Pongamol Prevents Neurotoxicity via the Activation of MAPKs/Nrf2 Signaling Pathway in H2O2-Induced Neuronal PC12 Cells and Prolongs the Lifespan of Caenorhabditis elegans.

Despite tremendous advances in modern medicine, effective prevention or therapeutic strategies for age-related neurodegenerative diseases such as Alzheimer's disease (AD) remain limited. Growing evidence now suggests that oxidative stress and apoptosis are increasingly associated with AD as promising therapeutic targets. Pongamol, a flavonoid, is the main constituent of pongamia pinnata and possesses a variety of pharmacological activities such as antioxidant, anti-aging and anti-inflammatory. In the present study, we investigated the antioxidant effects and mechanisms of pongamol in H2O2-induced PC12 cells and Caenorhabditis elegans (C. elegans). Our findings revealed that pongamol reduced cellular damage and apoptosis in H2O2-induced PC12 cells. Furthermore, pongamol reduced levels of apoptosis-related proteins Bax, Cyto C, Cleaved Caspase-3, and Cleaved PARP1, and increased the level of anti-apoptotic protein Bcl-2. Pongamol also effectively attenuated the level of oxidative stress markers such as glutathione (GSH) and reactive oxygen species (ROS) in H2O2-induced PC12 cells. Additionally, pongamol possessed antioxidant activity in H2O2-induced PC12 cells through the MAPKs/Nrf2 signaling pathway. Furthermore, pongamol exerted neuroprotective and anti-aging effects in C. elegans. All together, these results suggested that pongamol has a potential neuroprotective effect through the modulation of MAPKs/Nrf2 signaling pathway.

Monolithic 2-µm single-frequency linearly-polarized gain-switched distributed feedback fiber laser by femtosecond laser direct-writing.

We report a single-frequency, linearly polarized gain-switched, distributed feedback (DFB), 2-µm thulium doped silica fiber laser (TDFL), with an effective cavity length of 2.5 mm. The cavity is based on a heavily thulium doped non-polarization-maintaining silica fiber and composed of a π-phase-shifted fiber Bragg grating (FBG) with a total FBG length of 35 mm. The DFB FBG was written by femtosecond-laser point-by-point (PbP) method. In-band pumping scheme is chosen with a 1550 nm nanosecond pulsed erbium-doped silica fiber laser pump. Single-longitudinal, linearly polarized, gain-switched TDFL at 2002 nm, with a recorded shortest pulse duration of 4.7 ns, a repetition rate of 20 kHz, a maximum peak power of 170 W, and single pulse energy of 0.8 µJ, has been obtained, benefitting from the ultrashort DFB cavity made by the femtosecond laser direct-writing method.

Triple-wavelength laser from a femtosecond laser directly-written fiber cavity for microwave generation.

We report an all-fiber triple-wavelength single-frequency distributed Bragg reflector (DBR) laser for 1-10 GHz microwave (MW) generation. The DBR cavity contains a non-polarization-maintaining Er fiber and a pair of fiber Bragg gratings (FBGs) made using a femtosecond laser line-by-line (LbL) direct-writing method. Such a configuration combining a short cavity and radial asymmetry leads to frequency locking and phase controlling of multi-wavelength fiber lasers. A 1.59-µm triple-wavelength laser with high coherence, spectrum purity, and polarization purity has been demonstrated; 3-6 GHz triple-frequency MW was generated.

Flexible chalcogenide glass large-core multimode fibers for hundred-watt-level mid-infrared 2-5 µm laser transmission.

