Limits to the NEP of an intracavity LiNbO3 upconverter
Y. C. See, Shekhar Guha, and Joel Falk
Limits to low noise equivalent power (NEP) operation of a lithium niobate upconverter are investigated.Upconversion is achieved inside the optical cavity of an Ar-ion laser. Limits to NEP are imposed by limitsto conversion efficiency and by noise present in the upconversion process. Conversion efficiency is limitedby thermal effects in the lithium niobate. Thermally induced wedging, focusing, and aberrations are causedby the lithium niobate absorption at the 514.5-nm argon pump wavelength. The primary component ofnoise in the upconverter is due to upconversion of thermal radiation from the lithium niobate crystal. Thelowest NEP, at X = 3.4,um, achieved in this study was 8.9 X 10-14 W/Hz 1 /2 .
1. Introduction
Applications of IR viewing are numerous. They in-clude night viewing, remote sensing, and astronomy.Early efforts at IR viewing employed direct detectionusing semiconductor materials. These devices requirecryogenic cooling for lowest noise performance. Evenunder optimum conditions these detectors are sub-stantially noisier than are visible detectors. During thepast fifteen years, IR detection by parametric upcon-version has received increased attention.14 The workreported in this paper emphasizes the understandingof the limits to cw upconversion. In particular, we re-port on the operation of an intracavity upconverter.Upconversion efficiency is proportional to laser pumppower, and an intracavity upconverter can, in principle,use the large pump power available inside the laser'soptical cavity.
In previous work4 we demonstrated cw operation ofan intracavity LiNbO3-argon upconverter operating inthe 3-5-,um region. In the work reported here, we in-vestigated the limits to the NEP of the intracavitysystem. Two optical configurations have been inves-tigated, one employing a three-mirror optical cavity andthe second using four mirrors.
NEP in the upconverter is limited by noise andquantum efficiency. We find that the dominant noisecomes from upconverted thermal radiation from the hot
All authors are with University of Pittsburgh, Pittsburgh, Penn-sylvania 15261; Shekhar Guha is in the Department of Physics &Astronomy, and the other authors are in the Department of ElectricalEngineering.
Received 13 November 1979.0003-6935/80/091415-04$00.50/0.
(600-K) LiNbO 3 crystal. This noise is minimized by theuse of spatial and spectral filtering. The limit to theupconversion efficiency is imposed by the LiNbO3 ab-sorption at the 514.5-nm pump wavelength. The ab-sorption of argon pump light not only reduces the up-converter's efficiency but also causes thermal lensing,thermal focusing, and thermal wedging.5
In this paper, we discuss the limits to conversion ef-ficiency in both the three- and four-mirror intracavityunconverters and sources of noise in the upconversionprocess.
A. Lithium Niobate Three-Mirror Upconverter
A schematic of this upconverter is shown in Fig. 1.The laser cavity is formed by mirrors M1 , M2, and M3.The light from the IR He-Ne laser (Xi = 3.39 jim) ismixed with the argon laser light (p = 514.5 nm) togenerate an output at X = 446.7 nm. The mixing me-dium is a 5-cm long a-cut lithium niobate crystal, whichis heated to near 600 K for phase matching. At high IRinput powers, the He-Ne is chopped at 1 kHz, and the1-kHz signal output is observed on an oscilloscope. Atlow input powers or when the upconverter noise per-formance is recorded, pulse counting electronics areused.
Noise in the upconverter is due to leakage of laserlight, phototube dark current, laser plasma light, andupconversion of thermal radiation. Five dielectric fil-ters eliminate residual laser light while providing a 90%loss at the signal wavelength. The phototube, an EMI9789QA cooled to -20'C, produces 2-counts/sec darkcurrent. Plasma light is eliminated by a calcite prismand a dichroic filter located internally to the laser cavityand a spatial filter located outside the laser's opticalcavity. The spatial filtering is designed for minimumtransmission of noise consistent with near-maximumsignal throughput. The filtering system is shown as
1 May 1980 / Vol. 19, No. 9 / APPLIED OPTICS 1415
SpatialFilter
To PMT
446.7nmF2 - F4
Fig. 1. Three-mirror upconverter. The 514.5-nm and 3.39-,um beamsizes in the lithium niobate are 50 Am and 250 im, respectively.
" 4.7 w
M1 2 L1 PUP REJECTION FILTERS
cL2
A ONL E 5 ' .5R m3391 rm
Fig. 2. Four-mirror upconverter. The spatial filter shown in Fig.1 was also used in this upconversion configuration.
part of Fig. 1. The prism and the spatial filter togetherreduce the plasma noise by a factor of 1010. The re-maining 260 counts/sec of plasma light are eliminatedby the intracavity blue-absorbing filter.
