Source: https://patents.google.com/patent/EP0147251B1
Timestamp: 2018-03-23 01:30:36
Document Index: 55629886

Matched Legal Cases: ['arts 91', 'art 91', 'art 92', 'art 93', 'art 94', 'art 93', 'art 93', 'art 94', 'art 94', 'art 93', 'art 94', 'art 93', 'art 94']

EP0147251B1 - Method and apparatus for measuring chromatic dispersion coefficient - Google Patents
EP0147251B1
EP0147251B1 EP19840401930 EP84401930A EP0147251B1 EP 0147251 B1 EP0147251 B1 EP 0147251B1 EP 19840401930 EP19840401930 EP 19840401930 EP 84401930 A EP84401930 A EP 84401930A EP 0147251 B1 EP0147251 B1 EP 0147251B1
EP19840401930
EP0147251A3 (en )
EP0147251A2 (en )
The present invention relates to a method and apparatus for measuring the chromatic dispersion coefficient, of particularly that of an optical fiber.
Advances in performance of optical fibers have led to the realization of actual long-distance fiber optical communication systems. A fiber optical communication system, particularly one using a single mode optical fiber for the optical transmission line, has the advantage of extremely small expansion of the pulse width during transmission of the pulses from the transmitter side to the receiver side. Accordingly, such a system is suitable for the transmission of high frequency data, i.e., large amounts of data. With a transmitter using a semiconductor laser as a light source, however, the emission spectrum distribution may be considerably wide and jitter of the pulses may occur. In such a case, with a transmission distance over 50 km, reliable data transmission at a high transmission speed of over 400 Mb/s cannot be ensured since both the expansion of the pulse width and variations in the optical pulses, caused by so-called "chromatic dispersion", reach non-negligible levels. That is, inter symbol interference and mode dispersion noise increase significantly.
"Chromatic dispersion" specifically refers to the dispersion of the optical pulse distribution at the receiver side and is caused by the inevitable differences in arrival times of various optical pulses at the receiver side in accordance with their respective wavelengths À1, A2, À3 and so on. For example, when a semiconductor laser is used as the light source, three or four wavelengths of light are generated therefrom.
As will be shown hereinafter, in one of the known systems based on the Raman effect, it is almost impossible to make a measurement of an optical fiber already laid; another system of this kind requires two light sources having different wavelengths, which is cumbersome. A known system using a frequency sweep method gives rather inaccurate measurements and another similar system involves the use of a very expensive measuring instrument.
The above object is attained by a measuring system using an optical transmitter, an optical spectrum analyzing part receiving an input optical signal branched from the optical transmitter, an optical receiver for connecting with, via an optical fiber to be measured, the optical transmitter, and a data processing part operative to produce a chromatic dispersion coefficient to be measured, the method comparing a calculated baseband characteristic, derived from data concerning the optical transmitter and the optical spectrum analyzing part with an actual baseband characteristic obtained in the optical receiver.
Fig. 3 is a graph of a light source emission spectrum distribution in terms of wavelength λ and amplitude a,;
Fig. 5 is a vector diagram of a receiving optical signal A expressed by using complex notation in amplitude;
Fig. 6A is a graph of a first example of both the measured baseband characteristic and calculated baseband characteristic;
Fig. 6B depicts a distribution of a light source spectrum which is used for obtaining the graph shown in Fig. 6A;
Fig. 7A is a graph of a second example of both the measured baseband characteristic and calculated baseband characteristic;
Fig. 7B depicts a distribution of a light source spectrum which is used for obtaining the graph shown in Fig. 7A;
Fig. 8 is a graph for schematically explaining a calculation algorithm of a least square approximation procedure;
Fig. 11A is a graph of a third example of both the measured baseband characteristic and calculated baseband characteristic;
Fig. 11 B depicts a distribution of a light source spectrum which is used for obtaining the graph shown in Fig. 11A;
Fig. 13A and 13B are flow charts of operations achieved in the chromatic dispersion coefficient measuring apparatus shown in Fig. 10.
