Laser Diode Characterization

Single Mode

Longitudinal modes

The term laser mode is often used in different contexts with different meanings. In a normal Fabry Perot laser a large number of longitudinal modes arise by the constructive resonance of the full internal reflection between the facets. The intermodal spacing that can be measured as described above with a spectrometer, allows us to determine the optical length of the cavity, since λ02 / Δλ = 2 L n, being λ0 the lasing wavelenght, L the length of the cavity length and n the diffraction index of the cavity.

In the context of communication lasers, a very efficient suppression of the next side mode of the lasing wavelenght is required and very complex structures on the cavity are fabricated (Distributed Feedback Laser = DFB) to suppress the side modes. For communication lasers this type of “single-mode” is mandatory and refers to the emission of only one lasing wavelength.

The longitudinal modes refer to the wavelength. With high resolution of the spectrum analyzer the individual modes of a laser diode can be read out. Single mode in this context requires a large power difference between the main mode and the next wavelength mode (not shown).

Polarization Modes

On lasers that are not intended to be used for communication, the longitudinal modes are not even shown in its full resolution in the datasheets. The measurement is done with a coarse wavelength resolution because it is faster. The term laser mode is also used to distinguish between the transverse electric (TE) and the transverse magnetic (TM) polarization modes, which propagate along each other in the cavity. Both modes are inherent linked to each other and it is just a very analytic way of describing the light wave. On edge-emitting laser diodes the laser is inherently strongly polarized and knowing the plane of either the TE or TM mode allows to understand the how the laser diode is mounted in its package. For vertical emitting laser diodes the polarization plane is not inherently designed and it is usually not a specified parameter.

Transversal modes

Finally, the term laser mode is also used to depict the lateral modes, that are the lasing modes that build up due to the lateral dimensions of the waveguide. In cases when higher laser output power is required so-called multi-mode laser diodes are built that have a large lateral waveguide guiding. In this case the emission pattern is poor but the existence of multiple modes leads to a large optical power output overall.

The vertical confinement in edge-emitting lasers is achieved with cladding layers that are deposited together with the heterojunction and therefore are very efficient in creating a very small vertical lateral mode. To create a tight horizontal lateral mode in an edge emitting laser the methods of current guiding through electrode patterning and other such as index guiding are somewhat more elaborate but sometimes unavoidable to create a lateral single mode also in the plane perpendicular to the P/N junction. Sometimes these lasers are referred to as a single-mode too, while here not the wavelength is meant, but a confined Gaussian far field.

Transversal Gaussian single mode pattern.

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