WO2014014534A2

WO2014014534A2 - Monitoring radiated infrared

Info

Publication number: WO2014014534A2
Application number: PCT/US2013/038276
Authority: WO
Inventors: Tim Wihl
Original assignee: Xyratex Technology Ltd.
Priority date: 2012-04-26
Filing date: 2013-04-25
Publication date: 2014-01-23
Also published as: WO2014014534A3

Abstract

A photometer system is provided. The photometer system includes an optical filter having a narrow stopband. The optical filter is positioned to receive a combination of narrowband light and infrared light, wherein the infrared light is radiated from an article illuminated by the narrowband light. The bandwidth of the stopband overlaps a bandwidth of the narrowband light. The system includes an infrared detector positioned to receive filtered light from the optical filter with the narrowband light reduced by the optical filter. The infrared detector is operative to detect the infrared light radiated from the article as passed by the optical filter.

Description

MONITORING RADIATED INFRARED

[0001] This application claims benefit of priority from US Provisional Application No.

61/639,019 filed April 26, 2012, and benefit of priority from US Provisional Application No.

61/801,300 filed March 15, 2013, which are hereby incorporated by reference.

BACKGROUND

[0002] An instrument that measures light intensity is called a photometer. A photometer can be specific to a wavelength of light or a range of wavelengths, such as ultraviolet light, visible light or infrared light. Infrared photometers are especially useful in measuring radiated light from objects. An infrared imager, which captures images of objects radiating infrared light, may be thought of as having many infrared photometers, one per pixel.

[0003] There is a need for detecting, observing or measuring radiated infrared light in very small objects. For example, a near field transducer (NFT) produces an intense near field amplitude from light coupled to the near field transducer by a waveguide. The near field transducer receives and emits light in one wavelength or a narrow range of wavelengths, but also radiates infrared light as a result of heating. It would be advantageous to measure the amount of radiated infrared light in order to gauge efficiency of the near field transducer.

However, commercially available and military grade infrared imagers are very expensive and difficult to adapt for use in photometry of very small objects, and may lack sensitivity and/or resolution to make effective measurements at microscopic scales. Therefore, there exists a need in the art for a solution which overcomes the drawbacks described above.

SUMMARY

[0005] In some embodiments, a method for measuring radiated light is provided. The method includes directing narrowband light at an article, wherein the article radiates infrared light. The method includes filtering a source light spectrum out of light outbound from the article, to create filtered outbound light and detecting a portion of the infrared light, in the filtered outbound light.

[0006] In some embodiments, a photometer system is provided. The photometer system includes an optical filter having a narrow stopband. The optical filter is positioned to receive a combination of narrowband light and infrared light, wherein the infrared light is radiated from an article illuminated by the narrowband light. The bandwidth of the stopband overlapping a bandwidth of the narrowband light. The system includes an infrared detector positioned to receive filtered light from the optical filter with the narrowband light reduced by the optical filter. The infrared detector is operative to detect the infrared light radiated from the article as passed by the optical filter.

[0007] In some embodiments, a photometer system is provided. The system includes a narrowband light source operable to illuminate an article with narrowband light and an infrared detector positioned so that there is an optical path from the article to the infrared detector. The system includes a lens positioned along the optical path between the article and the infrared detector and a narrow band-stop optical filter positioned along the optical path, between the lens and the infrared detector. A stopband of the narrow band-stop optical filter overlaps a spectrum of the narrowband light source. The system includes an infrared camera and a first splitter positioned along the optical path between the narrow band-stop optical filter and the infrared detector, the first splitter producing a further optical path to the infrared camera.

[0008] Other aspects and advantages of the embodiments will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The described embodiments and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. These drawings in no way limit any changes in form and detail that may be made to the described embodiments by one skilled in the art without departing from the spirit and scope of the described embodiments.

[0010] Fig. 1 is a schematic diagram of a photometer system for monitoring infrared in accordance with an embodiment of the invention.

[0011] Fig. 2 is a schematic diagram of a variation of the photometer system of Fig. 1.

[0012] Fig. 3A is a schematic diagram of an illumination system for a device under test, suitable for use in the photometer system of Fig. 1.

[0013] Fig. 3B is a schematic diagram of another illumination system for a device under test, suitable for use in the photometer system of Fig. 1.

[0014] Fig. 3C is a schematic diagram of another illumination system for a device under test, suitable for use in the photometer system of Fig. 1.

[0015] Fig. 4A is a schematic diagram of a detector that can be used in the photometer system of Fig. 1.

[00 6] Fig. 4B is a schematic diagram of another detector that can be used in the

photometer system of Fig. 1.

[0017] Fig. 4C is a schematic diagram of another detector that can be used in the photometer system of Fig. 1.

[0018] Fig. 4D is a schematic diagram of another detector that can be used in the photometer system of Fig. 1.

