PHTN1400: Principles Of Laser Systems
First-Pulse Suppression in Q-Switched Lasers (2017)

Introduction

This lab demonstrates the "giant first pulse" phenomenon in q-switched lasers as well as techniques to control or limit it.

Introduction

In this part of the lab the "quirkier" characteristics of a "standard" industrial DPSS Q-Switched laser will be investigated and the observed characteristics related back to physical principles in the laser such as population of the ULL.

Several characteristics will be investigated including how pulse energy varies with pulse frequency (and hence pump time of the rod: longer with a lower pulse frequency), the "ding-dong" effect, and the "giant first pulse" effect which may be corrected through various schemes (required when a harmonic generator is used).

PreLab

Lab Experiment

The Lee LDP-20 Laser is a modern, diode-pumped, Q-switched YAG laser capable of outputs of 20W at 1064nm or 10W at 532nm. For this experiment the laser is setup for 1064nm operation. The laser will be operated in both CW and Q-switched modes.


DANGER: This is a class-IV laser with EXTREMELY high peak powers capable of ocular damage with only one pulse. The particular danger here is the Q-switched infrared output at 1064nm since hazards presented by specular reflections are not obvious. Ensure the beam is intercepted as close as possible to the laser and pay attention to spurious refections from optical elements in the beam path.

SAFETY GLASSES MANDATORY - DO NOT REMOVE THEM WHEN THE LASER IS OPERATING ... Q-Switched YAG lasers are responsible for more eye injuries than any other types of laser combined!


Part 1: Pulse Power vs. Repetition Rate

The output of the laser is monitored by an internal power meter (the LDM-50) and by a fast external p-i-n detector. The internal LDM-50 meter works by intercepting the output beam (i.e. when the meter is on no output beam will be found) and measures average output power. The p-i-n detector is fast and can measure the amplitude of individual laser pulses down to 10ns. Turn the alignment HeNe on and ensure the output beam will strike the external p-i-n detector before proceeding.

The laser is configured for 1064nm. Start the laser according to the SOP by starting the cooling pump. Set the Q-switch to Internal modulation at 5KHz, close the internal shutter on the laser (the switch is on the power supply), turn the LDM-50 power meter ON, turn the main power on, set the diode current for 10.0A, and open the shutter. The laser should be seen to operate at a power of about 1.5W on the LDM-50 meter. Wait two minutes for the output power to stabilize then set the diode current SLOWLY until an output of 1.5W is seen on the meter.

With 1.5W of average output power seen (with Q-switching enabled), turn the LDM-50 power meter on, and turn the Q-switch driver off so that CW output is produced. Measure the average power.

Now, turn the Q-switch driver on, set for 5KHz and free-running, and measure the average power (on the LDM-50). Turn the meter off and the individual pulse amplitude (using the p-i-n detector) while operating the Q-switch at rates of 5 to 50KHz in increments of 5KHz. Capture the output of the laser (on the scope) at low (10KHz) and high (50KHz) repetition rates and photograph the screen for analysis and inclusion into the report. To measure the amplitude on the scope, set the trigger level on the oscilloscope to obtain a stable display. If output pulses vary wildly between pulses, capture the output.

In summary, make a table of Q-Switch rate (Hz), and Pulse Amplitude (mV, arbitrary units, from the scope). As well, two screen captures from the scope are required. Observe the actual pulses produced for consistency: do they all appear or are come missing? If pulses are missing, the likely cause is 'ding dong' which you will research as part of the lab write-up.

YAG output at 10KHz
A series of "clean" pulses at 10KHz as measured using the fast detector. As frequency increases, consistency of the pulses suffers as you will discover in this lab

Part 2: Giant First Pulse and Suppression Techniques

Now, investigate the "giant first pulse" effect which will be covered in lectures - this effect is evident when the Q-switch is closed for a long duration (for example between cuts on a materials-processing laser) and the Q-switch is suddenly opened ... the first pulse in a string of repetitive pulses that follow will be much larger than the rest.

