They are used for Q switching of solid-state lasers. The AOM, called Q switch, then serves to block the laser resonator before the pulse is generated. In most cases, the zero-order (not diffracted) beam is used under lasing conditions, and the AOM is turned on when lasing should be prohibited. This requires that the caused diffraction losses (possibly for two passes per resonator round trip) are higher than the laser gain. For high-gain lasers (for example, fiber lasers), one sometimes uses the first-order diffracted beam under lasing conditions, so that very high resonator losses result when the AOM is turned off. However, the losses in the lasing state are then also fairly high.
AOMs can also be used for cavity dumping of solid-state lasers, generating either nanosecond or ultrashort pulses. In the latter case, the speed of an AOM is sufficient only in the case of a relatively long laser resonator; an electro-optic modulator may otherwise be required.
Active mode locking is often performed with an AOM for modulating the resonator losses at the round-trip frequency or a multiple thereof.
An AOM can be used as a pulse picker for reducing the pulse repetition rate of a pulse train, e.g. in order to allow for subsequent amplification of pulses to high pulse energies.
In laser printers and other devices, an AOM can be used for modulating the power of a laser beam. The modulation may be continuous or digital (on/off).
An AOM can shift the frequency of a laser beam, e.g. in various measurement schemes, or in lasers which are mode-locked via frequency-shifted optical feedback.
In some cases one exploits the effect that the diffraction angle depends on the acoustic frequency. In particular, one can scan the output beam direction (at least in a small range) by changing the modulation frequency.
Various aspects can be essential for the selection of an acousto-optic modulator for some application:
The material should have a high transparency at the relevant wavelengths, and parasitic reflections should be minimized e.g. with anti-reflection coatings.
In many cases, a high diffraction efficiency is important. For example, this matters when using the AOM as a Q switch in a high-gain laser, and even more so for cavity dumping.
The required RF power influences both the electric power demands and cooling issues.
The switching time is critical for some applications.
For frequency shifters, the device often has to be used in a wide range of RF frequencies.
High optical peak powers require a suitable material and a large open aperture, allowing for a high damage threshold.
Due to various trade-offs, quite different materials and operation parameters are used in different applications. For example, the materials with highest diffraction efficiencies are not those with the highest optical damage threshold. A large mode area can increase the power handling capability, but requires the use of a larger crystal or glass piece and a higher drive power, and also increases the switching time, which is limited by the acoustic transit time. For fast acousto-optic beam scanners, a large mode area is required for achieving a high pixel resolution, whereas a smaller mode area is required for a high scanning speed.
Adjacent Channel Power Ratio; a measurement of the amount of power in the adjacent frequency channel. ACPR is usually defined as the ratio of the average power in the adjacent frequency channel to the average power in the transmitted frequency channel. It describes the amount of power generated in the adjacent channel due to nonlinearities in RF components. The ACPR measurement is part of the UMTS standard but is not a part of the cdmaOne standard. [Tel05] ACR
Adjacent Channel Rejection; the measure of how well a receiver can receive the transmitted signal in the presence of a high-level interfering signal in the adjacent channel. It is measured as a frame error ratio (FER) or packet error rate (PER) at specified interferer power levels. [Agi00] ACRR
Adjacent Channel Rejection Ratio; the ratio of the average power in a specific offset frequency to the average power in the transmitted frequency. [Bar06] Active
A device or circuit capable of some dynamic function, such as amplification, oscillation, or signal control, and which usually requires a power supply for operation.[Arr11] Also, in computer parlance, a service flow is said to be “active” when it is permitted to forward data packets. A service flow must first be admitted before it is active. Active Circuit
Powered circuitry containing transistors, such as amplifiers, power supplies or converters.
A functioning satellite that receives and transmits or retransmits radio-communication signals to or from a base station. [Tim11] Active Service Flow
An admitted Service Flow from the Cable Modem (CM) to the Cable Modem Termination System (CMTS) which is available for packet transmission.
Advanced Communications Technology Satellite; a NASA experimental satellite project to demonstrate the use of the Ka-Band (30/20 GHz) services. [Sat07]
Advertising spots available to a cable operator to insert local advertising on a cable network.
A fitting used between two sets of equipment to provide a transition from one to another when they cannot be directly interconnected. Adaptive Smart Antenna
Combines multiple antenna elements with signal processing algorithms to optimize its radiation and/or reception pattern automatically in response to the signal environment.
Retry with exponential timeout: first attempt — 1 sec and the last attempt — 16 sec.
Additional Call Offering (ACO)
An ISDN feature that allows multiple calls to be placed simultaneously to the same telephone number. A serving switch is programmed with the number of lines on the receiving telephone equipment. The switch will offer an additional call if there is a line available to accept it. Sometimes erroneously called FCO or Flexible Call Offering.
The number dialed by a calling party which identifies the party called. Also a location or destination in a computer program.
Address Resolution Protocol (ARP)
A protocol of the IETF for converting network (IP) addresses to 48-bit Ethernet addresses.
Able to signal from the headend or hub in such a way that only the desired subscriber's receiving equipment is affected. In this way, it is possible to send a signal to a single subscriber and effect changes in the subscriber's level of service.
