Monday 6 June 2011
Sunday 8 May 2011
Wednesday 6 April 2011
If you aren’t afraid of flying yet, you should be
The Indian aviation industry has witnessed remarkable growth in recent years, with major contributions from the civil aviation segment.
Currently the ninth largest in the world, the civil aviation market is anticipated to register more than 16 per cent growth between 2010-2013.
But this phenomenal expansion faced the inevitable challenge of insufficient safety apparatus. While we crowed over our burgeoning fleet size and airport ratings, we paid little attention to safety surveillance. "With the growth of aviation, it is essential to enhance safety systems. In India the reverse happened", alleged Sanat Kaul, Chairman of IFFAAD.
The result has been devastating: 158 passengers died in the Mangalore air crash, which safety advocates say was completely avoidable. But before blaming it all on the infrastructure, we need to consider the more appalling aspects of the problem.
According to a DGCA report, pilot error caused the Mangalore crash. Data retrieved from the cockpit voice recorder shows that the captain was on the wrong flight path. Moreover, Captain Zlatko Glusica delayed in taking corrective measures, despite being requested by his co-pilot to go in for a "go-around".
Pilot error has been the leading cause of airline tragedies by a wide margin for many years. The Indian Commercial Pilot Association believes 78 per cent of all crashes have taken place due to human error.
On February 14, Yahoo! India News had carried the story of an Indigo pilot who just couldn't land right. An inquiry conducted later by the Director General of Civil Aviation revealed that on 15 to 20 occasions, Captain Parminder Kaur Gulati had landed the aircraft at an angle indicating that the nose wheel may have touched the tarmac first.
Gulati was arrested on March 9 for allegedly using a forged marksheet to get a licence from the country's airline regulator. The fake pilot licence scandal broke following Gulati's arrest. So far, seven pilots have been nabbed.
Meanwhile, CNN IBN has brought to light the story of Arjun Giare, who was caught cheating by an US examiner in 2000. The US Federal Aviation then cancelled his licence. But that did not stop Giare from getting a commercial pilot's licence in India.
The DGCA is now scrutinising the licences of 3,000 to 4,000 pilots. Will Indian aviation set a record of sacking 4,000 fake pilots? Highly possible!
As if fake pilots were not enough, over the past two years, 57 pilots have been caught reporting for duty drunk. Ten were fired and others have had their licences suspended for short periods.
What's happening in a country expected to be the fastest growing civil aviation market in the world by 2020? Unless the DGCA checks the credentials of every pilot, and ensures that no more Arjun Giares sneaks in, you could be entrusting your life to a spurious pilot.
Monday 21 March 2011
Crystalline resonators add properties to photonic devices
Crystalline whispering-gallery-mode resonators are becoming practical optical tools in radio frequency photonics, enabling novel components.
17 February 2010, SPIE Newsroom. DOI: 10.1117/2.1201002.002536
Mainstream broadband applications greater than 1GHz currently dominate radio frequency (RF) photonics, driven by expanding digital communications needs. However, narrowband applications represent important opportunities in analog communications, high-spectral purity RF and microwave signals, photonic front ends, and narrow passband (1–100MHz) RF signal processing. Our work is focused on using the advantages of ultra-high quality factor (Q) crystalline whispering-gallery-mode (WGM) resonators in such applications (see Figure 1), and we have made several recent advances in functional device integration.
True single-sideband modulators
Crystalline WGM resonators fabricated with electro-optic (EO) materials are particularly useful as narrowband modulators (see Figure 2). The extremely high intrinsic optical Q and small mode volume of WGMs provide for both narrow bandwidth and unprecedented effective interaction lengths of optical and RF fields. Modulators operating with saturation powers as low as −26dBm (corresponding to effective Vπ≃18mV) have been demonstrated in X- and Ka-band modulator prototypes at optoelectronic (OE) waves. Resonator-based modulators can also efficiently operate at any desired wavelength within the transparency window of the EO material. These devices play key roles in micro-optic implementation of microwave photonics functions.
Figure 2. Mechanical model of an early optoelectronic wave prototype of an injection-locked, distributed feedback laser-based narrowband photonic receiver. The prototype is equipped with gradient index lens fiber coupling ports for external laser diagnostics and baseband signal retrieval using an external photodetector. WGM: Whispering-gallery-mode.
