The online explanation of chirp is definitely indispensable: the word “chirp” comes from the bird call in ancient Chinese.
Another common explanation: it is actually a FM signal. I don't know how many good young women have been misled by this sentence. In most people's cognition, FM refers to that high frequency represents 1, low frequency represents 0, and different frequencies are used to represent digital signals. And "chirp" does not refer to the modulation of the frequency components in the signal spectrum analysis, but the modulation of the instantaneous frequency.
Concept distinction: instantaneous frequency & frequency component
There is an angle modulation signal ej [ω 0 t + ϕ (t)] e^{j[\omega_0t+\phi(t)]}ej [ ω0t + ϕ ( t ) ] It corresponds to the time domainsin {ω 0 t + ϕ (t)} sin\{\omega_0t+\phi(t)\}s i n { ω0t+ϕ(t)}，其中 ω ( t ) = ω 0 + d ϕ ( t ) d t \omega(t)=\omega_0+\frac{d\phi(t)}{dt} ω ( t )=ω0+dtdϕ(t)It represents the rate of change of the phase at a certain moment and has the dimension of frequency, which is called the instantaneous frequency.
If sin [ϕ (t)] sin[\phi(t)]s i n [ ϕ ( t ) ] is a periodic signal, then the angle modulation signalej ϕ (t) e^{j\phi(t)}ej ϕ ( t ) is also a periodic signal, which can be expanded according to a Fourier series, and each item in the expansion is a frequency component.
Although the frequency has high and low, it actually describes how fast the angle of 2pi is turned, then the time derivative of the passing angle (phase is actually the angle) should be a description of the physical quantity of frequency.
When you get a complete signal, you can determine its frequency spectrum through Fourier transform (periodic signals are simple frequency components), which is an overall grasp of the signal. But the instantaneous frequency is a microscopic and detailed grasp.
It's like your teacher has a complete grasp of a project, as if everything can be done, but you ask him "Teacher, do you think this line of code is written wrong?" Your teacher is confused.
It is not a simple matter to request the instantaneous frequency of the signal, and some special methods are needed.
The reason for the chirp of the light source is that when the light source is modulated, the current of the light source is usually changed, which leads to the change of the temperature of the laser. The output wavelength of the laser is related to the temperature, which leads to the change of the wavelength of the laser. . If the direct modulation method is used to carry the electrical signal on the light, the chirp is often accompanied.