Area of Dominant Influence; in the measurement of television audience data, geographic area composed of all the counties influenced by originating stations in a particular television market. For example, the New York City ADI is composed of all the counties in New York and New Jersey where the New York City television stations are viewed. Each county in the United States is allocated exclusively to one ADI. [All11] Adjacent Channel
Any of two TV channels are considered adjacent when their view carriers, either off-air or on a cable system, are 6 MHz apart. FM signals on a cable system, two channels apart are adjacent when their carriers are 400 to 600 kHz apart. The channel (frequency band) immediately above or below a specific channel.[Arr11] Adjacent Channel Rejection (ACR)
The measure of how well a receiver can receive the transmitted signal in the presence of a high-level interfering signal in the adjacent channel. It is measured as a frame error ratio (FER) or packet error rate (PER) at specified interferer power levels. A sampling of standards that specify ACR:
IEEE Std 802.15.4™-2006, Specific requirements--Part 15.4: Wireless MAC and PHY Specifications for Low-Rate WPANs
ANSI/IEEE Std 802.11 First Edition 1999-00-00, Receiver adjacent channel rejection
NTC TS 003 – 2548, “Radiocommunication Equipment Used in Aeronautical Mobile Services in the VHF Frequency Band”, Technical Standards for Telecommunication Equipment , Office of the National Telecommunications Commission, 87 Phahon Yothin 8 Road, Samsennai, Phayathai, Bangkok 10400, www.ntc.or.th
The adjacent channel rejection (ACR) measurement is very important because it tells you how well your device will perform under real-world conditions. The alternate channel rejection test is very similar to this test. The test setup, according to the IEEE standards, must have two independent signal sources for the in-band and the adjacent channel signals. The two signal sources cannot be coherent in either phase or frequency. You will need a signal combiner for the test signals. It is recommended that you use an isolator for each source in order to avoid injection locking. [Agi00]
ACR Diagram courtesy of Peder Rand, Texas Instruments (TI), EE Times, 04Aug08, “What's Required for RF4CE?” http://www.eetimes.com/design/microwave-rf-design/4018987/What-s-Required-for-RF4CE-?pageNumber=1
Adjacent Channel Rejection Ratio (ACRR)
The ratio of the average power in a specific offset frequency to the average power in the transmitted frequency. [Bar06] Admitted
A service flow is said to be “admitted” when the Cable Modem Termination System (CMTS) has reserved resources (e.g., bandwidth) for it on the Data Over Cable Service Interface Specification (DOCSIS®) network. Admitted Service Flow
A Service Flow, either provisioned or dynamically signaled, which is authorized and for which resources have been reserved but is not active.
Advanced Design System; electronic design automation (EDA) software for high-frequency and high-speed system, modeling, and RF circuit design applications, sold by Agilent® Technologies under the moniker “EEsof® EDA”. [Agi11] ADSL Asymmetric Digital Subscriber Line
Advanced Television (ATV)
Digital television technology that provides better audio and video quality than the analog NTSC standard television Broadcast system. High Definition TV (HDTV) is a form of ATV. [Tim11] Advanced Television Enhancement Forum (ATVEF)
Advanced Television System Committee (ATSC)
An organization founded in 1983 to research and develop a digital TV standard for the U.S.; an international organization of 200 members that is establishing voluntary technical standards for advanced television systems.
Cable suspended in the air on poles or other overhead structures. Usually implies the use of a “messenger strand” to which the cable is lashed for support.[Arr11] Aerial Plant
Cable that is suspended in the air on telephone or electric utility poles.
A broadcast station that airs a network. [Tim11] AGC
Automatic Gain Control; a process or means by which gain is automatically adjusted in a specified manner as a function of input level or another specified parameter.[Fib111]
AGC Diagram courtesy of Fiber Optics Info, http://www.fiber-optics.info/fiber_optic_glossary/a
The level of input current, voltage, or RF power at which the automatic gain control (AGC) circuit becomes active. AGC Time Constant
The amount of time it takes to achieve the required AGC level; also the amount of time it takes to recover from AGC.
At the FCC, a term that refers to the document which is to be voted on by the Commission at an agenda meeting. The final document is released to the public the same day or as soon as possible. [Tim11] Aggregator
Any person or business that, in the normal course of business, provides a public telephone for the use of patrons through an Operator Service Provider (OSP). [Tim11] Agile Component
A component which can be tuned to any desired channel. [Cha07] AGL
The ratio of the amount of electromagnetic energy reflected by a surface to the amount of energy incident upon it, often expressed as a percentage. Example: the albedo of the Earth is 34 percent. Also, the reflectivity of a body as compared to that of a perfectly diffusing surface at the same distance from the Sun, and normal to the incident radiation. Albedo may refer to the entire solar spectrum or merely to the visible portion. [Tim11]earthobservatory.nasa.gov540 × 313 - Global Albedo : Image of the Day
540 × 313 - Global Albedo : Image of the Day
Global albedo image courtesy of http://earthobservatory.nasa.gov/IOTD/view.php?id=2599