Abstract:
A fiber optic communication system is provided that includes a light source adapted to emit a system optical signal and an electrical signal source adapted to provide a data input electrical signal. Additionally, the fiber optic communication system includes a feed forward photonic modulation circuit adapted to receive the data input electrical signal and the system optical signal and output a final modulated optical signal substantially free from residual error.

Description:
FIELD OF INVENTION  
       [0001]     The invention relates generally to signal transmissions within a mobile platform communication system. More specifically, the invention relates to maintaining signal integrity of optical signals over a wide frequency range and a wide amplitude range within a mobile platform fiber optic communications system.  
       BACKGROUND OF THE INVENTION  
       [0002]     At least some links within communication systems of a mobile platform, such as an aircraft, bus, ship or train, transmit signals at very high frequencies, e.g. greater than 1 GHz. Signals at such high frequencies can not be digitally sampled and therefore must be transmitted as an analog signal. Additionally, these high frequency signals often require a very high degree of transmission accuracy between various points of the mobile platform. For example, electronic warfare systems of an aircraft require a high degree of transmission accuracy. Currently, coaxial cable is typically used to provide such communication links. However, coaxial cable is costly and very heavy and thus adds production costs and weight to the mobile platform. To reduce this cost and weight, attempts have been made to incorporate fiber optic links in some known mobile platform communications systems. To date, implementation of fiber optics has been impeded by the inability to maintain linearity, i.e. transmission accuracy, between the high frequency electrical signal input to the communication system and the optical signal output from the communication system. That is, fiber optic communication systems within the mobile platform are generally not capable of converting such high frequency electrical signals to optical signals without degradation of the signal.  
         [0003]     More specifically, in order to modulate a laser source at high frequencies, an external modulator is generally employed, for example a Mach-Zehnder interferometer. When using a modulation device, such as a Mach-Zehnder interferometer, the optical signal is modulated across one arm of the interferometer, thereby delaying the phase of the optical signal through that arm with respect to the other arm of the interferometer. As a result of the constructive or destructive interference, the optical signal output from the interferometer is amplitude modulated. However, the modulation of optical signals at high frequencies within a broad range of amplitudes is generally non-linear, i.e. the optical signals are generally distorted with respect to the electrical signal used to modulate the optical signal.  
         [0004]     Therefore, it would be desirable to employ a fiber optic communication system within a mobile platform, wherein optical signals can be modulated within a wide range of frequencies and amplitudes without distortion. Thus, there would be very little degradation or distortion of the optical signals with respect to the electrical signal use to modulate the optical signal. Employing such fiber optics in certain mobile platform communication systems would save costs and considerably reduce the payload of the mobile platform.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     In one preferred embodiment of the present invention, a fiber optic communication system is provided that converts a data input electrical signal to a final modulated optical signal with a high degree of accuracy. More specifically, the system includes a feed forward photonic modulation (FFPM) circuit that converts the data input electrical signal to the final modulated optical signal. The FFPM utilizes a feed forward technique to correct for degradation and/or distortion in the final modulated optical signal. Thus, the FFPM converts the data input electrical signal such that the final modulated optical signal has a high degree of linearity with respect to the data input electrical signal.  
         [0006]     The FFPM circuit includes a first portion that receives the data input electrical signal and a system optical signal. The FFPM first portion utilizes the input electrical signal and the system optical signal to generate a first modulated optical signal having a first wavelength. The FFPM first portion then splits the first modulated optical signal into a first segment and a second segment, each having the first wavelength. The FFPM circuit additionally includes a second portion that receives the data input electrical signal, the system optical signal and the second segment of the first modulated optical signal. The FFPM second portion utilizes the data input electrical signal, the system optical signal and the second segment to generate a second modulated optical signal having a second wavelength. The FFPM second portion then combines the second segment with the second modulated optical signal to generate a summed optical signal.  
         [0007]     The FFPM circuit further includes a third portion that receives the summed optical signal and the system optical signal. The FFPM third portion utilizes the summed optical signal and the system optical signal to generate a corrective modulated optical signal having a third wavelength. Further yet, the FFPM circuit includes a fourth portion that combines the first segment of the first modulated optical signal with the corrective modulated optical signal to generate the final modulated optical signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will become more fully understood from the detailed description and accompanying drawings, wherein;  
         [0009]      FIG. 1  is a block diagram of a fiber optic communication system, in accordance with one preferred embodiment of the present invention; and  
         [0010]      FIG. 2  is a schematic of one preferred embodiment of feed forward photonic modulation (FFPM) circuit shown in  FIG. 1 . 
