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Summary Background Emergency abdominal surgery is associated with poor patient outcomes. We studied the effectiveness of a national quality improvement (QI) programme to implement a care pathway to improve survival for these patients. Methods We did a stepped-wedge cluster-randomised trial of patients aged 40 years or older undergoing emergency open major abdominal surgery. Eligible UK National Health Service (NHS) hospitals (those that had an emergency general surgical service, a substantial volume of emergency abdominal surgery cases, and contributed data to the National Emergency Laparotomy Audit) were organised into 15 geographical clusters and commenced the QI programme in a random order, based on a computer-generated random sequence, over an 85-week period with one geographical cluster commencing the intervention every 5 weeks from the second to the 16th time period. Patients were masked to the study group, but it was not possible to mask hospital staff or investigators. The primary outcome measure was mortality within 90 days of surgery. Analyses were done on an intention-to-treat basis. This study is registered with the ISRCTN registry, number ISRCTN80682973. Findings Treatment took place between March 3, 2014, and Oct 19, 2015. 22 754 patients were assessed for elegibility. Of 15 873 eligible patients from 93 NHS hospitals, primary outcome data were analysed for 8482 patients in the usual care group and 7374 in the QI group. Eight patients in the usual care group and nine patients in the QI group were not included in the analysis because of missing primary outcome data. The primary outcome of 90-day mortality occurred in 1210 (16%) patients in the QI group compared with 1393 (16%) patients in the usual care group (HR 1·11, 0·96–1·28). Interpretation No survival benefit was observed from this QI programme to implement a care pathway for patients undergoing emergency abdominal surgery. Future QI programmes should ensure that teams have both the time and resources needed to improve patient care. Funding National Institute for Health Research Health Services and Delivery Research Programme.

Index of refraction profiles of optical fibers are obtained by a new interference technique closely related to Mach-Zehnder microscope interferometry. Data are obtained from specially prepared samples using a conventional reflected light microscope. Excellent agreement is obtained with the results using the Mach-Zehnder method but at a considerable reduction in the cost and complexity of instrumentation.

Measurements have been made of Rayleigh scattering in CCl(4) and C(2)Cl(4) as a function of wavelength between 6000 A and 10,600 A. The scattering loss rate is 25 dB/km for CCl(4) and 68 dB/km for C(2)C(4) at 6328 A. The scattering loss rate follows a (wavelength)(-4) dependence over the entire spectrum observed. The Rayleigh scattering was measured by placing the liquid sample in a hollow fused-quartz fiber and measuring the light scattered by the liquid through the fiber wall. It appears that this is the first time that the expected (wavelength)(-4) dependence of Rayleigh scattering has actually been observed in liquids over a wide spectral range. This technique is simple and accurate and may be applied to the measurement of the scattering coefficient of any liquid whose index of refraction exceeds the cladding index of refraction (for fused quartz n(D) = 1.458).

A dc-biased Kerr cell light modulator has been operated continuously at 5 Mc/sec. An analysis has been made showing the generation of harmonics as a function of applied bias and rf voltages. This shows how these may be chosen to optimize generation of a selected harmonic. Experimental data in excellent agreement with theory have been obtained. Modulation in excess of 50% at first harmonic has been obtained with rf voltages of the order of 15% of the dc extinction voltage.

Narrowband optical filters were made by direct deposition of thin films of TiO/sub 2/ and SiO/sub 2/ on the ends of optical fibers. Single-cavity etalons with bandwidths of 1.1 nm, insertion loss of 1 dB, and 2.25 nm, insertion loss of 0.5 dB, were made. It is necessary to use fibers with mode fields larger than standard single mode fiber. Thermal expansion was used to diffuse the core dopant and produce tapered-core fibers with mode-field diameters of 22 mu m. >

We have used an interferometric technique to measure first- and second-order dispersion of a particular type of dispersion-shifted fiber. The values of beta and beta near the dispersion minimum are substantially the same as those of a conventional fiber except for being shifted from the 1.3-microm to the 1.5-microm wavelength range. We discuss the implications of this observation on the bandwidth of dispersion-shifted fibers.

