Abstract:
The invention discloses a tunable long period fiber grating structure and fabrication method, which produces an expected microbending deformation by adjusting the temperature and making use of the unmatched coefficients of thermal expansion between different materials, and thus creating a phenomenon of mode coupling effect occurred in traditional optical gratings in order to provide the structure and fabrication method for having an effect of tuning spectra by controlling the temperature. This invention not only can be applied to the optical communications and wavelength dependent multiplexed devices and equipments, but also can be applied to the temperature and deformation detection. The tunable long period fiber grating structure in accordance to the present invention comprises a substrate; a periodic structure being disposed thereon and having a first coefficient of thermal expansion; a coupling agent, having a second coefficient of thermal expansion for fixing a fiber to the periodic structure; wherein the first and second coefficients of thermal expansion are different.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a tunable long period fiber grating structure and a fabrication method capable of producing an expected microbending deformation by adjusting the temperature and making use of the unmatched coefficients of thermal expansion between different materials, and thus creating a phenomenon similar to the mode coupling effect occurring in traditional optical gratings in order to provide the structure and fabrication method for having an effect of tuning wave spectra by controlling the temperature. 
   BACKGROUND OF THE INVENTION 
   Long period fiber grating is an important element in the optical communications market, and its main commercial applications include the use of a phase mask fabrication method to produce a grating on fibers, which serves as a band rejection filter, a mode converter, a band rejection filter or a gain equalizer in various applications. The long period fiber grating has its advantages of being an all-fiber device and exhibits very low loss when connected to fibers, a low insert loss and a low back reflection, etc. In addition, the long period fiber grating is highly sensitive to high temperature, curvature, twist, and load, and thus is applicable for the development of optical communications and sensors. However, the long period fiber grating produced by photolithography has different properties according to the different conditions of the manufacturing process and needs accurate control. Once a fiber is written, it cannot be recovered or revised anymore. 
   In view of the foregoing shortcomings, an external force type tunable simple-structured long period fiber grating becomes the key point for the development of long period fiber grating. Two U.S. patents regarding the long period fiber grating filters are described below: 
   The U.S. Pat. No. 6,282,341 B1 disclosed a tunable mechanically induced long-period fiber grating filter, and this filter comprises a pair of periodic mechanical structures (which could be a periodic structure plus a plate) for mounting the fiber inside, ridges of upper periodic and lower periodic structures being engaged and aligned with each other for directly applying pressure on the surface of a fiber as to define a grating by means of directly inducing perturbations in the refractive index caused by the photoelastic effect. By changing the pressure applied on the periodic structures and controlling the perturbations of the refractive index, the frequency spectrum (loss) can be tuned. With the design of the shape and arrangement of the periodic structures, the wavelength and spectral form can be changed and the polarization dependence loss can be eliminated. The idea of such patent resides on the photoelastic effect caused by the perturbations of the refractive index, which is a result of applying pressure on the fibers, and thus it is necessary to prevent the microbending. Besides, the fiber must be jacketed, or else it will have a lower efficiency and additional loss (1˜2 dB). 
   The U.S. Pat. No. 6,408,117 B1 also disclosed mechanically induced long period fiber gratings. Unlike the foregoing patent, it only has a periodic structure on one side, but the other side is attached to a plate made of an elastic material for mounting a fiber between both sides. Screws are used for fixing and adjusting the tightness. The purpose of using an elastic material for the plate resides on producing a continuous microbending mode under the condition of applying an even pressure on the fiber to support the periodic structure when both sides of the periodic structure are engaged in order to produce a fiber grating. The objective of tuning the frequency spectrum can be achieved by adjusting the tightness of the screws, the selection of the elastic material, and the design, arrangement and change of the periodic structure. The idea of such patent emphasized on the fiber grating produced by the microbending, but the fiber must be jacketed (wrapped with plastics having a low refractive index), otherwise the intensity, the reliability and the depth of the frequency spectrum (loss) will be unsatisfactory. 
