High-power 1645nm femtosecond fiber laser based on nonlinear gain modulation

Abstract

The invention discloses a high-power and high-coherence 1645nm femtosecond fiber laser based on nonlinear gain modulation, the output wavelength is adjustable, the whole U wave band (1620-1680nm) can be covered, and a system comprises an erbium-doped mode-locked laser, an optical circulator, a fiber bragg grating, an erbium-doped fiber amplifier, a single-frequency laser, a coupler, a nonlinear fiber, an optical splitter and a pulse compressor. An ultrafast pulse generated by the mode-locked laser is filtered and amplified by the fiber bragg grating, the ultrafast pulse and 1645nm single-frequency laser are injected into the nonlinear fiber, nonlinear gain modulation is performed on the single-frequency laser, so that the time domain of the single-frequency laser is shaped into the ultrafast pulse, and femtosecond pulse output can be realized after compression. The tuning single-frequency laser can realize continuous tuning of an output spectrum, and covers the whole U wave band. The U-band femtosecond pulse generating device generates U-band femtosecond pulses with the power of the magnitude of several watts, and fills the gap of erbium and thulium ion emission spectra. The high-coherence and high-power U-band femtosecond light source is suitable for a plurality of fields such as atmospheric remote sensing, imaging, communication, radar and the like.

Description

Technical Field

[0001] The present invention relates to the field of ultrafast fiber laser technology, and particularly to a high-power 1645nm femtosecond fiber laser based on nonlinear gain modulation. Background Art

[0002] The rapid development of ultrafast laser pulse generation technology has promoted great progress in scientific research and industrial fields, and is widely used in various fields such as ultrafast process imaging, remote sensing detection, lidar, industrial processing, and communication. The 1645nm ultrafast pulse is located in the U band, where there is absorption information of various greenhouse gases. Located in the atmospheric communication window, it can be widely used in atmospheric multi-component remote sensing, three-photon imaging, U-band communication, and free-space communication.

[0003] Currently, one of the main methods for generating ultrafast pulses is the active / passive mode-locking technology. The mode-locking technology realizes the in-phase wide-spectrum output by locking the phases of each longitudinal mode in the emission spectrum of rare-earth ions, corresponding to the ultrafast pulse in the time domain. However, the U band where 1645nm is located is at the edge of the emission spectra of erbium ions and thulium ions, and it is impossible to obtain net gain through the mode-locking technology to achieve femtosecond laser output. Currently, the application light sources for ultrafast pulses in the U band mainly use nonlinear frequency conversion technologies such as supercontinuum generation, soliton self-frequency shift, and self-phase modulation. These methods can transfer the ultrafast pulse energy in other bands to 1645nm, but the conversion efficiency is lower than 20%, and the average power of the generated 1645nm ultrafast pulse is typically in the milliwatt range. Also, due to the fact that 1645nm is at the edge of the rare-earth ion emission spectrum, it is impossible to use a fiber amplifier to achieve gain amplification in this band, making it difficult to develop various ultrafast laser applications in the U band. Therefore, generating high-power and highly coherent ultrafast pulses that can be practically applied at 1645nm remains a difficult problem in the field of light sources. Summary of the Invention

[0004] In order to solve the problem that the mode-locking technology cannot generate ultrafast pulses in the 1645nm band, and the traditional nonlinear frequency conversion methods have low conversion efficiency and cannot be practically applied, the present invention is based on the nonlinear optical gain modulation technology and uses the 1550nm chirped pulse amplification technology. Taking the 1550nm band as the pump source, the 1645nm single-frequency laser is nonlinearly gain-modulated by stimulated Raman scattering, and the single-frequency laser is modulated into an ultrafast laser with the same repetition frequency as the 1550nm pump light. This technology uses an all-fiber structure, is easy to integrate, the system is simple, the generated 1645nm ultrafast pulse has high coherence, and the energy conversion efficiency can be as high as more than 60%, and it can be practically applied to various fields.

