A complete investigation of ion lasers including optical characteristics and power supply circuitry (gas processing will be covered in the high vacuum/thin-film course). This is a two week lab in which students in small groups will complete both parts alternately. One lab report is required for both parts, which have separate questions to be answered.

PART A: Optical and Operational Characteristics

A Coherent Innova-90 argon-ion laser, featuring both multiline and single-line (wavelength selective) optics, is utilized to investigate the characteristics of an ion laser. In the setup for this lab experiment the beam passes through a neutral density filter which reduces the power to a manageable level. The beam then passes through a prism which separates it into individual components. Several beamstops are evident in this photograph which absorb stray reflections.

Introduction

This lab introduces students to the operating procedures of a large-frame ion laser as well as provides an investigation into the sensitivity of the output spectrum to plasma tube current.

PreLab (Part A only)

• Print the LAB WORKSHEET for Part A
• Read the Standard Operating Procedure (SOP) for the Innova-90 laser
• Review Chapter 9 of Csele specifically on ion lasers
• Research the energy levels and transitions in singly ionized argon (Ar⁺). (see the SP Intro pdf)

The laser employed is a class-IV laser producing over 6 Watts of power. Safety glasses suitable for an argon (i.e. with an OD > 4 at 488nm and 514nm) are required. Be careful with the position of the beam since it will start fires as well as burn flesh if carelessly positioned! There will undoubtedly be stray beams in the room from reflections off various optical components such as filters, the prism, etc. for this reason ...

• KEEP THE DOOR CLOSED when the laser is operating
• SAFETY GLASSES are mandatory
• DO NOT run any other laser in the lab at the same time
• Block stray beams using beamstops

Experiment Setup

An alternate view of the setup showing all beams including stray reflections from various components

For some configurations (in our new lab in V15, 2007 onward) the setup will omit the neutral density filter. In this case, component beams must be limited to 300mW each maximum, the rating of the meter. A 500mW multiline output will meet this criteria.

Part I: Laser Controls (Light / Current)

Consider the following excerpt from the Operator’s Manual Innova® 90C Series Ion Laser (Coherent P/N: 0170-078-00, Rev B) which outlines the operation of each mode:

Begin by familiarizing yourself with the controls of the laser, specifically, light and current controls. First, ensure the beam is terminated by a beamstop then begin by setting the light control to maximum (fully clockwise) and vary the current control to regulate the tube current to 20 A. Now, gently vary the wavelength selector control to misalign the HR and observe the effect on laser output power. Make a chart of values showing output power (W/mW) vs. tube current (A) in this mode - use a step size of about 20mW. Will tube current ever exceed the preset value of 20A?

Now optimize for highest laser output and decrease the light control until the output is 500mW. Set the current control to maximum (fully clockwise). Again, gently misalign the HR optic via the wavelength selector control and observe how tube current varies. Make a chart of values showing output power (W/mW) vs. tube current (A) in this mode - use s step size of about 1A. Plot this data for the lab report.

In practice, the maximum tube current (30A, to be practical) is usually set with the shutter closed (i.e. no light output) then the shutter is opened and the laser operated in 'constant light' mode by setting the light contol to the desired level (why do we usually use 'constant light' mode ? Longevity or Stability ?). For the remainder of this experiment, though, we will utilize the 'light' control to keep maximum optical output power to 500mW (to protect meters and optical elements) and will vary the current control primarily - we will allow optical power to hence vary from zero to a maximum of 500mW.

Part II: Multiline Output

With the multiline optics installed (as they should be when the lab begins), optimize the alignment of the resonator. Project the output beam through a prism and onto a screen of cardboard so that individual components are visible. Now decrease the current to the minimum possible value and operate the laser in current regulation mode (i.e. use the light control setting strictly to limit the maximum output power to 500mW) - at this point the tube stays lit but lasing will cease.

Increase the tube current gradually and stop when an output line appears. The first line will be the 488nm cyan line. Record the current at this point. Using a pencil, draw a mark on the screen to show the position of this beam (to identify it for later). Continually increase the current and note the currents at which new lines appear until the maximum optical power is reached. Be sure to identify the wavelength of the line as well (this can be done later when single-line optics are used, using the marks on the screen as a guide).

