A high power laser seed source based on a germanium saturable absorber for multi-modal medical imaging

Abstract

The application discloses a kind of high-power laser seed sources for multimodal medical imaging based on germanium saturable absorber, it is characterized in that, including first pump light source, second pump light source and laser resonator, the laser resonator is by first wave division multiplexer, second wave division multiplexer, erbium-doped optical fiber, polarization controller, polarization independent isolator, germanium saturable absorber, single-mode optical fiber, optical coupler sequentially connected, the first pump light source is connected with first wave division multiplexer, and second pump light source is connected with second wave division multiplexer.The germanium saturable absorber prepared in the application has good self-starting effect on pulse, the preparation method is simple, and the preparation cost is not high.And the stability of the obtained saturable absorber is good, and it can be used for passive mode locking for a long time.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of imaging technology, in particular to a high-power laser seed source based on germanium saturable absorber for multi-modal medical imaging. BACKGROUND

[0002] With the progress of science and technology, various modalities of new imaging technologies such as optical coherence tomography imaging and terahertz imaging have been developed, which play an extremely important role in assisting physicians in clinical diagnosis and treatment. For optical-based medical imaging systems, the key core component is the laser light source, and the performance parameters of the light source directly determine the imaging quality of the medical imaging system. For example, the sensitivity of the imaging system depends on the size of the output optical power. At present, for optical coherence tomography imaging of ophthalmology and blood vessels and terahertz imaging for material analysis and defect detection, low-cost and high-power laser seed sources are needed to improve the light source, thereby improving the sensitivity of the imaging system. In recent years, high-power mode-locked Er-doped fiber lasers based on saturable absorbers have attracted widespread attention and development due to their high output power and low cost. However, so far, the maximum average output power has not broken through 100mW. Therefore, there is still a lot of room for improvement in high-power laser seed source technology for medical imaging.

[0003] In the document "Investigations of mode-locked Er-doped oscillators with record high-pulse energies", two pump light sources are used, although a larger output power is achieved, but no saturable absorber is connected, which cannot achieve self-starting and is unstable. Therefore, there is an urgent need for a laser seed source with high output power, self-starting function and high stability. SUMMARY

[0004] In view of the above prior art, the purpose of the present application is to provide a high-power laser seed source based on germanium saturable absorber for multi-modal medical imaging.

[0005] To achieve the above purpose, the present application adopts the following technical solutions:

[0006] The present application provides a high-power laser seed source based on germanium saturable absorber for multi-modal medical imaging, comprising a first pump light source, a second pump light source and a laser resonant cavity, the laser resonant cavity is connected by a first wavelength division multiplexer, a second wavelength division multiplexer, an erbium-doped optical fiber, a polarization controller, a polarization-independent isolator, a germanium saturable absorber, a single-mode optical fiber and an optical coupler in sequence, the first pump light source is connected with the first wavelength division multiplexer, and the second pump light source is connected with the second wavelength division multiplexer.

[0007] Preferably, the germanium saturable absorber is prepared by the following method:

[0008] S1: 1-2 g of germanium powder is added to 10-20 mL of alcohol to prepare a germanium solution, and the germanium solution is ultrasonically treated for 5-7 hours to obtain a germanium dispersion solution;

[0009] S2: A polyvinyl alcohol solution is added to the germanium dispersion solution, and the volume ratio of the polyvinyl alcohol solution to the germanium dispersion solution is 1:1, and the mixture is ultrasonically treated for 2-4 hours to obtain a germanium-polyvinyl alcohol dispersion solution;

[0010] S3: The germanium-polyvinyl alcohol dispersion solution is coated into a culture dish, dried at room temperature, and evaporated for 10-15 hours to obtain a translucent germanium-polyvinyl alcohol film;

[0011] S4: The germanium-polyvinyl alcohol film is cut into small pieces and placed on the end face of one fiber jumper, and a fiber sleeve is connected in the middle to form a sandwich structure with the end face of another fiber jumper to make a germanium saturable absorber.

[0012] Preferably, the output end of the first pump light source is connected to the first input end of the first wavelength division multiplexer, the output end of the second pump light source is connected to the first input end of the second wavelength division multiplexer, the output ends of the first and second wavelength division multiplexers are respectively connected to the two ends of the doped fiber, the second input end of the first wavelength division multiplexer is connected to one end of the polarization controller, the other end of the polarization controller is connected to one end of the polarization independent isolator, the other end of the polarization independent isolator is connected to one end of the germanium saturable absorber, the other end of the germanium saturable absorber is connected to one end of the single-mode fiber, and the other end of the single-mode fiber is connected to the input end of the optical coupler, and the 20% output end of the optical coupler is connected to the second input end of the second wavelength division multiplexer.

