Assessing polarization-dependent loss

John Kim

The increasing deployment of dense wavelength-division multiplexing (DWDM) fiberoptic networks, and the resulting knowledge of crucial performance parameters, are driving the demand for new testing methods for systems and components. Among these parameters, polarization-dependent loss (PDL) measurement is now mandatory for the proper characterization of DWDM components in fiberoptic systems.

The distributed feedback (DFB) lasers used as network transmitters in DWDM systems produce a highly linear polarized output. However, the transmission fibers, optical amplifiers, demultiplexers, and other ancillary devices in DWDM systems do not maintain this polarization. While polarization-stabilization products are currently available, it is cumbersome and costly to actively control the state of polarization (SOP) throughout a fiberoptic network by implementing many of these devices.

Since the state of polarization is essentially unknown at various points along the network, manufacturers must screen all incorporated devices to determine the polarization sensitivity of the components used. By quantifying the variations for any possible SOP, a worst-case specification can be established and the corresponding loss power budget can be factored into the overall system design.

The need for polarization screening fuels the demand for reliable, efficient, and cost-effective measurement techniques to obtain PDL characterization. Several methods are currently in use to characterize PDL, each with its own advantages and disadvantages.

Definitions

Polarization-dependent loss can be an elusive parameter to measure. In most cases, this elusiveness is not attributable to changes in the actual polarization sensitivity of the device, but is instead due to unknown, random variations in the input signal SOP. In the simplest sense, PDL can be defined as the peak-to-peak power throughput variation measured at the output of a device while the input is exposed to all possible states of polarization (see Fig. 1).

PDL appears as a random or unrepeatable variation in the overall insertion loss of the device. Furthermore, the PDL of a component may vary depending on the input wavelength of the source.

Virtually all fiberoptic components exhibit some degree of polarization sensitivity. Typical PDL values for various components range from tenths of a decibel to over 30 dB (see table).

PDL measurement methods

The first three PDL-measurement techniques described in this article require the use of an active polarization controller. Both fiber-loop and rotating waveplate-based polarization controllers are commercially available. The fourth method requires the use of a polarization analyzer to obtain the actual state of polarization of the signal.

The first, all-states method, uses a polarization controller to generate all possible states of polarization while observing the output power of the device under test (see Fig. 2). While this method is thorough, it is also inefficient, since the user must single-step through each possible polarization state. However, this method, in theory, offers the most measurement accuracy, since the user is assured that the device has been exposed to all possible states of polarization.

An alternate method to reduce the required measurement time is to randomize the input polarization while monitoring the output power. By obtaining a sufficient number of samples, the PDL can be determined within an amount of statistical certainty. Increasing the number of samples allows more random polarization states to be covered, thus increasing the likelihood of observing the conditions under which maximum and minimum power occur.

The scan rate of the polarization controller should be fast enough to minimize the overall measurement time. Similarly, to obtain the PDL at each state of polarization, that state should remain constant long enough for the receiver to obtain the optical power level. Thus, the optical power meter used should be capable of performing measurements quickly. For commercially available instruments, a typical scan time of 30 seconds corresponds to a PDL error of less than 2%. The randomization technique drastically reduces the required measurement time in comparison to the all-states method.

A third measurement technique, known as Mueller-Stokes analysis, involves the application of well-defined polarization states to the device under test. Specifically, optical power is measured while the device is exposed to linear horizontal, linear vertical, linear diagonal, and right-hand circular states. The significant advantage of this approach lies in its simplicity and ease of automation. Using a programmable polarization controller and an optical power meter, the PDL of a device may be efficiently characterized through a remote acquisition system. Therefore, this method is well-suited for use in automated testing systems, such as those often found in component manufacturing and qualification environments. Furthermore, since only four polarization states are required, the measurement time can be quite short. At the time of writing, the Mueller-Stokes method was proposed for adoption as a working standard.

