A TUNABLE MID-INFRARED LASER SOURCE FOR REMOTE SENSING

Norman P. Barnes NASA Langley Research Center Hampton, Virginia 23665, USA

ABSTRACT

Many remote sensing needs can be effectively addressed with a tunable laser source in the mid infrared. One potential laser source is an optical parametric oscillator and amplifier system pumped by a near infrared solid state laser. Advantages of such a system and progress made at NASA Langley to date on such a system are described.

Keywords: Remote sensing, Mid infrared source, Optical parametric oscillator, Optical parametric amplifier

Many remote sensing applications could be effectively ad- dressed with a tunable laser source in the mid infrared. Among the applications are the remote sensing of trace at- mospheric constituents such as CO, CH4, 0 3 , and NO,. In addition, more common atmospheric constituents, such as H 2 0 and C02, could also be measured. Quantitative mea- surements on these constituents could be made by using the tuning capabilities of this device in conjunction with the Dif- ferential Absorption Lidar, or DIAL, technique. A primary advantage of this technique is the ability to measure concen- tration as a function of range by using a pulsed source and time of flight information. Another advantage is constituent identification which can be achieved by spectrally scanning the source. Other remote sensing applications could be ad- dressed without resorting to tuning. Here a primary appli- cation is the remote sensing of wind velocity. Wind velocity could be measured using Doppler techniques. Range infor- mation could again be obtained by using the time of flight technique.

Remote sensing in the mid infrared has several advantages over remote sensing in the visible or near infrared. One of the primary advantages is the existence of strong absorption features in this region. Atmospheric constituents, such as H20 and 0 2 , have strong absorption features in the near infrared, either because of their strong dipole moment or because of their abundance. Many trace constituents have the benefit of neither. To overcome this lack, the strong absorption features in the mid infrared can be utilized. Remote sensing in the mid infrared also benefits from the eye safe aspects of the radiation. Wavelengths longer than about 1.4 pm are considered to be eyesafe since these wavelengths are not, t,raasmit,ted by the vitreous hnmor of t,he eye. Consequently, the radiation is absorbed throughout the volume of the eye rather than being focused on the retina.

Nonlinear optics appears to be a practical method of achiev- ing a tunable mid infrared laser source. Although near in- frared solid state laser sources could be used as a pump

source for the system, mid infrared solid state lasers have several problems. One of the problems is associated with the lifetime of the upper laser level. Solid state lasers which operate in the mid infrared, such as Dy:YLF, tend to have short upper laser level lifetimes. Short upper laser level life- times require short pump pulses which, in turn, tends to limit the laser output energy or the efficiency. In addition, Lanthanide series solid state lasers, such as Er:YAG, suffer from limited tuning. On the other hand, transition metal solid state lasers, such as Co:MgF2, would often suffer from low gain. Other types of lasers which operate in the mid in- frared have other problems. Chemical lasers, such as HF or CO, have exhibited relatively high efficiency. However, these lasers tend to be only line tunable and have problems asso- ciated with the lifetime and handling of the often noxious gasses. Laser diodes can also produce mid infrared wave- lengths. However, they tend to be low power devices rather than the high energy per pulse devices preferred for remote sensing.

Optical parametric oscillators present advantages over other nonlinear optical techniques such as harmonic generation or stimulated Raman scattering. Harmonic generation pro- duces frequency multiples of a laser source. Thus, a tun- able source of mid infrared radiation would require a tun- able laser source in the far infrared. A CO2 laser could serve as such a source. However, a high pressure CO2 laser would be required to broaden the vibrational-rotational lines of this laser to produce continuous tuning. While the 4.2 to 5 . 3 pm region could be readily addressed using this tech- nique, the shorter wavelengths would entail successive har- monic processes. As each harmonic process has less than unity efficiency, usually less than 0.5, the process of gen- erating shorter wavelengths would become increasingly less efficient. In addition, the CO2 laser has lifetime limitations associated with the high peak currents needed for pumping and the dissociation of CO2. Stimulated Raman scatter- ing, on the other hand, would utilize a near infrared laser as the primary source. Since the Raman scattering process produces a fixed frequency shift, a tunable laser would be re- quired. If a short wavelength near infrared laser were used, such as Ti:A1203, several successive Raman shifts would be required. Successive Raman shifts would restrict the effi- ciency. If a long wavelength near infrared laser, such as Co:MgF2, were used, a low gain would make it difficult to produce the high peak power needed for efficient stimulated Raman scattering.

