Figure - available from: Optics Letters
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(a) Carrier signal of the optical heterodyne setup; (b) FFS of the carrier signal; (c) background noise spectrum of the optical heterodyne setup by demodulating the carrier signal.
Source publication
We present an all-fiber frequency shifter (AFFS) consisting of a fiber Bragg grating (FBG) modulated via an acoustic flexural wave for optical heterodyne measurement. The AFFS can efficiently generate the frequency-shifted signal due to the resonance peak with a high-reflection efficiency and being completely separated from the reflection spectrum...
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Acoustic source emission rate is generally low at low frequencies. In this work, we propose a simple design of ‘LEGO’-type acoustic metamaterial that can significantly enhance the low frequency emission rate of an acoustic monopole source. Such enhancement is resulted from the coupling between resonances of a cavity and a dual grating comprised of...
Citations
... However, lower frequency shifts are a challenge in some application scenarios, such as laser feedback interference technology [17], where two AOMs with different shifts are used to achieve a small frequency-shift amount, leading to high cost and complexity. Recently, several AOFSs in fibers with several MHz have been reported that usually involve AOI by tapering and etching the fiber based on an evanescent wave [18,19] or cladding mode coupling [20]. ...
We demonstrate two all-fiber low-frequency shift schemes based on the acousto–optic interaction in a few-mode fiber (FMF). Two acoustically induced fiber gratings (AIFGs) are cascaded in reverse to achieve an efficient cycle conversion between LP11 and LP01 core modes in the FMF while obtaining a frequency shift of 1.8 MHz. In addition, a long-period fiber grating (LPFG) is employed to replace the AIFG, which achieves a lower frequency shift of 0.9 MHz, and its tunable wavelength range exceeds 100 nm. Both schemes show the characteristics of an upward frequency shift. Moreover, we also present a heterodyne detection system based on the above frequency shift schemes, which is verified in response to micro-vibration signals ranging from tens to hundreds of kilohertz, as well as speech signals in a lower frequency range. The experimental results show that these all-fiber frequency shift schemes have potential applications, such as in fiber optic hydrophones, laser speech detection, and fiber optic sensors.
... The first appearance of an AIFG regarding frequency-shifting experiment was implemented by Kim et al. in 1986 [22]. Since then, AIFGs have been widely used in fields such as high-order mode coupling [23], vortex beam generation [24] and optical heterodyne detection [25]. As shown in Fig. 1(a), the AIFG is composed of a piece of FMF, aluminum horn and piezoelectric transducer (PZT). ...
We demonstrate a programmable high-order mode control method that can be implemented in high-power fiber lasers. 2 W average-power mode-locked pulses are obtained based on a mode-locked fiber laser working in dissipative soliton resonance regime. The fundamental mode (LP01) is fully or partially converted to the high-order modes (LP11a/b) via an acoustically-induced fiber grating. The mode-superposition fields are recorded using an optical 4f system, and mode components are subsequently analyzed by a mode decomposition algorithm. Our experiments suggest that the mode patterns are stable and dynamically switchable. The method is expected to possess good application value in optical tweezers, fiber communication, laser material processing and other research fields.
... We have previously developed a new type of fiber-based acousto-optic frequency shifter (AOFS) by employing a fiber Bragg grating (FBG) under modulation of traveling acoustic wave for heterodyne displacement measurement with subnanometer accuracy [19]. The acoustic transducer cone tip was bonded to the end facet of the FBG fiber, resulting in a robust structure. ...
... Reflection at peak A involves coupling between LP 01 and LP 31 modes, i.e., n cl = 1.4603 and m = 2. The frequency shift results from the ALPG in an amount of mf sig due to the Doppler effect, i.e., f ac = 2f sig = 2.103 MHz and f sig at peak A and peak B, respectively [19]. In order to ensure high power transmission of the acoustic wave on the fiber, the grating region of FBG1 was etched in HF acid to reduce the fiber diameter to 30 μm, as denoted in the circular inset of Fig. 1(a). ...
