Results

Fiber optic light coupling

Once the light was optimized, we were able to observe the following modes by varying the orientation of the fiber (x, y, z, Q, F).

            LP₀₁                                                    LP₁₁

            LP₂₁                                                 LP₀₂?

Core Diameter Calculation

Observed l_cutoff = 791+/- 3nm. 

Therefore, calculated core diameter = 6.03 +/- .022 um.

True core diameter = 6.0 (+/- ?) um. http://www.lasercomponents.de/pdf/ofs/fi_smm-b.pdf

Mode Analysis

The red line represents the V at which we exclusively optimized the throughput of each mode.

We hypothesized that as the V number approached the cutoff wavelength for the LP₂₁ mode, the intensity would significantly increase.  Simplistically, it should continue to grow (or at least remain steady) past the cutoff wavelength.  The LP₁₁ mode has no cutoff in this wavelength region, therefore we thought it should be unaffected.

The results displayed here appear to be too complicated to be explained by our hypothesis.  The LP₂₁ mode does increase as it passes through the cutoff V (3.832).  However, once past the cutoff wavelength and the wavelength at which it was optimized, the intensity decreases, which we cannot completely explain.

Further, the LP₁₁ mode displays the same effect as V approaches the wavelength at which it was optimized, but it continues to increase with V above that point.

This analysis is further complicated by the fact that the intensity of the laser varies as a function of wavelength.  These data are displayed after having been referenced to the laser's intensity for each wavelength.  

Though the LP₂₁ mode cutoff appears as predicted observationally, the intensity dependence seems to be multi-factorial.  The difficulty arises in that the ability of a mode to propagate is dependent not only on the orientation of the light entering the fiber, but also on the wavelength of the light.  

Thus, though we can optimize the orientation such that only the LP₂₁ mode is transmitted at a given wavelength, as we measure the intensity of light as a function of wavelength, more than the LP₂₁ mode may be able to propagate at all but the single, optimized wavelength.

We have not been able to fully elucidate this wavelength and orientation dependence.  The following video displays a mode transition between LP₂₁ and LP₁₁ over a period of about 5 nm.  LP₂₁-LP₁₁ transition

(There are two images due to the fact that the beam splitter has two reflective surfaces)

Conclusions