894 nm high temperature operating vertical-cavity surface-emitting laser and its application in Cs chip-scale atomic-clock system

Abstract

In this study, an 894 nm high temperature vertical-cavity surface-emitting laser (VCSEL) is reported. Furthermore, a Cs chip-scale atomic clock (CSAC) system experiment based on this VCSEL is carried out.To achieve low threshold/power consumption under high temperature condition, the VCSEL epitaxial structure is optimized. Especially, the so-called gain cavity-mode detuning technology is utilized to improve the temperature sensitivity of the device output characteristics. The relationship between the structure of quantum well and the gain is simulated by using the commercial software PICS3D. In order to achieve high gain and low threshold properties, the thickness of the quantum well is optimized. Based on the theory of transmission matrix, the VCSEL cavity mode (etalon) is calculated. Finally, a -15 nm quantum well gain-cavity mode offset is utilized to achieve relatively stable cavity mode gain, which can guarantee the temperature-insensitivity of the VCSEL output characteristics.The output performance of the VCSEL device we fabricated is investigated experimentally. The VCSEL lightcurrent (L-I) characteristic is tested under different temperatures. It is found that benefiting from the gain-cavity mode offset design, the threshold can be maintained at 0.200.23 mA when the temperature increases from 20 ℃ to 90 ℃. Meantime, the output power of more than 100 W is achieved at different temperature levels. By comparing with the results at room temperature, No dramatic degradation of the VCSEL high temperature L-I characteristics is found, which means that the VCSEL output characteristic is relatively temperature-insensitive. The wavelength of the VCSEL is 890.4 nm at a temperature of 20 ℃. When the temperature increases up to 85.6 ℃, the VCSEL wavelength is red-shifted to 894.6 nm (Cs D1 line), corresponding to a red shift ratio of 0.064 nm/℃. According to the polarization requirement of CSAC applications, the polarization properties of the VCSEL are studied and the results are as follows: under an injected current of 1 mA and operation temperature of 20 ℃, Pmax = 278.2 W and Pmin = 5.9 W, corresponding to a polarization ratio of 47:1; at a temperature of 85.6 ℃, Pmax = 239.2 W and Pmin = 4 W, corresponding to a polarization ratio of 59:8:1.Using the VCSEL reported in this paper as a laser source, the CSAC experiment is carried out. At 4.596 GHz of modulated frequency, the output laser of the VCSEL is collimated and interacts with Cs atoms. Finally the closed-loop frequency locking atomic clock is demonstrated. The Cs laser absorption spectrum for laser frequency stabilization, as well as the CPT signal for Cs CSAC microwave frequency stabilization is obtained.