Frequency-doubled Diode Laser Research Articles

Detection of elemental mercury using a frequency-doubled diode laser with wavelength modulation spectroscopy

We demonstrate a new method for elemental mercury sensing by wavelength modulation spectroscopy (WMS) using a tunable ultraviolet laser generated through a process of second-harmonic generation (SHG). The WMS is implemented by fast modulating the injection current of the Fabry–Perot-type green diode laser equipped with a Littrow grating to increase the laser-mode power density. The technique of correlation spectroscopy is exploited to deal with the signal variations due to mode hops and guarantee the measurement accuracy. According to the performance evaluation, the SHG-WMS system exhibits a better sensitivity (0.15 µg/m3 for 1-m pathlength with an integration time of 10 s) and a comparably high linearity (R2 = 0.9995 within the range of 60 µg/m2) compared with the direct absorption scheme. The employment of WMS significantly simplifies the data processing for extraction of small mercury absorption signals from the large and complex SHG light background, and, thus, give robust measurement results. High-harmonic (4f and 6f) detections are also carried out, showing a great potential for suppression of large residual amplitude modulation background. The proposed SHG-WMS system shows great promise for rapid and sensitive mercury sensing in industrial fields.

Optical frequency synthesizer for precision spectroscopy of Rydberg states of Rb atoms

We have developed an optical frequency synthesizer for the precision spectroscopy of highly excited Rydberg states of Rb atoms. This synthesizer can generate a widely tunable 480 nm laser light with an optical power of 150 mW and an absolute frequency uncertainty of less than 100 kHz using a high-repetition-rate (325 MHz) Er fiber-based optical frequency comb and a tunable frequency-doubled diode laser at 960 nm. We demonstrate the precision two-photon spectroscopy of the Rydberg states of 87Rb atoms by observing the electromagnetically induced transparency in a vapor cell, and measure the absolute transition frequencies of 87Rb to nD (n = 53–92) and nS (n = 60–90) Rydberg states with an uncertainty of less than 250 kHz. It is the first direct frequency measurements of these transitions using an optical frequency comb.

Focus on lasers and imaging

Focus on lasers and imaging

Frequency-doubled DBR-tapered diode laser for direct pumping of Ti:sapphire lasers generating sub-20 fs pulses

For the first time a single-pass frequency doubled DBR-tapered diode laser suitable for pumping Ti:sapphire lasers generating ultrashort pulses is demonstrated. The maximum output powers achieved when pumping the Ti:sapphire laser are 110 mW (CW) and 82 mW (mode-locked) respectively at 1.2 W of pump power. This corresponds to a reduction in optical conversion efficiencies to 75% of the values achieved with a commercial diode pumped solid-state laser. However, the superior electro-optical efficiency of the diode laser improves the overall efficiency of the Ti:sapphire laser by a factor > 2. The optical spectrum emitted by the Ti:sapphire laser when pumped with our diode laser shows a spectral width of 112 nm (FWHM). Based on autocorrelation measurements, pulse widths of less than 20 fs can therefore be expected.

Structure and NLO property relationship in a novel chalcone co-crystal

Single crystals of a chalcone co-crystal (C18H19NO4/C17H16NO3Br; 0.972/0.028) have been grown by slow evaporation from solution. The powder second harmonic generation (SHG) efficiency of this chalcone co-crystal is 7 times that of urea. The dependence of second harmonic (SH) intensity on particle size revealed the existence of phase matching direction in this crystal. The large SHG efficiency observed is mainly due to the unidirectional alignment of molecular dipoles, in which the dipole moment of each molecule adds to establish a net polarization. The weak N–H⋅⋅⋅O hydrogen-bond interactions help to stabilize the noncentrosymmetric crystal packing and also contribute partly to the SHG. The better thermal stability, transparency and high laser damage resistance (>1.5 GW cm−2 at 532 nm, 8 ns) of this chalcone co-crystal indicate that it is a promising material for frequency doubling of diode lasers down to 470 nm. This molecule also shows a third-order NLO response and good optical limiting property of 8 ns laser pulses at 532 nm. The mechanism for optical limiting in this chalcone was attributed to two-photon induced excited state absorption that leads to reverse saturable absorption. The structure–property relationship in this chalcone and related compounds is discussed based on the experimental results and semiempherical hyperpolarizability calculations.

