MHz Fiber Research Articles

Nonlinear amplification based on a tightly phase locked 750 MHz Yb:fiber frequency comb

In this paper, a nonlinear amplification system for a low noise 750 MHz ytterbium-doped fiber frequency comb is developed with a repetition rate stabilized to a continuous wave laser by employing a bulk electro-optic modulator. By managing the dispersion prior to the nonlinear amplifier, the proposed high-power frequency comb light source can deliver over 10 W of <90 fs pulse trains with an integrated relative intensity noise instability of 0.04%. Moreover, the noise characteristics of the proposed amplified comb, where the seed laser is a tightly stabilized 750 MHz fiber frequency comb, are analyzed in several representative states of different launched pump powers and pre-chirping group delay dispersions. The optical fiber frequency combs produced by nonlinear amplification may serve as an alternative for achieving high-power and ultrafast combs with good phase stability.

Efficient high harmonics generation by enhancement cavity driven with a post-compressed FCPA laser at 10 MHz

Efficient high harmonics generation (HHG) was demonstrated at 10 MHz repetition rate with an external femtosecond enhancement cavity, seeded by a ${\sim}70~\text{fs}$ post-compressed 10 MHz fiber chirped pulse amplifier (FCPA) laser. Operation lasting over 30 min with 0.1 mW outcoupled power at 149 nm was demonstrated. It was found that shorter pulse was beneficial for alleviating the nonlinear plasma effect and improving the efficiency of HHG. Low finesse cavity can relax the plasma nonlinearity clamped intra-cavity power and improve the cavity-locking stability. The pulse duration is expected to be below 100 fs for both 1040 nm and 149 nm outputs, making it ideal for applications such as time-resolved photoemission spectroscopy.

3.5-GHz intra-burst repetition rate ultrafast Yb-doped fiber laser

We report on an all-fiber Yb laser amplifier system with an intra-burst repetition rate of 3.5GHz. The system is able to produce minimum of 15-ns long bursts containing approximately 50 pulses with a total energy of 215μJ at a burst repetition rate of 1kHz. The individual pulses are compressed down to the subpicosecond level. The seed signal from a 108MHz fiber oscillator is converted to approximately 3.5GHz by a multiplier consisting of six cascaded 50/50 couplers, and then amplified in ten stages. The highly cascaded amplification suppresses amplified spontaneous emission at low repetition rates. Nonlinear interactions between overlapping pulses within a burst is also discussed.

Macrophage with gold nanorod visualized by optical-resolution and acoustic-resolution photoacoustic microscopes.

Macrophages play a key role in inflammation and they are frequently observed in vulnerable atherosclerotic plaque. In the present study, macrophages phagocytosing gold nanorod (AuNR) were observed by optical-resolution (OR) and acoustic-resolution (AR) photoacoustic microscope (PAM). The OR-PAM consisted of diode laser optically focused to 60 micron and planar ultrasonic transducer with the central frequency of 8 MHz placed under the object. AR-PAM consisted of concave ultrasonic transducer with the central frequency of 20 MHz and optical fiber through the center hole of the transducer for laser irradiation. First, PA signal from gold, silver and copper wire were measured in order to determine the best metal substrate for enhancing PA contrast. Gold generated largest PA signal. AuNR with the resonance wavelength of 1064 nm was co-cultured with the macrophages for phagocytosis. PA signal was successfully detected from macrophages with AuNR by both OR-PAM and AR-PAM. PA imaging of the macrophages with AuNR indicates inflammation in the vulnerable plaque and AR-PAM method would be applicable for clinical settings.

Coherent pulse interrogation system for fiber Bragg grating sensing of strain and pressure in dynamic extremes of materials.

A 100 MHz fiber Bragg grating (FBG) interrogation system is described and applied to strain and pressure sensing. The approach relies on coherent pulse illumination of the FBG sensor with a broadband short pulse from a femtosecond modelocked erbium fiber laser. After interrogation of the FBG sensor, a long multi-kilometer run of single mode fiber is used for chromatic dispersion to temporally stretch the spectral components of the reflected pulse from the FBG sensor. Dynamic strain or pressure induced spectral shifts in the FBG sensor are detected as a pulsed time domain waveform shift after encoding by the chromatic dispersive line. Signals are recorded using a single 35 GHz photodetector and a 50 G Samples per second, 25 GHz bandwidth, digitizing oscilloscope. Application of this approach to high-speed strain sensing in magnetic materials in pulsed magnetic fields to ~150 T is demonstrated. The FBG wavelength shifts are used to study magnetic field driven magnetostriction effects in LaCoO3. A sub-microsecond temporal shift in the FBG sensor wavelength attached to the sample under first order phase change appears as a fractional length change (strain: ΔL/L<10-4) in the material. A second application used FBG sensing of pressure dynamics to nearly 2 GPa in the thermal ignition of the high explosive PBX-9501 is also demonstrated. Both applications demonstrate the use of this FBG interrogation system in dynamical extreme conditions that would otherwise not be possible using traditional FBG interrogation approaches that are deemed too slow to resolve such events.

