Optical metasurfaces are endowed with unparallel flexibility to manipulate the light field with a subwavelength spatial resolution. Coupling metasurfaces to materials with strong optical nonlinearity may allow ultrafast spatiotemporal light field modulation. However, most metasurfaces demonstrated thus far are linear devices. Here, we experimentally demonstrate simultaneous spatiotemporal laser mode control using a single-layer plasmonic metasurface strongly coupled to an epsilon-near-zero (ENZ) material within a fiber laser cavity. While the geometric phase of the metasurface is utilized to convert the laser’s transverse mode from a Gaussian beam to a vortex beam carrying orbital angular momentum, the giant nonlinear saturable absorption of the ENZ material enables pulsed laser generation via the Q-switching process. The direct integration of a spatiotemporal metasurface in a laser cavity may pave the way for the development of miniaturized laser sources with tailored spatial and temporal profiles, which can be useful for numerous applications, such as superresolution imaging, high-density optical storage, and three-dimensional laser lithography.
We report on the first experimental investigation on the dissipative soliton pulsation in a Mamyshev oscillator. We observe period-2 pulsation in a single pulse state as well as a bound soliton (soliton molecule) state when the filter separation (4 nm) is relatively small. Single-shot spectra measured by the time-stretch dispersive Fourier transform (TSDFT) method also reveal the spectral bandwidth breathing during pulsation. Period-3 pulsation of a soliton state comprising two wide spaced DSs has also been observed, where the 2 DSs pulsate in different patterns. Our observations enrich possible pulsating dynamics in mode-locked fiber lasers.
We report the periodic intensity fluctuations and period-timing bifurcation in a spatiotemporal mode-locked multimode fiber laser. By adjusting the intracavity waveplate or the pump power, the intensity pattern of the spatiotemporal modelocked output could transit from uniformity to period-2, then to period-4. We further find that, in the state of multipleperiod pulsations, the extent of intensity fluctuation of different transverse modes varies. This indicates that not only the entire energy, but also the shape of the output spatial profile alters with multiple-period. A simple model is proposed to interpret the output pulse dynamics of multimode fiber lasers. This investigation would contribute to the understanding of the complex nonlinear dynamics in spatiotemporal mode-locked multimode fiber lasers.
Nonlinear microscopies based on coherent anti-Stokes Raman scattering (CARS) and stimulated Raman
scattering (SRS) have demonstrated important capabilities for
non-invasive label-free imaging of biological
samples [1, 2]. In contrast to other nonlinear microscopies, Raman imaging is best performed with
excitation pulses in the 2-7 ps range, which overlap spectrally with the desired Raman resonances. There is
interest in performing CARS and SRS imaging with femtosecond pulses or picosecond/femtosecond
combinations, or with chirped femtosecond pulses. This article will outline the challenges associated with
the demonstration of a picosecond fiber source for Raman microscopies, and assess approaches to such a
source.
A new method to achieve wideband continuously tunable delays in an all-fiber system is proposed, which uses dispersion in the optical fiber and spectrum filtering of supercontinuum generation of the ultra-short signal pulse in highly nonlinear fiber. A delay of 3.175 ns is experimentally demonstrated, and the potential delay up to 800 ns can be achieved using large dispersion fiber and wideband supercontinuum (SC) generation up to several hundreds of nanometers.
We present an ultrafast depolarizer based on four-wave mixing (FWM) in
highly nonlinear optical fiber (HNLF), in which a completely polarized laser beam with
any fixed state of polarization (SOP) mixed with an unpolarized pump can be
instantaneously converted into an unpolarized idler wave. The degree-of-polarization
(DOP) of the idler wave is experimentally measured to be 0.33 when the DOPs of the
polarized signal and unpolarized pump are 0.986 and 0.036, respectively. We analyze the
beam couplings between a polarized beam and a completely unpolarized pump in the
ultrafast depolarizer by using vector FWM theory. The nonzero DOP of the idler wave
ascribes to the polarization dependence of the FWM conversion efficiency. The speed of
the depolarizer is determined by the intrinsic nonlinear response of silica, which could be
just a few femtoseconds.
A multi-channel surface-enhanced Raman scattering (SERS) probe based on a multi-core photonic crystal fiber (PCF) is
investigated. The multi-core fiber contains seven hollow core PCFs arranging in a compact hexagon pattern that one PCF
is in the center with six other surrounded, and the total diameter including the protective jacket is less than 200μm. The
seven PCFs can guide light respectively, providing seven channels for sensing. The excitation light is coupled into each
core from one end (measuring-tip) of the fiber while the sample entrance is at the other end (probing-tip). Analyte
solution mixed with the silver nanoparticles enters each core via the capillary effect, and the silver nanoparticles serve as
the SERS substrate. The excitation light transmitting in each hollow core can interact directly with the analyte and the
silver nanoparticles in the air cores along the fiber axis. The SERS signal scattered by the sample propagates through the
fiber back to the measuring-tip; then couples out of the fiber into the Raman spectrometer. Comparing to a single core
PCF SERS probe, the multi-core probe not only takes the advantages of high efficiency of light usage and large
interaction space for SERS in each channel, but also is more robust and could provide multi-data. Basing on the seven
data of the analyte from the seven channels, an accurate average result could be achieved with less instability. Different
concentration Rhodamine 6G solutions have been used as test samples, and the multi-channel sensing idea has been
demonstrated by the proof-of-concept experiments.
