Associative Grammar Combination Operators for Tree-Based Grammars

Polarized unification grammar (PUG) is a linguistic formalism which uses polarities to better control the way grammar fragments interact. The grammar combination operation of PUG was conjectured to be associative. We show that PUG grammar combination is not associative, and even attaching polarities to objects does not make it order-independent. Moreover, we prove that no non-trivial polarity system exists for which grammar combination is associative. We then redefine the grammar combination operator, moving to the powerset domain, in a way that guarantees associativity. The method we propose is general and is applicable to a variety of tree-based grammar formalisms.     

Low-noise operation modes of a passively mode-locked fiber laser

Optimum low-noise operation modes of a passively mode-locked fiber laser are described and characterized. The laser produces 60 fs bandwidth limited pulses with root mean square amplitude fluctuations of 0.4% in a frequency-band from 30 Hz to 100 kHz. It is shown that low-noise operation up to 5 dB below the amplitude noise of the pump laser is possible with overall negative cavity group-velocity dispersion and a large difference between the intracavity loss under free-running and mode-locked laser operation. Additional frequency stabilization reduces the timing jitter to 4 fs in a frequency band from 1 kHz to 100 kHz and 110 fs in the 30 Hz to 100 kHz band     

Unification Grammars and Off-Line Parsability

Unification grammars are known to be Turing-equivalent; given a grammar G and a word w, it is undecidable whether w L(G). In order to ensure decidability, several constraints on grammars, commonly known as off-line parsability (OLP), were suggested, such that the recognition problem is decidable for grammars which satisfy OLP. An open question is whether it is decidable if a given grammar satisfies OLP. In this paper we investigate various definitions of OLP and discuss their interrelations, proving that some of the OLP variants are indeed undecidable. We then present a novel, decidable OLP constraint which is more liberal than the existing decidable ones.     

Characterisation of laser ignition in hydrogen–air mixtures in a combustion bomb

Laser-induced spark ignition of lean hydrogen–air mixtures was experimentally investigated using nanosecond pulses generated by Q-switched Nd:YAG laser (wavelength 1064 nm) at initial pressure of 3 MPa and temperature 323 K in a constant volume combustion chamber. Laser ignition has several advantages over conventional ignition systems especially in internal combustion engines, hence it is necessary to characterise the combustion phenomena from start of plasma formation to end of combustion. In the present experimental investigation, the formation of laser plasma by spontaneous emission technique and subsequently developing flame kernel was measured. Initially, the plasma propagates towards the incoming laser. This backward moving plasma (towards the focusing lens) grows much faster than the forward moving plasma (along the direction of laser). A piezoelectric pressure transducer was used to measure the pressure rise in the combustion chamber. Hydrogen–air mixtures were also ignited using a spark plug under identical experimental conditions and results are compared with the laser ignition ones.     

A Hebrew verb–complement dictionary

We present a verb–complement dictionary of Modern Hebrew, automatically extracted from text corpora. Carefully examining a large set of examples, we defined ten types of verb complements that cover the vast majority of the occurrences of verb complements in the corpora. We explored several collocation measures as indicators of the strength of the association between the verb and its complement. We then used these measures to automatically extract verb complements from corpora. The result is a wide-coverage, accurate dictionary that lists not only the likely complements for each verb, but also the likelihood of each complement. We evaluated the quality of the extracted dictionary both intrinsically and extrinsically. Intrinsically, we showed high precision and recall on randomly (but systematically) selected verbs. Extrinsically, we showed that using the extracted information is beneficial for two applications, prepositional phrase attachment disambiguation and Arabic-to-Hebrew machine translation.     

Language resources for Hebrew

We describe a suite of standards, resources and tools for computational encoding and processing of Modern Hebrew texts. These include an array of XML schemas for representing linguistic resources; a variety of text corpora, raw, automatically processed and manually annotated; lexical databases, including a broad-coverage monolingual lexicon, a bilingual dictionary and a WordNet; and morphological processors which can analyze, generate and disambiguate Hebrew word forms. The resources are developed under centralized supervision, so that they are compatible with each other. They are freely available and many of them have already been used for several applications, both academic and industrial.

Experimental study of additive-pulse mode locking in an Nd:glasslaser

Self-starting additive-pulse mode locking (APM) has been investigated experimentally in a continuous-wave Nd:glass laser. Stress has been put on the study of the self-starting process and relaxation oscillation instabilities. An intensity threshold for self-starting APM is observed and related to the linewidth of the first beat note of the power spectrum of the free-running laser output. Under steady-state conditions, two distinct operating regimes are obtained: mode locking with repetitive self-Q switching and pure mode locking. Increasing the intracavity power gives rise to a strong chirp developing on the mode-locked pulses and to a simultaneous disappearance of self- Q switching     

Laser ablation of dental tissues with picosecond pulses of 1.06-microm radiation transmitted through a hollow-core photonic-crystal fiber

Sequences of picosecond pulses of 1.06-microm Nd:YAG laser radiation with a total energy of approximately 2 mJ are transmitted through a hollow-core photonic-crystal fiber with a core diameter of approximately 14 microm and are focused onto a tooth's surface in vitro to ablate dental tissue. The hollow-core photonic-crystal fiber is shown to support the single-fundamental-mode regime for 1.06-microm laser radiation, serving as a spatial filter and allowing the laser beam's quality to be substantially improved. The same fiber is used to transmit emission from plasmas produced by laser pulses onto the tooth's surface in the backward direction for detection and optical diagnostics.