High energy thulium-doped fiber laser based on self-mode-locking and dispersion management

Abstract

The application relates to the field of optical information technology, and particularly relates to a high-energy thulium-doped fiber laser based on self-locking mode and dispersion management technology, wherein a pump laser is connected with one end of a pump protector, the other end of the pump protector is connected with an a end of a wavelength division multiplexer, a b end and a c end of the wavelength division multiplexer are respectively connected with an i end of a three-port circulator and one end of a thulium-doped gain fiber, the other end of the thulium-doped gain fiber is connected with a d end of an optical coupler, an e end of the optical coupler is connected with one end of a polarization controller, the other end of the polarization controller is connected with one end of a single-mode optical fiber, the other end of the single-mode optical fiber is connected with a g end of the three-port circulator, an h end of the three-port circulator is connected with one end of a collimator, and the other end of the collimator is provided with a dispersion compensation assembly, high-energy ultrashort pulses can be generated, the structure is optimized, and the manufacturing cost is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optical information technology, and particularly relates to a high-energy thulium-doped fiber laser based on self-mode-locking and dispersion management technology. BACKGROUND

[0002] Traditional mode-locking techniques mainly include active mode-locking and passive mode-locking. The active mode-locking technique is to insert a modulator or inject external pulses into a laser resonant cavity to actively modulate light waves to achieve mode-locking. The advantage is that the operation is intuitive and the repetition frequency is tunable, but complex electrical control equipment needs to be introduced, increasing the system cost and complexity. The passive mode-locking technique is simpler and more efficient, which generates ultra-short pulses by adding a saturable absorber in the laser resonant cavity. Traditional saturable absorbers include nonlinear optical ring mirrors, nonlinear polarization rotation and low-dimensional nanomaterials such as carbon nanotubes, graphene and black phosphorus; however, the use of these traditional saturable absorbers in mode-locked fiber lasers also increases the complexity and cost of the laser, especially, these traditional saturable absorbers generally have instability or low damage threshold problems under high-energy single-pulse operation.

[0003] The self-mode-locking technique enables the laser to achieve mode-locking without adding mode-locking devices in the laser resonant cavity, which reduces the complexity and cost of the laser relative to traditional mode-locking techniques. In a high-doped thulium-doped fiber, the upconversion and reabsorption processes of thulium ions can induce saturable absorption, so that the high-doped thulium-doped fiber can simultaneously play the role of gain medium and saturable absorber, thereby enabling the thulium-doped fiber laser to have the ability of self-mode-locking. Current research on self-mode-locked thulium-doped fiber lasers mainly focuses on the realization of self-mode-locking and the tunability of its gain bandwidth, and the potential of directly outputting high-energy ultra-short pulses has not been tapped. SUMMARY

[0004] In order to solve the problem that the existing self-mode-locked thulium-doped fiber laser lacks the ability to directly output high-energy ultra-short pulses, the application provides a high-energy thulium-doped fiber laser based on self-mode-locking and dispersion management technology, which can generate high-energy ultra-short pulses.

[0005] The technical scheme of the application is as follows:

[0006] The high-energy thulium-doped fiber laser based on self-mode-locking and dispersion management technology comprises a pump laser, a pump protector, a wavelength division multiplexer, a thulium-doped gain fiber, an optical coupler, a polarization controller, a single-mode fiber, a three-port circulator and a collimator.

[0007] The pump laser is connected with one end of a pump protector, the other end of the pump protector is connected with a port a of a wavelength division multiplexer, a port b and a port c of the wavelength division multiplexer are connected with a port i of a three-port circulator and one end of a thulium-doped gain fiber respectively, the other end of the thulium-doped gain fiber is connected with a port d of an optical coupler, a port e of the optical coupler is connected with one end of a polarization controller, the other end of the polarization controller is connected with one end of a single-mode fiber, the other end of the single-mode fiber is connected with a port g of the three-port circulator, a port h of the three-port circulator is connected with one end of a collimator, and the other end of the collimator is provided with a dispersion compensation assembly.

