A time-domain amplification system for mode-locked laser pulses

Abstract

The application discloses a mode-locked laser pulse time domain amplification system, and aims to solve the technical problem that the time domain waveform of an output ultrashort optical pulse of a passively mode-locked fiber laser is difficult to be measured in real time. The system comprises a passively mode-locked fiber laser, a fiber coupler, a photoelectric detector, a band-pass filter, a power divider, a first phase shifter, a second phase shifter, a third phase shifter, a first microwave power amplifier, a second microwave power amplifier, a third microwave power amplifier, a first dispersion compensation fiber, a first electro-optic phase modulator, a second electro-optic phase modulator, a third electro-optic phase modulator and a second dispersion compensation fiber. The system can amplify the time domain waveform of the output ultrashort optical pulse of the passively mode-locked fiber laser by 100-500 times, has the advantages of simple operation, stable performance and convenient parameter control, and can be applied to real-time measurement of the time domain characteristics of a mode-locked laser pulse.

Description

Technical Field

[0001] This invention relates to the measurement of the time-domain characteristics of mode-locked laser pulses, and more specifically to the design of a time-domain amplification system for mode-locked laser pulses. Background Technology

[0002] Passively mode-locked fiber lasers (FLPS) can output ultrashort optical pulses with narrow width, high peak power, and broad spectrum, making them irreplaceable in laser processing, biomedicine, high-speed optical communication, and the generation of optical frequency combs. Measuring the temporal characteristics of mode-locked optical pulses is crucial for the application and research of FLPS and has long been a focus of researchers. With the development of FLPS, increasingly higher demands are placed on the accurate measurement of the temporal characteristics of optical pulses, especially the real-time observation of their transient temporal properties, which remains a challenging problem.

[0003] Traditional measurement methods, such as autocorrelators, can only measure the average time-domain characteristics of the ultrashort pulses output by passively mode-locked fiber lasers over a large number of pulse periods, but cannot observe the transient time-domain evolution of the ultrashort pulses in real time. Although high-speed real-time oscilloscopes can measure the output pulse sequence in real time, their time resolution is limited to tens of picoseconds (ps) due to bandwidth limitations, which is insufficient to resolve the evolution details of ultrashort pulses in the picosecond or femtosecond (fs) range output by passively mode-locked fiber lasers.

[0004] Therefore, we propose a mode-locked laser pulse time-domain amplification system, which amplifies the ultrashort optical pulses output by a passively mode-locked fiber laser by hundreds of times in the time domain, and then realizes the real-time measurement of the time-domain characteristics of the ps or fs mode-locked laser pulses through a real-time oscilloscope. Summary of the Invention

[0005] The purpose of this invention is to solve the technical problem that it is difficult to measure the time-domain waveform of ultrashort pulses on the order of ps or fs output by passively mode-locked fiber lasers in real time, and to propose a time-domain amplification system for mode-locked laser pulses.

[0006] The technical solution of the present invention is as follows: a mode-locked laser pulse time-domain amplification system, comprising a passive mode-locked fiber laser, an optical fiber coupler, a photodetector, a bandpass filter, a power divider, a first phase shifter, a second phase shifter, a third phase shifter, a first microwave power amplifier, a second microwave power amplifier, a third microwave power amplifier, a first dispersion compensation fiber, a first electro-optic phase modulator, a second electro-optic phase modulator, a third electro-optic phase modulator, and a second dispersion compensation fiber.

[0007] The passive mode-locked fiber laser, fiber coupler, first dispersion compensation fiber, first electro-optic phase modulator, second electro-optic phase modulator, third electro-optic phase modulator, and second dispersion compensation fiber are connected in sequence to form the optical path of the system.

[0008] The fiber coupler, photodetector, bandpass filter, and power divider are connected in sequence to generate a cosine radio frequency (RF) signal that is synchronized with the output optical pulse signal of the passively mode-locked fiber laser.

