Laser Design and Analysis

Over the years Vigitek, Inc. developed various custom high-performance lasers and components. Here we present highlights from several projects.

1. An Indian company required a Diode-Pumped TEMoo Nd:YAG Laser for the cutting of natural diamonds. Being a highly humid environment the client required the laser with no water cooling to avoid condensation on optical surfaces. While a fiber-laser appears at first like a viable candidate, – the requirement of variable repetition rate and necessity for the high-contrast, fixed orientation polarization makes diamond cutting with fiber laser a very inefficient process.

Vigitek developed a special air-cooled mount for the Nd:YAG rod capable of handling up to 40 Watts of diode pump power at 808 nm. With intra-resonator LBO frequency-doubling we were able to achieve direct output of 10 Watts at 532 nm. In an intra-cavity frequency-tripling configuration the laser produced direct output of 4.5 Watts at 355 nm. This end-pumped design required short resonator length in order to achieve mode-matching and therefore produced short pulsewidth and high peak power. This created minor sub-surface micro-cracking; however any sort of cutting-induced defects must be brought to an absolute minimum considering high cost of material.

Currently Vigitek is in the process of developing a side-pump version of the Nd:YAG with single-stage intracavity frequency-tripling, which is expected to deliver ~12-15 Watts @355 nm. In addition, side-pumping will allow for a longer resonator and pulsewidth, thus virtually eliminating possibility of diamond micro-cracking.

2. Q-switched Alexandrite Laser has a multitude of applications in aesthetic dermatology, ranging from treatment of melasma to removal of certain color tattoos. However, combination of a low gain, inconsistent optical quality and specifics of GRM unstable resonators, makes a design of the laser itself very challenging.

To overcome the expected problems, Vigitek hired a Senior Laser Scientist to assist us in this project. This scientist spent over 15 years developing software to model advanced resonator design, based on 2D Fast-Fourier Transform Method. We present design of the GRM coating, as well as projected laser beam profile for various resonator parameters and expected relative output energy extraction in Q-switched mode.

For computer modeling purposes all coating parameters were made variable.

 

 

 

 

 

 

 

 

 

The goal was to find a suitable compromise between resonator magnification, energy extraction, pulse-to-pulse stability and flatness of the beam profile both inside of the resonator (to protect coated optical components) and 1500 mm from the output coupler (articulated arm exit) to insure treatment uniformity). While in Nd:YAG the window of suitable parameters is fairly wide, Alexandrite with its low gain leaves very narrow margin of error. Despite 3 design iterations, which included fabrication of custom output couplers, we couldn’t find a design solution, where all requirements can be satisfied.

Therefore we had to choose the design version that resulted in the highest energy extraction (800 mJ/50-70 nsec) at the expense of middle-field beam quality. The issue of treatment uniformity was resolved by placing at the exit of the articulated arm a beam-homogenizing handpiece based on refractive microlens array. This resulted in a square pattern with variable width and high uniformity, which simplified beam overlap issues during skin treatment procedures. The advantage of this approach is that beam-homogenizer delivers consistent output beam quality regardless of the profile of incoming beam. 

In addition we were able to find a solution, in which by re-arranging of optical components the handpiece delivered a Fractional pattern of variable size and TMZ (thermal micro-zones) density. Total loss of energy through the entire optical arrangement was <10%. The tests of Fractional handpiece for treatment of certain skin conditions are under way. 

Since 7-mirror articulated arm is > $3,000 item, we intend to develop a fiber optics beam delivery arrangement, which should be considerably less expensive. Plus the fiber automatically “scrambles” the beam thus flattening the output profile. However, reliability issues for any medical device always play a prevailing role, and currently most of fibers experience frequent damage when transmitting the beam of a Q-switched Alexandrite laser (despite its pulsewidth being an order of magnitude longer than in Q-sw. Nd:YAG).  In addition, after fiber-optic beam delivery it is difficult to obtain Fractional pattern with TMZ diameter much less than 40% of the fiber core, and so clinical efficacy with this approach would probably suffer.

3. Er:YAG Laser due to its high absorption in water is an ideal modality for fine wrinkle treatment. Its capabilities can be greatly improved and expanded with the addition of a Fractional handpiece, which must produce fairly high fluence in order to insure deep penetration (1000 µm or more) for clinically efficacy. However in multimode Er:YAG with its long wavelength and high divergence obtaining small thermal micro-zones (“pixels”) is very challenging. Two most widely used methods that employ 2D scanning and diffraction optical elements (“stamp-mode”) can deliver individual pixels in 300-400 µm range, which are too large.

In order to improve clinical efficacy we developed a unique optical arrangement, for which we filed patent application. It consists of 3 critical components:

  • Focusing lens to vary pixel density;
  • Short-focus refraction microlens array to create a Fractional pattern with
  • Long focal length, multi-lens Focusing objective with virtually diffraction-limited performance to transfer the image of a Fractional pattern on the patient’s skin, while preserving the pixel size.

This arrangement allows achieving Fractional pattern of variable size and pixel density with 150 µm individual TMZ diameter. According to our knowledge this arrangement is one of its kind at least in Medical Laser Industry. Currently we are trying to find ways to vary the shape of the Fractional Pattern as well.

 

4. Nd:Glass Pump Chamber design.