Detection of biological samples in low concentration is of great significance to the basic research in science, the
development of medical technology and many other fields related to our lives. Surface-Enhanced Raman Scattering
(SERS), well-known as a powerful analytical tool with high sensitivity, is especially suitable for biomolecule detection
as it enables near infrared (NIR) excitation and label-free detection. SERS probe made of conventional optical fiber
provides better flexibility in detection; however, it requires a complicated fabrication process and doesn't serve as a
well-set detecting platform. In this talk we propose and demonstrate a photonic crystal fiber (PCF) based SERS probe,
which has the new advantages of simplicity in fabrication, better light confinement and increased light-analyte
interaction volume. The PCF-based SERS probes are prepared in three different ways: mixed solution of sample and
gold nanoparticles filled in air holes of PCF, sample solution dried in gold coated air holes and sample solution filled in
gold coated air holes, respectively. Sample solution of adenine is in concentration of about 10-6M. Almost every
characteristic peak of adenine can be observed in the spectra detected by each of the three probes.
We demonstrate, for the first time to our knowledge, a hollow core photonic crystal fiber
(HCPCF) surface-enhanced Raman probe. The probe consists of a HCPCF (also known as a
holey fiber) with a layer of Au nanoparticles coated on the inner surface of the air holes serving
as the substrate of surface enhanced Raman scattering (SERS). The sample being tested enters
the air holes by the capillary effect. The excitation light is coupled into the fiber core from one
end (measuring-tip) while the sample entrance is at the other end (probing-tip) of the fiber. The
SERS signal scattered by the sample propagates through the fiber core back to the measuring-tip;
then is coupled out of the fiber into the Raman spectrometer. The advantages of such probes
include, the confinement of light inside the HCPCF provides a higher light efficiency; and the
nanoparticles coated inside the air holes offers a larger interaction area for SERS. Both
experimental results and theoretical analysis are presented and discussed.
Polarization mode dispersion (PMD) is becoming major system impairment in high speed and long distance optical fiber transmission systems. As the bit rate climbs from 10 to 40Gb/s per channel and beyond, optical pulses are increasingly distorted by 1st and higher order PMD. We report on the experimental mitigation of pulse distortion due to 1st and higher order PMD effect based on one tunable differential group delay (DGD) element, which is a compact concatenation via six magneto-optic polarization rotators (Faraday rotators) of six YVO4 birefringence crystals whose lengths decrease in a binary power series. Two different experiments are carried out, with and without an electric polarization controller set before the tunable DGD element. Optical pulses with width of 41ps are broadened and distorted by the PMD emulator which generates 1st and 2nd order PMD with mean magnitude of 30.28 ps and 483.31 ps2, respectively, and then reshaped by the compensation device. Degree of polarization (DOP) is used as the feedback signal, which is significantly increased from around 0.15 to around 0.85. The experiment results show that pulse distortion due to 1st and higher order PMD is successfully mitigated.
A database is founded to estimate the nonlinear impairments of single channel return-to-zero (RZ) transmission systems with arbitrary parameters. Our link setup consists of multi-span transmission fiber and dispersion compensation component, with dispersion slope full compensating. The total transmission distance and the transmission fiber could be arbitrary, i.e. with arbitrary nonlinear coefficient, attenuation and dispersion. The parameters of input signal could also be arbitrary, i.e. with arbitrary transmission rate, input power, and any duty cycle of RZ pulse. Even the amplification schemes could be single-stage erbium-doped fiber amplifier (EDFA), or double-stage EDFA, or hybrid EDFA-Raman amplification. The database is obtained by numerous numerical simulations, neglecting the amplified spontaneous emission noise, polarization mode dispersion. The performance of transmission system impaired by nonlinear effects is represented by eye opening penalty (EOP). One can use the database to estimate the EOP of single channel transmission systems with bit rate up to 160 Gb/s, and can also evaluate the maximum transmission distance for given system impairment conveniently. For high-bit-rate transmission systems, intra-channel nonlinearity dominates the nonlinear impairments, therefore, the database to asses the nonlinear impairment of single channel transmission link is also valid for high-bit-rate wavelength-division multiplexed (WDM) systems. The accuracy and the limitation of the database are also discussed.
We demonstrate that all the inter- and intra-channel nonlinear impairments can be eliminated simultaneously by optical phase conjugation (OPC) in a power-symmetry system. However, for practical systems without power-symmetry, it is found that the effects of OPC on various nonlinearities are different in the same link. Even some nonlinearities are suppressed and some are enhanced. Therefore, optimizing the transmission link with OPC to suppress the dominant
nonlinearity is demanded. By using 1-km-long highly nonlinear fiber (HNLF) and tuning the pump wavelength near the zero dispersion wavelength of the HNLF, we experimentally generate the phase conjugation of the dispersed ~300 fs pulses. OPC with conversion efficiency of about -16 dB and conversion bandwidth of about 38 nm is obtained.
