Fiber Optic Beam Delivery for Laser Medicine
By Karl Jalbert, Bishop & Associates Inc.

Laser technology has found its way into a broad range of medical specialties, from gynecology, gastroenterology, dentistry, dermatology, urology, ophthalmology, and neurology to ear, nose, and throat (ENT) surgery, and cancer treatment. Surgical removal of tissue with a laser is a physical process similar to industrial laser drilling. Carbon dioxide (CO₂) lasers are able to burn away tissue because their infrared beams are strongly absorbed by the water that make up the bulk of living cells. A laser beam cauterizes the cuts and stops bleeding in blood-rich tissues, such as the female reproductive tract or the gums. Laser wavelengths, near 1,000 nanometers (nm), can penetrate the eye, welding a detached retina back into place, or cutting internal membranes that often grow cloudy after cataract surgery. Ophthalmologists surgically correct visual defects by removing tissue from the cornea, reshaping the transparent outer layer of the eye with intense ultraviolet pulses in LASIK surgery. Another medical application for lasers is in the treatment of skin conditions. Pulsed lasers can bleach certain types of tattoos, as well as dark-red birthmarks called port wine stains. Cosmetic laser treatments include removing unwanted body hair and wrinkles.

For the laser to do its job, the laser’s “beam” or output must be focused onto the tissue to be treated. A conventional beam delivery system uses a rigid but articulating arm in which there is a system of lenses and mirrors that focus the beam through it. For example, a beam expander, called an upcollimator, expands the diameter of the beam and reduces its divergence. Mirrors are used to direct the beam past each articulation point. At the end of the articulating arm, an objective lens focuses the laser beam onto the cutting tip, or directly onto the tissue to be treated.

The difficulty with this type of delivery system is an inherent trait of all light beams, laser or otherwise—divergence. As a laser beam travels through space, it diverges (expands) and causes two difficulties. First, the longer the beam travels, the larger its diameter becomes, requiring a larger diameter of the hardware components it passes through. In long distances, the objective lens must be larger and larger with every increase in distance. Secondly, large objective lenses limit the minimal focal distance and can increase aberrations, and also increase the minimum spot size, thus affecting optical performance. At this point, the only solution to maintaining a minimal spot size is to keep the optics fixed and then move the tissue. In large objects, such as the human body, this can be difficult or impossible. For these applications, a flexible optical system is highly desirable to delivery of the laser beam. Optical fiber technology holds high promise for these applications. Optical fibers offer:

• 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

Laser Light Transmitted Through Fiber Optics
Fiber optics is the contained transmission of light through long, thin strands of very pure glass about the diameter of a human hair. The light travels by a process of total internal reflection. In Figure 7, you can see that the optical fiber 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 refraction. This construction allows light traveling through the core, at an angle that is less than the critical angle (n_c/n_f), to be totally reflected whenever it hits the core-clad interface (similar to the luminiferous light of an aquarium produced from the light bulb above).

Total internal reflection allows the beam to be continuously reflected back into the core and propagated along the length of the fiber, even around corners, 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 microns (400 µ) to 1,000 microns (1000 µ). The fiber is typically enclosed in an armor jacket having a diameter of about 8mm to protect it from damage. A typical refractive index (RI) for the core material is 1.457, and 1.440 for the cladding. These values produce a critical angle of about 81.2 degrees, which means that rays striking the end of the fiber, at an angle of 12.8 degrees or less, will be accepted into the end of the fiber, but sharper angles will be rejected into the cladding, where they will be absorbed by the buffer material. This angle is the acceptance half-angle which is referred to as the numerical aperture (NA), and is the sine of the angle. For this fiber, the NA is sin (12.8°), or .22 NA. In simple terms, the Numerical Aperture (NA) at the end of the fiber can be thought of as water spraying out from a garden hose when the nozzle is adjusted to a cone (but obviously traveling in the opposite direction—in, not out).

In summary, every optical fiber has the following structure:

• Core—Center of the fiber where the light travels.

• Cladding—Outer optical material surrounding the core that reflects the light back into the core.

• Buffer—High-RI plastic coating that protects the fiber from damage and moisture, and removes unwanted cladding light.

Optical fibers come in two types:

• Multimode—The first type of optical fiber to be manufactured. As the name implies, multimode fibers can accept hundreds or even thousands of different waterfronts of light (typically called light rays or modes for theoretical proposes). Hence, they have large cores, with diameters from about 50m up to 1000m, and wider numerical apertures. Modes result from the fact that light will only propagate in the fiber core at discrete angles within the cone of acceptance (NA). Three types of modes travel down the fiber. One mode travels straight down the center of the core (axial ray). A second mode of rays travels at a steep angle and bounces back and forth by total internal reflection. The third mode exceeds the critical angle and refracts into the cladding. Intuitively, it can be seen that the second mode travels a longer distance than the first mode, causing the two modes to arrive at separate times. This is referred to as modal dispersion, and as a result, multimode fibers have less bandwidth than their singlemode counterparts. Multimode fibers can transmit wavelengths, from the ultraviolet into the mid-infrared spectrum, from many different sources, depending on the types of glass used in their construction.

• Singlemode—Singlemode fibers have tiny cores of about 9m in diameter and transmit only one light ray of light (the axial ray), an emitted and infrared diode-laser at pre-set wavelengths of 1310 nm, 1490 nm, and 1550 nm for maximum efficiency. Singlemode fibers have no modal dispersion because there is only one ray traveling through the core. This results in a nearly infinite bandwidth. Because of this, they are used exclusively in long distance communications and cable television. Single-mode fiber has its disadvantages. The smaller core diameter makes coupling light entering the core more difficult. The tolerances for single-mode connectors and splices are also much more demanding. Singlemode fiber is never used in medical fiber optics.

