CN107946893A - The saturable absorber device of gradual change multimode single mode structure based on microcavity built in single mode - Google Patents

Abstract

The invention discloses a saturable absorber device based on a single-mode-built-in microcavity graded multimode-single-mode structure, which adopts an all-fiber structure and consists of a first single-mode fiber, a second single-mode fiber and a built-in microcavity. Combination of graded index multimode fiber. The characteristics of the saturable absorber are determined by the graded index multimode fiber with built-in microcavity. Due to the specific loss relationship related to the light intensity caused by its strong nonlinear multimode interference effect, the two wings of the pulse are lost and the The peak can pass through, thus obtaining ultra-short mode-locked pulses. The saturable absorber has the advantages of low cost, simple structure, and excellent mechanical properties. At the same time, it can greatly reduce the energy density in the fiber and increase the damage threshold. It can be further applied to high-power ultrashort pulse mode-locked fiber lasers. .

Description

Saturable absorbers with graded multimode-single-mode structure based on single-mode-built-in microcavity device

technical field

The invention relates to the fields of laser technology and nonlinear optics. In particular, a saturable absorber based on a single-mode-built-in microcavity graded multimode-single-mode structure.

Background technique

Saturable absorber devices are the most critical optical devices for passively mode-locked ultrashort pulse fiber lasers. This kind of device can meet a specific loss relationship, that is, when the pulse energy passing through the device increases, its optical loss becomes smaller and the transmittance increases. At present, commonly used saturable absorber devices include semiconductor saturable absorber mirror (SESAM), carbon nanotubes, new two-dimensional materials represented by graphene, and equivalent saturable absorbers based on the nonlinear polarization rotation (NPR) effect. . However, these common saturable absorbers still have certain defects. For example, the response time of SESAM can only reach the picosecond level, which makes them not ideal for generating ultrashort pulses of femtosecond level. The large coupling loss between fibers will limit the improvement of laser conversion efficiency. Saturable absorber materials such as carbon nanotubes and graphene are easily contaminated, and material defects such as poor long-term reliability and low damage threshold also greatly limit the output power and service life of lasers. NPR-based saturable absorbers are sensitive to disturbances in the external environment and are generally used in laboratory environments. Reliable saturable absorbers need to have good properties such as fast response time (relaxation time), high stability, long-term reliability, ease of use, appropriate saturation flux, and low loss.

In recent years, the multimode interference effect in multimode fiber has been deeply studied and successfully applied to various fiber optic devices, including beam shapers, sensors and filters in linear applications. Among them, graded-index multimode fiber has attracted more and more attention due to its advantages of less self-excited high-order modes and small modal dispersion. Since the graded-index multimode fiber can reduce the modal dispersion, and the propagation constants of its modes are equidistant, the self-focusing length of the optical signal can be very short, even less than 1 mm. Therefore, the nonlinearity in the graded-index multimode fiber Effects such as four-wave mixing, self-phase modulation, and cross-phase modulation are abundant. At present, in terms of nonlinear effects, a variety of applications such as Bessel beams and supercontinuum have been realized based on graded-index multimode fibers. These applications provide theoretical support for the use of graded-index fibers to fabricate saturable absorbers. Moreover, saturable absorbers made of graded-index multimode fibers, such as two-dimensional materials such as graphene, and semiconductor saturable absorber mirrors are simpler in structure and have a higher damage threshold. The high-quality saturable absorber is not easy to age, and it is a very reliable material for making a saturable absorber.

Contents of the invention

The invention designs a saturable absorber with low cost, simple structure and stable performance, provides a saturable absorber device based on a graded-index multimode optical fiber with a built-in microcavity and a preparation method thereof, and the saturable absorber device The saturable absorber device is applied to an all-fiber mode-locked fiber laser to obtain ultrashort pulse output. One of the purposes of the present invention is to solve the technical problems existing in the existing saturable absorber, and to provide a saturable absorber device with a wide band and high damage threshold; another purpose is to use the saturable absorber device to realize passive Mode-locked fiber lasers to obtain ultrashort pulse output.

In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a saturable absorber device for a graded-index multimode fiber with a built-in microcavity, including the first single-mode fiber (1-1) which is sequentially welded, and a microhole on the end surface A graded-index multimode fiber (2), and a second single-mode fiber (1-2). Wherein, the first single-mode optical fiber is welded with the graded-index optical fiber with microholes to form a microcavity (3).

As a preferred solution, the first single-mode fiber and the second single-mode fiber are the same type of single-mode fiber.

As a preferred solution, the graded-index multimode optical fiber adopts an optical fiber with a core/cladding ratio of 62.5 μm/125 μm or 50 μm/125 μm.

As a preferred solution, the length of the graded-index multimode optical fiber is greater than 15 cm.