   SUMMARY OF THE INVENTION 
   The primary objective of the present invention is to provide a tunable long period fiber grating structure and fabrication method that controls the temperature to solve the problem of the fiber being damaged by using external mechanical forces to continuously produce microbending or photoelastic effect in the prior art. 
   The secondary objective of the present invention is to provide a tunable long period fiber grating structure and fabrication method that uses a simple combination of a small number of components to substitute a large number of devices used in the prior art so as to simplify the complicated manufacturing process and cost. 
   The present invention relates to a tunable long period fiber grating structure comprises a substrate; a periodic structure being disposed thereon and having a first coefficient of thermal expansion; a coupling agent, having a second coefficient of thermal expansion for fixing a fiber to the periodic structure; wherein the first and second coefficients of thermal expansion are different, and thus will cause the fiber to have a microbending phenomenon due to the different coefficients of thermal expansions of the two materials (which are the substrate and the coupling agent) to produce an effect similar to the fiber grating. 
   To make it easier for our examiner to understand the objective of the invention, its structure, innovative features, and performance, we use a preferred embodiment together with the attached drawings for the detailed description of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustrative view of the tunable long period fiber grating structure according to the present invention. 
       FIGS. 2A and 2B  are views of a first preferred embodiment of the present invention. 
       FIGS. 3A and 3B  are views of a second preferred embodiment of the present invention. 
       FIG. 4  is a perspective view of the substrate according to the present invention. 
       FIG. 5  is a perspective view of the periodic structure according to the present invention. 
       FIG. 6  is the frequency spectrum chart of the coupling agent that adopts heating for solidification according to the present invention. 
       FIG. 7  is the frequency spectrum chart of the coupling agent that adopts room temperature for solidification according to the present invention. 
       FIG. 8  is an illustrative view of three coupling modes of the coupling agent, periodic structure and fiber according to the present invention. 
       FIG. 9  is an illustrative view of the displacement of the most preferable mode of the coupling agent, periodic structure and fiber according to the present invention. 
       FIG. 10  is an illustrative view of controlling the external resultant force of the substrate according to the present invention. 
       FIG. 11  is a flow chart of manufacturing a tunable long period fiber grating according to the present invention. 
       FIG. 12  is an illustrative view of a general periodic structure according to a preferred embodiment of the present invention. 
       FIG. 13  is an illustrative view of a chirping structure according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The detailed description and technical characteristics of the present invention are described together with the drawings as follows. 
   Please refer to  FIG. 1  for an illustrative view of the tunable long period fiber grating structure of the present invention. The tunable long period fiber grating structure of the present invention comprises a substrate  100  having a periodic structure  150  with a first coefficient of thermal expansion; a coupling agent  200  for fixing a whole single-mode fiber  300  with no coating layer onto the periodic structure  150 , and having a second coefficient of thermal expansion; and a thermoelectric cooling chip  121  for providing a temperature change effect for the tunable long period fiber grating structure of the present invention; or other devices such as a heating plate and a heating wire to achieve the same effect of controlling temperature; wherein the first coefficient of thermal expansion and the second coefficient of thermal expansion are different for producing an effect similar to the fiber grating by the microbending phenomenon of the fiber caused by different coefficients of thermal expansion of the two materials which are the substrate and the coupling agent under a temperature change. The temperature change and the designed periodic structure can change the waveform of the penetrating frequency spectrum such as the depth and resonance wavelength. The foregoing substrate is made of a metal, a semiconductor, glass material, ceramic material, composite material or plastic material; and the coupling agent is a high-temperature solidified coupling agent or a room-temperature solidified coupling agent. 
