Patent Grant
7145715
This invention relates generally to multiple pump parametric apparatuses and, more particularly, to a multi-pump parametric apparatus having no idler signal broadening.
Parametric processes in highly nonlinear fiber (HNLF) are used in parametric amplification, wavelength conversion, ultrafast optical sampling and multiple channel penalty mitigation. A unique characteristic of two-pump parametric interaction in HNLF enables multispectral switching, which is not attainable by conventional one-pump devices. While significant progress in advancing the art of parametric design has been made to date, a number of practical issues still need to be resolved. The occurrence of Brillouin scattering represents a chief obstacle towards higher parametric performance since it limits the usable pump powers, and consequently, the available parametric gain/efficiency. In parametric fiber applications requiring high continuous wave (cw) optical power, SBS (stimulated Brillouin scattering) must be suppressed. This is commonly accomplished by frequency modulation, digital phase modulation, or analog phase modulation of the cw light to suppress the carrier component and broaden the pump spectrum. Frequency modulation of the pump light is typically accomplished by repetitively dithering the frequency of the source laser. Digital phase modulation is accomplished by means of an external (separate device) phase modulator driven repetitively by an electronic signal such as a PRBS (pseudo-random bit stream), typically at fairly high data rates (>1 Gb/s). Analog phase modulation is accomplished by means of an external phase modulator driven repetitively by an electronic signal composed of multiple harmonics. While applications requiring high cw optical power use pump spectral broadening in order to increase the Brillouin threshold, in one-pump parametric architectures this causes excessive idler spectral broadening, which cause a fundamental impairment. This basic limitation can be, in principle, completely removed in the two-pump architecture by counter phasing the modulation of the pumps, when using PRBS or multiple-harmonic pump phase modulation or frequency modulation. In such an arrangement, when the phase of one pump is increased the phase of the other pump is decreased and vice versa, thereby maintaining a constant average pump optical frequency, a condition that eliminates idler broadening. For digital phase modulation, the two pumps are phase modulated by separate phase modulators driven synchronously by a PRBS sequence and its complementary sequence, PRBS bar, respectively, or by another suitable sequence together with its complementary sequence.
A fiber parametric amplifier is an example of an application requiring high cw power in which a high-power pump amplifies an input signal(s) and creates one or more idlers at new wavelengths. However employing phase modulation to broaden the pump spectrum in order to suppress SBS has the undesirable consequence that the idlers experience broadening. This can degrade the idlers in various transmission and filtering environments. In U.S. Pat. No. 5,386,314 a technique was disclosed to prevent idler broadening in fiber parametric amplifiers. This patent disclosed that the distortion effects resulting from pump modulation used to raise the SBS threshold can be eliminated by using the same modulation, but opposite in sign, on the two pump signals. As a result, the phase conjugate mixing product of the input signal and the two modulated pump beams will therefore not have the low frequency distortion which normally accompanies SBS suppression by pump modulation.
More recently, the article by S. Radic, C. J. McKinstrie, R. M. Jopson, J. C. Centanni, A. R. Chraplyvy, C. G. Jorgensen, K. Brar and C. Headley, “Selective Suppression of Idler Spectral Broadening in Two-Pump Parametric Architectures”, IEEE Photon. Technol. Lett, V. 15, p673, May 1, 2003, discloses that while co-phased pump modulation allows the use of a single phase modulator, pump counter-phasing inherently requires two synchronized phase modulators. However, the electrooptic responses of two nominally identical high-speed phase modulators driven by nominally identical electronic amplifiers are never exactly the same. These differences in electronic and electrooptic responses can lead to degradation of the idler(s) because the pumps are not perfectly counter-phased.
Thus, there is a continuing need to reduce or eliminate SBS in fiber parametric applications to eliminate SBS without degrading idler(s).
