The basic principles of photonics
Chapter 1: Exploring and controlling the light
Why can we see objects?
In Ancient Greece, we used to say that the light came from the eyes to identify objects as seen in cartoons.
This statement is false. Indeed, we can see objects because they reflect a portion of the light that is received. We cannot see an object that receives no light as it will be unable to reflect a light to our eyes. This is the case of an object in the dark.
Yet, we can see certain objects because they emit light. This is the case of the sun, stars, lights, candles and flames: it’s what we call primary sources of lights.
What is light?
In 1666, during his optical experiments, Newton passes the white sunlight through a prism. He notices that the output of the prism, that white light, is divided into several different color lights. These are the rainbow colors.
Newton understands with this experience that the white light is separated into its components because each color beam is deflected differently by the glass of the prism. Thus, he notices for example that the red light is always less deviated than purple one.
Newton therefore understands that when white light passes from one transparent medium (such as air) to another (such as glass), its components are deflected a first time by color and when they emerge in air, they are again deflected, creating a luminous rays spread from red to purple, like the colors of the rainbow.
Why can we see a rainbow in the sky after the rain?
When the sun illuminates a thin mist (e.g. near water falls) or the charged atmosphere of thin droplets, you can see a rainbow. In reality, the droplets act as mini-prisms. White light enters inside the round droplet, reflects and springs. It is “decomposed.”
How does the light react when it runs into an object?
When incident light enters into the interface between two environment such as air and water, we see two things:
- A small portion of the light is reflected: this is the partial reflection phenomenon.
- A major portion of the beam enters in the water with a change of direction; the beam appears decomposed. This phenomenon whereby light changes direction when it passes from one transparent medium to another is called refraction.
Moreover, there are two types of reflection:
● The first type is: specular reflection. This reflection appears when the radiation reflected by the surface occurs in a single direction. This is done only on smooth surfaces such as mirrors.
The rays are reflected in a direction related to the angle at which they hit the object, much like a billiard ball hitting on a bar.
The second type is diffuse reflection. When the surfaces are rough with asperities, the reflection is diffuse. The radiation is reflected in all directions because of the heterogeneity of the environment, usually with a preferred direction where you can observe more reflection then.
Why an object that is illuminated by white light takes a particular color?
The color of objects around us depends on the light they diffuse.
To summarize, in the diffusion phenomenon, an object receives light and returns a small part in all directions. When diffusing a portion of the received white light is absorbed, the other is returned and gives color to the object: a red object absorbs all the colors of white light except the red, green object absorbs all the lights except the green, etc.
Thus, an object which is illuminated by a white light has the color of light it does not absorb.
How does a white subject react when illuminated by white light?
By illuminating a white surface, almost all the light rays of white light are reflected.
How does a black subject react when illuminated by a white light?
By illuminating a black surface, there is substantially no reflected light because the surface will absorb the radiation. The absence of light at the observer’s eye level is translated by the brain as a black color.
How does a color object react when illuminated by white light?
By illuminating a blue surface (or other), the material absorbs the other colors and transforms them into heat. Only scattered blue light can reach the observer’s eye. This object then seems blue.
Chapter 2: Light behaves both as a particle and as a wave
What is light??
Light is a form of energy like electricity. It is composed of tiny particles of light called “photons” moving in the form of an electromagnetic wave.
What is the light made of?
White light is composed of colored lights: the primary colors such as blue, green and red as the secondary colors such as yellow, cyan and magenta. The colors diffuse along different waves (one per color).
What is a wave?
A wave is the propagation of a disturbance, which produces on its way a reversible change in physical properties of the local environment. It moves with a given speed which depends on the characteristics of environment. A wave carries energy without transporting matter.
The light, which is an electromagnetic wave, can be easily spread in the space (vacuum). In vacuum and in the air, the light waves are propagated at a speed of 300 000km/s.
What is the electromagnetic spectrum?
The electromagnetic spectrum includes the set of all the electromagnetic waves according to their wavelength and frequency.
Located between the infrared and ultraviolet, visible spectrum occupies only a small place in the full electromagnetic spectrum. Other animals or insects do not have the same vision as us: their visibility spectrum can be shifted into the IR (mosquitoes) or UV (bees).
How is light generated?
Everything around us is made of tiny particles: the atoms.
An atom consists of a nucleus around which electrons move as clouds. Atom is often represented like our solar system, the sun (nucleus) is surrounded by planets (electrons).
As a simple picture, we can imagine that inside an atom, the electrons rotate around the nucleus in circular orbits. The electrons that revolve on a close orbit near the nucleus have lower energy than those who revolve on a more distant orbit.
Inside an atom, an electron orbit may change under certain conditions. For example, an electron can move from a high orbit to a low orbit. It moves from a state where it has a lot of energy towards a state where it has less (the electron releases the energy). This amount of energy released, which must be somewhere (in physics, no energy “disappears”), takes the form of an “energy package”, called “photon”.
A light source uses the phenomenon of photon generation on a large scale.
Chapter 3: Mirrors and lenses
What is a lens?
