All-polymer fiber-optic pH sensor.

A novel all-polymer fiber-optic pH sensor using a UV-cured pH-sensitive hydrogel, poly(ethylene glycol) diacrylate (PEGDA), coated on a polymer fiber Bragg grating was developed. The PEGDA increased in volume according to the pH value of the surrounding fluid, which subsequently induced a lateral stress in the polymer fiber Bragg grating. The proposed pH sensor exhibits a pH sensitivity of up to -0.41 nm/pH and a fast response time of 30 s.

Novel fiber Bragg grating fabrication system for long gratings with independent apodization and with local phase and wavelength control.

We proposed and demonstrated a novel practical fiber Bragg grating (FBG) fabrication setup constructed with high performance linear stages, piezoelectric translation (PZT) stages, and a highly stable continuous wave laser. The FBG fabrication system enables writing of long FBGs by a continuous translate-and-write process and allows implementation of arbitrary chirp and apodization. A key innovation is that the local Bragg wavelength is controlled by a simple movement of the phase mask by a PZT in the direction perpendicular to its surface. The focus position of the two writing beams is not changed during the Bragg wavelength change, an intrinsic feature of the design, ensuring simplicity, robustness and stability. Apodization can be achieved by vibrating the phase mask in the direction parallel to its surface by a PZT. Phase steps can also be inserted in FBGs at any desired locations by stepping the same PZT. A long uniform FBG and a linearly chirped FBG are written to demonstrate the performance of the setup.

A robust and dither-free technique for controlling driver signal amplitude for stable and arbitrary optical phase modulation.

We propose a robust and dither-free technique using a delay line interferometer, a balanced detector and simple signal processing to adjust the amplitude of the driver signal of an optical phase modulator automatically for stabilizing the modulated phase of an optical carrier at any arbitrary value. The technique is analytically shown to be robust against practical device imperfections. A stable 45 degrees phase shift with deviation less than ± 0.8 degrees is experimentally demonstrated.

High-pressure and high-temperature characteristics of a Fabry-Perot interferometer based on photonic crystal fiber.

A fiber-optic Fabry-Perot interferometer was constructed by splicing a short length of photonic crystal fiber to a standard single-mode fiber. The photonic crystal fiber functions as a Fabry-Perot cavity and serves as a direct sensing probe without any additional components. Its pressure and temperature responses in the range of 0-40 MPa and 25°C-700°C were experimentally studied. The proposed sensor is easy to fabricate, potentially low-cost, and compact in size, which makes it very attractive for high-pressure and high-temperature sensing applications.

Signed frequency offset measurement for direct detection DPSK system with a chromatic dispersion offset.

We demonstrated a method for the measurement of signed frequency offset between optical source and delay interferometer (DI) for 10 Gb/s DPSK signals based on asynchronous delay-tap sampling technique with a chromatic dispersion (CD) offset. The demodulated DPSK signals show asymmetrical property and amplitude shoulder appears on the waveforms with frequency offset and a fixed CD offset together. The delay-tap sampling scatter plots also show the asymmetry related to the asymmetrical signal distortion. Our proposed method cannot only realize the measurement of the magnitude of frequency offset but also the polarity. The measurement range is from -2 GHz to +2 GHz and the sensitivity can reach ±100MHz. The simulation and experimental results are demonstrated and in good agreement.

Switchable UWB pulse generation using a polarization maintaining fiber Bragg grating as frequency discriminator.

We propose and successfully demonstrate a novel approach to optically generate ultrawideband (UWB) pulse with switchable shape and polarity by using a polarization-maintaining fiber Bragg grating (PM-FBG) as frequency discriminator. Depending on the shape of the reflective spectrum of the PM-FBG, the system can function as a first- or second-order differentiator for the generation of Gaussian UWB monocycle or doublet pulses. Consequently, the shape and the polarity of the generated UWB pulse can be switched by simple adjustment of a polarization controller (PC). Gaussian monocycle and doublet pulses were successfully obtained with fractional bandwidths of about 188% and 152%, respectively. Higher-order UWB pulses with spectrum covering from 2.9 GHz to 9.8 GHz have also been obtained through adjustment of the PC.

Signed chromatic dispersion monitoring of 100Gbit/s CS-RZ DQPSK signal by evaluating the asymmetry ratio of delay tap sampling.

