阅读说明：本技术 基于外部时钟同步的高速调制锁模掺钬光纤激光器 (High-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization ) 是由 马万卓 林鹏 王天枢 赵德胜 刘润民 姜会林 于 2019-11-22 设计创作，主要内容包括：基于外部时钟同步的高速调制锁模掺钬光纤激光器,属于光信息技术领域,为了解决现有技术的激光器结构难以应用到空间光通信的问题,该激光器包括掺铥光纤激光器、掺铥光纤放大器与第一波分复用器的反射端依次连接,第一波分复用器公共端与掺钬光纤、第一电光调制器与第一耦合器的一分路连接连接,第一耦合器的另一个分路作为激光输出端,第一耦合器的公共端、第一光滤波器与第一波分复用器透射端连接,构成全光纤谐振腔结构；射频信号源具有两个输出端,其中一输出端与微波放大器连接,微波放大器与第一电光调制器连接,射频信号源另一输出端、第二电光调制器、第二耦合器、眼图仪与第二电光调制器连接,作为调制信号反馈,从而实现外部时钟同步。(The laser comprises a thulium-doped fiber laser, a thulium-doped fiber amplifier and a reflection end of a first wavelength division multiplexer, wherein the reflection end of the first wavelength division multiplexer is sequentially connected, the common end of the first wavelength division multiplexer is connected with a holmium-doped fiber, a first electro-optic modulator and a branch of a first coupler, the other branch of the first coupler is used as a laser output end, and the common end of the first coupler and a first optical filter are connected with the transmission end of the first wavelength division multiplexer to form an all-fiber resonant cavity structure; the radio frequency signal source is provided with two output ends, one of the output ends is connected with the microwave amplifier, the microwave amplifier is connected with the first electro-optical modulator, and the other output end of the radio frequency signal source, the second electro-optical modulator, the second coupler and the eye pattern instrument are connected with the second electro-optical modulator and used as modulation signal feedback, so that the synchronization of an external clock is realized.)

1. The high-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization is characterized in that a thulium-doped fiber laser (1) is connected with a thulium-doped fiber amplifier (2), the thulium-doped fiber amplifier (2) is connected with a reflection end of a first wavelength division multiplexer (3), a common end of the first wavelength division multiplexer (3) is connected with a holmium-doped fiber (4), the holmium-doped fiber (4) is connected with a first electro-optical modulator (5), the first electro-optical modulator (5) is connected with a branch of a first coupler (6), the other branch of the first coupler (6) is used as a laser output end, the common end of the first coupler (6) is connected with a first optical filter (7), and the first optical filter (7) is connected with a transmission end of the first wavelength division multiplexer (3) to form an all-fiber resonant cavity structure;

the radio frequency signal source (8) is provided with two output ends, one output end is connected with the microwave amplifier (9), the microwave amplifier (9) is connected with the first electro-optical modulator (5) to realize the modulation signal loading of mode locking pulses, the other output end of the radio frequency signal source (8) is connected with the second electro-optical modulator (10), the second electro-optical modulator (10) is connected with the second coupler (11), the second coupler (11) is connected with the eye pattern instrument (12), and the eye pattern instrument (12) is connected with the second electro-optical modulator (10) to serve as modulation signal feedback, so that the synchronization of an external clock is realized.

2. The high-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization according to claim 1, wherein the thulium-doped fiber laser (1) is formed by sequentially connecting a first pump source (1-1), a second wavelength division multiplexer (1-2), a first thulium-doped fiber (1-3), a first optical isolator (1-4), a polarization controller (1-5), a second optical filter (1-6) and a third coupler (1-7), and one beam splitting end of the third coupler (1-7) is used as laser output.

3. The high-speed modulation mode-locked holmium-doped fiber laser based on external clock synchronization according to claim 1, characterized in that the output wavelength of the thulium-doped fiber laser (1) is in the range of 1850-1950nm and is located in the high absorption band of the holmium-doped fiber (4).

4. The high-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization according to claim 1, wherein the thulium-doped fiber amplifier (2) is formed by sequentially connecting a second pump source (2-1), a third wavelength division multiplexer (2-2), a second thulium-doped fiber (2-3) and a second optical isolator (2-4), and the other end of the second optical isolator (2-4) is used as an output end.

5. The high-speed modulation mode-locked holmium-doped fiber laser based on external clock synchronization according to claim 1, wherein the maximum output power of the thulium-doped fiber amplifier (2) is more than 1W so as to meet the mode-locked pulse starting condition.

