CN115865210A - Octave microwave signal generation device and method - Google Patents

Abstract

The invention discloses a frequency octave microwave signal generating device and method, and belongs to the technical field of microwave photonics. The octave frequency microwave signal generating device comprises a tunable laser, a radio frequency signal source, a polarization modulator, an erbium-doped optical fiber amplifier, a polarization beam splitter, a double-parallel Mach-Zehnder modulator, a first photoelectric detector and a second photoelectric detector; the tunable laser is used for outputting an optical carrier to the polarization modulator, and the radio frequency signal source is used for outputting a low-frequency reference signal to the polarization modulator; the polarization modulator is used for generating a polarization modulation optical signal and outputting the polarization modulation optical signal to the erbium-doped fiber amplifier. The octave microwave signal generating device and the octave microwave signal generating method do not need any phase shifter and optical filter, generate octave microwave signals by the polarization modulator and cascade the polarization modulator and the double parallel Mach-Zehnder modulator, and have the characteristics of high frequency, rapidness and real-time wide tuning.

Description

Octave microwave signal generation device and method

Technical Field

The invention relates to an octave frequency microwave signal generating device and method, and belongs to the technical field of microwave photonics.

Background

The microwave signal source is used as the heart of radar, communication and other electronic systems, and provides local oscillation signals, clock signals, reference radio frequency signals and the like for various electronic systems. The quality of the microwave signal source determines the upper limit of the system. Along with the development of technologies such as radar, communication and the like to a high-frequency broadband direction, a high-frequency microwave signal is urgently needed, but the traditional electronic technology is limited by an electronic bottleneck, frequency multiplication needs to be carried out on a fundamental frequency signal for generating a high-frequency signal, not only is the system more and more complex and the cost more and more high, but also the phase noise of the generated signal is degraded by 20lgN dB each time of frequency multiplication, wherein N is a frequency multiplication coefficient. Therefore, there is an urgent need to develop a novel high-frequency microwave signal generation method.

Microwave photonic technology combines the advantages of photonic technology broadband and microwave technology flexibility and adjustability, and is considered to be an effective way for generating high-frequency microwave signals. The method for generating the high-frequency microwave signal is various, and the core idea is to generate the microwave signal by using two optical signal beats. In theory, any frequency band microwave signal can be generated, but in practice the quality of the generated microwave signal is affected by the coherence of the two optical signals. In order to improve the correlation between the two optical signals, technologies such as a dual-wavelength laser, optical injection locking, an optical phase-locked loop, an optical injection phase-locked loop, an optical frequency comb, a mode-locked laser, external modulation and the like can be adopted. Where dual wavelength lasers are highly thresholded to manufacture, it is desirable to design and manufacture lasers that lase two relevant modes. The optical injection locking needs to use two coherent optical sideband injection locking two slave lasers, and the system is complex and high in cost. The long-term stability of the optical phase-locked loop and the optical injection phase-locked loop system is good, but a complex phase-locked automatic control system is needed, the structure is complex, and the cost is high. The generation of the optical frequency comb requires a cascade of several modulators and control of parameters such as the modulation factor of each modulator. In addition, a specially designed optical filter is required to select the two comb teeth. The passive mode-locked laser has a simple structure, does not need a reference radio frequency signal and a modulator, but has poor phase noise of a generated microwave signal. In addition, a passive mode-locked laser cannot generate a high-frequency microwave signal due to the limitation of the cavity length of the passive mode-locked laser. The active mode-locked laser eliminates the disadvantages of a passive mode-locked laser, can generate high-frequency microwave signals, but needs a high-speed electro-optical modulator and a photoelectric detector. The external modulation technique utilizes two coherent optical sidebands to beat to generate a high frequency microwave signal. Compared with other technologies, the external modulation technology is considered to be the most reliable photo-generated microwave technology due to the advantages of low cost, low phase noise of generated microwave signals and the like, and becomes an important research direction in the field of signal generation.