Now decrease the current to a relatively low value where both 488nm and 514nm both lase. Measure the power of each line using a power meter (be sure to isolate each individual line from the others). Now increase the current to a relatively high value (the maximum allowed by the light regulation mode) and measure the power of each line again. Although the 488nm line appears first, the ultimate output power of the 514nm line may well exceed that of the 488nm line.

When done, turn the current down to 10A

Part III: Single-Line Output

Install a beamstop in front of the prism assembly but DO NOT misalign the prism.

Remove the rear multiline optic and replace with the single-line optic (follow the procedure in the SOP carefully to avoid the need for a major realignment exercise!). Increase the tube current to 20A. Lasing may be achieved easily by rotating the wavelength control upwards to 800nm, past this setting to 400nm, and up again to 560nm. Now, rotate the horizontal adjustment slowly while rocking the mirror mount gently via the hole provided on the top of the laser case (see the SOP for the alignment procedure). When lasing is achieved, set the wavelength control for the 488nm line, and tweak the horizontal for maximum power. You can identify the 488nm line via the position on the cardboard screen made earlier (remove the beamstop now in order to do this) - do not rely on the wavelength markings on the control as it may not be calibrated.

Set for 488nm output and optimize the alignment of the resonator. Reset the identification ring so that it reads '488'. Now examine each line as follows: Set the wavelength selector for the desired wavelength and increase the current until the laser beam appears (we are limited, still, to 500mW but not to any particular tube current). 'Tweak' the aligment for maximum output at this wavelength (both horizontal and wavelength controls). Now decrease current until the beam just disappears. Note this current (the threshold current for this individual line). Continue the procedure for all lines on the laser (with single-line optics there are up to ten, not five or six) determining the threshold current for each line to appear. Since you now know the wavelength from the selector, the exact wavelength of the lines from part B can now be assigned as well (by the position on the screen that you recorded earlier).

Finally, remove the wavelength selector and install the multiline optic again. Adjust for maximum power output

Shut down the laser allowing the water to run for one minute after shutdown to remove latent heat in the plasma tube as per the SOP

Analysis of Results:

To explain the behaviour of the 488/514 lines (the anomaly where the 488nm line shows up first, meaning it has the highest gain, but the 514nm line ultimately has higher output power), consider first the gain of the lines: the 488nm line has a much higher gain. Next consider the power-to-gain ratio as outlined in SAM's Laser FAQ (laserarg.htm#argwse2). As for the "families" of levels they talk about: Find a very detailed reference to the quantum levels of the argon-ion laser transitions (the one in Csele is not detailed enough as you need descriptions like "4p²S⁰" for each level - the "SPIntro" document will provide this) and you'll see that 514nm (plus one other line) have different ULL's (QUOTE THEM).

DON'T WAIT until after next week to write-up the lab! There is a fair bit of research required on your part to complete this lab - start now to ensure you complete it on time!

PART B: Power Supply Circuitry

Introduction

In this part of the lab, students will investigate the high-current circuitry of an argon laser power supply. Large-frame ion laser supplies, such as the one used in this lab, operate from three-phase AC current and can produce regulated DC output for a laser tube at voltages of up to 400V at currents as high as 70A. In this lab, reduced voltages will be used to allow investigation of circuits without the dangers normally associated with "poking around" inside such power supplies.

PreLab (Part B Only)

• Read chapter 9 of Fundamentals of Light Sources and Lasers by Csele, specifically details on the ion laser supply
• Read SAM's Laser FAQ, specifically on ion power supply design: "Three-phase 208V front end" and "Series Pass Regulators"
• Research how three-phase voltage is measured, specifically, what is phase-to-phase voltage and what is 'phase-to-neutral' voltage? Our lab has 3-phase 208V service, how is single-phase 120V service derived from this for receptacles around the room?
• Print the circuit diagram Basic Schematic to guide you during the investigation
• Read over this lab, highlighting the tasks you need to accomplish during the lab period (many tasks are to be reserached and completed during the lab write-up, not during the lab period).

