Optically stabilized tunable diode-laser system for saturation spectroscopy

We present an optically stabilized lead-salt diode-laser system which is the nucleus of a very-high-resolution instrument for sub-Doppler molecular spectroscopy in the mid-infrared. By application of external optical feedback, we have narrowed the diode-laser linewidth by two orders of magnitude, yielding a spectral width of less than 200 kHz. The diode- laser frequency is stabilized and controlled via the external reflector by variable-frequency offset-locking the diode-laser to a CO laser frequency. This substantial improvement in the spectral properties enabled us to perform a Lamb-dip experiment on a carbonyl sulfide (OCS) absorption line near 1985 cm^–1. We were able to detect a saturated dispersion signal at low pressure (5 Pa) with a signal-to-noise ratio of several thousand. The present paper describes the unique features of the optically stabilized tunable diode-laser system and its use as a spectroscopic tool for sub-Doppler applications.     

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEM_oo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm⁻¹) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10⁻¹⁰ cm⁻¹ for 1 s averaging over scans, with a dynamic range of four orders of magnitude.     

Spectroscopy of the 689 nm intercombination line of strontium using an extended-cavity InGaP/InGaAIP diode laser

A visible diode laser emitting at 690 nm at room temperature has been frequency-stabilized using a simple scheme based on optical feedback from a diffraction grating. The possibilities offered by these lasers for high-resolution spectroscopy are demonstrated by recording the sub-Doppler signal of the 689 nm intercombination line of strontium and resolving the hyperfine structure of the close P63 R70 (8, 4) transitions of iodine.     

Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

Digital, phase-sensitive detection for in situ diode-laser spectroscopy under rapidly changing transmission conditions

A new harmonic detection scheme for fully digital, fast-scanning wavelength-modulation spectroscopy (DFS-WMS) is presented. DFS-WMS is specially suited for in situ absorption measurements in combustion environments under fast fluctuating transmission conditions and is demonstrated for the first time by open-path monitoring of ambient oxygen using a distributed-feedback diode laser, which is doubly modulated with a fast linear 1 kHz-scan and a sinusoidal 300 kHz-modulation. After an analog high-pass filter, the detector signal is digitized with a 5 megasample/s 12-bit AD-converter card plugged into a PC and subsequently – unlike standard lock-ins – filtered further by co-adding 100 scans, to generate a narrowband comb filter. All further filtering and the demodulation are performed completely digitally on a PC with the help of discrete Fourier transforms (DFT). Both 1f- and 2f-signals, are simultaneously extracted from the detector signal using one ADC input channel. For the 2f-signal, a linearity of 2% and a minimum detectable absorption of 10^-4 could be verified experimentally, with the sensitivity to date being limited only by insufficient gain on the 2f-frequency channel. Using the offset in the 1f signal as a transmission ‘probe’, we could show that the 2f-signal can be transmission-corrected by a simple division by the 1f-background, proving that DFS-WMS provides the possibility of compensating for transmission fluctuations. With the inherent suppression of additive noise, DFS-WMS seems well suited for quantitative in situ absorption spectroscopy in large combustion systems. This assumption is supported by the first measurements of oxygen in a high-pressure combustor at 12 bar. Received: 29 April 2002 / Revised version: 31 May 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-6221/545-050, E-mail:volker.ebert@pci.uni-heidelberg.de     

High-resolution microwave spectroscopy on trapped ion clouds

Ion traps are particular usefully devices for precision spectroscopy on ionic ground states in the microwave domain. Although ultimate precision is achieved only with laser-cooled single ions, in many cases the precision obtained using large uncooled clouds of ions is sufficient for many requirements in atomic physics. The stronger signal in this case makes possible experiments on forbidden transitions or on systems with complex spectra and many substates. Recent examples of laser-microwave double resonance spectra on Pb⁺ and Eu⁺ are presented along with attempts to laser-cool a large ion cloud in order to reduce uncertainties from the second-order Doppler effect.     

