CN116184052A - Device and method for measuring relative intensity noise characteristics of extremely low background noise - Google Patents
Device and method for measuring relative intensity noise characteristics of extremely low background noise Download PDFInfo
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Abstract
The device and the method for measuring the relative intensity noise characteristic of extremely low noise floor adopt a high-saturation photoelectric detector and low-thermal noise spectrum analysis to realize the measurement background of-171 dBc/Hz, greatly improve the performance of a test system, solve the interference of a frequency influence curve on a measurement result in an extremely low noise floor test environment, reduce the frequency response fluctuation of the system which is originally +/-2 dB to +/-0.7 dB through data processing, and further reduce the interference of the system frequency response to the measurement result.
Description
Technical Field
The invention relates to the technical field of lasers, in particular to a device and a method for measuring relative intensity noise characteristics of extremely low noise floor.
Background
The narrow linewidth single-frequency laser has very important roles in the fields of coherent optical communication, coherent laser radar, microwave photon, optical fiber sensing and the like, and the intensity noise of the single-frequency laser is an important index for evaluating the performance of the single-frequency laser. The maximum transmission rate of a high-speed communication system is determined by the channel bandwidth and the signal-to-noise ratio, the current optical transmission technology is very close to the shannon limit, the improvement of the SNR is a key factor for improving the transmission rate, and the intensity noise of a light source is a focus of attention. Both the intensity noise in a microwave photonic system and the generated shot noise of the photocurrent may be high enough that excessive thermal noise becomes a major factor limiting system performance.
The relative intensity noise is typically used to characterize the intensity noise, i.e. the fluctuation in optical power, at the laser. The measurement of the relative intensity noise with lower noise floor is of great importance for evaluating and solving the system performance limitation caused by the relative intensity noise, and meanwhile, the accurate evaluation of the relative intensity noise of the single-frequency laser is the basis and premise of improving and optimizing the laser.
One of the prior art: singley J M, diehl J, urick V J. Characation of Lasers for Use in Analog Photonic Links [ J ]. Characterization of lasers for use in analog photonic links,2011. Noise testing was performed on a typical laser applied to an analog photon link, the intensity noise testing system reached the 10 kHz-40 GHz measurement band, -165dBc/Hz measurement background index, and the intensity noise characteristics of different types of typical lasers were presented. In order to meet the requirement of higher intensity noise test, the measurement background of the measurement system needs to be further reduced on the basis of the research.
The second prior art: hong Jun, zhang Songhua, luo Ze, etc. based on measurement of laser relative intensity noise by microwave optical link [ J ]. Chinese science: information science, 2015 (5): 8. An intensity noise measurement method is reported, the relative intensity noise measurement of the frequency band of 1 MHz-8 MHz is realized, the measurement background of-162 dBc/Hz is realized, and the performance of the measurement frequency band and the measurement background is further improved on the basis.
Third prior art: zhang, wei Shanshan, liu, liu Yuanhuang, yao Bo, mao Qing and. Single frequency laser broadband frequency and intensity noise measurement technique [ J ]. Chinese laser, 2021,48 (3): 0301002. An intensity noise test technique of 1mHz-50MHz is reported, a shot noise limit of-154 dBc/Hz is realized, and further improvement on the performance of the measurement frequency band and the measurement background is required on the basis of the research.
Various researches and reports of the intensity noise test build up single-frequency laser relative intensity noise test systems with different performances, test analysis is carried out on noise characteristics of various single-frequency lasers under different frequency bands, and further development is needed in the aspect of considering performance indexes of wide frequency band and extremely low measurement background, so that wider test frequency band and lower measurement background requirements in the fields of high-speed communication, microwave photons and the like can be met.
Disclosure of Invention
In order to overcome the defects of the prior art and lower the noise background, the invention provides a relative intensity noise characteristic measuring device which can be applied to the fields of lasers, coherent optical communication, coherent laser radars, microwave photons, optical fiber sensing and the like.
