- Email: [email protected]
Measurements on high-bandwidth optical waveguides
Volume 25, number 1
MEASUREMENTS
April 1978
OPTICS COMMUNICATIONS
ON HIGH-BANDWIDTH
OPTICAL WAVEGUIDES
D.B. KECK Corning Glass Works, Corning, N. Y., USA
and R. BOUILLlE Centre National d’Etudes des Telecommunications,
Lannion, France
Received 4 November 1977 Revised manuscript received 29 December 1977 Frequency response measurements on six high-bandwidth optical waveguides made by the doped deposited silica process are reported. One of these had an extrapolated bandwidth in excess of 3 GHz-km. From plane wave measurements the actual or optimum waveguide profile may be determined given the other. Data taken on a germanium borosilicate waveguide is in agreement with interference measurements.
1. Introduction Because the low loss of present optical waveguides will allow large repeater spacings, many optical waveguide systems being contemplated will push to the limit present graded-index waveguide bandwidth manufacturing capability. The bandwidth limitation is primarily imposed by the quality of the radial index gradient and its ability to reduce propagation delay differences (intermodal dispersion) between the transmitted modes. Estimates of the maximum bandwidth which should be achievable have been made [ 1,2] . It is most important, however, to have existence proofs at various stages in the technological evolution that in fact these are practically achievable. In this paper several waveguides are described which address this question, one of which has an optical power -3 dB bandwidth in excess of 3 GHz over a 1-km length. This is covered in section 2. To enable progress toward routinely achieving these higher bandwidths, deeper quantitative understanding about the operation of waveguide must be obtained. This includes things such as information on the differential mode attenuation [3] and how it affects bandwidth, information on the optimum profile [4] and how it is affected by material
composition and processing [5,6] and information on the actual waveguide profile including perturbations, both azimuthal and axial. Plane wave pulse measurements made on one of these high-bandwidth waveguides gives an additional source of information for the last two and is covered in section 3.
2. Waveguide bandwidth
measurements
Six graded-index optical waveguides prepared by Corning Glass Works were measured at the French Centre National d’Etudes des Telecommunications (CNET). All guides were prepared by the “outside vapor phase oxidation” (OVPO) doped deposited silica process [7] and consist of a germanium-borosilicate radially graded core with a borosilicate cladding. The bandwidth measurement is done by injecting short light pulses into the waveguide and directly recording the output frequency response with a spectrum analyzer [8] . As shown in fig. 1, light from an experimental Standard Telecommunications Laboratory gallium aluminum arsenide laser diode emitting at -850 nm is launched into a - IO-meter section of waveguide. A pulse generator with about a 30-MHz repetition rate 43
Volume 25, number 1
OPTICS COMMUNICATIONS
April 1978 FREQUENCY (GHr)
PULSE GENERATOR
SPECTRUM ANALYZER WAVEGUIDE WAVEGUIDE
El
(a)
COMPUTER
Fig. 1. Schematic of the bandwidth measurement apparatus used at CNET. A mode coupling waveguide is used to more uniformly excite the waveguide under test.
is used to drive the laser diode whose output is regulated by feedback circuitry. The source spectral bandwidth is about 0.4 run. The output from the IO-meter waveguide section is butt-coupled to the test waveguide by use of a micropositioner . Mode coupling in the lometer section is used to more uniformly excite the mode spectrum of the test waveguide, thus the measurement realistically corresponds to long distance system behavior. The transmitted light is detected using a Radio Technic Compelec silicon-pin diode and fed directly to a spectrum analyzer. The use of a pin diode was essential for measurement of these high-bandwidth waveguides. The spectrum analyzer is driven by a digital computer which sweeps the frequency spectrum and directly records the wave-guide frequency response from about 50 to 1700 MHz. This is shown plotted in figs. 2a and 2b for two of the highest-bandwidth waveguides tested. For both of these the -6 dB electrical power frequency (-3 dB optical power frequency) is greater than the measurement limit of the equipment. The computer is used to fit the amplitude data to a power law; dB (j) = AfB. This is an empirical function which matches the data quite well and provides an analytic form for the frequency response. The coefficient A is governed by the waveguide length, profile, and index difference. For a gaussian response the coefficient B = 2. Using the power law extrapolation of the measured data, -6 dB points of 1800 and 3100 MHz are obtained for these two fibers. Of the remaining four guides, all had -6 dB bandwidths greater than 1000 44
-6
L
\ FREQUENCY
0 01
B 0
01
(GHzl IO
-I-
(b) -5-
-6-
Fig. 2. Electrical response (dB) as a function of frequency of two Corning OVPO optical waveguides as measured at CNET. The guides are each 1 km in length. The solid curve is the measured response while the dotted curve is the best-fit response using a form dB = AfB. The -6 dB frequencies are (a) 3100 MHz and (b) 1800 MHz.
