AWGs for High Energy Lasers

Today laser intensities reached levels where relativistic effects dominate in laser-matter interaction.
High-energy lasers are pulsed lasers which emit light pulses with relatively high pulse energy.

There is no universal definition of how high the pulse energy must be, but usually one compares with ordinary Q-switched lasers, which are mostly solid-state lasers, and considers pulse energies of 100 mJ or more as high-energy lasers.
New applications of high pulse energy lasers emerge in various disciplines ranging from fundamental physics to materials research, life sciences and aerospace & defense.
High-energy lasers can also be used for research, such as in fusion energy research with systems like NIF, which is the world’s largest and most energetic laser.

The Arb Rider AWG-7000 is the right tool for these cutting-edge experiments that demand fast rise time, high amplitude and low jitter signals.

Types of High Energy Lasers

Q-Switched Lasers
In most cases, these are Q-switched solid-state lasers based on laser crystals or some laser glasses, and the pulse duration is somewhere in the nanosecond regime.
Optical pumping can in principle be done with laser diodes (diode-pumped lasers), but this is often not practical for very high pulse energies, since the required pumping energy needs to be supplied within a time which is roughly limited by the upper-state lifetime of the laser gain medium; that is most often between some hundreds of microseconds and a couple of milliseconds.

For example, a pulse energy of 1 J may require a pumping energy around 2 J, and if that should be supplied within ≈100 μs, that translates into a required pump power of 2 J / 100 μs = 20 kW. Although laser diodes can be used in quasi-continuous-wave operation for pulse pumping with increased power (beyond what the diodes could deliver in continuous-wave), that increase is usually quite limited.
The higher required pump power then translates into a high cost for laser diodes and it is also difficult to combine the radiation from many laser diodes into the laser gain medium. For those reasons, one often uses lamp-pumped lasers with flash lamps (pulsed gas discharge lamps), which can provide a high pulse energy at quite moderate cost, although that approach leads to a quite low power conversion efficiency of the laser, mostly due to the undirected and broadband lamp emission.

In the Figure 1, the Arb Rider AWG-4000 Series is used to drive an acousto-optic modulator (AOM) for Q-switching in laser experiments by providing a precise, high-amplitude up to 12 Vpp and rapidly switched Radio Frequency (RF) signal to the AOM’s piezoelectric transducer.
This signal generates high-power acoustic waves within the AOM, which diffract the laser light, allowing for the controlled accumulation and release of energy, resulting in high-energy, short pulses characteristic of Q-switching.

In these experiments, the AWG provides the fast digital on/off switching required for rapid control of the AOM, high-amplitude pulses necessary to drive the AOM and achieve the substantial RF power needed for efficient Q-switching.

The “Arbitrary” nature of the AWG allows for the generation of complex patterns to precisely control the laser’s Q-switching behavior; moreover the AWG can provide synchronized signals for other components in the experimental setup, which is crucial for coordinating the laser operation.

Free-Running Lasers
Q switching is the most frequently used technique for pulse generation and it typically leads to nanosecond pulse durations. However, some high-energy lasers are operated in free-running mode, without a Q-switch in the laser resonator.

This leads to substantially longer pulses with durations of the order of the pump pulse duration; the latter may be determined, for example, by the used flash lamp in conjunction with the driver electronics. The laser output peak power is correspondingly lower.

Amplified Sources (MOPAs)
Large laser devices very often face severe stability problems associated with large electrical power inputs, optical damage problems, mechanical vibrations, cooling and heat-dissipation problems, acoustic noise, etc.

One common way to obtain high laser power output, simultaneously with good beam quality, short pulse length, excellent frequency stability and good beam control, is to generate a stable input laser signal from a small but well-controlled laser oscillator. This signal can then be amplified through a chain of laser amplifiers, in what is commonly known as a master-oscillator-power-amplifier or MOPA system.

