Multi-ring cavity high power single-longitudinal-mode laser based on dynamic grating

Abstract

This invention relates to the field of laser technology and provides a high-power single-longitudinal-mode laser based on a dynamic grating multi-ring cavity. The laser comprises multiple components, including: a semiconductor laser, a beam combiner, a 2.5m long erbium-ytterbium co-doped double-clad fiber, a cladding power stripper, a fiber circulator, a 3m long low-doped polarization-maintaining erbium-doped fiber, two 2×2 polarization-maintaining fiber couplers with a split ratio of 20:80, a polarization controller, a 1×2 polarization-maintaining output coupler with a split ratio of 10:90, and a high-reflectivity fiber Bragg grating. This invention proposes a high-power single-longitudinal-mode laser based on a dynamic grating multi-ring cavity structure and a double-clad pumped erbium-ytterbium co-doped fiber, achieving single-longitudinal-mode laser output with a linewidth in the kHz range and a power output in the hundreds of milliwatts. Based on optimized intracavity parameters, this combination is a reliable method for achieving output power exceeding the W range in the 1550nm band continuous-frequency laser.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, and particularly relates to a high-power single-longitudinal-mode laser with multiple ring cavities based on dynamic gratings. Background Technology

[0002] The 1.4μm-2.1μm band is the "eye-safe" band, which represents the atmospheric transmittance of near-infrared lasers. The 1.55μm band is a commonly used near-infrared transmission window for atmospheric transmission. This band has high atmospheric transmittance and low loss.

[0003] The damage threshold of the human eye at the 1.55μm wavelength is four orders of magnitude higher than that at the 1064nm wavelength. In addition, the maximum permissible exposure of the human eye at a wavelength of 1.5μm is ten times higher than that of laser at the 2μm wavelength.

[0004] Since fiber optic devices in the 1.55μm band are more mature than those in other bands, it is easier to realize an all-fiber structure, which has advantages such as compact structure, small size, and easy transportation. However, existing erbium-doped single-mode lasers have disadvantages such as weak environmental stability and low output power. Summary of the Invention

[0005] The purpose of this invention is to provide a high-power single-longitudinal-mode laser with multiple ring cavities based on dynamic gratings, aiming to solve the problem of low output single-longitudinal-mode laser power in existing single-resonant-cavity fiber lasers.

[0006] To achieve the above objectives, this invention provides a multi-ring cavity high-power single-longitudinal-mode laser based on a dynamic grating, comprising multiple devices, which are as follows:

[0007] A semiconductor laser, wherein the semiconductor lasers are connected in series as a pump source;

[0008] A beam combiner for coupling a pump source into the cavity;

[0009] Erbium-ytterbium co-doped double-clad fiber, which serves as a gain fiber to provide inverted particles for the laser, and whose double-clad structure allows for clad pumping.

[0010] A cladding power stripper, wherein the cladding power stripper is used to filter out unabsorbed pump light in the cladding;

[0011] An optical fiber circulator is used as a unidirectional device for a ring cavity laser. The laser enters from port 1 to port 2, is reflected by a high-reflectivity fiber grating, and is output from port 3 of the circulator.

[0012] The low-doped polarization-maintaining erbium-doped fiber is in an unpumped state. The laser is transmitted in opposite directions through a high-reflectivity grating and a circulator, and a dynamic grating is formed in the low-doped fiber to play a narrowband filtering role.

[0013] A polarization-maintaining fiber coupler, used to construct a parallel connection between a sub-ring cavity and a main ring cavity;

[0014] A polarization controller is used to adjust the polarization state within the cavity, thereby regulating the stable oscillation of a single longitudinal mode.

[0015] The polarization-maintaining output coupler outputs 90% of the laser beam outside the resonant cavity, while the remaining 10% continues to oscillate and amplify within the resonant cavity.

[0016] A high-reflectivity fiber Bragg grating is used to reflect laser light into the cavity.

