Patent Publication Number: US-2004057735-A1

Title: Optical transmitter using highly nonlinear fiber and method

Description:
FIELD OF THE INVENTION  
       [0001] The field of the present invention relates generally to optical fiber (lightwave) communication systems. More particularly, the invention relates to a direct modulation optical transmitter using highly nonlinear fiber for signal distortion compensation.  
       BACKGROUND INFORMATION  
       [0002] There is a growing interest in high speed optical data transmission, particularly for data rates greater than 10 Gbps. To accommodate this high speed data transmission, cost effective means of lightwave modulation have been explored. One such method is the addition of an external modulator to the optical transmitter. However, external modulators add expense, complexity and/or bulk to the communication system and require additional amplifiers in the transmission line to compensate for the limited output power of the optical transmitter. Hence, an attractive alternative to external modulation is to incorporate direct modulation to a high power optical transmitter. One such direct modulation technique is to incorporate a directly modulated laser diode system (such as a distributed feedback laser diode “DFB-LD”) within the optical transmitter as shown in FIG. 1.  
       [0003] The advantages of the directly modulated laser diode system include its small size, low cost, low driving voltage and high output power characteristics. With direct modulation of a laser diode, an amplification-free design is achievable for long transmission distances. However, the disadvantage of this conventional system is the frequency chirp characteristic of the directly modulated laser diode system which distorts the signal and significantly limits the maximum achievable transmission distance. As shown in the example given in FIG. 1, prior art optical transmitter may include a laser driver  2  to amplify and/or reshape the input signal  6  and a distributed feedback laser diode  3  for modulation. The signal  6  is transmitted through a single mode fiber  4  and received by an optical receiver  5 . The prior art system would suffer from the disadvantage noted above.  
       [0004] One way to analyze data transmission systems is through a generated display called an eye pattern. An eye pattern may be created by applying the received wave to the vertical deflection plates of an oscilloscope. Additionally, a sawtooth wave is applied to the horizontal deflection plates. The waveforms are then translated into a one interval display on the oscilloscope, resulting in an eye pattern similar to the one illustrated in FIG. 2. An eye pattern may also be synthesized via computer simulation. The interior region of the eye pattern is called the eye opening. The larger the width of the eye opening, the greater the time interval over which the received wave can be sampled without error from intersymbol interference. Additionally, the slope of the eye opening defines the sensitivity of the system to timing error while the height of the eye opening defines the margin over noise. See “ Communication Systems ”, Simon Haykin, Second Edition, pp. 496-497.  
       [0005] In this regard, FIG. 2 illustrates 10 Gbps output waveform signal quality after 40 km fiber transmission for the prior art optical receiver  5  by means of its simulated eye pattern discussed above. It is clear from FIG. 2 that the low eye opening height and timing jitter spreading in the eye pattern indicates that poor bit error rate performance will occur over this distance and at this data rate. Thus, the solution of the directly modulated laser diode optical transmitter has created a problem of degradation of signal quality over long distance and high speed transmission.  
       [0006] Prior art solutions to this problem have taken three approaches. First, a dispersion compensation fiber (DCF) (including negative dispersion fiber) can be added to the transmission line to compensate the signal distortion. However, to be an effective solution, the length of the DCF needs to be matched to the length of the conventional fibers already installed. Thus, customizing the length of the DCF for each existing fiber system is required. An alternative is to reinstall all new fibers in the transmission path with negative dispersion fiber. Either alternative is expensive. Second, installing a narrow optical bandpass filter just after the distributed feedback laser (DFB-LD) will suppress the frequency chirping of the DFB-LD. But, the narrow bandwidth requirement needed by the bandpass filter increases the sensitivity to temperature variations and causes passband stability problems. Additionally, a narrow bandwidth limits the quantity of data transmission which is not desirable. Third, optical amplifiers and/or regenerators may be added to the transmission path to overcome the dispersion penalty. However, this solution greatly increases cost, complexity and/or bulk to the transmission system.  
