CA2108097C - Optical soliton generator - Google Patents
Optical soliton generatorInfo
- Publication number
- CA2108097C CA2108097C CA002108097A CA2108097A CA2108097C CA 2108097 C CA2108097 C CA 2108097C CA 002108097 A CA002108097 A CA 002108097A CA 2108097 A CA2108097 A CA 2108097A CA 2108097 C CA2108097 C CA 2108097C
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- CA
- Canada
- Prior art keywords
- modulator
- output
- pulse
- input
- optical
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/25077—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion using soliton propagation
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F2203/00—Function characteristic
- G02F2203/26—Pulse shaping; Apparatus or methods therefor
Abstract
An optical pulse generator for producing a soliton pulse train having a modulator which amplitude and phase modulates an optical frequency output of a continuous optical wave source. The modulator is driven by first and second harmonically related continuous sinusoidal waves from first and second CW voltage sources and the generator includes a pulse compression means for producing at the output of the generator a soliton pulse train.
Description
OPTICAI. SOl~ITON r,~M~ ~0R.
F~F.T.n OF lNV~ ION
This invention relates to optical pulse generation and, more particularly, to an optical pulse generator for use with the soliton propagation mode of intelligence bearing optical pulse trains.
P~Al'R~ ~n OF T}~ uv~ N
In the propagation of optical pulses over extreme distances, such as, for example, transoceanic distances, numerous parameters associated with such transmission limit the capacity of the optical fiber cable system through which the optical pulses are transmitted and, hence, the amount of intelligence that can be transmitted. The most obvious of such parameters is fiber loss, which degrades and diminishes the pulses passing therethrough. In general, such loss can be compensated by periodic pulse regeneration and amplification. Thus, in an extremely long cable transmission system, spaced regenerators are used for periodically restoring the pulses to a viable condition.
Most recently, the use of erbium doped fiber amplification incorporated in the transmitting optical fibers has made it possible to transmit multi-gigabit per second pulse signals over extreme distances, such as transoceanic, without the necessity of regeneration of the signals. Thus, cable or fiber loss can be substantially eliminated as a factor in the transmission of pulse signals, leaving the parameters of fiber dispersion, fiber nonlinearities and amplifier noise as the limiting factors. As discussed in U.S. patent 5,157,744 of Korotky, optical fibers are not strictly linear, but have a small amount of nonlinearity in their transmission characteristics which make possible a pulse transmission mode in the optical fiber that is effectively ',.'.. i,.. .: "", ',~ ", . ,," ' ", ,~ " ,;,j,"""",,, ,"~ " ,, ,,,;
F~F.T.n OF lNV~ ION
This invention relates to optical pulse generation and, more particularly, to an optical pulse generator for use with the soliton propagation mode of intelligence bearing optical pulse trains.
P~Al'R~ ~n OF T}~ uv~ N
In the propagation of optical pulses over extreme distances, such as, for example, transoceanic distances, numerous parameters associated with such transmission limit the capacity of the optical fiber cable system through which the optical pulses are transmitted and, hence, the amount of intelligence that can be transmitted. The most obvious of such parameters is fiber loss, which degrades and diminishes the pulses passing therethrough. In general, such loss can be compensated by periodic pulse regeneration and amplification. Thus, in an extremely long cable transmission system, spaced regenerators are used for periodically restoring the pulses to a viable condition.
Most recently, the use of erbium doped fiber amplification incorporated in the transmitting optical fibers has made it possible to transmit multi-gigabit per second pulse signals over extreme distances, such as transoceanic, without the necessity of regeneration of the signals. Thus, cable or fiber loss can be substantially eliminated as a factor in the transmission of pulse signals, leaving the parameters of fiber dispersion, fiber nonlinearities and amplifier noise as the limiting factors. As discussed in U.S. patent 5,157,744 of Korotky, optical fibers are not strictly linear, but have a small amount of nonlinearity in their transmission characteristics which make possible a pulse transmission mode in the optical fiber that is effectively ',.'.. i,.. .: "", ',~ ", . ,," ' ", ,~ " ,;,j,"""",,, ,"~ " ,, ,,,;
immune from the aforementioned pulse degradation factors.
As will be discussed more fully hereinafter, this mode, known as "soliton propagation", effectively balances out the aforementioned factors provided the pulses have a required power level and hence velocity of propagation and a pulse shape in both the time and frequency domains that is optimum for compensating dispersion at a given power level and for reducing interference and cross-talk among pulses. Coupled with erbium doped amplification, which maintains the power required for soliton propagation, it is possible to transmit over large distances with minimum pulse degradation and loss resulting from the aforementioned factors as well as cross-talk. In soliton propagation, there is very little interaction among differing wavelength channels, hence, an additional significant advantage of such propagation is that wavelength division multiplexing (WDM) may be used to increase further the fiber capacity.
Prior art arrangements for generating pulses for use in soliton transmission have most often comprised some form of mode-locked laser. Such a pulse generator produces pulses having a bit rate that is tied to the time of a round trip of the laser cavity. It can be appreciated that extreme precision in sizing the cavit~ resonator is required for a given frequency, and tunability of repetition rate and pulse width is not easily realized. As a consequence, such a mode-locked laser pulse generator is itself a limiting factor in the transmission system.
The prior art pulse generator as herein discussed may comprise a semi-conductor injection laser that has a continuous wave (CW) non-modulated output. The output is then fed to an optical modulator having a switching characteristic to which is applied a pulse waveform modulatiny voltage. A Mach-Zehnder interferometric type of modulator has generally been preferred for such systems.
However, a high-repetition-rate electrical pulse wave~orm is difficult to produce and propagate.
:
~ 3 ~ 2~0~97 The present invention is, therefore, directed towards the reductlon or elimination of the use of complex wave forms in an improved soliton pulse generator.
