CN104767570B - The unrelated coherent receiver of one kind polarization - Google Patents

Abstract

The invention belongs to the technical field of optical communication, and discloses a polarization-independent coherent receiver, comprising: a local oscillator light source, which generates linearly polarized local oscillator light; a local oscillator light modulation unit, which converts the local oscillator light into modulated local oscillator light; frequency mixing The device receives and modulates the local oscillator light and the signal light, and performs mixing to generate the mixed signal light; the photoelectric conversion module receives the mixed signal light and converts it into an intermediate frequency signal for output; wherein, the local oscillator light modulation unit includes: a polarization beam splitter , phase modulator and polarization beam combiner; polarization beam splitter receives linearly polarized local polarization light and splits it into first linear polarization and second linear polarization with equal power and polarization states perpendicular to each other; phase modulator and polarization beam splitter connected to receive the first linearly polarized light and adjust its phase to generate the third linearly polarized light; the third linearly polarized light and the second linearly polarized light generate a modulated phase difference; the polarization beam combiner receives the third linearly polarized light and the second linearly polarized light and combines the two, Generate modulated local oscillator light. The invention has simple structure, easy control and low cost.

Description

A Polarization-Independent Coherent Receiver

technical field

The invention relates to the technical field of optical communication, in particular to a polarization-independent coherent receiver.

Background technique

In the field of optoelectronic communication, larger bandwidth, longer transmission distance, and higher receiving sensitivity are always the development goals of optoelectronic communication technology. Due to its huge trend in sensitivity, coherent optical detection technology can greatly extend the relay distance, effectively guarantee the bandwidth advantage of optical carrier, greatly improve the communication capacity, and alleviate the problem of traditional optical communication bandwidth and power limitation. Become the main development direction of the optical communication system in the future.

However, as the core device of a coherent optical communication system, an optical coherent receiver has very strict requirements for coherent reception, which greatly increases the cost of optical and electronic components of the system and limits the wide-scale application of coherent communication technology. Polarization sensitivity is an inherent difficulty in coherent detection. Only when the polarization states of the signal light and the local oscillator light are exactly the same, the coherent receiver can obtain the maximum coherent gain and good receiving sensitivity. The practical application in coherent communication systems In the optical channel, the polarization state of the transmitted signal light will be randomly disturbed under the influence of the environment, which further increases the polarization matching difficulty of the coherent receiver. At present, the solution adopted in commercial use is to realize the polarization independence of the coherent receiver through the polarization diversity technology, but the complexity of the system is greatly increased, and the application range is limited.

Contents of the invention

The technical problem to be solved by the present invention is to provide a coherent receiver that improves anti-disturbance transmission, reduces matching difficulty, improves coherent gain and obtains good sensitivity.

In order to solve the above technical problems, the present invention provides a polarization-independent coherent receiver, including:

a local oscillator light source, which generates linearly polarized local oscillator light;

a local oscillator light modulation unit, the local oscillator light modulation unit is connected to the local oscillator light source, and adjusts the polarization state of the local oscillator light to adapt to signal lights of different polarization states;

a frequency mixer connected to the local oscillator light modulation unit, receives the local oscillator light, and mixes it with the signal light to generate a mixed signal light;

A photoelectric conversion module, the photoelectric conversion module is connected to the mixer, receives the mixed signal light, converts it into an electrical signal output, and converts the polarization information of the signal light into the intensity information and phase information of the output signal ;

Wherein, the local oscillator light modulation unit includes: a polarization beam splitter, a phase modulator, and a polarization beam combiner;

The polarization beam splitter receives the linearly polarized light and splits it into first linearly polarized light and second linearly polarized light with equal power and perpendicular polarization states;

The phase modulator is connected to the polarization beam splitter, receives the first linearly polarized light and adjusts its phase to generate a third linearly polarized light;

The polarization beam combiner receives the third linearly polarized light and the second linearly polarized light, and combines them into one beam to generate the modulated local oscillator light.

