CN105099570B - Orthogonal multiple carrier light source and PDM-QPSK sender units - Google Patents
Orthogonal multiple carrier light source and PDM-QPSK sender units Download PDFInfo
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Abstract
The invention discloses a kind of orthogonal multiple carrier light source and PDM QPSK sender units.Wherein, the orthogonal multiple carrier light source includes：Sinusoidal radio frequency signal source, for exporting the sinusoidal radio frequency signal of prearranged signals frequency to power divider；Power divider, for the sinusoidal radio frequency signal of input to be divided into left sinusoidal radio frequency signal and right wing sinusoidal radio frequency signal；First electrical power amplifier, for carrying out power amplification to left sinusoidal radio frequency signal；Phase-shifter, for being adjusted to right wing sinusoidal radio frequency signal；Second electrical power amplifier, for carrying out power amplification to the right wing sinusoidal radio frequency signal after phase-shifter adjusts；Electroabsorption Modulated Laser, for generating optical signal under the driving for the left sinusoidal radio frequency signal that the first electrical power amplifier exports；Phase-modulator generates Frequency Locking and orthogonal multicarrier under the driving for the right wing sinusoidal radio frequency signal that the second electrical power amplifier exports, being modulated to the optical signal of input.
Description
Technical field light
The invention relates to the field of coherent communication, in particular to an orthogonal multi-carrier light source and a Polarization multiplexed quadrature phase shift keying (PDM-QPSK) signal transmitting device.
Background
In the field of optical communications, a frequency-locked and flat-power orthogonal multi-carrier generation technology is widely applied, and the technology can be used for microwave photonics, all-optical signal processing, optical arbitrary waveform generation, a Wavelength Division Multiplexing (WDM) ultra-wideband light source and the like. Especially when multiple carriers are used as WDM coherent ultra-wideband light source, the orthogonal multiple carrier generation technology is considered as a key enabling technology of future Tbit/s optical communication.
The current main technical solutions in the aspect of orthogonal multi-carrier generation technology include: (1) a cascade scheme based on a Phase Modulator (PM) and an Intensity Modulator (IM); (2) a cascade scheme based on a phase modulator and its frequency multiplication drive; (3) an I/Q modulator based generation scheme; (4) a phase modulation based loop multi-carrier generation scheme; (5) based on the generation scheme of IQ-modulators in combination with frequency shift loops (RFS), etc. The above schemes can generate orthogonal multi-carriers with tunable wavelength, frequency locking and flat power, but have the disadvantages of high insertion loss and high cost.
In the related art, a multi-carrier generation scheme based on a direct modulation distributed feedback laser (DML) and a phase modulator cascade is also proposed, which can effectively overcome the disadvantages of the five schemes such as high cost and simple structure, but the sub-carrier generated by the scheme has a relatively wide line width (about 25MHz), and thus can only be used for modulating an intensity modulated optical signal, and is different from being used for modulating a PDM-QPSK modulated signal.
However, coherent detection PDM-QPSK modulated signals have higher spectral efficiency and are also increasingly widely used compared to direct detection of intensity modulated signals. Therefore, it is particularly important for the orthogonal multi-carrier generation technology that can be applied in the coherent optical detection system of PDM-QPSK modulation signal.
In view of the problems of high insertion loss and high cost in generating PDM-QPSK modulation signals in the related art, no effective solution has been proposed.
Disclosure of Invention
The invention provides an orthogonal multi-carrier light source and a PDM-QPSK signal transmitting device, aiming at the problems of high insertion loss and high cost in the generation of PDM-QPSK modulation signals in the related art, and at least solving the problems.
According to an aspect of the present invention, there is provided an orthogonal multicarrier optical source comprising: an electroabsorption modulated laser, a phase modulator, a sinusoidal radio frequency signal source, a power divider, a phase shifter, a first electrical power amplifier, and a second electrical power amplifier, wherein; the sinusoidal radio frequency signal source is used for outputting a sinusoidal radio frequency signal with a preset signal frequency to the power divider; the power divider is used for dividing the input sinusoidal radio-frequency signal into a left sinusoidal radio-frequency signal and a right sinusoidal radio-frequency signal, inputting the left sinusoidal radio-frequency signal to the electric power amplifier, and inputting the right sinusoidal radio-frequency signal to the phase shifter; the first electric power amplifier is used for carrying out power amplification on the left sinusoidal radio frequency signal and inputting the left sinusoidal radio frequency signal after power amplification to the electro-absorption modulation laser; the phase shifter is used for adjusting the right sinusoidal radio frequency signal to enable the right sinusoidal radio frequency signal to be synchronous with the left sinusoidal radio frequency signal, and outputting the adjusted right sinusoidal radio frequency signal; the second electric power amplifier is used for performing power amplification on the right sinusoidal radio frequency signal adjusted by the phase shifter and inputting the right sinusoidal radio frequency signal subjected to power amplification to the phase modulator; the electroabsorption modulation laser is used for generating an optical signal under the driving of the left-path sinusoidal radio frequency signal output by the first electric power amplifier and inputting the generated optical signal to the phase modulator; the phase modulator is used for modulating the input optical signal under the driving of the right-path sinusoidal radio frequency signal output by the second electric power amplifier, and generating frequency-locked and orthogonal multiple carriers.
