Phase noise characterization of a QD-based diode laser frequency comb.

We measure, simultaneously, the phases of a large set of comb lines from a passively mode locked, InAs/InP, quantum dot laser frequency comb (QDLFC) by comparing the lines to a stable comb reference using multi-heterodyne coherent detection. Simultaneity permits the separation of differential and common mode phase noise and a straightforward determination of the wavelength corresponding to the minimum width of the comb line. We find that the common mode and differential phases are uncorrelated, and measure for the first time for a QDLFC that the intrinsic differential-mode phase (IDMP) between adjacent subcarriers is substantially the same for all subcarrier pairs. The latter observation supports an interpretation of 4.4ps as the standard deviation of IDMP on a 200µs time interval for this laser.

All-optical short pulse translation through cross-phase modulation in a VO2 thin film.

VO2 is a promising material for reconfigurable photonic devices due to the ultrafast changes in electronic and optical properties associated with its dielectric-to-metal phase transition. Based on a fiber-optic, pump-probe setup at 1550 nm wavelength window, and by varying the pump-pulse duration, we show that the material phase transition is primarily caused by the pump-pulse energy. For the first time, we demonstrate that the instantaneous optical phase modulation of probe during pump leading edge can be utilized to create short optical pulses at probe wavelength, through optical frequency discrimination. This circumvents the impact of long recovery time well known for the phase transition of VO2.

Nanophotonic modal dichroism: mode-multiplexed modulators.

As the diffraction limit is approached, device miniaturization to integrate more functionality per area becomes more and more challenging. Here we propose a strategy to increase the functionality-per-area by exploiting the modal properties of a waveguide system. With such an approach the design of a mode-multiplexed nanophotonic modulator relying on the mode-selective absorption of a patterned indium-tin-oxide (ITO) is proposed. Full-wave simulations of a device operating at the telecom wavelength of 1550 nm show that two modes can be independently modulated, while maintaining performances in line with conventional single-mode ITO modulators reported in the recent literature. The proposed design principles can pave the way to a class of mode-multiplexed compact photonic devices able to effectively multiply the functionality-per-area in integrated photonic systems.

Near-infrared electro-optic modulator based on plasmonic graphene.

We propose a novel scheme for an electro-optic modulator based on plasmonically enhanced graphene. As opposed to previously reported designs where the switchable absorption of graphene itself was employed for modulation, here a graphene monolayer is used to actively tune the plasmonic resonance condition through the modification of interaction between optical field and an indium tin oxide (ITO) plasmonic structure. Strong plasmonic resonance in the near infrared wavelength region can be supported by accurate design of ITO structures, and tuning the graphene chemical potential through electrical gating switches on and off the ITO plasmonic resonance. This provides much increased electro-absorption efficiency as compared to systems relying only on the tunable absorption of the graphene.

Bandwidth efficient coherent lidar based on phase-diversity detection.

Bandwidth efficient coherent lidar based on phase-diversity detection is reported for the first time, to the best of our knowledge, which allows the doubling of bandwidth efficiency through the simultaneous utilization of the in-phase (I) and quadrature (Q) components. By maintaining RF phase continuity between linearly frequency-chirped I and Q components through digital signal processing, the range resolution of the lidar system can be improved.

Fiber-optic acoustic pressure sensor based on large-area nanolayer silver diaghragm.

A fiber-optic acoustic pressure sensor based on a large-area nanolayer silver diaphragm is demonstrated with a high dynamic pressure sensitivity of 160 nm/Pa at 4 kHz frequency. The sensor exhibits a noise limited detectable pressure level of 14.5 µPa/Hz(1/2). Its high dynamic pressure sensitivity and simple fabrication process make it an attractive tool for acoustic sensing and photo-acoustic spectroscopy.

Femtosecond multi-beam interference lithography based on dynamic wavefront engineering.

A method for precise multi-spot parallel ultrafast laser material structuring is presented based on multi-beam interference generated by dynamic spatial phase engineering. A Spatial Light Modulator (SLM) and digitally programming of phase masks are used to accomplish the function of a multi-facet pyramid lens, so that the laser beam can be spatially modulated to create beam multiplexing and desired two-dimensional (2D) multi-beam interference patterns. Various periodic microstructures on metallic alloy surfaces are fabricated with this technique. A method of preparing extended scale periodic microstructures by loading dynamic time-varying phases is also demonstrated. Scanning electron microscopy (SEM) reveals the period and morphology of the microstructures created using this technique. The asymmetry of interference modes generated from the beams with asymmetric wave vector distributions is equally explored. The flexibility of programming the period of the microstructures is demonstrated.

Single fiber laser based wavelength tunable excitation for CRS spectroscopy.

