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When I was growing up, two tech-nologies captivated both science

and science fiction: robots and lasers.Both started out expensive and compli-cated, but today, these technologiesare within reach on any budget. This isespecially true of lasers, which just ageneration ago were laboratorycuriosities and the stuff of adventure

novels. Now they are such an integralpart of our lives we have all but forgotten about them. We’ve lost theappreciation of how useful they can be.

Thanks to advances in semiconduc-tor technologies, you can purchase afully functioning laser for just a fewdollars. Given their low cost, and theunique properties and capabilities oflaser light, you may want to considercombining these two stalwarts of sci-fidom into your next project. Whatfollows are some ideas to pique yourinterest and, of course, a listing ofonline sources you can check out tofurther your education and experimen-tation into the world of lasers.

Lasers 101Though there are many types of

lasers, they all do pretty much the samething: Lasers amplify a source of photonsinto an intense beam of light. The wavelength of the light varies across the visible, infrared, and ultraviolet spectrum. Most people are familiar withthe ruby-red light of the common laserpointer. These operate at about 650-670nanometers (nm), depending on theirdesign. A newer class of laser pointersput out a green beam (about 530 nm),

which is useful because the human eye is most sensitive to light of this wave-length. The green lasers are more expen-sive to manufacture, so they cost more.

Many devices such as CD and DVDplayers use infrared lasers that put out aninvisible beam in the 750-780 nm range.While you can’t see the beam, aninfrared laser is nevertheless quite useful,especially when combined with sensorsthat are most receptive to light in theinfrared region. These include most typesof phototransistors and photodiodes, andboth CMOS and CCD image arrays.

And, of course, there are lasersthat emit light in the deep blue andeven ultraviolet region. These are fairlyexpensive, finding typical uses in such things as high definition DVDplayers, and validating the authenticityof paper currency.

The vast majority of lasers todayare the semiconductor variety. They aretypically constructed of a sandwich ofsemiconducting material that has beencleaved at exactly the right angle toallow a pinpoint of amplified laser lightto be emitted. At low currents, thelaser operates like a light emittingdiode (LED); with the proper operatingcurrent, the device emits true laserlight, described below.

Semiconductor lasers can be furtherclassified by their operating mode. Mostof the devices we are most familiar withare designed for continuous light output. These are operated within a con-trolled region of current; if the current istoo low, the light that is emitted lacksthe laser characteristics. If the current istoo high, the device will overheat and

burn up. To maintain the proper operat-ing output, a sensor inside the laser collects a portion of the emitted lightand a control circuit varies the current tokeep it within the prescribed range.

Other semiconductor lasers —capable of much higher light outputs —operate in a pulsed mode. They areoperated by applying a series of pulses,each one of a short enough durationthat the device will not overheat. Theintensity of the laser is controlled byvarying the duty cycle — the ratio of ontime versus off time — of the pulses.

Diode lasers may be self con-tained, or they may require separatedriver electronics. Self-contained diodelasers include the laser itself, as well asits control circuitry. You need onlyapply power. This is the case with laserpointers. Diode lasers without circuitryrequire a separate driver board. Theboard provides the correct voltage andcurrent to the laser diode at all times.

One advantage to getting a laserdiode and separate driver board is theextra flexibility in operating the laser.With a separate driver board, you oftenhave more control over the intensity ofthe laser output. The better driverboards have a separate modulationinput that allows you to use an externalsignal to turn the laser on and off veryquickly. Modulation speeds of 5-10 kHzare common.

The older style of laser (still foundon the surplus market) uses a tubefilled with various gasses. A familiarversion is the helium-neon laser, whichemits a red beam of 632 nm. The laseritself is constructed of a glass tube

Using Lasers WithYour Robots

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filled with a mixture of helium andneon gasses. A high voltage is appliedto terminals on either end of the tube.Carefully positioned mirrors on eitherend serve to amplify the light thatbounces back and forth. One of themirrors is fully reflective; the other ispartially reflective. Once suitably ampli-fied, the light escapes the partiallyreflective mirror and exits the tube.

There are yet more methods ofproducing lasers, including various crys-tals such as ruby and YAG, plasma, andeven Jello. I’ll leave it to you to researchthese if the subject is of interest to you.

The Properties ofLaser Light

Laser light is special for a numberof reasons. First is that unlike most lightsources, the beam from a typical laser iscomposed of a single wavelength, orcolor. The single wavelength makes itpossible to isolate the color, and ignoreall others. For example, if you’re designing a sensor that is only sensitiveto the light of your laser source, you canfilter out all but that light. You knowwhatever remaining light your sensor ispicking up is probably from your laser.

(In actuality, many lasers emit sev-eral specific wavelengths, called main-lines. These may be selectively filteredor split to obtain the desired color. Forexample, an argon gas laser emits botha green and a weaker blue light. A simple prism may be used to separatethe mainlines of a laser, while notreducing the light output of the beam.)

