Photoelectric hybrid feedback laser based on Fano resonance in crystal echo wall cavity

Abstract

The invention discloses a photoelectric hybrid feedback laser based on Fano resonance in a crystal echo wall cavity, which comprises an original laser, an optical signal emitted by the original laser sequentially passes through an optical fiber circulator and an optical fiber coupler, an optical signal output by the optical fiber coupler is divided into two beams, one beam of optical signal is transmitted to a laser output end, and the other beam of optical signal is transmitted to a laser output end. The other beam of optical signal enters the prism crystal cavity coupling system to generate Fano resonance, the Fano optical signal output from the prism crystal cavity coupling system is divided into two paths, one path of Fano optical signal returns to the optical fiber circulator through the optical fiber delay line and then is re-injected into the original laser, and the feedback loop is an optical feedback loop; and the other path of Fano optical signal passes through a photoelectric detector to generate an error electric signal for active frequency locking, the electric signal is transmitted to a PID servo controller, and the electric signal passing through the PID servo controller is transmitted to an original laser to form an electric feedback loop. The laser is high in feedback signal strength, simple in overall system structure, low in noise and high in reliability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lasers, and particularly refers to an optoelectronic hybrid feedback laser based on Fano resonance in a crystal whispering gallery mode (WGM) cavity. Background Art

[0002] Ultra-low noise and ultra-narrow linewidth lasers have been widely used in the fields of coherent optical communication, gravitational wave detection, optical clocks, high-resolution spectroscopy, laser cooling, etc. due to their high coherence and high frequency stability.

[0003] In recent years, high-quality factor (Q-value) whispering gallery mode (WGM) cavities have been gradually applied to the field of low-noise lasers due to their extremely narrow mode linewidth and single-piece small volume. Among them, a type of crystal (such as CaF2, MgF2) WGM cavity has an extremely high Q-value (>10 9 ) and excellent thermal performance, and has successfully realized ultra-low noise and ultra-narrow linewidth lasers with a small volume. Currently, there are mainly two types of low-noise laser technologies based on WGM cavities: self-injection locking type and PDH active locking type.

[0004] The self-injection locking technology couples a part of the light of the original laser through the WGM cavity and then feeds it back to the master oscillation cavity, and then locks the laser frequency of the master oscillation cavity to a certain resonance mode of the WGM cavity, and finally can greatly narrow the linewidth of the original laser. The PDH frequency locking scheme uses electronic means such as modulation and mixing to "differentiate" the originally symmetric Lorentz line-shaped transmission whispering gallery mode, so as to generate an error signal linearly related to the frequency for locking the laser frequency. In comparison, the self-injection locking technology shifts the overall laser noise in all frequency bands downward. Due to the inherent thermo-mechanical noise of the optical path structure, a certain degree of low-frequency noise will be retained. The active locking technology stably locks the laser frequency to the resonance of the WGM cavity through a control circuit, so the low-frequency laser noise can be reduced to the thermal noise limit of the WGM cavity, but high-frequency noise in the frequency locking electronic circuit will be introduced. The core of the above two types of technologies depends on the extremely narrow linewidth and frequency-stable optical mode provided by the high-Q whispering gallery cavity. At present, both types of technologies face some problems: the feedback signal in the self-injection locking laser technology comes from the backscattering on the surface of the WGM cavity. This signal is a "band-pass" filtering type and can be used for feedback locking, but the intensity is very weak (usually only 10 -3 ~10 -4 ), and it is uncontrollable. In the active locking PDH technology, complex frequency locking circuits such as electro-optic modulation and mixing are required, which will not only introduce additional electronic noise such as residual amplitude modulation noise (RAM), but also make the overall system complex and reduce the reliability. Summary of the Invention

[0005] The object of the present invention is to provide an optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity, which has a large feedback signal intensity, a simple overall system structure, low noise and high reliability.

[0006] To achieve the above object, an optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity of the present invention includes a primitive laser. The optical signal emitted by the primitive laser sequentially passes through an optical fiber circulator and an optical fiber coupler. The optical signal output by the optical fiber coupler is divided into two beams. One beam of the optical signal is transmitted to the laser output end, and the other beam of the optical signal enters a prism crystal cavity coupling system to generate Fano resonance. The Fano optical signal output from the prism crystal cavity coupling system is divided into two paths. One path of the Fano optical signal returns to the optical fiber circulator through an optical fiber delay line and then is reinjected into the primitive laser, and this feedback loop is an optical feedback loop; the other path of the Fano optical signal generates an error electrical signal for active frequency locking after passing through a photodetector. This electrical signal is transmitted into a PID servo controller, and the electrical signal after passing through the PID servo controller is transmitted into the primitive laser to form an electrical feedback loop.

