Analysis of dispersion compensation by chirped fiber grating in directly and externally modulated CATV systems

The dispersion compensation effect of the chirped fiber grating (CFG) is analyzed theoretically, and analytic expressions are derived for composite second-order (CSO) distortion in analog modulated sub-carrier multiplexed (AM-SCM) cable television (CATV) systems with externally and directly modulated transmitters. Simulations are given for the two kinds of modulations and for standard single mode fiber and non-zero dispersion shift fiber (NZDSF) systems. The results show that CFG could be used as a dispersion compensator in directly modulated systems, but its dispersion coefficient should be adjusted much more precisely than the externally modulated system. The requirements for the NZDSF system could be loosened much. It is proposed that directly modulated source may be used as a transmitter in CATV systems combined with tunable CFG dispersion compensator being adjusted precisely, which may be more cost-effective than externally modulation technology. (c) 2006 Elsevier GmbH. All rights reserved.

Optical generaton of millimeter-wave pulses using a fiber Bragg grating in a fiber-optics system

A scheme is proposed to transform an optical pulse into a millimeter-wave frequency modulation pulse by using a weak fiber Bragg grating (FBG) in a fiber-optics system. The Fourier transformation method is used to obtain the required spectrum response function of the FBG for the Gaussian pulse, soliton pulse, and Lorenz shape pulse. On the condition of the first-order Born approximation of the weak fiber grating, the relation of the refractive index distribution and the spectrum response function of the FBG satisfies the Fourier transformation, and the corresponding refractive index distribution forms are obtained for single-frequency modulation and linear-frequency modulation millimeter-wave pulse generation. The performances of the designed fiber gratings are also studied by a numerical simulation method for a supershort pulse transmission. (c) 2007 Optical Society of America.

Studies of bacteriophage T4 tail fibers and tail fiber genes

The distal half of the bacteriophage T4 tail fiber interacts with the surface of the bacterium during adsorption. The largest polypeptide in this half fiber is the product of gene 37 (P37). During assembly of the tail fiber, P37 interacts with the product of gene 38 (P38). These two gene products are incompatible with the corresponding gene products from the related phage T2. T2 P37 does not interact with T4 P38 and T2 P38 does not interact with T4 P37. Crosses between T2 and T4 phages mutant in genes 37 and 38 have shown that the carboxyl end of P37 interacts with P38 and with the bacterial surface. In the corresponding region of gene 37 and in gene 38 there is no recombination between T2 and T4. In the rest of gene 37 there are two small regions with relatively high recombination and a region of low recombination.

When T2/T4 heteroduplex DNA molecules are examined in the electron microscope four nonhomologous loops appear in the region of genes 37 and 38. Heteroduplexes between hybrid phages which have part of gene 37 from T4 and part from T2 have roughly located gene 37 mutations in the heteroduplex pattern. For a more precise location of the , mutations a physical map of gene 37 was constructed by determining the molecular weights of amber polypeptide fragments on polyacrylamide gels in the presence of sodium dodecyl sulfate. When the physical and heteroduplex maps are aligned, the regions of low recombination correspond to regions of nonhomology between T2 and T4. Regions with relatively high recombination are homologous.

The molecular weight of T2 P37 is about 13,000 greater than that of T4 P37. Analysis of hybrid phage has shown that this molecular weight difference is all at the carboxyl end of P37.

An antiserum has been prepared which is specific for the distal half fiber of T4. Tests of the ability of gene 37 hybrids to block this antiserum show that there are at least 4 subclasses of antigen specified by different parts of P37.

Observations in the electron microscope of the tailfiber - anti- body complexes formed by the gene 37 hybrids and the specific anti- serum have shown that P37 is oriented linearly in the distal half fiber with its N-terminus near the joint between the two half fibers and its C-terminus near the tip of the fiber. These observations lead to a simple model for the structure of the distal half fiber.

The high recombination in T4 gene 34 was also investigated. A comparison of genetic and physical maps of gene 34 showed that there is a gradient of increasing recombination near one end of the gene.

Specklegram in a multiple-mode fiber and its dependence on longitudinal modes of the laser source

A specklegram in a multimode fiber (MMF) has successfully been used as a sensor for detecting external disturbance. Our experiments showed that the sensitivity in the sensor with a multiple longitudinal-mode laser as its source was much higher than that with a single longitudinal-mode laser. In addition, the near-field pattern observations indicated that the coupling between different transverse modes in the MMF is quite weak. Based on the experimental results, a theoretical model for the speckle formation is proposed, taking a bend-caused phase factor into consideration. It is shown in the theoretical analysis that the interferences between different longitudinal modes make a larger contribution to the specklegram signals. (C) 2007 Optical Society of America.

