Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2001-04-06
2003-05-13
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C359S332000, C372S044010
Reexamination Certificate
active
06563627
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to an optical wavelength converter. In particular, the invention relates to an optical wavelength converter incorporated into an optical switching element.
2. Background Art
Converting the wavelength of a light signal has always presented a challenge, particularly in optical communications systems in which an optical carrier is modulated with a data signal and it is desired to convert the wavelength of the optical carrier while maintaining the data signal without the need to convert to an intermediate electrical signal corresponding to the data.
Wavelength conversion has been suggested for greatly increasing the capacity and flexibility of switched optical networks. Wavelength division multiplexing (WDM) has long been known in which, for example, a silica optical fiber carries a number of discrete optical carriers in the 1525 to 1575 nm band, each impressed with its separate data signal. Electronic data signals are presently limited to about 10 to 40 gigabits per second (Gbs), but if these data rates are multiplied by the number W of WDM channels, where W is reaching 80 and higher, the total data capacity of a single fiber exceeds a terabit per second (Tbs). Such high data rates are anticipated to be needed as more visual forms of data begin to dominate the communications networks.
However, modem communication systems are based upon a complex network connected at its edges to users with typically many switching nodes separating a pair of users. A primary example is the Internet based upon the Internet Protocol (IP). Signals need to routed through the nodes of the network, which requires routers at the nodes to perform such selective routing. Heretofore, routers have been based on electronic switches, typically crossbars. As a result, WDM signals need to be optically demultiplexed, electrically detected, electrically switched, impressed upon respective optical carriers, and optically multiplexed before the components signals are sent in their respective proper directions. That is, regeneration of the optical signals needs to be performed with currently available routers. This design does not scale well with vastly increased optical channels.
All-optical WDM communication networks have been proposed in which wavelength cross connects (WXCs) located at nodes in the network redirect the separate WDM signals according to their wavelength. However, conventional WXCs require switching times that are much longer than the duration of packets typical of IP networks and other flexible communications networks. Further, WXCs maintain the carrier wavelength of the data signal so that using a wavelength identifier to route a signal between pairs of a number of users presents a significant network management problem for large networks in reusing wavelengths in different parts of the network.
Many of these problems can be overcome by the use of wavelength converters. In “Wavelength conversion technologies for WDM network applications,
Journal of Lightwave Technology,
vol 14, no. 6, June 1996, pp. 955-966, I have described some of the network applications for wavelength converters and different ways of implementing them. In U.S. Provisional Application, Ser. No. 60/185,640, filed Feb. 29, 2000 and in U.S. patent application Ser. No. 09/654,384, filed Sep. 1, 2000, I describe an optical router implemented with wavelength converters capable of throughput capacity of petabits per second (10
15
bits per second). This patent application is incorporated herein by reference in its entirety.
A switching fabric
10
illustrated in
FIG. 1
can be used in such a high-speed optical router in which WDM signals on K input optical channels
12
, typically optical fibers, are switched to any of K output optical channels
14
. Optical demultiplexers
16
associated with each of the input fibers
12
separate the optical WDM signal into its W wavelength components. Input wavelength converters
18
convert the wavelength of the optical carriers on each of the demultiplexers outputs to a selected one of a number of wavelengths before these signals are input to a WK×WK wavelength router
20
. Such a wavelength router
20
may be implemented as one or more array waveguide gratings (AWGs) and may be a passive device in which the switching direction of an input signal from any input port to any output port is determined completely by the carrier wavelength of the input signal as impressed by the input wavelength converters
18
. Note that the size of the wavelength router
20
may be reduced if some limitations are imposed on the number of allowed switching paths. The output ports of the wavelength router
20
are connected to respective output wavelength converters
22
which freely convert the carrier wavelengths to new values determined by the WDM wavelength comb of the output channels
14
. Optical multiplexers
24
receive W wavelength carriers and combines them onto associated optical output channels
14
.
In order to achieve the capacity and flexibility of the optical router described in the aforecited patent application, the wavelength converters
18
,
22
must be able to switch between any of the WDM wavelengths and should be able to do this in a time not much greater than the duration of the packet length, which in a typical design is a random length, for example 48 bytes or 1.5 kilobytes. Some signal delay is tolerated at the nodes, but it should be minimized for low signal latency on the network.
One type of wavelength converter applicable to optical switching is a gated wavelength converter, which may be implemented in a Mach-Zehnder interferometer converter
30
, described briefly in my technical article cited above and illustrated in schematic plan view in FIG.
2
. The converter
30
is integrated on, for example, an InP opto-electronic chip
32
formed with a Mach-Zehnder interferometer
34
having optical waveguides formed into two arms
36
,
38
. Forward biased optical semiconductor amplifiers (SOAs) are formed in active region
40
,
42
of the both arms
36
,
38
. Forward biasing means that a positive voltage +V is applied to the p-type side of the semiconductor diode relative to the n-type side, which is typically grounded. All the illustrated waveguide, including that in the active regions
40
,
42
is single-mode through the band of the optical carriers. An optical data signal at carrier wavelength &lgr;
1
is input to a waveguide coupler at one end of the upper arm
36
configured to supply the data signal only to the upper arm
36
. An unmodulated probe signal at wavelength &lgr;
2
is input to another coupler at the other end of the upper arm
36
. Note that the probe signal counter-propagates relative to the &lgr;
1
data signal. That coupler divides the probe signal between the upper and lower arms
36
,
38
. The probe signal, after propagating through the two arms
36
,
38
is recombined by the optical coupler on the left to an output waveguide
44
.
The optical amplifiers in both arms
36
,
38
are operated near or in optical saturation so that when the power of the on-state of the &lgr;
1
optical data signal passes through the upper active region
40
, it causes a change of phase relative to the lower active region
42
, which does not experience the extra &lgr;
1
signal power. In the off-state of the &lgr;
1
optical data signal, there is no phase difference between the two active regions
40
,
42
. Sometimes differential biasing imposes a time-invariant phase difference between the two arms
40
,
42
. As a result of the induced temporally varying phase difference, the &lgr;
2
probe signal will experience a phase differential in the two Mach-Zehnder arms
34
,
36
depending upon the modulation state of the &lgr;
1
optical data signal. The waveguide coupler at the end of the Mach-Zehnder interferometer opposite the &lgr;
2
probe source receives both &lgr;
2
signals and combines them onto the output waveguide
44
. Depending upon the relative phase differences between the
Guenzer Charles S.
Lee John D.
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