The Fabry Perot (FP) laser, also known as the Fabry Perot Interferometer, is a versatile laser technology widely utilized in telecommunications, astronomy, and various other fields. Its primary application is in low-data-rate short-distance transmission over distances of up to 20 kilometers. The FP laser serves as a more advanced alternative to the DFB (Distributed Feedback) laser.
Key Differences Between FP and DFB Lasers
The fundamental distinction between DFB and Fabry Perot lasers lies in their spectrum width. The Fabry Perot laser features a wide spectrum width and operates as a multi-longitudinal mode laser, while the DFB laser boasts a narrow spectrum width and functions as a single longitudinal mode laser.
Working of FP Laser
The FP laser is the most basic type of semiconductor laser, consisting of an active region in the center and two parallel mirrors on either side. This setup forms a Fabry-Perot cavity, where the mirrors reflect light back and forth through the active medium, amplifying the light.
A laser comprises two mirrors and the active medium between them. The mirrors provide positive feedback, returning stimulated photons to the active medium to stimulate more photons, thereby amplifying the light. This feedback mechanism is crucial for sustaining laser action.
The frequency of FP lasers is controlled by the spacing of mirrors at each end of the laser. This allows lasers to oscillate at specific frequencies determined by the cavity length and the spacing of the mirrors. Adjusting this spacing can lead to mode hopping, where the laser shifts between different oscillation modes, affecting its output wavelength and spectral properties.
Resonator Dynamics
The two mirrors form a resonator with a length– L. A random wave is reflected at m2 when it travels from one mirror (m1) to another (m2). The wave undergoes a 180-degree phase shift and a phase break.
When a stable wave passes through the FP laser’s resonator, it undergoes a 180-degree phase shift and continues to propagate. This wave continues to travel with the same phase shift as at mirror m1. In other words, a stable wave produces a stable pattern known as a standing wave.
The wavelength is the primary distinction between the random wave and the stable wave. The resonator can only support waves of a specific wavelength – waves that form a standing pattern. These specific wavelengths are determined by the dimensions and characteristics of the resonator. Longitudinal modes are those whose wavelength is determined by the resonator’s physical length and boundary conditions. A resonator can support an infinite number of such waves as long as they form a standing wave pattern within the cavity. Each of these modes corresponds to a specific resonant frequency, contributing to the overall spectral output of the laser.
Gain and Feedback Mechanism
The active medium in the Fabry-Perot laser only provides gain within a narrow wavelength range. Since laser radiation is generated by the interaction of a resonator and an active medium, only a few resonant wavelengths within the gain curve can be radiated. This is because the gain medium amplifies light only over a limited spectral range, determined by its gain profile.
Furthermore, light production begins only when the gain exceeds the loss in the cavity. Only resonant wavelengths that fall within the gain curve and are not suppressed by the loss curve will eventually be amplified. The FP laser generates lumped feedback due to the continuous nature of the cavity’s mirror reflections, resulting in a large number of longitudinal modes and, eventually, a relatively wide spectral width. This broad spectral output is a consequence of the multiple wavelengths supported by the resonator’s feedback mechanism.
Although this laser can generate high powers, they are typically multimode at higher operating currents. That is how the FP laser works.
About Inphenix
Inphenix is a light source manufacturing company based in the United States that specializes in a range of optical devices such as swept source lasers, Fabry Perot lasers, gain chips, distributed feedback lasers, and VCSELs. Inphenix products are cutting-edge, affordable, and compatible with a wide range of devices.
