Walking the beam

"Walking the beam" is a staple of any laser-using experimental lab. Simply put,

These are the steps, which will be explained below. We'll assume each mirror has two knobs (i.e., two angles can be adjusted). If this isn't the case then well, you can guess what kind of time you'll have (or you will need four mirrors with two different kinds of mounts).

1. Mark the line in space you want the laser beam to occupy (the optical axis) at two points using irises (adjustable circular apertures).

2. Place one mirror (M2) very close to the first iris (I1), and another some distance away, closer to the laser.

3. Reflect the laser off of the two mirrors into the vicinity of the irises. This is a rough alignment, probably by just shuffling the position of the laser and mount around by hand.

4. Adjust the mirror closest to the laser (M1) to get the position of the beam right in the center of the first iris (I1).

5. Adjust the mirror closest to the first iris (M2) to center the beam on the second iris (I2).

6. Repeat steps 4-5 until the beam passes through both irises.

Why does this work? The beam itself travels in a straight line and can be described fully by 4 positional variables in a coordinate system defined by your line. One set of possible coordinates are, at a given point of the line, its x and y coordinates and the angles between the line and the z axis and each of the x and y axis. Alternatively, a line is just defined by two (x,y,z) coordinates.

If you could place the first iris right at the second mirror, it would be impossible to adjust the position of the beam on this iris using this mirror. By placing the first iris near the second mirror, you are making the second mirror the "angle controller" of the beam at the first iris (and the position of the beam on the second iris). The closer I1 and M2 are to each other, the more this is true. M1, on the other hand, will control the position of the beam on the first iris. Then,  "walking the beam" is essentially the iterative optimization of the angles and x,y coordinates of the beam at the location of I1. The process converges faster if the inter-mirror and inter-iris separations are large and if distance between I1 and M2 is very small.

There are some other tips to keep in mind when doing this.

First, any optical axis you choose should probably be parallel to the optical table you are mounting all your equipment on. There are at least two reasons for this:
1) if your line has a large angle relative to the table, the beam is probably aimed at eye-height somewhere away from where you are working (which is hopefully well-below that height!)
2) most optical mounts which are fixed to the table are designed to operate with an optical axis (i.e., your beam line through space) that is parallel to the table. This makes alignment of refractive and diffractive components much easier, since you can work in a more-or-less 2D world if you mount them properly. If you are aligning a diffraction grating or, worse still, a sequence of them with some lenses thrown in (such a sequence is found in a grating dispersive stretcher) and your beam path is not confined to some plane parallel to the table, you can guess what kind of time you're gonna have.

Second, you want your irises to be very precise and rigid. In the typical case, one would walk the beam and then use the subsequent, well-defined beam in a much more complicated optical sequence. One can use the irises as a tool to re-align or to put a different beam through the sequence, since they define the optical axis of the whole assembly. If they are aligned in a bit of haphazard way, somewhere down the line you'll eventually notice that the optical axis you wanted is not the optical axis you've got. This is probably the result of having I1 and I2 too close together and having them at different heights. Deciding on which height to use is generally one that requires having planned out the entire assembly. As for rigidity, if something in your assembly is bumped, rigid irises will allow you to re-adjust the bumped component back into alignment. If you only have two irises and one of them is bumped out of alignment, you can re-align it to the beam as long as the beam is still aligned. If the two go out of alignment at once, well...


Third, try to keep the angle of the mirrors from getting too large and keep the beam in more or less the center of them. Even though increasing the inter-mirror distance makes the convergence faster, it is sometimes better to not get too ridiculous with this, since at vast inter-mirror distances even the slightest adjustment of M1 will deflect the beam off of M2. If the angle of the mirrors is very large, it is almost impossible to avoid clipping part of the beam. 


In a complicated assembly, you will want to define the optical axis in at least a couple places. The reason for this is that, if 3000 knobs control the position of your focus at the end of a long chain of optics, any one knob can and will make that position wrong. If you have an optical axis which is defined at multiple points throughout the sequence, you can check approximately which knob you need to correct based on where you see your optical axis and the beam diverging (i.e., where the first iris that the beam is misaligned on is).

