Fiber ring laser with a feedback mirror

We describe the spectral and power features of a ytterbium-doped double-clad photonic crystal fiber laser that is operated in a ring configuration with an external mirror that feeds back only one of its two output beams. We compare the operation of the laser with and without an external feedback mirror. We find that the feedback mirror reduces significantly the spectral and power fluctuations. It is also responsible for an interesting spectral phenomenon: The laser frequency is drifting periodically over 9 nm at a rate of 2 nm/s from a short wavelength to a longer wavelength and vice versa.     

Fiber laser with passive Q-switching based on photoinduced fiber resonance oscillations

Automodulation oscillations in the intensity of a fiber laser in the regime of passive Q-switching based on resonance bending oscillations of a fiber segment representing an element of the laser cavity have been studied. It is shown that the proposed scheme can be used for the creation of fiber-optic transducers.     

An upper limit to the frequency noise associated with the relaxation oscillation of a monolithic Nd:YAG ring laser

Abstract

A measurement of the frequency noise at the relaxation oscillation frequency of a Nd:YAG monolithic non-planar ring laser is presented. By careful monitoring of various applied calibration signals, a limit to the frequency noise level of ～ 6.3 × 10^?2 Hz/√Hz is set over the relaxation oscillation bandwidth and it is shown that this is approximately at the level to be expected, based on the relaxation oscillation induced time varying refractive index.     

Relaxation oscillations in a laser with a Gaussian mirror

We present an analysis of the relaxation oscillations in a laser with a Gaussian mirror by taking into account the three-dimensional spatial field distribution of the laser modes and the spatial hole burning effect. In particular, we discuss the influence of the Gaussian mirror peak reflectivity and a Gaussian parameter on the damping rate and frequency of the relaxation oscillation for two different laser structures, i.e., with a classically unstable resonator and a classically stable resonator.     

Picosecond absorption relaxation measured with nanosecond laser photoacoustics

Picosecond absorption relaxation-central to many disciplines-is typically measured by ultrafast (femtosecond or picosecond) pump-probe techniques, which however are restricted to optically thin and weakly scattering materials or require artificial sample preparation. Here, we developed a reflection-mode relaxation photoacoustic microscope based on a nanosecond laser and measured picosecond absorption relaxation times. The relaxation times of oxygenated and deoxygenated hemoglobin molecules, both possessing extremely low fluorescence quantum yields, were measured at 576 nm. The added advantages in dispersion susceptibility, laser-wavelength availability, reflection sensing, and expense foster the study of natural-including strongly scattering and nonfluorescent-materials.     

Efficient double-clad thulium-doped fiber laser with a ring cavity

We have constructed a novel ring cavity for a double-clad thulium-doped fiber laser, by placing the fiber's 45 degrees angle-polished output end before the input end and relaunching the pump and the laser power into the fiber. This design can reduce reabsorption by using short fibers without loss of pump efficiency. The dependence of the laser's performance on the fiber's length and the output coupler's reflectivity is investigated experimentally and theoretically. With an 80-cm-long fiber, 2.7-W single-mode continuous-wave output is generated for 11.5 W of launched pump power.     

Unexpected power-law stress relaxation of entangled ring polymers

After many years of intense research, most aspects of the motion of entangled polymers have been understood. Long linear and branched polymers have a characteristic entanglement plateau and their stress relaxes by chain reptation or branch retraction, respectively. In both mechanisms, the presence of chain ends is essential. But how do entangled polymers without ends relax their stress? Using properly purified high-molar-mass ring polymers, we demonstrate that these materials exhibit self-similar dynamics, yielding a power-law stress relaxation. However, trace amounts of linear chains at a concentration almost two decades below their overlap cause an enhanced mechanical response. An entanglement plateau is recovered at higher concentrations of linear chains. These results constitute an important step towards solving an outstanding problem of polymer science and are useful for manipulating properties of materials ranging from DNA to polycarbonate. They also provide possible directions for tuning the rheology of entangled polymers.     

Mode-locked ring laser with output pulse width of 0.4 ps

The output pulse width of a mode-locked ring laser composed of an erbium-doped fiber amplifier, Mach-Zehnder optical modulator, and optical band-pass filter depends largely on the repetition frequency and the wavelength characteristics of these optical circuit elements. In previous experiments, the output pulse width was in the order of 5 ps at a repetition frequency of 5 GHz. The principal reason was that the narrow passage band of the optical circuit elements made it extremely difficult to generate an ultra-short optical pulse. Consequently, we examined how to narrow the optical pulse width by flattening the wavelength characteristics of these optical circuit elements. Furthermore, we drove the optical modulator in the cavity using a frequency multiplier to operate at an effectively higher frequency By widening the wavelength passage band of all the devices in the optical circuit, we achieved an output pulse width of 0.4 ps at a repetition frequency of 5 GHz; the pulse peak power was more than +23 dBm, and the time-bandwidth product was 0.34. We successfully tested an ultra-short optical pulse source with an output pulse width of 0.4 ps with no external pulse compression using a mode-locked ring laser     

Errors in reproducing the planar-angle unit with a ring laser

Translated from Izmeritel'naya Tekhnika, No. 12, pp. 11–12, December, 1989.     

Orthogonal linear polarization tunable-beat ring laser with a superluminescent diode

An orthogonal linear polarization operated ring laser with a superluminescent diode has been demonstrated to generate a tunable optical beat signal. The ring cavity contains a superluminescent diode as the optical gain medium, Faraday rotators, and a variable phase retarder (Babinet-Soleil compensator). By controlling the retarder, we changed the beat frequency in the range from a few tens of megahertz to 100 MHz.     

