Fiber Optic Beam Delivery Systems
Conventional Laser Beam Delivery Systems
For material processing, the output of the laser must be focused onto the material surface. Conventional beam delivery systems utilize lenses and mirrors to accomplish this purpose. Specifically, the following elements are used:
An upcollimator (beam expander) is used to increase the size of the beam, and reduce its divergence (see below)
One or more mirrors are used to direct the beam towards the material
An objective lens focuses the beam onto the sample
The difficulties with this type of system stem in part from a basic characteristic of all lasers. As a laser beam travels through space, it expands (or diverges). This expansion causes two difficulties:
For delivery over long distances, the beam can become very large, requiring commensurate increases in the diameters of the optical elements1. In the case of the objective lens, increasing the diameter limits the minimum focal length, and may introduce aberrations in the optical performance. Both of these factors increase the minimum focused spot size.
As the distance between the laser and the objective lens changes, the focused spot size also changes. The only way to maintain a constant spot size is to keep the optics fixed, and move the material. For large objects, this may be difficult or impossible.
In addition to the problems caused by the laser beam divergence, conventional beam delivery systems are rather inflexible. Changing the relative positions of any of the elements can cause misalignment problems, especially if any rotations are required (such as welding or cutting contour surfaces). For these applications, delivery of the laser radiation through a flexible optical system is highly desirable. Ideal characteristics of this system include:
Constant beam diameter over a range of distances
Flexibility (position and orientation) in positioning the focused spot
Complete enclosure of the beam, for safety reasons.
For these applications, optical fiber technology holds high promise.
Optical Fiber Technology
An optical fiber (Figure 1) consists of two concentric layers: a core surrounded by a cladding. The core and cladding are typically both fused silica, but with slightly different indices of refraction2. This construction allows light traveling through the core at less than a critical angle3 to be totally reflected whenever it hits the core-clad interface. This "total internal reflection" allows the beam to be propagated along the length of the fiber, with all of the beam energy contained within the core. A typical optical fiber used to deliver laser radiation has a core diameter of 400 µm to 1000 µm, and a cladding diameter of 1100 µm. The fiber is typically enclosed in an armor jacket (diameter 8 mm) to protect it from damage. Typical indices of refraction are 1.457 for the core, 1.440 for the cladding. These values result in a critical angle4 of about 81.2°. This in turn means that rays striking the end of the fiber at an angle of 12.8° or less will be propagated. This angle is often referred to as the acceptance half angle. The acceptance half-angle of the fiber is often expressed in terms of numerical aperture (NA), which is the sin of the angle5. For this fiber, the NA is sin (12.8°), or 0.22. To avoid confusion, it should be noted that the critical angle (which is referenced to the surface normal of the core-clad interface) is a minimum angle for total internal reflection, while the acceptance angle (which is referenced to the surface normal of the fiber end face) is a maximum angle.
Figure 2. Optical Fiber
The above description refers to a particular type of fiber, referred to as straight core, step index. Other types of fibers (gradient index, dual clad, and tapered core) are also available, and offer different combinations of parameters.
Once energy has entered the core (subject to the angle constraints discussed above), it is propagated, with the only losses due to absorption or scattering within the core material. These losses, referred to as attenuation losses, are very low: the attenuation factor is typically < 5 db/km, which corresponds to a power loss of only 11% through a 100 meter long fiber.
As discussed above, as long as the angle of incidence is greater than the critical angle, the beam will be propagated within the core. Bends in the fiber may change the angle of incidence, and potentially allow some of the energy to escape. However, for bend radii as small as 150 mm, this effect is negligible.
In summary, optical fibers have the following properties which make them appealing for delivery of high power laser radiation:
Optical fibers are thin and highly flexible
Optical fibers transmit radiation over long distances with minimal energy loss.
The optical fiber completely contains the laser beam within its core, keeping the beam diameter constant. It thus eliminates the problems of beam divergence over long distances, and reduces the risk of personnel exposure.
