help with photodiode receiver

I hope I am posting in the correct forum, if not please forgive the newbie mistake.
Attached is a circuit used to detect a pulsed red laser 635 - 650nm. Could some one please explain the reasoning behind the use of the inductor. and what effects would it have if I changed the inductor to a higher value. The output signal is fed into an lm324 for appropriate signal amplification.

Currently the ambient light is filtered using an optical filter, would it be possible to implement an electronic filter for the said wavelengths without getting too complex.

The circuit works as it is, I am just trying to learn some of the basics. any possible suggestions to improvement of the circuit are extremely welcomed.

cheers

The circuit is designed to detect the changes in lighting, rather than the level of lighting. That is how IR remote control work, as the pulses are distinctive and can detected.

A photodiode will generate current that depends on the lighting level. If no current is taken from the photodiode, the current that it generates will just charge up its own internal capacitance, up to the forward voltage, and that will mean that the voltage does not vary very much as the light level changes, and those variations will be slow.

The inductor shorts out the photodiode for steady current, that correspond to background light, but any changes in lighting will result in fast changes in current, so large changes in voltage.

If you increased the value of the inductor you would increase the output voltage. However, you can't get more than the open circuit voltage of the SFH206, which is about 0.3 V. Also, larger inductors have larger stray capacitance and larger resistance so you might run into problems there.

Those photodiodes are often used reverse-biased, where a large supply voltage, 5 - 40 V is put across them and the leakage current depends on the light level. They may be more sensitive like that.

You cannot filter the wavelength electrically. The electrical signal depends on the lighting level, and the wavelength. If the lighting level is steady, the electrical signal is steady as well. The fact that the light has a wavelength, and therefore a frequency, has no effect at all on the electrical behaviour. 640 nm represents a frequency of 470 THz (Terahertz) which can't even go down a wire as an electrical signal. It's also a million times faster (literally) than most electronics engineers will ever deal with. Optical filtering is your only way if you need such a narrow band.