Articles

PANDA-style fibers move beyond telecom

August 02, 2004

By: A. Carter and B. Samson

Advances in design and fabrication techniques have brought PANDA-style polarization-maintaining fibers into use in fiberoptic gyroscopes, as well as in linearly polarized fiber lasers.

Despite being a technology more than 20 years old, PANDA-style polarization-maintaining (PM) fibers are undergoing a new lease on life. (Panda stands for polarization-maintaining and absorption-reducing.)

Originally developed for the telecommunications industry, PM fibers filled the need for low-cost, high-volume, high-reproducibility fiber. Recently, specialty-fiber companies have introduced a variety of product developments aimed at diverse markets such as fiberoptic gyros and high-power fiber lasers. The technology is now being applied outside telecom to a new family of fiber products, which includes high-birefringence fibers for fiberoptic sensors, such as the fiber gyro, as well as polarizing-fiber designs for linearly polarized fiber lasers.

PANDA-PM fiber technology

Light waves can be defined as oscillating electric and magnetic fields that are perpendicular to each other and to the direction of propagation. In a standard single-mode fiber, guided light propagates as an ensemble of discrete waves, with their electric and magnetic fields randomly oriented and fluctuating in response to changes in environmental conditions that include temperature, pressure, and mechanical stresses. Fibers are never perfectly symmetric, and propagating light is thereby subject to a small but nonetheless significant modal birefringence. Consequently, standard single-mode fiber can be thought of as a bimoded fiber with each mode having orthogonal linear polarization. As the propagation characteristics of these two modes are quite similar, it is easy to transfer power between them, thereby altering the state of polarization of a guided wave.

For several fiberoptic-based sensing applications (such as fiberoptic gyroscopes) and optical signal-processing applications (including coherent communications) the polarization state of the guided wave must be conserved over long fiber lengths. In response, fibers with stress-induced birefringence were developed such that the fiber was capable of either supporting only a single polarization state or allowing the two modes to be discretely decoupled.¹ In these fibers, birefringence is induced by the incorporation of boron-doped stress regions on either side of the fiber's core.

During the draw process, the preform is heated and these two regions are in thermal equilibrium. On cooling, however, differences in their respective coefficients of thermal expansion create residual stress anisotropy across the core of the resultant fiber. The composition, location, and geometry of the stress members determine the birefringence in the fiber. More specifically, the magnitude of induced birefringence is proportional to the difference in thermal-expansion coefficients, and inversely proportional to the size and distance between the stress-inducing regions.

FIGURE 1. PANDA, elliptical-clad, and bow-tie PM fibers are all in use.

During the past 20 years, different techniques have been developed to affect the incorporation of these stress-inducing regions, resulting in several slightly different PM-fiber designs. As a result, three PM-fiber designs are in common use today: bow-tie, Panda, and elliptical-clad (see Fig. 1). Each of these three designs has advantages from a manufacturing and/or applications perspective. While its inherent uniformity and reproducibility has allowed the PANDA-type fiber to be the long-preferred technology for all telecom applications, it is fundamentally more difficult to achieve the extreme stress-rod dopant concentrations and close proximity to the core that is demanded by applications requiring very high birefringence. Consequently, until very recently, this fiber design has not been available to specialty-fiber applications other than telecom.

Fiberoptic gyroscopes

In fiberoptic-gyroscope (FOG) applications, it is critical to manufacture PM fibers with extremely high birefringence and commensurately short beat lengths. Further complicating these fiber designs is the smaller cladding diameter (80 µm as compared with 125 µm for telecom), resulting in smaller stress regions. As a result, FOG manufacturers have been restricted to a choice between bow-tie and elliptical-clad fiber, and have been forced to deal with the lack of uniformity, processing difficulties, and difficulty in production scalability inherent to these two design types. By taking advantage of compositional analyses, Nufern has commercially developed PANDA-type PM fibers with beat lengths of less than 1.2 mm at 633 nm.^{2, 3}

FIGURE 2. A Nufern PANDA-based fiberoptic-gyroscope fiber exhibits an enhanced temperature range with respect to polarization crosstalk.

