Frequency shift generating apparatus and method for super high speed polarization multiplexed optical signal

Abstract

The application belongs to the technical field of space satellite laser communication and high-speed coherent light communication, and particularly relates to a frequency shift generating device and method of super-high-speed polarization multiplexed optical signal. The device comprises an input light amplification processing unit, a polarization diversity common path frequency shift ring unit, a quadrature radio frequency driving unit and a bias voltage generator. In the polarization diversity common path frequency shift ring unit, a fiber polarization beam splitter / combiner decomposes the input light signal into X and Y polarized light, and injects the two light signals into a quadrature optical modulation loop composed of two pairs of couplers, modulators and optical phase shifters in clockwise and counterclockwise directions. Under the unified radio frequency driving and bias control, the two light signals simultaneously generate carrier suppressed single sideband frequency shift, and are combined into a polarization multiplexed signal after frequency shift by the fiber polarization beam splitter / combiner. The application ensures that the frequency shift amounts of the two polarizations are completely equal based on physical reciprocity, and saves hardware cost and volume, and can be used for space laser Doppler simulation and coherent light communication test.

Description

Technical Field

[0001] This invention belongs to the fields of space satellite laser communication and high-speed coherent optical communication technology, specifically relating to a frequency shifting generation device and method for ultra-high-speed polarization multiplexed optical signals. Background Technology

[0002] When conducting ground simulations and experimental verifications of high-speed inter-satellite and satellite-to-ground optical communication links, it is usually necessary to introduce precise optical frequency shifts into the optical signals in the laboratory to accurately simulate the time-varying Doppler frequency shift effect caused by the relative high-speed motion between satellites.

[0003] Existing optical frequency shift generation methods typically include frequency shifting using acousto-optic modulators or single-sideband frequency shifting of single-polarized optical signals based on single-polarization IQ modulators. These methods are relatively mature for processing single-polarized optical signals, but they face significant technical bottlenecks when dealing with ultra-high-speed dual-polarization (DP) optical signals such as 100Gbps DP-QPSK, 200Gbps DP-16QAM, and 400Gbps DP-64QAM. If the combined signal is processed only with a single-polarization device, it will directly lead to aliasing and loss of orthogonal polarization information.

[0004] Furthermore, to achieve frequency shifting of polarization-multiplexed optical signals using traditional techniques, polarization demultiplexing is typically performed, followed by processing the X-polarization and Y-polarization separately using two independent single-polarization IQ modulation frequency shifting devices, and finally combining the two polarized optical signals. However, this approach requires two RF sources, RF amplifiers, a 90-degree RF bridge, an optical IQ modulator, and a bias controller. Since the two devices have independent RF cable lengths, amplifier amplitude-frequency responses, modulator half-wave voltages, fiber connection lengths, temperature drift coefficients, and DC bias drift characteristics, inconsistencies arise in several parameter characteristics between the two polarized optical signals. These inconsistencies introduce problems such as frequency shift deviations, phase noise differences, inter-polarization group delay differences, optical power imbalances, different residual carrier suppression depths, and inconsistent sideband suppression ratios. This causes the constellation diagram of the output polarization-multiplexed signal to rotate, spread, or experience polarization crosstalk, making it difficult to meet the requirements of ultra-high-speed coherent optical communication systems for polarization orthogonality, phase continuity, and frequency shift consistency. Simultaneously, the system size and hardware cost increase exponentially.

[0005] Therefore, it is necessary to actively explore ultra-high-speed polarization multiplexed optical signal frequency shift generation devices and methods that overcome the limitations of single-polarization frequency shifting methods and achieve time synchronization, power equality, and frequency shift equality of two orthogonal polarization components under the same optical link and the same RF driving reference, without destroying the original polarization multiplexing information. Specifically, the device should ensure that X-polarized light and Y-polarized light share the same frequency shift signal source and the same set of orthogonal RF drives, guaranteeing equal frequency shifts from the source; it should ensure that the two polarization components pass through the same optical devices in opposite directions within the same orthogonal optical modulation loop, guaranteeing equal optical paths from a physical reciprocity perspective; and it should also use fiber amplifiers, RF attenuators, RF phase shifters, and modulator bias voltage control to uniformly balance optical power, RF amplitude, RF phase, and modulator operating point, ensuring that the two reconstructed polarization components maintain consistency in power, phase, and information delay. Summary of the Invention

[0006] This invention aims to overcome the problems in existing technologies for frequency shifting of polarization-multiplexed optical signals, which typically employ two independent modulation devices. These devices suffer from inconsistencies in frequency shift, phase, time delay, and power, leading to polarization crosstalk and significantly increasing size and cost. The invention provides a frequency shifting device and method for ultra-high-speed polarization-multiplexed optical signals that features polarization diversity common-path frequency shifting, physical reciprocity ensuring complete equality of frequency shift and phase, and saves on hardware costs and size.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] A frequency shifting generation device for ultra-high-speed polarization multiplexed optical signals, comprising:

[0009] The system includes an input optical amplification and processing unit, a polarization diversity common-path frequency shifting loop unit, a quadrature radio frequency drive unit, and a bias voltage generator.

