Abstract:
An optical transmission system that includes an optical amplifier coupled to the input of an optical modulator having at least two complementary output ports for providing complementary modulated optical output signals can be used such that the relative intensity noise (RIN) associated with the optical amplifier is coupled into the modulated optical output signals as common mode noise and can therefore be eliminated using a differential detection scheme. Removing the RIN associated with the optical amplifier advantageously increases the carrier to noise ratio (CNR) for the optical transmission system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical transmission systems, and more specifically to noise reduction in fiber optic transmission systems carrying broadband signals. 
     2. Description of the Related Art 
     Fiber optical transmission is commonly used for the economical distribution of broadband signal content (such as multi-channel cable television (CATV) systems) as a means to transmit amplitude modulated/frequency division multiplexed (AM/FDM) signals over long distances, with or without repeaters. Such optical fiber links apply optical intensity modulation to a light source, sending the modulated light signal via fiber to a receiver which converts the optical intensity signal back to an electrical signal. The modulating electrical signal is a composite signal with the AM video channels multiplexed in frequency. 
     Externally modulated differential detection transmission systems, such as the systems described in U.S. Pat. No. 5,253,309, entitled “Optical Distribution of Analog and Digital Signals Using Optical Modulators with Complementary Outputs,” by Nazarathy et al., and incorporated herein by reference in its entirety, are known to reduce two signal degrading effects associated with the above described optical transmission, namely, phase-to-amplitude noise conversion and fiber-induced composite second order distortion (CSO). In addition, the use of differential detection raises the net carrier-to-noise ratio (CNR) by taking advantage of complementary carrier power that would otherwise be lost at the optical modulator. 
     FIG. 1 illustrates a prior art externally modulated differential detection transmission system  100 . Optical source  110 , typically a laser such as a distributed feedback laser (DFB), provides an optical carrier to modulator  120 , typically a dualoutput lithium niobate (LiNbO 3 ) electro-optic modulator. When properly biased by bias voltage  122 , modulator  120  will produce two lightwave signals (e.g., complementary output signals  130  and  140 ), encoded with the same RF information (as provided by RF input  124 ), and 180° out of phase with each other. These signals can propagate through optical media such as optical fibers  150 , and subsequently be detected by a balanced receiver  160  which includes, for example, two distinct photodetectors. The detected signals can then be recombined in the RF domain to provide RF output  170 . As long as the total path lengths of the two transmission links (including both the optical fiber and coaxial cable elements of those paths) are within (c/n)/ν of each other, where v is the highest frequency CATV channel and c/n is the speed of light in the fiber, the RF carriers will add coherently, generally increasing by 6 dB over the carrier power for a single transmission link. Additionally, most of the noise from the two photodetectors will add incoherently, thereby increasing by 3 dB. The net benefit is a 3 dB increase in the CNR. Common mode noise and distortion will cancel in the recombined signal. 
     Three examples of common mode noise and distortion canceled by differential detection are relative intensity noise (RIN) associated with the DFB optical source, phase-to-amplitude noise conversion, and CSO. Since the complementary output signals  130  and  140  of modulator  120  are derived from the same optical source, the RIN caused by intensity fluctuations in the DFB is common to both output signals, and appears with equal amplitudes and in the same direction with respect to the quadrature point to which the modulator is linearized. Thus, upon subtraction of the two detected signals at balanced receiver  160 , the RIN is canceled. In contrast, the carrier signals in the two complementary output signals  130  and  140  have equal amplitudes but opposite directions with respect to the quadrature point to which the modulator is linearized. Consequently, subtraction of the two signals at the balanced receiver results in reinforcing the resultant signal to double the value of the modulation signals in each transmission link. 
     CSO is generated by the interaction of self-phase modulation with dispersion in the optical fiber. The effect generally increases as the square of the fiber length, dispersion, and launch power, and is most severe at high channel frequencies. The use of differential detection both cancels the fiber-induced CSO and minimizes the effect of the external phase modulation on the CSO. 