[0019] Fig. 5A is a cross-section view of a waveguide, which is suitable for use as a device under test, or a portion of a device under test, in the photometer system of Fig. 1.

[0020] Fig. 5B is a perspective view of a near field transducer, which is suitable for use as a device under test, or a portion of a device under test, in the photometer system of Fig. 1.

[0021] Fig. 6 is a plot showing blackbody radiation, which can be detected by some embodiments of the photometer system of Fig. 1. [0022] Fig. 7 is a plot showing filtering of source light, so that infrared can be detected in some embodiments of the photometer system of Fig. 1.

[0023] Fig. 8 is a flow diagram of a method for monitoring radiated light in accordance with some embodiments of the photometer system of Fig. 1.

DETAILED DESCRIPTION

[0024] A photometer system herein disclosed monitors radiated infrared light from a device under test (DUT). Embodiments of the photometer system can be used as a device test or characterization system. Laser light, monochromatic light, or other narrowband source light is directed at the device under test. The device under test radiates infrared light, with a spectrum that approximates blackbody radiation. Outbound light from the device under test is a combination of source light, which may be reflected, attenuated, concentrated or otherwise manipulated by the apparatus and/or the device under test, and radiated infrared light from the device under test. A splitter is applied on the outbound light path, i.e. on the path of the light outbound from the device under test. The splitter splits the outbound light into two paths. The first split path is directed towards an infrared detector. The second split path is directed towards a camera or other imaging device. A filter is inserted in the outbound light path, between the device under test and the splitter. The filter removes the source light spectrum. In the case where the source light is laser light, for example infrared laser light, the filter can be a very narrowband notch filter, centered on the frequency of the laser light. The infrared detector thus receives the radiated infrared light, as the source light has been removed by the filter. The infrared detector measures the radiated infrared light. On the second path, the camera or imaging device produces an image from the radiated infrared light. The device under test can then be analyzed based on the results of the measurements and images. For further analysis, some embodiments of the photometer system include additional detectors and an additional camera or imaging device as described below.

[0025] An optical power detector measures the combined source light and radiated infrared light. The optical power detector is along an outbound light path that does not include the narrowband filter. Comparing the measured radiated infrared light with the measured combined source light and radiated infrared light allows a determination of characterization data of the device under test. For example, if the device under test is more efficient at coupling the inbound source light and therefore re-radiates less infrared light, this is one factor that can be determined by the comparison. Perhaps the device under test has an efficiency of conversion that changes in response to some other stimulus, in which case this can be measured by the apparatus. Further uses for the comparison and the characterization data can be devised.

[0026] Some embodiments of the photometer system include an illuminator, to illuminate the device under test for imaging, and a camera, to perform imaging of the device under test. A polarized output power detector can also be included, in order to measure intensity of polarized light. This is useful when a polarized light source is used. A spectrometer can be included, to measure the spectrum of outbound light. Some embodiments of the photometer system include a computer for control and/or computation. These and other aspects of the system and an example device under test will be further described below.

[0027] Detailed illustrative embodiments are disclosed herein. However, specific functional details disclosed herein are merely representative for purposes of describing embodiments. Embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein.

[0028] It should be understood that although the terms first, second, etc. may be used herein to describe various steps or calculations, these steps or calculations should not be limited by these terms. These terms are only used to distinguish one step or calculation from another. For example, a first calculation could be termed a second calculation, and, similarly, a second step could be termed a first step, without departing from the scope of this disclosure. As used herein, the term "and/or" and the " symbol includes any and all combinations of one or more of the associated listed items.

[0029] As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "includes", and/or "including", when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Therefore, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0030] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.

[0031] Fig. 1 is a schematic diagram illustrating an embodiment of a photometer system 100, shown acting as a testing mechanism for a device under test that emits infrared. In various examples disclosed herein, the device under test is a waveguide or light pipe, a near field transducer, or a near field transducer combined with a waveguide, although other devices or combinations of devices can act as devices under test in the photometer system 100. The photometer system 100 has several functions. One function is to record and analyze the laser light from the exit aperture of the light pipe or waveguide being processed. For instance the camera may obtain an image of the waveguide's exit aperture and the area around it when the waveguide is illuminated at its input aperture by the instrumentality's laser in order to assess optical power leaking away from the waveguide, as disclosed elsewhere in this application. In one embodiment the laser light wavelength is□ = (830 +/- 5) nanometers and in other embodiments it could be anywhere in the range of 400 to 1600 nanometers. Another function is to obtain a camera image of the waveguide's exit aperture and the area around it using light from an illuminator pointed at the surfaces surrounding the exit aperture. In this embodiment the illuminator's wavelength is 645 nanometers and other wavelengths are feasible. The photometer system 100 can perform said functions sequentially or in parallel under user control. The ability to perform functions simultaneously can be useful when trying to determine the exact location of an exit aperture in relation to its neighbouring features. The photometer system 100 and the data acquisition and control systems are also capable of performing the combined function of taking a camera image of the light from the waveguide's exit aperture and then overlaying it with a camera image of the surfaces surrounding the exit aperture taken while the illuminator is on and the waveguide's laser light source is off. It is advantageous to ensure that the laser and illuminator wavelengths are different so that they are distinguishable by the camera.

