Laser linewidth test system integrating long and short delay self-heterodyne beat frequency

Abstract

The utility model discloses a laser line width test system integrating long and short delay self-heterodyne beat frequency in the technical field of laser line width measurement. Comprising a laser, an optical isolator, a first optical coupler, a first variable optical attenuator, a second variable optical attenuator, a third variable optical attenuator, a long delay optical fiber, a frequency shifter, a modulation driver, a short delay optical fiber, a second optical coupler, a photoelectric detector and a spectrum analyzer. According to the invention, two schemes for measuring the laser line width are integrated, the long-delay beat frequency method and the short-delay beat frequency method can be used for measuring the laser line width, and when the laser is extremely narrow and exceeds the measurement limit of the long-delay beat frequency method, the short-delay beat frequency method can be used for measuring the laser line width without replacing a measurement device, so that the measurement efficiency is improved. According to the scheme, tedious operation of replacing the device is reduced, and errors caused by manual replacement of the device can be avoided as much as possible.

Description

Technical Field

[0001] The utility model relates to the technical field of laser line width measurement, in particular to a laser line width test system integrating long and short delay self-heterodyne beat frequency. Background Art

[0002] Narrow linewidth lasers have extremely high spectral purity, extremely high peak spectral density, ultra-long coherence length and extremely low phase noise. Therefore, as core light sources, they have important applications in gravitational wave detection, optical clocks, coherent optical communications, lidar, optical precision measurement and other fields. The performance of the above optical coherence systems, such as their range, accuracy, sensitivity and noise, largely depends on the linewidth of the laser linewidth. Therefore, how to accurately measure the laser linewidth is very important.

[0003] The best commercial spectrometers in the visible and near-infrared bands have a frequency resolution of GHz, and Fabry–Pérot interferometers can only reach MHz. However, laser technology is developing rapidly, and sub-Hz ultra-narrow linewidth lasers have been realized. Traditional measurement methods are no longer suitable for the measurement needs of narrow linewidth lasers. We need more convenient and more accurate methods.

[0004] The above two commercial instruments no longer meet our needs. The fiber self-heterodyne beat frequency method based on Mach-Zehner interferometer is currently the most commonly used method. This method is extremely simple and convenient for suitable lasers, but due to the requirements for delay time, when the line width of the laser to be measured is very narrow (the coherence time of the laser is very long), the required delay fiber length must reach more than 1,000 kilometers, making this solution difficult to implement, and too long delay fiber will introduce 1 / f noise, affecting the measurement accuracy.

[0005] Another method is to replace the long-delay fiber in the above method with a short-delay fiber. At this time, the beat frequency signal spectrum is no longer a Lorentz line type, but a modulated coherent envelope spectrum. Our research found that it has a certain relationship with the laser line width and the length of the delay fiber. We use the characteristic peaks and valleys of the coherent envelope to extract the laser line width.

[0006] Both of the above methods have certain advantages and disadvantages. When using these two methods for linewidth measurement, the measurement system is too large, the delayed optical fiber needs to be replaced, and the power of the delayed light and the frequency-shifted light needs to be adjusted to be equal. In this process, the flanges at the connection of each device need to be adjusted, which affects the measurement accuracy and convenience. Based on this, we propose a laser linewidth test system that integrates long and short delay self-heterodyne beat frequency. Utility Model Content

[0007] The object of the present utility model is to overcome the above problems existing in the prior art, and to provide a laser linewidth measurement system integrating long and short delay self-heterodyne beat frequencies, which is convenient for solving the existing technologies and measurement methods, the cumbersome operation of replacing measurement devices and the measurement errors brought about when replacing devices, and reducing the accuracy of our measurement of the linewidth.

[0008] To achieve the above technical objectives and reach the above technical effects, the present utility model is realized through the following technical solutions:

[0009] A laser linewidth measurement system integrating long and short delay self-heterodyne beat frequencies, comprising a laser, an optical isolator, a first optical coupler, a first variable optical attenuator, a second variable optical attenuator, a third variable optical attenuator, a long delay optical fiber, a frequency shifter, a modulation driver, a short delay optical fiber, a second optical coupler, a photodetector and a spectrum analyzer.

