General

The head finder program allows you to select appropriate Ophir Power/Energy heads and displays based on laser parameters you input. The program calculates the laser power and energy densities, calculates the power and energy density capabilities of each absorber based on the laser wavelength, pulse length, rep rate and other relevant parameters and decides if the head withstands the laser. It also compares all the other requirements such as maximum and minimum power, energy beam size etc.


Power vs. Power and Energy

Two classes of heads are examined depending on whether you choose to measure power only or power and energy. If you are only interested in measuring average power and enter "Power Only" then the program allows all heads that can measure the average power except pyroelectric heads which are assumed to be reserved for users wanting to measure energy per pulse. If you enter "Power and Energy", then it is assumed you want to be able to measure energy per pulse. Then if you enter a pulse rate of 0.2s or less (5 sec per pulse), then thermopile heads are allowed since they can measure at this rate or slower. If you enter a higher rate, then only heads able to measure energy at that rate are allowed. So if you are interested in measuring single shot energy, enter 0.2s or less for the repetition rate.


Order of Selection

The heads are selected in terms of cost effectiveness and ease of use, i.e. first photodiode heads and thermopiles are selected and then pyroelectric and RP heads. If you want power only then pyro and RP heads will not be selected even if they could operate. If you select a repetition rate consistent with single shot energy (<=0.2Hz), then RP heads will not be selected even if they could operate in this regime.


Aperture

The heads are selected so that the head aperture is always at least 10% larger than the beam. This is so as not to allow the beam to fill the entire aperture which is not practical. For rectangular beams, the beam corners are allowed to touch the aperture.

If no Heads are Chosen

If there are no heads available for your input parameters then by somewhat relaxing the limits there may be a solution. You can:

1. Enlarge the laser beam diameter thereby lowering the power and energy density.
2. Reduce the maximum power and energy
3. Reduce the maximum repetition rate if you want to measure energy.

Optional Criteria

There are also additional optional criteria:

1. In addition to specifying the maximum power and energy, you can specify the minimum. This will eliminate heads which are not sensitive enough for the lower powers and energies.
   If you do not specify a minimum power or energy, the program will assume that the minimum is 50% of the maximum. 
   
2. If you only plan to measure the laser power for short periods at a time or intermittently, then there are some more compact heads for intermittent use.
   These will be allowed if you specify the duty cycle, i.e. the maximum time the head will measure at a time. Such a head will also be allowed to measure repeatedly intermittently if the (time on)/(time off)*(power) <= max CW rating of the head.
   For instance if you have a laser on for 1minute at 100 Watts and off for 4 minutes, then the 30(150)A head will be able to be used since 1 minute is within the time limit for intermittent exposure and (1 minute)/(4 minutes)*100W = 25W which is less than the continuous rating of the head which is 30 Watts.
3. If there are space limitations, you can enter these and only heads smaller than the dimensions given will be allowed.

Damage Threshold

All heads which are within specification for the laser parameters input are shown. Some heads are closer, however, to the laser damage threshold than others. Since the damage threshold is not a firm line and can vary somewhat from case to case and is also cumulative, we have listed how close a particular solution is to the damage threshold. The percent of damage threshold that is given is the higher between the power and energy damage threshold. Other things being equal, the user should prefer a head which is less than 50% of the damage threshold.

Displays

The program also lists the displays which can operate with the heads listed. Note that the Orion PE can measure only energy so if you choose "power and energy" and select a pyroelectric head, you will only be able to measure energy and not average power.