Repetition-frequency-adjustable ultrafast fiber laser rare earth Raman hybrid amplification system

Abstract

The invention discloses a repetition-frequency-adjustable ultrafast fiber laser rare earth Raman hybrid amplification system which is composed of a repetition-frequency continuously-adjustable picosecond pulse laser, a single-frequency continuous laser, a pumping source, a laser beam combining device, a rare earth doped fiber, a collimator, an optical filter and a space grating pair. The laser beam combining device couples repetition-frequency continuously adjustable picosecond laser, a pumping source for amplifying the picosecond laser and single-frequency continuous laser into the rare earth doped optical fiber. Picosecond laser is firstly subjected to gain amplification by rare earth, and then nonlinear optical gain is provided for single-frequency continuous laser through a Raman effect, so that Raman pulse of which the central wavelength corresponds to the wavelength of the single-frequency continuous laser is generated, and the width of the Raman pulse is compressed by utilizing a space grating pair. The repetition frequency of the Raman pulse can be controlled by changing the repetition frequency of the picosecond laser. The system has the advantages of being simple in design, good in adjustability and the like, and can meet the application scene needing repetition frequency continuously adjustable and special wavelength ultrafast laser.

Description

Technical Field

[0001] The invention relates to the technical field of ultrafast optical fiber lasers, in particular to an ultrafast optical fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate. Background Art

[0002] Ultrashort pulse fiber lasers have attracted widespread attention due to their compact structure, good beam quality, simple operation, and high cost-effectiveness. They have broad application prospects in material processing, biomedicine, basic research, and optical communications. Due to the fixed ion energy level interval and limited energy level width of rare-earth-doped optical fibers, lasers can only be generated within a discrete spectral range, which limits the application of ultrafast fiber lasers in fields such as bioimaging, basic scientific research, and optical metrology, which require pulses of specific wavelengths (see JQ Zhou, WWPan, WAQi, et al., "Ultrafast Raman fiber laser: a review and prospect," PhotoniX 3, 1-23 (2022)). Stimulated Raman scattering is an effective way to generate lasers in a wavelength range that rare-earth-doped fiber lasers cannot cover. It does not depend on the energy level of the ions and has a wide gain spectrum in the optical fiber. In theory, in the transmission medium, Raman lasers of any target wavelength can be obtained by a suitable pump wavelength.

[0003] At present, there are three main categories of ultrafast Raman fiber lasers, namely mode-locked, synchronously pumped, and nonlinear optical gain modulation. Mode-locked or synchronously pumped Raman fiber lasers require a resonant cavity structure and cannot continuously and widely change the repetition frequency of Raman pulses. Nonlinear optical gain modulation technology uses a single-frequency continuous laser as a seed source and avoids the resonant cavity configuration and simplifies the structure of the system through ultrafast laser pulse pumping (see WAQi, JQZhou, SZCui, et al., "Femtosecond pulse generation by nonlinear optical gain modulation," Adv.Photon.Res.3, 2100255 (2021)). Moreover, femtosecond lasers in special bands can be easily obtained through nonlinear optical gain modulation technology. In this technology, the repetition rate of the newly generated Raman pulse depends only on the repetition rate of the ultrafast pump pulse.

[0004] At present, the ultrafast pump source used in Raman lasers based on nonlinear optical gain modulation technology is mainly a mode-locked laser, and the repetition rate tuning range is limited, which greatly limits the application scenarios of this technology. Gain switching diodes are an effective technical means to achieve laser pulses with a large range of continuously adjustable repetition rates. Compared with traditional repetition rate adjustable light sources based on acousto-optic or electro-optic modulation, they can achieve continuous adjustment of repetition rates and have important application prospects in many applications that require frequency sweeping. However, the pulses generated by gain switching diodes currently have problems such as strong noise, large pulse width, and fixed wavelength (see W. Fu, L G Wright, F W Wise, "High-power femtosecond pulses without a model-locked laser," Optica 4 (7), 831-834 (2017)). Therefore, applying gain switching diodes to nonlinear optical gain modulation technology can make the two technologies complementary and produce ultrafast laser light sources with flexible and adjustable parameters such as repetition rate, pulse width, and wavelength.

[0005] Traditional Raman amplifiers all use passive optical fibers to provide Raman gain (see Q. Zhao, WWPan, X. Zeng, et al., "Partially coherent noise-like pulse generation in amplified spontaneous Raman emission," Appl. Opt. 57(9), 2282-2286(2018)), which increases the problem of splicing and matching between active and passive optical fibers, increases losses, and increases system complexity, posing risks to the long-term stability of the system.

