Applied diode laser spectroscopy and characterization of optical fiber nonlinearity

This thesis describes work and progress on accurate nonlinearity measurements of optical fibers, design and characterization of external cavity diode lasers, and spectroscopic measurement of air temperature and humidity for accurate determination of the refractive index of air.The first part of the thesis describes measurement of the nonlinear coefficient of standard and erbium-doped single-mode fibers, commonly used in telecommunications. A simulation tool was developed to model the previously neglected effects of dispersion in the continuous-wave self-phase modulation method. The simulation can be included in already existing measurement set-ups increasing their versatility and reducing their uncertainty. It is shown that reliable erbium-doped fiber nonlinearity measurements are possible even for very short fibers when the whole measurement system is carefully characterized for nonlinearity. With the help of the dispersion simulation and a carefully optimized fiber optic power measurement, an expanded uncertainty of 2.0 % (k =2) was achieved for the nonlinearity of a single-mode fiber. The Expanded uncertainty for measurement of an erbium-doped fiber was found to be 3.0 % (k =2). Applied diode laser spectroscopy is covered in the second part of this thesis. External-cavity diode laser based on non-dispersive holographic volume grating was designed and characterized in this work. The use of a non-dispersive element for feedback eliminates beam directional variations and enables compact design with good wavelength reproducibility. Laser designs for applications in metrology, molecular spectroscopy and for multicomponent absorption spectroscopy were developed.This thesis describes accurate measurement of temperature and humidity using diode laser spectroscopy, which is crucial for refractive index compensation in demanding interferometric length measurements. The measurement system was tested both in laboratory and outdoor environment successfully over distances up to 130 m. The standard deviation of temperature measurement in laboratory environment was 7 mK using a 120 s sample time, which is the best spectroscopic value ever reported. Performance of the system was found to be excellent when a commercial interferometer was compensated in an environment with local temperature variations, demonstrating the suitability of the method for industrial dimensional measurements. A portable and robust temperature measurement set-up was developed for long-distance geodetic applications. The set-up was tested successfully in harsh outdoor conditions.