In optical communication systems, the need may arise to multiplex different wavelength signals onto a single fiber. For example, wavelength multiplexing is one method for achieving full bidirectional transmission on a single fiber. In its simplest form, a bidirectional system may comprise two stations, S1 and S2, which transmit information at wavelengths λ1 and λ2, respectively. Thus, station S1 needs a transmitter, which operates at λ1 and a receiver, which is tuned to wavelength λ2. Station S2, obviously, has the opposite requirements. Each station also needs a duplexing element to inject both wavelengths onto the single transmitting fiber. Although simple in theory, such an arrangement is cumbersome in implementation. For example, each station comprises a separate transmitter, receiver, and duplexer. Therefore, some sort of optical coupling must also be provided, for example, by using optical waveguides. Such coupling requires many expensive and time-consuming adjustments to achieve optimum alignment. Additionally, the optical losses attributed to this coupling, including attachment between the duplexer, fiber, transmitter and receiver, may degrade the overall performance of the station to an unacceptable level.
An alternative to this straightforward implementation is disclosed in U.S. Pat. No. 4,592,619 issued to E. Weidel on Jun. 3, 1986. Weidel discloses an optical coupling element utilizing a variety of microoptic elements with spherical and plane surfaces for collimating, focusing and redirecting transmitted/received light waves. Although an improvement over the prior art, the Weidel arrangement utilizes at least one optical element, which must be traversed twice by a received light signal. Further, Weidel is necessarily limited to providing coupling between both a transmitter and receiver to an optical fiber. However, there exist situations wherein a pair of transmitters, operating at different wavelengths, must be coupled over the same fiber (unidirectional transmitter).
Thus, a need remains in the prior art for a dual wavelength optical coupler, which is robust in design and is capable of operating in either a bidirectional mode (transmitter and receiver) or unidirectional mode (two transmitters or two receivers).
An attempt to solve the problems that occur in the system of U.S. Pat. No. 4,592,619 is made in U.S. Pat. No. 4,904,043 issued in 1990 to R. Schweizer. This technique was developed in AT&T Bell Laboratories. In this patent, Schweizer describes a device, which practically is the nearest prototype of modern bidirectional transceivers as it contains all elements that found use in subsequent devices of this type. Dual wavelength coupling is achieved utilizing a set of three lenses and a dichroic filter, all held in a precision die-cast housing with the active devices. In one embodiment, the coupler may be used as a bidirectional transceiving device that includes a LED operating on a first wavelength and a PIN receptive to a different wavelength. In another embodiment, the coupler may be used as a unidirectional device, including either a pair of LEDs at different wavelengths or a pair of PINs at different wavelengths. One of the objects of the design described by Schweizer is to avoid active alignment of the components forming the coupler. By careful choice of the lenses, alignment tolerances may be minimized to the extent that the filter and lenses may be merely placed in their proper locations within the housing. Another aspect is to provide a coupler design, which is flexible enough to be utilized with a number of different lenses, as well as different transmitting and receiving wavelengths.
However, a main disadvantage of the device disclosed in U.S. Pat. No. 4,904,043 consists in that a photoreceiver should always be optically coaxial with an optical axis of one of two light sources that generate light of a working wavelength λ1 or λ2. Such a design is inconvenient for suppression of crosstalk, e.g., a parasitic signal with the wavelength λ2 when a photodiode receives a useful signal with wavelength λ1.
All further developments in the field of bidirectional transceivers had design close to the aforementioned device developed by AT&T Bell Laboratories, but with positions of the photodiode and the second light source reversed for obviating the inconvenience inherent in the device described in U.S. Pat. No. 4,904,043.
