HK1192659B - Method for generating an optimized return-to- zero pulse shape against aggressive optical filtering and an optical transmitter implementing the method - Google Patents

Description

Method for generating optimized return-to-zero pulse forms for invasive optical filtering and optical transmitter for implementing the method

Technical Field

The present invention relates generally to the field of optical communications, and more particularly to a method for generating optimized return-to-zero pulse forms for invasive optical filtering.

Background

With the rapid growth of emerging demand-distributed network services, it is highly desirable that next generation Dense Wavelength Division Multiplexing (DWDM) optical transmission techniques that employ multi-level modulation formats carry as many information bits as possible over the existing limited bandwidth ITUT-T channels. Polarization-multiplexed return-to-zero quadrature phase shift keying (PM-RZ-QPSK) with digital coherent detection has been recognized as a next generation optical transmission network standard that mitigates optical link impairments by multiplexing individual data branches at a lower bit rate, thereby allowing DSP-oriented coherent receivers to easily process them.

At present, the existing 50GHz DWDM channel spacing can hardly carry 112Gb/s PM-QPSK signals. But since the use of soft decision Forward Error Correction (FEC) with higher overhead makes the line rate of new generation PM-QPSK products potentially as high as 128Gb/s, insufficient channel bandwidth can lead to significant performance degradation that will be even more problematic when considering that a series of on-line optical filters, such as reconfigurable optical add-drop multiplexers (ROADMs) along the optical transmission path, can cause bandwidth narrowing effects.

Return-to-zero (RZ) pulses are generally more tolerant of filtering and nonlinear degradation than non-return-to-zero (NRZ) pulses. But if a channel spacing of 25GHz or less is used to support future gigabit Nyquist-WDM superchannels, then the conventional RZ pulses do not work well when ten 128Gb/s subchannels are mixed with such aggressive optical filtering.

SUMMARY

Accordingly, the present invention is directed to a novel method for generating optimized return-to-zero pulse forms for aggressive optical filtering using established optical transmitters (e.g., PM-QPSK transmitters).

In some embodiments, a method of generating a signal having an optimized return-to-zero pulse form using an optical modulator having an input terminal and an output terminal is provided. The method comprises the following steps: applying a clock signal and a drive voltage to the optical modulator, wherein the drive voltage has a bias point at a predetermined reference voltage level; modifying the driving voltage by lowering a bias point of the driving voltage by a predetermined offset from a predetermined reference voltage level; receiving an optical signal at an input terminal, wherein the optical signal includes an x-polarization branch and a y-polarization branch, the y-polarization branch being pulse-to-pulse aligned with the x-polarization branch; modulating the optical signal using the modified drive voltage; and outputting the modulated optical signal at an output terminal.

In some embodiments, an optical transmitter includes: a first and second set of optical in-phase-quadrature modulators, an integratable tunable laser assembly, a first polarizing beam splitter, a second polarizing beam splitter, and an optical modulator; each set of optical in-phase and quadrature modulators is coupled to a respective pair of electronic amplifiers for receiving two respective input signals; the integratable tunable laser assembly is configured to generate a continuous waveform optical signal; the first polarization beam splitter is configured to split the continuous waveform optical signal into x-and y-polarization branches, wherein each of the x-and y-polarization branches is modulated by one of the first and second sets of optical in-phase-quadrature modulators in accordance with two respective input signals applied to respective pairs of electronic amplifiers; the second polarization beam splitter is configured to combine the modulated x-polarization branch and the y-polarization branch into one optical signal; the optical modulator is configured to modulate the combined optical signal with a drive voltage having a bias point that is reduced by a predetermined offset from a predetermined reference voltage level.

