Abstract:
An optical circuit is disclosed, which may include a semiconductor optical amplifier (SOA); an optical filter operable to filter light emerging from the SOA; and a PIN for converting the light output from the optical filter into an electrical signal, wherein the gain profile of the optical filter is configured to maximize throughout of signal energy within a predetermined wavelength range (in-band), and to impose an insertion loss (L oob ) of less than 20 dB on signal energy outside the predetermined wavelength range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/896,683, filed Mar. 23, 2007, entitled “Optical Component and Method of Fabrication”, the entire disclosure of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Passive optical networks (PONs) have been deployed worldwide. A network roll-out requires significant investment, and once constructed, should be able to be upgraded economically. Recent activities in both the Full Service Access Network (FSAN) organization, and the IEEE 802.3av 10G-EPON (Ethernet Passive Optical Network) study group have studied next-generation access networks. Both organizations have considered PONs (Passive Optical Networks) operating at 10Gb/s (10 Gigabits-per-second—“10G communication”) rates. It would be desirable to have an upgrade which can leverage the existing infrastructure without requiring outside plant adjustment or changes in customer premises equipment. 
     The 10G-EPON group has specifically addressed the issue of “coexistence” of a new 10G-EPON standard with an existing GE-PON (Gigabit Ethernet Passive Optical Network). By using a combination of Wavelength Division Multiplexing (WDM) and Time-Division Multiple Access (TDMA), it is possible for both a legacy 1G PON and new 10G PON to operate on the same network. 
     This scheme is illustrated schematically in  FIG. 1 . Network  100  may include 1G/10G Optical Line Terminal (OLT), dual rate polarization mode dispersion (PMD) device  110 , 1G Optical Network Unit (ONU)  104 , second 1G ONU  106 , and 10G ONU  108 . OLT  102  includes a Media Access Control (MAC) which controls both the legacy 1G ONUs  104 ,  106  and new 10G ONU  108 . 
     A large link budget in the downstream direction is not a problem since the OLT  102  can use either a high power Distributed Feedback Laser (DFB) laser or a DFB laser in combination with an SOA (Semiconductor Optical Amplifier) to meet the link budget. In both the specifications for (Ethernet Passive Optical Networks—EPON—IEEE 802.3ah) and for (Gigabit capable Passive Optical networks—GPON—ITU-T G.984), the upstream wavelength is defined to range between 1260 and 1360 nm (1310+/−50 nm) and the downstream wavelength is defined to lie within the 1480-1500 nm band. (1490+/−10 nm). With regard to 10G communication, the 1571 nm wavelength was the working wavelength chosen by the IEEE 10G-EPON study group (IEEE 802.3av) for 10G downstream communication, that is communication from OLT  102  to various 10G ONUs, such as ONU  108 . 
     Thus, one limiting performance factor in the system of  FIG. 1  occurs in the upstream communication direction (that is, toward the OLT  102 ). The optical powers of the legacy 1G ONU  104 ,  106  transmitters are fixed, while it is desirable to use a lower power transmitter in the 10G ONU  108  to keep system costs to a minimum. The challenge for the OLT  102  receiver is meeting a large (e.g. 29 dB) link budget for both the 1G and 10G data communication streams. Legacy 1G ONUs  104 ,  106  must be used, and the 1G signals operate over a specified wavelength range of 1260-1360 nm (nanometers), without Forward Error Correcting (FEC), in the upstream direction (toward the OLT  102 ). Since the 10G ONU PMD is not yet defined, the wavelength may be specified with more precision within the 1260-1360 window, and FEC may be available to meet required link budgets. Some existing approaches are discussed below. 
     There are two basic approaches to using an Avalanche Photo-Diode (APD) as a dual-rate receiver, which are illustrated in  FIGS. 2 and 3 , respectively. One goal when using an APD at both 1G and 10G rates is optimizing the bandwidth of the Trans-Impedance Amplifier (TIA). 
       FIG. 2  shows network  200  which includes APD  202 , resistance  204 , TIA  206 , 1G Band Pass Filter (BPF)  208 , and 10G BPF  210 . The resistance value “R” of resistance  204  is the resistance across the TIA  206 . The bandwidth of the TIA  206  will be proportional to 1/R while the (thermal noise current) 2  of the receiver will also be proportional to 1/R. If the bandwidth of the TIA  206  is large enough to enable throughput of both 10 Gb/s (10G) and 1.25 Gb/s (1G) signals, then the thermal noise current of TIA  206  will be (10/1.25) 1/2 , or 4.5 dB higher than under ideal circumstances for the 1.25 Gb/s signal. 
     For thermal-limited receivers (such as PIN, and APD to a lesser extent), the receiver sensitivity is proportional to 1/(thermal noise current). With such a static TIA, the 1G sensitivity in the dual-rate receiver will be 4.5 dB lower (i.e. worse) than in an APD-TIA combination optimized for 1G operation. 
