Minimizing Thermal Lensing in Ultrafast Systems

Helping Ultrafast Systems Keep their Cool

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.

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.

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.

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.

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.