One of several techniques available for modulating the output beam of a laser is Q-switching. Modulation can be done using either electro-optic (E-O) or acousto-optic (A-O) materials (see Laser Focus World, May , p. 127). Previous discussions on electro-optic modulation touched briefly on E-O Q-switching. This article will look at acousto-optic methods of Q-switching. The traditional markets for these switches are flashlamp-pumped solid-state lasers, but the growth today is mainly in the various applications for Q-switched diode-pumped solid-state (DPSS) lasers.
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This Product Focus briefly reviews the theory of Q-switching. Many versions of A-O Q-switches are available commercially, and custom-designed solutions are also possible (see Fig. 1). There are important criteria involved in choosing an A-O Q-switch, and these will be reviewed. The article also covers some of the uses of Q-switches, looking at solid-state lasers, both diode-pumped and flashlamp-pumped systems. A representative listing of suppliers of acousto-optic Q-switches follows on p. 151. A more comprehensive inventory is available in the Laser Focus World Buyers Guide, beginning on p. 282.
In Q-switching, the energy is stored in the population inversion of the lasing medium, building up in the laser cavity until the Q-switch is activated. Once activated, the stored energy is then released in a single pulse. There are several types of Q-switches, including A-O, E-O, mechanical, and dye. An A-O Q-switch consists of a block of optical material that is transparent at the desired lasing frequency. Quartz, fused silica (SiO2), flint glass, and tellurium dioxide are all materials that have been used commercially for Q-switches. Special uses for some of these materials will be reviewed later.
A piezoelectric transducer is bonded to the side of the optical block. The transducer material is usually a crystalline material such as lithium niobate. The bonding can be done by epoxy or vacuum metallic bonding. The acoustical signal is generated by the radio-frequency (RF) driver. When the signal is generated, it creates a sound wave through the medium, acting as a disturbance to the incoming beam. The beam is defracted in a predictable pattern out of the laser cavity, reducing the quality, or "Q," of the resonator, allowing the energy to build up. When the sound wave stops, the beam is no longer diffracted. Then the energy escapes the laser in a single pulse.
Selection of a particular type of Q-switch is dependent upon the type of laser, its characteristics, and performance parameters. The first criterion is that the laser be a type that can be Q-switched. Only lasers with an upper-state lifetime that is long enough to prevent spontaneous energy emissions can be Q-switched. These are solid-state lasers, generally Nd:YAG. Other lasers that can be Q-switched include the traditional ruby and glass as well as newer crystalline materials such as neodymium-doped vanadate (Nd:YVO4), neodymium-doped yttrium lithium fluoride (Nd:YLF), and holmium. Gas lasers, such as CO2 or ion lasers, are not usually Q-switched.
The second criterion for using an A-O Q-switch is that the laser must be a low-gain laser. The diffraction pattern generated by the acousto-optic switch does not remove all of the light from the cavity. If the laser gain is great enough, then even a small amount of feedback can override the Q-switch, causing the laser to lase. Once it is clear that an A-O Q-switch is both possible and desirable, then other criteria come into play. These include whether the beam is multimode or single-mode, polarized or unpolarized, and how divergent the beam is.
Different Q-switch designs are available to accommodate each choice. Diode-pumped solid-state lasers are treated differently than flashlamp-pumped systems, because of the smaller beam diameter, the higher gain, and the tight packaging requirements.
Intended usage of the laser/Q-switch system is another important criterion. Some systems will be used in industrial applications 24 hours a day, seven days a week. These systems must be reliable and durable. In many cases, they must also be able to handle high laser power. Other applications are in research laboratories, where continuous, demanding usage may not be as significant a concern.
Next come the actual performance specifications of the Q-switch. The user should be aware that an A-O Q-switch allows for a much lower insertion loss but can accommodate a much lower gain than an E-O Q-switch. It is important to convey to the manufacturer how much power the Q-switch will have to accommodate. Cooling methods are also a consideration because some systems require water cooling to remove the excess heat, while for others air cooling is sufficient. There are also choices as to whether the optics have an antireflection coating or if they are mounted at Brewster`s angle for minimal reflection. Damage thresholds of the coatings also affect performance and must be considered for high-power systems.