       [0007] In view of the above drawbacks, it would be desirable to have a low cost, less complex, direct modulation optical transmitter system that uses a laser diode without significant signal distortion caused by frequency chirp. It would also be desirable to have an external modulation optical transmitter system which provides greater transmission distances but without significant signal distortion caused by transmitter frequency chirp.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention addresses the drawbacks of the prior art by providing a direct modulation optical transmitter system using highly nonlinear fiber to compensate for transmitter frequency chirping. The present invention overcomes the signal distortion problem without the use of dispersion compensation fibers which require DCF length customization for each system or alternatively, replacement of existing standard fibers. The present invention compensates the signal distortion problem without the use of narrow optical band pass filters which limit data transmission at high speed. Additionally, the present invention avoids the usage of expensive and complex optical amplifiers and/or regenerators in the transmission path to avoid the distance limitation.  
       [0009] According to one aspect of the invention, the optical transmitter of the present invention includes an input for accepting a signal, a laser diode for direct modulation, a highly nonlinear fiber to compensate for the frequency chirp generated by the laser diode and an output for sending the optical signal to the transmission link. In a preferred embodiment, the laser diode is a distributed feedback laser diode and the highly nonlinear fiber is a highly nonlinear dispersion shifted fiber.  
       [0010] In another aspect of the invention, the present invention is an optical transmitter for providing signal distortion compensation which includes an input for accepting a signal, a distributed feedback laser diode for signal modulation, a highly nonlinear dispersion shifted fiber for compensating the frequency chirping caused by the distributed feedback laser diode; and an output for sending the optical signal. In a preferred embodiment, the optical transmitter also includes a laser driver for providing amplification to the input signal. As needed, the laser driver may reshape the input signal.  
       [0011] In yet another aspect of the invention, the present invention is a transmitter with a modulated input signal, the transmitter includes a highly nonlinear dispersion shifted fiber for compensating the frequency chirping in the signal. In a preferred embodiment, the transmitter includes a driver for amplifying and/or reshaping the signal.  
       [0012] In yet another aspect of the invention, the present invention is an optical transmitter system which includes an input for accepting a signal, an external modulator for signal modulation, a highly nonlinear fiber for inducing proper frequency chirping for the external modulator; and an output for sending the optical signal. In a preferred embodiment, the external modulator also includes a distributed feedback laser diode.  
       [0013] In yet another aspect of the invention, the present invention is optical transmission system having an optical transmitter (which includes an input for accepting a signal, an output for sending the signal, a laser diode for signal modulation and a nonlinear fiber for signal distortion compensation), an optical receiver for receiving the signal, and a transmission fiber for transmitting the signal from the optical transmitter to the optical receiver.  
       [0014] In yet another aspect of the invention, the present invention is an optical transmitter which includes an input for accepting a signal, a laser driver for amplifying and/or reshaping the signal, a distributed feedback laser diode for signal modulation, a highly nonlinear fiber for compensating the frequency chirping caused by the distributed feedback laser diode; and an output for sending the optical signal. In a preferred embodiment, the highly nonlinear fiber is a highly nonlinear dispersion shifted fiber.  
       [0015] In yet another aspect of the invention, the present invention is an optical transmitter which includes an input for accepting a signal, an modulator having a distributed feedback laser diode, an optical amplifier for amplifying the optical signal, a highly nonlinear fiber for inducing proper frequency chirping for the external modulator; and an output for sending the optical signal. In a preferred embodiment, the optical amplifier is a Raman amplifier, a semiconductor optical amplifier or an erbium doped fiber amplifier.  
       [0016] In yet another aspect of the invention, the present invention is a method for transmitting a signal by generating a signal, modulating the signal with a modulator and compensating distortion to the signal by passing the signal through a highly nonlinear fiber.  
       [0017] In yet another aspect of the invention, the present invention is a method for transmitting a signal by generating a signal, modulating the signal with a distributed feedback laser diode and compensating distortion to the signal by passing the signal through a highly nonlinear dispersion shifted fiber.  
       [0018] Other and further objects and advantages of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019]FIG. 1 is a block diagram of a prior art direct modulation optical transmission system.  
     [0020]FIG. 2 illustrates the simulated 10 Gbps receiver output waveform eye pattern after 40 km fiber transmission using the prior art optical transmission system.  
     [0021]FIGS. 3 a  and  3   b  are two block diagrams of two embodiments of optical transmission systems in accordance with the present invention.  
