A 1550nm high stability femtosecond fiber laser

Abstract

The application relates to a 1550nm high-stability femtosecond fiber laser, which belongs to the technical field of optoelectronic devices and mainly comprises a single-chip microcomputer module (1), a microwave signal source module (2), a lithium niobate modulator driver (3), a high-speed photoelectric converter (20), an amplitude self-adaptive adjustment circuit (21), a repetition frequency detection circuit (22), a repetition frequency change rate detection circuit (23) and the like. The application realizes all-fiber chirped pulse amplification suitable for a femtosecond fiber laser system, improves the stability of output femtosecond light pulses, and improves the electro-optical conversion efficiency of the whole system.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic equipment technology, and specifically relates to a 1550nm high-stability femtosecond fiber laser. Background Technology

[0002] Femtosecond lasers can rapidly and accurately concentrate the energy of a laser onto a target area, thus finding wide applications in micro-nano fabrication, precision optical measurement, and biomedicine. Femtosecond fiber lasers are considered ideal femtosecond laser sources due to their miniaturization, portability, lack of water cooling, and high stability.

[0003] In existing technologies, femtosecond fiber lasers typically use either passively mode-locked or actively mode-locked fiber lasers as seed sources. The output pulse widths of these two types of mode-locked fiber lasers are on the picosecond scale. Chirped pulse amplification is required outside the resonant cavity of the mode-locked fiber laser to further compress the output pulses, thereby obtaining femtosecond pulses with peak power reaching megawatts. A common method for chirped pulse amplification is to initially broaden the pulse using a spatially structured high-dispersion grating, then amplify it, and finally further compress it using the grating. The shortcomings of existing femtosecond fiber lasers are: firstly, the system employs numerous spatial optical components, making its structure complex, unstable, and difficult to integrate; secondly, the chirped pulse amplification process is completed outside the resonant cavity of the mode-locked fiber laser, which is an uncontrolled open-loop pulse compression method, resulting in poor stability of the output femtosecond pulses and thus shortening the time for the femtosecond laser to maintain stable operation. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the above-mentioned shortcomings of existing femtosecond fiber laser systems and provide a femtosecond fiber laser that can generate more stable peak power and repetition frequency, has a longer continuous stable working time, and has higher electro-optical conversion efficiency.

[0005] The technical problem of this invention is solved by the following technical solution:

[0006] A 1550nm high-stability femtosecond fiber laser has the following structure: the RJ45 network port output of a microcontroller module 1 is connected to the input of a microwave signal source module 2; the output of the microwave signal source module 2 is connected to the input of a lithium niobate modulator driver 3; the output of the lithium niobate modulator driver 3 is connected to the modulation input of a lithium niobate modulator 4; the optical output of the lithium niobate modulator 4 is connected to one end of a dispersion compensation fiber 5; the other end of the dispersion compensation fiber 5 is connected to the input of a second optical isolator 6; the output of the second optical isolator 6 is connected to one end of a second erbium-doped fiber 7; the other end of the second erbium-doped fiber 7 is connected to the common end of a second wavelength division multiplexer 8; the 980nm end of the second wavelength division multiplexer 8 is connected to the optical output of a second 976nm pump source 9; and the control input of the second 976nm pump source 9 is connected to the UART serial port output of the microcontroller module 1.

[0007] The structure is characterized by the following: the 1550nm end of the second wavelength division multiplexer 8 is connected to the optical input end of the second tunable optical filter 10; the control input end of the second tunable optical filter 10 is connected to the second USB output end of the microcontroller module 1; the optical output end of the second tunable optical filter 10 is connected to the input end of the first optical isolator 11; the output end of the first optical isolator 11 is connected to one end of the first erbium-doped fiber 12; the other end of the first erbium-doped fiber 12 is connected to the common end of the first wavelength division multiplexer 13; the 980nm end of the first wavelength division multiplexer 13 is connected to the optical output end of the first 976nm pump source 14; and the output optical power of the first 976nm pump source 14 can be preset. Since the input is a fixed value, its control input does not need to be connected. The 1550nm end of the first wavelength division multiplexer 13 is connected to one end of the dispersion delay line 15, and the other end of the dispersion delay line 15 is connected to one end of the photonic crystal fiber 16. The other end of the photonic crystal fiber 16 is connected to the input of the 50:50 optical coupler 17. One 50% output of the 50:50 optical coupler 17 is connected to the optical input of the first tunable optical filter 18. The control input of the first tunable optical filter 18 is connected to the first USB output of the microcontroller module 1. The optical output of the first tunable optical filter 18 is connected to the optical input of the lithium niobate modulator 4. The other 50:50 output of the 50:50 optical coupler 17 is connected to the optical input of the first tunable optical filter 18. The 50% output terminal is connected to the input terminal of the first 10:90 optocoupler 19. The 10% output terminal of the first 10:90 optocoupler 19 is connected to the optical input terminal of the high-speed photoelectric converter 20. The SMA interface output terminal of the high-speed photoelectric converter 20 is connected to the SMA interface input terminal of the amplitude adaptive adjustment circuit 21. The Vadjgain-OUT output terminal of the amplitude adaptive adjustment circuit 21 is connected to the Vfdetect-IN input terminal of the repetition frequency detection circuit 22. The repetition frequency detection circuit 22 is connected to the microcontroller module 1 via the SPI bus. The Vfdetect-OUT output terminal of the repetition frequency change rate detection circuit 22 is connected to the repetition frequency change rate detection circuit 22. The Vfvardetect-IN input terminal of the repetition rate change detection circuit 23 is connected to the Vfvardetect-OUT output terminal of the repetition rate change detection circuit 23, which is connected to the A / D input pin of the microcontroller module 1. The 90% output terminal of the first 10:90 optocoupler 19 is connected to the input terminal of the second 10:90 optocoupler 24. The 10% output terminal of the second 10:90 optocoupler 24 is connected to the optical input terminal of the laser energy meter module 25. The output terminal of the laser energy meter module 25 is connected to the UART serial port input terminal of the microcontroller module 1. The 90% output terminal of the second 10:90 optocoupler 24 is the optical pulse output terminal of the femtosecond fiber laser system.

