How to couple fiberoptics

H O W T O C O U P L E F I B E R O P T I C S

1.) Mikropositioner:

The Mikropositioner sets the proper position of the input collimator lens assy in reference to the input laserbeam, so that the focal point is placed accuratelly onto the fiberoptic core center. The Mikropositioner HMP-2 is optimizing direction angle in combination with the Position horizontal and vertical. All of these 4 paramters are individually adjustable.

First place the HMP-2 mechanically on your optical table or rail system. Insure that the beam position of the laser is best possible centered to the aperture of the HMP-2, also the direction of the beam is best possible in line with the beam path of the Mikropostioner.

By optimizing the mounting position you insure that during the coupling optimation the adjustable range of the angle and position settings is not overranged. Make the mounting as stable as possible, remember you reference the beam onto a fiber core of a few Mikrons ! Unstable mounts may cause misadjustments with permanent damage of the fiber at high power operation.

Set the adjustments position and angle of the Positioner to balanced middle position to have both same capability of forwad or backward adjustments with same range. Also thighten the corner Lock screw slightly to fix position of the input adapter.

After you have checked visually the mounting position and angle to be well centered to the beam you plug in the collimator with the connected fiber. The cylindrical adapter is very precise so that the connected can be repeated serveral times without the need to optimize adjustments. The collimator is key coded for rotation reference - the nose of the input cylinder must match the gab in the collimator housing, also the collimator is fixed in plugged conditions by the screwring - the ring should be smoothly thightened to fix the parking.

When you want to adjust X/Y position you should open the corner lock screw by approx. 2 turns which gives you free traveling range of approx +/-1.5mm, the lock screw at the corner is spring loaded to press the collimator traveling flange against the X/Y setting spindels. Once the position is optimized you should lock its calibration by thighten smoothly the lock screw. The optimation of the input fiber coupling should be started with low power, approx 0.2 Watt at the beginning. Sequentelly optimize positionX and positionY with angleX and angleY

-best possible start with the X/Y angle adjustments as they have the highest coupling sensitivity. Optimation should be made upon your preferences of best mode or highest coupling efficiency. During optimation you should monitor both parameters of interest:

the output power/efficiency and mode/beamdiameter. Please note that there may be different optimations for best mode or highest efficiency on particular fibers which have an autointerfearing effect of output beam concentration. This effect is preffered to produce good beam divergency and diameter at the output with fibers of high NA and core size - mainly typically on gradient index fibers.

Once the Mikropositoner is roughly optimized you can go to slightly higher power levels up to max. 2 Watts when it is insured that the focal length of the collimator is setted in proper range to the fiberfacett.

2.Fiber Input Collimator:

The fiber input collimator is a dust free sealed assembling which remains onto the fiber for quick, clean and accurate connections with high repeatability. The fiber is connected with the collimator by an precise SMA905 compatible connector. The focal adjustment remains perfectly stable in reference to the connected fiber and can be kept when the input laser beam parameters are constant. The focal position of the fiber to the collimator lens is an important factor which mainly determs power capability, efficiency and sensibility of misadjustments. The focal position can be varied by open the locknut of the connector ferrule and rotate the fiber with the ferrule clockwise or counterclockwise to move the fiber input facett toward or away from the lense. For access to the ferrule rotate the transparent protection tube counter clockwise and slip it backward.

Best possible the focal spot diameter should be approx. 80% of the fiber core diameter, to have enough tolerance to misadjustments and variations on one side and high power capability on the other side.

It is of importance that you consider not to place high concentrated focal spots ( which can come down to <6 Micron with TEM00 input laser beam ) onto fiber facett as this may dramatically and permanent damage the quarz fiber material.

There are different standard versions of input collimators available depending on lense types focal lengths and apertures ( Achromatic:f=16mm / f=25mm Biconvex:f=22mm) Achromatic lenses should be preferered due to virtuall no chromatic and spherical aberration but limited to their power capability of absolute max. 10W/CW. Biconvex lenses have higher power ratings (typ. 20Watts) but you should consider a visible reduction in effeciency and also wavelength dependency of focal optimation.

When you disconnect input collimator from the Positioner attach the dust cap to avoid contaminations of the collimator lense. You should disconnect the fiber only in case of service checks from the collimator to avoid fiber facett contaminations in dirty field enviroments.