Time-averaged and time-resolved spectra of single-mode, optically pumped, ultrashort-cavity semiconductor film lasers have been studied in detail. The time-averaged spectra are broad (3-17 nm). Time-resolved spectra show that the lasing wavelength increases (chirps) during the picosecond-time-scale duration of the pulse. The chirp is due to free-carrier plasma effects on the refractive index of the highly excited semiconductor (10(20) carriers/ cm(3)). Film-laser pulses are compressed by a factor of 2 in an 8-m length of single-mode optical fiber because of the linear group-velocity dispersion of the fiber, producing a pulse of 3.7-psec duration at 1.264 microm.

A new Raman-gain medium consisting of molecular hydrogen dissolved at high pressure in low-loss solid-silica optical fibers is described. Stimulated Raman gain at 0.88-microm wavelength was measured in a half-kilometer length of this H(2)-diffused fiber by using a krypton-laser pump and an injection-laser probe. The gain coefficient determined from these experiments at 0.88 microm is 5 x 10(-12) cm/W. The overtone absorption spectrum of high-pressure hydrogen in silica was observed.

A low-loss triangular-profile single-mode fiber is reported. The fiber loss at 1.3-1.55microm wavelengths is below 0.4 dB/km, and its zero-chromatic-dispersion wavelength is 1.402 microm.

Ultrashort-cavity, thin-film lasers from In(1-x)Ga(x)As(y)P(1-y) of five different compositions, including InP and In(0.53)Ga(0.47)As, have been made to lase between 0.83 and 1.59 microm. The multitude of lasing wavelengths observed had line-to-line separations of less than 10 nm. The lasers were pumped with 1-psec pulses from a mode-locked dye laser. An output pulse of 6-psec duration was measured at a wavelength of 1.16 microm.

For the frequency-division-multiple-access-frequency-shift-keying (FDMA-FSK) network, the FSK signal is converted to amplitude-shift-keyed (ASK) format by a tunable fiber Fabry-Perot (FFP) filter that acts simultaneously as a demodulator and demultiplexer. Frequency-tunable two-electrode distributed-feedback (DFB) laser transmitters produce distorted FSK spectra due to nonuniform FM response for modulation frequencies above approximately 200 Mb/s. The frequency-domain properties of the laser are related directly to the time-domain properties of the transmitted data stream. The critical dependence on the FFP passband width and center frequency location for undistorted signal recovery is shown. By optimizing these parameters, the data transmission rate can be increased while keeping the power penalty due to signal distortion below 1 dB. >

An optical cross-connect switch using the star-coupler-based frequency-division-multiplex technique are discussed. Two specific tunable receivers have been implanted. The first is a heterodyne receiver with a tunable laser as the local oscillator (LO) and the second is a tunable filter followed by a direct-detection receiver. In the heterodyne receiver, the tunable LO was a monolithic three-section multiple-quantum-well distributed Bragg laser capable of a 1000-GHz tuning range. Receiver sensitivity was measured to be -38 dBm at 1 Gb/s (BER=10/sup -10/). The power margin in the system substantiated feasibility for a 400*400 switch. In the tunable-filter receiver, the tunable filter is a tunable two-stage optical fiber Fabry-Perot filter design consisting of a narrowband filter followed by a wideband filter. The tuning of the filters is computer controlled, and the combined filter has a tuning range of 15000 GHz with a finesse of approximately=5170. Therefore, it is capable of covering over 1000 channels of 2.9 GHz each. >

Results obtained using a fiber optical star network using densely-spaced frequency-division multiplexing (FDM) and heterodyne detection techniques are presented. The system consists of three optical sources frequency-shift keyed at 45 Mb/s and spaced by 300 MHz. Frequency selection of the desired channel is done by a heterodyne FM receiver at a bit-error rate of 10/sup -9/ is -61 dBm or 113 photons/b, which is 4.5 dB from the shot-noise limit. Cochannel interference is found negligible for this channel spacing and modulation rate. Frequency stabilization of the FDM signals to a comb of equally-spaced frequencies, imperative in such a densely-spaced FDM system, is also demonstrated. The results indicate that this system can provide a throughput of 4500 Gb/s. >

