基于微结构光纤的10 GHz超过1100信道的平坦超连续谱光源

报道了一种基于微结构光纤的宽带、平坦超连续谱(SC)光源。利用锁模半导体激光器产生的1.6ps,重复率为10GHz的光脉冲,通过一段80m的色散平坦高非线性微结构光纤(HNL-MF),在1.55μm波长区域产生了谱宽超过100nm的平坦超连续谱。实验中采用的微结构光纤的非线性系数约为11W-1·km-1。光纤具有小的正常色散和平坦的色散特性,在1550nm波长处,光纤的色散值约为-0.58ps·nm-1·km-1,而在1500～1650nm波长范围内,光纤的色散值变化小于1.5ps·nm-1·km-1。实验中获得的宽带、平坦超连续谱在1503～1593nm宽达90nm的波长范围内,具有±2.5dB的平坦度。该宽带、平坦超连续谱能同时提供波长间隔为10GHz,超过1100路的多波长载波信道。通过对光谱滤波,获得了速率为10Gbit/s的多波长脉冲序列。这样的超连续谱光源在波分复用(WDM)光通信系统、光波长变换等方面都有重要的应用。     

大模场面积掺镱双包层光纤的色散测量

大模场面积掺镱双包层光纤的色散特性在高功率超连续谱的产生中具有重要影响。搭建了一套基于马赫-曾德尔干涉仪和超连续谱光源的超宽波段、高精度的色散测量系统。干涉仪的测量臂插入待测光纤,参考臂通过高精度步进电机调节光程。通过此系统,仅使用27.2 cm长的样品对大模场面积掺镱双包层光纤的色散特性进行了精确测量。实验记录了700~1 600 nm范围内不同波长的干涉条纹,计算得到光纤的色散曲线。使用全矢量有限元法对光纤的色散进行了数值模拟,模拟结果与实验结果一致,证明了实验方法与系统的精确性。     

Broad-band mid-span spectral inversion without wavelength shift of1.7-ps optical pulses using a highly nonlinear fiber Sagnacinterferometer

We demonstrate broad-band mid-span spectral inversion without wavelength shift of short optical pulses using a highly nonlinear fiber Sagnac interferometer. The 1.55-μm optical pulses with a 1.7-ps pulsewidth are transmitted over 40 km of standard fiber, showing the suitability of this technique for broad-bandwidth second-order dispersion compensation in high-bit-rate optical fiber transmission systems     

Complete dispersion compensation for 400-fs pulse transmission over10-km fiber link using dispersion compensating fiber and spectral phaseequalizer

We have demonstrated essentially complete dispersion compensation for 400-fs pulses over a 10-km fiber link using dispersion compensating fiber and a programmable femtosecond pulse shaper functioning as a spectral phase equalizer. The pulse shaper impresses adjustable quadratic and cubic phases onto the spectrum and removes all the residual dispersion and dispersion slope in the dispersion compensated fiber link. Our work shows that the pulse shaper technique provides a powerful and convenient tool for programmable fiber dispersion compensation over broad optical bandwidth. This allows distortion-free femtosecond pulse transmission over a fiber link in excess of 10 km without requiring the exact trimming of the dispersion-compensating fiber     

Measurement of propagation constants related to material properties in high-bandwidth optical fibers

The material contribution to group index and material dispersion were measured in high-bandwidth graded-index optical fibers. A shuttle-pulse technique provided measurements of group index with precisions and accuracies of 0.1 and 0.2 percent using 5 m lengths of optical fiber. Material dispersion in fibers was measured over the0.8-0.9 mum wavelength region using different wavelength, short-pulse laser diodes. The influence of material dispersion on fiber bandwidth measurements was evaluated for laser diode sources. Limitations arising from source linewidth were experimentally determined from measurements on a fiber with high microbending enhanced bandwidth.     

