All-optical phase noise inhibition method for coherent detection

A phase noise and coherent detection technology, applied in the field of coherent optical communication, to achieve the effects of simple implementation, low cost and simple system structure

Active Publication Date: 2010-12-15
HUAZHONG UNIV OF SCI & TECH
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[0003] The present invention proposes an all-optical phase noise suppression method suitable for coherent detection, aiming at the current situation in which ultra-low phase noise lasers are used as signal light sources and local light sources or complex digital circuit processin...
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Abstract

The invention relates to an all-optical phase noise inhibition method for coherent detection. Aiming at the current situation that the local oscillation coherent reception phase noise characteristic is improved through an ultra-low noise performance light source and complex circuit processing technology at present, the all-optical method is provided to inhibit the phase noise and improve the receiving performance. The method comprises the steps that: through a transmitting terminal, one path of a pair of light carrier waves with relevant properties is loaded with information to form signal light, while the other path is used as coherent light and is subjected to wave combination with the signal light to enter optical fibers for transmission; and a receiving terminal performs frequency mixing on the extracted signal light and coherent light, and photovoltaic conversion and signal demodulation are performed in turn. In the method, the phase noise in the coherent detection is inhibited by utilizing the characteristic that homologous generated light carrier waves have relativity without using a local light source and a complex signal processing process, so the structure is simplified and the cost is reduced. In addition, multiple groups of light carrier waves can be acquired by using the technology of combining a wide-spectrum and multi-wavelength light source with spectral partitioning, so the inhibition of the phase noise in a multi-channel coherent light communication system is easy to realize.

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  • All-optical phase noise inhibition method for coherent detection
  • All-optical phase noise inhibition method for coherent detection
  • All-optical phase noise inhibition method for coherent detection

