Raman-enhanced adjustable multi-soliton mode-locked fiber laser capable of directly outputting super-continuum spectrum

Abstract

The invention discloses a Raman-enhanced adjustable multi-soliton mode-locked fiber laser capable of directly outputting a super-continuum spectrum. The Raman-enhanced adjustable multi-soliton mode-locked fiber laser comprises a laser diode, a wavelength division multiplexer, a gain fiber, a polarization controller, a polarization dependent isolator, a Raman gain medium and an optical coupler, the laser diode is pumped through the wavelength division multiplexer, and the polarization dependent isolator and a polarization controller form a nonlinear polarization rotation NPR mode locking device. The ytterbium-doped fiber is adopted as a gain medium, meanwhile, the single-mode fiber is introduced into the cavity to serve as a Raman gain medium, the Raman gain effect of the long fiber is utilized, the efficient stimulated Raman scattering process is achieved, the threshold value of Raman laser is reduced, and therefore the wavelength range of laser output is effectively expanded. A non-linear polarization rotation (NPR) mode locking mechanism is formed by adopting a three-rotation polarization controller and a polarization dependent isolator and serves as an artificial saturable absorber (SA) to realize passive mode locking operation of the laser.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber lasers, and particularly to a Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting an ultra-wide continuous spectrum. Background Art

[0002] Ultra-wide continuous spectrum light sources are widely used in spectroscopy, fiber sensing, precision metrology, biomedical imaging, optical coherence tomography, lidar, ultrafast optics and other fields due to their broad and continuous spectral characteristics, high spatial and temporal coherence. Traditional ultra-wide continuous spectrum light sources usually adopt the master oscillator power amplification structure of "seed source + amplifier + nonlinear fiber", and realize the output of the ultra-wide continuous spectrum by amplifying the seed laser source through one or more stages and then coupling it into the nonlinear fiber. However, this solution requires multiple amplification stages, resulting in a complex system, large volume, high cost, and poor pulse stability.

[0003] In recent years, researchers have proposed the concept of intracavity ultra-wide continuous spectrum generation, that is, directly realizing the output of the ultra-wide continuous spectrum through a nonlinear mechanism in the laser cavity. This solution has the advantages of simple structure, small volume, low cost, etc., and has higher spectral conversion efficiency and more stable pulse output. Currently, the common solutions for realizing ultra-wide continuous spectrum output include the following two categories: extracavity ultra-wide continuous spectrum generation and intracavity ultra-wide continuous spectrum generation. Although intracavity ultra-wide continuous spectrum generation has certain advantages, the existing technologies still have problems such as insufficient spectral width, poor spectral flatness, limited output power, and poor pulse stability. Specifically:

[0004] 1. High system complexity: The traditional "seed source + amplifier + nonlinear fiber" solution requires multiple stages of amplification, with a complex system and large structure, which limits its integrated and miniaturized applications.

[0005] 2. High setup cost: High-cost components such as multi-stage amplifiers and photonic crystal fibers result in high overall equipment cost, making it difficult to promote in low-cost application scenarios.

[0006] 3. Limited pulse characteristics: The time-domain characteristics of existing ultra-wide continuous spectrum output devices are poor, and there are often problems such as complex pulse shapes and unequal pulse intervals.

[0007] 4. Insufficient spectral flatness: Existing intracavity ultra-wide continuous spectrum generation devices often cannot effectively control the balance between nonlinear effects, resulting in the spectral flatness and coverage range being difficult to meet expectations.

[0008] Therefore, a Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting an ultra-wide continuous spectrum is proposed. Summary of the Invention

[0009] To solve the problems existing in the prior art, the present invention provides a Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting an ultra-continuous spectrum, so as to solve the technical problems proposed in the above-mentioned background art section.

[0010] To achieve the above object, the present invention provides the following technical solutions:

[0011] A Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting an ultra-continuous spectrum proposed by the present invention includes a laser diode, a wavelength division multiplexer, a gain fiber, a polarization controller, a polarization-dependent isolator, a Raman gain medium, and an optical coupler;

[0012] The laser diode is pumped through the wavelength division multiplexer, and the polarization-dependent isolator and a polarization controller form a non-linear polarization rotation NPR mode-locking device.

[0013] Preferably, the gain fiber is one of ytterbium-doped fiber YDF, erbium-doped fiber EDF, and praseodymium-doped fiber PDF.