Broadband Mach–Zehnder interferometer design using microstructured optical fibers for multi-channel DPSK demodulation

Microstructured optical fibers (MOFs) are typically all silica optical fibers with air holes. Tremendous interest in these fibers has been generated because of potential applications in optical communications, remote sensing, frequency metrology and optical coherence tomography. A feature of MOFs is that waveguide dispersion can be modified significantly by means of geometrical parameters, namely, the positions and sizes of the different holes. We propose here an innovative MOF design whose waveguide dispersion inversely matches the material dispersion of silica to obtain broadband zero group velocity dispersion from 1400 to 1650 nm. One application of this fiber is in the demodulation of differential phase shift keying (DPSK) signals. We propose a novel Mach–Zehnder interferometer design using a small section of zero-dispersion fiber providing a wavelength-independent time delay and address all practical considerations. The constant free spectral range of the interferometer would allow the demodulation of multiple evenly spaced wavelength-multiplexed DPSK-encoded channels in a telecommunication link instead of one demodulator per channel.     

Chromatic dispersion measurements over a 50-km single-mode fiber

Chromatic dispersion has been measured over a 50-km-long single-mode fiber, using five sinusoidally modulated laser diodes. Results indicate that chromatic dispersion characteristics for single-mode fibers with 33-dB optical loss can be evaluated in the1.2-1.6-mum spectral region.     

Simultaneous Measurements of Dispersion Parameter and Fiber Length Using the Self-Seeding Laser Oscillation of a Fabry–PÉrot Laser Diode

A simultaneous measurement technique of the dispersion parameter and the length of an optical fiber has been demonstrated by using the self-seeding laser oscillation of a Fabry-Perot laser diode. We measured the dispersion parameter and the length of an optical fiber from the modulation frequency changes required to induce single-mode laser oscillations through an optical closed-loop path. The dispersion and fiber length measurements were within 1.5% and 0.2%, respectively, of the values measured by commercial instruments.     

Long-haul WDM transmission using higher order fiber dispersionmanagement

We propose a fiber dispersion management scheme for large-capacity long-haul wavelength division multiplexing (WDM) transmission systems that considers not only second- but also third-order dispersion characteristics using transmission fibers with opposite dispersion signs. It eliminates the waveform distortion of WDM signals that originates from the existence of third-order dispersion, which is a constraint placed on WDM capacity in conventional dispersion management, while reducing the interchannel interaction caused by the interplay of fiber nonlinearity and second-order dispersion. Design concept of the scheme is discussed to show the feasibility of using actual fiber parameters. An experimental investigation on transmission performance regarding the signal pulse format, nonreturn-to-zero (NRZ) and return-to-zero (RZ), and interchannel interaction caused by four-wave mixing (FWM) and cross-phase modulation (XPM) is described for optimizing WDM system performance. It is experimentally shown that RZ pulse transmission is possible without significant spectral broadening over a wide wavelength range in dispersion managed fiber spans. Using these results together with a wideband optical amplifier gain-bandwidth management technique, yields long-distance WDM transmission with the capacity of 25×10 Gb/s over 9288 km     

Coherent optical frequency domain reflectometry (OFDR) using afiber grating external cavity laser

An optical frequency domain reflectometry (OFDR) system containing a narrow linewidth fiber grating external cavity laser is demonstrated to have 62-dB of sensitivity when detecting Fresnel backreflection and 2 m of resolution at a 115 m range in optical fiber. With this system we were able to detect Rayleigh backscattering in optical fiber with 20-dB signal-to-noise ratio. The phase noise limitation on the distance range for the OFDR was investigated, and the measured signal-to-noise ratio (SNR) data followed the theoretical simulation over the ranges measured. This technique has potential to be applied to the OFDR at 1550 mm with very high dynamic range by using an erbium doped fiber laser     

Nondestructive position-resolved measurement of the zero-dispersionwavelength in an optical fiber

We describe a new technique for mapping the distribution of the zero-dispersion wavelength in a span of optical fiber. With this technique, which is based on four-wave mixing of short pulses, we obtained a spectral accuracy of ±0.2 nm and a spatial resolution of 700 m. Theoretical analysis agrees well with experimental results and points to an order of magnitude improvement in spatial resolution using shorter pulses. Spatial resolutions less than 100 m may be necessary because we find, by destructive measurements, variations in lambda-zero by as much as 0.5 nm over spans of 500 m     

