Vertical-cavity surface-emitting laser
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The vertical-cavity surface-emitting laser (VCSEL; [v'ɪxl]) is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer.
There are several advantages to producing VCSELs when compared with the production process of edge-emitting lasers. Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not work, whether due to bad contacts or poor material growth quality, the production time and the processing materials have been wasted. VCSELs however, can be tested at several stages throughout the process to check for material quality and processing issues. For instance, if the vias have not been completely cleared of dielectric material during the etch, an interim testing process will flag that the top metal layer is not making contact to the initial metal layer. Additionally, because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three inch Gallium Arsenide wafer. Furthermore, even though the VCSEL production process is more labor and material intensive, the yield can be controlled to a more predictable outcome.
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[edit] Structure
The laser resonator consists of two distributed Bragg reflector (DBR) mirrors parallel to the wafer surface with an active region consisting of one or more quantum wells for the laser light generation in between. The planar DBR-mirrors consist of layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivities above 99%. High reflectivity mirrors are required in VCSELs to balance the short axial length of the gain region.
In common VCSELs the upper and lower mirrors are doped as p-type and n-type materials, forming a diode junction. In more complex structures, the p-type and n-type regions may be buried between the mirrors, requiring a more complex semiconductor process to make electrical contact to the active region, but eliminating electrical power loss in the DBR structure.
In laboratory investigation of VCSELs using new material systems, the active region may be pumped by an external light source with a shorter wavelength, usually another laser. This allows a VCSEL to be demonstrated without the additional problem of achieving good electrical performance; however such devices are not practical for most applications.
VCSELs for wavelengths from 650 nm to 1300 nm are typically based on gallium arsenide (GaAs) wafers with DBRs formed from GaAs and aluminium gallium arsenide (AlxGa(1-x)As). The GaAs/AlGaAs system is favored for constructing VCSELs because the lattice constant of the material does not vary strongly as the composition is changed, permitting multiple lattice matched epitaxial layers to be grown on a GaAs substrate. However, the refractive index of AlGaAs does vary relatively strongly as the Al fraction is increased, minimizing the number of layers required to form an efficient Bragg mirror compared to other candidate material systems. Furthermore, at high aluminum concentrations, an oxide can be formed from AlGaAs, and this oxide can be used to restrict the flow of current in a VCSEL, enabling very low threshold currents.
Recently the two main methods of restricting the current flow in a VCSEL were characterized by two types of VCSELs: Ion Implanted VCSELs and Oxide VCSELs.
In the early 1990s, telecommunications companies tended to favor Ion Implanted VCSELs. Ions, (often hydrogen ions, H+), were implanted into the VCSEL structure everywhere except the aperture of the VCSEL, destroying the lattice structure around the aperture, thus inhibiting the current flow. In the mid to late 1990s, companies moved towards the technology of oxide VCSELs. The current flow is confined in an oxide VCSEL by oxidizing the material around the aperture of the VCSEL. A high content aluminum layer that is grown within the VCSEL structure is the layer that is oxidized. Oxide VCSELs also often employ the ion implant production step. As a result in the oxide VCSEL, the current path is confined by the ion implant and the oxide aperture.
The initial acceptance of oxide VCSELs was plagued with concern about the apertures "popping off" due to the strain and defects of the oxidation layer. However, after much testing, the realibilty of the structure has proven to be robust. As stated in one study by Hewlett Packard on oxide VCSELs, "The stress results show that the activation energy and the wearout lifetime of oxide VCSEL are similar to that of implant VCSEL emitting the same amount of output power." [1]
A production concern also plagued the industry when moving the oxide VCSELs from research and development to production mode. The oxidation rate of the oxide layer was highly dependent on the Aluminum content. Any slight variation in Aluminum would change the oxidation rate sometimes resulting in apertures that were either too big or too small to meet the specification standards.
Longer wavelength devices, from 1300 nm to 2000 nm, have been demonstrated with at least the active region made of indium phosphide. VCSELs at even higher wavelengths are experimental and usually optically pumped. 1310 nm VCSELs are desirable as the dispersion of silica-based optical fiber is minimal in this wavelength range.
