The invention relates to an optical device which carries multiple optical signals where the optical device has a plurality of distal waveguides some of which may be configured to control insertion loss among the multiple optical signals.
There is an increasing demand for telecommunication capacity as a result of increased Internet traffic, a growing number of telephone lines for telephones, fax, and computer modems, and an increase in other telecommunication services. This increasing demand is being addressed through the combination of multiple telecommunication signals for concurrent transmission through telecommunication lines to increase telecommunication capacity. One way of increasing capacity is by combining multiple signals through the use of wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM). WDM involves combining or multiplexing a plurality of optical signals having a predetermined difference in their wavelengths. The multiplexed signals are transmitted over a single optical fiber. An optical wavelength multiplexer/demultiplexer is essential to the transmission of a multiplexed signal as a multiplexer/demultiplexer can combine optical signals of different wavelengths or separate a multiplexed signal into several optical signals based upon the respective wavelength of each signal. An arrayed-waveguide grating is one standard device used as an optical wavelength multiplexer/demultiplexer.
A multiplexer combines multiple optical signals having different wavelengths into a multiplexed signal. This multiplexed signal is then transmitted through the optical system, which can include amplifiers, optical fiber, receivers, and other optical components typically used in WDM systems. When the multiplexed signal reaches its destination, a demultiplexer uses the different wavelengths of the signals within the multiplexed signal to separate signals thereby allowing routing of the individual signals to their particular destination.
Multiple routing functions including multiplexing or demultiplexing are customarily integrated on a silicon wafer to form a planar lightwave circuit (PLC). PLC""s are integrated optic devices made using tools and techniques developed by the semiconductor industry. Although integrating multiple components on a PLC lowers the manufacturing, packaging, and assembly costs per function, challenges remain to increase performance of the PLC.
An arrayed-waveguide grating (AWG) integrated optical device for wavelength demultiplexing includes at least one input waveguide for receiving a wavelength division multiplexed signal from the optical system. As discussed above, the wavelength division multiplexed signal comprises a plurality of signals each of which has a unique wavelength The AWG contains an input slab waveguide for expanding the wavelength division multiplexed signals coupled from the input waveguide into the input slab waveguide. The device also has a phased array of waveguides comprising a plurality of waveguides, each of which has a predetermined length. The xe2x80x9cphased arraysxe2x80x9d may also be called a xe2x80x9cgrating region.xe2x80x9d The lengths of each waveguide in the phased array differs from each other by a predetermined amount and correspond to the difference between the wavelength of the signals. The difference in lengths of these waveguides causes the light travelling through a waveguide to leave the waveguide with a phase difference from light travelling through another waveguide of a different length. Next, the AWG has an output slab waveguide which focuses the signals of different wavelengths leaving the waveguides of the phased array into a plurality of predetermined positions in accordance with the respective wavelength differences. The AWG also contains a plurality of output waveguides. The output waveguides each have a first and second end where the first end is connected to an external device and the second end of each output waveguide is arranged at a distinct position where each signal separated by its respective wavelength is coupled one of the output waveguides and may be transmitted to the external device.
In operation of the device, the input waveguide of the chip routes the multiplexed signal to the input lens. Then, the signal expands into the free propagation region of the input slab. Next, the expanded wavefront is coupled to the individual waveguides of the phased array. The waveguides of the phased array are arranged radially along an arc of the input slab waveguide. As discussed above, each waveguide of the phased array has a length which is different from another waveguide in the phased array. As the wavefront exit the waveguides of the phased array, each signal has a predetermined phase delay resulting from the length differences of the waveguides and the differences in wavelengths of the signals. The phase delay causes the signals to be refracted to different region in the output slab waveguide. Since the phase delay depends on the wavelength of the signal, each signal with a different wavelength has a different phase delay which causes the signal to be focused into different positions along an arc of the output slab waveguide. The output waveguides are arranged in order along the arc of the output slab waveguide. Accordingly, each signal, having its respective phase delay, is transmitted into a particular output waveguide. A description of the operation of these devices may be found in Katsunari Okamoto, Fundamentals of Optical Waveguides (Academic Press, 2000) the entirety of which is hereby incorporated by reference.
The structures of a multiplexer and a demultiplexer are often similar with the device mode of operation depending upon the application. For example, a multiplexer may simply be the reverse operation of the above described demultiplexer. In other words, using the example described above, a multiplexer receives a plurality of signals at an end of the device (in the above example, the output end) and the signals are combined into a multiplexed signal which leaves the opposite end (in the above example, the input end.) Other applications require specialized device functional parameters, which can only be realized through design of the individual parts of the device.
The transmission properties of entire optical system dictate the design parameters for the multiplexer/demultiplexer component. The basic device structure of the AWG provides certain characteristic spectral signatures. For instance, as a signal emerges from each output waveguide of the AWG, the signal experiences a decrease in the amount of optical power relative to the amount of optical power entering the AWG for each individual channel. This loss is referred to as xe2x80x9cinsertion lossxe2x80x9d (IL) and is an important example of such a spectral signature. IL may result from device fabrication methods, and/or coupling of the grating waveguides to/from the input and output lenses. Other sources causing IL may also exist.