Further, the photoelectric conversion module includes: a balanced detector; the balanced detector is connected to the mixer, and receives the mixed signal and converts it into an electrical signal.

Further, the mixer is a 90° mixer;

The photoelectric conversion module includes:

Four photodetectors PD and four low-pass filters LPF corresponding thereto, the four photodetectors PD and four low-pass filters LPF corresponding thereto receive the mixed output of the 90° mixer Frequency signal light to generate four signals of I ₁ , I ₂ , Q ₁ and Q ₂ ;

Wherein, the I ₁ signal is multiplied by the I ₂ signal, and the Q ₁ signal is multiplied by the Q ₂ signal; the two products are superimposed to cancel the intermediate frequency part, obtain the OOK signal, and realize the phase-independent sum of the OOK signal Frequency is irrelevant.

Further, the polarization-independent coherent receiver also includes: an intensity and phase modulation module; the intensity and phase modulation module is connected between the local oscillator light source and the polarization beam splitter to compensate for power and phase deviation.

Further, when the signal light is linearly polarized signal light, the local oscillator light is circularly polarized local oscillator light, and the phase difference between the third linearly polarized light and the second linearly polarized light is 90°.

Further, when the ellipticity of the signal light is known, the phase of the first linearly polarized light is adjusted by the phase modulator, so that the phase difference between the third linearly polarized light and the second linearly polarized light is 90° °, to obtain elliptical local oscillator light, which is mixed with the signal light.

Further, when the polarization state of the signal light is non-linearly polarized light, a dynamic periodic scanning voltage is applied to the phase modulator to control the phase difference between the third linearly polarized light and the second linearly polarized light at - Dynamically change within the range of π～π to realize phase scanning;

Wherein, the output signal is the average value of the signal intensity in a single cycle; the cycle of the dynamic voltage is synchronized with the light cycle of the signal or is an integer multiple of the light cycle of the signal.

The polarization-independent coherent receiver provided by the present invention converts the local oscillator light into modulated local oscillator light through the local oscillator light modulation unit, which is used as carrier and signal light for frequency mixing detection, and converts the polarization information of the signal light into the change of the intensity of the output electrical signal. Amplitude and phase information, using the control of the phase change of the intensity information, realizes the polarization independence of heterodyne reception in the application of intensity modulation, thereby avoiding the adverse effects caused by polarization disturbance in the transmission process, greatly reducing the matching difficulty and improving the Coherent gain to obtain good sensitivity; at the same time, based on the high-speed OOK optical fiber communication system, a 90° coherent mixer is used to multiply the two in-phase signals received by the photodetector after mixing through a low-pass filter, and the The multiplication signal and the multiplication signal of the orthogonal signal are superimposed to realize the polarization-independent, phase-independent and frequency-independent characteristics of the high-speed OOK coherent receiver, which greatly reduces the complexity of the system while maintaining good coherent gain and sensitivity; this device The structure is simple, easy to control, low in cost and easy to be widely used.

Description of drawings

FIG. 1 is a structural schematic diagram of a heterodyne coherent receiver based on modulated local oscillator light provided by an embodiment of the present invention;

Fig. 2 is a diagram of the generation mechanism of modulated local oscillator light provided by the embodiment of the present invention;

FIG. 3 is a schematic diagram of local oscillator light polarization control used to detect signal lights with different polarization states provided by an embodiment of the present invention;

FIG. 4 is a schematic diagram of a high-speed near-homodyne OOK receiver provided by an embodiment of the present invention.

FIG. 5 is a schematic diagram of an output waveform of a polarization-independent coherent receiver for nonlinearly polarized signal light according to an embodiment of the present invention.

detailed description

Referring to FIG. 1 and FIG. 2 , a polarization-independent coherent receiver provided by an embodiment of the present invention includes: a local oscillator light source, a local oscillator light modulation unit, a mixer, and a photoelectric conversion module.

The local oscillator light source is used to generate linearly polarized local oscillator light as a transmission carrier.

The local oscillator light modulation unit is connected with the local oscillator light source, and converts the local oscillator light into modulated local oscillator light; it is beneficial to realize the transmission of signal light polarization information to electrical signal phase information.

