Fiber laser frequency stabilization system and method

Abstract

The present invention discloses a fiber laser frequency stabilization system and method, which includes an electro-optic modulator for receiving a PDH modulation signal and performing phase modulation on the laser; a radio frequency signal source for applying a PDH modulation signal to the electro-optic modulator and providing a local reference signal; a mixer for mixing a reflected laser electrical signal with frequency jitter information with a local reference signal to obtain a dispersion signal; a low-pass filter for filtering out high-frequency components in the dispersion signal and retaining a DC part as an error signal; and a PID-based digital circuit module controller for receiving an error signal and outputting an opposite voltage signal according to the deviation between the signal information and a set value to adjust the driving signal of the laser to suppress the frequency fluctuation of the laser and lock it on an ultra-stable cavity. The system can effectively suppress the influence of the laser itself and external environmental factors on its frequency stability, and can realize rapid tracking and locking of an ultra-narrow linewidth ultra-stable cavity, avoiding the loss of lock caused by cavity length drift.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular to a fiber laser frequency stabilization system and method. Background Art

[0002] Currently, the second quantum revolution represented by quantum information technology is underway. Using the basic principles of quantum mechanics and quantum control technology, quantum information technology can break through the classical limits of information technology. For example, quantum computers have extremely strong computing power and extremely low energy consumption, quantum communication can provide an unconditionally secure communication method, and quantum precision measurement can provide extremely high sensitivity and measurement accuracy. This revolution is expected to achieve breakthroughs in the fields of quantum communication, quantum computing, quantum simulation, etc., and become a powerful means to solve major challenges in the fields of information, energy, environment, medicine, etc. Among the various quantum systems currently available, laser-cooled ultracold ions are one of the most promising systems for realizing large-scale general quantum computing, and are also one of the important candidate systems for realizing quantum precision measurement and quantum sensing.

[0003] At present, the types of ultracold ions mainly include calcium ions and ytterbium ions. For calcium ions, 729nm lasers are the most important lasers for manipulating the quantum state of ions, which require performance parameters such as high power and narrow line width. At present, the mainstream 729nm lasers mainly include semiconductor lasers and titanium sapphire lasers. There are problems such as low power and unstable operation of semiconductor laser tubes in 729nm semiconductor lasers. The power of 729nm titanium sapphire lasers can meet the demand, but because it has many optical and mechanical components and a complex structure, it is overall unstable, has many failures, and requires additional professional maintenance.

[0004] Fiber laser is a laser that uses optical fiber as gain medium and resonant cavity, and has the advantages of high efficiency, high power, high stability, high reliability, low noise and low cost. However, the frequency of fiber laser will also be affected by many factors and cause jitter and drift, such as temperature fluctuation, mechanical vibration, environmental noise, nonlinear effect and mode competition, so effective technical means are needed to achieve line width narrowing and control. Summary of the invention

[0005] The purpose of the present invention is to overcome the above-mentioned deficiencies of the prior art and to provide a fiber laser frequency stabilization system and method to effectively suppress the influence of the laser itself and external environmental factors on its frequency stability.

[0006] To achieve the above object, the technical solution of the present invention is:

[0007] In a first aspect, the present invention provides a fiber laser frequency stabilization system, comprising:

[0008] An electro-optical modulator, used to receive a PDH modulation signal and perform phase modulation on the laser;

[0009] A radio frequency signal source, used for applying a PDH modulation signal to the electro-optical modulator and providing a local reference signal;

[0010] The mixer mixes the reflected laser electrical signal with frequency jitter information with the local reference signal to obtain a dispersion signal; the low-pass filter filters out the high-frequency components in the dispersion signal and retains the DC part as the error signal;

[0011] The PID-based digital circuit module controller is used to receive the error signal and output an opposite voltage signal according to the deviation between the signal information and the set value to adjust the driving signal of the laser to suppress the frequency fluctuation of the laser and lock it on the ultra-stable cavity.