[0010] The input optical amplification and processing unit is used to receive the polarization multiplexed optical signal to be processed, and to perform power amplification, unidirectional transmission and port routing selection on the polarization multiplexed optical signal.

[0011] The polarization diversity common-path frequency shifting loop unit includes an optical fiber polarization beam splitter / combiner, a first optical fiber coupler, a second optical fiber coupler, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a first optical phase shifter, and a second optical phase shifter. The optical fiber polarization beam splitter / combiner decomposes the input polarization multiplexed optical signal into X-polarized light and Y-polarized light, and injects the X-polarized light clockwise and the Y-polarized light counterclockwise into an orthogonal optical modulation loop composed of the first optical fiber coupler, the second optical fiber coupler, the first Mach-Zehnder modulator, the second Mach-Zehnder modulator, the first optical phase shifter, and the second optical phase shifter.

[0012] The orthogonal radio frequency driving unit is used to generate two radio frequency driving signals with a 90-degree phase difference at the same frequency, and feeds them into the radio frequency ports of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator, respectively.

[0013] The bias voltage generator is used to apply bias voltages to the first Mach-Zehnder modulator and the second Mach-Zehnder modulator respectively, so that the first Mach-Zehnder modulator and the second Mach-Zehnder modulator operate at the carrier suppression point; at the same time, it is used to control the first optical phase shifter and the second optical phase shifter respectively to generate the required 90-degree optical phase difference, so that the X-polarized light and Y-polarized light transmitted in opposite directions both undergo single-sideband frequency shift, and are then combined into a frequency-shifted polarization multiplexed optical signal by the fiber polarization beam splitter / combiner.

[0014] Preferably, the input optical amplification processing unit includes an optical fiber amplifier, an optical isolator, and an optical fiber circulator connected in sequence; the first port of the optical fiber circulator is connected to the input optical path, the second port of the optical fiber circulator is connected to the optical fiber polarization splitter / combiner and receives the frequency-shifted signal after beam combining, and the third port of the optical fiber circulator serves as the output end of the frequency-shifted polarization multiplexed optical signal.

[0015] Preferably, the orthogonal optical modulation loop includes an upper arm and a lower arm that are parallel to each other. The upper arm is provided with a first Mach-Zehnder modulator and a first optical phase shifter, and the lower arm is provided with a second Mach-Zehnder modulator and a second optical phase shifter. One end of the upper arm and one end of the lower arm are coupled to a first fiber coupler, and the other end of the upper arm and the other end of the lower arm are coupled to a second fiber coupler.

[0016] Preferably, the quadrature RF drive unit includes a frequency shift signal source, an RF amplifier, and a 90° RF bridge connected in series; the two quadrature output branches of the 90° RF bridge are respectively connected in series with an RF attenuator and an RF phase shifter for balancing amplitude and phase.

[0017] This invention also provides a method for frequency shifting to generate ultra-high-speed polarization multiplexed optical signals, comprising the following steps:

[0018] S1, the ultra-high-speed polarization multiplexed optical signal to be frequency shifted is input into the fiber amplifier, and after being amplified by the fiber amplifier, it is output to the optical isolator, and then injected into the fiber polarization splitter / combiner by the fiber circulator.

[0019] S2, the fiber polarization splitter / combiner orthogonally decomposes the polarization multiplexed optical signal into X-polarized light and Y-polarized light, and sends the X-polarized light in a clockwise direction and the Y-polarized light in a counterclockwise direction into the orthogonal optical modulation loop;

[0020] S3, the orthogonal radio frequency driving unit generates two radio frequency driving signals with a 90-degree phase difference at the same frequency, which are used to drive the first Mach-Zehnder modulator and the second Mach-Zehnder modulator respectively;

[0021] S4, the bias voltage generator locks the first Mach-Zehnder modulator and the second Mach-Zehnder modulator at the carrier suppression bias point, and controls the first optical phase shifter and the second optical phase shifter to stabilize the optical phase difference of the orthogonal optical modulation loop at 90 degrees, so that the X-polarized light and Y-polarized light transmitted in opposite directions undergo carrier suppression single-sideband modulation at the same time, and obtain equal and same optical frequency shift;

[0022] S5, the frequency-shifted X-polarized light and Y-polarized light are combined into a frequency-shifted polarization multiplexed light signal by the fiber polarization splitter / combiner, and then output by the input optical amplification processing unit.

