spectral control of an alexandrite laser for an airborne water-vapor differential absorption lidar...

Spectral control of an alexandrite laserfor an airborne water-vapor differentialabsorption lidar system

Patrick Ponsardin, Noah S. Higdon, Benoist E. Grossmann, and Edward V. Browell

A narrow-linewidth pulsed alexandrite laser has been greatly modified for improved spectral stability inan aircraft environment, and its operation has been evaluated in the laboratory for making water-vapordifferential absorption lidar measurements. An alignment technique is described to achieve theoptimum free spectral range ratio for the two talons inserted in the alexandrite laser cavity, and thesensitivity of this ratio is analyzed. This technique drastically decreases the occurrence of modehopping, which is commonly observed in a tunable, two-intracavity-6talon laser system. High spectralpurity (> 99.85%) at 730 nm is demonstrated by the use of a water-vapor absorption line as a notchfilter. The effective cross sections of 760-nm oxygen and 730-nm water-vapor absorption lines aremeasured at different pressures by usingthis laser, which has a finite linewidth of 0.02 cm 1 (FWHM). Itis found that for water-vapor absorption linewidths greater than 0.04 cm-' (HWHM), or for altitudesbelow 10 km, the laser line can be considered monochromatic because the measured effective absorptioncross section is within 1% of the calculated monochromatic cross section. An analysis of theenvironmental sensitivity of the two intracavity 6talons is presented, and a closed-loop computer controlfor active stabilization of the two intracavity talons in the alexandrite laser is described. Using awater-vapor absorption line as a wavelength reference, we measure a long-term frequency drift ( 1.5 h)of less than 0.7 pm in the laboratory.

Key words: Alexandrite laser, differential absorption lidar, intracavity etalons, wavelength stabiliza-tion, spectral purity.

1. IntroductionSeveral experimental differential absorption lidar(DIAL) systems for making measurements of atmo-spheric water vapor have been developed and testedover the last decade.'- 6 These early experimentspointed out the extreme importance of having atransmitter fully optimized to the stringent require-ments of the water-vapor DIAL technique. Amongthe narrow-band, pulsed lasers capable of tuning overa spectral region having a selection of water-vapor

When this work was performed, P. Ponsardin and B. E. Gross-mann were with Old Dominion University Research Foundation,Norfolk, Virginia 23508. P. Ponsardin is now with the ScienceApplications International Corporation, Hampton, Virginia 23666,and B. E. Grossmann is now with Thomson-TRT D6fense, 78283Guyancourt, France. N. S. Higdon and E. V. Brownell are withthe Atmospheric Sciences Division, NASA Langley Research Cen-ter, Hampton, Virginia 23681.

Received 29 December 1992; revised manuscript received7 October 1993.

0003-6935/94/276439-12$06.00/0.© 1994 Optical Society of America.

absorption lines, few candidates are readily availableexcept for the dye and solid-state vibronic lasers.The problems encountered with the amplified sponta-neous emission from dye lasers, as well as the pros-pect of using solid-state lasers in future spacebornesystems, led NASA Langley Research Center to de-velop an airborne water-vapor DIAL system7 incorpo-rating an alexandrite laser. In the first phase of thisdevelopment, which is discussed here, the systemuses the alexandrite laser8 as the on-line DIAL trans-mitter, while a Nd:YAG-pumped dye laser is used forthe off-line transmitter. The second phase of thisdevelopment will permit the system to evolve towardan all-solid-state laser configuration.

The atmospheric water-vapor absorption lines at730 nm have the characteristic of being extremelynarrow-band transitions. They typically range from5 pm (FWHM) at an altitude of 8 km to 12 pm atground level. This characteristic renders the accu-rate measurements of the atmospheric distribution ofthe water vapor a complex process. 9 This is reflectedby the stringent spectral properties required for theon-line transmitter.

20 September 1994 / Vol. 33, No. 27 / APPLIED OPTICS 6439

First, a precise positioning of the laser wavelengthrelative to the water-vapor absorption line center isrequired for each laser pulse, if one assumes aline-center absorption cross section in the data-reduction process. This implies a good shot-to-shotspectral stability of the laser and a good wavelength-scanning capability for the initial setting. Whenusing the line-center cross section, one obtains aDIAL water-vapor measurement error of less than5% at any altitude between ground level and 5 km ifthe laser-wavelength deviation from the water-vaporabsorption line center is restricted to 0.8 pm.9

This deviation includes the long-term drift as well asthe shot-to-shot jitter of the laser wavelength. As away to achieve this kind of laser-line stability, theimplementation of an optical servo system to lock thespectral output of the laser on a wavelength referenceis suitable.

Second, a spectral output that can be consideredmonochromatic with respect to the absorption line isnecessary. This requires the use of a laser having anarrow-band output and a high degree of spectralpurity (more than 99.5%). The finite laser spectrallinewidth is a source of systematic error9,10 that canbe neglected if the ratio of the absorption linewidthdivided by the laser linewidth is greater than four.In this case, the actual cross section of the water-vapor absorption line will agree to within a fewpercent of the absorption-line cross section obtainedwith a monochromator. The spectral purity is de-fined as the percentage of laser energy transmittedwithin a given spectral interval compared with thetotal laser energy. This interval is defined here asthe spectral range containing three longitudinal modesof the laser cavity, i.e., 1.3 pm. This value of thespectral interval is chosen to be consistent with thevalue of the laser linewidth: the linewidth require-ment defines the number of intracavity modes thatare included in the laser spectral line. Here thespectral purity is a measure of the amount of energycontained in all the unwanted parasitic modes outsidethe laser line, resulting from a misalignment or fromthe design of the intracavity line-narrowing filter.A poor spectral purity (less than 99.0%) produceslarge systematic errors in water-vapor DIAL measure-ments9 that are difficult to correct.

All these spectral characteristics are required forthe on-line transmitter because we use the line-centerabsorption cross section in the data-reduction process.These requirements could be relaxed only if thesimultaneous quantification of all these parameterswas conducted shot to shot during the lidar measure-ments.

