Fiber-delivered picosecond source for coherent Raman scattering imaging

We demonstrate a two-color, fiber-delivered picosecond source for coherent Raman scattering (CRS) imaging. The wavelength-tunable picosecond pump is generated by nonlinear spectral compression of a prechirped femtosecond pulse from a mode-locked titanium:sapphire (Ti:S) laser. The 1064?nm picosecond Stokes pulse is generated by an all-fiber time-lens source that is synchronized to the Ti:S laser. The pump and Stokes beams are combined in an optical fiber coupler, which serves not only as the delivery fiber but also as the nonlinear medium for spectral compression of the femtosecond pulse. CRS imaging of mouse skin is performed to demonstrate the practicality of this source.     

Coherent anti-Stokes Raman scattering imaging with a laser source delivered by a photonic crystal fiber

We demonstrate laser-scanning coherent anti-Stokes Raman scattering (CARS) imaging with two excitation laser beams delivered by a large-mode-area photonic crystal fiber. The group-velocity dispersion and self-phase modulation effects are largely suppressed due to the large mode area of the fiber and the use of picosecond pulses. The fiber delivery preserves the signal level and image spatial resolution well. High-quality images of live spinal cord tissues are acquired using the fiber-delivered laser source. Our method provides a basic platform for developing a flexible and compact CARS imaging system.     

Use of multimode optical fibers for fiber-based coherent anti-Stokes Raman scattering microendoscopy imaging

A multimode fiber (MMF) was used for both delivery of excitation lasers and collection of returned coherent anti-Stokes Raman scattering (CARS) signals in a CARS microendoscopy prototype imaging system. We demonstrated a polarization-based scheme for suppression of four-wave mixing (FWM) signals in delivery fibers. Our experimental results showed that this polarization-based FWM-suppressing scheme can dramatically reduce FWM signals generated in MMFs, and MMFs can be used to produce CARS images in this microendoscopy system. The proposed MMF-based CARS microendoscopy imaging system with the polarization-based FWM-suppressing scheme offers a potential platform for building fiber-based CARS microendoscopes that can effectively suppress FWM background noises.     

Broadly tunable dual-wavelength light source for coherent anti-Stokes Raman scattering microscopy

The signal and idler beams from a picosecond, synchronously pumped optical parametric oscillator (OPO) provide the two colors necessary for coherent anti-Stokes Raman scattering (CARS) microscopy. The OPO provides a continuously tunable frequency difference between the two beams over a broad range of Raman shifts (100-3700 cm(-1)) by varying the temperature of a single nonlinear crystal. The near-infrared output (900-1300 nm) allows for deep penetration into thick samples and reduced nonlinear photodamage. Applications of this light source to in vivo cell and ex vivo tissue imaging are demonstrated.     

Coherent optical phonon generation in Bi3Ge4O12

Time-domain spectroscopy of coherent optical phonons in bismuth germinate (Bi4Ge3O12) is presented. Utilizing both impulsive stimulated Raman scattering and time-domain terahertz spectroscopy, more than 12 unique vibrational states ranging in frequency from 2 to 11 THz are identified, each with coherent lifetimes ranging from 1 to 20 ps. These modes are highly sensitive to crystal orientation and demonstrate frequency shifts on picosecond timescales consistent with an anharmonic lattice potential.     

Amplification of picosecond pulses and gigahertz signals in bismuth-doped fiber amplifiers

We demonstrate the capability for amplification of picosecond pulses in two bismuth-doped alumosilicate fibers. A spectrally filtered supercontinuum source is used to provide a train of picosecond pulses at discrete wavelengths within the gain bandwidth of bismuth fiber amplifiers. With a 30 m length of active fiber, a small signal gain at 1160 nm of over 20 dB is observed. In addition, we assess the viability of amplification of high repetition rate signals in such amplifiers, applying a 10 GHz modulation to a continuous wave Raman fiber laser operating at 1178 nm, finding that such signals are amplified without noticeable distortion.     

Single-laser light source for CARS microscopy based on soliton self-frequency shift in a microstructured fiber

A femtosecond light source comprising two high-intensity beams at different wavelengths is applied to coherent anti-Stokes Raman scattering microscopy. One wavelength is supplied by a Titanium?:?Sapphire oscillator, while the other is derived from that via soliton-self-frequency shift in a microstructured fiber. Clear CARS images are acquired with the frequency difference adjusted to resonances of polystyrene and lipids.     

