DIODES STORM THE TUNABLE LASER RANKS

Low-cost, rugged, compact sources of tunable laser light support the rapid growth of applied spectroscopy, from fetal monitoring to fruit sweetness quantification.

Tunable lasers first became commercially available around thirty years ago, revolutionizing the field of spectroscopy. Since that time, these lasers have enabled an ever-widening range of diverse applications, from laser-induced fluorescence of biopolymers to Doppler-free measurements of atomic hyperfine structure. Until very recently, however, the cost, size, and complexity of tunable laser systems have limited their use to the research laboratory, whereas lamp-based spectroscopy has long been appropriately packaged for field use. This is unfortunate, as laser-based spectroscopy is generally far superior, and can often provide an excellent answer to the ever-growing demand for sensitive, noninvasive, real-time diagnostics.

The situation is now changing due to the development of the tunable diode laser , a completely new type of tunable laser source. These rugged, compact lasers are typically only 1/20th the size of traditional tunable laser systems; they offer true turnkey simplicity for 1/10th the cost of their complex predecessors, yet deliver equivalent or better performance. Based on the simple semiconductor chips used in telecommunications and compact disk players, these lasers are now opening whole new areas of applied spectroscopy, from quantifying the sweetness of fruit to monitoring fetal brain oxygenation during delivery.

Tunable Diode Lasers

Laser diodes are small, monolithic solid-state devices that efficiently convert electricity into laser light. Manufactured by standard semiconductor fabrication techniques, they offer low cost in high volume. In addition to their long lifetime, they require only a low-voltage power supply and no water-cooling. Laser diode chips are now available at most wavelengths between 630 nm and 2.4 microns, and most of these devices are capable of emitting laser radiation over several tens of nanometers. In principle, therefore, laser diodes are virtually ideal sources of tunable laser light in the red and near-infrared spectral regions.

As with any tunable laser medium (e.g., dye, titanium sapphire), the wavelength of a laser diode must be actively controlled. Otherwise, the output will be unstable, consisting of a rapidly varying cluster of wavelengths (modes) centered on the wavelength of highest gain. With laser diodes, this control is best provided by utilizing an external cavity that incorporates wavelength-selective feedback. Tunable monolithic devices, such as DFB (distributed feedback) lasers, are available for telecommunications systems, but their scan range is generally far too narrow for most spectroscopic purposes.

Figure 1 schematically illustrates the main elements of an external-cavity diode laser, Newport's Model 2010 series tunable diode laser. The output wavelength is controlled by a diffraction grating. In this so-called Littman-Metcalf cavity, the wavelength is adjusted by rotating the tuning mirror, thereby effectively changing the feedback angle of the diffraction grating. Using this approach, the series 2010 lasers from Newport Corporation deliver a linewidth of 100 kHz and a maximum scan speed of 25 nm/sec, yet the entire laser head measures less than 175 cm x 100 cm x 75 cm. The power supply is similarly compact.

Two other aspects of this design merit special mention. First, the laser beam always exits the cavity at the same angle, no matter what the output wavelength. Among other benefits, this permits highly efficient fiber coupling. Just as important, the laser diode and collimating lens are assembled in a low-cost module. Kinematic "plug-and-play" mounting enables simple field interchange of these modules, allowing the wavelength range to be quickly changed without any cavity realignment.

Taking to the Field

While these lasers are fast replacing traditional tunable lasers for many types of demanding laboratory experiments, it is in field applications that they are having their most revolutionary impact. These applications include those that formerly used lamp-based spectrometers, as well as new applications that were hitherto impossible.

All these applications involve some type of absorption measurement, and a high-performance tunable laser offers several important advantages over a lamp-based spectrometer in this area.First,these lasers provide extremely high spectral brightness, over 30,000 times greater than a typical lamp spectrometer. Together with the low amplitude and frequency (wavelength) noise, this brightness allows weakly absorbing systems to be measured with unprecedented signal-to-noise ratio and short data-acquisition times. In addition, the laser's coherence allows a whole new type of absorption measurement, using a so-called "ring-down" cavity.

