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Section 3: Fiber Optics in Medicine - Coupling Light in Fiber Optics - INSTRUCTORNancy Donaldson, Ph.D. and Charles Gosselin, Rockhurst University; Mary Lowe, Ph.D. and Alex Spiro, Ph.D., Loyola University Maryland; Mark Nadeau, M.D., University of Texas Health Science Center– San Antonio_________________________________________________________________________________

Learning objectives

1. Students will view an animated laser endoscopic procedure to blast bile duct stones and be able to understand how to couple laser light into the proximal end (physician’s end) of an optical fiber. Students will understand the consequences of imperfect coupling.

2. Students will understand how to couple diverging light from an LED into an optical fiber to maximize the amount of light that travels down the fiber.

A. Coupling light into the fiber

Now we will move on to think about how light transfers from one optical device to another, i.e. from the light source to the fiber or from fiber to fiber - this is called coupling.

Coupling is a concept you have seen used commonly in your everyday world. A limited analogy to fiber optics coupling can be made with the use of hoses to transport water over longer distances. It is important to use the right size coupler (center of figure below) between two hoses so as to keep all water moving along to the second hose. If water leaks out at the coupler, there is less water coming out at the distal end.

Similarly, manufacturers of fiber optics use couplers to join fibers together or to connect the light source to the optical fibers. However, the coupling is much more complex. For efficient coupling and transfer of light, not only do we need to keep all the light traveling from one fiber to another, we also need to

http://commons.wikimedia.org/wiki/File:Garden_hose.jpg

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direct the light from one optical device to another within the correct acceptance angle so that the light continues to travel by TIR down the coupled fiber.

Just as in hose coupling, for the best light transfer and greatest power at the output (distal) end, we want to reduce coupling loss. Whenever light leaves an optical system through inefficient coupling, there is a loss of power of transmitted light.

How can inefficient coupling result in loss of light intensity at the distal end of an optical fiber?

1. Misalignment of fiber/fiber or laser/fiber coupling: rays are not aimed into the fiber correctly.2. For laser/fiber coupling, there may be a mismatch between NA of optics and NA of fiber: rays

enter fiber outside of acceptance cone.3. Light spot is larger than the fiber diameter.4. At the coupling interface, there may be reflections.

Coupling is a very important concept in the use of fiber optics in medicine. Since our goal will be to optimally deliver light to the patient for medical procedures, we have to consider how to get light from the light source into the optical fiber and what we want the light to do when it comes out at the distal end. These decisions will guide the type of light we use, and the coupling at the proximal (close to physician’s eye) and distal (patient) ends of the optical fibers used in the endoscope.

In this activity, we will investigate how to direct light into a fiber: coupling.

B. Patient scenario

There are different uses for fiber optic light. In the colonoscopy and appendectomy scenarios, a fiber optic is used to transmit visible light to illuminate and view the inside of the body. Fiber optics is also used to deliver laser light into the body for laser surgery when physicians want to cut, blast, vaporize or irradiate tissue. Let’s look at another patient scenario:

Patient scenarioOscar, age 64, had severe pancreatitis characterized by intense upper abdominal pain radiating to his back, nausea, and vomiting that worsened when he ate. His gastroenterologist suspected that he had developed a large bile duct stone that was obstructing his biliary duct. The MRI (below) shows the bile duct stone. To remove the stones, the gastroenterologist performed an endoscopic procedure called ERCP (endoscopic retrograde cholangiopancreatography.)

couplercouplerLaser or lamp

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Background information on ERCPERCP is an endoscopic procedure that uses visible light to illuminate and view the bile and pancreatic ducts and may use forceps to cut away at the stones or sound waves to blast the stones. The approach chosen depends on the size and location of the stone or stones.

Holmium laser lithotripsy technology is combined with ERCP and uses a powerful infrared solid state holmium:YAG laser ( = 2.1 m) to blast the stones. This laser therapy uses the SpyGlass® System, a slim endoscope inside of the illuminating/viewing endoscope that allows the gastroenterologist to apply the

(Left) Magnetic resonance imaging scan shows a common bile duct stone (red arrow).

