VCSEL:
Vertical Cavity Surface-Emitting Lasers

Vertical Cavity Surface Emitting Lasers (VCSEL) are semiconductor lasers which emit perpendicular to their p-n junction, and feature circular, low-divergence beams.
Their single-mode properties are attractive for applications such as spectroscopy.

The spectroscopist needs a light source which is tuneable, easy to focus, stable, insensitive to interference and with a narrow emission linewidth. Traditional edge-emitting diode lasers only partially achieve these requirements. Such lasers have elliptical, divergent beams which must be optically corrected in order to collimate the beam even over short distances.
Furthermore it is often necessary to isolate the laser from back-reflection of the emitted light into the
resonator, which can lead to changes in the laser’s output characteristics. The divergent, elliptical beam is a result of diffraction at the rectangular emission area of a conventional diode laser resonator. The divergence varies inversely with the size of this emission area. Consequently the beam divergence perpendicular to the junction is significantly greater than parallel to it (see Fig. 1 a).

Fig. 1: Designs and beam profiles of a) Edge-emitter, b) VCSEL

Aperture of a VCSEL as seen by a Scanning Electron Microscope

Vertical Resonator

In order to fully control the emitted beam profile it is necessary to define the geometry of the emission area. This is only possible with a vertical resonator, perpendicular to the p-n junction, since the active layer is parallel to the emission area (Fig 1 b). To obtain a round, low-divergence beam, a circular output complex may be applied to the entire emitting area.

In the case of a vertical resonator the length of the gain medium is defined by the thickness of the p-n junction. In order to achieve laser operation, this must be compensated by the use of highly efficient Distributed Bragg Reflectors as resonator mirrors. The lower resonator mirror is made up of forty alternating layers of AlxGa(1–x)As and AlAs, each layer l/4 thick, which give a reflectivity in excess of 99,99%. The output coupler, with twenty-five l/4 layers, has a reflectivity of 99.9%.

The high resonator efficiency and small gain medium volume combine to give threshold currents of only a few mA, which means that the low operating currents make VCSELs suitable for any application.

Special Features

A further advantage of this mirror design is that any light reflected back towards the laser from any other part of the optical system can not re-enter the resonator.
VCSELs are effectively isolated against such reflections due to their DBR output coupler structure. The resonator design also means that the laser emission is single-mode, an important characteristic for spectroscopy. Unlike conventional edge emitters, the gain profile of a VCSEL contains only one longitudinal mode, because of the short cavity length (see Fig. 2). The desired wavelength is achieved by using the correct layer structure.

Fig. 2: Gain profiles g(E)
a) Edge-emitter: several modes, b) VCSEL: only one mode above threshold

LASER COMPONENTS’s VCSELs also feature a special mesa structure which ensures that all additional transverse modes are suppressed. The result of this is predominantly single-mode emission with a correspondingly narrow emission linewidth (< 50 MHz at 780 nm, for example).

This mode will tune within the gain profile as the laser temperature is varied, which allows VCSELs to be temperature-tuned over a range of several nanometers, without mode hopping. The typical tuning rate, ca. 0.4 nm/mA, is significantly higher than in an edge-emitter, another advantage of the VCSEL for the spectroscopist.

The vertical structure has a further significant advantage: the laser may be tested before bonding and whilst still on the wafer. Furthermore, VCSELs may be easily incorporated into ICs and are also highly suitable for use in 2-dimensional arrays. LASER COMPONENTS currently offers VCSELs with wavelengths in the range 760 – 960 mm. Single-mode output powers are typically 0.3 – 0.5 mW, with 5 mW multi-mode versions also available.

Further Information:
Donna Dickie: 01245 491 499

Datasheet:

VCSELs (114 kB, 3 pages, PDF-file)

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