[0007] As a further solution of the present invention: the prism crystal cavity coupling system includes an input GRIN lens, an output GRIN lens, a prism, a crystalline WGM cavity, and a base. The input pigtail is connected to the input end of the input GRIN lens, and the output pigtail is connected to the output end of the output GRIN lens. The prism is an isosceles right triangular prism. The right-angled surface of the prism is located between the input GRIN lens and the output GRIN lens and is spaced apart by a certain distance. Behind the hypotenuse surface of the prism is the crystalline WGM cavity; the optical signal is focused by the input GRIN lens from the input pigtail to the hypotenuse surface of the prism for total internal reflection. Part of the optical signal is coupled into the crystalline WGM cavity in the form of an evanescent field, and the other part of the optical signal is directly totally reflected. These two parts of the optical signal will interfere to generate a Fano resonance mode and are simultaneously collected by the output GRIN lens and output through the output pigtail;

[0008] The input GRIN lens, the output GRIN lens, the prism, and the crystalline WGM cavity are fixedly arranged on the base. A slit is arranged at a position near the rear of the base. The crystalline WGM cavity and the prism are respectively located on both sides of the slit. The input GRIN lens, the output GRIN lens, and the prism are on the same side of the slit. A piezoelectric ceramic is arranged on the front side surface of the base to precisely control the gap size of the slit.

[0009] As a further solution of the present invention: the Fano optical signal transmitted between the optical fiber delay line and the optical fiber circulator is a "band-pass filtering type" transmission spectrum; the error electrical signal is a "vertically and horizontally anti-symmetric type" transmission spectrum.

[0010] As a further solution of the present invention: the material of the crystal WGM cavity is magnesium fluoride crystal or calcium fluoride crystal or quartz crystal.

[0011] As a further solution of the present invention: the control bandwidth of the PID servo controller is less than 100 Hz.

[0012] As a further solution of the present invention: the original laser uses a multi-longitudinal mode FP laser or a single-longitudinal mode DFB semiconductor laser.

[0013] As a further solution of the present invention: the base is a flexible adjustment metal base.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: the "band-pass filtering type" transmission spectrum is used as the feedback signal of the self-injection locking type laser. This feedback signal can retain the extremely narrow bandwidth of the crystal WGM cavity, and the intensity is much greater than the traditional backscattering signal, which is more conducive to locking the frequency of the original laser and reducing the laser linewidth; the Fano resonance can realize a laser frequency locking mechanism without modulation, eliminating the electronic circuits such as electro-optic modulation, demodulation, and mixing in the traditional PDH frequency locking technology, and realizing an "all-optical" error signal for laser frequency locking. This method can eliminate the residual amplitude modulation noise, and the system is simpler, has lower power consumption, and higher stability; the adopted crystal WGM cavity has lower inherent thermal noise, can more effectively reduce the low-frequency noise of the laser after frequency locking, and the single crystal WGM has the advantages of small volume and stable mechanical properties; the prism crystal cavity coupling system can effectively excite the Fano resonance mode, and different line-shaped Fano line types can be generated by adjusting the phase delay of the optical feedback loop and the electrical feedback loop, realizing two kinds of feedbacks at the same time. Since the feedback signal is generated by the same optical mode of the crystal cavity, the electrical feedback loop can effectively compensate the laser frequency drift and improve the stability of the optical feedback, with high reliability. Description of the Drawings

[0015] Figure 1 is a schematic structural diagram of the optoelectronic hybrid feedback laser based on Fano resonance in a crystal whispering gallery mode cavity of the present invention.

[0016] Figure 2 is the Fano signal generated in the present invention, where Figure 2 (a) is the "band-pass filtering type" Fano signal, Figure 2 (b) is the "up and down anti-symmetric type" Fano signal.

[0017] Figure 3 is a schematic structural diagram of the prism crystal cavity coupling system in the present invention, where Figure 3 (a) is a three-dimensional view of the prism crystal cavity coupling system, Figure 3 (b) is a top view of the prism crystal cavity coupling system.