Brief Description of Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:

FIG. 1A illustrates an optical transmitter and its various components, including an optical modulator, according to some embodiments of the present invention;

FIG. 1B illustrates an x-polarization arm and a y-polarization arm produced by an optical transmitter according to some embodiments of the present invention;

FIG. 1C illustrates shifting a bias point of a drive voltage of an optical modulator by a predetermined offset, according to some embodiments of the invention;

FIG. 1D illustrates increasing the amplitude of an offset drive voltage of a light modulator by a predetermined amount, according to some embodiments of the invention;

FIG. 1E illustrates an optical signal transmission system including an optical transmitter, an optical filter, and an optical receiver according to some embodiments of the invention;

FIG. 1F illustrates a simulated amplified spontaneous emission spectrum of an optical filter according to some embodiments of the present invention;

FIGS. 2A-2C illustrate, respectively, simulated time domain waveforms of an optical signal having a non-return-to-zero (NRZ) pulse form before a Gaussian optical filter, and their associated spectra before and after the Gaussian optical filter, according to some embodiments of the present invention;

FIGS. 2D-2F respectively illustrate a pulse form at 8/16V with a 50% return to zero (RZ) pulse form according to some embodiments of the invention_πA simulated time domain waveform of the biased optical signal before the Gaussian optical filter and its associated spectra before and after the Gaussian optical filter;

FIGS. 2G-2I respectively illustrate a pulse shape at 11/16V with a 50% return to zero (RZ) pulse form according to some embodiments of the invention_πA simulated time domain waveform of the biased optical signal before the Gaussian optical filter and its associated spectra before and after the Gaussian optical filter;

FIGS. 2J-2L each illustrate a pulse shape at 12/16V with a 50% return to zero (RZ) pulse form according to some embodiments of the invention_πA simulated time domain waveform of the biased optical signal before the Gaussian optical filter and its associated spectra before and after the Gaussian optical filter;

FIGS. 2M-2O respectively illustrate a pulse shape at 13/16V with a 50% return to zero (RZ) pulse form according to some embodiments of the invention_πA simulated time domain waveform of the biased optical signal before the Gaussian optical filter and its associated spectra before and after the Gaussian optical filter;

FIGS. 2P-2R respectively illustrate a pulse shape at 14/16V with a 50% return to zero (RZ) pulse form according to some embodiments of the invention_πA simulated time domain waveform of the biased optical signal before the Gaussian optical filter and its associated spectra before and after the Gaussian optical filter;

FIGS. 2S-2U respectively illustrate a pulse shape at 15/16V with a 50% return to zero (RZ) pulse form according to some embodiments of the invention_πA simulated time domain waveform of the biased optical signal before the Gaussian optical filter and its associated spectra before and after the Gaussian optical filter;

FIG. 3 illustrates a block diagram of the error rate performance of an optical transmitter as a function of the OSNR when the drive voltage has an original magnitude but a different bias point and is optically filtered, according to some embodiments of the present invention;

FIG. 4 illustrates a block diagram of the error rate performance of an optical transmitter as a function of OSNR with or without optical filtering with increased magnitude and different bias points of the drive voltage, according to some embodiments of the present invention;

FIG. 5 illustrates a block diagram of the error rate performance of an optical transmitter after 22GHz aggressive optical filtering at the transmitter side and transmission over 1040km SMF-28 fiber, according to some embodiments of the invention; and is

Fig. 6 is a block diagram illustrating a process for generating a signal having an optimized return-to-zero pulse form using an optical modulator having an input terminal and an output terminal according to some embodiments of the present invention.

Detailed Description

Reference will now be made in detail to the various embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous non-limiting specific details are set forth in order to provide an understanding of the subject matter presented herein. It will be apparent, however, to one skilled in the art that various alternatives may be used without departing from the scope of the invention, and that the invention itself may be practiced without these specific details. For example, it will be clear to one of ordinary skill in the art that the main content presented herein may be implemented on other types of optical signal transmission systems.

Some experimental results show that in the case of narrow-band optical filtering, the sensitivity of the optical receiver can be improved by optimizing the RZ pulse form so that the phase form of the signal is optimized when its spectrum shows a "zero" dip at the center wavelength, i.e. a flat spectrum appears over the signal bandwidth. However, the foregoing explanation may not be correct, since the end-to-end channel response is similar to a low pass filter, which requires the transmit signal to be high pass filtered to the extent that the RF spectrum of the received signal is equalized.