[0032] Components, and interrelationships among the components, of the photometer system 00 as shown in Fig. 1 will now be described. It should be appreciated that variations of the photometer system 100 can be devised by changing the order of the components, i.e., rearranging portions of the system. These rearrangements may add or delete instruments, or change the splitting of the light path so as to introduce an instrument earlier along a light path or later along a light path. It should further be appreciated that the photometer system 100 or components thereof can be enclosed in light tight housings or operated under dark conditions. Embodiments of the photometer system 100 can include electronics for stage control, instrumentation control, positioning of optics and switching in or out of optical elements, analog- to-digital conversion of detector outputs, and so on.

[0033] The article or device under test 102 can be mounted to a movable stage, for example a stage that can be moved manually or under computer control. Such a stage could include micro positioners or nano positioners, as readily understood and devised. The stage could scan in three orthogonal directions, e.g., using an XYZ positioner. Characterization data recorded as a result of XYZ scanning of a device under test relative to the apparatus can be used to pinpoint alignment criticalities, coupling resonances and other component aspects.

[0034] The device under test 102 could be illuminated from a separate light source, or could include an integral light source. In one example, the device under test includes a near field transducer located at or near the focal point of a waveguide, with the waveguide optically coupled to an infrared laser. Such a device, and illumination arrangements, will be further described with regard to Figs. 3A-3C and Figs. 5A-5B.

[0035] Continuing with reference to Fig. 1 , outbound light from the device under test 102 is gathered by an objective lens A. In the embodiment shown, the objective lens A has a high numerical aperture, and accepts light over a wide angle. Generally, a lens with a high numerical aperture can intercept a larger cone of light and can resolve finer detail than can a lens with a lower numerical aperture. The ability to receive large cone of light is advantageous when observing radiated light from a device under test, as such light may be radiated over a large angle. The finer resolution enabled by a high numerical aperture lens is advantageous in observations of radiated light from a very small object, such as a near field transducer. The outbound light, gathered by the objective lens A, is shown as an outbound beam of light. It should be appreciated that additional lenses, for example a collimator or a multi-lens objective, could be introduced.

[0036] Downstream from the objective lens A, the outbound beam of light is split by a splitter M. One split beam from the splitter M is then split by another splitter B. One beam from the splitter B is then split by another splitter C. One beam from the splitter C goes to an output power detector D. The output power detector D measures the power of the outbound light, i.e. measures the intensity of the portion of the outbound light reaching the power detector D.

Measurements made by the power detector D can be compared to measurements made by the power detector P and/or the polarized output power detector H, scaled according to light reductions through splitters, in order to determine efficiency or other metrics of a device under test 102.

[0037] An illuminator E provides light to the other light path of the splitter C. The illuminator can thus provide illumination light to the device under test 102 via the splitter C, the splitter B, the splitter , and the objective lens A. In variations, the illuminator E can include a white light source, a monochromatic light source, a filtered light source or other light source.

[0038] The other beam from the splitter B is then split by another splitter F. One beam from the splitter F passes through a polarization selective filter G and goes to a polarized output power detector H. In one type of test, polarized infrared laser light is coupled into a waveguide and near field transducer, which then produces a polarized output. The polarization selective filter G can be rotated to selectable angles. The polarized output power detector H can thus measure intensity of the outbound light at various polarization angles. Such measurements are used in calculations of efficiency of the device under test. For example, one test could look for polarized light at a right angle of polarization to the original source light, to determine

inefficiencies in coupling, or misalignment or undesired polarization effects. Particularly, for a device under test that is supposed to efficiently couple to polarized light, appearance at the polarized output power detector H of high levels of polarized light can indicate inefficiency.

[0039] The other beam from the splitter F is then split by another splitter I. One beam from the splitter I goes to a spectrometer J. The spectrometer J can measure the spectrum of outbound light. For example, one test can look to see if a waveguide or a near field transducer is producing light at wavelengths other than the wavelength of the laser light illuminating the device under test. The spectrometer J can also be used to measure or verify the wavelength of the source light itself.

[0040] The other beam from the splitter I goes through an attenuator K and to a camera L or other imaging device. For example, the camera could be a visible light imaging device for imaging the device under test 102 when under visible light illumination from the illuminator E. The attenuator K could include crossed polarizers, an iris, a fixed gradient or variable gradient filter, or other known attenuation device.

