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  • 1
    Language: English
    In: Applied optics, 10 February 2018, Vol.57(5), pp.1247-1253
    Description: A precise 3D surface measurement method for large stepped structures without height ambiguity is proposed based on optical-frequency-comb-referenced frequency-sweeping interferometry and Fourier-transformed fractional phase retrieval. Unlike other interferometry that depends on the absolute phase value for several certain wavelengths, this method obtains results from the phase change during frequency sweeping and thus remains free from the confined non-ambiguity range. By reference to an optical frequency comb, the relative uncertainty from the tunable laser frequency was reduced by three orders of magnitude, and the sweeping frequency range can be precisely determined. Besides, the fractional phase can be rapidly retrieved in only one step using a Fourier transform method, with advantages of high accuracy and immunity to light intensity fluctuation and mechanical vibration noise. Samples of step heights from 1 μm to 1 mm were measured, and the standard uncertainty was 45 nm. This permits applications such as quality assurance in microelectronics production and micro-electro-mechanical system (MEMS) manufacture.
    Keywords: Sweeping ; Immunity ; Phase Retrieval ; Interferometry ; Tunable Lasers ; Fourier Transforms ; Luminous Intensity ; Quality Assurance ; Optical Frequency ; Variations ; Interferometry ; Ambiguity ; Uncertainty ; Fourier Transforms ; Phase Transitions;
    ISSN: 1559128X
    E-ISSN: 1539-4522
    E-ISSN: 21553165
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  • 2
    Language: English
    In: Applied Optics, March 1, Vol.54(7), p.1681(7)
    Description: We present a hybrid mode-locked Er-fiber oscillator whose repetition rate can be tuned from 101.3 to 103.0 MHz. By incorporating saturable absorption and nonlinear polarization evolution in a ring cavity, mode-locked states can be easily accessed with a 3 dB spectral range wider than 50 nm. In addition, by the combination of a computer-controlled optical delay line and a piezo actuator to change the length of the cavity, the repetition rate can be stabilized at any frequency in the tunable range for the long term. When phase locking to an Rb clock for 14 h continuously, the standard deviation of the fundamental repetition rate is 0.2 mHz with a gate time of 1 s. In addition, the feedback bandwidth can be extended to ~400??Hz by stabilizing the 10th harmonic instead of the fundamental repetition rate. Furthermore, the linewidth of the comb modes is investigated by beating with a wavelength-tunable diode laser.
    Keywords: Oscillations – Measurement ; Absorption – Analysis ; Polarization (Light) – Analysis ; Wavelength – Measurement
    ISSN: 1559-128X
    E-ISSN: 21553165
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  • 3
    Language: English
    In: Applied optics, 01 December 2016, Vol.55(34), pp.D29-D34
    Description: We present an optically stabilized Erbium fiber frequency comb with a broad repetition rate tuning range based on a hybrid mode-locked oscillator. We lock two comb modes to narrow-linewidth reference lasers in turn to investigate the best performance of control loops. The control bandwidth of fast and slow piezoelectric transducers reaches 70 kHz, while that of pump current modulation with phase-lead compensation is extended to 32 kHz, exceeding laser intrinsic response. Eventually, simultaneous lock of both loops is realized to totally phase-stabilize the comb, which will facilitate precision dual-comb spectroscopy, laser ranging, and timing distribution. In addition, a 1.8-MHz span of the repetition rate is achieved by an automatic optical delay line that is helpful in manufacturing a secondary comb with a similar repetition rate. The oscillator is housed in a homemade temperature-controlled box with an accuracy of ±0.02  K, which not only keeps high signal-to-noise ratio of the beat notes with reference lasers, but also guarantees self-starting at the same mode-locking every time.
    Keywords: Repetition ; Optical Fibers ; Locks ; Tuning ; Pumps ; Oscillators ; Lasers ; Erbium ; Lasers, Optics, and Electronics (So) ; Optics (Ea) ; Optics (Ah);
    ISSN: 1559128X
    E-ISSN: 1539-4522
    E-ISSN: 21553165
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  • 4
    Language: English
    In: Applied optics, 10 August 2016, Vol.55(23), pp.6321-30
    Description: Dual-comb spectroscopy (DCS) has shown unparalleled advantages but at the cost of highly mutual coherence between comb lasers. Here, we investigate spectral degradation induced by laser frequency instabilities and improvement benefited from active laser stabilization. Mathematical models of DCS in the cases of direct radio-frequency (RF) locking and optical phase stabilization were separately established first. Numerical simulations are utilized to study the impact of laser intrinsic stability and the improvement by different locking strategies on spectral performance in the following. Finally, both simulations are proven by corresponding experiments. It shows that an optically phase-stabilized system owns a better immunity of laser frequency fluctuations than a direct RF-stabilized one. Furthermore, the performance improvement by the feedback servos is also more effective in the optically phase-stabilized system. In addition, the simulations could instruct optimal design and system improvement.
    Keywords: Mathematical Models ; Computer Simulation ; Stabilization ; Locking ; Lasers ; Stability ; Spectroscopy ; Spectra ; Lasers, Optics, and Electronics (So) ; Optics (Ea) ; Optics (Ah);
    ISSN: 1559128X
    E-ISSN: 1539-4522
    E-ISSN: 21553165
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  • 5
    Language: English
    In: Applied Optics, Feb 10, 2018, Vol.57(5), p.1247(7)
    Description: A precise 3D surface measurement method for large stepped structures without height ambiguity is proposed based on optical-frequency-comb-referenced frequency-sweeping interferometry and Fourier-transformed fractional phase retrieval. Unlike other interferometry that depends on the absolute phase value for several certain wavelengths, this method obtains results from the phase change during frequency sweeping and thus remains free from the confined non-ambiguity range. By reference to an optical frequency comb, the relative uncertainty from the tunable laser frequency was reduced by three orders of magnitude, and the sweeping frequency range can be precisely determined. Besides, the fractional phase can be rapidly retrieved in only one step using a Fourier transform method, with advantages of high accuracy and immunity to light intensity fluctuation and mechanical vibration noise. Samples of step heights from 1 ?m to 1 mm were measured, and the standard uncertainty was 45 nm. This permits applications such as quality assurance in microelectronics production and micro-electro-mechanical system (MEMS) manufacture.
    Keywords: Interferometry – Usage ; Wavelength – Research
    ISSN: 1559-128X
    Source: Cengage Learning, Inc.
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