[0009] The power divider, the first phase shifter, the first microwave power amplifier, and the first electro-optic phase modulator are connected in sequence to form a first phase modulation link; the power divider, the second phase shifter, the second microwave power amplifier, and the second electro-optic phase modulator are connected in sequence to form a second phase modulation link; the power divider, the third phase shifter, the third microwave power amplifier, and the third electro-optic phase modulator are connected in sequence to form a third phase modulation link; the first, second, and third phase modulation links are respectively used to apply time-dependent secondary phase modulation to the optical pulse signal passing through the first, second, and third electro-optic phase modulators.

[0010] Preferably, the passively mode-locked fiber laser is an erbium-doped mode-locked fiber laser operating in the net negative dispersion or full negative dispersion region, with an output pulse width on the order of ps or fs, a center wavelength of 1550 nm, and a repetition frequency on the order of MHz.

[0011] Preferably, the fiber optic coupler is a 1×2 coupler with an output coupling ratio of 99:1.

[0012] Preferably, the bandwidth of the photodetector is 50 GHz.

[0013] Preferably, the center frequency of the bandpass filter is 16 GHz and the bandwidth is 6 MHz.

[0014] Preferably, the power divider is a three-way power divider, with the output power ratio of the three output ports being 1:1:1.

[0015] Preferably, the phase shifting range of the first phase shifter, the second phase shifter, and the third phase shifter is 360°, and the phase shifting accuracy is 0.2°.

[0016] Preferably, the first microwave power amplifier, the second microwave power amplifier, and the third microwave power amplifier can amplify the input microwave power to 33dBm.

[0017] Preferably, the dispersion coefficients of both the first dispersion-compensating fiber and the second dispersion-compensating fiber at 1550 nm are 180 ps. 2 / km, wherein the length of the first dispersion compensation fiber is 9.3m, and the length of the second dispersion compensation fiber can be selected between 930 and 4650m.

[0018] Preferably, the half-wave voltage of the first electro-optic phase modulator, the second electro-optic phase modulator, and the third electro-optic phase modulator is 3.5V, and the bandwidth is 20GHz.

[0019] The beneficial effects of this invention are:

[0020] (1) A cosine RF signal synchronously generated by the optical pulse signal output by the passive mode-locked fiber laser enhances the applicability of the system.

[0021] (2) All the components used in this invention are commercially available and easy to purchase, making the system of this invention easy to build;

[0022] (3) The present invention has the advantages of simple operation, stable performance and convenient parameter control. Attached Figure Description

[0023] Figure 1 This invention provides a mode-locked laser pulse time-domain amplification system.

[0024] Figure 2 This is a time-domain shape diagram of the ultrashort pulse output by the passively mode-locked fiber laser according to an embodiment of the present invention.

[0025] Figure 3 The image shows the amplified pulse time-domain shape of the second dispersion compensation fiber when it is 1860m long, as shown in this embodiment of the invention.

[0026] Figure 4 The amplified pulse time-domain shape diagram output through the second dispersion compensation fiber is shown in the embodiment of the present invention when the second dispersion compensation fiber is selected to be 4650m long.

[0027] Explanation of reference numerals in the attached figures: 1—Passive mode-locked fiber laser, 2—Fiber optic coupler, 3—Photodetector, 4—Bandpass filter, 5—Power divider, 6—First phase shifter, 7—Second phase shifter, 8—Third phase shifter, 9—First microwave power amplifier, 10—Second microwave power amplifier, 11—Third microwave power amplifier, 12—First dispersion compensation fiber, 13—First electro-optic phase modulator, 14—Second electro-optic phase modulator, 15—Third electro-optic phase modulator, 16—Second dispersion compensation fiber. Detailed Implementation

[0028] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0029] This invention provides a mode-locked laser pulse time-domain amplification system, such as... Figure 1As shown, the included devices are a passive mode-locked fiber laser, a fiber coupler, a photodetector, a bandpass filter, a power divider, a first phase shifter, a second phase shifter, a third phase shifter, a first microwave power amplifier, a second microwave power amplifier, a third microwave power amplifier, a first dispersion-compensating fiber, a first electro-optic phase modulator, a second electro-optic phase modulator, a third electro-optic phase modulator, and a second dispersion-compensating fiber.