(Lower left) Endoscopic retrograde cholangiopancreatography (ERCP)

(Lower right) Liver, pancreas, and bile duct.

h ttp://commons.wikimedia.org/wiki/ File:CBD_stones.JPGhttp://commons.wikimedia.org/wiki/Category:ERCP#mediaviewer/File:Diagram_showing_an_endoscopic_retrograde_cholangio_pancreatography_(ERCP)_CRUK_097.svghttp://commons.wikimedia.org/wiki/File:Pancrease_biliary1.png

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most efficient stone breaking technology available to these difficult stones in the bile and pancreatic ducts. The efficiency of holmium laser lithotripsy means that in difficult stone cases the gastroenterologist can, in many cases, eliminate the need for a repeat ERCP or for an open surgical intervention to remove the stone. ERCP has been shown to be safe and effective (Dasari et al, 2013; Aawsaj et al, 2015). Holmium laser lithotripsy has also been used for fragmenting calculi in urological procedures (Anup et al, 2015).

View the video to see this procedure: Spy Animation w clinical_Hi Res_QT.mov

To learn how a laser can break stones in the body, we need to study the optics to get the light from the source to the stones. In order to do the surgery inside the biliary system, several things need to happen. The laser system needs to:

Optimally couple the laser source to the optical fiber Travel through the fiber with minimal loss (TIR) Be focused in the proper place to do its job

We have already talked about how light travels down the optical fiber by TIR. We’ll now tackle the other issues using our knowledge of geometric optics. We’ll start by investigating how to get the light from the laser source into the optical fiber.

C. Getting light into an optical fiber

In a medical endoscope used for laser surgery, the laser is contained in a large box, and the laser beam is coupled into the fiber for the surgeon’s use in the patient (see figure below).

Depending on the type of laser, the diameter of the beam ranges between 0.5 mm and 3 mm (500 m – 3000 m). However, a typical medical glass fiber for surgery has a core diameter of 200-1000 m. For imaging, the core diameter is even smaller; each fiber in the bundle is only ~10 m in diameter.

By what factor is the diameter of a typical surgical medical glass optical fiber smaller than the diameter of the laser beam? Surgical fiber is an example of multimode fiber.dfiber = 300 m = .3 mmdlaser = 3 mmfiber is 1/10 dlaser.

Since the laser beam is so much larger in diameter than the fiber, how do we get it into the fiber without losing much of the light to the surroundings (like a badly coupled hose)?

To investigate, we’ll use a macroscopic representation, where a ¼” acrylic rod will serve as the optical fiber. Visible light from an LED light source will be directed into the rod.

Lecture on intensity and power.

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Activity 1: Getting white LED light into a macroscopic “fiber” consisting of a ¼” acrylic rod. Application: Illumination inside the body.

Materials:

Vernier Combination 1.2 m dynamics track/optics bench Parts from Vernier: Optics Expansion kit, specifically LED light source, 10 cm and 20 cm

converging lenses, aperture set to 1/4”, screen. ¼” acrylic rod with polished ends, 18” length - used to represent the “fiber.” Parts from Thorlabs to support acrylic rod: small kinematic V-clamp mount (KM100V), 2” post

(TR2), 2” post holder (PH2), base (BA2). Parts from McMaster-Carr: screw and square nut to mount base to optics bench, specifically

knurled-head thumb screw, 1/4x20, 5/8” L (#94567A540), and square nut, 1/4x20 (#94855A247).

Index card – used for aligning the beam and positioning the acrylic rod Optional: Baby powder, chalk dust or Fog Spray (Arbor Scientific).

Turn on the LED light source. Recalling your work in geometric optics, what path does the light take as it exits the light source? Does it diverge or does it travel in parallel rays? Draw a ray diagram.

Look at the acrylic rod mounted at the end of your optics bench. It has a ¼” aperture in front of it that allows us to look at just the light that goes into the fiber (light around it is absorbed). Check the intensity of the light exiting the rod by placing your screen 5 cm behind the rod.

o Compare the power of the light exiting the rod to the power entering the rod.o Compare the power of the light exiting the rod to the power of the LED.

Photodynamic therapy. A patient is given a photosensitive drug containing cancer killing substances which are absorbed by cancer cells. The figure shows a close-up of the surgeon’s hands. During the surgery, the light beam is positioned at the tumor site, which then activates the drug that kills the cancer cells. Photosensitive drugs can also be given intravenously. Light can be delivered to the target area using an endoscope (Huang, 2005).

http://commons.wikimedia.org/wiki/File:Photodynamic_therapy.jpg

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Light diverges. The LED is almost like a point source.A small fraction of the light from the LED enters the rod.

The power exiting the rod = power entering the rod because of TIR (ignore scattering).The total power of the light exiting the rod is far less than what the LED produces.

