Diode-pumped Novel All-solid-state Yb Femtosecond Lasers

All-solid-state ultrafast lasers with a directly diode-pumped scheme have opened many new applications in scientific research, military, agriculture and industry, human engineering and so on due to its small volume, low cost, high efficiency, simple operation and easy maintenance. In recent years, Yb-doped materials have been widely used in ultrafast lasers as gain media for generating near-infrad laser. Because they have many advantages, such as simple production process, capability of large-size fabrication, low cost, excellent optical performance and so on. Up to now, most of all-solid-sate Yb ultrafast lasers generate femtosecond pluses by using of a semiconductor saturable absorber mirror （SESAM）. The bandwidth and damage threshold of SESAM not only restrict the laser pulse duration, but also limit the life and long-term stability of oscillator. In this paper, we studied diode-pumped all-solid-state Yb femtosecond lasers. By assisting of the third-order nonlinear effect in ytterbium doped laser materials, the bandwidth of mode-locked laser pulses is no longer restricted by SESAM. So the laser pulses duration is further compressed and the structure of the cavity is further simplified. The theory and practice prove that the Kerr-lens mode-locked laser can be continuous long-term operation and generate laser pulses with narrower duration. In this paper, we studied the continous-wave and mode-locked laser characteristics of several new Yb-doped laser materials. We demonstrated Kerr-lens mode-locked femtosecond lasers based on Yb-doped laser materials, generating laser pulses with pulse duration of 33 fs. Now we summarize the main research works and the innovative results as follows:1. We first studied the continous-wave laser characteristics of Yb:YCOB crystals with different Yb concentrations and cutting directions. The maximum average output power of contious-wave laser was as high as 2 W. And the tunable range of the contious-wave laser was about 93 nm （1007-1100 nm）. Then, we used a semiconductor mirror as the saturable absorber to start and keep stable passively mode-locking. A passively mode-locked Yb:YCOB （20 at.%, X-cut） ultrafsat laser was accomplished. The pulse duration, spectrum width and the average output power of the mode-locked laser were 150 fs,9.6 nm and 430 mW, respectively. We also designed a commertial Yb:YCOB ultrafast oscillator with many avantages, such as compact size, high efficiency, low cost, and high reliability and so on. Related work has been published on Chinese Physics B,2014,23（5）:054207.2. We characterized the Yb:CGA laser medium in continuous-wave and passively mode-locked operations under output couplers with different transmittances. A maximum output power of the continuous-wave laser was as high as 1.02 W. And the wavelength tuning range of the continuous-wave laser as broad as 68.5 nm （1006-1074.5 nm）. In passively mode-locked operation, by finely designing and adjusting the cavity, a dioed-pumded passively mode-locked Yb:CGA femtosecond laser was obtained with a SESAM. The pulse duration and spectrum width of the stable passively mode-locked laser were 185 fs and 7.2 nm, respectively.3. We demonstrate a stable Kerr-lens mode-locked Yb:YCOB oscillator for the first time. Under the 3-W pump power, the maxium output power of the stable mode-locked laser was about 70 mW. The spectrum width, pulse duration, central wavelength, and repetition rate of the stable Kerr-lens mode-locked laser pulses were 19 nm,73 fs,1043 nm, and 110 MHz, respectively. By improving the cavity and using high Yb concentration （30 at.%） sample, stable laser pulses were generated. The pulses duration and spectrum width of the mode-locked laser pulses were 39 fs and 35 nm, respectively. From this work, we make sure that Yb:YCOB is an ideal candidate for producing sub-50 fs laser pulses. Related work has been published on Optics Letters,2014,39（20）:5870-5872.4. We developed a stable Kerr-lens mode-locked femtosecond Yb:CGA laser with broad tunable wavelength. By finley adjusting the cavity and slightly tilting the end mirror, the wavelength tuning range of the stable Kerr-lens mode-locked laser was from 1043.5 to 1076 nm, as broad as 32.5 nm. The pulses duration, spectrum width, central wavelength of the stable mode-locked laser were 60 fs,21.5 nm,1043.8 nm, respectively. A broad wavelength tunable femtosecond laser based on a Yb:CGA crystal is expected to become a commercial all-solid-state ultrafast laser with stability, high efficiency and long life. Related work has been published on Laser Physics Letters,2016,13:015302.5. With the help of a laser diode, we developed a stable Kerr-lens mode-locked Yb:CYA femtosecond laser for the first time. The Yb:CYA oscillator could be generate one of shortest laser pulses in the world. Under the absorbed pump power of 1.24 W, the Yb:CYA oscillator could work at stable Kerr-lens mode-locking operation. The pulse duration, spectral bandwidth, central wavelength, and repetition rate of the Yb:CYA oscillator were 33 fs,49 nm,1059 nm, and 115 MHz, respectively. To the best of our knowledge, this is one of the shortest pulses generated from Yb-doped bulk oscillators. Related work has been published on Photonics Research,2015,3（6）:335.6. We developed a Kerr-lens mode-locked Yb.GYSO oscillator for the first time. By adjusting the cavity, the oscillator could work at different states. When the stable laser pulses could operate at the central wavelength of 1094 nm, the pulses duration, spectrum width, average output power of the Yb:GYSO oscillator were 141 fs,10.1 nm, and 237 mW, respectively. When the stable laser pulses could operated at the central wavelength of 1054 nm, the pulses duration, spectrum width, average output power of the Yb:GYSO oscillator were 55 fs,23.5 nm, and 27 mW, respectively. In this experiment, the double wavelength performance of Yb:GYSO was detailed excavated. Related work has been published on Applied Sciences,2015,5:817-824.7. We developed a Kerr-lens mode-locked Yb:(Y_0.9La_0.1)₂0₃ ceramic laser for the first time. Under 4-W pump power and output coupler with transmission of 0.8%, the stable mode-locked laser could work at 1074.5 nm. The spectra width, pulse duration and maxium output power were 17 nm,85 fs and 80 mW, repectively. Under 4.9-W pump power and output coupler with transmission of 1.5%, the stable mode-locked laser could work at 1072.8 nm. The spectra width, pulse duration and maxium output power were 13.4 nm,106 fs and 120 mW, repectively. Under 6-W pump power and output coupler with transmission of 2.0%, the stable mode-locked laser could work at 1072.5 nm. The spectra width, pulse duration and maxium output power were 8 nm,187 fs and 260 mW, repectively. The above experimental results indicate that the mode-locked pulse duration reduces with a smaller transmission of the OC at the expense of a smaller output power. Related work has been published on Laser Physics Letters,2014,11（11）:115302.8. With the help of a diode laser, we developed a stable Kerr-lens mode-locked Yb:YAG ceramic femtosecond laser. This Yb:YAG ceramic oscillator could generate sub-100 fs laser pulses. When the stable laser pulses could be operate at the central wavelength of 1049 nm, the pulses duration, spectrum width, average output power, and pulse energy of the mode-locked Yb:YAG ceramic oscillator were 97 fs,13 nm,320 mW, 2.8 nJ, respectively. This also prove that the Yb：YAG laser ceramic is a kind of excellent ultrafast laser material for generating ultrashort laser pulses with high power and high efficiency. Related work has been published on Chinese Physics B,2016,25（2）:024204.