Fiber Based Light Sources Development for Spectroscopy and Microscopy Applications

Abstract

Ultrafast laser technology has become an important tool to push the edge of scientific knowledge and industrial applications. Many ultrafast lasers have been developed and successfully used in research and in industry. Currently, Ti:sapphire solid state lasers could be the most important and widespread apparatus to begin with. However, having an ultrafast laser source with a compact size, stable performance, alignment- and maintenance-free becomes a desirable feature for scientists and industrial developers, especially for the applications in advanced spectroscopy and microscopy. In these ultrafast laser applications, system movability becomes a challenge since a spectroscopy or microscopy setup is supposed to be able to move to where the sample of interest is located. This practical requirement makes femtosecond fiber lasers the best candidate. In this dissertation, we have developed three ultrafast fiber laser sources for a variety of spectroscopy and microscopy applications. The first source is a free-running compact ultra-broadband fiber laser for dual-comb spectroscopy (DCS). We start from a free-running bidirectional mode-locked fiber laser (FRBML) delivering two outputs, in clockwise and counter-clockwise directions, near 1550 nm. The two outputs exhibit mutual coherence since they are generated from a single laser cavity. They also have a slight difference in repetition rate, which provides the required mechanism for DCS. This laser design concept has been proven working by HCN absorption measurements which was reported in the past by our group. The wavelength coverage of this source is then extended here for broader detection ability. A piece of highly-nonlinear fiber was used for supercontinuum (SC) generation in each output arm of the FRBML. This mutually coherent SC light sources cover a wavelength range from 1 μm to 2 μm. The water vapor absorption resonances in the range from 1380 nm to 1850 nm measured with this newly developed source show good agreements with the HITRAN data. Wavelength extension for this free-running DCS source has also been done by using an all-fiber bidirectional optical parametric oscillator (FOPO). This FOPO delivers two outputs with a tuning rage from 1600 nm to 1650 nm. Single-shot DCS measurements performed with the source and a CH4 gas cell indicate good absorption frequency accuracy when comparing to the HITRAN data. The second source is related to another application of the FRBML in terahertz time-domain spectroscopy (THz-TDS). The amplified FRBML outputs (mentioned above) are used as the excitation source on a pair of terahertz emitter and receiver. The repetition rate difference of the outputs naturally provides the constant time-delay of the optical sampling during the measurement. The terahertz wave generated from an advanced plasmonic enhanced terahertz emitter is from 0.1 to 1.5 THz. The generated terahertz wave has been demonstrated to be useful in high signal-to-noise ratio free-running THz-TDS. This trigger-free, free-running THz-TDS performance was characterized with the water vapor absorption and the results matched well with the HITRAN data. The third source is a high power all-fiber optical parametric chirped-pulse amplifier (FOPCPA) developed for deep-tissue multiphoton microscopy (MPM). We first generate a SC to get a seed around 1300 nm. A piece of Corning SMF-LS fiber was used as the parametric gain medium. With a 7 W pump power at 1550 nm, about 40 dB gain at 1300 nm was achieved. As the result, Watt-level output power with a 12-nm bandwidth (supporting ~300 fs pulses) centered at 1308 nm is obtained by the FOPCPA. This light source has been integrated with our lab-built multiphoton microscope. Multiphoton imaging of a variety of samples carried out with this light source has shown a good signal to noise ratio.