mW Average Power Research Articles

Efficient and flexible laser ultrasound generation using spatial light modulators

Efficient production of surface acoustic waves using optical distributions controlled by spatial light modulators is demonstrated. The measurements presented used only 40 mW average optical power on the sample surface. It is shown that a spatial light modulator produces light distributions which control the surface acoustic wave generation on the sample surface; this can be used to produce similar light distributions to those formed by computer generated holography.

Picosecond surface-emitting semiconductor laser with > 200 mW average power

A passively modelocked diode-pumped surface-emitting semiconductorlaser at 950nm with a 2GHz repetition rate is reported. Compared tothe first device of this kind, which the authors recently reported, agreatly improved average output power of 213mW and a reduced pulseduration of 3.2ps are achieved. The device consists of an opticallypumped semiconductor gain structure and a semiconductor saturableabsorber mirror (SESAM) in an external cavity. Introduction: Pulsed laser sources with multi-GHz repetition rates andaverage powers > 100mW are required for applications such as opticalclocking of integrated circuits, optical testing of semiconductor elec-tronics and telecommunication components, particle accelerators, andanalogue-to-digital conversion. Edge-emitting semiconductor lasershave been passively modelocked for quite some time [1] and ultra-shortpulses [2] and extremely high repetition rates [3] have been reported.The average and peak power is limited in these devices, because stablemodelocking requires fundamental transverse mode operation, whichrestricts the size of the facet. With optically pumped vertical-external-cavity surface-emitting semiconductor lasers (VECSELs [4]) these con-straints are eliminated, and much higher powers are possible withaccordingly increased mode areas, while diffraction-limited output isenforced by the external cavity. Pulsed operation has been achieved withsynchronous pumping [5, 6] and with active modelocking [7]. Recently[8], we demonstrated the first VECSEL which is continuously pumpedand passively modelocked with a semiconductor saturable absorber mir-ror (SESAM [9, 10]). With a new gain structure optimised for a lowthermal impedance and a smooth gain spectrum we have obtained tentimes more average power and six times shorter pulse duration. Gain structure: In the previously described gain structure [8], the gener-ated heat is removed through the GaAs substrate, which introduces asignificant thermal impedance. We have eliminated this problem by fab-ricating a structure where the active region is separated from the copperheat sink only by a semiconductor Bragg mirror. This is achieved by epi-taxial lift-off (ELO): first, we grew the gain structure in reverse order ona GaAs substrate with three intermediate etch-stop layers. We used afluxless indium soldering process similar to the one published by So etal . [11] to solder a dice to a copper heat sink and finally removed theGaAs substrate by etching. The additional effort of the epitaxial lift-off is justified by two impor-tant improvements. First, the reduced thickness of the semiconductormaterial leads to a greatly reduced thermal impedance, and the nearlyone-dimensional heat flow into the heat sink makes the device power-scalable: the output power can, for example, be doubled by applyingtwice the pump power to twice the mode area without raising the tem-perature of the gain structure, as long as the heat sink is able to removethe additional power. The second improvement is that we eliminate thepreviously observed etalon effect from the interference of reflectedwaves from the Bragg mirror and the soldered surface. This effect lim-ited the bandwidth and disturbed the modelocking in the previousdevice. The gain structure consists of a bottom mirror, an active region, andan anti-reflective section grown by metal-organic chemical vapour depo-sition (MOCVD). For the bottom mirror we used seven pairs of GaAs/AlAs and 23 pairs of Al

Intracavity frequency-tripling of actively mode-locked diode-pumped Nd:YAG laser

We report the first diode-pumped solid-state laser operating in cw-mode-locked regime and simultaneously achieving intracavity frequency-tripling. This laser provide UV picosecond pulses (λ=355 nm) of 10 ps duration with 0.5 mW average power at 150 MHz repetition rate. A different set of adjustments gave rise to a Q-switched mode-locked regime. Trains of hundred UV pulses of 60 ps duration and 4 W peak power were produced in this latter case at 50 kHz repetition rate.

