The Laser Diode as a Light Source for Atomic Absorption Spectroscopy

Introduction The development of laser diodes has increased the variety of practical applications for lasers as light sources in analytical spectroscopy. Lasers are excellent radiation sources due to their monochromaticity, good beam focusing capabilities, and small line-widths but have traditionally been expensive and difficult to operate, limiting their applications to specialized fields. Because the stability of the light source for atomic absorption spectroscopy is critical, tunable dye lasers and optical parametric oscillators are not well suited despite their high power densities. Due to their low cost, tunability of several nanometers, stable output power, compactness and ease of operation, diode lasers are finding an increasing number of uses in analytical spectroscopy. Some of these applications include laser-enhanced ionization spectroscopy, laser-excited atomic fluorescence spectroscopy, resonance ionization mass spectrometry, and specifically, atomic absorption spectroscopy. Diode laser technology is based on the optical emission of semi-conducting materials when population inversions are created with applied electrical potentials. The emission wavelength is mainly dependent on the semi-conducting material (Table 1). Diode lasers used in analytical spectroscopy are typically small (300 µm x 300 µm x 150 µm), have low power (mW), and convert electrical power to produce optical radiation with high (10-30%) efficiency. Currently, diode lasers used for analytical spectroscopy are commercially available in the 630-1600 nm range. Second harmonic generation using non-linear crystals can yield 0.1 µW between 335 and 410 nm and as much as 1-3 µW in the 410-430 nm range. Due to poor conversion efficiencies, power losses are significant 3 with harmonic generation. Elemental transitions with energies matching ultraviolet photons energies will be accessible by sum frequency generation, a nonlinear photon conversion technique that will be able to yield ultraviolet radiation in the 10-100 nW range in the near future.[1] The limited wavelength ranges available with diode lasers, along with the poor conversion efficiencies of nonlinear optics limits the elements accessible by diode laser atomic absorption spectroscopy. Devices emitting in the yellow, green and blue regions have been developed but have had problems with short lifetimes (sometimes minutes or hours). Robust diode lasers emitting in the blue region are becoming commercially available.[2] For example, Melles Griot has recently introduced a violet diode laser with output at 405 nm and 440nm, producing 6 mW and 1 mW of power, respectively. When atoms combine to form a solid (in this case, the semi-conducting material in a diode laser), the previously distinct energy levels of …