Abstract:
Improved dispersion compensating circuits for optical transmission systems are disclosed. According to the improved method, there is provided a compensation circuit comprising a varactor diode network. The network is preferably inserted between a source of laser modulating signal and the laser. A low-pass filter or all pass filter constructs the network. The network preferably includes an inductor or inductors and a combined circuit, which includes varactors. The network preferably provides an amplitude dependent delay of the modulating signal applied to the laser or to the optical receiver as post dispersion correction circuitry. In a first embodiment, a fixed capacitor is in series with a varactor and connected to a DC bias through inductor. Additional embodiments, using multiple varactors in different circuit configurations, with particular advantages for various applications identified.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to fiber optic transmission systems and dispersion compensating circuits associated with optical transmission systems. In particular the present invention compensates for the laser chirp and chromatic dispersion distortions to enable effective broadband transmission and extended fiber link reach. 
         [0003]    2. Background of the Invention 
         [0004]    Optical transmission systems are widely used to transmit data on a broadband network. In a typical optical transmission system, a laser provides an optical signal at a predetermined frequency, which is typically modulated to provide an optical transmission data signal. 
         [0005]    In Broadband Passive Optical Network (B-PON) or Gigabit-Passive Optical Network (G-PON) applications, the longest distance is 20 km. So the dispersion compensation needs only to be adequate for fiber lengths up to 20 km. 
         [0006]    In Hybrid Fiber Coaxial (HFC) networks and narrowcast overlay digital applications, where analog and digital channels are combined at the HFC hub site, operators are requesting Dense Wavelength Division Multiplexing (DWDM) narrowcast transmitters, to transmit at higher optical power, e.g., 10 dBm, and to carry wider bandwidth of digital payload for longer reach, e.g., up to 100 KM of single mode fiber. Extending the link reach will produce Low Frequency Noise Rise (LFNR) due to fiber dispersion in the analog channels band. 
         [0007]    Normally a 1550 nm (nanometer) optical signal is assigned for video signal transmission. Typically, an externally modulated laser is selected as the optical source because it has much lower laser chirp than a directly modulated laser transmitter. Laser chirp is the shift in the laser output wavelength/frequency resulting from the modulating signal. A directly modulated laser transmitter, especially a high power laser transmitter, may have a total laser chirp up to several GHz because of the large laser modulation Radio Frequency (RF) current. The large total laser chirp helps improve the Stimulated Brillouin Scattering (SBS) suppression optical power level, but as a result of fiber dispersion, introduces serious second order distortions, such as Composite Second Order (CSO) distortion. 
         [0008]    When the chirped optical frequencies pass through a fiber, different optical wavelengths propagate at different group velocities, which causes delay dispersion at the receiving end and often causes distortions in the communication signal. CSO distortions often occur in the low −40 dBc range. For the B-PON or G-PON laser transmitter to be effective, the laser transmitter CSO distortions should be better than −60 dBc. A narrowcast laser transmitter, which transmits for example 100 MHz of digital payload of 256 QAM, and depending on the laser chirp and fiber link reach, could produce more than 5 dB of LFNR in the analog channels frequency domain, which greatly degrades the transmission of analog channels in case of a narrowcast overlay application. Accordingly, a high degree of CSO correction ability in the high frequencies and very accurate adjustable distortion compensation is needed in order to use a directly modulated laser. 
         [0009]    The problem of fiber dispersion compensation has been investigated in great detail and various techniques have been used to solve this problem. The solutions were in both optical domain and electronic domain. 
         [0010]    Dispersion compensation fiber (DCF) or chirp fiber Bragg grating (CFBG) can be used for the compensation devices in the optical domain. DCF is an optical fiber that has exactly the opposite dispersion effect as a regular single mode fiber. CFBG is a component that reverses the group delay compared to the ordinary fiber group delay between wavelengths. 
