CN111693255A - Device and method for measuring frequency drift of laser light source - Google Patents
Device and method for measuring frequency drift of laser light source Download PDFInfo
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Abstract
The invention discloses a device and a method for measuring frequency drift of a laser light source, wherein the device comprises a semi-reflecting and semi-transmitting mirror, a frequency divider and a frequency divider, wherein the semi-reflecting and semi-transmitting mirror is used for uniformly dividing laser to be measured output by the laser light source to be measured into two beams; a signal generator for generating a sine wave of a predetermined frequency and amplitude; the frequency drift measuring circuit comprises a scanning confocal cavity F-P interferometer and a first photoelectric detector, wherein the scanning confocal cavity F-P interferometer receives a beam of sine wave generated by a light and signal generator to obtain a multi-beam interference signal, and the first photoelectric detector detects the multi-beam interference signal to obtain a transmission signal of the multi-beam interference signal; the optical power compensation circuit comprises a second photoelectric detector and a second optical power compensation circuit, wherein the second photoelectric detector is used for receiving another beam of light and detecting to obtain a light intensity signal of the beam of light; and the digital signal processing system is used for receiving the sine wave, the transmission signal and the light intensity signal and obtaining the frequency drift amount of the laser light source by using a digital phase-locked amplification demodulation method. The method has high measurement precision, is simple and reliable, and can be widely applied to quantitative evaluation and qualitative analysis of the narrow-linewidth laser light source.
Description
Technical Field
The invention relates to the photoelectron technology, in particular to a device and a method for measuring frequency drift of a laser light source.
Background
The narrow linewidth laser has the excellent characteristics of long coherence length, narrow linewidth, good beam quality and the like, and plays an increasingly important role in the fields of laser interferometry, laser communication, laser radar, laser remote sensing, resonant fiber optic gyroscope and the like. In many application fields, laser light not only requires a single frequency output, but also requires a high stability of the laser output frequency. However, the long-term frequency stability of the narrow linewidth laser is generally not good, because the narrow linewidth laser is easily affected by various external factors (such as temperature, etc.), the output frequency of the narrow linewidth laser changes along with the working time, and this phenomenon is called the output frequency drift of the laser. In actual work, the drift of the laser frequency can have a great influence on the result, for example, in precision interferometry, the laser wavelength is used as a measuring 'scale', the wavelength changes, physical quantities such as length, angle, displacement and the like measured according to the optical interference principle all change, and the accuracy of the laser frequency directly influences the measurement precision. In a resonant fiber optic gyroscope system, when signal processing is performed, the frequency of a laser needs to be locked to the resonant frequency of one of the loops, and at this time, the frequency of the laser drifts, which directly affects the stability and the final frequency locking precision of the whole loop, resulting in output signal deviation.
The currently common measuring methods for the output frequency drift of the laser mainly include a direct measuring method, a double-beam beat frequency method, a single-beam self-heterodyne method, a frequency standard reference method and the like. Direct measurement methods are divided into two categories, one is to directly measure the output frequency of a laser by using a spectrum analyzer, and the measurement accuracy is limited by the frequency resolution of the spectrum analyzer. The other type is that the wavelength change is measured by a wavelength meter to obtain the frequency drift, and the existing products on the market, such as HP86120B, HP86120C and other optical wavelength meters of Agilent company, have the measurement accuracy of +/-0.3 pm, are only suitable for the situation that the laser frequency stability is low, and have a slow measurement speed. The double-beam beat frequency method is one of the most common methods in engineering, and is to beat frequency between the laser to be measured and the reference light source with high stability to obtain the frequency of the light to be measured by measuring the beat frequency. The single-beam self-heterodyne method is mostly to split the laser to be measured and measure the frequency drift characteristic of the laser by using the principle of optical fiber delay self-timing, and compared with the single-beam self-heterodyne method, the single-beam self-heterodyne method is simple in system and easy to implement, but in order to achieve sufficiently high measurement accuracy, the method usually needs to adopt an optical fiber delay line of hundreds of meters or even kilometers, and an overlong optical fiber can bring loss and instability of a laser signal and is also easily influenced by an external environment. The frequency standard reference method is a hot spot of current research, and the principle is to use a standard reference tool to obtain a scale for measuring the frequency. For example, the frequency drift is measured by using the optical frequency comb principle, but it is very difficult to achieve high measurement accuracy by using the method, and the higher the measurement accuracy is, the higher the requirements on the instrument are, and the more complicated the measurement system is.
