Given the lack of an accuracy standard for measuring a laser’s M-square value, what can a laser user utilize that will instil confidence that the M-squared measurement being made is as accurate as possible? While the ISO 11146-1 provides a method for making a measurement, there is no means to insure the M-square measurement is the most accurate possible. Most mainstream M-square measurement systems have higher then desirable variability in their measurements and consequently puts into question the accuracy of the result. Variabilities of 5 to 10% are not uncommon and therefore undesirable for many users. As there is no perfect laser for which an accuracy standard could perhaps be utilized, the repeatability of a measurement can therefore be the next best metric in providing the upper and lower limits in the measurement’s accuracy. The predominate source of variability should be limited to that of the inherent stability of the laser and not the variability caused by the measurement device itself. The mainstream M-square measurement systems have higher than desirable variability due the constantly changing attenuation and camera exposure times from motion of optics through the beam caustic and the resulting time averaging of the measurement. Having an instrument operating in real-time with fixed attenuation/exposure time and thus eliminating any time averaging of beam caustic data points, would therefore provide the best repeatability possible and limit any variation to that of the inherent stability of the laser source.
M-square measurements since the inception of the ISO 11146-1 measurement standard of 1996 has been one that has been difficult even for a seasoned veteran of such measurements. Variations of more than 10% are not uncommon for the same measurement tool on the same laser being measured. Much of the variation comes from alignment, the motion involved (time averaged based), complex attenuation techniques which often include variable neutral density filters and the type of sensors employed. Moreover, setup times for the instrument can take hours and the measurements themselves many minutes. Measurement of a laser or a laser systems' M-square should be as simple as measuring the power of the laser. In that one aligns the laser to the device; put the device in self calibration mode; make a measurement.
In 2012 the authors developed a passive optical design that provided real-time M-square measurement of a laser or laser system but nevertheless still required calibration of the key optics within the system: a Fabry-Perot etalon pair and their spacing in order to obtain an accurate M-square result. Using existing data from the sensor along with a simple ray tracing technique, the etalon spacing can be determined with high accuracy through the deconvolution of the data from the sensor; thereby eliminating a separate time consuming calibration. The key calibration information can now be obtained in a fraction of a second without any effort on the part of the user.
The ISO 11146-1 standard for measurement of a laser’s M-square requires the minimum measurement of five (5) spatial profiles within the first Rayleigh range and an addition five (5) outside the second Rayleigh range. The first five spatial profiles within the first Rayleigh range establish the beam waist and its location; the second five beyond the second Rayleigh range establish the divergence or convergence from the focusing lens for the M-square computation. The majority of methods used to date are all time averaged and as such are incapable of a real time M-square measurement. We present an ISO 11146-1 compliant method for measuring single shot M-square or beam parameter product values or the measurement of continuous wave sources at rates greater than five frames per second utilizing a pair of GigE based CMOS sensors. One GigE CMOS sensor is setup to measure the minimum of five spots within the first Rayleigh range for the establishment of the beam waist and its location. A second GigE CMOS sensor is setup to measure the five spatial profiles beyond the second Rayleigh range for the determination of the beam divergence from the focusing lens. Both GigE cameras utilize optics that passively create multiple spatial time slices of the beam and superimpose these time slices on the CMOS sensor in real time resulting in the ability to make single pulse measurements or continuous wave measurements at speeds of greater than five frames per second with full ISO 11146-1 compliance.
Industrial high power laser systems are often evaluated based upon spatial profile of the beam before they are brought to focus for processing materials. It is therefore often assumed that if the raw beam profile is good that the focus is equally as good. The possibility of having good optics and poor alignment or bad optics and good alignment and therefore not achieve a good focal spot is quite high due to the fact that a raw beam spatial profile does not manifest third order aberrations. In such instances the focal spot will contain aberrations when there are slightly misaligned, poor quality, high power optics in the system such as a beam expander or eye piece and objective of a 3-axis galvo. Likewise, if the beam itself is not on axis, the third order aberrations of astigmatism and coma are likely to appear but again not be seen in the unfocused beams spatial profile. The third order aberrations of astigmatism, coma and spherical aberration can significantly alter both the size and spatial profile at the focus resulting in out of spec performance. The impact of beam and zoom expanders and their alignment in beam delivery systems is investigated by measuring both the far field unfocused and the far field focus beams using an all passive beam waist analyzer system.
