Kerr soliton frequency comb generation by tuning the coupling coefficient in coupled nonlinear microcavities.

Kerr soliton frequency comb generation in nonlinear microcavities with compact configurations are promising on-chip sources. Current Kerr comb generation by using a single microcavity with a tunable CW pump laser or high-power femtosecond pulse pump are difficult to be integrated on chip. In this paper, we propose an on-chip soliton comb generation scheme by tuning the coupling coefficient of two coupled microcavities instead of tuning the wavelength of the cw pump laser or using a pulsed pump laser in a single microcavity. The two microcavities are assumed to be identical. We showed by numerical simulation that Kerr comb generation is possible in both the blue and red detuned regions of the main microcavity in the coupled cavity system. We further found that the range and boundary of the soliton generation region of the couple microcavities depend on the coupling coefficient between the coupled cavities. To ensure that the modes being coupled have identical optical paths, we designed a Sagnac loop structure which couples the clockwise and counterclockwise modes in a single microcavity and demonstrated Kerr comb generation in both the blue and red detuned regions by tuning the coupling coefficient. The proposed Kerr comb generation scheme can be utilized for chip-scale integrated soliton comb sources, which will contribute to the development of on-chip applications.

On-Chip Reconfigurable and Ultracompact Silicon Waveguide Mode Converters Based on Nonvolatile Optical Phase Change Materials.

Reconfigurable mode converters are essential components in efficient higher-order mode sources for on-chip multimode applications. We propose an on-chip reconfigurable silicon waveguide mode conversion scheme based on the nonvolatile and low-loss optical phase change material antimony triselenide (Sb2Se3). The key mode conversion region is formed by embedding a tapered Sb2Se3 layer into the silicon waveguide along the propagation direction and further cladding with graphene and aluminum oxide layers as the microheater. The proposed device can achieve the TE0-to-TE1 mode conversion and reconfigurable conversion (no mode conversion) depending on the phase state of embedded Sb2Se3 layer, whereas such function could not be realized according to previous reports. The proposed device length is only 2.3 µm with conversion efficiency (CE) = 97.5%, insertion loss (IL) = 0.2 dB, and mode crosstalk (CT) = -20.5 dB. Furthermore, the proposed device scheme can be extended to achieve other reconfigurable higher-order mode conversions. We believe the proposed reconfigurable mode conversion scheme and related devices could serve as the fundamental building blocks to provide higher-order mode sources for on-chip multimode photonics.

Highly coherent, flat, and broadband time-stretched swept source based on extra-cavity spectral shaping assisted by a booster semiconductor optical amplifier.

We demonstrate a flat broadband time-stretched swept source based on extra-cavity spectral shaping. By adjusting the polarization-dependent gain profile and driving current of the booster optical amplifier (BOA), extra-cavity spectral shaping is optimized to generate output with a 1-dB bandwidth of ∼100 nm, 3-dB bandwidth of ∼140 nm and output power of ∼21.4 mW. The short-term and long-term stabilities are characterized. The average cross correlation of 183,485 round trips is 0.9997 with a standard deviation of 2×10-5, indicating high single-shot spectral similarity and high coherence. The noise floor of relative spectral energy jitter is -141.7 dB/Hz, indicating a high short-term spectral energy stability. The proposed highly stable flat broadband time-stretched swept source is applied to an optical coherence tomography (OCT) system. The axial resolution is 10.8 µm. The proposed swept source can serve as excellent light sources in ultra-fast coherent detection systems for high precision sensing and imaging.

Fourier Domain Mode Locked Laser and Its Applications.

The sweep rate of conventional short-cavity lasers with an intracavity-swept filter is limited by the buildup time of laser signals from spontaneous emissions. The Fourier domain mode-locked (FDML) laser was proposed to overcome the limitations of buildup time by inserting a long fiber delay in the cavity to store the whole swept signal and has attracted much interest in both theoretical and experimental studies. In this review, the theoretical models to understand the dynamics of the FDML laser and the experimental techniques to realize high speed, wide sweep range, long coherence length, high output power and highly stable swept signals in FDML lasers will be discussed. We will then discuss the applications of FDML lasers in optical coherence tomography (OCT), fiber sensing, precision measurement, microwave generation and nonlinear microscopy.

