Superfluorescence from Yb- and Yb–Er-doped phosphotellurite glass fibres

Short length, air-clad, multimode fibres were fabricated using high concentration Yb- and Yb–Er-doped phosphotellurite glass. Superfluorescent light sources are obtained by optically pumping the fibres using a 980 nm laser diode. Ytterbium-doped fibre showed amplified spontaneous emission (ASE) in the 1000–1100 nm wavelength region, whereas Yb–Er-codoped fibre in both 1000–1100 nm (due to Yb³⁺ ions) and 1450–1650 nm (due to Er³⁺ ions) wavelength regions. The ASE spectra from the fibres are compared for different fibre lengths and pump powers. The observed change in the ASE spectral shape and the shift in peak wavelength with the length of the optical fibre is discussed.     

Passive phase demodulation and laser-noise compensation scheme forfibre interferometers

A scheme for passive phase demodulation and laser intensity noise suppression for fibre interferometers is presented. The scheme uses a matched Ronchi grating filter to transform the interference fringe pattern formed between the reference and the signal fibres into intensity variation that can be related to the optical differential phase in the interferometer. By dividing the grating filter into four sectors with their fringe positions slightly shifted to introduce an equivalent of π/2 phase difference between the signals detected by individual sectors, one can apply cross-subtraction on diagonal sectors to remove intensity noise. In addition, the phase information due to the detected sample vibration can be recovered by taking the root-mean-square of the differential signals. The new scheme greatly reduces the complexity of practical fibre interferometers and an experimental setup has been implemented to demonstrate its expected merits     

Characteristics of phase-matched sum-frequency light in optical fibres

Characteristics of phase-matched sum-frequency light in optical fibres is investigated to determine the input power dependence and fibre length dependence. The output power at 0.54 ?m increases in proportion to the square root of the input power, increasing almost exponentially with increases in the fibre length from 4 m to 17 m and becoming nearly constant above 17 m.     

RF Amplitude and Phase-Noise Reduction of an Optical Link and an Opto-Electronic Oscillator

In this paper we examine the optical sources of noise that degrade high-performance microwave photonic links. In particular, we study the residual phase noise due to laser frequency fluctuations and the detector nonlinearity on microwave signals transmitted on an optical fiber, or generated in the opto-electronic oscillator (OEO). Based on experimental findings, we identify a significant reduction of the relative intensity noise of the laser if the received optical power saturates the photodiode. Furthermore, we suggest the use of a semiconductor optical amplifier in saturation as yet another means to reduce the phase noise induced by laser intensity fluctuations. We also identify the use of multiple photodetectors to reduce the influence of associated 1/f noise, which adds to the phase noise of a transmitted microwave signal, and is the ultimate limitation to the phase noise of the high-performance OEO. Reduction of noise that is due to optical interferences is also addressed.     

Source-induced noise in optical systems driven by low-coherencesources

High-power quasimonochromatic light sources may deliver enough power into optical and fiber-optic systems to make the source noise dominant at the system output. With an RMS value proportional to light intensity, this noise limits the system sensitivity and dynamic range to levels which cannot be improved by injecting more light into the system. Source-related noise may originate not only from the source intensity fluctuations, but also from a phase-to-intensity conversion process which is characteristic of many single-spatial-mode multiple-path optical systems. The shape of the power spectral density of the source-induced noise, being critically dependent on the physical structure of the system, is analyzed for a self-homodyne Mach-Zehnder structure and for a recirculating delay line. For single-path communication systems, it is shown that source-originated noise exceeds both shot and thermal noise for a received optical power of only a few tens of microwatts     

Polarisation characteristics in long length v.a.d. single-mode fibres

Polarisation characteristics in long single-mode fibres are investigated. Single-mode fibres used in the experiment have been fabricated by the v.a.d. (vapour-phase axial deposition) method. It was shown from the measurements on a 21 km single-span v.a.d. fibre that the state of polarisation of the incident light beam can be maintained over a long length when the incident plane of polarisation coincides with the major or minor axis of the single-mode optical fibre.     