MHz/km, the smallest being 1240 MHz/km. It is noted that the narrow spectral bandwidth of the source used for these measurements should have a material dispersion limit in excess of 10 GHz-km. The zero frequency attenuation rate of all six of these waveguides is very low so that mode coupling and/or differential attenuation effects are not expected to play a significant role in generating these large bandwidths. Thus the measured values are indicative of how well the average radial index profile along the wave, guide matches the optimum profile [9]. For the highest bandwidth waveguide, the numerical aperture of -0.18 would predict a step profile bandwidth of ap-
Volume 25. number 1
April 1978
OPTICS COMMUNlCATIONS
proximately 17 MHz-km. Thus a profile pulse reduction factor of about 175 exists for this waveguide. The highest factors previously reported are less than 100. [ 10-121. For the pure o-class of profiles, the theoretical maximum pulse reduction factor should be ~1 O/A [4]. For the above waveguide, this is e1300. (This factor in principal can be made nearly 5 times higher, however, by careful correction to the a-class profile [2]). Nevertheless, the present waveguide is an existence proof that waveguides with profile-limited bandwidths in excess of 3 GHz-km can be fabricated by the OVPO process.
3. Plane wave measurements 3.1. Experiment The extremely high bandwidth exhibited by the above waveguide makes it a particularly good candidate for further study. A second waveguide drawn from the same OVPO preform but several kilometers removed from it has been examined at Corning. This waveguide also had an extremely large bandwidth. However, due to the approximately 4-nm spectral bandwidth of the Corning pulse dispersion measurement equipment gallium arsenide source, the bandwidth was limited by material dispersion at about 1200 MHz-km and thus no intemrodal dispersion could be measured. In the analysis at Corning, several parameters were measured on this waveguide. The profile was determined by both interference [S] and near-field [13] measurements. These gave an average profile parameter of 2.01 (at 750 nm) and 2.03 (at 900 nm), respectively. (For the near-field measurement, corrections for leaky modes were necessary and thus this value is somewhat questionable.) Additionally, pulse broadening and relative arrival time measurements were made for plane wave excitation at three different wavelengths, 900 nm, 799 nm, and 676 nm, using a gallium arsenide pulse laser (RCA 30025) and a mode-locked Kr laser respectively. These had input pulse widths of -500 and -300 ps, respectively. The general experimental setup has been previously described [4,14]. A quasi-plane wave is injected into the waveguide at various angles and the resulting pulse broadening and arrival time are detected with a silicon APD (Mitsubishi PD-020B) and recorded on a sampling oscilloscope. The oscilloscope is inter-
faced to a digital computer with CAMAC instrumentation to allow for computation of the relative pulse arrival time and rms pulse width as a function of input angle. In addition, the Fourier transform may be calculated to yield the amplitude and phase in the frequency domain. Figs. 3a, 3b, and 3c show the relative pulse output at the three wavelengths for O”, 4”, 6”, g”, and 10” plane wave inputs. It is noted that in all cases the higher angle (higher order mode) inputs arrive progressively earlier in time. This means that the actual profile of the waveguide is slightly below the optimum. The magnitude of the earlier arrival becomes progressively greater as the wavelength is decreased. This is generally the expected behavior based on the material properties [5] and the fact that the profile was optimized for 900 nm. At 900 nm very little difference in the arrival time with input angle is observed. 3.2. Analysis This type of measurement gives additional data to confirm actual and optimum profile measurements made by other techniques such as near-field [ 131 and interference microscope [5] and wavelength-dependent numerical aperture [6] measurements. The a-class profile is assumed where the radial refractive index is given by [ 1] n2(r) =n*(O)[l-2A(r/a)cY] = n2(0)[l-2A]
rda (1)
r >a.
