We proposed the multipolar inserting zeros (MIZ) code for direct-sequence optical code division multiple access (OCDMA) systems. This code is designed to correspond with the judgment based on linear convolution. Simulation results showed that it can provide a lower multiuser interference than the conventional Gold sequence. With the same number of users, the bit error rate in the OCDMA system coded by MIZ can be reduced by several decuples than that coded by Gold, and the source power needed to reach a BER of 1×10?9 can also be reduced by several decibels. So the performance of OCDMA can be improved.
A chirped, phase-shifted structure is demonstrated for compact multi-wavelength DFB fiber laser at room temperature for the first time. The chirped structure provides separated resonance cavities and then the stable multi-wavelength operation. The equivalent phase shift method is demonstrated to realize the desired chirp and phase shifts simply and flexibly. A 44pm-spaced, dual-wavelength DFB fiber laser is then achieved experimentally, which is the narrowest spacing ever reported for a compact multi-wavelength fiber laser.
In this paper, an optical code division multiple access (OCDMA) system is demonstrated. The ultrashort light pulse is encoded and then decoded by amplitude sampled fiber Bragg gratings with equivalent phase shift (EPS). Compared with traditional superstructured fiber Bragg grating (SSFBG) with real phase shift (RPS), FBG with EPS is much easier to fabricate. In our experiment, it shows its full ability to perform encoding and decoding in OCDMA systems, and good encoding/decoding performance is achieved.
Polarization mode dispersion can decrease the performance of the fiber-optic transmission seriously. Thus the compensation of Polarization mode dispersion is a critical issue in fiber optics. In this paper, a novel Polarization mode dispersion compensator is suggested and demonstrated based on a special-design fiber Bragg grating. A polarization mode dispersion compensation grating with 10-156ps dynamical span and flat-top response is first reported based on the combination of reconstruction algorithm and the equivalent-chirp method. A 10-Gb/s system experiment using the tunable PMD compensator shows the power penalty of BER at is about 1.2dBwhen the PMD of the system is 60ps.
A cost-effective tunable dispersion compensator using reconstruction-equivalent-chirp method is fabricated. Only uniform phase mask, sub-micron precision and uniform thin metal film are required in the fabrication. The group delay ripple is less than 14 ps during the whole tuning range. An experiment in 40-Gb/s system is demonstrated with a power penalty of 0.7dB.
A pure silica microstructured optical fiber(MOF) with seven dual cores is designed for chromatic dispersion compensation by a finite difference frequency domain(FDFD) method with a perfect matched layer(PML) boundary condition. The multi-core structure fiber is presented for the first time. The negative chromatic dispersion peak value of the designed microstructured fiber is -4500ps/nm.km and the full width at half maximum (FWHM) is evaluated at 12nm. Furthermore the effective area of the inner core fundamental mode can reach 65mm2 at 1550nm wavelength, which is three times that of a conventional dispersion compensating fiber (DCF).
We design a novel tunable interchannel dispersion-slope compensator using a single broad-band nonchannelized sampled fiber Bragg grating (FBG) with chirp in the sampling period. Tunability of the dispersion-slope can be achieved by stressing or heating the grating uniformly, which has a third-order time-delay variation with wavelength, thereby causing a second-order variation in dispersion. By using the equivalent-chirp method, i.e. by chirping the sampling function, we get the time-delay within -1st order Fourier reflection band and consequently realize dispersion-slope compensator.
Based on a compact 2-D Finite-Difference Time-Domain(FDTD) method, We study the effect of imperfection of transverse periodicity on the leakage loss in photonic crystal fibers(PCFs). Random fluctuations of holes radius and position in PCFs are introduced to analyze their effect on the guiding ability.
In this paper, with the superimposed technique a 4-channel chirped grating in a fiber for multi-channel dispersion compensation is been written. We have performed a dispersion compensation experiment with a system using a 10Gbit/s externally modulated laser. The experimental results show that measured BER of the all channels has the power penalty less than ±0.3dB when the grating is inserted through a circulator in front of the 100km transmission fiber.
We demonstrate the simple relationship between the refractive index modulation, the chirp coefficient and the
reflectivity of the large chirped Bragg grating. When applying it to design the gain flattened filter, we can get almost the
same profile as the custom requests. At last, we propose the procedure using this method to write the gratings.