A key component of Wavelength Division Multiplexed (WDM) networks is a tunable broadband wavelength demultiplexer, with good selectivity and high tuning speed. In this communication we report low waveguide loss InGaAsP Fabry-Perot filters, grown by Metal Organic Vapor Phase Epitaxy, with tuning ranges up to 3.07 nm (380 GHz). These filters have waveguides with bandgap wavelengths between 1.3 and 1.48 /spl mu/m so that they are transparent at zero bias and at low tuning currents ( >

The authors discuss results of research aimed at understanding the use of monolithic tunable lasers and fiber Fabry-Perot filters in implementing an optical cross-connect switch. The switch architecture uses optical frequency division multiplexing where laser signals are transmitted at different frequencies and then summed in a star coupler. At each star coupler output, a tunable receiver is used to retrieve any input signal of choice. The experimental receiver consists of an optical tunable filter followed by a direct-detection receiver. The laboratory demonstration consists of four frequency-locked transmitters with monolithic tunable lasers, each dithered with a slightly different frequency for the dual purpose of frequency locking and positive channel identification upon signal reception. The tunable filter is a tunable two-stage fiber Fabry-Perot filter design consisting of a narrow filter followed by a wide filter. The tuning of the filters is computer controlled, and the combined filter has a tuning range of 15000 GHz with a finesse approximately=5170. Therefore, it is capable of covering over 1000 channels of 2.9-GHz each. Receiver sensitivity was measured to be -30 dBm at 1.7 Gb/s and -29 dBm at 2.5 Gb/s (bit error rate=10/sup -10/). >

The authors report an 8-Gb/s lightwave transmission system operating at 1.53- mu m wavelength through 130 km of single-mode fiber having a chromatic dispersion of zero at 1.3- mu m wavelength. A receiver sensitivity of -34.2 dBm (365 photons/bit) was achieved using an erbium-doped fiber preamplifier pumped at 980-nm wavelength. A sensitivity of -31.9 dBm (620 photons/bit) was reached by a preamplifier pumped at a 1480-nm wavelength. By employing a Ti:LiNbO/sub 3/ external modulator and a fiber Gires-Tournois interferometer to equalize fiber chromatic dispersion, the penalty for 130 km transmission was held to only 0.9 dB. >

We report measurements of intensity as a function of both time and frequency for frequency modulation and switching of a tunable semiconductor laser. Because of the uncertainty principle limitations, the measured time‐frequency signal can have a complex structure and does not show the simple‐minded picture of a laser spectrum whose center frequency varies in time. The observations are explained by a theory of the time‐dependent spectral measurements, well known in the field of speech analysis. We discuss implications for channel switching speed and channel interference in switched, frequency‐multiplexed optical networks.

FiEnd (fiber end) etalon filters have been made by multilayer deposition directly on the beveled end facets of fibers. Both standard depressed-cladding single-mode fibers and beam-expanded fibers made by thermal diffusion of the core dopant of standard fibers were used. The fiber end facets were polished at appropriate bevel angles to eliminate back reflection. It was found that, if a standard fiber is used, the bandpass of the FiEnd etalon filter is asymmetric and skewed toward shorter wavelengths. It was also shown that to make reflection free FiEnd etalon filters on beveled fiber and facets with bandpasses smaller than about 5 nm, it is necessary to use fibers with mode field diameters larger than those of standard fibers. >

A fiber Fabry-Perot resonator containing a Faraday-active medium and measured nonreciprocal resonance-enhanced transmission for circularly polarized light was built. The results show that an 18- mu m length of material producing only 2.1 degrees of Faraday rotation could achieve an optical isolation of 10 dB. The isolation would be larger for interferometers with higher finesse or more single-pass Faraday rotation. The device could also serve as a nonreciprocal wavelength filter transmitting different sets of wavelengths in the two directions. >