[0012] Furthermore, the fiber laser frequency stabilization system further comprises:

[0013] A first component, the first component comprising a first λ / 2 wave plate, a first polarization beam splitter, a first reflector, a second λ / 2 wave plate, a first λ / 4 wave plate, a first lens, a first pair of reflectors, a first coupling head, a single-mode polarization-maintaining optical fiber, a second coupling head, a second λ / 4 wave plate, and a Glan-Taylor prism;

[0014] The laser becomes vertically polarized light after passing through the first λ / 2 wave plate, and then passes through the first polarization beam splitter prism, the first reflector, the second λ / 2 wave plate, the first λ / 4 wave plate, the first lens, the single-mode polarization-maintaining optical fiber, the third λ / 2 wave plate, the Glan-Taylor prism, and then the electro-optic modulator.

[0015] Furthermore, the first λ / 2 wave plate and the first polarization beam splitter prism are combined to adjust the polarization direction of the laser to make it linearly polarized light;

[0016] The first λ / 4 wave plate and the second λ / 2 wave plate are used to control the polarization of the laser to become stable linear polarization;

[0017] The first lens is used to adjust the laser spot size to increase the maximum coupling efficiency into the optical fiber;

[0018] The first pair of reflectors is used to adjust the relative position of the laser spot and the coupling head to improve the maximum coupling efficiency of the laser beam entering the single-mode polarization-maintaining optical fiber. The laser beam is emitted through the single-mode polarization-maintaining optical fiber and passes through the third λ / 2 wave plate and Glan-Taylor prism combination to adjust the polarization of the laser beam entering the electro-optic modulator to meet the incident polarization requirement of the electro-optic modulator.

[0019] Furthermore, the fiber laser frequency stabilization system further comprises:

[0020] The second component includes a second polarization beam splitter prism and a second λ / 4 wave plate; the laser light from the electro-optic modulator passes through the second polarization beam splitter prism and the second λ / 4 wave plate to generate circularly polarized light.

[0021] Furthermore, the fiber laser frequency stabilization system further comprises:

[0022] The second pair of reflecting mirrors generates circularly polarized light through the second polarization beam splitter prism and the second λ / 4 wave plate, and then the circularly polarized light passes through the second pair of reflecting mirrors and vertically enters the ultra-stable cavity from the laser incident port of the vacuum chamber; the second pair of reflecting mirrors is used to adjust the mode of the laser to match the mode of the ultra-stable cavity.

[0023] Furthermore, the optical path of the laser is U-shaped.

[0024] Furthermore, the two polarization beam splitters and the second λ / 4 wave plate are used to adjust polarization so that the power reflected by the ultra-stable cavity is transmitted through the second polarization beam splitter to a minimum; the second pair of reflectors is used to adjust the resonance between the laser and the cavity to eliminate other high-order modes.

[0025] Furthermore, the reflected laser with frequency jitter information is generated by the following components:

[0026] λ / 4 wave plate, polarization beam splitter, photoelectric detector; the laser with frequency jitter information reflected from the vacuum chamber passes through the λ / 4 wave plate and the polarization beam splitter, and the linearly polarized light passes through the photoelectric detector to obtain an electrical signal, which is input into a PID-based digital circuit module controller.

[0027] In a second aspect, the present invention provides a method for stabilizing the frequency of a fiber laser PDH, comprising:

[0028] The single-frequency laser output by the fiber laser is passed through an electro-optic modulator to generate two sidebands around it with a frequency difference of a certain value;

[0029] The modulated laser is injected into an ultra-stable optical cavity. When the laser frequency matches the resonant frequency of the ultra-stable optical cavity, a standing wave is formed in the ultra-stable optical cavity.