   Please refer to  FIG. 5  for a perspective view of the periodic structure of the present invention. A linear cutting fabrication method is used to produce a V-shaped groove  151  on a steel material with a periodic width of 650 microns, a depth of approximately 284 microns and an angle of 90 degrees. The round corner is created by the path of the linear cutting (diameter 200 microns). The periodic width of 650 microns is to satisfy resonance wavelength at the phase match conditions of 1530 nm and 1585 nm. The phase matching conditions and the basic characteristics of the long period fiber grating are described below. After a light wave passes through a long period fiber grating, a diffraction is produced to couple a core mode propagating in a positive direction to a cladding mode propagating in a positive direction and carry a level of a diffused light to satisfy the phase match conditions of the wavelength in the vacuum, the propagating constant of the core mode and the n th  level clapping mode, and the periodic interval. If the clapping wavelength propagated into the core satisfies the phase match conditions (resonance wavelength), then the light will be guided into the cladding and cause a light energy loss in the cladding to form a loss peak. Such characteristic is the foundation of a wave. 
   The manufactured substrate  100  comprises a designed periodic structure  150 , but the periodic structure  150 , the coupling agent  200  and the single-mode fiber  300  on the substrate  100  are coupled with each other as shown in the following two embodiments including the fabrication method of filling up the periodic structure with the coupling agent and the fabrication method of filling a portion of the periodic structure with the coupling agent. These two fabrication methods are described as follows. 
   Refer to  FIGS. 2A and 2B  for the fabrication method of filling up the periodic structure with the coupling agent according to the first preferred embodiment of the present invention. In  FIG. 2A , the coupling agent  200  is filled up in a groove  151  and then a fiber  300  is placed perpendicularly on the periodic structure  150 . The fiber  300  will be completely adhered onto the periodic structure automatically due to the viscosity of the coupling agent  200  and the capillary effect. After the temperature is lowered and the structure is solidified, the coefficient of thermal expansion of the coupling agent  200  and the coefficient of thermal expansion of the substrate  100  are not equal, and thus the downward contraction force of the coupling agent  200  as indicated by fiber the arrow Y in the figure, the horizontal contraction force of the substrate  100  as indicated by the Arrows X 1 , X 2  simultaneously form a microbending. On the other hand, if the temperature rises, the curvature of the fiber according to this embodiment is reduced. Refer to  FIG. 2B  for the embodiment adding a plurality of temperature control circuit  111 . In  FIG. 2B , the temperature control circuit  111  can be fixed into the groove  151  of the periodic structure  150 , which can concurrently serve as a pivot of the fiber  300  and a device for adjusting the temperature as to control the temperature of the coupling agent  200  and the substrate  100  more directly. In this embodiment of filling up the periodic structure with the coupling agent, a better result will be achieved if the second coefficient of thermal expansion is defined to be larger than the first coefficient of thermal expansion. 
   Refer to  FIGS. 3A and 3B  for the fabrication method of filling a portion of the periodic structure with the coupling agent according to the second preferred embodiment of the present invention. In  FIG. 3A , the coupling agent  200 ′ is coated on the top of the periodic structure  150 ′ and the top of the periodic structure  150 ′ is sharp, in an arc shape or in any other shape, or the coupling agent  200  can be coated onto the external side of the periodic structure  150 ′. The groove  151 ′ is not filled up, but only the end point in contact with the fiber  300 ′ is coupled. When the temperature of the solidified structure drops, the main effects of the horizontal contraction force of the substrate  100 ′ indicated by the Arrows X 1 ′, X 2 ′ and the contraction at the end point of the coupling agent  200 ′ as indicated by the Arrows XY 1 ′, XY 2 ′ constitute a microbending. On the other hand, the curvature will be reduced if the temperature drops. Refer to  FIG. 3B  for the embodiment that adds a plurality of temperature control circuits  111 ′. The temperature control circuit  111 ′ of this embodiment corresponds to each end point where the coupling agent  200 ′ and the groove  151 ′ for directly adjusting the temperature of the coupling agent  200 ′. 