[0041] Moving back to the splitter M, the other beam from the splitter M goes through a filter N and is then split by another splitter O. In the embodiment shown, the filter N is a narrow band-stop filter, i.e., an optical notch filter that notches out or filters out the source light spectrum. The stopband of the narrow band-stop optical filter overlaps the spectrum of the narrowband light source illuminating the device under test 102. Specifically, in one embodiment the light source is an infrared laser, and the filter N filters out the wavelength or frequency of the infrared laser light. Generally, the stopband of the filter N is approximately centered on the wavelength or frequency of the light source and overlaps the bandwidth of the light source. Thus, the bandwidth of the stopband of the filter N should be wider than the bandwidth of the source light, and should overlap the bandwidth of the source light in both directions, i.e., in the direction of both lower and higher frequencies or longer and shorter wavelengths.

[0042] One beam from the splitter O goes to a power detector P. Examples of detectors are described below with reference to Figs. 4A-4D. The power detector P can measure the intensity of the radiated infrared light from the device under test 102. In the case where the device under test 102 is illuminated by an infrared laser, the power detector P receives the radiated infrared light from the device under test 102, with the source light i.e., the infrared laser light, removed from the outbound light by the filter N. Measurements made by the power detector P can be compared with measurements made by the output power detector D, to characterize device efficiency. Particularly, a ratio of output power to power lost as infrared radiation can be used as a metric of device efficiency for a device that is not supposed to lose efficiency through infrared radiation, or a metric of conversion efficiency for a device that is supposed to radiated infrared radiation.

[0043] The other beam from the splitter O goes to a camera Q or other imaging device. The camera Q forms an image from the infrared light radiated by the device under test 102, as filtered through the filter N. Radiation patterns, hotspots, leakage and other aspects of the response of the device under test 102 can be analyzed using the image.

[0044] With continued reference to Fig. 1 , an operating example is given as applied to a waveguide under test. In this operating example, the splitter M is moved out of the outbound light path. An example waveguide, suitable for use as a device under test or a portion thereof, is further described below regarding Fig. 5A. When the photometer system 100 is used to capture or analyze light from the waveguide exit aperture, the illuminator E is off. Light from the waveguide's exit aperture is captured and collimated by objective lens A. Objective lens A has a high numerical aperture (NA) in order to ensure that all, or a representative majority of, the light from the exit aperture is collected. In one embodiment the NA of the objective lens A is 0.8 or higher. The collimated light is split 50:50 by non-polarizing beam splitter (NPBS) cube B (i.e., splitter B). Following splitter B, 50% of the captured light is split once again by 50:50 NPBS cube C (i.e., splitter C) such that 25% of the captured light is fed into the optical power detector D. The electrical signal from the optical power detector D is fed into the photometer system 100 computational system. The purpose of the cube C (i.e., splitter C) is to allow light from the illuminator E to illuminate the area of and immediately around the exit aperture of the waveguide (i.e., the device under test 102 in this example). In one embodiment the NPBS C (i.e., splitter C) for example splits the light in the ratio 90: 0 such that 45% of the light from the exit aperture of the device under test 102 is delivered to the output power detector D. In yet another embodiment a mechanical arrangement moves the NPBS C (i.e., splitter C) into and out of the optical path as required such that 50% of the light from the exit aperture of the device under test 102 is delivered to the output power detector D. In further embodiments, other splitters could be moved into and out of the optical path.

[0045] Continuing with the example, 50% of the light entering splitter B is fed to NPBS F (i.e., splitter F) such that 25% of the light from the waveguide's exit aperture is fed through a polarization selective filter G. In this embodiment it is advantageous to select the plane of polarization to be orthogonal to the light delivery system's plan of polarization (i.e. the polarization of the light to the device under test 102). Losses in the polarization selective filter G are low, in one embodiment. The electrical signal from polarized output power detector H is fed into the photometer system 100 computational system.

[0046] Continuing with the example, 50% of the light entering splitter F is fed to NPBS I (i.e., splitter I) such that 12.5% of the light from the waveguide's exit aperture (i.e., from the device under test 102) is fed through to the spectrometer J, where the wavelength of the incoming light is measured. The measured value of the wavelength of the laser light is transmitted to the photometer system 100 computational system.

[0047] Finally, 12.5% of the light from the exit aperture (i.e., from the device under test 102) is fed through optical attenuator K to the camera L. Attenuator K can be moved in or out of the light path as desired. In this embodiment the camera L has a 100 micrometer times 100 micrometer field of view and a 100 nanometer times 100 nanometer pixel size. Other embodiments may employ a higher-resolution camera in order to obtain a larger field of view.

[0048] It is clear to a person skilled in the art that the function of the photometer system 100 may be optimized by using NPBS elements or splitters with split ratios other than those stated above. For example it may be advantageous to deliver more of the polarized light from the waveguide's exit aperture to polarized output detector H. [0049] With continued reference to Fig. 1 , when the photometer system 100 is used to take a camera image of the waveguide's exit aperture and the area around it, the illuminator E is on. Light from the illuminator E is reflected by the splitter C and the splitter B, then travels through splitter M and through objective lens A onto the waveguide's exit aperture and the area immediately around it (i.e., onto the device under test 102). Light from the illuminator E is reflected by the surface of the exit aperture and the area around it back through objective lens A, splitter M, splitter B, splitter F, splitter I and attenuator K to the camera L. The image from the camera is transmitted to the photometer system 100 computational system. The whole of the photometer system 100 is built into a light-tight box for ease of mounting, protection of the optical surfaces and the prevention of light pollution from external sources, in one embodiment.