6. The high-speed modulation mode-locked holmium-doped fiber laser based on external clock synchronization according to claim 1, wherein the modulation signal emitted by the radio frequency signal source (8) is a sine, triangle or square wave radio frequency signal with any waveform, and the modulation frequency is an integral multiple of the longitudinal mode spacing of the resonant cavity.

Technical Field

The invention relates to a high-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization, and belongs to the technical field of optical information.

Background

The mode-locked fiber laser is used for generating ultrashort pulse laser with the time domain pulse width smaller than picosecond magnitude, and can improve pulse energy and peak power by several magnitudes, so that the mode-locked fiber laser has important application in the fields of material processing, laser medical treatment, optical communication, remote sensing, generation of intermediate infrared light sources and the like.

Compared with other types of mode-locked lasers, the mode-locked fiber laser has the obvious advantage of generating stable ultrashort pulse laser with high repetition frequency, and the mode-locked fiber laser with the repetition frequency above gigahertz is an ideal light source of a high-speed digital transmission system. In addition, the mode-locked pulse has higher peak power and can better resist the transmission loss of laser in the atmosphere, so the high repetition frequency mode-locked fiber laser has irreplaceable application value in the field of free space optical communication.

At present, there are three schemes for generating high repetition frequency mode-locked ultrashort pulses in an all-fiber resonant cavity: 1. a passive fundamental frequency mode locking structure with a short cavity length is adopted; 2. a high-order harmonic passive mode locking structure is adopted; 3. the high-order harmonic active mode locking structure adopting electro-optical modulation is adopted. Scheme 1 can increase the repetition frequency of fundamental mode-locked pulses by shortening the cavity length, however, due to the inherent tail fiber length of the optical fiber device, the cavity length cannot be infinitely reduced, so that the repetition frequency of fundamental pulses is difficult to break through gigahertz magnitude, and the capacity requirement of an optical communication system is difficult to meet. In the scheme 2, high-order harmonic mode locking pulses are obtained by increasing pumping power or nonlinear accumulation and the like, so that the repetition frequency of the passive mode locking pulses can be greatly improved, however, when the passive mode locking laser works in a high-order harmonic mode locking state, the passive mode locking laser is influenced by supermode noise, and the obtained kilohertz order mode locking pulses have a lower signal-to-noise ratio, so that the scheme is difficult to meet the stability requirement of an optical communication system. In scheme 3, the longitudinal mode in the cavity is actively modulated by adopting an electro-optical material such as lithium niobate and the like, so that stable mode locking pulses above gigahertz repetition frequency can be generated, however, in an actual communication system, the time domain synchronization of the mode locking pulses and modulation signals is difficult to realize, and the modulation signals are difficult to load on the mode locking laser. In addition, erbium-doped and ytterbium-doped optical fibers are mostly adopted in the laser structures of the three schemes as gain media, the obtained mode locking pulse works in the wave bands of 1.55 micrometers and 1 micrometer, the corresponding wave bands are usually suitable for the traditional optical fiber communication system, and for a space optical communication system taking the atmosphere as a transmission medium, the lasers in the two wave bands have stronger scattering in the atmosphere transmission and are easily influenced by turbulence.

Disclosure of Invention

The invention provides a high-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization, aiming at solving the problem that the laser structure in the prior art is difficult to apply to space optical communication, and the high-speed modulation mode-locking holmium-doped fiber laser realizes the time domain synchronization of mode-locking pulses and modulation signals, expands the output laser wave band to 2.05 microns and can be used as an ideal light source for high-speed space laser communication.

The invention adopts the following technical scheme:

the high-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization is characterized in that the thulium-doped fiber laser is connected with a thulium-doped fiber amplifier, the thulium-doped fiber amplifier is connected with a reflection end of a first wavelength division multiplexer, a common end of the first wavelength division multiplexer is connected with a holmium-doped fiber, the holmium-doped fiber is connected with a first electro-optical modulator, the first electro-optical modulator is connected with one branch of a first coupler, the other branch of the first coupler serves as a laser output end, the common end of the first coupler is connected with a first optical filter, and the first optical filter is connected with a transmission end of the first wavelength division multiplexer to form an all-fiber resonant cavity structure; the radio frequency signal source is provided with two output ends, one output end is connected with the microwave amplifier, the microwave amplifier is connected with the first electro-optical modulator to realize the modulation signal loading of the mode locking pulse, the other output end of the radio frequency signal source is connected with the second electro-optical modulator, the second electro-optical modulator is connected with the second coupler, the second coupler is connected with the eye pattern instrument, and the eye pattern instrument is connected with the second electro-optical modulator to be used as the modulation signal feedback, so that the external clock synchronization is realized.