A 5-Gb/s lightwave transmission experiment demonstrates the effectiveness of an all-pass-type fiber Fabry-Perot optical equalizer for eliminating an error floor caused by laser chirp and chromatic dispersion. >

Recently, we proposed a spectrum-sliced fiber amplifier light source for multi-channel WDM applications [1]. The high-power ASE from an EDFA could be efficiently divided into many channels using an integrated optic WDM demultiplexer [2]. Thus, this "spectrum-sliced" ASE could be used as light sources for WDM systems rather than several wavelength-selected DFB lasers. However, these incoherent light sources would suffer from excess noise caused by the beating of the various Fourier components within the broad spectrum. In particular, this excess noise becomes dominant over the electrical noise when the optical bandwidth per channel is significantly reduced, as in the proposed WDM light source. Thus, the SNR of the spectrum-sliced ASE source at the receiver is given.

We discuss experimental results on a photonic switch architecture using dense WDM where laser signals are transmitted at distinct but close frequencies and then summed in a star coupler. The switching function isperformed by selecting a particular frequency at each star coupler output. Two types of receivers wereimplemented: a tunable optical filter followed by a direct-detection receiver and a heterodyne receiver with a tunable laser as local oscillator. To avoid performance degradation due to frequency drifts, the laser transmittersare frequency-locked to a fixed reference frequency comb using a dithering technique with distinct tones. At the receiver side, the tunable filter is computer controlled and locks to the selected channel using the channeldithering tone. Likewise, the heterodyne receiver uses a frequency discriminator in combination with the dithering tone to lock the local oscillator at the appropriate distance from the selected channel. 1. INTRODUCTIONAs transmission rates continue to climb into the multigigabit-per-second region due to the introduction ofmature optical fiber communications technology in networks, there is an increasing need for high-capacityswitches [1 ,2J. Photonic switching has the potential to offer this high capacity because of its inherenttransparency to data rates [3]. Among the candidates for photonic switching, we opted to investigate a

A temperature-insensitive, nontunable fibre Fabry-Perot etalon is demonstrated. This compensation has been obtained for an etalon with a cavity length of 740 μm (free spectral range = 135 GHz), and the principle may be applied for etalons of arbitrary length. This device may prove useful in setting frequencies for WDM applications.

Single-mode-fibre output couplers for diode lasers have been fabricated to have long working distances. The fibres have (i) tapered cores expanded at one end to 25 μm diameter by thermal diffusion of the core dopant and (ii) outside diameters etched down to 30–60 μm. Hemispherical lenses were made by melting the ends. The resulting working distances are 35–40 μm, and coupling losses, as low as 3dB, are mostly between 3.5 and 5.5dB.

1.2 ps transform-limited pulses at 1557 nm have been produced by passively mode locking an Er-doped fibre laser. An InGaAs/GaAs-on-GaAs superlattice was used as a fast saturable absorber. The all-fibre laser is designed to be in a ring configuration. The low pump power requirement allowed pumping by a semiconductor laser diode.

An all fibre, diode-pumped, electrically tunable ring laser is reported. Gain is provided by an erbium-doped fibre and tuning by a fibre Fabry-Perot etalon. The threshold at l.566 μm is 2.9mW, the slope efficiency is 0.15 and the output power is 4.2 mW with 32 mW of pump power. The output wavelength can be tuned from 1.525 to 1.586 μm with a variation in power of less than 3.5 dB.

The performance of fibre Fabry–Perot interferometers has been improved by using different mirror coatings. Finesse of 1000 with insertion loss of 4dB has been demonstrated.

A diode-pumped, broadly tunable, single frequency erbium-doped fibre ring laser is reported. Tuning and single longitudinal mode selection are accomplished by use of two fibre Fabry-Perot etalons. The threshold for 1.48 μm pumping is 8.8 mW and the slope efficiency is 11.5%. The laser can be tuned to wavelengths between 1.530 and 1.575 μm and operates singlemode throughout this range. Its linewidth is measured at 1.56 μm to be less than 5.5 kHz.