[0050] As shown in Fig. 2, the splitter M can be moved out of the light path in some embodiments. Alternatively, the embodiment shown in Fig. 2 could be implemented without a splitter M, filter N, splitter O, power detector P, and camera Q. The variation shown in Fig. 2 has less attenuation of the outbound light, since the light output is not reduced by splitting through the splitter M. The variation shown in Fig. 2 could be used for characterization or test of a device without measurement of radiated infrared light, e.g., as described in one of the above operating examples.

[0051] Figs. 3A-3C show illumination systems applicable to a device under test 102 in the photometer system 100 of Fig. 1. Generally, a laser 302 provides a laser light beam 306, which is then coupled to the device under test. In Fig. 3A, the coupling is direct or air-based. That is, there are no lenses in the laser light beam 306, which is aimed directly at the device under test. In this example, the device under test is a waveguide 304, about which more will be discussed regarding Fig. 5A.

[0052] In Fig. 3A, the light beam 306 is coupled into the waveguide 304. For example, the waveguide 304 may include a grating coupler. The exit aperture of the waveguide 304 may act almost like a point source of light, and produce an exit cone of light 308. Some waveguides produce a dispersion of light spanning about 180° at the exit aperture, i.e., hemispherical wavefronts at the far field. In the example shown, a blocking device 324 is placed in line with the exit aperture of the waveguide 304, so as to block laser light proceeding through the exit aperture of the waveguide 304 directly from the laser light beam 306. The exit cone of light 308 is gathered by the high numerical aperture objective lens A, which directs this light into a wide beam 310 of light. The wide beam 310 passes through the filter N. A filtered wide beam 312 from the filter N passes into the power detector P. In embodiments where the stopband of the filter N overlaps the frequency of the laser light beam 306, the filtered wide beam 312 has radiated infrared light from the device under test, but the laser light is greatly attenuated by the filter N.

[0053] In Fig. 3B, the source laser light beam 306 is expanded and then shaped into a diminishing cone 316 by a lens system 314. For example, the lens system 314 could include a diverging lens, a collimator, and a converging lens. The diminishing cone 316 may better couple into the waveguide 304.

[0054] In Fig. 3C, a fiber-optic guide 318 couples the laser 302 to waveguide 304. A laser beam 320 may proceed over a small distance between an exit of the fiber-optic guide 318 and the entrance to the waveguide 304. Other fiber-optic couplings and other types of couplings are readily devised. For example, combinations of mirrors and/or lenses could be used, in a variation.

[0055] Figs. 4A-4D show detectors that can be used in or as the power detector P in embodiments of the photometer system 00. Embodiments can use a single infrared detector, as shown in Figs. 4A and 4B, multiple infrared detectors as shown in Fig. 4C, or an imaging device as shown in Fig. 4D. Suitable infrared detectors include photodiodes, phototransistors, thermocouples, thermopiles, photoconductive materials, a Golay cell, a Bolometer, photovoltaic materials, and pyroelectric materials.

[0056] Fig. 4A shows a single power detector P that includes a narrow bandpass filter 402. For example, the narrow bandpass filter could have a passband centered at 1000 nanometers (nanometers) wavelength, which is in the infrared band of light. The width of the passband should be sufficiently narrow so that any remaining light from the infrared laser illumination source, even after filtering by the filter N, is further attenuated and does not disrupt the measurement made by the power detector P. The version shown in Fig. 4A allows

measurement of the radiated infrared light at one wavelength, dictated by the passband of the narrow bandpass filter 402.

[0057] Fig. 4B shows a single power detector P that includes multiple optical filters 402, 404, 406, 408. In this embodiment, the optical filters can be moved so that, for a given measurement, a single one of the optical filters 404 intercepts the light heading into the detector. For example, the optical filter 402 could be a bandpass filter and pass light of 1000 nanometers, the optical filter 404 could be a bandpass filter and pass light of 2000 nanometers, the optical filter 406 could be a bandpass filter and pass light of 3000 nanometers, and the optical filter 408 could be a bandpass filter and pass light of 5000 nanometers. Other combinations of wavelengths for passbands, and other numbers of filters, can be devised. The filters could be moved in a linear arrangement, a rotary arrangement or other arrangement as readily devised. The movement of the filters could be under manual or computer control.

