BACKGROUND

Raman spectroscopy has proven to be a highly valuable portable analytical tool for accurate chemical identifications. In the most common implementation, a Raman spectrometer requires the operator to hold the device close (e.g., less than couple of centimeters away) to the substance under investigation, illuminating the substance with a pump laser and collecting the Raman scattering. With typical portable Raman instruments, the laser hazard zone is half a meter or larger and laser training is required for the operators of such a tool since it is typically classified as a Class 3B instrument. Standoff Raman systems with a meter or more of standoff distance from sample also exist but the laser hazard zone is much larger (e.g., a few meters) preventing easy adoption of such systems in the field in addition to burdening the operators and manufacturers with a more onerous laser safety training class.

For visible or near-infrared (NIR) Raman systems laser hazard is due to ocular damage, in particular thermal damage to the retina, from the laser used as the Raman pump. The damage is a function of radiance (W/m2/sr), distance of Raman laser source to the eye, wavelength, and exposure time. For a standoff Raman system, the laser power exits the system output aperture and converges to a spot outside the system, on a target sample.

SUMMARY

Examples of the devices and methods described here aim at increasing or maximizing the allowable output laser power of an instrument of a certain laser class, while also mitigating risks (e.g., explosion) or other issues (e.g., perturbation of the kinetics of a reaction) linked to local heating of the analyte. A spectroscopic laser-based instrument with an assigned class of 3B may be downgraded to a class of 3R (or, similarly, an instrument with an assigned class of 3R may be downgraded to class 1) with the use of the devices and methods presented here. A spectroscopic laser-based instrument that can ignite some analytes may be made to reduce or eliminate the risk of igniting these analytes using the devices and methods presented here. A spectroscopic laser-based instrument that causes significant local heating of an analyte may be made to cause nominal local heating of the analyte using the devices and methods presented here.

Here we present a method for implementing an eye-safe (Class 3R or lower) Raman system using beam scanning. The system operates at a standoff distance of a few centimeters to a few meters with a laser output power that would correspond to a higher-class system (from an eye safety point of view) if beam scanning was not used (e.g., the system would be class 3B without scanning and can be made as safe as a class 3R instrument with proper beam scanning).

The signal to noise ratio (SNR) of a given Raman system increases as the Raman pump power increases. There is thus a competition between the desire for a high SNR system, providing rapid spectral acquisition for fast chemical analysis, and the desire for an eye-safe system. A second trade-off is with regard to the etendue of the spectrometer: a spectrometer with a large etendue can stare at a wider area on the sample. This is often used to improve eye safety by widening the Raman pump spot size on the sample, thereby reducing the radiance. However, increasing the etendue of the spectrometer generally reduces its resolution. We present here a system that allows to maintain a small etendue (e.g. smaller than 0.2 mm2) and corresponding potential for high resolution, together with a high Raman pump power (e.g. larger than 5 mW) for good SNR and fast spectral acquisition, while reducing the risks of ocular damage.

A high radiance on the sample is also sometimes associated with explosion or ignition risks of the material analyzed. Alternatively, the sample may heat up so much under the high radiance of the Raman pump that it emits an incandescent glow that interfere with or altogether prevent the measurement. From this point of view, there is also a need to reduce the radiance on the sample, which is addressed by the system presented here.

If the laser spot is scanned on a sample over a scan area with a high enough speed, the effective ocular laser spot size approaches that of the scan area, significantly increasing the threshold for eye-safe laser operation. (The scan is an areal scan that probes a finite volume or layer of material since the beam has a finite depth of focus and there is scattering inside the sample.) Whether this effective average exposed area or the instantaneous scanned spot should be considered depends on the exact scan parameters and may be evaluated as outlined in the laser safety standards. The instantaneous spot size, scan area, and scan speed, as well as the divergence of the laser beam can be optimized or selected to produce such enhanced eye-safe standoff Raman system. In particular the scan pattern can be optimized for homogeneous illumination to prevent “hot regions” in the scan area. If the laser spot dwells for too long in a region, the ocular damage threshold can be reached, which means the irradiance on the retina can exceed the threshold for damage, for example thermal damage. Empty or hollow scan patterns can have small effective scan areas. For example, a fast circumferential scan produces an effective scan area which is approximately the product of diameter of laser spot and the circumference of the scanned circle. If the inner area of the circle is scanned as well, the total scan area (and hence the effective spot size) may approach the area of the circle, which is larger than the effective area of the circular scan.

By properly choosing the scan parameters, as explained below, to produce large effective area for ocular damage, the temperature rise on the sample surface is also reduced. Both ocular damage and sample temperature rise are due to concentrated laser power dwelling for sufficient time on the same spot. The thermal diffusion of retinal tissue would likely be different than the thermal diffusion of the sample but in both cases a reduced temperature rise can be obtained with the use of appropriate scanning.

Eye-safe lasers are typically understood to be lasers that emit less than 5 mW of output power (e.g., class 1, 2 and 3R lasers, but not class 3B lasers), Eye-safe lasers can be operated without special safety training. Class 3R instruments are considered eye-safe when handled carefully and have a small hazard potential for accidental exposure. Some commercial laser pointers are class 3R instruments.

