[Aritomi, Yaochin, Yuhang]
Yesterday we found that loss from dichroic mirror is 6.7%, so we investigated loss of dichroic mirror. We used two dichroic mirrors: one is HBSY11 from thorlabs which we are using and another one is from newport.
We measured BAB transmission of dichroic mirror and tried to minimize it. To remove green generated by BAB inside OPO, we put OPO temperature in the region with no parametric gain. This time we set 9.36kOhm of OPO temperature. We tweaked angle of dichroic mirrors a lot, but reflection power almost didn't change. By looking at transmission of dichroic mirror, we somehow found the minimum point and measured loss. We measured BAB power in two ways: one is to scan OPO and measure the BAB peak height, another one is to lock OPO by hand. For both cases, we had still ~3% loss from dichroic mirrors.
To characterize HBSY11, we checked reflectivity of HBSY11 with LO. We measured reflection from HBSY11 with power meter and PD. The result is as follows. Reflection power with power meter didn't depend on angle of HBSY11, but reflection power with PD changed from 380mV to 392mV by tweaking angle. Note that distance between HBSY11 and PD is 4cm.
| power meter | |
| injection | 129 mV |
| reflection | 128 mV |
| reflectivity | 99.2% |
| PD | |
| injection | 384 mV |
| reflection | 380 - 392 mV |
| reflectivity |
99 - 102 % |
For both cases, reflectivity of HBSY11 is consistent with specification which is 99.3%. From these results, we concluded that dichroic mirror (HBSY11) is fine.
Then we put HBSY11 back to after OPO and measured reflection and transmission of HBSY11 at the same time. The result is as follows and shown in Pic.1. The sum of reflection and transmission doesn't match with injection just after OPO.
| injection (just after OPO) | 429 uW |
| reflection | 408 uW |
| transmission | 0.14 uW |
| injection (after removing dichroic mirror) | 399 uW |
We thought some scattering light comes from OPO and the scattering light is also measured just after OPO. So we removed dichroic mirror and measured OPO transmission at further point to remove the scattering as shown in Pic.2. BAB transmission after removing dichroic mirror is 399uW. BAB transmission is lost by 7% between two points. The loss seems scattering from OPO, but we are not sure the reason. Yuhang will check if the aperture of power meter is large enough.
Conclusion: Dichroic mirror is fine. Loss is coming from OPO transmission.
