Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-08-28
2003-05-20
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S031000, C385S147000, C065S031000
Reexamination Certificate
active
06567588
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical fiber Bragg gratings and, in particular to an improved method for writing masks for use in fabricating in-fiber chirped Bragg gratings.
BACKGROUND OF THE INVENTION
Optical fibers are essential components in modern telecommunications systems. Comprised of thin strands of glass, optical fibers enable the transmission of light, or optical signals, over long distances with little loss. An optical fiber typically has a core of glass having a specific index of refraction surrounded by a glass cladding having a lower index of refraction. Thus, light entering the fiber is retained within the core by internal reflection.
In applications wherein a single optical fiber carries signals on more than one wavelength, such as in wavelength division multiplexing (WDM), fiber Bragg gratings are used for controlling the specific wavelengths of light within the fiber. Fiber Bragg gratings also have other applications such as in fiber lasers. A typical Bragg grating comprises a length of optical fiber having periodic modulations in the index of refraction in its core, spaced equally along the length of the grating.
Several methods have been developed to fabricate fiber Bragg gratings. For example, the holographic, or interferometer method uses the interference patterns created at the intersection of two coherent light beams to induce index modulations directly in the optical fiber. A second fabrication method involves the use of a phase mask positioned close and parallel to an optical fiber on which the grating is to be formed. For example, by placing the phase mask between a coherent light source, such as a laser, and an optical fiber, the diffraction caused by the mask replicates the function of an interferometer, creating a plurality of divergent light beams that interfere with each other in a predictable pattern, resulting in periodic alterations in the refractive index of the exposed core of the optical fiber. Typically, fiber Bragg grating fabrication using a phase mask requires stripping away the cladding before exposure of the core, although gratings can be printed onto an unstripped fiber having a cladding that is transparent to the wavelength of light passing through the phase mask.
Because of their ability to selectively reflect specific wavelengths in a narrow bandwidth, while allowing the remaining wavelengths to pass essentially unimpeded, Bragg gratings are used as filters, stabilizers, dispersion compensators and for other applications in fiber optic systems. It is desirable, however, under certain circumstances to broaden the range of wavelengths affected by a Bragg grating. To accomplish this, a technique known as chirping is applied wherein the spacing between the periodic modulations in the refractive index (pitch) of an ordinary Bragg grating is gradually increased or decreased along the length of the grating. Thus a chirped fiber Bragg grating has a wider active bandwidth and a wavelength-dependent time delay because it has a wider range of spacings.
Although the characteristics of chirped fiber Bragg gratings are desirable, fabrication of such gratings has proven difficult and time-consuming. Particularly, the fabrication of chirped phase masks has been challenging. For example, a typical chirped Bragg grating phase mask may have an array of between 100 to 200 grating segments, each between 0.5 and 1.0 mm in length and with a pitch change between segments measured on the picometer scale. The pitch of each successive segment varies continuously from, and must be “stitched” or placed precisely relative to, the preceding segment. Conventional lithography tools such as an electron beam (e-beam) tool (for example, the MEBES III or MEBES 4500, both manufactured by Applied Materials, Inc. of Santa Clara, Calif.) have been unable to achieve the accuracy necessary to produce a phase mask for a chirped Bragg grating in a time period that makes them competitive.
A lithography tool uses an image writing element such as a laser beam or an electron beam to print an image, such as that of a phase mask onto a substrate. Thus exposed, the substrate can be processed such that a grating pattern comprising, for example, an array of alternating lines and spaces is etched into the substrate. On a MEBES tool in particular, phase mask fabrication has been attempted using a “scale factor” approach and is therefore particularly complex. Specifically, a basic unscaled segment, or grating pattern, is established having a predetermined address unit defining its size. The grating pattern is rescaled as needed by applying scale factors to the address unit. Scale factors are dimensionless values that are applied by the MEBES tool to the address unit to achieve the desired reduction or magnification of the grating pattern. When applied across an entire mask, a specific chirp, or rate of change in the grating period of the finished phase mask is the result.
FIG. 1
is a block diagram of the steps in fabricating a chirped fiber Bragg grating from a phase mask produced applying the scale factor method on lithography tool such as a MEBES tool which uses an electron beam as an image writing element. The first step, shown as
10
, is to provide a photoresist coated substrate. As is common in the art, the substrate is often approximately 152×152×6.35 mm and is typically formed of amorphous quartz, due to its ability to transmit ultraviolet light, or of some other substantially transparent material. One of the major surfaces of the substrate is typically coated with 3000 to 5000 angstroms of photoresist material such as PBS or ZEP7000 over 1000 angstroms of a Cr/Cr oxide layer.
The second step, shown as
20
in
FIG. 1
, is to provide a grating pattern and the necessary scale factor and address unit values to the MEBES tool. The scale factor value to be applied to the grating pattern for each grating segment to be written onto the mask is established during the design and is known prior to the fabrication of the mask.
TABLE 1
Scale Factor jobdeck example
CHIP
1, (1, PHASEDE-MO-TK, AD = 0.125, SF = 1.0738)
ROWS
62500/13805.6585
CHIP
2, (1, PHASEDE-MO-TK, AD = 0.125, SF = 1.0738027,
GC = 1)
ROWS
62500/14304.97612775
CHIP
3, (1, PHASEDE-MO-TK, AD = 0.125, SF = 1.0738054,
GC = 1)
ROWS
62500/14804.295011
CHIP
4, (1, PHASEDE-MO-TK, AD = 0.125, SF = 1.0738081,
GC = 1)
ROWS
62500/15303.61514975
CHIP
5, (1, PHASEDE-MO-TK, AD = 0.125, SF = 1.0738108,
GC = 1)
ROWS
62500/15802.936544
* * *
CHIP
199, (1, PHASEDE-MO-TK, AD = 0.125, SF = 1.0743346,
GC = 1)
ROWS
62500/112695.034811
CHIP
200, (1, PHASEDE-MO-TK, AD = 0.125, SF = 1.0743373,
GC = 1)
ROWS
62500/113194.60102775
Table 1 is an excerpt of a typical jobdeck of the commands issued to a MEBES III or 4500, illustrating the commands instructing it to write the first five and last two of 200 grating segments on a particular substrate. The MEBES jobdeck addresses each segment as CHIP followed by the segment number. Referring to the jobdeck shown in Table 1, PHASEDE-MO-TK is the name arbitrarily given to the particular grating pattern from which the grating segments written to the substrate are modelled, AD is the address unit in microns prior to scaling and SF is the scale factor.
The address unit is chosen at the design stage, as with the scale factors, prior to the fabrication of the mask. AD is an integral divisor of the unscaled pitch of the grating pattern. A smaller value for AD results in increased accuracy and resolution, whereas a larger value results in an improved write time. For an unscaled pitch of 1.0 micron, the address unit is typically either 0.1 micron or 0.125 micron.
The location of the center of the grating segment follows the ROWS command, given as Y/X coordinates on an axis fixed relative to the substrate. As is well known in the art, a typical jobdeck provides all of these
Amster Rothstein & Ebenstein
Bovernick Rodney
Pak Sung
Photronics, Inc.
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