Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-01-23
2003-11-25
Stafira, Michael P. (Department: 2877)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06654521
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
The complexity of Fiber Bragg Grating (FBG) structures continually increases, for example in non-linear chirped FBGs for tunable dispersion compensation, dispersion-free FBGs in the DWDM networks, multi-channel sampled FBGs, DFB fiber lasers, gain-flattening filters (GFF) in Erbium doped fiber amplifiers (EDFAs), and other FBG applications.
There are applications in which it is desired to make FBGs with multiple bands of reflectivity in the telecommunications band around 1,550 nm, for example a band of reflectivity 1 nanometer wide at 1,550 nm, and then another at 1,552 nm. These periodically extend for example throughout the C-Band from about 1,530 nm to 1,565 nm, in which the erbium doped fiber amplifier has gain. The ITU standard grid either at 100 gigahertz spacing (roughly 0.8 nm), or 50 gigahertz spacing (roughly 0.4 nanometers) is where typical telecommunication laser transmitters operate that send voice and data information over telephone systems today. A standard set of frequencies or wavelengths in this 1,535 nm to 1,565 nm band has been selected by standards committees, and it is therefore of interest to fabricate FBGs which operate at those wavelengths. In such an FBG there is an underlying grating period of about 0.5 micron with slower superimposed modulations that produce effects like chirp and non-linear chirp, which are fundamentally important to particular applications such as dispersion compensation.
The reflectivity peak of a Bragg grating occurs at a wavelength equal to twice the index of refraction of the fiber core times the physical period of the index grating. Typically, the period of the index grating is about 0.5 micron and the index of refraction is about 1.5, such that twice 1.5 times 0.5 microns results in 1.5 microns or reflectivity in the band around 1500-1550 nanometers.
One approach is to make a fiber Bragg grating that, rather than reflecting in a wide continuous wavelength band which can result in a FBG which is impractically long, reflects specifically in multiple channels located at periodically spaced frequencies (or wavelengths). Each channel reflects in a certain bandwidth around its central wavelength, and in this bandwidth the FBG can provide a number of filtering functions, such as tunable dispersion compensation. A method called sampling imposes a periodic superstructure on the underlying 0.5 micron basic Bragg grating period, producing a multiplicity of Bragg reflection peaks in the spectrum surrounding the underlying Bragg reflection wavelength. The superstructure can be understood in terms of a Fourier transform argument. The underlying grating reflects at a certain wavelength, and the imposed superstructure has a Fourier transform which represents a comb function of regularly spaced peaks, one for each channel, and having a certain envelope that determines the spectral distribution or uniformity of the reflectivities of those channels. It is sufficient to understand that the multiplicity of channels is determined by the Fourier spectrum of the periodic superstructure, which can periodically vary the underlying FBG either in phase, i.e. the locations of the index modulations, or amplitude, i.e. the magnitude of the index modulations. The period of such a superstructure can be about a millimeter to generate channels with a 100 gigahertz spacing in the telecommunications band, much longer than the fundamental 0.5 micron structure of the grating.
To write a grating of periodic index variation in the core of a fiber, one way is to propagate UV light into that core. Where the UV light exposes the core, the index of refraction increases slightly, and where it does not, it does not change at all. If the basic exposure pattern has a period of about 0.5 microns, that will create reflectivity in the telecommunications band around 1,500 nanometers.
A phase mask is widely used in manufacturing fiber Bragg gratings (FBG). The side-writing systems using phase masks in close proximity to the fiber are less critical to alignment, vibration and UV beam coherence than are imaging or holographic direct write systems that demand interferometric accuracies. Accordingly, phase masks are particularly suitable for industrial fabrication. In addition, the nanometer scale structures required by the FBG are built into the phase mask, benefiting from high accuracy lithographic mask technology.
In prior art side-writing technology a fiber is placed as close as possible to the mask, which is a slab having a periodically varying surface grooves. When UV light propagates through the mask, it splits into multiple diffraction orders. The mask is manufactured such that the zeroth diffraction order, which ordinarily goes straight through, is suppressed, for example by a standard technique of adjusting the depth of the grooves in the mask. The groove depth is chosen for a particular mask groove period, such that the zeroth order is suppressed. The FBG is formed from the interference between the plus first and minus first orders diffracted from the phase mask. About 35 per cent to 40 per cent of the incident light is diffracted by the mask into each of the plus and minus first orders. Higher diffraction orders typically do not contribute to formation of the relevant Bragg index modulation in the fiber and are thus ignored, and in some circumstances are eliminated by inserting additional optics.
The interference between the two UV beams diffracted from the mask creates an intensity modulation in the core of the fiber, which modulates the index in the photosensitive fiber core. The UV writing beam may have a wavelength of typically around 244 nanometers, although the method may be used at any wavelength at which the fiber exhibits sufficient photosensitivity. For FBGs in the 1550 nm telecommunication band, the period of the mask is selected to be about 1070 nm, which produces an angle of diffraction of the first order beams of roughly +/−13 degrees, so that the two diffracted beams propagate at 26 degrees with respect to each other. When two beams are at 26 degrees to each other, they create an intensity interference pattern with a 535 nm basic period, which then generates a Bragg reflection in the band around 1550 nm. This side writing method is the standard prior art that many FBG manufacturers use, e.g. see U.S. Pat. No. 5,367,588, issued Nov. 22, 1994.
Additionally, prior art U.S. Pat. No. 6,081,640, issued Jun. 27, 2000, describes a periodic superstructure that can be either in phase, amplitude, pitch of the grating, anything that varies periodically and is recognized to create multiple channels, but does not disclose in detail how to incorporate variation of pitch or phase. One method described in U.S. Pat. No. 6,081,640 uses amplitude sampling, in which the mask has a periodic amplitude superstructure. To create a large number of reflective channels using amplitude sampling requires a very small duty factor. That is, for example, a periodic rectangular wave amplitude superstructure pattern where the “on” section is extremely short and the “off” section is very long, generates many channels, but that periodic superstructure is “off” most of the time, such that there is no grating in most of the fiber. That is, a small section of grating is followed by a long section with no grating present, which is then is followed by another small section of grating. To achieve significant reflectivity the light must interact for a reasonably long path length with the grating. The way Bragg reflection works is that each reflection from a single period of index variation is extremely small, on the order of 10
−3
reflection amplitude from each index period, and at the Bragg resonance wavelength they add constructively to generate a high reflectivity band.
Therefore, amplitude sampling is extremely inefficient, since most of the fiber has no grating present. In contrast, phase sampling, i.e., periodically var
Li HongPu
Rothenberg Joshua E.
Sheng Yunlong
Wang Ying
Zweiback Jason
Fulbright & Jaworski L.L.P.
Stafira Michael P.
Teraxion Inc.
Valentin, II Juan D.
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