We demonstrate coherent Raman spectroscopy (CRS) using a tunable excitation source based on a single femtosecond fiber laser. The frequency difference between the pump and the Stokes pulses was generated by soliton self-frequency shifting (SSFS) in a nonlinear optical fiber. Spectra of C-H stretches of cyclohexane were measured simultaneously by stimulated Raman gain (SRG) and coherent anti-Stokes Raman scattering (CARS) and compared. We demonstrate the use of spectral focusing through pulse chirping to improve CRS spectral resolution. We analyze the impact of pulse stretching on the reduction of power efficiency for CARS and SRG. Due to chromatic dispersion in the fiber-optic system, the differential pulse delay is a function of Stokes wavelength. This differential delay has to be accounted for when performing spectroscopy in which the Stokes wavelength needs to be scanned. CARS and SRG signals were collected and displayed in two dimensions as a function of both the time delay between chirped pulses and the Stokes wavelength, and we demonstrate how to find the stimulated Raman spectrum from the two-dimensional plots. Strategies of system optimization consideration are discussed in terms of practical applications.

Embedded optical waveguides fabricated in SF10 glass by low-repetition-rate ultrafast laser.

Symmetric embedded waveguides were fabricated in heavy metal oxide SF10 glass using slit-shaped infrared femtosecond laser writing in the low-repetition frequency regime. The impact of the writing parameters on the waveguide formation in the transverse writing scheme was systemically studied. Results indicate that efficient waveguides can be inscribed in a wide parameter space ranging from 500 fs to 1.5 ps pulse duration, 0.7-4.2 µJ pulse energy, and 5 µm/s to 640 µm/s scan speed and pointing out the robustness of the photoinscription process. The refractive index profile reconstructed from the measured near field pattern goes up to 10(-3). In addition, propagation losses of the waveguides are tolerable, with the lowest propagation loss estimated at 0.7 dB/cm. With a 5 µm/s scan speed and 3.5 µJ pulse energy in a high-dose regime, few-mode guiding was achieved in the waveguide at 800 nm signal injection wavelength. This is due to a combination of increased refractive index in the core of the trace and the appearance of a depressed cladding.

Complex-optical-field lidar system for range and vector velocity measurement.

A coherent lidar system based on the measurement of complex optical field is demonstrated for the first time. An electro-optic in-phase/quadrature (I/Q) modulator is used in the lidar transmitter to realize carrier-suppressed complex optical field modulation in which the positive and the negative optical sidebands can carry independent modulation waveforms. A fiber-optic 90° hybrid is used in the lidar receiver for coherent heterodyne detection and to recover the complex optical field. By loading a constant modulation frequency on the lower optical sideband and a wideband linear frequency chirp on the upper sideband, vector velocity and target distance can be measured independently. The wide modulation bandwidth of this lidar system also enabled unprecedented range resolution and the capability of measuring high velocity unambiguously.

Polarization behavior of femtosecond laser written optical waveguides in Ti:Sapphire.

Ultrashort pulsed laser photoinscription of Ti:Sapphire crystals may result in the self-organization of nanoscale material redistribution regions in regular patterns within the laser trace and stress-induced birefringence around the laser trace. We report on the formation of anisotropic optical waveguides in Ti:Sapphire by a procedure that involves femtosecond laser inscription of adjacent nonguiding birefringent traces with nanopatterned crosssections and the accumulation of stress birefringence in the region between. Double parallel line structures with a separation of 25µm with vertical and horizontal nanoscale arrangements were written with a choice of orthogonal polarizations. Due to anisotropic light scattering on periodic nanostructures and stress-induced birefringence in the central zone, remarkable polarization dependent guiding effects were observed as a function of the microscopic geometry of the structures. Building on this polarization sensitivity, several structure such as 3 × 3 waveguide arrays, diamond and hexagon patterns are also investigated.

Stressed waveguides with tubular depressed-cladding inscribed in phosphate glasses by femtosecond hollow laser beams.

We report on the single-step fabrication of stressed optical waveguides with tubular depressed-refractive-index cladding in phosphate glasses by the use of focused femtosecond hollow laser beams. Tubelike low index regions appear under direct exposure due to material rarefaction following expansion. Strained compacted zones emerged in domains neighboring the tubular track of lower refractive index, and waveguiding occurs mainly within the tube core fabricated by the engineered femtosecond laser beam. The refractive index profile of the optical waveguide was reconstructed from the measured transmitted near-field intensity.

Digital subcarrier multiplexing for flexible spectral allocation in optical transport network.

We demonstrate a spectrally efficient digital subcarrier multiplexed (DSCM) coherent optical system for optical transport network. In the proposed system, mutually orthogonal subcarrier channels are digitally generated, which allows a high degree of flexibility in bandwidth allocation and scalability in data rate granularity. The receiver can also dynamically change the number of DSCM channels for detection without changing the system configuration. We experimentally investigate the transmission performance of a 22.2 Gb/s DSCM system with 10 subcarrier channels using QPSK modulation. The impacts of channel spacing and time mismatch between subcarrier channels are also explored.

Theoretical and experimental investigation of compatible SSB modulation for single channel long-distance optical OFDM transmission.