[0017] Furthermore, the length of the erbium-ytterbium co-doped double-clad optical fiber is 2.5m.

[0018] Furthermore, the length of the low-doped polarization-maintaining erbium-doped fiber is 3m.

[0019] Furthermore, the number of polarization-maintaining fiber couplers is two, and the splitting ratio of the two polarization-maintaining fiber couplers is 20:80.

[0020] Furthermore, the polarization-maintaining output coupler is a single unit with a beam splitting ratio of 10:90.

[0021] Furthermore, the erbium-ytterbium co-doped double-clad fiber has an absorption coefficient of 3.47 dB / m for 976 nm pump light.

[0022] Furthermore, the total cavity length of the multi-ring cavity high-power single longitudinal mode laser based on dynamic grating is 12m. Multiple components in the laser are fixed to a water-cooling plate with high-temperature tape. The water chiller temperature is set to 12 degrees Celsius to circulate water and cool the entire laser system.

[0023] In summary, this invention proposes a high-power single-longitudinal-mode laser based on a multi-ring cavity structure with dynamic gratings and a double-clad pumped erbium-ytterbium co-doped fiber. This results in a single-longitudinal-mode laser output with a linewidth in the kHz range and a power output in the hundreds of milliwatts range. Based on the optimization of the intraring cavity parameters, this combination is a reliable method for achieving output power exceeding the W range in the 1550nm band continuous-frequency laser.

[0024] This invention addresses the low output power limitation of single-mode longitudinal fiber lasers with a single oscillator stage by optimizing the laser's structural parameters. The laser employs a dynamic grating combined with a multi-ring cavity structure. Considering the filtering effect of the dynamic grating and the vernier effect of the multi-ring cavity, the longitudinal mode spacing is increased. Experiments were conducted to determine the optimal length of the low-doped polarization-maintaining erbium-doped fiber for generating the dynamic grating and the optimal optical path difference between the sub-ring cavities. Ultimately, a maximum output single-mode laser power of 287mW was achieved at a pump power of 4.2W, with an optical-to-optical conversion efficiency of 6.8% and a slope efficiency of 9.3%. The output power stability (standard deviation / average) was 0.9% within one hour, the output wavelength was 1550.036nm, and the linewidth of the single-mode laser measured using the time-delay self-heterodyne detection method was 1.66kHz. This single-oscillator stage laser, based on a dynamic grating combined with a multi-ring cavity structure, achieves single-mode laser output power in the hundreds of milliwatts range. At this point, only some components inside the cavity are polarization-maintaining structures. Further optimization of the cavity into a fully polarization-maintaining structure can effectively reduce resonant cavity losses, improve power while preventing mode hopping, and is expected to achieve single-longitudinal-mode laser output in the W range. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention.

[0026] Figure 1 This is a structural diagram of the multi-ring cavity high-power single-longitudinal-mode laser based on a dynamic grating according to the present invention;

[0027] Figure 2 This is a graph showing the relationship between the output power and the pump speed of this invention.

[0028] Figure 3 This is the output spectrum of the multi-ring cavity single-longitudinal-mode fiber laser based on a dynamic grating according to the present invention;

[0029] Figure 4(a) shows the beat frequency signal detected by the oscilloscope at 80 MHz;

[0030] Figure 4(b) shows the beat frequency spectrum detected by the spectrum analyzer at 80 MHz;

[0031] Figure 5 This is a measurement diagram of the single-longitudinal-mode laser linewidth at the highest output power of this invention.