The alexandrite laser1"12 is tunable over the spec-tral range 725-785 nm, giving access to a goodportion of the 4u absorption band of water vapor (i.e.,717-738 nm). The rod is inserted in a Fabry-Perotlaser resonator, and an acousto-optic device is used toQ-switch the cavity. Pulses of 200 ns duration at arepetition rate of 10 Hz are transmitted with anaverage energy of 30 mJ. The energy was purposely

limited to 30 mJ as a way to avoid any internal opticaldamage. Three intracavity tuning elements narrowthe spectral output linewidth down to 1 pm (FWHM).They are scanned synchronously over a 150-pm range(3 cm-') anywhere in the tuning range of the laser byusing a processor-controlled routine. The broad-band intracavity spectral element is a five-plate bire-fringent tuner, and the two other filters are Fabry-Perot 6talons.

In Section 2 the characteristics of the intracavityspectral elements are analyzed in detail, and thetechnique used to adjust the Fabry-Perot free spec-tral range (FSR) ratio is described. Section 3 reportsthe measurements we conducted in the laboratory toassess the degree of laser monochromaticity. InSection 4 the closed-loop computer control of the twointracavity dtalons that is used to frequency-stabilizethe laser is described, and the measured laser-wavelength stability is reported.

2. Intracavity Spectral SelectionThe alexandrite crystal is a low-gain medium that hasthe characteristics of a four-level homogeneouslybroadened laser transition. However, when this me-dium is inserted in a Q-switched laser cavity, multi-mode operation is observed. This results from thenonsaturation of the gain during the pulse buildupphase, which permits a high number of longitudinalcavity modes to be amplified. Intracavity longitudi-nal mode selection is therefore required to achieve thespecified spectral output linewidth. Before analyz-ing the sensitivity of this mode selector, one musthave an understanding of the spectral narrowing andside-mode suppression processes to be able to definethe alignment tolerances on this device.

A. Spectral NarrowingThe choice of the high-resolution 6talon (HRE) thick-ness results from the estimation of the spectralnarrowing achieved during the pulse buildup time.The resulting spectral width (FWHM) 8X of the pulseis given with a good approximation' 3 by

8 = _2 aresin -[ R (21/2q,rrnt cos(O) arciFR~

- 1)1/2 , (1)

where n is the refractive index of the interplatemedium, 0 is the internal angle that is equal to the tiltangle in the case of an air-spaced 6talon, t is thethickness, R is the HRE reflectivity, and q is thenumber of round trips that can be made by a photonduring the buildup time before the gain reachessaturation.

The selected HRE consists of two flat mirrors eachwith 30% reflectivity, separated by 10 mm andmounted inside a cylindrical, air-filled cell. Thespace between the two mirrors can be finely con-trolled by three piezoelectric ceramic bars. The re-flectivity R has purposely been limited to 30% foroptical damage as well as insertion-loss reasons. In'this configuration the number of cavity round trips q

6440 APPLIED OPTICS / Vol. 33, No. 27 / 20 September 1994

can be reasonably estimated 3"14 to be at least 100.Using a value of 100 for q, we find that the calculatedlaser linewidth produced by a single peak of the HREis consistent with the DIAL requirement of 1 pm.

B. Side-Mode Suppression

The sole purpose of all the other spectral elementsinserted in the cavity is to suppress all the unwantedside modes of the HRE, leaving only the main HREtransmission peak. The side-mode suppression ra-tio (SMSR) is defined as the relative strength of agiven mode n compared with the main mode, ex-pressed in decibels: SMSR - 10 log(Pn/P 0). Theminimum value of the SMSR is determined by theacceptable level of spectral impurity (i.e., less than0.5%). To quantify the SMSR, we will use theanalysis developed by Sooy' 5 for saturable absorber Qswitching. This analysis can also be applied to anyQ-switched laser that has a large buildup time.

When the Q switch is opened the different HREmodes will grow independently of each other, untiltheir combined power is sufficient to start depletingthe alexandrite population inversion. The relativemode strengths that have been established at thattime are indicative of which modes will appear in theoutput pulse. If we consider the spectral emission inthe vicinity of the HRE main mode then we canassume that the side modes are suppressed by trans-mission-loss differentiation rather than by gain differ-entiation, because in the 730-nm region the alexan-drite gain slowly increases with wavelength at alinear rate of less than 2% per nanometer. Therelative strengths15 of mode n and mode m are

Pm (To) (2)

where Tn is the single-pass transmission term for thenth HRE mode. If we define To to be the single-passtransmission of the main HRE mode, the modesuppression ratio SMSR is given by

SMSR = -20q log(T) (3)

It is found that a single-pass transmission ratio Tn /Toof 0.95 provides a SMSR of 44 dB, which is consistentwith the spectral purity criterion. The require-ments on the additional spectral elements inserted inthe cavity are therefore to reduce the single-passtransmission for all the HRE side modes by a factor ofat least 0.95 relative to the HRE main mode. This isaccomplished by inserting into the cavity a thinFabry-Perot talon in conjunction with a birefrin-gent filter.

The thin 6talon, or low-resolution 6talon (LRE), is a1-mm-thick 6talon with a single-surface reflectivity of30%. The ratio R = 6.5 between the 6talon's FSR issuch that only the HRE modes that are separated by13 high-resolution talon FSR's overlap the LREmodes [see Fig. 1(a)]. All the HRE modes that arenot overlapping a LRE mode have a single-pass

1.0

0.8

0

Cl)

0.6

0.4

0.2

0.0

1.0

0.8

0.3Ml

0.6

0.4

0.2

0.0

730.2 730.4 730.6 730.8 73

Wavelength (nm)(a)

0

......................... ... ............................ ...I..............I........... .............I.......... .. ..... .... ........ .. .........

5<!0

...... .......... 7

730.2 730.4 730.6

Wavelength (nm)

1.0

730.8 731.9

(b)Fig. 1. Calculated single-pass transmission of the two Fabry-Perot 6talons (R = 6.5) and the birefringent filter: (a) individualtransmissions, (b) combined transmission. The correspondingvalues of n2 are shown for one FSR.

transmission of less than 0.95, resulting in good modesuppression. The effect of this second element istherefore to increase the FSR of the intracavity-modeselector from 26 to 320 pm, maintaining the resolu-tion provided by the HRE main-mode linewidth.

