Sisi Tan

Pulsing Manipulation in a 1.55-

It is difficult to manipulate the pulse distribution of a multipulse passively mode-locked fiber laser, and an effective solution is yet to be demonstrated. Here, we all-optically manipulate the pulsing of a mode-locked fiber ring laser at a telecommunication window (1.5 μm) by an optical pattern at 1 μm. It can be demonstrated as an effective solution by examining different optical patterns modulated on an external laser source. In particular, the minimum pulse separation being manipulated by the external optical pattern can be shorter than 20 ps. It is noted that, with such a large wavelength separation between the cavity and the external sources, i.e., ~1550 and 1060 nm, respectively, our scheme can simultaneously serve as an ultra-wideband wavelength convertor.

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The wavelength sweeping technology has gained its popularity in various research areas for the high resolution and high throughput capabilities. Illuminating with continuously wavelength-swept spectra, traditional spectrally encoded optical systems show low detection sensitivities in either time or spectral domains, due to the optical power divergence. In addition, they can also deliver a nontrivial sampling rate when fast line scan is performed, easily go beyond 50 GS/s, which overwhelms the conventional data processing system. In this paper, we demonstrate a 15-MHz discretely swept source at a bandwidth of ∼70 nm particularly for high-speed spectrally encoded applications. The wideband discretely swept laser exhibits higher peak power, which enhances the detection sensitivity of optical system by more than 3 dB. The discretely sweeping characterization of the proposed laser is also proved to have the potential of reducing the data stream for fast processing without compromising the line-scan rate. It is believed that the efforts made in this paper provide a promising resolution for in situ ultrafast optical diagnosis at a higher sensitivity.

Mode-Locked Fiber Laser by a 1-

The temporally magnified tomography system is further improved in terms of resolution and imaging stability. We simplify the system configuration and improve the axial resolution simultaneously by utilizing a stabilized all-fiber broadband source. The highly stable spectrum of the source assisted by a phase-locked loop guarantees an improved imaging quality. In addition, the impact of the repetition-rate fluctuation of the source to the system stability is analyzed, which also applies to other temporal imaging systems. Achieving a 90-μm in-air resolution at 89-MHz A-scan rate and improved stability, we are taking one major step toward the practical application of this new optical tomographic modality.

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The fiber optical parametric amplifier (FOPA) has been well investigated and widely adopted at the telecommunication window, and outstanding progress has been achieved in areas such as high gain, wide bandwidths, and even flexible gain-spectrum shape. In contrast, a FOPA at the bio-favorable window, 1.0 μm, has been largely underexploited, especially for its relatively limited bandwidth. Here, we demonstrate an all-fiber single-pump FOPA at 1.0 μm with versatile performances, including ultrahigh gain (∼52  dB), wide bandwidth (∼110  nm), and good gain-spectrum flatness (∼3  dB). To showcase the practical applications, the FOPA is utilized to amplify the broadband optical image signal from a spectrally encoded microscopy, yielding a sensitivity enhancement of 47 dB. Thus, it is promising that this all-fiber versatile FOPA works well as an add-on module in boosting sensitivity for existing optical systems at a 1.0 μm window.

Optical Pattern

We report a high-speed wavelength-swept source operating at 2.0 μm through advanced time-stretch technology. It sweeps over 30 nm at a speed of 3.3×109  nm/s and a repetition rate of ∼19  MHz. To the best of our knowledge, this is the first time that a megahertz-stable swept source has been implemented at such a long wavelength. A wide bandwidth is enabled by a simple mode-locked fiber laser that covers a wavelength range of ∼60  nm. The all-optical wavelength sweeping is realized by a chirped fiber Bragg grating (CFBG), which shows a superior temporal stability and power efficiency, compared with commonly used dispersive fibers, particularly in the 2.0 μm wavelength window. To showcase its specialties, here we employ it to perform high-speed spectrally-encoded microscopy (i.e., time-stretch imaging) through a scattering medium at a line-scan rate of up to ∼19  MHz. Better image quality is achieved, compared with a conventional imaging window at 1.0 μm. It is believed that the potential applications of this new high-speed swept source will benefit the transient diagnosis that requires deep penetration through a scattering medium.

An Ultrafast Wideband Discretely Swept Fiber Laser

We proposed a sensitivity enhancement method of the interference-based signal detection approach and applied it on a swept-source optical coherence tomography (SS-OCT) system through all-fiber optical parametric amplifier (FOPA) and parametric balanced detector (BD). The parametric BD was realized by combining the signal and phase conjugated idler band that was newly-generated through FOPA, and specifically by superimposing these two bands at a photodetector. The sensitivity enhancement by FOPA and parametric BD in SS-OCT were demonstrated experimentally. The results show that SS-OCT with FOPA and SS-OCT with parametric BD can provide more than 9 dB and 12 dB sensitivity improvement, respectively, when compared with the conventional SS-OCT in a spectral bandwidth spanning over 76 nm. To further verify and elaborate their sensitivity enhancement, a bio-sample imaging experiment was conducted on loach eyes by conventional SS-OCT setup, SS-OCT with FOPA and parametric BD at different illumination power levels. All these results proved that using FOPA and parametric BD could improve the sensitivity significantly in SS-OCT systems.

Compact and stable temporally magnified tomography using a phase-locked broadband source.

We report an ultrawide C- and L-band erbium-doped fiber ring laser with 92-nm bandwidth centered at 1550 nm and 44.5-MHz repetition rate. Furthermore, it was applied to ultrafast time-stretch microscopy.

110 nm versatile fiber optical parametric amplifier at 1.0 μm.

We experimentally observe dissipative soliton resonance in a thulium-doped fiber laser operating in the anomalous dispersion regime. The laser output exhibits broad quasi-Gaussian spectra (~38 nm) centered at 1970 nm. The system is subsequently applied to spectrally encoded confocal microscopy.

High-speed wavelength-swept source at 20 μm and its application in imaging through a scattering medium

We demonstrate a spectrally encoded confocal microscopy system using a wideband supercontinuum source at a central wavelength of 1870 nm over a bandwidth of 180 nm. It is illustrated to achieve a better depth of field (DOF) and penetration depth at 1.9 μm, compared with the 1.5-μm spectral window when using the same experimental setup. Our scheme shows the potential of optical source operating ~2-μm wavelength range to be applied in other established imaging modalities, such as time-stretch imaging for the imaging of highly scattering materials with a large depth range, where a large DOF and penetration depth are required.

Sensitivity enhancement in swept-source optical coherence tomography by parametric balanced detector and amplifier

A semi-classical model is proposed theoretically and demonstrated experimentally on the optical receiver sensitivity enhancement by single-band (signal or idler) and dual-band (signal and idler) fiber optical parametric amplifier (FOPA). The sensitivity enhancement by single-band is determined by the gain of FOPA and the transmission loss of signal and idler, and it can be further improved by up to 3-dB using amplified signal and phase-conjugated idler together at dual-band configuration. The theoretical results are experimentally verified in both fiber communication and biomedical imaging applications. This detection sensitivity enhancement scheme can be potentially applied in the scenarios where ultrafast broadband signal at low-power level is being handled.