Lenses are made from a transparent material (glass or plastic). It can be observed that all these lenses are delimited by two smooth surfaces of which at least one is a curved surface. They are all symmetrical with respect to an axis called the optical axis.
The lenses can be classified into two types:
- Those whose centre is thicker than the edges are converging lenses (bottom drawing on the left).
- Those whose edges are wider than the centre are divergent lenses (on the right).
In optics, a focus is a point where the light rays emitted from a source converge after passing through an optical system (eg.lens).
What is a converging lens?
By illuminating a converging lens with a parallel light beam, we can focus the light into a small spot almost reduced to a point. If we place a piece of paper at this point, it ignites. This is what we call the focus of lens.
What is a diverging lens?
By illuminating a diverging lens with a parallel beam of light, we can see that the light beam is deflected when it passes through the lens.
How an image is formed with a lens?
When an illuminated object is at a distance from the lens greater than the focal length f, the lens forms its image inverted, at a distance; this image can be observed on a screen.
What is a mirror?
Chapter 4: The concept of the laser
What is an excited atom?
An atom is in its ground state if electrons are located on lower orbits (the closest to the nucleus). In this case the atom is in its idle state and has the lowest possible energy.
If an electron partially beats the attraction of the nucleus and goes to a higher orbit, then we say that the atom is in an excited state. It has a higher energy than the ground state.
What is the spontaneous emission?
An excited atom can spontaneously lose his excitement when the electron moves from a high orbit to a lower orbit via a high orbit to a low orbit. In this case it loses energy by emitting a photon.
What is absorption?
In an atom, an electron can also mount from a low orbit to a higher orbit if given enough energy.
What is stimulated emission?
If an atom that is already in an excited state (electron on an upper orbit) receives a photon of the right wavelength, the electron “pushed” by the incident photon releases a second photon exactly equal to the first and falls in its lower energy state. The excited atom plays the role of a “photon copy machine”. For one photon received it releases two.
What is the laser effect?
Now imagine that we use the two identical photons from the stimulated emission (see above) to interact with two other excited atoms, then we get four identical photons. In turn, those four photons might hit four excited atoms and generate four additional photons. We get an avalanche effect, it is the laser effect.
In the laser effect, an amplification of light intensity occurs and all the resulting photons are in the same state which means the light is coherent. All photons are in the same state (direction, frequency, polarization, and phase). Since the frequency of the light wave corresponds to a colour (in the visible spectrum), the highly directional beam is of a very pure single colour. The colour in question depends on the atom which was used in the laser process, with its individual energy gap between the two involved energy levels between which the electrons ‘jump’.
How do you make a laser?
The laser effect is based on stimulated emission. Imagine a material, the amplifying medium, where the atoms are mostly in an excited state. Sooner or later, an atom will emit a photon by spontaneous emission, and this photon will induce a cascade stimulated emission: the laser beam is born.
To ensure an emission in the selected direction, and increase the effect of the stimulated emission, the amplifying medium is placed between two mirrors so that the light runs back and forth many times before being transmitted: thanks to this cavity optical amplification gains in efficiency.
What are the different applications of the laser?
Laser applications are numerous. The laser has replaced many processes by improving them while being less invasive. Here is an incomplete list:
The laser in medicine
The laser beam can indeed be so thiin that it reches only one cell. therefore, it can be used in several applications in medicine for manipulating a single cell.
Laser in Ophthalmology
One of the best known applications of laser in medicine is the treatment of myopia. It enables a rapid and painless surgical procedure to fix the lens
Laser in dermatology
The laser is very useful in dermatology for the treatment of haemangioma, rosacea, age spots and scars.
Laser in dental surgery
The laser has become an important tool in dental surgery: precision, safety, absence of bleeding and postoperative pain, better healing.
Laser in industry
In industry, lasers are used for their power and precision in tagging operations, welding, drilling or blasting. Unlike conventional tools, they have the advantage not to wear out.
Laser in cutting
Laser cutting is a manufacturing process that uses a laser to cut the material (metal, wood) due to the large amount of energy concentrated on a very small surface.
Laser in drilling
Accurate and fast, laser drilling allows obtaining small holes of varying geometries, for applications as glass on metal or plastic.
Laser in engraving and marking
A laser beam which energy is concentrated on a very small surface destroys the material by burning or vaporization. It is the cavity obtained by removing the material that creates the engraving. Even with low-power lasers, a beam focus on a very small area allows local destruction by vaporization. This marking method is very accurate, it allows the engraving of photos or complex designs on all types of materials.
Laser in military technology
Missile (airborne or ground-based)
Laser designator: laser light source for guiding a projectile (missile) or facilitating the targeting by means of a laser pointing on light arms.
Laser in the information technologies
Optical fibers are made of a transparent material, glass or plastic, and have the ability to conduct light like a pipe conducts water. Optical fibers can transmit information over computer networks and are increasingly popular as internet access because of their high-speed connectivity.
CD / CD ROM
The readout of the CD is done using an infrared laser spot.