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Generation of ultrashort pulses from chromium doped cunyite laser

Posted on:2011-09-16Degree:Ph.DType:Thesis
University:City University of New YorkCandidate:Jeanty, MicheletFull Text:PDF
GTID:2448390002967864Subject:Physics
Abstract/Summary:
This thesis focuses on the generation of ultra short pulses from the chromium-doped Cunyite laser. The various principles and operation of Cunyite laser systems capable of generating femtosecond pulses in the near infrared are described. Self-starting mode-locking was successfully engineered and implemented with the assistance of semiconductor absorber mirrors (SESAMs).;The broad tunability of Cr4+: Ca2GeO4 laser indicates its potential as a source of ultrafast light generation. The spectral range between 1.3 mum and 1.5mum is both important for optical communications and the eye-safe 1.45mum wavelength range. If the entire laser bandwidth of Cr4+: Ca2GeO4 are utilized, pulses as short as sub-20 fs of Cr4+: Ca2GeO 4 may be attainable.;Cr4+: Ca2GeO4 laser (Cunyite) is developed at the Institute for Ultra-fast Spectroscopy and Lasers of City College of New York.The setup is formed by a standard confocal X-shaped cavity composed of the active material in the focus, and a broadband output coupler in combination with several flat mirrors used for dispersion compensation. So far, a semiconductor saturable absorber mirror (SESAM) is necessary to sustain mode-locking. Early attempts to generate mode-locked pulses led to the generation of full width at half maximum 60 picosecond pulses using an intracavity quantum-well-based semiconductor saturable absorber mirror. The SESAM is made of a thin narrow band gap absorption region, which is sandwiched between a cap layer and a spacer layer placed on the top of a high reflectivity semiconductor saturable absorber mirror. The SESAM is prepared by stacking pairs of quarter-wavelength layers that are composed of semiconductors with alternating high and low refractive indices. It consists of 24.5 periods of 123-nm AlAs low-index-104.9nm GaAs high-index quarter-wave layers for 1.43mum. The pulse width was further reduced to 8.6ps and subsequently to 365fs by using a highly-doped crystal. The tuning range of the mode-locked Cunyite laser was determined to be 134 nm. The CW output wavelength was approximately 1430 nm for all output couplers. The corresponding threshold pump power and the slope efficiency with respect to pump power were 0.5W, 1W, 1.5W and 3.125%, 4.58%, 3.75% respectively. The best laser performance was obtained with the 2.5% output coupler (OC). Using this output coupler, the laser produced 240mW of output power with 5W pump power. Subsequently, pulses with duration of 223 fs pulses were generated by this means. In conjunction with the measured spectra, this indicates that the nonlinear refractive index of this material is indeed sufficient to support Kerr-lens mode-locking.;A four-mirror astigmatically X-cavity was found to be more appropriate for mode-locking. The passive mode-locking was achieved using a semiconductor saturable absorber mirror, so-called SESAM. The laser performances concerning the pulse duration and the output power are quite different using different SESAMs. To explain the experimental results, the key parameters of the SESAMs must be known.;Time-bandwidth measurements indicated the presence of chirp in the output pulses. Numerical calculation of the phase characteristics of various optical materials indicated that a quartz glass plate was used in the cavity in order to compensate for the chirp. The results obtained indicate that the noise like mode is not significantly affected by the cavity dispersion. To improve the situation, a careful selection of the optical material is needed if the pulse duration is to be minimized. For Cr4+:Ca 2GeO4, the optimum choice turned out to be quartz plates, which, by minimizing the "Third-Order-Dispersion" in the cavity will allow the generation of nearly bandwidth-limited pulses of sub-100fs in duration around 1.4mum. In fact, extremely broad double-peaked spectra centered at 1.4mum could be obtained. However, the quartz glass plates have the advantage that the desired negative second-order dispersion can be adjusted continuously.;Self-starting mode-locking was demonstrated directly from the laser oscillator with double chirped mirrors in combination with fused silica glass plates. Numerical calculations of the cubic phase characteristics used indicated that the SF10 prisms compensated for quadratic phase but introduced a large cubic phase term. Numerical evaluation of other optical glasses indicated that a fused silica glass plates introduced a smaller cubic phase but the same quadratic phase term as the SF10.
Keywords/Search Tags:Pulses, Laser, Generation, Semiconductor saturable absorber mirror, Cubic phase, Glass plates, SESAM, Output
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