316. Terahertz radiation

Terahertz radiation

Terahertz waves lie at the far end of the infrared band, just before the start of the microwave band.

In physics, terahertz radiation refers to electromagnetic waves sent at frequencies in the terahertz range.
It is also referred to as submillimeter radiation, terahertz waves, terahertz light, T-rays, T-light, T-lux and THz.
The term is normally used for the region of the electromagnetic spectrum between 300 gigahertz (3x10¹¹ Hz) and 3 terahertz (3x10¹² Hz), corresponding to the sub millimeter wavelength range between 1 millimeter (high-frequency edge of the microwave band) and 100 micrometer (long-wavelength edge of far-infrared light).

Introduction

Like infrared radiation or microwaves, these waves usually travel in line of sight.
Terahertz radiation is non-ionizing submillimeter microwave radiation and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. It can also penetrate fog and clouds, but cannot penetrate metal or water.

Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the range of 1 to 3 THz of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm.

The Earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short, limiting its usefulness for communications. In addition, producing and detecting coherent terahertz radiation was technically challenging until the 1990s.

Sources

Terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin.
While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies.
Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona.
Planned telescopes operating in the submillimeter include the Atacama Large Millimeter Array and the Herschel Space Observatory.
The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.
As of 2004^[update] the only viable sources of terahertz radiation were:

• the gyrotron,
• the backward wave oscillator ("BWO"),
• the far infrared laser ("FIR laser"),
• quantum cascade laser,
• the free electron laser (FEL),
• synchrotron light sources,
• photomixing sources, and
• single-cycle sources used in Terahertz time domain spectroscopy such as photoconductive, surface field, Photo-dember and optical rectification emitters.

The first images generated using terahertz radiation date from the 1960s; however, in 1995, images generated using terahertz time-domain spectroscopy generated a great deal of interest, and sparked a rapid growth in the field of terahertz science and technology. This excitement, along with the associated coining of the term "T-rays", even showed up in a contemporary novel by Tom Clancy.
There have also been solid-state sources of millimeter and submillimeter waves for many years. AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1000 GHz with solid state sources and detectors. Nowadays, most time-domain work is done via ultrafast lasers.
In mid-2007, scientists at the U.S. Department of Energy's Argonne National Laboratory, along with collaborators in Turkey and Japan, announced the creation of a compact device that can lead to a portable, battery-operated sources of T-rays, or terahertz radiation. The group was led by Ulrich Welp of Argonne's Materials Science Division.
This new T-ray source uses high-temperature superconducting crystals grown at the University of Tsukuba, Japan. These crystals comprise stacks of Josephson junctions that exhibit a unique electrical property: when an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage; this phenomenon is known as the Josephson effect. These alternating currents then produce electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage – around two millivolts per junction – can induce frequencies in the terahertz range, according to Welp.
In 2008 engineers at Harvard University announced they had built a room temperature semiconductor source of coherent Terahertz radiation. Until then sources had required cryogenic cooling, greatly limiting their use in everyday applications.
In 2009 it was shown that in addition to X-rays, T-waves are produced when unpeeling adhesive tape. The observed spectrum of this terahertz radiation exhibits a peak at 2 THz and a broader peak at 18 THz. The radiation is not polarized. The mechanism of terahertz radiation is tribocharging of the adhesive tape and subsequent discharge.

Theoretical and technological uses under development

Medical imaging:

Terahertz radiation is non-ionizing, and thus is not expected to damage tissues and DNA, unlike X-rays. Some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content (e.g. fatty tissue) and reflect back. Terahertz radiation can also detect differences in water content and density of a tissue. Such methods could allow effective detection of epithelial cancer with a safer and less invasive or painful system using imaging.

Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate and safer than conventional X-ray imaging in dentistry.

Security:

Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. This is of particular interest because many materials of interest have unique spectral "fingerprints" in the terahertz range. This offers the possibility to combine spectral identification with imaging. Passive detection of Terahertz signatures avoid the bodily privacy concerns of other detection by being targeted to a very specific range of materials and objects.

Scientific use and imaging:

Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.

Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to perform measurements on, and obtain images of, samples which are opaque in the visible and near-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance, since it is very difficult to distinguish changes in the THz pulse caused by the sample from those caused by long term fluctuations in the driving laser source or experiment. However, THz-TDS produces radiation that is both coherent and broadband, so such images can contain far more information than a conventional image formed with a single-frequency source.

A primary use of submillimeter waves in physics is the study of condensed matter in high magnetic fields, since at high fields (over about 15 teslas), the Larmor frequencies are in the submillimeter band. This work is performed at many high-magnetic field laboratories around the world.

