When designing a high-power laser delivery cable, there are three main components that must be considered: the fiber material, the input connector, and the mode stripper (if necessary).
The first thing to consider is the fiber and the fiber-to-laser interface. Common to many high-power fibers is a step-index fused silica core and a fluorine-doped fused silica cladding. A high-purity fused silica core is capable of handling huge amounts of laser energy.
The challenge to getting the energy into the fiber is the air-silica interface, which occurs at the input connector. The quality of the polish will maximize the amount of power that the interface will handle.
Laser polishing of the fiber after the normal polishing step has been shown to increase the power handling by as much as 10–15%. This is done with a CO2 laser that scans across the face of the fiber and re-melts the top surface, removing any subsurface micro-cracking or scratches left behind from the polishing media. The failure mode for continuous wave (CW) lasers is thermal, caused by microscopic irregularities in the air-silica interface that absorb energy.
For pulsed lasers, the failure mode can either be thermal or a dielectric breakdown at the atomic level, depending on the laser’s characteristics. In either case, there is a maximum amount of power per unit of area that can be coupled into the fiber referred to as the laser damage threshold (LDT). For CW lasers, this is expressed in W/cm2 , also known as irradiance. For pulsed lasers it is J/cm2 and is known as fluence. The reason for the difference is that CW lasers run, as the name suggests, as a continuous stream of unbroken energy, while pulsed lasers operate as a series of pulses, or bursts of repeating energy.
Determining if a laser will damage a fiber involves calculating the irradiance or the fluence for the laser by dividing the CW power or the energy per pulse, by the area of the beam where it makes contact with the fiber. The LDT for a CW laser and at the air-silica interface is 1.5 MW/cm2 (at 1064 nm) and for a pulsed laser, 16.0 J/cm2 (at 1064 nm and 1 ns). As a rule, lower wavelengths will require larger fibers or lower powers to couple into the fiber.
For a pulsed laser, it may require lower power or a combination of lower power and longer pulses. There are a few common connectors that are used for high-power assemblies. One is the SMA, another is the D80, but what make them work well are a few common characteristics. The connectors are precision machined and feature an epoxy free termination at the tip. This is done by what is called an air-gap or a cantilevered fiber tip.
Epoxies and other organic materials, if left on the fiber face, will cause the fiber to burn when a laser strikes it. So, an air gap removes epoxy from the front where a laser might contact it and cause it to burn or heat up to the point that it fails and causes outgassing on the fiber face. See Figure below for a view of an air-gap interface.
ysical size and construction constraints when materials (organics) break down. The connectors may utilize extra features to remove excess heat such as heat sinks or water-cooling to prevent the breakdown to the extent possible. Input connectors can be designed with mode stripping capabilities.
Mode strippers are designed to remove energy from the cladding of the optical fiber and dissipate it in the form of heat. For a variety of reasons such as poor alignment or poor beam quality, energy can be coupled into the cladding instead of all of it going into the core of the fiber. When this happens, it can cause heating in the connector and other beam quality issues.
A mode stripper is a predictable way to remove the energy from the cladding at the input connector. Depending on the amount of heat to be removed, mode stripping may be done with an air-cooled heat sink or with a watercooled housing.
When the amount of power needed is higher than the laser damage threshold of the base fiber, the designer will use an end cap as the air-fiber interface. This allows the user to couple higher power into the fiber core by reducing the power density at the air-silica interface. An end cap is designed to optimize the laser launch conditions with the fiber size and numerical aperture. End-cap size to fiber size ratios can range from 2:1 to 10:1 or even greater as the need requires.