Objective
1. Understand the Basic Structure of He-Ne Laser tube;
2. Know the working principle of He-Ne laser;
3. Learn how to adjust lasers with cross-hairs, and other related adjusting methods.
Experimental equipment
Full-External He-Ne Laser、power supply、cross-hairs、laser meter.
Experimental principle
Since the lasers came out in the 1960s, various types of lasers have been successfully developed. Because of their unique properties - high brightness, good directivity, monochromaticity and coherence, lasers have been widely used in industry, agriculture, national defense, metrology, medical and other industries. Among them, gas lasers are the most widely used. He-Ne laser is the most representative of gas lasers. It is easy to make and reliable to operate. So we take He-Ne laser as a typical example to carry out structural analysis and experiments.
The three principal elements of a laser are Gain medium (laser glass tube or discharge glass tube)、Resonating cavity、pump source (high voltage power supply)
The Gain medium of He-Ne Laser is Helium and Neon which over 99.99% purity. Neon is the gas that can excite laser, helium is the gas that provides the condition of light amplification (which produces the inversion of the number of particles). They charge the glass discharge tube in a certain proportion and at a certain pressure. In order to increase the energy to ignite the gas, an anode and a cathode are installed on it.
The resonator is mainly composed of a cavity, a mirror and a capillary. Their combination guarantees the oscillation and amplification of light in the cavity, and finally obtains the laser output. The technical requirements are: the straightness of the capillary (discharge tube), the parallelism of the two mirrors and the verticality of the mirror and the capillary. Capillary is not only strict in straightness, but also has special requirements in Inner diameter. There are two mirrors, one is a full-reflection concave mirror, reflectivity is better than 99.85%, the other is a plane mirror with a certain transmittance, reflectivity is about 98.5%.
The pump source of He-Ne laser is usually DC high voltage, which is called He-Ne laser power supply. It will use 220V AC to convert to DC high voltage, and according to the characteristics of gas discharge, realize the normal operation of high voltage electricity. The electrical parameters of He-Ne laser are: start-up voltage, working voltage and optimum working current. The starting voltage is higher than the working voltage. The starting voltage of the laser used in the experiment is 4500V, the working voltage is about 1200V, and the optimum working current is about 5mA.
Adjustment method:
To adjust the laser is to adjust the capillary straightness, the parallelism between the two mirrors and the perpendicularity between the capillary and the mirror (hereinafter referred to as straightness, parallelism and perpendicularity), so as to make the laser in the best statement and obtain satisfactory performance.
Several methods are described as below:
This method is suitable for adjusting the misalignment of the perpendicularity between the reflective mirror and the capillary axis, which results no light. The adjusting tool is called cross light target. It consists of a light screen and a light lamp. The screen is made of aluminium or iron plates, about 6cm in size, with white paint on one side, a small hole about 1mm in the middle, and a black cross wire dotted at the center of the hole. When used, the white screen must be illuminated by a lamp, so a light bulb can be installed beside it to form a whole for easy use, as shown in Figure 2. In case of emergency, a thick white paper sheet can be used to make a light screen, and a desk lamp or a flashlight can be used to illuminate it. It also becomes a simple light target.
Firstly, the laser is ignited to make the discharge tube lighted. The cross face is about 10 cm away from the output of the laser. Using your eyes to observe the axis of the capillary through the holes in the light target.
If too bright, you can put a filter between the light target and the eye or wear protective glasses. The color is light red or light green. If the power of the tuned laser is high, in order to protect the eyes, a baffle can be inserted into the cavity, and the baffle can be placed between the window and the lens in the unadjusted section of the cavity. It can also be put into transparent green light sheet and so on. As long as it is placed in the cavity, the laser oscillation can be suppressed, and the sudden emergence of light during the adjustment can be prevented, so as to directly shoot the eyes.
The hole of the light target is aligned with the axis of the capillary, and move the position of the light target . The bright spot (key point) in the center of the capillary is selected by eye observation. When the capillary is observed by the light target through the end mirror, it is easy to see the bright spot on the inner diameter of the capillary. Do not mistake the bright spot on the inner hole of the capillary as the bright spot. The main points to see this bright spot are: we must look along the axis of the capillary; we must look at the circle of the capillary hole. If the diameter of this bright spot is less than 0.5 mm, it is the axis of the capillary, as shown in Figure 4. When a small bright spot is seen, the light target is slightly moved to make the bright spot is on the center of the capillary bright spot. At this time, it is not necessary to locate the cross-wire image of the tube, as long as it reaches the state shown in Figure 4, that is to say, the hole of the light target has been placed on the axis of the capillary.
