Emitting angle of laser diode

Laser diodes exhibit asymmetric beam divergence, typically 30°–45° in the fast axis and 5°–15° in the slow axis.

Understanding Laser Diode Divergence

The emission angle of a laser diode, also called the beam divergence, describes how the laser beam spreads as it propagates in free space. It is not a local property but a characteristic of the beam as a whole, measured in the far field from the beam waist . Laser diodes typically produce an elliptical beam due to the asymmetry of the emitting region .

Fast Axis vs. Slow Axis

• Fast Axis: Perpendicular to the diode's junction plane. The emission region is extremely narrow (1–2 µm), resulting in large divergence angles, often 30°–45° or more . This high divergence is due to diffraction from the small aperture.
• Slow Axis: Parallel to the junction length. The emission region is much wider (hundreds of microns), producing smaller divergence angles, typically 5°–15° . The beam spreads more gradually along this axis. This asymmetry is a key consideration in optical system design, requiring fast-axis collimation (FAC) and slow-axis collimation (SAC) optics to shape the beam for applications like fiber coupling or free-space propagation .

Physical Basis

The divergence angle is determined by the waveguide structure and the size of the emission facet. According to diffraction theory, smaller apertures produce larger divergence. For Gaussian beams, the 1/e² half-angle divergence is given by:

where $\lambda$ is the wavelength and $w_0$ is the beam waist radius . The product of beam waist and divergence, called the beam parameter product (BPP), remains constant for a diffraction-limited beam.

Practical Implications

• Collimation: Highly divergent beams require lenses with high numerical apertures to collimate without truncation .
• Fiber Coupling: Asymmetric divergence limits coupling efficiency into optical fibers, necessitating precise alignment and specialized optics .
• Beam Quality: Higher divergence generally indicates lower beam quality, affecting focusing and long-distance propagation . In summary, laser diode emission angles are inherently asymmetric, with a wide fast-axis divergence and a narrow slow-axis divergence, and understanding these angles is crucial for designing optical systems and achieving efficient beam control .