[0013] Preferably, the first pump light source and the second pump light source are both semiconductor fiber lasers with a central wavelength of 980 nm.

[0014] Preferably, the working wavelengths of the first and second wavelength division multiplexers are 980 / 1550 nm, and the pigtails are both ordinary single-mode fibers.

[0015] Preferably, the gain medium of the resonant cavity is a 61 cm long doped fiber with the model Er-80.

[0016] Preferably, the polarization controller uses a three-coil rotating polarization controller.

[0017] Preferably, the optical coupler uses a 20:80 coupling ratio, and the 80% signal light output is used for data measurement.

[0018] As a preference, the devices of the laser resonant cavity are all connected by single-mode optical fibers.

[0019] The process of laser generation: the first pump light source and the second pump light source provide pump light, which is coupled into the ring cavity through the first wavelength division multiplexer and the second wavelength division multiplexer, and then passes through the erbium-doped fiber gain, the polarization controller, the polarization-independent isolator, the germanium saturable absorber, the single-mode optical fiber, and the optical coupler. The coupling ratio of the optical coupler is 20:80, wherein 80% of the output is used for data measurement, and the remaining 20% continues to operate in the laser resonant cavity. The polarization-independent isolator ensures the unidirectional transmission of the light in the cavity. By adjusting the values of the first pump light source and the second pump light source and the polarization controller, a stable high-power mode-locked pulse output is finally obtained. After adjusting the mode-locked state, the laser is turned off. The next time the laser is turned on, the mode-locked state can be achieved without further adjusting the polarization state of the laser.

[0020] There are mainly two methods for generating pulses in a laser: active modulation and passive modulation. Active modulation requires an external modulator (acousto-optic / electro-optic modulator) to be implemented, but this increases the cost and reduces the portability of the system. Passive modulation only requires the addition of an amplitude self-modulation device in the cavity, without the need for any external device, and can be automatically adjusted. Currently reported passive modulation mode-locked fiber lasers are mainly based on nonlinear polarization evolution and saturable absorbers. However, the modulation and mode-locked laser based on nonlinear polarization evolution technology is sensitive to the polarization change in the cavity, making it difficult to apply to mature laser products. Using a saturable absorber for passive modulation and mode-locking operation is a convenient and low-cost method to obtain pulsed laser.

[0021] A saturable absorber has an absorption characteristic related to light intensity. Under the action of strong light, it can be saturated instantaneously and become transparent, i.e. "bleaching". When the incident light weakens below the saturation light intensity, it can recover instantaneously to have a large light absorption coefficient and a very low transmittance. In the initial stage of laser pumping, the initial noise pulse formed by spontaneous radiation becomes stronger after multiple round trips until a dominant peak with the maximum light intensity is sufficient to saturate the saturable absorber. The transmittance at the peak of the pulse is the highest, and the transmittance at the front and back edges of the pulse is the lowest. After passing through the saturable absorber each time, the pulse is compressed once. After multiple cycles in the resonant cavity, the initial pulse is continuously compressed and narrowed, and finally a stable mode-locked pulse is formed.

[0022] In a laser, the periodicity of light's round-trip propagation within the laser cavity causes phase correlations between some longitudinal modes, resulting in beats and sharp intensity fluctuations in the laser beam. The saturable absorber within the laser cavity selectively absorbs these fluctuations. Fluctuations exceeding a certain intensity are amplified by the saturable absorber and the gain medium. During this round-trip propagation, the pulse's leading and trailing edges are attenuated, while the middle of the pulse is amplified, continuously compressing the pulse width. This pulse compression process is the establishment of phase correlations between the longitudinal modes within the cavity, also known as mode-locking.

[0023] The beneficial effects of this invention are:

[0024] The germanium saturable absorber prepared in this invention exhibits good self-starting capability for pulses. After adjusting the mode-locking state and turning off the laser, mode-locking can be achieved the next time the laser is turned on, without needing to readjust the laser's polarization state. The preparation method is simple and the preparation cost is low. Furthermore, the obtained saturable absorber has good stability and can be used for passive mode-locking for extended periods.

