This exercise will introduce you to the optical laboratory and some of its equipment. You will begin to use optical mounts and practice some beam alignment skills. You will also investigate the characteristics of a commercial optical power meter and use it to calibrate optical filters. A written report is not required, but you will need some of the values you measure for subsequent experiments.
You should take some time to explore the laboratory and learn what is there. The yellow bins under the table hold a variety of mounting hardware. The parts are all based on common dimensions for mounting, like an erector set or Legos. They can be assembled in many different configurations; feel free to play with them. Later, you may need to mount something in an unusual position while providing angular or translational motion, and it helps to know what is available.
The manuals for all the equipment are in the laboratory, either in the file cabinet or on the shelves. If you want to check one out to read at home, ask me. For this experiment you should read the manual for the Newport optical power meter, Model 1815-C, and the detector head 818-SL (the only one we have). For a good general reference on lasers, see Verdeyen or Siegman
The world's most powerful laser, used at Lawrence Livermore Laboratory to initiate nuclear fusion reactions, has a Safety Hazard Classification 4. You will be working with a Helium Neon (HeNe) laser of moderate power that has a Hazard Classification of 3a. Don't panic; the close numerical ratings say more about the classification system than about the dangers of our laser. There is a low, {\em but non-zero}, risk of eye damage with a Class 3a laser, even for direct exposure to the full beam, because your natural blink response and head movement will limit the time of exposure. Even so, if the beam should enter your eye, you will not be able to see anything for awhile; it's like having your picture taken with a {\em really bright}\/ flash camera, and it is not pleasant. Plus, there is some chance of permenant damage.
More information on laser hazards and safe practices can be found in the booklet Laser Safety Guide published by the Laser Institute of America \cite{LIA93}. There is a copy in the lab and you should read it.
Finally, not everything is dangerous. This laser beam will not damage normal skin, and there is no danger in looking at the image of the beam on a scattering surface, such as a card, paper, or the wall. The uniqueness and beauty of laser light is one of the pleasures of working in optics, so enjoy it, but protect yourself.
You will use the Newport 1815--C Optical Power Meter with the 818--SL detector head to measure the power of the HeNe laser and to calibrate attenuating filters. The basic setup is shown in Fig.~\ref{Setup-2mirror}. The alignment is not critical for these experiments; you just have to get the laser beam through the filter and into the detector. However, it is convenient to align the laser beam, which defines the experimental axis, parallel to the table surface and along a line of tapped holes, because that is the way all the standard mounting hardware is designed. Establishing the optical axis is the function of the two ``steering'' mirrors M1 and M2. While it is possible to so align the laser directly, it is not easy. Also, the laser is less likely to be dropped and damaged when it is fixed in place. Of course, many lasers are too big to be moved easily, making the steering mirrors essential.
Defining the experimental optical axis consists of setting up two targets for the beam that can be used to align the laser. Many different types of targets can be used; sometimes the axis is already defined by another piece of (immovable) equipment, or by the beam from another laser, which may be invisible. In some situations, a good deal of ingenuity is required in devising and positioning suitable targets. In our case, there are no such restrictions and we will use two adjustable apertures to define an axis:
Now your task is to align the laser beam through the two apertures. The adjustable apertures are nice because you can open the first one wide initially to get the beam roughly through and see what the error is, and then close it down to get more accuracy as the alignment gets better. You should place a screen at the far end of the table, past the second iris, so you can see the beam position easily---and to confine the beam to the table, a good safety practice.
Since the value of two steering mirrors may not be obvious, first try using just one. Besides, you need the practice.
Do you see the problem? The alignment is quite difficult because with this setup, there is no fine motion for translation, and when you move the laser, or the steering mirror mount, you destroy rather completely any angular adjustments you may have made. You need four degrees of adjustment freedom that are independent and reasonably orthogonal. The second steering mirror provides this.
