AU2246501A - Positively chirped signals in optical communication systems - Google Patents

Description

WO 01/19003 PCT/US0O/23314 1 Positively Chirped Signals in Optical Communication Systems This application claims the benefit of U.S. Provisional Application Number 60/152,626, filed September 7, 1999, U.S. Provisional Application 5 Number 60/178,394, filed January 27, 2000, U.S. Provisional Application Number 60/186,407, filed March 2, 2000 and U.S. Provisional Application Number 60/186,796, filed March 3, 2000. Background of the Invention 10 1. Field of the Invention The present invention relates generally to an optical waveguide fiber communications system, and particularly to such a communication system comprising negative dispersion waveguide fiber and a source of positively 15 chirped signals. 2. Technical Background As the demand for bandwidth continues to increase in telecommunication systems, strategies are being sought to enhance system 20 performance even while cost is decreased. Cost is a very important consideration in intra-city links, where the density of nodes is high. A promising strategy is one that involves matching system components in such a way that a particular property of one component compensates a deficiency in another component. Preferably, the component matching WO 01/19003 PCT/USOO/23314 2 strategy is one in which a given component is designed to allow another component to operate more efficiently or effectively. Such compensation schemes have been effective, for example, in reducing dispersion penalty by adding a dispersion compensating module to the end of a communications link, 5 thereby recovering a desired signal to noise ratio or signal pulse shape. Another example of effective compensation is the use of large effective area waveguide fiber in communications systems in which non-linear effects are a major source of signal degradation. Recently, workers have discovered that the positive chirp that results 10 from directly modulating a laser, for example, a distributed feedback (DFB) semiconductor laser, can be offset by using optical waveguide fiber having negative total dispersion over a desired range of operating wavelengths. The effect of the negative dispersion fiber does not depend upon the type of light source or modulation scheme. The beneficial effect is achieved when the 15 negative dispersion fiber is used in conjunction with a signal having a positive chirp. Matching a waveguide fiber to a positively chirped DFB semiconductor laser is a cost effective combination, especially because, in addition to being low in cost, the DFB semiconductor laser has relatively high power output and good longevity. Furthermore, direct modulation is simpler and less expensive 20 than external modulation schemes. However, even given the efficiency improvement of compensating laser chirp through use of negative total dispersion waveguide, certain system links are still bandwidth limited. There is therefore a need for further optimization of a positively chirped source of optical signals for use in a telecommunication 25 system including negative dispersion waveguide fiber. Definitions -In this application we are primarily concerned with waveguide fiber chromatic dispersion. Chromatic dispersion arises because different frequency 30 components of a pulse or signal travel with different group velocities in the fiber. Chromatic dispersion is characterized by the so called dispersion parameter D which is expressed in units of ps/nm-km. The value of the WO 01/19003 PCT/USOO/23314 3 dispersion parameter D is the sum of two terms, one arising from the fiber material and the other from geometric characteristics of the waveguide. - The product of the length of a waveguide fiber and the dispersion parameter D (including material and waveguide contributions) will be subsequently called 5 the dispersion product. A link may be made of waveguide fiber within which the chromatic dispersion changes along the fiber length. The sum of dispersion products for a fiber length is the algebraic addition of the individual dispersion products associated with the individual fiber lengths. In symbols, the sum of dispersion products is 7i Di x Li, where, Di is the dispersion 10 parameter over the length Li, and the total length of the fiber is 7l Li. The dispersion parameter of a waveguide fiber is, by convention, positive when shorter wavelength light propagates at a higher speed than light of longer wavelength. The converse is the definition of negative dispersion waveguide fiber. 15 - A waveguide fiber telecommunications link, or simply a link, is made up of a transmitter of light signals, a receiver of light signals, and a length of waveguide fiber having respective ends optically coupled to the transmitter and receiver to propagate light signals therebetween. A link can include additional optical components such as optical amplifiers, optical attenuators, optical switches, 20 optical filters, or multiplexing or demultiplexing devices. One may denote a group of inter-connected links as a telecommunications system. - The Q of a waveguide fiber link is the difference between the mean photodetector current, i 1 , when receiving a '1' or mark bit and the the mean photodetector current, io, when receiving a '0' or space bit, divided by the sum 25 of the respective standard deviations of the two noise currents, ai and 0o, associated with the mark and space respectively. That is Q = (i 1 - io)/(o 1 + Go). See Fiber Optic Communications Systems 2 Edition, Agrawal, pages 172 173. In the figures, Q is expressed in dB so that the equation is written Q (dB) = 10 log 1 o Q. A higher Q value represents a link having a superior (i.e. lower) 30 bit error rate.

