There is no debate that fiber optic cables are a much swifter, lightweight, flexible, and reliable mode of data transfer over mainstream copper cables. As the demand for data and high-speed communication continues to rise, fiber optic will become ubiquitous.

However, building a long-distance network of optical fiber cable comes with its challenges. Such installation is carried out by joining two fiber ends with a fiber cable connector or a fusion splicer. Naturally, where there is a connection, there will be some form of fiber splicing attenuation i.e., optical signal loss. If designers are not careful, splice loss in optical fiber could be a big hindrance to network performance.

In this blog, we will take a closer look at the fiber splicing loss phenomenon, answering questions like:

• What is optical fiber splicing?
• What is optical fiber splice loss?
• How is optical fiber splice loss measured?
• What parameters control optical fiber splice loss?

What is Optical Fibre Splicing?

Before we dive headfirst into all sorts of numbers and equations, let us paint a clearer picture of what splicing a fiber optic cable is.

Fiber splicing refers to the process of joining two optical fiber cable to create a longer link for optical signal. Thus, fiber splicing is what makes long-distance optical fiber communication possible. As such, fiber splicing involves couplers to which the end of one fiber bundle and the starting point of another optic fiber bundle are connected. The goal of fiber splicing is to ensure that any light passing through does not get reflected back from the splice point. Thus, adequate splicing can only occur if the two optic fiber bundles align together with proper geometry and strength.

Now, there are two broad fiber splicing types or splicing methods:

• Fusion splicing – You might imagine that fusion fiber splicing indicates fusing together optic fiber cables. That is correct. It is one of the best fiber splicing techniques that can produce a permanent connection. Fusion splicing involves thermally joining optic fiber cable ends using an electric arc. One of the first fusion splicing steps is proper alignment and butting of the two fibers inside a holder. Then, the heat from the electric arc melts the ends of the fiber, fusing them together permanently. A polyethylene jacket covers the burnt area to protect the connetion.
• Mechanical splicing – In mechanical splicing, there are two possible techniques: V-grooved splicing and elastic tube splicing. In the first method, a V-shaped substrate is used to hold the two fibers inside a groove and bond them using an adhesive or index matching gel. In the second method, fibers of different diameters are aligned using symmetrical force inside a rubber elastic material with much smaller hole diameter.

Fusion fiber splicing is the preferred splicing method by designers and installers as it has a low optical loss range between 0.05-0.10 dB. It is also equally efficient for both single mode and multi-mode fibers. On the other hand, mechanical splicing (especially V-groove splicing) experiences greater splice loss due to its non-permanent nature and slight off alignment.

Now, the more the number of splices in an optical fiber connection, the more splice loss. But what exactly is splice loss in optical fiber?

What is Optical Fibre Splice Loss?

As we have already read, an optical fiber beam travelling through the core of an optical fiber cable will gradually lose its signal strength. This phenomenon is called attenuation or fiber optic loss. While there are many ways it could occur, attenuation also happens in the form of optical fiber spice loss.

Splice loss in optical fiber is defined as the part of optical power that is not transmitted through the splice and is radiated out of the fiber instead. It is measured in decibels (dB) and is given by the formula:

α_splice = 10log₁₀ P_in/P_trans

Here:

α_splice = Fiber splicing loss

P_in = Total power incident on the fusion splice

P_trans = Desirable portion of the optical power transmitted across the fusion splice

Given that the total power that a fusion splice receives is always greater than the power it should transmit ideally, splice loss is always in positive. However, in fusion splicing technique, this loss is usually very negligible, often to the tune of 0.05 to 0.1 dB.

The two main reasons why fusion splice loss occurs are due to internal factors, also termed as intrinsic fiber core attenuation, and external factors, also known as extrinsic fiber attenuation. We shall read about these towards the end of the blog. In any case, it is important for manufacturers and optic fiber installers to reduce the splice loss as much as possible. Doing so enables much more effective transmission of the optical signal over longer distances.

