New mechanism for self-adaptive ultra-high spectral purity laser

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A new publication from Opto-Electronic Advances, DOI 10.29026/oea.2023.210149, discusses ultra-high spectral purity laser derived from weak external distributed perturbation.

Laser has the characteristics of high coherence, pure monochromaticity and good directionality, which is different from ordinary light sources. Since the advent of lasers, scientists have been working on optimizing laser parameters to promote the development of scientific technologies and industrial applications. Among them, spectral purity is a key factor in determining laser coherence. The physical essence of affecting the spectral purity is that the laser will inevitably be affected by the intensity and phase disturbance of the spontaneous radiation during the operation of the laser. As a result, the output laser cannot achieve the ideal single frequency in the frequency domain, but always presents a certain linewidth. Likewise, the laser operation is also easily interfered by other factors, such as the power jitter of the pump source, temperature variation, vibration, and lattice defects of the luminescent gain medium, which will greatly affect the linewidth of the laser, thereby reducing the coherence of the laser. So far, the servo-electrical feedback technology based on frequency stabilization control and the fixed external-cavity optical feedback technology based on photon lifetime extension are the common means to realize narrow linewidth laser output. However, the servo-electrical feedback technology requires high precision and high sensitivity external optical frequency detection and control devices, the core of which is an expensive and bulky high-stability frequency reference source, which is not conducive to industrial applications. In addition, the fixed external-cavity optical feedback technology is difficult to deeply compress laser linewidth under the premise of single-longitudinal mode operation, and cannot automatically match the change of the main cavity laser wavelength, so it does not have the ability of wavelength self-adaptation. Therefore, how to achieve deep compression of laser linewidth and self-adaptive wavelength under normal conditions is still a difficult problem and challenge for current scientific research and industrial applications.

In light of this, the authors of this article investigated the wavelength self-adaptive spectral purification mechanism of ultra-narrow linewidth laser, and proposed a new idea that the spontaneous radiation of laser cavity can be effectively suppressed by the weak external distributed perturbation signal, so as to realize the laser spectral deep purification under normal conditions. Based on this, a new laser configuration combining the main cavity and the external cavity with distributed weak feedback is proposed, and the linewidth compression experiment is carried out using the semiconductor DFB laser as the typical laser carrier. Eventually, a self-adaptive ultra-narrow linewidth laser output of 10 Hz or less is achieved under normal conditions. The key role of the distributed weak feedback to deeply compress the laser linewidth is to slow down the coupling rate of the laser cavity with the spontaneous radiation during operation, so that the laser intrinsic linewidth can be greatly reduced. In addition, the single-point feedback intensity of the distributed weak feedback is lower than that of the spontaneous radiation, so the laser phase can be adaptively and continuously modify in the time domain. It can also avoid the laser phase mutation and multi-longitudinal mode oscillation formed by the feedback of the fixed external cavity, while slowing down the spontaneous emission coupling rate, so as to ensure the continuous operation of the single longitudinal mode and achieve the extreme compression of the laser linewidth. This work provides an effective theoretical and experimental basis for the realization of self-adaptive ultra-high spectral purity lasers under normal conditions. The ideas and laser configurations proposed by the research group have opened up new horizons for improving and obtaining other types of high-coherence laser light sources, and also have important reference significance for realizing the extreme control of other laser parameters. At present, the next step of the research group will further research the extreme regulation and control of laser time-frequency-spatial parameters on the basis of high coherence, and promote the development of the field of laser precision measurement towards higher precision, faster speed and wider range.

Article reference:

Dang LY, Huang LG, Shi LL, Li FH, Yin GL et al. Ultra-high spectral purity laser derived from weak external distributed perturbation. Opto-Electron Adv 6, 210149 (2023). doi: 10.29026/oea.2023.210149

Keywords: high-coherence / weak external distributed perturbation / narrow linewidth / laser.

The Laboratory of Photonic Devices and Systems is an integral part of the “Key Laboratory of Optoelectronic Technology and Systems of the Ministry of Education” of Chongqing University in China. The research group mainly studies the optical laws and optical characteristics of the interaction of lasers with novel low-dimensional materials and local structures at the conventional, mesoscopic and nanoscale level. Here, the research of time-domain, frequency-domain, and spatial-domain controllable laser is carried out to break the bottleneck of precision measurement technology such as distributed fiber sensing, intelligent sensing, and its related cost and engineering application. The laboratory is headed by Prof. Tao Zhu, a recipient and winner of the China National Fund for Distinguished Young Scholars. In the past five years, the research group has undertaken more than 20 scientific research projects, published more than 100 papers, and authorized more than 20 domestic and foreign invention patents..

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