The technical scheme of the invention is as follows:
in one aspect, the invention provides a relative intensity noise characteristic measuring device with extremely low noise floor, which comprises a photoelectric detector, a spectrum analyzer and a computer, and is characterized by further comprising a bias device and a digital multimeter; the saturated photocurrent of the photodetector is greater than 40mA; the thermal noise of the spectrum analyzer is less than-160 dBm/Hz;
outputting an optical signal by the laser to be tested, inputting the optical signal into the photoelectric detector after passing through an optical fiber jumper to generate an electric signal corresponding to photocurrent, and dividing the electric signal into two paths, namely an alternating current component and a direct current component, through the biaser; the alternating current signal is subjected to spectrum analysis by a spectrum analyzer and is transmitted to a computer, and the direct current signal is detected by a digital multimeter and is transmitted to the computer.
On the other hand, the invention also provides a method for measuring the relative intensity noise characteristic of the laser, which is characterized by comprising the following steps:
s1, obtaining corresponding photocurrent by adopting a photoelectric detector with saturated photocurrent larger than 40mA;
s2, separating an alternating current component and a direct current component through a bias device;
s3, measuring a direct current component by using a digital multimeter, measuring an alternating current component by using a spectrum analyzer with thermal noise smaller than-160 dBm/Hz, and respectively setting a plurality of measuring frequency bands and corresponding resolution bandwidths and video bandwidths by controlling the spectrum analyzer through a computer when measuring the alternating current component;
s4, calculating a relative intensity noise curve according to the received direct current component and alternating current component;
s5, smoothly splicing the acquired relative intensity noise curves, and calibrating the frequency response curve under the test of the extremely low background noise of the frequency band of 1 GHz-40 GHz.
Further, the calibration is specifically that a plurality of lasers are tested for a relative intensity noise curve, and relative intensity noise test result data of the lasers under the condition that shot noise of 1 GHz-40 GHz is limited is collected; and subtracting the intensity noise curve calculated by the shot noise formula from the actual measurement curve to obtain a frequency response curve of the whole test system, and applying the frequency response curve to frequency response calibration during measurement of other lasers.
Compared with the prior art, the invention has the beneficial effects that:
1. by adopting a high-saturation photoelectric detector and low-thermal noise spectrum analysis to realize the measurement background of-171 dBc/Hz, the performance of a test system is greatly improved, and the measurement background of the existing intensity noise test instrument is commonly at-160 dBc/Hz.
2. The interference of the frequency influence curve on the measurement result under the extremely low background noise test environment is solved, the original + -2 dB frequency response fluctuation of the system is reduced to + -0.7 dB through data processing, and the interference of the system frequency response on the measurement result is further reduced.
Drawings
FIG. 1 is a schematic diagram of an embodiment of a device for measuring relative intensity noise characteristics with very low noise floor according to the present invention;
the relative intensity noise spectra of the various lasers of fig. 2;
fig. 3 is a comparison of intensity noise curves before and after frequency response calibration.
Detailed Description
The invention is further illustrated in the following examples and figures, which should not be taken to limit the scope of the invention.
Referring to fig. 1, fig. 1 is a schematic structural diagram of an embodiment of a measuring device for noise characteristics of relative intensity with very low noise floor according to the present invention, as shown in the figure, a high saturation photoelectric detector with 50mW saturated light power and responsivity greater than 0.8A/W is used as a photoelectric conversion device, a maximum of 40mA photocurrent can be generated under saturated input, the shot noise limit reaches-171 dBc/Hz, the photoelectric detector is directly powered by a 12V lithium battery or a dry battery, and a switching power supply or other control circuits should be avoided to avoid noise introduced by the power supply. The 40G bias device and the digital multimeter are connected behind the photoelectric detector to serve as direct current test branches, and meanwhile, the direct current component of the radio frequency signal meets the requirement of DC coupling input of the spectrometer. A spectrum analysis mode with extremely low system noise is used, and an N9040B spectrum analyzer of a keyweight company is used as an analysis instrument, so that the system has a measurement frequency range of 2Hz to 40GHz, a system noise floor of less than-160 dBm/Hz and a resolution bandwidth of 30MHz at maximum, and can meet the requirement of 40G test. The spectrometer can control and analyze parameters such as frequency band, resolution bandwidth and the like in real time through software, meets the requirement of the test system on segmented acquisition under broadband test, and adjusts measurement parameters suitable for the current frequency band. The system noise limit of the spectrometer is less than-171 dBc/Hz under the condition of 40mA photocurrent. The invention can measure the relative intensity noise of the single-frequency laser from 40kHz to 40GHz, the saturated input light power is 50mW, the saturated light current is 40mA, and the measuring limit of-171 dBc/Hz is realized under the condition of saturated input.
