Schematic setup of a non-resonant acousto-optic modulator. A transducer generates a sound wave, at which a light beam is partially diffracted.
The most common type is an acousto-optic modulator (AOM). The transmission losses through some crystal or glass piece are small as long as the acoustic wave is switched off, whereas strong Bragg reflection occurs with the acoustic wave switched on, so that the losses are typically of the order of 50% per pass, corresponding to 75% per double pass in a linear laser resonator. For generating the acoustic wave, an electronic driver is required with an RF power of the order of 1 W (or several watts for large-aperture devices) and a radio frequency (RF) of the order of 100 MHz. There are various kinds of trade-offs. For example, tellurium dioxide material with its high elasto-optic coefficients requires small acoustic powers, but has a moderate damage threshold. Higher optical intensities can be tolerated by crystalline quartz or fused silica, but at the cost of higher acoustic powers (and thus also RF powers). The acoustic power required also depends on the optical aperture of the device: large aperture devices, as required for high-power lasers, require higher acoustic powers. The heat generation in the Q switch can then be so strong that water cooling is necessary. At lower power levels, conductive cooling is sufficient. The switching speed (or modulation bandwidth) is finally limited not by the acousto-optic transducer, but by the acoustic velocity and the beam diameter. To suppress reflections from the optical surfaces, anti-reflection coatings are frequently used. There are also Q switches where the active element is operated at Brewster's angle.
Electro-optic Q Switches
For particularly high switching speeds, as required e.g. in Q-switched microchip lasers, an electro-optic modulator (EOM) can be used. Here, the polarization state of light can be modified via the electro-optic effect (or Pockels effect), and this can be turned into a modulation of the losses by using a polarizer. Compared with an acousto-optic devices, much higher voltages are required (which need to be switched with nanosecond speeds), but on the other hand no radiofrequency signal.
Mechanical Q Switches
Particularly in the early days of Q-switched lasers, mechanical Q switches were often used – mostly in the form of rotating mirrors. Here, a small laser mirror is mounted on a quickly rotating device. The mirror is used as an end mirror in a linear laser resonator. A pulse builds up when the mirror is in a position where it closes the laser resonator. This approach is simple and very robust, suitable particularly for high-power lasers with relatively long pulse durations.
Passive Q Switches
Passive Q switches are saturable absorbers which are triggered by the laser light itself. Here, the losses introduced by the Q switch must be small enough to be overcome by the laser gain once sufficient energy is stored in the gain medium. The laser power then first rises relatively slowly, and once it reaches a certain level the absorber is saturated, so that the losses drop, the net gain increases, and the laser power can sharply rise to form a short pulse. For a passively Q-switched YAG laser, a Cr4+ : YAG crystal typically serves as the passive Q switch. There are other possible materials, such as various doped crystals and glasses, and semiconductor saturable absorber mirrors are particularly suitable for small pulse energies.
For the selection of a suitable Q switch, the following aspects have to be considered:
the operation wavelength, which influences e.g. the required anti-reflection coating
the open aperture
the losses in the high-loss state (particularly for high gain lasers) and low-loss state (influencing the power efficiency)
the switching speed (particularly for short pulse lasers)
the size of the setup (particularly for compact lasers)
Of course, the electronic driver must be selected to fit to the Q switch. [Enc11] QAM Quadrature Amplitude Modulation; a method of modulating digital signals onto a radio-frequency carrier signal involving both amplitude and phase coding. A modulation scheme used by telecommunications providers. More advanced modulation offers increased capacity (e.g., 256 QAM offers greater capacity/transmission speeds than 64 QAM).
Quaternary Dispersion Supported Transmission. See DST. [Fib111]
Quality-of-Service; Flow or a Service Class.