[0008] The length of the dispersion compensation fiber 5 needs to meet the following condition: Let the length of all erbium-doped fibers in the femtosecond fiber laser be L. gain Its second-order dispersion coefficient is The length of the dispersion-compensating fiber is L DCF Its second-order dispersion coefficient is The sum of the lengths of all ordinary single-mode optical fibers is L. SMF Its second-order dispersion coefficient is The average second-order dispersion coefficient of the resonant cavity In this invention, the length of the dispersion compensation fiber 5 must be adjusted to ensure... It is a positive number;

[0009] The amplitude adaptive adjustment circuit 21 has the following circuit structure: pins 2 and 5 (COM1) and pins 10, 11, 12, 14, and 17 (COM2) of chip U1 are grounded; pin 8 (IPBS) is grounded through capacitor C11; pin 9 (CPBS) is grounded through capacitor C12; pin 1 (VPS1) is connected to a +5V power supply and grounded through parallel capacitors C9 and C10; pin 3 (INHI) is connected to the positive terminal of the SMA interface through capacitor C1; and the negative terminal of the SMA interface is grounded. Pin 4, INLO, is grounded via capacitor C2. Pin 15, CPLO, is connected to +5V power via inductor L2 and grounded via capacitor C7. Pin 13, VPS2, is connected to +5V power and grounded via parallel capacitors C5 and C6. Pins 18-22, VPS2, and pin 23, ENBL, are connected to pin 7, VPOS, of chip U2, to +5V power, and grounded via parallel capacitors C3 and C4. Pin 16, OPHI, is connected to +5V power via inductor L1. Connect the CP terminal (pin 6) of chip U3 to capacitor C8; connect the INHI terminal (pin 1) of chip U2 to the CP terminal (pin 6) of chip U3 via capacitor C13; connect the VOUT terminal (pin 5) to the GAIN terminal (pin 24) of chip U1; connect the TADJ terminal (pin 6) to ground via resistor R1; connect the INLO terminal (pin 8) to ground via capacitor C14; connect the VSET terminal (pin 4) to ground via resistor R2 and connect to a +3.3V power supply via potentiometer W1; connect the CLPF terminal (pin 3) to ground via capacitor C15; connect the... The COMM pin is grounded; pins 1, 4, 5, 8, 9, 12, and 14 (GND), pin 2 (RP), pin 3 (RN), pin 7 (CN), and pin 10 (QN) of chip U3 are grounded; pins 13 and 16 (VCC) are connected to a +3.3V power supply and grounded through parallel capacitors C17 and C18; pin 15 (VR) is connected to a +3.3V power supply through capacitor C16; and pin 11 (QP) serves as the Vadjgain-OUT output of the amplitude adaptive adjustment circuit 21.

[0010] The repetition frequency detection circuit 22 has the following circuit structure: pins 21 and 22 (N / C terminals) and pin 23 (VTUNE terminal) of chip U4 are connected to pin 6 (inverting input) and pin 7 (output) of operational amplifier U5B; pins 24 and 26 (N / C terminals), pin 25 (VCC2 terminal), and pin 27 (VCC1 terminal) are connected to a +3.3V power supply and grounded through parallel capacitors C27 and C28; pins 18-20 (N / C terminals) and pin 17 (CEN terminal) are connected to a +3.3V power supply and grounded through parallel capacitors C19 and C20, and through a resistor... R3 is connected to pin 16 (DVDD3V). Pin 15 (XREEP) is connected to the Vfdetect-IN input of the repetition frequency detection circuit 22 via capacitor C33. Pin 10 (3VRVDD) is connected to a +3.3V power supply and grounded via capacitor C34. Pin 7 (VDDLS) is grounded via capacitor C21, connected to pin 3 (VPPCP), grounded via parallel capacitors C35 and C36, and grounded via series inductor L3 and capacitor C37. Pins 5 and 6 (N / C) are grounded. Pin 1 (AVDD) is connected to a +3.3V power supply and grounded via... Overcapacitor C22 is connected to pin 40 (BIAS), pin 39 (VCCPD), pin 36 (VCCPS), and pin 35 (VCCHF), and grounded through parallel capacitors C23 to C26. Pins 30 (SEN), 31 (SDI), 32 (SCK), and 33 (SDO) are connected to the four general-purpose I / O pins of microcontroller module 1 to form an SPI bus for transmitting control data. Pin 4 (CP) is connected to one end of resistor R5 and capacitor C30 through resistor R4. The other end of capacitor C30 is connected to the first pin of operational amplifier U5A. The output terminal of pin 1 and the inverting input terminal of pin 2 are connected together. The other end of resistor R5 is connected to the non-inverting input terminal of pin 3 of op-amp U5A and grounded through capacitor C29. The output terminal R6 of pin 1 of op-amp U5A is connected to one end of resistor R7 and capacitor C32. The other end of capacitor C32 is connected to the output terminal of pin 7 and the inverting input terminal of pin 6 of op-amp U5B. The other end of resistor R7 is connected to the non-inverting input terminal of pin 5 of op-amp U5B and grounded through capacitor C31. The output terminal of pin 7 of op-amp U5B serves as the Vfdetect-OUT output terminal of the repetition frequency detection circuit 22.

[0011] The repetition frequency change rate detection circuit 23 has the following circuit structure: the non-inverting input terminal of pin 3 of operational amplifier U6A is grounded through resistor R9 and connected to a +3.3V power supply through resistor R8; the output terminal of pin 1 is connected to the inverting input terminal of pin 2 and connected to the non-inverting input terminal of pin 5 of operational amplifier U6B through resistor R11; the inverting input terminal of pin 6 of operational amplifier U6B is connected to the Vfvardetect-IN input terminal of the repetition frequency change rate detection circuit 23 through a series resistor R12 and capacitor C38, connected to the output terminal of pin 1 through a series potentiometer W2 and resistor R10, connected to the output terminal of pin 1 through capacitor C39, and connected to the positive terminal of Zener diode D1. The negative terminal of Zener diode D1 is connected to the negative terminal of Zener diode D2, and the positive terminal of Zener diode D2 is connected to the output terminal of pin 1. The output terminal of pin 1 serves as the Vfvardetect-OUT output terminal of the repetition frequency change rate detection circuit 23.

