Why is the fiber damaged?

Fiber Damage Mechanism

Damage mechanisms for unterminated (bare), terminated fibers, and other laser light source-based fiber components are detailed below, including damage mechanisms at the air-glass interface (free-space coupling or when using splice) and damage mechanisms within the fiber glass. . Fiber optic components such as bare fiber, fiber patch cords, or splice couplers may be subject to a variety of potential damage (eg, splices, fiber end faces, and the device itself). The maximum power available for a fiber is always limited by the minimum of these damage mechanisms.

While scaling relationships and general rules can be used to estimate damage thresholds, the absolute damage threshold for an optical fiber is highly application- and user-specific. Users can use this tutorial as a guide to estimate safe power levels that minimize the risk of damage. If all proper preparation and suitability guidelines are followed, the user should be able to operate fiber optic components below the specified maximum power level; if any component does not have a specified maximum power, the user should adhere to the “practical safety levels” described below in order to safely Operate related components. Factors that may reduce power capability and cause damage to fiber optic components include, but are not limited to, misalignment of the fiber coupling, contamination of the fiber end face, or defects in the fiber itself.

Damage to the air-glass interface

There are several potential damage mechanisms at the air/glass interface. Light is incident on this interface when free-space coupling or using an optical splice to match two fibers. If the light intensity is high, it will reduce the suitability of the power and cause permanent damage to the fiber. On the other hand, for terminated fibers that use epoxy to hold the splice to the fiber, the heat from the high-intensity light can melt the epoxy, leaving a residue on the surface of the fiber in the optical path.

Undamaged fiber end face  Damaged fiber end face

Damage mechanism of bare fiber end face

Damage mechanisms on fiber endfaces can be modeled as large optical components, and industry-standard damage thresholds for UV fused silica substrates apply to silica-based fibers (refer to the table to the right). But unlike large optics, both the surface area and beam diameter associated with the fiber air/glass interface are very small, especially when coupled to single-mode (SM) fibers, so for a given power density, the incident light beam with a smaller beam diameter is very small. The power requirement of the fiber is relatively low.

The table above lists two optical power density thresholds: a theoretical damage threshold and a “practical safety level”. In general, the theoretical damage threshold represents an estimate of the maximum power density that can be incident on the fiber end face without risk of damage, provided the fiber end face and coupling conditions are very good. The “practical safe level” power density represents the lowest risk of fiber damage. It is also possible to operate fibers or components beyond practical safety levels, but the user must follow proper suitability instructions and verify performance at low power before use.

Calculate the effective area of single-mode and multi-mode fibers
The effective area of a single-mode fiber is defined by the mode field diameter (MFD), which is the cross-sectional area of light passing through the fiber, including the core and part of the cladding. When coupling to a single-mode fiber, the diameter of the incident beam must match the MFD of the fiber to achieve good coupling efficiency.

For example, the mode field diameter (MFD) of SM400 single-mode fiber at 400 nm is approximately Ø3 µm, while the MFD of SMF-28 Ultra single-mode fiber at 1550 nm is Ø10.5 µm. Then the effective area of the two fibers can be calculated according to the following:SM400 Fiber:Area = Pi x (MFD/2)² = Pi x (1.5 µm)² = 7.07 µm² = 7.07 x 10^-8 cm²
SMF-28 Ultra Fiber: Area = Pi x (MFD/2)² = Pi x (5.25 µm)² = 86.6 µm² = 8.66 x 10^-7 cm²To estimate the applicable power level on the fiber end face, multiply the power density by the effective area. Note that this calculation assumes that the beam has a uniform intensity distribution, but most laser beams in single-mode fibers are Gaussian, making the center of the beam more dense than at the edges, so these calculations will be slightly Power above the damage threshold or the actual safe level. Assuming a continuous light source, from the estimated power density, the corresponding power level can be determined:

SM400 Fiber: 7.07 x 10^-8 cm² x 1 MW/cm² = 7.1 x 10^-8 MW = 71 mW (Theoretical damage threshold)
     7.07 x 10^-8 cm² x 250 kW/cm² = 1.8 x 10^-5 kW = 18 mW (Actual safety level)

SMF-28 UltraFiber: 8.66 x 10^-7 cm² x 1 MW/cm² = 8.7 x 10^-7 MW = 870 mW (Theoretical damage threshold)
     8.66 x 10^-7 cm² x 250 kW/cm² = 2.1 x 10^-4 kW = 210 mW (Actual safety level)

The effective area of a multimode (MM) fiber is determined by the core diameter, which is generally much larger than the MFD value of an SM fiber. For optimal coupling, Thorlabs recommends that the spot size of the beam be focused to 70 – 80% of the core diameter. Due to the large effective area of multimode fiber, the power density of the fiber end face is reduced, so higher optical power (generally on the order of kilowatts) can be coupled into multimode fiber without damage.

Ferrule/connector termination related damage mechanisms

Fibers with terminated connectors need to consider more power applicable conditions. Optical fibers are typically epoxied into ceramic or stainless steel ferrules. When light is coupled into the fiber through the splice, light that does not enter the core and propagates in the fiber is scattered to the outer layer of the fiber and then into the ferrule, where the epoxy is used to hold the fiber in place. If the light is strong enough, it can melt the epoxy, vaporize it, and leave a residue on the surface of the joint. In this way, local absorption points appear on the end face of the fiber, resulting in reduced coupling efficiency, increased scattering, and

The damage associated with epoxy is wavelength dependent for several reasons. In general, short wavelength light scatters more strongly than long wavelength light. Since the short-wavelength single-mode fiber has a smaller MFD and generates more scattered light, the shift in coupling is also larger.

