Does anyone have any references/examples for detailed circuitry for time-of-flight laser distance measurements - schematics, appnotes etc.?
I also wondered how these affordable devices work.
Searching for 'laser distance measurement patent' I came across: http://www.freepatentsonline.com/3733129.html . Note that this patent was filed already in 1973 so I could imagine that today's laser distance measurement devices use some other method.
From what I understood is this using the target as one of the reflectors of the laser cavity and then modulates the laser trying to find a resonance frequency which seems to be related to the wavelength of an electromagnetic wave with this frequency (e.g. 1 meter seems to correspond to 150 MHz, bringing this down to a more 'manageable' frequency range).
I have to admit that I did not fully understand the details in the patent abstract though.
I'm not convinced though how well this actually works with 'real world surfaces'...I would think that e.g. a wall surface alone is not reflective enough to keep the system lasing.
EDIT: I found another page here: http://www.acuitylaser.com/resources/principles-measurement.shtml . Looking at the 'Time of flight' section, they are a bit more specific: the reflected laser light is focused onto a light diode whose (inverted) signal is then used to modulate the laser. This will form an oscillator. The phase shift is determined by the distance to be measured (apart from fixed delays inside the device), so the frequency of this 'oscillator' is determined by the distance and can be measured.
(Again, the time it takes to travel to a object at e.g. 1.5 m distance and back is 3m/3E8 m/s = 10ns which corresponds to an oscillation frequency of 100 MHz).
Contact Maxim-IC and ask for their whitepaper number "HFRD40". It describes a laser rangefinder using time-of-flight. Straightforward to build. It basically uses a time-to-voltage converter (charge a cap) to measure the TOF of a bunch of laser pulses (to average out noise) then uses an ADC to measure the voltage on the cap. Their whitepaper has a complete design. They sent me one of their test units a while back, because I was looking at building one for a scanning lidar system, but ran out of time. You can see some more discussion here: http://forums.trossenrobotics.com/showthread.php?t=4357
You can look at current state of the art for Lunar Distance measurement http://en.wikipedia.org/wiki/Apache_Point_Observatory_Lunar_Laser-ranging_Operation I think that this materials are public property. There is another link there to Single Photon Avalanche Photodiodes