A Distributed Bragg Reflector (DBR) Laser is a semiconductor laser with a p-n junction as an active medium and contains one or two Bragg reflectors on each side of the active region. Bragg reflectors are gratings that act as mirrors or end reflectors with reflectivity optimized at a particular wavelength and it narrows the laser linewidth. In DBR, the grating is outside the active region where no current flows. The emission wavelength of DBR lasers can typically range from 700 nm to 4000 nm, depending on the specific design and intended application. They typically have several hundred milliwatts of output power and operates in single TEM₀₀ mode. They are suitable for raman spectroscopic applications since their emission linewidth is less than 4 MHz. These lasers are widely used in telecommunications, sensing, and scientific research applications.

Figure 1: Schematic of a DBR Laser

The schematic of a distributed bragg reflector laser is shown in figure 1.

By changing the current or temperature, DBR can be continuously tuned over a range of 2 nm approximately. These lasers are very stable and have low noise. DBR laser uses high reflectivity surface grating and a high index contrast. They have lower defect levels resulting in higher powers and improved stability. 

Principles of DBR Lasers

DBR lasers are based on the principle of distributed feedback, where the feedback mechanism is provided by a corrugated structure formed using a periodic variation of the refractive index in the active region of the laser. This periodic variation is achieved by alternating layers of two different materials with different refractive indices. The most commonly used materials for DBR lasers are GaAs and AlGaAs.

The DBR structure is located on either side of the active region, and it acts as a highly reflective mirror for the laser light. When a bias voltage is applied to the device, electrons and holes are injected into the gain medium, leading to the emission of photons that resonate within the cavity. Then the Bragg reflectors selectively reflect light back into the cavity, creating a positive feedback loop that amplifies the laser emission and leads to lasing action.

Structure of DBR Laser

Figure 2: Structure of DBR Laser

The DBR laser structure consists of three main regions: the active region, the DBR mirror regions, and the waveguide region. 

The active region contains a quantum well or quantum dot structure, which provides the gain necessary for lasing. This region is usually located in the center of the DBR laser structure and is surrounded by the DBR mirror regions.

The DBR mirror regions are made up of multiple layers of two different semiconductor materials with different refractive indices, arranged in a periodic structure. The periodicity of the structure is designed to reflect a specific wavelength range, which determines the lasing wavelength of the laser. The DBR mirror regions act as highly reflective mirrors that reflect the laser light back and forth through the active region, creating a feedback mechanism that enables laser oscillation.

The waveguide region is located between the active region and the DBR mirror regions. It serves as a guiding structure that confines the laser light within the active region. The waveguide region is typically a single layer of a semiconductor material with a lower refractive index than the DBR mirror regions.

Advantages of Distributed Bragg Reflector (DBR) Lasers

• Narrow linewidth
• High output power
• Low threshold current

Disadvantages of DBR Laser

• Maximum output power is limited around 150 mW 

Applications of DBR Lasers

DBR lasers have a number of applications in different fields. One of the most important applications of DBR lasers is in telecommunications. DBR lasers are used as light sources in fiber optic communication systems, which enable high-speed data transfer over long distances. DBR lasers are also used in wavelength division multiplexing (WDM) systems, where multiple signals are transmitted over a single fiber using different wavelengths. 

Another important application of DBR lasers is in sensing. DBR lasers can be used as high-resolution spectroscopy and sensing tools to detect and measure various physical and chemical parameters such as temperature, pressure, and gas concentration. DBR lasers can also be used as optical biosensors for medical diagnosis and drug discovery.

These lasers also have applications in scientific research. They are used in laser cooling experiments to cool and trap atoms or molecules to near absolute zero temperatures. They are also used in quantum information processing experiments to generate and manipulate entangled states of photons and atoms.