The Apollo-22HD ruby laser firing. The red glow is light emitted from the ends of the rod when pumped with several thousand Joules of energy. The small cylinder at the left of the lamp housing is the Pockel's cell EO modulator employed as a Q-switch. The optic in the center of the photo is a water-cooled etalon used as both an OC and a single-frequency lock.

Introduction

This is a two part lab: in the first part students will determine the half-wave voltage of the EOM as well as the threshold conditons of the laser (allowing computation of g₀ and g_th which define the inversion limits in the formula in Csele 7.4. During the week, students will develop a spreadsheet model predicting the EOM voltage for the first pulse. In the second part of the lab, students will prove/disprove the model by detemining the required voltage to produce equal pulses.

PreLab

• Read chapter 7 in Fundamentals of Light Sources and Lasers by Csele on Electro-Optic modulators (a topic covered previously in PHTN1400)
• Read the SOP for the Apollo/Polaris 22HD ruby laser on this site. Specifically read the sections on basic operation of the controls as well as Timing Controls, Fire Mode, and Double-Pulse Operation

Experiment Setup

Install the QE-4 Joulemeter in front of the ruby laser with a ceramic scatter-plate in front to attenuate the high-power pulse. The QE-4 is connected to the BNC connector and held in a three-point mount.

Connect the EXT TRIG input on the scope to the MASTER SYNC output on the laser via a BNC-to-BNC cable. Connect the cable from the QE-4 joulemeter to the CH1 input on the scope. Now, set the timebase to 250μsec and channel 1 to 50mV/div. Select SINGLE SEQuence on the scope and, after the laser is CHARGED before each firing, ready the scope by pressing the SINGLE SEQ button again. The scope status will read "READY" (on the top of the screen) at which point the laser may be fired via the front panel control and the output will be recorded (as seen to the right).

The Experiment

Ruby is aluminum oxide (Al₂O₃) doped with a small percentage (0.05%) of chromium Cr3+ ions. When undoped, the material is sometimes called 'sapphire'. It has a density of 3.98 g/cm³ (source: www.matweb.com) and an absorption coefficient if 0.02/cm (i.e. 98% transmission through 1 cm). The oscillator rod in the Apollo 22HD ruby laser is 3.0 inches in length by 0.375 inches diameter (convert these to cm before continuing). The doping density is 1.58*10²⁵ ions/m³.

The optical cavity consists of an essentially perfect HR (100% R) and an 85% reflecting OC (determined using an Airy function since the OC is actually an etalon).

PART A

Determine, in the lab, the following parameters ...

1. The half-wave voltage of the EOM. Start around 6kV and increment/decrement the Q-switch voltage until the optimal output is found (and hence the actual half-wave voltage).

Analysis: Even though it is specified as 8.5kV, the polarizer and the rod (analyzer) are not at precisely right angles and so a new formula for transmission must be developed. The formula will incorporate an angular offset of the form T=T₀ sin² ((π/2) * (V_actual/V_half +φ)).

Develop a new formula for transmission of the EOM (with applied voltage as a variable).

2. Determine the maximum gain (g₀) of the laser amplifier in a similar manner to that used in PHTN1300 except that the EOM is the inserted loss. Knowing the formula for transmission of the EOM (from above), and the minimum voltage to threshold the laser (determine this experimentally in the lab), the gain may be determined.
3. Calculate the threshold gain of the system with the Q-switch fully-open (g_th). This will be a theoretical value based on optical losses (and the maximum transmission of the EOM).
4. Convert all gain figures into inversion densities (ΔN in m^-3) using the cross-section of the laser transition.

Before next week, use the experimentally determined values of g₀ and g_th to produce a model predicting where the mid-point for ΔN and then gain should be for equal pulses. Convert to a voltage for the QS1 Q-switch pulse. This model will be discussed in lectures

PART B

1. Now, produce two equal pulses as follows: Using the procedure from the SOP, adjust the laser to produce two equal pulses 100μs apart. Save an oscilloscope trace showing the equal pulses as well as a trace showing a significantly larger first pulse and a third showing a significantly larger second pulse. In each case ensure you record the parameters used to produce these pulses.