The mixer receives the modulated local oscillator light and the signal light, and performs mixing to generate the mixed signal light;

Photoelectric conversion module, the photoelectric conversion module is connected with the mixer, receives the mixed signal light, converts it into an electrical signal output; in the process of converting the optical signal into an electrical signal, successfully converts the polarization information of the signal light into the intensity of the output signal Information and phase information, by using the control of the phase change of the intensity information, the polarization-independent characteristics are realized, and the anti-disturbance ability is greatly improved. The photoelectric conversion module includes: a balanced detector; the balanced detector is connected to the mixer, receives the mixed signal light and converts it into an electrical signal, and converts the polarization information of the signal light into the amplitude and phase information of the intensity change of the electrical signal; realizes polarization independence .

Wherein, the local oscillator optical modulation unit includes: a polarization beam splitter, a phase modulator and a polarization beam combiner;

The polarization beam splitter receives the linearly polarized light and divides it into the first linearly polarized light and the second linearly polarized light with equal power and perpendicular polarization states; the phase modulator is connected with the polarization beam splitter to receive the first linearly polarized light and adjust its The phase generates the third linearly polarized light; the polarization beam combiner receives the third linearly polarized light and the second linearly polarized light and combines them into one beam to generate modulated local oscillator light.

In the case of using the heterodyne detection mode, the photoelectric conversion module includes: a balanced detector; the balanced detector is connected to the mixer, receives the mixed signal light and converts it into an electrical signal, and converts the polarization information of the signal light into the intensity change of the electrical signal amplitude and phase information.

Referring to FIG. 4, in the case of the near homodyne detection mode, for the OOK homodyne coherent receiving mode, this embodiment also proposes that the mixer is a 90° mixer; the photoelectric conversion module includes:

Four photodetectors PD and corresponding four low-pass filters LPF, four photodetectors PD and corresponding four low-pass filters LPF receive the mixing signal output by the 90° mixer Light, generating four signals of I ₁ , I ₂ , Q ₁ and Q ₂ ;

Among them, the I ₁ signal is multiplied by the I ₂ signal, and the Q ₁ signal is multiplied by the Q ₂ signal; the two products are superimposed, and the trigonometric function part is offset, that is, the intermediate frequency part is offset to obtain the OOK signal, and the phase-independent sum of the OOK signal is realized. Frequency is irrelevant.

Referring to Figure 3, the polarization-independent coherent receiver also includes: an intensity and phase modulation module; the intensity and phase modulation module is connected between the local oscillator light source and the polarization beam splitter to compensate for the power and phase deviation caused by device loss, making the system easy to control , to achieve better polarization independence.

In a high-speed optical fiber communication system, because the optical fiber used as the transmission medium has various bending and rotation, the polarization state of the optical signal propagating through the optical fiber will change greatly. Polarization matching technology is also very important in coherent optical fiber communication. . Both homodyne coherent reception and heterodyne coherent reception have their own advantages and disadvantages. For the homodyne coherent receiver, it is necessary to ensure that the phase and frequency of the signal light and the local oscillator light must be completely consistent, which poses a great challenge to the system control; for the heterodyne coherent receiver, the phase and frequency of the local oscillator light The phase and frequency are not too high, but the heterodyne intermediate frequency is more than twice the signal bandwidth to obtain good results, which greatly increases the requirements for the response bandwidth of the high-speed photodetector itself

In actual communication, the division is made according to the polarization state of the optical signal, including: linear polarization signal, circular polarization signal and elliptical polarization signal; in the face of the randomness of signal light, the local oscillator light must have corresponding modulation measures, which is suitable for mixing Frequency demodulation needs.

When the signal light is linearly polarized signal light, the local oscillator light is circularly polarized local oscillator light, and the phase difference between the third linearly polarized light and the second linearly polarized light is 90°.

