Conventional test of the peak wavelength of a laser used to be applied immediately after a device is injected current. However, the results can not be considered as an accurate description to temperature characteristic. This passage puts forward a concept of relaxation time in wavelength texts, mainly based on the experiment of 940 nm strain quantum well laser, confirming that under constant current, wavelength will get through a process of rising, and then, reach the limit. This process brings the effect on spectral characteristics of a device which cannot be ignored and the accumulated heat in relaxation time will gradually impact the emission wavelength of the laser, even crest split to form bimodal phenomenon.
 G. R. He, W. J. Shen, Q. Wang, W. H. Zheng and L. H. Chen,“Temperature Characteristics of 980 nm High Power Vertical Cavity Surface Emitting Lasers,”Infrared and Laser Engineering, Vol. 1, 2010, pp. 57-60.
 H. W. Qu, X. Guo, L. M. Dong, J. Deng, X. L. Da, Z. T. Xu and G. D. Shen, “Study on the Temperature Characteristics of Vertical Cavity Surface Emitting Laser,” Infrared and Laser Engineering, Vol. 2, 2005, pp. 83-86.