GB2382247A

GB2382247A - RZ (return to zero) optical transmission using chirped NRZ

Info

Publication number: GB2382247A
Application number: GB0221715A
Authority: GB
Inventors: Steve Webb; Steve Desbrulais
Original assignee: Ditech Networks Inc
Current assignee: Ditech Networks Inc
Priority date: 2001-09-24
Filing date: 2002-09-18
Publication date: 2003-05-21
Anticipated expiration: 2022-09-18
Also published as: GB0221715D0; GB2382247B

Abstract

A chirped NRZ (non-return to zero) encoded optical signal is passed though a dispersive element to cause the light to bunch into packets centred on a respective clock cycle so as to be detectable as an RZ (return to zero) encoded signal. The optical NRZ signal generated by modulator 11 is transformed into a chirped RZ format by applying optical phase or frequency modulation at the data clock rate 12-14, and subsequently passing it through a linear chromatic dispersive element 16. The architecture 13 can be implemented using a single Lithium Niobate substrate incorporating the two modulation functions 11, 12. The dispersive element e.g. a Fibre Bragg Grating (FBG) may be placed anywhere between the output of the modulators and a receiver, or it may be replaced by the net dispersion of a transmission line. The modulators 11, 12 may be Mach Zender (MZ) modulators.

Description

CHIRPED NRZ OPTICAL TRANSMISSION Background to the Invention Long haul optical data transmission at 10Gbit/s often utilises a Chirped RZ modulation format. A Chirped RZ format improves the detection of the optical signal since distortions to the signal during transmission are generally less severe than for NRZ A known architecture used to generate such an RZ signal from NRZ electrical data uses three concatenated Lithium Niobate modulators. The first two are Mach Zehnder (MZ) modulators and the third is a phase modulator. In this arrangement, the phase modulation is via a clock signal adjusted so that one half of a data pulse is red shifted and the other half blue shifted. This is termed"clock pre- chirp". Although the architecture provides high performance transmission quality, the number of separate modulators required to generate an RZ signal with clock pre-chirp makes the design expensive and complicated.

It is possible to integrate more than one function onto a single Lithium Niobate substrate, but at 10Gbit/s this is limited to two modulation functions with current technology. The objective is to achieve high quality chirped RZ format at the receiver with one modulator package to reduce component count, assembly size and cost.

Summary of the Invention According to a first aspect of the present invention, a method of transmitting optical data comprises the step of passing a chirped NRZ encoded optical signal through a dispersive element to cause the light to bunch into packets centred on a respective clock cycle so as to be detectable as an RZ encoded signal In one embodiment the dispersive element is an integral component of an optical transmitter. In another embodiment an optical transmission path within a transmission system forms at least part of the dispersive element. A transmission system inevitably contains some dispersion, which is known from a dispersion map, and this can replace or be used in combination with a specific dispersive element.

In fact, rather than incorporating a dispersive element into the transmitter, the degree of chirp applied to an NRZ encoded optical system can be altered in dependence on the known dispersion of a transmission path in order to obtain an RZ encoded signal at a receiver.

Preferably, the chirped NRZ encoded optical signal is generated by passing an NRZ optical signal through a phase modulator driven by a clock. Altematively,

the chirped NRZ encoded signal is generated by applying a clock to a laser source to produce a frequency modulated output which is then modulated by an NRZ electrical data signal.

According to a second aspect of the present invention, an optical transmission system for generating an optical data signal having an RZ format, comprises a signal modulator that generates a chirped NRZ optical signal, the output of the signal modulator being coupled to a dispersive element that perturbs the optical signal so that each bit of data bunches into a packet centred on a respective clock cycle.

Preferably, the signal modulator comprises a Mach Zender (MZ) modulator driven by an NRZ electrical data signal, and a phase modulator driven by a clock signal that applies clock pre-chirp.

Preferably, the clock signal is derived from the electrical data signal.

The dispersive element may be a length of dispersion compensation fibre.

Alternatively, the dispersive element may be a fibre Bragg grating (FBG).

A transmission system inevitably contains some dispersion and this can replace or be used in combination with a specific dispersive element. Accordingly the dispersive element may comprise all the optical elements coupled between the output of the signal modulator and a receiver. Furthermore, rather than incorporating a dispersive element into the transmitter, the degree of chirp applied to an NRZ encoded optical system can be altered in dependence on the known dispersion of a transmission path in order to obtain an RZ encoded signal at a receiver.

Preferably, the dispersive element introduces a degree of dispersion given by the equation:

c Dispersion =----xiooo-.., 2 (/LB) Equation 1 where: A is the wavelength (m) B is the bit rate (Bits 5.') c is the speed of light (mus'') Preferably, the phase amplitude ( < 1 > a) is adjusted so that

Equation 2 4 4 In the present invention, an optical NRZ signal may be transformed into an RZ format by applying optical phase or frequency modulation at the data clock rate, and subsequently passing it through linear chromatic dispersion. The architecture required to perform this function can be implemented using a single Lithium Niobate substrate incorporating two modulation functions.

