Journal of the Korean Physical Society, Vol. 73, No. 11, December 2018, pp. 1637∼1643 Frequency-Modulated Pulse-Ampliﬁcation Method for Reducing Pulse Shape Distortion Jihoon Jeong, Seryeyohan Cho, Seungjin Hwang and Tae Jun Yu ∗ Department of Advanced Green Energy and Environment, Handong Global University, Pohang 37554, Korea (Received 5 June 2018, in ﬁnal form 31 July 2018) To reduce the laser pulse shape distortion accompanying the ampliﬁcation process and achieve an intended output pulse shape in the Nd:YAG ampliﬁer chain, we propose a frequency-modulated pulse-ampliﬁcation method. Assuming carrier-frequency-modulated seed pulses, we numerically simulate the pulse ampliﬁcation in an Nd:YAG ampliﬁer chain where severe distortion occurs. For the calculation, we develop a modiﬁed Frantz-Nodvik equation, which enables two inputs with diﬀerent carrier frequencies. The simulation results indicate that the temporal contrast of the seed pulse needed to obtain a ﬂat output pulse shape is reduced by 16 - 25 dB when frequency modulation is applied. PACS numbers: 42.55.-f, 42.55.Rz, 42.60.By Keywords: Pulse shaping, Pulse distortion, Nd:YAG, Frequency modulation DOI: 10.3938/jkps.73.1637 I. INTRODUCTION After the invention of lasers, considerable eﬀort was directed towards increasing the pulse energy. In addi- tion, temporal shape control is important for many ap- plications, such as optical pumping [1,2], frequency dou- bling [3], and laser plasma applications [4]. Although the target shape varies depending on the application, a ﬂat pulse shape is generally desired. The master-oscillator power-ampliﬁer conﬁguration can be used to achieve a high pulse energy and a shaping capability simultane- ously [5–9]. Typically, a pulse shaper is placed in the os- cillator section, where the pulse energy is low and, there- fore, relatively easy to handle. However, when the pulse passes through the power ampliﬁer, the shape is distorted because of the gain depletion eﬀect [10]. The depletion eﬀect causes pulse shape narrowing as the energy stored in the medium is eﬃciently extracted. Serious temporal shape narrowing may damage the optics. Consequently, the pulse shape distortion not only hinders pulse shape control but also limits the energy can be extracted from the ampliﬁer. A pre-compensated seed pulse shape can be used to overcome the distortion problem and obtain a ﬂat out- put pulse shape. The pre-compensated shape is usually generated by lowering the front part of the pulse, which will experience higher gain. For proper compensation, larger distortion in the ampliﬁer yields a higher required ∗ E-mail: taejunyu@handong.edu; Fax: +82-54-260-1781 temporal contrast between the rear and the front parts of the seed pulse. Therefore, a pulse shaper with a high extinction ratio is essential for the ampliﬁer chain, where severe distortion occurs. However, there are two prob- lems. First, the seed pulse shape must be extremely de- formed to compensate for severe distortions. Although such a seed pulse shape yields a ﬂat pulse shape for the ﬁnal output, a spiky shape with a high peak power can be formed inside the ampliﬁer. This can cause nonlin- ear phenomena and induce damage inside the ampliﬁer. Second, the extinction ratio of a pulse shaper is typically limited to approximately 30 dB [11,12]. In addition, a serial connection of only a few pulse shapers is allowed because of the high insertion loss of the modulator (3.5 - 5.5 dB [11,12]). Therefore, if a ﬂat (or intended) output pulse shape and a high extraction eﬃciency are to be achieved simultaneously, a method for reducing the dis- tortion inside the ampliﬁer chain must be developed. In this paper, we propose a frequency-modulated pulse-ampliﬁcation (FMPA) method that eﬀectively ma- nipulates the gain cross sections by modulating the car- rier frequency and reduces the pulse shape distortion in- side the ampliﬁer chain. To evaluate the eﬀectiveness of the method, we performed a comparative simulation be- tween conventional pulse ampliﬁcation and FMPA. For the simulation, we develop a modiﬁed Frantz-Nodvik equation that allows an ampliﬁcation calculation for the frequency-modulated pulse in a double-pass ampliﬁer. In a simulation involving a multistage double-pass Nd:YAG ampliﬁer wherein severe pulse shape distortion occurs, the temporal contrast of the seed pulse required for pISSN:0374-4884/eISSN:1976-8524 -1637- c 2018 The Korean Physical Society