Narrow linewidth lasers have important applications in optical communication and sensing. To accurately measure the linewidth, we built a laser linewidth measurement system based on 3×3 coupler. It uses the demodulation algorithm of Differential Cross Multiplication to get the phase noise, and then obtains the linewidth value according to the β-separation line method. At the same time, we introduced wavelet denoising in the signal processing in order to reduce the influence of low frequency noise. Two wavelet bases, db4 and sym11, which were more effective for this system were identified through simulation, and experiments were conducted using a 2 μm band laser. The results prove that the frequency noise power spectral density and linewidth values are reduced after using wavelet denoising.
A lateral microdisplacement sensor based on tapered fiber tips, which is suitable for measuring lateral displacement with high sensitivity, is proposed. A jacket-off single-mode fiber (SMF) is tapered down to tens of microns and then cleaved in the middle of the waist region to get two identical tapered fiber tips. The lateral microdisplacement can be measured by monitoring the transmission intensity of a tip-tip structure. The beam propagation characteristics of such tapered fiber tips with different waist diameters are analyzed in detail. The tip-tip structure, together with the structure composed by one tip and one cleaved SMF, is investigated both experimentally and numerically. Experimental and numerical results show that this kind of lateral microdisplacement sensing structure can provide remarkable performance. A sensitivity of about −1.0 dB/μm with a measuring range of 28 μm is obtained experimentally.
Signal processing is studied for Sagnac fiber distributed sensors. Wavelet de-noising and wavelet decomposition
are comparatively used to analyze signals in frequency domain to give smoothed frequency spectra, based on which null
frequencies of the spectra are found more accurately. The analysis is helpful to more accurately locate the disturbance.
A novel fibre Bragg grating (FBG) hydrophone system is introduced in the paper. The influence of the sound pressure on
the FBG is transformed to light intensity measurement with tuned laser. Elastic material and matched FBGs are
employed to enhance the sensitivity of the hydrophone system. The hydrophone system can operate in a wide acoustic
frequency range from 100Hz to 3kHz and good linear relationship is observed between the output light intensity and the
sound pressure.
The transmission characteristics of a nonuniform photonic fiber Bragg grating (PFBG) are studied in detail by FEM
furnished with 1st - order BGT - like TBC and transfer matrix method. The effects of the air hole size on PFBG's
transmission properties such as effective index, Bragg reflective wavelength, normalized bandwidth and centre
wavelength shift are investigated. It is believed the study can provide useful information for PFBG's design and
optimization with a more effective and accurate method.
System performance degradation caused by group delay ripples of chirped fiber Bragg grating dispersion compensators is analyzed in detail with considerations for the ripple period, amplitude and phase offset. And the induced different kinds of signal distortions are also shown and explained.
We fabricated linearly chirped fiber gratings by using uniform phase mask instead of chirped mask. The chirp of the grating is realized by precisely setting the distance between the fiber and the phase mask at every point of the fiber. In experiments we derived linearly chirped fiber grating which has dispersion -1102ps/nm, time delay ripple is 17ps. And also the asymmetry high order apodization method is used successfully to reduce the time delay ripple. The experiment results consistent with the simulation results. We can fabricate gratings with different chirp extent use one uniform phase mask conveniently by only changing the curve function of the fiber.
The dispersion of 8×10Gb/s wavelength division multiplex (WDM) system has been compensated by the cascaded chirped fiber Bragg gratings(CFBGs), with ITU-T standard wavelengths and wavelength grid. The ASE of the EDFA could be reduced, the OSNR of the transmitted signal could be increased and the fluctuation of the EDFA gain could be controlled in the certain scope by the dispersion compensated CFBGs' WDM system. Impact of cascaded CFBGs' delay ripple on dispersion compensation has been analyzed. Experiment of error-free 8×10Gb/s 2015km transmission without forward error correction (FEC) and electronic repeaters were demonstrated. In the transmission, simplex CFBGs compensators were used and no other form of dispersion compensators were adopted. The experiment result showed that the consistency of the dispersion compensating in each channel is perfect over 2015km optical fiber transmission. The experiment result does agree with the theoretic analysis.
We demonstrate a 10.7Gb/s-line-rate L-band WDM loop transmission over 1890km standard single-mode fiber (SSMF) with 100km amplifier spacing as well as non-return-to-zero (NRZ) format. For the first time, dispersion compensating fiber (DCF) plus chirped fiber Bragg grating (CFBG) is employed for hybrid inline dispersion compensation. The power penalty of each channel is less than 3dB after three loop transmission. The experimental results show that high-performance-CFBGs can be successfully used in ultra-long haul (>1000km) WDM systems. We also point out that all-CFBG compensation scheme is not suitable for re-circulating loop transmissions.
The effect of source wavelength instability on the performance of a system using chirped fiber Bragg gratings as dispersion compensators is numerically investigated, by which it's shown that source wavelength instability will surely induce an additional penalty for the system. And quantified relations of the EOP induced by group delay ripples and reflection ripples with the extent of wavelength instability is given.