[0012] Beneficial effects:

[0013] 1. This invention utilizes a first optical isolator 11, a first erbium-doped fiber 12, a first wavelength division multiplexer 13, a first 976nm pump source 14, a dispersion delay line 15, and a photonic crystal fiber 16 to realize all-fiber chirped pulse amplification suitable for the femtosecond fiber laser system of this invention.

[0014] 2. This invention incorporates the chirped pulse amplification process into the resonant cavity of a mode-locked fiber laser, thereby achieving closed-loop pulse compression and improving the stability of the output femtosecond optical pulse.

[0015] 3. The present invention incorporates a first tunable optical filter 18 into the resonant cavity of a femtosecond fiber laser, thereby improving the electro-optical conversion efficiency of the entire system through feedback control of the average output optical power.

[0016] 4. The present invention adds a second tunable optical filter 10 to the resonant cavity of the femtosecond fiber laser, and improves the stability of the repetition frequency of the output femtosecond optical pulse by feedback control of the rate of change of the output optical pulse repetition frequency.

[0017] 5. The present invention designs an amplitude adaptive adjustment circuit 21, which can adaptively amplify the small signal with variable amplitude output by the high-speed photoelectric converter 20 into a pulse signal with fixed amplitude (this value can be adjusted by potentiometer W1), thereby improving the detection accuracy of the subsequent repetition frequency detection circuit 22.

[0018] 6. The present invention designs a repetition frequency detection circuit 22, which can realize the accurate measurement of the repetition frequency of femtosecond-level optical pulses.

[0019] 7. The present invention designs a repetition frequency change rate detection circuit 23, which can convert the voltage signal representing the repetition frequency of the femtosecond laser output by the repetition frequency detection circuit 22 into a voltage signal representing the repetition frequency change rate of the femtosecond laser output. Attached image description:

[0020] Figure 1 This is an overall structural block diagram of the 1550nm high-stability femtosecond fiber laser of the present invention.

[0021] Figure 2 This is the schematic diagram of the amplitude adaptive adjustment circuit 21.

[0022] Figure 3 This is the schematic diagram of the repetition frequency detection circuit 22.

[0023] Figure 4 This is the schematic diagram of the repetition frequency change rate detection circuit 23.

[0024] Figure 5 This is a schematic diagram of the center wavelength scanning range and optimal filtering position of the first tunable filter 18. Detailed Implementation

[0025] The working principle of the present invention will be further explained below with reference to the accompanying drawings. The component parameters listed in the various embodiments are preferred parameters, but are not intended to limit the scope of protection of the present invention.

[0026] Example 1: Overall Structure of the Invention

[0027] like Figure 1As shown, the overall structure of this invention includes: the RJ45 network port output of the microcontroller module 1 (STMicroelectronics' STM32 microcontroller system with embedded UART serial port, USB, RJ45 and A / D modules) is connected to the input of the microwave signal source module 2 (Signal Hound's TG44A vector signal source module); the output of the microwave signal source module 2 is connected to the input of the lithium niobate modulator driver 3 (Optilab's MD-20-M type modulator driver); the output of the lithium niobate modulator driver 3 is connected to the modulation input of the lithium niobate modulator 4 (Optilab's IMC-1550-20 type lithium niobate modulator); the optical output of the lithium niobate modulator 4 is connected to one end of the dispersion compensation fiber 5 (Chengdu Beiyi Fiber Technology Co., Ltd.'s ADCM-050 type dispersion compensation fiber); and the other end of the dispersion compensation fiber 5 is connected to the second optical isolator 6 (Chengdu Beiyi Fiber Technology Co., Ltd.'s PFSI-1). The input terminal of the -55-1-1 optical isolator is connected to the input terminal of the second optical isolator 6. The output terminal of the second optical isolator 6 is connected to one end of the second erbium-doped fiber 7 (FT-R37004 type erbium-doped fiber from Chengdu Beiyi Fiber Technology Co., Ltd.). The other end of the second erbium-doped fiber 7 is connected to the common terminal of the second wavelength division multiplexer 8 (FT-93-SMWDM9815 type wavelength division multiplexer from Chengdu Beiyi Fiber Technology Co., Ltd.). The 980nm terminal of the second wavelength division multiplexer 8 is connected to the optical output terminal of the second 976nm pump source 9 (FT-LSM-PUMP-976 type pump source module from Chengdu Beiyi Fiber Technology Co., Ltd.). The control input terminal of the second 976nm pump source 9 is connected to the UART serial port output terminal of the microcontroller module 1. In the above structure, if the 1550nm end of the second wavelength division multiplexer 8 is connected to the optical input end of the lithium niobate modulator 4 to form a resonant cavity, a traditional active mode-locked fiber laser system can be constructed. The dispersion compensation fiber 5 is used to ensure that the resonant cavity is filled with dissipative solitons. The repetition frequency and single pulse energy of the system output optical pulse can be adjusted by the microcontroller module 1, which controls the lithium niobate modulator 4 and the second 976nm pump source 9, respectively.

