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A New Clarity

Tue, 04/15/2014 - 9:16am
Paul Livingstone

A dynamic shift in how spectroscopy optics are developed and built is transforming the performance of scientific instrumentation.

The STS line of microspectrometers from Ocean Optics are designed specifically for OEM applications. Small size of less than 50-mm square and a weight of less than 70 g are achieved through CMOS-based optical architecture.Few areas of instrumentation have prompted as much development or efforts toward innovation as spectroscopy. Without sophisticated approaches to handling light, spectrometers as we know them would not function, and we would be without a deep understanding of the chemical nature of the world around us.

Microscopes, of course, depend on the power of optical magnification to reveal the domain of the small. But spectrometers are just as dependent on a variety of optical solutions, including gratings, mirrors, spectrographs and monochromators. The push to interpret radiative light energy as a function of its wavelength in order to learn chemical information has produced a dizzying variety of spectroscopic techniques, all of them supported by a number of key optical components.

In recent years, these components, which also include detector arrays, laser diodes and fiber optics, have been improved, miniaturized and standardized to cut cost. This has given way to a push for modular, miniaturized components that can be used almost anywhere while still providing laboratory-grade performance.

In spectrometry, modular means miniature
Ocean Optics, Dunedin, Fla., a leader in the OEM optics marketplace for spectroscopy, has helped lead the way toward more compact yet powerful optics solutions for vendors. Ocean Optics was at the forefront of development for the first miniature spectrometers, and has continued to expand and improve these offerings. Its latest example is the USB2000+, a general-purpose spectrometer for absorption, transmission, reflectance, emission, color and other applications. Compact enough to fit into the palm of a hand, this OEM-ready device covers the ultraviolet (UV) and visible light regions, from 200 to 1,100 nm on the electromagnetic spectrum. Based on fiber-optics technology, the spectrometer accepts light through a single strand and disperses it via a fixed grating across a linear 2,048-element CCD array detector. It accepts a custom wavelength range and optical resolution changes, and can be synchronized to other devices for triggering functions. Accessories such as light sources, probes, collimators and sample holders can be integrated into the light path.

Ocean Optics works with more than 100 OEM customers, and so has a formalized approach to supplying its optics and spectroscopy. According to Bill DeCosta, the Western U.S. OEM sales manager for Ocean Optics, more than 220,000 of the company’s optics products have been deployed worldwide, giving the company plenty of experience in how to work with and supply customers. A standard Ocean Optics spectrometer, he says, can be built in more than 40,000 configurations. Typical OEM product specifications that Ocean Optics can directly address include communications protocols, firmware and optical interfaces. This capability helps ensure consistency and ease of integration over the product lifecycle.

“If a more customized solution is desired, we can partner with a customer at several levels of integration, from modules that integrate customized optics, light sources and spectro-analysis, to full turnkey products,” says DeCosta.

A typical OEM supply item is the company’s line of STS spectrometers. Designed for handheld and point-of-use applications, the highly reproducible instrument is just a 50-mm2 square, weighing only 68 g. Despite the size, an optical resolution of about 1.5-nm FWHM makes it comparable to a number of larger spectrometers. Available in visible light, UV and NIR versions, the STS spectrometers all make use of a Panavision-made 1,024-element CCD array. Ocean Optics applies a coating to this detector, which allows them to vary to the optical sensitivity.

Aside from size, the attraction to OEMs for this instrument is the interface. Power can be supplied through USB, or through a GPIO port. Because the STS is computer-controlled, all operating parameters are specified through software.

Component innovations
Some technologies that seemed impossible just a few years ago are now finding their way to the OEM marketplace. One prime example is the TAG Optics TAG Lens 2.0, an R&D 100 Award-winning varifocal device that uses acoustics to shape light. Designed to change focus by using the small density changes that occur in fluids exposed to sound waves, this lens offers a substantial increase in focusing speed. It also promises reliability because it has no moving parts.

At Photonics West 2014, TAG Optics launched the beta version of its TAG Lens HP. Similar to the TAG Lens 2.0, the new device is specialized for laser material processing applications and can accept more than 100 W of power density.

According Christian Theriault, president and CEO of TAG Optics, Princeton, N.J., several trends apparent in the photonics and optics industries continued to make their presence felt at Photonics West: the rise of solid-state/LED lighting for illumination, the use of ultra-short pulsed lasers instead of continuous-wave lasers, adaptive lenses instead of motion systems and new resolution/tunable detectors for hyperspectral imaging and spectroscopy.

“Additionally, I do believe there is a slow, but constant shift to what I would call ‘smart optics’ as the industry gains more trust and exposure to these non-traditional components,” says Theriault.

With a width of 22 mm and travel ranges up to 26 mm, the piezo-actuated LPS-22 linear stage is one of the smallest available on the market. Manufacturing advances like these have helped OEMs make their instruments more compact. Image: PIThe optical component industry is also heavily supported by a number of OEMs who specialize in support mechanisms for the core light-handling components. These include makers of enclosures, power supplies and positioning tools.