The birefringent filter is a five-plate design thatuses a plate ratio of 1:2:2:10:10. We calculated thethickness of the thinnest crystal quartz plate, d, inorder to produce a path-length difference of 6 be-tween the ordinary and extraordinary rays at thedesign wavelength (i.e., d = 0.5 mm). This providesa FSR larger than the alexandrite gain spectral width.Tuning is accomplished by rotating the plate aroundan axis perpendicular to its surface, thus maintainingthe Brewster orientation of the plate in the lasercavity. The width of its single-pass transmission iscalculated to be 1.5 nm (FWHM). Figure 2 showsthe calculated single-pass transmission of the birefrin-gent filter by using the Jones matrix formalism.'6The birefringent-filter transmission for the HRE sidemodes overlapping a LRE mode (i.e., 320 pm apart) isless than 0.90 for the two modes adjacent to the HREmain mode and less than 0.50 for all the other modes.This filter therefore suppresses all the remainingHRE modes that were overlapping a LRE peak, and itleaves only the main HRE mode. This single HREmode is narrowed down to 1 pm and is consistent with


.V �.V�..V .v .. ......... ...V ..._-V .. .... ... - .. ... .. .. ... - .V _

1:2:2:10:10

................. ............................. .. ....... ........................................................................................................................................5. ..

