CN114649732B

CN114649732B - Saturated absorption spectrum laser frequency locking method and device and frequency locking laser

Info

Publication number: CN114649732B
Application number: CN202011520857.8A
Authority: CN
Inventors: 彭翔; 郭弘; 王海东; 王博闻
Original assignee: Peking University
Current assignee: Peking University
Filing date: 2020-12-21
Publication date: 2024-07-12
Anticipated expiration: 2040-12-21

Abstract

The invention provides a saturated absorption spectrum laser frequency locking method, which is characterized in that a beam of laser is used as pumping light to be incident into an atomic air chamber, and the part of the pumping light emitted from the atomic air chamber, which is transmitted by a partial reflector, carries Doppler broadening back information and is converted into a first electric signal by a first photoelectric detector; the part of the pumping light emitted from the atomic gas chamber, which is reflected by the partial reflector, is detection light carrying Doppler broadening back information and saturated absorption spectrum information, and is converted into a second electric signal by a second photoelectric detector; the differential amplifier receives and processes the first electric signal and the second electric signal and outputs a third electric signal with Doppler spread background information eliminated; the phase-locked amplifier receives and processes the third electric signal, outputs an error signal, and the error signal is received and converted into a frequency-locked signal by the negative feedback controller and is fed back to the laser. The invention can eliminate Doppler broadening back information in saturated absorption spectrum information by only using one laser beam, realizes laser frequency locking and has simple optical path design.

Description

Saturated absorption spectrum laser frequency locking method and device and frequency locking laser

Technical Field

The invention relates to the technical field of laser frequency locking, in particular to a saturated absorption spectrum laser frequency locking method and device and a frequency locking laser.

Background

The ruby laser is used as an artificial light source and has the characteristics of high intensity, good coherence and the like. The laser is widely applied to the fields of military, medical treatment, industrial production, precise measurement and the like, such as laser guidance, laser cosmetology, laser welding, laser marking, optical frequency standard, atomic magnetometer and the like. In the field of precision measurement, lasers are used as pump sources or detection means for polarization of atomic ensembles or for perception of atomic states. The frequency, intensity, polarization stability of the laser determine the level of precision measurement. In practical applications, the laser frequency is shifted due to interference of external environment, such as vibration and temperature change. The slow drift of the center frequency of a free running laser can reach tens of megahertz within 5 minutes. For optical precision measurements, the frequency fluctuations of the laser severely affect the sensitivity of the measurement. Active frequency locking is required in addition to ensuring constant temperature and preventing external vibration.

There are various methods for locking the frequency of the laser, and one method that is relatively commonly used is to lock the frequency of the laser at a suitable atomic transition frequency, so as to obtain a laser with high spectral purity, narrow linewidth and high frequency stability. The saturated absorption spectrum frequency locking technology is one of the common laser frequency locking technologies. The atomic energy level transition frequency is used as a reference, a frequency discrimination error signal is obtained by comparing the laser frequency with a reference frequency, and then the laser frequency is corrected by feedback, so that the laser frequency is locked at the atomic transition reference frequency, and the laser frequency is stabilized.

The traditional saturated absorption spectrum frequency locking is to divide the laser output by the laser into two beams (pumping light with stronger light intensity and detection light with weaker light intensity) and simultaneously inject the two beams into the atomic gas chamber in opposite incidence directions. When the laser frequency is appropriate, only atoms with a velocity of zero in the direction of light propagation can interact with both lasers at the same time. Because the intensity of the pumping light is higher, a large amount of particles distributed at the lower energy level are pumped to the upper energy level, and when weaker detection light passes through, the absorption capacity of atoms on the detection light is weakened, so that a convex transmission peak is obtained on the optical path of the transmission atom air chamber through detection, and the convex transmission peak is called a saturated absorption peak. The frequency corresponding to the highest point of the saturation absorption peak is the center frequency of the laser. The purpose of the laser frequency locking is to lock the center frequency of the laser. The Doppler absorption spectrum formed by the velocity distribution of atoms obeys the Boltzmann distribution is wide in back. The saturated absorption peak is the transmission peak on the back of the large doppler absorption. The Doppler background affects the signal to noise ratio of the signal and affects the laser frequency locking. In order to eliminate the influence of the Doppler background, to highlight the saturated absorption spectrum information, obtain a higher signal-to-noise ratio and more conveniently lock the highest point of the transmission peak, a method for eliminating the Doppler background is generally introduced.

