OSRAM PL530 OPSL Laser
The inspiration for the project comes from my earlier experimentation with making holograms using red laser diodes. Researching more on holographic lasers, I was recommended by a member on a holographic group to have a look at the Osram PL530, an Optically Pumped Semiconductor Laser (OPSL) that is not only an inexpensive device but capable of excellent beam characteristics that are ideal for holography.
Originally, these Lasers were developed for Pico laser projectors and mounted into an RGB "Light Engine". The PL530 can be "coaxed" into Single Longitudinal Mode (SLM) operation through very careful control of temperatures.
SLM output laser sources produce a very narrow wavelength bandwidth and increased coherence length, ideal for metrology, interferometry, and holography, therefore, this project undertaken was to construct a stable, Single Longitudinal Mode (SLM) laser source for exposing holographic film plates such as the LitiHolo C-RT20 instant films.
The PL530 is an incredibly small and very impressive device measuring only 14.15mm x 6.75mm x 4.3mm, and capable of stable 50mW-120mW SLM 530nm (Green) output.
To achieve stable output, the laser must be carefully controlled. The operating requirements are 1.8V at 450mA for the pump diode. The PPLN (Periodically Poled Lithium Niobate) crystal temperature is controlled using an internal heater element which has a resistance around 25Ω (range may be between 27Ω - 32Ω or higher). The PPLN is the most crucial element that needs to be fine-tuned and kept stable. The PPLN Heater (PTC) current must not exceed 80mA. Finally, the device needs to be mounted onto a thermal-electric cooler (TEC), and feedback provided by a thermistor (NTC) to maintain correct temperature, in total, 3 drivers are required to operate the laser.
Having been recommended to look at the OSRAM PL530 Laser, I searched the internet and came up with various resources where many hobbyists and those using them for holography, had wired up all sorts of separate drivers to making these units work.
I came across one website, W's Laser-Projects page, with perhaps most concise information on the PL530 Laser.
These devices were made for integration into white-light engines for small Pico Projectors such as Microvision's ShowWX Pico Projectors, which use tiny MEMS mirrors to scan the laser beam (pixel by pixel) (Laser Beam Scanning) to produce the output projection video.
Tech Insights published a small write-up "A Look Inside MicroVision's PicoP Projector Technology".
Searching on Ebay, I found a seller who had some of the lasers available, and on 9 February 2022, I ordered two of the devices.
A few weeks passed, and the lasers had arrived. I was amazed at the small size of these devices, and with knowledge of the optics inside, are incredible feat of optical engineering.
The lasers then would sit for a while as I researched drivers.
In early August, while searching on Ebay, I came across a driver board that was designed to operate these lasers. Investigating further, I narrowed down the manufacture, Power Drive Controls, where after some discussions with the owner, Dasheng Pan, placed an order for a driver board as well as a TEC and thermistors, the order was received within two weeks of dispatch.
I have to say, that dealing with Dasheng at Power Drive Controls has been one of the best customer service experiences that I have ever encountered from a US company, and the immense support and guidance has been invaluable.
The driver board that was selected is the LHD-94. This features 3 PID drivers (closed loop) to control the Laser Diode current, PPLN Heater and external TEC. The board measures 94mm x 29mm.
I began setting out on the project development on 24th August 2022 researching how to mount the very tiny thermistor as well as the overall mounting of the laser module, TEC and driver board. The Design and construction of the base of the module commenced on 22nd September 2022. Two aluminium sections; a 32mm x 120mm for the main base, and 15mm x 32mm for the small plinth. M2.5 threads for screws.
LHD-94 Driver board
Some considerable time was spent researching thermal interface materials (TIM) required for mounting the Laser, TEC and Thermistor (NTC) to a base and then a heatsink. One of the critical specifications is the Thermal Conductivity in W/m.K, the efficiency of the material to conduct heat.
When received the thermistors, the first reaction is the incredibly small size of the device, less than 1mm diameter, with wires several times smaller.
Attaching the NTC would become a challenging part of the project as well as the fine soldering required.