The first prior art is based on cascading two dual parallel mach-zehnder modulators to generate octave frequency microwave signals. But the signal-to-noise ratio of the generated octave microwave signal is influenced by the output phase fluctuation of the radio frequency phase shifter. When the radio frequency phase shifter accurately keeps 45-degree phase difference, the signal-to-noise ratio of the octave microwave signal is the maximum and reaches 30dB; when the phase difference of the radio frequency phase shifter fluctuates, the suppression capability of the system on the carrier wave is rapidly deteriorated, the signal-to-noise ratio of the octave microwave signal is deteriorated, and particularly when the output phase of the radio frequency phase shifter becomes 30 degrees or 60 degrees, the optical carrier suppression ratio is close to zero, and the 8-frequency-multiplication scheme fails.

The dual-purpose optical phase shifter replaces the radio frequency phase shifter in the first prior art, but octave microwave signals generated by the system are still limited by the change of the output phase of the optical phase shifter, and when the output phase of the optical phase shifter fluctuates, the system generates clutter, so that the spectral purity of the generated octave microwave signals is reduced.

The third prior art is based on the generation of a six-fold frequency microwave signal by a polarization modulator in combination with an optical notch filter. Due to the non-ideal roll-off coefficient of the optical filter, the tuning range of the generated six-time frequency microwave signal is limited by the passband broadband of the optical filter. Furthermore, the generated six-fold frequency microwave signal is also difficult to tune in real time because the optical filter cannot be reconstructed in real time.

Therefore, it is a problem to be solved in the art to develop a microwave signal with high frequency multiplication coefficient and real-time wide tuning.

Disclosure of Invention

The invention aims to overcome the defects in the prior art and provides a frequency octave microwave signal generation device and a frequency octave microwave signal generation method.

In order to achieve the above purpose/to solve the above technical problems, the present invention is realized by adopting the following technical scheme:

in a first aspect, the invention provides an octave microwave signal generating device, which comprises a tunable laser, a radio frequency signal source, a polarization modulator, an erbium-doped fiber amplifier, a polarization beam splitter, a double-parallel Mach-Zehnder modulator, a first photoelectric detector and a second photoelectric detector;

the tunable laser is used for outputting an optical carrier to the polarization modulator, and the radio frequency signal source is used for outputting a low-frequency reference signal to the polarization modulator;

the polarization modulator is used for generating a polarization modulation optical signal and outputting the polarization modulation optical signal to the erbium-doped fiber amplifier;

the erbium-doped fiber amplifier is used for amplifying the polarization modulation optical signal and outputting the amplified polarization modulation optical signal to the polarization beam splitter;

the dual parallel Mach-Zehnder modulator includes a first sub-modulator and a second sub-modulator;

the polarization beam splitter is used for dividing the amplified polarization modulation optical signal into B, C, wherein the B path optical signal outputs a frequency-doubled microwave electrical signal to the first sub-modulator through the photoelectric conversion of the first photoelectric detector; the C path of optical signals are transmitted into the double parallel Mach-Zehnder modulator after being branched and divided into an upper branch and a lower branch, wherein the upper branch of optical signals are transmitted to the first sub-modulator, and the lower branch of optical signals are transmitted to the second sub-modulator; and the second photoelectric detector is used for performing photoelectric conversion on the optical signal output by the double parallel Mach-Zehnder modulator and outputting a octave microwave electrical signal.

The power amplifier is arranged between the first photoelectric detector and the double parallel Mach-Zehnder modulator and is used for amplifying the microwave electric signals output by the first photoelectric detector.

Further, the polarization modulation system also comprises a first auxiliary polarization modulator and a second auxiliary polarization modulator;

the first auxiliary polarization modulator is used for controlling the polarization state of the light carrier wave output by the tunable laser, and the second auxiliary polarization modulator is used for controlling the polarization state of the polarization modulation light signal.

Further, the first auxiliary polarization modulator is arranged between the tunable laser and the polarization modulator, and the second auxiliary polarization modulator is arranged between the polarization modulator and the erbium-doped fiber amplifier.

Further, the polarization-modulated optical signal output by the polarization modulator includes ± 1 st-order optical sidebands and an optical carrier.

Further, the B-path optical signal includes a ± 1 st order optical sideband, and the C-path optical signal includes an optical carrier.

Further, the phase difference between the first sub-modulator and the second sub-modulator is 180 °.

