Determination of a property of an optical device

Details Determination of a property of an optical device Determination of a property of an optical device

2001-08-28

2003-08-12

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

Using fiber or waveguide interferometer

C356S073100, C356S491000

Reexamination Certificate

active

06606158

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to determination of a property of an optical device under test, e.g. the determination of the elements of the Jones Matrix of an optical device. The Jones matrix contains information about the optical properties of the device under test, which can be a fiber or an optical component. Knowledge of the Jones matrix of an optical component enables determination of many important optical properties of the component, such as insertion loss, reflectivity or transmissivity, polarization dependent loss (PDL), and polarization mode dispersion (PMD).
PMD is a fundamental property of single mode optical fiber and components in which signal energy at a given wavelength is resolved into two orthogonal polarization modes of slightly different propagation velocity. The resulting difference in propagation time between polarization modes is called the differential group delay, or DGD. The term PMD is used to denote the physical phenomenon in general and the mean, or expected, value of DGD in particular. The attributes that define PMD are DGD, and the principal states of polarization, or PSP. Both are generally functions of wavelength in single mode fiber systems. Long fibers typically exhibit random polarization coupling, and consequently, PMD scales with the square root of fiber length for fibers longer than several kilometers. State of the art fiber may be limited to a few tenths of picoseconds of DGD per root kilometer. Additionally, state of the art components for such fiber communication systems may exhibit only tenths of picoseconds of DGD.
PMD causes a number of serious capacity impairments, including pulse broadening. In this respect its effects resemble those of a chromatic dispersion, but there is an important difference. Chromatic dispersion results from a variation in propagation delay with wavelength caused by the interplay of fiber material and dimensions and is a relatively stable phenomenon. The total chromatic dispersion of a communications system can be calculated from the sum of its parts, and the location and value of dispersion compensators can be planned in advance. In contrast, the PMD of single mode optical fiber at any given signal wavelength is not stable, forcing communications system designers to make statistical predictions of the effects of PMD and making passive compensation impossible. Moreover, PMD becomes a limiting factor after chromatic dispersion has been sufficiently reduced.
This is because the channel PMD, i.e. the mean value of DGD for the fiber over wavelength and time, also called the expected value, can often be in excess of 20 ps. This value is within the bit resolution of a 40 Gbit/s communication system, and as a result, the communication system is adversely affected by the PMD. Additionally, in state of the art communication systems components are often introduced in cascades, e.g. by introducing a cascade of a great number of Bragg gratings in the fibers. Although the single component of such a cascade may exhibit only tenths of picoseconds of DGD the total cascade may exhibit DGDs which reach the resolution of the transmission rate. Therefore, it becomes more and more necessary to be able to gain exact information about the PMD of each single component.
The aforementioned problem has inspired the development of many measurement methods to measure PMD. In the following a few methods of the known methods shall be discussed.
In the fixed analyzer PMD measurement method PMD is determined statistically from the number of peaks and valleys in the optical power transmission through a polarizer as wavelength is scanned. A polarizer placed directly before a detector is referred to as an analyzer, hence the name of the method. The fixed analyzer response may be Fourier transformed to yield a spectrum that gives insight into the degree of mode coupling and allows calculation of PMD from a Gaussian fit or from the second-moment algorithm. The problem with the fixed analyzer method is that it is not possible to measure the PMD of components which exhibit band widths which are smaller than the variation in the optical power transmission over wavelength.
Another method is the interferometric method, which determines PMD from the electric field autocorrelation function using a broad band source. The value of PMD is computed with an algorithm based on the second moment. The problem of this method is that it only produces exact values of PMD when the PMD is caused by pure birefringence. However, this method is not able to produce useful PMD values when the PMD is wavelength dependent.
Another method is the so called Poincaré arc or SOP (state of polarization) method, which uses a polarimeter to capture the arc traced out on the poincaré sphere by the output polarization of the test device over a series of wavelength increments. However, if the polarized light is coupled accidentally into the main state of polarization of the test device no PMD can be measured. Another problem is that a high-resolution polarimeter is necessary which kind of polarimeters tend to be very expensive. Moreover, with this method no chromatic dispersion can be measured.
Another method is the so called Jones matrix eigenanalysis or JME method. This method determines DGD and PSP as functions of wavelength from measurements of the transmission matrix at a series of wavelengths. Again, this method uses an expensive polarimeter. This method gives no information about chromatic dispersion, neither.
Finally, there are methods known which measure the PMD more or less on a direct way. These methods, e.g. the modulation phase method and the pulse-delay methods determine PMD from measurements of the change in modulation phase and the change in pulse arrival time, respectively, between the principal states of polarization. The drawback of these methods is the pulse shape dependency of the results.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to enable an improved determination of properties of an optical device.
The object is solved by the independent claims.
An advantage of the present invention is the possibility of deriving transmissive and reflective properties, e.g. the PMD of the device under test (DUT) just by determining the elements of the Jones matrix (which will be explained in more detail further below) of the DUT without need to make use of an expensive polarimeter, and the possibility of simultaneously measuring the chromatic dispersion of the DUT. So all the above-mentioned problems in the prior art can be avoided by the present invention. Moreover, it is possible to derive additional information from the derived Jones Matrix of the DUT. The present invention is capable of determining from the Jones matrix of a DUT as a function of wavelength during just one measurement. From the measured Jones matrix, it is possible to determine the insertion loss, reflectivity or transmissivity, polarization dependent loss (PDL), group delay, chromatic dispersion, differential group delay, principal states of polarization, and higher-order PMD parameters.
It is preferred to use a Jones Matrix. The information in the Jones matrix can be represented in several different ways, one alternative is called the Mueller matrix, which carries nearly the same information as the Jones matrix but represents it in a 4×4 matrix with all real elements. However, absolute phase properties and consequently chromatic dispersion cannot be described by the Mueller matrix. There are other ways to achieve the same goal. One topic of the invention is a way to measure PMD, PDL, PSPs, DGD, group delay, chromatic dispersion, etc. with the interferometric method and apparatus described. However, it is possible to perform the inventive derivation with other tensors than with the Jones Matrix.
The term “coherent” in this application means that the coherence length of the incoming light beam is bigger than the difference of lengths of the paths of the light beams to be superimposed.
In a preferred embodiment of the invention the apparatus cont

Affiliated with

Also associated with

Rating

Determination of a property of an optical device does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Determination of a property of an optical device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Determination of a property of an optical device will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3123152