PANDA configurations afford the FOG user exceptional fiber-crosstalk performance over a broad operating-temperature range without any noticeable hysteresis (see Fig. 2). This performance, coupled with high birefringence and exceptional dimensional control, is critical to the manufacture of high-performance FOGs. The availability of high-performance PANDA-style PM fibers for gyro applications has enabled guidance-system manufacturers to leverage the array of low-cost, PANDA-style PM components developed for the telecommunications industry. This access affords the guidance industry higher-performance FOGs at lower costs, opening new opportunities for FOG-based guidance systems.

High-power fiber lasers

In fiber lasers, random fluctuations in the polarization state result in instability in the laser output. The amplitude and frequency of a continuous-wave fiber laser fluctuates, and modelocking of pulsed fiber lasers becomes complicated. Consequently, it is desirable that a fiber laser or amplifier provides output light with a defined polarization. Many applications—such as medical, materials processing, and defense applications—require a particular wavelength of light that can best be achieved by frequency-shifting the output, or require output powers that can best be achieved by coherent combination of the output of several laser cavities.

While a number of recent technological breakthroughs in the design of active doped fibers have resulted in an exponential increase in the reported output powers of continuous-wave and pulsed fiber sources, historically only a few PM active doped fibers have been reported in the literature; typically, these fibers had low dopant concentrations and/or low birefringence.

The complication in manufacturing such fibers arises primarily as a result of the need to deposit an active core region and stress-inducing elements. Of the three fiber types, the PANDA-style fiber has an advantage in that the production of the two components can be decoupled; they can be manufactured in two processing steps and combined just before drawing the fiber. However, extremely high laser powers can only be achieved in large-mode-area double-clad fibers (LMA DCFs) in which the core and cladding geometries are very different from those in standard telecommunications-type PM fibers.

In LMA DCFs, the large diameter of the core negatively affects the achievable birefringence. Consequently, while passive PM fibers have been commercially available for many years, active doped PM fibers have not been available until recently.^{4, 5} In fact, an amplifier using ytterbium-doped PM-DCF was first reported in 2000. If PM-LMA DCFs were to be feasible, considerable research had to be performed to optimize the compositional and the geometrical design of the stress members. In 2003, Nufern announced the results of such detailed experimental and theoretical analyses. The analyses showed that through careful manipulation of the stress-member composition and location, it was possible to manufacture PM-DCFs with core diameters of around 20 µm and still achieve a birefringence similar to that of standard communications-type Panda fibers, namely greater than 3.5 × 10^-4.

FIGURE 3. Bend loss can be used to induce a loss in one axis of a PM fiber, making the fiber a polarizing element.

Nufern researchers discovered that, with further optimization of the stress-rod size and location, it is possible to move from a regimen in which the fiber simply maintains the linear polarization state into one in which the fiber actually has a significant loss for one of the polarization axes, essentially creating a polarizing behavior—for example, resulting when the bend-induced loss for an optimized PM large-mode-area ytterbium-doped fiber is different for each of the two polarization axes (see Fig. 3).⁶ By coiling the fiber into a near-optimum diameter (say around 10 cm), the light polarized along the fast axis experiences a substantial loss (around 10 dB/m), while the other polarization experiences negligible loss (less than 0.1 dB/m).

FIGURE 4. A 300-W linearly polarized grating-based fiber laser incorporates a Nufern polarization-selective fiber (inset, fiber cross section) to control the polarization. The coil diameter of the Nufern fiber was approximately 9 cm.

A 300-W linearly polarized fiber Bragg grating–based laser has been demonstrated with very high slope efficiency using this technique, delivering around 19-dB polarization extinction ratio simply though coiling of the optimized fiber (see Fig. 4). Polarization control in the fiber laser is simplified, reducing or removing the need for external polarizing elements and allowing monolithic cavity designs to be achieved even at these very high power levels.

REFERENCES

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2. K. Tankala et al., Photonics West 2003, 4974 (San Jose, CA).
3. D. P. Machewirth et al., Solid State Laser Conference, New Mexico (2003).
4. K. Tajima, Electronics Lett. 26, 18, 1498 (1990).
5. D. A. V. Kliner et al., Optics Lett. 26(4), 184 (2001).
6. U. Manyam et al., Proc. ASSP 2004, paper MA4.