In a second aspect, the present invention provides a method of operating an apparatus for generating frequency octave microwave signals, the method comprising:

the tunable laser outputs a light carrier to the polarization modulator, the radio frequency signal source outputs a low-frequency reference signal to the polarization modulator, the first auxiliary polarization modulator is adjusted to enable the polarization state of the first auxiliary polarization modulator to be 45 degrees with the main shaft of the polarization modulator, and the second auxiliary polarization modulator is adjusted to enable the light carrier to be separated from the +/-1 order optical sideband and to be amplified and output through the erbium-doped fiber amplifier;

the erbium-doped fiber amplifier amplifies the polarization modulation optical signal and outputs the amplified polarization modulation optical signal to the polarization beam splitter, and the polarization beam splitter divides the amplified polarization modulation optical signal into a plus or minus 1-order optical sideband of a path B and an optical carrier of a path C;

the optical signal of the B path is detected by a first photoelectric detector to generate a frequency-doubled microwave electrical signal, and the frequency-doubled microwave electrical signal is amplified by a power amplifier and then loaded on a double-parallel Mach-Zehnder modulator;

and the optical carrier of the C path enters the double-parallel Mach-Zehnder modulator, the direct-current bias voltage on the double-parallel Mach-Zehnder modulator is adjusted, the first sub-modulator is positioned at the maximum working point, the phase difference of output signals between the first sub-modulator and the second sub-modulator is 180 degrees, optical signals output by the double-parallel Mach-Zehnder modulator are detected by the second photoelectric detector, and octave microwave electrical signals are generated through photoelectric conversion.

Further, the output wavelength of the tunable laser is 1549.97nm, and the output power of the tunable laser is 3dBm.

Further, the output frequency of the radio frequency signal source is 2.5GHz, and the output power of the radio frequency signal source is 10dBm.

Compared with the prior art, the invention has the following beneficial effects:

according to the octave frequency microwave signal generating device and method provided by the invention, any phase shifter and optical filter are not needed, the polarization modulator is used for generating octave frequency microwave signals, the polarization modulator and the double parallel Mach-Zehnder modulator are cascaded to generate the octave frequency microwave signals, and the octave frequency microwave signal generating device and method have the characteristics of high frequency, rapidness and real-time wide tuning;

according to the octave frequency microwave signal generation device and method provided by the invention, the erbium-doped optical fiber amplifier is adopted, so that the link loss is compensated, and the polarization modulation optical signal output by the polarization modulator is amplified, so that the +/-1-order optical sideband input into the first photoelectric detector is higher in power, the microwave signal output by the first photoelectric detector is higher in power, and the requirement on the gain coefficient of the radio frequency power amplifier is reduced.

Drawings

Fig. 1 is a schematic structural diagram of an apparatus for generating octave microwave signals according to an embodiment;

FIG. 2 is a schematic diagram of an apparatus for generating octave microwave signals according to an embodiment;

FIG. 3 is a spectrum diagram of an output signal of a polarization modulator according to a second embodiment;

FIG. 4 is a spectrum diagram of an output signal of the erbium-doped fiber amplifier of the second embodiment before use;

FIG. 5 is a graph showing the output signal spectrum of the erbium-doped fiber amplifier of the second embodiment after use;

FIG. 6 is a spectrum diagram of optical signals before and after the double parallel Mach-Zehnder modulator in the second embodiment;

FIG. 7 is a spectrum diagram of an octave microwave signal according to the second embodiment;

FIG. 8 is a frequency spectrum diagram of an octave microwave signal according to a second embodiment;

FIG. 9 is a schematic diagram of phase noise of the medium and low frequency reference signal, the frequency-doubled and frequency-octaved microwave signal according to the second embodiment;

fig. 10 is a spectrum diagram of the second embodiment when the octave microwave signal is tuned.

Detailed Description

The invention is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.

In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention. Furthermore, the terms "first", "second", etc. are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first," "second," etc. may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified.

In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms in the present invention can be understood by those of ordinary skill in the art through specific situations.

