These four technologies are discussed together because they all directly affect the output characteristics of the laser resonant cavity.
1. Mode selection:
Mode selection is actually frequency selection. Most lasers use longer resonant cavities to obtain larger output energy, which makes the laser output multi-mode. However, compared with higher-order modes, the fundamental transverse mode (TEM00 mode) has the characteristics of high brightness, small divergence angle, uniform radial light intensity distribution, and single oscillation frequency. It has the best spatial and temporal interference. Therefore, a single fundamental transverse mode laser is an ideal coherent light source, which is very important for applications such as laser interferometry, spectral analysis, and laser processing. In order to meet these conditions, measures to limit laser oscillation must be adopted to suppress the operation of most resonant frequencies in multi-mode lasers, and use mode selection technology to obtain single-mode single-frequency laser output.
Mode selection is divided into two ways: one is the selection of laser longitudinal mode; the other is the selection of laser transverse mode. The former has a greater impact on the output frequency of the laser and can greatly improve the coherence of the laser; the latter mainly affects the uniformity of the light intensity of the laser output and improves the brightness of the laser.
1) Longitudinal mode selection: To improve the monochromaticity and coherence length of the beam, the laser needs to work in a single longitudinal mode. However, many lasers often have several longitudinal modes oscillating at the same time. Therefore, to design a single longitudinal mode laser, a frequency selection method must be used. Common methods include: short cavity method, Fabry-Ploy etalon method, three-mirror method, etc.
2)Transverse mode selection: The condition for laser oscillation is that the gain coefficient must be greater than the loss coefficient. The losses can be divided into line emission losses related to the transverse mode order and other losses independent of the oscillation mode. The essence of selecting the fundamental transverse mode is to make the TEM00 mode reach oscillation conditions and suppress the oscillation of higher-order transverse modes. Therefore, we only need to control the line emission loss of each high-order mode to achieve the purpose of selecting transverse modes. Generally speaking, as long as the TEM01 mode and TEM10 mode oscillations that are one order higher than the fundamental transverse mode can be suppressed, the oscillations of other higher-order modes can be suppressed. Common methods include: aperture method, focusing aperture method and intra-cavity telescope method, concave-convex cavity, using Q-switched mode selection, etc.
2. Frequency stabilization:
After the laser obtains single-frequency oscillation through mode selection, the resonant frequency will still move within the entire linear width due to changes in internal and external conditions. This phenomenon is called "frequency drift." Due to the existence of drift, the problem of laser frequency stability arises. The purpose of frequency stabilization is to try to control these controllable factors to minimize their interference with the oscillation frequency, thereby improving the stability of the laser frequency.
Frequency stability includes two aspects: frequency stability and frequency reproducibility. Frequency stability refers to the ratio of the frequency drift of the laser to the oscillation frequency within a continuous working time. The smaller the ratio, the higher the frequency stability. Frequency reproduction is the relative change in frequency when the laser is used in different environments. Frequency stabilization methods are divided into two types: passive and active. Specific frequency stabilization methods include: Lamb sag method and saturation absorption method.
3. Q-switching:
Generally, the light pulses output by solid-state pulse lasers are not single smooth pulses, but a sequence of small peak pulses with varying intensities and widths in the microsecond range. This light pulse sequence lasts for hundreds of microseconds or even milliseconds, and its peak power is only tens of kilowatts, which is far from meeting the needs of practical applications such as laser radar and laser ranging. For this reason, some people have proposed the concept of Q-switching, which has improved the output performance of laser pulses by several orders of magnitude, compressed the pulse width to the nanosecond level, and the peak power is as high as gigawatts.
Q refers to the quality factor of the laser resonant cavity. The specific formula is Q=2T"Energy stored in the resonant cavity/Energy lost per oscillation cycle.
At this time, the principle of laser oscillation Q-switching: a certain method is used to make the resonant cavity in a high-loss and low-Q value state at the beginning of pumping. The threshold of oscillation is very high, and even if the particle density inversion number accumulates to a very high level, it will not produce oscillation; when the particle inversion number reaches the peak value, the Q value of the cavity is suddenly increased, which will cause the gain of the laser medium to greatly exceed the threshold, and oscillation will occur extremely quickly. At this time, the energy of the particles stored in the metastable state will be quickly converted into the energy of photons, and the photons will increase at an extremely high rate. The laser can output a laser pulse with high peak power and narrow width.
Because the loss of the resonant cavity includes reflection loss, absorption loss, radiation loss, scattering loss and transmission loss, different methods are used to control different types of losses to form different Q-switching technologies. At present, common Q-switching technologies are: acousto-optic Q-switching, electro-optic Q-switching and dye Q-switching.
4. Mode locking:
Q-switching can compress the laser pulse width and obtain laser pulses with a pulse width of the order of microseconds and a peak power of the order of gigawatts. Mode locking technology is a technology that further modulates the laser in a special way, forcing the phases of the various longitudinal modes oscillating in the laser to be fixed, so that each mode can be coherently superimposed to obtain ultrashort pulses. Using mode locking technology, ultrashort laser pulses with a pulse width of the order of femtoseconds and a peak power higher than the order of T watts can be obtained. Mode locking technology makes the laser energy highly concentrated in time and is currently the most advanced technology for obtaining high peak power lasers.
Mode locking principle: Generally, non-uniformly broadened lasers always produce multiple longitudinal modes. Since there is no definite relationship between the frequency and initial phase of each mode, each mode is incoherent with each other, so the light intensity output by multiple longitudinal modes is the incoherent addition of each longitudinal mode. The output light intensity fluctuates irregularly over time. Mode locking allows multiple longitudinal modes that may exist in the resonant cavity to oscillate synchronously, keeps the frequency intervals of each oscillation mode equal and keeps their initial phases constant, so that the laser outputs a short pulse sequence with regular and equal intervals in time.
Mode locking technology is divided into active mode locking and passive mode locking. Active mode locking: insert a modulator with a modulation frequency v=c/2L into the resonant cavity to modulate the amplitude and phase of the laser output to achieve synchronous vibration of each longitudinal mode. Passive mode locking: insert a dye box with saturated absorption characteristics into the laser cavity. The absorption coefficient of the dye box with saturable absorption characteristics will decrease with the increase of light intensity. In the laser, as the optical pump excites the working material, each longitudinal mode will occur randomly, and the light field will fluctuate in intensity due to their superposition. When some longitudinal modes are coherently enhanced by chance, parts with stronger light intensity appear, while other parts are weaker. These stronger parts are absorbed less by the dye and the loss is not large. The weaker parts are absorbed more by the dye and become weaker. As a result of the light field passing through the dye many times, the strong and weak parts are clearly distinguished, and finally these longitudinal mode coherent enhancement parts are selected in the form of narrow pulses. Passive mode locking has certain requirements on the optical properties of the dye box: the absorption line of the dye must be very close to the laser wavelength; the line width of the absorption line must be greater than or equal to the laser line width; and the relaxation time must be shorter than the time it takes for the pulse to travel back and forth in the cavity.
