The goal of laser structures is to maintain a low threshold with high efficiency and a focused output beam. Gain guiding and Index guiding are two fundamental laser design strategies. 

A gain-guided laser is a type of laser in which the laser light is confined within the active medium rather than by external mirrors. Gain-guided lasers have a broad stripe geometry, where the active region spans over the entire width of the waveguide. A gain-guided laser is simple to make and has a flat layer of the active layer. The schematic of a gain guided laser is shown in figure 1. Insulating regions at the top of the chip in gain-guided laser prevents current from flowing to either side in a complex double-heterojunction laser. A narrow stripe in the middle that runs over the entire length of the chip helps the current flow through a single passage. The current flows through the narrow stripe and undergoes recombination of current carriers. Hence population inversion occurs and as a result, gain occurs only in that zone. Even though there is no physical boundary separating the stripe from the rest of the active layer, those regions do not emit light since there is no gain.

These lasers have a threshold current greater than 100 mA. The light confinement is low for this laser and has an advantage in generating high powers ranging from a few milliwatts to several kilowatts as a result of spreading light over a larger area that reduces the possibility of optical damage to the emitting surface.  

Figure 1: Schematic of a Gain-guided laser

Index-guided lasers have a narrow stripe geometry, where the active region is much smaller than the waveguide width. By implementing stripes of semiconductor material beside the active region, the light confinement is improved in these lasers. The stripe geometry that creates a narrow stripe through the middle is called a buried heterostructure. In this structure, the active region is bounded on all sides by a material of a lower refractive index forming mirrored surfaces and this serves to guide the light better.

Index-guided lasers are semiconductor lasers or laser diodes that confine the output beam to the active layer using a built-in refractive index profile formed either in the active layer, surrounding layer, or both. They emit light at a wavelength of 1300 nm, deliver an output power of 20-25 mW, and operate in a single lateral mode. The light is confined by surrounding the stripe in the active layer with a lower refractive index material. Index-guided lasers have a complex structure. They require a low threshold current ranging from 10-20 mA.

A schematic of an index-guided laser is shown in figure 2. Here, in this case, a narrow strip containing the GaAs junction layer act as the active medium that runs through the length of the chip. An n-type GaAlAs is deposited on both sides of the stripe and it is covered with an insulator and then a metal contact is deposited on it. The insulator is used to confine the current flow through the boundaries. The confinement of light in the index-guided laser is better and they produce better beam qualities. Therefore, they are mostly used for diode laser applications. Index-guided lasers are also used for applications that require diffraction-limited output.

Figure 2: Schematic of an Index-guided laser

Optical confinement in Index-guided lasers can be of two types: positive index-guided and negative index-guided. In positive index-guided lasers, the central region has a higher refractive index and in negative index-guided, the central region has a lower refractive index.

Figure 3: Graph between current and light intensity for Gain-guided laser and Index-guided laser

Index-guided lasers overcome most disadvantages of a gain-guided structure like high threshold current, low differential quantum efficiency, and nonlinear kinks in the current. Figure 3 shows the current versus light output graph for gain-guided laser and index-guided laser. The active region of an index-guided laser is surrounded by a region of lower refractive index that confines the photons to a narrow stripe, in both the transverse and vertical directions. The reduced contact area of these lasers decreases the threshold current and the reduced emission area makes it easy to couple light to optical fibers.

Applications of Gain guided lasers & Index guided lasers

One of the most common applications of index-guided lasers is in telecommunications. The high power and stability of these lasers make them ideal for use in fiber optic communication systems. They are used to transmit data over long distances through optical fibers and are also used in the development of new technologies such as wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM).

Another important application of index-guided lasers is in medicine. These lasers are used in laser surgery, where their high power and stability make them ideal for cutting and removing tissue. They are also used in laser therapy, where they can be used to treat conditions such as cancer and skin disorders.

Index-guided lasers are also used in manufacturing, particularly in the semiconductor industry. They are used to pattern and etch semiconductor materials, and are also used in the production of optoelectronic devices such as laser diodes and photodiodes.

Index-guided lasers are also becoming increasingly popular in the field of lidar, which is a technology that uses laser beams to measure distance, velocity, and other properties of objects. This technology is used in autonomous vehicles, 3D mapping, and robotic applications.