.................... ... .................. ............ ........ 15 m

~~~~~~~~~~~~~~~~~~~~~~~~~~~~. ..... .... .. ....... ...... ..A

740 750 760 770 780

Wavelength (nm)

Fig. 2. Calculated single-pass transmission of the five-plate bire-fringent filter for a tuning angle corresponding to a peak maximumat 728 nm.

the water-vapor DIAL requirements. Figure 1(b)shows the calculated single-pass combined transmis-sion of the three elements when they are perfectlyaligned.

C. Sensitivity of the FSR Ratio Adjustment

The birefringent filter is the broadest spectral ele-ment, and it is easily aligned to the LRE so that onlyone LRE mode overlapping a HRE mode is selected.The LRE to HRE alignment is more complex, and onemust make a careful FSR ratio adjustment to ensurethat there will be no mode hopping caused by a slightrelative drift between the two 6talons and to achieve acontinuous scanning range of 150 pm. In this sec-tion we discuss the sensitivity and the measurementof this ratio.

1. Sensitivity AnalysisThe FSR ratio adjustment is a twofold process. Oneadjustment consists of setting the FSR ratio R asclose as possible to 6.5 by changing the thickness ofthe HRE invar spacer. This is defined as the macro-adjustment AR and is typically of the order of 10-'.The other adjustment consists of aligning a givenHRE mode to a LRE mode by changing the HREpiezoelectric spacer voltage. This is defined as themicroadjustment bM and is typically of the order of10-5.

The combination of two Fabry-Perot 6talons inseries can be treated as one optical element having amore complex transmission function. It is referredto as the compound-6talon system (CES).17 Its spec-tral resolution is defined by the HRE resolution, andthe FSR is given by the ratio S. In the followingdiscussion we use the formalism developed by Mogh-rabi and Gaume'8 for the general treatment of two6talons in series with the same reflectivity. Thetransmission through the CES is given'8 by

m - COS(pl) m - COs(P 2)

Here qPl and YP2 are the phase retardations of the laserbeam in the LRE and HRE, respectively; 'pl and P2can be defined in terms of the integer numbers N, andN2 of FSR's of each talon necessary for mutualoverlap of modes to occur. We have NAul = N2A2c2with N2 > N1, where Au, is the FSR of the LRE, andAcr 2 is the FSR of the HRE. In this case we haveN, = 2 and N2 = 13. We also define n2 as the relativeorder of a HRE mode between two consecutive HREmodes overlapping a LRE mode (see Fig. 1): n2 is aninteger and we have 0 < n2 < N2. A CES mainmode is defined by n2 = 0; the other values of n2correspond to the CES parasitic peaks. We want todetermine the required precision in R needed to keepthe ratio Tn2 /T, below 0.95 for all the parasitic peaks.If R deviates from its half-integer value of 6.5, theparasitic peaks that will increase will be the onescorresponding either to n2 = 6 or to n2 = 7 dependingon the direction of this deviation. The transmissionof the parasitic mode n2 when To = 1 is given' 8 by

rn-i

= - cos[-2Tr + (2'Trn 2N1/N2)] (5)

Figure 3 shows the parasitic peak transmissionwhen To = 1 for n2 = 6 and n2 = 7 versus thedeviation AM corresponding to a change in N2 from12 to 14. It is found that AR has to be smaller than0.18 to have a mode suppression of the parasitic peaksconsistent with the spectral purity requirements (i.e.,Tn2 is less than 0.95) once bM is adjusted to have theHRE main mode overlapping a LRE mode. Thistolerance on the macroadjustment of R sets the levelof maximum attainable suppression of the strongestparasitic peaks when 8M is varied.

The tolerance on the macroadjustment of ' has tobe reduced to allow for the inevitable relative drift

1.00

.H

CD

0)

.,

_41E

C,,.4

0.98

0.96

0.94

0.92

0.90

0.886.00 6.25 6.50

FSR Ratio6.75 7.00

1 + R 2 Fig. 3. Single-pass transmission for the parasitic peaks correspond-with m = 2R ing to n2 = 6 and n2 = 7 versus the FSR ratio R of the two

etalons. For each value of5, n12 0 corresponds to a total overlap(4) of the respective peaks of each 6talon.


1.0

0 0.8

0

U)

M 0.6

U 0.4

E- 0.2

0.0700 710 720 730

............6 2nn=2

. ................. . \ ............................. . . . . ./

_ . . . ..,. . . . . . . . . .

between the transmission peaks of the two 6talons.This relative drift corresponds to a microchange 8Min that is caused by individual changes of theoptical thickness of each 6talon, and most of all that iscaused by the nonsynchronicity of the tuning of the6talons during a wavelength-scanning procedure.Mode hopping is defined as a sudden increase ofT02/To for a parasitic peak above the limit of 0.95, andwe define &Wmax as the maximum microchange in -that will not cause a mode hop; &-max is dependent onthe initial value of A. To be able to reduce themode-hopping occurrence during a wavelength-scanning procedure, we want the largest possiblevalue of &9?max.

For a given initial value of - corresponding to amutual overlap of the two 6talon modes, a change Swill cause To to depart from unity, and T0 2 for thestrongest parasitic peaks will increase. It can beshown that the dependence of T02 /To with respect to8M is given by

T__ m - cos[2iTk0 /(' +;g)]T. m - cos[2,rrk/(M + Ad)] (6)

where ko and k 0 are the interference order of the twoHRE modes corresponding to the CES main modeand the CES parasitic peak of relative order n2 ,respectively. Figure 4(a) shows the values of Tn2/Tofor n2 = 6 and n2 = 7 versus the microdeviation from-W for three values of R and for k = 27391. Figure4(b) reports the values of 8Rma. versus the initialvalue of M. It is found that 'max is reduced by afactor of 2 when the initial value of R deviates by 0.1from its optimum value of 6.5. It is therefore suit-able to restrict the tolerance on the macroadjustmentof W to ±0.1.

When the birefringent-filter transmission is per-fectly aligned with the selected LRE mode, the require-ments calculated for the FSR ratio adjustment arerelaxed as a result of the lower transmission experi-enced by the LRE modes that are adjacent to theselected LRE mode. However, most of the time thebirefringent filter is slightly misaligned with the mainLRE mode. The assumption in this analysis thattwo consecutive LRE modes have an equal transmis-sion (i.e., it corresponds to a birefringent-filter rela-tive drift of 80 pm) ensures that the calculated FSRratio sensitivity will not be increased by the relativedrifts of the birefringent filter.

2. Experimental ProcedureWe developed an experimental procedure to measurethe value of precisely. Using this measuredvalue, we can correct the HRE spacing to set - asclose as possible to its half-integer value. An argonlaser-pumped cw ring dye laser (Spectra Physics,380D) was used to scan simultaneously in parallel thetransmissions of both talons. This extremely nar-row-linewidth laser (10-4 cm-') can be scanned over arange of 2 cm-' (100 pm). Two successive scanswere therefore necessary to cover the whole FSR ofthe LRE (3.3 cm-'). The laser beam was divided into

1.4

1.3

1.2

E0

E-

1.1

1.0

0.9

0.8

- =6.4_ =6.5

- =6.6

0.7 - I W I | I |-40 -30 -20 -10 0 10 20 30 40

Deviation fromR, &X (x 10-)

(a)4

0

,X7

5

0

P4

aE

S

3

2

o '0.00 0.05 0.10 0.15 0.

43

3 '

1 y

t11

J o.20

Initial Ratio Deviation, AM(b)

