Abstract:
Devices and methods for processing signals using a Burst-Mode TIA that meets EPON and GPON specifications are disclosed herein. A signal provided by a power detector is processed with the appropriate gain by using a gain selector which includes a feedback circuit to choose the gain internally, thereby eliminating the need for an external control. Further embodiments of the invention include power detectors featuring a low-pass filter, a peak detector, and/or an envelope detector. Further embodiments of the invention include a Freeze function circuit for maintaining a current gain. Further embodiments of the invention apply appropriate gains when bursts with substantially different power levels are received consecutively, and prevent the gain from being changed during a burst. In this method, a two-pole low-pass filter with an undamped response function is used.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to Burst-Mode Trans-Impedance Amplifiers, which can have controlled- or uncontrolled-gain, for point-to-multipoint communication and fast optical switching applications.  
         [0002]     Fiber-optic communication systems require an amplifier at the receiver to amplify the weak currents generated by the detector diode. These amplifiers must provide sufficient bandwidth, sensitivity, dynamic range, and output signal level to achieve good system performance. The most common amplifier in the fiber-optic field is known as a Trans-Impedance Amplifier (TIA). It is part of almost every optical transceiver. It consists of a high-gain amplifier and a feedback resistor.  
         [0003]     Some new fiber-optic communication technologies, like point-to-multipoint links and switches, require fast signal “lock-in”, in addition to the common requirements of a TIA. In this case, the off-the-shelf components cannot meet the requirements. The reason for this is that it takes a relatively long period of time for a TIA to output a stable signal.  
         [0004]     The majority of applications for a Burst-Mode TIA are in FTTH (Fiber-to-the-Home) networks in which a point-to-multipoint topology is used. For this application, we define burst mode to mean a transmission mode where data is transmitted in bursts rather than in continuous streams. In addition, fast optical switching applications require a Burst-Mode TIA in order to quickly output a reliable signal after switching has occurred.  
         [0005]     A Burst-Mode optical receiver (or TIA) with fast response is required for Passive Optical Networks (PON) [see e.g. IEEE 802.3ah Draft Standard, p. 358, ITU-T Recommendation G.984.2 p. 27, and Maeda et al.,  IEEE Communications Magazine , vol. 40, p. 126-132, December 2001]. In PON systems, an optical line terminal (OLT) receives a burst of packet data with different optical powers due to point-to-multipoint communication. The receiver in the OLT must handle this type of packet data. The receiver requires high sensitivity, wide dynamic range, and quick response. Low cost and high reliability are also required in such PON networks.  
         [0006]     Supporting the wide dynamic range is achieved by several existing methods. One of these methods utilizes high-speed Automatic Gain Control (AGC) [Yamashita et al., IEEE J. Solid-State Circuits, vol. 37, p. 881-886, July 2002; Le et al., ISSCC Dig. Tech. Papers, p. 474-475, February 2004]. The drawback of AGC is that it requires long acquisition time (hundreds of bits), making it unsuitable for applications that require fast acquisition.  
         [0007]     Another approach utilizes DC cancellation from the input signal [Ota et al.,  IEEE J Lightwave Technol ., Vol. 12, No. 2, p. 325-331, February 1994]. This may improve the dynamic range by a nominal amount (˜3 dB), but it also degrades the sensitivity of the receiver (1-3 dB degradation).  
         [0008]     Another approach utilizes nonlinear gain [Nakamura et al.,  IEEE J Solid - State Circuits , vol. 33, p. 1179-1187, August 1998; Brigati et al.,  IEEE J. Solid - State Circuits , vol. 37, p. 887-894, July 2002]. This approach is hard to implement with silicon circuit fabrication technology, and degrades the sensitivity performance as well (1-2 dB degradation).  
         [0009]     A further method utilizes programmable gain [Nakamura et al.,  ISSCC  2005 , Optical Communication , Session 12.4]. This method involves selecting two or more gain values based on the input value. This method seems to be the best choice for the above-mentioned applications. It does not degrade the sensitivity performance and widens the dynamic range by a factor of approximately two (in dB) in the case of selecting between two gains. It is also fast (on the order of ten bits) and easy to implement.  
