Semiconductor lasers will support both TE and TM modes

Typically, for lasers in optical communication systems, waveguide designs are used to achieve a single transverse mode. By adjusting the thickness of the surrounding area of the cladding layer and the etching depth of the ridge in the ridge waveguide device, a single mode device can be obtained. The importance of lasers is reflected in the following aspects:

A chip without ridge waveguide design and narrow ridge waveguide chip B. For coherent light sources, the far-field pattern is essentially the Fourier transform of the near-field pattern (mode shape in the device).
The far field pattern of a single mode is a moderate 30 ° divergence angle for a ridge waveguide device, while the far field pattern of a large area device is stretched very long, emitting several degrees in the plane and very much out of the plane. It is not difficult to couple to optical fibers in the later stage.

The second reason why lasers require single mode is that it is necessary for devices to achieve true single wavelength. DFB laser is a single-mode laser prepared using periodic gratings, which is based on the effective refractive index to reflect a single wavelength. Different transverse modes have different effective refractive indices, so multimode waveguides with DFB gratings can have more than one wavelength output.

In reality, dielectric waveguides are simply first-order models of the actual waveguides of semiconductor lasers. The waveguide region of the laser is also the gain region, so the refractive index has a complex part associated with the gain (or the loss component in the absence of current).

The optical mode becomes "gain oriented" and refractive index oriented, without the need for a truly accurate optical cut-off design. The trend of this gain oriented is to favor the propagation of a single mode. In practice, the far-field and mode structure details calculated based on the refractive index distribution may differ significantly from the measured values of manufactured devices.

As a waveguide, semiconductor lasers will support both TE and TM modes, with TE being the transverse electric field and TM being the transverse magnetic field. However, in semiconductor quantum well lasers, the light emitted is mainly TE polarized. This is based on the different reflection coefficients of TE and TM modes at the cavity surface, and most lasers are inherently highly polarized.

For TE and TM modes, only certain discrete angles can become guiding modes, thereby propagating along the waveguide. Just as the light in a etalon must undergo phase length interference to support a specific wavelength, the light in a waveguide must also undergo phase length interference to allow a specific "mode" to exist, corresponding to a specific incident angle.

In the analysis of waveguides, the typical approach is to fix the wavelength and naturally choose the angle of its propagation. The reason is the same, assuming that the plane wave in the cavity originates from all points on the bottom edge. If the round-trip distance is not an integer multiple of the wavelength, the destructive interference will ultimately cause the light wave to disappear.

Source: Chip Process Technology