Focusing dependence of the efficiency of a singly resonant optical parametric oscillator

Received: 15 February 1998/Revised version: 6 April 1998     

The use of a consumer grade photo camera in optical-digital correlator for pattern recognition and input scene restoration

In this work an optical-digital correlator for pattern recognition and input scene restoration is described. Main features of the described correlator are portability and ability of multi-element input scenes processing. The correlator consists of a consumer grade digital photo camera with a diffractive optical element (DOE) inserted as a correlation filter. Correlation of an input scene with a reference image recorded on the DOE are provided optically and registered by the digital photo camera for further processing. Using obtained correlation signals and DOE’s point spread function (PSF), one can restore the image of the input scene from the image of correlation signals by digital deconvolution algorithms.The construction of the correlator based on the consumer grade digital photo camera is presented. The software procedure that is necessary for images linearization of correlation signals is described. Experimental results on optical correlation are compared with numerical simulation. The results of images restoration from conventionally and specially processed correlation signals are reported. Quantitative estimations of accuracy of correlation signals as well as restored images of the input scene are presented.     

The filtering characteristics of simple grating optical low-pass filters

We studied the characteristics of a two-dimensional grating optical low-pass filter (GOLF) theoretically and experimentally. The modulation transfer function (MTF) of an optical system that consists of a lens and a GOLF is theoretically derived by taking all orders of diffracted beams into consideration. The MTFs of a two-phase chess-board-type GOLF and a three-phase GOLF were calculated for various phase differences and compared with that of a birefringent low-pass filter (BLF). The three-phase GOLF with nine center beams of equal strength removes most moiré fringes, but the resolution degradation is severe compared to the BLF. The two-phase GOLF with a phase difference of 180°, which is similar to the BLF in term of beam distribution, has a medium characteristic somewhere between those of the three-phase GOLF and the BLF. Samples of two GOLFs are made and experimented on by attaching them to a digital camera. The experimental result coincides with the theoretical development. Received: 31 October 2001 / Revised version: 4 March 2002 / Published online: 2 May 2002     

Optimal Truncation of Element Beam in a Coherent Fiber Laser Array

The beam quality of a coherent fiber laser array often suffers from the low fill factor of the Gaussian laser array. One simple and effective approach to improve the fill factor is to truncate the array element properly. An analytical expression for far-field intensity distribution of a truncated coherent fiber laser array is derived. Optimal truncation of the element beam in different coherent fiber laser arrays is obtained by using energy encircled in the far-field central-lobe as the beam quality criterion. By optimal truncation, energy encircled in the central-lobe can be 97% compared with the ideal case. The shift in optimal truncation parameter in the case of phase noise is also analyzed.     

First-order optical systems with real eigenvalues

It is shown that a lossless first-order optical system whose real symplectic ray transformation matrix can be diagonalized and has only real eigenvalues, is similar to a separable hyperbolic expander in the sense that the respective ray transformation matrices are related by means of a similarity transformation. Moreover, it is shown how eigenfunctions of such a system can be determined, based on the fact that simple powers are eigenfunctions of a separable magnifier. As an example, a set of eigenfunctions of a hyperbolic expander is determined and the resemblance between these functions and the well-known Hermite-Gauss modes is exploited.     

Mach-Zehnder joint transform correlator with multi-channel quantized reference functions for color pattern recognition

Based on the Mach-Zehnder interferometer, we introduce a novel construction of the optical correlator for color pattern recognition. This system could remove the zero order term, which decreases the discrimination ability of the system, in only one step. Since each color can be separated into 3 channels, we propose the multi-level quantized reference functions (MQRF) for each color channel, and utilize them in spatial domain. Consequently, a total of 3 MQRFs and 3 color channels of the test image could be implemented at the input plane of liquid crystal spatial light modulator (LCSLM) of Mach-Zehnder joint transform correlator. Each LCSLM is modulated along the real-valued axis.     

