An optoelectronic hybrid feedback laser based on fano resonance in a crystal whispering gallery cavity

Abstract

This invention discloses a hybrid optoelectronic feedback laser based on Fano resonance within a crystal whispering cavity. It includes a primary laser, whose emitted optical signal sequentially passes through a fiber optic circulator and a fiber optic coupler. The optical signal output from the fiber optic coupler is split into two beams. One beam is transmitted to the laser output end, while the other enters a prism-crystal-cavity coupling system to generate Fano resonance. The Fano optical signal output from the prism-crystal-cavity coupling system is split into two paths. One Fano optical signal returns to the fiber optic circulator via a fiber delay line and is re-injected into the primary laser; this feedback loop is the optical feedback loop. The other Fano optical signal passes through a photodetector to generate an error electrical signal for active frequency locking. This signal is then sent to a PID servo controller, and the signal after passing through the PID servo controller is sent back to the primary laser, forming an electrical feedback loop. This laser exhibits high feedback signal strength, a simple overall system structure, low noise, and high reliability.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically referring to a photoelectric hybrid feedback laser based on the Fano resonance in a crystal whispering cavity. Background Technology

[0002] Ultra-low noise, ultra-narrow linewidth lasers have wide applications in coherent optical communication, gravitational wave detection, optical clocks, high-resolution spectroscopy, laser cooling and other fields due to their high coherence and high frequency stability.

[0003] In recent years, high-quality-factor (Q-factor) whispering-gallery (WGM) cavities have been increasingly used in low-noise laser applications due to their extremely narrow mode linewidths and small monolithic size. One type of crystal WGM cavity (such as CaF2 and MgF2) possesses an extremely high Q-factor (>10). 9 With its excellent thermal properties, WGM cavity-based low-noise lasers with ultra-low noise and ultra-narrow linewidth have been successfully realized. Currently, there are two main types of low-noise laser technology based on WGM cavities: self-injection locked type and PDH active locked type.

[0004] Self-injection locking technology couples a portion of the original laser light through a WGM cavity and feeds it back to the main oscillator cavity, locking the laser frequency of the main oscillator cavity to a specific resonant mode of the WGM cavity, ultimately significantly narrowing the linewidth of the original laser. PDH frequency locking schemes use modulation, mixing, and other electronic means to "differentiate" the originally symmetrical Lorentz-shaped transmission whispering-gallery mode, generating an error signal linearly related to the frequency for locking the laser frequency. In contrast, self-injection locking technology shifts all laser noise across all frequency bands downwards. Due to the inherent thermomechanical noise of the optical path structure, a certain degree of low-frequency noise is retained. Active locking technology uses control circuitry to stably lock the laser frequency to the WGM cavity resonance, thus reducing low-frequency laser noise to the thermal noise limit of the WGM cavity, but introducing high-frequency noise into the frequency locking electronic circuitry. Both of these technologies rely on the extremely narrow linewidth and frequency-stable optical modes provided by high-Q whispering galvanic cavities. Currently, both technologies face some challenges: the feedback signal in self-injection locked laser technology originates from backscattering from the surface of the WGM cavity. This signal is a "bandpass" filtered signal, which can be used for feedback locking, but its intensity is very weak (typically only 10). -3 ~10 -4 Furthermore, it is uncontrollable. In active-locked PDH technology, complex frequency-locking circuits are required, such as electro-optic modulation and mixing, which not only introduce additional electronic noise, such as residual amplitude modulation noise (RAM), but also make the overall system more complex and reduce its reliability. Summary of the Invention

[0005] The purpose of this invention is to provide a photoelectric hybrid feedback laser based on the Fano resonance in a crystal whispering cavity. This laser has a large feedback signal intensity, a simple overall system structure, low noise, and high reliability.

[0006] To achieve the above objectives, this invention provides a hybrid optoelectronic feedback laser based on Fano resonance within a crystal whispering cavity. The laser includes a primary laser. The optical signal emitted by the primary laser sequentially passes through a fiber optic circulator and a fiber optic coupler. The optical signal output from the fiber optic coupler is split into two beams. One beam is transmitted to the laser output end, while the other beam enters a prism crystal cavity coupling system to generate Fano resonance. The Fano optical signal output from the prism crystal cavity coupling system is split into two paths. One Fano optical signal returns to the fiber optic circulator via a fiber delay line and is then re-injected into the primary laser; this feedback loop is an optical feedback loop. The other Fano optical signal passes through a photodetector to generate an error electrical signal for active frequency locking. This electrical signal is then sent to a PID servo controller, and the signal after passing through the PID servo controller is sent back to the primary laser, forming an electrical feedback loop.

