Microwave Photonic Links: ASK-Modulated RF Subcarrier Fiber-Optic Transmission

Tool Used: OptSim

This application note demonstrates transmission of an ASK-modulated RF sub-carrier over a standard singlemode fiber.

Subcarrier multiplexing helps carry multiple RF data streams over a single optical carrier, reducing the number of lasers. The RF data can be amplitude modulated (ASK), frequency modulated (FSK) or phase modulated (PSK). The modulated RF is then used to modulate the optical carrier. The detected electrical signal at the receiver is demodulated using the appropriate demodulation technique depending on the type of modulation used at the transmitter [1-3]. This application note uses amplitude modulation of the RF signal.

Figure 1 shows the schematic layout.

Simulation setup schematics for this application note

Figure 1. Simulation setup schematics for this application note

The RF carrier at 60GHz is amplitude modulated by the digital data signal carrying 1Gbps binary data. Figure 2 shows NRZ baseband spectrum of the data stream at the upper input of the mixer. 

Figure 2. Baseband spectrum of the NRZ data stream at the mixer input

The spectrum of the ASK-modulated RF at the mixer output is shown in Figure 3.

Figure 3. ASK-modulated RF spectrum at the mixer

The ASK-modulated RF subcarrier is used to modulate an optical carrier at 1550nm. The MZM is biased at null to get all-optical half-wave rectification. The signal travels through a single-mode fiber whose length is scanned from 21 km to 41 km in steps of 2 km. The detected baseband signal at the receiver is low-pass filtered to obtain demodulated ASK.

Figure 4 shows BER versus transmission distance. The inset eye diagrams show received eye diagrams at various distances.

Figure 4. BER as a function of transmission distance in km

The deterioration in BER is due to increases fiber impairments as the transmission distance increases.

References

1.       Preetpaul S. Devgan, John F. Diehl, Vincent J. Urick, Christopher E. Sunderman, and Keith J. Williams,

"Even-order harmonic cancellation for off-quadrature biased Mach-Zehnder modulator with improved RF

metrics using dual wavelength inputs and dual outputs," Optics Express, vol. 17, no. 11, May 2009.

2.       Kamau Prince, Idelfonso Tafur Monroy, Jorge Seoane, and Palle Jeppesen, "All-optical envelope detection for Radio-over-Fiber links using external optical injection of a DFB laser," Optics Express, vol. 16, no. 3, Feb. 2008.

3.       Stefano Camatel, and Valter Ferrero, "Homodyne Coherent Detection of ASK and PSK Signals Performed by a Subcarrier Optical Phase-Locked Loop," IEEE Photonics Technology Letters," vol. 18, no. 1, January 2006.