[0005] The solution of the present invention is as follows:

[0006] A high-power 1645 nm femtosecond fiber laser based on nonlinear gain modulation, comprising a 1550 nm erbium-doped mode-locked fiber laser, an optical circulator, a fiber Bragg grating, an erbium-doped fiber amplifier, a 1645 nm single-frequency laser, a coupler, a nonlinear fiber, a splitter and a pulse compressor. The 1550 nm erbium-doped mode-locked fiber laser generates femtosecond pulses covering 1550 nm as a femtosecond pulse seed source, which is introduced into the fiber Bragg grating by the optical circulator. The fiber Bragg grating reflects the narrowband spectrum centered at 1550 nm, provides pulse broadening under the Fourier limit for chirped pulse amplification, and eliminates the spectral components that are contaminated with subsequent stimulated Raman scattering. The remaining spectral components are output through the first output port. The reflected narrowband spectrum is again guided by the optical circulator (2) to the erbium-doped fiber amplifier. After being amplified by the erbium-doped fiber amplifier, it is used as pump light and injected into the nonlinear fiber together with the single-frequency laser output from the 1645 nm single-frequency laser as signal light through the coupler. In the nonlinear fiber, the 1550 nm femtosecond pulses transfer energy and time-domain characteristics to the 1645 nm single-frequency laser through the stimulated Raman scattering effect, making it modulated into an ultrafast pulsed laser. Subsequently, the remaining 1550 nm pump light is separated from the 1645 nm ultrafast pulsed laser by the splitter. The 1645 nm ultrafast pulsed laser is compressed to the femtosecond level by the pulse compressor and then output through the second output port, and the remaining 1550 nm pump light is output through the third output port.

[0007] The working process of this laser is that the 1550 nm erbium-doped mode-locked fiber laser first generates a femtosecond pulse train. After being broadened by narrowband filtering of the optical circulator and the fiber Bragg grating, the power is increased by the erbium-doped fiber amplifier, and then it is coupled with the single-frequency laser generated by the 1645 nm single-frequency laser into the nonlinear fiber. In the nonlinear interaction region, the 1550 nm pulses transfer energy efficiently to the 1645 nm band through stimulated Raman scattering, and at the same time, due to self-phase modulation, its time-domain characteristics are transformed into ultrafast pulses. The remaining 1550 nm pump light is filtered out by the splitter, and the 1645 nm pulses are further optimized in time-domain characteristics by the compressor, and finally, high-power and narrow-pulse-width femtosecond 1645 nm laser is output.

[0008] The tuning method of the laser repetition frequency output from the second output port is achieved by adjusting the cavity length of the 1550 nm erbium-doped mode-locked fiber laser; the method for adjusting the initial pulse width of the laser output from the second output port is achieved by adjusting the reflection bandwidth of the fiber Bragg grating; the tuning method of the central wavelength of the laser output from the second output port is achieved by adjusting the wavelength of the 1645 nm single-frequency laser or replacing the single-frequency laser with different wavelength bands; the method for adjusting the spectral width of the laser output from the second output port is achieved by adjusting the length of the nonlinear fiber or the pump light power output from the erbium-doped fiber amplifier. Since the stimulated Raman scattering efficiency of the silicon-based fiber in the frequency shift range of 9 - 15 THz exceeds 50%, this scheme can cover the wavelength tuning requirements of the entire U band (1620 - 1680 nm) and has the spectral regulation ability of wide range and high flexibility.

[0009] The repetition frequency of the 1550 nm erbium-doped mode-locked fiber laser is 102.3 MHz, and the central wavelength is 1550 nm.

[0010] The wavelength tuning range of the 1645 nm single-frequency laser is 1620 - 1680 nm, covering the entire U band. The output laser linewidth of the 1645 nm single-frequency laser is less than 10 MHz, and the coherence of the 1645 nm femtosecond pulse output from the second output port after pulse width compression inherits from the 1645 nm single-frequency laser; when adjusting the wavelength of the 1645 nm single-frequency laser, the spectrum of the 1645 nm femtosecond pulse output from the second output port undergoes continuous translation, and the spectral shape and intensity are maintained.

[0011] The optical circulator is a three-port fiber circulator, and the working band is 1550 nm.

