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Advantages Of Positive Feedback Amplifier.What Is Positive Feedback Amplifier

Advantages Of Positive Feedback Amplifier.What Is Positive Feedback Amplifier

September 18 , 2023 0

Positive Feedback Amplifier

Definition: The presence of a resonant cavity in an optical amplifier, so that an optical pulse can circulate through the cavity many times before being coupled out.

Positive feedback amplifiers are devices used to amplify optical pulses, usually ultra-short pulses on the order of picoseconds or femtoseconds (see Ultrafast Amplifier). Placement of a gain medium in a resonant cavity, which can be passed through the gain medium (usually a solid state medium) many times, coupled with an optical switch, is achieved using electro-optical modulators and polarisers. The number of turns that can be cycled through the resonant cavity can be controlled by the optical switch and can be very large so that the overall amplification factor can be very high.

Despite the fact that the pulse passes through the gain medium several times in a positive feedback amplifier, a multipass amplifier refers to a device with a defined number of channels, obtained according to the geometrical nature of the beam path, rather than using optical switches. It is also quite natural to think of a positive feedback amplifier as a multi-pass amplifier.

Working Principle of Positive Feedback Amplifier

The working principle of positive feedback can be understood as follows:

First, the gain medium is first pumped for a period of time where some energy can be collected.
Then, the first beam of pulses enters the resonant cavity through a port that is briefly open (less than the time it takes for the pulse to circulate through the cavity once) controlled by an electro-optic effect switch (or acousto-optic effect).
This pulse is amplified to a very high energy level by circulating many times (possibly hundreds of times) in the resonant cavity.
Finally, the pulse is released from the resonant cavity. This can be done by employing a second electro-optical switch, or it can be done by employing a switch that couples it in.
This principle of operation allows high gain and pulse energies on the order of millijoules to be obtained with medium-sized amplifiers, and even higher energies with larger devices. Typical pulse repetition rates are in the order of 1 kHz (and may be in the order of several hundred kHz), and the highest pulse energies are obtained at lower repetition rates.

Note that seed lasers are usually mode-locked lasers, which have very high pulse repetition rates, e.g. several hundred megahertz, and only a very small number of seed pulses are used in amplifiers.

For high repetition rate cases (tens of kHz or even higher), the amplifier gain medium is usually pumped continuously. For low repetition rate cases (especially where the pulse period is much larger than the upper energy level lifetime), pulse pumping (e.g., with a Q-modulated laser) is more favourable. A common construction is to use a frequency-doubled YAG laser to pump a positive-feedback titanium sapphire amplifier.

Other Problems

Due to the limited gain bandwidth of the amplifier medium the pulse width is reduced and the amplified pulse length can become larger. This is known as the gain narrowing effect and can be reduced by adding a suitable spectral filter to the amplifier, so that the net gain spectrum becomes wider and flatter.

In femtosecond devices, the dispersion caused by the bubbleglass box needs to be compensated for. Nonlinear effects can also exist, and chirped-pulse amplification or split-pulse amplification is needed to effectively suppress these effects. If no measures are taken, the obtained pulse energy will be limited by the nonlinear effects and even produce optical damage phenomena.

The power efficiency of positive feedback amplifiers is greatly reduced by loss effects in the cavity (especially in electro-optical switches). The sensitivity to these losses is increased when the monoturn gain is low, as it is then necessary to increase the number of turns that the pulse circulates through the loop in order to obtain a certain amplification factor.

Positive feedback amplifiers also reduce their gain and pumping efficiency because of the limited lifetime of the low-energy state, so that there is a large number of Bougues in the low-energy level when amplifying the pulse, and laser jumps occur as a result of reabsorption. This problem occurs mainly when the amplifier amplifies ultrashort pulses, but not during chirp amplification, when the amplifier pulse length is relatively long.

In some applications, it is necessary to maintain the companion pulse energy at a relatively low level, i.e., to obtain a high pulse contrast. The companion pulse can be generated in several ways, e.g., imperfect switching of the bubblegum box or parasitic reflections in the amplifier. Additional pulse pickups need to be employed to suppress the preceding or following pulses.

When the pulse repetition rate is high, the pulse amplification process may have a strong rise and fall. Multiple period bifurcations can also occur, i.e. the pulse energy varies between two values, followed by between four or even eight values. At even higher pulse repetition rates, chaotic effects are induced [8,9,12]. The following picture shows a situation

A commonly used gain medium for positive feedback amplifiers is titanium sapphire, which has a wide gain bandwidth and thermal conductivity. Other gain media, such as ytterbium- or neodymium-doped media, are suitable for diode-pumped situations, which have a high energy storage capacity due to the long lifetime of the energy levels on them, but produce longer pulses (typically several hundred fs or more) due to the smaller gain bandwidth.
Applications for positive feedback amplifiers include materials processing (e.g. cutting metal with mJ pulse energy) and scientific experiments, e.g. for generating high harmonics in high energy physics.