Image transfer by modulation of short light pulses

This paper shows that the Fourier-processing based modulation of short laser pulses can be successfully used for all-optical image transfer. The system under consideration consists of a transmitter, where the transferred image modulates the frequency profile of a carrier pulse, and a receiver, where after the reverse processing an original image is restored. The main limitations are discussed and the information capacity of a single pulse is obtained. The results from preliminary experiments using a CPM-ring dye laser as a source of carrier pulses are presented.     

Spectral properties of a Tm:Ho:YAG laser in active mirror configuration

3+ :Ho³⁺:YAG laser is operated in active mirror configuration. This laser is characterised by a rather thin crystal (3 mm thickness in our experiment) and a very short resonator (about 3.2 mm). Therefore this laser is well suited for operation in a single longitudinal mode. Single-frequency operation is demonstrated with and without an intracavity etalon. It is further shown that single-frequency emission is reached with a stable cw emission. With a slight detuning of the outcoupling mirror, spiking can be achieved, resulting in an emission on different lines. The assignment of the observed laser lines to the known transitions between Stark sublevels of the ⁵I₇ and the ⁵I₈ level is discussed and compared with predictions in the literature. Received: 2 December 1997     

Controlled two-pulse generation in Q-switch mode from a single laser using Q-modulator with frustrated total internal reflection

In this work a cavity design for double-pulse generation in Q-switched mode from a single laser is proposed, based on the construction of a second laser channel using a FTIR (frustrated total internal reflection) Q-modulator. A time interval between the two pulses from 500 ns to 120 μs is obtained in a Nd:YAG laser. A comparison with other methods and cavity designs for double-pulse generation is presented. The case when this technique is applied on a tunable laser with metastable upper laser level (Cr:LiCAF, Cr:LiSAF, Alexandrite, Co:MgF₂ or other) is also considered. Even though the method presented in the paper does not rely only on the cavity configuration proposed, some advantages can be obtained – both polarization and wavelength-independent tuning without polarization and wavelength restrictions in combination with the possibility of different wavelengths and polarizations in each pulse. Moreover, by using an active medium generating wavelengths around and up to 3 μm, the Pockels-cell-Q-switch optical transmission problems can be avoided. Received: 9 May 2001 / Revised version: 2 August 2001 / Published online: 15 October 2001     

Real-time nondestructive imaging with THz waves

We present a real-time imaging measurement in the terahertz (THz) frequency region. The dynamic subtraction technique is used to reduce long-term optical background drift. The reflective images of two targets, a Nikon camera’s lens cap and a plastic toy gun, are obtained. For the lens cap, the image data were processed to be false-color images. For the toy gun, we show that even under an optically opaque canvas bag, a clear terahertz image is obtained. It is shown that terahertz real-time imaging can be used to nondestructively detect concealed objects.     

Vector model analysis in nondestructive imaging by using the photothermal deflection methods

The noncontact imaging of the buried structures is carried out in the open-air atmosphere by using the photothermal deflection (PTD) method. We applied these techniques to the layered samples. Besides the PTD images for the optically opaque buried structures, the parameters of the materials such as thermal diffusivity can also be calculated from the PTD amplitude and phase signal in the PTD scanning images. When the PTD signals at two different modulation frequencies are used, the thermal diffusivity of the buried structure can be obtained from the PTD signal outside of the sample nondestructively. Received: 27 November 1998 / Accepted: 18 March 1999 / Published online: 7 July 1999     

Imaging and post-processing of laser-induced fluorescence from NO in a diesel engine

Received: 11 July 1996/Revised version: 12 November 1996     

Energetic electronic ensembles: new rules for the interaction of radiation with matter

2 α, where Z is the number of cooperatively acting electrons, which leads to anomalously strong coupling between radiation and matter. Received: 13 October 1997     

Optimization of grating size in chirped-pulse-amplification. laser system

We performed calculations for a chirped-pulse-amplification laser system with a sub-100-fs pulse duration including the finite size of gratings in an aberration-free stretcher/compressor design. Both the higher-order phase dispersion and the spectral clipping, which is due to the finite size of the optics in the stretcher and the compressor, are taken into account in the calculation. We discuss the influence of both phase dispersion and spectral clipping on the important parameters of the recompressed pulse, such as pulse duration, pulse contrast, and peak intensity, employing different phase-dispersion compensation methods. The calculations show that the use of a small compressor grating reduces the higher-order phase errors and smoothes the sharp spectral cut-off caused by spectral clipping in the stretcher, which consequently improves the pulse contrast ratio of the main pulse to the pedestal. Received: 1 April 2000 / Published online: 8 November 2000     

