Suppression of Q-switching instabilities in broadened-waveguide monolithic mode-locked laser diodes

It is predicted theoretically that broadening the optical confinement layer in monolithic mode-locked semiconductor lasers may suppress Q-switching instability, by increasing the carrier transport time, and lead to emission of shorter, more stable optical pulses.     

Precise operation-frequency control of monolithic mode-locked laser diodes for high-speed optical communication and all-optical signal processing

We describe a method for pulse-repetition-frequency tuning of mode-locked laser diodes (MLLDs) monolithically integrated with a distributed Bragg reflector (DBR). The pulse-repetition frequency, i.e., the cavity-roundtrip time, is tuned through loss-induced change in the effective length of the DBR. The frequency-tuning range as large as the chip-to-chip frequency deviation caused by cavity-length fabrication variation of 10 μm has been confirmed experimentally, and the MLLDs operate at SDH (synchronous digital hierarchy) frequencies of 9.953, 19.906 and 39.813 GHz. Synchronization with an external system-clock through the hybrid mode-locking operation reduces the timing jitter of the optical pulses to less than 0.3 ps. As an optical pulse source for optical communication, error-free 20-Gbps transmission over 3000 km has been demonstrated, confirming that the MLLD properties satisfy the requirements for use in real systems. The novel application of MLLDs to all-optical clock extraction, one of the essential functions required in all-optical signal processing, has been demonstrated at the 40-GHz SDH frequency.     

High frequency pulse trains from a self-starting additive pulse mode-locked all-fiber laser

In this paper, a coupled-cavity Er-doped fiber laser is experimentally developed and analyzed. The proposed scheme has the advantage of an all-fiber configuration. Two similar fiber Bragg gratings are employed as reflective components of the main cavity containing the gain medium. The second cavity is generated, in one side, by the reflective flat end of a standard fiber optic pigtail of variable length and, in the other, by one of the Bragg gratings belonging to the main cavity. Depending on the ratio between the lengths of both cavities, trains of stable and short pulses were obtained with a repetition frequency larger than the frequency of the main cavity. The repetition rate of the pulse trains experimentally obtained was as high as 780 MHz (15 times the main cavity frequency) and the pulse width was ∼110 ps. Prediction of the possible repetition rates for each cavities lengths ratio and the upgrading possibilities of this laser system are analyzed.     

Sub-100-as timing jitter optical pulse trains from mode-locked Er-fiber lasers

We demonstrate sub-100-as timing jitter optical pulse trains generated from free-running, 77.6 MHz repetition-rate, mode-locked Er-fiber lasers. At -0.002(±0.001) ps2 net cavity dispersion, the rms timing jitter is 70 as (224 as) integrated from 10 kHz (1 kHz) to 38.8 MHz offset frequency, when measured by a 24 as resolution balanced optical cross correlator. To our knowledge, this result corresponds to the lowest rms timing jitter measured from any mode-locked fiber lasers so far. The measured result also agrees fairly well with the Namiki-Haus analytic model of quantum-limited timing jitter in stretched-pulse fiber lasers.     

Generation of 10-GHz duty-cycle tunable square optical pulse in an SOA-based mode-locked fiber laser

We demonstrate the generation of 10-GHz optical square pulses by injecting a picosecond pulse train into an SOA-based mode-locked fiber laser. The novel scheme exploits nonlinear effects and gain saturation phenomenon in the semiconductor optical amplifier (SOA). This technique uses gain-compression dynamics between the input pulses and the generated ones in gain-saturated SOA to form square pulses. The center wavelength of the generated optical square pulse can be tuned from 1530 to 1570 nm by adjusting the center wavelength of the optical band pass filter (OBPF) in the SOA-based mode-locked fiber ring laser. The duty cycle of the output pulse can be tuned from 12.7 to 88.4%, which strongly depends on the input power and intra-cavity power.     

Ultralow-noise mode-locked optical pulse trains from an external cavity laser based on a slab coupled optical waveguide amplifier (SCOWA)

We report the generation of optical pulse trains with 8.5 fs timing jitter (10 Hz to 10 MHz) from a mode-locked semiconductor laser, with a slab coupled optical waveguide amplifier used as the gain element. This is, to our knowledge, the lowest residual timing jitter reported to date from an actively mode-locked laser.     

Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.     

Generation and propagation of pulse trains with ultrashort pulse separation

We investigate the nonlinear Schrödinger equation with variable coefficients by employing perturbation method. The analysis solution of the harmonic form is presented. The solution is one of forms to describe pulse trains with ultrashort pulse separation, which is about two orders of magnitude shorter than one of sech-type solitons considered before. And we could systematically adjust the perturbation parameter to obtain different pulse separation. As an example, we consider a nonlinear dispersive system with spatial parameter variations, and the results show that, the pulse train with ultrashort pulse separation presented by analysis solution may keep its shape even if the velocity is changed. The stability of the solution is discussed numerically, and the results reveal that the finite initial perturbations, such as white noise could not influence the main character of the solution. In addition, the stability of the solution is also discussed under more general conditions.     

Sub-microdrilling with ultrafast pulse laser interference

A novel sub-microdrilling technique utilising the phenomenon of ultrafast pulse laser interference is reported. This technique overcomes the feature size limit of conventional laser micromachining. The method of first interfering with the laser light, and then using the central bright fringe of the interfered beam to machine has been proven to effectively reduce the effective ablation spot size and, subsequently, to reduce the size of the drilled features. Preliminary results show a 300% reduction in drilled feature size with the interfered laser beam compared with the conventional non-interfered laser beam. 300 nm holes were successfully drilled on a 1000 Å thick Gold film using the interfered laser beam compared to 1 m holes ablated using the conventional non-interfered laser beam at the same pulse energy. PACS 42.62.Cf; 42.15.Eq; 42.25.Hz     

Nearly transform-limited optical pulse from a passively mode-locked laser diode

An optical ultra-short pulse train with a duration of 2.9 ps was successfully generated from a passively mode-locked laser diode. The time-bandwidth product was 0.43, and it was very close to the transformlimited value of a Gaussian waveform. The highest peak power of 10 mW in an InP-based passively mode-locked laser has been achieved. The laser is promisng as an optical source for an ultra-high-speed bit rate transmission system, especially for the optical time division multiplexing (OTDM) system.     

250 mW, 1.5 microm monolithic passively mode-locked slab-coupled optical waveguide laser

We report the demonstration of a 1.5 microm InGaAsP mode-locked slab-coupled optical waveguide laser (SCOWL) producing 10 ps pulses with energies of 58 pJ and average output powers of 250 mW at a repetition rate of 4.29 GHz. To the best of our knowledge, this is the first passively mode-locked slab-coupled optical waveguide laser. The large mode and low confinement factor of the SCOWL architecture allows the realization of monolithic mode-locked lasers with high output power and pulse energy. The laser output is nearly diffraction limited with M2 values less than 1.2 in both directions.     

Experiment on the optical tunneling process of electrons from a gold surface induced by mode-locked CO2 laser pulse trains

Using intense mode-locked CO₂ laser pulse trains we verified the occurrence of the laser-induced, new-type dynamic optical tunneling of electrons from a gold surface. The results are in accordance with the theoretical predictions and our previous experiments carried out for both atoms and gold surface using single longer (ns) CO₂ laser pulses. Unknown properties of the development of the CO₂ laser train are revealed based on our higher-order coherence detection of electron emission.     

Optical and RF stability transfer in a monolithic coupled-cavity colliding pulse mode-locked quantum dot laser

We report a novel quantum dot based laser design where a stable high-Q master laser is used to injection lock a passively mode-locked monolithic colliding pulse slave laser. Coupling between the crossed orthogonal laser cavities is achieved through a common monolithically integrated saturable absorber, which results in the locking and hence reduction of the timing jitter as well as the long-term frequency drift of the slave laser. A stable 30?GHz optical pulse train is generated with more than 10?dB reduction in the RF noise level at 20?MHz offset and close to 3 times reduction in the 10?dB average optical linewidth of the slave laser.     

Short optical pulse generation from laser diodes with directional coupler external cavity

We demonstrate picosecond optical pulse (POP) generation from semiconductor laser diodes (LDs) operating in a novel external cavity configuration. The structure uses a looped directional coupler to form an optical cavity whose end reflectivity is controlled by the coupling constant. The end reflectivity can be varied continuously between 0 and 100%, and the cavity length can be adjusted by changing the loop length. A theoretical model and experimental verification of results on the generation of high-repetition POPS are presented.     

