In English

Novel Interferometric Methods for Characterization of Microscale Components for External-Cavity Semiconductor Lasers

Anders Camenius
Göteborg : Chalmers tekniska högskola, 2011.
[Examensarbete på avancerad nivå]

External cavity lasers (ECL:s) have found widespread use in various applications, and many different wavelength tuning techniques have been demonstrated over the years. A novel wavelength tuning concept, utilizing longitudinal cavity dispersion provided by a diffractive optical element (DOE) has been invented by Dr. Kennet Vilhelmsson [1]. The laser design has been demonstrated and tested, but various aspects of the laser performance have not yet been fully understood. The work carried out in the master thesis project presented in this report has been focused on the characterization of the critical cavity components of the ECL functional prototype. In the report, an overview of the field of external cavity lasers is given, as well as a presentation of different wavelength tuning methods found in the literature. The new concept for wavelength tuning (see [1]) is explained. The results from characterization of the external mirror and spectral measurement equipment are described. The external mirror properties are shown to fulfil the design criteria. However, the power reflectance is only about 87%, and the laser threshold could probably be reduced by choosing a mirror with slightly higher reflectance. In this work, the first main issue has been to study the gain chip used in the laser, which was known to produce an astigmatic and elliptical beam. The design, assembly and test of a lateral shear interferometer for beam characterization is presented. The white-light compensated interferometer can be utilized to measure the astigmatism and beam parameters of the semiconductor gain medium. Test results from the interferometer are given, proving the functionality of the equipment. A method for extraction of the wavefront radius of curvature from captured interferograms by means of the Gabor transform was implemented. The full characterization of the gain chip still remains to be carried out, when the laser is assembled, so that the gain medium can be studied under lasing conditions. The second important task of the project has been to study the properties of the diffractive optical element (DOE) used to provide the necessary longitudinal dispersion of the cavity. An experimental method for determination of the DOE focal length is introduced. The results show that the relative focal lengths of the four different elements available are in very good agreement with the design criteria. When the measured focal lengths are normalized to the measurement on the DOE with the longest focal length, and compared to normalized design values, the agreement is almost perfect. When absolute focal lengths are considered, a discrepancy of 7-10% between the measured and intended values is detected. This observation can probably be explained from systematic errors in the measurement setup and difficulties in locating the focal points correctly from simple ocular inspections of camera images. The most extensive part of the project has been to quantify the diffraction efficiency of the DOE. Only the DOE having the longest focal length has been subject to this investigation. A direct measurement technique, measuring the optical power in the principal diffraction order and comparing to the total power, has been tested. The estimated diffraction efficiency is found to be 77+/-5% (including one Fresnel reflection). The value is rather uncertain due to limitations of the measurement method. The need for a more robust way to measure the DOE efficiency was obvious, and led to the development of a self-interferometric measurement method. The idea is to measure the interference pattern formed by the overlapping diffraction orders from the DOE. A theoretical model of the DOE has been set up, which can be adjusted until the agreement with the measured data is as good as possible. In this way, the DOE profile can be determined, and the efficiency can be simulated when the micro-scale shape of the DOE is known. This method led to an estimation of the DOE diffraction efficiency of 82+/-2% (including one Fresnel reflection). The measurement method can be modified to work for diffractive optical elements of arbitrary shape, as long as their diffraction orders overlap spatially. Finally, the more robust DOE model mentioned above was used to simulate the behavior of the DOE focal length as a function of the wavelength, thus verifying the paraxial model of the DOE. The paraxial model is shown to overestimate the DOE focal length by about 3%. Also, data from the interferometric measurement were used to calculate the focal length for one of the DOEs, and after compensation for the 3% overestimation introduced by the paraxial model, the focal length is found to be only 0.75% from its design value. Hence, the combined results from this investigation and the previous focal length measurement indicate that the focal lengths of all four DOEs are very close to the intended design values.

Publikationen registrerades 2012-02-06. Den ändrades senast 2013-04-04

CPL ID: 154776

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