In the present application, two methods for optimizing RZ pulse form for optical filtering are disclosed, which are referred to as "type-I" and "type-II" optimized RZ (orz), respectively. According to some embodiments of the present application, a "6.26 dB" drop at the center waveform may make a 128Gb/s PM-QPSK signal in the form of a type-I ORZ pulse conform to a 22-GHz Gaussian channel with low optical signal-to-noise ratio (OSNR) performance degradation. Receiver sensitivity can be further improved by employing type-II ORZ pulse form. Both ORZ schemes can effectively overcome the filtering effect by adjusting the bias point of the drive voltage of the optical modulator in the optical transmitter and/or increasing the drive voltage of the optical modulator in the optical transmitter without introducing additional complexity to the already established PM-QPSK optical transmission system.

Fig. 1A illustrates an optical transmitter 10 and its various components, including an optical modulator 130, according to some embodiments of the present invention. As shown in FIG. 1A, a fully integrated optical transmitter 10 generally includes an Integratable Tunable Laser Assembly (ITLA)140, two sets of optical in-phase (I) quadrature (Q) modulators 120-1 and 120-2, two Polarization Beam Splitters (PBSs) 110-1 and 110-2, a Mach-Zehnder (MZ) return-to-zero (RZ) modulator 130, and four electronic amplifiers (EZ)100-1 through 100-4.

In some embodiments, a Continuous Wave (CW) optical signal emitted from ITLA 140 is split by PBS110-1 into x-polarized and y-polarized orthogonal branches that are modulated by data symbols at I-Q modulators 120-1 and 120-2, respectively. Note that the I-Q modulators 120-1 and 120-2 are provided with data symbols via two Electronic Amplifiers (EA). Inside the optical transmitter 10, the two optical paths for the polarized signal are made substantially equal so that the x-polarization and y-polarization output branches are substantially aligned pulse-to-pulse as shown in FIG. 1B after passing through the MZ modulator 130, as shown in FIG. 1B, which illustrates the x-polarization branch and the y-polarization branch produced by the optical transmitter according to some embodiments of the present invention. MZ modulator 130 is controlled by clock signal 150 and voltage source 160. As described below, the output of MZ modulator 130 has a measured BER performance for optical filtering by adjusting the bias point and amplitude of the driving voltage at voltage source 160, respectively. A typical RZ pulse regime is achieved by biasing the Mach-Zehnder optical modulator (MZM) at its 50% transmission (i.e., its positive focal point), which produces an output optical pulse having a full width at half maximum (FWHM) of 50% bit width, referred to as 50% RZ.

To enhance tolerance against fibre channel effects caused by low pass filtering, fig. 1C illustrates a type-I ORZ pulse form that shifts the bias point of the drive voltage of the optical modulator by a predetermined offset, according to some embodiments of the present invention. The type-I method proposes that the bias of the drive voltage can be shifted from the original bias point of 50% RZ towards the minimum transmission by Δ V while maintaining the amplitude and clock frequency of the drive voltage. In this way, the optical carrier will be suppressed to a degree that helps equalize the frequency response of the carried signal. Although the type-I method has better filtering tolerance, the optimized RZ (orz) pulses exhibit a smaller extinction ratio, ER, than the 50% RZ pulses, which can reduce receiver sensitivity if the filtering effect is not taken into account.

FIG. 1D illustrates shifting the amplitude of the offset drive voltage of the light modulator by a predetermined amount 2 Δ V from A according to some embodiments of the present invention₁To A₂type-II ORZ pulse form. As shown in fig. 1D, the type-II method generates an ORZ pulse having an output ER equal to that of a 50% RZ pulse by increasing the driving voltage. Thus, the proposed increased amount of peak-to-peak drive voltage can take full advantage of the modulation swing between 0% and 100% transmission of the MZM. As a result, the output type-II ORZ pulse can achieve better filtering margin without paying the cost of ER reduction.

