METHOD AND APPARATUS FOR TEMPERATURE MEASUREMENT IN OPTICAL FIBER FUSION SPLICING

Abstract

The present invention relates to a method and an apparatus for measuring the temperature of optical fibers during fusion splicing or thermal processing, said method comprising: a) measuring, using an interferometric method, a change in an optical path length in an optical fiber due to temperature dependent properties of the optical fiber during fusion splicing or thermal processing; and b) determining the temperature of the optical fiber based on the measured changes in the optical path length.

Claims

1. A method for measuring temperature of optical fibers during fusion splicing or thermal processing, said method comprising: a) measuring, using an interferometric method, a change in an optical path length in an optical fiber due to temperature dependent properties of the optical fiber during fusion splicing or thermal processing; and b) determining the temperature of the optical fiber based on the measured changes in the optical path length.

2. The method according to claim 1, wherein said optical fiber is made of a material selected from the group consisting of silica-based glass, telluride glass, chalcogenide glass, fluoride glass, sapphire crystal, quartz crystal, and crystalline silicon.

3. The method according to claim 1, wherein said temperature dependent properties of the optical fiber are selected from the group consisting of thermal expansion and refractive index.

4. The method according to claim 1, wherein the temperature measured is in a range of from 100° C. to 3000° C.

5. The method according to claim 1, wherein said interferometric method comprises measuring the change in an optical path length using a Fabry-Perot or Mach-Zehnder type interferometer.

6. The method according to claim 1, wherein the method further comprises: c) using the temperature determined in step b) as feedback during splicing to enable controllable temperature.

7. The method according to claim 6, wherein the method further comprises the step: d) analyzing the optical fiber after fusion splicing using an interferometric method to determine quality of the splice through detection of variations in diameter or refractive index of the optical fiber.

8. The method according to claim 1, wherein the temperature measurement is not sensitive to changes in temperature and humidity in an atmosphere surrounding the optical fiber.

9. A fusion splicing apparatus for fusing or welding optical fibers together, said apparatus comprising means for measuring temperature of the optical fibers during fusion splicing or thermal processing, wherein said means comprise an optical microcavity interferometer configured to measure a change in an optical path length in an optical microcavity due to temperature dependent properties of the optical microcavity, and wherein said optical microcavity interferometer is arranged to use an optical fiber being subjected to fusion splicing or thermal processing in the fusion splicing apparatus as the optical microcavity.

10. The fusion splicing apparatus according to claim 9, wherein the optical microcavity interferometer is a Fabry-Perot or Mach-Zehnder type microcavity interferometer.

11. The fusion splicing apparatus according to claim 10, wherein the apparatus is configured to use the measured temperature as feedback during splicing to enable controllable temperature.

12. (canceled)

13. The fusion splicing apparatus according to claim 10, wherein the apparatus is configured to use the measured temperature as feedback during splicing to enable controllable temperature.

14. The method according to claim 2, wherein said temperature dependent properties of the optical fiber are selected from the group consisting of thermal expansion and refractive index.

15. The method according to claim 1, wherein the temperature measured is in a range of from 1000° C. to 3000° C.

16. The method according to claim 2, wherein said interferometric method comprises measuring the change in an optical path length using a Fabry-Perot or Mach-Zehnder type interferometer.

17. The method according to claim 3, wherein said interferometric method comprises measuring the change in an optical path length using a Fabry-Perot or Mach-Zehnder type interferometer.

18. The method according to claim 3, wherein the method further comprises: c) using the temperature determined in step b) as feedback during splicing to enable controllable temperature.

19. The method according to claim 5, wherein the method further comprises: c) using the temperature determined in step b) as feedback during splicing to enable controllable temperature.

20. The method according to claim 7, wherein said optical fiber is made of a material selected from the group consisting of silica-based glass, telluride glass, chalcogenide glass, fluoride glass, sapphire crystal, quartz crystal, and crystalline silicon; and wherein the temperature measurement is not sensitive to changes in temperature and humidity in an atmosphere surrounding the optical fiber.