     
    
       [0011]     Corresponding reference numerals indicate corresponding parts throughout the several views of drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  is a block diagram of a fiber optic communication system  10 , in accordance with one preferred embodiment of the present invention. The system  10  generates a final modulated optical signal  14  having a high degree of linearity with respect to a data input electrical signal  18  used to modulate a system optical signal  22 . More specifically, the system  10  includes a feed forward photonic modulation (FFPM) circuit  26 . The FFPM circuit  26  is especially well suited for converting high frequency analog data input electrical signals  18 , e.g. radio frequency signals, within a very broad range of amplitudes to the final modulated optical signals  14 . The final modulated optical signals  14  generated by the FFPM circuit  26  have a high degree of linearity with respect to the high frequency data input electrical signals  18 . That is, the final modulated optical signals  14  have little or substantially no degradation with respect to the high frequency data input electrical signals  18 . Thus, the system  10  is suitable for use in any environment where it is desirable to utilize optical signals to communicate data from one point to another, for example within a mobile platform, such as an aircraft, bus, boat or train.  
         [0013]     The system  10  additionally includes a light source  30 , such as a laser, for emitting the system optical signal  22 , and an electrical signal source  34  that provides the data input electrical signal  18  to the FFPM circuit  26 . In one preferred embodiment, the data input electrical signal  18  is a high frequency analog electrical signal. The light source  30  emits the system optical signal  22  at a substantially constant amplitude. The electrical signal source  34  can be any device that communicates information or data via electrical signals. For example, the electrical signal source  34  can be a component of an electronic warfare system onboard a military aircraft. In which case the electrical signal source  34  may generate one or more data input electrical signals  18  to communicate data to a pilot regarding the vulnerability of the aircraft to hostile weapons, e.g. missiles and aerial rockets.  
         [0014]     The FFPM circuit  26  includes a first portion  38  that receives the data input electrical signal  18  and the system optical signal  22 . The FFPM first portion  38  utilizes the input electrical signal  18  and the system optical signal  22  to generate a first modulated optical signal  40  (shown in  FIG. 2 ) having a first wavelength λ 1 . The FFPM first portion  38  splits the first modulated optical signal  40  into a first segment  42  and a second segment  46 , each having the first wavelength λ 1 . The FFPM circuit  26  additionally includes a second portion  50  that receives the data input electrical signal  18 , the system optical signal  22  and the second segment  46  of the first modulated optical signal  40 . The FFPM second portion  50  utilizes the data input electrical signal  18 , the system optical signal  22  and the second segment  46  to generate a second modulated optical signal  52  (shown in  FIG. 2 ) having a second wavelength A 2 . The FFPM second portion  50  then combines the second segment  46  of the first modulated optical signal  40  with the second modulated optical signal  52  to generate a summed optical signal  54 .  
         [0015]     The FFPM circuit  26  further includes a third portion  58  that receives the summed optical signal  54  and the system optical signal  22 . The FFPM third portion  58  utilizes the summed optical signal  54  and the system optical signal  22  to generate a corrective modulated optical signal  62  having a third wavelength λ 3 . Further yet, the FFPM circuit  26  includes a fourth portion  66  that combines the first segment  42  of the first modulated optical signal  40  with the corrective modulated optical signal  62 , thereby generating the final modulated optical signal  14 . The final modulated optical signal  14  has a high degree of linearity, i.e. little or substantially no degradation, with respect to the data input electrical signal  18 .  
         [0016]      FIG. 2  is a schematic of one preferred embodiment of the feed forward photonic modulation (FFPM) circuit  26 , shown in  FIG. 1 . The first portion  38  of the FFPM circuit  26  includes a first electrical amplifier that amplifies the data input electrical signal  18  to a first level and outputs a first amplified electrical signal  74 . The first portion  38  additionally includes a first optical modulating sub-circuit  78 . The first optical modulating sub-circuit  78  uses the first amplified electrical signal  74  to modulate the system optical signal  22  and output the first modulated optical signal  40  having the wavelength λ 1 . The first optical modulating sub-circuit  74  includes a first laser diode  82  that has the first wavelength λ 1 . The system optical signal  22  is adjusted to have the first wavelength of λ 1  as it passes through the first laser diode  82 .  
         [0017]     The system optical signal  22  having the first wavelength λ 1  is then input to a first optical modulator  86 , e.g. a Mach-Zehnder interferometer. The first optical modulator  86  modulates the system optical signal  22  in accordance with the first amplified electrical signal  74  and outputs the first modulated optical signal  40  having the first wavelength λ 1 . The first portion  38  further includes a splitter  90  that splits the first modulated optical signal  40  into the first segment  42  and the second segment  46 , wherein each of the first and second segments  42  and  46  comprise a portion of the first optical signal  40 . The ratio of first segment  42  to the second segment  46  can be any desirable ratio depending on the application of the fiber optic communications system  10 . For example, the first segment  42  could comprise 90% of the first optical signal  40  and the second segment could comprise 10% of the first optical signal  40 . The first and second segments  42  and  46  each have the first wavelength λ 1 .  
