An investigation of the optical characteristics of the argon-ion laser including wavelength-selective optics and operating modes.

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

PreLab

• Print the LAB WORKSHEET
• 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 a 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), review the concepts of gain you had covered in BATP9301. According to the CRC Handbook, for the 488nm line A=0.78*10⁸s^-1, and for the 514nm line A=0.095*10⁸s^-1. Using formulae from section 5.8 of Csele one may find the cross-section of each transition (it is not even necessary to solve the numerical value for this, only understand the proportionality) - remember that t_sp = 1/A. Solving for cross section of the transitions, one may now deduce the gain for the transitions (5.8.1 and assume DN is the same for both transitions). Compute the RATIO of the gain of these line to see which has higher gain, and by how much higher. Logically, the higher gain line will then appear first. Report the relative gain difference and use this to justify results.

So why does the 514nm ultimately produce more power? The answer is 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).

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 "... when should light mode be used ..." is not simply "most times" but rather a few sentences describing why and when it is desirable with, perhaps, an example application. Simple numeric answers like "29.4mW", or a simple "yes"/"no" answer to a question asking "when" or "how" is insufficient for full marks: without explanation of any kind, this 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 - answers can be found in the text (chapter 9), in the downloads, and in Sam's Laser FAQ

Ion Laser Characteristics
1. Hand-in a completed lab worksheet
2. Hand-in a graph of output power vs. tube current for light-regulation mode.
   • When should current mode be used?
   • When should light mode be used?
   • Assume a laser was capable of producing 500mW at 30A tube current, is currently operating in light mode, and is producing 500mW. What happens if the resonator misaligns slightly due to thermal expansion of the resonator, 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 30A and is limited by the power supply. How does the laser behave now?

Quantum Mechanics
3. Describe and EXPLAIN the physics behind the behaviour of the 488/514nm lines ... explain why the 488nm line appears first and why the 514nm line ultimately yields higher output powers. Be sure to cite the physical characteristics (like gain, cross section, saturation intensity, whatever is required) of each transition in order to explain why these two lines behave in this manner.
4. In the argon-ion laser energy level structure there are shared upper (ULL) and shared lower (LLL) energy levels. In general (not specifically an ion laser, any laser really) ...
   • When a LLL is shared by multiple transitions, what is the overall effect on the output of each transition - will output power increase or decrease on each line 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 even simultaneously?
   • Cite a different gas laser, other than an argon or krypton, in which this situation (shared ULL) exists and describe if the situation in [e] holds true in this laser (i.e. do multiple transitions from the shared ULL laser simultaneously or not)?
   • 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)?
   • In the argon-ion laser the 454.5nm and 476.5nm transitions share an ULL - if broadband optics are employed what would be the expected optical output of the laser on these two lines? Will they lase simultaneously?

Plasma Tube Engineering
5. 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.
6. 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)
7. The pressure conditions inside operating 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).

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