The invention has the beneficial effects that:

the invention adopts an independently designed external clock synchronization structure, realizes the time domain synchronization of mode locking pulse and modulation signal by using only one radio frequency signal source under the working state of high modulation frequency, improves the problem that the modulation structure of the traditional mode locking laser in an optical communication system is complex, has simple structure and high reliability, and can be directly applied to various high-speed optical communication systems.

According to the invention, the holmium-doped optical fiber is used as a gain medium, the thulium-doped optical fiber laser is used as a corresponding pumping source, the output laser wavelength is between 2 microns and 2.1 microns and is in a high-transmittance window of atmospheric transmission, so that the laser has irreplaceable advantages and wide application prospects in the fields of space laser communication, laser radar and the like.

Drawings

Fig. 1 is a schematic structural diagram of a high-speed modulation mode-locking holmium-doped fiber laser based on external clock synchronization.

Fig. 2 is a schematic structural diagram of a thulium-doped fiber laser.

Fig. 3 is a schematic structural diagram of a thulium-doped fiber amplifier.

Fig. 4 is an output laser spectrum.

FIG. 5 is a 2.02GHz mode-locked pulse time-domain sequence.

FIG. 6 is a graph of mode-locked pulse autocorrelation.

FIG. 7 is a mode locked pulse spectrum.

FIG. 8 is a time domain sequence of mode-locked pulses modulated at a modulation frequency of 1 GHz.

Fig. 9 is a mode-locked pulse eye diagram after modulation and atmospheric transmission.

Detailed Description

The present invention will be described in detail with reference to the accompanying drawings.

As shown in fig. 1, the high-speed modulation mode-locked holmium-doped fiber laser based on external clock synchronization includes a thulium-doped fiber laser 1, a thulium-doped fiber amplifier 2, a first wavelength division multiplexer 3, a holmium-doped fiber 4, a first electro-optical modulator 5, a first coupler 6, a first optical filter 7, a radio frequency signal source 8, a microwave amplifier 9, a second electro-optical modulator 10, a second coupler 11 and an eye chart 12.

The thulium-doped fiber laser 1 is connected with a thulium-doped fiber amplifier 2, the thulium-doped fiber amplifier 2 is connected with a reflection end of a first wavelength division multiplexer 3, a public end of the first wavelength division multiplexer 3 is connected with a holmium-doped fiber 4, the holmium-doped fiber 4 is connected with a first electro-optical modulator 5, the first electro-optical modulator 5 is connected with a branch of a first coupler 6, another branch of the first coupler 6 is used as a laser output end, the public end of the first coupler 6 is connected with a first optical filter 7, the first optical filter 7 is connected with a transmission end of the first wavelength division multiplexer 3, and an all-fiber resonant cavity structure is formed.

The radio frequency signal source 8 is provided with two output ends, one of the output ends is connected with the microwave amplifier 9, the microwave amplifier 9 is connected with the first electro-optical modulator 5 to realize the modulation signal loading of the mode locking pulse, the other output end of the radio frequency signal source 8 is connected with the second electro-optical modulator 10, the second electro-optical modulator 10 is connected with the second coupler 11, the second coupler 11 is connected with the eye pattern instrument 12, and the eye pattern instrument 12 is connected with the second electro-optical modulator 10 to be used as the modulation signal feedback, so that the external clock synchronization is realized.

Adjusting the output wavelength and power of the thulium-doped fiber laser 1, amplifying the power through the thulium-doped fiber amplifier 2, injecting the amplified pump light into the holmium-doped fiber 4 through the first wavelength division multiplexer 3, enabling the generated gain light with the wave band of 2.1 microns to firstly enter the first electro-optic modulator 5, periodically modulating the intensity of the gain light, continuously entering the first coupler 6, providing intracavity feedback and laser output, enabling the feedback light to enter the first optical filter 7 for super-mode noise suppression, and forming the mode-locked pulse with high repetition frequency. Two output ends of a radio frequency signal source 8 emit radio frequency signals with the same waveform, wherein one path of radio frequency signals is electrically amplified through a microwave amplifier 9 so as to drive a first electro-optical modulator 5, the other path of radio frequency signals is directly input to a second electro-optical modulator 10 to realize remodulation of mode locking pulses, the remodulated mode locking pulses are incident to an eye pattern instrument 12 through a second coupler 11 to carry out photoelectric conversion, and the converted mode locking pulse electrical signals are fed back to the second electro-optical modulator 10 to realize clock synchronization of the mode locking pulses and the modulation signals.

