RF Over Fiber and Photonic Links Analog Photonic Links Informational

What is the noise figure of a typical RF photonic link and how does it compare to a coaxial cable?

Q: Does fiber length affect NF?

Yes, indirectly. Fiber loss reduces the optical power at the photodetector: less optical power = less photocurrent = lower signal power and lower shot noise. The thermal noise remains constant. As fiber loss increases: the link gain decreases, and the NF increases. For a 20 km link at 1550 nm: 4 dB fiber loss. The NF increases by approximately 2 × fiber_loss(dB) = 8 dB compared to a zero-length link (because link gain ∝ optical_power², so 4 dB optical loss = 8 dB RF loss). Mitigation: use an EDFA before the photodetector to restore the optical power (but the EDFA adds its own noise: NF_EDFA ≈ 4-6 dB in the optical domain).

Q: What about digital RFoF noise?

Digital RFoF does not have a traditional noise figure. Instead: the signal is digitized (ADC) at the antenna. The quantization noise of the ADC sets the noise floor: SNR_ADC ≈ 6.02 × ENOB + 1.76 dB. For a 14-bit ADC: SNR ≈ 86 dB. The digital data is transmitted over fiber with essentially zero degradation (digital signals are immune to fiber noise and distortion). At the remote end: the DAC reconstructs the analog signal with noise limited by the DAC resolution. The effective NF of a digital RFoF link: limited by the ADC performance, not the fiber. Typical effective NF: 15-25 dB (determined by the ADC noise and the full-scale input power).

The noise figure of an analog photonic link is significantly higher than a coaxial cable, which is the primary disadvantage of RFoF for receive-side applications: (1) Coaxial cable NF: the noise figure of a passive coaxial cable equals its loss (in dB). A 100 m cable with 15 dB loss at 10 GHz: NF = 15 dB. A 10 m cable with 5 dB loss: NF = 5 dB. The cable adds thermal noise proportional to its loss but does not introduce additional noise sources. (2) Analog photonic link NF: typical directly modulated link: NF = 25-40 dB. Typical external modulation (MZM) link: NF = 20-35 dB. High-performance link (high-power laser, low V_π): NF = 10-20 dB. Research links (optimized components): NF = 3-7 dB (approaching the shot-noise limit). (3) Noise sources in a photonic link: laser RIN (relative intensity noise): the dominant noise source at high optical power. Appears as a flat noise floor proportional to I_PD². Typical RIN: -155 to -165 dB/Hz. Shot noise: fundamental quantum noise from the discrete nature of photons. N_shot = 2qI_PD (A²/Hz). This is the theoretical minimum noise floor. Thermal noise of the detector load: k_B T / R_load. Dominates at low optical power. Amplifier noise: if an RF amplifier is used at the photodetector output, its noise figure adds to the link NF. (4) Comparison for a 500 m link at 10 GHz: coaxial cable (LMR-400): loss ≈ 20 dB. NF = 20 dB. Photonic link: NF = 25-30 dB. The photonic link has 5-10 dB worse NF. At 500 m: the coaxial cable is marginally better (if the loss is tolerable). At 2 km: coaxial loss would be 80+ dB (unusable), while photonic NF remains 25-30 dB. (5) Mitigation: place an LNA at the antenna (before the RFoF transmitter). The LNA sets the system NF: NF_system = NF_LNA + (NF_link - 1) / G_LNA. With NF_LNA = 1 dB and G_LNA = 30 dB: NF_system = 1 + (300 - 1) / 1000 ≈ 1.3 dB. The high photonic link NF is masked by the LNA gain.

Category: RF Over Fiber and Photonic Links

Updated: April 2026

Product Tie-In: Fiber Components, Modulators, Photodetectors

Photonic Link vs Coaxial Noise Figure

The high noise figure is the fundamental trade-off of analog photonic links. Understanding when it matters (and when it does not) is critical for system design.

When NF Does Not Matter

(1) Transmit applications: the photonic link carries a high-level transmit signal from the signal source to the antenna. The signal level is well above the noise floor of the link. NF is irrelevant for the transmitted signal quality. (2) High-signal-level receive applications: if the received signal at the antenna is strong (e.g., close-range communications, test range instrumentation), the photonic link NF does not limit the system. (3) LNA-preceded links: with an LNA at the antenna (NF = 1 dB, G = 30 dB), the system NF is dominated by the LNA (1.3 dB), regardless of the photonic link NF. This is the standard architecture for receive RFoF.

NF Comparison

Coax NF = cable loss (dB)
Photonic NF: 20-40 dB (typical)
Coax at 500m/10GHz: NF ≈ 20 dB
Photonic at 500m: NF ≈ 25-30 dB
LNA + link: NF_sys ≈ NF_LNA + (NF_link-1)/G_LNA

Common Questions

Frequently Asked Questions

Can photonic link NF be below 10 dB?

Yes, with optimized components: high-power laser (200+ mW) with low RIN (< -165 dB/Hz). Low V_π modulator (< 2V). High-responsivity photodetector (0.9 A/W). Research demonstrations have achieved NF = 3-5 dB (near the shot-noise limit). Commercial modules are available with NF = 10-15 dB (at premium cost: $5,000-20,000). The key is maximizing the optical power at the photodetector (to make the signal power dominate all noise terms).

Does fiber length affect NF?

Yes, indirectly. Fiber loss reduces the optical power at the photodetector: less optical power = less photocurrent = lower signal power and lower shot noise. The thermal noise remains constant. As fiber loss increases: the link gain decreases, and the NF increases. For a 20 km link at 1550 nm: 4 dB fiber loss. The NF increases by approximately 2 × fiber_loss(dB) = 8 dB compared to a zero-length link (because link gain ∝ optical_power², so 4 dB optical loss = 8 dB RF loss). Mitigation: use an EDFA before the photodetector to restore the optical power (but the EDFA adds its own noise: NF_EDFA ≈ 4-6 dB in the optical domain).

What about digital RFoF noise?

Digital RFoF does not have a traditional noise figure. Instead: the signal is digitized (ADC) at the antenna. The quantization noise of the ADC sets the noise floor: SNR_ADC ≈ 6.02 × ENOB + 1.76 dB. For a 14-bit ADC: SNR ≈ 86 dB. The digital data is transmitted over fiber with essentially zero degradation (digital signals are immune to fiber noise and distortion). At the remote end: the DAC reconstructs the analog signal with noise limited by the DAC resolution. The effective NF of a digital RFoF link: limited by the ADC performance, not the fiber. Typical effective NF: 15-25 dB (determined by the ADC noise and the full-scale input power).

Keep Reading