Fig. 4. (a) Single-pass transmission ratio of parasitic peaks(corresponding to n2 = 6 and n2 = 7) over main peak (correspondingto n2 = 0) versus deviation bM from S. The calculation has beencarried out for three different initial values of -A. (b) Maximumratio drift 8,Wma. permissible without mode hop versus the initialratio deviation A from the half-integer value of R (i.e., 6.5).8&max is converted to the corresponding wavelength drift on theright axis.

four separate beams. Two of these beams were usedto scan in parallel the CES talons, one was used toscan a reference Fabry-Perot (FSR = 0.016 cm-')that provides a relative wavelength calibration, andthe last one was used to monitor the laser power.

The relative measurement of both FSR's in paral-lel, expressed in terms of the number of referenceFabry-Perot FSR's, provides a direct measurementof the ratio M. From this measurement, the spacerthickness adjustment Bt needed for the thick 6talon inorder to make this ratio close to 6.5 is determined by

R - 6.5at = 2A ,

2Aul

where R is the measured FSR ratio and Au, is thethin talon FSR (cm-'). After we adjust the spacing,we perform a second scan to check the final FSR ratio.


N1

(7)

Wavelength (a.u.)

Fig. 5. Simultaneous measurement of the transmissions of thetwo Fabry-Perot 6talons with a tunable ow dye laser. The lowertrace is the reference Fabry-Perot transmission curve (FSR =

0.016 cm-').

The value of R can be measured with an error ofless than 1% by using the above technique. Theerror is mostly due to the inaccuracy in determiningthe center position of a transmission peak. In thiscase the uncertainty was estimated to be less thanhalf a FSR of the reference Fabry-Perot (0.016 cm-').Using the above experimental technique, we reachedan acceptable ratio in the first attempt (i.e., =6.53 ± 0.05, see Fig. 5). The CES was inserted intothe laser cavity, and after the relative position of thetwo elements was changed, there was successivelasing of the two outer modes. Then there was aphase where no lasing occurred; finally, only thecentral mode was lasing. The simultaneous lasing oftwo adjacent HRE modes was never observed. Inaddition, a large continuous wavelength-scanningrange was obtained (i.e., in excess of 150 pm). Thisbehavior experimentally verifies the importance ofproperly choosing the value of R in order for only onemode of the HRE to be selected.

3. Characterization of the AlexandriteLaser MonochromaticityThe design of the cavity mode selector has beenanalyzed in terms of mode suppression and outputlinewidth. These two design criteria will ensurethat the laser output is consistent with the DIALrequirement of monochromaticity with respect to thewater-vapor absorption line. In order to confirmthis, we made measurements of the effective crosssection for different values of the absorption line-width of both oxygen and water vapor. The spectralpurity of the alexandrite laser at 760 and 730 nm wasevaluated by using a strong gas absorption line as anotch filter.

A. Effective Cross-Section Measurements

We used a long-path absorption cell (White cell type)filled with gas to obtain the desired absorption fea-ture to evaluate the laser output (see Fig. 6). Thecell is 3 m long, and the path length can be adjustedup to 300 m. The gas density was measured in thecell by using a temperature-controlled transducer(MKS-Baratron) operating in the 0-1000 Torr rangewith a specified accuracy of 0.15%. An energy ratio-meter (Laser Precision RJ7200) and two pyroelectricenergy probes (Laser Precision RJP-735) were used to

Fig. 6. Experimental apparatus used to assess the pulsed laserspectral characteristics: M's, mirrors; D's, detectors; BS, beamsplitter.

measure the White cell transmission. We performedempty cell scans to check that the cell mirror reflectiv-ity remained constant over the spectral scan range ofthe laser.

The effect of the finite laser linewidth, as well as thelaser jitter, on the cross-section measurements wasobserved in conjunction with the oxygen and water-vapor absorption lines at different pressures (i.e.,different absorption linewidths). From this observa-tion it is possible to deduce the minimum ratio of theabsorption linewidth to the laser linewidth to meetthe DIAL requirements on the laser monochromatic-ity with respect to the absorption line. The oxygenline (RQ 7,8) at 760.6765 nm with a strength of9.06 x 10-24 cm2 cm-1 molecule-' (where molec is themolecular weight) was used,19 and the path length ofthe White cell was adjusted to 16 m. The tempera-ture was maintained at 298 K while the oxygen pres-sure was varied from 100 to 1038 Torr. This re-sulted in a variation in the absorption linewidth from0.0182 to 0.0706 cm-' (HWHM). An absorption lineof the water-vapor molecule at 728.7378 nm with aline strength of 14.95 x 10-24 (cm-1/molec)/cm 2 wasalso used.20 The path length was adjusted to 80 m,and the temperature was maintained at 324 K. Thewater-vapor pressure was varied from 13.8 to 64.2Torr, which corresponds to a variation in the absorp-tion linewidth of 0.0255 to 0.0475 cm-' (HWHM).

The measured cross sections were then comparedwith the calculated cross sections deduced from themost recent spectroscopic data' 920 (see Fig. 7). Forabsorption linewidths greater than 0.04 cm-'(HWHM), the ratio of the measured cross section tothe calculated cross section stays constant. Thisindicates that in this case the finite laser linewidthand the laser jitter do not have any significant effecton the measured cross section. For absorption line-widths greater than 0.04 cm-', the effective crosssection can therefore be directly deduced from theavailable spectroscopic data to within 3%. This un-certainty is understandable when one considers thatthis absolute comparison is done with two differentsets of data, each one having an accuracy of betterthan 2%. Cahen and M6gie10 have shown theoreti-


Absorp./Laser Linewidth Ratio

0

-I

0.)

Q)co

M0

0C)S

V

'_

1.04 3

1.00 .

0.96 I

5 7

0.92 F

0.88 _0.01

00.03 0.05

00.07

Absorption LinewidthFig. 7. Ratio of the measured to calculated cross sections for twodifferent molecular species versus the molecular absorption line-width (cm-'). The corresponding ratio of the absorption to laserlinewidth is reported on the upper axis.

cally that the effect of finite linewidth is negligible(< 1%) as long as the ratio of the absorption width(HWHM) to the laser linewidth (HWHM) is greaterthan 3.3. This model result is consistent with ourexperimental data. In this study the ratio for whichthe effect of finite laser linewidth or nonmonochroma-ticity begins to be noticeable is approximately 4. Forwater-vapor DIAL measurements in the 720-nm re-gion, with this kind of laser the effect of nonmonochro-maticity will be negligible for average absorption linesbelow an altitude of 10 km. For altitudes above 10km, the appropriate correction for the effective crosssection used in the data reduction should be made.

To check the consistency of the measurements, weused the results from Fig. 7 to make an indirectmeasurement of the laser linewidth by using deconvo-lution techniques. For simplicity a Gaussian profilefor the laser line and a Voigt profile for the absorptionline were assumed, and the laser linewidth (assumedGaussian) was calculated to be 0.7 pm (0.014 cm-',FWHM). This result is consistent with the mea-sured value of 1 pm (FWHM) for the width of theactual three-modal laser spectral output.