[0023] Preferably, the input optical amplification processing unit includes an optical fiber amplifier, an optical isolator, and an optical fiber circulator connected in sequence; the optical fiber amplifier amplifies the power of the input signal to compensate for the insertion loss of subsequent links, the optical isolator suppresses the reflected light of the subsequent stage; the optical fiber circulator allows the input polarization multiplexed optical signal to enter the second port from its first port, and allows the frequency-shifted polarization multiplexed optical signal to be output from the second port through the third port.

[0024] Preferably, in step S2, the X-polarized light and the Y-polarized light pass through the same set of optical devices in opposite directions in the orthogonal optical modulation loop, sharing the same optical path.

[0025] Preferably, in step S3, the frequency shift signal source output angular frequency in the quadrature radio frequency driving unit is . The radio frequency signal is amplified by the radio frequency amplifier and then split into two orthogonal signals by a 90° radio frequency bridge. The two orthogonal signals are corrected for amplitude and phase by the radio frequency attenuator and the radio frequency phase shifter, respectively, and then fed into the radio frequency ports of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator, so that the two modulators obtain the driving conditions of equal amplitude and 90 degrees phase difference.

[0026] Preferably, in step S4, the bias voltage generator uses bias voltage feedback to keep the DC phase difference between the two arms of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator near π to suppress the original optical carrier; the bias voltage generator is also used to cooperate with the RF attenuator, RF phase shifter and the first optical phase shifter and the second optical phase shifter in the quadrature RF drive unit so that the electrical 90-degree phase difference and the optical 90-degree phase difference together satisfy the single-sideband selection condition.

[0027] Preferably, in step S5, the frequency-shifted X-polarized light and Y-polarized light respectively carry the original electric field complex envelope information, and the angular frequencies of the X-polarized light and the Y-polarized light are determined by... Unified translation to or The polarization multiplexed optical signal after beam combining maintains the polarization orthogonality of the two orthogonal polarization channels and the original modulation information.

[0028] Compared with the prior art, the beneficial effects of this invention are: (1) ingenious structure to achieve polarization diversity common path frequency shift: This invention ingeniously combines the polarization diversity loop architecture with the orthogonal optical modulation structure, and utilizes the bidirectional propagation characteristics to make X-polarized and Y-polarized beams propagate in opposite directions in the same set of physical hardware; this design of polarization common optical path ensures from the physical bottom layer that the device performance, path loss and external disturbance experienced by the two signals are completely consistent, and the structure is simple; (2) physical reciprocity to ensure that the frequency shift and phase are completely identical: Based on the physical reciprocity of the optical path, the two polarization components are in the same set of physical hardware. Synchronous modulation under RF driving reference; fundamentally eliminates the random errors caused by RF source differences, bias point drift and fiber path jitter in the traditional dual-device scheme, realizes constant frequency shift and initial phase difference, greatly improves the consistency and stability of frequency shift; (3) Halved scale, significantly reducing system volume and cost: dual polarization synchronous frequency shift is achieved by single device, reducing the number of key hardware such as core optical IQ modulator, RF amplifier, 90-degree RF bridge and bias controller by half; reduces hardware cost and engineering cost, and has high engineering practical value. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of a frequency shifting generation device for ultra-high-speed polarization multiplexed optical signals in this invention. Detailed Implementation

[0030] To more clearly illustrate the embodiments of the present invention, specific implementation methods will be described below with reference to the accompanying drawings. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings and other implementation methods can be obtained based on these drawings without any creative effort.

[0031] like Figure 1 As shown, the present invention provides a frequency shifting generation device for ultra-high-speed polarization multiplexed optical signals, comprising:

[0032] The system includes an input optical amplification and processing unit, a polarization diversity common-path frequency shifting loop unit, a quadrature radio frequency drive unit, and a bias voltage generator.

[0033] The input optical amplification and processing unit is used to receive the polarization multiplexed optical signal to be processed, and to perform power amplification, unidirectional transmission and port routing selection on the polarization multiplexed optical signal.

