Our need is to adapt commercially available and procurable Blu-ray Optical Pickup Unit (OPU) technologies for micro and nanoscale applications.
The critical point about OPUs is that they actually make incredibly fast (100 MHz +) analogue distance measurements rather than simple binary ones. This means that they can be repurposed to act as extremely sensitive, very high bandwidth and surprisingly cheap displacement sensors for micro and nanoscale applications.
The challenge is to identify a readily available and purchasable Blu-ray OPU and then design and test an electronic circuit to power the OPU laser and photodiodes and digitise the photodiode signals at the best possible signal-to-noise.
The digitised photodiode signals from the main quadrant photodiode (typically labelled as A, B, C and D) must be digitised at sampling rates of at least 10 megasamples per second.
BACKGROUND ON OPUs:
Optical pickup units (OPUs) are the core component of any optical disc drive be it CD, DVD or Blu-Ray. These units contain the laser diode to generate the light used for reading and writing data to disc, along with the necessary focusing lenses and optical components to collect the reflected light from the disc and shine it on a quadrant photodiode – ready for interpretation as binary 1s (from the flat disc) and 0s (from pits in the surface of the disc).
In order to determine whether the focussed light is falling on the flat surface or into a pit the OPU actually makes a continuous, analogue measurement of the distance of the reflector (the disc) from the focussing lens. As the light falls into a pit this distance is increased and the electronics attached to the OPU interpret this as a binary 0.
The critical point is that OPUs actually make incredibly fast (100 MHz +) analogue distance measurements rather than simple binary ones. This means that they can be repurposed to act as extremely sensitive, very high bandwidth and surprisingly cheap displacement sensors for micro and nanoscale applications.
The key challenge for anyone attempting to use an OPU for this application lies in the fact that they are not available for purchase by normal consumers, instead they are sold in bulk to disc drive manufacturers or used in-house for the same purpose. This means that obtaining the information necessary to correctly power and read the analogue displacement data back from OPUs is very difficult – not because it is IP protected, but because the information is proprietary and not publicly available.
We have previously been highly successful in operating DVD OPUs correctly. However, DVD OPUs are becoming harder to source in bulk and are also less performant (i.e. have lower measurement bandwidths etc.) than Blu-Ray OPUs.
We seek Solvers who can help us identify and operate Blu-Ray optical pickup units at their best performance levels. The best Solver will be one who identifies a readily purchasable Blu-Ray OPU and then designs and tests an electronic circuit to power the OPU and obtain the best signal-to-noise possible from both the laser diode and the photodiode.
Key considerations and optimisations we have discovered during our work with DVD OPUs are:
1) The laser diode should be powered via an automatic power control (APC) circuit, making use of the laser intensity monitoring diode built into the laser diode itself to maintain a stable output intensity. In our experience this APC circuit is occasionally detailed in an appendix of an OPU datasheet (see our example DVD OPU datasheet).
2) The laser diode output should be oscillated via a high-frequency modulator (HFM) to achieve the best intensity output. This component is often included on the OPU itself (see our example DVD OPU datasheet).
3) Any circuit path lengths from the photodiode (the detector) segments to the analog-to-digital convertors must be kept short to maintain good signal-to-noise.
The ideal outcome is that the Solver includes 100 MS/s or greater analogue to digital conversion of all four signals in their circuit (alongside a suitable digital communication protocol). However, a good part solution would be to provide practical access points (BNC, SMB, test points etc.) for us to perform our own digital conversion in external hardware.
• The proposed solution must use available, affordable technology from a sustainable and reliable supply chain.
• The proposed solution must enable access to the four main photodiode segments, usually labelled A, B, C and D, so that we can derive a displacement signal from them.
• Identification of selected OPU technology and supplier (including OPU datasheets).
• Design & test of electronic circuit to power the laser and photo diodes and to read back the four main photodiode segments. The four photodiode signals should be digitised at 16-bit resolution with a sampling rate of at least 10 MS/s. A suitable digital communication protocol to read back these digitised signals should also be selected.
• Simultaneously recorded measurements of all four photodiode segments so that the achieved signal-to-noise ratio can be assessed.
• Willingness for collaborative support, in the form of being available for virtual discussions, so we can work with you until the technology has been fully adopted and satisfies our micro and nanoscale measurement application requirements.
Based on a 1-10 scale (1 is not important - 10 is critical)