Analysis and Simulation of Optical Spectral Phase Encoding using an Array of Fabri-Perot Interferometers and Code Division Multiplexing

This paper proposes a method for a scheme of spectral phase encoding with resolution of up to 10 MHz and addressability of 40GHz, for a typical wide-band optical data carrying signal, by means of optical instruments and prior knowledge about the signal. The setup includes an array of Fabri-Perot Interferometers (FPI) which bypass the grating limitations, and a phase-only spatial light modulator (SLM) to properly encode the diffracted light. Their arrangement along with the method of Optical Code Division Multiple Access (OCDMA) was simulated, and promised fine results for various encoding schemes.

In this paper we present a new “dynamical” spectral encoding method for the optical code division multiple access (OCDMA) indoor wireless communication system. In contrast with the classical OCDMA systems, where one user transmits all the information bits using the same spreading code sequence, in the proposed “dynamical” case one user transmits a fixed number of bits using one code sequence, and thereafter the code sequence is changed. According to obtained results, the performances of the system are better because of multipath interference reduction.

Chromatic and temperature induced dispersion can both severely affect incoherent high data rate communications in optical fibre. This is certainly also true for incoherent optical code division multiple access (OCDMA) systems with multi-wavelength picosecond code carriers. Here, even a relatively small deviation from a fully dispersion compensated transmission link can strongly impact the overall system performance, the number of simultaneous users, and the system cardinality due to the recovered OCDMA auto-correlation being strongly distorted, time-skewed, and having its full width at half maximum (FWHM) value changed. It is therefore imperative to have a simple tunable means for controlling fibre chromatic or temperature induced dispersion with high sub-picosecond accuracy. To help address this issue, we have investigated experimentally and by simulations the use of a semiconductor optical amplifier (SOA) for its ability to control the chirp of the passing optical signal (OCDMA codes) and to exploit the SOA ability for dispersion management of a fibre link in an incoherent OCDMA system. Our investigation is done using a 19.5 km long fibre transmission link that is exposed to different temperatures (20 °C and 50 °C) using an environmental chamber. By placing the SOA on a transmission site and using it to manipulate the code carrier’s chirp via SOA bias adjustments, we have shown that this approach can successfully control the overall fibre link dispersion, and it can also mitigate the impact on the received OCDMA auto-correlation and its FWHM. The experimental data obtained are in a very good agreement with our simulation results.

A detailed analysis of the scalability of two-dimensional optical codes is presented and two network deployment architectures for optical code division multiple access (OCDMA) are discussed. We present the architecture and code design for a highly scalable, 2.5 Gb/s per user OCDMA system. The system is scalable to 100 potential and more than 10 simultaneous users, each with BER of less than 10-9. The system architecture uses fast wavelength-hopping, time-spreading codes. Unlike phase sensitive coherent OCDMA systems, this architecture utilizes standard on-off-keyed optical pulses allocated in the time and wavelength domains. This incoherent OCDMA approach is compatible with existing WDM optical networks and utilizes off-the-shelf components. We discuss a novel optical subsystems design for encoders and decoders that enable the realization of a highly scalable incoherent OCDMA system with rapid reconfigurability. We demonstrate the operation of 4 simultaneous users operating at OC-48 (~2.5 Gbit/s) with a power penalty of <;0.5dB and the reduction of multiple-access interference using ultra-fast all-optical sampling with newly proposed TOAD-based OCDMA receiver.