In accordance with the apparatus and operating method of the present invention, a multi-pump phase modulator apparatus is disclosed that uses only one phase modulator to eliminate SBS without degrading the idler(s). Since only one phase modulator is used, the resulting modulation on the pumps is more easily made to be exactly opposite in sign and thus the phase distortion effects resulting from phase modulation used to raise the SBS threshold are more easily eliminated. More specifically in an embodiment of our invention, a multi-pump phase modulator apparatus comprises
means for receiving multiple optical wavelength pump signals;
a single phase modulator for phase modulating the multiple pump signals to form phase modulated multiple pump signals using a modulation signal having a predetermined modulation period;
a wavelength selective signal splitter for separating the phase modulated multiple pump signals into at least two wavelength groups;
a delay apparatus for delaying a first wavelength group by the predetermined modulation period relative to other wavelength groups; and
combiner for combining the delayed first wavelength group with the other wavelength groups to form a combined modulated multiple pump signal.
In another embodiment, a multi-pump phase parametric apparatus comprises
means for receiving multiple optical wavelength pump signals;
a single phase modulator for phase modulating the multiple pump signals to form phase modulated multiple pump signals using a modulation signal having a predetermined modulation period;
a wavelength selective signal splitter for separating the phase modulated multiple pump signals into at least two wavelength groups;
a delay apparatus for delaying a first wavelength group by the predetermined modulation period relative to other wavelength groups;
a combiner for combining the delayed first wavelength group with the other wavelength groups to form a combined modulated multiple pump signal; and
a parametric apparatus for receiving the combined modulated multiple pump signal and for amplifying an input signal S, and for outputting a signal selected from a group including an amplified input signal and one or more idlers generated by the parametric apparatus.
Other embodiments include the use of the multi-pump phase parametric apparatus as part of an Optical Switch, Sampler, Tributary Extractor, and Regenerator.
One feature of the invention is directed to a method of operating a parametric apparatus, comprising the steps of:
receiving multiple optical wavelength pump signals;
using a single phase modulator for phase modulating the multiple pump signals to form phase modulated multiple pump signals using a modulation signal having a predetermined modulation period,
separating the phase modulated multiple pump signals into at least two wavelength groups;
delaying a first wavelength group by the predetermined modulation period relative to other wavelength groups; and
combining the delayed first wavelength group with the other wavelength groups to form a combined modulated multiple pump signal.
The present invention will be more fully appreciated by consideration of the following Detailed Description, which should be read in light of the accompanying drawing in which:
In the following description, identical element designations in different figures represent identical elements. Additionally in the element designations, the first digit refers to the figure in which that element is first located (e.g., 112 is first located in
The present invention is directed to a multi-pump phase modulation apparatus using one phase modulator. A multi-pump parametric apparatus is formed by combining the multi-pump phase modulation apparatus with a parametric amplifier apparatus. Our multi-pump phase conjugator apparatus may be used with two or more pumps. However, for illustrative convenience only the two-pump and three-pump embodiments are described. It should be understood that embodiments using four or more pumps are a straightforward extension of the two-pump and three-pump embodiments described.
Two-Pump Embodiment
Initially, only the two-pump phase modulator and two-pump parametric apparatuses is described. Thus, in the following description of
Shown in
As shown, the output of a two-pump parametric apparatus has four primary bands including inner bands (1+ and 2−) and outer bands (1− and 2+). Inner bands are those bands (1+ and 2−) that are located between the two pump wavelengths and the outer bands (1− and 2+) are those that are not located between but are outside of the two pump wavelengths P1 and P2. Signal amplification of an input signal S is accompanied by the generation of two conjugated idlers and one non-conjugated idler. As shown by 110, when the signal S is in the inner-band 1+, it generates one conjugated idler I1 and one non-conjugated idler I3 in outer-band idlers 1− and 2+, respectively, and one conjugated idler I2 in the inner-band 2−. As shown by 120, when the signal S is in the outer-band 1−, it generates a conjugated idler I1 and a non-conjugated idler I2 in inner-band idlers 1+ and 2−, respectively, and one conjugated idler I3 in outer-band idler 2+. While the signal S is shown as including a plurality of wavelengths f1–f9, the signal S may more generally include one or more wavelengths. Note that when the input signal S is applied to the other bands 2− and 2+ it would produce similar output signal spectra when the pumps P1 and P2 are on.
