Miniaturization pushes spectrometers forward

Applications that were once thought impossible using real-time, in situ spectroscopy are now nearly common place. From the bottom of the sea to the moon and from the rainforest to inside the human body, miniature spectrometers have revolutionized the measurement of light-matter interactions in the last 16 years.

Every user wins
An important factor in the spectrometer's success story was its suitability for mass production. In the past it could take one to six months to build a spectrometer, today hundreds of miniature spectrometers are manufactured by several companies in one week, with the benefits being felt by users in academia and industry alike.

Figure 1

Today's miniature spectrometers have, for example, liberated researchers from their laboratories and given them the opportunity to take measurements in the field (see figure 1). Industrial users have been quick to adopt the technology to lower their analysis and quality-control costs.

Figure 1a

Until the advent of this new technology, expensive laboratory spectrophotometers were commonly used to monitor the colour or chemical composition of a finished product. Today, several inexpensive spectrometer modules can be deployed on manufacturing lines for real-time monitoring and control of the process parameters. This modular approach to spectroscopy makes multiple-point sampling and redundant sensing much simpler to integrate into applications, which reduces per-sensor costs and results in more reliable data.

Original equipment manufacturers (OEMs) are the other major beneficiaries of this technology. The wide availability of mass-produced miniature spectrometers allows OEMs to add value by integrating these products into their own analytical instruments and optical engines. Impressive combinations of spectrometers, accessories and light sources all packaged and running on proprietary software have seen OEMs develop innovative products for many applications. These include everything from hand-held Raman chemical identification systems and devices to analyse nucleic acid concentrations, to new ellipsometer designs for thin-film measurements and simple colour analysers.

Early technology advances
The original miniature spectrometer (the S1000) was based on a 1024 pixel CCD detector initially designed for fax machines. The S1000 had the capacity to detect wavelengths from 200 to 1100 nm at resolutions down to 0.7 nm (FWHM) using a crossed Czerny-Turner design. The product could be configured with 14 different gratings, seven different slits and other optics, which resulted in more than 200 factory-set custom-configurations depending on the user application.

Figure 2

Figure 2 charts the resolution improvements that we have witnessed in fibre-optic miniature spectrometers from their invention to the present day. It is also important to note that the spectral ranges of these devices have improved with time.

The first devices were only able to detect wavelengths from 350 to 1100 nm due to the physical impediments of the CCD. However, major improvements in thin-film technology and optics allowed lenses and coatings to be integrated in the optical bench, which brought the detection capability down to 190 nm.

Figure 3

Different kinds of semiconductor-based detectors, such as InGaAs, have pushed the technology into the near-infrared range. Now, with the ability to combine several spectrometer modules, it is possible to create a multichannel system that can operate over the 190–2500 nm spectral range, as shown in figure 3.

Vacuum ultraviolet
The advance of miniature spectrometers is generally dictated by the development of new detectors and optics. Recent innovations and improvements in back-thinned CCD detectors, optical materials and thin-film depositing processes have been exploited to create a new generation of spectrometers capable of detecting deep and vacuum ultraviolet (VUV) radiation. These new VUV miniature spectrometers can detect radiation down to 153 nm.

The term vacuum in VUV comes from the need to evacuate the optical path of the spectrometer system by applying vacuum or purging the system with gases that do not absorb UV radiation. Due to the small size of the new systems, this evacuation procedure is now affordable and simple to do. It can be as easy as placing the spectrometer and light source inside a commercially available Ziploc plastic bag purged with a mixture of nitrogen and argon gases.

Today's new generation of miniature VUV spectrometers feature a 101.6 mm focal length optical bench with the typical Czerny-Turner design, an innovative back-thinned CCD detector and specially designed VUV optical components. The spectrometer incorporates a purging port to evacuate the optical bench of any VUV absorbing gases. Achieving this frontier is opening the door to similar substantial advances in applications that until today were restricted to fundamental research.

This new technology has the potential to impact on a number of applications, including spectroscopy where measuring the emission lines of halogen gases, carbon, phosphorus, sulphur and rare-earth elements can be problematic. It will also influence the field of lithography, due to this market's requirement for optical characterization in the VUV spectral range.

Another important user of VUV characterization is the semiconductor industry. Thin-film characterization through ellipsometric techniques will also benefit from low-cost measurements in the VUV spectral range. The main advantage here will be the possibility of placing the spectrometer directly in the ellipsometer's detection path, which until now was only possible using large, intricate and expensive devices.

Worldwide access
History is repeating itself and breakthroughs in optics and detectors are now coinciding with the latest advances in the telecommunication industry, such as wireless access. With the latest technologies, we are now able to deploy a network of spectrometers around the world, each collecting real-time information that is instantly accessible via the internet using either an Ethernet or a wireless connection.

These innovative spectrometers are small spectral sensors that consume little power and include built-in powerful microprocessors and communications protocols, which allow the user to control and send information over the internet. Such spectrometers can be used as a "web of sensors", which could have an enormous impact on process control, remote and unmanned research, with implications spanning everything from how we combat the outbreak of disease to what crops we plant and when.

Figure 4

Spectroscopy-based web systems like the Jaz (see figure 4) provide the framework for such networks. The Jaz has all the common advantages, such as multichannel capability, but also comes with innovative features such as a microprocessor and onboard display that eliminates the need for a PC.

The Jaz is made of stackable and autonomous instrument modules that make it simple to customize the system to changing application needs. Options include Ethernet connectivity for remote operation, several sources of power including rechargeable batteries and power over Ethernet and loadable application packages.

Recent Raman advances
Recently, miniature spectrometers have been adopted as optical engines in medical sensing devices. The spectrometers are used in Raman spectroscopic systems for non-invasive, real-time diagnosis of cancer in tissues ranging from tooth dentine to breast, skin and even the eyes.

Compared with other common techniques such as reflectometry and fluorescence, Raman spectroscopy provides detailed molecular and chemical information about the sample. Another advantage is that Raman can be used for in vivo measurements as it is non-destructive and has a better penetration depth compared with other methods. Proper chemometric models can be designed to differentiate important parts of the tissue such as collagen, fat or inorganic abnormalities such as calcium hydroxyapatite and calcium carbonate.

In other medical applications, Raman spectroscopy is used as an online process control sensor for blood dialysis monitoring. This technique allows the dialysis technicians to differentiate and monitor the amount of urea in blood while the blood is circulating through the dialysis machine.

Conclusion
Giant leaps in technology are continuing to build new functionality around a core spectroscopy technique – and this is just one example from a single manufacturer. Every day, optical designers are developing instrumentation that modernizes the miniature spectrometer industry and its applications.

The evolution of this technology will continue thanks to advances in photonics, optics and sensors. New detectors based on microelectromechanical systems and complementary metal–oxide semiconductor technologies are starting to appear in spectrometers opening the door for spectrometric imaging applications. Without doubt these advances will further revolutionize the field and will fulfill the ever-growing demand to understand and measure the interaction of light and matter.

Jorge Macho is the director of Total Technical Service at Ocean Optics. For more information, visit www.oceanoptics.com or e-mail jorge.macho@oceanoptics.com.

• This article originally appeared in the September 2008 issue of Optics & Laser Europe magazine.

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