Abstract:
The invention pertains to optical fiber transmission systems, and is particularly relevant to transmission of large volumes of data over long distances at high rates. An apparatus and method for improved bit-error-rate (BER) performance when transmitting optical data over long distances using is disclosed. In particular, the improvement teaches the proper decision threshold setting of the optical signal at the receiver module.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 60/385,947, entitled “Apparatus and Method for Controlling the Decision Threshold in an Optical Network”, by Eiselt, et al., filed Jun. 4, 2002, the content of which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to optical fiber transmission systems, and is particularly relevant to transmission of large volumes of data over long distances at high rates. An apparatus and method for improved bit-error-rate (BER) performance when transmitting optical data over long distances is disclosed. In particular, the improvement teaches the proper decision threshold setting of the optical signal at the receiver module. 
     BACKGROUND OF THE INVENTION 
     A goal of many modem long haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time division multiplexing (TDM) and wavelength division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, are co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport. 
     The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two level binary modulation scheme is predominantly employed. 
     Often, the modulator is separate from the laser diode. This allows for a carrier signal with higher spectral purity and higher reach and capacity performance. One modulator may be used to directly encode the data onto the laser signal. For example, one modulator may be used to achieve a non-return-to-zero (NRZ) format. In a non-return-to-zero format, the instantaneous power of a high optical signal does not return to the low value between adjacent high data bits. 
     For best long haul transmission performance, the return-to-zero (RZ) performance is used. RZ signals, however, exhibit a larger bandwidth than NRZ signals. In practice, a two stage modulator may also be used to achieve this improved performance. For example, a first modulator may be used to shape a train of all high optical pulses with good contrast to the low value between pulses. A second modulator may then be used to encode the data onto this stream of pulses, effectively attenuating those bits that are to be encoded as zeros. 
     During transmission, attenuation in the optical fiber is compensated by in line optical amplifiers. The optical signal propagates through a span of fiber that may be 60-120 km long. At the end of each span, the signal is amplified in an optical amplifier. This process may be repeated over 60 times, for a total system reach of approximately 6000 km. The limit to the number of times this process may be repeated is determined by the optical noise from the optical amplifier. 
     The receiver is located at the opposite end of the optical fiber, from the transmitter. The receiver is typically comprised of a semiconductor photodetector, electrical amplifier, filter and decision circuitry. The role of the decision circuitry is to determine whether a bit is a zero (low) or a one (high) as accurately as possible in the presence of noise, or uncertainty in the level of the received bit. The resultant electrical noise in the received electrical signal stems from mixing of the optical noise with the signal power. Therefore the amount of noise in the (higher power) one rail is larger than the noise on the zero rail. Typically, the standard distribution of the probability density of ones is three times the standard deviation of the probability density of zeros. The decision process leads to the concept of a minimum or optimum bit-error-rate (BER). Erred ones are transmitted signal ones that are mistakenly detected as zeros. Erred zeros are transmitted signal zeros that are mistakenly detected as ones. The rate at which these errors occur is the BER. Typical BERs for optical transport equipment are in the 10 −12  to 10 −15  range. A BER of 10 −15  implies that one erroneous reading is made in 10 15  bits sent. 
     Current decision circuitry control in the art assumes that the number of erred ones are equal to the number of erred zeros for optimum BER. Since the standard deviations of the probability densities are different, this assumption is sub-optimal, and leads to an unnecessarily high BER. There is a need for a decision apparatus and method that sets the optimal threshold. Further there is a need for a decision apparatus and method that sets the optimal threshold in light of unequal standard deviations in the probability densities. 
     SUMMARY OF THE INVENTION 
     In the present invention, improvements to the receiver module of a fiber optic data transmission system are taught for improved signal-to-noise performance. An apparatus and method for improved bit-error-rate (BER) performance when transmitting optical data over long distances using are disclosed. In particular, the improvement teaches the proper decision threshold setting of the optical signal at the receiver module. 
     In one embodiment of the invention, an optical receiver is taught with an optimum decision threshold. 
     In another embodiment of the invention, an optical receiver is taught with a decision threshold that may be optimized. 
     In another embodiment of the invention, a method for optimizing the decision threshold of an optical receiver is taught. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
         FIG. 1  is a schematic illustration of a prior art multiplexed optical transport system. 
         FIG. 2  is a schematic illustration of an optical receiver with an optimum decision threshold. 