[0028] This invention, based on a traditional actively mode-locked fiber laser resonator, incorporates an all-fiber chirped pulse amplification system, consisting of a first optical isolator 11, a first erbium-doped fiber 12, a first wavelength division multiplexer 13, a first 976nm pump source 14, a dispersion delay line 15, and a photonic crystal fiber 16, into the laser resonator. To ensure stable optical pulse circulation within the resonator, a first tunable optical filter 18 and a second tunable optical filter 10 are inserted at both ends of the all-fiber chirped pulse amplification system. The former extracts the smooth portion of the spectrum and improves the electro-optical conversion efficiency of the entire system through feedback control of the output average optical power. The latter counteracts the output optical pulse repetition frequency shift caused by the former, improving the stability of the system's output femtosecond optical pulse repetition frequency through feedback control of the output optical pulse repetition frequency change rate. The structure is characterized by the following: the 1550nm end of the second wavelength division multiplexer 8 is connected to the optical input end of the second tunable optical filter 10 (LT-WLTF-FC-1550 type tunable optical filter from Chengdu Beiyi Fiber Technology Co., Ltd.); the control input end of the second tunable optical filter 10 is connected to the second USB output end of the microcontroller module 1; and the optical output end of the second tunable optical filter 10 is connected to the first optical isolator 11 (PFSI-1-55-1-1 optical isolator from Chengdu Beiyi Fiber Technology Co., Ltd.). The input end of the first optical isolator 11 is connected to the input end of the first erbium-doped fiber 12 (FT-R37004 type erbium-doped fiber from Chengdu Beiyi Fiber Technology Co., Ltd.), and the output end of the first erbium-doped fiber 12 is connected to one end of the first wavelength division multiplexer 13 (FT-93-SMWDM9815 type wavelength division multiplexer from Chengdu Beiyi Fiber Technology Co., Ltd.). The 980nm end of the first wavelength division multiplexer 13 is connected to the optical output end of the first 976nm pump source 14. The output optical power of pump source 14 (FT-LSM-PUMP-976 pump source module from Chengdu Beiyi Fiber Technology Co., Ltd.) can be preset to a fixed value, so its control input does not need to be connected. The 1550nm end of the first wavelength division multiplexer 13 is connected to one end of the dispersion delay line 15 (FT-73-10-S9-1-FC dispersion delay line from Chengdu Beiyi Fiber Technology Co., Ltd.), and the other end of the dispersion delay line 15 is connected to the photonic crystal fiber 16 (PB fiber from Chengdu Beiyi Fiber Technology Co., Ltd.). One end of the photonic crystal fiber 16 is connected to the optical input of a 50:50 optical coupler 17 (FT-73-SMFC-1550-1-50-1-0-1-1 type optical coupler from Chengdu Beiyi Fiber Technology Co., Ltd.). A 50% output of the 50:50 optical coupler 17 is connected to the optical input of a first tunable optical filter 18 (LT-WLTF-FC-1550 type tunable optical filter from Chengdu Beiyi Fiber Technology Co., Ltd.).The control input terminal of the first tunable optical filter 18 is connected to the first USB output terminal of the microcontroller module 1. The optical output terminal of the first tunable optical filter 18 is connected to the optical input terminal of the lithium niobate modulator 4. The other 50% output terminal of the 50:50 optocoupler 17 is connected to the input terminal of the first 10:90 optocoupler 19 (FT-73-SMFC-1550-1-10-1-0-1-1 type optocoupler from Chengdu Beiyi Fiber Technology Co., Ltd.). The 10% output terminal of the first 10:90 optocoupler 19 is connected to the optical input terminal of the high-speed photoelectric converter 20 (FT-93-LDPD45220R type high-speed photoelectric converter from Chengdu Beiyi Fiber Technology Co., Ltd.). The SMA interface output terminal of the high-speed photoelectric converter 20 is connected to the SMA interface input terminal of the amplitude adaptive adjustment circuit 21. The Vadjgain-OUT output terminal of the amplitude adaptive adjustment circuit 21 is connected to the Vfdetect-IN input terminal of the repetition frequency detection circuit 22. The repetition frequency detection circuit 22 is connected to the microcontroller module 1. The components are connected via an SPI bus. The Vfdetect-OUT output of the repetition frequency detection circuit 22 is connected to the Vfvardetect-IN input of the repetition frequency change rate detection circuit 23. The Vfvardetect-OUT output of the repetition frequency change rate detection circuit 23 is connected to the A / D input pin of the microcontroller module 1. The 90% output of the first 10:90 optocoupler 19 is connected to the input of the second 10:90 optocoupler 24 (FT-73-SMFC-1550-1-10-1-0-1-1 type optocoupler from Chengdu Beiyi Fiber Technology Co., Ltd.). The 10% output of the second 10:90 optocoupler 24 is connected to the optical input of the laser energy meter module 25 (PM102 type laser energy meter module from THORLABS, USA). The output of the laser energy meter module 25 is connected to the UART serial port input of the microcontroller module 1. The 90% output of the second 10:90 optocoupler 24 is the optical pulse output of the femtosecond fiber laser system.

[0029] Example 2: Amplitude adaptive adjustment circuit 21 in the output optical pulse repetition frequency feedback loop

[0030] The structure of the amplitude adaptive adjustment circuit 21 is as follows: Figure 2As shown: Pins 2 and 5 (COM1) and pins 10, 11, 12, 14, and 17 (COM2) of chip U1 are grounded; pin 8 (IPBS) is grounded through capacitor C11; pin 9 (CPBS) is grounded through capacitor C12; pin 1 (VPS1) is connected to a +5V power supply and grounded through parallel capacitors C9 and C10; pin 3 (INHI) is connected to the positive terminal of the SMA interface through capacitor C1, and the negative terminal of the SMA interface is grounded; pin 4 (INLO) is connected to a capacitor... C2 is grounded. Pin 15, CPLO, is connected to +5V power through inductor L2 and grounded through capacitor C7. Pin 13, VPS2, is connected to +5V power and grounded through parallel capacitors C5 and C6. Pins 18-22, VPS2, and pin 23, ENBL, are connected to pin 7, VPOS, of chip U2, to +5V power, and grounded through parallel capacitors C3 and C4. Pin 16, OPHI, is connected to +5V power through inductor L1 and to pin 6, CP, of chip U3 through capacitor C8. Pin 1, INHI, of chip U2 is connected to pin 6, CP, of chip U3 through capacitor C13. Pin 5, VOUT, is connected to pin 24, GAIN, of chip U1. Pin 6, TADJ, is grounded through resistor R1. Pin 8, INLO, is grounded through capacitor C14. Pin 4, VSET, is grounded through resistor R2 and connected to +3.3V power through potentiometer W1. Pin 3, CLPF, is grounded through capacitor C15. Pin 2, COM... The M terminal is grounded; pins 1, 4, 5, 8, 9, 12, and 14 (GND), pin 2 (RP), pin 3 (RN), pin 7 (CN), and pin 10 (QN) of chip U3 are grounded; pins 13 and 16 (VCC) are connected to a +3.3V power supply and grounded through parallel capacitors C17 and C18; pin 15 (VR) is connected to a +3.3V power supply through capacitor C16; and pin 11 (QP) serves as the Vadjgain-OUT output of the amplitude adaptive adjustment circuit 21.