Before describing the preferred embodiments, a discussion will be made of the prior art measuring system for reference purposes.
where L denotes a length in km of an optical fiber transmission line, Δλ a difference between wavelengths in mm and corresponding to the aforesaid differences in wavelengths, i.e., |λ1-λ2| |λ2-λ3|, and so on, and Δτ the difference in arrival times in ps at the receiver side between each two signals having the wavelengths. The unit of M is ps/km×nm. That is, M defines that an arrival time delay of Δτ (ps) per 1 km of the transmission line would be induced if the wavelength λ deviates by 1 nm.
Fig. 1 is a general view of an example of a prior art apparatus for measuring the chromatic dispersion coefficient. In Fig. 1, reference numeral 14 indicates an optical fiber to be measured for chromatic dispersion coefficient. At the optical input side of the optical fiber 14, a yttrium aluminum garnet (YAG) laser source 11, a short optical fiber 12, and a spectroscope 13 are located. At the optical output side thereof, a measuring apparatus 15 is located. Both the YAG laser source 11 and the optical fiber 12 form, as one body, a fiber Raman laser utilizing the Raman effect, where oscillations of various frequencies take place with the wavelengths of λ1, À2, λo, and so on. Then, lights having these various wavelengths are selected one by one by means of the spectroscope 13, so that an arrival time t1 for the light of λ1, an arrival time t2 for the light of h2, and so on are sequentially obtained by means of the measuring apparatus 15. Thus, Δτ and Δλ for defining the above recited equation (1) are actually measured, and, then the chromatic dispersion coefficient M can be obtained with the known value of L.
The measuring system of Fig. 1 has the serious shortcoming that it is almost impossible to measure the coefficient when the optical fiber 14 is actually laid. First, it is essential to maintain synchronization between the sending time of the light from the spectroscope 13 and the measuring time for the sent light at the measuring apparatus 15. Accordingly, both ends of the optical fiber 14 must be located close to each other, preferably at the same site. This means the measurement is only effective for inspections of the manufactured optical fiber before shipment from the factory. Second, the fiber Raman laser system (11, 12, 13) is too heavy for operators to carry.
Because the coefficient M measured in a pre-shipment inspection differs from that measured in the field, it is meaningless to find the coefficient M in the factory. The difference between the coefficient M measured in the factory and the field results from the difference in stresses applied to the optical fiber laid in the factory and installed the field, the difference in the arrangements of the optical fiber laid in the factory and installed the field.
In another prior art measuring system based on the Raman effect, two light source having different wavelengths are used and the difference in arrival times between two optical signals therefrom is obtained by measuring the differences in phase therebetween. It is clear that a single light source is preferable for the measurement in view of cost and easy setting of the measuring system.
Another prior art measuring apparatus utilizes the frequency sweep method. There are two types of this apparatus. The first type assumes that the light source spectrum exhibits a so-called Gaussian distribution. In the system, the following well known equation is used for deriving M:
where, f6dB denotes a modulation frequency at which the value of the baseband characteristic is reduced by 6 dB, and Δλ1/2 denotes a wavelength pitch in the light source spectrum with which an amplitude of light is halved with respect to the amplitude observed at the center of the Gaussian distribution, i.e., a half width of the spectrum.
The second type of apparatus takes note of the fact that modulation frequencies at which the attenuation reaches almost 0 dB occur periodically, and derives M by the following equation:
where fT denotes a frequency at which the attenuation reaches almost 0 dB again in the high frequency range, and δλ an oscillation mode interval (corresponding to difference in wavelength between two adjacent line spectrums) with which the line spectrums appear periodically.