In addition to plasma light, another important noisesource is upconverted thermal background. Thisbackground includes 300-K room-temperature black-body radiation as well as oven and LiNbO 3 thermalradiation. The LiNbO3 upconverter appears to belimited by this upconverted noise. This noise accountsfor approximately 30 counts/sec of photoelectron outputin the absence of 3.39-jim input and limits the NEP to3 X 10-13 W/Hz1/2 . A detailed description of this noiseis given later in this paper.
The conversion efficiency in the LiNbO3 upconverteris limited by absorption in lithium niobate. Absorption
at 514.5 nm (a = 0.025 cm-') limits intracavity pumppower to about 0.6 W. The NEP quoted above is ac-complished with a conversion efficiency of 4 X 10-5 andwith a pump power of 0.5 W.6 In an effort to overcomethe effects of absorption we place the upconversioncrystal in an auxiliary optical cavity that is stronglycoupled to the main laser cavity. We refer to this con-figuration as a four-mirror intracavity upconverter.
B. Lithium Niobate Four-Mirror Upconverter
The four-mirror cavity provides enhanced intracavitypower in spite of optical loss due to the upconversioncrystal. Our experiments represent the first applicationof this optical cavity to cw parametric upconversion.
The basic components of this upconverter are shownin Fig. 2. The four-mirror cavity provides (1) an en-hancement of the pump power due to feedback from thefourth mirror; and (2) a reduction in the effects of lossand distortion of the LiNbO3. This reduction is due tothe partial isolation of the LiNbO3 from the main lasercavity. As shown in Fig. 2, mirrors M1 and M2 form themain laser cavity; mirrors M2, M3, and M4 form anauxiliary cavity.
Lens L1 is chosen to provide a small beam waist (wo= 50 jm) in the LiNbO3 crystal and a large collimatedbeam in the argon plasma tube. The cavity optics aredesigned so that the TEMoo modes of each cavitycompletely overlap in the region between mirrors Mland M2.
Using the configuration of Fig. 2, we have achieveda cw conversion efficiency6 of 0.023% at pumping in-tensities of 1.3 X 104 W/cm2 . This corresponds well totheoretical predictions.4 We find that cw conversioiefficiency is limited by thermally induced wedging andlensing in the lithium niobate caused by the LiNbO3absorption at 514.5 nm. This distortion is visuallyobserved at pump powers greater than -1.5 W. Toverify the conjecture that high average argon powercauses distortion, we operate the argon laser in a pulsemode with a low (1:8) duty cycle. This minimizesheating due to 5145-A absorption. Peak argon powersof up to 4 W (Pp/A = 5.0 X 104 W/cm2) are applied in0.4-msec duration pulses. The upconverter's measuredpeak conversion efficiency is linear with pump powerfor P < 3 W (Fig. 3), suggesting that the limit to cw ef-ficiency is due to average power heating effects, not peakpowers. At Pp 3 W, the upconverter's peak efficiencydecreases due to the breaking of phase matching (dueto 514.5-nm absorption during the 0.4-msec durationpulse). Heating during the pump pulse is observed asa change in the upconverter's output pulse shape as thecrystal's nominal temperature is varied through phasematching. \ If the crystal's temperature (T) is set justbelow phase matching temperature (Tm), the upcon-verter output appears as a rising ramp. At T > Tm, theoutput is a decaying ramp. These pulse variations areshown in Fig. 4.
The upconverter's NEP was measured when theupconverter was operating in the 1:8 duty cycle. Aspatial filter identical to the one used in the three-mirrorupconverter reduced noise while providing minimal loss
1416 APPLIED OPTICS / Vol. 19, No. 9 / 1 May 1980
M4 / I
L4 A2f = 15cm d AlOOm
Ald 1.6mm
L,f =20cm >
- 6.00
*4.0 I
a, 40G 2.0 .
0
a0 0 1.0 2.0 3.0 4.0
Peak Pump Power (Watts)
Fig. 3. Four-mirror upconverter. Peak conversion efficiency vs peakpump power.
to the signal. With peak pump powers of 1.7 W (Pav -
0.21 W), the peak NEP was 1.1 X 10-14 W/I/ 1 2 . Thiscorresponds to an average NEP of 8.9 X 10-14 W/Hz'12
Phototube output in the absence of 3.39-tum idler inputwas 7 counts/sec due primarily to upconvertecblackbocly radiation from the hot LiNbO 3 . The averageconversion efficiency measured during this noise mea-surement was 6.6 X 10-5.6
C. Noise Sources in the Lithium Niobate Upconverter
The understanding of noise sources in the upcon-verter is important because the noise generated by theupconverter is a major determinant of the minimum IRpower that can be detected by the system. Filters in-side and outside the LiNbO3 upconverter remove twolarge sources of noise, plasma light and residual pumplight (5145 A). The remaining noise is at least partiallydue to upconverted thermal emission from the hot(e600-K) LiNb03. Smith investigated an upconvertersimilar to ours and found that he can account for onlyabout one-third of the system's noise as due to upcon-verter thermal radiation. Smith -postulates a newsource of quantum noise as the cause of the additionalnoise.7 A major goal of our noise studies is to provideexperimental data that either helps corroborate or in-validate Smith's theoretical explanation.