Fiber Optic Beam Delivery Systems
A Fiber Optic Beam Delivery (FOBD) System includes more than the optical fiber. Referring to Figure 2, the system includes three additional subsystems:
Input Coupling Optics
Fiber End Connections
Output Coupling Optics
Figure 3. Fiber Optic Beam Delivery System
Input Coupling Optics
The purpose of this optical assembly is to couple the energy from the laser into the core of the fiber. The input coupling optics generally include an upcollimator (which expands the laser beam6), and a focusing lens assembly, which focuses the beam into the fiber. To function properly, the system must meet the following criteria:
All of the energy must be focused into the core of the fiber. Energy that is focused into the cladding or outside of the fiber can cause catastrophic failure near the end of the fiber, especially at high power levels. Therefore, the diameter of the focused spot must be smaller than the core diameter of the fiber, and the spot must be aligned to the center of the core.
None of the energy can arrive at an angle greater than the acceptance angle of the fiber. Any energy arriving at a greater angle will not be completely reflected at the first core-clad intersection; the energy escaping into the cladding will be lost, and may also cause catastrophic failure. Therefore, the cone angle of the input beam (determined by the size of the beam at the focusing lens, and the focal length of the lens) must be less than the acceptance angle of the fiber.
Fiber End Connections
The fiber end connections serve several purposes:
Since the fiber core diameter and the size of the focused spot are quite small (< 1 mm), alignment and stability are critical, if catastrophic failure is to be avoided. At the same time, easy replacement of fibers is required, ideally without the need for realignment. A properly designed connector accomplishes both.
At a glass-to-air interface (such as the end of the fiber), a percentage of the laser power can be reflected from the surface (this reflection is also referred to as Fresnel losses). Typically, the reflected power is about 4% of the incident power (for 2000 watts input, about 80 watts is reflected). The connection system must be capable of dissipating the reflected energy without either damaging the fiber or causing it to change position.
The ideal connection system will employ a method to reduce the Fresnel losses at the surface. This increases the amount of power delivered to the material to be processed, and it also reduces the requirements to dissipate the reflected energy.
The fiber end connection typically consists of a mechanical connector (with mating socket) which rigidly holds the fiber. Possible methods to reduce the Fresnel losses include depositing an anti-reflection (AR) coating on the fiber ends (this technique is routinely used for fixed optics, but until recently has not been feasible for optical fibers).
Output Coupling Optics
The purpose of the output coupling optics is to collect the radiation leaving the fiber, and re-focus it onto the material to be processed. The parameters of the focused beam, which vary with the specific application, include spot size, beam profile, depth of focus, and working distance.
The output coupling optics generally includes two separate lens assemblies. The first assembly collimates the beam leaving the fiber. Its f-number7 must be low enough to collect all of the radiation leaving the fiber8. The second lens assembly focuses the collimated beam onto the workpiece. The final spot size is a function of the fiber core diameter, the clear aperture of the focusing optics, the working distance of the focusing lens assembly, and any optical aberrations.
1. Sterling, Donald J. Jr., Technician's Guide to Fiber Optics, Second Edition, Delmar Publishers, 1993.
2. Marcuse, Dietrich, Theory of Dielectric Optical Waveguides, Second Edition, Academic Press, 1991.
1 The divergence of a 2000 watt CW Nd:YAG laser is roughly 25 mrad, or 1.4°. The beam from this laser would expand about 25 mm per meter of travel. An upcollimator (beam expander) reduces this value by the expansion ratio, but it also increases the initial beam diameter. For a beam path of 5 meters, the diameter of the raw beam would be 130 mm; a 3:1 upcollimator would reduce this to about 60 mm. In either case, standard laser optics (with diameters from 20 to 50 mm) could not be used.
2 The index of refraction (n) of a material is the ratio of the velocity of light in free space to the velocity of light in the material.
3 The critical angle ac is defined as arcsin (nc/nf), where nc is the index of refraction of the core material, and nf is the index of refraction of the cladding
4 The acceptance half-angle Qc is calculated as arcsin [nf N sin (90 - ac)].
5 The NA of a fiber can be calculated directly from the indices of refraction of the core (nf) and the cladding (nc): NA = [(nf)2 - (nc)2]´
6 The upcollimator, by expanding the laser beam, reduces its divergence. This, in turn, allows the beam to be focused to a smaller spot.
7 The f-number of a lens (or optical system) is the ratio of its focal length to the diameter of its clear aperture.
For a fiber with NA of 0.22, the
f-number of the collimating lens
must be 2.2 or lower