An optical system may use an optical fibre as a communication medium and light as an information carrier. For instance, an optical signal may be a beam of light modulated to represent binary information.
Optical signals may be employed in digital communication systems that exchange information as a series of binary digits called "bits". The reception of binary information is primarily concerned with the correct recognition of the value of a bit. Communication quality may be measured in terms of a bit error rate, a ratio of incorrectly recognized bits to correctly recognized bits.
Due to practical limits to the length of optical fibre, a continuous fibre may not reach from a sender to a receiver. As well, attenuation reduces the amplitude of an optical signal over a distance. To maintain an effective signal level, an optical communication link between a sender and a receiver may be broken into smaller individual links, called spans. To maintain a strong signal, an amplifier may be installed where each span is connected to a subsequent span.
The connection between the amplifier and the optical fibre may not be perfect. As a result, a small amount of the incident optical signal may reflect away from the destination end of a span rather than transmitting. Part of the reflected signal may then be reflected away from the source end of the span and may appear as a component of the original signal when received at the destination end. The detection of ones and zeroes of the original signal is made difficult by the combination of the original signal with the twice reflected interfering signal, referred to as multiple path interference (MPI). This increased detection difficulty elevates the bit error rate. Reflections may also be introduced in optical systems due to imperfections in individual devices such as the amplifiers, isolators, filters, optical switches, components within amplifiers, etc. Accordingly, methods and devices used to assess the effects of optical devices on MPI are known.
Mathematical predictions of the effect of MPI on an optical system often test the predictions using systems in which a controlled amount of MPI is introduced. In Gimlett and Cheung, "Performance Implications of Component Crosstalk in Transparent Lightwave Networks," (1994), J. Lightwave Technology, vol. 7, pp.888-895, a power penalty is derived dependent on a reflection coefficient that is representative the of amount of MPI relative to the original signal. In Goldstein et al., "Effects of Phase-to-Intensity Noise Conversion by Multiple Reflections on Gigabit-per-Second DFB Laser Transmission Systems," (1989), IEEE Photonics Technology Letters, vol. 6, pp.657-660., a power penalty is approximated dependent on a ratio of crosstalk (MPI) to signal power at a receiver. In both of the above, the power penalty is the increase in power necessary to obtain an error rate equivalent to the error rate in the absence of MPI.
U.S. Pat. No. 5,617,200 issued Apr. 1, 1997 to Vance describes a method for measuring the secondary path intensity of an optical device. The method includes applying a pulse of light to a first end of the device and detecting the light exiting from the second end of the device. The exiting light is analyzed in the time domain to determine a primary pulse intensity and a secondary pulse intensity. The ratio of the secondary pulse intensity to the primary pulse intensity provides a measure of the device's secondary path intensity.
The above methods however, do not take into account real in-field conditions to which a device may be subject. Specifically, in-field reflected optical signals may be present at both inputs and outputs of the device, that may be further reflected by the device.
Moreover, the measurement of a power penalty due to MPI may not be as indicative of the quality of the device as a measurement of the magnitude of MPI attributable to the device.
Finally, a measurement of MPI in the frequency domain may be preferable to an average of measurements of MPI made in the time domain.
The present invention accordingly attempts to address some of the shortcomings of known ways to measure MPI.