Frequency stabilization and output power undulations of diode lasers with feedback by volume holographic gratings

The mode-hop behavior and the power characteristics of a laser diode with wavelength-selective optical feedback are experimentally investigated. The feedback is provided by external volume holographic gratings, also called ‘Bragg mirrors’, at normal incidence. We demonstrate that a Bragg mirror forces a laser diode to operate only within a narrow wavelength range, and that the emission wavelength of the laser diode is stabilized against variations of the injection current. Moreover, we present periodic undulations of the power characteristics of the laser, depending on the driving current. They can be qualitatively explained with a simple model which takes into account that the threshold gain in the laser system strongly depends on the wavelength.     

Tunable dual-wavelength operation of an external cavity semiconductor laser

Tunable dual-wavelength operation of a laser diode has been achieved using a dual-period holographic element. The holographic element was positioned in Littrow configuration in an external cavity. The design of this holographic element was based on the superposition of a constant-period grating and a variable-period grating on the same location in a photoresist. We observed simultaneous emission at two wavelengths from a visible semiconductor laser operated with this holographic element. The spectral separation of the dual-wavelength output was varied from 0.76 to 6.27 nm by a simple translation of the holographic element.     

Modelling of experimentally measured Q-switched pulsations in InGaAs/GaAs diode lasers

A numerical calculation of a passively Q-switched two-section ridge-waveguide InGaAs/GaAs diode laser is presented in this study. The author has modelled the output power-current (L-I) variation under cw conditions, time evolution of photon numbers under transient conditions and Q-switched pulses of the device for various reverse bias voltages to the absorber section. Resulting simulations of the L-I characterisation and Q-switched pulsations are compared quantitatively with experimental results. Simulated Q-switched pulse profiles have been obtained in the absence and presence of noise. In both cases, proposed model shows that a tail occurring at the end of the Q-switched pulse is eliminated at −7.5 V reverse bias voltage, which is confirmed by experiment. As a result, experimentally obtained tail-free and single peak picosecond Q-switched pulses with peak powers of ∼1 W and durations of typically tens of picoseconds are also demonstrated theoretically. Simulations show consistency with the experimental data.     

Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

Near-infrared trace-gas sensors based on room-temperature diode lasers

2 monitor designed for field applications using room-temperature diode lasers are presented. Near-infrared DFB lasers operating at 1.57 μm and around 2.0 μm have been used for CO₂ measurements. At ambient concentration levels a resolution of more than two orders of magnitude has been demonstrated at 1.57 μm, at 2 μm the precision is in the order of 0.1 ppm CO₂, and for trace analysis a detection limit of 10 ppb has been obtained. The measurements demonstrate the capability of near-infrared DFB diode lasers for the precise determination of CO₂ concentrations as required for climatological, medical, or industrial applications. Received: 24 February 1998/Revised version: 27 April 1998     

Frequency noise suppression in a diode laser locked to a travelling wave resonator incorporating a Brewster prism

A polarisation locking technique was applied to stabilise an extended cavity diode laser using a travelling wave resonator incorporating a Brewster prism. Despite the fact that the employed unbalanced detection was sensitive to optical power fluctuations, the in-loop photodetector measured 90 dB of noise suppression at 10 Hz in comparison to the free running frequency noise spectrum. Excess intensity noise measured with an out-of-loop detector, indicated the presence of correction-correlated noise in the output of the stabilised diode laser.     

Diode-pumped Nd:YAG laser using reflective pump optics

We have examined the performance of a diode-laser side-pumped Nd:YAG laser using elliptical mirrors to focus the output of 6 × 10 W laser-diode arrays into the Nd: YAG rod. The multimode cw output power was 14 W with an optical to optical efficiency of 29%. With a resonator designed for TEM₀₀ mode operation 12 W of output was achieved.     

Measurement and analysis of the electric field in semiconductor lasers by continuous-wave electro-optic probing

The electric field distribution in semiconductor lasers are detected experimentally by using CWEOP (Continuous-Wave Electro-Optic Probing). The paper briefly describes the experimental results. The obtained results reflect several characteristics of the lasers such as injection current, carrier confinement, and the variation of the distribution of the electric field corresponding to different bias conditions.     