More generally, the devices and methods presented here lower the eye safety classification of a given system using laser beam scanning. For example, a class 3B system can be transformed into a class 3R system using laser scanning. Or a Class 3R system may be transformed into a Class 1 system. Or a Class 4 system may be transformed into a Class 3B system.

While the inventive devices and methods are presented here in the context of Raman spectroscopy, implying the use of a pump laser illuminating a sample, and whereby the scattered light is collected by an optical system and focused into a spectrometer entrance slit, the inventive devices and methods are applicable to other laser spectroscopy techniques since they address in general increasing eye safety for instruments requiring the illumination of a sample with a bright focused beam of light. Other forms of Raman spectroscopy (coherent Raman spectroscopy, resonant Raman spectroscopy, difference Raman spectroscopy, spatially offset Raman spectroscopy, surface-enhanced Raman spectroscopy, etc.) can benefit similarly from the devices and methods described here as well as other laser-based techniques, such as laser induced fluorescence spectroscopy.

The devices and methods presented here are applicable to laser-based instruments in general. They can be implemented without an enclosure or shield to prevent the beam from exiting a certain perimeter. They are of particular utility for standoff instruments with a certain length of open air (e.g., >2 cm) between the instrument and the sample. The technology presented here involves laser beam scanning, together with the implementation of fail-safe monitoring and automatic shut-off mechanisms. It allows the use of higher power laser than would be otherwise allowed without laser beam scanning, within a certain laser class.

An inventive Raman system may have a standoff distance larger than 2 centimeters. The system includes several features designed to mitigate the unique challenges due to the increased standoff distance, and the interference of ambient light and the signal variations resulting from the operator's hand movements, as well as challenges common to most Raman systems, such as sample fluorescence, risk of sample explosion or ignition, and eye safety risks.

An example Raman system can be implemented as a handheld standoff instrument, a Raman microscope, or a general chemical analysis tool, for example, used to monitor chemical reactions, identify materials, confirm a material composition, or quantify relative concentrations in a mixture of several materials. It may have one or more of the following features:The system may measure a sample located at a distance greater than 2 centimeters from the instrument;The system may allow for handheld operation and thus assume a dynamic signal, within particular variations in ambient light background, sample fluorescent background, as well as Raman signal during the measurement;The system may use at least two Raman lasers in a difference Raman mode of operation to mitigate the effects of sample (or substrate) fluorescence. A tunable laser may be used instead of several lasers since the spectra are acquired successively with different Raman pump wavelength;The system may also use difference Raman technology to remove the background spectrum due to blackbody emission from a hot sample;The laser or lasers used as Raman pump laser may be uncooled, allowing to reduce the system weight and power consumption. A small amount of the Raman laser light may be leaked in the Raman probe towards the spectrometer, causing the zero-Stokes shift line (Rayleigh scattering) to be visible in the Raman spectrum. The zero-Stokes shift line provides a reference to compute the Stokes shifts even if the Raman laser wavelength varies over time;The system may use short integration frame times with interleaved Raman frames and background frames to provide robustness against signal variations;The system may be compatible with laser beam scanning, which increases eye safety of the instrument and reduces the risk of sample ignition or explosion for a given laser power output and spot size;The timing of the different processes (Raman laser modulation or switching, spectral acquisition, laser beam scanning) may be synchronized and chosen in a manner aimed at reducing or minimizing measurement fluctuations due to laser beam scanning or potential ambient light modulation;The system may include a camera imaging the sample;The system may use the visual information from the camera for image stabilization;The system may allow the user to indicate a feature of interest, for example, by pressing a touchscreen displaying an image of the area surrounding a sample, to indicate a desired measurement spot. The system can lock onto this spot, e.g., by automatically steering the telescope optics to focus the Raman probe to that spot and maintaining that aim. The system maintains this aim by correcting automatically for the hand movements of the operator using information from the camera.The system may have an adjustable focus distance;The system may have an autofocus system to automatically adjust the focus distance and increase or maximize Raman signal;The auto-focus system may allow for an offset setting so that the system may be focused a short distance away from a first scattering surface;The auto-focus may be able to detect multiple interfaces along the optical axis and sequentially tune the instrument focus to realize measurements at or near these interfaces, or at any point between consecutive interfaces.

Unless noted otherwise, the laser safety considerations presented here are derived from the 2014 revision of the IEC 60825-1 standard since US Federal Laser Notice 50 allows classification according to IEC 60825-1, instead of compliance with 21 CFR 1040.10 and 1040.11. However, since these eye-safety standards refer to the same bio-physical constraints, they tend to present similar derivations and the strategy presented here is applicable in general to increase eye-safety of a laser-based instrument. Details of some bio-physical considerations underlying the eye-safety standards can be found in Francois C. Delori, Robert H. Webb, and David H. Sliney, “Maximum permissible exposures for ocular safety (ANSI 2000), with emphasis on ophthalmic devices,” J. Opt. Soc. Am. A 24, 1250-1265 (2007), which is incorporated herein by reference in its entirety.

It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. All combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein.

