This page serves as a hub for information on the MEC LPL seed laser and pulse shaping. This is a living document.
Overview
- Continuous-wave (CW) output from NP Photonics "The Rock" is coupled by fiber to a lithium niobate-based electro-optic modulator (EOM)
- EOM provides amplitude shaping to slice up the CW input into seed pulses at 10Hz repetition rate; seed pulses can be up to 35ns in duration and require various shapes in order to provide the types of pulse needed on-target
- Pulse shape/duration is defined by the arbitrary waveform generator (AWG), which can define 140 equally-spaced amplitude heights over a 35ns window (→ spacing of 250ps/pixel); heights are chosen to pre-compensate for the amplitude distortion arising due to gain saturation effects throughout the rest of the system
Further details
Product info here (rack-mounted version here)
Brain dump:
- We have two of these, with the rack-mounted version serving as back-up; located in rack 64B; the back-up has been tested for functionality previously, so it should be plug and play
- These can be wavelength tuned, but it's a little finnicky and laggy (especially on the rack-mounted version, which is tuned using a pot on the front of the unit), so it's best not to touch them; in the ancient past, these units got shuffled around with the factory supposedly in order to get the tuning range set appropriately for the system it would be seeding
- There is a way to read out information from these units from the back panel, but there is no way to actually control these units from the back panel – no hope for writing an IOC
- Only plugs in the back should be power and interlock
- System should show temp stable and second mode suppressed or else there will be major performance/stability issues; depending on how long it's been off or how unstable the environment is, this may be fine right away or it may take ~30-60min
- Front display should show ~135-140mW and 1053.045nm when running
- Past experimenting didn't show any effect of using ACC vs. APC mode nor RIN suppression on vs. off
- Upgrade ideas:
- This is essentially the only laser of its kind I know of that doesn't use a fiber pre-amplifier somewhere before the free-space amplifier. Sometimes this is located before or after the EOM (or both), but it can provide way better performance this way.
Representative/historical pictures:
Product info here
Brain dump:
- EOM is mounted on the mezzanine inside the Highland T400B AWG, with fibers coupled to the back panel.
- Besides the two fiber connections, the EOM has three electrical connections too:
- Shaped RF input taken from the output of the T400B's AWG output
- Bias voltage input taken from the output of the MBC
- On-board photodiode signal out, which is usually not plugged in but its BNC connector is hanging out loose on the front side of the T400B; not very useful except when doing basic troubleshooting to see if there's any energy coming through the EOM at all
- The input power is rated to be <100mW, but they've said there shouldn't be any problem putting in what we've been doing. They aren't a particular useful lot when it comes to getting troubleshooting advice from them, but often it centers on trying to attenuate a bunch to see what happens – we've just found work-arounds for our problems instead, but maybe that could be done differently in the future with more time and effort.
- Once upon a time, Michael Greenberg got a TEC for temperature stabilizing the unit to help it run for stably, but especially after relocating the T400B into the controls rack the performance doesn't seem to be affected too much so we never put it together, even though Marcio or someone of the like wrote an IOC for the TEC at some point
- The extinction is supposed to provide at least a polarization contrast of >1000:1; however, we have tried measuring this in the past as we've noticed that there's more light than normal sneaking through the EOM when it's "off" and causing a pre-pulse/pedestal between the slow opening of the Pockels cell in the YFE and the true rising edge of the laser pulse. We don't have a good way to measure the amount of leakage at very low power levels, so I would calibrate the strength of signal at full throughput (MUCH lower than the ~140mW supposedly getting injected into the EOM) with a power meter, a CCD, and a calibrated filter attenuator. From there, I could estimate that at the zero output case the attenuation was closer to 23dB than >30dB.
- We used this measurement also to estimate the amount of pulse energy actually making it out of the EOM – it's probably on the 100pJ-scale.
- Upgrade ideas:
- Many other facilities use two-stage EOMs instead of single-stage. In this case, there's a second input that can be used with a high-bandwidth square pulse (for example) to sharpen up the rising and falling edges of the optical pulse AND to improve the polarization extinction on the front edge. In fancier cases they can pull tricks to make a pulse that already has a bit of a rising edge in order to not waste pixel depth of the AWG on the front side of the pulse (i.e. if you know the rising edge needs to be at 10%, if your pulse starts at 12% then you have a lot more pixel range to tune with than if you start at 100% and need to get down to 10%).
Representative/historical pictures:
