This research work was supported by a Paul Instrument Fund grant (September 1993 – October 1995). The aim was to implement an optoelectronic configuration [1-3,6,8-10] of a clipped correlator, based on an original idea of prof. D. A. Jackson and J. D. C. Jones. Ryan Harding studied for a PhD on this project and added his excellent computing expertise and patience in fusing fibre.

The aim was to expand the number of channels and the speed of a previous version of an 8 channel, 10 ns correlator built in Kent. The research was very challenging in terms of high speed electronics. We designed and built very fast photodetector modules on PTFE board (amplifiers up to 45 dB, 4.5 GHz), 85 ps step recovery diode shapers, high speed powerful distributors (based on GaAs powerful transistors, logical circuits using GaAs ICs (125 ps) and switchable delay line blocks (0.2 ns). The PTFE boards where produced either outside using state of the art gold plating and metallic through holes or in house using a milling machine for microwave circuits, specially purchased to assist this research. As the majority of electronic circuits in the correlator used surface mount components, the work was very demanding in terms of miniaturisation, adjustment and maintenance.

At the end of the project, an original hybrid configuration of the fastest multichannel digital correlator with 0.25 ns lag time and 640 channels [1] was completed. The remarkable high speed was achieved with a relatively low cost by an in-house design and implementation of very fast electronic AND gates based on double-gate GaAs transistors [6]. We also showed [2] how the Fibre Optic Digital Correlator configuration (potentially very fast but rigid in terms of channel bin) could be used in tandem with an electronic clipped correlator (versatile in terms of channel bin but potentially slower) for extended versatility in terms of delays and number of channels for time bins over 10 ns.

We demonstrated the retrieval of signal from noise at up to 2 GHz (the fastest reported at the time in a multichannel configuration and highly appreciated by the participants at the OSA Conference “Photon correlation and scattering ’96” [9].)

A comparative analysis of the fibre fixed delay correlator and the clipped electronic correlator is presented in [5] and we showed that this configuration does not necessarily need a derandomiser to recover a periodic signal buried in noise [1].

We experimentally demonstrated that a self-pulsating laser with optoelectronic feedback can be used [4] for the adjustment of the delays.

We devised two altered versions of the correlator which could act as a multichannel time of flight analyser for time-resolved imaging and investigation of human tissue, such as babies’ heads and breast tissue (unpublished results).

Publications

Journals
1. A. Gh.Podoleanu, R. K. Harding, D. A. Jackson, “Practical implementation of a high-speed multichannel correlator with fiber-optic delays”, Appl. Opt. (1997), Vol. 36, No.30, pp.7523-7530.
2. A. Gh.Podoleanu, D. A. Jackson, “Comparative Studies of an Electronic Clipped Correlator and an Optical Fiber Fixed Delays Correlator”, Opt. Engineering, 35, (6), pp.1565-1572, (1996).
3. A. Gh.Podoleanu, R. K. Harding, D. A. Jackson, “Low Cost High-speed Multichannel Fiber-Optic Correlator”, Opt. Lett., vol.20, pp. 112-114, (1995).
4. A. Gh.Podoleanu, R. K. Harding, D. A. Jackson, “Delay Measurements Using a Self-pulsating Laser with Optoelectronic Feedaback2, Meas.Sci.Technol., 6, pp. 371-375, (1995).
5. A. Gh. Podoleanu, D. A. Jackson, “Simple Very Low Cost High Speed 2 Input AND Gate”, Rev. Sci. Instrum.66 (9), pp. 4698-4702, (1995)
6. A. Gh. Podoleanu, D. A. Jackson, “Combined Electronic Clipped Correlator and Fibre-Optic Correlator”, Electron Lett., vol.31, No.17, pp. 1492-1494, (1995).
7. S. S. G. Pasca, A. Gh. Podoleanu L. I. Plesea, “One Counter Photon Arrival Timer”, Measurement Science and Technology, vol.4, pp. 1404-1409, (1993).

Conference Proceedings:
8. A.Gh.Podoleanu R. K. Harding D.A.Jackson 40-channel 1-nanosecond Digital Fiber Optic Correlator Intern. Conf. on Applic. Of Photon Technology (ICAPT’96), G.A.Lambropoulos ed., Montreal Canada, 29 July-1 Aug. 1996.
9. A.Gh.Podoleanu, D.A.Jackson, “160-Channel 0.5 nanosecond Hybrid Digital Correlator Based Upon Fiber Optic Delay Lines”, Technical Digest of Photon Correl. & Scatter., OSA Confer., Aug. 21-24, Capri, pp. FB3-3/111-113, 1996.
10. A.Gh.Podoleanu, R.K.Harding and D.A.Jackson Very Fast Optoelectronic Correlator 6th Joint EPS-APS Intern. Conf. on Physics Computing 94, R. Gruber, M. Tomassini eds., Lugano, Aug. 22-26, pp. 531-534, (1994).