The present invention pertains to optical communications. More particularly, this invention relates to an optical multi-plexing device with passively aligned molded optics which spatially disperses collimated multi-wavelength light from an optical fiber into individual wavelength bands. In certain preferred embodiments, the improved multiplexing device of the present invention is particularly suited for wavelength division multiplexing systems for the fiber optic data-communications and telecommunications systems.
In wavelength division multiplexed optical communication systems, many different optical wavelength carriers provide independent communication channels in a single optical fiber. Future computation and communication systems place ever-increasing demands upon communication link bandwidth. It is generally known that optical fibers offer much higher bandwidth than conventional coaxial communications; furthermore, a single optical channel in a fiber waveguide uses a microscopically small fraction of the available bandwidth of the fiber (typically a few GHz out of several tens of THz). By transmitting several channels at different optical wavelengths into a fiber [i.e., wavelength division multiplexing (WDM)], this bandwidth may be more efficiently utilized.
Prior art optical multiplexers and demultiplexers include the Nosu et al U.S. Pat. No. 4,244,045. Nosu utilizes a series of wavelength sensitive filters arranged in a zigzag optical pathway at a predetermined oblique angle to the light beam. Nosu positions filters on both sides of substrate 60. It is significant that the fabrication of the Nosu device is expensive and time consuming. As described at column 4, lines 33-52, the multi-layer filters are separately grown on the substrate and have as many as ten layers each. The couplers and lenses must be separately aligned in a tedious, expensive post-fabrication process.
The Scobey U.S. Pat. Nos. 5,583,683 and 5,786,915 teach an eight channel multiplexing device in which a continuous variable thickness interference filter is deposited onto the surface of an optical block. This design has inherent weaknesses. First, each filter must necessarily integrate the signal over its width, since its thickness (and wavelength sensitivity) varies across its width; resulting in less precise filtering. Secondly, the interference filter thickness may vary from device to device, also degrading its filtering performance. Thirdly, each of the individual couplers such as item 62 of FIG. 3 must be separately aligned after fabrication.
The Scobey et al U.S. Pat. No. 5,859,717 teaches the use of a precision optical block which is opaque (col. 4, lines 20-29) and in which slots or gaps are machined or formed to provide an optical path. Any machining required adds to the expense of producing the recesses, gaps and slots. Furthermore, the separate collimators (6,24,46) and filters 32 must be separately aligned after fabrication of the device, adding to the time and expense of production.
The Jayaraman U.S. Pat. No. 5,835,517 teaches an optical multiplexer having multiple optical cavities which must each be "tuned" after fabrication by adjusting its length. Such "tuning" adds significantly to cost of production, and is expressly avoided with the present invention.
The Lemoff et al U.S. Pat. 5,894,535 teaches an optical multiplexer in which a plurality of dielectric channel waveguides are embedded in a cladding region, which process is considerably more expensive than the monolithic formed parts of the present invention. Furthermore, the filters must be separately aligned with the channel waveguides after fabrication. Perhaps the greatest inherent disadvantage of the Lemoff et al design is the difficulty, expense and time of mounting the mirror 36 in channel 50 and properly mounting the filters 45a through 45d. The channel 50 and the filter mounting surface must be cut with an expensive excimer laser or microtome, yielding a relatively rough mounting surface. The rough mounting surface creates bonding and alignment problems. The filters, for example, are angularly sensitive and must be mounted precisely vertically and parallel with each other as shown in FIG. 3. Additionally, since the waveguides are so broad, the waveguides have large angular content, which causes a "rolled off" or variable filter response. The present invention avoids the bonding and alignment problems of Lemoff et al as well as the variable filter response. The present invention uses preformed and/or premolded extremely flat surfaces for carrying the reflective mirror surface and the filters, so that the mirror and filters are properly bonded and aligned. The present invention has much smaller divergence which, together with proper filter alignment, results in much cleaner separation of the n wavelength beams by the filters.
An optical multiplexing device combines or separates multiple light signals with varying optical frequencies. The optical multiplexing device has applications for both dense and coarse wavelength division multiplexing (DWDM and CWDM) for both multi-mode and signal-mode fiber optic data communications and telecommunications. Multiple wavelength light sources are combined into a single optical path for transmission or multi-wavelength light travelling in a single optical path is separated into multiple narrow spectral bands that is focused onto individual fiber optic carriers or detectors.
Current wavelength division multiplexed (WDM) devices are designed for operation in single-mode optical fiber telecommunications systems, where performance over long distances (100 km) is the primary factor and cost and size are secondary. As bandwidth demands within the Networking Industry [Local Area Networks (LAN) and Wide Area Networks (WAN)] increase, compact inexpensive wavelength division multiplexed systems will become necessary in order to utilize the full bandwidth of the optical fiber. The WDM device described herein utilizes plastic-mold injection and inexpensive dielectric filters to create a compact device capable of multiplexing or demultiplexing multiple optical wavelengths.
One of the key features and a primary object of the present invention is to provide a compact and cost effective optical multiplexer and demultiplexer for both single-mode or multi-mode fiber optic communication systems wherein the device includes preformed and premolded passively aligned optics.
A further object of the invention is to provide an optical multiplexer/demultiplexer having only two preformed parts and a series of filters, the filters being sandwiched between the preformed parts.
Another feature of the present invention is to minimize optical loss due to divergence of light between the source coupling optics and the fiber optic connector coupling optics.
Another feature of the present invention is to integrate a fiber optic connector within the optical module to passively align an optical fiber to the coupling optic.
A further feature of the present invention is to allow demultiplexed multi-wavelength light to couple directly to a photodetector array without any intervening optical fiber.
The present invention is comprised of a single plastic molded coupling module, a single precision optical block with a reflective coating on one side, an array of discrete multi-wavelength Fabry-Perot transmission filters in one embodiment, and index matching optical adhesive to bond said components together.
The plastic molded coupling module is formed by integrating an aspherical off-axis collimating lens array, filter mechanical alignment features, a redirectional mirror, a coupling lens, a fiber optic connector, and mechanical features for passive alignment of the optical part to either an array of light sources, a detector array, or an optical fiber array, all within a single part.
A precision optical block can be formed of any transparent optical material capable of transmitting light over the desired spectral region and being formed or polished to the correct thickness. A reflective surface can be placed on the optical block by a number of techniques, to include; dielectric interference coatings, metallic coatings, etc.