Encyclopedia of Laser Physics and Technology
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 Encyclopedia of Laser Physics and Technology

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Acousto-optic modulators

 

(Acronym: AOM)

Definition: optical modulators based on the acousto-optic effect

An acousto-optic modulator (AOM) is a device which allows to control the power, frequency or spatial direction of a laser beam with an electrical drive signal. It is based on the acousto-optic effect, i.e., the modification of the refractive index by the oscillating mechanical pressure of a sound wave.

The key element of an AOM is a transparent crystal (or piece of glass) through which the light propagates. A piezoelectric transducer attached to the crystal is used to excite a high-frequency sound wave (with a frequency in the order of 100 MHz). Light can then experience Bragg diffraction at the periodic refractive index grating generated by the sound wave; therefore, AOMs are sometimes called Bragg cells. The scattered beam has a slightly modified optical frequency (increased or decreased by the frequency of the sound wave) and a slightly different direction. The frequency and direction of the scattered beam can be controlled via the frequency of the sound wave, while the acoustic power allows to control the optical powers. For sufficiently high acoustic power, more than 50% of the optical power can be diffracted.

The acoustic wave may be absorbed at the other end of the crystal. Such a traveling-wave geometry allows to achieve a broad modulation bandwidth of many megahertz. Other devices are resonant for the sound wave, exploiting the strong reflection of the acoustic wave at the other end of the crystal. The resonant enhancement can greatly increase the modulation strength (or decrease the required acoustic power), but reduces the modulation bandwidth.

Some applications of acousto-optic modulators are:

1.      Q switching of solid state lasers (where the AOM typically serves to block the laser cavity before the pulse is to be generated)

2.      active mode locking (where the AOM modulates the cavity losses with the round-trip frequency or a multiple thereof)

3.      modulating the power of a laser beam, e.g. for laser printing

4.      shifting the frequency of a laser beam, e.g. in various measurement schemes

Adiabatic soliton compression

 efinition: a pulse compression technique based on the adaptation of solitons to slowly varying propagation parameters

Adiabatic soliton compression is a technique for the temporal compression of ultrashort pulses in a fiber. The principle is as follows. For a fundamental soliton pulse in a fiber, the product of pulse energy and pulse duration is proportional to the dispersion divided by the nonlinearity of the fiber. Thus, the pulse duration must be reduced if the dispersion is reduced while keeping constant the pulse energy. Significant pulse compression can thus be obtained by propagating the pulses through a dispersion-decreasing fiber. However, the following conditions must be fulfilled:

1.      The initial pulses must fulfill the soliton condition at the input fiber end.

2.      The fiber dispersion must be varied sufficiently slowly to allow adiabatic adaptation of the pulses to the fiber parameters. (Otherwise, the pulses can get distorted.) More precisely stated, the dispersion must not vary significantly over a length scale of a soliton period. As the latter scales with the square of the pulse duration, rather long fibers are required if the initial pulses are longer than e.g. a picosecond.

3.      The fiber dispersion must stay sufficiently constant over the whole bandwidth range of the compressed pulses. (However, it has been shown that slightly normal dispersion in the wings of the generated pulse spectrum can be beneficial.)

Even though the method is rather elegant and powerful, it suffers from the need to use a dispersion-decreasing fiber. The latter requirement is eliminated by a variant of the method, where the fiber has constant dispersion but contains a laser-active dopant which allows to amplify the pulses: an increasing pulse energy for constant dispersion also results in a temporal compression.

Amplification factor

 

The amplification factor (or gain factor) of an optical amplifier is the factor by which the input power is amplified. An alternative specification of gain is in terms of decibels, calculated as 10 times the logarithm of base ten of the amplification factor.

 

Amplified spontaneous emission

 

(Acronym: ASE)

Definition: a process where spontaneously emitted radiation (fluorescence) is amplified

In a laser medium with large gain, the fluorescence from spontaneous emission can be amplified to high levels. This amplified fluorescence may be used in applications where light with low temporal coherence but good spatial coherence is required.

In lasers and particularly in high gain amplifiers, amplified spontaneous emission is usually an unwanted effect. It tends to limit the achievable gain in a single stage of a fiber amplifier to the order of 40 dB; higher gains are possible e.g. for amplification of pulses, if several amplifier stages are used, which are separated by filters, isolators, and/or optical modulators.

Even if amplified spontaneous emission in an amplifier is not strong enough to extract significant power, it can significantly contribute to the noise of the amplified signal.