As noted, the two-pump parametric apparatus produces phase conjugate replicas of the input signal or non-conjugated replicas of the input signal. As shown by 110 and 120, depending in which band the input signal S is located different one(s) of the idlers I1, I2, I3 will be conjugated or non-conjugated relative to input signal S. Consequently, the output from the two-pump parametric apparatus can be selected to be the signal S or one of the idler signals I1, I2, I3, thus the output signal can be a mirrored version (wavelength shifted and conjugated) or translated version (wavelength shifted and non-conjugated) of the signal spectrum S.
The table below summarizes the idler phasing in different output bands for an input signal S in different bands. As shown the first column indicates the input band, the second column the output signal band, and the third column the idler phasing. The first row shows an input band 1−, when the input signal S has a frequency below the frequency of pump signal P1. The input band 1+ is when the input signal S has a frequency above that of pump signal P1 but below the mean of the frequencies of pump signals P1 and P2. The input band 2− is when the input signal S has a frequency above the mean of the frequencies of pump signal P1 and P2 and below that of pump signal P2. Finally, the input band 2+ is when the input signal S has a frequency above that of pump signal P2. The different row entries in the table below show for a given input signal S band, which output band should be used to obtain an output signal that is either a non-conjugate or a conjugate to the input signal S.
With reference to the example 110, if the input signal S is in band 1+, the resulting three idler bands are I1, I2, and I3. As shown, idler I3 is a non-conjugate signal 111 in band 2+ and idlers I1 and I2 are conjugate signals obtained in bands 1− and 2−, respectively. Thus non-conjugate idler 111 has signals f1′–f9′ which have the same relative wavelength amplitude distribution f1′–f9′ as the input signal S, while conjugate idlers I1 and I2 have the reverse spectral distributions, f1″–f9″ and f1′″–f9′″ respectively, as the input signal S. Thus with reference to 110, for an input signal S in band 1+, if a non-conjugate output signal is desired then the idler signal I3 in band 2+ is selected as the output signal and if a conjugate output signal is desired then either idler signal I1 or I3 in bands 1− and 2−, respectively, is selected. As shown, when the output signal, e.g., I3, is a non-conjugate replica of the input signal, S, the wavelength amplitude distribution 112 (f1–f9) of the input signal S and the spectral distribution 111 (f1′–f9′) of the output signal, I3 vary in the same manner. When the output signal, e.g., I2, is a conjugate replica of the input signal, S, the spectral distribution 112 (f1–f9) of the input signal S and output signal I2. i.e., 113, vary in the reverse manner.
With reference to the example 120, when the input signal S is in band 1− the resulting three idler bands are I1, I2, and I3 are located, respectively, in bands 1+, 2−, and 2+. Thus, when the input signal S is in band 1−, a non-conjugate output signal 113 can be obtained using idler I2 in band 2− and conjugate output signal can be obtained using idler I1 or idler I3 in bands 1+ and 2+, respectively.
Under ideal conditions as shown by examples 110 and 120 of
Shown in
Phase modulation circuit 203 includes two phase modulated sources 210 and 220 with nearly identical responses whose output signals are combined in coupler CP, 230, and become the counter-phased modulated pump signals 202. The phase modulated source 210 includes a pump signal λ1 coupled via a polarization controller PC1 to a phase modulator PM1 that is modulated by a waveform D1. The output of phase modulator PM1 is delayed in delay circuit τ1 and coupled via polarization controller PC2 to coupler 230. Phase modulated source 220 is similar to phase modulator 210 and includes a pump signal λ2 coupled via a polarization controller PC2 to a phase modulator PM2 that is modulated by a waveform D2. Ideally, phase modulator PM2 is identical to PM1. Note that the polarization of pumps λ1 and λ2 are maintained orthogonal.
