Abstract:
In one embodiment a system and method pertain to generating a pump from a received optical signal, inputting the generated pump into a phase-sensitive oscillator, and amplifying a carrier component of the pump to generate an optical carrier having the same phase and polarity of an optical carrier of the received optical signal.

Description:
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with Government support under Contract/Grant No. DAAD1702C0097, awarded by the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0002]    Coherent optical communication has attracted renewed interest in recent years. In coherent optical communication, an incoming optical signal is combined with a local oscillator signal to generate an interference signal that can be used to detect the data contained in the incoming data signal. 
         [0003]    In order for the two signals to properly interfere or “beat,” the two signals must be coherent, i.e., have the same frequency, phase, and polarization. In order to produce a coherent local oscillator signal, the phase and polarization of the optical carrier of the incoming optical signal must be recovered. Unfortunately, carrier phase and polarization recovery remains a significant challenge of coherent optical communication. As a result, coherent optical communication is not frequently used. 
         [0004]    Although various methods have been proposed for determining the phase and polarization of the optical carrier of incoming signals, such methods have proven to be complex and/or unreliable. Moreover, known methods are incapable of simultaneous recovery of both phase and polarization, further increasing complexity. Therefore, it can be appreciated that it would be desirable to have less complex and/or more reliable systems and methods for optical carrier phase and polarization recovery. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The disclosed systems and methods can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. 
           [0006]      FIG. 1  is a block diagram of an embodiment of a system for optical carrier recovery. 
           [0007]      FIG. 2  is a block diagram of an embodiment of an optical carrier synchronizer shown in  FIG. 1 . 
           [0008]      FIG. 3  is a block diagram of an embodiment of a nonlinear Mach-Zehnder interferometer. 
           [0009]      FIG. 4  is a block diagram of an example experimental system incorporating a phase-sensitive oscillator shown in  FIG. 2 . 
           [0010]      FIG. 5A  is a graph of an optical spectrum of a pump used in the system of  FIG. 4 . 
           [0011]      FIG. 5B  is a graph of optical spectra of a transmitter laser (CW), a recovered optical carrier, and a binary phase-shift keying signal that resulted from the system of  FIG. 4 . 
           [0012]      FIG. 6A  is a graph of a self-heterodyne signal of a transmitter laser used in the system of  FIG. 4 . 
           [0013]      FIG. 6B  is a graph of a heterodyne signal between the 100 Mhz-shifted transmitter laser and an optical carrier recovered using the system of  FIG. 4 . 
           [0014]      FIG. 7  is an eye diagram of a homodyne-demodulated signal that resulted from the system of  FIG. 4 . 
           [0015]      FIG. 8  is a block diagram of an embodiment of an alternative phase-sensitive oscillator that can be used in the optical carrier synchronizer shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    As described above, desired are less complex and/or more reliable systems and methods for optical carrier phase and polarization recovery, i.e., optical carrier synchronization. As described in the following, such systems and methods can be achieved through use of a phase-sensitive oscillator. More particularly, when the received optical signal is used as a pump for a phase-sensitive oscillator, the optical carrier, and its phase and polarization, can be recovered. 
         [0017]    With reference now to the figures, in which corresponding reference numerals identify like components,  FIG. 1  is a block diagram of an embodiment for a system  100  for optical carrier phase and polarization recovery. As indicated in  FIG. 1 , an optical transmitter  102  transmits an optical signal to an optical carrier synchronizer  104 , which comprises a phase-sensitive oscillator  106 . As described in greater detail below, use of the phase-sensitive oscillator  106  results in a recovered optical carrier being output from the optical carrier synchronizer  104 . 