The random deviation of the periodicity of the gratings will affect the performance of the fiber gratings. The random errors would not accumulate when the gratings were cascaded. But we found a kind of fabricating system errors induced by the method for the side writing of fiber gratings, which would accumulate when cascaded. So laser with the less pulse energy should be used to write the gratings to developing the system's performance.
Based on the theory of Jones Matrix, a model for the analysis of non-PM fiber ring lasers polarization
characteristics was developed. The model was further tested by experiments. From the numerically and
experimentally analysis results, it was found that lasers composed by non-PM fiber can achieve stable output but
its polarization varied with external perturbation. If a polarizer was inserted into the laser cavity, though the
polarization state was more stable, the instability of output was even more serious. So special polarization control
must be performed to obtain single polarization laser output using non-PM fiber.
We have succeeded in transmitting the signal as long as 1400km with G.652 fiber using chirped fiber gratings. Asymmetrical apodization was adopted to reduce the grating’s GDR (group delay ripple). Power and dispersion management was adopted here to suppress the nonlinear effects.
For the first time, we study the transmission performance of 10Gbps PRBS data stream over ultra-high polarization mode dispersion (PMD) dispersion compensating fiber (DCF), which PMD coefficient is 237.95ps/km1/2. The simulation has been done by coupled nonlinear Schrodinger equations. Also transmitted experiment has been done by DCF with a PMD coefficient of 237.95ps/km1/2. The result of the simulation is consistent with that of the experiment.
Dispersion-compensating chirped fiber gratings provide a compact low-loss means of compensating fiber dispersion.
They are potential candidates for per-channel tunable dispersion compensation devices [2]. Group delay ripple (GDR) is
the principle reason restraining the practical implementation of chirped fiber Bragg gratings and many papers indicate
how the GDR impairs the systems performance [1, 3, 4]. S.Jamal has indicated that the period of the group delay ripple
has much important influence on the systems performance [1]. But the ripple period of the chirped fiber grating was not
consistent along side the grating, and Michael Sumetsky has explained the cutoff phenomenon of the high frequency in
[2]. Because the period of the chirped fiber grating changed with the wavelength and the components of different
frequency overlapped with each other, the time-frequency analysis was needed to descript the time-frequency distribution
of the GDR of the grating. [5]
Delay ripples of chirped fiber Bragg gratings (CFBGs), which a CFBG can compensate 200 km long optic fiber's dispersion, have been analyzed in detail. A numerical simulation of cascaded grating delay ripples has been done by Schroedinger equation and compared with experiment of 1,000 km transmission over G652 fiber by 5 groups of CFBGs dispersion compensation. The research shows that the system degradation depends on the delay ripple period, which is 0.01~0.1nm through a lot of experiments, and amplitude of delay ripple. We had experimentally studied fluctate of power penalty depend on ripple perod of CFBGs when source wavelength changed +/-20GHz around CFBGs center wavelength, the results of theory agree well with these of experiment.
In this paper, we focus on the improvement of accuracy of phase-shift methods to measure the fiber dispersion. By the analysis of experiment results and possible error sources, the more suitable measurement conditions are provided to decrease the measurement error to a great extent. Except keeping a constant temperature and isolating the device from mechanical vibrations when measurement is carried out, as large as 1nm measurement wavelength step and large wavelength span at several tens nm level are preferred to obtain more
accurate and repeatable measurement.
With a new apodisation method and special package material, we have made high-quality chirped fiber Bragg gratings (CFBGs) with the fluctuating error of reflectivity less than 1dB, the delay ripple less than +/-15ps and the temperature coefficient being as low as 0.5‰onm/°C. Using such self-made high-quality CFBGs, we present, for the first time, the experiment results of dispersion compensation of 2x10Gb/s 1000km WDM system with less than 1 dB power penalty for each channel.
We have shown the polarization mode dispersion (PMID) of chirped fiber Bragg gratings (FBG) written on different photosensitive optical fibers. PMD of FBGs on special photosensitive fibers and hydrogenated single mode fiber (SMF) was studied and compared. Experiment results showed that the chirped FBG written on hydrogenated SMF has very low differential group delay (DGD), and it adapt in long-haul optically amplified lightwave systems. The PMD of chirped FBG is determined by the birefringence of the fiber and the dispersion of the chirped FBGs. The birefringence of FBG involved intrinsic and extrinsic or photo-induced birefringence. At the end of this paper, we presented the PMID of the dispersion compensation system using CFBG as dispersion compensator.
It is first time to study on Gauss pulse transmission over ultra-high PMD fiber. Gauss pulse is broken into a series of deformed pulse when it transmits over ultra-high PMD fiber. He has explained that the walk-off deformed pulses cause by ultra-high PMD. Transmitted experiment has been done using fiber with PMD coefficient 237.95ps/km1/2. The simulated result is consistent with experiment.
We used UV laser to write Bragg gratings on the polarization maintaining fiber (PMF). Because of the high-birefringence of the fiber, the Bragg wavelength of the grating on the two polarization axes was different. So that it could introduce great polarization mode dispersion (PMD). When the input was adapted to the axis of the grating, it can serve as the polarization compensation. We measured the polarization of the grating at the end of the article.
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