Laser diode that can be modulated (current or voltage) - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: December 19, 2020, 02:19:22 am
content: Hi,I am looking for a laser diode with lens assembly which I can modulate the intensity/out-power through current source or voltage control.- Modulation up to 200KHz (which I is very low for any LED so I assume even more so for a laser diode).- Power 1mW max, similar to laser pointers- with lense that I can adjust the focus/beam-collimation (i.e. beam opening or closing slightly with distance)The modules I came across say they include the driver and you supply 3-5V so I assume it cannot be modulated like any other LED. But I am not sure.Any feedback on that?Also any suggestions of part numbers?Thank you

author: james_s
author_rank: Super Contributor
timestamp: December 19, 2020, 02:36:04 am
content: In some cases the "driver" is nothing more than a resistor. You can easily remove it too, or take apart a cheap laser pointer, the cheaper it is, the more likely it has a simple current limiting resistor instead of a proper drive. Bare laser diodes and modules with lenses are readily available.

author: ricko_uk
author_rank: Super Contributor
timestamp: December 19, 2020, 03:17:20 am
content: Thank you James,but some have some electronics in it with a tiny IC (often just 3 legs like a current source). Is that a current source or more complex electronic?Either way, can I just module it if it has such tiny modules connected?Thank you

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: December 19, 2020, 03:31:27 am
content: I don't think this is trivial. See Section 2 ofhttps://www.thorlabs.com/NewGroupPage9_PF.cfm?ObjectGroup_ID=1832I use acousto-optic modulators which require RF signals (e.g.,https://intraaction.com/wp-content/themes/Divi/pdf/AOM-AF1_Series_04053.pdf)What's your application and budget?

author: moffy
author_rank: Super Contributor
timestamp: December 19, 2020, 07:59:55 am
content: The following laser diode:https://odicforce.com/Osram-10mW-515nm-Green-Laser-Diode-56mm-PLT5510has up to 100MHz modulation bandwidth. The place seems to sell housings and lenses so you can make you own module. You just need a variable current source for the control, which can be a transistor an opamp and some resistors/capacitors.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 19, 2020, 05:22:48 pm
content: Laser diodes themselves are quite easily modulated well into the GHz region. The limit usually comes from the drive circuits. Small lasers are a bit fiddly to drive. They can be damaged instantly from excess power output or reverse overvoltage. The output power and threshold also vary quite a bit with temperature. This for laboratory use it's common to use either constant current or constant power drivers which are slow and super slow, respectively, especially when driving the diode at the end of a 2m cable.The normal solution is to use a bias tee and set the DC operating point with a nice constant current or constant power drivers then modulate through the RF port of the bias tee. It also helps a lot to have the driver directly attached to the laser to minimize lead inductance.If you don't care about an accurate DC level you can feed it with almost whatever you want as long as you use resistors to limit the current to a safe value. Make sure to take into account turn on transients.

author: Pack34
author_rank: Frequent Contributor
timestamp: December 19, 2020, 05:50:33 pm
content: Does wavelength matter?

author: moffy
author_rank: Super Contributor
timestamp: December 20, 2020, 11:44:49 am
content: Quote from: Pack34 on December 19, 2020, 05:50:33 pmIf you are asking about modulation frequency response it is strangely more related to efficiency of the laser diode. The higher the efficiency the longer the hole/electron lifetimes are and the lower the modulation frequency.

author: ricko_uk
author_rank: Super Contributor
timestamp: December 20, 2020, 06:43:49 pm
content: Thank you all for the feedback and infos!Pack34,if you are talking about wavelength of the laser then I am thinking of trying:1) red2) infrared3) blue-violetThank you

author: Pack34
author_rank: Frequent Contributor
timestamp: December 20, 2020, 07:30:20 pm
content: Quote from: ricko_uk on December 20, 2020, 06:43:49 pmWavelength and wavelength stability matter a lot depending on what you're using the laser diode for. There's suppliers like Ondax and IPS that sell modules and the raw diodes but they're pricy. Expensive due to how accurate the laser is. This is important if the end application is spectroscopy.If you're just looking for colors like for a laser pointer, then they would be overkill.If spot-size maters with the laser, you'll likely be wanting to limit your search towards single-mode instead of multi-mode. You can get higher power with MM and it's much harder to break them but there's less to worry about optics-wise.