Example one

As shown in fig. 1, the present invention provides an octave microwave signal generating apparatus, which includes a tunable laser TLS, a radio frequency signal source RF, a polarization modulator PolM, an erbium-doped fiber amplifier EDFA, a polarization beam splitter PBS, a dual parallel mach-zehnder modulator DPMZM, a first photodetector PD1, and a second photodetector PD2;

the tunable laser TLS is configured to output an optical carrier to the polarization modulator PolM, and the radio frequency signal source RF is configured to output a low-frequency reference signal fx to the polarization modulator PolM;

the polarization modulator PolM is used for generating a polarization modulation optical signal and outputting the polarization modulation optical signal to the erbium-doped fiber amplifier EDFA;

the erbium-doped fiber amplifier EDFA is used for amplifying the polarization modulation optical signal and outputting the amplified polarization modulation optical signal to the polarization beam splitter PBS;

the dual parallel Mach-Zehnder modulator includes a first sub-modulator and a second sub-modulator;

the polarization beam splitter is used for dividing the amplified polarization modulation optical signal into B, C, wherein the B path optical signal outputs a frequency-doubled microwave electrical signal to the first sub-modulator through the photoelectric conversion of the first photoelectric detector; the C path of optical signals are transmitted into the double parallel Mach-Zehnder modulator after being branched and divided into an upper branch and a lower branch, wherein the upper branch of optical signals are transmitted to the first sub-modulator, and the lower branch of optical signals are transmitted to the second sub-modulator; and the second photoelectric detector is used for performing photoelectric conversion on the optical signal output by the double parallel Mach-Zehnder modulator and outputting a octave microwave electrical signal.

In the above technical solution, the radio frequency signal fx performs polarization modulation on the optical carrier through the polarization modulator PolM, the output of the polarization modulator PolM is a polarization modulated optical signal, and the erbium-doped fiber amplifier EDFA is used for compensating link insertion loss and amplifying the polarization modulated signal.

In some embodiments, to amplify the microwave electrical signal output by the first photodetector PD1, a power amplifier PA is further included, and the power amplifier PA is disposed between the first photodetector PD1 and the dual parallel mach-zehnder modulator DPMZM and is configured to amplify the microwave electrical signal output by the first photodetector PD 1.

In some embodiments, in order to make the polarization state TE of the optical carrier output by the tunable laser TLS 45 ° to the polarization modulator principal axis TM, a first auxiliary polarization modulator PC1 and a second auxiliary polarization modulator PC2 are further included;

the first auxiliary polarization modulator PC1 is configured to control a polarization state of an optical carrier output by the tunable laser TLS, and the second auxiliary polarization modulator PC2 is configured to control a polarization state of a polarization-modulated optical signal.

In some embodiments, to enable control of the tunable laser TLS and the polarization modulated optical signal by the first auxiliary polarization modulator PC1 and the second auxiliary polarization modulator PC2, said first auxiliary polarization modulator PC1 is arranged between the tunable laser TLS and the polarization modulator PolM, and said second auxiliary polarization modulator PC2 is arranged between the polarization modulator PolM and the erbium doped fiber amplifier EDFA.

In some embodiments, the polarization-modulated optical signal output by the polarization modulator PolM includes ± 1 st order optical sidebands and an optical carrier.

In some embodiments, the B-path optical signal includes a ± 1 st order optical sideband and the C-path optical signal includes an optical carrier.

In some embodiments, the C optical carrier enters the dual parallel mach-zehnder modulator DPMZM and is divided into two paths, the upper optical carrier is modulated by the double-frequency microwave electrical signal output by the first photodetector PD1 after passing through the first sub-modulator, so as to adjust the dc bias voltage on the dual parallel mach-zehnder modulator DPMZM, and the second sub-modulator is used for outputting the lower optical carrier, so that the phase difference between the first sub-modulator and the second sub-modulator is 180 °.

In the technical scheme, the optical carrier sequentially enters the DPMZM through the C points of the PolM, the EDFA and the PBS. Loading a low-frequency local oscillation signal with the center frequency of 5GHz and the power of 10dBm to a sub-modulator MZMa in the DPMZM without loading any radio-frequency signal on the polarization modulator; the other sub-modulator MZMb is not loaded with any radio frequency signal. Adjusting the direct current Bias voltage (i.e. Bias in fig. 1) on the DPMZM to make the MZMa be at the maximum working point, and the phase difference between the output signals of the MZMa and the MZMb is 180 °, so as to generate a quadruple frequency microwave signal; the output spectrum of the DPMZM is shown in FIG. 6, and line A in FIG. 6 represents the optical carrier entering the DPMZM; line B represents the optical signal output by the DPMZM.

