An ultra-narrow linewidth laser system

Abstract

The present invention discloses an ultra-narrow linewidth laser system, which relates to the field of ultra-narrow linewidth lasers. In the system, a beam splitting module, an electro-optic modulator, an adjustment module, a quarter-wave plate, and an optical reference cavity are arranged in sequence. The beam splitting module is arranged on the laser output optical path of the laser. The laser generates a reflected beam and a transmitted beam through the beam splitting module. The electro-optic modulation module is used to perform phase modulation on the transmitted beam. The adjustment module is used to adjust the optical power coupled into the optical reference cavity. The transmitted light of the adjustment module enters the optical reference cavity through the quarter-wave plate. The reflected light and the transmitted light of the optical reference cavity pass through the quarter-wave plate, the adjustment module, and the photodetector in sequence. The electro-optic modulator driver is connected to the electro-optic modulator. The mixer is respectively connected to the photodetector, the electro-optic modulator driver, and the servo control system. The servo control system is also connected to the electro-optic crystal. The present invention overcomes the problems of tuning hysteresis behavior and laser output power jitter when using piezoelectric ceramics.

Description

Technical Field

[0001] The present invention relates to the field of ultra-narrow linewidth lasers, and in particular to an ultra-narrow linewidth laser system. Background Art

[0002] As a precision measurement tool, an ultra-narrow linewidth laser with an extremely narrow linewidth and extremely high frequency stability has important applications in scientific research and engineering application fields such as ultra-precision laser spectroscopy, measurement of basic physical quantities, optical atomic clocks, and gravitational wave detection. Generally, the laser frequency emitted by a laser is locked to the resonance frequency of an optical reference cavity through the Pound-Drever-Hall (PDH) frequency stabilization technique to obtain an ultra-narrow linewidth laser.

[0003] External cavity semiconductor lasers are inexpensive and easy to use, have a high emission power and can cover a wide wavelength range, and are the most commonly selected lasers for ultra-narrow linewidth laser systems. In a traditional ultra-narrow linewidth laser system, the laser frequency is resonated with the optical reference cavity by scanning the voltage on a piezoelectric ceramic, and the obtained error signal is fed back and adjusted to the drive current of the laser through a servo control circuit, thereby realizing the locking of the laser frequency and obtaining an ultra-narrow linewidth laser.

[0004] In this process, the hysteresis behavior of the traditional component piezoelectric ceramic in changing the laser frequency will cause the correspondence between the laser frequency and the applied voltage to be inaccurate, as shown in Figure 1 (b) in the appendix. Figure 1 (b) in the appendix is a graph showing the relationship between voltage and frequency change when using a piezoelectric ceramic to achieve frequency adjustment. Thus, when the ultra-narrow linewidth laser system loses lock and then relocks, the parameters of the servo control circuit need to be readjusted, increasing the maintenance cost. Especially for an unattended automatic locking system, it will increase the complexity of the automatic locking algorithm and thus prolong the locking time. On the other hand, by locking the laser frequency by feedback-adjusting the drive current of the laser, the change in the drive current of the laser diode will cause the output optical power of the laser to jitter, as shown in Figure 2 shown, thereby affecting the performance of the ultra-narrow linewidth laser. Summary of the Invention

[0005] The object of the present invention is to provide an ultra-narrow linewidth laser system to overcome the tuning hysteresis behavior and the jitter of the output power of the laser when using a piezoelectric ceramic.

[0006] To achieve the above object, the present invention provides the following solution:

[0007] An ultra-narrow linewidth laser system, comprising: a laser, a beam splitting module, an electro-optic modulator, an adjustment module, a quarter-wave plate, an optical reference cavity, an electro-optic modulator driver, a photodetector, a mixer, and a servo control system;

[0008] The beam splitting module is arranged on the laser output optical path of the laser; the beam splitting module, the electro-optic modulator, the adjustment module, the quarter-wave plate and the optical reference cavity are arranged in sequence; the laser generates a reflected beam and a transmitted beam through the beam splitting module; the reflected beam is output as an ultra-narrow linewidth laser; the electro-optic modulation module is used for phase modulation of the transmitted beam; the adjustment module is used to adjust the optical power coupled into the optical reference cavity; the laser after phase modulation generates a transmitted light through the adjustment module; the transmitted light generated by the adjustment module enters the optical reference cavity through the quarter-wave plate; the reflected light and transmitted light of the optical reference cavity sequentially pass through the quarter-wave plate, the adjustment module and the photodetector; the electro-optic modulator driver is connected to the electro-optic modulator; the mixer is respectively connected to the photodetector, the electro-optic modulator driver and the servo control system;

[0009] The frequency adjustment and feedback device in the laser is an electro-optic crystal; the servo control system is also connected to the electro-optic crystal.