Coherent imaging of extended objects

When used with coherent light, optical imaging systems are inherently unable to reproduce both the amplitude and the phase of a two-dimensional field distribution. This is because their impulse response function varies slowly from point to point, a property known as non-isoplanatism. For sufficiently small objects, this usually results in a phase distortion and has no impact on the measured intensity. Here, we show that the intensity distribution can be dramatically distorted when extended objects are imaged. We illustrate the problem using two simple examples: the pinhole camera and the thin lens. The effects predicted by our theoretical analysis are confirmed by experimental observations.     

Distributed imaging using an array of compressive cameras

We describe a distributed computational imaging system that employs an array of feature specific sensors, also known as compressive imagers, to directly measure the linear projections of an object. Two different schemes for implementing these non-imaging sensors are discussed. We consider the task of object reconstruction and quantify the fidelity of reconstruction using the root mean squared error (RMSE) metric. We also study the lifetime of such a distributed sensor network. The sources of energy consumption in a distributed feature specific imaging (DFSI) system are discussed and compared with those in a distributed conventional imaging (DCI) system. A DFSI system consisting of 20 imagers collecting DCT, Hadamard, or PCA features has a lifetime of 4.8× that of the DCI system when the noise level is 20% and the reconstruction RMSE requirement is 6%. To validate the simulation results we emulate a distributed computational imaging system using an experimental setup consisting of an array of conventional cameras.     

Rb atomic absorption line reference for single Sr+ laser cooling systems

85 Rb, 5s²S_1/2(F”=2)→6p²P_1/2(F’=2,3) absorption resonance with the ⁸⁸Sr⁺, 5s²S_1/2→5p²P_1/2 transition is exploited to provide a simple, effective frequency reference for a laser cooling/fluorescence excitation source applied to single Sr⁺ ions. A modulation-free frequency stabilization system has been designed which uses the differential signal from two frequency-displaced beams traversing a Rb cell and which probe the Doppler-broadened Rb S–P lineshape at microwatt power levels. The method is applied to frequency lock a 422-nm frequency-doubled diode laser system that is used for excitation of a single ⁸⁸Sr⁺ ion. Stable, long-term laser cooling and fluorescence are achieved using the frequency-stabilized 422-nm source resulting in observed ion confinement times without adjustment of over 8 h, together with an improvement in single-ion loading efficiency. Received: 12 February 1998     

High-power intracavity frequency doubling of a Ti:sapphire femtosecond oscillator

We demonstrate intracavity frequency doubling of a standard femtosecond Ti:sapphire oscillator. The cavity is extended with a pair of focusing mirrors and a 0.5-mm-thick BBO crystal. We achieve a repetition rate of 50 MHz and simultaneously generate 22 mW of 55-fs pulses at 810 nm and 200 mW of 73-fs pulses at 405 nm, which corresponds to 4 nJ per pulse. We create a total of 330-mW, 405-nm light when pumping the Ti:sapphire crystal with 5.7 W from an Ar-ion laser, corresponding to a conversion efficiency of 5.7%. No saturation is found, which implies that higher outputs can be achieved with higher pump rates. Preliminary results from the use of blue pulses as pump in an optical parametric amplifier seeded by pulses from a photonic crystal fiber are presented. Received: 27 January 2003 / Revised version: 27 March 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +45-861/96199, E-mail: tva@chem.au.dk     

Short-wavelength far-infrared laser cavity yielding new laser emissions in CD3OH

A significantly improved far-infrared laser has been used to generate optically pumped laser emissions from 26 to 150 μm for CD₃OH. Using an X–V-pumping geometry, several new laser emissions have been found for CD₃OH. In addition, an increase in power, by factors from 10 to 1000, for many of the previously known shorter-wavelength laser lines, below 100 μm, has been observed. Frequency measurements for several lines have also been performed and have been reported to a fractional uncertainty up to ±2×10^-7, permitting the spectroscopic assignment of the laser transition. One of the frequency-measured lines, 44.256 μm observed using the 10R34 pump, has confirmed the assignment of the previously reported FIR emission (n,K;J)=(1,7;20)?(0,8;20)A in the ground vibrational state. Received: 26 October 2000 / Published online: 7 February 2001     

