The overall goal of this procedure is to create an optimized supercontinuum in the mid-infrared through tapering a calgene eye fiber. This is accomplished by first constructing the fiber tapering setup, which allows for spectral monitoring to determine the level of broadening during the tapering of a calcite fiber. The second step is to prepare a short arsenic tris sulfide fiber to be tapered.
Next, the optical fiber is inserted into the tapering setup with the pump source coupled to the fundamental mode of the fiber. The final step is to heat the fiber and begin the tapering process, terminating the tapering when the spectral measurement is maximized. Ultimately a coherent and instantaneously broadband supercontinuum in the mid-infrared will be generated after passing through the in C two tapered fiber.
The main advantage of this technique over existing methods where tapering to a set fiber diameter is determined by simulations, is that an optimally broadened spectrum can be obtained in a single fiber taper experiment. We first had the idea for this method when we realized that efficient super continuum generation in a taper called cogen fiber could be adversely affected by the imperfect reproducibility of the tapering process. So we thought we could experimentally optimize the super continuum generation by observing the spectrum during the tapering process To build the tapering setup, secure two motorized linear stages approximately in the center of a 12 inch by 12 inch breadboard.
The stages should be in contact and oriented to translate towards and away from one another along a line. Prepare the fiber mounts by attaching identical optical posts to each of the linear stages using the holes that are closest. Then attach bare optical fiber mounts to the tops of the posts.
Make sure the V grooves of the fiber are aligned. The height of the V grooves will be approximately the beam height for the experiment. Next, prepare the input and output coupling elements.
Attach a linear translation stage to each of the motorized linear stages for input coupling mount, an anti-reflective coated zinc selenide lens, a focal length 12.7 millimeters on a pedestal in an optical amount with X and Y translation as shown here. Place the lens on the input translation stage for output coupling. Place an identically mounted uncoated zinc selenide lens of focal length 20 millimeters on the output translation stage.
Make sure the center of each lens is at the same height as the V grooves with the fiber clamps. The last step is to position the heating element. The heating element is an aluminum block machine to accommodate the fiber, temperature monitoring devices and cartridge heaters.
Secure the heater to an X, Y, Z linear stage and mount the stage on the breadboard. Ensure that the hole in the heater for the fiber can be centered with the V grooves of the fiber clamps. Translate the heater away from the clamps before inserting a fiber to produce a fiber for the experiment.
Soak at least 8.5 centimeters of jacketed arsenic tric sulfide fiber per desired taper in acetone until the jacket becomes soft about 10 minutes. Gently remove no more than five centimeters of the softened jacket at a time with a delicate task wipe. Once done, clean the bare fiber with isopropanol on a delicate task wipe now.
Use a beaver tail cleaver to cleat the end of the fiber. Avoid contact with the cleaved end. If the cleave is good.
Measure a length of fiber at least six point 35 centimeters long from the cleaved end, and break the fiber at that point. Once again, use the beaver tail cleaver and cleave the second end of the fiber while avoiding contact with the center of the fiber and the cleaved ends. Place the fiber between the fiber clamps of the tapering setup.
The first step in the tapering procedure is to couple a mid-infrared pump source to the fundamental mode of the fiber with the anti-reflective coated sink selenide lens. Use the uncoated lens to image the output of the fiber with a pyro cam to ensure the power is mostly in the fundamental mode. Make sure the pump beam is propagating along the axis of the fiber or the coupling will change when the assembly begins to move.
Next place a chopper in front of the pump source. Then use one uncoated calcium fluoride lens to direct the output of the fiber to the monochromator. Use another identical lens to couple the monochrome output to the indium antimonide detector.
Rotate the grading of the monochrome to allow the long wavelength side of the spectrum to pass through. Continue until the transmitted signal is barely above the noise floor at a wavelength of about 3.9 micrometers. In the case of the 3.1 micrometer pump source used here, now translate the aluminum heater until the fiber can slip through the slit.
Center the fiber in the heater's fiber hole. Gently press the resistance temperature detector against the aluminum heater so that it is level with one of the cartridge heaters. Ensure that the detector is fully in contact with the heater or risk causing the fiber to break during tapering.
Make sure the signal to the monochrome has not decreased to reduce airflow and improve temperature stability. Cover the setup with a box with holes for the input and output beams. Connect the temperature detector and the cartridge heaters and turn on the temperature controller.
Set the temperature to about 200 degrees Celsius. Once the temperature is stable around the set point, start the software that translates each motorized stage at about 10 micrometers per second away from the other. Monitor the spectral measurement signal.
Once the detector signal reaches its maximum value, stop the motorized stages and turn off the temperature controller. Wait for about 10 minutes for the fiber to solidify the detector signal will decrease a little during this time. Remove the covering box and translate the heater Block along the fiber away from the tapered region.
Then translate the heater block away from the fiber. Using the slit to allow the fiber to pass the fiber is now ready for characterization of its super continuum generation using spectral measurements with the monochrome, rotate the grading of the monochrome to capture the super continuum on the detector. Keep in mind, different parts of the spectrum may need to be captured on different detectors to prevent overlap with higher order deflections of the grading.
The taper waste of the fiber is too small to observe by eye. These scanning electron microscope images of a fiber broken at the taper give an idea of the potential scales of the feature. On the left is an arsenic tris sulfide fiber that has been tapered to a diameter of about 2.3 micrometers close to the optimal for supercontinuum generation with a 3.1 micrometer pump source.
The right image shows the smallest taper diameter created using the setup about 760 nanometers. This data compares the normalized spectra of the input pump in red and the output super continuum In blue, note that the generated bandwidth of the output is about three times broader than the input at 40 decibels below the peak. The dip in the output spectrum around 4, 200 nanometers corresponds to absorption by carbon dioxide in the atmosphere here.
The normalized output power after the monochrome is shown as a function of the increase in the separation between the fiber mounds after tapering referred to as the pulling length. The pulling length can be related to the fiber diameter shown across the top. With the monochrome set at 3.9 micrometers, the output power begins to rise dramatically after 17 millimeters of pulling length.
The maximum signal occurs at close to 18 millimeters of pulling length or fiber diameter of about 2.3 micrometers. Following the institute fiber tapering procedure, other fiber devices fabricated through fiber tapering such as fiber couplers and dispersion compensators can be optimized as well. After watching this video, you should have a good understanding of how to produce an optimized supercontinuum in the infrared by using in PS tapering of coco optical fibers.
