Integrated spectral encoder/decoder for optical CDMA...

Details Integrated spectral encoder/decoder for optical CDMA... Integrated spectral encoder/decoder for optical CDMA...

2000-07-14

2004-10-19

Chan, Jason (Department: 2633)

Optical communications

Multiplex

Code division multiplexing

C398S077000, C398S087000, C398S183000, C398S201000, C398S214000, C385S024000, C385S037000

Reexamination Certificate

active

06807372

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to optical telecommunication technology, particularly to an optical spectral encoder/decoder for a Code Division Multiple Access (CDMA) communication system.
(2) Brief Description of the Related Art
Fiber-optics communication links are gaining great popularity because the transmission medium has a comparatively unlimited bandwidth and excellent attenuation properties.
In a broadcast and select communication system, it is desirable that a number of end-user stations be able to interconnect their respective communication links through a common bus. This ability is known as multiple access. Two common multiple access systems are the time division multiple access (TDMA) and the Code Division Multiple Access (CDMA). In a TDMA system, each user is assigned a time slot. In a CDMA system, each user is assigned a special code. Signals are separated using a correlator that accepts only signal energy from the key variable binary (sequence) code used at the transmitter.
An optical CDMA (O-CDMA) system is amenable to optical communication, because the codes can be represented by different wavelengths (colors) of light. Signals can be spectrum-encoded.
Recently there has been considerable interest in CDMA systems as an efficient protocol for local area broadcast networks. Both coherent and incoherent systems have been investigated. Coherent approaches require pico- or femtosecond pulsed lasers and phase detection systems that are generally complex, expensive, and cumbersome. Several different incoherent schemes have been proposed and demonstrated, some of which implemented bipolar coding as used in radio frequency CDMA and spread spectrum systems. The most successful incoherent systems employ complementary spectral keying (CSK), in which one spectral pattern is transmitted for data “1” and the complementary pattern is transmitted for data “0”. CSK is a very powerful and general modulation technique that, independent of the particular coding structure used, can provide significant noise immunity and signal to noise ratio improvements. Although our discussion here is based on using a broadband incoherent source, CSK can also use coherent sources, either a short pulsed laser or an array of CW laser sources. To date most implementations of CSK, including CDMA, have used free-space bulk optics, and are not compatible with the compact packaging requirements, stability, and integration needed by telecommunications industry. In this invention, we present a design for an integrated, programmable, CSK encoder/decoder in a compact reflective symmetric structure as a new device for optical CDMA application.
The first bipolar coding technique applied to an incoherent spectrum-encoded optical CDMA system was demonstrated by James Young's group at Rice University. (Young, et al, U.S. Pat. No. 5,760,941) (
FIG. 3
)
FIG. 3
shows the basic spectral encoder/decoder of the present invention. As an encoder, a super-fluorescent fiber source (SFS) of light
50
is incident on a spectral demultiplexer (an optical grating in this case)
51
, which spreads the light into a band
52
of light of different wavelengths (colors). The light band
52
is reflected by a first mirror
55
to pass through a coded mask
53
. The mask
53
allows certain wavelengths of the reflected light from the mirror
55
to be transmitted and other wavelengths of light to be reflected. The transmitted wavelengths are reflected by a second mirror
54
and focused on the spectral multiplexer (grating)
51
to generate a single light beam
56
as spectral code for digital 1's. The reflected wavelengths from the mask
53
are reflected by the first mirror
55
and focused on the grating
51
to generate a single light beam
57
as spectral code for digital 0's. These digital codes are modulated
FIG. 3
can also be used as a decoder. In this case, the incident light
50
for an encoder is replaced as an incoming signal, which is coded. The functions of the rest of the components are the same as that for an encoder. Only the incoming coded signal which correlates with the code of the mask
53
can yield 1's output as ray
56
and 0's output as ray
57
.
The use of bulky optics introduces mismatch problems among encoder and decoder that affects the correlation detection process, such as the resolution and precision of the gratings, coding masks, etc.
In this invention, we utilize the planar lightwave circuit technology to implement an integrated device for this application. We will use the above system as an example to explain our concept, our invention is suitable for other types of systems stated above as well. The essential components for all required functions of a typical O-CDMA encoder/decoder can be described in FIGS .
1
-
2
.
FIGS. 1-2
show the basic components and system of an O-CDMA system.
FIG. 1
shows an O-CDMA encoding scheme. A broadband light source
10
is incident on an array waveguide grating (AWG)
11
to spread the light source into many different colors. Since the light source intensity throughout the spectrum may not be uniform, the different color lights from the grating
11
are equalized in intensity by the attenuators
12
before passing through the coded mask
13
(indicated as “Encoding Control”). The coded mask
13
transmits certain wavelengths of light and reflects other wavelengths of light. The transmitted wavelengths may represent digital 1's and the reflected wavelengths may represent digital 0's. Each set of 1's light is focused by grating
14
to generate a single light beam
16
, which has a spectral code for digital “1”. The set of 0's light is focused by a grating
15
to generate a single light beam
17
for spectral code “0”. The output of the digital communication signal is done through the section and transmission of either a “1” signal or a “0” signal for each clock cycle. This splitting into two groups is referred to as a 1×2 switch. These two groups of spectral codes are alternately switched (modulated) with digital data
19
in a high-speed switch (modulator)
18
. The output of the high speed switch
18
the coded signal
20
sent out to be transmitted.
At a receiving station, the coded signal can be decoded as shown in FIG.
2
. The coded signal
30
irradiates a spectral demultiplexer
31
to spread the signal into different wavelengths. The different color signals pass through a coded mask
33
, which correlates the incoming signal with the particular code of the mask
33
. Certain wavelength components of the correlated signal are transmitted as digital 1's in one path. Other wavelength components are transmitted as digital 0's in another path. The 1's signals are combined on a spectral multiplexer
34
to generate a light beam
36
for spectral code “1”. The 0's signals are combined by another multiplexer
35
to generate a light beam
37
for spectral code “0”. The 1′ light beam
36
irradiates a photodiode P
1
. The 0's light beam
37
irradiates a photodiode P
2
. The outputs of P
1
and P
2
are fed to a balanced detector
38
, which yields a data output
40
. Any uncorrelated signals appear as a noise and is canceled by the balanced detector
38
.
The scheme presented in
FIGS. 1-2
offers a bipolar signaling transmission for optical CDMA systems. However, using individual gratings, attenuators and switches will make devices too complex to be integrated. We take advantage of the symmetric structures and realize the encoder/decoder in the description of the invention.
SUMMARY OF THE INVENTION
An object of this invention is to integrate an O-CDMA system on a monolithic chip. Another object of this invention is to produce uniform spectral amplitude in the processing the optical signal of un-uniform spectrum. Still another object of this invention is to provide programmable capability of the O-CDMA system. A further object of this invention is to provide a combined optical multiplexer/demultiplexer for encoding and decoding for

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