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Hello everyone, this is Colin from Fiber Optics For Sale.
In this tutorial, I will explain what is a Fabry-Perot Laser, also called FP laser. So let's get started.
FP lasers are named after the French scientists Charles Fabry and Alfred Perot. FP laser is the simplest form of semiconductor laser. It has a active region at the center, and two parallel mirrors on either end. This is also called a Fabry-Perot Resonator.
The right side picture shows the structure of a typical FP laser.
Now let's take a look at how a FP laser works.
The two mirrors and the active medium between them form a laser. This is shown again in the first figure. We need mirrors to provide positive feedback, that is, the return of stimulated photons to an active medium to stimulate more photons. The two mirrors themselves form a resonator with length L.
Let a random wave travel from the left-hand mirror to the right-hand mirror, as the second picture shows. At the right-hand mirror, this wave is reflected; hence, the wave experiences a 180 degree phase shift. As you can see, the wave should have a break in its phase, which is impossible here. In other words, this resonator does not support this wave.
Now let another wave, as shown in the third figure, travel inside a resonator. At the right-hand mirror, the wave experiences a 180° phase shift and continues to propagate. At the left-hand mirror, this wave again has the same phase shift and continues to travel. Thus, the second wave yields a stable pattern called a standing wave.
The only difference between the two waves is their wavelengths. Thus, a resonator can support only a wave with a certain wavelength, the wave that forms a standing-wave pattern.
This resonator supports many wavelengths that can form a standing wave. Wavelengths selected by a resonator are called longitudinal modes.
How many longitudinal modes can a laser diode radiate? A resonator can support an infinite number of waves as long as they form a standing wave. However, the active medium provides gain within only a small range of wavelengths. Since a laser radiation is the result of the interaction of a resonator and an active medium, only several resonant wavelengths that fall within the gain curve might be radiated. This is shown in this figure.
Of course, light generation starts only when gain exceeds loss. Thus, eventually only those resonant wavelengths that are within the gain-over-loss curve will actually be radiated. Compare these two figures, only these three waves will be the actual laser output, the other two modes are not generated since their loss is bigger than their gain.
FP laser provides lumped feedback, which results in many longitudinal modes and, eventually, in a relatively large spectral width. This picture shows the output spectrum of a 1.55um FP laser.
FP lasers are capable of producing high powers, but are typically multimode at higher operating currents.
So there you have it. Please leave your comment below if you'd like to see other topics.
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I hope you learned a thing or two from this tutorial. I will see you in the next video!