In addition, a method of measuring a frequency drift using a fabry-perot (F-P) interferometer as a frequency reference has been receiving more and more attention. The Shanghai technical and physical research institute of the Chinese academy of sciences in 2014, Liuhao and the like disclose a method (CN 103855599A) for realizing laser frequency drift measurement and locking by utilizing a scanning confocal cavity F-P interferometer, the method utilizes a time measurement technology to measure the frequency change of a slave laser by measuring the time difference change of output laser of a master laser and the slave laser in the scanning F-P cavity, and obviously the resolution and the precision of the method are limited by the resolution and the precision of a time measurement chip.
Disclosure of Invention
The embodiment of the invention aims to provide a device and a method for measuring frequency drift of a laser light source, and aims to solve the problems that in the prior art, when the frequency drift of a laser is accurately measured, a required device is complex and measuring conditions are harsh.
In order to achieve the above purpose, the technical solution adopted by the embodiment of the present invention is as follows:
in a first aspect, an embodiment of the present invention provides a device for measuring frequency drift of a laser light source, which is used for measuring frequency drift of the laser light source to be measured, and includes:
the semi-reflecting and semi-transmitting lens is used for uniformly dividing the laser to be detected output by the laser light source to be detected into two beams of light;
a signal generator for generating a sine wave of a predetermined frequency and amplitude;
the frequency drift measuring circuit comprises a scanning confocal cavity F-P interferometer and a first photoelectric detector, wherein the scanning confocal cavity F-P interferometer receives one beam of light and the sine wave generated by the signal generator to obtain a multi-beam interference signal, and the first photoelectric detector detects the multi-beam interference signal to obtain a transmission signal;
the optical power compensation circuit comprises a second photoelectric detector and a second optical power compensation circuit, wherein the second photoelectric detector is used for receiving another beam of light and detecting to obtain a light intensity signal of the beam of light;
and the digital signal processing system is used for receiving the sine wave, the transmission signal and the light intensity signal and obtaining the frequency drift amount of the laser light source by using a digital phase-locked amplification demodulation method.
Further, the frequency drift measurement path further comprises a shaping lens, and the shaping lens is arranged on an optical axis between the semi-reflecting and semi-transparent mirror and the scanning confocal cavity F-P interferometer.
Furthermore, the output end of the signal generator is connected with a piezoelectric ceramic ring attached to the scanning confocal cavity F-P interferometer.
Furthermore, the scanning confocal cavity F-P interferometer is a confocal cavity F-P interferometer with the cavity length controlled by a piezoelectric ceramic ring attached to the end face of the cavity.
Further, the signal generator is a sine wave generator.
Furthermore, the digital signal processing system comprises a first AD converter, a second AD converter, a third AD converter and a digital signal processing chip, wherein the output end of the frequency drift measuring circuit is connected with the signal input end of the digital signal processing chip through the first AD converter, the output end of the optical power compensating circuit is connected with the signal input end of the digital signal processing chip through the second AD converter, and one output end of the signal generator is connected with the signal input end of the digital signal processing chip through the third AD converter.
In a second aspect, an embodiment of the present invention further provides a method for measuring a frequency drift of a laser light source, including:
acquiring laser to be detected output by a laser light source to be detected;
dividing laser to be detected into two beams, shaping one beam, carrying out phase modulation on the shaped beam by a scanning confocal F-P cavity interferometer applying sinusoidal modulation, and detecting a transmission light signal of the scanning confocal F-P cavity interferometer in real time by a first photoelectric detector;
detecting the other beam of light by a second photoelectric detector to obtain a light intensity signal of the beam of light for subsequent light power compensation;
receiving the transmitted light signal and the light intensity signal, converting the transmitted light signal and the light intensity signal into a digital signal, dividing the light intensity signal by the transmitted light signal, and performing digital phase-locked amplification and demodulation by combining with a sine modulation signal applied to a scanning confocal F-P cavity interferometer to obtain a demodulation result Vout；
According to the demodulation result VoutAnd finally, calculating the frequency drift of the light source according to the obtained demodulation result by the linear relation of the light source frequency drift delta f.
Further, the demodulation result VoutThe mathematical model of the linear relationship with the source frequency drift Δ f is as follows:
wherein:
wherein VoutRepresenting the phase-locked output signal, G representing the resonant transmission, F representing the definition, U representing the amplitude of the sinusoidal modulation signal, NADRepresenting the D/A conversion coefficient, D1、D2Respectively representing the conversion coefficient between the input light intensity and the output voltage of the first and second photodetectors, f0Representing the laser center frequency, c representing the speed of light in vacuum, f representing the exiting laser frequency, n representing the refractive index of the medium in the confocal cavity, L being the fixed cavity length of the interferometer after the sinusoidal modulation is applied, and Δ L being the maximum variation of the cavity length. J. the design is a square1(2πf04n Δ L/c) denotes a first order Bessel function at 2 π f 04 n.DELTA.L/c, g(1)(f0) Representing g (f) function at f0First derivative of (c), g (f)0) The expression function is at f0The value of (c).
