High-speed swept fiber-optic light source

Abstract

This invention discloses a high-speed sweeping fiber optic light source. The specific device includes: a mode-locked pulse light source, an optical fiber amplifier, a highly nonlinear optical fiber, and a dispersion-compensating optical fiber. The passively mode-locked pulse light source outputs a high repetition rate, wide-spectrum mode-locked pulse. Its dispersion propagation follows a parabolic differential equation similar to one-dimensional paraxial diffraction, analogous to Fraunhofer diffraction. The pulse is amplified outside the cavity by an optical fiber amplifier to increase the laser pulse energy and peak power. Then, it is broadened by the highly nonlinear optical fiber to obtain a supercontinuum light source. Finally, the pulse is stretched by a highly dispersive medium provided by the dispersive optical fiber, and the pulse frequency domain spectrum is mapped to a time domain waveform, thereby realizing a high-speed sweeping frequency light source in the time domain.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology, and in particular relates to a high-speed sweeping fiber light source. Background Technology

[0002] Optical coherence tomography (OCT) is a novel method for high-resolution cross-sectional imaging of transparent and semi-transparent samples and tissues, introduced in 1991. In recent years, it has been widely applied in many fields, especially in biomedicine. As a crucial component of OCT systems, swept-frequency light sources have broad development prospects and applications; therefore, the research and development of high-speed swept-frequency light sources is highly feasible and of practical significance.

[0003] The main frequency-sweeping light sources used in OCT systems include: short-cavity lasers, swept-frequency vertical-cavity surface-emitting lasers (VCSELs), and Fourier-domain mode-locked lasers (FTLS). Short-cavity lasers have discrete wavelengths, and their coherence length decreases with increasing scan rate. Increasing the sweep rate degrades laser performance, thus limiting these lasers to sweep rates of a few hundred kHz. Swept-frequency VCSELs are essentially Fabry-Perot filters with integrated gain media; their maximum scan rate is limited by the mechanical properties of the filter driven by the microelectromechanical system (MEMS). Fourier-domain mode-locked lasers are all-fiber lasers, and their sweep rate is limited by the tuning of the FP-tunable filter. The tuning elements in both swept-frequency VCSELs and Fourier-domain mode-locked lasers limit the resonant frequency to below MHz during tuning. None of these frequency-sweeping light sources can produce high-quality OCT imaging at high speeds.

[0004] This invention achieves fiber output of ultrashort laser pulses based on passive mode-locking and realizes time-domain separation of wavelengths by using extracavity dispersion time stretching, thereby realizing high-speed sweep frequency light source output and providing a high-quality sweep frequency light source for OCT systems. Summary of the Invention

[0005] The purpose of this invention is to provide a high-speed sweeping fiber optic light source that outputs a high repetition rate, wide-spectrum mode-locked pulse through a passive mode-locked pulse light source. This pulse is stretched by dispersion time outside the cavity to achieve high-speed sweeping in the time domain.

[0006] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0007] A high-speed sweeping fiber optic light source includes a high-repetition-rate mode-locked pulse output from a mode-locked pulse source, and has a wide spectrum. Its dispersion propagation follows a parabolic differential equation similar to one-dimensional paraxial diffraction, analogous to Fraunhofer diffraction.

[0008] The high repetition rate mode-locked pulse is amplified by an optical fiber amplifier outside the cavity to increase the laser pulse energy and peak power, and then broadened by a highly nonlinear optical fiber to obtain a pulsed supercontinuum light source.

[0009] The pulsed supercontinuum light source provides a pulsed frequency domain spectrum by stretching a highly dispersive medium through a dispersive fiber. The pulsed frequency domain spectrum is then mapped onto a time domain waveform, thereby realizing a time-domain swept light source.

[0010] Furthermore, the mode-locked pulse source includes a laser, which generates a monochromatic laser source as a pump source, which flows into a gain medium through a wavelength division multiplexer (WDM) for gain.

[0011] A polarization-dependent isolator (PS-ISO) is used to suppress backfeedback in order to ensure unidirectional operation of the ring cavity laser;

[0012] The polarization state of the light wave in the laser is adjusted by a polarization controller (PC). When the pulse propagates into the PC and PS-ISO, the pulse is further narrowed in the time domain under the effect of nonlinear polarization rotation (NPR), and the two wings of the frequency domain are also filtered out at the same time.

[0013] Using single-mode fiber (SMF) as the connection, and with the output coupler (OC) coupling ratio selected as 90:10, the laser can achieve self-starting when the pump power exceeds a certain threshold by appropriately adjusting the polarization state of the PC under the NPR effect.