[0034] The polarization diversity common-path frequency shifting loop unit includes an optical fiber polarization beam splitter / combiner (which splits the beam in the forward direction and combines it in the reverse direction, thus achieving both functions), an optical fiber coupler 1, an optical fiber coupler 2, a Mach-Zehnder modulator 1, a Mach-Zehnder modulator 2, an optical phase shifter 1, and an optical phase shifter 2. The optical fiber polarization beam splitter / combiner decomposes the input polarization multiplexed optical signal into X-polarized light and Y-polarized light, and injects the X-polarized light clockwise and the Y-polarized light counterclockwise into an orthogonal optical modulation loop composed of optical fiber coupler 1, optical fiber coupler 2, Mach-Zehnder modulator 1, Mach-Zehnder modulator 2, optical phase shifter 1, and optical phase shifter 2.

[0035] The orthogonal RF drive unit is used to generate two RF drive signals with a 90-degree phase difference at the same frequency, and feeds them into the RF ports of Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2, respectively.

[0036] The bias voltage generator is used to apply bias voltages to Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2 respectively, so that Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2 operate at the carrier suppression point; at the same time, it is used to control optical phase shifter 1 and optical phase shifter 2 to generate the required 90-degree optical phase difference, so that both X-polarized light and Y-polarized light transmitted in opposite directions undergo single-sideband frequency shift, and are then combined into frequency-shifted polarization multiplexed optical signals by the fiber polarization beam splitter / combiner.

[0037] Specifically, the input optical amplification processing unit includes an optical fiber amplifier, an optical isolator, and an optical fiber circulator connected in sequence. The optical fiber amplifier is used to compensate for the insertion loss generated by the subsequent optical isolator, optical fiber circulator, optical fiber polarization beam splitter / combiner, and orthogonal optical modulation loop, so that the optical power entering the polarization diversity common path frequency shifting loop unit is within the operating range of Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2. The optical isolator is used to suppress reflected light from subsequent optical devices. The first port of the optical fiber circulator is connected to the input optical path, the second port of the optical fiber circulator is connected to the optical fiber polarization beam splitter / combiner and receives the frequency-shifted signal after beam combining, and the third port of the optical fiber circulator serves as the output terminal of the frequency-shifted polarization multiplexed optical signal.

[0038] Specifically, the orthogonal optical modulation loop includes an upper arm and a lower arm that are parallel to each other. The upper arm is provided with the Mach-Zehnder modulator 1 and the optical phase shifter 1, and the lower arm is provided with the Mach-Zehnder modulator 2 and the optical phase shifter 2. One end of the upper arm and one end of the lower arm are coupled to an optical fiber coupler 1, and the other end of the upper arm and the other end of the lower arm are coupled to an optical fiber coupler 2.

[0039] Specifically, the quadrature RF drive unit includes a frequency shift signal source, an RF amplifier, and a 90° RF bridge connected in series; the two quadrature output branches of the 90° RF bridge are respectively connected in series with an RF attenuator and an RF phase shifter for balancing amplitude and phase.

[0040] This invention also provides a method for frequency shifting to generate ultra-high-speed polarization multiplexed optical signals, comprising the following steps:

[0041] S1, the ultra-high-speed polarization multiplexed optical signal to be frequency shifted is input into the fiber amplifier, and after being amplified by the fiber amplifier, it is output to the optical isolator, and then injected into the fiber polarization splitter / combiner by the fiber circulator.

[0042] S2, the fiber polarization splitter / combiner orthogonally decomposes the polarization multiplexed optical signal into X-polarized light and Y-polarized light, and sends the X-polarized light in a clockwise direction and the Y-polarized light in a counterclockwise direction into the orthogonal optical modulation loop;

[0043] S3, the orthogonal radio frequency driving unit generates two radio frequency driving signals with a 90-degree phase difference at the same frequency, which are used to drive Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2 respectively.

[0044] S4, the bias voltage generator locks the Mach-Zehnder modulator 1 and the Mach-Zehnder modulator 2 at the carrier suppression bias point, and controls the optical phase shifter 1 and the optical phase shifter 2 to stabilize the optical phase difference of the orthogonal optical modulation loop at 90 degrees, so that the X-polarized light and Y-polarized light transmitted in opposite directions undergo carrier suppression single-sideband modulation at the same time, and obtain equal and same optical frequency shift.

[0045] S5, the frequency-shifted X-polarized light and Y-polarized light are combined into a frequency-shifted polarization multiplexed light signal by the fiber polarization splitter / combiner, and then output by the input optical amplification processing unit.

[0046] Specifically, the input optical amplification processing unit includes an optical fiber amplifier, an optical isolator, and an optical fiber circulator connected in sequence; the optical fiber amplifier amplifies the power of the input signal to compensate for the insertion loss of subsequent links, the optical isolator suppresses the reflected light of the subsequent stage; the optical fiber circulator allows the input polarization multiplexed optical signal to enter the second port from its first port, and allows the frequency-shifted polarization multiplexed optical signal to be output from the second port through the third port.