         FIG. 3  is a method for optimizing the decision threshold of an optical receiver 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments described herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
       FIG. 1  is an illustrative block diagram of an optical transport system  110  for data and/or voice transmission used to support the present invention. Typical long haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 10 Gbps (gigabit per second) channels across distances of 3000 to 6000 km in a single 30 nm spectral band. Shown in the FIGURE is a duplex system in which traffic is both transmitted and received between parties at opposite end of the link. The optical carrier is generated using laser transmitters  120 . In current DWDM long haul transport systems laser transmitters  120  are DFB lasers stabilized to specified frequencies on the ITU frequency grid. In many systems, the carrier is externally modulated using a modulator  121 . A single stage modulator is sufficient for an NRZ modulation format. With current devices, this single modulator may be a lithium niobate modulator which would be external to the laser. Alternatively, an external, electro-absorptive modulator may be integrated with the laser. Alternatively, in short reach systems, the laser may be modulated by direct modulation of the excitation injection current. 
     In a DWDM system, different channels operating at distinct carrier frequencies are multiplexed using a multiplexer  122 . Such multiplexers may be implemented using array waveguide (AWG) technology or thin film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber  123  for transmission to the receiving end of the link. The total link distance may in current optical transport systems be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the signal is periodically amplified using an in line optical amplifier  124 . Typical span distances between optical amplifiers  124  is 50-100 km. Thus, for example, 30 100 km spans would be used to transmit optical signals between points 3000 km apart. Examples of inline optical amplifiers  124  include erbium doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs). 
     At the receiving end of the link, the optical channels are demultiplexed using a demultiplexer  125 . Such demultiplexers may be implemented using array waveguide grating (AWG) technology or thin film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers  126 . 
     It is a purpose of this invention to teach improved receiver modules for improved BER performance, and these improvements will be discussed in more detail below. 
     It should be noted that  FIG. 1  depicts an optical transport system  110  supporting duplex operation wherein each endpoint can both send and receive voice and data traffic. This is important to achieve a typical conversation. In  FIG. 1 , duplex operation is shown to use two distinct fibers, the both together often referred to as a fiber pair.  FIG. 1  is by restrictive in this or in many other instances. For example, optical transport systems are sometimes deployed with bidirectional traffic providing duplex service on a single fiber. 
     Other common variations include the presence of post-amplifiers and pre-amplifiers just before and after the multiplexer  122  and demultiplexer  125 . Often, there is also included dispersion compensation with the in line amplifiers  124 . These dispersion compensators adjust the phase information of the optical pulses in order to compensate for the chromatic dispersion in the optical fiber while appreciating the role of optical nonlinearities in the optical fiber. Another variation that may be employed is the optical dropping and adding of channels at cities located in between the two end cities. The invention disclosed herein, would find application in any of these variations, as well as others. For example, the improved receiver module taught herein would benefit short reach, or metro applications which may not include an inline optical amplifier  124 . 
     In  FIG. 2  is shown a schematic representation of receiver  126  in accordance with the invention. Receiver  126  is shown in relation to demultiplexer  125 . As shown in  FIG. 2  receiver  126  comprises photoreceiver  210 , electronic amplifier  212 , electronic filter  214 , decision circuitry  216  and forward error correction (FEC) circuitry  218 . In a preferred embodiment, photoreceiver  210  is realized by a semiconductor photodetector, and converts received optical data into high speed electrical signals. In a preferred embodiment electronic amplifier  212  may be realized by a stripline RF FET amplifier. In a preferred embodiment electronic filter  214  is a low pass filter and may be realized by stripline RF capacitors and RF inductors. Functionally, electronic amplifier  212  amplifies said high speed electrical signals. Functionally, low pass electronic filter  214  rejects high frequency components that disproportionately contribute to noise. Decision circuitry  216  optimally converts the analog signal to either a 0 or a 1 binary digital signal. The decision circuitry may include clock recovery in order to extract a clock signal synchronous with and at the rate of the received signal, a quantizer wherein voltages above a threshold are assigned a mark, or a one, and voltages below the threshold are assigned a space or a zero, and a latch to retime the quantized data signal with the recovered clock to produce a data signal with minimum amplitude and phase distortion. In a preferred embodiment FEC circuitry  218  is an integrated circuit that performs a Reed-Solomon or other FEC algorithm that allows for additional optical transport link budget. Also shown in  FIG. 2  is controller  220 . In a preferred embodiment controller  220  comprises a microcontroller, memory and integrative logic. 
