Making Fiber Act like a Pockels Cell

Breck Hitz

Researchers at Acreo AB in Stockholm, Sweden, have recently demonstrated an all-fiber device that, like a Pockels cell, rotates the polarization of light passing through it when a voltage is applied. Although the physics is completely different — the Swedish device depends on thermally induced stress birefringence generated in the fiber, not the linear electro-optic effect — the result is important from a practical perspective. Pockels cells make very efficient Q-switches for conventional bulk lasers, but in a fiber laser they are awkwardly bulky and inefficient. The new device offers the advantages of a conventional Pockels cell in an all-fiber architecture.

Figure 1. A resistive heating element inside a microstructured fiber’s airholes induced enough thermal birefringence to rotate the polarization of light passing through the fiber. Images reprinted with permission of Optics Letters.

The investigators fabricated their device from an ~10-cm-long microstructured polarization-maintaining fiber from Acreo FiberLab in Hudiksvall, Sweden. The 125-μm-diameter fiber had an 8-μm core and a pair of ∼28-μm airholes on either side of the core (Figure 1 inset). Into these holes the researchers poured a molten BiSn alloy (melting temperature 137 °C), which solidified to form an ~7-cm-long resistive heating element inside the fiber. They polished small depressions in the side of the fiber to make electrical contact with the BiSn heater (Figure 1). They report that, because the metal did not reach to the ends of the fiber, the device could readily be spliced to conventional fibers.

To evaluate the device performance, they placed it between all-fiber polarizers and applied electrical pulses to the in-fiber heating element. They observed full 90° rotation of the polarization from electrical pulses in the kilovolt range, lasting for several nanoseconds, at a 20-Hz repetition frequency (Figure 2). The optical rise time was approximately 8 ns, and the fall time was several hundred microseconds.

Figure 2. Placed between polarizers, the device in Figure 1 produced a full 90° polarization rotation, indicated by the 100 percent switching shown here. The optical rise time was 8 ns, and the decay time was ~300 μs (inset).

Like a Pockels cell or an ordinary waveplate, the fiber device achieves polarization rotation by splitting incoming light into two orthogonal linear polarizations and retarding one polarization with respect to the other. To achieve a 90° rotation, the induced birefringence must produce a 180° phase difference between the two polarizations. In a laser resonator, the Q-switch is typically placed adjacent to a resonator mirror, so the light makes a double pass through it, and a phase retardation of only 90° per pass is adequate for 100 percent switching.

Figure 3. Switching rates of up to 10 kHz could be optained by operating the device at a higher temperature and mounting it on a Peltier cool

As is the case for any new device, the all-fiber polarization rotator had a few kinks that needed to be worked out. One issue was the slow repetition rate of a laser Q-switched with the device, imposed by the slow decay time shown in Figure 2. The decay time depends on heat diffusion, and it shrank as the operating temperature increased. However, at higher operating temperatures — corresponding to approximately 2-kHz pulses, or an average of 2 W of power, applied to the heating element — the BiSn heating element melted. The researchers finessed this problem by substituting an AuSn heating element (melt temperature 280 °C) and operating the device on a Peltier cooler. Under these conditions, they obtained full switching at rates of up to 10 kHz (Figure 3). In lifetime testing, the scientists drove the device containing AuSn heaters at 3 kHz for 100 hours (approximately 10⁹ pulses) and observed no degradation.

Figure 4. The acoustic vibrations of the fiber were most severe in a four-hole fiber with only one hole filled with a resistive heating element (left). When the fiber was side-polished to break the radial symmetry, the acoustic vibrations were markedly damped (right).

Another issue was acoustic vibrations, which are minimized but still visible in Figure 1. The researchers hypothesized that these vibrations, which would diminish the performance of a Q-switch, arise from radial mechanical oscillation modes of the 125-μm fiber. They achieved a significant damping of the oscillations by breaking the fiber’s radial symmetry (Figure 4).

Optics Letters, March 15, 2007, pp. 614-616.