[TC News] New laser promises to make internet faster

Scientists have developed a new laser
that holds the potential to increase by
orders of magnitude the rate of
data transmission on the internet.

WASHINGTON: Scientists have developed a new laser that holds the potential to increase by orders of magnitude the rate of data transmission in the optical-fibre network - the backbone of the internet. 

The laser is the result of a five-year effort by researchers at the California Institute of Technology (Caltech). 

Light is capable of carrying vast amounts of information - approximately 10,000 times more bandwidth than microwaves, the earlier carrier of long-distance communications. 

To utilize this potential, the laser light needs to be as spectrally pure - as close to a single frequency - as possible. The purer the tone, the more information it can carry, and researchers have been trying to develop a laser that comes as close as possible to emitting just one frequency. 

Today's worldwide optical-fibre network is still powered by a laser known as the distributed-feedback semiconductor (S-DFB) laser, developed in mid-1970s, researchers said. 

The S-DFB laser's unusual longevity in optical communications stemmed from its, at the time, unparallelled spectral purity - the degree to which the light emitted matched a single frequency. 

The S-DFB laser managed to attain such purity by using a nanoscale corrugation within the laser's structure that acts like a filter. 

Although the old S-DFB laser had a successful 40-year run in optical communications, the spectral purity, or coherence, of the laser no longer satisfies the ever-increasing demand for bandwidth, researchers said. 

The old S-DFB laser consists of continuous crystalline layers of materials called III-V semiconductors - typically gallium arsenide and indium phosphide - that convert into light the applied electrical current flowing through the structure. 

Since III-V semiconductors are also strong light absorbers - and this absorption leads to a degradation of spectral purity - the researchers sought a different solution for the new laser. 

The high-coherence new laser still converts current to light using the III-V material, but in a fundamental departure from the S-DFB laser, it stores the light in a layer of silicon, which does not absorb light. 

Spatial patterning of this silicon layer - a variant of the corrugated surface of the S-DFB laser - causes the silicon to act as a light concentrator, pulling the newly generated light away from the light-absorbing III-V material and into the near absorption-free silicon. 

This newly achieved high spectral purity - a 20 times narrower range of frequencies than possible with the S-DFB laser - could be especially important for the future of fibre-optic communications, researchers said. 

The study was published in the Proceedings of the National Academy of Sciences.

Source: http://timesofindia.indiatimes.com

[TC] Home Made Movie Maker

Like real movies, this circuit makes use of a characteristic of the human eye and brain known as the persistence of vision. A sequence of still pictures is projected onto a screen in rapid succession. The pictures differ slightly from one another and the brain interprets the succession of still pictures as continuous motion.

Here the pictures are shadows cast by low-voltage lamps. There are four Lamps in all, which glow in sequence cyclically. This gives the illusion of a simple but realistic movie.

Fig. 1 shows the circuit for the movie maker. It is driven by clock pulses provided by NAND gates N1 and N2. The flickering frequency is adjustable through preset VR1. A suitable rate for perceiving continuous motion is 16 Hz. The clock pulses are fed to counter IC CD4022 (IC2). IC2 has eight outputs, but only the first four (0-3) are used in this circuit. The outputs go high one at a time, in sequence. The fifth output (output 4) is connected to the reset input so that the counter is immediately reset at the fifth count and the first output (output 0) goes high.

The counter outputs are fed to CD4049 hex buffer (IC3). The buffer outputs drive transistors T1 through T4 in a sequence. As each transistor conducts, the lamp connected to it glows. The lamps are rated at 0.3A so these provide enough light to operate the movie show in a dimly-lit room.

Fig. 1: Circuit for movie maker

Assemble the circuit on a general-purpose PCB. Power-on the circuitusing switch S1 and make sure that the outputs of IC2 (0 through 3) are normally low but briefly go high three-four times within a second. Also ensure that the lamps flash one at a time in a repeating sequence. If the sequence appears to be wrong or any of the lamps fails to glow, check the wiring. The light shield and film holder can be made of a thin card, sheet metal or plywood. Strictly adhere to the various dimensions as shown in Fig. 2. Otherwise, the shadow images may fail to register properly when projected.

Use a plastic cabinet as shown in Fig. 3 to hold the circuit board and battery. Owing to the power requirements of the lamps, it is more economical to use four 1.5V cells in a battery box. Else, you can use a 6V power adaptor. 

Fig. 2: Assembly arrangement

There are two ways of mounting the lamps. The more satisfactory but more expensive method is to bolt the four lamps. Alternatively, drill four 1cm dia. holes on the front of the cabinet, wedge the base of the lamps in these holes and solder wire to the bases.

Fig. 3: Plastic case with assembled circuit 

The easiest way to prepare the film frames is to photocopy the desired drawings onto transparent films. Alternatively, trace them on a transparent acetate film or draughtsman's film, using a fine marker pen. Align all the drawings on the frames and project onto the screen. 

  

Fig. 3: Flim making

Working of the circuit is simple. First of all, fix the clock frequency at about 16 Hz. Place the film on the holder. Ensure a distance of 12 cm between the screen and the assembled unit and power-on the circuit using switch S1. Now you can see your drawings as a short movie clip on the screen.

EFY note. We have tested this circuit without the mechanical arrangement.

Source: 
  http://electronicsforu.com/