Fiber optic cable with monitoring of backward propagating radiation

Abstract

The fiber optic cable includes an optical fiber that transports a forward-propagating laser beam. The optical fiber includes a core, a cladding, and an output end face that emits the forward-propagating beam. The fiber optic cable also includes a mode stripper along a section of the optical fiber that couples outward backward-propagating radiation that is coupled into the cladding at the output end face. The fiber optic cable further includes a waveguide having a waveguide body with a bore that contains at least a portion of the section of the optical fiber. The bore is defined by an inward-facing surface that directs at least a percentage of the backward-propagating radiation coupled outward of the cladding by the mode stripper from the output end face toward a rear opening of the distal-most bore. Additionally, the fiber optic cable includes one or more sensors or fiber ports that receive a portion of the backward-propagating radiation that emerges from the rear opening.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of U.S. Provisional Application No. 63 / 440,975, filed January 25, 2023, the entire contents of which are incorporated herein by reference.

[0002] (Technical field of the invention) The present invention relates to the laser processing of materials using laser light delivered by a fiber optic cable, and in particular to the integration of sensors within the fiber optic cable for the purpose of monitoring the laser process. [Background technology]

[0003] (Discussion of Background Art) High-power laser radiation beams are used to machine, weld, and otherwise process a wide range of materials, including metals, plastics, and glass. Common industrial laser processes include cutting, scribing, drilling, marking, welding, heat treating, and annealing. A typical laser processing system includes a laser source that generates a laser beam and a processing head that focuses the laser beam onto a workpiece and steers it as needed. The processing head has several free-space optical elements for focusing and steering the laser beam. However, frequently, the laser beam is transported from the laser source to the processing head via a fiber optic cable.

[0004] Most industrial laser processes require high laser power. Average laser power can be as much as tens of kilowatts. The fiber optic cable used to transport the laser beam must therefore be capable of handling the high laser power. Connectors at the input and output ends of the fiber optic cable can experience significant thermal loads from partial reflections (e.g., at the interface between the fiber end face and the end cap), unintentional light leakage from the optical fiber (e.g., caused by mechanical strain and stress), and intentional mode stripping to remove portions of the laser beam propagating in the cladding modes of the optical fiber. The connector at the output end of the fiber can also experience thermal loads from laser radiation that is back-reflected from the workpiece. Some industrial fiber connectors are water-cooled to manage the thermal load and may include sensors to monitor the condition of the fiber optic cable.

[0005] Radiation propagating backward from the workpiece through the processing head can provide useful information about the laser process. Such backward-propagating radiation includes back-reflected laser radiation, luminescence radiation, and thermal radiation. Back-reflected laser radiation results from specular reflection and other elastic scattering of the forward-propagating laser beam (the beam used to process the workpiece). Back-reflected laser radiation has the same wavelength as the forward-propagating laser beam. Luminescence radiation results from inelastic scattering of the forward-propagating laser beam and typically spans a much wider wavelength range than the forward-propagating laser beam. Thermal radiation is caused by the forward-propagating laser beam heating a target area on the workpiece. Many industrial laser processes heat the target area to thousands of degrees Kelvin, resulting in thermal radiation peaking in the near-infrared or even red portion of the visible spectrum. Summary of the Invention [Means for solving the problem]

[0006] Disclosed herein is a fiber optic cable having functionality for monitoring backward-propagating radiation. The fiber optic cable is useful for fiber delivery of laser beams in industrial laser processing applications such as laser machining and laser welding, and the monitoring functionality can be used to monitor radiation propagating backward from a target area on a workpiece. The fiber optic cable is configured to selectively monitor backward-propagating radiation coupled into the cladding of the optical fiber. The present invention's selective monitoring of cladding-coupled backward-propagating radiation helps ensure that the monitored radiation originates from a target location on the workpiece.

[0007] In the optical fiber cable of the present invention, backward-propagating cladding-coupled radiation is at least partially stripped from the cladding by a mode stripper and then directed toward one or more sensors. A section of optical fiber equipped with a mode stripper is installed inside a waveguide. At least a portion of the extracted radiation propagates backward through the waveguide and is collected for detection at the back end of the waveguide. A sensor may be positioned at the back end of the waveguide for direct detection of the backward-propagating radiation. Alternatively, or in addition, backward-propagating radiation emerging from the back end of the waveguide may be directed to the sensor(s) by one or more reflectors. The mode stripper, waveguide, sensor(s), and any included reflector(s) may also be implemented directly within the optical fiber cable connector. All elements required to direct the extracted radiation from the waveguide to the sensor(s) can be aligned during assembly of the fiber connector, without the need for further alignment during installation in the laser processing equipment and subsequent use in a laser processing task. Without compromising this inconsistent quality of the design of the present invention, any one of the sensors (singular or plural) can be replaced by a fiber port, whereby a dedicated optical fiber can transmit a portion of the extracted backpropagating radiation to an external sensor instead of the integrated sensor replacement.