[0014] Preferably, the non-linear polarization rotation NPR is replaced by one of a non-linear optical loop mirror NOLM or a multimode interference MMI.

[0015] Preferably, the Raman gain medium is one of a single-mode fiber, a highly non-linear fiber, and a photonic crystal fiber.

[0016] More preferably, the Raman gain medium is a single-mode fiber.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] The technical solution of the present invention can effectively solve the following technical problems:

[0019] 1. Simplify the system structure: By directly realizing the output of the ultra-continuous spectrum in the laser cavity, the use of multi-stage amplifiers is avoided, and the system complexity is greatly reduced.

[0020] 2. Reduce costs: Cancel the expensive external amplification stage and photonic crystal fiber to achieve low-cost ultra-wideband spectral output.

[0021] 3. Improve the spectral width and flatness: The output spectral range covers 1008 nm to 1087 nm, with better spectral flatness and higher power spectral density.

[0022] 4. Tunable multi-soliton pulse output: By simply rotating the PC vane, the present device can obtain multi-soliton pulse trains with different numbers and different intervals, greatly improving the tunability of the output pulses. Moreover, multi-soliton pulse trains in different states can be maintained stable for a long time during the experiment. Description of the Drawings

[0023] Figure 1 Schematic diagram of the structure of the Raman-enhanced ytterbium-doped fiber laser in the present invention;

[0024] Figure 2 Output spectrum diagram of the Raman-enhanced ytterbium-doped fiber laser in the present invention;

[0025] Figure 3 Output pulse diagram of the Raman-enhanced ytterbium-doped fiber laser in the present invention;

[0026] Figure 4 Slope efficiency diagram of the Raman-enhanced ytterbium-doped fiber laser in the present invention.

[0027] In the figure: 1. Laser diode; 2. Wavelength division multiplexer; 3. Gain fiber; 4. Polarization controller; 5. Polarization-dependent isolator; 6. Raman gain medium; 7. Optical coupler. Specific embodiments

[0028] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0029] As Figure 1 shown, the present invention provides a Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting an ultra-wide continuous spectrum, including a laser diode 1, a wavelength division multiplexer 2, a gain fiber 3, a polarization controller 4, a polarization-dependent isolator 5, a Raman gain medium 6 and an optical coupler 7. An ytterbium-doped fiber is used as the gain medium, and at the same time, a single-mode fiber with a length of about 235 m is introduced into the cavity as the Raman gain medium 6. By utilizing the Raman gain effect of the long fiber, an efficient stimulated Raman scattering process is realized, the threshold of the Raman laser is reduced, and thus the wavelength range of the laser output is effectively expanded. A non-linear polarization rotation (NPR) mode-locking mechanism is formed by using a three-rotation polarization controller 4 and a polarization-dependent isolator 5 to realize the passive mode-locking operation of the laser as an artificial saturable absorber (SA). By adjusting the PC rotating plate, the polarization state in the cavity of the mode-locked ytterbium-doped fiber laser is optimized, and an ultra-wide continuous spectrum output with an octave-wide spectrum can be realized without external amplification and additional non-linear fiber media. This design significantly simplifies the system structure of the ultra-wide continuous spectrum output device, reduces costs, and can improve the width, flatness of the output spectrum and the performance of the output pulse.

[0030] Pulse stabilization mechanism: By precisely adjusting the polarization controller, the polarization state in the cavity is optimized, enabling the stable operation of mode-locked pulses in the ytterbium-doped fiber laser ring cavity, ensuring the pulse width and stability of the mode-locked pulses, and thus guaranteeing the high peak power output of the laser. Spectral broadening and flattening technology: The mechanism of SC generation in normal dispersion is well-known. First, Raman peaks appear cascadingly, and then the space between each Stokes wave is filled by self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), etc., until a flat broad-spectrum output is achieved. This device can indirectly balance various nonlinear effects in the laser resonator by adjusting the polarization state in the cavity through the polarization controller, improving the flatness and power spectral density of the spectrum, and making the power at different wavelengths of the output spectrum more uniform.

