Mid-infrared Femtosecond Optical Parametric Amplifier

As the most important laser source for ultrashort pulses, the optical parametric amplifiers are tunable in broad band between ultraviolet, visible and infrared light. It means much for the research and application of ultrashort pulses.Since the femtosecond pulses came forth, people has always been bending themselves to compress the pulse width. It's just because the femtosecond pulses especially optical period-width pulses that only include one or several optical period are indispensable to the research on the physical ultra-fast phenomena and pursuing process of the photochemistry and photobiology action. It's very important how to get optical period-width pulses with enough energy.With the development of ultra-fast technology and the importance of nonlinear Spectroscopy research on oscillation dynamics in a large of physics, chemistry and biology systems, more and more attension are paid to the middle infrared optical parametric amplification technology. But because of invisible and the limit of measure equipment and materials, it's development and application is restricted. How to develop MIR femtosecond laser technology is very crucial to the development of international laser technology in the future.In the thesis we lay emphasis on the theory and technology of MIR femtosecond OPA. Firstly, the femtosecond OPA is theoretically studied, and the nonlinear-coupled wave equations which show the law of optical wave coupling is deduced within slowly varying approximation. At the same time the research and application about using the quadratic nonlinear interaction to develop femtosecond laser technology during the 1um wave band is discussed too. Thirdly the experimental research on MIR OPA is discussed in detail. Lastly the optical parametric amplification to the unchirp femtosecond pulses is researched theoreticaly. Our work and conclusions are mainly follows:1. Theoretically study and systemically analyse the femtosecond OPA.Basing on the nonlinear-coupled wave equations which is deduced within slowly varying approximation, we designed a program for the numeric simulation. Some problems were analyzed and numerical simulated such as phase match, GVM, GVD and noncollinear phase match.. At the same time, it is also studied that using the quadratic nonlinear process such as OPA, SHG to develop 1 Mm femtosecond laser technology.2. Put forward and realize a kind of novel broadly tunable technology.Making use of the special phase match character of LiNbO3 OPA, we adopt the combined tune method which combine the collinear and small angle noncollinear phase match , and design a set of characteristic MIR femtosecond OPA system. The large range tuning between 2.0-4.5μm is realized using only one type of crystal and one set of device.3. Construct successfully a set of MID LiNbO₃ femtosecond OPA that can realize continuous tunable character within broad spectrum range.Construct successfully a set of MID OPA that can operate steadily. Using the Ti:sapphire femtosecond laser to provide pumping pulse with 800nm center wavelength, we select the near infrared component of the continuum white light as the seed pulse of our system. The system consist of two stages amplifier which each include a different thickness LiNbO₃ crystal. The continuous tunable character within broad spectrum range is realized by the collinear and noncollinear phase match separately. We obtain these parameters of the output pulse: continuous tunable between the 2.0～4.5um wavelength range; the maximum attainable energy at 30uJ; 1kHZ repetition rate; the bandwidth up to several hundreds of nanometers, and pulse width 100fs. The system provide a applied MID source for the research of MID spectrum, and accelerate the development speed of ultrashort and ultra-high intensity pulse technique in the MID field. Now the system is applied excellently to the MID spectrum measurement and the other relevant application.4. In order to increase the energy of MIR femtosecond laser pulse, put forward and study a kind of novel femtosecond OPA.Study numerically a novel ultra-broadband optical parametric amplifier which directly amplifies non-chirped femtosecond signal pulses and is pumped by picosecond pulses with a wavelength corresponding to the second-harmonic of the signal. The OPA is capable of supporting amplification of femtosecond pulses with duration of ¹0-fs and can be highly efficient due to the sweep of the signal within the picosecond pump pulse resulted from the group-velocity mismatch. The pulse-swept OPA may greatly improve the contrast-ratio of the signal pulses, conversion efficiency～30%, the parametric gain coefficient～104. Before the limitations set by the Kerr nonlinearity and group-velocity dispersion of the nonlinear crystal go into effect, the pulse-swept OPA may be capable of supporting efficient amplification of femtosecond pulses with duration of～10-fs.