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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 serve as key indicators of their performance. To enhance these aspects, closed-loop control with dual-beam paths, such as saturated absorption spectrum spatial stabilization, is commonly employed. The signal-to-noise ratio (SNR) of the control beam detection significantly impacts the control precision. Investigating parameters that influence this SNR and analyzing their relationships hold great engineering significance for further improving the tuning precision and frequency stabilization accuracy of lasers.
To increase the SNR, this article 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 of 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 chapter provides 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. Building on theoretical analysis, it also simulates optical phenomena involving half-wave plates with different tilt and rotation angles, as well as dual-frequency Gaussian elliptically polarized light under various modulation parameters.
Theoretical analysis indicates that the intensities of p and s polarization light undergo periodic variations as the angles between the half-wave plate's optical axis and the PBS's slow-axis direction and between the linear-polarization direction and the half-wave plate's optical axis change. The positions of the extreme values of these intensities shift with variations in PER. 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 implies that as time progresses (corresponding to the center wavelength being tuned to different values), the interference intensity exhibits periodic changes from constructive interference to destructive interference and back to constructive interference. Consequently, abnormal dips and peaks may appear in the optical signal intensity.
The experiment employed a 633-B-A81-SA-PZT laser from LD-PD INC with a 10mW output. Simulation used a true zero-order half-wave plate model centered at 633 nm. The laser wavelength was tunable within 633 nm±10 pm, with a 10kHz sine-wave current modulation under wavelength-current tuning coefficient of 1 pm/mA. After an isolator, a 90:10 coupler split the beam into a 9mW output and a 1mW experiment beam, which was collimated and adjusted by a polarizer, a true zero-order half-wave plate, and a PBS to set the p and s light power ratio. Two Thorlabs FDS100 detectors captured the beams, with signals collected via a data acquisition card. PD1 and PD2 signals showed significant differences under certain conditions, and the p and s light signals varied periodically with half-wave plate rotation. Adding a polarizer at the laser exit and adjusting its angle improved signal consistency. After alignment, the SNR rose by 10 dB to 31 dB .
In this study, wavelength tuning of a 633nm semiconductor laser was performed using a saturated absorption spectrum ring light path. Under different modulation conditions, inconsistencies in the two-beam intensity signals were observed. Polarization control raised the SNR to 31 dB, confirming the theoretical model. Additionally, time domain analysis of stray light from the wavelength-tuned source revealed that reducing the wavelength tuning range and modulation frequency effectively suppresses high frequency noise.-
Keywords:
- Lasers and laser optics /
- Wavelength modulation /
- Elliptical light interference /
- Semiconductor laser
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