• Primer on diodes and how capacitors smooth output from a rectifier into constant DC (single-phase)
• Rectifier Circuits covering both single and three phase

SAFETY NOTE #1:
Normally, ion laser power supplies operate from 208V (or higher, up to 440V for one of our lasers) three-phase supply voltages. In this lab, low voltages and isolation transformers are used to allow safe investigation of circuits - these safeguards are NOT employed in a normal "live" ion laser supply so never be so curious or tempted as to try the investigations of this lab with a "live" power supply.

SAFETY NOTE #3:
Despite the fact that virtually all large power supplies contain safety features such as interlock switches on access doors and bleeder resistors on capacitors do NOT rely on the proper operation of such features to protect against death. Put bluntly: Would you trust your LIFE to a 50-cent resistor ?. ALWAYS short capacitors with a proper grounding rod (if provided) or a screwdriver before assuming they are discharged!

Equipment Required

• Safety glasses Bring your own from your first-year kit
• A basic, digital, Volt-Ohm meter
• A potentiometer for driving the passbank (in V15 already)
• Jumper wires (for jumpering interlocks, etc)

The Investigation

In this investigation you will examine the following elements of the ion laser supply (a Coherent Supergraphite CR-6 medium-frame ion laser):

• Safety Interlock and Contactor
• Autotransformer (not present on all laser supplies)
• Three-phase Rectifiers and Capacitor Bank
• Linear Passbank

Pace Yourself: by the one-hour mark you should have started the linear passbank section of the investigation

NOTE: Normal multimeters measure RMS voltage on the "AC" setting. For this lab, all AC voltages quoted are RMS.

1. Component Identification:

Given a basic shematic of the CR-6 ion laser power supply, identify all major physical components including the contactor, autotransformer (three-phase), fuses (three), rectifiers (six of them, for three-phase), main capacitor bank (four, in series-parallel arrangement), and control transformer.

Download a High Resolution Photo of the power supply.

2. Safety Interlocks and Contactor: Powering-up the supply

The laser power system works by feeding a control transformer from the incoming (always live) supply and producing a 120V supply used for interlocks. Identify the control transformer and the fuses which feed the control transformer from the incoming three-phase supply (these are NOT the three large fuses rated at >40A, they are small glass fuses). These small fuses, when blown, will prevent the interlock system from running (one problem to look for in a 'dead' power supply). Only when all interlocks are closed, and the ON button is pressed, will the main contactor pull-in powering-up the main autotransformer. An auxiliary 120V winding from that transformer then powers all control systems for the laser (the control transformer does not power the control system such as the current regulator and the delay relay - it powers the interlock system only).

In order to safely test the laser, it is not desired to apply full 208V/Three-phase power to the entire system: faults are not only spectacular but also can also be FATAL. In our case, we will power-up the slightly modified power supply on lower voltage 30V/three-phase so that instead of 208VAC on the input to the autotransformer, 30VAC will appear there. The control transformer has been replaced with a step-up transformer to generate the necessary 120VAC supply for the interlock system (the normal transformer would step-down 208V input to 120V, this one steps 30V input up to 120V).

Now, turn on the control system by switching the main three-phase supply on (the main shutoff switch).

With the feed supply on, the insides of this power supply now feature 120VAC on the control system (although this supply is isolated from the AC line) - be careful not to touch exposed terminals to avoid an electric shock. Determine which interlocks must be jumpered (including two door switches, one external interlock connector on the rear panel, the tube housing interlock on the top of the unit which would normally be on the cover of the laser head, and the water flow interlock which must be jumpered). It will be necessary to bypass the safety interlocks on the doors of the power supply as well as other interlocks on the system. Dorr interlocks may be pulled outward to put defeat them for service. A single jumper will be required for the water-flow sensor (to show the flow is 'on' and allow the system to operate).

Jumper/bypass the interlocks and ensure the contactor operates via the ON and OFF pushbuttons on the front panel. When these buttons are pressed they will cause the contactor to operate and apply three-phase power to the autotransformer. If it fails to operate, use the multimeter to check for voltage at the control transformer.