What the hell is the internet anyway? Part 2

Later, from about 12 through 16, I became heavily involved in what may be the most esoteric hobby possible and, equally or even more so, in an online community auspiciously devoted to it. Brickfilms.com, then a growing and active repository of Lego stop motion films and a forum for hobbyists of all ages, was frequented by maybe a few hundred or so aspiring directors from nearly the world over. Then at the critical size of maybe 30-100 very actively communicating contributors, it was a community as bona fide as any I've encountered.

Lego-based stop motion films are a topic for another reflection (although the parallels with experimental optics are so vivid as to make that reflection a likely one). Brickfilms.com was, at the time, as much a social entity as it was a topical discussion forum. In particular, the chat room which became an attached feature of the discussion board became a regular hang-out for many forum members, more than a few who had all but given up (in some cases before even starting) on actually participating in the hobby. For all its peculiarities it is, in retrospect, remarkable how very close to "real life" it was, how for those 3-5 crucial years of my life Brickfilms.com was, in a undeniable way, my de facto home on the internet and how the apparently radical differences between an internet forum and any other social vehicle were really, pretty minor.

Many brickfilms went far beyond this level of sophistication, though an equal number followed the lower road

On one hand, there was all manner of usual adolescent ribaldry (tempered, but less than you'd imagine, by the overwhelming fraction of the community which was evangelically homeschooled), with many discussions of girls and other decidedly non-Lego-movie topics. On the other was Brickfilms' diversity of ages and origins, a harmony of individuals that may have never been possible in a pre-internet age. And for all the darkness and baseness of the internet, it is this that actually serves as among the best evidence for the more-or-less goodness of people. There were as many adults actively participating in the community, both socially and as filmmakers, as teenagers (not to mention actual children). To be sure, there were many possibly single or at least childless men, ages 20-50, who were playing with Lego and who were also shooting the breeze with teenagers in a way that was, in retrospect, both admirably patient and cognizant. While no doubt much of the misanthrope-fuel of the internet is driven by a small number of participants, it is probably as much the anonymity that pushes the content to the top. Here was a community with a size where anonymity essentially vanished in lieu of actualized online identities. Reddit has, in a much more contrived but probably sincere way, attempted to reproduce this and has had some success (the relative ease with which one can slip into an anonymous account, fully separated from any established identity, is one weakness). Nonetheless, I think it is not a unique experience to become engaged in an online community with a diversity that seems to defy societal norms and yet functions in a ,well, functional way. Somehow, people from a staggering number of countries, of a large range of ages and with some obviously stark differences in philosophy, all carried on in a way with only the most trivial of conflicts. While Brickfilms.com may be a bit of a rarity, online role playing games seem to actively aim to foster this kind of interaction.

Around this time, my friends in the "real world" were becoming excited about the coming of World of Warcraft. Driven mostly by their enthusiasm and less by my own, I agreed to be a shaman or druid without much complaint and, by then feeling some historical significance to it all, joined them in the first open beta release of the game. Although I persisted for several months, dutifully and somewhat reluctantly playing the role of the healer in our adventures, I spent the majority of my playing time in WoW with my friends and, while the economy and, as the game was revised, increasing ease of interaction with others was fascinating, I eventually retired my account. My friends carried on the drive, however, and would appear to have become engaged in a community not unlike the one I experienced with Brickfilms (albeit less diverse at the time). For all the deserving criticisms WoW has accumulated since its debut, it also contains some of the purest goodness of the internet. In guilds, it has one of the most powerful mechanisms for fostering cross-cultural, cross-class friendships within organized communities with definite online identities and at a size where these identities have actual meaning (i.e., typically well below 100 to less than 10). It is hard to defensibly wax romantic about a game that I not only did not like, but which has since garnered as much hate as love. But to say it is one of the most powerful new objects to enter the internet cannon is possibly understating its significance.

Eventually, two other juggernauts became apparent. The first, Wikipedia, worked its way into my psyche around the same time the myspace craze was spreading amongst my peers. Myspace, being perhaps more open and increasingly spam-ridden, gave way to Facebook, an auspiciously cleaner and closed alternative.