   Refer to  FIG. 4  for the perspective view of the substrate according to the present invention. The numerals are the same as those adopted in  FIGS. 2A ,  2 B and  3 . The stainless steel substrate  100  comprises a groove  151  of the periodic structure  150  and a guide groove  131 , and the guide groove  131  is a shallow semicircle (with a radius of 100 microns) for maintaining the fiber  300  in a position perpendicular to the groove  151  when it is adhered onto the surface of the periodic structure  150  and keeping the interval constant, and achieves the special change for the intervals by intentionally causing the fiber and the periodic structure to show a specific included angle or curvature. Different kinds of coupling agents are used for the hardening according to the foregoing embodiment can give different frequency spectrum characteristics as elaborated as follows: 
   If the coupling agent adopts heating for its solidification, such coupling agent is called a high-temperature solidification coupling agent such as the EPO-TEK353ND-T, which will be solidified by heating it at 150° C. for one hour. By that time, the fiber is maintained straight and level at a high temperature, and will be gradually cooled down to room temperature as the object cools after it is solidified. By the stress produced by the foregoing unmatched coefficients of thermal expansion, a progressively increased bending effect is obtained. The experiment result as shown in  FIG. 6  shows the frequency spectrum chart of the coupling agent being solidified by heating according to the present invention. In  FIG. 6 , the high-temperature wave spectrum is flat at the temperature approaching 150° C., but the temperature will drop gradually and the two resonant wave peaks will become obvious. 
   Taking a periodic structure having a length of 30 periods for example, if the temperature reaches room temperature around 25° C., its second resonance mode loss is up to −16 dB. 
   If the room-temperature solidification coupling agent (i.e. the room temperature coupling agent) such as the ChemitechU-425 is adopted, it is necessary to expose the coupling agent under ultraviolet beams for several minutes for the hardening. By that time, the fiber is maintained straight and level at a constant temperature, and will be gradually cooled down to room temperature as the object cools after it is solidified. By the stress produced by the foregoing unmatched coefficients of thermal expansion, a progressively increased bending effect is obtained.  FIG. 7  shows the wave spectrum of the room-temperature coupling agent of the present invention. In  FIG. 7 , the experiment result shows the room-temperature wave spectrum is flat at a constant temperature, but when the temperature gradually drops to −15° C., the second resonance mode loss peaks will be −14 dB for the periodic structure having a length of 28 periods. 
   In the foregoing experiments performed by two different types of coupling agents, the following two conclusions will be drawn:
     1. A wave filter having a high-temperature solidification coupling agent is heated to lower its wave efficiency as to have the wave filter effect at room temperature and the waveform is flat at a high temperature as shown in  FIG. 6 .   2. A wave filter having a room-temperature solidification coupling agent drops its temperature to improve its wave filter efficiency. Therefore the wave filter has a flat waveform at room temperature and a wave filter effect only at a low temperature as shown in  FIG. 7 . Therefore, the two can be controlled to have the expected results by controlling the temperature.   

   In view of the description above, there are two ways of changing the wave filter effect, either by changing the method of coating the coupling agent on the periodic structure or using different types of coupling agents. Therefore, these two ways are used alternately to develop a plurality of feasible combination modes to comply with all kinds of situations. 
   Refer to  FIG. 8  for the illustrative view of three coupling modes for the coupling agent, the periodic structure and the fiber. In  FIG. 8 , the left figure shows that the coating height of the coupling agent  200  reaches exactly on the lower edge of the fiber  300 . In other words, the coating height is the same as the height of the periodic structure  150 . The figure in the middle shows that the coating height of the coupling agent  200  is exactly one half of the length of the fiber  300 . The figure on the right shows that the coating of the coupling agent  200  submerges the fiber  300 . Since the thickness of the coating of the agent coupling and the contact area of the fiber can change the periodic curvature of the fiber, an appropriate thickness (approximately half of the height of the fiber) can provide the best bending efficiency. 
   Refer to  FIG. 9  for the illustrative view of the displacement of the most preferable mode of the coupling agent, periodic structure and fiber of the present invention. In  FIG. 9 , the design for the structure, profile and appearance of the periodic structure filled up with the coupling agent can be optimized to make each periodic curvature of the fiber and the contraction path of the coupling agent consistent to minimize the stress at the interface of the coupling agent and the fiber. 
