composite laser crystals (original) (raw)
Definition: laser crystals consisting of several parts of different materials or with different chemical compositions (e.g. doping concentrations)
Alternative terms: hybrid laser crystals, bonded laser crystals
Categories: 
optical materials, 
laser devices and laser physics
- laser gain media
- laser crystals
* laser rods
* composite laser crystals
* neodymium-doped crystals
* ytterbium-doped crystals
* erbium-doped crystals
* thulium-doped crystals
* holmium-doped crystals
* praseodymium-doped crystals
* chromium-doped crystals
* titanium-doped crystals
* (more topics)
- laser crystals
Related: laser crystalslaser gain mediaceramic laser gain medianeodymium-doped laser gain mediaytterbium-doped laser gain mediathermal lensinghigh-power lasers
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DOI: 10.61835/30j Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
What are Composite Laser Crystals?
Composite laser crystals (sometimes called hybrid laser crystals) are laser crystals which have been fabricated by combining different parts. Often, one combines parts with and without a laser-active dopant, or with different dopant concentrations, to achieve certain advantages in a laser design.
Typically, adhesive-free diffusion bonding of carefully prepared crystal surfaces is used, e.g., to combine an Nd:YAG or Yb:YAG crystal with an undoped YAG crystal. The same can be done e.g. with Nd:YVO4. Another possibility is to bond a Cr:YAG crystal (a saturable absorber material for passive Q-switching) to Nd:YAG.
In other cases, a nonlinear crystal material for nonlinear frequency conversion is bonded to a laser crystal.
Composite gain media can also be made of ceramics. The fabrication techniques for ceramics introduce a lot of freedom for composite structures, including doping gradients. It is also possible to combine single crystals and ceramics, e.g. to grow undoped ceramic around a doped single crystal.
The optical quality of bonded interfaces is essential. Different processes have been developed for obtaining high-quality bonds. Some of these operate at high temperatures, while others can be performed also at room temperature. One may, for example, use irradiation with high-energy ions in a vacuum to remove any disturbing surface layers before bonding. In any case, preparing very flat surfaces is essential.
Examples of Using Composite Laser Crystals
In the following, some examples of the use of composite gain media are given:
- Undoped end caps on a short laser rod can reduce thermal effects by extracting some of the heat through the endfaces of the doped part. This reduces the peak temperature, the tendency for thermal fracture, and (for several reasons) the strength of thermal lensing. Such crystals improve e.g. the performance of quasi-three-level neodymium lasers, such as Nd:YAG operated at 946 nm, where the heat generation is substantially stronger than for the usual 1064-nm wavelength. (While the quantum defect is smaller, high pump intensities need to be applied, and the high excitation level leads to increased losses by quenching effects.)
- With an undoped cap on the disk of a thin-disk laser, power scalability can be maintained up to very high power levels. This is because the undoped cap helps to suppress amplified spontaneous emission (ASE) and parasitic lasing, if it has a similar refractive index. At the same time, the undoped cap can provide additional mechanical stabilization, avoiding stress fracture and also making the handling easier.
- When plates with different doping concentrations are bonded together (multi-segmented rods), the density of absorbed pump power (which normally decays rapidly in the pumping direction) can be smoothed. This leads to a more uniform temperature rise in an end-pumped laser, particularly when the crystal is pumped from one side only. With suitable laser designs, this feature can be converted into improved overall performance, in particular for a higher output power, power efficiency, and beam quality.
- Another approach is to use a core-doped rod, where pump light is absorbed only in the region covered by the laser beam. This is suitable also for side pumping of lasers, as it avoids pumping regions of the crystal which cannot be accessed by the lasing modes. It may therefore be possible to obtain enhanced efficiency and possibly a better beam quality. The doped part may even act as a waveguide, as the doping often somewhat increases the refractive index.
- In a composite crystal with different dopants, one part can act as the gain medium and the other one as a saturable absorber for passive Q-switching. Such parts are used in, e.g., passively Q-switched microchip lasers.
- In other situations, undoped end caps which are properly shaped (e.g. conically) can act as ducts for pump radiation.
- In some single-frequency ring lasers, an undoped section at a point of beam reflection can eliminate spatial hole burning effects, if it is thick enough to avoid interference in the doped section.
Frequently Asked Questions
What is a composite laser crystal?
A composite laser crystal, also called a hybrid laser crystal, is a gain medium fabricated by combining different parts. This often involves joining a laser-active (doped) section with an undoped section or with parts having different dopant levels.
How are composite laser crystals fabricated?
They are typically made using a technique called adhesive-free diffusion bonding. This process requires extremely flat and clean crystal surfaces to create a high-quality optical interface between the different parts.
Why are undoped end caps added to laser crystals?
Undoped end caps are used for thermal management. They help extract heat from the ends of the doped crystal section, which reduces the peak temperature, the risk of thermal fracture, and the strength of thermal lensing.
How can composite crystals improve laser pumping?
A multi-segmented rod with varying dopant concentrations can create a more uniform pump absorption profile. Alternatively, a core-doped rod confines pump absorption to the area used by the laser beam, improving efficiency.
Can composite crystals be used for Q-switched lasers?
Yes. A gain medium like Nd:YAG can be bonded directly to a saturable absorber material such as Cr:YAG. This creates a monolithic component for passively Q-switched lasers, particularly microchip lasers.
Are composite gain media made only from single crystals?
No, composite gain media can also be made from glasses and laser ceramics. Ceramic fabrication techniques offer significant flexibility for creating complex composite structures, including those with doping gradients.
Suppliers
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The composite (or diffusion-bonded) laser crystals consist of two crystals that are fixed together to form a monolithic design. The most popular combination includes Nd3+:YAG as the laser medium and Cr4+:YAG as the passive Q-switch. Most of the composite laser crystals like Nd3+:YAG + Cr4+:YAG, Yb3+:YAG + Cr4+:YAG, Nd3+:YAG + V3+:YAG, Er3+:YAG + YAG, Nd3+:YVO4 + YVO4 are available from stock or with customer’s specifications with AR, PR or HR coatings.
⚙ hardware🧩 accessories and parts🧴 consumables🔧 maintenance, repair📏 metrology, calibration, testing💡 consulting🧰 development
Diffusion-bonded laser crystals are composite laser crystals consisting of two, three, or more parts crystals with different doping compositions, where the constituent parts are combined through a technique called diffusion bonding. Diffusion bonding laser crystals are usually applied to decrease thermal lensing effects, improve beam quality, and make compact laser modules.
Shalom EO offers diffusion-bonded crystals with copious composition options of combining YAG, Nd:YAG, Cr:YAG, Nd:Ce:YAG, YVO4, Nd:YVO4. A diameter range of 2–18 mm and a length range of 2–200 mm is available. Doping percentages and coatings could be tailored upon request.
Bibliography
| [1] | R. Zhou et a l., “Continuous-wave, 15.2 W diode-end-pumped Nd:YAG laser operating at 946 nm”, Opt. Lett. 31 (12), 1869 (2006); doi:10.1364/OL.31.001869 |
|---|---|
| [2] | R. Wilhelm et al., “Power scaling of end-pumped solid-state rod lasers by longitudinal dopant concentration gradients”, IEEE J. Quantum Electron. 44 (3), 232 (2008); doi:10.1109/JQE.2007.911702 |
| [3] | Y. T. Chang et al., “Comparison of thermal lensing effects between single-end and double-end diffusion-bonded Nd:YVO4 crystals for 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions”, Opt. Express 16 (25), 21155 (2008); doi:10.1364/OE.16.021155 |
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