[0008] In one aspect of the present invention, a fiber optic cable with backward-propagating radiation monitoring includes an optical fiber for transporting a forward-propagating laser beam. The optical fiber includes a core, a cladding, and an output end face configured to emit the forward-propagating laser beam. The fiber optic cable also includes a mode stripper within or on the cladding along a first longitudinal section of the optical fiber for coupling out of the cladding backward-propagating radiation coupled into the cladding at the output end face. The fiber optic cable further includes a hollow waveguide including a waveguide body having a bore that contains at least a portion of the first longitudinal section of the optical fiber. The bore has a back opening distal most from the output end face. The bore is defined by an inward-facing surface to guide at least a percentage of the backward-propagating radiation coupled out of the cladding by the mode stripper in a backward direction toward the back opening. Additionally, the fiber optic cable includes one or more optical receivers for receiving individual portions of the backward-propagating radiation emerging from the back opening. Each optical receiver includes a sensor or an optical fiber port.

[0009] In another aspect of the present invention, a fiber optic cable with backward-propagating radiation monitoring includes an optical fiber for transporting a forward-propagating laser beam. The optical fiber includes a core, a cladding, and an output end face configured to emit the forward-propagating laser beam. The fiber optic cable also includes a mode stripper within or on the cladding along a longitudinal section of the optical fiber for coupling out of the cladding backward-propagating radiation coupled into the cladding at the output end face. The fiber optic cable further includes a waveguide containing at least a portion of the longitudinal section of the optical fiber. The waveguide is configured to guide at least a percentage of the backward-propagating radiation coupled out of the cladding by the mode stripper in a backward direction away from the output end face. The waveguide has a back end distal to the output end face. Additionally, the fiber optic cable includes one or more optical receivers for receiving individual portions of the backward-propagating radiation emerging from the back end of the waveguide. Each optical receiver includes a sensor or an optical fiber port. [Brief explanation of the drawings]

[0010] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate generally preferred embodiments of the present invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0011] [Figure 1A] 1A and 1B illustrate a fiber optic cable having functionality for monitoring backward-propagating radiation, according to one embodiment. The backward-propagating radiation is extracted from the cladding of the cable's optical fiber inside a hollow waveguide. One or more reflectors direct the extracted backward-propagating radiation emerging from the rear end of the hollow waveguide toward one or more sensors. [Figure 1B] 1A and 1B illustrate a fiber optic cable having functionality for monitoring backward-propagating radiation, according to one embodiment. The backward-propagating radiation is extracted from the cladding of the cable's optical fiber inside a hollow waveguide. One or more reflectors direct the extracted backward-propagating radiation emerging from the rear end of the hollow waveguide toward one or more sensors.

[0012] [Figure 2] FIG. 2 illustrates the fiber optic cable of FIG. 1 in more detail, including some optional features.

[0013] [Figure 3] FIG. 3 shows an exemplary reflector / detector configuration for the fiber optic cable of FIG. 1 having two reflectors.

[0014] [Figure 4] FIG. 4 shows an exemplary reflector / detector configuration for the fiber optic cable of FIG. 1 having four reflectors.

[0015] [Figure 5]FIG. 5 illustrates another fiber optic cable having functionality for monitoring backward propagating radiation utilizing capillary waveguiding, according to an embodiment.

[0016] [Figure 6] FIG. 6 illustrates a modification of the fiber optic cable of FIG. 1 in which one or more sensors are positioned directly at the back opening of the hollow waveguide, according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] (Detailed Description of the Invention) Referring now to the drawings, in which like components are designated by like numerals, FIGS. 1A and 1B illustrate, in cross-section, one fiber optic cable 100 having functionality for monitoring backward-propagating radiation. FIG. 1A shows the fiber optic cable 100 in an exemplary scenario in which the fiber optic cable 100 is mounted in a laser processing apparatus together with a processing head 170 to perform a laser process on a workpiece 180. Only a portion of the fiber optic cable 100 near its output end is depicted in FIG. 1A. FIG. 1A is not to scale. For example, the depicted portion of the fiber optic cable 100 is greatly enlarged relative to the processing head 170. The fiber optic cable 100 includes an optical fiber 110, a mode stripper 120, a hollow waveguide 130, one or more reflectors 140 (at least two in the depicted embodiment), and one or more sensors 150 (at least two in the depicted embodiment). Mode stripper 120, hollow waveguide 130, reflector(s) 140, and sensor(s) 150 are implemented within output connector 102 of fiber optic cable 100. Figure 1B is an expanded view of the end of output connector 102, more clearly showing the features of optical fiber 110 and the light coupling into and out of the end of optical fiber 110. In each of Figures 1A and 1B, the depicted cross-section is parallel to the YZ plane of Cartesian coordinate system 198 shown in Figure 1A.

