10 Gbit/s optical CDMA encoder-decoder BER performance using HNLF thresholder
ABSTRACT We demonstrate bit-error-rate performance of a 10 Gbit/s optical code division multiple access system based on a spectral-phase encoding scheme. A highly nonlinear fiber based thresholder discriminates between correctly and incorrectly decoded pulses.
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ABSTRACT: A new technique for encoding and decoding of coherent ultrashort light pulses is analyzed. In particular, the temporal and statistical behavior of pseudonoise bursts generated by spectral phase coding of ultrashort optical pulses is discussed. the analysis is motivated by recent experiments that demonstrate high-resolution spectral phase coding of picosecond and femtosecond pulses and suggest the possibility of ultrahigh speed code-division multiple-access (CDMA) communications using this technique. The evolution of coherent ultrashort pulses into low intensity pseudonoise bursts as a function of the degree of phase coding is traced. The results are utilized to analyze the performance of a proposed CDMA optical communications system based upon encoding and decoding of ultrashort light pulses. The bit error rate (BER) is derived as a function of data rate, number of users, and receiver threshold, and the performance characteristics are discussed for a variety of system parameters. It is found that performance improves greatly with increasing code lengthJournal of Lightwave Technology 04/1990; · 2.56 Impact Factor
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ABSTRACT: This paper reports comprehensive experimental results on a femtosecond code-division multiple-access (CDMA) communication system test bed operating over optical fiber in the 1.5 μm communication band. Our test bed integrates together several novel subsystems, including low-loss fiber-pigtailed pulse shapers for encoding-decoding, use of dispersion equalizing fibers in dispersion compensated links for femtosecond pulse transmission and also in femtosecond chirped pulse amplification (CPA) erbium doped fiber amplifiers (EDFAs), and high-contrast nonlinear fiber-optic thresholders. The individual subsystems are described, and single-user system level experimental results demonstrating the ability to transmit spectrally encoded femtosecond pulses over a 2.5-km dispersion compensated fiber link followed by decoding and high contrast nonlinear thresholding are presentedJournal of Lightwave Technology 12/1998; · 2.56 Impact Factor
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ABSTRACT: A 10 Gbit/s, 810 fs, return-to-zero signal is spectrally encoded, transmitted over a 40 km dispersion shifted fibre, and decoded using a photonic spectral encoder and decoder pair that uses high resolution arrayed-waveguide gratings and phase filters. A 255 bit binary phase code with the maximum length sequence is used for spectral codingElectronics Letters 08/1999; · 1.04 Impact Factor
10 Gbit/s optical CDMA encoder-decoder
BER performance using HNLF thresholder
Kebin Li, Wei Cong, V. J. Hernandez, Ryan P. Scott, Jing Cao, Yixue Du,
J. P. Heritage, Brian H. Kolner, S. J. B. Yoo
Department of Electrical and Computer Engineering, University of California, Davis, California 95616
Abstract: We demonstrate bit-error-rate performance of a 10 Gbit/s optical code division multiple
access system based on a spectral-phase encoding scheme. A highly nonlinear fiber based
thresholder discriminates between correctly and incorrectly decoded pulses.
2004 Optical Society of America
OCIS codes: (060.4510) Optical Communication; (060.2330) Fiber optics communication
Conventional optical access networks typically use wavelength-division multiplexing (WDM) and/or time-division
multiplexing (TDM) techniques which require wavelength or time domain processing. For RF wireless networks,
code-division multiple access (CDMA) has become the dominant approach because of significant advantages. For
high-capacity access networks, optical code-division multiple access (O-CDMA) may provide significant rewards.
Instead of relying on WDM or TDM technologies alone, O-CDMA can utilize optical codes to achieve truly flexible
and reconfigurable access of large network capacity. In addition, it has the potential for achieving enhanced security
in the optical layer, decentralized network control, and increased reliability and survivability.
Different O-CDMA schemes have been proposed using various laser sources, coding schemes and detection
methods [1-3]. The Spectral Phase Encoded Time Spreading (SPECTS) scheme exploits relatively simple all-optical
pulse shaping to achieve optical encoding and decoding of information. The pulse shaping action of the encoder on
ultrashort pulses spreads the pulse in time, and the decoder can reconstruct the original pulse if the matching code is
used. Otherwise, the decoder will generate another pulse spread in time that will appear as low intensity noise-like
bursts. By introducing optical threshold detection, properly decoded pulses can be selected against the noise.
Several different approaches for optical thresholding have been proposed [4-6], however most of them require
high peak pulse powers incompatible with current fiber optic communication technologies operating at multi-
gigabit/s data rates. Here, we demonstrate the effectiveness of using highly non-linear fiber (HNLF) in one of these
schemes  instead of DSF fiber to increase the sensitivity of the thresholder by more than 40 times in order to
measure the bit-error-rate (BER) at the 10Gbit/s data rate.
