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     

Two-wavelength self-seeded pulsed tunable laser

A compact self-seeded pulsed tunable laser is described. Its optical cavity comprises a diffraction grating, operating at a grazing angle of incidence as a spectral selector and narrow-band spectral gate. The grating couples two partially overlapping laser channels: a highly selective master laser and a nonselective slave laser. Due to the implemented efficient intracavity self-seeding the laser emits spectrally pure single-longitudinal-mode radiation at two independently tunable wavelengths, with an efficiency and output energy that are typical for nonselective lasers. Results of the experimental investigation of the laser’s output characteristics are presented. Received: 12 March 2001 / Revised version: 10 December 2001 / Published online: 7 February 2002     

A dispersionless Michelson interferometer for the characterization of attosecond pulses

A novel application of a free-standing transmission grating as a beam splitter in a Michelson-type interferometer is described. The arrangement can operate in the XUV and soft X-ray spectral region and, therefore, it is well suited for the characterization of attosecond pulses. Using ray-tracing codes, we have analyzed three different setups in which spherical mirrors are employed in conjunction with the transmission grating and have investigated in detail their dispersive characteristics. It is shown that such an arrangement can be made to exhibit group-delay dispersion of ∼1 as² while it provides two co-propagating and converging beams. Received: 12 October 2001 / Revised version: 17 December 2001 / Published online: 7 February 2002     

Excimer laser-induced etching of sub-micron surface relief gratings in fused silica using phase grating projection

Phase grating projection using a Schwarzschild objective is presented in conjunction with a process that allows laser-induced etching of transparent materials at the interface of liquids, in order to generate periodical surface relief structures with sub-micron resolution in fused silica. The achieved sinusoidal gratings exhibit a period of 780 nm and a depth of up to 180 nm, having a roughness lower than 5 nm r.m.s. The depth and roughness of the gratings are related to the applied laser fluence and pulse number. In addition to the grating formation, an overall removal of material at higher laser fluences was found. Received: 3 December 2001 / Accepted: 5 December 2001 / Published online: 11 February 2002     

Multiple-order adaptive dispersion compensation using polynomially-chirped grating devices

Devices such as the planar arrayed-waveguide grating or the distributed Bragg reflector (AWG and DBR, respectively) are assuming increasing importance in the areas of fibre point-to-point communication and networking. In the particular context of dense wavelength-division multiplexing (DWDM), these devices play a well-established role as wavelength-selective elements. More recently, chirped variants have found use as dispersion compensators, offering wideband reduction of both basic and higher-order departures from constant group delay. However, up to the present time, the existence of a systematic approach to higher-order dispersion compensation has not been recognised. Additionally, we have identified a comprehensive isomorphism between AWG and DBR devices that allows the design characteristics of the former transverse geometry to be mapped on to the latter longitudinal structure. Here, we present a systematic study of these important passive optical grating-based devices which considers, firstly, analytic solutions for 2nd-order dispersion compensation (DC), and then 3rd- and 4th-order dispersion compensators with 100 GHz bandwidth. We then review optimisation strategies for DC, 3 dB bandwidth, and passband dispersion ripple, as determined by device chirp, apodisation, coupling strength and polyphase transfer function superposition. We conclude with a preliminary discussion of grating parametric sensitivity as evidenced by Monte Carlo analysis. Received: 22 May 2001 / Revised version: 31 July 2001 / Published online: 2 November 2001     

Manifestation of Curie–Weiss law for optical phase transition

Considerable slowing down is observed for both the temporal development of the coherent oscillation slightly above the threshold and the refractive index grating decay slightly below the threshold for a semilinear photorefractive oscillator with two counter-propagating pump waves. It is shown that in the vicinity of the threshold the reciprocal characteristic time is a linear function of deviation from the threshold coupling strength. This behaviour is similar to an empirical Curie–Weiss law and points to the analogy of the oscillation threshold to a second-order phase transition. Received: 2 April 2001 / Revised version: 23 August 2001 / Published online: 7 November 2001     

Reflection-type photorefractive gratings in tin hypotiodiphosphate

Reflection photorefractive gratings recorded by nearly counterpropagating light waves in the near infrared are studied in tin hypothiodiphosphate. The ratios are established for certain electrooptic tensor components responsible for reflection grating recording, and the Debye screening length is evaluated. Reflection holograms of binary objects are recorded. Received: 26 June 2001 / Revised version: 3 September 2001 / Published online: 29 November 2001     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Efficient self phase matched third harmonic generation of ultrashort pulses in a material with positive dispersion

An ultrafast transient refractive index grating, produced in barium fluoride, a material with positive dispersion, promotes very efficient third harmonic generation. The significant enhancement of the generation up to a conversion efficiency of about 3% is due to self phase matching, involving the instantaneous grating. At the same time several diffraction orders of the third harmonic signal are observed behind the sample. Received: 17 October 2000 / Published online: 21 February 2001     

Detection of small phase modulation using two-wave mixing in photorefractive crystals

Two-wave mixing of phase-modulated beams in photorefractive crystals under a dc electric field is studied both theoretically and experimentally. Based on the vectorial theory of light diffraction in crystals of cubic symmetry, we derived an analytical expression that describes the phase demodulation in crystals with an arbitrarily oriented electric field. The phase-demodulation technique was used for estimation of the space-charge field created in photorefractive crystals. It is shown that the space-charge-field grating created in GaP and Bi₁₂TiO₂₀ crystals is much smaller than that predicted from the one-level band-transport model of the photorefractive effect. Received: 12 December 2000 / Revised version: 8 February 2001 / Published online: 27 April 2001     

