Please Help me with the Laser Diode driver Circuit Design

Thread Starter

nanji

Joined Feb 17, 2024
8
Thank you for your patience and kind help!

I am currently designing a driving circuit for a 690nm, 30mW laser diode named HL6738MG. Its operating current is 85mA, and I require a VCO (Voltage-Controlled Oscillator) circuit capable of modulating its frequency at 100MHz. Could you recommend an integrated VCO circuit suitable for this application? Additionally, I plan to use IC-HK and IC-WK for the laser diode and modulation switch circuits, respectively. Are these choices appropriate?

Thank you so much for your help!
 

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Ron H

Joined Apr 14, 2005
7,063
I don't think you'll find one VCO with an FM range that is that wide. You MIGHT find one with that tuning range.
 
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Thread Starter

nanji

Joined Feb 17, 2024
8
I don't think you'll find one VCO with an FM range that is that wide. You MIGHT find one with that tuning range.
Thank you Ron

The frequency I need to modulate is around 100MHz. I can choose from a frequency range of 50MHz to 1000MHz, just to clarify.
 

Ron H

Joined Apr 14, 2005
7,063
What FM range do you need, as a percentage of center frequency. For example, if you have a 100 MHz center frequency, do you want to modulate that by ±1%, or ±10%, or??? BTW, I have no experience with lasers, but I do have a little with VCOs.
 

Thread Starter

nanji

Joined Feb 17, 2024
8
What FM range do you need, as a percentage of center frequency. For example, if you have a 100 MHz center frequency, do you want to modulate that by ±1%, or ±10%, or??? BTW, I have no experience with lasers, but I do have a little with VCOs.

Thank you Ron!

1% should be enough.
 

LowQCab

Joined Nov 6, 2012
4,075
Why Do You need to Modulate the Frequency ?
Why does the Frequency need to be so high ?
What is the purpose of the Laser-Diode and it's driving-Circuitry ?

If the purpose is "Laser-Etching", all You need is PWM-Modulation, plus, Current-Control,
both of which the iC-HK, iC-HKB 155 MHz LASER SWITCH is capable of providing.
But if "Laser-Etching" is the purpose of this project,
I would not attempt to use such high-Frequencies, they are not necessary,
unless your goal is mind-bending-production-speeds, with super-high-resolution.

As an "off-hand-guess",
I would say that a ~10kHz PWM-Frequency would be far more than adequate,
and, super-high Frequencies can create all sorts of unforeseen problems,
so in this case, it would be better to avoid them.
The only real challenge I see with this Chip is "on-the-fly" Current adjustments.
But, with some study of the Spec-Sheet,
Current-Adjustments can probably be easily accomplished with an external-"add-on"-Circuit.

Your options will probably depend on the Programming of the Micro-Controller that You will be using.
What is the Character and Format of it's Outputs ?
.
.
.
 

Thread Starter

nanji

Joined Feb 17, 2024
8
Why Do You need to Modulate the Frequency ?
Why does the Frequency need to be so high ?
What is the purpose of the Laser-Diode and it's driving-Circuitry ?

If the purpose is "Laser-Etching", all You need is PWM-Modulation, plus, Current-Control,
both of which the iC-HK, iC-HKB 155 MHz LASER SWITCH is capable of providing.
But if "Laser-Etching" is the purpose of this project,
I would not attempt to use such high-Frequencies, they are not necessary,
unless your goal is mind-bending-production-speeds, with super-high-resolution.

As an "off-hand-guess",
I would say that a ~10kHz PWM-Frequency would be far more than adequate,
and, super-high Frequencies can create all sorts of unforeseen problems,
so in this case, it would be better to avoid them.
The only real challenge I see with this Chip is "on-the-fly" Current adjustments.
But, with some study of the Spec-Sheet,
Current-Adjustments can probably be easily accomplished with an external-"add-on"-Circuit.

Your options will probably depend on the Programming of the Micro-Controller that You will be using.
What is the Character and Format of it's Outputs ?
.
.
.
Thank you so much LOWQCAB!

The frequency modulation in the context you're describing is typically used for Frequency Domain Diffuse Optical Spectroscopy (FD-DOS), which is a technique for measuring the optical properties of tissue, such as in brain imaging or breast cancer detection. The high frequency, such as 100MHz, is necessary because it allows for the capture of dynamic changes in the tissue's optical properties due to physiological events, like blood oxygenation levels.

The laser diode is part of an optode, which is a probe used to emit and collect light from the tissue. The driving circuitry for the laser diode is crucial because it must precisely control the laser's output to ensure accurate measurements.

For laser etching, PWM (Pulse Width Modulation) and current control are indeed sufficient, and high frequencies like 155MHz are not necessary. However, for medical imaging purposes like FD-DOS, these higher frequencies are used for modulating the light to penetrate the tissue and collect meaningful data.

Regarding the iC-HK, iC-HKB switch, while it is capable of fast switching up to 155MHz, it may not be suitable for modulating a sine wave at 100MHz if modulation is not one of its features. The switch is designed for turning the laser on and off at high speeds but does not inherently modulate the waveform of the laser's output. So, in this case, I should find another IC circuit to replace the IC-HK switch, right?(Because the IC-HK can only be a switch on and off up to 155MHz)
 

MisterBill2

Joined Jan 23, 2018
18,568
Based on very bitter experience as the only failed project in my career, I suggest that if you do not have the resource of one very skilled in assembling lasers of that style with the required optics I would avoid lasers having that form factor if beam shape or alignment matter for project success.
And see that the driver that uses temperature feedback may support modulation UP TO 2 KHZ.
One additional question based on the information about the application: Is the modulation needed to be linear intensity modulation? Or simply ON/OFF switching. My experience with "Frequency Domain Diffuse Optical Spectroscopy (FD-DOS)" is lacking.
The type of modulation will have a major effect on the required circuits.
 
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