WO2014114329A1

WO2014114329A1 - Optical comb generator

Info

Publication number: WO2014114329A1
Application number: PCT/EP2013/051189
Authority: WO
Inventors: Le Nguyen Binh; Bangning Mao; Ming Chen; Changsong Xie
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2013-01-23
Filing date: 2013-01-23
Publication date: 2014-07-31
Also published as: CN104919361A

Abstract

The invention relates to an optical comb generator (500) for generating a plurality of optical sub-carriers (507) from a primary lightwave carrier (506), the optical comb generator comprising an optical pulse-shaping circuit (502) arranged to modulate the amplitude of the primary lightwave carrier (506) in response to an electrical driving signal (509) so as to form a sequence of optical pulses (512), an optical phase modulation circuit (504) arranged to modulate the phase of the sequence of optical pulses in response to a phase-shifted electrical driving signal (510) so as to form the plurality of optical sub-carriers (507), wherein the optical comb generator further comprises a phase shift unit (516) adapted to generate the phase-shifted electrical driving signal (510) by shifting the phase of a copy of the electrical driving signal (509), the phase shift of the phase-shifted electrical driving signal (510) with respect to the electrical driving signal (509) being such that the operation of the optical pulse-shaping circuit (502) is synchronized with the operation of the optical phase modulation circuit (504).

Description

DESCRIPTION

Optical comb generator BACKGROUND

The present invention relates to an optical comb generator for generating a plurality of optical sub-carriers and to a method for generating a plurality of optical sub-carriers from a primary lightwave carrier.

Optical networking using lightwaves whose frequencies are allocated into a uniform grid, the dense wavelength division multiplexing (DWDM), is well known over the last half of century. A DWDM channel is formed and transmitted by lightwaves generated in a single longitudinal frequency semiconductor laser over which the information is superimposed by manipulating the intensity and/or phase via a modulating device operating in the optical frequency region. An optical receiver subsequently converts the lightwave into electric current by mixing with a local laser whose amplitude and phases are preserved in the electrical domain.

Due to fast growth of information, the demands on lightwaves networks are intense in terms of capacity and efficiency that can be increased significantly by grouping information data into a single symbol of a system of multi-level in amplitude and phase, the multi-level modulation formats. The number of levels in phases can be switched or shifted from one state to the others. This technique is called phase shift keying (PSK). A phase state can be formed by two vectorial components in a Euclidean coordinate system, the in-phase or real part and the quadrature or imaginary part.

Thus the lightwaves at the transmitter generated from the laser are mapped into a number of states corresponding to the combination of the binary bits into one state of the multi-level system of phase and amplitude, the in-phase and quadrature system, the quadrature phase shift keying (QPSK) modulation format. Thence these states of the lightwaves are transmitted over the transmission medium, the single optical fibers. These signals are then entered into a receiver in which they are mixed with a laser locally positioned so that the beating lightwaves between the signals and this laser gives the recovery states of the original modulated states at the output of the transmitter. This is the coherent detection and is referred as Co-QPSK. Other modulation formats by manipulating amplitude and phase of a number of lightwave carriers can be used to increase the information capacity carried by a single lightwave source. Orthogonal frequency division multiplexing can also be implemented on a lightwave by generating several sub-carriers in the electrical domain by using the fundamental relationships of the individual sinusoidal or co-sinusoidal functions. Once transferred to the optical domain, that is the frequency spectral region in order of 193.51 THz, these sub- carriers are orthogonal in the passband of the lightwave. The modulation schemes can be superimposed on several lightwave carriers which are generated from a lightwave source. They are arranged in such a fashion that they are orthogonal to each other. In another word the maximum of one channel is positioned exactly on the zero of the adjacent channel spectra. The optical carriers generated from a single frequency lightwave source named as secondary lightwave carriers, the modulated secondary carriers are called secondary channels.

The secondary channels are arranged to meeting the conditions for orthogonality in the frequency domain. They are named as orthogonal primary carrier channels.

The generation of a set of secondary carriers or sub-carriers allows the modulation and carrying multiple information channels over one primary lightwave source. That is the total information capacity of the primary lightwave source is equivalent to the summation of the capacity of all the secondary sub-carriers.

The set of subcarriers generated in a fashion that they are equally spaced between adjacent sub-carriers and their amplitude are nearly equal with each other can be called as comb- structure sub-carriers. The optical and electrical circuitry that generates these sub-carriers is called the comb generator.

Optical modulation format for high spectral efficiency based on multi-level in-phase and quadrature phases and multicarrier have already been demonstrated for 100 Gigabit per second (Gb/s), 1 12 Gb/s or 224 Gb/s and 448 Gb/s per secondary carriers. Comb generation of multi sub-carriers are employed in these optical transmission systems. Transmission of Tera bits per second (Tb/s) capacity of information is critical to Tera bits information networks due to extensive demands of information hungry societies. The limit of electronic speed circuit hinders the increase of the transmission capacity. Generation of comb-structure sub-carriers and the uses of these subcarriers to carry information channels over the optical transmission lines are reported in literatures and granted patents. These devices for generation of comb-structure sub-carriers, however, are complex and the flatness is not well equalized.

SUMMARY

It is an object of the invention to provide a concept for an optical comb generator which is simple to realize and which provides an improved flatness of the comb spectrum at a high optical carrier-to-noise ratio (OCN ). This object is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

The invention is based on the idea that two optical modulators, an optical pulse shaper and an optical phase modulator are simultaneously excited, so that diffraction and equalization can occur at the same time to achieve an equally spaced and equalized amplitude comb structure.

