A cavity tuning method based on cavity mirror spot morphology monitoring

Abstract

The present invention provides a cavity tuning method based on cavity mirror spot morphology monitoring, in which a monitor and a photodetector (PD) are used in the cavity measurement optical path to respectively analyze the light fields inside and outside the cavity, so as to realize rapid diagnosis of cavity misalignment and complete high-efficiency precision cavity tuning. The monitor is used to monitor the spot morphology and position on the cavity mirror, and the photodetector is used to record the transmission signal intensity of the laser light source after passing through the cavity. In the coarse adjustment stage of the cavity, the cavity mirror spot morphology and position are used as auxiliary cavity tuning criteria to adjust the cavity parameters so that the spot mode is in the fundamental transverse mode and is located at the center of the cavity mirror. In the fine adjustment stage, the transmission light field signal intensity recorded by the photodetector is used as a cavity tuning evaluation index, and the cavity is continuously adjusted so that the transmission signal intensity value reaches the maximum to complete the cavity adjustment. This method can quickly diagnose the cavity misalignment state by directly monitoring the spot morphology on the cavity mirror, and has a simple structure, a novel method, convenient operation, and high cavity tuning efficiency.

Description

Technical Field

[0001] The invention relates to the field of laser technology, and in particular to a cavity tuning method based on cavity mirror spot morphology monitoring. Background Art

[0002] An optical resonant cavity (abbreviated as optical cavity) is a cavity in which light waves are reflected back and forth to provide positive feedback of the axial light wave mode. Among them, a working medium without gain or a working medium without considering its gain is called a passive resonant cavity (abbreviated as passive cavity). (Zhou Bingkun, Gao Yizhi, Chen Tirong, et al. "Laser Principles" (7th Edition) [M]. Beijing: National Defense Industry Press, 2014). Passive cavities are widely used in the fields of optical cavity ring-down technology and optical fiber sensing technology. The precise adjustment of passive cavities, namely the cavity tuning method, is of great significance to the application of related technologies. Taking the application of optical cavity ring-down technology in high reflectivity measurement as an example, the accuracy and efficiency of cavity tuning have an important impact on the true reliability of its measurement results and the difficulty of measurement. At present, in the process of cavity tuning in the field of cavity ring-down technology, if the laser light source is in the non-visible light band, an additional indicator light source in the visible light band needs to be introduced to assist in cavity tuning. If the laser light source is in the visible light band, it is difficult to tune the cavity in the initial stage of making the laser resonate in the cavity. Alternatively, in order to record the distribution of the transmitted light field of the cavity to assist in cavity tuning, a camera is installed behind the cavity mirror of the cavity to collect the transmission light spot information. (Xue Ying, Du Xinghu, He Xing, et al., Cavity ring-down tuning method based on transmission light spot morphology monitoring, Chinese Laser, 2020, 47(5): 0504001). However, these methods lead to an increase in light source devices, forming a high threshold for cavity tuning experience, making it difficult to measure the weaker extra-cavity transmission signal when the cavity is not fully resonant, which affects the application effect of related technologies in practical applications.

[0003] Therefore, based on the observation and study of the cavity mirror spot morphology, a monitor is introduced into the resonant cavity measurement optical path to monitor the spot morphology and position on the cavity mirror as an auxiliary cavity tuning criterion, and a photoelectric detector is used to synchronously record the transmission signal intensity of the laser light source after passing through the resonant cavity, and the resonant cavity parameters are adjusted to achieve the cavity mirror spot morphology in the fundamental transverse mode and located at the center of the cavity mirror and the transmission signal intensity reaches the maximum, thus completing the cavity tuning. Compared with the traditional method, this method can quickly diagnose the cavity misalignment state by directly monitoring the spot morphology on the cavity mirror, without the introduction of additional indicator light source devices, without restrictions on the light source wavelength, simple structure, novel method, convenient operation, and high cavity tuning efficiency. Summary of the invention

[0004] The technical problem to be solved by the present invention is that the current passive resonant cavity tuning process is limited by the laser wavelength, the initial tuning stage is difficult, and the transmitted light field signal outside the cavity is weak. As a result, the current tuning process requires the introduction of additional light source devices, the tuning experience threshold is high, and the tuning efficiency is low, which brings a series of technical challenges in actual engineering applications.

