JPH03171036A - Optical fiber communicating method and system - Google Patents

Abstract

PURPOSE:To allow the transmission of a light signal up to a long distance by applying at least one time of proper frequency chirping to signal light pulses at the intermediate points of transmission paths. CONSTITUTION:An optical signal transmission section 1 which forms the signal light pulses, optical fibers 21 to 23 which transmit the signal light pulses outputted from the optical signal transmission section 1 and at least one piece of optical amplifiers 31, 32 which amplify the signal light pulses disposed on the way of the optical fiber transmission paths. Further, at least one piece of optical phase modulators 61 which are disposed on the way of the optical fiber transmission paths and apply the proper quantity of the frequency chirping to the signal light pulses, a driving circuit 62 which impresses electric pulses in synchronization with the signal light pulses, and an optical signal receiving on section 5 which receives the transmitted signal light pulses are provided. The transmission distance is increased in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical fiber communication method using an optical fiber as a transmission medium and an optical fiber communication system for carrying out the method. (Prior art) In optical fiber communication systems currently in practical use, a photodetector converts a signal light pulse attenuated by optical fiber propagation into an electrical signal, amplifies this electrical signal, and then converts the amplified electrical signal into an electrical signal. Long-distance transmission is achieved by so-called regenerative relaying, in which a semiconductor laser is driven by a signal and regeneration is performed at the level of the signal light [for example, IE, Journal of Lightwave Technology ( IEEE Journal of
Lightwave Technology), Volume 3 (1985), 1005 Baseco. This method has the disadvantage that it can only support bit rates determined by electrical circuits because the relay process involves optical-to-electrical-to-optical conversion.
On the other hand, recently, research has been actively conducted on optical amplification and relay communication, in which signal light pulses are amplified and relayed in the optical state using optical amplifiers and transmitted.
Volume 8 (1989), pages 282-290. In this type of optical amplification relay communication, the bit rate can be easily changed because there is no optical-to-electrical-to-optical conversion described above. It also has the advantage of being able to flexibly respond to future technological innovations such as bidirectional optical fiber communications and wavelength multiplexed optical fiber communications. Here, as an optical amplifier, so far, 1) those using a semiconductor laser medium, 2) those using an optical fiber doped with rare earth elements such as Er in the core, and 2) those using stimulated Raman scattering, stimulated Brillouin scattering, etc. of the optical fiber. Some methods using nonlinear optical effects have been reported. (Problem to be Solved by the Invention) The maximum transmission distance that can be achieved in optical fiber communication using conventional optical amplification repeaters is due to deterioration of the signal-to-noise ratio (SN ratio) due to accumulation of optical noise or pulse broadening due to optical fiber dispersion. Mainly limited. Of these factors,
Regarding the deterioration of the S/N ratio, it is not a particularly big problem for transmission up to several thousand kilometers. On the other hand, pulse broadening due to optical fiber dispersion increases rapidly in proportion to the square of the bit rate, and becomes a major limiting factor for transmission distance at bit rates in the Gb/s band. For example, a typical single mode optical fiber has a dispersion of about 20 Ps/nm/km at a wavelength of around 1.55 μm. Therefore, if we consider the case where signal light in this wavelength range is transmitted using a single mode optical fiber, the maximum transmission distance is bit rate} 2
Approximately 1000km in Gb/s, 10Gb/s
It will only be about 40km. Recently, as a method to alleviate the limitation on transmission distance due to the above-mentioned pulse spread,
An optical soliton that utilizes self-phase modulation of an optical fiber has been demonstrated at the laboratory level [for example, Optics Letters]
) magazine, Volume 13 (1988), page 675, and Electronics Letters (Electroni
csLetters), Volume 25 (1989, 1
99 pages]. However, in this method using optical solitons, the signal light pulse not only requires a high peak power of several tens of milliwatts to about IW, but also requires strict control of the optical power to generate optical solitons. The disadvantage is that it does not. Furthermore, when such high-power signal light pulses are amplified, the optical amplifier cannot obtain sufficient gain due to gain saturation, etc., and as a result, the optical amplification repeater interval becomes short. As described above, conventional optical fiber communication systems have issues that need to be resolved regarding transmission distance. The purpose of the present invention is to solve the problems remaining in the conventional optical fiber communication system using optical amplification and repeating as described above, and to create an optical fiber that can transmit optical signals over a much longer distance than before. The objective is to provide a communication method and a system for implementing it. (Means for Solving the Problems) The optical fiber communication method of the present invention is an optical fiber communication method that uses an optical fiber as a transmission medium, amplifies and relays the light as it is in the middle of the transmission path, and transmits signal light pulses. The method is characterized in that an appropriate amount of frequency chirping is applied to the signal light pulse at least once in the middle of the transmission path. The optical fiber communication system of the present invention includes: an optical transmitter that generates a signal light pulse; an optical fiber that transmits the signal light pulse output from the optical transmitter; at least one optical amplifier that amplifies the optical pulse; at least one optical phase modulator disposed on the optical fiber transmission line that imparts an appropriate amount of frequency chirping to the signal optical pulse; and the optical phase modulator It is characterized by comprising a drive circuit that applies an electric pulse in synchronization with the signal light pulse, and an optical receiver that receives the signal light pulse transmitted through the optical fiber. (Function) In a normal single mode optical fiber, there is a zero dispersion wavelength near the wavelength of 1.3 μm where the material dispersion and the fiber separation are balanced.