At present, a method for introducing a reference light path is generally adopted to eliminate the Doppler back. The specific method is to divide the laser emitted by the same light source into three beams. Two of the weaker light beams pass through the atomic gas cell in parallel along different spatial positions, one beam being called reference light and one beam being called probe light. The other beam of stronger light is called pumping light, and after passing through a proper light path, the pumping light reversely enters from the atomic gas chamber and forms correlation with the detection light, so that a saturated absorption spectrum structure is formed. And the reference light directly passes through the atomic gas chamber and is incident to the photoelectric detector to carry Doppler broadening back information. And finally, the saturated absorption spectrum information obtained by the detection light is subjected to difference with the Doppler background signal obtained by the reference light, so that the saturated absorption spectrum information of the background without Doppler broadening is obtained.

However, the method of introducing the reference light path eliminates the complex structure of the device used by the Doppler back, which is unfavorable for application, and particularly under the condition of compact requirement, the multiple light beams increase the complexity and volume of the device.

Disclosure of Invention

In order to solve the existing problems, the Doppler broadening background information in the saturated absorption spectrum information can be eliminated by only one laser beam, the laser frequency locking is realized, and the optical path design is simple. Meanwhile, the method for locking frequency by utilizing the differential signal can eliminate common mode interference of the outside on a frequency locking system or intensity fluctuation of the laser, and effectively reduces the requirement on the performance of the laser, in particular to the laser with obvious light intensity fluctuation.

One embodiment of the invention provides a saturated absorption spectrum laser frequency locking method, which comprises the following steps:

After the incident laser provided by the laser is split by the beam splitter, one beam of laser is used as pump light and is incident into the atomic gas chamber;

pump light emitted from the atomic gas chamber: the part transmitted by the partial reflector carries Doppler broadening back information and is detected by a first photoelectric detector and converted into a first electric signal; the part reflected by the partial reflector is detection light, and the detection light passes through the atomic gas chamber, carries Doppler broadening back information and saturated absorption spectrum information, and is detected by a second photoelectric detector and converted into a second electric signal;

The first electric signal and the second electric signal are received and processed by a differential amplifier, and a third electric signal is output, wherein the third electric signal contains saturated absorption spectrum information with Doppler spread background information eliminated;

the third electric signal is received and processed by a phase-locked amplifier, and an error signal is output, and the error signal is received and converted into a frequency locking signal by a negative feedback controller and is fed back to the laser.

Further, the portion transmitted by the partial reflector carries doppler broadening backing information, and is detected by the first photodetector and converted into a first electrical signal, specifically:

The part transmitted by the partial reflector carries Doppler broadening back information, is attenuated by the optical filter to a light intensity range which can be detected by the first photoelectric detector, is focused to the first photoelectric detector by the first lens, and is detected by the first photoelectric detector and converted into a first electric signal.

Further, after the incident laser provided by the laser is split by the beam splitter, one of the laser beams is used as pump light and is incident into the atomic gas chamber, specifically:

The incident laser provided by the laser is split by a first component consisting of a first 1/2 wave plate and a first prism, wherein one beam of laser reflected by the first prism is standby laser, and one beam of laser transmitted by the first prism is completely transmitted by a second component consisting of a second 1/2 wave plate and a second prism and then is used as pump light to be incident into an atomic gas chamber.

Further, the part reflected by the partial reflector is detection light, and after the detection light passes through the atomic gas chamber, the detection light carries doppler broadening back information and saturated absorption spectrum information, and is detected by a second photoelectric detector and converted into a second electric signal, specifically:

the part reflected by the partial reflector is detection light, the detection light passes through the atomic gas chamber, carries Doppler broadening back information and saturated absorption spectrum information, is totally reflected by a third component consisting of a 1/4 wave plate and a second prism, which are arranged on any side of the atomic gas chamber, is focused to a second photoelectric detector by a second lens, and is detected by the second photoelectric detector and converted into a second electric signal.

Further, the first photodetector and the second photodetector are photocells capable of responding to an optical signal of the center wavelength of the optical device; wherein the optical device comprises: the first 1/2 wave plate, the first prism, the second 1/2 wave plate, the second prism and the partial reflector.

Further, the first prism and the second prism are both polarization beam splitting prisms.

Further, the atomic gas chamber is made of pyrex glass.

Further, the negative feedback controller is a proportional-integral-derivative controller.

The embodiment of the invention provides a saturated absorption spectrum laser frequency locking device which comprises a laser, an atomic gas chamber, a photoelectric detector, a differential amplifier, a phase-locked amplifier, a proportional-integral-differential controller and related optical devices, wherein the saturated absorption spectrum laser frequency locking device is used for realizing the saturated absorption spectrum laser frequency locking method of any one of the above.

An embodiment of the present invention provides a frequency-locking laser, where the frequency-locking laser is a tunable laser, so as to implement any one of the saturated absorption spectrum laser frequency-locking methods.