Fig. 2 is a general view of an example of an apparatus for measuring the chromatic dispersion coefficient according to the present invention. In Fig. 2, an oscillator 21 and a selective level meter 24 are connected, via an electro-optic converter (E/0) 22 and an opto-electric converter (O/E) 23, to the ends of the optical fiber 14 to be measured. The oscillator 21 produces an AC signal, such as a sine wave signal, with a variable frequency f. The AC signal modulates the optical output in the E/O 22, which is made of, for example, a semiconductor laser.
Now, the term "baseband characteristic" is generally defined as an amplitude response characteristic of received light with respect to a variation of the modulation frequency f, which received light is measured at the optical output side of the optical fiber 14 when an optical signal, which is the transmitted light from the semiconductor laser and directly modulated in amplitude by the oscillator 21, is launched onto the optical input side of the optical fiber 14. To be more specific, if there is a certain optical fiber which has a vector Pin(t) as an input and a vector Pout(t) as an output thereof, these vectors can be expressed as follows:
where w denotes an angular frequency. Since the signal Pin(t) is input to the optical fiber with the angular frequency ω and then the signal Pout(t) is output therefrom, transfer function H(t) of the optical fiber can be expressed, in terms of the above equations (4) and (5), as follows:
Thus, the attenuation of the signal having the angular frequency w can be defined as follows:
Generally, the signal attenuation given from the above equation (7) is a function of the angular frequency w and is called the "baseband characteristic". That is, the baseband characteristic of the optical fiber is given by the following expression (8)
where ω' denotes an arbitrary angular frequency, but ω'≠0. The second term in the above expression represents the optical power loss.
According to the present invention, to suppress deterioration of linearity due to the wide variation in the optical power level at the O/E 23, an optical attenuator (ATT) 101 is introduced in the apparatus and level adjustment, i.e.,
is achieved thereby in advance. Therefore, the value of the second term in the above expression (9) finally becomes zero.
The relationship between a light source spectrum and the baseband characteristic will be detailed below. The term "light source spectrum" is defined as an optical spectrum of the output light from the E/O 22 of Fig. 2. Considering here an optical emission spectrum distribution of the light source and the chromatic dispersion characteristic, these exhibits graphs as shown in Figs. 3 and 4. Fig. 3 is a graph of a light source emission spectrum distribution in terms of wavelength λ and amplitude a,. Fig. 4 is a graph of a chromatic dispersion characteristic in terms of wavelength A and the chromatic dispersion characteristic. The light source emission spectrum distribution is expressed by ai(λi), where i=0, ±1, ±2 ... The chromatic dispersion coefficient m is approximated to be a constant (mo) in the range of the light source spectrum, because the expansion of the light source spectrum, in case of a laser diode (LD), is very narrow, e.g., several nm at most.
In Fig. 4, the solid line curve 41 exhibits an actually obtained characteristic. From an equation m=mo, the difference t, in time delay between an arrival time of a signal consisting of λ0 and an arrival time of a signal consisting of λ, both at a receiver side, is expressed as follows:
where Δλi is represented by λi-λ0. A signal having a spectrum intensity of the aforesaid a is modulated by the sine wave signal produced from the oscillator 21 at the E/O 22 and transmitted along the optical fiber 14. Then, the transmitted signal reaches the O/E 23 as a received optical signal A. The signal A can be expressed as follows by using the aforesaid angular frequency ω(=2πf):
where ti denotes a delay time of the signal in the i-th spectrum mode. From the above equation (11),
Fig. 5 is a vector diagram of a received optical signal A expressed by using complex notation regarding amplitude. Each vector shown in Fig. 5 is expressed with a signal of wavelength λ0 and amplitude ao, as a reference vector. In Fig. 5, the abscissa R represents a real component and the ordinate I an imaginary component.
Accordingly, the baseband characteristic of the above recited expression (8) is then expressed as follows:
This equation (13) provides a theoretically approximated function. On the other hand, the theoretically approximated function can be expressed as the following equation (14) in relation to the selective level meter 24:
As seen from equation (14), the amplitude is taken into consideration as a at every oscillation mode, therefore enabling highly accurate measurement never before obtained.