[0032] In the attached diagram: 1. Semiconductor laser; 2. Beam combiner; 3. Erbium-ytterbium co-doped double-clad fiber; 4. Cladding power stripper; 5. Fiber circulator; 6. Low-doped polarization-maintaining erbium-doped fiber; 7. Polarization-maintaining fiber coupler; 8. Polarization controller; 9. Polarization-maintaining output coupler; 10. High-reflectivity fiber Bragg grating. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0034] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0035] like Figure 1 As shown, in this embodiment of the invention, a multi-ring cavity high-power single-longitudinal-mode laser based on a dynamic grating is provided. This laser includes multiple devices, which are as follows:

[0036] A 976nm semiconductor laser 1 (LD), a 976 / 1550nm (2+1)×1 combiner 2, a 2.5m long erbium-ytterbium co-doped double-clad fiber 3 (SM-EYDF, 10 / 125), a cladding power stripper 4 (CPS), a fiber circulator 5 (CIR), a 3m long low-doped polarization-maintaining erbium-doped fiber 6 (PM-EDF), two 2×2 polarization-maintaining fiber couplers 7 (couple1, couple2) with a split ratio of 20:80, a polarization controller 8 (PC), a 1×2 polarization-maintaining output coupler 9 (PM-OC) with a split ratio of 10:90, and a high-reflectivity fiber Bragg grating 10 (HR-FBG).

[0037] Furthermore, in this embodiment of the invention, the 976nm semiconductor laser (LD) is used as a pump source by being connected in series;

[0038] Furthermore, in this embodiment of the invention, the 976 / 1550nm (2+1)×1 combiner is used to couple the pump source into the cavity;

[0039] Furthermore, in this embodiment of the invention, the 2.5m long erbium-ytterbium co-doped double-clad fiber (SM-EYDF, 10 / 125) serves as the gain fiber, providing inverted particles to the laser. Its double-clad structure allows for clad pumping. The absorption coefficient for 976nm pump light is 3.47dB / m.

[0040] Furthermore, in this embodiment of the invention, the cladding power stripper (CPS) is made using single-mode fiber (SM-GDF-10 / 130) from Nufern, and is used to filter out unabsorbed pump light in the cladding.

[0041] Furthermore, in this embodiment of the invention, the fiber optic circulator (CIR) is used as a unidirectional device for the ring cavity laser. The laser enters from port 1 to port 2, and after being reflected by the high-reflectivity fiber grating, it is output from port 3 of the circulator.

[0042] Furthermore, in this embodiment of the invention, the high reflectivity fiber Bragg grating (HR-FBG) allows the laser to enter the cavity after being reflected by the high reflectivity grating.

[0043] Furthermore, in this embodiment of the invention, the 3m long low-doped polarization-maintaining erbium-doped fiber (PM-EDF) is in an unpumped state. The laser is transmitted in opposite directions through the high-reflectivity grating and circulator, reaching the point where a dynamic grating is formed in the low-doped fiber, thus playing a narrowband filtering role.

[0044] Furthermore, in this embodiment of the invention, the two 2×2 polarization-maintaining fiber couplers (couple1, couple2) with a splitting ratio of 20:80 are selected with a splitting ratio of 20:80 to accommodate the construction of two sub-ring cavities and the main ring cavity in parallel.

[0045] Furthermore, in this embodiment of the invention, the polarization controller (FiberPolarizer) is used to adjust the polarization state within the cavity, thereby regulating the stable oscillation of the single longitudinal mode.

[0046] Furthermore, in this embodiment of the invention, the 1×2 polarization-maintaining output coupler (PM-OC) with a beam splitting ratio of 10:90 outputs 90% of the laser beam outside the resonant cavity, while 10% of the laser beam continues to oscillate and amplify within the resonant cavity.

[0047] Furthermore, in this embodiment of the invention, the total cavity length of the multi-ring cavity laser is 12m, and each component in the laser is fixed to the water-cooling plate with high-temperature tape. The water chiller is set to a temperature of 12 degrees Celsius to circulate water and cool the entire laser system.

[0048] like Figure 2 , Figure 3 Figure 4(a), Figure 4(b) and Figure 5 As shown, the multi-ring cavity high-power single-longitudinal-mode laser based on dynamic grating provided by this invention was tested using a power meter, wavelength meter, photodetector, oscilloscope, spectrum analyzer, and heterodyne detection system to record the average output power, laser wavelength, beat frequency signal, beat frequency spectrum, and 1550nm laser linewidth of the single-longitudinal-mode laser, and the laser's operating status was monitored.