WO 01/19003 PCT/USOO/23314 4 - Extinction ratio is defined as the ratio of the transmitted power P 1 when the transmitter is in the on state (a one bit is transmitted) to the transmitted power Po when the transmitter is in the off state (a zero bit is transmitted). - A positively chirped signal source produces pulses in which the carrier 5 frequency varies along the pulse time axis. That is, the frequency of the electric field of the output pulse is red or blue shifted compared to the frequency of the laser at the threshold. For example, a conventional single electrode DFB semiconductor laser produces pulses with an average blue shift, herein defined as positive chirp. The chirp in a signal, for example a signal 10 from a directly modulated DFB laser, can be expressed approximately as a combination of adiabatic chirp and transient chirp. - Adiabatic chirp is proportional to the output power of the signal. - Transient chirp is proportional to the derivative of the output power of the signal and so is present only in the time periods when the signal power is in 15 transition between a 0 and a 1 (or a 1 to a 0). - Dispersion power penalty of a link is the reduction of the link power budget due to dispersion-induced distortion of the signal. In this application, the dispersion penalty is expressed as eye closure penalty. The eye diagram is known in the art as the eye shaped opening formed when adjacent signal 20 pulses begin to overlap. As overlap increases, the eye is said to close. This is a convenient way to refer to the reduction in signal to noise ratio due to pulse dispersion. - Gain compression factor, also known as the nonlinear gain parameter, refers to a semiconductor laser and is a proportionality constant that relates 25 semiconductor laser material optical gain of the active region of the laser to the number of photons in the active region. In the relationship, G = f(EP), G is the gain of the laser, e is the gain compression factor, P is number of photons in the active region (which is directly related to the laser output power) and f is a function. See Fiber Optic Communications Systems 2"d Edition, Agrawal, page 30 113.

WO 01/19003 PCT/USOO/23314 5 Summary of the Invention One aspect of the present invention is a telecommunications link that includes an optical signal transmitter, an optical signal receiver, and a length of optical waveguide fiber optically connected between the transmitter and 5 receiver to carry the light signals from the transmitter to the receiver. At least a portion of the waveguide fiber of the telecommunications link has negative dispersion and the optical signals of the transmitter are positively chirped. In addition, the signal chirp is predominately adiabatic. In an embodiment of the link, the signal source is a directly modulated 10 laser included in the transmitter. In a preferred embodiment the laser is a directly modulated DFB semiconductor laser. In another embodiment of the link, the positively chirped signal pulses originate in a continuous wave source of optical power that is externally modulated. 15 Adiabatic laser chirp is characterized by a relatively high gain compression factor, e.g., one in the range of 4 x 10-23 m 3 to 30 x 10-23 m 3 . Furthermore, adiabatic chirp is favored in laser operating conditions in which the extinction ratio is not greater than about 20 dB. Extinction ratios which fall in the range of 5 dB to 11.5 dB are preferred. An extinction ratio as high as 20 20 dB is contemplated in communications links that include forward error correction. Forward error correction is a scheme by which bits that become corrupted during data transmission can be corrected. This requires electronics to encode the data before transmission and to decode the data after reception. The forward error correction electronics is known in the art and need not be 25 discussed further here. Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, 30 the claims, as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are WO 01/19003 PCT/USOO/23314 6 intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings 5 illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. Brief Description of the Drawings Fig. 1 is a theoretical graph of the optical output power versus time for a 10 directly modulated adiabatic chirp dominated DFB semiconductor laser operating at the OC-48 (2.5 Gb/s) rate. Fig. 2 is a theoretical graph of the optical output power versus time for a directly modulated transient chirp dominated DFB semiconductor laser 15 operating at the OC-48 (2.5 Gb/s) rate. Fig. 3 is a theoretical graph of the optical frequency deviation versus time for a directly modulated adiabatic chirp dominated DFB semiconductor laser operating at the OC-48 (2.5 Gb/s) rate. 20 Fig. 4 is a theoretical graph of the optical frequency deviation versus time for a directly modulated transient chirp dominated DFB semiconductor laser operating at the OC-48 (2.5 Gb/s) rate. 25 Fig. 5 is a theoretical graph of simulated eye closure penalty versus the dispersion product for a directly modulated adiabatic chirp dominated DFB semiconductor laser operating at the OC-48 (2.5 Gb/s) rate. Fig. 6 is a theoretical graph of simulated eye closure penalty versus the 30 dispersion product for a directly modulated transient chirp dominated DFB semiconductor laser operating at the OC-48 (2.5 Gb/s) rate.