The invention has the following working principle:
(1) The single-frequency laser is used as the laser to be measured, the laser to be measured generates photocurrent after passing through the high-saturation photoelectric detector, and the direct-current component is read by the digital multimeter after passing through the biaser;
(2) And a high saturation input photocurrent and high photoelectric conversion efficiency of the high saturation photoelectric detector are utilized to generate a larger photocurrent, so that the shot noise limit is reduced.
The shot noise is taken as white noise, and the current spectral density of mean square noise output by the high-saturation photoelectric detector is
Where e is the charge constant, I dc Is the high saturation photo-detector photo-current. Photocurrent I for a given photodetector dc ，Z out For the output impedance of the detector, the power spectral density of shot noise is calculated as
N shot ＝2eI dc Z out
The characteristics of the laser intensity noise are mainly determined by the laser, and cannot be calculatedThe formula is given directly. In practical application, laser noise is derived from relaxation oscillation, spontaneous emission, mode competition, cavity structure, temperature control state and other factors. The Relative Intensity Noise (RIN) is typically used to characterize the laser intensity noise, which is the signal carried by the laser itself, based on the spectral density of the noise power generated by the laser at the detector output, so the laser intensity noise power N laser The relative intensity noise level of the measurement signal does not change with the change of the photocurrent of the test system in proportion to the square of the photocurrent. From this, the relative intensity noise component RIN of the laser itself can be calculated laser Is that
According to the power spectrum density calculation formula of the shot noise, the intensity noise component RIN of the shot noise can be further calculated shot Is that
According to the formula of the relative intensity noise components of the three noise sources, the RIN component of the shot noise becomes smaller along with the increase of the photocurrent, the shot noise limit of the test system is reduced, and when the photocurrent reaches 40mA, the test system can reach the shot noise limit of-171 dBc/Hz.
The photocurrent signal passes through the bias device and then another path of alternating current signal is output, and the alternating current signal is detected by the low-noise spectrum analyzer. The system noise is the noise floor measured by the test system without input, and consists of thermal noise, 1/f noise and other disturbances. The extremely low noise spectrum analyzer can reduce the system noise limit of the test, and the system noise limit is further reduced under the high saturation current.
The relative intensity noise component RIN of the system noise is calculated in the same way as the shot noise limit calculation sys Is that
In the process of increasing the photocurrent, the system noise limit can be reduced, when a low-noise spectrum analyzer with thermal noise smaller than-160 dBc/Hz is selected, the test system can realize the system noise limit of-171 dBc/Hz under the 40mA photocurrent condition, so that the interference of the system noise to the measuring process is reduced to a great extent, the measuring precision is improved, and the measuring background noise is reduced.
(3) The computer is used for controlling the spectrum analyzer to read noise signals in three frequency bands of 40 kHz-100 MHz,100 MHz-18 GHz and 18 GHz-40 GHz, parameters such as resolution bandwidth and the like in the test are adjusted to control the sweep frequency time, the computer receives data and carries out smooth splicing calibration on the data to form a complete relative intensity noise measurement curve, the test system in the frequency band of 1 GHz-40 GHz has larger interference caused by frequency response, and the test result is calibrated in the stage. Before calibration, a plurality of typical lasers are tested for a relative intensity noise curve, and relative intensity noise test result data of the typical lasers under the condition of 1 GHz-40 GHz shot noise limitation are collected. Since shot noise is white noise, it should be a flat straight line in the test result, regardless of the measurement frequency, and can be calculated by a formula. The intensity noise curve calculated by the shot noise formula is subtracted from the actual measurement curve to obtain the frequency response curve of the whole test system, and the frequency response curve is used for frequency response calibration during measurement of other lasers.
