Introduction

Precision Mechanics and Optics

The most important element of fiber coupler systems is the fiber coupler. This is designed to couple laser light with high efficiency and the correct direction of the polarization axis in the optical fibers. For this purpose, the LINOS fiber coupler incorporates a lens and is equipped with diverse adjustment elements.

Construction of the LINOS Fiber Coupler

The LINOS fiber coupler is designed as a tilting coupler; i.e., the fiber coupler lens and the fibers are permanently connected to one another and are tilted around the beam axis by three fine adjustment screws (arranged at less than 120°). The restoring force needed is generated by three fillister-head screws with six conical spring washers, which are each offset by 60° from the fine adjustment screws, which means the spring washers are located opposite from the screws. Tilting the fiber coupler lens and fiber light guide with respect to the laser beam causes a lateral shift in the focus on the end of the fiber. The resulting reduction ratio in the leverage allows single-mode fibers with small core diameters to be precisely adjusted. Using the three screws that generate restoring force, the position set can be permanently fixed by increasing the force. The tilting adjustment range is 2° from the parallel basic position. 

In addition to being able to be tilted, the fiber coupler lens can be shifted together with the fiber in the x and y direction perpendicularly to the beam axis. This can also be accomplished using the two fine adjustment screws whose counterpressure can be adjusted by two spring bolts with thumbscrews. Here, the contact points are on a conical plane so that inadvertent tilting while shifting the lens and fiber in the X or Y direction can be virtually ruled out.

Shifting the X-Y direction of the fiber coupler lens and fiber changes the angle at which light enters the fiber. The position set can be permanently secured using an additional locking screw. The fiber coupler lens can be shifted together with the fiber by ±1 mm. As the LINOS fiber coupler has been specially designed for use with polarization-maintaining fibers, it is possible to optimally couple the linear polarization of the laser in one of the two main fiber axis (in this case, the slow axis) by turning the fiber along with the fiber coupler lens by 360° around the beam axis. In the process, centering is accomplished by using the adjustment elements for X-Y shifting.

Moreover, the LINOS fiber coupler has been designed so that the fiber cable can be detached. This is important, for example, if you need to disconnect both components for transportation. Moreover, this enables a defective fiber cable to be exchanged independently of the fiber coupler. For this reason, the fiber cables have an FC male connector can be attached to the matching FC female connector on the fiber coupler.

To optimize coupling, the fiber coupler offers the possibility of changing the distance between the lens and the fiber by ±0,5 mm from the center position.This is done by shifting the mounted lens in the Z direction via a laterally attached set collar using a fine adjustment screw. The counter pressure required for this is generated by a pressure spring. The lens mount can be secured in its position by two setscrews. This adjustment mechanism allows the high transmission of the fiber coupler system to be attained again even after exchanging the fiber guide. To prevent the setting of the fine adjustment screw from being accidentally changed, access to this screw is sealed off by a protective plug. Furthermore, the setscrews for securing the lens mount are not accessible using the adjustment tools.

Two Allen wrenches with a size of 2 mm (for fine adjustment screws) and 1.5 mm (for screws with conical spring washers) are needed to adjust the fiber coupler. To attach the fiber coupler to a laser using M3 fillister-head screws, a 2.5-mm Allen wrench is required.

Fiber Coupler Lens

The main component of the fiber coupler is the lens that is used to focus the laser beam input on the fiber core. Two major criteria must be met in selecting the appropriate fiber coupler lens:

• The image-side numeric aperture (NA) of the lens must not be larger than the NA of the fiber.
• The diameter of the focused laser beam must not be greater than the mode field diameter (MFD) of the fiber.

Special requirements on fiber coupler systems for multi-line applications, in other words, simultaneous operation for several different wavelengths in the particular spectral range, give rise to problems with chromatic aberration (different focal lengths for different wavelengths). For this reason, LINOS uses an achromatic lens, which has (nearly) the same focal length for the various wavelengths, in its fiber coupler.