My criticisms of Wikipedia could fill another post this size and even then only in summary form. Its very interconnectedness, scope and ease of use are also its greatest weaknesses. Its quality drops off exponentially as one leaves the central mass of carefully-pruned common knowledge. With every fault being amplified several fold as one moves outwards through the layers of specialization, Wikipedia can quickly become a frustrating and embarrassing mess. But even the most particular and narrow Wikipedia article has generally been written, if not coherently and if not with any particular mind to pedagogy or a logical structure, with some genuine enthusiasm and care. The external links and references, while a minefield of their own, often riddled with the whims of a particular self-interested party (usually one can make a clear guess as to the institutional affiliation of the primary author by a cursory glance of the references and external links), are often a entranceway into a much more useful source. But my early years with Wikipedia had all the unfiltered love of a young romance and as much as her disgusting quirks irritate me now, Wikipedia is the greatest website in existence. It is among the pillars of the internet and on almost pure knowledge alone. For casual (i.e., non-contributing) users, Wikipedia employs only the cheap trick of being an easy and enormous wealth of knowledge. Except perhaps in the most perverted magnification, Wikipedia is a rare internet entity that is nothing if not a sturdy thrust away from misanthropy.

Facebook is more divisive and more pervasive and, for the majority of internet users, is probably the closest thing to the de facto e-community I reminisced about above. Whether or not Facebook has been constructed and evolved in a calculating way or has simply been the organic following-ones-nose it appears to be, it is an almost enigmatically functional collection of features and interconnectedness that seem to really dig into the fibre of social fabric in a way that seems extraordinarily visceral. I wish I could say more about Facebook, especially because it probably represents the starting point for most new internet users, but it blurs the line too much between the online and offline world for me to feel entirely comfortable with it. Entirely online interactions and interpersonal relationships have a framework of manners and a protocol which is probably some combination of ad libbed, inheritance from offline interaction and e-necessary adaptations. On Facebook, the online and offline expectations and etiquette are fused in an unpredictable and haphazard way. Facebook is World of Warcraft but with the "game" replaced by the offline world and with the veil of anonymity and fantasy replaced by a more-or-less e-representation of ones actual self. Add to this that Facebook stretches long-dead personal relationships into an uncomfortable and almost surreal length and intimacy. Having participated in both, I find Facebook much more jarring, more intrinsically challenging, weird and unsettling.

But as an experiment in removing one crucial aspect of the internet, anonymity, from the equation, Facebook is worth reflecting on. Probably my earliest motivations to join were the usual ones. It is striking how quickly it became impolite to not be a friend-able member of the site. Very soon, my peers became a bit suspicious of anyone who was not available for scrutinization on the website.  Whether my life has been improved by it or not, I can at least agree that it is now not unreasonable to expect to be able to "check someone's social credentials".

On the one hand, the removal of full-on anonymity would apparently remove some of the darkest layers of the internet. But just as anonymity makes it easier to do wrong it makes it excruciatingly hard to be nice -particularly in the increasingly prey-mentality, psycho-phobic culture that has emerged in the internet era.  Some of the most unadulterated acts of kindness I have ever experienced were through the supreme anonymity of the internet. Even in WoW, where one has a quasi-identity, true kindness is much easier to give and receive. As much as I once hoped the slightly increased distance Facebook adds to any interaction might make politeness, kindness and compassion easier to implement, it seems to actually amplify the suspicious and awkward response these tend to receive in the offline world. In WoW, I found the world is a bit simpler and the threats and hysteria are so much further from actuality. Even if an act of kindness is "perpetrated" with ulterior motives, the worst consequences of those motives are so relatively minor that the impediment to just accepting kindness is markedly lessened. It is just as markedly easier to be make that act. And while it is similarly easier to be nasty and inconsiderate, it is just as markedly easier to flippantly move on without much offence. On Facebook, not only are most potential acts of kindness bound to be in the public record, for all to scrutinize and make judgements on, but they are in an arena which is perhaps even more full of caution and where the apparent ultimate consequences of any ulterior motive seem far more real. It is much easier to be kind for its own sake with other men (or presumably, with other women), but my experiments with this have ended badly with enough frequency to make me reluctant to continue. Most relatives are all pretty safe bets: Facebook is great in this respect. Overt kindness of any sort to the opposite sex within some range of your own age is very difficult. How precisely something is wrongly interpreted no doubt varies depending on the attractiveness of the parties involved but it seems rare that any exchange can just be kindness for its own sake. Offline, body language and intonation can be used to make a compliment or favour just "nice" instead of "creepy". On Facebook, creepy and nice intonations are practically indistinguishable. In all, Facebook essentially represents the internet with a bandpass filter: most of the worst of the internet is removed but most of the best is dampened equally.