[0014] Furthermore, the laser includes an active mode-locked pulsed laser, a nonlinear polarization rotation mode-locked pulsed laser, a saturable absorber mode-locked pulsed laser, a Fourier transform mode-locked pulsed laser, and a nonlinear fiber ambient pulsed laser.

[0015] Furthermore, the fiber amplifier includes: a rare-earth-doped fiber amplifier, a Raman fiber amplifier, and a semiconductor optical amplifier.

[0016] Furthermore, the highly nonlinear optical fiber includes: thin-core optical fiber, nanowire waveguide, and photonic crystal fiber.

[0017] Furthermore, the dispersive fiber is a medium of sufficient length to provide large dispersion, used for dispersion modulation of highly nonlinear fibers, adjusting the spectral width and flatness of the output pulse supercontinuum, and thus adjusting the sweep frequency range of the sweep frequency light source.

[0018] Furthermore, the dispersion of the dispersive fiber is changed by adjusting its length, which is used to adjust the sweep speed and frequency resolution of the output sweep light source.

[0019] Furthermore, the dispersion fiber includes dispersion-shifting fiber, dispersion-compensating fiber, and single-mode fiber.

[0020] The present invention has the following advantages: (1) The light source is an all-fiber structure with no optical coupling device, and the structure is compact and easy to install and adjust. (2) The spectral width obtained by nonlinear broadening is large, which can realize a wide range of spectral scanning. (3) The frequency sweep range of the light source can be adjusted by the dispersion control of the nonlinear fiber. (4) The frequency sweep speed of the light source can reach the order of hundreds of MHz, which is far superior to traditional frequency sweep light sources. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the device of the present invention.

[0022] Figure 2 This is a schematic diagram of the present invention with a pump light source.

[0023] Figure 3 This is a diagram of the laser output pulse sequence of the present invention.

[0024] Figure 4 This is the spectrum in logarithmic coordinates according to the present invention.

[0025] Figure 5 This is a comparison of the spectrum under linear coordinates and the pulse broadening after dispersion stretching under DFT according to the present invention.

[0026] Figure 6 This is the fundamental frequency spectrum diagram of the present invention.

[0027] Figure 7 This is a spectrum diagram of higher harmonics. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0029] like Figure 1-2As shown, the device structure consists of a pulse seed source, an optical fiber amplifier, a highly nonlinear optical fiber, and a dispersion-compensating optical fiber. The pulse seed source comprises a wavelength division multiplexer (WDM), a gain medium, a polarization-dependent isolator (PS-ISO), a polarization controller, a single-mode optical fiber for connection, and an output coupler. In this example, a monochromatic laser source with a working wavelength of 980 nm is used as the pump source, which is then amplified by a WDM and fed into a 10 m long low-erbium-doped optical fiber. The device uses a polarization-dependent isolator (PS-ISO) to suppress backfeedback, ensuring unidirectional operation of the ring cavity laser. A polarization controller (PC) is used to adjust the polarization state of the light wave in the laser. After the pulse propagates into the PC and PS-ISO, the pulse is further narrowed in the time domain due to the NPR effect, and the two flanks of the frequency domain are also filtered out. The device uses a single-mode optical fiber (SMF) for connection, with a total length of 1.6 m. The output coupler (OC) has a coupling ratio of 90:10. Under the influence of the NPR effect, due to factors such as saturated absorption and self-phase modulation in the laser, the laser can achieve self-starting when the pump power exceeds a certain threshold by appropriately adjusting the polarization state of the pump circuit (PC). In this example, when the laser has run for approximately 1500 revolutions, the energy within the laser reaches the mode-locking condition, achieving NPR mode-locking and forming a stable mode-locked pulse output, resulting in pulse spectrum broadening. At this point, the pump power is 47mW. The output signal is observed using an oscilloscope, and the observed... Figure 3 The pulse sequence shown has a pulse period T of 64.6 ns. The device uses an external fiber amplifier to amplify the output laser pulse power, employs a high nonlinear fiber (HNLF) to enhance the nonlinear effect of the fiber, and uses a 5 km long dispersion-compensating fiber (DCF) to broaden the laser pulse output from the output coupler, thereby realizing the output of a high-speed sweep frequency light source.

