CN-121977450-A - High-accuracy narrow linewidth laser linewidth measurement system

CN121977450ACN 121977450 ACN121977450 ACN 121977450ACN-121977450-A

Abstract

The invention provides a high-accuracy narrow linewidth laser linewidth measuring system, which can directly quantify 1/f noise of a delay optical fiber in a traditional delay self-heterodyne linewidth measuring device, compares thermal noise of delay optical fibers with different lengths by changing lengths of two arms of the optical fiber of an interferometer under the condition that noise influence of the laser is identical, obtains normalized instantaneous phase noise of the laser by using a short optical fiber interferometer, and obtains integral linewidth by carrying out signal autocorrelation and wiener Xin Qin theorem, thereby avoiding linewidth broadening caused by the thermal noise of the optical fiber introduced by long optical fiber in the traditional delay self-heterodyne linewidth measuring method, solving the problems of affected measured differential phase noise, integral linewidth broadening and inconsistent measuring results of the optical fibers with different lengths, and further improving linewidth measuring accuracy of the narrow linewidth laser.

Inventors

GAO CHUNQING
JIANG ZIHAN
ZHANG YIWEI
WANG HAO
WANG YUHONG

Assignees

北京理工大学

Dates

Publication Date: 20260505
Application Date: 20260202

Claims (5)

1. The high-accuracy narrow linewidth laser linewidth measurement system is characterized by comprising a laser (1) to be measured, a first optical fiber coupler (2), an acousto-optic modulator (3), a first delay optical fiber (4), a second delay optical fiber (5), a second optical fiber coupler (6), a coherent detection system (7), a data acquisition module (8) and a data processing module (9), wherein the lengths of the first delay optical fiber (4) and the second delay optical fiber (5) are adjustable, and the optical path difference between the first delay optical fiber (4) and the second delay optical fiber (5) is kept unchanged; The laser signal output by the laser to be tested (1) is incident to a first optical fiber coupler (2), one output end of the first optical fiber coupler (2) is connected with a first delay optical fiber (4) through an acousto-optic modulator (3), the other output end of the first optical fiber coupler is directly connected with a second delay optical fiber (5), output optical signals of the first delay optical fiber (4) and the second delay optical fiber (5) are combined through a second optical fiber coupler (6) and then enter a coherent detection system (7) to be converted into an electric signal, the electric signal is forwarded to a data processing module (9) through a data acquisition module (8) to be demodulated, the sum of differential phase noise of the laser and the optical fiber or the normalized instantaneous differential phase noise power spectrum density of the laser to be tested (1) is obtained, and the data processing module (9) obtains the integral linewidth of the laser to be tested (1) according to the normalized instantaneous differential phase noise of the laser to be tested (1).
2. A high accuracy narrow linewidth laser linewidth measurement system as in claim 1 wherein when the length of the first delay fiber (4) is different from the length of the second delay fiber (5), the data processing module (9) directly demodulates the electrical signal to obtain the sum of the differential phase noise of all delay fibers and the differential phase noise accumulated by the laser itself in the difference between the lengths of the two delay fibers.
3. A high accuracy narrow linewidth laser linewidth measurement system as in claim 1 wherein the data processing module (9) demodulates and normalizes the electrical signal to a normalized instantaneous phase noise power spectral density of the laser under test (1) itself when the first delay fiber (4) length is 0 and the second delay fiber (5) length is a short fiber no greater than 100 m.
4. The high-accuracy narrow linewidth laser linewidth measurement system according to claim 1, wherein the method for obtaining the power spectral density of the normalized instantaneous differential phase noise of the laser (1) to be measured comprises the following steps: The length of the first delay optical fiber (4) is adjusted to 0, the length of the second delay optical fiber (5) is set to L, the length of L does not influence the measurement result, and L is not more than 100m, and the differential phase noise accumulated in the optical path difference of the two delay optical fibers of the laser (1) per se to be measured, the power spectral density of the sum of the thermal noise of the first delay optical fiber (4) and the thermal noise of the second delay optical fiber (5) can be obtained by directly demodulating in the data processing module (9) ; And then to the differential phase noise power spectral density Performing normalization operation according to the following formula to obtain the power spectral density of the normalized instantaneous differential phase noise : Wherein, the For the operating frequency of the laser (1) to be measured, For measuring the time of passage of time light through a second delay fiber (5) of length L, The light propagation time corresponding to the 1m optical path is set.
5. A high accuracy narrow linewidth laser linewidth measurement system as claimed in claim 1 wherein the data processing module (9) obtains the integrated linewidth of the laser (1) to be measured according to the normalized instantaneous differential phase noise of the laser (1) to be measured by: According to the normalized instantaneous differential phase noise power spectral density of the laser (1) itself to be measured Acquiring an autocorrelation function of an optical signal output by the second optical fiber coupler (6) : Wherein, the For the operating frequency of the laser (1) to be measured, Is the light splitting ratio of the first delay optical fiber (4) and the second delay optical fiber (5), Is the offset of the acousto-optic modulator (3), For the amplitude of the laser signal output by the laser (1) to be measured, Is the corresponding delay time of the second delay optical fiber (5), A time parameter customized for the autocorrelation function; for autocorrelation function And performing fast Fourier transform to obtain the integral linewidth of the laser (1) to be tested.