Photolysis of Caged Calcium in Femtoliter Volumes Using Two-Photon Excitation

A new technique for the determination of the two-photon uncaging action cross section ( δ u) of photolyzable calcium cages is described. This technique is potentially applicable to other caged species that can be chelated by a fluorescent indicator dye, as well as caged fluorescent compounds. The two-photon action cross sections of three calcium cages, DM-nitrophen, NP-EGTA, and azid-1, are studied in the range of excitation wavelengths between 700 and 800 nm. Azid-1 has a maximum δ u of ∼1.4 GM at 700 nm, DM-nitrophen has a maximum δ u of ∼0.013 GM at 730 nm, and NP-EGTA has no measurable uncaging yield. The equations necessary to predict the amount of cage photolyzed and the temporal behavior of the liberated calcium distribution under a variety of conditions are derived. These equations predict that by using 700-nm light from a Ti:sapphire laser focused with a 1.3-NA objective, essentially all of the azid-1 within the two-photon focal volume would be photolyzed with a 10- μs pulse train of ∼7 mW average power. The initially localized distributions of free calcium will dissipate rapidly because of diffusion of free calcium and uptake by buffers. In buffer-free cytoplasm, the elevation of the calcium concentration at the center of the focal volume is expected to last for ∼165 μs.

29 GHz modelocked miniature Nd:YVO4 laser

A miniature Nd:vanadate (Nd:YVO4) laser is presented which was passively modelocked at the very high repetition rate of 29 GHz using a semiconductor saturable absorber mirror (SESAM). An 81 mW average output power was achieved, which corresponds to 2.8 pJ pulse energy, and a pulse duration of 6.8 ps.

Progress in the growth of mid-infrared InAsSb emitters by metal-organic chemical vapor deposition

We report on recent progress and improvements in the metal-organic chemical vapor deposition (MOCVD) of mid-infrared InAsSb emitters using a high speed rotating disk reactor (RDR). The devices contain AlAsSb claddings and strained InAsSb active regions. These emitters have multi-stage, type I, InAsSb/InAsP quantum well active regions. A semi-metal GaAsSb/InAs layer acts as an internal electron source for the multi-stage injection lasers and AlAsSb is the electron confinement layer. These structures are the first MOCVD multi-stage devices. Growth in an RDR was necessary to avoid the previously observed Al memory effects found in a horizontal reactor. Broadband LEDs produced 2 mW average power at 3.7 μm and 80 K and 0.1 mW at 4.3 μm and 300 K. A multi-stage, 3.8–3.9 μm laser structure operated up to T=180 K. At 80 K, peak-power >100 mW/facet and a high slope-efficiency (48%) were observed in these gain guided lasers.

Self-starting passively mode-locked tunable Cr4+: yttrium–aluminum–garnet laser with a single prism for dispersion compensation

A self-starting passive mode-locked Cr4+:yttrium–aluminum–garnet laser that uses a saturable Bragg reflector with a single prism for dispersion compensation instead of the standard prism pair is demonstrated. We observed over 230 mW average output power with a pulse width of 400 fs at center wavelengths shorter than 1.5 μm. The tuning range for cw laser operation extends from 1345 to 1557 nm. The mode-locked pulses are self-starting and tunable from 1420 to 1510 nm by means of lossless prism tuning without aperture.

5-GHz repetition-rate dual-wavelength pulse-train generation from an intracavity frequency-modulated Er–Yb:glass laser

We report the experimental demonstration of nearly transform-limited dual-wavelength pulse trains at a 5-GHz repetition rate that were generated by spectral filtering of an intracavity 2.5-GHz frequency-modulated Er-Yb bulk-glass laser operating at the 1533-nm wavelength. Highly stable dual-wavelength pulse trains with approximately 165-GHz frequency separation, approximately 48-ps pulse duration, and approximately 1-mW average single-mode fiber-coupled power were obtained.

Ultraviolet single-frequency pulses with 110 mW average power using frequency-converted passively Q-switched miniature Nd:YAG ring lasers

We demonstrate a compact source of ultraviolet single-frequency pulses with high average output power realized by subsequent single-pass frequency doubling and quadrupling of a diode-pumped passively Q-switched miniature Nd:YAG ring laser. Pulse widths of 2 ns (full width at half maximum), 20 μJ single pulse energy, and 10 kW peak power are achieved at a wavelength of 266 nm with kilohertz repetition rates. A single-pass energy conversion efficiency of 22% with respect to the fundamental light at 1064 nm and an overall efficiency of 2.8% with respect to the diode pump power are obtained.

