S-108.3110 OPTICAL COMMUNICATIONS
Experiment 1: Fiber attenuation and detection of light
Experiment 1:
Fiber attenuation and
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.
Figure 1. Structural diagram of the photodiode.
S-108.3110 OPTICAL COMMUNICATIONS
Experiment 1: Fiber attenuation and detection of light
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
(nm)
Voltage (V)
Voltage Bias (V)
Photocurrent Responsivity
(mA)
(A/W)
1500
1510
1520
1530
1540
1550
1560
1570
1580
1590
1600
0.3
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)
1500
1520
1550
1580
1600
ΔPmW
ΔPdBm
Loss(dB/km)