         [0018]      FIG. 2  illustrates an example embodiment for the optical carrier synchronizer  104  shown in  FIG. 1 . As indicated in  FIG. 2 , the synchronizer  104  comprises a pump generator  202  that provides a pump signal, or “pump,” to a phase-sensitive oscillator  204  via an optical coupler  206 . In the embodiment of  FIG. 2 , the phase-sensitive oscillator  204  includes a phase-sensitive amplifier  208  that generally comprises an nonlinear optical fiber loop  210  (i.e., a nonlinear loop mirror) provided with a polarization controller  212 . The phase-sensitive amplifier  208  is configured as a nonlinear optical loop mirror. Notably, the phase-sensitive amplifier  208  may also be considered to include the coupler  206  or a portion thereof. The phase-sensitive oscillator  204  further includes a reflector  214 , in the form of a fiber Bragg grating (FBG), that is placed in optical communication with the phase-sensitive amplifier  208  via a further optical fiber  216 . Therefore, the phase-sensitive oscillator  204  comprises a nonlinear optical loop mirror (NOLM) and the FBG reflector  214 . Provided along the optical fiber  216  is an optical stabilizer  218  and a further polarization controller  220 . 
         [0019]    In operation, the pump generator  202  is driven by the received optical signal. The pump generator  202  amplifies the received optical signal and outputs the pump onto an upper branch  222  of the phase-sensitive oscillator  204 . The pump passes through the coupler  206  and into the optical fiber loop  210  of the phase-sensitive amplifier  208 , traverses that loop, and exits the loop via the upper branch of the coupler  206 . An optical signal begins to grow from the spontaneous parametric fluorescence in the oscillator  204  formed by the reflector  214  and the phase nonlinear optical loop mirror  208 . Specifically, a carrier component of the pump is output along a lower branch  224  of the phase-sensitive oscillator  204  travels along optical fiber  216  to the reflector  214 , which reflects the carrier component back into the phase-sensitive amplifier loop  210 , at which point the carrier component again traverses the loop and travels back again to the reflector. As can be appreciated from the above, through the combined use of the phase-sensitive amplifier  208  and the reflector  214 , the carrier component is repeatedly fed into the phase-sensitive amplifier. 
         [0020]    As described below, amplifiers provide the highest gain to the carrier component of the pump when they are operated as phase-sensitive amplifiers. Therefore, within the phase-sensitive oscillator, or cavity, the carrier component grows dominantly because of the gain advantage against other optical frequency components. Because the gain process depends on the polarization of the pump, such that the oscillating carrier in the cavity aligned to the polarization of the pump, the oscillating carrier in the cavity only comprises the recovered carrier from the pump in terms of carrier phase and polarization. This recovery process is called carrier synchronization. 
         [0021]    Through the operation of the optical carrier synchronizer  104  described above, the pump is, in essence, the input optical signal. Therefore, only the signal components having the same phase and polarization of the input optical signal are amplified by the phase-sensitive amplifier  208 . That is, only the components that are coherent with the input optical signal exhibit gain and the remaining components drop out. Accordingly, in a single operation, an optical carrier automatically results that has both the same phase and polarization of the input optical signal. That optical carrier, i.e., the recovered optical carrier of the input optical signal, can then be output from the phase-sensitive oscillator  204  using a port  226  connected to the lower branch  216  of the oscillator. By way of example, the port  226  draws a small portion, e.g., 5%, of the power of the optical carrier. The recovered optical carrier can then be used as a local oscillator for various purposes, such as homodyne detection, all-optical phase-shift keying (PSK) signal regeneration, characterization of coherent optical signals, and so forth. 
