The laser range finder is becoming an increasinglyvital component of high-precision targeting engage-ments for the individual soldier. Precise and accu-rate range-to-target information is an essentialvariable in the fire control solution of today’s sophis-ticated weapons. This range information is readilyprovided by a laser range finder, such as the currentfielded system, the Miniature Eyesafe Laser InfraredObservation Set. In the next generation of laserrange finders for the individual soldier, the goal is toproduce a smaller, lighter, low-cost version that iseasily weapon mountable and readily integrated intoother systems. The U.S. Army Communications-Electronics Command Night Vision and ElectronicSensors Directorate is addressing these laser rangefinder issues with the development of the microlaserrange finder ~mLRF!.1
The most expensive subsystem of a solid-state laserrange finder is the laser transmitter. Typical solid-state laser transmitters used in today’s military sys-
The authors are with the Night Vision and Electronic SensorsDirectorate, U.S. Army Communications-Electronics Command,R&D and Engineering Center, 10221 Burbeck Road, Suite 430,Fort Belvoir, Virginia 22060. The email address for B. W. Schill-ing is [email protected].
Received 7 September 1999; revised manuscript received 21 Jan-uary 2000.
2428 APPLIED OPTICS y Vol. 39, No. 15 y 20 May 2000
tems incorporate complicated optical benches thatcontain a large number of components, both opticaland mechanical for the resonator assembly. Thesecomponents must be individually fabricated, ma-chined, mounted, andyor aligned by skilled labor, amajor contributing factor to the overall cost of the sys-tem. To address these issues, we have developed anovel laser resonator, the monoblock, to simplify themanufacture of a 1.5-mm solid-state laser transmitterby reducing the number of components and simplifyingalignment procedures. This is accomplished in twoways. By applying the proper optical coatings di-rectly to the crystal faces, as in monolithic lasers, onecan eliminate some physical components, such as mir-rors, from the device. In conjunction with this mono-lithic approach, the crystals are fabricated with precisetolerances for crystal face parallelism and crystal face-to-crystal side perpendicularity, allowing the crystaledges to be used as registration surfaces. Underthese conditions, laser resonator alignment is essen-tially automatic, greatly simplifying the mounting re-quirements for resonator components. The crystalsare simply pushed against a horizontal and verticalstop for alignment, as shown in Fig. 1. The preciselyfabricated laser crystals can be easily bonded to thelaser pallet, with little or no alignment, resulting in acompact, one-piece, functioning laser resonator.
As mentioned, the monoblock laser makes use ofconcepts that originated in the development of mono-lithic or one-piece lasers. An early example of this
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type of laser is the monolithic ring laser, introducedin 1985 by Kane and Byer.2 The idea behind amonolithic laser is to cut precisely, polish, and coatthe gain medium so that the resonator is containedwithin a single crystal. Later, this technique wasapplied to optical parametric oscillator ~OPO! crys-tals, and laser designs employing monolithic OPO’swere reported.3–5 An early version of a monolithicOPO was demonstrated by use of LiNbO3,3 and, morerecently, Ketteridge et al. incorporated monolithiccomponents into their miniature eyesafe rangefinder, a precursor to the mLRF.4
Another relevant technology is the microchip laser,also commonly referred to as a monolithic or semi-monolithic device.6,7 A microchip laser typicallycontains several individual laser pieces sandwichedand bonded together into a one-piece, functioning la-ser. However, when microchip lasers typically con-sist of millimeter-sized laser pieces, the monoblocklaser incorporates larger-scale laser crystals, result-ing in a much higher output energy device.
Our 1.54-mm laser transmitter is based on a flash-amp-pumped, Nd-doped yttrium aluminum garnetNd:YAG! laser rod, with a saturable absorber Cr-oped YAG ~Cr41:YAG! Q switch and an intracavityotassium titanyl arsenide ~KTA! OPO for frequency
conversion. Operating characteristics of the mono-block laser are described, and the experimental re-sults of a prototype mLRF field test are presented.
2. Design
A block diagram of the monoblock laser transmitterdesign is shown in Fig. 2. The gain medium is asquare Nd:YAG laser rod with a 3 mm 3 3 mm crosssection that is approximately 25 mm long at its long-est point. Nd:YAG is the chosen gain medium be-cause it is readily available, relatively inexpensive,
Fig. 1. Monoblock approach used in the mLRF laser transmitter.