Optionally, the method further comprises: and the 2 frequency multiplier is connected between the phase shifter and the second electric power amplifier and is used for realizing the 2 frequency multiplication of the right sinusoidal radio frequency signal output by the phase shifter and inputting the right sinusoidal radio frequency signal after the 2 frequency multiplication into the second electric power amplifier.
Optionally, the electroabsorption modulated laser comprises: the distributed feedback laser is used for outputting optical signals; and the electric absorption modulator is used for carrying out optical modulation on the optical signal output by the distributed feedback laser under the driving of the left-path sinusoidal radio-frequency signal output by the first electric power amplifier and outputting the modulated optical signal.
Optionally, the operating current of the distributed feedback laser is greater than the threshold current of the distributed feedback laser.
Optionally, the bias voltage of the electroabsorption modulator is within a linear modulation region of the electroabsorption modulator.
Optionally, the electroabsorption modulated laser further comprises: and the semiconductor optical amplifier is used for performing entrance compensation on the optical signal output by the electric absorption modulator so as to compensate the insertion loss of the electric absorption modulator and output the compensated optical signal.
Optionally, the linewidth of the electroabsorption modulated laser is 1.9 MHz.
Optionally, the phase modulator is further configured to increase the number of generated orthogonal subcarriers by increasing the amplitude of the radio frequency signal driving the phase modulator.
Optionally, the electroabsorption modulated laser is further configured to flatten the amplitude of the generated subcarrier by adjusting the amplitude of the radio frequency signal driving the electroabsorption modulated laser.
According to another aspect of the present invention, there is also provided a polarization multiplexing quadrature phase modulation PDM-QPSK signal transmitting apparatus, including: the system comprises an orthogonal multi-carrier light source, a photon carrier selection module and a PDM-QPSK optical signal transmitting module which are sequentially connected; the orthogonal multi-carrier light source is the orthogonal multi-carrier light source; the photonic carrier selection module comprises: the optical add-drop multiplexer is used for dividing the multi-carrier output by the orthogonal carrier light source into an odd part and an even part and inputting the odd path of multi-carrier or the even path of multi-carrier into the tunable optical filter; the tunable optical filter is used for filtering input multiple carriers to obtain required optical carriers by adjusting the bandwidth and wavelength of the tunable optical filter; the PDM-QPSK optical signal transmitting module comprises: the I/Q modulator is used for generating and outputting a QPSK signal under the drive of the optical carrier output by the photon carrier selection module, and the phase difference of the upper arm and the lower arm is pi/2; the polarization multiplexer is used for dividing the optical QPSK signal output by the I/Q modulator into two branches, delaying one path of optical signal, carrying out power equalization on the other path of optical signal, then combining the two paths of optical signals, simulating polarization multiplexing of the signals, generating a PDM-QPSK optical signal, and transmitting the PDM-QPSK optical signal through an optical fiber link.
Optionally, the optical add/drop multiplexer is at a frequency of 12.5/25-GHz.
Optionally, the polarization multiplexer comprises: the polarization maintaining optical coupler is used for dividing an input QPSK optical signal into two branches, wherein one branch of signal is input to the optical delay line, and the other branch of signal is input to the optical attenuator; the optical delay line is used for generating a delay of 150 symbol lengths for the input QPSK optical signals through simulation, and inputting the delayed QPSK optical signals to the polarization beam combiner; the optical attenuator is used for adjusting power of QPSK optical signals, balancing power of the QPSK optical signals in the two branches, and inputting the adjusted QPSK optical signals to the polarization beam combiner; the polarization beam combiner is used for combining two paths of input optical signals, simulating polarization multiplexing of the signals and generating the PDM-QPSK signals.
By adopting the orthogonal multi-carrier light source based on cascade connection of the electro-absorption modulated laser (EML) and the Phase Modulator (PM), the invention not only can generate a certain number of sub-carriers with good flatness, but also effectively overcomes the defect of overlarge line width of the generated sub-carriers in a DML and PM cascade connection scheme, thereby being capable of receiving coherent light of a high-speed PDM-QPSK modulated signal.
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 application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention. In the drawings:
fig. 1 is a schematic structural diagram of an orthogonal multi-carrier light source according to an embodiment of the invention;
FIG. 2 is a schematic diagram of a spectrum of an orthogonal multi-carrier light source output according to an embodiment of the invention;
fig. 3A is a schematic structural diagram of a PDM-QPSK signal transmitting apparatus according to an embodiment of the present invention;
fig. 3B is a schematic structural diagram of a coherent optical receiving system for PDM-QPSK signals according to an embodiment of the present invention;
fig. 4 is a schematic structural diagram of a photonic carrier selection module according to an embodiment of the present invention;
fig. 5 is a spectrum diagram of odd-numbered multi-carriers output by the optical add/drop multiplexer of the photonic carrier selection module according to the embodiment of the present invention;
FIG. 6 is a diagram of a spectrum of a desired photonic carrier output from a polarization maintaining tunable optical filter using a photonic carrier selection module in an embodiment of the present invention;
fig. 7 is a schematic structural diagram of a PDM-QPSK optical signal transmitting module according to an embodiment of the present invention;
fig. 8 is a schematic structural diagram of a homodyne coherent light detection module according to an embodiment of the present invention.