         [0022]    The principles underlying the optical carrier synchronizer  104  can be explained with reference to a nonlinear Mach-Zehnder interferometer (NMZI)  300  illustrated in  FIG. 3 . The NMZI, like the NOLM shown in  FIG. 2 , utilizes four-wave mixing (FWM) and can provide phase-insensitive (non-degenerate FWM) gain. As a result, when the NMZI (or NOLM) is placed in a cavity to form an oscillator, only the amplification process with a larger small-signal gain or lower pump threshold will oscillate. The small-signal gains for phase-sensitive and phase-insensitive amplification can be found by solving the coupled-mode equations of the electric fields in the NMZI: 
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         [0000]    where γ is the nonlinear coefficient and the subscript n−1 (or 2) indicates the upper (or lower) branch of the NMZI. E a , E s , and E p  are the fields of idler, signal, and pump, respectively. Energy conservation requires that Δω=ω s −ω p =ω p −ω a , where ω p , ω s , and ω a  are the angular frequencies of the pump, signal, and idler, respectively. It is assumed that the optical frequencies of the signal and idler are very close to the pump so that ΔkL=β 2 Δω 2 L□ 1, where β 2  is the group velocity dispersion at ω p . The initial conditions at z=0 are given by E a,n (0)=0,E s,n (0)=i 2−n E s (0)/√{square root over (2)}E p,n (0)=i n−1 E p (0)/√{square root over (2)}, and P p,n =|E p,n (0)| 2 . 
         [0023]    For small-signal gain under the undepleted-pump approximation, the fields at z=L are given by: 
         [0000]        E   p,n   =E   p,n (0) e   iγP     p,n     L    
         [0000]        E   s,n =(1 +iγP   p,n   L ) E   s,n (0) e   iγP     p,n     L    
         [0000]        E   a,n   =−iγP   p,n   LE   s,n (0) e   −2i(φ     s0     −φ     po     )   e   iγP     p,n     L   (Equation 2) 
         [0000]    where φ po  and φ so  are the initial phases of the pump E p (0) and the signal E s (0), respectively. The electric fields at the signal output port just after the 3-dB coupler (z=L) are given by 
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         [0024]    From Equation 3, the small-signal gain of the signal E s  for phase-insensitive amplification is 1+G 2 , where G=(½)γP p L=(½)γ|E p (0)| 2  L. 
         [0025]    The small-signal gain of the phase-sensitive amplification process can be found from Equation 3 by setting the signal and idler optical frequency to be the same (degenerate FWM process). The sum of the two fields in Equation 3, which is now the output signal for the degenerate case, is: 
         [0000]        E   s ( L )= ie   iγP     p     L/2 (| E   s (0)| e   iφ     s0   +φ NL   |E   p (0)| e   iφ     p0   )  (Equation 4) 
         [0000]    where the phase-sensitive nonlinear phase shift Φ NL  is γL|E p (0)||E s (0)|sin(φ po −φ so ). The phase-sensitive amplifier is an amplifier based on this phase-sensitive amplification process. From Equation 4, the maximum small-signal gain is given by 1+2G 2 +2G√{square root over (1+G 2 )} when the initial phase difference between the pump and signal is π/2 or 3π/2. The phase-sensitive gain is larger than phase-insensitive gain. Therefore, when the NMZI/NOLM is placed in the cavity to form an oscillator, the phase-sensitive amplification process is dominant because it has a lower pump threshold. At the same time, the polarization of the oscillating signal is aligned to the pump since FWM is polarization sensitive. 
         [0026]    From Equation 4, it is observed that the phase-sensitive amplifier gain has a two-fold symmetry with respect to the pump phase. Specifically, the output signal E s (L) has the same output phase and amplitude for the pump phases of φ po  and φ po +π. Therefore, a continuous-wave (CW) signal can be amplified by the phase-sensitive amplifier with either a CW pump or a data-modulated binary PSK (BPSK) pump. 
         [0027]    Experiments were performed to confirm the viability of the optical carrier phase and polarization recovery using the methodology described above in relation to  FIGS. 2 and 3 . In the experiment, the system  400  shown in  FIG. 4  was used. As indicated in  FIG. 4 , the system  400  included a phase-sensitive oscillator  402  having the configuration shown in  FIG. 2 . The phase-sensitive amplifier  404  of the phase-sensitive oscillator  402  comprised a bismuth-oxide-based nonlinear fiber (Bi-NLF)  406  having a nonlinear coefficient of ˜1200/kmW and a length of ˜5.5 m. A cavity was formed using an FBG  408  having a 3-dB bandwidth of 3.3 GHz as an end mirror. The cavity length including the phase-sensitive amplifier  404  was less than 20 m, and the total insertion loss of the Bi-NLF patch (including spliced points) was approximately 7 dB. Stabilization was achieved using a fiber stretcher  410  by monitoring the output power. Ports  412  and  414  were used to monitor the pump and the recovered optical carrier, respectively. The pump threshold for the phase-sensitive oscillator  402  was approximately 2 W. 