[0047] Specifically, in step S2, the X-polarized light and the Y-polarized light pass through the same set of optical devices in opposite directions in the orthogonal optical modulation loop, sharing the same optical path. The path delay, device temperature drift, and modulation phase conditions obtained by the two are highly consistent.

[0048] Specifically, in step S3, the frequency shift signal source output angular frequency in the quadrature radio frequency driving unit is The radio frequency signal is amplified by the radio frequency amplifier and then split into two orthogonal signals by the 90° radio frequency bridge. The two orthogonal signals are corrected for amplitude and phase by the radio frequency attenuator and the radio frequency phase shifter, respectively, and then fed into the radio frequency ports of Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2, so that the two modulators obtain the driving conditions of equal amplitude and 90 degrees phase difference.

[0049] In this step, the frequency of the frequency shift signal source... The final optical frequency shift is determined by the angular frequency satisfying the following conditions: The radio frequency amplifier boosts the voltage output from the frequency-shifted signal source to the drive amplitude required to approach the half-wave voltage of the modulator.

[0050] Specifically, in step S4, the bias voltage generator uses bias voltage feedback to keep the DC phase difference between the two arms of Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2 near π to suppress the original optical carrier; the bias voltage generator is also used to cooperate with the radio frequency attenuator, radio frequency phase shifter and optical phase shifter 1 and optical phase shifter 2 in the quadrature radio frequency drive unit so that the electrical 90-degree phase difference and the optical 90-degree phase difference together satisfy the single-sideband selection condition.

[0051] Since X-polarized light and Y-polarized light propagate towards each other within the same orthogonal optical modulation loop and are modulated by the same set of radio frequency fields, they achieve identical polarization. Frequency shifting will not produce random phase differences caused by different radio frequency sources, different bias points and different optical paths in two independent devices.

[0052] Specifically, in step S5, the frequency-shifted X-polarized light and Y-polarized light respectively carry the original electric field complex envelope information, and the angular frequencies of the X-polarized light and the Y-polarized light are changed by... Unified translation to or The polarization multiplexed optical signal after beam combining maintains the polarization orthogonality of the two orthogonal polarization channels and the original modulation information.

[0053] For example Figure 1 As shown, the complete workflow of the ultra-high-speed polarization multiplexed optical signal frequency shifting generation device provided by the present invention is as follows:

[0054] The 1.5μm band polarization-multiplexed optical signal to be processed (e.g., DP-QPSK or DP-64QAM signal) is first input to the input optical amplification unit. The optical signal enters the fiber amplifier for power boosting to meet the power threshold required for normal operation of the subsequent modulator; the amplified optical signal then passes through the optical isolator and enters the first port of the fiber circulator.

[0055] The optical signal output from port 2 of the fiber optic circulator is injected into the core node of this invention—the fiber optic polarization beam splitter / combiner. This fiber optic polarization beam splitter / combiner forcibly demultiplexes the mixed polarization optical signal into mutually orthogonal X-polarized light and Y-polarized light, and sends them respectively into a bidirectional polarization diversity orthogonal optical modulation loop: X-polarized light branch (clockwise propagation): X-polarized light is output from port 2 of the fiber optic polarization beam splitter / combiner and enters the orthogonal optical modulation loop in the forward direction. This loop is sequentially composed of fiber coupler 1, Mach-Zehnder modulator 1, optical phase shifter 1, optical phase shifter 2, and fiber coupler 2; Y-polarized light branch (counterclockwise propagation): Y-polarized light is output from port 3 of the fiber optic polarization beam splitter / combiner and injected in the reverse direction. Due to optical reciprocity, Y-polarized light and X-polarized light completely share the same set of optical devices and optical paths.

[0056] On the quadrature RF drive unit side, the frequency-shift signal source generates a high-frequency sinusoidal continuous wave, which is then boosted by an RF amplifier to the half-wave voltage level required for electro-optic modulation. The amplified RF signal is decomposed into I-channel and Q-channel orthogonal RF signals by a 90° RF bridge. To overcome the non-ideal characteristics of the actual physical device, the two signals are then finely adjusted by an RF attenuator and an RF phase shifter to eliminate amplitude and phase imbalances, and finally fed into the RF drive electrodes of Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2, respectively.

[0057] Simultaneously, the output voltage of the bias voltage generator locks the DC bias points of Mach-Zehnder modulators 1 and 2 at the lowest transmittance point (Null point) to achieve carrier suppression; at the same time, it drives and controls optical phase shifters 1 and 2 to maintain a tight 90° optical phase difference between the two arms of the modulation loop. At this time, the forward-passing X-polarized light and the backward-passing Y-polarized light undergo an optical frequency shifting process in the same optical path and device. After the frequency shifting is completed, the X-polarized light and Y-polarized light are repolarized and combined at the fiber polarization splitter / combiner, reconstructing into a single dual-polarization multiplexed optical signal. This signal returns to the second port of the fiber circulator via the first port of the fiber polarization splitter / combiner, and is finally output from the third port of the fiber circulator.