In laser imaging detection, the detection array detects the intensity information of each pixel. When the laser is irradiated on the surface of the measured object, due to the difference in the surface state of the measured object, the polarization state of the signal light fed back from different regions may be different. The coherent gain is inconsistent, affecting the final image quality.

The heterodyne receiving scheme of modulating local oscillator light can make the signals detected by the detection array unit have the same coherence gain, and the intermediate frequency signal processing circuit has been established, and the processing circuit is less, which reduces the difficulty of improving the detection system.

In space optical communication, multi-beam technology is widely used. Due to the different transmission channels of multiple beams, not only amplitude information and phase information fluctuations will be caused, but also polarization state changes will be caused. For multiple light beams, corresponding multiple polarization control systems are required, which greatly increases the complexity of the system and affects the application of space coherent communication of the multi-beam technology.

The polarization-independent coherent reception method of circularly polarized local polarization light is used to convert the polarization information of the linearly polarized signal light into a part of the phase information of the intermediate frequency signal output by the detection, that is, to convert the noise caused by the polarization state jitter of the signal light into phase noise A component of the phase noise processing scheme, the phase change caused by the polarization state change can be compensated by the phase compensation method in the electrical field. Therefore, the front-end optical processing part of the final coherent receiver will be greatly simplified, reducing the implementation cost of the multi-beam coherent receiver.

According to the generation mechanism of circularly polarized light, the local oscillator light is composed of two mutually perpendicular linearly polarized lights, then the vector of the light field of the local oscillator circularly polarized light is defined as:

Here, A _LO , ω _L , represent the optical field amplitude, frequency and phase of the initial light, respectively. Ideally,

Taking the x-axis as the reference, the signal light is linearly polarized light, and its polarization angle is θ. Taking OOK intensity modulation as an example, the vector of the optical field of the signal light is defined as:

Here, A _S , ω _S , represent the optical field amplitude, frequency and phase of the signal light, respectively.

When the signal light is mixed with the circularly polarized local oscillator light, the output signal can be regarded as the superposition of the x-axis component and the y-axis component, and the mixing part is represented by the sum of the two components, and the final output signal of the intermediate frequency part is:

ω _IF ＝ω _L -ω _S

From formula (3), it can be clearly seen that the polarization angle information of the signal light is transformed into the phase information of the IF signal, and the amplitude of the output IF signal has nothing to do with the polarization angle θ, that is, any polarization state fluctuation of the signal light does not It will affect the amplitude demodulation of the intermediate frequency signal, and will not reduce the receiving sensitivity of the heterodyne detection. From the structural point of view, the optical path processing required by the system is reduced, especially for the heterodyne detection system, the circuit processing part can use the basic intermediate frequency signal processing circuit, which greatly reduces the system cost.

According to the above calculation process, even if the signal light is non-linearly polarized light, if the ellipticity of the signal light is known, the phase of the first linearly polarized light is adjusted by the phase modulator to make the phase difference between the third linearly polarized light and the second linearly polarized light For 90°, the elliptical local oscillator light is obtained and mixed with the signal light. Polarization independence can still be achieved.

For the homodyne detection mode, combined with the circular polarization local oscillator technology, the polarization information of the signal light is converted into the phase information of the intermediate frequency signal. Through this near homodyne OOK demodulation scheme, polarization-independent, phase-independent and The frequency-independent OOK signal reduces the accuracy requirements on the device performance of the receiver system, greatly simplifies the structure of the receiver system, and reduces the matching difficulty of the coherent receiver.

In OOK modulation, the signal information is loaded on the amplitude, not the phase and frequency. Ideally, the signal light is still linearly polarized light, combined with the circular polarization conversion scheme of the local oscillator light, so that the receiver output signal is as follows:

For the non-ideal situation, the signal light is elliptically polarized light _{, and} the ellipse angle of the signal light is φ, then

Then the final output signal is expressed as:

For ideal linearly polarized signal light,

Referring to Figure 5, in the case where the polarization state of the signal light is non-linearly polarized light, that is, when the polarization state of the signal light is unknown, in order to find a suitable local oscillator light as carrier frequency mixing and demodulation; apply a dynamic Periodically scan the voltage to control the phase difference between the third line polarized light and the second line polarized light to change dynamically within the range of -π~π to realize phase scanning;

Wherein, the output signal is the average value of the signal intensity in a single period; the period of the dynamic voltage is synchronized with the light period of the signal or an integer multiple of the light period of the signal.