Detailed Description
The invention will be described in detail hereinafter with reference to the accompanying drawings in conjunction with embodiments. It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.
According to an embodiment of the invention, an orthogonal multi-carrier light source is provided.
Fig. 1 is a schematic structural diagram of an orthogonal multi-carrier light source according to an embodiment of the present invention, and as shown in fig. 1, the orthogonal multi-carrier light source mainly includes: electro-absorption modulated laser (EML), Phase Modulator (PM), sinusoidal radio frequency signal source, power divider, Phase Shifter (PS), and 2 electrical power amplifiers (EA).
Wherein the sinusoidal radio frequency signal source is configured to output a sinusoidal radio frequency signal of a predetermined signal frequency (e.g., 12.5GHz) to the power divider. The power divider is used for dividing an input sinusoidal radio frequency signal into a left path and a right path, inputting the left path signal into the first electric power amplifier (EA), and inputting the right path signal into the phase shifter. And the first electric power amplifier is used for carrying out power amplification on the left sinusoidal radio frequency signal output by the power divider and inputting the signal after power amplification into the electro-absorption modulated laser (EML) as a radio frequency driving signal. In the embodiment of the invention, the main function of the electric power amplifier is to amplify the power of the left sinusoidal radio frequency signal, so that the proper number of subcarriers and the flatness can be generated by reasonably adjusting the amplitude of the left radio frequency signal output by the electric power amplifier. And the phase shifter adjusts the right sinusoidal radio frequency signal to synchronize the right sinusoidal radio frequency signal with the left sinusoidal radio frequency signal and outputs the adjusted right sinusoidal radio frequency signal. And the second electric power amplifier is used for carrying out power amplification on the right sinusoidal radio frequency signal adjusted by the phase shifter and inputting the right sinusoidal radio frequency signal subjected to power amplification to the phase modulator. The electro-absorption modulation laser is used for generating an optical signal under the driving of the left-path sinusoidal radio frequency signal output by the first electric power amplifier and inputting the generated optical signal to the phase modulator. The phase modulator is used for modulating the input optical signal under the driving of the right path sinusoidal radio frequency signal output by the second electric power amplifier, and generating frequency-locked and orthogonal multiple carriers.
In an optional implementation manner of the embodiment of the present invention, as shown in fig. 1, the orthogonal multi-carrier light source may further include a frequency multiplier 2, which is connected between the phase shifter and the second electrical power amplifier, and the right sinusoidal rf signal first passes through the phase shifter, then passes through the frequency multiplier 2 to realize frequency multiplication of the signal frequency by 2, and finally passes through the second electrical amplifier to perform power amplification, and then is input as an rf driving signal to the phase modulator. In this alternative embodiment, the primary function of the frequency 2 multiplier is to achieve frequency 2 multiplication of the right sinusoidal radio frequency signal. Since the phase modulator has a relatively large modulation bandwidth, in this embodiment, a higher frequency-2 multiplication radio frequency signal is used to drive the phase modulator, and a lower frequency-single multiplication radio frequency signal is used to drive the electro-absorption modulated laser. Performing a frequency 2 multiplication operation on the rf signal driving the phase modulator helps to further increase the number of generated subcarriers.
In an alternative implementation of the embodiment of the present invention, as shown in fig. 1, the electroabsorption modulated laser may be formed by integrating a Distributed Feedback (DFB) laser and an electroabsorption modulator (EAM), and optionally, the linewidth of the EML is only 1.9MHz (much less than the linewidth of the DML by 25 MHz). Optionally, the working current of the distributed feedback laser needs to be larger than the threshold current of the distributed feedback laser, and the working current of the electro-absorption modulated laser is provided by a direct current power supply (DC); and the optical signal output by the distributed feedback laser is injected into the electric absorption modulator driven by one radio frequency signal, and the bias voltage of the electric absorption modulator needs to be in the linear modulation region of the electric absorption modulator. In the linear modulation range, increasing the bias voltage of the electroabsorption modulator will expand the optimal operating area range of the electroabsorption modulated laser. However, when the bias voltage of the electro-absorption modulator is too high, the average power of the output signal of the electro-absorption modulated laser will be less than-10 dBm due to the introduction of large insertion loss. It is contemplated that an integrated Semiconductor Optical Amplifier (SOA) may be used to compensate for the insertion loss of the modulator. Optionally, the modulation factor of the electroabsorption modulated laser is defined as follows: the ratio of the amplitude of the radio frequency signal driving the electroabsorption modulated laser to the bias voltage of the electroabsorption modulator. On the premise of a certain bias voltage, the adjustment of the power flatness of the output subcarrier can be effectively realized by improving the amplitude of the radio frequency signal for driving the electro-absorption modulated laser. Along with the increase of the amplitude of the radio frequency signal, the power difference of the generated subcarriers is reduced, namely the power flatness is improved; however the number of generated sub-carriers will remain the same.
