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Photonics Handbook® > Laser Beam Measurement
Laser Beam Measurement
Slit-Based Profilers for Pulsed Beams
Although slit-based profilers were designed to measure continuous-wave (CW) lasers, they can also work for pulsed-mode lasers.
Measuring pulsed-beam lasers has generally required the use of a CCD array profiler. This is a reasonable solution for low-power lasers in the UV and visible wavelength range, but these require external attenuation when used with CCD cameras. Once the lasers leave the UV-VIS range, array cameras become extremely expensive.

Although low-frequency pulsed lasers operating in the 1 to 1000 Hz range have no real alternative to the array profiler, it is possible to measure kilohertz-frequency lasers with a slit-based profiler. This will allow the inherent advantages of minimal beam attenuation and a broad range of available wavelength response. Photon’s NanoScan profiler incorporates the peak-connect algorithm and software-controlled variable scan speed on all scanheads to enable the measurement of these pulsed lasers. This makes it useful for measuring Q-switched lasers and lasers operating with pulse width modulation (PWM) power control. Lasers with pico- and femtosecond pulse durations are being used in many applications. Although they add some additional complication to the measurement techniques, a slit profiler can be well suited to measure them.

PWM lasers

Many lasers, especially CO2 lasers, use pulse width modulation to control laser power levels. This is not true pulsed operation, but rather a reduction of the duty cycle to lower the average power. The beam operates as if it were continuous wave, and many operators do not realize that the laser is pulsing. However, when attempting to measure a PWM laser with a scanning slit profiler, it must be treated as a pulsed laser source.

To use the pulsed mode of a slit-based profiler, the laser’s pulse frequency must be at least several kilohertz, and the combination of the frequency and beam size must provide a sufficient number of pulses across the beam to generate a meaningful profile. Fifteen pulses are a reasonable minimum.

PWM lasers usually operate around 10 kHz. The relationship of the beam size and frequency is a fairly simple mathematical model. With the NanoScan, for instance, drum speed is software controlled from 1.25 to 20 Hz. There are two available drum sizes; the standard head has a drum diameter of 42 mm, and the large-aperture and high-power heads use a drum with an 84 mm diameter.

TABLE 1.
MINIMUM BEAM SIZE PER PULSE FREQUENCY

  NanoScan                                       
Normal Drum                                                   
Large Drum (HP)
  Rotation Rate
(Hz)
        
   1.25      
   2.50      
   5.00         
 10.00          
   20.00         
   1.25
        
   2.50         
   5.00
        
   10.00
                                       
  Slit Speed
(µm/ms)
        
116.63           
233.25        
466.50         
933.01          
1866.01         
233.25
         466.50         
933.01
        
1866.01
                                       
  Data Points
(per Profile)
        
      15           
    15        
      15         
      15          
        15         
      15
               15         
      15
        
        15
                                       
 
Pulse Frequency
(kHz)
                    
Minimum Beam Diameter
(um)
                                
Minimum Beam Diameter
(um)
                                       
  1         
 1749      
  3499      
   6998         
 13995         27990         
   3499
        
6998
   13995
      27990
  2         
   875      
  1749      
   3499         
   6998          
13995         
   1749
        
3499
     6998
        3995
  3
        
   583      
  1166         2333         
   4665          
  9330        
   1166
        
2333
     4665
        9330
  4
        
   437      
    875      
   1749         
   3499          
  6998         
     875
        
1749
     3499
        6998
  5
        
   350      
    700      
   1400         
   2799          
  5598         
     700
        
1400
     2799
        5598
  6
        
   292      
    583      
   1166         
   2333          
  4665         
     583
        
1166
     2333
        4665
  7
        
   250      
    500      
   1000         
   1999          
  3999         
     500
        
1000
     1999
        3999
  8
        
   219      
    437      
    875         
   1749          
  3499         
     437
        
  875
     1749
        3499
  9
        
   194      
    389      
    778         
   1555          
  3110         
     389
        
  778
     1555
        3110
  10
        
   175      
    350      
    700         
   1400          
  2799         
     350
        
  700
     1400
        2799
  11
        
   159      
    318      
    636         
   1272          
  2545         
     318
        
