Hello, everyone.
In this post, I would like to share with you The Laser Diodes used as a light source in communication systems.
Laser diodes (LD) are used in applications in which longer distances and higher data rates are required.
Because an LD has a much higher output power than an LED, it is capable of transmitting information over long distances. Consequently, and given the fact that the LD has a much narrower spectral width, it can provide high-bandwidth communication over long distances.
The LD’s smaller N.A. also allows it to be more effectively coupled with single-mode fiber.
The difficulty with LDs is that they are inherently nonlinear, which makes analog transmission more difficult. They are also very sensitive to fluctuations in temperature and drive current, which causes their output wavelength to drift.
In applications such as wavelength division multiplexing in which several wavelengths are being transmitted down the same fiber, the stability of the source becomes critical. This usually requires complex circuitry and feedback mechanisms to detect and correct for drifts in wavelength. The benefits, however, of high-speed transmission using LDs typically outweigh the drawbacks and added expense.
Laser diodes can be divided into two generic types depending on the method of confinement of the lasing mode in the lateral direction.
• Gain-guided laser diodes work by controlling the width of the drive-current distribution; this limits the area in which lasing action can occur. Because of different confinement mechanisms in the lateral and vertical directions, the emitted wavefront from these devices has a different curvature in the two perpendicular directions. This astigmatism in the output beam is one of the unique properties of laser-diode sources. Gain-guided injection laser diodes usually emit multiple longitudinal modes and sometimes multiple transverse modes. The optical spectrum of these devices ranges up to about 2 nm in width, thereby limiting their coherence length.
• Index-guided laser diodes use refractive index steps to confine the lasing mode in both the transverse and vertical directions. Index guiding also generally leads to both single transverse-mode and single longitudinal-mode behavior. Typical linewidths are on the order of 0.01 nm. Index-guided lasers tend to have less difference between the two perpendicular divergence angles than do gain-guided lasers.
Single-frequency laser diodes are another interesting member of the laser diode family. These devices are now available to meet the requirements for high-bandwidth communication. Other advantages of these structures are lower threshold currents and lower power requirements.
One variety of this type of structure is the distributed-feedback (DFB) laser diode.
With the introduction of a corrugated structure into the cavity of the laser, only light of a very specific wavelength is diffracted and allowed to oscillate. This yields output wavelengths that are extremely narrow—a characteristic required for DWDM systems in which many closely spaced wavelengths are transmitted through the same fiber.
Distributed feedback lasers have been developed to emit light at fiber optic communication wavelengths between 1300 nm and 1550 nm.
That's all, I hope you like it.
Thank you!