Fiber grating sensors for high-temperature measurement

Two fiber grating sensors for high-temperature measurements are proposed and experimentally demonstrated. The interrogation technologies of the sensor systems are all simple, low cost but effective. In the first sensor system, the sensor head is comprised of one fiber Bragg grating (FBG) and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured based on the wavelength-shifts of the FBG induced by the strain. In the second sensor system, a long-period fiber grating (LPG) is used as the high-temperature sensor head. The LPG is very-high-temperature stable CO2-Aaser-induced grating and has a linear function of wavelength-temperature in the range of 0 - 800 degrees C. A dynamic range of 0 - 800 degrees C and a resolution of 1 degrees C have been obtained by either the first or the second sensor system. The experimental results agree with theoretical analyses. (c) 2007 Elsevier Ltd. All rights reserved.

Control of polarization signal distortion by frequency domain phase conjugation in optical fiber systems

Optical frequency domain phase conjugation (FDPC) is based on phase conjugation of spectrum of an input signal. It is equivalent to the phase conjugation and the time reversal of the temporal envelope of an input signal. The use of FDPC to control polarization signal distortion in birefringent optical fiber systems is proposed. Evolution of polarization signals in the system using midway FDPC is analyzed theoretically and simulated numerically. It is shown that the distortion of polarization signals can be controlled effectively by FDPC. The impairments due to dispersion and nonlinear effects can be suppressed simultaneously.

A novel fiber Bragg grating high-temperature sensor

A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is proposed and experimentally demonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head is comprised of one FBG and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG induced by strain. A dynamic range of 0-800 degrees C and a resolution of 1 degrees C have been obtained by the sensor system. The experiment results agree with theoretical analyses. (c) 2007 Elsevier GmbH. All rights reserved.

On a fiber grating sensor system with the capacity of cross-sensitivity discrimination

A novel fiber Bragg grating (FBG) sensor system based on an interrogating technique by two parallel matched gratings was designed and theoretically discussed. With an interrogation grating playing the role of temperature compensation grating simultaneously, the wavelength drifts induced by temperature and strain were discriminated. Additionally, the expressions of temperature and strain were deduced for our solution, and dual-value problem and cross sensitivity were solved synchronously through data processing. The influence of the FBG's parameters on the dynamic range and precision was discussed. Besides, the change of environment temperature cannot influence the dynamic range of the sensor system through temperature tuning. The system proposed in this paper will be of great significance to accelerate the real engineering applications of FBG sensing techniques. (c) 2007 Elsevier GmbH. All rights reserved.

Specklegram in a grapefruit fiber and its response to external mechanical disturbance in a single-multiple-single mode fiber structure

Specklegram in multimode fiber has successfully been used as a sensor for detecting mechanical disturbance. Speckles in a multimode pure silica grapefruit fiber are observed and compared to that of a step-index multimode fiber, showing different features between them. The sensitivities to external disturbance of two kinds of fiber were measured, based on single-multiple-single mode (SMS) fiber structure. Experimental results show that the grapefruit fiber shows higher sensitivity than does the step-index multimode fiber. The transmission spectrum of the grapefruit fiber was measured as well, showing some oscillation features that are significantly different from that of a step-index multimode fiber. The experiments may provide suggestions to understand the mechanisms of light propagation in grapefruit fibers. (D 2008 Optical Society of America.

Ultrafast double pulses ablation of Cr film on glass

Ultrafast lasers ablation of Cr film was investigated by using double-pulse method. Experimental results show that there exists a temporal ablation window effect with each of the double pulses adjusted just smaller than the threshold. When the delay between the double pulses is within the order of 400 ps, the ablation of Cr film could happen. When the delay between the double pulses is beyond the order of 400 ps, the ablation of Cr film would not happen, and the reflectivity from the surface of the Cr film shows a sharp rise at the same time. The two-temperature model was developed into the form of double pulses to explain the experimental phenomena. Furthermore, microbump structures were formed on the surface of Cr film after ablation by ultrafast double pulses. Their heights exhibit an obvious drop between 1 and 10 ps double pulses delay, which is involved with the electron-phonon coupling process according to the numerical simulation. These results should be helpful for understanding the dynamic processes during ultrafast lasers ablation of metal films. (C) 2008 Elsevier B.V. All rights reserved.

Characterization of a dissolved oxygen sensor made of plastic optical fiber coated with ruthenium-incorporated solgel

A dissolved oxygen sensor made of plastic optical fiber as the substrate and dichlorotris (1, 10-phenanthroline) ruthenium as a fluorescence indicator is studied. Oxygen quenching characteristics of both intensity and phase were measured; the obtained characteristics showed deviation from the linear relation described by the Stern-Volmer equation. A two-layer model is proposed to explain the deviation, and main parameters can be deduced with the model. (C) 2009 Optical Society of America

Repetition rate stabilization of the SBS Q-switched fiber laser by external injection

Repetition rate fluctuation is one of the main drawbacks of the low-threshold stimulated Brillouin scattering (SBS) Q-switched fiber laser. A method to stabilize the repetition rate is proposed in this paper by injecting a square-wave modulated light. It is measured experimentally that variance of the repetition rate can be improved from similar to 20% to similar to 1% of the period. It is also found that effectiveness of the method depends on modulation frequency and duty cycle of the injection. Its working mechanism is analyzed qualitatively. (C) 2009 Optical Society of America