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Introduction
 
Home / Products / PROBES / PATCHCORDS / Power / Power Patchcords/ Introduction
 
 

High power delivery through optical fiber has created many possibilities in many applications (Laser welding or cutting, printing, marking, medical probes, sensing, amplification, etc...). However injecting very high optical power into fibers implies to solve a number of problems. Poor alignment, focus or spot quality can quickly destroy the optical fiber or its connector. Consequently standard connectors and assembly techniques are not appropriate for such applications. Fibers and protection tubes also require a lot of attention.
The first step is to select the fiber according to the power output of the laser, laser spot size, core size and NA of the fiber. If the core size of the fiber is too small, some laser energy may be launched into the cladding, which may result in fiber failure. It is generally recommended that the input spot diameter fills no more than 2/3 of the fiber core.
The wavelength, the numerical aperture and the proper core, cladding and coating materials will also highly influence the power acceptance of the assembly. The best suited fibers are silica/silica fibers.

ALL SILICA FIBERS

The laser power damage level threshold of pure silica is about 1 GW/cm2 for pulsed laser and 2 MW/cm2 for continuous but to achieve maximum power transmission the beam must be focussed properly in the core of the fiber. Because power levels are constantly increasing a part of laser energy can be launched into the cladding. All silica fibers (silica core and silica cladding) are particularly suited for high power delivery. These fibers are optimized for transmission in the UV + Visible or Visible + IR region according to the core and cladding material (doped or not) and high or low water content. High OH fibers are typically preferred for 700nm and shorter wavelengths (see HCG and TCG fibers). Low OH fibers are suggested for longer wavelengths (see HCL and TCL fibers).

HCG and HCL fibers are protected by a thin polymer buffer (15µm) for mechanical resistance, plus ETFE coating. TCG and TCL are protected with a polyimide coating. Polyimide is a high T° material and fibers with this coating can be used up to 400°C Note that polyimide is difficult to remove so it is recommended to keep it to connectorize

   
 
 

Connectors

For high power injection the fiber end must not be in contact with epoxy or metal. Consequently standard connectors with epoxy around the fiber up to the optical face will burn easily. Several connector solutions have been developed mainly derived from the SMA standard:

• SuperNova & Nova SMA: New design with a copper ferrule and large open space around the fiber end and with heat dissipator in option.

• “PowerShot” : Recent development has led to a complete redesign of the SMA connector to meet the cost objective of “Disposable probes”. Priority has been given to reducing manufacturing cost of the connector. But the real challenge was to reduce the cost of assembly on the fiber. The result is a global concept offered under the names “One Shot” and “Power Shot” for power applications.

•  "4Power" : a connector for very high optical power.
SEDI-ATI Fibres Optiques has introduced a new connector for very high power laser delivery systems used in industrial or medical applications and compatible to the Mitsubishi standard. The technology derives from the “high power SMA” connector with a free standing fiber end creating an air gap between the fiber and the metallic ferrule. A number of improvements have been made for better heat dissipation. The ferrule is larger than the SMA’s with a 4mm diameter allowing fibers up to 1000 µm, the material of the ferrule is copper and the body of the connector forms a heat dissipator. Besides the connector allows SEDI’s crimp and cleave special assembly technology which has much better performances than polishing. The connector can be supplied with or without indexation as well as electrical continuity with the metallic tube of the patch cord if required. 

The main idea is to keep a free space around the fiber end to dissipate heat and energy not focused in the fiber core. These specific power SMA connectors are fully compatible with the SMA standard interface and consequently with the mechanical parts of the lasers using SMA bulkheads.

 
 
 

Assembly

The most difficult part is the way to assemble the connector to the fiber (end face quality, stress, gluing, cable retention, etc.). Consequently these connectors are not proposed as a discrete component but only assembled (pigtailed or as patchcord).

• Optical face: they can be polished or cleaved. It is important to note that the highest power is reached with cleaved end faces. Polished end faces even very carefully made have small scratches due to the abrasion process. Not perfectly transparent they burn more easily when high power is injected. The cleaving process on the contrary creates an optical end face which follows the molecular structure of silica and offers better transparency. Our assemblies are consequently made by cleaving on all fibers from 200µm to 600µm. For bigger fibers the polishing process is used. The optical end face is situated in the same position than a standard SMA connector.

• Retention: a distinction must be made between the retention of the connector to the fiber and the retention of the connector to the protection tube (cable, metal, etc.). Both can generate stress on the fiber and consequently creates a hot spot which will eventually burn the fiber. The retention systems are a trade-off between the pulling force which can be applied to the assembly and the maximum stress acceptable by the fiber.

• These high power assemblies can be made with 200µm to 1000µm fibers

 
 
 
 

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