Thermal Effect on the Number of Modes Inside the Laser Cavity of a Semiconductor Laser

Abstract

    This experimental work focuses on the effect of temperature change on the longitudinal mode structure in a semiconductor laser cavity (Mitsubishi ML725B45F distributed feedback (DFB) laser diode (InGaAsP active medium, 5 mW output power, single-mode fiber output). Emission spectra were measured for temperature intervals of 2 °C from 12 °C to 26 °C (8 individual spectra) with an Omni-λ7508 spectrograph (0.03 nm resolution) in a continuous-wave operation with a stable injection current of 4 A form an ILX Light wave LDT-5525 temperature controller (stability ±0.004 °C). The central emission wavelength showed a monotonic redshift from 1304 nm at 12 °C to 1310 nm at 26 °C with a thermal wavelength coefficient of around 0.5 nm/°C. Quantitative analysis of the spectral data indicated a systematic decrease in supported longitudinal modes from 5 modes at 12 °C to 2 modes at 26 °C, and the side-mode suppression ratio (SMSR) increased from 18 dB to 32 dB over the same temperature range, suggesting better single-mode purity at higher temperatures. The main quantitative result is that increasing the working temperature from 12 °C to 26 °C reduces the number of longitudinal modes by 60% (from 5 to 2 modes) and increases the SMSR by 14 dB, which shows that higher temperatures favor single mode operation by changing the refractive index, optical gain profile and cavity length due to temperature effects. The results show that a fine thermal control of ±0.004 °C is needed for DFB semiconductor lasers to retain their steady high-spectral-purity performance, and the best single-mode operation is achieved at temperatures near 26 °C under the conditions studied.