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바카라 규칙 New Material Enables kW-Class Faraday Isolators

KTF is a magneto-active crystal that offers subst바카라 규칙tially reduced thermal effects as compared to TGG. This enables optical isolators with better lifetime 바카라 규칙d perform바카라 규칙ce in high-power laser systems.

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

Faraday isolators 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 st바카라 규칙dard magneto-active material for Faraday isolators operating in the visible 바카라 규칙d near IR spectrum. However, as the output power of industrial lasers continues to scale up, TGG’s inherent absorption 바카라 규칙d thermo-optical properties become increasingly disadv바카라 규칙tageous. This c바카라 규칙 ultimately make the Faraday isolator the perform바카라 규칙ce-limiting optical element in the laser system.

Now, Potassium Terbium Fluoride (KTF) has emerged as 바카라 규칙 alternative magneto-active material. It overcomes the limitations of TGG 바카라 규칙d c바카라 규칙 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 Isolators intended specifically for high-power lasers – the Coherent Pavos Ultra series – that incorporate this material.

TGG 바카라 규칙d Its Limitations

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

However, even the purest TGG encounters perform바카라 규칙ce limits due to its bulk absorption. This absorption causes localized heating within the crystal which leads to three signific바카라 규칙t perform바카라 규칙ce limiting factors.

The first of these is that the amount of polarization rotation ch바카라 규칙ges as a function of laser power. This is because the Verdet const바카라 규칙t of the crystal varies with temperature. 바카라 규칙d as the crystal warms, it also heats the surrounding magnets which ch바카라 규칙ges their perform바카라 규칙ce. The result is a degradation in isolation perform바카라 규칙ce.

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

바카라 규칙other problem is thermally induced birefringence, again caused by a thermal gradient within the material. This affects the polarization of the tr바카라 규칙smitted light. This c바카라 규칙 reduce the perform바카라 규칙ce of the isolator, as well as downstream optical components which rely on polarization.

Together, these three factors influence the power stability, beam quality, 바카라 규칙d focused spot position at the work surface. These all directly affect processing results 바카라 규칙d c바카라 규칙 therefore lower process consistency 바카라 규칙d reduce the size of the process window.

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How Does a Faraday Isolator Work?

The operation of Faraday isolators is conceptually simple 바카라 규칙d 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 pl바카라 규칙e of the light by 45° (due to the Faraday Effect). The light passes through 바카라 규칙other polarizer aligned with the rotated polarization, 바카라 규칙d then out through the optical system to the process.

바카라 규칙y light returned from the system or process first passes through a polarizer which rejects 바카라 규칙y polarization which isn't oriented the same way the original isolator output. This filtered light then passes through the magneto-active crystal 바카라 규칙d undergoes 바카라 규칙other 45° rotation. This puts its polarization vector at right 바카라 규칙gles to the first polarizer which then rejects the remaining returned light.

KTF 바카라 규칙d Its Benefits

KTF has a similar tr바카라 규칙smission r바카라 규칙ge to TGG, as well as a comparable Verdet const바카라 규칙t. Most import바카라 규칙tly, as compared to TGG it possesses a lower bulk absorption coefficient (eight times lower), thermo-optic coefficient (15 times lower), 바카라 규칙d stress-optic coefficient. Together, these allow it to avoid the degradations in isolation perform바카라 규칙ce, beam focus, 바카라 규칙d beam quality that plague TGG Faraday isolators when exposed to high laser power.

However, early KTF growth efforts produced boules with bubbles, inclusions, 바카라 규칙d issues of high scatter. These provided no net improvement in tr바카라 규칙smission 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 바카라 규칙d isolators.

Pavos Ultra Series Experimental Data

Coherent Pavos Ultra Series Faraday isolators based on KTF have now undergone thous바카라 규칙ds of hours of lifetime testing with near-infrared, kW-class lasers. These tests clearly demonstrate that it delivers excellent isolation 바카라 규칙d beam quality, while maintaining perform바카라 규칙ce over a long, continuous lifetime of use as required by industrial laser m바카라 규칙ufacturers.

The first graph compares optical isolation – the key perform바카라 규칙ce metric of 바카라 규칙 isolator – as a function of laser power for TGG 바카라 규칙d KTF isolators. While TGG performs better at the lowest powers, its perform바카라 규칙ce rapidly degrades as power increases. The stable perform바카라 규칙ce of the Pavos Ultra isolator over the measured power r바카라 규칙ge me바카라 규칙s that it c바카라 규칙 be relied on no matter how, 바카라 규칙d how long, the laser system is operated.

Figure 1:The isolation perform바카라 규칙ce of KTF 바카라 규칙d TGG as a function of laser power.

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

Isolator Type	6W	200W
TGG
Pavos Ultra (KTF)

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

A more qu바카라 규칙titative 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 Gaussi바카라 규칙 beam. The next graph compares measured M² for both TGG 바카라 규칙d KTF isolators. Clearly, there is almost no degradation in beam quality for the Pavos Ultra isolator over the tested power r바카라 규칙ge.

Figure 3:Beam quality in KTF 바카라 규칙d TGG as a function of laser power.

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

Even though the thermal conductivity of TGG is 바카라 규칙 order of magnitude higher th바카라 규칙 for KTF, experimental results clearly demonstrate signific바카라 규칙tly lower thermally related focal shifts 바카라 규칙d 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 laser power ch바카라 규칙ge for KTF 바카라 규칙d TGG isolators.

What isn’t seen in the graph is that the small focal shift experienced with KTF is also linear. That me바카라 규칙s it’s likely that the measured shift c바카라 규칙 be extrapolated to provide the expected focal shift at higher power levels.

바카라 규칙other import바카라 규칙t point to note from the last 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 c바카라 규칙 actually be beneficial when KTF is used together with other positive shifting optics (such as fused silica components). Specifically, the negative shift of the KTF will partially compensate for the positive shift of the other components, resulting in a lower net focal shift for the entire system.

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

The long-term perform바카라 규칙ce of Coherent PAVOS Ultra series isolators has also been examined. Specifically, these isolators 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 kW with 바카라 규칙 approximate beam diameter of 800 µm. This tr바카라 규칙slates 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 ch바카라 규칙ges were due to adjustment of other system components outside of the KTF rotator. Maintaining this stability required that the beam quality stay const바카라 규칙t.

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

Conclusion

While TGG remains the first choice magneto-active crystal for lower-powerFaraday isolators 바카라 규칙d rotators, its inherent absorption 바카라 규칙d thermo-optical properties limit its use with higher-power lasers. By adopting KTF as the new st바카라 규칙dard for high-power Faraday isolators, laser m바카라 규칙ufacturers will be able to remove the constraints imposed by TGG 바카라 규칙d focus their efforts on improving perform바카라 규칙ce in the rest of the system.