One of the best things about blogging is the stuff you stumble upon while researching something else. Take, for instance, my recent Google search on optical fiber sensors. It led me, rather improbably, to the official Website and blog of Jean-Michel Jarre, a French composer, performer and music producer who has sold some 80 million records worldwide, making him one of the most successful musicians I've never heard about. (Jen-Luc Piquant, on the other hand, has far more wide-ranging tastes and has been savoring the YouTube videos of Jarre's live performances for quite some time.) Why did Jarre's name come up? Well, while reading about optical fiber sensors, I stumbled upon a mention of the laser harp: an electronic musical instrument that uses numerous laser beams, blocked at various lengths, to produce audible sounds. Jarre is the most prominent musician to use this instrument in his music. (He also uses various other cutting-edge synthesizers and electronic instruments, including the theremin.)
Laser harps have been around since 1976, when the first working prototype was invented by Geoffrey Rose, who built it out of a matrix of 5x5 laser beams in an octagonal frame. The laser harp is usually connected to synthesizer or computer to produce audible notes. The instrument only needs a single laser, although some of the newer versions employ multiple lasers that can be individually controlled by pulsing on and off. In the single-laser version, the beam is split into an array of parallel beams, usually spreading outward like a fan. (See this YouTube video of Jarre in concert to get an idea of how it looks and sounds. There have been rumors that the instrument is fake, but Jarre has just as many defenders. Regardless, the visual impact is pretty impressive!) 
Whenever a beam is blocked, the change is detected by a photodiode connected to the electronics, and the computer activates the relevant note. (That's in the frameless version favored by Jarre. The framed laser harp usually has an array of photodiodes embedded in the upper part of the frame.) By matching the timing of the reflected beam, the computer can determine which beam is being blocked, and therefore which note needs to be played.
Not just any old laser can be used to build a laser harp. You need one with at least 20 mW of power just to produce visible beams, but to get the best results, a laser with 500 mW of power or more is best. This means there's a high risk of suffering skin or eye damage unless one takes precautions: Jarre for instance, wears gloves when "plucking" the "strings" of his laser harp, and other users have been known to don protective glasses. Artist Jen Lewin has been known to use laser harps in her art installations. most notably at at Lincoln Center in 2000 and Burning Man 2005. And for those interested in perhaps building their own photonic instrument,, there are several online resources available.
Why on earth was I Googling optical fiber sensors in the first place? You might ask. Well, sometime last year, I came across a group of scientists at the Ecole Polytechnique de Montreal in Quebec -- led by a professor there named Raman Kashyup --who've created a novel musical instrument based on optical fiber. They call it a "photocello." It's not something that seems to have captured much attention in the scientific community -- I wasn't able to find hardly any mention of the photocello on the Intertubes -- but I still find the concept charming. The photocello employs a vibrating optical fiber to reproduce vibrations that mimic those of a stringed acoustic musical instrument (like a normal cello, for instance). Rather than several different strings, the photocello just needs the one optical fiber, and one common detector (a photodiode).
Optical fiber sensors are used in lots of different applications these days: to measure strain, temperature, pressure, or other useful parameters. These sensors detect a change in one of the properties of the optical fiber, and then translate that detected change into a signal that can be "read" or otherwise used in some way. For instance, optical fiber sensors are used in hydrophones used in seismic or SONAR applications; they are also used to measure temperature and pressure in deep oil wells -- an environment that is not especially hospitable to standard semiconductor sensors. And the Boeing 767 uses an optical fiber as a sensor in its optical gyroscope.
The Montreal scientists exploited an intriguing feature of optical fiber: a single one can represent the "richness of the multitude of harmonic frequencies," which is why the photocello only needs a single optical fiber sensor instead of several different strings. The optical fiber "string" can produce many different notes depending on which section of the stretched fiber one chooses to focus on. Perturbing a stretched optical fiber -- by plucking it, or otherwise causing it to vibrate, in any given segment -- creates a change in the interference pattern, which can be detected by the photodiode and then, through computer processing, demodulated into a reproduction of the harmonic vibrations one would find a stringed musical instrument. The sound is barely audible, although it can be easily amplified to recreate specific musical notes.