Submillimetre astronomy.

Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old building, without harming the artwork.

Communication:

Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite, or satellite to satellite.

Manufacturing:

Many possible uses of terahertz sensing and imaging are proposed in manufacturing, quality control, and process monitoring. These generally exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods.

Terahertz versus submillimeter waves

The terahertz band, covering the wavelength range between 0.1 and 1 mm, is identical to the submillimeter wavelength band. However, typically, the term "terahertz" is used more often in marketing in relation to generation and detection with pulsed lasers, as in terahertz time domain spectroscopy, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.

Safety

The terahertz region is between the radio frequency region and the optical region generally associated with lasers. Both the IEEE RF safety standard and the ANSI Laser safety standard have limits into the terahertz region, but both safety limits are based on extrapolation. It is expected that effects on tissues are thermal in nature and, therefore, predictable by conventional thermal models. Research is underway to collect data to populate this region of the spectrum and validate safety limits.

315. RADIO FREQUENCIES FOR SPACE COMMUNICATION

RADIO FREQUENCIES FOR SPACE COMMUNICATION

INTRODUCTION
To be useful satellites and spacecraft must communicate, sometimes to relay communications between two points, sometimes to transmit data they have collected. Although there have been some experiments in optical communications using lasers, most satellite communication is accomplished by radio, one part of the electromagnetic spectrum. Radio frequencies must be shared with terrestrial radio services, and international frequency assignment is essential to avoid interference between all the different uses made of the radio spectrum.
The International Telecommunications Union (ITU) is the global body that assigns radio frequency allocations. In doing this they divide the world into three regions, regions I, II and III. Australian lies in region III. The general frequency assignments may be found at the ITU web site, and Australian allocations may be seen at the ACMA web site. The ITU unfortunately, charges horrendous prices for any of its publications. However, the Australian Communications and Media Authority (ACMA) makes all spectral allocation information available for free download. This includes a book, an attractive wall poster and a simplified spectrum graphic.
This note discusses the frequencies that are used for space communications.

THE ELECTROMAGNETIC SPECTRUM
There are four, and only four known forces in the universe (although the so-called dark energy hints at another). These are, in order of strength, the nuclear strong force, the electromagnetic force, the nuclear weak force and the gravitational force. The two nuclear forces exert their influence over only very very short (nuclear) distances, and apart from holding all matter together do not directly influence us in everyday life. It is gravity and particularly electromagnetism that are of direct concern to us in our daily interactions.
Gravity springs from the property of matter we call mass, while electromagnetic effects derive from the property we call charge. When a charge is stationary, it has around it an electrostatic field. If it moves with a constant velocity it produces a magnetic field, and when it accelerates or declerates it generates electromagnetic radiation.
Electromagnetic radiation is a coupled oscillation of electric and magnetic fields that propagates through space with a velocity of about 3 x 10⁸ metres per second. The properties of this electromagnetic radiation vary markedly depending on the frequency of the oscillation. This gives rise to what we know as the electromagnetic spectrum.
The chart below shows the major divisions of the electromagnetic spectrum. An electromagnetic wave may be characterised by its frequency f (the number of times per second the signal undergoes a complete oscillation at a specified point in space) or its wavelength λ (the distance between successive extremal values of the wave at a specified time).

Note:

1 For clarity the bands are not shown with uniform frequency or wavelength spacing
2 The visible spectrum occupies only a very small part of the total EM spectrum
3 Bands also have subdivisions (this is particularly true of the radio spectrum)
4 The band divisions are not as sharp as shown, but rather fuzzy, merging into one another
5 In the frequency scale T=10¹², P=10¹⁵, E=10¹⁸
6 In the wavelength scale μ=10^-6, n=10^-9, p=10^-12

THE RADIO SPECTRUM
The radio spectrum is a subset of the electromagnetic spectrum. It extends from frequencies below 1 Hz up to around 3000 GHz or 3 THz, where it gives way to the infrared spectrum. Different frequencies have different uses because of different propagation, generation and general properties. The radio spectrum is divided into many different bands.
This table shows the usually accepted division of the radio spectrum. The left hand column lists the frequency (f), the centre column the band designator and the right column the wavelength (λ). The relationship between frequency and wavelength is given by the expression:

f = c / λ

where c = 3 x 10⁸ metres per second is the speed of light (and other EM radiation) in free space.