Next, referring to fig. 2, the generation process of the octave microwave signal is described, and as shown in fig. 2, the first auxiliary polarization modulator PC1 is adjusted to make the polarization state TE of the optical carrier output by the tunable laser TLS and the principal axis TM of the polarization modulator at 45 °. The RF signal source RF outputs a low frequency reference signal fx that is polarization modulated by PolM onto the optical carrier. The polarization modulated optical signal output by PolM contains 1 st order optical sidebands (indicated by ± fx at point a in fig. 2) and an optical carrier (indicated by TLS at point a in fig. 2). As can be seen from the point A in FIG. 2, the polarization states of the + -1 order optical sidebands are the same and are in the TM direction, the polarization state of the optical carrier is in the TE direction, and the polarization state of the + -1 order optical sidebands is perpendicular to the polarization state of the optical carrier. The polarization modulation optical signal output from point a is injected into EDFA after passing through PC2 and amplified, and the optical carrier and the ± 1-order optical sideband are divided into two paths by PBS, as shown by points B and C in fig. 2, where point C only contains the optical carrier; point B contains only the ± 1 st order optical sidebands.

After the B-spot light signal is detected by the PD1, the PD1 outputs a frequency-doubled microwave signal (fy =2 fx) through photoelectric conversion. The double frequency (fy) is amplified by a Power Amplifier (PA) and loaded on the first sub-modulator MZMa of the DPMZM.

The C point optical carrier enters the DPMZM and is divided into two paths, the upper branch optical carrier is modulated by the fy signal output by the PD1 after passing through the first sub-modulator MZMa, the direct current bias voltage on the DPMZM is adjusted, the first sub-modulator MZMa is positioned at the maximum working point, the odd-order optical sideband is inhibited, and only the optical carrier and the even-order optical sideband (+/-2 fy) are left; the lower branch optical carrier is output through a second sub-modulator MZMb (no rf signal is loaded on the MZMb), the dc bias voltage on the third MZM in the DPMZM, i.e., the dc bias voltage on the PMc in fig. 1, is adjusted to make the phase difference between MZMa and MZMb be 180 °, so that the optical carriers passing through MZMa and MZMb are equal in magnitude and opposite in phase, and cancel each other out at the output end of the DPMZM, the optical signal output by the DPMZM only contains an optical sideband with a frequency of ± 2fy, and the frequency of the microwave signal output by the PD2 is 4fy through photoelectric conversion by the PD2, and the frequency of the microwave signal output is 4 × 2 × fx=8fx because fy =2fx, thereby generating an octave microwave signal (indicated by point E in fig. 2).

In summary, the octave microwave signal generating device provided in this embodiment generates the octave microwave signal by cascading the polarization modulator and the dual-parallel mach-zehnder modulator without any phase shifter and optical filter, and has the characteristics of high frequency, rapidness, and real-time wide tuning.

Example two

The invention provides an operation method of an octave frequency microwave signal generating device, which comprises the following steps:

the tunable laser TLS outputs an optical carrier to the polarization modulator PolM, the radio frequency signal source RF outputs a low-frequency reference signal fx to the polarization modulator PolM, the first auxiliary polarization modulator PC1 is adjusted to enable the polarization state of the first auxiliary polarization modulator PC1 to be 45 degrees with the main shaft of the polarization modulator PolM, and the second auxiliary polarization modulator PC2 is adjusted to enable the optical carrier to be separated from the +/-1-order optical sideband and amplified and output through the erbium-doped fiber amplifier EDFA;

the erbium-doped fiber amplifier EDFA amplifies the polarization modulation optical signal and outputs the amplified polarization modulation optical signal to the polarization beam splitter PBS, and the polarization beam splitter PBS divides the amplified polarization modulation optical signal into a plus or minus 1-order optical sideband of a B path and an optical carrier of a C path;

the optical signal of the B path is detected by a first photoelectric detector PD1 to generate a frequency doubling microwave electrical signal, and the frequency doubling microwave electrical signal is amplified by a power amplifier PA and then loaded on a double-parallel Mach-Zehnder modulator (DPMZM);

and the optical carrier of the C path enters the double-parallel Mach-Zehnder modulator (DPMZM), the direct-current bias voltage on the double-parallel Mach-Zehnder modulator (DPMZM) is adjusted, the first sub-modulator is positioned at the maximum working point, the phase difference of output signals between the first sub-modulator and the second sub-modulator is 180 degrees, the optical signal output by the double-parallel Mach-Zehnder modulator (DPMZM) is detected by the second photoelectric detector (PD 2), and octave microwave electrical signals are generated through photoelectric conversion.

