The power reduction frequency point is a very important concept in pulsed lasers. It is defined as the lower limit of the repetition frequency when the laser can output the maximum rated average power under a specific pulse width. In other words, below this frequency point, even if the power is set to 100%, the actual output power will be lower than the rated power. The existence of the reduced power frequency point is determined by the dynamic process of the laser itself. An accurate understanding of the reduced power frequency point is helpful to correctly guide the practice of laser processing technology. JPT MOPA nanosecond pulsed fiber laser's manual gives the power reduction frequency points corresponding to each pulse width. Careful reading of these data can help you find the direction of process optimization more quickly.
In pulsed lasers, the existence of power reduction frequency points is mainly limited by two physical effects. One is the saturation amplification effect in the gain medium, which limits the maximum pulse energy that can be output, and the other is the nonlinear effect, which limits the maximum output that can be output. Pulse peak power.
Above the power-down frequency point, the pump of the laser amplifier works continuously to excite the doped rare-earth ions from the ground state to the excited state within the adjacent pulse interval (T) to achieve pump energy storage. At this time, there is no signal light to extract energy, and the energy storage in the gain medium increases, just like the water level of a dam increases. After the signal pulse passes, through the stimulated emission process, the ions at the upper energy level are induced to emit at the same wavelength and direction to achieve pulse amplification, just like a dam opens a gate to release water, forming a flood peak.
Figure 1 Laser stimulated absorption, spontaneous emission and stimulated emission process
As shown in Figure 1, in addition to pump stimulated absorption and signal stimulated emission, ions in the excited state will continue to be lost through spontaneous radiation, so the energy storage cannot increase indefinitely. The physical quantity that limits the cross-sectional energy storage density in the direction of light is called It is the saturated flux (Fsat), which is mainly determined by the absorption cross-section (σabs) and emission cross-section (σem) at the signal laser frequency (v) position. The product of the saturated flux and the optical fiber area (Af) is the saturated energy (Esat). Physically, Est is the input pulse energy when the laser gain is reduced to 1/e (~37%) of the small signal gain.
When the signal pulse energy is much lower than Estat, the laser gain is high and the laser works in the small signal amplification zone. When it is larger than Estat, the laser gain decreases gradually to zero, and the laser works in the saturation amplification zone. In the Ytterbium fiber laser, the absorption cross section and emission cross section at 1064nm are 0.0064pm2 and 0.3978pm2, respectively. For a 20μm core fiber, the saturation energy is about 0.6mJ, and for a 30μm core fiber, the saturation energy is about 1.3. About mJ, the saturation energy is 14.5mJ for a super-large mode field fiber of 100μm. It can be seen that the maximum output energy of the current pulsed fiber laser is basically equivalent to the saturation energy. Above the saturation energy, the pulse can still be amplified, but the gain will get smaller and smaller, slowly approaching zero. Estimated with a pulsed laser with a rated power of 30W, the typical optical efficiency is 60%, that is, the required pump power is 50W, and the quantum efficiency from 915nm to 1064nm is 86%. The time required to complete 2mJ of energy storage is 2mJ/ 86%/50W = 46μs. Taking 46μs as the pulse period, the corresponding frequency is 22kHz.
In fact, in MOPA pulsed lasers, generally only long pulses of more than 200ns can reach the nominal maximum energy output. The shorter the pulse width, the smaller the maximum output energy. Under the same energy, the shorter the pulse width, the higher the peak power, and the peak power higher than a certain threshold will cause significant nonlinear effects in the fiber. Therefore, in addition to limiting the maximum pulse energy, MOPA lasers also have an important limitation that is the peak value. Power, the typical value is 10kW.
In conventional MOPA fiber lasers, common nonlinear effects include self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), and stimulated Raman scattering (SRS). The direct consequence of these nonlinear effects is to broaden the spectrum. The laser with too broad spectrum passes through an optical system without achromatic design, and the focal position and focal spot size of different wavelengths are different, which directly affects the processing effect. In addition, nonlinear effects may also cause deterioration of the laser beam quality and physical damage. By rationally optimizing the laser design scheme and device selection, Guangzhi Technology can customize lasers with peak power far higher than conventional models for users.
Figure 2 Schematic diagram of pulse laser power reduction frequency points
As shown in Figure 2, the significance of the frequency point of pulse laser power reduction is summarized as follows:
(1) The laser can work at the maximum average power (Pmax) below the maximum working frequency (fmax) of the laser and above the power reduction frequency point (f0). The pulse energy (E) output in this section is equal to Pmax/f. The higher the frequency, The lower the single pulse energy.
(2) Below the power-down frequency point, the maximum power (P) that the laser can output will decrease approximately linearly with the actual working re-frequency, that is, P = Pmax*f/f0, for example, the working re-frequency is reduced to half of the power-down frequency point , The corresponding maximum output power is only half of the rated power, and the single pulse energy E = P / f = Pmax / f0, that is, the single pulse energy remains basically unchanged compared to when working above the reduced power frequency point. It should be pointed out that this linearity is only approximate, the actual situation may be more complicated, but it is sufficient as a simple estimate.
(3) The power-down frequency of long pulses is smaller than that of short pulses. For example, the YFL-PN-GM-80-L laser of Guangzhi Technology, the power-down frequency point corresponding to more than 250ns is 53kHz, and the pulse is more Shorter, the greater the frequency of power reduction. This is because limited by the peak power, the single pulse energy that can be extracted with a long pulse is higher.
In the actual process optimization process, when will the re-frequency parameters below the power reduction frequency point be used? Simply put, it is the occasion where pulse energy or peak power is required instead of excessive average power, that is, single pulse energy or peak power is required to ensure the processing effect, but the average power must be reduced to avoid excessive thermal effects. For example, in the process of breaking the oxide layer of anodized aluminum sheet, which is often used in the processing of 3C products, a certain pulse energy is required to destroy the oxide layer. However, if the average power is too high, the thermal effect will cause the material to deform and form a convex hull on the back.
More often, in order to maximize the pulse energy effect, such as laser cleaning, the laser can be directly operated at the reduced power frequency. In the proofing experiment of some samples, it is also possible to start the verification from the power reduction frequency point. If the effect can be achieved or the processing is over-processed, the energy should be appropriately reduced until the boundary parameters that meet the process requirements are found.
Except for MOPA pulsed lasers, Q-switched lasers or ultrafast lasers rarely mention power-down frequency points. This is because such lasers have only a fixed pulse width and only open the working frequency band above the power-down frequency point. For example, Q-switched lasers generally work at 30kHz to 80kHz, while ultrafast lasers work at 100kH to 1MHz. The lower frequency limit here is also close to the power-down frequency point of such lasers. MOPA pulsed laser has a large repetition frequency adjustable range, and it can basically ensure that the single pulse energy and peak power remain unchanged at the reduced power frequency. It is an important advantage. Correctly mastering and flexibly applying this performance advantage will help customers better Give full play to the performance of the laser.