     Dispersion in the optical fiber will convert phase noise into amplitude noise. This effect is most severe at high channel frequencies. At high frequencies (e.g., greater than 400 MHz) differential detection provides additional CNR improvement of approximately 1.5 dB beyond what would be expected for uncorrelated noise. This effect is due to the cancellation of phase-to-amplitude noise, which is correlated between the two fiber links because it originates in the noise of the common DFB laser. 
     Balanced receiver  160  can be implemented in a variety of ways, as illustrated by the examples shown in FIGS. 3A-3B. In FIG. 3A, complementary output signals  130  and  140  are optically coupled to identical photodiodes  310  and  320 , respectively. Photodiodes  310  and  320  convert the incoming optical signals into photocurrents. For the RF signals typically produced by photodiodes  310  and  320 , capacitors  330  and  340  essentially provide a short circuit and inductor  350  essentially provides an open circuit. Reactive impedance matching circuit  360  is then used to extract a differential RF output signal  170 . A balanced receiver may also be realized by taking a the RF outputs from pair of standard optical receivers modules, applying a phase shift of 180° degrees to one RF leg, then combining the resulting RF signal in phase. 
     In FIG. 3B, complementary output signals  130  and  140  are optically coupled to identical photodiodes  315  and  325 , respectively. Photodiodes  315  and  325  convert the incoming optical signals into photocurrents. These photocurrents are amplified by amplifiers  370  and  380 . Differential amplifier  390  is used to subtract the amplified signals from each other and thereby produce RF output signal  170 . 
     As illustrated by FIG. 1, many externally modulated differential detection transmission systems do not need additional optical amplification. However, when the length of the fiber link is extended, or when other optical power losses must be compensated for (e.g., insertion losses at the external modulator) optical amplifiers are often used to amplify the optical signal. The availability of efficient erbium doped fiber amplifiers (EDFAs) operating in the 1550 nm wavelength region, where standard telecommunication fiber exhibits its minimum attenuation, has motivated the development of broadband transmitters compatible with the gain bandwidth of EDFAs. FIG. 2 illustrates a transmission system  200  (similar to transmission system  100  of FIG. 1) that includes EDFAs  210 . The optical power is amplified by an EDFA which is downstream from the external modulator. Thus the information bearing light signal enters the fiber optic span with an optical signal determined by the saturated output power of the EDFA. Note that additional EDFAs  220  and fiber lengths  230  can be added to transmission system  200  to extend its overall length. 
     However, a closer look at the CNR of such a transmission system demonstrates that EDFAs added to the system contribute additional noise. CNR is a function of the optical modulation index m, the photocurrent at the receiver I photo , electrical bandwidth B e  (typically 4 MHz for NTSC based CATV systems), shot noise at the receiver  2 eI photo , thermal equivalent noise i th , and the RIN of the signal:              CNR   =           1   2          m   2          I   photo   2           B   e          [       i   th   2     +     2      e                   I   photo       +       (   RIN   )          I   photo   2         ]         .             (   1   )                                
     Note that in equation 1, RIN is the sum of RIN from a variety of sources including the DFB laser. For example, each EDFA inserted into the transmission system contributes:                  RIN   EDFA     =       (     NF   =     1   G       )            2                 hv       P   in           ,           (   2   )                                
     where NF is the EDFAs noise figure, P in  is the optical input power to the EDFA, hν is the photon energy, and G is the net gain of the EDFA. 
     CNR is a primary figure of merit in optical transmission systems, and thus raising the CNR for a given system is desirable. Accordingly, it is desirable to have an optical transmission system that includes optical amplifiers, while reducing some of the noise contributed by those amplifiers. 
     SUMMARY OF THE INVENTION 
     It has been discovered that an optical transmission system that includes an optical amplifier coupled to the input of an optical modulator having at least two complementary output ports for providing complementary modulated optical output signals can be used such that the RIN associated with the optical amplifier is coupled into the modulated optical output signals as common mode noise and can therefore be eliminated using a differential detection scheme. Removing the RIN associated with the optical amplifier advantageously increases CNR for the optical transmission system. 
