Build-up Cavity Research Articles

Integration of a high finesse cryogenic build-up cavity with an ion trap

We report on the realization of a hemispherical optical cavity with a finesse of F = 13 000 and sustaining inter-cavity powers of 10 kW, which we operate in a closed-cycle cryostat vacuum system close to 4 K. This was designed and built with an integrated radio-frequency Paul trap in order to combine optical and radio-frequency trapping. The cavity provides a power build-up factor of 2300. We describe a number of aspects of the system's design and operation, including low-vibration mounting and locking and thermal effects at high powers. Thermal self-locking in the high intracavity power regime was observed to enhance passive stability below 1 kHz. Observations made over repeated cool-downs over the course of a year show a repeatable shift between the ion trap center and the cavity mode.

Simple technique of coupling a diode laser into a linear power buildup cavity for Raman gas sensing.

We report a novel, to the best of our knowledge, and simple technique to lock a 642 nm multi-quantum well diode laser to an external linear power buildup cavity by directly feeding the cavity reflected light back to the diode laser for enhancement of gas Raman signals. The dominance of the resonant light field in the locking process is achieved by reducing the reflectivity of the cavity input mirror and thus making the intensity of the directly reflected light weaker than that of the resonant light. Compared with traditional techniques, stable power buildup in the fundamental transverse mode TEM00 is guaranteed without any additional optical elements or complex optical arrangements. An intracavity exciting light of 160 W is generated with a 40 mW diode laser. Using a backward Raman light collection geometry, detection limits at the ppm level are achieved for ambient gases (N2, O2) with an exposure time of 60 s.

Characterization of high-harmonic emission from ZnO up to 11 eV pumped with a Cr:ZnS high-repetition-rate source.

We report the measurement of high-order harmonics from a ZnO crystal with photon energies up to 11eV generated by a high-repetition-rate femtosecond Cr:ZnS laser operating in the mid-infrared at 2-3μm, delivering few-cycle pulses with multi-watt average power and multi-megawatt peak power. High-focus intensity is achieved in a single pass through the crystal without a buildup cavity or nanostructued pattern for field enhancement. We measure in excess of 108 high-harmonic photons/second.

Cavity-enhanced Thomson scattering measurements of electron density and temperature in a hollow cathode discharge.

A cavity-enhanced Thomson scattering (CETS) diagnostic has been developed to perform electron density and temperature measurements in low-density weakly ionized discharges. The diagnostic approach is based on generating a high-power beam in an optical build-up cavity and using the beam as a light source for Thomson scattering from plasma housed within the cavity. In our setup, a high-power (∼5 W) fiber laser at 1064nm allows an intra-cavity power of 11.7kW in a two-mirror cavity for measurements in the plume of a BaO hollow cathode discharge. A study of plasma density and temperature was performed at various operating conditions. Electron densities and temperatures in the range of ∼1012 cm-3 and ∼3 eV were measured, respectively. The high signal-to-noise ratio (SNR) of the present measurements (SNR=1100) suggests the ability to measure significantly lower density plasmas in the range of ∼3×109 to 3×1010 cm-3, thereby extending current laser Thomson scattering diagnostic capabilities.

Measurements of the energy distribution of a high brightness rubidium ion beam

The energy distribution of a high brightness rubidium ion beam, which is intended to be used as the source for a focused ion beam instrument, is measured with a retarding field analyzer. The ions are created from a laser-cooled and compressed atomic beam by two-step photoionization in which the ionization laser power is enhanced in a build-up cavity. Particle tracing simulations are performed to ensure the analyzer is able to resolve the distribution. The lowest achieved full width 50% energy spread is (0.205 ± 0.006) eV, which is measured at a beam current of 9 pA. The energy spread originates from the variation in the ionization position of the ions which are created inside an extraction electric field. This extraction field is essential to limit disorder-induced heating which can decrease the ion beam brightness. The ionization position distribution is limited by a tightly focused excitation laser beam. Energy distributions are measured for various ionization and excitation laser intensities and compared with calculations based on numerical solutions of the optical Bloch equations including ionization. A good agreement is found between measurements and calculations.

Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams

A two-step photoionization strategy of an ultracold rubidium beam for application in a focused ion beam instrument is analyzed and implemented. In this strategy the atomic beam is partly selected with an aperture after which the transmitted atoms are ionized in the overlap of a tightly cylindrically focused excitation laser beam and an ionization laser beam whose power is enhanced in a build-up cavity. The advantage of this strategy, as compared to without the use of a build-up cavity, is that higher ionization degrees can be reached at higher currents. Optical Bloch equations including the photoionization process are used to calculate what ionization degree and ionization position distribution can be reached. Furthermore, the ionization strategy is tested on an ultracold beam of $^{85}$Rb atoms. The beam current is measured as a function of the excitation and ionization laser beam intensity and the selection aperture size. Although details are different, the global trends of the measurements agree well with the calculation. With a selection aperture diameter of 52 $\mu$m, a current of $\left(170\pm4\right)$ pA is measured, which according to calculations is 63% of the current equivalent of the transmitted atomic flux. Taking into account the ionization degree the ion beam peak reduced brightness is estimated at $1\times10^7$ A/(m$^2\,$sr$\,$eV).

Efficient continuous wave second harmonic generation of 872 nm diode laser radiation using KNbO3 with high stability

We report on a highly stable laser system at a wavelength of 436 nm for the coherent excitation of the transition in mercury based on efficient second harmonic generation in a potassium niobate crystal. The system delivers up to 220 mW of blue light with a peak-to-peak stability of 2.3%. This stability is achieved by setting the whole build-up cavity inside a massive and temperature stabilized aluminum block. Absolute frequency stability to 480 kHz peak-to-peak is achieved by polarization spectroscopy. With a short time linewidth of 59.9 kHz and a tunability of more than 6 GHz the system is suitable for experiments requiring coherent excitation as well as applications in spectroscopy.

Trapping cold ground state argon atoms.

We trap cold, ground state argon atoms in a deep optical dipole trap produced by a buildup cavity. The atoms, which are a general source for the sympathetic cooling of molecules, are loaded in the trap by quenching them from a cloud of laser-cooled metastable argon atoms. Although the ground state atoms cannot be directly probed, we detect them by observing the collisional loss of cotrapped metastable argon atoms and determine an elastic cross section. Using a type of parametric loss spectroscopy we also determine the polarizability of the metastable 4s[3/2](2) state to be (7.3±1.1)×10(-39) C m(2)/V. Finally, Penning and associative losses of metastable atoms in the absence of light assisted collisions, are determined to be (3.3±0.8)×10(-10) cm(3) s(-1).

Cavity-enhanced optical feedback-assisted photo-acoustic spectroscopy with a 10.4 μm external cavity quantum cascade laser

An ultra-sensitive photo-acoustic spectrometer using a 10.4 μm broadly tunable mid-IR external cavity quantum cascade laser (EC-QCL) coupled with optical feedback to an optical power buildup cavity with high reflectivity mirrors was developed and tested. A laser optical power buildup factor of 181 was achieved, which corresponds to an intra-cavity power of 9.6 W at a wavelength of 10.4 μm. With a photo-acoustic resonance cell placed inside the cavity this resulted in the noise-equivalent absorption coefficient of 1.9 × 10−10 cm−1 Hz−1/2, and a normalized noise-equivalent absorption of 1.1 × 10−11 cm−1 W Hz−1/2. A novel photo-acoustic signal normalization technique makes the photo-acoustic spectrometer’s response immune to changes and drifts in the EC-QCL excitation power, EC-QCL to cavity coupling efficiency and cavity mirrors aging and contamination. An automatic lock of the EC-QCL to the cavity and optical feedback phase optimization permitted long wavelength scans within the entire EC-QCL spectral tuning range.

Fourth harmonic generation of continuous-wave lasers in coupled-cavity configuration

Frequency quadrupling of a continuous-wave laser is carried out in coupled cavities with the stabilization to the resonance. The fourth harmonic generation in the coupled cavities is expected to be more efficient than in the two build-up cavity configuration in tandem, because input couplers, which increase the optical loss of the cavity, are not required. The cavity length is stabilized by the feedback of the error signal obtained by the cavity length modulation. In the present demonstration, the ultraviolet beam of 6μW at 238nm is obtained by the fourth harmonic generation of the near infrared laser of 180mW at 952nm. This output power is expected to become higher (∼1.7mW) when the cavity setup is optimized.