(4) Fig. 2 shows a sample of the test method and the test system for testing a typical laser, which respectively select several types of commercial lasers commonly used in the fields of optical communication, microwave photons and the like to measure the relative intensity noise, wherein the lasers are respectively a planar waveguide grating external cavity semiconductor laser (PWECL) manufactured by RIO corporation of united states, a fiber grating bragg negative feedback ultra-narrow line width fiber laser (DFB FL) manufactured by denmark NKT Photonics corporation, a 1782 distributed feedback semiconductor laser (DFB LD) manufactured by Emcore corporation, and a self-grinding non-planar annular cavity solid laser (NPRO), and can completely and clearly embody the intensity noise characteristics of various lasers in the frequency range of 40kHz to 40 GHz.
(5) FIG. 3 shows the comparison of the intensity noise curves before and after the frequency response calibration of the test system by the test method. The noise floor of the system, a fiber Bragg grating negative feedback ultra-narrow line width fiber laser (DFB FL) of Denmark NKT Photonics company and a self-developed non-planar annular cavity solid laser (NPRO) are selected for relative intensity noise measurement, and the frequency response floating of +/-2 dB introduced by a test system is reduced to +/-0.7 dB, the fluctuation of a test result caused by the frequency response is obviously reduced, and the test result is flatter.
Claims (3)
1. The device for measuring the relative intensity noise characteristics of the extremely low noise floor comprises a photoelectric detector (1), a spectrum analyzer (3) and a computer (5), and is characterized by further comprising a biaser (2) and a digital multimeter (4); the saturated photocurrent of the photodetector (1) is greater than 40mA; the thermal noise of the spectrum analyzer (3) is less than-160 dBm/Hz;
outputting an optical signal by the laser to be tested, inputting the optical signal into the photoelectric detector after passing through an optical fiber jumper to generate an electric signal corresponding to photocurrent, and dividing the electric signal into two paths, namely an alternating current component and a direct current component, through the biaser; the alternating current signal is subjected to spectrum analysis by a spectrum analyzer and is transmitted to a computer, and the direct current signal is detected by a digital multimeter and is transmitted to the computer.
2. The method for measuring the relative intensity noise characteristic of the laser is characterized by comprising the following steps:
s1, obtaining corresponding photocurrent by adopting a photoelectric detector with saturated photocurrent larger than 40mA;
s2, separating an alternating current component and a direct current component through a bias device;
s3, measuring a direct current component by using a digital multimeter, measuring an alternating current component by using a spectrum analyzer with thermal noise smaller than-160 dBm/Hz, and respectively setting a plurality of measuring frequency bands and corresponding resolution bandwidths and video bandwidths by controlling the spectrum analyzer through a computer when measuring the alternating current component;
s4, calculating a relative intensity noise curve according to the received direct current component and alternating current component;
s5, smoothly splicing the acquired relative intensity noise curves, and calibrating the frequency response curve under the test of the extremely low background noise of the frequency band of 1 GHz-40 GHz.
3. The method for measuring the relative intensity noise characteristics of the lasers according to claim 2, wherein the calibration is specifically that a plurality of lasers are tested in a relative intensity noise curve, and the relative intensity noise test result data of the lasers under the condition of 1 GHz-40 GHz shot noise limitation is collected; and subtracting the intensity noise curve calculated by the shot noise formula from the actual measurement curve to obtain a frequency response curve of the whole test system, and applying the frequency response curve to frequency response calibration during measurement of other lasers.
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