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Reduction of Multiple Access Interference in a DS-OCDMA
System via Two-Photon Absorption
P.J.Maguire, K. Bondarczuk, and L.P.Barry
Research Institute for Networks and Communications Engineering, Dublin City University, Dublin 9, IRELAND.
Tel: +353 (0)1 700 6401, Fax: +353 (0) 1 700 5508 maguirep@eeng.dcu.ie
C.Tian, Z.Zhang, M.Ibsen, D.J.Richardson and P.Petropoulos
Optoelectronics Research Centre, University of Southampton, Southampton, UNITED KINGDOM.
cht@orc.soton.ac.uk
Abstract: We demonstrate a technique to suppress multi-access interference in an OCDMA
system by using the nonlinear process of Two-Photon Absorption. This allows the extinction ratio
to be improved by 5dB when compared to linear detection.
©2006 Optical Society of America
OCIS codes: (060.4510) Optical communications; (190.4360) Ultrafast nonlinear devices
1. Introduction
Due to the bursty packet-based nature of Internet usage, it is necessary to consider alternative, more flexible optical
multiplexing techniques, to improve the spectral efficiency and reduce operational cost instead of the more stringent
provisioning of dedicated wavelengths per user commonly employed today. A prime candidate is Optical CodeDivision Multiple Access (OCDMA), where each channel is assigned a specific optical code that can be used to
label bits that are transmitted by, or intended for, a specific user [1].
Fig 1. Block diagram of a basic OCDMA system
A block diagram of the basic OCDMA topology is shown in Figure 1. It consists of a transmitter, fibre and a
receiver. The major components of the transmitter are a modulated laser pulse source and an optical encoder, with
the receiver containing an optical decoder, thresholder and detector. As mentioned, data bits that are transmitted are
encoded using a user specific optical code, with multiple users accommodated by assigning different minimally
interfering codes to different user pairs [2]. The optical encoding can be preformed in the time domain, frequency
domain or utilising a hybrid of the two. There are a number of different ways in which to generate the required
coding sequences to represent each channel, with one promising technique, especially for time-domain encoding
(i.e. direct sequence OCDMA – DS-OCDMA), employing superstructured fibre Bragg grating (SSFBG) technology.
The encoded data bits that are transmitted by each user in the network are incident on the receiver, but the decoder
in any one receiver matches the code of only one transmitter. Therefore, only the encoded data bits of one
transmitter that are intended for a particular receiver get properly decoded back to the original transmitted format,
and the encoded data bits transmitted by all the other transmitters remain as improperly decoded pseudonoise
signals, contributing to the decoded signal as multi-access interference (MAI) noise.
The presence of the temporally dispersed MAI necessitates the use of a fast receiver at the output of the system
(ideally with a bandwidth matching the chip rate of the code) to avoid integration of the MAI power that can lead to
false interpretation of the unmatched signals [3]. However, this would increase the cost of the receiver and
complicate signal reception. Alternatively, an optical thresholder can be used at the output of the decoder, which
performs the task of distinguishing between the correctly decoded signals and the equally energetic MAI signals [24]. An optical nonlinearity is generally employed in the thresholder as this enables the successful separation of the
desired user from the MAI, thereby permitting multiple users to be transmitted [3]. The output from the thresholder
can then be converted back into electronic form by an optical detector that matches the data rate of the signal. A
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number of different techniques have been demonstrated for use as an optical thresholder in an OCDMA network
including the use of nonlinear effects in fibre followed by a long pass filter [2, 5] and second harmonic generation in
a PPLN waveguide [3]. These methods still rely on the use of a separate optical thresholder followed by an optical
detector to convert the signal back into electrical form. Here an alternative low-cost method based on the nonlinear
optical-to-electrical conversion process of Two-Photon Absorption (TPA) in a semiconductor is presented that can
carry out simultaneous optical thresholding and detection.
2. Two-Photon Absorption-based Optical Thresholder and Detector
TPA is a nonlinear process where two photons are absorbed in the generation of a single electron-hole pair [6]. The
generated photocurrent is proportional to the square of the intensity, and it is this nonlinear response that enables the
use of TPA for various optical signal processing applications including optical demultiplexing, optical sampling and
optical thresholding. In terms of optical thresholding for DS-OCDMA networks, the nonlinear response of TPA
would permit differentiation between correctly coded optical pulse signals and pseudorandom noise bursts, by
clearly detecting the contrast in their peak intensities. It has already been shown that TPA can be used for optical
thresholding at average powers as low as 0.1µW [7]. This paper demonstrates simultaneous optical thresholding and
channel detection using a single TPA device that can be used in the receiver of an OCDMA system.
3. Experimental Set Up
Fig. 2. Experimental OCDMA set up using a TPA device for simultaneous optical thresholding and detetion.
We investigated the applicability of the TPA optical thresholder for OCDMA applications in a simple 2-channel
back-to-back OCDMA experiment (Fig.2). The transmitter in our experimental set up comprised an electroabsorption modulator-based pulse carver, which generated 28 ps pulses at 10 GHz at the wavelength of the OCDMA
encoders (1561.6 nm). These pulses were gated down to 155 MHz and data-modulated with a pseudorandom bit
sequence, before being split into two paths and fed to the encoding SSFBGs representing each of the OCDMA
channels. The SSFBG’s (each having a total length of 77.5 mm) assigned two 31-chip, 40 Gchip/s quaternary phaseshift keyed codes to the pulses. The phase shifts on the codes were written on a continuous (rather than a discrete)
fashion, thus limiting the spectral extent of the OCDMA signals for WDM applications [8]. However, this came at
the expense of a slightly degraded correlation response during decoding, making the use of an optical thresholder
even more appropriate.