3. Autotransformer:

This laser features an autotransformer to boost the line voltage. On many of our other lasers, 208V is rectified to about 300VDC however a longer ion tube requires higher voltage to operate (Our Innova-200, for example, runs from 440VAC power since the very long plasma tube runs at much higher voltages than available with a 208V supply). This particular supply, the CR-6, features an autotransformer to boost the 208V input supply to a higher voltage as required - multiple taps allow selection of operating voltage.

Check the taps on the transformer - the input is currently tapped at 210V - and calculate the DC voltage available on the capacitors for a nominal input of 208V AC (i.e. with no transformer used, simply 208V rectified directly like most other laser power supplies do). What is the absolute maximum DC voltage available if the taps of the transformer were set for maximum voltage available from the transformer and assuming a 208VAC input? Review: For a single-phase rectifier, the peak DC voltage is V_rms*2^0.5. For a three-phase rectifier, the peak DC voltage is V_rms*2.34 / 3^0.5.

Verify the operation of the autotransformer by measuring the phase-to-phase input voltage (normally 208VAC but about 30VAC here) and the voltage on various output taps (190V, 240V, and 355V taps) and ratio. If, for example, the transformer was fed with 208VAC at the 210V tap and the 355V tap was used to feed the rectifiers, the expected AC voltage would be 208*(355/210) or 352VAC. Report the actual input voltage measured, the expected voltage at each tap (by ratioing the rated voltages), and the measured voltage at each tap.

4. Rectifier and Capacitor Bank:

First, determine how the capacitors are wired (physically connected) and the value of the capacitor bank as it appears across the DC bus - you may have to loosen a capacitor mount to read the value on it do this ("MFD" is actually MicroFarads). The schematic shows a single capacitor but the physical wiring is quite different. Draw a diagram of the capacitor arrangement, compute the total capacitance of the bank, and show how the capacitance was computed. Knowing the maximum DC voltage available (above, by using the maximum tapped voltage available on the transformer), determine why the capacitor bank is wired as it is and why a single capacitor (or even all four in parallel) cannot be used. If the maximum voltage is used (i.e. the 355V tap on the autotransformer with 208V input), what is the expected value across each individual capacitor in the bank?

Measure the AC voltage output from the autotransformer (i.e. the input to the rectifier - you can measure this at the three large fuses inside the lower portion of the supply) as well as the DC voltage as measured across the capacitor bank. Measure the voltage across each individual capacitor as well.

Question: what is the function of the resistors across each capacitor? What would happen if they were simply not there? To find out, place a meter across the capacitor bank (i.e. on the DC bus), turn the system OFF via the pushbutton on the front panel and observe how the DC voltage decreases - if the resistors were not there what would be expected?

For a single-phase system, the maximum ripple is 100% (between zero volts and the peak DC voltage). Determine the approximate AC ripple, with no capacitor, for a three-phase rectifier (see section 9.12 in Csele). If the system were powered via a three-phase supply one could measure this by setting a DVM to "AC Volts" and measuring the AC component across the capacitor bank (in other words, the ripple). The significance: (i) a three-phase system requires a lot less capacitance to filter the DC bus and (ii) when capacitors fail (as they sometimes do from age), AC ripple on the DC bus can increase drastically - In a three-phase system where ONE phase fails, ripple increases drastically (in other words, the laser is operating almost as if fed from a SINGLE PHASE source).

The significance of this situation: When ONE phase fails (usually due to a single blown fuse or a blown feed from the transformer ... we've had this happen to our I-90 before in our lab), the laser may well still operate but will not operate properly due to large AC ripple on the main capacitor bank.

5. Diagnosing a Passbank:

The system uses a load resistance (one ohm, 225W) to simulate the laser plasma tube (via connector J101 on the rear of the unit). Connect a potentiometer to drive the passbank between DC+ and DC-, with the output (wiper) connected to the DRIVE line on the passbank. All of these connections are shown in the Notes document. Turn the potentiometer to minimum (towards the DC- side of the potentiometer) and turn the supply ON - the LASER AMPS meter should read zero at this point.