FINAL-FOCUS SYSTEM AND COLLISION SCHEMES FOR A 5-TeV W-BAND LINEAR COLLIDER

A 3-km long high-gradient W-band switched matrix linac may, in parallel channels, accelerate multiple electron and positron bunches to an energy of 2.5 TeV, with a tight control on the intra-bunch energy spread. In this report, we describe a final-focus system for such an accelerator, whose length is restrained by eliminating chromatic correction. The interaction point (IP) spot size is limited by synchrotron radiation in the last quadrupole (Oide effect). The energy loss due to beamstrahlung is optionally suppressed by combining bunches of opposite charge and colliding the neutral beams. We present two different high-luminosity multiple-collision schemes, which can provide a luminosity of up to 1035 cm -2 s -1, with only about 1 MW average beam power. In the first scheme, batches of equally-charged bunches are combined into superbunches which, possibly after charge compensation, are collided head-on with the opposing beam. In the second scheme, 25 charge-neutral electron-positron bunch pairs of one beam are each collided with 25 neutral bunch pairs of the other beam. These multiplexed collisions are facilitated by a crossing angle and by crab cavities upstream of the electron-positron combiner; however they also require focusing channels (e.g., a crystal) preserving IP beam size between the collision points, a difficult if not impossible construct. We describe the challenges posed by each approach.

Femtosecond optical parametric oscillators based on periodically poled lithium niobate

We describe two configurations of collinearly pumped femtosecond optical parametric oscillator based on periodically poled lithium niobate and tunable in the infrared from 975 nm to 4.98 μm. Maximum output powers of 240 mW for the signal and 106 mW for the idler were recorded with 25 mW of average power measured at 4.88 μm. An overall conversion efficiency of 35% and slope efficiencies for the signal of 46% at a wavelength of 1.04 μm and 70% at 1.1 μm were measured. Interferometric autocorrelations of the signal and idler pulses at various wavelengths within the tuning range have been obtained and imply nearly transform-limited pulse durations of about 140fs for the signal and about 190fs for the idler.

Femtosecond optical parametric oscillators based on periodically poled lithium niobate

Abstract We describe two configurations of collinearly pumped femtosecond optical parametric oscillator based on periodically poled lithium niobate and tunable in the infrared from 975 nm to 4.98 μm. Maximum output powers of 240 mW for the signal and 106 mW for the idler were recorded with 25 mW of average power measured at 4.88 μm. An overall conversion efficiency of 35% and slope efficiencies for the signal of 46% at a wavelength of 1.04 μm and 70% at 1.1 μm were measured. Interferometric autocorrelations of the signal and idler pulses at various wavelengths within the tuning range have been obtained and imply nearly transform-limited pulse durations of about 140fs for the signal and about 190fs for the idler.

NaCl:OH − color center laser modelocked by a novel bonded saturable Bragg reflector

We present the design and modelocking results of a novel saturable Bragg reflector constructed from separate Bragg stack and quantum well semiconductor wafers, bonded by the technique of atomic rearrangement. This device was used to modelock a NaCl:OH − color center laser, producing 200 fs pulses in the 1.60 μm range. The laser is self-starting, requires no dedicated dispersion compensating optics, and maintains a highly stable, nearly transform limited output pulse train with up to 500 mW average output power.

Design and flight performance of a space borne 3He refrigerator for the infrared telescope in space

We report the design, space-qualification and flight performance of a 3He refrigerator that has achieved the first sub-Kelvin temperatures in orbit. The refrigerator has been used to cool the bolometric detectors of the Far Infrared Photometer (FIRP) on the Infrared Telescope in Space (IRTS). The refrigerator has no moving parts, and can be re-cycled indefinitely in zero-g with >95% duty cycle using simple control electronics. It provides 10 μW of cooling power at T<310 mK with ≤2 mW average power dissipation to a 2.0 K heat sink for about 10 days.

Efficient frequency doubling of 1.5 μm femtosecond laser pulses in quasi-phase-matched optical fibers

Second-order nonlinear gratings in optical fibers have been produced for efficient quasi-phase-matched frequency conversion around 1.5 μm. Periodic poling was achieved by defining a patterned electrode on a D-shape fiber via standard lithography and applying high voltage (4–5 kV) at elevated temperature (270–280 °C). This fabrication technique has allowed us to produce gratings uniform over 7.5 cm, as indicated by the shape and the bandwidth of the phase-matching curve. By frequency doubling ∼100 fs pulses in a grating ∼4 cm long and with a period of 57.15 μm, ∼1.05 mW average power at 768 nm has been generated, with an average conversion efficiency ∼1.2%.