         [0010]     There is thus a widely recognized need for, and it would be highly advantageous to have, a TIA that would have a wide dynamic range without sacrificing sensitivity performance, signal integrity, or response time. Furthermore, the need to provide these features and operate in burst-mode to accommodate multi-source packet data is finding an increasing number of industrial applications.  
         [0011]     The present invention shows two different new architectures, using a programmable gain, that enable fast selection of the appropriate gain and keep the gain constant during a burst of data.  
       SUMMARY OF THE INVENTION  
       [0012]     It is the purpose of the present invention to provide a device (i.e. internal circuitry components and interface) for producing a Burst-Mode TIA with a programmable gain. The wide dynamic range of the present invention is achieved by using the programmable gain.  
         [0013]     According to one aspect of the present invention, there is provided a device which can be used for applications in Ethernet Passive Optical Networks (EPON). This aspect provides a relaxed acquisition time because of the line-coding (like “8b10b”). “8b10b” line-coding, for example, is a coding scheme which translates 8-bit data into 10-bit data and prevents long sequences of 1&#39;s and 0&#39;s; therefore, no external control is used. We will refer to this as an uncontrolled-gain architecture.  
         [0014]     According to another aspect of the present invention, there is provided a device which can be used for applications in Giga-Bit Passive Optical Networks (GPON). This aspect requires fast acquisition and no data line-coding like “8b10b” is used. There is a necessity to support a large number of consecutive identical digits. These are large sequences, usually up to 72 bits of 1&#39;s or 0&#39;s, which make tracking difficult; therefore, external control is used. We will refer to this as a controlled-gain architecture.  
         [0015]     To contrast the present invention with the prior art of Nakarnura, the prior art assumes apriori knowledge of the time that each burst starts. It assumes an “external reset” signal which resets the TIA before or in the beginning of a burst. There are two drawbacks to this approach: First, in some applications, an “external reset” adds an external control pin. The addition of such a pin makes the solution expensive and inappropriate for crosstalk and noise reasons. Second, the “external reset” brings the TIA to a “known state”; therefore, the entire gain selection process is done automatically within the TIA. Thus, external control of the gain selection is not possible. This makes the design very sensitive to process changes, and consequently, not robust.  
         [0016]     The present invention provides solutions to each of the above-mentioned drawbacks. In applications where the timing requirements are relaxed (e.g. EPON or others), an uncontrolled-gain architecture is suggested. When more strict timing is required, the present invention implements a different approach. In this case, an external Freeze signal is used. This Freeze signal can put the TIA in one of two states: (1) Not Freeze—in this state, the TIA continually adapts the gain to the input signal level, (2) Freeze—in this state, the TIA keeps its last selection (i.e. before it was switched to the Freeze state). The method of the present invention enables external control of the TIA while making the TIA more robust than the prior art of Nakamura.  
         [0017]     Therefore, according to the present invention, it is now disclosed for the first time a TIA for processing signals which includes: (a) a TIA core for providing an appropriate gain to a detector output signal, (b) a coupler for transferring a part of a TIA output signal without introducing distortion or noise, (c) a power detector for obtaining a desired signal level from the signal part provided by the coupler, (d) a feedback circuit which regulates a gain selector for choosing the appropriate gain internally based on the desired signal level from the power detector, and (e) a gain control for setting the appropriate gain obtained from the gain selector.  
         [0018]     According to further features in preferred embodiments of the invention described below, the power detector includes a low-pass filter for filtering the signal part provided by the coupler.  
         [0019]     According to further features in preferred embodiments of the invention described below, the power detector includes a peak detector for detecting and measuring a peak of the signal part provided by the coupler.  
         [0020]     According to further features in preferred embodiments of the invention described below, the power detector includes an envelope detector for detecting and measuring an envelope of the signal part provided by the coupler.  
