Device and method for optical performance monitoring in an...

Optical waveguides – With optical coupler – Plural

Reexamination Certificate

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Reexamination Certificate

active

06577786

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to monitoring optical signals, more particularly, to a spectrometer and corresponding diffraction grating having improved performance.
BACKGROUND OF THE INVENTION
The telecommunications industry has grown significantly in recent years due to developments in technology, including the Internet, e-mail, cellular telephones, and fax machines. These technologies have become affordable to the average consumer such that the volume of traffic on telecommunications networks has grown significantly. Furthermore, as the Internet has evolved, more sophisticated applications have increased data volume being communicated across telecommunications networks.
To accommodate the increased data volume, the telecommunications network infrastructure has been evolving to increase the bandwidth of the telecommunications network. Fiber optic networks that carry wavelength division multiplexed optical signals or channels provide for significantly increased data channels for the high volume of traffic. The wavelength division multiplexed optical channels or polychromatic optical signals comprise narrowband optical signals. The wavelength division multiplexed optical channels carry packets containing information, including voice and data. Contemporary optical networks can include forty or more narrowband optical channels on a single fiber and each narrowband optical channel can carry many thousands of simultaneous telephone conversations or data transmissions, for example. An optical component often utilized in performing a number of operations in optical networks is a diffraction grating.
Wavelength division multiplexed optical systems, such as systems utilizing diffraction gratings for performing multiplexing and demultiplexing operations, have the advantage of parallelism in transmitting optical signals. This yields higher performance and lower cost for high channel count systems. In particular, a diffraction grating is a device that diffracts light by an amount varying according to its wavelength. For example, if sunlight falls on a diffraction grating at the correct angle, the sunlight is broken up into its individual component colors (i.e., rainbow).
Gratings work in both transmission (where light passes through a material with a grating written on its surface) and in reflection (where light is reflected from a material with a grating written on its surface). In optical communications, reflective gratings have a widespread use. A reflective diffraction grating includes a very closely spaced set of parallel lines or grooves made in a mirror surface of a solid material. A grating can be formed in most materials wherein the optical properties thereof are varied in a regular way, having a period that is relatively close to the wavelength. Incident light rays are reflected from different lines or grooves in the grating. Interference effects prevent reflections that are not in-phase with each other from propagating.
There are two primary groove profiles in conventional diffraction gratings, blazed gratings and sinusoidal gratings. The blazed grating includes a jagged or sawtooth shaped profile. The sinusoidal grating has a sinusoidal profile along the surface of the grating.
The diffraction equation for a grating is generally described by
Gm&lgr;=n
(sin (&agr;)+sin (&bgr;))
where, G=1/d is the groove frequency in grooves per millimeter and d is the distance between adjacent grooves, m is the diffraction order, &lgr; is the wavelength of light in millimeters, &agr; is the incident angle with respect to the grating normal, &bgr; is the exiting angle with respect to the grating normal, and n is the refractive index of the medium above the grooves.
FIG. 14A
is a representative pictorial showing optical characteristics of a blazed diffraction grating in reflecting a narrowband optical signal. The blaze diffraction grating
900
is defined by certain physical parameters that effect optical performance. These physical parameters include the reflection surface material, the number of grooves g per millimeter, blaze angle &thgr;
B
, and the index of refraction of an immersed grating medium
902
. The reflection surface
905
typically resides on a substrate
910
.
As shown on
FIG. 14A
, the groove spacing is defined by d. An incident narrowband optical signal with a center wavelength &lgr;
1
has an incident angle &agr;
1
(measured from the grating normal N
g
) and a reflection angle &bgr;
1
(also measured from the grating normal N
g
). The angle between the grating normal N
g
and the facet normal N
f
defines the blaze angle &thgr;
B
.
As previously discussed, when light is incident on a grating surface, it is diffracted in discrete directions. The light diffracted from each groove of the grating combines to form a diffracted wavefront. There exists a unique set of discrete or distinct angles based upon a given spacing between grooves that the diffracted light from each facet is in phase with the diffracted light from any other facet. At these discrete angles, the in-phase diffracted light combine constructively to form the reflected narrowband light signal.
In practice, narrowband light signals or beams are not truly monochromatic, but rather a tight range of wavelengths. Each signal is defined by a narrow passband and has a center wavelength which is the representative wavelength to which an optical signal is associated. Each center wavelength is generally predefined, and may correspond with an industry standard, such as the standards set by the International Telecommunication Union.
A sinusoidal diffraction grating is similarly described by the equation above. When &agr;=&bgr;, the reflected light is diffracted directly back toward the direction from which the incident light was received. This is known as the Littrow condition. At the Littrow condition, the diffraction equation becomes

m*&lgr;=
2*
d*n*
sin (&agr;),
where n is the index of refraction of the immersed grating medium
902
in which the diffraction grating is immersed.
FIG. 14B
is a representative pictorial showing optical characteristics of a sinusoidal diffraction grating. Sinusoidal gratings, however, do not have a blaze angle parameter, but rather have groove depth (d). An immersed grating medium
955
resides on the sinusoidal grating surface
950
having a certain index of refraction, n. The diffraction grating equation discussed above describes the optical characteristics of the sinusoidal diffraction grating based upon the physical characteristics thereof.
FIG. 14
c
shows a polychromatic light ray being diffracted from a blazed grating
960
. An incident ray (at an incident angle &thgr;
i
to the normal) is projected onto the blazed grating
960
. A number of reflected and refracted rays are produced corresponding to different diffraction orders (values of m=0, 1, 2, 3 . . . ). The reflected rays corresponding to the diffraction order having the highest efficiency (i.e., lowest loss) are utilized in optical systems.
An important component of the fiber optic networks is an optical performance monitor (OPM) for monitoring the performance of the optical system. The OPM provides a network/system operator the ability to monitor the performance of individual narrowband optical signals. The optical performance of the individual narrowband optical signals may include the following metrics, for example, power levels, center wavelength, optical signal-to-noise ratio (OSNR), interference between channels such as crosstalk, and laser drift. By monitoring these metrics, the optical network operator can easily identify and correct problems in the optical network so as to improve the performance of optical communication therein.
One form of an OPM employs a diode array spectrometer that generally includes optical lenses, a dispersion component, and an optical sensor. The optical lenses process the polychromatic optical signal and cause the polychromatic optical signal to be incident to the dispersion component preferably at a near-Littro

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