At shorter wavelengths, it takes a positive dispersion value (normal dispersion), and at longer wavelengths it takes a negative dispersion value (abnormal dispersion). In the wavelength region of normal dispersion, long wavelength light propagates farther through the optical fiber than short wavelength light, and conversely, short wavelength light propagates slower through the optical fiber than long wavelength light. Therefore, if an optical pulse is propagated through an optical fiber by applying linear frequency chirping (frequency change with a constant variation over time) whose sign is different from the dispersion value, the pulse width of this optical pulse will change at a certain point in the optical fiber. It exhibits a behavior of narrowing until then and then widening. Therefore, by appropriately selecting the optical fiber length and frequency chirping, it is possible to equalize the width of the input optical pulse and the output optical pulse. The present invention focuses on the optical fiber propagation characteristics of optical pulses having such linear frequency chirping, and provides an appropriate amount of frequency chirping to the signal optical pulse to compensate for pulse broadening due to wavelength dispersion of the optical fiber. It is something that is transmitted. As a result, in the present invention, the bit rate is several Gb
/s or more, the transmission FI@ distance is several 1000K
The advantage is that it is possible to construct an optical fiber communication system that reaches m. In addition, unlike conventional methods using optical solitons, the method of the present invention does not require strict control of signal light power, and by changing the sign of frequency chirping, it is possible to control whether normal dispersion or anomalous dispersion occurs. It has the advantage of suppressing pulse broadening even in the wavelength range. Here, according to the analysis of the propagation characteristics of optical fiber, the length is L
(km), and the dispersion value is D (ps/nm/km
), in the case of a single mode optical fiber with 1/(
DL) By adding linear frequency chirping of nm/ps, the input and output waveforms can be made almost the same. In addition, when frequency chirping is generated using an optical phase modulator as in the optical fiber communication system of the present invention, the optical frequency generally changes in proportion to the differential of the driving voltage, so the optical phase modulator has a parabolic curve. It is most preferable to apply a shaped voltage pulse to add linear frequency chirping. (Example) Next, the optical fiber communication method of the present invention and the system for implementing the same will be explained in more detail with reference to the drawings. In the following, while explaining the elliptical structure of optical fiber communication systems, we will also explain optical fiber communication methods. 1st
The figure is a configuration diagram of a first embodiment of an optical fiber communication system according to the present invention. In this figure, 1 is an optical transmitter;
6, 41.42 is a frequency chirping imparting section including an optical phase modulator and its driving circuit, 21, 22.23 is an optical fiber serving as a transmission path, 31.32 is an Er-doped optical fiber amplifier, and 5 is an optical receiving section. It is. Here, since 31 and 32 and 41 and 42 are the same, the details of the S configuration are omitted. In this embodiment, one semiconductor laser l1 has a wavelength of 1.536+
The czm InGaAsP/I nP distributed feedback semiconductor laser and the optical intensity modulator 12 have a modulation frequency band of approximately 10G.
Hz traveling wave LiNbO, Matsuha Zehnder optical modulator,
Optical fibers 21, 22, and 23 are all 100 km long.
, a single mode serial fiber with a loss of 0.2 dB/km at a wavelength of 1.536 μm and a dispersion value of -16 ps/nm/km (anomalous dispersion). traveling wave type LiNbO,
The optical phase modulator and excitation light source 314 have a maximum output of 100
■nGaAsP/I with a wavelength of 1.48 μm that can obtain mW
nP fabribe type semiconductor laser, optical multiplexer/demultiplexer 312,
313 is excitation light with a wavelength of 1.48 μm and a wavelength of 1.536
Single mode optical fiber coupler for wavelength multiplexing capable of multiplexing and demultiplexing with μm signal light, Er-doped optical fiber 31
1 has a core diameter of 7 μm, a length of 20 m, and an Er concentration of 300 ppm.
