Fabry-Perot (FP) Laser
Fabry-Perot (FP) Laser (or Fabry-Perot Laser Diode) is a fundamental type of semiconductor laser widely used in photonics.
A Fabry-Perot (FP) laser is a laser diode that uses a Fabry-Pérot resonant cavity (or interferometer) as its optical resonator. This cavity consists of two parallel, partially reflecting mirrors (facets) at the ends of the semiconductor gain medium, which provide optical feedback for lasing. It is the simplest and most common type of laser diode.
Technical Info:
Structure: The device features a semiconductor p-n junction with an active region (gain medium, often quantum wells) where stimulated emission occurs. The cavity is formed by cleaving the semiconductor crystal to create two parallel end facets, which act as mirrors due to the refractive index difference between the semiconductor and air (typical reflectivity ~30% without coatings). High-reflectivity coatings can be applied to one or both facets for better performance.
Operation: When forward-biased, current injects carriers into the active region, leading to population inversion and gain. Light bounces back and forth between the mirrors, amplifying via stimulated emission. Only wavelengths that form standing waves (resonant modes) within the cavity experience sufficient feedback to lase. The mode spacing (free spectral range) is determined by the cavity length L and refractive index n: Δλ ≈ λ² / (2 n L) (where λ is the wavelength). This results in multiple longitudinal modes (multimode output), with a typical spectral width of several nm, unlike single-mode lasers.
Key Characteristics:
Simple and low-cost fabrication.
Output power from mW to hundreds of mW (or higher in arrays).
Temperature-sensitive wavelength (shifts ~0.3–0.5 nm/°C) and threshold current.
Can operate in continuous wave (CW) or pulsed modes.
Often edge-emitting, with elliptical beam divergence.
Compared to Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR) lasers, FP lasers lack internal gratings for mode selection, making them less spectrally stable but cheaper and suitable for many applications.
Applications:
Telecommunications: Used in short-reach optical links, fiber-optic networks, and as low-cost sources in wavelength-division multiplexing (WDM) systems (often with injection locking for improved performance).
Sensing and Spectroscopy: Gas sensing (especially with external cavities), optical coherence tomography (OCT), and range finding.
Consumer and Industrial: Laser pointers, printers, barcode scanners, optical mice, and pumping other lasers or amplifiers.
Medical and Scientific: Low-coherence sources for imaging, research in photonics, and as seed lasers.
Other: Security systems, material processing, and testing/measurement equipment.
FP lasers balance simplicity, cost, and performance, making them a workhorse in photonics despite their multimode nature. For applications needing narrow linewidth or high stability, more advanced designs like DFB lasers are preferred.