US20180292262A1 - Laser speckle reduction and photo-thermal speckle spectroscopy - Google Patents
Laser speckle reduction and photo-thermal speckle spectroscopy Download PDFInfo
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Definitions
- the present invention relates to laser speckle reduction and to a method for photo-thermal speckle spectroscopy.
- IR lasers such as tunable quantum cascade lasers (QCLs)
- QCLs quantum cascade lasers
- SNR signal-to-noise ratio
- speckle When coherent light (such as light from a laser source) illuminates a rough surface, the resulting image contains a granular pattern called speckle (Goodman, Speckle Phenomena in Optics, Roberts & Company, Englewood Colorado (2007)).
- a typical speckle pattern is shown in FIG. 1 .
- the speckle grain size and distances from neighboring grains is often very sensitive to changes in sample geometry, optical configuration, and wavelength of light used. Speckle can be modeled by considering the speckle pattern to be due to interference from a collection of scattering centers.
- FIG. 2 illustrates this concept.
- speckle is a nuisance as it obfuscates the real signal (e.g. reflectance, transmittance etc.) from the sample.
- speckle contrast as defined by the fluctuations of speckle intensity
- it can reach levels where the signal to noise ratio of the detection system is speckle limited. Therefore, it is imperative to reduce speckle contrast but without an associated reduction in signal-to-noise due to lower laser power throughput. This is very hard to do as “de-speckling” invariably comes with a decrease in the optical power of the de-speckled laser light.
- a speckle-reduction optical setup will allow for tuning the amount of speckle reduction.
- Spatial averaging involves combining multiple pixels to wash-out speckle contrast.
- Temporal averaging involves collecting signal over a longer time or combining consecutive camera frames.
- Spectral averaging involves reducing spectral resolution by smoothing, which reduces speckle contrast.
- Polarization averaging involves combining the signal from illuminating with two polarization states of the laser.
- a de-speckling procedure can involve a coherent illumination wavefront becoming an incoherent wavefront.
- the present invention provides for laser speckle reduction and for exploiting speckle from rough surfaces for the detection of trace amounts of chemicals.
- Laser speckle is very sensitive to small movements in the imaging setup. Heating the speckle-inducing substrate causes it to expand and the resulting speckle to change. The amount of speckle change is proportional to the increase in temperature, which is in turn proportional to the infrared (IR) absorption spectrum of the analyte to detect.
- IR infrared
- FIG. 1 shows a typical speckle pattern
- FIG. 2 is an illustration of speckle pattern formation and its modeling.
- FIG. 3 shows images from a thermal imaging camera of an infrared speckle off a rough surface. Speckle exhibits the peculiar property that it appears focused in an otherwise defocused image.
- FIG. 4 is a schematic drawing of a speckle-reducing setup.
- FIGS. 5A-5C shows images of direct speckle observation by imaging the output of the multi-mode fiber onto the beam profiler.
- FIG. 5A shows an image with the diffuser with no spinning.
- FIG. 5B shows an image of the shiny side (without the diffuser) with spinning.
- FIG. 5C shows an image with a diffuser with spinning.
- a dramatic improvement in speckle reduction is achieved by spinning the Infragold® diffuser. Also, it is demonstrated that having a good diffusive reflectance properties of the spinning diffuser is essential (Infragold vs. its shinier, but still somewhat rough side).
- FIGS. 6A and 6B show images after illuminating a rough surface with light from the speckle-reduction unit. As shown in FIG. 6A , with the diffuser motor off, the speckle is still present. As shown in FIG. 6B , with the diffuser spinning, speckle is eliminated.
- FIGS. 7A-7C compare the effect of different illumination modes on speckle formation. As shown in FIG. 7A , using a free-space laser beam results in a fully developed speckle. As shown in FIG. 7B , using a fiber-coupled laser beam results in a dense speckle pattern with lower speckle contrast. As shown in FIG. 7C , using the speckle reduction approach of a spinning diffuser prior to fiber coupling results in no evidence of speckle.
- FIG. 8 shows an IR reflectance microscopy setup. For testing purposes, a 127 ⁇ m thick platinum wire was used.
- FIGS. 9A-9C show microscope images of a thin wire.
- FIG. 9A shows a thermal image of a hot wire.
- FIG. 9B shows an active, laser reflectance micrograph using a speckle reduction unit with the spinning diffuser stopped.
- FIG. 9C shows an active, laser reflectance micrograph using a speckle reduction unit with the diffuser spinning (for maximum speckle reduction potential).
- FIGS. 10A and 10B show the influence of changing wavelength on the reflectance micrographs. As shown in FIG. 10A , when the illumination is less coherent, the micrographs are nearly identical. However, as shown in FIG. 10B , without good speckle reduction, the images change with wavelength considerably.
- FIG. 11 shows a photo-thermal speckle spectroscopy setup.
- the prevent invention relates to speckle reduction approaches and their ability to reduce speckle contrast while at the same time preserving a high optical throughput.
- Multi-mode fibers, integrating spheres, and stationary and moving diffusers may be used for speckle reduction.
