Abstract:
A method for detecting defects in an optical device. The method includes obtaining an optical device that is a component of an optical communications system carrying information signals at a communications wavelength. Infrared light is applied to the optical device. The infrared light has a wavelength corresponding to the communications wavelength. An image of the optical device is obtained while the optical device is transmitting the infrared light. The image provides multidimensional data including positional values and intensity values. The image is then analyzed to detect defects in the optical device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a method and system for inspecting optical devices. 
     2. Description of Related Art 
     Inspection of optical devices, such as optical fiber, has been performed in a variety of ways. One technique is visual inspection performed by a human operator using a microscope. This type of human, visual inspection is a tedious and time-consuming task. Certain types of defects, such as a scratch in an optical fiber, are difficult to detect with the human eye. 
     A visual laser fault locator is an inexpensive and simple device to detect a defect in optical fiber. However, because the laser signal from the device is weak, the defect position is difficult to detect, the result is often inaccurate and the range of inspection is short. This type of device provides for quick inspection of short optical fibers. 
     An optical time domain reflectometer (OTDR) finds a fault location by measuring attributes of the input and output of the optical signal. One of advantage of the OTDR is that it provides a quantified result of the loss along the fiber. An OTDR, however, cannot detect fiber loss if isolators are inserted in the fiber path. The only way to inspect fiber paths including isolators with an OTDR is to cut the fiber and measure the loss. Locating the position of a defect in a complicated system such as an optical fiber cassette is difficult with an OTDR. Lastly, the OTDR is a relatively costly device. 
     SUMMARY OF THE INVENTION 
     A method for detecting defects in an optical device. The method includes obtaining an optical device that is a component of an optical communications system carrying information signals at a communications wavelength. Infrared light is applied to the optical device. The infrared light has a wavelength corresponding to the communications wavelength. An image of the optical device is obtained while the optical device is transmitting the infrared light. The image provides multidimensional data including positional values and intensity values. The image is then analyzed to detect defects in the optical device. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a block diagram of a system for inspecting optical devices; 
     FIG. 2 is an exemplary image of an optical fiber; 
     FIG. 3 depicts intensities in the image of FIG. 2; 
     FIG. 4 is a flowchart of an exemplary process for detecting defects; 
     FIG. 5 depicts exemplary characteristics of a peak in the intensity; 
     FIG. 6 depicts an exemplary optical subassembly; 
     FIG. 7 is a flowchart of an exemplary process for detecting defects in the optical subassembly; 
     FIG. 8 is a block diagram of an alternate system for inspecting optical devices; 
     FIG. 9 is an end view of an optical connector; and 
     FIG. 10 is an end view of an optical connector. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof. 
     FIG. 1 is a block diagram of an exemplary system for inspecting optical devices. The system includes a camera  10  coupled to a processor  12 . The camera  10  is preferably an infrared camera that obtains an image of the optical device under test  16 . The processor  12  may be implemented using a general-purpose computer implementing the processes described herein. The general-purpose computer may operate in response to a computer program stored in a storage medium  13  (e.g., hard drive) accessible by the computer. A display  14  is provided to allow a user to view images generated by camera  10  and to provide a graphical user interface to operate the system. Suitable input peripherals (not shown) such a keyboard, mouse, etc. may be coupled to processor  12 . 
     The optical device under test  16  receives infrared light from an IR source  18  or has IR source internally. The optical device may be a component (e.g., a fiber, a spliced fiber, an optical filter, etc.) or may be an optical assembly (e.g., amplifier, transceiver, etc.) including a plurality of optical components. In an exemplary embodiment, the optical device under test is designed to be part of an optical communications system carrying an information signal at or about a communications wavelength. The IR source emits infrared light at a wavelength substantially equal to the communication wavelength. In an exemplary embodiment, the IR source  18  is a laser generating infrared light at a wavelength of 1550 nm. 
     A second IR source  18 ′ may be used to test transmission characteristics of the device under test at a second wavelength. Again, the optical device under test is designed to be part of an optical communications system. This second wavelength is used in the optical communications network as a service channel, transmitting commands and other control information between network elements. In an exemplary embodiment, the second IR source  18 ′ generates infrared light at a wavelength of about 1310 nm. Alternatively, the second IR source  18 ′ generates infrared light at a wavelength of about 1625 nm. 
     The inspection system may be used to inspect a variety of optical devices for defects. One type of optical device that may be inspected is optical fiber. Certain types of fiber (such as recoated fiber) may include defects such as incomplete filling of epoxy, scratches, air bubbles, and/or contamination that cause the infrared light to leak out. The system of FIG. 1 may be used to detect such defects. 