A global design of high power Nd-Yb co-doped fiber lasers

A global optimization method - niche hybrid genetic algorithm (NHGA) based on fitness sharing and elite replacement is applied to optimize Nd³⁺-Yb³⁺ co-doped fiber lasers (NYDFLs) for obtaining maximum signal output power. With a objective function and different pumping powers, five critical parameters (the fiber length, L; the proportion of pump power for pumping Nd³⁺, η; Nd³⁺ and Yb³⁺ concentrations, N_Nd and N_Yb and output mirror reflectivity, R_out) of the given NYDFLs are optimized by solving the rate and power propagation equations. Results show that dividing equally the input pump power among 808 nm (Nd³⁺) and 940 nm (Yb³⁺) is not an optimal choice and the pump power of Nd³⁺ ions should be kept around 10-13.78% of the total pump power. Three optimal schemes are obtained by NHGA and the highest slope efficiency of the laser is able to reach 80.1%.     

Temporal synchronization of independently tunable single longitudinal mode hybrid CO2 lasers

We report a quantitative study of temporal synchronization of two independently tunable, single longitudinal mode, hybrid CO₂ lasers, sharing a common high pressure section. The theoretical estimates for the dependence of the laser pulse build up time on the cw section pressure are in satisfactory agreement with the experimental results. Further, we show that temporal synchronization over a larger frequency range with no significant degradation of peak power or longitudinal mode selection is possible, by utilising the cw section gain length and cavity Q as additional control parameters.     

High dielectric anisotropy compound doped transmission gratings of HPDLC

Effects of high dielectric anisotropy compound (MLC) and oligomer molecular weight on the grating formation and electro-optical properties of transmission holographic polymer dispersed liquid crystal (HPDLC) have been studied. Addition of MLC reduced the response time and switching voltage with the increased formulation viscosity which introduced a sequence of phenomenon, i.e. slow diffusion, polymerization, phase separation, grating formation, and droplet coalescence giving small droplet size and enhanced diffraction efficiency of the film. Apparently, increase in oligomer molecular weight gave effects similar to those of increasing MLC content since both variables control droplet coalescence through different mechanisms.     

Passive compensation of the thermal drift of magnetostriction based Q-switched fiber lasers

The authors propose and demonstrate a method to compensate the thermal drift of magnetostriction based Q-switched fiber lasers, which is caused by the eddy currents induced in the Terfenol-D magnetostrictive actuators. The consequent wavelength detuning between the fiber gratings of the laser is passively compensated by the use of Monel 400 as thermal actuator of the non-modulated grating. A highly stable pulsed signal is achieved in the range of 1 Hz-5 kHz, with a wavelength detuning between gratings maintained below 10 pm. Furthermore, an optimization of the use of the pump power is proposed, utilizing part of it for simultaneously pumping a fiber optic based amplification stage.     

A time-compensated spectral filter for transform-limited tunable picosecond pulse generation from spectro-temporal-selection dye lasers

Diffraction and transform-limited picosecond tunable pulses are generated from Spectro-temporal-Selection (STS) dye lasers by using a new extra-cavity filter. This filter is based on a grazing-incident grating and arranged in the configuration of a folded dispersive delay line. Thus, it provides both high spectral selectivity and controllable temporal compensation for elimination of pulse broadening. Direct production of diffraction- and transform-limited picosecond dye laser (10 µJ, 50 ps) pulses spectrally adjustable between 398 and 702 nm is demonstrated in a compact device, with 8 ns pump pulses from a nanosecond nitrogen laser.     

Second-order grazing-angle scattering in uniform wide holographic gratings

Grazing-angle scattering (GAS) is a type of Bragg scattering in slanted wide periodic gratings. It occurs when the diffracted order satisfying the Bragg condition (scattered wave) propagates at a grazing angle to the grating boundaries. Previous research has been concerned only with first-order GAS, which has been shown to be a highly unusual type of scattering characterised by a strong resonant increase of amplitudes of the scattered and incident waves in the grating. In this paper, a rigorous numerical study of second-order GAS is presented for the case of bulk TE electromagnetic waves in planar holographic gratings. A highly unusual pattern of strong resonances in the grating, which is strongly different from that for first-order GAS, is predicted, described, and discussed. Physical interpretations of the predicted results are presented. In particular, a special new type of eigenmodes in a slanted wide periodic grating with large amplitude is predicted. These eigenmodes are shown to be guided by the grating alone without any conventional guiding effect in the structure. The typical field structure in such eigenmodes is investigated and discussed. Received: 16 September 2002 / Revised version: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +61-7/3864-9079, E-mail: d.pile@osa.org     