In an optional implementation manner of the embodiment of the present invention, the output optical signal of the electroabsorption modulated laser is further input to the phase modulator driven by the other path of 2-times frequency multiplication radio frequency signal to generate multiple carriers with frequency locking and orthogonality, and a frequency interval between the multiple carriers is 12.5 GHz. Optionally, the modulation factor of the phase modulator is defined as follows: the ratio of the amplitude of the radio frequency signal driving the phase modulator to the half-wave voltage of the phase modulator. On the premise that the half-wave voltage is constant, the adjustment of the number of output subcarriers can be effectively realized by improving the amplitude of the radio frequency signal for driving the phase modulator. As the amplitude of the radio frequency signal increases, the number of generated subcarriers will increase. Therefore, in practical applications, the amplitude of the rf signal driving the electro-absorption modulated laser and the phase modulator should be properly adjusted to produce as many multi-carriers as possible with good power flatness. Thus, optionally, the phase modulator is further arranged to increase the number of generated orthogonal subcarriers by increasing the amplitude of the radio frequency signal driving the phase modulator. The electroabsorption modulated laser is also used for flattening the amplitude of the generated subcarrier by adjusting the amplitude of a radio frequency signal for driving the electroabsorption modulated laser.
The multi-carrier generation in the technical solution provided by the alternative embodiment of the present invention will be explained in detail below with reference to fig. 1 by taking an orthogonal multi-carrier light source including an electro-absorption modulated laser, a phase modulator, a sinusoidal radio frequency signal source, a power divider, a phase shifter, a 2-frequency multiplier, and an electrical amplifier as an example.
Firstly, the sinusoidal radio frequency signal source outputs a sinusoidal radio frequency signal with a signal frequency of 12.5GHz to the power divider.
Secondly, the power divider divides the radio frequency signals into two paths: one path of single frequency multiplication radio frequency signal is subjected to power amplification through the electric amplifier and then drives the electric absorption modulation laser; and the other path of the radio frequency signal passes through the phase shifter firstly and then the frequency multiplier 2 to realize the frequency multiplication of the signal frequency by 2, and finally the frequency multiplication radio frequency signal of 2 passes through the electric amplifier to carry out power amplification and then drives the phase modulator.
The phase shifter adopted by the second branch circuit mainly has the function of realizing the synchronization of two paths of radio frequency signals.
The electroabsorption modulated laser is formed by integrating a distributed feedback laser and an electroabsorption modulator. A DC power supply provides operating current for the DFB laser. Preferably, the operating current is greater than the threshold current of the laser.
And thirdly, inputting the optical signal output by the distributed feedback laser into the electric absorption modulator driven by one single-frequency radio frequency signal for optical modulation, wherein the bias voltage of the electric absorption modulator needs to be in a linear modulation region of the device.
Then, the optical signal outputted by the electroabsorption modulator is further inputted into a polarization-maintaining erbium-doped fiber amplifier to compensate the modulation loss introduced by the electroabsorption modulator.
Then, the power amplified optical signal output by the polarization maintaining erbium-doped fiber amplifier is further input into the phase modulator driven by another 2-frequency multiplication radio frequency signal, so as to generate frequency-locked and orthogonal multiple carriers, and the electrical domain expression of the output signal is as follows:
wherein R1 is the modulation factor of the electro-absorption modulated laser, which is defined as follows: the ratio of the amplitude of the radio frequency signal driving the electro-absorption modulated laser to the bias voltage of the electro-absorption modulator; r2 is a phase modulator modulation factor, which is defined as follows: the ratio of the amplitude of the radio frequency signal driving the phase modulator to the half-wave voltage of the phase modulator. As can be seen from the above formula, the second and third terms in formula 1 not only achieve power flattening for the output multi-carrier, but also introduce a new frequency component, so that the frequency interval between adjacent sub-carriers is halved from the frequency of the 2-times signal actually driving the phase modulator to the frequency of the single-frequency signal.
In practical application, the electrical amplifiers of the two branches shown in fig. 1 can be adjusted respectively to realize reasonable adjustment of the single frequency radio frequency signal for driving the electroabsorption modulated laser and the 2-frequency multiplication radio frequency signal for driving the phase modulator. Experiments prove that the orthogonal multi-carrier light source based on the EML and PM cascade connection can finally output 25 frequency locking orthogonal multi-carriers with the power difference smaller than 5dB at the output end of the phase modulator, and the frequency interval between the sub-carriers is 12.5GHz as shown in figure 2.
Optionally, as shown in fig. 1, the orthogonal multi-carrier light source may further include an optical amplifier (PM-EDFA) disposed between the EML and the PM cascade to amplify the optical signal.
According to the embodiment of the invention, the invention also provides a PDM-QPSK signal transmitting device.
Fig. 3A is a schematic structural diagram of a PDM-QPSK signal transmitting apparatus according to an embodiment of the present invention, and as shown in fig. 3A, the apparatus mainly includes: the device comprises an orthogonal multi-carrier light source, a photon carrier selection module and a PDM-QPSK optical signal transmitting module which are sequentially connected.
The transmitting device provided by the embodiment of the invention adopts an orthogonal multi-carrier light source based on EML and PM cascade to generate multi-carriers, and adopts a PDM-QPSK optical signal generating module to generate a high-speed PDM-QPSK modulation signal
The above-described modules will be described below.
The orthogonal multi-carrier light source is the orthogonal multi-carrier light source shown in fig. 1 provided in this embodiment, and specific reference is made to the above description, which is not described herein again.
