Showing papers on "Ring laser published in 2004"

A fast low-power optical memory based on coupled micro-ring lasers.

Abstract: The increasing speed of fibre-optic-based telecommunications has focused attention on high-speed optical processing of digital information1. Complex optical processing requires a high-density, high-speed, low-power optical memory that can be integrated with planar semiconductor technology for buffering of decisions and telecommunication data2. Recently, ring lasers with extremely small size and low operating power have been made3,4,5,6,7, and we demonstrate here a memory element constructed by interconnecting these microscopic lasers. Our device occupies an area of 18 × 40 µm2 on an InP/InGaAsP photonic integrated circuit, and switches within 20 ps with 5.5 fJ optical switching energy. Simulations show that the element has the potential for much smaller dimensions and switching times. Large numbers of such memory elements can be densely integrated and interconnected on a photonic integrated circuit: fast digital optical information processing systems employing large-scale integration should now be viable.

Direct measurement of diurnal polar motion by ring laser gyroscopes

Abstract: [1] We report the first direct measurements of the very small effect of forced diurnal polar motion, successfully observed on three of our large ring lasers, which now measure the instantaneous direction of Earth's rotation axis to a precision of 1 part in 108 when averaged over a time interval of several hours. Ring laser gyroscopes provide a new viable technique for directly and continuously measuring the position of the instantaneous rotation axis of the Earth and the amplitudes of the Oppolzer modes. In contrast, the space geodetic techniques (very long baseline interferometry, side looking radar, GPS, etc.) contain no information about the position of the instantaneous axis of rotation of the Earth but are sensitive to the complete transformation matrix between the Earth-fixed and inertial reference frame. Further improvements of gyroscopes will provide a powerful new tool for studying the Earth's interior.

Multisoliton states and pulse fragmentation in a passively mode-locked fibre laser

Abstract: We report the observation of a novel feature in the pulse emission characteristics of a passively mode-locked fibre ring laser. In both anomalous and normal path-averaged dispersion regimes, we show switching from stationary multisolitons, which comprise a small number of pulses, to large soliton trains. The abrupt transition between the two emission regimes is manifested in both ways.

Ultrafast semiconductor-based fiber laser sources

Abstract: In this paper, a novel ring laser platform is presented that uses a single active element, a semiconductor optical amplifier (SOA), to provide both gain and gain modulation in the optical cavity. Gain modulation is achieved by an externally introduced optical pulsed signal. This signal periodically saturates the amplifier gain and forces the ring laser to mode lock. Using this laser platform, we demonstrate picosecond pulsetrain generation at repetition rates up to 40 GHz, either in single or multiwavelength operation mode. In particular, using rational harmonic mode locking, 2.5-ps pulses were obtained up to a 40-GHz repetition rate, while output pulses and output power were constant over a 20-nm tuning range. In addition, a multiwavelength optical signal was obtained using the same laser platform with the addition of a Fabry-Pe/spl acute/rot filter for comb generation. Multiwavelength oscillation is possible due to the broad gain spectrum of the SOA used and its inhomogeneous line broadening. To this end, 48 oscillating wavelengths were obtained at the laser output, with 50-GHz line spacing. Combining both modes of operation, it was possible to mode lock the oscillating multiwavelength signal and to obtain at the output ten wavelength channels, simultaneously mode locked at a 30-GHz repetition rate. The mode-locked channels are temporarily synchronized and exhibit almost identical spectral and time characteristics.

Tunable multiwavelength fiber ring lasers using a programmable high-birefringence fiber loop mirror

Abstract: We demonstrate a comb filter with digitally tunable wavelength spacing using a programmable high-birefringence (HiBi) fiber loop mirror (FLM). We then use the HiBi-FLM to implement a tunable multiwavelength semiconductor optical amplifier (SOA)-based fiber ring laser and a tunable multiwavelength all-fiber Raman ring laser. Both lasers exhibit stable room-temperature operation and have a wavelength spacing that is digitally tunable between 1.6 and 3.2 nm. With the SOA-based configuration, we obtain six wavelengths with a spacing of 3.2 nm and 11 wavelengths with a spacing of 1.6 nm; with the all-fiber Raman-based configuration, we obtain four wavelengths with a spacing of 3.2 nm and five wavelengths with a spacing of 1.6 nm.