[0010] Preferably, the laser, the optical isolator and the first optical coupler are sequentially connected in order;

[0011] The first output port of the first optical coupler is connected to the first variable optical attenuator, one end of the long delay optical fiber is connected to the first variable optical attenuator, and the other end of the long delay optical fiber is connected to the first input port of the second optical coupler;

[0012] The second output port of the first optical coupler is connected to the second variable optical attenuator and the frequency shifter, and is driven by a modulation driver; the other end of the frequency shifter is connected to the second input port of the second optical coupler;

[0013] The third output port of the first optical coupler is connected to the third variable optical attenuator, one end of the short delay optical fiber is connected to the third variable optical attenuator, and the other end of the short delay optical fiber is connected to the third input port of the second optical coupler;

[0014] The output end of the second optical coupler is connected to the photodetector, and the output end of the photodetector is connected to the spectrum analyzer.

[0015] Preferably, the long delay optical fiber is longer than 10 km.

[0016] Preferably, the short delay optical fiber is shorter than 150 m.

[0017] Preferably, the first optical coupler and the second optical coupler are 1*3 optical couplers, and the splitting ratio is 3:1:1.

[0018] In summary, the present utility model includes at least one of the following beneficial effects:

[0019] The utility model provides a laser linewidth measurement system integrating long and short delay self - heterodyne beat frequencies. The long - delay optical fiber and the short - delay optical fiber are integrated together through a first optical coupler and a second optical coupler. When measuring using long - delay beat frequency, the optical power of the short - delay optical fiber path is attenuated to 0 through a third variable optical attenuator. When measuring using short - delay beat frequency, the optical power of the long - delay optical fiber path is attenuated to 0 through a first variable optical attenuator. When using different measurement methods, there is no need to replace the delay optical fiber. When independently using long - and short - delay beat frequency tests, the first variable optical attenuator and the second variable optical attenuator are used in cooperation, and the third variable optical attenuator and the second variable optical attenuator are used in cooperation for power adjustment, without the need to use a flange for adjustment. Compared with the original technology, the utility model integrates long - and short - delay optical fibers into one body, reducing the cumbersome operation of replacing measurement devices, reducing the errors caused by replacing measurement devices, and the above - mentioned devices use broadband devices, improving the wavelength range of measurable lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a structural block diagram of a laser linewidth measurement system integrating long and short delay self - heterodyne beat frequencies of the present utility model;

[0021] Figure 2 is the power spectral density curve of the long - delay optical fiber measured by a laser linewidth measurement system integrating long and short delay self - heterodyne beat frequencies of the present utility model;

[0022] Figure 3 is the power spectral density curve of the short - delay optical fiber measured by a laser linewidth measurement system integrating long and short delay self - heterodyne beat frequencies of the present utility model;

[0023] In the drawings, the list of components represented by each reference numeral is as follows:

[0024] 1 - Laser, 2 - Optical isolator, 3 - First optical coupler, 4 - First variable optical attenuator, 5 - Second variable optical attenuator, 6 - Third variable optical attenuator, 7 - Long - delay optical fiber, 8 - Frequency shifter, 9 - Short - delay optical fiber, 10 - Second optical coupler, 11 - Photoelectric detector, 12 - Spectrum analyzer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The following further describes the present utility model in detail Figures 1-3 with reference to the accompanying drawings.

[0026] Refer to Figure 1, the present utility model discloses a laser linewidth measurement system integrating long and short delay self-heterodyne beat frequency, which includes a laser 1, an optical isolator 2, a first optical coupler 3, a first variable optical attenuator 4, a second variable optical attenuator 5, a third variable optical attenuator 6, a long delay optical fiber 7 (longer than 10 km), a frequency shifter 8, a modulation driver, a short delay optical fiber 9 (shorter than 150 m), a second optical coupler 10, a photodetector 11 and a spectrum analyzer 12;

[0027] The laser 1, the optical isolator 2 and the first optical coupler 3 are sequentially connected in order;

[0028] The first output port of the first optical coupler 3 is connected to the first variable optical attenuator 4, one end of the long delay optical fiber 7 is connected to the first variable optical attenuator 4, and the other end of the long delay optical fiber 7 is connected to the first input port of the second optical coupler 10;