SU~!I~Z~RY OF T~ N V ~ lON
The present invention is a pulse generator for producing pulses appropriate for soliton mode transmissions which has controllable pulse outputs over a wide range of frequencies. In a preferred embodiment, the pulse generator comprises a CW laser, the output of which is applied to an amplitude modulator driven by a CW sinusoidal source. The modulator has a COS2 transfer function as will be discussed more fully hereinafter, and its output is a transform-limited return-to-zero (RZ) pulse train having a ratio of pulse period T=1/f to pulse full width at half maximum (FWHM) ~ of three (T/~=3) and approximately 100%
modulation depth. Here f is the pulse repetition frequency. The output of the amplitude modulator is applied to a phase modulator which is also driven from a source of CW sinusoidal voltage to frequency chirp the input pulses, thereby broadening the optical spectral width of the pulse train.
Connected to the output of the phase modulator is a compression fiber having a group velocity dispersion characteristic at the operating wavelength which, in conjunction with the phase modulator, as will be discussed hereinafter, compresses the pulses to narrow widths. The ratio T/~ becomes greater than five (T/~>5) as a consequence of this compression. Thus, the output of pulse generator of the invention is a pulse train having waveforms in both the time and frequency domains which approach or equal a squared hyperbolic secant (sech2) in shape, which is an ideal shape for soliton propagation.
The numerous features and advantages of the present invention will be more readily understood from the following detailed description read in conjunction with the accompanying drawings.
i'7 BE~T~ D~ClCl2TPTION OF T~l~ DRAWINGS
FIG. 1 iS a block diagrammatic view of a soliton pulse generator embodying the principles and features of the present invention;
FIG. 2 iS a diagram of predicted time and frequency domain waveform outputs of the circuit of Fig. l; and FIGS. 3~, 3B, and 3C are actual time and frequency domain waveform outputs of the circuit of Fig. 1.
~IG. 4 iS a view of one embodiment of a combined amplitude/phase modulator for use in the circuit of Fig. 1 r~;!'r~TT.l;~n T~ CE2T~'TION
Soliton transmission requires pulses of a shape (sech2 intensity envelope in time and frequency) and peak power such that the non-linearity of the optical fiber index exactly cancels the dispersive broadening characteristics of the fiber. When this condition obtains, soliton pulses can travel over extremely long distances without dispersion induced distortion in either the time or frequency domains.
Transform-limited optical pulses propagating through an optical fiber undergo pulse spreading as a result of group velocity dispersion arising from the dependence of the linear index of refraction nO of the fiber on frequency.
In addition, there is a non-linear component n2 of the fiber ;ndex of refraction such that the overall index of refraction n is given by -(1) n=nO+n2I ' ';
where I is the light intensity. It can be shown that the peak power P1 of a soliton pulse in a single mode optical fiber having an effective mode field area A~f iS glven by 2 1 ~ 7 Pl= ~;n where (3) ~=0 322 2C~2 where ZO is the soliton period, D is the dispersion coefficient, ~ is the pulse width, l is the optical wavelength and c is the velocity of light. It can be seen from equations (2) and (3) that n2 and D have the effect of canceling each other, thereby insuring a virtually distortion free soliton pulse. A detailed explanation of solitons is given in "Transmission of Stationary Nonlinear Pulses in Dispersive Dielectric Fibers" by A. Hasegawa and F. Tappert, ~pplied Phip. Lett. Vol. 23, No. 3, (1973), at pp. 142-144.
When used to transmit information in optical form along an optical fiber transmission path, a soliton is an optical pulse signal that has: a special wave shape in the time domain, a specific optical frequency content in the frequency domain, no chirp, and an intensity that matches the dispersion characteristics of the optical transmission ; path.
In Fig. 1, there is shown a block diagram of the soliton pulse generator 10 of the present invention.
Generator 10 comprises a continuous wave (CW) laser source 11 of unmodulated optical signals.~ Laser 11 may be, for example, an InGaAsP distributed feedback type laser having an output wavelength A=1.55-m. The output of laser 11 is optically connected to a modulator 15, shown in dashed lines, which comprises an amplitude modulator 12 and a phase modulator 18. Modulator 12 may take any one of a :
,- . .
- 6 - 2~8~7 number of forms such as, for example, an electrorefractive modulator, an electroabsorption modulator, a waveguide interferometer or directional coupler, or a Mach-Zehnder type of interferometric modulator, all of which are well known in the art. It has been found that a Y-branch Mach-Zehnder type of interferometer produces excellent results.
The modulator 12 has applied thereto a CW sinusoidal voltage of frequency f/2 from a source 1~ through an electrical connection 16 to produce an output optical signal from modulator 12. This output signal is a transform limited, return to zero (RZ) pulse train having a ratio of pulse period T to pulse width (FWHM)~ of (4) T~3 , and virtually 100% modulation depth.
In practice, the amplitude modulator 12 has a COS2 transfer characteristic that is symmetrical about zero such that the output signal from modulator 12 is given by (5) Llght out = cog ( V"X2) where Va is the switching voltage. Thus, with the Mach-Zehnder modulator biased at zero the signal is sinusoidally modulated between the negative switching voltage and the positive switching voltage, thus in one cycle of RF voltage of frequency f/2, two pulses are produced hence the pulse repetition frequency becomes f.
The GutpUt of modulator 12 is connected through optical connector means 17 to the input of phase modulator 18. Phase modulator 18 may also take any of a number of form such as, for example, an compensated electrorefractive or electroabsorption phase modulator or a wave guide phase '':-,'~,.
:
:: i~ - . ~ .: .: ~ . . . :
.. . . .. . .
_ 7 - 21~ 7 modulator, which are all well known. In the present embodiment of the invention, a Y-branch Mach-Zehnder interferometric phase modulator has been used to good effect. A CW sinusoidal wave modulating voltage of frequency f, phase locked to the voltage output of source 14, is supplied from a source 19 through electrical connection 21 to phase modulator 18. Modulator 18 functions to chirp the optical frequency of each pulse from amplitude modulator 12, thereby broadening the spectral width of the pulse train. ~hile f and f/2 have been designated as the modulator frequencies, the broader case simply requires that they be harmonically related with the higher frequency being applied to modulator 18.