  636
     1272
        2545
  12
        
   146      
    292      
    583         
   1166          2333         
     292
        
  583
     1166
        2333
  13
        
   135      
    269      
    538         
   1077          
  2153         
     269
        
  538
     1077
        2153
  14
        
   125      
    250      
    500         
   1000          
  1999         
     250
        
  500
     1000
        1999
  15
        
   117      
    233      
    467         
    933          
  1866         
     233
        
  467
       933
        1866
  16
        
   109      
    219      
    437         
    875          
  1749         
     219
        
  437
       875
        1749
  17
        
   103      
    206      
    412         
    823          
  1646         
     206
        
  412
       823
        1646
  18
        
    97      
    194      
    389         
    778          
  1555         
     194
        
  389
       778
        1555
  19
        
    92      
    184      
    368         
    737          
  1473         
     184
        
  368
       737
        1473
  20
        
    87      
    175      
    350         
    700          
  1400         
     175
        
  350
       700
        1400
  21
        
    83      
    167      
    333         
    666          
  1333         
     167
        
  333
       666
        1333
  22
        
    80      
    159      
    318         
    636          
  1272         
     159
        
  318
       636
        1272
  23
        
    76      
    152      
    304         
    608          
  1217         
     152
        
  304
       608
        1217
  24
        
    73      
    146      
    292         
    583          
  1166         
     146
        
  292
       583
        1166
  25
        
    70      
    140      
    280         
    560          
  1120         
     140
        
  280
       560
        1120
  50
        
    35      
     70      
    140         
    280          
    560         
       70
          140
       280
         560
  100
        
    17      
     35      
     70         
    140          
    280         
       35
        
    70
       140
         280
  150
        
    12      
     23      
     47         
     93          
    187         
       23
        
    47
       93
         187

On the 42 mm drum at the 1.25 Hz rotation rate, the slits travel at around 116.6 mm/s or 116.6 µm/ms. At a 10 kHz laser repetition rate, a 175 µm beam would have 15 pulses during the time that the slit was traversing it. This would provide enough data to generate a meaningful profile. A smaller beam would require a faster pulse rate, a larger one could perhaps run at a lower repetition rate. For example, a 1.0 mm beam could be measured with a pulse rate as low as 2 kHz and still provide a profile.

It is recommended that the 1.25 Hz scan speed be used for pulsed beams. However, if the beam sizes are large enough or the pulse rates are fast enough, the measurement can be sped up by increasing the scan speed to 2.5 Hz or above. The NanoScan software will generate a warning if the scan rate is set too high for the pulse rate or beam size. This warning algorithm is based on having at least 15 pulses across the beam to provide a minimum of 2 percent accuracy.

Calculations

Table 1 lists calculated minimum beam diameters at a given pulse frequency for each of the drum sizes and for a desired number of pulses per profile. The more pulses per profile, the more accurate the measurement is likely to be. The formula is fairly simple. Because of the 45° angle of the slits to the direction of rotation, the actual speed of the slits is the drum speed divided by the square root of two.

Equation1.gif

Where:

  • v = drum velocity in microns per millisecond
  • f = pulse frequency in kilohertz
  • N = pulses per profile
  • Dmin = minimum beam diameter in microns

The pulsed beams can be measured at any rotation rate, but it is recommended that the scan rate be 1.25 or 2.5 Hz unless the laser repetition rate is above 50 kHz. The larger drum used in the large-aperture and high-power versions of the NanoScan causes the slits to move faster at any given rotation rate because of the larger circumference. For this reason, the minimum beam sizes are larger for the large drum. The peak-connect algorithm finds the highest peak pulse. Using the frequency value entered by the operator, it finds the other peaks and connects them to generate a smooth beam profile. It is important that the exact pulse frequency be entered into pulse acquisition parameters.

The earlier instruments only allowed the measurement of pulsed beams with the pyroelectric detector. NanoScan provides this capability with all scanheads and detectors. Beams with average powers that were too low to be measured with the pyroelectric detector can now be profiled using silicon or germanium scanheads. At very high laser repetition rates (e.g., >100 kHz) it may be better to operate the profiler in CW mode and let the filter smooth the beam. When this is preferable is dependent on the individual laser’s pulse performance. If inconsistent results are seen with a high-repetition-rate laser, it may be advisable to try the measurement both ways.

