CN115632299A

CN115632299A - High-energy mode-locked fiber pulse laser

Info

Publication number: CN115632299A
Application number: CN202211301838.5A
Authority: CN
Inventors: 刘军; 曾琼; 王振洪; 张春香
Original assignee: Shenzhen University; Shenzhen Technology University
Current assignee: Shenzhen University; Shenzhen Technology University
Priority date: 2022-10-24
Filing date: 2022-10-24
Publication date: 2023-01-20

Abstract

The invention discloses a high-energy mode-locked fiber pulse laser, which comprises a pumping source, a wavelength division multiplexer for forming an optical loop, a gain fiber for realizing population inversion and generating stimulated radiation amplification of light, a fiber coupler for outputting the laser, a fiber polarization controller for changing the polarization state in a resonant cavity, a polarization-independent isolator for ensuring unidirectional transmission of the laser and a graded-index multimode fiber for forming a saturable absorber structure, wherein the graded-index multimode fiber and the single-mode fibers on two sides form the saturable absorber structure. The high-energy mode-locking optical fiber pulse laser adopting the structure has the advantages of low cost, simple manufacturing process, stability and reliability, and can generate stable high-energy pulses and realize the output of ultrashort pulses.

Description

High-energy mode-locked fiber pulse laser

Technical Field

The invention relates to the technical field of lasers, in particular to a high-energy mode-locked fiber pulse laser.

Background

Mode-locked laser pulses are favored by researchers in the fields of materials science, biomedicine, photonics, etc., because of their superior characteristics of ultra-short duration, high peak power, wide spectral bandwidth, and high beam quality. As a core device in a mode-locked laser, a mode-locked modulator can be divided into a loss modulator based on an electro-optic effect or an acousto-optic effect and a saturable absorber based on self-amplitude modulation according to different laser systems. Because the fundamental frequency and the pulse width of the active mode-locked laser are limited by the modulation capability of the additional electro-optical modulator, the generation of ultrashort pulses is researched by using a passive mode-locking technology.

For implementation of a passively mode-locked fiber laser, a key component, the Saturable Absorber (SA), is necessary, which converts continuous light into pulsed light and shapes the pulse. To date, mainly two types of saturable absorbers have been reported: artificial SA and material-like SA. The artificial saturable absorber is characterized in that the optical fiber laser adopts a special structure, and mode locking can be realized by introducing a similar saturable absorption effect; the material saturable absorber realizes a saturable absorption effect by utilizing the interaction of a photoelectric material with excellent optical properties and laser in a laser cavity so as to realize passive mode locking. Such as nonlinear polarization rotation, nonlinear magnifying toroidal mirrors, and nonlinear optical toroidal mirrors, all belong to artificial saturable absorbers; the common application of the material saturable absorber is that other two-dimensional materials such as a semiconductor saturable absorber mirror, graphene, a topological insulator, black phosphorus and the like are used in the optical fiber laser. Researchers in the field of ultrafast photonics have never stopped looking for better performing SAs to improve the performance of ultrafast lasers, including better stability, higher repetition rate, and higher pulse energy.

The passive mode locking is realized by using a multimode fiber structure based on nonlinear multimode interference, the fiber can work in a higher peak power state and has a quicker response time, and some problems of a traditional saturable absorber in a mode locking fiber laser, such as low damage threshold, instability, easy oxidation and the like, can be solved. At present, two methods are used for generating mode locking pulses in a multimode fiber resonant cavity structure: one is to adopt a cavity structure based on nonlinear multimode fiber, needs longer cavity length to increase nonlinear phase shift, and has higher mode locking threshold; the other mode is that a cavity structure based on mixed multimode fibers is adopted, multimode fibers with different fiber core diameters are welded together, so that the mode locking threshold is reduced, and the type of the output mode locking pulse is single.

Disclosure of Invention

The invention aims to provide a high-energy mode-locked fiber pulse laser which is low in cost, simple in manufacturing process, stable and reliable, and capable of generating stable high-energy pulses and outputting ultrashort pulses.

In order to achieve the above object, the present invention provides a high-energy mode-locked fiber pulse laser, which includes a pump source, a wavelength division multiplexer for forming an optical loop, a gain fiber for realizing population inversion and generating stimulated radiation amplification of light, a fiber coupler for laser output, a fiber polarization controller for changing polarization state in a resonant cavity, a polarization-independent isolator for ensuring unidirectional laser transmission, and a graded-index multimode fiber for forming a saturable absorber structure, the pump source, the wavelength division multiplexer, the gain fiber, the fiber coupler, the fiber polarization controller, the polarization-independent isolator, and the graded-index multimode fiber are connected in sequence by single-mode fibers and form an annular cavity structure, and the graded-index multimode fiber and the single-mode fibers on both sides form the saturable absorber structure.