The rapidly-developed high-power mid-infrared 2-5 µm laser technology requires a compact, flexible low-loss glass fiber for power delivery or laser generation. With the broadest bandwidth of low-loss transmission window in mid-infrared region amongst all mid-infrared glass fibers, chalcogenide glass fiber is the best candidate covering the whole 2-5 µm range. Multi-hundred-watt high-power delivery for 5.4-µm CO laser was previously demonstrated in a multimode chalcogenide fiber with a 1-mm-diameter large core, at the cost of giving up one of the most desirable fiber advantages, the flexibility. Indeed, chalcogenide glass fibers with decent flexibility have never exhibited hundred-watt-level power transmitting capability in the 2-5 µm range. In this paper, we have experimentally demonstrated 100-watt-level power transmission in multimode As2S3 chalcogenide fibers, using a customized high-power 2-µm thulium doped silica fiber laser source. With effective forced cooling, the multimode As2S3 fiber with 200 µm core diameter can resist incident laser power of 120 W and deliver transmitted power of 63 W. Nano-sized scattering center related laser damage mechanism and the cylindrical heat transfer model have been proposed to explain the high-power damage process of chalcogenide glass fibers. The calculation is in good agreement with the experiments. It is promising to further enhance the transmitted power above 100 W in flexible chalcogenide glass large-core fibers.

Few-moded ultralarge mode area chalcogenide photonic crystal fiber for mid-infrared high power applications.

We demonstrate a novel few-moded ultralarge mode area chalcogenide glass photonic crystal fiber for mid-infrared high power applications. The numerical simulation indicates that the fiber has ultralarge mode areas of ∼10500 µm2 and ∼12000 µm2 for the fundamental mode LP01 and the lowest higher-order mode LP11, respectively. Dual-moded operation is confirmed experimentally at 2 µm, in good agreement with the numerical simulation. By selectively launching technique, low bending loss of 0.7 dB/m, equivalent to 0.55 dB/turn, has been observed in the fiber with a small bending radius of ∼12 cm, indicating excellent bending resistance of the few-moded fiber with such a large mode area. The fiber has been demonstrated to sustain an incident power density up to 150 kW/cm2 under 2-µm CW laser irradiation, showing the potential of the fiber for high-power applications in mid-infrared.

High-precision fiber Bragg gratings inscription by infrared femtosecond laser direct-writing method assisted with image recognition.

For achieving high efficiency fiber Bragg gratings (FBGs) utilizing infrared femtosecond laser point-by-point inscription method, it is crucial to make the inscribed periodic structure perfectly in phase. It requires a perfect alignment between the micrometer-sized laser spot with the fiber along the length. Here we report the highly precise fabrication of FBGs by infrared femtosecond laser point-by-point direct-writing method. Image recognition technique is applied to for automatically aligning the trace of the laser spot with the referenced central axis of the fiber along the whole FBG length. FBGs inscription with high spatial precision is confirmed by multiple approaches, including microscopic photographing and FBG spectroscopic measurement. 50 mm-long uniform FBGs with high reflectivity have been successfully demonstrated in a small-core single-mode silica fiber using auto-aligning technique.

All-solid mid-infrared chalcogenide photonic crystal fiber with ultralarge mode area.

In this Letter, we report, to the best of our knowledge, the largest effective single-mode mid-infrared chalcogenide (ChG) fiber. Two thermally matched ChG glasses with a large index contrast of ∼0.35 are chosen for constructing all-solid photonic crystal fiber (PCF) with two rings of "holey" microstructured cladding. Single-mode operation is confirmed in the fabricated ChG PCF with a core diameter of 91.2 µm, from both near-field and far-field observation at 4 µm. Ultralow numerical aperture of <0.025 has been observed at wavelengths between 2.5 and 4.0 µm. The fundamental mode area is calculated to be 5200 µm2 at 4 µm, where the propagation loss of the ChG PCF is measured to be 5.2 dB/m. Broadband 3.5 and 7.5 µm supercontinuum has been generated in a 35-cm-long ultralarge-mode-area PCF.

Versatile mode-locked fiber laser with switchable operation states of bound solitons.

