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Optics & Laser Technology 39 (2007) 359–367

www.elsevier.com/locate/optlastec

A simple scanning semiconductor diode laser source and itsapplication in wavelength modulation spectroscopy around 825 nm

Ayan Ray, Amitava Bandyopadhyay, Sankar De, Biswajit Ray, Pradip N. Ghosh�

Department of Physics, University of Calcutta, 92 A.P.C. Road, Calcutta 700 009, India

Received 21 January 2005; received in revised form 12 May 2005; accepted 13 July 2005

Available online 15 September 2005

Abstract

A very simple and inexpensive tunable semiconductor diode laser controller is designed for stable operation of the diode laser. The

diode laser controller is stable within +/�8mA and +/�10mK, respectively. The noise spectrum of the current controller is studied

by FFT analysis. We have used our home-made diode laser system in a tunable diode laser absorption spectrometer (TDLAS) to

probe weak overtone transitions of water vapour molecule. The diode laser wavelength is coarsely tuned by changing the operating

temperature to probe (2, 1, 1)’(0, 0, 0) band overtone transitions of water vapour within 818–835 nm. To demonstrate line shape

study, seven transitions are scanned by ramping the drive current of the diode laser (at constant operating temperature) under

different perturber (laboratory air) pressures within the sample cell. A balanced detector and a lock-in amplifier are used for phase

sensitive detection purpose. Small current modulation amplitude, balanced detection and proper adjustment of the lock-in amplifier

help to obtain a S/N ratio ranging from �100 to �7 using a small sample path length of 1.5m. Experimentally obtained derivative

spectrum is numerically integrated to reveal the original line shape and fitted with a nonlinear least squares fitting program to extract

air broadening coefficients and line strength parameters. The spectroscopic line parameters are compared with the results from

HITRAN database.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Diode laser; Wavelength modulation; Lock-in amplifier

1. Introduction

Semiconductor diode laser is inexpensive, small in sizeand easy to handle. Some of the major application areasof this type of laser include frequency modulationspectroscopy (FMS), optical communications, hologra-phy, interferometry, etc. Typically diode laser emissionwavelength changes by 3GHz/mA and 30GHz/1C.Overheating may damage the diode laser. The diodelaser is extremely sensitive to electromagnetic radiation.To ensure safe and stable operation of diode laser,

e front matter r 2005 Elsevier Ltd. All rights reserved.

tlastec.2005.07.002

ing author. Tel.: +9133 2350 8386;

1 9755.

esses: ayan_ray_in@rediffmail.com (A. Ray),

diffmail.com (A. Bandyopadhyay),

hoo.com (P.N. Ghosh).

precision current and temperature controllers areneeded. In free running mode, diode laser is operatedunder temperature and current controlled condition. Bychanging operating temperature of the diode laser, it ispossible to tune over a large wavelength region. But thespectral profile of the laser is often disturbed by modebreaks. It is possible to choose a single mode region byadjusting the operating temperature and drive current ofthe diode laser. FMS with a diode laser is a usefultechnique in spectroscopy. Depending on frequency ofmodulation voltage (superimposed over laser drivecurrent) the FMS can be divided into two sub-regions—(I) wavelength modulation spectroscopy(WMS) (modulation frequencyo1MHz) and (II) con-ventional FMS (modulation frequency4100MHz).Doppler widths of weak overtone band transitions ofsample gases like acetylene, ammonia, water vapour,

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Fig. 1. Side view of the laser head mount. The thermo electric cooler

(TEC) is from Melcor (1.4-127-06L).

A. Ray et al. / Optics & Laser Technology 39 (2007) 359–367360

carbon dioxide, etc. are �1GHz. A diode laser system,which can probe these transitions should have tempera-ture stability �30mK and current stability �few mA.