     [0022]FIG. 4 illustrates the simulated 10 Gbps receiver output waveform eye pattern after 40 km fiber transmission using an optical transmission system with a highly nonlinear fiber at its output in accordance with the present invention.  
     [0023]FIG. 5 is a power profile versus time graph of a laser diode output waveform simulation.  
     [0024]FIG. 6 is a frequency chirp profile versus time graph of a laser diode output waveform simulation comparing the effects of no chirping compensation and with chirping compensation as introduced by a highly nonlinear dispersion shifted fiber.  
     [0025]FIG. 7 illustrates the bit error rate (BER) characteristics for three transmission scenarios utilizing the self phase modulation of a highly nonlinear dispersion shifted fiber.  
     [0026]FIG. 8 illustrates the power penalty characteristics of a highly nonlinear dispersion shifted fiber for two single mode fiber lengths at a bit error rate (BER) of 10 −9 .  
     [0027]FIG. 9 illustrates the bit error rate (BER) characteristics for various pseudo random bit sequence (PRBS) lengths using highly nonlinear dispersion shifted fiber.  
     [0028]FIG. 10 is a block diagram of another embodiment of an optical transmission system with external modulation in accordance with the present invention.  
     [0029]FIG. 11 is a block diagram of yet another embodiment of an optical transmission system with external modulation in accordance with the present invention.  
     [0030]FIG. 12 is a block diagram of yet a different embodiment of an optical transmission system with external modulation in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0031] The present invention is directed to optical transmitters using highly nonlinear fiber to compensate for or modify transmitter frequency chirping. The present invention compensates for transmitter frequency chirping without using either dispersion compensation fibers in the transmission path or narrow optical band pass filters. The present invention uses highly nonlinear fiber in the optical transmitter to reduce transmitter frequency chirp and improve performance.  
     [0032]FIG. 3 a  is a block diagram of a first embodiment of an optical transmission system in accordance with the present invention. In a preferred embodiment, the optical transmitter  100  comprises a laser driver  110 , a distributed feedback laser diode (DFB-LD)  120  and a highly nonlinear fiber (HNLF)  130 . The nonlinearity characteristic of the highly nonlinear fiber  130  is desirable for compensating unwanted chirping.  
     [0033] The highly nonlinear fiber  130  is characterized by its self phase modulation (SPM) properties, which introduces a negative frequency chirp versus the transmitted optical pulse power level, to compensate the positive frequency chirping of the distributed feedback laser diode. The placement of the highly nonlinear fiber at the output of the distributed feedback laser diode eliminates the need to customize fiber length for each system even if the transmission fiber length varies from system to system. Additionally, there is no need to customize the fiber nonlinearity for each communication system. Rather, to optimally achieve compensation for the chirping distortion of the distributed feedback laser, certain parameters of the highly nonlinear fiber are preferred. In a preferred embodiment, the product of the length L of the highly nonlinear fiber multiplied by the nonlinearity coefficient y of the highly nonlinear fiber material is preferred to be in the range of 200-400W −1 . Two examples of materials suitable for highly nonlinear fibers are tellurite and chalcogenide glasses. Although these two fiber materials are mentioned, it will be appreciated that they are presented only as examples and the invention is not limited thereby.  
     [0034] The laser driver  110  amplifies and/or reshapes the input signal  10  and feeds the input signal to the distributed feedback laser diode  120  which further amplifies and performs signal modulation. The distributed feedback laser diode  120  may or may not include cooling, depending on the application. The modulated signal is then passed through the highly nonlinear fiber  130  before being outputted from the optical transmitter  100  to the transmission fiber  140  and finally to the optical receiver  150 . Frequency chirping is a byproduct of the distributed feedback laser diode  120  which results in transmitter signal distortion. If the chirping distortion is not corrected, the distorted signal characteristics at the output of the optical receiver  150  are evident from the eye pattern shown in FIG. 2. However, in the optical transmitter of the present invention, the highly nonlinear fiber  130  compensates for the frequency chirp generated by the distributed feedback laser diode  120  and a less distorted signal (as evident by the clearer eye pattern shown in FIG. 4) is passed through the transmission fiber  140  and received by the optical receiver  150 .  
