Holographic Master Record System and Preparations (continued)

There are three main reasons for producing multiple transverse modes:

1 The design of the optical resonant cavity is irrational;

2 Attach dust to mirrors and Brewster windows;

3 The resonance of the resonator is not good.

The adjustment method is as follows: First, turn on the power and let the laser light emitted from the laser shine on a white paper to observe whether it is a single transverse mode. If it is not a single transverse mode, analyze the reason. If it belongs to the first case, the external cavity helium neon laser can be replaced by a mirror with a radius of curvature compatible with the discharge tube. If it cannot be changed, you can finely adjust the parallelism of the mirror by observing the laser output pattern until the single transverse mode output is obtained, then fix it, or put a needle with a diameter of about 2 to 3 mm between the discharge tube and the mirror. Hole, the laser beam through the pinhole to the mirror to adjust. For the ruby ​​laser, 2 to 4 mm pinholes can be inserted between the ruby ​​rod and the mirror, and the position of the holes can be adjusted and fixed by observing the laser oscillation pattern. Pinhole material available thin metal plate. The size of the pinhole is determined by the diameter of the ruby ​​rod and the size of the oscillation, so the optimum value will be different. All kinds of pinholes with different diameters should be prepared for use.

If a complex pattern is present in the spot, dust is often attached to the outside of the laser output mirror. If it belongs to the second case, you can use a blower to blow the air onto the mirror surface of the reflector and the window of the Brewster window, or use compressed nitrogen to blow off the dust. If dust adheres to other filth, you can use cotton ball or lens paper to dip the acetone or ethanol, ethyl ether mixture (ethanol, ether 6:4 or 5:5) to force the excess liquid to be lighter. Wipe the mirror, and be careful not to scratch the mirror. Replace the mirror paper or cotton ball after one wipe.

In the third case, as in the first case, by observing the laser oscillation pattern, the mirror of the resonator is finely adjusted so that the two mirrors are parallel. In single transverse mode, the laser output power is maximum. If the two mirrors are still not parallel, their parallelism can be adjusted by the following method.

1 Make a hole with a diameter of 1 to 2mm with a suitable thickness of paper, and draw several concentric holes with a spacing of 1mm on it.

2 Discharge tube, ND filter, mirror, flashlight, and pinhole are configured as shown in Figure 5-9. Turn on the power and discharge the discharge tube. Put the pinhole in the position where the center of the discharge tube is completely visible, and observe whether the concentric image drawn on the pinhole board reflected on the mirror is clear. If it is not clear, use a flashlight to illuminate the pinhole cardboard, and then adjust the mirror so that the concentric image and the discharge tube are concentric (ie, coaxial). In this case, the discharge tube is perpendicular to the reflector. By adjusting the mirror on the other side in the same way, the parallelism of the resonator can be adjusted.

Note: Because the above method is to adjust the parallelism of the resonant cavity by the way of observing the mirror by eyes, when the mirror is adjusted to be parallel, it will produce a strong laser output to injure the eyes. Therefore, when approaching the required parallelism, remove the eye and adjust the direction of the mirror movement as you wish. Adjust the adjustment screw of the mirror. For an external cavity laser, you can insert an ND filter or wear protective glasses to adjust between the Brewster window and the mirror. If it is an internal cavity helium-neon laser, the discharge tube is deformed due to thermal expansion, so that the transverse mode of the laser beam changes to reduce the output power. At this time, the installation state of the discharge tube can be adjusted slightly to reach the normal output power. .

Check if the resonator has light leakage other than the desired output laser light. Use thick black cloth for light shielding. If the light leakage is blue-white, this is the light produced by the discharge of the discharge tube, and it can also be eliminated by setting a small aperture stop at the exit of the laser.

