CN106654823A - Mode-locking fiber laser system capable of switching wavelength through collimators - Google Patents

Abstract

The invention provides a mode-locked fiber laser system through which the wavelength is switched by a graphene reflector, and the system includes a sequentially connected pump source, a wavelength division multiplexing fiber coupler, an erbium-doped gain fiber, an isolator, and a first optical fiber A collimator, a transmission window, a graphene saturable absorber, a second fiber collimator, an output coupler and a rotating frame, wherein the graphene saturable absorber is attached to the surface of the transmission window, and the second fiber collimator is fixed The rotating frame of the detector rotates at a certain angle along the direction perpendicular to the incident laser light.

Description

Mode-locked fiber laser system with wavelength switching by collimator

technical field

The invention relates to the field of laser technology, in particular to a mode-locked fiber laser system and a wavelength switching method for switching wavelengths through a collimator.

Background technique

Fiber lasers have the advantages of small size, light weight, high conversion efficiency, and good output laser beam quality, so they have developed rapidly in recent years. In particular, mode-locked fiber lasers have broad prospects in many fields such as detection and diagnosis, biomedicine, precision microprocessing, and military affairs due to their ability to produce ultrashort pulse lasers. Mode-locking technology can be mainly divided into active mode-locking, passive mode-locking, self-mode-locking and hybrid mode-locking technology. Among them, the passive mode-locking technology has become a research hotspot because it does not require an additional external modulation source and is easy to realize the advantages of full fiber optics, and has important practical application significance.

The basic principle of passive mode-locked fiber laser technology is to combine the dispersion of the fiber in the resonator, the nonlinear effect of the laser, and the balance between the gain and loss of the fiber to the laser, and through the passive mode-locked element to adjust the laser intensity or phase The phase locking of the laser is realized by the nonlinear absorption effect, so as to obtain the ultrashort pulse laser output. Usually, fiber laser technologies for passive mode-locking include semiconductor saturable absorber mirror (SESAM), carbon nanotube (SWNT) and other technologies, but both of these technologies have deficiencies. The manufacturing process of SESAM is complicated, the production cost is high, and the range of saturable absorption spectrum is relatively narrow. SWNTs are not universal due to their selectivity to laser wavelengths. Recently, graphene (Graphene) material was found to be useful as a new type of saturable absorber, which can be used for mode locking of fiber lasers. Graphene is a new carbonaceous material with a two-dimensional honeycomb lattice structure formed by the precise accumulation of single-layer carbon atoms. As a saturable absorber, graphene has a wide working range of wavelengths, is simple to manufacture, and has a variety of processes, which can be realized by physical and mechanical stripping and chemical deposition.

Fully polarization-maintaining mode-locked fiber laser is a laser system that can realize linearly polarized ultrashort pulse laser output. The gain fiber and transmission fiber in the laser cavity are composed of fibers with transversely anisotropic refractive index, such as Panda fiber. Compared with ordinary mode-locked fiber lasers, the fiber birefringence characteristics of fully polarization-maintaining mode-locked fiber lasers are not easily affected by ambient temperature and torque, and the output mode-locked laser is more stable. Moreover, the output linearly polarized laser has better application value in many fields, such as fine micromachining, scientific research and other fields.

Contents of the invention

Different from the above NPR technology to achieve dual-wavelength mode-locked laser output, the present invention directly realizes laser mode-locked output at central wavelengths of 1532nm and 1558nm by adjusting the azimuth angle of the approximately linearly polarized laser output from the polarization-maintaining fiber. The invention provides an ultra-short pulse fiber laser system which is more convenient for dual-wavelength adjustment and can realize linearly polarized laser output with a high extinction ratio. Mode-locked fiber laser system and wavelength switching method for wavelength switching by collimator

The technical solution of the present invention is: a mode-locked fiber laser system through which the wavelength is switched by a graphene reflector, the system includes a sequentially connected pump source, a wavelength division multiplexing fiber coupler, an erbium-doped gain fiber, an isolation device, a first fiber collimator, a transmission window, a graphene saturable absorber, a second fiber collimator, an output coupler and a rotating frame, wherein the graphene saturable absorber is attached to the surface of the transmission window, fixed with The rotating frame of the second fiber collimator rotates at a certain angle along the direction perpendicular to the laser incident; the pump light from the pump source enters the erbium-doped gain fiber through the pump end of the wavelength division multiplexing fiber coupler, and the generated signal The light oscillates counterclockwise through the isolator and enters the first fiber collimator to generate collimated light, then it is incident on the transmission window, after being absorbed by the graphene saturable absorber, it is incident on the second fiber collimator, and passes through the fixed The rotation of the rotating frame of the second fiber collimator can fine-tune the polarization azimuth angle of the approximately linearly polarized light emitted from the second fiber collimator, and the adjusted outgoing light enters the coupler and splits a certain proportion of power laser output.