[0007] As a further aspect of the present invention: the prism-crystal cavity coupling system includes an input GRIN lens, an output GRIN lens, a prism, a crystal 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-angled triangular prism, with its right-angled face located between the input and output GRIN lenses at a certain distance. The crystal WGM cavity is located behind the hypotenuse of the prism. The optical signal is focused by the input pigtail through the input GRIN lens to the hypotenuse of the prism, resulting in total internal reflection. Part of the optical signal is coupled into the crystal WGM cavity in the form of an evanescent field, while the other part of the optical signal is directly subjected to total internal reflection. 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 crystal WGM cavity are fixedly mounted on the base. A slit is provided near the rear of the base. The crystal WGM cavity and the prism are located on opposite sides of the slit. The input GRIN lens, the output GRIN lens, and the prism are on the same side of the slit. Piezoelectric ceramics are provided on the front side of the base for precise adjustment of the slit gap size.

[0009] As a further aspect of the present invention: the Fano optical signal transmitted between the fiber delay line and the fiber optic circulator is a "bandpass filtered" transmission spectrum; the error electrical signal is an "anti-symmetric" transmission spectrum.

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

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

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

[0013] As a further aspect of the present invention, the base is a flexible adjustable metal base.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: the "bandpass filter type" transmission spectrum serves as the feedback signal of the self-injection locked laser. This feedback signal can retain the extremely narrow bandwidth of the crystal WGM cavity and its intensity is much greater than that of the traditional backscatter signal, which is more conducive to locking the original laser frequency and reducing the laser linewidth; the Fano resonance can realize the modulation-free laser frequency locking mechanism, eliminating the electronic circuits such as electro-optic modulation, demodulation, and mixing in the traditional PDH frequency locking technology, and realizing the use of the "all-optical" error signal for laser frequency locking. This method can eliminate residual amplitude modulation noise and is simpler, consumes less power, and is more stable. The crystal WGM cavity used has lower inherent thermal noise, which can more effectively reduce the low-frequency noise of the laser after frequency locking. In addition, a single crystal WGM has the advantages of small size and stable mechanical properties. The prism crystal cavity coupling system can effectively excite the Fano resonance mode and generate different Fano line shapes by adjusting the phase delay of the optical feedback loop and the electrical feedback loop. It can realize two types of feedback 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 for the laser frequency drift, improve the stability of the optical feedback, and has high reliability. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the optoelectronic hybrid feedback laser based on the intracavitary Fano resonance of the crystal whispering galvanic cavity according to the present invention.

[0016] Figure 2 It is the Fano signal generated in this invention, wherein Figure 2 (a) is a "bandpass filtered" Fano signal. Figure 2 (b) is a “top-bottom antisymmetric” Fano signal.

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

[0018] Figure 4 (a) is the "bandpass filtered" Fano signal generated in the embodiment of the present invention. Figure 4 (b) is the “top-bottom antisymmetric” Fano signal generated in the embodiment of the present invention.

[0019] Figure 5 This is a schematic diagram of the laser noise power spectral density according to an embodiment of the present invention.

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

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

[0022] like Figure 1 As shown, a hybrid optoelectronic feedback laser based on Fano resonance within a crystal whispering cavity includes a primary laser 1. The optical signal emitted by the primary laser 1 passes sequentially through an optical fiber circulator 2 and an optical fiber coupler 3. The optical signal output from the optical fiber coupler 3 is split into two beams. One beam is transmitted to the laser output end 4, and the other beam 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 split into two paths. One Fano optical signal returns to the optical fiber circulator 2 via an optical fiber delay line 7 and is then re-injected into the primary laser 1, thereby achieving self-injection locking of the laser. This feedback loop is an optical feedback loop. The other Fano optical signal passes through a photodetector 6 and generates an error electrical signal as active frequency locking. This electrical signal is sent to a 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 sent back to the primary laser 1, forming an electrical feedback loop.

