AIM:
To study the mode characteristics of fiber optic cable and observe the lower order Linearly Polarized (LP) modes
EQUIPMENTS REQUIRED :
1.LASER Source (633 nm – 1mW)
2.Source to Fiber Coupler
3.Single Mode Fiber
4.Fiber Holding Stand
5.Opaque Screen
THEORY :
The central spot carries 95% of the intensity for laser beams with Gaussian profile. I = Ioe-2(r / w) ^2 where e = 2.718 is the base of the natural logarithm. An accepted definition of a radius of a Gaussian beam is t 0.135 times its peak value Io. This radius is called spot size. The spot diameter is w. Spot Diameter (d) micron = Focal length of the Lens (f) mm x Laser beam full divergence angle (DA) mrad. In order to achieve maximum coupling efficiency, the fiber core diameter has to be bigger than the spot diameter. NA rays = Laser Beam Diameter (B.D.) 2 x Lens Focal Length (f) The source coupler is comprised of two base plates. One of the base plates contains a focusing lens and a female connector receptacle. The other base plate is attached onto the laser. An O-ring is sandwiched between the base plates. Threaded screws interconnect the two base plates. A screw driver to alter the angular orientation of one base plate relative to the other can then adjust the screws. The number of modes propagating through the fiber depends on V-number. If the fiber whose number is less than 2.045, it allows to propagate single mode through it, so it is called as Single Mode fiber. A Multimode fiber, V-number is slightly greater than 2.045 but the number of allowed modes is small enough that they may be individually identified when the output of the fiber is examined. When V < 2.045, then only a single mode may propagate in the fiber waveguide. This mode is HE11 mode or LP01 – Linearly Polarized mode. When V > 2.045, other modes may propagate, when V is slightly greater than 2.045 i.e. V = 4.91 then 4 Linearly Polarized modes will propagate through fibe
PROCEDURE:
1.Keep optical bread board onto original and flat table surface, so that it will not toggle.
2.Fix the pre-fitted cylindrical head of the He-Ne laser source on to the surface of the bread board. Confirm the rigid ness of the mount
3.Fix the laser to the fiber coupler mount on to the bread board with base plate orientation of it towards He-Ne laser exit.
4.Turn on the He-Ne laser and locate the beam spot on the central portion of the laser-fiber coupling lens assembly by adjusting the vertical and horizontal travel arrangement provided with the mount. Tighten the screws of the vertical and horizontal slots.
5.Now look for the back reflection of the He-Ne laser spot from the rod lens of the coupler. In case if you found the back spot, away from the exit of the cylindrical laser head of the laser, adjust the back-reflected spot going back in exit hole by slowly moving the four screws provided for the laser mount.
6.Confirm the central alignment of the laser beam at the exit of the laser fiber coupler by putting a white card sheet and zooming the spot on to it. In case the spot is found of center, adjust it to the center by slightly moving the screws of the laser mount.
7.Put the multimode optical patch cord on to the laser fiber coupler exit and fix the other end of the fiber in the fiber holding stand by moving the grub screws provided with the holder.
8.Notice the bright laser beam spot coming out of the fiber. Adjust the height of exit tip of the fiber to about 50mm. Min. from the white sheet of the paper.
9.Observe the bright round shape circular spot with laser speckle pattern on to the screen. Multimode pattern can be refined by screws provided with laser-fiber coupler. Slightly adjusting or moving the screws on the laser mount, view the change in pattern of this multimode spot. After observing the multimode pattern, change multimode fiber optic patch cord with single mode fiber patch cord.
10.For single mode patch cord, the blur pattern of the various single mode patterns will appear on the screen. That is, single circular two lobes, three lobes and four lobes patterns can be very well observed by slightly adjusting the Allen screws of the laser-fiber coupler
RESULT: