All-fiber unmodulated frequency discriminator with long absorption range

Abstract

The application discloses a full-fiber unmodulated frequency discriminator with long absorption spectrum, which is used for obtaining an error electric signal capable of indicating the relative central frequency deviation of the current output frequency of a laser, and can be applied to a laser frequency stabilization system. The frequency discriminator is based on the gas chamber absorption spectrum difference principle, and comprises a laser to be stabilized, a frequency shifter, a gas chamber, a balanced detector and a transmission optical fiber. The frequency discriminator realizes double gas absorption spectrum while saving cost and reducing the system volume through the internal optical structure design. The frequency discriminator has simple structure and is easy to integrate. The process of obtaining the error electric signal does not need modulation and demodulation, and high frequency discrimination sensitivity is realized through long absorption spectrum.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a long absorption range all-fiber modulation-free frequency discriminator for frequency stabilization systems. Background Technology

[0002] Currently, single-frequency lasers are widely used in fields such as laser coherent radar, gravitational wave detection, and laser cooling. The application demands of these fields also place higher requirements on the performance of single-frequency lasers. Therefore, taking artificial measures to obtain high-frequency stable laser output is of great significance for single-frequency lasers. Among these methods, active frequency stabilization based on molecular spectral lines has the advantages of being unaffected by external environmental factors and having high long-term stability. Furthermore, using a gas cell as a reference signal source has the advantages of small size, simple structure, and ease of miniaturization.

[0003] A frequency stabilization system mainly consists of three parts: a frequency discriminator, a controller, and a regulator. The frequency discriminator is the core of the frequency stabilization system, used to obtain an error electrical signal that can identify the magnitude and direction of the deviation of the laser's current output frequency from the center frequency. However, most of the frequency discriminators in commonly used molecular spectral line frequency stabilization systems require modulation of the laser output signal and demodulation of the signal using an electrical phase-locked loop after a photodetector. This makes the system complex and introduces additional noise, reducing the frequency stabilization effect. If a modulation-free frequency stabilization method is used, the commonly used modulation-free frequency stabilization system based on the Zeeman effect and acousto-optic frequency shift is easily affected by environmental interference, which is not conducive to system integration and long-term frequency stability. Summary of the Invention

[0004] This invention addresses the shortcomings and deficiencies of existing technologies by proposing a long-absorption-range, all-fiber modulation-free frequency discriminator for laser frequency stabilization systems. The all-fiber design facilitates system integration and long-term frequency stability. The long-absorption-range gas cell structure achieves a long absorption range without increasing the number of gas cells (saving costs) or increasing the gas cell length (reducing system volume), thereby improving frequency discrimination sensitivity and further enhancing the overall system stabilization performance. Based on the modulation-free frequency stabilization principle of gas cell absorption spectral line shifting and difference, no additional noise is introduced.

[0005] The technical solution of the present invention is as follows:

[0006] 1. A long absorption range all-fiber modulation-free frequency discriminator, characterized in that it comprises a laser to be stabilized (1), a frequency shifter (2), a gas cell assembly (3), and a balance detector (4), with each component connected by optical fibers. The output frequency of the laser to be stabilized (1) exhibits jitter and drift, and the frequency discriminator is used to measure the frequency stability of the laser—converting the frequency fluctuations into fluctuations in an electrical signal that can be used for subsequent processing. The output of the laser to be stabilized (1) is split into an output light and a frequency-stabilized light by a first fiber beam splitter (102) after passing through an isolator (101). The output light is output through an output fiber (103), and the frequency-stabilized light is split into two paths by a second fiber beam splitter (104).

[0007] The frequency shifter (2) is connected to the first output port of the second fiber beam splitter (104) for frequency shifting of the beam;

[0008] The air chamber assembly (3) includes a first fiber optic circulator (301), a second fiber optic circulator (302), a first air chamber (303), and a second air chamber (304), wherein: the first port of the first fiber optic circulator (301) is connected to the output port of the frequency shifter (2), the second port is connected to the first port of the first air chamber (303), and the third port is connected to the first input port of the balanced detector (4); the first port of the second fiber optic circulator (302) is connected to the second output port of the second fiber optic beam splitter (104), the second port is connected to the first port of the second air chamber (304), and the third port is connected to the second input port of the balanced detector (4); the second port of the first air chamber (303) is connected to the second port of the second air chamber (304), thereby forming a bidirectional optical path, so that light can pass through the first air chamber (303) and the second air chamber (304) in sequence to achieve a long absorption range;

[0009] The balanced detector (4) is used to perform photoelectric conversion on the two input lights and to calculate the difference between the obtained electrical signals to generate an error electrical signal. This error electrical signal is used to adjust the output frequency of the laser (1) to be stabilized.