[0030] The laser reflected from the ultra-stable optical cavity is mixed with the driving signal of the electro-optic modulator through a photodetector to obtain an error signal containing frequency detuning information;

[0031] The error signal is fed back to the piezoelectric ceramic and fiber-integrated acousto-optic modulator built into the laser through a PID-based digital circuit module controller, and the low-frequency and high-frequency compensation of the laser frequency is performed to achieve the locking of the ultra-stable cavity.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention applies a PDH modulation signal to the electro-optic modulator to phase modulate the laser. The error signal obtained by the photodetector is fed back to the PID-based digital circuit module controller to suppress the frequency fluctuation of the laser and lock it on the optical cavity, thereby narrowing the laser line width and stabilizing the frequency for a long time. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the 729nm laser PDH optical path and corresponding frequency-locking electronic components provided in an embodiment of the present invention. Among them: HWP (Half Wave Plat): λ / 2 wave plate; PBS (Polarization Splitting Prism): polarization splitter prism; L (Len): lens; Coupler: coupling head; PMF (Single-mode Polarization-maintaining Fiber): single-mode polarization-maintaining fiber; GTP (Grant-Taylor Prism): Grant-Taylor prism; QWP (Quarter Wave Plat): λ / 4 wave plate; EOM (Electrooptical Modulator): electro-optical modulator; HR (High Reflector): high reflector; PD (Photo-Detector): photodetector; Monitor: oscilloscope; PS (Phase Shifter): phase shifter; Mixer: mixer; LPF (Low-Pass Filter): low-pass filter; SG (Signal Generator): RF signal source; Ultra-stable Cavity: ultra-stable cavity (Fabry-Perot optical resonant cavity);

[0035] Figure 2 Schematic diagram of error signal obtained by scanning frequency of 729nm laser near fundamental mode;

[0036] Figure 3 Schematic diagram of the error signal after 729nm laser locking. DETAILED DESCRIPTION

[0037] Example:

[0038] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0039] PDH (Pound-Drever-Hall) frequency stabilization is a technology that uses an ultra-stable optical cavity as a frequency reference and can achieve precise control of the laser frequency through electro-optical modulation and phase demodulation. The principle of the fiber laser PDH frequency stabilization method in the present invention is as follows:

[0040] First, the single-frequency laser output by the fiber laser is passed through an electro-optic modulator to generate two sidebands around it with a certain frequency difference.

[0041] The modulated laser is then injected into an ultrastable optical cavity, which has high finesse and low loss. A standing wave can only be formed in the cavity when the laser frequency matches the resonant frequency of the cavity. The laser reflected from the cavity is then mixed with the driving signal of the electro-optic modulator through a photodetector to obtain an error signal containing frequency detuning information.

[0042] Finally, the error signal is fed back to the piezoelectric ceramic and fiber-integrated acousto-optic modulator built into the laser through a servo control system to compensate for the low and high frequencies of the laser, thereby achieving locking of the ultra-stable cavity.

[0043] In addition, an embodiment of the present invention provides a fiber laser frequency stabilization system, which mainly includes an electro-optic modulator, a radio frequency signal source, a mixer, a low-pass filter, and a digital circuit module controller based on.

[0044] The electro-optic modulator is used to receive the PDH modulation signal and perform phase modulation on the laser; the RF signal source is used to apply the PDH modulation signal delta to the electro-optic modulator. m (δ m The PID-based digital circuit module controller is used to receive the error signal, output the opposite voltage signal according to the deviation between the signal information and the set value, and adjust the laser drive signal to suppress the frequency fluctuation of the laser and lock it on the ultra-stable cavity.

[0045] It can be seen that this system performs phase modulation on the laser by applying a PDH modulation signal to the electro-optic modulator. The error signal obtained by the photodetector is fed back to the PID-based digital circuit module controller, which gives a feedback signal based on the error signal obtained to control the frequency jitter of the laser, thereby suppressing the frequency fluctuation of the laser and locking it on the optical cavity, thereby narrowing the laser line width and stabilizing the frequency for a long time.

[0046] In a specific embodiment, the reflected laser with frequency jitter information is generated by the following components:

[0047] λ / 4 wave plate, polarization beam splitter, photoelectric detector; the laser with frequency jitter information reflected from the vacuum chamber passes through the λ / 4 wave plate and the polarization beam splitter, and the linearly polarized light passes through the photoelectric detector to obtain an electrical signal, which is input into a PID-based digital circuit module controller.

