In this lab you will fabricate a 2 ´ 2 fiber optic coupler using 1-mm-diameter plastic fiber. The coupler can be used for a variety of applications including wavelength division multiplexing and power splitting, which will be outlined in this lab.

Equipment List

The following equipment is needed to complete this laboratory

2 1-foot sections of 1-mm diameter plastic-jacketed fiber (part #2705FIBOPT)¹

1 razor blade

1 heat gun

1 4" piece of heat-shrink tubing

2 high-brightness LEDs (1 green, 1 red)

2 plastic fiber connectors (part #2400228087-1)¹

2 plastic fiber LED mounts (part #2400228040-1)¹

4 multimode ST-connectors for 1-mm fiber (part #F1-0065)²

1 electronic breadboard with +5 volt supply

1 850-nm fiber optic source with ST adapter (part #9050-0000)²

1 850-nm fiber optic detector with ST adapter (part #F1-8513HH)²

1 low-cost diffraction grating (part #J01-307)³

1 1-meter patch cord (terminated with ST connectors)

1 fiber optic termination kit (includes scissors, alcohol wipes, crimp tool, fiber inspection microscope, razor blades, etc.)¹

(Notations 1, 2, 3: See sources in APPENDIX.)

Procedure

PART I:  Making a Fiber Optic Coupler

1. With the razor blade, carefully strip off approximately 3" of the fiber jacket in the middle of the fiber (see Figure 8-43).
   

   Figure 8-43

2. Where the fiber has been stripped, twist the two fibers together.

3. On each end of the stripped area, place a small weight (i.e., paperweight, book) to hold the fiber in place (see Figure 8-44).
   

   Figure 8-44

4. Using the heat gun on the low setting, apply heat to the twisted area. Move the heat gun gently back and forth to uniformly melt the fiber. CAUTION: Do not hold the heat gun stationary because the fiber will melt quickly!

5. As the fiber is heated, you will notice that it will contract a bit. This is normal. When the contraction subsides, remove the heat gun and let the fiber cool for a minute.

6. With a laser pointer or fiber optic source, shine light into port 1 of the coupler. You should observe a fair amount of coupling (~20–30%) into port 3 of the coupler. If more coupling is needed, repeat the heating process until the desired coupling is obtained.

PART II:  Wavelength-Division Multiplexing Demonstration

1. Apply the AMP plastic fiber connectors to the two input fibers (ports 1 and 4) according to manufacturer’s specifications. Polish the ends if necessary. Also polish the ends of the unterminated fibers if necessary.

2. On the electronic breadboard, set up the circuit shown in Figure 8-45. Depending on the type of LED, you may have to use epoxy to secure the LED in the mount.
   

   Figure 8-45

3. When the circuit is complete, connect the fibers to the LEDs and observe the output of port 2. The red and green colors will be mixed.

4. To separate the colors, observe the output of port 2 through the diffraction grating. You should observe a central bright spot (coming from the fiber) and two identical diffraction patterns—one on either side—with the red and the green separated (see Figure 8-46). To ensure that the two signals are indeed independent, turn off the LEDs one at a time and observe the output of port 2 through the diffraction grating.
   

   Figure 8-46

Part III:  Measuring Coupler Loss

1. Repeat steps 1–6 (Part I) for fabrication of a 2 ´ 2 coupler.

2. “Connectorize” each port of the coupler using ST-multimode connectors and polish if necessary. (Instructions for termination are supplied with the connectors when purchased.)

3. Measure the output of your fiber optic source at the output of the patch cord. This will be the input power to the coupler. Record the power in Table 8.6.

4. Measure the output power at each of the ports and record in Table 8.6.

5. Calculate the throughput loss using the following equation:

              L_th = –10 log (P₂/P₁)

6. Calculate the tap loss using the following equation:

              L_tap = –10 log (P₃/P₁)

7. Calculate the directionality loss using the following equation:

              L_dir = –10 log (P₄/P₁)

8. Calculate the excess loss using the following equation:

              L_ex = –10 log (P₂ + P₃)/P₁

9. Repeat steps 3–9 (Part III) using each port as the input. Record the results in Table 8.6.