An all-fiber frequency shifted feedback laser is developed to export mode-locked pulses with a narrow spectral bandwidth (238 MHz). A simple cavity structure is established by employing a fiber Bragg grating-based acousto-optic frequency shifter. An exotic reflection peak in a large spectral separation from the Bragg reflection peak is generated by applying flexural acoustic waves, and is utilized for narrow-band filtering and frequency shifting in the laser cavity. Stable mode-locked pulse output can be obtained with the cavity length as short as 16.5 m, whereas increasing the cavity length can reduce the pump power to 37 mW for mode-locked pulse output. A maximum power conversion efficiency of 17% with an average output power of 103 mW were obtained experimentally at a pump power of 600 mW, with corresponding pulse width, pulse energy and peak power of 2 ns, 88 nJ and 4.5 W, respectively. A time-bandwidth product of mode locked pulses of 0.476 denotes nearly Fourier-transform-limited performance. This work provides a new technique to implement efficient, compact and low-cost all-fiber FSFLs for narrow spectral bandwidth mode-locked pulse output.
... A typical longitudinal acoustic wave was applied to modulate a fiber Bragg grating (FBG) and induced an all-fiber acousto-optic super-lattice modulation structure [17][18][19][20], in which the input light could be reflected by the modulated narrow side band of the FBG and be converted to the backward core mode with a frequency shift. To achieve reasonably good efficiency, the FBG used had to be up to 50 mm, which was difficult to prepare and therefore expensive [21,22]. An acoustic flexible wave was applied universally in an all-fiber acoustic-optic structure [23][24][25] because of its simple structure and high efficiency. ...
An all-fiber acousto-optic frequency shifter (AOFS) based on an acousto-optic tunable filter (AOTF) cascaded with a packaged tapered fiber (TF)-coupled microsphere was proposed and demonstrated in both theory and experiment. The configuration has the advantages of easy alignment, robustness, compact size, and low cost, which will improve its further application, such as in the optical heterodyne detection technique (OHDT).
... Besides, as an alternative to advanced digital signal processing algorithms that have the disadvantages of complex nonlinear digital operation and incomplete signal-to-signal beating interference (SSBI) mitigation, such as Kramers-Kronig receiver [12] and SSBI iterative cancellation [13], the mode-selective coupler (MSC) can achieve the SSBI cancellation in optical domain [14]. Recently, W Zhang proposed two kinds of all-fiber frequency shifter to construct the optical heterodyne setup [15]- [17] with high measurement accuracy. But these schemes need the use of the evanescent wave coupling technique which may decrease the stability of the optical heterodyne system, or need an accurate adjustment of the incident wavelength to match the resonant peak. ...
We experimentally obtain a lower frequency shift based on mode conversion by using a mode-selective coupler (MSC) and an acoustically induced fiber grating (AIFG) for optical heterodyne micro-vibration measurement. Through efficient conversion between LP
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and LP
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core modes in a few-mode fiber, a mode conversion frequency shifter (MCFS) can be achieved and the magnitude of frequency shift is several hundred kHz, which is generally challenging for traditional bulk acousto-optic frequency shifter. Moreover, the signal-to-signal beating interference (SSBI) can be eliminated by using the MCFS without requirement of guard band. The scheme of optical heterodyne micro-vibration measurement based on the MCFS has a high-quality signal-to-noise ratio (SNR) over 85 dB. An all-fiber Bragg grating (FBG) scheme is also designed to improve the measurement sensitivity of heterodyne detection, which has a measurable vibration frequency range of 1 Hz to 300 kHz and minimal detectable amplitude of 0.019 nm. The all-fiber MCFS has the advantages of compactness, easy integration and can be applied directly to satellite ground station and laser feedback tomography.
Flexural acoustic waves can induce optical reflection peaks on one side of the Bragg wavelength in the reflection spectrum of a fiber Bragg grating, thus laying a cornerstone of active acoustic-optic fiber devices. The acoustically-induced refractive index distribution of FBG is deduced. The mechanism of forming acoustically-induced reflection peaks is investigated using mode coupling theory in vector mode presentation. A precise picture is depicted with the selection rules for mode coupling and observation of phase matching, such that the two-step and the three-step processes for acoustically-induced reflection peaks are identified with experimental confirmation. While the three-step coupling theory is discovered, our theoretical framework also refines the former scalar-mode two-step coupling theory. Empowering the theoretical prediction to perfectly match experiment, this work will facilitate the design and application of novel active fiber-optic devices based on acoustic-optic interactions.