Ring-laser optical flip-flop memory with single active element

Abstract: We present a novel optical flip-flop configuration that consists of two unidirectional ring lasers with separate cavities but sharing the same active element unidirectionally. We show that in such a configuration light in the lasing cavity can suppress lasing in the other cavity so that this system forms an optical bistable element. Essential for obtaining the bistability is the presence of an additional feedback circuit that is shared by both lasers. We show experimentally that the flip-flop can be optically set and reset, has a contrast ratio of 40 dB and allows low optical power operation. We also present a model based on roundtrip equations. Good agreement between theory and experiments is obtained.

Integration of multiwavelength lasers with fast electro-optical modulators

Abstract: Photonic Integrated Circuits (PICs) are of key importance in Wavelength-Division Multiplexing (WDM) networks because of their reduced volume and packaging costs compared to discrete components. The research described in this thesis was focussed on the integration of WDM-lasers and Radio-Frequency (RF) modulators. The WDM-lasers are based on an array of Semiconductor Optical Amplifiers (SOAs) and an Arrayed-Waveguide Grating (AWG). These lasers can be operated as multiwavelength lasers for simultaneous generation of multiple wavelengths, or as tunable laser in which case their digital control is an advantage over precise analog control needed in for instance sampled-grating distributed Bragg reflector lasers. The RF modulators are based on a traveling-wave Mach-Zehnder structure. The integration of both components on one single chip involves engineering trade-offs relating to the optimization of microwave, electrical, optical and fabrication characteristics. An important aspect of the fabrication of PICs is the selection of a suitable integration technology to realize different waveguide types for active (e.g. amplifier) and passive (e.g. AWG) elements. InP epitaxial wafers containing high-quality integrated active and passive regions were developed by JDS Uniphase and the COBRA Research Institute using a three-step metal-organic vapor-phase epitaxy re-growth process. Our PICs use a ridge waveguide design, for which a reactive ion etch process was developed the COBRA cleanroom. As a first step towards the integration of lasers and modulators, we focused on the fabrication of stand-alone devices in compatible structures. The Mach-Zehnder modulator structure was realized in two versions. The first version employed 4-µm-wide phase shifters. This waveguide width enabled a tolerant fabrication process but severely limited the modulator bandwidth due to a high microwave attenuation and a velocity mismatch. A velocity match is important to have an efficient interaction between the modulating microwaves and the optical carrier. Also the modulator impedance of ?? 21W was not matched well to a 50W driver. An additional impedance mismatch caused a standing wave pattern limiting the modulator 3dBe bandwidth to 5GHz. In a next design, we addressed an increase of the modulator impedance and a reduction of both the microwave attenuation and index to achieve a velocity match. All three issues could be accomplished simultaneously, mainly by tuning one design parameter: the waveguide width. The optimum width of 1µm forced us to develop new fabrication steps in order to realize 2- µm-wide metal lines on top of such narrow phase shifters. The result was a traveling-wave Mach-Zehnder modulator that was both velocity- and impedance-matched. The switching voltage was measured to be lower than 5V and the static extinction ratio better than 20dB at 1550nm. The simulated 3-dBe relative optical response was over 50GHz for a device with 2-mm-long phase shifters. The bandwidth deduced from electrical measurements was reduced by a poor quality plated gold to 34GHz. The optical bandwidth measured with a photodiode was reduced to 9GHz by a high contact resistance. These two problems were solved in a second fabrication run. There, a small velocity mismatch limited the modulator bandwidth extracted from S-parameter measurement to 34GHz, enough for 40Gb/s operation. A number of multiwavelength lasers was developed separately in various configurations of three main components: passive waveguides, semiconductor amplifiers (SOA)s and one or more AWGs. We realized a multiwavelength 4-l laser using the basic configuration of an SOAarray and a single AWG in a linear Fabry-Perot cavity. If no simultaneous operation is required, AWG-based lasers can also be applied as discretely tunable lasers. Then, the properties of the AWG can be exploited to increase the number of generated wavelengths over the number of integrated amplifiers. Using two AWGs and eight SOAs, we realized two linear 16-l digitally tunable lasers with a channel spacing of 100GHz. One of these lasers was measured to have an side-mode suppression ratio of over 40dB and an output power of ?? 1mW at 100mA bias current. This power level was substantially higher than that of earlier published AWG-based digitally tunable lasers with an increased number of wavelengths. This was accomplished by coupling two cavities with a multimode interference coupler into one output waveguide. As an alternative for AWG-based multiwavelength lasers in a linear cavity, we realized several WDM ring lasers, which were the first of their kind. Two 4-l AWG-based ring lasers, a 7-l and a 9-l ring laser were all fabricated in the same technology. One of these ring lasers hold the smallest device size of an AWG-based laser to date (1×1.8mm2). As integrated ring lasers had not been used earlier for multiwavelength operation, we made an extensive study of the stability and the mode-competition mechanism in such lasers. From our investigations it can been concluded that the stability properties of these lasers can be good, but that it is difficult to tap sufficient power out of the ring. AWG-based multiwavelength lasers can be applied as an integrated continuous-wave source for a wavelength converter or a modulator. Integrated modulators are a relevant option since direct laser modulation is limited due to the long laser cavity. We designed and fabricated a 4-l multiwavelength laser integrated with a high-speed Mach-Zehnder modulator. For a device consisting of a 4-l multiwavelength laser with four integrated modulators, we applied a novel concept using a single AWG (de)multiplexer. The realization of these integrated components has been accomplished and we came close to operating devices.