[0006] Compared with the traditional Raman amplifier, the rare earth Raman amplifier in the present invention has the following characteristics: (1) The energy amplification and Raman conversion of the picosecond laser with continuously adjustable repetition rate are completed simultaneously in the rare earth doped optical fiber, avoiding the fusion matching problem of the rare earth doped optical fiber and the passive optical fiber under high power; (2) The coupling of the picosecond laser with continuously adjustable repetition rate and the single-frequency continuous laser requires the use of a wavelength division multiplexer or an optical coupler, which may become a bottleneck for high-power ultrafast Raman fiber lasers. The single-frequency laser and the picosecond laser with continuously adjustable repetition rate in the present invention are optically coupled at a lower power, avoiding the use of this device. Summary of the invention

[0007] In order to solve the problem of fusion splicing between rare-earth-doped optical fiber and passive optical fiber under high power, avoid using coupling devices under high power, and obtain Raman pulses with continuously adjustable repetition frequency, the present invention provides an ultrafast fiber laser rare-earth Raman hybrid amplification system with adjustable repetition frequency, the purpose of which is to simply and stably obtain an ultrafast Raman pulse light source with flexible repetition frequency and wavelength.

[0008] The solution of the present invention is as follows:

[0009] The invention discloses an ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate, which is composed of a picosecond pulse laser, a single-frequency continuous laser, a continuous light pump source, a laser beam combining device, a rare earth doped optical fiber, a collimator, a filter and a spatial grating pair. The picosecond laser with adjustable repetition rate generated by the picosecond pulse laser, the single-frequency continuous laser generated by the single-frequency continuous laser and the pump laser generated by the continuous light pump source are coupled by the laser beam combining device and injected into the rare earth doped optical fiber to simultaneously complete the rare earth amplification and Raman conversion process of the picosecond laser. The generated Raman pulse laser is collimated by the collimator, and then the lasers of other bands are separated or filtered out by the filter to retain only the Raman pulse laser, and then the Raman pulse width is compressed by the spatial grating pair, and finally the ultrafast Raman pulse with adjustable repetition rate is output. Except for the collimator, the filter and the spatial grating pair, the system is an all-fiber structure.

[0010] The picosecond laser with continuously adjustable repetition rate generated by the picosecond pulse laser is amplified by the rare earth gain in the rare earth doped fiber, and then used as a pulse pump source in the nonlinear optical gain modulation process to amplify the single-frequency continuous laser generated by the single-frequency continuous laser within the nonlinear optical gain window time (corresponding to the pulse width of the picosecond laser). In this process, the single-frequency continuous laser is transformed into a Raman pulse laser with the same repetition frequency as the picosecond laser. In the entire section of rare earth doped fiber, the continuous light pump source provides energy for the picosecond pulse laser. In the process of nonlinear optical gain modulation, while the picosecond pulse laser transfers energy to the Raman laser through the stimulated Raman scattering effect, the continuous light pump source also continuously transfers energy to the picosecond pulse laser.

[0011] Methods for improving Raman conversion efficiency include: increasing the average power of single-frequency continuous laser, optimizing the average power of continuous light pump source, and optimizing the length of rare-earth doped optical fiber.

[0012] Furthermore, the picosecond pulse laser is a combination of a gain switching diode and a rare earth-doped fiber amplifier, which can generate picosecond laser with continuously adjustable repetition frequency, with a repetition frequency range of 10kHz to 1GHz and a pulse width range of 1ps to 500ps.

[0013] Furthermore, the line width of the single-frequency continuous laser generated by the single-frequency continuous laser is less than 10 MHz, and the output wavelength λ1 and the output center wavelength λ2 of the picosecond laser with continuously adjustable repetition rate generated by the picosecond pulse laser device should satisfy:

[0014]

[0015] Where Δν is the frequency shift corresponding to the peak value of the nonlinear gain coefficient, and c is the speed of light in a vacuum.

[0016] Furthermore, the laser beam combining device is a single device or a combined device of an optical coupler, a wavelength division multiplexer and a beam combiner.

[0017] Furthermore, the rare earth doped optical fiber doping ions include Er 3+ , Yb 3+ 、Tm 3+ , Gd 3+ 、Ho 3+ 、Dy 3+ , Tb 3+ He Lu 3+ A combination of one or more of .

[0018] Furthermore, the filter is a dichroic mirror or an edge pass filter.

[0019] Furthermore, the collimator is used to collimate the laser output from the rare-earth-doped optical fiber, and no passive optical fiber is included between the collimator and the rare-earth-doped optical fiber.

