Threshold current density is denoted by the symbol J th and is determined by dividing the experimentally obtained threshold current value I th by the area of the laser. As a result, when comparing the threshold current values of different devices, it is more appropriate to talk about threshold current density rather than threshold current. A laser that is wider or longer obviously requires more electrical power to reach the onset of laser action than a laser of a smaller area.
This is because the area of the device can be large. One laser diode could demonstrate a much higher threshold current than another device and yet be considered a much better laser. However, threshold current is also dependent on the size and area of the laser device. Threshold current is dependent on the quality of the semiconductor material from which the device is fabricated, and also the general design of the structure of the device waveguide. The inset schematically shows a broad area (100 µm wide stripe) laser diode emitting radiation from both its front and back mirror facets. curve, denoted by the change in output power over the change in current (ΔP/ΔI). The efficiency of the laser in converting electrical power to light power is determined by the slope of the L.I. I th represents the threshold current at which the device begins to lase. Such devices open up a wide range of new possibilities in such applications as industrial welding and precision cutting of metals and various other materials. In this fashion, the need to use electrical amplifying circuits is eliminated and the optical signals are directly amplified, with greater efficiency, and without the need to convert the light signal to an electrical signal and back.īy stacking several high-power laser diode bars on top of one another, it is possible to make stacked laser diode arrays (Figure 7) with output powers potentially in the range of kilowatts. Such optical amplifiers are used in direct optical amplification of the 1550 nm wavelength telecommunication signals propagating along the long haul telecommunication lines. In these applications, a high-power laser diode operating at the wavelength of 980 nm is used as a pump source for erbium doped fiber amplifiers. In addition to applications involving the pumping of solid-state laser rods, high-power laser diodes are also very useful for fiber optic telecommunication purposes. By modulating the drive current, the output of the laser diode is modulated with frequencies up to several GHz in high-speed data communications. Operation at shorter blue and UV wavelengths makes smaller spot sizes possible, consequently allowing more information to be stored on optical disks at a higher density.Īnother advantage of laser diodes is that they can be directly modulated at high frequencies. The size of the resultant spot is dependent on the wavelength of the laser - the shorter the wavelength of light, the smaller the size of the spot that can be generated. The resulting ruggedness and small size allow laser diodes to be used in environments and spaces in which other types of lasers cannot operate.Ĭoherence and single wavelength characteristics of laser diodes enable the outputs of these devices to be focused to a diffraction limited spot size.
Since laser diodes are made of semiconductor materials, they do not require the fragile glass enclosures or mirror alignment typical of gas lasers. Overall efficiencies greater than 30% are typical in the case of laser diodes. Most laser diodes operate with voltage drops of less than 2 V with power requirements determined by their current setting. In addition, compared to other types of lasers, laser diodes use very little power.
This characteristic makes these devices suitable for cable TV transmission, high definition TV (HDTV) development, and medical applications. Today, hundreds of watts of power are commercially available from laser diodes operating under continuous wave (CW) conditions in packages as small as a few cubic inches. The output of laser diodes is very bright considering their small size.
The optical characteristics, small size, and ruggedness of laser diodes have allowed many new uses to be commercialized. The commercial and industrial use of laser diodes has dramatically increased recently.