         [0018]     In one preferred embodiment, the first electrical amplifier  70  is capable of handling analog electrical signals over a very wide range of frequencies and amplitudes. Likewise, the first laser diode  82 , the first optical modulator  86 , and the splitter  90  are capable of handling analog optical signals over a very wide range of frequencies and amplitudes. For example, the frequency ranges may include electrical and/or optical signals with frequencies greater that 1 GHz.  
         [0019]     The second portion  50  of the FFPM circuit  26  includes a second electrical amplifier  94  that amplifies the data input electrical signal  18  to a second level and outputs a second amplified electrical signal  98 . The second portion  50  additionally includes a second optical modulating sub-circuit  102 . The second optical modulating sub-circuit  102  uses the second amplified electrical signal  98  to modulate the system optical signal  22  and output the second modulated optical signal  52  having the wavelength λ 2 . The second optical modulating sub-circuit  102  includes a second laser diode  106  that has the second wavelength λ 2 . The system optical signal  22  is adjusted to have the second wavelength of λ 2  as it passes through the second laser diode  106 .  
         [0020]     The system optical signal  22  having the second wavelength λ 2  is then input to a second optical modulator  110 , e.g. a Mach-Zehnder interferometer. The second optical modulator  110  modulates the system optical signal  22  in accordance with the second amplified electrical signal  98  and outputs the second modulated optical signal  52  having the second wavelength λ 2 . The second portion  50  further includes a first dichroic mirror  114  that combines the second segment  46  of the first modulated optical signal  40  with the second modulated optical signal  52 . The output of the first dichroic mirror  114  is the summed optical signal  54 . The summed optical signal  54  comprises the second segment  46  having the first wavelength λ 1  and the second modulated optical signal having second wavelength λ 2 .  
         [0021]     In one preferred embodiment, the second electrical amplifier  94  is capable of handling analog electrical signals over a very wide range of frequencies and amplitudes. Likewise, the second laser diode  106 , the second optical modulator  110  and the dichroic mirror  114  are capable of handling analog optical signals over a very wide range of frequencies and amplitudes. For example, the frequency ranges may include electrical and/or optical signals with frequencies greater that 1 GHz.  
         [0022]     The first electrical amplifier  70  and the second electrical amplifier  94  each have a specified gain, wherein the gain of the second electrical amplifier  94  is less than the gain of the first electrical amplifier  70 . For example, the first electrical amplifier  70  may have a gain of +10 while the second electrical amplifier  94  may have a gain of −1. Since the second electrical amplifier  94  has a smaller gain, the second amplified electrical signal  98  incurs less distortion. That is, the second amplified electrical signal  98  is a more accurate, i.e. more linear, signal with respect to the data input electrical signal  18 . Generally, the greater the difference in the gains of the first and second electrical amplifier  70  and  94 , the more accurate the second amplified electrical signal  98  will be in relation to the first amplified electrical signal  40 . Additionally, as described above, the ratio of first segment  42  to the second segment  46  can be any desirable ratio depending on the application of the fiber optic communications system  10 . However, in one preferred embodiment, the ratio of the first segment  42  to the second segment  46  equals the ratio of the gains of the first and second electrical amplifiers  70  and  94 . For example, if the gains of the first and second electrical amplifiers  70  and  94  are respectively +10 and −1, then the splitter  90  will spit the first modulated optical signal  40  such that the first segment  42  is 90% of the first modulated optical signal  40  and the second segment  46  is 10%.  
         [0023]     The third portion  58  of the FFPM circuit  26  includes an optical detector  118 . The optical detector  118  receives the data input electrical signal  18  and converts the summed optical signal  54  into a corrective electrical signal  122 . The third portion  58  also includes a third electrical amplifier  126  that amplifies the corrective electrical signal  122  and outputs a corrective amplified electrical signal  130 . The third portion  58  additionally includes a third optical modulating sub-circuit  134 . The third optical modulating sub-circuit  134  uses the corrective amplified electrical signal  130  to modulate the system optical signal  22  and output the corrective modulated optical signal  62  having the third wavelength λ 3 . The third optical modulating sub-circuit  134  includes a third laser diode  138  that has the third wavelength λ 3 . The system optical signal  22  is adjusted to have the third wavelength of λ 3  as it passes through the third laser diode  138 .  