It is known to drive a phase modulator with a pulse train, or with a periodic parabolic shaped voltage waveform, to achieve chirping. In the generator of Fig. 1, it has been found that a sinusoidal voltage properly phase related to the amplitude modulator sinusoidal driving voltage provides the desired frequency chirping.
Although the output of the amplitude modulator 12 is shown serially connected to the input of phase modulator 18, they may be reversed so that the output of modulator 18 is connected to the input of modulator 12. It is also possible for both modulators 12 and 18 to be part of a unitary modulator structure where amplitude and phase modulation take place simultaneously yet are independently controllable.
The output of phase modulator 18 is connected by an optical fiber or waveguide 22 to a pulse compression means 23. Compression means 23 may take any of a number of well known forms such as, for example, an optical material having a group velocity dispersion characteristic such as a dispersive optical fiber, a pair of interconnected optical gratings, or a chirped grating. In the embodiment of Fig. 1, an optical fiber having a group velocity dispersion D of approximately 18 ps/km-nn at the operating wavelength has been used. The relative phases of the - 8 - 2 ~ ~ 8 0 ~ 7 amplitude modulator voltage and the phase modulator voltage can be chosen such that the frequency chirp produced by phase modulator 18 causes -the optical dispersion characteristic of the compression means 23 to compress the pulses. This occurs because the chirping instantaneously changes the optical frequency within the pulse broadening the optical spectrum, so that the trailing edge of the pulse tends to overtake the leading edge, thereby compressing the pulse. As a result, the pulse output from the pulse generator 10 has a ratio T/~>3. The value of this ratio is preferably 5 or better.
In Fig. 1, the pulse compression means 23 is shown as following both modulators 12 and 18. Means 23 may also be inserted between the two modulators 12 and 18, especially where phase modulator 18, precedes amplitude modulator 12.
The output of the pulse compression means 23 may be applied to a suitable encoder 24 for signal encoding the pulse train and/or to a suitable amplifier. Encoder 24 may also be connected between laser 11 and the modulator, or between the modulator 12 and 18, or between the modulator 15 and the compression means 23 as a means of compensating optical loss.
Fig. 2 depicts the predicted pulse wave forms in both the time and frequency domains for the pulse generator of Fig. 1 for peak-to-peak sinusoidal phase modulation (PMpp) of 0.75~ radians. The dashed line in the frequency domain indicates the shape of a sech2 curve. It can be seen that the frequency spectrum of the pulses conforms to the dash line shape. The pulses in the time domain almost exactly coincide with the sech2 curve, hence the latter is masked by the solid lines.
Figs. 3A, 3B, and 3C indicate actual experimental results for 15 GHz pulse repetition rate for differing values of PMpp. It can be seen in Fig. 3A that the PMpp value of zero, the pulses in the time domain are too broad, i.e., FWHM equals approximately 20ps, and the peak amplitude is too low. Fig. 3B, on the other hand, shows ~ :: :
,:, . ~ . .. . : . . . :
9 - 2~ ~$~97 that, for PM~p equals 0.75~, the pulses in both the time and frequency domains correspond closely to the predicted results shown in Fig. 2 with FWHM equal to 14ps. For a PMpp of 1.0~, the FWHM equals approximately lOps, as shown in Fig. 3C. Thus, the results shown in Flgs. 3B and 3C are pulses that fall within the soliton constraints.
As illustrated in Fig. 1, the various components of the pulse generator 10 are discrete, separate entities. In the present state of the art, it is possible to combine two or more of the discrete functions in a single unitary device. In Fig. 4, there is shown such a device 26 which functions as a combined amplitude and phase modulator with both modulations occurring simultaneously. Device 26 is configured in the form of a Y-branch Mach-Zehnder modulator having an input optical waveguide 27 which branches into two Y-branches 28 and 29, which pass along traveling-wave electrodes 31 and 32, respectively. Electrode 31 has an RF
input electrode 33 connected to source 14 and 19, not shown, and an RF output electrode 34. In like manner, electrode 32 has an RF input electrode 36 also connected to RF source 14 and 19, not shown, and an RF output electrode 37. The voltage from source 19 is applied to input electrodes 33 and 36 in phase, while the voltage from source 14 applied to input electrode 36 is delayed relative to that applied to electrode 33 so that it is 180~ out of phase relative thereto. Beyond the traveling wave regions defined by electrodes 31 and 32, optical wave guide arms 28 and 29 recombine into a single output waveguide 38. In the structure of Fig. 4, electrodes 31 and 32, which are in the form of coplaner waveguides, are totally independent of each other and are, as shown, individually addressable.
If the ratio of phase modulator (PM) to amplitude modulator (AM) is designated as a, it can be shown that (6) (1+~
-2 ~ 9~
where (7) I~ ~B
where V~A is the change in propagation constant in arm 28 and V~ is the change in propagation constant in arm 29.
Thus, the value of a may be tuned to produce the preferred optical signal shape at output 38 for input into the compression fiber 23.
The device 26 comprises, for example, a lithium niobate (LiNbO3) crystal substrate 39 having titanium diffused therein with a silicon dioxide (SiO2) buffer layer 41 thereon. The electrodes are gold plated on the buffer layer 41. For a more complete description of the device 26, reference may be made to the Technical Digest of the Topical Meeting on Integrated Photonic Research Montery, Calif., Paper TVG2, Optical Society of America, 1991, at pp. 53-54 "High Speed, Low Power Optical Modulator With Adjustable Chirp Parameter" by Korotky, et al. U.S. patent 5,157,744 of Korotky, referred to hereinafter, also includes a description of such a modulator. It is also possible, with the present state of the art, to integrate virtually the entire circuit of Eig. 1 onto a single substrate.