Photorefractive multichannel correlator based on stoichiometric LiTaO3

We demonstrate a Vander Lugt type correlator based on photorefractive stoichiometric LiTaO₃ as the volume holographic material. Accurate correlator output was obtained for binary phase-modulated input images. The correlator is tested with binary amplitude- and phase-modulated input images. Phase-modulated images show a high-quality correlation, whereas amplitude-modulated images suffer in part from a photorefractive self-focusing effect. Using 100 previously stored holograms as filters, a rate of 10000 correlations/s are demonstrated. A paraxial theoretical analysis of the correlator, giving analytic expressions for the shift invariance, is also described in detail. Received: 22 November 2000 / Revised version: 9 February 2001 / Published online: 20 April 2001     

The modeling and the optimization of a grating optical low-pass filter

Through the one-dimensional analysis of the diffraction of a phase grating, we calculated the optical transfer function (OTF) of a phase grating. By a 0th-order approximation of the optical transfer function, we analyzed the feasibility of using it as a spatial frequency filter and obtained the structure of a grating for the optimum-filtering characteristics. The OTF of a phase grating as an optical low-pass filter is optimal when the incoming light is equally divided into three beams. The distance of those beams must be equal to the pixel size. The theoretical development was verified through an experiment. Received: 21 May 2001 / Revised version: 16 July 2001 / Published online: 15 October 2001     

Accurate method for the measurement of absorption cross sections of solid-state saturable absorbers

A rigorous approach taking into account both the temporal and spatial distributions of the pulsed probe beam used for transmission measurements of solid-state saturable absorbers is applied to characterise these absorbers accurately. The use of this method can significantly influence the resulting value for the ground-state absorption cross section. It also permits us to avoid the introduction of an artificial excited-state absorption in systems where, according to energy-level diagrams, no such loss should occur. This is illustrated in the case of passive Q-switchers for 1.5-μm lasers such as Co²⁺-doped LaMgAl₁₁O₁₉ and MgAl₂O₄ and Co²⁺- or Cr²⁺-doped ZnSe and ZnS. Received: 21 November 2001 / Revised version: 22 January 2002 / Published online: 14 March 2002     

20-Hz operation of an eye-safe cascade Raman laser with a Ba(NO3)2 crystal

Operation of a 1.598-μm eye-safe third-Stokes Raman laser with a Ba(NO₃)₂ crystal pumped by a 1.064-μm Nd:YAG laser is described. We observed a substantial decrease in the output energy during the first 50 s of the continuous operation at 20 Hz. The energy drop is ∼76% of the initial third-Stokes output. We confirmed negative thermal lensing and thermally induced birefringence in the crystal. With a concave cavity mirror at a matched curvature to the thermal lensing, we obtained an output energy of 11 mJ at 20 Hz. TEM₀₀ output was also obtained with a smaller pump-beam diameter with a highest conversion efficiency of 15.5% for a pumping power of only 45 MW/cm² (0.9 J/cm²). Received: 20 November 2001 / Revised version: 20 February 2002 / Published online: 2 May 2002     

Design and characterization of a near-diffraction-limited femtosecond 100-TW 10-Hz high-intensity laser system

We present the design and characterization of a femtosecond high-intensity laser system emitting a near-diffraction-limited beam. This system was dimensioned in order to reach intensities in excess of 10²⁰ W/cm² at a high repetition rate for ultrahigh-field physics experiments. We describe the improvements that were added to a conventional chirp pulse amplification configuration in order to decrease the deleterious effects of gain narrowing, gain shifting, thermal focusing in the amplifier stages, and spatial degradation due to multipass amplification processes. Received: 4 January 2002 / Published online: 14 May 2002     

Generation of subpicosecond infrared pulses tunable between 5.2 μm and 18 μm at a repetition rate of 76 MHz

2 (2 mm thick) and GaSe (1 mm thick) crystals are applied as nonlinear media. GaSe has a larger tuning range and is more efficient in the whole spectral range than AgGaS₂. The average IR power reaches up to 2.0 mW at 8.5 μm when the GaSe crystal is used, and up to 1.3 mW at the same wavelength when AgGaS₂ is applied for the frequency conversion. Received: 17 February 1997/Revised version: 14 July 1997     

