Measuring fiber position errors from spectral data

Precise fiber positioning is crucial to a wide field, multi-fiber spectroscopic survey such as the Guoshoujing Telescope （the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, LAMOST）. are based on CCD photographic and Nowadays, most position error measurements image processing techniques. These methods only work for measuring errors orthogonal to the telescope optical axis, but there are also errors that lie parallel to the optical axis of the telescope, such as defocusing, and errors caused by the existing deviation angle between the optical axes of a fiber and the telescope. Directly measuring the two latter types of position errors is difficult for an individual fiber, especially during observations. Possible sources of fiber position errors are discussed in brief for LAMOST. By constructing a model of magnitude loss due to the fiber position error for a point source, we propose an indirect method to calculate both the total and systematic position errors for each individual fiber from spectral data. Restrictions and applications of this method are also discussed.

A new demodulation technique for optical fiber interferometric sensors with[3×3]directional couplers

Optical fiber interferometric sensors based on [3×3] couplers have been used in many fields. A new technique is proposed to demodulate output signals of this kind of sensors. The technique recovers the signal of interest by fitting coefficients of elliptic （Lissajous） curves between each fiber pair. Different from other approaches, this technique eliminates the dependence on the idealization of [3×3] coupler, provides enhanced tolerance to the variance of photoelectric converters, and is anti-polarization in a certain extent. The main algorithm has been successfully demonstrated both by numerical simulation and experimental result.

Suppression of backscattering induced noise by the sideband locking technique in a resonant fiber optic gyroscope

Polarization fluctuation induced noise and backscattering-induced noise are the dominant noises in resonant fiber optic gyroscopes. This Letter proposes a new method to suppress the carrier and backscattering induced noise by the sideband locking technique. Besides choosing an optimized modulation depth and different clockwise and counterclockwise modulation frequencies, the sideband is locked to the cavity resonance. With the proper modulation frequency, the carrier frequency component locates at a position far away from the resonant frequency, and then it is suppressed by the cavity itself, which can be taken as a bandpass filter. The amplitude of the carrier frequency can be suppressed by 20–25 d B additionally by the cavity and the total intensity suppression ratio can reach 115.74 d B. The backscattering induced noise can be eliminated for the adoption of different frequencies. The method can realize a stable and high suppression ratio without high requirements for parameter accuracy or device performance.

Novel Technique for All-Optical Clock Extraction Using Fiber Bragg Gratings

We demonstrate a simple passive technique, using phase shifted fibre Bragg gratings, for extraction of optical clock signals from RZ optical data signals. The generation of an optical clock signal at 12.5 GHz from a 12.5 Gbs RZ optical data stream is experimentally demonstrated with simulation of optical clock extraction at 10 GHz from 40 Gbs OTDM data streams.

Bismuth-doped germanate glass fiber fabricated by the rod-in-tube technique

Bismuth （Bi）-doped laser glasses and fiber devices have aroused wide attentions due to their unique potential to work in the new spectral range of 1 to 1.8 μm traditional laser ions, such as rare earth, cannot reach. Current Bi-dopcd silica glass fibers have to be made by modified chemical vapor deposition at a temperature higher than 2000℃. This unavoidably leads to the tremendous loss of Bi by evaporation, since the temperature is several hundred degrees Celsius higher than the Bi boiling temperature, and, therefore, trace Bi （-50 ppm） resides within the final product of silica fiber. So, the gain of such fiber is usually extremely low. One of the solutions is to make the fibers at a temperature much lower than the boiling temperature of Bi. The challenge for this is to find a lower melting point glass, which can stabilize Bi in the near infrared emission center and, meanwhile, does not lose glass transparency during fiber fabrication. None of previously reported Bi-doped multicomponent glasses can meet the prerequisite. Here, we, after hundreds of trials on optimization over glass components, activator content, melting temperature, etc., find a novel Bi-doped gallogermanate glass, which shows good tolerance to thermal impact and can accommodate a higher content of Bi. Consequently, we successfully manu- facture the germanate fiber by a rod-in-tube technique at 850℃. The fiber exhibits similar luminescence to the bulk glass, and it shows saturated absorption at 808 nm rather than 980 nm as the incident power becomes higher than 4 W. Amplified spontaneous emissions are observed upon the pumps of either 980 or 1064 nm from ger- manate fiber.

Preparation and laser properties of Yb^3＋-doped microstructure fiber based on hydrolysis-melting technique

The Yb^(3+)-doped silica glass was prepared by the Si Cl_4 hydrolysis doping and powder melting technology based on high frequency plasma. The absorption and emission characteristics of the Yb^(3+)-doped silica glass are studied at room temperature. The integrated absorption cross section,stimulated emission cross section and fluorescence lifetime are calculated to be 8.56×10~4 pm^3,1.39 pm^2 and 0.56 ms,respectively. The Yb^(3+)-doped microstructure fiber(MSF) was also fabricated by using the Yb^(3+)-doped silica glass as fiber core. What's more,the laser properties of the Yb^(3+)-doped MSF are studied.