For applications that only require measurements on a single absorption feature, such as sugar in fruit (or blood), the laser is far superior to the use of two or three fixed filters. Specifically, its fast tuning allows these weak absorption signals to be distinguished from background variations, due to scatter and other phenomena, by wavelength-derivative measurement.

The range of applications that has been developed, or is under development, is already amazingly broad. A few examples include trace-gas analysis to monitor combustion emission, toxic gas leaks, and moisture in gases used for semiconductor fabrication (to the parts-per-billion level and beyond). In medicine, these lasers are being used to noninvasively monitor the oxygen level in a fetal brain during birth (by measuring the hemoglobin/oxyhemoglobin ratio in the 750-nm spectral region) and to characterize the state of arterial plaque. In agriculture and foodstuffs, measurement of hydrocarbon- and/or water-absorption features enables everything from quantifying dry matter in onions to determining water content in long-shelf-life baked goods.

To more fully appreciate the benefits of tunable diode lasers, it is useful to examine two of these applications, namely trace-gas analysis and agricultural produce, in a little more detail.

Two groups at the forefront of trace-gas spectroscopic analysis are Southwest Sciences Inc. (Santa Fe, NM) and Professor Kevin Lehman's team at Princeton University (Princeton, NJ). Mark Paige, a senior research scientist at Southwest Sciences, says that "of the gases that can be detected with diode lasers, trace-water-vapor detection holds the greatest industrial and government interest. With a multipass Herriott cell, we have measured sub-parts-per-billion water-vapor concentrations. This detection level is required for monitoring semiconductor process streams. We have also licensed diode laser technology to Ametek, Inc. for detecting hydrogen fluoride leaks in oil refineries and for monitoring ammonia in industrial stacks." He further notes that Southwest Sciences has measured absorbances as low as 10^-7 using diode lasers. "We're able to measure such small absorbances because diode lasers can be rapidly wavelength-modulated through current modulation. Second-derivative detection of the signal provides a background-free spectrum that also discriminates against light-scattering events."

(See Figure 2.)

Rather than use a long optical path or a multipass cell, Lehman's group is using a relatively new technique to measure ultralow concentrations: ring-down spectroscopy. Here the laser beam "bounces" between two high-reflectance mirrors that define a resonant cavity. When the laser is abruptly switched off using a modulator, the light intensity in the cavity has a characteristic decay, or ring-down, time. Even a small absorption within the cavity can measurably shorten this time. Here the laser's narrow linewidth is critical in maximizing signal-to-noise. With this method, Lehman has now measured water vapor in nitrogen below the parts-per-billion level, and is collaborating to commercialize this technique with Meeco Inc., Warrington, PA, a leader in moisture sensing.

Agricultural Innovations Inc., Athens, GA, is a company that specializes in the use of near-infrared spectroscopy for nondestructive, noninvasive characterization of agricultural products. Gerald Dull, the president, notes, "The selling price of bulk produce is determined by several established quantitative measurements. With melons and other fruit, it's sugar content, with onions it's percent dry matter, and with potatoes for the chip industry it's usually both these parameters." Until recently, Agricultural Innovations' product line was built around conventional spectrometers and halogen tungsten lamps, but they are now developing diode-laser-based alternatives. Dull explains, "We are dealing with high-optical-density, high-moisture products and a large dynamic signal range. It may seem amazing, but we have passed measurable light from a conventional spectrometer through a full-size, unskinned honeydew melon, but not with acceptable signal-to-noise. With a diode laser, however, even a large melon can be characterized with no damage." These prototype instruments use 2f detection of the 910-nm carbohydrate band and the water peak at 958 nm.

Thirty years ago, the advent of continuously tunable laser light began a revolution in research spectroscopy. Now rugged, compact, economical tunable diode lasers have begun a similar revolution in applied spectroscopy, and are already performing sensitive diagnostics and measurement in many fields.

For more information, contact the author of this article, Michael Lang, Product Manager, Newport Corporation (formerly Environmental Optical Sensors Inc.), 6395 Gunpark Drive, Boulder, CO 80301; mlang@eosi.com; www.eosi.com.