Effect of Packaging Stress in a Laser Chip on Its Radiation Spectrum

A wavelength division multiplexing system is a basic infrastructure for the next coming optical fiber network. Since the band gap of a laser chip that determines the wavelength of a radiated laser beam is varied by strain in a laser chip, packaging-induced stress may change a radiation spectrum of a mounted laser chip. The final residual strain in an active layer of a laser chip varies significantly depending on the thin film deposition process for interconnections and mirrors, and thus, changes the radiation spectrum of the laser chip. Thus, the change of residual stress caused by packaging was analyzed by a finite element method. The estimated change reached about several hundreds MPa depending on materials of a substrate, adhesion material, and deflection of the package. In order to confirm the effect of such an additional strain on a radiation spectrum of a laser beam, mechanical stress (strain) was applied directly to a laser chip and the change of the spectrum of the laser chip under the applied strain was measured. A four-point bending test was used for applying mechanical stress to a laser chip. It was confirmed that the radiation spectrum of a laser chip, such as wavelength and width of the spectrum, changes almost linearly with the amplitude of the applied uni-axial strain. The wavelength of the laser chip decreased and a width of the spectrum of the laser chip at the threshold current increased with the increase of the magnitude of the applied strain. The change rate of the wavelength was about 0.4-nm/micro-strain. Therefore, it is very important to control the final residual stress or strain in an active layer of a laser chip to assure the precise optical characteristics of the laser chip.