     FIG. 2 depicts and exemplary two-dimensional image  5  of a spliced and recoated fiber  7 . As described above, IR source  18  is coupled to the fiber and the camera  10  acquires an image  5  as shown in FIG.  2 . In other words, image  5  shown in FIG. 2 is the raw or base image from camera  10  and clearly shows the IR light leaking through the fiber. The image  5  is processed by processor  12  to locate areas of high intensity that indicate that a defect is present. When the infrared light passes through the fiber and encounters a defect (e.g., poor recoating, scratches, air bubbles, contamination) the infrared light is leaked or scattered causing an emission or bright spot in the image. Processor  12  process the image  5  to detect such defects. 
     FIG. 4 is a flowchart of a process implemented by processor  12  to detect a defect in an image  5  of a fiber  7  such as that shown in FIG.  2 . The process begins at step  110  where the image  5  is acquired. The inspection process may be performed in real time as the image  5  is acquired by camera  10  or may be performed on images stored in storage device  13 . The image  5  is generated by applying an IR source to the fiber and then imaging the length of the fiber  7  with camera  10 . 
     An image plot  6 , such as that shown in FIG. 3, is obtained at step  110 . The image plot  6  provides a three-dimensional representation of the raw data in the form of a plot of IR intensity (i.e., a z component) for x, y locations in the image  5 . 
     At step  112 , peaks in the image plot  6  are located which may correspond to defects in the fiber  7  because the peaks correspond to the maximum IR light leakage areas. This may be performed through a profile analysis routine that analyzes profiles in the image plot  6  and derives profile characteristics. For example, as shown in FIG. 5, profiles may be located by applying a threshold to the image  6 . FIG. 5 is a simplified representation showing one dimensional axis and intensity such as in image plot  6 . The profile analysis is preferably performed on three-dimensional data, namely x, y position and intensity. Profile characteristics such as peak height, peak width, height-to-width ratio, leading edge slope, trailing edge slope etc., may be derived to characterize the profile as either a defect or noise at step  114 . Defects such as poor recoating, scratches, air bubbles, and contamination may be detected based on the profile of the infrared signal. A report may then be generated at step  116  notifying an operator of the defect and the results stored in a database. 
     The processing described above with respect to FIGS. 2-4 may be applied to a section of fiber, a fiber splice or a fiber splice within a splice protection device. Often, spliced fibers are positioned within a splice protection device which protects the fiber splice from, for example, tension on the splice region. By using an IR source  18  with sufficient power (e.g., 50 mw), defects in a splice encased in an IR-permeable splice protection device (e.g., plastic) can be imaged by camera  10 . 
     In another embodiment of the invention, the system of FIG. 1 may be used to inspect more complex optical devices such as an optical subassembly. FIG. 6 depicts an exemplary optical device under test in the form of an optical subassembly  30 . Optical subassembly  30  includes incoming fibers  31  that are wrapped around spools  32  to store excess fiber. The incoming fibers are spliced to input fibers for amplifiers  36  at splices  34 . The output of amplifiers  36  are coupled by additional fiber to output ports  38 . 
     Testing the optical subassembly  30  using conventional techniques would be difficult. Conventional techniques would involve applying an input at incoming fibers  31  and monitoring an output at output ports  38 . If the signal is degraded, this indicates that a defect exists in the optical subassembly  30 , but the location of the defect cannot be determined. To individually test the splices  34 , each end of the spliced fiber must be destroyed and each splice must be inspect one by one. To individually test amplifiers  36  and output ports  38 , the transmission path would need to be interrupted proximate to each component which is time-consuming. 
     Using the system of FIG. 1, optical devices such as optical subassembly  30  can be tested quickly and non-invasively. The inspection process is depicted in the flowchart of FIG.  7 . The process begins at step  210  where the image is acquired. The inspection process may be performed in real time as the image is acquired by camera  10  or may be performed on images stored in storage device  13 . The image is generated by applying an IR source to the optical device and imaging the optical device with camera  10 . Additionally, if the device under test is an active device, the device is activated (e.g., power is applied). The image provides three-dimensional data in the form of x, y coordinates defining locations in the image and a third dimension (e.g., z coordinate) of pixel intensity proportional to the infrared light detected by camera  10 . In the example shown in FIG. 6, the IR source is applied to incoming fibers  31 . 
     At step  212 , three-dimensional pattern analysis is performed to distinguish noise from potential defects. The pattern analysis step locates profiles in the three-dimensional data to isolate potential defects. The profile information of the data such as peak height, peak width, height-to-width ratio, leading edge slope, trailing edge slope etc., may be derived to characterize a profile as either a defect or noise at step  212 . Noise peaks are eliminated from further consideration. 
