Optical channel selector

Details Optical channel selector Optical channel selector

1999-01-27

2003-06-17

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C385S024000, C385S037000, C385S032000, C385S027000, C359S199200

Reexamination Certificate

active

06580534

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to optical communication systems, and more particularly, to channel selectors useful for wavelength division multiplexed (WDM) optical communication systems.
DESCRIPTION OF THE RELATED ART
Optical communication systems typically include a variety of devices (e.g., light sources, photodetectors, switches, optical fibers, modulators, amplifiers, and filters). For example, in the optical communication system
1
shown in
FIG. 1
, a light source
2
generates optical signals
3
. Each optical signal
3
comprises a plurality of wavelengths. The optical signals
3
are transmitted from the light source
2
to a detector
5
. Typically, an optical fiber
4
transmits the optical signals
3
from the light source
2
to the detector
5
. The optical fiber
4
has amplifiers (not shown) and filters (not shown) positioned along its length. The amplifiers and filters propagate the optical signals
3
along the length of the optical fiber
4
from the light source
2
to the detector
5
.
Some optical communications systems include a single channel. Single channel optical communications systems transmit optical signals at a single specified channel wavelength. The transmission rate of a single channel is about 2.5 Gbits/sec (Gigabits/second). Consequently, the transmission capacity of single channel optical communication systems is limited to about 2.5 Gbits/sec.
One technique used to increase the transmission capacity of optical communication systems is wavelength division multiplexing (WDM). WDM optical communication systems are multi-channel systems. Each channel of the multi-channel system transmits optical signals using wavelengths that are different from one another. In WDM systems, optical signals are simultaneously transmitted over the multiple channels.
WDM systems are desirable because their total transmission capacity increases relative to single channel systems as a function of the number of channels provided. For example, a four-channel WDM optical communication system can transmit optical signals at a rate of about 10 Gbit/sec (4×2.5 Gbit/sec). Since a single channel system only transmits optical signals at a rate of about 2.5 Gbit/sec, the four-channel WDM system has about 400% greater capacity than the single-channel system.
In a WDM optical communication system, the optical signals are multiplexed over multiple channels. Thereafter, the multiplexed optical signals are transmitted over a waveguide (e.g., optical fiber). At the receiving end, the multiplexed optical signals are demultiplexed such that each of the multiple channels is routed individually to a designated receiver by a channel selector. Typically, the channels are routed using mode couplers (e.g., multi-mode interference couplers or star couplers) or diffraction gratings.
In order for the channel selector to route each channel to the designated receiver, it is desirable that the wavelengths of the selected channels correspond to the passband of the channel selector. The term passband as used in this disclosure refers to the band of wavelengths transmitted by the channel selector, denoted as
10
in FIG.
2
A. When the wavelengths of the selected channels and the passband of the channel selector do not correspond, the channel selector can not route the selected channels to the designated receivers. Additionally, it is desirable for the channel selector to have a passband with sharp cut-off regions. The cut-off regions of the passband refer to the transition region from the passband to the stopband, denoted as
12
in FIG.
2
A. The term stopband as used in this disclosure refers to the band of wavelengths not transmitted by the channel selector, denoted as
11
in FIG.
2
A. Sharp cut-off regions have steep slopes. When the passband of the channel selector does not have cut-off regions with steep slopes adjacent channels potentially interfere (crosstalk) with each other. Interference between adjacent channels, denoted as
14
in
FIG. 2B
, causes transmission errors.
Oda, K. et al., “A Wide-Band Guided-Wave Periodic Multi/Demultiplexer with a Ring Resonator for Optical FDM Transmission Systems”,
IEEE J. Light. Tech.,
Vol. 6, No. 6, pp. 1016-1022 (1988) describes a channel selector for use in a WDM optical communication system. The channel selector has a structure which combines a single Mach-Zehnder Interferometer (MZI) with a ring resonator. In Oda et al., the coupling ratio between the ring resonator and the MZI has a fixed value of 8/9. This fixed value for the coupling ratio limits the passband width of the Oda et al. channel selector to about half the free spectral range (FSR). Typically, the FSR describes the band of multiplexed channel wavelengths transmitted in a WDM system. Since the passband width of the channel selector only corresponds to half the wavelengths of the FSR, the Oda et al. channel selector is useful for selecting only a limited number of multiplexed channels.
Another channel selector for a WDM optical communication system, an autoregressive moving average (ARMA) filter, is described in Jinguji, K., “Synthesis of Coherent Two-Port Optical Delay-Line Circuit with Ring Waveguides”,
IEEE J. Light. Tech.
, Vol. 14, No. 8, pp. 1882-1898 (1996). Autoregressive moving average filters include both poles and zeros in their transfer functions. The Jinguji ARMA filter includes a structure that has a cascade of MZIs with a single ring resonator on one arm of each MZI. Both the MZIs and the ring resonators also include phase shift controllers. Such a filter architecture is complex and difficult to fabricate since a large number of MZIs are needed for channel selection. Additionally, all Jinguji filter architectures must be ring resonator based structures.
Thus, channel selectors for use in WDM systems that have advantages over prior art channel selectors are sought.
SUMMARY OF THE INVENTION
The present invention is directed to a channel selector useful for a multi-channel WDM optical communications system. The channel selector selects one or more channels from input multi-channel optical signals for routing to predetermined destinations within the optical communication system. Multi-channel optical signals include two or more channels, wherein each channel transmits optical signals of a specified wavelength. The channel selector selects one or more channels by first splitting (dividing) portions of the input multi-channel optical signals across two or more optical paths. Thereafter, a desired phase response is applied to the split portions of the multi-channel optical signals. The desired phase response is applied to the split portions of the multi-channel optical signals so the channels therein interfere with each other either constructively or destructively when the split portions of the multi-channel optical signals are combined.
The desired phase response is applied to the split portions of the multi-channel optical signals by shifting the phase of each channel in the multi-channel optical signal as a function of frequency. Phase-shifted channels which have approximately the same phase constructively interfere with each other when the sum of their magnitudes is greater than zero. Phase-shifted channels destructively interfere with each other when the sum of their magnitudes approximates zero. Channels which interfere with each other constructively are selected by the channel selector. Channels which interfere with each other destructively are not selected by the channel selector.
The channel selector of the present invention has a structure which includes a plurality of input ports, a plurality of output ports, a splitter, a combiner, one or more all-pass optical filters, and a plurality of optical paths. Each optical path has one input and one output. The plurality of input ports are coupled to the inputs of the plurality of optical paths via the splitter. The outputs of the plurality of optical paths are coupled to the plurality of output ports via the combiner. The one or more all-pass optical filters are coupled to the

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