114 nm broadband all-fiber nonlinear polarization rotation mode locked-laser and time-stretch optical coherence tomography.

We propose and demonstrate an all-fiber Er-doped mode-locked laser with a 3-dB spectrum of 114 nm by using nonlinear polarization rotation (NPR), which to the best of our knowledge is the first realization to date of such a broad spectrum without any spatial optical devices. The repetition rate and pulse width of the laser are 183.6 MHz and 3.7 ps, respectively. Such an all-fiber NPR mode-locked laser is then applied in time-stretch optical coherence tomography. The axial resolution is 12.1 µm. The all-fiber high speed broadband swept laser based on the time stretching technique has compact structure and high stability, which is a promising source for frequency metrology and high resolution optical coherence tomography.

Suppression and revival of single-cavity lasing induced by polarization-dependent loss.

For most photonics devices and systems, loss is desperately averted, since it will increase the power consumption and degrade the performance. However, in some non-Hermitian systems, loss can induce a modal gain when the parity-time symmetry is broken, which offers a new way to manipulate the lasing of active cavities. Here we experimentally observe the counterintuitive phenomenon in a single laser cavity assisted by the polarization-dependent loss. A parity-time symmetric system is constituted by the two orthogonally polarized photonic loops in a single laser cavity, which can guarantee the consistency of two coupling loops. The measured output power of the cavity depends on the cross-polarization loss, which reveals virtually opposite relationships before and after the critical point. It provides a novel, to the best of our knowledge, understanding of polarization loss and shows great potential for lasing manipulation in a single cavity with polarization control.

400 MHz ultrafast optical coherence tomography.

An ultrafast time-stretched swept source with a sweep rate of 400 MHz is demonstrated based on the buffering of a 100 MHz femtosecond laser pulse train. To the best of our knowledge, this is the highest sweep rate of swept sources for optical coherence tomography (OCT) that has been reported. With a 10 dB sweep range of ∼100nm, an axial resolution of 19 µm is obtained in the OCT. OCT imaging of high-speed rotating disks is demonstrated. A composite complex apodization method is proposed and demonstrated to enhance the signal to noise ratio in the OCT imaging.

Ultracompact optical fiber acoustic sensors based on a fiber-top spirally-suspended optomechanical microresonator.

Acoustic wave sensors with a high sensitivity and small size are highly desired for a wide variety of important and emerging applications such as photoacoustic gas sensing and bio-imaging. Here we present an ultracompact optical fiber acoustic sensor based on an optomechanical resonator that is directly in situ printed on the end face of a standard single-mode optical fiber by using an optical 3D µ-printing technology. The fiber-top optomechanical microresonator is composed of a microscale suspended polymer micro-disk that forms a Fabry-Perot interferometric cavity, together with the optical fiber end face, and acts as the acoustic wave-sensitive micromechanical resonator simultaneously. The microbeams for suspending the micro-disk are devised with a spiral structure to overcome the small-size imposed low deflection amplitude so as to improve its sensitivity to acoustic waves. The sensor with a high sensitivity of 118.3 mV/Pa and low noise equivalent acoustic signal level of 0.328µPa/Hz1/2 at audio frequency is experimentally demonstrated. Moreover, with a resonance amplification mechanism, the sensitivity can be enhanced by 40.1 times when the frequency of the acoustic wave matches with the natural resonance frequency of the optomechanical resonator. Such an ultrasmall fiber-tip acoustic sensor has not only a miniaturization-induced broad bandwidth, but also a structure-enhanced ultrahigh sensitivity and thus is very promising in various acoustic wave-based sensing, imaging, and testing applications.

