Linewidth Characteristics Of Single-Frequency Fiber Lasers

Single-frequency fiber lasers have a very narrow limiting linewidth with a Lorentzian spectral line shape, which is more significantly different from single-frequency semiconductors. The reason for this is that single-frequency fiber lasers have longer laser resonant cavities and longer intracavity photon lifetimes. This means that single-frequency fiber lasers have lower phase and frequency noise than single-frequency semiconductor lasers.
The linewidth test results of single-frequency fiber lasers are related to the integration time. This integration time is often difficult to understand, in fact, can be simply understood as "observation test" single-frequency fiber laser time, during which we shoot the frequency means of measuring the spectral phase noise to calculate the linewidth. Take the self-absorbing non-equilibrium M-Z interferometer as an example, the length of the delay fiber is 50km, single-mode fiber core refractive index is assumed to be 1.5, the speed of light in a vacuum for 3x108 m / s, then the light in a single-mode fiber transmission of about 1 meter will produce a delay of about 4.8ns, after 50km of the fiber is equivalent to the production of a delay of 240us.
Let's take the single-frequency laser to be tested after the 1:1 optical splitter imagine into 2 billets with identical characteristics, one of the billets than the other one ran 240us more, when the two billets and then through the second 1:1 optical coupler merge, 240us more than a billet with phase noise, because of the influence of the phase noise, re-merged single-frequency laser and did not set off before the state of the laser compared to the There is a certain width in the spectrum, a little more professional, this process is called phase noise modulation, because the modulation caused by the spreading of the bilateral band, so the phase noise spectrum width is to be measured single-frequency laser line width of 2 times. In the spectrum to calculate the spectral width of this broadening needs to use the means of integration, so this time is called the integration time.
Through the above explanation, we will be able to understand the "integration time" and single-frequency fiber laser measurement line width must have a relationship between. The shorter the "integration time", the smaller the effect of phase noise brought about by the split, the narrower the measurement line width of the single-frequency fiber laser.
To put it another way, what is the linewidth describing? It is the frequency noise and phase noise of the single-frequency laser. These noises have always existed, the longer the cumulative time, the more obvious the noise, so the longer the "observation test" single-frequency fiber laser frequency noise and phase noise, the measured line width will be larger. Of course, the time said here is actually very short, such as nanoseconds, microseconds, milliseconds, up to the second level, which is a common sense test measurement of random noise.
The narrower the spectral linewidth of a single-frequency fiber laser, the cleaner and prettier the spectrum is in the time domain, with a very high Side Mode Suppression Ratio (SMSR), and vice versa. Mastering this point can be in the absence of line width test conditions to determine the single-frequency performance of single-frequency lasers, of course, due to the spectrometer (OSA) of its own technical principles as well as the resolution limitations, single-frequency fiber laser spectra can not be quantitatively or accurately reflect its phase noise and frequency noise, this judgment is quite rough, and sometimes will bring the wrong results.
Single-frequency semiconductor lasers are generally higher than the actual line width of single-frequency fiber lasers, although some manufacturers of single-frequency semiconductor lasers to mention the line width of the very beautiful indicators, the actual test shows that single-frequency semiconductor lasers than single-frequency fiber lasers to the limit of the line width of the single-frequency fiber lasers to be wider than the frequency noise and phase noise indicators are also poorer, which is a single-frequency laser resonance cavity structure and length of the decision. Of course, the continuous development of single-frequency semiconductor technology by greatly increasing the length of the outer cavity, extend the photon life, control the phase to improve the resonant cavity standing wave conditions formed by the threshold and other ways to continuously inhibit the phase noise, narrowing the linewidth of the single-frequency semiconductor laser. We will introduce the single-frequency semiconductor laser technology in our future product technology seminars.