Femtosecond stimulated Raman scattering picosecond molecular thermometry in condensed phases

We demonstrate the capability of femtosecond stimulated Raman scattering (FSRS) data to measure the temperature of condensed matter at the molecular vibrational level. We report the temperature dependence of Raman loss to Raman gain ratios for low frequency modes (<300 cm(-1)) in a CaCO3 single crystal from cryogenic to room temperature, which is shown to be in agreement with theoretical predictions. We also report the measurements of nonequilibrium time evolution of mode specific vibrational temperatures in the CaCO3 single crystal to demonstrate that FSRS can measure temperature on picosecond time scales. Finally, we point out the unique origin of this temperature dependent anti-Stokes to Stokes ratio in stimulated Raman, which is not present in other coherent Raman spectroscopies. These measurements require no material dependent parameters or prior calibration.     

High power tunable picosecond green laser pulse generation by frequency doubling of an Yb-doped fiber power amplifier seeded by a gain switch laser diode

A compact tunable high power picosecond green laser pulse source based on frequency doubling of an Yb-doped fiber amplifier seeded by a gain switch laser diode has been developed. The fiber amplifier generates the picosecond infrared pulses with average power of 10.3 W, repetition rate of 1 MHz, pulse duration of 150 ps, and tunable range of 20 nm around 1064 nm. For underwater use, the tunable output infrared pulses are frequency doubled into picosecond green laser pulses, which can be tuned from 527 to 537 nm with average power of more than 1.1 W, corresponding to an overall conversion efficiency of 10.7% by a BBO nonlinear crystal. This kind of laser source will have potential application for underwater optical communication.     

Four-wave mixing: Photon statistics and the impact on a co-propagating quantum signal

We reconstruct the photon statistics of a single-photon source based on the stimulated four-wave mixing (FWM) process. We find that the statistics of the source goes from thermal, at a low power regime, to Poissonian, at a high power regime. We also study the impact of the FWM process on a co-propagating coherent quantum signal in a wavelength-division multiplexed (WDM) lightwave system. The statistics of the quantum signal changes from Poissonian to thermal when a low power is used, and remains Poissonian for a higher power. A multithermal regime is also observed when a moderate power is used.     

Fiber parametric oscillator for the 2 μm wavelength range based on narrowband optical parametric amplification

We describe a widely tunable synchronously pumped coherent source based on the process of narrowband parametric amplification in a dispersion-shifted fiber. Using an experimental fiber with a zero-dispersion wavelength of 1590 nm and pump wavelengths of 1530 to 1570 nm yields oscillations at 1970 to 2140 nm-the longest reported wavelength for a fiber parametric oscillator. The long-wavelength oscillations are accompanied by simultaneous short-wavelength oscillations at 1200 to 1290 nm. The parametric gain is coupled to stimulated Raman scattering. For parametric oscillations close to the Raman gain peak, the two gain processes must be discriminated from each other. We devised two configurations that achieve this discrimination: one is based on the exploitation of the difference in group delay between the wavelengths where Raman and parametric gain peak, and the other uses intracavity polarization tuning.     

Analysis of bottlenecks in DWDM fiber optic communication system

This paper deals with the limiting factors like Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Four Wave Mixing (FWM) in DWDM Fiber Optic Communication. The variation of SBS threshold power level for the spectral width of the source is analyzed and it is found that addition of spectral width in the source lowers the SBS effect. By reducing the SBS effect, we could transmit more optical power in a single channel and maintain high OSNR. Similarly the effect of phase mismatching (dispersion) and wavelength spacing on both FWM and SRS are analyzed. This work aims at finding the optimal value of spectral width, dispersion and channel spacing, thereby exploring the possibilities of increasing the overall capacity of the fiber optic trunk line.     

A picosecond near-infrared laser source based on a self-seeded optical parametric generator

We report on the design and development of a new type of near-IR laser source. The source comprises of an optical parametric generator (OPG) and a second harmonic generator (SHG) pumped by an 80-MHz, 1064-nm, 7-ps Nd:YVO₄ laser. The OPG is self-seeded with a fraction of its own signal output, which significantly enhances its conversion efficiency. The SHG doubles the frequency of OPG signal to produce a coherent output. The final output beam has a tunable wavelength near 800 nm, an average power of over 1 W, and a pulse duration around 5 ps. The M²-factor of the output beam can reach 1.1 after spatial filtering. With the new laser source, we have successfully demonstrated coherent anti-Stokes Raman scattering microscopy on 1 μm polystyrene beads, which shows that it has the potential to be a substitute for a picosecond optical parametric oscillator in certain microscopy or spectroscopy applications.     

Generation of a compact high-power high-efficiency normal-dispersion pumping supercontinuum in silica photonic crystal fiber pumped with a 1064-nm picosecond pulse

Broadband normal dispersion pumping supercontinuum （SC） generation in silica photonic crystal fiber （PCF） is investigated in this paper. A 1064-nm picosecond fiber laser is used to pump silica PCF for the SC generation. The length of PCF is optimized for the most efficient stimulated Raman scattering process in the picosecond pump pulse region. The first stimulated Raman Stokes peak is located in the anomalous dispersion regime of the PCF and near the zero dispersion wavelength; thus the SC generation process can benefit from both a normal dispersion pumping scheme and an anomalous dispersion pumping scheme. The 51.7-W SC spanning from about 700 nm to beyond 1700 nm is generated with an all-fiber configuration, and the pump-to-SC conversion efficiency is up to 90%. In order to avoid the output fiber end face damage and increase the stability of the system, an improved output solution for the high power SC is proposed in our experiment. This high-efficiency near-infrared SC source is very suitable for applications in which average output power and spectral power density are firstly desirable.     

