An adjustable terahertz wave source based on resonant sideband of soliton mode-locked fiber laser

Abstract

The application discloses a kind of adjustable terahertz wave sources based on soliton mode-locked fiber laser resonant sideband, including sequentially arranged fiber laser, fiber amplifier, aspheric lens, photoconductive antenna and terahertz wave filter along optical path, the lens is bonded at the back of the photoconductive antenna, fiber laser is connected with fiber amplifier, the light output by fiber amplifier enters aspheric lens, the output beam of aspheric lens is irradiated on photoconductive antenna, and outputs electrical signal, and terahertz wave is filtered out after electrical signal passes through terahertz wave filter.The application utilizes the tunability of resonant sideband spacing in soliton mode-locked fiber laser, and obtains adjustable terahertz signal by optical mixing, avoids the complexity of the previous terahertz wave generation system pumped by two lasers, and the instability of output terahertz wave caused by double-wavelength laser as pump source, so that the system structure is more compact, adjustment is convenient, and the conversion efficiency of optical to terahertz wave of terahertz wave generation system is improved.

Description

Technical Field

[0001] This invention relates to a tunable terahertz wave source, and more particularly to a tunable terahertz wave source based on the resonant sideband of a soliton mode-locked fiber laser. Background Technology

[0002] Terahertz waves are electromagnetic waves with frequencies between 0.1 THz and 10 THz, corresponding to wavelengths ranging from 3 mm to 30 μm. In the electromagnetic spectrum, the terahertz wave band lies between millimeter waves and the infrared band. Although terahertz waves exist in nature, their intensity is relatively weak due to significant absorption by the Earth's atmosphere. Because the generation and detection of terahertz waves are more difficult than other bands, this frequency band has only received widespread attention and development in recent decades. This special band of terahertz waves shows broad application prospects in fields such as high-speed wireless communication, secure imaging, materials spectroscopy, astronomy, and biomedicine.

[0003] Terahertz waves can be generated using electronic and photonic technologies. In electronics, there are methods for generating terahertz waves based on back-wave tubes and Gunn diode oscillators. On the other hand, generating terahertz waves using photonic technology is an important research direction in terahertz wave generation. For example, terahertz waves can be generated by exciting photoconductive antennas with femtosecond laser pulses, using intra-difference frequency converters in nonlinear crystals, or using terahertz parametric oscillators. Furthermore, with the development of ultrafast optics, ultrafast optoelectronic devices, and nonlinear optical crystal materials, terahertz wave generation technology has also developed rapidly.

[0004] With the expansion of terahertz wave applications, higher demands are being placed on the performance of terahertz wave generation systems, such as tunability, stability, and power. Currently, there are reports of obtaining continuously tunable terahertz waves by mixing two laser beams generated by a dual-wavelength fiber laser. However, the incoherence of the two laser beams leads to instability in the generated terahertz waves. Terahertz wave generation systems based on mode-locked fiber lasers can produce stable terahertz waves, but the optical energy utilization rate of these systems is not high. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to propose a tunable terahertz wave source based on the resonant sideband of a soliton mode-locked fiber laser, which generates a broadband tunable terahertz wave while maintaining the stability of the output terahertz wave.

[0006] Technical solution: The present invention includes a fiber laser, a fiber amplifier, an aspherical mirror, a photoconductive antenna, and a terahertz wave filter arranged sequentially along the optical path. A lens is attached to the back of the photoconductive antenna. The fiber laser is connected to the fiber amplifier. The light output from the fiber amplifier enters the aspherical mirror. The output beam of the aspherical mirror illuminates the photoconductive antenna and outputs an electrical signal. The electrical signal is filtered out as a terahertz wave by the terahertz wave filter.

[0007] The fiber laser is a soliton mode-locked fiber laser capable of generating resonant sidebands.

[0008] The fiber laser adopts a ring cavity structure, which includes a wavelength division multiplexer, a first optical fiber, an optical circulator, a second optical fiber, an optical isolator, an optical coupler, and a saturable absorber connected in sequence from end to end. The wavelength division multiplexer is connected to the pump source.

[0009] The first optical fiber is a gain fiber, and the gain fiber is an erbium-doped fiber.

[0010] The output end of the gain fiber is connected to the input end of the optical circulator, the reflective end of the optical circulator is connected to the grating stress tuning device, and the output end of the optical circulator is connected to the second fiber.

[0011] The grating stress tuning device is a chirped fiber grating stress tuning device, which includes a chirped fiber grating. The two ends of the chirped fiber grating are fixed by fiber optic clamps, and the fiber optic clamps are connected to the displacement platform through a bracket. One of the brackets has a displacement knob at the bottom.

[0012] The displacement knob is used to adjust the stress on the chirped fiber grating to change the spacing of the resonant sidebands of the soliton mode-locked fiber laser, thereby changing the frequency at which the terahertz wave is generated.

[0013] The second optical fiber is a single-mode optical fiber, which is used to adjust the length of the laser cavity and ensure the stability of the laser output.