To simulate the BER performance of optimized RZ pulse shaping according to either the type-I or type-II modes, fig. 1E illustrates an optical signal transmission system including an optical transmitter 10, an optical filter 20, and an optical receiver 30 having various pulse forms (including type-I and type-II pulse forms) according to some embodiments of the present invention. For example, the system includes a 128Gb/s PM-QPSK transmitter 10 with ORZ pulse shaping, a digital coherent receiver 30 with 20GHz ADC bandwidth and 8 oversampling rate, and a second order Gaussian optical filter 20 with 22GHz 3-dB bandwidth located between the transmitter 10 and receiver 30 to simulate the effects of a bad channel. FIG. 1F illustrates a simulated amplified spontaneous emission spectrum of a Gaussian optical filter 20 according to some embodiments of the present invention.

Fig. 2A illustrates a simulated time domain waveform of a sampled optical signal prior to a gaussian optical filter 20, with a conventional non-return-to-zero (NRZ) pulse form. 2B-2C illustrate the correlation spectra of the time domain waveform before and after a Gaussian optical filter, respectively, according to some embodiments of the invention.

FIG. 2D illustrates a pulse form at 8/16V with a 50% return to zero (RZ) pulse_πA sampled analog time domain waveform of the biased optical signal prior to the gaussian optical filter 20. Fig. 2E and 2F illustrate the correlation spectra of the time domain waveform before and after gaussian optical filter 20, respectively, according to some embodiments of the present invention. Note that V_πIs the half-wave voltage of the MZM.

FIG. 2G illustrates a pulse form at 11/16V with a 50% return to zero (RZ) pulse_π(i.e., 3/16V offset from the 50% return-to-zero pulse form_π) A sampled analog time domain waveform of the biased optical signal prior to the gaussian optical filter 20. Fig. 2H and 2I illustrate the correlation spectra of the time domain waveform before and after gaussian optical filter 20, respectively, according to some embodiments of the present invention.

FIG. 2J illustrates an exemplary return-to-zero (RZ) pulse form at 12/16V with a 50% return-to-zero (RZ) pulse shape_π(i.e., 4/16V offset from the 50% return-to-zero pulse form_π) The analog time domain waveform of the biased optical signal before the gaussian optical filter 20. FIGS. 2K and 2I illustrate respectively before and after Gaussian optical filter 20, according to some embodiments of the inventionOf the time domain waveform.

FIG. 2M illustrates an exemplary return-to-zero (RZ) pulse shape at 13/16V with a 50% return-to-zero (RZ) pulse shape_π(i.e., 5/16V offset from the 50% return-to-zero pulse form_π) The analog time domain waveform of the biased optical signal before the gaussian optical filter 20. Fig. 2N and 2O illustrate the correlation spectra of the time domain waveform before and after gaussian optical filter 20, respectively, according to some embodiments of the present invention.

FIG. 2P illustrates an exemplary return-to-zero (RZ) pulse shape at 14/16V with a 50% return-to-zero (RZ) pulse shape_π(i.e., 6/16V offset from the 50% return-to-zero pulse form_π) The analog time domain waveform of the biased optical signal before the gaussian optical filter 20. Fig. 2Q and 2R illustrate the correlation spectra of the time domain waveform before and after gaussian optical filter 20, respectively, according to some embodiments of the present invention.

FIG. 2S illustrates an exemplary return-to-zero (RZ) pulse shape at 15/16V with a 50% return-to-zero (RZ) pulse shape_π(i.e., 7/16V offset from the 50% return-to-zero pulse form_π) The analog time domain waveform of the biased optical signal before the gaussian optical filter 20. Fig. 2T and 2U illustrate the correlation spectra of the time domain waveform before and after gaussian optical filter 20, respectively, according to some embodiments of the present invention.

From the spectra in the form of the ORZ pulse before the gaussian optical filter as shown in fig. 2H, 2K, 2N, 2Q and 2T, respectively, it can be seen that the optical carrier rejection in the form of a dip near the center of the spectrum becomes more pronounced with increasing bias voltage offset, which shifts the bias point of the drive voltage towards minimum transmission.

