Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2001-09-20
2004-09-28
Prasad, Chandrika (Department: 2839)
Optical waveguides
With optical coupler
Switch
C385S019000
Reexamination Certificate
active
06798941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to optical switches. In particular, the invention relates to optical switches used in multi-channel optical communications networks and having controlled transmissivity for different channels.
2. Background Art
Modern communications networks are increasingly based on silica optical fiber, which offers very wide bandwidth including several transmission bands usable for communications. In a conventional point-to-point optical communications link, at the transmitter, an electrical data signal is used to modulate the output of a semiconductor laser emitting, for example, in the 1550 nm band, and the modulated optical signal is impressed on one end of the silica optical fiber. Other transmission bands at 850 nm and 1310 nm are also available. On very long links, the optical signal may be amplified along the route by one or more optically pumped erbium-doped fiber amplifiers (EDFAs) or other optical amplifiers. At the receiver, the optical signal from the fiber is detected, for example, by an optical p-i-n diode detector outputting an electrical signal in correspondence to the modulating electrical signal. The transmission bandwidth of such systems is typically limited by the speed of the electronics and opto-electronics included in the transmitter and receiver. Speeds of 10 gigabits per second (Gbs) are available in fielded systems, and 40 Gbs systems are reaching production. Further increases in the speed of the electronics will be difficult. These speeds do not match the bandwidth inherent in the fiber, which is well in excess of one terabit per second. Furthermore, such fast optical transmitters and receivers are expensive and may require special environmental controls.
Transmission capacity of fiber systems can be greatly increased by wavelength division multiplexing (WDM) in which the optical signal is generated in a transmitter including multiple semiconductor lasers emitting at different respective wavelengths within the transmission band. The 1550 nm transmission band has a bandwidth of about 35 nm, determined by the available amplification band of an EDFA. Other amplifier types and amplification bands are being commercialized so that the available WDM spectrum is growing each year. In a WDM system, each laser is modulated by a different electrical data signal, and the different laser outputs are optically combined (multiplexed) into a multi-wavelength optical signal which is impressed on the optical fiber and which together can be amplified by an EDFA without the need to demultiplex the optical signal. At the receiver, an optical demultiplexer, such as one based on a diffraction grating, an arrayed waveguide grating, or a thin-film filter array, spatially separates the different wavelength components, which are separately detected and output as respective electrical data signals. For an N wavelength WDM wavelength grid, the fiber capacity is increased by a factor of N using electronics of the same speed. Dense WDM (DWDM) systems are being designed in which the WDM comb includes 40, 80 or more wavelengths with wavelength spacings of under 1 nm. Current designs have wavelength spacings of between 0.4 and 0.8 nm, that is, frequency spacings of 50 to 100 GHz. Spectral packing schemes allow for higher or lower spacings, dictated by economics, bandwidth, and other factors.
Point-to-point WDM transmission systems as described above enable very high transmission capacity in networks having simple connectivity. However, a modem communications network
10
, such as that illustrated in
FIG. 1
, tends to be more complex and requires that the WDM concept be expanded to cover not only transport but also switching in a complex communication network. This network
10
has multiple switching nodes
12
switching signals between multiple terminals
14
at the edges of the network
10
. Fiber optic links
16
interconnect the switching nodes
12
and terminals
14
. The switching nodes
12
should be capable of switching single-wavelength WDM channels in different directions with the directions being changeable over some time period. The network diagram of
FIG. 1
is highly conceptual but emphasizes the switching requirement of the illustrated complexly connected network. Such a WDM network
10
achieves high capacity through multi-wavelength channel transport along complexly fiber-interconnected routes. To achieve a flexible dynamic interconnection within the network
10
, it is desirable that the fiber infrastructure be wired differently for different wavelengths. As a result, the optical switches
12
should not only be dynamically reconfigurable between multiple ports but also the switch state should be able to be wired simultaneously, yet independently in each wavelength channel. That is, the cross-connects
12
should be wavelength selective. Channel paths
20
are illustrated in
FIG. 1
in which three wavelength channels at wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
can enter a switching node
12
on a single fiber
16
but be switched at its outputs to three different fibers
16
. Also note that this architecture allows the reuse of wavelengths between different pairs of terminals
14
, for example, the wavelength &lgr;
1
as illustrated. Frequency reuse further increases the traffic capacity of the network
10
. Such switching requirements apply as well to a more regularly configured ring network having switching nodes distributed around a fiber ring with a terminal associated with each node.
The practice in the recent past has been to form each link
16
in the network
10
as a separate point-to-point WDM system so that each switching node
12
includes an optical receiver and an optical transmitter for each wavelength channel. The electrical data signals derived from the optical receiver are spatially switched by conventional electronic switches and then converted back to optical form for transmission on the next link. However, a point-to-point design does not integrate well into a complex network such as in FIG.
1
. The required number of optical receivers and transmitters become very expensive. Also, such a system requires the electronics and opto-electronics at each switching node to be operating at the highest data rate supported by the network. Such a system is unduly costly when some end-to-end links require only modest data rates, and the system is difficult to upgrade since all the nodes must be upgraded at the same time.
For these and other reasons, there is much interest in all-optical communication networks in which each switching node demultiplexes the multi-wavelength WDM signal from an input fiber into its wavelength components, spatially switches the separate single-wavelength beams in different directions, and multiplexes the switched optical signals for retransmission on one or more output fibers. Thus, a wavelength-routing node will generally switch WDM channels in an all-optical manner unless there is a specific need to electrically regenerate a specific subset of channels, for example, to remove accumulated optical noise through electrical regeneration or to perform wavelength conversion. Indeed, the goal is to reduce the number of conversions from optical to electrical and back to optical at each intervening node in a fiber optic end-to-end link.
An all-optical wavelength-selective switching node
12
may be implemented by a wavelength cross connect (WXC) such as a 3×3 WXC
22
, represented in the simple schematic diagram of
FIG. 2
, coupling three input ports
24
to three output ports
26
, each port being typically equated with a transmission fiber in the network. The WXC
22
has the capability of switching any wavelength channel on any input port
24
to the corresponding wavelength channel on any output port
26
. The design is not limited to a 3×3 cross connect with three input and three output ports and may be generalized to a W×W cross connect with W input ports and W output ports, where W is greater than 1. Systems under development have value
Farhan Fariborz
Golub John E.
Smith David A.
Guenzer Charles S.
Movaz Networks, Inc.
Prasad Chandrika
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