An arrayed-waveguide grating (AWG) with branched output waveguides could simultaneously monitor wavelengths in multiple WDM channels, report Jae-Hoon Lee and others at Kwangwoon University (Seoul, Korea).1 Because wavelength drift can create havoc in WDM systems, an ideal monitoring system would be inexpensive, compact, and allow simultaneous measurement of different channels.
Another group of researchers suggested that AWGs could be used for this application by exploiting of the AWG crossover transmission property.2 Crossover is the point at which the spectral curve of one channel crosses the spectral curve of an adjacent channel; at this wavelength, the transmittance in each curve is the same. Each WDM channel wavelength is locked to one of the crossover wavelengths. Advantages of this method are that it is insensitive to the channel power variation, does not need extra channel modulation, and can be made using low-speed electrical circuits.
Typical AWGs, however, also have large transmission losses at these crossover wavelengths, which results in a severely attenuated signal at the AWG output compared with the amplified spontaneous emission (ASE) noise. This noise adds error to the measured wavelengths. The design of the branched AWG allows it to accurately monitor channel wavelengths despite the presence of ASE.
Lee's group overcame the ASE problem by designing an AWG with branched output waveguides (see figure). Each output waveguide branches at a small angle (about 1° in some devices). Because the spatial field distributions at the waveguide inputs are wavelength-dependent, the transmitted wavelengths at adjacent output ports are slightly different. This reduces the losses at crossover wavelengths, which also reduces the measurement errors due to ASE.
According to their simulation, the device can monitor 100-GHz-spaced WDM channels with less than 3 dB of excess loss for all crossover wavelengths. The slight branching suppresses the transmitted ASE power.
For more information contact Jae-Seung Lee at firstname.lastname@example.org.
- J.-H. Lee et al., IEEE Photon. Tech. Lett. 13(11), p. 1185 (November 2001).
- M. Teshima, M. Koga, and K. I. Sato, J. Lightwave Technol. 14(10), p. 2277 (October 1996).