[0010] Optionally, the laser further includes a laser diode, a collimating lens, an interference filter and a partial mirror; the laser diode, the collimating lens, the electro-optic crystal, the interference filter and the partial mirror are arranged in sequence.

[0011] Optionally, the partial mirror is a partial reflection film with a reflectivity of 28% plated on the front end face; the reflectivity is the reflectivity of the laser output by the laser.

[0012] Optionally, the beam splitting module specifically includes a first half-wave plate and a first polarization beam splitter prism arranged in sequence; the first polarization beam splitter prism is used to split the laser output by the first half-wave plate into a reflected beam and a transmitted beam.

[0013] Optionally, the adjustment module specifically includes a second half-wave plate and a second polarization beam splitter prism arranged in sequence; the reflected light and transmitted light of the optical reference cavity sequentially pass through the quarter-wave plate and the second polarization beam splitter prism and are input to the photodetector.

[0014] Optionally, the ultra-narrow linewidth laser system further includes a voltage amplifier; the voltage amplifier is arranged between the electro-optic crystal and the servo control system; the voltage amplifier is respectively connected to the electro-optic crystal and the servo control system.

[0015] Optionally, the collimating lens is an aspherical lens.

[0016] Optionally, the bandwidth of the interference filter is 0.3 nm; the transmittance of the interference filter at the central wavelength is 93%; the angle between the interference filter and the vertical direction is 6°; the vertical direction is perpendicular to the output direction of the laser.

[0017] Optionally, the electro-optic crystal is a lithium niobate crystal.

[0018] According to the specific embodiments provided by the present invention, the following technical effects are disclosed:

[0019] In the present invention, the beam splitting module is arranged on the laser output optical path of the laser; the beam splitting module, the electro-optic modulator, the adjustment module, the quarter-wave plate and the optical reference cavity are arranged in sequence; the laser generates a reflected beam and a transmitted beam through the beam splitting module; the reflected beam is output as an ultra-narrow linewidth laser; the electro-optic modulation module is used for phase modulation of the transmitted beam; the adjustment module is used to adjust the optical power coupled into the optical reference cavity; the laser beam after phase modulation generates transmitted light through the adjustment module; the transmitted light generated by the adjustment module enters the optical reference cavity through the quarter-wave plate; the reflected light and the transmitted light of the optical reference cavity pass through the quarter-wave plate, the adjustment module and the photodetector in sequence; the electro-optic modulator driver is connected to the electro-optic modulator; the mixer is respectively connected to the photodetector, the electro-optic modulator driver and the servo control system; the frequency adjustment and feedback device in the laser is an electro-optic crystal; the servo control system is also connected to the electro-optic crystal. The present invention uses an electro-optic crystal to change the laser frequency to overcome the tuning hysteresis behavior that occurs when using a piezoelectric ceramic, and the correspondence between the voltage and the laser frequency is accurate; it is beneficial to the locking of the ultra-narrow linewidth laser system; for the ultra-narrow linewidth laser system with electro-optic crystal feedback, the electro-optic crystal is modulated to avoid the jitter of the output optical power of the laser. Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 It is a relationship diagram of voltage and frequency changes when using a lithium niobate crystal and a piezoelectric ceramic to achieve frequency adjustment;

[0022] Figure 2 It is a laser power jitter diagram when using current feedback and electro-optic crystal feedback;

[0023] Figure 3 It is a schematic diagram of the ultra-narrow linewidth laser system provided by the present invention;

[0024] Figure 4The figure shows the result of testing the laser linewidth after locking.