     Additionally, if the optical amplifier is, for example, an EDFA, coupling the amplifier between an optical source and the optical modulator (as opposed to coupling the amplifier to the output of the optical modulator), typically increases the optical input power P in  for the amplifier. As can be seen from equation 2, a higher P in  means that the EDFA can have a larger noise factor (NF) and still maintain the same RIN value. In general, EDFAs with high NF are less expensive than EDFAs with low NF. 
     This optical transmission system also takes advantage of the power increase inherent when using complementary modulated optical output signals, even when those signals are not subsequently differentially detected. 
     Accordingly, one aspect of the present invention provides an optical transmission system including an optical amplifier and an optical modulator. The optical amplifier is operable to receive an optical signal from an optical source and provide an amplified optical signal. The optical modulator has an input port coupled to the optical amplifier, a modulation port operable to receive a modulation signal including information for transmission, and a plurality of complementary output ports for providing a plurality of complementary modulated optical output signals in response to the modulation signal and the amplified optical signal. 
     In another aspect of the invention, a method of transmitting an optical signal is taught. An optical signal from an optical source is provided. The optical signal is amplified with an optical amplifier wherein the amplifying introduces noise into the optical signal. The optical signal is modulated in accordance with a modulation signal having information for transmission, and the modulating produces first and second complementary modulated optical output signals. The first and second complementary modulated optical output signals are transmitted through a first and a second optical medium, respectively. The first and second complementary modulated optical output signals are received at a balanced receiver, thereby forming first and second detected signals, respectively. At least a part of the noise is removed by taking the difference between the first and second detected signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
     FIG. 1 illustrates a prior art optical transmission system using differential detection. 
     FIG. 2 illustrates a prior art optical transmission system including optical amplifiers and using differential detection. 
     FIGS. 3A-3B show two prior art balanced receivers. 
     FIG. 4 illustrates an optical transmission system in accordance with the present invention. 
     FIGS. 5-7 illustrate additional optical transmission systems in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one having ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to unnecessarily obscure the present invention. 
     Optical transmission system  400  (as illustrated in FIG. 4) includes an optical amplifier  410  between optical source  110  and modulator  120 . By virtue of amplifying the optical signal before modulating it, any RIN contributed by optical amplifier  410  appears as common mode noise, i.e., it is identical in both output signals  130  and  140 . As noted above, EDFAs are typically used for optical amplifiers in optical transmission systems, however those having ordinary skill in the art will readily recognize that a variety of different optical amplifiers can be used for amplifier  410 . Fiber amplifiers based on other rare earth dopants and host glasses (e.g., Nd in ZBLN glass) are possible. Other examples of optical amplifiers include: semiconductor optical amplifiers (SOAs) based on single-pass amplification in semiconductor structures similar to those used in semiconductor laser diodes; and Raman amplifers using glass fibers as the active medium and exploiting the stimulated Raman effect. 
     To illustrate the CNR benefits associated with optical transmission system  400 , assume that optical source  110  is a 5 mW 1550 nm DFB laser, the complementary fiber links include no additional EDFAs (such as optional amplifier  220 ), and the fiber links are equal length 50 km single mode fibers (SMFs) having 11 dB loss at 1550 nm. Optical source  110  produces 7 dBm, that is supplied to optical amplifier  410  (in this case an EDFA). The saturated output power of optical amplifier  410  is 21 dBm (typical for EDFAs), which is supplied to optical modulator  120 , which is in this example a LiNbO 3  modulator having 7 dBm of loss. Thus, the optical signal power received at balanced receiver  160  is 3 dBm in each of the fiber links. If each photodetector of receiver  160  produces 1 mA of current for 1 mW of received signal, then the 3 dBm of optical power yields a photocurrent I photo =2 mA. If additional assumptions are made regarding typical system properties (e.g., DFB laser RIN, EDFA noise figure, there are only RIN contributions associated with the laser and the EDFA, etc.) the noise observed at a single detector is calculated to be −139 dB, and the noise after differential detection is calculated to be −142.5 dB. 
