Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-05-24
2004-06-15
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000, C359S569000
Reexamination Certificate
active
06751381
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to fiber optical communication technologies and more specifically to a system and method for embodying amplitude information into phase masks for writing fiber Bragg gratings.
BACKGROUND
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 make fibers in which the refractive index varies regularly along their length. These fibers are called fiber gratings because they interact with light like diffraction gratings. Their effects on light passing through them depend very strongly on the wavelength of the light.
A diffraction grating is a row of fine parallel lines, usually on a reflective surface. Light waves bounce off of the lines at an angle that depends on their wavelength, so light reflected from a diffraction grating spreads out in a spectrum. In fiber gratings, the lines are not grooves etched on the surface, instead they are variations in the refractive index of the fiber material. The variations scatter light by what is called the Bragg effect, hence fiber Bragg gratings (FBGs). Bragg effect scattering is not exactly the same as diffraction scattering, but the overall effect is similar. Bragg scattering reflects certain wavelengths of light that resonate with the grating spacing while transmitting other light.
FBGs are 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 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. 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.
In some applications it is desired to make FBGs which have multiple spectral bands of operation (channels). One method to make such devices is to further modulate the FBG with a period longer than the underlying grating period. This method of providing a superimposed structure or super-structure may sometimes be referred to as sampling. This super-structure may involve either modulation of the FBG amplitude or period (or phase). Examples of these type of FBG devices are described in the U.S. patent application Ser. No. 09/757,386, entitled “EFFICIENT SAMPLED BRAGG GRATINGS FOR WDM APPLICATIONS”.
FBGs are typically created in one of two manners. The first manner is known as the direct write FBG formation In this manner two ultraviolet beams may be impinged onto the fiber, in such a manner that they interfere with each other and form an interference pattern on 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. The fiber or the writing system is moved with respect to the other such that the FBG is scanned, or written, into the fiber. 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 beam separator and part of the focusing system is 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 Stepanov et al., WO 99/63371, which is hereby incorporated herein by reference in its entirety, and teaches 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.
The second manner for creating FBGs uses a phase mask. The phase mask is a quartz slab that is patterned with a grating. This grating is typically a row of finely spaced parallel lines, or grooves, with a duty cycle typically in the forty to sixty percent range. These lines are usually etched lithographically onto the surface of the quartz slab (mask). The mask is placed in close proximity with the fiber, and ultraviolet light, usually from an ultraviolet laser, is shined through the mask and onto the fiber. As the light passes through the mask, the light is primarily diffracted into two directions, which then forms an interference pattern on the fiber. At this point, the FBG is formed in the same way as the direct write manner. See also Kashyap, “Fiber Bragg Gratings”, Academic Press (1999), ISBN 0-12-40056-8, which is hereby incorporated herein by reference in its entirety.
Each manner has advantages and disadvantages when compared with each other. For example, 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.
When making an FBG there is a need to combine two pieces of information. A phase profile, provided by variation of the period, commonly referred to as chirp, of a phase mask and an amplitude profile (i.e. the magnitude of the index modulation of the core of the fiber) provided by varying the light exposure of the mask or other FBG creation mechanism. For complex FBG designs this creates an opportunity for errors. Assuming a perfect phase mask, manufactured in accordance with prior art methods, a phase mask only has a part of the information needed to write an FBG on a fiber, the phase information. The other information necessary for proper function of the FBG is the amplitude, which provides a profile for magnitude of a spatially varying oscillatory index modulation of the grating that is written into the core of the fiber.
In the prior art, the amplitude information is provided by separate data used to modulate the laser beam intensity or by other methods, such as rapidly vibrating the mask, fiber, or aiming mechanisms of the laser beam. Thus, in the prior art the amplitude information is controlled separately from the phase information that is incorporated into the mask. Therefore, great care has previously been required such that variation of the amplitude is controlled during the FBG writing process so as to be precisely spatially synchronized with the phase information incorporated into the mask in order to insure proper function of the grating. In addition, in the prior art, changes in the FBG amplitude made by modulating the laser intensity may cause changes in the average index of refraction of the fiber, which effectively leads to errors in the intended chirp.
One prior art attempt to combine phase and amplitude information for FBGs uses two masks. A phase mask and an amplitude mask are stacked or sandwiched. A window is cut out of a chrome layer of the amplitude mask. The light beam is focused through the amplitude mask and thus through the phase mask, focusing on the core of the fiber. This is undesirable as modulation of the mean index of refraction modifies the desired FBG. Ideally, a uniform mean index of refraction with symmetric o
Popelek Jan
Rothenberg Joshua E.
Fulbright & Jaworski LLP
Palmer Phan T. H.
Teraxion Inc.
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