150m of SMF was included in one of the paths to ensure that the two data signals were uncorrelated and not
coherently interfering when they were recombined into a single fiber, whereas an optical delay line was used in one
of the paths to allow adjustment of the relative timing of the two codes at the output of the transmitter. The two
OCDMA signals were combined into a single fiber using a 3-dB coupler, and this signal was fed into the decoding
SSFBG, which was matched to one of the two codes. The output of the optical decoder then entered an inline power
meter / attenuator which allowed the average power of the optical signal falling on the TPA device to be constantly
monitored during the course of the experiment. The decoded signal then entered a polarisation controller before
being incident on a 1.3µm laser diode (InGaAsP laser diode, BW~1GHz) acting as the TPA detector. The resulting
photocurrent generated from the laser diode was electrically amplified with a low-noise RF amplifier, before being
displayed on a digital communications analyser (DCA), with a bandwidth of approximately 622MHz.
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4. Experimental Result
Prior to carrying out the 2-channel OCDMA experiment, a photocurrent measurement as a function of varying
incident optical peak power was carried for the TPA detector, with the results of this shown in Figure 3 (a). As
clearly shown, there is approximately a square dependence of the photocurrent on the incident optical intensity,
evidencing the TPA process. Figure 3 (b, c) displays overlaid DCA traces obtained at the receiver for the back-toback two-channel OCDMA system of Figure 2 operating at an individual channel rate of 155Mb/s. In order to obtain
a clear view of the performance improvement offered by the TPA device, we first detected the signal at the output of
the SSFBG decoder using a conventional linear photodetector, and displayed the results on a DCA with a bandwidth
of 622 MHz (Fig.3b). This was then compared to the signal detected at the output of the TPA thresholder using the
same DCA (Fig.3c). The signal detected with the linear detector shows two distinct responses, corresponding to the
auto- and cross-correlation signals respectively. This can be compared to the result for the nonlinear TPA detector
that only shows a response for the auto-correlation signal. Note, because of the relatively low bandwidth of two
detectors compared to the chip rate of the codes, the fine features of the correlation signals are not resolved. The
extinction ratio between the auto- (desired) and the cross-correlation signal is just 5.3 dB. After application of the
TPA detector this figure was improved to more than 10 dB, which was the dynamic range of our detection system.
Fig. 3. (a) Plot of photocurrent as a function of incident optical signal for TPA detector used for optical thresholding and detection
experiments; DCA traces at 155Mb/s using: (b) a linear detector; (c) a nonlinear TPA detector.
5. Conclusion
This paper has presented a technique to improve the performance of an optical thresholder in an OCDMA receiver
by suppressing the effects of MAI. By using a nonlinear TPA detector, the extinction ratio between the desired autocorrelation signal and the cross-correlation noise signal was improved to more than 10dB when compared to the
extinction ratio of 5.3dB for a linear detector. This shows that TPA is ideally suited for simultaneous optical
thresholding and detection of the desired user in OCDMA systems, and should permit improved performance in
such systems.
6. References
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Based on Fibre Grating Technology”, European Conference on Optical Communications (ECOC2005) 4,867-870 (2005).
[2] H.P.Sardesai, C.C.Chang and A.M.Weiner, “A Femtosecond Code-Division Multiple-Access Communication System Test Bed”, IEEE Light.
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[3] Z.Jiang, S.Yang, D.E.Leaird, R.V.Roussev, C.Langrock, M.M.Fejer and A.M.Weiner, “Four-User 10-Gb/s Spectrally Phase-Coded OCDMA
System Operating at ~30fJ/bit”, IEEE Photon. Technol. Lett. 17, 705-707 (2005).
[4] H.P.Sardesai and A.M.Weiner, “Nonlinear fibre-optic receiver for ultrashort pulse code division multiple access communications”,Electron.
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[5] W.Cong, C.Yang, R.P.Scott, V.J.Hernandez, N.K.Fontaine, B.H.Kolner, J.P.Hertiage and S.J.Ben Yoo,“Demonstration of 160- and 320-Gb/s
SPETS O-CDMA Network Testbeds”, IEEE Photon. Technol. Lett. 18, 1567-1569 (2006).
[6] H.Folliot, M.Lynch, A.L.Bradley, T.Krug, L.A.Dunbar, J.Hegarty, J.F.Donegan and L.P.Barry, “Two-photon-induced photoconductivity
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[7] L.P.Barry, B.C.Thomsen, J.M.Dudley and J.D.Harvey, “Autocorrelation and Ultrafast Optical Thresholding at 1.5µm using a Commercial
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[8] Z.Zhang, C.Tian, P.Petropoulos, D.J.Richardson and M.Ibsen, “Fibre Bragg Grating Based Continuous-Phase Encoder-Decoders for OCDMA
Networks”, European Conference on Optical Communications (ECOC2006), paper We3.P.38 (2006).
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