Optical fibers are manufactured in two profiles:

• Step-Index—A step-index fiber has a uniform refractive index within its core, and a uniform (but lower) refractive index through its cladding, producing a sharp decrease in its refractive index at the core-cladding interface (Figure 4). Singlemode fiber uses the step-index profile. Multimode specialty fiber, such as that used in fiber optic beam delivery (FOBD) systems, is always step-index type. A step-index fiber is characterized by the core (n₁) and cladding (n₂) refractive indices. Step-index optical fiber is generally made by doping high-purity fused silica glass (SiO₂) with different concentrations of materials like titanium, germanium, or boron. Quartz glass is used for delivery of laser light in the ultraviolet spectrum.

• Graded-Index—A graded-index fiber is an optical fiber that has a core with a graduated refractive index that decreases with increasing radial distance from the fiber axis. An analogy could be the concentric rings in the cross-section of a tree trunk. With each larger ring in the tree trunk, the refractive index (RI) would decrease until reaching the tree’s bark. In the actual fiber, the layers of glass closer to the fiber axis have a higher refractive index than the parts near the cladding, allowing the outer rays to travel progressively faster in order to keep up with the inner rays that have less distance to travel. Because of this, light rays are bent a little more through each progressing outward layer they pass through, until they eventually reach the critical angle and begin to reverse their direction, thus creating sinusoidal paths down the fiber. The advantage of the graded-index fiber, compared to multimode step-index fiber, is the considerable decrease in modal dispersion, and thus a substantial increase in bandwidth capacity. This type of fiber is normalized by the International Telecommunications Union ITU-T. It is the choice for fiber optic LANs. Graded–index fiber is never used in FOBD systems.

Some optical fibers can be made from plastic, for less demanding applications with lower bandwidth requirements. These fibers, referred to as Plastic Optical Fiber (POF), have a large core (1mm diameter) and transmit visible red light (wavelength of 650 nm) from LEDs. They are typically found in automotive, avionics, entertainment, and factory floor automation, especially in wet environments. In the early days, plastic fiber was only available in a step-index profile. Today, high-performance GI POF is available from several manufacturers.

Optical Fiber Used for Optical Beam Delivery Systems (FOBD)
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 percent through a 100-meter-long fiber. In optical fiber glasses, such as germanium sulfide, the removal of the hydroxyl radical OH impurities are critical since they quench the fluorescence. OH impurity removal is essential for good optical performance. The optical attenuation characteristics are quite different for high-OH and low-OH fiber core material. The OH content of the core’s fused silica must be formulated into the raw preform material before the optical fiber is made. The choice is dependent on the user’s application. The low-OH-type optical fiber has very low attenuation throughout the near-IR wavelength range, from 700 nm to beyond 1800 nm, except for a small peak at 1385 nm. Several manufacturers, such as Polymicro Technologies, Fiberguide Industries, CeramOptec (pictured above), and Stocker Yale, manufacture high-purity silica step-index multimode fiber for transmission in the deep UV spectrum from 190 to 1600 nm. These products have high laser damage thresholds and are suitable for medical laser delivery systems.

FOBD and Connectors
A FOBD system includes more than the optical fiber. Referring to Figure 9, the system includes three additional subsystems:

• Input Coupling Optics

• Fiber End Connections

• Output 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 beam), 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 numerical aperture of the fiber. Any energy arriving at a greater angle will not be completely accepted by the fiber. 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 NA of the fiber.

The connectors serve several purposes:
Since the fiber core diameter and the size of the focused spot are quite small (<1mm), alignment and stability are critical, if catastrophic failure is to be avoided. At the same time, easy replacement of fibers is required, without the need for realignment. A properly designed connector accomplishes both. The fiber end connection typically consists of a mechanical connector with a corresponding mating socket, which rigidly holds the fiber. 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 called Fresnel losses. Typically, the reflected power is about four percent of the incident power. For example, 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 connector system will employ a method to reduce the Fresnel losses at the surface. This increases the amount of power delivered to the tissue to be processed, and it also reduces the requirements to dissipate the reflected energy. Possible methods to reduce the Fresnel losses include angle-polishing or depositing an anti-reflection (AR) coating on the fiber ends, a technique routinely used for fixed optics that until recently has not been feasible for optical fibers.

Larger diameter fibers used in the FOBD system limit the type of connector that can be used. Off-the-shelf connectors with ceramic ferrules are designed primarily for standard 125m diameter communications fiber. The logical choices for connectors for the FOBD system include the standardized SMA (considered by many to be obsolete, nowadays), ST, and SC connectors with metal or thermoplastic composite ferrules. Because these ferrules can be precision-drilled, they can be sized to any hole diameter necessary to accommodate the larger diameter fibers used in the FOBD.

Karl Jalbert Director of Military & Industrial Technology, Bishop & Associates, Inc. Applications engineer, product manager, and marketing manager, Karl Jalbert’s tenure in the connector industry has spanned over 20 years. In 1985, Jalbert joined the applications engineering team and later became western regional sales manager for Optical Fiber Technologies Inc. (OFTI), which became part of the AMP Inc. product line in 1992. Throughout the 1990s, Jalbert was a senior applications engineer for M/A-Com in Boston, and senior product management specialist for AMP in Harrisburg, PA. Prior to joining Bishop & Associates, Jalbert was a market development director for FCI Electronics USA, focusing on the industrial and embedded computing market sectors.   Jalbert holds a BSME from Central New England College.