[0048] In a preferred embodiment, the above-mentioned fiber laser frequency stabilization system further includes:

[0049] The first component includes a first λ / 2 wave plate, a first polarization splitter prism, a first reflector, a second λ / 2 wave plate, a first λ / 4 wave plate, a first lens, a first pair of reflectors, a first coupling head, a single-mode polarization-maintaining optical fiber, a second coupling head, a second λ / 4 wave plate, and a Glan-Taylor prism.

[0050] The laser becomes vertically polarized light after passing through the first λ / 2 wave plate, and then passes through the first polarization beam splitter prism, the first reflector, the second λ / 2 wave plate, the first λ / 4 wave plate, the first lens, the single-mode polarization-maintaining optical fiber, the third λ / 2 wave plate, the Glan-Taylor prism, and then the electro-optic modulator.

[0051] The polarization beam splitter prism transmits horizontal polarization and reflects vertical polarization. The first λ / 2 wave plate and the first polarization beam splitter prism are combined to roughly adjust the polarization direction of the laser to make it linearly polarized light. The first λ / 4 wave plate and the second λ / 2 wave plate accurately control the polarization of the laser to become stable linear polarization. The lens is used to adjust the size of the laser spot to increase the maximum coupling efficiency entering the optical fiber. The first pair of reflectors is used to adjust the relative position of the laser spot and the coupling head to increase the maximum coupling efficiency entering the optical fiber. For single-mode polarization-maintaining optical fiber, the position of the frequency-stabilizing cavity is far from the position of the laser, and needs to be pulled there by optical fiber. The laser is emitted from the other end of the optical fiber and passes through the third λ / 2 wave plate and Glan-Taylor prism combination to finely adjust the polarization entering the electro-optic modulator to meet the strict incident polarization requirements of the electro-optic modulator.

[0052] In a preferred embodiment, the above-mentioned fiber laser frequency stabilization system also includes a second component, which includes a second polarization beam splitter prism and a second λ / 4 wave plate; the laser from the electro-optic modulator passes through the second polarization beam splitter prism and the second λ / 4 wave plate to generate circularly polarized light.

[0053] In a preferred embodiment, the above-mentioned fiber laser frequency stabilization system also includes a second pair of mirrors, which generates circularly polarized light through a second polarization beam splitter prism and a second λ / 4 wave plate, and then passes through the second pair of mirrors and vertically enters the ultra-stable cavity from the laser incident port of the vacuum chamber; the second pair of mirrors is used to adjust the mode of the laser to match the mode of the ultra-stable cavity.

[0054] Since the laser resonates with the ultra-stable cavity, the beam waist radius of the laser needs to match that of the ultra-stable cavity. The distance is long, starting from the optical fiber output and ending at the beam waist of the ultra-stable cavity. Due to limited space, a U-shaped optical path is used. The second polarization beam splitter prism and the second λ / 4 wave plate are used to adjust the polarization so that the power reflected by the ultra-stable cavity is transmitted through the second polarization beam splitter prism to a minimum. The second pair of mirrors is used to adjust the resonance between the laser and the cavity and eliminate other high-order modes.

[0055] The working principle of the fiber laser frequency stabilization system of the present invention is further explained below by taking a laser with a wavelength of 729 nm as an example. Figure 1 As shown, the specific steps include:

[0056] Step 1: The laser with a wavelength of 729nm passes through the λ / 2 wave plate HWP1 and the polarization beam splitter PBS1 to select vertical polarized light, and the horizontal polarized light is used for the power requirements of other optical paths. The vertical polarized light passes through the reflector HR1, the λ / 2 wave plate HWP2, the λ / 4 wave plate QWP1, the lens L1, a pair of reflectors HR2, HR3, and the coupling head Coupler1 in sequence to couple into the single-mode polarization-maintaining fiber PMF.

