Differential multiplexing of fiber bragg gratings by means of optical time domain refractometry

A differential method of detection and multiplexing of signals from fiber Bragg gratings based on optical time domain reflectometry is developed. The method is based on probing of a system of gratings by means of short laser pulses and measurement of the power of the resulting reflected pulses by means of standard fiber-optic reflectometry. The proposed method may find extensive use in the area of monitoring of deformation and temperature distributions by means of fiber-optic sensors based on Bragg gratings.     

Measuring temperature profiles in high-power optical fiber components

We demonstrate a new method for measuring changes in temperature distribution caused by coupling a high-power laser beam into an optical fiber and by splicing two fibers. The measurement technique is based on interrogating a fiber Bragg grating by using low-coherence spectral interferometry. A large temperature change is found owing to coupling of a high-power laser into a multimode fiber and to splicing of two multimode fibers. Measurement of the temperature profile rather than the average temperature along the grating allows study of the cause of fiber heating. The new measurement technique enables us to monitor in real time the temperature profile in a fiber without the affecting system operation, and it might be important for developing and improving the reliability of high-power fiber components.     

Nondestructive measurement of an optical fiber refractive-index profile by a transmitted-light differential interference contact microscope

A novel transmitted-light differential interference contrast (DIC) system is used for nondestructive measurement of the refractive-index profile (RIP) of an optical fiber. By means of this system the phase of a measured light beam can be modulated with an analyzer, and the phase distribution of a fiber is obtained by calculation of the various interference patterns. The measurement theory and structure and some typical applications of this system are demonstrated. The results of measuring RIPs in graded-index fiber are presented. Both the experimental results and theoretical analysis show that the system takes the advantage of high index resolution and of sufficient measurement accuracy for measuring the refractive index of the optical fiber. The system has strong ability to overcome environmental disturbance because of its common-path design. Moreover, one can use the system to measure the RIP along the fiber axis and acquire an image of the three-dimensional RIP of the fiber.     

Raman散射型光纤测量太阳能电池的温度

为了测试太阳能电池温度,提出了光纤测量太阳能电池温度的方法。在分析温度与太阳能电池输出功率关系的基础上,利用光纤反StokesRaman背向散射温度效应建立了光纤太阳能电池测温系统。系统中波长为905nm的激光器产生的激光,经过波分复用器、光纤传到太阳能电池中,其反射光经过光纤时受到温度调制,再通过波分复用器照射到光电探测器上,通过光电探测器检测出电池的温度。应用该系统测试出来的温度比间接测量的温度精度提高了2.5%,用该温度修正太阳能电池的输出功率,克服了温度造成的功率损失。结果表明,此方法将太阳能电池的输出功率提高了4%,为太阳能的广泛应用提供了基础。     

Ultra-high-speed phase-sensitive optical coherence reflectometer with a stretched pulse supercontinuum source

We demonstrate an ultra-high-speed phase-sensitive time-wavelength-domain optical coherence reflectometer with a stretched pulse supercontinuum source. A pulsed fiber laser operating at 10 MHz repetition rate was used to generate a pulsed supercontinuum of 30 ps pulse duration by using a nonlinear optical fiber. The supercontinuum pulses are stretched into 70 ns pulses with a highly dispersive fiber. With this stretched pulse source, we have built a phase-sensitive optical coherence reflectometer that measures the spectral interferogram of reflected light. By using the linear relation between the wavelength and the temporal position in a linearly chirped pulse, ultra-high-speed spectrum measurement can be obtained with this method in the time domain. We have demonstrated ultra-high-speed two-dimensional surface profiling for a standard image target and high-speed single-point monitoring for a fixed point under vibrational motion. It is shown that the measurement speed for the position of a single point can be as fast as 2.5 MHz, while the position accuracy can be better than 4.49 nm.     

Chromatic dispersion measurement using a multiwavelength frequency-shifted feedback fiber laser

In this paper, we present the realization of a fiber laser source emitting simultaneously over 17 wavelengths spread over the whole C-band. An acoustooptic frequency shifter is placed in the laser ring cavity to suppress the cross-gain saturation effects of the erbium-doped fiber. The emitted wavelengths are fixed by a set of fiber Bragg gratings (FBGs). A power uniformity reaching 6 dB is achieved by inscribing the FBGs while monitoring the laser output. We demonstrate the reliability of this laser as a source for characterization of optical components and networks by the measurement of optical fiber chromatic dispersion. The measurement is performed over the whole telecommunication C-band (1530-1560 nm) using the time-of-flight method. We perform the measurement on three different fibers with different levels of dispersion, namely a standard fiber, a nonzero dispersion shifted fiber, and a dispersion compensating fiber. The results are compared with measurements obtained using a standard network analyzer. The agreement between the two methods is better than /spl plusmn/1%, thus proving the suitability of the developed laser source for this application.     

