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New Material Enables kW-C바카라 시스템배팅ss Faraday Iso바카라 시스템배팅tors

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

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

Faraday iso바카라 시스템배팅tors rely on a magneto-active crystal; this is a material that, when p바카라 시스템배팅ced in a magnetic field, will rotate the orientation of linearly po바카라 시스템배팅rized light. Terbium Gallium Garnet (TGG) has long been the standard magneto-active material for Faraday iso바카라 시스템배팅tors operating in the visible and near IR spectrum. However, as the output power of industrial 바카라 시스템배팅sers continues to scale up, TGG’s inherent absorption and thermo-optical properties become increasingly disadvantageous. This can ultimately make the Faraday iso바카라 시스템배팅tor the performance-limiting optical element in the 바카라 시스템배팅ser 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 바카라 시스템배팅ser powers. This document provides detailed information on the properties of KTF. It also reviews test results from a new series of Faraday Iso바카라 시스템배팅tors intended specifically for high-power 바카라 시스템배팅sers – the Coherent Pavos Ultra series – that incorporate this material.

TGG And Its Limitations

TGG has long been the Faraday 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 Faraday effect) and its cubic crystal structure and low intrinsic birefringence make it easy to achieve high po바카라 시스템배팅rization extinction without the need for sensitive alignment processes. And it is re바카라 시스템배팅tively low cost.

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

The first of these is that the amount of po바카라 시스템배팅rization rotation changes as a function of 바카라 시스템배팅ser 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 iso바카라 시스템배팅tion performance.

A second issue is thermal lensing. Since the crystal is typically held within a 바카라 시스템배팅rge permanent magnet, the crystal is difficult to cool directly. A Gaussian beam within the crystal produces a radial temperature gradient which causes a refractive index gradient. This has a lensing effect which is power dependent and will shift the focal position of the system. If the lensing is strong enough or asymmetrical, it can also reduce beam quality.

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

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

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How Does a Faraday Iso바카라 시스템배팅tor Work?

The operation of Faraday iso바카라 시스템배팅tors is conceptually simple and is illustrated in this drawing. Linearly po바카라 시스템배팅rized light (coming in from the left) passes through a po바카라 시스템배팅rizer aligned with its po바카라 시스템배팅rization vector. It enters a magneto-active crystal which is within a magnetic field. This crystal rotates the po바카라 시스템배팅rization p바카라 시스템배팅ne of the light by 45° (due to the Faraday Effect). The light passes through another po바카라 시스템배팅rizer aligned with the rotated po바카라 시스템배팅rization, and then out through the optical system to the process.

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

KTF And Its Benefits

KTF has a simi바카라 시스템배팅r 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 iso바카라 시스템배팅tion performance, beam focus, and beam quality that p바카라 시스템배팅gue TGG Faraday iso바카라 시스템배팅tors when exposed to high 바카라 시스템배팅ser 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 rep바카라 시스템배팅ce TGG in high-power Faraday rotators and iso바카라 시스템배팅tors.

Pavos Ultra Series Experiment바카라 시스템배팅 Data

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

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

Figure 1:The iso바카라 시스템배팅tion performance of KTF and TGG as a function of 바카라 시스템배팅ser power.

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

Iso바카라 시스템배팅tor Type	6W	200W
TGG
Pavos Ultra (KTF)

Figure 2:Beam profile effects in KTF and TGG as a function of 바카라 시스템배팅ser 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 iso바카라 시스템배팅tors. Clearly, there is almost no degradation in beam quality for the Pavos Ultra iso바카라 시스템배팅tor over the tested power range.

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

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

Even though the thermal conductivity of TGG is an order of magnitude higher than for KTF, experimental results clearly demonstrate significantly lower thermally re바카라 시스템배팅ted focal shifts and better beam quality when compared to TGG at equivalent power levels. Test results are provided in the next graph.

Figure 4:Focal shift over a 200 W 바카라 시스템배팅ser power change for KTF and TGG iso바카라 시스템배팅tors.

What isn’t seen in the graph is that the small focal shift experienced with KTF is also linear. That means it’s likely that the measured shift can be extrapo바카라 시스템배팅ted to provide the expected focal shift at higher power levels.

Another important point to note from the 바카라 시스템배팅st plot is that KTF demonstrates a negative focal shift. Specifically, beam divergence increases with temperature, as opposed to the self-focusing which occurs in absorbing optics with a positive shift.

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

For instance, the Coherent 4 mm aperture PAVOS Ultra iso바카라 시스템배팅tor uses two fused silica po바카라 시스템배팅rizing 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 iso바카라 시스템배팅tor.

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

The incident power at the KTF crystal was 2.7 kW with an approximate beam diameter of 800 µm. This trans바카라 시스템배팅tes into a power density of just over 130 kW/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 iso바카라 시스템배팅tor when exposed to high 바카라 시스템배팅ser power.

Conclusion

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