Nd:YVO_4 amplifier for ultrafast low-power lasers

An Nd:YVO4 amplifier consisting of two modules end pumped at 808 nm at 30 W total absorbed power has been designed for efficient, diffraction-limited amplification of ultrafast pulses from low-power seeders. We investigated amplification with a 50 mW, 7 ps Nd:YVO4 oscillator, a 2 mW, 15 ps Yb fiber laser, and a 30 mW, 300 fs Nd:glass laser. Output power as high as 9.5 W with 8 ps pulses was achieved with the 250 MHz vanadate seeder, whereas the 20 MHz fiber laser was amplified to 6 W. The femtosecond seeder allowed extracting Fourier-limited 4 ps pulses at 7 W output power. To our knowledge, these are the shortest pulses from any Nd:YVO4 laser device with at least 7 W output power. This suggests a novel approach to exploit the gain bandwidth of vanadate amplifiers with high output power levels. Such amplifier technology promises to offer an interesting alternative to high-power thin disk oscillators at few picoseconds duration, as well as to regenerative amplifiers with low-repetition-rate fiber seeders.

A Thermodynamic Analysis of Energy Flow in Optical Fiber Communication Systems

A thermodynamic analysis of energy flow in different elements of an optical fiber communication system is presented. The analysis depends on a previously introduced entropy approach that postulated the flow of electric charges as a flow of electromagnetic waves. Such a postulate is introduced here to find a plausible explanation of the electro-optic Kerr effect. Following the same approach, the transmitter and receivers of the optical communication-systems are explained according to measurement results as converters of electric waves into light waves or converters of light waves into electric waves. Similarly, pumping of optical energy or electrical energy into the elements of a communication system is found equivalent to pumping of entropy. Such results are introduced to derive energy equations that model the flow of energy and different processes in an optical communication-system. The introduced approach that depends on the postulated definition of electric current is compared to a classical approach in an analysis of energy flow into 80 MHz fiber laser.

Optimization of a 100 MHz fiber optic hydrophone for medical ultrasound applications using nanometer scale gold coating.

This work describes a novel numerical model developed to optimize the design of a broadband, spatial averaging free fiber optic hydrophone probe (FOHP) for characterization of ultrasound fields in the frequency range 1–100 MHz. The design included minimization of the active sensor dimensions, so its cross-section is comparable with the half acoustic wavelength at the highest frequency of interest and improvement of voltage-to-pressure sensitivity of the probe measured in V/Pa. Initial simulations based on the assumption that bulk refractive index of sputtered material could be used indicated that a thin film gold coating (1–35 nm) could indeed enhance the voltage sensitivity of the FOHP by 16–30 dB; however, a follow up analysis revealed that determination of the coating thickness influence on the FOHP performance would require an introduction of the complex (as opposed to bulk) index of refraction of the sputtered film. The input parameters to the model, their selection criteria, and implementation of the coupled acousto-optic interaction of gold layer will be discussed. The measured prototypes produced unprecedented voltage sensitivity between −234 and −245 dB re 1 V per uPa or 2 and 560 mV/MPa, respectively. [The authors wish to acknowledge support of the NIH R01EB007117 grant.]

Non-invasive monitoring of fouling in hollow fiber membrane via UTDR

Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system.

Tunable pulses from below 300 to 970 nm with durations down to 14 fs based on a 2 MHz ytterbium-doped fiber system

A noncollinearly phase-matched optical parametric amplifier pumped by a commercial 2 MHz fiber laser is presented and discussed. The pump system allows the direct generation of a seed continuum from a sapphire plate. Clean pulses with up to 860 nJ energy and down to 14 fs pulse length can be obtained over a fundamental tuning range from 620 to 970 nm. Conversion by second- and third-harmonic generation as well as sum frequency mixing results in an extended tuning range down to well below 300 nm.

Sputter deposited piezoelectric fiber coatings for acousto-optic modulators

All-fiber optical phase modulators were fabricated by coating standard telecommunication optical fibers with thin films of piezoelectric ZnO. Piezoelectric optical fiber coatings of PbZrxTi1−xO3(PZT) were also produced, and the actuation capability necessary for making optical fiber modulators is demonstrated. Both the ZnO and the PZT fiber coatings were deposited by reactive direct current magnetron sputtering of metal targets. The geometry of the sputtering system, fiber rotation, and reactive sputtering processes that result in sufficiently high deposition rates for deposition of axially symmetric coatings between 0.5 and 7 μm thick were studied. Two types of piezoelectric fiber optic modulator structures, Cr/Au/ZnO/Cr/Au and Ti/Pt/PZT/Au, are currently being investigated. Results on crystalline phase formation, microstructure, and piezoelectric and optical behavior of the coated optical fiber devices are presented. Impedance and phase angle measurements are used to demonstrate how device geometry affects the resonance response of ZnO coated devices. Interferometric measurements have shown that the ZnO-based modulators can produce optical phase shifts as high as 0.70 rad when the modulator is driven at the 196.5 MHz fiber radial resonance.