B. Spectral Purity EstimationWith the relatively small optical depth used in theseeffective cross-section measurements, the transmis-sion was mostly sensitive to the laser-line profile andto the laser linewidth. By an increase in the pathlength and the strength of the absorption line, theeffective cross-section measurement becomes sensi-tive to any out-of-band laser emission. The strongabsorption line is used as a notch filter to suppressthe laser-line contribution to the long-path cell trans-mitted energy. The only remaining contribution tothe energy transmitted through the cell will be causedby the out-of-band laser emission that can therefore

be quantified. This method to measure the spectralimpurity was first described by Schwemmer et al.

2 'A strong oxygen absorption line (RQ 13,14) at

760.0493 nm with a strength of 6.44 x 10-24 cm 2

cm-' molecule-' was used in the first set of measure-ments. An oxygen pressure of 650 Torr was used,giving a total linewidth of 5 pm (FWHM). The pathlength was adjusted to 138 m, and the laser wasscanned so we could find the maximum absorption.An optical thickness of 14 was obtained, and theminimum transmitted radiation was below 0.01%(i.e., the sensitivity threshold of the energy ratiom-eter) at the center of the line. Thus, with more than99.99% of the laser-pulse energy absorbed by thestrong, narrow absorption line, the high degree ofspectral purity of the alexandrite laser was demon-strated in the 760-nm region.

We also conducted spectral purity measurements inthe wavelength region near 730 nm by using a strongwater-vapor absorption line. With 37 Torr of watervapor (H 20) and 240 m of path length, an absorptionof 99.85% was measured when the laser was tuned tothe H20 line at 726.5594 nm with a strength of30.061x10-24cm 2 cm-molecule-'. Thecalculatedabsorption at the center of the line for this opticalthickness is 99.95%, and the linewidth is 3.8 pm(FWHM). With the available long-path cell, it wasnot possible to increase the path length further.The difference between the measured absorption andthe calculated absorption is equivalent to a differenceof 13% between the measured cross section and thecalculated cross section. This cannot be attributedto the previously discussed effect of the finite laserlinewidth, because the ratio of the absorption widthto the laser linewidth is close to 3.8. Scatteringeffects in the cell as well as other instrumental effectscan be neglected, because the purity measurementconducted with oxygen demonstrated that the experi-mental apparatus was not introducing any significanterror. In addition, it is not possible to concludeunequivocally that this difference in the cross sec-tions is due to a small degradation of the spectralpurity of the laser when it is operating at 730 nm.Among the several other factors that can account forthis underestimation of the measured cross section,the most significant is the modification of the laserspectral profile by the water-vapor absorption.9 Foran optical depth of 7.5 and a ratio of the absorptionwidth to the laser linewidth of 3.8, this effect can leadto an error of 10% in the measured cross section.This measured absorption value of 99.85% at 730 nmcan be considered as a lower limit of the spectralpurity estimation that demonstrates the still-highspectral purity of the laser in this wavelength region.

Considering the worst case, i.e., the 0.15% ofresidual transmission that is due to an out-of-bandcomponent of the laser output, Ismail and Browellfound that the error introduced in the DIAL measure-ment by using an uncorrected cross section wasacceptable. 9 Therefore, below 10 km this laser canbe assumed to be monochromatic in the DIAL mea-


Oxygen line----------- -a

5 Water- Vapor Line

I

, II ,4, I1

IIw ,II,

II

l

surements. To correct fully the lidar returns for theerror introduced by the laser spectral output above 10km, one must carry out an extremely precise andcomplete mapping of the laser spectral distribution.

4. Wavelength StabilizationIn addition to monochromaticity, the deviation of thelaser wavelength relative to the absorption line centershould be minimized for accurate DIAL measure-ments. Consequently, a stabilization device has beendesigned to correct for the frequency drift caused bylong-term mechanical drifts, pressure variations, andtemperature variations that are encountered in anaircraft environment. Before describing the wave-length stabilization device, we present a quantifica-tion of the environmental effects on the cavity modeselector. This will clarify the need for a stabilizationdevice and the choice of the wavelength-locking tech-nique selected.

A. Sensitivity Analysis of the Wavelength Drift

1. Birefringent FilterA birefringent filter model22 based on the Jonesformalism' 6 has been used to analyze the behavior ofthe birefringent filter with temperature and pressure.To tune this filter in wavelength one must rotate theoptical axis of the crystal,23 which is in the planedefined by the plate surface. The sensitivity of thisadjustment is found to be 4.7 nm/degree of arc. Themechanical drive resolution of 1/1000 degree of arccorresponds to a wavelength resolution of less than 5pm, which is therefore sufficient to align this filteraccurately. The temperature sensitivity2 4 caused bythe combined effect of the change in the materialrefractive index as well as the material thermalexpansion has been estimated to be 85 pm/ 'C. Thepressure sensitivity caused by the change in therefractive index of air causing a variation in therefracted angle has been estimated to be less than 0.2pm/Torr. This effect can therefore be neglected.If the birefringent filter drifts by 96 pm, the single-pass transmission for an adjacent HRE mode selectedby a LRE mode (i.e., 320 pm apart) will increase from0.89 to 0.95, thus causing a mode hop. The birefrin-gent-filter transmission drift should therefore berestricted to be less than 95 pm. Temperature con-trol of the birefringent filter has not been imple-mented because the temperature changes are slowenough to require only an infrequent manual adjust-ment of the filter transmission wavelength.

2. Fabry-Perot EtalonsTo assess the performance of the compound Fabry-Perot 6talon in a varying environment, we havecalculated the effect of variations in the pressure andtemperature on its peak wavelength. The wave-length of an 6talon transmission peak is given by

kX = 2nt cos(O), (8)

where k is the order of interference, t is the thickness,n is the refractive index of the interplate medium, and0 is the internal angle. By differentiation, one ob-tains an expression for the change in wavelength2 5

caused by small changes in n, t, or 0:

8X = X + t- tan(0)80 , (9)

with 0 = O + 80/2 ( being the initial angle).Usually 0 is set small as a way to reduce the insertionlosses associated with an intracavity talon.26 Forsmall angles, 0 = nair/n, with nair as the refractiveindex of air and 4 as the 6talon tilt angle.

In terms of temperature changes T, pressurechanges 8P, humidity changes 8H, spacing variations8tvar, and tilt-angle variations 854),va, the general for-mula for the wavelength change 8X for an air-spaced6talon is given by

, Ii anar nair a nar8X= A[- S- T + a SP + - HI

Kr aa aH /

+ t t 6T + 8tvar - tan())85)var] (10)

