Single-mode vs Multimode Fabry-Perot Laser Diodes

Author: Mike McGoff, Sales Application Engineer, Sheaumann Laser, Inc.

The Fabry-Perot (FP) laser diode is the most common type of laser diode commercially available in the market today and is utilized in a wide-range of applications.

Fabry-Perot Laser Diode – Single-Mode vs Multimode

FP lasers are edge-emitting devices, and the basis for their operation is the laser resonator cavity known as a Fabry-Perot resonator. A Fabry-Perot resonator is a cavity with two mirrors. The front and back facet of the laser diode serve as the mirrors. Only the photons with a frequency of the resonant mode can travel in the cavity. These photons are reflected back and forth between the two facets and amplified by the semiconductor gain medium inside the cavity. In the fabrication of laser devices, highly efficient reflections may be achieved by applying a combination of a high-reflector coating on one end and an antireflective- coating on the other end. The laser beam is emitted from the antireflective coated edge.

fig. 2

FP laser diodes are sometimes categorized as single-mode or multimode, which refers to single spatial mode or multi-spatial mode. The key contrasting difference between the two types is the far field distribution in the lateral direction (slow axis). A single-mode laser shows a bell shaped far field distribution with only one peak, while a multimode laser exhibits a distribution with multiple peaks. Also, single-mode lasers are typically lower power but higher brightness. The principal factor in determining these properties relates to the emitter width – the width of the emission region [identified in fig. 1]. The output beam of the laser has an elliptical shape with the longer dimension known as the fast axis and the shorter dimension being known as the slow axis [fig. 2]. 

The properties of the FP laser diode output beam are classified in one manner as to how tightly the laser beam can be focused by the measurement of beam radius. In terms of M2, lasers can be split into three main types: 1) a laser with beam quality <1.3 is defined as a “pure” single-mode with a value of 1 considered pure single-mode, 2) 1.3 < x <2 is considered semi-single-mode, and 3) >2 is considered multimode. M2 is known as the beam propagation ratio and is a commonly accepted measure of laser output beam quality. Laser beam output has two points of interest with the difference being the distance from the edge of the device. The point closest to the device is known as the near field and the more commonly measured point is the far field. The below Fig. 3 and fig. 4 contrast the differences in far field of the beams from a single-mode and multimode FP laser. Comparing the two it is readily apparent that the difference exists in the shorter (slow axis) axis where the multimode laser exhibits a “rabbit-ear” effect caused by more than one mode being supported in the gain cavity. Note: The far-field mode shape of multimode laser diodes can change significantly as a function of driving current or rather temperature.

fig. 3 (Click Image To Enlarge)
Multimode Far Field Illustration
fig. 4 (Click Image To Enlarge)

Other factors come into play that affect laser diode operation and beam quality including the temperature stability of the device junction temperature. Temperature influences the band gap and consequently causes wavelength shifting, though it does not alter the single-mode versus multimode property.
At the application level, a FP laser device may be utilized in a free-space or fiber-coupled configuration. Depending on if it is single-mode or multimode, different challenges are faced due to beam properties described in this application brief.

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