     It is significant to note that the noise level actually falls by 3.5 dB. This compares very favorably with prior art systems in which noise increases by 3 dB under differential detection. For a typical optical modulation index (e.g., m=3%) the single receiver CNR is 52 dB and the differential detection CNR is 61.1 dB, an 8.9 dB increase in CNR compared to the previously noted approximately 3 dB increase in CNR for optical transmission systems like those of FIGS. 1 and 2. Experiments under high EDFA noise conditions (e.g., low EDFA input power) demonstrate that differential detection combined with amplifier location before the modulator yields CNR increases in excess of 7 dB for all channels in the CATV broadband. 
     FIG. 5 illustrates an optical transmission system  500  including an optical amplifier  410  between optical source  10  and modulator  510 . Note that modulator  510  is not a dual output modulator like modulator  120 , and consequently no balanced receiver is shown, i.e., there is no differential detection. Nevertheless, optical transmission system  500  benefits from the location of amplifier  410  with respect to modulator  510  in that the optical input power P in  for the amplifier is increased. As can be seen from equation 2, a higher P in  allows the EDFA to tolerate a larger noise factor (NF) and still maintain the same RIN value. In general, EDFAs with high NF are less expensive than EDFAs with low NF. 
     FIG. 6 illustrates an optical transmission system  600  similar to optical transmission system  400 , but differing in that it is not intended for use with differential detection. Nevertheless, optical transmission system  600  benefits from the same EDFA advantages of optical transmission system  500 . Moreover, because optical transmission system  600  utilizes a dual output modulator, it possesses the net power increase inherent when using complementary modulated optical output signals, as opposed to using only a single output modulator. FIG. 7 illustrates a tree-and-branch architecture optical transmission system  700  similar to those used by the cable industry for video distribution. Splitters  720  divide the optical signal for multiple instance distribution. Additionally fiber lengths  730  and  760 , and amplifiers  710  and  740  allow the signal distribution to be further extended. Note that each final destination would include a receiver  770  producing an RF output signal. However, in contrast to prior art tree-and-branch architectures, system  700  includes optical amplifier placement and a dual output modulator as in system  600 , thereby extending the aforementioned benefits to system  700 . One would typically like to distribute many optical fibers from the headend of a cable network, so it is advantageous to find methods of reducing the number of transmitters used to modulate the video signals onto the outgoing multiple fibers. 
     Those having ordinary skill in the art will readily recognize that a variety of different components and techniques can be substituted for those previously described. The optical sources described are typically semiconductor DFB lasers, but a variety of different types of lasers can be used, for example Nd:YAG lasers. A variety of different types of EDFAs. including polarization preserving EDFAs can be used. Lithium niobate electro-optic modulators are the typical example for optical modulators illustrated in this application, but a variety of other optical crystals and modulating materials can be used (e.g., InP), or even non-electro-optic modulators (e.g., acousto-optic modulators). Moreover, different modulator designs can be chosen to provide complementary modulator outputs, such as Mach-Zelhnder interferometers, directional couplers, Y-fed directional couplers. balanced-bridge interferometers, and a phase-shifting interferometers. These and other modulator designs are described in greater detail in U.S. Pat. No. 5,253,309, previously incorporated by reference in its entirety. The optical medium through which optical signals are transmitted is typically a SMF fiber, but other types of fiber (e.go., dispersion shifted, polarization preserving, multi-mode), optical wave guides, and indeed air (e. g., an above ground line of sight transmission system) can be used with the invention(s) described in this application. 
     The description of the invention set forth herein is illustrative and is not intended to limit the scope of the invention as set forth in the following claims. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.