Multi-longitudinal mode fiber laser digital vibration-sensing system based on a multi-carrier modulation/demodulation technique.

A multi-longitudinal mode fiber laser sensor (MLMFLS) system based on a digital modulation/demodulation technique is proposed. To the best of the authors' knowledge, this is the first digital vibration-demodulation system ever reported by a MLMFLS system. Multiple beat frequency signals (BFS) generated by the MLMFLS work as signal carriers of applied vibration signals. The vibration signals modulated on several BFS at different frequencies are simultaneously demodulated based on a multi-channel digital down-conversion technique by utilizing the digital universal software radio peripheral (USRP). The demodulated vibration signals from different signal channels are superposed by the USRP. Signal-to-noise ratio (SNR) of output vibration signal is highly improved, which increases sensing stability and accuracy of the system. This is really important for sensing of vibration signals that have an extremely low frequency or weak energy. Measurement results demonstrate that the sensing system has an excellent low frequency vibration-sensing capability, simple structure, high SNR, stability and accuracy.

Multi-longitudinal mode fiber laser sensor system with high signal-to-noise ratio based on laser modes control.

A multi-longitudinal mode fiber laser sensor (MLMFLS) system with high signal-to-noise ratio (SNR) is proposed. To the best of authors' knowledge, this is the highest SNR ever reported by MLMFLS system. The laser sensor consists of a narrow-band fiber Bragg grating (FBG), erbium-doped fiber (EDF) and an optical reflector. The FBG controls the number of laser modes which excited by laser sensor. Beat frequency signals (BFS) modulated by applied measurand are generated by those laser modes and their SNR are controlled by the FBG. Measurement results show the BFS have SNR of 65.0 dB, accuracy of ± 2.0 µÎµ, and stability of 0.4 kHz, which demonstrate its high SNR, accuracy and stability.

Synthesis, Crystal Structures and Urease Inhibition of N'-(2-Bromobenzylidene)-2-(4-nitrophenoxy) acetohydrazide and N'-(4-Nitrobenzylidene) -2-(4-nitrophenoxy)acetohydrazide.

Two new hydrazone compounds, N'-(2-bromobenzylidene)-2-(4-nitrophenoxy)acetohydrazide (1) and N'-(4-nitrobenzylidene)-2-(4-nitrophenoxy)acetohydrazide (2), were prepared and structurally characterized by elemental analysis, IR, UV-Vis and (1)H NMR spectroscopy, and single-crystal X-ray diffraction. Compound 1 crystallizes in the monoclinic space group P2(1)/n with unit cell dimensions of a = 5.3064(5) Å, b = 18.202(2) Å, c = 15.970(2) Å, ß = 95.866(3)º, V = 1534.4(2) Å(3), Z = 4, R(1) = 0.0457, and wR(2) = 0.0975. Compound 2 crystallizes in the monoclinic space group P2(1)/c with unit cell dimensions of a = 4.6008(7) Å, b = 14.451(2) Å, c = 23.296(3) Å, ß = 93.620(2)º, V = 1545.8(4) Å(3), Z = 4, R(1) = 0.0441, and wR2 = 0.0985. Structures of the compounds are stabilized by hydrogen bonds and π···π interactions. The urease inhibitory activities of the compounds were studied. Both compounds show strong urease inhibitory activities, with IC(50) values of 8.4 and 20.2 µM, respectively.

Femtosecond laser-assisted deep anterior lamellar keratoplasty for keratoconus and keratectasia.

AIM: To describe the initial outcomes and safety of femtosecond laser-assisted deep anterior lamellar keratoplasty (DALK) for keratoconus and post-LASIK keratectasia. METHODS: In this non-comparative case series, 10 eyes of 9 patients underwent DALK procedures with a femtosecond laser (Carl Zeiss Meditec AG, Jena, Germany). Of the 9 patients, 7 had keratoconus and 2 had post-LASIK keratectasia. A 500 kHz VisuMax femtosecond laser was used to perform corneal cuts on both donor and recipient corneas. The outcome measures were the uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA), corneal thickness, astigmatism, endothelial density count (EDC), and corneal power. RESULTS: All eyes were successfully treated. Early postoperative evaluation showed a clear graft in all cases. Intraoperative complications included one case of a small Descemet's membrane perforation. Postoperatively, there was one case of stromal rejection, one of loosened sutures, and one of wound dehiscence. A normal corneal pattern topography and transparency were restored, UCVA and BCVA improved significantly, and astigmatism improved slightly. There was no statistically significant decrease in EDC. CONCLUSION: Our early results indicate that femtosecond laser-assisted deep anterior lamellar keratoplasty could improve UCVA and BCVA in patients with anterior corneal pathology. This approach shows promise as a safe and effective surgical choice in the treatment of keratoconus and post-LASIK keratectasia.

Acousto-optic tunable second harmonic generation in periodically poled LiNbO3.

Acousto-optic (AO) tunable second harmonic generation (SHG) was proposed in periodically poled lithium niobate (PPLN). The acoustic wave could either be induced from an external transducer or self-generated in PPLN driving with a cross-field radio frequency field. The reciprocal vector of PPLN compensates the SHG wave-vector mismatch when quasi-phase- matching (QPM) condition is satisfied, while phonons with suitable frequencies may affect it by scattering photons to different polarization state. The QPM SHG and AO polarization rotation are coupled together. Second harmonic waves' intensities, polarization states and even phases thus could be manipulated instantly through AO interaction.

Liquid crystal modulator with ultra-wide dynamic range and adjustable driving voltage.

We demonstrated a reflective-type liquid crystal (LC) intensity modulator in 1550 nm telecomm band. An effective way to compensate the residual phase of a LC cell is proposed. With the adjustment of a true zero-order quarter wave plate and enhanced by total internal reflection induced birefringence, over 53 dB dynamic range was achieved, which is much desired for some high-end optical communication, infrared scene projection applications. In addition, the driving voltages were decreased and adjustable. Mechanical and spectral tolerance measurements show that our LC modulator is quite stable. Further applications of our experimental setup were discussed including bio-sensors and high speed modulators.