[edit] Special Forms
- Multiple active region devices (aka bipolar cascade VCSELs). Allows for differential quantum efficiency values in excess of 100% through carrier recycling
- VCSELs with tunnel junctions. Using a tunnel junction (n+p+), an electrically advantageous n-n+p+-p-i-n configuration can be built that also may beneficially influence other structural elements (e.g. in the form of a Buried Tunnel Junction (BTJ)).
- Widely tunable VCSEL with micromechanically (MEMS) movable mirror
- Wafer-bonded or wafer-fused VCSEL: Combination of semiconductor materials that can be fabricated using different types of substrate wafers
- Monolithically optically pumped VCSELs: Two VCSELs on top of each other. One of them optically pumps the other one.
- VCSEL with longitudinally integrated monitor diode: A photodiode is integrated under the back mirror of the VCSEL.
- VCSEL with transversally integrated monitor diode: With suitable etching of the VCSEL's wafer, a resonant photodiode can be manufactured that may measure the light intensity of a neighboring VCSEL.
- VCSELs with external cavities, knows as VECSELs or semiconductor disk lasers. VECSELs are optically pumped with conventional laser diodes. This arrangement allows a larger area of the device to be pumped and therefore more power can be extracted - as much as 30W. The external cavity also allows intracavity techniques such as frequency doubling, single frequency operation and femtosecond pulse modelocking.
- Vertical-cavity semiconductor optical amplifiers, known as VCSOAs . These devices are optimized as amplifiers as opposed to oscillators. VCSOAs must be operated below threshold and thus require reduced mirror reflectivities for decreased feedback. In order to maximize the signal gain, these devices contain a large number of quantum wells (optically pumped devices have been demonstrated with 21-28 wells) and as a result exhibit single-pass gain values which are significantly larger than that of a typical VCSEL (roughly 5%). These structures operate as narrow linewidth (tens of GHz) amplifiers and may be implemented as amplifying filters.
[edit] Characteristics
Because VCSELs emit from the top surface of the chip, they can be tested on-wafer, before they are cleaved into individual devices. This reduces the fabrication cost of the devices. It also allows VCSELs to be built not only in one-dimensional, but also in two-dimensional arrays.
The larger output aperture of VCSELs, compared to most edge-emitting lasers, produces a lower divergence angle of the output beam, and makes possible high coupling efficiency with optical fibers.
The high reflectivity mirrors, compared to most edge-emitting lasers, reduce the threshold current of VCSELs, resulting in low power consumption. However, as yet, VCSELs have lower emission power compared to edge-emitting lasers. The low threshold current also permits high intrinsic modulation bandwidths in VCSELs (Iga 2000).
The wavelength of VCSELs may be tuned, within the gain band of the active region, by adjusting the thickness of the reflector layers.
While early VCSELs emitted in multiple longitudinal modes or in filament modes, single-mode VCSELs are now common.
[edit] Applications
- Optical fiber data transmission
- Analog broadband signal transmission
- Absorption spectroscopy (TDLAS)
- Laser printers
- computer mouse
[edit] History
The first VCSEL was presented in 1979 by Soda, Iga, Kitahara and Suematsu (Soda 1979), but devices for CW operation at room temperature were not reported until 1988 (Koyama 1988). The term VCSEL was coined in a publication of the Optical Society of America in 1987[citation needed]. Today, VCSELs have replaced edge-emitting lasers in applications for short-range fiberoptic communication such as Gigabit Ethernet and Fibre Channel.
[edit] References
- Iga, Kenichi (2000), "Surface-emitting laser—Its birth and generation of new optoelectronics field", IEEE Journal of Selected Topics in Quantum Electronics 6 (6): 1201–1215
- Soda et al, Haruhisa (1979), "GaInAsP/InP Surface Emitting Injection Lasers", Japanese Journal of Applied Physics 18 (12): 2329–2330. doi:10.1143/JJAP.18.2329
- Koyama et al, Fumio (1988), "Room temperature cw operation of GaAs vertical cavity surface emitting laser", Trans. IEICE, E71(11): 1089-1090