In this paper, we theoretically and experimentally demonstrated the residual chromatic dispersion (CD) monitoring of 100-Gbit/s carrier suppress return-to-zero differential quadrature phase shift keying (CS-RZ DQPSK) signals by evaluating the asymmetry ratio of delay tap asynchronous sampling. This scheme can easily differentiate the positive and negative residual CD of the fiber link. The resolution of this scheme is better than 8 ps/nm and the measurable range is around +/- 24 ps/nm for 100 Gbit/s CS-RZ DQPSK signals. We can also simultaneously realize both signed CD monitoring and demodulation of CS-RZ DQPSK signal based on only one demodulator.

Observation of symmetrical reflection sidebands in a silica suspended-core fiber Bragg grating.

We have observed symmetrical sidebands in reflection from Bragg grating written in a silica suspended-core fiber, which are caused by longitudinal periodic refractive index modulation in the Ge-doped suspended-core fiber with a core diameter of approximately 1.3 microm. Our simulation shows that the effective refractive index of the guided mode varied by 0.023% along the fiber with a period of approximately 650 microm. The periodic index variation can lead to amplitude modulation of fiber Bragg gratings, which can be studied by observing the spectra of a fiber Bragg grating written in the Ge-doped core. In addition, we have also characterized the temperature and strain responses of the fiber Bragg gratings, and showed that both responses in the suspended-core fiber are 20 to 25% lower than that of a fiber Bragg grating written on a conventional fiber.

Dissipative vector solitons in a dispersionmanaged cavity fiber laser with net positive cavity dispersion.

We report on the experimental observation of gain-guided vector solitons (GGVSs) in a dispersion-managed fiber laser mode-locked with a semiconductor saturable absorber mirror (SESAM). Both the frequency-locked and phase-locked GGVS were observed. In addition, formation of multiple GGVSs and GGVS harmonic mode-locking were also experimentally revealed. Numerical simulations confirmed the GGVS formation in the fiber lasers.

C-band single-longitudinal mode lanthanum co-doped bismuth based erbium doped fiber ring laser.

We propose and demonstrate a stable, tunable and narrow linewidth C-band lanthanum co-doped bismuth based erbium doped fiber (EDF) ring laser with single longitudinal mode (SLM) operation. A free space thin film filter acts as a wavelength discriminative component selecting a few oscillating modes while a Lyot filter formed by a polarization maintaining (PM) fiber and a linear polarizer further discriminates and selects SLM efficiently. A power stability of < or = 0.05 dB, central wavelength variation of < or = 0.02 nm, a side-mode suppression ratio (SMSR) of at least > 43 dB, and a linewidth of about 1.3 kHz have been experimentally demonstrated.

Observation of spectral enhancement in a soliton fiber laser with fiber Bragg grating.

We demonstrate that soliton generation in a fiber laser containing a fiber Bragg grating exhibits spectral enhancement near the Bragg resonance wavelength. The Bragg grating leads to a spectral hole on the soliton spectrum while the observed enhancement is always located at the long wavelength side of the Bragg resonance.

Multiplexing of polarization-maintaining photonic crystal fiber based Sagnac interferometric sensors.

Three multiplexing schemes are presented for polarization-maintaining photonic crystal fiber based Sagnac interferometric sensors. The first technique is wavelength division multiplexing using coarse wavelength division multiplexers (CWDMs) to distinguish signals from each multiplexed sensor in different wavelength channels. The other two schemes are to multiplex sensors in series along a single fiber link and in parallel by using fiber-optic couplers. While for the CWDM scheme, the multiplexed sensing signal can be obtained by direct measurement; for the other two multiplexing techniques, the sensing signal is more complex and cannot be easily demultiplexed. Thus, some signal processing methods are required. In this regard, two mathematical transformations, namely the discrete wavelet transform and Fourier transform, have been independently and successfully implemented into these two schemes. The operating principles, experimental setup, and overall performance are discussed.

Coexistence of polarization-locked and polarization-rotating vector solitons in a fiber laser with SESAM.

We report the experimental observation of the coexistence of polarization-locked vector solitons (PLVSs) and polarization-rotating vector solitons (PRVSs) in a fiber laser mode locked with a semiconductor saturable absorber mirror. It was found experimentally that interaction between the PLVSs in a fiber laser could lead to formation of bound states of PRVSs. Moreover, the bound PRVSs as a unit have the same group velocity as that of the PLVSs in the cavity.