In some embodiments, the output wavelength of the tunable laser TLS is 1549.97nm, and the output power of the tunable laser TLS is 3dBm.

In some embodiments, the output frequency of the radio frequency signal source RF is 2.5GHz, and the output power of the radio frequency signal source RF is 10dBm.

The effects of the present invention can be illustrated by the following experiments:

the devices used were as follows:

TLS(Agilent 81989A)；

PD1 (PTHS 992-003, S/N02010241, 3dB bandwidth 10 GHz);

PD2 (FINISAR u2t XPDV2120RA,3dB bandwidth 50 GHz);

PA (Mini-circles, bandwidth 5-18GHz, noise figure 7dB, gain 18 dB);

high precision spectrometer (OSA, FINISAR Wave-Analyzer 1500s, minimum resolution 125 MHz);

a radio frequency signal Source (RF Source, tektronix AWG 5014C);

spectrometer (ESA, ROHDE SCHWARZ, FSW-SIGNAL ANALYZER-2Hz-26.5 GHz).

The output wavelength and power of TLS were fixed at 1549.97nm and 3dBm, and PC1 was adjusted to have a polarization state 45 ° to the principal axis of the polarization modulator. The radio frequency signal source outputs a low-frequency reference signal of 2.5GHz and 10dBm and loads the low-frequency reference signal on the polarization modulator. The optical signal output by the polarization modulator is divided into two paths by the PBS, and the PC2 is adjusted to separate the optical carrier from the ± 1 order sidebands, and the spectra at the B point and the C point are shown in fig. 3, as can be known from fig. 3: only +/-1 order optical sideband signals exist at the point B; point C has only an optical carrier signal present. In the C-point spectrum, there is a spurious signal (spur) in addition to the optical carrier, where the spurious signal is another longitudinal mode that fails to start oscillation in the TLS cavity. FIG. 4 is a graph of the spectrum of point B without the use of an EDFA; fig. 5 is a graph of the spectrum of point B using an EDFA. Comparing fig. 4 and 5, it can be seen that the use of an EDFA simultaneously amplifies the ± 1 st order optical sidebands, the residual optical carrier and the system bottom noise (simply referred to as "noise floor") in the B-point. However, because the EDFA has polarization-selective amplification characteristics, the gains obtained by optical signals in different polarization states are different, for example, the power of the residual optical carrier is increased from-85.4 dBm to-62.1 dBm, which is increased by 23.3dB; the power of the +/-1 order optical sideband is increased from-65.9 dBm to-50.4 dBm, and is increased by 15.5dB; the noise floor of the system increased from-90 dBm to-81 dBm, increasing by 9dB. The use of the EDFA reduces the power difference between the +/-1 st order optical sideband and the residual optical carrier from 19.5dB to 11.7dB. The reduction in the power difference results in a reduction in the spectral purity of the output doubled frequency signal at PD1 (as the EDFA is used, in addition to the power increase of the 2-frequency multiplied signal generated by the required 1 st order sideband beat, the residual optical carrier and the fundamental frequency components generated by the 1 st order sideband beat increase, thereby reducing the spectral purity). In theory, EDFAs should be avoided. However, in the experiment, due to the limitation that the maximum output power of the used laser and the radio frequency signal source is not enough to compensate the insertion loss of the link, the EDFA is required to be adopted. The EDFA compensates link loss and amplifies the polarization modulation optical signal output by the PolM, so that the +/-1-order optical sideband power of the input PD1 is higher, the power of the microwave signal output by the PD1 is higher (the optical power of the input PD1 is increased by 3dB, and the power of the microwave signal output by the PD1 is increased by 6 dB), and the requirement on the gain coefficient of a radio frequency Power Amplifier (PA) is reduced. According to the analysis in the first embodiment, the optical signal output from the point B can generate a frequency-2 multiplied microwave signal after being detected by the PD 1.