Beam Combining
For obtaining highest pulse energies, one sometimes applies techniques of beam combining:

• Spectral beam combining works with multiple laser sources of somewhat different optical wavelength. One may, for example, use a diffraction grating. Unfortunately, many high-energy lasers cannot be easily realized with different output wavelengths, since their gain media have a rather small emission bandwidth.
  A notable exception are fiber lasers, which however are far more limited in output peak power than solid-state bulk laser.
• Coherent beam combining requires mutual coherence of the optical fields of multiple lasers. This is also not easy to obtain from high-energy lasers; it generally requires diffraction-limited beams, with optimal beam quality.

Pulse Duration

Pulse durations as short as possible are often desired, basically because they lead to higher peak powers. However, it is more challenging to achieve that specifically for high-energy lasers for various reasons:

• The high intracavity peak powers increase the risk of optical damage.
• For a Q-switched laser, the round-trip time of the laser resonator should be kept short for achieving short pulses. However, lamp-pumped lasers (or lasers pumped with a large number of laser diodes) require a rather long gain medium and therefore long laser resonators.
• Short pulses require relatively fast operation of the Q-switch. If that is an acousto-optic modulator, that is possible only for relatively tight focusing of the laser radiation in the device because the utilized sound wave has a limited velocity in the material. That, however, leads to problems with laser-induced damage. In the case of an electro-optic modulator (Pockels cell), a problem is the large electrical capacitance which usually results from a large crystal size.

The highest pulse energies are achieved without any Q-switch, but then the pulse durations are far longer.

Typical Applications of High-energy Lasers

Nuclear Fusion Extremely high optical intensities are required to drive nuclear fusion, where they create a combination of extremely high temperature and pressure for a short moment (inertial confinement fusion). Huge stationary devices have been constructed for experimental tests of that principle. Most notably, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in Livermore, California, has been constructed since 1997, and after many years of development it succeeded in generating megajoule fusion yields by applying multi-megajoule laser pulses with few picosecond durations.

It is based on huge amplifier chains and laser beams from a large number of such sources are concentrated on a single small target containing the nuclear fuel.
The main purpose is research on nuclear fusion reactors based on laser ignition.

Laser Material
Processing In the material processing of glass, the femtosecond laser micromachining technique allowed to process complex transparent structures.
Now bottom-up milling of glass, thin glass cutting, laser based Bessel-beam scribing techniques and through glass Vias fabrication can be precisely fabricated by selectively removing material through drilling, cutting, and milling.

In the processing of metal, femtosecond lasers enable the production of complex shapes and features, while also providing the capability to perform black/white marking and coloring without the need for chemical additives.

Polymers are widely used in various applications, including automotive, medicine, and consumer electronics. However, due to their inherent property of low heat conductivity, polymers are quite sensitive to heat. High thermal loads can lead to significant heat affected zones, warping, and carbonization. Femtosecond lasers, with their very short pulse durations, offer a solution to this problem by enabling the precise machining of polymers while preserving process quality.

Remote Sensing
Various methods of remote sensing can be applied over relatively large distances provided that high enough pulse energies are available. Examples are chemical analysis based on laser-induced breakdown spectroscopy (LIBS) and various LIDAR methods.

High-power laser diodes are an integral part of how LiDAR works. These diodes must emit high light output within a short current driving time, also known as pulse width.

High-power laser diodes emit a concentrated beam of light to illuminate objects. The LiDAR system then measures the time it takes for the reflected light to return, thereby determining the distance and shape of the object. The power of the laser diode is critical in this process; a stronger and more focused light beam yields longer-range detection and higher-resolution imaging.

Medical Applications
Certain medical treatments like in dermatology, require laser pulses with relatively high energy. In that way, the radiation can be applied to relatively large areas on the skin, and reasonable processing speeds are achieved.

Scientific Research Various areas of physics require laser pulses of rather high energy. The requirements concerning not only the pulse energy, but also pulse duration, wavelength and other parameters differ very much between different applications. Some examples:

• The generation of hard ultraviolet light or X-rays by high harmonic generation requires femtosecond pulses with extremely high power and high beam quality.
• Other applications in high field physics have similar requirements with peak powers reaching multiple terawatts or even petawatts.
• Chemical analysis based on laser-induced breakdown spectroscopy (LIBS) can be done on a relatively large distance, if a sufficiently high pulse energy is available.
  