基于双向二级喇曼抽运的偏振控制研究

为了发展具有实用价值的受激喇曼全光偏振控制方法,根据光纤中受激喇曼散射矢量理论和光纤随机双折射效应设计了基于双向二级喇曼抽运的全光偏振控制方案,构建了偏振吸引理论模型,采用严格数值迭代算法对该偏振控制方案进行了仿真分析,并完成了偏振控制方案最佳工作性能的优化设计。结果表明,当信号光波长为1550nm、功率为0.1mW、一级抽运光和二级抽运光的功率分别为1W和4W、其偏振态都为（0,1,0）、光纤长度为3km时,信号光偏振度可达0.85以上,且信号光强度波动低于35%。该方案有效降低了作用光波功率、减弱了信号光输出强度噪声,并提高了信号光输出偏振度。     

Photoelectric instability in optical fibre resulting in spontaneous second harmonic generation

The self-organised second harmonic generation (SHG) in optical fibres is explained as a consequence of a convective instability leading to spontaneous formation of the chi /sup (2)/ grating due to the growth of small fluctuations. The authors show that the observed SHG should be interpreted as amplification of noise.<>     

Experimental study on noise characteristics of a gain-saturatedfiber optical parametric amplifier

A fiber optical parametric amplifier (OPA) has a unique saturation property such that, with increase in signal input, the signal output power increases, reaches a peak level, then decreases. Around the output peak condition, a negative feedback mechanism works to suppress intensity fluctuations in the signal output. This paper experimentally studies noise characteristics of such a gain-saturated fiber OPA. Two kinds of incident signals are injected into a fiber OPA. One is intentionally fluctuated light with large excess noise and the other is excess-noise-free (or shot-noise-limited) light. It is demonstrated that the noise suppression has an effect both on the excess noise and on beat noise due to amplified spontaneous emission generated inside the amplifier     

光纤CATV中调相法抑制相位干涉噪声的研究

本文从理论上研究了附加相位调制抑制相位干涉噪声的方法 ,研究表明 :在光纤AM CATV外调制传输系统中 ,对传输光载波信号引入附加相位调制 ,当相位调制信号的频率为CATV频带的两倍以上时 ,可以有效地抑制相位干涉噪声的发生 ,抑制程度与相位调制度有关。     

Reliability investigation of light-emitting diodes via low frequency noise characteristics

Investigation of changes of operation and noise characteristics during aging process of light-emitting diodes (LEDs) has been carried out. Several groups of different design (different optics) LEDs based on different materials (nitride-based blue and white LEDs, phosphide-based red LEDs) have been investigated. It is found that leakage current components appear due to LED’s defects and their affect is observed as increase of both the low frequency electrical noise intensity and non-ideality factor of current-leakage characteristic in small current region. No considerable changes of light intensity characteristics during LEDs aging have been observed. Noise modeling, spectral and correlation analysis of optical and electrical fluctuations show on partly correlated optical and electrical fluctuations caused by defects in the active region of the LED. Degradation processes of investigated LEDs foremost occur in the diode chip and lead to the leakage current that has important affect to the electrical fluctuation level, but practically has a weak influence to the light emission properties of LED. Phosphorous layer of white LEDs and additional optical elements have no significant influence to the reliability of investigated LEDs under given aging conditions.     

Bruit relatif d’intensité des lasers à semiconducteur

The intensity fluctuations of the laser diodes light, characterized by the so-called relative intensity noise, begin occupying one paragraph in optoelectronic semiconductor data sheets and worrying users of optical fibers in AM analog transmission techniques. In this paper we first consider how noise is characterized. Then we describe the measurements techniques and give results on some components.     

Absorption loss in optical fibres due to hydrogen

It is shown that OD formation in optical fibres due to the deuterium molecule can become a barrier to OH formation in optical fibres exposed to hydrogen, although it cannot be a barrier to the absorption due to the hydrogen molecule in a fibre. The mechanism is discussed.     