Leave your steering mirror where it is, but move the laser and add another steering mirror so the table looks like Fig. 1:
This is such a common task in an optics laboratory that it is worth some thought about how the arrangement works. Adjustments to the second mirror, M2, change the {\em angle} of the beam, pivoting it about a fixed point on the mirror surface. Adjustment of the first mirror has two effects. Most importantly, it moves the location of the pivot point on the second mirror, which is a translation of the beam. Unfortunately the translation is not pure, because adjustments to the first mirror also change the angle of the beam entering, and thus leaving, the second mirror. The degree of mixing is related to the spacing of the mirrors. A skilled experimenter can achieve a pure horizontal or vertical beam translation (or an effective pivoting about a fixed external point) by adjusting both mirros simultaneously at the proper rates, and in the proper directions, one with each hand. This takes practice, and is called ``walking the beam'' in optics jargon. Thus, in all experiments the two steering mirrors should be placed within an easy arms reach of each other.
When aligning the beam to a particular axis, as through two apertures, or a laser tube, it is easiest to align the beam to the reference point nearest M2 first (the beam pivot point), using the first mirror (primarily a translation). Then observe the resulting angle and correct it with M2; of course, adjusting the angle will move the beam off the first reference point as well, which is where ``walking the beam'' comes in. It is an iterative process, and while it is not trivial, it is a lot easier with two steering mirrors than with just one, or none. One can construct or buy beam steering mechanisms that give true independent and orthogonal degrees of freedom, but they are elaborate, expensive, and require quite a bit of alignment themselves. Sometimes they are necessary, but usually the arrangement you have built here is sufficient.
The Newport Model 1815-C power meter contains electronics suitable to both semiconductor and thermopile detectors. The basic operation of the unit is described in the handout from the manual. You will be using a 818-SL detector head, which uses a semiconductor photodiode in the unbiased, photovoltaic mode. This combination is quite simple and easy to use, yet is surprisingly versatile. Look at the dial and note the range of powers that can be measured: from 2~watts to 20~nanowatts, a range of 10^8! Of course, 10^3 of that is achieved using an attenuator, but still, a linear dynamic range of 10^5 is very large for any type of sensor. A large dynamic range is one of the hallmarks of photodiode detectors.
Make sure the meter is off and then unscrew the attenuator on the front of the detector head so you can see the actual photodiode. It is the shiny black circle in the center. When making measurements with the attenuator, remember that the active area is considerably smaller than the input area of the attenuator. One common measurement error is to fail to collect all the light you think you are measuring.
While you have the attenuator off, explore the ultimate sensitivity of the detector and get a feel for what a few nanowatts of light is. It is not much, so you will need to shield the detector from as much room light as possible. A handy way to do this is to put a cardboard box over the detector head. I know it sounds crude, but boxes are readily available, easy to cut to the right shape with your handy pocket knife, and you can make holes wherever necessary for beams to enter. Every working optics laboratory I've seen has a collection of ``good'' cardboard boxes stashed away somewhere. Look around this lab and find a box of the appropriate size. It will probably have a few holes in it from previous use; cover them up with black masking tape---another secret tool of optical experimenters.
Put your ``custom light shield'' over the detector head; set the meter to the most sensitive scale, and turn on the meter. You will find that the meter is off scale and you cannot zero the reading using the Zero control: there is still too much light reaching the meter. Turn off the overhead lights and you should then be able to use the Zero control to set the meter reading close to zero. Now shine the flashlight around to see how sensitive the detector is. Turn on the room lights and change the scale on the meter to read the actual light leakage of your shield in watts; about how many photons per second are striking the detector? If we really needed better shielding (some experiments require a background of only a few photons per second), we could use black tape to seal the seams in the box and to seal it to the table, then maybe throw a black cloth over the whole thing, and turn off the lights, too. Sometimes one even has to disable or block the panel lights on instruments. That's really working in the dark!
