RF Over Fiber – Taking RF Signals from a Few Meters to Kilometers with Fiber Optics

In an increasingly connected world, the demand for high-speed, high-capacity signal transmission is pushing the limits of traditional coaxial cable-based systems. Recently there has been an ever-increasing interest in Radio Frequency over Fiber (RFoF), a technology that merges the low-loss, high-bandwidth advantages of optical fiber with the versatility of RF communication (Figure 1). By transmitting RF signals over optical fiber, RFoF systems enable long-distance, interference-free signal delivery across a wide range of applications—from satellite ground stations and remote antenna deployments to 3G-5G infrastructure and defense systems. This article explores the fundamentals of RFoF system design.

Figure 1: RFoF key features. (Image source: NuPhotonics)

Going the distance - signal strength

Coaxial cables offer varying performance based on cable configuration. Typical dielectric SMA cables offer about 0.25 dB/m insertion loss (at 2 GHz). Air filled cables achieve slightly better performance, but at a drastically higher cost. This high loss is the driving force to use RFoF for transmission distances over 50 meters. RFoF most often utilizes two wavelengths 1310 nm and 1550 nm. 1310 nm loses about 0.35 dB/km of optical signal, 1550 nm loses only 0.25 dB/km. As can be seen this is drastically lower than compared to coaxial cables.

DigiKey and NuPhotonics enabling ease component sourcing

DigiKey has been a global leader in enabling key components to be easily sourced. DigiKey is used by hobbyists, students, professionals, and large corporations. As a leader in RF & optoelectronic device industry, it made sense that NuPhotonics and DigiKey partner to help supply industry with easy to use and easy to access components (Figure 2).

Figure 2: NuPhotonics 10G pin photodiode pigtail FC/APC. (Image source: NuPhotonics)

There are some commercially available solutions, but they often don’t make financial sense. This article will go over-standard design which will enable users to develop low-cost specialized solutions with NuPhotonics parts. The products and solutions discussed here are available from DigiKey for easy ordering.

RFoF transmitter design – 10G DFB laser

The first part of designing an RFoF system is developing the transmitter. For RFoF architecture, a data-carrying RF signal is imposed on a Lightwave signal before being transported over the optical link. A distributed feedback laser (DFB) Laser can be directly modulated by the RF signal which makes it an ideal component to transform the electrical RF signal into an optical signal. A basic diagram can be seen in Figure 3. Since the laser is biased on the anode side, this is also the input for RF frequency. For system safety, the circuit incorporates a DC blocking capacitor (C2). The value of C2 will be fined tuned by the desired lower frequency cut off point. Resistor R1 in the circuit is used for impedance matching of the 10 Ω DFB laser to a 50 Ω system. The higher the value of R1, the better matching of the link with the adverse effect of increasing the optical link insertion loss. This enables precise level control for the desired impedance matching and insertion loss. Resistor R2 in the circuit is the current limiting resistance used to limit the current to the laser. Inductor L acts as a high impedance path for the RF signal while acting as a minimal resistance current path for the DC bias of the laser. Capacitor C1 is an optional filtering capacitance used to filter out power supply noise on the bias T.

Figure 3: 10G DFB laser with Bias-T and impedance matching. (Image source: NuPhotonics)

RFoF receiver design – 10G pin photodiode

The optical light in the fiber needs to be converted to an electrical signal that is more usable. For this, a photodiode is used. When photons of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. These holes move toward the anode (+) and electrons toward the cathode (-). This effect produces a photocurrent. Since the circuit deals with broadband operation, the photodiode will be operated in reverse bias. When reverse biased, current will only flow through the photodiode with incident light creating a photocurrent. This also has the added benefit of increasing the linearity of the photodiode. The reverse bias response time is reduced by increasing the size of the depletion layer. This increased width reduces the junction capacity and increases the drift velocity of the carriers in the photodiode. The transit time of the carriers is reduced, improving the response time.

