In 1982, the Consultative Committee for Length (CCDM) of the International Bureau of Weights and Measures recommended several laser spectral lines as wavelength standards, such as the wavelength of 339 pm for the methane-stabilized helium-neon laser in a vacuum, and 633 pm for the iodine-stabilized helium-neon laser, also in a vacuum.
The International Bureau of Weights and Measures establishes standards based on the wavelengths of several laser spectral lines. By using a high-resolution interferometer to compare these wavelengths against established standards, highly precise measurements of light wavelengths can be achieved.
Overview of Light Waves
Light exhibits wave-particle duality (meaning a substance simultaneously possesses both wave-like and particle-like properties): in other words, when viewed at the microscopic level, it consists of photons and behaves as particles; yet, from a macroscopic perspective, it manifests as a wave.
The essence of light is an electromagnetic wave, and its color is determined by wavelength and frequency. Among visible light, violet has the highest frequency and shortest wavelength, while red light is exactly the opposite.
Infrared, ultraviolet, X-rays, and others all belong to the category of invisible light.
Infrared frequencies are lower than those of red light, and their wavelengths are longer.
Ultraviolet rays, X-rays, and other types of radiation have higher frequencies and shorter wavelengths than violet light.
Light waves are a type of electromagnetic wave.
Optical communication: A method of communication that uses light as the carrier wave.
Fiber-optic communication is a method of transmitting information using light waves as the carrier signal and optical fibers as the transmission medium. It operates in the near-infrared region, with wavelengths ranging from 0.8 μm to 1.8 μm—and corresponding frequencies between 167 THz and 375 THz. At the heart of fiber-optic communication systems are lasers (as light sources) and photodetectors, along with optical fibers themselves.
Classification of light propagation modes: Based on whether there is an electric field component or a magnetic field component along the direction of propagation, light can be categorized into three types. In fact, any light can be represented as a combination of these three wave types.
TEM wave: There are no electric or magnetic field components in the direction of propagation; hence, it is called a transverse electromagnetic wave.
TE wave: A transverse electric wave, characterized by having a magnetic field component but no electric field component in the direction of propagation.
TM wave: A wave with an electric field component in the direction of propagation but no magnetic field component is called a transverse magnetic wave.
The particular solutions of the wave equation in the form of simple harmonic waves are classified—based on how their amplitude varies with spatial position—into plane waves, spherical waves, and cylindrical waves.
Measurement of light wavelength
Wavelength refers to the distance a wave travels during one complete vibration cycle. Specifically, it’s the distance between two consecutive points along the direction of wave propagation that are in the same phase of vibration. The wavelength λ is equal to the product of the wave speed V and the period T, expressed as λ = VT.
The wavelength of a light wave is simply the light's wavelength.
Currently, precise measurement of optical wavelengths is primarily achieved by comparing high-resolution interferometers with established wavelength standards. Commonly used interferometers include the Michelson interferometer and the Fabry-Perot interferometer, among others. When measuring wavelengths with an interferometer, at a given optical path difference, the laser wavelength is inversely proportional to either the rate of change in interference order (as in the case of the Michelson interferometer) or to the actual interference order itself (as seen in the Fabry-Perot interferometer). Therefore, the wavelength ratio can be determined by accurately measuring either the interference order or the rate at which the interference order changes.
The principle of the precision wavelength measurement system is illustrated in the figure. In the diagram, Lo represents the optical wavelength standard, while Lx denotes the laser whose wavelength is being measured. The two laser beams are collimated and overlapped using a precise alignment device, ensuring their optical axes coincide before entering the interferometer. This setup generates two independent interference signals at the output of the interferometer. By processing these interference signals through a dedicated measurement unit, the system determines either the interference order or the corresponding change in interference order. From this information, the ratio of the wavelengths from the two lasers is calculated, and finally, the actual wavelength of the laser under test is determined by referencing the known wavelength value of the standard reference laser.       
Internationally, the speed of light has been defined as a constant, so the wavelength of light can also be determined by measuring its frequency and then using the formula λ = c/f. Here, λ represents the wavelength of light; f is the frequency of the light wave; and c is the speed of light.