1. Field of the Invention
The present invention relates to transmission of optical signals, and more particularly to wavelength division multiplexers and demultiplexers for optical signals.
2. Description of Related Art
Telecommunications systems using optical media offer far larger information transmission capacity than traditional electrical communications media. Telecommunication systems using optical media perform the same basic functions as copper wires or coaxial cables in conveying information, but they transmit light instead of electrical signals. Optical media can carry voice, data, or video information. An optical transmission system is similar to a conventional transmission system, except that the transmitter converts electrical information signals to light signals while the receiver converts the light back into electrical signals.
The term "optical media" is used to describe a wide variety of media. For instance, optical fibers are thin, flexible strands of clear glass or plastic that can carry up to several gigabits of information per second. Optical media offer many distinct advantages over older transmission media, including larger bandwidth, small size and light weight, electromagnetic immunity, and low transmission loss. Regarding bandwidth, modem optical fibers, for instance, can have bandwidths 10 to 100 times greater than the best coaxial cables. If properly utilized, this bandwidth offers far larger transmission capacity than traditional electrical communications media. One of the proposed methods to exploit more efficiently the high potential bandwidth of optical media is by wavelength division multiplexing (WDM).
With the WDM technique, a large number of communication channels may be transmitted simultaneously over a single optical medium. Such a technique is necessary because information sent over optical fibers is usually encoded onto a single carrier, i.e., a single wavelength of light which is modulated to encode the information. Optoelectronic transmitters and receivers are therefore generally capable of transmitting and receiving only one wavelength of light, or a very narrow band of wavelengths. This causes an inefficiency in optical communications system, since optical media are generally capable of supporting a broad band of wavelengths.
Consequently, a scheme has been devised to increase the amount of information sent over one optical medium. The scheme, called wavelength division multiplexing (WDM) and demultiplexing (WDDM), uses multiple carriers (i.e., wavelengths) to enhance the transmission bandwidth of optical communications systems and sensor systems by simultaneously transmitting multiple communication channels over a single optical medium. Multiplexing requires focusing various carriers into one medium while demultiplexing requires a separation of carriers according to wavelength.
Transmission of multiple carriers is analogous to the use of multiple channels in the AM broadcast band: the carriers in an optical fiber are denoted according to wavelength, and the modulation scheme for each carrier is amplitude-modulation, either analog or digital. Systems that transmit information on multiple carriers in a single optical medium must take several limitations into account. First, optoelectronic transmitters have a single wavelength output. Second, each carrier requires a separate receiver for its detection. These two limitations result in the need, when transmitting multiple carriers over a single medium, for devices that multiplex (combine) multiple carriers at the transmitting end and demultiplex (separate) the carriers at the receiving end of an optical fiber link.
The most common implementation of the WDM (wavelength division multiplexer) is to accept light from each of several input ports, with each port coupled to accept light of predominantly one wavelength from an optical medium such as an optical fiber. The WDM then multiplexes (combines) the light from each of these several input ports into a single output port, which is usually coupled to one optical medium, such as a single optical fiber, for transmission to a remote location. In the terminology of electrical engineering, we might consider the WDM as a passive device with a plurality of input ports, and one single output port.
The WDDM (wavelength division demultiplexer) is the inverse device of the WDM described above. A WDDM accepts light of several different wavelengths from one input port and demultiplexes (divides) the light from the single source, such as an optical fiber, according to by wavelength. The WDDM routes each of the separated light beams into a different output port, each of which is usually coupled to one of several output optical fibers for transmission to an optoelectronic receiver.
In some implementations, the output optical signals from a WDDM may be coupled directly into optoelectronic receivers for conversion to electrical signals at a single location. In this case, the WDDM has some similarity to a spectrometer. In some implementations, a WDM can serve as a WDDM by reversing the direction of light through the device. Some of the embodiments presented in this disclosure are reversible, others are not.
Various physical implementations of WDMs and WDDMs have been proposed and demonstrated. These can be categorized into several types, based on the physical mechanisms used therein. These mechanisms include interference filters, waveguides, and holographic diffraction gratings. Various aspects of these categories are described in the following publications, each of which is incorporated herein by reference:
Maggie M. Li and Ray T. Chen, "Two-channel surface-normal wavelength division demultiplexer using substrate guided waves in conjunction with multiplexed waveguide holograms," App. Phys. Lett. 66(3), 262(Jan. 16, 1995); PA0 Maggie M. Li and Ray T. Chen, "Five-channel surface-normal wavelength-division demultiplexer using substrate-guided waves in conjunction with a polymer-based Littrow hologram," Optics Lett. 20(7), 797(Apr. 1, 1995); PA0 Charles C Zhou, Ray T. Chen, Boyd V. Hunter, and Paul Dempewolf, "Axial-Graded-Index (AGRIN) Lens-Based Eight-Channel Wavelength Division Demultiplexer for Multimode Fiber-Optic Systems," IEEE Photon. Technol. Lett. 10(4) 564(Apr. 4, 1998); PA0 Jian Liu and Ray T. Chen, "A Planarized Two-Dimensional Multi-Wavelength Routing Network with 1-to-many Cascaded Fanouts," White Paper, 1998. PA0 Jian Liu and Ray T. Chen, "Two-Dimensional Dual-Wavelength Routing Network with 1-to-10 Cascaded Fanouts," IEEE Photo. Technol. Lett. 10(2), 238(1998); and PA0 Charles C. Zhou, Zhenhai Fu, Ray T. Chen, and Brian M. Davies, "Dispersion correction of surface-normal optical interconnection using two compensated holograms," Apl. Phys. Lett. 72(25), 3249(Jun. 22, 1998).