The photonic carrier selection module may include an optical add/drop multiplexer (IL) and a tunable optical filter (PM-TOF). Alternatively, as shown in FIG. 4, the photonic carrier selection module consists of an optical add/drop multiplexer (IL) consisting of a 12.5/25-GHz and a polarization maintaining tunable optical filter (PM-TOF). The optical add-drop multiplexer is used for dividing the multi-carrier output by the orthogonal carrier light source into an odd part and an even part and inputting the odd path of multi-carrier or the even path of multi-carrier into the tunable optical filter; the tunable optical filter is used for filtering input multiple carriers to obtain required optical carriers by adjusting the bandwidth and wavelength of the tunable optical filter. In the embodiment of the invention, the multi-carrier output by the orthogonal multi-carrier light source based on EML and PM cascade is firstly divided into two parts of odd and even number by the optical add-drop multiplexer, and the frequency interval between the odd number path multi-carrier and the even number path multi-carrier is increased to 25 GHz. Then inputting the odd or even multi-carrier output by the optical add/drop multiplexer into the polarization maintaining tunable optical filter, and filtering out the required optical carrier by adjusting the bandwidth and wavelength of the tunable optical filter.
Alternatively, the tunable optical filter may be a polarization maintaining erbium doped fiber amplifier.
In the embodiment of the invention, the orthogonal multi-carrier input generated by the orthogonal multi-carrier light source is divided into an odd part and an even part by the optical add-drop multiplexer. Optionally, the optical add/drop multiplexer device parameters are 12.5/25-GHz in order to match the rf signal source frequency of 12.5 GHz. The intersection adopts the alternative of other add-drop multiplexer device parameters, and the parameter setting can realize the optimal separation of the optical multi-carrier signals. The frequency separation between the odd or even multiple carriers will thus increase to 25 GHz. The spectrum diagram of the odd-numbered multi-carrier output by the optical add/drop multiplexer is shown in figure 5. The odd number of multiple carriers are then input into the polarization maintaining tunable optical filter. Optionally, the bandwidth and wavelength of the tunable optical filter should be consistent with the desired optical carrier. Here, the tunable optical filter also simultaneously achieves the filtering of the polarization maintaining erbium-doped fiber amplifier ASE noise used by the orthogonal multi-carrier light source to compensate the modulation loss of the electro-absorption modulator. Optionally, a plurality of bandpass filters with a determined center frequency may be used to implement filtering of the corresponding optical carrier, but the tunable optical filter may simplify the system structure on one hand, and on the other hand, may be more flexible and convenient in adjusting the center frequency of the filter. The spectrum of the desired photonic carrier output by the polarization maintaining tunable optical filter is shown in fig. 6. And finally, the photon carrier output by the polarization maintaining tunable optical filter is subjected to power amplification through a polarization maintaining erbium-doped optical fiber amplifier, and is input into a PDM-QPSK optical signal transmitting module to serve as an optical carrier signal to realize QPSK optical signal modulation.
The PDM-QPSK optical signal transmission module may include an I/O modulator and a polarization multiplexer. The I/O modulator is used for driving the optical carrier output by the photon carrier selection module to generate an optical QPSK signal, and the phase difference of the upper arm and the lower arm is pi/2; the polarization multiplexer is used for dividing the optical QPSK signal output by the I/O modulator into two branches, delaying one path of optical signal, performing power equalization on the other path of optical signal, then combining the two paths of optical signals, simulating polarization multiplexing of the signals, generating a PDM-QPSK optical signal, and transmitting the PDM-QPSK optical signal to the homodyne coherent optical detection module through an optical fiber link.
Alternatively, as shown in fig. 7, the PDM-QPSK optical signal transmitting module is composed of an I/Q modulator and a polarization multiplexer. The I/Q modulator consists of two parallel mach-zehnder modulators (MZMs), and the mach-zehnder modulators are both biased at a null point and driven in full-wave. The phase difference of the upper arm and the lower arm of the I/Q modulator is controlled to be pi/2. The optical carrier filtered out by the photonic carrier selection module is driven by a 28G wave special electrical binary signal through the I/Q modulator to generate an optical QPSK signal, and the electrical binary signal is generated in a code pattern generator (PPG). The polarization multiplexer consists of a polarization maintaining optical coupler (PM-OC), an optical Delay Line (DL), an optical attenuator and a Polarization Beam Combiner (PBC). The polarization maintaining optical coupler firstly divides an input optical QPSK signal into two branches, wherein one branch of the signal generates a delay of 150 symbol lengths through the simulation of the optical delay line, and the other branch of the signal realizes the balance of the power of the two branches of the optical signal through the optical attenuator. And finally, combining the two paths of optical signals through the polarization beam combiner, simulating polarization multiplexing of the signals, and generating the PDM-QPSK signal.
In this embodiment, one optical carrier signal and one 28 gbaud electrical binary signal are input to the I/Q modulator together for optical modulation, and a QPSK modulated optical signal is output. The electrical binary signal is composed of a pseudorandom binary sequence of length 223-1 and is generated by a pattern transmitter. Preferably, the I/Q modulator consists of two parallel mach-zehnder modulators above and below and with a pi/2 phase difference, the mach-zehnder modulators being biased at null and driven in full wave. Compared with other Mach-Zehnder modulator parameter setting schemes, the setting can realize optimal phase modulation with zero chirp and pi phase jump.