An efficient and completely tunable mid infrared source could be made using a near infrared laser as the pump and an optical parametric oscillator and amplifier system. One such combination would utilize a Ho:YAG or Ho:YLF

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laser as the pump and an AgGaSe2 or ZnGeP2 crystal as the nonlinear material. By using a sensitizer, such as Tm, in the laser crystal along with the active atom, Ho in this case, GaAlAs laser diode pumping could be employed. Laser diode pumping of the Ho:YAG or Ho:YLF laser would provide for a long lifetime device having high efficiency. By using GaAlAs laser diode technology, development of the laser diode arrays could be minimized since this is the same technology used for diode pumped Nd:YAG. A demonstrated high quantum efficiency, approaching 2:1, associated with such lasers would enhance the efficiency. A 2:l quantum efficiency implies that for each laser diode pump photon, at about 0.78 pm, two excited Ho atoms, each capable of producing a 2.1 pm photon, could be produced.

Efficiency of the optical parametric oscillator and amplifier is enhanced by using a long wavelength near infrared pump laser, one having a wavelength around 2.0 pm. An optical parametric oscillator can be considered to be a photon splitter. That is, each pump photon can be split into two photons. The energy of the two generated photons must equal the energy of the incident pump photon. In essence, this results from the conservation of energy. Efficiency of the optical parametric oscillator is limited by the ratio of the energy of the generated photon to the energy of the pump photon. An implication of this is that the efficiency of these devices is limited to the ratio of the pump wavelength to the generated wavelength. By using a long wavelength near infrared pump laser, this ratio is kept high. Furthermore, use of these long wavelength near infrared pump lasers allows nonlinear crystals with large nonlinearities, such as AgGaSe2 and ZnGeP2, to be utilized.

An optical parametric oscillator which tunes across the en- tire mid infrared can be constructed using a single nonlin- ear crystal and a long wavelength near infrared pump laser. Tuning is accomplished through the conservation of momen- tum. In essence, of the many combinations of photons which can satisfy conservation of energy, only one combination can simultaneously satisfy conservation of momentum. Coqser- vation of momentum also serves as a coarse tuning mecha- nism for the nonlinear interaction. Since the momentum of a photon depends on the refractive index, this parameter can be varied to tune the nonlinear interaction. In birefringent crystals, the refractive index can be varied by changing the direction of propagation in the nonlinear crystal. Varying the refractive index to produce conservation of momentum is often referred to as phase matching. Phase matching curves for both AgGaSe2 and ZnGePz indicate that the entire mid infrared, from 2.5 to about 12.0 pm, can be covered with a single nonlinear crystal. Having an extremely large tun- ing range enhances the versatility of the optical parametric oscillator.

A tunable mid infrared system could consist of a Ho:YAG or Ho:YLF pump laser and an AgGaSez or ZnGePz optical parametric oscillator followed by one or more optical para- metric amplifiers. A possible system configuration is shown in the figure. Since narrow spectral bandwidth operation of the optical parametric oscillator is desired, the pump laser should also operate in a single frequency mode. A single fre- quency near infrared laser could also serve as a laser trans- mitter for wind velocity measurements. Single frequency operation of this pump laser could be achieved efficiently by employing injection seeding from a single frequency continu- ous wave Ho:YAG laser. Output of the pump laser would be used to pump the optical parametric oscillator and amplifier. Single frequency operation of the optical parametric oscilla- tor could also be achieved by employing injection seeding. If injection seeding of the pulsed optical parametric oscillator were used, a continuous wave optical parametric oscillator could be used as the seed source. It could be pumped by the same continuous wave Ho:YAG laser used to injection seed the pulsed Ho:YAG laser. Whether or not injection seeding of the optical parametric oscillator is employed, the output

of this device would be amplified to the desired energy level in one or more optical parametric amplifiers. Having defined the general system, the level of development of the various components can be investigated.

Single frequency operation of a Ho:YAG laser pumped by GaAlAs laser diodes has been demonstrated for the first time at NASA Langley. Both single frequency and multi- ple frequency operation have been demonstrated. Multiple frequency operation of this device has demonstrated the po- tential for a 2:1 quantum efficiency, even when operated near room temperature. By employing a modest amount of cool- ing, operation at about 250 K, the quantum efficiency has been demonstrated to exceed 1.5. Quantum efficiency is ac- tually in excess of 1.5 since the losses in the laser resonator cannot be measured and taken into account. The marked dependence of this laser on the operating temperature re- sults from the quasi three level nature of this laser and the strong dependence of the deleterious up conversion process on temperature. A quasi three level laser implies that the lower laser level has a nonnegligilbe thermal population. Up conversion, on the other hand, reduces the population inver- sion by exciting Ho atoms out of the upper laser level.