Figure 4 represents the basic circuit to operate the photodiode. Similarities can be seen between the photodiode circuit and the laser circuit. Capacitor C is the DC blocking capacitor which protects the RF port. Inductor L is a low impedance DC path to ground and allows the current to flow from the DC Bias pin to ground since the DC blocking capacitor C will not allow a direct path to ground. Components R1 and C1 are selected to help improve the high frequency impedance matching.

Figure 4: 10G pin photodiode with Bias-T and impedance matching. (Image source: NuPhotonics)

PCB layout – RF design considerations

Designing PCBs for RF applications involves far more than routing signals and placing components, it's a discipline where electromagnetic behavior dominates and small layout choices can make or break performance. To achieve desired performance, careful attention needs to be paid to impedance control and the ground return paths to ensure resonances are not present. The first step will be to select a PCB material. In this case, a dielectric material that has a εr ~ 3 and a tan-δ <0.01 ensures that the RF signal is not being attenuated due to PCB dielectric losses. Once a material is selected, the traces need to be designed. For RF trace design, there are a few approaches. It is preferable to utilize a coplanar waveguide (CPW) as it will offer better isolation, better confining of the electromagnetic field, as well as smaller ground return paths to help ensure minimal resonances. In Figure 5, a basic circuit layout for the circuits from in Figures 3 and 4 can be seen. A CPW was utilized with plenty of ground VIAs to ensure minimal return paths for the RF signal. DigiKey’s DKRed would be a great option for quick turn PCBs to begin testing the circuit.

Figure 5: 10G DFB laser board and 10G PIN photodiode board. (Image source: NuPhotonics)

PCB assembly

The TO-56 laser and photodiode are easily soldered directly to the PCB. This makes NuPhotonics devices easily to incorporate into standard PCBs and makes them a desirable choice for both hobbyist to industry professionals. Figure 6 shows the assembled PCBs from Figure 5.

Figure 6: Assembled Photodiode & Laser PCB. (Image source : NuPhotonics)

RF results – RFoF link

With the devices mounted on the PCBs allowing easy connection with SMA connectors, the device performance can be measured. RF tests were performed on a Vector Network Analyzer. The tests performed will be specifically looking at the S-parameters S₁₁ and S₂₁. S₁₁ will show how well matched the DFB laser is. The 1550 nm is a 10 Ω series device, so broadband matching the device is a challenge. S₂₁ is the amount of loss or attenuation seen in the link. Below 0 dB S₂₁ means the link is losing some signal and above 0 dB the link is adding gain to the input RF signal. Figure 7A shows the S₂₁ of the link where it can be seen that the overall system has a flat response up to 3 GHz and a 3 dB bandwidth of 6+ GHz. Figures 7B and 7C show the S₁₁ matching of the photodiode and laser respectively. The overall link gain is -2 dB over the entire 6 GHz frequency band. The results show this method is an easy approach to transmitting electrical signals over long distances with fiber optic cables. NuPhotonics products provide an easy PCB mountable solution that hobbyists to industry professionals can incorporate into their systems.

Figure 7A: Link S₂₁ (dB). (Image source: NuPhotonics)

Figure 7B: Photodiode S₁₁ (dB) matching. (Image source: NuPhotonics)

Figure 7C: Laser S₁₁ (dB) matching. (Image source: NuPhotonics)

Conclusion

This article highlights how easy RFoF link design can be with NuPhotonics products that are readily available from DigiKey for prototyping while only scratching the surface of RFoF link design. RFoF enables the seamless integration of radio-frequency systems with the low-loss, high-bandwidth, and interference-resistant advantages of optical fiber. As wireless networks, satellite links, and defense applications demand higher frequencies, wider bandwidths, and longer reach, RFoF offers a scalable and future-proof solution. Ongoing research ensures improvements in linearity, noise performance, and cost-effectiveness are key factors for unlocking the full potential of 5G, 6G, advanced radar, and next-generation communication systems.