QPSK Quadrature Phase-Shift Keying; a phase modulation technique that transmits two bits in four modulation states. See PSK, phase modulation and QAM. This modulation produces two signals that transport the information, one is sinusoidal, or quadrature (Q), and the other one is cosinusoidal, or in-phase (I). The coding is made according to the phase of these signals. Two phases are possible (180 ° phase difference) for both signal (I and Q), that enables realizing 4 different symbols (in the following example, signal/data line). QPSK temporal diagram below is courtesy of http://commons.wikimedia.org/wiki/File:QPSK_timing_diagram.png
Timing diagram for QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown on top and the total, combined signal at the bottom. Note the abrupt changes in phase at some of the bit-period boundaries.
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These 4 symbols can be represented in the form of an I-Q constellation diagram:
QPSK constellation diagram assuming Gray coding
QPSK modulation is often used in satellite transmission because it exhibits relative insensitivity to interference as compared to higher density amplitude and phase modulation techniques. QPSK has a symbol rate of four (4) with two (2) data bits assigned per symbol. These characteristics are important as the signals emitted by the satellite have to cover path links up to 36000 km (>22k miles). QPSK is also specified by Cable Labs within all DOCSIS® specifications as the last available default modulation when all other assignable modulation techniques result in unacceptable link performance. For example, a DOCSIS® performance management function may detect that response time is degrading due to a high number of uncorrected frames, and may issue a configuration management change to modify the modulation type from various forms of QAM to QPSK.
QoS Parameter Set
The set of Service Flow Encodings that describe the Quality of Service attributes of a Service
Quad Antenna Array
Four identical off-air antennas, mounted and aligned in such a way so as to improve the gain of weak broadcast signals and in many instances eliminate or reduce picture ghosts. [Arr11] Quad Shield Cable
A type of coaxial drop cable where the protective covering over the center conductor and dielectric and underneath the jacket consists of a foil-braid-foil-braid combinations. [Arr11] Quadrapower
An amplification technique whereby four output devices (or two power doubling devices) are operating in parallel to increase output capability. [Arr11] Quadrature Phase-Shift Keying (QPSK)
A method of modulating digital signals onto a radio-frequency carrier signal using four phase states to code two digital bits. QPSK is a four level use of digital phase modulation (PM). Quadrature signal representations involve expressing an arbitrary phase sinusoidal waveform as a linear combination of a cosine wave and a sine wave with zero starting phases.
Guarantees network bandwidth and availability for applications. Any real time media stream that crosses a DOCSIS® access link needs to be given prioritized traffic management treatment in order to assure the best user-perceived quality end-to-end. DOCSIS® 1.1 and PacketCable provide several potential methods for classifying traffic (ranging from DIFFSERV to flow-classification) and several access-link traffic management functions, which can be applied to such classified traffic (priority, CBR real-time polling, header compression, stream specific modulation methods). PacketCable enables on-demand real-time bandwidth management of DOCSIS® QoS sessions.
The process of converting the voltage level of a signal into digital data before or after the signal has been sampled. [Fib111] Quantizing Error
Inaccuracies in the digital representation of an analog signal. These errors occur because of limitations in the resolution of the digitizing process. [Fib111] Quantizing Noise
Noise which results from the quantization process. In serial digital video, a granular type of noise that occurs only in the presence of a signal. [Fib111] Quantum Efficiency
In a photodiode, the ratio of primary carriers (electron-hole pairs) created to incident photons. A quantum efficiency of 70% means seven out of ten incident photons create a carrier. [Fib111]
Quantum-dot Semiconductor Optical Amplifiers
Optical amplifiers having nano-sized semiconductor particles, called quantum dots; show attractive features such as an ultrawide operating wavelength range, suppressed waveform distortion in high power output, and capability of noise reduction (signal regeneration) by limiting amplification. With these features, the quantum-dot devices have been developed targeting applications in optical communication systems such as inline, booster, and preamplifiers, and are presently in the stage of commercialization. Their application is not limited to optical amplifiers, but also includes the light sources for sensors, gyroscopes, optical coherence tomography, etc., and the gain elements integrated into wavelength-tunable lasers and mode-locked lasers. [TAk06]