Table of Contents

1. What are Fiber Optic Attenuators
2. Fiber Optic Attenuator Package Types
3. Working Theory of the Fiber Optic Attenuator
4. Three Major Types of Fiber Optic Attenuator
5. What is dB - Decibels (dB)
   • Calculating dB Power Loss and Power Gain
   • Understanding dB in Percentages
   • The Rule of Thumb to use dB
   • Absolute Power and dBm
   • dB is Used in More Than Fiber Optic Field
6. Calculating the Attenuation Values with Attenuators
7. Using Female to Male Fiber Optic Attenuators to Test Receiver Sensibility

What are fiber optic attenuators

Fiber optic attenuator, or named optical fiber attenuator, refers to a passive fiber optic component that is used in the fiber optic link to reduce the light power to a pre-set value (this value is often described as a certain dB). Fiber optic attenuators are generally used in single mode long distance transmissions (there are indeed multimode types fiber optic attenuators for some applications), by reducing the light power via the attenuator, it helps avoid too high overload at the receiving end.

The pre-set value is usually 0dB,1dB,2dB,3dB,4dB,5dB,6dB,7dB,8dB,9dB,10dB,11dB,12dB,13dB,14dB,15dB,16dB,17dB,18dB,19dB,20dB,21dB,22dB,23dB,24dB,25dB,26dB,27dB,28dB,29dB,30dB and other custom values. dB is also one of the commonly used terms to describe the quality of a certain fiber optic signal. dB is short for decibel, which was originally used to measure the strength of sound. dB means 1/10 of a bel. For a fiber optic attenuator, the most important terms to describe it is its connector type, such as LC/UPC, LC/APC, SC/UPC, SC/APC, FC/UPC, FC/APC, ST/UPC, ST/APC, MU/UPC, MU/APC and its corresponding dB level.

Fiber optic attenuator package types

There are a few different types of fiber optic attenuators based on its package, including the bulkhead attenuator, inline fiber optic attenuator, hand-held fiber optic attenuator, variable fiber optic attenuator.

Bulkhead fiber optic attenuator is also named female to male attenuator, it is a small size component looks like a female to male fiber optic adapter, when you plug the connector from the female side, the light power goes through the attenuator will be reduced, it is totally plug and play, simple to use. Because they have unchanging level of attenuation, they are also called fixed value fiber optic attenuator.

The other fixed value attenuator is a patch cord type, which is called inline fiber optic attenuator. These inline fiber optic attenuators are generally used to link from the rack / patch panel interface to the other equipment. It has pre-defined fixed attenuation value, and it has connectors at each end of the cable. Inline fiber optic attenuators can be various length which is made according to your needs.

Working theory of the fiber optic attenuator

Based on the different working theories, we will explain the commonly used attenuator types which are fixed attenuation value attenuators, stepwise variable fiber optic attenuators, and continuously variable optical fiber attenuators.

GAP-loss attenuator is a typical inline fiber optic attenuator. It is usually placed very close to the transmitter. Only in this way can it achieve the preset attenuation value more accurately. The farther it is placed from the transmitter side, the less accurate it will work.

We have drawn a working principle illustration picture of the gap loss attenuator. As can be seen from the picture, the gap-loss attenuator artificially creates a gap in the fiber, so that when light passes through the gap from one fiber to another fiber, Due to the existence of the gap and the divergence of light, the purpose of attenuation is achieved. The gap loss attenuator is not recommended if you want to achieve precise attenuation at the far end of the transmitter.

Absorptive Principle is a popular way to make fiber optic attenuators. Traditionally, it is the low-attenuation, high-quality fiber that makes long-distance fiber-optic communication possible. But when making attenuators, we can instead use imperfect fibers to achieve the desired attenuation. We can use a piece of material in the fiber attenuator to absorb the energy of the fiber. When the light passes through, the material absorbs the energy of the fiber and converts it into heat, so that the energy of the light is reduced, this is how the absorptive attenuators work.

Light scattering is one of the main causes of light attenuation in optical communication, and this characteristic is also used in the manufacture of optical fiber attenuators. In this type of attenuator, one uses an attenuating material so that only a portion of the light that matches one's pre-designed values can propagate through it, so the total amount of light that passes is reduced to achieve attenuation.

Three major types of fiber optic attenuator

Fixed fiber attenuators are available in LC, SC, ST, FC and MU styles with APC and UPC polishes. Usually, attenuators are not needed for multimode systems because multimode source VCSELs rarely have enough power output to saturate the receiver. Instead, single-mode systems, especially short links, are often too powerful and require attenuators.