In the above relation, frequency is given in Hertz (Hz) when wavelength is specified in metres (m).
Note that the designated band qualifiers are not in the same order going toward lower frequencies as they are going toward higher frequencies. Also note that the division between ELF and ULF is not universally agreed. It can be placed anywhere from 1 Hz to 100 Hz. The one shown (1 Hz) is that employed by geophysicists.
There is no lower limit to the ULF band and magnetic signals with periods of years can be identified.
Microwave is a term that was historically applied to signals with wavelengths less than one foot (30 cm), and this region has been subdivided into letter bands. However, there are several schemes of designation for microwave bands. Two of these, which we shall call traditional and new, are given below. Despite the efforts of many engineers to have the 'new' division adopted, the 'traditional' scheme seems to be firmly entrenched among space communicators.
WINDOWS TO SPACE
Not all of the electromagnetic spectrum can pass through the Earth's atmosphere. Obviously, visible light can - we can see the stars at night, at least when there is no cloud. However, ultraviolet and higher frequencies are mostly absorbed by different components of the atmosphere.
There are in fact only two main windows of the EM spectrum that are open to space. One is the visible spectrum, as mentioned above, and the other is the radio spectrum. However, not all of the radio spectrum is useable for space communication. The available window spans from about 30 MHz to 30 GHz, although these are not absolute end frequencies.
Below 30 MHz, the ionosphere, at altitudes from around 100 to 500 km, absorbs and reflects signals. Above 30 GHz, the lower atmosphere or troposphere, below 10 km, absorbs radio signals due to oxygen and water vapour. Even between 20 and 30 GHz, there are some absorption bands that must be avoided.

HISTORICAL SPACE FREQUENCIES
The first satellite to orbit the Earth was Sputnik 1, launched by the Soviets in October 1957. It carried two radio beacons on frequencies of 20.005 and 40.01 MHz.
The Soviets continued to use frequencies around 20 MHz and even some around 15 MHz for many subsequent missions.
The first satellite launched by the USA (Explorer 1) carried beacons on 108.00 and 108.03 MHz. This lay just above the terrestrial FM broadcast band (from 88 to 108 MHz) and just inside the civil aviation band which extends from 108 to 136 MHz. This frequency had been specified by an international committee for the International Geophysical Year (IGY - 1957/8) as the one to be used for all scientific satellites launched in pursuit of IGY objectives. The Soviets had chosen to ignore this recommendation and use the much lower frequencies previously mentioned.

SPACE COMMUNICATION BANDS
The following is a list of some of the more heavily used frequency bands for space communication. Specific frequencies may be found in the links provided at the end of this note.

VHF Band

136 - 138 MHz
This band was used heavily by many different types of satellites in the past. Today (2012), most activity is restricted to 137-138 MHz (which is the current allocation) and consists of meteorological satellites transmitting data and low resolution images, together with low data rate mobile satellite downlinks (eg Orbcomm)

144 - 146 MHz
One of the most popular bands for amateur satellite activity. Most of the links are found in the upper half of the band (145 - 146 MHz).

148 - 150 MHz
This tends to be used for uplinks of the satellites that downlink in the 137 - 138 MHz band.

149.95 - 150.05 MHz
This is used by satellites providing positioning, time and frequency services, by ionospheric research and other satellites. Before the advent of GPS it was home to large constellations of US and Russian satellites that provided positioning information (mainly to marine vessels) by use of the Doppler effect). Many satellites transmitting on this band also transmit a signal on 400 MHz.

240 - 270 MHz
Military satellites, communications. This band lies in the wider frequency allocation (225 - 380 MHz) assigned for military aviation.

UHF Band

399.9 - 403 MHz
This band includes navigation, positioning, time and frequency standard, mobile communication, and meteorological satellites. Around 400 MHz is a companion band for satellites transmitting on 150 MHz.

432 - 438 MHz
This range includes a popular amateur satellite band as well as a few Earth resources satellites.

460 - 470 MHz
Meteorological and environmental satellites, includes uplink frequencies for remote environmental data sensors.

L Band

1.2 - 1.8 GHz
This frequency range includes a very diverse range of satellites and encompasses many sub-allocations. This range includes the GPS and other GNSS (Global Navigation Satellite Systems - Russian Glonass, European Galileo, Chinese Beidou). It also hosts SARSAT/COSPAS search and rescue satellites which are carried on board US and Russian meteorological satellites. It also includes a mobile satellite communication band.

1.67 - 1.71 GHz
This is one of the primary bands for high resolution meteorological satellite downlinks of data and imagery.

S Band

2.025 - 2.3 GHz
Space operations and research, including 'deep space' links from beyond Earth orbit. This encompasses the Unified S-band (USB) plan which is used by many spacecraft, and which was also used by the Apollo lunar missions. It also includes military space links including the US Defense Meteorological Satellite Program (DMSP). Many Earth resources (remote sensing) satellites downlink in this band.