Description

High-accuracy narrow linewidth laser linewidth measurement system Technical Field The invention belongs to the technical field of lasers, and particularly relates to a high-accuracy narrow linewidth laser linewidth measuring system. Background Linewidth is a key parameter that characterizes the coherence of a narrow linewidth laser. The linewidth of a laser is often represented by an intrinsic linewidth, which represents the linewidth of the laser due to spontaneous radiation, and an integral linewidth, which accounts for such factors as laser cavity variations (e.g., cavity vibration, temperature variations, etc.). In coherent detection applications, the integrated linewidth determines parameters such as resolution and accuracy of the coherent detection system, and thus the integrated linewidth of the laser is generally more of a concern in applications. Currently, the most common line width measurement method for integrated line width is the delay self heterodyne method. For example, the line width measuring device disclosed in chinese patent CN112432767a is a typical delay self-heterodyne measuring system, one arm of the device is an acousto-optic frequency shift arm, and the other arm is an optical fiber delay arm, and the basic principle of the method is that one path of optical signal output by a laser is delayed by a long delay optical fiber (usually needs to be delayed to be above the coherence time), and beat frequency is carried out on the optical signal and the other path of reference optical signal frequency-shifted by an acousto-optic frequency shifter, and the laser line width is estimated by analyzing the frequency spectrum of the generated radio frequency signal. However, this classical delayed self heterodyne method has its inherent limitation that the length of the delay fiber must be greater than the coherence length of the laser, typically requiring the use of fiber lengths of tens of km (long fibers) for narrow linewidth lasers with linewidths on the order of kHz. The long optical fiber can generate remarkable 1/f frequency noise due to thermodynamic effects such as ambient temperature fluctuation. The noise introduced by the measuring system can be superimposed on the intrinsic noise of the laser to be measured, so that the linewidth spectrum line measured finally is widened, and the measuring result deviates from the actual linewidth value. In order to avoid noise introduced by long optical fibers, a measurement scheme of short optical fiber delay is also commonly adopted for linewidth measurement of ultra-narrow linewidth lasers at present. For example, chinese patent CN105571830B discloses a method for measuring linewidth of ultra-narrow linewidth laser. The method is also based on a self heterodyne detection system, and only uses short optical fibers to replace long optical fibers, so that the noise problem caused by the long optical fibers is avoided to a certain extent. However, this short fiber method can only reflect its intrinsic linewidth, and cannot obtain an integrated linewidth containing all frequency noise components, which is more important in practical applications. Disclosure of Invention In order to solve the problems, the invention provides a method and a system for measuring the line width of a narrow line width laser with higher measurement accuracy, which can eliminate the influence of optical fiber thermal noise on a measurement structure, thereby improving the accuracy of measuring the line width of the narrow line width laser. The high-accuracy narrow linewidth laser linewidth measurement system comprises a laser (1) to be measured, a first optical fiber coupler (2), an acousto-optic modulator (3), a first delay optical fiber (4), a second delay optical fiber (5), a second optical fiber coupler (6), a coherent detection system (7), a data acquisition module (8) and a data processing module (9), wherein the lengths of the first delay optical fiber (4) and the second delay optical fiber (5) are adjustable, and the optical path difference between the first delay optical fiber (4) and the second delay optical fiber (5) is kept unchanged; The laser signal output by the laser to be tested (1) is incident to a first optical fiber coupler (2), one output end of the first optical fiber coupler (2) is connected with a first delay optical fiber (4) through an acousto-optic modulator (3), the other output end of the first optical fiber coupler is directly connected with a second delay optical fiber (5), output optical signals of the first delay optical fiber (4) and the second delay optical fiber (5) are combined through a second optical fiber coupler (6) and then enter a coherent detection system (7) to be converted into an electric signal, the electric signal is forwarded to a data processing module (9) through a data acquisition module (8) to be demodulated, the sum of differential phase noise of the laser and the optical fiber or the normalized instantaneous different