[0031] The preferred parameters for each component in the amplitude adaptive adjustment circuit 21 are as follows: capacitors C1, C2, C3, C5, C7, C8, C9, C16, and C17 are all 100pF; capacitors C4, C6, and C10 are all 100nF; capacitors C11 and C12 are all 10nF; capacitors C13 and C14 are all 1nF; capacitor C15 is 220pF; capacitor C18 is 4.7uF; resistor R1 is 18KΩ; resistor R2 is 510Ω; inductors L1 and L2 are both 120nH; potentiometer W1 is 1KΩ; chip U1 is ADL 5331; chip U2 is AD8319; and chip U3 is HMC749.

[0032] The amplitude adaptive adjustment circuit 21 works as follows: The VOUT pin of the AD8319 provides voltage to the GAIN pin of the ADL5331 from low to high to change the amplification factor of the ADL5331 amplifier. The output pulse waveform of the OPHI output pin of the ADL5331 is output to the INHI pin of the AD8319, and the AD8319 detects its peak value. The AD8319 compares the detected peak value with the input DC voltage on its VSET pin. If they are equal, the increase in the output voltage of the VOUT pin stops. This ensures that no matter how large the amplitude of the pulse waveform input to the INHI pin of the ADL5331 is, the peak value of the pulse waveform output to the OPHI pin is equal to the set voltage of the VSET pin of the AD8319. The OPHI pin of the ADL5331 is also connected to the HMC749 chip to convert its output ultra-short pulse signal into a square wave signal, so that the subsequent repetition frequency detection circuit can work more accurately and stably.

[0033] Example 3: Repetition frequency detection circuit 22 in the output optical pulse repetition frequency feedback loop

[0034] The structure of the repetition frequency detection circuit 22 is as follows: Figure 3As shown: Pins 21 and 22 (N / C), and pin 23 (VTUNE) of chip U4 are connected to pin 6 (inverting input) and pin 7 (output) of operational amplifier U5B. Pins 24 and 26 (N / C), pin 25 (VCC2), and pin 27 (VCC1) are connected to a +3.3V power supply and grounded through parallel capacitors C27 and C28. Pins 18-20 (N / C) and pin 17 (CEN) are connected to a +3.3V power supply, grounded through parallel capacitors C19 and C20, and connected to pin 16 (DVDD3V) through resistor R3. Pin 15 (XREEP) is connected to the Vfdetect-IN input of the repetition frequency detection circuit 22 via capacitor C33. Pin 10 (3VRVDD) is connected to a +3.3V power supply and grounded via capacitor C34. Pin 7 (VDDLS) is grounded via capacitor C21, connected to pin 3 (VPPCP), grounded via parallel capacitors C35 and C36, and grounded via series inductor L3 and capacitor C37. Pins 5 and 6 (N / C) are grounded. Pin 1 (AVDD) is connected to a +3.3V power supply and connected to pin 40 via capacitor C22. The BIAS pin is connected to pin 39 (VCCPD), pin 36 (VCCPS), and pin 35 (VCCHF), and grounded through parallel capacitors C23 to C26. Pins 30 (SEN), 31 (SDI), 32 (SCK), and 33 (SDO) are connected to the four general-purpose I / O pins of microcontroller module 1 to form an SPI bus for transmitting control data. Pin 4 (CP) is connected to one end of resistor R5 and capacitor C30 through resistor R4. The other end of capacitor C30 is connected to the output pin 1 of operational amplifier U5A. The other end of resistor R5 is connected to the inverting input terminal of pin 2, and to the non-inverting input terminal of pin 3 of op-amp U5A, and grounded through capacitor C29; the output terminal R6 of pin 1 of op-amp U5A is connected to one end of resistor R7 and capacitor C32, the other end of capacitor C32 is connected to the output terminal of pin 7 and the inverting input terminal of pin 6 of op-amp U5B, the other end of resistor R7 is connected to the non-inverting input terminal of pin 5 of op-amp U5B, and grounded through capacitor C31; the output terminal of pin 7 of op-amp U5B serves as the Vfdetect-OUT output terminal of repetition frequency detection circuit 22.

[0035] The preferred parameters for each component in the repetition frequency detection circuit 22 are as follows: capacitors C19, C36, and C37 are all 4.7uF; capacitors C20, C21, C24, C25, C28, C34, and C35 are all 0.47uF; capacitor C22 is 1nF; capacitors C26 and C27 are both 22pF; capacitors C29 and C31 are both 10pF; capacitor C30 is 12pF; capacitor C32 is 68pF; capacitor C33 is 100nF; resistor R3 is 51Ω; resistors R4 and R5 are both 6.2KΩ; resistors R6 and R7 are both 2.5KΩ; inductor L3 is 47nH; operational amplifiers U5A and U5B are AD8042; and chip U4 is HMC1033.

[0036] The working principle of the repetition frequency detection circuit 22 is as follows: a square wave signal is generated by the voltage-controlled oscillator in HMC1033, and the frequency of the signal is compared with that of the input signal (Vfdetect-IN input terminal). The frequency difference between the two signals is used to control the voltage-controlled oscillator, so that the output frequency of the voltage-controlled oscillator is exactly the same as the frequency of the input signal. At this time, the control voltage of the voltage-controlled oscillator (Vfdetect-OUT output terminal) can reflect the frequency of the input signal.

