In order to improve the bit-error-rate (BER) performance of free-space optical (FSO) communication systems employing binary phase-shift keying subcarrier intensity modulation (BPSK-SIM), an innovative hybrid modulation scheme called PPM-MSK-SIM is proposed, which is based on pulse position modulation (PPM) and minimum shift keying (MSK) subcarrier intensity modulation. Subsequently, the BER performance of PPM-MSK-SIM is studied in detail for an FSO system over log-normal turbulence channels with avalanche photodiode detection. The results of the numerical simulation show that PPM-MSK-SIM has the advantages of improving the BER performance compared with BPSK-SIM and PPM. For example, at the same received irradiance of À2.1 dBm and the same strength of turbulence C 2 n ¼ 7:5 Â 10 À15 m À2=3 , the BER performance of 2-PPM-MSK-SIM can decrease to 1:02 Â 10 À9 , whereas those of 2-PPM and BPSK-SIM are just 1:19 Â 10 À7 and 2:29 Â 10 À6 , respectively. This makes PPM-MSK-SIM a favorable candidate for the modulation technique in FSO communication systems.Index Terms: Free-space optical communication, subcarrier intensity modulation, atmospheric turbulence, atmospheric attenuation, bit-error rate (BER).

We demonstrate a new method for fabrication of rare-doped silica-based glass, which is liquid doped with non-chemical vapor deposition (non-CVD) melting process. The Ytterbium doped silica-based glass with the ratio of 98SiO2-0.5Yb2O3-1.5Al2O3wt% is achieved by using this method. The refractive index (nD) and the density(ρ) of the glass are 1.519 and 2.62g/cm3 respectively. In measurement experiment, it shows that it has strong absorption in the wavelength range from 850nm to 1050nm, and the main absorption peak is at 978nm and the secondary absorption peak is at 919nm. Using the laser with wavelength of 978nm as pumping source, the emission peak is at 1018nm in the fluorescence spectrum with a 71.8nm fluorescence half-line width. Meanwhile, the integrated absorption cross section, stimulated emission cross section, fluorescence time, saturated pump intensity, minimum pump intensity and other parameters are calculated. Test results indicate that the doped silica-based glass has good thermal stability and large power threshold, which is an ideal gain medium for large power photonic crystal fiber lasers.

The optical properties of photonic crystal fiber cladding knot among the three air holes are analyzed. The mode area, nonlinear coefficient and dispersion characteristics of the core and cladding knot are contrasted. Cladding knot of photonic crystal fiber has a small core and highly nonlinear characteristics. For larger cladding air holes, double zero dispersion curves are obtained. According to the dispersion curve, phase-matching features are analyzed for dispersive wave generation. Variation rules of the central wavelength of the dispersive wave with pump power and wavelength are achieved. The photonic crystal fiber designed is fabricated. The visible and infrared broadband dispersive waves above 300 nm are obtained in experiment. Experimental and theoretical results are completely consistent with each other. These are foundation for wavelength conversion and supercontinuum broadband light source.

Surface-enhanced Raman scattering (SERS) technology can effectively enhance the Raman signal of sample molecules. It has a higher sensitivity to detect biomolecule and thus has many potential applications in biochemistry. The combination of hollow-core microstructured fiber and SERS technology not only enables remote real-time and distributed detection, but also can increase the effective action area between the light field and the object to be measured, and further reduce silica glass background signal that is unavoidable in traditional fiber probes. In this paper, the hollow-core microstructure fiber Raman probes with excellent performance are investigated from the aspects of fiber preparation and SERS experi-mental testing. First, we design and manufacture a kind of hollow-core microstructured fiber with multi-bands in the visible and near-infrared wavelength. The fibers show good light guide performance and thus can fully meet the requirements for surface-enhanced Raman excitation and signal transmission. At the same time, the large core size facilitates the coupling of excitation light, and provides enough room for the test object and the light field. Then, this hollow-core microstructured fiber is used in surface-enhanced Raman experiment. A layer of nano-Ag film is modified on the inner surface of the hollow-core microstructure fiber to prepare the SERS probe by the vacuum physical sputtering method, and Rhodamine 6G (R6G) alcohol solutions with different concentrations are prepared by the dilution method. The hollow-core microstructured fiber deposited with the Ag nano-film is immersed in R6G alcohol solution for 2 min. The alcohol solution of R6G is sucked into the air hole of the hollow-core microstructured fiber by the capillary effect. Then this fiber with R6G alcohol solution is placed in a drying oven at 40 ℃ for 3 h until the alcohol solvent in the air hole is completely volatilized. After that, this fiber is taken out and tested under a detection environment full with air. The fiber SERS probes are tested by microscopic confocal Raman spectroscopy, then the Raman spectra of R6G alcohol solvents with different concentrations are obtained. An R6G Raman signal with a concentration as low as 10-9 mol/L is successfully detected on the front side of the probe. In the far-end back-side detection mode, the detected concentration of SERS probe can be less than 10-6 mol/L. The designed hollow-core microstructured fiber probe has a simple structure and is easy to prepare and test. Compared with the traditional optical fiber, it has advantages of large effective area for the test object and the light field, small interference from the silica glass background signal. This hollow-core microstructured fiber probe has wide application prospects in biochemical detection and other fields.

In this paper, a novel four-mode micro-structured fiber with low-level crosstalk and high mode differential group delay is proposed to solve the large transmission capacity and low crosstalk problems in the mode division multiplexing system. Electromagnetic field distribution, crosstalk, mode differential group delay and dispersion of the fiber are studied by using the full-vector finite element method. To determine the particular parameters of the micro-structured fiber, the performances of the inter-core crosstalk and mode differential group delay (MDGD) are considered comprehensively under different conditions. Simulation results show that this fiber can support four-mode transmission with 19 cores over the whole C+L wavelength band when the cladding diameter is 125 μm. The inter-core crosstalks of LP01 mode, LP11 mode, LP21 mode and LP02 mode are -131.01, -96.36, -63.32, -49.96 dB respectively and the mode differential group delays are high as all of them are more than 160 ps/m. Therefore, compared with the previous work, this fiber has the lower inter-core crosstalk and larger MDGD. Owing to the large index difference between core and cladding, the n_eff differences between the linearly polarized modes are all larger than 10-3across the whole operating wavelength band, which is beneficial to low inter-mode corsstalk. Furthermore, the fabrication of this fiber is simple due to its preforming only need stacking technique to adjust the hexagonal structure geometry size without complex modified chemical vapor deposition process involved. The designed fiber can be used in short-distance and large-capacity transmission system, and it has potential applications in making the corresponding high power devices.