         [0021]     According to further features in preferred embodiments of the invention described below, the gain selector includes a multiplexer for selecting an input signal based on the desired signal level from the power detector.  
         [0022]     According to further features in preferred embodiments of the invention described below, the gain selector includes a logic unit for selecting a gain, by the feedback circuit, for both the TIA core and the input signal of the multiplexer.  
         [0023]     According to further features in preferred embodiments of the invention described below, the TIA core includes a high-gain amplifier and feedback resistors for setting the appropriate gain.  
         [0024]     According to further features in preferred embodiments of the invention described below, the TIA core further includes switches for engaging or disengaging the feedback resistors.  
         [0025]     According to further features in preferred embodiments of the invention described below, the TIA operates in a burst mode.  
         [0026]     According to further features in preferred embodiments of the invention described below, the TIA further includes a freeze function circuit for controlling the gain selector.  
         [0027]     According to further features in preferred embodiments of the invention described below, the freeze function circuit can maintain the current gain of the gain selector.  
         [0028]     According to further features in preferred embodiments of the invention described below, the freeze function circuit includes an interface for remotely activating or deactivating the freeze function circuit.  
         [0029]     According to further features in preferred embodiments of the invention described below, the freeze function can override the logic unit.  
         [0030]     According to the present invention, it is now disclosed for the first time a TIA for processing signals which includes: (a) a TIA core for providing an appropriate gain to a detector output signal, (b) a coupler for transferring a part of a TIA output signal without introducing distortion or noise, (c) a power detector for obtaining a desired signal level from the signal part provided by the coupler, (d) a feedback circuit which regulates a gain selector for choosing the appropriate gain internally based on the desired signal level from the power detector, (e) a gain control for setting the appropriate gain obtained from the gain selector, and (f) a freeze function circuit for controlling the gain selector.  
         [0031]     According to the present invention, it is now disclosed for the first time a method for processing a signal using a TIA which includes: (a) obtaining a pre-processed signal from a power detector at the input of the TIA, (b) using a gain selector of the TIA to choose an appropriate nominal gain (Gain i ), (c) setting a nominal input gain (G i ) to the multiplexer of the TIA, and (d) controlling values of each Gain i  and G i  through a mathematical relation of Gain i  to G i , thereby providing a desired processing of the signal.  
         [0032]     According to further features in preferred embodiments of the invention described below, the step of controlling values of each Gain i  and G i  through a mathematical relation of Gain i  to G i  includes prescribing the product of Gain i  and G i  to equal a constant.  
         [0033]     According to further features in preferred embodiments of the invention described below, the step of controlling values of each Gain i  and G i  through a mathematical relation of Gain i  to G i  includes prescribing the product of Gain i  and G i  to equal an i-dependent nominal constant.  
         [0034]     According to further features in preferred embodiments of the invention described below, the method is performable by the TIA operating in a burst mode.  
         [0035]     According to the present invention, it is now disclosed for the first time a method for processing signals using a TIA which includes: (a) providing a consecutive pair of burst signals from a detector output, where the consecutive signals having substantially different power levels, (b) filtering each burst signal with a response function to provide an output signal with an appropriate gain amplification, and (c) using the output signal to obtain an accurate signal peak for each burst signal, thereby providing a desired processing of the burst signals.  
         [0036]     According to further features in preferred embodiments of the invention described below, the step of filtering each burst signal with a response function includes filtering each burst signal with an undamped response function.  
         [0037]     According to further features in preferred embodiments of the invention described below, the step of filtering each burst signal with a response function includes filtering each burst signal using a two-pole low-pass filter with the following response function:  
           H   ⁡     (   s   )       =         a   ⁢           ⁢   s     +   b         ξ   2     +     2   ⁢     ξω   n     ⁢   s     +     s   2           ,       
 
         [0038]     where ξ is the damping factor, ω n  is the natural frequency, a and b are constants, s is an arbitrary complex variable, and H(s) is the Laplace transform of the response of the detector, which is defined as follows:  
           H   ⁡     (   s   )       =       ∫   0   ∞     ⁢       ⅇ     -   st       ⁢     h   ⁡     (   t   )       ⁢     ⅆ   t           ,       
 
         [0039]     where h(t) is the impulse response of the filter.  