The invention provides a method for preparing a saturable absorber device based on a single-mode-built-in microcavity gradient multimode-single-mode structure, comprising the following steps:

Step 1: making a microhole at the fiber core on one of the end faces of the graded-index multimode fiber;

Step 2: using a fusion splicer to weld the end of the graded-index multimode fiber end face with the microhole to the first single-mode fiber to form a microcavity;

Step 3: Splicing the other end of the graded-index multimode fiber and the second single-mode fiber with a fusion splicer.

As a preferred solution, the microholes on the end face of the graded-index multimode optical fiber can be obtained by hydrofluoric acid etching or femtosecond laser micromachining.

As a preferred solution, the diameter of the microcavity in the saturable absorber is less than or equal to 45 μm.

The main principle of the present invention: due to nonlinear effects such as self-phase modulation and cross-phase modulation in the graded-index multimode fiber, the equivalent refractive index will change, and finally the self-phase modulation under high peak power in the graded-index multimode fiber The focal length differs from that at low peak power. However, the mode field distribution of the graded index multimode fiber is different, which will affect the intensity of the nonlinear effect, so the perturbation of the graded index multimode fiber with a microcavity on the end face will affect the mode field of the optical signal entering the graded index multimode fiber distributed. When the ratio of the fundamental mode reaches an appropriate level, the high peak power optical signal of the laser from the graded index multimode fiber to the single mode fiber is just self-focused to the core of the single mode fiber, while the low peak power The optical signal of high power enters the cladding of the single-mode fiber and is only attenuated by loss. This property corresponds to a saturable absorber.

Compared with the prior art, the saturable absorber device based on the "single-mode-built-in microcavity graded multi-mode-single-mode structure" saturable absorber device of the present invention has the following beneficial effects:

(1) The diameter of the microcavity in the above-mentioned saturable absorber can be precisely controlled by changing the specific parameters of corrosion or femtosecond processing, thereby improving the characteristics of the saturable absorber, and then optimizing the output parameters of the laser.

(2) The above-mentioned saturable absorber has the advantages of low cost, simple structure, excellent mechanical properties, etc., and is easy to be mass-produced on a large scale.

(3) The above-mentioned saturable absorber has very excellent optical properties. The use of multimode fiber can greatly reduce the energy density in the fiber, and the damage threshold is high, and can be further applied to high-power ultrashort pulse mode-locked fiber in the laser.

(4) Through the reasonable design of the length and core size of the multimode fiber, the above-mentioned saturable absorber device can achieve low-loss transmission in a wide spectral range.

Description of drawings

Fig. 1 is a schematic diagram of the structure of the saturable absorber of the present invention.

Fig. 2 is a measurement chart of saturated absorption characteristics of a single-mode-built-in microcavity gradient multimode-single-mode saturable absorber.

Fig. 3 is a schematic structural diagram of the fiber laser of the present invention.

Figure 4 is the pulse sequence output of a passively mode-locked fiber laser based on a single-mode-built-in microcavity with a graded multimode-single-mode saturable absorber.

Fig. 5 is a pulse width measurement diagram of the laser in the pulsed state of Fig. 4 .

Detailed ways

The main idea of the present invention is: due to nonlinear effects such as self-phase modulation and cross-phase modulation in the graded-index multimode fiber, the equivalent refractive index will change, and finally the graded-index multimode fiber under high peak power The autofocus length differs from that at low peak power. However, the mode field distribution of the graded index multimode fiber is different, which will affect the intensity of the nonlinear effect, so the perturbation of the graded index multimode fiber with a microcavity on the end face will affect the mode field of the optical signal entering the graded index multimode fiber distributed. When the ratio of the fundamental mode reaches an appropriate level, the high peak power optical signal of the laser from the graded index multimode fiber to the single mode fiber is just self-focused to the core of the single mode fiber, while the low peak power The optical signal of high power enters the cladding of the single-mode fiber and is only attenuated by loss. This property corresponds to a saturable absorber.

The specific process of preparing the "single-mode-gradient multi-mode-single-mode structure with built-in microcavity" saturable absorber of the present invention is as follows:

1. Cleave both ends of a 23 cm section of graded-index optical fiber (62.5/125 μm, Corning) with a core of 62.5 μm flat.

2. Corrode one of the end faces with hydrofluoric acid for 5 minutes. Since hydrofluoric acid corrodes the fiber cladding and fiber core at different rates, a micropore will be formed at the fiber core after the multimode fiber is corroded.

3. Use a fusion splicer to fuse the corroded end of the multimode fiber with the single-mode fiber. The discharge time of the welding machine is 800ms, and a microcavity with a diameter of 45 μm is formed after welding.

4. Splice the other end of the graded index multimode fiber with a section of single mode fiber to obtain a saturable absorber with a graded multimode-single-mode structure of single-mode-built-in microcavity.