21. The method according to claim 20, wherein said interferometric method comprises measuring the change in an optical path length using a Fabry-Perot or Mach-Zehnder type interferometer; and wherein the temperature measured is in a range of from 100° C. to 3000° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Referring now to the drawings, which are exemplary embodiments, and wherein:

[0045] FIG. 1a is a schematic view of a fusion splicing apparatus according to the invention comprising a Fabry Perot type microcavity interferometer;

[0046] FIG. 1b is a schematic view of a fusion splicing apparatus according to the invention comprising a Mach Zehnder type microcavity interferometer;

[0047] FIG. 2a is a schematic view of a Fabry Perot type microcavity interferometer;

[0048] FIG. 2b is a schematic view of the optical path in a Fabry Perot type microcavity interferometer;

[0049] FIG. 3a is a schematic view of a Mach Zehnder type microcavity interferometer;

[0050] FIG. 3b is a schematic view of the optical path in a Mach Zehnder type microcavity interferometer;

[0051] FIG. 4 is a calibration curve showing change in measured phase change of an SM optical fiber with change in temperature ΔT 0 to ˜1500° C., using a HeNe laser.

[0052] FIG. 5 is a diagram showing the temperature of an optical fiber measured during and directly after fusion splicing using the inventive method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0053] The correlation between temperature and measured changes in an optical path length in an optical fiber due to temperature dependent properties of the optical fiber during fusion splicing or thermal processing can be used in an optical fiber fusion splicing apparatus of the automatic type, which is schematically shown in FIG. 1a. The apparatus (100) has clamps (101,102), in which end portions of optical fibers (103,104) are placed and retained as their position is adjusted in the welding process. The clamps are displaceable in a direction parallel to the longitudinal direction of the fibers. Possibly, the clamps can also be displaceable in directions perpendicular to the fiber longitudinal direction in order to align the fibers with each other. Generally, however this alignment is produced by placing the fiber ends in V-grooves or similar fixed mechanical guides. The clamps are operated along suitable mechanical guides by means of control motors (not shown). Electrical lines (105,106) to electrodes (107,108) extend from a driver circuit (109).

[0054] The apparatus (100) further includes a Fabry Perot type microcavity interferometer (110). The interferometer (110) is comprised of a light source (111), a detector (112), a reference detector (113) and suitable optical components. The Fabry Perot type microcavity interferometer (110) is described more in detail with reference to FIGS. 2a and 2b. The light source (111) is connected to a driver circuit (114) and the detector (112) is connected to a detector interface (115).

[0055] The apparatus (100′) of FIG. 1b is identical to the apparatus of FIG. 1a, with the exception that the interferometer (110′) is of the Mach Zehnder type. The interferometer (110′) is comprised of a light source (111′), a detector (112′) and suitable optical components. The Mach Zehnder type microcavity interferometer (110′) is described more in detail with reference to FIGS. 3a and 3b. The light source (111′) is connected to a driver circuit (114′) and the detector (112′) is connected to a detector interface (115′).

[0056] The electrode and light source driver circuits (109,110,110′), and the detector interface (115,115′), are connected to a control unit (116), specifically a microprocessor. In particular, a signal from the detector (112,112′) is provided to a data processing and temperature analysis feedback loop (117) of the control unit (116). The data processing and temperature analysis feedback loop (117) analyses the detector signal in order to determine among other things the temperature of the optical fiber. The data processing and temperature analysis feedback loop (117) uses values provided by the user, including the optical fiber diameter as well as inherent temperature dependent properties of the material of the optical fiber being processed, specifically thermal expansion and refractive index, to determine the optical path length. A change in optical path length of light interacting with the optical fiber can then be used to determine a temperature of the fiber with great accuracy. Temperature is extracted by analyzing the phase/intensity of the interfering beams.

[0057] The data processing and temperature analysis feedback loop (117) may further compare the determined temperature to different set values stored in a memory (not shown).

[0058] The temperature, as well as other parameters determined by the temperature analysis feedback loop (117) can be displayed on a monitor (118).

