Optical pulses

Optical pulses

Bursts of electromagnetic radiation of finite duration. Optical pulses are used to transmit information or to record the chronology of physical events. The simplest example is the photographic flash. This was probably first developed by early photographers who used flash powder that, when ignited, produced a short burst of intense light. This was followed by the flash lamp, in which a tube filled with an inert gas such as xenon is excited by a brief electrical pulse. A great advance in the creation of short optical pulses came with the invention of the laser. Lasers are now the most common and effective way of generating a variety of short optical pulses, of different durations, energies, and wavelengths. See Laser

Pulses of millisecond (10-3 s) duration are very simply generated by mechanically modulating a constant light source such as a lamp or a continuous-wave laser. This can be done, for example, by placing a rotating disk with holes in it in front of the light source. Shorter laser pulses, of microsecond (10-6 s) or nanosecond (10-9 s) duration, are generated by using a technique known as Q-switching. A modulating device is incorporated inside the laser cavity that allows the buildup of the laser radiation inside the cavity and then switches it out in an instant. The modulating device is usually controlled by external electrical pulses. Semiconductor diode lasers, which are used to transmit information (voice or data) over a fiber-optic cable, are pumped by electricity and can be directly pulsed by applying to them a pulsed electrical signal. See Optical fibers

Ultrashort laser pulses, with durations of the order of picoseconds (1 ps = 10-12 s) or femtoseconds (1 fs = 10-15 s), are generated by using a general principle known as mode locking, whereby several frequency modes of the laser structure are made to resonate simultaneously and with a well-orchestrated relationship so as to form a short-duration pulse at the laser output.

Pulses as short as 11 fs have been produced directly by a passively mode-locked titanium:sapphire laser. The titanium:sapphire laser has also allowed the extension of ultrashort optical pulses to other wavelength ranges, such as the near-infrared (2–10 μm). Dye lasers, based on organic dyes in solution, have achieved durations as short as 27 fs. Ultrashort diode laser pulses have been obtained by active and passive mode locking and produce pulses as short as a few hundred femtoseconds. They are more commonly operated so as to give rise to pulses in the picosecond range, appropriate for optical communication systems.

The generation of ultrashort laser pulses has been motivated by the quest for ever better resolution in the study of the temporal evolution and dynamics of physical systems, events, and processes. Such laser pulses are capable of creating snapshots in time of many events that occur on the atomic or molecular scale, a technique known as time-resolved spectroscopy. This stroboscopic aspect of ultrashort laser pulses is their most important scientific application and is used in physics, engineering, chemistry, and biology. For example, ultrashort pulses can excite and take snapshots of molecular vibrations and deformations. They can track the passage of charge carriers through a microscopic semiconductor device. This ability to understand the dynamics of the more elemental building blocks of nature can in turn make it possible to build ever faster devices for use in information processing and information transmission, in addition to providing a better understanding of the physical world. See Laser spectroscopy

References in periodicals archive ?
The transmitter module converts the electrical signals from a DisplayPort source device to optical pulses that are transported over the fiber optic cable to the receiver.
This material can be made to assume an amorphous state, like glass, or a crystalline state, like a metal, by using either electrical or optical pulses.
To address this problem, one widely used method to detect eavesdropping is to artificially mix optical pulses with different intensities (decoy states).
In the next stage, the team demonstrated the ability to weaken this circuitry by stimulating the same nerves with a memory-erasing, low-frequency train of optical pulses.
Roberto Malinow, PhD, said that they can form a memory, erase that memory and then they can also reactivate it according to their requirement by affecting a stimulus that selectively strengthens or weakens synaptic connections and re-stimulating the same nerves with a memory-forming, high-frequency train of optical pulses.
Inside the connectors, there are tiny optical-to-electronic transceivers that convert the high-speed video signal into optical pulses that are carried over plastic optical fiber.
Femtosecond lasers emit optical pulses of extremely short duration in the domain of femtoseconds, as short as one-quadrillionth of a second.
Attosecond optical pulses are a laser-like light that is redefining ultra-fast physics and chemistry.
December 2006, IBM scientists demonstrated silicon nanophotonic delay line that was used to buffer over a byte of information encoded in optical pulses - a requirement for building optical buffers for on-chip optical communications.
It is capable of generating optical pulses in the ultrafast duration of 3 picoseconds (1 picosecond = one-trillionth of a second), with ultrahigh output peak power of 100 watts and repetition frequency of 1 gigahertz.
Optical pulses only a few femtosecond long have been used for several years now in high resolution time domain spectroscopy, but a number of processes in the natural world are much faster yet, so attosecond laser pulses have now been developed to investigate them.