High brightness, tunable biphoton source at 976 nm for quantum spectroscopy

A compact all solid state continuous-wave biphoton source, tunable around 488 nm, for quantum spectroscopic applications based on a frequency doubled diode laser system is presented. Copolarized photon pairs in the fundamental transversal mode could be generated at 976 nm by spontaneous parametric down conversion inside a type-0 quasi phase matched periodically poled lithium niobate waveguide crystal with an efficiency of 8-10(-6). A high flux rate greater than 10(7) photon pairs per second has been achieved at pump powers in the muW range resulting in more than 7-10(9) photon pairs/s-mW. Further a detailed investigation of the spectral behavior and the flux rate as a function of the detuning from the degenerated case is presented.

Controlled photoionization loading of 88Sr+ for precision ion-trap experiments

We report on a photoionization scheme for controlled loading of 88Sr+ ions into a radio-frequency (rf) ion trap. The two-step doubly-resonant process drives the 88Sr atom on the 5s21S0 – 5s5p1P1 transition at 461 nm using a frequency-doubled diode laser, and then excites the atom to the autoionizing (4d2+5p2) 1D2 state with 405 nm light from a free-running diode laser. This method of trap loading is quantitatively compared to electron bombardment loading, and shown to reduce the Sr vapour pressure required to load by four orders of magnitude. It also provides more than an order of magnitude reduction in the day-to-day variation of the voltages required to compensate micromotion. We additionally introduce a two-step atomic source consisting of an oven and hotplate, which reduces the number of impurities evaporated towards the trap. This is shown to significantly reduce fluctuations in required micromotion compensation voltages. The demonstrated reduction in flux and increased source purity are expected to improve the reliability of trap operation for precision ion-trap experiments.

Tunable diode laser spectroscopy of copper, cadmium, and indium at 325 nm

A tunable frequency-doubled external-cavity diode laser has been recently developed for atomic absorption spectroscopy in the wavelength range from 320 to 327 nm. The line width of the laser is 0.003 pm. In this work the light source has been applied to the laser absorption spectroscopy of copper at 324.754 nm, cadmium at 326.106 nm, and indium at 325.609 nm and 325.856 nm. The Cu transition was measured in three different environments: in a direct current plasma, a diffusion flame, and in a low-pressure hollow-cathode discharge. Both the plasma and the flame were at atmospheric pressure. The Cd and In transitions were measured in the diffusion flame. A Voigt profile was fit to the measured spectra taking into account the hyperfine structure. From the measured absorption lines spectral reference data for Cu is extracted.

Pressure broadening parameters of the hydroxyl radical A 2 ?+ (??=0) ? X 2 ?3/2 (?=0) transitions at ca. 308 nm

Pressure broadening widths and pressure-induced frequency shifts for the hydroxyl radical Q11([see full textsee full text]), Q21([see full textsee full text]), Q11(2), Q21(2) absorption lines in the A $^{2}{\rm \Sigma} ^{+}\; (\upsilon' =0) \leftarrow {\rm X} \; ^{2}{\rm \Pi} _{3/2}\;(\upsilon''=0)$ band at ca. 308 nm have been determined in the presence of a variety of gases (N2, Ar, Ne and He). Hydroxyl radicals were formed using either a microwave discharge or a vacuum ultra-violet photolysis of water vapour and were detected using direct absorption spectroscopy with a bandwidth normalised noise equivalent absorption sensitivity of 5 ×10-5 Hz-1/2. A tunable external cavity diode laser at 835 nm and a single-frequency continuous-wave intracavity frequency-doubled diode laser at 488 nm were used to produce tunable continuous-wave light at 308 nm by sum-frequency generation in a β-BaB2O4 crystal.

Frequency-stabilized 1520-nm diode laser with rubidium 5S_1/2 → 7S_1/2 two-photon absorption

A 760-nm light source of >10 mW is obtained from a frequency-doubled external-cavity diode laser by use of using an erbium-doped fiber amplifier and a periodically poled lithium niobate waveguide. The 5S(1/2) --> 7S(1/2) two-photon transitions of rubidium are observed with such a light source. This laser frequency is locked to the Rb two-photon transitions with an instability of 10 kHz (1 s). Our experimental scheme provides a compact, high-performance frequency, standard in the S band (1480-1530 nm) for fiber-optic communication and sensing applications.