[0018] Optical fiber 110 has a core 112 and a surrounding cladding 114. Cladding 114 may have two or more distinct concentric cladding layers. Core 112 may include several cores, for example, two concentric cores. Optical fiber 110 may also include additional cladding layers that are radially inward from cladding 114, for example, between the central core and the annular core of core 112.

[0019] In operation, the optical fiber 110 transmits a forward-propagating laser beam 190. The forward-propagating laser beam 190 propagates primarily within the core of the optical fiber 110 and emerges from the output end face 116 of the optical fiber 110. The 1 / e 2 1A and 1B also show an acceptance envelope 194A of the cladding 114, shown using a dashed line. Backward-propagating radiation 194 propagating towards the optical fiber 110 within acceptance envelope 194A may be coupled into its cladding.

[0020] 1A , processing head 170 includes beam-steering optics 172 (adjustable mirrors) and an objective lens formed by two lenses 174 and 176. Processing head 170 steers and focuses forward-propagating laser beam 190 onto a target location on workpiece 180. This scenario is easily generalized to other processing heads that include (a) an objective lens that focuses the forward-propagating laser beam onto the workpiece, and, optionally, (b) one or more beam-steering elements for directing the focused forward-propagating laser beam to a desired target location on the workpiece. In such a scenario, backward-propagating radiation 194 coupled into optical fiber 110 originates from a location on workpiece 180 that, at least approximately, coincides with the target location for forward-propagating laser beam 190.

[0021] The backward-propagating radiation 194 may include back-reflected laser radiation 192B, luminescent radiation 192L, and thermal radiation 192T. The intensity and spectral distribution of the backward-propagating radiation 194 may provide useful information about the laser processing of the workpiece 180 performed with the forward-propagating laser beam 190. The strict spatial selectivity imposed by the optical fiber 110 in coupling the backward-propagating radiation 194 into its core or cladding minimizes contamination of the backward-propagating radiation 194 with radiation originating from areas other than the target location on the workpiece 180, thereby maximizing the integrity of the information conveyed by the backward-propagating radiation 194. The fiber optic cable 100 extracts at least a portion of the backward-propagating radiation 194 that is coupled into the cladding of the optical fiber 110 and directs at least a portion of this portion of the backward-propagating radiation 194 to the sensor(s) 150. The measurements taken by the sensor(s) 150 are then used to evaluate the laser process performed by the forward-propagating laser beam 190, and modifications to the laser process may be made based on this evaluation. Potential modifications include modifications to (a) the operation of the laser source generating the forward-propagating laser beam 190, (b) the operation of the processing head 170, and (c) changes to the local environment at the workpiece 180. In one scenario, measurements obtained by the sensor(s) 150 during the processing of a particular workpiece 180 are used to modify the state of, for example, the laser source and / or the processing head 170 during the processing of that same workpiece 180. In another scenario, continuous measurements obtained by the sensor(s) 150 during the processing of a series of workpieces 180 are used to detect drift in one or more process parameters and make corrections accordingly.

[0022] Extraction of the cladding-coupled backward-propagating radiation 194 from the optical fiber 110 is performed by a mode stripper 120. The mode stripper 120 is implemented within or on the cladding 114. The mode stripper 120 is schematically shown as a sawtooth line in FIGS. 1A and 1B. In one implementation, the mode stripper 120 is a roughened section of the cladding 114. Such roughening can be produced by chemical etching. In another implementation, the mode stripper 120 is a non-random structure within the surface of the cladding 114, produced, for example, by laser machining. While the roughening tends to result in omnidirectional light scattering, the non-random structure can be directional and configured to preferentially direct the extracted backward-propagating radiation 194 in the backward direction. The mode stripper 120 can also be in the form of an additional material disposed on the cladding 114, having refractive index and / or surface properties that promote coupling of radiation out of the cladding.

[0023] The waveguide 130 includes a waveguide body 132 having a bore 134 that contains a longitudinal section of the optical fiber 110 having the mode stripper 120. As used herein, "longitudinal" refers to the dimension along the optical fiber 110, and "transverse" and "radial" refer to dimensions perpendicular to the longitudinal axis of the optical fiber 110. The bore 134 is defined by an inwardly facing surface 138 that is at least partially reflective. The waveguide 130 thereby confines the net propagation of the extracted backward-propagating radiation 194 essentially along the optical fiber 110. A portion of the extracted backward-propagating radiation 194 propagates backward from the output end face 116 of the optical fiber 110 toward the farthest back opening 136 of the bore 134, reaching the back opening 136 after various reflections by the inwardly facing surface 138. At a shallow angle that is approximately parallel to the optical fiber 110, backward-propagating radiation 194 that is coupled out of the optical fiber 110 may reach the back opening 136 without being reflected by the inward-facing surface 138. An exemplary propagation path of the backward-propagating radiation 194 that is coupled out toward the back opening 136 and thereafter toward the sensor 150 is shown by the dashed line in FIG.