How is Optical Fibre Splice Loss Measured?

Now that we know what is fiber splicing and splice loss in optical fiber, are you curious to know how it is measured? The answer is Optical Time Domain Reflectometer (OTDR). It is commonly used to measure fusion splice loss.

The operation of the OTDR is simple. During optical fiber testing, it injects a series of optical pulses into the fiber from one end. Then, it extracts any backscattered light from the same end and measures its strength as a function of both time and the length of the fiber.

Thus, during fusion splicing and measuring with OTDR, when two fibers with different geometric properties and MFD values are spliced, the OTDR captures different volumes of backscattered light. This results in either an apparent gain or loss artifact at the surface. This has been depicted in the illustrations below.

Before the actual splice loss is determined, the error component of the measured splice loss is also determined by the following formula:

α_OTDR = 10.log [ω₁/ω₂]

Here, ω₁ and ω₂ represent the respective fiber mode-field radius of the first and second fiber.

The total unidirectional OTDR measured splice loss is then calculated using the following equation:

α_measured = [α_others + α_MFD]_{splice loss} + α_OTDR

where α_measured = Unidirectional OTDR loss

α_others = core to core offset, tilt, and other loss mechanisms

α_OTDR = Error component of OTDR loss

Now, in order to remove the error component from the splice loss measurement, the OTDR takes a bidirectional reading. In this method, the measurements are taken from both the sides of spliced fibers and consequently measuring the step or gain (A1 and A2) at the splice point. The final splice loss is calculated by taking the average of the bidirectional OTDR traces. This is given by the following formula:

Splice loss = [A1 + A2]/2 = [(Δω + δ) + (-Δω + δ)]/2 = δ

where Δω = α_OTDR

δ = [α_others + α_MFD]_{splice loss}

Parameters Controlling Optical Fibre Splice Loss

When it comes to splicing fiber optic cable, the splice loss in optical fiber is controlled by two main parameters: intrinsic splice loss and extrinsic splice loss.

• Intrinsic – Fusion splice loss caused due to differences in the optical characteristics of the fibers, such as aperture and core diameter.
• Extrinsic – Fusion splice loss caused due to unevenness of splicing, such as core to core offset, tilt mismanagement and deformation at the splice point.

Intrinsic Parameters

Let us suppose that a set of two single mode fibers are being spliced. Now, since intrinsic parameters deal with loss due to the properties of the fibers themselves, here we can observe that the main difference between the two fibers is their different modal field radius.

To calculate modal field radius, we use the formula:

ω ≈ a_core (0.65 + 1.6/v^3/2)

where a_core = fiber core radius

v = generalised wave number = (2π/λ) a_coreNA

Ultimately, the splice loss is a result of the difference in the modal radius of the two fibers at a given wavelength. This loss due to Modal Field Diameter (MFD) mismatch is given by the formula:

α_MFD = -20 Log₁₀ [2 ω₁ω₂/ ω₁2 + ω₂2]

The following graph depicts splice loss variation when MFD is kept constant for one fiber and changed for the other. For an MFD difference of 1µm, intrinsic splice loss can reach as high as 0.05 dB.

The second graph, on the other hand, depicts experimentally measured splice loss for G.652.D fibers where one test fiber with a fixed MFD value is spliced to fibers having a range of MFD values

Beyond the usual splice loss, intrinsic attenuation also comprises of absorption losses, dispersion losses and scattering losses.

Extrinsic Parameters

Fiber splicing loss is also dependent upon a host of extrinsic parameters that emerge during the process of splicing. These include:

• lateral and angular alignment
• contamination of fiber ends
• core deformation due to un-optimised heating and pressing

Unoptimised splicing parameters, such as the ones depicted in the below graphic, can induce a splice loss as high as 0.04 dB in two identical fibers. Thus, the importance of using skilled splicing operators and automated equipment cannot be understated.

