Laser Frequency Conversion by Stimulated Raman Scattering in the Near Infrared Spectral Region
Published in June 2013
Book
192 pages
978-3-86387-319-6 (ISBN)
Description
The present thesis is divided into three main parts. The following two chapters provide the theoretical background of the investigated physical effects. Chapter 2 deals with the spectral and spatial properties of solid-state lasers which are of particular interest in terms of trace gas detection. At the beginning, several mechanisms aiming at longitudinal mode control are introduced where the emphasis is placed on the method of injection-seeding. Afterwards, thermally induced phase distortions which impose severe limitations on the spatial beam quality are discussed together with techniques to overcome these difficulties such as birefringence compensation and phase conjugation.
The theoretical description of spontaneous and stimulated Raman scattering is presented in chapter 3. While a quantum mechanical approach gives insight into the relationship between both phenomena, the classical treatment allows for the derivation of Stokes wave equations as well as for the explanation of coupled-wave problems such as Raman fourwave mixing. In the course of the theoretical elaboration, the Raman gain coefficient is derived both quantum mechanically and classically paying special attention on consistency with the International System of Units (SI).
The second part of the work is closely related to the experimental results of the DFG project mentioned above. In chapter 4, the spectroscopic analysis of 17 crystalline materials using a mode-locked Nd3+:Y3Al5O12 laser is demonstrated, yielding the identification of numerous participating Raman-active vibrational modes as well as their respective interaction strength with the incident laser radiation. The manifestation of nonlinear interactions is examplarily shown for the mineral topaz which also forms the basis of computational simulations on ultra-short pulse synthesis from SRS frequency combs.
In the following chapter, the development of powerful solid-state lasers operating in the near infrared (NIR) spectral range is extensively studied. Two different Nd3+:Y3Al5O12 pump laser systems emitting at 1.06415 µm are used in combination with diverse external barium nitrate (Ba(NO3)2) Raman lasers in order to realize laser operation at 1.599 µm. The mechanisms described in chapter 2 are applied to optimize the spectral and spatial properties of the NIR laser sources, thus fulfilling the requirements for CO2 detection.
The latter is illustrated in chapter 6. Prior to the experiments with the Raman laser, tunable diode laser absorption spectroscopy is performed by means of a low power diode laser to determine the absorption characteristics of CO2 in the spectral region of interest. Then, the developed barium nitrate Raman laser is employed to detect CO2 using a multi-pass absorption cell. The experimental investigations are accompanied by numerical simulations based on HITRAN database parameters.
Finally, chapter 7 is devoted to the investigation of SRS in bulk silicon single crystals. Besides temperature-dependent measurements of the SRS threshold, the realization of Raman laser operation in bulk silicon is demonstrated. The strong defocusing of the pump tokes radiation due to the free carriers lens at high pump fluence is theoretically modelled. Numerical simulation of the the free carrier distribution along the silicon crystal allows to adapt the resonator design and, in turn, to compensate for the defocusing effect. In conclusion, this PhD work intends to highlight the relevance of SRS in crystalline materials regarding both basic and applied research.
The theoretical description of spontaneous and stimulated Raman scattering is presented in chapter 3. While a quantum mechanical approach gives insight into the relationship between both phenomena, the classical treatment allows for the derivation of Stokes wave equations as well as for the explanation of coupled-wave problems such as Raman fourwave mixing. In the course of the theoretical elaboration, the Raman gain coefficient is derived both quantum mechanically and classically paying special attention on consistency with the International System of Units (SI).
The second part of the work is closely related to the experimental results of the DFG project mentioned above. In chapter 4, the spectroscopic analysis of 17 crystalline materials using a mode-locked Nd3+:Y3Al5O12 laser is demonstrated, yielding the identification of numerous participating Raman-active vibrational modes as well as their respective interaction strength with the incident laser radiation. The manifestation of nonlinear interactions is examplarily shown for the mineral topaz which also forms the basis of computational simulations on ultra-short pulse synthesis from SRS frequency combs.
In the following chapter, the development of powerful solid-state lasers operating in the near infrared (NIR) spectral range is extensively studied. Two different Nd3+:Y3Al5O12 pump laser systems emitting at 1.06415 µm are used in combination with diverse external barium nitrate (Ba(NO3)2) Raman lasers in order to realize laser operation at 1.599 µm. The mechanisms described in chapter 2 are applied to optimize the spectral and spatial properties of the NIR laser sources, thus fulfilling the requirements for CO2 detection.
The latter is illustrated in chapter 6. Prior to the experiments with the Raman laser, tunable diode laser absorption spectroscopy is performed by means of a low power diode laser to determine the absorption characteristics of CO2 in the spectral region of interest. Then, the developed barium nitrate Raman laser is employed to detect CO2 using a multi-pass absorption cell. The experimental investigations are accompanied by numerical simulations based on HITRAN database parameters.
Finally, chapter 7 is devoted to the investigation of SRS in bulk silicon single crystals. Besides temperature-dependent measurements of the SRS threshold, the realization of Raman laser operation in bulk silicon is demonstrated. The strong defocusing of the pump tokes radiation due to the free carriers lens at high pump fluence is theoretically modelled. Numerical simulation of the the free carrier distribution along the silicon crystal allows to adapt the resonator design and, in turn, to compensate for the defocusing effect. In conclusion, this PhD work intends to highlight the relevance of SRS in crystalline materials regarding both basic and applied research.