[0029] The second output port of the first optical coupler 3 is connected to the second variable optical attenuator 5 and the frequency shifter 8, and is driven by the modulation driver; the other end of the frequency shifter 8 is connected to the second input port of the second optical coupler 10;

[0030] The third output port of the first optical coupler 3 is connected to the third variable optical attenuator 6, one end of the short delay optical fiber 9 is connected to the third variable optical attenuator 6, and the other end of the short delay optical fiber 9 is connected to the third input port of the second optical coupler 10;

[0031] The output end of the second optical coupler 10 is connected to the photodetector 11, and the output end of the photodetector 11 is connected to the spectrum analyzer 12;

[0032] The first optical coupler 3 and the second optical coupler 10 are 1*3 optical couplers (the splitting ratio is 3:1:1). The light is split into three paths by the first optical coupler 3 and enters the first variable optical attenuator 4 and the long delay optical fiber 7; the second variable optical attenuator 5 and the frequency shifter 8; the third variable optical attenuator 6 and the short delay optical fiber 9 respectively. The three paths of light perform beat frequency on the second optical coupler 10;

[0033] When measuring the linewidth by the long delay beat frequency method, the third variable optical attenuator 6 (attenuation degree 0.8 - 60 dB) can attenuate the optical power of the short delay optical fiber 9 path to 0; when measuring the linewidth by the short delay beat frequency method, the first variable optical attenuator 4 (attenuation degree 0.8 - 60 dB) can attenuate the optical power of the long delay optical fiber 7 path to 0, realizing independent linewidth measurement by the long delay beat frequency method and the short delay beat frequency method. And in cooperation with the second variable optical attenuator 5 (attenuation degree 0.8 - 60 dB), during the two measurement methods, the optical powers of the two paths of light are balanced, making the test results more accurate.

[0034] Working principle:

[0035] 1. Long-delay self-heterodyne beat frequency method:

[0036] Reference Figure 1 , the laser emitted by laser 1 passes through optical isolator 2 and is split into three paths by first optical coupler 3. The first output end of first optical coupler 3 is connected to first adjustable optical attenuator 4 and then to long-delay optical fiber 7. The second output end is connected to second adjustable optical attenuator 5 and then to frequency shifter 8. The third output end is connected to third adjustable optical attenuator 6 and then to short-delay optical fiber 9. We use third adjustable optical attenuator 6 to attenuate the light passing through short-delay optical fiber 9 to zero, and use first adjustable optical attenuator 4 and second adjustable optical attenuator 5 to balance the light passing through long-delay optical fiber 7 and frequency shifter 8, so that the optical powers of the two paths of light are equal. The light output from long-delay optical fiber 7 and the light output from frequency shifter 8 respectively pass through the first input end and the second input end of second optical coupler 10 to beat on second optical coupler 10, and are output from the output port of second optical coupler 10 and input to photodetector 11. Then the signal of photodetector 11 is output to spectrum analyzer 12, and through the analysis of spectrum analyzer 12 and simple calculation, the linewidth of the laser to be measured can be obtained.

[0037] Based on the theory of long-delay self-heterodyne beat frequency method, the long-delay self-heterodyne beat frequency method can be measured by the above steps. It can be known from the theory that the laser linewidth is proportional to the linewidth output by the spectrum analyzer. The power spectral density curve output by spectrum analyzer 12 is as Figure 2 shown.

[0038] 2. Short-delay self-heterodyne beat frequency method:

[0039] Reference Figure 1 and Figure 3 , the laser emitted by laser 1 passes through optical isolator 2 and is split into three paths by first optical coupler 3. The first output end of first optical coupler 3 is connected to first adjustable optical attenuator 4 and then to long-delay optical fiber 7. The second output end is connected to second adjustable optical attenuator 5 and then to frequency shifter 8. The third output end is connected to third adjustable optical attenuator 6 and then to short-delay optical fiber 9. We use first adjustable optical attenuator 4 to attenuate the light passing through long-delay optical fiber 7 to zero, and use third adjustable optical attenuator 6 and second adjustable optical attenuator 5 to balance the light passing through short-delay optical fiber 9 and frequency shifter 8, so that the optical powers of the two paths of light are equal. The light output from short-delay optical fiber 9 and the light output from frequency shifter 8 respectively pass through the third input end and the second input end of second optical coupler 10 to beat on second optical coupler 10, and are output from the output port of second optical coupler 10 and input to photodetector 11. Then the signal of photodetector 11 is output to spectrum analyzer 12, and the power spectral density curve is obtained through the analysis of spectrum analyzer 12. Through the analysis of the relationship between the power spectral density curve, the optical fiber length, and the linewidth of the laser to be measured, the linewidth of the laser to be measured is obtained by substituting into the calculation formula.