To do this, set QS2 to be optimal (half-wave) such that it will utilize any ULL population left after the first pulse. Set QS1 to be your predicted value and experimentally determine the actual voltage required for equal pulses from there.

How to proceed ...

As per the SOP, allow the power supply (with twin internal thyratrons) to preheat for ten minutes. The cooling system must be turned on as well during this time to allow it to reach operating temperature. Before use, flush out DI water remaining in the system (as per the SOP) and refill with fresh DI from the lab system. Turn on the water circulator/heater and allow the coolant to heat. Turn the alignment HeNe on and ensure the output beam will strike the detector.

To determine gain, set the system to operate as a single pulse laser by setting the first pulse (QS1) to zero (zero volts) and use the SECOND pulse (fixed at 1ms) to generate the single-pulse output. Begin with a timing of 100μs for the oscillator lamp and 0μs for the unused amplifier lamp. Channel 1 of the oscilloscope monitors the output pulse from the detector and the trigger output from the power supply connects to the oscilloscope EXT TRIG input. The preferred detector is a high-speed Gentec QE-4 detector with a ceramic scatter-plate in front acting as an attenuator, alternately a semiconductor detector can be used. Pulses are only 10ns in length and are separated by 150μs. Set the scope for 100 microseconds/division, and set the trigger position such that MPOS is 800 microseconds - the pulse will appear on the third grid from the right. Set for external trigger at 1.60V (rising edge) and use single sequence mode. Set the oscillator lamp voltage to 4.8kV and the unused amplifier lamp voltage to zero.

Verify the half-wave voltage by adjusting the Q-switch voltage (QS1) to obtain the maximum output power - adjust the voltage in increments of 1 kV and test-fire the laser. Continually increase or decrease the Q-switch voltage in finer amounts (down to 0.1 kV) until the maximum output is seen. This _should_ be the half-wave voltage of the Pockels cell (which you must calculate from data in the SOP) however if it does NOT agree consider what might happen if the polarizer and analyzer are not exactly 90 degrees apart. Compute the 'offset angle' and formulate a new equation for the transmission of the cell.

To obtain a reading of the amplitude of the pulse the oscilloscope which captures the output from the detector must be set to TRIGGER on an external rising slope and set to SINGLE SHOT mode. Immediately before the laser is fired (after it indicates that the capacitor banks are charged and ready to fire) the trigger of the oscilloscope is ARMED using the RUN/STOP and SINGLE SEQ buttons on the oscilloscope (READY condition indicates armed status). Following the laser pulse the shape of the light pulse (amplitude in time) will be recorded on the scope. It may be SAVED to a USB key.

Decrease the QS1 voltage until the laser ceases to lase and no pulse is seen at 1ms - first, in coarse increments of '1' and later in finer increments. You will now have determined the voltage allowing the threshold of lasing.

Before the second week, use the experimentally determined values of g₀ and g_th to produce a model predicting where the mid-point for ΔN, and then gain, should be for equal pulses. Convert to a voltage for the QS1 Q-switch pulse. This model will be discussed in lectures

Lab Report

Hand in the following ...

1. A basic outline of how an EOM Q-switch works
2. The experimental method used to determine actual half-wave voltage (outline how an EOM _should_ work, optimally, and how the one on the laser is actually aligned)
3. Outline the new formula for the EOM transmission, and explain how a new formula was produced based on data from the lab
4. Determine g₀ for this laser (explain how, show all data and calculations)
5. Determine g_th for this laser (explain how, show all data and calculations)
6. An outline of the model used to determine optimal parameters for equal pulses, specifically:
   • The formula used to determine energy of each pulse
   • An explanation of how your model works to obtain equal pulses
   • Mathematical output from the model showing where equal pulses were obtained
   • How conversion of model output was made to produce a VOLTAGE for QS1
7. The experimental results of the experiment to produce double-pulse output