Compared with the prior art, the embodiment of the invention has the beneficial effects that:

1. According to the saturated absorption spectrum laser frequency locking method provided by the invention, a beam of laser is used as pumping light to be incident into an atomic air chamber, and the part of the pumping light emitted from the atomic air chamber, which is transmitted by a partial reflector, carries Doppler broadening back information and is detected by a first photoelectric detector and converted into a first electric signal; the part of the pumping light emitted from the atomic gas chamber, which is reflected by the partial reflector, is detection light, and after the detection light passes through the atomic gas chamber, the detection light carries Doppler broadening back information and saturated absorption spectrum information and is detected by a second photoelectric detector and converted into a second electric signal; the differential amplifier receives and processes the first electric signal and the second electric signal and outputs a third electric signal, wherein the third electric signal contains saturated absorption spectrum information with Doppler spread background information eliminated; the phase-locked amplifier receives and processes the third electric signal and outputs an error signal; the error signal is received by a negative feedback controller and converted into a frequency locking signal to be fed back to the laser; the invention can eliminate Doppler broadening back information in saturated absorption spectrum information by only using one laser beam, realizes laser frequency locking and has simple optical path design.

2. The invention provides a saturated absorption spectrum laser frequency locking method, which utilizes a 1/4 wave plate and a second prism to fully reflect detection light so as to prevent the detection light from being injected into a laser and damaging the laser.

3. The saturated absorption spectrum laser frequency locking method provided by the invention can eliminate common mode interference of the outside on a frequency locking system or intensity fluctuation of a laser by utilizing the differential signal frequency locking method.

4. The saturated absorption spectrum laser frequency locking device provided by the invention effectively reduces the requirements on the performance of the laser, in particular to a laser with obvious light intensity fluctuation.

Drawings

In order to more clearly illustrate the technical solutions of the present invention, the drawings that are needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and that other drawings can be obtained according to these drawings without inventive effort for a person skilled in the art.

FIG. 1 is a flow chart of a method for frequency locking a saturated absorption spectrum laser according to an embodiment of the present invention;

FIG. 2 is a flow chart of a method for frequency locking a saturated absorption spectrum laser according to another embodiment of the present invention;

FIG. 3 is a schematic diagram of a saturated absorption spectrum laser frequency-locking device according to an embodiment of the present invention;

Fig. 4 is a block diagram of a saturated absorption spectrum laser frequency locking device according to another embodiment of the present invention.

Detailed Description

The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

It should be understood that the step numbers used herein are for convenience of description only and are not limiting as to the order in which the steps are performed.

It is to be understood that the terminology used in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in this specification and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

The terms "comprises" and "comprising" indicate the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The term "and/or" refers to any and all possible combinations of one or more of the associated listed items, and includes such combinations.

In order to solve the technical problems that the device structure is complex and the application is not facilitated because Doppler broadening back information in absorption spectrum information is eliminated through multiple beams of light in the prior art, the Doppler broadening back information in saturated absorption spectrum information can be eliminated by only one beam of laser, the laser frequency locking is realized, and the light path design is simple.

In a first aspect.

Please refer to fig. 1 and 3. An embodiment of the present invention provides a saturated absorption spectrum laser frequency locking method, including:

S10, after the incident laser provided by the laser is split by the beam splitter, one laser beam is used as pump light and is incident into the atomic gas chamber.

The laser 10 is a device capable of emitting laser light, and is not limited to a fiber laser or a semiconductor laser.

The beam splitter 20 is used for splitting the laser beam provided by the laser, and the beam splitter 20 is not limited to an optical device such as a polarization beam splitter prism.

The atomic gas cell 30 is used to provide the reference frequency required for laser frequency locking. In one embodiment of the present invention, the atomic gas chamber 30 may be a cylindrical glass bubble with a bottom diameter of 20mm and a height of 40 mm. In another embodiment of the present invention, the atomic gas chamber 30 may be selected from pyrex glass. In another embodiment of the invention, the interior of the atomic gas chamber is filled with helium (⁴ He) atomic gas.

S20, pumping light emitted from the atomic air chamber: the part transmitted by the partial reflector carries Doppler broadening back information and is detected by a first photoelectric detector and converted into a first electric signal; the part reflected by the partial reflector is detection light, and after passing through the atomic gas chamber, the detection light carries Doppler broadening back information and saturated absorption spectrum information and is detected by a second photoelectric detector and converted into a second electric signal.