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Example Embodiment

[0011] The present invention will be described in detail below with reference to the accompanying drawings.
[0012] figure 1 It is a schematic flow chart of the present invention. It is an all-optical phase noise suppression method suitable for coherent detection. figure 2 It is an example diagram of an application of the present invention. It is a single-channel heterodyne coherent optical communication system using the present invention. image 3 It is another application example diagram of the present invention. It is a multi-channel heterodyne coherent optical communication system using the present invention.
[0013] The all-optical phase noise suppression method for coherent detection of the present invention comprises the following steps:
[0014] Step 1: After demultiplexing the output light wave of the light source at the transmitting end of the system, a pair of optical carriers with frequency and phase correlation characteristics are obtained. For the homodyne system, the light source is used to emit a single longitudinal mode light wave with stable frequency. The demultiplexing process is to divide the power of the light wavelength to obtain a pair of optical carriers with the same center frequency. For the heterodyne system, a multi-wavelength light source is used to output two frequency intervals. For stable longitudinal mode light waves, and the center frequency difference between the two must be greater than the required bandwidth of the signal, the demultiplexing process is to separate the two longitudinal modes to obtain the required optical carrier.
[0015] The method can be applied to a single-channel heterodyne coherent optical communication system, in which the light source can be selected from a multi-wavelength light source 1, and the demultiplexing process can be implemented by an arrayed waveguide grating 2. The center frequency of the output end of the arrayed waveguide grating 2 corresponds to the multi-wavelength light source 1. The center frequencies of the two longitudinal modes of , as figure 2 shown.
[0016] Step 2: One optical carrier is loaded with information to obtain signal light through the modulation process, and the other optical carrier is subjected to an appropriate polarization state control process to obtain coherent light, and then the signal light and the coherent light are coupled into the same optical fiber through a multiplexing process for transmission.
[0017] For the homodyne system, the coherent light is obtained after the above-mentioned other optical carrier is rotated by 90° polarization to form a polarization state orthogonal to the signal light; for the heterodyne system, the coherent light can be obtained by the above-mentioned other optical carrier by 90° polarization rotation. After obtaining the polarization state orthogonal to the signal light, it can also directly enter the wave combining process without going through a 90° polarization rotation, but maintaining the polarization state consistent with the signal light. But the premise of the latter case is that the center frequency separation of the signal light and the coherent light is greater than the minimum frequency separation required for the filtering process when separating the two.
[0018] When the method is applied to a single-channel heterodyne coherent optical communication system, the signal light and the coherent light transmitted in the optical fiber can be in orthogonal polarization states. The device that realizes the 90° polarization rotation process can choose the 90° optical rotator 4, and the device that realizes the wave combination process can choose the polarization beam combiner 5, such as figure 2 shown.
[0019] Step 3: After the coupled signal light and coherent light are transmitted through the optical fiber to the receiving end, the corresponding wavelength division or polarization state mode is selected to perform demultiplexing processing on the signal light and the coherent light according to the difference in frequency or polarization state. The separated signal light and coherent light are obtained. If the polarization states of the obtained signal light and coherent light are orthogonal, the coherent light is rotated by 90° to obtain a polarization state consistent with the signal light, and then the two paths of light are mixed; if the polarization states of the obtained signal light and coherent light are If they are consistent, the coherent light is directly mixed with the signal light without going through the 90° polarization rotation process. The optical signal obtained after mixing is then converted into photoelectricity by a photodetector to obtain an electrical signal with suppressed phase noise.
[0020] Finally, according to whether the coherent optical communication system is homodyne or heterodyne and the modulation format of the signal, the corresponding demodulation process is selected to restore and process the baseband electrical signal to achieve information extraction.
[0021] When this method is applied to a single-channel heterodyne coherent optical communication system, the demultiplexing process at the receiving end corresponds to the optional polarization beam splitter 7, and the 90° polarization rotation process of the coherent light can be realized by using the optical rotator 4. The mixing process can optionally be implemented with a 90° optical mixer 8 having two inputs and four outputs. The photodetector that implements the photoelectric conversion process is a set of four identical PIN photodiodes 9 . The processing flow of the demodulation process is to first input the four-way differential electrical signals output by the PIN photodiode 9 into the two subtractors 22 respectively; The intermediate frequency carrier generated by the multiplier 11 is multiplied; then the output of the multiplier 12 is processed by the filter 13 to obtain the in-phase and quadrature components of the baseband signal (that is, the I, Q signals); finally, the I and Q signals are respectively sampled and judged Signal demodulation can be achieved, such as figure 2 shown.
[0022] The principle analysis of the above invention is as follows:
[0023] The current expression obtained by the photodetector can be expressed as:
[0024] I ( t ) = R ( P S + P L ) + 2 R P S P L cos ( ω IF t + φ )
[0025] ω IF =ω S -ω L , φ=φ S -φ L
[0026] In the formula, R is the photoelectric conversion factor; P S is the signal optical power; P L is the coherent optical power; ω S is the angular frequency of the signal light; ω L is the angular frequency of coherent light; φ S is the signal light phase; φ L is the coherent light phase; ω IF and φ are the angular frequency and phase corresponding to the intermediate frequency electrical signal, respectively.
[0027] when ω S =ω L When , corresponding to the homodyne system, the electrical signal is a baseband signal; when ω S ≠ω L When , corresponding to a heterodyne system, the electrical signal is an intermediate frequency signal, and the baseband signal is obtained by secondary demodulation in a synchronous or asynchronous manner.
[0028] Phase φ of signal light and coherent light in coherent detection S and φ L It can be regarded as two random variables, the variance of the phase difference φ (electrical signal phase noise) and φ S Variance with φ L Variance (light source phase noise) relationship is: In the method of the present invention, φ S and φ L are two random variables with close to linear correlation characteristics, that is, ρ is close to 1, cancel out The value is extremely small, which is a relatively common local oscillator coherent receiving method. Under the condition of the same light source phase noise and no complex circuit processing technology, the phase noise of the electrical signal in the homodyne or heterodyne system can be better suppressed.
[0029] It should be noted that the method of the present invention can also be applied to a wavelength division multiplexing system. Among them, a wide-spectrum multi-wavelength light source is used to generate several frequency-spaced stable longitudinal modes that meet the requirements of the number of channels. For a homodyne system, the number of required longitudinal modes is equal to the number of channels; for a heterodyne system, the number of required longitudinal modes is equal to twice the number of channels, and then multiple groups of optical carrier assignments with related characteristics can be simultaneously obtained through spectral division technology to multiple channels. The wavelength division/multiplexer is used to realize the channel synthesis and separation, and each channel can realize the phase noise suppression of the multi-channel coherent optical communication system by using the method of the above steps.
[0030] The method is specifically applied in a multi-channel heterodyne coherent optical communication system, and the wide-spectrum multi-wavelength light source can be selected as the supercontinuum light source 15 . The wavelength division/multiplexers 16, 18 and 19 used in the division/combination can be of the same type or different types, and the number of ports, central wavelength value and bandpass characteristics can meet the system requirements, such as image 3 shown.
[0031] figure 1 , figure 2 and image 3 The description of the present invention is intended to be descriptive and not restrictive. E.g figure 2 The arrayed waveguide grating in can also be one or more cascaded other types of demultiplexing devices; figure 1 The 90° polarization rotation process can be realized by a single 90° optical rotator, or it can be realized simultaneously by other integrated devices combined with the splitting and combining processes, for example, a polarizing beam splitter with a crystal structure can be used in a homodyne system. Simultaneously realize the power division process in step 1 and the 90° polarization rotation process of one optical carrier to obtain two optical carriers with orthogonal polarization states; figure 2 The PIN photodiodes in can also be other types of photodetectors.
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Semiconductor device

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Owner:SEMICON ENERGY LAB CO LTD
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