We recently introduced a new class of WGM-based single sideband (SSB) modulators that results in a high RF return. They also exhibit center frequency tunability over a very wide range (exceeding an octave). As such, they represent a new class of highly efficient devices that simultaneously provide a narrow modulation bandwidth over a very wide RF range. These properties make them easier to use in current applications and enable new applications such as widely tunable RF photonic receivers and oscillators.
Optoelectronic oscillators
Optoelectronic oscillators (OEOs) are used to generate spectrally pure RF signals using photonics.1 A generic OEO includes a laser, an amplitude EO modulator (EOM), one or multiple optical delay lines, a fast photodiode, a narrowband RF filter, and an RF amplifier. The EOM modulates the continuous wave light emitted by the laser. The modulated signal passes the optical delay line(s) and is transformed into an RF signal with the photodiode. The RF signal is subsequently filtered, amplified, and fed back to the modulator. As a result, a closed active RF circuit is produced that will generate self-sustained oscillation when the RF amplification exceeds the circuit's integral loss. Some circuit elements can be merged or replaced. For instance, a directly modulatable laser and an optical bandpass filter can be used instead of the RF filter, but the oscillation principles remain intact. High-Q crystalline WGM resonators can be used to replace the EOM or for buffering the RF signal and cleaning the OEO's supermode spectrum.2 The tunable resonant SSB modulators enable tunable, compact OEOs.3 We have developed various types of ultra-compact OEOs based on high-Q WGM resonators. These oscillators operate in the X-, Ku-, and Ka-bands and are able to generate spectrally pure RF signals characterized with less than −120dBc/Hz phase noise at 100kHz from the carrier (see Figure 3).4 The phase noise floor (<−140dBc/Hz) is limited by the signal's shot noise received at the photodiode. We have demonstrated both tunable and fixed frequency oscillators.
Figure 3. Typical phase noise of a tunable opto-electronic oscillator based on a WGM tunable electro-optical modulator.
RF photonic receivers
Photonic RF receivers have large dynamic range and high sensitivity at high frequencies. These compact devices also use relatively little power for operation. Direct processing of high-frequency RF signals with conventional electronic approaches is hindered by the absence of efficient RF amplifiers, detectors, and methods for up and down conversion of the received signals. RF photonic receivers enable conversion of the RF signals to an intermediate frequency (IF) or to baseband, where subsequent efficient detection is feasible. We recently developed a Ka−band photonic receiver based on all-resonant interaction of light and RF radiation in solid-state WGM resonators5, 6 (see Figure 4). The core of the receiver, a mixer based on material nonlinearity of the EO WGM resonator, operates well for frequencies ranging from several GHz to 100GHz. The coherent photonic receiver can have a spurious free dynamic range exceeding 55dB and sensitivity of better than −100dBm in the 10MHz reception band. (The sensitivity does not degrade with increasing RF frequency.) It also enables separation of detecting and processing RF signals in space when combined with high-quality optical links used to transmit the up-converted RF signals.
Figure 4. Schematic diagram (top) of coherent photonic receiver and packaged optoelectronic wave, Ka-band photonic receiver prototype on an RF test board. IF: Intermediate frequency. LO: Local oscillator. The entire optical bench (laser, resonator, photodiode, and coupling optics) is incorporated into the surface-mount RF package (bottom) inside the interposer (center, lid removed).
RF photonic notch filters
Notch filters are used to reflect electromagnetic radiation within a selected spectral region while allowing high transmission outside of it. Tunable RF photonic filters benefit phase array and other kinds of radar. Our work led to a novel, highly efficient photonic WGM notch filter that is based on a dual-polarization interferometer with coincident optical paths in the arms. We developed a 10MHz filter with 5.5dB insertion loss and 45.5dB of rejection.7 The measured rejection value is limited by the finite (3kHz) line width of our laser.
Our work suggests that RF photonics can strongly benefit from the use of crystalline WGM resonators. Our research has already resulted in several novel WGM-based photonic devices with advanced functions. Future work in the field will focus on improved packaging of the resonators to produce devices that withstand severe environmental conditions. We expect that maturing fabrication, handling, and coupling techniques for resonators, together with adaptation of assembly methods and components from broadband photonics, will result in successful deployment of unique narrowband, high-sensitivity, and tunable functions of WGM resonators in compact manufacturable devices.
Highly Oblate Microspheroid as an Optical Resonator
Large values of resonance quality factor and finesse have been observed.
