Fiber Optic Beam Delivery System Using Step Index Fiber

In a fiber optic beam delivery system, the output optical assembly (end effector) consists of a dual lens imaging system. The first lens (collimator lens - L1) takes the rapidly diverging beam from the fiber exit, and straightens, or collimates, it. This lens should be placed at a distance exactly equal to its focal length from the fiber exit face. If this is not done, the beam will not be collimated, and the imaging convention described below is void. The second lens (L2) acts as an objective, and focuses the beam to form an image of the fiber face.

If the laser energy is delivered to the end effector through a step index fiber, the beam, as it passes through the fiber, expands to fill the fiber core. Therefore, the beam profile, at the exit face of the fiber, is a top hat, with a diameter equal to the fiber core diameter. This phenomenon is independent of the diameter of the beam when it enters the fiber.

When the fiber optic beam delivery system is configured as described above, the diameter of the focused image is a function of the fiber core diameter, and the ratio of the focal lengths of the two lenses:

If lenses of identical focal length are used, the ratio will be 1:1, and the focused spot diameter will equal the fiber core diameter. If a longer focal length lens is used as the objective lens, then the ratio will be >1, and the focused spot diameter will be greater than the fiber core diameter.

For example, if a 60 mm collimating lens and a 75 mm objective lens are used with a 0.6mm (600 µm) core fiber, then the diameter of the focused spot is 0.75 mm (750 µm, or ~.030"):

Placement of the Objective lens in relation to the collimator lens is not critical, but in general should be 6" or less.

In selecting the collimating lens (L1) focal length, careful consideration should also be given to the diameter of the lens, so as not to overfill it. Step index fiber used by U.S. Laser Corp. generally has a numerical aperture of 0.20. The numerical aperture (NA) of a fiber, which is defined by the index of refraction of its core and cladding, determines the maximum solid angle (2) at which a beam can be launched into it.

Using a 0.20 NA fiber, this acceptance angle is ~23E. This is the maximum angle that a beam can be safely launched into, and propagated thru a fiber, and will also be the maximum angle at the exit of the fiber. Knowing the maximum angle of the beam exiting the fiber, we can then calculate the required clear aperture (A) of the collimating lens to transmit the beam without loss.

For example, if we want to use a 60mm focal length lens as a collimating lens with a 0.20 NA fiber, then:

Thus the clear aperture of the lens must be at least 25.5mm to transmit the beam.

If long working distances and/or small spot diameters are to be achieved, it is then obvious from the above that longer focal length collimating lenses are desirable. However, as the collimator focal length increases, so does the clear aperture requirement, as shown in the following table:

Lens Focal length (Fl_L1)	50mm	60mm	75mm	90mm	100mm
Minimum Clear Aperture	22mm	25.5mm	32mm	39mm	42.5mm

Note: Since the lens must be held in a mechanical mount, the actual lens diameter should be at least 2mm larger than the clear aperture to allow for mounting.

If short focal length lenses are to be used, caution should be taken, as optical distortions could be a significant factor. Where lenses are to be used where the lens F# (the ratio of the lens focal length to the lens diameter) is less than 2, optical aberrations can be significant. These aberrations can be of sufficient magnitude that they become the limiting factor in determining the ultimate focused spot size. In general, short focal length lenses should be avoided. If they must be used, aberration corrected multi-element lenses should be used.

The above discussions have focused on the maximum (or worst case) limits of the fiber optic beam delivery system. Depending on the type of laser, its operating level, and the optics used to couple the beam into the fiber, performance better than described above may be possible. For example, it may be possible to couple the beam into the fiber at angle less than the maximum acceptance angle. Since the angle of the beam exiting the fiber would also be less than the acceptance angle, this would possibly allow the use of a longer focal length collimating lens . This can only be done when the laser output characteristics have been carefully measured (see U.S. Laser Technical Note on M² measurement). If done incorrectly, catastrophic damage to the fiber can occur.