Coherent anti-Stokes Raman scattering microscopy with a photonic crystal fiber based light source

A coherent anti-Stokes Raman scattering microscope based on a Ti:sapphire femtosecond oscillator and a photonic crystal fiber is demonstrated. The nonlinear response of the fiber is used to generate the additional wavelength needed in the Raman process. The applicability of the setup is demonstrated by imaging of micrometer-sized polystyrene beads.     

Near-infrared coherent anti-Stokes Raman scattering microscopy using supercontinuum generated from a photonic crystal fiber

Coherent supercontinuum generated from a photonic crystal fiber is applied to near-infrared coherent anti-Stokes Raman scattering (NIR-CARS) microscopy. A clear CARS image of polystyrene beads has been successfully obtained at a wavenumber difference resonant with the CH-stretching vibrational mode. PACS 87.64.Je; 39.30.+w; 87.64.Vv     

Broadband coherent anti-Stokes Raman scattering spectroscopy using soliton pulse trains from a photonic crystal fiber

Pulse trains of fundamental soliton pulses with different center wavelengths and delay times from a photonic crystal fiber were generated and used as Stokes optical pulses in coherent anti-Stokes Raman scattering (CARS) spectroscopy. The pulse trains were created by shaping optical pulses with a pulse shaper and their waveforms were measured by a cross-correlation frequency-resolved optical gating method. By the use of pulse trains, the time required for obtaining broadband CARS signals was reduced to be about one third compared with our previous study without using pulse trains. With this setup, broadband CARS signals between 500 and 3100 cm⁻¹ of a single polystyrene bead sample have been measured and the most of the Raman peaks in this frequency range of samples have been observed clearly.     

Coherent anti-Stokes resonance Raman scattering

The intensity of coherent anti-Stokes resonance Raman scattering (CARRS) is calculated taking into account the resonant absorption of laser and anti-Stokes waves and changes in the molecule population. The resonant absorption causes an alteration of the phase of the laser wave leading to a decrease and periodic behaviour of the anti-Stokes intensity. This phase change can be compensated by the choice of a definite wave vector mismatchK, which determines the optimum condition for the CARRS. The optimum angle between the Stokes and the laser propagation direction and the optimum cell length are calculated. The line shape of the CARRS is discussed and used for the explanation of the observed dependence of the CARRS spectra on the laser intensity and the frequency and on the molecule number density. In the case of a O-O excitation the vibrational frequencies of the ground and the excited level are generated simultaneously in the CARRS spectra.     

Coherent anti-Stokes Raman scattering microspectroscopy of silicon components with a photonic-crystal fiber frequency shifter

Coherent anti-Stokes Raman scattering (CARS) microspectroscopy of silicon components is demonstrated with pump and probe fields delivered by a mode-locked Cr:forsterite laser and the frequency-shifted soliton output of a photonic-crystal fiber as a Stokes field. CARS microspectroscopy is shown to allow a visualization of microscale features and defects on the surface of silicon wafers, offering much promise for online diagnostics of electronic and photonic silicon chip components.     

Coherent anti-Stokes Raman scattering velocimetry with nearly degenerate pumps

The time evolution of the anti-Stokes signal produced from the non-linear interaction of a short Stokes pulse and two long pump pulses that are nearly degenerate in frequency has been investigated. It is shown that this approach allows us to specify the accuracy of CARS (coherent anti-Stokes Raman scattering) velocimetry and to extend the range of operation of the method. In addition, an original optical scheme capable of delivering short visible pulses with good spatial and spectral properties is reported. The optical bench has been used for the characterisation of a low-pressure laminar Mach-10 flow. Received: 24 October 2001 / Revised version: 8 January 2002 / Published online: 14 March 2002     

Coherent anti-Stokes resonance Raman scattering of excited molecules

A theoretical investigation of the coherent anti-Stokes resonance Raman scattering (CARRS) of excited molecules is given. The calculation of the anti-Stokes intensity predicts the simultaneous appearance of the vibrational frequencies of the ground and the excited electronic state in the CARRS spectra. The line shape of these spectra and the optimum conditions for the anti-Stokes amplification are discussed.     

Broad spectral photonic crystal fiber surface enhanced Raman scattering probe

A broad spectral surface enhanced Raman scattering sensor is developed using the solid core holey photonic crystal fiber with silver nanoparticles cluster. This SERS probe offers an operational excitation wavelength range overlaying visible light and near infrared light. The PCF SERS sensing is demonstrated in the detection of the 4-Mercaptobenzoic acid (10⁻⁶ M) solution with 514.5 and 785 nm excitation. In this structure of PCF sensor, the related analysis shows that leakage modes also make an important contribution in the SERS activity not only by the evanescent field way.     

Multiwavelength fiber laser based on a photonic crystal fiber loop mirror with cooperative Rayleigh scattering

In this work, a multiwavelength fiber Raman laser based on a highly birefringent photonic crystal fiber loop mirror is presented. A laser resonator is formed when the Raman amplification with cooperative Rayleigh scattering in a dispersion-compensating fiber is used as a distributed mirror and combined with a photonic crystal fiber loop mirror filtering structure. Stable multiwavelength lasing at room temperature is achieved due to the low temperature sensitivity of the highly birefringent photonic crystal fiber.     

