Dr. Zhen Xu Profile

Dr. Zhen Xu

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SPIE Journal Paper | 10 April 2024

Modeling of self-injection locking bandwidth of a whispering gallery mode microresonator

Zhen Xu, Suhui Yang

OE, Vol. 63, Issue 04, 046102, (April 2024) https://doi.org/10.1117/12.10.1117/1.OE.63.4.046102

KEYWORDS: Microresonators, Laser frequency, Modeling, Semiconductor lasers, Whispering gallery modes, Optical microcavities, Optical engineering, Backscatter, Reflection, Laser stabilization

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Whispering gallery mode microresonators are promising in applications where narrow linewidth laser and optical frequency comb are required. We analyze the influence of the parameters of a microresonator on the locking bandwidth of the self-injection locking effect. Both single-mode and multimode cases are discussed. In the single-mode state, we discuss the effects of the spatial gap between the coupler and the microresonator on the locking bandwidth. The variation of the locking bandwidth with respect to the gap is simulated under various parameters of both the microcavity and laser diode. The results show that the maximum locking bandwidth does not always occur at the critical coupling point. Consequently, the method of determining the intrinsic quality factor of the microresonator by measuring locking bandwidth can result in an underestimation of the value. In the case of the multimode state, the locking bandwidth may be affected by adjacent modes. Therefore, the multimode locking bandwidth is studied further. The result shows that when the initial phase case is zero, both the forward and backward scanning locking bandwidth will not exceed the frequency interval between the adjacent modes because of the overlapping of the locking ranges of the adjacent modes.

Proceedings Article | 21 December 2022 Presentation + Paper

Improvement of underwater ranging accuracy in turbid medium by using a spiral phase plate

Yingqi Liao, Suhui Yang, Hao Yan, Jian Song, Xinyu Liu, Zhen Xu

Proceedings Volume 12314, 123140N (2022) https://doi.org/10.1117/12.2641312

KEYWORDS: Light scattering, Ranging, Spiral phase plates, Signal attenuation, Optical vortices, Scattering, Modulation, Spatial coherence, Mie scattering, LIDAR

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Lidar has been widely used in underwater detection, survey, and seabed mapping. However, the performance of underwater lidar is deteriorated by scattering. Scattering not only contributes to attenuation but also worsens ranging accuracy or imaging contrast. Therefore, it was important to suppress scattering in underwater detection. We present investigations of applying a spiral phase plate (SPP) as a spatial filter to suppress scattering in an underwater lidar system. An SPP was inserted in the echo path to separate target-reflected signals from scattering clutters, because, in the echo, the target-reflected light maintained spatial coherence and was converted into an optical vortex ring after passing through the SPP, while the scattering clutters lost their spatial coherence and cannot be converted to the optical vortex but remained a centrally stronger distribution according to the Mie scattering law. A mask was produced by coating a glass sheet with opaque paint, leaving only a transparent ring for light on the optical vortex ring to pass through. Experimentally, the response of the light in the center of the vortex to the RF modulation frequency decreased with the increase of attenuation length, so it was mainly scattered light, while the light on the vortex was measured to maintain RF frequency modulation at high attenuation length, so it was dominated by signal light. The ranging results showed that the ranging error was significantly reduced in a turbid medium by blocking scattering clutters inside and outside the vortex. Moreover, a high-order SPP was more effective in reducing ranging errors. We reduced the ranging error from 30 cm to 6.6 cm with a 24-order SPP when the attenuation length was 15.

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