What is an Optical Spectrum Analyzer?

An Optical Spectrum Analyzer (OSA) is a device that measures and shows the amount of light being emitted at different wavelengths. It can split a light signal into different colors or wavelengths. This allows the user to see the different wavelengths comprising the signal, along with their respective intensities. The obtained information is displayed on a graph, where the horizontal axis represents the wavelength and the vertical axis represents the signal strength. 

With an OSA, it is possible to separate the different modulated signals that are transmitted through a single fiber using a dense wavelength division multiplexing (DWDM) system, so that each signal can be analyzed individually. This can provide important information about the interaction of signals with each other at different wavelengths.

Block Diagram of Optical Spectrum Analyzer

The incoming optical signal to be measured undergoes a process where it is transmitted through a wavelength tunable filter. This specialized filter has the ability to distinguish and separate various spectral components of the signal. After that, a photodetector is employed to convert the optical signal into its electrical equivalent. The magnitude of the resulting electrical current is directly proportional to the intensity of the incident optical power.

For further analysis, the current is then converted to an equivalent voltage utilizing a transimpedance amplifier. This voltage signal is then digitized by an ADC converter within the optical spectrum analyzer. The resulting digitized signal is used to represent the amplitude of the optical signal on the vertical part of the display unit.

In order to adjust the wavelength tunable filter, a ramp generator is employed. This ramp generator generates a ramp signal that serves the purpose of tuning the filter so that its resonant wavelength corresponds to the horizontal position being measured.

Types of Optical Spectrum Analyzers

There are several types of Optical Spectrum Analyzers (OSAs) based on different principles of operation. Some of the commonly used types of OSAs are:

• Grating-based OSA
• Fabry-Perot Interferometer-based OSA
• Michelson Interferometer-based OSA

Optical Spectrum Analyzer based on Diffraction Grating

The operation of an optical spectrum analyzer is mainly based on the principles of diffraction gratings, which are used to separate the different wavelengths in an optical signal. The device typically consists of a light source, a diffraction grating, and a photodetector. When light is incident on the grating, it is diffracted, and the diffracted light is focused onto a photodetector. The photodetector measures the intensity of the diffracted light at different wavelengths, producing a spectrum of the input signal. 

Optical spectrum analyzers that depend on diffraction gratings are often used for examining the spectra of lasers and LEDs. The resolving power of these devices generally spans from 0.1 nm to 10 nm. A tunable optical filter can be created by fitting monochromators with diffraction gratings in this particular type of optical spectrum analyzer. The monochromator breaks down the light into its individual wavelengths and allows only those specific wavelengths that are being targeted by the optical spectrum analyzer to reach its photodetector. The outcome of this process is enhanced resolution of the wavelengths being analyzed.

Optical Spectrum Analyzer based on Fabry-Perot Interferometer

Some optical spectrum analyzers use Fabry-Perot interferometers to analyze light. In a Fabry-Perot interferometer-based OSA, the input signal is directed into the interferometer, where it undergoes multiple reflections between the mirrors. These multiple reflections give rise to a series of interfering beams within the interferometer. The interference between these beams creates a pattern of resonances, which correspond to the different wavelengths present in the signal. The intensity of these resonances is detected by a photodetector, and the resulting spectrum is displayed on a screen.

They can only measure within a specific frequency range, and the results given have a limited amount of detail. These analyzers are good at measuring sudden changes in the color of lasers, but they may have limitations in terms of analyzing a wide range of colors compared to other types of analyzers. Sometimes, different colors of light can pass through the analyzer simultaneously, which can affect the accuracy of the measurements. To enhance the analysis of multiple colors, a special filter called a monochromator can be added after the analyzer.

Optical Spectrum Analyzer based on Michelson Interferometer

In a Michelson interferometer-based OSA, the input signal is directed into the interferometer, where it is split into two beams that travel different lengths before recombining. The interference between the two beams creates a series of fringes that correspond to the different wavelengths present in the signal. The intensity of the fringes is detected by a photodetector, and the resulting spectrum is displayed on a screen. 

The main difference in principles between optical spectrum analyzers based on fabry-perot interferometer and those based on michelson interferometer is the method used to separate and measure the wavelengths of light. Optical spectrum analyzers that use Michelson interferometers are used to make very precise measurements of the length of the coherence of light waves, as well as to accurately measure the wavelength of light. Direct measurement of coherence length is not possible with other types of spectrum analyzers.

Applications of Optical Spectrum Analyzer

Optical spectrum analyzers are used in a wide range of applications, including optical communication systems, fiber optic sensing, and spectroscopy. They are particularly useful in the testing and development of optical communication systems, where they play a critical role in characterizing the spectral properties of optical signals. These analyzers can be used to measure the center wavelength, spectral width, and power of an optical signal, as well as to detect the presence of any unwanted noise or interference in the signal. They can also be used in the development of biomedical instruments for the analysis of biological samples. It can help scientists and engineers develop new technologies and products.

In fiber optic sensing applications, OSAs can be used to measure the spectral properties of light reflected or scattered from a fiber optic sensor. This information can be used to determine the characteristics of the sensing element, such as its temperature, strain, or pressure.

OSAs are also used in spectroscopy applications, where they can be used to measure the absorption or emission spectra of a sample. Hence it can be used to identify the chemical composition of the sample or to monitor the progress of a chemical reaction.