After PD1 detects and generates a 2-frequency-doubled microwave signal, a low-frequency reference signal of 2.5GHz and 10dBm is loaded on a polarization modulator, the 2-frequency-doubled signal output by PD1 is amplified by PA and then loaded on MZMa, the direct-current bias voltage on DPMZM is adjusted to enable MZMa to be in a maximum working point, the phase difference between the output signals of MZMa and MZMb is 180 degrees, at the moment, the output spectrum of DPMZM is shown in FIG. 7, the spectrum of the microwave signal output on PD2 is shown in FIG. 7Shown in fig. 8. As can be seen from FIG. 7, the + -2 order optical sidebands (+ -2 f) _y ) Power ratio optical carrier (TLS) and + -1 order sidebands (+/-f) _y ) The power is 10dB higher, namely the optical sideband suppression ratio is 10dB. Wherein f is _y Is the frequency of the 2-times frequency microwave signal output on PD1, f _y ＝ω _y And/2 pi. Except for f _y And. + -. 2f _y Outside the optical sideband, there is also an unwanted sideband signal, shown as unwanteded sidebands, in FIG. 7. They are generated by loading the PD1 output fundamental signal (generated by the remnant optical carrier and the ± 1 order sideband beat in B-spot) onto MZMa. As can be seen from fig. 8, the power of the octave microwave signal output by the PD2 is much higher than that of the microwave signals of other frequencies, i.e. the spurious suppression ratio of the electrical signal is 10dB.

As shown in FIG. 9, line X represents the phase noise of a 2.5GHz, 10dBm low frequency reference RF signal; line Y represents the phase noise of the frequency-doubled microwave signal (frequency 5 GHz) output by PD 1; line Z represents the phase noise of the PD2 output octave microwave signal (frequency 20 GHz). It can be seen from the figure that at 10KHz frequency offset, the octave signal phase noise is 18.6dB lower than the local oscillator signal phase noise. The experimental results are essentially in accordance with theoretical expectations (the multiplied signal phase noise degrades at 20lgN, N is the multiplication factor, 20lg8= 18db). However, at low frequency offset (1 kHz-5 kHz) and high frequency offset (100 kHz-1000 kHz), the phase noise of the octave microwave signal is significantly degraded and much larger than the theoretical calculation.

The phase noise expression of the microwave signal generated by the external modulation technology is as follows:

10log ₁₀ [S(f)]＝10log ₁₀ [S _res (f)]+10log ₁₀ [m ² *S _e (f)]

where m denotes the multiplication factor, S _res (f) Which represents additive noise or system residual phase noise, such as laser RIN noise, shot noise of PD1, PD2, spontaneous emission noise of EDFA and PA. S _e (f) Representing the phase noise of the low frequency local oscillator microwave signal. According to the formula, the residual noise of the system and the phase noise of the low-frequency local oscillation signal influence the phase noise of the octave microwave signal. Therefore, the phase noise at the low frequency offset (1 kHz-5 kHz) of the octave microwave signal is mainly from the flicker noise of the system, while the phase noise at the high frequency offset (100 kHz-)1000 kHz) comes mainly from EDFAs.

Because the octave microwave signal generating device provided by the invention does not need any phase shifter and optical filter, the generated octave signal has the tuning characteristics of high speed (the reference radio frequency signal changes, the octave microwave signal changes along with the change, the octave microwave signal and the reference radio frequency signal are linked, and no waiting time exists), wide band (the tuning range of the octave microwave signal is only limited by the PD2 wide band because no filter and phase shifter are used, but the limitation can be ignored because a commercial photoelectric detector with 100GHz bandwidth is available at present), the output frequency of the low-frequency local oscillator signal is changed, and the DPMZM output spectrum is shown in figure 10.

As can be seen from fig. 10, when the frequency of the low-frequency local oscillation signal is changed from 2.5GHz to 3GHz, the frequency octave output microwave signal is changed from 20GHz to 24GHz. Therefore, the octave frequency microwave signal generating device provided by the invention has the characteristics of high frequency, high speed and real-time wide tuning.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.