The invention as disclosed herein in the illustrative embodiment is a soliton pulse generator which is tunable and which does not require the use of complex voltage shapes in forming the pulses, relying instead upon simple sinusoidal waveforms to produce the desired soliton pulse train.
The foregoing description of an embodiment of the invention is for purposes of illustrating the principles, features, and advantages thereof. Numerous other arrangements or configurations may be implemented by :: - i : , , : :: . . . : . , : , 11 - 2~ 08~97 workers skllled in the art without departure from the spirit and scope of the invention.
., ~
~: :
: ~ ''.~''''''.,~
;'- '"'~ ~' '. '
As will be discussed more fully hereinafter, this mode, known as "soliton propagation", effectively balances out the aforementioned factors provided the pulses have a required power level and hence velocity of propagation and a pulse shape in both the time and frequency domains that is optimum for compensating dispersion at a given power level and for reducing interference and cross-talk among pulses. Coupled with erbium doped amplification, which maintains the power required for soliton propagation, it is possible to transmit over large distances with minimum pulse degradation and loss resulting from the aforementioned factors as well as cross-talk. In soliton propagation, there is very little interaction among differing wavelength channels, hence, an additional significant advantage of such propagation is that wavelength division multiplexing (WDM) may be used to increase further the fiber capacity.
Prior art arrangements for generating pulses for use in soliton transmission have most often comprised some form of mode-locked laser. Such a pulse generator produces pulses having a bit rate that is tied to the time of a round trip of the laser cavity. It can be appreciated that extreme precision in sizing the cavit~ resonator is required for a given frequency, and tunability of repetition rate and pulse width is not easily realized. As a consequence, such a mode-locked laser pulse generator is itself a limiting factor in the transmission system.
The prior art pulse generator as herein discussed may comprise a semi-conductor injection laser that has a continuous wave (CW) non-modulated output. The output is then fed to an optical modulator having a switching characteristic to which is applied a pulse waveform modulatiny voltage. A Mach-Zehnder interferometric type of modulator has generally been preferred for such systems.
However, a high-repetition-rate electrical pulse wave~orm is difficult to produce and propagate.
:
~ 3 ~ 2~0~97 The present invention is, therefore, directed towards the reductlon or elimination of the use of complex wave forms in an improved soliton pulse generator.
SU~!I~Z~RY OF T~ N V ~ lON
The present invention is a pulse generator for producing pulses appropriate for soliton mode transmissions which has controllable pulse outputs over a wide range of frequencies. In a preferred embodiment, the pulse generator comprises a CW laser, the output of which is applied to an amplitude modulator driven by a CW sinusoidal source. The modulator has a COS2 transfer function as will be discussed more fully hereinafter, and its output is a transform-limited return-to-zero (RZ) pulse train having a ratio of pulse period T=1/f to pulse full width at half maximum (FWHM) ~ of three (T/~=3) and approximately 100%
modulation depth. Here f is the pulse repetition frequency. The output of the amplitude modulator is applied to a phase modulator which is also driven from a source of CW sinusoidal voltage to frequency chirp the input pulses, thereby broadening the optical spectral width of the pulse train.
Connected to the output of the phase modulator is a compression fiber having a group velocity dispersion characteristic at the operating wavelength which, in conjunction with the phase modulator, as will be discussed hereinafter, compresses the pulses to narrow widths. The ratio T/~ becomes greater than five (T/~>5) as a consequence of this compression. Thus, the output of pulse generator of the invention is a pulse train having waveforms in both the time and frequency domains which approach or equal a squared hyperbolic secant (sech2) in shape, which is an ideal shape for soliton propagation.
The numerous features and advantages of the present invention will be more readily understood from the following detailed description read in conjunction with the accompanying drawings.
i'7 BE~T~ D~ClCl2TPTION OF T~l~ DRAWINGS
FIG. 1 iS a block diagrammatic view of a soliton pulse generator embodying the principles and features of the present invention;
FIG. 2 iS a diagram of predicted time and frequency domain waveform outputs of the circuit of Fig. l; and FIGS. 3~, 3B, and 3C are actual time and frequency domain waveform outputs of the circuit of Fig. 1.
~IG. 4 iS a view of one embodiment of a combined amplitude/phase modulator for use in the circuit of Fig. 1 r~;!'r~TT.l;~n T~ CE2T~'TION
Soliton transmission requires pulses of a shape (sech2 intensity envelope in time and frequency) and peak power such that the non-linearity of the optical fiber index exactly cancels the dispersive broadening characteristics of the fiber. When this condition obtains, soliton pulses can travel over extremely long distances without dispersion induced distortion in either the time or frequency domains.
Transform-limited optical pulses propagating through an optical fiber undergo pulse spreading as a result of group velocity dispersion arising from the dependence of the linear index of refraction nO of the fiber on frequency.
In addition, there is a non-linear component n2 of the fiber ;ndex of refraction such that the overall index of refraction n is given by -(1) n=nO+n2I ' ';
where I is the light intensity. It can be shown that the peak power P1 of a soliton pulse in a single mode optical fiber having an effective mode field area A~f iS glven by 2 1 ~ 7 Pl= ~;n where (3) ~=0 322 2C~2 where ZO is the soliton period, D is the dispersion coefficient, ~ is the pulse width, l is the optical wavelength and c is the velocity of light. It can be seen from equations (2) and (3) that n2 and D have the effect of canceling each other, thereby insuring a virtually distortion free soliton pulse. A detailed explanation of solitons is given in "Transmission of Stationary Nonlinear Pulses in Dispersive Dielectric Fibers" by A. Hasegawa and F. Tappert, ~pplied Phip. Lett. Vol. 23, No. 3, (1973), at pp. 142-144.
When used to transmit information in optical form along an optical fiber transmission path, a soliton is an optical pulse signal that has: a special wave shape in the time domain, a specific optical frequency content in the frequency domain, no chirp, and an intensity that matches the dispersion characteristics of the optical transmission ; path.