Pulse-train nonuniformity in a fiber soliton ring laser mode-locked by using the nonlinear polarization rotation technique

Abstract: We report on the experimental observation of periodical intensity fluctuations on the output of a fiber soliton ring laser passively mode-locked by using the nonlinear polarization rotation technique. It is found that the appearance of such intensity fluctuations is independent of the orientation of the polarizer in the cavity, but closely related to the pump intensity. We have also numerically simulated the pulse dynamics of the laser. Our numerical simulations confirmed the experimental observations and showed that the soliton pulse nonuniformity is caused by the interaction between the nonlinear polarization rotation and the polarizer in the cavity.

Strong optical bistability in a simple L-band tunable erbium-doped fiber ring laser

Abstract: We demonstrate strong optical bistability in a widely tunable L-band erbium-doped fiber ring laser pumped by a 980-nm laser diode. The bistable region is as much as 150-mW wide and can be controlled by the lasing wavelength or the length of erbium-doped fiber. The mechanism for the bistability is not typical cross gain saturation in a two-mode operation, but rather saturable absorption strongly enhanced in the proposed scheme.

Dispersion-managed breathing-mode semiconductor mode-locked ring laser: experimental characterization and numerical simulations

Abstract: A dispersion-managed breathing-mode mode-locked semiconductor ring laser is studied. The working regime and pulse evolution at the key cavity points are experimentally characterized and numerically simulated. Linearly chirped, asymmetric exponential pulses are generated and externally compressed to 274 fs, which is within 10% of the bandwidth limit. The close agreement between the simulated and the measured results verifies our ability to control the physical mechanisms involved in pulse formation and shaping within the ring cavity.

Efficient single-axial-mode operation of a Ho:YAG ring laser pumped by a Tm-doped silica fiber laser

Abstract: Efficient single-frequency operation of a Ho:YAG ring laser at room temperature with a traveling-wave TeO2 acousto-optic modulator to enforce unidirectional operation is reported. By use of a 2-at. % Ho3+-doped 10-mm-long Ho:YAG rod, end pumped by a cladding-pumped tunable Tm-doped silica fiber operating at 1.9 microm, the Ho:YAG ring laser yielded 3.7 W of single-frequency output at 2.1 microm in a diffraction-limited TEM00 beam with M2 < 1.1 for an incident pump power of 8.8 W. The rf power required for unidirectional operation was 0.3 W and corresponded to an increase in cavity loss for the lasing direction (due to diffraction) of only 0.5%. The prospects for further improvement in efficiency are discussed.