Fig. 2. Schematic of the resonator design.
and well understood. The Nd:YAG laser rod has 1%Nd doping. The back face of the Nd:YAG crystal iscoated to be highly reflective ~.99%! at 1.06 mm,acting as the rear reflector of the laser cavity. Key tothe monoblock concept, the back mirror is fabricatedto be perpendicular to the side of the crystal to atolerance of less than 610 arc sec. In addition, theface is given a concave radius of curvature of approx-imately 1 m for a stable resonator design. The sec-ond face of the Nd:YAG laser rod is cut at theBrewster angle to facilitate linear polarization in thecavity, which is necessary for effective wavelengthconversion by the OPO. It was undesirable to workwith a deviated laser resonator, so a small piece ofNd:YAG, referred to as an endcap, was incorporatedinto the design to bring the resonator back into astraight line. The endcap resembles a miniatureversion of the Nd:YAG square laser rod. It has a 1%Nd-doping concentration, a 3 mm 3 3 mm cross sec-tion and is approximately 10 mm long ~at its longestpoint!. The endcap has a Brewster cut on the firstface to complement the laser rod and is antireflectioncoated at 1.06 mm on the second face. Although theendcap face does not contain a resonator mirror coat-ing, a poorly cut endcap will introduce loss that is dueto wedging. Therefore, the faces of the endcap arecut to tolerances that are similar to those of the laserrod.
Although much attention has been given to diodepumping of solid-state lasers in recent years,8–10 welected to use a single flash lamp as the optical pumpource for the monoblock laser. A flash lamp waselected over diode pumping based on cost consider-tions, the robust and mature nature of the technol-gy, and its ability to function well over a broad rangef temperatures. The commercial photographicashtube from a Kodak MAX flash disposable cameraas an inexpensive and altogether acceptable device
or pumping the laser. The flash lamp is placed be-ide the Nd:YAG gain material and fastened to theaser pallet by epoxy. We constructed the flash-amp-pump chamber by placing a plastic hemicylin-rical cover, internally covered by a piece of copperape, over the flash lamp and Nd:YAG laser rod.
For the short-pulse operation needed for applica-ions such as range finding, the laser employs a pas-ive Q switch. The availability and robustness ofhe passive Q switch that operates at 1.06 mm is therimary reason we employ the intracavity OPO de-ign, as opposed to the 1.5-mm erbium-based laser.he passive Q switch is a Cr41-doped YAG crystal
with a nominal optical density of 0.3. The Q switchis antireflection coated at 1.06 mm on both faces, hasa 3 mm 3 3 mm cross section, and is approximately3 mm long. Although the Q switch does not containa resonator mirror coating, a poorly cut or positionedQ switch will introduce cavity losses that are due towedging and cause alignment difficulties. Again, byhaving the crystal fabricated to precise tolerances,similar to that of the laser rod, these problems areminimized.
We achieved wavelength shifting from 1.06 mm to
20 May 2000 y Vol. 39, No. 15 y APPLIED OPTICS 2429
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1.54 mm by using a monolithic KTA OPO. Coatingsere applied to the KTA crystal to allow for an in-
racavity OPO configuration. The first face has aichroic coating, which is antireflective at 1.06 mm
and highly reflective at 1.54 mm. The second face,hich acts as the laser output coupler, also has aichroic coating that is highly reflective at 1.064 mmnd 55% reflective at 1.54 mm. When incorporatednto the design shown in Fig. 2, the resonator is anntracavity design in which the 1.06-mm radiationscillates between the first face of the Nd:YAG crystalnd the second face of the KTA OPO. The 1.54-mmadiation oscillates between the parallel faces of thePO crystal itself. Again, the parallelism of these
aces is critical and is specified to within 610 arc sec.As has become common in modern use of OPO’s, we
mploy the intracavity design to take advantage ofhe greater fluence level inside the 1.06-mm laseravity. The intracavity OPO scheme was first pro-osed and studied in the early 1970’s.11,12 For sim-
ple resonator designs, the fluence inside the cavity isgreater than the fluence outside the cavity ~as withxtracavity OPO! by a factor of ~1 1 R!y~1 2 R!,
where R represents the reflectivity of the output mir-ror.13 Making use of this relationship to estimatethe difference in our case, the fluence inside the cav-ity is approximately 3.4 times that of the fluenceoutside the cavity. This greater fluence level makesit easier to exceed the OPO threshold for wavelengthconversion, therefore offering more robust perfor-mance. In addition, the intracavity design acts as a1.06-mm energy limiter. When levels of 1.06-mm en-rgy build to the point where the wavelength conver-ion threshold of the OPO is reached, the energy isonverted to 1.54 mm and output through the outputoupler before resonator damage can occur.