     One potential solution to minimize this penalty is to vary the value of “R” in time such that the TIA  206  bandwidth is optimized for either 1G or 10G traffic. However, this approach adds significant complexity, as the receiver must implement a high-speed dynamic TIA  206  as well as communicate with the MAC in OLT  102  to track the bit-rate of the incoming signals. 
     Another approach is to use two separate APDs, as shown in  FIG. 3 .  FIG. 3  is a block diagram of network  300  that includes a 3 dB optical splitter  302  (labeled with “3 dB” in  FIG. 3 ), APDs  304 ,  306 , a 1G TIA and BPF  308 , and a 10G TIA and BPF  310 . However, with this approach, performance suffers due to the insertion loss of the optical splitter  302 , which effectively halves the power that is directed along each of the two branches to the right of splitter  302 . 
     In the following, it is noted that sensitivities having measurements in “dBm” having larger negative numbers are most beneficial for operation of an optical communication network. 
     Good APDs provide −34 dBm sensitivity with a Bit Error Rate (BER) of 10 −12  at 1G and −25 dBm sensitivity at 10G. A single APD having the foregoing specifications, if used as a dual-rate receiver, in the network of  FIG. 2  would yield a sensitivity of −29.5 dBm at 1G and −25 dBm at 10G, while providing −31 dBm and −22 dBm sensitivities for the 1G and 10G data streams, respectively, if deployed within the network shown in  FIG. 3 . 
     The sensitivity disparities between the 1G and 10G data communication streams described above are undesirable. The performance of the networks of  FIGS. 2 and 3  could result in the 1G communication operating well, and the 10G communication malfunctioning to an unacceptable degree. Accordingly, there is a need in the art for an improved system and method for receiving data at multiple data rates. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the invention is directed to an optical circuit which may include a semiconductor optical amplifier (SOA); an optical filter operable to filter light emerging from the SOA; and a PIN for converting the light output from the optical filter into an electrical signal, wherein the gain profile of the optical filter is configured to maximize throughout of signal energy within a predetermined wavelength range (in-band), and to impose an insertion loss (L oob ) of less than 20 dB on signal energy outside the predetermined signal wavelength range, wherein signal energy at wavelengths outside the predetermined wavelength range is out of band signal energy. 
     Other aspects, features, advantages, etc. will become apparent to one skilled in the art when the description of the preferred embodiments of the invention herein is taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purposes of illustrating the various aspects of the invention, there are shown in the drawings forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
         FIG. 1  is a block diagram of a new 10G (10 Gigabit/second) passive optical network overlaid on, and co-existing with legacy 1G (one gigabit/second) passive optical network; 
         FIG. 2  is a block diagram of a network using a single TIA and a resistor connected in parallel across the TIA; 
         FIG. 3  is a block diagram of a network that splits an incoming signal into two paths, with each path having a separate APD, TIA, and band-pass filter; 
         FIG. 4  is a block diagram of an optical circuit in accordance with an embodiment of the present invention; 
         FIG. 5  is a plot of gain versus wavelength occurring at various points along the optical circuit of  FIG. 4 ; 
         FIG. 6  is a graph of receiver sensitivity plotted against out-of-band Loss for 1G and 10G communication obtainable using the optical circuit of  FIG. 4  in accordance with an embodiment of the present invention; 
         FIG. 7  is a test setup in accordance with an embodiment of the present invention; and 
         FIGS. 8A ,  8 B,  8 C,  8 D,  8 E, and  8 F are plots of sensitivity measurements obtained using the circuit of  FIG. 7 , in which  FIG. 8A  is a plot for a 1G signal without the use of a filter,  FIGS. 8B and 8C  are plots for a 1G signal with simulated soft filters,  FIG. 8D  is a plot for a 10G signal with no filter;  FIG. 8E  is a plot for a 10G signal with a simulated soft filter; and  FIG. 8F  is a plot for a 10G signal with a notch filter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one having ordinary skill in the art that the invention may be practiced without these specific details. In some instances, well-known features may be omitted or simplified so as not to obscure the present invention. Furthermore, reference in the specification to phrases such as “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of phrases such as “in one embodiment” or “in an embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
       FIG. 4  is a block diagram of an optical circuit  400  in accordance with an embodiment of the present invention. Circuit  400  may include SOA  402 , filter  404 , PIN photodiode  406 , TIA  408 , 1G band-pass filter  410 , and/or 10G band-pass filter  412 . 
     SOA  402  is a semiconductor optical amplifier, preferably providing amplification for light having a range of wavelengths from 1260 to 1360 nm. However, SOA  402  is not limited to the providing amplification for the stated range of wavelengths. Filter  404  is an optical filter. PIN  406  is a photodiode. TIA  408  is preferably a single, static trans-impedance amplifier. BPFs  410  and  412  are band-pass filters configured to process 1G and 10G data communication, respectively. 
