Mode hop
A discontinuous jump in lasing wavelength as a laser's preferred longitudinal mode switches between adjacent cavity modes.
A mode hop is the abrupt transition of a laser from one longitudinal cavity mode to an adjacent one. It occurs when operating-condition changes (drive current, temperature, optical feedback) shift the gain envelope or cavity resonances such that the adjacent mode becomes more favorable for lasing than the current dominant mode.
Observational signatures:
- Discrete jump in peak wavelength of one FSR, typically 0.5–2 nm for typical Fabry–Pérot devices
- Hysteresis: mode hops occur at different conditions during forward vs reverse current or temperature sweeps
- SMSR drops near the hop boundary, often falling below 20 dB at the transition
- Output power discontinuity at the hop
- Linewidth broadens dramatically during the transition
Conditions producing mode hops:
| Mechanism | Effect |
|---|---|
| Current increase (CW) | Active region heats, gain peak shifts to longer wavelength faster than the cavity mode |
| Temperature change | Cavity modes and gain peak shift at different rates |
| Optical feedback | External cavity creates beat conditions that can favor different modes |
| Aging | Cavity loss changes over device lifetime |
For multi-mode Fabry–Pérot lasers, the device is always partially mode-hopping — power continuously redistributes between several longitudinal modes as conditions change.
For DFB lasers, the Bragg grating locks the lasing wavelength against gain-peak drift, so mode hops are suppressed within the normal operating range. Mode hops can still occur at the extreme limits — when the gain peak drifts so far that even the strong grating selectivity cannot maintain single-mode operation at the design Bragg wavelength. This defines the temperature endpoints of mode-hop-free operation.
For distributed Bragg reflector (DBR) and external-cavity diode lasers, mode-hop-free continuous tuning requires synchronous adjustment of multiple controls (current, temperature, external cavity length) to keep the longitudinal mode aligned with the gain peak.
Mode-hop-free operation across the rated current and temperature range is a standard acceptance criterion for telecom DFB laser qualification (see DFB Laser Characterization Workflow).