[0025] This invention can obtain high-power pulses in the 1.5μm band, achieving a breakthrough in the index of 100mW. It can be used as a high-power laser seed source for multimodal medical imaging such as optical coherence tomography and terahertz imaging, thereby improving the sensitivity of medical imaging. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the all-fiber laser structure provided in Example 1 of the present invention;

[0027] Figure 2 This is a graph showing the relationship between output power and pump power in Embodiment 1 of the present invention;

[0028] Figure 3 This is the spectrum of Embodiment 1 of the present invention;

[0029] Figure 4 This is a pulse sequence diagram of Embodiment 1 of the present invention;

[0030] Figure 5 This is a single pulse diagram of Embodiment 1 of the present invention;

[0031] Figure 6 This is a spectrum diagram of Embodiment 1 of the present invention;

[0032] The figure shows: 1. First pump source, 2. Second pump source, 3. First wavelength division multiplexer, 4. Second wavelength division multiplexer, 5. Erbium-doped fiber, 6. Polarization controller, 7. Polarization-independent isolator, 8. Germanium saturable absorber, 9. Single-mode fiber, 10. Optical coupler. Detailed Implementation

[0033] It should be noted that the following detailed description is illustrative only, and is intended to provide further description in order to provide a thorough understanding of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0034] As described in the background, based on this, the present application provides a high-power laser seed source based on a germanium saturable absorber for multi-modal medical imaging, comprising a first pump light source 1, a second pump light source 2 and a laser resonant cavity, the output end of the first pump light source 1 is connected with the first input end of a first wavelength division multiplexer 3, the output end of the second pump light source 2 is connected with the first input end of a second wavelength division multiplexer 4, the output ends of the first wavelength division multiplexer 3 and the second wavelength division multiplexer 4 are connected with the two ends of a doped fiber 5 respectively, the second input end of the first wavelength division multiplexer 3 is connected with one end of a polarization controller 6, the other end of the polarization controller 6 is connected with one end of a polarization independent isolator 7, the other end of the polarization independent isolator 7 is connected with one end of a germanium saturable absorber 8, the other end of the germanium saturable absorber 8 is connected with one end of a single-mode fiber 9, the other end of the single-mode fiber 9 is connected with the input end of an optical coupler 10, and the 20% output end of the optical coupler 10 is connected with the second input end of the second wavelength division multiplexer 4. The first pump light source 1 and the second pump light source 2 are both semiconductor fiber lasers with a center wavelength of 980 nm. The working wavelengths of the first wavelength division multiplexer 3 and the second wavelength division multiplexer 4 are 980 / 1550 nm, and the pigtail fibers of both are ordinary single-mode fibers. The gain medium of the resonant cavity is a 61 cm long doped fiber with a model of Er-80. The polarization controller 6 adopts a three-coil rotating polarization controller. The optical coupler 10 adopts a coupling ratio of 20:80, and the 80% signal light output is used for data measurement. The devices of the laser resonant cavity are all connected through single-mode fibers.

[0035] The germanium saturable absorber is prepared by the following method:

[0036] S1: 1-2 g of germanium powder is added to 10-20 mL of alcohol to prepare a germanium solution, and the germanium solution is ultrasonically treated for 5-7 hours to obtain a germanium dispersion solution;

[0037] S2: A polyvinyl alcohol solution is added to the germanium dispersion solution, and the volume ratio of the polyvinyl alcohol solution to the germanium dispersion solution is 1:1, and ultrasonic treatment is performed for 2-4 hours to obtain a germanium-polyvinyl alcohol dispersion solution;

[0038] S3: The germanium-polyvinyl alcohol dispersion solution is coated into a culture dish, dried and evaporated at room temperature for 10-15 hours to obtain a translucent germanium-polyvinyl alcohol film;

[0039] S4: cut the germanium-polyvinyl alcohol film into small pieces and place it on the end face of one fiber jumper, connect it in the middle with a clean fiber sleeve, and form a sandwich structure with the end face of another fiber jumper to make a germanium saturable absorber.

