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Optical Frequency Range

A frequency range or frequency band is a range of wave frequencies. It most often refers to either a range of frequencies in sound or a range of frequencies in electromagnetic radiation, which includes light and radio waves.
Many radio devices operate within a specified frequency range which limits the frequencies on which it is allowed to transmit, or is able to receive (for example the 80m band on a particular radio might refer to a frequency range of 3.5MHz to 4MHz).

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Thefrequency range of the human ear refers to those frequencies that people can detect sound, even if with diminished sensitivity; often taken to be 20Hz to 20kHz by engineers (although the range varies from person to person, and lower frequencies may very well be "felt" by a listener down to a few Hz).

The frequency range of a loudspeaker (and some other transducers) usually is a more flattering specification than frequency response in that the decibel range is either not given ormay be many decibels, as it refers to the range of frequencies that maybe used with some success, rather than the range for with the amplitudeof the signal barely varies.
Frequency standards are devices for producing or probing frequencies. Among all physical quantities, the frequency (or time) is the one which can be measured with by far the highest precision. Optical frequency standards are required e.g. for optical clocks, but also for optical fiber communications.
Active and Passive Frequency Standards

Anactive optical frequency standard is a kind of laser source emitting light with a very well-defined and known optical frequency, or sometimesa set of a few or even many well-defined optical frequency components in a frequency comb. Combined with an optical clockwork, such a frequency standard can form the basis of an optical clock. Other application areas of ultraprecise optical frequency standards are high-precision spectroscopy, global positioning systems, tests of the theory of relativity, and gravitational wave detection.

A passiveoptical frequency standard is a passive device with a well-defined frequency response, which can be used to build an active standard. Important examples are high-Q reference cavities and devices such as gascells for probing certain optical transitions.
Standards Based on Optical Transitions

Anoptical frequency standard is usually based on some optical transition (usually a forbidden transition) with narrow bandwidth of certain atoms (e.g. Ca, Rb, Sr, Yb, Mg, or H), ions (Hg+, Sr+, Yb+, In+, Al+), or molecules (I2 = iodine, CH4 = methane, C2H2 = acetylene). This transition is used to stabilize the frequency of a single-frequency laser to the transition frequency. In order to reduce inhomogeneous broadening by thermal movement (Doppler broadening) and collisions, the particles' density and relative velocities have to be minimized. One possibility is to keep the particles in a trap (e.g. a Penning trap or an optical trap) within a vacuum chamber and to apply laser cooling in order to reduce the temperature strongly. This allows for very precise spectroscopic measurements on the clock transition. Alternatively, such measurements can be done on laser-cooled atom beams. Simple gas cells, probed e.g. with Doppler-Free spectroscopy, are used when a lower precision is sufficient.

To serve well in a high-precision optical atomic clock, an atom, ion or molecule should meet a number of requirements:

    * The clock transition should have a very narrow linewidth (high Q factor).
   * The optical clock frequency should be convenient, so that a suitable interrogation laser is available and further processing (for example theconnection to an optical clockwork) is convenient.
    * There should be other transitions suitable for laser cooling.
    * The clock transition should be very insensitive to external disturbing factors such as electric or magnetic fields.

Inthe case of ions, it is often advantageous to use only a single ion in order to remove disturbances from ion–ion interactions and let the ion sit exactly at the center of the trap. Laser cooling in a trap down to the quantum-mechanical ground state of the ion's motion is then often possible. Interrogation of the clock transition is possible while the trap potential is turned on. Single-ion frequency standards are nearly free from systematic frequency shifts. However, the signal-to-noise ratio for the interrogation is fairly small with a single particle. Thisis detrimental for the stability of the frequency standard, because short-term deviations of the oscillator can not be well suppressed.

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Neutralatoms can be used in much larger numbers, such as a million, to improvegreatly the signal-to-noise ratio, thus allowing for very high stability of the frequency standard. However, collisions between the atoms lead to uncontrollable frequency shifts. Also, the magneto-opticaltrap often has to be switched off while interrogating the clock transition, because the light field of the trap introduces systematic frequency shifts which are difficult to eliminate. Switching off the trap limits the interaction time and introduces Doppler-related frequency shifts. These problems can be solved by loading the atoms intoan optical lattice [5, 9], as can be generated with superimposed laser beams, and by operating the trap with an optical frequency which is adjusted such that it its effects on the upper and lower energy level exactly cancel [9]. Such an optical lattice clock allows the systematic frequency shifts to be largely eliminated.
Reference Cavities as Flywheel Oscillators

Asthe signal-to-noise ratio for the interrogation of a weak clock transition is typically small (particularly for ion traps), it is important to use a well-stabilized laser as a flywheel oscillator. The laser is typically stabilized to a stable reference cavity with high Q factor, which gives good short-term stability (and can itself be considered as a frequency standard). The clock transition is then used to provide the long-term stability, which the cavity can not guarantee due to various kinds of drifts.