[0057] Step 2: The single-mode polarization-maintaining fiber PMF introduces the light into the frequency-locked optical path module at a distance, and then emits it through the coupling head Coupler2, passes through the λ / 2 wave plate HWP3, and the Glan Taylor prism GTP; the laser passes through the fiber-coupled continuous electro-optic modulator EOM (using Thorlabs, EO-PM-NR-C1), and the PDH modulation signal δ is applied to the EOM through the RF signal source SG. m In this embodiment, δ m =20MHz,δ m is the modulation frequency (PDH modulation frequency 0-50MHz);

[0058] Step 3, the phase-modulated laser generates elliptically polarized light through a second polarization beam splitter prism PBS2 and a second λ / 4 wave plate QWP2;

[0059] Step 4: Finally, the laser passes through a pair of reflectors HR4 and HR5 and is vertically incident into an ultra-stable cavity, wherein the pair of reflectors HR4 and HR5 are used to adjust the mode of the laser to match the mode of the ultra-stable cavity;

[0060] Step 5. The laser reflects back and forth many times in the ultra-stable cavity. When the laser frequency is inconsistent with the laser frequency matched by the ultra-stable cavity, the laser will be reflected by the concave cavity mirror and then transmitted by the planar cavity mirror. This part of the laser with frequency jitter information will pass through the λ / 4 wave plate QWP2 and the polarization beam splitter prism PBS2 again to become linearly polarized light, and the signal will be detected by the photodetector PD1.

[0061] Step 6. The AC part of the signal detected by the photodetector PD1 is input into the PID-based digital circuit module controller, and the DC part is input into the oscilloscope to monitor the change of the reflected photoelectric signal. The photodetector PD2 is used to monitor the transmitted photoelectric signal, and the CCD is used to observe and detect the laser mode. The AC part of the signal input into the PID-based digital circuit module controller is mixed with the local RF signal source inside the digital circuit module controller, and the phase of the phase shifter PS is adjusted to obtain the modulated and transferred dispersion signal, and then the error signal is obtained by passing through the low-pass bandwidth filter LPF, such as Figure 2 As shown. The PDH signal used as the frequency discrimination signal continuously adjusts the laser drive signal according to the deviation between the feedback information and the set value, thereby suppressing external interference and stabilizing the laser frequency. Figure 3 It can be clearly seen that the error signal value after locking is 70mV. This small value indicates that the system is very stable after locking, and the locking time is as long as 12 hours.

[0062] It can be seen that the 729nm fiber laser involved in the present invention has a simple structure, an all-fiber optical path, is stable and reliable, and can achieve true maintenance-free.

[0063] In summary, compared with the prior art, the present invention has the following technical advantages:

[0064] 1. It can effectively suppress the influence of the laser itself and external environmental factors on its frequency stability, such as temperature, vibration, current noise, etc.;

[0065] 2. It can realize fast tracking and locking of ultra-narrow linewidth ultra-stable cavities, avoiding the loss of lock caused by cavity length drift;

[0066] 3. The ultrastable cavity can be used as an absolute frequency standard to achieve accurate measurement and calibration of the laser frequency;

[0067] 4. No additional frequency shifting device is required. The laser can be frequency modulated and locked on an optical ultra-stable cavity, thereby narrowing the laser line width. It is suitable for fields such as precision spectroscopy, precision measurement, and quantum information.

[0068] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made based on the essence of the content of the present invention should be included in the protection scope of the present invention.