Embedded dots inside glass for optical film using UV laser of ultrafast laser pulse

Microstructures are usually fabricated on the surface of optical sheets to improve the optical characteristics. In this study, a new fabrication process with UV (ultraviolet) laser direct writing method is developed to embed microstructures inside the glass. Then the optical properties of glass such as reflection and refraction indexes can be modified. Single- and multi-layer microstructures are designed and embedded inside glass substrate to modify the optical characteristics. Both luminance and uniformity can be controlled with the embedded microstructures. Thus, the glass with inside pattern can be used as a light guide plate to increase optical performance. First, an optical commercial software, FRED, is applied to design the microstructure configuration. Then, UV laser direct writing with output power 2.5-2.6 W, repetition rate 30 kHz, wave length: 355 nm, and pulse duration 15 ns is used to fabricate the microstructures inside the glass. The effect of dot pattern in the glass such as the dot pitch, the layer gap, and the number of layer on the optical performance is discussed. Machining capacity of UV laser ranges from micron to submicrometer; hence with this ultrafast laser pulse, objectives of various dimensions such as dot, line width, and layers can be easily embedded in the glass by one simple process. In addition, the embedded microstructures can be made with less contamination. Finally, the optical performance of the glasses with various configurations is measured using a Spectra Colorometer (Photo Research PR650) and compared with the simulated results.     

Generation of time-dependent ultra-short optical pulse trains in the presence of self-steepening effect

Starting from the extended nonlinear Schr?dinger equation in which the self-steepening effect is included, the evolution and the splitting processes of continuous optical wave whose amplitude is perturbed into time related ultra-short optical pulse trains in an optical fibre are numerically simulated by adopting the split-step Fourier algorithm. The results show that the self-steepening effect can cause the characteristic of the pulse trains to vary with time, which is different from the self-steepening-free case where the generated pulse trains consist of single pulses which are identical in width, intensity, and interval, namely when pulses move a certain distance, they turn into the pulse trains within a certain time range. Moreover, each single pulse may split into several sub-pulses. And as time goes on, the number of the sub-pulses will decrease gradually and the pulse width and the pulse intensity will change too. With the increase of the self-steepening parameter, the distance needed to generate time-dependent pulse trains will shorten. In addition, for a large self-steepening parameter and at the distance where more sub-pulses appear, the corresponding frequency spectra of pulse trains are also wider.     

Mode-locked laser pulse trains with subfemtosecond timing jitter synchronized to an optical reference oscillator

We independently phase lock the repetition rates of two femtosecond lasers at their approximately 456, 000th harmonic to a common optical oscillator. The timing jitter of each individual laser relative to the optical reference is only 0.45 fs in a 100-Hz bandwidth. Our method takes advantage of the tremendous leverage that is possible when stability is transferred from the optical to the microwave domain. The low timing jitter is commensurate with the independently measured fractional frequency instability in the repetition rates of < or = 2.3 x 10(-15) in 1-s averaging time, limited by the measurement system. The microwave signals at 1 GHz that are extracted by photodetection of the pulse trains have a 10-times-greater instability, confirming the presence of excess noise in the photodetection.     

External shaping of laser pulse trains

Two simple passive systems are presented for modifying the gaussian pulse train envelope produce by a dye-mode-locked Nd: YAG laser oscillator. The first system truncates the gaussian to produce a flat-topped envelope; the second system sharpens the envelope to a triangular form. The second system is analyzed as an intensity-dependent filter.     

A short pulse mode-locked Nd-glass laser

A mode-locked Nd-glass laser with a ring cavity configuration has been made. With this laser, using a resonator made with prisms instead of mirrors, a stable TEM₀₀ mode is obtained. Two photon measurements performed on the whole pulse train consistently yield a pulsewidth of 3.5 ps, with the proper contrast ratio 3:1.     

Timing jitter eater for optical pulse trains

Using a single phase modulator and dispersive fiber, we demonstrate a 12-dB reduction in the phase noise of a train of 6.5-ps pulses at a repetition rate of 10 GHz. A prechirp fiber is shown to improve performance.