The space between the two reflective plates con-tains ambient air. The changes of index caused byhumidity variations can be neglected because theyaccount for a change in wavelength of less than 0.04pm/Torr of water vapor. A change in ambient pres-sure or temperature will cause a change in therefractive index of the interplate medium' 7 accordingto

( anair = (n,-1)3.8753 x 1-3 (1+ 0.003661T) 2 ,aT J= 760

(11)

( ani) = (n. -1) (1.3149 x 10-3 + 1.626 x 10- 9P),aP =1

where nO is the index of standard air (i.e., for 1 atm,15 'C and 730 nm, nO = 1.0002751). The pressurechange of air will account for a wavelength change of0.26 pm/Torr, and the combined effect of tempera-ture on the refractive index of air and the 6talonspacing (i.e., 50 nm/ °C) will account for a wavelengthchange of 3 pm/ 'C. The extreme sensitivity of thethick 6talon to environmental changes is readilyapparent. In addition, the sensitivity of the 6talon'swavelength with respect to the plate spacing is calcu-lated to be 0.07 pm/nm, and assuming an intracavityinsertion angle of 1 degree of arc, we calculate thesensitivity of the 6talon's wavelength with respect tothe overall 6talon tilt angle to be 12.5 pm/mrad.

For a solid 6talon, the general formula for thewavelength change for small environmental changes


and small mechanical variations is given by

[ an 1 at 1 nairX=A XI- T + -TT- -tan -4)[n T t T n n

Ianair an ~X ( ST + 4 ai SP

aT aP

+ (i anair SH + nair8(l)var)1 (13)

The change in the refracted angle, caused by changesin the refractive index of air associated with changesin temperature, pressure, and humidity, can be ne-glected because these effects account for SX of lessthan 10-4 pm/ C, -5 x 10-5 pm/Torr and air, and-5 x 10-6 pm/Torr of water vapor, respectively.The combined effect of temperature on the refractiveindex of the material (i.e., fused silica) and thethermal expansion of the material accounts for a SX of7 pm/ C. In the same manner as for the air-spaced6talon, one should control the tilt angle to minimizethe associated wavelength changes. The sensitivityin 4) is equal to the air-spaced talon tilt sensitivitymultiplied by the talon's refractive index ratio (i.e.,nair/n), resulting in a calculated sensitivity of 16.7pm/mrad.

B. Stabilization MethodWith no control on the HRE, we observed laserwavelength drifts as large as 3 pm/h in the laboratoryenvironment, where the temperature can fluctuate byseveral degrees. The wavelength drift of the LREdoes not directly produce a wavelength drift of thealexandrite laser output, because the LRE acts onlyas a thick talon order selector and therefore is onlyresponsible for mode hopping. With two separateopen-air thermal enclosures, the talon's tempera-ture can be controlled in the laboratory with aprecision of 0.2 pm. Because of the precise adjust-ment of the FSR ratio, a large tolerance in thespectral position of the thin 6talon with respect to thethick talon is permitted (i.e., 4 pm), and modehopping was rarely observed in the laboratory.Nevertheless, spectral control of the LRE has to beimplemented in anticipation of the more hostile air-craft environment: the temperature can fluctuaterapidly over several degrees, and the cabin pressuredrops to 620 Torr for a typical flight altitude of 6 km.As a way to achieve the required wavelength stability,the talons should be freed from their extreme sensi-tivity to the enviromental conditions.

This can be realized by enclosing both talons in atemperature-stabilized pressure vessel. For reasonsof system flexibility and development time and cost,we did not select this approach. Another methodconsists of using the measured wavelength of thelaser output to generate a feedback signal as a way tostabilize the wavelength.27-29 This method can beimplemented for a single wavelength-tuning intracav-ity element. In the present cavity configuration, the

laser wavelength results from the superposition ofthree tuning elements that have their own separateoptical, mechanical, and thermal responses, whichmakes corrections for mode hops an additional prob-lem. Therefore, an active 6talon-transmission stabi-lization technique was selected. We control the sepa-rate optical thicknesses by using an externalwavelength as a reference. A stable laser outputwavelength will result from the insertion of a wave-length-stabilized mode selector into the jittering lasercavity.

1. Experimental ApparatusWe used a frequency-stabilized cw He-Ne laser as anexternal reference to control actively the LRE andHRE (see Fig. 8). The He-Ne laser used in thissystem has 1 mW of low-amplitude noise outputpower, is linearly polarized, and has a long-termfrequency stability better than 2 Mhz/24 h (UltraStable Laser from Laboratory for Science). Theintensities of the reflected He-Ne laser beam fromthe HRE and transmitted He-Ne laser beam throughthe LRE are kept constant by the control circuit.The optical thicknesses of the two 6talons are continu-ally readjusted to hold these intensities constant,thereby stabilizing the transmissions of the talonsrelative to the He-Ne laser wavelength. The paththrough each talon is initially adjusted so that theHe-Ne laser wavelength corresponds to the half-maximum of each of the Fabry-Perot peaks. At thispoint the slope is maximum, which in turn providesmaximum sensitivity for the monitoring of the Fabry-Perot transmission.

The He-Ne laser output is divided into threeseparate beams. One of these linearly polarizedbeams goes through a X/4 wave plate. The resultingcircularly polarized beam is reflected by the thick6talon through the rear mirror of the laser cavity.

BeamSplitter

Fig. 8. Active stabilization of the optical thicknesses of thealexandrite laser intracavity talon, using an external He-Ne laseras a wavelength reference: A/D, analog-to-digital; D's, detectors;BRT, birefringent tuner; P's, prisms; M's, mirrors; BS, beamsplitter.


The reflected opposite-handed circularly polarizedbeam is restored to a linear polarization orthogonal tothe incident one after passing through the same /4wave plate. This reflected beam is directed toward aphotodiode by a polarizing beam splitter. One of theother He-Ne laser beams goes directly through thethin 6talon and then to a second photodiode. A thirdphotodiode is used as a He-Ne laser-power reference.

These three signals are sent to a data-acquisitionand control board (IBM), where they are digitized to12-bit accuracy and read every 2 ms by the stabiliza-tion software. The appropriate voltage to be appliedto the thick 6talon piezoelectric spacers is generatedas a way to maintain constant the ratio of thereflected beam power and the reference power, thusmaintaining constant the optical thickness of the6talon. In the same fashion, transistor-transistorlogic pulses are sent to the thin 6talon stepper-motordriver whenever the angle has to be adjusted tocorrect for any drift in the optical thickness.

2. Computer-Controlled Operation of the Etalons

The stabilization software that controls the 6talonswas written in PASCAL. The software controls thethick 6talon optical path by adding or subtracting therequired amount of voltage to or from the highvoltage applied to the piezoelectric spacers by theramp generator (Burleigh). By sending a 12-bit wordthrough the data-acquisition and control system, onegenerates a voltage between -5 and +5 V. Thissignal is amplified with a gain of 125 and added to thehigh voltage applied to the piezoelectric bars. Thepiezoelectric spacers have a coefficient of expansion of+2.5 nm/V, and the voltage resolution of the errorsignal is 0.3 V, thus permitting the spacing to becontrolled with a resolution of 0.75 nm. The wave-length resolution SX of the transmission-peak posi-tion control corresponding to this is

