What is an Optical Time-Domain Reflectometer and Its Working

In the late period of the 1990s, OTDR administrative representatives and customer community introduced an exclusive data technique for data storage and analysis of OTDR fiber information. The main intention behind this development was to be genuinely universal. But they identified a few of the irregularities in the format. After resolving all the communication issues and enabling cross-utilization between various manufacturers, the device came into establishment in the year 2011. Now, this article provides detailed information on optical time-domain reflectometer working, specification, advantages, and disadvantages.

What is OTDR (Optical Time-Domain Reflectometer)?

The acronym for Optical Time-Domain Reflectometer is OTDR. It is the optoelectronic device utilized to distinguish an optical fiber. This is the device that is optically similar to the electronic time-domain reflectometer. The main purpose of this instrument is to find or observe dispersed or back mirrored light via an optical fiber that happens because of any imperfections and crusts in the fiber. An OTDR generally observes the propagation of the optical fiber signal.


Also, an OTDR is utilized to analyze a few of the factors like splice losses, fiber attenuation, and signal reflectance angle. When there is a signal transmission from the optical fiber, then there will be some reflection in the signal. This outcome in the signal attenuation which essentially happens because of faults in the cable. So, an OTDR is also utilized for assessing tools in the optical communication systems so as to know the level of signal loss.

Working of OTDR

An optical time-domain reflectometer is the testing equipment that is utilized to assess the signal loss inside the fiber by sending out pulses into the fiber and calculates the level of the dispersed signal. With the below figure, the optical time-domain reflectometer working principle can be easily understood.

The device is included with a light source which is called a laser, a receiver that is connected either to a circulator or coupler. The fiber and coupler connection is done under examination using a front panel connector. The laser generates a small and heavily intensified light beam and these pulses move into the fiber link using the optic coupler. As because of this, there will be no transmission of all the signals into the fiber.

Still, in spite of utilizing a coupler, when a circulator is used, then the loss in signal transmission can be eliminated. Because circulator is considered as the extreme directional instruments those direct the entire signal into fiber. Also, circulators send the dispersed signal inside the detector. Using a circulator in the optical time-domain reflectometer enhances the dynamic range of the device.

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Operation of Optical Time Domain Reflectometer
Operation of Optical Time Domain Reflectometer

But the insertion of circulators augments the device cost when compared to that of coupler insertion. As a result, at the time of light propagation in the fiber, because of absorption and Rayleigh dispersion, few losses happen in the transmitted signals. In addition to these, few losses are introduced because of splicers. In a few cases, the difference in the refractive index also triggers light reflection. This reflected light moves towards OTDR and it identifies the fiber link characteristics.

Optical Time-Domain Reflectometer Specifications

A few of the specifications of OTDR are discussed as below:

Dead Zone

It is the main factor to be observed in the OTDR device. This is considered as the dead zone because at this distance the cable does not hold the ability to detect imperfections exactly. But there might arise the question that why there will be an occurrence of the dead zone in OTDR?

In the situation, when more amount of the transmitted wave gets reflected, then the power that got delivered at the photodetector is more than that of the back dispersed amount of power. This drenches the device with the light and so it requires few amounts of time to prevail over saturation.

During this recovery period, the instrument does not hold the ability to identify the back dispersed reflection. Because of this, the dead zone is formed in the optical time-domain reflectometer.

Trace of OTDR

The light that gets reflected is traced on the reflectometer’s screen. With the below picture, the reflected power in the OTDR device can be observed:

OTDR Trace
OTDR Trace

In the picture, the x-axis signifies the distance that is between calculation points of the fiber connection. Whereas the y-axis signifies the optical level of power that is in the reflected wave. By the representation of optical time-domain reflectometer, few of the observed points are stated as follows:

  • The positive points in the OTDR trace are because of Fresnel reflection those occur at fiber link connections and at the defects in the fiber.
  • Because of losses those take place at the fiber connections, there happen shifts in the OTDR trace
  • The deteriorated portions in the OTDR are the result of Rayleigh scattering. This dispersion is the outcome of the instabilities in the fiber’s refractive index. This stands as a crucial reason for the signal’s attenuation in the fiber.

Optical Time-Domain Reflectometer Performance Parameters

The performance parameter of OTDR can be known by measuring mainly two crucial parameters and those are dynamic and measurement ranges.

Dynamic Range – In general, this is the difference that is in between back dispersed optical power that is at the front-end connector and the maximum peak level at the fiber’s another end. With the evolution of the dynamic range, the maximum amount of losses in the fiber link can be known.

Measurement Range – This parameter calculates the distance where the fiber links can be known by the OTDR. This value is based on the transmitted pulse width and also the attenuation.

With these, we can finalize that OTDR is the most crucial device that is utilized in the optical communication networks. But there exist a few disadvantages of the optical time-domain reflectometer such as the OTDR dead zone.

Types of OTDR

Few of the types in the OTDR are

Full-features OTDR’s

These are of conventional type and they have extremely rich features, larger and have minimal portability. These are employed in laboratories and they get powered either through batteries or AC.

Hand-held OTDR’s

These are constructed to analyze and resolve issues in fiber networks. These are easily operated, and minimal weight type of OTDR’s.

So, by the implementation of perfect OTDR as per the requirement will offer ultimate results and provide answers for troubleshooting that makes sure of good performance of the device. So, this article clearly clarifies the optical time-domain reflectometer working, specifications, parameters, and principle behind it. In addition to these also know what are the advantages of optical time-domain reflectometer?

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