3. Experiments
The primary goal of the microlaser program is thedevelopment of a low-cost laser transmitter thatwould be suitable for use in a future military laserrange finder or related system. For this application,the goal is a laser that can emit from 1 to 3 mJ of1.5-mm energy in a relatively short pulse ~tens of
anoseconds or less! and at a low repetition rate ~1hot every 15 s!. The components described in Sec-ion 2 were procured, and limited testing of the indi-idual components was performed. The componentsere subsequently arranged on the ceramic laser pal-
et, and the entire transmitter was characterized.inally, the monoblock laser transmitter was incor-orated into a breadboard as a laser range finder.he results of these laboratory and field tests areeported in this section.
Limited characterization of the flash lamp from theodak MAX flash disposable camera was performed
o determine its suitability as an optical pump for ourow-cost laser. The broadband output energy perulse of the MAX flash lamp was found to be ;2.7 Jn an ;125-ms ~FWHM! pulse.
Slope efficiency of the monolithic KTA OPO waseasured as part of the component level testing for
430 APPLIED OPTICS y Vol. 39, No. 15 y 20 May 2000
he laser. The OPO slope efficiency is importantrom a systems point of view because it relates to theverall lasing efficiency and therefore to the system’sverall power requirement. To measure the slopefficiency of the monolithic KTA OPO, we pumped therystals with a variably powered 1.06-mm source andeasured the resulting output energy at 1.54 mm
KTA!. We used the Surelite II laser that operatest a 5-Hz repetition rate, has a pulse width of 10 ns,nd has a 2.5-mm beam diameter as the optical pumpor these experiments. By employing a half-wavelate and polarizer, one can continuously adjust theutput power from the Surelite II. Preliminaryeasurements were made on the monolithic KTAPO, and input–output energy curves were gener-ted from the data, as shown in Fig. 3. Figure 3hows that the monolithic KTA OPO has a slope ef-ciency of 33%. By extending the linear curve fit inig. 3 to the x axis, the threshold energy of the mono-
ithic KTA OPO is estimated to be approximately 13.5J, resulting in a threshold fluence of 68.8 mJycm2.Based on the intracavity OPO design shown in Fig.and the monoblock approach as illustrated in Fig. 1,
everal prototype laser transmitters have been as-embled to date. As indicated above, the flat faces ofhe rectangular crystals were fabricated to preciseolerances and coated in such a way that the individ-al components can be placed on a flat, ceramic plat-orm and pushed against a vertical stop forlignment. The first transmitter was assembled byand in this manner, using no special tools or equip-ent. With the individual components placed on
he ceramic laser pallet as shown in Fig. 1, the Nor-and Optical Adhesive 61 UV-curing epoxy was usedo cement the pieces in place. The flash lamp wasositioned close to the Nd:YAG square laser rod andpoxied to the laser pallet in a similar fashion. Inhis configuration, without the use of optical mountsr alignment fixtures, the transmitter was functionals a 1.54-mm source, emitting pulses of 3 mJ. Re-idual 1.06-mm radiation was measured to be ;300J out of the transmitter. For eye safety, we re-oved the 1.06-mm output by a system-level filter.y using a fast photodetector, we measured theonoblock laser’s FWHM pulse width. A typical
Fig. 3. Input–output relationship for the monolithic KTA OPO.
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pulse was found to be approximately 27 ns in dura-tion, as shown in Fig. 4.
In subsequent transmitters, the Nd:YAG rod, end-cap, and Q switch were placed on the laser pallet,pushed against the vertical stop, and epoxied as be-fore. However, an alignment jig was utilized beforewe applied epoxy to the final component, the OPOcrystal. Having the ability to align the output cou-pler ~on the final face of the OPO crystal! improvedoverall output energy, from around 3 mJ to more than7 mJypulse, with an average of approximately 4.5mJypulse. Thus it is concluded that some alignmentmay be desirable for maximum output, but it is notnecessary for laser range finding at modest ranges~see ranging analysis in Section 4!.