     While shown in one possible arrangement, the invention is not limited to the particular arrangement shown. For instance, the optical amplifier and filter may each be located at any location within an optical network that includes optical circuit  400 . Both filter  404  and amplifier  402  could be placed between a splitter and PMD  110  ( FIG. 1 ) outside of a central office (in the outside plant). In this case, the filter  404  and the amplifier  402  would be a product in the same box. 
     Alternatively, the filter could be at the receiver in the central office (CO), and the optical amplifier  402  could be outside. In other alternative embodiments, the optical amplifier  402  and filter  404  could have other optical element (boxes) between them such as optical switches, splitters, and/or optical monitor ports. 
     The following is an alternative approach to providing a multiple data rate receiver which employs a combination of SOA  402 , filter  404 , and PIN  406  to achieve acceptable sensitivity for both 1G rate and 10G rate optical communication. Effectively, the circuit of  FIG. 4  enables a tradeoff to be effected between the sensitivity of the 1G communication and the 10G communication. More specifically, some amount of 1G sensitivity may be sacrificed to improve 10G sensitivity to an acceptable level. The various design choices are shown in  FIG. 6 . 
     In one embodiment, the circuit of  FIG. 4  preferably enables a single, static TIA  408  to process a signal spanning a wavelength range incorporating both 1G and 10G signal energy. This differs from existing optical receiver circuits as described above. The existing circuit of  FIG. 2  employs a variable resistor to vary the characteristics of TIA  206  to accommodate either 1G or 10 g signals. The TIA  206  of circuit  200  of  FIG. 2  is therefore not “static.” The separate TIAs within circuits  308  and  310 , of circuit  300  of  FIG. 3 , separately accommodate 1G signals and 10G signals, respectively. Thus, circuit  300  is not able to use a single TIA to process a signal including signal energy for both 1G and 10G communication. 
       FIGS. 5 and 6  are described below, since reference is made thereto in the discussion of the operation of  FIG. 4 . FIG.  5  is a graph of Amplified Spontaneous Emission (ASE) gain plotted against wavelength occurring at various points in the optical circuit of  FIG. 4 .  FIG. 6  shows calculated receiver sensitivity at 10-12 BER as a function of L oob . Parameters: hυ=−155.3 dBm/s (at 1310 nm); η=0.85 A/W, i th =18 pA/√Hz, Be(1G)=0.875 GHz, Be(10G)=7.0 GHz, SOA noise figure=7 dB, SOA gain=20 dB, filter width=12 nm, transmitter extinction ratio=10 dB. Hollow/solid circles on the vertical lines at the right of  FIG. 6  show the 1G/10G sensitivities for the conventional architectures illustrated in  FIGS. 2 and 3 , respectively, as illustrated at the right of  FIG. 6 . 
     One possible drawback to using SOA  402  as a preamplifier in circuit  400  is that although the 10G ONU transmitter wavelength can be specified within a narrow range (by using an un-cooled DFB laser, for example), the legacy 1G ONT transmitter is specified more broadly. Specifically, the 1G signal is indicated as lying within a 100 nm window, between 1260 nm and 1360 nm. Accordingly, the use of a narrow band noise blocking filter in this situation is undesirable since the 1G signal energy would be blocked. 
     In the embodiment of  FIG. 4 , a “soft” filter  404  is proposed as a solution to this problem, as illustrated in  FIGS. 4 and 5 . By way of further introduction to  FIG. 5 , plot  502  shows the gain of SOA  402 . Plot  504  shows the attenuation imposed by filter  404 . Plot  506  shows the resulting gain of the combination of the SOA  402  and the filter  404 . 
     After the “soft filter”  404 , the net gain is G within the 10G wavelength band (the notch in plot  506 ) and G-L oob  outside of the band. The choice of the L oob  value enables an optimization of the relative sensitivity for the 10G and the 1G signals for the dual-rate receiver, as illustrated in  FIG. 6 . 
     Existing notch filters commonly impose insertion losses of about 40 dB or more on out-of-band signals, thus minimizing the throughput of signal energy outside a defined band. In this situation, the use of such a filter would essentially eliminate the 1G signal energy throughput which is not desired. Instead, the soft filtering of filter  404  herein still maximizes the throughput of signal energy within the 10G band (which may be within a narrow range of wavelength on either side of 1310 nm). However, filter  404  preferably optimizes rather than minimizes the throughput of signal energy outside the 10G signal energy band, but still within the range of 1260 to 1360 nm. 