The effective reflectance of multilayer high reflectance mirrors applied to fiber ends is determined by a shuttle pulse method. Light from a pulsed laser diode at 1.53-μm wavelength is injected into a fiber with mirrors at both ends, and the emergent pulse train amplitude decrement is used to obtain the reflectance.

A method is described for measuring transmittance, reflectance, and loss spectra in thin optical films. The method is applied to measure the properties of multilayer coatings of Si and SiO(2) used to make mirrors and antireflection coating in the 1.0-1. 7-microm wavelength region. Mirrors with reflectance up to 99.5% with nine quarter-wavelength layers and two-layer antireflection coatings with reflectance of0.2% have been made.

The enhancement of Faraday rotation in a rare-earth iron garnet plate inside a fibre Fabry-Perot resonator is demonstrated. An enhancement factor of seven is obtained with a resonator finesse of 10.

A three-mirror, all-fibre, Fabry–Perot filter was built having a second-order near Butterworth bandpass response, which is characterised by a fiat top and steep skirts.

Multilayer thin-film filters have been deposited on the end faces of optical fibres. Single and double cavity bandpass filters and low-ripple long-pass filters have been made with good optical quality. Reflections are reduced by depositing on bevelled end faces.

The effects of inherent fiber birefringence, packaging stresses, and stress tuning on the behavior of a fiber Fabry-Perot interferometer are examined. A polarization doublet due to birefringence is observed. For birefringence of 1 ps/km, the doublet spacing is 40 MHz, at 1.5- mu m wavelength, independent of the FFP finesse or free spectral range, and is due mainly to the inherent fiber birefringence. It was found that annealing depressed-cladding single-mode fiber reduced the fiber birefringence, for example, from 40-80 MHz to 15-25 MHz. Where the bandpass of the FFP is greater than the birefringence doublet, this device should be polarization insensitive for most applications. >

The formation of hydroxyl due to reaction of a silica glass made by the modified chemical vapor deposition process with hydrogen has been studied for temperatures between 200 and 900 °C. The glass is treated under 1 atm of hydrogen, which diffuses into and reacts with the glass. For treatment temperatures between 900 and 500 °C, spatial hydroxyl concentration profiles are measured. For treatment at 400 °C and below, only equilibrium OH concentrations are measured. Three temperature regimes are identified for the reaction: T>700 °C, 700 °C>T>400 °C, and T

Two-electrode distributed-feedback (DFB) lasers show promise for combining high speed and frequency tunability for frequency-division-multiplexed-frequency-shift-keyed (FDM-FSK) networks. The FM and FSK responses of such lasers have been measured up to modulation frequencies of approximately 1 GHz. Using these lasers in a noncoherent detection system in which a fiber Fabry-Perot tunable optical filter converts an FSK signal into an amplitude-shift-keyed (ASK) format, a 10/sup -9/ BER was observed up to 1 Gb/s. Nonuniform FM response and consequent tone broadening of the optical FSK spectra can lead to system power penalties due to optical-filtering effects. Thus, for a given FM response, the behavior of these lasers in FSK optical systems can be projected. >

Fiber Fabry-Perots have been built with a finesse of 500 and throughput of 30-40 percent. The devices are piezoelectrically scanned and operate at 1.52μm, and have free spectral ranges of 1000, 500, 333, and 250 MHz, with linewidth resolution as small as 0.5 MHz. The sources of loss which limit the attainable finesse have been identified and analyzed.