[0024] The reflector(s) 140 are positioned adjacent the back opening 136 and adjacent to the optical fiber 110. At least a portion of the extracted backward-propagating radiation 194 is incident on the reflector(s) 140. Each reflector 140 is arranged to reflect the extracted backward-propagating radiation 194 away from the optical fiber 110 toward one or more sensors 150 positioned further away from the optical fiber 110. In one embodiment, each reflector 140 has a reflective surface oriented at approximately 45 degrees relative to the longitudinal axis of the optical fiber 110, e.g., 35-55 degrees relative to the longitudinal axis of the optical fiber 110. The reflective surface of the reflector 140 can be planar or curved. The focused power of a curved reflector surface can help direct the extracted backward-propagating radiation 194 onto the sensor(s) 150. However, a planar reflector surface is simpler to manufacture.

[0025] In embodiments of the fiber optic cable 100 that include multiple reflectors 140, some or all of these reflectors 140 may be different individual reflective surfaces of a single piece. For example, two faces may be cut / polished on the end of a glass cylinder at 45 degrees to the cylinder axis and then coated to form two reflectors 140.

[0026] The output connector 102 may include a connector housing 160 that contains and structurally supports the waveguide 130, the reflector(s) 140, the sensor(s) 150, and a portion of the optical fiber 110. The connector housing 160 may be mated to a fiber port of a processing head 170. Advantageously, the waveguide 130, the reflector(s) 140, and the sensor(s) 150 may be aligned and secured during assembly of the output connector 102 such that subsequent use of the fiber optic cable 100 is alignment-free. Furthermore, the connector housing 160 may be hermetically sealed so that propagation of the extracted backpropagating radiation 194 from the mode stripper 120 inside the connector housing 160 to the sensor(s) 150 occurs within a protected environment.

[0027] FIG. 2 illustrates the fiber optic cable 100 in more detail, including several optional features. The length axis of the bore 134 can be parallel to the optical fiber 110 and can be coincident with the longitudinal axis of the optical fiber 110, as shown in FIG. 2. The inwardly facing reflective surface 138 can be parallel to the optical fiber 110, as also shown in FIG. 2. In one such example, the bore 134 is cylindrical. The bore 134 of the waveguide 130 has a length 288 along the longitudinal axis of the optical fiber 110 and a transverse region 286. The transverse region 286 can be less than 10 millimeters (mm), e.g., in the range of 3 to 10 mm, and the length 288 can be in the range of 20 to 100 mm. In one embodiment, the length 288 is at least five times the maximum value of the transverse region 286. For example, when the bore 134 is cylindrical, the length of the cylindrical bore can be at least five times its diameter. The mode stripper 120 extends a distance 280 along the optical fiber 110. In one embodiment, the distance 280 is between 10 and 50 mm.

[0028] In some usage scenarios, the average power of the forward-propagating laser beam 190 is high enough that cooling is required to manage the heat load, for example, from partial reflections and unintended light leakage from the optical fiber 110, as well as from components of the forward-propagating laser beam 190 propagating within the cladding 114 and extracted by the mode stripper 120. The average power of the forward-propagating laser beam 190 can be from hundreds of watts to tens of kilowatts. The waveguide body 132 can act as a heat sink for such heat load and, for this purpose, can include one or more liquid cooling channels 238. Although not shown in FIG. 2 , the liquid cooling channel(s) 238 can be contained such that the cooling liquid does not enter the propagation path of the extracted backward-propagating radiation 194 to the sensor(s) 150. The waveguide body 132 can be made substantially of metal to achieve high thermal conductivity. When the average power of the forward-propagating laser beam 190 is high, the backward-propagating radiation 194 may likewise be coupled into the optical fiber 110 at a relatively high average power. Typically, the back-reflected laser radiation 192B constitutes a majority of the power of the backward-propagating radiation 194, due in part to the reflective properties of the workpiece 180, but also due to the optical components of the process head 170, which are typically selected for optimal transmission at the wavelength of the forward-propagating laser beam 190. In high average power scenarios, partial absorption of the extracted backward-propagating radiation 194 by the waveguide body 132 may contribute significantly to the heat load. To effectively manage the heat load from either (a) the extracted backward-propagating radiation 194 and (b) the extracted component of the forward-propagating laser beam 190 propagating within the cladding 114, the mode stripper 120 may terminate at a non-zero distance 282 from the front opening and a non-zero distance 284 from the back opening of the bore 134. Each of the non-zero distances 282 and 284 may be at least 5 mm.

