CN114978330A - Feedforward post-compensation linearization radio frequency optical transmitter and improvement method thereof - Google Patents

Abstract

The invention discloses a feedforward post-compensation linearization radio frequency optical transmitter and an improvement method thereof, which comprises a beam splitter, a modulator, a detector and an amplifying circuit, wherein a composite modulator is utilized to complete the post-compensation function of a feedforward optical path, the feedforward is that light emitted by a laser is divided into two parts, one part enters a modulator chip, the other part enters an optical detector and the amplifying circuit to be subjected to phase inversion processing, and then enters the modulator, and second-order distortion signals in two paths of signals have the same time delay, the same amplitude and the opposite phase so as to realize mutual cancellation, so that the second-order nonlinear post-compensation of light in the laser is realized, the linearity of the optical transmitter can be effectively improved, and the SFDR non-stray dynamic range of the optical transmitter is improved.

Description

Feedforward post-compensation linearized radio frequency optical transmitter and improvement method thereof

Technical Field

The invention relates to the technical field of photoelectric communication, in particular to a feedforward post-compensation linearization radio frequency optical transmitter and an improvement method thereof.

Background

With the rapid development of the demand of high-capacity information technology, the drawbacks of microwave communication become more obvious, and the microwave transmission medium has great loss when transmitting high-frequency microwaves for a long distance, so that the high-frequency expansion of the use frequency is limited. The radio frequency optical fiber transmission has the advantages of high bandwidth, high sensitivity, strong anti-interference performance, long transmission distance and high confidentiality, and is widely applied to the fields of backbone network communication, television broadcast signal transmission and the like.

The radio frequency signal optical fiber transmission comprises three parts, namely an electric/optical conversion device (an optical transmitter), a transmission medium (an optical fiber) and an optical/electric conversion device (an optical receiver). At the transmitting end, a radio frequency optical transmitter modulates a radio frequency signal onto an optical carrier and outputs an optical wave through an optical fiber; at a receiving end, an optical signal carrying a radio frequency signal enters a detector, the radio frequency signal is demodulated out, and finally output after a series of signal processing, so that electric/optical-optical/electric conversion is realized.

Radio frequency fiber transmission has also attracted a great deal of attention in antenna array networking systems due to its significant advantages. The radio frequency signal optical fiber transmission technology is realized in an analog modulation mode, and is an analog communication technology, so strict requirements are imposed on parameters such as linearity and dynamic range of a transmitter, otherwise serious distortion of a microwave radio frequency signal is caused, and the nonlinear line of a laser at a light transmitting end has great influence on the radio frequency signal demodulated by a receiving end, so that the strict requirements of a radio frequency optical fiber transmission system applied in the fields of antenna array systems and the like on indexes such as harmonic wave, stray suppression and the like of the radio frequency signal become key factors for restricting the application of radio frequency optical fiber transmission.

In the prior art, US5132639A discloses a predistorter for linearization of electronic and optical signals, which employs a predistortion circuit to improve indexes, but compensates second-order distortion of an optical path through a diode device circuit, so that corresponding indexes cannot be completely matched, and problems of complex structure, difficult adjustment, unsatisfactory compensation effect and the like exist. With the continuous improvement of application requirements, the index requirements for second-order distortion compensation are higher, and the existing predistortion circuit compensation scheme cannot meet the requirements of professional application fields. In order to solve this problem, a linearizer that is simple and reliable, easy to adjust, and has good compensation effect is needed.

Disclosure of Invention

The invention aims at the technical problems and provides a feedforward post-compensation linearization radio frequency optical transmitter and an improvement method thereof, which meet the practical requirements of high linearity and high dynamic range.

In order to achieve the above purpose, the invention provides the following technical scheme:

a feedforward post-compensation linearization radio frequency optical transmitter comprises a laser unit, a composite modulator unit, a predistortion compensation unit, an SBS suppression unit, a bias control unit and a beam splitter; the laser unit comprises a temperature controller and a laser; the composite modulator unit comprises a beam splitter, an optical detector, an amplification and equalization circuit and a modulator chip, the composite modulator unit and a laser form a post-compensation optical path, the nonlinearity of a modulator cosine-like modulation curve is utilized to compensate the second-order nonlinearity of the laser, an optical carrier signal from the laser is divided into two parts by the beam splitter, one part enters the modulator chip, the other part enters the amplification and equalization circuit through the optical detector, the two parts enter the modulator after being subjected to phase inversion processing, the two parts entering the modulator chip have the same optical amplitude and opposite phases, and the amplitude end of the modulator chip is connected with a bias control unit and is used for adjusting a second-order MAX point of the modulator; the drive amplifying circuit unit consists of a preamplifier circuit, a second-order and third-order predistortion compensation circuits and a drive amplifying circuit, and a part of the optical carrier signal entering the amplification equalizing circuit enters the drive amplifying circuit after being subjected to phase inversion processing.

