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One way to address the ever-increasing demand for more bandwidth is to simultaneously transmit multiple data streams through the fiber optic network. This can be done by using different laser wavelengths. When 4 or 8 or 12 different wavelengths are used, this is called course wavelength division multiplexing (CWDM). With 16 or more wavelengths, it is dense wavelength division multiplexing (DWDM). As an example of how the technique is applied, a DWDM system that uses 40 laser wavelengths over the range from 1480 nm to 1620 nm, with each laser operating at 10 gigabits per second, would yield a system with a total available bandwidth of 40 x 10 Gbps = 400 Gbps.
To optimize system performance, it is necessary to monitor and control the power at each individual wavelength. This is because transmitted power can vary from laser to laser, and also because attenuation will vary by wavelength over long distances. Monitoring is thus crucial for dynamic gain equalization. In terms of detector types, compact InGaAs photodiodes (or photodiode arrays) with high linearity are ideally suited to this purpose.
In a DWDM system, it is also necessary to monitor each wavelength to make sure it is operating within a narrow spectral range. This type of monitoring is often performed with a spectroscopic measurement technique. Hamamatsu's InGaAs linear image sensors are designed specifically for this task.
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