[0014] The aspherical mirror is mounted on the top of the support.

[0015] Beneficial effects: This invention utilizes the tunability of the resonant sideband spacing within a soliton mode-locked fiber laser and obtains a tunable terahertz signal through optical mixing. This avoids the complexity of previous terahertz wave generation systems that used two lasers to pump the terahertz wave, as well as the instability of the output terahertz wave caused by using dual-wavelength lasers as pump sources. This makes the system structure more compact and easier to adjust. On the other hand, thanks to the high intensity of the resonant sideband within the output laser, more optical energy is concentrated within the resonant sideband, improving the optical energy utilization rate and thus enhancing the light-to-terahertz wave conversion efficiency of the terahertz wave generation system. Attached Figure Description

[0016] Figure 1This is a schematic diagram of the adjustable terahertz wave source structure of the present invention;

[0017] Figure 2 This is a schematic diagram of the soliton mode-locked fiber laser that generates resonant sidebands in this invention;

[0018] Figure 3 This is the chirped fiber grating stress tuning device in this invention. Detailed Implementation

[0019] The invention will now be further described with reference to the accompanying drawings.

[0020] like Figure 1 As shown, the present invention includes a fiber laser 15, a fiber amplifier 16, an aspherical mirror 17, a photoconductive antenna 18, and a terahertz wave filter 23 arranged sequentially along the optical path. The fiber laser 15 is a soliton mode-locked fiber laser capable of generating resonant sidebands. The fiber laser 15 is connected to the fiber amplifier 16 via a single-mode fiber. The fiber amplifier 16 is an erbium-doped fiber amplifier. The output light of the erbium-doped fiber amplifier enters the single-mode fiber. The fiber head at the other end of the single-mode fiber is connected to the aspherical mirror 17 on the top of the bracket 24 via a fiber flange. The fiber flange is placed at the front focal point of the aspherical mirror, and its output beam 20 illuminates the photoconductive antenna 18. A super-hemispherical lens 19 is attached to the back of the photoconductive antenna 18 to converge the beam and output an electrical signal 21. The electrical signal 21 is transmitted in free space and reaches the terahertz wave filter 23, which filters out the desired terahertz wave 22.

[0021] like Figure 2The image shows the soliton mode-locked fiber laser that generates resonant sidebands. It is an all-fiber ring cavity structure with a total dispersion of 0.4 ps / nm to 25 ps / nm. The ring cavity includes, in sequence, a wavelength division multiplexer 2, a gain fiber 3, an optical circulator 8, a single-mode fiber 4, an optical isolator 5, an optical coupler 6, and a saturable absorber 20. The wavelength division multiplexer 2 is connected to the pump source 1, which is a fiber-coupled semiconductor laser with an output wavelength of 976 nm and an adjustable pump power from 0 mW to 600 mW. The wavelength division multiplexer 2 is a 980 nm / 1550 nm wavelength division multiplexer, and its operating wavelength should be consistent with the stimulated emission wavelength of the gain fiber 3, which is an erbium-doped fiber. Pump source 1 pumps the erbium-doped fiber through wavelength division multiplexer 2. The output end of the erbium-doped fiber is connected to the input end of optical circulator 8. The reflecting end of optical circulator 8 is connected to grating stress tuning device 7 through single-mode fiber. The output end of optical circulator 8 is connected to single-mode fiber 4. Changing the length of single-mode fiber 4 can change the length of the laser cavity, thereby ensuring the stability of the laser output. The output end of single-mode fiber 4 is connected to optical isolator 5, which is used to ensure unidirectional transmission of light within the fiber loop. The coupling ratio of optical coupler 6 is 30:70. 70% of the output end of optical coupler 6 is connected to saturable absorber 20, and 30% of the output end is used as the laser output. Saturable absorber 20 serves as the actual passive mode-locking device within the laser cavity. It can also be replaced by artificial saturable absorbers such as nonlinear polarization rotation (NPR) or nonlinear ring magnifying glass (NALM). The saturable absorber 20 is fused to the ring cavity via an integrated optical fiber pigtail. The input optical fiber of the saturable absorber 20 is connected to the output optical fiber of the optical coupler 6, and the output optical fiber of the saturable absorber 20 is connected to the reflection optical fiber of the wavelength division multiplexer 2.

[0022] like Figure 3 As shown, the grating stress tuning device 7 is a chirped fiber grating stress tuning device, including a chirped fiber grating 9 with a wavelength of 1544nm to 1556nm. Both ends of the chirped fiber grating 9 are fixed by fiber clamps 10, which are connected to a displacement platform 11 via a bracket. A displacement knob 12 is located at the lower part of the outermost bracket. Adjusting the distance between the two fiber clamps 10 using the displacement knob 12 adjusts the stress on the chirped fiber grating 9, thereby changing the spacing of the resonant sidebands of the soliton mode-locked fiber laser and thus changing the frequency of the generated terahertz wave. In practical applications, adjusting the position of the fiber clamps within a 3mm displacement range can obtain tunable terahertz waves ranging from 0.2THz to 0.9THz. The tunable range of the terahertz wave frequency is limited by the bandwidth of the chirped fiber grating and its maximum withstand stress.

