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Optical Spectrum Analyzer (OSA): Function and Application

optical spectrum analyzer osa optical instrument dwdm fiber optic

This page covers an application note on the Optical Spectrum Analyzer (OSA). It describes the function of an Optical Spectrum Analyzer (OSA), including a block diagram, and also mentions popular vendors.

As we know, an RF spectrum analyzer is used to view and diagnose RF signals over frequency bands. Here, the X-axis represents electromagnetic radio frequency, and the Y-axis represents power. Similarly, for the measurement of optical signals, an Optical Spectrum Analyzer (OSA) is needed. It is basically an optical instrument.

It measures the properties of light over a range of the electromagnetic spectrum. The measured variable of an OSA is usually light intensity, but it can also be the polarization state.

Optical Spectrum Analyzer Figure-1 depicts Optical Spectrum Analyzer model number AQ6375B from Yokogawa Electric Corporation.

The main application of an OSA is to troubleshoot problems in DWDM systems. Refer to the DWDM tutorial to learn more about DWDM.

Optical Spectrum Analyzer Block Diagram Figure-2 depicts a simple block diagram of an Optical Spectrum Analyzer.

Functions of OSA Modules

Let’s understand the functions of each of the modules of the OSA:

  • Wavelength Tunable Filter: The incoming optical signal to be measured is passed through a wavelength-tunable filter. This optical filter resolves different spectral components individually.
  • Photodetector: The photodetector converts the optical signal into an equivalent electrical signal. The electrical current magnitude is proportional to the incident optical power.
  • Transimpedance Amplifier: The current is converted into an equivalent voltage using a transimpedance amplifier.
  • ADC Converter: Later, this voltage is digitized using an ADC converter in the Optical Spectrum Analyzer. This signal is applied to the vertical part of the display unit as amplitude.
  • Ramp Tuning: A ramp tunes the optical filter such that its resonant wavelength is proportional to the horizontal position.

Optical Spectrum Analyzer Vendors

There are different vendors of Optical Spectrum Analyzers (OSAs), including YOKOGAWA, Keysight Technologies, Thorlabs, Anritsu, Finisar, EXFO, APEX technologies, etc.

The following table summarizes popular vendors or manufacturers of OSAs.

VendorsModel, Features or Specifications
YOKOGAWAModel#1: Long wavelength (1200 to 2400 nm) OSA AQ6375B Model#2: Visible wavelength (350nm to 1200nm) OSA AQ6373B Model#3: Telecom OSA AQ6370B (600nm to1700nm) , for single and multi-mode optics
FINISARModel: WaveAnalyzer 1500S It has 4 measurements per sec across C-band or 10 Hz across any 200GHz frequency window
AnritsuModel: MS9740A Benchtop OSA Wavelength: 600 nm to 1750 nm
EXFOModel: IQS-5240S-BP Supports 1250 nm to 1650 nm