Q-switched lasers

Another type of pulsed laser, operating in the kilohertz pulse rate regime, is the Q-switched laser. These use the pulsing to increase, rather than decrease, their effective power. Because the laser power is concentrated into a short pulse, the peak power of each pulse increases while maintaining a low average power. In order to measure these lasers, the same mathematical relationship of pulse rate to beam diameter applies, but there is an additional complication; the peak power of the pulses may exceed the damage thresholds of the NanoScan even though the average power remains within the operating space.

CW beams are measured as power (P) in watts; pulsed beams as energy (E) in joules. Therefore it is necessary to understand the beam’s energy (Epulse) to determine whether the unattenuated beam can be directly measured with the NanoScan.

Equation2.gif

Therefore a beam with an average power of 300 W with a pulse frequency of 8 kHz will have energy as follows:

Equation3.gif

The power density per pulse is also a function of the pulse duration τ. This is also important in understanding the potential damage to the profiler. Taking the above example, if the pulse duration is 1 ms, then:

Equation4.gif

Pico- and femtosecond lasers

When the pulse duration of the laser becomes very short, such as with pico- and femtosecond lasers, the peak power of the pulses can become very large. This creates some added complications when determining the type of scanhead that can safely measure these beams. In addition to the average power of the beam, which is used to determine the proper operating space of a given scanhead, it is important to know the energy density of the pulses. The energy density must be below the damage threshold for the aperture material, and the average power must fall within the operating space of the scanhead for it to be possible to measure the beam without additional attenuation. To determine the energy density, first use the above formula for Epulse:

Equation5.gif

Most pico- and femtosecond lasers have both a high repetition rate and a fairly low average power. They use the short pulse duration to amplify the effective power of the laser beam. A typical laser that one might encounter would have an average power of 1.0 W and a repetition rate of 80 kHz. For this laser the Epulse would be:

Equation6.gif

Using this value, calculate the energy density for a given beam diameter by the following formula. Note that the energy density is presented as joules per square centimeter; therefore the beam area needs to be converted to centimeters in the formula. Unless the beam is wildly different from round, it is easiest to consider that the area will be that of a circle:

Equation7.gif

For a 100 µm beam at 12.5 µJ:

Equation8.gif

Once the energy density is calculated, it can be compared with the damage threshold for the aperture type and the wavelength range for the aperture material. The standard blackened slit material can only handle 10 mJ/cm2 before the blackening starts to ablate. For this reason, scanheads intended for use with these pico- and femtosecond lasers should have the reflective slits, regardless of the detector type or the average power of the lasers.

The wavelength of the laser also influences the energy density that the aperture material can withstand. For standard nickel alloy slits, the maximum energy density is 600 mJ/cm2 for the range of 190 to 400 nm; for 400 nm and above, the value is 1.0 J/cm2. For high-power copper slits, the values are 2.5 J/cm2 from 700 nm to 3 µm and 5 J/cm2 above 3 µm. Copper slits are not recommended for use below 700 nm. However, in some experiments we have seen better performance in the UV (at 355 nm) from copper slits. This may be attributed to the better heat dissipation of the copper material or the fact that the copper aperture material is thicker than the nickel alloy.

Figure1.jpg
Figure 1. Damage threshold curves showing a comparison of energy per pulse at a given beam diameter with the appropriate threshold line for the aperture material and wavelength of use.

Figure 1 can be used in lieu of the calculation to compare the energy per pulse at a given beam diameter with the appropriate threshold line for the aperture material and wavelength of use. For the above case, 12.5 µJ energy at 100 µm would be below the 600 mJ damage line, but would certainly be well above the damage level for blackened apertures.

These estimates of damage threshold are primarily based on the relative reflectivity of the slit material. There are many other factors that may influence interaction of the laser beam and the aperture. At some level of power and pulse duration, this interaction may become nonlinear. In addition, surface finish, roughness, contamination, tarnish or oxidation can also affect the reflectivity of the materials. For this reason, these damage threshold values can only serve as a guideline, not an absolute guarantee. Use caution when measuring any new or unfamiliar laser system.
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