2. Experimental setup
Fig. 1. Diagram of experimental setup for Bit-Error-Rate measurement of SPECTS O-CDMA testbed. EDFA; Erbium
Doped Fiber Amplifier, DCF; Dispersion Compensation Fiber, HNLF, Highly Nonlinear Fiber, L1 & L2; Lenses, M1 &
The experimental setup is shown in Fig. 1. The mode-locked fiber laser (optical clock) generates 2.4 ps pulses
centered at 1550 nm with a repetition rate of 10 GHz. A synchronized LiNbO3 Mach-Zehnder modulator modulates
the pulse train at 10 GHz with a 223 – 1 PRBS. The modulated pulses are amplified by an Erbium doped fiber
amplifier (EDFA), and then compressed to 0.4 ps by a nonlinear-fiber-based pulse compressor. This pulse train is
spectrally phase encoded by a pulse shaper and is then decoded by an identical pulse shaper, followed by a threshold
detection system composed of a dispersion compensated EDFA and 500 m of highly nonlinear fiber (HNLF). Bit
error rate testing was performed using a 10 Gb/s receiver with clock recovery and a HP 70843B test set.
The inset in Fig.1 shows the details of a SPECTS encoder or decoder. A spatial light phase modulator (SLM) is
located in the Fourier plane of a zero dispersion pulse compressor consisting of a pair of diffraction gratings and
lenses. This arrangement allows the individual spectral components to be phase modulated (0 or π phase shift) by
the SLM with a m-sequence code.
3. Results and discussion
To verify the proper operation of the encoder and decoder, we made cross correlation measurements at several key
locations in the system, as shown in Fig.2. Notice that the correctly decoded pulse is only slightly broadened when
compared with the input pulse (Fig. 2a). While the encoded (Fig. 2b) or incorrectly decoded pulse (Fig. 2c) is
broadened approximately 30 times.
Time Delay (ps)
Time Delay (ps)
0 10 2030 4050
Time Delay (ps)
Amplitude ( a.u.)
Fig. 2. Cross correlation traces (using 0.4 ps pulse as the reference) showing the optical pulse state at various points in the
system. (a) Input pulse to the encoder (gray) and decoder output pulse (black). (b) Encoded pulse with a 31 bit m-sequence
on the SLM of encoder. (c) Incorrectly decoded output pulse from the decoder.
Even though the incorrectly decoded pulse is very broad, it will be interpreted the same as the short pulse in
typical optical receivers because of their limited temporal responses. This mandates the use of a threshold detector
which acts as a discriminator. The threshold detector takes advantage of fiber non-linearity which generates
additional frequency components through self-phase modulation for shorter (higher peak power) pulses but not for
longer ones. Figure 3a shows the input spectrum to the encoder and the decoder output spectrum. The narrowing of
the output spectrum is caused by spectral windowing from the SLM. After passing through the HNLF, the spectrum
is significantly broadened (Fig.3b) with the properly decoded pulse showing significantly more spectrum at the
longer wavelength when compared to the improperly decoded pulse (rejected) spectrum. When a 1 nm bandpass
filter is used at 1561 nm, we measured contrast ratios in excess of 7 dB. Figure 3c shows the output spectra from the
thresholder for correctly and incorrectly decoded pulses.
Fig. 4 shows the bit error rates for the back-to-back signal and the correctly decoded signal that passes through
the HNLF-based thresholder. The inset shows the eye diagrams of back-to-back, correctly decoded, and incorrectly
decoded signals. It clearly shows that the eye diagram of the correctly decoded signal is open widely while the eye
of the incorrectly decoded signal is completely closed. The degradation of the incorrectly decoded signal prevents
synchronization for a BER measurement at the available powers. In contrast, less than 10-11 BER was achieved
with a correctly decoded signal. The BER of the incorrectly decoded signal is around 0.4 due to the failure of
synchronization while the BER of correctly decoded signal below 10-11.
Spectral Power (dBm)
Fig. 3. Optical spectra at various locations in the system. (a) The encoder input spectrum and the decoder
output spectrum. (b) Output spectra of the 500 m long HNLF for correctly and incorrectly decoded pulses.
(c) Output spectra of the thresholder for correctly and incorrectly decoded pulses.
Fig. 4. Bit Error Rate and eye diagrams for back-to-back and decoded signals
This paper presented 10 Gb/s results achieved on a SPECTS O-CDMA testbed. The experimental results showed
successful encoding and decoding, and selective detection of correctly decoded pulses, while rejecting the
incorrectly decoded pulses by using a HNLF thresholder. The 10 Gb/s BER were below 10-11 with reasonably low
power detection of -14 dBm. Multiple user access network BER performance study is in progress.
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This work was supported in part by DARPA and SPAWA under agreement number N6601-02-1-8937..