A six sigma laser processing system is proposed that utilizes real time measurement of ISO 11146 and ISO 13694 laser beam parameters without disrupting the process beam and with minimal loss. If key laser beam parameters can be measured during a laser process, without a disruption to the process, then a higher level of process control can be realized. The difficulty in achieving this concept to date is that most accepted beam measurement techniques are time averaged and require interruption of the laser beam and therefore have made it impractical for real time measurement which is necessary to consider six sigma process control. Utilizing an all passive optical technique to measure a laser’s beam waist and other parameters for both focused and unfocused beams, the direct measurement of the ISO laser beam parameters are realized without disruption to the process and with minimal loss. The technique is simple enough to be applied to low and high power systems well into the multi-kilowatt range. Through careful monitoring of all laser beam parameters via software control of upper and lower limits for these parameters, tighter quality control is possible for achieving a six sigma process. In this paper we describe the optical design for both low and high power laser systems and how six sigma laser processing may be realized.
An all passive optical design laser beam waist analyzer has been developed which can analyze in real time a focused beam waist independent upon its polarization state and facilitates a wide dynamic range of neutral density adjustment for optimizing the intensity on the sensor in an extremely compact size. The technique is applicable from the UV to the far infrared and tests in the visible and the far infrared are presented.
At the core of the design is a short Fabry-Perot resonator which produces spatial time slices of a focused laser beam and post the resonator is a pair of wire grid polarizers. The Fabry-Perot resonator optics provides a ~ 4.0 optical density for the incoming laser beam. As the intensity of the light following the Fabry-Perot resonator is sufficiently low, a very efficient and compact arrangement of a pair of wire gird polarizers are introduced to provide a wider dynamic range of focus intensity at the sensor plane without the need to add additional neutral density filters. This simple, but unique combination of optics, makes for a very compact and efficient means to evaluate focused laser beams from the ultra violet to the far infrared.
A laser beam waist analyzer system has been developed that permits the real time focal spot measurement of a high
power fiber laser in excess of ten of kilowatts and the ability to monitor thermal lensing of a laser and its optical system.
The analyzer can provide a laser's spatial profile, circularity, centroid, astigmatism and M-squared values inclusive of
the optics used in the process application. The system is very compact, measures in real time with no moving parts by
incorporating an all passive optical design to obtain spot diameter, M-squared, Rayleigh length, beam waist position at
power levels in excess of 20 kW in less than one second from laser off to laser on.
A laser beam analysis system, with all passive optical components, has been developed that permits the real time
measurement of a high power laser beam in the tens of kilowatts which can provide the laser's spatial profile, circularity,
centroid, astigmatism and M-squared values using all the optics of a process application, including the focus lens and
cover glass. At the heart of the technique is a Fabry-Perot resonator used with a focusing lens that provide a means to
both attenuate and provide a multiplicity of focused laser spots each representing a spatial slice of the focused beam
waist of interest onto a single CCD or CMOS camera. This arrangement provides real time data on the laser system's
beam properties and is the basis upon which this work it done. The coatings of the Fabry-Perot resonator provide a high
degree of attenuation of the input beam so that thermal lensing is not a factor in the measurement. By adjusting incident
angle and spacing between the mirrors of the Fabry-Perot resonator, a large number of spatial cross sections can be seen
on the detector. This permits then the possibility of evaluating any focusing objective whether long or short in focal
Athermalization of focusing objectives is a common technique for optimizing imaging systems in the infrared where
thermal effects are a major concern. The athermalization is generally done within the spectrum of interest and not
generally applied to a single wavelength. The predominate glass used with high power infrared lasers in the near infrared
of one micron, such as Nd:YAG and fiber lasers, is fused silica which has excellent thermal properties. All glasses,
however, have a temperature coefficient of index of refraction (dn/dT) where as the glass heats up its index of refraction
changes. Most glasses, fused silica included, have a positive dn/dT. A positive dn/dT will cause the focal length of the
lens to decrease with a temperature rise. Many of the fluoride glasses, like CaF2, BaF2, LiF2, etc. have a negative dn/dT.
By applying athermalization techniques of glass selection and optical design, the thermal lensing in a laser objective of a
high power laser system can be substantially mitigated. We describe a passive method for minimizing thermal lensing of
high power laser optics.