         [0011]    The advantage of optical technique is its precision. However, the DCF is costly, adds attenuation, needs additional amplifications and is difficult to adjust/readjust. The CFBG optical attenuation is low, but optical bandwidth is limited to about one nm, thus reducing laser source choices and introducing the need to stabilize laser wavelength. 
         [0012]    Electronic compensation techniques are significantly more cost effective. In the electronic domain, pre-chirp compensation techniques have been widely used in digital applications. 
         [0013]    One method is described in U.S. Pat. No. 5,115,440 to Hermann Gysel, et al. In this patent, a single varactor-tuned delay line network is inserted between the modulating signal source and the laser. A varactor is a type of diode designed to function as a variable capacitor, the varactor&#39;s capacitance is a function of the instantaneous voltage impressed on its terminals. The delay line network provides an instant amplitude-dependent delay of the positive portion of the modulating signal applied to the laser and compensates the fiber delay caused distortion so that the CSO distortion can be reduced. This electrical compensation technique is not sensitive to the transmitted optical wavelength, it works for all 1550 Dense Wavelength Division Multiplexing (DWDM) and Coarse Wavelength Division Multiplexing (CWDM) system applications, and is easy to adjust electronically. 
         [0014]    This approach worked very well for low optical power (1-3 mw) lasers with relatively large laser chirp (1.8 Ghz/ma). However, modern 1550 nm lasers have much lower laser chirp and may have much larger optical power, e.g., laser chirp now may be between 0.03-0.1 Ghz/ma and optical power may up to 10-20 mw, i.e., 10-13 dBm. 
         [0015]    Large optical power is important for B-PON and G-PON applications as well as for narrowcast transmitters in HFC networks. A large optical output power laser usually has more total laser chirp, so the SBS suppression optical power level will be larger. Further, when light having a larger optical power transmits through an Erbium Doped Fiber Amplifier (EDFA), it improves the systems&#39; signal-to-noise ratio. 
         [0016]    A directly modulated laser transmitter has advantages for use in a B-PON or G-PON system, if CSO distortion problems can be eliminated. Directly modulated laser transmitters are much cheaper than externally modulated laser transmitter. Reliability and temperature stability of the directly modulated laser transmitters are much better than the externally modulated laser transmitters. In the directly modulated laser transmitter, the Optical Modulation Index (OMI) is usually at least 1-2 dB higher than for an externally modulated laser transmitter. Thus the signal-to-noise ratio of the B-PON system using directly modulated laser transmitter can be 1-2 dB higher. By using the directly modulated laser transmitter, due to the large laser total chirp, the SBS suppression optical power level can be higher than for the externally modulated laser transmitter. This is very useful for B-PON and G-PON applications. 
         [0017]    Reducing and eliminating LFNR in DWDM narrowcast transmitters is important in the transmission success of DWDM narrowcast overlay applications. 
         [0018]    Large optical power usually requires larger RF drive voltages. For example, for most 1550 nm lasers with a power range from 10-13 dBm, the peak RF drive voltage will be 4-8 volts. For older, low optical power lasers, peak RF driving voltages were less than one volt. Driving the varactor with large RF voltages greatly increases the nonlinearity of the capacitance change with voltage. Due to the large RF driving voltage, compared to the prior art, the RF driving voltage needs to be pre-distorted in order to provide a linear change in capacitance. Also, a very smooth control of dispersion compensation method is needed for CSO correction. 
         [0019]    What is needed is a varactor network for distortion compensation to be used with 1550 nm lasers that have a chirp between 0.03-0.1 Ghz/ma and optical power up to 10-13 dBm. 
       BRIEF SUMMARY 
       [0020]    The present invention provides circuitry that improves the capabilities and performance of fiber optic transmission systems by improving the CSO distortion caused by fiber dispersion in the transmitter side or at the receiver side of an optical CATV communication system. The present invention may be used as post-dispersion circuitry in a CATV HFC optical receiver module, where this circuitry could be placed after an optical receiver and an RF amplifier to compensate for fiber dispersion and improve CSO performance and hence extend fiber link reach. 