[0020] Compared with the prior art, the technical effects of the present invention are:

[0021] 1) Use a picosecond pulse laser with continuously adjustable repetition frequency as the gain driver of the ultrashort pulse laser to amplify the single-frequency continuous optical signal within the gain window time range. The newly generated Raman pulse has the same repetition frequency as the picosecond pump source, so the repetition frequency of the Raman pulse can be changed by simply manipulating the repetition frequency of the picosecond pump source.

[0022] 2) The repetition rate of the generated Raman pulses is continuously adjustable and has a wide adjustment range. The picosecond pulses generated by the gain switching diode can achieve continuous adjustment of the repetition rate by changing its trigger signal, so the generated Raman pulses also have the property of continuously adjustable repetition rate.

[0023] 3) Compared with traditional Raman pulse fiber lasers, this system has the following advantages: the rare earth amplification and Raman conversion processes of the picosecond pulse pump source of this system are completed in the rare earth-doped optical fiber, which avoids the use of coupling devices at high power on the one hand, and avoids the fusion problem of rare earth-doped optical fiber and passive optical fiber on the other hand, thereby improving the long-term stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The invention is a schematic diagram of the framework of an ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate.

[0025] Figure 2 The present invention discloses an output spectrum diagram after a collimator of an ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate when the repetition rates are 10 MHz, 20 MHz and 30 MHz.

[0026] Figure 3 The present invention discloses an autocorrelation curve of a Raman pulse of an ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate when the repetition rate is 20 MHz.

[0027] Figure 4 The present invention discloses an autocorrelation curve of a Raman pulse after passing through a spatial grating pair when the repetition rate of an ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate is 20 MHz.

[0028] Figure 5 The invention discloses a time domain sequence of Raman pulses when the repetition rate of an ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate is 20 MHz.

[0029] Figure 6 The invention discloses a radio frequency spectrum of Raman pulses of an ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate when the repetition rate is 10 MHz. DETAILED DESCRIPTION

[0030] The present invention is further described below with reference to an example and accompanying drawings, but this should not limit the protection scope of the present invention.

[0031] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0032] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0033] It should be understood that when a unit is referred to as being "connected" or "coupled" to another unit in this article, it can be directly connected or coupled to another unit, or an intermediate unit can exist. In contrast, when a unit is referred to as being "directly connected" or "directly coupled" to another unit in this article, it means that there is no intermediate unit.

[0034] This example provides a repetition rate adjustable ultrafast fiber laser rare earth Raman hybrid amplification system, the system structure is similar to Figure 1The invention is similar to the invention, and comprises: a picosecond pulse laser 1 composed of a 1065nm band gain switching diode and a rare earth-doped fiber preamplifier, used to generate a picosecond laser with a continuously adjustable repetition rate; a single-frequency continuous laser 2, used to provide a seed source for a nonlinear optical gain modulation process; a continuous optical pump source 3 for generating a 976nm pump laser; a laser beam combining device 4 composed of a wavelength division multiplexer and a beam combiner, wherein the wavelength division multiplexer is used to couple the picosecond laser with a continuously adjustable repetition rate and the single-frequency continuous laser, and the beam combiner is used to couple the above-mentioned coupled laser and the 976nm pump laser; The rare earth-doped fiber 5 is a double-clad ytterbium-doped fiber with a length of 2.4 m. The further amplification and Raman conversion process of the picosecond laser are simultaneously completed in the rare earth-doped fiber 5. The collimator 6 directly connected to the double-clad ytterbium-doped fiber is used to collimate the output laser of the double-clad ytterbium-doped fiber. The collimated laser passes through the filter 7, which is a long-wave pass filter with a cutoff wavelength of 1100 nm. The unconverted 1065 nm picosecond pulse laser and the remaining 976 nm pump laser are filtered out. The converted Raman pulse is compressed by adjusting the distance between the spatial grating pair 8. Except for the collimator 6, the filter 7 and the spatial grating pair 8, the system is a full polarization-maintaining fiber structure.

[0035] The picosecond pulse laser 1 is a combination of a gain switching diode and an ytterbium-doped fiber amplifier. The gain switching diode can output a picosecond laser with continuously adjustable repetition rate, and the picosecond laser is pre-amplified by the ytterbium-doped fiber amplifier. In this example, the adjustment range is 10MHz to 30MHz, the output pulse width is 19.0ps, and the central wavelength is 1064.97nm.

[0036] The single-frequency continuous laser 2 is a semiconductor laser with a central wavelength of 1120.47 nm and an average power of 15 mW.

[0037] The continuous light pump source 3 is a multi-mode semiconductor laser, which generates a 976nm pump laser with a maximum output power of 27W.