[0008] Preferably, the dispersion compensation assembly comprises a focusing lens, a grating and a high reflection mirror, and the laser passes through the focusing lens, the grating and the high reflection mirror in sequence.

[0009] Preferably, the pump laser is a 1550nm continuous laser.

[0010] Preferably, the length of the single-mode fiber is 1km.

[0011] Preferably, the wavelength division multiplexer is a 1550 / 1950nm wavelength division multiplexer.

[0012] Compared with the prior art, the present application has the following beneficial effects:

[0013] (1) The high-doped thulium-doped fiber can act as a saturable absorber while serving as a gain medium, so that the laser resonant cavity can realize self-mode-locking without additional embedding of a traditional material saturable absorber as a mode-locking device, which not only overcomes the problem of low damage threshold of the material saturable absorber, supports high-power operation of the laser, but also optimizes the structural complexity of the laser and reduces the manufacturing cost.

[0014] (2) The introduction of a longer negative dispersion single-mode fiber and a grating-based tunable dispersion assembly realizes dispersion management of the laser resonant cavity, improves the mode-locking pump power threshold and pulse splitting energy threshold of the fiber laser, so as to realize higher energy single-pulse laser output, and regulates the width of the output pulse.

[0015] (3) Compared with the traditional high-energy ultrashort pulse fiber laser, the high-energy thulium-doped fiber laser proposed in the present application can directly output picosecond pulses with micro-joule pulse energy without additional amplification and compression, and has a simpler structure, a more compact size and a lower cost. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is a schematic diagram of the overall structure of the present application;

[0017] In the figure: 1-pump laser, 2-pump protector, 3-wavelength division multiplexer, 4-thulium-doped gain fiber, 5-optical coupler, 6-polarization controller, 7-single-mode fiber, 8-three-port circulator, 9-collimator, 10-focusing lens, 11-grating, 12-high-reflectivity mirror. DETAILED DESCRIPTION

[0018] The application will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0019] Reference Figure 1 The high-energy thulium-doped fiber laser based on self-mode-locking and dispersion management technology comprises a pump laser 1, a pump protector 2, a wavelength division multiplexer 3, a thulium-doped gain fiber 4, an optical coupler 5, a polarization controller 6, a single-mode fiber 7, a three-port circulator 8, and a collimator 9. The pump laser 1 is connected to one end of the pump protector 2, the other end of the pump protector 2 is connected to the a end of the wavelength division multiplexer 3, the b end and the c end of the wavelength division multiplexer 3 are respectively connected to the i end of the three-port circulator 8 and one end of the thulium-doped gain fiber 4, the other end of the thulium-doped gain fiber 4 is connected to the d end of the optical coupler 5, the e end of the optical coupler 5 is connected to one end of the polarization controller 6, the other end of the polarization controller 6 is connected to one end of the single-mode fiber 7, the other end of the single-mode fiber 7 is connected to the g end of the three-port circulator 8, and the h end of the three-port circulator 8 is connected to one end of the collimator 9, forming a laser resonant cavity.

[0020] The other end of the collimator 9 is provided with an adjustable dispersion assembly.

[0021] The pump laser 1 is used to generate a 1550 nm continuous laser light source, which is generally composed of a 1550 nm continuous laser seed and an erbium-doped fiber amplifier.

[0022] The pump protector 2 is arranged at the output end of the pump laser and can isolate the return light to avoid the high-energy laser signal in the laser resonant cavity from returning to the pump laser 1 to cause damage to the pump laser 1.

[0023] The a end of the wavelength division multiplexer 3 is connected to the output end of the pump protector, and the pump laser is introduced into the laser resonant cavity through the c end.

[0024] The thulium-doped gain fiber 4 is connected to the c end of the wavelength division multiplexer, and is used to generate 1.9 μm band gain laser under the action of the pump laser.