         [0021]    In accordance with an aspect of the present invention, an apparatus comprising a capacitive structure may be used with an input signal. The capacitive structure includes a capacitor and a varactor, the combination of which linearizes the capacitance of the capacitive structure under a large voltage and RF signal with one bias control. Accordingly, the apparatus may be used with 1550 nm lasers that have a chirp between 0.03-0.1 Ghz/ma and optical power up to 10-13 dBm. 
         [0022]    In accordance with another aspect of the present invention, an apparatus comprising a capacitive structure may be used with an input signal. The capacitive structure includes a varactor and varactor bank (i.e. more than one varactor, e.g., two or three varactors), the combination of which linearizes the capacitance of the capacitive structure under a large voltage and RF signal with two separate bias controls. Accordingly, the apparatus may be used with 1550 nm lasers that have a chirp between 0.03-0.1 Ghz/ma and optical power up to 10-13 dBm. 
         [0023]    A system in accordance with an aspect of the present invention includes an apparatus for use with an input signal. The apparatus comprises a capacitive structure and an inductor or inductors. The capacitive structure and the inductor or the inductors are arranged as a low-pass filter or all pass filter for the input signal and are arranged to provide an output signal. The capacitive structure comprises a varactor and a capacitor. The varactor is disposed in series with the capacitor. 
         [0024]    The present invention offers compensation methods for the nonlinear varactor capacitance change thereby permitting larger amplitude RF driving voltage. Thus, linearized delay time compensation can be obtained even with large RF drive voltage. This compensating delay can be easily adjusted so that different length fibers can be precisely compensated for with high degree of accuracy. The CSO improvement, for 20 km fiber lengths, can be over 20 dB at high frequencies at 10-13 dBm optical output power. 
         [0025]    The capacitive structure comprises a first varactor and a varactor bank. The first varactor is disposed in series with the varactor bank. The varactor bank comprises a second varactor and a possible third varactor, arranged in parallel and have opposite polarity to the first varactor. Exemplary embodiments of this aspect may further comprise a first DC bias source and a second DC bias source. The first varactor is controlled by both DC bias sources, whereas the varactor bank is controlled only by the second DC bias source. These two DC bias source have opposite polarity. 
         [0026]    Additional advantages and novel features of the invention are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
         [0027]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate exemplary embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
           [0028]      FIG. 1  illustrates an exemplary embodiment of a dispersion compensating circuit; 
           [0029]      FIG. 2  illustrates an exemplary embodiment of a dispersion compensating circuit in accordance with the present invention; 
           [0030]      FIG. 3  illustrates another exemplary embodiment of a dispersion compensating circuit in accordance with the present invention; and 
           [0031]      FIG. 4  illustrates an exemplary use of a post-dispersion compensating circuit in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The capacitance of a varactor at any voltage can be expressed by: 
         [0000]        C ( V )= C   j0 (1− V/V   bi ) −γ   =C   j1 ( V   bi   −V ) −γ ,  (1) 
         [0000]      where  C   j1   =C   j0 ( V   bi ) −γ .  (2) 
         [0033]    Here C(V) is the varactor capacitance at any instant voltage. C j0  is the varactor capacitance measured at zero voltage. V bi  is the varactor junction voltage. γ is the varactor doping profile parameter. For example, γ=0.5 for an abrupt junction, γ=1 for a hyper-abrupt junction. 
         [0034]    The group delay for the delay-compensating network may be given by: 
         [0000]        T   gd =( L   1   *C ( V )) 1/2 ;  (3) 
         [0000]      δ T   gd =(½)*(δ C/C )* T   gd .  (4) 
         [0035]    Here T gd  is the varactor network delay time, L 1  is the inductance in the delay network, and δT gd  is the delay time difference selected for the dispersion compensation. 