[0037] Example 4: Repetition frequency change rate detection circuit 23 in the output optical pulse repetition frequency feedback loop

[0038] The structure of the repetition frequency change rate detection circuit 23 is as follows: Figure 4 As shown: the non-inverting input of pin 3 of operational amplifier U6A is grounded through resistor R9 and connected to a +3.3V power supply through resistor R8. The output of pin 1 is connected to the inverting input of pin 2 and connected to the non-inverting input of pin 5 of operational amplifier U6B through resistor R11. The inverting input of pin 6 of operational amplifier U6B is connected to the Vfvardetect-IN input of the repetition frequency change rate detection circuit 23 through a series resistor R12 and capacitor C38, connected to the output of pin 1 through a series potentiometer W2 and resistor R10, connected to the output of pin 1 through capacitor C39, and connected to the positive terminal of Zener diode D1. The negative terminal of Zener diode D1 is connected to the negative terminal of Zener diode D2, and the positive terminal of Zener diode D2 is connected to the output of pin 1. The output of pin 1 serves as the Vfvardetect-OUT output of the repetition frequency change rate detection circuit 23.

[0039] The preferred parameters for each component in the repetition frequency change rate detection circuit 23 are as follows: capacitor C28 is 100pF, capacitor C39 is 5pF, resistors R8 and R9 are both 50KΩ, resistors R11 and R12 are both 50Ω, potentiometer W2 is 2KΩ, resistor R10 is 200Ω, Zener diodes D1 and D2 are both 1N4370, and operational amplifiers U6A and U6B are AD8042.

[0040] The working principle of the repetition rate change detection circuit 23 is as follows: when the voltage at the input terminal of Vfvardetect-IN remains constant, the voltage at the output terminal of Vfvardetect-OUT remains constant at 1.65V. When the voltage at the input terminal of Vfvardetect-IN changes by ΔV, the voltage at the output terminal of Vfvardetect-OUT equals 1.65V ± (W2 + R10) * C38 * ΔV, with the sign determined by the direction of the voltage change. Therefore, the stability of the repetition rate of the output optical pulses of the femtosecond fiber laser can be determined by detecting whether the voltage at the output terminal of Vfvardetect-OUT equals 1.65V.

[0041] Example 5: Working principle of the present invention

[0042] The working principle of the present invention will be explained in conjunction with the above embodiments and accompanying drawings.

[0043] In a structure consisting of a microcontroller module 1, a microwave signal source module 2, a lithium niobate modulator driver 3, a lithium niobate modulator 4, a dispersion-compensating fiber 5, a second optical isolator 6, a second erbium-doped fiber 7, a second wavelength division multiplexer 8, and a second 976nm pump source 9, a conventional active mode-locked fiber laser system can be constructed by connecting the 1550nm end of the second wavelength division multiplexer 8 to the optical input end of the lithium niobate modulator 4 to form a resonant cavity. The dispersion-compensating fiber 5 is used to ensure that dissipative solitons operate in the resonant cavity. The repetition frequency and single-pulse energy of the system output optical pulse can be adjusted by the microcontroller module 1, controlling the lithium niobate modulator 4 and the second 976nm pump source 9, respectively. The resonant cavity of this system operates with dissipative solitons, and its pulse width is on the picosecond scale. However, the present invention aims to output optical pulses on the femtosecond scale, therefore a pulse compression stage is required. This invention designs an all-fiber chirped pulse amplification system, consisting of a first optical isolator 11, a first erbium-doped fiber 12, a first wavelength division multiplexer 13, a first 976nm pump source 14, a dispersion delay line 15, and a photonic crystal fiber 16, to achieve optical pulse compression. Amplification is accomplished through the first optical isolator 11, the first erbium-doped fiber 12, the first wavelength division multiplexer 13, and the first 976nm pump source 14, while two-stage pulse compression is achieved through the dispersion delay line 15 and the photonic crystal fiber 16. To achieve closed-loop optical pulse compression and make the output femtosecond optical pulse more stable, this invention places the pulse compression process inside the resonant cavity of the femtosecond fiber laser. However, pulse compression cannot be directly applied to the resonant cavity of a mode-locked fiber laser. This is because the stable evolution of the optical pulse within the resonant cavity requires a balance between dispersion and nonlinear effects. After compression, the peak power of the optical pulse becomes extremely high, approaching megawatt levels, and the spectrum broadens to hundreds of nanometers. At this point, the nonlinear effect far outweighs the dispersion effect, preventing the formation of a stable optical pulse within the laser resonant cavity. To address this issue, after the optical pulse is narrowed by the photonic crystal fiber 16, a first tunable optical filter 18 is added to the resonant cavity. This filter filters out a portion of the broadened spectrum, allowing it to continue propagating within the resonant cavity. The filtered portion exhibits a wider pulse width and lower peak power, thus maintaining the balance between dispersion and nonlinear effects within the resonant cavity and ensuring the femtosecond fiber laser maintains a stable operating state. The principle of spectral filtering is detailed in the attached manual. Figure 5 As shown. (Attached to the instruction manual) Figure 5It is known that during the process of optical pulse compression and spectral broadening, a very large fluctuation occurs near the center wavelength of the spectrum. If this spectrum is filtered out, the time-domain waveform of the resulting optical pulse will not be stable. Therefore, the filtering center wavelength of the first tunable optical filter 18 must be deviated from the center wavelength of the femtosecond fiber laser. Since the amplitude of the broadened spectrum is not smooth but fluctuates, the distance between the center wavelength of the first tunable optical filter 18 and the center wavelength of the femtosecond laser determines the average power of the output femtosecond optical pulse. Accordingly, this invention uses the microcontroller module 1 to control the first tunable optical filter 18 to scan within the range of 1550nm to 1600nm after each adjustment of the output power of the second 976nm pump source 9 or the repetition frequency of the lithium niobate modulator 4 in the femtosecond laser system. Based on the average output power of the laser fed back by the laser energy meter module 25, the wavelength offset that maximizes the average output optical power is found, thereby improving the electro-optical conversion efficiency of the femtosecond fiber laser. Inserting a first tunable optical filter 18 into the resonant cavity of a femtosecond fiber laser causes a shift in the spectral center wavelength of the optical pulses within the resonant cavity, resulting in a change in the repetition frequency of the optical pulses in the time domain. Therefore, this invention introduces a second tunable optical filter 10 into the resonant cavity. The direction of the filter's center wavelength deviation relative to the laser's center wavelength is opposite to that of the first tunable optical filter 18, used to counteract the output optical pulse repetition frequency shift caused by the first tunable optical filter 18. Furthermore, a repetition frequency change rate detection system, composed of a first 10:90 optical coupler 19, a high-speed photoelectric converter 20, an amplitude adaptive adjustment circuit 21, a repetition frequency detection circuit 22, and a repetition frequency change rate detection circuit 23, achieves feedback control of the output optical pulse repetition frequency change rate, improving the stability of the system's output femtosecond optical pulse repetition frequency. Finally, a femtosecond optical pulse capable of long-term stable output is obtained at the 90% output end of the second 10:90 optical coupler 24.