The present invention can be implemented in two ways. In an optical transmitter so as to generate an RZ encoded optical signal from an NRZ encoded signal prior to transmission over an optical transmission line or optical network, or in an optical transmission system (i. e. a transmitter, a transmission path and a receiver) so as to generate an RZ encoded optical signal at the receiver from a chirped NRZ (CNRZ) signal at the transmitter. The signal may take a variety of forms at different points along the transmission path. The degree of chirp applied to the NRZ signal in the transmitter can be altered to match the net dispersion of the transmission system so as to generate an RZ encoded optical signal at the receiver.

Brief Description of the Drawings Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows an architecture for generating a chirped NRZ (CNRZ) optical signal ; and, Figures 2A and 2B are an eye diagram and signal waveforms, respectively, for an RZ signal generated using 680 ps nm-1 of pre-dispersion compensation; Figures 3A and 3B are an eye diagram and signal waveforms, respectively, for an RZ signal generated using 340 ps nm-1 of pre-dispersion compensation; Figure 4 is a schematic diagram of a transmission system; Figure 5 shows a typical dispersion map and signal evolution down a system ; Figures 6a, 6b and 6c are eye diagrams showing the evolution of a signal down a system with net dispersion; Figure 7 shows how dispersion compensation error can be corrected by reducing phase modulation magnitude; Figure 8 shows the evolution of a signal down a system with net dispersion with the last compensation element removed; and,

Figure 9 shows the evolution of a signal down a system with net dispersion with the transmit compensation moved to the receive end.

Detailed Description Two schemes for conversion of optical NRZ signals to RZ optical signals according to the present invention are presented in Figure 1.

The preferred implementation consists of a CW laser 10, for example a DFB laser, that is coupled to an integrated MZ modulator 11 and phase modulator 12 component 13 (implemented using an MZ modulator device on the same substrate).

NRZ electrical data is applied to the MZ modulator 11. A clock derived from the original data signal is then applied to the phase modulator after it has passed through adjustable attenuation and delay elements 14 and 15 respectively. The optical signal at the output of the MZ component 13 then undergoes linear dispersion of positive or negative sign. Typically, the dispersive element 16 is arranged to be fibre with a low optical power to ensure non-linear signal transforms are prevented. It is possible to use other dispersive elements such as chirped Fibre Bragg Gratings (FBG's) or a combination of dispersive elements. The dispersive element 16 is shown surrounded by a dotted line to signify that it may be placed anywhere between the output of the modulator 13 and a receiver or it may be replaced by the net dispersion of a transmission line. This is described in more detail later on.

The amplitude and phase of the clock signal with respect to the data signal are arranged to produce the correct optical clock pre-chirp such that a CNRZ to RZ signal conversion is achieved after the dispersive element 16. The process is due to the optical red/blue shifts on alternate halves of each data bit with dispersion causing the light to bunch into packets centred on a clock cycle.

A dispersion map is used to achieve optimal transmission in any long haul high bit-rate system. Typically, this involves pre-and post-dispersion compensation at the system beginning and end, as well as regular compensation throughout the transmission link. In this design the pre-dispersion element is used as an NRZ to RZ transformation component. Our experimental investigations show that at a wavelength of 1555nm and 10 Gbit/sec bit rate, the degree of dispersion required is 680 ps nm-1. The effect of this is shown in Figures 2A and 2B. The effect of reducing this amount of dispersion by half is indicated in Figures 3A and 3B. Here the conversion is less optimal and increased phase modulation is required to achieve an RZ signal. The net result is a reduced pulse width.

More generally, the ideal degree of dispersion (in ps nm-') required is given by the following equation:

where c is the speed of light, X is the wavelength of the optical signal, and B is the bit rate of the optical signal.

The phase amplitude (0.) ils adjusted so that:

The alternative implementation intimated in Figure 1 (by the dotted lines) involves applying a clock signal to the CW laser in order to frequency modulate it. Typically, a standard DFB laser will exhibit about 1GHz optical wavelength shift per mA of injection current applied (due to changes in the internal refractive index). The required currents are sufficiently small that very little unwanted amplitude modulation will be seen. The frequency modulation applied is an analogy of the prechirp via phase modulation described above As shown in Figure 1, the optical signal may then pass through an amplifier 17 to boost the signal powers before entering a transmission system. Typically, the signal will undergo non-linear transformations during propagation for which the format has been shown to be robust against. Balancing of the exact phase and amplitude is required to achieve optimal transmission, which may be controlled via a feedback loop (not shown) As mentioned above, in another embodiment a dispersive element in the transmitter which converts a CNRZ signal to an RZ signal prior to the signal entering a transmission system can be replaced or supplemented by the net dispersion of the transmission line. The important feature is the generation of an RZ signal at a receiver.

Figure 4 is a schematic representation of an optical transmission system. A transmitter T is connected via an optical network 20 to a receiver R. The transmitter T corresponds to the Transmitter architecture shown in Figure 1. The transmission path includes a number of optical nodes 21. The nodes may comprise various optical components such as amplifiers, switches, optical add-drop multiplexers and

dispersion compensators. Each component in the optical path has an associated dispersion and it possible to construct a dispersion map showing the net dispersion between the transmitter and any point on the transmission path.