In order to describe the invention in detail, the following terms, abbreviations and notations will be used:

OCNR: Optical Carrier to Noise Ratio;

RZ-OOK: Return -to-zero On/Off Keying;

RZ-ASK: Return -to-zero Amplitude Shift Keying;

CSRZ-OOK Carrier-Suppressed Return-to-Zero On/Off Keying;

CS RZ-ASK: Carrier-Suppressed Return-to-Zero Amplitude Shift Keying;

N RZ-OOK: Non-Return-to Zero On/Off Keying; NRZ-DPSK Non-return-to-zero Differential Phase Shift Keying;

NRZ-DQPSK Non-return-to-zero Differential Quadrature Phase Shift Keying;

RZ-DPSK: Return-to Zero Differential Phase Shift Keying;

RZ-DQPSK: Return-to Zero Differential Quadrature Phase Shift Keying;

CSRZ-DPSK Carrier-Suppressed Return-to-Zero Differential Phase Shift Keying;

CS-DQPSK Carrier-Suppressed Differential Quadrature Phase Shift Keying;

QPSK Quadrature Phase Shift Keying;

PSK Phase Shift Keying;

OOK On/Off Keying; ASK Amplitude Shift Keying;

RF: Radio Frequency;

ECL: External cavity laser;

MZIM: Mach-Zehnder Interferometer Modulator;

IQ-Modulator: Inphase-Quadrature-Modulator; OFDM: Orthogonal Frequency Division Multiplex;

DAC: Digital to analog converter.

According to a first aspect, the invention relates to an optical comb generator for generating a plurality of optical sub-carriers from a primary lightwave carrier, comprising: an optical pulse-shaping circuit modulating the amplitude of the primary lightwave carrier to form a sequence of optical pulses, the optical pulse-shaping circuit in response to an electrical driving signal; an optical phase modulation circuit modulating a phase of the sequence of optical pulses to form the plurality of optical sub-carriers in response to a phase-shifted electrical driving signal; and a phase shift unit for phase shifting the electrical driving signal to obtain the phase-shifted electrical driving signal to synchronize an operation of the optical pulse-shaping circuit and the optical phase modulation circuit.

The first aspect of the invention describes an apparatus to generate secondary carriers, the sub-carriers, from the principal light source by simultaneously modulating the continuous lightwave in both amplitude and phase with specific shapes for diffraction by chirping the phase and equalizing the amplitudes in the distribution of the generated sub-carriers in the spectral domain.

The comb generator generates sub-carriers from a primary lightwave source which has a characteristic like a single frequency lightwave source. The generation is implemented by simultaneous modulation in amplitude and phase of the lightwave and may be performed by a simple design as described below with respect to Figures 1 to 3.

In a first possible implementation form of the optical comb generator according to the first aspect, the plurality of optical sub-carriers are equally spaced in frequency.

The amplitude envelope is shaped specifically to a super-Gaussian like and then the lightwave carrier under this shaped optical sequence is phase manipulated with some design distribution so that the chirping of the phase can be achieved and thence diffraction of such lightwaves to achieve a set of equally spaced and flat-amplitude sub-carriers.

In a second possible implementation form of the optical comb generator according to the first aspect as such or according to the first implementation form of the first aspect, the optical comb generator comprises an electrical signal generator for generating the electrical driving signal.

The electrical driving signal may be generated internally in the comb generator or may be taken from an external signal generator. In a third possible implementation form of the optical comb generator according to the second implementation form of the first aspect, the electrical driving signal is sinusoidal. In a fourth possible implementation form of the optical comb generator according to the second implementation form or according to the third implementation form of the first aspect, a frequency of the electrical driving signal is adjustable, the frequency of the electrical driving signal is determining a frequency spacing of the optical sub-carriers.

The comb generator is flexible in adjusting to different environments where different sub- carrier characteristics are required. In a fifth possible implementation form of the optical comb generator according to one of the second to the fourth implementation forms of the first aspect, the optical comb generator comprises an electrical signal splitter for splitting the electrical driving signal into a first and a second electrical signal path. In a sixth possible implementation form of the optical comb generator according to the fifth implementation form of the first aspect, the electrical driving signal drives the optical pulse- shaping circuit and the phase-shifted electrical driving signal drives the optical phase modulation circuit such that both driving signals are simultaneously and synchronously entering electrodes of the optical pulse-shaping circuit and the optical phase modulation circuit and travelling along the electrodes at the same speed.

In a seventh possible implementation form of the optical comb generator according to the fifth implementation form or according to the sixth implementation form of the first aspect, the phase shift unit is implemented as a first electrical phase shifter in the second electrical signal path.

In an eighth possible implementation form of the optical comb generator according to the seventh implementation form of the first aspect, a phase-shift of the first electrical phase shifter is adjustable.

In a ninth possible implementation form of the optical comb generator according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the optical pulse-shaping circuit is configured to modulate the amplitude of the primary lightwave carrier according to a super-Gaussian pulse-shape. In a tenth possible implementation form of the optical comb generator according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the optical pulse-shaping circuit comprises an optical splitter for splitting the primary lightwave carrier into a first and a second optical path.

In an eleventh possible implementation form of the optical comb generator according to the tenth implementation form of the first aspect, a second electrical phase shifter is

implemented in the optical pulse-shaping circuit to phase shift the electrical driving signal obtaining a second phase-shifted electrical driving signal.