Physik Instrumente (PI), Auburn, Mass., has been filling the needs of positioning for decades, and has an extensive portfolio of miniature piezoelectric positioning stages that can align optical components, gratings and lenses, such as those used in spectroscopy and microscopy. The company recently introduced a new generation of positioning products that are driven by a ceramic motor and use no energy to hold a position, meaning there is less heat dissipation and better stability on a microscopic and nanoscopic level.

The new miniature stages are available for linear and rotary motion—with linear travel from 0.25 to 1.0 in—and provide resolution down to the nanometer range. They can be stacked for multi-axis arrangements and controlled from a computer.

According to Stefan Vondran, VP for marketing at PI, one of the key drivers for recent improvements in his company’s products has been the miniaturization of optics, which requires smaller and high-precision mechanisms for test and assembly.

“The need for smaller and higher-precision mechanisms has driven our R&D department to come up with new motion technologies, more powerful control and alignment algorithms and smaller positioning products,” says Vondran.

PI’s piezo mechanisms and 6-axis hexapods provide the precision and flexibility. PI hexapods, Vondran says, can move in all six degrees of freedom and allow the user to program the center of rotation anywhere in space: at a focal point of a lens, at the waist of a beam or the tip of an optical fiber.

Optics transform mass spectrometry
Plug-and-play optics don’t always work for scientific instrumentation, and this holds true not just for customized or single-purpose devices built in a single laboratory. High-volume instrument vendors also work to develop their own optics solutions, which they standardize for use throughout the instrumentation line, or license for outside manufacture.

For laboratory researchers, optics components are indispensable in two key instrumentation families: spectroscopy and microscopy. While microscopy relies on more traditional optics solutions—including lenses, beamsplitters and photomultipliers ubes—spectroscopy requires gratings and mirrors. Both also use imaging solutions including charge-coupled devices (CCD) and CMOS-based detector arrays.

For spectrometer development, miniaturization is playing a key part in how vendors’ R&D efforts are being focused. Handheld Raman imaging instruments have been on the market for several years now, and high-performance mass spectrometry techniques are about to make an appearance as well.

Advances in optics technologies are also a major enabling factor for improved performance in mass spectrometry, particularly with regard to detection range. This is an important feature of infrared spectrometers because it allows the instruments to perform a wider variety of measurements without the need for additional analysis or secondary equipment, such as additional beamsplitters or detector exchanges.

At the same time, this performance must be carried over to instruments that have been considerably reduced in size. Recent introductions in the product line at BaySpec Inc., San Jose, Calif., reflect this balance of development. A company that specializes in the type of spectral optics required in spectrometers and Raman imaging instruments, BaySpec introduced several new imaging products at Pittcon 2014 that rely on optics technologies developed in-house.

For the benchtop, a new system was introduced that combines near-infrared (NIR) spectroscopy with Raman imaging. NIR light sources penetrate samples deeper and greatly reduce noise from fluorescence and other background lights, as compared to visible light sources. Both Raman and NIR spectroscopy are non-destructive. NIR spectroscopy is based on absorptions or reflections in the NIR spectral region by molecular bond vibrations from the sample when illuminated by a white light source. Raman spectroscopy is also based on the molecular bond vibrations, but the Raman system instead disperses stokes-shifted photons scattered from the sample when illuminated by a single-wavelength laser.

The combination of a new plasma torch design, high-density holographic grating and 1-m focal length optics combine to provide high-resolution, full-wavelength coverage for a wide range of elemental analyses. Image: Horiba ScientificBaySpec’s NIR/Raman Amphi-Spec System is the first dual-band combined NIR/Raman system that can perform transmission, reflectance, attenuated total reflection and customized Raman probing. Two core components that make this instrument possible are the SuperGamut spectral engine and the PeakFinder fiber-optic probe. SuperGamut employs a highly efficient Volume Phase Grating (VPG) and a deep-cooled -60 C ultra-sensitive CCD or InGaAs array detector, providing high-speed parallel processing and continuous spectral measurement. The PeakFinder fiber-optic probe features optical filtering of 106 for efficient attenuation of the Rayleigh line for background-free Raman spectra. The probe is lightweight, compact and flexible. It can be readily used with a sample holder for routine measurements of liquids and solids, or simply as a point-and-shoot approach. The Rayleigh filtering can be turned on and off to switch between Raman and NIR spectral measurements. All components in the system utilize polymer-encased fiber-optic cables with standard connectors for coupling to various light sources, lasers and spectrographs, permitting flexible sample configurations.

More integration is on display with BaySpec’s Dual-Band Agility Raman Analyzer, a spectrometer that, for the first time, integrates a computer and sampling chamber into the instrument. The design, which has self-aligning sample holders, allows the analyzer to occupy a small footprint. With battery power, analysis can be performed in any location in the laboratory. With dual wavelengths, the analyzer has a wide range of potential applications, from homeland security to food and beverage analyses to quality control.