Bound states of two solitons are among the typical forms of bound states and can be observed in various operation states of mode-locked fiber lasers. We experimentally investigated bound solitons (BSs) in a passively mode-locked erbium-doped fiber laser based on a semiconductor saturable absorber mirror, whose operation states can be switched among multiple pulses, passively harmonic mode-locking, and "giant pulses" by simply adjusting the in-line polarization controller with the pump power fixed. Up to four pulses, fourth-order harmonic mode-locking (HML), and a "giant pulse" with four BSs were obtained with increasing pump power. Experimental results showed a correlative relationship among those operation states (N pulses/Nth-order HML/"giant pulses" of N bound solitons) at different pump power levels. The birefringence induced by the erbium-doped fiber inside the laser cavity played a vital role in the transitions of those operation states.

Tunable passively harmonic mode-locked Yb-doped fiber laser with Lyot-Sagnac filter.

A novel passively harmonic mode-locked dissipative soliton Yb-doped fiber laser with all normal dispersion is proposed and experimentally demonstrated based on a semiconductor saturable absorption mirror and tunable Lyot-Sagnac filter. By only tuning the bandwidth of the filter at fixed pump power, the repetition rate of 9.87 to 167.8 MHz (corresponding to 17th-order harmonic) is obtained. This is the highest repetition rate and harmonic order for a passively harmonic mode-locked dissipative soliton Yb-doped fiber laser with all-normal dispersion to the best of our knowledge. The signal-to-noise ratio and super-mode suppression ratio for all harmonic orders are higher than 65 and 35 dB, respectively, which shows the high stability of the fiber laser.

Investigation on Nyquist pulse generation using a single dual-parallel Mach-Zehnder modulator.

The generation of Nyquist pulses with a dual parallel Mach-Zehnder modulator (DPMZM) driven by a single RF signal is demonstrated theoretically and experimentally. A complete theoretical analysis is developed and the limitation of the proposed scheme is also discussed. It is theoretically proved that Nyquist pulses with a spectrum of 5 flat comb lines can be generated using a single DPMZM, which is also verified with simulation. 7 flat comb lines in frequency domain can also be obtained if a large RF driving voltage is applied to DPMZM but the generated waveforms won't present a sinc-shape. This scheme is further investigated experimentally. 40 GHz Nyquist pulses with full-width-at-half-maximum (FWHM) less than 4.65 ps, signal-to-noise ratio (SNR) better than 29.5 dB, and normalized root-mean-square error (NRMSE) less than 2.4% are generated. It is found that a tradeoff exists between the insertion loss of the DPMZM and the deviation of generated pulses. The tunability of repetition rate is experimentally verified by generation of 1 GHz to 40 GHz Nyquist pulses with SNR better than 28.4 dB and NRMSE less than 6.15%.

Detailed study of four-wave mixing in Raman DFB fiber lasers.

We both experimentally and numerically studied the ultra-compact wavelength conversion by using the four-wave mixing (FWM) process in Raman distributed-feedback (R-DFB) fiber lasers. The R-DFB fiber laser is formed in a 30 cm-long commercially available Ge/Si standard optical fiber. The internal generated R-DFB signal acts as the pump wave for the FWM process and is in the normal dispersion range of the fiber. Utilizing a tunable laser source as a probe wave, FWM frequency conversion up to ~40 THz has been demonstrated with conversion efficiency > -40 dB. The principle of such a wide bandwidth and high conversion efficiency in such a short R-DFB cavity has been theoretically analyzed. The simulation results match well with the experimental data.

Halo-tellurite glass fiber with low OH content for 2-5µm mid-infrared nonlinear applications.

We report the fabrication of new dehydrated halo-tellurite glass fibers with low OH content (1ppm in weight) and low OH-induced attenuation of 10dB/m in 3-4 µm region. It shows halo-tellurite glass fibers a promising candidate for nonlinear applications in 2-5µm region.

Ultrawide-range four-wave mixing in Raman distributed-feedback fiber lasers.