According to the technical scheme, the device is simple, a high-stability reference light source is not needed, the scanning confocal cavity F-P interferometer and the digital phase-locked amplification demodulation method are combined to measure the frequency drift, and the limit of the resolution and the precision of time domain measurement is broken through. In addition, through the optical power compensation circuit, the measurement drift problem caused by unstable power of the light source is eliminated. The invention can be used for the output characteristic evaluation of the narrow linewidth laser. The invention overcomes the defects of high stability of a reference light source, harsh measurement conditions, complex device, high cost and the like in the prior art for measuring the frequency drift of the laser, and can be used for testing and screening the laser light source in the system application with high frequency stability requirement.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention. In the drawings:
fig. 1 is a schematic structural diagram of a device for measuring frequency drift of a laser light source according to an embodiment of the present invention;
fig. 2 is a schematic diagram of the overall construction of a measuring device for frequency drift of a laser light source provided by the embodiment of the invention;
fig. 3 is a schematic flowchart of a method for measuring frequency drift of a laser light source according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of the linear variation interval in the sine curve employed in the present embodiment;
FIG. 5 is a graph of the relationship between the laser frequency drift and the phase-locked demodulation output in the present embodiment;
FIG. 6 is a measurement result of frequency shift of the laser source in the case that the power of the laser source is unchanged with time and the frequency shifts with time in the present embodiment;
FIG. 7 is a measurement result of frequency shift of the laser source in the case where the power of the laser source is shifted with time while the frequency of the laser source is unchanged with time in the present embodiment;
fig. 8 shows the measurement result of the frequency shift of the laser light source in the case where the power and the frequency of the light source are shifted with time in this embodiment.
Detailed Description
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
It should be noted that the terms "first," "second," and the like in the description and claims of this application and in the drawings are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the application described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
Referring to fig. 1, the present embodiment provides a device for measuring frequency drift of a laser light source, which is used to measure frequency drift of the laser light source 1 to be measured, and includes:
the semi-reflecting and semi-transmitting lens 2 is used for uniformly dividing the laser to be detected output by the laser light source 1 to be detected into two beams of light 31 and 41;
a signal generator 6 for generating a sine wave of a predetermined frequency and amplitude;
the frequency drift measuring circuit 3 comprises a scanning confocal cavity F-P interferometer 34 and a first photoelectric detector 37, wherein the scanning confocal cavity F-P interferometer 34 receives one beam of light 31 and the sine wave generated by the signal generator 6 to obtain a multi-beam interference signal, and the first photoelectric detector 37 detects a transmission signal of the multi-beam interference signal;
the optical power compensation circuit 4 comprises a second photodetector 42 for receiving another beam of light 41 and detecting to obtain a light intensity signal of the beam;
and the digital signal processing system 5 is used for receiving the sine wave, the transmission signal and the light intensity signal and obtaining the frequency drift amount of the laser light source by using a digital phase-locked amplification demodulation method.
In this embodiment, the frequency drift measurement path 3 further includes a shaping lens 32, and the shaping lens 32 is disposed on the optical axis between the half-reflecting and half-transmitting mirror 2 and the scanning confocal cavity F-P interferometer 34. The light beam 31 split by the half-reflecting and half-transmitting mirror 2 is shaped by the shaping lens 32, the shaped light beam 33 is injected into the scanning confocal cavity F-P interferometer 34 to generate multi-beam interference, a transmission signal 36 of the multi-beam interference is received by the first photoelectric detector 37, and the output end of the first photoelectric detector 37 is directly connected with the input end of the digital signal processing system 5.
In this embodiment, the output end of the signal generator 6 is connected to a piezoelectric ceramic ring 35 attached to the scanning confocal cavity F-P interferometer 34. The scanning confocal cavity F-P interferometer 34 here is a confocal cavity F-P interferometer whose cavity length is controlled by a piezoelectric ceramic ring 35 attached to the end face of the cavity.
In this embodiment, the signal generator 6 is a sine wave generator. The output end of the signal generator 6 is connected with a piezoelectric ceramic ring 35 attached to the scanning confocal cavity F-P interferometer 34, and the other output end of the signal generator 6 is connected with the other input end of the digital signal processing system 5.