Several electronic circuits related to current control ofdiode laser have been reported earlier [1–6]. To ourknowledge the design schematic presented in Ref. [1] iseasier to understand and to fabricate with mostinexpensive electronic components compared to allother designs. However, there are two shortcomings inthis design [1]. They are: (1) lack of a current limiter and(2) lack of a modulation input with modulationbandwidth 45 kHz. We have followed the currentshunt feedback loop and diode laser protection mea-sures prescribed by Bradley et al. [1] and have modifiedthe circuit to remove the shortcomings stated above. Anover current protection unit based on variable Zenercircuit [7] and an external modulation input are added inthe current controller board. The temperature control-lers described in earlier reports [1,2,4] are based onthermistor and are suitable for long-term stabilizationpurpose only. But it is possible to tune over a largespectral region by changing the temperature of diodelaser a few tens of degrees about room temperature [8].For this purpose thermistor sensors are not suitablebecause of non-linearity in their response. A tempera-ture controller based on sensor like AD590 [9,10] ismore suitable [11] when thermal tuning over a widerange and temperature stabilization both are requiredfrom a temperature controller. The temperature con-troller designed by us fulfills both these requirements.

Precise knowledge of water vapour radiative proper-ties in the infrared (IR) and near infrared (NIR) regionis needed for many applications starting from study ofplanetary atmosphere to atmospheric sounding experi-ments, radiative sensing of combustion and propulsionprocesses and optical diagnostics of gas dynamic andaerodynamic studies [12]. Measurement of line strengthparameter and line broadening coefficient is useful foratmospheric remote sensing of water vapour using theDifferential Absorption Lidar (DIAL) technique [13].Baumann and Mecke [14] observed and assigned watervapour transitions in the 810–840 nm region. Farmer[15] published line strength parameters of 41 watervapour transitions within 814–823 nm. Nakano et al.[16] worked on eight transitions within 819–826 nmusing a GaAs diode laser. Adler-Golden et al. [17]worked in the 817–824 nm region. Lucchesini et al. [8]and Ponsardin et al. [18] worked around 820 nm region.Recently Bruno et al. [19] worked on the fundamental n1band in the 3 mm region using a difference frequencygeneration laser spectrometer. H2 and He broadening ofwater vapour transitions have been performed at380–600 cm�1 region by Steyert et al. [20]. More recentlywe reported air-broadened line shape parameters ofwater vapour in the 822–832 nm region [12]. Detailed listof line assignments and line parameters for water

vapour transitions are available at HITRAN database[21]. In this work, we characterized our home-madediode laser controllers (both temperature and currentcontrollers) and standardized the diode laser spectro-meter system by applying it in wavelength modulationspectroscopy of seven water vapour rovibrationaltransitions in the 818–835 nm wavelength region. Theextracted line shape parameters of these transitions arepresented in tabular form. We believe that our reportwill be helpful in developing a very simple andinexpensive TDLAS, which can be applied for precisionexperiments like derivative spectroscopy, gas sensing,remote sensing, etc.

2. Tunable semiconductor diode laser system

The diode laser is placed in a home-built mount (Fig.1). The Thermo Electric Cooler (TEC) is mountedbetween two brass plates. Thermal grease is used toimprove the heat transfer. Attaching the plates withteflon screws does compression mounting of TEC, whichhelps in thermal isolation of the plates too. One of theplates is further mounted on a heat sink. The diode laseris placed on the front plate with the help of plasticclamp. The AD590 sensor is pressure fitted into a holewithin 10mm distance from diode laser so that it cancorrectly probe the laser temperature. Connections tothe diode laser and AD590 sensor are made throughtransistor sockets. To mount the home-built collimator(f ¼ 6mm, NA ¼ 0.5) we have used a brass plate, whichis screwed to the ebonite base and stands just in front ofthe diode laser. A finely threaded hole is made in the

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Table 1

study of AC noise of current controller measured across 10O feedback

resistance

Noise type Current noise (rms)

(mA)

Fourier frequency

bandwidth

Broad 0.152 10 kHz

Band

Broad 0.239 100 kHz

Band

Broad 0.415 1MHz

Band

00:00 00:14 00:28 00:43 00:57 01:12 01:26-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

Err

or v

olta

ge (

Vol

ts)

Time (hr:min)

Fig. 3. Response of error voltage when the diode laser temperature is

stabilized at 18 1C. The ambient temperature is 24 1C. A temperature

stability of +/�10mK is observed after a period of 50min.

A. Ray et al. / Optics & Laser Technology 39 (2007) 359–367 361

plate keeping the laser position at the centre. Theoutside of the cylindrical metal holder of the collimatinglens is also finely threaded to make finer progressthrough the hole. The complete setup is placed on anebonite base and enclosed in an aluminium cover to getrid of ambient air currents. The laser emission iscontrolled with the help of home-built current andtemperature controllers (see Appendix). Three differentdiode lasers of HL8311 E series and a HL8314E diodelaser [22] are used in this work.

