Burst Mode Lasers
Pulsed lasers can operate in different modes concerning the temporal patterns of the generated light pulses. Most common are (a) single-pulse operation, also called pulse on demand, and (b) repetitive pulsing with a fixed pulse repetition rate. However, there are also burst mode lasers, emitting bursts (bunches, groups) of pulses (see Figure 1). Each burst consists of some number of pulses, sometimes also called micro-pulses, while a whole burst may be considered as a macro-pulse.
Such a burst mode can be characterized with the following parameters:
- Each single pulse may be characterized with a pulse energy, duration and peak power, and possibly with additional details like center wavelength, bandwidth, etc. Those pulse parameters may all be constant in the simplest case, apart from some amount of random fluctuations and systematic variations. In some cases, however, one has substantial systematic variations, for example a decay of pulse energy within each burst, or some other time dependence of the pulse energy tailored for some application (see Figure 2).
- Each burst contains a certain number of pulses. The pulses have a certain pulse repetition rate inside each burst. This is also called intra-burst repetition rate. It is the inverse of the temporal pulse spacing within the burst. (There may also be an irregular pulse spacing, but regular spacings inside each burst are most common.)
- In many cases, the bursts are repeated with a fixed burst repetition rate, which is often orders of magnitude lower than the pulse repetition rate within a burst. In other cases, bursts may be triggered on demand.
- In some cases, the total burst energy (macro-pulse energy) is specified, which should of course not be confused with the lower micro-pulse energy.
There will usually be no coherence between subsequent pulses in a burst, except for certain sources based on a mode-locked laser as described below.
Depending on the kind of laser source and the application, the burst parameters can take very different values. For example, the repetition rate inside a burst is only a few kilohertz in some cases, and multiple gigahertz in others. The burst repetition rates may be very low (e.g. 1 Hz) or much larger, e.g. hundreds of kilohertz.
Applications of Burst Mode Lasers
The use of a burst mode may be motivated by the requirements for a certain laser application (e.g. in laser material processing and optical metrology) or largely by the properties of the used type of laser, if only a burst mode allows for sufficiently good performance.
Note that different applications require very different burst parameters, which can be achieved with very different laser architectures as described further below.
An important application of burst mode lasers is in laser material processing, for example in the form of laser drilling or other methods involving laser ablation. Here, micro-pulse energies range from nanojoules (for laser micromachining) to millijoules or even higher, while pulse durations vary from femtoseconds to nanoseconds. One pulse burst may be used for drilling one hole, and during the time between two bursts the laser spot or the workpiece can be moved to the location of the next hole.
Inside each burst, the pulse spacing for systems with Q-switched lasers (microseconds to milliseconds) will often still be so large that the residual heating from one pulse to the next one, considered within the laser spot, is weak or even negligible. So the amount of ablated material will depend only on the number of applied pulses, but not on their temporal spacing. However, the situation changes for bursts with very high intra-burst repetition rate (megahertz to gigahertz), where the temperature rise of the material at the laser spot largely remains from one pulse to the next one. In that case, the ablation efficiency (in terms of removed mm3 per joule of optical energy) may be substantially improved, comparing with a method where regular pulses with a similar energy but a much lower repetition rate are used. Burst mode operation may then also allow one to use lower pulse energies and peak powers. The duration of the bursts must be kept short enough to avoid the formation of a too large heat affected zone.
Other applications are in high-speed imaging. For example, fluid flow diagnostics with particle image velocimetry and other methods can utilize intense pulses with a high repetition rate (multiple megahertz) in order to record data at least for the limited duration of a burst. While it may seem preferable to continuously image with such repetition rates, the available laser power may be insufficient, or the imaged objects cannot tolerate such high average power.
Types of Burst Mode Lasers
Burst mode lasers can be realized in very different ways, with very different burst parameters, and for very different reasons. Such laser types, including various types of master oscillator power amplifier (MOPA) devices, are explained in the following sections.
Some Q-switched lasers are operated in a burst mode where within each burst one has an average output power which is well beyond what the laser could generate over longer times, for example because the laser crystal or the pump source would be overheated. For short enough bursts, however, such performance can be achieved. If the time between the bursts is long enough, it allows the laser parts to cool down sufficiently such that the pulse energy can be much higher than for regular repetitive operation with that repetition rate. Frequently, the time between the bursts is much longer than the duration of each burst. However, there can still be limitations, for example set by the threshold of laser-induced damage.
Typically, optical pumping in such a laser is either continuous during each burst (e.g. with a long pulse from a flash lamp) or occurs in the form of one pump pulse before each laser pulse, while no pumping occurs between the bursts. Note that the energy in the laser crystal (or other type of gain medium) needs to be replenished before each pulse; one can generally not extract multiple pulses without pumping the gain medium between them. Therefore, the pulse repetition rate within a bunch usually cannot be very high.
One may also employ cavity dumping for obtaining shorter pulses, and/or an additional amplifier stage for higher pulse energies .
Mode-locked Laser Plus Pulse Picker and Amplifier
Some laser systems contain
- a continuously operating mode-locked laser, usually with a pulse repetition rate between tens of megahertz and several gigahertz,
- a pulse picker or possibly an optical modulator with a continuously variable transmissivity, and
- some kind of optical amplifier, e.g. based on a laser crystal (possibly with an optical arrangement for a multipass amplifier) or (for smaller pulse energies) a rare-earth-doped fiber. One may also use a sequence of such amplifiers, for example a fiber amplifier followed by a power booster with a laser crystal.