[0058] The detailed working principle and rigorous mathematical derivation of this invention are as follows:

[0059] Let the optical field of the input 1.5μm band ultra-high-speed polarization multiplexed optical signal be E. in (t), whose carrier angular frequency is It is composed of the superposition of two orthogonal complex envelope signals in space: taking the QPSK signal as an example, the X-polarization and Y-polarization carry independent digital symbol sequences. Let the two bits of information of the X-polarization in the k-th symbol period be... and The two bits of Y-polarization information are and ,in When using QPSK mapping, complex symbols can be represented as: (1); (2);

[0060] in, This represents the k-th QPSK complex symbol for X-polarization; This represents the k-th QPSK complex symbol for Y-polarization.

[0061] The complex envelope of the two polarization channels can be written as: (3); (4);

[0062] Where p(t) is the shaping pulse, T s The symbol period.

[0063] Therefore, the input polarization multiplexed optical field can be further explicitly written as: (5);

[0064] The above equation shows that the digital information of the polarization-multiplexed optical signal is directly contained in the complex envelope of the two orthogonal paths. The frequency-shifting objective of this invention is to change only the common carrier term exp(jω0t) without changing s. X (t) and s Y The digital modulation information in (t) is given by the signal, where j is negative and t is time. If the envelope information imparted by higher-order QAM modulation to the signal is expressed as amplitude... and phase If we express this, then the light field can also be equivalently written as: (6);

[0065] In the formula, and It is an orthogonal polarization unit vector.

[0066] The input light first undergoes power enhancement via an fiber amplifier. Let the average input optical power be P. inThe power gain of the fiber amplifier is G. A G is expressed in dB. A,dB ,but The average optical power after amplification is Considering the total insertion loss of the optical isolator, fiber circulator, and connector before entering the fiber polarization splitter / combiner, let L be... pre,dB Corresponding to linear loss The optical field and power reaching the input of the fiber polarization beam splitter / combiner are respectively: (7); (8);

[0067] After passing through the fiber polarization splitter / combiner, the optical field is physically split into two orthogonally polarized light signals. The X-polarized light field then enters the modulator for forward propagation. Y-polarized light field propagating in the opposite direction They are respectively: (9); (10);

[0068] In the quadrature RF drive unit, the frequency shift signal source outputs an angular frequency of: (corresponding frequency shift) microwave signals Among them, after power boosting by the RF amplifier (gain is ), The amplifier output voltage can be expressed as: (11);

[0069] An ideal 90-degree RF bridge distributes the signal evenly into two channels with amplitudes of The two signals are orthogonal signals with a phase difference of π / 2. However, in a practical bridge circuit, there are inconsistencies in amplitude and phase errors. Let the amplitude error coefficients of the two signals be α1 and α2, and the phase errors be ε1 and ε2, then the bridge output can be expressed as: (12); (13);

[0070] The two signals pass through radio frequency attenuators (attenuation coefficient) respectively. ) and radio frequency phase shifter (additional phase shifter) , Then, the final RF voltage injected into Mach-Zehnder modulator 1 and Mach-Zehnder modulator 2 is: (14); (15);

[0071] By adjusting and This can correct the inherent amplitude and phase inconsistencies of the bridge to achieve equal amplitude orthogonal drive required for orthogonal optical modulation.

[0072] (16); (17);

[0073] The bias voltage controller precisely biases the DC operating points of Mach-Zehnder modulators 1 and 2 at the point of lowest transmittance (Null point). For push-pull Mach-Zehnder modulators, this bias is applied at a DC bias difference. The equivalent output with the RF modulation index m can be written as: (18);

[0074] The bias voltage generator locks the modulator precisely at the null point ( At this point, the coefficient of the unmodulated carrier is That is, the original optical carrier cancels out.

[0075] Will Expanding using the Bessel function, we have the following: Because of the null bias The residual carrier terms cancel each other out, while the combination of quadrature RF drive and 90° optical phase shift results in the cancellation of the first-order lower sideband and the coherent superposition of the first-order upper sideband. Taking forward-propagating X-polarized light as an example, using a Bessel function of the first kind for small-signal expansion, the optical fields output by the upper and lower arms of the modulation loop are as follows: (19); (20);

[0076] in, Let be the modulation index. The optical phase shifter is controlled to induce a π / 2 optical phase shift in the optical path of Mach-Zehnder modulator 2 relative to Mach-Zehnder modulator 1. Therefore, the electric field of X-polarized light at the fiber polarization splitter / combiner is: (twenty one);

[0077] Substituting equations (19) and (20) and simplifying, the negative first-order edge bands cancel each other out, while the positive first-order edge bands are enhanced, resulting in: (twenty two);

[0078] The above mathematical results show that X-polarized light retains the original digital modulation envelope. and At the same time, its optical carrier frequency was shifted by +ω.