WO 01/19003 PCT/USOO/23314 7 Fig. 7 is a graph of Q versus dispersion product for a laser operated at an extinction ratio of 6.2 dB and the OC-48 (2.5Gb/s) rate. Fig. 8 is a graph of Q versus dispersion product for a laser operated at an 5 extinction ratio of 8.8 dB and the OC-48 (2.5Gb/s) rate. Fig. 9 is a graph of Q versus dispersion product for a laser operated at an extinction ratio of 11.3 dB and the OC-48 (2.5Gb/s) rate. 10 Fig. 10 is a schematic drawing of an exemplary multi-channel optical communication link. Detailed Description of the Invention Reference will now be made in detail to the present preferred 15 embodiments of the invention. The invention comprises, in combination, a positively chirped laser in which the chirp is predominately adiabatic and an optical waveguide optical fiber having negative dispersion. The term laser in this application is generally used to describe a source of optical power suitable for use in an optical waveguide link which is positively chirped and which can 20 be directly modulated. However, the invention includes any continuous wave source of optical power which is externally modulated and which exhibits positive chirp. An example of a positively chirped laser is the distributed feedback semiconductor laser which is directly modulated. Chirp can be characterized as adiabatic which means the chirp is 25 proportional to the optical output power of the laser. In contrast, transient chirp is proportional to the rate of change of optical output power with time. In the case of the directly modulated DFB lasers, the chirp is predominantly adiabatic when the laser is always operated well above threshold with low extinction ratios (e.g. 6dB). However with present technology the chirp becomes 30 predominantly transient when the laser is operated closer to threshold, where the extinction ratios becomes much higher (e.g. 12dB). The exact extinction ratio or drive condition under which a laser's chirp switches from predominantly WO 01/19003 PCT/USOO/23314 8 adiabatic to predominantly transient depends upon the exact parameters of the laser itself. It is contemplated that laser design parameters can be found that provide adiabatic chirp at an extinction ratio as high as 12 dB. The characteristics of a directly modulated laser exhibiting 5 predominately adiabatic chirp are illustrated in Figs. 1 and 3. A particular laser which is described by these figures is the directly modulated DFB semiconductor laser. The figures are derived from the exemplary case in which the bit rate is 2.5 Gb/s, the OC-48 rate. It will be understood that the figures could be modified to describe a higher or lower bit rate. Also, it will be 10 understood that the bit rate can be achieved by any of several means known in the art, including time or wavelength division multiplexing, the latter of which is shown in Fig. 10. In Fig. 1 is shown a sequence of digital 1's, segments 6 of the chart, and O's, segments 2 of the chart. The ringing 8 (the oscillation of the laser 15 signal about a steady state value) of the 1's, and the ringing 4 of the O's is seen to be small. This is to be compared to the O's, 14, and 1's, 16, of the transient chirp dominated laser sequence of laser signals shown in Fig. 2. In Fig. 2 the ringing, for example chart segments 18 and 20, is much larger for the transient chirp dominated laser output. As is seen in the optical frequency