U.S. Pat. No. 5,485,538 issued on Jan. 16, 1996 to T. Bowen et al. discloses a typical bidirectional transceiver with orthogonal arrangement of two light beams with wavelength λ1 and λ2. The device comprises a compact optical transceiver that includes a ceramic mounting block with a laser diode abutting a first end of the mounting block for generating light of a first wavelength. A holographic optical element (HOE) is positioned adjacent a laser diode and acts as a hologram lens which receives and focuses the generated light to the end face of an optical fiber which is attached to the compact optical transceiver. A glass element is mounted on the mounting block between the diode and the optical fiber end face and includes a dichroic beam splitter that passes light of a first wavelength λ1 and deflects light of a second wavelength λ2. The dichroic beam splitter is mounted in an angular positioning groove of the mounting block and receives and passes the generated light of the first wavelength λ1 which has been focused by the HOE. From a remote transmitter, light of a second wavelength λ2 is then transmitted through the optical fiber to the compact optical transceiver from a direction opposite to that of the light generated by the laser diode. The light transmitted from the optical fiber is then transmitted and passed through a section of the fiber supported by a ferrule attachable to the optical fiber and attached in a second V-shaped positioning groove of the ceramic mounting block. The light of the second wavelength is sent to the beam-splitter and is deflected through a bore hole in the ceramic mounting block to a detector abutting the bottom of the mounting block. A blocking filter can be included for blocking light of wavelengths other than the first and second wavelengths from the detector.
One disadvantage of the transceiver of U.S. Pat. No. 5,485,538 consists in that all optical elements, including HOE, used for beam management transform the beams into converging or diverging beams. This creates significant problems for alignment of the beams because all the optical elements must be adjusted simultaneously. Another disadvantage, which is inherent in all orthogonal bidirectional transceivers, consists in that their geometry is unsuitable for use in conjunction with high-speed controllers.
Another U.S. Pat. No. 5,487,124 issued on Jan. 23, 1996 to the same applicants as the previous patent describes a bidirectional transceiver that differs from the one described in U.S. Pat. No. 5,485,538 by the fact that the HOE was replaced by a GREEN lens. It can be clearly seen from FIG. 2 of U.S. Pat. No. 5,487,124 that the beams of the light source are transformed into converging and diverging beams. Therefore the device of this patent entails all disadvantages of the previously described design, including speed limitations due to geometry.
It should be noted that both designes disclosed in two previous patents were developed at the Whitaker Corporation, Wilmington, Del. In an attempt to solve problems associated with difficulties of alignment mentioned in two previous patents, the Whitaker Corporation developed a new design described in U.S. Pat. No. 5,621,573 issued on Apr. 15, 1997 to W. Lewis, et al. This patent describes a bidirectional link that allows sequential or simultaneous transmission and reception of optical signals using conventional components. To effect the relatively simple alignment of the devices and components, the emitter or transmitter is disposed in a subassembly having a sub-housing with the required optical focusing beam splitting elements disposed therein. This subassembly is optically aligned in a relatively simple active alignment process, and the subassembly is then inserted into the main housing of the bidirectional link. At this stage, a detector is mounted in the main housing and is aligned optically with the pre-aligned elements of the sub-assembly described above. The detector is then fixed using common adhesive and the assembly of the bidirectional link is complete. In this device, the inventors for the first time used so-called canted fibers and lens subassemblies that comprise a lens and a fiber preassembled in a common ferrule-type sub-housing that simplifies manipulation and adjustment. The fiber end face can be slightly inclined from a perpendicular to the optical axis of the lens for decreasing reflection and thus for improving optical coupling. Although the bidirectional transceiver of U.S. Pat. No. 5,621,573 partially simplifies the alignment procedure due to the use of preassembled units, the limitations by speed remain unsolved due to the use of practically the same geometry as in all previously known orthogonal arrangements.
In subsequent years, designs of orthogonal bidirectional transceivers were improved with a new technique developed by Lucent Technologies Inc. This new technique was aimed at improved alignment, more efficient optical coupling, and suppression of crosstalk. Thus, U.S. Pat. No. 5,796,899 issued to T. Butrie et al. on Aug. 18, 1998 describes an optical transceiver assembly for use in a bidirectional system that includes a beam splitter to direct an incoming signal to a photodiode. An outgoing signal from a laser diode is partially transmitted and partially reflected by the splitter. The reflected signal, which may reach the photodiode, constitutes crosstalk which is reduced by means of a cavity positioned to receive the reflected signal and an oblique surface within the cavity adapted to prevent much of the reflected signal from reaching the photodiode.
