What is a Fabry-Perot (FP) Laser Diode?

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- GoPhotonics

Jul 17, 2023

The Fabry-Perot (FP) laser diode is a type of laser diode that utilizes a Fabry-Perot resonant cavity to control the modes of operation and achieve wavelength stability. FP laser cavity functions as a Fabry-Perot interferometer, which is based on the fundamental principle of multiple beam interference. It is an edge-emitting laser diode that produce highly monochromatic laser output. This laser diode emits light at multiple distinct wavelengths in a range of 379 nm – 1610 nm and delivers output powers from 2 mW to 1W. Its compact design and precise emission characteristics have contributed to its extensive utilization in various applications, including telecommunications, sensing, and scientific research.

Construction of Fabry-Perot Laser Diode

Fabry-Perot laser diodes are constructed using semiconductor materials, typically gallium arsenide (GaAs) or indium phosphide (InP). The laser diode consists of several layers, including the active region where light emission occurs. Its structure resembles that of a PIN diode, with the active region sandwiched between the p-type and n-type semiconductor layers. The active region serves as the region where optical gain is achieved, leading to laser output generation.

The intrinsic region within the laser diode has a higher refractive index compared to the surrounding n-type and p-type regions. This higher refractive index allows for lateral confinement of light, effectively acting as a waveguide that confines the light to propagate along the active region.

The Fabry-Perot laser diode, in its simplest form, consists of an active region enclosed within a Fabry-Perot cavity. Both ends of the active region are cleaved facets that are parallel to each other and perpendicular to the length of the active region. One facet serves as a highly reflective mirror (HR), while the other facet acts as a partially reflective mirror (PR). These facets are created by precisely cleaving the semiconductor material, resulting in surfaces with specific reflectivity characteristics. The use of the Fabry-Perot resonant cavity in an FP laser diode enables precise control over the longitudinal modes of the laser output, resulting in a highly monochromatic beam.

Fabry-Perot Resonant Cavity

Fabry-Perot resonant cavity is an optical structure that consists of two partially reflecting mirrors placed at the ends of the Fabry-Perot laser diode's gain medium. Only photons with frequencies matching the resonant mode can propagate within the cavity. These photons undergo multiple reflections between the facets and experience amplification from the semiconductor gain medium within the cavity. In this design, significant light reflections take place at both ends of the laser diode, while minimal reflections occur within the gain medium.

Working Principle of Fabry-Perot Laser Diode

Upon applying a voltage across the laser diode, an injection current is driven through it. This current causes electrons from the n-region and holes from the p-region to enter the active region and recombine, resulting in the emission of photons. These photons then stimulate the emission of additional photons through the process of stimulated emission, leading to the generation of coherent laser output.

Resonant Modes and Output Spectrum

In an FP resonant cavity, only certain wavelengths that can form a standing wave mode within the cavity are allowed to oscillate. These wavelengths correspond to integral multiples of half the laser wavelength and match the dimensions of the cavity. This characteristic results in the FP resonance spectrum, which consists of a series of discrete resonant modes.

Furthermore, only the wavelengths within the gain medium spectrum, which represents the range of wavelengths where there is sufficient gain in the active region of the laser, will be present in the output. This means that not all the resonant modes supported by the FP cavity will actually contribute to the laser output.

For a Fabry-Perot resonator, the resonant condition is given by:


    n is the refractive index of the medium inside the cavity,

    L is the length of the cavity,

    m is an integer representing the mode number, and

    λ is the wavelength of light.

This equation represents the condition for the light waves to constructively interfere and form a resonant mode within the FP cavity. The resonant modes occur when the path length difference between the reflected waves from the two mirrors is an integer multiple of the wavelength.

The resonant condition equation allows for the determination of the allowed wavelengths or resonant modes supported by the FP laser diode. By adjusting the cavity length or the refractive index of the medium, different resonant modes can be achieved. This provides control over the specific wavelengths or frequency ranges at which the laser diode can emit coherent light.

The number of output laser wavelengths supported by the FP cavity is limited and determined by the combination of the FP resonance spectrum and the gain medium spectrum. The resonant modes that fall within the gain medium spectrum and have sufficient gain will be the ones that are actually emitted as laser output. The superposition of the FP resonance spectrum and the gain medium spectrum defines the specific wavelengths that contribute to the output of the FP laser.

Applications of Fabry-Perot Laser Diode

Fabry-Perot (FP) laser diodes find a wide range of applications across various fields due to their precise emission characteristics and compact design. Some of the notable applications of FP laser diodes include:

  • Telecommunications: FP laser diodes play a crucial role in fiber optic communication systems, serving as optical transmitters and receivers. They enable long-distance communication by providing highly monochromatic and focused laser beams.
  • Sensing and Metrology: In fiber optic sensors, interferometry, and spectroscopy, they are used for precise measurements of parameters such as distance, displacement, strain, and pressure.
  • Optical Data Storage: They are integral components in optical storage systems like CDs, DVDs, and Blu-ray drives. It provides the laser beam necessary for reading and writing data on optical discs.
  • Biomedical Applications: In the biomedical field, FP laser diodes are utilized in laser surgery, tissue analysis, flow cytometry, and DNA sequencing. Their coherent and monochromatic output enables precise and controlled medical procedures.
  • Scientific Research: FP laser diodes are extensively used in research laboratories for spectroscopy, microscopy, laser-induced fluorescence, and other scientific techniques requiring a stable.