LC/UPC female to male fiber attenuator

LC/APC female to male fiber attenuator

SC/UPC female to male fiber attenuator

SC/APC female to male fiber attenuator

MU/UPC female to male fiber attenuator

FC female to male fiber attenuator

ST female to male fiber attenuator

Therefore, fixed fiber attenuators are usually of the single-mode type. The most commonly used types are LC single-mode female-to-female and female-to-male fixed fiber optic attenuators and SC single-mode female-to-female and female-to-male fixed fiber optic attenuators. Different from the common female-to-male type mentioned above, the female-to-female fixed fiber optic attenuator has a female fiber optic adapter at both ends, so it is also called a fiber optic adapter type attenuator. They can be used as adapters and attenuators at the same time.

The step variable attenuator has a real-time variable fiber attenuation value, which can range from 0.1dB, 0.5dB, 1dB to a larger desired range. The step variable fiber optic attenuator can be used when there are multiple light sources. For example, if you have 4 inputs light source, you may need to adjust the attenuation of the signal for each input. In addition, the step variable attenuator can also be used to adjust the output signal. At this time, your input signal is stable, but the output signal needs to be adjusted. In either case, before choosing to use attenuators, it is necessary to confirm the performance parameters of the inputs, outputs and equipment.

The Variable Fiber Optic Attenuator is widely used in optical communication, and its primary function is to reduce or control the optical signal. The most fundamental characteristic of an optical network should be adjustable, especially with the application of DWDM transmission system and EDFA in optical communication. For the applications where the input and/or the output need continually adjust and without interruption of the transmission, the VOA (variable optical attenuator or variable fiber optic attenuator) is an indispensable key device. Typical variable fiber optic attenuators in the market include manually variable fiber optic attenuators and electronic variable fiber optic attenuators, these variable fiber optic attenuators work with single mode, multimode and PM fibers.

What is dB - Decibels (dB)

In the process from the light passing through the transmitting end to the receiving end, the signal will be attenuated, that is to say, the light will lose some energy in the process. This loss of energy can be caused by many reasons, such as insufficiently pure fibers, damaged fibers, or gaps at the connections between components, etc.

The receiving end of the fiber needs enough energy to work, and we need to be able to measure the total energy loss of the light from the transmitting end to the receiving end. Also, being able to know and find out where and how much energy the light is losing primarily in the link is critical to our troubleshooting.

In optical communication, we introduce the unit dB to describe the change of optical signal. For example, we often see how many dB is the fiber attenuator, and how many dB is the insertion loss of the fiber optic patch cord. what is dB? The full writing of dB is decibel, deci means one tenth in English (actually in Latin). So this unit dB was originally from the unit Bel. However, because the energy difference required to achieve a numerical comparison of a bel is usually relatively large and is not commonly used in fiber optics, one-tenth of a bel, that is, the unit decibel, is more commonly used. The unit Bel is to commemorate Alexander Graham Bell (March 3, 1847 - August 2, 1922) who was an American inventor and entrepreneur. He patented the world's first usable telephone, created the Bell Telephone Company (predecessor of AT&T). The unit dB was originally used to measure the sound heard by the human ear.

In fact, it means that when you encounter two energies (signals), dB is a unit of representation we use to express the difference between these two energies. It is not an independent absolute unit (such as Volt, Ampere, etc.) itself. The appearance of the dB unit means that there are two units of energy (or signals) of the same nature being compared.

Calculating dB Power Loss and Power Gain

In optical fiber communication, dB is mainly used to describe optical power, that is, signal power. An optical power meter is a special device used to test optical power. With the maturity of technology and components, the price of optical power meters has been reduced to a much lower level than before.

In the 1960s, the initial goal was to achieve a fiber attenuation of 20dB per kilometer, so that optical fiber communication could become a reality from theory. Actually, 20dB per kilometer attenuation is a very large energy loss, it means that almost 99% of the energy is lost in this transmission process.

Fiber attenuation in today's era has reached such low levels that fiber amplifiers are no longer required in most cases. In optical fiber communication, we use the unit of dB to calculate the optical loss in the entire link. In the case of using a fiber amplifier, we can also use dB to calculate the signal power gain provided by the fiber amplifier.

We use the following equation to accurately calculate the dB value of the loss or gain in optical power: dB= 10*Log10(Pout/Pin)

The logarithm base 10 of a number is its logarithm to the base of 10 where Log (1) = 0.