     For example, the insertion loss of signal energy outside the “notch” or band intended to correspond to 10G signal energy, but still within the 1260 to 1360 nm wavelength range may be set lower than 40 dB, such as at 20 dB, 15 dB, 10 dB, or still lower, if beneficial to an embodiment of the invention. 
     In addition to providing a notch to provide zero or minimum attenuation within the 10G signal energy range, and providing moderated attenuation (insertion loss) for signal energy outside this band (L oob  values are shown in  FIG. 6 ), this embodiment of filter  404  may provide a curved attenuation profile, as shown in  FIG. 5 . The curved portion of filter  404  attenuation profile  504  may be operable to counteract at least a portion of the curve in the gain profile  502  of SOA  402  shown in  FIG. 5 . The product of the SOA gain  502  and the filter attenuation  504  is shown as curve  506 , which represents the gain in effect in circuit  400  in between filter  404  and PIN  406 , and thus at the input to PIN  406 . 
       FIG. 6  illustrates receiver sensitivity for 1G and 10G upstream communication for a range of values of out-of-band Loss (L oob ). At right, the receiver sensitivities for 1G and 10G communication of the circuits of  FIG. 2  and  FIG. 3  are shown, and are labeled with the figure numbers illustrating the respective circuits. 
     Effectively, the graph of  FIG. 6  may be employed as a design-phase tool to enable an optical circuit designer to selected the best available combination of 1G and 10G receiver sensitivity values for a given package of equipment. The optical equipment used for  FIG. 6  was identified above. It will be appreciated that the 10G and 1G curves shown in  FIG. 6  may vary with varying performance characteristics of the optical equipment of  FIG. 4 . 
     The  FIG. 2  and  FIG. 3  performance data (shown at the right of  FIG. 6 ) show that the 1G communication performance tends to fall within an acceptable range. However, the circuits of  FIG. 2  and  FIG. 3  tend to suffer from undesirably poor 10G sensitivity. The curves in the main portion of  FIG. 6  show how the 1G and 10G receiver sensitivities vary with L oob . The value of L oob  may be varied through the selection of the attenuation characteristics of filter  404 , one example of which attenuation is shown with curve  504  of  FIG. 5 . 
     Again directing attention to  FIG. 6 , it may be seen that as L oob  increases from 0 to 10 the sensitivity of 10G communication gets progressively better, and the sensitivity of 1G communication gets progressively worse. Thus, varying the filter attenuation characteristics of filter  404  effectively enables trading off 1G sensitivity for 10 sensitivity. More specifically, 1G sensitivity may be worsened within acceptable bounds in order to bring 10G sensitivity within an operationally acceptable range. In the embodiment shown in  FIGS. 4 and 6 , an L oob  value of about 6 dB provides a desirable combination of 1G and 10G sensitivity values. However, other L oob  values may be selected. 
     Thus, in one embodiment, upon viewing the design-phase graph of  FIG. 6 , the filter attenuation  504  of filter  404  over the wavelength range shown in  FIG. 5  could be selected so as to provide about 6 dB of attenuation for signal energy outside the “notch” region, which notch area may correspond to the wavelength range of 10G communication signal energy. However, it will be appreciated that with other equipment and other signal types, the optimal L oob  value could be greater or less than the 6 dB shown in  FIG. 6 . 
     It is clear that a better range of 10G sensitivities are available using the circuit of  FIG. 4 , compared to the APD architectures of  FIG. 2  and  FIG. 3 . In addition, the use of the SOA  402  as a preamplifier may enable the PIN  406  to operate in the RIN (Relative Intensity Noise)-limited noise regime. Thus, the PIN  406  is preferably insensitive to thermal noise. Consequently, there is no dual-rate penalty. 
     An experimental setup is illustrated in  FIG. 7 . A simulation was provided by using a pair of SOAs as illustrated in  FIGS. 7-8 .  FIG. 8  shows the observed spectrum at the PIN photodiode  714 . The SOAs were model SOAM-02P426 manufactured by Alphion Corporation. The 10G (1312 nm) transmitters  702  and receiver  714  were from a commercial multi-protocol 10 km XFP transceiver with a measured extinction ratio of 4.7 dB, while the (1310 nm) 1G transmitter was from a commercial GE-PON ONU transceiver with a measured extinction ratio of 16 dB. Various optical filters  710 ,  712  were used in the 1288-1300, 1302-1314, and 1320-1232 nm (3 dB bandwidth) ranges. Receiver  714  sensitivities were calculated taking into account the overall shape of the measured ASE profiles shown in  FIG. 8D . 
     The sensitivity measurements taken in the situation shown in  FIG. 8  (parts A-F) agreed with calculation with a standard deviation of less than 1 dB. 
     The above has demonstrated that with the use of an appropriate optical filter along with an SOA and PIN photodiode, one can create a dual-rate, multi-band OLT receiver that provides higher sensitivity than APD-based receivers. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.