2.5 - 2.67 GHz
Fixed (point-to-point) communication and broadcast satellites, although the broadcast allocation is only used in some Asian and Middle-eastern countries.

C Band

3.4 - 4.2 GHz
Fixed satellite service (FSS) and broadcast satellite service (BSS) downlinks. International TV broadcast uses this allocation heavily.

5.9 - 6.4 GHz
This is the FSS/BSS uplink for the 3.4-4.2 GHz downlink band.

X band

8 - 9 GHz
This is used heavily for space research, deep space operations, environmental and military communication satellites. Many satellites/spacecraft carry complementary S and X band transmitters.

Ku band

10.7 - 11.7 GHz
Fixed satellite services (FSS)

11.7 - 12.2 GHz
Broadcast satellite service (BSS) downlinks. This band is used for domestic TV programs.

14.5 - 14.8 GHz
The uplink for the previous Ku downlink band.

17.3 - 18.1 GHz
An alternate 'Ku' band BSS uplink.

'Ka' band

23 - 27 GHz
A region that will be used increasingly by a variety of fixed link, broadcast, environmental and space operations satellites in the future as more bandwidth is required than can be provided in the lower bands. The disadvantage of this band is the increased absorption due to water vapour and rain. Not very useful for tropical regions of the Earth.

SPECIFIC SPACE COMMUNICATION FREQUENCIES

Russia

Russian manned spacecraft use 143.625 and 121.5 MHz FM for voice communications. Other frequencies used on manned missions include 166 and 923 MHz. The Russian ISS (International Space Station) module uses the band from 628 - 632 MHz.

China

China uses 180 MHz for weather satellite downlinks, and possibly for manned missions. Meteorological satellites also use 480 MHz for downlink.

North Korea

North Korea has now (May 2012) made three unsuccessful attempts to launch an orbiting satellite. They have stated that communications will be on 27 MHz (morse code slogan), 470 MHz (propaganda song) and 8 GHz (imagery).

Amateur Satellites

Many satellites have been launched which use the amateur radio bands for downlinking data, telemetry and imagery, and for providing relay communications and store and forward communications. Most of the amateur radio frequency bands have a satellite allocation sub-band. The most popular bands for these satellites are the 144-146 and 435-438 MHz bands. The Russians have often used the HF bands at 21 and 29 MHz for amateur communications.
A very popular frequency for many amateur satellites is 145.825 MHz.
ARISS (Amateur Radio on the International Space Station) typically uses frequencies in the 144 - 146 band.

314. PETAFLOP

PETAFLOP

A petaflop is a measure of a computer's processing speed and can be expressed as a thousand trillion floating point operations per second.

FLOPS are floating-point operations per second. Floating-point is considered to be a method of encoding real numbers within the limits of finite precision available on computers.

Using floating-point encoding, extremely long numbers can be handled relatively easily. A floating-point number is expressed as a basic number, an exponent, and a number base which is usually ten but may also be 2.

 Petaflop Arrival

To get a perspective on how far we are from a petaflop machine (1 quadrillion mathematical computations per second), the world's fastest supercomputer today, the Blue Gene Supercomputer in Livermore, California, has a top speed of 360 trillion operations a second.

Scientists predict we will see a petaflop computer by the year 2010, others claim it could be as early as 2006.

 Petaflop Race

Japan's Earth Simulator supercomputer shocked Washington a few years ago and many believed that the United States could lose its lead in many areas, just as it did in climate science.

The Earth Simulator Center reportedly negotiated deals with Japanese automakers to use time on the world's fastest computer to boost their quality and productivity; that's when the race became heated!

IBM developed the Blue Gene computer in 2004 which may not hold the lead if reports are accurate; Its said that Japan is working on a petaflop computer that can operate 10 quadrillion calculations per second (10 petaflops). This petaflop computer would be over 70 times as fast as the Blue Gene and could run $750 million dollars.

• PETAFLOP is expressed as a
  thousand trillion operations per second.
  
  A petaflop computer is expected in 2006.

 Blue Gene The world's fastest computer (getting close to petaflop speed) is IBM's Blue Gene with a top speed of 360 trillion operations a second.

IBM and its petaflop partners are exploring a growing list of applications including hydrodynamics, molecular dynamics, quantum chemistry, climate modeling and financial modeling.

• Earth Simulator The world's fastest computer was the Earth Simulator in Yokohama, Japan.
  
  It had a top speed of 40 trillion operations a second.