Glass Bead NTC (0.7mm lead for scale)
Researching various interface materials including epoxy / adhesives, and thermally conductive tapes, it was found that the best thermal conductive materials are heat transfer pasts followed by tapes.
Most thermal adhesives I found, their thermal conductivity typically around 0.6W/m-k to at best 4W/mk, and are very expensive, even more difficult when only a very tiny amount is required.
For attaching the PL530 to the TEC and TEC to Aluminium base, I found that our local supplier, Jaycar Electronics stocks a 100mm x 100mm x 0.5mm sheet of L37-5 Silicon thermally conductive pad with very good thermal conductivity of 1.6W/m.K, and easily available. The part number: NM2790.
Perhaps one of the most daunting tasks was dealing with the shear small scales of the connections required both on the PL530 laser but more so the NTC thermistor with its wires of less than 0.1mm.
Initially, the NTC was mounted in contact with the PL530 silicon baseplate and thermal tape attached, however, later would prove problematic.
Initial NTC Mounting
With the components wired, it was time to apply power and begin testing of the laser (27 September 2022).
With power applied, and after considerable work, I was greeted with the 530nm green output, however the challenges soon became reality as the laser was not stable and power cycling took over.
Over the course of the next week, I spent troubleshooting and diagnosing issues together with a wealth of information and support from the manufacture of the board, Dasheng from Power Drive Controls.
One of the first suggestions was to remove the thermal tape as would cause reading errors and to increase input voltage to 4.5V at 2A. I then replaced the thermal tape with Kapton tape around the thermistor.
NTC Rework
In the tests that followed, the thermal cycling issues had not been resolved. Further email discussions with Dasheng provided other suggestions included adding an additional 100uF or 200uF capacitor across C22, (some clients using upto 800uF). Increasing to 400uF did not settle cycling. Reading further on the laser, I concluded that perhaps the location of the thermistor was causing further errors.
As part of reworking the thermistor, I looked further into thermal pasts, and decided to obtain some "Arctic Silver 5" which has a thermal conductivity of 8.9W/m.K.
Over the next few days, I played around with the positioning of the thermistor, however still not achieving any stability. As I began to take note of current draw, and thermistor positioning, I found some glimpse of stability and concluded that defiantly was a case of a thermal runaway.
As I began to take note of current draw, and thermistor positioning, I found some glimpse of stability and concluded that defiantly was a case of a thermal runaway.
Going over thermistor positioning, it was determined that the thermistor must only contact the ceramic part of the TEC and NOT the baseplate of the laser.
I carefully reworked the thermistor and then decided to place the entire laser assembly on cold block (made with ice wrapped in foil and paper towel in between).
Remounted Thermistor & Wiring
Following considerable work, and significant learning curve, stable output was achieved with the laser module sitting on a cold block.
Laser Spot at 7 meters from the output.
With the laser now showing stable operation, the next step was to mount the module onto a large finned heatsink. On power up, there was observed changes in output behaviour to a stable point, then slowly would begin reducing. Current draw from the power supply began to climb where after a couple minutes, the laser would then cut out (thermal runaway?).
I then touched the top of the laser, and noticed the laser would begin picking up, the current draw from the supply also began to reduce, curious to the behaviour I then experimented with placing a small block of Aluminium on top of the laser case, and found that over the course of several minutes the laser stabilises solid with a very bright beam, and the power supply current draw locks in tight at 520mA in the first minute, dropping to 515mA, then dropping by 5mA each subsequent minute.
At 5 minutes, we see current solid at 496mA. At 6 minutes observe current flutter between 496mA to 503ma. At 10 minutes, power supply current stable at 515ma to 520mA any deviation due to air current over the laser, the circuit corrects back.
Using a laser power meter, the laser output was measured at 85mW.
Additional Heatsink Block
So, the question is, why does placing the block on top of the laser completely stabilise the system?
Analysing the behaviour of the laser, it's clear that very careful thermal control is paramount to stable operation.