[0012] The fiber Bragg grating is used for narrowband filtering, with a central reflection wavelength of 1530 - 1560 nm, a reflection bandwidth of 0.1 - 0.5 nm, and a reflectivity of 100%.

[0013] The erbium-doped fiber amplifier is a dedicated amplifier for the 1550 nm band, with the power amplification characteristics of a wide dynamic range, and the output power has a continuously adjustable function to meet the precise control of different gain requirements of the system.

[0014] The coupler is a coarse wavelength division multiplexer, and the splitter is a coarse wavelength division multiplexer.

[0015] The nonlinear fiber is Raman gain fiber, and its Raman gain coefficient is greater than 2.5 (W·km) when the pump wavelength is 1550 nm -1 , the dispersion is -20 ps / (nm·km), and the dispersion slope is 0.02 ps / (nm 2 ·km).

[0016] The pulse width compressor uses a negative dispersion passive optical fiber.

[0017] Compared with the prior art, the technical effects of the present invention are as follows:

[0018] 1) The present invention generates 1645 nm ultrafast pulses through the nonlinear optical gain modulation technology, providing a full-fiber, low-cost, and high-conversion efficiency 1645 nm ultrafast fiber laser solution. Using the chirped pulse amplification technology in the 1550 nm band, through the stimulated Raman scattering effect in the nonlinear optical fiber, the 1550 nm pump energy is transferred to the 1645 nm single-frequency laser and its time domain is shaped into ultrafast pulses. Compared with the 1645 nm ultrafast pulses obtained by the nonlinear frequency conversion technology relied on by current researchers, the optical power, optical coherence, environmental stability, and adjustable freedom degree of the ultrafast pulses generated by the present invention are all significantly improved.

[0019] 2) The conversion efficiency of the generated 1645 nm ultrafast pulses can be as high as over 60%, achieving an average power output of several watts. The cost of the system and the robustness of the full fiber both meet the requirements of practical applications. It fills the gap in the emission spectra of erbium ions and thulium ions, solves the problem of low conversion efficiency of existing nonlinear frequency transfer methods such as self-phase modulation, soliton self-frequency shift, and supercontinuum, and increases the average power of the U-band ultrafast pulse output from the current several milliwatts to several watts, which can be practically applied to fields such as free space optical communication and long-distance atmospheric remote sensing.

[0020] 3) The spectral position of the generated ultrafast pulses can be continuously tuned. By tuning the single-frequency laser, the overall spectrum of the output ultrafast pulses is translated and the spectral shape and intensity are basically maintained. The conversion efficiency of stimulated Raman scattering is greater than 50% within the frequency transfer range of 9 - 15 THz, enabling the high-conversion efficiency tuning range to cover the entire U-band.

[0021] 4) The spectral shape and width of the generated ultrafast pulses can be continuously tuned. By adjusting the reflection bandwidth of the Bragg fiber grating, the pump optical power, and the length of the nonlinear optical fiber, the spectral shape and width of the output can be continuously adjusted, and the output spectrum can be accurately predicted through simulation based on the nonlinear Schrödinger equation.

[0022] 5) From the perspective of energy conversion, the generated ultrafast pulses are obtained by amplifying the narrow linewidth single-frequency laser through stimulated Raman scattering. Therefore, the output ultrafast pulses inherit from the single-frequency laser and enhance coherence during the stimulated Raman scattering amplification process. Traditional optical amplification structures will introduce amplified spontaneous emission and noise to destroy coherence. The generated ultrafast pulses can be applied to fields such as coherent detection and U-band communication.

[0023] 6) The generated ultrafast pulses result from the gain modulation of the pump pulse, enabling the single-frequency laser to expand sidebands through self-phase modulation and develop into a broadband light source. The interval between each longitudinal mode is strictly equal to the repetition frequency of the pump pulse. By adjusting the cavity length of the erbium-doped mode-locked laser that provides the pump pulse, continuous adjustment of the repetition frequency of the output pulse can be achieved.