[0029] In a modified version of the output connector 102, the mode stripper 120 extends outside the bore 134 in either or both the forward direction (positive Z direction) and the backward direction (negative Z direction). Extending the mode stripper 120 in the backward direction may not significantly affect the outcoupling of radiation from the cladding 114. Most of the cladding-coupled backward-propagating radiation 192 may be extracted near the forward-most end of the mode stripper 120, and the cladding-coupled portion of the forward-propagating laser beam 190 may be extracted before reaching the portion of the optical fiber 110 inside the output connector 102. Extending the mode stripper 120 in the forward direction may have a more significant effect on the behavior of the output connector 102, because most of the cladding-coupled backward-propagating radiation 192 may be extracted near the forward-most end of the mode stripper 120, as described above. However, when the backward propagating radiation 192 is partially extracted from the cladding 114 by a portion of the mode stripper 120 forward of the bore 134, at least some of such extracted backward propagating radiation 192 may enter the bore 134 and ultimately propagate at a sufficiently shallow angle to reach the sensor(s) 150.

[0030] 2, the fiber optic cable 100 may include a second mode stripper further from the output end face 116 than the mode stripper 120 for the purpose of extracting the component of the forward-propagating laser beam 190 propagating within the cladding 114. The second mode stripper may be implemented outside the output connector 102, for example, within an input connector at the input end of the fiber optic cable 100, or between the output connector 102 and the end of the optical fiber 110 that is spliced ​​to a laser source that generates the forward-propagating laser beam 190.

[0031] Some embodiments of the output connector 102 include an end cap 270 at the output end face 116 of the optical fiber 110. The end cap 270 helps prevent damage to the output end face 116 that may occur if contaminants are present on the output end face 116. The end cap 270 may be fused directly to the output end face 116. Alternatively, for example, if the optical fiber 110 is a hollow-core fiber that cannot be fused directly to the end cap 270, the end cap 270 may be a short distance away from the output end face 116, and the local environment between the output end face 116 and the end cap 270 is sealed. In either case, the end cap 270 may include an anti-reflective coating to reduce Fresnel losses in the transmission of the forward-propagating laser beam 190. The minimum value of the lateral area 272 of the end cap 270, e.g., the diameter of a cylindrical end cap 270, may exceed the maximum value of the lateral area 286 of the bore 134. The connector housing 160 may be sealed to the end cap 270 .

[0032] As previously mentioned, the backward-propagating radiation 194 may include a wide range of spectral components. To obtain different types of information from the backward-propagating radiation 194, the output connector 102 may include several sensors 150, each configured to detect a different spectral component of the backward-propagating radiation 194. In one embodiment, the output connector 102 includes one sensor 150 that detects radiation at the same wavelength as the forward-propagating laser beam 190 and at least one other sensor 150 that detects radiation that does not have the same wavelength as the forward-propagating laser beam 190. For example, in one embodiment configured for a forward-propagating laser beam 190 at a near-infrared wavelength λ, the output connector 102 includes a first sensor 150 that detects back-reflected laser radiation 192B at this near-infrared wavelength λ, a second sensor 150 that detects luminescent radiation 192L in one or more portions of the visible and / or ultraviolet spectrum, and a third sensor 150 that detects infrared (e.g., near-infrared) thermal radiation 192T away from the wavelength λ. Regardless of how many sensors 150 are included and which wavelengths are detected by each sensor 150, the output connector 102 may include one or more optical filters to improve detection selectivity. Spectral filter(s) 246 may be disposed between the reflector(s) 140 and the sensor(s) 150, where the reflector(s) 140 may be spectrally selective. Additionally, one or more apertures 248 may restrict the acceptance cone of the sensor(s) 150 to limit the range of propagation angles of the detected backward-propagating radiation 192 that passes through the spectral filter(s) 246. This restriction of the propagation angle range may improve the performance of the spectral filtering achieved with the spectral filter(s) 246. The composition of the backward-propagating radiation 192 may be dependent on the back-reflected laser radiation 192B. The output connector 102 may therefore include at least one spectral filter 246 configured to suppress detection of the back-reflected laser radiation 192B by the associated sensor 150. An associated aperture 248 may help maximize the suppression of back-reflected laser radiation 192B by this spectral filter 246.The spectral filter 246 may be wedge-shaped or arranged at non-normal incidence, and the aperture 248 may include a beam trap.

[0033] FIG. 3 illustrates one exemplary reflector / detector configuration 300 of the output connector 102. FIG. 3 illustrates the reflector and sensor along the negative Z-axis direction. The configuration 300 includes two reflectors 340 positioned on opposite sides of a central plane 380 containing the longitudinal axis of the optical fiber 110. Each reflector 340 deflects the extracted backward-propagating radiation 194 incident thereon away from the central plane 380, as depicted in FIG. 1A, for example. Although shown as separate from one another in FIG. 3, the reflectors 340 may be different parts of a single, integrally formed structure, with ridges rather than gaps between the reflectors 340. The shape of the reflectors 340 may deviate from that depicted in FIG. 3.

