1. Field of the Invention
This invention relates to optical systems and, more particularly, to amplifier systems for use in optical receivers.
2. Description of the Related Art
Fiber optic WDM (Wavelength Division Multiplexing) and DWDM (Dense WDM) systems are increasingly using more channels, narrower channel spacing, and wider wavelength ranges. As fiber optic systems such as these become more complex, it becomes increasingly important to characterize the wavelength-dependent optical properties of both broadband network components (e.g., couplers, combiners, splitters, attenuators, isolators, circulators, etc.) and narrowband network components (e.g., multiplexers, demultiplexers, interleaves, wavelength filters, etc.). By characterizing the optical properties of components, properties such as wavelength flatness and polarization-dependent loss that may adversely affect network system performance may be identified. In narrowband devices, it may be particularly useful to measure the center wavelength, crosstalk, and/or channel spacing in order to improve system performance. Since narrowband and broadband devices may be used over the entire wavelength band of a WDM or DWDM system, it may also be useful to test each device over the entire wavelength band (as opposed to just testing over the channel of intended operation for each particular device). Characterization systems may also be used to provide real-time feedback while adjusting the placement and/or orientation an optical component in a system.
One way in which optical components may be characterized is by measuring the throughput power as a function of wavelength. This may provide information about a component's center wavelength, channel spacing, wavelength flatness, filter transfer function, and/or crosstalk. A common type of measurement system is a step-and-measure system. In a step-and-measure system, an operator tunes a tunable laser source to each wavelength used to characterize the component under test and then detects and measures the component's output at that wavelength. One consequence of the increasing wavelength range of optical systems is that optical components need to be tested over these larger wavelength bands. Additionally, the narrower channel spacing may make higher resolution in the measurements desirable. Since more data points may be measured in systems that have larger wavelength ranges and/or higher resolution, testing using a step-and-measure system may take a undesirable amount of time. For example, in a 50-GHz DWDM system with 80 channels and channel spacing of 0.4 nm, component performance may need to be characterized over a wavelength span of greater than 35 nm. Properly characterizing the component may also require a resolution of 0.01 nm. Consequentially, a measurement for the device may involve at least 3,500 data points. In a step-and-measure system, this measurement may take hours to complete.
In order to improve measurement times, swept-wavelength systems have been developed in which the wavelength of the tunable laser source is automatically swept across a range of wavelengths while the output from the component under test is recorded. The wavelength sweep may occur very quickly, allowing a component to be characterized in significantly less time than it could in a step-and-measure system. However, because of the increased speed at which swept-wavelength systems operate, receivers used in such systems need to have enough dynamic range to capture the optical component's output over the sweep of the laser while still operating quickly enough to provide the desired resolution.
Optical receivers used with swept-wavelength systems may need to handle a large dynamic range relatively quickly. However, in some receiver components (e.g., analog logarithmic amplifiers), there may be a tradeoff between speed and dynamic range. For example, amplifiers designed to operate at speeds commensurate with those used in swept-wavelength systems may not provide the dynamic range needed to capture an optical component's output over the entire sweep of the laser. Conversely, logarithmic amplifiers designed to have the needed dynamic range may not operate quickly enough to provide the desired measurement resolution.