When coupling of the laser beam, it is focused into the fiber core, while a high power density occurs on the front surface. Although the anti-reflective (AR) coating of this surface reduces the Fresnel loss, it does not tolerate high power density even despite a large core diameter. A fiber end face without AR coating allows greater performance, but simultaneously leads more heat into the connector on the output side. This must be considered in terms of heat management.

For small fiber core Diameters with high power density dominate fiber connectors with a cylindrical end cap made of quartz glass currently. This end cap is larger than the fiber core and is spliced to the optical fiber. In doing so, the laser beam is focused onto the fiber core via the end surface of the quartz cap, where, at the same laser power, the power density is significantly lower. When, for instance, coupling 500 W of laser power in a 200 micron fiber, a 3 mm long quartz cap changes the energy density at the surface by the factor of 30. This allows the use of AR coating also in the kilowatt range. The method requires robust and low-loss splicing that guarantees a high damage threshold at the border between the end cap and the fiber.

Technical Specifications:

• Connector Type: D80, HP-SMA
• End Cap-Diameter: 1,5 mm
• End Cap-Length:
  - 3,8 mm - for fiber core 100µm und 200µm
  - 2,5 mm - for fiber core > 400µm
• End Cap-Material: Fused Silica
• AR-Coating optional:
  - Only by L 3,8mm
  - AR<0.7% in range of 750nm and 1070nm, also AR <30% (Transmission >70%) at 670nm +-15nm
• Coating absorption: <15ppm@1064nm
• Fiber Type circular: 100/660, 200/500, 400/480, 600/720, 800/880, 1000/1100
• Fiber Type non circular: 100x100, 200x200, 400x400

During coupling into a singlemode fiber, the laser beam is focused into the fiber core, resulting a high power density on the facet. This is higher the smaller the fiber core and larger the laser power. To reduce the power density, an end cap will be spliced on the fiber facet. The laser beam is focused on the fiber core over the end face of this end cap, where at the same laser power, the power density is significantly lower.

The length of the end cap is adjusted exactly to the fiber core. For singlemode fibers this starts at 100 μm. The diameter of the end cap is adjusted to the cladding diameter of the fiber. The singlemode fibers with spliced end cap are terminated with standard connectors (SMA, FC …) but also with application-specific ferrules.

Technical Specifications:

• Connector Type: SMA, FC/PC, FC/APC, Special Ferrule
• End Cap-Diameter: The same like Cladding Diameter
• End Cap-Length: from 100 µm
• End Cap-Material: Fused Silica
• Fiber Type: Singlemode, Polarisation Maintaining PM Fiber

To obtain a good transfer performance, the beam must be optimally coupled into the fiber.
This requires that the focus on the fiber core generally should not to be more than 85% to 90% of the fiber core diameter. At the same time its divergence angle must be less than numerical aperture of the fiber. This is especially important for diode lasers that have a rectangular beam profile. If the laser beam couples not in the core only, but also in the cladding, the modes are also guided in the cladding. In order to remove them from the cladding, the specially developed for this purpose mode stripping is used. This mode stripper is installed in the connector body and removes the cladding modes over a certain length. The required length is defined according to the coupling properties (spot size and NA-difference). The mode stripper ensures that the beam profile looks perfect at the fiber output. The removed cladding modes are converted into the heat in the connector body. If, in this case, the connector is not sufficiently cooled, the fiber might burn off in the connector.

For this reason, different requirements may be imposed on the length of the mode stripper. This is why FCC has developed its own software to determine the mode stripper length.

To monitor the temperature of the fiber, the connectivity and the fiber break, a double-wound copper wire is used. Over connectivity of the fiber connector the circuit is closed. The principle is based on applying voltage at temperature-dependent resistors, which is proportional to the respective resistance. If the voltage falls below the lower or the upper limit, a relay switches, interrupting power transfer to the laser. In monitoring the fiber, the upper limit voltage serves to protect against fiber breakage, while the lower limit voltage bridges the contacts for connectivity. In monitoring the fiber connector temperature, only the lower limit voltage is used to prevent the fiber connector from overheating.

Technical Specifications:

• Operating Voltage: 5 V DC
• Load Bearing Capacity: 10 W
• Operating Temperature: 10 °C bis 30 °C
• Shutdown Temperature: +60 °C
• Tolerance of the Shutdown Temperature: +/- 2 °C
• Lifetime of the Relay: 100000 Schaltzyklen
• Switching Time of the relay: <7 ms