Surface emitter

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The surface emitter or VCSEL [ v'ɪxl ] (from English vertical-cavity surface-emitting laser ) is a laser diode in which the light is emitted perpendicular to the plane of the semiconductor chip, in contrast to the edge-emitting laser diode in which the light is emitted at one or two Flanks of the chip emerges.

features

Decisive features of the surface emitters are the lower manufacturing costs compared to edge emitters and the better beam quality with, however, lower output power. VCSELs are characterized by the fact that they are available in single mode and the wavelength can be determined by the structure. The coupling efficiency in optical waveguides is high due to the small construction, the good beam quality and the good focusability.

Applications

Surface emitters are used as optical transmitters for fiber optic data transmission and are suitable for analog broadband signal transmission. Surface emitters are also used for absorption spectroscopy ( Tunable Diode Laser Absorption Spectroscopy , TDLAS). Laser printers are another area for VCSEL .

structure

Scheme of a simple surface emitter structure

The laser resonator is formed by two Bragg mirrors arranged parallel to the plane of the wafer , in which an active zone (usually with two-dimensional quantum wells , also called quantum film) is embedded for generating the laser radiation. Such planar Bragg mirrors are made up of layers with alternating low and high refractive indices , each of which has an optical path length l of a quarter of the laser wavelength in the material: l = λ / (4n) with n = refractive index of the medium. This achieves the degree of reflection of over 99 percent required in the surface emitter . Due to the small dimensions of the actual laser of only a few micrometers in each direction, surface emitters have lower output powers compared to edge emitters.

A distinction is made between optically pumped surface emitters, in which the active zone is excited from the outside to emit light with a shorter wavelength, and electrically pumped surface emitters, which are a pin diode .

Surface emitters for wavelengths between 650 nm and 1300 nm are based practically exclusively on GaAs wafers, while at wavelengths between 1300 nm and 2000 nm (long-wave surface emitters ) at least the active zone is based on InP or GaInAsN. Surface emitters with an even larger wavelength are currently in the experimental stage and are mostly optically pumped.

Special forms

Special forms of surface emitters:

  • Surface emitter with tunnel contact. Here, the tunnel contact ( n + p + ) creates an electrically advantageous nn + p + - pin configuration that can also have a positive effect on various other structural details, e.g. B. at the Buried Tunnel Junction (BTJ).
  • Widely tunable surface emitter with a micromechanically movable mirror, also called Vertical External Cavity Surface Emitting Laser (VECSEL).
  • Wafer-bonded or wafer-fused surface emitter: A combination of semiconductor material that can be manufactured on different types of wafer substrates.
  • Monolithic optically pumped surface emitters: Two surface emitters placed on top of one another, in which one optically pumps the other.
  • Surface emitter with longitudinally integrated monitor diode: A photodiode is integrated under the rear mirror.
  • Surface emitter with transversely integrated monitor diode: A resonant photodiode for light from an adjacent surface emitter can be produced from the wafer of a surface emitter by suitable etching.

Historical

Soda, Iga, Kitahara and Suematsu presented the first surface emitter in 1979, but components with a threshold current below 1 mA were only presented in 1989. Today, surface emitters have replaced edge emitters in short-range optical connections such as Gigabit Ethernet or Fiber Channel .

Web links

Individual evidence

  1. ↑ Haruhisa Soda, Ken-ichi Iga, Chiyuki Kitahara, Yasuharu Suematsu: GaInAsP / InP Surface Emitting Injection Lasers . In: Japanese Journal of Applied Physics . tape 18 , no. 12 , 1979, pp. 2329-2330 , doi : 10.1143 / JJAP.18.2329 .