Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
2001-07-17
2004-02-24
Chang, Audrey (Department: 2872)
Optics: measuring and testing
By light interference
Spectroscopy
C356S328000
Reexamination Certificate
active
06697159
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus and method for analyzing an optical signal and, more particularly, for analyzing an optical spectrum of a Dense Wave Division Multiplexing (DWDM) system or a Frequency Division Multiplexing (FDM) system sing a tunable optical filter.
BACKGROUND OF THE INVENTION
Dense Wave Division Multiplexing (DWDM) is widely used in fiber optic transmission systems to expand the capacity of the fiber optic system. In a DWDM network a plurality of optical channels, each operating at a specific wavelength are transported in single fiber. Each wavelength is separated by a channel spacing in the order of, for example, 0.4 nm. As many as 160 channels are transmitted over a single fiber.
A necessary part of network management includes performance monitoring to guarantee the quality of service. Conventional link performance monitoring (LPM) is performed in the transport layer of the network in the electronics domain and on a per-channel basis. The use of DWDM necessitates optical domain performance monitoring, which measures the optical signal-to-noise-ratio, wavelength, power of each channel and other characteristics of each channel. Traditional diffraction-grating-based Optical Signal Analyzers (OSAs) are generally large in size in order to achieve a reasonable optical resolution. Photo-diode array-based OSAs are compact in size but they generally provide poor spectral resolution.
As shown in prior art
FIG. 1
, a depiction of a prior art analyzer
100
indicates the use of a reflective grating
102
receiving a parallel light beam from a collimator
104
and refracting that light beam to a photo diode array
106
. The photo diode array
106
detects the amplitude of light signals
108
and converts them into electronic signals
110
for receipt by the signal processor
112
for processing. A data processing module
114
provides for data acquisition and processing.
Typically, the analyzer
100
provides low spectral resolution. The increase of resolution has two fundamental limitations. First, the limited number of photo diodes in the photo diode array negatively affects resolution. As the performance monitoring is provided over the 256-element photo diode array
106
, only 256 effective samples are taken over the entire spectral range. The resolution may be increased by implementing a 512-element or greater photo diode array
106
, but this increase in elements presents challenges in technology and manufacturing. Hence, additional elements in the photo diode array
106
, in order to improve resolution, become cost prohibitive.
Second, the resolution of the reflective grating
102
is not fine enough for high resolution of the analyzer
100
. For example:
N=&lgr;/&Dgr;&lgr;,where:
N=Number of lines in reflective grating,
&lgr;=Wavelength, and
&Dgr;&lgr;=Reflective grating resolution.
If the prior art analyzer
100
is desired to have a reflective grating resolution of &Dgr;&lgr;=0.01 nm, and with a wavelength of 1550 nm, then:
N
=&lgr;/&Dgr;&lgr;=1550 nm/0.01 nm=155,000 lines.
If the density of the reflective grating
102
is 600 lines/mm, then the size of the reflective grating is 155,000/600 lines/mm=258 mm which is not a feasible solution to increase resolution for a small spectrum analyzer.
Therefore, a solution is needed that would provide for a wide bandwidth monitoring and a better wavelength resolution.
BRIEF SUMMARY OF THE INVENTION
There is, therefore, provided in the practice of the invention a novel optical domain signal analyzer, for providing high resolution spectrum analysis over a wide optical bandwidth. The optical domain signal analyzer broadly includes an optical filter for providing wavelength samples of a received optical signal, a dispersing element for receiving the samples and dispersing the samples, a detector for receiving the dispersed signal and for providing electrical signals representative of the dispersed sample, and a processor for receiving the electrical signal and calculating the characteristics of the spectrum. In a preferred embodiment, the detector is a linear imaging sensor. Also, in a preferred embodiment, the optical filter is a Fabry-Perot interferometer (FPI) comprised of two parallel mirrors, each of which only partially transmit light. If constructive interference occurs in the FPI cavity, then the light with that particular frequency is transmitted from the FPI.
Accordingly, it is an object of the present invention to provide an improved optical domain signal analyzer for providing for high resolution and wide bandwidth testing of an optical signal.
REFERENCES:
patent: H1152 (1993-03-01), Korendyke
patent: 5589970 (1996-12-01), Lyu et al.
patent: 5689333 (1997-11-01), Batchelder et al.
patent: 5894362 (1999-04-01), Onaka et al.
patent: 5969834 (1999-10-01), Farber et al.
Webpage: www.burleigh.com/Pages/fabryTheory.htm, dated Mar. 8, 2001,Fabry Perot Interferometers Theory, Brian P. Samoriski, Ph.D.
Webpage: www.intl-light.com/handbook/ch04.html, dated Sep. 27, 2001,Manipulating Light.
Webpage: http://fusioned.gat.com/Teachers/Curriculum/Curriculum-HTML/T03S-CD-diffract.html, dated Sep. 27, 2001,The Compact Disk as Diffraction Grating.
Fiber Optic Networks, by Paul E. Green, © 1993 by Prentice-Hall, Inc., Englewood Cliffs, New Jersey, pp. 108-117.
Allen Denise S.
Blackwell Sanders Peper Martin LLP
Chang Audrey
The University of Kansas
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