CN108362388B - Method for measuring phase noise of dual-channel differential laser - Google Patents
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Abstract
The invention relates to a method for measuring phase noise of a dual-channel laser, which mainly solves the problems that the stability of the existing laser phase noise measuring system is not high, and the influence of system low-frequency noise on the measuring precision is large. The invention adopts a measuring device of phase noise of a dual-channel differential laser, and a hardware platform of the measuring device comprises: the device comprises a laser to be measured, a variable optical attenuator, a DPSK electro-optic modulator, two photoelectric detectors, two low-noise amplifiers, a data acquisition circuit, a computer and a DPSK modulator control circuit, wherein the mature and stable electro-optic modulator is used for replacing an optical fiber interferometer to measure the phase noise of the laser, and a dual-channel differential signal receiving mode is adopted, so that the stability of a measuring system is improved, and partial influence of the background noise of the system on the measuring result is eliminated.
Description
Technical Field
The invention belongs to the field of measurement, and particularly relates to a method for measuring phase noise of a dual-channel differential laser.
Background
The photoelectric technology is an indispensable part in the modern high and new technology field, penetrates the aspects of national life, and plays an important role in national defense construction, the phase noise of the laser is a parameter for representing the wavelength jitter of the laser, and is also an important factor influencing the signal-to-noise ratio of the system in an optical fiber sensing system and an optical carrier microwave transmission system, and the measurement of the phase noise of the laser is very critical to the screening and process control of laser devices and the evaluation of the influence of the phase noise on an application system.
A common method for measuring the phase noise of a laser is a frequency shift heterodyne method, which uses an optical fiber interferometer, and introduces a long delay and an acousto-optic frequency shift into one arm to realize the measurement of the phase noise of the laser, wherein the measurement accuracy of the optical fiber interferometer mainly depends on the stability of laser frequency, and the common method is to stabilize the frequency of the laser and has higher requirements on working environment conditions.
The invention provides a mature and stable electro-optical modulator to replace an optical fiber interferometer, and the measurement of the phase noise of the laser is realized by using a dual-channel difference method, so that the stability of the system is improved, and the influence of the background noise of the system on the measurement precision is eliminated.
Disclosure of Invention
The technical problem to be solved by the invention is to overcome the existing defects, and to provide a novel method for measuring the phase noise of a dual-channel differential laser, which utilizes an electro-optical modulator to replace an optical fiber interferometer to measure the phase noise of the laser, adopts dual-channel differential receiving signals to improve the stability of a measuring system, eliminates partial influence of the background noise of the system on the measuring precision,
in order to solve the technical problems, the technical scheme adopted by the invention is as follows: the invention adopts a method for measuring phase noise of a two-channel differential laser, which comprises a laser to be measured, an adjustable optical attenuator, a DPSK electro-optic modulator, a photoelectric detector, a low-noise amplifier, a data acquisition circuit, a computer and a DPSK modulator control circuit, wherein light emitted by the laser to be measured enters the DPSK electro-optic modulator with double outputs after passing through an adjustable optical attenuator, the DPSK modulator adjusts the optical path difference between two paths of transmission optical signals in the DPSK modulator through the control circuit, two paths of optical signals output by the control circuit of the DPSK modulator have certain optical delay difference, the two paths of signals respectively enter two photoelectric detectors which are positioned in different transmission channels, two electric signals output by the two photoelectric detectors respectively enter the data acquisition circuit after being amplified through the two low-noise amplifiers, the two low-noise amplifiers are positioned in different transmission channels, the data acquisition circuit is connected with a computer, the computer acquires data, performs differential processing on the two paths of signals, eliminates the influence of system background noise on a measurement result, and transmits the data to the DPSK modulator control circuit (201), wherein the background noise mainly comes from a photoelectric detector and a low-noise amplifier.
In order to solve the technical problems, the technical scheme adopted by the invention further explains the working principle adopted by the invention, and the specific measurement principle is as follows:
the optical field of the output of the DPSK modulator may be represented as:
where f is the frequency of the light source, Ein(f) Tau is the differential time delay between two paths of signals output by the DPSK modulator for the light field distribution on the incident near detector. The above equation can be approximated as:
where δ f is the laser frequency jitter, which directly translates to the phase noise of the laser. In the above formula
Within the wavelength bandwidth of the laser, the time delay difference of two arms of an interferometer in DPSK or the wavelength of the laser is adjusted, so that a determined optical frequency f can be obtained0Is provided with
Accordingly, formula (2) is:
the DPSK photoelectric modulator output signal is converted into an electric signal after passing through a photoelectric detector, and after low-noise amplification, the jitter of the electric signal output by the photoelectric detector, which is introduced due to laser phase noise, is as follows:
Vnoise＝πτIinRκGδf+Vsystem(4)
in the above formula IinFor the light intensity entering the photodetector, R is the detector output load, kappa is the responsivity of the photodetector, G is the amplification factor of the low noise amplifier, and V issystemTo test the noise floor of the system. The dc signal output by the photodetector may be:
after the two paths of signals are acquired by the data acquisition circuit, the two paths of signals are subjected to differential processing, so that the background noise of the test system can be eliminatedVsystemThe effect on the measurement results is mainly due to the noise of the photodetector and the low noise amplifier.