deviations of 20 segments 10 of Fig. 3 as compared to the optical frequency deviations of segments 12 in Fig. 3, the frequency difference between O's and 1's in the adiabatic case is pronounced. This difference in optical frequency is to be compared to that in Fig. 4 where segments 22 are frequency deviation for the signal O's and segments 24 are frequency deviation for the signal 1's. 25 The primary difference between the laser characterized by Figs. 1 and 3 and the laser characterized by Figs. 2 and 4, is that the gain compression factor, defined above, was 5 x 10~2 m3 for the former laser and 1 x 10~2 m3 for the latter laser. Gain compression factor for a particular laser structure may be measured by using fitting techniques described, for example, in L. A. Coldren 30 and S. W. Corzine, "Diode lasers and photonic integrated circuits", Wiley, 1995, p.211, 'Intensity modulation and chirp of 1.55um MQW laser diodes: modeling and experimental verification', K. Czotscher et. al., IEEE Journal of WO 01/19003 PCT/USOO/23314 9 Selected Topics in Quantum Electronics, vol. 5, no. 3, May/June 1999, or, 'Extraction of DFB laser rate equation parameters for system simulation purposes', J.C. Cartledge et. al., IEEE Journal of Lightwave Technology, vol. 15, no. 5, May 1997. 5 It is also well-known that operating a laser at lower extinction ratios, so that the laser is always well above threshold, helps significantly in reducing transient ringing and allowing the laser to operate with predominately adiabatic chirp and very small transient chirp. In general, a system designer maintains extinction ratio as high as possible consistent with the desired system 10 operating parameters. The decrease in signal overshoot, the reduction in ringing amplitude and duration, and the marked difference in optical frequency between O's and 1's of the predominately adiabatic chirped laser provide a telecommunications link in which distance between signal regenerators is large in comparison to the case of a predominately transient chirped laser. 15 The improved performance of the laser having predominately adiabatic chirp carries over into link performance as can be seen in Fig. 5 where simulated eye closure penalty is charted versus accumulated waveguide fiber dispersion at 2.5 Gb/s. Acceptable eye closure penalty is preferred to be no more negative than -2 dB. For laser pulses, having predominately adiabatic 20 chirp, propagating in a negative dispersion waveguide fiber, curve 26 of Fig. 5 shows an accumulated dispersion of -5000 ps/nm when eye closure penalty reaches -2 dB. A typical dispersion shifted negative dispersion waveguide can have a total dispersion at 1550 nm of about -3.5 ps/(nm-km). Thus, at a signal wavelength of 1550nm, the link length traversed by the pulse, without 25 electronic regeneration, before incurring a -2 dB eye closure penalty is about 1430 km. The benefit of pulse compression afforded by the negative dispersion waveguide can be seen by comparing curve 26 to curve 28 in Fig. 5. Curve 28 of Fig. 5 corresponds to a link identical to that of curve 26 except that positive dispersion waveguide fiber is used. Curve 28 shows that the -2 dB eye 30 closure penalty is reached at an accumulated dispersion of only about 1200 ps/nm which corresponds to a link length of about 350 km of a positive dispersion fiber with the same absolute value of dispersion parameter.