[0010] Among them, the isolator (101), the first fiber optic bundle splitter (102), the second fiber optic bundle splitter (104), the frequency shifter (2), the first fiber optic circulator (301), the second fiber optic circulator (302), the first air chamber (303), and the second air chamber (304) are all fiber optic devices and are connected through the transmission fiber to form an all-fiber integrated structure.

[0011] 2. According to the present invention, a long absorption range all-fiber modulation-free frequency discrimination device is provided, wherein the frequency shifter (2) includes a frequency shifter driver and a signal generator, wherein the frequency shifter driver is connected to the frequency shifter, the signal generator is connected to the frequency shifter driver, the frequency shifter (2) realizes spectral line shifting, and the device for realizing frequency shifting is an electro-optic modulator or an acousto-optic modulator.

[0012] 3. According to the present invention, when a frequency shifter (2) is added to both optical paths after the fiber beam splitter (104), the modulation frequency of the frequency shifter (2) in the two paths should be different to ensure that there is a shift between the absorption spectrum lines of the two signals after passing through the gas cell.

[0013] 4. According to the long absorption range all-fiber modulation-free frequency discrimination device provided by the present invention, when the modulation frequency of the frequency shifter (2) is different, the amplitude and slope of the obtained error signal are different. The modulation frequency can be selected according to the broadening of the selected frequency reference spectrum and the frequency fluctuation of the laser to be stabilized.

[0014] 5. According to the present invention, a long absorption range all-fiber modulation-free frequency discriminator is provided, wherein the first air chamber (303) and the second air chamber (304) provide frequency references.

[0015] 6. According to the present invention, a long absorption range all-fiber modulation-free frequency discriminator is provided, wherein the long absorption range structure of the air cell assembly (3) further includes a bidirectional optical path composed of a first fiber circulator (301), a second fiber circulator (302), a first air cell (303), a second air cell (304), a first fiber Bragg grating (305), and a second fiber Bragg grating (306); the second port of the first air cell (303) is connected to the first fiber Bragg grating (305), and the second port of the second air cell (304) is connected to the second fiber Bragg grating (306); the beam passing through the frequency shifter (2) is input from the first port of the first fiber circulator (301) and output from the second port. One beam passes through the first air chamber (303), is reflected by the first fiber Bragg grating (305), and then passes through the first air chamber (303) again. It is input through the second port and output through the third port of the first fiber circulator (301), finally entering the first input port of the balanced detector (4). The other beam is input through the first port and output through the second port of the second fiber circulator (302), passes through the second air chamber (304), is reflected by the second fiber Bragg grating (306), and then passes through the second air chamber (304) again. It is input through the second port and output through the third port of the second fiber circulator (302), finally entering the second input port of the balanced detector (4). The first fiber Bragg grating (305) and the second fiber Bragg grating (306) serve as beam reflectors.

[0016] Technical effects of the present invention:

[0017] This invention provides a long absorption range all-fiber modulation-free frequency discriminator for laser frequency stabilization systems. The discriminator is used to convert the frequency fluctuations of the laser to be stabilized into an electrical signal that can be used for subsequent processing. After processing, the electrical signal can be used to adjust the output frequency of the laser to be stabilized in real time, thereby achieving laser frequency stabilization.

[0018] This invention is based on the principle of dual-color laser frequency stabilization. The gas cells have different absorption coefficients for beams of different frequencies. A frequency shifter is used to shift the incident light frequency, and the harmonic signal of the absorption curve is obtained by differentially analyzing the output light power, serving as the frequency discrimination signal. Compared to traditional modulation and demodulation frequency stabilization methods, this invention eliminates the need for an electrical phase-locked loop after the photodetector to demodulate the signal, avoiding the introduction of additional laser intensity and frequency noise caused by modulation and demodulation. Furthermore, the two gas cells in this invention achieve a bidirectional optical path internally. Compared to the traditional dual-color laser frequency stabilization method where the beam passes through the gas cells in one direction, this invention achieves twice the light-matter interaction distance without increasing the number of gas cells (saving costs) or increasing the gas cell length (reducing system volume), thus improving frequency stabilization accuracy. In addition, the components of this invention are connected using optical fibers, making the system less susceptible to external interference and easier to integrate.

