US20040028417A1 - Arrangement for generating chirped return to zero (crz) optical data signals - Google Patents
Arrangement for generating chirped return to zero (crz) optical data signals Download PDFInfo
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- US20040028417A1 US20040028417A1 US10/205,643 US20564302A US2004028417A1 US 20040028417 A1 US20040028417 A1 US 20040028417A1 US 20564302 A US20564302 A US 20564302A US 2004028417 A1 US2004028417 A1 US 2004028417A1
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- 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
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
-
- 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
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference
- G02F1/225—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference in an optical waveguide structure
-
- 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
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
- H04B10/5051—Laser transmitters using external modulation using a series, i.e. cascade, combination of modulators
-
- 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
- H04B10/50—Transmitters
- H04B10/508—Pulse generation, e.g. generation of solitons
-
- 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
- G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
- G02F2201/12—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 electrode
- G02F2201/126—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 electrode push-pull
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Optics & Photonics (AREA)
- Nonlinear Science (AREA)
- General Physics & Mathematics (AREA)
- Optical Communication System (AREA)
- Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
Abstract
Optical chirped return-to-zero (CRZ) data signals are generated without the need for a separate phase modulator, by using a dual-drive Mach-Zehnder modulator for RZ pulse carving that is driven with two typically sinusoidal signals of either unequal amplitude or unequal relative phase, i.e. of non-vanishing phase difference.
Description
- This invention relates generally to the field of optical telecommunications, and in particular, to a method and apparatus for generating optical chirped return-to-zero (CRZ) data signals.
- Optical CRZ signals, i.e., signals in which (a) the optical intensity returns to zero during each bit period, and (b) the optical phase varies (by a desired amount) within each bit period, are important for ultra long distance optical fiber communications. Such signals can be advantageously used e.g., in undersea lightwave systems. For this class of systems, CRZ has been shown to yield high tolerance towards impairments caused by fiber nonlinearities. See, for example, N. S. Bergano, “Undersea Communication Systems”, inOptical Fiber Telecommunications IV B, Academic Press, 2002.
- One known arrangement for generating CRZ signals uses three optical modulators, as shown in FIG. 1. The first
optical data modulator 102 imprints data modulation (among many other possible formats, e.g., non-return-to-zero on-off keying (NRZ-OOK), or NRZ-differential phase shift keying (NRZ-DPSK)) on the light emitted by a continuously operatinglaser 101. Thesecond modulator 103 is a return-to-zero (RZ) pulse carver that carves RZ pulses out of the optical (usually NRZ) data signal. As indicated in FIG. 1,modulator 103 is frequently realized as a dual-drive Mach-Zehnder modulator using sinusoidal drive signals at either the data rate or at half the data rate. Thethird modulator 104 is a sinusoidally driven phase modulator that generates the desired amount of bit-synchronuous phase modulation, i.e. a phase term - exp[j m cos(2πRt)], (1)
- where m is called the modulation index.
- Another known way of generating CRZ can be found in a paper by F. Liu et al., “Chirped return-to-zero source used in 8×10 Gbit/s transmission over 2000 km of standard singlemode fiber”,Electron. Lett. 36 (16), 1399-1400, 2000, where a single Mach-Zehnder modulator is used to generate the data modulation, carve RZ pulses, and generate chirp. However, the maximum phase excursion obtainable from that scheme is limited to ˜0.5π without significantly degrading the data modulation.
- In accordance with the present invention, optical chirped return-to-zero (CRZ) data signals are advantageously generated without the need for a
separate phase modulator 104, as required in the prior art, by using a dual-drive Mach-Zehnder modulator (such asmodulator 103 in FIG. 1) for RZ pulse carving that is driven with two typically sinusoidal signals of either unequal amplitude or unequal relative phase (i.e. with some non-vanishing phase difference). - The present invention will be more fully appreciated by consideration of the following detailed description, which should be read in light of the drawing in which:
- FIG. 1 is a block diagram of a prior art arrangement for generating CRZ signals that uses three optical modulators;
- FIG. 2 is a block diagram of an arrangement in accordance with the present invention, for generating CRZ signals that does not need a separate phase modulator, as required in the prior art arrangement of FIG. 1;
- FIG. 3 is a diagram illustrating the optical intensity and chirped optical phase waveforms of signals generated using the arrangement of FIG. 2, in which the pulse carver is sinusoidally driven at the data rate R to generate RZ with around 50% duty cycle;
- FIG. 4 is similar to FIG. 3, but the pulse carver is sinusoidally driven at half the data rate (R/2) and biased for zero transmission, carrier-suppressed RZ (CSRZ), and
- FIG. 5 is similar to FIG. 3, but the pulse carver is sinusoidally driven at half the data rate (R/2) and biased at maximum transmission to generate RZ with 33% duty cycle.