The pump light is emitted from the atomic gas chamber 30, carries Doppler broadening back information, and then is emitted to the partial reflector 40; the Doppler broadening backing information is carried by the transmission portion of the partial mirror 40 and detected by the first photodetector 50 and converted into a first electrical signal, i.e., the first electrical signal carries the Doppler broadening backing information; the portion reflected by the partial reflecting mirror 40 is a probe light, which carries doppler broadening backing information and saturated absorption spectrum information after passing through the atomic gas chamber 30 in a direction opposite to the pump light, and the probe light is detected by the second photodetector 60 and converted into a second electrical signal, that is, the second electrical signal carries doppler broadening backing information and saturated absorption spectrum information.

It should be noted that, by adjusting the transmittance of the partial mirror 40, a large portion of the light intensity of the pump light is transmitted through the partial mirror 40, and a small portion of the light intensity is reflected from the mirror surface of the partial mirror 40, and the reflected light is defined as the probe light.

In an embodiment of the present invention, the first photodetector 50 and the second photodetector 60 are both photocells, and are both configured to convert optical signals into electrical signals and read information carried by the optical signals; that is, the first photodetector 50 converts the optical signal of the pump light into a first electrical signal, reads the doppler spread background information carried by the pump light, and the second photodetector 60 converts the optical signal of the probe light into a second electrical signal, reads the doppler spread background information and the saturated absorption spectrum information carried by the probe light.

S30, the first electric signal and the second electric signal are received and processed by a differential amplifier, and a third electric signal is output, wherein the third electric signal contains saturated absorption spectrum information with Doppler spread background information eliminated.

The first electric signal carrying the Doppler stretching background information and the second electric signal carrying the Doppler stretching background information and the saturated absorption spectrum information are subjected to operation processing through a differential amplifier 70 to obtain a third electric signal, wherein the third electric signal contains the saturated absorption spectrum information with the Doppler stretching background information eliminated.

And S40, the third electric signal is received and processed by a phase-locked amplifier, and an error signal is output, and the error signal is received and converted into a frequency locking signal by a negative feedback controller and is fed back to the laser.

And the lock-in amplifier 80 is used for demodulating the third electric signal to determine the center frequency point of the laser. The negative feedback controller 90 is a proportional-integral-derivative controller (also called PID controller) for closing the loop of the frequency locking system to lock the laser frequency. It should be noted that the lock-in amplifier 80 and the negative feedback controller 90 may also be implemented by software programming.

According to the saturated absorption spectrum laser frequency locking method provided by the invention, a beam of laser is used as pumping light to be incident into an atomic air chamber, and the part of the pumping light emitted from the atomic air chamber, which is transmitted by a partial reflector, carries Doppler broadening back information and is detected by a first photoelectric detector and converted into a first electric signal; the part of the pumping light emitted from the atomic gas chamber, which is reflected by the partial reflector, is detection light, and after the detection light passes through the atomic gas chamber, the detection light carries Doppler broadening back information and saturated absorption spectrum information and is detected by a second photoelectric detector and converted into a second electric signal; the differential amplifier receives and processes the first electric signal and the second electric signal and outputs a third electric signal, wherein the third electric signal contains saturated absorption spectrum information with Doppler spread background information eliminated; the phase-locked amplifier receives and processes the third electric signal and outputs an error signal; the error signal is received by a negative feedback controller and converted into a frequency locking signal to be fed back to the laser; the invention can eliminate Doppler broadening back information in saturated absorption spectrum information by only using one laser beam, realizes laser frequency locking and has simple optical path design.

Referring to fig. 2 and 4, in one embodiment:

the step S10 specifically comprises the following steps:

S11, the incident laser provided by the laser is split by a first component formed by a first 1/2 wave plate and a first prism, wherein one beam of laser reflected by the first prism is standby laser, and one beam of laser transmitted by the first prism is completely transmitted by a second component formed by a second 1/2 wave plate and a second prism and then is used as pump light to be incident into an atomic gas chamber.

The first 1/2 wave plate 22 and the first prism 23 are used for dividing the laser light output by the laser 10 into two beams according to a preset light intensity ratio or a preset light splitting ratio, wherein one beam is used for standby or various specific laser application scenes, which can be called standby laser, and the other beam is injected into the second component. The preset light intensity proportion or the light splitting proportion can be adjusted according to actual needs, and the specific operation is as follows: the first 1/2 wave plate 22 is rotated so that the first prism 23 divides the light beam into two light beams according to a preset light intensity ratio. The first prism 23 is not limited to a polarization splitting prism. It should be noted that the purpose of the first component is to split light, not limited to the two devices, and if the laser is an optical fiber output, a fiber splitter may be used to split light.