The waveform D2 is either an identical or complementary waveform to waveform D1. The output of phase modulator PM2 is delayed in delay circuit τ2 and coupled via polarization controller PC4 to coupler 230. The two-pump parametric apparatus of
Undesirably, the synchronized pump modulation technique of
The combined pump signal is modulated by a phase modulator (PM), 302 driven using a periodic electrical waveform D1D2, corresponding to a predetermined phase pattern to be impressed on both pump signals (λ1 and λ2). Illustratively, a BERT (bit error rate test set) 310 is used to generate the predetermined phase patterns D1 and D2, which may be pseudo random bit sequences (PRBSs). The BERT 310 may be implemented in a well-known manner using shift registers. Such a BERT is available in various 2N−1 bit sizes, where N is 7, 10, 15, 23, etc. Since delay element 306 is typically implemented using a length of fibers N is selected by determining the practical length of the fiber needed to implement the delay element 306, considering as well the desired spectral characteristics of the modulated pump signals. It should be noted that while our two pump phase conjugator apparatus preferably uses predetermined phase patterns D1 and D2 which use a digital PRBS signal, in other embodiments non-PRBS digital signals or analog signals may be used.
The periodic electrical waveform subpatterns (D1, D2) identical durations (τD1=τD2=τD1D2/2). The modulated pump signal Output of phase modulator 302 is demultiplexed by bandsplitting element (CP2), 303, into a modulated λ1 signal and a modulated λ2 signal.
The modulated λ1 signal is passed through polarization controller PC3A, 305 which can be used to achieve a desired polarization relationship between the modulated λ1 and λ2 signals. The modulated 2 signal is passed through a delay element, 306, that delays the signal by time τ. As will be discussed in a later paragraph, the τ delay element 306 is chosen to match the duration of the modulation subpattern: τ=τD1=τD2=τD1D2/2. The τ delay element 306 may be implemented using a length of single mode fiber (SMF).
The delayed modulated λ1 and λ2 pump signals are combined into a modulated multiple pump signal by combiner coupler (CP3), 307 prior to insertion into amplifier 308 that amplifies the combined signal. The output of amplifier 308 is coupled to parametric apparatus 250. As previously discussed in
The co- or counter-phased pump scheme used in
The τ optical delay 306 introduced to the modulated λ2 pump signal after it is demultiplexed by element CP2, 303, is ideally exactly the length of time of the D1 or D2 sequence (also referred to as the modulation period). Thus the τ delay of element 306 is the flight or travel time (nL/c) that corresponds to the length, L, of the fiber section used to implement the τ delay and ideally equals the duration of the modulation subpattern (τD1=τD2). Here, c/n is the group velocity in the fiber of signals having wavelength λ2. The co- and counter-phased pump modulation corresponds to complementary (D1={overscore (D2)}) and concatenated (D1=D2) waveforms. Specifically, both schemes use a 2N−1 bit long PRBS word generated by the pattern generator: the complementary scheme repeats the PRBS word and its complement, whereas the concatenated scheme simply repeats the PRBS word. The physical length L of the fiber delay (τ) is determined by the duration of the PRBS word:
L=(2N−1)×c/nf,
where N is the PRBS order, c/n is the group velocity of the delayed pump (λ2) and f is the phase modulation frequency. In accordance with one aspect of the present invention, rather than precisely adjusting the length of the fiber used in the τ optical delay 306 in order to match the pattern length and pump group velocity, the required optical delay 306 is adjusted or preset to within about 1 cm (equivalent to about 50 ps delay). The exact delay is then achieved by tuning (or adjusting) the phase modulation frequency, f, (bit rate) of the BERT pattern generator 310. This procedure allows for a large length error (˜cm) in the length of the fiber used for the τ optical delay 306 to be compensated by a small frequency detuning, even for a short PRBS length (N=7): 10 GHz pump modulation requires approximately +/−40 MHz adjustment for each centimeter of the fiber delay mismatch. More importantly, this method does not require precise knowledge of the delayed pump group velocity.