[0049] In this embodiment of the invention, a fiber circulator (CIR), a 3m long low-doped erbium-doped polarization-maintaining fiber (PM-EDF), and a high-reflectivity fiber Bragg grating (HR-FBG) form a dynamic grating for narrowband filtering; two 2×2 polarization-maintaining fiber couplers (couple1, couple2) with a split ratio of 20:80 construct two passive sub-ring cavities, using the vernier effect to increase the longitudinal mode spacing to obtain single longitudinal mode laser output; a polarization controller (FiberPolarizer) is used to control the intracavity polarization environment and suppress the oscillation of other longitudinal modes; and a 1×2 polarization-maintaining output coupler (PM-OC) with a split ratio of 10:90.

[0050] In the multi-ring cavity high-power single longitudinal mode laser based on dynamic grating provided by the present invention, a 976nm pump source is connected to the resonant cavity via a beam combiner, and then sequentially connected to a 2.5m double-clad erbium-ytterbium co-doped fiber, a cladding optical stripper, a circulator, a low-doped polarization-maintaining erbium-doped fiber, a high-reflectivity fiber grating, two 20:80 polarization-maintaining couplers used to construct two sub-ring cavities and connected to the main ring cavity, a polarization controller, and the output end of the output coupler (10%) connected to the pigtail of the beam combiner to form a closed ring cavity.

[0051] Furthermore, doped fibers can not only be used as gain materials for lasers, but also, with proper configuration and modulation of the light absorption and emission of doped fibers, can be used as saturable absorbers and other devices to form dynamically induced narrowband gratings, which can be used in single-frequency or narrow-linewidth fiber lasers.

[0052] Furthermore, when erbium-doped fiber is in an unpumped state, it exhibits saturable absorption of signal light; that is, the greater the light intensity, the smaller the absorption coefficient, and vice versa. In a saturable absorber, two beams of light with the same frequency propagating in opposite directions interfere to form a standing wave field. The light intensity is greatest and the absorption coefficient is smallest at the antinodes of the standing wave; at the nodes, the light intensity is smallest and the absorption coefficient is largest. Therefore, the absorption coefficient exhibits periodic modulation along the fiber axis.

[0053] Furthermore, according to the Kramers-Kroning relationship, the refractive index gradually decreases as the absorption coefficient increases, so the refractive index also exhibits a periodic change, which is equivalent to writing a narrowband Bragg grating into the gain fiber [47,50]. The stronger the light intensity, the smaller the absorption coefficient, the smaller the cavity loss, the higher the gain, and the easier it is to start oscillation, giving it an advantage in mode competition. Therefore, the self-written grating has the function of automatically tracking the dominant mode, that is, it has the function of automatically tracking the center wavelength of the fiber grating. Single longitudinal mode output is achieved when the reflection bandwidth of the saturable absorber is smaller than the free light range of the resonant cavity.

[0054] Therefore, the full width at half maximum (FWHM) of the PM-EDF reflectance spectrum is:

[0055] λ=2n eff Λ; where κ, Λ, and N are the coupling coefficients, period, and number of periods of the FBG, Lg is the length of the FBG, and n eff n is the effective refractive index of the optical fiber eff =1.46, Δn is the maximum refractive index difference induced by erbium-doped fiber Δn = 2.6056 × 10 -7 λ is the center wavelength of the Bragg grating (FBG).

[0056] The free spectral range of a unidirectional ring cavity can be expressed as: In the formula, nl is the optical cavity length of the ring laser.

[0057] When Δf < Δv, single-mode operation can theoretically be achieved. Calculations show that when the length of the unpumped erbium-doped fiber PM-EDF is greater than 2m, the reflection bandwidth of the dynamic grating is less than the free spectral range of the laser.