In a twelfth possible implementation form of the optical comb generator according to the eleventh implementation form of the first aspect, the optical pulse-shaping circuit comprises a dual drive Mach-Zehnder Interferometer Modulator having a first electrode driven by the electrical driving signal and a second electrode driven by the second phase-shifted electrical driving signal.

In a thirteenth possible implementation form of the optical comb generator according to the twelth implementation form of the first aspect, the optical pulse-shaping circuit comprises a biasing circuit for biasing the electrodes of the dual drive Mach-Zehnder Interferometer Modulator.

According to a second aspect, the invention relates to a method for generating a plurality of optical sub-carriers from a primary lightwave carrier, comprising: modulating an amplitude of the primary lightwave carrier to obtain a sequence of optical pulses, the amplitude modulating being driven by an electrical driving signal; phase shifting the electrical driving signal to obtain a phase-shifted electrical driving signal; and/or modulating a phase of the sequence of optical pulses to obtain the plurality of optical sub-carriers, the phase

modulating being driven by the phase-shifted electrical driving signal, wherein the phase shifting the electrical driving signal is such that the amplitude modulating and the phase modulating are synchronized.

The second aspect of the invention describes a technique to generate secondary carriers, the sub-carriers, from the principal light source by simultaneously modulating the continuous lightwave in both amplitude and phase with specific shapes for diffraction by chirping the phase and equalizing the amplitudes in the distribution of the generated sub-carriers in the spectral domain. The generated comb spectrum according to the aforementioned aspects of the invention offers the following performances and advantages:

- Frequency spacing of the sub-carriers or secondary carriers is 50GHz when driving with a sine wave electrical signal simultaneously to both the pulse shaper and the optical phase modulator with the same wave frequency.

- The optical to carrier noise ratio (OCN ) can reach 60 dB offering the highest ever ratio due to non-integrated optical amplifying device. This is in significant contrast to other types of comb generators.

- The number of comb-structure sub-carriers can reach 14 (fourteen) and higher if desired, with appropriate equalization using pulse shaping and electrical driving signals to the phase modulator.

- Equalization of the amplitudes of the sub-carrier comb spectrum can be achieved with only 0.5 dB ripple over the entire spectrum of 650 GHz. Wider comb spectrum can be achieved if a respective electrical wave is available for driving the optical devices.

The integration of these modulated multi-carrier channels leads a total capacity of Tb/s per wavelength carrier. The optical channels are transported over optical fiber links that are arranged in orthogonal fashion in both transmitting and receiving sub-systems.

The comb generator generates several secondary carriers from a primary lightwave carrier of equally space distribution over very wide spectral range. The amplitudes of the secondary carriers are equalized with very small difference in amplitudes. The secondary carriers are equally spaced. The frequency spacing between the secondary carriers is adjustable.

The comb generator may comprise a single lightwave source which generates single frequency like spectrum. The lightwave generated from this primary source is a continuous or oscillating wave with a specific period which might not vary. Thus this can be considered as a coherent electromagnetic source. In an implementation form, the single lightwave source is provided externally.

The primary lightwave is fed through integrated optical circuits through which the lightwave can be guided through optical wave guiding paths, wherein the lighwave can be modulated by high speed electrical waves fed through travelling microwave electrodes. The amplitude of the primary lightwave is shaped by an optical-wave shaper which shapes the envelope of the primary carrier with some specific super-Gaussian shape. This shaped optical pulse sequence is then fed through an optical modulation circuit through which the phase of the optical pulse sequence is modulated or chirped by some specific electrical wave.

In accordance with another aspect of the invention, a method for phase modulation of the amplitude-shape optical pulse sequence is presented so that chirping of the phase of these lightwave pulses can be achieved. These phase modulation and amplitude shaping modulation are simultaneously excited, whereby the diffraction and splitting of the primary lightwave can occur so that a set of sub-carriers can be generated.

In accordance with another aspect of the invention, a method for equalization of the amplitudes of the secondary carriers is provided. The amplitudes of the secondary carriers are equalized by matching the shape of the electrical wave applied to the optical phase modulator and/or the shape of the amplitude of the shaped pulse optical sequence.

In accordance with another aspect of the invention, a method for using these secondary carriers to carry information channels of ultra-high speed is provided so as to generate an optical transmission system that carries information capacity reaching several Tb/s over an optical guided transmission line. The total capacity of all channels of the secondary carriers can reach several Tb/s.

The comb generator can be applied in the field of optical communications for:

- Carrying ultra-high bit rate information per sub-carrier with the overall capacity of higher than 1.0 Tb/s per wavelength channel (i.e. the primary carrier).

- The spacing of the subcarriers of the comb generator is tuneable so that their separation can be shorten to overlapping the spectra of the information channels carried by the sub- carriers, thus this leads to carrying high capacity of information over a primary wavelength carrier.