We present a novel optical code division multiple access (OCDMA) scheme based on spatial optical heterodyning for free-space optical communication systems. In this technique, in particular, the decoding process is established by means of a spatial optical heterodyne receiver. The spatial heterodyning OCDMA introduced can be considered to be in the class of spread-space techniques, which implies that there is no structural limitation on the shape of time domain signals, e.g., digital or analog modulation. For the sake of simplicity and practicality we considered on-off keying modulation. However, there is no limitation on using more advanced modulation schemes such as pulse position modulation. In this scheme a set of independent orthogonal spatial channels at both transmitting and receiving apertures is needed. Thus this class of OCDMA is applicable only to near-field free-space systems. The structures of the encoder and decoder are presented, and the wave propagation is completely analyzed by means of the Fresnel approximation for the free-space kernel. Only the effect of multiuser interference is considered. The effects of background noise and atmospheric turbulence on the performance of the system are avoided in order to highlight the main capabilities and features of spatial optical heterodyning OCDMA. A thorough statistical analysis is carried out, and the characteristic function of the sampled output is computed. The bit error rate is obtained by means of Gaussian and saddle-point approximations. Results show that the signal-to-interference ratio is approximately a function of processing gain and the number of users, as expected. The optical spatial heterodyning OCDMA technique can prove to be of importance in optical switching and free-space optical communication systems.

The transmission of both optical code division multiple-access (OCDMA) and wavelength division multiplexing (WDM) users on the same band is investigated. Code pulses of spectral amplitude coding (SAC)/optical code division multiple-access (CDMA) are overlaid onto a multichannel WDM system. Notch filters are utilized in order to suppress the WDM interference signals for detection of optical broadband CDMA signals. Modified quadratic congruence (MQC) codes are used as the sig- nature codes for the SAC/OCDMA system. The proposed system is sim- ulated and its performance in terms of both the bit-error rate and Q-factor are determined. In addition, eavesdropper probability of error-free code detection is evaluated. Our results are compared to traditional nonhybrid systems. It is concluded that the proposed hybrid scheme still achieves acceptable performance. In addition, it provides enhanced data confiden- tiality as compared to the scheme with SAC/OCDMA only. It is also shown that the performance of the proposed system is limited by the interference of the WDM signals. Furthermore, the simulation illustrates the trade- off between the performance and confidentiality for authorized users.

The construction of a special family of 2n codes, so-called &apos;2n prime-sequence&apos; codes, for optical code division multiple access (CDMA) has recently been investigated. Since the codes pose the algebraic properties of the prime-sequence and 2n codes, those optical coding architectures, which are especially designed for either of the two codes, can be jointly employed to optimally cope with various constraints (e.g. power budget and cost) imposed by the systems using optical CDMA. As a result, an optical coding architecture, which is particularly attractive for ultrafast optical signal processing and waveguide implementation for the future, high capacity all-optical CDMA networks with potentially 3 dB ideal power ideal, has been proposed. However, the cardinality of the codes is very limited and shown to approach zero when the code weight is sufficiently large. A generalised family of 2n prime-sequence codes, which has the same cardinality as the original prime-sequence codes, is constructed and supported with complete mathematical proofs.

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.

Optical code division multiple access has been proposed as an alternative multiple access scheme for multiuser fiber-optic communication systems. Many previous works on the receivers for such systems have focused on the correlation receiver which is optimal in the single user case. However, it has been shown that the performance of the correlation receiver is unsatisfactory when there is a large number of interferers. Multiuser detection becomes an appealing method to improve the system performance. In this paper, a new multiuser detector for optical code division multiple access communication systems, referred to as the multistage decorrelating receiver, is proposed. Its structure is much simpler than the optimal multiuser detector at the expense of slight performance degradation. Both simulation and analysis of the receiver are carried out. Comparison of the proposed detector with several other receivers is also presented.

We experimentally demonstrate what we believe to be a novel detection scheme for interfacing asynchronous optical code division multiple access (CDMA) signals with an electronic clock and data recovery system that operates only at the baseband bandwidth. This allows using a large optical bandwidth expansion factor in which the optical chip rate is much larger than the bandwidth of the optoelectronic receiver. The received optical CDMA signal is launched into a four-wave-mixing-based wavelength-aware all-optical front end that rejects multiaccess interference, followed by an amplitude-noise suppression stage comprised of a semiconductor optical amplifier. The clean signal is then converted into a non-return-to-zero-like signal by a baseband receiver. Using the proposed detection scheme, asynchronous transmission and detection of optical CDMA signals is implemented. With the novel detection scheme, the classic CDMA near-far problem is mitigated, and error-free detection is easily obtained.