One also needs to account for the thermal input from the pump laser that will cause a temperature rise in the laser case, together with the PPLN heater. The temperature of the PPLN must be carefully set (tuned) and very stable to optimise its optical performance (and the output characteristics of the laser) A balance needs to be struck, where the thermal input of the pump is nulled out, (but cooled to ensure correct operation) and the correct operating temperature of the PPLN crystal.
With knowledge some additional heatsinking of the laser would completely bring the system into stable operation, I decided to experiment with a small finned heatsink.
On 7 October 2022, I added a proper finned heatsink to the laser, where full stable output was observed, when measured the output of the laser, I was very impressed to see an output at 105mW, a phenomenal output from such a device, the chosen heatsink providing better efficiency again.
Power input: 4.5V @ 534mA - 538mA, the laser now ready for further detailed characterisation.
Finned Heatsink on Laser
One interesting observation is the resultant strong contract fringes produced as the laser bounces off a (rear surface) mirror on one side of the room and back onto the wall, a travel of 7.5 meters. The Interference pattern produced is a result of interference of light from front and rear surface of the mirror (also known as the Modulation Transfer Function in optics).
With the laser now showing very stable output and operation, it was time to begin characterising the laser and fine tune any parameters.
Details of Laser characterisation data, including observations of power sweet spots form part of the project document.
Following successful tests on the laser, the time arrived to build the enclosure of the laser.
An off the shelf instrument enclosure available from Jaycar Electronics, HB6034, measuring 150mm x 80mm x 30mm has been employed to enclose the laser module.
The bottom section of the enclosure has a cut-out to side over the laser module and is attached to the main heatsink via M2.5 screw.
As continued to undertake testing, found that the laser power had increased to 125.8mW.
The next day, work continued, to install the plugs as well as a cut-out for access to the switch.
A breakout port also provisioned to access GND, K_1 (Heater Monitor) and K_2 (Laser Diode Current) for external measurements as required.
In future, some fine adjustment to the heater current may be required to compensate for any drift, however so far, the laser has been operating with very consistent and predictable results.
The next task was to install the shutter; however it was found after provisioning a 3.5mm TRS socket and then testing the shutter, that its operation works by pulsing it with 3V + (to open), then reverse the polarity to close (it is not under spring return), therefore to enable correct operation, would need to provision 4 connections (two for power, and two for switched side.
With this information, I had to order two TRRS 3.5mm (4 contact) panel sockets, the sockets had to be ordered from Digikey, P/N: 839-54-00173-ND.
The order was placed on 12 October 2022 and arrived on 17 October 2022. With the TRRS plug installed, the shutter now operable via a momentary switch with centre-off position.
Finally, all wiring secured, and an extension piece added to the switch to enable it to be operated once the cover is on.
On 20 October 2022, the remote switch for the shutter was completed. This using a smaller version of the same enclosure measuring 90mm x 50mm x 24mm available from Jaycar Electronics, PN: HB6031.
The final components to add are two double concave beam expansion lenses to allow the beam to be used to fill a holographic plate.
On 22 November 2022 a B&WTek BTC11-S Spectrometer was used to measure the laser spectrum. The wavelength measured at 529.35nm, (the device specifications 528nm to 535nm) as can be observed, the width of the curve is very narrow, with a precise wavelength, a characteristic of Single Longitudinal Mode quality.
We will see from the curves that the measured linewidth is less than 2nm!
Click on a Spectrograph to enlarge;
Documentation
Following considerable work to achieve stable output, characterisation of the laser was completed by tracking optical output power and electrical input parameters. The board includes monitoring points for the heater (K_1) and laser current (K_2) which have been broken out and accessible via a 3.5mm TRS socket installed on the side of the laser enclosure.
The journey of this project has been collated and published as a Technical Document.
Download: OSRAM PL530 OPSL Laser Datasheet (English).
Download documents as compressed archive FRS_PL530_TechDoc.zip.
To purchase the driver boards and lasers including the datasheets, head to Power Drive Controls
- Flavio Spedalieri -
Written: 22 November 2022
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