Edge Emitting Lasers (EELs) and Vertical-Cavity Surface-Emitting Lasers (VCSELs) have grown in popularity since their introduction in the late 1900s. This is largely due to their ease of manufacturability and utility in a wide range of fields including optical communication, automotive LiDAR, consumer electronics, and more. EELs and VCSELs are laser diode devices that share several common attributes due to their underlying semiconductor fabrication techniques. They are both constructed using layered structures of semiconductor materials, allowing precise control over their electrical and optical properties as required by their final applications.
These semiconductor-based lasers are inherently compact structures with very small footprints, making EELs and arrays of VCSELs suitable for integration into various systems but more challenging to test individually. The biggest practical difference between EEL and VCSEL is the direction and divergence of the laser emitted from the device. As suggested by its name, EEL devices emit and sense signals from the side or “edge” of the DUT (Detail A) while VCSEL devices emit and sense from the top surface of the DUT (Detail B).
Device Access Is (nearly) Everything
EELs and VCSELs convert electrical energy into coherent light and possess the ability to modulate the intensity of the emitted laser over short distances. Note the keyword here is emitted, where the light source originates from the DUT, implying the need for a clear optical path at the socket/lid level for the laser diode’s source and, in some cases, an integrated photodiode sensor to measure a return path.
You’ll see us talk a lot about our low-profile open-cavity test sockets and lids in the context that the DUT requires exposure to external stimuli or direct access for probing and inspection tools. But what happens when the device itself is responsible for both emitting and sensing a high-frequency optical signal from an angle above or beside the device? Similar design rules apply where low profile unobstructed access to one or more surfaces of the DUT is maintained during test through open air beside the fixture or via a wide-angle opening in the lid
In the case of testing Near Infrared (NIR) and Short Wavelength Infrared (SWIR) using EELs or VCSELs, RTI’s solutions are designed to provide clearance for optical transmission to and from the DUT itself while mitigating crosstalk and maintaining an acceptable operating temperature. Although some design considerations are consistent with traditional package testing, there are unique characteristics of these devices that require more technical solutions at the socket and fixture level.
Test Solutions for EEL Devices
Test solutions for EEL devices require access to the DUT from the side of the package while simultaneously seating, aligning, contacting, powering, and regulating the temperature of the DUT during test. Since the light source emanates from the side, the anode is placed on the top surface and the cathode on the bottom surface of the DUT. Conventional test sockets that contact only the bottom side of a DUT won’t work here.
This custom burn-in fixture for EEL Diodes seats four independent sockets, each with multiple test sites, flush along the test edge of the PCB. Sockets align each diode (DUT) by 3 sides to allow the laser unobstructed area when lasing out the side of the package. Detail A above shows the electrical connection to the top anode pad on the DUT using a spring pin and a thin conductive gold surface on the board to contact the bottom cathode side of the DUT. Additional hot plates are mounted directly to the PCB to stiffen the assembly and draw further heat away from the cathode terminal pad. Precise torsion on the mounting hardware that presses the socket against the DUT and secures it to the PCB is essential to the success. Therefore, specialized hardware and socket designs are required to avoid overstress during device loading.
Test Solutions for VCSEL Devices
Test solutions for VCSEL devices require access to the DUT from the top of the package with a wide angle of visibility and a short working distance. Arrays of VCSEL devices can run much hotter and demand more power than their EEL counterparts. Since the DUT may also be responsible for sensing reflected light, a thin non-reflective coating is applied to the lid surface to minimize glare towards the sensor.
These VCSEL test sockets are manufactured with separated gold-plated copper bodies on the anode and cathode sides for additional heat spreading through the socket material. Since the anode and cathode pads on the device are relatively larger than traditional IC contacts like balls or leads, multiple pogo pins can be evenly distributed across each terminal pad for increased power delivery and simultaneous heat dissipation. (see Detail B)
EELs and VCSELs produce NIR and SWIR nanometer wave pulses at high power reaching several amperes thereby generating a lot of heat with extended use. The performance of these devices can be greatly impacted by sudden changes in device and environmental temperatures. As a result, EELs and VCSELs are both susceptible to crosstalk, misalignment, and interference at the die level as things heat up resulting in unwanted nonlinear effects if not managed properly.
Testing these devices reliably requires maintaining stable temperature conditions under load to ensure accurate and repeatable results, imperative for sensing applications in medical and automotive where failure is not an option. For this reason, RTI developed high-performance solutions with power, thermal expansion, and heat dissipation in mind. Carefully positioned arrays of spring pins at the anode and cathode combined with heat-spreading materials, machined pockets for airflow, and integrated RTD or Thermocouple sensors for device-level temperature monitoring can help you maintain appropriate operational temperatures during test.
Scalable solutions that provide ease of use and long-term reliability require advanced design and manufacturing techniques provided by companies like Robson Technologies, Inc. Contact us today to speak with an engineer about customized test solutions for your next VCSEL array or EEL test program.