         [0036]    The RF signal swing at the varactor causes the capacitance of the varactor to change. δC/C is the varactor capacitance change due to the RF voltage swing. The dispersion compensation ability is proportional to the varactor capacitance change δC/C caused by the RF voltage swing. 
         [0037]    Dispersion caused by signal amplitude delay as the optical signal propagates through along the fiber is equal in amplitude but opposite in sign for the RF positive swing and negative swing. From Equation 1, it can be seen that when the RF voltage swing is large (amplitude 4-8 volts), the capacitance change will be nonlinear. Total capacitance change is unequal under the RF voltage positive swing versus negative swing. An unequal total capacitance change in the RF swing will cause the delay compensation to be unequal, which may introduce compensation errors. A method of equalization is needed. 
         [0038]      FIG. 1  illustrates an exemplary embodiment of a dispersion compensating circuit for optical transmission system in accordance with the present invention. In this embodiment, the polarity of the varactor is controllable for versatility. 
         [0039]    In  FIG. 1 , circuit  100  includes a capacitor  103 , an inductor  105 , an inductor  106 , a DC bias  107 , an inductor  108 , a capacitor  109  and capacitive structure  110 , which includes a varactor  102  and capacitor  101 . Capacitor  101  is used to linearize the overall capacitance change of varactor  102  under large RF voltage swing. The value of capacitor  101  may be selected roughly equal to the largest capacitive value of varactor  102  permitted under the RF voltage swing. 
         [0040]    The effect of capacitor  101  can be understood by the following explanation. When the capacitance of varactor  102  is equal to the capacitance of capacitor  201 , half of the RF voltage is dropped at varactor  102 . When the RF voltage swing is in the positive direction, the capacitance of varactor  102  increases and more of the RF voltage will be dropped at capacitor  101 . When the RF voltage swings negatively, the capacitance of varactor  102  decreases and most of the RF voltage will be dropped at the varactor  102 . Changing the value of capacitor  101  can reduce the RF voltage change on varactor  102  during the positive RF swing. This procedure can make the total capacitance change for the positive voltage swing and the negative swing equal in amplitude. Inductors  105  and  106  and a combination of capacitor  101  in series with capacitor  102  form a low-pass Tchebyscheff filter with a 0.1 dB ripple, which extends the frequency response up to 1.5 GHz. Capacitor  110  provides a linearizing function for the output signal. 
         [0041]    Inductor  108  (it is not shown on the graph) is RF blocking inductor used to supply DC bias  107  to varactor  102 . The polarity of varactor  102  can be reversed, depending on application conditions. For example, if the RF output is used for a laser driver, the polarity of varactor  102  depends on the laser grounding condition. Alternatively, if the RF output is used at the optical receiver side, the polarity of varactor  102  depends on the number of RF amplifiers before the signal reached the circuit RF input side. 
         [0042]    One of the advantages of circuit  100  is that the bias voltage provided by DC bias  107  on varactor  102 , is the only adjustment required. Further, as mentioned above, capacitor  101  linearizes the overall capacitance change under large RF voltage swing. Absent capacitor  101 , the capacitance of varactor  102  becomes very non-linear with large voltages. Because larger voltages may be used with the inclusion of capacitor  101 , the varactor circuit may be used with larger powered lasers for longer transmission. 
         [0043]    Reference values for an exemplary embodiment of circuit  100  are as follows: 
         [0044]    the inductance of inductor  105 =the inductance of inductor  106 =8.2 nH; 
         [0045]    the capacitance of capacitor  101 =3.9 pf, 
         [0046]    varactor  102  is Toshiba varactor 1SV 239; 
         [0047]    the capacitance of capacitor  103 =the capacitance of capacitor  109 =0.1 μf; and 
         [0048]    the inductance of inductor  108 =10 μH. 
         [0049]    If circuit  100 , using the parameters discussed above, is used at the optical receiver side, the input RF power level should be at 38-42 dBmv/channel. Total RF drive power is about 10 dBm with 2 dB power in error. 