- The orthogonality of the modulated secondary-carrier channels generated from the

primary source permits the recovery of individual secondary channels with high fidelity of minimum error rate. The comb generator can be applied in the field of wireless communications and radio astronomy. The subcarriers are frequency locked to a reference wavelength which is that of the primary carrier. Thus they can be locked in the phase domain and thus a set of phase locked and frequency locked lightwaves is possible. These phase- and frequency-locked waves can be employed:

- to carry radio waves to distribute to phase array antennae for directional transmission of radiation electromagnetic waves in wireless communication systems or

- to phase array antenna for radio astronomical systems such as the square kilometre antenna array.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

Fig. 1 shows a block diagram of an optical comb generator according to an implementation form;

Fig. 2 shows a block diagram of an optical comb generator according to an implementation form;

Fig. 3 shows a block diagram of an optical pulse-shaping circuit according to an

implementation form;

Fig. 4 shows a graph depicting an optical signal generated by an optical pulse-shaping circuit according to an implementation form;

Fig. 5 shows a graph depicting a shaped electrical driving signal according to an

implementation form;

Fig. 6 shows a graph depicting a spectrum of an output signal of an optical comb generator according to an implementation form;

7 shows a graph depicting a spectrum of an output signal of an optical comb generator according to an implementation form; Fig 8 shows a block diagram of an optical transceiver comprising an optical comb generator according to an implementation form;

Fig 9 shows schematic diagrams and a modulation diagram of a modulation using an

optical comb generator according to an implementation form;

Fig. 10 shows a block diagram of an optical transmitter comprising an optical comb generator according to an implementation form; Fig. 1 1 shows a schematic diagram of an optical network comprising an optical transceiver with a comb generator according to an implementation form; and

Fig. 12 shows a schematic diagram of a method for generating a plurality of optical sub- carriers according to an implementation form.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig. 1 shows a block diagram of an optical comb generator 500 according to an

implementation form. A lightwave 506 in continuous form is generated from an external cavity laser acting as a primary lightwave source 501 . In an implementation form, the external cavity laser is a single frequency external cavity laser. The lightwave which serves as the primary lightwave carrier 506 is coupled into an optical pulse shaping circuit 502 and then to an optical phase modulation circuit 504. Thus, the primary lightwave carrier 506 is amplitude-modulated, i.e. pulse-shaped, by the optical pulse shaping circuit 502 which forms a sequence of optical pulses 512 and this sequence of optical pulses 512 is then phase- modulated by the optical phase modulation circuit 504.

In an implementation form, the optical pulse-shaping circuit 502 shapes the main carrier of the lightwave, i.e. the primary lightwave carrier 506. The optical waves are modulated simultaneously by the electrical signals generated from a radio frequency (RF) signal generator 503. In an implementation form, the RF signal generator generates an electrical RF sinusoidal excitation wave with frequency spacing. The electrical signal at the output 508 of the RF signal generator 503 which generates an electrical driving signal 509 is split equally into two signal paths by an electrical signal splitter 505. The first signal path from the signal splitter 505 to the pulse shaping circuit 502 carries the electrical driving signal 509 and the second signal path from the signal splitter 505 to a phase shifter 516 phase-shifts the electrical driving signal 509 to obtain a phase-shifted electrical driving signal 510. Thus, the phase shifter 516 phase-shifts or electrically delays the second electrical signal path by an amount such that the two electrical signal paths act on the modulation of the lightwaves simultaneously and synchronously at the same position of the lightwaves passing through the pulse shaper 502, i.e. the optical pulse shaping circuit 502, and the phase modulator 504, i.e. the optical phase modulation circuit 504, to generate comb-structured sub-carriers, i.e. a plurality of optical sub-carriers 507 at the output of the optical comb generator 500. The optical phase modulation circuit 504 can be seen as a temporal imager superimposing images of the sequence of optical pulses 512 to generate the comb-structured sub-carriers 507.

In an implementation form, the optical pulse-shaping circuit 502 shapes the primary lightwave carrier 506 such that a flat top pulse sequence is generated. In an implementation form, the optical pulse-shaping circuit 502 shapes the primary lightwave carrier 506 such that a Carrier-Suppressed Return-to-Zero (CSRZ) optical pulse sequence is generated. In CSRZ the field intensity drops to zero between consecutive bits (Return-to-Zero, RZ), and the field phase alternates by π between neighbouring bits, so that if the phase of the signal is e.g. 0 in even bits (bit number 2n, wherein n is an integer equal or lager than zero (0)), the phase in odd bit slots (bit number 2n+1 ) will be ττ, the phase alternation amplitude. In an implementation form, the optical pulse-shaping circuit 502 comprises a Mach-Zehnder modulator (MZM) for generating a CSRZ signal. In an implementation form, the optical pulse-shaping circuit 502 comprises a single Mach-Zehnder modulator, driven by two sinusoidal waves at half the bit rate B_R, and in phase opposition. This gives rise to characteristically broad pulses, e.g. with duty cycle 67%. In an implementation form, the optical pulse-shaping circuit 502 shapes the primary lightwave carrier 506 such that an Alternate-Phase Return-to-Zero (APRZ) optical pulse sequence is generated. The signal format APRZ can be viewed as a generalization of CSRZ in which the phase alternation can take up any value ΔΦ (and not necessarily only ττ) and the duty cycle is also a free parameter. In an implementation form, the optical pulse-shaping circuit 502 uses CSRZ to generate specific optical modulation formats, e.g. CSRZ-OOK, in which data is coded on the intensity of the signal using a binary scheme (light on=1 , light off=0, or vice versa), or CSRZ- DPSK, in which data is coded on the differential phase of the signal, etc. CSRZ is often used to designate APRZ-OOK. The characteristic properties of an CSRZ signal are those to have a spectrum similar to that of an RZ signal, except that frequency peaks (still at a spacing of BR) are shifted by B_R/2 with respect to RZ, so that no peak is present at the carrier and power is ideally zero at the carrier frequency. Compared to standard RZ-OOK, the CSRZ- OOK is considered to be more tolerant to filtering and chromatic dispersion, thanks to its narrower spectrum.