Preferably, the signal transmission fiber in the wavelength division multiplexing fiber coupler includes a polarization maintaining fiber.

Preferably, the pump source includes a laser and the pigtail is a single-mode fiber.

Preferably, the output coupler is a polarization maintaining fiber coupler with an output ratio of 30:70.

Preferably, the rotating frame is set to be able to rotate within ±30 degrees along the direction perpendicular to the incident laser light.

Preferably, the transmission window is a flat glass plate with a near-infrared light transmittance greater than 90%.

Preferably, the transmission window and the graphene saturable absorber are placed on the first fiber collimator and the second fiber collimator, and the intervals therebetween are respectively between 0.1-2 millimeters.

Preferably, the layer thickness of the graphene saturable absorber is between 100nm-10um

Preferably, the layer thickness of the graphene saturable absorber is between 800nm and 1um.

Preferably, the metal reflector is a gold-plated or silver-plated reflector with a reflectivity greater than 90%.

The present invention has the following advantages:

1. The present invention uses a polarization-maintaining fiber as the ring resonator of a mode-locked fiber laser to achieve linearly polarized laser output with a high extinction ratio.

2. The present invention changes the azimuth angle of the incident laser polarization state by directly adjusting the fiber collimator, and realizes two central wavelength laser mode-locked outputs, with simple structure and convenient operation.

3. The present invention adopts a multilayer graphene saturated absorber as a mode-locking device, which reduces manufacturing cost and process difficulty, and is easy to realize industrialization.

It should be understood that both the foregoing general description and the following detailed description are exemplary illustrations and explanations, and should not be used as limitations on the claimed content of the present invention.

Description of drawings

With reference to the accompanying drawings, more objects, functions and advantages of the present invention will be clarified through the following description of the embodiments of the present invention, wherein:

FIG. 1 is a structural diagram of a graphene passive mode-locked fiber laser system of the present invention with two center wavelength mode-locked pulsed fiber lasers that can be adjusted by switching wavelengths through a collimator.

Figure 2 is a cross-sectional structure diagram of a Panda polarization maintaining fiber.

Fig. 3 is a spectrum diagram of the mode-locked laser measured by a spectrometer with a resolution of 0.02nm.

Fig. 4 is a time-domain diagram of the mode-locked laser pulse output measured by an oscilloscope with a bandwidth of 1 GHz.

detailed description

Referring to FIG. 1, the present invention provides a pulsed fiber laser system with two center wavelengths mode-locked that can be adjusted by switching wavelengths through a collimator. The system 100 includes a sequentially connected pump source 1, a wavelength division multiplexing fiber coupler 2. Erbium-doped gain fiber 3, isolator 4, first fiber collimator 5, transmission window 6, graphene saturable absorber 7, second fiber collimator 8, output coupler 9 and rotating frame 10. Wherein the graphene saturable absorber 7 is attached to the surface of the transmission window 6 .

Among them, the pump light from the pump source 1 enters the erbium-doped gain fiber 3 with a length of 1 m through the pump end of the wavelength division multiplexing fiber coupler 2, and the generated signal light in the C+L band passes through the isolator 4 and inverts The hour hand oscillates amplified. The isolator 4 completely isolates the signal light in the clockwise direction, and the advantage of this design is that the remaining pump light in the signal light can be completely filtered out. After the isolated light enters the first fiber collimator 5 to generate collimated light, it is incident on the transmission window 6 , and after being absorbed by the graphene saturable absorber 7 , it is incident to the second fiber collimator 8 . Through the rotation of the rotating frame 10 on which the second fiber collimator 8 is fixed, the polarization azimuth angle of the approximately linearly polarized light emitted from the second fiber collimator 8 can be fine-tuned. The conditioned exit light exits into the coupler. Finally, the mode-locked and amplified laser is output through the output coupler 9 with a certain proportion of power.