Here n(0) is the axial refractive index, A is the fractional index difference and a is the core radius. By a straightforward summation over the propagation delays for the various waveguide rays excited by the plane wave, the relative arrival time and pulse broadening may be calculated. Referenced to the zero angle input to first order in A they are found to be
W(O)=&
NLA
o-o0 -&2 ) q*
(2)
and cr(T+J(O) = y
Ia-cuOl (o+l)‘+Y+2)2
2
n ’
(3)
where N is the axial group refractive index, L is the 45
Volume 25, number 1
OPTICS COOMUNICATIONS
April 1978 /O”
799nm
0 NANOSEC
I.OOr
0
1.00
2.00
3.00
4.00 50 NANOSEC
7.00
8.00
9m
10.00
Fig. 3. Measured pulse output as a function of time for various plan: wave angles fo;three different wavelengths: (a) 676 nm, (b) 799 nm and (c) 900 nm. Guide length was 1 km. In all cases, the 10 precedes the 0 average energy arrival.
waveguide length, c is the velocity of light, Q is the actual waveguide profile, cr” is the optimum profile, and 77= sin0/sin8, relates the plane wave angle to the waveguide critical angle. To this order the relative arrival time and rms pulse broadening depend upon the square of the normalized plane wave angle. The measured dependences are shown plotted as a function of v2 in figs. 4 and 5 respectively. The straight lines are least-squares fits to the measured data. For 900-nm IO
r
Fig. 4. Average arrival time at a given plane wave angle relative to the zero degree input plotted as a function of the normalized input angle squared for various wavelengths; q 676 nm, A 799 nm, o 900 nm.
46
and 799-mn data, the fits are quite good. For 676 nm there is much greater uncertainty in the fits. At this wavelength the pulse shape for low angles is different than is predicted based on equal excitation. This departure from the theoretical dependence could be due to differential attentuation effects or small departures from the a-profile class.
Fig. 5. RMS pulse broadening at a given plane wave angle relative to the zero degree pulse broadening plotted as a function of the normalized input angle squared for various wavelengths; q 676 nm, A 799 mn, o 900 nm.
OPTICS COMMUNICATIONS
Volume 25, number 1
Given either the actual waveguide profile parameter or the optimum profile, the slope of the lines in figs. 4 and 5 can be related to the other profile parameter through eqs. (2) and (3). In this case, the actual profile parameter IY= 2.01 from interference data was assumed and the optimum profile (YOwas calculated as a function of wavelength. Slightly different values of 01’ were obtained from the pulse arrival time data than for the pulse broadening data. The average of these values of CY’at the three wavelengths is shown in fig. 6. Also shown is the optimum profile calculated for this germanium-borosilicate (CBS) waveguide from optimum profile measurements on germanium-silicate (GS) and borosilicate (BS) waveguides using the following approximation for the optimal profile,
o&-2
=A --L
{(o& -2)AGs+(o;s-2)A,s).