UV cavity enhanced absorption spectroscopy of the hydroxyl radical

We present the application of a continuous-wave ultra-violet tuneable light source for detection of the hydroxyl radical (OH) using cavity-enhanced absorption spectroscopy of the Q11(2) and Q21(2) absorption lines in the A2Σ+(v′=0)←Χ2Π3/2(v′′=0) band at ca. 308 nm. A tuneable infra-red diode laser operating at 835 nm and either an Ar+ laser or a single frequency continuous-wave intracavity frequency-doubled diode laser, both operating at ca. 488 nm, were used to produce 0.1–0.5 μW of tuneable radiation at ca. 308 nm by sum frequency generation in a βBaB2O4 crystal. Cavity enhanced absorption spectroscopy was used to detect OH generated by UV photolysis of water vapour in argon, nitrogen, neon and helium at atmospheric pressure. A noise-equivalent (1σ) absorption sensitivity of 2.1×10-7 cm-1Hz-1/2 measured over 128 scans in a time of 1.16 s was demonstrated with mirrors of reflectivity 0.9963 in a cavity of length 58.5 cm for a ∼2 cm-1 scanning range at a UV power of ∼0.5 μW. An OH detection limit (1σ) of 3.84×109 molecule cm-3 was estimated in argon at atmospheric pressure. OH collisional broadening in humidified N2, Ar, Ne and He was determined at atmospheric pressure .

Generation of twin beams from an optical parametric oscillator pumped by a frequency-doubled diode laser.

Quantum-correlated twin beams were generated from a triply resonant optical parametric oscillator with an a-cut KTP crystal pumped by a frequency-doubled diode laser. A total output of 5.1 mW was obtained in the classical-nonclassical light-conversion system driven by a 50-mW diode laser at 1080 nm. A quantum-noise reduction of 4.3 dB (63%) in the intensity difference between the twin beams was successfully observed at the detection frequency of 3 MHz.

Frequency-doubled diode laser for ultraviolet absorption spectroscopy at 325 nm

A frequency-doubled tunable diode laser for absorption spectroscopy in the ultraviolet is described. The device is based on an external cavity diode laser with a transmission grating. Second-harmonic generation at 320–327 nm is carried out using a lithium iodate crystal. The measured single-pass cw ultraviolet power is 60 nW and the system has a 40-GHz continuous mode-hop-free tuning range in the ultraviolet. The laser was demonstrated by measuring the 324.75-nm 42S1/2–42P3/2 transition of copper at atmospheric pressure with a single continuous scan.

The treatment of leg telangiectasia

Leg telangiectasia is more difficult to treat than facial telangiectasia. Investigations may be needed to elucidate the origin of the telangiectasia. In cases of venous insufficiency, investigation of venous haemodynamics is desirable. Treatment is primarily by sclerotherapy, including foam sclerotherapy. Light and laser treatments are also effective, especially for telangiectasia up to 1 mm in diameter. New developments incorporate longer pulse durations, such as frequency-doubled neodynium YAG and diode lasers as well as intense pulsed light. After all treatment methods, compression for 48 h using pads, bandages or hosiery is recommended. As with all cosmetic procedures, patients need to be tutored comprehensively about anticipated results and possible complications or side-effects.

Indirect laser‐induced fluorescence detection for capillary electrophoresis using a frequency‐doubled diode laser

A blue (452 nm) frequency-doubled diode laser with a quasi-cw optical output power of 10 microW is used for indirect laser-induced fluorescence detection in combination with the capillary electrophoretic separation of inorganic anions. As fluorescing probe ion the anion of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) was selected having an absorption maximum of 454 nm in alkaline medium. Employing a capillary coated with linear acrylamide, baseline separation of eight inorganic anions was possible within 5 min. With a separation buffer containing 50 micromol.L(-1) HPTS and 10 mmol.L(-1) lysine the limits of detection for sulfate, nitrite, nitrate, azide, thiocyanate, and chlorate were between 0.9 and 4.7 micromol.L(-1). Separation of chloride and sulfate was achieved by adding 0.25 mmol.L(-1) calcium hydroxide to the separation buffer. Inorganic anions in several mineral and tap water samples have been determined with the technique developed and results are compared to data obtained by ion chromatography in combination with conductivity detection after conductivity suppression.

