The dependence of the filtering bandwidth on the mode locking technique was studied in details in two types of mode locked fiber laser cavities; one utilizes SESAM, while the other employs NPE. The results show that for the two cases, below certain value of filtering bandwidth, no mode locking can be observed and the cavities will be in a lossy region. Moreover, NPE-based all fiber cavities can support narrower spectral bandwidth compared to SESAM-based cavities. Hence, NPE-based cavities produce shorter pulse width than SESAM-based ones. Experimental investigation was carried out to verify our simulation results and good agreement was achieved.
The present work demonstrates the integration of hollow core photonic crystal fibers (HC-PCF), microfluidics, and statistical analysis for monitoring biomolecules using Raman spectroscopy. HC-PCF as a signal enhancer has been proven by many researchers. However, there have been challenges in using HC-PCF for practical applications due to limitations such as coupling, stability, evaporation, clogging, consistent filling, and reusing the same fiber. This limited the potential of HC-PCF to detect low concentrations of liquid samples, which is why HC-PCF still hasn’t transcended the lab barriers. The current device is based on an H-design lay-out which uses the pressure difference between the two ends of the fiber for filling and flushing the liquid samples. This mitigated several issues related to device performance by allowing us to fill the fiber with liquid samples consistently, rapidly and reproducibly. The resulting Raman signals were significantly more stable as various concentrations of ethanol in water were sequentially introduced into the fiber. The scheme also allowed us to overcome the barrier of predicting low concentrations by applying Partial Least Square (PLS) technique which was done for the first time using HC-PCF. Thus, the present scheme paves path for the inclusion of HC-PCF in the main stream point-of-care technology.
Heparin is the most widely used anti-coagulant for the prevention of blood clots in patients undergoing certain types of surgeries including open heart surgeries and dialysis. The precise monitoring of heparin amount in patients’ blood is crucial for reducing the morbidity and mortality in surgical environments. Based upon these considerations, we have used Raman spectroscopy in conjunction with partial least squares (PLS) analysis to measure heparin concentration at clinical level which is less than 10 United States Pharmacopeia (USP) in serum. The PLS calibration model was constructed from the Raman spectra of different concentrations of heparin in serum. It showed a high coefficient of determination (R 2 >0.91 ) between the spectral data and heparin level in serum along with a low root mean square error of prediction ∼4 USP/ml . It enabled the detection of extremely low concentrations of heparin in serum (∼8 USP/ml ) as desirable in clinical environment. The proposed optical method has the potential of being implemented as the point-of-care testing procedure during surgeries, where the interest is to rapidly monitor low concentrations of heparin in patient’s blood.
An increase in energy of pulses generated in a similariton mode-locked femtosecond fiber laser is shown experimentally
by increasing the length while reducing the doping of the ytterbium-doped fiber gain medium. Mode-locking is achieved
by nonlinear polarization rotation evolution in a cavity using a combination of fiber and bulk optical components. The
level of doping of the gain medium is varied by using lengths of differently doped ytterbium fiber. Experimental results
verify that an increase in length of gain medium with a lower doping results in an increase in the output pulse energy.
We demonstrate the operation of a novel portable, fibre delivery miniaturized multimodal microscope (exoscope) for
coherent anti-Stokes Raman scattering and two-photon excitation fluorescence imaging using a single Ti:sapphire
femtosecond pulsed laser. This microscope features a large mode area photonic crystal fibre for light delivery, as well as
biaxial scanning microelectromechanical system mirrors and custom miniaturized optics corrected for chromatic
aberration. We demonstrate imaging of polystyrene beads, two photon excitation fluorescence beads in both forward and
backward (epi) directions. This miniaturized exoscope will enable in-vivo imaging of rat spinal cord.
The present work explores the feasibility of using surface enhanced Raman scattering (SERS) for detecting the
neurotransmitters such as glutamate (GLU) and gamma-amino butyric acid (GABA). These amino acid neurotransmitters
that respectively mediate fast excitatory and inhibitory neurotransmission in the brain, are important for neuroendocrine
control, and upsets in their synthesis are also linked to epilepsy. Our SERS-based detection scheme enabled the detection
of low amounts of GLU (10-7 M) and GABA (10-4 M). It may complement existing techniques for characterizing such
kinds of neurotransmitters that include high-performance liquid chromatography (HPLC) or mass spectrography (MS).
This is mainly because SERS has other advantages such as ease of sample preparation, molecular specificity and
sensitivity, thus making it potentially applicable to characterization of experimental brain extracts or clinical diagnostic samples of cerebrospinal fluid and saliva. Using hollow core photonic crystal fiber (HC-PCF) further enhanced the
Raman signal relative to that in a standard cuvette providing sensitive detection of GLU and GABA in micro-litre
volume of aqueous solutions.
We discuss the design and implementation of a novel multimodal coherent anti-Stokes Raman scattering (CARS)
miniaturized microscope for imaging of injured and recovering spinal cords in a single living animal. We demonstrate
for the first time, the use of a biaxial microelectromechanical system (MEMS) mirror for scanning and diffraction
limited multiple lens miniaturized objective for exciting a CARS signal. The miniaturized microscope design includes
light delivery using a large mode area photonic crystal fiber (PCF), and multimode fiber for collection of the nonlinear
optical signal. The basic design concept, major engineering challenges, solutions, and preliminary results are presented.