WO 01/19003 PCT/USOO/23314 10 Referring now to Fig. 6, which pertains to a system bit rate of 2.5 Gb/s, the advantage afforded by adiabatic chirp as compared to transient chirp is seen by comparing curve 26 (predominately adiabatic chirp) of Fig. 5 to curve 30 (predominately transient chirp) of Fig. 6. For laser pulses having 5 predominately transient chirp, the -2 dB eye closure penalty limit is reached at an accumulated dispersion of about -1500 ps/nm, corresponding to a link length of about 430 km. As was stated above, the beneficial effect of this combination also pertains to bit rates at 10 Gb/s and higher. The 2.5 Gb/s rate is used as an illustration only. Comparing curves 30 and 32 of Fig. 6 again 10 shows the advantage of negative dispersion waveguide fiber in combination with a positively chirped laser. The positive dispersion waveguide fiber of corresponding to curve 32 shows an acceptable accumulated dispersion of only about 1000 ps/nm, which yields a typical link length of about 285 km of a positive dispersion fiber with the same absolute value of dispersion parameter. 15 The use of a laser having predominantly adiabatic chirp in combination with a negative dispersion fiber increases unregenerated link length by about a factor of four. Additional benefit is achieved by adjusting the operating point of the laser having predominately adiabatic chirp. A bit error rate of 1012 20 corresponds to a link Q value greater than or equal to 8.5 dB. In systems in which a higher bit error rate can be tolerated, Q values greater than or equal to 6 dB are acceptable. In systems which make use of forward error correction electronics a Q value greater than or equal to 3 dB is acceptable. Curve 34 of Fig. 7 shows the performance of a link operating at 2.5 Gb/s, in terms of Q 25 value versus accumulated dispersion, having an extinction ratio no greater than 6.2. The accumulated link dispersion is not less negative than -12000 ps/nm. Again assuming a total dispersion at 1550 nm of -3.5 ps/nm-km, one finds a corresponding link length of nearly 3500 km before electronic signal regeneration is needed. At the higher extinction ratio, a ratio no greater than 30 about 9 dB, curve 38 of Fig. 8, also pertaining to a 2.5 Gb/s rate, the acceptable accumulated dispersion of a pulse propagating in a negative dispersion waveguide fiber is not less negative than -6000 ps/nm, WO 01/19003 PCT/USOO/23314 11 corresponding to an unregenerated link length at 1550 nm of about 1700km. Curve 42 of Fig. 9 shows link performance at an extinction ratio not greater than 11.3 dB. In this link, identical to the 2.5 Gb/s links used to generate curves 34 of Fig. 7 and 38 of Fig. 5 except for laser extinction ratio, at a Q 5 value greater than or equal to 8.5 dB, the accumulated dispersion not less negative than -1950 ps/nm, corresponding to a link length for a 1550 nm signal of about 560 km. The measurement of Q can be made using transmitters and receivers known in the art. Example receivers suitable for use in the telecommunications 10 links disclosed and described herein are Alcatel 1916 SDH, Receiver STM 16/OC-48 for D-WDM, Alcatel, 12030 Sunrise Valley Drive, Reston, VA, 22 091, and, 1320-Type Lightwave Receiver, Lucent Technologies, 555 Union Boulevard, Room 30L-15P-BA, Allentown, PA, 18103. Example transmitters suitable for use in the telecommunications links disclosed and described herein 15 are D2570, D2526, D2555 Wavelength-Selected Laser 2000, Lucent Technologies (see address above), and LC155W-20A, WDM DFB Laser Module, Optoelectronics, Brixham Road, Paignton, Devon, TQ4 7BE. It will be understood that link length can be reduced to allow higher waveguide fiber total dispersion. Also link length can be increased in systems 20 that can tolerate a higher value of Q. Curve 36 of Fig. 7, curve 40 of Fig. 8, and curve 44 of Fig. 9 were generated using links essentially identical to those of links used to generate corresponding curves 34, 38, and 42, except that waveguide fiber having positive total dispersion was used. The advantage of using negative dispersion 25 waveguide fiber in combination with a laser of predominantly adiabatic chirp is evident. The advantage becomes more pronounced as extinction ratio of the laser is decreased. An exemplary link, ideally suited to an intra-city or other shorter length, high data rate system, is illustrated schematically in Fig. 10. The link includes 30 a plurality 46 of sources of light pulse signals, for example, a plurality of directly modulated distributed feedback semiconductor lasers, having positive, predominately adiabatic chirp and operating in a wavelength division WO 01/19003 PCT/US0O/23314 12 multiplexing mode. The directly modulated DFB laser is a simple, low cost, and reliable signal source. The plurality 46 of lasers are coupled into waveguide 52 via optical multiplexer 48. Optical amplifiers 50 are inserted into the waveguide fiber path at pre-selected intervals to maintain a desired signal 5 amplitude. The signal then passes through optical demultiplexer 54 which separates the wavelengths and delivers a particular wavelength to one of the plurality of receivers 56. At least a portion of the waveguide fiber 52 linking the transmitters and receivers has negative chromatic dispersion to compress the positively chirped laser pulses. Spacing between transmitters and receivers 10 can be a few kilometers, or tens of kilometers, or hundreds of kilometers. The laser is operated at an extinction ratio of 5 dB to 10 dB, with 20 dB extinction ratios being workable for lasers having the required chirp characteristic at a higher pulse power. The link can support 2.5 Gb/s or 10 Gb/s data rates. No electronic regeneration is required. 15 It will be apparent to those skilled in the art that various modifications and variations of the present invention can be made without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims (13)