In equation (14), the parameters a,, f, Δλi, and L are all known values. The chromatic dispersion coefficient mo is unknown. Under these circumstances, on one hand, a measured baseband characteristic is obtained by plotting data produced from the selective level meter, while, in the other hand, a calculated baseband characteristic is obtained through computer simulation by inputting a variety of mo to equation (14) sequentially. Specifically, the calculated baseband characteristic an be obtained with the use of a processor 25 shown in Fig. 2. Next, a search is run for the calculated baseband characteristic with a profile most suited to a profile of the measured baseband characteristic. Then, a chromatic dispersion coefficient M to be finally obtained is derived from m, which specifies the thus searched calculated baseband characteristic.
Fig. 6A is a graph of a first example of the measured baseband characteristic and calculated baseband characteristic. Fig. 6B depicts a distribution of a light source spectrum used for obtaining the graph shown in Fig. 6A. Similarly, Fig. 7A is a graph of a second example of the measured baseband characteristic and calculated baseband characteristic. Fig. 7B depicts a distribution of a light source spectrum which is used for obtaining the graph shown in Fig. 7A.
In Fig. 6A (same for Fig. 7A), the abscissa represents a modulation frequency f (MHz) in logarithmic indication, and the ordinate an attenuation loss (Loss) in dB. The readings of the ordinate correspond to the readings of the selective level meter 24 shown in Fig. 2. In the graph, each small circle (o) indicates the measured data. A curve H represents a curve calculated in accordance with the above recited equation (14) while varying the value mo and corresponds to a curve most approximated to the measured data. Accordingly, the value mo which simulates the curve H, becomes the chromatic dispersion coefficient M to be finally obtained.
In Fig. 6B (same for Fig. 7B), the ordinate represents the amplitude a, and the abscissa the wavelength À. Regarding the wavelength, each graduation defines 1 nm. Therefore, the Δλ in Fig. 6B equals about 0.73x(i-1) nm and, in Fig. 7B, about 1.43x(i-1) nm.
Computer simulation is most effective for searching for a desired curve H having a profile closest to a profile defined by the measured data displayed in Figs. 6A and 7A. Concretely speaking, the closest curve can be found by, for example, a least square approximation method. That is, a certain value m is searched which will make the value of an appreciation function AF minimum. The appreciation function AF is defined as follows:
where Fk denotes a baseband value (corresponding to the value of Loss in Figs. 6A and 7A) measured at the modulation frequency fk, N is a number of measuring points, and H(fk) denotes a calculated value, at that frequency fk, according to the theoretically approximated function by equation (14). Incidentally, it should be understood that no expensive measuring unit is needed, because the frequency fk is about 1000 MHz (1 GHz) at most.
Fig. 8 is a graph for schematically explaining the calculation algorithm of a least square approximation procedure. In Fig. 8, the abscissa represents the chromatic dispersion coefficient m, while the ordinate represents the value of the appreciation function AF given by equation (15). The unit amount for varying the chromatic dispersion coefficient is indicated as Am. The procedure in Fig. 8 is basically as follows.
(1) If the value of AF under the present value of m is smaller than the value of AF under the preceding value of m, the related calculation is continued under the following value of m; m-m+Am.
(2) If the value of AF under the present value of m is larger than the value of AF under the preceding value of m, the related calculation is continued under the following value of m; m→m-Δm', where Am' is reduced to Am/N, where N is an arbitrary coefficient preferably selected to be a value on the order of 2 through 10.
(3) If Δm/m<10-3 stands, it is considered that the related calculation has converged.
