Study On The Near-infrared External Cavity Semiconductor Laser Of Littrow Configuration

Lasers have important applications in many fields including atomic physics,high resolution spectrum,coherent optical communication and laser radar,which require that the output beam of the laser can be tuned to one or more specific wavelengths.Therefore,tunable semiconductor lasers with excellent output characteristics have become indispensable optical devices.Monolithic integrated lasers such as distributed feedback(DFB)lasers,distributed Bragg reflection(DBR)lasers and vertical cavity surface emitting lasers(VECSL)achieve tunable wavelengths through temperature or injection current.The tunable range is only several nanometers and it is difficult to achieve continuous tuning over a wide range.Tunable external cavity semiconductor laser(ECDL)can effectively avoid the above problems.By tuning the optical element in the external cavity,the maximum continuous tunable range can be up to several tens of nanometers.And the linewidth of the output beam is effectively narrowed.The external cavity semiconductor laser has the advantages of simple structure,wide tunable range,high photoelectric conversion efficiency and high side mode surpression ratio(SMSR).An external cavity semiconductor laser of Littrow configuration with different output bands has been fabricated in our paper.By designing the special structure of gain chip or a tunable optical element,the output characteristics of the laser are optimized,which provides the theoretical basis and experimental evidence for the extensive applications of tunable external semiconductor lasers.The specific research content and experimental results are as follows:(1)The InGaAs/GaAs dual quantum dots are fabricated by epitaxial growth with a waveguide structure of a gradient refractive index separation limit heterostructure.Based which,the quantum dot external cavity semiconductor laser is fabricated.The laser can achieve the first excited state excitation with high output power.We attribute the excellent output characteristics of the QD ECDL to carrier radiation recombination in high energy states,which has higher degeneracy than low energy states and can accommodate more carriers.When the injection current is 500 mA,the maximum output power reaches 120 mW and the tunable range is 28.9 nm(970.1 nm~999 nm).The spectrum linewidth is 0.2 nm at the output wavelength of 988.3 nm,and the SMSR is 35 dB.When the tuning wavelength of the quantum dot external cavity semiconductor laser is 982.6 nm,the threshold current is the lowest and the minimum threshold current is 2.75 kA/cm~2.With the diffraction grating position unchanged and the current increasing gradually,the quantum dot external cavity semiconductor laser has good wavelength stability.When the tuning wavelength is995 nm,the wavelength change rate is about 0.7 nm/A.(2)We reaserched the influence factors on the output characteristics of 1550-nm Littrow outer cavity semiconductor laser.Firstly,we rotated the grating along the axis of the incident beam,so that the polarization direction of the beam changes from vertical to the groove direction of grating(strong feedback)to at a certain angle with the groove direction of grating(weak feedback).The rotation of the grating reduces the first-order diffraction efficiency and the intensity of the feedback light.It was observed from the experiment that the output light spectrum has obvious residual feedback under the condition of strong feedback.The residual feedback leads to a low SMSR of the output light at low injection current.The maximum SMSR is only 47 dB.The residual feedback phenomenon can effectively eliminate by polarization mismatch of the weak feedback mode.The SMSR reaches 54 dB.Both output modes can achieve single longitudinal mode and wide range tunable.At an injection current of 300 mA,the maximum tuning range of the weak feedback mode is 130.9 nm,and the minimum threshold current is 84 mA.Due to the weak optical feedback intensity,the maximum output power is 5.5 mW.The maximum tuning range of the strong feedback mode is 161.2 nm,the minimum threshold current is 50 mA,and the maximum output power can reach 49.9 mW.We believe that the weak feedback mode will benefit many applications,where clean and pure single-frequency optical spectrum with wide wavelength tunable range is needed.Secondly,the effect of grating constant on the performance of external cavity semiconductor lasers is researched.When the parameter of the grating is increased from 600 lines/mm to 1200 lines/mm,the SMSR of the laser is increased from 47 dB to 65 dB,and the tunable range of the laser is increased from 160.1 nm to 209.9 nm.The linewidths of the two lasers were 0.07 nm and 0.05 nm,respectively.The larger the groove density of the grating,the larger the angular dispersion,which significantly improves the grating resolution.The external cavity semiconductor laser has higher SMSR and wider tunable range.