Stable multigigahertz pulse-train formation in a short-cavity passively harmonic mode-locked erbium/ytterbium fiber laser

We demonstrate a short-cavity erbium-ytterbium fiber laser that is passively mode locked by a saturable Bragg reflector with a fundamental repetition rate of 235 MHz . The laser operates in the soliton regime and under passive harmonic mode locking with 11 pulses in the cavity and produces output pulse trains at 2.6 GHz with transform-limited 270-fs pulses and 1.6 mW of average power. Within the cavity the multiple pulses form a stable pattern with fixed, nearly equal pulse-to-pulse temporal spacings, causing the output pulse train to have timing offsets of less than 15 ps. A slow gain-recovery model is proposed to explain the pulse-train self-organization.

Pulse propagation of accumulated photon echoes in Nd3+ ion-doped silicate glass fiber

Pulse-propagation phenomena of accumulated photon echo signals was investigated for the F3/24(1)–I9/24 transition of the Nd3+ ion doped with 20 ppm concentration in silicate glass fiber (core diameter of 6 μm) at 4.2 K using an 82 MHz mode-locked dye laser. The intensity of the transmitted probe pulses from the Nd3+ fiber was measured as a function of the Nd3+ fiber length. The fiber length for the maximum probe-pulse intensity induced by the pump pulses was determined to be 4 m when the pump pulses of 0.50 mW average power and the probe pulses of 0.25 mW average power were used. We derived the pulse area equations which take into account the coherent interaction between the pulses and the Nd3+ fiber. The fiber-length dependence of the signal intensity was explained well by the pulse area equations.

440 nm light generation by frequency tripling in germanosilicate glasses

Generation of 120 μW average power blue-violet light at 440 nm through efficient frequency tripling of 1.319 μm Nd:YAG laser pulses in bulk GeO2-SiO2 and Er3+-doped GeO2-SiO2 glasses is reported. Third-harmonic signal light enhancement owing to two- and three-photon resonant pumping of erbium ions in doped samples is also reported.

Low-threshold picosecond optical parametric oscillation in quasi-phase-matched lithium niobate

We report a singly resonant optical parametric oscillator in periodically poled lithium niobate, synchronously pumped by the second harmonic of an amplified 10 μs pulse train from a continuous wave mode-locked Nd:YLF laser. Pulses of ∼2 ps duration have been generated over the tuning range from 883 to 1285 nm with a typical threshold of 200 mW average power within the 10 μs envelope. The M2 beam quality factor for the generated signal was ∼1.1, indicating absence of any significant photorefractive damage.

Real time frequency domain fibreoptic temperature sensor using ruby crystals

The excited state phosphorescence lifetime of ruby crystals is used to monitor temperature in the physiological range from 15° to 45°C with precision and accuracy less than 1°C, in real time. Precision of 0.1°C is attained with 3 min integration times. A 500 μm cubic ruby crystal bounded to the distal end of an optical fibre of similar core dimensions is excited with pulsed Ne-He laser light of about 9 μW average power. The instrument uses a sampler for data acquistion, and frequency domain methods for data fitting. The instrument amplifies the a.c. components of the detector output and band limits the signal to 800 Hz. The fundamental frequency of the excitation is set to 24.41 Hz to obtain 32 or less harmonics. This band-limited signal is sampled and averaged between 20 and 100 cycles to obtain temperature measurements in real time. The frequency domain representation of the data is obtained by employing fast Fourier transform algorithms. The phase delay and the modulation ratio, of each sampled harmonic, is then computed. Five to 32 values of the phase and modulation are averaged before computing the sensor lifetime. The technique is capable of measuring precise and accurate excited state lifetimes from subpicowatt luminescent signals in plastic optical fibres. A least squares fit yields the lifetimes of single exponentials. A component of zero lifetime is introduced to account for the backscatter excitation seen by the photodetector leaking through optical interference filters. The phosphorescence lifetimes measured reproducibly to about six parts in 1000, with a 2 s integration time, are used to monitor physiological temperature. Temperatures are computed employing empirical polynomials. The system drift is 3% over 5 h of continuous operation. The instrumentation and methods allow 2.7 s update times and 50 s full response times.