NASA's Jet Propulsion Laboratory, Pasadena, California
Experiments have shown that a highly oblate microspheroid made of low-dielectric-loss silica glass can function as a high-performance optical resonator. The shape of this resonator (see figure) is intermediate between that of (1) microdisk or microring resonators and (2) microsphere resonators, which have been described in a number of previous NASA Tech Briefs articles. As described below, the oblate spheroidal shape results in large values of both resonance quality factor (Q) and finesse. Large values of these parameters are favorable for single-mode operation of a laser or an optoelectronic oscillator.A microsphere resonator exploits the circulation of light by total internal reflection, in "whispering-gallery" (WG) modes characterized by large values of Q. In contrast, the Q values of microring and microdisk resonators are limited because of significant scattering losses on their flat surfaces.
The preferred WG modes of a microsphere resonator are those in which light circulates by propagating along the equator. As a consequence of spherical symmetry, a microsphere resonator is characterized by a large spectral density of modes because, along with the equatorial modes, some modes with small propagation-vector components transverse to the desired equatorial circulation are also coupled to an input/output device. A large spectral density of modes is not favorable for single-mode operation.
The highly oblate microspheroid resonator is not subject to the disadvantages of microsphere, microdisk, or microring resonators. In the highly oblate microspheroid resonator, the greater curvature of the surface in the direction transverse to the desired equatorial circulation effectively decouples the partly transverse modes from the input/output device. As a result, the resonator can be operated in a regime similar to that of single-longitudinal mode Fabry-Perot etalons. The free spectral range (FSR) [the difference in frequency between successive modes] is defined by successive integer numbers of wavelengths packed along the equatorial round-trip light path. For a highly oblate spheroid with an equatorial diameter (corresponding to D in the figure) of the order of hundreds of microns and a typical wavelength of 1.55 μm, an FSR as large as 1 THz is expected; in contrast, for a microsphere of approximately equal parameters, the FSR can be expected to be much smaller (typically between 2 and 10 GHz).
At the same time that it affords a much greater FSR, the highly oblate microspheroid resonator retains the high Q (up to about 108) typical of microspheres. This high Q corresponds to a resonance bandwidth of a few megahertz. Consequently, the resonator is characterized by very high finesse (finesse � FSR/resonance bandwidth): typical values of finesse range from 104 to 105. Heretofore, such high values of finesse were available only in relatively large Fabry-Perot resonators.
If resonators like this one were utilized in simple diode-laser frequency-locking schemes, robust single-mode operation should be possible because the FSRs of the WG modes would be compatible with the gain�bandwidth of typical diode lasers. For spectral-analysis applications, resonators like this one offer a highly attractive combination of miniaturization and unprecedented spectral resolution. For optoelectronic oscillators, resonators of this type could provide convenient sideband frequency references in the terahertz range, provided that appropriate detectors and modulators for this frequency range were also developed.
The Highly Oblate Spheroidal portion protruding from the cylindrical portion of this object acts as a high-finesse optical resonator. This object was fabricated by heating a sphere of low-loss silica glass to the softening point and squeezing it between flat cleaved tips of an optical fiber.This work was done by Vladimir Iltchenko, X. Steve Yao, and Lute Maleki of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Technology Reporting Office
JPL Mail Stop 249-103 4800 Oak Grove Drive Pasadena, CA 91109 (818) 354-2240 Refer to NPO-20951, volume and number of this NASA Tech Briefs issue, and the page number.
Operation of opto electronic oscillator
Most OEOs utilize the transmission characteristics of a modulator together with a fiber-optic delay line to convert light energy into stable, spectrally pure RF/microwave reference signals. Light from a laser is introduced into an E/O modulator, the output of which is passed through a long optical fiber and detected with a photodetector. The output of the photodetector is amplified and filtered and fed back to the electric port of the modulator. This configuration supports self-sustained oscillations, at a frequency determined by the fiber delay length, the bias setting of the modulator, and the band pass characteristics of the filter. It also provides for both electric and optical outputs. The conditions for self-sustained oscillations include coherent addition of partial waves each way around the loop and a loop gain exceeding losses for the circulating waves in the loop. The first condition implies that all signals that differ in phase by some multiple of 2π from the fundamental signal may be sustained. Thus the oscillation frequency is limited only by the characteristic frequency response of the modulator and the setting of the filter, which eliminates all other sustainable oscillations. The second condition implies that, with adequate light input power, self-sustained oscillations may be obtained without the need for the RF/microwave amplifier in the loop.
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