sweep range and speed, and critically depending on the coherence properties. One example is the compression of the FDML output to short pulses by compensating the spectral phase. For a sweep range of 100 nm around a central wavelength of 1310 nm, the theoretically obtainable pulse duration is a few 10 fs. Furthermore, potential pulse energies in the µW range directly from the laser oscillator and the potential to shape the pulse by adjusting the frequency sweep function make such a concept appealing for applications such as material processing. A further improvement of the linewidth is critical to approach the theoretical limit of compression. Our theoretical model will be instrumental in identifying the effects limiting the linewidth and in a further design improvement. Frequency combs have attracted a lot of attention recently, since they can be used to measure unknown frequencies with unprecedented precision. They are typically generated with mode-locked lasers producing ultrashort pulses. However, also the optical output of FDML lasers is ideally periodic, consisting of frequency-swept laser pulses. The existence of a frequency comb structure in the FDML spectrum is particularly interesting from a fundamental point of view, since it is directly linked to the question if the optical output exhibits interference with that of the previous roundtrips. Considering that the cavity length is typically a few km, this would indicate remarkable stability properties of the FDML laser. Our theoretical model will be instrumental for the theoretical investigation of the stability properties and the associated frequency comb structure. Recently, a regime of ultrastable operation has been discovered for FDML lasers, characterized by an extremely narrow instantaneous spectral linewidth and very low intensity noise. This regime is highly interesting for OCT applications to further increase the imaging depth, and would also be very attractive for other potential applications, such as the pulse compression and the frequency comb discussed above. However, this effect has up to now only been observed over a small wavelength range, and the reason for the emergence of ultrastable operation is unclear. Our theoretical description will be instrumental in identifying the internal stabilization mechanisms responsible for the emergence of ultrastable operation. In FDML lasers, typically semiconductor optical amplifiers are used as an optical gain medium. However, there are several potential advantages of using fiber amplifiers, such as potentially higher output powers, polarization insensitive, robust laser setups, and reduced amplified spontaneous emission noise. Fiber amplifiers can show a much faster (for Raman amplifiers) or much slower (for doped fiber amplifiers) gain recovery dynamics than semiconductor optical amplifiers, largely affecting the FDML characteristics. Theoretical modeling will be crucial for a detailed comparison of the properties of FDML lasers based on different types of gain media, and for a design optimization of fiber-amplifier-based FDML lasers.
Publications
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“A theoretical description of Fourier domain mode locked lasers”, Opt. Express 17, 24013 (2009)
C. Jirauschek, B. Biedermann, and R. Huber
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“Analysis of the Optical Dynamics in Fourier Domain Mode-Locked Lasers,” in Optical Sensors, OSA Technical Digest (CD) (Optical Society of America, 2010), paper SWC4
S. Todor, B. Biedermann, R. Huber, and C. Jirauschek
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“Linewidth Optimization of Fourier Domain Mode-Locked Lasers,” in Conference on Lasers and Electro- Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CMW7
S. Todor, C. Jirauschek, B. Biedermann, and R. Huber
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“Instantaneous lineshape analysis of Fourier domain mode-locked lasers”, Opt. Express 19, 8802 (2011)
S. Todor, B. Biedermann, W. Wieser, R. Huber, and C. Jirauschek
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“Semi-Analytic Theory of Similariton Amplifiers and Laser Oscillators Using a Shape-Adaptive Model Pulse,” in Nonlinear Optics: Materials, Fundamentals and Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper NThC2
C. Jirauschek and F. Ö. Ilday
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“Semianalytic theory of self-similar optical propagation and mode locking using a shape-adaptive model pulse”, Phys. Rev. A 83, 063809 (2011)
C. Jirauschek and F. Ö. Ilday
