Bi-directional transport of light

[bluetooth]

In several fiber-optic-based probes in medical imaging fields, the light travels towards an object through an optical fiber (or even free space), interacts with the object and then travels back through the same fiber (or the same path in free space) and is captured by a camera or photodetector, etc. How come the light that is returning from the object does not interact with the light that is traveling toward the object?
In case of fluorescence imaging, the excitation beam reflects off of a dichroic mirror, passes through set of focusing lenses, falls on a sample and induces fluorescence in the sample. The fluorescence (higher wavelength) - captured by the lenses - travels 'backwards', passes through the dichroic mirror and is then captured by a camera. The two wavelengths (excitation and emission) travel the same path in opposite directions. Why do they not interact with each other?
Thanks.

 

[mfb]

Light does not interact significantly with light in free space and most materials - why should it? There are materials and setups where those interactions are relevant (this is the field of nonlinear optics), but those are not used if undisturbed bi-directional transmission of light is required.

 

[bluetooth]

I think my confusion stems from lack of clear understanding of light transport at quantum level. Generally, I do not know what happens when two beams travel in along the exact same path in the opposite direction. Do the photons traveling toward each other never collide ? Is there never a constructive or destructive interference for light beams traveling in opposite direction? What will happen if you launch light (of same wavelength) into both ends of a fiber optic?

 

[bluetooth]

I understand that the light traveling through free space will not interact too much with the surrounding light. I am mainly talking about the light within the imaging system. How is the light traveling "toward" the specimen is distinguished from that returning "from" the specimen? Why do photons traveling in different directions not interfere with each other?

 

[Andy Resnick]

light scattering light (in a vacuum) cannot occur classically because Maxwell's equations are linear. To be sure, nonlinear optics, a classical phenomenon, allows for light-light scattering in matter through altered material properties- for example, the Pockels and Stark effects. Light-light coupling in a vacuum is purely quantum and is described as the creation and subsequent annihilation of a electron-positron pair from the two incident photons.

A detailed calculation of the photon-photon scattering amplitude is carried out in Landau & Lifshitz, CTP vol. 4 (Quantum Electrodynamics), pp 566-573.

 

[mfb]

I think my confusion stems from lack of clear understanding of light transport at quantum level. Generally, I do not know what happens when two beams travel in along the exact same path in the opposite direction. Do the photons traveling toward each other never collide ? Is there never a constructive or destructive interference for light beams traveling in opposite direction? What will happen if you launch light (of same wavelength) into both ends of a fiber optic?

There is simply no possible "collision process" (to an extremely good approximation). Light interacts with charged particles only, and light is not charged. There is interference if you put a screen there, but otherwise it does not matter.

How is the light traveling "toward" the specimen is distinguished from that returning "from" the specimen?

By their phases at different points.