Glossary entry

Vertical-cavity surface-emitting laser (VCSEL)

A semiconductor laser with its optical cavity oriented perpendicular to the wafer surface, emitting through the top or bottom of the chip. Dominant short-wavelength datacom source.

A VCSEL has its laser cavity oriented vertically through the epitaxial layers, with light emitted from the top (or bottom) surface rather than from a cleaved edge. The cavity is short ( $\sim 1$ μm, $\sim$ one wavelength) and bounded by two high-reflectivity distributed Bragg reflectors (DBR mirror stacks) above and below the active region.

Standard structure:

Layer	Function
Top DBR (p-type)	20 – 30 pair AlAs/GaAs (or similar) at $R \gtrsim 99.5$ %
Oxide aperture	Selective oxidation of one layer to provide current and optical confinement
Active region	One to several quantum wells
Bottom DBR (n-type)	30 – 40 pair AlAs/GaAs at $R \gtrsim 99.9$ %
Substrate	GaAs (most VCSELs) or InP (1300+ nm long-wavelength VCSELs)

The high mirror reflectivities are required because the short cavity length gives low single-pass gain — typical mirror loss is $\alpha_m \approx \ln(1/R)/L_\text{cav}$ which must be balanced by the gain–length product.

Performance characteristics:

Parameter	Multimode 850 nm	Single-mode 1310/1550 nm
Threshold current	0.3 – 1 mA	1 – 5 mA
Max single-ended output	2 – 5 mW	1 – 3 mW
Modulation bandwidth	15 – 30 GHz	10 – 20 GHz
Beam divergence (FWHM)	15° – 25°	8° – 15°
RIN	$-130$ to $-140$ dB/Hz	$-140$ to $-150$ dB/Hz
Linewidth	$\sim$ 50 – 100 MHz	$\sim$ 5 – 50 MHz

VCSELs dominate the short-reach datacom market: 850 nm multimode VCSELs over OM3/OM4 fiber drive 10/25/100/400 GbE switch interconnects, optical mice, and 3D depth sensors. Long-wavelength VCSELs (1310/1550 nm) compete with edge-emitting DFB lasers in some access-network and silicon-photonic co-packaged optics applications.

Distinguishing features vs edge-emitting lasers:

Circular output beam (vs elliptical) — easier fiber coupling
Lower threshold current — direct modulation at very low drive
On-wafer testing possible (no cleaving)
Easily packaged into 2D arrays for parallel optical links
Limited output power vs edge emitters
Limited yields at long wavelengths (1310/1550 nm) due to difficulty growing matched DBRs on InP