Delayed-self-heterodyne measurement of laser frequency fluctuations

The two-sample (or Allan) variance of frequency fluctuations was used for laser frequency-noise characterization at Fourier frequencies below 1 MHz. The delayed self-heterodyne technique was used for the variance measurement. Analytical calculations ensure that just twice the variance values were observed at τ<0.3τ_d, where τ is the averaging time for the frequency measurement and τ_d is the delay time. The variance measurements of 1.5-μm distributed Bragg reflector and distributed feedback laser diodes were demonstrated at 6.0×10^-7⩽τ⩽1.0×10^-4 [s] by using an 81.5-km fiber delay line. Observed flicker fluctuations (or 1/ f noise) are discussed     

Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

PLL propagation delay-time influence on linewidth requirements ofoptical PSK homodyne detection

Phase-locked loop (PLL) propagation delay-time influence on optical homodyne detection was investigated both theoretically and experimentally. Applying the Pade approximation, which is often used in the control system, to the calculation of the phase-error variance with the nonzero loop delay time, a high-accuracy analytic expression phase-error variance is obtained. The linewidth requirement with the nonnegligible loop delay time for phase-shift-keying (PSK) homodyne detection is obtained as δν=2.04×10^-3/τ where δν (hertz) is beat linewidth and τ (seconds) is the loop delay time. The linewidth requirement with small delay time approaches δν=6.2×10^-4 R_b where R_b (bits-per-second) is the system bit rate. Results were confirmed by a 10-GB/s optical PSK homodyne detection experiment using external cavity laser diodes. Receiver sensitivity degradations due to loop delay time and beat linewidth are in good agreement with theoretical results     

Excited-state absorption studies of Cr4+ ions in severalgarnet host crystals

An experimental study of saturable absorption and excited-state absorption (ESA) in several inorganic saturable absorbers, Cr⁴⁺ :YAG, Cr⁴⁺:GGG, and Cr⁴⁺:YSGG, is presented. We provide the theoretical background of absorption characteristics in saturable absorbers that exhibit ESA, with some new results: approximate analytical solutions are proposed for the optical transmission in the case of a slow absorber, and for various light intensity conditions of spatially or temporally Gaussian beams in fast and slow absorbers. Experimentally, partial bleaching of the first excited state itself could be observed in Cr⁴⁺:YAG at λ=1064 nm, yielding the higher excited-state lifetime as τ*=(0.55±0.1) ns. The regular transmission bleaching curve was measured in Cr⁴⁺:GGG, for the first time in this material, yielding σ_ga=(58±5)×10^-1 cm², and σe_s=(13±2)×10^-19 cm² at λ=1064 nm, ESA spectra were measured for the three materials between ~700 and 900 nm. All three exhibit crossing between saturable absorption at longer wavelengths and inverse saturable absorption at shorter wavelengths     

Laser-pumped rubidium frequency standards: new analysis andprogress

We have achieved a stability of 3·10^-13 τ ^-1/2 for 3<τ<30 s with a laser-pumped rubidium gas-cell frequency standard by reducing the effects due to noise in the microwave and laser sources. This result is one order of magnitude better than the best present performance of lamp-pumped devices     

Experimental determination of electron drift velocity in 4H-SiC p+-n-n+ avalanche diodes

4H-SiC p⁺-n-n⁺ diodes of low series resistivity (<1×10^-4 Ω·cm²) were fabricated and packaged. The diodes exhibited homogeneous avalanche breakdown at voltages U_b=250-270 V according to the doping level of the n layer. The temperature coefficient of the breakdown voltage was measured to be 2.6×10^-4 k^-1 in the temperature range 300 to 573 K. These diodes were capable of dissipating a pulsed power density of 3.7 MW/cm² under avalanche current conditions. The transient thermal resistance of the diode was measured to be 0.6 K/W for a 100-ns pulse width, An experimental determination of the electron saturated drift velocity along the c-axis in 4H-SIC was performed for the first time, It was estimated to be 0.8×10⁷ cm/s at room temperature and 0.75×10⁷ cm/s at approximately 360 K     