[0030] The passive mode-locked fiber laser, fiber coupler, first dispersion compensation fiber, first electro-optic phase modulator, second electro-optic phase modulator, third electro-optic phase modulator, and second dispersion compensation fiber are connected in sequence to form the optical path of the system.

[0031] The fiber coupler, photodetector, bandpass filter, and power divider are connected in sequence to generate a cosine RF signal that is synchronized with the output optical pulse signal of the passively mode-locked fiber laser.

[0032] The power divider, the first phase shifter, the first microwave power amplifier, and the first electro-optic phase modulator are connected in sequence to form a first phase modulation link; the power divider, the second phase shifter, the second microwave power amplifier, and the second electro-optic phase modulator are connected in sequence to form a second phase modulation link; the power divider, the third phase shifter, the third microwave power amplifier, and the third electro-optic phase modulator are connected in sequence to form a third phase modulation link; the first, second, and third phase modulation links are respectively used to apply time-based secondary phase modulation to the optical pulse signal passing through the first, second, and third electro-optic phase modulators.

[0033] In this embodiment, the passively mode-locked fiber laser is an erbium-doped mode-locked fiber laser operating in the net negative dispersion or full negative dispersion region. Its output pulse width is on the order of ps or fs, the center wavelength is 1550 nm, and the repetition frequency is on the order of MHz.

[0034] In this embodiment, the fiber optic coupler is a 1×2 coupler with an output coupling ratio of 99:1.

[0035] The photodetector in this embodiment can be a photodetector from Finisar, France, with a bandwidth of 50 GHz.

[0036] The bandpass filter in this embodiment can be a bandpass filter from ComMet, with a center frequency of 16 GHz and a bandwidth of 6 MHz.

[0037] In this embodiment, the power divider can be a three-way power divider from Lier Microwave, with the output power ratio of the three output ports being 1:1:1.

[0038] In this embodiment, the phase shift range of the first phase shifter, the second phase shifter, and the third phase shifter is 360°, and the step is 0.2°.

[0039] In this embodiment, the first microwave power amplifier, the second microwave power amplifier, and the third microwave power amplifier can be microwave power amplifiers manufactured by Beijing Jitai Optoelectronic Technology Development Co., Ltd., which amplify the input microwave power to 33dBm.

[0040] In this embodiment, the first and second dispersion compensation fibers can be dispersion compensation fibers manufactured by Yangtze Optical Fibre and Cable, with a dispersion coefficient of 180 ps at 1550 nm. 2 / km, the length of the first dispersion compensation fiber is 9.3m, and the length of the second dispersion compensation fiber can be selected between 930 and 4650m.

[0041] In this embodiment, the first, second, and third electro-optic phase modulators are electro-optic phase modulators (EOSPACE PM-OS5-40-PFA-LV) manufactured by Eospace, with a half-wave voltage of 3.5V and a bandwidth of 20GHz.

[0042] The main working principles involved in this invention are as follows:

[0043] We set the output of the passively mode-locked fiber laser 1 to a hyperbolic secant pulse with a pulse width of 1.5 ps, and its time-domain waveform is as follows. Figure 2 As shown, the fundamental frequency of the pulse is 20MHz. The laser output pulse is split by an optical fiber coupler 2 with an output coupling ratio of 99:1, and its 1% port is input to a photodetector 3 to generate a cosine-type RF signal synchronized with the optical pulse signal. Specifically, the output signal of the photodetector 3 is filtered by a bandpass filter 4 with a center frequency of 16GHz, filtering out a cosine-type RF signal synchronized with the laser output optical pulse signal at the 800th harmonic of the optical pulse fundamental frequency. The bandwidth of the bandpass filter is 6MHz, which is less than the pulse repetition frequency. The 16GHz RF signal obtained after filtering is split into three paths by a power divider 5. The first RF signal passes sequentially through a first phase shifter 6 and a first microwave power amplifier 9 as the RF drive signal for a first electro-optic phase modulator 13; the second RF signal passes sequentially through a second phase shifter 7 and a second microwave power amplifier 10 as the RF drive signal for a second electro-optic phase modulator 14; and the third RF signal passes sequentially through a third phase shifter 8 and a third microwave power amplifier 11 as the RF drive signal for a third electro-optic phase modulator 15. The phase shifter in the system adjusts the initial phase of the cosine RF signal so that the optical pulse signal is positioned at the peak or trough of the cosine signal when it passes through the electro-optic phase modulator, ensuring that the optical pulse signal is subjected to secondary phase modulation in time. The microwave power amplifier is used to amplify the RF signal power to the required 33dBm.