  

  High power diodes for medical applications

  BURST MODE LASER

  Burst-mode lasers generate a string of closely spaced short pulses in a lower repetition, achieving high repetition rate and high peak power simultaneously.
  Some applications of high-energy and high-peak-power burst-mode lasers include materials processing, precision surgery, high-resolution detection, high-power microwave generation, aerospace & defense applications and so on.

  A novel application in recent years for burst-mode lasers is high-power pulsed agile microwave generation.
  The frequency spectrum and peak power of the microwave depend on that of the laser pulse.

  In particular, a laser working in the burst-mode is beneficial to generate high-frequency (over GHz level), high-power and agile frequency microwave burst signals, for which some output characteristics need to be considered. For one thing, the even distribution of the burst envelope affects the effective microwave burst duration. For another, the intra-burst repetition rate needs to be tuned flexibly to yield a frequency adjustable microwave signal.
  Therefore, an approximate uniform envelope and tunable frequency for the burst-mode laser are required.

  Moreover, in order to adapt burst-mode lasers for other different application requirements, in addition to increasing the burst energy and peak power, it is necessary to promote the tuning performance in the time domain, including the repetition rate and duration of the intra-burst pulse.
  The fourth method is to use an electro-optical modulator (EOM) to modulate at a very fast rate, which could achieve the widest tuning range of ps–ns pulse duration and an over-GHz repetition rate, in theory.

  However, because of high insertion loss and the single-mode fiber input, it is difficult to amplify the weak signals from the EOM while maintaining the uniformity of the envelope.
  In recent studies and applications, it has been used a high-pulse-energy and high-peak-power allfiber burst-mode laser system whose repetition rate and duration of the intra-burst pulse are tunable. There are different schemes that can be used to obtain a burst envelop that is approximately uniform in the seven stages of the fiber amplifier.

  In this example we will analyze a double-pre-compensation structure made of an arbitrary waveform laser diode (LD) driver, an AOM, an EOM, three Single-Mode Amplifiers and four stages of multimode amplifier.

  

  Burst Mode Laser

  
  
  

  Figure 10 – Burst Mode Laser System

  A schematic diagram of the laser system setup is depicted in the Figure 10.

  The laser system is mainly segmented into five parts: a pulsed seed, three stages of single-mode pre-amplifier, a high-repetition-rate modulation part, a secondary pre-compensation modulation part and four stages of multi-mode amplifier.
  One arbitrary waveform LD driver is utilized to directly modulate the seed LD into the precompensated waveform with the repetition rate of 100 Hz.

  There are three stages for the single-mode pre-amplifiers, whose gain media are all highly doped single-mode Yb 3 + doped fibers (YDFs) with the absorption coefficient of 250 dB/m at 976 nm and pump sources are single-mode 976 nm LDs.

  In this application, the Arbitrary Waveform Generators are used for three main purposes:

  1. the Laser Driver generates a trigger signal for the AWG to synchronize the waveform signal that drive the Electro-optic modulator with the laser pulse. The AWG-7000 and AWG-5000 Series Arbitrary Waveform Generator are able to trigger an analog signal with an extremely low jitter, in the range of few pico-seconds.

  2. The high-repetition-rate modulation part includes an EOM with bandwidth of 5-10 GHz, and it needs to be controlled by an AWG that generates high-speed electric signals. The AWG-7000 and AWG-5000 can drive directly the EOM without needing to use an external amplifier. They can deliver high voltage amplitude pulses up to 5Vpp with fast rise/fall time up to 50 ps. Moreover they can generate signals up to 10 GHz.

  3. The secondary pre-compensation modulation part includes an AOM with bandwidth of 200 MHz or lower and another AWG is needed to control it.

  The AOM not only reforms the pulse shape to the pre-compensation waveform but also blocks the amplified spontaneous emission (ASE) pedestal from the pre-amplifiers in the time domain.
  The AWG-5000 and AWG-4000 Series can generate signals for the AOM that usually needs a lower frequency bandwidth in the range of 200 MHz.