We present the first experimental demonstration of compatible single-sideband (compatible-SSB) modulated OFDM optical system at 11.1Gb/s data rate for long distance transmission over 675 km uncompensated standard single-mode fiber. Compatible-SSB modulation employing a simple dual-drive Mach-Zehnder modulator (MZM) in the transmitter and direct-detection at the receiver provides an OFDM system implementation with reduced complexity. It does not require a guard-band between the carrier and the OFDM sideband and so makes full use of available digital-to-analog converter (DAC) bandwidth. We demonstrate digital pre-compensation applied in the transmitter to correct MZM nonlinear transfer function and to improve the system transmission performance. We show that optimum modulation index value decreases with increasing transmission distance. We have also investigated the impact of self-phase modulation (SPM) on the system performance, and show that a compatible-SSB modulated system is less vulnerable to SPM in comparison to the conventional offset-SSB modulation at the same data rate.

OFDM system implementation using compatible SSB modulation with a dual-electrode MZM.

Single-sideband (SSB) direct-detection optical orthogonal frequency division multiplexing (OFDM) transmission is experimentally demonstrated in a system with 125 km fiber length, which does not need a guard band between the carrier and the OFDM band. The waveforms of compatible SSB modulation are analytically derived and realized through a single dual-electrode Mach-Zehnder modulator. The required optical signal-to-noise ratios for different modulation indices on the OFDM band are discussed for optimal operation.

[Review on label-free optical bio-sensing technology based on whisper-gallery-mode].

Optical biosensors are becoming an important tool for drug research and life science, and the label-free optical biosensor based on whisper-gallery-mode (WGM) is reviewed in the present paper. The WGM-based sensors are categorized into three types according to the microcavity structure. The biosensor using microsphere got extensive research because of high quality factor, and its response to protein, virus, and bacteria had been studied. The models based on single photon resonant state and perturbation theory were established. The biosensor using microdisk was proposed early since it can make use of mature lithography technology; however, the quality factor was increased greatly only after the thermal reflow process was introduced and single molecule measurement was then realized. The biosensor using microring has simpler mode structure and materials such as polymer, silicon nitride and silicon-on-insulator had been used for sensor fabrication. As a 3-dimension expansion, sensor using microtube can combine the optical channel and fluidic channel, which attracting more and more attention.

Dual-band OOFDM system based on tandem single-sideband modulation transmitter.

We demonstrate an optical orthogonal frequency division multiplex (OOFDM) transmitter using a simple dual-electrode Mach- Zehnder modulator (MZM) to perform tandem single-sideband optical modulation. This allows a doubling of transmission capacity. A commercial optical transmitter card, equipped with an on-board arbitrary waveform generator, is used for the system test. A proof-of-concept transmission experiment is conducted using a fiber-optic re-circulating loop.

PMD monitoring in traffic-carrying optical systems and its statistical analysis.

Differential group delay (DGD) experienced by the optical signal in in-service terrestrial optical fiber systems has been monitored for the first time without the requirement of looping-back, in which the live traffic carried in the fiber was used as the probing signal. The relationship between the measured DGD using this technique and the actual fiber PMD parameter is formulated and verified by field experiments.

Two-frequency CARS imaging by switching fiber laser excitation.

To fully exploit the power of coherent Raman imaging, techniques are needed to image more than one vibrational frequency simultaneously. We describe a method for switching between two vibrational frequencies based on a single fiber-laser source. Stokes pulses were generated by soliton self-frequency shifting in a photonic crystal fiber. Pump and Stokes pulses were stretched to enhance vibrational resolution by spectral focusing. Stokes pulses were switched between two wavelengths on the millisecond time scale by a liquid-crystal retarder. Proof-of-principle is demonstrated by coherent anti-Stokes Raman imaging of polystyrene beads embedded in a poly(methyl methacrylate) (PMMA) matrix. The Stokes shift was switched between 3,050 cm-1 , where polystyrene has a Raman transition, and 2,950 cm-1 , where both polystyrene and PMMA have Raman resonances. The method can be extended to multiple vibrational modes.

Two-photon microscopy with wavelength switchable fiber laser excitation.

Two-photon scanning fluorescence microscopy has become a powerful tool for imaging living cells and tissues. Most applications of two-photon microscopy employ a Ti:sapphire laser excitation source, which is not readily portable or rapidly tunable. This work explores the use of two-photon fiber laser excitation (TP-FLEX) as an excitation source for scanning two-photon microscopy. We have further demonstrated the use of a photonic crystal fiber (PCF) for facile tuning of the excitation wavelength over the range from 810 nm to 1100 nm. We generated two-photon fluorescence images at excitation wavelengths from 850 nm to 1100 nm detected on a scanning-stage microscope. By PCF wavelength tuning the dye BODIPY fl was selectively excited at 1000 nm whereas MitoTracker red was excited preferentially at 1100 nm. We discuss the potential for fiber laser sources coupled with PCF wavelength tuning as an attractive tunable excitation source for two-photon scanning fluorescence microscopy.