Prediction: Discuss with your group - what are two things you could do to increase the amount of light from the LED light source that enters the rod? (This will also increase the amount of light coming out of the fiber).

1. Move the rod closer to the light source - put the rod right up against the light source. 2. Use a converging lens to collect more light from the LED and focus the light into the rod.

Let’s try one technique to increase the amount of light entering the rod:

Move the light source so that it rests near the center of the acrylic rod. Keep the screen distance 5 cm from the far end of the fiber. Observe the light on the screen that is coming out of the end of the rod. Do you see a change in the amount of light exiting the fiber when you move the LED close to the acrylic rod? Why? Explain in terms of power and intensity, and write an equation for the power entering the rod. Note, however, that a lot of the LED light is wasted.

More light exits the rod.Not all of the light from the LED enters the rod.The area-of-interest is the area of the endface of the acrylic rod. When the acrylic rod is close to the LED, the intensity is greater, and the power entering the rod is greater. Power entering rod = (power LED/4r2)*(area of endface), where r is the distance between the LED and the endface.

Let’s try another method for increasing the amount of light entering the rod:

The acrylic rod is a macroscopic version of an optical fiber. In a real fiber optics scenario, we have to get the maximum amount of light – within the correct acceptance cone – into the fiber. Therefore, our ultimate goal in this experiment will be to “couple” the diverging LED light to our acrylic rod “optical fiber” without loss of power at the distal end of the acrylic rod. In order to do so, we’ll go through several steps to make this happen:

Start with a ray diagram. Our task will be to collect as much light from the LED and converge it to the smallest possible diameter. Think about how to do this; you will need two lenses.

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Implement your idea with the equipment.o What is the diameter of the parallel beam you created? What is dictating the diameter?o Does it matter where you place the 20 cm lens?

Place a 10 cm converging lens about 10 cm in front of the light source. This will place the LED light at the focal point of the 10 cm lens. Roughly speaking, the light from the LED will be collimated after the 10 cm lens. The collimation is imperfect because the LED is not actually a point source.

Move the paper back and forth behind the 10 cm converging lens so that the light from the 10 cm converging lens remains the same diameter. Adjust the position of the lens as necessary. To check more completely, measure the diameter with a ruler and make sure the light spot stays approximately the same size as you move the screen. At this point, you have created a parallel (collimated) beam from a diverging source.

Diameter of beam is the diameter of the lens. D = ###

If the beam is perfectly collimated, placement of 20 cm lens is not critical.

Technically, an image of the LED is formed. For the purposes of this module, one can think of this image as being a point located at the focal point of the +20 cm lens.

Note: when two lenses are used, the equivalent focal length is given by:f = f1 f2 / (f1 + f2 – d) where the position of the equivalent lens is approximately in between the two lenses when d is small.

In this experiment, f1 = 10 cm and f2 = 20 cm. If d is 10 mm, the equivalent focal length is about 69 mm. If the object (i.e. LED light source) is 10 cm from the equivalent lens, the magnification is about -2. So this system is producing an image of the LED that is about twice as large as the LED. In terms of size, it would be better to make the image ½ the size of the LED by interchanging the two lenses. However, the problem with using the 20 cm lens as the collimating lens is that it will collect 4x less light, so the image will have less power. All in all it is better to use the lens combination that will get as much light into the rod as possible. Therefore, the setup described in the procedure is used because it collects enough light and produces an image that best fits the diameter of the rod.

The point where the rays converge on the far side of the 20 cm converging lens gives us the best concentration of light rays from the LED light source. This is the optimal location to place the acrylic rod “optical fiber” so that the maximum amount of light enters the fiber. Let’s test that you have found that point:

1. As you move the 20 cm lens back and forth towards and away from the acrylic rod, watch the light on the aperture. When the diameter of the light beam is equal to or less than the size of the acrylic rod, you are getting the maximum amount of light into the acrylic rod “fiber.”

2. Place the screen directly at the distal end of the rod. Observe the intensity of light as you move the 20 cm lens back and forth. Comment on the spot size, intensity, and light power.The spot size is fixed at Aendface so the intensity is proportional to the light power entering the rod.

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Intensity = Prod/Aendface

Power varies as 20 cm lens is shifted because light may not be focused onto the proximal end.Intensity at the focal point of 20 cm lens is greater than intensity off-focal point, but power is the same.