[0024] 7) The initial pulse width of the ultrafast pulses generated by the present invention inherits from the pump pulse. Therefore, the initial pulse width of the output pulse can be adjusted by adjusting the reflection bandwidth of the fiber Bragg grating. Moreover, the output pulse is linearly chirped and can be compressed to near the Fourier transform limit through a dispersion element or a negative-dispersion passive fiber and output as femtosecond pulses. Description of the Drawings

[0025] Figure 1 It is a schematic structural diagram of a high-power 1645-nm femtosecond fiber laser based on nonlinear gain modulation according to the present invention.

[0026] Figure 2 It is the output spectrum, pulse train sequence, and pulse width of a high-power 1645-nm femtosecond fiber laser based on nonlinear gain modulation according to the present invention under a pump power of 4.3 W.

[0027] Figure 3 It is the output spectrum during the tuning of the single-frequency laser of a high-power 1645-nm femtosecond fiber laser based on nonlinear gain modulation according to the present invention under a pump power of 4.3 W.

[0028] Description of the Reference Numerals:

[0029] 1 - 1550-nm erbium-doped mode-locked fiber laser, 2 - optical circulator, 3 - fiber Bragg grating, 4 - erbium-doped fiber amplifier, 5 - 1645-nm single-frequency laser, 6 - coupler, 7 - nonlinear fiber, 8 - optical splitter, 9 - pulse width compressor, A1 - first output port, A2 - second output port, A3 - third output port. Detailed Embodiment

[0030] The present invention will be further described below in conjunction with an example and the drawings, but the protection scope of the present invention should not be limited thereby.

[0031] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0032] It should be noted that the terms used herein are only for describing the specific embodiments and are not intended to be limiting.

[0033] It should be understood that when a unit is referred to as "connected" or "coupled" to another unit herein, it can be directly connected or coupled to the other unit, or an intermediate unit may exist. In contrast, when a unit is referred to as "directly connected" or "directly coupled" to another unit herein, it means that no intermediate unit exists.

[0034] This embodiment provides the above-mentioned high-power 1645 nm femtosecond fiber laser based on nonlinear gain modulation, and the system structure is the same as Figure 1 the following. It includes: an erbium-doped mode-locked fiber laser 1 at 1550 nm generates ultrafast laser covering 1550 nm, enters a fiber Bragg grating 3 through an optical circulator 2. The fiber Bragg grating 3 reflects a narrowband spectrum, provides pulse broadening under the Fourier transform limit, and is used for chirped pulse amplification. A wide spectral range will cause problems such as difficult amplification and uneven stimulated Raman scattering gain. The remaining spectral components are output from the first output port A1. The reflected narrowband light is then introduced into an erbium-doped fiber amplifier 4 by the optical circulator 2 for amplification; an optical coupler 6 is used to couple the amplified 1550 nm pulsed light and the 1645 nm single-frequency laser generated by a 1645 nm single-frequency laser 5 into a nonlinear fiber 7; a nonlinear optical gain modulation process occurs in the nonlinear fiber 7, shaping the single-frequency laser into an ultrafast pulse in the time domain and transferring the energy to the 1645 nm single-frequency laser; a beam splitter 8 is used to separate the generated 1645 nm ultrafast pulse from the remaining 1550 nm pump light. The remaining 1550 nm pump light is output from the third output port A3, and the generated 1645 nm ultrafast pulse is compressed by a pulse compressor 7 and then output from the second output port A2; the system is a fully polarization-maintaining fiber structure.

[0035] The used erbium-doped mode-locked fiber laser 1 at 1550 nm has a repetition frequency of 102.3 MHz, a central wavelength of 1550 nm, a 10 dB bandwidth of 91 nm, and an output power of 5 mW.

[0036] The used optical circulator 2 is a 1550 nm optical circulator produced by the Opto-Electronics Co., Ltd., and is a three-port fiber optical circulator.

[0037] The used fiber Bragg grating 3 has a reflection bandwidth of 0.33 nm, a central reflection wavelength of 1550 nm, and a reflectivity of 100%.

[0038] The used erbium-doped fiber amplifier 4 amplifies the band at 1550 nm, can amplify the 1550 nm pulsed light with an average power of 5 μW to a maximum average power of 4.3 W, and the amplified 1550 nm output power is continuously adjustable.