         [0050]    Circuit  100  can be used both for laser transmitter delay correction and for delay correction at the optical receiver side. When circuit  100  is used for laser transmitter delay correction, network RF output should be connected to the laser. When circuit  100  is used for the optical receiver side dispersion correction, the network RF output should be connected to the RF amplifier. 
         [0051]    Additional embodiments will now be described with reference to  FIGS. 2 and 3 . 
         [0052]    The exemplary embodiments illustrated in  FIGS. 2 and 3  use a full-pass filter, and employ multiple varactors in order to greatly expand capability beyond the bandwidth and/or transmission distance required for current B-PON or G-PON applications. One of skill in the art would select the appropriate varactor type for an associated specific application. 
         [0053]    In  FIG. 2 , circuit  200  includes an input capacitor  202 , an output capacitor  204 , a first bias source  206 , a resistor  208 , an inductor  210 , a second bias source  212 , a resistor  214 , an inductor  216 , a varactor bank  218 , which includes varactors  220 ,  222  and  224  in parallel, a varactor  226 , and a center-tapped inductor  228 . 
         [0054]    In circuit  200 , two bias sources  206  and  212  provide controlled bias to varactor  226 . Inductor  210  is an RF blocking inductor used to isolate a DC bias from circuit  200 . Bias source  212  provides a controlled bias to varactor bank  218 . Inductor  216  is an RF blocking inductor used to isolate a DC bias from circuit  200 . First bias source  206  and second bias source  212  may be separately controlled 
         [0055]    In an exemplary embodiment, each of bias source  206  and bias source  212  is a voltage source, which can be electronically set, in combination with a remotely controllable Digital to Analog Converter (DAC). As such, bias source  206  may be individually set and/or adjusted, via the remotely controllable DAC, to establish a bias voltage for the varactor  226  and varactor bank  218  to compensate for changing parameters within circuit  200  as a result of temperature, age or fiber link reach. Similarly, bias source  212  may be individually set and/or adjusted, via the corresponding remotely controllably DAC, to establish a bias voltage for the varactor bank  218  to compensate for changing parameters within circuit  200  as a result of temperature, age or fiber link reach. 
         [0056]    By applying different bias voltages, the circuit can be precisely tuned to balance the positive and negative capacitance swings of the input RF signal. 
         [0057]    In one embodiment, bias source  206  is a bias source that can be set via a remotely controllable DAC. Accordingly, by monitoring the output of the circuit by known methods, the circuit may be tuned by adjusting at least one of the amplitude and the polarity of the bias signal from bias source  206  by known methods, for example by the remotely controllable DAC. 
         [0058]    In another embodiment, bias source  212  is a bias source that can be set via a remotely controllable DAC. Accordingly, by monitoring the output of the circuit by known methods, the circuit may be tuned by adjusting at least one of the amplitude of the bias signal from bias source  206  by known methods and by adjusting at least one of the amplitude of the bias signal from bias source  212  by known methods, for example by the remotely controllable DAC. 
         [0059]    In yet another embodiment, bias source  206  and bias source  212  are remotely controllable bias sources as discussed above. Accordingly, by monitoring the output of the circuit by known methods, the circuit may be tuned by adjusting at least one of the amplitude and the polarity of the bias signal from bias source  212  by known methods, for example by the remotely controllable DAC. 
         [0060]    A working example of a circuit as illustrated in  FIG. 2  included the following parameters: 
         [0061]    the resistance of resistor  208 =10 KΩ; 
         [0062]    the inductance of inductor  210 =1 μH; 
         [0063]    the capacitance of capacitor  202 =0.1 μF; 
         [0064]    the inductance of center-tapped inductor  228 =14 nH; 
         [0065]    the capacitance of output capacitor  204 =0.1 μF; 
         [0066]    the resistance of resistor  214 =10 KΩ; 
         [0067]    the inductance of inductor  216 =1 μH; and 
         [0068]    each of varactors  220 ,  222 ,  224  and  226  comprises a Toshiba varactor 1SV 239. 