The second 1/2 wave plate 24 and the second prism 25 are used for controlling the intensity of the laser, and specifically operate as follows: the second 1/2 wave plate 24 is rotated so that the laser light is completely transmitted from the second prism 25 without being reflected, and the transmitted light is used as pump light.

Step S20 includes:

S21, the part transmitted by the partial reflector carries Doppler broadening back information, is attenuated by the optical filter to a light intensity range which can be detected by the first photoelectric detector, is focused to the first photoelectric detector by the first lens, and is detected by the first photoelectric detector and converted into a first electric signal.

The pump light is emitted from the atomic gas chamber 30 and carries doppler spread back information, then is emitted to the partial reflector 40, and is emitted to the optical filter 41 through a part of the partial reflector 40, attenuated by the optical filter 41 to a light intensity range which can be detected by the first photodetector 50, emitted to the first lens 42, focused by the first lens 42 to the first photodetector 50, and detected by the first photodetector 50 and converted into a first electric signal carrying doppler spread back information.

The filter 41 is a neutral density filter, and the absorption rate of the neutral density filter 41 is adjusted so that the intensity of the pump light transmitted from the partial mirror 40 is attenuated to a light intensity range that can be detected by the first photodetector 50. The first lens 42 is a first plano-convex lens, and the distance between the first plano-convex lens 42 is adjusted to focus the pump light on the sensing element of the first photodetector 50, and the optical signal of the pump light is converted into a first electrical signal.

And S22, the part reflected by the partial reflector is detection light, the detection light passes through the atomic air chamber and carries Doppler broadening back information and saturated absorption spectrum information, the detection light is totally reflected by a third component consisting of a 1/4 wave plate and the second prism, which are arranged on any side of the atomic air chamber, and the detection light is focused to a second photoelectric detector by a second lens, and the detection light is detected by the second photoelectric detector and is converted into a second electric signal.

The pump light is emitted from the atomic gas chamber 30, carries Doppler broadening back information, and then is emitted to the partial reflector 40, and the part reflected by the partial reflector 40 is the probe light. In a specific embodiment, after the probe light passes through the atomic gas chamber 70 in a direction opposite to the pump light, the probe light carries doppler broadening backing information and saturated absorption spectrum information, and after passing through the third component formed by the 1/4 wave plate 26 and the second prism 25, the probe light is totally reflected, and then is directed to the second lens 61, focused by the second lens 61 to the second photodetector 60, and detected by the second photodetector 60 and converted into a second electrical signal. In another embodiment, the detected light passes through the atomic gas chamber 30 after passing through the 1/4 wave plate 26, carrying doppler broadening backing information and saturation absorption spectrum information, and the detected light exiting the atomic gas chamber 30 passes through the second prism 35, is totally reflected by the third component composed of the 1/4 wave plate 26 and the second prism 25, passes through the second lens 61, is focused by the second lens 61 onto the second photodetector 60, and is detected by the second photodetector 60 and converted into a second electrical signal.

The invention provides a saturated absorption spectrum laser frequency locking method, which utilizes a 1/4 wave plate and a second prism to fully reflect detection light so as to prevent the detection light from being injected into a laser and damaging the laser. The common mode interference of the external world to the frequency locking system or the intensity fluctuation of the laser can be eliminated by utilizing the differential signal frequency locking method.

The second aspect.

Referring to fig. 4, an embodiment of the present invention provides a saturated absorption spectrum laser frequency locking device, which includes:

The first 1/2 wave plate 22 and the first prism 23 are used for dividing the output laser of the laser 10 into two beams, one beam is specially used for laser frequency locking, and the other beam is used for various specific laser application scenes. The purpose of this structure is to split light, not limited to the two devices, if the laser 10 is an optical fiber output, a fiber beam splitter may be used to split light, and the splitting ratio may be adjusted according to practical needs.

A second 1/2 wave plate 24, a second prism 25 for controlling the intensity of the pump light, and the combination of the second prism 25 and the 1/4 wave plate 26 makes the probe light back-propagating from the other end of the optical path totally reflected from the second prism 25, so as to realize the splitting of the probe light and prevent the back-propagating probe light from damaging the laser 10.

The atomic gas chamber 30 is used for generating saturated absorption spectrum information by interaction with the pump light and the probe light, and it should be noted that the atomic gas chamber 30 is disposed behind the second prism 25, and the 1/4 wave plate 26 may be disposed in front of or behind the atomic gas chamber 30 (fig. 4 shows that the 1/4 wave plate is disposed in front of the atomic gas chamber), so that the final frequency locking effect is not affected.

A partial reflector 40 for reflecting a part of the pump light back to the atomic gas cell 30, defined as probe light, and transmitting another part of the pump light through the partial reflector, carrying doppler spread back information. The proportion of the reflection of the partial mirror 40 is adjusted according to the light intensity requirement in order to improve the signal-to-noise ratio.