With a suitable electronic control, one can then form bursts with any number of pulses and burst repetition rate, while the pulse repetition rate inside each burst can only be the pulse repetition rate of the mode-locked laser or some integer fraction of that. If one uses a suitable kind of optical modulator in the system, one can also tailor the evolution of pulse energy within each burst. Other pulse properties such as their duration and center wavelength can usually not be modified. The pulse duration is in the femtosecond or picosecond regime.
Another possibility of forming high repetition rate bursts is in principle to operate a mode-locked laser such that multiple pulses are circulating in its resonator . However, it is then generally difficult to control the distribution of power, timing and pulse quality. Therefore, this approach is not further considered here.
As for such systems the pulse repetition rate within each burst is normally very high (tens of megahertz or more), there is not much time for re-pumping the amplifier between the micro-pulses. (Typically, re-pumping requires a time of the order of the upper-state lifetime, which is microseconds to milliseconds for the usual solid-state gain media.) Therefore, pumping is essentially done just before each burst, or continuously if the burst repetition rate is high. A consequence of that is that the amplifier gain more or less drops within each bunch (→ gain saturation), depending on how much of the stored energy is extracted by the pulses.
Particularly in cases with small burst repetition rate, one will usually want to extract much of the stored energy by each burst, because that energy would otherwise be lost in the time to the next burst. Consequently, one will obtain a strong decay of gain within each burst. For a constant input pulse energy for the amplifier, this would lead to a corresponding decay of pulse energy within each burst. That decay of pulse energy will be particularly pronounced in cases where the amplifier gain (and consequently the reduction of that gain) is high.
If the decay of pulse energy is undesirable, one may try to compensate it by creating a burst pattern before the amplifier where the pulse energy appropriately increases from pulse to pulse. Particularly for a high-gain amplifier (e.g. a fiber amplifier), that approach is just limited by how strong the input pulse modulation can be in practice. If a preamplifier is used, one may want to optimize that for weak gain saturation (e.g. by using a large mode area fiber) even if that somewhat compromises the power conversion efficiency.
The high intra-burst pulse repetition rate can also be very relevant for applications. For example, laser ablation may lead to a substantially increased ablation rate, as was already explained above.
Mode-locked Laser Plus Pulse Splitter
For obtaining both of our short pulses with a very high burst repetition rate (many megahertz or even gigahertz), one can combine a mode-locked laser with some kind of pulse splitter, which converts each output pulse of the laser into a sequence of e.g. 2, 4 or 8 pulses. (Typically, the number of pulses is a power of 2, since one subsequently doubles the number of pulses.)
Such sources can be used for studying rather fast processes, for example in the context of laser ablation .
Q-switched Mode Locking
A passively mode-locked laser can under certain circumstances (e.g. low pump power) get into the regime of Q-switched mode locking. Here, it naturally generates bursts of ultrashort pulses, part of which can exhibit substantially higher pulse energies than for continuous mode locking (Figure 3, red lines).
This might therefore seem to be a simple solution for realizing a burst mode for ultrashort pulses. However, this approach comes with serious limitations. It is not only that one obtains a smooth modulation of the pulse energy rather than bursts with constant pulse energy as shown in Figure 1. In particular, the burst repetition rate needs to be rather high, because if the pulses fully “die out” between the bursts, the pulse generation becomes unstable: the pulses can become longer and exhibit substantial fluctuations of their duration and other temporal and spectral details. For obtaining much higher maximum pulse energies, however, one would need to go exactly into that regime of low burst repetition rates.
Seed Laser Diode Plus Amplifier
A very flexible way of producing seed pulses is to use a gain-switched laser diode as the seed laser for an amplifier system (often a master oscillator fiber amplifier). One can then very freely tailor the input pulse pattern. One may want to use an additional modulator for even more freedom, for example for modifying the evolution of pulse energy in each burst without changing the pulse duration and other pulse parameters. Also, it is possible to use multiple laser diodes with different wavelengths, which can be interesting for some applications.
Compared with a mode-locked laser, such a seed source is of course much more compact and cheap. On the other hand, it is substantially more limited in terms of pulse energy and duration. Typically, one would require one more fiber amplifier stage in order to achieve a sufficiently high gain for a given output pulse energy. In some cases, it is also relevant that mutual coherence of the pulses can then not be obtained.
Microchip Lasers and Other Miniature Lasers
An actively or passively Q-switched microchip laser can also be suitable for a burst mode laser system with or without an optical amplifier. For example, one may use a passively Q-switched laser which is pumped for long enough time intervals to generate multiple output pulses, which typically have approximately equal energies and temporal spacing. Such a device can be suitable for the laser-induced plasma ignition of combustion engines .
The achievable pulse durations can be in the nanosecond or picosecond domain, while the pulse energies can typically reach microjoules – much higher than for pulsed laser diodes, for example.
Due to the substantial peak power, it is generally not difficult to equip burst mode lasers with an efficient device for nonlinear frequency conversion to other wavelengths. In the simplest case, this is a frequency doubler, but frequency tripling and quadrupling are also possible. Pumping an optical parametric oscillator is another option, provided that the pulse build-up in the OPO is fast enough. If not, an optical parametric generator (OPG) should normally be fast enough for such an operation mode, but it normally requires more intense pump pulses.
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