Drive current fluctuations affect the emission fre-quency of the diode laser. Earlier Gerginov et al. [5]studied the performance of the current controller bymonitoring the drop across the feedback resistance aftershorting the output to ground. Libberecht and Hall [4]studied the same by using a temperature-controlledprecision resistance as load. Compared to them, ourstudy reveals drift and noise of the drive current in amore practical situation because we have used a HL8311E diode laser as dynamic load. The DC drift of thecurrent controller is +/�8 mA. Fig. 2 shows the noisepower spectral density (SDn) of the current controller{obtained by fast Fourier transformation (FFT)} over1MHz Fourier frequency (f) bandwidth. A YOKOGA-WA DL 1620 storage oscilloscope is used with asampling speed of two Mega samples/s for this purpose.The root mean square (rms) fluctuations of the acvoltage (Durms) can be expressed as:

Du2rms ¼

Z 10

SDuðf Þ df . (1)

The data obtained from FFT analysis (Fig. 2) is used tocalculate rms noise voltage over different Fourierfrequency bandwidths. The rms current noise of thecontroller is calculated from the rms voltage fluctuationsobtained across 10O feedback resistance of the currentcontroller (Fig. 8). It has been found that noise increaseswith increasing bandwidth (Table 1).

104 105 10610-20

10-19

10-18

10-17

10-16

10-15

10-14

10-13

Test condition

Noi

se S

pect

ral D

ensi

ty (

V2 /H

z)

Diode laser used: HL8311 EDrive current: 80 mA (T=25°C)

Fourier frequency (Hz)

Fig. 2. Noise power spectral density of the current controller measured

across 10O feedback resistance of the current controller (Appendix,

Fig. 8). Hanning window function is used here.

Thermal instabilities introduce drift in diode laseremission frequency. The AD590 sensor acts in anexponential way to a step temperature change given by[9]

TðtÞ ¼ T eq � ðTeq � T0Þe�t=t, (2)

where, Teq is the equilibrium temperature of the metalmedium sensed by AD590, T0 is the start temperatureand t is the time constant for a particular medium,which is 0.6 s for aluminium. From Eq. (2) it is clear thatset temperature will be reached in an approximatelylinear way with time if its value is set near the ambienttemperature. Recording of the error signal of theproportional-integrator type temperature controllershows error voltage fluctuations p200 mV after a periodof 40min when diode laser temperature is stabilized 6 1Cbelow the room temperature (Fig. 3). This indicatesp+/�10mK thermal stability of diode laser systemwhen sensitivity of AD590 is 10mV/1C. The errorvoltage is measured when a HL8311E diode laser [22] isused as an active load. The diode laser dissipates heat

ARTICLE IN PRESSA. Ray et al. / Optics & Laser Technology 39 (2007) 359–367362

during current conduction. As a result residual thermalfluctuations are observed in the error voltage responsegraph (Fig. 3).

The diode laser wavelength is coarsely tuned over awide region by changing the base plate temperature ofthe laser head with the help of TEC. Thermal scan isuseful in revealing the mode structure of a diode laser.To perform finer scan of diode laser wavelength, lasertemperature is stabilized at a particular set-point. Aramp voltage is superimposed over the diode laser drivecurrent to perform current scan at a fixed lasertemperature. Typically HL8311 E [22] diode laser hastuning rates of �3.3GHz/mA, �23.5GHz/1C andHL8314 E [22] diode laser has tuning rates of�3.5GHz/mA, �24GHz/1C.

3. Observation of the absorption spectra

To detect the overtone band transitions, the drivecurrent of the diode laser is modulated at 5 kHz, +/�10mA peak to peak with the help of a local oscillator.Low modulation amplitude reduces the residual ampli-tude modulation (RAM) noise of the diode laserconsiderably. A quarter waveplate is used to partiallyprevent optical feedback from different optical compo-nents to the laser cavity. The laser beam is divided intofour parts. Two of them are allowed to pass through thesample cell and a reference cell (Fig. 4). Transmitted laserbeams are detected with a balanced detector, which hastwo identical UDT 455 photodiodes followed bydifferential output section composed of OP177 opera-tional amplifier, which has low offset (�10mV at roomtemperature), very low drift (�0.1mV/1C) and �130 dBcommon mode rejection ratio (CMRR) [12,23]. AnITHACO 3962A single phase lock-in amplifier is usedto obtain the derivative spectrum. One of the remaining