[0005] The technical solution adopted by the present invention to solve its technical problems is: a cavity tuning method based on cavity mirror spot morphology monitoring, using a monitor and a photodetector (PD) in the resonant cavity measurement optical path to respectively record and analyze the light fields inside and outside the resonant cavity, thereby realizing rapid diagnosis of cavity misalignment, adjusting the resonant cavity parameters, and completing high-efficiency and precise cavity tuning.

[0006] The specific implementation steps are as follows:

[0007] Step (1), constructing a resonant cavity, introducing a laser beam into the cavity of the resonant cavity, using a monitor to monitor the shape and position of the light spot on the cavity mirror of the resonant cavity, and using a photodetector (PD) to record the intensity of the transmission signal of the resonant cavity;

[0008] The resonant cavity is a stable resonant cavity, and its structure is a straight cavity or a folded cavity.

[0009] The laser beam has no restrictions on beam diameter and wavelength; it can also be limited to a laser beam of a specific wavelength and a specific diameter.

[0010] The monitor can be an external camera that responds to the wavelength of the light beam in the cavity, and is used to directly display the spot image on the cavity mirror; or it can be directly observed by human eyes. The cavity mirror monitored by the monitor can be any cavity mirror that constitutes the resonant cavity.

[0011] Step (2), during the coarse adjustment of the resonant cavity, a monitor is used to collect the light spot image on the cavity mirror as an auxiliary cavity adjustment criterion, the cavity misalignment state of the resonant cavity is quickly diagnosed, and the resonant cavity parameters are adjusted to make the light spot mode in the fundamental transverse mode state and at the center of the cavity mirror.

[0012] The adjusted resonant cavity parameters are cavity axis offset, cavity length, cavity mirror tilt, including tilt in both horizontal and elevation dimensions.

[0013] Step (3), the resonant cavity fine tuning stage, after the light spot shape on the cavity mirror is adjusted to the fundamental transverse mode and is at the center of the cavity mirror, the resonant cavity transmission signal intensity recorded by the photodetector is used as the cavity tuning evaluation index, and the resonant cavity parameters are further fine-tuned to maximize the transmission signal intensity value, and the cavity tuning is completed.

[0014] The adjusted resonant cavity parameters are cavity axis offset, cavity length, cavity mirror tilt, including tilt in both horizontal and elevation dimensions.

[0015] This method can be applied in a variety of scenarios that require cavity tuning, including fiber optic sensing systems, large laser systems, and cavity ring-down systems.

[0016] The principle of the present invention is: using a monitor and a photodetector (PD) to record and analyze the light field inside and outside the resonant cavity respectively. In the coarse adjustment stage of the resonant cavity, the monitor is used to monitor the shape and position of the light spot on the cavity mirror as an auxiliary cavity adjustment criterion, and the parameters of the resonant cavity are adjusted so that the light spot mode is in the fundamental transverse mode state and is at the center of the cavity mirror. Then, entering the fine adjustment stage of the resonant cavity, the light intensity of the resonant cavity transmission signal recorded by the photodetector is used as the cavity adjustment evaluation index, and the resonant cavity parameters are further finely adjusted to maximize the transmission signal intensity value and complete the cavity adjustment.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] Compared with traditional methods, this method can quickly diagnose the cavity misalignment state by directly monitoring the spot morphology on the cavity mirror. There is no need to introduce additional indicator light source devices, there is no restriction on the wavelength of the light source, and the experience threshold for cavity tuning is lowered. It has a simple structure, a novel method, is easy to operate, and has high cavity tuning efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the structure of the three-cavity mirror folded resonant cavity of the present invention;

[0020] Figure 2 The cavity mirror spot shape and the cavity output signal of the present invention when the cavity is not adjusted;

[0021] Figure 3 It is the cavity mirror spot shape and the cavity output signal when the cavity tuning is completed in the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described below with reference to the accompanying drawings and specific embodiments.

[0023] like Figure 1 As shown, a cavity tuning method based on cavity mirror spot morphology monitoring of the present invention is specifically implemented as follows:

[0024] Step (1), build a three-cavity mirror folded resonant cavity, introduce a laser beam with a wavelength of 635nm and a diameter of ≤5mm into the cavity of the resonant cavity, use an external camera (Nikon, D5300) that responds to the 635nm beam wavelength to monitor the spot shape and position on the cavity mirror M2 of the resonant cavity, and use a photodetector (PDA8A / M, Thorlabs) to receive and record the intensity of the resonant cavity transmitted beam signal.

