2D Electronic Spectroscopy Using Rotating Optical Flats

Paper describing the 2D spectrometer design was published in The Journal of Physical Chemistry A (DOI: 10.1021/acs.jpca.0c00285). The accepted version of the manuscript can be downloaded here, along with the supporting information.

2D Electronic Spectrometer Layout
Layout of the 2D spectrometer. CM: concave mirrors, DO: 2-dimensional diffractive optic, ND: neutral density filter, DG: delay glass, CW: chopper wheel, ROFs: rotating optical flats, BB: beam block. The positions of the laser pulses are labeled with kn and LO. The third-order signal (red line) is emitted from the sample on the same path as the LO (dotted line).

Power supply replacement for Spectra-Physics Millennia Prime J80

Our Millennia Prime stopped working, tripping the circuit breakers. I’m hoping it’s just power supply issues, and it’s out of warranty, so let’s take a look. Just don’t think about how much this thing costs…

The Spectra-Physics J80 laser diode pump for the Millennia Prime unit, minus top panel.

Getting to the power supply unit doesn’t look like fun… It is mounted on the bottom left, underneath a couple of layers of circuitry. At this point it’s impossible to even see what sort of unit it is.

Let’s fast forward through the disassembly and see what it is. This is somewhat awkward and time consuming, but not hard once you know how. (Follow in reverse to see how to do the disassembly.)

That looks a bit scary, but the power supply is out!

So it’s a commercial unit, a Vicor PFC MegaPAC.

Vicor PFC MegaPAC power supply unit from the Spectra-Physics J80 diode pump laser for the Millennia Prime.

These units are customisable, using a switch mode front-end to produce 300 V DC, which is then converted down to the required voltages using up to 8 DC-DC converter ModuPAC modules. These modules are easily removable.

Vicor ModuPAC modules used in the Spectra-Physics J80 diode pump laser for the Millennia Prime.

The unit is set up to use two high-current 3.3 V rails to run the two laser diodes, and a single 24 V rail to run the thermoelectric cooling system and other electronics. The “B” in the model name indicates they are a “current booster” module, and are connected in parallel to the equivalent non-B module. Note that the J40 system only has a single diode, so I would expect it to only have one 3.3 V rail, and perhaps one fewer of the 24 V boosters.

So the Vicor PFC MegaPAC can be bought new, but they are pretty expensive. We’re looking at over $4000 to replace this unit as configured, with at least a 6-week wait time. The DC-DC converter modules are probably fine though, and can be swapped into another front-end… and there’s a few options on eBay. We ended up getting one shipped from the US for just over $300 (yes, about a third of that was shipping costs). The DC converters swapped over fine and a quick check with a multimeter indicates it’s working OK and no longer tripping the circuit breakers.

So here’s the reassembly (the disassembly process is just the reverse of this). Bolt the power supply back into the chassis from the side and bottom.

The Vicor PFC MegaPAC is bolted into the J80 frame from the side and underneath.

Attach the DC terminals and the voltage trim pot connectors (the small Molex plugs with red and black wires).

DC connections to the Vicor PFC MegaPAC. Two 3.3 V rails and one 24 V rail. The bus bars connect the booster modules to the matching master modules.

Screw in the mounting bracket for the diode driver circuitry onto the top of the power supply.

Mounting bracket is screwed onto the top of the power supply.

Mount the diode driver circuit boards and plug in the connectors at the back. The black foam is to seal this chamber to direct airflow through the power supply and heatsinks on the diode driver boards. Try not to destroy it during disassembly!

The two diode driver modules of the J80. The J40 would just have one of these.

Connect the high-current 3.3 V lines and diodes to the driver boards. Screw in the ground connection to the power supply, and plug in the connector on the left (this is the connection to the logic board to independently switch the 3.3 V rails on or off).

3.3 V and diode connections to the driver boards.

Fit the rear fan assembly using the nuts on the bottom, then re-attach the back panel. The fibre optic cables are armoured, but be careful here and don’t twist or kink them.

Back panel and fan assembly fitted to the J80.

The logic board is then fitted on top of the diode driver boards. Connect the various Molex plugs.

Logic board fitted to the J80.

Note that the rear panel DB-15 connector and the black wires (bottom-right and right side of the above photo, top-right and top of below photo) are actually attached to the front panel. The DB-15 needs to be unscrewed from the rear panel during disassembly.

Logic board fitted to the J80, showing the single-board computer sub-board and 24 V DC-DC power supply module.

As an aside, you can see the single-board computer and 24 V power converter here. That cylinder on the right is a PS/2 keyboard socket. We’ll resist the urge to try hacking it…

Reattach the front panel and connectors to the power switch/power supply, LCD and indicator diodes. The red switch part of the key lock pops out of the lock assembly.