Fig. 4. Experimental block diagram of the tunable diode laser

spectrometer. DL, diode laser; LO, local oscillator; LIA, lockin

amplifier; D, photodector; PC, computer; M, mirror; BS, beam splitter;

ET, etalon; SC, sample cell; RC, reference cell; QWP, quarter

waveplate; PG, Baratron pressure gauge.

two parts of the laser beam is passed through a Fabry-Perot air spaced Etalon (FSR—5GHz) while the otherpart is detected by a Coherent wavemeter (Resolution0.01 cm�1) for frequency marking purpose. A SR510lock-in amplifier is used to record the Etalon fringes. ANI488.2 GPIB card has been used for data acquisition(DAQ). The digital to analog converter (DAC) output ofthe DAQ unit is used for ramping the diode laser current.

The glass cells we used are 1.5m long (Fig. 4) and areevacuated to 10�2 Torr by a double stage rotary pumpwith pumping speed 100 l/s. Vapour of distilled water isintroduced in the sample cell. At room temperature thewater vapour pressure settles at �20Torr. We rampedthe drive current to record individual spectroscopictransitions. Diode laser dissipates heat due to currentconduction. We have found that when the diode lasertemperature is set more than 7–8 1C away from roomtemperature, the integration time constant of thetemperature controller should be in the order ofmilliseconds to maintain constant temperature while fastcurrent scan of the diode laser is carried out [12]. Butsmaller time constant of the temperature controller maycause short term cycling of laser temperature aroundpreset level. We used ramp time of 45 s to scan the lasercurrent while the integrator time constant of thetemperature controller is held at �2 s. As a resulttemperature set-point remains fixed without any short-term drift and the current scan is conducted almostquasistatically because the ramp time is set far larger thantemperature controller time constant [12]. The currentscan is repetitive and is used to calibrate the time axis towavelength scale. The reference cell is maintained at10�2Torr. By subtracting the reference signal from thesample signal, the balanced detector removes thecommon mode background (etalon fringes introducedby optical components, intensity noise of laser beam, etc.)and hence improves the (S/N) ratio of spectrum. The airperturber is introduced into the sample cell in steps of�30Torr. For cell pressure measurement we have used aMKS Baratron pressure gauge (accuracy 0.1Torr). Thetotal leak rate within the vacuum manifold is less than1Torr/h as is measured by a Piranni gauge. The observedwater vapour transitions are well in accordance with thelist published in the HITRAN database [21]. The (S/N)ratio of the derivative spectrum obtained here is rangingfrom �100 to �7. The first derivative spectra around12144 cm�1 region is shown in Fig. 5. The lock-in timeconstant is kept at 100ms during recording time.

4. Lineshape study

According to Beer–Lambert’s law, the transmittedintensity (IT) through a sample cell is given by

ITðoÞ ¼ I0ðoÞ exp½�aðoÞpl�, (3)

ARTICLE IN PRESS

-1

0

1

2

3

4

5

5 GHz|-------------|

Frequency

Inte

nsity

in a

rb. u

nit

12144.862 cm-1

12144.914 cm-1

12144.795 cm-1

12145.279 cm-1

12145.444 cm-1

1.75

1.80

1.85

1.90

1.95

2.00

Fig. 5. A part of the current scan of first derivative spectrum of water

vapour transitions while diode laser temperature is fixed at 20 1C. S/N

ratio of the transition at 12145.444 cm�1 is �100 and of the transition

at 12144.862 cm�1 is �7. The sample path length is 1.5m and sample

gas pressure is �20Torr. Inset shows spectral baseline of scan.

12038.657cm-1

(b)(a)

0.048cm-1120 Torr

60 Torr

0 Torr

Inte

nsi

ty in

arb

. un

it

Frequency

Fig. 6. Experimental profiles (a) and residuals after fitting (b) for the

transition at 12038.657 cm�1 are presented in same scale for three

different air pressures. Sample pressure is �20Torr.