         [0040]     These and further embodiments will be apparent from the detailed description and examples that follow. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0042]      FIG. 1  shows a schematic block diagram of an electrical circuit used in an uncontrolled-gain, Burst-Mode TIA;  
         [0043]      FIG. 2  shows a schematic block diagram of an electrical circuit used in a controlled-gain, Burst-Mode TIA;  
         [0044]      FIG. 3  shows a schematic block diagram of an electrical circuit showing a high-level description of the controlled-gain TIA core;  
         [0045]      FIG. 4  shows a schematic block diagram of an electrical circuit showing a controlled-gain TIA core in which the different resistances are achieved by the opening or closing of various switches;  
         [0046]      FIG. 5A  shows a schematic block diagram of an electrical circuit showing a high-level description of a power detector including an envelope detector;  
         [0047]      FIG. 5B  shows a schematic block diagram of an electrical circuit showing a high-level description of a power detector including low-pass filter;  
         [0048]      FIG. 5C  shows a schematic block diagram of an electrical circuit showing a high-level description of a power detector including a peak detector;  
         [0049]      FIG. 6  shows a schematic block diagram of an electrical circuit showing a gain selector in uncontrolled-gain mode;  
         [0050]      FIG. 7  shows a schematic block diagram of an electrical circuit showing a gain selector in controlled-gain mode;  
         [0051]      FIG. 8  shows a schematic block diagram of a typical sequence of bursts received at the CO (Central office) in a typical point-to-multipoint topology;  
         [0052]      FIG. 9  shows a schematic diagram of an operational problem that can occur with a typical TIA;  
         [0053]      FIG. 10  shows a schematic diagram of a solution to an operational problem that can occur with a typical TIA.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]     The present invention is of a device for Burst-Mode signal processing. The principles and operation of a Burst-Mode TIA with programmable gain (both a controlled-gain and uncontrolled-gain TIA) according to the present invention may be better understood with reference to the drawings and the accompanying description.  
         [0055]     Referring now to the drawings,  FIG. 1  is a schematic block diagram of an electrical circuit used in an uncontrolled, Burst-Mode TIA. Signal comes in from a photodiode at a Signal In  20  to a controlled-gain TIA core  22 . Controlled-gain TIA core  22  provides the appropriate gain for the signal based on a gain control  24  which is chosen by a gain selector  26 . Gain selector  26  obtains the output of a power detector  28  to determine which gain to select. Power detector  28  receives the signal from a coupler (CPL  30 ) which transfers part of the signal with minimal distortion to the output signal which is obtained at an output, TIA Out  32 .  
         [0056]      FIG. 2  is a schematic block diagram of an electrical circuit used in a controlled-gain, Burst-Mode TIA. A signal comes in from a photodiode (Signal In  20 ) to controlled-gain TIA core  22 . Controlled-gain TIA core  22  provides the appropriate gain for the signal based on gain control  24  which is chosen by gain selector  26 . Gain selector  26  obtains the output of power detector  28  to determine which gain to select. Gain selector  26  can be controlled as well by a Freeze  34  function. Power detector  28  receives the signal from a coupler (CPL  30 ) which transfers part of the signal with minimal distortion to the output signal which is obtained at output, TIA Out  32 .  
         [0057]      FIG. 3  is a schematic block diagram of an electrical circuit showing a high-level description of controlled-gain TIA core  22 . The circuit comprises a high-gain amplifier  38  and a feedback resistor, R n , where gain control  24  is provided. Gain control  24  is controlled by selecting different resistances.  