(4)
GBS
Here Ai is the fractional index difference attributable to the ith dopant taken with silica (Ai is always positive), and olpis the optimum profile for the ith binary silicate. Eq. (4) predicts that the optimum profile of the germanium-borosilicate will always be less than that of the binary germanium silicate. Based on present binary silicate measurements a value oEBS be-
w 2.1 -I iiz & 0 I I’ E 2.0
CALCULATED
0
I
‘-300
I
tween 1.93 and 1.97 is predicted at 900 nm. The fairly large uncertainty in the ois which can arise from processing conditions accounts for the wide range of predicted c&$S. The optimum Q’ measured by the present plane-wave technique is seen to have the same slope as that predicted from the binary silicate measurements; however, a value o&S = 2.02 is predicted. Failure to achieve better agreement than the systematic difference of -0.07 is probably attributable to any of a number of things: processing variations which give rise to slightly different optimum profiles for the various constituents, the theoretical approximation that for the plane wave there is equal mode excitation and attenuation, uncertainties in measuring the relative arrival time and pulse broadening, uncertainties in previous measurements of o” for the binary silicate constituents, nonlinear dependence of An on composition, and possible variations in cr along the waveguide length. The data appears to indicate a slightly higher o&S than predicted from the binary silicate data [5]. There are probably too many uncertainties in the various fabrication, measurement and analysis techniques which have gone into the present analysis to warrant that judgment at the present time. Nevertheless, the technique provides a direct measurement of the optimum profile once the profile of the waveguide is known or vice versa. This may provide an expedient way of measuring and specifying the profile of the waveguide. Such a technique may provide a useful specification for optical equalization [9]. As the measurement precision required increases with higher performance waveguides, correlation between several measurement techniques may be required to know when an accurate measurement of a waveguide parameter has been made.
4. Conclusion
-
600 WELENGTH
April 1978
I 600 (nm)
I lDoo
Fig. 6. Optimum profile as a function of wavelength for a germaniumborosilicate waveguide. The measured data points from the plane wave analysis are shown by o. The shaded region was predicted from binary silicate measurements, and the solid point is an interference measurement of the actual waveguide profile.
In this paper, a waveguide made by the OVPO doped deposited silica process has been shown to have a pulse reduction factor of about 175 which demonstrates an extrapolated optical power -3 dB bandwidth slightly in excess of 3 GHz-km. This is the highest reported to date. Five other waveguides all had bandwidths in excess of 1200 MHz and are now part of a 6-km 140 Mbit/s link installed underground at CNET [ I.51 . When all six waveguides were connected in series, the link bandwidth was measured to be 300 MHz. 47
Volume 25, number
1
A companion waveguide made from the same OVPO preform but approximately 9 kilometers removed from the highest-bandwidth waveguide also exhibits an extremely high bandwidth. A plane wave measurement and analysis of several wavelengths indicates the profile is nearly optimum for 900-nm operation. A change in the profile parameter along the preform of only -0.02, however, could optimize it for 850-nm operation. This is within manufacturing precision and is believed to account for the extremely high bandwidth observed in the original waveguide at CNET. The plane wave analysis indicates that either the actual effective waveguide profile or the optimum profile may be obtained given the other. Within the present experimental uncertainties, the optimum profile from this data is in agreement with that predicted by interference measurements on binary silicate glass compositions.
Acknowledgments The authors wish to acknowledge the comments and help of Dr. R. Olshansky, Mr. T.A. Cook and Mr. M.G. Blankenship.
48
April 1978
OPTICS COMMUNICATIONS References
[II D. Gloge and E.A.J. Marcatili, Bell. Syst. Tech. J. 52 (1973) 1563. [21 J.S. Cook, Bell Syst. Tech. J. 56 (1977) 7 19. [31 R. Olshansky, S.M. Oaks, D.B. Keck, Topical Meeting on Optical
fiber transmission II, Williamsburg, and D.B. Keck, Appl. Optics and H. Presby, Appl. Optics
141 R. Olshansky [51 I.P. Kaminow
Va. (1977). 15 (1976) 483 15 (1976)
3029.