     [0035] In contrast to FIG. 2, FIG. 4 illustrates 10 Gbps output waveform signal quality after 40 km fiber transmission for the optical receiver  150 , with a highly nonlinear fiber following laser diode  120 , by means of its simulated eye pattern. The clean height of the eye pattern indicates low bit error rate performance is possible over this 40 km distance and at the specified data rate given the chirp compensation provided by the highly nonlinear fiber  130  at the output of the distributed feedback laser diode  120 .  
     [0036] In another embodiment, a specific type of highly nonlinear fiber known as a highly nonlinear dispersion shifted fiber (HNL-DSF)  230  is placed at the output of a distributed feedback laser diode  220  as shown in FIG. 3 b.  Highly nonlinear dispersion shifted fibers, with its enhanced nonlinearity, have been developed as one of the optical functional fibers. The enhanced nonlinearity characteristic is desirable for compensating unwanted transmitter chirping. The properties of a highly nonlinear dispersion shifted fiber are described in “ Silica - Based Functional Fibers With Enhanced Nonlinearity and Their Applications ”, Okuno et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 5, September/October 1999, the disclosure of which is incorporated in its entirety herein by reference thereto. The optical communication system shown in FIG. 3 b  is similar to that shown in FIG. 3 a  except that the highly nonlinear fiber  130  is replaced by a highly nonlinear dispersion shifted fiber  230 . Specifically, the optical transmitter  200  comprises a laser driver  210  for amplifying and/or reshaping the input signal  20  and then feeds the input signal into the distributed feedback laser diode  220  for signal modulation. Signal  20  is transmitted through a highly nonlinear dispersion shifted fiber  230  where its signal distortion is compensated for the frequency chirp added by the distributed feedback laser diode  220 . At this point, the signal  20  is then output from the optical transmitter  200  for transmission through a transmission fiber  240  and received by the optical receiver  250 .  
     [0037] FIGS.  5 - 9  are performance graphs for the present invention shown in FIG. 3 b.  FIG. 5 is a power profile versus time graph of a distributed feedback laser diode output waveform simulation at a bit rate of 10 Gbps. FIG. 6 is a frequency chirp profile versus time graph which shows the results of the distributed feedback laser diode output waveform simulation and the chirping compensation introduced by the highly nonlinear dispersion shifted fiber  230 . The upper graph displays the chirping characteristics versus time at the output of the distributed feedback laser diode  220 , without the chirping compensation provided by the highly nonlinear dispersion shifted fiber  230 . The lower graph displays the chirping characteristics versus time at the output of the highly nonlinear dispersion shifted fiber  230 . Comparison of the upper and lower graphs indicates that the highly nonlinear dispersion shifted fiber  230  effectively reduces the frequency chirp to minimize signal distortion.  
     [0038] The bit error rate (BER) performance of the present invention shown in FIG. 3 b  is tested for various transmission scenarios. FIG. 7 illustrates the bit error rate (BER) performance for three transmission scenarios utilizing the self phase modulation of the highly nonlinear dispersion shifted fiber  230  in a laboratory setup that emulates the present invention shown in FIG. 3 b  in a field setting. As shown in FIG. 7, the digital bit error rate performance versus receiver input power for three fiber transmission link scenarios is summarized: back to back (zero length fiber), 25 kilometer (km) single mode fiber (SMF) and 50 km SMF. Each scenario is characterized by an HNL-DSF input power of 15 dBm. The results indicate that the best performance is obtained for the 50 km SMF case. As indicated by FIG. 6, the frequency chirp at the output of the distributed feedback laser diode  220  is reduced (i.e., compensated) after signal  20  passes through the highly nonlinear dispersion shifted fiber  230 . The frequency chirp characteristics at the output of the optical transmitter  200  affect the transmission delay in the transmission SMF fiber  240 . Two factors will affect how the transmission SMF fiber  240  will distort an optical signal passing through it: the SMF fiber length and the frequency chirp characteristics at the output of the optical transmitter  200 . In the three scenarios illustrated in FIG. 7, the frequency chirp characteristics at the optical transmitter  200  output plus the transmission SMF fiber  240  length results in the lowest bit error rate performance for the 50 km SMF case.  
