There is an ever-increasing demand for greater data transmission capacity in optical communications networks, i.e. for greater bandwidth. WDM (wavelength-division-multiplexing) systems provide a method for increasing the channel capacity of existing fiber optic links without physically modifying the optical fiber, by allowing multiple wavelengths to be transmitted and received over a single optical fiber. Dense WDM (DWDM) systems can be utilized to further increase information transmission capacity. DWDM systems increase the capacity of an embedded fiber by first assigning incoming optical signals to specific wavelengths within a designated wavelength band, and then multiplexing the resulting signals onto one fiber. DWDM systems combine multiple optical signals so that they can be amplified as a group, and transported over a single fiber to increase capacity. Each signal can be carried at a different rate, and in a different format. DWDM systems can multiplex and demultiplex large numbers of discrete communication channels onto a single optical fiber, and transmit these channels over long distances.
Channel routing and switching is an important function performed by DWDM network components, and allows service providers to have optical access to data at desired nodes on the network. Optical switches direct one or more wavelengths of light from an input port to an output port. All-optical switches can steer light pulses among different fiber spans without converting them into electrical signals at any point, and can therefore eliminate the need for repeated optical-electrical-optical (OEO) conversions in the network. Channel routing involves extracting and inserting of optical signals, i.e. add/drop multiplexing. For example, selected channels may be extracted or “dropped” from a multiplexed signal, and routed to desired nodes. Alternatively, extracted signals, or newly generated signals, may be inserted or “added” into the multiplexed signal.
Currently, many techniques are being developed, in the pursuit of all-optical channel switching configurations, such as switchable optical cross-connects and all-optical add-drop multiplexers. These techniques include opto-mechanical, opto-magnetic, electro-optic, acousto-optic, thermo-optic, liquid crystal, and electro-holographic devices, and bubblejet devices. Each of these techniques possess advantages and disadvantages, depending on factors such as speed, size, scalability, reliability, and integrability. None of the techniques currently under development, however, offer the potential for an optical network with increased wavelength channel density. At present, the available channel spacings (i.e. the minimum frequency separation between two different multiplexed signals) are as large as about 50 GHz.
In recent years, micro-resonators have generated significant interest for application to optical switching and channel multiplexing. In particular, optical microcavity resonators show great promise for optical communication applications such as filtering, multiplexing, and switching. Most investigations have used ring and disk-type whispering-gallery-mode (WGM) resonator fabricated from silica or silicon-based materials by chemical-vapor-deposition (CVD) and photolithographic methods. The small size of these devices (˜10 um diameter) allows for mass-production efficiency and high device density. However, the methods used to prepare microrings and microdisks, combined with bending losses for such small resonators, limit the cavity quality factor (Q) of these resonators. Sidewall roughness in wafer-processed integrated-optics resonator elements is very significant due to required etching. Such resonators typically have Q-factors of 103 to 105 in the 1550 nm wavelength region.
Silica microsphere resonators are an especially promising type of micro-optical cavity. Silica microsphere resonators have quality factors that are several orders of magnitude better than typical surface etched resonators, because these microcavities can be shaped by natural surface tension forces during a liquid state fabrication. Silica microspheres are three-dimensional WGM resonators, typically 50-500 μm in diameter, which can be fabricated by simply melting the tip of an optical fiber. Surface tension shapes the molten silica into a near-perfect sphere before it hardens. The result is a clean, smooth silica surface with low optical loss and negligible scattering. These microcavities are inexpensive, simple to fabricate, and are compatible with integrated optics.
For these optical microcavity resonators, measured Qs as large at 1010 have been reported. The high-Q resonances encountered in these microcavities are due to optical whispering-gallery-modes (WGM) that are supported within the microcavities. The fabrication process creates an extremely smooth surface, which contributes directly to low optical WGM propagation losses. Furthermore, the spherical curvature perpendicular to the optical path in microspheres focuses the WGMs, reduces the mode volume, and thus increases the cavity Q. The total optical loss experienced in microsphere resonators is exceptionally low and Q-factors as high as 108 to 1010 have been demonstrated.
Because the ultra-high Q values of microcavities are the result of energy that is tightly bound inside the cavity, optical energy must be coupled in and out of the high Q cavities, without negatively affecting the Q. The use of SPARROW waveguides for efficiently coupling light into the microcavity resonant modes is described in detail in U.S. patent application Ser. No. 09/893,954 (hereinafter the “'954 application”), entitled “Optical Microcavity Resonator System,” which is commonly owned by the present assignee and which is incorporated herein by reference.
Using microsphere optical resonators coupled to SPARROW waveguides, channel spacings of approximately 100 MHz or less can be readily obtained. The high Q-factor translates directly into resonant optical linewidths of several MHz or less. This provides the potential for ultra-dense wavelength division multiplexing optical channel networks.
There is currently a need for optical switches that can be used in dense all-optical communications networks. In particular, there is a need for an improved system for performing all-optical channel switching, as part of ultra-dense wavelength division multiplexing optical communications networks having very narrow channel spacings. Because of the properties of optical microsphere resonators, described above, optical microsphere resonators have the potential to provide superior performance in numerous applications, including applications such as all-optical switches in DWDM communications systems that call for ultra-narrow linewidths.