Research On External Cavity Feedback Spectrum-improvement Of High Power Semiconductor Lasers

With a limited spectrum characteristics and beam quality, the semiconductorlasers with high power is handicapped in its application, such as cooling atoms,magneto-optical trap atoms, Raman spectroscopy and diode laser pump solid lasers.In order to meet the strict requirements of scientific studies and practical utilization,several improving techniques were developed in the last thirty years. External-cavityfeedback can significantly compress the spectrum width, stabilize the wavelengthand realize the wavelength continuous tuning.The study was aimed to understand the influence of external-cavity feedbackon high power semiconductor lasers spectrum characteristics, explore theexperimental conditions of realizing spectral width compression and wavelengthtuning. For adopting external-cavity feedback to improve the output properties ofhigh power semiconductor lasers laid a solid foundation.Firstly, this thesis established the rate equations for describing a semiconductorlaser optically isolated from the outside world, which are derived from the Maxwellequations. Then we extended the rate equations to the case of optical injection andfeedback by considering the Fabry-Perot resonator model. By means of numericalcalculation, we theoretically analyzed the impact of optical injection parameters andfeedback parameters on diode laser dynamics and output properties.Finally, based on the studies on improving output characteristics of diode lasersby means of optical feedback, we used two experimental schemes as followed toimprove the spectral characteristic: blazed grating full-feedback, two externalcavities feedback consisting of blazed grating and conventional plane mirror. In thecase of grating full-feedback, we studied the wavelength tuning range and the effectof injection current, feedback rate and external cavity length on the spectralproperties of a Broad-Area diode laser. In this scheme, when the injection currentand external cavity length were set to2A and34.50cm, the spectrum width wascompressed to0.28nm from1.3nm; the spectrum width compression was21.54%.The drift rate of wavelength with current was0.29nm/A, comparing to0.59nm/A ofthe free running, the stability of wavelength was doubled. In addition, we alsorealized wavelength continuously tuning with range of3.07nm. In the scheme of twocavity feedback, when the two cavity length were quite different and the injectioncurrent was greater than3A, two peaks wouldl appear in diode laser’s spectrum. Butwhen the difference is diminishing, we achieved a good spectrum characteristic.Beyond this, we also studied the relationship between spectrum and the ratio ofplane mirror feedback in this scheme. We found that the effect of the two cavity feedback was proportional to the feedback ratio of plane mirror. Although we canimprove the spectrum of the diode laser through these two programs, the low outputpower of the zero-order diffraction grating was not conducive to practical utilization.So we based on the research on the two above schemes, we proposed anotherprogram, the grating half-feedback. In this program, we improved the spectrumcharacteristics of semiconductor laser efficiently. At the same time, we got a higheroutput power. In this part, we studied the effect of the cut-ratio of plane mirror andthe cavity length on the spectrum. As shown in experimental results, when the ratioof plane mirror feedback and the cavity length were set respectively to1/2and34.50cm, we got the most efficient spectrum width compression. Under thiscondition, when the injection current was varied from1A to3.5A, the wavelengthdrift rate was0.20nm/A, it was more stable than that of grating full-feedback. Theexperimental results indicated that the grating half-feedback not only given a betterimprovement of the spectrum properties but also significantly increased the outputpower. In this program, when the injection current was set to2A, the spectrum widthwas compressed to0.28nm, the wavelength continuously tuning range was5.18nmand the output power reached321mW, when the injection current was set to4.5A,the spectrum width was compressed from1.55nm to0.6nm, and the output powerreached988mW.