Continue to observe the cross-filament image illuminated by the lamp on the mirror through the small hole. At this time, the small hole of the light target has been placed on the capillary axis. If the mirror and the capillary axis are in a misaligned state, it is not vertical. At this time, the intersection of the cross-filament image observed on the mirror deviates from the bright spot, as shown in Figure 5. Then the mirror needs to be adjusted.
The mirror is adjusted by a knob. The knob is mounted on the end face of the laser. Generally, it is designed to be orthogonal adjustment. When adjusting the two knobs, the mirror oscillates around X axis and Y axis respectively. When adjusting, it just rotates and exerts pressure. By adjusting a knob at one end of the knob, the horizontal movement of the crosshair can be observed, and the position of the vertical line coincides with the position of the central bright spot. When adjusting another knob, the horizontal movement of the crosshair image should also coincide with the central bright spot, as shown in Figure 6. Note: In the case of fast coincidence, the eyes move slightly so that they can't see two points, so that even light does not illuminate the eyes. Repeatedly adjust to the crossing point coincides with the small highlights, and the verticality is adjusted.
If the laser is only misaligned at one end, it should light. If both ends of the laser are out of tune, the above methods are used to adjust the two ends separately. When adjusting, it is difficult to adjust the light successfully at one time because of observation error. Therefore, in the case of both ends are out of balance, it is generally necessary to adjust repeatedly.
2. Scanning method
If the laser is only misaligned at one end, it should emit light. If both ends of the laser are out of tune, the above methods are used to adjust the two ends separately. When adjusting, it is difficult to adjust the light successfully at one time because of observation error. Therefore, in the case of both ends are out of balance, it is generally necessary to adjust repeatedly.
When operating, the two knobs are adjusted at the same time. One knob rotates slowly or step by step in situ, while the other knob rotates faster in situ. This is equivalent to sweeping the lens and tube axis in the basic vertical position to find the best vertical position to make it light. In the course of sweeping, once the laser flashes out, the rotation should stop immediately. Then, the power meter is used to monitor and adjust the knob carefully and repeatedly to increase the output laser power.
This method is suitable for the case of verticality misalignment, but the misalignment is not large, and there is laser output. Although the laser output, the power is not high, and then adjust the reflective mirror separately, the power does not rise, can not reach the original power index. This is because although the parallelism of the two mirrors has been adjusted, the perpendicularity still has a small misalignment. At this time, although there is laser output, the power is low, as shown in Figure 7.
When the mirror is adjusted, the power will decrease no matter which end is adjusted, As long as the parallelism of the two mirrors is destroyed, the power will decrease. The problem of the perpendicularity between the mirror and the capillary axis can not be solved by adjusting only one mirror.
In this state, the pattern of output spot is not circular, uneven and asymmetrical.
In the above case, it is difficult to determine the direction of the laser misalignment when the perpendicularity misalignment of the laser is not large. Because of the output laser, it can be monitored and adjusted by a power meter. Firstly, a knob at the end of the laser is adjusted, and a small angle is rotated clockwise to destroy its parallelism intentionally. At the same time, the laser power is observed and the output power is reduced to 1/4 of the original power value. Then adjust the knob corresponding to the front end of the laser. If the back end adjusts the knob that the mirror rotates around the X-axis, the front end should also adjust the knob that the mirror rotates around the X-axis. In fact, the front-end lens is used to track the back-end lens to restore the parallelism of the two lenses. At this time, the observed output power value, such as power is lower than the original power, shows that the verticality is worse, should be changed to counter-clockwise rotation of the rear knob, the front-end tracking adjustment. If the power is on the rise, it will continue to track and adjust until the perpendicularity is the best, while the parallelism is not destroyed, so that the power reaches the maximum. Adjust the other pair of knobs in the same way, so that the power can rise to the maximum. In order to obtain the best perpendicularity, it is necessary to adjust the two sets of knobs repeatedly by this method, and finally achieve the maximum laser power. At this time, the pattern of the spot is also the best, and the intensity distribution is gaussian.
Experiments
1. Recognize the three components of the laser according to the physical and structural sketches.
2. Turn on the laser power supply to ignite the gas in the capillary. If the resonator is in normal condition, there will be laser output.
3. Adjusting the mirror knob at one end to make it misaligned, that is, the parallelism of the two mirrors is destroyed, then light can not be reflected and amplified back and forth in the medium, and laser can not be produced.
4. Autocollimation( Cross light target method) experiment was used.
5. Continue to adjust to make the output power of He-Ne laser largest.
