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About laser emitter and receiver

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honggexjq

New Member
Hi everyone,I'm new here and this is my first thread.
I'm recently undertaking a laser ranging project,using phase-shift measurement.I was trying to directly modulate the laser beam of a laser diode using a sinusoidal wave with frequency around 10MHz and then build a receiving circuit to convert this modulated beam into an electric signal.I have already built an emitting circuit here,injecting a bias current together with a modulated current into the diode.I'm always trying to make the circuit as simple as possible so as to reduce mistakes,since i don't have a good circuit design sense.As for the receiving part,my basic idea is to use a PIN photodiode followed by a pre-ampifier but i find it hard to capture the modulated signal.I don't know what's wrong here,but the desired signal just won't come out.Could you give me some advice on that?Thx
 

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ericgibbs

Well-Known Member
Most Helpful Member
Hi everyone,I'm new here and this is my first thread.
I'm recently undertaking a laser ranging project,using phase-shift measurement.I was trying to directly modulate the laser beam of a laser diode using a sinusoidal wave with frequency around 10MHz and then build a receiving circuit to convert this modulated beam into an electric signal.I have already built an emitting circuit here,injecting a bias current together with a modulated current into the diode.I'm always trying to make the circuit as simple as possible so as to reduce mistakes,since i don't have a good circuit design sense.As for the receiving part,my basic idea is to use a PIN photodiode followed by a pre-ampifier but i find it hard to capture the modulated signal.I don't know what's wrong here,but the desired signal just won't come out.Could you give me some advice on that?Thx
hi,:)
Do you have a datasheet for the SLD device.

Is the circuit at this stage just a 'simulation' or actual circuit that dosnt work.?
 

honggexjq

New Member
hi,:)
Do you have a datasheet for the SLD device.

Is the circuit at this stage just a 'simulation' or actual circuit that dosnt work.?
in actual circuit i use a LD RLD65MPT3,and i think it's more or less the same with the SLD,and here's the datasheet:eek:
the simulation works,but i have no idea whether the actual circuit works or not.I simply can't get a good result,but i don't know where the problem occurs,whether in the emitting part or the receiving one?Can you tell me a good way to check this?THX
 

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ericgibbs

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Thx for your test.I'm not familiar with LTspice,but in pspice it works:)
hi,
The sim indicates that you are over driving the LD.

Use the PD in a feedback loop to control the max current thru the LD.

If you have a spare LD/PD module, use the PD section in a receiving amp to detect the output from the master LD.

LT Spice, is the free lite version of PSpice.
 

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honggexjq

New Member
hi,
The sim indicates that you are over driving the LD.

Use the PD in a feedback loop to control the max current thru the LD.

If you have a spare LD/PD module, use the PD section in a receiving amp to detect the output from the master LD.

LT Spice, is the free lite version of PSpice.
I see and I'll try.BTW,can you give me some advice on the receiving circuit design?It's relatively easy to design a receiving circuit to detect constant light,but in my case i don't think it's the same.
 

ericgibbs

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I see and I'll try.BTW,can you give me some advice on the receiving circuit design?It's relatively easy to design a receiving circuit to detect constant light,but in my case i don't think it's the same.
hi,
This is a high speed receiver, from the web.

You need a 'ac' coupled amp as you are pulsing the TX with a 10MHz mod.
 

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honggexjq

New Member
hi,
This is a high speed receiver, from the web.

You need a 'ac' coupled amp as you are pulsing the TX with a 10MHz mod.
Many thx,so i guess there's no good ic that can meet my needs and i need to build a circuit using these discrete components?
 

ericgibbs

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Many thx,so i guess there's no good ic that can meet my needs and i need to build a circuit using these discrete components?
hi,
On the RX side, what sort of output do you want from the amp.
I guess you would want to maintain the 10MHz sinewave form.?

You can get very fast OPA's, could be pricy.???

What sort of laser range/distance are you working with.?
 

honggexjq

New Member
hi,
On the RX side, what sort of output do you want from the amp.
I guess you would want to maintain the 10MHz sinewave form.?

You can get very fast OPA's, could be pricy.???

What sort of laser range/distance are you working with.?
yep,that's what i want.i've tried several fast OPAs but the output don't come so good.
i'm working on a distance about 1m and need a resolution of about 1mm
 
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ericgibbs

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yep,that's what i want.i've tried several fast OPAs but the output don't come so good.
i'm working on a distance about 1m and need a resolution of about 1mm
I guess your going to use the phase shift on the 10Mhz signal.?? to try and get the distance.?

A 1mtr distance [2mtr path length] is only 6.6nSec! and 1mm resolution is in the order of pSecs.

Let me know how it goes.:)
 

Mr RB

Well-Known Member
Definitely not from a hobby level device. Even the very expensive pro devices have a few mm error which they try to average out over time by taking a longer reading.

Is there any reason you want to use a sinewave?? :eek:
 

honggexjq

New Member
Definitely not from a hobby level device. Even the very expensive pro devices have a few mm error which they try to average out over time by taking a longer reading.

Is there any reason you want to use a sinewave?? :eek:
hi,the 1mm error is not a strict requirement and if i can make a sample device with a 10mm error,that'll be OK.
I decided to use a sinewave cuz i want to use the phase-shift method to measure the distance.Can you give me some advice if a loose error requirement is allowed?
 

ericgibbs

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hi,the 1mm error is not a strict requirement and if i can make a sample device with a 10mm error,that'll be OK.
I decided to use a sinewave cuz i want to use the phase-shift method to measure the distance.Can you give me some advice if a loose error requirement is allowed?
hi,
Can you post your phase shift measurement circuit.?:)
 

honggexjq

New Member
hi,
Can you post your phase shift measurement circuit.?:)
sorry, but now i'm only at the emitting and receiving stage,and haven't reached the following phase-detect stage yet.Anyway,the basic idea is to measure the phase shift between the emitted sine wave and the received sine wave.As you can see,my first mission is to produce a sine wave and then capture an acceptable sine wave:)
 

ericgibbs

Well-Known Member
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sorry, but now i'm only at the emitting and receiving stage,and haven't reached the following phase-detect stage yet.Anyway,the basic idea is to measure the phase shift between the emitted sine wave and the received sine wave.As you can see,my first mission is to produce a sine wave and then capture an acceptable sine wave:)
hi,
OK.
How do you plan to construct the TX module.?
Is the TX module going to be moving or stationary?

Also can you say what the target that reflects the laser beam will be.
 
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