A diode-laser optical frequency standard based on laser-cooled Ca atoms: Sub-kilohertz spectroscopy by optical shelving detection

We report an optical frequency standard at 657 nm based on laser-cooled/trapped Ca atoms. The system consists of a novel, compact magneto-optic trap which uses 50 mW of frequency-doubled diode laser light at 423 nm and can trap >10⁷ Ca atoms in 20 ms. High resolution spectroscopy on this atomic sample using the narrow 657 nm intercombination line resolves linewidths (FWHM) as narrow as 400 Hz, the natural linewidth of the transition. The spectroscopic signal-to-noise ratio is enhanced by an order of magnitude with the implementation of a “shelving" detection scheme on the 423 nm transition. Our present apparatus achieves a fractional frequency instability of in 1 s with a potential atom shot-noise-limited performance of and excellent prospects for high accuracy. Received 2 November 1998     

Verification of X-ray line coincidences by high-resolution spectroscopy

Photo-resonant pumping of X-ray lasers requires close coincidence of a strong emission line with a suitable absorbing transition. We use a high-resolution crystal spectrometer to verify five coincidences, one of which has apparently not yet appeared in published work. All but one of the coincidences are found to lie within one linewidth. With respect to X-ray laser photo-pumping three of the five coincidences seem to be promising candidates.     

Multiple frequency stabilization of lasers using double saturation spectroscopy

We have developed a method of stabilizing multiple lasers based on double saturation spectroscopy. Compared with other laser-stabilization methods based on conventional saturation spectroscopy, ours provides numerous reference spectra constructed with several velocity groups of atoms. Two independent laser sources can be simultaneously stabilized by using combined optical-pumping processes associated with a single Rb reference. We analyzed the stability of the feedback loop taking the correlation effect of the saturation spectra into consideration. We experimentally demonstrated two stabilized laser sources with a frequency difference of 6,109 MHz in the ⁸⁷Rb D₂ line. The results were in good agreement with theory within errors in measurement.     

Absorption cross-section fluctuations driven by continuous and discrete laser frequency variations

Though the operational definition of a photon’s absorption cross-section is straightforward, the definition has a fundamental and well-known subtlety. In order to make contact with theory, the incident photons must be monoenergetic and should impinge on the target atom or molecule at a constant rate. To some extent, however, these two constraints are always violated: no field is perfectly monochromatic and laser photons bunch (for both technical and fundamental reasons). Thus, the textbook definition of cross-section must be understood in terms of a mean, in which case the quantum nature of a field and the practicability of its realization must be recognized as sources of cross-section variance. Here, we consider the role that continuous and discrete random frequency fluctuations play in determining the variance of an absorption cross-section. In particular, we investigate the differing roles of phase diffusion and mode-partition noise, finding that discontinuous mode variations have a significant and not easily discerned effect on an absorption cross-section’s variance. As a by-product of our studies, agreement between theory and experiment highlights the utility of a hybrid absorption cross-section for use in the quasi-static approximation, and also suggests an exponential relationship between the rate of mode hops and injection current in cleaved-facet diode lasers.     

Isotope shifts and hyperfine structure of erbium,dysprosium, and gadolinium by atomic-beam diode-laser spectroscopy

A commercial diode laser system, without external frequency stabilization, has been used to study the isotope shifts and hyperfine structure in a near infrared transition of erbium, dysprosium and gadolinium. Specific mass shifts and relative values for the change in the radial nuclear charge parameters of the isotopes were obtained for erbium and dysprosium that compare well with literature values. The hyperfine structure constants measured for erbium fell within the range predicted from the results of other workers. The isotope shifts in the gadolinium transition investigated were too small for the spectrum to be resolved.     

Current spreading and series resistance of proton-implanted vertical-cavity top-surface-emitting lasers

In this paper, the current flow through the whole volume of the proton-implanted Vertical-Cavity top-Surface-Emitting Lasers (VCSELs) is analysed in detail. A simple approximate analytical relation was derived for a radial distribution of the current density entering active regions of those lasers. This distribution is nearly uniform in the case of VCSELs with a very small active region, but is becoming more and more non-uniform with an increase in its size. In VCSELs with very large active regions, current is flowing practically only within a narrow annular area close to the active-region perimeter. The VCSEL series electrical resistance is determined as a function of its active-region radius.     