x atax = A--V, (14)

where t is the 6talon spacing (mm), SV is the voltageresolution (V), and X is the wavelength of interest(nm). For an average spacing of 10 mm, a voltagestability of 0.3 V, and a wavelength of 730 nm, thewavelength position of a transmission peak of thethick 6talon can theoretically be controlled with aresolution of less than 0.1 pm.

We tested the software program by plotting theintensity of the thick 6talon reflected beam on astrip-chart recorder. The intensity was steady innormal conditions (less than 2% of variation, whichcorresponds to a wavelength stability of 0.18 pm), andit exhibited a small deviation that was recovered inless than 1 s when a sudden temperature change wasapplied to the thick 6talon.

C. Wavelength-Stability Measurement

The experimental apparatus previously described inthe spectral purity measurements was used to ob-serve the frequency drift of the laser. Its output

wavelength was tuned to an oxygen line located in the760-nm region (A band). The laser scan was thenstopped, and the stabilization program was turnedon. By the observation of the temporal evolution ofthe White cell transmission, the laser-frequency driftwas determined. 21 This can be done only when amode hop does not occur, because the measuredtransmission change results from the combined ef-fects of wavelength drift and mode hops. When nomode hop occurs, the wavelength drift from linecenter, 8X1,, is given by

ln(R/Rbs) 1]2 (15)

where R is the transmission of the cell (outputenergy/input energy), Rba, is the baseline or 0% ofabsorption, R0 is the transmission of the cell at thecenter of the line, and y is the HWIM of the line(cm-').

The main advantages of using an oxygen lineinstead of a water-vapor line are the relatively smalllinewidth (5 pm at 1 atm), the capability of having ahigh optical thickness with reasonable path lengths,and the absence of wall adsorption. These resultsobtained at 760 nm should be a good indication of thewavelength stability at 730 nm, because here we aremainly testing the effect of inserting a wavelength-stabilized Fabry-Perot filter in a jittering laser cavity.The change in the gain and the associated builduptime between these two regions should not affect themeasurement of the long-term wavelength drift ofthe laser. The oxygen line used (RR 23, 23) is at759.5765 nm, and it has a strength of 0.761 x 10-24cm2 cm-' molecule-'. The oxygen pressure was 650Torr, giving a FWHM of 4.6 pm. The pathlengthwas adjusted to 240 m.

We first performed an energy ratiometer stabili-tytest to determine its precision in this configuration.We evaluated the ratiometer precision AT/T to be 1%by looking at the reproducibility of the measurementswith a nonscanning laser and an evacuated Whitecell. The corresponding precision on the molecularabsorption cross-section measurement is given by

Au 1-- In 1

or Tr

AT 1 ATT TT

(16)

where T is the optical thickness of the cell. If theabsorption line shape is assumed to be Lorentzian,then the relative wavelength deviation from thecenter of the line AX,, resulting from a variation Au ofa, is given by

AX1 C = 2( u0 )Y1/ (17)

For a path length of 240 m, the optical thickness forthis oxygen line is 3.3. Thus Au/u = 3.10-3 andAX,, = 0.12 pm. In this experimental configurationthe precision of the frequency-stability measurementis therefore estimated to be better than 0.2 pm.


10 20Time (min.)

33 43 53 63Time (min.)

c)-24

3_7.r

76 86

pm length was capable of being maintained within 0.7 pm.: 0 from the peak of a molecular absorption line for

-3-~~ A periods as long as 1.5 h. High spectral purity was0 observed for the modified alexandrite laser at 730 and

0.7 p 760 nm. The effect of the finite laser linewidth onU the cross-section measurement was found to be negli-P gible for absorption linewidths greater than 0.04

30 cm-', which corresponds to altitudes below 10 km in

96

Time (min.)

Fig. 9. Absorption cross-section variations versus time. Thecorresponding wavelength drift is reported on the right axis.

In Figure 9 the cross-section variations are con-verted to wavelength drift. Each point representsthe average of 10 pulses, thus reducing the effectcaused by the short-term frequency jitter of thealexandrite laser. During the 1.5-h test, 99% of thepoints were within 0.7 pm from the center of the line.The stabilization program had to correct the voltageapplied to the thick 6talon piezoelectric spacers by 20V, which corresponds to a drift of 4 pm. At times 45and 85, mode hopping caused by a birefringent-tunerdrift was corrected by rotating this tuner. In eachcase the birefringent-filter drift corresponded to anambient-air temperature change of 3 C. For theDIAL measurements, the relevant parameter is therelative wavelength deviation from line center. Thedirection of this deviation and the total absolute driftof the laser wavelength do not need to be explicitlyquantified in this measurement. This maximumdeviation of 0.7 pm from line center ensures that ifone uses the line-center absorption cross section inthe DIAL data-reduction process, the DIAL measure-ment error associated with this detuning will be lessthan 4% for any altitude between ground level and 5km.

5. Conclusion

The capability of the alexandrite laser to be used as areliable on-line transmitter for a water-vapor DIALsystem is demonstrated. The laser output wave-

water-vapor lidar measurements. The techniquesdeveloped to adjust and stabilize the intracavityelements of the laser should be applicable to othersimilar tunable pulsed lasers. This laser system hasbeen integrated into the airborne DIAL system andmade the first airborne water-vapor DIAL measure-ments in March 1989. The stabilization device wasfound to perform satisfactorily in the aircraft environ-ment (NASA/Wallops Lockheed Electra aircraft).During a flight test, the stabilization device had tocorrect the voltage applied on the thick 6talon piezo-electric spacers by 40 V for a drift of 8 pm to beavoided. In addition, the angle of the thin talonwas corrected by the program to compensate for adrift of 19 pm, thus avoiding two successive modehops. The second phase of this development is inprogress, with the objectives of improving the laseroutput energy and incorporating a second alexandritelaser as the off-line transmitter.

The authors thank B. L. Meadows of NASA Lan-gley Research Center for his technical assistance andK. Leslie of Allied Military Lasers Products for help-ful discussions. This research was supported by theNASA Langley Research Center under contract NAS1-18584.

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cm 2 .1 o-24r 6.44 -0P 6.24 -c) a) 6.04 -

U) -

5.84 -(I) -

O 5.64 -

L) 5.441

0

cm 2 .1 o-24

0.)C)IU)

rn

Ur

V0C)

6.44

6.24

6.04

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5.44

cm2

.11

We 6.440_ 6.2405) 6.04

U)5.84

M

O 5.64

C 5.4466

I

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spectral control of an alexandrite laser for an airborne water-vapor differential absorption lidar...

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