Pyroelectric Technology in Laser Sensors and Beam Analyzers

Ophir Photonics uses pyroelectric detectors in a number of their products, both for beam profiling and for laser power measurement. The Photon and Spiricon brands are laser beam profilers based on scanning slit or array technologies; Ophir brand products are laser power measurement instruments.
 
Spiricon Pyrocam III
The Pyrocam™ III is a pyroelectric array camera that can be used to profile lasers of very short wavelength UV light or Infrared from the near IR wavelengths to the very far IR and even Terahertz wavelengths.

The Pyrocam detector consists of a LiTa03 pyroelectric crystal mounted with indium bumps to a solid-state readout multiplexer. This sensor, developed for the original Pyrocam I, has proven to be the most rugged, stable, and precise IR detector array available. Light impinging on the pyroelectric crystal is absorbed and converted to heat, which creates charge on the surface. The multiplexer then reads this charge onto the video line. For use with short laser pulses, the firmware of the camera creates a very short electronic shutter to accurately capture the thermally generated signal.
 
The Pyrocam III measures the beam profile of both pulsed and CW lasers. Since the pyroelectric crystal is an integrating sensor, pulses from femtosecond to 12.8ms can be measured. The pyroelectric crystal only measures changes in intensity, and so is relatively immune to ambient temperature changes.

 

Figure 1. Spiricon Pyrocam™ III

Because CW laser beams must be chopped to create a changing signal, the Pyrocam III contains an integral chopper as an option.
 
The Pyrocam III is an essential tool in the maintenance of industrial infrared lasers, especially CO2. The beam profiler replaces non-electronic mode burns and acrylic blocks by providing higher definition electronic recording of data and analysis of short-term fluctuations.
Photon NanoScan
The NanoScan is a slit-based profiler that can be equipped with silicon, germanium, or pyroelectric detectors. With the pyroelectric detector, the NanoScan can measure lasers of nearly any wavelength. The system comprises a drum with two orthogonal slits that rotates at software controllable rates from 1.25 Hz to 20 Hz. The pyroelectric sensor is a single element detector that responds to the light passing through the slits, generating a beam profile of the x and y axes of the beam.
 
Advantages of this measurement technique include the ability to measure very small beams, including beams at focus down to 20um diameter; measurement of many beams directly without the need for attenuation; very accurate determination of the waist location; and beam pointing and position.
 
The pyroelectric detector equipped NanoScans are available in standard and high power versions. The standard version has nickel alloy slits and an aluminum drum; it is nominally rated to handle powers up to ~100W. The high power versions have copper slits and a copper clad drum and can handle several kW of 10um (CO2 Laser) power. Maximum power capabilities for either of these configurations are wavelength and power density dependent. The detectors in both configurations are identical.

NanoScans are normally used in applications where feedback about the laser beam is necessary, especially involving adjustment of a laser system, focusing or aligning laser optics, or making measurements of beams much smaller that are compatible with the Pyrocam array systems. Because they can be used with wavelength they are popular with users who are working with multiple lasers or with the harmonics of the Nd:YAG lasers (1064nm, 532nm, 355nm, and 266nm).
Ophir Energy Measurement
Pyroelectric energy sensors are based on the principle that heat polarizes a pyroelectric crystal thereby causing the generation of an electric charge proportional to the heat absorbed in the crystal. The simplest form of pyroelectric energy detector will then be a thin pyroelectric crystal, metalized on both faces to collect the charge generated, with a parallel capacitor to produce a voltage proportional to the energy, and a parallel resistor to bleed off the generated charge to be ready for the next pulse.

 

Figure 3. Photon NanoScan Beam Profiler

The above system works well for short duration pulses and low repetition rates i.e. low duty cycle, where the time between pulses is long compared to the duration of the pulse, at least 10:1 and preferably much more than that. If, however, the duty cycle is shorter, then nonlinear effects come in as shown in the second picture of Fig 4.
Figure 4. Pyroelectric Power Sensor  

We see that several problems happen:

  • The voltage can start decaying before the laser pulse is finished thus causing a low reading of energy.
  • The voltage has not decayed before the next pulse comes in so the reading starts from a nonzero point.
  • The crystal is hot from previous pulses and is cooling off during the time till the next pulse is read so the reading again will be lower than it should be.

Ophir has attacked these problems in a sophisticated electronic circuit built into the sensor head. The solution is that instead of a fixed resistor to bleed off the charge, the circuit keeps high impedance during the pulse so the charge will not bleed off, and low impedance between pulses so the crystal will be ready for the next pulse immediately after the end of the previous pulse. This is accomplished by a complex circuit, illustrated schematically in Figure 4 by a switch opening and closing.

This method results in dramatic improvements in performance. Instead of 5-10% duty cycle, we are able to achieve close to 50% duty cycle. Instead of a fixed tradeoff between maximum pulse width capability and maximum repetition rate, we can arrange the circuit to measure long pulses at low repetition rates and short pulses at high repetition rates with a user selectable pulse width setting. In the same sensor we can get repetition rates as high as 10KHz and pulse widths as long as 5ms. For a high repetition rate detector, this is more than 100 times the maximum pulse width available from traditional non-switchable methods.
Table 1. Available settings for a Pyroelectric Power sensor
The above method also optimizes noise behavior because for short pulses the integration time can be short, minimizing noise. This results in a larger dynamic range of maximum to minimum measurable energy, as well.
 
The absorbing surfaces used in pyroelectric sensors must face the challenge of standing up to high energy nanosecond pulses without damage. If a laser pulse is absorbed on the surface of material within, say, the first 0.1um of the thickness, then during the short time of the pulse, a very thin layer of material is heated to very high temperature. At relatively low energy densities of ~0.1J/cm² or less, the material can discolor or even vaporize. In order to increase the resistance to damage, we can make the layer thicker and partially transparent so the laser beam is absorbed over a much greater thickness (e.g.,~ 5um). On the other hand, if the thickness is too great, then the heat takes time to migrate into the crystal and the repetition rate of the sensor will be low.
Figure 6. Examples of Different Types of Pyroelectric Energy Sensors

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