Patent Application
20070003291
1. Field of the Invention
The invention relates to optical receivers, and in particular to transimpedence amplifiers that have a non-linear operating characteristic and provide an interface between a photodiode that receives an optical signal from an optical fiber, such as used in fiber optical communications links, and electronic network terminal units.
2. Description of the Related Art
A variety of optical communications systems are known in the art which include an optical transmit portion that converts an electrical signal into a modulated light beam that is coupled to an optical fiber, and a receive portion that receives an optical signal from an optical fiber and converts it into an electrical signal. Traditionally, optical receive sections include an optical assembly to focus or direct the light from the optical fiber onto a photodetector, which in turn, is connected to an amplifier/limiter circuit on a circuit board. The photodetector or photodiode is typically packaged in a hermetically sealed package in order to protect it from harsh environmental conditions. The photodiodes are semiconductor chips that are typically a few hundred microns to a couple of millimeters wide and 100-500 microns thick. The package in which they are mounted is typically from three to six mm in diameter, and two to five mm tall and has several electrical leads coming out of the package. These electrical leads are then soldered to the circuit board containing the amplifier/limiter and other circuits for processing the electrical signal.
Fiber optic transmission has been used for many years for the distribution of analog CATV signals. Optical receivers for analog CATV signals typically consist of a packaged photodiode electrically connected to a separately-packaged low-noise electronic amplifier, with the photodiode package usually consisting of a TO can with an optical window cap or a ball lens cap to allow light from an optical fiber to be coupled to the photodiode.
A disadvantage of the above arrangement is that there are significant parasitic capacitances associated with the separate photodiode and electronic amplifier packages. These parasitic capacitances play a role in limiting the performance of the optical receiver, resulting in lower bandwidth and higher noise, and contributing to higher power dissipation required to meet a particular target specification.
For digital optical receivers, the standard practice is to package the electronic amplifier (known as a transimpedance amplifier or TIA) inside the TO can next to the photodiode with no intervening packaging. In this way, parasitic capacitances are minimized and performance is improved. However, to date this has not been an option for analog CATV receivers because the power dissipation of the analog TIA has been too high for TO cans to dissipate. This high power dissipation occurs because analog CATV systems require extremely high linearity and low electrical distortion, which is achieved by operating the amplifying transistors with high quiescent currents and voltages so that the signal swing is limited to a small fraction of the available current or voltage excursion. These high quiescent voltages and currents lead to the high DC power dissipation of analog TIAs relative to digital TIAs.
Another technique that is common for analog TIAs is the use of a push-pull or balanced amplifiers, which cancel even-order distortion products, particularly second-order distortion, and make achieving the required linearity much easier. Push-pull amplifiers also come at the price of higher power dissipation, however, because two amplifiers are needed instead of just one single-ended amplifier, and also at the price of balanced-to-unbalanced transformers needed to interconnect to later single-ended stages.
Examples of specific TIAs known in the prior art include U.S. Pat. No. 4,998,012, which depicts a single GaAs FET input transistor configured as a transimpedance amplifier. Such circuit achieves low distortion by operating the transistor with a high drain voltage (15 V supply shown in
Other prior art examples include U.S. Pat. No. 5,013,903; U.S. Pat. No. 5,179,461; and U.S. Pat. No. 5,347,389. Such prior art uses two transimpedance amplifier stages connected to opposite ends of the photodiode, thereby amplifying approximately identical signals that are 180 degrees out of phase from each other. The outputs of the two transimpedance amplifier stages are combined together, typically by means of a transformer, so that the signals are added together in phase but even order distortion products subtract and are cancelled.
As noted above, however, the high power dissipation in all the prior art examples discussed precludes co-packaging of the TIA with a photodiode inside of a TO can or other low-cost package. The separate packaging of the photodiode and the TIA adds parasitic capacitance which reduces bandwidth, and increases noise and distortion.
Prior to the present invention, the high power dissipation of analog CATV TIAs, which is generally at least a watt or more, has made it impossible to package the TIA and photodiode together in a cost-effective way that would reduce the parasitic capacitance.
