Semiconductor lasers or laser diodes generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers bounded at their side ends by cleaved surfaces that act as mirrors. Semiconductor laser diodes typically include parallel facets that are formed when semiconductor laser diodes are formed by cleaving a semiconductor crystal along the crystal's natural cleavage planes. The demand for semiconductor light-emitting diodes (LEDs) that emit light in the visible spectrum has increased with an increase in the necessity of high density information writing. As a result, semiconductor laser diodes of various types which emit lasers in the visible spectrum have appeared. Among them, a nitride semiconductor LD of an III-V group has attracted special attention because of its direct transfer method where a probability of a laser oscillation is high and its possible blue laser oscillation. Nitride based semiconductors, also known as group III nitride semiconductors or Group III-V nitride semiconductors, comprise elements selected from group II, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as indium gallium aluminum nitride (InGaAlN). Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the ultraviolet spectrum. Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Nitride semiconductor laser diodes that generate laser with wavelength from green to an ultraviolet region, are widely applied in high-density optical information storing and reproducing, high-resolution laser printers, and projection TVs. Excitation of the lasing medium can be accomplished in any of a variety of ways, including optical pumping, current injection, or the use of an electrical discharge. A laser diode utilizes current injection, and such a laser is typically fabricated initially from an intrinsic semiconductor heterostructure grown on a substrate.

Semiconductor laser diodes have many advantages, including low cost, small size, high mechanical stability, potential for substantial output power, and very good efficiency. Laser diodes are small in that the widths of their active regions which are typically submicron to a few microns and their heights are usually no more than a fraction of a millimeter. The length of their active regions is less than about a millimeter. A semiconductor laser diode has low threshold current and high efficiency. Semiconductor laser diodes are comparatively small and threshold current for laser oscillation of the semiconductor laser diode is smaller than that of a conventional laser device. Semiconductor laser technology also presents one of the most efficient and adaptable methods of generating coherent laser radiation at different wavelengths. By varying the type of semiconductor alloy from which the semiconductor lasers is made, semiconductor laser diodes can produce radiation at a range of wavelengths. Because of their numerous advantages, semiconductor laser diodes are used in many laser applications, such as in laser printing, optical data storage, long-haul fiber communication, spectroscopy, metrology, barcode scanners, and fiber amplifier pump sources. Semiconductor laser diodes have been widely using as devices for high speed data transmission or high speed data recording and reading in communications or players in which optical discs are used.

Laser diodes are used in a wide range of commercial, medical and military applications. These applications include materials processing (soldering, cutting, metal hardening), medical imaging (MRI) and surgical procedures (tissue fusion, dermatology, corneal shaping, photodynamic therapy), satellite communication, display technology/graphics, remote sensing, and inertial fusion confinement/energy. Laser devices are commonly used in medical fields, such as incision of skin during surgery, coagulation of hemorrhage sites, tissue welding during anastomosis of tissues such as nerves and blood vessels, tissue vaporization, photodynamic diagnosis and treatment, resurfacing, revasculariziation, hair removal, and removal of artificial pigments. Over the years, laser diode apparatuses have demonstrated their usefulness especially for hair removal, dermatology treatment and endoscopic laser surgery. In the case of hair removal, the laser diode technology is generally considered by dermatologists as one of the most efficient permanent hair removal techniques. Laser diode arrays also find application in high-speed laser printing. Due to the fundamental limits of polygon rotation speed, laser turn-on times and laser power, printing at high speeds and at high resolution requires laser arrays. Dual-spot red lasers and quad-spot infrared lasers have been used for laser printers. Using the laser diode light directly in laser machining and product marking may increase the efficiency and reduce the cost for such systems. Laser diodes are continually finding new applications in the commercial, military, medical and other sectors.

Laser diodes have various uses, but have been mainly used in optical pickups for writing and reading data on high-capacity recording media. Such high-capacity recording media include magnetic optical disc drives (MODD), digital versatile discs (DVDs), digital versatile disc-read only memory (DVD-ROM) drives, and DVD-random access memory (DVD-RAM) drives. High-density recording media have been significantly developing in line with advances in information technologies. Such optical recording apparatuses must be optimized with high-accuracy depending on the type of optical recording medium used. As high-density information recording is increasingly being required, the demand for a visible light semiconductor laser diode is increasing. The reading and writing operations in such optical disks are performed through the medium of light. In order to ensure accurate recording/playback of data, such optical recording apparatuses have an automatic laser diode power control (APC) circuit for controlling the power applied to a laser diode. A laser diode is used for a light source of the above optical disk apparatus and optical pickup. A laser diode used in an optical disk apparatus or an optical pickup device built therein or optical communications equipment is of the stripe structure.

Semiconductor lasers are used as a light source in fiber optic communication systems. Today, optical fibers form the backbone of a global telecommunication system. A telecommunication trunk generally uses an optical telecommunication system in which optical fibers carry information traffic in the form of optical signals. Optical fibers and lasers have dramatically increased the capacity of the telephone and data systems. Laser diode modules are employed as signal light sources of main trunk systems/CATV systems, and light-excitation light sources of fiber amplifiers. Optical fiber lasers and fiber amplifiers have become indispensable components in building optical networks for the transmission and amplification of optical signals. Semiconductor laser diodes have been used as a pumping light source for optical fiber amplifiers and as a signal light source in the fiber optic systems. A laser diode element and an optical fiber for allowing the light from the laser diode element to propagate therethrough are optically coupled in a module. Semiconductor laser diodes can be used as the optical pumping source for fiber-optic networks and communication systems. Laser modules which include a light-emitting circuit having a laser diode (LD) and a light-receiving circuit having a photodiode (PD) are known in the field of optical communication. The laser diode of the light-emitting circuit produces a predetermined optical output power when a bias current is added to a pulse current which is responsive to input data, and also outputs monitoring light for APC.