In this embodiment, the optical power compensation circuit 4 includes a second photodetector 42. The light beam 41 split by the half-reflecting and half-transmitting mirror 2 is directly received by the second photodetector 42, and the output of the second photodetector 42 is output as the optical power compensation path 4.
In this embodiment, the digital signal processing system 5 includes a first AD converter 51, a second AD converter 52, a third AD converter 53 and a digital signal processing chip 54, the output end of the frequency drift measuring circuit 3 is connected to the signal input end of the digital signal processing chip 54 through the first AD converter 51, the output end of the optical power compensating circuit 4 is connected to the signal input end of the digital signal processing chip 54 through the second AD converter 52, and one output end of the signal generator 6 is connected to the signal input end of the digital signal processing chip 54 through the third AD converter 53.
The embodiment also provides a method for measuring frequency drift of a laser light source, which includes:
acquiring laser to be detected output by a laser light source 1 to be detected;
dividing laser to be detected into two beams, shaping one beam 31, carrying out phase modulation on the shaped beam by a scanning confocal F-P cavity interferometer 34 applying sinusoidal modulation, and detecting a transmission light signal of the scanning confocal F-P cavity interferometer 34 in real time by a first photoelectric detector 37;
the second photodetector 42 detects another light beam 41 to obtain a light intensity signal of the light beam for subsequent optical power compensation;
receiving the transmitted light signal and the light intensity signal, converting the transmitted light signal and the light intensity signal into a digital signal, dividing the light intensity signal by the transmitted light signal, and performing digital phase-locked amplification and demodulation by combining with the sine modulation signal applied to the scanning confocal F-P cavity interferometer 34 to obtain a demodulation result Vout；
According toDemodulation result VoutAnd finally, calculating the frequency drift of the light source according to the obtained demodulation result by the linear relation of the light source frequency drift delta f.
The sine modulation is applied by generating a sine signal with a preset amplitude and a preset modulation frequency by the signal generator 6, and applying the sine signal to the piezoelectric ceramic ring 35 of the scanning confocal cavity F-P interferometer 34 to realize that the cavity length of the confocal cavity F-P interferometer changes according to the sine at the modulation frequency.
The demodulation result VoutThe mathematical model of the linear relationship with the source frequency drift Δ f is as follows:
wherein:
wherein VoutRepresenting the phase-locked output signal, G representing the resonant transmission, F representing the definition, U representing the amplitude of the sinusoidal modulation signal, NADRepresenting the D/A conversion coefficient, D1、D2Respectively representing the conversion coefficient between the input light intensity and the output voltage of the first and second photodetectors (37, 42), f0Representing the laser center frequency, c representing the speed of light in vacuum, f representing the exiting laser frequency, n representing the refractive index of the medium in the confocal cavity, L being the fixed cavity length of the interferometer after the sinusoidal modulation is applied, and Δ L being the maximum variation of the cavity length. J. the design is a square1(2πf04n Δ L/c) denotes a first order Bessel function at 2 π f 04 n.DELTA.L/c, g(1)(f0) Representing g (f) function at f0First derivative of (c), g (f)0) The expression function is at f0The value of (c).
The following describes a specific principle of measuring the frequency drift of the laser light source:
the laser source 1 to be measured outputs laser, and the central wavelength of the laser is set to be lambda. The light beam 31 is shaped by the lens 32 and injected into the scanning confocal cavity F-P interferometer 34, and at this time, the signal generator 6 outputs a specific sinusoidal signal, as shown in the following formula (2), for modulating the cavity length of the scanning confocal cavity F-P interferometer 34.
Vsin＝U·sin(ωt)+B (2)
VsinRepresenting the output signal of the signal generator 6, U representing the sine wave amplitude and B representing the dc offset.
The transmitted light intensity I of the confocal cavity F-P interferometer with the cavity length modulated by sine1Can be expressed as:
in the above formula, G represents resonance transmittance, and F represents resolution, both being constant. I is0Is half of the light intensity of the light source, omega is the sinusoidal modulation frequency, and t is the scanning time.
