Fiber Optic Lasers: Understanding Lasers in Optical Transceivers

Fiber optic lasers are the core component of fiber optical transceivers which convert electrical data into optical signals for transmission over the fiber network.

They account for a large part of the cost of an optical transceiver and the laser used is directly linked to the transmission distance.

Different designs of fiber optic laser are also available, which also have an influence on the cost of production as well as the types of application they can be used in.

The most commonly used are, FP (Fabry Perot Laser), VCSEL (Vertical-Cavity Surface-Emitting Laser), DFB (Distributed Feedback Laser) also known as DML (Directly Modulated Lasers), and EML (Electro-absorption Modulated Lasers).

Fiber Optic Lasers 101 – How they work

Laser is an acronym for “Light Amplification by Stimulated Emission of Radiation”. A fiber optic laser consists of three components, an energy/pump source, a gain medium and an optical resonator which is formed by two or more optical lenses. In very simple terms, the laser works by applying an electrical current to the optical gain medium which excites electrons, then, as the electrons lose energy, they release photons. The photons (light particles) are then reflected through the optical resonator which creates the output laser beam.

There are different designs of fiber optic laser: edge-emitting semiconductor lasers (also known as in-plane lasers) which emit the laser beam along the same direction as the wafer surface of the semiconductor chip; and, surface-emitting lasers, which emit the light in a perpendicular direction to the wafer surface. Edge-emitting lasers were the first semiconductor lasers and are still in use today.

Edge-emitting semiconductor laser.

Surface-emitting semiconductor laser (e.g. VCSEL)

Lasers used in fiber optical transceivers

Laser Type	Wavelength	Maximum Transmission Distance	Maximum Bandwidth
VCSEL	850nm	Up to 500 meters	Up to 400G (QSFP-DD)
FP	1310 and 1550nm	500 meters to 10 km	Up to 1000M (SFP)
DFB	1310 and 1550nm	Up to 40 km	Up to 200G
EML	1310 and 1550nm	Up to 40 km	Up to 400G (QSFP-DD and OSFP)

Fabry-Pérot Edge Emitter

FP (Fabry Pérot Laser)

Fabry Perot (FP) lasers transmit at specific wavelengths (1310nm and 1550nm) and are suitable for transmission distances below 40km. They have a high modulation rate and large output power, however, the smaller divergence angle and narrow spectrum of the FP lasers mean that thate are not able to be used for Wavelength Division Multiplexing (WDM).

FP is typically used in SFP.

Example fiber optical transceiver:
100M SFP BiDi LX, 20 km, Tx1550/Rx1310 nm, DDM, LC Simplex, Singlemode >

VCSEL Emitter

VCSEL (Vertical-Cavity Surface-Emitting Laser)

As the name suggests the VCEL emits the laser beam perpendicular to the surface of the semiconductor wafer. This gives it some advantages over edge-emitting lasers during the production process which reduces the fabrication costs whilst ensuring quality. The way that VCSELs are constructed means that it is possible to test for quality and processing issues at different stages throughout the production process, whereas edge-emitting lasers cannot be tested until the end of the production and are wasted if they are found to be of low quality. In addition, their design means that thousands of VCELs can be processed simultaneously.

Transmitting at 850nm for up to distances of 500 metres over multimode fiber, VCSELs have a lower power consumption than edge-emitting semiconductor lasers and are suitable for high-speed communications in networks and enterprise datacenters.

VCSELs are typically used in CFP, SFP, SFP+, SFP28, SFP56, QSFP and QSFP-DD optical transceivers.

Example fiber optical transceivers:
400G QSFP-DD SR8, 100 m, 850 nm, DDM, MPO, Multimode >
100M SFP FX, 2 km, 850 nm, DDM, LC Duplex, Multimode >

DFB Edge Emitter

DML (Directly Modulated Lasers)/DFB (Distributed Feedback Lasers)

Directly Modulated Lasers (DML), also known as Distributed Feedback (DFB) lasers because of how they are constructed, consist of a single chip making them suitable for transceiver designs requiring a small footprint and low power consumption. The DFB passes only specific wavelengths (1310nm and 1550nm) through a grating, transmitting at distances below 40 km.

DML/DFB is typically used in SFP, SFP+, SFP28, QSFP, XFP, QSFP-DD.

Example fiber optical transceivers:
200G QSFP56 FR4, 2 km, 1271-1331 nm, DDM, LC Duplex, Singlemode >
100M SFP CWDM EX, 40 km, 1430 nm, DDM, LC Duplex, Singlemode >

EML (Electro-absorption Modulated Laser)

The EML design integrates a laser diode with an electro-absorption modulator (EAM) on a single chip, and features lower chromatic dispersion, so it is better suited to higher data rate transmission over longer distances.

EML is typically used in SFP+, SFP28, QSFP, XFP, OSFP.

Example fiber optical transceivers:
400G QSFP-DD FR4, 2 km, 1271-1331 nm, DDM, LC Duplex, Singlemode >
10G SFP+ CWDM ER, 40 km, 1450 nm, DDM, LC Duplex, Singlemode >

DML/DFB vs EML

DML/DFB is generally used for lower data rate applications over shorter distances than EML up to around 10km (although 100M data rates are available for up to 40km). EML can handle longer distances at higher data rates.

To find out more about optical transceivers, including information on the type of fiber optic laser used, take a look at the Pro Optix Customer Portal >