Preferably, the single-mode fiber is a standard g.652 type fiber, the single-mode fiber, the pump source, the wavelength division multiplexer, the gain fiber, the fiber coupler, the fiber polarization controller, the polarization-independent isolator, and the graded-index multimode fiber are respectively fusion-spliced by using a fiber fusion splicer, and a fusion loss is lower than 0.1dB.

Preferably, the pumping source is a semiconductor laser, the output wavelength band of the pumping source is 980nm, and the output power range of the pumping source is 0-700mW.

Preferably, the wavelength division multiplexer is a 980nm/1550nm type optical fiber device, and the insertion loss is less than 0.2dB.

Preferably, the gain fiber is a single-mode erbium-doped fiber, and the output laser of the gain fiber is 1550nm waveband.

Preferably, the fiber coupling ratio of the fiber coupler is 10.

Preferably, the optical fiber polarization controller adopts a three-paddle optical fiber polarization controller or a squeezing type optical fiber polarization controller.

Preferably, the core size of the GI multimode fiber is 50 μm or 62.5 μm.

Therefore, compared with other mode locking devices, the multimode fiber saturable absorber based on the graded index has the advantages of high power transmission bearing capacity, high damage threshold, no performance degradation along with time, low cost, simple structure and the like, and provides wider application prospect for realizing a new generation of mode locking fiber laser with a high-power and high-stability all-fiber structure.

The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.

Drawings

FIG. 1 is a schematic structural diagram of an embodiment of a high-energy mode-locked fiber pulse laser according to the present invention.

Fig. 2 is a schematic diagram of the principle of the SMF-GIMF-SMF structure as a saturable absorber according to an embodiment of the present invention.

FIG. 3 is a non-linear saturable absorption curve of a GI multimode fiber material according to an embodiment of the invention.

FIG. 4 is a diagram of the output characteristics of a high-energy mode-locked pulse laser with a time-based frequency-locked pulse at a pump power of 55.5 mW; FIG. 4 (a) is a pulse spectrogram; FIG. 4 (b) is a timing diagram; FIG. 4 (c) is a radio frequency diagram; fig. 4 (d) is an autocorrelation diagram.

FIG. 5 is a graph showing the output characteristics of a high-energy mode-locked pulse laser with a time-frequency mode-locked pulse at a pump power of 215 mW; FIG. 5 (a) is a pulse spectrogram; FIG. 5 (b) is a timing diagram; FIG. 5 (c) is a radio frequency diagram; fig. 5 (d) is an autocorrelation diagram.

FIG. 6 shows the output power and pulse energy of a high-energy mode-locked pulsed laser according to an embodiment of the present invention at different pump powers.

Reference numerals

1. A pump source; 2. a wavelength division multiplexer; 3. a gain fiber; 4. a fiber coupler; 5. an optical fiber polarization controller; 6. a polarization independent isolator; 7. a graded-index multimode optical fiber; 8. a single mode optical fiber.

Detailed Description

The technical solution of the present invention is further illustrated by the accompanying drawings and examples.

Examples

As shown in fig. 1, a high-energy mode-locked fiber pulse laser includes a pump source 1, a wavelength division multiplexer 2, a gain fiber 3, a fiber coupler 4, a fiber polarization controller 5, a polarization-independent isolator 6, and a graded-index multimode fiber 7, which are connected in sequence via a single-mode fiber 8, to form an annular cavity structure. The single mode optical fiber 8 is a standard g.652 type optical fiber, and has the least loss at 1550nm band, which is also one of the most commonly used optical fibers. And the single-mode optical fiber 8 and the devices are welded by adopting an optical fiber welding machine, and the welding loss is lower than 0.1dB.

The pumping source 1 is a semiconductor laser with an output wave band of 980nm, and the output power range is 0-700mW; the wavelength division multiplexer 2 is used for forming an optical loop, and adopts a 980nm/1550nm type optical fiber device, so that the insertion loss is less than 0.2dB; the gain optical fiber 3 is used for realizing population inversion and generating stimulated radiation amplification of light, the gain optical fiber 3 adopts a single-mode erbium-doped optical fiber, a nLIGNT Er80-8/125 optical fiber is selected, the length of the highly-doped gain optical fiber 3 is 34cm, the output laser is 1550nm waveband, and the dispersion coefficient at 1550nm is 15.7 ps/(nm-km). The pump source 1 introduces pump laser into the erbium-doped fiber through the wavelength division multiplexer 2, the pump laser is absorbed by the erbium-doped fiber to generate spontaneous radiation in 1550nm waveband, and then oscillation is formed in the ring cavity, so that laser is generated.