[0044] On the other hand, the 99% end of the fiber coupler 2 is first connected to the first dispersion compensation fiber 12, which has a dispersion coefficient of 180 ps. 2 / km, under the current system parameters, the length of the first dispersion compensation fiber 12 is taken as 9.3m, and the total dispersion D in 1.674ps 2 Let the time-domain expression of the pulsed optical field input to the first dispersion-compensating fiber 12 be A(τ), and the frequency-domain expression be... Therefore, the frequency domain expression of the optical field after passing through the first dispersion-compensating fiber 12 is:

[0045]

[0046] Where g1(ω)=exp(-iaω) 2 Let be the frequency domain response function of the first dispersion-compensating fiber, a = D. in / 2. Subsequently, the optical pulse signal passes sequentially through the first electro-optic phase modulator 13, the second electro-optic phase modulator 14, and the third electro-optic phase modulator 15, applying secondary phase modulation to the time variable of the optical pulse. The frequency domain response function after passing through the three phase modulators is:

[0047]

[0048] In the formula,

[0049]

[0050] Among them, V π V is the half-wave conversion voltage of the phase modulator (i.e., the voltage required for the phase modulator to achieve a π phase shift), N is the number of modulators (equal to 3), and V m and ω m Let the amplitude and angular frequency of the modulation voltage be respectively. Then, the frequency domain expression of the light field after passing through the phase modulator is:

[0051]

[0052] Here, * denotes convolution. It's worth mentioning that cascading three electro-optic phase modulators can triple the phase modulation amplitude compared to a single electro-optic phase modulator, which helps to increase the pulse width range of ultrashort pulses that the optical pulse amplification system can amplify. It's also worth noting that in this scheme, the modulation amplitude can be increased by cascading more phase modulators and appropriately increasing the RF signal power, further expanding the pulse width range of ultrashort pulses that the system can amplify. Finally, the optical pulse signal is broadened through the second dispersion-compensating fiber 16, with a dispersion coefficient of 180 ps. 2 / km, lengths selectable between 930 and 4650m, total dispersion D out Between 167.4 and 837 ps 2Between these points, the frequency domain response function is g2(ω)=exp(-ibω) 2 ), where b = D out / 2, then the output pulse light field of the system is:

[0053]

[0054] In the above formula, if a + c = c 2 / (b+c), that is:

[0055]

[0056] Let the amplification factor of the pulse time-domain amplification system be M:

[0057]

[0058] From equations (5), (6) and (7), we can obtain:

[0059]

[0060] As can be seen from the above formula, the pulse output from the second dispersion-compensating fiber 16 is an image of the pulse input to the first dispersion-compensating fiber 12 magnified M times in time. The pulse envelope shape remains unchanged, only the amplitude changes to that of the pulse input to the first dispersion-compensating fiber 12. The optical pulse amplification system outputs an optical pulse signal that is a time-domain amplified version of the direct output optical pulse signal from a passively mode-locked fiber laser. The amplification factor is the ratio of the dispersion of the second dispersion compensation fiber 16 to the first dispersion compensation fiber 12. Under the current system parameters, the amplification factor is 100–500, and the width of the time-domain amplified optical pulse is 150–750 ps, ​​which can be measured in real time using a real-time oscilloscope.