Claims

1. A fiber laser frequency stabilization system, characterized in that: include: An electro-optical modulator, used to receive a PDH modulation signal and perform phase modulation on the laser; A radio frequency signal source, used for applying a PDH modulation signal to the electro-optical modulator and providing a local reference signal; The mixer mixes the reflected laser electrical signal with frequency jitter information with the local reference signal to obtain a dispersion signal; the low-pass filter filters out the high-frequency components in the dispersion signal and retains the DC part as the error signal; A PID-based digital circuit module controller is used to receive the error signal and output an opposite voltage signal according to the deviation between the signal information and the set value to adjust the laser drive signal to suppress the frequency fluctuation of the laser and lock it on the ultra-stable cavity; Also includes: A first component, the first component comprising a first λ / 2 wave plate, a first polarization beam splitter, a first reflector, a second λ / 2 wave plate, a first λ / 4 wave plate, a first lens, a first pair of reflectors, a first coupling head, a single-mode polarization-maintaining optical fiber, a second coupling head, a second λ / 4 wave plate, and a Glan-Taylor prism; The laser light becomes vertically polarized light after passing through the first λ / 2 wave plate, and then passes through the first polarization beam splitter prism, the first reflector, the second λ / 2 wave plate, the first λ / 4 wave plate, the first lens, the single-mode polarization-maintaining optical fiber, the third λ / 2 wave plate, the Glan-Taylor prism, and then passes through the electro-optic modulator. The first λ / 2 wave plate and the first polarization beam splitter are combined to adjust the polarization direction of the laser to make it linearly polarized light; The first λ / 4 wave plate and the second λ / 2 wave plate are used to control the polarization of the laser to become stable linear polarization; The first lens is used to adjust the laser spot size to increase the maximum coupling efficiency into the optical fiber; The first pair of reflectors is used to adjust the relative position of the laser spot and the coupling head to improve the maximum coupling efficiency of the laser beam entering the single-mode polarization-maintaining optical fiber. The laser beam is emitted through the single-mode polarization-maintaining optical fiber and passes through the third λ / 2 wave plate and Glan-Taylor prism combination to adjust the polarization of the laser beam entering the electro-optic modulator to meet the incident polarization requirement of the electro-optic modulator.

2. The fiber laser frequency stabilization system according to claim 1, characterized in that: Also includes: a second component, the second component comprising a second polarization beam splitter prism and a second λ / 4 wave plate; The laser light from the electro-optic modulator passes through a second polarization beam splitter prism and a second λ / 4 wave plate to generate circularly polarized light.

3. The fiber laser frequency stabilization system according to claim 2, characterized in that: Also includes: The second pair of reflecting mirrors generates circularly polarized light through the second polarization beam splitter prism and the second λ / 4 wave plate, and then the circularly polarized light passes through the second pair of reflecting mirrors and vertically enters the ultra-stable cavity from the laser incident port of the vacuum chamber; The second pair of reflecting mirrors is used to adjust the mode of the laser to match the mode of the ultra-stable cavity.

4. The fiber laser frequency stabilization system according to claim 1, characterized in that: The optical path of the laser is U-shaped.

5. The fiber laser frequency stabilization system according to claim 3, characterized in that: The two polarization beam splitters and the second λ / 4 wave plate are used to adjust polarization so that the power reflected by the ultra-stable cavity is transmitted through the second polarization beam splitter at a minimum; the second pair of reflectors is used to adjust the resonance between the laser and the cavity and eliminate other high-order modes.

6. The fiber laser frequency stabilization system according to claim 1, characterized in that: The reflected laser with frequency jitter information is generated by the following components: λ / 4 wave plate, polarization beam splitter, photoelectric detector; the laser with frequency jitter information reflected from the vacuum chamber passes through the λ / 4 wave plate and the polarization beam splitter, and the linearly polarized light passes through the photoelectric detector to obtain an electrical signal, which is input into a PID-based digital circuit module controller.

7. A fiber laser PDH frequency stabilization method, based on the fiber laser frequency stabilization system according to claim 1, characterized in that: include: The single-frequency laser output by the fiber laser is passed through an electro-optic modulator to generate two sidebands around it with a frequency difference of a certain value; The modulated laser is injected into an ultra-stable optical cavity. When the laser frequency matches the resonant frequency of the ultra-stable optical cavity, a standing wave is formed in the ultra-stable optical cavity. The laser reflected from the ultra-stable optical cavity is mixed with the driving signal of the electro-optic modulator through a photodetector to obtain an error signal containing frequency detuning information; The error signal is fed back to the piezoelectric ceramic and fiber-integrated acousto-optic modulator built into the laser through a PID-based digital circuit module controller, respectively, to compensate the low and high frequencies of the laser to achieve locking of the ultra-stable cavity.

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