Claims (10)

1. A octave frequency microwave signal generating device is characterized by comprising a tunable laser, a radio frequency signal source, a polarization modulator, an erbium-doped fiber amplifier, a polarization beam splitter, a double parallel Mach-Zehnder modulator, a first photoelectric detector and a second photoelectric detector;

the tunable laser is used for outputting an optical carrier to the polarization modulator, and the radio frequency signal source is used for outputting a low-frequency reference signal to the polarization modulator;

the polarization modulator is used for generating a polarization modulation optical signal and outputting the polarization modulation optical signal to the erbium-doped fiber amplifier;

the erbium-doped optical fiber amplifier is used for amplifying the polarization modulation optical signal and outputting the amplified polarization modulation optical signal to the polarization beam splitter;

the dual parallel Mach-Zehnder modulator includes a first sub-modulator and a second sub-modulator;

the polarization beam splitter is used for dividing the amplified polarization modulation optical signal into B, C, wherein the B path optical signal outputs a frequency-doubled microwave electrical signal to the first sub-modulator through the photoelectric conversion of the first photoelectric detector; the C path of optical signals are transmitted into the double parallel Mach-Zehnder modulator after being branched and divided into an upper branch and a lower branch, wherein the upper branch of optical signals are transmitted to the first sub-modulator, and the lower branch of optical signals are transmitted to the second sub-modulator; and the second photoelectric detector is used for performing photoelectric conversion on the optical signal output by the double parallel Mach-Zehnder modulator and outputting a octave microwave electrical signal.

2. The octave frequency microwave signal generating device according to claim 1, further comprising a power amplifier disposed between the first photodetector and the dual parallel mach-zehnder modulator for amplifying the microwave electrical signal output by the first photodetector.

3. The octave frequency microwave signal generating apparatus of claim 1 further comprising a first auxiliary polarization modulator and a second auxiliary polarization modulator;

the first auxiliary polarization modulator is used for controlling the polarization state of the light carrier wave output by the tunable laser, and the second auxiliary polarization modulator is used for controlling the polarization state of the polarization modulation light signal.

4. The apparatus according to claim 3, wherein the first auxiliary polarization modulator is disposed between the tunable laser and the polarization modulator, and the second auxiliary polarization modulator is disposed between the polarization modulator and the erbium-doped fiber amplifier.

5. The apparatus of claim 1, wherein the polarization-modulated optical signal output by the polarization modulator comprises ± 1 order optical sidebands and an optical carrier.

6. The frequency octave microwave signal generating apparatus of claim 5 wherein the B-path optical signal comprises a ± 1 st order optical sideband and the C-path optical signal comprises an optical carrier.

7. The frequency octave microwave signal generating apparatus of claim 6 wherein the phase difference between the first and second sub-modulators is 180 °.

8. A method of octave microwave signal generation for an apparatus according to any of claims 1 to 7, comprising:

the tunable laser outputs a light carrier to the polarization modulator, the radio frequency signal source outputs a low-frequency reference signal to the polarization modulator, the first auxiliary polarization modulator is adjusted to enable the polarization state of the first auxiliary polarization modulator to be 45 degrees with the main shaft of the polarization modulator, and the second auxiliary polarization modulator is adjusted to enable the light carrier to be separated from the +/-1 order optical sideband and to be amplified and output through the erbium-doped fiber amplifier;

the erbium-doped fiber amplifier amplifies the polarization modulation optical signal and outputs the amplified polarization modulation optical signal to the polarization beam splitter, and the polarization beam splitter divides the amplified polarization modulation optical signal into a plus or minus 1-order optical sideband of a path B and an optical carrier of a path C;

the optical signal of the B path is detected by a first photoelectric detector to generate a frequency doubling microwave electric signal, and the frequency doubling microwave electric signal is amplified by a power amplifier and then loaded on a double parallel Mach-Zehnder modulator;

and the optical carrier of the C path enters the double-parallel Mach-Zehnder modulator, the direct-current bias voltage on the double-parallel Mach-Zehnder modulator is adjusted, the first sub-modulator is positioned at the maximum working point, the phase difference of output signals between the first sub-modulator and the second sub-modulator is 180 degrees, optical signals output by the double-parallel Mach-Zehnder modulator are detected by the second photoelectric detector, and octave microwave electrical signals are generated through photoelectric conversion.

9. The method of claim 8, wherein the tunable laser has an output wavelength of 1549.97nm and an output power of 3dBm.

10. The method of claim 8, wherein the output frequency of the RF signal source is 2.5GHz and the output power of the RF signal source is 10dBm.

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