In Fig. 1, there is shown a block diagram of the soliton pulse generator 10 of the present invention.
Generator 10 comprises a continuous wave (CW) laser source 11 of unmodulated optical signals.~ Laser 11 may be, for example, an InGaAsP distributed feedback type laser having an output wavelength A=1.55-m. The output of laser 11 is optically connected to a modulator 15, shown in dashed lines, which comprises an amplitude modulator 12 and a phase modulator 18. Modulator 12 may take any one of a :
,- . .
- 6 - 2~8~7 number of forms such as, for example, an electrorefractive modulator, an electroabsorption modulator, a waveguide interferometer or directional coupler, or a Mach-Zehnder type of interferometric modulator, all of which are well known in the art. It has been found that a Y-branch Mach-Zehnder type of interferometer produces excellent results.
The modulator 12 has applied thereto a CW sinusoidal voltage of frequency f/2 from a source 1~ through an electrical connection 16 to produce an output optical signal from modulator 12. This output signal is a transform limited, return to zero (RZ) pulse train having a ratio of pulse period T to pulse width (FWHM)~ of (4) T~3 , and virtually 100% modulation depth.
In practice, the amplitude modulator 12 has a COS2 transfer characteristic that is symmetrical about zero such that the output signal from modulator 12 is given by (5) Llght out = cog ( V"X2) where Va is the switching voltage. Thus, with the Mach-Zehnder modulator biased at zero the signal is sinusoidally modulated between the negative switching voltage and the positive switching voltage, thus in one cycle of RF voltage of frequency f/2, two pulses are produced hence the pulse repetition frequency becomes f.
The GutpUt of modulator 12 is connected through optical connector means 17 to the input of phase modulator 18. Phase modulator 18 may also take any of a number of form such as, for example, an compensated electrorefractive or electroabsorption phase modulator or a wave guide phase '':-,'~,.
:
:: i~ - . ~ .: .: ~ . . . :
.. . . .. . .
_ 7 - 21~ 7 modulator, which are all well known. In the present embodiment of the invention, a Y-branch Mach-Zehnder interferometric phase modulator has been used to good effect. A CW sinusoidal wave modulating voltage of frequency f, phase locked to the voltage output of source 14, is supplied from a source 19 through electrical connection 21 to phase modulator 18. Modulator 18 functions to chirp the optical frequency of each pulse from amplitude modulator 12, thereby broadening the spectral width of the pulse train. ~hile f and f/2 have been designated as the modulator frequencies, the broader case simply requires that they be harmonically related with the higher frequency being applied to modulator 18.
It is known to drive a phase modulator with a pulse train, or with a periodic parabolic shaped voltage waveform, to achieve chirping. In the generator of Fig. 1, it has been found that a sinusoidal voltage properly phase related to the amplitude modulator sinusoidal driving voltage provides the desired frequency chirping.
Although the output of the amplitude modulator 12 is shown serially connected to the input of phase modulator 18, they may be reversed so that the output of modulator 18 is connected to the input of modulator 12. It is also possible for both modulators 12 and 18 to be part of a unitary modulator structure where amplitude and phase modulation take place simultaneously yet are independently controllable.
The output of phase modulator 18 is connected by an optical fiber or waveguide 22 to a pulse compression means 23. Compression means 23 may take any of a number of well known forms such as, for example, an optical material having a group velocity dispersion characteristic such as a dispersive optical fiber, a pair of interconnected optical gratings, or a chirped grating. In the embodiment of Fig. 1, an optical fiber having a group velocity dispersion D of approximately 18 ps/km-nn at the operating wavelength has been used. The relative phases of the - 8 - 2 ~ ~ 8 0 ~ 7 amplitude modulator voltage and the phase modulator voltage can be chosen such that the frequency chirp produced by phase modulator 18 causes -the optical dispersion characteristic of the compression means 23 to compress the pulses. This occurs because the chirping instantaneously changes the optical frequency within the pulse broadening the optical spectrum, so that the trailing edge of the pulse tends to overtake the leading edge, thereby compressing the pulse. As a result, the pulse output from the pulse generator 10 has a ratio T/~>3. The value of this ratio is preferably 5 or better.
In Fig. 1, the pulse compression means 23 is shown as following both modulators 12 and 18. Means 23 may also be inserted between the two modulators 12 and 18, especially where phase modulator 18, precedes amplitude modulator 12.
The output of the pulse compression means 23 may be applied to a suitable encoder 24 for signal encoding the pulse train and/or to a suitable amplifier. Encoder 24 may also be connected between laser 11 and the modulator, or between the modulator 12 and 18, or between the modulator 15 and the compression means 23 as a means of compensating optical loss.
Fig. 2 depicts the predicted pulse wave forms in both the time and frequency domains for the pulse generator of Fig. 1 for peak-to-peak sinusoidal phase modulation (PMpp) of 0.75~ radians. The dashed line in the frequency domain indicates the shape of a sech2 curve. It can be seen that the frequency spectrum of the pulses conforms to the dash line shape. The pulses in the time domain almost exactly coincide with the sech2 curve, hence the latter is masked by the solid lines.
Figs. 3A, 3B, and 3C indicate actual experimental results for 15 GHz pulse repetition rate for differing values of PMpp. It can be seen in Fig. 3A that the PMpp value of zero, the pulses in the time domain are too broad, i.e., FWHM equals approximately 20ps, and the peak amplitude is too low. Fig. 3B, on the other hand, shows ~ :: :
,:, . ~ . .. . : . . . :
9 - 2~ ~$~97 that, for PM~p equals 0.75~, the pulses in both the time and frequency domains correspond closely to the predicted results shown in Fig. 2 with FWHM equal to 14ps. For a PMpp of 1.0~, the FWHM equals approximately lOps, as shown in Fig. 3C. Thus, the results shown in Flgs. 3B and 3C are pulses that fall within the soliton constraints.