Single-mode giga-hertz optical pulses generated by a self-modulation of the relaxation oscillation frequency in a semiconductor laser with subharmonic polarization-rotated optical feedback

In this research, single-mode 3.76 GHz optical pulses were generated in a Fabry-Perot type single-mode semiconductor laser with polarization-rotated optical feedback (PROF) at a round-trip feedback distance of 50.8 cm, corresponding to a feedback frequency of 0.59 GHz. Experimental results and numerical simulations revealed that the pulse generation mechanism involved a self-modulation of the laser’s relaxation oscillation frequency so that the oscillation frequency approximated to an integer multiple of the PROF round-trip feedback frequency. This effect is very different from similar experiments reported by many researchers before, in which the laser’s output was amplitude modulated by the feedback frequency and an ultra-short feedback distance was required to generate giga-hertz optical pulses. Investigations about some characteristics of the self-modulation mechanism will be reported in the paper.     

Modulation-free acetylene-stabilized lasers at 1542 nm using modulation transfer spectroscopy

We report frequency stabilization of diode lasers using modulation transfer spectroscopy of an acetylene transition (¹³C₂H₂, ₁+₃, P(16)) at 1542 nm. We realize modulation-free acetylene-stabilized lasers with a frequency stability of about 10^–11 and an absolute frequency accuracy of about 20 kHz.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Passively modelocked surface-emitting semiconductor lasers

This paper will review and discuss pico- and femtosecond pulse generation from passively modelocked vertical–external-cavity surface-emitting semiconductor lasers (VECSELs). We shall discuss the physical principles of ultrashort pulse generation in these lasers, considering in turn the role played by the semiconductor quantum well gain structure, and the saturable absorber. The paper will analyze the fundamental performance limits of these devices, and review the results that have been demonstrated to date. Different types of semiconductor saturable absorber mirror (SESAM) design, and their characteristic dynamics, are described in detail; exploring the ultimate goal of moving to a wafer integration approach, in which the SESAM is integrated into the VECSEL structure with tremendous gain in capability. In particular, the contrast between VECSELs and diode-pumped solid-state lasers and edge-emitting diode lasers will be discussed. Optically pumped VECSELs have led to an improvement by more than two orders of magnitude to date in the average output power achievable from a passively modelocked ultrafast semiconductor laser.     

80 Gb/s All-optical logic AND operation using Mach-Zehnder interferometer with differential scheme

An all-optical logic AND gate is demonstrated by using a semiconductor optical amplifier (SOA) based Mach-Zehnder interferometer (MZI). The AND results are numerically analyzed by solving the rate equation of SOA. Q-factor values have been calculated. The operation of the AND logic gate is experimentally demonstrated at 80 Gb/s. Operation at higher data rates is feasible using SOAs with shorter phase recovery time.     

Phase Noise of Optically Generated Microwave Using Sideband Injection Locking

Optically generated 20-GHz microwave carriers with phase noise lower than -75 dBc/Hz at 10 kHz offset and lower than -90 dBc/Hz at 100 kHz offset are obtained using single- and double-sideband injection locking. Within the locking range, the effect of sideband injection locking can be regarded as narrow-band amplification of the modulation sidebands. Increasing the current of slave laser will increase the power of beat signal and reduce the phase noise to a certain extent. Double-sideband injection locking can increase the power of the generated microwave carrier while keeping the phase noise at a low level. It is also revealed that partially destruction of coherence between the two beating lights in the course of sideband injection locking would impair the phase noise performance.     

Quasi-three-level Nd:YAG laser under diode pumping directly into the emitting level

We present theoretical and experimental investigations on ground-state direct pumping at 869 nm into the emitting level ⁴F_3/2 of end-pumped quasi-three-level Nd:YAG lasers operating at 946 nm. We have demonstrated, what we believe is for the first time, a Nd:YAG laser at 946 nm directly pumped by diodes and obtained 1.6 W of output power.     

Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser

Cavity-enhanced absorption spectroscopy is explained in terms of the transmission function of a rapidly swept interferometer, and the integrated transmission is shown to be proportional to the cavity ringdown time. The technique is demonstrated on the b¹Σ_g ⁺-X³Σ_g ^-  (1,0) band in molecular oxygen at 687 nm using a tunable diode laser and a relative-ly high-Q optical cavity (finesse ≈4000). A detection limit of 3×10^-8 cm^-1 s^1/2 is achieved for a 0.8 cm^-1 scanning range. Received: 24 June 2002 / Revised version: 5 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: peverall@physchem.ox.ac.uk