[0061] Numerical simulations were performed on the principle of the mode-locked laser pulse time-domain amplification system provided by this invention, and the results are as follows:

[0062] Figure 1 This invention provides a mode-locked laser pulse time-domain amplification system.

[0063] Figure 2 The image shows the time-domain shape of the output optical pulse from a passively mode-locked fiber laser. The pulse width is 1.5 ps, and the time-domain shape is a hyperbolic secant.

[0064] Figure 3 The image shows the time-domain shape of the output pulse when the second dispersion-compensating fiber is selected to be 1860m long. At this time, the pulse is a distortion-free amplification of the direct output pulse of the passively mode-locked fiber laser, with an amplification factor of 200 and an output pulse width of 300ps.

[0065] Figure 4 The image shows the time-domain shape of the output pulse when the second dispersion-compensating fiber is 4650m long. The amplification factor is 500, and the output pulse width is 750ps.

[0066] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A mode-locked laser pulse time-domain amplification system, characterized in that, The included devices are a passive mode-locked fiber laser (1), a fiber coupler (2), a photodetector (3), a bandpass filter (4), a power divider (5), a first phase shifter (6), a second phase shifter (7), a third phase shifter (8), a first microwave power amplifier (9), a second microwave power amplifier (10), a third microwave power amplifier (11), a first dispersion compensation fiber (12), a first electro-optic phase modulator (13), a second electro-optic phase modulator (14), a third electro-optic phase modulator (15), and a second dispersion compensation fiber (16). The passive mode-locked fiber laser (1), fiber coupler (2), first dispersion compensation fiber (12), first electro-optic phase modulator (13), second electro-optic phase modulator (14), third electro-optic phase modulator (15) and second dispersion compensation fiber (16) are connected in sequence to form the optical path of the system. The fiber coupler (2), photodetector (3), bandpass filter (4) and power divider (5) are connected in sequence to generate a cosine radio frequency signal that is synchronized with the output optical pulse signal of the passive mode-locked fiber laser (1). The power divider (5), the first phase shifter (6), the first microwave power amplifier (9), and the first electro-optic phase modulator (13) are connected in sequence to form a first phase modulation link; the power divider (5), the second phase shifter (7), the second microwave power amplifier (10), and the second electro-optic phase modulator (14) are connected in sequence to form a second phase modulation link; the power divider (5), the third phase shifter (8), the third microwave power amplifier (11), and the third electro-optic phase modulator (15) are connected in sequence to form a third phase modulation link; the first, second, and third phase modulation links are used to apply time-based secondary phase modulation to the optical pulse signal passing through the first electro-optic phase modulator (13), the second electro-optic phase modulator (14), and the third electro-optic phase modulator (15).

2. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The passive mode-locked fiber laser (1) outputs a pulse with a center wavelength of 1550 nm, a repetition rate in the MHz range, and a pulse width in the picosecond or femtosecond range.

3. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The fiber optic coupler (2) is a 1×2 coupler with an output coupling ratio of 99:

1.

4. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The bandwidth of the photodetector (3) is 50 GHz.

5. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The center frequency of the bandpass filter (4) is 16 GHz and the bandwidth is 6 MHz.

6. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The power divider (5) is a three-way power divider, with the output power ratio of the three output ports being 1:1:

1.

7. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The phase shift range of the first phase shifter (6), the second phase shifter (7) and the third phase shifter (8) is 360° and the phase shift accuracy is 0.2°.

8. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The first microwave power amplifier (9), the second microwave power amplifier (10), and the third microwave power amplifier (11) can amplify the input microwave power to 33dBm.

9. The mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The dispersion coefficients of both the first dispersion-compensating fiber (12) and the second dispersion-compensating fiber (16) at 1550 nm are 180 ps. 2 / km.

10. A mode-locked laser pulse time-domain amplification system according to claim 1, characterized in that, The first electro-optic phase modulator (13), the second electro-optic phase modulator (14), and the third electro-optic phase modulator (15) have a half-wave voltage of 3.5V and a bandwidth of 20GHz.

Images