[61 F.M.E. Sladen, D.N. Payne and M.J. Adams, Topical Meeting on Optical fiber transmission 11, Williamsburg, Va. (1977). [71 R.D. Maurer, Proc. of IEEE 61 (1973) 452. 181 R. Bouillie, J.C. Bizeul, M. Guibert, Second European Conf. on Optical fiber communications, Paris (1976). [91 M. Eve, Opt. Quantum Elect., submitted. [lOI L.G. Cohen, G.W. Tasker, W.G. French and J.R. Simpson, Appl. Phys. Lett. 28 (1976) 391. press release, Electronics 49 (1976) illI Philips Laboratories 65. [I21 S. Sentsui, K. Yoshida, Y. Furui and T. Kuroha, 1977 Intern. Conf. on Integrated optics and optical fiber communication, Tokyo, 1977. F.M.E. Sladen, D.N. Payne and M.J. Adams, Appl. Phys. Lett. 28 (1976) 255. D.B. Keck, Appl. Optics 13 (1964) 1882. R. Bouillie, Third European Conf. on Optical communications, Munich (1977).
MEASUREMENTS
April 1978
OPTICS COMMUNICATIONS
ON HIGH-BANDWIDTH
OPTICAL WAVEGUIDES
D.B. KECK Corning Glass Works, Corning, N. Y., USA
and R. BOUILLlE Centre National d’Etudes des Telecommunications,
Lannion, France
Received 4 November 1977 Revised manuscript received 29 December 1977 Frequency response measurements on six high-bandwidth optical waveguides made by the doped deposited silica process are reported. One of these had an extrapolated bandwidth in excess of 3 GHz-km. From plane wave measurements the actual or optimum waveguide profile may be determined given the other. Data taken on a germanium borosilicate waveguide is in agreement with interference measurements.
1. Introduction Because the low loss of present optical waveguides will allow large repeater spacings, many optical waveguide systems being contemplated will push to the limit present graded-index waveguide bandwidth manufacturing capability. The bandwidth limitation is primarily imposed by the quality of the radial index gradient and its ability to reduce propagation delay differences (intermodal dispersion) between the transmitted modes. Estimates of the maximum bandwidth which should be achievable have been made [ 1,2] . It is most important, however, to have existence proofs at various stages in the technological evolution that in fact these are practically achievable. In this paper several waveguides are described which address this question, one of which has an optical power -3 dB bandwidth in excess of 3 GHz over a 1-km length. This is covered in section 2. To enable progress toward routinely achieving these higher bandwidths, deeper quantitative understanding about the operation of waveguide must be obtained. This includes things such as information on the differential mode attenuation [3] and how it affects bandwidth, information on the optimum profile [4] and how it is affected by material
composition and processing [5,6] and information on the actual waveguide profile including perturbations, both azimuthal and axial. Plane wave pulse measurements made on one of these high-bandwidth waveguides gives an additional source of information for the last two and is covered in section 3.
2. Waveguide bandwidth
measurements
Six graded-index optical waveguides prepared by Corning Glass Works were measured at the French Centre National d’Etudes des Telecommunications (CNET). All guides were prepared by the “outside vapor phase oxidation” (OVPO) doped deposited silica process [7] and consist of a germanium-borosilicate radially graded core with a borosilicate cladding. The bandwidth measurement is done by injecting short light pulses into the waveguide and directly recording the output frequency response with a spectrum analyzer [8] . As shown in fig. 1, light from an experimental Standard Telecommunications Laboratory gallium aluminum arsenide laser diode emitting at -850 nm is launched into a - IO-meter section of waveguide. A pulse generator with about a 30-MHz repetition rate 43
Volume 25, number 1
OPTICS COMMUNICATIONS
April 1978 FREQUENCY (GHr)
PULSE GENERATOR
SPECTRUM ANALYZER WAVEGUIDE WAVEGUIDE
El
(a)
COMPUTER
Fig. 1. Schematic of the bandwidth measurement apparatus used at CNET. A mode coupling waveguide is used to more uniformly excite the waveguide under test.