[0025] Among them, cavity mirror M1 and cavity mirror M2 are both plano-concave mirrors with a curvature radius of about 1m, cavity mirror M3 is a plane mirror, and the cavity length is 0.7m, which meets the stable resonant cavity conditions. Both cavity mirrors M1 and M2 are installed in a two-dimensional adjustable mirror mount (VM1, adjust resolution: 0.005°, Thorlabs).

[0026] Step (2), during the coarse adjustment of the resonant cavity, an external camera is used to capture the light spot image on the cavity mirror M2 as an auxiliary cavity adjustment criterion, the cavity misalignment state of the resonant cavity is quickly diagnosed, and the inclination of the resonant cavity cavity mirror M1 and cavity mirror M2 in the horizontal and pitch dimensions is adjusted to make the light spot mode in the fundamental transverse mode state and at the center of the cavity mirror M2.

[0027] Step (3), during the resonant cavity fine tuning stage, after the light spot shape on the cavity mirror M2 is adjusted to the fundamental transverse mode and is at the center of the cavity mirror M2, the intensity of the resonant cavity transmission signal recorded by the photodetector PD, i.e., the cavity output signal, is used as the cavity tuning evaluation index, and the inclination of the cavity mirrors M1 and M2 in the horizontal and elevation dimensions is further fine-tuned so that the transmission signal intensity value reaches the peak value and no longer increases, and the cavity tuning is completed.

[0028] The above description is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any person familiar with the technology can understand and think of any changes or substitutions within the technical scope disclosed by the present invention, which should be included in the scope of the present invention.

Claims

1. A cavity tuning method based on cavity mirror spot morphology monitoring, characterized in that: The implementation steps are as follows: Step 1: Build a resonant cavity, introduce a laser beam into the cavity of the resonant cavity, use a monitor to monitor the shape and position of the light spot on the light-entering side of the cavity mirror of the resonant cavity, and use a photodetector (PD) to record the intensity of the signal transmitted by the resonant cavity; Step 2: During the coarse adjustment of the resonant cavity, the monitor is used to collect the spot image on the light incident side of the cavity mirror as an auxiliary cavity adjustment criterion, and the cavity misalignment state of the resonant cavity is quickly diagnosed. The resonant cavity parameters are adjusted to make the spot mode in the fundamental transverse mode state and in the center of the cavity mirror; Step 3, during the resonant cavity fine tuning stage, after the light spot shape on the light incident side of the cavity mirror is adjusted to the fundamental transverse mode and is at the center of the cavity mirror, the intensity of the resonant cavity transmission signal recorded by the photodetector is used as the cavity tuning evaluation index, and the resonant cavity parameters are further fine-tuned to maximize the transmission signal intensity value, and the cavity tuning is completed.

2. The cavity tuning method based on cavity mirror spot morphology monitoring according to claim 1 is characterized in that: The resonant cavity described in step 1 is a stable resonant cavity, and its structure is a straight cavity or a folded cavity.

3. The cavity tuning method based on cavity mirror spot morphology monitoring according to claim 1 is characterized in that: The laser beam described in step 1 has no restrictions on beam diameter and wavelength; or is limited to a laser beam of a specific wavelength and a specific diameter.

4. The cavity tuning method based on cavity mirror spot morphology monitoring according to claim 1 is characterized in that: The monitor described in step 1 is an external camera that responds to the wavelength of the light beam in the cavity, and is used for intuitive display of the light spot image on the light incident side of the cavity mirror; or directly observed by the human eye; the cavity mirror monitored by the monitor is any cavity mirror that constitutes the resonant cavity.

5. The cavity tuning method based on cavity mirror spot morphology monitoring according to claim 1 is characterized in that: In the coarse adjustment phase of the resonant cavity described in step 2 and the fine adjustment phase of the resonant cavity described in step 3, the adjusted resonant cavity parameters are the cavity axis offset, the cavity length, and the cavity mirror tilt in both horizontal and elevation dimensions.

6. The cavity tuning method based on cavity mirror spot morphology monitoring according to claim 1 is characterized in that: This method can be applied to optical fiber sensing systems, large laser systems or optical cavity ring-down systems.

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