[0018] Figure 4 (a) is the "band - pass filtering type" Fano signal generated in the embodiment of the present invention, Figure 4 (b) is the "up - down anti - symmetric type" Fano signal generated in the embodiment of the present invention.

[0019] Figure 5 It is a schematic diagram of the laser noise power spectral density in the embodiment of the present invention.

[0020] In the figure: 1. Original laser, 2. Optical fiber circulator, 3. Optical fiber coupler, 4. Laser output end, 5. Prism - crystal cavity coupling system, 6. Photoelectric detector, 7. Optical fiber delay line, 8. PID servo controller, 51. Crystal WGM cavity, 52. Prism, 53. Input - end GRIN lens, 54. Input - end tail fiber, 55. Output - end GRIN lens, 56. Output - end tail fiber, 57. Base, 58. Piezoelectric ceramic. Detailed implementation manners

[0021] The present invention will be further described below with reference to the accompanying drawings.

[0022] As Figure 1 shown, a photoelectric hybrid feedback laser based on Fano resonance in a crystal whispering - gallery - mode cavity includes an original laser 1. The optical signal emitted by the original laser 1 sequentially passes through an optical fiber circulator 2 and an optical fiber coupler 3. The optical signal output by the optical fiber coupler 3 is divided into two beams. One beam of the optical signal is transmitted to the laser output end 4, and the other beam of the optical signal enters the prism - crystal cavity coupling system 5 to generate Fano resonance. The Fano optical signal output from the prism - crystal cavity coupling system 5 is divided into two paths. One path of the Fano optical signal returns to the optical fiber circulator 2 through the optical fiber delay line 7 and is then reinjected into the original laser 1, thereby realizing self - injection locking of the laser. This feedback loop is an optical feedback loop; the other path of the Fano optical signal generates an error electrical signal for active frequency locking after passing through the photoelectric detector 6. This electrical signal is transmitted to the PID servo controller 8 to adjust the laser current in real time to compensate for long - term laser frequency drift. The electrical signal after passing through the PID servo controller 8 is transmitted to the original laser 1 to form an electrical feedback loop.

[0023] As Figure 2 (a) shown, the Fano optical signal transmitted between the optical fiber delay line 7 and the optical fiber circulator 2 is a "band - pass filtering type" transmission spectrum; as Figure 2 (b) shown, the error electrical signal is an "up - down anti - symmetric type" transmission spectrum. These two different spectra can be achieved by adjusting the phase delay of the optical feedback loop or the electrical feedback loop.

[0024] As Figure 3 (a), (b) shown, taking Figure 3(b) The front is indicated below the top view. The prism crystal cavity coupling system 5 includes an input GRIN lens 53, an output GRIN lens 54, a prism 52, a crystal WGM cavity 51, and a base 57. The input pigtail 54 is connected to the input end of the input GRIN lens 53, and the output pigtail 56 is connected to the output end of the output GRIN lens 55. The prism 52 is an isosceles right triangular prism. The right-angle face of the prism 52 is located between the input GRIN lens 53 and the output GRIN lens 55 with a certain distance in between. The crystal WGM cavity 51 is arranged behind the hypotenuse face of the prism 52. The optical signal is focused by the input GRIN lens 53 from the input pigtail 54 onto the hypotenuse face of the prism 52 to undergo total internal reflection. Part of the optical signal is coupled into the crystal WGM cavity 51 in the form of an evanescent field, and the other part of the optical signal is directly totally reflected. These two parts of the optical signal will interfere to generate a Fano resonance mode, and are simultaneously collected by the output GRIN lens 55 and output through the output pigtail 56. The Fano resonance mode can transform the transmission mode of the crystal WGM cavity 51 originally having a "band-stop filtering" line shape into a "band-pass filtering" line shape. Therefore, it can be used as a feedback signal for a self-injection locking type laser. This feedback signal can retain the extremely narrow bandwidth of the crystal WGM cavity 51 and has a much greater intensity than the traditional backscattering signal, which is more conducive to locking the original laser frequency and reducing the laser linewidth. The Fano resonance generates a new type of laser frequency locking mechanism without modulation, eliminating the electronic circuits such as electro-optic modulation, demodulation, and mixing in the traditional PDH frequency locking technology, and realizing an "all-optical" error signal for laser frequency locking. This method can eliminate the residual amplitude modulation noise, and the system is more concise, has lower power consumption, and higher stability.

