Study Of Blue High-power GaN-based Grating-coupled External Cavity Tunable Laser

Along with the developments of semiconductor material growth and device fabrication technology,high-power GaN-based laser diodes(LDs)have been commercialized,and they have important applications in many fields.However,due to the influence of gain material and device structure,the output spectrum of the traditional edge-emitting semiconductor laser is wide,showing multi-longitudinal modes,and the lasing wavelength is not easy to be tuned.In order to meet the demand of high-power,tunable,and narrow-linewidth blue laser source for atomic cooling and trapping,high-resolution spectroscopy,and high-capacity holographic data storage,grating-coupled Littrow-type and Littman-type external cavity(EC)lasers have been constructed,and their performances are analyzed.The main work done in this paper is as follows:1.Based on a 450 nm-high-power commercial GaN-based edge emitting LD,a Littrow-type grating-coupled EC laser has been fabricated.Moreover,the influence of the placement of gain device relative to the grating,on the performance of EC laser has been studied.In the experiments,two kinds of EC configurations with the grating lines perpendicular(A configuration)and parallel(B configuration)to the p-n junction are evaluated.Good performance has been demonstrated for the EC laser with B configuration due to the better mode selection effect induced by the narrow feedback wavelength range from the grating.Under an injection current of 1100 mA with B configuration,the spectral linewidth is narrowed significantly down to?0.1 nm from?1 nm(the free-running width).Meanwhile,a tuning bandwidth of 3.6 nm from 443.9 nm to 447.5 nm is realized with output power of 1.24 W,EC coupling efficiency of 80%,and a higher than 35 dB-amplified spontaneous emission suppression ratio at the central wavelength.2.The influence of grating parameters,including the diffraction efficiency and ruling density,on the performance of high-power blue EC lasers has been studied.In the experiments,three gratings with different parameters are used as the light feedback elements in the EC laser system.It is found that the high zeroth-order diffraction efficiency of grating benefits the high output power,and the high first-order diffraction efficiency contributes to the wide wavelength tuning range.By using a grating with a zeroth-order diffraction efficiency of 76%and a first-order diffraction efficiency of 11.5%,the external light extraction efficiency can reach up to 80%with an output power of more than 1200 mW and a tuning bandwidth of 3.6 nm under an injection current of 1100 mA.When a grating with a first-order diffraction efficiency of 58.5%and a zeroth-order diffraction efficiency of 28.6%is used,the threshold current is reduced by 46%at the central Fabry-Perot resonance wavelength.Moreover,a tuning bandwidth of 8.3 nm ranging from 442.2 nm to 450.5 nm with an output power of more than 500 mW is achieved under an injection current of 1100 mA.Besides,we also find that the high ruling density is helpful in obtaining narrow lasing linewidth,especially for the EC laser system injected with low current.By using a grating with a ruling density of 2400 grooves/mm,the spectral linewidth of the EC laser can be narrowed to be 0.02 nm with an operating current of 300 mA.As the injection current is increased,the lasing linewidth is little affected by the ruling density of the grating due to the broadened feedback spectrum by the flattening of gain spectrum.3.A Littman-type EC laser has been constructed by using high-power GaN-based LD,diffraction grating,and reflecting mirror.The influence of incident angle of the output light from the gain device with respect to the normal of the grating,on the performance of the Littman-type EC laser is studied.The experimental results combined with the grating diffraction beam analysis show that,for the 450 nm-Littman-type EC laser,the incident angle in the range of 10.4 to 39.8° can effectively reduce the loss of the diffracted light,which is beneficial to obtain a good EC laser output performance.When the incident angle is selected as 35°,a waveleng tuning range of 3.1 nm from 453.7 nm to 456.8 nm is realized with output power of 260mW and spectral linewidth of 0.08 nm under an injection current of 1500 mA.