[0058] The longer the fiber, the narrower the bandwidth of the dynamic grating, which is more conducive to selecting a single longitudinal mode. However, the absorption loss of the signal light will also increase, resulting in a decrease in the output power and slope efficiency of the laser. At the same time, the threshold power of the laser will also increase. Therefore, a 3m long gain fiber was selected as the dynamic grating in the experiment.

[0059] A multi-ring cavity is a resonant cavity composed of multiple ring cavities with different cavity lengths. The longitudinal mode spacing is increased through the vernier effect to obtain single-mode laser output. For the simplest erbium-doped fiber laser with a single ring cavity, a longer cavity length typically results in a wider Ernst-Hyperdentic curve. 3+ A large number of longitudinal modes are generated within the gain bandwidth.

[0060] Furthermore, by combining a conventional fiber grating (FBG) with a typical reflection bandwidth of 0.11 nm (~13.7 GHz, 1550 nm), the longitudinal mode number can be reduced accordingly. Assuming the ring cavity length is L, the master free spectral range (FSRP) is... Where c is the speed of light in vacuum, n is the effective refractive index, and the length of the main ring is set to approximately 12 meters, the calculated FSR is 17.1 MHz, and a large number of longitudinal modes can oscillate within the FBG bandwidth. To obtain single-frequency lasers and effectively increase the free spectral range, a common approach is to select several sub-resonators with very small differences in cavity length, and combine these resonators with free spectra on the order of MHz into a multi-ring cavity with a free spectrum on the order of GHz. This allows for effective single-longitudinal-mode oscillation within a fiber grating resonator with a reflection bandwidth of approximately 0.1 nm. The disadvantage is that the multiple ring cavities reduce system stability; adding a polarization controller to the main cavity can enhance system stability. The analysis of coupled cavity resonators with two or more sub-ring cavities is quite complex and tedious. To simplify the fiber length selection criteria used for practical guidance, we will take a two-ring cavity as an example and analyze it based on the constructive interference conditions of SLM operation. Assuming the lengths of the two sub-rings are L1 and L2, based on simplified modeling and analysis, the resonant cavity has no particular requirements for the individual length of L1 (or L2), which has little impact on the performance of the fiber laser. Therefore, we only need to focus on the difference in sub-ring lengths. Under ideal conditions, when the two sub-cavities are at interference orders m and m+1 respectively, constructive interference can occur simultaneously.

[0061] Furthermore, the combined FSR of the two coupled subcavities can be expressed as:

[0062]

[0063] Effective FSR is broadened by reducing the length difference. The laser's emission frequency is located at the reflection center of the fiber grating, and two side modes at the edge of its 3dB bandwidth will eventually be suppressed. Therefore, when the size of FSRcp is equal to half the fiber grating bandwidth, the maximum length difference can be approximately estimated.

[0064] If an FBG with a bandwidth of ~13.7GHz is selected, the maximum length difference between the two sub-rings is estimated to be ~3cm based on the formula. The lengths of the two sub-rings, L1 and L2, are set to 2.40m and 2.37m respectively, corresponding to free spectral ranges of 85.2MHz and 86.2MHz. Combined, their effective FSRcp is ~6.82GHz, ensuring single-mode laser operation according to our standards.

