July 2022
I picked up on eBay an Ando AQ6140 optical spectrum analyzer which, in what constitutes a rare demonstration of honesty from an eBay seller of this kind of product, was described as "powering up but unable to boot". After arrival and a quick investigation, the reason for the boot failure appeared to be a faulty system CF card inside. The embedded computer was working fine otherwise, as it could run OpenBSD from a new CF card.
Unfortunately, the original CF card was totally unresponsive and could not be read at all from another computer; its controller (a proprietary and unobtainium Sandisk chip) appears to be completely dead. The flash chip inside is also a proprietary and obscure Sandisk model and the position of ground and power pins did not match any common NAND pinout, so data extraction sounded difficult (pictures: 1 2). Contacting Yokogawa/Ando to obtain a replacement firmware image resulted in a pure stream of B.S. along the lines of "the internal reference laser is dead" (it wasn't), "the internal reference laser can no longer be manufactured because it needs helium" (orders of magnitude less than a party balloon), "why don't you buy our newest model?" (I'd rather buy a small yacht), etc.
Opening the top cover of the unit reveals the main two electronic boards. They can be easily removed to access the other components of the device: the main power supply, a high-voltage power supply, a Vexta/Oriental Motor stepper control board, and the optical block where the magic happens. The optical block is made of a cast iron machined block that holds all the other components with the optics in place. Inside is a traditional scanning Michelson interferometer with two corner cubes on a moving carriage and a HeNe laser as reference (which is working fine contrary to what Yokogawa/Ando claimed), which is very similar to Michelson wavemeter designs described in the literature. The most remarkable thing is that the moving carriage is directly connected to a stepper motor with a synchronous belt; there is apparently not much being done to isolate the carriage from vibrations or make its motion smooth, and they certainly did not take extreme measures such as the air bearings described in some wavemeter papers.
Overall, the unit appears to be exceptionally well-built and maintainable (unlike e.g. the Burleigh WA20 which is totally janky, ridiculously frustrating to align, and an electrocution and fire hazard), it is a shame that Yokogawa/Ando does not care at all about repairs and tells lies to try to sell their new products.
The two photodiodes for the reference and measurement interferograms are mounted on a small circuit board, part of the optical block, which also contains two transimpedance amplifiers. The outputs are carried over coax cables to the DSP board. There, the edges of the reference interferogram are used by an analog circuit to clock an ADC that digitizes the measurement inteferogram (synchronous sampling). This and Nyquist's theorem probably have to do with the shortest wavelength that the AQ6140 is advertised as being capable of measuring (1270nm, which is close to 2*633nm=1266nm). So far so good, but things go downhill quickly from there, which is why I eventually gave up trying to make use of the original electronics.
The ADC is interfaced through an Altera FLEX FPGA to a TMS320 DSP whose sole purposes appear to be filling a large bank of SRAM with the samples, and communicating with a second TMS320 DSP over a pair of dual-ported (and expensive-looking) SRAM chips. This second TMS320 DSP performs the Fourier transform and communicates with the embedded computer on the ISA bus using a second Altera FLEX FPGA and more of that pricy dual-port SRAM. Dealing with the TMS320 is very much a pain in the neck, with antiquated development tools and a proprietary equivalent of JTAG for which debug probes are only available from dodgy vendors at ridiculous prices (I didn't buy). The computer board, based on a CARD-586 module from Epson, is also a monstrosity with a third FPGA dedicated to making the CF card look like a floppy disk during boot so that the BIOS can access it, a full 6502/UVPROM/SRAM mini-computer whose sole purpose is to scan the front panel keys and emulate a PC keyboard, and an ASIC (!) to generate pulses for the stepper driver and for which they apparently ran out of signals on the ISA bus since some of the ASIC's control and address lines had to go through an ISA GPIO chip. I'm not certain if it would have been fully doable considering the lousy ISA DMA bandwidth available at the time and Conway's law considerations, but it looks like the entire digital part of this system (other than the CARD-586) could have been advantageously replaced with a single FPGA. Ironically the CARD-586 has more FPU power (and much nicer development tools) than the TMS320 and its FPU probably just sat unused. But one thing is for sure: manufacturing those boards must have cost a lot of money!
I eventually managed to obtain optical spectra by digitizing the two signals from the TIAs using an oscilloscope and sending the data to a computer for a bit of processing with Python/NumPy/SciPy. The stepper motor and its driver (PMD07CV) are quite peculiar with a motor winding topology that seem to be used only in Japan; luckily the driver was in good working order and Oriental Motor emailed me its datasheet (in Japanese). The rest was pretty straightforward. Analyzing the combined output of two CWDM lasers produced peaks at the expected positions on the spectrum. Even incoherent light can be analyzed, with limited resolution, using this setup when the carriage crosses the point of zero optical path difference. Connecting a low-cost EDFA with the firmware configured in ACC mode (constant pump current) and no signal at its input produced the textbook spectrum of erbium-doped glass ASE. Considering the very high price of NIR CCD sensors (including linear ones), this is probably what typically needs to be done instead of trying to use a grating-based spectrometer in this wavelength region; additionally, for coherent light sources, the resolution is higher than what could be achieved with a simple and compact grating-based spectrometer.