is used to drive the laser diode whose output is regulated by feedback circuitry. The source spectral bandwidth is about 0.4 run. The output from the IO-meter waveguide section is butt-coupled to the test waveguide by use of a micropositioner . Mode coupling in the lometer section is used to more uniformly excite the mode spectrum of the test waveguide, thus the measurement realistically corresponds to long distance system behavior. The transmitted light is detected using a Radio Technic Compelec silicon-pin diode and fed directly to a spectrum analyzer. The use of a pin diode was essential for measurement of these high-bandwidth waveguides. The spectrum analyzer is driven by a digital computer which sweeps the frequency spectrum and directly records the wave-guide frequency response from about 50 to 1700 MHz. This is shown plotted in figs. 2a and 2b for two of the highest-bandwidth waveguides tested. For both of these the -6 dB electrical power frequency (-3 dB optical power frequency) is greater than the measurement limit of the equipment. The computer is used to fit the amplitude data to a power law; dB (j) = AfB. This is an empirical function which matches the data quite well and provides an analytic form for the frequency response. The coefficient A is governed by the waveguide length, profile, and index difference. For a gaussian response the coefficient B = 2. Using the power law extrapolation of the measured data, -6 dB points of 1800 and 3100 MHz are obtained for these two fibers. Of the remaining four guides, all had -6 dB bandwidths greater than 1000 44
-6
L
\ FREQUENCY
0 01
B 0
01
(GHzl IO
-I-
(b) -5-
-6-
Fig. 2. Electrical response (dB) as a function of frequency of two Corning OVPO optical waveguides as measured at CNET. The guides are each 1 km in length. The solid curve is the measured response while the dotted curve is the best-fit response using a form dB = AfB. The -6 dB frequencies are (a) 3100 MHz and (b) 1800 MHz.
MHz/km, the smallest being 1240 MHz/km. It is noted that the narrow spectral bandwidth of the source used for these measurements should have a material dispersion limit in excess of 10 GHz-km. The zero frequency attenuation rate of all six of these waveguides is very low so that mode coupling and/or differential attenuation effects are not expected to play a significant role in generating these large bandwidths. Thus the measured values are indicative of how well the average radial index profile along the wave, guide matches the optimum profile [9]. For the highest bandwidth waveguide, the numerical aperture of -0.18 would predict a step profile bandwidth of ap-
Volume 25. number 1
April 1978
OPTICS COMMUNlCATIONS
proximately 17 MHz-km. Thus a profile pulse reduction factor of about 175 exists for this waveguide. The highest factors previously reported are less than 100. [ 10-121. For the pure o-class of profiles, the theoretical maximum pulse reduction factor should be ~1 O/A [4]. For the above waveguide, this is e1300. (This factor in principal can be made nearly 5 times higher, however, by careful correction to the a-class profile [2]). Nevertheless, the present waveguide is an existence proof that waveguides with profile-limited bandwidths in excess of 3 GHz-km can be fabricated by the OVPO process.