Then, the QPSK modulated optical signal output by the I/Q modulator is input into a polarization multiplexer to generate a PDM-QPSK signal, and the PDM-QPSK optical signal is transmitted through an optical fiber link. The polarization multiplexer consists of a polarization maintaining optical coupler, an optical delay line, an optical attenuator and a polarization beam combiner. The polarization maintaining optical coupler firstly inputs a QPSK modulated optical signal and divides the QPSK modulated optical signal into two branches, preferably, one of the two branches is simulated by the optical delay line to generate a delay of 150 symbol lengths, and the other branch is simulated by the optical attenuator to achieve power equalization of the two branches. And finally, combining the two paths of optical signals through the polarization beam combiner, and simulating polarization multiplexing of the signals. There is an alternative to directly using an integrated optical polarization multiplexer module, but the analog optical signal polarization multiplexing module has advantages in terms of the cost of experimental devices on the one hand, and is more flexible and convenient because the optical delay line can be directly adjusted on the other hand. The generated 112-Gb/s optical PDM-QPSK signal is then transmitted to the receiving end via an optical fiber link.
According to the embodiment of the invention, the invention also provides a coherent light receiving system of the PDM-QPSK signal.
Fig. 3B is a schematic structural diagram of a coherent optical receiving system for PDM-QPSK signals according to an embodiment of the present invention, and as shown in fig. 3B, the system includes a receiving apparatus and the PDM-QPSK signal transmitting apparatus. As shown in fig. 3B, the receiving apparatus uses a homodyne coherent optical detection module to implement coherent detection and data recovery of the local oscillator light and the signal light signal. The 112-Gb/s optical PDM-QPSK signal generated by the transmitting device is then transmitted to the receiving device via an optical fiber link consisting of 80km of standard single mode fiber-28 (SMF-28).
Optionally, the homodyne coherent light detection module of the receiving apparatus may include: the device comprises a polarization diversity and phase diversity optical coherent detection module and a digital signal processing unit. The polarization diversity and phase diversity optical coherent detection module comprises an external cavity laser, two polarization beam splitters, two 90-degree optical mixers, four photodiodes and four high-speed analog-to-digital converters, wherein the external cavity laser is used as a local oscillator and used for receiving the PDM-QPSK optical signals, the local oscillator and the PDM-QPSK optical signals pass through one polarization beam splitter respectively, the polarization beam splitter separates the local oscillator and the PDM-QPSK optical signals into two orthogonal polarization state optical signals, and the local oscillator and the PDM-QPSK optical signals in the same polarization state are input into one 90-degree optical mixer; the 90-degree optical mixer is used for generating phase shifts of 0 degrees, 90 degrees, 180 degrees and 270 degrees on an input optical signal and then carrying out beat frequency on the input optical signal and the PDM-QPSK optical signal to realize coherent detection and then output the optical signal; the photodiodes are used for carrying out balanced detection on four paths of coherent detection optical signals output by the two 90-degree optical mixers and respectively inputting four paths of output optical currents into the four high-speed analog-to-digital converters; and the adjusting analog-to-digital converter is used for carrying out Nyqusit sampling on the input photocurrent to convert the input photocurrent into a sampling signal. And the digital signal processing unit respectively carries out data recovery on the sampling signals obtained by the high-speed analog-to-digital conversion sampling.
Alternatively, as shown in fig. 8, the homodyne coherent optical detection module is composed of a polarization diversity plus phase diversity coherent detection module and a Digital Signal Processing (DSP) unit. The polarization diversity and phase diversity optical coherence detection module comprises an External Cavity Laser (ECL), two Polarization Beam Splitters (PBS), two 90-degree optical mixers, four Photodiodes (PD) and four high-speed analog-to-digital converters (AEC). The external cavity laser acts as a local oscillator light source (LO) and the received signal light transmitted by the optical fiber link is separated into two orthogonal polarization states by one polarization beam splitter; then, the local oscillator light and the signal light in the same polarization state are input into one 90-degree optical mixer, and the 90-degree optical mixer has the main function of enabling the local oscillator light to generate phase shifts of 0 degrees, 90 degrees, 180 degrees and 270 degrees and then beat with the signal light to realize coherent detection; then four paths of coherent detection light signals (X polarization direction in-phase component and orthogonal component; Y polarization direction in-phase component and orthogonal component) output by the two 90-degree light mixers are respectively input into the four photodiodes for balanced detection, and four paths of output light currents are respectively input into the four high-speed analog-to-digital converters for Nyqusit sampling and converting into sampling signals. The digital signal processing unit has the main functions of realizing data recovery of a sampling signal obtained by high-speed analog-to-digital conversion sampling, and comprises the following steps: and performing signal retiming, dispersion compensation, constant modulus algorithm equalization, carrier recovery, differential decoding and bit error rate calculation.
