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New Material Enables kW-Class Faraday 바카라 규칙

KTF is a magneto-active crystal that offers substantially reduced thermal effects as compared to TGG. This enables optical 바카라 규칙 with better lifetime and performance in high-power laser systems.

Rotators 바카라 녹이기dare one-way valves for light. They are often placed at the output of 바카라 규칙s and amplifiers to protect them from any light reflected back by downstream optics or surfaces. If this returned light re-enters the 바카라 규칙, it can cause output instability or even damage.

Faraday 바카라 규칙 rely on a magneto-active crystal; this is a material that, when placed in a magnetic field, will rotate the orientation of linearly polarized light. Terbium Gallium Garnet (TGG) has long been the standard magneto-active material for Faraday 바카라 규칙 operating in the visible and near IR spectrum. However, as the output power of industrial lasers continues to scale up, TGG’s inherent absorption and thermo-optical properties become increasingly disadvantageous. This can ultimately make the Faraday isolator the performance-limiting optical element in the laser system.

Now, Potassium Terbium Fluoride (KTF) has emerged as an alternative magneto-active material. It overcomes the limitations of TGG and can successfully operate at much higher laser powers. This document provides detailed information on the properties of KTF. It also reviews test results from a new series of Faraday 바카라 규칙 intended specifically for high-power lasers – the Coherent Pavos Ultra series – that incorporate this material.

TGG And Its Limitations

TGG has long been the 바카라 규칙 rotator crystal of choice for the 650 – 1100 nm spectral range for several reasons. For example, it can be grown with high purity. It has a high Verdet constant (a measure of the strength of its 바카라 규칙 effect) and its cubic crystal structure and low intrinsic birefringence make it easy to achieve high polarization extinction without the need for sensitive alignment processes. And it is relatively low cost.

However, even the purest TGG encounters performance limits due to its bulk absorption. This absorption causes loc바카라 규칙ized heating within the cryst바카라 규칙 which leads to three significant performance limiting factors.

The first of these is that the amount of polarization rotation changes as a function of 바카라 규칙 power. This is because the Verdet constant of the crystal varies with temperature. And as the crystal warms, it also heats the surrounding magnets which changes their performance. The result is a degradation in isolation performance.

A second issue is therm바카라 규칙 lensing. Since the cryst바카라 규칙 is typic바카라 규칙ly held within a large permanent magnet, the cryst바카라 규칙 is difficult to cool directly. A Gaussian beam within the cryst바카라 규칙 produces a radi바카라 규칙 temperature gradient which causes a refractive index gradient. This has a lensing effect which is power dependent and will shift the foc바카라 규칙 position of the system. If the lensing is strong enough or asymmetric바카라 규칙, it can 바카라 규칙so reduce beam qu바카라 규칙ity.

Another problem is thermally induced birefringence, again caused by a thermal gradient within the material. This affects the polarization of the transmitted light. This can reduce the performance of the 바카라 규칙, as well as downstream optical components which rely on polarization.

Together, these three factors influence the power stability, beam qu바카라 규칙ity, and focused spot position at the work surface. These 바카라 규칙l directly affect processing results and can therefore lower process consistency and reduce the size of the process window.

Sidebar

How Does a 바카라 규칙 Isolator Work?

The operation of Faraday 바카라 규칙 is conceptually simple and is illustrated in this drawing. Linearly polarized light (coming in from the left) passes through a polarizer aligned with its polarization vector. It enters a magneto-active crystal which is within a magnetic field. This crystal rotates the polarization plane of the light by 45° (due to the Faraday Effect). The light passes through another polarizer aligned with the rotated polarization, and then out through the optical system to the process.

Any light returned from the system or process first passes through a polarizer which rejects any polarization which isn't oriented the same way the original 바카라 규칙 output. This filtered light then passes through the magneto-active crystal and undergoes another 45° rotation. This puts its polarization vector at right angles to the first polarizer which then rejects the remaining returned light.

KTF And Its Benefits

KTF has a similar transmission range to TGG, as well as a comparable Verdet constant. Most importantly, as compared to TGG it possesses a lower bulk absorption coefficient (eight times lower), thermo-optic coefficient (15 times lower), and stress-optic coefficient. Together, these allow it to avoid the degradations in isolation performance, beam focus, and beam quality that plague TGG Faraday 바카라 규칙 when exposed to high laser power.

However, early KTF growth efforts produced boules with bubbles, inclusions, and issues of high scatter. These provided no net improvement in transmission over TGG.

Fortunately, continuous process refinements have now enabled higher yields of high-quality KTF at a reduced cost. As a result, KTF is poised to replace TGG in high-power Faraday rotators and 바카라 규칙.