[0058] Fig. 4C shows three power detectors 418, 410, 412. Each power detector has a respective narrow bandpass filter. The first power detector 418 has a first narrow bandpass filter 402, for example passing light of 2000 nanometers. The second power detector 410 has a second narrow bandpass filter 404, for example passing light of 4000 nanometers. The third power detector 412 has a third narrow bandpass filter 406, for example passing light of 6000 nanometers. Other numbers of power detectors and filters, and wavelengths for passbands, can be devised.

[0059] Fig. 4D shows an imaging device 416, such as a charge coupled device (CCD) serving as a photodetector. Incoming light is diffracted by a diffraction grating 414, spreading out into a spectrum that is then detected by the imaging device 416. This is an infrared spectrum, and a charge coupled device sensitive to infrared, in the case of detecting radiated infrared light. The power detector of Fig. 4D is a type of infrared spectrometer. Other types of infrared spectrometers could be used.

[0060] In Figs. 5A and 5B, examples of a device under test are shown. Fig. 5A shows a waveguide 304, into which the near field transducer 530 of Fig. 5B can be introduced or mounted. These devices can be tested separately or in combination, in embodiments of the photometer system 100. A brief theory of operation and a set of example dimensions are presented below.

[0061] One proposed application of the waveguide 304 and the near field transducer 530 is in the heat assisted magnetic recording (HAMR) head for hard disk drives. In operation in a disk drive, optical power from a source laser diode is coupled into a waveguide such as the waveguide 304, and the waveguide 304 focuses the laser light onto a near field transducer, such as the near field transducer 530. The near field transducer focuses the optical power at a small distance from an air bearing surface (ABS). The thickness of the air bearing is directly related to the flying height of the disk drive head. This optical power heats up a tiny region on the hard disk. The heated region on the hard disk is then susceptible to magnetizing, for writing a "1" or "0" bit onto the hard disk. A byproduct of the near field transducer's function is the near field transducer heating up. The heating of the near field transducer generates optical power in infrared wavelengths. Embodiments of the photometer system 100 use the detection of this radiated infrared power (primarily), and any other optical power emitted and detected

(secondarily) outside of the narrow band provided by the optical source, to determine optimum coupling and characterize the efficiency of the device under test in energizing the near field transducer.

[0062] The waveguide 304, in Fig. 5A, is a solid immersion parabolic reflector. Incoming rays of light 510 are focused to the focal point 522 of the parabolic reflector. In one

embodiment, the entrance aperture 504 is 2 microns across, i.e. 2000 nanometers. The exit aperture 506 is 20 nanometers across, which is much less than the wavelength of visible light. Light 516 emerging from the exit aperture acts as if the exit aperture is a point source of light. Light from the incident cone 508 of light (e.g., in Fig. 3B) may pass through transparent or semitransparent walls of the waveguide 304 and emerge as stray light 514. The waveguide 304 may heat up and emit infrared radiation 518. In one embodiment, the light rays 512 passing straight through the exit aperture 506 are blocked by the blocking device 324, as discussed regarding Fig. 3A.

[0063] The near field transducer 530, in Fig. 5B is in the shape of a tiny lollipop. In one example of a near field transducer, the "lollipop" disk diameter is about 350 nanometers, the width of the peg or stem 532 is about 60 nanometers, and the thickness 534 is about 22 nanometers. Placing the near field transducer 530 at the focal point 522 of the waveguide 304, and coupling polarized infrared laser light at a wavelength of 830 nanometers into the waveguide 304, results in the near field transducer 530 developing an intense field at the tip of the stem 532. However, the near field transducer 530 also heats up rapidly to temperatures in the range of 600 to 700° Kelvin. When it heats up, the near field transducer radiates infrared radiation in a spectrum that approximates blackbody radiation. It would be advantageous to measure the infrared radiation to determine temperature of the device and efficiency of the device. A device that is more efficient at coupling the laser light, i.e., a more efficient transducer, absorbs less of the laser light and heats up less. Too high a temperature risks melting of the gold from which one embodiment of the near field transducer 530 is made.

[0064] Referring back to Fig. 5A, the spatial relationship between the entrance pupil of the waveguide 304, the exit pupil of the waveguide 304 and structures making up a disk drive head, may cause significant optical power to exit the surface from or near the exit pupil that did not pass through the waveguide or light pipe as intended. This "Context Light" can exceed the power transmitted along the designed optical path, which tends to be inefficient by nature of design requirements. This un-wanted signal can lead to a failure to determine the designed coupling point and mischaracterize the device under test. Removal of the source light, through application of the filter N, enables and greatly improves accuracy of measurement of the radiated infrared light.

[0065] Fig. 6 is an illustration of blackbody radiation at various temperatures, for comparison purposes. As temperature of a blackbody increases, the intensity peak of the blackbody radiation moves to shorter wavelengths. As temperature of a blackbody decreases, the intensity peak lowers and moves to longer wavelengths, and the overall spectrum spreads out. To gauge temperature of a blackbody or other radiating body, the wavelength of the intensity peak and the shape of the spectrum should be investigated. With reference back to Figs. 4A- 4D, applying an infrared detector with measurements at more wavelengths or finer resolution across a spectrum could result in a more accurate estimation of the intensity versus wavelength characteristics of radiation from a radiating body.