[0059] The different steps in the procedure are controlled by the control unit (116). The microprocessor further controls the displacement of the fiber ends (103,104) in relation to each other via the motors, and the time, when a heating of the fiber ends is to be made, via the driver circuit (109) to provide the welding electrodes (107,108) with an electric voltage, so that a suitable electrode current flows between the electrodes (107,108), and it also controls those time periods, during which different electrode currents are to be supplied.

[0060] The automated splicing process will typically include a series of steps. A prefuse cycle is used to remove any dirt on the fiber ends and preheat the fibers for splicing. The fibers (103,104) are aligned using the clamps (101,102), mechanical guides and control motors. The fibers are fused by an automatic arc cycle that heats them in an electric arc and feeds the fibers together at a controlled rate. When fusion is completed, the splice is inspected to estimate the optical loss of the splice.

[0061] During all these time periods also the temperature can be automatically analyzed by the interferometer (110,110′) and data processing and temperature analysis feedback loop (117) of the control unit (116). The data processing and temperature analysis feedback loop (117) uses values provided by the user, including the optical fiber diameter as well as inherent temperature dependent properties of the material of the optical fiber being processed, specifically thermal expansion and refractive index, to determine the optical path length. A change in optical path length of light interacting with the optical fiber can then be used to determine a temperature of the fiber with great accuracy. Temperature is extracted by analyzing the phase/intensity of the interfering beams.

[0062] The temperature of the fiber ends is determined according to the method by measuring changes in an optical path length, as shown in FIGS. 2b and 3b, in an optical fiber due to temperature dependent properties of the optical fiber (103,104).

[0063] The temperature dependent properties of the optical fiber are converted into temperature values using a calibration curve for the specific material of the optical fiber. An example of a calibration curve for an SM fiber, using a HeNe laser, is shown in FIG. 4.

[0064] The signal from the detector (112,112′) is analyzed by the data processing and temperature analysis feedback loop (117) of the control unit (116), and the temperature of the fiber ends is determined. An example of a temperature monitoring during fiber splicing is shown in FIG. 5.

[0065] The optical fiber material should be optically transparent/semitransparent in at least one wavelength region. The wavelength region should be transparent or semitransparent within the temperature range of interest. The fiber cross section should be circular symmetric, i.e in the form of a cylinder. When heated, the optical fiber changes in dimension and refractive index due to thermal expansion and the thermo-optical properties of the material.

[0066] Generally, the microcavity interferometer consists of the optical microcavity (i.e. the optical fiber being spliced), a light source (e.g. a laser), optical components (e.g. mirrors, lenses, beam splitters and polarization optics), detector components (e.g. homodyne detection, hereodyne detection, spectral detection, or imaging/fringe analysis), and equipment for processing, presenting, and/or storing the data/results obtained from the measurement.

[0067] The light source should have a wavelength/wavelengths overlapping with wavelength region of transparency/semitransparency of the optical fiber, and a coherence length of the order of, or greater than the diameter of the optical fiber.

[0068] FIG. 2a is a schematic representation of a Fabry Perot type microcavity interferometer (FP interferometer). The FP interferometer (110) consists of a light source (111) in the form of a laser, a beam splitter (119), and a focusing lens (120) for focusing the beam (121) on the optical fiber microcavity (103,104).

[0069] FIG. 3a is a schematic representation of a Mach Zehnder type microcavity interferometer (MZ interferometer). The MZ interferometer (110′) consists of a light source (111′) in the form of a laser and a focusing lens (122) for focusing the beam (123) on the optical fiber microcavity (103,104).

[0070] The optical interferometers of the invention is based on a micro-cavity design having circular symmetry. Impinging light will be partially reflected and scattered at the interfaces of the micro-cavity due to changes of the refractive index between the ambient air and the micro-cavity, i.e. Fresnel reflections. By using a beam of light positioned in such a way that different Debye series modes of scattering are co-aligned and overlap, the modes can be made to interfere at a position outside the cavity, as shown in FIG. 2b for a FP interferometer and in FIG. 3b for an MZ interferometer.