The rotation of the rotating frame 10 is controlled by a precise three-dimensional translation stage, so as to achieve high coupling efficiency of laser light into the fiber collimator 8 . The approximate linearly polarized light polarization azimuth angle emitted from the collimator 8 can be fine-tuned through the rotating frame 10, which is equivalent to changing the θ angle in the following formula (1), and also changing the fast axis and slow axis components and phase delay of the optical fiber to achieve Adjust the gain and loss of laser light at different wavelengths. Finally, the laser mode-locked output can be realized at the center wavelength of 1532nm and 1558nm respectively. Figure 2 is a cross-sectional structure diagram of a Panda polarization maintaining fiber.

In the above formula, T represents the transmission coefficient, θ represents the deflection angle, Represents the analysis angle, Δφ _PC , Δφ _LB and Δφ _NL represent the phase delay caused by the polarization controller, fiber birefringence and nonlinear effects, respectively, L and B _m represent the laser cavity length and fiber normalized birefringence, respectively. Among them, the phase delay caused by fiber birefringence changes inversely with the wavelength, which can realize the filtering function.

Preferably, the pump source 1 includes a laser and the pigtail is a single-mode fiber;

The erbium-doped gain fiber is a high-concentration polarization-maintaining erbium-doped fiber with high absorption ratio for pump light.

Preferably, the output coupler is a polarization-maintaining fiber coupler with an output ratio of 30:70, that is, the mode-locked amplified laser splits out 30% of the power laser output through the output coupler 7 .

The rotating frame 10 is arranged to be rotatable along the direction shown in FIG. 1 , ie along the direction perpendicular to the incident laser light, at a certain angle, such as the angle A shown in FIG. 1 , for example, between ±30 degrees.

The transmission window 6 is a flat glass plate with a light transmittance greater than 90% in the near-infrared band.

Preferably, the signal transmission fiber in the wavelength division multiplexing fiber coupler includes a polarization maintaining fiber.

According to the present invention, the core diameter of the erbium-doped gain fiber 3 is determined by the active fiber used. The core diameter of the cladding is preferably 125 μm, and the core diameter of the fiber core can be 4 μm, 8 μm or 10 μm, preferably 10/125 μm. According to a specific embodiment of the present invention, the type of optical fiber should match the pumping wavelength of the pumping source 1 .

The pump wavelength matched by the erbium-doped fiber can be 980nm or 1480nm, and the parameters of the wavelength division multiplexing fiber coupler 2 are further determined according to the wavelength and core diameter parameters. The final emitted laser wavelength is determined by the reflection wavelength of the Bragg fiber grating within a certain gain range of the active fiber (such as 1530-1560nm). The typical output wavelength of Erbium-doped fiber is 1064nm.

For example, in this embodiment, if an erbium-doped fiber with a core diameter of 10/125 μm is selected as the gain medium. The output wavelength of the pump source 1 is 976nm, and the laser output can be obtained within this range. In the experiment, if an erbium-doped fiber with a core diameter of 10/125 μm is selected as the gain medium, the pigtail fiber of the pump source 1 needs to select the same type of core diameter. The pump source 1 is a 915nm single-mode output, and the laser output can be obtained within this range.

As shown in FIG. 1 , the rotation of the rotating frame 10 is controlled by a precision three-dimensional translation stage, so as to achieve high coupling efficiency of laser light into the fiber collimator 8 . The polarization azimuth angle of the approximately linearly polarized light emitted from the collimator 8 can be fine-tuned by rotating the frame 10 .

The transmission window 6 and the graphene saturable absorber 7 are placed on the first fiber collimator 5 and the second fiber collimator 8, and the interval therebetween is, for example, between 0.1-2 mm, so as to increase the intensity of the reflected signal light as much as possible. coupled input.

The graphene saturated absorber 7 can prepare a single-layer graphene film by methods such as mechanical peeling and chemical synthesis, and then attach it to the metal mirror surface by wet transfer or dry transfer, etc., and can be prepared by layer-by-layer stacking. 5-10 layers of graphene saturated absorber. The layer thickness of the graphene saturable absorber 6 is, for example, between 100nm-10um, preferably between 800nm-1um.

The metal reflector is a gold-plated or silver-plated reflector with a reflectivity greater than 90%.