[0023] Working principle:

[0024] A soliton mode-locked fiber laser is used to generate resonant sidebands. By changing the stress on the chirped fiber grating within the laser cavity, the spacing of the resonant sidebands in the output spectrum is altered. The laser beam containing the resonant sidebands then illuminates a photoconductive antenna, where it is mixed. The resulting electrical signal is filtered by a terahertz wave filter to obtain a tunable terahertz wave. The working principle is explained using formulas.

[0025] The spacing between the two most intense resonant sidebands in a soliton mode-locked fiber laser can be expressed as:

[0026]

[0027] Where λ0 is the laser center wavelength, D is the average dispersion within the cavity, L0 represents the laser cavity length, τ is the pulse width, and c is the speed of light. This formula shows that the resonant sideband spacing is inversely proportional to the square root of the total dispersion within the laser cavity. Therefore, the resonant sideband spacing can be changed by altering the total dispersion of the laser cavity.

[0028] Compared to the total dispersion of the fiber within the laser cavity, the chirped fiber grating exhibits a larger dispersion. By placing the chirped fiber grating inside the laser cavity, it can be used to control the total dispersion of the laser cavity. The dispersion characteristics of the chirped fiber grating can be represented by the detuning function:

[0029]

[0030] In this equation, Λ0 is the spatial period of the center point of the chirped fiber grating, β is the propagation constant, and the third term on the right-hand side is the chirp function, which is related to the length L of the chirped fiber grating. By applying stress to the chirped fiber grating, the length L and the spatial period Λ0 can be changed, thereby altering its dispersion. Therefore, by adding a stress tuning device to the chirped fiber grating to change its stress, the dispersion of the chirped fiber grating can be tuned. Subsequently, by placing this structure within the cavity of a mode-locked fiber laser, the resonant sideband spacing can be adjusted.

[0031] When a photoconductive antenna is illuminated by a laser containing resonant sidebands, any two optical signals within the spectrum will mix within the photoconductive antenna. The two optical signals E1 and E2 are represented as follows: and Where ω1 and ω2 represent the angular frequencies of the photoelectric field. and This represents the initial phase. The electrical signal strength generated after mixing is:

[0032]

[0033] This equation shows that optical mixing produces a component ω with a frequency equal to the difference between the frequencies of the two optical signals. mix=ω1-ω2, and the intensity of this frequency component is proportional to the product of the intensities of the two incident light signals. Based on this principle, if the spacing between the two resonant sidebands generated by a soliton mode-locked fiber laser is on the order of terahertz, then a terahertz signal of the corresponding frequency will be generated after mixing. Furthermore, a terahertz wave filter needs to be placed after the photoconductive antenna to filter out unwanted electrical signals in the mixed signal.

Claims

1. A tunable terahertz wave source based on the resonant sideband of a soliton mode-locked fiber laser, characterized in that, The system includes a fiber laser, a fiber amplifier, an aspherical mirror, a photoconductive antenna, and a terahertz wave filter arranged sequentially along the optical path. A lens is attached to the back of the photoconductive antenna. The fiber laser is connected to the fiber amplifier. The light output from the fiber amplifier enters the aspherical mirror. The output beam of the aspherical mirror illuminates the photoconductive antenna and outputs an electrical signal. The electrical signal is filtered out by the terahertz wave filter. The fiber laser is a soliton mode-locked fiber laser capable of generating resonant sidebands. The fiber laser adopts a ring cavity structure. The ring cavity includes a wavelength division multiplexer, a first fiber, an optical circulator, a second fiber, an optical isolator, an optical coupler, and a saturable absorber connected in sequence. The wavelength division multiplexer is connected to the pump source. The first fiber is a gain fiber. The output end of the gain fiber is connected to the input end of the optical circulator. The reflective end of the optical circulator is connected to a grating stress tuning device. The output end of the optical circulator is connected to the second fiber. The grating stress tuning device is a chirped fiber grating stress tuning device, including a chirped fiber grating. The two ends of the chirped fiber grating are fixed by fiber clamps, which are connected to a displacement platform via a bracket. One of the brackets has a displacement knob at its lower part, which is used to adjust the stress on the chirped fiber grating. By changing the stress on the chirped fiber grating in the laser cavity, the spacing of the resonant sidebands in the output spectrum is changed. Then, the laser containing the resonant sidebands irradiates the photoconductive antenna and is mixed in the photoconductive antenna. The generated electrical signal is filtered by a terahertz wave filter to obtain a tunable terahertz wave.

2. The tunable terahertz wave source based on the resonant sideband of a soliton mode-locked fiber laser according to claim 1, characterized in that, The second optical fiber is a single-mode optical fiber.

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