Passbanks use multiple transistors in parallel to control large currents. Increase the passbank drive via the potentiometer slowly until a current of about 10A flows (as indicated on the LASER AMPS on the front panel). For each transistor in the bottom bank of the passbank (they are in parallel), measure the emitter current through each - do this by measuring the voltage across each emitter resistor (the gold resistors with metal heatsink fins surrounding them) and using the formula I=V/R (the resistance is marked on the resistor). Connect the negative terminal of the meter to DC- and connect the positive terminal to the emitter of each transistor in the passbank recording the voltage across each resistor.

In chart form, present the emitter current for each device as a percentage of maximum (the single device showing the highest current through it). This should help illustrate how, as current rises, the emitter resistor arrangement helps distribute current equitably through all devices (all 19, in parallel). One, or more, transistor(s) in the bank have failed: identify these transistors and describe how the determination was made.

Turn OFF the supply. Using a VOM set to the lowest resistance scale (200 ohms) measure the resistance between the emitter and the base of each transistor. Again, identify and failed transistors and correlate the 'static' reading you are making with the meter (with the supply OFF) to the previous current readings. Normally, with an operating supply, you would want to diagnose the passbank while OFF since lethal voltages are present on the passbank during operation so a simple method using a meter is desired here!

Now, consider the markings on the PASSBANK VOLTAGE meter. At low voltages across the passbank (25V), current may be set to full (50A) but at increased voltages, tube current will be limited (to avoid overloading the passbank). Assuming the supply is rated for 40A (full current), there is a point on the passbank voltage meter where current begins to be limited. Use this information to compute the power the passbank can dissipate knowing that P=I*V. It should become evident why the passbank is water-cooled along with the laser tube itself.

Now, if the tube pressure dropped and the voltage across it did as well, passbank voltage would rise (the voltage across the tube, and across the passbank, must equal the DC-bus supply voltage). If the passbank voltage were to rise to is 100V, what is the maximum current you can set without overloading the passbank (i.e. exceeding the power rating of the passbank you calculated)?

Practical Question: (Putting it all together ...) Assume you wish to use the CR-6 power supply to drive a newer Coherent plasma tube with a nominal rated tube voltage of 260VDC. Determine how to configure the power supply to use this plasma tube, specifically the transformer taps to use (there are many to choose from). Keep the passbank voltage around 30V for a "new" tube allowing variations to occur as the tube ages (e.g. the tube voltage will drop as gas pressure drops during use). Passbank voltage will rise as tube voltage drops - how low can the tube voltage drop before the maximum current allowed becomes limited?

Lab Submission:

Where a question requires an observation, each must be written with a few sentences describing each observation - For example, the answer the for item "... calculate the DC voltage available on the capacitors ..." is not simply "305V" but rather a few sentences describing the parameter and HOW YOU ARRIVED AT IT, something like "In order to compute the voltage on the main capacitor bank the phase-to-phase AC voltage, determined from the transformer taps, is 220V times xxxxx which yields a peak DC voltage of 305 volts.". A simple numeric answer for any point, without explanation of any kind, will yield a maximum mark of 50% for that answer!

Some answers require diagrams and figures to complete (as noted in the investigation). Many questions require research - answer can be found in the text (chapter 9), in the downloads,and in Sam's Laser FAQ

Prepare a lab report (word processed, never hand-written) as follows and submit the lab report in a folder or binder NOT simply a pile of loose, stapled papers!

Ion Laser Characteristics
1. Hand-in a completed lab worksheet for Part A
2. Hand-in a graph of output power vs. tube current for light-regulation mode.
   • When should current mode be used (applications/procedures)?
   • When should light mode be used (applications/procedures)?
   • Assume a laser was capable of producing 500mW at 30A tube current, the maximum tube current is 40A, and it is currently operating in light mode producing 500mW. What happens if the resonator misaligns slightly due to thermal expansion of the resonator (and so losses increase), as occurs during warm-up? (Explain the mode it will be in, expected output power, expected tube current, and explain WHY this will all happen).
   • Assume, now, in the above example that the maximum tube current is 32A as is set on the power supply. How does the laser behave now?