Wherein, the temperature controller comprises a temperature detector, a controller and a refrigerator. The modulator chip is integrated with a phase modulator, connected with the SBS suppression unit and used for SBS suppression of the optical link.

The invention also provides a method for improving the feedforward post-compensation linearization radio frequency light, which adopts the feedforward post-compensation linearization radio frequency optical transmitter and the following steps:

radio frequency input signal U ₃₀₁ The signal is input from a port and generates a second-order intermodulation signal U through a predistortion compensation unit and a laser unit ₁₀₁ Signal U entering the complex modulator unit ₁₀₁ After beam splitting by the beam splitter, a first branch signal U ₂₀₁ Modulating by a composite modulator, and generating a second-order phase signal U by the modulated signal ₂₄ The other part of the signal enters the amplification and equalization circuit through the optical detector and outputs a second branch signalU ₂₀₂ The two parts of light which are processed in an inverted way enter the modulator, the amplitudes of the two parts of light which enter the modulator are the same and the phases of the two parts of light are opposite, and the modulator obtains an output signal U ₂₀₃ 。

Wherein the radio frequency input signal U ₃₀₁ Expressed as:

U ₃₀₁ ＝Acosω ₁ t+Bcosω ₂ t

second order intermodulation signal U ₁₀₁ Expressed as:

U ₁₀₁ ＝a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)

the first branch signal is represented as:

U ₂₀₁ ＝C[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]

second order phase signal U ₂₄ Expressed as:

U ₂₄ ＝cC[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B2cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]-D(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)

second branch signal U ₂₀₂ Expressed as: u shape ₂₀₂ ＝(1-C)[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]

Output signal U ₂₀₃ Expressed as:

U ₂₀₃ ＝U ₂₄ +U ₂₀₂ 。

compared with the prior art, the invention has the beneficial effects that:

the invention discloses a feedforward post-compensation linearization radio frequency optical transmitter, which comprises a beam splitter, a modulator, a detector and an amplifying circuit, wherein a composite modulator is utilized to complete a feedforward optical path post-compensation function, the feedforward is that light emitted by a laser is divided into two parts, one part enters a modulator chip, the other part enters an optical detector and the amplifying circuit to be subjected to phase inversion processing, and then enters the modulator, second-order distortion signals in the two paths of signals have the same time delay, the same amplitude and the opposite phase so as to realize mutual cancellation, so that second-order nonlinear post-compensation of light in the laser is realized, the linearity of the optical transmitter can be effectively improved, and the SFDR non-stray dynamic range of the optical transmitter is improved.

The invention relates to a method for improving feed-forward post-compensation linearized radio frequency light, which carries out first compensation on second-order distortion and third-order distortion, and adopts a composite modulator to complete the feed-forward post-compensation function of a laser, and the second-order distortion compensation effect is obvious. Compared with a method for compensating the nonlinearity of the laser by only adopting a predistortion circuit, the scheme for performing post-compensation on the laser by using the optical modulator has the advantages of better index matching degree, more obvious linearization effect and wider application value.

Drawings

In order to more clearly illustrate the embodiments of the present application or technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments described in the present invention, and other drawings can be obtained by those skilled in the art according to the drawings.

Fig. 1 is a schematic diagram of a mechanism of a feed-forward post-compensation linearized rf optical transmitter according to an embodiment of the present invention.

Detailed Description

In order to make the technical solutions of the present invention better understood by those skilled in the art, the present invention will be further described in detail with reference to the accompanying drawings and examples.

The invention provides a high-dynamic low-distortion high-linearity feedforward post-compensation linearization radio frequency optical transmitter which comprises a laser unit 1, a composite modulator unit 2, a predistortion compensation unit 3, an SBS (stimulated Brillouin scattering) suppression unit 4, an offset control unit 5 and a beam splitter 6.