[0039] The single-frequency laser 5 used is a 1645 nm single-frequency laser produced by LD-PD Company, with a line width of 200 kHz, an output power of 50 mW at a driving current of 300 mA, and a tuning range of 5 nm achieved through temperature control.

[0040] The non-linear optical fiber 7 used is a Raman gain optical fiber produced by OFS Company. In this example, the length used is 10 meters.

[0041] The optical coupler 6 and optical splitter 8 used are 1550 nm / 1645 nm high-power coarse wavelength division multiplexers produced by Optical Library Company.

[0042] The pulse width compressor 9 used is a negative dispersion passive optical fiber.

[0043] The erbium-doped mode-locked fiber laser 1 generates ultrafast pulses covering 1550 nm and inputs them into the fiber Bragg grating 3 through the optical circulator 2. After being reflected by the fiber Bragg grating 3, the spectral 3 dB width is 0.33 nm, the pulse width is 33 ps, and the average optical power is 5 μW. Then it is introduced into the erbium-doped fiber amplifier 4 by the optical circulator 2. During the amplification process of the erbium-doped fiber amplifier 4, spectral broadening should be avoided, the spectral width should be controlled to be less than 1 nm, and four-wave mixing and Raman scattering should be avoided. The amplified pump pulse needs to maintain a good Gaussian line shape, without pulse splitting, and the pulse train is stable. After being amplified by the erbium-doped fiber amplifier 4, 1550 nm ultrafast pulsed light with an average power of 4.3 W is obtained. Nonlinear gain modulation is generated in the non-linear optical fiber 7, and the 1550 nm pump energy is transferred to the 1645 nm single-frequency laser through stimulated Raman scattering. The 1645 nm single-frequency laser expands into broadband light due to self-phase modulation, and 1645 nm ultrafast pulses are obtained, with a pulse width of 32 ps, which is the same as the pump pulse width. After separating the remaining 1550 nm pump light by the optical splitter 8, 1645 nm ultrafast pulsed light with an average power of 2.3 W, a repetition frequency of 102.3 MHz and strictly equal to the pump pulse repetition frequency is obtained. Through precise alignment of the optical fiber end face microscope, there is still a 15% splicing loss between the Raman gain optical fiber and the tail fiber of the wavelength division multiplexer. Considering the splicing loss, the optical-to-optical conversion efficiency reaches 63%. When the pump power is 4.3 W, the spectrum, pulse train, and pulse width of the output 1645 nm femtosecond pulse are as Figure 2 shown. The single-frequency laser 5 is modulated into broadband light by non-linear gain, and the spectral coherence width is about 17 nm; the intensity of the pulse train of the output 1645 nm ultrafast pulse is stable, and the period matches the repetition frequency; through the pulse width compressor 9, the 1645 nm ultrafast pulse is compressed to 234 fs, approaching the Fourier transform limit (about 220 fs). When the pump power is 4.3 W, the single-frequency laser 5 used is tuned, and the overall output spectrum is translated and maintains a stable spectral shape, as Figure 3As shown, it is confirmed that this method can generate high-energy and highly coherent 1645 nm femtosecond pulsed lasers, and when the single-frequency laser is tuned, the output spectrum maintains continuous translation of shape and intensity. It can fill the gap of being unable to output high-power and highly coherent femtosecond lasers in the U band and can be applied to multiple fields such as biological tissue imaging, coherent detection, lidar, multi-component atmospheric remote sensing, highly sensitive spectral detection, free-space optical communication, and U-band communication.