[0025] The input GRIN lens 53, the output GRIN lens 55, the prism 52, and the crystal WGM cavity 51 are fixedly arranged on the base 57 and cured using a very low thermal expansion glue. The base 57 is a flexible adjustment metal base. There is a slit provided at a position near the rear of the base 57. The crystal WGM cavity 51 and the prism 52 are respectively located on both sides of the slit, and the input GRIN lens 53, the output GRIN lens 55, and the prism 52 are on the same side of the slit. A piezoelectric ceramic 58 is provided on the front side of the base 57 for precisely regulating the gap size of the slit, so as to ensure the optimal coupling gap between the crystal WGM cavity 51 and the prism 52.

[0026] The material of the crystal WGM cavity 51 is magnesium fluoride crystal or calcium fluoride crystal or quartz crystal.

[0027] The control bandwidth of the PID servo controller 8 is limited to be less than 100 Hz to avoid interfering with the optical feedback loop.

[0028] The original laser 1 uses a multi-longitudinal mode FP laser or a single-longitudinal mode DFB semiconductor laser.

[0029] Example:

[0030] As Figure 1 shown, the original laser is a multi-longitudinal mode FP laser. First, the optical feedback loop is turned on, i.e., self-injection frequency locking. When the laser frequency reaches stability, the electrical feedback loop is then turned on to eliminate laser frequency drift. It should be noted that the control bandwidth of the electrical feedback loop should be limited to below 100 Hz to avoid interference with the optical feedback. The laser output from the laser output end 4 is measured for the central wavelength by a spectrometer, and the F-P interferometer is scanned to characterize whether it is in the single-longitudinal mode operation state (mainly monitoring whether the multi-longitudinal mode laser can select the single-longitudinal mode oscillation by the Fano feedback signal). The laser frequency noise measurement system is used to analyze the laser noise power spectral density.

[0031] According to Figure 3 shown, a copper or aluminum base is used and a flexible adjustment structure is designed to achieve precise position adjustment. In this embodiment, a copper base is used. The magnesium fluoride crystal WGM cavity 51 and the prism 52 are respectively placed on both sides of the slit of the base 57. Then, the input GRIN lens 53 and the output GRIN lens 55 are placed. After aligning the optical path to obtain the best Fano mode, all optical elements are cured with an ultraviolet glue with an extremely low thermal expansion coefficient. In subsequent experiments, due to factors such as temperature fluctuations, the slight displacement generated between the crystal WGM cavity 51 and the prism 52 can be compensated by the piezoelectric ceramic 58 placed on the side of the base 57. A TEC chip is attached to the bottom of the base 58 for precise temperature control. Then, the components above the base 58 are sealed with a lid (not shown in the figure), and only the pigtails on both sides are left for input and output. In the above packaging module design, there are basically no moving elements and the volume is small, so it has good mechanical stability. Previous experiments found that the prism crystal cavity coupling system 5 is the most sensitive to the external environment, which is the gap size between the crystal WGM cavity 51 and the prism 52. The gap size is mainly due to the thermal expansion and contraction of the metal base caused by temperature fluctuations. Therefore, the piezoelectric ceramic 58 is designed for micro-displacement compensation.

[0032] A swept-frequency semiconductor laser is used to test whether the prism crystal cavity coupling system 5 can generate Fano resonance. The output of the swept-frequency semiconductor laser is connected to Figure 3 the input pigtail 54 in it, and the output pigtail 56 is connected to a photodetector for photoelectric conversion, and then connected to an oscilloscope for testing. The central wavelength of the laser is set to 1550 nm, the sweep range is 10 GHz, and the sweep period is 20 ms. The incident laser polarization is adjusted to only excite a single-polarization crystal cavity mode. Figure 4 (a) and (b) respectively show the "band-pass filtering type" and "upper and lower anti-symmetric type" Fano transmission modes measured by the oscilloscope experiment, which can be used for the optical feedback loop and the electrical feedback loop proposed in the present invention respectively.

[0033] Figure 5 is the measured noise power spectral density of the laser of the present invention. It can be seen that the high-frequency white noise of the laser is as low as 0.1 Hz 2 / Hz, and this noise level is much lower than the noise of the original laser, indicating that the optical feedback effectively suppresses the noise of the laser.