Gain dispersion for dissipative soliton generation in all-normal-dispersion fiber lasers.

Properties of dissipative solitons generated in all-normal-dispersion fiber lasers through the gain dispersion effect are numerically studied by using a pulse-tracing technique that considers interaction between gain saturation, gain dispersion, cavity dispersion, fiber Kerr nonlinearity, and cavity boundary conditions. The numerical results qualitatively match with experimental observations and show that the finite gain bandwidth, together with the pump power, determines the properties of the generated dissipative solitons, which further dictates the performance of the pulse compression.

Pulse breaking recovery in fiber lasers.

Pulse breaking recovery is numerically demonstrated in dispersion-managed fiber lasers designed for generating high peak power ultrashort optical pulses. It is shown that due to the cavity boundary condition, local pulse breaking can be absorbed by the pulse propagation in erbium-doped fiber with normal dispersion. Consequently, high peak power transform-limited pulses beyond the gain-bandwidth limitation could be generated.

Soliton interaction in a fiber ring laser.

We have experimentally investigated the soliton interaction in a passively mode-locked fiber ring laser and revealed the existence of three types of strong soliton interaction: a global type of soliton interaction caused by the existence of unstable cw components, a local type of soliton interaction mediated through the radiative dispersive waves, and the direct soliton interaction. We found that the appearance of the various soliton operation modes observed in the passively mode-locked fiber soliton lasers are the direct consequences of these three types of soliton interactions. The soliton interaction in the laser is further numerically simulated based on a pulse tracing technique. The numerical simulations confirmed the existence of the dispersive-wave-mediated soliton interaction and the direct soliton interaction. Furthermore, it was shown that the resonant dispersive-wave-mediated soliton interaction in the laser always has the consequence of causing random irregular relative soliton movement and the experimentally observed states of bound solitons are caused by the direct soliton interaction. In particular, as the solitons generated in the laser could have a profile with long tails, the direct soliton interaction could extend to a soliton separation that is larger than 5 times the soliton pulse width.

Bound twin-pulse solitons in a fiber ring laser.

Bound states of twin-pulse solitons were experimentally observed in a passively mode-locked fiber ring laser. Similar to those of single-pulse solitons, the bound states of twin-pulse solitons are marginally stable and occur at some fixed, quantized soliton separations. Our experimental investigations revealed that the formation of such bound states might be resulted from the dispersive wave mediated long-range soliton interaction in the laser.

Induced solitons formed by cross-polarization coupling in a birefringent cavity fiber laser.

We report on the experimental observation of induced solitons in a passively mode-locked fiber ring laser with a birefringence cavity. Owing to the cross coupling between the two orthogonal polarization components of the laser, it was found that if a soliton was formed along one cavity polarization axis, a weak soliton was also induced along the orthogonal polarization axis, and depending on the net cavity birefringence, the induced soliton could have either the same or different center wavelengths to that of the inducing soliton. Moreover, the induced soliton always had the same group velocity as that of the inducing soliton. They formed a vector soliton in the cavity. Numerical simulations confirmed the experimental observations.

Intensity-modulated fiber Bragg grating sensor system based on radio-frequency signal measurement.

An intensity-modulated, fiber Bragg grating (FBG) sensor system based on radio-frequency (RF) signal measurement is presented. The RF signal is generated at a photodetector by two modulated optical signals reflected from the sensing FBG and a reference FBG. Wavelength shift of the sensing FBG changes intensity of the RF signal through changing the delay between the two optical signals, with temperature effect being compensated automatically by the reference FBG. It also exhibits important features including potentially high-speed measurement, low cost, and adjustable sensitivity. In the experiment, strain measurement with a maximum sensitivity of -0.34 microV/micro epsilon has been achieved.

Long-period grating fabricated by periodically tapering standard single-mode fiber.

We fabricated an asymmetric long-period grating (LPG) by periodically tapering a section of standard single-mode fiber using a resistive filament heating. The LPG exhibits large peak transmission attenuation of -30.31 dB with only 22 periods in a 1.0 cm long optical fiber and possesses unique characteristics for sensing applications. The bending and strain sensitivities are 1.74 nm m and 1.11 pm/mu epsilon, respectively. The polarization dependent loss is large, up to 11.65 dB, which is caused by an asymmetric index profile in the cross section of the tapered LPG.