The optical fiber coupler 4 is used for outputting the laser, the optical fiber coupler 44 adopts a coupling ratio of 30 to 70, wherein 30% of one end of the optical fiber coupler is used as the output end of the laser, and 70% of one end of the optical fiber coupler is connected with the polarization-independent isolator 6 and generates laser by circulating oscillation in the annular resonant cavity; the optical fiber polarization controller 5 obtains double refraction by bending and winding the optical fiber, changes the polarization state in the resonant cavity, and uses the three-paddle optical fiber polarization controller 5, the middle coil is used as a half-wave plate, and the two sides are respectively provided with a quarter-wave plate; the polarization-independent isolator 6 is used for ensuring unidirectional laser transmission, the maximum isolation degree is within 30dB, the insertion loss is less than 0.5dB, and the maximum power which can be borne is 300mW.

A long-fly 62.5/125 μm graded-index multimode fiber 7 is selected, the diameter of a fiber core of the fiber is 62.5 μm, the diameter of a cladding of the fiber is 125 μm, the attenuation is less than 0.6dB/km, and the selected length is 32.5cm. A common Single Mode Fiber (SMF) 8 is respectively fused to both ends of a Graded Index Multimode Fiber (GIMF) 7 to form a saturable absorber structure for maintaining and starting the mode locking. As fig. 2 shows the principle of SMF-GIMF-SMF structure as saturable absorber, low power signal light (Lowpower) is rapidly attenuated by cladding radiation, and High power signal light (High power) is transmitted in multimode fiber with minimum transmission loss and High transmittance.

The cavity length of the whole laser is about 20.34m, and the dispersion coefficient of each device of the laser is negative dispersion, so that the laser cavity is a full negative dispersion cavity.

The saturable absorption characteristic nonlinear curve of the GI multimode fiber 7 measured by the two-arm measurement method is shown in FIG. 3, and the modulation depth and the non-saturation loss obtained by fitting the following equation were 40.76% and 19.72%, respectively. The modulation depth of the saturable absorption device was measured in the experiment to be 22.33%.

The multimode fiber of the saturable absorber device has obvious saturable absorption characteristics, and the transmissivity of the device is gradually increased along with the increase of incident light power, and finally the device is saturated.

When the pump power is just over 50mW, a stable fundamental mode-locked pulse can be obtained by properly adjusting the fiber polarization controller 5, indicating that the laser has a lower mode-locking threshold.

Fig. 4 (a) - (d) show the output characteristic diagrams of the time-based frequency-locked mode pulse with the pump power of 55.5mW, which are respectively a spectrogram, a timing diagram, a radio frequency diagram and an autocorrelation diagram. As can be seen from the spectrum of fig. 4 (a), the mode-locked pulse has a center wavelength of 1560.5nm, a spectral bandwidth of 3db of 4.98nm, and a wide spectral width. Fig. 4 (b) shows the corresponding pulse sequence, and it can be seen from the time sequence that the adjacent pulses are spaced apart by about 101.6ns. Fig. 4 (c) shows the rf spectrum of the mode-locked pulse, which shows a repetition rate of about 9.84MHz and a signal-to-noise ratio (SNR) of about 46dB, indicating that the mode-locked state of the laser has better stability. Fig. 4 (d) shows an autocorrelation curve of the mode-locked pulse, where the full width at half maximum measured by autocorrelation is 2.45ps and the pulse width obtained by hyperbolic secant fitting is 1.59ps. Through calculation, the time-bandwidth product (TBP) of the mode-locked pulse is about 0.975, and since the devices used by the laser are all negative dispersion devices, the mode-locked pulse is correspondingly broadened, which is slightly larger than the transformation limit of the hyperbolic secant pulse of 0.315. Higher energy mode-locked pulses can be obtained as the pump power continues to increase.