Seven-terawatt Ti:sapphire laser system operating at 50 Hz with high beam quality for laser Compton femtosecond X-ray generation

We present a 7-TW Ti:sapphire laser system operating at 50 Hz for laser Compton femtosecond X-ray generation. This laser system delivers 8.4 W of average output power at a repetition rate of 50 Hz with a pulse width of 24 fs. It demonstrates successful management using a dynamically stable resonator in the regenerative amplifier and compensation for thermal lensing by a convex mirror in a ring-type four-pass power amplifier. We also present the results of closed-loop corrections for distorted wavefronts of amplified and compressed laser pulses, using an adaptive optical system consisting of a Shack–Hartmann-type wavefront sensor and a deformable mirror. This closed-loop correction results in dramatic improvements, reducing wavefront distortions below 0.05 λ rms. Received: 31 October 2002 / Revised version: 3 March 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-3349, E-mail: ito@festa.or.jp     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

Generation of high-quality 10-femtosecond laser pulses with intracavity spatial and spectral control

Spatial and spectral control, using an intracavity capillary and a slit, is applied to improve the output pulse quality of a Ti:sapphire laser. Satellite-free 10-fs optical pulses with a smooth spectral and spatial profile have been generated. Employing a root-mean-square formalism for pulse characterization, spatial, spectral and temporal intensity distributions are analyzed for laser pulses with a duration as short as three to four optical cycles. Received: 11 June 2001 / Published online: 18 July 2001     

Coupling laser radiation to a fabry-perot cavity with a single-mode optical fiber

Three aspects of coupling to Fabry-Perot cavities used in optical frequency standards are discussed: the use of a single-mode optical fiber to maintain coupling stability while improving vibration isolation of the cavity, the required stability of the coupling geometry, and the phase and polarization variations resulting from fiber movement. Optical fiber coupling should be useful when laser linewidths and stabilities at the Hertz level are desired.     

Diffractive effects in singly resonant continuous-wave parametric oscillators

I /k_P=0.33and0.50. It is found that the ratio of pump depletion to maximal depletion as a function of the ratio of pump power to threshold power agrees with the plane-wave prediction to within 5%, for a wide range of focusing conditions. The observed trends are explained as resulting from intensity- and phase-dependent mechanisms. Received: 19 January 1998/Revised version: 13 March 1998     

All-optical power limiting in waveguides with periodically distributed Kerr-like non-linearity

A slab waveguide with an embedded non-linear grating is considered as an all-optical power-limiting device. The limiting action is based on the power losses at boundaries between grating sections, where a mode-mismatch results from the self-focusing of the fundamental mode in each non-linear section. Time-harmonic and optical pulse propagation are both modelled numerically by finite-difference techniques to demonstrate the behaviour of the device. Received: 17 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Electron dynamics characteristics in high-intensity laser fields

This paper addresses the conditions under which the vacuum laser acceleration scheme CAS (capture and acceleration scenario), newly proposed by the authors (see, e.g., P.X. Wang et al., Appl. Phys. Lett. 78, 2253 (2001)), can be observed. Specifically, the laser intensity threshold (a₀)_th and the range of the electron incident momentum for the CAS scheme to emerge are examined. We found that (a₀)_th is critically dependent on the laser beam width w₀. At kw₀=60, (a₀)_th=8, which is an intensity obtainable using present laser systems. The required energy of the incident electron is in the range 5–15 MeV. This study is of significance in designing an experimental setup to test CAS and helpful in understanding the basic physics of CAS. Received: 4 March 2002 / Revised version: 25 March 2002 / Published online: 8 May 2002     

Statistical modeling of imaging process in active night vision systems with gate-light detection

Characteristics of object images observed through scattering media with an active vision system operating in the mode of spatial selection are estimated using the Monte Carlo method. The effect of optical and geometrical conditions of observation on the contrast in images of reflecting objects is considered. Interpretation is given to the obtained dependences. Received: 11 February 2002 / Revised version: 31 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-38/2225-9086, E-mail: belov@iao.ru