1. Spectral Linewidth
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum. A spectral line extends over a range of frequencies, not a single frequency (i.e., it has a nonzero linewidth). In addition, its center may be shifted from its nominal central wavelength.
Linewidth is width of the spectrum of a light beam or an absorption feature.
"Widely wavelength tunable lasers are key components for future reconfigurable dense wavelength-divisionmultiplexed (DWDM) optical networks. And they are also expected to replace fixed-wavelength light sources, thus reducing the spares inventory and system costs. Many types
of widely tunable lasers have already been proposed and developed , including distributed Bragg reflector (DBR) lasers , distributed feedback (DFB) laser arrays,
and external cavity lasers (ECLs)]. Of these, we have developed tunable DFB laser arrays (TLAs), because we think they are the most reliable. Their stable mode behavior is a great advantage for use in optical communication systems that require highly reliable components. We obtained a high fiber output power of more than 20 mW, a wide tuning range of more than 35 nm in both the C- and L-bands, and a high side-mode suppression ratio (SMSR) of more than 45 dB with
the TLAs. These features make TLAs extremely useful for current 10-Gbit/s DWDM applications." Hiroyuki Ishii, Member, IEEE, Kazuo Kasaya, and Hiromi Oohashi, Member, IEEE
2. Distributed feedback (DFB) lasers
Distributed Feedback (DFB) Lasers are single mode lasers containing an integrated grating structure. The result is a single mode emission at an outmost precise wavelength with an extremely narrow line width.
3. Optical Fiber Communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in core networks in the developed world. Optical fiber is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals.
The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal.
4. Optical Transmitters
The most commonly used optical transmitters are semiconductor devices such as light-emitting diodes (LEDs) and laser diodes. The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light. For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient, and reliable, while operating in an optimal wavelength range, and directly modulated at high frequencies.
In its simplest form, a LED is a forward-biased p-n junction, emitting light through spontaneous emission, a phenomenon referred to as electroluminescence. The emitted light is incoherent with a relatively wide spectral width of 30-60 nm. LED light transmission is also inefficient, with only about 1% of input power, or about 100 microwatts, eventually converted into launched power which has been coupled into the optical fiber. However, due to their relatively simple design, LEDs are very useful for low-cost applications.
Communications LEDs are most commonly made from Indium gallium arsenide phosphide (InGaAsP) or gallium arsenide (GaAs). Because InGaAsP LEDs operate at a longer wavelength than GaAs LEDs (1.3 micrometers vs. 0.81-0.87 micrometers), their output spectrum, while equivalent in energy is wider in wavelength terms by a factor of about 1.7. The large spectrum width of LEDs is subject to higher fiber dispersion, considerably limiting their bit rate-distance product (a common measure of usefulness). LEDs are suitable primarily for local-area-network applications with bit rates of 10-100 Mbit/s and transmission distances of a few kilometers. LEDs have also been developed that use several quantum wells to emit light at different wavelengths over a broad spectrum, and are currently in use for local-area WDM (Wavelength-Division Multiplexing) networks.
5. Tunable Lasers
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.
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