Fig. 5 (a) - (d) show the output characteristic diagrams of mode-locked pulse with the pump power of 215mW, which are respectively a spectrogram, a timing diagram, a radio frequency diagram and an autocorrelation diagram. From the spectrum of FIG. 5 (a), the center wavelength of the mode-locked pulse was 1558.6nm and the 3dB bandwidth was 3.31nm. Fig. 5 (b) shows the corresponding pulse sequence, and from the time series it can be seen that adjacent pulses are spaced by about 101.6ns, from which it can be calculated that the repetition frequency is about 9.84MHz. Fig. 5 (c) shows a plot of the rf spectrum of the mode-locked pulse, showing a repetition rate of 9.84MHz, which is consistent with a pulse spacing of 101.6ns and a cavity length of 20.34m, and a signal-to-noise ratio (SNR) of about 58dB, indicating that the mode-locked state of the laser has good stability. Fig. 5 (d) shows the autocorrelation curve of the mode-locked pulse, which has a pulse width (FWHM) of about 1.16ps.

With the pump power gradually increased, the laser can maintain the mode-locked pulse state, the intensity of the mode-locked pulse state increases linearly with the pump power, and the output power and the pulse energy at different pump powers are shown in fig. 6. When the pumping power is increased from 165mW to 665mW, the average output power is increased from 6.01mW to 32.4mW, and the pulse energy is further increased from 0.61 nanojoules (nJ) to 3.29 nanojoules (nJ), which exceeds the pulse energy of a typical conventional soliton (0.1 nJ).

Therefore, the saturable absorber of the high-energy mode-locked fiber pulse laser adopting the structure provides a new method for realizing the mode-locked fiber laser, the high-energy mode-locked fiber pulse laser can be suitable for a high-power mode-locked fiber laser, is of a full-fiber structure, can be realized only by fiber fusion, and has the advantages of simple manufacturing process, stability, reliability, low cost and the like. Compared with other mode locking devices, the high-energy mode locking fiber pulse laser based on the saturable absorber of the graded-index multimode fiber 7 not only can generate stable high-energy pulses, but also can realize the output of ultrashort pulses, has the advantages of high-power transmission bearing capacity, high damage threshold, no time degradation of performance, low price and the like, and can further promote the batch production and industrialization of the all-fiber mode locking laser.

Finally, it should be noted that: the above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and although the present invention is described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the invention without departing from the spirit and scope of the invention.

Claims

1. The utility model provides a high energy mode locking fiber pulse laser which characterized in that: including the pump source that sets gradually, be used for forming the wavelength division multiplexer of light circuit, be used for realizing the population reversal and produce the gain optic fibre of the stimulated radiation amplification of light, be used for the fiber coupler of laser output, be used for changing the fiber polarization controller of resonant cavity polarization state, be used for guaranteeing the polarization irrelevant isolator of laser unidirectional transmission and be used for forming the graded index multimode fiber of saturable absorber structure, the pump source the wavelength division multiplexer gain optic fibre the fiber coupler fiber polarization controller polarization irrelevant isolator with graded index multimode fiber adopts single mode fiber to be connected in proper order and constitutes the annular chamber structure, graded index multimode fiber and both sides single mode fiber constitutes the saturable absorber structure.

2. A high energy mode-locked fiber pulse laser as defined in claim 1, wherein: the single-mode optical fiber is a standard G.652 type optical fiber, the single-mode optical fiber, the pumping source, the wavelength division multiplexer, the gain optical fiber, the optical fiber coupler, the optical fiber polarization controller, the polarization-independent isolator and the graded-index multimode optical fiber are respectively welded by an optical fiber welding machine, and the welding loss is lower than 0.1dB.

3. A high energy mode-locked fiber pulse laser as defined in claim 1, wherein: the pumping source is a semiconductor laser, the output wave band of the pumping source is 980nm, and the output power range of the pumping source is 0-700mW.

4. A high energy mode-locked fiber pulse laser as defined in claim 1, wherein: the wavelength division multiplexer is a 980nm/1550nm type optical fiber device, and the insertion loss is less than 0.2dB.

5. A high energy mode-locked fiber pulse laser as defined in claim 1, wherein: the gain optical fiber is a single-mode erbium-doped optical fiber, and the output laser of the gain optical fiber is 1550nm waveband.

6. A high energy mode-locked fiber pulse laser as defined in claim 1, wherein: the fiber coupling ratio of the fiber coupler is 10.

7. A high energy mode-locked fiber pulse laser as defined in claim 1, wherein: the optical fiber polarization controller adopts a three-paddle optical fiber polarization controller or an extrusion type optical fiber polarization controller.

8. A high energy mode-locked fiber pulse laser as defined in claim 1, wherein: the core size of the GI multimode fiber is 50 μm or 62.5 μm.