Development of elliptical-core PM ZBLAN fiber

Last updated on 04/24/2021

This post introduces polarization-maintenance ability of an elliptical-core PM ZBLAN fiber that we manufactured recently.

Introduction

Polarization-Maintaining Fiber (PMF) is used when the maintenance of linear polarization is critical as light propagates through an optical fiber. The most commonly used one is a PMF by internal stress (see Figure 1), where two stress-applying parts positioned on both sides of the core produce thermally induced birefringence.

Non-PM fiber and different types of PM fibers

Figure 1: Non-PM fiber and different types of PM fibers.

Alternatively, a PM fiber can be made by making the shape of the core elliptical. The manufacturing process does not require drilling two holes (for inserting stress-applying parts) in the glass, and thus is much simpler. This feature is beneficial for production for ZBLAN-based PM fiber, as mechanical processing of ZBLAN glass is much more difficult than that of silica glass.

Fiber structure and characterization

The cross section of our elliptical-core ZBLAN fiber is shown in Figure 2. The size of the elliptical core is 8.2 µm × 3.2 µm. The core numerical aperture is 0.2.

Cross section of elliptical-core PM ZBLAN fiber

Figure 2: Cross section of elliptical-core PM ZBLAN fiber.

The ability of maintaining the polarization state can be quantified by polarization crosstalk (see Figure 3 for schematic and definition). We measured the polarization crosstalk of our PM ZBLAN fiber using an experimental setup shown in Figure 4. An output from an ASE source (wavelength: 1550 nm) was polarized by a polarizer at the input, and the polarization state was aligned to the long axis of the ZBLAN fiber. The light was then launched to the core of a three-meter-length PM ZBLAN fiber, and as the light propagated along the fiber, a tiny portion of the light coupled to the short axis by polarization coupling. The degree of polarization coupling was quantified by the follwoing three steps: (1) placing another polarizer (called analyzer) at the output, (2) rotating the polarizer, and (3) measuring the max and minimum transmitted power (i.e, measuring P0 and P1 in Figure 3).

Schematic and definition of polarization crosstalk

Figure 3: Schematic and definition of polarization crosstalk.

Experimental setup

Figure 4: Experimental setup.

Figure 5 displays the measured transmitted power after the analyzer. The oscillatory behavior with a period of 180 degrees clearly shows that the linear polarization was maintained in the fiber. The measured polarization crosstalk was less than -15 dB, showing that this fiber possesses sufficient capability as a PM fiber.

Measured polarization crosstalk

Figure 5: Measured polarization crosstalk.

Rare-earth-doped PM ZBLAN fibers for polarized light source

One main interest of such a ZBLAN fiber would be the construction of polarized fiber lasers and amplifiers, as ZBLAN fibers are inherently superior to silica fibers in terms of a wider range of emission wavelength from visible to the mid-IR.

In addition to passive (non-rare-earth-doped) PM ZBLAN fibers, we have already manufactured some thulium-doped elliptical core fibers and have them in stock. So please feel free to contact us if you are interested in our stock fibers, as well as a custom draw using other rare-earth dopants of your interest.

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