How does the relative intensity noise of the laser affect the noise floor of the photonic link?
Laser RIN in Photonic Links
Laser RIN is the ultimate noise floor of an analog photonic link. Once the link enters the RIN-limited regime, no amount of additional optical power will improve the noise figure.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Does RIN change with frequency?
Yes. The RIN spectrum is not flat: at low frequencies (DC to ~100 MHz): RIN may be elevated due to 1/f noise from the laser bias circuit. At the relaxation oscillation frequency (1-10 GHz for DFB lasers): RIN has a sharp peak (10-20 dB above the floor). This peak is the resonance in the laser photon-carrier interaction. Above the relaxation frequency: RIN drops rapidly (> 40 dB/decade). The RIN floor (far from the relaxation frequency) determines the link noise at most RF frequencies. The relaxation oscillation peak can degrade the NF in a narrow band around that frequency.
How does an EDFA affect RIN?
An EDFA adds its own noise: amplified spontaneous emission (ASE). The ASE-signal beating noise is equivalent to: NF_EDFA_opt = 2 × n_sp = 4-6 dB (for a well-designed EDFA). After the EDFA: the effective RIN at the photodetector is degraded by the EDFA noise figure. For a link using an EDFA to boost optical power: the total noise is the combination of laser RIN, EDFA noise, and shot noise. In many cases: the EDFA noise dominates (especially if the input optical power to the EDFA is low), and the laser RIN becomes irrelevant.
Can I measure RIN?
Yes. Standard measurement: connect the laser directly to a photodetector (no modulation). Measure the electrical noise power spectrum at the PD output using an RF spectrum analyzer. Subtract the thermal noise (measured with the laser off) and shot noise (calculated from the photocurrent). The remaining noise is the RIN. RIN = measured_noise_density / (I_PD² × R_load). Report in dB/Hz. Standard: IEC 61280-2-9 defines the RIN measurement method. Some laser datasheets specify RIN; for critical applications, measure it yourself (the datasheet value may be optimistic or measured under different conditions).