[0023] like Figure 2 As shown in (a), the Fano optical signal transmitted between the fiber delay line 7 and the fiber circulator 2 has a "bandpass filtered" transmission spectrum; as Figure 2 As shown in (b), the error electrical signal is an "anti-symmetric" 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] like Figure 3 As shown in (a) and (b), with Figure 3(b) The bottom of the top view indicates the front. The prism-crystal coupling system 5 includes an input GRIN lens 53, an output GRIN lens 55, 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-angled triangular prism. The right-angled face of the prism 52 is located between the input GRIN lens 53 and the output GRIN lens 55, and is spaced a certain distance apart. The crystal WGM cavity 51 is located behind the inclined face of the prism 52. The optical signal is focused by the input pigtail 54 through the input GRIN lens 53 to the inclined face of the prism 52 and undergoes 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 subjected to total internal reflection. The 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, which originally has a "band-stop filtered" line shape, into a "band-pass filtered" line shape. Therefore, it can be used as a feedback signal for a self-injection locked laser. This feedback signal can retain the extremely narrow bandwidth of the crystal WGM cavity 51 and its intensity is much greater than that of the traditional backscattered signal, which is more conducive to locking the original laser frequency and reducing the laser linewidth. The Fano resonance generates a novel modulation-free laser frequency locking mechanism, eliminating the electronic circuits such as electro-optic modulation, demodulation, and mixing in traditional PDH frequency locking technology, and realizing the use of "all-optical" error signals for laser frequency locking. This method can eliminate residual amplitude modulation noise and the system is simpler, consumes less power, and has higher stability.

[0025] The input GRIN lens 53, the output GRIN lens 55, the prism 52, and the crystal WGM cavity 51 are fixedly mounted on the base 57 and cured with ultra-low thermal expansion adhesive. The base 57 is a flexible adjustable metal base. A slit is provided near the rear of the base 57. The crystal WGM cavity 51 and the prism 52 are located on opposite sides of the slit. 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 precise adjustment of the slit gap size, which can 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, calcium fluoride crystal, or quartz crystal.

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

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

[0029] Example:

[0030] like Figure 1 As shown, the original laser is a multi-longitudinal-mode (FP) laser. First, the optical feedback loop is activated, i.e., self-injection frequency locking. Once the laser frequency stabilizes, the electrical feedback loop is activated to eliminate laser frequency drift. It is worth noting that the control bandwidth of the electrical feedback loop must be limited to below 100Hz to avoid interference with the optical feedback. The laser output from laser output terminal 4 has its center wavelength measured by a spectrometer, and the FP interferometer is scanned to characterize whether it is in single-longitudinal-mode operation (primarily monitoring whether the multi-longitudinal-mode laser can be selected for single-longitudinal-mode oscillation by the Fano feedback signal). A laser frequency noise measurement system is used to analyze the laser noise power spectral density.

[0031] according to Figure 3 As 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 prism 52 are placed on both sides of the slit of the base 57, respectively. Then, the input GRIN lens 53 and the output GRIN lens 55 are placed. After aligning the optical path to obtain the optimal Fano mode, all optical components are cured with ultraviolet adhesive with extremely low thermal expansion coefficient. In subsequent experiments, the slight displacement between the crystal WGM cavity 51 and prism 52 caused by factors such as temperature fluctuations can be compensated by the piezoelectric ceramic 58 placed on the side of the base 57. A TEC plate is attached to the bottom of the base 58 for precise temperature control. Then, the components above the base 58 are sealed with a cover (not shown in the figure), leaving pigtails on both sides for input and output. The above packaged module design contains virtually no moving components and is small in size, thus exhibiting good mechanical stability. Preliminary experiments revealed that the prism-crystal-cavity coupling system 5 is most sensitive to the external environment in terms of the size of the gap between the crystal WGM cavity 51 and the prism 52. The size of the gap is mainly due to the thermal expansion and contraction of the metal base caused by temperature fluctuations. Therefore, piezoelectric ceramic 58 was designed for micro-displacement compensation.

[0032] A swept-frequency semiconductor laser was used to test whether the prism-cavity coupling system 5 could generate Fano resonance. The output of the swept-frequency semiconductor laser was connected to... Figure 3 The input pigtail 54 and output pigtail 56 are connected to a photodetector for photoelectric conversion, and then connected to an oscilloscope for testing. The laser center wavelength is set to 1550nm, the frequency sweep range is 10GHz, and the sweep period is 20ms. The polarization of the incident laser is adjusted to excite only a single-polarization crystal cavity mode. Figure 4 (a) and (b) show the "bandpass filter type" and "top-bottom antisymmetric type" Fano transmission modes obtained by oscilloscope experiments, respectively. They can be used in the optical feedback loop and electrical feedback loop proposed in this invention.