Polarization mode dispersion is becoming major system impairment in high speed and long distance optical fiber transmission systems. As the bit rate climbs from 10 to 40 Gb/s per wavelength division multiplexed channel and beyond, optical pulses are increasingly distorted by polarization mode dispersion effect. We report on polarization mode dispersion compensation experiments in 10 Gb/s, 40 Gb/s optical communication systems. The polarization mode dispersion compensator used in the experiments is a compact variable differential group delay element base on concatenation via six magneto-optic polarization rotators (Faraday rotators) of six YVO4 birefringence crystals whose lengths decrease in a binary power series. Feedback scheme is used to optimize the performance of polarization mode dispersion compensation, using degree of polarization as the feedback signal. In the experiments in 10 Gb/s and 40 Gb/s optical transmission systems, eye-diagrams and bit error rate curves of the code sequences before and after polarization mode dispersion compensation are analyzed. The experimental results demonstrate that the polarization mode dispersion effect induced by the polarization mode dispersion emulator is feasibly mitigated. Separate experiment to reshape the 39ps pulses distorted by polarization mode dispersion is also carried out. The incident optical pulses with width of 39ps are broadened and distorted by polarization mode dispersion effect and then reshaped by the polarization mode dispersion compensator. The relationship between the feedback signal degree of polarization and differential group delay is also analyzed.
We investigate response flatting of cascaded sum- and difference- frequency generation (SFG/DFG)-based wavelength conversion in quasi-phase-matched (QPM) periodically poled lithium niobate waveguides for wavelength division multiplexing optical communication systems. The cascaded SFG/DFG-based configuration shows more robustly combined properties than conventional DFG-based and cascaded second-harmonic generation and DFG (SHG/DFG)-based configurations. The response fluctuation, generated while enhancing the conversion bandwidth, can be efficiently reduced by the method of pump detuning or QPM period detuning.
The PMD at a wide range of optical frequencies and statistics of all-order PMD, especially the first-, second- and third-order PMD, of a n-segment DGD units combination separated by PCs or PRs are fully analysed with Poincare sphere, Mueller matrix and Fourier transform methods. The results show that, to simulate all-order PMD in DWDM systems as truly as possible, the number of n should be as large as possible, the least value of 4 is preferred. The DGD units should be variable or unequal. Choosing PC or PR just affects the probability density function of PMD when n is not too much and the DGD values of the DGD units are unchangeable. At last, a simple, practical, and low cost all-order PMD emulator in DWDM systems is proposed.
We report on the birefringence of photonic crystal fibers as functions of fiber structure parameters and incident wavelength. It is found that the sign of the fiber birefringence can be changed by controlling the incident wavelength. The guided-mode coupling properties of the dual-core photonic crystal fibers are highly engineerable by tailoring the birefringence. New types of polarization splitters based on the photonic crystal fibers are presented.
We investigate wavelength conversions based on chirped optical superlattices, such as linearly chirped and sinusoidally chirped optical superlattices, through cascaded second-order nonlinear processes for wavelength division multiplexing application. They are compared with periodic optical superlattices in terms of bandwidth, efficiency and response flatness. The sinusoidally chirped optical superlattice shows more excellent overall performance than the periodic and linearly chirped optical superlattices.
We investigate the four-wave mixing (FWM) effect in a dispersion-managed transmission line. The dispersion-managed line consists of many repeated fiber spans, each of which includes a positive- and a negative-dispersion fiber. The analytical expression of the FWM is obtained. The influence of the channel spacing, the fiber spans’ number and the compensating dispersion parameter on the FWM effect is analysed. The dispersion-managed transmission line is optimized to depress the generated FWM noise.
We have demonstrated efficient amplification of 830 nm and 1.06 micrometers light in a ring resonator using Rh:BaTiO3. The power oscillating inside the ring exceeded the pump power by up to a factor of 2.3 at 1.06 micrometers . We have also showed that such an efficiently working photorefractive cavity is also sensitive to nanometer changes in its cavity length. We have also observed simultaneous, bi-directional and counterpropagating oscillations in a resonator pumped by a single 647 nm pump beam. The intensity of one oscillation beam was up to two orders of magnitude higher than the intensity of the other oscillation beam.
We propose and demonstrate a new configuration of mutual- coherent-pumped phase conjugation with Ce:BaTiO3. Two coherent beams are incident into a crystal and overlap inside the crystal. The two input beams undergo stimulated photorefractive backscatterings in separated regions, in which their corresponding phase conjugations are generated. Then the four waves in the crystal undergo four-wave mixing. The intensity ratio as well as the interference modulation of the two phase conjugations is characterized as a function of the intensity ratio of the input beams. The phase conjugator exhibits a large intensity dynamic range. We theoretically analyze the modulation transfer function of the mutual-coherent-pumped phase conjugator with Ce:BaTiO3. The theoretical analysis is in agreement with the experimental results.
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