Efficient normal mode operation of a Ho:YAG laser has been demonstrated at room temperature even when using rel- atively inefficient flashlamp pumping. Room temperature slope efficiencies approaching 0.03 have been achieved at NASA Langley and slope efficiencies of 0.05 by other re- searchers. By using laser diode pumping, the efficiency is expected to be substantially higher than this. However, a t room temperature, the Q-switched efficiency is more than an order of magnitude less than this.

Efficient Q-switched operation of a Ho:YAG laser can be obtained by cooling the laser. Data taken by NASA re- searchers indicates that the slope efficiency and threshold for normal mode and &-switched operation are quite simi- lar a t reduced temperature. At cryogenic temperatures, the slope efficiency of a fla.shlamp pumped Ho:YAG laser has been demonstrated to be in vicinity of 0.02 for both nor- mal mode and Q-switched operation. This efficiency was achieved in spite of the fact that the laser was restricted to operation in the TEMoo mode. While cryogenic tempera- tures were used in these experiments, a preliminary analysis indicates that temperatures this low need not be employed to achieve high efficiency. Achieving high efficiency will de- pend critically on optimizing the particular laser material, the concentration of both the active atom and sensitizer, and the operating temperature. Studies are underway to provide this optimization, including a search for better materials. A quantum mechanical model is being used to predict the spec- troscopic properties of new materials which are germane to laser operation.

An optical parametric oscillator operated at NASA Langley has demonstrated a threshold as low as 3.6 mJ and a slope efficiency of 0.31 at 1.5 times threshold. For this demonstra- tion, an Er:YLF pump laser was used with a 25 mm long AgGaSe2 crystal. A singly resonant optical parametric os- cillator was used for this demonstration. Highly repeatable operation of this device was achieved. The output wave- length in this case was 3.82 pm while the pump wavelength was 1.73 pm. Thus, the efficiency was limited to about 0.45. Consequently, a slope efficiency of 0.31 is about two thirds of the maximum performance. While the output energy of this device was relatively low, about 1.0 mJ, this energy can be increased through the use of optical parametric amplifiers.

Continuous tuning of this device between 2.6 and 5.1 pm was demonstrated using a single set of mirrors. As the resonant wavelength was tuned between 3.4 to 2.6 pm, the output wavelength varied between 3.5 to 5.1 pm. Tuning was limited by the reflectivity of the mirrors forming the optical parametric oscillator. Other wavelength ranges can

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be obtained using different sets of mirrors. Good agreement between the observed and predicted tuning curves can be obtained if the Sellmeier coefficients are rederived from the original refractive index data and the birefringence angle is taken into account.

A single pass gain in excess of 13 has been demonstrated in an AgGaSe2 amplifier at NASA Langley. An Er:YLF laser was used for these experiments. Using a slightly shorter piece of AgGaSe2, amplification of a 3.39 pm HeNe laser was demonstrated over the duration of the Er:YLF pump pulse. The gain of 13 corresponds to the amplification at the peak of the pump pulse. Amplification of plane waves is expected to proceed according to cosh2(l?!) where r depends on the pump intensity and ! is the length of the nonlinear crystal. However, since the pump cannot be well approximated by a plane wave, this amplification must be averaged over the pump beam cross section. After doing this, good agreement was obtained between theory and experiment.

While several of the requisite technologies have been demon- strated, further work has to be done. Principally, a diode pumped, high energy Ho:YAG or Ho:YLF laser must be demonstrated along with spectral narrowing of the optical parametric oscillator. While some cooling of the pump laser may be beneficial, new materials for a Ho laser may obviate the need for cooling. NASA Langley is actively pursuing de- velopment of Ho laser materials, both the continuous wave and the pulsed Ho lasers as well as the required nonlinear optical devices.

PULSED Ho:YAG

cw, Ho:YAG

SFO osc

TUNABLE MID INFRARED LASER SYSTEM

PULSED Ho:YAG

AMP

cw, AgGaSe 2

SFO

\ PULSED PULSED AgGaSe2 - AgGaSe2 4

osc AMP OUTPUT

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