(2) Selection and Adjustment of the Longitudinal Mode When using a small holmium laser as the recording light, it is not necessary to specifically perform the longitudinal mode selection work. If large holmium lasers, argon ion lasers, and ruby ​​lasers are used as the recording light source, especially when recording large scene objects requiring a long coherence length, the longitudinal mode must be selected and adjusted using a single longitudinal mode laser beam.

(3) Reduce noise If: 1 laser discharge current is not appropriate; 2 dust or air flow between the mirror and Brewster window; 3 multi-mode oscillation, the beat frequency is generated between the modules. In the output power of the laser, noise that changes over time is mixed in and the coherence of the laser beam deteriorates.

The noise reduction method is: If the noise is caused by the first reason, for a HeNe laser with a changeable discharge current, you can observe the laser output while observing the laser output with an oscilloscope connected to the optical galvanometer to adjust the discharge current until the laser output. Power is maximized and noise is reduced to a minimum. For a helium-neon laser with a fixed discharge current, a variable-resistor voltage regulator can be installed on the primary side of the excitation current to adjust the discharge current. If this is the second reason (only external cavity or semi-cavity type lasers have this kind of noise phenomenon), it can be solved by the method of blocking and external air convection.

If it is the third reason, it can be solved by the single longitudinal mode selection method.

In addition to the above three main reasons, if the excitation power supply is unstable, the excitation power supply voltage is not appropriate, the discharge tube and the mirror are not fixed, and the service life of the laser has reached, it is also a noise cause. Therefore, the reasons for this type of noise should be carefully analyzed and solved using different methods.

4. Optical components and other components

According to the recorded holographic original type and design requirements of the recording light path, draw a configuration diagram first. Then use the number of beam splitters, pinhole filters, convex lenses, and total reflection mirrors to prepare dry plate, second or 1/250 s shutter, and one laser power meter capable of measuring microwatts to ten microwatts. In addition, prepare diffuser plates such as frosted glass plates.

5. Configure recording light path

(1) When the direction of the light beam is arranged parallel to the work surface and the recording light path is arranged, the direction of the light beam should first be parallel to the work platform. The height of the laser beam should be moderate. If the height of the laser output is not suitable, adjust the lens to a suitable height using a lift (or drop) light mirror. Use a fine adjustment mechanism of a mirror or lifter and a movable bracket with a fixed height mark to adjust the laser beam to Countertops are parallel.
(2) Selection of split beam splitting ratio After adjusting the beam parallelism, a beam splitter is installed. The splitting ratio of the beam splitter is selected taking into account the size of the recorded object and the surface scattering ability. The ratio of the intensity of the reference beam to the object beam at the surface of the recording medium is kept in the range of 1 to 10.
(3) The general formula for the degree of modulation of interference fringes in the polarization direction of the laser beam is: where the reference and object beam ratios are the angles between the directions of vibration of the reference and object beams, and Ï„(t) is the reference beam and the object beam. Time correlation. The above formula shows that polarized light perpendicular to each other cannot produce bright and dark stripes. The external cavity laser has a Brewster window, the laser output is polarized light, and the polarization direction should be perpendicular to the incident surface of the light beam; the output laser beam of the internal cavity laser is not polarized, but there is a certain direction of vibration intensity is the maximum The characteristic, so should make this direction perpendicular to the incident surface.
(4) When the laser beam is to be installed through the beam expander and the center of the collimator, the beam expander should be adjusted up, down, or left and right (without pinholes) so that the center of the emitted spot coincides with the laser beam. When using the collimator, the position of the lens center should be coincident with the center of the laser beam before the laser beam is expanded. The method is to observe whether the series of light spots reflected on the two surfaces of the lens are located on the same line.
(5) Stable mechanical structure of various components The brackets for mounting various optical components have large adjustments of 2 to 4 degrees of freedom. When using them, check whether there is any looseness in each part of the mechanism. Each fixing screw should be tightened. The adjustment part is preferably installed in the middle position, and the bracket is generally made of a magnetic watch stand. When it is recorded, it should be fastened to the table top.