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

[0034] The magnesium fluoride crystal WGM cavity used in this invention has lower inherent thermal noise, which can more effectively reduce the low-frequency noise of the laser after frequency locking. Furthermore, a single-piece WGM crystal has advantages such as small size and stable mechanical properties, and shows great promise in realizing ultra-low noise and ultra-narrow linewidth lasers with simple structure and miniaturized size.

[0035] The prism-cavity coupling system of this invention can effectively excite the Fano resonance mode, and by adjusting the phase delay of the optical feedback loop and the electrical feedback loop, different Fano line shapes can be generated, achieving both types of feedback simultaneously. Since the feedback signal is generated from the same optical mode of the crystal cavity, the electrical feedback loop can effectively compensate for laser frequency drift and improve the stability of the optical feedback.

Claims

1. An optoelectronic hybrid feedback laser based on crystal whispering gallery cavity Fano resonance, characterized in that, The original laser (1) emits optical signals which pass through the optical fiber circulator (2) and the optical fiber coupler (3) in sequence. The optical signals output by the optical fiber coupler (3) are divided into two beams. One beam of optical signals is transmitted to the laser output end (4), and the other beam of optical signals enters the prism crystal cavity coupling system (5) to generate Fano resonance. The Fano optical signals output from the prism crystal cavity coupling system (5) are divided into two paths. One path of the Fano optical signals is returned to the optical fiber circulator (2) through the optical fiber delay line (7) and then re-injected into the original laser (1), forming an optical feedback loop. The other path of the Fano optical signals passes through the photoelectric detector (6) to generate an error electrical signal as active frequency locking. The error electrical signal is transmitted to the PID servo controller (8), and the electrical signal after passing through the PID servo controller (8) is transmitted to the original laser (1), forming an electrical feedback loop. The prism crystal cavity coupling system (5) comprises an input end GRIN lens (53), an output end GRIN lens (55), a prism (52), a crystal WGM cavity (51), and a base (57). The input end tail fiber (54) is connected to the input end of the input end GRIN lens (53), and the output end tail fiber (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 apart by a certain distance. The hypotenuse face of the prism (52) is followed by the crystal WGM cavity (51). The optical signals are focused by the input end GRIN lens (53) to the hypotenuse face of the prism (52) to occur total internal reflection. Part of the optical signals is coupled into the crystal WGM cavity (51) in the form of evanescent field, and the other part of the optical signals is directly totally reflected. The two parts of the optical signals will interfere to generate Fano resonance mode and are then collected by the output end GRIN lens (55) and output through the output end tail fiber (56). The Fano optical signals transmitted between the optical fiber delay line (7) and the optical fiber circulator (2) are "band-pass filter type" transmission spectrum. The error electrical signal is "up-down anti-symmetrical type" transmission spectrum. 2.The optoelectronic hybrid feedback laser based on Fano resonance of whispering gallery cavities according to claim 1, wherein, 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). The base (57) is provided with a slit near the rear position. The crystal WGM cavity (51) and the prism (52) are located on the two sides of the slit, respectively. The input end GRIN lens (53) and the output end GRIN lens (55) are located on the same side of the prism (52) as the slit. The front side of the base (57) is provided with a piezoelectric ceramic (58) for precisely regulating the gap size of the slit.

3. The optoelectronic hybrid feedback laser based on Fano resonance of whispering gallery cavities according to claim 2, characterized in that, The material of the crystal WGM cavity (51) is magnesium fluoride crystal or calcium fluoride crystal or quartz crystal. 4.The optoelectronic hybrid feedback laser based on Fano resonance of crystal whispering gallery cavity according to claim 1, wherein, The control bandwidth of the PID servo controller (8) is less than 100 Hz.

5. The optoelectronic hybrid feedback laser based on Fano resonance of whispering gallery cavities according to claim 1, characterized in that, The original laser (1) adopts a multi-longitudinal mode FP laser or a single-longitudinal mode DFB semiconductor laser. 6.The optoelectronic hybrid feedback laser based on Fano resonance of whispering gallery cavities according to claim 2, wherein, The base (57) is a flexible adjustable metal base.

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