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PZT MEMS-VCSEL for precise and high-speed wavelength tuning

Precise wavelength control of light sources is necessary for applications such as 3D sensing, OCT, and spectroscopy. Tunable MEMS-VCSELs are attractive candidates because of high-speed modulation and low manufacturing cost. However, there are problems such as non-linearity of swept-wavelength and decrease in activation range over MHz-speed for conventional electrothermally and electrostatic-driven MEMS-VCSELs, which leads to performance degradation. In this paper, high linearity of wavelength sweep has been demonstrated by using MEMS actuators which consists of piezoelectric PZT layers and meander structures. The 940 nm MEMS-VCSEL consists of MEMS and bottom-emitting half-VCSEL. As for MEMS, two kinds of actuator are employed, a large meander structure and a small membrane structure. The meander structures are utilized to move dielectric DBR mirrors deposited on MEMS linearly with an applied voltage, and perpendicularly to the chip surface. Half-VCSEL has n-DBR, active layers inserted in spacer layers and several pairs of p-DBR that is located between an air gap and spacer layers on n-GaAs substrate. MEMS and Half-VCSEL are integrated by thermal bonding via bonding layers. The Coefficient of determination between an applied voltage and swept wavelength is over 0.99 with the large MEMS actuator. Also, modulation frequency over 1.3 MHz with 2.98nm wavelength sweep is obtained with the small actuator. These actuators overcome the trade-off between high-speed wavelength modulation and precise adjustment of wavelength compared to conventional MEMS-VCSEL. There are remaining challenges for further improvement. Modifying the stiffness of actuators and reduction in mass enables faster resonance frequency more than a ten MHz order.