[0065] In summary, this invention addresses the low output power of single-mode fiber lasers with a single oscillator stage by optimizing the laser's structural parameters. The laser employs a dynamic grating combined with a multi-ring cavity structure. Considering the filtering effect of the dynamic grating and the vernier effect of the multi-ring cavity, the longitudinal mode spacing is increased. Experiments were conducted to determine the optimal length of the low-doped polarization-maintaining erbium-doped fiber for generating the dynamic grating and the optimal optical path difference between the sub-ring cavities. Ultimately, a maximum output single-mode laser power of 287mW was achieved at a pump power of 4.2W, with an optical-to-optical conversion efficiency of 6.8% and a slope efficiency of 9.3%. The output power stability (standard deviation / average) was 0.9% within one hour, the output wavelength was 1550.036nm, and the linewidth of the single-mode laser measured using the time-delay self-heterodyne detection method was 1.66kHz. This single-oscillator stage laser, based on a dynamic grating combined with a multi-ring cavity structure, achieves a single-mode laser output power in the hundreds of milliwatts range. At this point, only some components inside the cavity are polarization-maintaining structures. Further optimization of the cavity into a fully polarization-maintaining structure can effectively reduce resonant cavity losses, improve power while preventing mode hopping, and is expected to achieve single-longitudinal-mode laser output in the W range.

[0066] The above solutions are merely illustrative examples of preferred embodiments and are not intended to limit the scope of the invention. Appropriate substitutions and / or modifications can be made according to user needs when implementing this invention.

[0067] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for the present invention. Other modifications can be readily made by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and examples shown and described herein.

Claims

1. A high-power single-longitudinal-mode laser with multiple ring cavities based on a dynamic grating, characterized in that, It includes multiple devices, which are as follows: A semiconductor laser, wherein the semiconductor lasers are connected in series as a pump source; A beam combiner for coupling a pump source into the cavity; Erbium-ytterbium co-doped double-clad fiber, which serves as a gain fiber to provide inverted particles for the laser; A cladding power stripper, wherein the cladding power stripper is used to filter out unabsorbed pump light in the cladding; An optical fiber circulator is used as a unidirectional device for a ring cavity laser. The laser enters from port 1 to port 2, is reflected by a high-reflectivity fiber grating, and is output from port 3 of the circulator. A low-doped polarization-maintaining erbium-doped fiber, wherein the low-doped polarization-maintaining erbium-doped fiber is in an unpumped state, and lasers are transmitted in opposite directions through a high-reflectivity grating and a circulator to form a dynamic grating in the low-doped polarization-maintaining erbium-doped fiber. A polarization-maintaining fiber coupler, used to construct a parallel connection between a sub-ring cavity and a main ring cavity; A polarization controller, used to adjust the polarization state within the cavity; The polarization-maintaining output coupler outputs 90% of the laser beam outside the resonant cavity, while the remaining 10% continues to oscillate and amplify within the resonant cavity. A high-reflectivity fiber Bragg grating is used to reflect laser light into the cavity.

2. The high-power single-longitudinal-mode laser with multiple ring cavities based on a dynamic grating according to claim 1, characterized in that, The length of the erbium-ytterbium co-doped double-clad optical fiber is 2.5m.

3. The high-power single-longitudinal-mode laser with multiple ring cavities based on a dynamic grating according to claim 2, characterized in that, The length of the low-doped polarization-maintaining erbium-doped fiber is 3m.

4. The high-power single-longitudinal-mode laser with multiple ring cavities based on a dynamic grating according to claim 3, characterized in that, The number of polarization-maintaining fiber couplers is two, and the splitting ratio of the two polarization-maintaining fiber couplers is 20:

80.

5. The high-power single-longitudinal-mode laser with multiple ring cavities based on a dynamic grating according to claim 4, characterized in that, The polarization-maintaining output coupler is a single unit with a beam splitting ratio of 10:

90.

6. The high-power single-longitudinal-mode laser with multiple ring cavities based on a dynamic grating according to claim 5, characterized in that, The erbium-ytterbium co-doped double-clad fiber has an absorption coefficient of 3.47 dB / m for 976 nm pump light.

7. The high-power single-longitudinal-mode laser with a multi-ring cavity based on a dynamic grating according to any one of claims 2-6, characterized in that, The total cavity length of the multi-ring cavity high-power single longitudinal mode laser based on dynamic grating is 12m. Multiple components of the laser are fixed on a water-cooling plate with high-temperature tape, and the water-cooling machine temperature is set to 12 degrees.

Images