Semiconductor Lasers in Optical Communications Products: An Introduction
This article summarizes the lasers used in optical communications products.
The lasers used in optical communications products are mainly semiconductor lasers, so this article starts with the semiconductor bandgap.
Bandgap
Bandgaps are divided into direct bandgaps and indirect bandgaps.
After absorbing enough energy, an electron jumps from the valence band to the conduction band and becomes a free electron, producing current. This is the optical-to-electrical conversion process of a photodetector. Conversely, when an electron drops from the conduction band to the valence band, it releases energy; this is the electrical-to-optical conversion process of a laser. To conserve momentum, electron transitions in an indirect bandgap also involve phonons. Direct-bandgap semiconductor materials are therefore better suited for absorbing or emitting light.
Bandgap and wavelength can be converted with the following simple formula:
Eg = 1240/Lambda
where Eg is the bandgap in eV,
and Lambda is the wavelength in nm.
In the figure below (ref: http://web.tiscali.it/decartes/phd_html/node4.html), the solid black dots are direct-bandgap materials, most of which are III-V compound semiconductors.
Besides having a direct bandgap, III-V compound semiconductors let you tune the bandgap by adjusting element ratios, enabling different wavelengths. The table below (ref: https://www.ushio.eu/laser-explained/) lists common III-V compound semiconductor materials and their wavelengths.
Laser diode classification
Laser diodes can be classified by emission direction and by modulation method.
By emission direction:
Surface-emitting: VCSEL
Edge-emitting: Fabry-Perot, DFB, DBR
Process differences
After epitaxy, edge-emitting lasers require the wafer to be cleaved into bars and the facets coated to form mirrors, so their yield is lower than that of surface-emitting lasers.
Longitudinal mode differences
A laser works by having light resonate in a cavity while a gain medium amplifies its energy. Light that satisfies the resonant frequency is retained and produces lasing. These specific resonant frequencies are called modes — more precisely, longitudinal modes. The longer the cavity, the more modes satisfy the resonance condition. For optical communications products, you want the light to be as close to a single frequency as possible to reduce material dispersion in the fiber. For background, see: https://www.drflyout.com/post/fiberoptics
Surface-emitting: the cavity is parallel to the epitaxial growth direction and short, so single longitudinal mode output is easier to achieve.
Edge-emitting: the cavity is perpendicular to the epitaxial growth direction and long, so high output power is easier to achieve. But multiple longitudinal modes may appear, which is why the Distributed Feedback (DFB) laser was developed. Compared with a conventional Fabry-Perot laser, a DFB laser adds a Bragg grating in the active layer, enabling single longitudinal mode output.
Transverse mode differences:
Transverse modes depend mainly on the cross-sectional size of the gain medium; a smaller cross-section is less able to support higher-order transverse modes. The more single the transverse mode, the lower the modal dispersion and the easier it is to transmit over long distances. Multimode and single-mode here correspond to the multimode and single-mode fiber specifications.
Surface-emitting: usually multi-transverse-mode; single-mode output can usually be achieved by adding a microlens.
Edge-emitting: usually single transverse mode.
By modulation method:
Direct modulation: a laser driver biases the laser into its operating state, and the laser's optical intensity is modulated directly by varying the current.
Pros: direct and simple, with relatively low power consumption.
Cons: because the laser itself is modulated, its drive state changes constantly, which can lead to thermal or optical dispersion issues. Bandwidth is lower than EML.
External modulation: after the laser emits light, it first passes through a modulator for separate modulation.
Pros: can be used with higher-power lasers and supports higher-bandwidth applications.
Cons: the circuitry is more complex and therefore more expensive.
Common modulation methods are optical (interferometric) modulation and electro-absorption modulation.
Electro-absorption modulation uses voltage to change the material's absorption coefficient, which varies the output optical intensity.
Optical modulation, also called a Mach-Zehnder modulator (MZM), splits light into two paths; a voltage applied to one path shifts the relative phase, producing constructive and destructive interference at the output and thus intensity variation.
Finally, a bandwidth-vs-distance chart illustrates how laser diodes are applied in optical communications. The longer the distance and the higher the data rate, the more single longitudinal mode and single transverse mode transmission is required.
Multimode fiber carries multiple transverse modes and supports transmission only on the order of a hundred meters, mostly using VCSELs as the light source. At 50 Gbaud, the theoretical reach of VCSELs has already shrunk to under a hundred meters, and VCSEL development is still pushing for more bandwidth. For recent developments, see https://www.drflyout.com/post/_800g
Single-mode fiber carries a single transverse mode and supports transmission at the kilometer scale. EMLs have lower dispersion, so they can reach longer distances than DMLs.
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