Fig. 2 shows a block diagram of an optical comb generator 600 according to an

implementation form. The figure illustrates the electrical and optical circuit for generating an optical pulse sequence 612 of shapes required for further diffraction by the phase modulator circuit 603 following the pulse shaper 602. The pulse shaper 602 and the optical phase modulator 603 may correspond to the optical pulse-shaping circuit 502 and the optical phase modulation circuit 504 as described with respect to Fig. 1. The lightwaves are generated in a continuous fashion by an external cavity laser 601 whose frequency spectrum looks like a single frequency impulse of a line width of less than 100 kHz. This lightwave is then modulated by the pulse shaper 602. The pulse shaper 602 is an integrated optical modulator in which optical waveguide paths act as guiding lines of the lightwaves whose phases are then modulated by the applied electrical signals coming from the radio frequency (RF) signal generator 604. The RF signal generator generates an electrical driving signal 609 which may correspond to the electrical driving signal 509 as described with respect to Fig. 1.

The output of the RF signal generator 604 is split into two paths by an electrical signal splitter 615; one is fed into the pulse shaper 602 and the other to the signal

generator/shaper 605, the phase shifter 606 and the optical phase modulation circuit 603. In an implementation form, the electrical signal splitter 615 corresponds to the electrical signal splitter 505 as described with respect to Fig. 1 for splitting the output of the RF signal generator 604 into the two signal paths. The phase shifter 606 is used for synchronization of the two electrical signals, i.e. the electrical driving signal 609 and the phase-shifted electrical driving signal 610, which drive the pulse shaper 602 and the optical phase modulator 603 to ensure the simultaneous modulation of the lightwave carriers in order to generate the comb- structured sub-carrier signal 607 at the output port of the of the phase modulation circuit 603.

The electrical driving signal 609 is used as synchronization signal for the signal

generator/shaper 605 shaping the electrical driving signal 609 to obtain a rectified sinusoidal wave |sinoo_RFt| as depicted in Fig. 5 described below. This rectified sinusoidal wave is input to the phase shifter 606. In an implementation form, the signal generator/shaper 605 is a rectifier or a unit forming the absolute value of the electrical driving signal 609. The detailed structure for comb generation is as follows: the lightwaves 614 follow the sequence of: optical devices external cavity laser (ECL) 601 , Optical Pulse Shaper 602 (for Carrier Suppressed Return to Zero Modulation), Optical Phase Modulator 603 (simultaneously chirping the phases of the lightwaves under the shaped pulse sequence). By this comb sub-carriers are generated with an amplitude power variation <1 dB. In an implementation form, the pulse shaper 602 comprises a CSRZ optical modulator. In an implementation form, the pulse shaper 602 comprises a dual drive Mach-Zehnder

Interferometer Modulator (MZIM). Due to its flexibility, the Mach-Zehnder Interferometer Modulator is superior compared with the electro-absorption (EA) modulator or a direct modulation technique. This is due to the change in the refractive index of some materials when an electric field, usually via a bias and a travelling RF wave, is applied to the electrodes. This affects the material almost instantly the lightwaves passing through the modulator. Hence the electro-optic refractive index change is proportional to the voltage applied to the material. Delaying the light phase causes interference effects that modulate the output intensity, either by enhancing or cancellation the output intensity.

In another implementation form, the pulse shaper 602 is an electro-absorption modulator.

In an implementation form, the pulse shaper 602 comprises an IQ modulator. In an implementation form, the pulse shaper 602 is biased at Vpi/2 and -Vpi/2 (pi = π). In an implementation form, the pulse shaper 602 provides a signal having an amplitude swing of Vpi/2 on both sides.

In an implementation form, the optical phase modulator 603 is driven by a parabolic profile pulse shape. In an implementation form, the optical phase modulator 603 provides a temporal imaging. The optical phase modulator 603 is simultaneously chirping the phases of the lightwaves under the shaped pulse sequence 612. The chirp provides a frequency sweep imposed as a result of power change. The chirp interacts with the dispersion properties of the materials, increasing or decreasing total pulse dispersion as the signal propagates.

In an implementation form, the optical phase modulator 603 generates comb sub-carriers 607 with an amplitude power variation smaller than 1 dB.

In an implementation form, the pulse shaper 602 uses one of the amplitude modulation formats:

- Return -to-zero On/Off Keying (RZ-OOK or RZ-ASK) - Carrier-Suppressed eturn-to-Zero (CSRZ-OOK or CSRZ-ASK) and

- Non-Return-to Zero On/Off Keying (NRZ-OOK).

In an implementation form, the optical phase modulator 603 uses one of the phase modulation formats:

- Non-return-to-zero Differential Phase Keying (NRZ-DPSK) and NRZ-DQPSK

- Return-to Zero Differential Phase Keying(RZ-DPSK) and

- RZ-DQPSK or CSRZ-DPSK and CS-DQPSK. Fig. 3 shows a block diagram of an optical pulse-shaping circuit 700 according to an implementation form. The optical pulse-shaping circuit 700 may correspond to the optical pulse-shaping circuit 502 as described with respect to Figure 1 or to the pulse shaper 602 as described with respect to Figure 2. The optical pulse-shaping circuit 700 accepts the lightwaves 712 from the primary source and passes the lightwaves 712 to a dual drive Mach- Zehnder Interferometer Modulator (MZIM) 720. An optical splitter 705 splits the lightwaves 712 equally into two paths. The lightwaves of the two optical paths are then travelling along an electrode pair 702a, 702b of the dual-drive MZIM 720 and experience phase shifts due to the electro-optic effects under the applications of two electrical signals 709, 710. The electrical signals 709, 710 generated by the electrical signal generator 703 are applied to the electrode pair 702a, 702b of the dual drive Mach-Zehnder Interferometer Modulator (MZIM) 720.

