Experiment 1: Fiber Attenuation and Detection of Light

S-108.3110 OPTICAL COMMUNICATIONS Experiment 1: Fiber attenuation and detection of light

Fiber attenuation and Experiment 1: detection of light

The purpose of this laboratory exercise is to study the loss properties of standard single mode fiber widely used in telecommunications. Other part of the lab work focuses on the characteristics of optical photodetector used to convert optical signal into measurable current.

Background/Theory The invention of low-loss silica fibers was a revolutionary step to high bandwidth optical communications. Early fibers suffered from high losses (typically hundreds of dB/km). First modern low-loss fibers were demonstrated in 1986 by Sumitomo Electric Industries. Their fiber had median attenuation of 0.21 dB/km and minimum attenuation of 0.154 dB/km at the wavelength of 1550 nm. This wavelength is used in the present long-haul networks, because it is not attenuated critically in silica fibers, and more importantly, a suitable fiber amplifier, EDFA, operates around this wavelength region. Optical fiber has many benefits compared to other transmission media. Optical fiber provides low-loss transmission over a wide frequency range. Though the loss of the fiber is very low, it must be taken into account especially in the long-haul transmission systems where distances may be hundreds or thousands of kilometers. Main loss mechanisms are Rayleigh scattering and material absorption.

The determination of optical power is one of the fundamental measurements in optics. Optical detectors can be divided into two categories according to the operational principle: thermal detectors and quantum detectors. In this lab work we use semiconductor quantum detector since it is easy to use and compatible with external electric circuits. The operation principle of used photodiode, which is probably the most widely used detector, is simple. Incident photons are absorbed in the depletion region where free or mobile electron-hole pairs are created. Positively charged holes move towards the p-type region and negatively charged electrons towards the n-type region. This phenomenon gives rise to photocurrent, which depends on the quantum efficiency of the photodiode. S-108.3110 OPTICAL COMMUNICATIONS Experiment 1: Fiber attenuation and detection of light

Figure 1. Structural diagram of the photodiode.

List of equipment  External cavity diode laser(ECDL): Coherent light source capable of several mWs of stable optical output power within narrow bandwidth. The wavelength can be tuned using front panel controls.  Long and short standard single mode fiber to connect the ECDL to the photodetector.  Current-to-Voltage converter: Electrical instrument that converts (and amplifies) photocurrent to measurable voltage.  Multimeter: Instrument that measures voltage.  Biased Germanium (Ge) photodetector: Widely used photodetector made from germanium. Cheap and well known detector, which is not optimal near 1550nm because of its spectral properties. See Appendix 1 for more info.  BNC-BNC cable to connect Ge photodetector to the I/V converter and BNC-banana cable to connect I/V converter to multimeter.

Measurement goals 1. Study the responsivity of Ge detector at different wavelengths 2. Find out what is the attenuation of standard optical fiber at different wavelengths

Measurement 1: Responsivity of Ge photodiode 1. Complete the measurement set-up according to Fig. 2. Use short fiber (~2m) as fiber under test. Optical fibers should be connected and cleaned very carefully. 2. Turn on the laser using a key located at the lower left corner of the front panel. Set power to 0.5 mW using button P and numbers buttons. Press enter to confirm. Finally press enable to activate the output. 3. Turn Ge photodiode on. Now you see the biased output voltage. 4. Turn I/V converter on and set amplification to minimum (=1 mA/V).

Figure 2. Measurement setup for measuring spectral responsivity of Ge photodiode S-108.3110 OPTICAL COMMUNICATIONS Experiment 1: Fiber attenuation and detection of light

5. Turn on the multimeter and use number of power line cycles (NPLC) of 60. 6. At this point you should see stable voltage on the display of the multimeter. If not, consult assistant! 7. Set wavelength to 1500nm using lambda functions and number keys. 8. Wait for a while and then record the value from the multimeter to your measurement report. Remember to use NPLC 100. 9. Perform steps 7&8 to wavelengths 1510,1520,...,1600nm. 10. Disable the laser output by pressing enable. Now you see the bias voltage, which you should write down for further calculations. 11. Compare the responsivity curve to the one found in Appendix 1.

Measurement 2: Attenuation of optical fiber at different wavelengths 1. Use the same setup, but replace the fiber under test with long cable. Optical fibers should be connected and cleaned very carefully. 2. Set wavelength to 1550nm. Measure the power and write it down to your report. 3. Measure the power also at wavelengths 1500,1520,1580,1600nm. 4. Calculate the loss of the fiber at different wavelengths to your measurement report. S-108.3110 OPTICAL COMMUNICATIONS Experiment 1: Fiber attenuation and detection of light

APPENDIX 1 S-108.3110 OPTICAL COMMUNICATIONS Experiment 1: Fiber attenuation and detection of light

MEASUREMENT REPORT Date:______Group number:______Student IDs:______,______,______,______

Measurement 1: Fill in the table. Use responsivity at 1550 nm as reference. Photocurrent can be deduced from voltage and responsivity can be calculated assuming that the output power is stable. Remember A = P*R. Start from 1550nm to get the input power.

Lambda Voltage (V) Voltage - Photocurrent Responsivity (nm) Bias (V) (mA) (A/W) 1500 1510 1520 1530 1540 1550 0.3 1560 1570 1580 1590 1600

Responsivity of Ge photodiode as a function of wavelength

1500 1520 1540 1560 1580 1600 S-108.3110 OPTICAL COMMUNICATIONS Experiment 1: Fiber attenuation and detection of light

Measurement 2: Fill in the table. Use values from measurement 1 when calculating power difference.

Lambda (nm) Voltage - Bias(V) ΔPmW ΔPdBm Loss(dB/km)

1500 1520 1550 1580 1600