Hyun Minji, Jeon Chan-Gi, Kim Jungwon
School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Korea.
Sci Rep. 2021 Sep 8;11(1):17809. doi: 10.1038/s41598-021-97378-1.
The phase noise of microwaves extracted from optical frequency combs is fundamentally limited by thermal and shot noise, which is inherent in photodetection. Saturation of a photodiode due to the high peak power of ultrashort optical pulses, however, prohibits further scaling of white phase noise by increasing incident optical power. Here we demonstrate that the photocurrent pulse shaping via balanced photodetection, which is accomplished by replacing a single photodiode with a balanced photodetector (BPD) and delaying one of the optical pulses, provides a simple and efficient optical-to-electrical interface to increase achievable microwave power and reduces the corresponding thermal noise-limited phase noise by 6-dB. By analysing contributing noise sources, we also show that the thermal noise floor can reach - 166 dBc/Hz even at a low photocurrent of 2-mA (4-mW optical input per photodiode) when using a p-i-n BPD. This finding may be useful for on-chip microwave generation, which consists of standard p-i-n structure photodiodes with relatively low saturation optical power.
从光学频率梳提取的微波的相位噪声从根本上受到热噪声和散粒噪声的限制，这是光电探测中固有的。然而，由于超短光脉冲的高峰值功率导致光电二极管饱和，限制了通过增加入射光功率进一步降低白相位噪声。在此，我们证明了通过平衡光电探测实现的光电流脉冲整形，这是通过用平衡光电探测器（BPD）取代单个光电二极管并延迟其中一个光脉冲来完成的，它提供了一个简单而有效的光电接口，以增加可实现的微波功率，并将相应的热噪声限制的相位噪声降低6分贝。通过分析相关噪声源，我们还表明，当使用p-i-n BPD时，即使在2毫安的低光电流（每个光电二极管4毫瓦光输入）下，热噪声本底也能达到-166 dBc/Hz。这一发现可能对由具有相对较低饱和光功率的标准p-i-n结构光电二极管组成的片上微波产生有用。