The filter 41 is used for attenuating the intensity of the light beam transmitted from the partial reflector 40, and the filter 41 may be a neutral density filter.

A first lens 42 for focusing the light beam on the rear first photodetector 50, said first lens 42 may be a plano-convex lens.

The first photodetector 50 converts the optical signal of the pump light into a first electrical signal, and reads the Doppler broadening back information carried by the pump light.

The arrangement in order on the second prism 25 perpendicular to the direction of the outgoing light of the laser 10 includes:

A second lens 61 for focusing light onto the second photodetector 60, said second lens 61 may be a plano-convex lens.

The second photodetector 60 converts the optical signal of the detection light into a second electrical signal, and reads the saturated absorption spectrum information and the doppler spread background information carried by the detection light.

The differential amplifier 70 performs differential operation on the first electrical signal carrying the doppler spread background information and the second electrical signal carrying the saturation absorption spectrum information and the doppler spread background information to obtain a third electrical signal, where the third electrical signal includes the saturation absorption spectrum information from which the doppler spread background information is eliminated.

And the lock-in amplifier 80 is used for demodulating the third electric signal to obtain an error signal taking the transition frequency of atoms in the atomic gas chamber as a reference, so as to determine the center frequency point of the laser 10.

The negative feedback controller 90 is used for closing the system loop to perform laser frequency locking, and the negative feedback controller 90 can be a proportional-integral-derivative controller (also called a PID controller).

The lock-in amplifier 80 and the negative feedback controller 90 may also be implemented by software programming.

In a specific embodiment, the frequency locking method of the saturated absorption spectrum laser frequency locking device comprises the following steps:

s001, a beam of laser light emitted from a laser 10 modulated by a signal generator is divided into two beams by a first 1/2 wave plate 22 and a first prism 23 according to different light intensity ratios, wherein one beam is used for saturated absorption spectrum frequency locking, and the other beam is used for various practical applications.

The first prism 23 divides the light beam into two laser beams according to different light intensity ratios by rotating the first 1/2 wave plate 22.

S002, the pumping light for saturated absorption spectrum frequency locking is adjusted to be completely transmitted by the second 1/2 wave plate 24, is incident to the second prism 25 and converted into circularly polarized light by the 1/4 wave plate 26, and then enters the atomic air chamber 30 to pump atoms and enable an atomic sample to reach saturated absorption; the atomic gas chamber 30 can be used for first inputting the atomic gas, pumping the atomic gas to saturate the atomic sample, and then realizing polarization conversion through the 1/4 wave plate 26.

The second 1/2 wave plate 24 is used to adjust the linear polarization direction of the laser light so that the laser light is completely transmitted through the second prism 25 without being reflected by the second prism 25. The second prism 25 is used for separating the reflected detection light, the 1/4 wave plate 26 is used for converting the linear polarized light into circular polarized light, and the atomic gas chamber 30 is used for providing the reference frequency required by laser locking.

The second 1/2 wave plate 24 is rotated so that the laser light is completely transmitted from the second prism 25 without being reflected. The light beam completely transmitted through the second prism 25 at this time serves as pump light.

The pump light passes through the 1/4 wave plate 26, the polarization state of the pump light is changed from linear polarization to circular polarization, and then the pump light enters the atomic air chamber 30 to pump atoms, so that the atoms reach a saturated absorption state.

S003, the pump light emitted from the atomic gas chamber 30 is incident on the partial reflector 40, where a part of the pump light is transmitted through the partial reflector 40 to carry doppler broadening backing information, attenuated by the optical filter 41 to a light intensity that can be detected by the first photodetector 50, and focused by the first lens 42 onto the first photodetector 50 to provide a first electrical signal carrying saturated absorption spectrum information and doppler broadening backing information.

The transmissivity of the partial mirror 40 is adjusted such that a large part of the light intensity of the pump light is transmitted through the partial mirror 40, and a small part of the light intensity is reflected from the mirror surface of the partial mirror 40, which is defined as probe light.

The absorptivity of the filter 41 is adjusted so that the intensity of the pump light transmitted from the partial mirror 40 is attenuated to the detectable range of the first photodetector 50.

The distance of the first lens 42 is adjusted so that the pump light is focused on the sensing element of the first photodetector 50, and the optical signal of the pump light is converted into the first electrical signal.

S004, the pump light emitted from the atomic gas chamber 30 is reflected by the surface of the partial mirror 40, and is defined as probe light, and enters the atomic gas chamber 30 again in a direction opposite to the direction of the pump light. The probe light entering the atomic gas cell 30 carries saturated absorption information and Doppler broadening backing information through the atomic gas cell 30.