Recall from high school physicsthat while light is made up of photons,the photons travel as a wave. Becausea laser beam is made up of the samewavelength of light, the photons exitthe laser and travel in synchronism.This is called coherence. One strikingbenefit of coherence is the effect ofreflections of the laser light. Thesereflections cause the waves of light tointerfere with one another. What wasonce practically a “solid” beam of light is now a mish-mash of light raysthat commingle in measurable ways.Such so-called local interference forms the basis for a number of sens-ing techniques. I’ll cover a few in a bit.

Another useful property of laserlight is the limited degree to which itspreads as it travels through space. Thisis due to the nature of coherence,described above. All light eventuallyspreads out, but with the right laser andthe right optics, it’s possible to focuslaser light into an extremely thin beamthat stays thin for a longer distancethan regular light. Without this proper-ty, we would not have CDs or DVDs.

For robots, we can use this propertyto ensure a small pinpoint of lightregardless of the distance between thelight source and its target. The spotcaused by the laser beam will remain relatively small and compact whether thelaser is a foot away from the target or 20feet away. Simple collimating optics canfurther control the spread of the beam.

Last, and certainly not least, is thesheer intensity of a laser beam. A smallpocket laser, operating on a couple ofwatch batteries, can emit a light as brilliant as noon day sun. Of course,the area of the light is limited to a smallspot, but that works to our advantage.Even in average lighting conditions, it’srelatively easy for people and sensorsto spot the light of a laser beam.

Uses for Lasers inRobotics

Some applications for lasers inrobotics are obvious, and some are not.First to mind are decorative uses — dress-ing up the bot with colored lasers thatflash on and off as the machine drivesdown a darkened hallway. Combine alaser beam with a reflective diffractiongrating, and the beam is split into multiple sub-beams that dance aroundthe room as the robot travels. You can get metalized diffraction grating material at any party store. Just look forthe stuff that makes a rainbow whenyou look at it under ordinary light.

More practical applications for usinglasers with robots involve some type ofsensor. Light-based sensors are alreadypopular solutions in all types of robots,but the majority of these use standardnon-laser (i.e., non-coherent) infrared orvisible light. Such sensors work bydetecting the amount or direction oflight. The coherent nature of laser light

permits additional sensory techniques.One notable approach is to rely on

the local interference of a laser beamreflected off a surface. To the nakedeye, the local interference appears as“speckle,” a grain-like pattern thatmoves as the light or the surfacemoves. You could use this idea as away to measure movement and evendistance. Point a laser toward theground, and pick up the reflectionsusing a suitable sensor. As the robotmoves, the pattern of the speckle alsomoves in direct proportion to the direc-tion and distance of that movement.

Systems of these types that rely onlocal interference typically warrant amulti-cell sensor array. A single lightsensor is insufficient to detect themotion of the speckle pattern.However, sensor arrays, such as linearCCDs or even low-resolution CMOScamera chips, can be used to measurefinite differences in the speckle pattern.

Lasers also find use in various beacon and landmark systems used forrobotics. One or more lasers pointedupward from a stationary “lighthouse”are used to project a pattern of beamsor lines onto a white ceiling. A tradi-tional CMOS or CCD camera is pointedtoward the ceiling, and with the rightfiltering, sees only the dots/lines of thelaser. The unique orientation of thedots or lines reveals the location of therobot within the room. This is basicallythe same concept as the celestial navigation techniques used for centuries by mariners. It’s already usedin some commercial and experimentalrobotic navigation systems.

Recall above that because of theproperty of coherence, a laser beamwill keep its pencil-thin shape for alonger distance than ordinary light.With appropriate optics, a single sensorcan focus onto the same area that thelaser beam is being projected onto.Using a variety of timing techniques, it’spossible to construct a laser-based distance measurement device that canscan a room and build a 3D map ofobjects in front of the sensor. This is thebasic idea behind the expensive laser-based rangefinder systems build byGerman electronics manufacturer SICK.

Determining the distance between

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laser sensor and target can be accom-plished in a variety of ways. With fastenough electronics, it’s possible tomeasure distances using time of flightof the light itself, which travels about186,000 miles per hour. Perhaps amore common method that does notrequire fast switching electronics is to modulate the laser beam with a fairly low frequency sine wave. The difference between the phase of thesource laser beam modulation and itsreturned reflection indicates distance.

Laser SafetyThe light from a laser is highly

intense, and when aimed directly intosomeone’s eyes can cause severe painand optic damage. In the UnitedStates, lasers are regulated by the Foodand Drug Administration, or morespecifically, an FDA unit known as theCenter for Devices and RadiologicalHealth, or CDRH.

Lasers are roughly classified by thepotential damage they can cause; thisdamage is defined by relying on simplemetrics, such as whether the beam isvisible or invisible to the human eye, ifthe light of the laser is ever exposedoutside of the device it’s used in, the

power output of the beam (usuallyexpressed in milliwatts or watts), andwhether the beam is stationary or constantly in motion.