As shown in fig. 2, the thulium-doped fiber laser 1 is formed by sequentially connecting 1-7 a first pump source 1-1, a second wavelength division multiplexer 1-2, a first thulium-doped fiber 1-3, a first optical isolator 1-4, a polarization controller 1-5, a second optical filter 1-6 and a third coupler, wherein one of the beam splitting ends of the third coupler 1-7 is used as laser output.

As shown in fig. 3, the thulium-doped fiber amplifier 2 is formed by sequentially connecting a second pump source 2-1, a third wavelength division multiplexer 2-2, a second thulium-doped fiber 2-3 and a second optical isolator 2-4, and the right end of the second optical isolator is used as an output end.

The modulation signal sent by the radio frequency signal source 8 is a sine, triangle, square wave random waveform radio frequency signal, and the modulation frequency needs to be an integral multiple of the longitudinal mode interval of the resonant cavity.

The radio frequency signal source 8 is an external active device, the eye pattern instrument 12 is an external testing device, the two devices can be replaced by other commercial products, and the radio frequency signal source and the eye pattern instrument are not required to be packaged inside a prototype for reducing the size during system integration.

And starting the power of the first pump source 1-1 to 1W, adjusting the second optical filter 1-6 to fix the output laser wavelength of the thulium-doped optical fiber laser 1 at 1900nm, wherein the output power is in the milliwatt level, and at the moment, starting the power of the second pump source 2-1 to 5W, and amplifying the laser at 1900nm to more than 1W. The 1900nm laser is injected and pumped into the holmium doped fiber 4 by the first wavelength division multiplexer 3, resulting in a gain at the 2.1 μm band. The radio frequency signal source 8 transmits a sinusoidal analog signal with the repetition frequency of 2.02GHz, the frequency of the sinusoidal analog signal is integral multiple of the interval of longitudinal modes of the resonant cavity, and the radio frequency signal is amplified by the microwave amplifier 9 and drives the first electro-optical modulator 5, so that mode locking is realized. The other port of the radio frequency signal source 8 transmits a 1GHz pseudorandom radio frequency signal to the second electro-optical modulator 10 to realize remodulation of the mode locking pulse, the modulated mode locking pulse is subjected to photoelectric conversion at the eye pattern instrument 12, and the converted mode locking pulse electrical signal is fed back to the second electro-optical modulator 10 to realize clock synchronization.

Fig. 4 shows the output laser spectrum, the output laser wavelength can be tuned continuously within the range of 2035 and 2050nm by adjusting the first optical filter 7.

Fig. 5 is a 2.02GHz mode-locked pulse time domain sequence, and in the scanning range of 8ns, the mode-locked pulse sequence has better flatness and no loss-of-lock phenomenon, and the mode-locked pulse repetition frequency can be changed by adjusting the modulation frequency of the radio frequency signal source 8.

FIG. 6 is a graph of autocorrelation of mode-locked pulses, with a repetition frequency of 2.02GHz and a pulse width of the mode-locked pulses of 40 ps.

FIG. 7 shows the frequency spectrum of the mode-locked pulse, where the signal-to-noise ratio of the fundamental frequency spectrum is 44.85dB at a repetition frequency of 2.02GHz, which illustrates that the mode-locked pulse operates in a low noise environment.

Fig. 8 is a time domain sequence of mode-locked pulses modulated after the modulation frequency is synchronized with an external clock, and the mode-locked pulses can be modulated into a pulse envelope of any shape by a pseudo-random signal.

Fig. 9 is a modulated mode-locked pulse eye diagram, modulated mode-locked pulses are transmitted through an equivalent atmospheric channel with a length of 1 km, pulse signals at a detection end are accumulated in a time domain, and a signal-to-noise ratio can reach 18.4dB when being observed by an eye diagram instrument, which indicates that the modulated signals have lower noise, so that the high-speed modulated mode-locked holmium-doped fiber laser based on external clock synchronization can be directly used as a light source of a high-speed spatial optical communication system.