 

 

Amplifier chains

 

Definition: amplifiers consisting of several stages

An amplifier chain is a sequence of several amplifiers, which can subsequently amplifiy some signal. In case of just two amplifiers, one often calls the first one a preamplifier and the second one a power amplifier. Such schemes are common in electronics, but also in optics (as considered here), because the concept of an amplifier chain can have advantages compared to an approach where one tries to obtain the full performance in a single amplifier stage:

  • There are certain trade-offs, for example between obtaining a high saturated output power and a high gain. For example, a high saturated output power calls for a large mode area in the last section of the amplifier, while the preamplifier can have a better gain efficiency when the mode area is chosen small. Thus one can obtain better overall performance by splitting the amplifier into different amplifier stages which can be independently optimized. In ultrashort pulse amplifiers, the power amplifier may also need increased mode areas for reducing nonlinear effects. One can even use different kinds of amplifiers, e.g. a regenerative amplifier as a high-gain preamplifier and a multipass amplifier as power amplifier with moderate gain, or one can combine an optical parametric amplifier with a laser amplifier.
  • Sometimes it is necessary to insert certain optical elements between different stages of an amplifier chain. For example, in high gain amplifiers one often uses an optical isolator between the stages in order to reduce the sensitivity to amplified spontaneous emission (ASE) and back reflections. In amplifiers for pulse amplification, one sometimes uses an optical gain (e.g. an acousto-optic modulator) between the stages, which is opened only for some short time interval around that of the pulse passage.
  • The use of amplifier chains allows for a more modular approach where different performance numbers can be obtained simply by differently combining existing amplifier stages (modules).

Amplifier noise

Definition: noise introduced to a signal in an amplifier

Apart from amplifying the input, every optical amplifier also adds some excess noise to the output. This is often quantified with the so-called noise figure.

At least for phase-insensitive amplifiers (i.e., those where the gain does not depend on the phase of the input), quantum optics dictates some minimum level of the added noise. For example, for high gain applied to a shot-noise limited input, the output noise power must be at least twice that of a hypothetical noiseless amplifier, i.e., the minimum noise figure is 2, corresponding to 3 dB. For laser amplifiers, the unavoidable excess noise comes from spontaneous emission of the gain medium into the laser mode. For a four-level gain medium with a low-noise pump, the excess noise can approach the minimum quantum-mechanically allowed level. (Note that the gain medium acts as an energy reservoir, effectively damping the influence of high frequency pump noise.) In a non-degenerate optical parametric amplifier, the excess noise comes from vacuum fluctuations entering the idler port, and possibly also from the pump wave. A degenerate parametric amplifier does not need to add excess noise (it has no idler!), but its amplification is phase-sensitive.

Amplifiers

Definition: devices for amplifying the power of light beams

An optical amplifier is based on a gain medium. Typically, it receives a laser beam as input, and has an amplified beam as output. The amplification can also occur in an optical fiber.

Most amplifiers are laser amplifiers, where the optical amplification is based on stimulated emission in a gain medium such as a crystal or glass material which is doped with laser-active ions, or an electrically pumped semiconductor. However, there exist other physical mechanisms for optical amplification, namely mechanisms based on optical nonlinearities, such as e.g. parametric gain or Raman gain (→ Raman amplifiers).

In cases with weak amplification in a single pass through the gain medium, the effective gain may be increased by arranging for multiple passes of the radiation through the amplifier medium. This can be achieved with combinations of mirrors (for several passes with slightly different angular directions), or (for short pulses) with a regenerative amplifier.

For high input light intensities, the gain of a gain medium saturates, i.e., is reduced.

For high gain, weak parasitic reflections can cause lasing, which limits the achievable gain. Also, amplified spontaneous emission may then extract a significant power from an amplifier. A related effect is that amplifiers also add some excess noise to the output.

Important parameters of an optical amplifier include:

Typical applications of optical amplifiers are:

Amplitude-squeezed light

Definition: light with an intensity noise below the shot noise level

Amplitude-squeezed light is light with an intensity noise level below the shot noise level, i.e., below the standard quantum limit. This reduced intensity noise can be obtained only at the expense of increased phase noise.

Amplitude-squeezed light can be generated e.g. in a singly or doubly resonant frequency doubler, or in a laser diode which is driven with a rather quiet current.

Avalanche photodiodes

Definition: photodiodes with internal signal amplification through an avalanche process

An avalanche photodiode is a semiconductor-based photodetector (photodiode) which is operated with a relatively high reverse voltage (typically tens or even hundreds of volts), sometimes just below breakdown. In this regime, carriers (electrons and holes) excited by absorbed photons quickly get accelerated in the strong internal electric field, so that they can generate secondary carriers, as it is known in photomultipliers. The avalanche process effectively amplifies the photocurrent by a significant factor. In this way, avalanche photodiodes can be used to build quite sensitive detectors, which need less electronic signal amplification and are thus less sensitive to electronic noise. However, the avalanche process itself is subject to some noise, which can offset the mentioned advantage. Tentatively, their noise performance is better compared to ordinary p-i-n photodiodes in the high speed regime, but not for low detection bandwidths.

Silicon-based avalanche photodiodes are sensitive in the wavelength region of about 450-1000 nm, with the maximum responsivity occurring around 600-800 nm, i.e., at somewhat shorter wavelengths than for silicon p-i-n diodes. For longer wavelengths, APDs based on germanium or indium gallium arsenide (InGaAs) are used.

When operated with carefully designed electronics, avalanche photodiodes can be used even for photon counting with dark count rates below 1 kHz. Such detectors with optimized amplifier electronics are also available in CMOS integrated form.


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