Resonant optical cavity

Details Resonant optical cavity Resonant optical cavity

1999-11-22

2003-06-10

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By light interference

C356S519000

Reexamination Certificate

active

06577398

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to optical resonant cavities and more particularly to a resonant cavity that is angle insensitive to light launched into a cavity along a predetermined plane.
BACKGROUND OF THE INVENTION
Using optical signals as a means of carrying channeled information at high speeds through an optical path such as an optical waveguide i.e. optical fibres, is preferable over other schemes such as those using microwave links, coaxial cables, and twisted copper wires, since in the former, propagation loss is lower, and optical systems are immune to Electro-Magnetic Interference (EMI), and have higher channel capacities. High-speed optical systems have signaling rates of several mega-bits per second to several tens of giga-bits per second.
Optical communication systems are nearly ubiquitous in communication networks. The expression herein “Optical communication system” relates to any system that uses optical signals at any wavelength to convey information between two points through any optical path. Optical communication systems are described for example, in Gower, Ed. Optical communication Systems, (Prentice Hall, NY) 1993, and by P. E. Green, Jr in “Fiber optic networks” (Prentice Hall New Jersey) 1993, which are incorporated herein by reference.
As communication capacity is further increased to transmit an ever-increasing amount of information on optical fibres, data transmission rates increase and available bandwidth becomes a scarce resource.
High speed data signals are plural signals that are formed by the aggregation (or multiplexing) of several data streams to share a transmission medium for transmitting data to a distant location. Wavelength Division Multiplexing (WDM) is commonly used in optical communications systems as means to more efficiently use available resources. In WDM each high-speed data channel transmits its information at a pre-allocated wavelength on a single optical waveguide. At a receiver end, channels of different wavelengths are generally separated by narrow band filters and then detected or used for further processing. In practice, the number of channels that can be carried by a single optical waveguide in a WDM system is limited by crosstalk, narrow operating bandwidth of optical amplifiers and/or optical fiber non-linearities. Moreover such systems require an accurate band selection, stable tunable lasers or filters, and spectral purity that increase the cost of WDM systems and add to their complexity. This invention relates to a method and system for filtering or separating closely spaced channels that would otherwise not be suitably filtered by conventional optical filters.
Currently, internationally agreed upon channel spacing for high-speed optical transmission systems, is 100 Ghz, equivalent to 0.8 nm, surpassing, for example 200 Ghz channel spacing equivalent to 1.6 nanometers between adjacent channels. Of course, as the separation in wavelength between adjacent channels decreases, the requirement for more precise demultiplexing circuitry capable of ultra-narrow-band filtering, absent crosstalk, increases. The use of conventional dichroic filters to separate channels spaced by 0.4 nm or less without crosstalk, is not practicable; such filters being difficult if not impossible to manufacture.
In a paper entitled Multifunction optical filter with a Michelson-Gires-Turnois interferometer for wavelength-division-multiplexed network system applications, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device hereafter termed the MGT device provides some of the functionality provided by the instant invention. For example, the MGT device as exemplified in
FIG. 1
serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror
16
. The etalon
10
used has a 99.9% reflective back reflector
12
r
and a front reflector
12
f
having a reflectivity of about 10%; hence an output signal from only the front reflector
12
f
is utilized. A beam splitting prism (BSP)
18
is disposed to receive an incident beam and to direct the incident beam to the etalon
10
. The BSP
18
further receives light returning from the etalon and provides a portion of that light to the plane mirror
16
and a remaining portion to an output port. Although this known MGT device appears to perform its intended function, it appears to have certain limitations. Furthermore, in the MGT device a finite optical path difference is required in the interferometer to produce an output response that mimics the one provided by the device of the instant invention. Typically for a 50 GHz free spectral range (FSR) this optical path difference would be a few millimeters; in contrast in the instant invention the optical phase difference need only be approximately &lgr;/4 resulting in a more temperature stable and insensitive system. One limitation of the MGT device, which makes this device less than practicable, is its apparent requirement in the stabilization of both the etalon and the interferometer. Yet a further drawback to the MGT device is the requirement for an optical circulator to extract the output signal adding to signals loss and increased cost of the device.
This invention provides an optical circuit and a method of obviating the aforementioned limitations of the MGT device.
A Fabry-Perôt cavity is a well-known device generally having two spaced apart reflective surfaces between which light of a predetermined wavelength launched therein, will resonate. The free-spectral range (FSR) of a resonant cavity is related to the gap or space between its reflective surfaces.
The present invention provides a resonant cavity tunable along a line along a plane by launching an input beam at locations along that line while being substantially angle insensitive along a line along an orthogonal plane, wherein both planes are orthogonal to an input end face of the resonant cavity.
This device is particularly advantageous in the design of a temperature insensitive flat-top interleaver circuit. Prior art MGTs have not been commercially used in the past as they require independent tuning for the FSR of the GT and the OPD of the Michelson interferometer.
The independent tuning characteristics of the cavity in accordance with this invention allows the FSR of the GT resonator to be performed first followed by fine tuning the OPD in the interferometer with respectively vertical and horizontal angles. In essence a de-coupling is performed allowing tuning the GT resonator in one dimension without affecting the tuning of the OPD in the interferometer.
It is an object of this invention to provide an angle insensitive GT resonator having rotationally symmetric optics having little or no walk-off and little or no change in optical path length for a plurality of input angles.
It is a further object of an embodiment of this invention to provide a GT resonator that utilizes cylindrical optic wherein a line along a plane can be used to tune the path length of the GT to its appropriate value, while providing angle insensitivity an along an orthogonal line.
It is a further object of a preferred embodiment of this invention to provide a Michelson GT interferometer having a cylindrical GT is within the device, adjusted alone a first line by varying the input angle to its proper path length, while using angles along a second orthogonal line to finely tune the interferometer without affecting the GT's adjustments previously made.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided, a resonant optical cavity comprising:
a first at least partially reflective end and a second at least partially reflective end spaced a predetermined distance apart, the resonant optical cavity being substantially angle insensitive to a collimated beam of light launched into the cavity at a non-normal angle of incidence to the first input end face along a first line lying on a first plane orthogonal to the first input end face, wherein one of the

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