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The Need for Optical Spectrum Analyzer (OSA) in the Telecom Network

Optical spectrum analyzer: As multiple wavelengths are used in a DWDM system, it is important to know the parameters for each wavelength/channel: exact wavelength, power level, dynamic range or optical signal over noise ratio (OSNR). An Optical Spectrum Analyzer (or OSA) is a precision instrument designed to measure and display the distribution of power of an optical source over a specified wavelength span.

The optical spectrum analyzer (OSA) provides these (exact wavelength, power level, dynamic range or optical signal over noise ratio (OSNR)). parameters and a trace of power as a function of wavelength. The optical spectrum analyzer (OSA) can also provide, together with a broadband source at the opposite end, a spectral attenuation trace for fiber characterization, which is an important parameter for DWDM installation.

Optical spectrum analyzer

The Need for Optical Spectrum Analyzer (OSA) in the Telecom Network

Optical spectrum analyzer (OSA) methods

There are three different optical spectrum analyzer (OSA) methods that can be used in the telecom network. They are the-
Interferometric method, 
diffraction-grating method 
Fabry-Perot method.

Calibration of the OSA is defined by IEC 62129 “Calibration of optical spectrum analyzer”. “Generic requirements for portable wavelength division multiplexer analyzers” provides the main specifications of an OSA.

OSA with the interferometric method
The principle of the interferometric method is that the equipment counts the interference maximum and minimum amplitude which are produced by a fixed mirror and a moving mirror (Michelson interferometer).

Individual wavelengths can be selected through subsequent computation of the spectrum using a fast Fourier transform (FFT). This principle can be also used for multi-wavelength meters, instruments which are mainly providing wavelength and power levels information, and no dynamic range/OSNR values.

Benefits and limits
The benefits of this method are its wavelength range, accuracy and stability. It also has a good dynamic range and OSNR values, but these tend to be less than diffraction-grating based OSAs.

As there are moving parts, this method is not fully optimized for the field or outside plant applications, but more optimized for inside plant applications. This is also the most expensive technology.

OSA with the diffraction-grating method
The principle of the diffraction-grating method is based on the fact that the light is broken into its spectral colors by a grating. The grating rotates so that different wavelengths are brought to the detector at different times and analyzed. Such a combination can also be called a monochromator.

Double pass monochromators (the light is reflected twice to the grating) provide better accuracy and higher dynamic range than single-pass monochromators.

Benefits and limits
The benefits of thismethodare its wave-length range, good dynamic range and OSNR values, but it is limited for the resolution and wavelength accuracy parameters.

As there are moving parts, this method is not fully optimized for the field or outside plant applications, but more optimized for inside plant applications.

OSA with the Fabry-Perot method
The principle of the Fabry-Perot method is the use of a cavity resonator. It is made of two partially mirrored plates, arranged at an adjustable distance using piezo elements, thereby forming a resonant cavity. The selectivity is directly determined by the transfer properties of the Fabry-Perot filter.

It is transparent when all the sub-beams arising between the plates due to multiple reflections are constructively superimposed. At all other wavelengths high attenuation occurs.

Benefits and limits

This method provides good wavelength accuracy, but is limited as far as the dynamic range/OSNR and wavelength range values are concerned. If the wavelength range is extended, using similar component specifications, then the dynamic range will be reduced.

Itssmall filter bandwidth means very close channels can be detected (down to 12.5 Ghz). Even the modulation or laser chirp of a given channel can be seen. It has no moving parts, making it rugged (not sensitive to drop, vibration, etc) and ideal for field and outside plant use, but also for DWDM system monitoring purposes. It is also compact and lightweight, as it uses.only components, and does not need free-air mechanics. Moreover, it has a low power requirement making it ideal for battery-operated instruments.

Where can be used OSA?
OSA measurements are only used when DWDM systems are installed or maintained. Consequently it will be required for:
Fiber installation for spectral attenuation measurements.
Upgrade of classical TDM 1310/1550nm networks to DWDM applications.
Maintenance and troubleshooting of DWDM networks.
DWDM network monitoring and surveillance.

How to Work Optical Spectrum Analyzer (OSA)
Each OSA model has a spectral resolution of 7.5 GHz, or 0.25 cm-1. The resolution in units of wavelength is dependent on the wavelength of light being measured. The resolution of this type of instrument depends on the optical path difference (OPD) between the two paths in the interferometer. It is easiest to understand the resolution in terms of wavenumbers (inverse centimeters), as opposed to wavelength (nanometers) or frequency (terahertz).

The resolution of the OSA can be set to High or Low in the main window of the software. In high resolution mode, the retroreflectors translate by the maximum of ±1 cm (±4 cm in OPD), while in low resolution mode, the retroreflectors translate by ±0.25 cm (±1 cm in OPD).

The OSA is also designed so that it samples more points/OPD when the translation of the retroreflector assembly is slower. The data sampling is triggered by the reference signal from the internal stabilized HeNe laser. A phase-locked loop multiplies the HeNe period up to 128X for the highest sensitivity mode. This mode can be very useful when the measured light is weak and broadband, causing only a very short interval in the interferogram at the ZPD to contain all the spectral information.

The vertical axis of the spectrum can be displayed as Absolute Power or Power Density, both of which can be displayed in either a linear or logarithmic scale. In Absolute Power mode, the total power displayed is based on the actual instrument resolution for that specific wavelength; this setting is recommended to be used only with narrow spectrum input light. For broadband devices, it is recommended that the Power Density mode is used. Here the vertical axis is displayed in units of power per unit wavelength, where the unit wavelength is based upon a fixed wavelength band and is independent of the resolution setting of the instrument.

The interference pattern of the reference laser is used to clock a 16-bit analog-to-digital converter (ADC) such that samples are taken at a fixed, equidistant optical path length interval. The HeNe reference fringe period is digitized and its frequency multiplied by a phase-locked loop (PLL), leading to an extremely fine sampling resolution. I think you can better understand by below figure.
How to work OSA
The Optical path in OSA

The pressures imposed by a competitive market entail that service providers upgrade and maintain their networks continuously to ensure that they are capable of delivering higher-speed, higher-quality applications and services to customers. This creates
a need to verify and make sure that the network’s fiber infrastructure and equipment can meet exacting performance standards and operate reliably.

As a result of the emergence of DWDM networks, some important changes were made in the optical fiber characterization and system turn-up. Consequently new test tools and procedures were needed.

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