Superluminescent diode (SLD)

A superluminescent diode (SLD, also SLED) is a forward-biased semiconductor waveguide in which spontaneous emission is amplified by stimulated emission on a single pass and leaves through the output facet before any cavity can form. The output is amplified spontaneous emission: spatially single-mode, so it couples efficiently into single-mode fiber, but spectrally broad and of low temporal coherence. Commercial devices cover roughly 400 nm to 1700 nm; an 840 nm SLD typically emits a few to tens of milliwatts with a full width at half maximum (FWHM) of 20–50 nm, and 1310 nm devices reach bandwidths of 50–100 nm. The structure sits between a light-emitting diode, which has no useful gain, and a laser diode, which has strong facet feedback.

Structure and feedback suppression

The active region is the same as in a laser diode or semiconductor optical amplifier, with a ridge waveguide a few micrometres wide and typically 0.5–2 mm long. What distinguishes the SLD is the effort spent removing feedback. The waveguide is tilted by several degrees relative to the facet normal so that reflected light misses the guided mode, the facets carry an anti-reflection coating, and some designs add an unpumped absorbing section at the rear end. Together these bring the effective modal reflectivity to around $10^{-5}$ or below.

The requirement follows from the round-trip condition. With single-pass gain $G$ and facet reflectivities $R_1$, $R_2$, residual cavity feedback imposes a Fabry–Pérot ripple on the spectrum whose peak-to-valley ratio is

$$\frac{P_\text{max}}{P_\text{min}} = \left(\frac{1 + G\sqrt{R_1 R_2}}{1 - G\sqrt{R_1 R_2}}\right)^{2}.$$

For $G$ = 1000 (30 dB) and $R_1 = R_2 = 10^{-5}$, $G\sqrt{R_1R_2}$ = 0.01 and the ripple is 0.17 dB. At $R = 10^{-4}$ the ripple grows to 1.7 dB, and at $R = 10^{-3}$ the product reaches 1: the device is at lasing threshold. High-power SLDs therefore need the lowest reflectivities, since output power rises with $G$.

Coherence and measurement

The optical spectrum analyzer is the primary instrument: it gives centre wavelength, FWHM, spectral shape, and ripple, which appears as a periodic modulation at the chip's longitudinal mode spacing. Output power is read with a power meter after the fiber pigtail, and a light–current curve should rise superlinearly without the sharp knee that marks lasing.

For a Gaussian spectrum of centre wavelength $\lambda_0$ and FWHM $\Delta\lambda$, the axial resolution of optical coherence tomography (set by the source coherence length) is

$$\delta z = \frac{2\ln 2}{\pi}\,\frac{\lambda_0^2}{\Delta\lambda}.$$

An 840 nm source with 50 nm FWHM gives $\delta z$ = 6.2 µm in air; a 1310 nm source with the same 50 nm width gives 15.1 µm, which is why ophthalmic OCT favours the 800–870 nm band and why SLD makers combine several offset gain sections or quantum well compositions to widen the spectrum. Dividing by the tissue group index (about 1.35–1.4) gives the resolution in the sample.

Where SLDs are used

Time-domain and spectral-domain OCT, fiber-optic gyroscopes, low-coherence interferometry, and fiber sensor interrogation all use SLDs. In a gyroscope the short coherence length suppresses errors from Rayleigh backscatter and polarization cross-coupling, which interfere coherently with the signal only when their path difference is within the coherence length. SLDs are also used as broadband test sources for measuring the spectral response of filters and gratings, and as low-speckle illumination.

Pitfalls

Back-reflections from the fiber connector or the application optics act as an external mirror; a few hundred ppm of feedback into a high-gain SLD can produce spectral ripple, parasitic lasing, or facet damage. An optical isolator and angled (APC) connectors are standard. Spectral shape matters as much as width: a spectrum with a dip or shoulders produces side lobes in the OCT point-spread function. The centre wavelength shifts with temperature and drive current, so most modules include a thermoelectric cooler, and power should be reduced when the output is unloaded if the datasheet says so.

Common questions

What is the difference between an SLD and an LED?

Both emit spontaneous light, but an SLD amplifies it along a waveguide, so its emission is directional and single-mode, reaching milliwatts in single-mode fiber. A surface-emitting LED radiates into a wide cone and couples only microwatts into single-mode fiber. The SLD's spectrum also narrows somewhat as gain increases.

What is the difference between an SLD and a laser diode?

A laser diode has a resonator and above threshold emits into one or a few longitudinal modes with coherence lengths ranging from millimetres for a multimode Fabry–Pérot diode to metres or more for a single-frequency DFB laser. An SLD has no resonator by design, so its coherence length is roughly 10–30 µm and its output has no threshold kink.

Why is it called superluminescent?

"Superluminescence" refers to spontaneous emission enhanced by stimulated emission in an inverted medium. It has nothing to do with speeds exceeding that of light.

References: B. E. A. Saleh, M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019); L. A. Coldren, S. W. Corzine, M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012); W. Drexler, J. G. Fujimoto (eds.), Optical Coherence Tomography: Technology and Applications, 2nd ed. (Springer, 2015).