An all-solid-state laser source at 671 nm for cold-atom experiments with lithium

We present an all solid-state narrow line-width laser source emitting $670\,\mathrm{mW}$ output power at $671\,\mathrm{nm}$ delivered in a diffraction-limited beam. The \linebreak source is based on a fre-quency-doubled diode-end-linebreak pumped ring laser operating on the ${^4F}_{3/2} \rightarrow {^4I}_{13/2}$ transition in Nd:YVO$_4$. By using periodically-poled po-tassium titanyl phosphate (ppKTP) in an external build-up cavity, doubling efficiencies of up to 86% are obtained. Tunability of the source over $100\,\rm GHz$ is accomplished. We demonstrate the suitability of this robust frequency-stabilized light source for laser cooling of lithium atoms. Finally a simplified design based on intra-cavity doubling is described and first results are presented.

Collective transverse cavity cooling of a dense molecular beam

Based on a classical point particle description, we study light induced transverse collimation and phase space compression of a fast molecular beam traversing the crossover region of a high-Q optical cavity and a standing-wave laser beam. The molecules scatter pump laser light, far detuned from any molecular transition, into the resonant cavity mode. We show that despite a very small probability for this process for a single particle, collective enhancement can lead to significant transverse cooling and collimation above the self-organization threshold. The analytical formulae for this threshold derived from homogeneous one-dimensional (1D) models give surprisingly good estimates for the pulsed 3D case. We compare ring and standing wave cavity geometries and show that phase space compression can be achieved even for extremely small particle field coupling if compensated by a large density and pump power. In the vicinity of the critical point the compression time can be reduced by tailored seeding of the cavity mode or using a second buildup cavity for the pump field.

Resonator-enhanced optical guiding and trapping of Cs atoms

We demonstrate a 90 cm launch of Cs atoms guided by a one-dimensional (1D) optical lattice. The 1064 nm wavelength optical lattice is made in a 2 m long buildup cavity and provides a transverse guide depth of up to 125 microK. Before they reach the top of their trajectory, the atoms are stopped and cooled by optical molasses, becoming trapped in the 1D lattice. The lattice can be loaded with multiple launches, filling sites that extend over 5 cm. The trap is far from all magnetic sources and will be used for a precision measurement of the electron electric-dipole moment.

Improved Measurement of the1s2s S01−1s2p P13Interval in Heliumlike Silicon

Using colinear fast-beam laser spectroscopy with copropagating and counter-propagating beams we have measured the 1s2s 1S0-1s2p 3P1 intercombination interval in 28Si12+ with the result 7230.585(6) cm{-1}. The experiment made use of a dual-wavelength, high-finesse, power build-up cavity excited by single-frequency lasers at 1319 and 1450 nm. The result will provide a precision test of ab initio relativistic many-body atomic theory at moderate Z.

Simple annular-beam generator with a laser-diode-pumped axially off-set power build-up cavity

We have developed a novel, compact-mode selective annular-beam generator that employs a laser-diode (LD)-pumped, passively locked power build-up cavity (PBC). The PBC functions as a flexible and compact light source that is capable of producing TEM 0n high-order Hermite-Gaussian (HG) beams. The generated HG beams can then be converted to LG modes, particularly to annular-beams, using two cylindrical lenses. The system was theoretically studied and a mode-matching condition was derived. The tailoring position of the active semiconductor laser medium relative to the external cavity of the PBC enabled the production of single high-order HG modes. The optimum position was calculated by model calculations of the mode-coupling efficiency. The desired TEM 0n modes of HG beams were generated by translating the axial position of the external cavity of the PBC in increments of ∼100 μm relative to the LD emission surface. High quality TEM 01 to TEM 015 of the HG beams and their conversion to the desired LG modes having powers of the order of 1 mW was successfully demonstrated. This system is suitable for optical tweezers.

