The crosstalk performance of coherent time-spreading OCDMA system based on superstructured fiber Bragg grating
(SSFBG) is investigated with different input pulse width and grating refractive index modulation amplitude, and
corresponding bit error rate (BER) performance was considered at different data-rate. Simulation results show that the
crosstalk performance will be degraded with the increasing of input pulse width or grating refractive index modulation
amplitude. Moreover, with fixed code length N = 127 and receiver bandwidth limitation, lower data-rate system has
better tolerance to the increasing of input pulse width, while higher data-rate system has better tolerance to the increasing
of grating refractive index modulation amplitude.
We compared the bit-error-rate (BER) and throughput performance of coherent time-spreading (TS) optical
code division multiple access (OCDMA) systems with PPM
(pulse-position modulation) and OOK (on-off
keying) signaling. The results indicate that PPM-OCDMA with an appropriate pulse position multiplicity
performs better than OOK-OCDMA in the receivers' bandwidth limitation case.
KEYWORDS: Signal attenuation, Modulators, Modulation, Single sideband modulation, Radio over Fiber, Microwave radiation, Hybrid fiber radio, Single mode fibers, Dispersion, Signal detection
The effect of Mach-Zehnder modulator (MZM) direct current (DC) extinction ratio on single sideband
(SSB) radio over fiber (RoF) link is analyzed and simulated, finite extinction ratio and drive signal
unbalanced ratio of dual-drive MZM (DD-MZM) are considered. Our results show that power variation of
target signal increase and harmonic suppression ratio decrease with the decline of extinction ratio. However,
for a nonideal modulator with low extinction ratio, optimum drive signal unbalanced ratio could be applied
to obtain a high quality microwave signal.
KEYWORDS: Signal attenuation, Modulators, Radio over Fiber, Microwave radiation, Modulation, Hybrid fiber radio, Signal generators, Dispersion, Signal detection, Double sideband modulation
The impact of Mach-Zehnder modulator (MZM) chirp on carrier suppressed double sideband (CS-DSB) radio
over fiber (RoF) link is analyzed and simulated. Finite extinction ratio and drive signal unbalanced ratio
of nonideal MZM are considered, power variation of targeted microwave signal and harmonic
suppression are calculated and evaluated. The results show that drive signal unbalanced ratio could
be optimized to minimize the power variation owing to the finite extinction ratio in the CS-DSB RoF
link. However, harmonic suppress ratio declined rapidly with the decrease of extinction ratio,
although the unbalanced ratio could also be adjusted to improve the harmonic suppression. For a
nonideal modulator with low extinction ratio, unbalanced ratio could be optimized to overcome
power penalty caused by finite extinction ration, while an additional filter is required to suppress the
unwanted harmonic components in the CS-DSB RoF link.
We compared the theoretical bit-error-rate (BER) and throughput performance limits of 2-D time-wavelength
and coherent time-spreading optical code division multiple access (OCDMA) communication systems. The
results show that the BER performance of the coherent time spreading OCDMA scheme is much superior to 2-D
time wavelength scheme in the multiple access interference (MAI) noise limited case without considering beat
noise, and the beat noise affects the system performance seriously and appears as a dominant source of noise for
the two kinds of OCDMA systems. Nevertheless, the 2-D time wavelength scheme has better tolerance to beat
noise than coherent time spreading OCDMA scheme, and the superiority is more apparent with the increasing of
the code length N. While the coherent time spreading scheme has higher peak throughput, and it supports a
higher throughput than the 2-D time wavelength scheme at higher offered loads.
Differential phase shift keying (DPSK) data format is employed to combat noise in the bandwidth limited coherent time spreading (TS) optical code division multiplexing access (OCDMA) system. In addition, the performance of DPSK-OCDMA system with turbo coding is analysed and simulated. Theoretical results show that performance degradation due to bandwidth limitation could be effectively restrained by the DPSK-OCDMA scheme, and further performance improvement could be achieved by incorporating turbo coding into OCDMA system. It is confirmed that the number of active users is increased from K = 11 to K = 25 in bandwidth limited OCDMA system by employing DPSK format and turbo code for a given BER of 10-6.