S005, the detection light carrying the saturated absorption spectrum information and the Doppler broadening back information is injected into the 1/4 wave plate 26 in the direction opposite to the direction of the pump light, and the circularly polarized light is converted into linearly polarized light, and at the moment, the polarization direction of the linearly polarized light is mutually perpendicular to the polarization direction of the linearly polarized light when the linearly polarized light is injected along the direction of the pump light.

The detection light emitted from the 1/4 wave plate 26 is incident on the second prism 25, totally reflected by the second prism 25, and focused on the second photodetector 60 through the second lens 61, so as to provide a second electric signal carrying the saturation absorption information and the Doppler broadening background information.

S007, sending the first electric signal carrying the doppler broadening backing information and the second electric signal carrying the saturated absorption spectrum information and the doppler broadening backing information to the differential amplifier 70, subtracting the first electric signal carrying the doppler broadening backing information from the second electric signal carrying the saturated absorption spectrum information and the doppler broadening backing information to obtain a third electric signal, where the third electric signal contains pure saturated absorption spectrum information with the doppler broadening backing information eliminated, and the third electric signal also eliminates common mode interference.

The differential scaling factor of the differential amplifier 70 is adjusted to a suitable value, and the first electrical signal is subtracted from the second electrical signal, so that Doppler broadening background information and common mode interference in saturated absorption spectrum frequency locking information are eliminated.

S008, the third electric signal from the differential amplifier 70 is sent to the lock-in amplifier 80 for demodulation, the third electric signal is changed into an error signal, and the error signal is sent to the negative feedback controller 90 for servo feedback to the laser 10, so as to complete the closed-loop locking of the laser frequency.

The invention provides a saturated absorption spectrum laser frequency locking device which comprises a laser, an atomic air chamber, a photoelectric detector, a differential amplifier, a phase-locked amplifier, a proportional-differential integral controller and related optical devices, wherein the saturated absorption spectrum laser frequency locking device is used for realizing any one of the saturated absorption spectrum laser frequency locking methods. The saturated absorption spectrum laser frequency locking device provided by the invention effectively reduces the requirements on the performance of the laser, in particular to a laser with obvious light intensity fluctuation.

The following describes the working process and principle of the present invention with a specific example of single beam Doppler spread saturated absorption spectrum frequency locking based on helium (⁴ He) atoms:

1. The specific components selected are as follows:

the central wavelength of the first 1/2 wave plate and the second 1/2 wave plate is 1083nm. The first prism and the second prism are both polarization beam-splitting prisms with the center wavelength of 1083nm. The center wavelength of the 1/4 wave plate is 1083nm. The atomic gas chamber is a cylindrical glass bubble with the bottom surface diameter of 20mm and the height of 40mm, helium (⁴ He) atomic gas is filled in the atomic gas chamber, and the pressure is 1Torr. The center wavelength of the partially reflecting mirror was 1083nm. The filter is a neutral density filter with a central wavelength of 1083nm. The first lens and the second lens are plano-convex lenses with the central wavelength of 1083nm and the focal length of 10 mm. The first photodetector and the second photodetector are InGaAs photocells capable of responding to an 1083nm center wavelength optical signal. The differential amplifier is a signal differential circuit board containing a self-made preamplifier.

