in optical communication systems, such as 40 G bits per second (b/s) or 100 G b/s Ethernet for distances less than 40 km, data rates and bandwidth are required to be very high, and the size and cost of transceivers are expected to be very low. To reduce the cost, in many cases, multiple optical signals of different wavelengths are combined into a single optical fiber. By combining several signals, the capacity of optical communications is increased.
There are two types of optical combiners, i.e., power combiners and wavelength combiners. Multimode interference (MMI) based couplers are commonly used as power combiners for combining multiple optical signals of different wavelengths. At the input side of the MMI coupler, a number of waveguide ports are used to guide multiple input signals. At the output, a single port is used for an output signal, and all input signals are combined into this output signal. However, for each input signal, only a fraction of power is present in the output signal.
For example, a 4×1 MMI coupler can combine four optical signals, but with a power loss of at least 6 decibels (dB) for each signal. Examples of such devices are described by Besse et al. “Optical bandwidth and fabrication tolerance of multimode interference couplers,” IEEE J. Lightwave Technol., vol. 12, no. 6, pp. 1004-1009, June 1994.
Examples of wavelength combiners include arrayed-waveguide gratings, and ring-resonator based filters. Wavelength combiners are typically narrow hand, which can cause signal distortion. The wavelength combiners theoretically provide small power loss, but in practice the loss is much larger. The precise control of dimensions of the wavelength combiners is difficult and small error in the dimensions of the combiner can lead to a large insertion loss. The insertion loss is the loss of signal power resulting from the insertion of the combiner in the optical fiber, and is usually dB. Examples of such devices are described in U.S. Pat. No. 6,195,482.
Accordingly, there is a need for an optical combiner that minimizes the insertion loss of the power of the input signals.