An electrical signal splitter 722 splits the output path of the electrical signal generator 703, e.g. a sinusoidal wave generator of 25 GHz or higher, into two electrical paths, a first path carrying the electrical driving signal 709 and a second path carrying a phase-shifted electrical driving signal 710. Both paths are amplified by respective amplifiers 716a, 716b and biased by respective bias circuits 708a, 708b. On the second path, the electrical driving signal 709 is shifted by a first phase shifter 718 by pi/2 and then adjusted by a second variable phase shifter 706 to provide the phase-shifted electrical driving signal 710. This phase shifting and appropriate biasing allows the depletion of the carrier at the central region and the generation of two frequency components in the lower and upper half of the frequency band and thus allow two frequency shifted components depending on the phase shift amount. It thus provides the shaping of the pulse shape of the modulated carrier. The electrode pair 702a, 702b is biased by a biasing circuit 708a, 708b in opposite sign of the transfer transmittance of the device 720 and with a swing amplitude equal to the voltage level at which level a total differential phase equal to pi radians can be achieved. In the lower part of Fig. 3, the spectrum 709 of the signal at the output of the dual drive Mach-Zehnder Interferometer Modulator (MZIM) 720 which is the output of the optical pulse- shaping circuit 700 and corresponds to the sequence of optical pulses 512, 612 as described with respect to Figures 1 and 2, is illustrated. This comb-structured spectrum results from the double side band frequency shifting of the input lightwave carrier.

The electrical signal generator 703 may correspond to the signal generator 503 as described with respect to Fig. 1 or to the F generator 604 as described with respect to Fig. 2. The first phase shifter 718 is using the electrical driving signal 729 to generate a pi/2 phase-shifted signal. The second phase shifter 706 is used to synchronize the electrical driving signal 729 with the second phase shifted electrical driving signal 710 by variable adjusting the phase between both signals 729 and 710. By the arrangement of signals in electrical and optical domain as described in Fig. 3, the double side band carrier is suppressed due to the optical pulse shaping.

Fig. 4 shows a graph 800 depicting an optical signal 802 generated by an optical pulse- shaping circuit according to an implementation form. The optical signal 802 is generated from the principal carrier 712 after having passed the optical pulse-shaping circuit 700 described with respect to Fig. 3. The optical signal 802 is modulated in amplitude such that its flat top 801 occupies a width of about 70% of the period 803 of the pulse sequence. The period 803 begins at position 804 as depicted in Fig. 4. This period 803 is determined by the period of the sinewave generated by the electrical signal generator 703 of the optical pulse- shaping circuit 700 as described with respect to Fig. 3. In an implementation form, the shape of the envelope of the pulse sequence is approximated to follow a super-Gaussian shape.

Fig. 5 shows a graph 900 depicting a shaped electrical driving signal 901 according to an implementation form. The shaped electrical driving signal 901 is generated from the signal shaper 605 as described with respect to Fig. 2. The shaped electrical driving signal 901 is in synchronization with the electrical driving signal 609 generated by the RF generator 604 as described with respect to Fig. 2 and is used to modulate the phase of the sequence of optical pulses 612 coming out of the optical pulse shaper 602. The signal shape of the shaped electrical driving signal 901 looks like the rectified form of a sinusoidal wave. The shaped electrical driving signal 901 is then fed into the electrode input of the electro-optic modulator 603 through which the phase of the pulse sequence 612 is manipulated so that diffraction of the pulse shaped lightwaves occurs and sub-carriers 607 are generated. The shape of the shaped electrical driving signal 901 is tailored for driving the optical phase modulator 603. This phase modulation generates the chirping of frequency of the lightwaves coming from the pulse shaper 602 as described with respect to Fig. 2. This phase

modulation must be simultaneously performed in synchronization with the pulse shaping action to diffract the primary carrier into several secondary carriers which form the comb structure spectrum.

Fig. 6 shows a graph 1000 depicting a spectrum of an output signal of an optical comb generator according to an implementation form. The figure illustrates the spectrum of the generated sub-carriers which may correspond to the optical subcarriers 507 described with respect to Fig. 1 or to the optical subcarriers 607 described with respect to Fig. 2. The central position of the principal carrier 1001 is suppressed. Other outer most sub-carriers 1009 and 1010 set the lower and upper limit of the subcarriers whose magnitudes are within the 0.5 dB flatness of all the subcarriers. The frequency spacing between the subcarriers 1002 can be set by the frequency of the electrical sinusoidal signal 703 as described with respect to Fig. 3.

Fig. 7 shows a graph 1 100 depicting a spectrum of an output signal of an optical comb generator according to an implementation form. A frequency spacing of 50GHz between the lines of the subcarriers is achieved with a 25GHz sinusoidal wave driving the pulse shaper 601 and the rectified wave to the phase modulator 603 as described with respect to Fig. 2. 14 sub-carriers are generated over the entire spectral region 1 101 of 650 GHz. Spectral distribution of sub-carriers generated by the optical comb generator is +/-0.5dB over the +/-300GHz frequency region. The comb spectrum is obtained under the sine generator driving signal amplitude which is equal to 4*Vpi [abs(sin)], the spectrum is taken after PM1 , the CS Z-PM repetition frequency is equal to 2* RZ_freq = 25GHz resulting in a 50GHz frequency shift resulting in a spacing of adjacent sub-carriers of 13x50 GHz, i.e. 650GHz.

