Laser Spectroscopy

Lasers have enormous potentials for applications in spectroscopy – in several respects:

• They can exhibit a very large degree of temporal coherence. This is related to a narrow linewidth – in extreme cases of precision spectroscopy, sub-hertz linewidths are used. Note that a narrow line with does not only help in terms of measurement resolution, but also for improving sensitivity. For example, the absorption related to a specific absorption line is more easily seen if it applies to all the probe light and not only to a part of its spectral content.
• The spatial coherence of laser light is usually very high, and this leads to further improvements. For example, one can use multipass gas cells, where a large number of round trips can be realized only with a collimated beam having a low beam divergence.

• Laser wavelengths can often be tuned (→ tunable lasers).; this can be used for highly precise scanning of optical transitions. Some lasers can be tuned quite rapidly (→ wavelength-swept lasers) for quick acquisition of spectra.
• Some lasers can provide rather high optical powers; particularly pulsed lasers can generated very high peak powers. This is relevant, for example, for laser-induced breakdown spectroscopy.
• Ultrafast lasers can generate ultrashort pulses, which also have various interesting properties for spectroscopy. For example, they allow for time-resolved spectroscopic measurements with an extremely high temporal resolution. Also, extremely precise frequency measurements can be done based on frequency combs.

In the following, we consider a range of different techniques of laser spectroscopy, which utilize different properties of laser light. Note, however, that laser spectroscopy is a huge field of research and applications, making it difficult to provide a complete overview.

Laser Absorption Spectroscopy

Frequency Comb Spectroscopy

Raman Spectroscopy

Photoacoustic Spectroscopy

Ultrafast Laser Spectroscopy

Laser-induced Breakdown Spectroscopy

Due to the enormously wide range of methods for laser spectroscopy, there is also a wide range of different laser sources which are used for such purposes:

• Small single-frequency laser diodes can be used as cheap and compact wavelength-tunable sources. The emission wavelength is often tuned simply by varying the drive current, which affects the temperature. More refined schemes involve an external laser resonator containing wavelength-selective optical elements. Such external cavity diode lasers offer higher performance.
• Some broadband tunable solid-state lasers, such as titanium–sapphire lasers, chromium zinc selenide and sulfide lasers (Cr²⁺:ZnSe, Cr²⁺:ZnS, see chromium-doped gain media), Cr⁴⁺:MgSiO₄ (forsterite) lasers and erbium-doped fiber lasers can cover wavelength ranges of tens to hundreds of nanometers and often also provide a substantial output power and low laser noise. In some cases, Q-switched operation with the generation of nanosecond pulses is useful; important examples are laser-induced breakdown spectroscopy and LIDAR. High pulse energies combined with a narrow linewidth (normally in single-frequency operation) are often of interest.
• Fixed-wavelength solid-state lasers with relatively high power are used in Raman spectroscopy. A narrow linewidth is important.
• Mid-infrared radiation and terahertz radiation can be generated with quantum cascade lasers, often offering better performance and higher convenience than the previously often used lead salt lasers. Very wide wavelength regions are accessible with such sources.
• Mode-locked lasers, usually in the form of solid-state lasers, emitting trains of ultrashort pulses, are used for generating frequency combs (see above), often with highly stabilized optical frequencies.
• Dye lasers can access broad wavelength regions, particularly in the visible spectral range, but also in the near ultraviolet and near infrared. There are continuous-wave, pulsed and mode-locked versions.
• Optical parametric oscillators, pumped with various types of lasers in continuous-wave operation, with nanosecond, picosecond or femtosecond pulses, can often be tuned in very wide wavelength regions, and can access spectral regions which are not directly accessible with lasers. The extension to shorter wavelengths is possible with methods like frequency doubling and sum frequency generation, whereas difference frequency generation is a method for generating long-wavelength (often mid-infrared or even terahertz) radiation.
• Supercontinuum generation can be used as a method to generate broadband light as required for some spectroscopic measurements. This method provides enormously broad spectra, often in combination with high spatial coherence and a significant optical power.

This list of laser sources is not complete; even exotic types of lasers and nonlinear sources are sometimes used to meet certain requirements for spectroscopic applications.

A low level of laser noise is often important for precision spectroscopy. Various types of noise, such as intensity noise, phase noise (related to a finite optical bandwidth) or timing jitter, may be relevant. More or less sophisticated schemes for low-noise operation and for the stabilization of lasers are therefore often employed. These themselves often involve techniques of spectroscopy, as far as frequency stabilization is concerned. In extreme cases, a linewidth below 1 Hz is achieved.

Methods of laser spectroscopy are often used for detecting the composition of materials, often including quantitative measurements of concentrations. Some examples:

• Concentrations of trace gases in the atmosphere are measured e.g. with laser radar (LIDAR) methods in the context of environmental monitoring. Similarly, pollutants can be detected in water, and concentrations of medically active substances can be measured. Spectroscopic devices based on the analysis of laser-induced fluorescence are partly used even in space, e.g. for monitoring vegetation and optimizing agriculture.
• Biology, medicine and chemistry can profit from precise and rapid methods of material analysis, which are partly non-destructive.
• Typical security applications include the detection of explosives and drugs.
• The detailed composition of pieces of artwork like paintings can be analyzed e.g. for revealing their age and checking their authenticity.
• There are numerous applications in industrial process monitoring and quality control, and in fundamental research, e.g. the diagnostics of combustion processes.
• In various contexts, spectroscopy can be used for temperature measurements based on the temperature dependence of level populations.

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