Optical code-division multiple access (OCDMA) is an interesting subject of research because of its potential to support asynchronous, bursty communications. OCDMA has been investigated for local area networks, access networks, and, more recently, as a packet label for emerging networks. Two-dimensional (2-D) OCDMA codes are preferred in current research because of the flexibility of designing the codes and their higher cardinality and spectral efficiency (SE) compared with direct sequence codes based on on-off keying and intensity modulation/direct detection, and because they lend themselves to being implemented with devices developed for wavelength-division-multiplexed (WDM) transmission (the 2-D codes typically combine wavelength and time as the two dimensions of the codes). This paper shows rigorously that 2-D wavelength/time codes have better SE than one-dimensional (1-D) CDMA/WDM combinations (of the same cardinality). Then, the paper describes a specific set of wavelength/time (W/T) codes and their implementation. These 2-D codes are high performance because they simultaneously have high cardinality (/spl Gt/10), per-user high bandwidth (>1 Gb/s), and high SE (>0.10 b/s/Hz). The physical implementation of these W/T codes is described and their performance evaluated by system simulations and measurements on an OCDMA technology demonstrator. This research shows that OCDMA implementation complexity (e.g., incorporating double hard-limiting and interference estimation) can be avoided by using a guard time in the codes and an optical hard limiter in the receiver.

With avionic platforms moving toward integrated optical networks, platforms are replacing electrical components with their optical counterparts. Replacing electronic components and copper wire with photonic chips and optical fibers provide large means of reducing the overall Size Weight and Power (SWaP), which is highly desirable for deployed avionic platforms [1–2]. In an optical LAN, optical fibers provide a conduit for high data rates with low loss and ultra low crosstalk between Wavelength Division Multiplexing (WDM) channels used by the connected modules. However, as a LAN expands and evolves with demands of new functionality, the number of modules increase and the problem of efficiently communicating between each element with a high degree of confidentiality becomes exponentially more difficult. To address the interconnect demand, an optical cross-connect with ultra small SWaP specifications capable of reconfiguring paths to selectively connect modules is necessary. However, in LAN communication between modules, the security level may not necessarily be the same, and the optical extinction ratio between unsecure channels may not be enough for practical deployment. For military and commercial applications, where security between data channels is critical, Optical Code Division Multiple Access (OCDMA) can be employed to achieve the appropriate security between channels in a network [3–5]. In this paper we present a WDM reconfigurable bi-directional optical cross-connect using chip-scale silicon photonics, with functionality of serving as an encoding source for OCDMA. The cross-connect utilizes slow-light photonic crystal waveguides as directional couplers to reduce the size and power requirements for reconfigurable operation, while similar photonic crystal elements can be used to impose an encoding technique on the propagating optical data signals. This proposed architecture allows for fast, efficient, low power, bi-directional routing for WDM or OCDMA optical networks.

Multiple Access Interference (MAI) is the major reason for the system degradation in the Optical Code Division Multiple Access (OCDMA). MAI increases with the number of simultaneous users, resulting in deterioration in the performance and limits the maximum number of simultaneous users. Optical Hard Limiter (OHL) is known to improve the performance of the conventional OCDMA system. However, in the present of other sources of noise, the improvement is not considerable. Moreover, the need for dynamic threshold value in the conventional OCDMA receiver to recover the data is very demanding requirement particularly in the high speed LANs. To overcome these limitations, some studies have been conducted on the Optical Parallel Interference Cancellation (OPIC) through this paper. OHL is placed in front of the receiver side to mitigate the effect of MAI. The study focuses on the lowest threshold value i.e. Th=1. The results are compared to the conventional OCDMA and it shows that, OHL is effective to reduce the power of MAI and a better performance can be obtained by selecting the smallest threshold value.

A direct sequence optical code division multiple access technique is investigated. This paper introduces, first, the principles definitions of Code Division Multiple Access in radio communications. After that, the performances of Optical Code Division Multiple Access are evaluated in association with a realistic amplified optical channel model.

Green's function calculating of electric fields in surface-channel charge-coupled devices : R. M. Barsan. Int. J. Electronics44, (2) 177 (1978)