At the California Institute of Technology (Pasadena, CA), researchers have measured the bit-error rate (BER) of a taper- resonator-taper (TRT) add/drop structure, and found good performance with low power penalties. The CalTech device, which was designed to optimize the coupling and phase-matching in the tapered fibers and microsphere, was able to extinguish the dropped signals by 26 dB. The device also had an insertion loss below 3 dB. However, researchers say that resonant-mode spacing will have to be significantly increased before the structures can be considered for commercial applications.
The principle of the TRT device is simple (see Fig.1): the WDM signal propagates along a fiber, which includes a section that tapers and widens.1 Where the fiber is thinnest (and containment is weakest) it comes in contact with a microsphere (a resonant cavity) with a diameter that allows it to resonate at specific whispering-gallery modes: CalTech researchers used microspheres with a diameter of 30 to 50 µm (see Fig. 2). Where the incoming channel has a wavelength that corresponds to one of these modes, the optical power is coupled into the microsphere. Other wavelengths are unaffected.
Due to the geometry of the system, the light caught by the resonator is then coupled into a second, tangentially-arranged tapered fiber, placed at the opposite end of the microsphere. Thus the signal is dropped from the network. Likewise, researchers added to a signal by sending it through the add/drop fiber at the design wavelength. It is then transferred, via the microsphere, to the main fiber, where it can propagate with the main WDM signal.
The CalTech team has conducted BER experiments on these couplers and reports that the TRTs perform extremely well. For a device with an optical bandwidth of 3.8 GHz, BER performance was almost identical to that of a back-to-back coupler at 2.5 Gbit/s, with just a 2-dB increase in power required for 5 Gbit/s. For more information contact Kerry Vahala at email@example.com.
1. Ming Cai et al., IEEE Phot. Tech. Lett. 12 (9), pp. 1177-1179 (Sept. 9, 2000).
Sunny Bains is a scientist and journalist based in London, England.
FIGURE 1. The microsphere cavity is chosen so that its whispering-gallery modes include one corresponding to the signal to be add/ dropped. The upper fiber carries the main WDM signal, with the lower fiber bringing in and taking away the add/drop light.
FIGURE 2. The taper-resonator-taper structure consists of fibers aligned tangentially to, and at opposite sides of, the microsphere.