Diode-pumped ytterbium-doped Sr5(PO4)3F laser performance

The performance of the first diode-pumped Yb³⁺-doped Sr ₅(PO₄)₃F (Yb:S-FAP) solid-state laser is discussed. An InGaAs diode array has been fabricated that has suitable specifications for pumping a 3×3×30 mm Yb:S-FAP rod. The saturation fluence for diode pumping was deduced to be 5.5 J/cm ² for the particular 2.8 kW peak power diode array utilized in our studies. This is 2.5× higher than the intrinsic 2.2 J/cm ² saturation fluence as is attributed to the 6.5 nm bandwidth of our diode pump array. The small signal gain is consistent with the previously measured emission cross section of 6.0×10^-20 cm², obtained from a narrowband-laser pumped gain experiment. Up to 1.7 J/cm³ of stored energy density was achieved in a 6×6×44 mm Yb:S-FAP amplifier rod. In a free running configuration, diode-pumped slope efficiencies up to 43% (laser output energy/absorbed pump energy) were observed with output energies up to ~0.5 J per 1 ms pulse. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 μs pulses     

Carrier transport related analysis of high-power AlGaN/GaN HEMTstructures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm²/V/s and a sheet concentration of 9×10¹² cm at 300 K (7900 cm²/V/s and 8×10¹² cm^-2 at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n_SdH=7.8×10¹² cm^-2 and a high quantum scattering lifetime of τ_q=1.5×10^-13 s at 4.2 K compared to n_SdH=8.24×10¹² cm^-2 and τ_q=1.72×10^-13 s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance     

Second harmonic generation and sum frequency mixing of dualwavelength Nd:YAlO3 laser in flux grown KTiOPO4crystal

On the basis of accurately measured refractive indexes, the authors have obtained the Sellmeier's equations for flux grown KTiOPO ₄ (KTP) crystal and used them to calculate the phase matched angles (&thetas;_m, φ_m) and effective nonlinear coefficients (d_eff) for type I and III second harmonic generation (SHG) and sum frequency mixing (SFM) of radiations at 1.0795 and 1.3414 μm. The optimum phase matching conditions for 1.0795 and 1.3414 μm SHG are that &thetas;_m=86.88 and 58.88°, respectively, in an XZ plane (φ=0) and for SMF of 1.0795 and 1.3414 μm in the same plane 76.02°. The corresponding d_eff values calculated from &thetas; _ms are 18.07×10^-9 and 17.42×10^-9 esu     

Lasing mode behavior of a single-mode MQW laser injected with TMpolarized light

The lasing mode behavior of a multiple quantum well (MQW) distributed feedback (DFB) laser was measured when intensity-modulated orthogonally polarized transverse magnetic (TM) mode light was injected. The 3-dB bandwidth of the frequency response shows a trend different from that observed with conventional bias current modulation: at high bias currents, it decreases with increasing bias current. The maximum bandwidth of 3 dB was observed when the normalized bias current was 4, and it reached 16 GHz at this bias current. The gain saturation coefficients for the transverse electric (TE) and TM modes estimated from these results were ∈_p^E; 2.5×10^-17 cm³ and ∈_q^E 5.7×10^-18 cm³ for the TE mode, and ∈_p^M: 6.0×10^-17 cm³ and ∈_q^M: 2.0×10^-14 cm³ for the TM mode     

Diamond cold cathode

Diamond cold cathodes have been formed by fabricating mesa-etched diodes using carbon ion implantation into p-type diamond substrates. When these diodes are forward biased, current is emitted into vacuum. The cathode efficiency (emitted current divided by diode current) varies from 2×10^-4 to 1×10^-10 and increases with the addition of 10^-2-torr partial pressure of O₂ into the vacuum system. Current densities of 0.1 to 1 A-cm^-2 are estimated for a diode current of 10 mA. This compares favorably with Si cold cathodes (not coated with Cs), which have efficiencies of ~2×10^-5 and current densities of ~2×10^-2 A-cm^-2. It is believed that higher current densities and efficiencies can be obtained with more efficient cathode designs and an ultrahigh-vacuum environment     

Measurement of mode couplings and extinction ratios inpolarization-maintaining fibers

Polarization mode couplings in the axial direction are evaluated for polarization-maintaining fibers using optical heterodyne detection. To verify the validity of this approach for fibers with various coupling constants, the method is applied to three fibers with modal birefringence values of 3.0×10^-4, 1.1×10^-4 , and 1.5×10^-4, respectively. The coupling constants in the 1.7×10^-4 m^-1 to 6.4×10^-7 m^-1 range are evaluated with a length resolution of 1 m. The extinction ratios are obtained from the coupling constants averaged over the fiber lengths. These values are in good agreement with the values measured directly from power ratios between the orthogonally polarized modes     

