The Direct Diode Laser (DDL) is a laser oscillator that uses a prism and lenses to concentrate the laser beams coming from a Laser Diode (LD) stack module made up of semiconductor laser arrays. This produces a high-density light spot that is ideal for laser beam processing or machining. Laser beams from the LD stack module are focused by optical synthesis using a stripe mirror and edge filter, or by combining polarisation and wavelength, and are then shaped into beam patterns that are perfect for laser beam processing or machining by a number of lenses. The usual operating wavelength of a direct diode laser is 915 nm. They emit light in the near-infrared region. The output of the laser diode is directly used for applications. Figure 1 shows the structure of a direct diode laser.

Figure 1: Structure of Direct Diode Laser

Direct diode lasers have a simple construction and high conversion efficiency. The efficiency of DDL is more than that of an Nd:YAG laser. They are also more efficient than conventional fiber lasers due to the absence of a gain medium. They are the smallest and most reliable laser source with a high-quality beam. There is no conversion loss in these lasers and they also reduce electric energy consumption. These lasers are perfect for laser soldering and plastic laser welding applications. 

Direct diode lasers are made of gallium arsenide technology that allows wavelength emission between 0.8 μm and 1 μm. There are many applications in which a direct diode laser is used that require higher output power. Single diode bars with power in the order of 100 W are not just used for these applications, instead, diode bar arrays that contain multiple diode bars delivering power in the order of 1 kW or several kilowatts are used. Combining spectral beams is also possible with several diode stacks that operate at different wavelengths. The radiance of high-power laser diodes has improved by improving the design of diode bars and diode stacks and also by using improved methods and components for beam combining of many laser diodes.

Working of Direct Diode Laser

Figure 2: Schematic of a Direct Diode Laser

The schematic diagram of a direct diode laser is shown in figure 2. The laser light is generated by the diode and then it is focused using a transform lens to a diffraction grating with a periodic refractive index. The light is then sent to an output coupler from the grating. Since it is a direct process, direct diode lasers have a wall-plug efficiency between 40 % and 50 %. The laser beam from this laser is different from any other laser. The wavelength range of a direct diode laser is from 0.94 to 0.97 microns. In direct diode lasers, more energy is converted to heat and hence the absorption rate is high resulting in metal cutting at an uncontrollable speed.

Advantages of Direct Diode Lasers 

The beam quality of high-power direct diode lasers originally was too low for direct application in laser material processing, e.g. laser welding. But later, the technical progression increased their beam quality and radiance to higher levels and many direct diode applications are now possible. They are very compact and cheaper compared to other types of lasers. Direct diode lasers are available at power levels of 1500 watts to 8,000+ watts. This power level along with other features such as reliability, efficiency, and beam quality now allow direct diode lasers to be used in thicker material applications. These lasers have a high-edge quality which is not yet possible with fiber technology or disc technology. The direct diode laser can also either emit a free-space laser beam or can be fiber-coupled like a solid-state laser. The optical beam delivery through an optical fiber is very much needed for many applications, and it is possible only if the beam quality of the source is sufficiently high. To utilize highly efficient light, direct diode lasers use a simple optical system. 

Limitations of Direct Diode Lasers 

The spot size of the laser beam in direct diode lasers is rectangular in shape and not circular. Therefore, it is not suitable for some cutting applications where a circular beam spot is needed. Beam divergence can also be an issue in these lasers and poor optics and improper coupler may cause beam divergence.

Applications of Direct Diode Lasers 

High-power direct diode lasers are used in material processing for metal cutting and welding, especially for welding thin metal sheets. Applications such as laser soldering and brazing, laser cladding, and some laser surface treatments are also possible with direct diode lasers. They are used in medical applications for hair removal. Direct diode lasers are also used for packaging and heat treatment applications.