[0066] Fig. 7 is an illustration of application of an optical band-stop filter, showing how an embodiment of the filter N acts to remove source light from outbound light. Recall that the outbound light from a device under test includes some of the source light, in this example infrared laser light 704 at 830 nanometers wavelength. The outbound light also includes radiated infrared light 706, in this example shown as a wavelength distribution centered at about 1000 nanometers. A tail 708 (shown in dashed line) of a much lower temperature radiated light spectrum is shown for comparison. The optical band-stop filter has a pass characteristic that passes most wavelength of light except for a stopband notch 702, which filters out light centered at 830 nanometers. The width of the stopband is greater than the bandwidth of the infrared laser light 704. In other words, the stopband notch 702 stops not only light of 830 nanometers wavelength, but also light of slightly shorter and longer wavelengths. It should be appreciated that the intensities are not shown to scale, and that the radiated infrared light would generally be of much lower intensity than the inbound laser light.

[0067] A detector of a single frequency or wavelength, e.g. 1000 nanometers, would not be able to differentiate between the radiated infrared light 706 and the tail 708 of a much lower temperature radiated light spectrum, in the case where the measured intensity of each at 1000 nanometers is equal. This is why it is advantageous to have multiple infrared detectors, an infrared detector with multiple filters, or an infrared detector that includes a spectrometer, so that the shape of the radiated infrared spectrum can be reconstructed or approximated.

[0068] In operation of some embodiments of the photometer system 100, the filter N is applied to remove the source light spectrum, e.g., light at 830 nanometers, from the outbound light. A splitter M splits the outbound light and then provides the filtered light to the power detector P and the camera Q as illustrated. As shown in Fig. 7, the filter N cancels the peak at 830 nanometers but the peak at 1000 nanometers is unaffected. Thus, the peak at 1000 nanometers can be quantified to provide data as to the efficiency, or other parameters of the device under test or some other observation. It should be appreciated that the 1000

nanometers peak is indicative of infrared radiation and results from the heating of the device under test, for example the near field transducer for a magnetic head for a disk drive.

[0069] Fig. 8 is a method for monitoring radiated light. The method, and variations thereof, can be practiced on various embodiments of the photometer system of Fig. 1. Various ways of implementing the method are discussed below along with a description of the method itself.

[0070] After a start point, laser light is directed at a device under test, in an action 802. Various source couplings of laser light, as discussed regarding Figs. 3A-3C or variations thereof can be applied in directing laser light at a waveguide 304, a near field transducer 530 or other device under test. The device under test radiates infrared light, in an action 804. For example, the radiated infrared light could be from heating of the near field transducer 530 or portions of the waveguide 304. Radiated infrared light may approximate a blackbody spectrum or may have another spectrum.

[0071] In an action 806, the source light spectrum is filtered out on the radiated light path from the device under test. For example, the radiated light path can be along the path of light outbound from the device under test, as discussed regarding Fig. 1. The source light spectrum can be filtered out with a filter N, such as an optical filter with a narrow stopband. The stopband overlaps the bandwidth of the source light.

[0072] In an action 808, a splitter is applied on the radiated light path. For example, the splitter O can be placed between the filter N and the power detector P, as shown in Fig. 1. The radiated light is imaged after the filter, on a first split path, in the action 810. For example, the camera Q in Fig. 1 is downstream of the filter N and the splitter O, on one of the split paths.

[0073] Radiated light is measured after the filter, on a second split path, in an action 812. For example, the power detector P is downstream of the filter N and the splitter O, on one of the split paths. Various types and configurations for the power detector P are discussed regarding Figs. 4A-4D.

[0074] In an action 814, device under test characterization data is determined, based on image and/or measurement. For example, measurements of the intensity versus wavelength of the radiated infrared can be performed by the power detector P as shown in Fig. 1. An image can be captured by the camera Q as shown in Fig. 1. Characterization data can be determined based on either of these, in isolation or in combination with measurements from other instruments such as the output power detector D, the polarized output power detector H, the spectrometer J and the camera L of Fig. 1.

[0001] With the above embodiments in mind, it should be understood that the embodiments might employ various computer-implemented operations involving data stored in computer systems. These operations are those requiring physical manipulation of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. Further, the manipulations performed are often referred to in terms, such as producing, identifying, determining, or comparing. Any of the operations described herein that form part of the embodiments are useful machine operations. The embodiments also relate to a device or an apparatus for performing these operations. The apparatus can be specially constructed for the required purpose, or the apparatus can be a general-purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general- purpose machines can be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations.