Design of a dual-channel modelocked fiber laser that avoids multi-pulsing.

Multi-channel modelocked lasers and their design have attracted much attention. Here, we use the Swift-Hohenberg equation to study dual-channel simultaneous modelocking (DSML) in a fiber laser. When a quartic filter is added to the laser cavity, the stable dual-channel simultaneous modelocking can be obtained for a given filter bandwidth when frequency separation, ωs, is less than a calculated threshold, ωth. When ωs>ωth, a multipulsing instability occurs. We use a linear stability analysis to determine the limit that the multi-pulsing instability imposes on DSML, and we propose a cavity design that avoids the multi-pulsing instability.

Hybrid Graphene-Silicon Based Polarization-Insensitive Electro-Absorption Modulator with High-Modulation Efficiency and Ultra-Broad Bandwidth.

Polarization-insensitive modulation, i.e., overcoming the limit of conventional modulators operating under only a single-polarization state, is desirable for high-capacity on-chip optical interconnects. Here, we propose a hybrid graphene-silicon-based polarization-insensitive electro-absorption modulator (EAM) with high-modulation efficiency and ultra-broad bandwidth. The hybrid graphene-silicon waveguide is formed by leveraging multi-deposited and multi-transferred methods to enable light interaction with graphene layers in its intense field distribution region instead of the commonly used weak cladding region, thus resulting in enhanced light⁻graphene interaction. By optimizing the dimensions of all hybrid graphene-silicon waveguide layers, polarization-insensitive modulation is achieved with a modulation efficiency (ME) of ~1.11 dB/µm for both polarizations (ME discrepancy < 0.006 dB/µm), which outperforms that of previous reports. Based on this excellent modulation performance, we designed a hybrid graphene-silicon-based EAM with a length of only 20 µm. The modulation depth (MD) and insertion loss obtained were higher than 22 dB and lower than 0.23 dB at 1.55 µm, respectively, for both polarizations. Meanwhile, its allowable bandwidth can exceed 300 nm by keeping MD more than 20 dB and MD discrepancy less than 2 dB, simultaneously, and its electrical properties were also analyzed. Therefore, the proposed device can be applied in on-chip optical interconnects.

Deterministic generation of single soliton Kerr frequency comb in microresonators by a single shot pulsed trigger.

Kerr soliton frequency comb generation in monolithic microresonators recently attracted great interests as it enables chip-scale few-cycle pulse generation at microwave rates with smooth octave-spanning spectra for self-referencing. Such versatile platform finds significant applications in dual-comb spectroscopy, low-noise optical frequency synthesis, coherent communication systems, etc. However, it still remains challenging to straightforwardly and deterministically generate and sustain the single-soliton state in microresonators. In this paper, we propose and theoretically demonstrate the excitation of single-soliton Kerr frequency comb by seeding the continuous-wave driven nonlinear microcavity with a pulsed trigger. Unlike the mostly adopted frequency tuning scheme reported so far, we show that an energetic single shot pulse can trigger the single-soliton state deterministically without experiencing any unstable or chaotic states. Neither the pump frequency nor the cavity resonance is required to be tuned. The generated mode-locked single-soliton Kerr comb is robust and insensitive to perturbations. Even when the thermal effect induced by the absorption of the intracavity light is taken into account, the proposed single pulse trigger approach remains valid without requiring any thermal compensation means.

Optical Fiber-Tip Sensors Based on In-Situ µ-Printed Polymer Suspended-Microbeams.

Miniature optical fiber-tip sensors based on directly µ-printed polymer suspended-microbeams are presented. With an in-house optical 3D µ-printing technology, SU-8 suspended-microbeams are fabricated in situ to form Fabry⁻Pérot (FP) micro-interferometers on the end face of standard single-mode optical fiber. Optical reflection spectra of the fabricated FP micro-interferometers are measured and fast Fourier transform is applied to analyze the cavity of micro-interferometers. The applications of the optical fiber-tip sensors for refractive index (RI) sensing and pressure sensing, which showed 917.3 nm/RIU to RI change and 4.29 nm/MPa to pressure change, respectively, are demonstrated in the experiments. The sensors and their optical µ-printing method unveil a new strategy to integrate complicated microcomponents on optical fibers toward 'lab-on-fiber' devices and applications.