1. Objects of the Invention
It is an object of the present invention to provide an integrated photodiode/ amplifier for use with optical network system units such as receivers and transceivers.
It is also another object of the present invention to provide an optical receiver subassembly for use in an optical transmission system within an industry standard TO housing.
It is an object of the present invention to provide an improved optical receiver using a low power dissipation amplifier subassembly that allows the photodiode/amplifier to be packaged in a TO can.
It is also another object of the present invention to provide a distortion cancellation circuit for use in non-linear amplifiers used in analog RF transmission systems.
It is still another object of the present invention to provide reduced heat dissipation in an optical receiver by providing an amplifier capable of operating at low power dissipation.
It is still another object of the present invention to provide a transimpedance amplifier with at least first and second stages, in which distortion in the second stage is opposite that of the first stage.
It is still another object of the present invention to provide a more generally applicable amplification function for broadband analog RF signal by means of an integrated circuit the employs internal distortion cancellation
2. Features of the Invention
Briefly, and in general terms, the present invention provides an optical receiver for converting and coupling an information-containing optical signal from an optical fiber, including a photodiode subassembly in the housing for converting an optical signal into a modulated electrical signal corresponding to the optical signal; and an amplifier subassembly in the housing.
Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility.
These and other features of this invention will be better understood and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:
The novel features and characteristics of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to a detailed description of a specific embodiment, when read in conjunction with the accompanying drawings.
Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiments or the relative dimensions of the depicted elements, and are not drawn to scale.
For new Fiber-to-the-Home (FTTH) applications that include an analog optical CATV receiver at each customer premise, there is a strong desire on the part of network interface hardware manufacturers and service providers (such as telephone or cable operating companies) to reduce power dissipation and to lower hardware cost. To this end, the present invention provides a new design for a single-ended analog TIA with power dissipation low enough to allow TO packaging, but performance adequate to meet demanding CATV specifications. A key feature of this design is internal distortion cancellation, in which later amplifier stages have distortion characteristics that are the opposite of those of earlier stages, resulting in an overall amplifier with low distortion but without the high power dissipation that would be the result if each stage individually had to meet tight distortion requirements.
The invention provides a low-noise, low-distortion amplifier that can be used as either a preamplifier or postamplifier for the analog CATV portion of FTTH systems. The low power dissipation of the amplifier is unique in that it allows the preamplifier to be packaged together with a photodiode in a TO can, thereby reducing input capacitance and improving system performance. Low distortion is achieved by employing an output buffer stage with distortion properties that are the opposite of those of the input stage, resulting in distortion cancellation. Temperature compensation of the output-stage bias current can be used to optimize the performance. The invention provides competitive advantages in terms of noise, distortion, power dissipation , and cost relative to other FTTH analog receiver solutions.
A schematic diagram of this invention is shown in
During SPICE simulations of the above circuit, distortion properties were found to be strongly dependent upon transistor current and voltage bias conditions. Under optimal bias conditions, however, both the second and third order distortion were extremely low. The associated power dissipation was much lower than that required for similar distortion performance with single-transistor PHEMT amplifiers. It has been found through both simulation and measurements of actual circuits that second-order distortion can be minimized by choosing an optimum drain-source current for the output source follower, FET3. An example of typical results is shown in
The low power dissipation of the above circuit relative to current products would allow the analog TIA to be packaged in a TO can together with the photodiode, significantly improving the noise and bandwidth performance for a given power dissipation.
In addition to its application as an analog transimpedance amplifier, the invention can also be used as an RF amplifier. As such, it works as a postamplifier for FITH CATV systems and may be widely usable in a variety of CATV and general RF applications where high linearity and low power dissipation are required.
Attached modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only certain embodiments of the present invention, and is not intended to serve as limitations of alternate devices within the spirit and scope of the invention.
It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above.
While the invention has been illustrated and described as embodied in a amplifier circuit for an optical receiver in an optical communications network, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention, and therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.