Fig. 3 illustrates a block diagram of the error rate performance of an optical transmitter as a function of the OSNR when the drive voltage has a raw amplitude but different bias points and is optically filtered, in accordance with some embodiments of the present invention, particularly when the BER is given as 2 × 10^-2The required OSNR is shown in the following table:

compared with 50% RZ pulse form, has 14/16V_πThe maximum OSNR improvement of (17.59-15.77) ═ 1.82dB can be obtained for the type of bias point at-I ORZ (i.e., curve (f) in fig. 3). Furthermore, from having 11/16V_πtype-I ORZ (i.e., curve (c) in fig. 3) to have 14/16V of the bias point at_πThe OSNR improvement for type I ORZ (i.e., curve (f) in fig. 3) of the bias point at (I) is about (16.85-15.77) ═ 1.08 dB.

Fig. 4 illustrates a block diagram of the error rate performance of an optical transmitter as a function of OSNR with or without optical filtering with increased magnitude and different bias points of the drive voltage, according to some embodiments of the present invention. Note that both type-I and type-II ORZ pulse forms have the same 14/16V_πWhen the given BER is 2 × 10^-2The required OSNR is shown in the following table:

	NRZ	50％RZ	type-I ORZ	type-II ORZ
					Filtered through	18.8dB	17.79dB	15.76dB	15.5dB
Without filtering	14.46dB	14.02dB	14.78dB	15.32dB

In other words, in the more aggressive optical filtering case, the type-II ORZ outperforms the type-I ORZ (15.76-15.5) ═ 0.26dB, while the low pass filtering performance degrades to only (15.5-15.32) ═ 0.18 dB. In contrast, the 50% RZ worst-case suffers an optical filtering performance degradation of up to (17.79-14.02) — 3.77 dB.

Fig. 5 illustrates a block diagram of the bit error rate performance of an optical transmitter after 22GHz aggressive optical filtering at the transmitter side and transmission over 1040km SMF-28 fiber according to some embodiments of the present invention when BER 2 × 10^-2The received OSNR is shown in the following table:

	NRZ	50％RZ	type-I ORZ	type-II ORZ
					1040km SMF-28	20.51dB	19.93dB	17.69dB	17.52dB

Fig. 6 is a block diagram illustrating a process of generating a signal having an optimized return-to-zero pulse form using an optical modulator having an input terminal and an output terminal according to some embodiments of the present invention. The process begins by applying a clock signal and a drive voltage to the optical modulator (610), the drive voltage having a bias point at a predetermined reference voltage level. In some embodiments, the predetermined reference voltage level is about 50% of the original pre-offset size of the drive voltage. Next, the drive voltage applied to the light modulator is modified by decreasing the bias point of the drive voltage by a predetermined offset from a predetermined reference voltage level (620). In some embodiments, the predetermined offset is about 37.5% of the original pre-offset magnitude of the drive voltage. Sometimes, the post-offset magnitude of the drive voltage is also increased by about twice the predetermined offset. The optical modulator receives an optical signal (630) at an input terminal, the optical signal including an x-polarization arm and a y-polarization arm, the y-polarization arm being aligned between pulses of the x-polarization arm. The optical modulator then modulates the optical signal (640) using the modified drive voltage and outputs the modulated optical signal (650) at an output terminal of the optical modulator. In some embodiments, the optical modulator includes an optical path comprised of one or more polarization maintaining fibers, and the optical modulator has an optical signal-to-noise ratio (OSNR) that is a function of a predetermined offset at a given bit error rate.

In some embodiments, an optical transmitter according to the present invention comprises: a first and second set of optical in-phase-quadrature modulators, an integratable tunable laser assembly, a first polarizing beam splitter, a second polarizing beam splitter, and an optical modulator; each set of optical in-phase and quadrature modulators is coupled to a respective pair of electronic amplifiers for receiving two respective input signals; the integratable tunable laser assembly is configured to generate a continuous waveform optical signal; the first polarization beam splitter is configured to split the continuous waveform optical signal into x-and y-polarization branches, wherein each of the x-and y-polarization branches is modulated by one of the first and second sets of optical in-phase-quadrature modulators in accordance with two respective input signals applied to respective pairs of electronic amplifiers; the second polarization beam splitter is configured to combine the modulated x-polarization branch and the y-polarization branch into one optical signal; the optical modulator is configured to modulate the combined optical signal with a drive voltage having a bias point that is reduced from a predetermined reference voltage level by a predetermined offset.

