Laser Beam Diagnostics in GHz Applications

Dick Reiley, Sales Manager, Mid-Atlantic Region, Ophir

In nuclear magnetic resonance spectroscopy (NMRS), for example, it is critical to define the size, shape, and intensity of the actual power of the laser source. Measurements need to be made not only at the output of the source, but also at locations along the optical path before the beam is deposited into a cryogenically cooled magnet.

In NMRS, the beam is generated by a microwave synthesizer operating around 12.5 GHz. The output of the synthesizer enters an amplifier stage and then a frequency multiplier chain. The output is converted to 198 GHz. Output is 50 to 60 mW (at the source). The signal can be adjusted across 195 to 201 GHz and is typically operated at CW mode.

The optical path of the source prior to injections into the cryo magnet is complex and signal loss can be expected. Knowing what that loss is, where it exists, and what power is being delivered into the cryo magnet, the final element of the processing state, is essential. Without this information, the expected results will be compromised.

Some of the elements in the optical path after the source include an amplifier, a frequency multiplier, additional mirrors and lenses including a Faraday Rotator for polarity, additional polarizers, and a final reflective surface into the cryo magnet.

In this application, the question regarding the power measurement accuracy in the optical path started at the first measurement point, just after the frequency amplifier. After recalculating the output to watts, the source power generator indicated source power should be close to 60mW of average power. Using the Ophir 3A-P-THz thermal sensor, the value measured 51.9 mW.

When measuring a laser with short pulses of tens of μs or less, the heat is deposited in a short time and cannot flow during the pulse (see illustration A below). Therefore, a surface absorber, which absorbs the energy in a thin surface layer, is not suitable. All the energy is deposited in a thin layer and that layer is vaporized.

In this case, volume absorbers are used. These traditionally consist of a neutral density glass thermally bonded to a heat-conducting metallic substrate. The ND glass absorbs the light over a depth of 1-3 mm instead of fractions of a micrometer. Consequently, even with short pulses where there is no heat flow, the light and heat are deposited into a considerable depth of material and therefore the power/energy meter with a volume absorber is able to withstand much higher energy densities – up to 10 Joules/cm² (see illustration C).

A second power measurement was then structured to determine the full output power after the influence of all the optical elements in the path just prior to the injection of the beam into a cryo magnet.

This measurement yielded only 17.28 mW, a drop of 66% from the primary source location. There was a real question whether this loss of power was real and if there was any way to validate this measurement. The technology selected to validate this signal loss was the Spiricon Pyrocam IV, a long wavelength beam profiling camera and array suitable from wavelengths from 1um to 3000 um.

The Pyrocam consists of a LiTa03 pyroelectric crystal mounted with indium bumps to a solid-state readout multiplexer. This sensor, developed as part of Ophir's core technology for the 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 out this charge. For use with short laser pulses, the firmware in the camera creates a very short electronic shutter to accurately capture the thermally generated signal.

The camera features a high resolution A/D converter that digitizes deep into the camera noise. This enables reliable measurement and analysis of both large signals and low level signals in the wings of the laser beam. High resolution digitizing also enables accurate signal summing and averaging to pull weak signals out of noise. This is especially useful with fiber optics at 1.3μm and 1.55μm, and in thermal imaging.

The same measurements are taken at the primary source and at the output point to quantify both the quality of the beam image (i.e. profile) and the intensity of the beam using linear intensity counts via the software program.

From the source location, the Spiricon Pyrocam IV camera produced this image and calculated the intensity counts. This validated that the graphic display of the beam was less and confirmed that the quantitative intensity from the Source to the Output port had dropped by 48%.

The last set of measurements in this investigation involved understanding why the 3A-P-THz sensor showed a drop of 66% in power from Source to Output, vs the Pyrocam IV intensity drop of only 48%. Upon analysis with factory engineering, the 3A-P-THz, although designed for terahertz measurement applications, is only calibrated for wavelengths from 0.3THz to 10THz, or 300GHz to 1000GHz. This application was operating at 198Ghz. The Absorption curve for the 3A-P-THz is shown below.Calibration Curve for 3A-P-THz. At 300Ghz, this sensor has an absorption of approximately 68%, whereas the application required calibration at 198Ghz where the absorption was approximately 40-45%.

This calibration limitation of the Ophir 3A-P-THz largely explains the power and intensity variance between the measurement at the power source vs the power at the output port. By employing the Spiricon Pyrocam IV to display both the graphics of the two beams and, more importantly, the intensity values of the beams at these two locations, the signal loss was verified to be at least 48%, and possibly greater. The 3A-P-THz is being considered for recalibration to included 198Ghz for improved accuracy. Only through the use of both technologies — the Ophir thermal laser power sensor for terahertz requirements and the Spiricon PyroCam IV long wavelength camera — was a more accurate measurement of the signal loss able to be determined and verified.