Clock recovery by a fiber ring laser employing a linear optical amplifier

Abstract: We report on subharmonic optical clock recovery (OCR) at 10 GHz from 160-Gb/s optical time-division multiplexed signals. The OCR circuit is based on a passively mode-locked principle in a fiber ring laser that utilizes fast gain dynamics of a linear semiconductor optical amplifier. To decrease the jitter amount in the clock pulse considerably, postelectrical signal processing is performed. The recovered clock is a 1.8-ps 10-GHz pulse train with 0.37 pulsewidth-bandwidth product.

Intracavity frequency-tripled CW laser with traveling-wave ring-resonator

Abstract: A traveling-wave ring laser resonator includes one or more gain-elements for generating fundamental radiation and three optically nonlinear crystals. A portion of the fundamental radiation is converted to second-harmonic radiation in a first of the crystals. Remaining fundamental radiation and the second-harmonic radiation traverse a second of the optically nonlinear crystals where a portion of each is converted to third-harmonic radiation. Fundamental and second-harmonic radiation pass through the third of the optically nonlinear crystals where most of the second-harmonic radiation is converted back to fundamental radiation. The third-harmonic radiation can be delivered from the resonator as output radiation or mixed with the fundamental radiation in a fourth optically nonlinear crystal to generate fourth harmonic radiation. An optical parametric oscillator arrangement is also disclosed.

Regimes of operation states in passively mode-locked fiber soliton ring laser

Abstract: The principal of passively mode-locked fiber soliton ring lasers is summarized, including its three output operation states: normal soliton, bound–solitons and noise-like pulse. The experimental results of the passively mode-locked fiber soliton ring lasers developed by us are given. Bound–solitons with different discrete separations and three-bound–solitons state have been observed in our fiber laser for the first time. The relationship among three operation states in fiber soliton laser is analyzed.

Tunable laser fluid sensor

Abstract: A sensitive fluid sensor for detecting fluids and particularly trace fluids. The sensor may be adjustable for detecting fluids of various absorption lines. To effect such adjustment, a tunable laser may be used. The laser may be an edge emitting diode, a VCSEL or other tunable source. The detection apparatus of the sensor may incorporate a sample cell through which a laser light may go through. The sample cell may comprise a tunable ring-down cavity. The ring-down cavity may be a ring laser cavity like that of a ring laser gyroscope. There may be a photo detector proximate to the ring down cavity connected to a processor.

Bound twin-pulse solitons in a fiber ring laser.

Abstract: 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.

Pulse-amplitude equalization in a rational harmonic mode-locked semiconductor fiber ring laser using a dual-drive Mach-Zehnder modulator.

Abstract: We present and demonstrate a simple method of pulse-amplitude equalization in a rational harmonic mode-locked semiconductor ring laser, using a dual-drive Mach-Zehnder (MZ) modulator. Pulse-amplitude equalization was achieved by adjusting the voltages applied to both arms of the modulator, such that each mode-locked pulse experiences the same transmission coefficient in the transmission curve of the modulator. With this method, amplitude-equalized pulse trains with repetition rates of ~7.41GHz (third rational harmonic) and ~12.34GHz (fifth rational harmonic) were successfully obtained without any additional function to the ring laser itself.

Digitally tunable ring laser using ladder filter and ring resonator

Abstract: We propose a digitally tunable ring laser incorporating a ladder filter and a ring resonator. The widely tunable ladder filter consists of two input-output waveguides and a waveguide array, and it selects one channel from the periodic outputs of the ring resonator. With this device, the passband of the ladder filter is important in terms of obtaining stable lasing operation and it becomes narrower with increasing diffraction order. However, the free-spectral range of the filter is reduced with increasing diffraction order and this induces lasing mode instability. We therefore optimize the diffraction order of the ladder filter. The device is monolithically integrated by using the InP-InGaAsP material system. We achieved 37 channel 1000-GHz spacing digitally tunable laser operation. A promising way of improving the device performance is to use a chirped ladder filter because this filter has one dominant passband.