3. Plane wave measurements 3.1. Experiment The extremely high bandwidth exhibited by the above waveguide makes it a particularly good candidate for further study. A second waveguide drawn from the same OVPO preform but several kilometers removed from it has been examined at Corning. This waveguide also had an extremely large bandwidth. However, due to the approximately 4-nm spectral bandwidth of the Corning pulse dispersion measurement equipment gallium arsenide source, the bandwidth was limited by material dispersion at about 1200 MHz-km and thus no intemrodal dispersion could be measured. In the analysis at Corning, several parameters were measured on this waveguide. The profile was determined by both interference [S] and near-field [13] measurements. These gave an average profile parameter of 2.01 (at 750 nm) and 2.03 (at 900 nm), respectively. (For the near-field measurement, corrections for leaky modes were necessary and thus this value is somewhat questionable.) Additionally, pulse broadening and relative arrival time measurements were made for plane wave excitation at three different wavelengths, 900 nm, 799 nm, and 676 nm, using a gallium arsenide pulse laser (RCA 30025) and a mode-locked Kr laser respectively. These had input pulse widths of -500 and -300 ps, respectively. The general experimental setup has been previously described [4,14]. A quasi-plane wave is injected into the waveguide at various angles and the resulting pulse broadening and arrival time are detected with a silicon APD (Mitsubishi PD-020B) and recorded on a sampling oscilloscope. The oscilloscope is inter-
faced to a digital computer with CAMAC instrumentation to allow for computation of the relative pulse arrival time and rms pulse width as a function of input angle. In addition, the Fourier transform may be calculated to yield the amplitude and phase in the frequency domain. Figs. 3a, 3b, and 3c show the relative pulse output at the three wavelengths for O”, 4”, 6”, g”, and 10” plane wave inputs. It is noted that in all cases the higher angle (higher order mode) inputs arrive progressively earlier in time. This means that the actual profile of the waveguide is slightly below the optimum. The magnitude of the earlier arrival becomes progressively greater as the wavelength is decreased. This is generally the expected behavior based on the material properties [5] and the fact that the profile was optimized for 900 nm. At 900 nm very little difference in the arrival time with input angle is observed. 3.2. Analysis This type of measurement gives additional data to confirm actual and optimum profile measurements made by other techniques such as near-field [ 131 and interference microscope [5] and wavelength-dependent numerical aperture [6] measurements. The a-class profile is assumed where the radial refractive index is given by [ 1] n2(r) =n*(O)[l-2A(r/a)cY] = n2(0)[l-2A]
rda (1)
r >a.
Here n(0) is the axial refractive index, A is the fractional index difference and a is the core radius. By a straightforward summation over the propagation delays for the various waveguide rays excited by the plane wave, the relative arrival time and pulse broadening may be calculated. Referenced to the zero angle input to first order in A they are found to be
W(O)=&
NLA
o-o0 -&2 ) q*
(2)
and cr(T+J(O) = y
Ia-cuOl (o+l)‘+Y+2)2
2
n ’
(3)
where N is the axial group refractive index, L is the 45
Volume 25, number 1
OPTICS COOMUNICATIONS
April 1978 /O”
799nm
0 NANOSEC
I.OOr
0
1.00
2.00
3.00
4.00 50 NANOSEC
7.00
8.00
9m
10.00
Fig. 3. Measured pulse output as a function of time for various plan: wave angles fo;three different wavelengths: (a) 676 nm, (b) 799 nm and (c) 900 nm. Guide length was 1 km. In all cases, the 10 precedes the 0 average energy arrival.
waveguide length, c is the velocity of light, Q is the actual waveguide profile, cr” is the optimum profile, and 77= sin0/sin8, relates the plane wave angle to the waveguide critical angle. To this order the relative arrival time and rms pulse broadening depend upon the square of the normalized plane wave angle. The measured dependences are shown plotted as a function of v2 in figs. 4 and 5 respectively. The straight lines are least-squares fits to the measured data. For 900-nm IO
r
Fig. 4. Average arrival time at a given plane wave angle relative to the zero degree input plotted as a function of the normalized input angle squared for various wavelengths; q 676 nm, A 799 nm, o 900 nm.
46
and 799-mn data, the fits are quite good. For 676 nm there is much greater uncertainty in the fits. At this wavelength the pulse shape for low angles is different than is predicted based on equal excitation. This departure from the theoretical dependence could be due to differential attentuation effects or small departures from the a-profile class.
Fig. 5. RMS pulse broadening at a given plane wave angle relative to the zero degree pulse broadening plotted as a function of the normalized input angle squared for various wavelengths; q 676 nm, A 799 mn, o 900 nm.
