Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-01-24
2003-09-16
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06621960
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to method of fabricating fiber Bragg gratings, and in particular to method of fabricating multiple superimposed fiber Bragg gratings.
BACKGROUND OF THE INVENTION
Fiber Bragg grating is now a key device in the established and emerging fields of optical communication and optical fiber sensing. There are basically two methods to photo-induce gratings in photosensitive optical fiber wave-guides, internal writing method and external writing method. The internal writing method was first described by K. O. Hill et al. and was disclosed in U.S. Pat. No. 4,474,427. In this method, coherent light having a wavelength in the visible region is launched into the core of a Ge-doped fiber from one end of the fiber. The light is reflected from the other end of the fiber. The forward propagating light and the backward propagating light interfere with each other to produce a standing wave with a period corresponding to half the wavelength of the writing light. By a photosensitive effect in the fiber, a refractive index grating with the period of the standing wave is written into the core of the fiber. The main drawback of this internal writing method is that only gratings with a period similar to that of a one-half the wavelength of the writing light can be made. The application of those fiber gratings written by internal writing method is substantially limited due to this drawback, especially in fiber communication areas.
Three general approaches of external writing method have been developed to overcome the drawbacks of internal writing method and actually make the application of fiber Bragg gratings in optical fiber communication areas possible. The first approach of external writing method was demonstrated by Glen et al. and was disclosed in U.S. Pat. No. 4,807,950. In this approach, an interferometer is used to split an incoming UV light into two beams that were subsequently recombined to form an interference pattern that side exposes a photosensitive fiber, inducing a permanent refractive index modulation in the core. As the Bragg grating period (which is identical to the period of the interference fringe pattern) of a fiber Bragg grating written in the core depends on both the irradiation wavelength and the half angle between the intersecting UV beams, theoretically Bragg gratings at any desired wavelength can be inscribed. The disadvantages of this approach include a susceptibility to mechanical vibration, relative complexity of the system and requirement for a UV laser source with good spatial and temporal coherence.
U.S Pat. No. 5,104,209 discloses a point-by-point approach of external writing method for fabricating Bragg gratings by inducing a change in the index of refraction corresponding to a grating plane one step at a time along the core of the fiber. The main advantage of this approach is its flexibility to alter the Bragg grating parameters. However, the point-by-point approach is a tedious process requiring a relatively long process time. Errors in the grating spacing due to thermal and/or mechanical vibration can occur. This limits the grating to a short length.
One of the most effective and mature approaches of external writing method for inscribing Bragg gratings in photosensitive fiber is the phase-mask approach. U.S. Pat. No. 5,367,588 discloses a phase-mask approach that employs a phase-mask (a diffractive optical element) to spatially modulate the UV writing beam. Phase-mask may be formed either holographically or by electron-beam lithography. The phase-mask is created as a one dimensional periodic surface-relief pattern. The profile of the periodic surface-relief grating is selected such that when a UV beam is incident on the phase-mask, the zero-order diffracted beam is suppressed to less than a few percent of the transmitted power. In addition, each of the diffracted plus and minus first orders is maximized to typically contain more than 35% of the transmitted power. The interference of the diffracted plus and minus first order beams produces a near-field fringe pattern with a period that is one-half of that of the phase-mask. The fringe pattern photo-imprints a refractive index modulation in the core of a photosensitive optical fiber that is placed in contact with or in close proximity to the phase-mask. Since its original demonstration in 1993, the phase-mask approach has been developed to a stage where the inscription of a nearly 100% reflective grating is now routine. U.S. Pat. No. 5,903,689 discloses a phase-mask-based method for spatially controlling the period and amplitude when inscribing a fiber Bragg grating in a photosensitive fiber.
As only one optical element is used to provide a robust and inherently stable method for producing fiber Bragg grating, the phase-mask approach substantially reduces the complexity and the cost of a fiber grating fabrication system. Since the fiber is usually disposed directly behind the phase-mask in the near field of the diffracted UV beams, sensitivity to mechanical vibration and therefore stability problems are minimized. Also, low temporal coherence does not affect the writing capacity as compared to the interferometric approach. However, the spatial coherence still plays an important role in the fabrication of Bragg gratings.
Multiple superimposed fiber Bragg gratings have been of great interest as a device in optical communications, lasers and sensor systems because multiple Bragg gratings at the same location basically perform a comb function that is ideal for manipulating, e.g. multiplexing and de-multiplexing, signals with different wavelengths. Writing all gratings at the same location of a fiber is well suited for optical integrated technology, where the physical size of a device is always a concern. Another advantage is the simplicity and cost-effectiveness for the athermal package structure that is one of the key technologies in fiber Bragg grating area. One general package can compensate the temperature induced wavelength shifts at the same time in all superimposed gratings at the same location.
Chirped Bragg gratings are highly valued in dispersion compensation applications of high-speed optical communication system. However, meaningful multi-channel dispersion compensation needs to cascade a plurality of single fiber Bragg gratings (each of them has a unpacked length of more than 10 cm) together that will result in large physical size and more complicated structure. Superimposing several chirped Bragg gratings at a same location can effectively reduce the physical size of such devices and can also substantially simplify the structure.
Multiple superimposed fiber Bragg gratings can also be used for material detection where the multiple Bragg lines can be designed to match the signature frequencies of a given material.
U.S. Pat. No. 5,627,927 discloses the use of two or more Bragg gratings superimposed at a same location of an optical fiber for sensing environment effect such as strain and temperature. However, this prior art reference does not specifically teach how the multiple superimposed Bragg gratings used are inscribed in the core of a photosensitive fiber.
U.S. Pat. No. 6,275,511 teaches two methods to create multiple superimposed Bragg gratings (column 4, lines 59-66). Both methods are on the basis of the phase-mask approach. The first method photo-induces multiple superimposed Bragg gratings by overwriting each of the fiber Bragg gratings with a corresponding phase-mask in an optical fiber. This method uses as many phase-masks as the number of gratings to be written in an optical fiber. Due to the changing of masks between each two consecutive writings, the configuration of the writing system can easily be altered and the repeated calibrations make the writing procedure a tedious process. The second method photo-induces multiple superimposed Bragg gratings by overwriting all of the fiber Bragg gratings with a single specially designed phase-mask that can generate interference patterns for all fiber Bragg gratings at the same time. This met
Li Weizhong
Wang Qinglin
Wu Weiti
Lumen Intellectual Property Services Inc.
Oplink Communications Inc.
Sanghavi Hemang
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