Low Pump Threshold Research Articles

Theoretical study of the Ti:sapphire laser with low pump threshold

In this paper, a theoretical and numerical analysis of the factors that affect mode_locked pump threshold was made. The key effect of cavity mode in Kerr_lens mode_locking was illustrated. At the same time, a soliton model showed the rela tion between the pump threshold and dispersion in the cavity. Based on the gene ral analysis, a self_starting femtosecond Ti:sapphire laser with mode_locking th reshold as low as 390 mW was demonstrated.

Experimental study of femtosecond Ti:sapphire laser with low-threshold pump

A low-threshold self-starting Kerr-lens mode-locked Ti:sapphire laser was demons trated by use of a tight focusing cavity design and a semiconductor saturable-ab sorber mirror (SESAM). With a 3% and a 12% output couplers, we achieved mode-loc king thresholds as low as 390 and 600mW, respectively. Stable femtosecond laser pulses with average power of 114mW were generated for pump power at 12W, which corresponds to a typical duration of 17fs and bandwidth of 47nm. The pump power range for mode-locking covers from 600 mW to 48W at a 12% output coupler, an d the mode-locking stability is improved.

Low threshold and high power output of a diode-pumped nonlinear mirror mode-locked Nd:GdVO4 laser

We demonstrated nonlinear mirror mode-locking of a diode-pumped Nd:GdVO4 laser by using a 10 mm long KTP crystal. The stable CW mode locking can be reached with the relative low threshold pump power as low as 2.3W. The highest output power of 2.65W at the 10 W pump powers are generated with the 121-MHz repetition rate of pulses. It confirms the excellent laser performance of the Nd:GdVO4 for high-peak power of the pulse operation.

Resonantly pumped optical pumping injection cavity lasers

An optical parametric oscillator is tuned to the resonance wavelength of the etalon in an optical pumping injection cavity (OPIC) laser with a type-II “W” active region, thereby minimizing the threshold pump intensity and maximizing the output slope efficiency. Previous OPIC experiments employed fixed-wavelength sources with only limited tuning available by adjusting the incident angle. Low threshold pump intensities of 330W∕cm2 at 100K and 14kW∕cm2 at 300K (where the output wavelength is 3200nm) were achieved. The energy conversion efficiency is found to decrease by over a factor of 100 when the pump wavelength is tuned from the resonance condition (1822nm at 300K) to only slightly off resonance (e.g., 1808nm).

Organic laser based on thiophene derivatives

Laser action is obtained from two different neat films of electroluminescent oligothiophenes. The emissions are tunable, from the yellow to the red, on wavelength ranges up to 37 nm centered around 600 nm, in single-mode with a TEM 00 beam quality. The lasers exhibit low pump thresholds and efficiencies up to 1.7%.

Low threshold tunable lasing from a new substituted thiophene-based oligomer

We report on the lasing properties of spin coated films formed by a novel soluble substituted oligothiophene. The molecule shows a high gain cross section (6×10 -16 cm 2 ) and a low threshold (20 μJ cm -2 ) for line narrowing The laser shows single-mode TEM 00 emission tunable from yellow to red, over 37 nm, with a low pump threshold as low as 10 μJ cm -2 and an external efficiency up to 1.7%. These results are among the best so far reported for organic lasers.

Distributed feedback laser action in sol–gel glass symmetric waveguides

Symmetric waveguide lasers were prepared by the sol–gel method on glass substrate using rhodamine dye-doped zirconia films as the active guiding layer. The cover layer was titania–silica with a refractive index matching that of the glass substrate. Distributed feedback laser action was induced in the waveguide structure by the crossing beams of a frequency-doubled neodymium:YAG laser. Tunable laser action was achieved in the symmetric waveguide with guilding film thickness as small as 0.12µm. Lasing was not obtained for asymmetric waveguides of the same thickness. Lower pump threshold was generally observed for symmetric waveguide lasers. The dispersion characteristics of the waveguide lasers were in excellent agreement with thin-film optical waveguide theory, demonstrating the high optical quality of the sol–gel waveguide structure.