All of the methods previously mentioned rely on a polarization controller to generate either random or defined polarization states. An alternate method to obtain PDL values, known as the Jones Matrix, requires the use of a polarization analyzer. By measuring the state of polarization at the input and output of a component, the analyzer can obtain the Jones Matrix of the device. This complex 2 × 2 matrix represents the polarization transfer function of the component. Once known, a solution of the matrix will give the PDL of the component. The Jones Matrix is very efficient-since a large number of measurements are not required, the entire process can be completed in a few seconds.

Moreover, unlike the first three techniques, the Jones Matrix can give information regarding the actual polarization states present at the input and output of a component. However, the primary disadvantage of the Jones Matrix is its lack of economy. While other methods can be inexpensively implemented using a polarization state controller and an optical power meter, a polarization analyzer is a complex and expensive instrument. For many users, the employment of a polarization analyzer in conjunction with the Jones Matrix is not a cost-effective solution.

Implementation issue

Mueller-Stokes analysis provides an accurate and efficient method for characterizing the PDL of a component that is particularly suitable for high-volume production environments. Several implementation issues are of importance in ensuring optimal accuracy and performance using the Mueller-Stokes method.

High-performance connectors should be used at all temporary joints, such as the ends of the launch and receive cables. Since the Mueller-Stokes method is based on power transmission measurements, the overall system repeatability cannot exceed that of any individual connector. Ideally, all temporary joints should be made with fusion splices; however, this may not be feasible in high-volume manufacturing environments. In such situations, the use of precision fiberoptic connectors and "gold standard" reference cables will provide adequate performance. Proper connector care is also essential for accurate measurements.

The photodetectors used in the optical power meter should have a nominally low PDL value. Detectors with high PDL will cause incorrect power measurements when the four polarization states are generated. Angled detectors, which are offset to reduce back reflections, typically suffer from greater polarization sensitivity. Alternatively, a depolarizing filter may be inserted in-line with the photodetector to reduce its polarization sensitivity.

A highly stable laser source is required for accurate PDL measurements. Any short-term source power drift will directly distort the measured PDL value. This effect can be alleviated by splitting the laser output using a 1 × 2 coupler before the device under test and measuring one output branch directly. By measuring power ratios at each required Mueller-Stokes polarization state instead of optical powers, any source drift will be compensated.

Mechanical fiber stability is perhaps the most significant contributor to PDL measurement accuracy. Unfortunately, such stability can also be difficult to maintain. While the earlier concerns are important, their effects can be minimized using straightforward measures. As mentioned previously, standard single-mode fiber is usually not polarization-maintaining. However, polarization is generally stable in single-mode fiber, provided that the cable is not moved. Physical stress caused by movement will change the birefringence of the cable and shift the polarization randomly. This effect can be easily observed by twisting a single-mode fiber connected to a polarization analyzer. Several minutes are required for the single-mode fiber to relax from these stresses and for the polarization state to stabilize.

To obtain acceptable PDL measurements, all connecting fibers in the instrument chain must remain physically stable throughout the course of the entire measurement. Since the Mueller-Stokes method relies on measurements at four well-defined polarization states, the launch cable must remain stable in order to preserve the same relative positions of the states during referencing and testing.

To demonstrate the effect of fiber movement on performance, PDL measurements using the Mueller-Stokes method were repeated both with affixed and unsecured patchcords (see Fig. 3). The results of this experiment clearly illustrate the importance of physical patchcord stability.

It is important to consider the advantages and disadvantages of each current PDL characterization method prior to implementation. The Mueller-Stokes method is attractive because of its accuracy, fast measurement time, and simplicity. Experimental data acquired using Mueller-Stokes analysis correlates with results predicted according to theory and with figures obtained using other methods. However, careful consideration of the interconnections and proper handling of components used during testing is crucial for ensuring acceptable accuracy and repeatability. For designers of automated test systems, the challenge is not only to understand and implement the appropriate PDL measurement methodology, but also to provide an efficient system of interconnections to achieve optimum measurement performance.

FIGURE 3. Experimental results showing a comparison of Mueller-Stokes PDL using taped and untaped patchcords show the importance of keeping the patchcord stable.