Spectrum

The optical spectrum (or emission spectrum) of a light source or certain beams contains information about how light energy or power is distributed over different wavelengths. Usually, it is presented in the form of graphs where some of the spectral quantities are plotted as a function of wavelength or frequency of light. The amount plotted can be spectral flux, spectral intensity, or spectral radiance, but in many cases, the spectrum has no absolute scale. In some cases, people use (calibrated or uncalibrated) logarithmic scales, such as in dBm/nm.

Figure 1 shows the numerical simulation spectrum of a supercontinuous light source, a highly polychromatic light source. In contrast to very broad spectra, the spectrum of a single-frequency laser source typically has very narrow spectral lines – in extreme cases, a line width of 1 Hz, corresponding only to ≈ 3 * 10−12 nm (for a 1 μm center wavelength). The spectrum of other lasers consists of multiple lines, and some lasers (especially mode-locked lasers for ultrashort pulses) can have spectral widths of 100 nm or more, with frequency comb structures.

Spectra can be recorded with different types of spectrometers, such as spectrometers, which vary greatly in terms of the spectral range covered and spectral resolution. The spectrum is closely related to the temporal coherence properties of light. For example, the temporal coherence function completely determines the spectrum. The spectrum is also related to the Fourier transform of the electric field. Therefore, it is also known as the Fourier spectrum of the light field.

Optical bandwidth

Optical bandwidth essentially refers to the width of the optical spectrum. There are different definitions, but the one that is often used is the full width of the half-height (FWHM).

Measurement of the spectrum

Optical spectra can be recorded with an instrument known as an optical spectroscopy analyzer. They are usually a type of spectrometer, but there are other types of spectral analyzers based on completely different operating principles. For example, there are interferometry devices, using Michelson interferometers (→ Fourier transform spectroscopy) or Fabry Perot interferometers. It is also possible to combine spectral analysis with imaging.