Femtosecond regenerative amplification in Cr:forsterite at 5–10 kHz

We developed a high-power oscillator–regenerative amplifier femtosecond laser system. Based on chromium-doped forsterite. The system is operating near 1.25 μm at a 5–10 kHz repetition rate. Chirped-pulse amplification produced 0.86 W (0.75 W) of average power, or 465 mW (400 mW) after compression at 5 kHz (10 kHz). Nearly bandwidth-limited pulses of duration 135 fs (shortest) and 150 fs (typical) are available with an energy of 93 μJ and 40 μJ at 5 and 10 kHz, respectively. Received: 7 June 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-30/63921289, E-mail: petrov@mbi-berlin.de     

Quantitative measurements of OH concentration fields by two-dimensional laser-induced fluorescence

Received: 7 May 1996/Revised version: 21 October 1996     

Laser frequency measurements and spectroscopic assignments of optically pumped 13CH3OH

A three-laser heterodyne system was used to measure the frequencies of twelve optically pumped laser emissions from ¹³CH₃OH in the far-infrared (FIR) region. These emissions, ranging from 54 to 142 μm, are reported with fractional uncertainties up to ±2×10^-7 along with their polarization relative to the CO₂ pump. Using the 9P32 and 10R14 CO₂ lines, complete spectroscopic assignments for two laser systems were confirmed. Received: 31 May 2001 / Published online: 19 September 2001     

Portable fiber-coupled diode-laser-based sensor for multiple trace gas detection

Tunable narrowband mid-infrared radiation from 3.25 to 4.4 μm is generated by a compact fiber-coupled, difference-frequency-based spectrosopic source. A 20-mW external cavity diode laser (with a tuning range from 814 to 870 nm) and a 50-mW distributed-Bragg-reflector diode-laser-seeded ytterbium-doped fiber amplifier operating at 1083 nm are difference-frequency mixed in a multi-grating, temperature-controlled periodically poled LiNbO₃ crystal. A conversion efficiency of 0.44 mW/(W² cm) (corresponding to a power of ≈3 μW at 3.3 μm) represents the highest conversion efficiency reported for a portable device. Performance characteristics of such a sensor and its application to spectroscopic detection of CO₂, N₂O, H₂CO, HCl, NO₂, and CH₄ will be reported in this work. Received: 14 May / Revised version: 24 June 1999 / Published online: 30 September 1999     

Table-top KrF amplifier delivering 270 fs output pulses with over 9 W average power at 300 Hz

Applying the combination of a solid-state Ti:Sa laser system and a newly developed wide-aperture, discharge-pumped KrF amplifier, output pulses with over 9 W average power at 300 Hz have been achieved in a single output beam. The frequency-tripled seed pulses of the Ti:Sa system – delivering approximately 10 μJ energy at 248 nm – were amplified to over 30 mJ using a 3-pass off-axis amplification scheme. The optical set-up has been fitted to the amplifier’s parameters, and stored-energy measurements were carried out with different parameters in order to optimize the operational conditions of the device for the highest average power. Received: 10 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/503-599, E-mail: jbekesi@llg.gwdg.de     

Ambient formaldehyde detection with a laser spectrometer based on difference-frequency generation in PPLN

A laser spectrometer based on difference-frequency generation in periodically poled LiNbO₃ (PPLN) has been used to quantify atmospheric formaldehyde with a detection limit of 0.32 parts per billion in a given volume (ppbV) using specifically developed data-processing techniques. With state-of-the-art fiber-coupled diode-laser pump sources at 1083 nm and 1561 nm, difference-frequency radiation has been generated in the 3.53-μm (2832-cm^-1) spectral region. Formaldehyde in ambient air in the 1- to 10-ppb V range has been detected continuously for nine and five days at two separate field sites in the Greater Houston area operated by the Texas Natural Resource Conservation Commission (TNRCC) and the Houston Regional Monitoring Corporation (HRM). The acquired spectroscopic data are compared with results obtained by a well-established wet-chemical o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) technique. Received: 8 November 2000 / Revised version: 30 January 2001 / Published online: 21 March 2001     

A laser system producing 5×1019 W/cm2 at 10 Hz

19 W/cm² have been achieved. Received: 10 March 1997/Revised version: 26 April 1997