[0079] Furthermore, based on the principle of optical reciprocity, the Y-polarized light propagating in the opposite direction enters the same set of orthogonal modulation structures. The Y-polarized light and X-polarized light undergo the same modulation process, therefore the electric field can be represented as follows: (twenty three);

[0080] Finally, the frequency-shifted X-polarized light and Y-polarized light are orthogonally combined at the fiber polarization splitter / combiner, and the final polarization-multiplexed optical field output is: (twenty four); (25);

[0081] By comparing the initial input E in (t) and the final output E out The expression for (t) leads to the following physical conclusions: First, the carrier angular frequency achieves a unified shift from ω0 to ω0+ω; second, the complex envelope... , and phase , The mathematical form remains unchanged, proving that the frequency shifting process does not alter the original information.

[0082] This invention abandons the traditional dual-set independent modulation hardware, which not only reduces the system cost and size, but also physically ensures the absolute equality of the frequency shift of the dual polarization signals and the strict equality of the optical propagation path. It can be widely used in the fields of space laser Doppler simulation and high-speed coherent optical communication testing and verification.

[0083] The above description is merely a detailed explanation of preferred embodiments and principles of the present invention. For those skilled in the art, there may be changes in specific implementation methods based on the ideas provided by the present invention, and these changes should also be considered within the scope of protection of the present invention.

Claims

1. A frequency shifting generation device for ultra-high-speed polarization multiplexed optical signals, characterized in that, include: The system includes an input optical amplification and processing unit, a polarization diversity common-path frequency shifting loop unit, a quadrature radio frequency drive unit, and a bias voltage generator. The input optical amplification and processing unit is used to receive the polarization multiplexed optical signal to be processed, and to perform power amplification, unidirectional transmission and port routing selection on the polarization multiplexed optical signal. The polarization diversity common-path frequency shifting loop unit includes an optical fiber polarization beam splitter / combiner, a first optical fiber coupler, a second optical fiber coupler, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a first optical phase shifter, and a second optical phase shifter. The optical fiber polarization beam splitter / combiner decomposes the input polarization multiplexed optical signal into X-polarized light and Y-polarized light, and injects the X-polarized light clockwise and the Y-polarized light counterclockwise into an orthogonal optical modulation loop composed of the first optical fiber coupler, the second optical fiber coupler, the first Mach-Zehnder modulator, the second Mach-Zehnder modulator, the first optical phase shifter, and the second optical phase shifter. The orthogonal radio frequency driving unit is used to generate two radio frequency driving signals with a 90-degree phase difference at the same frequency, and feeds them into the radio frequency ports of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator, respectively. The bias voltage generator is used to apply bias voltages to the first Mach-Zehnder modulator and the second Mach-Zehnder modulator respectively, so that the first Mach-Zehnder modulator and the second Mach-Zehnder modulator operate at the carrier suppression point; at the same time, it is used to control the first optical phase shifter and the second optical phase shifter respectively to generate the required 90-degree optical phase difference, so that the X-polarized light and Y-polarized light transmitted in opposite directions both undergo single-sideband frequency shift, and are then combined into a frequency-shifted polarization multiplexed optical signal by the fiber polarization beam splitter / combiner.

2. The frequency shifting generation device for ultra-high-speed polarization multiplexed optical signals according to claim 1, characterized in that, The input optical amplification and processing unit includes an optical fiber amplifier, an optical isolator, and an optical fiber circulator connected in sequence; the first port of the optical fiber circulator is connected to the input optical path, the second port of the optical fiber circulator is connected to the optical fiber polarization splitter / combiner and receives the frequency-shifted signal after beam combining, and the third port of the optical fiber circulator serves as the output terminal of the frequency-shifted polarization multiplexed optical signal.

3. The frequency shifting generation device for ultra-high-speed polarization multiplexed optical signals according to claim 2, characterized in that, The orthogonal optical modulation loop includes an upper arm and a lower arm that are parallel to each other. The upper arm is provided with a first Mach-Zehnder modulator and a first optical phase shifter, and the lower arm is provided with a second Mach-Zehnder modulator and a second optical phase shifter. One end of the upper arm and one end of the lower arm are coupled to a first fiber coupler, and the other end of the upper arm and the other end of the lower arm are coupled to a second fiber coupler.