Revue des effets de l'interaction du deuterium avec des fibres optiques de silice. Les experiences ont montre qu'il est commode d'utiliser le deuterium pour realiser l'echange isotopique qui permet de convertir le groupe hydroxyde en groupe deuteroxyde. Cet echange permet de reduire l'attenuation dans la transmission et la dispersion de fenetre entre 1,3 et 1,6 μm des fibres optiques

The results obtained with a fiber-optical star network using densely spaced frequency-division-multiplexing (FDM) and heterodyne detection techniques are discussed. The system consists of three optical sources transmitting around 1.28 mu m, frequency-shift keying (FSK) modulated at 45 Mb/s and spaced by 300 MHz. A 4*4 optical coupler combines the three optical signals. The FDM signals, received from one of the four outputs of the coupler, are demultiplexed by a heterodyne FM receiver. The minimum received optical power needed to obtain a bit error rate (BER) of 10/sup -9/ is -61 dBm or 113 photons/bit, which is 4.5 dB from the shot noise limit. Cochannel interference is negligible for the above channel spacing and modulation rate. The results indicate that such a system has a potential throughput of 4500 Gb/s. The results obtained with two frequency stabilization circuits used to confine these three FDM optical signals to a comb of equally spaced frequencies are also presented. >

An optical frequency-division-multiple-access (FDMA) star network is analyzed and demonstrated experimentally using two 45-Mb/s frequency-shift-keyed (FSK) laser channels at 1.5 mu m. A tunable fiber Fabry-Perot (FFP) filter is used to select channels and convert FSK to intensity modulation for direct detection. The analysis predicts and experiment supports a minimum channel spacing of about six times bit rate B for a single FFP. These constraints are similar to those for more complex heterodyne demultiplexing. Estimates show that a network with 1000 users, independent of bit rate, is feasible with a tandem FFP. For B=1 Gb/s per channel the network capacity would be 1 Tb/s. >

An air-gap Fabry-Perot resonator with plane mirrors between closely spaced fiber ends may yield low throughput because of the poor match between the modes of typical single-mode fibers and the resonant mode in the air-gap cavity. The throughput can be improved by confining the resonant mode by a hollow dielectric tube placed inside the resonator. Short fiber-coupled Fabry-Perot resonators with and without an inserted hollow dielectric waveguide and expressions for their transmission losses are derived. It is shown that the throughput of both types of resonator can be improved significantly by using fiber with large mode size to couple to the resonator. The special fiber is then spliced to a conventional single-mode fiber. It is concluded that the resonator with an inserted hollow dielectric waveguide offers increased throughput for resonators with high finesse. >

This paper reviews all the interactions of hydrogen and deuterium with silica-based optical fibers. Included in the discussion are H 2 overtone absorption, OH formation, increased UV and IR absorption, radiation effects on H 2 -treated fibers, the gas-in-glass laser and applications, the reduction of OH absorption by isotope exchange OH → OD and other isotope-exchange related effects, and applications of isotope exchange and substitution for diagnostic purposes. Also included is a description of common sources of H 2 in materials used in fiber and cable manufacture, and countermeasures against hydrogen generation and intrusion into the fiber.

A high-finesse optical filter can be constructed by cascading two moderate-finesse Fabry-Perot filters of different free spectral ranges. Methods for controlling the undesirable interactions within the interfilter cavity are studied. In particular, the use of an isolator, an attenuator, or a length of fiber longer than the coherence length of the light sources used is considered. The use of a three-mirror filter, which avoids the offending cavity altogether, is also considered. The implications on the number of potential users of optical frequency-division multiple-access (FDMA), local-area networks (LANs) are also addressed. >

The problems associated with determining the angular distribution of continuous and discrete noise sources by means of a linear array are discussed. Among the problems considered are the effect of fluctuations in the received signal upon the reliability of the estimate of the angular distribution of the noise. It is shown that smoothing due to side lobe pickup results in a large reduction in the ability of an array to pick out small peaks in the angular distribution. The relationship is shown between optimum array length and the angular size and power of the noise sources making up the distribution. The discussion is given in terms of considering the array as a collection of correlators operating simultaneously. Special purpose arrays, such as Arsac‐type arrays, are discussed briefly. Extensive use is made of techniques and knowledge available from radio astronomy. (This work was supported by the Office of Naval Research.)