         [0024]     The gain of the third electrical amplifier  126  is adjusted so that the overall optical gain of the FFMP third portion  58  has the ratio of the split at the splitter  90 , only having a negative value. Thus, for example, the exemplary system in which the first and second segments  42  and  46  respectively contain 90% and 10% of the power of the first modulated optical signal  40 , the third electrical amplifier  126  will adjust the corrective modulated optical signal  62  to have 9 times the power of the summed optical signal  54 . Additionally, the corrective modulated optical signal  62  will have a negative value relative to the summed signal  54 . Thus, in this exemplary system the gain of the third electrical amplifier  126  will be determined as follows. If the first modulated optical signal  40  has a distortion of ‘e’, the first modulated optical signal  40  will have a coefficient of 1(1−e). Accordingly, the first segment  42  of the first modulated optical signal  40  will have a coefficient of 0.9(1−e) and the second segment  46  will have a coefficient of 0.1(1−e). The second modulated optical signal  52  will have a coefficient of −0.1 because the gain of the second electrical amplifier  94  is −{fraction (1/10)} of the gain of the first electrical amplifier  70  and the second modulated optical signal  52  is substantially undistorted. Therefore, the summed modulated signal  54  has coefficient 0.1(1−e)−0.1=−0.1e. The third amplifier  126  has its gain adjusted so that the FFMP third portion  58  has overall gain of −9, such that the corrective modulated optical signal  62  will have a coefficient of 0.9e. When the corrective modulated optical signal  62  is summed with the first segment  42  by the second dichroic mirror  146  the final optical signal  14  with have a coefficient of 0.9, relative to the first optical signal  40 . Therefore, the final optical signal  14  will be substantially undistorted.  
         [0025]     The system optical signal  22  having the third wavelength λ 3  is then input to a third optical modulator  142 , e.g. a Mach-Zehnder interferometer. The third optical modulator  142  modulates the system optical signal  22  in accordance with the corrective amplified electrical signal  130  and outputs the corrective modulated optical signal  62  having the third wavelength λ 3 . The wavelength of the corrective modulated optical signal  62 , i.e. λ 3 , can generally be any wavelength other than the wavelength of the first modulated optical signal  40 , i.e. λ 1 . That is, the third wavelength λ 3  can equal λ 2  or any other desirable wavelength, with the exception that the third wavelength λ 3  can not be equal to λ 1 .  
         [0026]     In one preferred embodiment, the optical detector  118  is capable of handling electrical and optical signals over a wide range of frequencies and amplitudes. Additionally, the third electrical amplifier  126  is capable of handling electrical signals over a wide range of frequencies. Likewise, the third laser diode  138  and the third optical modulator  142  are capable of handling optical signals over a wide range of frequencies and amplitudes. For example, the frequency ranges may include electrical and/or optical signals with frequencies greater that 1 GHz.  
         [0027]     The fourth portion  66  of the FFPM circuit  26  includes a second dichroic mirror  146  that combines the first segment  42  of the first modulated optical signal  40  with the corrective modulated optical signal  62 . The second dichroic mirror  146  outputs the final modulated optical signal  14 . The final modulated optical signal  14  is then input into an output fiber where the final modulated optical signal  14  is transmitted to a receiving device (not shown). The final modulated optical signal  14  comprises the first segment  42  having the first wavelength λ 1  and the corrective modulated optical signal  62  having the third wavelength λ 3 . The two signals comprising the final modulated optical signal  14 , i.e. the first segment  42  and the corrective modulated optical signal  62 , are summed when they reach the receiving device. Thus, the final modulated signal  14  received by the receiving device is an undistorted version of the data input electrical signal  18 . More specifically, the corrective optical signal  62  corrects any distortion, i.e. non-linearity, in the first modulated optical signal  40  with respect to the data input electrical signal  18 .  
         [0028]     In one preferred embodiment, the fourth portion  66  additionally includes a delay device  150 . The delay device  150  delays the first segment  42  of the first modulated optical signal  40  to compensate for any delay in the corrective modulated optical signal  62  caused by the third electrical amplifier  126 . In another preferred embodiment, the delay device  150  and the second dichroic mirror  146  are capable of handing optical signals over a wide range of frequencies and amplitudes. For example, the frequency ranges may include optical signals with frequencies greater that 1 GHz.  
         [0029]     It will be appreciated that the all the optical signals generated by the FFMP circuit  26 , e.g. optical signals  40 ,  42 ,  46 ,  52 ,  54 ,  62  and  14  are optically pumped by the system optical signal  22 .  
         [0030]     Thus, the fiber optic communications system  10  utilizes the FFPM circuit  26  to modulate the data input electrical input  18  such that the final optical signal  14  received by the receiving device has a substantially linear relationship with the data input electrical signal  18  over a wide range of frequencies and amplitudes.  
         [0031]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.