We demonstrate CARS and two photon excitation fluorescence microscopy in a benchtop setup with the miniaturized
optics and MEMS scanning. The light source is based on a single femtosecond laser (pump beam) and a supercontinuum
generated in a nonlinear PCF (Stokes beam). This is coupled using free space optics onto the surface of a resonantly
driven two dimensional scanning MEMS mirror that scans the excitation light in a Lissajous pattern. The novel design of
the miniaturized microscope is expected to provide significant new information on the pathogenesis of demyelinating
diseases such as Multiple Sclerosis and Spinal Cord Injury.
The performance of two different photonic crystal fibers (PCF) of identical lengths for implementation of the Stokes
source in a multimodal CARS microscopy and spectroscopy setup is compared. RIN measurements are performed to
experimentally determine the noise in the supercontinuum from the two fibers as well as in the CARS signal under
similar excitation conditions of the input pulse into the PCF. The RIN of the CARS signal is found to be higher than
the RIN of the corresponding Stokes signal, in both fibers. The implications for CARS microscopy of the SC spectrum
and its noise dependence on input pulse conditions in both fibers, are discussed.
We report on a novel simple configuration of an Yb-doped fiber laser cavity comprising only Yb-doped fiber and a
saturable absorber element. Numerical results show that stable mode-locked operation does exist in such a laser cavity
in limited range of parameters. The conditions to obtain stable pulses are investigated as a function of the Yb-fiber
bandwidth, length, and gain coefficient. The temporal and spectral behavior of the femtosecond pulses are also studied
for different input parameters. The instability dynamics of the mode-locked pulse is also elucidated.
A new method for using a non-selectively filled hollow-core photonic crystal fiber (HC-PCF) as a sensitive
Raman spectroscopy platform suitable for biosensing applications is presented. A 1550 HC-PCF was
completely filled with ethanol (core and cladding holes). Using a 785nm excitation laser, the Raman spectrum
of ethanol in the fiber core was obtained and compared with the equivalent Raman spectrum of an ethanolfilled
cuvette. Using a relatively short 9.5cm length of HC-PCF, a Raman signal enhancement factor of 40 over
a bulk solution of ethanol was observed under the same excitation conditions. The small sample volume
utilized and longer interaction length provides the potential for compact, sensitive, and low-power Raman
sensing of biological materials
We demonstrate cladding mode assisted supercontinuum generation in a solid core photonic crystal fiber.
Although the fiber is commonly considered as endlessly single mode, we show that coupling light into the
cladding excites much broader spectrum compared with the situation when only fundamental mode is excited.
Comparing experimental data with the results of the modal analysis and the simulations of the pulse propagation
in the fiber, we conclude that cladding mode has more favorable dispersion characteristics for the continuum
generation. We discuss possible usage of the cladding mode assisted supercontinuum in biosensor applications.
Mode-locked regimes of Yb-doped fiber laser with the photonic bandgap fiber (PBF) used for dispersion compensation in the laser cavity are studied using numerical modeling. The impact of the PBF's third-order dispersion on the dynamic instabilities of mode-locked regimes is elucidated. It is demonstrated that the instabilities leading to multi-period pulsing results from destabilization of resonant sidebands associated with dispersive waves. Novel type of stretched-pulse multi-period pulsing regime with symmetry-breaking features is identified. It is demonstrated that third-order dispersion suppresses the multi-period instabilities in the stretched-pulse regime. From the analysis of the spectral sidebands, we conclude that the suppression mechanism relies on resonant coupling of the phase-velocity matched and group-velocity matched dispersive waves with the main pulse spectrum.
We have recently demonstrated coherent anti-Stokes Raman scattering (CARS) microscopy and multiplex CARS spectroscopy of lipid-rich structures based on a single femtosecond Ti:sapphire laser. A nonlinear photonic crystal fiber (PCF) with two closely lying zero dispersion wavelengths is used to generate the Stokes pulse. Further optimization in terms of higher spectral resolution in the CARS spectra can be achieved by adding a second PCF to the pump arm to produce a spectrally compressed picosecond pulse. Theoretical predictions from modeling the propagation of the negatively chirped pump pulse in the PCF, are compared with experimental results. The effect of pulse duration, peak power and the length of the PCF in determining the bandwidth of the spectrally compressed pump pulse are considered. It is shown that for higher average output power and constant pulse duration, it is desirable to use shorter length of the PCF for attaining transform limited spectral width of the pump pulse.
Tunable distributed Bragg reflectors with comb-like reflection spectrum have been shown to allow semiconductor lasers tuning over about 100 nm wavelength range. Recently we proposed a concept of BSG as a multiwavelength reflector and looked into a number of key design issues for widely tunable lasers. In this paper we analyze the technical aspects of designing and implementing the BSG. We show that the calculated performance of the binary grating is at least as good as that of the existing approaches. However, BSG offers additional design freedoms in positions and amplitudes of reflectance peaks, which in turn allows extension of the tuning range of the laser or relaxing the tuning current requirements. The principles of widely tunable lasers have been adapted to a practical design which should significantly simplify the control requirements for the tuning. We show that a BSG reflector for 1.55 micrometers tunable laser can be successfully implemented with the smallest feature size as large as 0.12 micrometers , which is well within the capacity of electron beam lithography. A simulated tuning curve for a BSG laser predicts a high side- mode suppression ratio (> 35 dB).
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