Kashyup has also built a photonic guitar, replacing the nylon strings on the frame of a standard acoustic guitar with a multimode optical fiber. Hit one string, and it generates a wave, which can then be transformed into an electrical signal using a photosensor. As with the photocello, the signal can then be sent to a typical audio system with amplifiers and speakers to produce audible notes.
Kashyup is certainly a prolific inventor. Fresh on the heels of his photocello -- which was recently featured in a Quebecois business newspaper called Les Affaires -- he has developed a new scheme to increase substantially the output power of a fiber amplifier. Apparently, the amplification takes place in the doped fiber cladding, not in the core (which is how it's done in the usual method). More specific details weren't available (at least not easily), probably because there's a patent pending.
I still think the photocello concept is pretty cool, as is the photonic guitar. You can listen to a sample of Kashyup "playing" the latter instrument here. It's a bit rougher-sounding than an acoustic guitar, but who knows? Kashyup might be the harbinger of the future of musical instruments. Jean-Michel Jarre could be working the photocello into future live performances as I type -- it should complement the laser harp quite nicely.
This is awesome. Thanks for sharing. There are usually these sensors used at big rock and pop shows, however, I would never mind just attending a light show - it's awesome.
Posted by: costume jewelry | June 16, 2008 at 06:54 PM
There is a huge mistake in this article that should be corrected: It was not Geoffrey Rose, who invented the Laser Harp; it was the frenchman Bernard Szajner. See my Laser Harp Pages (http://www.laserharp.org) for the complete history, as well as an interview with Bernard.
Manuel Schulz
Posted by: Manuel Schulz | June 17, 2008 at 04:17 AM
Hmph! This is what you get for scorning my love of electronica, missy. I've known about Jarre for years. There's a lot of innovation in this genre. This is very cool though.
Posted by: Lee Kottner | June 17, 2008 at 09:16 AM
Raman Kashyap (not Kashyup) has done some incredible writing in the field of fiber optics. His book on Bragg gratings was informative and intriguing. He has roughly 100 US patents and is one of the truly smart guys in the world of optics.
Posted by: Charles Boyer | June 17, 2008 at 03:34 PM
Manuel, Id suggest correcting the Wikipedia entry for laser harps, which cites the inventor as Geoffrey Rose. Far more people will use Wikipedia as a first stop than my humble little blog. :)
Posted by: Jennifer Ouellette | June 17, 2008 at 03:54 PM
hi jennifer,
i do regulary. unfortunately, geoffrey rose is undoing my editing all the time. he claims to be the inventor, but he is not. he contacted me, urging me to change the laser harp pages naming him as the inventor. he has absolutely no evidence that he is the inventor. nor has he sent anything to prove it.
i am in very good touch with bernard szajner, the original inventor of the harp. he sent me tons of infos about his harp, and everything that happened to it, including the story of giving the permission to jarre for the china concerts to use a laser harp.
anyway, i enjoyed your article. if you have any questions about the harp, you are welcome.
best wishes
manuel schulz
Posted by: Manuel Schulz | June 18, 2008 at 03:44 AM
Wow! You'd never heard of Jean-Michel Jarre, I was a big fan in the late 70s early 80s, although never got to see his pioneering instruments used live. Just for your interest, he's the son of composer Maurice Jarre who composed the famous music for Lawrence of Arabia, and countless others. J-M's big hit was Oxygene, which as a budding chemist at the time, I could not help but be drawn to. I guess if you're a physicist you should have been attracted to the later Magnetic Fields album.
db
Posted by: David Bradley | June 18, 2008 at 05:25 AM
This reminds me of a talented cat I've seen...
http://www.youtube.com/watch?v=_bawn7i2pGg
Posted by: Dave Mosher | June 18, 2008 at 08:17 AM
"See this YouTube video of Jarre..."
Youtube says,
"The video you have requested is not available." :(
Posted by: Srinivasa Ramanujam | July 04, 2008 at 02:38 PM