[0034] The longitudinal protrusion 334 of the bore 134 along the Z-axis above the reflector 340 is indicated by a dashed outline. In the embodiment depicted in FIG. 3 , the longitudinal protrusion 334 is within the corresponding footprint of the reflector 340 to maximize the proportion of the extracted backward-propagating radiation 194 that is collected by the reflector 340. The reflector 340 has a central hole that allows the optical fiber 110 to pass through the reflector 340. This central hole, like any gaps or ridges between the reflectors 340, reduces the achievable area of ​​the reflector 340. Furthermore, it is possible to arrange the reflectors 340 so that at least 75% of the protrusion 334 of the bore 134 above the reflector 340 coincides with its achievable area.

[0035] Each reflector 340 may direct the extracted backward-propagating radiation 194 toward one or more sensors 150. In the example depicted in Figure 3, one reflector 340 directs the extracted backward-propagating radiation 194 toward a single sensor 150, while the other reflector 340 directs the extracted backward-propagating radiation 194 toward two sensors 150. This allows for separate detection of three different spectral components of the backward-propagating radiation 194 using only two reflectors.

[0036] Configuration 300 may be modified, for example, to include only a single reflector 340 configured to reflect backward-propagating radiation 192 in the positive Y-axis direction. In one such embodiment, this single reflector 340 is positioned entirely on the positive Y-axis side of mid-plane 380. Alternatively, the footprint of this single reflector 340 may be the same as the footprint of the pair of reflectors 340 shown in FIG. 3 , extending across mid-plane 380 and without, for example, a gap or ridge at mid-plane 380.

[0037] FIG. 4 shows another exemplary reflector / detector configuration 400 of the output connector 102 having four reflectors 440. FIG. 4 shows the configuration 400 in a view similar to that used for the configuration 300 in FIG. 3. The configuration 400 includes four reflectors 440 in a pyramidal arrangement, each arranged to reflect the extracted backward-propagating radiation 194 away from the optical fiber 110. A pair of reflectors 440 positioned on opposite sides of the optical fiber 110 reflect the extracted backward-propagating radiation 194 in positive and negative directions, respectively, generally along the X-axis. Another pair of reflectors 440 positioned on opposite sides of the optical fiber 110 also reflect the extracted backward-propagating radiation 194 in positive and negative directions, respectively, generally along the Y-axis. As with the reflectors 340 in the configuration 300, each reflector 440 can direct the extracted backward-propagating radiation 194 toward one or more sensors 150. It may also be possible to accommodate more sensors 150 with configuration 400 than with configuration 300.

[0038] The configuration 400 is easily generalized to a different number (3, 5, or more) of reflectors 440 distributed around the optical fiber 110 and each arranged to reflect the extracted backward propagating radiation 194 away from the optical fiber 110 and towards one or more sensors 150.

[0039] FIG. 5 is a cross-section of another fiber optic cable 500 and associated output connector 502 with functionality for monitoring backward-propagating radiation utilizing waveguiding by a capillary tube 580. The fiber optic cable 500 is similar to the fiber optic cable 100, except for (a) the waveguide 130 of the output connector 102, which is replaced by a heat sink 530 within the output connector 502 that is mechanically similar to the waveguide 130, and (b) the capillary tube 580, which is disposed within the bore 534 of the heat sink 530 such that a section of the optical fiber 110 having the mode stripper 120 is contained within the bore 584 of the capillary tube 580. The capillary tube 580 may be a glass tube. A radially outward-facing surface 588 of the capillary tube 580 may be coated to be at least partially reflective within one or more selected wavelength ranges. The capillary tube 580 therefore functions as a waveguide for the spectral components of the extracted backward-propagating radiation 194 within the selected wavelength range(s). Radiation stripped by the mode stripper 120 and having wavelengths outside the selected wavelength range(s) may pass through the coating of the capillary tube 580 and reach the heat sink 530. The body 532 of the heat sink 530 includes one or more liquid cooling channels 238, discussed above with reference to FIG. 2. The inward-facing surface 538 of the heat sink body 532 may be primarily light-absorbing. In one example, the inward-facing surface 538 is coated black to maximize light absorption.

[0040] The output connector 502 may be tuned for use with a forward-propagating laser beam 190 of a particular wavelength λ. In such an implementation, the radially outward-facing surface 588 of the capillary tube 580 is coated to reduce and / or minimize reflection at the wavelength λ while promoting and / or maximizing reflection in one or more other wavelength ranges that match at least some spectral components of the backward-propagating radiation 194. This implementation may be advantageous when the composition of the backward-propagating radiation 194 is dominated by the back-reflected laser radiation 192B, which is often the case. The coating on the radially outward-facing surface 588 of the capillary tube 580 may facilitate spectrally selective waveguiding of the extracted backward-propagating radiation 194, thereby improving the signal-to-noise ratio for detection of the luminescent radiation 192L and the thermal radiation 192T. The coating on the radially outward-facing surface 588 may be slightly reflective at wavelength λ so that the back-reflected laser radiation 192B can be detected using a dedicated sensor 150. In connection with detecting the back-reflected laser radiation 192B, it is also possible that some back-reflected laser radiation 192B is coupled out of the optical fiber 110 at a relatively shallow propagation angle and propagates directly into the reflector(s) 140.