[0040] The process of laser generation: the first pump light source 1 and the second pump light source 2 provide pump light, which is coupled into the ring cavity through the first wavelength division multiplexer 3 and the second wavelength division multiplexer 4, and then sequentially passes through the polarization controller 6, the polarization-independent isolator 7, the saturable absorber 8, the single-mode optical fiber 9 and the optical coupler 10, wherein the coupling ratio of the optical coupler 10 is 20:80, 80% of which is output for data measurement, and the remaining 20% continues to operate in the laser resonant cavity, the polarization-independent isolator 7 ensures the unidirectional transmission of the light in the cavity, and finally the stable high-power mode-locked pulse output is obtained by adjusting the values of the first pump light source 1 and the second pump light source 2 and the polarization controller 6. After adjusting the mode-locked state, the laser is turned off. The next time the laser is turned on, the mode-locked state can be realized without adjusting the polarization state of the laser again.

[0041] In order for those skilled in the art to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described in detail below in conjunction with specific embodiments.

[0042] The test materials used in the embodiments of the present application are all conventional test materials in the art and can be purchased through commercial channels. In the following examples, the pump light source, the wavelength division multiplexer, the polarization-independent isolator, the polarization controller and the optical coupler are all conventional choices in the industry and are not specially limited.

[0043] Embodiment

[0044] A preparation method of a germanium saturable absorber, comprising the following steps:

[0045] S1: 1 germanium powder is added into 10 mL of 30% alcohol solution to prepare a germanium solution, and the germanium solution is ultrasonically treated for 6 hours to obtain a germanium dispersion solution;

[0046] S2: a 5% polyvinyl alcohol solution is added to the germanium dispersion solution, the volume ratio of the polyvinyl alcohol solution to the germanium dispersion solution is 1:1, and ultrasonic treatment is performed for 3 hours to obtain a germanium-polyvinyl alcohol dispersion solution;

[0047] S3: 100 muL of uniformly dispersed germanium-polyvinyl alcohol solution is spin-coated into a clean culture dish, the solution is dried and evaporated at room temperature for 12 hours to obtain a translucent germanium-polyvinyl alcohol film;

[0048] S4: Cut the Ge-PVA film into 1mm x 1mm pieces and place it on the end face of one fiber jumper, connect it with a clean fiber ferrule in the middle, and then put it on the end face of another fiber jumper to form a sandwich structure, and make a Ge saturable absorber.

[0049] Laser configuration:

[0050] Combination Figure 1 A high-power laser seed source based on a Ge saturable absorber, comprising a first pump light source 1, a second pump light source 2 and a laser resonant cavity, the output end of the first pump light source 1 is connected with the first input end of a first wavelength division multiplexer 3, the output end of the second pump light source 2 is connected with the first input end of a second wavelength division multiplexer 4, the output ends of the first wavelength division multiplexer 3 and the second wavelength division multiplexer 4 are connected with the two ends of a doped fiber 5 respectively, the second input end of the first wavelength division multiplexer 3 is connected with one end of a polarization controller 6, the other end of the polarization controller 6 is connected with one end of a polarization independent isolator 7, the other end of the polarization independent isolator 7 is connected with one end of a Ge saturable absorber 8, the other end of the Ge saturable absorber 8 is connected with one end of a single-mode fiber 9, the other end of the single-mode fiber 9 is connected with the input end of an optical coupler 10, and the 20% output end of the optical coupler 10 is connected with the second input end of the second wavelength division multiplexer 4. The first pump light source 1 and the second pump light source 2 are both semiconductor fiber lasers with a center wavelength of 980nm. The working wavelengths of the first wavelength division multiplexer 3 and the second wavelength division multiplexer 4 are 980 / 1550nm, and the pigtail of each is a common single-mode fiber. The gain medium of the resonant cavity is a 61cm long doped fiber with a model of Er-80. The polarization controller 6 adopts a three-coil rotating polarization controller. The optical coupler 10 adopts a coupling ratio of 20:80, and the 80% signal light output is used for data measurement. The devices in the laser resonant cavity are all connected by single-mode fiber fusion.

[0051] Laser generation process:

[0052] Laser generation process: The first pump light source 1 and the second pump light source 2 provide pump light, which is coupled into the ring cavity through the first wavelength division multiplexer 3 and the second wavelength division multiplexer 4, and then passes through the polarization controller 6, the polarization independent isolator 7, the saturable absorber 8, the single-mode fiber 9 and the optical coupler 10 in turn. The coupling ratio of the optical coupler 10 is 20:80, and the 80% output is used for data measurement. The remaining 20% continues to operate in the laser resonant cavity. The polarization independent isolator 7 ensures the unidirectional transmission of the light in the cavity. By adjusting the values of the first pump light source 1 and the second pump light source 2 and the polarization controller 6, a stable high-power mode-locked pulse output is finally obtained.