- In accordance with the present invention, we have eliminated the need for a phase modulator for generating CRZ signals, by imbalancing the drive signals of the Mach-Zehnder modulator used for RZ pulse carving. As shown in FIG. 2, the light of a
laser 201 is modulated with, for example, NRZ data, using adata modulator 202, to produce a modulated optical data signal. (It is to be noted here thatmodulator 202 can be any known modulator arranged to generate a modulated optical data signal, such as an NRZ OOK modulator, an NRZ DPSK modulator, a duobinary or modified duobinary modulator, a vestigial sideband modulator or a QPSK modulator, etc.) The modulated optical data signal output frommodulator 202 is applied to aRZ pulse carver 203, which includes a Mach-Zehndermodulator 204 having first andsecond arms electrodes pulse carver 203 is the desired CRZ signal. - The invention is based on the following principle: The optical field eRZ(t) leaving the Mach-Zehnder
pulse carver 203 is generally given by - e RZ(t)=e NRZ(t)exp[jπ(u 1(t)+u 2(t))/(2V π)]cos[π(u 1(t)−u 2(t))/(2V π)+φbias]. (2)
- where eNRZ(t) denotes the NRZ modulated field generated by
NRZ modulator 202 entering the pulse carver, and u1(t) and u2(t) are thedrive voltage signals - There are basically three types of RZ pulse carver driving to be distinguished, as shown in FIGS. 3, 4 and5:
- A. Pulse Carver Driven at the Data Rate R
- If the pulse carver is sinusoidally driven at the data rate R with
- u 1(t)=U 1 cos[2πRt], and u 2(t)=U 2 cos[2πRt], (3)
- and φbias is adjusted for maximum pulse extinction, RZ pulses with duty cycles between 50% and 33% can be generated by varying the difference of the two drive amplitudes, UΔ=U1−U2, from Vπ/2 to 0. Usually, U1=−U2=Vπ/4 (or, equivalently, UΔ=Vπ/2) is chosen for chirp-free, 50% RZ pulse carving. Imbalancing the drive amplitudes under the constraint that their difference UΔ remains constant does not change the intensity waveforms of the RZ pulses, but generates a bit-synchronuous, sinusoidal phase modulation with maximum phase excursions at the pulse center, as a consequence of the exponential term in Equation (2), which then reads
- exp[jπ(U1+U2)/(2Vπ)cos(2πRt)]. (4)
- Comparing this expression to Equation (1), we identify the phase modulation index m as
- m=π(U 1 +U 2)/(2V π)=πUΣ/(2V π). (5)
- By changing the sum UΣ=U1+U2 of the drive amplitudes while leaving their difference UΔ constant, the amount of bit-synchronuous chirp can thus be easily tuned to any desired value. Also, by either changing the modulator bias φbias by π, or by exchanging the role of u1(t) and u2(t), the sign of the chirp can be changed from positive to negative. FIG. 3 illustrates this type of operation in terms of intensity (upper graph) and phase (lower graph) for three different amounts of chirp (m=0, m=π/2, m=2.5π).
- B. Pulse Carver Driven at R/2, Biased at Minimum Transmission
- If the pulse carver is sinusoidally driven at half the data rate (R/2) and biased for zero transmission, carrier-suppressed RZ (CSRZ) is generated. The pulse duty cycle can be continuously changed from 67% (shown in FIG. 4) to 50% by varying UΔ from Vπto 0. If, according to our invention, the amplitudes of the carver are imbalanced, we achieve alternate-chirp CSRZ (AC-CSRZ), as proposed in [R. Ohhira, D. Ogasahara, and T. Ono, “Novel RZ signal format with alternate-chirp for suppression of nonlinear degradation in 40 Gb/s based WDM”, Proc. OFC'01, paper WM2, 2001.] to mitigate nonlinear transmission effects. As for case A, we can choose any phase modulation index m by setting UΣ to the desired value. FIG. 4 illustrates this type of operation in terms of intensity (upper graph) and phase (lower graph) for three different amounts of chirp (m=0, m=π/2, m=2.5π).
- C. Pulse Carver Driven at R/2, Biased at Maximum Transmission
- If the pulse carver is sinusoidally driven at half the data rate (R/2) and biased at maximum transmission, RZ with 33% duty cycle is generated. As opposed to cases A and B, the pulse shape cannot be tailored without degrading pulse extinction. Also, chirp cannot be generated by imbalancing the drive amplitudes. However, by imbalancing the relative phase between the two driving signals, AC-CSRZ signals with variable phase modulation index can be generated. Using the Ansatz
- u 1(t)=U 1 cos[πRt], and u 2(t)=U 2 cos[πRt+δ], (6)
- where δ represents a phase angle between the two drive signals, we calculate the modulation index m from Equations (1) and (2) as
- m=πU 1 /V π cos(δ/2). (7)
- Note that for this kind of pulse carver operation, the drive amplitudes U1 and U2 have to be adjusted as U1=U2=Vπ sqrt[m2/π2+0.25] to assure good pulse extinction as well as no linear phase term that would cause pulse frequency shifts. FIG. 5 illustrates this type of operation in terms of intensity (upper graph) and phase (lower graph) for three different amounts of chirp (m=0, m=π/2, m=2.5π).
