980nm大功率垂直腔面发射激光器偏振特性

Polarization Characteristics of 980 nm High Power Vertical Cavity Surface Emitting Laser

大功率垂直腔面发射激光器单管器件出光口径大、横向模式多。随着注入电流和工作温度的改变出射光偏振态在两个正交偏振基态上转换。为分析输出光偏振特性,采用500μm出光口径980nm底发射器件,通过控制器件热沉温度,利用偏振分光镜分离正交偏振基态为透射波和反射波,半导体综合参数测试仪测量其功率、中心波长等参量。分析得出:两个偏振态的光功率温度特性与未加偏振分光镜时的总输出光的温度特性基本一致,中心波长差随温度升高缓慢增加。在温度低于328K时,随着注入电流的增大,反射波首先达到阈值,形成激射。但透射光波形成激射后其斜效率大于反射波。因此在达到某个电流后两个偏振态的功率变化曲线出现交替。当温度升高到328K以上时两个偏振态的功率曲线却没有明显的交替。根据对大尺寸VCSEL器件偏振特性的研究,提出通过外腔选频的方法来控制偏振的方案,分析计算后得出外腔腔长大约为0.45mm。

Vertical-cavity surface-emitting lasers （VCSEL） have many advantages such as low threshold current and single mode operation, as well as easy to be fabricated in high-density arrays and low cost in manufacturing. VCSELs are used in many areas, for example, optical communications, optical interconnections, and optical signal processing. But in some applications, the polarization orientation is sensitive, such as optic recording and laser frequency doubling. It is necessary to control the polarization characteristics of VCSEL effectively. In fact, the reliable control of the polarization orientation of single devices with small emission window is realized through many methods, in which the most popular one is the sub-wavelength surface grating technique. But due to the existence of multi-transverse modes in high power VCSEL devices with large emission window, the polarization orientation changes from one basic state to the other orthogonal one depending on the input current and temperature. The temperature dependence of the polarization characteristics of a 980 nm bottom-emitting laser with 500 μm emitting aperture is investigated. The temperature changes from 298 K to 368 K with a increasing step of 5 K. It is not easy to directly measure the temperature of the active region, so the temperature of the heatsink is used to characterize the temperature of the active region. A polarization beam splitter （PBS） element is used to split the two orthogonal polarization states into transmission wave and reflection wave, respectively. The output powers and center-wavelengths of these two orthogonal polarizations states are measured by using a semiconductor laser parameters test system. The temperature dependence of the output power of each polarization state is the same as that of the total output power of the device without the PBS. The center-wavelength difference between the two polarization states increases slowly with increasing ternperature. When comparing the polarization behavior of both states, we find that the reflection state reaches threshold of lasing before the transmission state when the device is kept at a temperature below 328 K. But the output power in the transmission state rises quickly than that in the reflection state. At every temperature, there is a certain current where the powers in the two states are equal with increasing the current. With increasing the current further, the power in the transmission state is higher than that in the reflection state. And when the de- vice is kept at a temperature above 328 K, there is no obvious alternation point between the two states, and the power in the transmission state is higher than that in the reflection state all the time. The reason may be that at low temperature and low current, the heat effect is not serious, the red shifts of the reflection wave and transmission wave are both small, but reflection one is a little more, so the reflection wave is closer to the center-wavelength of DBR. With increasing the current, the internal temperature of the device increases seriously, the red shift of reflection state increase faster than that of transmission state, so the transmission wave is closer to the center-wavelength of DBR. At high temperature the heat effect is serious, whatever at low current and high current, both of the red shifts are big, so the transmission wave is closer to DBR center-wavelength all the time. According to the detailed investigation on the polarization characteristics of large diameter VCSEL device, a method with an extemal resonator was proposed of about 0.45 nun. to realize a stable polarization orientation with a resonance Key words： vertical cavity surface emitting laser （VCSEL） ; PBS; polarization basic state length of about O. 45 ram.