Design and Fabrication of an Optical Fiber Made of Water

CAUTION: This experiment involves high voltage. It is the reader's responsibility to verify with the safety authorities that their experiment follows regulations before turning on the high voltage.

NOTE: Any kind of polar liquid can be utilized to produce liquid fibers, such as ethanol, methanol, acetone, or water. The polarity of the liquid dictates the stability and diameter of the created fiber^23,24. For best results, use deionized water with 18 MΩ resistance. Before choosing optical components, such as optical fibers and light sources, consult the literature to ensure a low absorption in the water/liquid fiber at the desired optical wavelength. The protocol can be paused at any given moment before filling the water reservoir (step 4.5).

1. Preparation of Water Reservoirs and Experimental Station

1. Manufacture two poly(methyl methacrylate) (PMMA) reservoirs with magnetic connectors for the pipette and the high voltage, according to Figure 1.
   1. Cut two PMMA plates to 60 x 50 x 10 mm in size, drill cavities of 8 mm in depth and 7 mm in diameter on the backside of the plates. Glue connector magnets inside the cavities.
   2. For the capillary clamp, cut a stripe of PMMA to 45 x 10 x 2 mm and glue two magnets on the top side of it.
   3. For the electric connector, wrap magnets in a small piece of metallic foil and connect it electrically with crocodile clamps to the high-voltage (HV) source. The reservoirs hold approximately 100 – 300 µL of water. Place the wrapped magnets in fluidic contact with the water in the reservoir.
      NOTE: Preferably, use magnetic connectors for clamps and high voltage. If possible, do not to use any kind of glue to attach the clamps or the connectors, as many types of glues dissolve under the influence of high voltage or in the presence of electric arcs and diminish the water fiber stability or optical quality.
2. Mount one PMMA reservoir on a 5-degree-of-freedom (DOF) micro-positioning stage.
3. Thoroughly clean all connectors and areas with isopropanol (spectral grade) followed by deionized water. Blow-dry with nitrogen. Cover the PMMA water reservoir and all clamps with polytetrafluoroethylene (PTFE) tape to avoid any leaking or dripping of water.
4. Position the set-up under an optical microscope for imaging. Use far-field objectives (5X, 0.14 NA, and 34 mm WD for long water fibers and 20X, 0.42 NA, and 20 mm WD lens for short water fibers) to avoid unwanted grounding between the HV area of the water fiber and the electrically conductive microscope set-up.
5. Set up two optical fiber clamps on linear transitional stages, one behind each water reservoir, according to Figure 1. Each fiber coupler should be able to move back and forward within its micropipette (discussed in the following section).

2. Choosing the Micropipettes and Voltage

1. The inner diameter of the micropipette ensures a maximum radius of the fabricated water fiber. To create a 5-µm radius water fiber, use 150-µm-inner-diameter pipettes, paired with 125-µm-diameter optical fibers. For thicker (20 – 90 µm) and longer (800 – 1,000 µm) water fibers, use micropipettes with an inner diameter of 850 µm.
   NOTE: As a rule of thumb, the water fiber maximum length is estimated by multiplying the maximal radius by 25. For details, refer to Table 1.
   1. Break the micropipette by hand over an edge to a length of 3 cm.
2. To create water fibers with a diameter of up to 110 µm, apply a voltage between the two water reservoirs between 1.5 kV and 3 kV. For water fibers reaching up to a millimeter in length, apply up to 8 kV. Compare with Figure 1 for electrical wiring suggestions.

3. Preparation of the Optical Couplers

NOTE: For the best transmission result, use a single-mode tapered fiber to launch laser light into the water fiber and a highly multimode reflowed fiber lens as the output coupler (core > 100 µm). However, for easy operation, use a low multimode fiber as the output coupler (for example, a 1550-nm single-mode fiber for a 780-nm wavelength).