- Various modifications of this 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 through which the art has been advanced are properly considered within the scope of the invention as described and claimed. For example, although the imbalanced signals have been described above as being sinusoidal, there may be instances in which non-sinusoidal drive signals may be preferred.
Claims (10)
1. Apparatus for generating optical chirped return-to-zero (CRZ) data signals, said apparatus comprising
a laser,
a first modulator for imprinting the output of said laser with a data signal to produce a modulated optical data signal, and
means for applying said modulated optical data signal to a Mach-Zehnder modulator having first and second arms that are driven by first and second drive signals, u1(t) and u2(t), respectively, wherein said first and second drive signals are imbalanced.
2. The invention defined in claim 1 where said drive signals have imbalanced amplitudes.
3. The invention defined in claim 1 where said drive signals have imbalanced phases.
4. The invention defined in claim 2 or 3 wherein said modulated optical data signal is generated by a first modulator chosen from the group consisting of an NRZ OOK modulator, a DPSK modulator, a duobinary or modified duobinary modulator, a vestigial sideband modulator and a QPSK modulator.
5. The invention defined in claim 2 , wherein the pulse carver is driven with two periodic signals of period 1/R or 2/R, where R is the data rate, and RZ pulses with desired shape and duty cycle are generated.
6. The invention defined in claim 5 wherein said periodic signals are u1(t)=U1 cos[2πRt], and u2(t)=U2 cos[2πRt].
7. The invention defined in claim 6 wherein RZ pulses with duty cycles between 50% and 33% are generated by selecting the difference of the two drive amplitudes, UΔ=U1−U2, in the range from Vπ/2 to 0.
8. The invention defined in claim 5 wherein said periodic signals are u1(t)=U1 cos[πRt], and u2(t)=U2 cos[πRt].
9. The invention defined in claim 8 wherein RZ pulses with duty cycles between 67% and 50% are generated by selecting the difference of the two drive amplitudes, UΔ=U1−U2, in the range from Vπ to 0.
10. The invention defined in claim 3 , wherein the pulse carver is driven at the data rate R/2, and wherein the drive signals are arranged such that u1(t)=U1 cos[πRt], and u2(t)=U2 cos[πRt+δ], where δ represents a non-zero phase angle between said two drive signals.
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Cited By (4)
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US20060008278A1 (en) * | 2004-06-30 | 2006-01-12 | Yuan-Hua Kao | Method and apparatus for CRZ-DPSK optical signal generation |
US20090074407A1 (en) * | 2007-09-14 | 2009-03-19 | Sierra Monolithics, Inc. | High-speed serializer, related components, systems and methods |
US20090255085A1 (en) * | 2008-04-09 | 2009-10-15 | Nixiface, A Series Of The Invention Machine, Llc | Over-the-door adjustable door stop |
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US7330666B1 (en) * | 2003-01-31 | 2008-02-12 | Ciena Corporation | Method and apparatus for controlling modulator phase alignment in a transmitter of an optical communications system |
US7305189B2 (en) * | 2002-04-15 | 2007-12-04 | Telefonaktiebolaget Lm Ericsson (Publ) | Phase modulation for an optical transmission system |
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US6714695B2 (en) * | 2001-09-17 | 2004-03-30 | Dorsal Networks, Inc. | Optical transmission system employing auto-synchronized chirped return-to-zero transmitter |
US20030133652A1 (en) * | 2002-01-15 | 2003-07-17 | Andrekson Peter A. | Method and apparatus for improving performance in noise limited optical transmission systems |
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2002
- 2002-07-25 US US10/205,643 patent/US6693929B1/en not_active Expired - Lifetime
Cited By (6)
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US20060008278A1 (en) * | 2004-06-30 | 2006-01-12 | Yuan-Hua Kao | Method and apparatus for CRZ-DPSK optical signal generation |
US7346283B2 (en) * | 2004-06-30 | 2008-03-18 | Lucent Technologies Inc. | Method and apparatus for CRZ-DQPSK optical signal generation |
US20090074407A1 (en) * | 2007-09-14 | 2009-03-19 | Sierra Monolithics, Inc. | High-speed serializer, related components, systems and methods |
US20090255085A1 (en) * | 2008-04-09 | 2009-10-15 | Nixiface, A Series Of The Invention Machine, Llc | Over-the-door adjustable door stop |
US20100086314A1 (en) * | 2008-10-02 | 2010-04-08 | Nec Laboratories America, Inc. | Method and Apparatus for an Optical Duty Cycle for an Optical Digital Coherent System |
US8358936B2 (en) * | 2008-10-02 | 2013-01-22 | Nec Laboratories America, Inc. | Method and apparatus for an optical duty cycle for an optical digital coherent system |
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