author: ricko_uk
author_rank: Super Contributor
timestamp: December 20, 2020, 09:40:39 pm
content: Thank you Pack34!Most laser diodes I came across have 3 pins connection like this:https://www.mouser.co.uk/datasheet/2/348/rld78nzm500a010-e-1843523.pdfwith 2 diodes is series (I assume the light is emitted at the junction of those two diodes?).Few questions:1) For modulation do you use the mid connection somehow or not? i.e. modulating that connection voltage perhaps?2) What is the mid connection used for? Is that for a different type of modulation than just current modulate both in series with a current source?3) The attached schematic uses only one of the diodes. Why?Thank you

author: Pack34
author_rank: Frequent Contributor
timestamp: December 20, 2020, 10:09:42 pm
content: In the schematic, there's a laser diode and a photodiode. The LD is the emitter and the PD allows for feedback. You can strive for constant current to ensure a constant output or strive for using the photodiode as feedback for constant output power. As the laser diode ages it can lose output power if you push it too hard. Too much current, noisy input, or overtemp can cause the diode to weaken.

author: ricko_uk
author_rank: Super Contributor
timestamp: December 21, 2020, 12:06:42 am
content: Thank you Pack34

author: amyk
author_rank: Super Contributor
timestamp: December 21, 2020, 04:40:53 am
content: Fiber optic transceiver?

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2020, 06:28:37 am
content: Early visible laser diodes were extremely fragile, the gap between where lasing just begins to start and where catastrophic optical damage occurs was very narrow so they had to be operated in closed loop using the internal photodiode to regulate the power. Modern laser diodes are far more robust and most can be run open loop. Many don't even have the third pin connected internally.

author: ricko_uk
author_rank: Super Contributor
timestamp: December 21, 2020, 04:22:41 pm
content: Thank you both.Amyk,unless I am missing something, fiber optic transceivers are digital? I need analog modulation.Thank you

author: LaserSteve
author_rank: Super Contributor
timestamp: December 21, 2020, 06:25:15 pm
content: May I suggest:edit, added:https://www.ichaus.de/keyword/Laser%20Diode%20and%20LED%20Drivershttps://www.thorlabs.com/navigation.cfm?guide_id=114https://www.teamwavelength.com/laser-diode-driver-basics/https://www.teamwavelength.com/product-category/laser-diode-drivers/For low cost diodes and collimators but not science grade drivers:https://sites.google.com/site/dtrlpf/Another source of collimators:https://www.mi-lasers.com/If you need one piece of an exotic wavelength:http://www.roithner-laser.com/---A bias tee may be in order, and if you need ultra stable lasers forprecision lab work:A low‐noise high‐speed diode laser current controllerReview of Scientific Instruments 64, 2133 (1993);https://doi.org/10.1063/1.1143949An ultrahigh stability, low-noise laser currentdriver with digital controlRev. Sci. Instrum. 79, 073107 (2008);https://doi.org/10.1063/1.2953597http://www.submm.caltech.edu/kids_html/DesignLog/DesignLog179/MillerMUSICReadoutDocs/HEMT%20Power%20Supply/Libbrecht%20and%20Hall,%20A%20Low%20Noise%20High%20Speed%20Diode%20Laser%20Current%20Controller.pdfLibbrecht and Hall above is a bit outdated, direct modulation and current regulation with a high side analog driver is now common.Protect your diode against static.. Put one of these within 6 cm of the device body:https://www.lasorb.com/Steve

author: Pack34
author_rank: Frequent Contributor
timestamp: December 21, 2020, 09:58:04 pm
content: Quote from: LaserSteve on December 21, 2020, 06:25:15 pmI second the icHaus chips. Simple to use and remarkably stable.As long as you're mindful of heat.If you're just pumping out 1mW from the diode then a passive heatsink should be fine but you'll likely want to have some active cooling involved with a TEC. Otherwise you won't be able to get rock-solid output power. The measured output of the laser can be as much of a product of thermal stability as wavelength stability.

author: ricko_uk
author_rank: Super Contributor
timestamp: December 22, 2020, 11:54:01 pm
content: Thank you all, lots of useful info!!