As illustrated in Fig. 1, the various components of the pulse generator 10 are discrete, separate entities. In the present state of the art, it is possible to combine two or more of the discrete functions in a single unitary device. In Fig. 4, there is shown such a device 26 which functions as a combined amplitude and phase modulator with both modulations occurring simultaneously. Device 26 is configured in the form of a Y-branch Mach-Zehnder modulator having an input optical waveguide 27 which branches into two Y-branches 28 and 29, which pass along traveling-wave electrodes 31 and 32, respectively. Electrode 31 has an RF
input electrode 33 connected to source 14 and 19, not shown, and an RF output electrode 34. In like manner, electrode 32 has an RF input electrode 36 also connected to RF source 14 and 19, not shown, and an RF output electrode 37. The voltage from source 19 is applied to input electrodes 33 and 36 in phase, while the voltage from source 14 applied to input electrode 36 is delayed relative to that applied to electrode 33 so that it is 180~ out of phase relative thereto. Beyond the traveling wave regions defined by electrodes 31 and 32, optical wave guide arms 28 and 29 recombine into a single output waveguide 38. In the structure of Fig. 4, electrodes 31 and 32, which are in the form of coplaner waveguides, are totally independent of each other and are, as shown, individually addressable.
If the ratio of phase modulator (PM) to amplitude modulator (AM) is designated as a, it can be shown that (6) (1+~
-2 ~ 9~
where (7) I~ ~B
where V~A is the change in propagation constant in arm 28 and V~ is the change in propagation constant in arm 29.
Thus, the value of a may be tuned to produce the preferred optical signal shape at output 38 for input into the compression fiber 23.
The device 26 comprises, for example, a lithium niobate (LiNbO3) crystal substrate 39 having titanium diffused therein with a silicon dioxide (SiO2) buffer layer 41 thereon. The electrodes are gold plated on the buffer layer 41. For a more complete description of the device 26, reference may be made to the Technical Digest of the Topical Meeting on Integrated Photonic Research Montery, Calif., Paper TVG2, Optical Society of America, 1991, at pp. 53-54 "High Speed, Low Power Optical Modulator With Adjustable Chirp Parameter" by Korotky, et al. U.S. patent 5,157,744 of Korotky, referred to hereinafter, also includes a description of such a modulator. It is also possible, with the present state of the art, to integrate virtually the entire circuit of Eig. 1 onto a single substrate.
The invention as disclosed herein in the illustrative embodiment is a soliton pulse generator which is tunable and which does not require the use of complex voltage shapes in forming the pulses, relying instead upon simple sinusoidal waveforms to produce the desired soliton pulse train.
The foregoing description of an embodiment of the invention is for purposes of illustrating the principles, features, and advantages thereof. Numerous other arrangements or configurations may be implemented by :: - i : , , : :: . . . : . , : , 11 - 2~ 08~97 workers skllled in the art without departure from the spirit and scope of the invention.
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Claims (18)
1. Apparatus for generating a soliton pulse train comprising:
a source of a continuous wave optical signal;
modulator means having an input for receiving the optical signal and producing an optical pulse output, said modulator means comprising means for amplitude modulating and phase modulating the optical signal;
a first voltage source for applying a first continuous wave sinusoidal modulating voltage to said modulator means;
a second voltage source for applying a second continuous wave sinusoidal modulating voltage harmonically related and phase locked to the first sinusoidal modulating voltage to said modulator; and optical pulse compression means having an input and an output for receiving the phase modulated optical signal to produce a pulse train at its output.
a source of a continuous wave optical signal;
modulator means having an input for receiving the optical signal and producing an optical pulse output, said modulator means comprising means for amplitude modulating and phase modulating the optical signal;
a first voltage source for applying a first continuous wave sinusoidal modulating voltage to said modulator means;
a second voltage source for applying a second continuous wave sinusoidal modulating voltage harmonically related and phase locked to the first sinusoidal modulating voltage to said modulator; and optical pulse compression means having an input and an output for receiving the phase modulated optical signal to produce a pulse train at its output.
2. The apparatus as claimed in claim 1 wherein said modulator means comprises an optical signal phase modulator having an input and an output and an optical signal amplitude modulator having an input and an output optically connected together in series.
3. The apparatus as claimed in claim 2 wherein said output of said amplitude modulator is optically connected to the input of said phase modulator.
4. The apparatus as claimed in claim 2 wherein the output of said phase modulator is connected to the input of said amplitude modulator.
5. The apparatus as claimed in claim 2 wherein said amplitude modulator is an interferometric type modulator.
6. The apparatus of claim 5 wherein said interferometric type modulator is a Y-branch Mach-Zehnder type modulator.
7. The apparatus as claimed in claim 2 wherein said phase modulator is an interferometric type modulator.
8. The apparatus as claimed in claim 7 wherein said interferometric type modulator is a Y-branch Mach-Zehnder type modulator.
9. The apparatus as claimed in claim 1 wherein said modulator means comprises a Y-branch interferometric type modulator with said first and second voltage source being electrically connected to one arm of the Y and said first and second voltage source being electrically connected to the other branch of the Y.
10. The apparatus as claimed in claim 1 wherein said optical pulse compression means comprises an optical material having a group velocity dispersion characteristic.