Claims

1. A 1550nm high-stability femtosecond fiber laser, comprising the following structure: the output terminal of the RJ45 network port of a microcontroller module (1) is connected to the input terminal of a microwave signal source module (2); the output terminal of the microwave signal source module (2) is connected to the input terminal of a lithium niobate modulator driver (3); the output terminal of the lithium niobate modulator driver (3) is connected to the modulation input terminal of a lithium niobate modulator (4); the optical output terminal of the lithium niobate modulator (4) is connected to one end of a dispersion-compensating fiber (5); and dispersion compensation... The other end of the optical fiber (5) is connected to the input end of the second optical isolator (6), the output end of the second optical isolator (6) is connected to one end of the second erbium-doped optical fiber (7), the other end of the second erbium-doped optical fiber (7) is connected to the common end of the second wavelength division multiplexer (8), the 980nm end of the second wavelength division multiplexer (8) is connected to the optical output end of the second 976nm pump source (9), and the control input end of the second 976nm pump source (9) is connected to the UART serial port output end of the microcontroller module (1). Its features are, The structure also includes the following: the 1550nm end of the second wavelength division multiplexer (8) is connected to the optical input end of the second tunable optical filter (10); the control input end of the second tunable optical filter (10) is connected to the second USB output end of the microcontroller module (1); the optical output end of the second tunable optical filter (10) is connected to the input end of the first optical isolator (11); the output end of the first optical isolator (11) is connected to one end of the first erbium-doped fiber (12); the other end of the first erbium-doped fiber (12) is connected to the common end of the first wavelength division multiplexer (13); the 980nm end of the first wavelength division multiplexer (13) is connected to the optical output end of the first 976nm pump source (14); and the output optical power of the first 976nm pump source (14) can be preset to [value missing]. A fixed value, therefore its control input terminal does not need to be connected. The 1550nm terminal of the first wavelength division multiplexer (13) is connected to one end of the dispersion delay line (15), the other end of the dispersion delay line (15) is connected to one end of the photonic crystal fiber (16), the other end of the photonic crystal fiber (16) is connected to the input terminal of the 50:50 optical coupler (17), a 50% output terminal of the 50:50 optical coupler (17) is connected to the optical input terminal of the first tunable optical filter (18), the control input terminal of the first tunable optical filter (18) is connected to the first USB output terminal of the microcontroller module (1), the optical output terminal of the first tunable optical filter (18) is connected to the optical input terminal of the lithium niobate modulator (4), and the 50:50 optical coupler (17) The other 50% output terminal is connected to the input terminal of the first 10:90 optocoupler (19), and the 10% output terminal of the first 10:90 optocoupler (19) is connected to the optical input terminal of the high-speed photoelectric converter (20). The SMA interface output terminal of the high-speed photoelectric converter (20) is connected to the SMA interface input terminal of the amplitude adaptive adjustment circuit (21). The Vadjgain-OUT output terminal of the amplitude adaptive adjustment circuit (21) is connected to the Vfdetect-IN input terminal of the repetition frequency detection circuit (22). The repetition frequency detection circuit (22) is connected to the microcontroller module (1) via the SPI bus. The Vfdetect-OUT output terminal of the repetition frequency detection circuit (22) is connected to the repetition frequency change... The Vfvardetect-IN input terminal of the repetition frequency change rate detection circuit (23) is connected, the Vfvardetect-OUT output terminal of the repetition frequency change rate detection circuit (23) is connected to the A / D input pin of the microcontroller module (1), the 90% output terminal of the first 10:90 optocoupler (19) is connected to the input terminal of the second 10:90 optocoupler (24), the 10% output terminal of the second 10:90 optocoupler (24) is connected to the optical input terminal of the laser energy meter module (25), the output terminal of the laser energy meter module (25) is connected to the UART serial port input terminal of the microcontroller module (1), and the 90% output terminal of the second 10:90 optocoupler (24) is the optical pulse output terminal of the femtosecond fiber laser system. The function of the first tunable optical filter (18) is to extract the smooth part of the spectrum and improve the electro-optical conversion efficiency of the fiber laser through feedback control of the average output optical power. The function of the second tunable optical filter (10) is to offset the output optical pulse repetition frequency shift caused by the first tunable optical filter (18) and improve the stability of the output femtosecond optical pulse repetition frequency of the fiber laser through feedback control of the output optical pulse repetition frequency change rate.