[0025] Symbol description:

[0026] 1 - Laser diode, 2 - Collimating lens, 3 - Electro - optic crystal, 4 - Interference filter, 5 - Partial mirror, 6 - First half - wave plate, 7 - First polarization beam splitter prism, 8 - Electro - optic modulator, 9 - Second half - wave plate, 10 - Second polarization beam splitter prism, 11 - Quarter - wave plate, 12 - Optical reference cavity, 13 - Electro - optic modulator driver, 14 - Photo - detector, 15 - Mixer, 16 - Servo control system, 17 - Voltage amplifier. Specific embodiments

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0028] The object of the present invention is to provide an ultra - narrow linewidth laser system to overcome the tuning hysteresis behavior and the jitter problem of the laser output power when using piezoelectric ceramics.

[0029] To make the above - mentioned objects, features and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0030] As Figure 3 shown, an ultra - narrow linewidth laser system provided by the present invention includes: a laser, a beam - splitting module, an electro - optic modulator 8, an adjustment module, a quarter - wave plate 11, an optical reference cavity 12, an electro - optic modulator driver 13, a photo - detector 14, a mixer 15 and a servo control system 16.

[0031] The beam splitting module is arranged on the laser output optical path of the laser; the beam splitting module, the electro-optic modulator 8, the adjustment module, the quarter-wave plate 11, and the optical reference cavity 12 are arranged in sequence; the laser generates a reflected beam and a transmitted beam through the beam splitting module; the reflected beam is output as ultra-narrow linewidth laser; the electro-optic modulation module is used for phase modulation of the transmitted beam; the adjustment module is used to adjust the optical power coupled into the optical reference cavity; the laser after phase modulation generates transmitted light through the adjustment module; the transmitted light generated by the adjustment module enters the optical reference cavity 12 through the quarter-wave plate 11; the reflected light and the transmitted light of the optical reference cavity 12 sequentially pass through the quarter-wave plate 11, the adjustment module, and the photodetector 14; the electro-optic modulator driver 13 is connected to the electro-optic modulator 8; the mixer 15 is respectively connected to the photodetector 14, the electro-optic modulator driver 13, and the servo control system 16.

[0032] The frequency adjustment and feedback device in the laser is the electro-optic crystal 3; the servo control system 16 is also connected to the electro-optic crystal 3. The electro-optic crystal 3 is a lithium niobate crystal. The electro-optic crystal 3 has two functions: continuously changing the frequency of the laser output by the laser when scanning the voltage, and serving as an actuator when locked.

[0033] In practical applications, the laser further includes a laser diode 1, a collimating lens 2, an interference filter 4, and a partial reflector 5; the laser diode 1, the collimating lens 2, the electro-optic crystal 3, the interference filter 4, and the partial reflector 5 are arranged in sequence. The collimating lens 2 is used to collimate the laser emitted by the laser diode 1 into parallel light. The partial reflector 5 has a partial reflection film with a reflectivity of 28% plated on the front end face; the reflectivity is the reflectivity of the laser output by the laser. The collimating lens 2 is an aspherical lens. The bandwidth of the interference filter 4 is 0.3 nm; the transmittance of the central wavelength of the interference filter 4 is 93%; the angle between the interference filter 4 and the vertical direction is 6°; the vertical direction is perpendicular to the output direction of the laser.

[0034] In practical applications, the beam splitting module specifically includes a first half-wave plate 6 and a first polarization beam splitter prism 7 arranged in sequence; the first polarization beam splitter prism 7 is used to split the laser output by the first half-wave plate 6 into a reflected beam and a transmitted beam. The first half-wave plate 6 and the first polarization beam splitter prism 7 are used in combination to divide the laser into a reflected beam and a transmitted beam, where the reflected beam is output as ultra-narrow linewidth laser, and the transmitted beam is used for system locking.