26 km-long polarisation-maintaining optical fibre

The letter describes the development of a long-length and large-diameter preform for polarisation-maintaining optical fibres. The preform was 470 mm in length and 48 mm in diameter. As a result, a 26 km-long PANDA fibre is developed. This fibre achieves crosstalk of ?22 dB, corresponding to h = 2.4 × 10-7m-1 and a transmission loss of 0.3 dB/km at a wavelength of 1.56 ?m. These fibres will be useful for future high-speed and medium-span coherent optical communications.     

All-optical inverter with one picojoule switching energy

An ultrafast inverter based on soliton dragging in optical fibres with a switching energy of only 1 pJ, which is the lowest switching energy of any all-optical device, is demonstrated. The inverter consists of a 30 m length of moderately birefringent fibre followed by a 2 km length of a polarisation-maintaining fibre. The switching energy is lowered by separating the soliton dragging mechanisms in two different fibres and optimising the parameters in a time domain chirp switch that is based on time shift keying logic.<>     

Fabrication of microstructured polymer optical fibres

A methodology is presented suitable for the fabrication of a wide range of microstructured polymer optical fibres (mPOFs), a recent development where light guidance is achieved through the incorporation of a pattern of air channels that run the entire length of the fibre. We show that the key to good mPOF fabrication is a secondary draw oven that has tight control over the length (15–20 mm) and temperature (C) of the hot-zone while maintaining fibre draw tension high enough (50–150 g) to ensure an acceptable level of partial hole collapse (30–40%) due to surface tension effects. Diverse mPOFs with diameters in the range 200–1200 μm have been fabricated with typical standard deviations of 2–3 μm.     

Single-mode-fibre point and extended temperature sensors

The response of single-mode fibres to time-varying thermal perturbations has been characterised by using a Mach-Zehnder interferometer. It is shown that single-mode fibres offer the possibility of high-speed, high-sensitivity remote temperature sensing with a minimum disturbance of the thermal environment and the capability to operate in an electro-magnetically active environment. In addition, fibre temperature sensors can be configured to measure not only the specific temperature at any given point, but also offer the unique feature of extended temperature measurements over the length of a fibre segment in which the device optically integrates any thermally induced fluctuations.     

Recent progress in microstructured polymer optical fibre fabrication and characterisation

Recent progress in microstructured polymer optical fibre fabrication and characterisation will be presented. A wide range of different optical functionalities can now be obtained by modifications of the microstructure, as is demonstrated by the fibres presented here. Microstructured fibres that are single-mode, highly birefringent or show twin-core coupling are described, in addition to graded-index microstructured fibres and hollow core fibres, the latter case being where light is guided in an air core. Microstructured polymer optical fibres are an exciting new development, offering opportunities to develop fibres for a wide range of applications in telecommunications and optical sensing.     

Double pass technique for fabricating compact optical fibre rockingfilters

By using the photosensitivity of Ge-doped birefringent fibres, efficient narrowband rocking filters have been fabricated that, to the authors' knowledge, are the shortest in length per unit beat length to date. This has been achieved by using a point-by-point external writing process in which a UV beam periodically exposes sections of an elliptically-cored fibre. Alternate sections are exposed from opposite sides of the fibre, effectively doubling the coupling strength per beat length. Alignment with the fibre axes is assisted by use of a keyed optical fibre     

Thermal phase noise in coherent optical-fibre systems

Coherent optical-fibre systems are sensitive to optical phase changes along the length of the fibre link. Thermal energy in the optical fibre can give rise to acoustic waves propagating along it and these result in optical phase changes. Thus, there is the possibility of a noise signal induced by thermal acoustic energy propagating along the fibre. This letter postulates a theoretical model for this interaction and estimates the magnitude of the effect. Conditions under which it may be a significant noise source are discussed.