Hyperbolic heat conduction due to a mode locked laser pulse train

In situations which involve repetitive pulsing of a material with a mode locked Nd:YAG laser, the pulse duration can be sufficiently small (i.e. in the picosecond range) that the classical parabolic heat conduction equation fails to adequately predict the resulting temperature distribution in the material. In such cases, the hyperbolic heat conduction equation, which accounts for the finite time to the commencement of heat flow, is appropriate. In the present work, the hyperbolic heat conduction equation is used to predict the temperature distributions in both semi-infinite and finite isotropic media due to a train of temporally rectangular pulses which approximate the Gaussian temporal profile of mode locked laser pulses. The energy carried in the pulses is assumed to be absorbed in the surface plane of the material. The spatial profile of the pulses can be either Gaussian, doughnut or a combination of the two. The parabolic and hyperbolic models are examined for selected pulse frequencies.     

Bipolar disorder dynamics: affective instabilities,relaxation oscillations and noise

Bipolar disorder is a chronic, recurrent mental illness characterized by extreme episodes of depressed and manic mood, interspersed with less severe but highly variable mood fluctuations. Here, we develop a novel mathematical approach for exploring the dynamics of bipolar disorder. We investigate how the dynamics of subjective experience of mood in bipolar disorder can be understood using a relaxation oscillator (RO) framework and test the model against mood time-series fluctuations from a set of individuals with bipolar disorder. We show that variable mood fluctuations in individuals diagnosed with bipolar disorder can be driven by the coupled effects of deterministic dynamics (captured by ROs) and noise. Using a statistical likelihood-based approach, we show that, in general, mood dynamics are described by two independent ROs with differing levels of endogenous variability among individuals. We suggest that this sort of nonlinear approach to bipolar disorder has neurobiological, cognitive and clinical implications for understanding this mental illness through a mechacognitive framework.     

Transformation of bistable chaotic oscillations in triggers of a ring system

Equations describing a ring system of three unidirectionally coupled Chua oscillators (subsystems) are presented. Numerical analysis is performed in the case in which only one subsystem is autooscillatory. It is shown that non-auto-oscillatory subsystems (simplified to single-shot trigger type) exhibit complication of circuit oscillations that is accompanied by multiplication of the oscillation frequencies and expansion of the generated spectrum.     

Spectral characteristics of a regenerative semiconductor optical amplifier mutually injection locked with a Fabry-Perot laser diode

The anomalous injection locking of a traveling-wave semiconductor optical amplifier (SOA) and a fiber-pigtailed Fabry-Perot laser diode (FPLD) linked with single-mode and improved side-mode suppression ratio output is demonstrated. We achieve this injection locking by driving the FPLD slightly below threshold and by feedback injecting the FPLD with fractional output of a closed-loop SOA. The SOA-FPLD link lases in a single FPLD longitudinal mode with a reduced linewidth of 0.013 nm and a maximum side-mode-suppressing ratio of 39.7 dB. A precise 3-dB linewidth of 45-50 MHz is also observed from the self-homodyne mode-beating spectrum. The optimized feedback-injecting power for the FPLD is approximately 2% of the SOA-FPLD linked output power of >400 microW. The variations in output power and in peak wavelength are not more than 0.54% and 0.06%, respectively. The injection-locked SOA-FPLD link is insensitive to the temperature fluctuation within +/- 0.25 degrees C.     

A semiconductor whispering-gallery-mode laser with ring cavity operating at room temperature

Whispering gallery mode (WGM) semiconductor lasers with ring and disk cavities (outside diameters 300, 200, and 100 μm) operating in a pulsed regime at room temperature on a λ = 2.4 μm wavelength have been created. A comparative study of the spectral and threshold characteristics of these lasers showed that ring lasers operate, like those with the disk cavities, on WGMs. The level of spontaneous emission in the ring lasers is lower by almost an order of magnitude than that in the disk lasers, which is due to the absence of recombination in the central part of the cavity. The threshold current density in ring lasers is slightly greater than that in the disk lasers, which is related to an increase in the current density at the edges of both cavities and to the appearance of surface leak currents on the inner surface of the ring-shaped cavity.     

Mode-hop-free single-longitudinal-mode erbium-doped fiber laser frequency scanned with a fiber ring resonator

We describe the experimental construction of a single-longitudinal-mode erbium-doped fiber ring laser equipped with a ring resonator filter. The system is designed to avoid mode hopping and can be frequency scanned for spectroscopic sensor applications. We modified a conventional ring resonator filter to have the capability to reduce transmittance of unwanted modes, which is essential for frequency scanning without mode hopping.     

Unidirectional single-mode Nd:YAG laser with a planar semimonolithic ring cavity

Unidirectional single-mode operation of a diode-pumped Nd:YAG laser with a planar semimonolithic ring cavity has been demonstrated at 1064 nm. The semimonolithic cavity consists of a laser active medium placed in a magnetic field, a crystal quartz plate, and an output coupling mirror, which form an optical diode by acting as a Faraday rotator, a reciprocal polarization rotator, and a partial polarizer, respectively. A single-mode output power of 155 mW and a slope efficiency of 17% were obtained with a 1.2-W diode laser at 809 nm. A laser linewidth of less than 100 kHz is inferred from a beat note frequency spectrum between two identical laser systems and continuous tuning to greater than 2 GHz was observed.