[0071] The recombined beams will interfere constructively or destructively depending on the phase difference between the two paths of the interferometer. The phase difference is given by the difference in optical path length, OPL=(length)×(refractive index), between the two paths. As the diameter and refractive index is temperature dependent and characterized by the thermal expansion coefficient and thermo-optic coefficient, respectively, a change in temperature will change the conditions of interference and signal detected.

[0072] Using a collimated beam with a diameter equal or greater than the micro-cavity, resulting in interference among all different scattering modes, making the interpretation more difficult. To simplify one can use a focused beam, limiting the angular distribution of scattering modes, which enables a higher degree of selection of the different scattering modes, approaching that of the ray tracing in FIGS. 2a and 2b.

[0073] Using a focused beam also ensures that the phase front of the light is parallel with the surface of reflection. This is true specifically for the FP interferometer (FIG. 2a). Using a focused beam with the focus placed at/near the center of the optical fiber, the phase front of the optical beam becomes parallel to the surface of the cylinder/sphere as in the case of a traditional Fabry-Perot interferometer.

[0074] In the MZ interferometer, incoming light is partially reflected at the surface (surface reflection), while part of the light is transmitted/refracted and can be reflected further within the cavity before exiting the cavity, as shown in FIG. 2b. For a specific position and angle of the incoming beam, the surface reflection and the 3rd order refracting beam are co-aligned and can be made to interfere. For silica glass the angle is ˜94 deg.

[0075] While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

EXAMPLES

[0076] The invention will now be described more in detail by way of the following non-limiting examples.

Example 1

Temperature as a Function of Phase

[0077] FIG. 4 shows the measured temperature as a function of phase when an SM fiber with a diameter of 125 μm was heated using a carbon-dioxide laser.

[0078] The measurement was performed by (i) measuring the change in phase by interferometry, using the optical fiber as the optical microcavity, and (ii) simultaneously measuring the temperature of the optical fiber, at the same position, using a previously calibrated high-temperature stable fiber Bragg grating as described in U.S. Pat. No. 6,334,018B1 (Optical material having periodically varying refractive index and method of making). Localized heating of the grating and interferometer optical microcavity was performed by laser irradiation using a carbon-dioxide laser operating at a wavelength of 10.6 μm.

[0079] To measure the change in phase (i) an optical setup similar to that shown in FIG. 2a was used. The light source was a Helium-Neon (HeNe) laser operating at 632 nm. Phase extraction was performed using fringe counting with signal detection comprised of recording the quadrature signals from the interferometer optical microcavity followed by phase unwrapping.

[0080] To correlate the phase change to a change in temperature, a high temperature stable optical fiber Bragg grating was positioned at the same location as the interferometer optical microcavity. The fiber Bragg grating was 1 mm long with a Bragg wavelength at room temperature of 1542 nm. The peak wavelength of the grating was monitored in reflection using a white light source, an optical fiber circulator and an optical spectrum analyzer. Heating was performed by irradiating the fiber for a duration of two seconds. The data points in FIG. 4 include heating and cooling dynamics of six separate measurements using different power levels.

Example 2

Temperature Measurement of Fiber During Fusion Splicing

[0081] FIG. 5 is a diagram showing the measured temperature changes in an SM optical fiber during and directly after heating of the optical fiber using the electric arc discharge from an Ericsson FSU-850 fusion splicer. The measurement was performed using standard parameter settings for SM optical fiber splicing.

[0082] Recording of changes in phase during heating was performed using a similar setup as shown in FIG. 2a, and as described in Example 1, with the interferometer optical microcavity positioned at the center of the electric arc discharge region. After recording the phase dynamics during heating, the measured phase changes were converted to corresponding temperature changes using the temperature-phase relation shown in FIG. 4, using extrapolation for temperatures in excess of 1500° C. The measurements shown in FIG. 5 include two consecutive measurements, with the corresponding cooling dynamics rescaled to start at time zero.