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Forgotten password? Forgotten password Use the form below to recover your username and password. New details will be emailed to you. Simply reserve online and pay at the counter when you collect. Available in shop from just two hours, subject to availability. Your order is now being processed and we have sent a confirmation email to you at. This item can be requested from the shops shown below. If this item isn't available to be reserved nearby, add the item to your basket instead and select 'Deliver to my local shop' at the checkout, to be able to collect it from there at a later date.

Preferred contact method Email Text message. When will my order be ready to collect? Following the initial email, you will be contacted by the shop to confirm that your item is available for collection. Call us on or send us an email at. Different approaches to improve the system throughput and their practical limits shown in a space of the signal channel multiplex number versus the signal bit rate for single channel.

O E TDM, optical electronic time division multiplexing. In order to achieve such an extremely high throughput in communication systems, WDM technique could be extensively used to further increase the system throughput.

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However, this approach would suffer from a variety of problems: the use of many lasers, each of which must be readily tuned to a specific wavelength channel, becomes difficult or even impractical as the channel number increases. This limit in the wavelength management and handling may restrict the total system throughput. Thus the development of ultrafast all-optical devices which can operate in the femtoseconds time scale is strongly desired for achieving very high throughput OTDM systems. A recent transmission experiment has shown 1.

Based on this recognition, extensive research and development are being conducted in the area of ultrafast physics, materials and devices by various research groups worldwide. The next section describes first the advantages of femtosecond all-optical devices and explains basic signal processing functions required for network systems. They include ultrafast devices for mode-locked pulse generation, pulse compression and dispersion compensation, and a few different types of all-optical switches using various device principles and ultrafast materials.

They include the advantage of fully utilizing the material's nonlinearity by an extremely high peak intensity of field in ultrafast pulses. This is essential in the development of all-optical switching and modulation devices with high efficiency without increasing the average power consumption. An ultrashort optical pulse occupies an extremely short distance in space and propagates at the light velocity, and this means a possibility to precisely control the delay time in a small dimension and the overall optical device and circuit can be very compact.

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Ultrashort pulse has a large spectral width due to the pulse shape-spectrum interdependence deduced directly from Fourier transform relationship, and this merits the use of various photonic functions in wavelength division, such as the extraction of multi-channel wavelengths from a ultrashort pulse, and also the wavelength conversion and pulse waveform shaping by applying this property. These can contribute to combine both OTDM and WDM techniques to increase further the overall throughput of optical signal transmission and processing systems.

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Figure 2. Characteristic features and potential applications of ultrashort optical pulses indicated for three dimensions: time, space and wavelength. In a transmitter, generation and coding functions of ultrafast pulses are required. The pulse is compressed or shaped if necessary and then transmitted through a fibre or other transmission medium. The propagation loss and dispersion effect being experienced in the transmission medium can be compensated by an optical repeater including either 2R function, which stands for reamplification and reshaping, or 3R function, which adds the retiming function to 2R.

The function of optical nodes is to receive and drop necessary signals by using a demultiplexing circuit DEMUX and to add slow rate signals prepared externally by using a multiplexing circuit.

The clock recovery circuit is necessary to extract clock timing out of the incoming series of data signals, and wavelength conversion may be required for directing signals to other WDM network channels. Figure 3. Simplified schematic diagram of the OTDM network system. Routing function is achieved basically in a header recognition circuit by recognizing the routing header pulses which are placed just before the pay load signal pulses within a signal packet which is fed into the router.

Recognition is carried out by comparing the header signal pattern and the reference keyword pattern by using an AND gate.

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Figure 4. Schematic diagram of an optical router used in a packet switching network. Input signal packet is sent to either the cut-through port or the drop port depending on the header recognition result. In the header recognition circuit, the routing header signal pattern is compared with the reference keyword pattern at ultrafast speed.

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A variety of such optical circuit-level functions are required for completing network functions, and they should be realized by combining basic device-level functions such as all-optical switch, modulator, low-level logic AND, OR, EXOR, etc , delay and memory, and also other optical device functions such as pulse waveform shaper, wavelength filter and converter, etc.

Among these, the most basic, indispensable device function for ultrafast optical signal processing systems should be the generation and switching of ultrafast optical signals. Ultrafast signal transmission and processing functions demonstrated so far have been achieved by using devices based on the nonlinearity of optical fibres. In order to establish a technical platform for ultrafast optical systems however, realization of compact, stable and cost-effective femtosecond all-optical devices is a prerequisite.

For realizing those ultrafast devices, new materials and device structures must be developed. For example, conventional optoelectronic devices consist of semiconductor materials and their operation speed is limited by the carrier recombination lifetime, which is usually in the ns or sub-ns range. To overcome this limit, various approaches are being exploited for developing new device concepts, which include novel device structures such as the Mach—Zehnder interferometer structure, new ultrafast phenomena such as spin relaxation and intersubband transition ISBT in quantum wells QWs , or new materials including quantum dots QDs and organic thin films , as will be discussed in the following sections.

Compact ultrafast lasers are indispensable for generating clock and data signals as well as control pulses for driving other all-optical devices. Ultrashort pulsewidth, sufficiently high optical power and repetition rate with low timing jitter, stable operation and compact size are basic requirements of ultrafast light sources for practical application in OTDM systems.

A combination of gain-switched laser and pulse compressor is another method of generating ultrafast pulses, and this technique is advantageous in flexibility of selecting the repetition rate. The semiconductor monolithic laser structures are advantageous to generate pulses with transform-limited waveforms and a very high repetition rate owing to the very wide optical gain bandwidth and extremely short photon round trip time, which are characteristic of micro-cavity semiconductor laser structures.

Passive mode-locking, which requires no external electronic control circuits, is advantageous for high repetition rate operation and has been studied using discrete semiconductor lasers and then integrated laser structures. Greatest technical issues in semiconductor mode-locked lasers are how to design structures for generating ultrashort pulses with high extinction ratio at high repetition rate, and how to stabilize the timing and frequency of pulse generation.

A semiconductor mode-locked laser basically consists of a gain medium and a saturable absorber placed within a waveguide cavity. Saturable absorber is composed of a reverse-biased heterostructure waveguide, most often the same as that used for the active gain medium. To obtain ultrashort pulse width and high repetition rate, it is essential to have a very fast absorption recovery in the saturable absorber.

Both of these methods are effective to rapidly sweep out photo-generated carriers in the absorber material. For further shortening the pulse width, the saturable absorber length must be sufficiently short for reducing the pulse propagation time inside it.

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Another important factor for shortening the pulse width is to maximize the ratio of pulse compression, which is caused by the nonlinearity within the saturable absorber. In order to maximize the compression ratio, it has been shown to be very effective to reduce the background absorption in the laser cavity.

To obtain a high repetition frequency, short cavity length is effective. Advantages of using CPM configuration are both shorter round trip time and lower absorption bleaching energy. The active layer consists of a 0. This is also in agreement with the spectrum in that the longitudinal modes are enhanced every other two mode. The pulse width and spectral width are 0. The averaged optical power is 1. Figure 5. Figure 6. Synchronization to external clock signals and low timing jitter characteristics are important in practical application of mode-locked lasers.

Various synchronization techniques including electrical and optical injection have been investigated. The most promising technique for ultrafast OTDM is sub-harmonic optical synchronous mode-locking SSML using sub-harmonic frequency optical pulse injection. This technique can be applied by using low-repetition-rate optical pulses and no ultrafast driving source is required. SSML stabilized operation over the range 8.