Microinterferometric optical phase tomography for measuring small, asymmetric refractive-index differences in the profiles of optical fibers and fiber devices

A new technique, microinterferometric optical phase tomography, is introduced for use in measuring small, asymmetric refractive-index differences in the profiles of optical fibers and fiber devices. The method combines microscopy-based fringe-field interferometry with parallel projection-based computed tomography to characterize fiber index profiles. The theory relating interference measurements to the projection set required for tomographic reconstruction is given, and discrete numerical simulations are presented for three test index profiles that establish the technique's ability to characterize fiber with small, asymmetric index differences. An experimental measurement configuration and specific interferometry and tomography practices employed in the technique are discussed.     

Ultrawide spectral range group-velocity dispersion measurementutilizing supercontinuum in an optical fiber pumped by a 1.5 μmcompact laser source

Group velocity dispersion (GVD) measurement is presented utilizing supercontinuum (SC) white pulses generated in an optical fiber by 15 μm compact laser sources. This provides 1) ultrawide continuous spectral measurement range >600 nm from a single optical source without the use of interpolation formulae and 2) stable far-end measurements by the simultaneous multi-wavelength nature of the SC pulses. A novel method that is independent of the detector bandwidth is proposed which measures λ-dependent phase shifts of one of the Fourier components of a short pulse train. Fiber GVD's of unusual dispersion characteristics were measured using SC pulses extended over the spectral range of 1150-1770 nm. It is shown that fiber lengths of up to 130 km can be measured with a group delay resolution of 0.01 ps/km     

Eigenmode geometry in a high-birefringence optical fiber

A new method for determining the weights of eigenmodes in a weakly multimode optical fiber with strong linear anisotropy is developed using the geometric approach. The proposed method is based on the measurement of geometric parameters of the lines of equal intensity (isolines) in the vicinity of a zero point of the field intensity distribution pattern at the output face of the fiber. Using this method, the weights of eigenmodes in a fiber have been determined upon computer processing of the experimental images of intensity distributions observed upon displacement of a probing laser beam relative to the center of the input face. The results are compared to theoretically calculated weights of the eigenmodes.     

一种新型全光纤弹速测量系统的研制

研制了一种新型的利用激光光束反射原理的全光纤弹速测量系统。该系统采用了全光纤结构和光纤耦合器等无源器件,以输出光功率1mW, 工作波长1300nm的半导体激光器作为测试系统的光源,用光纤耦合器进行分光,实现了在一根光纤中同时传输光源和接受目标反射的信号光,避免了复杂的调节和准直过程。该系统结构简单、可靠性高,利用它,成功地测量了霍普金森杆发射的子弹速度,结果表明其速度测量相对不确定度小于1%。     

Optical time-domain reflectometry in optical fiber with reflection delay time matched to the period of the optical frequency modulation

A method of optical time-domain reflectrometry in optical fiber is described that uses a single distributed feedback diode laser and a reference reflector. When the period of the frequency modulation of the laser matches the time difference between the reference reflection and the reflection of interest an increase in the noise at the detector occurs. The locations of reflections within the fiber are then mapped to frequencies at which the noise at the detector increases. A sinusoidal frequency modulation is analyzed, and an experiment is described in which the system is used to measure the location and the wavelength of two Bragg gratings located 10 cm apart in an optical fiber. Wavelength measurement is accomplished by temperature tuning the diode laser.     

Frequency-domain intermodal interferometer for the bandwidth measurement of a multimode fiber

A new bandwidth measurement technique for a multimode optical fiber (MMF) using a frequency-domain intermodal interferometer is proposed. We have demonstrated that the relative modal delay (RMD) of a MMF can be obtained easily and accurately based on an optical frequency-domain reflectometry (OFDR) technique by using an intermodal interference signal among the excited modes of a MMF. As an example, a photonic crystal fiber with a few modes is prepared and its RMD is measured by using our proposed measurement technique. Measurement results are compared with those from a previously reported frequency-domain method. We have also measured the RMD of a commercial MMF as a practical application and compared our result with the one obtained from a well-known time-domain differential mode delay measurement technique.     