The optical splitter 412 is a single mode optical fiber coupler with a branching ratio of 20:1 at a wavelength of 1.536 μm, the photodetector 413 is an InGaAs photodiode, and the signal light receiver 51 is an Er-doped single mode optical fiber. This is an InGaAs avalanche photodiode that can respond up to 10GHz or higher. In the optical fiber communication system of this example,
First, the signal light output from the InGaAsP/InP distributed feedback semiconductor laser 11 is inputted from the information signal input terminal 10 and amplified by the amplifier circuit 13 at 5 Gb/s.
, L i N b to which a voltage pulse of RZ code is applied
Os is modulated by a Mach-Zehnder optical modulator 12. This 5 Gb/s signal light pulse is further subjected to frequency chirping by a traveling wave type LfNbO optical phase modulator 61, and then sent to a single mode serial fiber 21. The signal light pulse transmitted through this single mode silica fiber is combined with the pump light from the pump light R314 by the optical fiber coupler 312. The combined light output from the optical fiber coupler 312 is E
The light is input to the r-doped optical fiber 311, and the light is amplified by about 20 dB. The signal light pulse amplified in this Er-doped optical fiber is separated from the excitation light by an optical fiber coupler 313 and applied to an optical splitter 412. Optical splitter 4
12 inputs 95% of the power of the input signal light pulse to a traveling wave type LiNbO optical phase modulator 411. Here, the @ optical pulse is given frequency chirping again and sent to the next single mode optical fiber 22. On the other hand, approximately 5% of the power of the signal light pulse is received by the InGaAs photodiode 413, and the signal light pulse and the traveling wave type L
It is used to synchronize with the voltage pulse applied to the INbO3 optical phase modulator 411. Follow the same steps below,
The signal light pulse propagates through the Er-doped optical fiber amplifier 32, the frequency chirping section 42, and the single mode optical fiber 23, and is received by the InGaAs avalanche photodiode 51. The information signal is then reproduced by the demodulation circuit 52 and output from the information signal output terminal 50. Here, in the frequency chirping imparting sections 41 and 42,
The timing extraction circuit 414 generates a sine wave voltage synchronized with the signal light pulse. Then, in the drive circuit 415,
This sine wave voltage is half-wave rectified and amplified to drive the optical phase modulator 411 to give it a pseudo-parabolic shape. FIG. 2 shows the waveform of the signal light pulse input from the optical splitter 412 to the optical phase modulator 411, its frequency and the waveform of the drive voltage applied from the drive circuit 415, and the signal output from the optical phase modulator 411. This shows the frequency of the optical pulse. In this example, the dispersion of a single mode optical fiber with a length of 100 km is −1 6 0 0 p
s/nm, and the frequency chirping amount is 6 × 10-'nm/p so that the input and output waveforms are equal.
s [=0. 1 1 GHz/ps]. At this time, traveling wave type LiNbO, optical phase modulator 411
The required driving voltage was approximately 18V. FIG. 3 shows the dependence of the power and pulse width of the signal light pulse on the transmission distance in the optical fiber communication system of the above embodiment. In this example, the transmitted optical power is OdBm, and it is attenuated at a rate of 0.2 dB/km when propagating through a single mode optical fiber, but the optical power is amplified by about 20 dB at every 100 km interval, so the received optical power after transmission is is about -22d
It is Bm. In addition, since frequency chirping is applied to the pulse width, the behavior is compressed up to the midpoint of each 100 km long optical fiber and then widened, but the pulse width at the time of reception is 100 ps.
It is kept the same as when it was sent. Due to the characteristics described above, in this example, a 5 Gb/s signal can be
It was possible to transmit data over a distance of 0 km with an error rate of IQ-12 or less. On the other hand, for comparison, when the frequency chirping imparting sections 6, 41, and 42 were removed as in the conventional case, no transmission was possible due to intersymbol interference due to pulse broadening. FIG. 4 shows a second optical fiber communication system according to the present invention.