You have seen that the "bare'' detector is too sensitive to use in room lights. Even with the 10^3 attenuator, the detector can measure the ambient light, but this value can usually be subtracted from measurements using the Zero control, if the ambient light is constant.
Turn on the HeNe laser. It will take only a few seconds to turn on, but you should let it warm up for 30 minutes or so to stabilize. As the temperature of the laser head rises, the length of the optical resonator changes and the number and relative intensity of the resonating optical modes varies. Thus, the output varies in a pseudo-periodic manner. I hope you have read this far in advance so that you have turned on the laser earlier and blocked it, so it is already warmed up.
If not, you can first observe the HeNe power fluctuations using the power meter, and then spend some time learning about the other equipment in the lab. In particular, learn how to operate the Hewlett Packard 54502A digital oscilloscope and the Stanford Research Systems (SRS) DG535 digital delay/pulse generator. Setup the SRS generator to produce a 1kHz rate, 4~volt output pulse with a width of 5 microsec, and check it on the oscilloscope. Can you narrow the pulse (go in steps) to 5 ns and still observe it?
When the laser is warmed up, you are ready to make measurements:
The third major cause of measurements is detector saturation: exceeding the dynamic range of the detector and operating at a level where the response is nonlinear. This is a very common error because it is easy to do and easy to miss; there is no obvious indication that something is wrong. In particular, when you are measuring a time-varying signal, the detector may be saturated even for very weak input signals by a dc ambient light that you are not setup to measure at all. Imagine trying to measure the v-i characteristic of a diode using a sinusoidal signal, but with a large and unknown dc bias. Obviously your results would depend strongly on the bias level you set.
The only real way to check for detector saturation is to change the input light to the detector a known amount, like a factor of 2, and see if the indicated reading changes the correct amount. Saturation is not a problem with this laser and this detector, but you should get in the habit of being suspicious and checking.
In future laboratories you will need to attenuate light by several orders of magnitude, more attenuation than is available on the New Focus wheel. Besides, as our "laboratory standard,'' the wheel should not be used for routine measurements, where it is subject to damage, but used only to calibrate other filters. The filter wheel cost about $250; uncalibrated filters cost only a few dollars apiece and are expendable. Expendable does not mean disposable; you should still handle them with care.
You will find a number of filters in mounts on the table labeled with letters. Look through them toward a bright surface or the overhead lights and try to guess their attenuation value. Your eye is a handy detector, and this is a useful skill, but remember your eye is logarithmic, just like OD values. If you look closely at the filter holders you will probably find a notation in pencil indicating their approximate OD. But, of course, the real way to measure them is to use the calibrated power meter:
There is another type of filter on the table that you should measure: a 633 nm "spike'' filter. A spike filter is sequence, or stack, of many thin film coatings that has a reasonably high transmission at one particular wavelength and a very low transmission for other wavelengths. The index of refraction of the layers alternates between high and low values; usually just two different materials are used, such a SiO2 and TiO2. The wavelength sensitivity of the stack results from interference between the reflections from the various layers, each of which is delayed by the travel time through different layers. In this case, the transmission peak is located at 633 nm, the HeNe laser wavelength. The filter is useful for detecting a weak HeNe signal by eliminating all other wavelengths of light. A spike filter is really a miniature, fixed wavelength monochrometer, the optical analog of a very high Q tuned electrical circuit. The half power bandwidth of the filter is about 10 nm; what is the equivalent Q?
The heading says it all. When you are finished, leave the lab the way you found it. In particular, you should not leave the apertures, steering mirrors, and laser all set up and thereby deny the next group the pleasure of aligning them. Be sure to turn off all the equipment, especially the power meter and the HeNe laser.
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This work is licensed by Bill Wilson and Jim Young under a Creative Commons Attribution License (CC-BY 1.0), and is an Open Educational Resource.
Last edited by Bill Wilson on Jan 19, 2005 1:59 pm -0600.