We report ultrawide-range and highly efficient wavelength conversion by exploiting four-wave mixing (FWM) in Raman distributed-feedback (R-DFB) fiber lasers. The lasers are 30 cm long center π phase-shifted DFB gratings UV written in commercially available germano-silica (Ge/Si) single-mode fibers (PS980 from Fibercore Ltd., and UHNA4 from Nufern). The R-DFB lasing signal acts as a pump wave for the FWM process within the DFB cavity, and the obtained FWM conversion efficiency is around -25 dB with a maximum wavelength conversion range of 112 nm.

Laser-induced crystalline optical waveguide in glass fiber format.

We report on the first fabrication of a glass fiber based laser-induced crystalline waveguide. The glass and crystal are based on the stoichiometric composition of (La,Yb)BGeO(5). A laser induced waveguide has been fabricated on the surface of a ribbon glass fiber using milliwatt-level continuous wave UV laser radiation at a fast scanning speed. Evidence of crystallinity in the created structure was observed using micro-Raman spectroscopy and scanning electron microscopy. Preliminary investigations on the waveguiding behavior and the nonlinear performance in the crystalline waveguide are reported.

Phase regeneration of DPSK signals in a highly nonlinear lead-silicate W-type fiber.

We experimentally demonstrate phase regeneration of a 40-Gb/s differential phase shift keying (DPSK) signal in a 1.7-m long highly nonlinear lead silicate W-type fiber using a degenerate two-pump phase-sensitive amplifier (PSA). Results show an improvement in the Error Vector Magnitude (EVM) and a reduction of almost a factor of 2 in the phase noise of the signal after regeneration for various noise levels at the input.

Sub-watt threshold, kilohertz-linewidth Raman distributed-feedback fiber laser.

We report a low-threshold, narrow linewidth Raman distributed-feedback (R-DFB) fiber laser at 1109.54 nm based on a 30 cm long center π phase-shifted Bragg grating written directly in a commercially available germano-silica (Ge/Si) fiber. The R-DFB was pumped by a continuous-wave (CW) linearly polarized fiber source at 1064 nm, and the threshold power, full width at half-maximum (FWHM) linewidth and slope efficiency with respect to the incident pump power are measured to be 440 mW, <2.5 kHz (measured with a 29.75 km fiber delay line) and 13.5%, respectively. An rf spectrum analyzer confirms that the R-DFB fiber laser exhibits single-frequency performance.

Highly efficient Raman distributed feedback fibre lasers.

We demonstrate highly efficient Raman distributed feedback (DFB) fibre lasers for the first time with up to 1.6 W of continuous wave (CW) output power. The DFB Bragg gratings are written directly into two types of commercially available passive germano-silica fibres. Two lasers of 30 cm length are pumped with up to 15 W of CW power at 1068 nm. The threshold power is ~2 W for a Raman-DFB (R-DFB) laser written in standard low-NA fibre, and only ~1 W for a laser written in a high-NA fibre, both of which oscillate in a narrow linewidth of <0.01 nm at ~1117 nm and ~1109 nm, respectively. The slope efficiencies are ~74% and ~93% with respect to absorbed pump power in the low-NA fibre and high-NA fibre respectively. Such high conversion efficiency suggests that very little energy is lost in the form of heat through inefficient energy transfer. Our results are supported by numerical simulations, and furthermore open up for the possibility of having narrow linewidth all-fibre laser sources in wavelength bands not traditionally covered by rare-earth doped silica fibres. Simulations also imply that this technology has the potential to produce even shorter R-DFB laser devices at the centimetre-level and with mW-level thresholds, if Bragg gratings formed in fibre materials with higher intrinsic Raman gain coefficient than silica are used. These materials include for example tellurite or chalcogenide glasses. Using glasses like these would also open up the possibility of having narrow linewidth fibre sources with DFB laser oscillating much further into the IR than what currently is possible with rare-earth doped silica glasses.

Phase sensitive amplification in a highly nonlinear lead-silicate fiber.

We experimentally demonstrate phase-sensitive amplification in a highly nonlinear and low-dispersion lead-silicate W-type fiber. A phase-sensitive gain variation of 6 dB was observed in a 1.56-m sample of the fiber for a total input pump power of 27.7 dBm.