Measuring Pulsed Beams with a Slit-based Profiler

Transcription

1 Measuring Pulsed Beams with a Slit-based Proiler Pulse Rates, Power, and Damage Considerations Although the NanoScan was designed originally to measure continuous wave (CW) beams, many s are operated in the d mode. Measuring these d beams has generally required the use o a CCD array proiler. This is a reasonable solution or low power s in the UV and visible wavelength range, but these will require external attenuation. Once the s leave the UV-VIS range, array cameras become extremely expensive. Although low requency d s operating in the 1Hz to 1000Hz range have no real alternative to the array proiler, the NanoScan can measure khz requency s. The NanoScan proiler incorporates the peak connect algorithm and sotwarecontrolled variable scan speed on all scanheads to enable the measurement o these d s. The NanoScan is ideal or measuring Q-switched s and s operating with width modulation power (PWM) control. In the past ew years, s with pico- and emtosecond durations have begun to be used in many applications. Although these s add some additional complication to the measurement techniques, the NanoScan is well suited to measure them, too. We will discuss the measurement o all these types o d s below. PWM Lasers Many s, especially CO s, use width modulation (PWM) to control the power level o the. This is not true, d operation, but rather a reduction o the duty cycle to lower the average power. The beam operates as i it were CW, and many operators do not even realize that the is pulsing. However, when attempting to measure a PWM with a scanning slit proiler, it must be treated as a d source. To use the d mode o the NanoScan the s requency must be at least several khz, and the combination o the requency and beam size must provide a suicient number o s across the beam to generate a meaningul proile. ight to ten s are a reasonable minimum. PWM s usually operate around 10kHz. The relationship o the beam size and requency is a airly simple mathematical model. The NanoScan drum speed is sotware controlled rom 1.5Hz to 0Hz. There are two available drum sizes or the NanoScan; the standard head has a drum diameter o 4mm and the large aperture and high power heads use a larger drum with 84mm diameter. On the 4mmdrum at the 1.5Hz rotation rate the slits travel at around 116.6mm per second or 116.6µm per millisecond. At a 10kHz repetition rate, a 175µm beam would have 15 s during the time that the slit was traversing it. This would provide enough data to generate a meaningul proile. A smaller beam would require a aster rate, a Photon Inc. June 007

2 Measuring Pulsed Beams with a Slit-Based Proiler Page larger one could perhaps run at a lower repetition rate. For example, a 1.0mm beam could be measured with a rate as low as khz and still provide a proile. There is a table o minimum beam sizes and requencies or the large and small hubs and scan speeds at the end o this document. It is recommended that the 1.5Hz scan speed be used or d beams, however, i the beam sizes are large enough, or the rates ast enough, the measurement can be sped up by increasing the scan speed to.5hz or above. The NanoScan sotware will generate a warning i the scan rate is set too high or the rate or beam size. This warning algorithm is based on having at least 15 s across the beam to provide a minimum o % accuracy. Q-Switched Lasers Another type o d, operating in the khz rate regime is the Q-Switched. These s use the pulsing to increase, rather than decrease, their eective power. By concentrating the power into a short, the peak power o each increases while maintaining a low average power. In order to measure these s the same mathematical relationship o rate to beam diameter applies, but there is an additional complication; the peak power o the s may exceed the damage thresholds o the NanoScan even though the average power remains within the operating space. CW beams are measured as power (P) in Watts; d beams as energy () in Joules. Thereore it is necessary to understand the beam s energy ( ) to determine whether the unattenuated beam can be directly measured with the NanoScan. Pavg Thereore a beam with an average power o 300 Watts with a requency o 8kHz will have energy as ollows: Pavg 300W 37. 5mJ Hz The power density per is also a unction o the duration τ. This is also important in understanding the potential damage to the proiler. Taking the above example, i the duration is 1ms, then: 37.5mJ P 37. 5W 3 τ 1 10 s Pico- and Femtosecond Lasers When the duration o the gets very short, such as with pico- and emtosecond s, the peak power o the s can become very large. This creates some added complications when determining the type o scanhead that can saely measure these beams. In addition to the average power o the beam, which is used to determine the proper operating space o a given scan head, it is important to know the energy density o the s. The energy density must be below the damage threshold or the aperture material, and the average power must all within the operating space o the scan head or