         [0058]     One preferred detailed implementation of the circuit in  FIG. 3  is shown in  FIG. 4 .  FIG. 4  shows a schematic block diagram of an electrical circuit showing controlled-gain TIA core  22  in which different resistances (R 1  through R n ) are achieved by the opening or closing of various switches (S 1  through S n ). Based on the signal to gain control  24 , the gain of controlled-gain TIA core  22  is one of n gains, Gain 1  to Gain n .  
         [0059]     Embodiments of the present invention with different configurations of power detector  28  are possible.  FIG. 5A  shows a schematic block diagram of an electrical circuit showing a high-level description of power detector  28  including an envelope detector (ED  42 ). In this embodiment, power detector  28  comprises two components, a low-pass filter (LPF  40 ) and ED  42 . Power detector  28  samples the output of controlled-gain TIA core  22  (via CPL  30 ), and passes its processed, output signal level  46  to gain selector  26 , which selects the gain according to the level sent from power detector  28  (i.e. signal level  46 ).  
         [0060]      FIG. 5B  shows a schematic block diagram of an electrical circuit showing a high-level description of power detector  28  including a low-pass filter (LPF  40 ). In this embodiment, power detector  28  comprises only LPF  40 . The signal is routed in the same way as in  FIG. 5A .  
         [0061]      FIG. 5C  shows a schematic block diagram of an electrical circuit showing a high-level description of power detector  28  including a peak detector (PD  44 ). In this embodiment, power detector  28  comprises two components, LPF  40  and PD  44 . The signal is routed in the same way as in  FIGS. 5A and 5B .  
         [0062]     The combination of LPF  40  and PD  44  in power detector  28  (depicted in  FIG. 5C ) can be used for the controlled-gain Burst-Mode TIA embodiment of the present invention (as in  FIG. 1 ). However, for an uncontrolled-gain, Burst-Mode TIA embodiment of the present invention (as in  FIG. 2 ), there is an additional requirement of hysteresis. This requirement is necessary to keep the gain constant during the signal burst. It also ensures that small fluctuations, like noise and other degradations, will not change the gain. This will be explained in more detail below.  
         [0063]      FIG. 6  shows a schematic block diagram of an electrical circuit showing gain selector  26  in uncontrolled-gain mode. The output of a logic unit  50  is used to select both the gain of controlled-gain TIA core  22  (one of n nominal gains, Gain i ) and the gain at the input of a multiplexer MUX  48 , which selects one of the inputs (G i ) according to the input control signal. Thus, we have a feedback loop from logic unit  50  back to MUX  48  which allows gain selector  26  to choose an appropriate gain internally without the necessity of an external control.  
         [0064]      FIG. 7  shows a schematic block diagram of an electrical circuit showing a gain selector  26  in controlled-gain mode. This circuit functions identically to the one shown in  FIG. 6  except for the control functions. Freeze signal  34  (when activated) keeps the current gain of controlled-gain TIA core  22  (Gain i ) and the selected gain (G i ) regardless of what the input to gain selector  26  is.  
         [0065]     There are two alternatives to set the values of the gain pair (Gain i , G i ) in both uncontrolled-gain mode ( FIG. 6 ) and controlled-gain mode ( FIG. 7 ). One option is where the gain product, Gain i ·G i , equals a constant, which is independent of i (where i is the index of the gain from one to n). The second option is where the gain product, Gain i ·G i , equals a Δ i  (where Δ i  is an i-dependent nominal constant, i.e. a nominal constant for each i index).  
         [0066]     In the embodiment of the present invention where the gain product (Gain i ·G i ) equals a constant, hysteresis can be achieved by choosing two thresholds, TH 1  and TH 2  (i.e. m=2 in  FIGS. 6 and 7 ). A higher threshold is set for low signals (which means high Gain i ), and a lower threshold is set for high signals (which means low Gain i ). Comparators (CMP 1  and CMP 2 ) provide the input to logic unit  50  based on the output of MUX  48  and the thresholds (TH 1  and TH 2 ).  