11. The apparatus as claimed in claim 10 wherein said optical material is a dispersive optical fiber.
12. A pulse generator having an output comprising:
a source of a continuous wave optical signal;
a first modulator having an input connected to the output of said source and an output;
a first source of a continuous wave sinusoidal modulating voltage at a frequency f electrically connected to said first modulator means;
a second modulator having an input and an output and having a cosine2 transfer function;
a second source of a continuous wave sinusoidal modulating voltage at a frequency f/2 electrically connected to said second modulator means;
the modulating voltages from said first and second sources being phase-locked together;
pulse compression means having an input and an output with its input being connected to at least one of the said outputs of said first and second modulator means producing at the output of the pulse generator a pulse train having a ratio of pulse period T to pulse full width at half maximum ? greater than three (T/?>3).
a source of a continuous wave optical signal;
a first modulator having an input connected to the output of said source and an output;
a first source of a continuous wave sinusoidal modulating voltage at a frequency f electrically connected to said first modulator means;
a second modulator having an input and an output and having a cosine2 transfer function;
a second source of a continuous wave sinusoidal modulating voltage at a frequency f/2 electrically connected to said second modulator means;
the modulating voltages from said first and second sources being phase-locked together;
pulse compression means having an input and an output with its input being connected to at least one of the said outputs of said first and second modulator means producing at the output of the pulse generator a pulse train having a ratio of pulse period T to pulse full width at half maximum ? greater than three (T/?>3).
13. A pulse generator as claimed in claim 12 wherein said first and second modulator means are serially optically connected together.
14. A pulse generator as claimed in claim 12 wherein the output of said first modulator is optically connected to the input of said second modulator.
15. A pulse generator as claimed in claim 12 wherein the output of said second modulator is optically connected to the input of said first modulator.
16. A pulse generator as claimed in claim 12 wherein said first modulator comprises a Y-arm Mach-Zehnder type of interferometric modulator.
17. A pulse generator as claimed in claim 12 wherein said second modulator comprises a Y-arm Mach-Zehnder type of interferometric modulator.
18. A pulse generator as claimed in claim 12 wherein said first modulator and said second modulator are incorporated into a single unitary structure.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US056,033 | 1993-04-30 | ||
| US08/056,033 US5477375A (en) | 1993-04-30 | 1993-04-30 | Optical soliton generator |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2108097A1 CA2108097A1 (en) | 1994-10-31 |
| CA2108097C true CA2108097C (en) | 1997-09-23 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002108097A Expired - Fee Related CA2108097C (en) | 1993-04-30 | 1993-10-08 | Optical soliton generator |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US5477375A (en) |
| EP (1) | EP0622916B1 (en) |
| JP (1) | JPH06326387A (en) |
| CA (1) | CA2108097C (en) |
| DE (1) | DE69428298T2 (en) |
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| US5477375A (en) * | 1993-04-30 | 1995-12-19 | At&T Corp. | Optical soliton generator |
| WO1995004952A1 (en) * | 1993-08-10 | 1995-02-16 | British Telecommunications Public Limited Company | Optical system |
| US5473458A (en) * | 1994-12-27 | 1995-12-05 | At&T Corp. | Soliton data transmission using non-soliton transmitter |
| JP3345214B2 (en) * | 1995-03-20 | 2002-11-18 | ケイディーディーアイ株式会社 | Short light pulse modulation method and apparatus |
| CN1080491C (en) * | 1995-03-31 | 2002-03-06 | 英国电讯公司 | Dark pulse generation and transmission |
| FR2738431B1 (en) * | 1995-09-01 | 1997-11-21 | Thomine Jean Baptiste | SYSTEM FOR THE TRANSMISSION, ESPECIALLY A LONG DISTANCE, OF RZ PULSES THROUGH AN AMPLIFIED OPTICAL LINE |
| US5631758A (en) * | 1995-10-26 | 1997-05-20 | Lucent Technologies Inc. | Chirped-pulse multiple wavelength telecommunications system |
| IT1279248B1 (en) * | 1995-12-21 | 1997-12-09 | Pirelli Cavi S P A Ora Pirelli | MODULATED AND DEPOLARIZED OPTICAL SIGNALS TRANSMISSION SYSTEM |
| JP3464867B2 (en) * | 1996-02-23 | 2003-11-10 | Ｋｄｄｉ株式会社 | Optical transmitting apparatus, wavelength multiplexing optical transmitting apparatus and optical transmission system using the same |
| US6396604B1 (en) | 1996-02-26 | 2002-05-28 | British Telecommunications Plc | Dark pulse TDMA optical network |
| FR2751150B1 (en) * | 1996-07-10 | 1998-09-18 | Alcatel Submarcom | SOLITON FORMAT DATA OPTICAL TRANSMISSION SYSTEM |
| JP3027944B2 (en) * | 1996-08-16 | 2000-04-04 | 日本電気株式会社 | Optical duobinary signal light generation method and optical transmitter |
| US6556326B2 (en) * | 1996-12-20 | 2003-04-29 | Tyco Telecommunications (Us) Inc. | Synchronous amplitude modulation for improved performance of optical transmission systems |
| US20040161245A1 (en) * | 1996-12-20 | 2004-08-19 | Bergano Neal S. | Synchronous amplitude modulation for improved performance of optical transmission systems |
| CA2197624C (en) * | 1997-02-14 | 2003-07-29 | Robert J. Davies | Hybrid single sideband optical modulator |
| US6072615A (en) * | 1997-06-13 | 2000-06-06 | Lucent Technologies Inc. | Phase modulator-based generation of high-quality high bit rate return-to-zero optical data streams |