In summary, the present invention proposes two new methods to produce an optimized return-to-zero pulse form for aggressive optical filtering by adjusting the bias point of the drive voltage of the optical modulator and/or increasing the drive voltage of the optical modulator. Both methods can support any type of DWDM optical transmission, are not limited by channel spacing and modulation standards, and do not introduce additional complexity to the established PM-QPSK system. It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims (14)

1. A method of generating a signal having an optimized return-to-zero pulse form using an optical modulator having an input terminal and an output terminal, the method comprising:

applying a clock signal and a drive voltage to the optical modulator, wherein the drive voltage has a bias point at a predetermined reference voltage level;

modifying the driving voltage by decreasing a bias point of the driving voltage by a predetermined offset from the predetermined reference voltage level;

receiving an optical signal at the input terminal, wherein the optical signal comprises an x-polarization arm and a y-polarization arm, the y-polarization arm being pulse-to-pulse aligned with the x-polarization arm;

modulating the optical signal using the modified drive voltage; and

outputting the modulated optical signal at the output terminal.

2. The method of claim 1, wherein the predetermined reference voltage level is about 50% of an original pre-offset magnitude of the drive voltage.

3. The method of claim 1, wherein the predetermined offset is about 37.5% of an original pre-offset magnitude of the drive voltage.

4. The method of claim 1, wherein the offset magnitude of the drive voltage is also increased by about twice the predetermined offset.

5. The method of claim 1, wherein the optical modulator is a Mach-Zehnder modulator.

6. The method of claim 1, wherein the optical modulator comprises an optical pathway comprised of one or more polarization maintaining fibers.

7. The method of claim 1, wherein an optical signal-to-noise ratio of the optical modulator is a function of the predetermined offset for a given bit error rate.

8. An optical transmitter, comprising:

a first set of optical in-phase and quadrature modulators and a second set of optical in-phase and quadrature modulators, each set of optical in-phase and quadrature modulators being coupled to a respective pair of electronic amplifiers for receiving two respective input signals;

an integratable tunable laser assembly configured to generate a continuous waveform optical signal;

a first polarizing beam splitter configured to split the continuous waveform optical signal into an x-polarized tributary and a y-polarized tributary, wherein each of the x-polarized tributary and the y-polarized tributary is modulated by one of the first set of optical in-phase-quadrature modulators and the second set of optical in-phase-quadrature modulators in accordance with the two respective input signals applied to the respective pair of electronic amplifiers;

a second polarizing beam splitter configured to combine the modulated x-polarizing tributary and the modulated y-polarizing tributary into one optical signal; and

an optical modulator configured to modulate the combined optical signal using a drive voltage;

wherein the drive voltage has a bias point at a predetermined reference voltage level, the drive voltage being modified by decreasing the bias point of the drive voltage by a predetermined offset from the predetermined reference voltage level.

9. The optical transmitter of claim 8, wherein the predetermined reference voltage level is about 50% of an original pre-offset magnitude of the drive voltage.

10. The optical transmitter of claim 8, wherein the predetermined offset is about 37.5% of an original pre-offset magnitude of the drive voltage.

11. The optical transmitter of claim 8, wherein the offset magnitude of the drive voltage is also increased by about twice the predetermined offset.

12. An optical transmitter according to claim 8, wherein the optical modulator is a Mach-Zehnder modulator.

13. The optical transmitter of claim 8, wherein the optical modulator comprises an optical path comprised of one or more polarization maintaining optical fibers.

14. The optical transmitter of claim 8, wherein an optical signal-to-noise ratio of the optical modulator is a function of the predetermined offset for a given bit error rate.