Noise as reliability screening for semiconductor lasers

The low-frequency electrical noise in semiconductor lasers is measured and used for device-reliability screening, which is a sensitive and non-destructive method. In the experiment, we developed some approaches to improve the validity of reliability screening by using noise criteria. A new method of determining the threshold level of noise criteria is given. The experimental results show that this method is effective. Received: 26 September 2002 / Revised version: 28 January 2003 / Published online: 9 April 2003 RID="*" ID="*"Corresponding author. Fax: +86-431/462-7013, E-mail: huguijun@sina.com     

Strain Compensated AlInGaAs/InGaAs/InAs Triangular Quantum Wells for Lasing Wavelength beyond 2μm

The subband energy and lasing wavelength of compressively strained triangular Ino.53Ga0.47As/InAs quantum well are calculated and compared with the conventional rectangular ones with the same strain contents. The strain compensation using Al0.33In0.36Ga0.31As barrier is introduced. The results show that lasing wavelength can be extended dramatically to beyond 2.8μm by changing the energy band from the conventional rectangular shape to a triangular one, the realization of such a structure using molecular beam epitaxy technology is also discussed.     

Electrically Pumped Room-Temperature Pulsed InGaAsP-Si Hybrid Lasers Based on Metal Bonding

A pulsed InGaAsP-Si hybrid laser is fabricated using metal bonding. A novel structure in which the optical coupling and metal bonding areas are transversely separated is employed to integrate the silicon waveguide with an InGaAsP multi-quantum well distributed feedback structure. When electrically pumped at room temperature, the laser operates with a threshold current density of 2.9 kA/cm^2 and a slope efficiency of 0.02 W/A. The 1542nm laser output exits mainly from the Si waveguide.     

Pulse evolution in injection mode locked TECO2 lasers

The pulse train evolution in an injection mode locked TECO₂ laser has been investigated for a wide range of input pulse widths e.g., ranging from the cavity round-trip time to near the bandwidth-limited value for the system. Regimes in which pulse narrowing and pulse broadening occur have been identified and are discussed. The combined use of injection and saturable-absorber mode-locking techniques has produced reliable subnanosecond pulse trains from a large aperture TEACO₂ laser.     

Recent advances in lead-chalcogenide diode lasers

The fundamental material properties of Pb_1−xSn_xTe, PbS_1−xSe_x and Pb_1−xSn_xSe are reviewed. Expressions for the temperature and compositional dependences of the band parameters and dielectric constants based on recently published data are presented. As far as device technology is concerned, crystal growth techniques and diode fabrication procedures which are in use today are reviewed and compared. A comprehensive summary of laser properties like threshold current density, output power, efficiency, maximum operating temperature and tuning range of different diode structures are presented. Application related aspects such as long term stability are treated. Recent progress in laser theory is applied to explain experimentali _th vs.T curves. The various laser applications are reviewed briefly. A new technique for monitoring gas concentrations using pulsed lasers and an integral method for signal processing is discussed and compared with the differential absorption, derivative spectroscopy. A long-path trace-gas monitoring system incorporating this new technique is presented.     

All-fiber actively Q-switched fiber laser tuned by a pair of temperature controlled fiber Bragg gratings

We report an all-fiber actively Q-switched erbium-doped fiber laser, where the linear laser cavity mirrors are composed of two fiber Bragg gratings (FBGs). The laser oscillation wavelength could be tuned by this pair of temperature controlled FBGs. The Q-switching is achieved by an all-fiber phase modulation device. Using this system, we could obtain stable Q-switched laser pulses output, which could be optimized by tuning the reflection wavelengths of the two FBGs to be adjacent to each other. Instead of being modulated by the FBG filter in high-speed oscillation, this fiber laser system is operating in the Q-switched regime using an all-fiber phase modulator, producing a more stable laser output spectrum.