The design and performance of the RF driver should merit some consideration as well. If the driver has CE approval (certified for sale in the European Union), then it will most likely meet shielding standards for emissions at the primary and harmonic electronic frequencies. Some drivers provide a level of diagnostics, informing users when the temperature has gone too high or when the power levels are outside of acceptable ranges. Key features are the amount of RF power required to drive the Q-switch and the risetime/ falltime of the RF pulse. In some designs, the RF driver is integrated with the Q-switch, reducing both cost and space requirements while improving performance.
The most common use for the acousto-optic Q-switch is still the flashlamp-pumped Nd:YAG system. As much as 80% of the A-O Q-switches sold are for this use, either with new lasers or as replacement parts for existing systems. Many of these systems are for industrial applications, that is, cutting, trimming, and machining of metals including, for example, marking parts in an automotive assembly plant. The lasers are in operation for three shifts a day, every day. The Q-switches are in dirty environments; it is difficult to keep the lasers sealed and clean, however, the Q-switches do not fail in a mechanical or electrical manner. The debris and dust in the environment are burnt onto the optics, rendering the switch useless. These shops will usually replace the Q-switch at the same time that they replace damaged optics, because the engineer or service person has already made the trip, and the system is down.
In DPSS systems, A-O Q-switched lasers have the predominant market share for pulsed products. Commercial electronics applications include precision micromachining and thin-film trimming. The trimming applications include the well-established memory repair, as well as trimming the sensor for air bags and gold coatings on quartz watches. Newer applications that have been receiving much interest include laser texturizing of magnetic recording disks and rapid prototyping for model building (see Fig. 2). The rapid-prototyping application requires a frequency-tripled, Q-switched diode-pumped Nd:YVO4 laser, run at such high repetition rates, 20-40 kHz, that the liquid polymer reacts to the pulsed laser as if it were the output from a CW source.In DPSS systems, A-O-switched lasers have the predominant market share for pulsed products. Commercial electronics applications include precision micromachining and thin-film trimming. The trimming applications include the well-established memory repair, as well as trimming the sensor for air bags and gold coatings on quartz watches. Newer applications that have been receiving much interest include laser texturizing of magnetic recording disks and rapid prototyping for model building (see Fig. 2). The rapid-prototyping application requires a frequency-tripled,-switched diode-pumped Nd:YVOlaser, run at such high repetition rates, 20-40 kHz, that the liquid polymer reacts to the pulsed laser as if it were the output from a CW source.
Published: April 28,
Optical modulators are key building blocks for many optical systems and functions, including data transmission, laser printing, Q-switching, active-mode locking, shifting beam spectral frequency and much more. In this article we will discuss the two major modulator types for laser Q-switching: electro-optic or acousto-optic modulators. While both allow for producing high-power, ultra-short laser pulses, system designers should know the strong and weak points of each technology to choose the best option for their application.
Q-switching is a common technique to produce ultra-short laser pulses. It involves increasing resonator loss with the use of a modulator, which leads to increase of accumulated electron population difference in the lasing medium as the pump is continuously delivering power. When the modulator is set to &#;open&#; mode, the resonator quality factor Q rapidly increases which results in a high-intense, ultra-short laser pulse. Not every laser can be Q-switched and the technique is mostly used with Nd:YAG and other solid state lasers, while gas lasers usually depend on other solutions.
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Fig. 1. Optical modulators &#; Pockels cells
While it is possible to build Q-switches based on mechanical or dye modulators, the acousto-optic and electro-optic options provide the best performance in terms of possible on/off frequencies and resulting output power. The ease of transforming signal from electrical to optical form is another factor which contributes to popularity of these devices.
Acousto-optic modulators take advantage of the phenomena in which an acoustic wave propagating in an optical medium produces a periodic modulation of the optical medium refractive index. The resulting phase grating leads to diffraction of incident light and the effect is used for spatial, temporal, and spectral light modulation. The structure of acousto-optic modulator consists of a block of optical material, such as Quartz, fused silica, flint glass or tellurium dioxide, with an attached piezoelectric transducer to its side. A radio-frequency driver generates acoustic signal which scatters passing beam in a predictable manner. The amount of light scattered is dependent on the amplitude of the piezo-induced sound wave within the optical material.
