What is a hologram?

When it comes to taking pictures, everyone is familiar with it. From babies born to old people with crutches and beards, people want to leave a good record in their life journey. In the past, when you walked into a photo studio, the waiter would ask you whether to take a black-and-white photo or a color photo. In the near future, the waiter will ask you, "Do you want a hologram?"

Holography is the most successful example of using the excellent coherence characteristics of laser. On the traditional photography base, we can generally see the image of the subject. But because it only records the light reflection intensity distribution on the surface of the object, it can't record the depth of the object, so it loses its three-dimensional sense. However, from the laser holographic backplane, there is no image of the photographed object at all, only fingerprint-like dense stripes. It is these stripes that not only record the amplitude of the object wave (the light wave reflected by the object on the back plate), but also record the phase of the object wave, thus reflecting the depth of the object. In other words, laser holography can record all the appearance information about the object, so it is called holography.

The recording process of holographic photography is different from traditional photography. A "camera" does not need a lens, but makes the photosensitive plate directly face the object irradiated by the laser and receive the reflected light waves for exposure. At this time, the laser beam is divided into two parts by the beam splitter. One part is used to illuminate the object, which is diffusely reflected by the object into an object wave, and the other part is reflected by the mirror and directly directed to the bottom plate to form a reference wave. The object wave interferes with the reference wave on the holographic bottom plate, forming dense coherent fringes. Generally speaking, when two beams of light are in phase, the vibration is enhanced; When the phases are opposite, the vibration is weakened. The phase of the two beams of light changes with the position of the object, so the enhancement or attenuation of light vibration changes with the position, thus producing bright and dark stripes at the intersection of the two beams of light. The contrast between light and dark stripes reflects the light intensity (because the light intensity is proportional to the square of the light wave amplitude); The distribution and shape of fringes reflect the change of light wave phase. It can be seen that the fringes produced by interference phenomenon can record all the changes of amplitude and phase well. Because laser photography can effectively distinguish different amplitudes and phases, as long as it is shot from different angles, the amplitude and phase of reflected light will also change, so that different phases and amplitudes can be recorded on the same floor, and even overlapping shooting of different objects will not affect each other.

From the recording process of holography, it can be seen that the essence of holography is interference recording of light, and the coherence of light source is highly required during recording. Therefore, lasers with excellent coherence characteristics are duty-bound to shoulder this important task. Laser holography not only has vivid images, but also has a strong three-dimensional effect. What is particularly wonderful is that when watching a hologram, the audience can see the scenery in different positions in the photo by changing the observation angle. Even more amazing, even if most of a hologram has been damaged and only a corner is left, it can still reproduce all the scenery.