Front panel connections to the J80.

Now put on the top panel, plug it in and fire it up. Yes, it is now working perfectly again!

Delay Calibrations

Here is a quick rundown on how the delay stages are calibrated to relate the rotation angle to the delay time of the laser pulse.

An interferometer is built by inserting a mirror/beamsplitter combination after the delays to interfere pairs of the four beams. A Helium-Neon (HeNe) laser is used as the light source, as the atomic emission at 632.8 nm is very well defined — a requirement to make a nice interference pattern and for use in the calibration fit equation.

Delay Calibration - HeNe Laser
A HeNe laser is used for calibration of the delays.

Delay Calibration - Vertical Pair
Three of four HeNe laser spots used for delay calibration.

Here we show the vertical configuration of the beamsplitter/mirror combination, which interferes the top and bottom beams, directing them downwards towards the table. The beam is directed to a fibre-optic which feeds the spectrograph and camera.

Overview of back end of interferometer used for delay calibration, in the vertical configuration.

I think this is the first look here at some of the spectrometer software! The intensity at the 632.8 nm laser line is collected as a function of one of the rotational stage angles. The rotational stage is swept between 0° (glass perpendicular to beam) to around 45°.  An interference pattern is generated.

Delay calibration interference pattern and fit.

The grey trace is the raw intensity data as a function of rotational stage angle. The yellow trace is with a Savitzky–Golay noise filter applied. The red trace, and the yellow curve in the bottom panel show the theoretical angle-to-delay relationship. As the data is collected, or when the “Fit” button pressed, the fit is performed and the three fitting parameters (α, β, γ) are determined. The fit can also be adjusted in realtime as the parameters are changed manually.

2D spectrometer software - delay calibration
Zoom into part of the collected delay calibration interferogram.

The theoretical model of the glass angle vs delay match the data very well across the entire range of rotation angles. That’s good, as it’s pretty fundamental to the whole spectrometer design!

Spectrograph and Camera

This is the back end of the spectrometer arms. The sample vial is visible, as well as the mirrors and lens used to direct light from the sample into the detector. The spectrograph is an Andor Shamrock, with an Andor Newton CCD camera as the detector. This is able to capture 500 spectra per second.

Spectrograph and Camera
The output end of the spectrometer, showing the sample and the Andor Shamrock spectrograph and Newton camera used as the detector.

2D Spectrometer Delay Stages

These are the three pairs of rotational stages and glass used to control the arrival times of the laser pulse sequence for the 2D spectrometer. The stages are computer controlled, and can rotate with an accuracy of one-thousandth of a degree. By rotating the glass, the effective thickness increases, delaying the laser pulse with control on the attosecond time scale. A pair of glass plates are used for each beam, rotated at opposite angles to correct for the path deviation.

Delay stages
The input side of the spectrometer, showing the three pairs of rotational stages and glass used to control the relative arrival times of the laser pulse sequence.

Pulse Compressor

Here’s the layout of the pulse compressor. It consists of both grating and prism stages.

Pulse compressor
Two stage pulse compressor, using diffraction grating and prism sections. The laser pulses from the NOPA output are highly chirped with a duration of 400 fs or more. This compensates for the chirp, producing pulses of 20 fs or less.

Non-colinear Optical Parametric Amplifier (NOPA)

Here’s the layout of the non-colinear optical parametric amplifier (NOPA). It’s a tunable light source which can produce laser light anywhere across the visible spectrum. It’s fed by the output of our titanium-sapphire regenerative amplifier (Spitfire, Spectra Physics), which provides 100 fs duration pulses with a 1 kHz repetition rate, centred at a wavelength of 800 nm. We split that to produce a white-light continuum seed and a 400 nm pump beam. The precise angle and timing of the overlap of these beams on the nonlinear crystal (beta barium borate, BBO) will change the wavelength of the NOPA output. Currently this is generating light centred around 600 nm.

Non-colinear optical parametric amplifier (NOPA)
Non-colinear optical parametric amplifier (NOPA). A tunable light source which can produce laser light anywhere across the visible range.

The output is strongly affected by the quality of the white-light seed. The white light is produced by focusing a small amount of the 800 nm laser onto a pure sapphire crystal. The produced white light radiates out and needs to be collected and collimated using a precisely placed curved mirror.

NOPA white light
Sapphire plate and off-axis parabolic mirror used to generate and collimate the white-light continuum.

NOPA White Light
Mounting for the off-axis parabolic mirror used to provide precise 4-axis positioning.