Fig. 9 is a general block diagram of a practical apparatus for measuring the chromatic dispersion coefficient according to the present invention. The construction of Fig. 2 can actually, as well as practically, be built as shown by Fig. 9. In Fig. 9, an apparatus 90 for measuring the chromatic dispersion coefficient is classified into four parts 91, 92, 93, and 94. The first part 91 is an optical transmitter in which an amplitude-modulation optical signal modulated by an AC signal of frequency f is generated. The amplitude-modulation optical signal being launched onto one end of the optical fiber 14 whose chromatic dispersion coefficient M is to be measured. The second part 92 is an optical receiver for detecting the baseband characteristic of an optical signal radiated from the other end of the optical fiber 14. The third part 93 is an optical spectrum analyzing part in which spectral decomposition is performed with respect to the optical signal supplied from the optical transmitter 91 and then analysis of an optical spectrum is made for the spectrally decomposed optical signal. The fourth part 94 is a data processing part for calculating the chromatic dispersion coefficient M in cooperation with the optical transmitter 91, the optical receiver 92, and the optical spectrum analyzing part 93.
As seen from Fig. 9, the optical transmitter 91 contains therein the oscillator 21 and the electro-optic converter (E/0) 22 driven by the oscillator 21, which generates the AC signal of variable frequency f, the value of which is used as a first data D1. The optical receiver 92 contains therein the opto-electric converter (O/E) 23 for transducing the received optical signal into an electric signal and the selective level meter 24 which is connected to the opto-electric converter (O/E) 23 and detects, from the electric signal, an amplitude level at each frequency f, the vlaue of which amplitude is used as a second data D2. The optical spectrum analyzing part 93 contains therein the spectroscope 13 for achieving spectral decomposition with respect to the optical signal produced from the optical transmitter 91 and an optical detector 95 which interlocks with the spectroscope 13 to detect both the wavelength and the amplitude, at each oscillation mode, which are used as a third data D3. The data processing part 94 receives the first, second, and third data D1, D2, and D3 and executes an arithmetic operation therewith so as to calculate the chromatic dispersion coefficient M. The above-mentioned spectroscope 13 functions to extract optical output, at each oscillation mode, corresponding to À-2, A-11 λ0 ... in Fig. 3 or to each line spectrum shown in Figs. 6B and 7B. The above-mentioned optical detector 95 functions to detect, at each oscillation mode, the wavelength (ÀI) and the amplitude level (a). Further, the above-mentioned data processing part 94 contains therein the processor 25 (also shown in Fig. 2), a console for control, printers (both not shown), and so on.
Fig. 10 is a general block diagram of an actually built apparatus based on the apparatus 90 shown in Fig. 9. In the optical transmitter 91, the oscillator 21 is a product of Ando Electric Co., Ltd., referenced as "GET-42P", and the electro-optic converter 22 is an improved version of a product of Ando Electric Co., Ltd., referenced as AQ-1309. In the optical receiver 92, the optical attenuator (ATT) 101 is a product of Fujitsu Ltd., referenced as "H72M-2016-M001 (variable optical attenuator), an amplifier (AMP) 102 is a product of B & H Ltd., referenced as AC-3002H, and the selective level meter 24 is a product of Ando Co., Ltd., referenced as "SLM-42 SP". In the optical spectrum analyzing part 93, both the spectroscope 13 and the optical detector 95 shown in Fig. 9 are formed, as an integral structure, with a product of Ando Electric Co., Ltd., referenced as FSM-01 (No. 105), i.e., an optical spectrum analyzer. Both the optical isolator 103 and the optical switch 104 are formed, as an integral structure, with a product of Fujitsu Ltd., referenced as H74M-5208-J003, i.e., a magneto-optic application switch. In the data processing part 94, the processor 25 is a product of Epson Ltd., referenced as HC-20.