Polarized fiber optical parametric amplification in randomly birefringent fibers: erratum.

We correct the value of coefficient Γ presented in Opt. Express23, 32747 (2015)10.1364/OE.23.032747, which describes the relationship between the mean output degree of polarization and the average gain of fiber optical parametric amplifiers.

High-order modulation on a single discrete eigenvalue for optical communications based on nonlinear Fourier transform.

In this paper, we experimentally investigate high-order modulation over a single discrete eigenvalue under the nonlinear Fourier transform (NFT) framework and exploit all degrees of freedom for encoding information. For a fixed eigenvalue, we compare different 4 bit/symbol modulation formats on the spectral amplitude and show that a 2-ring 16-APSK constellation achieves optimal performance. We then study joint spectral phase, spectral magnitude and eigenvalue modulation and found that while modulation on the real part of the eigenvalue induces pulse timing drift and leads to neighboring pulse interactions and nonlinear inter-symbol interference (ISI), it is more bandwidth efficient than modulation on the imaginary part of the eigenvalue in practical settings. We propose a spectral amplitude scaling method to mitigate such nonlinear ISI and demonstrate a record 4 GBaud 16-APSK on the spectral amplitude plus 2-bit eigenvalue modulation (total 6 bit/symbol at 24 Gb/s) transmission over 1000 km.

Experimental generation of discrete ultraviolet wavelength by cascaded intermodal four-wave mixing in a multimode photonic crystal fiber.

In this Letter, we demonstrate experimentally for the first time, to the best of our knowledge, discrete ultraviolet (UV) wavelength generation by cascaded intermodal FWM when femtosecond pump pulses at 800 nm are launched into the deeply normal dispersion region of the fundamental guided mode of a multimode photonic crystal fiber (MPCF). For pump pulses at average input powers of Pav=450, 550, and 650 mW, the first anti-Stokes waves are generated at the visible wavelength of 538.1 nm through intermodal phase matching between the fundamental and second-order guided mode of the MPCF. The first anti-Stokes waves generated then serve as the secondary pump for the next intermodal FWM process. The second anti-Stokes waves in the form of the third-order guided mode are generated at the UV wavelength of 375.8 nm. The maximum output power is above 10 mW for Pav=650 mW. We also confirm that the influences of fiber bending and intermodal walk-offs on the cascaded intermodal FWM-based frequency conversion process are negligible.

Deep-ultraviolet second-harmonic generation by combined degenerate four-wave mixing and surface nonlinearity polarization in photonic crystal fiber.

Deep-ultraviolet (UV) second-harmonics (SHs) have important applications in basic physics and applied sciences. However, it still remains challenging to generate deep-UV SHs especially in optical fibers. Here, for the first time, we experimentally demonstrate the deep-UV SH generations (SHGs) by combined degenerate four-wave mixing (FWM) and surface nonlinearity polarization in an in-house designed and fabricated air-silica photonic crystal fiber (PCF). When femtosecond pump pulses with average input power P av of 650 mW and center wavelength λ p of 810, 820, 830, and 840 nm are coupled into the normal dispersion region close to the zero-dispersion wavelength of the fundamental mode of the PCF, the anti-Stokes waves induced by degenerate FWM process are tunable from 669 to 612 nm. Then, they serve as the secondary pump, and deep-UV SHs are generated within the wavelength range of 334.5 to 306 nm as a result of surface nonlinearity polarization at the core-cladding interface of the PCF. The physical mechanism of the SHGs is confirmed by studying the dependences of the output power P SH of the SHs on the PCF length and time. Finally, we also establish a theoretical model to analyze the SHGs.

