In most cases the emitted light from the laser is polarized. Usually linearly polarized, that is, the electric field oscillates in a specific direction perpendicular to the direction of propagation of the laser beam. Some lasers (eg, fiber lasers) do not produce linearly polarized light, but other stable polarization states, which can be converted to linearly polarized light using a suitable combination of waveplates. In the case of broadband radiation, and the polarization state is wavelength dependent, the above method cannot be used.
A superradiance light source (also known as an ASE light source) is a superradiance-based broadband light source (white light source). (It is often mistakenly called a superluminescent light source, which is based on a different phenomenon called superfluorescence.) Generally, a superluminescent light source contains a laser gain medium that is excited to radiate light and then amplified to emit light.
Fiber polarization controllers create stress birefringence by wrapping the fiber around two or three circular disks, thereby forming independent waveplates that change the polarization state of light propagating in a single-mode fiber.
Femtosecond lasers are lasers that can emit optical pulses with a duration of less than 1 ps (ultrashort pulses), that is, in the femtosecond time domain (1 fs = 10−15 s). Therefore, such lasers are also classified as ultrafast lasers or ultrashort pulse lasers. For generating such short pulses, a technique called passive mode locking is often used.
Photodiodes are often used as photodetectors. Such devices contain a p-n junction and usually have an intrinsic layer between the n and p layers. Devices with intrinsic layers are called PIN-type photodiodes. The depletion layer or the intrinsic layer absorbs light and generates electron-hole pairs, which contribute to the photocurrent. Over a wide power range, the photocurrent is strictly proportional to the absorbed light intensity.
People utilized this ASE process to make broadband ASE light source which is essential for many different telecommunications, fiber sensing, fiber optic gyroscope, and test & measurement applications.
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