[0038] The laser beam combining device 4 is a combination of a fully polarization-maintaining 1064 / 1120nm wavelength division multiplexer and a beam combiner. The 1064 / 1120nm wavelength division multiplexer is used to couple the picosecond laser with continuously adjustable repetition rate and the single-frequency continuous laser. The beam combiner is used to further couple the above-mentioned coupled laser and the 976nm pump laser.

[0039] The rare earth-doped optical fiber 5 is a double-clad ytterbium-doped optical fiber with a total length of 2.4 m.

[0040] The collimator 6 is used to collimate the laser light outputted from the double-clad ytterbium-doped optical fiber, and no passive optical fiber is included between the ytterbium-doped optical fiber and the collimator.

[0041] The filter 7 is a long-wave pass filter with a cut-off wavelength of 1100 nm, which is used to filter out the unconverted 1065 nm picosecond pulse laser and the remaining 976 nm pump laser.

[0042] The repetition frequencies were set to 10, 20, and 30 MHz, respectively, and the total power of the 976 nm pump source 3 was adjusted to 21.1 W. The single pulse energies directly output from the collimator 6 were 1.41 μJ, 0.72 μJ, and 0.49 μJ, respectively. The corresponding output spectra are shown in Figure 2 As shown in the figure, the Raman conversion efficiencies are 70.0%, 66.4% and 53.0% respectively, which proves that the system can still maintain a high Raman conversion efficiency when the repetition rate changes greatly. Figure 3 This is the autocorrelation curve of the Raman pulse output when the repetition rate is 20MHz. The measured pulse width is 14.3ps. Figure 4 This is the autocorrelation curve of the Raman pulse after passing through the spatial grating pair 8. The pulse width can be compressed to 576fs. Figure 5 It is a pulse sequence with a pulse interval of 50ns, corresponding to a set repetition rate of 20MHz. Figure 6 For the pulsed radio spectrum, the signal-to-noise ratio can reach 68.9dB, which proves the stability of the invention.

Claims

1. A repetition-rate adjustable ultrafast fiber laser rare earth Raman hybrid amplification system, comprising a picosecond pulse laser (1), a single-frequency continuous laser (2), a continuous optical pump source (3), a laser beam combining device (4), a rare earth doped optical fiber (5), a collimator (6), an optical filter (7) and a spatial grating pair (8), wherein the picosecond laser with a repetition rate continuously adjustable generated by the picosecond pulse laser (1), the single-frequency continuous laser generated by the single-frequency continuous laser (2) and the pump source (3) After being coupled by a laser beam combining device (4), the Raman laser is injected into a rare-earth doped optical fiber (5) to simultaneously complete the rare-earth amplification and Raman conversion process of the picosecond laser. The generated Raman pulse laser is collimated by a collimator (6), retained by a filter (7), and then compressed by a spatial grating pair (8) to output an ultrafast Raman pulse with a continuously adjustable repetition frequency. Except for the collimator (6), the filter (7) and the spatial grating pair (8), the system is an all-fiber structure.

2. The ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate according to claim 1, characterized in that: The laser beam combining device (4) is a single device or a combined device of an optical coupler, a wavelength division multiplexer and a beam combiner.

3. The ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate according to claim 1, characterized in that: The optical filter (7) is a dichroic mirror or an edge-pass filter.

4. The ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate according to claim 1, characterized in that: The picosecond pulse laser (1) is a combination of a gain switching diode and a rare earth doped optical fiber amplifier. The laser can generate picosecond laser with continuously adjustable repetition frequency, the repetition frequency range is 10kHz to 1GHz, and the pulse width range is 1ps to 500ps.

5. The ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate according to claim 1, characterized in that: The rare earth doped optical fiber (5) is doped with ions including Er 3+ , Yb 3+ 、Tm 3+ , Gd 3+ 、Ho 3+ 、Dy 3+ , Tb 3+ He Lu 3+ A combination of one or more of .

6. The ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate according to claim 1, characterized in that: The single-frequency continuous laser line width generated by the single-frequency continuous laser (2) is less than 10 MHz, and the output wavelength λ1 and the repetition rate continuously adjustable picosecond laser output center wavelength λ2 generated by the picosecond pulse laser device (1) should satisfy: Where Δν is the frequency shift corresponding to the peak value of the nonlinear gain coefficient, and c is the speed of light in a vacuum.

7. The ultrafast fiber laser rare earth Raman hybrid amplification system with adjustable repetition rate according to claim 1, characterized in that: The collimator (6) is used to collimate the laser light output from the rare-earth-doped optical fiber (5), and no passive optical fiber is included between the collimator (6) and the rare-earth-doped optical fiber (5).

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