Vol. 25, No. 3 | 6 Feb 2017 | OPTICS EXPRESS 2285
Integration of polarization-multiplexing and phase-shifting in nanometric two dimensional self-mixing measurement YUFENG TAO, WEI XIA, MING WANG,* DONGMEI GUO,
AND
HUI HAO
Jiangsu Key Laboratory on Opto-electronic Technology, Department of Physics Science and Technology, Nanjing Normal University, Nanjing, 210023, China *
[email protected]
Abstract: Integration of phase manipulation and polarization multiplexing was introduced to self-mixing interferometry (SMI) for high-sensitive measurement. Light polarizations were used to increase measuring path number and predict manifold merits for potential applications. Laser source was studied as a microwave-photonic resonator optically-injected by double reflected lights on a two-feedback-factor analytical model. Independent external paths exploited magnesium-oxide doped lithium niobate crystals at perpendicular polarizations to transfer interferometric phases into amplitudes of harmonics. Theoretical resolutions reached angstrom level. By integrating two techniques, this SMI outperformed the conventional single-path SMIs by simultaneous dual-targets measurement on single laser tube with high sensitivity and low speckle noise. In experimental demonstration, by nonlinear filtering method, a custom-made phase-resolved algorithm real-time figured out instantaneous two-dimensional displacements with nanometer resolution. Experimental comparisons to lock-in technique and a commercial Ploytec-5000 laser Doppler velocity meter validated this two-path SMI in micron range without optical cross-talk. Moreover, accuracy subjected to slewing rates of crystals could be flexibly adjusted. © 2017 Optical Society of America OCIS codes: (260.5430) Polarization; (050.5080) Phase shift; (130.3730) Lithium niobate; (060.5625) Radio frequency photonics; (140.1340) Atomic gas lasers.
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#281038 Journal © 2017
https://doi.org/10.1364/OE.25.002285 Received 17 Nov 2016; revised 16 Jan 2017; accepted 16 Jan 2017; published 27 Jan 2017
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1. Introduction Inspired by the seminal work of Lang and Kobayashi [1] at 1980, self-mixing interference (SMI), threshold laser gain was modulated by optical injection reflected by one rough target to alter the laser intensity, spectrum and slope efficiency, became a non-contact cost-effective sensing method with high sensitivity to phase, easy demodulation and compactness. As a
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competitive fitting-to-embedded-situation laser technique, tiny laser diode SMIs in-built with photo diodes had applied from general physical sensing (distance, velocity, vibration, bending, displacement, liquid surface, thickness, pressure, refraction index [2], ranging...) to more interdisciplinary studies (mechanics, chemistry, nanoparticle [3], medical biomedical signal [4], vapor [5], laser micro-drilling [6], terahertz imaging [7], material and even nanometer damping of mechanical cantilever [8]...) in last two decades. Recently, more and more excellent SMIs were reported for single target utilizing diodes, such as differential structural SMI [9], phase shifting SMI [10,11], edge-filter enhanced SMI [12,13], and frequency modulated SMI [14]. Nevertheless, as studied by S. Donati [15], diodes subjected to speckle effect [16] from diffusive surfaces would generated pseudo phase and amplitude as a systematic speckle-pattern random error [17]. Hence, diode SMIs in practical application required Aspheric lens for coupling beam to distant object, or performed in short target-tolaser distance (1m target-tolaser distance under poor reflection condition and the ameliorated phase algorithm spends less than 20 milliseconds to retrieve two-dimensional phases, which is faster and more sensitive than laser wavelength scanning method using expensive optical spectrum analyzers [26]. In arrangement, we firstly expound the basic conception of integrating two techniques. Then, the observed SMI signal containing multi-harmonics distribution in frequency domain is analyzed to confirm theory. Subsequently, a feasible signal processing is presented with nanometric two-dimensional displacement measurement under laboratory room. At last, experimental comparisons to lock-in technique and a laser Doppler velocity meter (LDV) conclude a one-magnitude resolution improvement than diode SMIs [14] with increased path number, which is applicable for real time simultaneous parameters measurement. 2. Principle Significant feature of integration of PM and PS is two independent polarization-based optical paths. The schematic in Fig. 1 is mainly implemented on an inexpensive dual longitudinal mode laser, electro-optic modulators (EOM) at perpendicular polarizations and a polarization beam splitter (PBS). Laser source (Atomic He-Ne gas laser, Class-II) driven by constant current (5mA) emits circularly polarized light (632.8nm) to a common polarization beam splitter (PBS) with 45° incident angle, which also receives back-scattered lights for selfmixing. The used PBS is composed by a pair of rectangular prisms coated with multi-media layers, thus, incident circularly polarized light is decomposed into two linearly polarized lights due to fast axis and slow axis of PBS, o-light and e-light, which are rigidly orthogonal in polarization. In experiment, beam waist radii of o and e lights are both less than 1.2mm even at distance of 1m away making impinged areas on targets much narrower than Gauss beam emitted by diodes, therefore, speckle effect is significantly weakened. In optical path, olight transmits PBS and is back-scattered by moving targeto, e-light is refracted by PBS and back-scattered by targete. EOMs for PS consisted of magnesium-oxide doped lithium niobate crystals offer a wide spectral-transparency window, broadband range (0~0.25GHz), low insertion losses (100MHz), will obtain angstrom-scale resolution theoretically. Compared to frequency-shifting within 100MHz by
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acoustic-optics modulator [25], EOMs (4002nf, Newfocus) cover a wider range from DC to 0.25GHz, if phase shifting is realized by a low-voltage resonant or waveguide EOM, a GHz level phase-shifting yields a much better resolution than lock-in on frequency modulated SMI [18]. 3. Experimental investigation
Experimental investigation is conducted using the Fig. 1 setup. A dual-output, atom-gas, twolongitudinal mode, He-Ne laser works as optical source [31] lasing at 632.8nm. PBS is placed at 10cm next to emission port of laser for splitting out two polarizations. EOMs are installed at perpendicular platforms to keep their intrinsic polarizations consistent to light polarizations respectively. Sinusoidal driven voltages are applied across the sealed crystal electrodes to shift phase through high-voltage drivers (3211, Newport). When measured targets are 3m away, impinged areas are still highlighted points with 1.5mm beam waist radius. Compared with light spot by using diodes, speckle noises are weakened significantly. Therefore, small diffusive angles (