Figure 5 shows a typical dispersion map and signal evolution down a transmission system. Dispersion is shown on the vertical axis and distance is shown on the horizontal axis. Typically a transmission line has an associated dispersion and the transmission system is dispersion compensated periodically with distance to maintain the net dispersion around a mean value. As already described the combination of clock chirp and residual dispersion may combine to form RZ like data from NRZ data. However, as shown in Figure 6, since the dispersion progressively changes with distance the data will steadily evolve from RZ in Figure 6a (with negative dispersion for example), back to NRZ at zero dispersion in Figure 6b and then disintegrate with positive conversion in Figure 6c. The required dispersion to achieve the NRZ to RZ conversion can be either lumped or distributed.

There is no distinction between the dispersion at the transmitter and that over the complete system.

In the idealistic dispersion map shown in Figure 5, dispersion is added at the end of the system in order to arrange for an RZ format to be formed there, and thereby improve reception.

Alternatively or in addition the amount of applied chirp can be tuned to generate an RZ signal at a particular point. Due to the dispersion slope typically exhibited by fibre, different wavelengths in a WDM link will not be optically compensated leading to a dispersion error, as shown in Figure 7. In this case the phase modulation amplitude for that wavelength may be adjusted to optimise the formation of RZ data at the receiver. In the example shown the phase modulation amplitude will be reduced.

This technique is made possible because the NRZ to RZ conversion is fairly insensitive to the exact value of dispersion given by equation 1. Furthermore, adjusting the phase modulation amplitude to optimise the performance of the system helps mitigate the effects of SPM and XPM and significantly improves the dispersion tolerance It has been found that the optimum phase modulation amplitude both corrects the residual dispersion error and produces an RZ waveform at the receiver.

By applying clock-related phase modulation onto all data bits they will each undergo the same transmission distortions. With unchirped NRZ this is not true since runs of ones may suffer corruption at the beginning and ends from SPM for example so some data bits are more likely to produce errors than others Transmission benefits in the modulation scheme also stem from the fact that at

certain points in the system the data is brought back to an RZ form. This will reduce interactions in a WDM system since wavelengths will walk through each other more rapidly under the influence of dispersion.

It is possible to leave out the final dispersion compensator of Figure 4 and invert the polarity of the phase modulation as shown in Figure 8. The transmitter does not launch an RZ signal but an RZ signal is received and one is generated at intermediate points. Similarly it is possible to remove the transmit end dispersion compensator and move it to the receive end. This is shown in Figure 9. The phase modulation polarity and magnitude is then adjusted to achieve RZ data at the receiver. In the example shown in Figure 9 removal of the receive end compensation element will achieve NRZ data with lower receiver performance.

Claims

1. A method of transmitting optical data comprising the step of: passing a chirped NRZ encoded optical signal through a dispersive element to cause the light to bunch into packets centred on a respective clock cycle so as to be detectable as an RZ encoded optical signal.

2. A method according to claim 1, wherein the dispersive element forms part of an optical transmitter.

3. A method according to claim 1, wherein a transmission system forms part of the dispersive element.

4. A method according to any preceding claim, wherein the chirped NRZ encoded optical signal is generated by passing an NRZ optical signal through a phase modulator driven by a clock.

5. A method according to any one of claims 1 to 3, wherein the chirped NRZ encoded signal is generated by applying a clock to a laser source to produce a frequency modulated output which is then modulated by an NRZ electrical data signal.

6. A method according to claim 4 or 5, wherein the degree of chirp applied to the NRZ encoded signal is altered in dependence on the net dispersion of a transmission path.

7. An optical transmission system for generating an optical data signal having an RZ format, comprising : a signal modulator that generates chirped NRZ optical signal, the output of the signal modulator being coupled into a dispersive element that perturbs the optical signal so that each bit of data bunches into a packet centred on a respective clock cycle.

8. An optical transmission system according to claim 6, wherein the signal modulator comprises a Mach Zender modulator dnven by an NRZ electrical data signal ; and, a phase modulator driven by a clock signal that applies clock pre-chirp.

9. An optical transmission system according to claim 8, wherein the clock signal is derived from the electrical data signal.

10. An optical transmission system according to any one of claims 7 to 9, wherein the dispersive element includes a length of dispersion compensation fibre.

11. An optical transmission system according to any one of claims 7 to 10, wherein the dispersive element includes a fibre Bragg grating.

12. An optical transmission system according to any one of claims 7 to 11, wherein the dispersive element comprises all optical elements coupled between the output of the signal modulator and a receiver.

13. An optical transmission system according to any one of claims 7 to 12, wherein the degree of chirp applied by the signal modulator is dependent on the net dispersion of the optical transmission system.

14. An optical transmission system according to any one of claims 7 to 13, wherein the dispersive element introduces a degree of dispersion given by the equation:

15. An optical transmission system according to any one of claims 7 to 14, wherein the phase amplitude (Ca) is adjusted so that:

16. An optical transmission system according to any one of claims 7 to 15, comprising a single Lithium Niobate substrate incorporating two modulation functions.