In this alternative embodiment, the PDM-QPSK optical signal output by the transmitter is transmitted via 80km standard single-mode fiber-28 as the received signal light to be input to the polarization diversity plus phase diversity optical coherence detection module. The polarization diversity and phase diversity optical coherent detection module comprises an external cavity laser, two polarization beam splitters, two 90-degree optical mixers, four photodiodes and four high-speed analog-to-digital converters.
And then, the input received signal light and the input local oscillation light are respectively subjected to separation of two orthogonal polarization states through one polarization beam splitter, and the local oscillation light source is realized by the external cavity laser.
And then, the local oscillator light and the signal light with the same polarization state are input into one 90-degree optical mixer together, and the 90-degree optical mixer has the main function of enabling the local oscillator light to generate phase shifts of 0 degree, 90 degrees, 180 degrees and 270 degrees and then to perform beat frequency with the signal light to realize coherent detection.
Then four paths of coherent detection light signals (an in-phase component and an orthogonal component in the X polarization direction; an in-phase component and an orthogonal component in the Y polarization direction) output by the two 90-degree light mixers are respectively input into the four photodiodes for balanced detection, and four paths of light currents are output.
And finally, respectively inputting the four paths of photocurrents into the four high-speed analog-to-digital converters for Nyqusit sampling and converting the four paths of photocurrents into sampling electric signals.
Although the amplitude and phase information carried in the received signal optical domain can be completely retained in the sampling electrical signal after photoelectric conversion through the polarization diversity plus phase diversity optical coherent detection, the frequency and phase of the sampling electrical signal are disturbed by the frequency and phase of the local oscillator optical frequency because the frequency between the local oscillator light source and the carrier of the transmitting-end optical is difficult to keep completely consistent and the line width of the local oscillator light source introduces corresponding phase offset. In addition, in the coherent detection process, there are sampling clock mismatch at the transceiving end, channel static damage caused by fiber dispersion, polarization mode dispersion effect and other signal damages. Therefore, the digital signal processing unit needs to be introduced to estimate and compensate the above losses respectively, so as to complete the recovery regeneration and recovery of the original transmission signal. As shown in fig. 7, preferably, the digital signal processing unit includes: and performing signal retiming, dispersion compensation, constant modulus algorithm equalization, carrier recovery, differential decoding and bit error rate calculation. The method mainly comprises the steps of retiming a signal, mainly solving the problem of clock misalignment caused by unmatched ADC sampling clocks, mainly eliminating the damage of optical fiber dispersion and polarization mode dispersion to the signal by dispersion compensation and constant modulus algorithm equalization, mainly eliminating the influence of phase offset to the signal by carrier recovery, and finally carrying out differential decoding on a correctly recovered signal constellation diagram to recover a 0-1 bit sequence and evaluating the overall performance of the system through bit error rate calculation.
In the system provided by the embodiment of the invention, because the orthogonal multi-carrier light source based on the cascade connection of the EML and the PM is adopted, the output sub-carrier can be effectively output, so that the line width of the transmitting end can be controlled to be 1.9MHz, and the product of the line width of the optical carrier and the symbol duration time is ensured to meet the condition (the product is less than 1 multiplied by 10 < -4 >) of implementing the coherent reception of the 28 Gbaud high-speed PDM-QPSK signal, and the implementation of the embodiment 3 becomes possible.
From the above description, it can be seen that, in the embodiment of the present invention, an orthogonal multi-carrier light source based on cascade connection of EML and PM is provided, and the multi-carrier light source is applied to a PDM-QPSK modulated signal transmitting apparatus and a coherent light detection system. The orthogonal multi-carrier light source not only can generate a certain number of sub-carriers with good flatness, but also effectively overcomes the defect of overlarge line width of the generated sub-carriers in a DML and PM cascading scheme, so that coherent light receiving of high-speed PDM-QPSK modulation signals becomes possible. In addition, the orthogonal multi-carrier light source provided by the invention has the characteristics of small volume, low power consumption and easy integration, thereby having wide application prospect in an actual system.
It will be apparent to those skilled in the art that the modules or steps of the present invention described above may be implemented by a general purpose computing device, they may be centralized on a single computing device or distributed across a network of multiple computing devices, and alternatively, they may be implemented by program code executable by a computing device, such that they may be stored in a storage device and executed by a computing device, and in some cases, the steps shown or described may be performed in an order different than that described herein, or they may be separately fabricated into individual integrated circuit modules, or multiple ones of them may be fabricated into a single integrated circuit module. Thus, the present invention is not limited to any specific combination of hardware and software.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (13)
1. An orthogonal multi-carrier light source, comprising: an electro-absorption modulated laser, a phase modulator, a sinusoidal radio frequency signal source, a power divider, a phase shifter, a first electrical power amplifier, and a second electrical power amplifier, wherein,
the sinusoidal radio frequency signal source is used for outputting a sinusoidal radio frequency signal with a preset signal frequency to the power divider;
the power divider is used for dividing the input sinusoidal radio-frequency signal into a left sinusoidal radio-frequency signal and a right sinusoidal radio-frequency signal, inputting the left sinusoidal radio-frequency signal to the first electric power amplifier, and inputting the right sinusoidal radio-frequency signal to the phase shifter;
the first electric power amplifier is used for carrying out power amplification on the left sinusoidal radio frequency signal and inputting the left sinusoidal radio frequency signal after power amplification to the electro-absorption modulation laser;
the phase shifter is used for adjusting the right sinusoidal radio frequency signal to enable the right sinusoidal radio frequency signal to be synchronous with the left sinusoidal radio frequency signal, and outputting the adjusted right sinusoidal radio frequency signal;
the second electric power amplifier is used for performing power amplification on the right sinusoidal radio frequency signal adjusted by the phase shifter and inputting the right sinusoidal radio frequency signal subjected to power amplification to the phase modulator;
the electroabsorption modulation laser is used for generating an optical signal under the driving of the left-path sinusoidal radio frequency signal output by the first electric power amplifier and inputting the generated optical signal to the phase modulator;
the phase modulator is used for modulating the input optical signal under the driving of the right-path sinusoidal radio frequency signal output by the second electric power amplifier, and generating frequency-locked and orthogonal multiple carriers.