Fig. 11A is a graph displaying a third example of both a measured baseband characteristic and calculated baseband characteristic. Fig. 11 B depicts a distribution of a light source spectrum which is used for obtaining the graph shown in Fig. 11A. The data was measured not by a prototype measuring appartus, but by an actual measuring apparatus having the arrangement shown in Fig. 10. The optical fiber 14 measured was an unprecedentedly long 48 km and the measuring frequency was several MHz. Thus, the data obtained was substantially the same as that of a commercial fiber optical communication system. The chromatic dispersion coefficient M to be obtained was derived as 2.1, i.e., M=2.1 ps/km/nm, from the data of Figs. 11A and 11B. In Fig. 11A, the small circles (o) represent the measured baseband characteristic, while the solid line curve represents the calculated baseband characteristic.
Fig. 12 is a graph of an example representing the actual relationship between the chromatic dispersion coefficient M and the wavelength λ. The data of Fig. 12 was obtained from an optical fiber having a length of 35 km, a core diameter of 8 pm, a differential specific refraction index of 0.3%, and a cut-off wavelength of 1.28 pm. Although only four data points are plotted (small circles) in the graph of Fig. 12, the solid line curve is believed to accurately reflect the chromatic dispersion coefficient throughout the range of the wavelength.
Figs. 13A and 13B are flow charts of operations achieved in the chromatic dispersion coefficient measuring apparatus 90 shown in Fig. 10. At step a, optical spectrum analysis is achieved at the optical spectrum analyzing part 93, based on the previously recited equation (14) so that the desired third data D3 is obtained, that is, a,, f, and Δλi are detected. At step b, the actual baseband characteristic is measured so as to obtain the desired second data, i.e., fk and Fk defined by the previously recited equation (15). At step c, the unknown number mo is initially determined, which mo is defined by the previously recited equation (14). Thereafter, the unknown number mo is varied and m" m2, and m3 are sequentially selected so as to determine the calculated baseband characteristic (refer to the solid line curve in Fig. 11A) having a profile which is closest to the profile of the measured actual baseband characteristic. Then, the number m2 which specifies the thus determined baseband characteristic is determined to obtain the desired chromatic dispersion coefficient M, i.e., m2→M (refer to step p in Fig. 13B). It should be understood, however, that the procedure for searching M from m1, m2, and m3 is not limited to the manner as mentioned above with reference to the figures. For example, the calculated baseband characteristic closest to the actual baseband characteristic can also be obtained through selection from a plurality of calculated baseband characteristics determined corresponding to various provisional chromatic dispersion coefficients.
At step d, mi, m2, and m3 are defined to be m1=m0-Δm, m2=m0, and m3=m0+Δm, where Am corresponds to that shown in Fig. 8. At step e, the calculated baseband characteristic is created by the data processing part 94. The calculation is performed, according to equation (14), by substituting mo with m1, m2, and m3 sequentially. Thereby, in step f, the appreciation functions AF,, AF2, and AF3 are derived with respect to m1, m2, and m3, respectively.
Thereafter, in accordance with the relative sizes of AF,, AF2, and AF3 (refer to steps g and h in Fig. 13B), the values of m, AF3 and AF2 are changed and a new value for AF1 is derived (refer to steps i, j, and k), or similarly a new value is derived for AF3 (refer to steps I, m, and n). Step o starts when AF2≤AF1, AF3 is obtained, wherein if the accuracy is not sufficient, i.e., |Δm/m|>10-3 is obtained, then, in step q, mo and Am are renewed. Then, the operation returns to step d in Fig. 13A. In step q, mo is determined by -b/2a and Am is shifted in value by Δm/10. The parameters a and b of the term -b/2a are identical to the coefficients a and b used in a second order curve, i.e., am2+bm+c, which corresponds to the solid line curve shown in Fig. 8. The coefficient c is not used here for the related calculation.
The above coefficients a and b are given by the following equations (16) and (17), respectively, pursuant to equation (15):
If |m/m|≤10-3 stands at step o, the value M is finally fixed to m2 at step p.