Use this equation to calculate loss in dB

Let's illustrate how to use this equation to calculate the loss of a fiber optic link. If the transmitter sends 10uW into the link and at the receiver end we get 4uW. Using the above equation, we can get

dB= 10*Log10 (4/10)

dB= 10*Log10 0.3

Loss =-3.98dB

From the above calculation result we get a negative value, which means it is a loss. When we use it to express loss, we remove the preceding minus sign. Therefore, we say that the optical loss is 3.98dB.

Use this equation to calculate gain in dB

Through this equation, we can also calculate the gain of the fiber amplifier. Usually, it is not necessary to use the amplifier in the fiber link, because the attenuation caused by the modern fiber is low enough, but when the transmission distance is relatively long, there is still fiber amplifiers may be required. Let's do a simple calculation with an example, assuming that the amplifier input power is 1uW and the output power is 20uW.

dB= 10*Log10 (20/1)

dB= 10*Log10 20

Gain =13.01dB

Use this equation to calculate power if we already know dB value

We can also transform the dB equation so that when we know the dB value, we can know the ratio of input power to output power, or their exact values.

Because dB= 10*Log10(Pout/Pin)

then

(Pout/Pin)= antilog (dB/10)

Let's take an example to illustrate the usage of this equation. We need to remember that dB is the ratio of output power to input power. If we know that the loss of the fiber is 3dB.

(Pout/Pin)= antilog (dB/10)

(Pout/Pin)= antilog (-3/10)

(Pout/Pin)= antilog -0.3

(Pout/Pin)= 0.501

If one value of input power and output power is known, and the dB value is known, we can calculate the other value. We continue the above equation calculation, if we already know that the input power is 12mW, then we calculate the input power: (Pout/Pin)=0.501

Pout= 0.501* Pin

Pout= 0.501* 12mW

Pout= 6.01mW

Understanding the dB in percentages

It is important to keep this in mind when dB is used to measure the gain or loss of a signal, it describes a ratio relative to the original signal. The dB value allows us to intuitively know the rate of change of the signal. For example, 0.1dB means about 98% of the signal power remaining. And if the dB value reaches 3, then it means that only 50% of the signal power remains.

In actual work, 3dB and 10dB are two values to keep in mind, because if the loss value is 3dB, it means that the signal is attenuated by half, and the loss value of 10dB means that the signal is attenuated by 90%. If the value of gain is 3dB, it means that the signal strength is twice as high as before, and if the value of gain is 10dB, it means that the signal strength is 10 times as strong as before. So 3dB and 10dB are figures with more typical reference meanings.

This relationship is not linear but logarithmic, which means that if the dB value of loss increases by 10dB, the signal will be attenuated by one-half of the previous value. The gain's dB value increases by 10dB each time, the signal will be enhanced to 10 times of the original.

The Rule of Thumb to use dB

In addition to the two values of 3dB and 10dB that we have mentioned before, we often use the value of 7dB in our work. 7dB loss means 80% signal attenuation, and 7dB gain means signal strength is 5 times stronger than before. The 3dB, 7dB and 10dB are the rule of thumb of to use dB in practical work. We can perform quick and easy calculations using these three values.

Below we explain how to use these rules of thumb. Because dB can be added and subtracted algebraically, You can use a combination of dB values to calculate total gain or total loss. For example, we know that the input of the fiber optic amplifier is 10uW and its gain is 13dB, then we first use the 10dB rule, multiply 10uW by 10. Then we can use the 3dB rule, multiply the result we just got by 2. This is what we finally get the remaining output value.

Pout= 10uW x 10

Pout= 100uW

Pout= 100uW x 2

Pout= 200uW

So the output of the amplifier is 200uW.

Of course, these rules of thumb do not apply to all situations. For example, when the gain of the amplifier is 18dB, we cannot use these rules to get accurate results. But we can still estimate an approximate range, because according to the example we gave above, we can calculate through the rules of thumb that a gain of 17dB corresponds to 500uW, and 20dB corresponds to 1mW, so we can still estimate that 18dB The corresponding output value should be between 500uW and 1mW.

Absolute Power and dBm

By itself, dB is just a relative number, it only has meaning when referenced to a known input or output power. But sometimes we still need to know the absolute value, in this case, the dB is referenced to 1mW (milliwatt), we add a lowercase m or mW after dB. 1mW is 0dBm.

When using dBm as the unit, power levels below 1mW are negative and power levels above 1mW are positive. -3dBm means that the signal attenuation of 1mW is half of the previous one, which is 500uW. And 3dBm means that the signal of 1mW is enhanced by 2 times of the previous one, which is 2mW.