The laser unit 1 comprises a temperature controller 11 and a laser 12, wherein the temperature controller 11 comprises a temperature detector, a controller and a refrigerator, so as to ensure that the luminous power of the laser is stable under the full-temperature environment and long-time use.

The complex modulator unit 2 includes a beam splitter 21, a photodetector 22, an amplification and equalization circuit 23, and a modulator chip 24. Due to the nonlinear optical characteristic of the modulator, the modulator and the laser 12 can form a post-compensation optical path, and the second-order nonlinearity of the laser 12 is compensated by utilizing the nonlinearity of the modulator cosine-like modulation curve. A phase modulator is integrated on the modulator 24 and connected to the SBS suppression unit 4 for SBS suppression of the optical link.

The optical carrier signal from the laser 12 is divided into two parts by the beam splitter 21, one part enters the modulator chip 24, the other part enters the amplification and equalization circuit 23 through the optical detector 22, the two parts enter the modulator 24 after being subjected to phase inversion processing, the two parts of light entering the modulator chip 24 have the same amplitude and opposite phases, the second order can be completely offset under the condition that the two parts of light enter the modulator chip 24 at the same time, and even if the second order index can be reduced by more than 15-20 dB due to delay errors. The amplitude terminal of the modulator chip 24 is connected to the bias control unit 5 for adjusting the second order MAX point (maximum value) of the modulator 24. In conclusion, the feed-forward and post-compensation function of the transmitter system is completed, the influence of the second-order index is effectively reduced, and the linearity and the dynamic range of the transmitter system are improved.

The driving amplification circuit unit 3 is composed of a pre-amplification circuit 31, second and third order pre-distortion compensation circuits 32, and a driving amplification circuit 33. The optical carrier signal that has entered the amplifier-equalizer circuit 23 is inverted, and a part of the inverted signal enters the driver amplifier circuit 33.

The nonlinear distortion of the laser is mainly a second-order intermodulation product, which is also the problem mainly solved by the scheme.

Second order term: k ² (Acosω ₁ t+Bcosω ₂ t) ² This formula is expanded to:

＝K ² [A ² /2+B ² /2+A ² /2cos ² ω ₁ t+B ² /2cos ² ω ₂ t+ABcos(ω ₁ ±ω ₂ )t]

in the formula A ² /2、B ² The second harmonic component is a second harmonic component (called a second harmonic product), and the fifth and sixth components are beat components (called beat products). In summary, the latter four terms are all newly generated frequency terms that create intermodulation interference as long as they fall within the normal signal frequency range. These frequency components are called second order intermodulation products, and the sum of several products is commonly referred to as combined second order distortion, IMD2 for short.

On the other hand, the invention also provides a feedforward post-compensation linearization radio frequency light improvement method adopting the radio frequency optical transmitter ₃₀₁ By U ₃₀₁ ＝Acosω ₁ t+Bcosω ₂ t denotes a second order intermodulation signal U which is input from the port 301 and is generated by the signal through the predistortion compensation unit 3 and the laser unit 1 ₁₀₁ The signal 101 entering the complex modulator unit 2 can be expressed as:

U ₁₀₁ ＝a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)；

after being split by the splitter 21, the signal at the first branch signal 201 can be represented as:

U ₂₀₁ ＝C[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]；

the first branch signal U201 is modulated by the complex modulator 24, and the modulated signal generates a second order phase signal with opposite phase:

U ₂₄ ＝cC[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B2cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]-D(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)；

the other part of the signal enters the amplification and equalization circuit 23 through the optical detector 22, and a second branch signal U is output ₂₀₂ ，U ₂₀₂ The signal is represented as: u shape ₂₀₂ ＝(1-C)[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]；

Make the signal at 202 have a sum of U ₂₄ The signals have the same amplitude but are 180 degrees out of phase.

The output signal at 203 can be expressed as:

U ₂₀₃ ＝U ₂₄ +U ₂₀₂ 。

since the value of C is generally small, U ₂₄ Middle cCb (A) ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t) this second order term has little effect on the signal and is negligible. I.e. when D ═ b (1-C), the signal U is output ₂₀₃ The second order term in (b) can be eliminated for the purpose of post-compensation of the modulator.

