# Light and Lasers

## Light

We need to introduce some details about nature of light and related effects in order to understand how Laser Diodes work and what they are. This guide is a brief introduction to light and lasers. The light can be described as an oscillating phenomena called electromagnetic wave. Radio waves, microwaves and X Rays have all the same behavoir of the light. Each of these waves has a different effects depending on its wavelenght, that is the distance between two repetitions of the same wave. Radio waves, such as those used in television broadcasting, have wavelengths in the magnitude between the meter and the centimeter. Visible light oscillation frequency is much higher, so the resulting wavelenght is between 400 and 700 billionths of meter (nanometer, nm). When our eye perceives a single color, this is the result of a single wavelenght. The graph below shows the perceived color related to its light source. Visible spectrum is between 380nm and 760nm. Ultraviolet light is under 380nm, infrared above 700nm. Both infrared and ultraviolet rays are not visible to the uman eye (to have an idea, all bodies releasing heat to ambient are emitting infrared radiation), however the threshold between visible and invisible light depends on several factor and changes from person to person, as the color perception.

## Note

Wavelenght is inversely proportional to frequency. In the XX century, experiments shown that the speed of light is not related to relative speed between emission source and observer (this behavoir poses the base to the Special Relativity theory). This means that, in vacuum, electromagnetic waves have the same and uniform speed of 299792 km/s, it travels more than 7 times aroud the earth on each second, and it does not depend if we are approaching or moving away from the light source. The frequency of an electromagnetic wave is the ratio between the speed (distance traveled on each second) and wavelenght (distance traveled on each oscillation). A 500nm light source has a frequency of 600 000 billion Hz (repetitions per second) or 600 THz.

## Heat and absorbed energy

We perceive a single color when our eye catches a light of a single wavelength. We perceive a white light when several waveforms are combined together. When we see a black object, it means that is is absorbing all the wavelenghts of the visible spectrum. While most objects around us do not emit light, we can see them because their surface can reflect some of the waveforms of the light it receives. Ideally, a completely black body does not reflect light, while a white body reflects the entire radiation. However, the same white object may absorb some of the wavelenghts out of our visible spectrum.

Absorbed light is mostly transformed in heat. This is the reason why two objects of a different color, both exposes to the same strong light, will reach two different temperatures. In the same manner, body tissues will react in different way depending on the light they receive. This is the reason why different wavelengths are needed for different tissues. In Medical Applications section we describe how different tissues react to different wavelenghts.

## Nature of light

We described the light as an oscillating phenomena. This is confirmed by several experiments, showing that it has the same diffraciotn and interference behavoid that we can observe in sound propagation and with fluids. However, we may also describe some of the characteristics of the light as if it were a beam of particles: the photoelectric effect is a typical example (emission of electrons due to light excitation of a certain surface). Theories about quantum mechanics shows that we can identify a minimum light particle called, or quantum, called “photon”. The picture below shows how we can explain the energy transfer that happens when a certain atom is hit by a photon. When the energy carried by the photon is sufficient to move the electron to a higher orbital, the atom can release its energy later by emitting another photon the same wavelength (color) that precisely depend on the distance between the two orbitals.

With the oscillating model we can explain diffraction, such as what happens on a CD surface where the light is decomposed.

## Note

Both wave and particle representations are description of the real nature of the light, or models that allows us to explain their behavoir. Quantum mechanics theory explains that the photon (and other sub-atomic particles) appears really different from the mechanics we are used to. It tells that we cannot get the full status of a particle in the moments before we measure them (because we disturb the system by measuring it). Some theories not only say that it’s impossible to measure both position and speed of a sub-atomic particle (the more precise if the former, the lesser is the other), but also that when we measure a certain parameter we cause the value of itself to be finite, while it had just a probability (or, it did not have a precise speed or position) before.

## Laser

The Laser (Ligh Amplification by Stimulated Emission of Radiation) is a monochromatic and coherent light source (same phase, or same wave position on the same moment). It is quite new: the first device was shown to the public in 1960.

A generic lamp emits different wavelengths (we perceive this light as mostly white), in several direction, and light beams are not coherent. Conversely, laser light is coherent and monochrome. Without collimation, laser light is dispersed with an angle that depends on the device. There are different technologies that were developed in order to generate laser light. Most laser sources make use of diodes; they generate light by exciting a semiconductor junction where two different areas are designed so that the electric current causes a spontaneous emission of photons. With maximum simplification, the image below shows the concept:

An electric current is passing between the junction (in the image the arrow shows the direction of the electrons). When the current passes, some photons are emitted (as it happes for LED light). These photons will trigger the emission of new photons with the same wavelenghts of the neighbouring atoms. The central transparent area, with thickness that is a multiple of the emitted wavelenght, creates a resonant effect, where light emission is stimulated with the same phase. Under most circumnstances laser emitted light is used to focus a large amount of light in a small spot. A special configuration of the circuit allows the system to generate very short pulses for special applications.