[0030] Figure 4 The spectrum is a logarithmic coordinate spectrum measured by a spectrometer, with a 3dB bandwidth of 54.15nm, achieving wide-spectrum mode-locking and a flat mode-locked spectrum; Figure 5 The pulse broadening spectrum is obtained by measuring with an oscilloscope and then performing a Discrete Fourier Transform (DFT). Since the dispersion stretching of the wavelength is linear, the shape of the pulse can correspond to the spectrum under linear coordinates, and the two have roughly the same shape.

[0031] Depend on Figure 6 It is known that the fundamental frequency pulse has a narrow structure, a center frequency of 15.37MHz, and a signal-to-noise ratio (SNR) of over 75dB, exhibiting extremely stable mode-locking; Figure 7It can be seen that the height of the higher harmonic pulses remains consistent within the maximum frequency range, meaning that the laser source has a narrow pulse width, is an ultrashort pulse, and has high energy. In summary, this example demonstrates a high-speed all-fiber sweeping fiber optic source with a sweep frequency of 15.37MHz, a sweep range of 54.15nm, and a sweep accuracy of 0.13nm.

[0032] The preferred embodiments of the present invention disclosed above are only for the purpose of illustrating the present invention, and are intended to better explain the principles and practical applications of the present invention, so that those skilled in the art can better understand and utilize the present invention.

Claims

1. A high-speed swept-frequency fiber optic light source, characterized in that: It includes high-repetition-rate mode-locked pulses output by a mode-locked pulse source, and has a wide spectrum. Its dispersion transmission obeys the parabolic differential equation of Fraunhofer diffraction. The high repetition rate mode-locked pulse is amplified by an optical fiber amplifier outside the cavity to increase the laser pulse energy and peak power, and then broadened by a highly nonlinear optical fiber to obtain a pulsed supercontinuum light source. The pulsed supercontinuum light source provides a pulsed frequency domain spectrum by stretching a highly dispersive medium through a dispersive fiber. The pulsed frequency domain spectrum is then mapped onto a time domain waveform, thereby realizing a time-domain swept light source.

2. The high-speed swept fiber-optic source of claim 1, wherein: The mode-locked pulsed light source includes a laser, which generates a monochromatic laser light source as a pump light source. The laser light source is amplified by a wavelength division multiplexer (WDM) and then coupled through an OC coupler to output the mode-locked pulsed light source. The coupling ratio of the OC coupler is selected as 90:

10.

3. The high-speed swept-frequency fiber optic light source according to claim 2, characterized in that: The coupler (OC) includes a second output terminal, which uses a polarization-dependent isolator (PS-ISO) to suppress backfeedback, thereby ensuring unidirectional operation of the ring cavity laser. A polarization controller (PC) is used to adjust the polarization state of the light wave in the laser. When the pulse propagates into the PC and PS-ISO, the pulse is further narrowed in the time domain under the effect of nonlinear polarization rotation (NPR), and the two wings of the frequency domain are also filtered out at the same time. Using single-mode fiber (SMF) as the connection, under the influence of the NPR effect, by appropriately adjusting the polarization state of the PC, the laser can achieve self-starting when the pump power exceeds a certain threshold.

4. The high-speed swept-frequency fiber optic light source according to claim 2, characterized in that: The lasers include active mode-locked pulsed lasers, nonlinear polarization rotation mode-locked pulsed lasers, saturable absorber mode-locked pulsed lasers, Fourier transform mode-locked pulsed lasers, and nonlinear fiber ambient pulsed lasers.

5. The high-speed swept-frequency fiber optic light source according to claim 1, characterized in that: The fiber amplifiers include: rare-earth-doped fiber amplifiers, Raman fiber amplifiers, and semiconductor optical amplifiers.

6. The high-speed swept-frequency fiber optic light source according to claim 1, characterized in that: The highly nonlinear optical fiber includes: thin-core optical fiber, nanowire waveguide, and photonic crystal fiber.

7. A high-speed swept-frequency fiber optic light source according to claim 1 or 2, characterized in that: The dispersive fiber is a medium of sufficient length to provide large dispersion, used for dispersion modulation of highly nonlinear fibers, adjusting the spectral width and flatness of the output pulse supercontinuum, and thus adjusting the sweep frequency range of the sweep frequency light source.

8. The high-speed swept-frequency fiber optic light source according to claim 7, characterized in that: The dispersion fiber changes the dispersion by adjusting its length, which is used to adjust the sweep speed and frequency resolution of the output sweep light source.

9. A high-speed swept-frequency fiber optic light source according to claim 1, 7, or 8, characterized in that: The dispersion fiber includes dispersion-shifted fiber and dispersion-compensated fiber.

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