4. The frequency shifting generation device for ultra-high-speed polarization multiplexed optical signals according to claim 1, characterized in that, The quadrature RF drive unit includes a frequency shift signal source, an RF amplifier, and a 90° RF bridge connected in series. The two quadrature output branches of the 90° RF bridge are respectively connected in series with an RF attenuator and an RF phase shifter for balancing amplitude and phase.

5. A method for generating ultra-high-speed polarization-multiplexed optical signals by frequency shifting, based on the device for generating ultra-high-speed polarization-multiplexed optical signals according to any one of claims 1-4, characterized in that, The frequency shifting method for generating the ultra-high-speed polarization multiplexed optical signal includes the following steps: S1, the ultra-high-speed polarization multiplexed optical signal to be frequency shifted is input into the fiber amplifier, and after being amplified by the fiber amplifier, it is output to the optical isolator, and then injected into the fiber polarization splitter / combiner by the fiber circulator. S2, the fiber polarization splitter / combiner orthogonally decomposes the polarization multiplexed optical signal into X-polarized light and Y-polarized light, and sends the X-polarized light in a clockwise direction and the Y-polarized light in a counterclockwise direction into the orthogonal optical modulation loop; S3, the orthogonal radio frequency driving unit generates two radio frequency driving signals with a 90-degree phase difference at the same frequency, which are used to drive the first Mach-Zehnder modulator and the second Mach-Zehnder modulator respectively; S4, the bias voltage generator locks the first Mach-Zehnder modulator and the second Mach-Zehnder modulator at the carrier suppression bias point, and controls the first optical phase shifter and the second optical phase shifter to stabilize the optical phase difference of the orthogonal optical modulation loop at 90 degrees, so that the X-polarized light and Y-polarized light transmitted in opposite directions undergo carrier suppression single-sideband modulation at the same time, and obtain equal and same optical frequency shift; S5, the frequency-shifted X-polarized light and Y-polarized light are combined into a frequency-shifted polarization multiplexed light signal by the fiber polarization splitter / combiner, and then output by the input optical amplification processing unit.

6. The method for frequency shifting to generate ultra-high-speed polarization multiplexed optical signals according to claim 5, characterized in that, The input optical amplification processing unit includes an optical fiber amplifier, an optical isolator, and an optical fiber circulator connected in sequence; the optical fiber amplifier amplifies the power of the input signal to compensate for the insertion loss of subsequent links, the optical isolator suppresses the reflected light of the subsequent stage; the optical fiber circulator allows the input polarization multiplexed optical signal to enter the second port from its first port, and allows the frequency-shifted polarization multiplexed optical signal to be output from the second port through the third port.

7. The method for frequency shifting to generate ultra-high-speed polarization multiplexed optical signals according to claim 5, characterized in that, In step S2, the X-polarized light and the Y-polarized light pass through the same set of optical devices in opposite directions in the orthogonal optical modulation loop, sharing the same optical path.

8. The method for frequency shifting to generate ultra-high-speed polarization multiplexed optical signals according to claim 5, characterized in that, In step S3, the frequency shift signal source output angular frequency in the quadrature radio frequency drive unit is The radio frequency signal is amplified by the radio frequency amplifier and then split into two orthogonal signals by a 90° radio frequency bridge. The two orthogonal signals are corrected for amplitude and phase by the radio frequency attenuator and the radio frequency phase shifter, respectively, and then fed into the radio frequency ports of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator, so that the two modulators obtain the driving conditions of equal amplitude and 90 degrees phase difference.

9. The method for frequency shifting to generate ultra-high-speed polarization multiplexed optical signals according to claim 5, characterized in that, In step S4, the bias voltage generator uses bias voltage feedback to keep the DC phase difference between the two arms of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator near π to suppress the original optical carrier. The bias voltage generator is also used to cooperate with the radio frequency attenuator, radio frequency phase shifter and the first optical phase shifter and the second optical phase shifter in the quadrature radio frequency drive unit so that the electrical 90-degree phase difference and the optical 90-degree phase difference together satisfy the single-sideband selection condition.

10. The method for frequency shifting to generate ultra-high-speed polarization multiplexed optical signals according to claim 5, characterized in that, In step S5, the frequency-shifted X-polarized light and Y-polarized light respectively carry the original electric field complex envelope information, and the angular frequencies of the X-polarized light and the Y-polarized light are changed by... Unified translation to or The polarization multiplexed optical signal after beam combining maintains the polarization orthogonality of the two orthogonal polarization channels and the original modulation information.

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