         [0069]    Other embodiments of the present invention may use a single varactor in place of varactor bank  218 . Further, other embodiments of the present invention may use two varactors in parallel as a varactor bank. 
         [0070]    Other embodiments of the present invention may include a capacitive unit disposed between varactor  226  and varactor bank  218 . Such a capacitive unit may include at least one of a capacitor or a varactor, and is used to enable independent adjustment of the bias on each of varactor  226  and varactor bank  218 . The embodiments may further include a shunt inductor, i.e. connected to ground, in order to bias varactor  226  independently of varactor bank  218 . 
         [0071]    The exemplary embodiment illustrated in  FIG. 3 , is an example of a varactor bank using two varactors in parallel in accordance with the present invention. Circuit  300  illustrated in  FIG. 3  differs slightly from circuit  200  of  FIG. 2 . Specifically, circuit  300  includes a varactor bank  302 , which includes a first varactor  304  arranged in parallel with a second varactor  306 . Further, circuit  300  includes separate wire wound inductors  308  and  310  in place of center-tapped inductor  228  used in  FIG. 2 . 
         [0072]    A working example of a circuit as illustrated in  FIG. 3  included the following parameters:
       the resistance of resistor  208 =10 KΩ;   the inductance of inductor  210 =1 μH;   the capacitance of capacitor  202 =0.1 μF;   the inductance of inductor  308 =6.2 nH;   the inductance of inductor  310 =6.2 nH;   the capacitance of output capacitor  204 =0.1 μF;   the resistance of resistor  214 =10 KΩ;   the inductance of inductor  216 =1 μH;   and each of varactors  304  and  306  were chosen from MA/COM with the part number MA4ST1200.       
 
         [0082]    One aspect of the present invention includes a capacitive structure including a first varactor connected in series with a varactor bank. The varactor bank includes a plurality of varactors that arranged in parallel and that are placed in opposite polarity to the first varactor. This arrangement linearizes the total capacitance of the capacitor structure. As such, the first aspect of the present invention enables the use of large voltage signals, which therefore enables the use of the circuit of driving large powered lasers. 
         [0083]    Those of skill in the art will appreciate that the present invention introduces a compensation method for the nonlinear varactor capacitance change under large RF driving voltage. Thus linearized delay time compensation can be obtained under large RF drive voltage. The compensating delay time can be easily and precisely controlled so that different fiber lengths can be easily compensated with a high degree of accuracy. At high optical output power of 10-13 dBm, for a 20 km fiber application, and at high frequencies, CSO improvements greater than 20 dB can be realized. 
         [0084]    In an HFC network, the present invention allows DWDM narrowcast directly modulated laser transmitters to carry wide bandwidth of digital payload and to extend the single mode fiber link reach by compensating for fiber dispersion and hence reduces the LFNR in the analog band (50-550 Mhz). 
         [0085]    The present invention also reduces the total reverse voltage on the varactor so that the varactor reverse voltage breakdown can be avoided even under the large RF drive voltage conditions. The present invention can also be used to improved delay correction for an externally modulated laser transmitter. 
         [0086]    The present invention can additionally be used for post-dispersion correction at the node receiver in a CATV HFC network to compensate for fiber dispersion, for example as illustrated in  FIG. 4 . In the figures, post-dispersion correction circuitry  404  is disposed to receive output from an optical receiver  402 . Optical receiver  402  includes a photo-detector  406  and a Radio Frequency (RF) amplifier  408 . Photo-detector  406  receives light signals and generates electrical signals corresponding thereto. RF amplifier  408  amplifies the electrical signals and provides the amplified electrical signals to post-dispersion correction circuitry  404 , which in accordance with the many embodiments of the present invention, modifies the output signal to compensate for the negative effects of dispersion. 
         [0087]    The foregoing description of various preferred embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments, as described above, were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.