     At step  214 , candidates for inspection are located in the image. These candidates are identified by profile pattern matching of three-dimensional data from the image that does not correspond to noise. The profiles from the image are analyzed and classified as defect candidates based on a database that stores information concerning patterns of defects. 
     At step  216 , the candidates located in step  214  are compared to reference patterns in a database. The candidates may correspond to individual components located in the optical subassembly  30 . For example, candidates may include splices  34 , amplifiers  36  and ports  38 . The optical subassembly  30  may include indicia  42  that define a coordinate system for the image from which areas of interest may be referenced. Thus, intensity peaks located outside predefined areas of interest may be ignored. 
     At step  220 , the degree of correlation between the candidate image and the reference image is used to detect defects. If the two images have a predetermined degree of correlation, then no defect is present. For certain components, an intensity peak may be expected. For example, amplifiers  36  may be erbium-doped fiber amplifiers (EDFA&#39;s) that typically emit light when the amplifier is operating properly. To properly detect defects, processor  12  compares the acquired image to a reference image stored in the storage device  13 . Differences between the acquired image and the reference image are indicative of a defect. 
     At step  222 , a report is generated identifying any defects that are present and where the defects are located. As noted above, the coordinate system for the image is detected so that defect location can be determined and associated with a component (e.g., a specific amplifier) in the optical device. This allows a technician to troubleshoot a single component rather than an entire subassembly. 
     FIG. 8 depicts an alternate embodiment where the device under test is an end face of an optical fiber connector  232  coupled to an optical fiber  230 . Camera  10  is directed towards the mating face of optical connector  232 . Shown in FIG. 8 is an exemplary power distribution  234  of the light emitted from connector  232 . Often such connectors are abutted face-to-face through an adapter to join to fiber paths. 
     In operation, an IR source  18  is applied to fiber  230  and the light emitted from connecter  232  captured by camera  10  and processed by processor  12 . A number of characteristics of the light emitted by the connector  232  may be determined. The system of FIG. 8 can detect a variety of characteristics of the light emitted by connector  232 . A first characteristic is the numerical aperture of the connector. Numerical aperture (NA) is associated with the angular spread of light from a central axis, as in exiting a fiber, emitting from a source, or entering a detector. As shown in FIG. 8, the NA may be expressed as sin(a). The detected NA may be compared to a reference NA to determine that the connector  232  meets standards. If two connectors having different NA&#39;s are abutted face-to-face, NA mismatch losses may occur. 
     The arrangement of FIG. 8 allows detection of other characteristics of connector  232 . FIG. 9 depicts an end view of connector  232  where the connector housing  240  is visible along with the cladding  242  and core  244  of optical fiber  230 . The core position is determined by light emitted from the end of connector  232  and concentricity of the core with respect to the cladding  242  can be detected. As seen in the example of FIG. 9, the light emitted from core  244  is offset from the center of the cladding  242 . A lack of concentricity between the light emitted by core  244  and the cladding  242  will result in signal loss when the connector  232  is mated with a similar connector. The system of FIG. 8 determines the concentricity of the light emitted by core  244  and reports an error if concentricity fails to meet a reference value. 
     Another characteristic detected using the system of FIG. 8 is ellipticity of light emitted by core  244 . Ellipticity is a measure of the distortion from a circular pattern of light emitted by the core  244 . Ellipticity may be determined based on known measurements, such as the ratio of the major and minor axis of the ellipse. As seen in the example of FIG. 10, the light emitted from core  244  is elliptical. Ellipticity in the light emitted by core  244  will result in signal loss when the connector  232  is mated with a similar connector. The system of FIG. 8 determines the ellipticity of the light emitted by core  244  and reports an error if ellipticity fails to meet a reference value. 
     A variety of defects may be detected using the system and methods described herein. As noted above, recoated fiber and/or spliced fibers may be inspected for defects. In addition, orientation of a splice may be confirmed by examining the infrared emissions of spliced fibers. In splices made up of two different types of fiber, one fiber will emit more light thus providing the ability to confirm the orientation of the splice. Fibers exceeding a minimum bend radius may also be detected as this condition causes the fiber to leak infrared light. 
     For certain components, the intensity of the image is used to provide quantitative information about the component. In one embodiment, the intensity of an image of a spliced fiber indicates that amount of signal loss at the splice. A high intensity in an image of a fiber splice indicates that a large amount of light is escaping the splice and thus signal loss is high. Measured brightness is correlated to signal loss through techniques such as mathematical formulae, look-up tables, etc. The measured intensity provides a measure of signal loss to determine if the fiber splice meets signal loss thresholds. 
     In addition to fibers, an optical subassembly containing a number of optical components may be inspected at one time. Operation of individual components may be confirmed through the inspection system (e.g., an operating amplifier emits certain amount of infrared light). 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the invention, and all such modificafions as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.