[0041] Output connector 502 may or may not include end cap 270, filter(s) 246, and aperture(s) 248. As is true for output connector 102, output connector 502 may implement a variety of reflector / detector configurations, including configuration 300 and configuration 400.

[0042] 6 illustrates yet another fiber optic cable 600 with functionality for monitoring backward-propagating radiation. Fiber optic cable 600 is a modification of fiber optic cable 100 in that the associated output connector 602 omits reflector(s) 140; instead, sensor(s) 150 are positioned adjacent to the optical fiber 110 and the back opening 136 of the waveguide 130. Optionally, one or more spectral filters 246 are positioned in front of the sensor(s) 150 at this location. Output connector 602 requires fewer optical elements than output connector 102 and may offer cost savings over output connector 102. However, when the lateral area of ​​bore 134 is in the millimeter range, it may be difficult to fit sensor(s) 150 closely enough to the optical fiber 110, especially in embodiments including three or more sensors 150, and optionally associated optical filters.

[0043] The modifications to fiber optic cable 100 and output connector 102 leading to fiber optic cable 600 and output connector 602 can also be applied to fiber optic cable 500 and output connector 502. In addition, each of output connectors 102, 502, and 602 can be modified for use with external sensors. In such modifications, one or more sensors 150 are replaced by individual fiber ports. A transport fiber can then be coupled to each such fiber port to transport the extracted backward-propagating radiation 194 to an external sensor that is not directly integrated within the output connector. Therefore, more generally, the sensor(s) 150 of output connectors 102, 502, and 602 can be replaced by individual optical receivers. Each optical receiver is in the form of a sensor or a fiber port.

[0044] The present invention is described above in terms of preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. 1. A fiber optic cable with monitoring of backward propagating radiation, the fiber optic cable comprising: an optical fiber for transporting a forward-propagating laser beam, the optical fiber including a core, a cladding, and an output end face configured to emit the forward-propagating laser beam; a mode stripper in or on the cladding along a first longitudinal section of the optical fiber for coupling out of the cladding backward-propagating radiation coupled into the cladding at the output end face; a hollow waveguide including a waveguide body having a bore containing at least a portion of the first longitudinal section of the optical fiber, the bore having a back opening distal most from the output end face, the bore being defined by an inwardly facing surface for directing at least a percentage of the backward propagating radiation coupled by the mode stripper to an outside of the cladding in a backward direction toward the back opening; one or more optical receivers for receiving individual portions of the backward-propagating radiation emerging from the rear opening, each optical receiver including a sensor or a fiber optic port; An optical fiber cable comprising:

2. The fiber optic cable of claim 1 , wherein the first longitudinal section is completely contained within the bore.

3. The fiber optic cable of any one of claims 1-2, further comprising an output connector housing containing said hollow waveguide, said optical fiber, and said one or more optical receivers.

4. An optical fiber cable as described in any one of claims 1 to 3, wherein the bore has a longitudinal area along the optical fiber and a maximum transverse area perpendicular to the optical fiber, the longitudinal area being at least five times the transverse area.

5. The fiber optic cable of any one of claims 1 to 4, wherein at least one of the optical receivers is positioned adjacent to the rear opening and the optical fiber.

6. The optical fiber cable of any one of claims 1-5, further comprising one or more reflectors positioned adjacent to the rear opening and the optical fiber, each of the one or more reflectors arranged to reflect the respective portion of the backward-propagating radiation toward at least one of the one or more optical receivers.

7. 7. The fiber optic cable of claim 6, further comprising, for at least one of the one or more optical receivers, a wavelength filter positioned in a propagation path of the individual portion of the backward-propagating radiation from one of the one or more reflectors to the optical receiver.

8. The fiber optic cable of claim 7, further comprising an aperture that limits an acceptance cone for said at least one of said one or more optical receivers.

9. The optical fiber cable of any one of claims 6-8, wherein the one or more reflectors are two reflectors arranged on opposite sides of a central plane containing the longitudinal axis of the optical fiber, and each of the two reflectors is arranged to deflect the backward propagating radiation incident on the two reflectors away from the central plane.

10. The optical fiber cable of any one of claims 6-8, wherein the one or more reflectors are a plurality of reflectors distributed around the optical fiber, each of the reflectors being arranged to deflect the backward propagating radiation incident on the reflector away from the optical fiber.

