What are Optical Wavelength Meters?
The optical wavelength meter, also known as an optical wavemeter, is a device used for the accurate measurement of wavelength. It has an interferometer-based approach where the two beams derived from the incident light are recombined to form a fringe pattern. Analyzing this pattern along with the pattern formed by a reference beam allows precise determination of the wavelength. It can measure the wavelength of both pulsed and continuous-wave optical signals. Precise determination of wavelength is very important for optical communication, spectroscopy, sensing, and metrology applications.
There are two types of wavemeters, i.e., scanning wavemeters and static wavemeters with no moving parts.
The scanning wavemeters work based on the Michelson interferometer setup. The system has a fixed length arm with a fixed mirror and a variable arm whose length can be varied smoothly using the movable scanning mirror. A beamsplitter is used to split the incident beam between the fixed arm and the variable arm. Both beams after reflection from the respective mirrors recombine at the beamsplitter to produce an interference pattern at the detector.
The wavemeter calculates wavelength using the equation: mλ = 2d, where m is the number of fringes in the interference pattern, and d is the displacement of the scanning mirror. Here, the displacement, d is an unknown value. To accurately measure it, a reference beam, whose wavelength is known, is introduced into the setup. The fringe pattern produced by the reference beam is analyzed to calculate the displacement, d of the scanning mirror. This value is then used to determine the wavelength of the incident beam using the above equation.
A static wavemeter has a Fizeau interferometer with two reflective plane surfaces at slight angles of a few arcsec forming a wedge. The incident laser beam illuminates the system and produces a parallel fringe pattern. The minima in the fringe pattern corresponds to the point where the integer multiple of the incident wavelength equals to the round-trip path between the reflective surfaces. This integer number denotes the order of the interference and hence the spacing between the fringe minima, i.e., its period, depends on the incident wavelength. So, by observing the fringe pattern, the incident wavelength can be calculated.
The wavemeter captures the fringe pattern using a photodiode array and analyzes it to determine the fringe period; thereby computing the wavelength of the incident beam. Fizeau interferometer-based wavemeters have better wavelength accuracy compared to scanning interferometers as the former have no moving parts.
Dense Wavelength Division Multiplexing (DWDM) systems
Stabilizing the operating wavelengths is very important for the proper working of DWDM systems. Wavemeters are used for accurate wavelength analysis and calibration of active DWDM components like transmitters or optical sources such as DFB lasers, tunable lasers, and VCSELs. It is also used for its optical performance monitoring and for optical spectrum analysis of other passive DWDM components.
Wavemeters in optical Systems
Optical wavelength meters are used for the characterization, calibration, and inspection of various optical systems like optical sensors, tunable lasers, laser diodes, and optical transceivers. Using wavemeters along with tunable lasers can detect and avoid undesired wavelength tuning or fluctuations.
For Sensing Applications
Optical wavemeters can be embedded into the fiber as in-line fiber optic wavemeters which helps in sensing or monitoring applications. They can be used for pressure, temperature, and strain sensing.