[0002] The embodiments can also be embodied as computer readable code on a computer readable medium. The computer readable medium is any data storage device that can store data, which can be thereafter read by a computer system. Examples of the computer readable medium include hard drives, network attached storage (NAS), read-only memory, random- access memory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and other optical and non- optical data storage devices. The computer readable medium can also be distributed over a network coupled computer system so that the computer readable code is stored and executed in a distributed fashion. Embodiments described herein may be practiced with various computer system configurations including hand-held devices, tablets, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers and the like. The embodiments can also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a wire-based or wireless network. [0003] Although the method operations were described in a specific order, it should be understood that other operations may be performed in between described operations, described operations may be adjusted so that they occur at slightly different times or the described operations may be distributed in a system which allows the occurrence of the processing operations at various intervals associated with the processing.

[0075] The foregoing description, for the purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the embodiments and its practical applications, to thereby enable others skilled in the art to best utilize the embodiments and various modifications as may be suited to the particular use contemplated. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.

Claims

What is claimed is:

1. A method for measuring radiated light, comprising:

directing narrowband light at an article, wherein the article radiates infrared light; filtering a source light spectrum out of light outbound from the article, to create filtered outbound light; and

detecting a portion of the infrared light, in the filtered outbound light.

2. The method of claim 1 , wherein the narrowband light is monochromatic.

3. The method of claim 1 , wherein the narrowband light is polarized.

4. The method of claim 1 , wherein the narrowband light is laser light.

5. The method of claim 1 , wherein filtering source light spectrum includes applying an optical notch filter.

6. The method of claim 1 , wherein the article radiates infrared light in response to being illuminated by the narrowband light.

7. The method of claim 1 , wherein the infrared light radiated by the article approximates blackbody radiation.

8. A photometer system comprising:

an optical filter having a narrow stopband, the optical filter positioned to receive a combination of narrowband light and infrared light, the infrared light radiated from an article illuminated by the narrowband light, the stopband overlapping a bandwidth of the narrowband light; and

an infrared detector positioned to receive filtered light from the optical filter with the narrowband light reduced by the optical filter, the infrared detector operative to detect the infrared light radiated from the article as passed by the optical filter.

9. The photometer system of claim 8, further comprising:

an infrared laser operable to provide the narrowband light and to direct the narrowband light to the article.

10. The photometer system of claim 8, wherein the infrared detector includes:

a bandpass optical filter; and

one or more from a set consisting of: a photodiode, a phototransistor, a thermocouple, a thermopile, a Golay cell, a Bolometer, a photoconductive material, a photovoltaic material, and a pyroelectric material.

11. The photometer system of claim 8, wherein the infrared detector includes:

a plurality of bandpass optical filters having differing selective wavelengths; and a detector, wherein each bandpass optical filter of the plurality of bandpass optical filters is selectably positionable in front of the detector.

12. The photometer system of claim 8, wherein the infrared detector includes:

a plurality of detectors; and

a plurality of bandpass optical filters of differing selective wavelengths, each detector of the plurality of detectors having a one of the plurality of bandpass optical filters.

13. The photometer system of claim 8, wherein the infrared detector includes:

a diffraction grating; and an imaging device operable to detect a spectrum of infrared light as produced by the diffraction grating.

14. A photometer system comprising:

a narrowband light source operable to illuminate an article with narrowband light;

an infrared detector positioned so that there is an optical path from the article to the infrared detector;

a lens positioned along the optical path between the article and the infrared detector; a narrow band-stop optical filter positioned along the optical path, between the lens and the infrared detector, a stopband of the narrow band-stop optical filter overlapping a spectrum of the narrowband light source;

an infrared camera; and

a first splitter positioned along the optical path between the narrow band-stop optical filter and the infrared detector, the first splitter producing a further optical path to the infrared camera.

15. The photometer system of claim 14 wherein the narrowband light source includes an infrared laser attached to the article. 6. The photometer system of claim 14, further comprising:

a second camera, operative to form a visible light image of the article; and

a second splitter positioned along the optical path between the lens and the narrow band-stop optical filter, the second splitter producing a further optical path to the second camera; and

an attenuator positioned between the second splitter and the second camera.

17. The photometer system of claim 14, further comprising; a second splitter positioned along the optical path between the lens and the narrow band-stop optical filter;

a third splitter; and

an illuminator positioned to illuminate the article via a series coupling of the third splitter and the second splitter.

18. The photometer system of claim 14, further comprising;

a second splitter positioned along the optical path between the lens and the narrow band-stop optical filter;

a fourth splitter; and

an output power detector positioned to receive light from the article via a series coupling of the second splitter and the fourth splitter.

19. The photometer system of claim 14, further comprising:

a fifth splitter;

a polarization selective filter; and

a polarized output power detector positioned to receive light from the article via a series coupling of the second splitter, the fifth splitter and the polarization selective filter.

20. The photometer system of claim 14, further comprising:

a sixth splitter; and

a spectrometer positioned to receive light from the article via a series coupling of the second splitter and the sixth splitter.