Vertical-cavity surface-emitting laser (VCSEL)

A vertical-cavity surface-emitting laser (VCSEL) is a semiconductor laser whose cavity is perpendicular to the wafer: the light travels up and down through the epitaxial layers, between two distributed Bragg reflectors grown above and below a thin active region, and leaves through the top surface or through the substrate. The cavity of an edge-emitting laser diode lies in the plane of the wafer, and the light leaves from a cleaved facet. The vertical geometry gives a round beam of low divergence, lets every laser be tested on the wafer before it is diced, and allows arrays of many lasers on one chip. In exchange, the light crosses only a few tens of nanometers of gain on each pass, and the mirrors have to make up for it.

Iga proposed the VCSEL in 1977, and the first one to run continuously at room temperature was reported in 1989 (Koyama and colleagues).

Why the mirrors reflect more than 99%

With three 8 nm quantum wells, a material gain of 2,000 cm⁻¹ and the factor of two gained by placing thin wells at a peak of the standing wave, one pass through the active region amplifies the light by only 1%. A 300 µm edge emitter with a confinement factor of 0.05 would amplify it twentyfold at the same gain, which is why cleaved facets reflecting 31% are enough there. The gain over a VCSEL's round trip, about 2% in this example, has to exceed what the two mirrors transmit and what the cavity absorbs, so the mirrors together transmit well under 2%.

Each mirror is a stack of quarter-wave layers of alternating index, whose reflections add in phase (Bragg condition). Each additional pair of layers cuts the mirror's transmission by a factor of about $(n_H\mathrm{/}{n}_L{)}^2$, 1.36 for indices of 3.5 and 3.0, so with abrupt interfaces 18 pairs reach 99.5% toward air and 27 pairs 99.9% toward the substrate, over a stop band 83 nm wide at 850 nm. Real mirrors grade the composition at each interface to lower the electrical resistance of the many heterojunctions the current crosses, which weakens each reflection, and so use more pairs. The high-index layers contain some aluminum, because GaAs absorbs light shorter than 870 nm.

Oxide aperture and threshold

Current is confined by an oxide aperture: a layer of AlGaAs with about 98% aluminum, next to the cavity, is oxidized from the sides of an etched mesa into insulating aluminum oxide, leaving a conducting opening in the center. The oxide's low index also guides the light, so the aperture sets the transverse modes. An aperture a few micrometers across keeps the laser in one transverse mode at the cost of power, while datacom VCSELs use larger apertures and emit several transverse modes, which suits multimode fiber. Oxide confinement, introduced in the mid-1990s as an alternative to proton implantation, brought thresholds below a milliamp and efficiencies above 50%: an oxide-confined 850 nm VCSEL reached a wall-plug efficiency of 57% in 1997 (Jäger and colleagues). The threshold is roughly the threshold current density times the area of the aperture, 0.57 mA for a 6 µm aperture at 2 kA/cm² (threshold current).

Spectrum and beam

The cavity is about a wavelength long, so its longitudinal modes are tens of nanometers apart, and only one of them falls within both the mirrors' stop band and the gain: a VCSEL has a single longitudinal mode by construction, and a single-mode VCSEL needs only its transverse modes controlled. The beam is round. A single-mode VCSEL with a mode 4 µm across at 850 nm spreads at a full angle of 9° at half maximum, and multimode devices spread further (far-field divergence). Nothing in the circular structure fixes the polarization, which can switch between two orthogonal directions as the current or temperature changes; devices that need a fixed polarization use an elliptical aperture, anisotropic strain or a surface grating.

Temperature

The lasing wavelength follows the cavity resonance, which moves by about 0.06 nm/K at 850 nm, while the gain peak moves by about 0.26 nm/K (wavelength temperature coefficient). The two drift apart by 0.2 nm/K, 12 nm between 25 and 85 °C, so VCSELs are designed with the gain peak on the short-wavelength side of the resonance at room temperature, where the two meet at the upper end of the operating range. Heating is strong in a device this small: the current crosses the resistance of the doped mirror and deposits its heat within a few micrometers, and at high current the output reaches a maximum and then falls, which is called thermal rollover.