Low-threshold Photonic Crystal Lasers from InGaAsP Free-standing Slab Structures

Photonic band gap structures have a high potential for nearly zero-threshold lasers. This paper describes new-types of low-threshold photonic crystal lasers fabricated in InGaAsP slab waveguides free-standing in air. Two-types of photonic crystal lasers are studied. One is a single-cell nano-cavity laser formed in a square array of air holes. This photonic band gap laser operates in the smallest possible whispering gallery mode with a theoretical Q >30000 and exhibits low threshold pump power of 0.8 mW at room temperature. The nther laser does not have any cavity structure and the lasing operation originates from the enhanced optical density of states near photonic band edges. A very low threshold of 35 $\mu$ W (incident pump power) is achieved from this laser at 80 K, one of the lowest values ever reported. This low threshold is benefited from low optical losses as well as enhanced material gain at low temperature.

A portable multi-purpose non-classical light source

A compact and portable multi-purpose non-classical light source (volume about 300×400×500 mm 3 ) has been designed and built. A laser diode-pumped intracavity frequency-doubled Nd:YAP/KTP laser and a semi-monolithic non-degenerate optical parametric oscillator (NOPO) are closely integrated on an invar steel base. Above the oscillation threshold of the NOPO (∼3 mW), the twin beams with an intensity quantum correlation of 5.9 dB were obtained; when a signal of subharmonic wave, which is out of phase to the pump field, was injected into the NOPO, bright two-mode quadrature amplitude-squeezed light of 2.1 dB and an EPR beam with amplitude anticorrelation and phase correlation was produced at a pump power of milliwatt order. The features of compactness, portablility, low pump threshold, and versatility make the device useful for developing practical non-classical light sources.

STIMULATED BRILLOUIN SCATTERING IN n-TYPE III–V PIEZOELECTRIC SEMICONDUCTORS

Based upon the hydrodynamic model of an one-component semiconductor plasma, a detailed analytical study of stimulated Brillouin scattering (SBS) processes in a doped weakly piezoelectric semiconductor has been made following the coupled mode approach. The nonlinear effective polarization and the corresponding third-order optical susceptibility at Stokes frequency for the SBS processes are obtained. The analysis deals with the qualitative behavior of threshold pump electric field required for the onset of SBS and the Brillouin gain with respect to the doping concentration well above threshold. Numerical analysis has been made for n-type InSb crystal duly irradiated by a nanosecond pulsed 10.6 μm CO 2 laser. The influence of piezoelectric properties of the crystal medium on both threshold pump field and Brillouin gain has been explored and it is found that SBS with significant gain at lower threshold pump field could be achieved in the weakly piezoelectric moderately doped n-type semiconductor. The Brillouin gain is noted to be significantly large only over a very narrow doping range. The theoretical analysis shows that the doped III–V semiconductors have a good potentiality in the fabrication of SBS based phase conjugate mirrors with high reflectivity well below the optical damage threshold.

Low threshold lasing from fluorene-based copolymer blends with liquid crystallinity

Efficient energy transfer and lasing characteristics of the blends comprising fluorene-based liquid crystalline and non-liquid crystalline photoluminescent polymers with different band gap energies were studied. Time-correlated single photon counting study showed an efficient Forster-type energy transfer from the liquid crystalline donor to the non-liquid crystalline acceptor. A liquid crystalline blend with 2.0 wt-% of the acceptor gave a light emission whose intensity was 13 times stronger than that of each component. Also, photo-pumped laser action from the waveguide device fabricated with the blend was observed with the very low threshold pump energy of 1 nJ/pulse.

Observations of multipass transverse modes in an axially pumped solid-state laser with different fractionally degenerate resonator configurations

We experimentally show, for the first time to our knowledge, that a diode-pumped Nd:YVO4 laser can operate with multipass transverse (MPT) modes that self-reproduce after several round trips in a plano–concave cavity that has fractionally degenerate resonator configurations when the pump beam waist is sufficiently smaller than that of the fundamental cavity mode. The MPT mode is found to exhibit multiple beam waists located at different positions and to experience a lower pumping threshold than the single-pass transverse mode. With off-axis pumping, the N-pass transverse mode forms a symmetric pattern for even N and an asymmetric pattern for odd N. This result can be explained as being due to the introduction of MPT modes but not to superposition of the standard cavity modes.

Photonic crystal distributed feedback fiber lasers with Bragg gratings

Two new types of optical fibers, where air-holes are running down their length, are considered for making fiber lasers with Bragg gratings. The mode areas for pump and signal in these fiber lasers may be either larger or smaller compared to the corresponding mode areas for fiber lasers based on standard step index fibers. This makes possible realization of fiber lasers with a low pump threshold (small mode area), and fiber lasers suitable for high-power applications (large mode area).

