Fiber bragg grating-based optical CDMA encoder/decoder

Details Fiber bragg grating-based optical CDMA encoder/decoder Fiber bragg grating-based optical CDMA encoder/decoder

2001-05-04

2003-09-02

Sanghavi, Hemang (Department: 2874)

Optical waveguides

With optical coupler

C359S199200, C359S199200, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06614950

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
FIELD OF THE INVENTION
This invention relates to an optical spectral coding scheme for a high-performance optical code-division multiple-access (OCDMA) network system. The encoder and decoder are structured with cascaded fiber Bragg grating (FBG) devices. The scheme can eliminate the multiple-access interference (MAI), and can promote the CDMA system capacity. Using the simple coder structure and low cost devices, the invention can be used on switching routers to connect local network computers, or on exchange modules for signal switching between network nodes. It is applicable to Asymmetric Digital Subscriber Loop (ADSL), or Cable Modem to connect with digital home network, local area network (LAN), and Internet etc.
BACKGROUND OF THE INVENTION
Optical networking is one way to provide a range of telecommunications services to meet the growing demands of an information-based society. However, existing multiple access implementations for LAN (local-area network) or MAN (metropolitan-area network) networks are apparently inadequate. Optical code-division multiple-access (OCDMA) offers versatile connection between numerous local network users, along with generous control on the wavelength stability or network synchronization requirements. Hence OCDMA techniques have drawn much attention in recent years.
Fiber Bragg gratings (FBG) are produced by exposure of photosensitive fiber to ultraviolet light. They have a refractive index that is spatially periodic along the propagation axis of a fiber. The desirable filtering characteristics of fiber Bragg gratings are disclosed by K. O. Hill and G. Meltz (
IEEE J. Lightwave Technology,
vol. 15 (8), pp. 1263-1276, 1997) and have been employed in some optical devices. For example, the Distributed Bragg Reflector (DBR) is designed with Bragg gratings in the laser resonant cavity. The DBR accumulates energy in the resonant cavity and emits light when the accumulated energy reaches a threshold. Fiber gratings have also been used for performing optical measurements. For example, due to the Bragg wavelength's change with tension or temperature, fiber Bragg gratings can measure the variation of external factors. In addition, the gratings can be used for the gain compensation of an Erbium-Doped Fiber Amplifier (EDFA), for the analyses improvement of spectral analyzer, and so on.
In optical fiber communications, fiber Bragg gratings can be employed as chromatic compensators. The propagation of long-wavelengths is slower than the propagation of short-wavelengths, causing the phenomenon of pulse broadening (or “dispersion”) in fiber transmission. This pulse broadening tends to reduce the data bit rate of digital communication. Fiber Bragg gratings provide “chirped apodization” by reflecting long-wavelength components at the front end of the grating and short-wavelength components at the rear end of the grating. This compensates for chromatic dispersion by ensuring that the full light band spends the same amount of time to pass through chromatic dispersion and grating compensation. Another popular application of FBG is the realization of light wave filter with the desired wavelengths on the reflective end. Such characteristics of fiber Bragg gratings are utilized in this invention to construct a new scheme of optical CDMA encoder/decoder devices.
J. A. Salehi et al. disclosed the technology of code-division multiple-access (CDMA) [
IEEE J. Lightwave Technology,
vol. 8 (3), pp. 478-491, 1990] for the application of optical fiber communications. In the early periods, bipolar codes with good correlation properties, such as M-sequence or Gold code, were adopted for the optical CDMA communications. At the transmitter end, data bits are encoded into unipolar optical signals. The receiver makes an Electrical/Optical (E/O) conversion and has a bipolar address decoding procedure in the electrical domain. Since after E/O conversions, the receiver must go through the procedure of Sequence Inverse Keying (SIK), the system is named as the SIK-CDMA. The SIK-CDMA system needs multiple electrical/optical and optical/electrical conversions and hence has serious limitation on the data transmission rate. Thereafter, research efforts have aimed at the all-optical signal processing to promote the data transmission rate.
In the light wave domain, the optical signal is inherently a unipolar system. As stated by F. R. K. Chung et al. [
IEEE Trans. on Inform. Theory,
vol. 35 (3), pp. 595-604, 1989], unipolar signature code such as optical orthogonal code (OOC) and modified prime code (MPC) can have the same correlation characteristics as those of the traditional bipolar sequences. Since the cross-correlation of unipolar codes is incapable of achieving complete orthogonality, the number of “1” in code sequences must be restricted to improve the correlation characteristic. Unfortunately, this means that a given code sequence length should have relatively few “1”. The number of unipolar code sequences is therefore far less than that of the traditional bipolar sequence codes.
One of the early demonstrations of OCDMA uses optical delay lines and optical orthogonal codes for OCDMA time domain coding. The time-encoded optical CDMA coder is as presented in FIG.
4
. In this delay line configuration of
FIG. 4
, the incoming signal is split into several independent paths in which each signal is delayed according to the specific delay elements of the desired optical orthogonal codes. The tapped delay line scheme suffers from high splitting loss due to intensity splitting among the optical delay lines. Also, to comply with the hasty growth on the number of users, one needs to substantially lengthen the code sequence to promote the system capacity. This increases system expenditures and is unsuited for the economical benefit.
On retracing the past technology, there were cases on utilizing the concept of optical phase coding to implement code-division multiplexing. These technologies need coherent, ultrashort pulses, with transmitter and receiver being wavelength and phase coherent. L. R. Chen et al. investigated ultrashort pulse propagation in fiber Bragg gratings [
IEEE Journal of Quantum Electronics,
vol. 34 (11), pp. 2117-2129, 1998] with applications on Wavelength Division Multiplexing (WDM) and Code-Division Multiple-Access (CDMA) systems. The adopted incident optical source is broadband ultrashort pulse. The coding scheme is the table-lookup type of frequency hopping. The decoder grating was arranged in a reverse order to that of the encoder grating to accomplish the same optical path for every component spectral chip.
A well-known frequency encoder for optical broadband sources is disclosed in the article by M. Kavehrad and D. Zaccarin [
IEEE J. Lightwave Technology,
vol. 13 (3), pp. 534-545, 1995], and is shown in FIG.
5
. This is the typical representative of incoherent broadband CDMA systems. The optical frequency coder of
FIG. 5
consists of a pair of diffraction gratings placed at the focal planes of a unit magnification confocal lens pair. The first grating spatially decomposes the spectral components present in the incoming optical signal with a certain resolution. A spatially patterned mask is inserted midway between the lenses at the point where the optical spectral components experience maximal spatial separation. After the mask, the spectral components are re-assembled by the second lens and second grating into a single optical beam. The mask can modify the frequency components in phase and/or in amplitude, depending on the coherence property of the incident optical source. The apparatus has been used with high-efficiency for temporal shaping of short pulses. An example has been illustrated by R. A. Griffin et al. [
IEEE J. Lightwave Technology,
vol. 13 (9), pp. 1826-1837, 1995] in an optical coherence coding scheme to implement optical frequency hopping CDMA. This article

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