Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-10-12
2003-07-22
Uliah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C359S199200
Reexamination Certificate
active
06597840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same, and particularly to a novel wavelength-locked loop servo-control circuit applied for writing control of Bragg gratings implemented in fiber optical links.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
As known, one optical network system element includes a Bragg grating which is a short section of optical fiber that has been slightly modified.
Particularly, as illustrated in
FIG. 1
, in a portion
100
of optic fiber implementing a Bragg grating comprises cladding layers
110
,
111
and, a core
112
forming an optical cavity having the fibre gratings
115
. To form the gratings, the optical fiber core
112
at that portion
100
is exposed to ultraviolet radiation in a regular pattern, which results in the refractive index
119
of the fiber core to be altered according to that regular pattern. If the fiber is then heated or annealed for a few hours, the index changes become permanent. As described in K. Hill, Fiber Bragg Gratings, Chapter 9 in Handbook of Optics vol. IV, OSA Press (2000) and, B. Poumellec, P. Niay, M. Douay et al., “The UV induced Refractive Index Grating in Ge:SiO2 Preforms: Additional CW experiments and the macroscopic origins of the change in index”, Journal Of Physics D, App. Phys. Vol. 29, p. 1842-1856 (1996), the contents and disclosures of which are incorporated by reference herein, this phenomena is known as “photosensitivity.” It is understood that the magnitude of the index change may depend upon many factors including: the irradiation wavelength, intensity, and total dose, the composition and doping of the fiber core, and any materials processing done either prior or subsequent to irradiation. For example, in germanium-doped singlemode fibers, index differences between 10
−3
and 10
−5
are achievable. Using this effect, periodic diffraction gratings can be written in the core of an optical fiber. Typically, the exposure is carried out using an interferometer or, through a phase mask with a periodic structure that permits writing of a periodically varying refractive index grating within the photorefractive media within the core. The reflectivity, bandwidth and central wavelength of such a Bragg structure are generally defined by the period and length of the phase mask and exposure time used.
Light traveling through these refractive index changes of optical fiber core having a fibre Bragg grating is reflected back, with a maximum reflection usually occurring at one particular wavelength known as the “Bragg wavelength”. That is, such gratings reflect light in a narrow bandwidth centered around the Bragg wavelength, &lgr;
B
, according to the following equation:
&Lgr;
B
=2N
eff
where &Lgr; is the spatial period, or pitch, of the periodic index variations and N
eff
is the effective refractive index for light propagating in the fiber core. Thus, the wavelength of light reflected back depends on the amount of refractive index change that has been applied and also on how distantly spaced the refractive index changes
119
are. If the spacing of the Bragg planes is varied across the length of the grating, it is possible to produce a chirped grating, in which different wavelengths can be considered to be reflected from different points along the grating.
These in-fiber Bragg gratings written with photorefractive interference techniques have become an important part of modern fiber optic data communication systems. Such gratings have been employed in many systems, and are especially attractive for dense wavelength multiplexing (DWDM) where they can serve as in-line filters. In this capacity, the fiber Bragg grating functions as a wavelength-selective optical filter.
As the grating is produced by direct optical writing in a photorefractive fiber media, it is often difficult to control the alignment between the grating period (wavelength responsivity) and the center wavelengths of the DWDM communication channels. This mismatch can result in excessive optical loss and poor link performance.
It would be highly desirable to provide a system and methodology that ensures wavelength alignment between the filter bandpass with the center wavelength of the DWDM channel by monitoring the transmission properties of the grating as it is formed in the optical fiber.
It would be further highly desirable to provide a system and methodology for forming a Bragg grating in a length of optical fiber that employs a feedback loop for adjusting the writing laser as required to optimize the grating properties of the optical fiber in the grate writing process.
As known, fibre Bragg gratings may additionally be implemented as narrowband retroreflectors for providing feedback at a specific wavelength in fibre lasers (both in short pulse and single frequency lasers); filters for multichannel wavelength-division multiplexed (WDM) communications systems; and, fibre dispersion compensators in fibre links, or spectral manipulators of optical pulses as in a chirped pulse amplification (CPA) system. Fiber dispersion is a phenomena that causes optical pulses to spread as they propagate through fibers, eventually causing intersymbol interference and bit errors. It is important that effective compensation techniques be provided as dispersion is a fundamental limitation on the maximum data rate in a fiber optic communication systems. Fiber Bragg gratings applied for dispersion compensation may be re-written in real time using various schemes including: photorefractive or photocehemically induced schemes, or electrorefractive schemes. One company, Southampton Photonics, Inc., produces electrically Fiber Bragg Grating filter devices (http://www.southamptonphotonics.com). Digilens Inc. (http://www.digilens.com/) has developed electrically-switchable Bragg gratings (S-bugs™) in liquid crystals rather than solid substrates such as silica and silicon. As the characteristics of the liquid crystal can be modified by applying an electric current, Digilens Bragg grating devices may split off a specific wavelength and then adjust its power or switch it in a single operation. This may significantly increase the unrepeated link distance and improve the bit error rate for channels running at 1 to 10 Gbit/s or beyond. Further, tunable fiber Bragg gratings provide the means to change the grating period in response to external optical signals.
It would be further highly desirable to provide an improved tunable Bragg grating technology that incorporates a novel feedback control loop that would permit the automatic adjustment of the grating properties over time and as a function of optical power and wavelength, effectively allowing the control loop to correct for all wavelength dependent absorption or dispersion properties of an DWDM fiber link.
SUMMARY OF THE INVENTION
It is therefore an object of t
DeCusatis Casimer Maurice
Jacobowitz Lawrence
International Business Machines - Corporation
Scully Scott Murphy & Presser
Townsend Tiffany L.
Uliah Akm E.
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