Optical spectrum analyzer

Details Optical spectrum analyzer Optical spectrum analyzer

1999-06-22

2002-09-17

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By light interference

C356S477000, C356S451000

Reexamination Certificate

active

06452681

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical spectrum analyzer (OSA) for measuring the optical power as a function of wavelength, and, more particularly, the spectral characteristics of optical channels in a wavelength division multiplexed (WDM) communication system.
BACKGROUND OF THE INVENTION
Current WDM communication systems strive for maximum transmission capacity by spacing optical channels as closely as possible, typically less than a nanometer (nm). As the channel spacing, however, decreases, monitoring the spectral characteristics of the channels becomes more critical in verifying system functionality, identifying performance drift, and isolating system faults. For example, such monitoring is critical in detecting wavelength drift, which can readily cause signals from one optical channel to cross into another. Also, with the recent use of optical amplifiers, real-time feedback to network elements has become increasing critical to ensure stable operation.
Optical instruments, called optical spectrum analyzers (OSAs), are known in the art, however, for measuring the optical power as a function of wavelength. Indeed, most conventional OSAs use a wavelength tunable optical filter, such as a Fabry-Perot interferometer or diffraction grating, to resolve the individual spectral components. In the latter case, light is reflected off the diffraction grating at an angle proportional to the wavelength. This is so, inasmuch as the grating lines cause the reflected rays to undergo constructive interference only in very specific directions. The spectrum of the light is then analyzed on the basis of the angle at which the light is diffracted using a detector array. Alternatively, the diffracted light is moved over a slit and then detected using a small detector.
Alternatively, a Fabry-Perot interferometer may be used consisting of two highly reflective, parallel mirrors that act as a resonant cavity, which transmits light only at a unique frequency (wavelength). Wavelength tuning may be accomplished by varying the mirror spacing or rotating the interferometer with respect to the incident light so as to provide an optical spectrum analysis.
Other OSAs known in the art are based on the Michelson interferometer, wherein the incident light is split into two paths. One path is fixed in length, and the other is variable so as to create an interference pattern between the signal and a delayed version of itself, known as an interferogram. The wavelength of the incident light can be determined by comparing the zero crossings in the interferogram with those for a known wavelength standard. The optical spectrum, however, is determined by performing a Fourier transform on the interferogram.
Although conventional OSAs perform acceptably, they are generally bulky as well as costly. Furthermore, it may take from a few seconds to a few minutes to obtain the optical spectrum, unsuited for next generation WDM communication systems that operate near or in excess of 10 Gbps, which require real-time monitoring.
Accordingly, it would be desirable to provide for a low cost, compact OSA capable of determining the spectral characteristics of the optical channels in real time.
SUMMARY OF THE INVENTION
The present invention is an optical spectrum analyzer (OSA) comprising a tree-structure of N-stage wavelength filters, a “wavelength slicer,” which “slices” the incident optical signal into desired groupings of individual “sliced” spectral components, each along a different output optical fiber. Cascaded fiber Bragg gratings and delay lines coupled to each output optical fiber then uniquely map the “sliced” spectral components into the time domain such that each spectral component is allocated a unique time slot.
In one embodiment, the optical spectrum analyzer comprises a three-stage, tree-structure of Mach-Zehnder interferometers which successively “slice” a WDM optical signal into eight (8) groupings of alternating spectral components. Each of the individual spectral components in each of the groupings is then allocated a unique time slot using two sets of delay lines, implemented preferably using fibers. A first set of delays delays each spectral component within each grouping of spectral components by a multiple integer time period. For each optical fiber, fiber Bragg gratings are also arranged such that the shortest wavelength spectral components are reflected first and the longest last. Then, second sets of delays disposed between each fiber Bragg grating, delay the reflections from each successive fiber Bragg grating so as to offset each new grouping of sliced spectral components uniformly in the time domain. In this latter manner, each sliced spectral component is thus allocated a unique slot within the time domain for conversion into an electrical signal by corresponding optical detectors.
In a second embodiment, a single stage is employed which, instead, “slices” the WDM optical signal into four consecutive bands of “sliced” spectral components. Similarly, each spectral component in each of the four bands of “sliced” spectral components is then allocated a unique time slot using two sets of delay lines. A first set of delays delays each band of spectral components by a multiple integer time period such that each of four bands is separated from one another in the time domain. Fiber Bragg gratings having corresponding transmission peaks are again arranged such that the shortest wavelength spectral components within each band are reflected first and the longest wavelength last. Additionally, second sets of delays delay the reflections from each successive fiber Bragg grating by an amount that offsets each desired sliced spectral component from one another in the time domain.


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