The instrument that most benefits from miniaturization design efforts at BaySpec, however, is the family of OCI hyperspectral imagers. These NIR instruments are the first handheld hyperspectral imagers weighing less than 1 lb.

Available as the OCI-1000 and OCI-2000, the imagers considerably improve the identification and classification of objects over today’s RGB or filter-based cameras. Snapshot (non-scanning) hyperspectral/multispectral imaging is a method of capturing spectral images during a single integration time of a detector array, so that no scanning is involved. One of the major advantages of snapshot imaging is it avoids motion artifacts, thereby simplifying the data processing and improving the image processing time.

In the OCI-1000, this capability is even further miniaturized, says Bergles, by integrating the hyperspectral imagers onto a sensor at the chip level. This eliminates bulky, complex optics formerly required on this type of imager. The result is an imager that weighs less than 0.5 lb and can be installed on conveyor belts in productions lines, or even on unmanned aerial vehicles.

According to Bergles, the availability of large-format, low-cost optical dispensing elements and detector arrays makes it now possible to bring hyperspectral imaging results to a wider audience of customers. He envisions them eventually being used in outpatients medical clinics for point-of-care diagnostics.

“OCI is a complete hyperspectral imager, not just a sensor. It contains the complete system for data acquisition, data process and data presentation,” says Bergles.

Advanced optical design elements, a cooled CCD and polymer-encased fiber optics have allowed BaySpec to recently introduce a high-performance spectrometer that can perform both Raman and NIR absorption/reflection spectroscopy in a single instrument. A variety of custom lenses are available that allow wide-angle to close-up views for enabling real-time hyperspectral/multispectral imaging at video rates. This opens numerous applications for the OCI family: colorimetry, process automation, forensic analysis and food and agricultural inspection.

In addition to miniaturization, the growing availability of full-wavelength optics is affecting the product lines of many spectrometry vendors. At Pittcon 2014, Horiba Scientific, Edison, N.J., introduced the Ultima Expert ICP-OES, which features a new optical design that integrates a high-density holographic grating with 1-m focal length optics. With thermal stabilization measures employed to help retain full long-term sensitivity, the optics help allow the Ultima to achieve a wavelength range of 120 to 800 nm, which extends its capability into the far UV.

Inductively-coupled optical-emission spectrometry (ICP-OES) is based on the spontaneous emission of photons from ions and atoms that have been excited in a radio frequency discharge. An inductively-coupled plasma (ICP) vaporizes the sample in aerosol form, and liberated analytes release photons as they relax to ground state. Samples can be directly injected if they are liquid or gas, but solids must be extracted or digested to reveal the analytes. The method is often used for trace element analysis because it delivers rich spectra, and it’s often used when a dispersive technique, such as atomic absorption spectrometry, can’t suffice.

Optics are a key component for this system, which collects a portion of the photons emitted by the ICP using the lens or concave mirror. This focusing lens forms an image of the ICP on a monochromator, which then selects the wavelength. A photodetector is typically used to convert this information to an electrical signal. The 70 elements commonly determined by the technique can produce as many as 70,000 emission lines or more in the 200- to 600-nm wavelength range. The spectral option requires elevated resolution, and this is typically achieved using plane grating monochromators with large focal lengths of 0.5 m or more.

Interference effects are induced using ruled gratings. Not long ago, resolutions of 0.01 nm were typically possible through a combination of gratings of various construction. Today, high-end ICP-OES achieves picometer-scale resolution in part because of advanced construction techniques that can produce high-density interference gratings with thousands of grooves per millimeter. In Horiba’s Ultima, a 2,400-g/mm grating is used in the first- and second-order optics. In combination with the 1-m focal length optics, it can deliver an optical resolution of less than 5 pm in the 120- to 320-nm range, and less than 11 pm in the 320- to 800-nm range. Optionally, a dual-grating system can be equipped to the Ultima, which features 4,320-g/mm gratings in the first order and 2,400 g/mm in the second order for less than 6 pm for 120- to 450-nm wavelengths and less than 11 pm in the 450- to 800-nm range.

A combination of the high-density holographic grating design, long focal length optics and the Total Plasma View system, which employs a vertical plasma torch design to permit radial viewing and lower matrix effects, the Ultima system can produce detection limits as low as 1.5 µg/L for potassium, 06 µg/L for sodium and 0.2 µg/L for chromium. The system’s far-UV capability makes it suitable for intensive halogen analyses in addition to general element analysis.

“As with prior Ultima ICP-OES systems,” says Matthieu Chausseau, ICP product manager for Horiba Scientific, “the new system has been designed for robust applications: mining, salt production, oil and petrochemical analysis and general metallurgical and chemical manufacturing.”

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