[0019] The relative advantages of this invention:

[0020] 1. The bidirectional optical path gas cell structure increases the absorption range of the gas for the light beam, improving frequency discrimination sensitivity and frequency stabilization accuracy without increasing the cost of frequency stabilization or the system size. It can also eliminate the impact of the different light beam absorption capabilities of the two gas cells in the traditional unidirectional gas cell scheme.

[0021] 2. The all-fiber frequency discriminator is no longer limited by the direction of the straight-line propagating optical path, making it easier to miniaturize and integrate the whole system, and it is less susceptible to interference from the external environment.

[0022] 3. The dual-color laser frequency stabilization principle eliminates modulation and demodulation, does not introduce additional noise, and can eliminate common-mode noise. Attached Figure Description

[0023] Figure 1 This is a system diagram of the all-fiber frequency discriminator of the present invention;

[0024] Figure 2 The absorption coefficient curve test results obtained by the first bidirectional optical path gas cell structure of the present invention;

[0025] Figure 3 The absorption coefficient curve test results obtained by the second bidirectional optical path gas cell structure of the present invention;

[0026] Figure 4 This is the test result of the frequency discrimination electrical signal obtained by the first bidirectional optical path air cell structure of the present invention;

[0027] Figure 5 This is the test result of the frequency discrimination electrical signal obtained by the second bidirectional optical path air cell structure of the present invention;

[0028] The reference numerals in the attached figures are listed below: 1-Laser to be stabilized; 2-Frequency shifter; 3-Gas cell assembly; 4-Balanced detector; 101-Isolator; 102-First fiber beam splitter; 103-Output fiber; 104-Second fiber beam splitter; 301-First fiber circulator; 302-Second fiber circulator; 303-First gas cell; 304-Second gas cell; 305-First fiber Bragg grating; 306-Second fiber Bragg grating. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0030] like Figure 1 As shown, a long absorption range all-fiber modulation-free frequency discrimination device is provided. The system consists of a laser to be stabilized (1), a frequency shifter (2), a gas cell assembly (3), a balance detector (4), and a transmission fiber. The laser output from the laser to be stabilized (1) is split by the first fiber beam splitter (102) after passing through the isolator (101), and part of it is used as output, while the other part is used for frequency stabilization. The light used for frequency stabilization is split by the second fiber beam splitter (104). The frequency shifter (2) is connected to the first output port of the second fiber beam splitter (104), the input port A of the gas chamber assembly (3) is connected to the frequency shifter (2), and the input port B of the gas chamber assembly (3) is connected to the second output port of the second fiber beam splitter (104). The balance detector (4) has two input ports, which are respectively connected to the two output ports of the gas chamber assembly (3). It performs photoelectric conversion on the two input lights and calculates the difference between the obtained electrical signals to obtain an error electrical signal that can indicate the deviation of the current output frequency of the laser from the center frequency. This error electrical signal can be used to adjust the output frequency of the laser to be stabilized (1) after subsequent processing to achieve laser frequency stabilization.

[0031] Specifically, the bandwidth and response band requirements of each device in the system must be matched.

[0032] Specifically, the frequency shifter (2) includes a frequency shifter driver and a signal generator.

[0033] Specifically, a frequency shifter (2) is used to shift the spectral lines. If both paths pass through the frequency shifter, the modulation frequencies of the two paths must be different, thereby shifting the absorption spectral lines of the gas cell.

[0034] Specifically, the modulation frequency of the frequency shifter (2) should be selected based on the spectral broadening of the selected reference and the frequency fluctuation of the laser to be stabilized.

[0035] Specifically, the first gas chamber (303) and the second gas chamber (304) selected are required to have rich spectral lines in the laser output band.

[0036] Specifically, both the first fiber optic circulator (301) and the second fiber optic circulator (302) are unidirectional devices. Based on the first fiber optic circulator (301), the second fiber optic circulator (302) and the transmission fiber can realize a bidirectional optical path in the air cell. The specific optical path direction of the optimized optical path structure is as follows: Figure 1 As shown in the first structure, the light beam passing through the frequency shifter (2) is input from the first port and output from the second port of the first fiber optic circulator (301), passes sequentially through the first gas chamber (303) and the second gas chamber (304), is input from the second port and output from the third port of the second fiber optic circulator (302), and finally enters the second input port of the balanced detector (4); another light beam is input from the first port and output from the second port of the second fiber optic circulator (302), passes sequentially through the first gas chamber (304) and the second gas chamber (303), is input from the second port and output from the third port of the first fiber optic circulator (301), and finally enters the first input port of the balanced detector (4). In this way, both light beams achieve twice the light-matter interaction distance, i.e., twice the gas absorption range, as is the case with the unoptimized light beam. Figure 2 The figure shows the absorption coefficient curve measured based on the hydrogen cyanide absorption spectrum using this bidirectional optical path structure, compared with the absorption coefficient curve of the traditional two-color laser frequency stabilization method using a unidirectional gas cell structure (without optimized structure).

[0037] Specifically, the first fiber Bragg grating (305) and the second fiber Bragg grating (306) are both reflective devices. Based on the first fiber circulator (301), the second fiber circulator (302), the first fiber Bragg grating (305), the second fiber Bragg grating (306), and the transmission fiber, a bidirectional optical path can be realized in the air cell. The specific optical path direction of the optimized optical path structure is as follows: Figure 1 As shown in the second structure, the light beam passing through the frequency shifter (2) is input from the first port and output from the second port of the first fiber optic circulator (301), passes through the first gas chamber (303), is reflected by the first fiber Bragg grating (305), passes through the first gas chamber (303) again, and is input from the second port and output from the third port of the first fiber optic circulator (301), finally entering the first input port of the balanced detector (4); another light beam is input from the first port and output from the second port of the second fiber optic circulator (302), passes through the second gas chamber (304), is reflected by the second fiber Bragg grating (306), passes through the second gas chamber (304) again, and is input from the second port and output from the third port of the second fiber optic circulator (302), finally entering the second input port of the balanced detector (4). In this way, both light beams achieve twice the light-matter interaction distance, i.e., twice the gas absorption range, as is the case with the unoptimized light beam. Figure 3 The figure shows the absorption coefficient curve measured based on the hydrogen cyanide absorption spectrum using this bidirectional optical path structure, compared with the absorption coefficient curve of the traditional two-color laser frequency stabilization method using a unidirectional gas cell structure (without optimized structure).

[0038] Specifically, because the fiber optic air cell has the characteristics of high pressure and small beam diameter, the bidirectional passage of the beam will not lead to saturation absorption. That is, this method can improve the frequency discrimination sensitivity while still maintaining a large frequency stability range.

[0039] like Figure 4 The diagram shows the frequency discrimination signal measured based on the hydrogen cyanide absorption spectrum using the first bidirectional optical path structure of the system of the present invention, compared with the frequency discrimination signal under the unidirectional gas cell structure in the traditional two-color laser frequency stabilization method.

[0040] like Figure 5 The diagram shows the frequency discrimination signal measured based on the hydrogen cyanide absorption spectrum using the second bidirectional optical path structure of the system of the present invention, compared with the frequency discrimination signal under the unidirectional gas cell structure in the traditional two-color laser frequency stabilization method.

[0041] In this embodiment of the invention, the P9 line (1549.73051nm) of the hydrogen cyanide molecular gas cell is selected as the frequency reference, based on the criterion of being as close as possible to the output wavelength (1550nm) of the laser to be stabilized.

[0042] In this embodiment of the invention, an acousto-optic modulator with a modulation frequency of 100MHz is selected as the frequency shifter, based on the criterion that the obtained stable frequency range is greater than the frequency fluctuation of the laser to be stabilized.

[0043] In this embodiment of the invention, a frequency shifter is used in only one path, which can be used to lock the laser output frequency on one side of the absorption spectrum and reduce costs.

[0044] In this embodiment of the invention, the expression for the obtained frequency discrimination electrical signal is: ,in To balance the sensitivity coefficient of the detector's photoelectric conversion, The amplitude of the electrical signal for frequency-stabilized light. This represents the gas cell absorption coefficient corresponding to the frequency of the laser output optical signal. The gas cell absorption coefficient corresponding to the frequency of the laser output optical signal after frequency shifting;

[0045] The long absorption range all-fiber modulation-free frequency discrimination device provided in this invention has the advantages of high frequency discrimination sensitivity, no modulation, and easy miniaturization. It makes up for the shortcomings of the current frequency discrimination devices used in laser frequency stabilization systems and has broad application prospects.

[0046] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Any modification or substitution of some of the technologies therein will not cause the essence of the corresponding technical solutions to deviate from the protection scope of the present invention.

Claims

1. A long absorption range all-fiber modulation-free frequency discriminator, characterized in that, It includes a laser to be stabilized (1), a frequency shifter (2), a gas chamber assembly (3), a balance detector (4), and a transmission optical fiber; The output of the laser to be stabilized (1) is split into output light and frequency-stabilized light by the first fiber beam splitter (102) after passing through the isolator (101). The output light is output by the output fiber (103), and the frequency-stabilized light is split into two paths by the second fiber beam splitter (104). The frequency shifter (2) is connected to the first output port of the second fiber beam splitter (104) and is used to shift the frequency of the beam. The gas chamber assembly (3) includes a first fiber circulator (301), a second fiber circulator (302), a first gas chamber (303), and a second gas chamber (304), wherein: The first port of the first fiber optic circulator (301) is connected to the output port of the frequency shifter (2), the second port is connected to the first port of the first air chamber (303), and the third port is connected to the first input port of the balance detector (4). The first port of the second fiber optic circulator (302) is connected to the second output port of the second fiber optic beam splitter (104), the second port is connected to the first port of the second air chamber (304), and the third port is connected to the second input port of the balance detector (4). The second port of the first air chamber (303) is connected to the second port of the second air chamber (304) to form a bidirectional optical path, so that light can pass through the first air chamber (303) and the second air chamber (304) in sequence to achieve a long absorption range; The balanced detector (4) is used to perform photoelectric conversion on the two input lights and to calculate the difference between the obtained electrical signals to generate an error electrical signal. This error electrical signal is used to adjust the output frequency of the laser (1) to be stabilized. Among them, the isolator (101), the first fiber optic bundle splitter (102), the second fiber optic bundle splitter (104), the frequency shifter (2), the first fiber optic circulator (301), the second fiber optic circulator (302), the first air chamber (303), and the second air chamber (304) are all fiber optic devices and are connected through the transmission fiber to form an all-fiber integrated structure.

2. The frequency discrimination device as described in claim 1, characterized in that, The frequency shifter (2) includes a frequency shifter driver and a signal generator. The frequency shifter shifts the absorption spectral lines of the gas cell by shifting the frequency of the light beam. The device that achieves the frequency shift is an electro-optic modulator or an acousto-optic modulator.

3. The frequency discrimination device as described in claim 1, characterized in that, The absorption capacity of the material inside the first gas chamber (303) and the second gas chamber (304) for light entering the gas chamber is related to the frequency of the incident light and can be used as a frequency reference.

4. The frequency discrimination device as described in claim 1, characterized in that, The long absorption path structure of the air cell assembly (3) also includes a bidirectional optical path composed of a first fiber circulator (301), a second fiber circulator (302), a first air cell (303), a second air cell (304), a first fiber Bragg grating (305), and a second fiber Bragg grating (306). The second port of the first air chamber (303) is connected to the first fiber Bragg grating (305), and the second port of the second air chamber (304) is connected to the second fiber Bragg grating (306); The light beam passing through the frequency shifter (2) is input from the first port and output from the second port of the first fiber optic circulator (301), passes through the first air chamber (303), is reflected by the first fiber Bragg grating (305), passes through the first air chamber (303) again, and is input from the second port and output from the third port of the first fiber optic circulator (301), and finally enters the first input port of the balanced detector (4). Another path of light is input from the first port and output from the second port of the second fiber optic circulator (302), passes through the second air chamber (304), is reflected by the second fiber Bragg grating (306), passes through the second air chamber (304) again, and is input from the second port and output from the third port of the second fiber optic circulator (302), and finally enters the second input port of the balanced detector (4); The first fiber Bragg grating (305) and the second fiber Bragg grating (306) serve to reflect the light beam.

5. The frequency discrimination device as described in claim 1, characterized in that, The frequency shifter (2) is added only to the first optical path after the second fiber beam splitter (104), or simultaneously to both optical paths after the second fiber beam splitter (104).

6. The frequency discrimination device as described in claim 1, characterized in that, The balanced detector (4) realizes photoelectric conversion, signal subtraction and error signal output.

7. A long absorption range all-fiber modulation-free frequency discriminator, characterized in that, Includes the frequency discrimination device as described in any one of claims 1-6.

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