Photorefractive properties of (1-10) and (111)-cut sillenite crystals when external electric field is applied along the direction of the optimum diffraction efficiency

We study the influence of the application of an external electric field on the grating formed in (1-10) and (111)-cut photorefractive sillenite crystals. Optimum conditions for application of the bias field are examined, taking into account the shift of the grating orientation for maximum diffraction efficiency along the crystal depth. The contribution of the piezoelectric effect is considered both in the electro-optic tensor and space charge field calculations. An analysis on the diffractive properties under these optimum conditions shows an increase of 30% in maximum diffraction efficiency in Bi₁₂GeO₂₀, and experimental verification for this is provided. Received: 5 November 2001 / Revised version: 18 April 2002 / Published online: 8 August 2002     

Optical limiting in cobalt-doped polyvinylpyrrolidone

The results of optical limiting investigations of cobalt-doped polyvinylpyrrolidone solutions are presented. The optical limiting studies have shown that this process is due to self-defocusing at the wavelength of 1064 nm and also due to reverse saturable absorption and self-defocusing at the wavelength of 532 nm. The results of measurements on the non-linear optical characteristics of organometallic complexes are presented. Received: 10 July 2001 / Revised version: 15 October 2001 / Published online: 29 November 2001     

Electric-field enhancement of beam coupling in Sn2P2S6

The influence of an external field on photorefractive recording in Sn₂P₂S₆ (SPS) crystals is studied. A large gain factor of more then 15 cm^-1 is achieved for a grating spacing of 12 μm at λ=0.9 μm. For an applied field exceeding ±200 V/cm a switching of the beam coupling direction is detected, exhibiting a pronounced hysteresis. Received: 25 October 2000 / Revised version: 18 January 2001 / Published online: 21 March 2001     

Threshold reduction of stimulated Brillouin scattering (SBS) using fiber loop schemes

SBS mirrors as self-pumped and easy to handle non-linear optical devices are frequently used in high-power laser systems for improving the beam quality based on optical phase conjugation. Because of the non-linear behaviour, a certain pulse energy or power of incident light is needed to generate enough reflectivity for practical purposes. Therefore, reducing this “threshold” is still a main topic in the development of new schemes for optical phase conjugation. In addition to the taper concept reported earlier, this paper deals with loop schemes for reducing the power requirements. A reduction of the so-called “threshold” by a factor of between two and four was obtained with the schemes investigated using liquids and fibers. Received: 4 September 2001 / Revised version: 22 October 2001 / Published online: 23 November 2001     

The characterization of plasma-polymerized C60 thin films

Plasma-polymerized C₆₀ thin films were investigated in the form of a field effect transistor (FET) structure. In the FET device, the C₆₀ polymer acts as a p-type semiconductor, whereas C₆₀ is an n-type semiconductor. Its conduction mechanism can be described as due to variable range hopping. As a sensing device, the C₆₀ polymer can behave as a gas sensor for electrophoric gases and can also be operated as a photo-sensing device in air. Received: 21 April 2001 / Accepted: 23 July 2001 / Published online: 2 October 2001     

Experimental study of pulse compression in a side-pumped Yb-doped double-clad mode-locked fiber laser

We have developed a side-pumped passively mode-locked Yb-doped double-clad fiber laser emitting sub-picosecond pulses at around 1.05 μm. Mode locking is achieved through a polarization additive-pulse mode-locking technique and compression is obtained with a grating pair inserted in the cavity. We have investigated the compression properties of this laser. High-energy pulses are emitted and different behaviors are observed, such as sideband generation, secant-like or Gaussian-like pulse emission. Received: 7 May 2001 / Revised version: 13 November 2001 / Published online: 17 January 2002     

Design, realisation and analysis of an apodised, film-loaded acousto-optic tunable filter

The design and realisation of a SiO₂ film-loaded Ti:LiNbO₃ integrated acousto-optic tunable filter are presented. An optimised Hamming apodisation is achieved through introducing an angular offset between the acoustic and optical waveguides along with carefully placed acoustic absorbers. The realised device has a 20-mm acoustic interaction length and achieves a 3-dB linewidth of 1.8 nm with 19-dB sidelobe suppression. The realised device is highly efficient, requiring a RF drive power of only 10 mW to achieve complete polarisation conversion. The enhanced drive efficiency of the device is analysed using both acoustic and optical mode analysis. Based on this analysis, possible explanations for the improved behaviour are presented and directions that may lead to further enhancements are discussed. Received: 18 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Squeezed light from microstructured fibres: towards free-space quantum cryptography

Amplitude-squeezed pulsed light has been produced using a microstructured silica fibre. By spectrally filtering after the non-linear propagation in the fibre a squeezing value of -1.7 dB has been measured. A quantum key distribution scheme based on squeezed light from such microstructured fibres is proposed. Received: 9 July 2001 / Published online: 23 November 2001     

Tunable twin beams generated by a type-I LNB OPO

We present a novel scheme to separate spatially twin beams generated by a type-I lithium niobate (LNB) optical parametric oscillator near frequency degeneracy. The system is based on a holographic diffraction grating acting as a beam splitter in a balanced detector. The fast and easy temperature tuning of LNB index of refraction allows an easy control of the twin-beam wavelength distance in a range of the order of ∼100 nm. We report correlation spectra measured for different twin-beam wavelength separations (15–60 nm) with a maximum noise reduction of 3.2 dB at 3.5 MHz. The described system exhibited a pump resonance stability longer than 6 h with infrared output power fluctuations within 4% around an average value of ≃2 mW in each beam. The measured oscillation threshold pump power was lower than 31 mW. Received: 7 June 2001 / Revised version: 17 July 2001 / Published online: 10 October 2001