     The signal flow through receiver  126  may now be understood in reference to  FIG. 2 . In a preferred embodiment, WDM optical signals flow into demultiplexer  125  where different channels are sorted to different output ports. As shown in  FIG. 2  one such output port leads to receiver  126 . In photodetector  210 , the optical signal is converted to an electrical signal. The electrical signal produced in photodetector  210  proceeds to electronic amplifier  212 , and the electronic signal is amplified. The amplified output of electronic amplifier  212  is sent to electronic filter  214  for low pass filtering. The output of electronic filter  214  proceeds to decision circuitry  216 , where bits above a threshold are decided to be ones, and bits below that threshold are decided to be zeros. The output of decision circuitry  216  proceeds to FEC circuitry  218 . As part of the FEC process of correcting erred ones and erred zeros in accordance to the FEC algorithm, the FEC circuitry also records the number of erred ones and erred zeros over a time span. This information is sent to controller  220 . 
     The optimum decision threshold apparatus in accordance with the invention may now be understood in reference to  FIG. 2 . Via feedback loop  230 , Controller  220  adjusts the threshold level in decision circuitry  216  in order to reduce and minimize the number of erred ones N 1  and the number of erred zeros N 0  in accordance with the following algorithm. Let the ratio R=N 1 /N 0 , and R opt  be a target ratio that provides the minimum number of erred ones N 1  and the minimum number of erred zeros N 0 . If R&gt;R opt , then too many ones are erred and the decision threshold needs to be reduced. Conversely, if R&lt;R opt  then too many zeros are erred and the decision threshold needs to be increased. 
     The value for R opt  for a particular transmission system depends mainly on the relative standard deviations of the one and zero rails, which is affected by many system parameters including but not limited to transmitter extinction ratio, fiber type and resulting amount of nonlinear impairments, and optical filter bandwidths. It also depends slightly on the system BER, which of course depends on all the same factors. Typically a value of R opt =3, the ratio of the standard deviation of the ones probability density function to the standard deviation of the zeros probability density function. In an alternate embodiment, R opt  is determined at channel turn-up by iteratively measuring the BER and adjusting R opt  for the minimum BER. 
     In an alternate preferred embodiment, an inverting receiver is used as photodetector  210  and electronic amplifier  212 . For an inverting receiver, the optical zeros have been transformed to a higher voltage than the optical ones. Therefore the standard deviation of the top rail, corresponding to transmitted zeros will be smaller than the lower rail, corresponding to transmitted ones. While the above procedure for optimum threshold tuning can still be followed, the value for R opt  needs to be ⅓ for the typical signal. The resulting optimum threshold will then be at a lower voltage than the point where the erred ones and erred zeros are equal. 
     In  FIG. 3  is a flow chart illustrating a method for optimizing the decision threshold of an optical receiver in accordance with the invention. In step  305  the decision threshold is set to 50% of the expected “eye opening”. The “eye opening” is well understood in the art as a measure of system performance. In step  310 , the number of erred ones, N 1 , are measured. In a preferred embodiment, N 1  is measured in the process of forward error correction. In step  315 , the number of erred zeros, N 0 , are measured. In a preferred embodiment, N 0  is measured in the process of forward error correction. In step  320 , the ratio R=N 1 /N 0 , is calculated. In a preferred embodiment the ratio R is calculated in controller  220 . In step  321 , the value of R is checked against R opt  by controller  220 . If R&gt;R opt , then too many ones are erred and the decision threshold needs to be reduced. At step  322  the decision threshold is reduced by a set amount dV. In a preferred embodiment dV=5 mV. The cycle is then repeated at step  310 . Conversely, if R&lt;R opt  then too many zeros are erred and the decision threshold needs to be increased. At step  323  the decision threshold is increased by dV. Control then returns to step  310  in the event that R=R opt  then control returns directly to step  310  without changing the decision threshold. The value for R opt  for a particular transmission system depends mainly on the relative standard deviations of the one and zero rails, which is affected by many system parameters including but not limited to transmitter extinction ratio, fiber type and resulting amount of nonlinear impairments, and optical filter bandwidths. It also depends slightly on the system BER, which of course depends on all the same factors. Typically a value of R opt =3, the ratio of the standard deviation of the zeros probability density function to the standard deviation of the ones probability density function. In an alternate embodiment, R opt  is determined at channel turn-up by iteratively measuring the BER and adjusting R opt  for the minimum BER. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.