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

Abstract: External cavity lasers (ECL:s) have found widespread use in various applications, and many different wavelengthtuning techniques have been demonstrated over the years. A novel wavelength tuning concept, utilizinglongitudinal cavity dispersion provided by a diffractive optical element (DOE) has been invented byDr. Kennet Vilhelmsson [1]. The laser design has been demonstrated and tested, but various aspects of thelaser performance have not yet been fully understood. The work carried out in the master thesis projectpresented in this report has been focused on the characterization of the critical cavity components of theECL functional prototype.In the report, an overview of the field of external cavity lasers is given, as well as a presentation of differentwavelength tuning methods found in the literature. The new concept for wavelength tuning (see [1]) isexplained. The results from characterization of the external mirror and spectral measurement equipmentare described. The external mirror properties are shown to fulfil the design criteria. However, the powerreflectance is only about 87%, and the laser threshold could probably be reduced by choosing a mirror withslightly higher reflectance.In this work, the first main issue has been to study the gain chip used in the laser, which was known to producean astigmatic and elliptical beam. The design, assembly and test of a lateral shear interferometer forbeam characterization is presented. The white-light compensated interferometer can be utilized to measurethe astigmatism and beam parameters of the semiconductor gain medium. Test results from the interferometerare given, proving the functionality of the equipment. A method for extraction of the wavefront radiusof curvature from captured interferograms by means of the Gabor transform was implemented. The fullcharacterization of the gain chip still remains to be carried out, when the laser is assembled, so that the gainmedium 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 fordetermination of the DOE focal length is introduced. The results show that the relative focal lengths of thefour different elements available are in very good agreement with the design criteria. When the measuredfocal lengths are normalized to the measurement on the DOE with the longest focal length, and comparedto normalized design values, the agreement is almost perfect.When absolute focal lengths are considered, a discrepancy of 7-10% between the measured and intendedvalues is detected. This observation can probably be explained from systematic errors in the measurementsetup 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 theDOE 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 beentested. The estimated diffraction efficiency is found to be 77+/-5% (including one Fresnel reflection). Thevalue 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 ofa self-interferometric measurement method. The idea is to measure the interference pattern formed by theoverlapping diffraction orders from the DOE. A theoretical model of the DOE has been set up, which can beadjusted until the agreement with the measured data is as good as possible. In this way, the DOE profile canbe determined, and the efficiency can be simulated when the micro-scale shape of the DOE is known. Thismethod 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, aslong as their diffraction orders overlap spatially.Finally, the more robust DOE model mentioned above was used to simulate the behavior of the DOEfocal length as a function of the wavelength, thus verifying the paraxial model of the DOE. The paraxialmodel is shown to overestimate the DOE focal length by about 3%. Also, data from the interferometricmeasurement 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 designvalue. Hence, the combined results from this investigation and the previous focal length measurementindicate that the focal lengths of all four DOEs are very close to the intended design values.