IL is not identical for all the output channels of an AWG. As described in Okamoto, the free spectral range of the grating order determines the IL uniformity. This intrinsic uniformity can differ from the desired system profile. In an AWG, signals leaving the output waveguides located more distantly from a symmetrical axis of the output slab waveguide experience higher IL than signals leaving the output waveguides located closer to the symmetrical axis. It may be desirable for an optical wavelength multiplexer/demultiplexer to minimize the difference in IL between signal with the highest IL and the signal with the lowest IL to achieve a uniform overall transmission loss for each of the signals of different wavelengths. In another example, it may be desirable to deliberately configure each output channels to have an IL that is designed for a selected response other than minimizing the EL between signals as described above.
A known technique for reducing the loss of each separated signal is discussed in U.S. Pat. No. 5,982,960 to Akiba et al. which teaches that each output channel waveguide at the PLC edge is provided with an outwardly tapered end. Each tapered end has a width at its end surface which is larger as the output channel waveguide becomes distant from the symmetrical axis of the output slab waveguide. In the device taught by Akiba et al., the coupling loss becomes lower as the width of the tapered end become larger. Therefore, Akiba et al. teaches adjusting the loss associated with each signal at the interface of a PLC and the optical fibers external to the PLC.
However, adjusting IL at the interface of a PLC and optical fibers may introduce additional problems. Lateral misalignment, defined as the offset between the central axis of the fiber and the central axis of the waveguide on the device, may introduce IL. In the use of a fiber array, lateral misalignment may occur in a direction described by a displacement vector that is perpendicular to the edge of the device but lies in the plane of the axes of the fibers. A problem may arise if waveguides of a device each have tapers of different sizes for each output. Depending upon the degree of lateral misalignment, the change in insertion loss as a result of the lateral misalignment may be different for each channel given the different widths of the channel at the interface. Accordingly, if the sizes of the waveguides at the PLC/fiber interface are the same, then the insertion loss for each channel will be the same given a particular lateral misalignment.
Moreover, it is known, for example, that large fiber arrays often experience bowing, which may also lead to a non-uniform IL at the output channel/fiber interface. In such a case, given the deviation between PLCs which may arise from such factors as bowing, it may be difficult to generate high production volumes of PLCs that consistently control insertion loss at the PLC/optical fiber interface.
While current attempts to achieve uniformity of IL show some promise, additional measures are required. A need remains to be able to achieve a high degree of control of IL without altering other aspects of the performance of the device. Accordingly, it may be desirable to control insertion loss within the PLC itself.
The invention provides an optical device for controlling insertion loss of wavelength-division multiplexed signals comprising a plurality of signals, each signal having a predetermined wavelength different from the remaining signals, the optical device comprising an phased array having a proximal end and a distal end, the phased array comprising a plurality of waveguides extending between the input and output ends, each the waveguide having a predetermined length different from another waveguide; at least one proximal waveguide having a first end and a second end; a proximal slab waveguide between the proximal end of the phased array and the second end of the proximal waveguide; a plurality of distal waveguides each having a first end and a second end, at least one of the plurality of distal waveguides includes at least one gap between the first and second ends; and a distal slab waveguide between the distal end of the phased array and the second end of the distal waveguide.
One variation of the invention includes varying the gap of each of said distal waveguides. The gaps may be varied with the largest towards a center axis of the distal slab waveguide. In another variation of the invention the gap of each distal waveguide is selected to introduce an insertion loss for each distal waveguide such that a difference in insertion loss between each of the plurality of distal waveguides is minimized.
In another variation of the invention, the gaps form an angle with the distal waveguide in a plane of the distal waveguide. The angle may be between 70 and 90 degrees when measured between a face of said distal waveguide adjacent to said gap and a side of said distal waveguide. In another variation, the gaps may form an angle with an axis orthogonal to a plane of the distal waveguide. This angle may also be between 70 and 90 degrees when measured between a face of the distal waveguide adjacent to the gap and the axis. In either of the above cases, the angle may be 82 degrees.
A variation of the invention includes the optical device wherein at least one of the distal waveguides has more than one gap.
In another variation of the invention the gaps may only extend partially through a waveguide. The distal waveguides which have a gap may includes a first portion and a second portion on either side of the gap where the first and second portions are misaligned by an offset distance.
The gap of the present invention may include a gap material that is placed within said gap. In one variation of the invention, the gap may have a width that is greater than a width of the gap material. The invention includes a variation where the gap material comprises an offset section of distal waveguide.
The invention may also include a variation where the optical device is a planar lightwave circuit.
The invention also provides a method controlling insertion loss between a plurality of input or output signals of a planar lightwave circuit comprising the act of transmitting at least one of the signals across at least one distal waveguide having a gap. The method may further include the act of transmitting the plurality of signals across a plurality of distal waveguides each having a respective gap which varies in width. The width of each respective gap may decrease as the waveguide is farther away from a center waveguide of the plurality of waveguides. Another variation of the method includes transmitting at least one of the signals across at least one distal waveguide having a gap where the gap forms an angle with the distal waveguide.