Side-mode suppression ratio (SMSR)

The side-mode suppression ratio (SMSR) of a laser is the power in its main lasing mode divided by the power in its strongest other mode, expressed in decibels:

$$\text{SMSR}=10{\log }_{10}\text{\hspace{0.17em}⁣}\left(\frac{P_{\text{main}}}{P_{\text{side}}}\right)\mathrm{.}$$

It is the standard measure of how single-mode a single-frequency laser is. A telecom DFB laser is typically specified at 35 dB or more at its operating point, and higher grades at 45 dB or more; at 35 dB the strongest side mode carries 0.03% of the main mode's power. On an optical spectrum analyzer the SMSR is read directly as the height difference between the two peaks: a main mode at +3.0 dBm and a strongest side mode at −39.5 dBm give an SMSR of 42.5 dB.

What the values mean

The same metric is quoted for DBR lasers, external-cavity and widely tunable lasers and single-mode VCSELs, each against the minimum in its own datasheet.

What sets it

The side modes of a single-mode laser sit below threshold. A DFB's grating, or a DBR's mirror, gives them a higher threshold gain than the main mode, by a margin $\mathrm{\Delta }\alpha$, and once the main mode lases the gain is clamped at its threshold value. Each side mode is then held a little below its own threshold and is fed only by spontaneous emission. In the simplest rate-equation picture a side mode holds $N_s=n_{\text{sp}}\mathrm{/}(v_g\mathrm{\Delta }\alpha {\tau }_p)$ photons, where $n_{\text{sp}}$ is the population inversion factor, $v_g$ the group velocity and ${\tau }_p$ the photon lifetime, so that

$$\text{SMSR}\approx \frac{N_m}{N_s}=\frac{N_m{v}_g\mathrm{\Delta }\alpha {\tau }_p}{n_{\text{sp}}},$$

with $N_m$ the number of photons in the main mode, assuming the two modes couple out equally. The ratio grows in proportion to the gain margin and to the power in the main mode. For a 300 µm, 1550 nm chip with a photon lifetime of 2.45 ps, group index 3.6 and $n_{\text{sp}}=1.6$, emitting 10 mW (about 240,000 photons in the cavity), a margin of 5 cm⁻¹ gives 16 photons in the side mode and an SMSR of 41.8 dB; a margin of 1 cm⁻¹ gives 34.9 dB. The same device gives 31.8 dB at 1 mW, since the SMSR rises by 3 dB for each doubling of the power.

The SMSR is therefore poor just above threshold, and a datasheet value applies only at the stated current. It also falls when the gain peak drifts, with temperature, toward a side mode until a mode hop occurs, and when spatial hole burning at high current erodes the gain margin of a phase-shifted DFB. Reflections returning into the laser can lower it sharply, and in the coherence-collapse regime of optical feedback the spectrum broadens altogether.

Why it matters

In a dispersive fiber link, power that fluctuates between the main mode and a side mode arrives at different times, and this mode partition noise sets an error floor that no increase in received power removes. Side modes also appear as spurious channels in a wavelength-division multiplexed system and as spurious lines in spectroscopy. For these reasons SMSR is part of the standard acceptance test for single-frequency lasers, alongside threshold, slope efficiency, linewidth and relative intensity noise.

Measuring it

The SMSR is measured with an optical spectrum analyzer at a resolution bandwidth narrow enough to separate the main mode from its neighbors, which in a DFB lie about 1 nm away, and it is the difference between the two peak readings in dB, taken at the same resolution bandwidth. The measurement is limited by the analyzer's dynamic range close to a strong signal, typically 50 to 60 dB at 0.5 nm and 60 to 70 dB at 1 nm from it on a benchtop grating instrument. A side mode below that floor is not seen, so the reading is then a lower bound, reported as, for example, "above 50 dB"; a tunable bandpass filter that removes the main mode lets the side mode be measured on its own. The laser's current, temperature and any modulation are recorded with the result, since each changes it. The procedure within a full device test is in the DFB characterization workflow, and the analyzer settings are in the optical spectrum analyzer article.

References: L. A. Coldren, S. W. Corzine, and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012), for the rate-equation treatment of side modes. G. P. Agrawal and N. K. Dutta, Semiconductor Lasers, 2nd ed. (Van Nostrand Reinhold, 1993), for mode partition noise and single-frequency laser design.