[Basic Optics Skills] Resonator mode matching and optical path adjustment skills

[Basic optical skills] Resonator mode matching and optical path adjustment skills

The resonator mode matching process requires the following instruments:

1. Beam quality analyzer;

2. Infrared observer;

3. Photodetector;

4. CCD camera;

5. Oscilloscope.

1. Pattern matching principles and steps

For a resonant cavity with given geometric parameters, the position and size of a waist spot can be determined according to the self-reproduction condition [1], which we generally call the intrinsic waist spot. The process of making the input beam focus at the same position and size as the cavity intrinsic waist spot after passing through the lens and mirror is light-cavity mode matching. The steps are as follows:

1) Determine the position and size of the intrinsic waist spot of the resonant cavity according to the parameters of the resonant cavity.

2) Determine the waist spot position and size of the input beam. Since the spot size of a Gaussian beam changes hyperbolically with the propagation distance, the spot size of the input beam at several different positions on the propagation path can be measured by a beam quality analyzer, and then the waist spot can be determined by curve fitting.

3) Select one or more lenses with appropriate focal length to focus the beam to the cavity intrinsic waist spot: Generally speaking, a lens group composed of two lenses can meet most applications. After the lens is placed, a beam quality analyzer can be used to check whether the waist spot distribution of the input light is close to the intrinsic waist spot of the cavity.

4) Adjust the height and left of a pair of mirrors in front of the cavity to allow the incident light to couple into the cavity and resonate, and then fine-tune the positions of the mirror and lens according to the state of the resonance signal to optimize the mode matching effect.

It should be noted that the above steps 1~3 are lens selection, which involves certain theoretical calculations. For specific formulas, please refer to reference [1]. The last step is to adjust the light path, that is, to adjust the light closure in the cavity to achieve resonance. Relevant experimental techniques will be given below.

2. Optical path adjustment skills

The working principle of the resonant cavity is based on the interference of multiple beams in the cavity, and a basic condition for interference is the spatial coincidence of the beams, which requires us to control the direction of the beam very accurately to couple into the resonant cavity. The steps of light-cavity coupling are as follows:

1) Collimate the input beam to the height of the resonant cavity, using the well-known "double aperture method", the schematic diagram is as follows. That is, two apertures are placed one behind the other in the beam propagation path, and the distance between the two apertures is as large as possible if space allows. The adjustment formula is "the far one is close, the near one is far away": adjust the reflector M1 to let the light pass through the aperture 1, adjust the reflector M2 to let the light pass through the aperture 2, and so on, until the light beam passes through the centers of the two apertures at the same time.

Figure 1 Schematic diagram of optical path alignment

2) Place the resonant cavity in the collimated optical path and adjust the closing of the resonant cavity. The linear cavity and the annular cavity will be explained separately below.

For linear cavities with good parallelism between the two cavity mirrors, the resonance signal can be observed by adjusting the direction of the incident light to make it perpendicularly incident. The method of adjusting the vertical incidence is similar to the method of collimating the optical path in the previous step. The key is to find the observation point, as shown in Figure 2 below: Use small pieces of paper (use a fluorescent board for invisible light, be careful not to block the incident light) and place them at the positions marked "1" and "2" in the picture respectively. Adjust the reflectors M1 and M2 in turn so that the incident light and the reflected light (dashed line) coincide with the positions of "1" and "2".

Figure 2 Linear cavity light path adjustment

Figure 3 Ring cavity light path adjustment

Different from the linear cavity, the incident light in the ring cavity is not vertically incident on the cavity mirror, so it is impossible to judge whether the light inside the cavity is closed outside the cavity. Therefore, it is necessary to select two observation points in the cavity, as shown as "1" and "2" in Figure

3. In addition, when the input light intensity is not large enough, the human eye cannot directly observe the weak light in the cavity. At this time, an infrared observer is needed to assist in adjustment. Similar to the above steps, adjust the reflectors M1 and M2 so that the input light spot at the "1" and "2" positions coincides with the rear light spot (dashed line) that has been transmitted in the cavity for one week, which can ensure that the resonant cavity is basically closed.

3) After the resonant cavity is basically closed, use a periodic signal to scan the cavity length or the incident laser frequency, and use a photodetector with appropriate wavelength and gain to receive the transmission (or reflection) signal of the cavity. After connecting to an oscilloscope, you should be able to observe the transmission peak (reflection peak) signal of the cavity. When pattern matching does not reach the optimal state, many high-order spatial patterns will be stimulated, as shown in Figure 4 below.

Figure 4 Transmission peak signal when mode matching is poor

Generally speaking, what we need is the fundamental mode (TEM00) resonance. Then we must first determine which transmission peak is the fundamental mode. The specific operation is to reduce the scanning voltage range until only one transmission peak is visible on the oscilloscope, and use a CCD camera to capture the spatial light intensity distribution of the transmission peak, as shown in the small picture in Figure

4. Then, the reflectors M1 and M2 are fine-tuned to increase the intensity of the fundamental mode while weakening the intensity of the higher-order modes, especially the intensity of TEM01 (TEM10). Slightly moving the position of the mode matching lens can also optimize the mode, which is generally helpful for suppressing TEM02 (TEM20) and higher-order modes.

High light-cavity mode matching efficiency is the prerequisite for making full use of the characteristics of the resonant cavity. In addition to certain theoretical guidance when selecting lenses, more work focuses on experimental operations. We need to continuously try to optimize and improve on the premise of mastering basic skills to meet experimental needs.

References

[1] Zhou Bingkun, Gao Yizhi, Chen Birong, Chen Jiahua. Laser Principles (6th Edition). Beijing: National Defense Industry Press, 2010.