  It is worth noting that the third stage of the single mode preamplifier is positioned after the EOM and before the AOM. This stage of the amplifier makes up the insertion loss and modulation of the EOM and provides the relatively high initial energy for the secondary pre-compensation of the AOM.

  This stage of the amplifier makes up the insertion loss and modulation of the EOM and provides the relatively high initial energy for the secondary pre-compensation of the AOM.

  An isolator/bandpass filter (IBP) with the 3 dB bandwidth of 2 nm is placed after the AOM. The multimode amplifiers in the system consist of four all-fiber double-clad amplifier stages, which are shown as the fourth to seventh amplifiers.
  The four stages of multi-mode amplifier boost the pulse energy to approximately mJ level and the output burst waveform is distributed approximately uniformly.

  

  Figure 11 – Burst Laser Waveforms

  The blue line and red line in Figure 11 illustrate the pre-compensation waveform from the AWG signal and the burst-mode pulse waveform after AOM modulation, respectively. The envelope of the burst-mode pulse generally resembles the pre-compensated waveform. Here, we obtain the precompensated burst-mode seed with a burst duration of approximately 60 ns.

  The burst-mode seed is introduced into four stages of the multi-mode amplifier to enhance the pulse energy. The maximum pulse energy achieves 13.3 mJ, which is the highest energy for an all-fiber GHz burst-mode pulse to our knowledge, and the calculated highest peak power reaches approximately 0.53 MW.

  The extraction efficiency is 31.4% at the maximum energy. The temporal shape evolution of the burst-mode pulse after the main amplifier is shown in Figure 11. The front part of the burst envelope lifts with the pump power increasing on account of the gain saturation effect, and the approximately uniform envelope is achieved at the maximum output energy, which indicates that the gain saturation effect is mitigated successfully by secondary pre-compensation. Because of the relatively slow rising and falling time of AOM modulation (~10 ns), it is unable to achieve a rapid edge for both sides of envelope.

  

  High Power PW Laser

  
  
  

  Figure 12: pulse shape during the pre-compenspesation process

  
  
  

  Figure 13: pulse generated with the Active Technologies AWG-7000 Series

  ANL AWG Laser Systems

  ANL AWG laser systems refer to high-energy Nd:YAG lasers that utilize an Arbitrary Waveform Generator (AWG) to shape the output pulses temporally. These lasers are designed for applications requiring precisely controlled pulse durations and waveforms, such as OPCPA pumping, plasma physics, and shock physics.

  The main laser feature is the ability to shape output pulses temporally which is accomplished by an electro-optical modulator driven by programmable arbitrary waveform generator (AWG).

  The front end of ANL AWG laser system is comprised of a single-mode CW laser which is amplified in a fiber amplifier in the next step. Later on, AWG driven modulator transmits pulses of required temporal shape and duration which are further amplified diode pumped regenerative amplifier or all-in-fiber amplifier in order to reach energies sufficient to amplify in single-pass diode and flash lamp pumped amplifiers. Pulse shaping resolution is 125 ps, while maximum pulse length is 500 ns. ANL series linear amplifiers are convenient solution for high energy nanosecond systems where pulses are amplified in a chain of flash lamp pumped amplification units up to required energy. During amplification spatial beam shaping is used in order to get a flat top shaped beam profile without hot spots at the system output.

  Angle-tuned non-linear crystals harmonic generators mounted in temperature stabilized heaters are used for second, third and fourth harmonic generation. Harmonic separation system is designed to ensure high spectral purity of radiation and direct it to the output ports.

An essential requirement in all those kind of appllication is the very low jitter in respect to an external trigger. Both the trigger signal and the reference clock are provided from the laser system and the trigger is locked to the reference clock.

A typical request regarding the jitter is <10ps RMS jitter between the external trigger and the output waveform.

Both the AWG-5000 and the AWG-7000 Series have a special feature called Low Jitter Trigger Mode that allow to minimize the jitter (less than 5ps) when the external trigger is synchronous with the reference clock signal.