Speed and uses

The small active volume makes the laser fast for little current. A datacom VCSEL reaches a relaxation-oscillation frequency of 10 GHz a few milliamps above its threshold, where a telecom DFB laser needs about 25 mA (D-factor), and 850 nm devices have reached a modulation bandwidth of 30 GHz while dissipating less than 100 fJ per bit at 25 to 50 Gbit/s (Haglund and colleagues).

• Data links. Directly modulated 850 nm VCSELs over graded-index multimode fiber are the standard short-reach link inside data centers, up to 100 m over OM4 fiber, with lanes at 25 Gbit/s NRZ or 50 and 100 Gbit/s PAM4 (pluggable optics).
• Sensing. Arrays at 940 nm, where water vapor absorbs much of the sunlight, illuminate scenes for 3D sensing and time-of-flight ranging, as in face recognition on phones.
• Atomic devices. Single-mode VCSELs, with their low power draw and current-tuned wavelength, are the light source of chip-scale atomic clocks, locked to a cesium or rubidium line.
• Long wavelengths. At 1310 and 1550 nm the InP-based materials give a smaller index step for the mirrors, and halving the index step roughly doubles the number of pairs needed, so long-wavelength VCSELs use dielectric mirrors or GaAs-based mirrors bonded to the InP active region, with tunnel junctions to inject the current.

VCSELs and edge emitters

Edge emitters keep the high-power and long-wavelength telecom roles, including the DFB lasers of single-mode fiber links, while VCSELs take the roles where low current, low cost per laser and arrays matter. A VECSEL replaces the top mirror with an external one to reach watts in a single mode, and a PCSEL uses a photonic crystal in the plane of the wafer for the same purpose.

Common questions

What is a VCSEL?

A vertical-cavity surface-emitting laser: a semiconductor laser whose cavity is perpendicular to the wafer, between two Bragg mirrors about a wavelength apart, so that it emits from the top surface. Its short cavity and high-reflectivity mirrors give it a low threshold, a round beam and a single longitudinal mode.

What is the difference between a VCSEL and an edge-emitting laser?

The direction of the cavity. An edge emitter's cavity lies in the plane of the wafer and is hundreds of micrometers long, and it emits an elliptical beam from a cleaved facet; a VCSEL's cavity is about a micrometer long, across the layers, and it emits a round beam from the surface. VCSELs need less current and can be tested on the wafer and made in arrays; edge emitters give more power per laser.

Why are VCSELs used in data centers?

Because they are inexpensive to make and test, run at a few milliamps, can be modulated directly at tens of gigabits per second, and couple easily into multimode fiber. 850 nm VCSEL links over OM4 fiber reach 100 m.

What wavelength do VCSELs emit?

Most are GaAs-based and emit between about 650 and 1100 nm, with 850 nm the usual wavelength for multimode data links and 940 nm for 3D sensing. InP-based and bonded designs reach 1310 and 1550 nm.

References: K. Iga, "Surface-emitting laser: its birth and generation of new optoelectronics field," IEEE Journal of Selected Topics in Quantum Electronics 6, 1201 (2000). F. Koyama, S. Kinoshita and K. Iga, "Room-temperature continuous wave lasing characteristics of a GaAs vertical cavity surface-emitting laser," Applied Physics Letters 55, 221 (1989). R. Michalzik (ed.), VCSELs: Fundamentals, Technology and Applications of Vertical-Cavity Surface-Emitting Lasers (Springer, 2013). A. Larsson, "Advances in VCSELs for communication and sensing," IEEE Journal of Selected Topics in Quantum Electronics 17, 1552 (2011). R. Jäger et al., "57% wallplug efficiency oxide-confined 850 nm wavelength GaAs VCSELs," Electronics Letters 33, 330 (1997). E. Haglund et al., "30 GHz bandwidth 850 nm VCSEL with sub-100 fJ/bit energy dissipation at 25–50 Gbit/s," Electronics Letters 51, 1096 (2015). L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012).