Due to the adoption of differential detection, the pi tau I in the formula (4) isinR κ G ═ K, formula (4) is represented as:
Vnoise＝Kδf+Vsystem(6)
obtaining the power spectral density S of the output voltage of the detector by performing Fourier transform on the voltage fluctuation value of the photoelectric detector obtained by converting the voltage fluctuation value of the photoelectric detector obtained by the formula (6) due to the phase noise of the laserν(f) Expressed as:
in the above equation, B is the signal bandwidth. The phase noise l (f) of the laser under test is expressed as:
the above equation characterizes the phase noise of the laser at a frequency offset of f. Therefore, the phase noise of the laser to be tested can be obtained according to the voltage fluctuation output by the photoelectric detector.
The invention has the beneficial effects that: the method adopts a mature and stable electro-optical modulator to replace an optical fiber interferometer to realize laser phase noise measurement, utilizes a double-channel differential receiving signal, can improve the stability and the measurement precision of a measurement system and can eliminate partial influence of background noise of the system on a measurement result compared with a traditional measurement scheme.
Drawings
The invention is further illustrated with reference to the following figures and examples.
FIG. 1 is a schematic diagram of a system framework for measuring phase noise of a dual-channel differential laser according to the present invention;
in the drawings:
101. laser 102 to be tested, adjustable optical attenuator 103 and DPSK electro-optical modulator
104. Photodetector 105, low noise amplifier 106, and photodetector
107. Low noise amplifier 108, data acquisition circuit 109 and computer
201. DPSK modulator control circuit
Detailed Description
The present invention will be further illustrated with reference to the accompanying drawings and specific embodiments, which are to be understood as merely illustrative of the invention and not as limiting the scope of the invention.
A method for measuring phase noise of a dual-channel differential laser utilizes a stable electro-optic modulator to replace an optical fiber interferometer to measure the phase noise of the laser and adopts a dual-channel differential receiving signal.
As shown in fig. 1, a hardware platform of the method for measuring the phase noise of the dual-channel differential laser includes a laser 101 to be measured, a tunable optical attenuator 102, a DPSK electro-optic modulator 103, a photodetector 104, a low-noise amplifier 105, a photodetector 106, a low-noise amplifier 107, a data acquisition circuit 108, a computer 109, and a DPSK modulator control circuit 201, where the laser 101 to be measured is turned on, and light emitted by the laser passes through the tunable optical attenuator 102 and then enters the DPSK electro-optic modulator 103 with dual outputs.
The DPSK electro-optical modulator 103 receives signals transmitted by the DPSK modulator control circuit 201 to adjust an optical path difference between two paths of transmission light in the DPSK electro-optical modulator 103, the DPSK modulator control circuit 201 is connected with the computer 109, the computer 109 is connected with the data acquisition circuit 108, the two paths of signal data are obtained by the data acquisition circuit 108 and then are subjected to differential processing, so that the influence of system background noise on a measurement result is eliminated, the background noise mainly comes from a photoelectric detector and a low noise amplifier, the computer 109 transmits the processed signals to the DPSK modulator control circuit 201, two paths of optical signals output by the output end of the DPSK electro-optical modulator 103 have a certain optical delay difference, and the two paths of optical signals generated by the DPSK electro-optical modulator 103 respectively enter the photoelectric detector 104 and the photoelectric detector 106.
The photodetector 104 and the photodetector 106 in the dual-channel differential signal receiving device are located in different transmission channels, the output light of the DPSK electro-optical modulator 103 is converted into an electrical signal after passing through the photodetectors 104 and 106, the electrical signals output by the two photodetectors are amplified by a low-noise amplifier 105 and a low-noise amplifier 107 respectively and then enter a data acquisition circuit 108, and the low-noise amplifier 105 and the low-noise amplifier 107 are located in different transmission channels.
The working principle adopted by the method for measuring the phase noise of the dual-channel laser comprises the following steps:
first, the optical field of the output of a DPSK modulator can be expressed as:
wherein f is the frequency of the light source, Ein(f) Tau is the differential time delay between two paths of signals output by the DPSK modulator for the light field distribution on the incident near detector. The above equation can be approximated as:
where δ f is the laser frequency jitter, which directly translates to the phase noise of the laser. In the above formula
Within the wavelength bandwidth of the laser, the time delay difference of two arms of an interferometer in DPSK or the wavelength of the laser is adjusted to ensure a determined optical frequency f0Is provided with
Accordingly, formula (2) is represented as:
the DPSK photoelectric modulator output signal is converted into an electric signal after passing through a photoelectric detector, and after low-noise amplification, the jitter of the electric signal output by the photoelectric detector, which is introduced due to laser phase noise, is as follows:
Vnoise＝πτIinRκGδf+Vsystem(4)
in the above formula IinFor the light intensity entering the photodetector, R is the detector output load, kappa is the responsivity of the photodetector, G is the amplification factor of the low noise amplifier, and V issystemTo test the noise floor of the system. The dc signal output by the photodetector can be expressed as:
after two paths of signals of the two channels are acquired by the data acquisition circuit, the two paths of signals are subjected to differential processing, and then the background noise V of the test system can be eliminatedsystemThe effect on the measurement results is mainly due to the noise of the photodetector and the low noise amplifier.