Wavelength selection in an integrated multiwavelength ring laser

Abstract: The wavelength selection mechanism of a compact integrated multiwavelength ring laser is demonstrated. The device contains four semiconductor optical amplifiers, a compact arrayed waveguide grating and passive waveguides integrated on a single InP wafer. The device can produce seven different wavelengths through biasing one or two out of the four amplifiers. Comparison of calculated and measured subthreshold laser spectra demonstrates the role of crosstalk in the arrayed waveguide grating in the laser and allows the crosstalk to be quantified. A rate-equation model of the laser and measurements are presented that describe the switching between wavelengths of the laser as a function of bias currents. A comparison between the measured data and the model is made.

A stabilized and tunable erbium-doped fiber ring laser with double optical filter

Abstract: We have proposed and experimentally demonstrated a stabilized and tunable erbium-doped fiber ring laser by using a Fabry-Pe/spl acute/rot laser and a fiber Fabry-Pe/spl acute/rot filter. The sidemode suppression ratio of >30 dB/0.1 nm and the output power of >2.2 dBm can be achieved while this ring laser is tuned from 1528.28 to 1559.64 nm with the step of 1.12 nm. The wavelength variation of less than 0.01 nm and the power fluctuation of /spl les/0.04 dB have also been obtained. When the constant output power is controlled by adjusting the bias current of 980-nm pump laser, the power variation of less than /spl plusmn/0.05 dB can be obtained over the entire tunable range.

Channel addition-removal response in all-optical gain-clamped lumped Raman fiber amplifier

Abstract: Application of all-optical gain-clamped (AOGC) lumped Raman fiber amplifier (RFA) for protection of surviving channels in multiwavelength networks is investigated experimentally and theoretically. Channel addition-removal was simulated by transmitting signals of two lasers through a counterdirectionally pumped RFA consisting of 16 km of dispersion compensating fiber. Light of one of the lasers was square-wave modulated at 500 Hz; power fluctuations of the other laser caused by cross-gain modulation of the RFA were monitored at the output of the amplifier with a digital oscilloscope. An all-optical feedback loop was implemented in the form of a ring laser. Theoretical analysis of the AOGC lumped RFA is based on numerical solution of coupled propagation equations for forward and backward-propagating pumps, signals, and spectral components of amplified spontaneous emission powers.

Doubled single-frequency Nd:YLF ring laser coupled to a passive nonresonant cavity.

Abstract: We demonstrate an original solution to obtain a single-frequency ring laser coupled to an external passive nonresonant ring cavity, which plays the role of an optical diode. This system provides more output power than systems with an intracavity unidirectional device. To the best of our knowledge, this work marks the first demonstration of a unidirectional planar ring laser at 1.3 μm. Using 12 W at 797 nm to pump a Nd:YLF laser, combined with intracavity second-harmonic generation, we achieve yields of 440 mW at 661.3 nm and 340 mW at 656.0 nm.

Discretely tunable erbium-doped fiber ring laser selecting ITU-T grids of 273 channels × 50-GHz spacing in C - and L -band regions

Abstract: We report the fabrication of a discretely tunable erbium-doped fiber ring laser with a novel scheme that can select ITU-T grids of 50-GHz spacing in both C and L bands. Wavelength selections of 273 channels are demonstrated with a signal to source spontaneous emission ratio of 60 dB/nm and excellent power flatness over 111-nm bandwidth. This was realized by incorporating both a solid Fabry–Perot interferometer and a fiber Fabry–Perot tunable filter into the ring laser. To our knowledge, our experimental result has the widest tuning range ever reported for a discretely tunable fiber laser with 50-GHz channel spacing that matches the ITU-T grids and is the first equipment that can operate in both C and L bands.

High-efficiency single-frequency Brillouin fiber laser with a tunable coupling coefficient

Abstract: We theoretically analyze a high-efficiency single-frequency Brillouin all-fiber ring laser at 1.5 μm wavelength, taking pump depletion into account. The output pump and Stokes intensities are calculated as functions of the cavity coupling coefficient and of the input pump intensity. Lasing threshold and pump-to-Stokes conversion efficiency are predicted. Furthermore, we demonstrate good agreement between model results and measurements. Applications to the improvement of optoelectronic links for radio-frequency signals by use of stimulated Brillouin scattering fiber lasers are also presented.