     [0039] The power penalty characteristics of the present invention shown in FIG. 3 b  (which is tested in a laboratory setup that emulates the present invention under various transmission scenarios) is illustrated in FIG. 8. FIG. 8 illustrates the power penalty [dB] characteristics of the highly nonlinear dispersion shifted fiber  230  for two transmission fiber lengths (25 km single mode fiber and 50 km single mode fiber), at an operating point of 10 −9  bit error rate, versus HNL-DSF input power [dBm]. The 50 km SMF fiber length has a lower power penalty than the 25 km SMF fiber length near the optimum input power level of around 15 dBm (at the input of the highly nonlinear dispersion shifted fiber  230 ). Power penalty is referenced relative to the back-to-back link scenarios at 0 dB. FIG. 8. also indicates the increased power penalty sensitivity for over pre-chirped signals versus under pre-chirped signals. To improve bit error rate performance, a forward error correction (FEC) technique (known to one of ordinary skill in the art) is combined with the pre-chirping technique of the present invention to correct transmitted signal errors. In one embodiment, the combination of forward error correction technique and the pre-chirping technique of the present invention improves communication performance. The combination of pre-chirping technique and forward error correction technique avoids the need for a booster amplifier which adds cost, complexity and/or bulk to the system.  
     [0040] Additionally, the present invention shown in FIG. 3 b  is tested (in a laboratory setup that emulates the present invention) to determine dependency on binary pattern lengths. FIG. 9 illustrates the bit error rate (BER) performance for various pseudo random bit sequence (PRBS) lengths for the present invention shown in FIG. 3 b.  FIG. 9 summarizes the digital bit error rate performance versus receiver input power for a wide range of pseudo random bit sequence lengths, ranging from 2 7 -1 bits to 2 31 -1 bits. These results demonstrate the performance insensitivity of the present invention to pseudo random bit sequence lengths.  
     [0041] In FIG. 10, a block diagram of another embodiment of an optical transmission system is shown. The transmission system includes optical transmitter  300  comprising an external modulator  310 , a distributed feedback laser diode  320  and a highly nonlinear fiber  330 . The external modulator  310  accommodates high speed data transmission (particularly for bit rates greater than 10 Gbps) over long transmission lines but adds distortion to the signal  30 . Since intrinsic frequency chirping is also a characteristic of transmitters using external modulators, the self phase modulation induced pre-chirping technique (discussed above with respect to FIGS. 3 a  and  3   b ) is applied to induce proper frequency chirping for the external modulators. In one embodiment, the highly nonlinear fiber  330  is placed at the output of the optical transmitter  300  to induce proper frequency chirping for the external modulator. In another embodiment, an optical amplifier  325  is included to meet the high power requirements of the external modulator  310 . Here, the highly nonlinear fiber  330  is placed at the last stage of the optical transmitter  300  (at the output of the optical amplifier  325 ). The self phase modulation characteristics of the highly nonlinear fiber  330  induces proper frequency chirping for the external modulator  310 . In a preferred embodiment, the optical amplifier  325  is a semiconductor optical amplifier (SOA) or a Raman amplifier. In one embodiment, a distributed feedback laser diode  320  is coupled to the external modulator  310 . In this embodiment, the highly nonlinear fiber  330  will induce proper frequency chirping for the external modulator  310 . In a preferred embodiment, as shown in FIG. 1, a specific type of highly nonlinear fiber  330 , a highly nonlinear dispersion shifted fiber  430  (not shown), is used for compensating the frequency chirp of optical transmitter  301 . Additionally, a specific type of optical amplifier, an erbium doped fiber amplifier  425  (EDFA), is used for amplification. In another embodiment, the optical amplifier in optical transmitter  302  includes a pump laser diode  525  as shown in FIG. 12. Although examples of optical amplifiers have been disclosed here, other forms of optical amplifiers (known to one of ordinary skill in the art) may be used with equal effectiveness in accordance with the present invention. The optical transmission systems illustrated in FIGS.  10 - 12 , with the inclusion of highly nonlinear fibers (such as highly nonlinear dispersion shifted fibers) in the transmitters, prophetically permit long distance transmission of over 80 km to 120 km (depending on the type of external modulator used) at high speed data transmission, particularly for bit rates greater than 10 Gbps.  
     [0042] While the present invention has been described in terms of the preferred embodiments, other variations which are within the scope of the invention as defined in the claims will be apparent to those skilled in the art.