A. Ray et al. / Optics & Laser Technology 39 (2007) 359–367 363

where I0 is the incident intensity. In general the lineshape function a is described by a Voigt function, whichis a convolution of Gaussian (Doppler broadened partof the transition) and Lorentzian (collision broadenedpart of the transition) functions. It is described by

av ¼ Ay

p

� �Z þ1�1

expð�t2Þ

y2 þ ðx� tÞ2dt. (4)

For simulation purpose, we used the Voigt function. t isdimensionless time parameter. The dimensionless para-meters x and y are described as x ¼ ðo2o0Þ=s, y ¼ G=sand A ¼ p1=2S=s. G is the collisional HWHM due tophase perturbing collisions. s is the Doppler half-widthof a transition of the sample gas at 1/e intensity and isdescribed by the relation s ¼ o0ð2kT=Mc2Þ1=2 at tem-perature T (in Kelvin). M is the molecular mass of thesample gas. The line shape function described by Eq. (4)is incorporated in Eq. (3) to derive the simulated lineshape. The standard Voigt profile has been computedusing the algorithm developed by Hui et al. [24].

Incorporation of instrumental linewidth into theore-tical simulation is another important issue. Noisespectrum of a precision frequency generator is repre-sented by one-sided power spectral density [25] Sy(f).The one-sided noise power spectral density is propor-tional to the frequency noise power spectral density{Sn(f)} of the frequency generator [25]. A free runningdiode laser’s output emission is mainly dominated by thewhite noise in frequency and the flicker noise infrequency [26]

Svðf Þ ¼ h0 þ h1=f . (5)

For a free running diode laser flicker noise dominatesover the white noise in the low frequency region of the

frequency noise spectrum. Fluctuations in drive currentand operating temperature contribute significantly toflicker noise. We have converted the time trace ofvoltage fluctuations at the output of the lock-inamplifier to optical frequency scale by using a firstderivative signal (of an overtone transition of watervapour at 12151.823 cm�1) as frequency discriminator[27]. We have obtained the long- term laser frequencyfluctuation �450MHz observed over a period of 35min[27]. As we have used a slow ramp (�45 s) to scan thelaser output, long-term laser frequency fluctuationsmainly contribute to instrumental width and areconsidered during simulation work. A Lorentzianprofile (HWHM 0.0075 cm�1) is convoluted to thetheoretical line shape profile (Eq. (3)) to take intoaccount instrumental contribution.

The first derivative of the experimental spectrum isnumerically integrated to obtain the original line shape.This original line shape is fitted with the simulated lineshape following the nonlinear least-squares fittingalgorithm based on Levenberg–Marquardt [28,29] pro-cedure. During the fitting procedure, G/P (pressurebroadening coefficient) and S (line intensity) are held asfloating parameters. The Doppler width is held fixed atits theoretical value (�0.02 cm�1 at e�1 intensity). Therecorded transition is normalized to the DC level of theempty reference cell before the fitting process starts.However, our fitting procedure is suitable for isolatedtransitions.

Plot of observed line shapes under different pressurecombinations and the residuals after fitting is presentedin Fig. 6 for the transition at 12038.657 cm�1. A plotof the collisional HWHM against the perturber pressureis created for the transitions at 12195.190 and12218.829 cm�1 (Fig. 7). Straight line fit of the plotyields a slope, which provides the air-broadeningcoefficient for each of the lines at room temperature.

ARTICLE IN PRESSA. Ray et al. / Optics & Laser Technology 39 (2007) 359–367364

The line strength parameter is evaluated from fittingprocedure for each pressure combination. A weightedaverage of these values yields the final value of strengthfor a particular transition (Table 2).

To determine the absorption cross-section (s) wechoose the transition at 11988.494 cm�1. Using themeasured values of the line strength and collisionalbroadening parameter (at 20Torr) for this transition atroom temperature, we obtained [30]:

s�6� 10�6 cm�1 Torr�1:

Utilizing this value of absorption cross-section, it isfound that the absorbance {(DI/I0), I0 is the intensity ofthe incident laser beam and DI ¼ I0�I, I is the intensityof the laser beam coming out of the sample cell} alongthe 1.5m long sample glass cell is �2% at line centre.Since in this work the line strengths of all the six watervapour overtone transitions are small, the directabsorption technique could not be followed to carry

