Most radio stations use frequency modulation (FM) to broadcast yet amplitude modulation (AM) ensures long distance modulation. The limitations of FM reception are the line of sight and the area of reception. These two parameters are much smaller in FM compared to AM which makes AM modulation have an added advantage over FM modulation. The results presented in this paper include; direct modulation at different bias currents and different transmission fiber lengths and the amplitude modulation using the Mach-Zehnder. The results show the possibility to transmit huge data at high speeds to over 100Gbps.
Direct modulation is very useful in high speed data transmissions though this method has several problems. The maximum bandwidth that can be modulated is just a few GHz and the maximum quantum efficiency \((\eta)\) is 100%, which places an upper limit on the slope efficiency and therefore the gain [1,2] shown in equation (1).
The experimental setup for the characterization of the Mach-Zehnder for different bias voltages is shown in Figure 1. When the bias voltage is varied, the power is varied so that the most suitable voltage at which transmission should be carried out is obtained. This is known as Mach Zehnder optimization.
Figure 2 shows the experimental set up that was used for modulation of signals at 10GBps and 8.5GBps for a fiber optic Mach-Zehnder interferometer based on Lithium Niobate.
In amplitude modulation also known as Amplitude-shift keying (ASK), digital data is represented as variations in the amplitude of the carrier wave. For an ASK system, the binary symbol 1 is represented by transmitting a fixed-amplitude carrier wave and a fixed frequency for a bit duration of T seconds. When the signal value is 1 it means that the carrier signal will be transmitted; otherwise, a signal value of 0 will be transmitted. This such carrier signal is shown in Equation (2).
For high speed data transmission over long distances, direct modulation may be inefficient and therefore the use of external modulation techniques for example the amplitude/ intensity and phase modulations using a Mach-Zehnder modulator.
Figure 3 shows the Mach-Zehnder bias voltage characteristic curve. From the two graphs it shows that the power increases with increase in the voltages both to the negative and positive biases.
A back to back (with no fiber) transmission was performed at different rates and the sensitivity determined. The results showed better sensitivity for an 8.5Gbps compared to the 10Gbps transmission as shown in Table 1. This means that the increase of the data transmission rates exposes the signal to greater errors (penalty) and hence the reduction in sensitivity.
| Rate | 8.5Gbps | 10Gbps |
|---|---|---|
| Sensitivity(dBm) | -19.7972 | -18.1763 |
The BER was also measured for the two data transmission speeds and the results are shown in Figure 4. The increase in data transmission rates increased the power penalty.
Figure 5 shows the Eye diagrams for data transmissions at 8.5GBps and 10GBps. There is a closure in the eye (10Gbps) compared to the eye (8.5Gbps) which signifies increased attenuation as the rate increases. For the case of 8.5Gbps the eye is clear implying it’s easier to distinguish between the zeros and ones by the receiver.
Very small power penalty was achieved at data transmission at a rate of 8.5GBps in a 74.91km single mode fibre which means that even with an increase in the rates to up to 100GBps, transmission at longer distances with minimal dispersion is possible.
