Development of all-fiber coherent supercontinuum ultrashort-pulse lasers and application to nonlinear optical microscopy

In this work of thesis titled “Development of all-fiber coherent supercontinuum ultrashort-pulse lasers and application to nonlinear optical microscopy” the aim is to develop architectures of ultra-broadband pulsed emission through the excitation of photonic crystal microstructured fibers and solid graded-index (GRIN) fibers with laser sources at different spectral windows: central wavelength at 1.06 μm and 1.5 μm. We also aim to study the temporal coherence between their pulses. Supercontinuum (SC) laser pulses are compressible to few- cycle temporal widths when there is temporal coherence between them.

Since research in the SC generation field started, SC versatility has been demonstrated for numerous applications: telecommunications, frequency metrology, spectroscopy, nonlinear optical (NLO) microscopy and biomedical imaging, among others. There are designs of architectures that use pump sources of different kinds: solid-state lasers, optical parametric oscillators, semiconductor lasers or optical fiber lasers, among others. They can be continuous wavelength emission sources or ultrashort pulse sources of temporal widths ranging from nanoseconds to femtoseconds. These sources are used to excite optical fibers of a wide variety of characteristics and different materials: tapered fibers, microstructured fibers, graphene waveguides or hollow core fibers filled with gas; fibers with anomalous, normal or all-normal dispersion curves, with zero dispersion values at different wavelengths, etc.

Some of the applications that make use of SC sources require that there is temporal coherence between the pulses emitted by the SC source. A high degree of coherence increases the stability of the emission, decreases the noise intensity and favors the temporal compression of the pulses to their Fourier transform limit (FTL). To avoid that the temporal coherence breaks during the process of XX SC generation, it is necessary to exploit nonlinear effects that do not imply solitonic phenomena. Solitonic effects appear when the pulses propagation happens at spectral regions where the chromatic dispersion of the fibers is anomalous. Effects such as self-phase modulation and four-wave mixing under normal chromatic dispersion conditions favor a temporally coherent broadening. It has been experimentally demonstrated that highly coherent SC can be generated exciting all-normal dispersion photonic crystal fibers (ANDi PCF).

In this work we are interested in obtaining commercial coherent SC sources that can be used in applications in which temporally coherent ultrashort pulse lasers are required. To do this, we design the sources from this work based on all-fiber architectures. Benefits of this characteristic are having laser sources permanently aligned, which are robust, do not need periodical maintenance and are easy to use by the end user. All this favors the use of the source and makes it easier to adopt temporally coherent SC sources in laboratories of different disciplines.

In this way, we have obtained an all-fiber coherent SC source exciting ANDi PCFs with an ultrashort pulse fiber laser at high repetition rate based on passive mode- locking at 1.06 μm central wavelength. On the one hand, we manufactured microstructured fibers with different geometrical parameters and studied the kind of spectral broadening that each one produced. With this, we defined the ranges of said parameters. On the other hand, we excited the ANDi PCFs without the need of a free-space coupling stage. We worked on the optimization of the transition between the laser fiber and the ANDi fiber to perform it through fiber splice. With this source we generated a 150 nm bandwidth SC. We studied the temporal coherence between its pulses using a dispersion-scan (d-scan) free- space compressor and confirmed that the temporal width of the compressed pulse, 12.6 fs, was practically its FTL. This development together with the proper electronical and mechanical design and a user-friendly software interface, has allowed to commercialize this source in the field of NLO microscopy. We have studied its performance as illumination source in multispectral and multimodal NLO microscopy. We have obtained different images of biological specimens through two-photon excitation fluorescence (2PEF), simultaneously in several fluorophores of absorption at different wavelengths. We have also obtained images combining this technique with second harmonic generation (SHG), allowing to observe neurons, muscle and pharynx from a C. elegans specimen in vivo. Our studies have shown that using this all-fiber coherent SC source allows XXI to image dispersive tissue specimens with cellular resolution and at deeper penetration lengths than with traditional sources.

In addition, we have developed an all-fiber SC source centered at 1.5 μm exciting solid core GRIN fibers with zero-dispersion wavelengths around 1535 nm. Experimentally we studied the generation of SC exciting fibers of different core diameters and lengths at different repetition rates, obtaining a SC spectrum of ~ 600 nm. Also, we simulated the SC generation depending on the length of the excited fiber and compared it with the experimental results finding a good correspondence, qualitatively, between them. We studied the compression of the pulses and measured the degree of temporal coherence between two consecutive pulses through an interferometric experiment.

In conclusion, we have developed and manufactured sources of SC emission at 1.06 μm y a 1.5 μm, respectively; the temporal coherence of the first has been demonstrated by the compression of the pulse to almost its FTL and the temporal coherence of the second has been measured. Finally, high resolution images of biological samples have been obtained through NLO microscopy techniques illuminating the samples with the 1.06 μm source.