Polarization of the residual noise was found to be thesame as the signal. The angular spread of the noise wasestablished by scanning the focal plane of lens L 3 (Fig.1) with a 200-jim pinhole. Distances in the focal planeof this lens correspond to angles at the output plane ofthe crystal.8 Figure 5 shows that the apparent idler IRangular width (FWHM) is -5° (7.2-mm linear extentin the focal plane).
The signal power produced from upconversion of theradiation from a blackbody thermal source can be cal-cualted approximately using a plane wave analysis ofblackbody upconversion. The signal intensity(watts/M2 ) can be written9'10 as Is = (parametric gain)X (blackbody density) X (effective spectral bandwidth)
(a) T < Tm
(b) T=Tm
(c) T >Tm
Fig. 4. Breaking of phase matching during a 0.4-msec pump pulse.Perfect phase matching is indicated by T = Tm (b). The horizontal
scale is 0.1-msec division.
10m AnguI Spreod
0 6
FWHM 7.2 mm
z 4
2
300 500 700Aperture Position (mils)
Fig.5. Angular extent of upconverter noise. A 200-pm pinhole wasused to scan the first focal plane in the spatial filter shown in Fig. 1.
X (blackbody solid angular acceptance.). Using thenumerical data appropriate to LiNbO3 we can calculatethe total signal power produced by a 600-K blackbody,P8 = 7.52 X 10-9 B2Pp, where Pp is the pump (argon)power, and B is the angular acceptance of the detectionsystem at the signal's wavelength.
1 May 1980 / Vol. 19, No. 9 / APPLIED OPTICS 1417
1LJ
The angular acceptance of our system is dl/(2f,),where d, is the diameter of the first aperture (A1 ) in thespatial filter shown in Fig. 1, and f, = 25 cm is the focallength of lens L1 . Hence for d1 = 1.5 mm, Pp = 0.5 W,and with the detection system shown in Fig. 1 (quantumefficiency = 8.5 X 10-3), -640 counts/sec should beproduced by upconversion of a 600-K blackbody. Notethat this plane wave anal~ysis does not account for anytransverse variation in signal intensity at the crystaloutput or in the plane of aperture A2. The implicitassumption is that the diameter of aperture A2 is large.For a comparison of this theoretical result to the ex-periment, aperture A2 was removed, and a measuredoutput of 80 counts/sec was recorded. Thus if theemissivity of the LiNbO 3 were 80/640 = 0.12, the re-sidual noise could be totally explained by upconvertedthermal radiation.
Thermodynamic considerations dictate that the valueof crystal emissivity be equal to the absorption of thecrystal. The results of seven measurements of ab-sorption (6 = 3.39 m) gave values varying from 12 to20%. This variation is believed to be indicative of thevariation of transmission with transverse position in theLiNbO 3. We note that the values of emissivity recorded(12-20%) are sufficient to explain the residual noise.
It should be stressed that the calculation of uncon-verted noise made above is based on a simple plane wavetheory, i.e., the effects of diffraction and transversebeam intensity variations are ignored. We are nowextending the theory of upconversion of thermal ra-diation to account for the Gaussian transverse profileof the pump. Before this theory is complete, it will beimpossible to determine with certainty if the noise ob-served has its origin in blackbody radiation.
This work was supported by NASA Goddard SpaceFlight Center. We have benefited from continuingdiscussions with T. Kostiuk and K. Ogilvie.
The work reported in this paper was presented at the1979 annual meeting of the Optical Society of America,Rochester, New York, paper FE-5.
References1. F. M. Johnson and J. A. Duardo, IEEE J. Quantum Electron.
QE-2, 296 (1966).2. J. E. Midwinter, Appl. Phys. Lett. 12, 68 (1968).3. J. Falk and J. M. Yarborough, Appl. Phys. Lett. 19, 68 (1971).4. J. Falk and Y. C. See, Appl. Phys. Lett. 32, 100 (1978).5. Y. C. See, S. Guha, and J. Falk, J. Opt. Soc. Am. 69, 1476A
(1979).6. Upconversion efficiency is defined as the signal power detected
by the phototube divided by the IR power incident on the up-conversion crystal.
7. H. Smith, Ph.D. Thesis, U. California, Berkeley, 1976 (unpub-lished).
8. This noise scan was taken with lens L1 (Fig. 1 or 2) located be-tween the folding mirror and the LiNbO3 crystal. This lensperforms the functions of 1I and L3 in Fig. 1.
9. J. Falk and W. B. Tiffany, J. Appl. Phys. 43, 3762 (1972).10. L. E. Estes, R. F. Lucy, J. Gunter, and K. Duval, J. Opt. Soc. Am.
64, 295 (1974).
1980
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