Frequency Modulating a Laser

[wookie]

Hi guys,

I'm making an FMCW Laser rangefinder, and I have most of the theory sorted, but I am unfamiliar with laser diodes.

I understand that attaching an antenna to an FM source will radiate the signal to be recieved. What I dont understand is how you can force a semi-conducting element to emit light.

Do you just have to offset the FM output to be above the forward voltage of the diode for it to work?

 

[alec_t]

What type of laser diode?
Have you read this about tunable lasers?

 

[wookie]

Hi alect_t,

Yes, I came across that page a while ago, unfortunately, tunable lasers are far beyond the scope of my project, I am looking to modulate a cheap "laser pointer" diode of 5mW peak power like **broken link removed**.

I do not know how to identify the type of laser diode this is.

 

[ronsimpson]

I don't think you can FM modulate that diode.
You can AM modulate the diode.
If you want to send audio then that complicated.
If you just want to send a 10,000hz tone then that is simple. Drive the power on/off 50% duty cycle at the frequency you want to transmit.

 

[wookie]

I don't think you can FM modulate that diode.

Why do you say that?

Just to give you an idea of my project, it goes like this:

Clock -> Rampwave Gen -> VCO -> Freq. Modulator -> Laser

The clock generates a 2kHz baseline for the system.
The Rampwave gen makes a sawtooth waveform, making the VCO "chirp" between 4-60kHz.
This chirp signal is to be modulated at 433MHz and transmitted by the laser diode.

At least... thats the plan

 

[gary350]

I used a mirror attached to a speaker to modulate a laser. Its does transmit sound from point A to point B.

 

[ronsimpson]

Why do you say that?

Just to give you an idea of my project, it goes like this:

Clock -> Rampwave Gen -> VCO -> Freq. Modulator -> Laser

The clock generates a 2kHz baseline for the system.
The Rampwave gen makes a sawtooth waveform, making the VCO "chirp" between 4-60kHz.
This chirp signal is to be modulated at 433MHz and transmitted by the laser diode.

At least... thats the plan

I can see the 4-60khz is changing frequency.
Maybe the 433mhz is FM. (don't know what type of modulation you are using)
The diode is not changing frequency. Red is red. I think you are AM modulation the diode with a FM signal.

 

[ericgibbs]

hi wookie,
All the laser pointers I have used have an inbuilt 'soft start' driver which makes it impossible to modulate the light output in amplitude or frequency.
E

 

[wookie]

hi wookie,
All the laser pointers I have used have an inbuilt 'soft start' driver which makes it impossible to modulate the light output in amplitude or frequency.
E

Correct, I would need to find a diode without inbuilt current regulation, and design my own to drive it.

I know this can be done, because there is a similar circuit here.

Edit1: Also, I may be getting confused with FM terminology. I included the 433MHz carrier wave as I thought it would be the crucial part of the transmitter... no?

 

[wookie]

I HAVE MADE A MISTAKE.

There is no "frequency modulation stage". I got confused with my own design. The laser driver is driven by the VCO output. The VCO is to operate linearly from 55MHz to 130MHz.

Still learning here

 

[ericgibbs]

hi,
I have been look thru the web and it appears that guys are making DIY laser amplitude modulators using laser pointers, without an internal driver.

Do you have any information for the variation of the laser light frequency versus operating voltage.?
E

 

[wookie]

No info on that, the laser light frequency is fixed.

Hmmm..... you can't change the frequency of the laser light, but can you use the frequency as a carrier for the "chirp" signal? more investigation needed

 

[steveB]

I thought I would just interject with a terminology comment. Often people will refer to the modulation of a laser as "intensity modulation". This distinguishes it from the very advanced "coherent communications" that is possible (although quite difficult) with lasers. It is possible to directly do AM, FM and PM modulation on the light carrier itself, and then all of the known radio detection techniques (heterodyne, homodyne, etc) can be used. However, it is far more common to do intensity modulation of the laser, which is often done with a brute force modulation of the current (external modulation is also possible).

There are some key differences between AM and intensity modulation. For example, intensity modulation is likely to modulate both the frequency (this is called chirp) and the amplitude of the light. But this does not matter for simple intensity modulation because of the way detection is done with a simple square-law photodiode detector. The other key difference is the detection process itself since we measure "power" with a photodiode, and not amplitude. The power is converted proportionally to current, so we sometimes forget that, from a communications-carrier point of view, we are dealing with a square-law detector.

 

[moffy]

Just use a current source as your laser diode driver. You have to have the current above the lasing threshold, then you can modulate with whatever signal you like. Just be aware that different lasers can have very different intensity modulation frequencies. Higher power/efficiency laser diodes have long carrier lifetimes and are therefore very low bandwidth.

 

[wookie]

I thought I would just interject with a terminology comment. Often people will refer to the modulation of a laser as "intensity modulation". This distinguishes it from the very advanced "coherent communications" that is possible (although quite difficult) with lasers. It is possible to directly do AM, FM and PM modulation on the light carrier itself, and then all of the known radio detection techniques (heterodyne, homodyne, etc) can be used. However, it is far more common to do intensity modulation of the laser, which is often done with a brute force modulation of the current (external modulation is also possible).

There are some key differences between AM and intensity modulation. For example, intensity modulation is likely to modulate both the frequency (this is called chirp) and the amplitude of the light. But this does not matter for simple intensity modulation because of the way detection is done with a simple square-law photodiode detector. The other key difference is the detection process itself since we measure "power" with a photodiode, and not amplitude. The power is converted proportionally to current, so we sometimes forget that, from a communications-carrier point of view, we are dealing with a square-law detector.

Okay, I think I understand. My chirp signal is not a true FM transmission, it is a hybrid of FM and AM, due to the nature of lasers in general - correct?

 

[steveB]

Okay, I think I understand. My chirp signal is not a true FM transmission, it is a hybrid of FM and AM, due to the nature of lasers in general - correct?

I would say that generally the natural laser frequency chirp will not interfere with what you are doing because you will be detecting the light with a photodiode, which is not sensitive to the light frequency changes that occur as current is modulated. If you introduce your own chirp signal at lower frequency, within the modulation bandwidth of the laser, then it depends on how you actually do it. You can modulate your frequencies with constant amplitude if you choose to.

Usually laser chirp does not matter unless you are going over very long distance where dispersion will cause the difference light frequencies to travel at different speeds. This is an issue for high speed telecommunications over optical fiber.

 

[wookie]

Hi Steve,

The plan is to measure the beat frequency of the light by comparing the outgoing sawtooth to the incoming sawtooth (reflected off the target).

I have discovered that to do this, a sawtooth waveform is not ideal, as the rise/fall times are uneven. a perfect triangle wave is now the preferred signal.

If i was to drive the VCO with a triangle wave, it would produce a varying frequency that changes linearly between, say, 55MHz and 130MHz.

If this signal was fed into the diode, what exactly am I transmitting?

 

[steveB]

... what exactly am I transmitting?

This is too difficult to answer. The "exact" signal you are transmitting depends on the details of the laser, but that does not really matter. The effective signal is more important and that has to do with how you detect the signals. Since detection can't follow the very high light frequencies, the effective signal is the one you just described. The electrical-to-light and light-to-electrical conversions will have minimal effects if you have good linearity (in the optical transmitter and receiver) and short propagation distance. There will be delays over the propagation distance, and Doppler effect off reflections too.

 

[wookie]

The delay over propagation distance is exactly what I am looking to determine, as it is the key to calculating distance traveled

How I actually compute that is a whole other kettle of fish... We'll cross that bridge when we come to it

Thank you steveB!