Double Trapezoidal Multimode Interference Coupler Used to Improve Electro-Optical Efficiency of Semiconductor Laser
Objective An active multimode interference(MMI)semiconductor laser(DTM-LD)with a double-trapezoidal multimode interference(DT-MMI)coupler structure is designed and fabricated.Based on the self-mapping distribution reconstruction effect of a double-trapezoidal MMI multimode waveguide,the laser has a shorter device cavity length while maintaining the same active area as the traditional rectangular active MMI laser(MMI-LD),which is beneficial for improving the electro-optical conversion efficiency of the device.A DTM-LD device with a double trapezoidal base width of 15 μm and a waist width of 30 μm is fabricated experimentally.When the active area is approximately 3.33×104 μm2,the cavity length of the laser device decreases by 4.2％compared to that of the MMI-LD.When the injection current is 1 A,the electro-optic conversion efficiency of DTM-LD increases from 14.5％to 16.3％,a relative increase of 12.4％,and its maximum output power slightly increases from 355 to 360 mW.When the width of the MMI multimode waveguide is reduced by 0.5 μm,the output power loss of DTM-LD reduces by 21.6％from 51 to 40 mW compared with that of the MMI-LD device,indicating that DTM-LD has lower sensitivity to manufacturing error.Methods A double-trapezoidal multimode interference coupler structure was developed,and it was found that the length difference between the rectangular MMI and DT-MMI could be increased by increasing the difference between Wbot and Wmid while maintaining the area of the multimode waveguide unchanged,as shown in Fig.3(a).However,the insertion loss of the device also increased significantly when the maximum light intensity was the output,as shown in Fig.3(b).The sensitivity of the output light intensity of the DT-MMI structure to manufacturing errors was less than that of the rectangular MMI structure when parameters Wbot and Wmid were changed owing to process errors.This provided a basis for parameter selection of the device.Therefore,the DT-MMI region was designed with a base width Wbot of 15 μm,waist width Wmid of 30 μm,length L of 1436 μm,single-mode waveguide width of 5 μm,and length of 100 μm;the active area of the device was approximately 3.33×104μm2.Semiconductor laser devices(DTM-LDs)had a waveguide structure etching depth of 1.4 μm.For comparative analysis,an MMI-LD device with a width of 21.7 μm and a length of 1508 μm with the same active area was fabricated for subsequent testing.Results and Discussion The DTM-LD and MMI-LD are tested and analyzed at a room temperature of 20 ℃.Fig.5 shows the power-current-voltage(P-I-V)characteristic curves of the DTM-LD and MMI-LD.It can be seen that the electro-optic conversion efficiency of DTM-LD slightly increases to 16.3％,which is 12.4％higher than that of the MMI-LD,indicating that reducing the cavity length of the active MMI semiconductor laser can improve the electro-optic conversion efficiency.Fig.6 shows the spot distributions of the MMI-LD and DTM-LD devices at 0.06,0.5,and 1 A.When the injection current increases from 0.06 to 0.5 A,both the MMI-LD and DTM-LD can maintain near-single-lobe output,as shown in Figs.6(a),(b),(d),and(e).However,there is a small sidelobe distribution on both sides of the main lobe of the far-field spot of the two laser devices because some photons from the input end directly reach the output end without interference gain from too many mode interference couplers,forming a Fabry Perot gain.With an increase in the injection current,the light spot exhibits an apparent segmentation phenomenon,as shown in Figs.6(c)and(f).Fig.7 shows the drift characteristic curve of the peak wavelength of the device at different operating temperatures.The wavelength temperature drift coefficient of the MMI-LD is 0.24 nm/℃,and the wavelength temperature drift coefficient of the DTM-LD is 0.23 nm/℃,indicating that the DTM-LD can maintain the same temperature drift performance as the MMI-LD.However,during the rise of 17℃ to 19℃,the refractive index of the MMI material changes,resulting in a significant increase in the central wavelength of the laser,which increases the temperature drift coefficient of the device.Fig.8 shows the relationship between the device output power and the variation in the multimode waveguide structural parameters in the presence of process errors.The figure shows that when the parameter changes by 0.5 μm,the maximum output power of the MMI-LD device decreases by 14.4％from 355 to 304 mW.The output power of the DTM-LD device decreases from 360 to 320 mW,a decrease of 11.2％.Compared with the MMI-LD,the output power of the DTM-LD decreases from 51 mW to 40 mW,a reduction of 21.6％.Therefore,the output power of the DTM-LD is less sensitive to manufacturing errors than that of the MMI-LD.Conclusions The mechanism of the double-trapezoidal MMI is analyzed theoretically to reduce the length and sensitivity to manufacturing errors,and a new type of active MMI semiconductor laser with a double-trapezoidal MMI structure is designed and fabricated.The test results show that the cavity length of the laser device decreases by 4.2％compared to that of the MMI-LD while maintaining an active area of about 3.33×104 μm2.When the injection current is 1 A,compared with MMI-LD,the electro-optic conversion efficiency of DTM-LD is increased from 14.5％to 16.3％,a relative increase of 12.4％,higher than that of the MMI-LD devices,and its maximum output power slightly increases from 355 mW to 360 mW.When the overall width changes by 0.5 μm,the output power loss of the DTM-LD decreases from 51 mW to 40 mW,which is 21.6％lower than that of the MMI-LD device,indicating that the DTM-LD has lower sensitivity to manufacturing error and lower process difficulty and preparation cost.
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