[0025] The optical coupler 5 has a beam splitting ratio of 50 / 50, the d end is connected to the second end of the thulium-doped gain fiber, the e end continues to transmit the laser signal in the laser resonant cavity, and the f end is used as the output end of the laser to output part of the laser signal outside the cavity.

[0026] The polarization controller 6 adjusts the polarization state of the laser in the cavity.

[0027] Single mode fiber 7: 1km length single mode fiber embedded in the laser cavity to introduce higher negative dispersion and reduce the repetition rate of the laser pulses;

[0028] Three-port circulator 8: g port is connected to the embedded single mode fiber, propagates the laser signal input to the g port to the h port and outputs, and can output the laser signal input to the h port from the i port;

[0029] Collimator 9: makes the laser couple into the single mode fiber with maximum efficiency.

[0030] In an embodiment of the present application, the tunable dispersion assembly includes a focusing lens 10, a grating 11 and a high reflection mirror 12, and the laser passes through the focusing lens 10, the grating 11 and the high reflection mirror 12 in sequence.

[0031] Among them, the focusing lens 10: a transmission lens, focuses the laser beam axially on the collimator, grating or high reflection mirror;

[0032] The grating 11: as a dispersion compensation element, compensates the dispersion in the laser cavity;

[0033] The high reflection mirror 12: reflects the laser acting on its surface.

[0034] The working principle of the present application will be described below in conjunction with a specific embodiment:

[0035] The pump laser 1 uses a 1550nm continuous laser, which contains a cooling module to ensure the stability of the output power; the pump laser 1 is connected with a pump protector 2 to avoid potential damage of the back light to the pump laser 1; the pump laser is introduced into the laser cavity through a wavelength division multiplexer 3 with a working wavelength of 1550 / 1950nm, and a 5m thulium-doped gain fiber 4 is used as the gain medium and saturable absorber of the laser cavity to realize self-mode locking of the fiber laser. The nonlinear absorption characteristics of the 4 thulium-doped gain fiber can be measured by the balanced dual detector method, and based on the simplified two-level energy model, the absorption coefficient can be expressed as:

[0036]

[0037] Among them, α0 and α ns are saturated absorption and unsaturated absorption, respectively, and I and I s are laser radiation intensity and saturation intensity, respectively.

[0038] The laser signal is extracted using a 5 optical coupler with a 50 / 50 splitting ratio. A larger splitting ratio requires a higher pump power for the laser cavity to accumulate enough intracavity laser energy to achieve mode-locking operation, which also leads to a higher output laser energy. A 6 polarization controller is inserted after the optical coupler to adjust the polarization state of the laser cavity.

[0039] According to the soliton area theorem, the pulse energy of a mode-locked fiber laser is positively related to the net cavity dispersion. Therefore, to obtain a higher single pulse energy, a single mode fiber and a tunable dispersion component can be inserted in the self-mode-locked thulium-doped fiber laser for dispersion management.

[0040] The soliton area theorem is as follows:

[0041] E∝|β2| / (γτ)

[0042] Wherein, E, β2, γ, τ represent the pulse energy, the second-order dispersion coefficient, the nonlinear coefficient, and the pulse width, respectively.

[0043] In this embodiment, a 1 km long single mode fiber 7 is added to the thulium-doped fiber laser cavity to increase the intracavity negative dispersion, so that the net dispersion of the entire laser cavity is in a larger negative dispersion domain. On the one hand, the stronger nonlinearity and dispersion introduced by the longer 7 single mode fiber requires stronger gain to balance, so as to promote the generation of laser pulses, thereby resulting in a higher mode-locking pump power threshold of the laser cavity. On the other hand, the increase in net negative dispersion caused by the introduction of the longer single mode fiber 7 leads to the time-domain broadening of the pulse during propagation, which affects the reduction of the pulse peak power, so that the accumulated nonlinear phase shift can be maintained within the tolerance range of single pulse mode-locking, and higher pulse energy can be accumulated without pulse splitting.