U.S. Pat. No. 5,838,859 issued on Nov. 17, 1998 to the same applicants as in U.S. Pat. No. 5,796,899 describes an optical transceiver assembly for use in a bidirectional system that includes a beam splitter to direct an incoming signal to a photodiode. An outgoing signal from a laser diode is partially transmitted and partially reflected by the splitter. The reflected signal, which may reach the photodiode, constitutes crosstalk, which is reduced by orienting the polarization direction of the splitter essentially parallel to that of the outgoing signal from the laser diode. In another embodiment, which enhances coupling efficiency, a single element aspheric lens is positioned between the laser diode and the splitter.
Another device developed by Lucent Technologies Inc. is a modular form that improves accuracy of alignment and makes the device suitable for mass production. This device is described in U.S. Pat. No. 5,841,562 issued on Nov. 24, 1998 to S. Rangwala, et al. In accordance with one aspect of this invention, a transceiver comprises a transmitter module and a receiver-splitter module, which is plugged into a self-aligning socket of the transmitter module. In one embodiment, the transmitter module includes a light source lensed to an opening in the socket, and the receiver-splitter module includes a ferrule, which is plugged into the socket. The ferrule carries an optical fiber so that one end of the fiber is optically coupled to the light source. This coupling enables an outgoing optical signal to be partially transmitted to a fiber pigtail located at the opposite end of the ferrule. A splitter is located at the other end of the fiber so that an incoming optical signal on the fiber pigtail is partially reflected to a light detector.
U.S. Pat. No. 6,075,635 issued on Jun. 13, 2000 to T. Butrie at al. describes a bidirectional optical transceiver developed at Lucent Technologies Inc. that includes a housing in which a light source, lens, beam splitter, photodetector and an optical fiber are mounted. The lens focuses an outgoing optical signal from the source through the splitter to the end face of the fiber. The splitter directs an incoming optical signal to the photodetector. In order to reduce reflections of the outgoing signal from the end face of the fiber, and hence crosstalk, without also sacrificing significantly the coupling efficiency to the fiber, the fiber end face is beveled at an acute angle φ to the normal to the common axis of the source, splitter and fiber, and the fiber is tilted at an acute angle θ to the same axis. In a preferred embodiment, which further enhances coupling efficiency, the fiber end face is beveled at an even smaller acute angle φ′ to the normal to the fiber axis, and the fiber axis is tilted at an acute angle θ to the common axis, where φ′ is about 2θ.
A disadvantage common to all four last-mentioned patents of Lucent Technologies Inc., as well to all preceding structures, is that the orthogonal geometry used in the bidirectional optical transceivers described in the aforementioned patents make it difficult to use such devices in high-speed systems with frequencies of 500 MHz or higher.
In fact, all known bidirectional transceivers, as well as those described above, are based on the use of standard commercially-produced laser diodes and photodiodes, such as, e.g., TO CAN packages. The orthogonal arrangement and geometrical dimensions of the aforementioned laser diodes and photodiodes required relatively long lead wires (not less than 10 mm) for commutation with the PC board circuitry. This limitation restricts the speed of transmission data through the bidirectional transceiver.
It is known that inductance depends on the length of the conductor. Therefore, if one takes as a LD series resistance 8 Ohms and accepts 1 nH/mm inductance, it is easy to evaluate that the integration time constant of LD (T=L/r) for 10 mm lead wire is greater than 1.25 ns and for 3 mm lead wire is greater than 0.375 ns, correspondingly. The first lead wire limits gigabit applications of the bidirectional transceiver, while the second one allows to operate at data rates coming to 2.5 Gb/s.