- Speckle-contrast can be measured directly by acquiring beam profiles of the illumination beam or, indirectly, by observing speckle formation from illuminating a rough surface (e.g. Infragold® coated surface) with an IR micro-bolometer camera.
- Speckle contrast reduction is characterized from spatial, temporal and wavelength averaging for both CW and pulsed QCLs. Examples of effect of speckle-reduction on hyperspectral images in both standoff and microscopy configurations are provided herein.
- Speckle was generated by using a tunable QCL (“MIRcat” by Daylight Solutions) tuned to approximately 8 ⁇ m.
- Direct speckle was observed by imaging of the output from an IR multi-mode optical fiber (“PIR400” by Newport; 400 ⁇ m core, 0.25 NA, 1 m, SMA terminated) by an optical beam profiler (“Pyrocam III” by Ophir/Spiricon).
- Indirect speckle was observed by imaging the speckle pattern formed by reflecting the beam off a rough (anodized aluminum part by 80/20 Inc.) by either a FLIR “E60” or FLIR “Photon Block 2” micro-bolometer.
- the micro-bolometers were equipped with wide-angle lenses. Both micro-bolometers have a built-in 7-14 ⁇ m bandpass filter.
- the laser beam needs to be conditioned.
- Our solution used a two-step approach: First, we reflected the IR laser off a rotating Infragold® diffuse surface. The rotation insured that we got a decrease in speckle contrast due to temporal averaging. Next, the diffusely reflected light was collected by a lens and coupled into a multi-mode optical fiber for further speckle reduction. A schematic of the setup is shown in FIG. 4 .
- FIG. 5A shows an image with the diffuser not spinning.
- FIG. 5B shows an image of the shiny side (without the diffuser) with spinning.
- FIG. 5C shows an image with the diffuser spinning. It can be seen from FIG. 5A that the diffuser alone is not enough to de-cohere the beam to a satisfactory level. It is only after the diffuser starts spinning that we get a good speckle reduction, as seen in FIG. 5C . Without the diffuser (when we flip it to its shiny side), the spinning alone cannot remove speckle, as seen in FIG. 5B . Therefore, it is only the combination of diffuser and the temporal averaging from its spinning that can reduce speckle to acceptable levels.
- the light was used to illuminate an anodized aluminum surface (80/20 Inc. part). Without the diffuser spinning, the speckle is clearly visible, as seen in FIG. 6A . With the diffuser spinning, speckle is eliminated, as shown in FIG. 6B .
- FIGS. 7A-7C we compare side-by-side the effect of different illumination modes (from completely coherent, free space illumination, to more incoherent illuminations) on speckle formation.
- FIG. 7A a fully developed speckle is seen when using a free-space laser beam.
- FIG. 7B a dense speckle pattern and lower speckle contrast is seen when using a fiber-coupled laser beam.
- FIG. 7C the speckle is completely eliminated, when using our speckle reduction approach of a spinning diffuser prior to fiber coupling.
- the schematic of the microscope used in this experiment is shown in FIG. 8 .
- the sample is illuminated with light from the de-speckling unit from a multi-mode fiber. This light covers a portion of the field view of the reflecting objective (bright-field illumination).
- the reflected light is viewed by a micro-bolometer array.
- FIG. 9A The performance of the microscope was tested by imaging a thin Pt wire (0.005′′ (127 ⁇ m) diameter wire). Passive (thermal emission) differential images of the wire heated by an ac power supply are shown in FIG. 9A .
- FIGS. 9B and 9C show laser reflectance micrographs using illumination from the speckle reduction unit with the spinning diffuser stopped ( FIG. 9B ) and moving ( FIG. 9C ).
- FIG. 10A shows a minimal change in the reflectance micrographs as the wavelength is tuned, indicative of good speckle reduction.
- FIG. 10B shows that without proper speckle reduction of the illumination, the micrographs are wavelength dependent and produce an image that is not consistent with the object being imaged.
- FIG. 11 shows a photo-thermal speckle spectroscopy (transmission geometry) set-up.
- An IR laser is toggled on/off, and a visible (red) laser is always on.
- the laser beams are focused on a foam, which causes the red laser to scatter.
- a visible CMOS camera (without lens) records the laser speckle pattern.
- a hypothesis is that when the foam heats up, the cells expand; thus, a change in speckle pattern is observed on the camera.
- the sample can be deposited (by solvent) into a foamy material (e.g. Vyon F polyethylene).
- a foamy material e.g. Vyon F polyethylene.
- the purpose of the foam is to have many scattering centers so it forms a good speckle pattern.
- the sample can also be a liquid (inside the pores), or even a gas or vapor inside the pores (in this case, the foam would have to be contained inside a containment cell with IR and visible light transmitting windows for access of the probe (mostly UV, visible or near IR) and pump (mid IR) beams.
- the foam will have to be made of IR transparent material and be of such thickness as to not absorb too much in the IR region. Thick foams will always absorb a lot, even though the foam material is transparent. Therefore, thinner foams are preferred.
- the foam cells are elongated and oriented such that the polarization state of the laser will decrease coupling into the foam.
- the analyte is more depolarized, so it will absorb at the same rate.