1. A telecommunications link comprising: a transmitter for providing optical signal pulses; a receiver for detecting said optical signal pulses; 5 a length of optical waveguide fiber having a first and a second end, the respective ends being optically connected to said transmitter and said receiver to guide optical signal pulses therebetween; wherein, at least a portion of the length of said waveguide fiber has a negative total dispersion, the optical signal provided by said transmitter is 10 positively chirped, and the chirp is predominately adiabatic.

2. The telecommunications link of claim 1 wherein said transmitter includes a semiconductor laser which provides the optical signal pulses, and said semiconductor laser is characterized by a non-linear gain compression factor 15 and the gain compression factor is in the range of 4 x 1021 m 3 to 30 x 10~21 M 3 .

3. The telecommunications link of claim 2 wherein said semiconductor laser is further characterized by an extinction ratio, said extinction ratio having a value no greater than 20 dB. 20

4. The telecommunications link of claim 3 wherein said extinction ratio is no greater than 12 dB.

5. The telecommunications link of claim 4 wherein said extinction ratio is in 25 the range of5 dB to 11.5 dB.

6. The telecommunications link of claim 3 further including forward error correction components, wherein the link is characterized by a bit rate of 2.5 Gb/s, a Q value, and a sum of dispersion products, wherein the sum of 30 dispersion products is not less negative than -1950 ps/nm, and said transmitter, said receiver, and said optical waveguide fiber are such that the Q of the link is greater than or equal to 3 dB. WO 01/19003 PCT/USOO/23314 14

7. The telecommunications link of claim 3, wherein the link is characterized by a bit rate of 2.5 Gb/s, a Q value, a sum of dispersion products, and an extinction ratio no greater than 11.3 dB and wherein the sum of dispersion 5 products is not less negative than -1950 ps/nm, and said transmitter, said receiver, and said optical waveguide fiber are such that the Q of the link is greater than or equal to 6 dB.

8. The telecommunications link of claim 7, wherein the Q of said link is 10 greater than or equal to 8.5 dB.

9. The telecommunications link of claim 3, wherein the link is characterized by a bit rate of 2.5 Gb/s, a Q value, a sum of dispersion products, and an extinction ratio no greater than 9.0 dB and wherein the sum of dispersion 15 products is not less negative than -6000 ps/nm, and said transmitter, said receiver, and said optical waveguide fiber are such that the Q of said link is greater than or equal to 6 dB.

10. The telecommunications link of claim 3, wherein the link is characterized 20 by a bit rate of 2.5 Gb/s, a Q value, a sum of dispersion products, and an extinction ratio no greater than 6.2 dB and wherein the sum of dispersion products is not less negative than -12000 ps/nm, and said transmitter, said receiver, and said optical waveguide fiber are such that the Q of said link is greater than or equal to 6 dB. 25

11. The telecommunications link of claim 1, wherein the link is characterized by a bit rate of 2.5 Gb/s, a sum of dispersion products not more negative than -5000 ps/nm, and the dispersion eye closure penalty of the link is in the range of about 0 to -2 dB. 30 WO 01/19003 PCTIUSOO/23314 15

12. The telecommunications link of any one of claims 1 through 11 wherein said transmitter includes a directly modulated, distributed feedback semiconductor laser. 5

13. The telecommunications links of claim 1 wherein the optical signal pulses provided by said transmitter originate in a continuous wave source of optical power, said continuous wave source being externally modulated. 10