As explained above in detail, in the present invention, first, the calculated baseband characteristic is obtained by means of the data processing part with data concerning the optical signal to be launched onto the optical fiber. Second, the actual baseband characteristic is obtained through the measurement of the receiving optical signal at the optical receiver. Third, the chromatic dispersion coefficient is determined from both the calculated and actual baseband characteristics. The above measurement is clearly different from the prior art measurement in which differences in arrival times among optical signals of different wavelengths are detected. Thus, the present invention can eliminate the troublesome operation to assure synchronization between the optical transmitter and the optical receiver. In addition, the present invention can measure an actually laid optical fiber.
Furthermore, in another prior art, two light sources having different wavelengths are used. The difference in arrival times between two optical signals therefrom is obtained by measuring the differences in phase therebetween. In the present invention, only one light source, such as a semiconductor laser, is enough to obtain the chromatic dispersion coefficient M. Thus, in view of the above, the present invention can be put into practical use easily and at a low cost.
(a) measuring an actual baseband characteristic of an optical fiber in accordance with a received optical signal level at an output end of the optical fiber, the received optical signal being given from an input optical signal launched onto an input end of the optical fiber, the input optical signal being modulated in amplitude with AC signals having a plurality of modulation frequencies;
(b) obtaining a calculated baseband characteristic having a profile closest to a profile of the actual baseband characteristic measured, by using data concerning wavelengths and amplitudes at each oscillation mode created in an optical spectrum of the input optical signal, data concerning the modulation frequencies, and data concerning a provisional chromatic dispersion coefficient; and
(c) setting the provisional chromatic dispersion coefficient corresponding to the calculated baseband characteristic as the actual chromatic dispersion coefficient of the optical fiber.
2. A method as set forth in claim 1, wherein, in step (b), the calculated baseband characteristic closest to the actual baseband characteristic is obtained through a comparison of these two characteristics while sequentially varying the value of the provisional chromatic dispersion coefficient.
3. A method as set forth in claim 1, wherein, in step (b), the calculated baseband characteristic closest to the actual baseband characteristic is obtained through selection from a plurality of calculated baseband characteristics each having a corresponding provisional chromatic dispersion coefficient.
4. A method as set forth in claim 2, wherein the calculated baseband characteristic is defined by a theoretical approximation function H(f), that is:
where a denotes an amplitude of an i-th oscillation mode among n oscillation modes created in the optical spectrum of said input optical signal, f said modulation frequency, Δλi a difference between the wavelength at the i-th oscillation mode and the wavelength at a first oscillation mode, m said provisional chromatic dispersion coefficient, and L a length of the optical fiber to be measured.
5. A method as set forth in claim 3, wherein the calculated baseband characteristic is defined by a theoretical approximation function H(f), that is:
where a denotes an amplitude of an i-th oscillation mode among n oscillation modes created in the optical spectrum of said input optical signal, f said modulation frequency, Δλ a difference between the wavelength at the i-th oscillation mode and the wavelength at a first oscillation mode, m said provisional chromatic dispersion coefficient, and L a length of the optical fiber to be measured.
6. A method as set forth in claim 4, wherein the chromatic dispersion coefficient is derived through computer simulation with the aid of a processor, the processor collecting and storing therein at least two data sets, in advance, a first data set available for specifying the theoretical approximation function and a second data set available for specifying the actual baseband characteristic measured.
7. A method as set forth in claim 5, wherein the chromatic dispersion coefficient is derived through computer simulation with the aid of a processor, the processor collecting and storing therein at least two data sets, in advance, a first data set available for specifying the theoretical approximation function and second data set available for specifying the actual baseband characteristic measured.
8. A method as set forth in claim 6, wherein said computer simulation is performed under a least square approximation method by employing an appreciation function AF, that is:
where Fk denotes the value of the actual baseband characteristic measured at the modulation frequency fk, and N a number of measuring points.
