Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-05-08
2003-12-30
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S011000, C385S014000, C385S024000, C385S037000, C385S039000, C385S041000, C385S131000, C385S132000, C398S053000, C398S065000, C398S083000, C398S084000, C398S085000
Reexamination Certificate
active
06671437
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally in the field of optical devices and relates to a method and device for the tunable frequency selective filtering of optical signals, particularly useful for adding or dropping channels in a wavelength division multiplexing optical communication system.
BACKGROUND OF THE INVENTION
Optical transmission systems, which are based on wavelength division multiplexing (WDM), achieve high information capacities by aggregating many optical channels onto a single strand of optical fiber. Tunable filters play a critical role in WDM communication systems. A tunable filter, which can redirect and route wavelengths, is used in conjunction with tunable lasers to create a tunable transmitter, midway in the fiber in wavelength for add and drop multiplexing applications, and at the receiving end in conjunction with a broad band detector for a tunable receiver.
In applications of add and drop multiplexing, the tunable filter is often termed a three (or more) port device, with an input, express, and drop (add) ports. In these applications, the network traffic enters the device at the input, with most of the channels leaving at the express port. The dropped channels are redirected to the drop port, while the added channels are input from the add port. During all times, the network is operational, and in particular, when tuning the filter from one channel to another, a critical feature of the filter is termed “hitless tuning”, which is the ability to tune from one channel to another without disturbing (“hitting”) any of the express channels, since this would constitute a traffic disruption in the network.
Tunable filters in state of art implementations fall under the following two categories:
(1) Tunable filters based on spatial distribution of the different channels and switching of the channels to be dropped. Here, tunability is achieved by applying spatially distinct switches, which switch different channels to the drop port.
(2) Tunable filters based on a change in the frequency of operation by physical changes in the optical filter medium. These are the so-called “scanning” tunable filters”, since they scan over frequencies.
Hitless tuning can easily be achieved in the first implementation. However, the first implementation suffers from many other drawbacks, especially energy loss, cross talk, and price, all of which make its use difficult for optical networks. The second type of filter is a preferred solution for most optical networks.
U.S. Pat. No. 6,292,299 describes a hitless wavelength-tunable optical filter, which includes an add/drop region and a broadband optical reflector adjacent thereto. The operation of the filter is based on selectively repositioning an optical signal in the add/drop region while adding or dropping an optical wavelength channel, and on the use of a broadband optical reflector, while tuning to a different optical wavelength channel.
“All fiber active add drop multiplexer” [IEEE Photonics Technology Letter, Vol. 9, No. 5 p 605] describes an architecture to be used as a reconfigurable router for exchanging channels between two fibers or as a reconfigurable add/drop multiplexing filter. The architecture consists of a Mach-Zender interferometer with identical gratings written in each arm, one pair of grating for each wavelength to be added or dropped. Each grating pair is also accompanied by a phase shifter, which is a thermo-optic heater.
SUMMARY OF THE INVENTION
There is a need in the art to facilitate the tuning of a frequency selective filter by providing a novel optical method and device for continuously flowing light through the frequency selective filter. The frequency selective filter may perform wavelength dropping or adding function.
The present invention utilizes separating at least a portion of the power of a selected frequency component from the remaining portion of the multi-frequency input light signal and directing the separated light components along two spatially separated optical paths, creating a phase delay between these optical paths by affecting the phase of the light component of said at least portion of the selected frequency band. This enables to either direct all the frequency components of the input light to a common first output channel with no power in a second output channel, or direct the entire power of the selected frequency band and all other frequencies along, respectively, the second and first output channels, depending on the phase delay between the two spatially separated optical paths. Thus, on one operational mode of the device according to the invention, all the input light is output in one channel, while the other output channel serving for dropping or adding function is inoperative, and in the other operational mode of the device, the selected frequency band is fully spatially separated from all other frequencies, and can therefore be either dropped or added to another optical signal.
The above is implemented by passing the input light through a first tunable frequency coupling element having two input ports, of which one is used for receiving the input multi-frequency light and the other is unconnected. The coupler further has two output ports associated with two spatially separated optical paths (waveguides). The optical path difference between the spatially separated waveguides can be adjusted by various well-established means. The phase difference can be adjusted between zero path difference, both waveguides having exactly the same optical length, and 1800 path difference, the optical length difference between the waveguides being equal to half the wavelength. The two waveguides are input into a second, reciprocal frequency-coupling element, which has two inputs and two outputs. The light input from both ports is recombined in the coupler, whereas in the first coupler, only one input port was active and in the second coupler both input ports are active and the combination of the two ports in the second coupler depends on the relative phase of the input ports.
The phase delay between the two spatially separated optical paths can be continuously adjustable up to 180°. The output at the second coupling element depends in a continuous manner on the phase delay between the two spatially separated optical paths, such that for a zero phase delay between the optical paths, the tunable selected frequency band of the input light is in one output channel of the device, and the remaining spectral content is in the other output channel, while for a 180° phase delay, substantially all the input light is in the same output channel.
At intermediate phase states, the amount of light at the selected frequency band is selectively variable. Selective dropping of a portion of the energy of a given frequency band is known as “optical broadcast functionality” and is useful in instances where the optical signal has to reach more than one destination node.
There is thus provided according to one aspect of the present invention, a method for controlling continuous propagation of input multi-frequency light through a tunable frequency selective optical filter device so as to selectively direct a selected frequency band of the input light to a dropping/adding output channel of the device, the method comprising:
(i) applying selective frequency coupling to the input light to split the input multi-frequency light into first and second light components propagating through first and second spatially separating optical paths, respectively, the first light component comprising at least a portion of power of the selected frequency band of the input light, and the second light component comprising a remaining portion of the selected frequency band and all other frequency bands of the input light;
(ii) selectively creating a phase delay between the first and second optical paths by adjusting the phase of said first light component;
(iii) depending on the phase of said first light component, either combining the first and second light components to propagate through a first output channel with subst
Healy Brian
Ladas & Parry
Lambda Crossing Ltd.
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