As noted on the FDA website, laserdevices are separated by class. Thelower class numbers are the least dan-gerous. Each of these classes has itsown warnings and restrictions for use.

• Class I products include laser printersand CD players where the laser radiationis usually contained within the product.

• Class II and IIa products include barcode scanners.

• Class IIIa products include laser pointers.

• Class IIIb and IV products includelaser light shows, industrial lasers, andresearch lasers.

The vast bulk of lasers available toconsumers is Class IIIa. Note that thelaser in a CD player is ordinarily a ClassI device (but that’s when it’s used insidethe player where its light is neverexposed). Used outside — you’vehacked a CD player and raided its optics— the laser is most likely a Class IIIa.

Also note that the FDA limits Class

IIIa devices to five milliwatts, as indicat-ed by a light meter specifically designedto measure laser output levels. It’s tech-nically possible to operate some laserpointers with a higher-than-normal voltage, or even to pulse them with significantly higher voltages. The resultis an increase in power output thatmakes these devices non-compliant.

Class IIIa and lower lasers are gen-erally considered safe, but only in theirintended application. Whether or notthe laser light may be harmful to peopleor animals depends on the outputpower of the laser, whether the laser isvisible, and if the beam is held station-ary for a long enough period of time. I’drecommend never using anything abovea Class IIIa laser in a robotics project,and then only use a visible light laser.

The beam of an infrared laser canbe damaging to the eyes, and whenstrapped to a robot, an unsuspectingperson or animal may be inadvertentlyexposed to the effects of the beam.When an infrared laser must be used,consider only employing it in a fashionwhere its beam is pointed down to theground, and not out or up.

If you must use a higher powerlaser, do so only with appropriateresearch and safety training, and besure to follow all laws and regulations.If your goal is to design a mobile lightshow robot, employing high power 10watt diode lasers, do so only after youhave fully immersed yourself into thestudy of laser safety.

Be aware that operating certainClass IIIb and above lasers in public without the appropriate safety measures may be against the law, andcould expose you to severe fines.

Finally, should you opt for olderfashioned tube lasers on your robot,know that these require high voltagesto operate. These voltages — typicallyin the 1-2 kilovolt range — can causenasty shocks. Be sure all wiring is covered. Lasers with glass tubes (likehelium neon) should be suitably protected in plastic or metal enclo-sures, to avoid the risks of broken glass.

SourcesIn addition to the sources listed

American Science and Surplus often carries optics and sometimes laser components.

here, be sure to check out the regularadvertisers in both SERVO and Nuts &Volts, as many of them carry surpluslasers and optics.

Almaz Opticswww.almazoptics.com

Even lasers need the occasionallens. This outfit sells optics for conven-tional and laser light applications.

American Science and Surpluswww.sciplus.com

Variable surplus merchandise, socheck their catalog. Often carriesoptics and sometimes laser compo-nents. Low prices.

Anchor Opticswww.anchoroptics.com

Low-cost optics and laser (diodeand gas) products.

Coherent, Inc.www.cohr.com

Leading manufacturer of industri-al, educational, and laboratory lasers.The site contains numerous applicationnotes and other useful information.

Industrial Fiber Opticswww.i-fiberoptics.com

Manufacturer of educationalgrade lasers. Check out their informa-tional pages.

Information Unlimitedwww.amazing1.com

Lasers, laser products, and opticsfor all sizes and types of interestingprojects.

Instaparkwww.instapark.com

Online retailer of laser pointersand diode laser modules. Fairly lowprices, even for the green lasers.

Jamecowww.jameco.com

Small — but impressive — selectionof diode lasers and laser pointers.

Laser Glowwww.laserglow.com

Red, green, yellow, and even bluelaser pointers and diode laser modules.

Laser Surplus Saleswww.lasersurplus.com

Laser Surplus Sales carries lasersand optics, at surplus prices. This includes variety of gas, solid-state,and crystal (e.g., ruby) lasers, and

upporting optics.

Melles Griotwww.mellesgriot.com

Maker of laser components,optics, and complete laser systems,

Jameco has a small — but impressive — selection of diodes and laser pointers.

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Another established company, Midwest offers a number of higher powereddiode lasers, including Class IIIb units of 40 mW and more.

from educational to industrial. Checkout their online tutorials, such as thesection on fundamental optics.

Meredith Instrumentswww.mi-lasers.com

Meredith Instruments is one of the

oldest names in hobby and educationallasers. Good selection of low-cost reddiode lasers, collimating lenses, andline-producing optics.

Midwest Laser Productswww.midwest-laser.com

Midwest Laser Products is anotherestablished company, offering a number of higher powered diodelasers, including Class IIIb units of 40 mW and more.

US Food and DrugAdministration CDRH websitewww.fda.gov/cdrh

Main portal to the CDRH pages atthe United States Food and DrugAdministration. Plenty of useful information and factoids about lasers,laser use, and laser safety. SV

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Gordon McComb can be reachedvia email at robots@robotoid.com

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