[0031] Specifically: The gain fiber 3 uses 30 cm of ytterbium-doped fiber, which is pumped by a laser diode 1 with a maximum pump power of 600 mW through a wavelength division multiplexer 2. An important NPR mode-locking device is composed of a polarization-dependent isolator 5 and a polarization controller 4, which simultaneously ensures the unidirectional transmission of optical pulses in the cavity and realizes the mode-locking operation. The polarization controller 4 adjusts the polarization state of the light in the laser cavity by arbitrarily rotating the polarizer to obtain the best mode-locking working state. A single-mode fiber (SMF) about 235 m long is used as the Raman gain medium 6 to ensure sufficient Raman gain can be generated in the laser cavity. The total cavity length of the laser is about 244 m. In the experimental system, except for the active fiber, the rest of the fiber and the pigtail of the device are all HI 1060 fiber. An 80:20 optical coupler (OC) is used to monitor the laser performance at the 20% output port. Then, the output laser is split by a beam splitter with a 30 / 70 coupling ratio, and the spectrum and pulse information are observed by a spectral analyzer and a digital oscilloscope respectively. The working stability of the cavity is studied using a spectral analyzer. The average output power is measured using an optical power meter.

[0032] As the pump power increases from 288 mW to 580 mW, the output spectrum of the laser is as Figure 2 (a) shown. The spectrum can generate and maintain stable first-order Stokes light during the power increase. Subsequently, when the pump power is 500 mW, the PC is appropriately adjusted to obtain a broad-spectrum output with a 20 dB spectral bandwidth greater than 66 nm, and the spectral range covers 1008 nm to 1087 nm, as Figure 2 (b) shown, successfully achieving the design goal of directly outputting supercontinuum spectrum in the laser cavity. In the later stage, the pump power can be continuously increased and the PC can be precisely adjusted to obtain higher-order Stokes light, broadening the spectrum while optimizing the spectral flatness.

[0033] In addition to the conventional single-pulse output, during the experiment, by adjusting the PC to change the polarization state of the laser, 2 - 5 split pulse trains can be observed at a constant pump power of 500 mW, as Figure 3 shown. In addition, when the laser outputs a pulse train, the PC rotating vane can be finely adjusted to obtain different pulse interval distributions, realizing the tunable multi-soliton output with both the number of pulses and the pulse interval adjustable. This device can obtain pulse train outputs with different numbers and different pulse intervals only by rotating the PC rotating vane, greatly improving the tunability of the output pulses and meeting various different actual engineering needs.

[0034] By increasing the pump power, different operating states of the fiber laser are as shown in Fig. 4. The output power increases linearly with the pump power, and the slope efficiency is 1.36%. When the pump power increases to 580 mW, the maximum output power in the single-pulse mode-locked state is 6.5 mW.

[0035] In the above, the ytterbium-doped fiber (YDF) used as the gain medium can be replaced by erbium-doped fiber (EDF) or praseodymium-doped fiber (PDF) to meet the requirements of supercontinuum output in different wavelength bands;

[0036] Selection of Raman gain medium: The 235 m single-mode fiber can be replaced by high-nonlinearity fibers or photonic crystal fibers of other lengths, but this will increase the system cost;

[0037] Mode-locking mechanism: Nonlinear polarization rotation (NPR) can be replaced by other mode-locking mechanisms such as nonlinear optical loop mirror (NOLM) or multimode interference (MMI). However, this replacement will increase the number of devices used in the cavity, increase the number of fusion splices during the laser setup process, and make the structure more complex.

[0038] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be encompassed within the present invention. Any reference signs in the claims should not be regarded as limiting the claims involved.

[0039] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting a supercontinuum, characterized in that: It comprises a laser diode (1), a wavelength division multiplexer (2), a gain optical fiber (3), a polarization controller (4), a polarization-dependent isolator (5), a Raman gain medium (6) and an optical coupler (7); The laser diode (1) is pumped by a wavelength division multiplexer (2), and the polarization-dependent isolator (5) and a polarization controller (4) constitute a nonlinear polarization rotation NPR mode-locking device.

2. The Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting supercontinuum according to claim 1, characterized in that: The gain optical fiber (3) is one of ytterbium-doped optical fiber YDF, erbium-doped optical fiber EDF and praseodymium-doped optical fiber PDF.

3. The Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting supercontinuum according to claim 1, characterized in that: The nonlinear polarization rotator NPR is replaced by one of a nonlinear loop mirror NOLM or a multi-mode interference MMI.

4. The Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting supercontinuum according to claim 1, characterized in that: The Raman gain medium (6) is one of a single-mode optical fiber, a highly nonlinear optical fiber, and a photonic crystal optical fiber.

5. The Raman-enhanced tunable multi-soliton mode-locked fiber laser capable of directly outputting supercontinuum according to claim 4, characterized in that: The Raman gain medium (6) is a single-mode optical fiber.

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