A novel apodization technique is presented using a combination of a phase mask with an amplitude mask of variable mark-space ratio for fabricating sampled Bragg grating (SBG) for the first time. This technique only needs single exposure process instead of double exposure to complete apodization and apodization compensation in SBG fabrication. From our numerical simulation and experimental results, it can be seen that this technique is promising in changing reflection spectrum and reducing the ripple of group velocity delay of the sampled fiber Bragg gratings.
A new instrument to measure stokes vectors using an electro-drived polarization controller and a polarizer based on "TDM" is demonstrated. Using this instrument as well as data fitting and interleaving technique higher-order PMD is detailed.
A method is presented for the short period and large chirped fiber grating fabrication. The relation of grating reflectivity with chirp parameter is discussed and various reflectivity gratings have been achieved under different UV exposal conditions. At last, a broad bandwidth filter with a free filter zone is obtained.
Numerical results show that only apodization compensation is necessary for apodized binary sampled Bragg grating, while sampling compensation is not necessary. A novel apodization compensation preconditioning approach is proposed. This process doesn't require the removal ofthe phase mask. We fabricate a 5 channels sampled Bragg grating with 0.25nm's bandwidth and —980ps/nm's dispersion. Dispersion compensation using the fabricated grating for standard single mode fiber 10Gb/s-80km transmission is demonstraded.
An effective method for photosensitizing fiber is presented in this paper. By adopting this method, the standard single- mode communication fibers are exposed to the flowing hydrogen gas at high temperature. We cannot only write the fiber gratings with different reflectivity, but also load hydrogen at different local position of fiber, which is very convenient for us to write the fiber gratings while the fiber is in fiber-based devices.
A sampled Bragg grating (SBG) with a chirp in sampling period (CSP) is fabricated. When stress gradient is applied in the SBG with CSP, the characteristics of the spectrum response are different from that of a SBG without CSP. These experimental phenomena are consistent of the previous predictions. The stress-gradient SBG with CSP may be used as a tunable compensator for both dispersion and dispersion slope.
A novel dispersion slope compensation by applying a strain gradient on a grating has been proposed. The strain gradient can produce both chirp in the grating period and chirp in the sampling period, which can be used to compensate for fiber dispersion and dispersion slope simultaneously in WDM system.
Optical limiting is an essential concept for nonlinear optics and its applications. Recently, the nonlinear response in a two-dimensional (2D) superlattice (a periodic structure with its refractive index modulated in two dimensions) was studied for its potential applications in new optical devices. The available results are only related to the transition between a low transmission state and a high transmission state in the allowed bands. Up to date, optical limiting is not explored in 2D case with one incident wave. In this paper, a novel structure, which is composed of two dissimilar superlattices, is proposed for the optical limiting. The novel structure is named hetero- superlattice-junction structure in this paper. In terms of real-space-matching concept, optical limiting is demonstrated in a 2D nonlinear hetero-superlattice-junction structure, while this phenomena is proved unavailable in a 2D nonlinear superlattice.
Optical superlattice LiNbO3 crystals with modulation periods were grown by Czochralski method. The generation of femtosecond pulses from violet to blue has been demonstrated in optical superlattice LiNbO3 crystals using third- order quasi-phase-matching. The output power of 9.0 mW at 390 nm, 11.5 mW at 402 nm, and 10.4 mW at 421 nm has been obtained. The dependence of the output power and the acceptance bandwidth on crystal length has been measured and theoretically discussed.
A new multifunctional active medium: optical superlattice Nd:MgO:LiNbO3, i.e., LiNbO3 crystal with periodic ferroelectric domain structures, was grown in our laboratory. The absorption and fluorescence spectra of a sample with the modulation period of 7.5 micrometer were carefully measured and show some differences from the results of common Nd:MgO:LiNbO3 with monodomain structure. Third-order quasi-phase-matched second harmonic generation was also demonstrated to characterize the nonlinear optical properties of the sample. These results indicate that the optical superlattice LiNbO3 has great potentials for self-frequency-doubling operation using the maximum nonlinear coefficient d33 of LiNbO3 and high gain pi polarization at the same time. Pumped by a pulsed dye laser, self-frequency-doubling green light was observed in a cavity with great loss.
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