Table 2

Air broadening coefficient Gair (cm�1/atm) and the line strength (cm/molecu

aObserved line J0 K0a K0c J Ka Kc Ga

Position (cm�1) (H

11988.494 6 0 6 7 0 7 0.0

12001.968 5 2 4 6 2 5 0.0

12037.515 4 0 4 5 0 5 0.0

12038.657 4 1 4 5 1 5 0.0

12155.526 6 6 1 6 6 0 0.0

12195.190 2 0 2 1 0 1 0.0

12218.829 3 2 2 2 2 1 0.0

aLine positions and assignments are extracted from HITRAN database [2

0 50 100 150 200 2500.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

Col

lisio

nal H

WH

M (

cm-1

)

Perturber Pressure (Torr)

(a)

(b)

Fig. 7. G (collisional half-width at half-maximum) vs. perturber

pressure plot for two transitions at (a) 12195.190 and (b)

12218.829 cm�1. The straight lines are the linear fit of data points

and the vertical bars are the error bars obtained from the fitting

program. Water vapour pressure inside the cell is �20Torr at room

temperature.

out any measurement for a single pass of radiationthrough a sample cell of 1.5m path length. So, weapplied the wavelength modulation technique to mea-sure the line shape parameters of these water vapourtransitions.

5. Conclusion

In this report, we have discussed fabrication of a verysimple and inexpensive tunable diode laser controllerand its use in wavelength modulation spectroscopy.The temperature controller based on AD590 sensoroffers wide thermal tuning of diode laser (by a fewtens of degrees about ambient temperature) with +/�10mK stability around set point. The currentcontroller is equipped with a current sweep input anda modulation input (bandwidth up to 100 kHz). The ACnoise of the current controller is studied through FFTanalysis. We have demonstrated spectroscopic use ofour diode laser system in probing weak overtone bandtransitions of water vapour around 825 nm. A 1.5mlong sample cell is used. A balanced detectoraccompanied by a lock-in amplifier is used for detectionpurpose. A (S/N) ratio ranging from 100 to �7 isobserved while scanning the transitions. This noisefigure indicates a significant improvement over earlierreport by Lucchesini et al. [8] where they had achievedS/N ranging from � 30 to �3 after using a 5msample path length. The experimental spectrum is fittedwith a standard Voigt profile following a nonlinearleast-squares program to extract line shape para-meters. The results are compared with the values quotedin HITRAN database. A stand-alone laser source,small sample path length and high (S/N) ratio ofspectrum have made our spectrometer a simple portabletool to be used for line shape measurements of gassamples. We strongly believe that the system can furtherbe upgraded to the level of standard industrial gassensors. Also we believe that the report presented herewill be helpful in airborne remote sensing of atmo-spheric water vapour.

le)

ir Gair Strength (� 1024) Strength (� 1024)

itran) (observed) (Hitran) (observed)

737 0.0765(4) 10.11 9.56(7)

833 0.0752(39) 8.29 7.26(6)

935 0.0805(16) 23.26 24.43(7)

778 0.0757(35) 8.41 8.58(6)

505 0.0523(28) 1.59 1.83(1)

985 0.0921(8) 35.44 35.96(6)

937 0.0822(21) 18.91 18.83(7)

1].

ARTICLE IN PRESSA. Ray et al. / Optics & Laser Technology 39 (2007) 359–367 365

Acknowledgements

The authors thank Mr. S. Bhattacharya of PhysicsDepartment, Calcutta University for his kind help inmechanical design and fabrication works. The authorsthank the Department of Atomic Energy, New Delhi forgranting a research project and A.B. thanks the Councilof Scientific and Industrial Research, New Delhi forResearch fellowship. S. De thanks U.G.C. for a researchfellowship.

Appendix A

A.1. Current controller

A FET input BFW16A transistor (Fig. 8) is used asthe current driver. The base voltage to the transistor issupplied from the output of an integrator. A currentshunt feedback loop is used for current stabilization [1].The feedback voltage is extracted from the drop across aprecision 10O resistance (3 ppm/1C) bought fromNeOhm. The feedback voltage is further amplified 6.6times and fed to the integrator to complete the feedbackloop. The input voltage supplied to the integrator iscontrolled by two 20 kO Helipots. One of them is usedfor coarse adjustment of drive current while the other is