[0034] The magnesium fluoride crystal WGM cavity used in the present invention has lower inherent thermal noise and can more effectively reduce the low-frequency noise of the laser after frequency locking. Moreover, the single crystal WGM has the advantages of small volume and stable mechanical properties, and has important prospects in the field of realizing ultra-low noise and ultra-narrow linewidth lasers with simple structure and miniaturized volume.

[0035] The prism crystal cavity coupling system of the present invention can effectively excite the Fano resonance mode, and generate different line-shaped Fano line types by adjusting the phase delays of the optical feedback loop and the electrical feedback loop, and can achieve both feedbacks simultaneously. Since the feedback signal is generated by the same optical mode of the crystal cavity, the electrical feedback loop can effectively compensate for the laser frequency drift and improve the stability of the optical feedback.

Claims

1. An optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity, characterized in that, It includes a primitive laser (1). The optical signal emitted by the primitive laser (1) sequentially passes through an optical fiber circulator (2) and an optical fiber coupler (3). The optical signal output by the optical fiber coupler (3) is divided into two beams. One beam of optical signal is transmitted to a laser output end (4), and the other beam of optical signal enters a prism crystal cavity coupling system (5) to generate Fano resonance. The Fano optical signal output from the prism crystal cavity coupling system (5) is divided into two paths. One path of Fano optical signal returns to the optical fiber circulator (2) through an optical fiber delay line (7) and then is reinjected into the primitive laser (1). This feedback loop is an optical feedback loop. The other path of Fano optical signal generates an error electrical signal for active frequency locking after passing through a photodetector (6). This electrical signal is transmitted into a PID servo controller (8), and the electrical signal after passing through the PID servo controller (8) is transmitted into the primitive laser (1) to form an electrical feedback loop.

2. The optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity according to claim 1, wherein The prism crystal cavity coupling system (5) includes an input end GRIN lens (53), an output end GRIN lens (54), a prism (52), a crystal WGM cavity (51), and a base (57). The input end pigtail (54) is connected to the input end of the input end GRIN lens (53), and the output end pigtail (56) is connected to the output end of the output end GRIN lens (55). The prism (52) is an isosceles right triangular prism. The right-angle face of the prism (52) is located between the input end GRIN lens (53) and the output end GRIN lens (55) and is spaced a certain distance. Behind the hypotenuse face of the prism (52) is the crystal WGM cavity (51). The optical signal is focused by the input end GRIN lens (53) from the input end pigtail (54) to the hypotenuse face of the prism (52) to undergo total internal reflection. Part of the optical signal is coupled into the crystal WGM cavity (51) in the form of an evanescent field, and the other part of the optical signal is directly totally reflected. These two parts of the optical signal will interfere to generate a Fano resonance mode and are simultaneously collected by the output end GRIN lens (55) and output through the output end pigtail (56). The input end GRIN lens (53), the output end GRIN lens (55), the prism (52), and the crystal WGM cavity (51) are fixedly arranged on the base (57). A slit is provided at a position near the rear of the base (57). The crystal WGM cavity (51) and the prism (52) are respectively located on both sides of the slit. The input end GRIN lens (53), the output end GRIN lens (55), and the prism (52) are on the same side of the slit. A piezoelectric ceramic (58) is provided on the front side of the base (57) for precisely regulating the gap size of the slit.

3. The optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity according to claim 1, wherein The Fano optical signal transmitted between the optical fiber delay line (7) and the optical fiber circulator (2) is a "band-pass filtering type" transmission spectrum; the error electrical signal is a "vertically and horizontally anti-symmetric type" transmission spectrum.

4. The optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity according to claim 2, wherein The material of the crystal WGM cavity (51) is magnesium fluoride crystal or calcium fluoride crystal or quartz crystal.

5. The optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity according to claim 1, wherein The control bandwidth of the PID servo controller (8) is less than 100 Hz.

6. The optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity according to claim 1, wherein The primitive laser (1) uses a multi-longitudinal mode FP laser or a single-longitudinal mode DFB semiconductor laser.

7. The optoelectronic hybrid feedback laser based on Fano resonance in a crystalline whispering gallery mode cavity according to claim 2, wherein The base (57) is a flexible adjustable metal base.

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