Due to the adoption of differential detection, the pi tau I in the formula (4) isinR κ G ═ K, formula (4) is represented as:
Vnoise＝Kδf+Vsystem(6)
obtaining the power spectral density S of the output voltage of the detector by performing Fourier transform on the voltage fluctuation value of the photoelectric detector obtained by converting the voltage fluctuation value of the photoelectric detector obtained by the formula (6) due to the phase noise of the laserν(f) Expressed as:
where B is the signal bandwidth. The phase noise l (f) of the laser under test is expressed as:
the above equation characterizes the phase noise of the laser at a frequency offset of f. Therefore, the phase noise of the laser to be tested can be obtained according to the voltage fluctuation output by the photoelectric detector.
Although the illustrative embodiments of the present invention have been described above to enable those skilled in the art to understand the present invention, the present invention is not limited to the scope of the embodiments, and it is apparent to those skilled in the art that all the inventive concepts using the present invention are protected as long as they can be changed within the spirit and scope of the present invention as defined and defined by the appended claims.
Claims (1)
1. A method for measuring phase noise of a dual-channel differential laser is characterized in that the specific measurement principle is as follows:
light emitted by a laser (101) to be tested enters a double-output DPSK electro-optical modulator (103) after passing through a variable optical attenuator (102), and the optical field output by the DPSK electro-optical modulator (103) can be represented as follows:
wherein f is the frequency of the light source, Ein(f) For the optical field distribution on the incident proximity detector, τ is the differential time delay between two paths of signals output by the DPSK electro-optic modulator, and the above formula can be approximately expressed as:
where δ f is the laser frequency jitter, which directly translates to the phase noise of the laser,
within the wavelength bandwidth of the laser, the time delay difference of two arms of an interferometer in the DPSK electro-optical modulator is adjusted or the wavelength of the laser is adjusted, so that a determined optical frequency f can be obtained0Is provided with
Accordingly, formula (2) is represented as:
the two paths of signals output by the DPSK electro-optical modulator respectively pass through the photoelectric detectors and the low-noise amplifiers in the respective channels in sequence, the signals are converted into electric signals after passing through the photoelectric detectors, and after the low-noise amplifiers, jitter of the electric signals output by the photoelectric detectors, which is caused by laser phase noise, can be expressed as:
Vnoise＝πτIinRκGδf+Vsystem(4)
in the formula IinFor the light intensity entering the photodetector, R is the detector output load, kappa is the responsivity of the photodetector, G is the amplification factor of the low noise amplifier, and V issystemTo test the local noise of the system, the dc signal output by the photodetector is expressed as:
after the two paths of signals are acquired by the data acquisition circuit, the two paths of signals are subjected to differential processing, and then the local noise V of the test system can be eliminatedsystemThe influence on the measurement result is caused by adopting differential detection and making pi tau I in the formula (4)inR κ G ═ K, formula (4) can be represented as:
Vnoise＝Kδf+Vsystem(6)
obtaining the power spectral density S of the output voltage of the detector by performing Fourier transform on the voltage fluctuation value of the photoelectric detector obtained by converting the voltage fluctuation value of the photoelectric detector obtained by the formula (6) due to the phase noise of the laserν(f) Can be expressed as:
where B is the signal bandwidth, the phase noise l (f) of the laser under test can be expressed as:
the above expression represents the phase noise of the laser when the frequency offset is f, so that the phase noise of the laser to be measured can be obtained according to the voltage fluctuation output by the photoelectric detector.
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| CN104682187A (en) * | 2015-03-09 | 2015-06-03 | 北京航空航天大学 | Automatic compensation device of phase noise of Raman laser system based on closed loop feedback and method thereof |
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| CN101909030A (en) * | 2010-08-12 | 2010-12-08 | 武汉光迅科技股份有限公司 | Dual-function optical differential phase shift keying format demodulator |
| CN102833007A (en) * | 2012-08-23 | 2012-12-19 | 诺方（哈尔滨）科技股份有限公司 | Differential phase-shift key-control demodulator |
| CN104682187A (en) * | 2015-03-09 | 2015-06-03 | 北京航空航天大学 | Automatic compensation device of phase noise of Raman laser system based on closed loop feedback and method thereof |
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