   Refer to  FIG. 10  for the illustrative view of controlling the external resultant force of the substrate according to the present invention. In  FIG. 10 , a plurality of external forces indicated by the arrows F 1 , F 2 , T 1 , T 2 , T 3 , T 4  respectively applied on the substrate  100  for changing the wave filter effect. Therefore, the present invention can help controlling external forces, in addition to its main purpose of controlling temperature. 
   Please refer to  FIG. 11  for a fabrication method of manufacturing a tunable long period fiber grating structure, which comprises the steps of:
     (1) preparing a substrate having a periodic structure made of a material such as a metal, a semiconductor, glass material, ceramic material, composite material or plastic material, and the substrate has a first coefficient of thermal expansion;   (2) preparing a coupling agent such as a high-temperature solidification coupling agent or a room-temperature solidification coupling agent, and the coupling agent has a second coefficient of thermal expansion;   (3) filling the coupling agent into a periodic structure such as a general periodic structure or a chirping structure, which is made by a mechanical machining, a semiconductor fabricating process, a metal molding, a plastic injection molding, a press molding or a stamping molding;   (4) disposing a whole single-mode fiber without a coating layer on the periodic structure;   (5) using the coupling agent to fix the fiber on the periodic structure;   (6) ending.
 
Where, the second coefficient of thermal expansion is different from the first coefficient of thermal expansion, and thus will produce an effect similar to the fiber grating by the microbending phenomenon of the fiber caused by the different coefficients of thermal expansion of the two materials including the substrate and the coupling agent under a temperature change. The temperature change and the designed periodic structure can change the waveform of the penetrating frequency spectrum such as the depth and resonance wavelength.
   

   As described above, the periodic structure is divided into a general periodic structure or a chirping structure. Please refer to  FIGS. 12 and 13  for the illustrative views of the general and chirping structures according to a preferred embodiment of the invention respectively. The periodic structure is an equidistant sharp serrate structure as shown in  FIG. 12A , an equidistant mixed structure as shown in  FIG. 12B  and an equidistant wavy structure as shown in  FIG. 12C . The chirping structure is a progressively changing equidistant sharp serrate structure as shown in  FIG. 13A , a progressively changing equidistant flat serrate structure as shown in  FIG. 13B , a progressively changing equidistant mixed structure as shown in  FIG. 13C  and a progressively changing equidistant wavy structure as shown in  FIG. 13D . In the meantime, the structure of the periodic structure can have different quantity of periods to cope with the required width and depth of the frequency spectrum. 
   In view of the description above, the present invention uses different matters with different coefficients of thermal expansion, the temperature or the external force is controlled to achieve a tunable long period fiber grating structure. Regardless of the structural cost and technical ideas, they are different from the prior art, not only does it save costs and is it easy to understand the simple principle of technology, but it achieves the effect of being extensively applied for different conditions. 
   Further, its continuous serially connected substrates (periodic structures) give a synergic effect on the penetrating wave spectrum as to achieve the shaping and gain equalizing effects. Since it is an all-fiber component, therefore it fits the development for optical communications and sensors and serves as a band rejection filter, a mode converter, a gain equalizer, and a sensor for detecting pressure, displacement and temperature. As to the novelty and utility of the present invention, the present invention is novel for the following reasons. (1) It uses a single-mode fiber for the manufacture and does not need to etch cladding, which has the characteristics of a traditional long period fiber grating. In the meantime, the wave spectrum can be tuned. (2) It adopts the unmatched coefficients of thermal expansion and the structural design to produce a periodic microbending for the fiber by a change of temperature and achieves the wave filter effect. (3) When the materials are coupled, the selection for hardening (high temperature/room-temperature hardening) is provided to offer additional wave spectrum characteristics (normally on/off). 
   The present invention is usable for the following reasons. (1) It makes the long period fiber grating tunable and also overcomes the shortcomings of the fiber being exerted by forces or having a poor reliability. (2) It gives wave efficiency up to −21 dB by a single-mode fiber that adopts 35 periods at a small range of temperature change (22° C.). Therefore, the present invention complies with the patent application requirements and is submitted to the Patent and Trademark Office for review and granting of the commensurate patent rights.