In the formula (3), z is,
are variables related to the output light frequency f of the laser, and are expressed as follows:
in the formula (4), c represents the speed of light in vacuum, n represents the refractive index of the medium in the confocal cavity, and L0Denotes the initial cavity length, d, of the scanning confocal cavity F-P interferometer 3433Represents the piezoelectric coefficient of the piezoelectric ceramic. It is believed that L represents the fixed cavity length of the scanning confocal cavity F-P interferometer 34 and Δ L represents the maximum change in cavity length when sinusoidal modulation is applied, where Δ L is required to avoid interference signal overlap during a scan cycle<λ/4。
According to the nature of trigonometric function, the transmitted light intensity I of the interferometer1The expressions may be organized as:
the light intensity I is detected in real time by the first photodetector 371The digital signal processing chip 54 utilizes the first AD converter 51 to process the real time signalThe output signal of the first photodetector 37 is collected, and the collection result can be expressed as:
wherein Vd1Representing the collected output signal, D, of the first photodetector 371Represents a conversion coefficient, N, between the input light intensity and the output voltage of the first photodetector 37ADRepresenting digital to analog conversion coefficients.
In contrast, the other light beam 41 is directly detected by the second photodetector 42, and the digital signal processing chip 54 uses the second AD converter 52 to acquire the response signal of the second photodetector 42 in real time, where the acquisition result can be expressed as:
Vd2＝NAD·D2·I0(7)
wherein Vd2Representing the collected output signal, D, of the second photodetector 422Representing the conversion factor between the input light intensity and the output voltage of the second photodetector 42.
In addition, the digital signal processing chip 54 also acquires the output signal of the signal generator 6 in real time by using the third AD converter 53:
Vd3＝NAD·U·sin(ωt)+NAD·B (8)
Vd3representing the output signal of the signal generator 6 as collected. Signal V to be acquiredd2Divided by the acquired signal Vd1To obtain the light intensity of elimination I0Signal V ofd4Therefore, the influence of the fluctuation of the power of the light source on the measurement result is eliminated. Vd4Can be expressed as:
the equation (9) is developed by using a Bessel function to obtain:
wherein m is an integer. Filter out Vd4Performing phase-locked demodulation after the medium-direct-current component, and outputting a modulation signal V by combining with the signal generator 6 acquired by the third AD converter 53 during demodulationd3Synchronous detection is performed, so that only sin (ω t) term in equation (10) is detected, and the final demodulation output result can be expressed as:
wherein VoutRepresenting co-frequency phase-locked output signals, J1(z) denotes the value of the first order Bessel function at z.
In this case, if the frequency of the laser light source shifts, a variable z relating to the laser output light frequency f,
will change as shown in the following formula:
for narrow linewidth lasers, the frequency drift Δ f is typically on the order of tens of megabits (10)6) Hertz. Δ L is nano (10)-9) Step constant, L is mm (10)-3) Stage constant, c 3 × 108m/s, it is therefore known that Δ z is of the order of 10-10For such small variations, J can be approximated1(z)≈J1(z0) Remain unchanged. While
Of the order of 10-4The variation is relatively large and is not negligible. By proper selection according to the nature of the sine function
Is of such a size that
Size and of
Is linear, i.e. is linear with the change in af. Thereby outputting signal V with same frequency phase lockoutAnd also linearly with the change in af. The linear relationship can be described by the following mathematical model:
in formula (13), g(1)(f0) Representing the function g (f) at the centre frequency f0First derivative of (c), g (f)0) The representing function being at the centre frequency f0The value of (c). As can be seen from equation (13), the phase-locked demodulation result is only linear with the frequency drift Δ f of the light source and independent of the optical power. And in formula (13) (D)2·F·U·NAD)/(2·D1·G)，J1(2πf0·4nΔL/c)，g(1)(f0)，g(f0) Are all constant, and therefore use the demodulated output VoutThe frequency drift quantity delta f of the laser light source can be accurately calculated, and a specific calculation model is shown as a formula (1).
The following is further illustrated by specific examples:
the measuring device for the frequency drift of the laser light source is shown in fig. 1, and the specific device selection and parameters are as follows:
1) laser light source 1 to be measured: the working wavelength is 1550nm, and the emergent power is 1 mW;
2) and (3) semi-reflecting and semi-transmitting mirror 2: the splitting ratio is 1: 1;
3) shaping lens 32: a biconvex lens, the wavelength range of 1050 nm-1700 nm, the wavelength reflectivity at 1550nm is lower than 0.1%;
4) scanning confocal cavity F-P instrument 34: the fineness is 250%, the cavity length is 50mm, and the reflectivity at the working 1550nm wavelength is more than 99%;
5) the first and second photodetectors 37 and 42 are self-made photoelectric detection modules in a laboratory, photosensitive materials of the photoelectric probes are all InGaAs, the light detection range is 1100-1700 nm, the responsivity is greater than 0.85, and the gain of a built-in amplifying circuit can be adjusted;
6) the signal generator 6 adopts an arbitrary function signal generator;
7) the digital signal processing system 5 adopts a self-made FPGA system board, wherein the digital processing chip 54 adopts an FPGA chip, the circuit board is further integrated with three AD converters 51, 52 and 53, and other electronic components are not described in more detail.