Lee Laser FPS Setup
The complete setup for the experiment. On the overhead two oscilloscopes are used to view output and RF input to the Q-Switch (right scope) and FPS and modulation signals (left scope). They have identical triggers from the signal generator and so can be configured to display the same tibmebase showing four signal simultaneously.

Ensure the FPS input is connected on the Q-switch driver (a BNC connector on the rear) to the low-frequency signal generator set to 100Hz, 5V p-p, Square wave. The Q-Switch driver should be set for 5KHz.

First, observe the Giant First Pulse phenomenon as described in section 6.10 of Laser Modeling by simply turning the FPS correction off. To do this, set the power reference (top trace, left scope) soi that a constant amplitude of +15V is seen (i.e.no FPS pulse can be distinguished) - ask the professor to demonstrate use of the two potentiometers at this point.

Set the timebase to display the first (giant) pulse and subsequent (tapering-off) pulses - until pulses reach a constant amplitude - all on the same screen (a 500μS timebase should be sufficient). Determine the height of the first pulse and compare to the amplitude of 'regular' pulses which eventually follow when they reach a constant amplitude. Save this scope trace (photograph the screen) showing the first pulse effect and the tapering-off of pulse energy.

Save a trace on the scope showing this first giant pulse (which will be much larger than the repetitive pulses that follow).

Now, the First-Pulse Suppression (FPS) scheme will be investigated. The Lee Q-switch driver has this feature built-in since these giant pulses will damage the second-harmonic generator when installed. In order to facilitate this investigation, the Q-switch driver has been modified to make all essential signals (available to a technician as test points on the controller board and used when setting-up the laser) available for external monitoring via an oscilloscope. One key signal is available on Test Point 4 TP4 (Power Ref) which allows the user to monitor the RF output power of the driver. With this signal at a level of +15V the RF generator will output 100% power which will, in turn, cause maximum DE of the AOM. The other key signal is TP2 (Modulation) which is normally used to pulse the laser with RF output varying from 100% to 0%. A positive-going +5V signal will set the RF output to zero so the AOM inserts a zero loss into the cavity and a Q-switched pulse is produced.

Lee Laser FPS Signals

The following signals are available for observation:

Normally, during the production of a stream of pulses, the RF power to the AOM is kept at 100% preventing oscillation and is then pulsed off (0%) temporarily to produce pulses. The modulation signal (TP2) serves this purpose and during normal pulses the RF level is kept at maximum (so a constant +15V signal will be seen on TP4). Verify this during operation.

If the laser is switched off for a long period of time, inversion will build to a large level and the first pulse will be very large (See Laser Modeling, section 6.10). To prevent this, the RF signal is modified before the first pulse to "bleed off" the large inversion - the normal pulses are suppressed during this time after which RF power is set to maximum and normal pulses are produced by pulsing TP2. Both the width of the FPS pulse (i.e. the time during which the inversion "bleeds down") and the RF level during this period are adjustable.

Relate both the RF power and the normal modulation pulses to the output pulse seen. Be sure to pay careful attention to the first few pulses and the relationship of these to the two signals mentioned. If you have questions, just ask during the lab.

Save a trace on the scope showing the optimal suppression of this first giant pulse.

In summary, you need two observations (or saved traces) for this part of the experiment showing the ralationships between the TP4 (RF power level) and TP2 (Modulation) signals to the output of the laser. You should have four traces in total now. To relate these signals, draw a multi-trace diagram relating all.

Question (discussion in the lab): the Q-switch driver allows the technician to adjust the width of the FPS pulse which occurs before the normal stream of pulses. Obviously, the pulse width must be sufficient to reduce the inversion to prevent that first giant pulse. If the pulse of TP4 was set very wide, say 1ms, what would the expected output be? Try this by varying the pulse width via the FPS controls (see below). Draw the expected output power signal from the laser.

Lee Laser FPS Controls Next, vary the width and depth of the FPS controls to determine the effect on laser output. To do this, insert a small flat screwdriver through the holes shown. Note how each adjustment affects the Power Reference pulse as well as the first output pulse seen from the laser.


