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Helping Ultrafast Systems Keep their Cool

 

Thermal lensing threatens to render some high-power ultrafast systems useless

 

There are limitations to making gain media less sensitive to thermal effects

 

Intracavity optics can be designed to minimize thermal lensing

 

Recent advances in highly-dispersive mirrors help solve this issue

The short pulse durations and high peak powers of ultrafast lasers make them ideal for a wide range of applications from materials processing, to medical lasers, to nonlinear imaging and microscopy. However, ultrafast lasers are particularly sensitive to thermal lensing, where heat causes either a gain medium or intracavity optics to deform or exhibit a refractive index gradient. This can be detrimental for ultrafast systems, even preventing them from mode-locking to produce a pulsed beam. Thankfully, advances in highly-dispersive intracavity mirrors from Edmund Optics’ partner UltraFast Innovations (UFI) enable the development of optics that experience negligible thermal effects.

 Why is Thermal Lensing a Hot Topic?

Thermal lensing can occur if an active gain medium is hotter along the beam axis compared to the rest of the medium, resulting in a transverse refractive index gradient. This can misalign the laser cavity and lead to different laser mode profiles and drifts in beam pointing. Intracavity mirrors are also a critical part of ultrafast oscillators, and thermal effects can distort them as well. When focusing a beam in a laser crystal, changes in a mirror’s radius of curvature can change the focus position and even prevent the ultrafast laser from mode-locking, therefore rendering it useless. While not much can be done to change the inherent thermal properties of a gain medium, there is an opportunity for carefully choosing the right intracavity mirrors to prevent thermal lensing.

 The Solution: Specialized Highly-Dispersive Mirrors

Recent developments in coating design have allowed for the development of low-loss, highly-dispersive mirrors with negligible thermal effects. This is achieved by careful manipulation of the different parameters of the optical coatings. In addition to thermal stability, the mirrors still need to provide high reflectivity and a negative group delay dispersion (GDD) to compensate for the positive GDD present in most optical media. Figure 1 shows the reflectivity and GDD of #17-070 (or UFI part number HD64), a highly-dispersive mirror with negligible thermal lensing designed for use with 1030nm ultrafast lasers. These mirrors are incredibly beneficial for high-power, solid-state lasers such as Yb:YAG, Nd:YAG, holmium, and thulium lasers.

Spectral and GDD performance of #17-070, or UltraFast Innovations part number HD64, a 1030nm highly-dispersive mirror with negligible thermal lensing
Figure 1: Spectral and GDD performance of #17-070, or UltraFast Innovations part number HD64, a 1030nm highly-dispersive mirror with negligible thermal lensing

Testing Thermal Performance

These new highly-dispersive mirrors were tested to characterize their level of thermal lensing. An infrared camera (FLIR SC305) was used to monitor the temperature of intracavity mirrors inside a Yb:YAG thin-disk laser in continuous wave (CW) operation. A typical highly-reflective mirror with a GDD of -3,000 fs2 without the new thermal lensing-reducing technology experienced a temperature rise of >50K (Figure 2). This caused the laser mode and oscillator stability to deteriorate. However, highly-dispersive mirrors with GDDs of -1,000 fs2 and -3,000 fs2 were tested in the same setup and experienced temperature changes of 10K and 20K, respectively (Figures 3 and 4). These mirrors showed no noticeable thermally-induced effects on mode or oscillator stability, and the laser was able to function as intended.

Stemmed Mirrors were shown to maintain surface flatness better than conventional mirrors by a factor of 2.
Figure 2: The highly-reflective mirror without new, low-thermal lensing coatings experienced a change in temperature of 57K, leading to a deterioration of system performance.
Stemmed Mirrors were shown to maintain surface flatness better than conventional mirrors by a factor of 2.
Figure 3: The low thermal lensing mirror with a GDD of -1,000 fs2 experienced a temperature change of 10K and did not suffer any detectable thermally-induced deterioration of performance.
Stemmed Mirrors were shown to maintain surface flatness better than conventional mirrors by a factor of 2.
Figure 4: The low thermal lensing mirror with a GDD of -3,000 fs2 experienced a temperature change of 20K and did not suffer any detectable thermally-induced deterioration of performance.

Low Thermal Lensing Ultrafast Mirrors at Edmund Optics®

UFI

UltraFast Innovations (UFI) 1030nm Highly-Dispersive Ultrafast Mirrors with Reduced Thermal Lensing

  • Ultrafast Highly-Dispersive Coating with Reduced Thermal Lensing
  • Highly Negative GDD up to -1000 fs2 at 5° AOI
  • >99.5% Minimum Reflection (P-Polarization) across 50nm Bandwidth
  • Ideal for the Generation of High-Power Ultrafast Laser Pulses
UFI

Custom Mirrors with Low Thermal Lensing

Need a mirror at a different wavelength or GDD? Let us know what specifications you need. Edmund Optics® and UltraFast Innovations will collaborate to manufacture custom mirrors tailored for your needs

References

  1. O. Pronin “Towards a Compact Thin-Disk-Based Femtosecond XUV Source”, Dissertation an der Fakultät für Physik der Ludwig–Maximilians–Universität, München, 2012.

FAQs

Dr. Vladimir Pervak, world-renowned expert in ultrafast coatings from UltraFast Innovations, answers several customer-inspired, frequently asked questions.

FAQ  Are low-thermal lensing mirrors needed for Ti:sapphire lasers?
No, there is typically not a high enough average power in Ti:sapphire lasers for thermal lensing to be an issue, so highly-dispersive mirrors without the thermally stable technology can be used. Low thermal lensing is, however, important for high-power, solid-state lasers such as Er:YAG, Nd:YAG, holmium, and thulium lasers.
FAQ  Are low-thermal lensing mirrors needed for fiber lasers?

No, fiber lasers do not have a solid-state cavity where these thermal effects can occur, so again highly-dispersive mirrors without the thermally stable technology can be used. Low thermal lensing is, however, important for high-power, solid-state lasers such as Yb:YAG, Nd:YAG, holmium, and thulium lasers.

FAQ  Are low-thermal lensing mirrors available for purchase from Edmund Optics?

Yes, Edmund Optics offers low-thermal lensing mirrors as standard products available for immediate shipping. If you need one of these mirrors, such as UFI part number HD73 or another with different specifications, contact us to discuss a custom design.

FAQ  Do all your highly-dispersive mirrors have negligible thermal lensing?

No, creating low-thermal lensing mirrors requires careful manipulation of coating parameters and tradeoffs with other specifications. If thermal effects are important for your application, they must be addressed with optics specifically designed for those conditions.

Technical Resources

Application Notes

Technical information and application examples including theoretical explanations, equations, graphical illustrations, and much more.

Ultrafast Dispersion
Read  

Highly-Dispersive Mirrors
Read  

LIDT for Ultrafast Lasers
Read  

Ultrafast Lasers – The Basic Principles of Ultrafast Coherence
Read  

Thermal Properties of Optical Substrates
Read  

Videos

Informative corporate or instructional videos ranging from simple tips to application-based demonstrations of product advantages.

Highly-Dispersive Ultrafast Mirrors for Dispersion Compensation 
Watch  

Webinars

Recorded webinars from Edmund Optics® experts on a wide range of optics and imaging topics.

Ultrafast Optics: Challenges and Solutions 
Watch  

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