         [0067]     In the embodiment of the present invention where the gain product (Gain i ·G i ) equals Δ i , when only two amplifiers are in use (i.e. G 1  and G 2  resulting from R 1 , R 2 , S 1 , S 2  in  FIGS. 4, 6  and  7  where n=2), hysteresis can be achieved with only one threshold, TH 1  (i.e. m=1 in  FIGS. 6 and 7 ). The gain product (Gain i ·G i ) is lower for low signals. As in the case described above, a comparator (CMP 1 ) provides the input to logic unit  50  based on the output of MUX  48  and the threshold (TH 1 ).  
         [0068]      FIG. 8  shows a schematic block diagram of a typical sequence of bursts received at the CO (Central Office) in a typical point-to-multipoint topology. The height of each burst is proportional to its power. The difference in power is caused by the fact that each unit can be located at a different distance from the CO. Each burst carries data signals (which are not shown in the figure). The received signal from Unit G is a low-power burst and is followed by a high-power burst from Unit H.  
         [0069]      FIG. 9  shows a schematic diagram of an operational problem that can occur with a typical TIA when a sequence of bursts as depicted in  FIG. 8  (i.e. signal from Unit G followed by Unit H) are received. An operational problem arises when an input signal  56  comprising a high-power burst  54  follows a low-power burst  52 . Since the first burst ( 52 ) is low-power, gain control  24  chooses a high gain  60  (i.e. first burst level ( 52 ) is lower than a low threshold  70 ). When the second burst ( 54 ) starts, gain control  24  attempts to choose a low gain  62  (i.e. second burst level ( 54 ) is higher than a high threshold  68 ).  
         [0070]     Because of the finite bandwidth of a detector, a detector output (DO)  58  crosses high threshold  68  (Case 1—DO  64 , and Case 2—DO  66 ), after a finite period of time (T 1  and T 2 , respectively). In the first case, input signal  56  is a Case 1—Very High  72 . The output of the detector, Case 1—DO  64 , is much higher than high threshold  68 ; and therefore, the output reaches high threshold  68  within T 1  seconds (which is faster than in the second case). In the second case, input signal  56  is a Case 2—High  74  (which is close to the level of high threshold  68 ). The output of the detector, Case 2—DO  68 , is higher than the high threshold  68 , but lower than Case 1—DO  64 , and therefore reaches high threshold  68  slower (within T 2  seconds).  
         [0071]     The closer detector output  58  of low-power burst  52  is to low threshold  70  and the closer detector output (Case 1—DO  66 ) of high-power burst  54  is to high threshold  68 , the longer the period of time (i.e. from T 1  to T 2 ) it takes to switch from high gain to low gain. This can result in unwanted occurrences of the gain changing in the middle of a burst, and a distortion of an output signal (e.g. Case 1—DO  64  and Case 2—DO  66 ). A similar problem can occur if a low-power burst follows a high-power burst as well.  
         [0072]      FIG. 10  shows a schematic diagram of a solution to an operational problem that can occur with a typical TIA. Detector output  58  should be as it is depicted in  FIG. 10 . Filtering detector output  58 , as in  FIG. 10 , generates a peak  80  which will quickly cross high threshold  68  and force a change in gain. Such filtering is achieved by using an undamped response control. One way of achieving such response control is by using a two-pole low-pass filter with the following response function:  
           H   ⁡     (   s   )       =         a   ⁢           ⁢   s     +   b         ξ   2     +     2   ⁢     ξω   n     ⁢   s     +     s   2           ,         
 where ξ is the damping factor, ω n  is the natural frequency, a and b are constants, s is an arbitrary complex variable, and H(s) is the Laplace transform of the detector response, which is defined as follows:  
           H   ⁡     (   s   )       =       ∫   0   ∞     ⁢       ⅇ     -   st       ⁢     h   ⁡     (   t   )       ⁢     ⅆ   t           ,         
 where h(t) is the impule response of the filter. The lower ξ is (e.g. ξ&lt;0.7), the higher peak  80  is in  FIG. 10 . 
 
         [0073]     All publications, patents, and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.  
         [0074]     While the present invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications, and other applications of the present invention may be made.