Over the past several years, Raman spectroscopy has become a popular tool for scientific research, industrial development, and process control in a wide variety of chemical, pharmaceutical, and bio-medical applications. However, the lack of a near-infrared laser with the necessary combination of performance, lifetime, reliability and cost of ownership characteristics has limited the use of Raman in the production environment. This limitation has been eliminated by a new external cavity 785-nm laser designed specifically to meet the needs of process Raman spectroscopy.
Laser sources
Because the intensity of the returned Raman signal is inversely proportional to the fourth power of the excitation wavelength, shorter wavelengths can provide stronger Raman scattering and better signal-to-noise ratio (SNR). As a result, frequency-doubled, diode-pumped Nd:YAG lasers with output at 532 nm are commonly used. The Raman shifted spectra resulting from 532-nm excitation fall within the peak responsivity region of silicon CCD detectors. Furthermore, the long lifetime and high reliability of these 532-nm lasers are particularly important for the process-control market.
Unfortunately, 532-nm excitation can cause sample fluorescence, which may swamp the faint Raman signal. Fluorescence occurs most commonly in complex organic molecules as found in polymers, pharmaceuticals, numerous synthetic products, and dyes.
The use of a near-infrared excitation source can eliminate sample fluorescence from most organic molecules. The wavelength of 785-nm has been found to be optimum for these applications, as it largely avoids fluorescence but still returns a Raman signal sufficient to enable detection by a CCD at a reasonable SNR.
Design requirements
Unlike the situation at 532 nm, however, until recently there were no commercial 785-nm lasers that achieved the combination of characteristics necessary for practical Raman with the ruggedness and reliability necessary for online process applications. In response to this situation, Kaiser developed a 785-nm laser (Invictus) specifically for fiber-coupled industrial process Raman spectroscopy. Design goals included long operating lifetime, elimination of periodic preventive maintenance, and suitability for industrial environments with ambient vibration and wide ranges of temperature and humidity. The laser also required a form factor compatible with existing equipment, and operational control through either a built-in keypad or through RS232 to a remote location. Optically, the design was targeted to deliver 200 mW into a 50-µm-core multimode fiber, and have a long-term wavelength stability of a fraction of a wavenumber (cm-1).
After analyzing the performance and failure mechanisms of several different products, Kaiser engineers settled on a design utilizing an external-cavity, 785-nm laser diode. Specifically, a high-power, broad-area laser diode is collimated and illuminates a reflection grating. The first diffracted order from the grating is directed back towards the collimator and re-images on the laser chip, providing strong external cavity feedback and forcing single wavelength operation. The zero-order diffracted beam forms the external cavity laser output, which is sent through a cylindrical telescope to reduce beam divergence in the slow axis of the diode.
This design enables the use of commonly available, mass-produced laser diodes run at a derated operating current, resulting in extended lifetimes. The laser resonator and cylindrical lenses are mounted on an actively cooled baseplate and sealed in a dry purged enclosure to further enhance lifetime and eliminate contamination. Active thermoelectric cooling helps stabilize output wavelength by stabilizing the mechanical dimensions of the external cavity structure. Cooling also enhances the operating life of the diode. Stability is further improved by the use of optical mounts specifically designed to eliminate mechanical creep.
Operational configuration
After exiting the sealed enclosure through a window, the beam then passes through steering optics, which include a holographic laser bandpass filter grating. The Fresnel reflection from this filter is sent to a monitor photodiode and used for active power stabilization. The first diffracted order, which contains nearly all of the light, is then directed to an optical isolator that blocks any retroreflections from a highly reflective sample, such as a polished silicon wafer, that can otherwise re-image to the diode and destabilize the laser.
The laser is normally configured with a fiber injector module, which focuses the laser into an optical fiber. The fiber acts as a spatial filter in conjunction with the holographic bandpass grating, rejecting any spontaneous emission from the diode that is not exactly at the lasing wavelength. The fiber injector module also is designed to accept accessories of interest in process applications, such as an electronic shutter, a laser sampling and tracking pickoff, and a fiber-breakage sensor.
The laser design provides variable output power from 0 to 200 mW into a 50-µm multimode fiber, with a linewidth of less than 30 GHz. Long-term wavelength stability has been measured at 0.02 nm (10 GHz) drift over 1000 hours. Thermal stability has been demonstrated at 0.0004 nm/°C. The laser maintains these specifications over 0°C to 50°C and from 5% to 95% relative humidity.
JIM TEDESCO is senior staff scientist and JOE SLATER is manager of new product development at Kaiser Optical Systems Inc., 371 Parkland Plaza, POB 983, Ann Arbor, MI 48106-0983; e-mail: [email protected].
Raman basics
When a material is illuminated, a small fraction of the light scattered by that material is shifted in frequency by an amount equal to the frequency of a molecular vibration. This frequency-shifted light is called Raman scatter. Most molecules have many different vibrations and produce Raman scattered light at many different frequencies. The spectrum of Raman shifted light, therefore, typically has several bands which can be related to molecular structure. The intensity of these bands is proportional to the number of molecules being observed. The physical state of the molecules can also influence Raman frequencies and intensities. As a result, a Raman spectrum can determine sample identity, concentration, and physical state (crystallinity, molecular orientation, for example).
Today, Raman spectrometers are available as rugged, compact, turnkey instruments. Their main functional elements are the laser source, fiber delivery system, imaging spectrometer (including a volume phase holographic notch filter and grating components), CCD camera and all associated control electronics. The Raman effect is weak—typically affecting one excitation photon in 108, so very high intensity illumination is needed to produce a measurable signal. Furthermore, since Raman scattered light is frequency shifted relative to the excitation, the illumination source must be monochromatic and stable in wavelength in order to reveal sharp Raman spectral features. Holographic notch filters are necessary to separate the weak Raman signal from much brighter unshifted illumination (Rayleigh scattered) light at the excitation wavelength.
Fiber delivery provides tremendous flexibility in how the device is implemented. In addition to remote operation, the modular fiber probes enable easy interchange between free-space delivery optics with various focal lengths and immersion probes for liquids, as well as simplifying coupling into a microscope.