Using the concepts developed in this module, you will be able to perform the following simple experimental projects as part of the laboratory exercise for this module.

Equipment List


Each of the projects identified above is described in detail below.

A. Multimode and single-mode optical fibers

Take 2-m lengths of multimode and single-mode optical fibers with cutoff wavelength less than 630 nm, so that the fibers are single moded at the HeNe laser wavelength of 633 nm. Couple light into the fibers and observe the output light as it falls on a screen. You should notice that the output from the multimode fiber is characterized by a speckle pattern while that of a single-mode fiber is uniform.

B. NA of a multimode optical fiber

Draw several concentric circles of increasing radii—say, from 0.5 cm to 1.5 cm—on a small paper screen. The screen is positioned in the far field (see Module 1-4, Basic Physical Optics) so that the axis of the fiber, at the output end, passes perpendicularly through the center of these circles on the screen (see Figure 7-23). The fiber end, which is mounted on an xyz-stack, is moved slightly toward or away from the screen so that one of the circles there just circumscribes the far-field radiation spot. The distance z between the fiber end and the screen, and the diameter D of the coinciding circle are measured accurately. The NA is calculated using the following equation

  NA = sin im = sin [tan–1 (D/2z)]

(7-37)

Figure 7-23 Measurement of the diameter D of the spot on a screen placed at a far-field distance z from the output end of a multimode fiber can be used to measure the NA of the fiber.

C. Attenuation measurement

A simple experiment can be performed for measuring the attenuation of the fiber at one specific wavelength. Take a length L (about 1 km) of the fiber and couple the beam from a laser into the fiber. Measure the power Po exiting at the output end of the fiber. Without disturbing the coupling system, cut off a reference length of 1 m of the fiber from the input end. Measure the power Pi exiting from the 1-m length of the fiber. This will be the input power to the longer portion of the fiber. The attenuation coefficient of the fiber at the wavelength of the laser is then given by



D. MFD measurement of an SMF

The MFD of a single-mode fiber can be easily determined by a measurement of the far-field radiation pattern emerging from the fiber. We have seen that the transverse field distribution of a single-mode fiber is approximately Gaussian in shape. When light exits the single-mode fiber, it diffracts as it propagates through air. The angle of diffraction is given approximately by

 

(7-38)

Thus a measurement of q at a specific chosen wavelength in the single-mode regime can give us the value of w.

To perform the experiment, take a 2-m length of a single-mode fiber, preferably at a visible wavelength such as 633 nm (HeNe laser). Couple light into the fiber using a microscope objective of magnification 20X. Place the photodetector (covered by a pinhole of about 0.1-mm diameter) of an optical power meter at a distance D of about 10 cm from the output end of the fiber. Adjust the detector to coincide with the center of the radiation pattern by maximizing the power. Note the power. Move the photodetector so that the power drops to about 1/e2 (~ 0.135) of the maximum. Measure the corresponding distance d moved from the center. Then

 

(7-39)

Thus MFD = 2w can be easily calculated from the measured values of d and D.

E. Splice loss across a multimode fiber joint

In Examples 7-21 and 7-22 we discussed the loss at a joint between a pair of step-index fibers and a pair of parabolic-index multimode fibers. A simple experiment to measure the variation of splice loss across a transverse misalignment can be performed and fitted using the formulas given. Take a multimode optical fiber of about 5-meter length and cut it into two almost equal pieces. Prepare the fiber ends by using a standard fiber cutter. Couple light from a white light source (such as a halogen lamp) into one of the fibers by focusing the light using a microscope objective. Place the other end of the fiber on a V-groove and couple this to the other fiber, which is placed on an xyz-stack. Bring the two ends of the fibers as close as possible—as shown in Figure 7-18—and adjust the transverse position of the fibers so that you couple the maximum amount of light. Now move the fiber in the transverse direction in steps of 10 mm and measure the power at each point. Measure the power exiting from the fiber end after removing the second fiber. Calculate the coupling efficiency (h) and plot as a function of the transverse misalignment. If you have knowledge of the fiber parameters, you can use Equations 7-33 and 7-34 to plot the theoretical variations and compare the experimentally measured variation with the theoretical plot. This experiment will show the sensitivity of the loss to transverse alignment of the two fibers. This is an important consideration while designing connectors for fibers.

A similar experiment can be carried out with single-mode fibers. However, you will need very precise movements to carry out the experiment, since single-mode fibers typically have mode-field diameters of about 5 to 10 mm. You will have to use Equation 7-32 to carry out the corresponding theoretical analysis.