So the internet is a supermarket for everything you can't find at a supermarket and where potato chips are pornography, profanity and piracy. It is a masked city on fast-forward and society on speed. It is a great throbbing mass of e-humanity with enough darkness to convert any rosy-eyed child into a raging misanthrope. But at all levels it contains a constant reassurance of the goodness of people,  even juxtaposed with the most stomach-churning gore, hate and ignorance we as a species can produce. It is just a hypothesis to be revealed true or false by a careful examination of a supermarket's balance sheet, but it may just be the case that to bring in the broccoli and blueberries, the grocer need to keep pushing the candy and potato chips.

What the hell is the internet anyway? Part 1

Answering a question with an implicit answer; a recollection.

It is safe to assert that I have grown up with, if not actually within, the internet. Like my earliest memories of life, my earliest memories of it are ill-formed and without any particular narrative. In elementary school, my class was walked through the process of acquiring an email address. The rationale for the then-novel exercise was that we would use our creatively chosen (read: ridiculous) "@hotmail.com" addresses to turn in electronic homework. In the neighbourhood of 8-10 years old at this time and with a level of web naivete (indeed, naivete in general!) which may be approaching extinction barely more than a decade later, myself and my colleagues rapidly fell prey to the most primal of internet predators; the chain letter. Complete with some of the most ruthless child-centric invocations for propagation (impending death, death of parents, siblings, animals, etc.), it was disturbingly contagious. As I recall with, no doubt, some embellishment, inboxes swelled to capacity as the exponential growth of asexually reproducing electronic messages overcame our essentially closed (save for probably a critical leak!) system of 20-30 children. Even as a child, I was begrudged to admit that the teacher's last-ditch effort to save our early e-lives was a comfort. Now I look back on her promise to save us from the chain letter's threats of death and destruction as as much a keen attempt to cure the epidemic using the same emotional and outlandish claims in reciprocation as it was the last gasp of a failed effort. We, the internet and I, have aged and grown in complexity (notice the purposeful avoidance of "matured")  but I would be hard-pressed to say the song does not remain the same.

Like this, but with more chain letter

Onward from that Lord of the Flies of the fledgling internet era, the next episode is less ripe for analogy with any classical morality piece and that is precisely the important point. Whereas chain letters have been successfully spread in the pre-internet era, Napster hid the mechanics of early file sharing in a veil of simplicity and an overwhelming abundance of free music. Surely piracy has been present before, even to children, with all the same moral ambiguity and controversy. Although it may have an apt place in a cycle of piracy and avenues for controversial and variously unpaid or "mis-paid" distribution schemes, Napster is perhaps the definitive, the resonant entry in this cycle. As has come to be a rule with the internet, ease of use and ease of distribution make moral ambiguities like the one posed by Napster especially ambiguous. Not only can something become rapidly mainstream but it can do so without any particular requirement for an understanding participant. It was not until I had already grown very attached, along with many of my colleagues in elementary school, to this new convenient musical distribution service that I even began to comprehend how the files were being proliferated and consider whether this was ethical. And to this, add anonymity and ubiquity. Then, as has become more clear in the evolution of the internet, add that the internet has become such a presence as to really and truly change the content of this and other debates. The accessibility and ubiquity of information, indeed of everything "not physical", non-negligibly changes it.

Napster 

But Napster was just a microcosm of the internet. At the same time, families were bringing the internet into their homes with no apparent conception of the vastness, the moral ambiguities, the  hearts of darkness and frustratingly beautiful potential it represented. Parents take their kids to the supermarket and watch them clamour over the candy and potato chips. On the internet, which is the essentially unsupervised (regardless of attempts one way or another) supermarket for literally everything else, pornography, piracy and profanity are potato chips. It is marvellous to think how thick the skin of the mind must be, now that we have grown up with the internet and borne witness to the darkest hearts and most ridiculous corners of humanity from, in all likelihood, the family living room. It is precisely this diversity and scope of morality and content that, far more accessible than potato chips from the corner store, makes it such an enigmatic behemoth. It is an extraordinary thing, as much as anything because it is so increasingly and actually ordinary. And to use but a single digital potato crisp, it is nothing if not really fucked up.

A whole new connotation to Lay's

In the second part, I will recall some other aspects of the internet through anecdotes about a forum for making animated Lego movies, my short-lived-but-not-entirely-unpleasant stay in the world of Azeroth and even Facebook. Also: humor on the internet is a bit odd.