EP-3954983-B1 - A METHOD OF MANUFACTURING AN INTERFEROMETRIC FIBER OPTIC REFRACTOMETER

Inventors

• VILLATORO BERNARDO, Agustín Joel
• Flores Bravo, José Ángel
• ZUBIA ZABALLA, JOSEBA ANDONI

Dates

Publication Date

20260506

Application Date

20200812

Claims (8)

1. A method (300) for manufacturing an interferometric fiber optic refractometer (100), characterized in that it comprises the following steps: introducing (301) a single mode fiber (103) with a flat end facet (104) into a longitudinal through hole (102) of a ferrule (101) until the flat end facet (104) reaches an end (107) of the ferrule (101), the flat end facet (104) being substantially perpendicular to a longitudinal axis (105) of the single mode fiber (103); depositing (302) a substance (108) onto the end (107) of the ferrule (101), the substance (108) having a refractive index that is different from a refractive index of a core (110) of the single mode fiber (103); moving (303) the single mode fiber (103) into the ferrule (101) a distance "d", such that the substance (108) fills a cavity (106) created inside the ferrule (101) by surface tension, the cavity (106) being defined by the distance "d"; at least partially solidifying (304) the substance (108); and polishing (305) the at least partially solidified substance (108) to form a flat end tip (109) of the ferrule (101) that is substantially parallel to the flat end facet (104) of the single mode fiber (103).
2. The method (300) for manufacturing an interferometric fiber optic refractometer (100), according to claim 1, comprising maintaining the single mode fiber (103) and the ferrule (101) in a vertical position during the depositing and moving steps.
3. The method (300) for manufacturing an interferometric fiber optic refractometer (100), according to claim 1 or 2, wherein the moving step is performed by a motorized translation stage.
4. The method (300) for manufacturing an interferometric fiber optic refractometer (100), according to any one of claims 1-3, wherein the substance (108) is a curable polymer and solidifying the substance comprises curing the curable polymer.
5. The method (300) for manufacturing an interferometric fiber optic refractometer (100), according to claim 4, wherein the curable polymer is a temperature curable polymer.
6. The method (300) for manufacturing an interferometric fiber optic refractometer (100), according to claim 4, wherein the curable polymer is a UV-curable polymer and curing the curable polymer comprises curing the UV-curable polymer with a UV light source.
7. The method (300) for manufacturing an interferometric fiber optic refractometer (100), according to any of the previous claims, wherein the distance "d" is within a range between 40 and 180 µm, more preferably within a range between 40 and 80 µm or within a range between 125 and 150 µm.
8. The method (300) for manufacturing an interferometric fiber optic refractometer (100), according to any of the previous claims, wherein a diameter of the longitudinal through hole (102) of the ferrule (101) and a diameter of the single mode fiber (103) is substantially the same.

Description

TECHNICAL FIELD The present invention refers to a method for manufacturing an interferometric optical fiber refractometer that has an increased measuring range and that is temperature independent. STATE OF THE ART Refractometers are devices that measure the refractive index of a sample. Said refractometers have been important in many industrial and scientific applications during centuries. Commercial refractometers measure the refractive index of a liquid deposited on a prism of known refractive index. Such refractometers generally operate at visible wavelengths (589 nm), but they can be modified to operate at infrared wavelengths. The measuring range of prism-based refractometers is limited to indices which are smaller than that of the prism, which is typically around 1.7. Commercial refractometers are precise because they use several opto-mechanical components and have a system to keep the sample at constant temperature during the measurements. Miniaturization of prism-based refractometers is complex and thus, they are not appropriate for refractometry in small spaces such as in microfluidic channels or in flowers in vivo. For these applications, refractometers based on optical fibers are good candidates. The challenge in this case is to achieve refractometers that have, not only miniaturized dimensions, but also broad measuring ranges, resolutions comparable or even higher to those of commercial refractometers and, ideally, temperature independence. In addition, the fabrication of the refractometer must be highly reproducible and the cost of the device must be low. To achieve all these features, the optical fiber sensing community has proposed a myriad of alternatives. Up to date, three different techniques have been proposed to measure the refractive index of a sample with optical fibers. In one of these techniques, the sample under test alters the guided light through direct interaction with evanescent waves or by means of thin layers. Alternatively, instead of evanescent waves or thin layers, tilted Bragg gratings have been also used. Refractometers based on evanescent wave interactions or on tilted Bragg gratings require an interaction length of several millimeters and can measure indices in a narrow range, typically between 1.3 and 1.45. Moreover, they are highly sensitive to temperature. Another technique exploits the Fresnel reflection of an optical fiber-sample interface, which depends on the refractive index of the fiber core and that of the sample. This technique requires a sophisticated interrogation system to compensate fluctuations of the light source or bending losses in the optical fiber. However, its main drawback is the fact that two values of refractive index can give the same result. In refractive index sensing, this issue is called ambiguity. Fabry-Perot Interferometry (FPI) is another powerful technique to measure refractive index of a sample. In this technique, a microscopic cavity is fabricated on the facet of an optical fiber. The sample under test can be placed inside the cavity or outside of the same. In either case, the interference pattern of the interferometer changes in proportion of the refractive index of the sample. Such changes can be quantified in different manners. The main drawbacks of most FPI-based refractometers reported until now include complex fabrication, hence, low reproducibility, limited measuring range, ambiguity issues, and temperature sensitivity. There is still a need in the state of the art for a manufacturing process that is easily reproducible, has low complexity and a low associated cost. US2003112443A1 discloses a method of manufacturing an interferometric fibre optic refractometer by inserting an optical fiber in a capillary tube and by filling a hydrogel into an end portion of the capillary tube and bringing it to gelation. DESCRIPTION OF THE INVENTION The solution herein disclosed refers to the manufacturing process of an interferometric optical fiber refractometer. The manufacturing process is easily reproducible and presents low complexity and cost. A first example outside the subject-matter of the claims is an interferometric fiber optic refractometer. The interferometric fiber optic refractometer comprises a ferrule having a longitudinal through hole and a Single Mode Fiber (SMF) having a flat end facet that is substantially perpendicular to the longitudinal axis of the SMF. The SMF is inserted within the longitudinal through hole of the ferrule with the flat end facet of the SMF being located at a distance "d" from the end (tip) of the ferrule. The distance "d" defines a cavity within the ferrule. The interferometric fiber optic refractometer further comprises a substance or material filing the cavity that forms a flat end with the tip of the ferrule (the flat outer surface of the substance or material is substantially aligned with the end surface of the ferule forming a flat end tip of the ferrule). This flat end tip of the ferrule, and mor