LASER = Light Amplification by Stimulated Emission of Radiation
A laser is actually an oscillator rather than a simple amplifier. The difference is that an oscillator has positive feedback in addition to the amplifier.
Light is understood in a general sense: electromagnetic radiation with a wavelength around 1 μm. Thus, one can have infrared, visible, or ultraviolet lasers.
The atomic medium with population inversion used in the laser is called the active medium. The positive optical feedback is obtained by placing the active medium between two mirrors. One of them (M1) totally reflects back the light (R1 = 1) to the active medium, while the other one (M2, called the output coupler) has a reflectivity less than unity (R2 < 1) and allows some of the light to be transmitted as the output of the laser. The two mirrors form a resonant cavity for the optical radiation.
Types of lasers
Lasers come in many shapes and sizes. They are classified by various criteria:
- Gain medium is solid, liquid, or gas
- Wavelength is in the infrared, visible, or ultraviolet spectral region
- Mode of operation is continuous or pulsed
- Wavelength is fixed or tunable
The present state of the art includes:
- Peak powers > 1012W
- Pulses shorter than 10-15s
- Cheap, efficient diode lasers available at blue (400 nm), red (620 nm-670 nm), and near-infrared wavelengths (700 nm–1600 nm)
- Other readily-available fixed-wavelengths include: