Modulated transfer frequency stabilization optical path device

Abstract

This invention discloses a high signal-to-noise ratio modulation transfer frequency stabilization optical path device, including a light source and a frequency stabilization optical path. The light source adopts an internally adjustable fiber collimator, with a 45° reflector behind it. The frequency stabilization optical path behind the 45° reflector includes a half-wave plate and a polarizing beam splitter (PBS). An atomic gas cell and two pyramidal reflectors are sequentially arranged on the probe light path of the PBS. A half-wave plate, a 45° reflector, an electro-optic modulator, another 45° reflector, and a half-wave plate are sequentially arranged on the pump light path. The polarizing beam splitter (PBS) is located at the convergence point of the two optical paths, and a 45° reflector, a frequency stabilization optical path lens, and a photodetector are sequentially arranged behind it. This invention cleverly utilizes a folded optical path design and fine-tunes the beam divergence angle to increase the length and area of ​​the laser-atomic interaction without increasing the system volume. At the same time, it actively controls the splitting ratio of the two frequency-stabilized beams, improving the stability of the frequency stabilization.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically relating to a modulation transfer frequency stabilization optical path device. Background Technology

[0002] In free-running conditions, laser performance is susceptible to external environmental factors such as temperature, air pressure, vibration, and noise. These factors can cause changes in the geometric length of the resonant cavity and the refractive index of the cavity medium, leading to frequency jitter and affecting the laser's stability, making it difficult to meet the precision requirements of specific applications. To ensure the stability of the laser wavelength, frequency stabilization technology becomes essential. The core of this technology lies in locking the laser's output frequency to a stable reference frequency, typically a selected atomic or molecular transition line.

[0003] Among numerous frequency stabilization techniques, including atomic bidirectional colorimetric stabilization (DAVLL), polarization spectroscopy, frequency modulation, Pound-Drever-Hall (PDH), saturated absorption spectroscopy, and modulation transfer spectroscopy (MTS), all have wide applications. In particular, modulation transfer spectroscopy (MTS) demonstrates unique advantages in providing frequency stabilization solutions. MTS can produce an effect similar to four-wave mixing, effectively transferring the modulation information of the pump light to the probe beam. This information is then captured by a photodetector and used to generate the desired error signal during demodulation. Because the modulation transfer process occurs only between atoms or molecules where the Doppler velocity is zero, the demodulated error signal does not contain Doppler noise, thus reducing the additional noise that might be introduced by directly modulating the laser. Due to its high sensitivity, high resolution, high error signal slope, and low sensitivity to temperature and power fluctuations, MTS has become an ideal choice for laser frequency stabilization.

[0004] In cold atom measurement techniques, the modulation-transfer frequency-stabilized optical path plays a crucial role. To achieve a higher error signal-to-noise ratio, current techniques typically involve increasing the beam diameter and extending the length of the atomic gas cell. However, these methods face several technical challenges.

[0005] In particular, due to the aperture limitations of acousto-optic modulators and electro-optic modulators, we can often only increase the beam diameter by setting up beam-expanding optical systems at both ends of the atomic gas cell. While this can improve frequency stabilization performance, it also increases the complexity of the frequency stabilization optical path and places higher demands on the precise alignment of the optical axis. Increasing the length of the atomic gas cell also limits the possibility of miniaturizing the frequency stabilization optical path.

[0006] In addition, the power ratio between the two laser beams in modulation transfer frequency stabilization has a significant impact on the error signal. However, due to the unstable polarization of the input light source, the splitting ratio of the laser power is prone to drift. This drift limits the long-term stability of modulation transfer frequency stabilization, and the parameters need to be readjusted after working for a period of time in order to continue locking.

[0007] These technical challenges urgently need to be addressed in order to promote the further development and widespread application of modulation-transfer frequency stabilization technology in cold atom measurement. Summary of the Invention

[0008] The purpose of this invention is to provide a modulation transfer frequency stabilization optical path device with significant advantages, addressing the shortcomings of existing technologies.

[0009] To achieve the above objectives, the technical solution adopted by the present invention to solve its technical problem is: a modulation transfer frequency stabilization optical path device, including a light source part and a frequency stabilization optical path, etc.; the light source part adopts an optical fiber collimator whose internal lens position can be finely adjusted, and a 45° reflector is arranged on the optical path behind the optical fiber collimator. Behind the 45° reflector are two sets of 1 / 2 waveplates and a polarizing beam splitter prism PBS, one set located in the light source part and the other set in the frequency stabilization optical path. The 1 / 2 waveplate of the frequency stabilization optical path behind the 45° reflector is mounted on a rotary motor. An atomic gas cell and two pyramidal reflectors are arranged sequentially on the probe light path led out by the polarizing beam splitter PBS. The polarizing beam splitter PBS is positioned in close proximity to the atomic gas cell and the pyramidal reflectors. The system is equipped with a power detector (PD). The frequency-stabilized optical path behind the 45° reflector includes a half-wave plate and a polarizing beam splitter (PBS). The probe light path led out by the polarizing beam splitter (PBS) has an atomic gas cell and two pyramidal reflectors arranged in sequence. The pump light path led out by the polarizing beam splitter (PBS) has a half-wave plate, a 45° reflector, an electro-optic modulator, another 45° reflector, and another half-wave plate arranged in sequence. The polarizing beam splitter (PBS) is located at the convergence point of the two optical paths. Behind the polarizing beam splitter (PBS), a 45° reflector, a frequency-stabilized optical path lens, and a photodetector are arranged in sequence. The PD is a photodetector and also includes a half-wave plate and a polarizing beam splitter (PBS) arranged in front of or behind the 45° reflector.

[0010] The modulation transfer frequency stabilization optical path device further includes a frequency shifting optical path disposed between the light source and the frequency stabilization optical path. The frequency shifting optical path adopts a common acousto-optic modulator double-pass optical path device, including a 1 / 2 wave plate II, a polarizing beam splitter PBS II, an acousto-optic modulator, a 1 / 4 wave plate, a frequency shifting optical path lens, and a 0-degree reflector arranged in sequence, so that the light beam passes through the acousto-optic modulator twice to obtain frequency shift, and then enters the atomic gas cell and the electro-optic modulator.

[0011] The aforementioned modulation-transfer frequency-stabilized optical path device has an acousto-optic modulator and a 0-degree reflector whose distances from the frequency-shifting optical path lens are close to the focal length of the frequency-shifting optical path lens.

[0012] The modulation transfer frequency stabilization optical path device described above uses a common alkali metal atomic gas cell, such as a rubidium atomic gas cell, with antireflection coatings on both surfaces of the gas cell and a window tilt angle of 2 to 3°.

[0013] The modulation transfer frequency stabilization optical path device described above has an electro-optic modulator (32) that is a phase modulator with a resonant frequency of about 10MHz.

[0014] The beneficial effects of this invention are as follows: By employing a slightly divergent quasi-parallel beam, this invention significantly expands the interaction area between the beam and atoms, thereby significantly improving the robustness of the frequency stabilization system. Furthermore, this invention ingeniously utilizes a folded optical path design, which not only doubles the beam's effective length but also improves frequency stability without increasing system volume. In addition, this invention improves the long-term stability of modulation transfer frequency stabilization through feedback control of the splitting ratio. These innovative improvements have practical value for enhancing the robustness of frequency stabilization optical paths, promoting integrated design, and applications in precision measurement. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of the first embodiment of the present invention;

[0016] Figure 2 This is a schematic diagram of the structure of the second embodiment of the present invention.

[0017] The labels in the attached figures are as follows: 1—Light source, 11—Fiber optic collimator, 12—Internal lens, 2—Frequency shifting optical path, 21—Acousto-optic modulator, 22—1 / 4 wave plate, 23—Frequency shifting optical path lens, 24—0-degree reflector, 3—Frequency stabilizing optical path, 31—Atomic gas cell, 32—Electro-optic modulator, 33—Frequency stabilizing optical path lens, 34—Photodetector, 35—Power detector PD, 36—Rotating motor, 41—1 / 2 wave plate one, 42—1 / 2 wave plate two, 51—Polarization beam splitter prism PBS one, 52—Polarization beam splitter prism PBS two, 6—45° reflector, 7—Corner cone reflector. Detailed Implementation

[0018] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that the disclosure of this application will be more thorough and complete. Example 1

[0019] like Figure 1 As shown, this embodiment of a modulation transfer frequency stabilization optical path device includes a light source section, a frequency stabilization optical path, etc. In this embodiment of the invention, the light source section 1 adopts an optical fiber collimator 11 whose front and rear positions can be finely adjusted with an internal lens 12. A 45° reflector 6 is arranged on the optical path behind the optical fiber collimator 11, and the frequency stabilization optical path 3 is located behind the 45° reflector 6. The frequency-stabilized optical path 3 mainly includes optical components such as an atomic gas cell 31, an electro-optic modulator 32, a half-wave plate 41, a polarizing beam splitter PBS-51, and a pyramidal reflector 7. The optical path is divided into two beams: a probe beam and a pump beam. The probe beam path is sequentially equipped with the atomic gas cell 31 and two pyramidal reflectors 7, and the probe beam directly enters the atomic gas cell 31. The pump beam path is sequentially equipped with the half-wave plate 41, a 45° reflector 6, an electro-optic modulator 32, a 45° reflector 6, and the half-wave plate 41. The polarizing beam splitter PBS-51 is located at the convergence point of the two beams. Behind the polarizing beam splitter PBS-51, the 45° reflector 6, the frequency-stabilized optical path lens 33, and a photodetector 34 are sequentially arranged. Behind the 45° reflector 6, there are two sets of half-wave plates 41 and polarizing beam splitters PBS-51, one fixed and one adjustable. The fixed set is located in the light source section 1, equivalent to... The polarizer is better polarized after passing through the polarizing beam splitter PBS-51, and the light power is more stable. The adjustable group is located in the frequency stabilization optical path 3. The half-wave plate 41 of the frequency stabilization optical path 3 behind the 45° reflector 6 is set on the rotary motor 36. The probe light path led out by the polarizing beam splitter PBS-51 is arranged with atomic gas cell 31 and two cornerstone reflectors 7 in sequence. Power detectors PD35 are arranged between the polarizing beam splitter PBS-51, the atomic gas cell 31 and the cornerstone reflectors 7. The pump light first passes through the electro-optic modulator 32, and then enters the atomic gas cell 31 in the opposite direction, and basically coincides with the probe light. The probe light enters the photodetector 34 through the frequency stabilization optical path lens 33. The half-wave plate 41 and the polarizing beam splitter PBS-51 on the front or back side of the 45° reflector 6 have mature circuit control solutions, so they are not specifically described in this invention.

[0020] The modulation-transfer frequency stabilization optical path device uses divergent light to replace the beam expander effect, increases the interaction surface between the atomic gas cell and the laser, and uses a folded optical path to increase the interaction length between the atomic gas cell and the laser, thereby improving the locking effect.

[0021] Considering that the error signal of modulation transfer frequency stabilization is related to the ratio of laser power of the two paths, and that the power ratio of the two beams is prone to fluctuation due to fiber polarization and other factors, thus affecting the signal-to-noise ratio of frequency locking and even causing loss of lock, this invention mounts the half-wave plate 41 before the polarization beam splitter PBS-51 on an electrically controlled rotating motor 36, and dynamically adjusts it according to the optical power wave of the two beams to ensure that the splitting ratio remains unchanged. This device will effectively improve the long-term stability of the frequency stabilization system.

[0022] The function of the rotating motor 36 is to control the rotation of the half-wave plate 41 to change the polarization direction of the beam, thereby changing the beam splitting ratio of the subsequent polarization beam splitter PBS-51. The function of the power detector PD35 is to sample the power of the two modulated, transferred, and frequency-stabilized laser beams. The control circuit feeds back the changes in the sampled signal to the drive signal of the rotating motor 36, controlling the rotation of the half-wave plate 41 to keep the power ratio of the two laser beams stable.

[0023] In this invention, the light source is an adjustable fiber collimator 11. After all optical paths are assembled and adjusted, the position of the internal lens 12 in the fiber collimator 11 is optimized so that the beam entering the frequency-stabilized optical path is slightly diverged. At the same time, the optimization is further performed based on the signal strength of the frequency stabilization error signal. During the optimization process, the laser amplitude and beam splitting ratio are adjusted to compensate for the reduction in laser power during the adjustment process.

[0024] The atomic gas cell 31 is a common alkali metal atomic gas cell, such as a rubidium atomic gas cell. Both surfaces of the gas cell are coated with antireflective films, and the window tilt angle is 2–3°. The electro-optic modulator 32 is a phase modulator with a resonant frequency of approximately 10 MHz. Example 2

[0025] like Figure 2 As shown, a modulation transfer frequency stabilization optical path device in this embodiment includes a light source part 1, a frequency shifting optical path 2, a frequency stabilization optical path 3, etc.

[0026] In this embodiment of the invention, a configuration of a light source section, a frequency-shifting optical path, and a frequency-stabilizing optical path is adopted. A frequency-shifting optical path 2 is set between the light source section 1 and the frequency-stabilizing optical path 3. The frequency-shifting optical path 2 adopts a conventional acousto-optic modulator 21 double-pass optical path device, including two sets of sequentially arranged 1 / 2 wave plates 42 and polarizing beam splitters PBS 52, as well as the acousto-optic modulator 21, 1 / 4 wave plates 22, a frequency-shifting optical path lens 23, and a 0-degree reflector 24. The 1 / 2 wave plates 42 are mounted on a rotary motor 36. Power detectors PD35 are set between the polarizing beam splitter PBS 52 and the atomic gas cell 31 and the pyramidal reflector 7, allowing the light beam to pass through the acousto-optic modulator 21 twice to obtain frequency shift, and then enter the atomic gas cell 31 and electro-optic... In modulator 32: After passing through acousto-optic modulator 21 for the first time, the beam passes through a quarter-wave plate 22, a frequency-shifting optical path lens 23, and a 0-degree reflector 24. The distance between the frequency-shifting optical path lens 23 and acousto-optic modulator 32 and the 0-degree reflector 24 is close to the focal length. After adjusting the 0-degree reflector 24, the returning beam passes through acousto-optic modulator 32 again and produces diffraction. The angle of the quarter-wave plate 22 in the optical path is adjusted so that the secondary diffracted light is polarized perpendicularly to the incident light at the polarization beam splitter PBS 52 and is thus output.

[0027] After completing the installation and adjustment of all optical paths, the position of the 0-degree reflector 24 is optimized to make the beam entering the frequency-stabilized optical path 3 slightly diverge. At the same time, further optimization is carried out based on the signal strength of the frequency stabilization error signal. During the optimization process, the laser amplitude and beam splitting ratio are adjusted to compensate for the reduction in laser power during the adjustment process.

[0028] The improvement of the frequency-shifting optical path 2 of this invention compared with the traditional modulation transfer optical path is that the position of the 0-degree reflector in the doublepass optical path of the acousto-optic frequency shifter is slightly adjusted so that the output beam is not parallel light, but quasi-parallel light with a certain divergence angle. At the same time, the optical path of the pump light injected into the atomic gas cell in the reverse direction is greater than the optical path of the probe light directly incident on the atomic gas cell. Therefore, both beams diverge to different degrees, and the diameter of the pump light beam is larger than that of the probe light. This optical path setting increases the interaction area between the beam and the atoms. Moreover, the diameter of the pump light beam is larger than that of the probe light, thereby increasing the robustness to optical axis deviation, improving the amplitude and stability of the frequency-stabilized signal, and the divergence angle can be adjusted according to the intensity of the frequency-stabilized signal.

[0029] The idea behind this invention is to achieve fine-tuning of the beam divergence angle by adjusting the position of the 0-degree reflector in the frequency-shifting optical path, which is equivalent to fine-tuning the lens inside the collimator. With the addition of a frequency-shifting optical path, the beam divergence angle input to the frequency-stabilized optical path can be changed by fine-tuning the position of the 0-degree reflector, thus eliminating the need to adjust the lens inside the collimator in the light source section.

[0030] The frequency-shifting optical path of the present invention has advantages such as increasing the frequency-locking range. In a specific optical path, it can also be omitted and the laser light source can be directly incident into the frequency-stabilizing optical path. For this optical path setting, the internal collimating lens of the fiber collimator can be finely adjusted so that the output beam is a slightly divergent quasi-parallel light. The specific size of the divergence angle can be optimized according to the intensity of the final frequency-stabilizing signal.

[0031] Another improvement of the frequency-shifting optical path of this invention compared with the traditional modulation transfer frequency-stabilizing optical path is that the propagation length of the light beam in the atomic gas cell is increased by a factor of two pyramidal reflectors. This folded optical path can increase the interaction length between atoms and laser without affecting the volume of the frequency-stabilizing optical path device.

[0032] This invention addresses a series of problems existing in traditional modulation-transfer frequency-stabilized optical paths by proposing an innovative improvement. This improvement, through careful design and optimization of the existing frequency-stabilized optical path, achieves the transformation of the collimated beam into a slightly divergent quasi-parallel beam, effectively expanding the interaction range between the beam and atoms without requiring an additional beam-expanding optical system. This improvement not only simplifies system design but also significantly reduces the technical difficulty of optical axis alignment, improving system stability and ease of operation.

[0033] Furthermore, this invention employs an innovative folded optical path design based on a pyramidal reflector, cleverly increasing the interaction length between atoms and the laser without affecting the system size. This unique design significantly improves the signal-to-noise ratio of frequency stabilization, while enhancing the robustness and integration of the frequency stabilization optical path, bringing a more compact, efficient, and easy-to-operate frequency stabilization optical path solution to the field of cold atom measurement.

[0034] In another embodiment of the present invention, the light source part 1 includes an optical fiber collimator 11 with a pigtail, a half-wave plate 41, a polarizing beam splitter PBS 51, and a 45° reflector 6, etc. The optical fiber collimator 11 transforms the output light source of the fiber laser into a collimated spatial beam, which has the advantage of good beam quality. The half-wave plate 41 and the polarizing beam splitter PBS 51 function as polarization filters, so that the output beam is linearly polarized light with a uniform polarization direction, which is beneficial to the stability of optical power.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A modulation transfer frequency stabilization optical path device, characterized in that: The light source section (1) and the frequency-stabilized optical path (3) are included. The light source section (1) adopts an optical fiber collimator (11) with an adjustable internal lens (12). The internal lens (12) is finely adjusted so that the output beam is a slightly divergent quasi-parallel light to expand the interaction range between the beam and the atoms. A 45° reflector (6) is set behind the optical fiber collimator (11). The frequency-stabilized optical path (3) behind the 45° reflector (6) includes a half-wave plate (41) and a polarizing beam splitter PBS (51). The half-wave plate (41) is set on a rotating motor (36). An atomic gas cell (31) and two pyramidal reflectors (7) are set in sequence on the probe light path led out by the polarizing beam splitter PBS (51). A power detector PD (35) is provided between S- (51), the atomic gas cell (31), and the corner cone reflector (7). A half-wave plate (41), a 45° reflector (6), an electro-optic modulator (32), a 45° reflector (6), and a half-wave plate (41) are arranged in sequence on the pump light path led out by the polarization beam splitter PBS- (51). A polarization beam splitter PBS- (51) is provided at the convergence point of the two optical paths. A 45° reflector (6), a frequency-stabilized optical path lens (33), and a photodetector (34) are arranged in sequence behind the polarization beam splitter PBS- (51). It also includes a half-wave plate (41) and a polarization beam splitter PBS- (51) arranged in front of or behind the 45° reflector (6).

2. The modulation transfer frequency stabilization optical path device according to claim 1, characterized in that, It also includes a frequency-shifting optical path (2) disposed between the light source part (1) and the frequency-stabilized optical path (3). The frequency-shifting optical path (2) adopts a sequence of 1 / 2 wave plate II (42), polarizing beam splitter PBS II (52), acousto-optic modulator (21), 1 / 4 wave plate (22), frequency-shifting optical path lens (23) and 0 degree reflector (24). The beam passes through the acousto-optic modulator (21) twice to obtain frequency shift, and then enters the atomic gas cell (31) and electro-optic modulator (32).

3. The modulation transfer frequency stabilization optical path device according to claim 2, characterized in that, The distances between the acousto-optic modulator (32) and the 0-degree reflector (24) and the frequency-shifting optical path lens (23) are close to the focal length of the frequency-shifting optical path lens (23).

4. A modulation transfer frequency stabilization optical path device according to claim 1, 2, or 3, characterized in that, The atomic gas chamber (31) is two rubidium atomic gas chambers with antireflective coatings on their surfaces and a window tilt angle of 2 to 3°.

5. The modulation transfer frequency stabilization optical path device according to claim 4, characterized in that, The electro-optic modulator (32) is a phase modulator with a resonant frequency of 10 MHz.

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