Experiment 3: fiber optics
1. Stripping and cleaving
2. Light coupling (HeNe to optical fiber)
3. Fusion splicing
4. Ben loss sensing (corrugated structure)
Introduction
In this lab we will evaluate basic techniques for preparing fibers for use in optical systems,
numerical aperture measurements, and coupling light into fibers. These procedures will be used in
most subsequent laboratories and will have a large effect on your experimental results. Therefore it
will be a good idea to take some time to become familiar with these methods.
In this class we will work with several different types of fiber including multi-mode, single mode,
and polarization preserving. Most of these are designed to operate in a particular wavelength range,
and we will later examine some of he consequences of deviating from the design parameters. Fiber is
usually purchased in large lengths that are coiled on spools. The fiber has a protective plastic sheath
surrounding the cladding. This plastic sheath must be removed prior to mounting in couplers and
other fixtures. The ends of the fiber are usually irregular unless they are prepared in some way.
Polishing or cleaving the ends of the fiber can accomplish this. This provides an optical surface that
allows light to be efficiently coupled into the fiber with little loss. Since fiber is an optical
component, it has a numerical aperture (NA) similar to a lens or other type of waveguide, and it will
be important to know what the value of this parameter is for efficient coupling. Therefore, in this lab
we will concentrate on developing fundamental skills for working with fibers.
Fiber Preparation and Inspection
Before a fiber can be cleaved the protective plastic jacket must be stripped from the glass
waveguide. This can be accomplished in several ways. The first is a mechanical strip with a razor.
Simply scratch the plastic away from the fiber by holding the razor at a slight angle to the fiber.
Another method is to chemically dissolve the plastic with a solvent like methylene chloride. Dip the
section into the solvent for 3-5 minutes, remove the fiber, and rub the plastic off with a tissue. Wear
gloves while doing this since this solvent is toxic. This method works better with small diameter
fibers.
Strip about 2” from each end of the fiber in preparation for cleaving. Cleaving forms an optical
surface for efficiently coupling light into the fiber. We have a decent cleaver for this lab. Read the
instruction sheet and practice making cleaves using MLD (multi-mode) fiber.
In order to determine the quality of the cleaved surface it is necessary to inspect the end faces of the
fiber. After cleaving the fiber end observe the tip under the microscope to determine if the face is
relatively flat. Illuminate the other end with a flashlight and turn the room lights off to help view the
fiber surface.
Task 1: Fiber Cleave and Inspection
1. Strip the plastic from the both ends of the MLD (multi-mode) fiber (about 3 inches).
2. Cleave both ends of the fiber.
3. Inspect the fiber ends with the microscope to ensure that they are flat using the microscope.
Task 2: Beam Coupling into a Fiber
Now that the fiber ends have been prepared its time to couple light into the fiber. In this experiment
we will be using a HeNe beam and a microscope objective for the coupling. For efficient coupling to
multi-mode fiber it is important to do the following:
1. Make the focused spot smaller than the core diameter.
2. Locate the focused spot near the center of the core.
3. Keep the NA of the beam less than that of the fiber.
Usually satisfying 3 immediately satisfies 1 for multi-mode fiber. It’s a little different for single
mode – we will discuss this later. Parameters for determining the focused spot size must be
determined using the Gaussian beam relations discussed in class. First measure the divergence angle
from the laser.
__Using the measured value for the divergence angle, determine the beam diameter at the
waist of the Gaussian – it can be assumed that the waist is at the output face of the laser.
Also remember that the size of the focused spot is inversely related to the size of the beam at
the lens- not the clear aperture of the lens.
__Calculate the position at which the output face of the laser should be placed relative to
the microscope objective to obtain the correct match to the NA of the fiber. The size of
the Gaussian beam at the lens and the focal length of the objective determine the NA of the
beam emerging from the lens. If the aperture of the 20X objective is completely filled the
effective NA is 0.4 that will overfill the acceptance angle of the fiber. Also remember that
2
these calculations determine the size of the beam at the 1/e points. Some of the beam power
2
resides beyond the 1/e points. Since we are limited in space (and time) the laser is set at a
fixed distance from the objective and aligned to the objective.
__Measure the coupling efficiency and record the data.
__Calculate the effective NA from the objective with the laser in this position. What is the
maximum coupling efficiency that you can get into the MLD fiber?
__Quantify the factors that will limit your coupling efficiency and compare to your
measured result.
Task 3: Fusion Splicer
Please follow the instruction provided by the TA.
Is your measurement matching the measurement indicated on the machine?
Task 3: Bend loss measurement
Please use the equation provided in the lecture notes and estimate the bend loss and compare with
your experiment measurement.