Referring to fig. 1 and 2, the overall construction process of the measuring device is as follows:
1) according to the step 601, fixing the laser light source 1 to be detected on an experiment table;
2) according to the step 602, a semi-reflecting and semi-transmitting lens 2 is arranged at the output end of the laser light source 1 to be detected, wherein an included angle of 45 degrees is formed between the output light propagation direction and the output light propagation direction, so that laser is divided into two uniform beams;
3) according to step 603, a shaping lens 32 is installed in the propagation direction of one of the beams of light 31 to shape the beam of light;
4) injecting the shaped beam 33 into the scanning confocal cavity F-P interferometer 34, according to step 604;
5) mounting a first photodetector 37 at the output of the scanning confocal cavity F-P interferometer 34 for receiving the interferometer transmission signal 36, according to step 605;
6) according to step 606, the output of the first photodetector 37 is connected to an input of the digital signal processing chip 54 via the first AD converter 51;
7) according to step 607, one output of the signal generator 6 is connected to the piezoelectric ceramic ring 35 on the scanning confocal cavity F-P interferometer 34;
8) according to step 608, the other output terminal of the signal generator 6 is connected to one input terminal of the digital signal processing chip 54 via the third AD converter 53;
9) according to step 609, a second photodetector 42 is installed in the propagation direction of the other beam of light 41 split by the half-reflecting and half-transmitting mirror 2 for directly measuring the power change of the light source;
10) according to step 610, the output terminal of the second photodetector 42 is connected to another input terminal of the digital signal processing chip 54 via the second AD converter 52;
11) as shown in step 611, the digital signal processing chip 54 processes the received data of the first AD converter 51, the second AD converter 52, and the second AD converter 53 in real time, and records and displays the measurement result.
The invention also discloses a method for measuring the frequency drift of the laser light source, which is shown in figure 3 and specifically comprises the following operations:
1) according to step 71, the laser light source 1 to be detected is turned on, and the laser light to be detected is injected into a subsequent device, as shown in fig. 1, the laser light to be detected is divided into two uniform beams by the half-reflecting and half-transmitting mirror 2, wherein one beam 41 is directly received by the second photodetector 42 for compensation of subsequent optical power. The other beam of light 31 is shaped by a shaping lens 32 and injected into a scanning confocal cavity F-P interferometer 34. At this time, the signal generator 6 generates a modulated sine wave, the output of the modulated sine wave is connected to the piezoelectric ceramic ring 35 on the scanning confocal cavity F-P interferometer 34, the cavity length of the scanning confocal cavity F-P interferometer 34 is subjected to sine modulation, and a transmission light intensity signal of the scanning confocal cavity F-P interferometer 34 is detected in real time through the first photoelectric detector 37 and used for subsequently calculating the frequency drift delta F of the laser light source.
2) According to step 72, the digital signal processing chip 54 synchronously receives the data collected by the first, second and third AD converters 51, 52 and 53. The received data are respectively represented as Vd1、Vd2、Vd3The formulae (6), (7) and (8).
3) According to step 73, the digital processing chip 54 receives the data Vd2、Vd1Is divided to obtain a signal Vd4As shown in equation (9), the measurement drift effect caused by the optical power fluctuation is eliminated.
4) According to step 74, the digital signal processing chip 54 processes the signal Vd4Performing phase-locked demodulation by combining the output signal V of the signal generator 6 collected by the third AD converter 53d3Synchronous detection is carried out, therefore Vd4Only sin (ω t) term is detected, and the final demodulation result is shown as a formula (11). From the foregoing theoretical analysis, the demodulation result can be described by the linear relationship shown in equation (1).
5) According to step 75, the digital signal processing chip 54 utilizes the linear relationship of equation (1) as defined byDemodulation output VoutThe frequency drift amount delta f of the laser source can be accurately calculated.
