Confused about reflected signal in a bow-tie optical cavity

Hello! I am building a bow-tie optical cavity, of finesse about 10,000. When I measure the transmitted signal out of the cavity, after aligning it, I see clear peaks, significantly higher than the background and pretty much single mode. I do see 2 other peaks, probably from other modes, but much smaller than the main one.

Also I am looking at the mode on a CCD camera and it is T##_{00}## as expected. (The linewidth of the peaks is 5-10 larger than expected theoretically so that is something I am still trying to figure out).

However, when measuring the reflected signal, I see a very very tiny (almost non-existent) signal (I can upload some pictures tomorrow). For example, the voltage on the oscilloscope due to the reflected light is flat around 500 mV, while the dip associated to the peak is barely 2-3 mV smaller. So basically my dip is ~0.5% of the reflected signal. Ideally the dip should be 100% (i.e. the dip should go down to zero). In practice I would expect something around 50-80%, definitely a lot more than what I am seeing.

It kinda looks like I am almost not coupling at all into the cavity, but then why do I get such a clear transmission signal? I didn't optimized the mode matching a lot, but I did try to make it as good as I could by maximizing the transmission peaks (I decided to look at the reflection signal after doing this) and I would definitely have expected a much higher value. Also, if I was not mode matched, I would have expected to see higher order modes in the transmission (on the CCD camera), too.

Does anyone have any idea about what is going on or how can I fix it (or at least how to properly investigate what is going on)?

EDIT: I am attaching below a picture of the signal on the oscilloscope (blue is the transmitted signal and blue is the reflected). At this scale (200 mV per square), the reflected signal is not visible at all.

Malamala said:

EDIT: I am attaching below a picture of the signal on the oscilloscope (blue is the transmitted signal and blue is the reflected). At this scale (200 mV per square), the reflected signal is not visible at all.

View attachment 325582

Maybe @Twigg can help?

Sorry for the late reply.

I have bad news for you. I take you've optimized your alignment and played with the beam diameter. Assuming that, the reason you see a low reflection contrast (the height of the dip in reflection) is most likely the same reason you have a broad linewidth.

What you're seeing is a large optical loss inside the cavity (or at one of the cavity mirrors). This affects both linewidth (finesse is approximately 1 / (loss + transmission)) and the coupling. Keep in mind that 10,000 finesse cavity is quite sensitive to small losses.

For example, a loss of 1 part in 10,000 is enough to halve your finessse. 1 part in 10,000 corresponds to a single piece of dust with a diameter of 10 microns landing on a cavity mirror where the beam diameter is 1mm. (This is because the loss from scattering would scale as the cross-sectional area of the dust particle).

You likely need to get your cavity mirrors cleaned.

One way to test this theory is to try to couple to higher order TEM modes (or LG modes, depending on your system). If you see that your linewidth is different on different TEM modes, you likely have particulates on your cavity mirrors.

A quick follow-up on this. I was just playing around with a cavity today, and found that compressed nitrogen can actually work for removing dust from cavity mirrors (it wasn't perfect, but it had an effect). I would avoid dust-off out of fear of spraying non-volatile residue on the mirrors.

For sure, be careful. In the worst case scenario, you could blast a piece of hard dirt into the mirror surface and scratch it. Then your cavity mirror will be truly dead. I would start by blowing the air from a distance and using an oblique angle. Good luck!

Sorry, one final final follow-up. Since you have a bowtie cavity and only 10,000 finesse, your mirrors are probably on kinematic mirror mounts, yes? Sorry, I'm used to much more annoying cavities where the mirrors are optically contacted onto rigid spacers, so a cleaning or re-contacting requires an expensive procedure done at a professional optics shop with the appropriate facilities. You're not in that situation though, so you have more options for repair.

If you're afraid of cleaning the mirrors (and that is a _totally_ legitimate fear), you can try rotating the mirrors in their mounts. That should hopefully move the dust out of the TEM00 mode.

Also, since you operate at high power, your issue might be a burn instead of dust. I know nominally you have probably designed the cavity to operate without damage at the intended power. However, with cavities, sometimes stuff just happens and you need to put your tinfoil hat on. Example: F = 13000 build-up cavity (see table II). This cavity should have been able to handle >4W input power, and yet its mirrors burned on two occasions after inserting 1.7W and 3.5W respectively.