Michael and Yuhang

Last week, we replaced two QUBIG PDs for FC GR lock. An RF output only PD is currently used for FC GR lock, which was found to have flat spectrum across tens of MHz as Fig.2. The QUBIG PD with DC and RF outputs is taken out and used for new OPO reflection, which has spectrum as Fig.1. Comparing Fig.1 and Fig.2, we can also find that the RF only PD has a better SNR. However, after a test of DC RF QUBIG PD for new OPO, we found it cannot read phase modulation taken from the reflection of OPO. Although from its specification, PD bandwidth (1-100MHz) is enough to detect 40MHz signal, we decide to not use it since we couldn't find by checking either spectrum directly or demodulated PDH signal. On the other hand, we will use it for monitoring DC response. Considering a BW of 100MHz, which has a very small rise time of 3.5ns.

A PDA05CF2 will be used for acquiring PDH signal for locking. This PD will be placed in the reflection of OPO. It has been proved to be able to provide large enough error signal.

We will use PDA36A-EC in the OPO transmission. If 0dB is enough to measure transmission signal, we can have 10MHz bandwidth, which gives 35ns rise time. However, NIR may have larger rise time since this PD is made of Si. According to Isogai paper, transmission doesn't need fast response. So maybe PDA36A-EC is enough. We will see this later. On the other hand, I took some dark noise measurement, which is summarized in Fig.3. The dB value in the legend shows different chosen gain of this PD. This measurement shows much less dark noise indicated in the specification of this PD. 0dB should be 300uV, 20dB should be 250uV, 40dB should be 340uV, 60dB should be 800uV. One issue is that the RMS I got is only from 10Hz to 3.2kHz. Higher frequency noise must be integrated but I didn't measure. But anyway, we can see that with a factor of 10 gain increase, the noise is not increased by the same factor.