Coherent beam combination of two fiber amplifiers with multi-dithering technique

Coherent beam combination of two W-level fiber amplifiers with multi-dithering technique is demonstrated.In the experiment,the master laser is split into two channels.Multi-dithering technique is used for phase control.One channel is modulated with a 100 kHz sine wave produced by signal generator,and DSP lock-in amplifier is used for electric signal processing in the feedback loop.When the phase control loop is off,the far field interference pattern is blurred and dynamic with 20% fringe contrast.When the feedback loop is closed,the far field interference pattern is clear and stable with 72% fringe contrast,and 75% coherent combination efficiency is achieved.

Pulse reference-based compensation technique for intensity-modulated optical fiber sensors

We propose a compensation technique based on pulse reference for intensity-modulated optical fiber sensors that can compensate the power fluctuation of the light source, the change of optical components transmission loss, and the coupler splitting ratio. The theoretical principle of this compensation technique is analyzed and a temperature sensor based on fiber coating-covered optical microfiber is carried out to demonstrate the compensation effect. The system noise is measured to be 0.0005 dB with the temperature sensitivity reaching -0.063 dB/℃, and the output drift is 0.006 dB in 2 h at room temperature. The output shows a slight variation （0.0061 dB） when the light source and the common liKht path suffer a 3 dB attenuation fluctuation.

A technique for enhancing the thermal stability of hydrogen-loaded fiber Bragg grating

Heat treatment with the presence of hydrogen (H2) that react with GeE' centers (Ge≡) at high temperature will weaken the refractive index modulation of grating fabricated in hydrogen-loaded normal germanosilicate fiber. Pre-annealing treatment of the above fiber was demonstrated to be able to enhance the grating's thermal stability effectively. 0.37-nm blue-shift of the reflected Bragg wavelength was observed.

Optical Measurement Techniques for Optical Fiber and Waveguide Devices

We describe three major optical characterization methods for fiber and fiber devices. A simple servo controlled scanning fiber-optic confocal microscope is proposed for determining the refractive index profile of an optical fiber. To measure the chromatic dispersion of a short length fiber a Mach-Zehnder fiber interferometer with a novel interferometric distance meter is introduced. At the end, a tomographic method is demonstrated for determining the 2-D stress profile of a fiber.

Thermal expanded core technique applied to high power fiber mode field adapter

A mode field adapter （MFA） fabricated by the thermal expanded core （TEC） technique is investigated. Firstly, the mode field characteristics of the TEC large mode area fiber （LMAF） are analyzed. Compared with the single-mode fiber （SMF）, the mode field diameter of the LMAF enlarged slower than that of the SMF. Secondly, the mode field characteristics of the different fibers with TEC treatment are discussed. Thirdly, the transmission efficiency of the MFA fabricated by the SMF and LMAF is also investigated. Finally, we used the 6/125 μm SMF and 15/130 μm LMAF to fabricate an MFA with transmission efficiency of 92% and the handling power as high as 100 W.

Effective generation of optical quadruple frequency millimeter-wave based on fiber laser using injection rational harmonic mode-locked technique

A method to generate the optical quadruple frequency millimeter-wave with high power efficiency is pro- posed and demonstrated based on the combination of the injection 2nd-order rational harmonic mode- locked fiber ring laser technique and the fiber grating notch filter. In this approach, the fiber Bragg grating notch filter is inserted into the laser cavity to prevent the undesired optical carrier, so that the pump power can be converted to 2nd-order harmonic wave more efficiently. In our experiment, the power efficiency of optical quadruple frequency millimeter-wave （40 GHz） generation is ten folds of that of our previous method based only on the rational harmonic mode-locked technique.

Low-noise and highly stable optical fiber temperature sensor using modified pulse-reference-based compensation technique

We modify the pulse-reference-based compensation technique and propose a low-noise and highly stable optical fiber temperature sensor based on a zinc telluride film-coated fiber tip. The system noise is measured to be 0.0005 dB, which makes it possible for the detection of the minor reflectivity change of the film at different temperatures. The temperature sensitivity is 0.0034 d B/℃, so the resolution can achieve 0.2℃. The maximum difference of the temperature output values of the sensor at 20℃ at different points in time is 0.39℃. The low cost, ultra-small size, high stability, and good repeatability of the sensor make it a promising temperature sensing device for practical application.