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

 

Introduction

Polarization-Maintaining (PM) fiber 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.

 

Figure 1: Non-PM 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.

 

Figure 2: Cross section of 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).

 

Figure 3: Schematic and definition of polarization crosstalk.

 

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.

 

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.