Claims

1. A high-power 1645nm femtosecond fiber laser based on non-linear gain modulation, characterized in that, It includes a 1550nm erbium-doped mode-locked fiber laser (1), an optical circulator (2), a fiber Bragg grating (3), an erbium-doped fiber amplifier (4), a 1645nm single-frequency laser (5), a coupler (6), a nonlinear optical fiber (7), a splitter (8) and a pulse width compressor (9). The 1550nm erbium-doped mode-locked fiber laser (1) generates femtosecond pulses covering 1550nm as a femtosecond pulse seed source, and is introduced into the fiber Bragg grating (3) through the optical circulator (2). The fiber Bragg grating (3) reflects the narrowband spectrum centered at 1550nm, and the remaining spectral components are output through the first output port; the reflected narrowband spectrum is guided to the erbium-doped fiber amplifier (4) again through the optical circulator (2), and after amplification, it is used as pump light and injected into the nonlinear optical fiber (7) together with the single-frequency laser output by the 1645nm single-frequency laser (5) as signal light through the coupler (6); in the nonlinear optical fiber (7), the 1550nm femtosecond pulses transfer energy and time-domain characteristics to the 1645nm single-frequency laser through the stimulated Raman scattering effect, so that it is modulated into an ultrafast pulsed laser; then the residual 1550nm pump light is separated from the 1645nm ultrafast pulsed laser by the splitter (8), and the 1645nm ultrafast pulsed laser is compressed to the femtosecond level by the pulse width compressor (9) and output through the second output port, and the residual 1550nm pump light is output through the third output port; The repetition frequency of the laser output from the second output port is tuned by adjusting the cavity length of the 1550nm erbium-doped mode-locked fiber laser (1); the central wavelength of the laser output from the second output port is tuned by adjusting the wavelength of the 1645nm single-frequency laser (3) or replacing the single-frequency laser with different wavelength bands; the initial pulse width of the laser output from the second output port is adjusted by adjusting the reflection bandwidth of the fiber Bragg grating; the spectral width of the laser output from the second output port is adjusted by adjusting the length of the nonlinear optical fiber (7) and the pump power output by the erbium-doped fiber amplifier (4).

2. The high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, The repetition frequency of the 1550nm erbium-doped mode-locked fiber laser (1) is 10MHz to 1GHz, and the central wavelength is 1550nm.

3. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that The wavelength tuning range of the 1645nm single-frequency laser (5) is 1620 - 1680nm, covering the entire U band. The output laser linewidth of the 1645nm single-frequency laser (5) is less than 10MHz, and the coherence of the 1645nm femtosecond pulses output from the second output port after pulse width compression inherits from the 1645nm single-frequency laser (5).

4. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, The fiber Bragg grating (3) is used for narrowband filtering, with a central reflection wavelength of 1530 - 1560nm, a reflection bandwidth of 0.1 - 0.5nm, and a reflectivity of 100%.

5. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, The coupler (6) is a coarse wavelength division multiplexer, and the splitter (8) is a coarse wavelength division multiplexer.

6. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, The non-linear optical fiber (7) is a Raman gain optical fiber, and its Raman gain coefficient is greater than 2.5 (W·km) when the pump wavelength is 1550 nm -1 , the dispersion is -20 ps / (nm·km), and the dispersion slope is 0.02 ps / (nm 2 ·km).

7. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, The pulse width compressor (9) uses a negative dispersion passive optical fiber.

8. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, After the 1550 nm femtosecond pulse enters the fiber Bragg grating (3) through the optical circulator (2) for narrowband filtering and returns, the femtosecond pulse is broadened into an ultrafast pulse with a pulse width of 10 - 50 ps, a spectral width of 0.1 - 0.5 nm, and an average power of 1 - 10 μW; after being amplified by the erbium-doped fiber amplifier (4), the average power is increased to 2 - 15 W; Through the stimulated Raman scattering effect, the energy conversion efficiency from the 1550 nm pump light to the 1645 nm signal light reaches more than 60%. After pulse width compression, the average power of the 1645 nm femtosecond pulse output from the second output port is 1 - 10 W, and the spectral width is 2 - 20 nm.

9. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, When adjusting the wavelength of the 1645 nm single-frequency laser (5), the spectrum of the 1645 nm femtosecond pulse output from the second output port undergoes continuous translation while maintaining the spectral shape and intensity.

10. A high-power 1645 nm femtosecond fiber laser based on non-linear gain modulation according to claim 1, characterized in that, The femtosecond fiber laser has an all-fiber structure, and all devices have polarization-maintaining characteristics.

Images