2. The orthogonal multicarrier light source of claim 1, further comprising: and the 2 frequency multiplier is connected between the phase shifter and the second electric power amplifier and is used for realizing the 2 frequency multiplication of the right sinusoidal radio frequency signal output by the phase shifter and inputting the right sinusoidal radio frequency signal after the 2 frequency multiplication into the second electric power amplifier.
3. The orthogonal multicarrier light source of claim 1, wherein said electroabsorption modulated laser comprises:
the distributed feedback laser is used for outputting optical signals;
and the electric absorption modulator is used for carrying out optical modulation on the optical signal output by the distributed feedback laser under the driving of the left-path sinusoidal radio-frequency signal output by the first electric power amplifier and outputting the modulated optical signal.
4. The orthogonal multicarrier light source of claim 3, wherein an operating current of said DFB laser is greater than a threshold current of said DFB laser.
5. The orthogonal multicarrier light source of claim 3 wherein a bias voltage of said electroabsorption modulator is within a linear modulation region of said electroabsorption modulator.
6. The orthogonal multicarrier light source of claim 3, wherein said electroabsorption modulated laser further comprises: and the semiconductor optical amplifier is used for performing entrance compensation on the optical signal output by the electric absorption modulator so as to compensate the insertion loss of the electric absorption modulator and output the compensated optical signal.
7. The orthogonal multi-carrier light source of any of claims 1-6, wherein the linewidth of the electroabsorption modulated laser is 1.9 MHz.
8. The orthogonal multicarrier optical source of any of claims 1 to 6, wherein said phase modulator is further configured to increase a number of generated orthogonal subcarriers by increasing an amplitude of a radio frequency signal driving said phase modulator.
9. The orthogonal multicarrier light source of any of claims 1 to 6, wherein said electroabsorption modulated laser is further configured to flatten the amplitude of the generated subcarriers by adjusting the amplitude of a radio frequency signal driving said electroabsorption modulated laser.
10. A polarization-multiplexed quadrature-phase modulated PDM-QPSK signal transmitting apparatus, comprising: the system comprises an orthogonal multi-carrier light source, a photon carrier selection module and a PDM-QPSK optical signal transmitting module which are sequentially connected; wherein,
the orthogonal multi-carrier light source is the orthogonal multi-carrier light source of any one of claims 1 to 9;
the photonic carrier selection module comprises:
the optical add-drop multiplexer is used for dividing the multi-carrier output by the orthogonal carrier light source into an odd part and an even part and inputting the odd path of multi-carrier or the even path of multi-carrier into the tunable optical filter;
the tunable optical filter is used for filtering input multiple carriers to obtain required optical carriers by adjusting the bandwidth and wavelength of the tunable optical filter;
the PDM-QPSK optical signal transmitting module comprises:
the I/Q modulator is used for generating and outputting a QPSK signal under the drive of the optical carrier output by the photon carrier selection module, and the phase difference of the upper arm and the lower arm is pi/2;
a polarization multiplexer for dividing the optical QPSK signal output by the I/Q modulator into two branches, delaying one path of optical signal, performing power equalization on the other path of optical signal, combining the two paths of optical signals, simulating polarization multiplexing of signals, and generating PDM-QPSK optical signal for transmission
11. The apparatus of claim 10 wherein the optical add/drop multiplexer is at a frequency of 12.5/25-GHz.
12. The apparatus of claim 10, wherein the polarization multiplexer comprises:
the polarization maintaining optical coupler is used for dividing an input QPSK optical signal into two branches, wherein one branch of signal is input to the optical delay line, and the other branch of signal is input to the optical attenuator;
the optical delay line is used for generating a delay of 150 symbol lengths for the input QPSK optical signals through simulation, and inputting the delayed QPSK optical signals to the polarization beam combiner;
the optical attenuator is used for adjusting power of QPSK optical signals, balancing power of the QPSK optical signals in the two branches, and inputting the adjusted QPSK optical signals to the polarization beam combiner;
the polarization beam combiner is used for combining two paths of input optical signals, simulating polarization multiplexing of the signals and generating the PDM-QPSK signals.
13. A polarization-multiplexed quadrature-phase-modulated PDM-QPSK signal coherent optical receiving system, comprising: the receiving apparatus and the transmitting apparatus according to any one of claims 10 to 12, wherein the receiving apparatus is configured to receive a PDM-QPSK signal transmitted by the transmitting apparatus.
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