3.) Fiber output collimator:

The fiber output collimator is identical in its usage amd calibration to the input collimator. The output collimator is in most cases different to the input collimator to optimize output beam diameter and divergency. As long the focal length of the installed lense is, as low is the divergency but as high is the output beam diameter. The output collimator with its focal length should be selected according to your preference of interest wether long or short distance projection and sizes of your scannermirrors.

4.) Fiber cable:

There are several fiber cable types available. In all cases every different fiber model has their advantages and also their disadvantages. Available fibers of interest are listed with their characteristics shortly:

Step index 30 Mikron: excellent output beam, good efficiency, very expensive low power to max. 5-8W, requires perfect coupling adjustments,

Step index 50 Mikron: good output beam, very good efficiency, very expensive, medium power to max. 8-10 W, easy coupling adjustments

Gradient index 50 Mikron: good output beam, moderate efficiency, low price, medium power to max. 8-10W, easy coupling adjustments

Gradient index 100Mikron: moderate output beam, moderate efficiency, low price high power to max. 15-20W, easy coupling adjustments

-high temperature ceramic cement

-free standing fiber center core

optimizes the connector to your specific application matching input power and ruggedness. In general burning damages and fiber breaks are in all cases not covered by warranty and the fibers are sold with their physical specific behavings as components , it is on the side of the client to decide which type of fiber matches best possible the application of interest.

GENERAL CONSIDERATIONS FOR HIGH POWER FIBER APPLICATIONS

In case of high power laser delivery especially in pulsed laser sources (e.g. YAG) fiber delivery applications may become sophisitcated and special care is required in setting up to avoid possible damage of the fiberoptics.

A.) Make absolutely shure that all optical components (lense surfaces an fibertips) are perfectly cleaned

B.) First you should verify the kind of your collimation element wether it is capable of handling the requested power levels:

-Standard lenses may handle power levels up to 50Watts and are also capable to withstand pulsed power to several 100mJ/Mikrosecond. You should also verify which kind of AR- coating is used and wether it withstands the peak power of the pulses. Coatings may have lower damage threshold for peak power pulses than the lense element itself.

-Selffoc gradient index lenses have low damage threshold around 1 Watt and low pulsed power capability

-Achromatic lenses are mostly two element glas compositions which are glued together via optical transparent UV-epoxy with low absorbtion. Average power should not exceed 10 Watts maximum.

There is no pulsed power capability for these lense types.

The fiberoptic part of the delivery system is the most critical part, as it handles the highest power density.

-Perfect optimized alignment of your beam input launcher is required to have the fiberoptic in the proper operation conditions. The fiber core center diameter is NOT determing the damage threshold of your fiber cable, the realized position and diameter of the spotsize is determing peak operation power level !

- it is the goal to have a spotsize 10% less of the fiber core center diameter. 10% in spare to have mechanical tolerances (vibration, long term stability, beam walking of lasersource, expansion coeff.) available.

-Sense with the Z-adjustment of your input collimator the proper focal condition of the input launcher carefully to optimize and match the focal spot size to your applicable fiber.

D.) There are several different damage szenarios for your fiberoptic cable:

1.) You may exceed damage threshold of the core center diameter which is for stepindex fibers pure Silica glas of very low absorbtion. This results in melting the fiber center area at the fibertip. This effects only a depth of a few 1/10 mm.

2.) You may exceed damage threshold of the core center diameter which is for gradient index fibers doted Silica glas of higher absorbtion. This results in material detoration at the particular atomic locations of the doted atoms. This damage threshold is particularely lower compared to stepindex fibers in pulsed and CW applications. This kind of damage may be affecting several meters of the gradient index fiber cable !

3.) You may enter with your spot size the high doted area of your fiberoptic which results in very high absorbtion due to the higher loss caused by the doted atoms. This can be done by accidentally displacing the spotsize in its position out of the symetrical center of the fiberoptic or having a too large spotsize compared to the center core diameter. The outer core area has much lower damage threshold compared to low doted center core area. In case of stepindex fibers this is the high doted reflective area, and in case of the gradient index fibers this is the saturated doting area where the conzentration of doting is much higher compared to the center area. This damage effect results in total burn out of these particular areas including the polymer cladding of the glas fiber which stabilizes mechanically the glass fiber. Also this damage occures to several meters of the fiber input.

4.) You may enter with your spot size the connector front surface by displacing the spotsize completely to the symetical center or you have a much much too large spotsize. This either burns the connector front surface material, or burns out the epoxy glue, or heats the connector up. Material (e.g. steel metal, epoxy,) is vaporized and contaminates the fiber front surface resulting in high absorbtion of the fiber front, which is secondarelly burning out later.