We measured the far-field, full-angle beam diver-gence by using a large-diameter spherical mirrorwith a focal length of f 5 914 mm. The laser beamprofile at the focal length of the spherical mirror,known as the far-field pattern, is shown in Fig. 5.We captured the image by using an Electro-Physics7290A vidicon camera in conjunction with Coherent’s
Fig. 4. Oscilloscope trace showing the 27-ns pulse width of the1.54-mm output.
BeamView Analyzer image capture software and la-ser beam diagnostics system. From the image,taken at the focal length of the mirror, the beam spotsize in the far field was measured as the circulardiameter that encompasses 86.5% ~1ye2 points! of theotal energy in the beam. The results for the mono-lock 1.54-mm laser under test were d ' 7.3 mm,
resulting in a beam divergence of '8 mrad.
4. Breadboard Range-Finder Testing and Results
To predict range performance, we modeled the1.54-mm mLRF using an internal software packagecalled Lasers95. The results are shown in Fig. 6.The curves in Fig. 6 show a 98% probability of detec-tion with a 0.001 false alarm rate for a 1-in. ~2.54-cm!nd 0.50-in. ~1.27-cm! receiver aperture under bothlear ~23-km visibility! and impaired ~7-km visibility!onditions. Other Lasers95 input parameters wereelected to reflect the mLRF system, such as a wave-ength of l 5 1.5 mm and the use of a p-i-n InGaAshotodetector as the low-cost receiver. The irregu-arities in the curves are due to rounding off theasers95 range predictions. As the graph in Fig. 6hows, to meet the goal of 3 km on a clear day, 2.5J of energy is required. Based on the Lasers95odel, the monoblock laser described in Section 3 is
uitable for incorporation into a mLRF. To demon-trate this suitability, we incorporated the monoblockaser transmitter into a breadboard laser rangender and conducted limited field tests.By use of an 83 telescope in front of the transmit-
er, the system beam divergence is nominally 1 mrad,s demonstrated by the far-field beam profile shownn Fig. 7. A p-i-n InGaAs photoreceiver with a 2.54-m-diameter receiver optic was optically boresightedo the transmitter. With the breadboard receivernd range-finder electronics integrated into the sys-em, we successfully ranged to several targets at dis-ances from 18 m to a highly reflective target ~whiteuilding! approximately 7 km away. Targets of op-ortunity consisted of vegetation, such as grassy hillsnd trees, and structures, such as wooden docks andarious buildings. The field test results are ingreement with the Lasers95 range modeling de-cribed above. These results demonstrate that the
Fig. 6. Lasers95 performance modeling for the mLRF.
20 May 2000 y Vol. 39, No. 15 y APPLIED OPTICS 2431
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monoblock laser transmitter is an appropriate sourcefor eyesafe range finding to at least 3 km.
5. Conclusions
A monoblock laser transmitter has been developedfor use in a small, lightweight, low-cost laser rangefinder. The significant reduction in size, weight,and cost is due to a palletized laser concept. Pre-cisely cut crystals of Nd:YAG, Cr41:YAG, and KTAre appropriately coated and arranged to form a pas-ively Q-switched, intracavity OPO laser that pro-uces pulses of ;27-ns duration with an energy ofominally 3.0 mJ at a wavelength of 1.54 mm. Therecise machining of the crystals allows the laserssembly to occur on a simple laser pallet, with onlyhe straight edges of the components to accomplishhe laser alignment.
In addition to a drastic reduction in the total num-er of parts in the laser transmitter, another advan-age of the monolithic approach to laser transmitteresign is elimination of the need for manual ~or ex-
tremely complicated mechanical! laser cavity align-ment during the fabrication of military lasers.Because of the inherent autoregistration quality ofthe monolithic approach, simple procedures can bedeveloped to fabricate laser resonators. In the fu-ture, some of these steps could be automated, furtherreducing the hands-on fabrication time.
In addition to range finding, future research anddevelopment of the monoblock approach to lasertransmitter fabrication will address other laser sen-sor applications ~e.g., laser radar and obstacle avoid-ance!. Future goals include a monoblock laser withincreased energy and repetition rate. To achievethese goals, a modeling effort is under way to further
Fig. 7. Far-field pattern of the mLRF system.
432 APPLIED OPTICS y Vol. 39, No. 15 y 20 May 2000
optimize resonator parameters, such as output cou-pler transmission, Q-switch optical density, and crys-tal dimensions. A thermal analysis is alsoimportant for operation at high repetition rates. Fi-nally, the monoblock approach can be applied to lasertransmitters based on other materials, further reduc-ing the complexity and cost of future laser systems.
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