When a counter-phased pump scheme is utilized (i.e., D1={overscore (D2)}), the τ optical delay 306 is set (and, if utilized, BERT pattern generator 310 may be adjusted) to cause the D2 modulated λ1 signal to arrive at combiner 307 at the same time as the D1 modulated λ2 signal. Thus, the modulation of the λ1 signal is the complement of the modulation of the λ2 signal. The net result is that the two modulated λ1 and λ2 pump signals enter the parametric amplifier 309 exactly counter-phased (i.e., when λ1 is modulated with D1, λ2 is modulated with D2 and vice-versa) and, because the modulated λ1 and λ2 pump signals were both modulated with the same modulator/drive electronics 302, there is no mismatch between the modulated λ1 and λ2 pump signals due to different frequency responses of electronic/electrooptic elements of the
While the present invention has been described as using a digital signal pattern D1, D2 for modulation of the pump signals, it should be understood that an analog signal may be used as the modulation signal. One such analog modulation signal containing four frequencies has been described in the article entitled “529 km unrepeatered transmission at 2.488 GBit/s using dispersion compensation, forward error correction, and remote post- and pre-amplifiers pumped by diode-pumped Raman lasers,” by P. B. Hansen; L. Eskilden, S. G. Grubb, A. M. Vengsarkar, S. K. Korotky, T. A. Strasser, J. E. J. Alphonsus, J. J. Veselka, D. J. DiGiovanni, D. W. Peckham, E. C. Beck, D. Truxal, W. Y. Cheung, S. G. Kosinski, D. Gasper, P. F. Wysocki, V. L. da Silva. J. B. Simpson, Electronics Letters, Vol. 31, Iss.17, 17 Aug. 1995 Pages:1460–1461. In one embodiment, these frequencies are spaced roughly by factors of three, for instance, 50 MHz, 150 MHz, 450 MHz, and 1350 MHz and their amplitude is adjusted so that the optical power after the phase modulator in each of the two phase modulation optical sidebands closest to the carrier is equal to the power in the carrier. In one embodiment the exact frequencies could be chosen to provide counter-phased pump modulation or co-phased pump modulation, as required.
The elimination of idler broadening depends on the alignment of the pump modulations at a point within the parametric amplifier, typically at the HNLF. For some embodiments, this alignment may differ from having the pump modulations aligned at combiner coupler (CP3), 307 as discussed in preceding paragraphs. Such difference may arise from the presence of chromatic dispersion, especially if the pumps are amplified or from additional splitting of the paths of the pumps. Where such difference exists, pump modulation can be aligned at any desired point by suitable choice of the length of the fiber used in the τ optical delay, 306, and the phase modulation frequency, f, (bit rate) of the BERT pattern generator 310.
However, changing a pump wavelength will shift the point at which the pump modulations are aligned. This is caused by chromatic dispersion between that point and phase modulator 302. For example, if the apparatus contains 50 meters of standard single mode fiber with a chromatic dispersion of 16 ps/nm, the pump modulation alignment λ1 change by 0.8 ps for every nanometer of change in the wavelength of one of the pumps, λ1 or λ2. This dispersion also causes distortion in the phase modulation imposed on the pumps. Both problems can be mitigated by dispersion compensation. Dispersion compensation is most conveniently provided by dispersion compensating fiber, but other means for compensating dispersion are well known. To completely eliminate distortion caused by chromatic dispersion, each pump should be compensated to provide a net dispersion of zero between the modulator and the desired point of alignment. This can be accomplished by separately compensating each pump in the paths between bandsplitting element 303 and combiner coupler 307 where it is demultiplexed from the other pumps. However, it will generally be more economical to provide most of the dispersion compensation in places such as between combiner 307 and amplifier 308, where the pumps are combined. If this compensation does not suffice small additional adjustments to the compensation can be made separately for each pump in the paths between bandsplitting element 303 and combiner coupler 307 or other paths where the pumps are demultiplexed. For very short pump modulation pattern lengths, the differential delay itself can be provided by dispersion compensation, eliminating the necessity of separately demultiplexing and remultiplexing the pumps.
