Methods used in measuring Attenuation
Some of the general methods used in measuring the attenuation are as follows:
- OTDR Method
- Total Fiber Attenuation
- Power ratio method
- Voltage Ratio Method
- Audio Frequency (AF) Substitution Method
- Intermediate Frequency (IF) Substitution Method
- Radio Frequency (RF) Substitution Method
OTDR(Optical Time Domain Reflectometer)
It is the most common instrument for the measurement of loss. It is essentially an optical radar. The laser source launches a narrow pulse of light inside the optical fiber and measures its reflection.
As the pulse travels inside the fiber it gets Rayleigh scattered. The backward scattered light is collected by the photodetector and displayed as a function of time. The pulse and the back scattered both get attenuated as they travel. The intensity of the back scattered light will be then twice the loss of the fiber. The location on the fiber can be found from the delay of the back scattered light. Since the Rayleigh scattering is very weak in the optical fiber, the back scattered light intensity is very small to be detected in the presence of electrical noise. Therefore, multiple pulses are transmitted and the received signal is integrated to improve the signal to noise ratio of the OTDR trace. Delay between the pulses has to be more than the round trip delay on the fiber.
The OTDR can also be used to monitor the status of the optical fiber while in operation. It can detect faults like tempering of the fiber as well as breakage in the fiber.
OTDR Block Diagram
Total Fiber Attenuation
Cut-back or differential method is the commonly used technique for determining the attenuation per unit length. A white light source is mechanically chopped at a low-frequency of a few hundred hertz. This allows the lock-in amplifier at the receiver to perform phase-sensitive detection. The chopped light is then passed through the monochromator which uses a prism to select the required wavelength at which the attenuation has to be measured. By using a microscope objective lens the light is filtered before focusing onto the fiber. A beam splitter is used to provide light for viewing optics and reference signals are used for compensating output power fluctuations. The fiber is put through a cladding mode stripper to remove light launched into the fiber cladding.
A mode stripper is used at the fiber end to remove any power which is scattered from the core to the cladding. The power at the end in detected using avalanche photodiode. At last the electric output from the photodiode is fed to a lock-in amplifier whose output is recorded.
The attenuation from this method can be found from the following mathematical equation,
= 10/(L1-L2)log10(P02/P01)
where,
- L1 is the original fiber length
- L2 is the cut-back fiber length
- P01 output optical power at a specific wavelength from original fiber length
- P02 output optical power at a specific wavelength from cut-back fiber length
= 10/(L1-L2)log10(V2/V1)
where,
- V1 is the output voltage from original fiber length
- V2 is the output voltage from cut-back fiber length
Cut-Back Block Diagram
Power Ratio Method
The first step in this method is to measure the input and output power of the microwave bench without the device is calculated. In the second step the input and output power of the microwave bench along with the device is calculated. The drawback of this method is that if the input power is low and attenuation is large then measurement may not be accurate.
Power Ratio Set up-1
Power Ratio Set up-2
Radio Frequency Substitution Method
In this method, the output power of the microwave bench is measured with the network whose attenuation has to be calculated. Again the output power is calculated by replacing the network with a precision calibrated attenuator. The attenuator is then adjusted to obtain the same power as the network. This method avoids the drawback of the power ratio method.
RF Substitution set up-1
RF Substitution set up-2
Voltage Ratio Method
In this method, a digital Voltmeter is used to measure the potential difference across a feed-through terminator, firstly directly connected to the matching attenuator and then the device is inserted and the difference is measured.
Voltage Ratio Setup
Audio Frequency Substitution Method
In AF substitution method, the attenuation of the device is measured by comparison with an audio frequency standard. The output voltage is measured first without the device under test from the signal generator. Then the device is connected and the output voltage is measured. The signal generator is set to the desired test frequency and the ratio of the voltages in two cases gives the attenuation at that frequency
AF Substitution Method
Intermediate Frequency Substitution Method
In this method, comparison is done with an IF attenuation standard like IF piston attenuator. In parallel IF substitution system, a reference signal is compared with the IF input signal. A turn phase sensitive detector is used is used to detect and display the difference between both the signals. When the device under test is inserted the system is brought to a null using the piston attenuator and the difference in two settings is used to find the attenuation.
IF Substitution Method
Attenuation
In this article, we will be discussing about attenuation. We will also discuss about attenuation in optical fiber and networking. We will look into the various factors that causes attenuation in optical fiber and networking. We will discuss about attenuation coefficient. We also will discuss how can we measure attenuation and the methods that are available to measure. In addition to this, we will discuss how can attenuation be prevented. We will also compare attenuation with amplification and see their differences. Later in this article, we will discuss about the various advantages, disadvantages and application of attenuation.
Table of Content
- What is Attenuation?
- Attenuation in Optical Fiber
- Methods
- How Attenuation can be Prevented?
- Attenuation Vs Amplification
- Advantages of Attenuation
- Disadvantages of Attenuation
- Applications of Attenuation
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