Here you have more information
Hi, jBeng,
First of all, thanks for your answer and also I appologize if my last post seemed to be rude. It was not my intention, far from it. I just wanted to express that I'm furious with this stuff about the modulation.
Let's see, there's a modulation in my circuit and I also think, as you say, that the laser diode frequency is fixed.
Here you have some text from the free ebook "Where am I?", pages 112-114, so that you can understand what kind of laser range finder I'm designing:
The phase-shift measurement (or phase-detection) ranging technique involves continuous wave
transmission as opposed to the short pulsed outputs used in TOF systems. A beam of amplitude modulated
laser, RF, or acoustical energy is directed towards the target. A small portion of this wave
(potentially up to six orders of magnitude less in amplitude) is reflected by the object's surface back
to the detector along a direct path [Chen et al., 1993]. The returned energy is compared to a
simultaneously generated reference that has been split off from the original signal, and the relative
phase shift between the two is measured as illustrated in Figure 4.21 to ascertain the round-trip
distance the wave has traveled. For high-frequency RF- or laser-based systems, detection is usually
preceded by heterodyning the reference and received signals with an intermediate frequency (while
preserving the relative phase shift) to allow the phase detector to operate at a more convenient lower
frequency [Vuylsteke, 1990].
Referring again to Figure 4.21, it can be seen that the total round-trip distance 2d is equal to some
integer number of wavelengths plus the fractional wavelength distance x associated with the
phase shift. Since the cosine relationship is not single valued for all of 1, there will be more than one
distance d corresponding to any given phase shift measurement [Woodbury et al., 1993]
Thanks for your help
P.S.: The attachment is a .doc file containing the figures referred in the text above.