Fig. 8 shows a block diagram of an optical transceiver 200 comprising an optical comb generator according to an implementation form. The optical transceiver 200, which is also called optical transponder, comprises a lightwave transmitter 205 and a lightwave receiver 207. The lightwave transmitter 205 is a Tbps optical transmitter and the lightwave receiver 207 is a Tbps optical coherent digital receiver. Lightwaves are coupled into an input port 202 of the transceiver 200 or are output from an output port 201 of the transceiver 200 via a bidirectional and wideband optical coupler 203. The bidirectional and wideband optical coupler 203 couples lightwaves from the transmitter to the output port 202 by the connection path 204 and from the input port 201 to the receiver 207 by the connection path 206. The lightwave transmitter 205 comprises a comb generator as described with respect to Fig. 1 or Fig. 2 for generating sub-carriers which are used to generate a modulated optical signal as described below with respect to Figures 9 and 10.

Fig. 9 shows schematic diagrams 300a, 300b, 300c and a modulation diagram 300d of a modulation using an optical comb generator according to an implementation form. A first schematic diagram 300a shows the spectrum of electrical signals of advanced modulation formats describing H- (horizontal polarized) 302 and V- (vertical polarized) 301 channels signal spectra. A second schematic diagram 300b shows the spectrum of H-secondary carriers 304 and V-secondary carriers 303 which correspond to sub-carriers generated by an optical comb generator as described with respect to Fig. 1 or Fig. 2. A third schematic diagram 300c describes modulation of the V-secondary carriers 303 and H-secondary carriers 304 of diagram 300b modulated with the V-polarized 301 and H-polarized 302 channels of diagram 300a resulting in H-polarized channels at secondary carriers 306 and V-polarized channels at secondary carriers 305. The Tbps lightwaves channels are generated by secondary optical carriers 303 from the primary carrier and then modulated by electrical signals of advanced modulation formats 301 and 302 so as to create multi-level manipulation of the phase, amplitude or frequency of the lightwaves of these sub-carriers. Furthermore, several multi-sub-carriers can also be modulated on top of a secondary carrier. The modulated signals whose lightwaves come from a comb generator as described with respect to Fig. 1 or Fig. 2 are of very narrow linewidth. This first set of comb-like subcarriers is multiplexed with another set of sub- carriers assigned in comb-like structure spectrum of 304. The information sequence of serial bits is encoded via the encoders embedded in 302. Each individual secondary carrier carries the phase variation and distribution in a constellation of a complex plane 307. Odd and even channels of the sub-carriers can have the constellation overlapped or slightly off set in amplitude 308. These channels can be distinguished by optical filtering at the optical processing unit prior to the optical-electronic conversion in the optical receiver.

Fig. 10 shows a block diagram of an optical transmitter 400 comprising an optical comb generator according to an implementation form. The transmitter 400 represents a

Tb/soptical transmitter employing the sub-carriers of the comb-like structure in which each sub-carrier is modulated by an optical modulator with some specific modulation format or the same format. Thus, each carrier carries the most possible information capacity and the combination of these channels form the total capacity of equal or higher than a Tb/s. The modulation formats for the data streams to be transported by the secondary carriers follow the following sequences: first encoding the data stream 410 into QPSK (Quadrature Phase shift keying) for odd channels 41 1 and offset by pi/4 QPSK for even channels 412. The encoded sequences are then formed into orthogonal frequency division multiplexed symbols via the OFDM formers 413 and 414. They are then converted into analog domain signals using the DACs (digital to analog converters) 415, 416 and 417, 418 to produce the inphase and quadrature signals to drive the optical modulators 420 whose input lightwaves come from the comb-structure lightwave sources 418 and resulting into modulated lightwave channels 419 which are then transmitted over optical guided medium such as single mode optical fibers. The comb-structure lightwave sources 418 may correspond to the optical comb generators 500 or 600 as described with respect to Figs. 1 and 2.

Fig. 1 1 shows a schematic diagram of an optical network 1 10 comprising an optical transceiver 102 with a comb generator according to an implementation form. A scale of the optical network 1 10 may be few hundreds or a few thousands to few tens of thousands of kilometers in metropolitan areas or terrestrial regions or intercontinental environment respectively which comprises of a number of optical network nodes 100. Several transponders 103 are employed as the network interfaces to the information connected by the transmission medium 101. These connections transfer the information channels to the nodes. The optical channels may fill up all the spectral regions from 1520 to 1620 nm wavelength of the low loss optical fibers. The fibers act as the guiding medium of all these optical channels. Each wavelength channel, called the primary carrier, carries a total capacity of information reaching at least 1 Tbps to several tens of Tbps. In each primary carrier source, there are several secondary carriers which are modulated with individual separate data streams. These Tbps optical channels are routed and distributed over the entire network 1 10 through the network nodes 100. The optical transmitting section of 103 acts as the transmitting end integrating all optical modulated secondary channels into Tbps groups. The transmitter is integrated with the Tbps receiver to form a Tbps transceiver 103 as described above with respect to Fig. 10.