Mirrorless lasing from mesostructured waveguides patterned by soft lithography

Mesostructured silica waveguide arrays were fabricated with a combination of acidic sol-gel block copolymer templating chemistry and soft lithography. Waveguiding was enabled by the use of a low-refractive index (1.15) mesoporous silica thin film support. When the mesostructure was doped with the laser dye rhodamine 6G, amplified spontaneous emission was observed with a low pumping threshold of 10 kilowatts per square centimeter, attributed to the mesostructure's ability to prevent aggregation of the dye molecules even at relatively high loadings within the organized high-surface area mesochannels of the waveguides. These highly processible, self-assembling mesostructured host media and claddings may have potential for the fabrication of integrated optical circuits.

Noncollinear-pumped KTP optical parametric oscillator

A theoretical analysis and an experimental study are presented on the phase-matching condition of noncollinearly pumped optical parametric oscillators (OPO's) based on a noncritical phase-matching potassium titanyl phosphate (KTP) OPO. It is shown that noncollinearly pumped, double-pass, singly resonant optical parametric oscillators maintain the advantage of round-trip parametric gain in a collinear pump, since in this case the two generated signal waves are coherent, which results in high conversion efficiency and low pump threshold. With this KTP OPO we achieved 31% energy conversion efficiency from a 1064-nm pump wave to 1572-nm eye-safe output. In addition, because the incident pump beam is not perpendicular to the OPO cavity mirrors and consequently no reflected beam flows back into the pump source, we can avoid employing optical isolators.

Stimulated soliton pulse formation and its mechanism in a passively mode-locked fibre soliton laser

We report on the stimulated formation of soliton pulses in a passively mode-locked fibre soliton laser. We found experimentally that if there are soliton pulses in the cavity, new soliton pulses can be generated one by one with very low pump threshold. We show that the mechanism for the soliton pulse generation is a soliton shaping of dispersive waves in the laser.

Observation of power drop and low threshold due to beam waist shrinkage around critical configurations in an end-pumped Nd:YVO 4 laser

Under pumping spot size less than that of the cavity mode, we observed a decrease of the fundamental mode power in an end-pumped solid-state laser when the cavity length was adjusted approaching certain critical resonator configurations where the transverse modes are degenerate. In the proximity of these configurations, we found, for the first time to our knowledge, that the laser exhibits a relatively low threshold pump power together with an unexpected far-field beam profile and shrinking beam waist.

Blue green stimulated emission of a high gain conjugated polymer

The stimulated emission properties associated with high optical gain is investigated in pure methyl-substituted poly-(paraphenylene) laddertype polymer (m-LPPP). Amplified spontaneous emission (ASE) in combination with waveguiding in the polymer film results in a highly directional linear polarised blue output at low pumping threshold of 3kW/cm2. Above a threshold value of 400 kW/cm2, additional narrow lines can be observed, that are assigned to amplified Stokes modes of stimulated Raman scattering (SRS). The output characteristics of different waveguides are investigated and explained by leaky wave propagation.

Ultrafast (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser for the 1.3 μm wavelength regime

(GaIn)(NAs) vertical-cavity surface-emitting lasers for room-temperature emission at 1.3 μm wavelength are designed and grown by metal-organic vapor-phase epitaxy using dimethylhydrazine and tertiarybutylarsine. Room-temperature operation at wavelengths up to 1.285 μm is achieved with low optical pumping thresholds between 1.6 and 2.0 kW/cm2. Stimulated emission dynamics after femtosecond optical pumping are measured and compare favorably with results on (GaIn)As/Ga(PAs)-based structures.

Demonstration of an InGaN/GaN-based optically pumped multiquantum well distributed feedback laser using holographically defined third-order gratings

We demonstrate an optically pumped InGaN/GaN-based multiquantum well distributed feedback laser in the blue spectral region. The third-order grating providing feedback was defined holographically and dry etched into the upper waveguiding layer by chemically assisted ion-beam etching. When aligning the stripe-shaped pump beam either parallel or perpendicular to the grating grooves, we found a considerably lower pumping threshold, higher slope efficiency, a slightly longer emission wavelength, and a much narrower linewidth for the geometry with the pump beam orthogonal to the grating lines. A nearly constant emission wavelength of 400.85 nm and a linewidth of 0.7 Å were observed under various pump intensities. To the best of our knowledge, this is the narrowest linewidth ever reported for an optically pumped device in this material system.