We use an equation to express the relationship between power and dBm, through which we can also convert between them.

dBm = 10Log10*(P/1mW)

If you compare this equation with the previous equation for calculating dB, you will find that they are very similar, the only difference is that the input power is a fixed value of 1mW. Using this equation, we can know the dBm value Calculate the optical power, if the optical power is known, the value of dBm can also be calculated.

To calculate optical power: Assuming the power meter gets -6dBm, the use the dBm equation, we can get:

-6 = 10Log10*(P/1mW)

-0.6= Log10*(P/1mW)

0.2512=(P/1mW)

P= 0.2512* 1mW

P=0.2512mW

To Calculate the dBm value: For example the power value is 20uW. Then we can use the dBm equation:

dBm =10Log10*(P/1mW)

dBm =10Log10*(20uW/1mW)

dBm =10Log10 (0.02)

dBm =10x -1.699

dBm = -16.99 the power value is -16.99dBm, roughly 17dBm. When using the dBm unit, the dBm value above 1mW is a positive number, the dBm value below 1mW is a negative number, and the dBm value corresponding to 1mW is 0.

The units dB and dBm are often confusing. We just need to remember that db describes loss and gain level, and dBm describes absolute power. In fact, if the loss is 3dBm, what you describe is that the loss of optical power in Watts is 2mW. means that the fiber link has half the signal attenuation (half the energy is lost from the initial state)

dB is used in more than fiber optic filed

The use of the unit dB is not only in the field of optical communication, as we said before, it was originally a unit used to express sound. dB is also used for electrical transmission performance, no matter what kind of field, the calculation method of dB is the equation we listed earlier. Also, it is important to remember that dB is a ratio, and the dB value depends on the ratio of input power to output power. Understanding the meaning of dB helps us compare the performance parameters of different products. For example, we get two identical cables, but their attenuation parameters are 16dB and 22dB. Generally speaking, we all know that the smaller the attenuation, the better. Of course, the 16dB cable will be a better choice. But how much better is it than 22dB? According to the knowledge of dB we have learned and the method of calculating dB, we will accurately understand that although the two cables appear to have only a difference of 6dB attenuation value, these 6 dB bring 75% more of the attenuation on the 22dB cable.

Calculating the attenuation values with attenuators

There are so many types of optical fiber attenuators. When choosing, we not only need to consider the application scenario and the type of optical fiber connector, but also need to calculate the required optical attenuation value.

The type of connector is obvious, here we give an example to illustrate how to simply calculate the attenuation value of the required attenuator. For example, we know that the maximum endurance of the fiber-optic receiver is -7dBm. If the received optical power is greater than this value, then the receiver will be overloaded. At the same time, we know that the output power of the transmitter is 4dBm, and the difference between the transmitter and the receiver is 4dBm. The total attenuation of the fiber and the entire link is already 6dB, so we can list a formula:

Transmitter side power = 4dBm

Receiver side max power = -7dBm

Fiber total loss = 6dBm

So the max attenuation you may need is: Receiver side max power+ Transmitter side power- Fiber total loss

In this example, it is -7dBm+6dB-4dBm= -5dB

It can be seen from this that we will need a 5dB fiber attenuator. Of course, it is possible that a fiber attenuator with a larger attenuation value may also work, as long as it does not cause excessive attenuation and affect signal transmission.

Using female to male fiber optic attenuators to test receiver sensibility

The fiber optic receiver works best when the input power it receives is in a suitable range. We need to test whether the fiber optic receiver can work optimally at the lowest input power indicated on the data sheet. We can use the female to male fiber optic attenuator to test in a few simple steps.

The first step is to use the optical power meter to calculate the output power of the optical fiber transmitter. What we need to pay attention to is that the transmitter and receiver used for testing need to be of the same type. This means that we should avoid using fast ethernet for transmission to match the receiving end of gigabit ethernet. We already know that for a specific network standard, the optical fiber industry has a clearly defined range for the power of the transmitting end and the receiving end.

In the second step, we connect the optical fiber transmitter and receiver, and we adjust the power of the transmitter to the maximum, because we want to test whether the receiver can work well under the minimum input power indicated by the equipment manufacturer's data sheet, we need to find out the value of this minimum input power.

The third step is to calculate the required fiber attenuation value and use a suitable female to male fiber optic attenuator. For example, we know that the power of the transmitter is -18dBm, and the minimum power required by the receiver is -34dBm, then we can calculate the required power The attenuation value of is their difference, which is 17dB. We choose a suitable 17dB optical fiber attenuator and place it in the optical fiber link before the receiving end, and restart the receiving end. If the receiver works fine after rebooting, then it complies with the standards indicated in the data sheet.