[0035] In practical applications, the adjustment module specifically includes a second half-wave plate 9 and a second polarization beam splitter prism 10 arranged in sequence; the reflected light and the transmitted light of the optical reference cavity 12 sequentially pass through the quarter-wave plate 11 and the second polarization beam splitter prism 10 and are input into the photodetector 14. The second half-wave plate 9 and the second polarization beam splitter prism 10 are used to adjust the optical power coupled into the optical reference cavity 12. Among them, after the laser passes through the second polarization beam splitter prism 10, transmitted light and reflected light are generated. In the present invention, the generated reflected light is not considered. A part of the transmitted light is directly reflected on the cavity surface of the optical reference cavity 12 to generate reflected light, which passes through the quarter-wave plate 11 and the second polarization beam splitter prism 10 and is input into the photodetector 14. Another part of the transmitted light is coupled into the optical reference cavity 12 and exits from the optical reference cavity 12 to generate transmitted light, which passes through the quarter-wave plate 11 and the second polarization beam splitter prism 10 and is input into the photodetector 14.

[0036] In practical applications, the ultra-narrow linewidth laser system further includes a voltage amplifier 17; the voltage amplifier 17 is arranged between the electro-optic crystal 3 and the servo control system 16; the voltage amplifier 17 is respectively connected to the electro-optic crystal 3 and the servo control system 16.

[0037] In practical applications, the reflected light and the transmitted light of the optical reference cavity 12 return along the original path, pass through the quarter-wave plate 11 and the second polarization beam splitter prism 10 and then enter the photodetector 14. The photodetector 14 receives this signal and demodulates it with another signal generated by the electro-optic modulator driver 13 through the mixer 15 to obtain the frequency discrimination signal for locking the laser. After the frequency discrimination signal is input into the servo control system 16, it is amplified by the voltage amplifier 17 and then modulates the electro-optic crystal 3 of the laser element, so as to realize the feedback control of the laser frequency and achieve the purpose of locking the laser frequency.

[0038] When an external voltage is applied to the electro-optic crystal 3, its refractive index will change, and its change value will change with the change of the external voltage. When the voltage applied to the electro-optic crystal 3 is V, the change in the equivalent cavity length of the external cavity of the laser is dL = λV / (2V π ), where λ is the wavelength of the laser, and V π is the half-wave voltage of the electro-optic crystal 3, and then the change in the output light frequency of the laser is: Among them, c is the speed of light in vacuum, and L is the equivalent cavity length of the external cavity.

[0039] Taking the ultra-narrow linewidth laser system with a wavelength of 698 nm as an example:

[0040] The central wavelength of the laser output by the laser diode 1 is 698 nm. The collimating lens 2 is an aspherical lens with a focal length of 4 mm, which is used to collimate the laser emitted by the laser diode 1 into parallel light. The electro-optic crystal 3 is a lithium niobate crystal (LiNbO3) with a cross-section of 4 mm × 3 mm and a length of 7 mm. The interference filter 4 has a bandwidth of 0.3 nm and a transmittance of 93% at the central wavelength, and the installation angle forms an angle of 6° with the vertical direction. The partial reflector 5 is coated with a partial reflection film with a reflectivity of 28% @ 698 nm on the front end face, and forms an external cavity of the laser with the semiconductor laser diode. The laser emitted by the laser passes through the first half-wave plate 6 and the first polarization beam splitter prism 7 to divide the laser into a reflected beam and a transmitted beam, where the reflected beam is the ultra-narrow linewidth laser output, and the transmitted beam is used for system locking; the electro-optic modulator driver 13 generates a signal with a driving frequency of 19 MHz to perform phase modulation on the laser; the second half-wave plate 9 and the second polarization beam splitter prism 10 are used to adjust the optical power coupled into the optical reference cavity 12; the finesse of the optical reference cavity 12 is 200,000, and the reflected light and transmitted light of the optical reference cavity 12 return along the original path, and after passing through the quarter-wave plate 11 and the second polarization beam splitter prism 10, enter the photodetector 14. The photodetector 14 receives this signal and demodulates it with another 19 MHz signal generated by the electro-optic modulator driver 13 through the mixer 15 to obtain the frequency discrimination signal for locking the laser. The frequency discrimination signal is input to the servo control system 16 and amplified by the voltage amplifier 17, and then modulates the electro-optic crystal 3 of the laser component, so as to realize the feedback control of the laser frequency and achieve the purpose of locking the laser frequency. Figure 4 The result of testing the laser linewidth after locking.

