Photodetector circuit

I am going to simulate a photodetector circuit within Orcard, but I don't understand the circuit clearly. can any 1 help me to analyse the circuit? the circuit is Fig7 in attached PDF.
And what's more, i want to know how can I simulate a photodiode when only have diodes and sources.
Many Thx!

Use LTSpice. Join the Yahoo LTSpice Users Group. This will give you access to the contributed library of Spice Models, which includes several photodiodes...