Fiber Optic Bending Overview
Fiber optic sensors have a small size, can be curved around the characteristics of the samples can be installed in a narrow space, too small a bending radius will bring optical signal loss, affecting the accuracy of the sensing measurement.
However, in many practical application scenarios, fiber optic sensor installation is unavoidable with a small bending radius, how to solve the optical fiber bending loss has become the focus of attention.
Let’s take a look at the following, ordinary optical fiber bending will bring what is bad and bend-resistant fiber optic small bending radius of the working principle.
Why Optical Fibers Should Not Be Overbent
Light from a medium to another medium, in the two media interface will produce refraction and reflection.
With the increase of the angle of incidence, the reflected light is getting stronger and stronger, the refracted light is getting weaker and weaker.
When the angle of incidence is large enough, the refracted light completely disappears, leaving only the reflected light, this phenomenon is called total reflection.
Optical fiber consists of three layers: core, cladding and coating.
The refractive index of the core is greater than that of the cladding, so that light can be transmitted in the core with total reflection.
The loss coefficient of general standard singlemode fiber at 1550nm wavelength is about 0.2dB/km, and the transmission loss is very low.
If the optical fiber is bent (macroscopic bending or microscopic bending), the light transmission does not satisfy the total reflection condition, and part of the light leaks out from the cladding, resulting in a decrease in optical power and loss.
How To Reduce Fiber Bending Losses
The main reason for the loss in the fiber optic link is that the fiber optic is installed and laid out in the process of fiber optic local position there is a large angle of bending, which is common in the fiber optic connector and fiber optic turning place.
Such bending loss is reversible, increase the bending radius of the optical fiber, the link loss will have a greater improvement.
Measuring a fiber link with an OFDR device results in an OFDR distribution curve (Distance-Intensity/Reflectivity), which reflects the loss at each location in the fiber link, which is mainly presented in the form of steps, as shown in the figure.
With the help of the OFDR curve, users can analyse to find the bending position and make adjustments.
Introduction To Bend-resistant Optical Fibers
Conventional single mode fibers (G.652D) are recommended to have a bending radius greater than 5mm (1cm diameter), otherwise there will be a significant loss of the optical signal, resulting in a reduction in the signal-to-noise ratio of the sensing measurement and an unstable measurement result.
Regarding the minimum bending radius, the rule of thumb is: for long-term applications, the bending radius should be more than 150 times the diameter of the fiber cladding; for short-term applications, it should be more than 100 times the diameter of the cladding.
For a conventional single mode fiber with a cladding diameter of 125 μm, these two minimum bend radii are 19 mm and 13 mm, respectively.
Bend-resistant optical fiber (G.657) is mainly designed to improve the bending resistance by changing the optical fiber structure.
There is a more common index for evaluating bending sensitivity in the industry: the MAC value.
The MAC value is the ratio of the mode field diameter to the cut-off wavelength in a near-step refractive index waveguide fiber.
The smaller the MAC value, the less sensitive the fiber is to bending.
Some of the basic methods for designing bend-insensitive fibers are simply to make bend-insensitive fibers by decreasing the mode-field diameter, or increasing the cut-off wavelength, or both decreasing the mode-field diameter and increasing the cut-off wavelength. Specific methods are:
- Decrease the mode field diameter to improve optical control. For example, reduce the core diameter or increase the core refractive index.
- Decrease the diameter of the fiber cladding to increase the bending characteristics. Existing bend-resistant optical fiber diameter from 125 microns to 80 microns, and even appeared in the 60 micron outer diameter of the optical fiber.
- Add a ring of low refractive index groove cladding. The function is similar to increasing the refractive index of the fiber core.
All of the above methods allow for a better controlled transmission of the light beam through the fiber core, thus reducing the effects of fiber bending in sensing measurements.
The commonly used type of bend-resistant optical fiber in the market is G.657B3, and its bending radius and bending loss parameters are listed in the table below for reference only.