Similarly, in order to get acceptable splice loss, it is important to prepare the fiber end for splicing through a good cleave. Here, it has been found that a cleave angle of less than 2 degrees induces acceptable splice loss.

Beyond fiber splicing loss, other kinds of extrinsic losses include connector loss or insertion loss and bending loss.

Bottom Line

Fiber splicing loss is inevitable – whether intrinsic or extrinsic. However, due diligence must always be done to ensure that it is minimised. That way, the output optical power is well received by the receiver while also leaving a margin of error for any performance degradation over time. Using better quality fiber and advanced splicing technique and skills is paramount.

FAQs

What causes loss in fiber optics?

As is the case with every form of energy or power transmission, fiber optics also suffers from many losses. In fiber optics, a beam of light carries an optical signal through the core of an optical fiber cable. As it propagates through the cable, the beam of light, and consequently the signal, loses its strength. This is due to a phenomenon called attenuation. Attenuation is caused due to many factors, both intrinsic and extrinsic. They include absorption of light, scattering of light, loss due to bending of fiber cable, or fiber splicing loss.

What are the losses in optical fiber communication?

There are multiple optical signal losses in optical fiber communication. They all lead to attenuation of the fiber optic signal, i.e., the reduction in the power output compared to the power input. The various losses in optical fiber communication are:

• Absorption
• Scattering
• Radiative loss that comprises macroscopic bends and microscopic bends
• Splice loss in optical fiber
• Dispersion loss
• Overamplification
• Attenuation fade
• Loss of timing
• Polarization
• Radiation exposure
• Fiber cable imperfections

What is the acceptable loss of the mating of two splice on connectors?

When it comes to designing optical fiber, it is important to balance performance with optical loss. As optical loss is inevitable, there is always a minimum level of acceptable loss involved in various processes. When it comes to mating of two splices on connectors, the acceptable fiber splicing loss is calculated to be around 0.7 to 1.5 dB per connector. In fusion splicing, the acceptable loss is reduced to around 0.1 to 0.5 dB per splice. Hence, it is not surprising to see fusion splicing being preferred in the industry.

What is acceptable dB loss for fiber?

As per the Fiber Optic Association (FOA), the acceptable dB loss for fiber is determined on the basis of what kind of optical fiber cable it is: multimode fiber cable or single-mode fiber cable. The multimode fiber loss is defined around 3 dB per km for an 850 nm light source and 1 dB per km for a 1300 nm light source. Similarly, a single-mode fiber cable experiences acceptable fiber loss of around 0.5 dB per km for 1310 nm optical power source and 0.4 dB per km for 1550 nm optical power source.  

How do you fix fiber loss?

While fiber loss, including splice loss in optical fiber, cannot be completely reduced to zero, there are certain ways through which it can be fixed to an acceptable degree. Combining planning, careful installation, using better quality material and components, and emphasizing on regular testing can help fix fiber loss to a great extant. Here are a few broad ways fiber loss can be fixed:

• Minimizing tight bends
• Using bend insensitive fiber cables wherever possible
• Never exceeding a cable’s maximum tensile load
• Installing the fiber optic cable with minimum stress on its surface
• Minimizing the number of splices

How is OTDR loss calculated?

OTDR stands for Optical Time Domain Reflectometer – an instrument used to calculate fusion splice loss in optical fiber. The total unidirectional OTDR loss is calculated using the following equation:

α_measured = [α_others + α_MFD]_{splice loss} + α_OTDR

where α_measured = Unidirectional OTDR loss

α_others = core to core offset, tilt, and other loss mechanisms

α_OTDR = Error component of OTDR loss

This works on the principle that different fibers will capture different backscattered light which result in different signals reaching the OTDR. Here, this equation governs the OTDR loss value for when two fibers with different MFD values are joined and measured. Actual OTDR splice loss can be measured by taking bidirectional measurements. The average of the two measurements gives the actual OTDR splice loss.