Amplifier 401 amplifies the modulated λ1 pump signal and amplifier 402 amplifies the modulated λ2 pump signal. In this arrangement, delay 306 is set to correct for differential delays such as the relative pump phasing (between λ1 and λ2) caused by the phase modulation pattern used, fiber chromatic dispersion, and path length differences in section 430 (including delay differences between amplifiers 401 and 402). In
As was described for
The two-pump phase conjugator apparatus 600 shown in
The two-pump phase conjugator apparatus 600 shown in
In another embodiment, the two-pump phase conjugator apparatus 600 also may be utilized as a Tributary Extractor with both pumps P1 and P2 on. The Tributary Extractor is used to extract every nth bit of a binary input signal S as the output signal OUT. The Tributary Extractor includes a Pump Intensity Modulator 602 that uses Pump Intensity Modulation Signal 612 to modulate the intensity of the combined modulated two-pump signal from the dispersion compensating unit 601. The bit sampler circuit 611 is used to sample the input signal S to recover the clock of the input signal S in order to generate a Pump Intensity Modulation Signal 612 with a pulse every n bits. The bit sampler circuit 611 is controlled by control signal 613 to select which bit is to be extracted by the Tributary Extractor. The bit sampler circuit 611 may be implemented as an opto-electronic circuit. In this Tributary Extractor, if desired, the coupler C2 may be used to sample the extracted output signal OUT as a Monitor signal.
Illustratively, if n=4, the Pump Intensity Modulation Signal 612 is turned on every 4th bit of the input signal S as shown in
In yet another embodiment, the two-pump phase conjugator apparatus shown in
Three-Pump Embodiment
With reference to
Alternatively, the additional circuit 321 may be a circuit with a delay equal to τ if λ3 and λ2 are to be co-phase signals and λ1 a counter-phase signal. In this case, the operation of three-pump phase conjugator apparatus for λ3 will be the same as that previously described for λ2. Note that when λ1 and λ2 are co-phased, the bandsplitter coupler CP2 can be made to split both λ1 and λ2 from λ3, so that both λ1 and λ2 would be processed by the same circuit of 320 that previously processed only λ1. In this manner, the additional separate λ2 circuits 321 would not have to be added. Similary, if λ3 and λ2 are co-phased, the bandsplitter coupler CP2 can be made to split both λ2 and λ3 from λ1, so that both λ2 and λ3 would be processed by the circuit 306 of 320 that previously processed only λ2. In either of these two cases, the additional separate λ3 circuits 321 would not have to be added.
The three-pump phase conjugator apparatus shown in
The three-pump phase conjugator apparatus shown in
The generalized three-pump phase conjugator apparatus shown in
While our multi-pump phase conjugator apparatus of the present invention has been discussed as using two or three wavelengths, it should be understood that it may use three or more wavelengths. Thus, what has been described is illustrative of the present invention. Various modifications of our invention will occur to those skilled in the art. Nevertheless all deviations from the specific teachings of this specification that basically rely upon the principles and their equivalents are properly considered within the scope of the invention as described and claimed.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5386314 | Jopson | Jan 1995 | A |
| 20040042060 | McKinstrie et al. | Mar 2004 | A1 |
| 20050146780 | McKinstrie et al. | Jul 2005 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20060061853 A1 | Mar 2006 | US |
