Abstract:
A lossless optical coupler includes at least one input waveguide, at least two output waveguides and an intermediate optical splitter. The optical splitter of the present invention further comprises an active section adapted to provide contemporaneous splitting and amplification of an input optical signal. The arrangement of the present invention provides an inexpensive and compact integrateable coupling device.

Description:
FIELD OF THE INVENTION 
   This invention relates to optical couplers and, more specifically, to integrated optical couplers having substantially no splitting loss. 
   BACKGROUND OF THE INVENTION 
   Optical couplers, such as mode couplers, are used extensively in integrated optics to split and to combine light signals propagating through an optical waveguide structure. Since an optical coupler splits an input into several outputs, each output will have less optical power than a corresponding input. This power drop is loosely termed as splitting loss. For example, a 1×2, 50-50 coupler has a 3-dB splitting loss beside other losses. This power drop or splitting loss fundamentally limits several useful system and device architectures that otherwise find many applications in electronic systems. For example, data bus architecture, which is ubiquitous in electronic computers and data networks, finds limited applications in optical systems mainly due to splitting losses. Filter designs based on finite or infinite impulse responses (FIR or IIR) also require a series of the splitting of an input signal. The applications in optics of such devices therefore are also limited due to splitting losses. 
   Current solutions for the splitting losses in conventional optical couplers include passive optical couplers having amplifiers at the inputs and/or the outputs to provide amplification for optical signals being split. However, the splitting and amplification of such optical couplers occur at different locations of the optical coupler and the transition between the active amplification process and the passive splitting require intricate and expensive manufacturing processes. In addition such optical couplers require a plurality of optical amplifiers and may comprise a relatively large form factor. 
   SUMMARY OF THE INVENTION 
   The present invention solves the deficiencies of the prior art by providing an inexpensive optical coupler having substantially no splitting loss and comprising a very small form factor. 
   In one embodiment of the present invention a lossless optical coupler includes at least one input waveguide, an optical splitter and at least two output waveguides. The optical splitter includes an active section adapted to provide contemporaneous splitting and amplification of an input optical signal. The optical splitter of the present invention may comprise an MMI coupler or may alternatively comprise a free space optical coupler such as a star coupler. 
   In an alternate embodiment of the present invention method for lossless optical coupling includes contemporaneous splitting and amplification of an input optical signal such that each component of the split optical signal comprises substantially the same optical power as the input optical signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  depicts a high-level block diagram of an embodiment of a lossless active optical coupler in accordance with the present invention; 
       FIG. 2  depicts a high level block diagram of an embodiment of the waveguide structures of the lossless optical coupler of  FIG. 1 . 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Although various embodiments of the present invention are being depicted herein with respect to a 1×2 MMI optical coupler, the specific embodiments of the present invention should not be treated as limiting the scope of the invention. It will be appreciated by one skilled in the art informed by the teachings of the present invention that the concepts of the present invention may be applied in other optical couplers, such as star couplers, having substantially any number of input and output ports. 
     FIG. 1  depicts a high-level block diagram of an embodiment of a lossless active optical coupler in accordance with the present invention. The lossless optical coupler  100  of  FIG. 1  illustratively comprises an MMI coupler  110 , a single input waveguide  105  and two output waveguides  120 ,  125 . In the optical coupler  100  of  FIG. 1 , the MMI coupler  110  comprises an active section such that the optical coupler  100  is capable of providing both splitting and amplification of an input optical signal. In the lossless optical coupler  100  of  FIG. 1 , the amplification and the splitting are accomplished at substantially the same location and at substantially the same time. Such a configuration provides a compact device that requires very few wire bonds. Although in the lossless optical coupler of  FIG. 1 , the coupler is depicted as an MMI coupler, other couplers, such as free space couplers including star couplers, may be implemented in place of the MMI coupler in accordance with the present invention. 
     FIG. 2  depicts a high level block diagram of an embodiment of the waveguide structures of the lossless optical coupler  100  of  FIG. 1 . The active waveguide structure of the lossless optical coupler comprises a channel waveguide structure consisting of a slab layer  210 , an index-loading rib layer  220  and an active layer  230 . In operation, the active layer  230  has a bias current applied to it (not shown). 
   In an optical coupler in accordance with the present invention, such as the optical coupler  100  of  FIG. 1 , an optical signal is input via the input waveguide  105 . The input optical signal is communicated to the MMI  110  wherein the optical power is distributed into different modes supported by the MMI  110  and refocused into the output waveguides  120  and  125  after propagating for a proper distance. During the propagation in the MMI  110 , the optical power in each supported mode also induces the stimulated emission of injected carriers provided by the bias current applied to the active layer  230  (i.e., coherent amplification). The recombination of the coherently amplified optical power in each MMI supported mode with proper phase accumulation along the propagation results in the substantially simultaneous splitting and amplification of an input optical signal. 
   An optical coupler in accordance with the present invention, such as the optical coupler  100  of  FIG. 1 , may be integrated using active-passive monolithic integration (APMI) techniques. For example, according to one fabrication technique, a shallow etched buried rib structure is used for forming the passive waveguides of the optical coupler of the present invention. Such a technique provides record low propagation loss in InP material systems. An active section is then formed by another thin layer of multi-quantum-wells (MQW) directly on top of the rib which is buried by the same re-growth that forms passive waveguides. As such, the optical coupler of the present invention may be used as an active device and a passive device. For example, an optical signal input into the optical coupler is amplified in the active section of the optical coupler in addition to being split by the MMI coupler. Preferably, InP/InGaAsP is the material of choice for an optical coupler in accordance with the present invention because this material allows for monolithic integration with other active components such as transmitters, receivers, optical amplifiers and the like. In this form, multiple waveguides may be placed in close proximity to each other such that each waveguide may be optimized for a specific optical function (e.g. active waveguide optimized for gain, passive waveguide optimized for ease of coupling, passive waveguide optimized for splitting, directional coupling or other passive devices). 
   Even further, an optical coupler in accordance with the present invention is capable of active-passive monolithic integration (APMI) using other techniques known in the art. For example, an optical coupler in accordance with the present invention may comprise optoelectronic integrated waveguide devices utilizing a tilted valence band quantum well semiconductor double hetero-structure with one growth of the same waveguide material. As such, an optical coupler of the present invention may be operated with no bias for normal passive operation or with reverse bias for operating as an active device. Such a technique is discussed generally in U.S. Pat. No. 5,953,479, issued to Zhou et al. on Sep. 14, 1999, which is herein incorporated by reference in its entirety. 
   In an experiment, the passive waveguide was confined with a low effective index contrast, Δ n , of 0.86%, while the active waveguide had a high Δ n  of 3.4%. Having a width of 6 um, the active waveguide supports seven (7) modes with a beating length, L π , of 91 um for symmetric excitation of an input optical signal. As such, the first-order 1×2 MMI coupler has a length of 45.5 um. For an input passive waveguide of 1.8 um wide, more than 96% of input power can be coupled into the active MMI section. The majority of the power loss is due to a slight vertical offset between the active mode and the passive mode. 
   Since tuning the bias current changes both the amplitude and the phase of the optical field in each MMI supported mode, the splitting ratio is expected also to be adjustable. In the extreme case, this would allow a fast switching of an input optical signal between two output waveguides. 
   While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.