Quantum Mechanics
3. In the argon-ion laser energy level structure there are shared upper (ULL) and shared lower (LLL) energy levels.
   • When a LLL is shared by multiple transitions, for example in an ion laser, what is the overall effect on the output of each transition - will output power increase or decrease on each line with a shared level and WHY?
   • Generally, can multiple transitions which share a single LLL lase simultaneously? Cite a different type of laser other than an argon or krypton in which this is the case.
   • How is the situation of any laser affected by the lifetime of the LLL and what is the ideal case for a LLL in terms of lifetime of the level (and WHY)?
   • When an ULL is shared by multiple transitions, what is the overall effect on the intensity of each transition?
   • Generally, can multiple transitions which share a ULL lase simultaneously? Consider the individual cases of the HeNe, ion, and CO2 lasers in your answer and explain WHY these shared transitions can/cannot lase simultaneously in each case.
   • Is the situation of shared ULLs affected by the lifetime of the ULL at all - will a longer lifetime help multiple transitions with a shared ULL oscillate simultaneously (and why)?

Plasma Tube Engineering
4. Some tubes have ceramic bores with tungsten inserts (discs) while some have graphite discs.
   • Aside from Thermal considerations, explain the disadvantages of graphite discs
   • Describe any tube structures required when graphite discs are used (i.e. those electrical structures not found on a tungsten-disc tube but found on graphite tubes). Again, ignore thermal considerations.
   • Describe the function of these structures and how they work.
5. A particular argon-ion tube (ours!) has an overpressure condition - the tube starts showing overpressure when the tube current is increased to operational levels (30A). Our autofill is currently disconnected so a malfunction of that system that is NOT the cause.
   • What is the likely cause of the excess gas pressure in the tube (i.e. how did the laser "get like this")? Cite the process (name it and describe it) by which normal gas pressure in the operating tube is reduced, and the process during normal operation of the tube by which pressure is increased in the tube to keep operational pressure correct.
   • Where (from what tube structures) does the excess gas in the tube evolve from when it is not operating ? What is the process called, and describe how this happens.
   • What is the cure for an overpressure condition (i.e. How can the laser be made 'normal' again through simple operational procedures ?)
   • Why does the cure above work at all (cite the physical principles and the mechanism involved which reduce pressure in the tube)? (the answer is not to repump the tube - the tube is never opened / modified / cooled cryogenically during this process)
6. The pressure conditions inside operating (modern) ion laser tubes are usually determined by monitoring the voltage across the tube: high pressure is seen as a higher tube voltage. Describe how to tell if a krypton-ion tube (running multi-line optics) is "high" or "low" pressure simply by observing the relative optical output of two key lines (one red, one yellow).


Ion Laser Power Supplies
7. Describe the Capacitor Bank:
   • Draw a schematic diagram of the capacitor arrangement (the electrical configuration)
   • Show how the total capacitance (in Farads, as connected across the DC bus) of the main bank was computed
   • Calculation of the DC voltage available on the capacitors for a nominal input of 208V AC (rms)
   • Determination of the absolute maximum DC voltage available if the autotransformer taps were set for maximum voltage available (leaving the input at the "210" tap)
   • Explain why the capacitor bank is wired as it is and why a single capacitor cannot be used (hint: consider the rating of each capacitor)
   • If this maximum voltage is used, what is the expected value across each individual capacitor in the bank
   • Explain the function of the resistors across each capacitor and what would happen if they were simply not there
   • What is the approximate voltage ripple, in percent, of both a single-phase and a three-phase rectifier WITHOUT a capacitor (i.e. how large would the ripple be if the capacitor failed in each case)?
   • What can excessive AC ripple do to the components of an ion laser tube (see SAM's for the answer)
8. Passbank:
• In chart form, show the current through each passbank transistor and the percentage of max for each
• Explain how the defective passbank transistors were found (both methods)
• Compute the power the passbank can dissipate based on voltage and current capability (show calculations and explain assumptions)
• If the passbank voltage is 100V, what is the maximum current you can set without overload
9. Detail the configuration of the CR-6 supply for the new plasma tube. Include required autotransformer taps. Explain how these were determined.

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