Combined time and wavelength multiplexing technique of optical fiber grating sensor arrays using commercial OTDR equipment

The use of a commercial optical time domain reflectometry device for simultaneous wavelength and time domain multiplexing of fiber-optic Bragg grating sensors is proposed and demonstrated. The realization of such systems provides an inexpensive way of interrogating several hundreds of sensors in a single fiber. Strain resolution of 30 /spl mu/m/m over a strain measuring range of 0.5% was obtained.     

Controlled core removal from a D-shaped optical fiber

The partial removal of a section of the core from a continuous D-shaped optical fiber is presented. In the core removal process, selective chemical etching is used with hydrofluoric (HF) acid. A 25% HF acid solution removes the cladding material above the core, and a 5% HF acid solution removes the core. A red laser with a wavelength of 670 nm is transmitted through the optical fiber during the etching. The power transmitted through the optical fiber is correlated to the etch depth by scanning electron microscope imaging. The developed process provides a repeatable method to produce an optical fiber with a specific etch depth.     

Optical generation 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.     

Spectrum-sliced Fourier-domain low-coherence interferometry for measuring the chromatic dispersion of an optical fiber

We present a novel spectrum-slicing method for measuring the chromatic dispersion of an optical fiber in Fourier-domain low-coherence interferometry. Broadband spectral interference data obtained from a low-coherence interferometer is sliced with Gaussian window functions. Each sliced spectral datum is used to calculate a relative group delay with Fourier transformation at the peak wavelength of a narrow window function. We have demonstrated that our proposed method is very powerful and simple for measuring chromatic dispersion and second-order dispersion in optical fibers and optical devices. Comparison of the proposed method with a conventional measurement method agrees within 0.5%.     

Step-type optical delay line using silica-based planar light-wavecircuit (PLC) technology

We successfully fabricated a step-type optical delay line using a silica-based planar light-wave circuit (PLC) technology with a total delay of 51.0 mm and an optical coherent crosstalk of -49.1 dB. For the purpose of applying it to a mode-locked Er³⁺-doped fiber ring laser, we aimed for: 1) an optical delay of several tens of mm so that any optical pulse repetition frequency could be set, 2) as-low-as-possible optical crosstalk so that the oscillation of the mode-locked ring laser would be stable, and 3) low optical insertion loss. We integrated an optical circuit consisting of eight optical delay units possessing three thermo-optic switches, which have a Mach-Zehnder interferometer configuration, and two waveguides. The difference in optical path length of the waveguides corresponds to the optical delay. We set the optical delay of each unit to 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, and 25.6 mm, respectively, to give a maximum delay of 51.0 mm. Moreover, the total insertion loss was 5.2 dB. Precision measurement of the optical delay of each unit gave good results, with a manufacturing error of <0.47%, Based on the above results, the fabricated PLC step-type optical delay line can be used in a mode locked ring laser     

Distributed measurement of the complex modulation of a photoinduced Bragg grating in an optical fiber

A method of measuring the complex modulation of a Bragg grating is derived from a one-dimensional model of light propagating in an optical fiber. Interference fringes between the Bragg grating and a reference air-gap reflector are measured, and a Fourier transform of the interference fringes generated as a laser is swept through the wavelength is used to compute the complex modulation function of the Bragg grating over a restricted domain. Supporting data, taken by temperature tuning a distributed feedback diode laser, are shown.     

Piezoelectric optical fiber stretcher for application in an atmospheric optical turbulence sensor

A piezoelectric optical fiber stretcher has been introduced for working point controlling and low-frequency noise suppression of a fiber-optic atmospheric turbulence sensor. It is actuated by a piezoelectric ceramic stack. The rational structure allows the fiber to extend only along the axial direction, which minimizes the nonlinear birefringence effects for the optical phase shift. What is believed to be a novel method has been proposed to measure its phase-shift coefficient. With the use of this device, the fiber-optic atmospheric turbulence sensor has been tested for the air refractive index measurement.     

Extrinsic fiber-optic Fabry-Perot interferometer sensor for refractive index measurement of optical glass

A simple fiber-optic sensor based on Fabry-Perot interference for refractive index measurement of optical glass is investigated both theoretically and experimentally. A broadband light source is coupled into an extrinsic fiber Fabry-Perot cavity formed by the surfaces of a sensing fiber end and the measured sample. The interference signals from the cavity are reflected back into the same fiber. The refractive index of the sample can be obtained by measuring the contrast of the interference fringes. The experimental data meet with the theoretical values very well. The proposed technique is a new method for glass refractive index measurement with a simple, solid, and compact structure.