The values of the individual variables in the model of formula (13) are obtained from the parameters of the measuring device. Wherein the central wavelength lambda of the emergent laser is 1550nm, the power of the light source is 1mW, and the conversion coefficient between the input light intensity and the output voltage of the two photodetectors satisfies (D)2)/(2·D1) 1, digital-to-analog conversion factor NAD＝212And 3.3, the refractive index n of the medium in the confocal cavity is 1, the confocal cavity definition F is 250, and the initial cavity length L of the confocal cavity050mm, resonant transmittance G0.2525. Sinusoidal modulation frequency fm200Hz, 6V, and d is satisfied by selecting the sine wave DC bias33B287.6 nm, so that
k is a positive integer, which is such that g (f)0)＝sin[(2πf0/c)·4nL]0, eliminating the intercept in formula (13), and making g(1)(f0) And the measurement sensitivity is improved to the maximum extent. As can be seen from FIG. 4, the magnitude of g (f) is proportional to the change in Δ f. The maximum change in cavity length Δ L is 115nm (less than λ/4 387.5nm), and z can be calculated0＝(2πf0C). 4n Δ L ≈ 1.8647. The resulting expression is:
Vout≈-0.01797·Δf (14)
wherein VoutRepresenting the same frequency phase lock output signal, and Δ f is in Hz. In order to verify the correctness of the formula (14), the above parameters are utilized to perform simulation based on the formula (11), and the simulation result is shown in fig. 5, and the result is basically consistent with the formula (14) after linear fitting. Therefore, the signal V is outputted by the same-frequency phase-locked in the formula (14)outCan be used to characterize the laser source frequency Δ f as shown in the following equation:
the measuring device of the invention can be used for carrying out simulation test on the frequency drift of the laser light source under the dynamic condition. A typical frequency drift of the laser light source is shown in fig. 6 (b), in which the abscissa is the laser usage time in hours (h) and the ordinate is the laser frequency drift amount in MHz, and (a) in fig. 6 is the variation of the laser light source power with time, in which the abscissa is the laser usage time in hours (h) and the ordinate is the laser output power in mW. Fig. 6 (c) is a graph showing the frequency drift of the laser measured by the method of the present invention, wherein the abscissa is the laser usage time in hours (h) and the ordinate is the frequency drift measurement in MHz. At the moment, the output power of the laser does not drift greatly, under the simulation condition, the measurement result is basically consistent with the frequency drift change of the laser, and the short-time random fluctuation and the long-time drift of the frequency of the laser can be well measured.
If the laser output power drifts with time, as shown in fig. 7 (a), fig. 7 (b) shows that the laser light source frequency does not change greatly with time, under this simulation condition, the light source frequency drift measurement result shown in fig. 7 (c) is obtained, and it can be seen that the frequency drift measurement result matches with the laser light source frequency drift change, and no measurement error is generated along with the fluctuation or drift of the light source power.
If the output power and the frequency of the laser drift with time, as shown in (a) and (b) of fig. 8, under the simulation condition, the light source frequency drift measurement result as shown in (c) of fig. 8 is obtained, it can be seen that the frequency drift measurement result is substantially consistent with the frequency drift change of the laser, and the drift of the laser frequency can still be accurately measured, which indicates that the fluctuation or the drift of the light source power does not affect the measurement result.
After the frequency drift amount delta f is obtained, the digital signal processing chip 54 records, displays or transmits the result to the upper computer, and the purpose of accurately measuring the laser frequency drift amount for a long time can be realized by utilizing the advantage that the upper computer stores and displays data in time. V in formula (15)outThe frequency drift measuring precision of the laser light source can reach 100 Hz.
The invention provides a narrow linewidth laser frequency drift measuring device and method based on a sine modulation scanning confocal cavity F-P interferometer. The invention overcomes the defects of the prior art that the measurement of the frequency drift of the laser needs a high-stability reference light source, the measurement condition is harsh, the device is complex, the cost is high and the like. The scanning confocal cavity F-P interferometer is combined with a digital phase-locked amplification demodulation method, the first harmonic component of the modulation frequency in the acquired signal is effectively extracted, and the frequency drift of the laser light source can be effectively calculated by utilizing the size of the first harmonic component of the modulation frequency in the acquired signal, so that frequency domain measurement is realized, the limitations of resolution and precision in the traditional time domain measurement method are broken through, and the measurement precision can reach 100 Hz. In addition, the invention also introduces an optical power compensation path, eliminates the measurement drift problem caused by the unstable power of the laser light source, and greatly improves the stability and the accuracy of the measurement. The invention has strong practicability and can be widely applied to laser light sources in system application for testing and screening high frequency stability requirements, such as a resonant fiber optic gyroscope system.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A measuring device for laser light source frequency drift is used for measuring the light source frequency drift of a laser light source (1) to be measured, and is characterized by comprising the following components:
the semi-reflecting and semi-transmitting lens (2) is used for uniformly dividing the laser to be detected output by the laser light source (1) to be detected into two beams of light (31, 41);
a signal generator (6) for generating a sine wave of a predetermined frequency and amplitude;
the frequency drift measuring circuit (3) comprises a scanning confocal cavity F-P interferometer (34) and a first photoelectric detector (37), the scanning confocal cavity F-P interferometer (34) receives one beam of light (31) and the sine wave generated by the signal generator (6) to obtain a multi-beam interference signal, and the first photoelectric detector (37) detects a transmission signal of the multi-beam interference signal;
the optical power compensation circuit (4) comprises a second photoelectric detector (42) for receiving another beam of light (41) and detecting to obtain a light intensity signal of the beam of light;
and the digital signal processing system (5) is used for receiving the sine wave, the transmission signal and the light intensity signal and obtaining the frequency drift amount of the laser light source by using a digital phase-locked amplification demodulation method.
