I am doing some measurements using an LG02 beam and I find that the inner diameter is decreasing with increasing beam energy. This has already been seen. This means that the valley of the LG beam must be crossing the threshold. My simulations show no increase in the valley height of the LG fluence profile. What is then causing the decrease of the inner diameter?
I have taken the information from the following paper:
Appl. Phys. Lett. 112, 211103 (2018); doi: 10.1063/1.5027661
The threshold for a mode of the laser is defined for the whole electromagnetic field (in interaction with the active medium) as an ensemble, and not as a local quantity. The fact that an annular -- or "doughnut" -- beam possesses a zero amplitude at its center is a structural property and does not imply local threshold crossing. One should not confuse the fact that there is a residual gain in the center of the beam (assuming the gain is homogeneous throughout the medium), unexploited by the particular mode, with threshold crossing.
The LG beams properties are independent of the intensity. So the inner radius of the LG beam will not change with intensity. May be you observed decrease in the inner radius with increasing intensity because of detector saturation. Please check it.
You can see this paper for vortex properties
Reddy, S.G., Permangatt, C., Prabhakar, S., Anwar, A., Banerji, J. and Singh, R.P., 2015. Divergence of optical vortex beams. Applied optics, 54(22), pp.6690-6693.
The threshold for a mode of the laser is defined for the whole electromagnetic field (in interaction with the active medium) as an ensemble, and not as a local quantity. The fact that an annular -- or "doughnut" -- beam possesses a zero amplitude at its center is a structural property and does not imply local threshold crossing. One should not confuse the fact that there is a residual gain in the center of the beam (assuming the gain is homogeneous throughout the medium), unexploited by the particular mode, with threshold crossing.
I feel certain Mr. Lecoeuche and Mr. Lippe are correct. The LG 02 mode probably isn't really changing. The overall sizes of the modes may be varying somewhat due to increased thermal lensing, and some of what you are seeing may be the whole mode getting smaller. However, based on your description it is more likely that what is actually happening is that you are adding other modes as the available gain increases.
The doughnut mode does not couple well to the gain in the center of the gain medium. Although the doughnut mode is favored (has less loss and more gain) in your cavity so that it is the dominant mode at low energy, when you increase the energy eventually there is sufficient gain for other modes to lase. Which modes are added depends on how much loss each mode encounters in the cavity (primarily determined by diffraction). However it also depends on how well they overlap the gain, so naturally modes which use the gain that the LG 02 mode doesn't, i.e. modes which have more intensity toward the middle, will tend to lase next. So your doughnut will tend to fill in as you increase the energy.
I agree with Mr. Albert that, as a consequence of thermal lensing, there may be a certain, weak amount of other modes which are excited as the pump is increased. If these modes do not drastically differ from the main one and their components are small, then it will be difficult to detect them even through fitting. We studied thermal lensing in a passive atomic vapour some years ago (T. Ackemann et al., Optics Communications 147, 411-428 (1998)). If you take a look at the paper you will see that the effects can be rather subtle but, when placed in a resonator, a lens can sufficiently change its properties to give transfer excitation to "unexpected" cold cavity modes.
Of course, I am excluding the possibility that the observed effect is not the simple inner radius reduction due to absolute thresholding in the measurement, rather than measuring at the 1/e point.
I hope firs Mr Sahoo has to comment on how he is obtaining LG02 mode. May be converting the mode externally ?
I am using a vortex wave plate to convert Linearly polarised TEM00 mode to LG02 mode.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Optics