9. A method as set forth in claim 7, wherein said computer simulation is performed under a least square approximation method by employing an appreciation function AF, that is:
10. A method as set forth in claim 1, wherein the data concerning wavelengths and amplitudes at each oscillation mode created in the optical spectrum of the input optical signal are obtained with the aid of an optical spectrum analyzer.
means for measuring an actual baseband characteristic of an optical fiber in accordance with received optical signal level at an output end of the optical fiber, the received optical signal being given from an input optical signal launched onto an input end of the optical fiber, the input optical signal being modulated in amplitude with AC signals having a plurality of modulation frequencies;
means for obtaining a calculated baseband characteristic having a profile closest to a profile of the actual baseband characteristic measured, by using data concerning wavelengths and amplitudes at each oscillation mode created in an optical spectrum of the input optical signal, data concerning the modulation frequencies, and data concerning a provisional chromatic dispersion coefficient; and
means for setting the provisional chromatic dispersion coefficient corresponding to the calculated baseband characteristic as the actual chromatic dispersion coefficient of the optical fiber.
12. An apparatus as set forth in claim 11, wherein said means are comprised, as a whole, of the following:
an optical transmitter operative to modulate an optical signal with AC signals having different modulation frequencies and to launch the thus modulated optical signal, as an input optical signal, onto one end of an optical fiber to be measured;
an optical receiver operative to detect a level of an output optical signal given from another end of the optical fiber at each of the modulation frequencies so as to measure an actual base baseband characteristic of the optical fiber;
an optic spectrum analyzing part which functions to detect the wavelengths and amplitudes at each oscillation mode created in an optical spectrum of the input optical signal; and
a data processing part which functions to derive a calculated baseband characteristic closest to the actual baseband characteristic measured in the optical receiver, by using data concerning the modulation frequency, the wavelength, and the amplitude produced from the optic spectrum analyzer, and a provisional chromatic dispersion coefficient and to set the provisional chromatic dispersion coefficient corresponding to the calculated baseband characteristic as the actual chromatic dispersion coefficient of the optical fiber.
13. An apparatus as set forth in claim 12, wherein said optical transmitter contains therein both an oscillator for producing the variable frequency AC signal and an electro-optic converter driven by the oscillator.
14. An apparatus as set forth in claim 13, wherein said electro-optic converter is provided with, at its output stage, an optical isolator.
15. An apparatus as set forth in claim 14, wherein an optical switch is located between said optical isolator and one end of the optical fiber, the optical switch operative to selectively supply the input optical signal to a spectroscope comprising said optical spectrum analyzing part or the optical fiber.
16. An apparatus as set forth in claim 12, wherein said optical receiver contains therein both an opto-electric converter for transducing the receiving optical signal from the optical fiber to the corresponding electric signal and a selective level meter which is supplied with said electric signal so as to detect the level of the output optical signal at each said modulation frequency.
17. An apparatus as set forth in claim 16, wherein said opto-electric converter is provided with, at its input stage, an optical attenuator.
18. An apparatus as set forth in claim 12, wherein said optical spectrum analyzing part contains therein both a spectroscope in which spectral decomposition is performed with respect to said input optical signal from said optical transmitter and an optical detector which interlocks with the spectroscope to detect both the wavelength and amplitude at each said oscillation mode.
19. An apparatus as set forth in claim 18, wherein both said spectroscope and said optical detector are formed, as an integral structure, to provide an optical spectrum analyzer.
EP19840401930 1983-10-31 1984-09-27 Method and apparatus for measuring chromatic dispersion coefficient Expired EP0147251B1 (en)
JP202706/83 1983-10-31
EP0147251A2 true EP0147251A2 (en) 1985-07-03
EP0147251A3 true EP0147251A3 (en) 1987-04-15
EP0147251B1 true EP0147251B1 (en) 1989-12-13
EP19840401930 Expired EP0147251B1 (en) 1983-10-31 1984-09-27 Method and apparatus for measuring chromatic dispersion coefficient
US6118523A (en) 2000-09-12 High-resolution zero-dispersion wavelength mapping in single mode fiber
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