고출력 펄스 생성을 위한 다채널 가간섭 펄스 결합 연구

Abstract

Since the development of a Ruby (Cr:Al2O3) laser in 1960, manifold researches for several types of lasers and increasing the output of lasers have been continuously investigated, and their application fields have been also expanded in science and industry. Recently, ultrashort pulse lasers have been noticeable as new optical sources in several research fields such as frequency metrology, dimensional metrology and laser physics. They also have been attractive to industrial precise machining, observation of instantaneous natural phenomena, generation of high harmonics for extremely short light waves and even laser induced nuclear fusion. Although each research field requires the specific demands such as pulse duration, repetition frequency of the pulse train and center wavelength, the common feature for their fulfilled conditions is definitely the high peak power or the high pulse energy. The typical amplification method for ultrashort pulse lasers is the chirped pulse amplification (CPA), which stretches the pulse to lower the peak power under the damage threshold level of the amplifying medium, increases the pulse energy and compresses the chirped pulse into the original ultrashort pulse. This conventional method has been widely used in high peak power femtosecond pulse lasers because of its robustness and well-developed techniques. However, the single CPA still has practical limitations in the view of safety if it is applied to high power lasers due to much heat generation. Furthermore, the amplifying medium and optics should be specially designed in order to avoid damage issues in the single CPA process. An efficient way to overcome the limitations of the single CPA is the divided chirped pulse amplification (DPA), which divides the chirped pulse into several sub-pulses, amplifies each pulse and combines the amplified sub-pulses as a single pulse. In the DPA process, the sub-pulses do not have to be amplified much strongly, which indicates the heat generation and damage issues can be mitigated compared to the single CPA process. The DPA can be categorized into two, temporal DPA (T-DPA) and spatial DPA (S-DPA), by the pulse division domain. In the T-DPA, the seed pulse is divided by time-delay lines and the coaxial sub-pulses can be generated with time intervals. These timely distributed sub-pulses are amplified by the same amplifying medium with the specific time difference and they are recombined with the coherent pulse combining technique. The main advantage of T-DPA is the efficiency enhancement of the amplifier to increase the output power. On the other hand, S-DPA spatially divides the seed pulse and the sub-pulses are amplified by each optical amplifier. Then, the output pulse can be generated by the coherent pulse combining and S-DPA can have the benefit to unite all amplification effects. The overall DPA combines these two principles in the single configuration and it can effectively obtain ultrashort high peak power and energy pulses with the aid of pulse compression. The most important issue in DPA is to completely combine the sub-pulses into the single pulse, which means the sub-pulses should be coherently combined as the constructive interference. If the sub-pulses are not overlapped with respect to perfectly constructive interference, the output pulse does not have high peak power and short pulse duration. Even, the pulse combining should be stabilized to obtain reliable experimental results in aforementioned research and industrial fields. Another subject considered in DPA is the size of realized optical configuration because most of previous researches has been implemented in laboratory and they have impractical limitation in size. In this investigation, we propose a miniaturized DPA module which divides, amplifies, and combines pulses temporally and spatially for industrial applications. Especially, the proposed T-DPA module has effectively long optical time delay line, which leads to a few ns delay time, because of the multi-path interferometric scheme and the single module can act a role of both dividing and combining pulses. The whole system consists of 3 parts; temporal pulse divider/combiner (T-DPA module), spatial pulse divider/combiner (S-DPA module) and pulse amplifiers. The seed pulse is incident to the T-DPA module and the pulse is temporally divided by multi-path interferometric principle. The temporal pulse train is then incident to the S-DPA module and it is divided into two pulse trains by the polarization state of pulses. These pulse trains go through pulse amplifiers and return back to the S-DPA and T-DPA modules. The amplified pulses are recombined in the system by the reverse operations of temporal and spatial division procedures and the single pulse with high energy is generated. To combine the divided pulses, the locking of optical coherent via single-detector electronic-frequency tagging (LOCSET) method based on modulation and demodulation techniques was used. The LOCSET method obtained the control signal based on the interference signal and it was feedback to the piezoelectric transducer (PZT) attached to a mirror through PID loop. In order to estimate the stability of pulse combining, the intensity signals were collected for approximately 13 hours and Allan deviation was calculated. As the result, the stability of pulse combining was 2.97 x 10-2 at 1 s integration time. In the experiments, however, the ytterbium doped fiber amplifiers (YDFAs) were used for the pulse amplification and the coupling loss between the free space and the fiber was significantly large. Therefore, most of the light was not delivered by the fiber. Because of the multi-path interferometric configuration constructed with bulk optics, the proposed system is more suitable for bulk optic amplifiers, which can generate ultra-high energy pulses. In this case, the thermal issues should be considered and the optical components such as PBSs should be replaced with the high damage threshold and highly reflection coatings. On the other hand, the proposed optical fiber based DPA module is suitable for the amplification by YDFAs. Compared to a bulk type of DPA module based on the multi-path interferometer, it has a relatively simple optical configuration and avoids the coupling loss because of splicing of all fiber components, advantageous for miniaturization. In this investigation, the proposed optical fiber based DPA module divides the pulses spatially, amplifies each pulse, and then combines them again. For combining the divided pulses, the LOCSET control method was also used and the PID control was performed. Then, the Allan deviation value was calculated as 1.09 × 10-3 (1 s.) to evaluate the stability. We expect the proposed DPA systems can be used for ultra-high energy pulses and they are commercially available for science and industrial fields.