Facet (laser and waveguide facets)

A facet is the end face of an optical waveguide: the cleaved or polished end of a semiconductor laser chip, the edge of a photonic chip where a waveguide terminates, or the end of a fiber. In an edge-emitting laser the two facets are the cavity mirrors. Semiconductor crystals of InP and GaAs split cleanly along their 110 planes, so a scribed wafer broken into bars gives atomically flat, parallel facets with no polishing. At normal incidence the Fresnel reflectance of such a facet is

$$R = \left(\frac{n-1}{n+1}\right)^2 ,$$

which is 0.31 for $n = 3.5$ and 0.27 for $n = 3.2$. The reflectance of the guided mode differs from this plane-wave value by a few percent, and the site's laser entries take $R \approx 0.3$. A cleaved silica fiber end ($n = 1.45$) reflects 3.4%, a return loss of 14.7 dB.

Facets as laser mirrors

A Fabry-Perot laser uses its two uncoated facets as mirrors, and their reflectance enters the threshold through the mirror loss, $\alpha_m = (1/2L)\ln(1/R_1R_2)$: 40 cm⁻¹ for a 300 µm chip with $R = 0.3$ on both ends. An uncoated chip emits equally from both facets. Production lasers usually carry a dielectric high-reflection (HR) stack on the rear facet, often around 90–95%, and a partially reflecting or anti-reflection coating on the front, typically a few percent. The share of output leaving the front facet is

$$F_1 = \frac{1-R_1}{(1-R_1) + \sqrt{R_1/R_2}\,(1-R_2)} ,$$

with $R_1$ the front reflectance: 0.96 for a 10%/90% pair and 0.99 for 5%/95%. The trade-offs among threshold, slope efficiency and facet intensity are worked through in Facet coatings and what they do to a laser diode.

Suppressing facet reflections

Amplifiers and broadband sources must not lase, so their facets are made as unreflective as possible. A semiconductor optical amplifier or superluminescent diode combines an AR coating with a waveguide tilted relative to the facet normal, commonly by about 7–8°, so that the reflected light misses the guided mode; together these bring the modal reflectance to around 10⁻⁵ or below. Fiber uses the same idea: an APC connector end face is polished at 8°, and the reflected light travels about 16° from the fiber axis, well outside the roughly 5° range of guided ray angles in the core of standard single-mode fiber (PC, UPC and APC connectors). A flat fiber cleave, by contrast, is held within about 0.5° of perpendicular.

Facets on photonic chips

On a photonic integrated circuit the facet is where light enters and leaves the chip edge. Silicon and silicon nitride chips are diced and then polished, or the facet is defined by a deep etch through the oxide and into the substrate, which also allows testing before dicing. An edge coupler expands the mode near the facet to match a fiber. A bare silicon facet in air reflects about 31% ($n = 3.48$), so chip facets are often AR coated or index matched to the fiber; otherwise the two facets of a chip form a parasitic Fabry-Perot cavity whose fringes appear in every transmission spectrum. On InP chips, etched facets make it possible to integrate lasers without cleaving each device.

Facet damage and passivation

The facet is the weakest point of a high-power diode laser. Surface states and oxide at a bare cleaved facet absorb a little of the light, the absorption heats the surface and narrows the bandgap locally, and above a critical intensity the process runs away into catastrophic optical damage, which melts the facet within nanoseconds to microseconds. The defences are passivation of the facet immediately after cleaving (often in vacuum, before oxidation), dielectric coatings that also seal the surface, and non-absorbing windows near the facet with a wider bandgap. The light just inside a facet is the sum of the forward wave and its reflection, so for the same output a cleaved facet carries a higher internal intensity than an AR-coated one.

Pitfalls

Dust, solder splash or epoxy on a facet changes its reflectance and can start damage. Light reflected back from fiber connectors or lenses into an AR-coated laser facet competes with a weak mirror and can destabilize the laser far more readily than it would with a cleaved facet; an optical isolator or angled interfaces prevent this.

Common questions

What is the reflectivity of a cleaved laser facet?

About 0.27–0.32 for III-V semiconductors with refractive indices of 3.2–3.6; 0.31 for $n = 3.5$. The modal reflectance of the guided mode differs from the plane-wave value by a few percent.

Why are SOA and SLD facets angled?

An angle of several degrees sends the reflected light out of the waveguide's acceptance, which suppresses feedback much further than an AR coating alone and prevents lasing.

References: L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012); G. P. Agrawal and N. K. Dutta, Semiconductor Lasers, 2nd ed. (Van Nostrand Reinhold, 1993); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).