When the phase modulator adds a sinusoidal voltage whose frequency is half of the signal frequency, and makes the phase difference ψ vary between -π and π, the average value of the receiver unit cycle output signal is equal to It is still independent of the polarization state of the signal light, therefore, through proper signal processing, a polarization-independent signal output can still be obtained.

Finally, a polarization-independent, phase-independent, and frequency-independent output signal can be obtained. In this way, nearly homodyne coherent reception can be achieved, that is, the phase and frequency of the signal light and the local oscillator light are not required to be completely consistent, and high-speed photoelectric detection is not required. The response bandwidth of the receiver itself is more than twice the signal bandwidth, which greatly simplifies the structure of the coherent receiver in the system.

Coherent receivers have good polarization independence for heterodyne detection in OOK modulation systems that are still widely used today.

The technical scheme of using circularly polarized local oscillator, its polarization independence is mainly determined by the local oscillator, which is restricted by two factors, that is, the amplitude difference and phase difference of the two lines of polarized light that constitute circularly polarized light. In an ideal situation, the polarization beam splitting is 1:1, and the phase difference is π/2, so the ideal value of the circularly polarized light generating device should be two linearly polarized lights, and the phase difference φ=π/2. In practical applications, although the optical device cannot achieve the ideal situation, and the value will have a certain deviation, it can be compensated by adding amplitude control and phase control to make the amplitude and phase difference meet the ideal requirements, which can avoid the influence of device accuracy errors on performance, and is easy to use. Control regulation.

For the non-ideal situation, the polarization state of the signal light is not a linear polarization state, but a random elliptical polarization state. At this time, a dynamic voltage can be applied to the phase modulator in the local oscillator light generating device to make the phase difference of the two components of the local oscillator light change continuously at a high speed from -π to π, which can be realized by using a sinusoidal signal or a triangular signal. In high-speed signal detection, high-speed phase control is required, and sinusoidal signals are easier to achieve high-speed signals, and the intensity information of the output signal can be expressed as:

The frequency f of the sinusoidal signal is selected to match the signal rate, each bit contains a complete period, and the average value within a single period is taken, which has nothing to do with the polarization state of the signal light. Therefore, through synchronous sampling, the average signal strength output in each sampling period is a relatively constant value. In this way, adjusting the dynamic voltage frequency of the phase modulator can realize polarization-independent coherent detection in the case of elliptically polarized signal light.

At the same time, the amplitude and center shift in a single cycle directly reflect the polarization ellipticity of the signal light, which can be used to measure the elliptical polarization state of the signal light.

The polarization-independent coherent receiver provided by the embodiment of the present invention converts the local oscillator light into modulated local oscillator light through the local oscillator light modulation unit, as a carrier wave and signal light mixing detection, and converts the polarization information of the signal light into the phase of the output electrical signal Information, realizing the polarization independence of heterodyne reception in intensity modulation and frequency modulation applications, thereby avoiding the adverse effects caused by polarization disturbance in the transmission process, greatly reducing the difficulty of matching, and improving the coherence gain to obtain good sensitivity; at the same time , based on the high-speed OOK optical fiber communication system, using a 90° coherent mixer to multiply the two in-phase signals received by the photodetector after mixing through a low-pass filter, and multiply the multiplied signal and the quadrature signal The signals are superimposed to realize the polarization-independent, phase-independent, and frequency-independent characteristics of the high-speed OOK coherent receiver, which greatly reduces the complexity of the system while maintaining good coherent gain and sensitivity; the device is simple in structure, easy to control, low in cost and easy to spread use. For random signal light, adjusting the dynamic voltage frequency of the phase modulator can realize polarization-independent coherent detection in the case of elliptically polarized signal light. Therefore, the detection system can be simplified, and the detection efficiency and accuracy can be improved.

Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention without limitation, although the present invention has been described in detail with reference to examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be carried out Modifications or equivalent replacements without departing from the spirit and scope of the technical solution of the present invention shall be covered by the claims of the present invention.

Claims (7)

1. the unrelated coherent receiver of one kind polarization, it is characterised in that including：

Local oscillator light source, the local oscillator light source produces the inclined local oscillator light of line；

Local oscillator light-modulating cell, the local oscillator light-modulating cell is connected with the local oscillator light source, adjusts the polarization of the local oscillator light State, to adapt to the flashlight of different polarization states；

Frequency mixer, is connected with the local oscillator light-modulating cell, receives the local oscillator light, and it is carried out into mixing generation with flashlight Mixed frequency signal light；

Photoelectric conversion module, the photoelectric conversion module is connected with the frequency mixer, receives the mixed frequency signal light, is converted into electricity Signal output, by the polarization information of the flashlight, switchs to the strength information and phase information of output signal；

Wherein, the local oscillator light-modulating cell includes：Polarization beam apparatus, phase-modulator and polarization beam combiner；

The polarization beam apparatus receives the inclined local oscillator light of line, and it is equal to be divided into power, polarization state orthogonal first Line polarisation and the second line polarisation；

The phase-modulator is connected with the polarization beam apparatus, receives first line polarisation and simultaneously adjusts its phase, produces the Three line polarisations；

The polarization beam combiner, receives the 3rd line polarisation and second line polarisation, and the two conjunction beam is produced into a branch of The modulation local oscillator light.

2. unrelated coherent receiver is polarized as claimed in claim 1, it is characterised in that the photoelectric conversion module includes：It is flat Weigh detector；The balanced detector is connected with the frequency mixer, receives the mixed frequency signal light and is converted into electric signal.

3. unrelated coherent receiver is polarized as claimed in claim 1, it is characterised in that：The frequency mixer is 90 ° of frequency mixers；

The photoelectric conversion module includes：

Four photoelectric detector PDs and corresponding four low pass filters LPF, four photoelectric detector PDs and Corresponding four low pass filter LPF receive the mixed frequency signal light of 90 ° of frequency mixers output, generate I₁、I₂、Q₁And Q₂ Four road signals；

Wherein, the I₁Signal and the I₂Signal multiplication, the Q₁Signal and the Q₂Signal multiplication；Two product superpositions, are supported Disappear intermediate-frequency section, obtains OOK signals, realizes that the phase of OOK signals is unrelated unrelated with frequency.

4. the unrelated coherent receiver of polarization as described in any one of claims 1 to 3, it is characterised in that also include：Intensity and phase Position modulation module；The intensity and phase modulation module are connected between the local oscillator light source and the polarization beam apparatus, compensation Power and phase deviation that device loss is caused.

5. unrelated coherent receiver is polarized as claimed in claim 4, it is characterised in that：It is the inclined flashlight of line in the flashlight In the case of, the local oscillator light is to justify inclined local oscillator light, and the phase difference of the 3rd line polarisation and second line polarisation is 90 °.

6. unrelated coherent receiver is polarized as claimed in claim 5, it is characterised in that：To know the ellipticity of the flashlight In the case of, the phase of first line polarisation is adjusted by the phase-modulator, makes the 3rd line polarisation and described the The phase difference of two line polarisations is 90 °, oval local oscillator light is obtained, with the signal photomixing.

7. unrelated coherent receiver is polarized as claimed in claim 4, it is characterised in that：It is non-in the polarization state of the flashlight In the case of line polarisation, apply cycle dynamicses scanning voltage on the phase-modulator, control the 3rd line polarisation with The phase difference of second line polarisation dynamic change in the range of-π~π, realizes phasescan；

Wherein, output signal is the average value of signal intensity in signal period；The cycle of the dynamic electric voltage and the flashlight Cycle synchronisation or the integral multiple for making the signal photoperiod.