[0041] An ultra-narrow linewidth laser system based on the feedback of the electro-optic crystal 3 of the present invention has the following advantages:

[0042] Figure 1 In (a) of [reference], it is the relationship diagram of the voltage and frequency change when using the lithium niobate crystal and the piezoelectric ceramic to realize frequency adjustment. As shown in (a) of Figure 1 [reference], when using the electro-optic crystal 3 to change the laser frequency, there will be no tuning hysteresis behavior that appears when using the piezoelectric ceramic, and the corresponding relationship between the voltage and the laser frequency is accurate; it is beneficial to the locking of the ultra-narrow linewidth laser system; for the ultra-narrow linewidth laser system with the feedback of the electro-optic crystal 3, modulating the electro-optic crystal 3 will not cause the jitter of the output optical power of the laser.

[0043] Each embodiment in this specification is described in a progressive manner. The key points of each embodiment are the differences from other embodiments. The same and similar parts between the embodiments can be referred to each other.

[0044] In this article, specific examples are used to elaborate on the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the device of the present invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manners and application scopes. To sum up, the content of this specification should not be construed as a limitation on the present invention.

Claims

1. A ultra-narrow linewidth laser system, characterized in that, Comprising: A laser, a beam splitting module, an electro-optic modulator, an adjustment module, a quarter-wave plate, an optical reference cavity, an electro-optic modulator driver, a photodetector, a mixer, and a servo control system; The beam splitting module is disposed on the laser output optical path of the laser; the beam splitting module, the electro-optic modulator, the adjustment module, the quarter-wave plate, and the optical reference cavity are sequentially disposed; the laser generates a reflected beam and a transmitted beam through the beam splitting module; the reflected beam is output as an ultra-narrow linewidth laser; the electro-optic modulator is used for phase modulating the transmitted beam; the adjustment module is used for adjusting the optical power coupled into the optical reference cavity; the laser after phase modulation generates transmitted light through the adjustment module; The transmitted light generated by the adjustment module enters the optical reference cavity through the quarter-wave plate; The reflected light and the transmitted light of the optical reference cavity sequentially pass through the quarter-wave plate, the adjustment module, and the photodetector; The electro-optic modulator driver is connected to the electro-optic modulator; The mixer is respectively connected to the photodetector, the electro-optic modulator driver, and the servo control system; The frequency adjustment and feedback device in the laser is an electro-optic crystal; the servo control system is also connected to the electro-optic crystal.

2. The ultra-narrow linewidth laser system according to claim 1, characterized in that, The laser further includes a laser diode, a collimating lens, an interference filter, and a partial mirror; the laser diode, the collimating lens, the electro-optic crystal, the interference filter, and the partial mirror are sequentially disposed.

3. The ultra-narrow linewidth laser system according to claim 2, wherein The partial mirror is a partial reflection film with a reflectivity of 28% coated on the front end face; the reflectivity is the reflectivity of the laser output by the laser.

4. The ultra-narrow linewidth laser system according to claim 1, characterized in that, The beam splitting module specifically includes a first half-wave plate and a first polarization beam splitter prism disposed in sequence; the first polarization beam splitter prism is used for splitting the laser output by the first half-wave plate into a reflected beam and a transmitted beam.

5. The ultra-narrow linewidth laser system according to claim 1, wherein, The adjustment module specifically includes a second half-wave plate and a second polarization beam splitter prism disposed in sequence; the reflected light and the transmitted light of the optical reference cavity sequentially pass through the quarter-wave plate and the second polarization beam splitter prism and are input to the photodetector.

6. The ultra-narrow linewidth laser system according to claim 1, wherein It further includes a voltage amplifier; the voltage amplifier is disposed between the electro-optic crystal and the servo control system; the voltage amplifier is respectively connected to the electro-optic crystal and the servo control system.

7. The ultra-narrow linewidth laser system according to claim 2, characterized in that, The collimating lens is an aspherical lens.

8. The ultra-narrow linewidth laser system according to claim 2, characterized in that, The bandwidth of the interference filter is 0.3 nm; the transmittance of the center wavelength of the interference filter is 93%; the angle between the interference filter and the vertical direction is 6°; the vertical direction is perpendicular to the output direction of the laser.

9. The ultra-narrow linewidth laser system according to claim 1, wherein, The electro-optic crystal is a lithium niobate crystal.

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