Comprehensive analysis of passive generation of parabolic similaritons in tapered hydrogenated amorphous silicon photonic wires.

Parabolic pulses have important applications in both basic and applied sciences, such as high power optical amplification, optical communications, all-optical signal processing, etc. The generation of parabolic similaritons in tapered hydrogenated amorphous silicon photonic wires at telecom (λ ~ 1550 nm) and mid-IR (λ ≥ 2100 nm) wavelengths is demonstrated and analyzed. The self-similar theory of parabolic pulse generation in passive waveguides with increasing nonlinearity is presented. A generalized nonlinear Schrödinger equation is used to describe the coupled dynamics of optical field in the tapered hydrogenated amorphous silicon photonic wires with either decreasing dispersion or increasing nonlinearity. The impacts of length dependent higher-order effects, linear and nonlinear losses including two-photon absorption, and photon-generated free carriers, on the pulse evolutions are characterized. Numerical simulations show that initial Gaussian pulses will evolve into the parabolic pulses in the waveguide taper designed.

Polarization-dependent intermodal four-wave mixing in a birefringent multimode photonic crystal fiber.

In this Letter, polarization-dependent intermodal four-wave mixing (FWM) is demonstrated experimentally in a birefringent multimode photonic crystal fiber (BM-PCF) designed and fabricated in-house. Femtosecond pump pulses at wavelengths ∼800 nm polarized along one of the principal axes of the BM-PCF are coupled into a normal dispersion region away from the zero-dispersion wavelengths of the fundamental guided mode of the BM-PCF. Anti-Stokes and Stokes waves are generated in the 2nd guided mode at visible and near-infrared wavelengths, respectively. For pump pulses at an average input power of 500 mW polarized along the slow axis, the conversion efficiencies ηas and ηs of the anti-Stokes and Stokes waves generated at wavelengths 579.7 and 1290.4 nm are 19% and 14%, respectively. For pump pulses polarized along the fast axis, the corresponding ηas and ηs at 530.4 and 1627 nm are 23% and 18%, respectively. We also observed that fiber bending and intermodal walk-off have a small effect on the polarization-dependent intermodal FWM-based frequency conversion process.

Optofluidic tunable mode-locked fiber laser using a long-period grating integrated microfluidic chip.

An optofluidic tunable mode-locked fiber laser using a microfluidic chip integrated with long-period grating (LPG) is presented. The microfluidic chip enables ultrafine adjustment of the liquid's refractive index and, thus, LPG's spectrum via tuning the mixing ratio of the microfluidic flows. With such an optofluidic spectrum-tunable filter, the central wavelength of the mode-locked laser can be tuned continuously, while the mode-locking state is steadily maintained. The mode-locked pulses are measured with a pulse duration of 0.9 ps and repetition rate of 12.14 MHz, respectively. Moreover, bound solitons with variable soliton separations are experimentally demonstrated.

Rapid, k-space linear wavelength scanning laser source based on recirculating frequency shifter.

We propose and successfully demonstrate a k-space linear and self-clocked wavelength scanning fiber laser source based on recirculating frequency shifting (RFS). The RFS is realized with a high speed electro-optic dual parallel Mach-Zehnder modulator operating at the state of carrier suppressed single sideband modulation. A gated short pulse is injected into an amplified RFS loop to generate the wavelength scanning pulse train. We find that the accumulation of in-band amplified spontaneous emission (ASE) noise over multiple scanning periods will saturate the erbium-doped fiber amplifier and impede the amplification to the pulse signal in the RFS loop. To overcome the degradation of temporal signal due to the accumulation of ASE noise over multiple scanning periods, we insert a modulated optical switch into the RFS loop to completely attenuate the in-band ASE noise at the end of each scanning period. The signal to noise ratio of the temporal pulsed signal is greatly enhanced. K-space linear and self-clocked wavelength scanning fiber laser sources in 6.1 nm/7.2 nm scanning range with 20 GHz/30 GHz frequency shifting are successfully demonstrated.