3 Measuring Pulsed Beams with a Slit-Based Proiler Page 3 it to be possible to measure the beam without additional attenuation. To determine the energy density irst use the above ormula or the : P Most pico- and emtosecond s have both a high repetition rate and a airly low average power. They use the short duration to ampliy the eective power o the beam. A typical that one might encounter would have an average power o 1.0 watt and a repetition rate o 80kHz. For this the would be: P avg 1W sec avg 1 1.5μJ Using this value calculate the energy density or a given beam diameter by the ollowing ormula. Note that the energy density is presented as J/cm ; thereore the beam area needs to be converted to cm in the ormula. Unless the beam is wildly dierent rom round, it is easiest to consider that the area will be that o a circle: For a 100μm beam at the 1.5μJ: 1.5μJ 100μm ( ) 0.16J / cm density density π 160mJ / cm Once the energy density is calculated, it can be compared to the damage threshold or the aperture type and the wavelength range or the aperture material. The standard blackened slit material can only handle 10mJ/cm beore the blackening starts to ablate. For this reason, scan heads intended or use with these pico- and emtosecond s should have the relective slits, regardless o the detector type or the average power o the s. The wavelength o the also inluences the energy density that the aperture material can withstand. For the standard nickel alloy slits the maximum energy density is 600mJ/cm or the range o 190nm to 400nm; or 400nm and above the value is 1.0J/cm. For the high power copper slits the values are.5j/cm rom 700nm to 3μm wavelength and 5J/cm above 3µm. Copper slits are not recommended or use below 700nm, however in some experiments we have seen better perormance in the UV (@355nm) rom copper slits. This may be attributable to the better heat dissipation o the copper material or the act that the copper aperture material is thicker than the nickel alloy. The chart below can be used in lieu o the calculation to compare the energy per at a given beam diameter with the appropriate threshold line or the aperture material and wavelength o use. For the above case the 1.5µJ energy at 100µm would be below the 600mJ damage line, but would certainly be well above the damage level or blackened apertures. πr

4 Measuring Pulsed Beams with a Slit-Based Proiler Page 4 These estimates o damage threshold are primarily based on the relative relectivity o the slit material. There are many other actors that may inluence interaction o the beam and the aperture. At some level o power and duration this interaction may become non-linear. In addition surace inish, roughness, contamination, tarnish or oxidation can also aect the relectivity o 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 unamiliar system.

5 Measuring Pulsed Beams with a Slit-Based Proiler Page 5 Calculating the Minimum Beam Diameter per Pulse Frequency The ollowing table gives a list o calculated minimum beam diameters at a given requency or each o the drum sizes and or a desired number o s per proile. The more s per proile the more accurate the measurement is likely to be. The ormula is airly simple. Due to the 45 o angle o the slits to the direction o rotation, the actual speed o the slits is the drum speed divided by the square root o two. where: v N D min v drum velocity in µm per msec requency in khz N s per proile Dmin minimum beam diameter in µm The NanoScan d operation can operate at any rotation rate, however it is recommended that the scan rate be 1.5 or.5hz unless the repetition rate is above 50kHz. The larger drum used in the large aperture and High Power versions o the NanoScan cause the slits to move aster at any given rotation rate due to the larger circumerence. For this reason the minimum beam sizes are larger or the large drum. The peak connect algorithm inds the highest peak, then using the requency value entered by the operator it inds the other peaks and connects them to generate a smooth beam proile. It is important that the exact requency be entered into acquisition parameters. The earlier BeamScan instruments only allowed the measurement o d beams with the pyroelectric detector. NanoScan provides this capability with all scan heads and detectors. Beams with average powers that were too low to be measured with the pyroelectric detector can now be proiled using silicon or germanium scanheads. At high repetition rates it may be better to operate the NanoScan in CW mode and let the auto ilter smooth the beam. When this is preerable is dependent on the individual s perormance. I inconsistent results are seen with a high rep rate (e.g., >80kHz), it would be advisable to try the measurement both ways.

6 Measuring Pulsed Beams with a Slit-Based Proiler Page 6 Minimum Beam Size per Pulse Frequency NanoScan Normal Drum Large Drum (HP) Rotation Rate (Hz) slit speed (um/msec) Data Points per Proile Pulse Frequency (khz) Minimum Beam diameter in μm Minimum beam diameter in μm