Research Lab Technologist
ChromaMatic2 Laser Tuning
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NOTICE: This webpage and associated files is provided for reference only. This is not a kit site! It is a collection of my work here at the University of Toronto in the Physics department. If you are considering using any schematics, designs, or anything else from here then be warned that you had better know something of what you are about to do. No design is guaranteed in any way, including workable schematic, board layout, HDL code, embedded software, user software, component selection, documentation, webpages, or anything.
All that said, if it says here it works then for me it worked. To make the project work may have involved undocumented additions, changes, deletions, tweaks, tunings, alterations, modifications, adjustments, waving of a wand while wearing a pointy black hat, appeals to electron deities and just plain doing whatever it takes to make the project work.
ChromaMatic2 is a tuneable laser. Although the name implies all the colours of the rainbow and more, tuning is limited to up to a ±8GHz range, limited by the laser, PLL and PD. In that range, wavelength can be tuned to within 1Hz, excluding the effects of line width. The basis of the tuning is to beat the tuneable laser against a reference laser, using a wideband photodiode (PD) to convert the beat frequency (the difference between the two frequencies) to electrical RF then using a PLL ( phase locked loop) to lock the beat frequency to a reference frequency derived by a DDS ( direct digital synthesis). For example, a reference laser is locked to a 40K absorption line at 767.6nm, or 390.5582THz. We want to be 1.2GHz higher, or 390.5594THz. Beat the tuneable and reference lasers on the PD. Divide the RF beat frequency by 16 to bring it into a manageable range, so we expect 75MHz. Generate a 75MHz reference from the DDS, feed this reference and the divided beat frequency into the PLL. The PLL and filters provide A) a 3KHz bandwidth error signal for gross tuning via the piezo on the external cavity of the tuneable laser, and, B) a 1MHz bandwidth error signal for fine tuning of the laser current which in turn tweaks the main cavity's index of refraction and wavelength.
The two lasers - reference and tuneable - are focused with the same polarization onto a polarization maintaining (PM) fibre. At the far end of the fibre is a Electro-Optics Technology ET-2030A-FC 2GHz amplified PD. Its RF output goes directly into a prescaler and PLL, Analog Devices' ADF4108 evaluation board . The 1GSPS DDS, Analog Devices' AD9858 evaluation board (I like having other people do the messy work), generates up to about 300MHz (Nyquist limited) for the RF reference, although the lowest practical RF reference is 10MHz because of the 1MHz PLL loop filter bandwidth. The whole mess is controlled by a Rabbit Semiconductor's RCM4200 module over UDP/IP from a host computer. The host calculates requirements, converts them to parameters compatible the DDS, then sends them to the Rabbit. Upon a signal from the ADWin realtime controller, the Rabbit executes the DDS sequence.
See the Laser Lock section for a description of the DDS. The microwave source uses a frequency generated by the DDS then multiplies it up to 1.3 ±0.1GHz using an Analog Device's ADF4360-5 VCO/PLL evalutaion board.
|Ref In frequency||
||Set in software for 10MHz|
|Ref In level||40mV
|Ref In PLL multiplier||for 1GSPS clock||100||Set in software|
|DDS frequency range||59.6Hz||up to ~250MHz||500MHz||Nyquist limited by 1GSPS clock|
|DDS frequency resolution||1GSPS clock||59.6Hz||From 1GHz / 2^24|
|Laser Lock, using ADF4108|
|PD input frequency||1GHz||8GHz|
|PD input level||125mV||-5 to +5dBm||400mV||RMS|
|DDS to lock transfer||Lock freq = 32 * DDS freq|
|PLL lock frequency||up to100MHz||104MHz|
|Microwave Source, using ADF4360-5|
|Microwave output level||???|
|DDS frequency multiplier||???||Set in software|
|Sorry, no more chance for asking direct questions, queries, broken links, problems, flak, slings, arrows, kudos, criticism, comments, brickbats, corrections or suggestions.|