The polarization azimuth angle of the approximate linearly polarized light exiting from the collimator 8 can be fine-tuned through the rotating frame 10, and the polarization state azimuth angle of the incident light and the phase delay of the orthogonal polarization state can be changed, as shown in the formula (2). The azimuth angle of the polarized light and the phase retardation of the orthogonal polarization state are related to the polarization state of the incident light.

tanα _r ＝Pe ^{-iΔφ tanα} _i (2)

In the above formula, α _r and α _i represent the azimuth angles of the reflected and incident laser polarization states, respectively, P represents the reflection coefficient, and Δφ represents the phase delay of the fast and slow axes. Therefore, by precisely adjusting the reflection angle of the rotating frame 10, the polarization azimuth angle of the reflected light and the phase delay of the orthogonal polarization component can be changed, and then, according to the formula (1), the gain and loss of different wavelength lasers can be adjusted. Finally, the laser mode-locked output can be realized at the center wavelengths of 1532nm and 1558nm respectively.

Fig. 3 is a spectrum diagram of the mode-locked laser measured by a spectrometer with a resolution of 0.02nm. Fig. 4 is a time-domain diagram of the mode-locked laser pulse output measured by an oscilloscope with a bandwidth of 1 GHz.

According to the erbium-doped fiber laser of the present invention, the rotatable frame 10 can be fine-tuned to transmit through the graphene saturated absorber layer to the exit angle of the collimator 8, thereby changing the polarization state azimuth angle of the incident light and the phase delay of the orthogonal polarization state, and can Two central wavelengths of 1532nm and 1558nm are mode-locked, so that the laser transmittance, gain and loss of the corresponding wavelength can be changed, and laser amplification of different wavelengths can be simultaneously mode-locked.

The invention adopts the polarization-maintaining fiber as the ring resonator of the mode-locked fiber laser to realize the linearly polarized laser output with high extinction ratio; change the azimuth angle of the polarization state of the incident laser to realize the mode-locked output of two central wavelength lasers, with simple structure and easy operation Convenience; the present invention adopts a multilayer graphene saturated absorber as a mode-locking device, which reduces manufacturing cost and process difficulty, and is easy to realize industrialization.

Other embodiments of the invention will be apparent to and understood by those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The description and examples are considered exemplary only, with the true scope and spirit of the invention defined by the claims.

Claims (10)

1. A mode-locked fiber laser system that switches wavelengths through graphene reflectors, said system includes sequentially connected pump sources, wavelength division multiplexing fiber couplers, erbium-doped gain fibers, isolators, and the first optical fiber collimator , a transmission window, a graphene saturable absorber, a second fiber collimator, an output coupler, and a rotating frame, where The graphene saturable absorber is attached to the surface of the transmission window, The rotating frame fixed with the second fiber collimator rotates at a certain angle along the direction perpendicular to the incident laser light; The pump light from the pump source enters the erbium-doped gain fiber through the pump end of the wavelength division multiplexing fiber coupler, and the generated signal light is oscillated and amplified counterclockwise by the isolator and enters the first fiber collimator to generate collimated light. , incident on the transmission window, after being absorbed by the graphene saturable absorber, it is incident on the second fiber collimator, and can be fine-tuned from the second fiber through the rotation of the rotating frame fixed with the second fiber collimator. The polarization azimuth angle of the approximate linearly polarized light emitted by the collimator, the adjusted outgoing light enters the coupler, and a laser output with a certain proportion of power is separated. 2. The mode-locked fiber laser system according to claim 1, wherein the signal transmission fiber in the wavelength division multiplexing fiber coupler comprises a polarization maintaining fiber. 3. The mode-locked fiber laser system of claim 1, wherein the pump source comprises a laser and the pigtail is a single-mode fiber. 4. The mode-locked fiber laser system according to claim 1, the output coupler is a polarization maintaining fiber coupler with an output ratio of 30:70. 5. The mode-locked fiber laser system according to claim 1, wherein the rotating frame is configured to be rotatable between ±30 degrees along a direction perpendicular to the incident laser light. 6. The mode-locked fiber laser system according to claim 6, wherein the transmission window is a flat glass plate with a light transmittance of more than 90% in the near-infrared band. 7. The mode-locked fiber laser system according to claim 1, the transmission window and the graphene saturable absorber are placed on the first fiber collimator and the second fiber collimator, and the interval therebetween is respectively at 0.1-2 between millimeters. 8. The mode-locked fiber laser system according to claim 1, the layer thickness of the graphene saturable absorber is between 100nm-10um. 9. The mode-locked fiber laser system according to claim 8, the layer thickness of the graphene saturable absorber is between 800nm-1um. 10. The mode-locked fiber laser system according to claim 1, wherein the metal reflector is a gold-plated or silver-plated reflector with a reflectivity greater than 90%.