Coherent anti-Stokes Raman scattering and normal anti-Stokes Raman scattering due to LO phonons in GaP using cw diode lasers

Coherent anti-Stokes Raman scattering (CARS) and normal anti-Stokes Raman scattering (NARS) have been measured in (001) GaP at room temperature due to the 403 cm⁻¹ LO phonons using a continuous wave (CW) 785.0 nm fixed-wavelength pump laser and a CW Stokes laser tunable in the 800-830 nm wavelength range. CARS measurements are normally made using pulsed lasers. The use of CW diode lasers allows a more accurate comparison between the measured and calculated values of the CARS signal. The pump and Stokes laser beams were linearly polarized perpendicular to each other, same as the pump and normal Stokes/anti-Stokes scattered light for the GaP sample used in this work. The pump and Stokes laser powers incident upon the GaP sample, located in the focal plane of a 20 mm effective focal length lens, were <20 and 50 mW, respectively. The diameter of the laser beams in the focal plane of the focusing lens was determined to 40±5 μm. The pump and Stokes laser beam intensities incident upon the 0.3 mm thick GaP sample were <2 and 5 kW cm², respectively. The powers of the CARS and NARS signals were measured using a Raman spectrometer. The signal output of the Raman spectrometer was calibrated using the pump laser and several neutral density filters. The Raman linewidth (full-width at half-maximum) of the LO phonons was determined to be 0.95±0.05 cm⁻¹, using the variation of the CARS signal with the wavelength of the Stokes laser. The measured powers of the CARS and NARS signals are about a factor of 5 and 1.5, respectively, smaller than those calculated from the corresponding theoretical expressions.     

All fiber supercontinuum light source using photonic crystal fibers pumped by nanosecond fiber laser pulses

A novel compact supercontinuum (SC) source using the single mode photonic crystal fibers (PCF) pumped with an all fiber MOPA fiber laser is demonstrated experimentally. A bandwidth of 700 nm is achieved by operating the pumping fiber laser at a wavelength of 1064 nm, pulse duration of 10 ns, repetition rate of 50 kHz and peak power of 1 kW. The SC generation is initiated through modulation instability (MI) which breakups the nanosecond pump pulses into picosecond or femtosecond pulses, and further broadened through nonlinear effects of PCF.     

Ultrabroadband background-free coherent anti-Stokes Raman scattering microscopy based on a compact Er:fiber laser system

We demonstrate a scheme for efficient coherent anti-Stokes Raman scattering (CARS) microscopy free of nonresonant background. Our method is based on a compact Er:fiber laser source. Impulsive excitation of molecular resonances is achieved by an 11 fs pulse at 1210 nm. Broadband excitation gives access to molecular resonances from 0 cm(-1) up to 4000 cm(-1). Time-delayed narrowband probing at 775 nm enables sensitive and high-speed spectral detection of the CARS signal free of nonresonant background with a resolution of 10 cm(-1).     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

Coherent and incoherent spectral broadening in a photonic crystal fiber

The coherence of the spectral broadening process is the key requisite for the application of supercontinua in frequency combs. We investigate the coherence of two subsequent supercontinuum pulses created in a photonic crystal fiber pumped by a femtosecond laser. We measure Young interference fringes from a Michelson-type interferometer at different wavelengths of the output spectrum and analyze their dependence on pump intensity and polarization. The visibility of these fringes is a direct measure of the coherence of the spectral broadening processes.     

Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen

We report on the generation of pure rotational stimulated Raman scattering in a hydrogen gas hollow-core photonic crystal fiber. Using the special properties of this low-loss fiber, the normally dominant vibrational stimulated Raman scattering is suppressed, permitting pure conversion to the rotational Stokes frequency in a single-pass configuration pumped by a microchip laser. We report 92% quantum conversion efficiency (40 nJ pulses in 2.9 m fiber) and threshold energies (3 nJ in 35 m) more than 1 x 10(6) times lower than previously reported. The control of the output spectral components by varying only the pump polarization is also shown. The results point to a new generation of highly engineerable and compact laser sources.     

Broadband coherent anti-Stokes Raman scattering light generation in BBO crystal by using two crossing femtosecond laser pulses

As broad as 12000 cm(-1) coherent anti-Stokes Raman scattering (CARS) light from ultraviolet to infrared was generated in a BBO crystal by using two crossing femtosecond laser pulses with 30% conversion efficiency. More than fifteenth-order anti-Stokes and second-order Stokes Raman sidebands were observed with nice Gaussian spatial mode. The effect of the crossing angle between two input beams on the spectrum and emitting angle of the Raman sidebands was studied in detail. Calculation shows that the phase-matching condition determines the frequencies and angles of the sidebands.     

Q-switched thulium-doped photonic crystal fiber laser

We report a novel, Tm-doped photonic crystal fiber (PCF) actively Q-switched oscillator that provides ~8.9 kW peak power with 435 μJ, 49 ns pulses at 10 kHz repetition rate at 2 μm wavelength. This fiber has a mode-field area >1000 μm2, the largest of any flexible PCF providing diffraction-limited beam quality to the best of our knowledge. As an application, the oscillator is used as pump to generate >350 nm broadening in ~50 m of SMF-28 fiber.     

High-contrast chemical imaging with gated heterodyne coherent anti-Stokes Raman scattering microscopy

A novel method is presented which substantially improves the signal-to-background ratio for coherent anti-Stokes Raman scattering (CARS) microscopy. It exploits the fixed phase relation between pump, Stokes and CARS fields together with the strong phase coherence in supercontinua generated by femtosecond lasers. Three phase-locked optical parametric amplifiers are used for the realisation of heterodyne signal detection. Proper pulse timing yields a gating mechanism which nearly completely suppresses solvent background signals. PACS 42.65.Dr; 42.65.Hw; 42.65.Yj     

Off-axis pumping of a photonic crystal fiber laser

We present a new method for free space pumping of double-clad photonic crystal fiber lasers and amplifiers. The method is based on off-axis injection of the pump beam, exploiting the relatively high numerical aperture of the photonic crystal fiber cladding. It prevents feedback from the laser into the pump source, does not require any special treatment to the fiber, and could lead to a more compact resonator. Experimental results with the new pumping method show improved laser performance.