2. The 1550nm high-stability femtosecond fiber laser according to claim 1, characterized in that, The amplitude adaptive adjustment circuit (21) has the following circuit structure: pins 2 and 5 (COM1) and pins 10, 11, 12, 14, and 17 (COM2) of chip U1 are grounded; pin 8 (IPBS) is grounded through capacitor C11; pin 9 (CPBS) is grounded through capacitor C12; pin 1 (VPS1) is connected to a +5V power supply and grounded through parallel capacitors C9 and C10; pin 3 (INHI) is connected to the positive terminal of the SMA interface through capacitor C1; and the negative terminal of the SMA interface is connected to... Pin 4 (INLO) is grounded via capacitor C2. Pin 15 (CPLO) is connected to +5V power supply via inductor L2 and grounded via capacitor C7. Pin 13 (VPS2) is connected to +5V power supply and grounded via parallel capacitors C5 and C6. Pins 18-22 (VPS2) and pin 23 (ENBL) are connected to pin 7 (VPOS) of chip U2, to +5V power supply, and grounded via parallel capacitors C3 and C4. Pin 16 (OPHI) is connected to +5V power supply via inductor L1. Connect pin 6 (CP) of chip U3 via capacitor C8; connect pin 1 (INHI) of chip U2 via capacitor C13 to pin 6 (CP) of chip U3, pin 5 (VOUT) to pin 24 (GAIN) of chip U1, pin 6 (TADJ) to ground via resistor R1, pin 8 (INLO) to ground via capacitor C14, pin 4 (VSET) to ground via resistor R2 and connected to a +3.3V power supply via potentiometer W1, pin 3 (CLPF) to ground via capacitor C15, pin 2... The COMM pin is grounded; pins 1, 4, 5, 8, 9, 12, 14 (GND), pin 2 (RP), pin 3 (RN), pin 7 (CN), and pin 10 (QN) of chip U3 are grounded; pins 13 and 16 (VCC) are connected to a +3.3V power supply and grounded through parallel capacitors C17 and C18; pin 15 (VR) is connected to a +3.3V power supply through capacitor C16; and pin 11 (QP) serves as the Vadjgain-OUT output of the amplitude adaptive adjustment circuit (21). Chip U1 is model ADL 5331, chip U2 is model AD8319, and chip U3 is model HMC749.

3. The 1550nm high-stability femtosecond fiber laser according to claim 1, characterized in that, The repetition frequency detection circuit (22) has the following circuit structure: pins 21 and 22 (N / C terminals) and pin 23 (VTUNE terminal) of chip U4 are connected to pin 6 (inverting input) and pin 7 (output) of operational amplifier U5B; pins 24 and 26 (N / C terminals), pin 25 (VCC2 terminal), and pin 27 (VCC1 terminal) are connected to a +3.3V power supply and grounded through parallel capacitors C27 and C28; pins 18-20 (N / C terminals) and pin 17 (CEN terminal) are connected to a +3.3V power supply and grounded through parallel capacitors C19 and C20 and through a resistor. R3 is connected to pin 16 (DVDD3V). Pin 15 (XREEP) is connected to the Vfdetect-IN input of the repetition frequency detection circuit (22) via capacitor C33. Pin 10 (3VRVDD) is connected to a +3.3V power supply and grounded via capacitor C34. Pin 7 (VDDLS) is grounded via capacitor C21, connected to pin 3 (VPPCP), grounded via parallel capacitors C35 and C36, and grounded via series inductor L3 and capacitor C37. Pins 5 and 6 (N / C) are grounded. Pin 1 (AVDD) is connected to a +3.3V power supply and grounded via... The capacitor C22 is connected to pin 40 (BIAS), pin 39 (VCCPD), pin 36 (VCCPS), and pin 35 (VCCHF), and grounded through parallel capacitors C23 to C26. Pins 30 (SEN), 31 (SDI), 32 (SCK), and 33 (SDO) are connected to the four general-purpose I / O pins of the microcontroller module (1) to form an SPI bus for transmitting control data. Pin 4 (CP) is connected to one end of resistor R5 and capacitor C30 through resistor R4. The other end of capacitor C30 is connected to the first pin of operational amplifier U5A. The output terminal of pin 1 and the inverting input terminal of pin 2 are connected to the output terminal of pin 3 of op-amp U5A and the other end of resistor R5 is connected to the non-inverting input terminal of pin 3 of op-amp U5A and grounded through capacitor C29; the output terminal R6 of pin 1 of op-amp U5A is connected to one end of resistor R7 and capacitor C32, the other end of capacitor C32 is connected to the output terminal of pin 7 and the inverting input terminal of pin 6 of op-amp U5B, the other end of resistor R7 is connected to the non-inverting input terminal of pin 5 of op-amp U5B and grounded through capacitor C31; the output terminal of pin 7 of op-amp U5B is used as the Vfdetect-OUT output terminal of the repetition frequency detection circuit (22); The operational amplifiers U5A and U5B are model number AD8042, and the chip U4 is model number HMC1033.

4. A 1550nm high-stability femtosecond fiber laser according to claim 1, characterized in that, The repetition frequency change rate detection circuit (23) has the following circuit structure: the non-inverting input terminal of the 3rd pin of the operational amplifier U6A is grounded through resistor R9 and connected to a +3.3V power supply through resistor R8; the output terminal of the 1st pin is connected to the inverting input terminal of the 2nd pin and connected to the non-inverting input terminal of the 5th pin of the operational amplifier U6B through resistor R11; the inverting input terminal of the 6th pin of the operational amplifier U6B is connected to the Vfvardetect-IN input terminal of the repetition frequency change rate detection circuit (23) through a series resistor R12 and capacitor C38, connected to the output terminal of the 1st pin through a series potentiometer W2 and resistor R10, connected to the output terminal of the 1st pin through capacitor C39, and connected to the positive terminal of Zener diode D1. The negative terminal of Zener diode D1 is connected to the negative terminal of Zener diode D2, and the positive terminal of Zener diode D2 is connected to the output terminal of the 1st pin. The output terminal of the 1st pin serves as the Vfvardetect-OUT output terminal of the repetition frequency change rate detection circuit (23). The op-amps U6A and U6B are model number AD8042.

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