Scrambling to Reduce Polarization-Related Impairments

Steve Yao, Yongqiang Shi and Jianwei Ma

Polarization is a fundamental property of light. In classical physics, light is modeled as a sinusoidal electromagnetic wave in which an oscillating electric field and an oscillating magnetic field propagate through space. Polarization is defined in terms of the pattern traced out in the transverse plane by the electric field vector as a function of time.

For unpolarized light, the plane of polarization fluctuates randomly around the direction of light beam propagation. Therefore, on average, no direction is favored. The rate of fluctuation is so fast that an “observer” or a detector cannot tell the state of polarization at any given instant. Natural light (sunlight, firelight) is unpolarized. Otherwise, the light beam is partially or fully polarized. The degree of polarization describes how much total light intensity is polarized. For instance, it equates to 100 percent for totally polarized light and zero for completely unpolarized light. Most high-performance lasers used in long-haul communications systems involve polarized light sources.

As the bit rate increases, fiber optic communications systems become increasingly sensitive to polarization-related impairments. Problems can arise related to polarization mode dispersion in optical fibers, polarization-dependent loss in passive optical components, polarization-dependent modulation in electro-optic modulators, polarization-dependent gain in optical amplifiers, polarization-dependent center wavelength in wavelength division multiplexing (WDM) filters, polarization-dependent response in receivers, and polarization-dependent sensitivity in sensors and coherent communications systems.

Scrambling principles

Polarization scrambling can help mitigate many of these issues. Scrambling occurs when the state of polarization of totally polarized light will vary randomly at a relatively low rate. Although the state is well-defined and the degree of polarization close to 100 percent at any instant, on a time average, the degree of polarization will approach zero. Therefore, the degree of polarization for scrambled light depends on the average time or the detection bandwidth of the observer.

A polarization scrambler actively changes the state of polarization using the polarization modulation method. Scramblers based on different technologies are available today, including LiNbO3, resonant fiber-coil- and fiber-squeezer-based systems.

LiNbO₃ scramblers use the electro-optic effect to modulate the state of polarization. For example, a phase modulator can work as a scrambler when the input state is linearly polarized 45° with respect to the applied modulation electric field. There will be a trade-off for the resulting high speed, though — in the forms of high insertion loss, polarization-dependent loss, residual amplitude modulation (activation loss), sensitivity to input polarization state, and cost — because a waveguide must be inserted in the fiber line. Multiple modulation sections with different electric field directions may help make the device less polarization-sensitive, but this is often at the expense of increased complexity and additional cost (Figure 1).

Figure 1. Types of polarization scramblers include devices based on LiNbO₃ (A), on fiber resonant coils (B) and on fiber squeezers (C).

A device based on a resonant fiber coil is constructed by winding fiber around an expandable piezoelectric cylinder. An applied electric field causes the cylinder to expand, which in turn induces birefringence in the fiber via the photoelastic effect. If the frequency of the electric field is in resonance with the piezoelectric cylinder, the induced birefringence will be large enough to cause sufficient polarization modulation with a relatively low applied voltage.

In practice, it is possible to cascade multiple fiber cylinders with different orientations to reduce polarization sensitivity. Compared with LiNbO₃ scramblers, this alternative has the advantages of low insertion loss, polarization-dependent loss and cost. On the other hand, it suffers from large size, low scrambling speed and large residual phase modulation as a result of significant fiber stretching when the fiber coil expands.

Squeezing fiber can induce enough birefringence to cause large polarization modulation if the input polarization is 45° from the squeezing axis. One way to make a polarization-insensitive scrambler is to cascade several fiber squeezers oriented 45° from each other (Figure 2). The device can operate resonantly at higher scrambling frequencies or nonresonantly at lower frequencies.

Figure 2. A polarization scrambler made with fiber squeezers is suitable for applications such as network equipment, fiber sensor systems, and test and measurement instruments.

Compared with the LiNbO₃ scrambler, the device has the benefit of low insertion loss, polarization-dependent loss and cost. Compared with the fiber coil scrambler, it offers scrambling flexibility and small size. In addition, it has an edge over both in terms of low residual phase modulation and residual amplitude modulation. Low residual phase modulation is important for avoiding interference-related noise, and low residual amplitude modulation is critical for using the scrambler for polarization-dependent loss and degree-of-polarization measurement of optical devices.

Engineers generally characterize a scrambler’s performance based on the degree of polarization of the scrambled light over a certain period of time and on the uniformity of the state-of-polarization Poincaré sphere coverage (Figure 3). In practice, the wavelength and temperature sensitivities of the scrambler’s performance also are important for real-world applications.

Figure 3. A multistage fiber squeezer scrambler is much less sensitive to wavelength and temperature changes than are its alternatives. Scrambling uniformity also is excellent, as shown on the Poincaré sphere developed for the scrambler board.