11. The fiber optic cable of any one of claims 1-10, wherein the waveguide body is made of metal and contains at least one liquid cooling channel.

12. The fiber optic cable of any one of claims 1-11, wherein the inwardly facing surface is cylindrical.

13. 13. The fiber optic cable of claim 12, wherein the inwardly facing surface has a diameter of 10 millimeters or less.

14. The optical fiber cable of any one of claims 1 to 13, wherein the mode stripper extends at least 10 millimeters along the optical fiber.

15. The optical fiber cable of any one of claims 1-14, wherein the mode stripper is a textured surface of the cladding.

16. The optical fiber cable of any one of claims 1-15, wherein each of the one or more optical receivers is a sensor.

17. The optical fiber cable of any one of claims 1-16, wherein the one or more optical receivers include three sensors.

18. 1. A fiber optic cable with monitoring of backward propagating radiation, the fiber optic cable comprising: an optical fiber for transporting a forward-propagating laser beam, the optical fiber including a core, a cladding, and an output end face configured to emit the forward-propagating laser beam; a mode stripper in or on the cladding along a longitudinal section of the optical fiber for coupling out of the cladding backward propagating radiation coupled into the cladding at the output end face; a waveguide containing at least a portion of the longitudinal section of the optical fiber and configured to guide at least a percentage of the backward propagating radiation coupled out of the cladding by the mode stripper in a backward direction away from the output end face, the waveguide having a back end distal most from the output end face; one or more optical receivers for receiving individual portions of the backward-propagating radiation emerging from the rear end of the waveguide, each optical receiver comprising a sensor or a fiber optic port; An optical fiber cable comprising:

19. 20. The fiber optic cable of claim 18, wherein the longitudinal section is completely contained within the bore.

20. the waveguide includes a capillary tube containing the longitudinal section of the optical fiber and having a radially outwardly facing surface that is lower in reflectivity at a first wavelength than within one or more wavelength ranges away from the first wavelength; The fiber optic cable further includes a heat sink and an inwardly facing surface, the inwardly facing surface (a) defining a bore containing the longitudinal section of the optical fiber and at least an associated longitudinal section of the hair portion, and (b) being primarily light absorbing. The optical fiber cable according to any one of claims 18-19.

21. 21. The fiber optic cable of claim 20, wherein the heat sink comprises liquid cooling channels.

22. The fiber optic cable of any one of claims 18-21, further comprising an output connector housing containing said waveguide, said optical fiber, and said one or more optical receivers.

23. An optical fiber cable as claimed in any one of claims 18-22, wherein the bore has a longitudinal area along the optical fiber and a maximum transverse area perpendicular to the optical fiber, the longitudinal area being at least five times the transverse area.

24. The fiber optic cable of any one of claims 18-23, wherein at least one of the optical receivers is positioned adjacent to the rear opening and the optical fiber.

25. The optical fiber cable of any one of claims 18-24, wherein the optical fiber cable further comprises one or more reflectors positioned adjacent to the rear opening and the optical fiber, each reflector arranged to reflect a respective portion of the backward-propagating radiation toward at least one of the one or more optical receivers.

26. 26. The fiber optic cable of claim 25, further comprising, for at least one of the one or more optical receivers, a wavelength filter positioned in a propagation path of the individual portion of the backward-propagating radiation from one of the one or more reflectors to the optical receiver.

27. 27. The fiber optic cable of claim 26, further comprising an aperture that limits an acceptance cone for said at least one of said one or more optical receivers.

28. 28. The optical fiber cable of claim 25, wherein the one or more reflectors are two reflectors arranged on opposite sides of a central plane containing the longitudinal axis of the optical fiber, and each of the two reflectors is arranged to deflect the backward-propagating radiation incident on the two reflectors away from the central plane.

29. 28. The optical fiber cable of claim 25, wherein the one or more reflectors are a plurality of reflectors distributed about the optical fiber, each of the reflectors arranged to deflect the backward propagating radiation incident on the reflector away from the optical fiber.

30. The optical fiber cable of any one of claims 18-29, wherein the waveguide body is made of metal and contains at least one liquid cooling channel.

31. The fiber optic cable of any one of claims 18-30, wherein the inwardly facing surface is cylindrical.

32. 32. The fiber optic cable of claim 31, wherein the inwardly facing surface has a diameter of 10 millimeters or less.

33. The optical fiber cable of any one of claims 18-32, wherein the mode stripper extends at least 10 millimeters along the optical fiber.

34. The optical fiber cable of any one of claims 18-33, wherein the mode stripper is a textured surface of the cladding.

35. The optical fiber cable of any one of claims 18-34, wherein each of the one or more optical receivers is a sensor.

36. The optical fiber cable of any one of claims 18-35, wherein the one or more optical receivers include three sensors.

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