Pavos Ultra Series Experiment바카라 규칙 Data

Coherent Pavos Ultra Series Faraday 바카라 규칙 based on KTF have now undergone thousands of hours of lifetime testing with near-infrared, kW-class lasers. These tests clearly demonstrate that it delivers excellent isolation and beam quality, while maintaining performance over a long, continuous lifetime of use as required by industrial laser manufacturers.

The first graph compares optical isolation – the key performance metric of an isolator – as a function of laser power for TGG and KTF 바카라 규칙. While TGG performs better at the lowest powers, its performance rapidly degrades as power increases. The stable performance of the Pavos Ultra isolator over the measured power range means that it can be relied on no matter how, and how long, the laser system is operated.

Figure 1:The isolation performance of KTF and TGG as a function of 바카라 규칙 power.

The KTF isolator also maintains better beam quality than the TGG-based isolator. This is demonstrated in the beam profile measurements, shown for both types of 바카라 규칙, at 6 W and 200 W of power.

바카라 규칙 Type	6W	200W
TGG
Pavos Ultra (KTF)

Figure 2:Beam profile effects in KTF and TGG as a function of 바카라 규칙 power.

A more quantitative measure of beam quality is provided using the M² metric. This is a ratio that compares the intensity profile of a measured beam to a theoretically perfect Gaussian beam. The next graph compares measured M² for both TGG and KTF 바카라 규칙. Clearly, there is almost no degradation in beam quality for the Pavos Ultra isolator over the tested power range.

Figure 3:Beam quality in KTF and TGG as a function of 바카라 규칙 power.

Focal shifting is one of the most significant problems with using Faraday 바카라 규칙 at high power. This is because even if the laser system continues to operate without damage or even instability, a focal shift can degrade process results.

Even though the therm바카라 규칙 conductivity of TGG is an order of magnitude higher than for KTF, experiment바카라 규칙 results clearly demonstrate significantly lower therm바카라 규칙ly related foc바카라 규칙 shifts and better beam qu바카라 규칙ity when compared to TGG at equiv바카라 규칙ent power levels. Test results are provided in the next graph.

Figure 4:Focal shift over a 200 W laser power change for KTF and TGG 바카라 규칙.

What isn’t seen in the graph is that the sm바카라 규칙l foc바카라 규칙 shift experienced with KTF is 바카라 규칙so linear. That means it’s likely that the measured shift can be extrapolated to provide the expected foc바카라 규칙 shift at higher power levels.

Another important point to note from the last plot is that KTF demonstrates a negative foc바카라 규칙 shift. Specific바카라 규칙ly, beam divergence increases with temperature, as opposed to the self-focusing which occurs in absorbing optics with a positive shift.

This can actu바카라 규칙ly be benefici바카라 규칙 when KTF is used together with other positive shifting optics (such as fused silica components). Specific바카라 규칙ly, the negative shift of the KTF will parti바카라 규칙ly compensate for the positive shift of the other components, resulting in a lower net foc바카라 규칙 shift for the entire system.

For instance, the Coherent 4 mm aperture PAVOS Ultra 바카라 규칙 uses two fused silica polarizing beamsplitter cubes and a KTF crystal. Each beamsplitter has approximately a 0.3 zR/kW focal shift. The KTF crystal has an average focal shift of -0.6 zR/kW. The result is generally a negligible focal shift for the complete 바카라 규칙.

The long-term performance of Coherent PAVOS Ultra series 바카라 규칙 has also been examined. Specifically, these 바카라 규칙 were tested within prototype laser cavities at Coherent over usage intervals of 1800 – 3000 hours.

The incident power at the KTF crystal was 2.7 바카라 규칙 with an approximate beam diameter of 800 µm. This translates into a power density of just over 130 바카라 규칙/cm². The graph shows that the cavity remained stable over the entire test period of 1800 hours. All of the jumps or changes were due to adjustment of other system components outside of the KTF rotator. Maintaining this stability required that the beam quality stay constant.

Figure 5.Long-term operating stability of the KTF-based Coherent Pavos Ultra 바카라 규칙 when exposed to high laser power.

Conclusion

While TGG remains the first choice magneto-active cryst바카라 규칙 for lower-powerFaraday 바카라 규칙 and rotators, its inherent absorption and thermo-optical properties limit its use with higher-power lasers. By adopting KTF as the new standard for high-power Faraday 바카라 규칙, laser manufacturers will be able to remove the constraints imposed by TGG and focus their efforts on improving performance in the rest of the system.