The ongoing technological advancements in terms of materials and broadband transducers have made acousto-optic modulators a reliable technology for laser beam control. The traditional application is in high-power Nd:YAG systems in industrial and military applications.
While acousto-optic modulators offer low insertion loss, they generally provide lower gain compared to electro-optic modulators. The frequency of operation ranges from 27 MHz to 1 GHz, which means they are less fit for high bandwidth applications. However, acousto-optic devices usually have an advantage of not requiring high driving voltage and are well suited for broader wavelength spectrum in IR, VIS & UV range.
Electro-optic modulators (EOM) are more flexible option in terms of light parameters they can modulate &#; they allow to control the amplitude, polarization or position of an optical beam. Electro-optic modulators provide a backbone to modern communication technologies, analog and digital processing, optical sensing and many more. What makes electro-optical modulators stand out from its acousto-optic or mechanical counterparts is much higher frequencies, in GHz range. This makes it a preferable solution for high-speed optical communication as it allows for data-transfer rates that are not matched by other modulation techniques. EOM-based Q-switching solutions offer high speed and high gain, although usually higher voltage is required compared to acousto-optic modulators. Hence the most common use is in the visible spectrum (as the half-voltage increases with wavelength). However Pockels cells for infrared (up to far IR) are also available on the market.
The electro-optic phenomena describes the change in materials&#; anisotropic optical properties under the external electric field. There are several technologies that make use of an electro-optic effect to modulate the light wave. The common principle is the ability to modulate optical properties of a material, e.g. index of refraction or natural birefringence, in a controlled manner by applying an external voltage.
Pockels Cells Optical Modulators
The linear electro-optic effect, known as Pockels effect, can be observed in crystals lacking a center of symmetry where birefringence is produced in a material proportionally to the applied electric field. The most common materials for Pockels cells are electro-optic crystals such as di-deuterium phosphate (KDDP), barium borate (BBO), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and ammonium dihydrogen phosphate (NH4H2PO4, ADP). Another alternative for a Pockels cell crystal is rubidium titanyl phosphate (RbTiOPO, RTP), giving an RTP Pockels cell, oftentimes used in neodynium lasers, such as Nd:YAG.
Pockels cells in combination with polarizers can be used as amplitude and phase modulators with the advantages of low drive voltages, low insertion loss and ability to handle high optical powers. The device is capable of reaching modulation frequencies of gigahertz range and can serve as shutter with response time of less than 1 nanosecond.
A sole Pockels cell itself can serve as a phase modulator, as it has a capability to change phase of the passing light when it travels in a direction of one of its optical axes without affecting its polarization. A common setup for light intensity modulation is a Pockels cell placed between two polarizers perpendicular to each other. At zero voltage the crystal does not change light polarization, causing the beam to be fully blocked by the second polarizer. Applying voltage induces a birefringence in the crystal, which in turn changes light polarization from linear to elliptical and the light can pass through the second polarizer. The amount of light passing depends on applied voltage, and specifically at the half-wave voltage the modulator in in open shutter mode transferring all the light.
Kerr Cells Optical Modulators
A similar phenomena is called quadratic electro-optic effect, also known as Kerr effect. Kerr effectcan be observed in practically all materials and describes a change in magnitude of the birefringence which is proportional to the square of the electric field.
This flexibility in a choice of material allows designers to align device to the specific needs of the target application, especially when there is a need to handle high optical powers. The Kerr modulators can operate with frequencies in a range of tens of gigahertz with driving voltages of up to tens of kilovolts. A Kerr cell placed between two crossed polarizers is a simple light intensity modulator, or shutter, that allows for light to pass through depending on the applied voltage.
In the article we summarized main optical modulators used for Q-switching, i.e. acousto-optic modulators and electro-optic modulators (Pockels cells, Kerr cells) as well as related design considerations for selecting either of the technologies. Naturally a selection is always a matter of a specific application and technical boundaries.
Solaris Optics manufactures Pockels cells (di-deuterium phosphate KDDP and lithium niobate LiNbO3) in our facilities in Poland. Should you have questions regarding optic modulators, please do not hesitate to contact us!
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