Operational lifetime is always an important consideration in industrial applications. Some users may question the lifetime of the fiber under stress in the fiber squeezers. Indeed, without proper treatment and protection, the fiber may break in a short time. Since 1996, General Photonics has put a great deal of effort into finding fiber failure mechanisms and corresponding methods for protection. Based on a proprietary and patented fiber protection recipe, fiber in a typical squeezer under maximum operational stress has an estimated mean time to failure of 2 billion years. Such a result is not surprising, considering that the stress in a polarization-maintaining fiber induced by the two stress rods is on the same order of magnitude as the stress applied to the fiber by the fiber squeezer. In continued endurance tests, fiber squeezers have passed 1 trillion (10¹²) activation cycles, and the number is expected to rise as the tests continue.

End users have access to a variety of polarization scramblers from several manufacturers. Examples include a stand-alone desktop instrument from ILX Lightwave Corp. of Bozeman, Mont.; a plug-in module from Exfo of Vanier, Quebec, Canada, that is part of a mainframe test instrument; and board-level scramblers designed for low-cost systems and OEM applications from General Photonics. These scramblers have their own intended market and their own advantages. Which type is most suitable for a particular user depends on the application, user preference and budget.

It also is possible to classify scramblers according to their driving frequency. To obtain the best result, the squeezers should not be set at the harmonic or subharmonic of each other. For some scramblers, the frequencies are factory-set and cannot be changed; therefore, their scrambling rate is fixed. To produce the highest possible scrambling rate, such devices generally rely on the resonant nature of the piezoelectric transducers.

Alternatively, scramblers can be constructed with user-selectable scrambling rates. An example is a fiber-squeezer-based miniature scrambler designed for handheld and field instruments. Its scrambling rate can be changed in increments from a few hertz to a few tens of kilohertz using either a switch or a computer command.

Applications

Polarization scramblers have numerous applications in optical communications networks, fiber sensor systems, and test and measurement systems (Figure 4). One example is to use the device at the transmitter side to minimize polarization-dependent gain or polarization hole burning of erbium-doped fiber amplifiers in ultralong-haul systems. For this application, the scrambling rate should be significantly faster than the inverse of the gain recovery time constant of the fiber amplifiers (on the order of 10 kHz).

Figure 4. A scrambler can be used at the transmitter side to minimize polarization-dependent gain or polarization hole burning of erbium-doped fiber amplifiers in ultralong-haul systems (A). It can also assist in monitoringpolarization mode dispersion in a WDM system (B), the elimination of the polarization fading of a fiber sensor (C), the effective elimination of polarization of an instrument such as a diffraction-grating-based optical spectrum analyzer (D), polarization-dependent loss measurement for a device under test (DUT) (E), and pump degree of polarization when combined with a digital scope (F).

Scramblers can also assist in the monitoring of polarization mode dispersion in a WDM system. Generally speaking, this involves measuring the degree of polarization of the optical data stream propagated through the fiber. A small value usually indicates a large polarization mode dispersion effect. However, such a measurement may be erroneous if the input state of polarization to the transmission fiber is substantially aligned with its principal state of polarization.

For such a situation, the measured degree of polarization is always large, no matter how large the differential group delay between the two principal states of polarization. A scrambler at the transmitter side can be used to effectively eliminate such an anomaly. Furthermore, it enables a polarimeter in the polarization mode dispersion compensator at the receiver side to identify the principal state of polarization, which in turn speeds up compensation. Signal-to-noise-ratio monitoring of WDM channels also is possible with a polarizer placed after a scrambler.

Eliminating fading

Another application uses scramblers to eliminate the polarization fading of a fiber sensor. In such a system, the envelope of the response curve is independent of the polarization fluctuation.

Placing a scrambler in front of a polarization-sensitive instrument, such as a diffraction-grating-based optical spectrum analyzer, can also effectively eliminate its polarization dependence if the scrambling rate is sufficiently faster than the detector speed in the instrument. In addition, the device can help to measure the polarization-dependent loss (PDL) of a device under test with the help of a digital scope. The resulting loss can be calculated as:

PDL = 10log (V_max/V_min)
where V_max and V_min are the maximum and minimum signals displayed by the digital scope.

Raman amplifiers generally exhibit strong polarization-dependent gain if the pump laser is highly polarized. Minimizing gain requires the use of a depolarized pump source. The degree of polarization of the pump source directly relates to the polarization-dependent gain of the amplifier and therefore must be carefully characterized.

It also is possible to use scramblers to accurately measure the degree of polarization (DOP) with the use of a digital scope. The degree of polarization of the light source is calculated using the following formula:

DOP = (V_max — V_min)/(V_max + V_min)
In summary, a polarization scrambler is an important device for fiber optic communications, fiber sensors, and fiber optic test and measurement applications.

Meet the authors

Steve Yao is chief technology officer at General Photonics Corp. in Chino, Calif.

Yongqiang Shi is the chief scientist, and Jianwei Ma is a senior applications engineer at the company.