Pulse restoration by filtering of self-phase modulation broadened optical spectrum

Restoration of distorted optical pulses is achieved using nonlinear fiber self-phase spectral broadening and subsequent optical band-pass filtering of a single sideband. Using this technique, the output pulsewidth is shown to remain constant for input pulse-widths between 9-20 ps. A detailed investigation of the signal-to-noise ratio shows that best performance is obtained by operating in normal fiber dispersion regime. This technique is also applied to restore 40 Gb/s RZ-data suffering distortion from polarization mode dispersion. The high-bandwidth fiber nonlinearity shows promise to scale to higher bit rate pulse distortion correction.     

A fiber-optic autocollimation refractometric method for dispersionmeasurement of bulk optical materials using a widely tunable fiber Ramanlaser

A simple and highly sensitive fiber-optic autocollimation method for refractive-index dispersion measurement of solid-state and liquid bulk optical materials is presented. The method is based on the use of a double-pass multimode fiber Raman laser which generates widely tunable emission in a broadband (0.54-1.01 μm) continuous spectral range at pumping with the second harmonic of a Q-switched Nd:YAG laser. The optical fiber is used in a specific multifunctional regime where, because of its micrometric core dimensions, it serves simultaneously as a point laser source for the formation of a collimated input emission, as a highly sensitive receiver of the autocollimation backreflectance, and as a medium for nonlinear frequency conversion and generation of a broad-band continuum spectrum. The experimental results obtained in the refractive-index dispersion measurements are fitted to the Sellmeier dispersion equation and used to determine the material dispersion and additional dispersive characteristics of the test optical material     

Impact of Optical Transmission on Multiband OFDM Ultra-Wideband Wireless System With Fiber Distribution

Multiband (MB) orthogonal frequency-division multiplexing (OFDM) ultra-wideband (UWB) wireless, which provides high data rate access, is required to be distributed by using optical fiber. The performance of MB-OFDM UWB over fiber transmission system is investigated considering optical modulation and demodulation impact. Theoretical analysis of the effect of fiber dispersion, optical transmitter, and optical receiver response on system performance is carried out considering amplitude and phase distortion. Experiments are conducted and verified by our theoretical analysis and good agreement is obtained. It is found that RF modulation index of $sim {hbox {4}}%$ is optimum for optical transmitter with Mach–Zehnder modulator, and optical receiver with Chebyshev-II response is the best for MB-OFDM UWB over fiber. Compared to back-to-back UWB over fiber, optical transmission is mainly limited by laser phase noise converted relative intensity noise and phase distortion induced by fiber dispersion when optimum modulation index is used. Higher modulation index is limited by amplitude and phase distortion to OFDM signal induced by optical transmitter and receiver response nonlinearities and fiber dispersion and the spectral mask. It is also found that highly received optical power is required for transmission of MB-OFDM UWB signal over fiber.      

Interferometric fiber-optic sensing based on the modulation ofgroup delay and first order dispersion: application tostrain-temperature measurand

We describe an extension of low coherence interferometry which uses the complete interferogram rather than just the interferogram envelope. We measure the phase across the entire source spectrum using dispersive Fourier transform spectroscopy, and obtain group delay and dispersion from the phase curve. We apply the technique to a simultaneous quasi-static strain-temperature measurand in which strain and temperature are recovered from the modulation of group delay and first-order dispersion. We show these parameters have a negligible cross-term, yield a particularly robust transformation to strain and temperature, are immune to 2 π phase ambiguity and insensitive to fiber geometry. With an ultra broadband thermal source we achieved simultaneous resolution of strain acid temperature to accuracies of 5 μstrain.m and 0.35 K over strain and temperature ranges of 1200 μstrain.m and 22 K. We present detail of a diffraction grating-coupled multiple SELED source and its operation in optical fiber DFTS. With this source we achieved resolutions of <1 μstrain in single parameter measurement of strain and a temperature insensitive measurement of strain to within 15 μstrain.m over a strain range of 1200 μstrain.m in the presence of a temperature fluctuation of 20 K     