The nonlinear optical loop mirror (NOLM) and the nonlinear amplifying loop mirror (NALM) based optical
thresholding in the high speed OCDM system are analyzed, the input pulse peak power of the NOLM is
optimized considering the thresholding decision function, pulse shaping and side-lobe suppressing of the NOLM
based on high nonlinear fiber (HNLF) are numerically simulated, and theoretical investigation is compared with
experimental result. The results show that the interference noise in the 40 Gbit/s OCDM system could be
suppressed effectively by the optical thresholding techniques based on the NOLM and the NALM. We show that
the NOLM and the NALM can act as a nonlinear processing element capable of reducing both the pedestal
associated with conventional matched filtering and the width of the associated code recognition pulse.
Correlation characteristics for phase-coding scheme utilizing superstructured fiber Bragg grating (SSFBG)
as encoder/decoder are evaluated. The impact of transmission impairments and the receiver's bandwidth
limitation in the coherent OCDMA system is investigated, and the performance improvement by using optical
thresholding is also analyzed.
40-Gbit/s optical code-division multiplexing (OCDM)/ wavelength-division multiplexing (WDM) system based
on supercontinuum (SC) and superstructured fiber Bragg grating (SSFBG) is experimentally demonstrated,
simultaneous multi-wavelength optical encoding of a single supercontinuum source using SSFBG is achieved. In
the receiver, a nonlinear amplifying loop mirror (NALM) is introduced to improve the system performance, both
the sidelobe and the pedestal are reduced, the waveform quality of decoded signal is enhanced. The peak power
of the decoded signal before the NALM is ~ 8 mW.
A seven-chip, 280-Gchip/s OCDM system incorporating quaternary phase coding and decoding is experimentally demonstrated. The encoder and decoder, consisting of superstructured fiber Bragg gratings, are fabricated using the equivalent-phase-shift method; only ordinary phase masks and submicrometer precision in control are required. Interchannel interference noise is also considered and evaluated; 40-Gbit/s×2-channel multiplexing is demonstrated. In addition, a nonlinear amplifying loop mirror (NALM) within the receiver is introduced to improve the system performance under two-channel operation. Switching is obtained for peak signal power less than 10 mW. We show that the NALM can act as a nonlinear processing element capable of reducing both the pedestal associated with conventional matched filtering and the width of the associated code recognition pulse.
A 7-chip, 280-Gchip/s quaternary phase-shift code is assigned to short pulses after reflection from a encoder, The code is then recognized by a decoder. The encoder and decoder used within our experiments are 4-phase shift SSFBGs. Each grating containing 7-chip has a uniform amplitude refractive index level along its length but in which discrete jumps in phase (0,∏/2,∏,or 3∏/2) are written into the grating at the boundaries of adjacent spatial chips. The 4-phase shift SSFBGs were fabricated using "equivalent phase shift" method, wherein the desired phase shift in one channel of Sampled Bragg grating (SBG) could be achieved by simply changing the sampling period. Only ordinary phase masks and submicrometer precision are needed to fabricate the encoding and decoding SSFBGs. The quaternary phase encoding and decoding of short pulses at 40Gb/s is experimentally demonstrated, to the authors' knowledge, this is the highest single channel rate obtained from FBG based OCDM system. Multiple user interference (MUI) is also considered and evaluated, we present an experimental demonstration 2-user 40Gb/s/user (2×40Gb/s) OCDM system employing 4-phase pulse pattern generation and recognition. In addition, a nonlinear optical loop mirror (NOLM) within the receiver is introduced to improve the system performance under multiuser operation, we show that the NOLM can act as a nonlinear processing element capable of reducing both the pedestal associated with conventional matched filtering and the width of the associated code recognition pulse. The system benefits of using the NOLM are experimentally demonstrated under 2-user operation.
Effective utilization of highly nonlinear fiber (HNLF) with relative large normal dispersion to further broaden optical spectrum of 40-GHz, 1.2-ps optical pulse train is investigated, in which an optimized scheme is presented. In the scheme, the pulse is launched into the HNLF of the first section to generate the spectrum broadened linearly up-chirped pulse, and recompressed after propagating through a suitable length of conventional single-mode fiber (SMF) of the second section for the pulse chirp being compensated. At last, the pulse travels through the second HNLF of the third section for further optical spectrum broadening. This new scheme applies the chirp pulse compensation technique and has scalability to the peak power of seed pulse. The experimental results show that ~1.7 nm increment of 3dB spectrum bandwidth is obtained when the new scheme is implemented given the same HNLF length and input seed pluses, and the coherence properties of the original pulses are maintained. This scheme provides a more effective means of pulse spectrum broadening in the HNLF when the peak power of seed pulse is confined by the EDFA output power and the high repetition rate of pulse laser.
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