- Other ways to minimize absorption through engineering the pores and/or shape and positioning of the scattering centers to minimize interaction of IR light with the substrate, but keep the analyte absorption at the same level are envisioned.
- Another possible way of decreasing the foam IR absorption would be to fill the pores with a liquid that has similar index of refraction in the infrared (but not at the wavelength of the probing visible laser) to the foam. This way the, IR light can pass though the substrate, and the only way it would get absorbed is if the analyte itself (which is stuck inside the pores, or even dissolved in the liquid) absorbs it.
- speckle pattern will be similar, no matter the analyte and its morphology. Also, the amount change in speckle pattern will then be proportional to the analyte concentration.
- Another possible substrate would be the thin polyethylene or Teflon cards used in FTIR spectroscopy. They have a milky color and diffuse visible light but are very transparent in the IR.
- Any specially design substrate made out of meta-materials or other engineered surface that suppresses IR absorption, but causes visible light to scatter (either in the reflected direction—surface scattering, or transmission—volume scattering).
- the substrate can also be a photo-thermal micro-bridge that flexes upon heating (like a bi-metal).
- the underside could be made rough to induce speckle that can be imaged by using a visible laser on the underside of the bridge.
- the speckle from the particles itself can be used. If substrate is also rough, the speckle will be a combination of the two. Given the different roughness of the substrate and particle spacing, these two patterns might be qualitatively different and therefore separable. For example, it is possible that the spatial frequency of the speckle pattern from the particles is different from that of the substrate (use 2D FFT to get spatial frequency map).
- Both reflectance and transmission geometry can be used for a system.
- UV or near IR (or longer wavelengths) of the probe laser would make the probe beam invisible.
- the IR heating laser can be modulated while the speckle images are collected.
- a demodulation at the frequency of the heating
- the previous method can be expanded such that multiple IR lasers can be used at the same time, but modulated at different frequencies. This multiplexing approach can speed up the collection.
- thermo diffusivity thermal diffusivity, conductivity etc.
- Speckle formation can be enhanced by cycling the probe light back into the substrate using beam splitters, cavities of similar.
- Speckle interferometry split the illuminating laser beam (using a beam-splitter) into two parts—one going through a substrate containing the analyte, the other through a clean substrate only. Then, recombine the two beams and observe the interfered beams with a camera. Process as described herein. In another version, propagate the second beam free-space (i.e. not through a clean substrate) and then recombine. In this case, to make the second beam the same size as the first, a beam expander may be needed.
- Tuning the IR laser in a continuous (e.g. linear) fashion within its tuning range and monitoring (by analyzing differences in speckle pattern, or other means) the differential heating (between closely spaced wavelengths) can recover the whole photo-thermal spectrum without the need to measure a list of wavelengths. There will be a need to normalize such a spectrum with the optical laser power delivered to the sample.
- the laser can be modulated (by chopper or electronically, opto-acoustically or other means) to get an “on-off” illumination pattern.
- Speckle can be imaged directly or by collecting the visible light with a suitable lens/objective. The goal is maximize speckle pattern change for the given induce temperature change. Additional polarizer, filters or any other optics to achieve this increased speckle pattern change can be utilized.
- the IR laser itself makes a speckle pattern—this could be viewed using an IR camera.
- the light doesn't penetrate the particles (or film) of analyte. Off resonance it does. Presumably, this would cause a change in the speckle pattern that can be distinguished from wavelength induced changes alone.
- the morphology of the speckle may be different.
- the inter-speckle grain distance may change. In this case, simple speckle pattern movements are not enough, as the wavelength would need to be changed.
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US16/983,578 US11262241B2 (en) | 2017-04-06 | 2020-08-03 | Laser speckle reduction and photo-thermal speckle spectroscopy |
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Cited By (3)
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CN110523776A (zh) * | 2019-08-27 | 2019-12-03 | 贵研铂业股份有限公司 | 一种三元扩散偶高通量制备和热处理方法 |
CN111142270A (zh) * | 2018-11-05 | 2020-05-12 | 青岛海信激光显示股份有限公司 | 一种激光散斑消除装置及其激光显示设备 |
WO2023089129A1 (fr) * | 2021-11-18 | 2023-05-25 | Rockley Photonics Limited | Membrane déformable pour atténuation de chatoiement |
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- 2018-04-06 JP JP2019554799A patent/JP2020519919A/ja active Pending
- 2018-04-06 EP EP18780450.5A patent/EP3607301A4/fr not_active Withdrawn
- 2018-04-06 CA CA3057148A patent/CA3057148A1/fr not_active Abandoned
- 2018-04-06 US US15/946,844 patent/US20180292262A1/en not_active Abandoned
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Also Published As
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WO2018187653A2 (fr) | 2018-10-11 |
US20200363264A1 (en) | 2020-11-19 |
EP3607301A2 (fr) | 2020-02-12 |
WO2018187653A3 (fr) | 2018-12-06 |
EP3607301A4 (fr) | 2020-12-16 |
JP2020519919A (ja) | 2020-07-02 |
CA3057148A1 (fr) | 2018-10-11 |
US11262241B2 (en) | 2022-03-01 |
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