High-order harmonic generation from a solid surface plasma by usinga picosecond laser

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a prepulse-free 2.2-ps 1054-nm laser pulse at maximum intensities of ~1×10¹⁵ W·cm^-2 . The harmonic intensities were strongest for p-polarized pump and dropped to about 50% in the case of s-polarized pumps. The ratio I_4wI_2w between the intensities of the fourth and second harmonics was 0.25 at Iλ²=6×10¹⁴ W·cm^-2·μm², which is relatively large when compared with the value predicted by simulations. The harmonics are collimated in the specular direction, with a transverse mode and divergence comparable to that of the driving laser     

Measurement of the stimulated emission cross section for the 4F3/2-4I13/2transition of Nd3+ in YAlO3 crystal

A method for the simultaneous measurement of the stimulated emission cross section and fluorescence lifetime by studying the relation between laser parameters and the laser relaxation oscillation frequency is discussed. The stimulated emission cross section for the ⁴F_3/2-⁴I_13/2 transition of Nd³⁺ ion in YAP crystal was measured to be (22±1)×10^-20 cm²     

2W reliable operation of λ=735 nm GaAsP/AlGaAs laser diodes

Reliable operation of 735 nm laser diodes based on a tensile-strained GaAsP quantum well embedded in an AlGaAs large optical cavity structure is reported. The 100 μm stripe width laser diodes were aged at a record high output power of 2W for 2000 hours. The degradation rates were 3.6×10^-5 h^-1     

Two-photon absorption coefficients of several liquids at 264 nm

The nonlinear absorption of water (H₂O), heavy water (D ₂O), ethanol, methanol, hexane, cyclohexane, 1, 2-dichloroethane, and chloroform at 264 nm was studied using femtosecond laser pulses. The two-photon absorption coefficients for these liquids were found to be between (34 ± 3) × 10^-11 cm/W and (95 ± 11) × 10^-11 cm/W     

Modulation characteristics of high speed (f-3 dB=20 GHz)tunneling injection InP/InGaAsP 1.55 μm ridge waveguide lasersextracted from optical and electrical measurements

High-performance multiquantum-well 1.55 μm InP-based tunneling injection lasers are fabricated using a conventional single mode ridge waveguide fabrication process and characterized. The lasers consist of an eight quantum-well strain-compensated gain region and a 30-Å InP tunneling barrier. The bandwidth of these lasers is measured to be 20 GHz with a damping limited bandwidth extracted from the K-factor (determined from optical modulation measurements) of 26 GHz. To our knowledge, this is the highest measured bandwidth recorded for an InP-based simple ridge waveguide structure. The differential gain is measured to be as high as 1×10^-15 cm², with a measured gain compression coefficient ϵ of 5×10^-17 cm³. It is shown that the K-factor can also be extracted solely from measurements of the small signal electrical impedance. The carrier escape time τ_esc is determined to be 0.5 ns, independent of bias. This high frequency performance is achieved with a very simple device structure at room temperature under constant drive currents     

High average and high peak brightness slab laser

A high average and high peak brightness Nd:YAG MOPA laser system composed of a laser-diode-pumped Nd:YAG master oscillator, flash-lamp-pumped slab power amplifiers and a phase conjugated mirror was developed. The system demonstrates an average output power of 235 W at a repetition rate of 320 Hz and a peak power of 30 MW at a pulse duration of 24 ns with M²=1.5. Both an average brightness of 7×10⁹ W/cm²·sr and a peak brightness of 1×10¹⁵ W/cm2·sr are achieved simultaneously. The system design rules that we confirmed suggest that by replacing lamp pumping in the amplifier with laser-diode pumping, an average output power of ~1 kW can be obtained at ~1 kHz with a higher average brightness of ~3×10¹⁰ W/cm²·sr and a higher peak brightness of ~3×10¹⁵ W/cm²·sr     

Demonstration of thermal poling in holey fibres

A second-order nonlinearity is induced for the first time in a holey fibre by thermal poling. Non-phase matched second harmonic generation with a ~10^-8/W conversion efficiency is observed and the electro-optic coefficient is measured to be ~0.02 pm/V