Measurement of the spatial distribution of birefringence in opticalfibers

We describe a technique for the measurement of the birefringence spatial distribution in a single-mode optical fiber with a resolution of 1 m. This technique is based on a polarization optical time-domain reflectometer using a rotary linear polarizer. We report results performed on different types of fibers: standard step-index and dispersion shifted fibers     

基于光学参量振荡器的高分辨率CARS光谱研究

提出了一种基于三硼酸锂（LBO）晶体的飞秒绿色泵浦光学参量振荡器（OPO）,其采用飞秒525 nm绿光光纤激光器作为泵浦源。OPO产生的信号光其可调谐的光谱范围由780～940 nm,光功率可达到250 mW以上;闲频光可调谐范围从1630～1190 nm,光功率可达到300 mW以上。利用本系统可在波数为1330 cm－1和6650 cm－1之间进行相干反斯托克斯拉曼散射（CARS）光谱测量。由于脉冲啁啾,光谱分辨率达到100 cm－1,与时间带宽限制脉冲激励源相比,分辨率达到后者的2.5倍。     

Fast Characterization of Dispersion and Dispersion Slope of Optical Fiber Links Using Spectral Interferometry With Frequency Combs

We demonstrate fast characterization ( $sim$1.4 $mu hbox{s}$) of both the dispersion and dispersion slope of long optical fiber links ( $sim$25 km) using dual quadrature spectral interferometry with an optical frequency comb. Compared to previous spectral interferometry experiments limited to fiber lengths of meters, the long coherence length and the periodic delay properties of frequency combs, coupled with fast data acquisition, enable spectral interferometric characterization of fibers longer by several orders of magnitude. We expect that our method will be useful to recently proposed lightwave techniques like coherent wavelength-division multiplexing and to coherent modulation formats by providing a real-time monitoring capability for the link dispersion. Another area of application would be in stabilization of systems which perform frequency and timing distribution over long fiber links using stabilized optical frequency combs.      

Heterodyne autocorrelation method for characterizing 1.55 μmoptical pulse train and for measuring dispersion and nonlinearity inoptical fibers

Heterodyne autocorrelation measurement of 1.55 μm optical pulses from an actively mode-locked external cavity diode laser is performed before and after transmission through an optical fiber. In heterodyne autocorrelation, optical spectrum is resolved electronically. This method is suitable for measurement of optical pulses with a spectral width of less than 100 GHz, and it gives not only the pulse width and chirp of the pulses, but also it is useful for determining the dispersion and optical Kerr constant of an optical fiber. Analytical formalism for deducing these quantities is given for Gaussian pulses. Principal measurement is performed using a mode-locked diode laser. Dispersion is measured for a conventional-dispersion fiber of 35 km. Also, self-phase modulation (SPM) is measured for a dispersion-shifted fiber of 15.83 km     

Azimuthal far-field analysis for the measurement of the effectivecutoff wavelength in single-mode fibers-effects of curvature, length,and index profile

The spectral analysis of the optical power transmitted through a bow tie slit rotating in the far-field pattern of a single-mode fiber provides an easy and accurate mean of determining the effective cutoff wavelength of the fiber. The principle of this technique is described. Theoretical considerations show that the ratio between the optical powers carried by the LP₁₁ mode and the fundamental mode, respectively, is directly related to the amplitude of two particular lines in the spectrum. The degree of coherence of the analyzed field is shown to be related to the amplitude of a third line. Owing to the sensitivity of the method, the detectable amount of the total power which is contained in the LP₁₁ mode is as small as 0.5%. The effects of length and curvature on the effective cutoff wavelength are measured over wide ranges for different kinds of single-mode fibers. The behavior of a quadruple-clad flattened dispersion fiber is shown to be somewhat different from that of a typical depressed cladding fiber