FIG. The difference from the first actual example shown in FIG. 1 is that the information signal input terminal 10 (not shown, but is a terminal provided in the optical transmitter 1 as in FIG. 1) is IOGb/s, RZ code voltage pulses are applied, and the number of frequency chirping imparting parts is increased. In this figure, optical fibers 71, 72, 73, 74, 75, and 76, which are transmission paths, are all single-mode silica fibers of the same type as in the first embodiment, but their length is 50 km. Other configurations are
Since this is the same as in Figure 1, the same elements are given the same numbers. As seen in FIG. 4, in this example, frequency chirping imparting sections are provided at five locations, which is more than in the example shown in FIG. This is to reduce the voltage. In other words, the drive voltage required to provide frequency chirping that makes the input and output waveforms equal increases in proportion to the square of the bit rate, so by halving the fiber length, the amount of dispersion in the optical fiber can be reduced. This reduces the increase in drive voltage. 10 Gb in this example
/s, the required driving voltage of a traveling wave LiNbOs optical phase modulator to make the input and output waveforms of a 50km long single mode optical fiber equal is approximately 36V. there were. FIG. 5 shows the transmission distance dependence of the power and pulse width of the signal light pulse in the second embodiment shown in FIG. It was possible to maintain the same 50 pS and transmit a 10 Gb/s signal over 300 km with an error rate of 10-'' or less.The above is the implementation of the optical fiber communication method and system according to the present invention. Although the present invention has been described using an example, the present invention is not limited to this embodiment, and several modifications can be made.For example, an Er-doped optical fiber amplifier was used as an optical amplifier in the embodiment, but in the present invention, an Er-doped optical fiber amplifier was used. It may be an amplifier that uses a semiconductor laser medium or an amplifier that uses the nonlinear optical effect of an optical fiber.Furthermore, the optical phase modulator may be an element that uses various semiconductor materials other than LiNbo, and basically a nearly linear frequency charger. Any means may be used as long as the chirping can be added to the signal light pulse.Furthermore, the number of optical amplifiers and frequency chirping adding parts, placement locations, etc. should be changed as appropriate according to the system design; Needless to say, the invention is not limited to this. Also, the optical transmitter, optical fiber, optical receiver, etc. may be any element or element as long as it has the performance. (Effects of the Invention) As explained above, the present invention The optical fiber communication method and system of the invention focuses on the optical fiber propagation characteristics of optical pulses with linear frequency chirping, and imparts an appropriate amount of frequency chirping to the signal optical pulses to be optically amplified and relayed to improve the wavelength dispersion of the optical fiber. As a result, in the present invention, even when the bit rate is several Gb/s or more, the transmission distance is several thousand kilometers.
The advantage is that it is possible to construct an optical fiber communication system that reaches . In addition, unlike conventional methods using optical solitons, the method of the present invention does not require strict control of signal light power, and by changing the sign of frequency chirping, it is possible to control whether normal dispersion or anomalous dispersion occurs. It has the advantage of suppressing pulse broadening even in the wavelength range.

[Brief explanation of the drawing]

FIG. 1 shows a first diagram of an optical fiber communication system according to the present invention.
FIG. 2 is an elliptic diagram showing an example of the first example, and shows the waveform and frequency of the signal light pulse input to the optical phase modulator 411, the waveform of the driving voltage, and the optical phase modulator 411 in the first example. FIG. 3 is a diagram showing the transmission distance dependence of the power and pulse width of the signal light pulse in the first embodiment, and FIG. 4 is a diagram showing the transmission distance dependence of the signal light pulse in the first embodiment. FIG. 5 is a block diagram showing a second embodiment of an optical fiber communication system according to the authors, and is a diagram showing the dependence of the power and pulse width of the signal light pulse on the transmission distance in the second embodiment. DESCRIPTION OF SYMBOLS 1... Optical transmitter, 10... Information signal input terminal, 11
... Semiconductor laser, 12... Light intensity modulator, 13.
...Amplification circuit, 21,22.23...Optical fiber, 3
1.32... Optical amplifier, 311... Er-doped optical fiber, 312.313... Optical multiplexer/demultiplexer, 314...
Excitation light source, 41, 42, 43, 44, 45.6... Frequency chirping imparting unit, 411.61... Optical phase modulator, 412... Optical splitter, 413... Photodetector,
414...timing extraction circuit, 415.62...
Drive circuit, 5... Optical receiver, 50... Information signal output terminal, 51... Signal optical receiver, 52... Demodulation circuit,
71, 712, 73, 74, 75, 76... optical fiber.

Claims (2)

[Claims] (1) In an optical fiber communication method in which an optical fiber transmission medium is used and a signal light pulse is transmitted by amplifying and relaying the light as it is in the middle of the transmission path, an appropriate amount of frequency is added to the signal light pulse at least once in the middle of the transmission path. An optical fiber communication method characterized by imparting chirping. (2) an optical transmitter that generates a signal light pulse; an optical fiber that transmits the signal light pulse output from the optical transmitter; and at least one disposed on the optical fiber transmission path that amplifies the signal light pulse. one optical amplifier; at least one optical phase modulator disposed on the optical fiber transmission line for imparting an appropriate amount of frequency chirping to the signal light pulse; and the optical phase modulator synchronized with the signal light pulse. What is claimed is: 1. An optical fiber communication system comprising: a drive circuit that applies electrical pulses to the optical fiber; and an optical receiver that receives signal light pulses transmitted through the optical fiber.