Introduction
The aim of this report is to introduce the process and the results of the laboratory experiments on fiber optics.
Taking into consideration the multitude of the fiber optics, it must be said that there are no universal standards of their form or functions. However, the scientists of the New York Institute of Electrical and Electronics Engineers (IEEE) highlight several main characteristics which have to be typical for fiber optics transmission. The components of photonic system in this work were selected according to the suggestions of IEEE.
Selecting Fiber Cable
The choice was to be made between two types of fiber cable: multimode and singlemode, which are used by the short and long links. The length of the waves can vary, and the most frequently used are: 850 nm, 1300 nm, 1550 nm. The choice of the fiber cable and the waves will define the theoretical optical loss.
The fallowing table gives the IEEE standards for the fiber optics:
It can be seen in the table, that the transmission of data along the distance of 40km will be possible with the help of a singlemode fiber cable and a wave which is 1550nm long.
When the choice of the fiber cable is made, the calculation of the losses is relevant. The main losses that should be counted in this work are: splices, fiber attenuation, and connector attenuation. The typical values of the losses for the fiber cable introduced by IEEE are shown in the next table:
A transmission can be embodied with a fiber optic system in case there is a minimum of one splice and two connectors. Under such circumstances, for every 10km we used one splice and two connectors. We also added extra 2dB to every value of fiber optic, considering the temperature and humidity, which may as well influence the result.
These calculations define the loss according to the table:
α op = Connectors_Loss+ Fibre_Loss +Splice_Loss+Safety_Margin_loss
α op = (2×1dB) + (40×0.2dB) + (4×0.1dB) + 2dB
α op = 2dB + 8dB + 0.4dB + 2dB
α op = 12.4dB
Having investigated the kinds of fiber optics, their characteristics and functions, we searched the Internet in pursuit of elements for our systems, which would meet the main requirements:
- Appropriate length
- Little noise level
- Singlemode cable
- The wave of 1550nm
The chosen fiber optic and its datasheet is added as Appendix1.
Selecting Transmitter
We chose the DFB Laser Diode, which has a distributed feedback. To meet the requirements for the transmission, which were a high speed and a long distance, and to have a dynamic single mode we needed to use optical cavities with selective reflections. The DFB employs a periodic longitudinal variation of the reflective index.
However, we have also considered selecting a laser diode with a slope effacing that can let our system gain the link, which is likely to cause a better Dynamic Range. Another demand for this component is ability to operate with the 1550 nm wavelength and a modulation frequency of 2 GHz.
Having studied different Laser Diodes, we looked for a Laser Diode with such characteristics:
- ability to operate with modulation frequency of 2 GHz or more
- ability to operate with 1500nm long waves
- sufficient slope efficiency η d
- distributed feedback
The chosen element is added as Appendix 2.
Selecting Receiver
Choosing between the two basic kinds of photodiodes, we took into consideration their quality, price and structure. Eventually it appeared that the PIN (Positive-Intersinc-Negative) diode is more simple and cheap in comparison with the APD (Avalanche Photodiode). In addition, the PIN diode is perfect for working in conditions of high frequencies.
Another point which had been considered was the ability of photodiode to increase the gain link of our system which, as it was mentioned before, can help to improve the Dynamic Range. The requirements for wavelength and modulation frequency for this device are the same as for the previous.
Having studied the different types of After photodiodes, we tried to find the device with such specifications:
- PIN Laser Diode
- ability to operate with 1500nm long waves
- ability to operate with modulation frequency of 2 GHz or more
- high responsive qualities
The selected photodiode and its datasheet is added as Appendix 3.
Summary
It can be said that the work of the photonic system can be improved with the means of reaching a lower noise level. The factor which can influence this level is the volume of the link gain. This defined the main principles of choosing the elements for our system. The demands for every device were different and were aimed at improving the work of the system.
The values of datasheet and assumptions for the calculations are as fallows:
λ = 1550 nm
I B = 110 mA
η d = 0.16 W/A
Rs = 5Ω
Ith = 40 mA
RIN = -157 dB/Hz = 1.995×10-16 W/Hz
m1dB = 0.85
m =
PL = 10 mA
α op = 12.4dB = 0.058
 = 0.85 A/W
I Dark = 0.6 nA
RL = 1.5 KΩ
Calculations of the average received power:
P = α op × PL = 0.058×10×10-3 = 0.58 mW
P = 10 Log(0.58) = -2.366 dBm
I P = 0.058×10-3 ×0.85 = 0.493mA
Calculation of the Link Gain:
GLink = (Âα op η d )2 RL/Rs = [ (0.85×0.058×0.16 )2×1500/5 ]= 0.019 = -17.2 dB
Calculation of the noise:
<i2RIN> = RIN×(IP)2 B = 1.995×10-16(0.493×10-3)2 ×1×106 = 48.48×10-18 A2
<i2Dark> = 2qIDarkB = 2 ×1.6×10-19 × 0.6×10-9×1×106 = 1.92×10-22 A2
<i2th >= 10.672×10-18 A2
<i2sh > = 2qIPB = 2×1.6×10-19×0.493×10-3×1×106 = 1.578×10-16 A2
<i2N-tot> = i2RIN +i2Dark + i2th + i2sh
<i2N-tot> = 48.48×10-18+1.92×10-22+10.672×10-18+1.578×10-16= 2.169×10-16A2
Ntot = <i2N-tot>RL = 2.169×10-16×1500 = 3.253×10-10mW = -94.87dBm
Selecting RF Amplifier
The amplifier is a very important component of our system, as it will serve the purpose of increasing the signal after passing the photodiode. To choose an appropriate device, we considered three main points. The first one was related to finding a matching resistor to the impedance of the amplifier and the photodiode. Such gadgets will theoretically improve the voltage standing wave ratio (VSWR) of the link. The second significant point was taking into consideration the thermal noise of the signal which can be also amplified. Finally, the third issue was choosing a model of an amplifier which can provide the system with an appropriate signal level. It means that the device had to have a sufficient gain value.
Having made a research into the different kinds of amplifiers, we looked on the web sites for a device which would meet our requirements. The amplifier which was chosen is added as Appendix 4.
Conclusion
Aim of our work was to design a photonic system which could transfer the data successfully. We were accurate in our choice of every component and considered all the requirements for their quality. During our work we learned how to operate with different devices; we also practiced calculation of the main values of the system (the average received power, the link gain, the noise figure etc.).
There were also some drawbacks in our work, such as missing some factors (for example, the price), and we should take them into consideration while doing similar assignments.
All in all, our laboratory experiments were very useful and it developed our professional skills.