[0044] A tunable dispersion component is introduced into the laser cavity by a three-port circulator 8, and the optical fiber port of the three-port circulator 8 is connected with a collimator 9, so that the reflected laser can be coupled into the single mode fiber 7 with maximum efficiency. The tunability of the tunable dispersion component is realized by a 11 transmission grating, a 10 lens mounted on a translation stage, and a 12 reflector mounted on another translation stage. By adjusting the distance between the grating 11 and the focusing lens 10, the continuous tuning of the net cavity dispersion can be realized.

[0045] The main functions of the tunable dispersion assembly in the high-energy thulium-doped fiber laser are as follows: on the one hand, when the negative dispersion provided by the introduced single-mode fiber 7 is insufficient, the negative dispersion compensation can be provided for the laser resonant cavity by adjusting the distance between the grating 11 and the focusing lens 10 of the tunable dispersion assembly, so as to avoid the pulse splitting of the output pulse under the high-energy operating condition; on the other hand, when the negative dispersion provided by the introduced single-mode fiber 7 is sufficient or excessive, the positive dispersion compensation can be provided for the laser resonant cavity by adjusting the distance between the grating 11 and the focusing lens 10 of the tunable dispersion assembly, so as to balance the dispersion, stabilize the mode locking and realize the pulse compression, and thus the laser resonant cavity can directly output the high-energy ultrashort pulse with the pulse width in the picosecond order and the pulse energy in the microjoule order.

[0046] The high-energy thulium-doped fiber laser described in the embodiment has the output pulse repetition frequency of about 204 kHz, and the single-pulse energy can be up to 1 muJ, so that the single-pulse energy directly output by the conventional fiber laser is improved by two orders of magnitude.

[0047] The above only describes the embodiments of the present application, and does not limit the patent scope of the present application, and any equivalent structure or equivalent flow transformation using the content of the specification and drawings of the present application, or direct or indirect application in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A high energy thulium doped fiber laser based on self-mode locking and dispersion management techniques, characterized in that, The pump laser, the pump protector, the wavelength division multiplexer, the thulium-doped gain fiber, the optical coupler, the polarization controller, the single-mode fiber, the three-port circulator and the collimator are connected in series to form a laser resonant cavity. The pump laser is connected with one end of the pump protector, the other end of the pump protector is connected with the a end of the wavelength division multiplexer, the b end and the c end of the wavelength division multiplexer are respectively connected with the i end of the three-port circulator and one end of the thulium-doped gain fiber, the other end of the thulium-doped gain fiber is connected with the d end of the optical coupler, the e end of the optical coupler is connected with one end of the polarization controller, the other end of the polarization controller is connected with one end of the single-mode fiber, the other end of the single-mode fiber is connected with the g end of the three-port circulator, the h end of the three-port circulator is connected with one end of the collimator, forming a laser resonant cavity. The other end of the collimator is provided with a tunable dispersion assembly.

2. The high energy thulium-doped fiber laser based on self-mode locking and dispersion management techniques according to claim 1, characterized in that, The tunable dispersion assembly comprises a focusing lens, a grating and a high reflection mirror, and the laser passes through the focusing lens, the grating and the high reflection mirror in sequence.

3. The high energy thulium-doped fiber laser based on self-mode locking and dispersion management techniques according to claim 1, wherein, The pump laser is a 1550nm continuous laser.

4. The high energy thulium doped fiber laser based on self-mode locking and dispersion management techniques according to claim 1, wherein, The length of the single-mode fiber is 1km.

5. The high energy thulium doped fiber laser based on self-mode locking and dispersion management techniques according to claim 1, wherein, The wavelength division multiplexer is a 1550 / 1950nm wavelength division multiplexer.

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