Measurement of the Stark shift within the6P1/2−7S1/2378-nm transition in atomic thallium

Using a thallium atomic beam apparatus and a stabilized, frequency-doubled diode laser system, we have measured the scalar Stark shift in the ${6P}_{1/2}\ensuremath{-}{7S}_{1/2}$ transition of thallium. We determine the Stark shift to be $\ensuremath{\Delta}{\ensuremath{\nu}}_{S}=\ensuremath{-}103.23(39)\mathrm{kHz}/(\mathrm{kV}/\mathrm{cm}{)}^{2},$ providing more than an order of magnitude improvement in precision over earlier measurements of this quantity. This measurement serves as a stringent test of recently published thallium parity nonconservation calculations.

Single-tone frequency-modulation spectroscopy with frequency-doubled current-modulated diode laser light

We demonstrate and characterize the generation of single-tone frequency-modulated and frequency-doubled radiation at 400 MHz and 430 nm. We obtained the radiation at 430 nm by frequency doubling light from a current-modulated 860-nm diode laser, using noncritical type I phase matching in a KNbO(3) crystal. The optical spectrum of the doubled light was found to be in keeping with our expectations based on the measured frequency- and amplitude-modulation indices of the fundamental radiation. The experimentally measured diode laser and crystal parameters were used to simulate the in-phase and quadrature signals that would be observed in a single-tone frequency-modulation spectroscopy experiment.

Wavelength-modulation spectroscopy using a frequency-doubled current-modulated diode laser

We have studied atomic absorption in an argon discharge by wavelength-modulation spectroscopy with a frequency-doubled diode laser. The tunable wavelength-modulated radiation at 430 nm was generated by frequency doubling a current-modulated 860-nm diode laser in a KNbO3 crystal. 2f-, 4f- and 6f-harmonic spectra as a function of diode laser modulation depth were measured on a Doppler-broadened sample of excited argon atoms produced in a capacitively coupled plasma chamber. Characterisation of the harmonic signals was accomplished. Minimum detectable absorbances of 7.7×10-5 and 1.9×10-4 based on a 3σ criterion (σ being the standard deviation of the noise) were estimated for 2f- and 4f-harmonic detection of the frequency-doubled radiation with a time constant of 0.1 s. The concentrations of argon in the 1s4 state were found to be in the range of 3×108 to 1.2×1011 cm-3 for the experimental conditions studied.

Frequency doubled diode laser in alternative extended cavity

We report on an alternative extended cavity scheme used with a 180 mW quantum well GaAlAs diode laser, operating near 850 nm. A dispersing prism and a thin glass plate are employed to enforce stable single-mode operation in the strong feedback regime, with no need for laser AR coatings. Compared to other configurations where a grating is used, the lower loss in the extended cavity allows higher fundamental power available for second harmonic generation. For example, by frequency doubling using potassium niobate in a power enhancement cavity, enough power can be generated from a single diode laser to decelerate and trap calcium atoms with radiation at 423 nm.

Violet light generation by frequency doubling of GaAlAs diode laser using a metallo-organic complex crystal ZnCd(SCN) 4

Violet 404-nm radiation is generated by type-I single-pass frequency doubling of the output of an GaAlAs diode laser in critically phase-matched ZnCd(SCN) 4 (ZCTC) crystal, a metallo-organic complex crystal. In this paper, its refractive index and second-harmonic generation phase-matching angles are reported. Using a 3-mm-long ZCTC crystal, a diode laser power of 473 mW generated 390 μW of second-harmonic radiation. More recently, 380-nm, 20 μW ultraviolet (UV) radiation is also realized by frequency doubling of the 100 mW output of a continuous-wave Ti:sapphire laser at 760 nm. ZCTC is a promising UV nonlinear optical candidate material for frequency doubling of diode lasers.