Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2001-09-18
2004-12-21
Dunn, Drew A. (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C385S031000, C385S037000
Reexamination Certificate
active
06833954
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to fiber Bragg gratings, and in specific to methods and apparatuses for producing masks that are used to create fiber Bragg gratings.
Normal optical fibers are uniform along their lengths. A slice from any one point of the fiber looks like a slice taken from anywhere else on the fiber, disregarding tiny imperfections. However, it is possible to modify fibers in such a way that the refractive index varies regularly along their length. These fibers are called fiber Bragg gratings (FBG). The periodic refractive index variation causes different wavelengths of light to interact differently with the fiber, with certain wavelengths being reflected and certain wavelengths being transmitted.
Whenever there is a change in the index of refraction within the fiber, there is a slight reflection from the transition. In an FBG there are many of these slight reflections. The locations of these reflections are arranged such that the reflections all interfere with each other to create a strong reflection at a certain wavelength. This is the so called Bragg condition, and is satisfied when the wavelength of light is equal to twice the period of the index modulation times the overall index of refraction of the fiber. Light that does not meet this Bragg condition will be transmitted.
FBGs can be used to compensate for chromatic dispersion in an optical fiber. Dispersion is the spreading out of light pulses as they travel on the fiber. Dispersion occurs because the speed of light through the fiber depends on its wavelength, polarization, and propagation mode. The differences are slight, but accumulate with distance. Thus, the longer the fiber, the more dispersion. Dispersion can limit the distance a signal can travel through the optical fiber because dispersion cumulatively blurs the signal. After a certain point, the signal has become so blurred that it is unintelligible. The FBGs are used to compensate for chromatic (wavelength) dispersion by serving as a selective delay line. The FBG delays the wavelengths that travel fastest through the fiber until the slower wavelengths catch up. The spacing of the grating is chirped, varying along its length, so that different wavelengths are reflected at different points along the fiber. These points correspond to the amount of delay that the particular wavelengths need to have so that dispersion is compensated. Suppose that the fiber induces dispersion such that a longer wavelength travels slower than a shorter wavelength. Thus, a shorter wavelength would have to travel farther into the FBG before being reflected back. A longer wavelength would travel less far into the FBG. Consequently, the longer and shorter wavelengths can be made coincidental, and thus without dispersion. FBGs are discussed further in Feng et al. U.S. Pat. No. 5,982,963, which is hereby incorporated herein by reference in its entirety. A circulator is used to separate the light reflected from the FBG onto a different fiber from the input. With a properly designed FBG, the group delay is a function of the wavelength of the reflected beam and has the desired shape to compensate for dispersion (group delay) accumulated in propagation through an optical communication transmission system. One practical problem encountered with such FBG devices is that the group delay fluctuates around the desired functional shape. This deviation shall be referred to as group delay ripple (GDR) and is generally deleterious to the quality of transmitted optical signals.
FBGs are typically fabricated in two manners. The first manner uses a phase mask. The phase mask is a quartz slab that is patterned with a grating. The mask is placed in close proximity with the fiber, and ultraviolet light, usually from an ultraviolet laser, is shined through the mask and into the fiber. As the light passes through the mask, the light is primarily diffracted into two directions, which then forms an interference pattern in the fiber. The interference pattern comprises regions of high and low intensity light. The high intensity light causes a change in the index of refraction of that region of the fiber. Since the regions of high and low intensity light are alternating, a FBG is formed in the fiber. See also Kashyap, “Fiber Bragg Gratings”, Academic Press (1999), ISBN 0-12-400560-8, which is hereby incorporated herein by reference in its entirety.
The second manner is known as the direct write FBG formation. In this manner two ultraviolet beams are impinged into the fiber, in such a manner that they interfere with each other and form an interference pattern in the fiber. At this point, the FBG is formed in the same way as the phase mask manner. One of the fiber or the writing system is moved with respect to the other such that the interference pattern is scanned and the fiber exposed. Note that the two beams are typically formed from a single source beam by passing the beam through a beam separator, e.g. a beamsplitter or a grating. Also, the two beams are typically controlled in some manner so as to allow control over the locations of the high and low intensity regions. For example, Laming et al., WO 99/22256, which is hereby incorporated herein by reference in its entirety, teaches that the beam separator and part of the focusing system are moveable to alter the angle of convergence of the beams, which in turn alters the fringe pitch on the fiber. Another example is provided by Glenn, U.S. Pat. No. 5,388,173, and Stepanov et al., WO 99/63371, both of which are hereby incorporated herein by reference in their entirety. Both teach the use of an electro-optic module, which operates on the beams to impart a phase delay between the beams, which in turn controls the positions of the high and low intensity regions.
Note that whichever manner is used, it is still difficult to manufacture FBGs. The period of the spacing of the index modulation of the fiber Bragg grating is typically about one-half micron. When a phase mask is used to fabricate an FBG, the period of the mask grating is chosen to be twice that of the FBG, or about 1 micron. Thus, the etched bars and spaces which comprise the phase mask are about five hundred nanometers in width. For example, one application of the FBG is dispersion compensation. In this application FBGs must have a chirp (a slow variation) in the period, which is typically a very small change (~1 nm) over the length of the FBG. Thus, the spacing would ideally need to be adjusted on a picometer scale to have the period change appropriately over the length of the grating. This presents a serious challenge in design of any grating writing system. Inaccuracies in forming the chirp can cause group delay ripple in the output of the FBG.
Each FBG writing manner has advantages and disadvantages when compared with each other. For example, the first manner, the phase mask manner, is relatively inflexible, as changes cannot be made to the mask. However, since the phase mask is permanent, the phase mask manner is stable, repeatable, and aside from the cost of the mask, relatively inexpensive to operate. On the other hand, the direct write manner is very flexible, and can write different gratings. However, this manner is less repeatable and is costly to operate. Also, the direct writing process must be very strictly controlled. Any variation will lead to differences between gratings. This is difficult because the coherence (i.e. the relative position of the index modulation on a nm scale) of the entire pattern, e.g. 20 cm or greater, must be maintained. Little changes in alignment, temperature, etc. can result in the loss of coherence.
Another problem with the phase mask manner resides in the fabrication of the masks. Masks are fabricated by lithographic or holographic techniques. More specifically, the exposure of the resist that coats the mask may be done holographically, as well as lithographically. In the lithographical method, a small beam (of width smaller than the minimum mask feature size—0.5 micron) is used to directly expose the resist wi
Caldwell Roger F.
Popelek Jan
Rothenberg Joshua E.
Zweiback Jason
Dunn Drew A.
Fulbright & Jaworski LLP
Pritchett Joshua L
Teraxion Inc.
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