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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 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 10mW 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 1mW 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 633nm 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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Keywords:
- lasers and laser optics /
- wavelength modulation /
- elliptical light interference /
- semiconductor laser
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图 1 (a)波长调谐激光光源退偏实验装置图; (b)传输过程中偏振态变化示意图
Figure 1. (a) Depolarization experimental setup for wavelength modulation laser; (b) schematic diagram of polarization changes during transmission.
图 3 不同$ \alpha $和$ \beta $下的透射光(左)和反射光强度(右)
Figure 3. Transmitted light (left) and reflected light intensity (right) under different $ \alpha $ and $ \beta $.
图 2 不同$ \alpha $和$ \beta $下的透射光、反射光强度之和
Figure 2. Sum of transmitted light and reflected light intensity under different $ \alpha $and $ \beta $.
图 4 饱和吸收实验装置图
Figure 4. Experimental setup for saturated absorption spectroscopy.
图 5 不同时刻下的双频同偏椭圆高斯光干涉信号
Figure 5. Interference signals of double frequency Gaussian light with the same polarization at different time.
图 6 不同时刻下的双频同偏椭圆高斯光相位差
Figure 6. Phase difference of double frequency Gaussian light with the same polarization at different time.
图 7 相位噪声与调制深度、光束直径、光程差、调制频率的关系
Figure 7. Relationship between phase noise and modulation depth, beam diameter, optical path difference and modulation frequency.
图 8 不同半波片旋转角度及调制参数下的p光、s光信号, 调制深度30%, 正弦波调制, 半波片转角0正弦波调, 30弦波调制, 60弦波调制, POL对准光轴(f); 调制深度60%, 半波片转角0半, 正弦波调制(d), 锯齿波调制(e)
Figure 8. Signals of p light & s light under different HWP rotation angles and modulation parameters. Modulation depth 30%, sine wave modulation, half-wave plate angle 0° (a), 30° (b), 60° (c), POL aligned to optical axis (f); modulation depth 60%, half-wave plate angle 0, sine wave modulation (d), sawtooth wave modulation (e).
图 9 探测器对p光(a)和s光(b)的响应信号
Figure 9. Response signals to p light (a) and s light (b).
图 10 不同半波片倾角下的相位延迟量
Figure 10. Phase-delay under different HWP angles.
图 12 激光器驱动电流稳定性监测(a)及工作温度稳定性监测(b)
Figure 12. Laser driver current stability monitoring (a) and operating temperature stability monitoring (b).
图 11 激光器波长-电流曲线和功率-电流曲线
Figure 11. Laser wavelength-current curve and power-current curve.
表 1 不同半波片旋转角和调制参数下的p光、s光信噪比
Table 1. Signals to noise ratio of p light and s light under different HWP rotation angles and modulation parameters.
SNR/dB p light s light Modulation
depth 30%Sine wave-angel 0° 25.4907 37.2894 Sine wave-angel 30° 21.2859 36.3941 Sine wave-angel 60° 31.6048 27.4752 Sine wave-with POL 30.6666 31.3396 Modulation
depth 60%Sine wave-angel 0° 18.1280 33.0986 Sawtooth wave-angel 0° 15.4966 37.8797 
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表 2 波长曲线与功率曲线的线性度分析
Table 2. Linearity analysis of wavelength curve and power curve.
Slope k Intercept b R2 Power curve 0.0399 –2.5156 0.9897 Wavelength curve 0.0024 632.4867 0.9483
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