Multi-wavelength laser source based on enhanced four-wave-mixing effect in a highly nonlinear fiber

We propose a multi-wavelength laser source based on an enhanced four-wave-mixing (FWM) effect which is achieved when a highly nonlinear fiber (HNLF) and an Erbium-doped fiber amplifier (EDFA) are employed in a fiber loop. A multi-wavelength laser source with totally 28 new wavelengths and a wavelength spacing of 0.8 nm is demonstrated. The wavelength spacing of the proposed multi-wavelength laser source can be tuned from 0.1 to 5.0 nm, and output spectrum of the multi-wavelength laser source with the wavelength spacing of 0.4, 0.8, and 1.6 nm are presented.     

Comparison of the compensation effects of fiber nonlinear impairments with mid-span optical phase conjugation between PDM CO-OFDM system and PDM QPSK system

The compensation effects of fiber nonlinearity in 112 Gb/s polarization division multiplexing(PDM) coherent optical systems by mid-span optical phase conjugation(OPC) based on four wave mixing(FWM) effect are studied. Comparisons of the compensation results between PDM coherent optical-orthogonal frequency division multiplexing(CO-OFDM)system and the single carrier(SC) PDM quadrature phase shift keying(QPSK) system are provided as well. The results demonstrate that nonlinear compensation effect with mid-span OPC in PDM CO-OFDM system is much more obvious than that in SC PDM QPSK system.     

Temperature-insensitive strain sensor based on four-wave mixing using Raman fiber Bragg grating laser sensor with cooperative Rayleigh scattering

A temperature-insensitive strain sensor based on Four-Wave Mixing (FWM) using two Raman fiber Bragg grating (FBG) lasers with cooperative Rayleigh scattering is proposed. Two FBG were used to form two linear cavities laser sensors based on Raman amplification combined with cooperative Rayleigh scattering. Due to the very low dispersion coefficient of the fiber, it is possible to obtain the FWM using the two lasers. This configuration allows the operation as a temperature-insensitive strain sensor where both sensors have the same sensitivity to temperature but only one of the FBG laser is sensitive to strain. The difference between the wavelengths of the signal sensor and the converted signal presents a strain coefficient sensitivity of 2?pm/??? with insensitivity to temperature. The FWM efficiency is also dependent on the applied strain, but it is temperature independent, presenting a maximum sensibility of 0.01?dB/???.     

Supercontinuum generated in all-normal dispersion photonic crystal fibers with picosecond pump pulses

The supercontinuum (SC) generation in all-normal dispersion (ANDi) photonic-crystal fiber (PCF) pumped by high power picosecond pulses are investigated in this paper. Our results show that an octave SC may be achieved by pumping the ANDi PCF with picosecond pump pulses. However, the PCF length required may have to be lengthened to several tens of centimeters, which is much longer than that with femtosecond pump pulses. The relatively long PCF gives rise to much higher Raman gain and stronger Raman frequency shift compared to those with femtosecond pump pulses, which in turn not only cause a distorted temporal waveform and an un-flattened spectrum, but also severely degrade the coherence of the generated SC.     

Picosecond ultrasonics time resolved spectroscopy using a photonic crystal fiber

We present a broadband picosecond ultrasonics time resolved spectroscopy. Detection of picosecond coherent acoustic phonons using a wavelength continuum generation in a photonic crystal fiber (PCF) with femtosecond laser pulses is developed. Measurements are performed for selected wavelengths of a broad wavelength probe pulse within a bandwidth of 250 nm with an 825 nm center wavelength on two samples made of tungsten and of gallium arsenide.     

Red picosecond pulses generated by frequency doubling a Raman amplified widely tunable 1.3 microm fiber ring laser

We generated 0.66 microm picosecond pulses by second-harmonic generation of the Raman amplified output of a 1.3 microm actively mode-locked fiber ring laser in a periodically poled potassium titanyl phosphate (PPKTP) waveguide. The ring laser produced 9 ps pulses at a 20 GHz repetition frequency, was tunable over 1284-1330 nm, and was based on a semiconductor optical amplifier and a Mach-Zehnder amplitude modulator. The Raman amplifier served both to amplify the ring laser and to compress the pulses as solitons. The spectral flexibility of the amplifiers and the modulator should enable similar configurations to be made at other wavelengths and facilitate efficient frequency doubling in waveguides to other visible wavelengths.