OPTICS COMMUNICATIONS
Volume 25, number 1
Given either the actual waveguide profile parameter or the optimum profile, the slope of the lines in figs. 4 and 5 can be related to the other profile parameter through eqs. (2) and (3). In this case, the actual profile parameter IY= 2.01 from interference data was assumed and the optimum profile (YOwas calculated as a function of wavelength. Slightly different values of 01’ were obtained from the pulse arrival time data than for the pulse broadening data. The average of these values of CY’at the three wavelengths is shown in fig. 6. Also shown is the optimum profile calculated for this germanium-borosilicate (CBS) waveguide from optimum profile measurements on germanium-silicate (GS) and borosilicate (BS) waveguides using the following approximation for the optimal profile,
o&-2
=A --L
{(o& -2)AGs+(o;s-2)A,s).
(4)
GBS
Here Ai is the fractional index difference attributable to the ith dopant taken with silica (Ai is always positive), and olpis the optimum profile for the ith binary silicate. Eq. (4) predicts that the optimum profile of the germanium-borosilicate will always be less than that of the binary germanium silicate. Based on present binary silicate measurements a value oEBS be-
w 2.1 -I iiz & 0 I I’ E 2.0
CALCULATED
0
I
‘-300
I
tween 1.93 and 1.97 is predicted at 900 nm. The fairly large uncertainty in the ois which can arise from processing conditions accounts for the wide range of predicted c&$S. The optimum Q’ measured by the present plane-wave technique is seen to have the same slope as that predicted from the binary silicate measurements; however, a value o&S = 2.02 is predicted. Failure to achieve better agreement than the systematic difference of -0.07 is probably attributable to any of a number of things: processing variations which give rise to slightly different optimum profiles for the various constituents, the theoretical approximation that for the plane wave there is equal mode excitation and attenuation, uncertainties in measuring the relative arrival time and pulse broadening, uncertainties in previous measurements of o” for the binary silicate constituents, nonlinear dependence of An on composition, and possible variations in cr along the waveguide length. The data appears to indicate a slightly higher o&S than predicted from the binary silicate data [5]. There are probably too many uncertainties in the various fabrication, measurement and analysis techniques which have gone into the present analysis to warrant that judgment at the present time. Nevertheless, the technique provides a direct measurement of the optimum profile once the profile of the waveguide is known or vice versa. This may provide an expedient way of measuring and specifying the profile of the waveguide. Such a technique may provide a useful specification for optical equalization [9]. As the measurement precision required increases with higher performance waveguides, correlation between several measurement techniques may be required to know when an accurate measurement of a waveguide parameter has been made.
4. Conclusion
-
600 WELENGTH
April 1978
I 600 (nm)
I lDoo
Fig. 6. Optimum profile as a function of wavelength for a germaniumborosilicate waveguide. The measured data points from the plane wave analysis are shown by o. The shaded region was predicted from binary silicate measurements, and the solid point is an interference measurement of the actual waveguide profile.
In this paper, a waveguide made by the OVPO doped deposited silica process has been shown to have a pulse reduction factor of about 175 which demonstrates an extrapolated optical power -3 dB bandwidth slightly in excess of 3 GHz-km. This is the highest reported to date. Five other waveguides all had bandwidths in excess of 1200 MHz and are now part of a 6-km 140 Mbit/s link installed underground at CNET [ I.51 . When all six waveguides were connected in series, the link bandwidth was measured to be 300 MHz. 47
Volume 25, number
1
A companion waveguide made from the same OVPO preform but approximately 9 kilometers removed from the highest-bandwidth waveguide also exhibits an extremely high bandwidth. A plane wave measurement and analysis of several wavelengths indicates the profile is nearly optimum for 900-nm operation. A change in the profile parameter along the preform of only -0.02, however, could optimize it for 850-nm operation. This is within manufacturing precision and is believed to account for the extremely high bandwidth observed in the original waveguide at CNET. The plane wave analysis indicates that either the actual effective waveguide profile or the optimum profile may be obtained given the other. Within the present experimental uncertainties, the optimum profile from this data is in agreement with that predicted by interference measurements on binary silicate glass compositions.
Acknowledgments The authors wish to acknowledge the comments and help of Dr. R. Olshansky, Mr. T.A. Cook and Mr. M.G. Blankenship.
48
April 1978
OPTICS COMMUNICATIONS References
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