[0040] Based on the theory of short-delay self-heterodyne beat frequency method, the short-delay self-heterodyne beat frequency method can be measured by the above steps, and the line width is measured by the calculation formula used in the theory.

[0041] In summary, a laser line width test system integrating long and short-delay self-heterodyne beat frequencies proposed by the present utility model has a scheme of integrating two methods to measure the laser line width. The long-delay beat frequency method and the short-delay beat frequency method can be used to measure the laser line width. When the line width of our laser is extremely narrow beyond the limit measured by our long-delay beat frequency method, we can use the short-delay beat frequency method to measure the laser line width without replacing the measuring device. This scheme not only reduces the cumbersome operation of replacing devices, but also can avoid the errors introduced by manual replacement of devices as much as possible.

[0042] The above are all the preferred embodiments of the present utility model, and the protection scope of the present utility model is not limited thereby. Therefore, all equivalent changes made according to the structure, shape, and principle of the present utility model shall be covered within the protection scope of the present utility model.

Claims

1. A laser linewidth test system integrating long and short delay self-heterodyne beat frequency, characterized in that: The invention comprises a laser (1), an optical isolator (2), a first optical coupler (3), a first adjustable optical attenuator (4), a second adjustable optical attenuator (5), a third adjustable optical attenuator (6), a long delay optical fiber (7), a frequency shifter (8), a modulation driver, a short delay optical fiber (9), a second optical coupler (10), a photodetector (11) and a spectrum analyzer (12); the laser (1), the optical isolator (2) and the first optical coupler (3) are connected in sequence; The first output port of the first optical coupler (3) is connected to the first adjustable optical attenuator (4), one end of the long-delay optical fiber (7) is connected to the first adjustable optical attenuator (4), and the other end of the long-delay optical fiber (7) is connected to the first input port of the second optical coupler (10); The second output port of the first optical coupler (3) is connected to the second adjustable optical attenuator (5) and the frequency shifter (8), and is driven by a modulation driver; the other end of the frequency shifter (8) is connected to the second input port of the second optical coupler (10); The third output port of the first optical coupler (3) is connected to the third adjustable optical attenuator (6), one end of the short-delay optical fiber (9) is connected to the third adjustable optical attenuator (6), and the other end of the short-delay optical fiber (9) is connected to the third input port of the second optical coupler (10); The output end of the second optical coupler (10) is connected to a photodetector (11), and the output end of the photodetector (11) is connected to a spectrum analyzer (12); The long delay optical fiber and the short delay optical fiber are integrated together by the first optical coupler and the second optical coupler. When the long delay beat frequency is used for measurement, the optical power of the short delay optical fiber is attenuated to 0 by the third adjustable optical attenuator. When the short delay beat frequency is used for measurement, the optical power of the long delay optical fiber is attenuated to 0 by the first adjustable optical attenuator. When different measurement methods are used, the delay optical fiber does not need to be replaced. The short-delay optical fiber (9) is shorter than 150 m; The splitting ratio of the first optical coupler (3) and the second optical coupler (10) is that the long delay splitting ratio is not less than 50%, and the short delay and frequency shifter splitting ratio is not more than 35%.

2. The laser linewidth test system integrating long and short delay self-heterodyne beat frequency according to claim 1, characterized in that: The long delay optical fiber (7) is longer than 10 km.

3. The laser linewidth test system integrating long and short delay self-heterodyne beat frequency according to claim 1, characterized in that: The first optical coupler and the second optical coupler are 1*3 optical couplers, and the splitting ratio is 3:1:

1.

4. The laser linewidth test system integrating long and short delay self-heterodyne beat frequency according to claim 1, characterized in that: The attenuation of the first adjustable optical attenuator (4), the second adjustable optical attenuator (5) and the third adjustable optical attenuator (6) is 0.8-60 dB.

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