| KR100279062B1 (en) * | 1997-07-25 | 2001-01-15 | 박호군 | Optical fiber loop short-wavelength pulsed laser apparatus and method of controlling width and repeatability of pulse using the same |
| US7010231B1 (en) | 1997-08-04 | 2006-03-07 | Cisco Photonics Italy S.R.L. | System and method of high-speed transmission and appropriate transmission apparatus |
| JPH1152313A (en) * | 1997-08-08 | 1999-02-26 | Mitsubishi Electric Corp | Optical modulation device |
| US6388786B1 (en) | 1997-08-15 | 2002-05-14 | Nec Corporation | Method for generating duobinary signal and optical transmitter using the same method |
| US6124960A (en) * | 1997-09-08 | 2000-09-26 | Northern Telecom Limited | Transmission system with cross-phase modulation compensation |
| US6163638A (en) * | 1998-10-29 | 2000-12-19 | Lucent Technologies Inc. | Article comprising an optical pulse compressor |
| GB2354598A (en) * | 1999-09-27 | 2001-03-28 | Cit Alcatel | An optical modulator |
| GB0005615D0 (en) * | 2000-03-09 | 2000-05-03 | Univ Southampton | An optical processing device based on fiber grating |
| AU2001247338A1 (en) * | 2000-03-09 | 2001-09-17 | Optium, Inc. | Apparatuses and methods for generating optical signals |
| US6735398B1 (en) * | 2000-03-15 | 2004-05-11 | Hughes Electronics Corporation | Generating methods for single and multi-channel wideband optical analog pulse positioned waveforms |
| FR2808095B1 (en) * | 2000-04-19 | 2002-08-09 | Algety Telecom | DEVICE FOR TRANSMITTING OPTICAL SIGNALS IN RZ FORMAT, ESPECIALLY OPTICAL SIGNALS WITH PULSES SOLUTIONS |
| US6760151B1 (en) | 2000-04-27 | 2004-07-06 | Jds Uniphase Corporation | Depolarized semiconductor laser sources |
| DE10196162B4 (en) * | 2000-05-08 | 2010-09-23 | IMRA America, Inc., Ann Arbor | Pulse generating device for generating parabolic pulses |
| US6954575B2 (en) | 2001-03-16 | 2005-10-11 | Imra America, Inc. | Single-polarization high power fiber lasers and amplifiers |
| US7068948B2 (en) * | 2001-06-13 | 2006-06-27 | Gazillion Bits, Inc. | Generation of optical signals with return-to-zero format |
| US20050203578A1 (en) * | 2001-08-15 | 2005-09-15 | Weiner Michael L. | Process and apparatus for treating biological organisms |
| US6671079B2 (en) | 2001-12-14 | 2003-12-30 | Lucent Technologies Inc. | Method and apparatus for transmitting a modulated optical signal |
| US6760142B2 (en) * | 2002-05-13 | 2004-07-06 | Lucent Technologies Inc. | Delay interferometer optical pulse generator |
| US7076120B2 (en) * | 2002-07-08 | 2006-07-11 | Lucent Technologies Inc. | Optical pulse generator for return-to-zero signaling |
| JP4444689B2 (en) * | 2004-02-26 | 2010-03-31 | 富士通株式会社 | Optical device for optical communication |
| DE102004037549A1 (en) * | 2004-08-03 | 2006-03-16 | Deutsche Telekom Ag | Device for generating and modulating a high-frequency signal |
| JP4328724B2 (en) * | 2005-01-17 | 2009-09-09 | 富士通株式会社 | Optical waveform measuring apparatus and optical waveform measuring method |
| US7761012B2 (en) * | 2006-01-12 | 2010-07-20 | Nec Laboratories America, Inc. | Optical communication system and method for generating dark return-to zero and DWDM optical MM-Wave generation for ROF downstream link using optical phase modulator and optical interleaver |
| CZ2012672A3 (en) | 2012-10-02 | 2014-06-04 | Rieter Cz S.R.O. | Method of generating light radiation and connection of LED radiation source LED in optical sensor for monitoring linear textile material |
| KR20140147321A (en) * | 2013-06-19 | 2014-12-30 | 한국전자통신연구원 | Wavelength swept source apparatus and operating method thereof |
| CN104283107B (en) * | 2014-10-23 | 2017-03-08 | 中国工程物理研究院激光聚变研究中心 | Radiation source generation system based on pulse period property phase-modulation |
| JP2016085396A (en) * | 2014-10-28 | 2016-05-19 | セイコーエプソン株式会社 | Short optical pulse generation device, terahertz wave generation device, camera, imaging device, and measurement device |
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| US4553810A (en) * | 1983-04-21 | 1985-11-19 | At&T Bell Laboratories | Traveling wave electrooptic devices |
| SE463740B (en) * | 1985-04-30 | 1991-01-14 | Ericsson Telefon Ab L M | ELECTROOPTIC MODULATOR |
| US5031235A (en) * | 1989-10-27 | 1991-07-09 | Hoechst Celanese Corp. | Cable system incorporating highly linear optical modulator |
| US5040865A (en) * | 1990-04-20 | 1991-08-20 | Hughes Aircraft Company | Frequency multiplying electro-optic modulator configuration and method |
| GB2245790B (en) * | 1990-07-03 | 1994-06-01 | Stc Plc | Optical transmitter |
| US5157744A (en) * | 1991-12-16 | 1992-10-20 | At&T Bell Laboratories | Soliton generator |
| US5303079A (en) * | 1992-04-09 | 1994-04-12 | At&T Bell Laboratories | Tunable chirp, lightwave modulator for dispersion compensation |
| IT1263613B (en) * | 1993-02-19 | 1996-08-27 | Pirelli Cavi Spa | FIBER OPTIC LASER GENERATOR WITH ACTIVE MODAL CHAIN |
| US5477375A (en) * | 1993-04-30 | 1995-12-19 | At&T Corp. | Optical soliton generator |
-
1993
- 1993-04-30 US US08/056,033 patent/US5477375A/en not_active Expired - Lifetime
- 1993-10-08 CA CA002108097A patent/CA2108097C/en not_active Expired - Fee Related
-
1994
- 1994-04-13 EP EP94302583A patent/EP0622916B1/en not_active Expired - Lifetime
- 1994-04-13 DE DE69428298T patent/DE69428298T2/en not_active Expired - Fee Related
- 1994-04-27 JP JP6089012A patent/JPH06326387A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| US5477375A (en) | 1995-12-19 |
| DE69428298D1 (en) | 2001-10-25 |
| JPH06326387A (en) | 1994-11-25 |
| EP0622916B1 (en) | 2001-09-19 |
| DE69428298T2 (en) | 2002-07-04 |
| EP0622916A1 (en) | 1994-11-02 |
| CA2108097A1 (en) | 1994-10-31 |
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