FPS RF Signal A view of the AOM signal on an oscilloscope. The top trace shows the output of the laser while the bottom shows the RF signal applied to the AOM. Visible in this shot is the FPS signasl (where the RF signal drops from maximum to an intermediate level such that the AOM is partially open) and several "normal" laser pulses in which the RF signal swings to essentially zero producing a pulse of maximum amplitude.


Lab Report

Hand In a WORD PROCESSED (not handwritten) lab report with contents as outlined below.

Lab Report The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, and the student's name and ID number

Answer each question as "1", "2", etc with each new question starting on a NEW PAGE so that question 2 starts on the top of a new page and question 3 starts at the top of a different page, etc. Where a question has multiple parts (e.g. 3a, 3b, 3c ...) you may answer those on the samepage however each part must begin in a separate paragraph with a title identifying the question in the form "3a., 3b., 3c. ...". Do NOT answer an entire question (e.g. question 3) as a single paragraph with nothing to denote section (a) from section (b), etc.

This format will assist you in ensuring EACH and EVERY question is answered since marks cannot be given for work not completed, nor would it be expected that you could complete the TEST QUESTIONS which will most certainly be similar to those you see here! (Hint !)

The lab must be submitted in a report cover (preferably either a three-hole punched cover or one with a clamp on the left side, not a binder), and NEVER as a stapled mass of loose papers!

Failure to follow this simple format, used for all condensed labs in this course, will result in deduction of marks

  1. Q-Switched Results:
    1. Include the chart of both individual pulse power (from the scope) and average beam power (from the Lee power meter) vs. repetition rate
    2. Include output traces (from the scope) at low and high repetition rates.
  2. You will notice that as the rate of the Q-switch increases, the pulse energy decreases. Explain this (in a paragraph or two) in terms of the build-up of inversion (ΔN) and the pump time between pulses.
  3. At high repetition rates, the pulses may be seen to be inconsistent. Research (on the Lee Laser site) the concept of "Ding Dong". Determine if and when it occurs in our system (and at what repetition rate ?), and determine if this is reasonable (i.e. under what conditions are we operating this laser that are an invitation for this according to the tech notes)? Include a screen shot from the oscilloscope showing "ding dong" occuring in our laser.
  4. "Giant" First Pulse:
    1. Show the giant first pulse effect as seen in the output of our laser: include a screen shot from the scope showing the first "giant" pulse occuring and subsequent pulses tapering to a nominal, constant, amplitude when manual gating is employed.
    2. These giant pulses can easily destroy a second-harmonic (SHG) generator so the Lee laser incorporates an option called FPS (First Pulse Supression) which can be found on their Tech Bulletins section. Describe (in a paragraph) the physics behind this method of correcting the first pulse so that it is of equal amplitude to all subsequent pulses - look at US Patent # 4,675,872 for a description of the technique as well as Sintec Optronics, a manufacturer of Q-Switch drivers which incorporate FPS. Describe physical parameters such as the population of specific YAG energy levels. Be careful to differentiate between the techniques of Pre-Pulse Kill (PPK) and First Pulse Supression (FPS) since both modify the RF signal ... only by investigating the actual RF signal applied to the AOM will the difference become apparent.
    3. From the description above (and the PDF files), deduce how RF power is applied to the AOM to cause FPS to occur. Include a sketch of the expected RF drive pulse (a graph of RF power on the y-axis vs. time on the x-axis) for the first few pulses in order to suppress the giant first pulse: specifically, describe the expected RF signal driving the Q-switch to produce a series of pulses during both "normal" operation and during "FPS" operation to contrast the two. (The diagram might look something like figure 6.23 in Laser Modeling except the top trace will now represent RF power (from 0% to 100%) and the bottom trace the expected output pulses).
    4. Show a scope shot showing the output similar to (a) above except with FPS enabled. Calculate the amplitude of the first pulse as a percentage of the nominal pulse amplitude for both the FPS condition and the normal (manually gated) condition.