Can a frequency laser diode be changed ??!!

[Electrolinux]

Hi everyone,
I'm designing a laser range finder using the technique of Amplitude Modulated Continuous Wave (AMCW). The range to be mesuared is up to 15 meters. As a photoreceiver, I have the well-known BPW77N phototransistor, but I'm afraid it doesn't have the sentivity enough to detect light from such a long distance. As far as I've read, I'd need an avalanche photodiode (APD) or a built in photodiode-amplifier-filter to do the job. Since these VERY high sensitive components aren't readily available to me (and they are not cheap !!), I wanted someone to advice me before ordering the component.

I don't understand how you can change the frequency of the laser pointer. Mine has a wavelength of 850 nm. Since c = wavelength x frequency and c is a constant, I deduce that the laser diode frequency cannot be changed. However, I know I have to modulate it with a 10 MHz pure sine wave. How is this possible, then ??

Thanks beforehand.

 

[Nigel Goodwin]

Modulation isn't changing the laser frequency, it's merely changing the amplitude of the emitted light - are you sure the laser will allow 10MHz amplitude modualtion?.

 

[Electrolinux]

I do know the signal is modulated, but which one ??

Like in the well-known radio AM, I'm raising the frequency of the signal up to 10 Mhz. That frequency cannot be lower than that value, because the measured range must be non-ambiguous. I mean, the phase difference must be limited to two x pi , that's why the signal frequency has to be increased depending on the maximum range to be measured. The thing is...... I don´t know exactly which signal we're talking about !!!!!

Thanks for your fast answer.

 

[jbeng]

I don't understand how you can change the frequency of the laser pointer.

You can't. The wavelength is determined at the time of manufacture by the semiconductor material used and physical parameters of the device, among other things.

Mine has a wavelength of 850 nm.

Probably more like 650 nm. 850 nm is in the infrared region.

... are you sure the laser will allow 10MHz amplitude modualtion?

Manufacturers can supply visible-wavelength units which have modulation capabilities (usually done with audio), but I doubt they can modulated at anywhere near 10MHz. Those units are specially made ones ... not just your ordinary cheap laser pointer.
JB

 

[Electrolinux]

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.

 

[hjames]

One point of confusion is basic physics - a monochromatic 880nm laser is actually a 340THz oscillator. This frequency is fixed and can't be (easily) modified or transformed.

The frequency that is used by the ranging technique is a frequency that is much lower than the laser's inherent 340THz frequency. With a 10MHz amplitude modulation, the laser does a bright-dim-bright flash within 100ns.

In order to modulate a laser at this frequency, you'll need to design your own driver circuit (or base it off a commercial chip that's meant to drive lasers at this frequency - check www.maxim-ic.com among others). Most prepackaged lasers aren't built or designed to be modulated, or are designed to be modulated in the KHz region.

As for the sensors you've mentioned, I don't have any experience designing any circuits with anywhere near this capability. (The circuits for the sensing will require someone with a lot of experience designing very low-noise, very high frequency analog circuits).

James