2. Working process and principle:

after the laser beam generated by any 1083nm laser which is introduced into small modulation by the signal generator passes through the combination of the first 1/2 wave plate and the first prism, the laser beam is divided into two beams of laser according to a specific light intensity proportion, one beam is used for outputting light and supplying specific laser application, and the other beam is used for saturated absorption spectrum frequency locking. The second 1/2 wave plate is rotated so that the laser light is completely transmitted from the second prism without being reflected, and the laser light transmitted through the second prism serves as pump light. The pump light passes through a 1/4 wave plate, the polarization state of the pump light is converted into a circular polarization state from a linear polarization state, and then the pump light enters an atomic gas chamber, and helium (⁴ He) atoms excited to a metastable state by a 38.88MHz radio frequency power source are contained in the atomic gas chamber. The pump light interacts with metastable helium (⁴ He) atoms, optically pumping the metastable energy levels and achieving saturated absorption. Subsequently, the pump light enters the partial mirror, the transmissivity of the partial mirror is adjusted (70% transmissivity is adopted in this example) so that most of the pump light is transmitted through the partial mirror, and a small part of the pump light is reflected from the partial mirror, which is defined as probe light, and the circular polarization direction of the probe light becomes the opposite circular polarization direction of the pump light. The pump light carries doppler spread back information which is attenuated to a suitable intensity and then focused on a first photodetector, in turn, through a neutral density filter of suitable optical density (optical density of 0.5 is used in this example) and a first lens of focal length 10mm, generating a first electrical signal. The detection light is reflected by the partial reflector, enters the atomic air chamber which has reached saturation absorption along the direction opposite to the direction of the pumping light, and forms a transmission peak on the Doppler broadening back, which is the saturation absorption peak and provides a frequency reference required by the saturation absorption spectrum frequency locking. The detection light emitted from the atomic gas chamber passes through the 1/4 wave plate, and the polarization state of the detection light is converted from circular polarization into linear polarization state which is mutually perpendicular to the previous linear polarization pump light. Thus, the probe light that becomes the linear polarization state is totally reflected by the second prism without any transmitted light. Further, the detection light passes through a second lens with a focal length of 10mm, is focused on a second photodetector, and generates a second electric signal. And inputting the two paths of photoelectric detection signals into a differential amplifier containing a preamplifier, adjusting a proper differential proportionality coefficient, subtracting the first electric signal carrying Doppler broadening backing information from the second electric signal carrying Doppler broadening backing information and saturated absorption spectrum information to obtain a third electric signal, wherein the third electric signal contains the saturated absorption spectrum information for eliminating Doppler broadening backing information and common mode interference. And (3) the third electric signal output by the differential amplifier is connected into the phase-locked amplifier for demodulation, then is subjected to servo feedback by the proportional-integral-differential controller, and finally is connected into the laser to complete the closed-loop locking of the laser frequency.

In a third aspect.

Another embodiment of the present invention provides a frequency-locking laser, where the frequency-locking laser is a tunable laser, so as to implement the saturated absorption spectrum laser frequency-locking method described in any one of the above.

It is to be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the application. Further, for convenience of description, only some, but not all, of the structures related to the present application are shown in the drawings. The step numbers used herein are also for convenience of description only, and are not limiting as to the order in which the steps are performed. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.

The terms "first," "second," and the like in this disclosure are used for distinguishing between different objects and not for describing a particular sequential order. Furthermore, the terms "comprise" and "have," as well as any variations thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, system, article, or apparatus that comprises a list of steps or elements is not limited to only those listed steps or elements but may include other steps or elements not listed or inherent to such process, method, article, or apparatus.

Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the application. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Those of skill in the art will explicitly and implicitly appreciate that the embodiments described herein may be combined with other embodiments.

The foregoing description is only of embodiments of the present application, and is not intended to limit the scope of the application, and all equivalent structures or equivalent processes using the descriptions and the drawings of the present application or directly or indirectly applied to other related technical fields are included in the scope of the present application.

Claims

1. A saturated absorption spectrum laser frequency locking method, which is characterized by comprising the following steps:

2. The method for locking frequency of saturated absorption spectrum laser according to claim 1, wherein the portion transmitted by the partial reflector carries doppler broadening backing information, and is detected by the first photodetector and converted into the first electrical signal, specifically:

3. The method for locking frequency of saturated absorption spectrum laser according to claim 1, wherein after the incident laser provided by the laser is split by the splitting device, one of the laser beams is used as pump light to be incident into the atomic gas chamber, specifically:

4. The method of claim 3, wherein the portion reflected by the partial reflector is detection light, and the detection light carries doppler broadening backing information and saturated absorption spectrum information after passing through the atomic gas chamber, and is detected by a second photodetector and converted into a second electrical signal, specifically:

5. A method of frequency locking a saturated absorption spectrum laser as claimed in claim 3 wherein said first photodetector and said second photodetector are photocells capable of responding to optical signals at the center wavelength of the optical device; wherein the optical device comprises: the first 1/2 wave plate, the first prism, the second 1/2 wave plate, the second prism and the partial reflector.

6. A method of frequency locking a saturated absorption spectrum laser as claimed in claim 3 wherein said first prism and said second prism are polarizing beam splitting prisms.

7. The method of claim 1, wherein the atomic gas cell is made of pyrex.

8. The method of claim 1, wherein the negative feedback controller is a pid controller.

9. A saturated absorption spectrum laser frequency locking device, comprising a laser, an atomic gas chamber, a photoelectric detector, a differential amplifier, a phase-locked amplifier, a proportional-integral-differential controller and related optical devices, wherein the saturated absorption spectrum laser frequency locking device is used for realizing the saturated absorption spectrum laser frequency locking method as claimed in any one of claims 1 to 8.

10. A frequency-locked laser, characterized in that the frequency-locked laser is a tunable laser for implementing the saturated absorption spectrum laser frequency-locking method according to any one of claims 1 to 8.