Glossary entry

Side-mode suppression ratio (SMSR)

The power ratio between the dominant lasing mode and the strongest side mode of a single-mode laser, in dB. Standard acceptance metric for DFB and DBR lasers.

The side-mode suppression ratio in decibels is

\text{SMSR} \;=\; 10 \log_{10}\!\left( \frac{P_\text{main}}{P_\text{side}} \right),

where $P_\text{main}$ is the optical power in the dominant longitudinal mode and $P_\text{side}$ is the power in the strongest non-target mode (typically the closest adjacent longitudinal mode).

High SMSR indicates clean single-mode emission with most power concentrated in one wavelength. Low SMSR indicates partial or full multi-mode operation.

Typical values:

Source	Typical SMSR
Commercial telecom DFB laser	35 – 50 dB
External cavity diode laser	40 – 60 dB
VCSEL (single-mode design)	20 – 40 dB
Fabry–Pérot laser (multimode)	$<$ 10 dB
Distributed Bragg reflector (DBR)	40 – 55 dB

Industry telecom DFB acceptance specifications typically require SMSR $> 35$ dB at the rated operating point. Higher-grade products specify $> 45$ dB.

SMSR degrades as the device approaches a mode hop: as the gain peak drifts toward an adjacent longitudinal mode, that mode begins to extract power, and SMSR drops. Mode-hop-free operation is defined as the range over which SMSR remains above the specified threshold.

Measurement requires an OSA with sufficient side-mode dynamic range — the spec is typically 50–80 dB for telecom-grade instruments, easily exceeding typical device SMSRs. The OSA resolution bandwidth must be narrow enough to resolve adjacent modes; for a typical laser with 1 nm mode spacing, 0.02–0.05 nm RBW is standard.

SMSR depends on operating current, temperature, and any external feedback. Reported SMSR values should specify these conditions.