To minimize the risk of melting the epoxy, an epoxy-free air-gap fiber splice can be constructed between the fiber and the ferrule near the fiber end face. Connectors of this design feature are used in our high power multimode fiber patch cables.

Determining power suitability with multiple damage mechanisms

Fiber optic patch cords or assemblies can be damaged in a number of ways (eg, fiber optic patch cords), and the maximum power applicable to the fiber is always limited by the minimum damage threshold associated with that fiber optic assembly.

The graph shows the approximate power applicable level for terminated single-mode silica fiber. Each line shows the estimated power level considering the specific damage mechanism. Maximum power applicability is limited by the lowest power level of all relevant damage mechanisms (represented by solid lines).

For example, the graph above presents an estimate of the limited power applicability of single-mode fiber patch cords due to fiber end-face damage and damage caused by optical splices. The total power available for a terminated fiber at a given wavelength is limited by the lesser of two limits (represented by the solid line) at any given wavelength. Single-mode fibers operating around 488 nm are primarily limited by fiber end-face damage (solid blue line), while fibers operating at 1550 nm are limited by damage caused by the splice (solid red line).

For multimode fibers, the effective mode field is determined by the core diameter, which is generally much larger than that of SM fibers. As a result, the power density on the fiber end face is lower, and higher optical powers (generally on the order of kilowatts) can be coupled into the fiber without damage (not shown in the figure). The damage limit of the ferrule/splicer termination remains unchanged, so that the maximum applicable power of the multimode fiber is limited by the ferrule and splice termination.

Note that the values on the curve are only rough estimates of power levels where damage is unlikely to be caused by reasonable handling and alignment steps. It is worth noting that optical fibers are often used at power levels exceeding the above-mentioned power levels. However, such applications generally require professional users and testing at lower power before use to minimize the risk of injury. But even so, these fiber optic components should be considered laboratory consumables if used at higher power levels.

Damage Threshold in Fiber

In addition to the damage mechanism of the air-glass interface, the damage mechanism of the fiber itself will also limit the power level used by the fiber. These limitations affect all fiber optic components as they exist in the fiber itself. Two types of damage in optical fibers include bending loss and photodarkening damage.

Bending Loss

The angle of light propagating into the core-clad interface in the fiber core is greater than the critical angle, so that it cannot be totally reflected, and the light will exit the fiber in a certain area, and bending loss will occur at this time. Light exiting the fiber typically has a high power density and can burn through the fiber coating and surrounding loose tubes.

There is a special fiber called double cladding that allows the fiber cladding (the second layer) to also act as a waveguide like the core, reducing the risk of bending damage. By making the critical angle of the cladding/cladding interface higher than the critical angle of the core/cladding interface, light exiting the core is confined within the cladding. This light leaks out over a distance of a few centimeters or meters rather than a localized point within the fiber, minimizing damage.

Photodarkening

The second damage mechanism in the fiber is called photodarkening or negative induction phenomenon, which generally occurs in ultraviolet or short-wavelength visible light, especially in fibers with germanium-doped cores. Fibers operating at these wavelengths have increased attenuation as exposure time increases. The cause of photodarkening is mostly unknown, but a series of measures can be taken to alleviate it. For example, it has been found that fibers with very low hydroxyl ion (OH) content resist photodarkening, and other dopants, such as fluorine, can also reduce photodarkening.

Even with the above measures taken, all optical fibers will suffer from photodarkening when used in UV or short wavelength light, so fibers used in these wavelengths should be considered consumables.

Precautions for using optical fibers at high power
In general, fibers and fiber components should operate within safe power level limits, but under ideal conditions (excellent optical alignment and very clean fiber endfaces), the power available for fiber components may increase. Users must first verify the performance and stability of the fiber within their system before increasing the input or output power, following all required safety and operational guidelines. The following items are some helpful suggestions for considering increasing optical power in a fiber or assembly.

Alignment steps are also important to prevent fiber damage from coupling light into the fiber. During the alignment process, light is easily focused on a part of the fiber other than the core before optimal coupling is achieved. If the high power beam is focused on the cladding or other parts of the fiber, damage can occur due to scattering

1. 1.Using a fiber fusion splicer to splicing fiber optic assemblies into the system increases the applicable power as it minimizes the possibility of air/fiber interface damage. The user should follow all appropriate instructions for preparation and quality fiber splicing. Poor splice quality can lead to scattering, or localized hot regions at the splice interface, which can damage the fiber.
2. After connecting the fiber or components, the light source should be tested at low power and aligned with the system. The system power was then slowly increased to the desired output power, while periodically verifying that all components were well aligned and that the coupling efficiency did not change relative to the optical coupling power.
3. Bending losses due to sharply bending the fiber may allow light to escape from the stressed area. When operating at high power, a large amount of light escapes from a very small area (the area under stress), creating localized high heat that damages the fiber. Please do not damage or abruptly bend the fiber during operation to minimize bending loss.
4. The user should select the appropriate fiber for the given application. For example, LMF fibers can be a good substitute for standard single-mode fibers in high-power applications because they provide better beam quality, larger MFD, and lower power density at the air/fiber interface.
5. Step-index silica single-mode fibers are generally not used for UV light or high peak power pulse applications, as these applications are associated with high spatial power densities.