Each category of WDDM is discussed separately below. (For ease of discussion, only WDDMs are discussed, although reversing the light path through the device results in wavelength division multiplexing for reversible methodologies.)
A relatively inexpensive type of WDDM is based on interference filters that select out a particular narrow band of wavelengths for transmission to an output port, thus selecting a single carrier. By using these interference filters at the output of a splitter, one may construct a WDDM. This implementation is not very efficient because much of the input light is lost unless the reflected light is sent to other output ports. These interference-based devices may be inexpensive, but suffer significant loss, and are essentially unrelated to the technique of the invention presented in this disclosure.
Using a different physical mechanism, one may also fashion WDDM devices based on optical waveguide techniques. Some of these are only applicable for a device with a small number of channels (2 or 3). However, the use of Fiber Bragg gratings in conjunction with directional couplers allows such devices to support more channels. The Fiber Bragg WDDM devices are currently the most efficient and provide the best performance, but are very expensive and are used only in very-long distance telecommunications applications such as undersea cables and cross-country links. Arrayed waveguide gratings are another highly efficient technology, but are also very expensive.
Closer to the physics of the present invention are devices based on diffraction gratings. WDDMs that use holographic gratings do exhibit high performance, but are difficult to manufacture and require high alignment tolerance. The physical concepts associated with WDDMs that utilize reflection gratings are nonetheless useful in understanding the concepts presented in this application. For instance, U.S. Pat. No. 4,926,412, entitled "High Channel Density wavelength Division Multiplexer with Defined Diffracting Means Positioning" which is incorporated herein by reference, discloses a dual-function simple lens to both collimate light from the end of a single optical fiber and also focus the diffracted light returned from a reflection diffraction grating, utilized in the Littrow geometry. Jannson '412 discloses at col. 3, lines 15-18 a desire to "maintain the relative positioning and alignment of the fiber optics, lens, and dispersion grating along optical axis." Jannson '412 presents a "compact, rugged" housing to maintain such positioning. This approach, however, leaves air space between the components and therefore still allows for misalignment due to outside agitation. Merely utilizing a rigid housing is therefore not sufficient to guard against misalignment over the life of the device.
U.S. Pat. No. 5,026,131 issued to Jannson et al. and entitled "High Channel Density, Broad Bandwidth Wavelength Division Multiplexer with Highly Non-Uniform Bragg-Littrow Holographic Grating," incorporated herein by reference, utilizes a diffraction grating with transmission capabilities similar to those of the present invention. Jannson '131, a continuation-in-part of Jannson '412, discloses a holographically-produced diffraction grating that both reflects and transmits diffracted light beams, rather than the reflection grating disclosed in Jannson '412. In addition to focusing reflected light beams back towards the source, the device disclosed in Jannson '131 focuses transmitted light beams to an array of fibers on the side of the grating opposite to that of the input fiber. This implementation suffers from sensitivity to misalignment, may be difficult to manufacture and package, and may suffer from low efficiency unless manufactured with high precision equipment. In addition, this implementation possesses significant air space between the elements within the housing and is therefore also subject to misalignment due to agitation from outside sources.
A U.S. patent that attempts to address vulnerability to vibration, thermal effects, and lack of rigidity is U.S. Pat. No. 5,682,255 issued to Friesem, et al, which is incorporated herein by reference. Friesem '255 incorporates a light-transmissive substrate as an essential part of the mechanical design of the WDDM. Friesem '255 accomplishes diffraction and redirection of light by using volume phase holographic transmission diffraction gratings that are bonded onto the rigid substrate. Friesem '255 does not provide a practical embodiment for a WDDM, however, because it embodies an impractical means of collimating light beams. The holograms disclosed in Friesem '255 accept both plane and spherical waves and theoretically perform the collimating function of the lens(es) disclosed in Jansson '131 and Jannson '412. While it is possible according to the theory of holograms to use a volume phase hologram to both to collimate and/or focus light while diffracting the light, such a scheme is difficult and impractical to implement. What is needed is a relatively rigid WD(D)M that uses practical, non-holographic elements to collimate and/or focus light.