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There is a resonant cavity in the optical amplifier, so the optical pulse can cycle many times inside the cavity before coupling out.
A positive feedback amplifier is a device used to amplify optical pulses, typically with ultra short pulse lengths in the picosecond or femtosecond range (see ultrafast amplifiers). Placing the gain medium in the resonant cavity can be achieved multiple times through the gain medium (usually a solid-state medium), coupled with an optical switch, using an electro-optic modulator and polarizer. The number of cycles in the resonant cavity can be controlled by an optical switch, which can be very large, resulting in a very high overall amplification factor.
Although pulses pass through the gain medium multiple times in a positive feedback amplifier, a multi pass amplifier still refers to a device with a determined number of channels obtained based on the geometric properties of the beam path, rather than using optical switches. It is also very natural to consider a positive feedback amplifier as a multi pass amplifier.
pictureoneDiagram of Picosecond Positive Feedback Amplifier Device. The Pockels box functions as an optical switch with quarter slides and thin film polarizers. Faraday rotation mirror is used to separate incident and outgoing pulses.
Working principle of positive feedback amplifier
The working principle of positive feedback can be understood from the following aspects:
- Firstly, the gain medium is pumped for a period of time to collect some energy.
- Then, the first pulse enters the resonant cavity through a port controlled by an electro-optic effect switch (or acousto-optic effect) that is briefly open (less than the time for the pulse to cycle once inside the cavity).
- This pulse circulates multiple times (possibly hundreds of times) in the resonant cavity and is amplified to a very high energy level.
- Finally, the pulse is released from the resonant cavity. This can be achieved by using a second electro-optical switch, or by using a switch that couples it in.
This working principle enables the use of medium-sized amplifiers to achieve high gain and pulse energy in the millijoule range, while larger devices can yield higher energy. The typical pulse repetition rate is1kHzScale (may also reach several hundred)kHz）At lower repetition rates, the highest pulse energy can be obtained.
Note that seed lasers are usually mode-locked lasers with very high pulse repetition rates, such as several hundred megahertz, and only a very small number of seed pulses are used in amplifiers.
For high repetition rate scenarios (several tens)kHzEven higher), the amplifier gain medium is usually continuously pumped. For low repetition rate situations (especially when the pulse period is much longer than the lifetime of the upper energy level), pulse pumping (for example, using modulation)QLaser is more advantageous. A commonly used structure is to use frequency doublingYAGLaser is used to pump the positive feedback titanium sapphire amplifier.
Other issues
Due to the limited gain bandwidth of the amplifier medium, the pulse width is reduced, resulting in an increase in the length of the amplified pulse. This is called the gain narrowing effect, which can be reduced by adding appropriate spectral filters to the amplifier, resulting in a wider and flatter net gain spectrum.
In femtosecond devices, the dispersion caused by Pockels boxes needs to be compensated for. Nonlinear effects may also exist, and it is necessary to use chirped pulse amplification or split pulse amplification to effectively suppress these effects. If no measures are taken, the obtained pulse energy will be limited by nonlinear effects, and even optical damage phenomena may occur.
The power efficiency of a positive feedback amplifier is greatly reduced due to the loss effect inside the cavity, especially in electro-optic switches. When the single loop gain is low, it is more sensitive to these losses because in order to obtain a certain amplification factor, it is necessary to increase the number of cycles of the pulse in the loop.
Positive feedback amplifiers can also reduce their gain and pump efficiency due to limited low-energy lifetime, resulting in a large population of low-energy levels when amplifying pulses, leading to reabsorption and laser transitions. This problem mainly occurs when the amplifier amplifies ultra short pulses, but it does not occur during chirp amplification because the pulse length of the amplifier is relatively long at this time.
In some applications, it is necessary to maintain the companion pulse energy at a relatively low level, that is, to obtain high pulse contrast. Companion pulses can be generated in several ways, such as imperfect switching in Pockels boxes or parasitic reflections in amplifiers. Additional pulse pickers are required to suppress the preceding or following pulses.
When the pulse repetition rate is high, there may be strong fluctuations in the pulse amplification process. Periodic bifurcations can also occur, where the pulse energy varies between two values, followed by four or even eight values. When the pulse repetition rate is higher, it will cause chaotic effects. picturetwoIt is a numerical simulation scenario.
picturetwoThe relationship between the pulse energy of a positive feedback amplifier and the number of cycles in the resonant cavity, utilizingRPFiber optic power software obtains simulation results.
The 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 doped or neodymium doped media, are suitable for use with diode pumping. Due to their longer upper level lifetime, they have strong energy storage capabilities. However, due to their smaller gain bandwidth, they generate longer pulses (usually several hundred)fsOr even longer).
The applications of positive feedback amplifiers include material processing (such as usingmJPulse energy cutting of metals and scientific experiments, such as in high-energy physics, are used to generate high-order harmonics.
Alternative products
An alternative to a positive feedback amplifier is a multi pass amplifier, where multiple channels are obtained by arranging many mirrors (each channel has a slightly different propagation direction). This scheme does not require the use of fast modulators, but it can be complex (and difficult to arrange) when there are many channels passing through the gain medium. According to empirical rules, a multi pass amplifier is more suitable for obtaining very high pulse energy when the gain is limited (because the incident pulse energy is already quite high), while a positive feedback amplifier is more suitable for very high gain situations. Of course, these two amplifiers can be combined together: the positive feedback amplifier provides high gain, and the terminal's multi pass amplifier increases pulse energy (see amplifier chain, ultrafast amplifier).
Another technique that can achieve high pulse energy, although not as high as in positive feedback amplifiers, is the use of tilted cavities in mode-locked lasers. This technology is particularly suitable for achieving high pulse repetition rates of up to several megahertz.