A build-up cavity for Fourier transform emission experiments

We have recorded electronic spectra of some diatomic species (I 2, K 2, and NaK), to illustrate the potential power of the combination of two high resolution techniques: intra-cavity laser induced fluorescence (ICLIF) and Fourier transform (FT) spectroscopy. Active and passive optical cavities have been used, working with visible continuous wave (cw) laser sources. The active cavity is a modified commercial ring dye laser, allowing for a sample up to 25 cm in length. Dispersed fluorescence spectra recorded on a Bomem Fourier transform spectrometer showed a signal enhancement of about 10 when a molecular source was placed within the resonator. The system was tested with a heatpipe source, producing alkali metal vapour at about 300 °C. These experiments illustrate enhanced cascade excitation mechanisms in K 2; the highest vibrational levels of the electronic ground state of K 2 can be observed with surprising ease. The increase in available power within the cavity has also led to the observation of fluorescence in NaK excited by a two-photon transition ( Q (66) 6 1Σ + ← X 1Σ + transition). Spatial limitations have driven us to build a more versatile ring cavity able to accommodate larger sources. This broad-band (590–650 nm) build-up cavity is locked by a Hänsch–Couillaud servo-loop to an input laser of (instantaneous) bandwidth ∼1 MHz. Power enhancement factors of around 30 have been obtained with a 2.6% input coupler. The performance of the build-up cavity has been tested by recording FT spectra of intra-cavity laser induced fluorescence of iodine. The technique clearly has useful applications for weakly absorbing species, or for those whose electronic states are inaccessible to single-photon absorption techniques. This paper describes the arrangement we have used, highlighting some of the advantages and describing some of the particular difficulties we have encountered.

Optical dipole trap using a Fabry–Perot interferometer as a power buildup cavity

We construct an optical dipole trap using a Fabry–Perot interferometer as a power buildup cavity. Large power enhancement allows us to produce 9 mK deep potential wells with 48 µm spot size and 100 nm detuning from the rubidium D1 resonance. The optical trap is loaded from a dark-spot magneto-optical trap which, in turn, is loaded from a low-velocity intense source of 85Rb atoms. Under typical experimental conditions, there are 1.4 × 106 atoms in 2000 antinodes of the optical trap. Average atom density is 1.1 × 1012 cm-3. The number of trapped atoms is limited by the atom density, or trapping volume. The standing-wave configuration with tight longitudinal confinement has much smaller trapping volume compared with the equivalent travelling wave. A method for converting the trap to a travelling wave-like configuration using phase modulation is proposed.

Trace Methane Detection Based on Raman Spectroscopy Using a High Finesse Optical Resonator

A Raman spectroscopy-based gas sensor employing a high-finesse simple diode laser pumped external cavity, Power Build-up Cavity (PBC), has been developed. Results show that a compact 10-cm-long PBC includes a simple diode-laser-pumped external cavity could achieve an intra-cavity power of 40 W with a photon lifetime of up to 29 μs. Raman scattering light from trace methane in the PBC was detected by a photo multiplier tube and a bandpass filter. Good linearity of the detected Raman signal was obtained in the 90-500 ppm methane concentration region with a response time of 1 sec.

Pulse picking by phase-coherent additive pulse generation in an external cavity.

We have implemented a simple method for generating an "amplified" phase-coherent light pulse in which a pulse train of phase-coherent, equidistant input light pulses from a mode-locked Ti:sapphire laser is coupled into a ring cavity resonator whose length is matched to the mode-locked pulse repetition frequency at 82 MHz. Pulses are thus coherently superimposed and added inside the buildup cavity and form an intense pulse that is switched out from the cavity via a fast acousto-optic modulator. The method thus provides a pulse train at a reduced and controlled repetition frequency and with higher pulse energies than the original mode-locked pulses.

Performance Characteristics of Power Build-Up Cavity for Raman Spectroscopic Measurement

Performance characteristics of power build-up cavity (PBC) as the light source of a Raman spectroscopy based gas sensor were studied. The key parameter to optimize stable and high intra-cavity power operation was beam diameter of the back reflected beam from external cavity to diode laser. The optimum diameter determined by an appropriate distance between the cavity and diode laser was found to be comparable with the waveguide cross section of diode laser for the effective spatial filtering, where inevitable cavity coupling loss caused by slight spatial mode mismatching existed. A PBC with a finesse of ∼10300 achieved a stable TEM00 mode in excess of intra-cavity power of 80 watts pumped by a 10 milliwatts diode laser. Simultaneously, the PBC wavelength is found to be passively locked effectively at 670 +/− 0.15 nm where the center of the gain region exists. A Raman spectrum of nitrogen measurement was demonstrated.