[0053] Experimental example

[0054] When there is no germanium saturable absorber in the laser, no matter how to adjust the pump power of the polarization controller, only continuous wave output is obtained, which excludes the possibility of mode-locked operation caused by nonlinear polarization rotation and Fabry-Perot cavity effect. After connecting the germanium saturable absorber to the fiber laser ring cavity, the pump power is appropriately increased and the polarization controller is adjusted, and the mode-locked pulse laser can be obtained.

[0055] The total pump power is increased from 1360mW to 1816mW, and the polarization controller is carefully rotated, and stable mode-locked pulses can be obtained. The output characteristics of the stable mode-locked operation under the pump power of 1816mW are shown in Figures 2 to 6 Figure 2 The output power and the pump power are linearly related, and the output power is 136.14mW when the pump power is 1816mW. Figure 3 The output spectrum is shown in Fig. 5, and the center wavelength and 3dB bandwidth are 1557.94nm and 2.134nm, respectively. Figure 4 The pulse sequence is shown in Fig. 6, and the time from one pulse to the next pulse is 6.06us. Figure 5 The single pulse diagram is shown in Fig. 7, and the duration of the diagram is 14.89ns. Figure 6 The frequency spectrum is shown in Fig. 8, and it can be seen that the basic repetition frequency is 168.13kHz, and the signal-to-noise ratio is about 50dB, which indicates that the obtained mode-locked pulse has good stability.

[0056] The above results show that the mode-locked pulse with high stability is obtained.

[0057] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.​

Claims

1. A high-power laser seed source based on a germanium saturable absorber for multimodal medical imaging, characterized in that, It includes a first pump source, a second pump source, and a laser resonant cavity, wherein the laser resonant cavity comprises a first wavelength division multiplexer, a second... The system comprises a wavelength division multiplexer (WDM), an erbium-doped fiber, a polarization controller, a polarization-independent isolator, a germanium saturable absorber, a single-mode fiber, and an optical coupler, connected sequentially. A first pump source is connected to a first WDM, and a second pump source is connected to a second WDM. The output of the first pump source is connected to the first input of the first WDM, and the output of the second pump source is connected to the first input of the second WDM. The outputs of the first and second WDMs are respectively connected to the two ends of the erbium-doped fiber. The second input of the first WDM is connected to one end of the polarization controller, and the other end of the polarization controller is connected to one end of the polarization-independent isolator. The other end of the ionizer is connected to one end of a germanium saturable absorber, the other end of which is connected to one end of a single-mode fiber. The other end of the single-mode fiber is connected to the input end of an optical coupler. The 20% output end of the optical coupler is connected to the second input end of a second wavelength division multiplexer. The operating wavelengths of the first and second wavelength division multiplexers are 980 / 1550 nm, and their pigtails are both ordinary single-mode fibers. The gain medium of the resonant cavity is a 61 cm long erbium-doped fiber, model Er-80. The optical coupler adopts a 20:80 coupling ratio, in which 80% of the signal light output is used for data measurement. All components in the laser resonant cavity are fused together using single-mode fibers. The germanium saturable absorber is prepared by the following method: S1: Add 1-2g of germanium powder to 10-20mL of alcohol to prepare a germanium solution. Sonicate the germanium solution for 5-7 hours to obtain a germanium dispersion solution. S2: Add the polyvinyl alcohol solution to the germanium dispersion solution, with a volume ratio of 1:1 between the polyvinyl alcohol solution and the germanium dispersion solution, and sonicate for 2-4 hours to obtain the germanium-polyvinyl alcohol dispersion solution. S3: Coat the germanium-polyvinyl alcohol dispersion solution into a petri dish, dry and evaporate at room temperature for 10-15 hours to obtain a translucent germanium-polyvinyl alcohol film; S4: Cut the germanium-polyvinyl alcohol film into small pieces and place them on the end face of an optical fiber jumper. Connect an optical fiber sleeve in the middle and form a sandwich structure with the end face of another optical fiber jumper to make a germanium saturable absorber.

2. The high-power laser seed source according to claim 1, characterized in that, Both the first and second pump sources are semiconductor fiber lasers with a center wavelength of 980nm.

3. The high-power laser seed source according to claim 1, characterized in that, The polarization controller is a three-coil rotary polarization controller.

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