Yes, single-mode laser diodes can experience mode skipping, commonly referred to as mode hopping, under certain conditions.

Understanding Mode Skipping

Mode skipping, or mode hopping, occurs when a laser abruptly switches from one longitudinal mode to another. Even in single-mode laser diodes, which are designed to emit light in a single spatial and longitudinal mode, this phenomenon can happen if the gain peak shifts relative to the cavity resonance. The result is a sudden change in the lasing wavelength, often accompanied by slight fluctuations in output intensity and increased relative intensity noise ( ).

Causes of Mode Hopping

Several factors can trigger mode skipping in single-mode diodes:

• Temperature variations: Changes in the diode junction or case temperature can shift the gain spectrum, allowing a competing mode to reach threshold and dominate ( ).
• Injection current fluctuations: Variations in current can alter carrier density and refractive index, affecting the alignment of the gain peak with the cavity mode ( ).
• Optical feedback: Reflections from external optics can destabilize the lasing mode, promoting mode hopping ( ).
• Cavity design limitations: A small free spectral range or insufficient mode discrimination can make the laser more prone to hopping ( ).

Mitigation Strategies

To minimize mode skipping in single-mode diodes:

• Temperature stabilization: Maintaining a constant junction temperature reduces shifts in the gain peak.
• Current control: Using low-noise, stable current sources helps prevent mode competition.
• Optical isolation: Faraday isolators or slight canting of reflective surfaces can reduce feedback-induced hopping.
• Cavity engineering: Designing the laser with a large free spectral range and strong mode selectivity (e.g., distributed feedback structures) enhances single-mode stability ( ).

Practical Implications

While single-mode diodes are optimized for stable, narrow-linewidth emission, mode skipping can still occur, especially under environmental or operational stress. In applications requiring continuous single-frequency operation, such as high-resolution spectroscopy, LiDAR, or fiber-optic communications, careful thermal and current management, along with proper optical isolation, is essential to maintain mode stability ( ). In summary, single-mode laser diodes are not immune to mode skipping, but with proper design and operational control, the occurrence can be minimized to maintain reliable single-mode performance.