1. Fabrication of a Tapered Fiber Coupler
   NOTE: See Figure 2.
   1. Strip the 780-nm single-mode fiber with a fiber stripper from its plastic cladding to expose an area of 10 – 15 mm of bare fiber. Clean the exposed area with delicate task wipes in combination with acetone. Pass the fiber through the desired micropipette before tapering it. Taper the fiber below the single-mode criteria with a slope smaller than 1/20.
   2. Use a hydrogen flame for tapering the fiber with a flow rate of 140 mL/min, while simultaneously pulling the taper from both sides at 0.06 mm/s.
      NOTE: The tapered portion is in total between 6 to 9 mm. If the fiber breaks before reaching the single-mode criteria, adjust the hydrogen flow to higher rates or place the fiber in a hotter area of the torch. If the area is longer, adjust the hydrogen flow to lower rates or place the fiber in a colder area of the torch.
   3. Turn off the flame and carefully increase the tension in the fiber until it breaks at its thinnest spot. Use this tapered fiber as the input coupler.
      CAUTION: The tapered fiber is fragile.
2. Fabrication of a Fiber Lens Coupler
   1. Strip the 1550-nm single-mode fiber tip with a fiber stripper and clean the exposed area with delicate task wipes in combination with acetone. Choose and prepare a pipette as described above and pass the fiber through it.
   2. Heat the tip with an electric fusion splicer or CO₂ laser at 15-W power, focused through a 200-mm lens, until the glass fiber end becomes liquid and forms a slightly rounded shape, known as a fiber lens.

4. Assembling

1. If not done yet, insert the fiber couplers into the desired micropipettes.
2. Clamp the micropipette, using the premanufactured, magnetic PMMA clamp, with the fiber couplers onto the PMMA reservoirs. The non-tapered side of the micropipettes should reach into the water reservoir. Clamp each of the fiber couplers on a linear positioning stage.
3. Connect the tapered fiber coupler to a 780-nm, continuous wave, fiber-coupled 10-mW laser source and the fiber lens coupler to a power meter. Fill the reservoir with water and ensure that no air bubbles are stuck in the micropipette. If necessary, push or pull them with the optical fiber coupler (from step 3.1 or, accordingly, from step 3.2).
   NOTE: At this stage, following the optical path, the stations are: the laser light source, the optical fiber, (and this fiber goes through) the fiber clamp on a linear stage, the water in the reservoir with electrical connection, the micropipette filled with water, the optical tapered fiber coupler, free space (later: water fiber), the fiber lens coupler (now the second fiber), the micropipette filled with water, the water reservoir with electric connection, the fiber clamp on a linear stage, and, lastly, the power meter.
4. Connect the ends of the mounted micropipettes by adjusting the 5-degre-of-freedom mount of the PMMA water reservoir to establish a fluidic contact between the micropipettes. Turn on the light source and the power meter. Adjust the fiber couplers to have a transmission with the help of the 5-DOF PMMA water reservoir mount.
   NOTE: Use appropriate laser safety equipment.
5. Connect the high voltage electrically with the water reservoir by placing the magnetic connectors wrapped in metallic foil over the magnetic counterparts in the PMMA water reservoir and attaching crocodile clamps to the metallic foil. Connect the crocodile clamps via electrical cables to the HV source (Figure 2a).

5. Running the Experiment

1. Increase the voltage to the desired value. A starting point for a very short and narrow bridge is 1.5 kV. Stable bridges with 100 µm and more in length can be achieved with 2.5 – 3 kV.
2. Slowly increase the distance between the micropipettes to the desired length according to the choice of the micropipettes (Figure 2b and 2c). Adjust couplers and pipettes with the 5-DOF stage and the 1-DOF stages to optimize the optical transmission.
3. Measure the coupling efficiency by taking a measurement on the power meter and taking the ratio of the coupled-in to the coupled-out laser power.
4. Disconnect the power meter and connect a photoreceiver to the output fiber coupler. Connect the photoreceiver to an oscilloscope. Record time trace measurements of the transmitted light, representing the capillary water fiber oscillations.
5. Convert the time trace measurements via Fast Fourier Transformation to frequency domain. Take the central frequency over full width at half maximum to receive the capillary quality factor.
   NOTE: Create a spectrogram to check for frequency jitter.
6. Use the top view microscope set-up to characterize the geometrical structure of the water fiber. The fiber radius is obtained at the thinnest portion of the water fiber.