Electrolinux
New Member
Hi to everyone:
I'm designing a laser range finder, but it's giving me many more problems than I could ever thought. First of all, it's not like an ordinary radar, in that the laser doesn't emit pulses. Instead, it's emitting all the time, what it's called "CW" (Continuous Wave). The range to be measured goes from 1 m up to 15 metres. All I've mentioned so far cannot be changed, they're some of the design requirements, let's say.
The technique I wanted to use is the so called "Phase Shift". It consists of the following:
A beam of amplitude modulated laser, RF, is directed towards the target. A small portion of this wave is reflected by the object's surface back to the detector along a direct path. 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 to ascertain the round-trip distance the wave has traveled. For high-frequency 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.
Extract from the free ebook Where am I? Sensors and Methods for Mobile Robot Positioning, by Borenstein and others.
In my case, there is no an intermediate frequency, much more simple, the laser diode (module) is just transmiting a 10 MHz sine wave. This is achieved by modulating the direct voltage of the power supply, which is about 4 V. On the other hand, the whole system must be eye-safe, so the laser diode is fewer than 15 mW. Very low power, indeed. Guess how much light the photoreceiver can get if the distance can be up to 15 meters. Almost nothing, right? That's why a VERY high sensitive photoreceiver is needed. According to what I have read, which it's been a lot, an avalanche photodiode (APD) is needed.
So far, so good. But here come the problems, when I have to choose a photodiode being able to detect such a low-level light intensity. Bear in mind that an ordinary photodiode area is about 2.29 x 2.29 mm (0.090 x 0.090 in).
Even though my laser diode module has a low beam divergence and provides a well collimated or small focused spot laser light source, its low beam power makes the detection a hard task to achieve. Taking a look at the integrated photodiode and amplifier OPT301 datasheet, I learn that the photodiode responsivity units are volts/uW or amperes/W. But how can I calculate the amperes/W received by an object in the worst case scenario, i.e. 15 metres away from the transmitter and a low reflectivity, quite rough material. This device costs more than 60 dollars/euros, so if something is able to do the job, this device might detect the light beam !!! Or maybe should I use a photodiode array ?
I forgot some data: the light is in the near IR region (650nm) and the whole system will not be operating outdoor.
Another problem is the bandwidth, since the modulation frequency used in my system is 10 MHz and most photodiodes don't support that frequency. Last question, I promise: do you think an optical filter is needed? I know this filter is integrated in the TSOP17xx series ICs, very used for remote controller receivers, where Pulse Code Modulation is used. But any optical filter doesn't seem to be integrated in the kind of photodiodes I've talked about.
Link to the laser diode module I'm using -----> **broken link removed**
Link to the OPT301 Burr-Brown photodiode -->
http://www.datasheetcatalog.com/datasheets_pdf/O/P/T/3/OPT301.shtml
I would greatly appreciate any kind of help. Thanks a lot
I'm designing a laser range finder, but it's giving me many more problems than I could ever thought. First of all, it's not like an ordinary radar, in that the laser doesn't emit pulses. Instead, it's emitting all the time, what it's called "CW" (Continuous Wave). The range to be measured goes from 1 m up to 15 metres. All I've mentioned so far cannot be changed, they're some of the design requirements, let's say.
The technique I wanted to use is the so called "Phase Shift". It consists of the following:
A beam of amplitude modulated laser, RF, is directed towards the target. A small portion of this wave is reflected by the object's surface back to the detector along a direct path. 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 to ascertain the round-trip distance the wave has traveled. For high-frequency 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.
Extract from the free ebook Where am I? Sensors and Methods for Mobile Robot Positioning, by Borenstein and others.
In my case, there is no an intermediate frequency, much more simple, the laser diode (module) is just transmiting a 10 MHz sine wave. This is achieved by modulating the direct voltage of the power supply, which is about 4 V. On the other hand, the whole system must be eye-safe, so the laser diode is fewer than 15 mW. Very low power, indeed. Guess how much light the photoreceiver can get if the distance can be up to 15 meters. Almost nothing, right? That's why a VERY high sensitive photoreceiver is needed. According to what I have read, which it's been a lot, an avalanche photodiode (APD) is needed.
So far, so good. But here come the problems, when I have to choose a photodiode being able to detect such a low-level light intensity. Bear in mind that an ordinary photodiode area is about 2.29 x 2.29 mm (0.090 x 0.090 in).
Even though my laser diode module has a low beam divergence and provides a well collimated or small focused spot laser light source, its low beam power makes the detection a hard task to achieve. Taking a look at the integrated photodiode and amplifier OPT301 datasheet, I learn that the photodiode responsivity units are volts/uW or amperes/W. But how can I calculate the amperes/W received by an object in the worst case scenario, i.e. 15 metres away from the transmitter and a low reflectivity, quite rough material. This device costs more than 60 dollars/euros, so if something is able to do the job, this device might detect the light beam !!! Or maybe should I use a photodiode array ?
I forgot some data: the light is in the near IR region (650nm) and the whole system will not be operating outdoor.
Another problem is the bandwidth, since the modulation frequency used in my system is 10 MHz and most photodiodes don't support that frequency. Last question, I promise: do you think an optical filter is needed? I know this filter is integrated in the TSOP17xx series ICs, very used for remote controller receivers, where Pulse Code Modulation is used. But any optical filter doesn't seem to be integrated in the kind of photodiodes I've talked about.
Link to the laser diode module I'm using -----> **broken link removed**
Link to the OPT301 Burr-Brown photodiode -->
http://www.datasheetcatalog.com/datasheets_pdf/O/P/T/3/OPT301.shtml
I would greatly appreciate any kind of help. Thanks a lot