         [0028]    The system  400  further comprised a radio frequency (RF) pattern generator  416 , an RF amplifier  418 , a transmitter laser  420 , a phase modulator  422 , and an optical amplifier  424 . Together, those components simulated a transmitter that generated a phase-modulated optical signal that simulated an incoming optical signal. In addition, the system  400  comprised a band-pass filter  426  that removed amplified spontaneous emission (ASE) noise, a further optical amplifier  428  that further amplified the signal, and an optical isolator  430  that prevented backward propagation of optical signals. Together, those components performed the function of a pump generator driven by the received optical signal. 
         [0029]    Pseudorandom 10-Gb/s BPSK data of length 2 15 −1 with an average power of 2 W was used as the pump. The optical spectrum of the pump is shown in  FIG. 5A , which does not have the optical carrier component as expected.  FIG. 5B  shows the optical spectra of the transmitter laser (CW), the recovered optical carrier, and the BPSK signal reflected from the FBG but without the cavity. The spectral profile of the recovered optical carrier is clearly closer to that of the transmitter laser than that of the reflected BPSK signal. 
         [0030]    To verify that optical carrier-phase recovery was achieved, the RF spectra of optical heterodyned signals were measured.  FIG. 6A  shows the RF spectrum of the optical self-heterodyne signal of the transmitter laser frequency-shifted by 100 MHz (using an AO modulator) and the transmitter laser itself.  FIG. 6B  is the heterodyne signal between the recovered carrier and the transmitter laser frequency-shifted by 100 MHz. In  FIG. 6B , spectral components due to data modulation have been suppressed revealing a noise profile almost identical to that of  FIG. 6A . To further verify successful carrier phase recovery, the BPSK signal was demodulated using the recovered carrier as the local oscillator (LO).  FIG. 7  shows the resulting homodyne-demodulated eye diagram consisting of 1800 data points. Those results clearly indicate successful all-optical carrier recovery using the phase-sensitive oscillator  304 . 
         [0031]      FIG. 8  is a block diagram of an alternative embodiment of a phase-sensitive oscillator  800  that can be used in the optical carrier synchronizer  104  of  FIG. 1 . As indicated in  FIG. 8 , the phase-sensitive oscillator  800  includes a phase-sensitive amplifier  802  having a configuration similar to the Mach-Zehner interferometer shown in  FIG. 3 . Therefore, the phase-sensitive amplifier  802  is an nonlinear interferometer-type phase-sensitive amplifier that comprises two input ports  804  and  806 , a first optical coupler  808  to which the input ports are coupled, upper and lower branches  810  and  812  of nonlinear optical fiber that are coupled to the first optical coupler, a second optical coupler  814  that is coupled to the upper and lower branches, and two output ports  816  and  818 . The output port  816  of the phase-sensitive amplifier  802  connects to a loop  820  of optical fiber. The loop  820  is also coupled to the input port  806  such that signals output from the phase-sensitive amplifier  802  are input back into the phase-sensitive amplifier to form an oscillator. As is further shown in  FIG. 8 , a stabilizer  822  and a polarization controller  824  are provided along the loop  820 . 
         [0032]    With the embodiment of  FIG. 8 , the pump is input into the phase-sensitive amplifier  802  via port  804  and the signal oscillates within the phase-sensitive oscillator  800  thereby generating the optical carrier of the optical signal (e.g., received optical signal) that was used to generate the pump. The recovered optical carrier can then be output on a port  826  also coupled to the loop  820 .