3. Place your screen 5 cm behind the acrylic rod and observe the intensity of light on the screen as you move the 20 cm lens back and forth. When is there more light power on the screen? Where or when is the light intensity the greatest?The spot size on the screen appears to vary probably because the rod is underfilled. The power exiting the rod decreases as the 20 cm lens is shifted because the focal point is not at the proximal end of the rod. The intensity at the distal end varies. Intensity is greatest in the center. Distinguish between power and intensity. A discussion is useful.

For a more quantitative method to find the optimal position of the 20 cm lens, you can use a light sensor at the distal end. The procedure is described below.

4. Ask your instructor to come by to test with fog – notice the location of the 3-dim cone of light when it narrows to the smallest diameter. If we assume the LED is a point source, then the narrowest location represents the focal point of the 20 cm lens. (Actually, since the LED is not a point source, the narrowest location is the image of the LED (object).) In addition, look at the light at the distal end. Do you see a cone?This is a cool demo. Baby powder also works but is messy.

If you remove the rod, the beam will look like an hourglass. At the narrowest point of the hourglass is a small image.

You can also look at the cone of light at the distal end.

5. Recall the acceptance angle of an acrylic rod with an air cladding. Is the acrylic rod “fiber” overfilled, underfilled, or filled to the maximum?Acceptance angle of acrylic rod clad with air = 90 degrees.Because the acrylic rod is underfilled, the cone of light at the distal end will be less than the NA of the rod.

3-dim cone of light: In the activity above, we observed that the light leaving the 20 cm converging lens exited in a 3-dim cone. The image of the LED light source is located at the narrowest portion of the beam. If we consider the LED light source to be approximately a point source, then the image is located at the focal point of the 20 cm lens. In addition there is a cone of light at the distal end of the rod.

D. Getting laser light into an optical fiberIn the activity above, we directed an LED light source into a ¼” acrylic rod that served as a model of a very large fiber. The general principles of this method are used to build the illumination system of an endoscope, but the components in the endoscope are more compact.

In Part B of this section, light was used to blast bile stones. Because an LED cannot produce sufficiently intense light, a laser must be used and into the fiber.

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The output of a laser is a collimated beam. Suppose you have a laser beam with a diameter of 3 mm and an optical fiber with a core diameter of 300 m. Draw a ray tracing diagram to design a system to couple laser light into a fiber. Use a converging lens to focus the laser beam to a spot. Place the end of the fiber at the focus.

Thinking about the acceptance cone for a fiber, what constraints does that place on the choice of lenses?To not lose light, the 3-dim cone of light produced by the converging lens must be smaller than the acceptance cone of the fiber, i.e. NA of the lens must be less than NA of the fiber. Reducing the NA of the lens can be accomplished by reducing the beam diameter or lengthening the focal length. However, reducing the NA of the lens will cause the focal spot size to become bigger, d f# = 1/(2 NA), so a balance must be achieved between the NA of the lens and the focal spot size.

Numerical aperture versus f-number (from Wkipedia)

Instead, the angular aperture of a lens (or an imaging mirror) is expressed by the f-number, written f/ or N, which is defined as the ratio of the focal length f to the diameter of the entrance pupil D: N = f/D

This ratio is related to the image-space numerical aperture when the lens is focused at infinity. Based on the diagram at the right, the image-space numerical aperture of the lens is:

NA = n sin = n sin[arctan(D/2f)] n(D/2f) for small angles.

thus N ≈ 1/ 2NA, assuming normal use in air (n = 1). For the 20 cm converging lens used in class, what is the NA of the

lens?

D = ______________f = 20 cmn=1 for airNA = n sin

If the coupling of the laser light to the fiber is imperfect, what are the consequences?If the fiber is overfilled (i.e. lens cone is larger than acceptance cone), then light will be lost in the cladding. This results in loss of light at the output of the fiber, and there is a risk that the fiber jacket will be damaged. If the fiber is underfilled, light will travel down the fiber without any loss of light. However there may be variability in the output of the fiber. If the fiber is filled to the maximum, but not overfilled, then there is no loss of energy, and the output is stable and corresponds to the NA of the fiber.

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Last, look at the length of acrylic rod when we have the maximum amount of light entering it. Note that although the converging lens system increased the intensity of light we are able to get out of the acrylic rod, we are still losing some light out of the rod as evidenced by the fact that we can see light coming out of the sides of the rod. If TIR were complete, we would get no light out of the sides of the rod - the only light would come out at the end. What factors would account for this loss of light out of the acrylic rod “fiber?”

o scattering from bubbles, defects, or impurities in the acrylico The manufacturing process of real fiber has undergone a lot of development to reduce

defects and impurities.