Fig. 12 shows a schematic diagram of a method 1200 for generating a plurality of optical sub-carriers according to an implementation form. The plurality of optical sub-carriers 507 are generated from a primary lightwave carrier 506. The method comprises modulating 1201 an amplitude of the primary lightwave carrier 506 to obtain a sequence of optical pulses 512, the amplitude modulating 1201 being driven by an electrical driving signal 509; phase shifting 1202 the electrical driving signal 509 to obtain a phase-shifted electrical driving signal 510; and modulating 1203 a phase of the sequence of optical pulses 512 to obtain the plurality of optical sub-carriers 507, the phase modulating 1203 being driven by the phase- shifted electrical driving signal 510. The phase shifting 1202 the electrical driving signal 509 is such that the amplitude modulating 1201 and the phase modulating 1203 are

synchronized. From the foregoing, it will be apparent to those skilled in the art that a variety of devices, methods, systems, computer programs on recording media, and the like, are provided.

The present disclosure also supports a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the performing and computing steps described herein.

The present disclosure also supports a system configured to execute the performing and computing steps described herein. According to an implementation form, a computer control system is provided to obtain the feedback signals from monitoring electronic signals from the monitoring photodetector embedded in the optical pulse shaper and the optical phase modulator so that control signals can be generated to control the modulation and synchronization with the phase modulation so as to enforce the equalization of the phase and amplitude of the generated light waves from the comb generator."

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present inventions has been described with reference to one or more particular

embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the inventions may be practiced otherwise than as specifically described herein.

Claims

CLAIMS:

1 . Optical comb generator (500) for generating a plurality of optical sub-carriers (507) from a primary lightwave carrier (506), comprising: an optical pulse-shaping circuit (502) modulating an amplitude of the primary lightwave carrier (506) to form a sequence of optical pulses in response to an electrical driving signal (509); a phase shift unit (516) for phase shifting the electrical driving signal (509) to obtain the phase-shifted electrical driving signal (510) to synchronize an operation of the optical pulse- shaping circuit (502) and the optical phase modulation circuit (504), and an optical phase modulation circuit (504) modulating a phase of the sequence of optical pulses to form the plurality of optical sub-carriers (507) in response to a phase-shifted electrical driving signal (510).

2. The optical comb generator (500) of claim 1 , wherein the plurality of optical sub- carriers (507) are equally spaced in frequency.

3. The optical comb generator (500) of claim 1 or claim 2, comprising an electrical signal generator (604) for generating the electrical driving signal (509).

4. The optical comb generator (500) of claim 3, wherein the electrical driving signal (509) is sinusoidal.

5. The optical comb generator (500) of claim 3 or claim 4, wherein a frequency of the electrical driving signal (509) is adjustable, the frequency of the electrical driving signal determining a frequency spacing of the optical sub-carriers (507).

6. The optical comb generator (500) of one of claims 3 to 5, comprising an electrical signal splitter (505) for splitting the electrical driving signal (509) into a first and a second electrical signal path.

7. The optical comb generator (500) of claim 6, wherein the electrical driving signal (509) drives the optical pulse-shaping circuit (502) and the phase-shifted electrical driving signal (510) drives the optical phase modulation circuit (504) such that both driving signals (509, 510) are simultaneously and synchronously entering electrodes of the optical pulse- shaping circuit (502) and the optical phase modulation circuit (504) and travelling along the electrodes at the same speed.

8. The optical comb generator (500) of claim 6 or claim 7, wherein the phase shift unit (516) is implemented as a first electrical phase shifter (606) in the second electrical signal path.

9. The optical comb generator (500) of claim 8, wherein a phase-shift of the first electrical phase shifter (606) is adjustable.

10. The optical comb generator (500) of one of the preceding claims, wherein the optical pulse-shaping circuit (502) is configured to modulate the amplitude of the primary lightwave carrier (506) according to a super-Gaussian pulse-shape.

1 1 . The optical comb generator (500) of one of the preceding claims, wheiein the optical pulse-shaping circuit (502) comprises an optical splitter (705) for splitting the primary lightwave carrier (712) into a first and a second optical path.

12. The optical comb generator (500) of claim 1 1 , wherein a second electrical phase shifter (718) is implemented in the optical pulse-shaping circuit (502) to phase shift the electrical driving signal (509) obtaining a second phase-shifted electrical driving signal (710).

13. The optical comb generator (500) of claim 12, wherein the optical pulse-shaping circuit (502) comprises a dual drive Mach-Zehnder Interferometer Modulator (720) having a first electrode (702a) driven by the electrical driving signal (709) and a second electrode (702b) driven by the second phase-shifted electrical driving signal (710).

14. The optical comb generator (500) of claim 13, wherein the optical pulse-shaping circuit (502) comprises a biasing circuit (708a, 708b) for biasing the electrodes (702a, 702b) of the dual drive Mach-Zehnder Interferometer Modulator (720).

15. Method for generating a plurality of optical sub-carriers (507) from a primary lightwave carrier (506), comprising:

Modulating (1201 ) an amplitude of the primary lightwave carrier (506) to obtain a sequence of optical pulses (512), the amplitude modulating (1201 ) being driven by an electrical driving signal (509); phase shifting (1202) the electrical driving signal (509) to obtain a phase-shifted electrical driving signal (510); and modulating (1203) a phase of the sequence of optical pulses (512) to obtain the plurality of optical sub-carriers (507), the phase modulating (1203) being driven by the phase-shifted electrical driving signal (510), wherein the phase shifting (1202) the electrical driving signal (509) is such that the amplitude modulating (1201 ) and the phase modulating (1203) are synchronized.