2. The device for measuring the frequency drift of the laser light source according to claim 1, wherein: the frequency drift measurement path (3) further comprises a shaping lens (32), and the shaping lens (32) is arranged on an optical axis between the semi-reflecting and semi-transparent mirror (2) and the scanning confocal cavity F-P interferometer (34).
3. The device for measuring the frequency drift of the laser light source according to claim 1, wherein: the output end of the signal generator (6) is connected with a piezoelectric ceramic ring (35) attached to a scanning confocal cavity F-P interferometer (34).
4. The device for measuring the frequency drift of the laser light source according to claim 1, wherein: the scanning confocal cavity F-P interferometer (34) is a confocal cavity F-P interferometer with the cavity length controlled by a piezoelectric ceramic ring (35) attached to the end face of the cavity.
5. The device for measuring the frequency drift of the laser light source according to claim 1, wherein: the signal generator (6) is a sine wave generator.
6. The device for measuring the frequency drift of the laser light source according to claim 1, wherein: the digital signal processing system (5) comprises a first AD converter (51), a second AD converter (52), a third AD converter (53) and a digital signal processing chip (54), wherein the output end of the frequency drift measuring circuit (3) is connected with the signal input end of the digital signal processing chip (54) through the first AD converter (51), the output end of the optical power compensating circuit (4) is connected with the signal input end of the digital signal processing chip (54) through the second AD converter (52), and one output end of the signal generator (6) is connected with the signal input end of the digital signal processing chip (54) through the third AD converter (53).
7. A method for measuring frequency drift of a laser light source is characterized by comprising the following steps:
acquiring laser to be detected output by a laser light source (1) to be detected;
dividing laser to be detected into two beams, shaping one beam of light (31), carrying out phase modulation on the shaped beam of light by a scanning confocal F-P cavity interferometer (34) applying sinusoidal modulation, and detecting a transmission light signal of the scanning confocal F-P cavity interferometer (34) in real time by a first photoelectric detector (37);
detecting another beam of light (41) by a second photoelectric detector (42) to obtain a light intensity signal of the beam of light for subsequent optical power compensation;
receiving the transmitted light signal and the light intensity signal, converting the transmitted light signal and the light intensity signal into a digital signal, dividing the light intensity signal by the transmitted light signal, and performing digital phase-locked amplification demodulation by combining with a sine modulation signal applied to a scanning confocal F-P cavity interferometer (34) to obtain a demodulation result Vout；
According to the demodulation result VoutAnd finally, calculating the frequency drift of the light source according to the obtained demodulation result by the linear relation of the light source frequency drift delta f.
8. The method for measuring the frequency drift of the laser light source according to claim 7, wherein: the demodulation result VoutThe mathematical model of the linear relationship with the source frequency drift Δ f is as follows:
wherein VoutRepresenting the phase-locked output signal, G representing the resonant transmission, F representing the definition, U representing the amplitude of the sinusoidal modulation signal, NADRepresenting the D/A conversion coefficient, D1、D2Respectively representing the conversion coefficient between the input light intensity and the output voltage of the first and second photodetectors (37, 42), f0Representing the laser center frequency, c representing the speed of light in vacuum, f representing the exiting laser frequency, n representing the refractive index of the medium in the confocal cavity, L being the fixed cavity length of the interferometer after the sinusoidal modulation is applied, and Δ L being the maximum variation of the cavity length. J. the design is a square1(2πf04n Δ L/c) denotes a first order Bessel function at 2 π f04 n.DELTA.L/c, g(1)(f0) Representing g (f) function at f0First derivative of (c), g (f)0) The expression function is at f0The value of (c).
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