Wavelength-tunable lasers play a crucial role in fields such as precision interferometry and ultra-stable laser applications. The precision of wavelength tuning and the accuracy of frequency stabilization in lasers are the key indicators of their performance. To improve these performance, closed-loop control with dual-beam paths, such as saturated absorption spectrum spatial stabilization, is commonly utilized. The signal-to-noise ratio (SNR) of the control beam detection significantly affects the control precision. Investigating the parameters that influence this SNR and analyzing their relationships are of great engineering significance for further improving the tuning precision and frequency stabilization accuracy of lasers.To increase the SNR, this work examines intensity noise in wavelength-modulation systems based on the polarizer-phase-delay-polarizer model. A polarization beam splitter (PBS) cannot achieve a zero polarization extinction ratio (PER), thus introducing intensity noise from the interference between p and s polarization light. Additionally, non-ideal stray light, such as back-reflected and scattered light from optical components, further reduces the SNR of the detection signal when it converges on the detector’s active area. This work carries out a detailed analysis of these two types of noise, exploring the effects of factors such as PER, wavelength-modulation range, beam diameter, laser polarization direction, and modulation frequency. Based on the theoretical analysis, it also simulates optical phenomena involving half-wave plates with different tilt angles and rotation angles, as well as dual-frequency Gaussian elliptically polarized light under various modulation parameters.The theoretical analysis indicates that the intensities of p and s polarization light undergo periodic variations as the angle between the half-wave plate’s optical axis and the PBS’s slow-axis direction, as well as the angle between the linear-polarization direction and the half-wave plate’s optical axis, changes. The extreme positions of these intensities move with the PER changing. At certain specific angles, destructive interference leads to extremely low intensities in both transmitted and reflected light. Furthermore, when the detector receives stray light of multiple frequencies, the synthesized phase varies periodically with wavelength tuning. This means that over time (corresponding to tuning the center wavelength to different values), the interference intensity exhibits periodic changes from constructive interference to destructive interference and then to constructive interference. Consequently, abnormal dips and peaks may appear in the optical signal intensity.A 633-B-A81-SA-PZT laser from LD-PD INC with a 10 mW output is used in the experiment. A true zero-order half-wave plate model centered at 633 nm is adopted in the simulation. The laser wavelength is tunable in the range of 633 nm±10 pm, and 10 kHz sine-wave current modulation is used, with a wavelength-current tuning coefficient of 1 pm/mA. After an isolator, a 90∶10 coupler splits the beam into a 9 mW output and a 1 mW experiment beam, which is collimated and adjusted by a polarizer, a true zero-order half-wave plate, and a PBS to set the ratio of p light power to s light power. Two Thorlabs FDS100 detectors capture the beams, with signals collected via a data acquisition card. The PD1 and PD2 signals show significant differences under certain conditions, and the p and s light signals vary periodically with half-wave plate rotating. Adding a polarizer at the laser exit and adjusting its angle can improve signal consistency. After alignment, the SNR increases from 10 dB to 31 dB.In this study, wavelength of a 633 nm semiconductor laser is tuned by using a saturated absorption spectrum ring light path. Under different modulation conditions, inconsistencies in the intensity signals of two beams are observed. Polarization control increases the SNR to 31 dB, confirming the theoretical model. Additionally, time domain analysis of stray light from the wavelength-tuned source shows that reducing the wavelength tuning range and modulation frequency can effectively suppress high frequency noise.
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