In the actual test of the invention, the DFB laser is used, the index of the IMD2 of the DFB laser is-43.3 dBm, the index of the IMD2 of the system after the DFB laser is compensated by the compensation circuit after the DFB laser passes through the modulator of the invention reaches-62.6 dBm, and the excellent correction effect is achieved.

The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: it is to be understood that modifications may be made to the technical solutions described in the foregoing embodiments, or equivalents may be substituted for some of the technical features thereof, but such modifications or substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (5)

1. A feedforward post-compensation linearization radio frequency optical transmitter is characterized by comprising a laser unit (1), a composite modulator unit (2), a predistortion compensation unit (3), an SBS suppression unit (4), a bias control unit (5) and a beam splitter (6); the laser unit (1) comprises a temperature controller (11) and a laser (12); the composite modulator unit (2) comprises a beam splitter (21), an optical detector (22), an amplification and equalization circuit (23) and a modulator chip (24), a post-compensation optical path is formed by the composite modulator unit (2) and the laser (12), the second-order nonlinearity of the laser (12) is compensated by utilizing the nonlinearity of a modulator cosine-like modulation curve, an optical carrier signal from the laser (12) is divided into two parts by the beam splitter (21), one part enters the modulator chip (24), the other part enters an amplification and equalization circuit (23) through a light detector (22), the light enters the modulator (24) after being subjected to phase inversion, the two parts of light entering the modulator chip (24) have the same amplitude and opposite phases, and the amplitude end of the modulator chip (24) is connected with the bias control unit (5) and used for adjusting a second-order MAX point of the modulator (24); the drive amplification circuit unit (3) is composed of a preamplifier circuit (31), a second-order and third-order pre-distortion compensation circuit (32) and a drive amplification circuit (33), and a part of optical carrier signals entering the amplification and equalization circuit (23) enter the drive amplification circuit (33) after being subjected to phase inversion processing.

2. A feed forward post-compensation linearized radio frequency optical transmitter as claimed in claim 1, characterized in that the temperature controller (11) comprises a temperature detector, a controller and a refrigerator.

3. A feed forward post compensation linearized radio frequency optical transmitter as claimed in claim 1, characterized in that a phase modulator is integrated on the modulator chip (24) and connected to the SBS suppression unit (4) for SBS suppression of the optical link.

4. A method for improving feedforward post-compensation linearized radio frequency light, characterized by using the feedforward post-compensation linearized radio frequency optical transmitter of any one of claims 1 to 3 and the following steps:

radio frequency input signal U ₃₀₁ The signal is input from a port (301) and generates a second-order mutual signal through a predistortion compensation unit (3) and a laser unit (1)Modulating signal U ₁₀₁ Signal U entering the complex modulator unit (2) and entering the complex modulator unit (2) ₁₀₁ After being split by a beam splitter (21), a first branch signal U ₂₀₁ Modulated by a complex modulator (24), the modulated signal producing a second order phase signal U ₂₄ The other part of the signal enters an amplifying and equalizing circuit (23) through a light detector (22) and outputs a second branch signal U ₂₀₂ The two parts of light which are processed in an inverted way enter the modulator (24), the amplitudes of the two parts of light which enter the modulator (24) are the same and the phases are opposite, and the modulator (24) obtains an output signal U ₂₀₃ 。

5. The method of claim 4, wherein the RF input signal U is a feedback signal ₃₀₁ Expressed as:

U ₃₀₁ ＝Acosω ₁ t+Bcosω ₂ t

second order intermodulation signal U ₁₀₁ Expressed as:

U ₁₀₁ ＝a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)

the first branch signal is represented as:

U ₂₀₁ ＝C[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]

second order phase signal U ₂₄ Expressed as:

U ₂₄ ＝cC[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B2cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]-D(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)

second branch signal U ₂₀₂ Expressed as:

U ₂₀₂ ＝(1-C)[a(Acosω ₁ t+Bcosω ₂ t)+b(A ² cos ² ω ₁ t+B ² cos ² ω ₂ t+2ABcosω ₁ tcosω ₂ t)]

output signal U ₂₀₃ Expressed as:

U ₂₀₃ ＝U ₂₄ +U ₂₀₂ 。

Images