Specific embodiments of the present invention are given below, and it should be noted that the present invention is not limited to the following specific embodiments, and all equivalent transformations done on the basis of the technical solutions of the present application all fall within the protection scope of the present invention.

Example 1

This example presents a saturable absorber based on a single-mode-built-in microcavity graded multimode-single-mode structure, with a microcavity in the saturable absorber, and a graded-index multimode fiber located between two sections of single-mode fiber between. As shown in Figure 1.

In this example, the non-linear effect and self-focusing effect in the saturable absorber with single-mode-built-in microcavity graded multimode-single-mode structure are used to measure the nonlinear effect and self-focusing effect, and the 1.5μm band femtosecond laser is used as the test light source. By adjusting the fiber laser Output power to measure the modulation depth of a saturable absorber As shown in Figure 2, as the incident light power increases, the transmittance of the material tends to saturate. The measurements were fitted with a simplified saturable absorption model: QUOTE ; Among them, ΔT, I _sat and T _ns represent the modulation depth, saturation power and unsaturated absorption coefficient, respectively, and the modulation depth of the material is 2.0% through fitting.

Example 2

This embodiment provides a passive mode-locked fiber laser based on a single-mode-built-in microcavity graded multimode-single-mode saturable absorber, including a pump source (4) with a wavelength of 1480nm, a wavelength division multiplexer ( 5), gain fiber (6), isolator (7), polarization controller (8), single-mode-graded multimode-single-mode saturable absorber with built-in microcavity (9) and fiber coupler (10), The fiber coupler (10) includes a 10% output port and a 90% output port, wherein the single-mode-gradient multimode-single-mode saturable absorber (9) with a built-in microcavity is the same as that in Embodiment 1.

Wavelength division multiplexer (5), gain fiber (6), isolator (7), polarization controller (8), single-mode-graded multimode-single-mode saturable absorber with built-in microcavity (9) and optical fiber 90% of the output ports of the coupler (10) are sequentially connected in a ring shape to form a ring laser resonator, as shown in FIG. 2 . The laser diode pumping source 1 is connected to the input end of the wavelength division multiplexer 2 .

As shown in Figure 3, the single-mode-built-in microcavity graded multimode-single-mode is used as a saturable absorber to realize passive mode-locked fiber laser operation. When the pump power is 92.3mW, the mode-locked pulse output is shown in Figure 4. It can be seen from the figure that the mode-locked pulse in the cavity works in a relatively stable state, the pulse interval is 70.9ns, and the corresponding pulse repetition frequency is 14.34MHz. Figure 5 shows the pulse width when the mode-locked fiber laser is in stable mode-locked pulse output, and its value is 528fs.

Claims (8)

1. A saturable absorber device based on a graded multimode-single-mode structure of a single-mode-built-in microcavity, characterized in that: a saturable absorber device of a graded-index multimode optical fiber with a built-in microcavity, including welding in sequence The first single-mode optical fiber (1-1), the graded-index multimode optical fiber with microholes on the end surface (2), and the second single-mode optical fiber (1-2). 2. Wherein, the first single-mode optical fiber and the graded-index optical fiber with microholes are fused to form a microcavity (3). 3. A kind of saturable absorber device based on a single-mode-built-in microcavity graded multimode-single-mode structure according to claim 1, characterized in that: the first single-mode fiber and the second single-mode fiber are the same A single-mode fiber. 4. A kind of saturable absorber device based on a graded multimode-single-mode structure of a single-mode-built-in microcavity according to claim 1, characterized in that: the graded-index multimode fiber adopts a core/cladding ratio 62.5μm/125μm or 50μm/125μm fiber. 5. A saturable absorber device based on a single-mode-built-in microcavity graded multimode-single-mode structure according to claim 1, characterized in that the length of the graded-index multimode fiber is greater than 15 cm. 6. A method for preparing a saturable absorber device based on a single-mode-built-in microcavity graded multimode-single-mode structure, characterized in that it comprises the following steps: A microhole located at the core is made on one of the end faces of the graded-index multimode optical fiber; Using a fusion splicer to weld the end face of the graded-index multimode optical fiber with the microhole to the first single-mode optical fiber to form a microcavity; Splice the other end of the graded-index multimode fiber to the second single-mode fiber with a fusion splicer. 7. the preparation method of a kind of saturable absorber device based on the graded multimode-single-mode structure of single-mode-built-in microcavity according to claim 5, it is characterized in that: the end face of the graded-index multimode optical fiber is micro Pores can be obtained by hydrofluoric acid etching or femtosecond laser micromachining. 8. The preparation method of a saturable absorber device based on a single-mode-built-in microcavity graded multimode-single-mode structure according to claim 5, characterized in that: the diameter of the microcavity in the saturable absorber is less than Equal to 45μm.