Setup and Procedure

Setup

The experimental setup was as diagrammed below:

Laser: Titanium Sapphire ring laser, pumped by a frequency doubled Nd:YAG laser, with an effective tuning range from 750-900 nm. 

Fiber Optic Coupler: Newport with aspheric lens (NA =.16).

Collimating lens: LA1951 plano-convex (f = 25.4mm)

Beam splitter

Focusing lens: LB1761 (f = 25.4mm)

Canon Camcorder (effective range out to 1 um)

Fiber: ThorLabs CF04246-30

Power Meters: Newport optical power meter and Melles-Griot thermal power meter.

Step Motor: 1.604 x 10^-4 nm/step.

Procedure

Fiber optic light coupling

• Cleave, examine, and mount fiber at both ends.
• Minimize stress on fiber.
• Match NA for objective lens and fiber. 
• Setup power meter to display total optical throughput.
• Maximize optical throughput by adjusting the orientation (x, y, z, Q, F) of the fiber in the Newport coupling mechanism.
• Setup camcorder to observe collimated image.
• Observe various modes by adjusting x, y, z, Q, F and the wavelength of the laser, l.

Mode Analysis

• Couple the light into a given mode (e.g. LP₂₁ or LP₁₁) by adjusting orientation of fiber and wavelength of light.
• Tune the laser to a wavelength sufficiently different from the l_o at which the mode was optimized. 
• Set the Lab View Step Motor VI to scan through the optimal wavelength, l_o.
• Position the step motor so that the motor will engage the gears on the laser's micrometer tuning gauge.
• Adjust the power meter level to optimize output reading on 0-1V scale by varying the ND filter setting before the Newport optical coupler.
• Run the VI.

Core Diameter Calculation

• Tune the laser to a wavelength regime in which the LP₂₁ mode can be observed.
• Adjust the orientation and position of the fiber to exclusively transmit light in the LP₂₁ mode. 
• Systematically increase the l of the laser, (lower the V) until the mode is no longer observable. 
• Record the cutoff l.

Results