Fig. 8. Circuit diagram o

used for fine adjustments in steps of 50mA. A LM399 [10]precision voltage reference is used to supply preset voltage.A BFW16A transistor is used in variable Zener config-uration [7] and used as current limiter. The ramp input isused to sweep the current for fine-tuning of laserwavelength. The current modulation input is ac coupledto the base of the driver transistor. The modulationbandwidth of our current controller circuit is 100kHz.Some precautionary measures [1] are followed to protectthe diode laser from reverse surge, voltage spike or faststart-up current. The diode laser module is connected inparallel to reverse biased high-speed switching diode and acapacitor–resistor combination. A Darlington pair is usedin the collector voltage supply section for slow start up ofdiode laser. Moreover to avoid power line voltage hazards,we used four lead-acid batteries as power supply. A 50mAconstant current charger [10] made of LM317 variablethree terminal voltage regulator is incorporated in thecircuit layout for recharging of battery cells.

A.2. Temperature controller

An AD590L grade device is used as the sensor with alinear current output of 1 mA/K [19]. We use AD590in series with a 9.76 kO resistance and a 500O trimpot(Fig. 9) [10]. The trimpot helps in calibrating the outputvoltage of AD590 to 2.98V at 25 1C. This one

f current controller.

ARTICLE IN PRESS

Fig. 9. Circuit diagram of the temperature controller.

A. Ray et al. / Optics & Laser Technology 39 (2007) 359–367366

temperature calibration of AD590L device helps indetecting the surrounding temperature with 0.11 accu-racy [9]. Under this configuration AD590 outputchanges by 10mV/1C. The output voltage fromAD590 is compared with a stable preset voltage togenerate error voltage. The diode laser is thermallytuned within a temperature interval of 50–121C. Coolingof diode laser below 12 1C is avoided because it maycreate a chance of condensation of moisture inside thelaser head mount. According to AD590 output voltagescale the maximum preset voltage is 3.23V at 50 1C andthe minimum preset voltage is 2.85V at 12 1C. Tominimize the drift of preset voltage, we have used metalfilm resistors and potentiometers with temperaturecoefficient of 50 ppm/1C. The final output of the errorsignal amplifier is fed to a control loop, which consistsof a proportional (P) amplifier and an integrator (I)amplifier. With integrator time constant set at �3 s andproportional gain set near 1.2, the decay ratio [11] of thesystem is little lower than 0.25, which indicates slightlyunder damped situation. The output power stage iscomposed of a bipolar push pull stage and the TEC. Toprotect the TEC from excess current drawing, a voltageclamp is used. The voltage drop across a 0.1Oresistance, which is in series with the TEC, providesthe approximate readout of the TEC current(0.1� ITEC) (Fig. 3). All the circuits are powered from

three terminal voltage regulators, which are used without-board pass transistors operating as current boosters.

References

[1] Bradely CC, Chen J, Hulet RG. Rev Sci Instrum 1990;61:2097.

[2] Wieman CE, Hollberg L. Rev Sci Instrum 1991;62:1.

[3] Telle HR. Spectrochim Acta Rev 1993;15:301.

[4] Libberecht KG, Hall JL. Rev Sci Instrum 1993;64:2133.

[5] Gerginov V, Laughman B, DiBerardino D, Rafac RJ, Ruggeiro

ST, Tanner CE. Opt Commun 2001;187:219.

[6] Lazar J, Jedlicka P, Cip O, Ruzicka B. Rev Sci Instrum

2003;74:3816.

[7] Hamilton TDS. Handbook of linear integrated electronics for

research. Maidenhead, Berkshire, England: McGraw-Hill; 1977.

[8] Lucchesini A, Gozini S, Gabbanini C. Euro Phys J D 2000;8:223.

[9] Analog Devices Data Sheet, 1997.

[10] Horowitz P, Hill W. The art of electronics. 2nd ed. Cambridge:

Cambridge University Press; 1989.

[11] Wavelength Electronics. Technical note 201, 1995.

[12] Ray A, Bandyopadhyay A, Ray B, Biswas D, Ghosh PN. Appl

Phys B 2004;79:915.

[13] Browell EV, Vaughan WR, Hall WM, Degnan JJ, Averill RD,

Wells JG, et al. In: Proceeding, 13th international laser radar

conference, Toronto, Canada; August, 1986. p. 6–9.

[14] Baumann W, Mecke R. Z Phys 1933;81:445.

[15] Farmer CB. Icarus 1971;15:190.

[16] Nakano K, Saito A, Ohashi N. J Mol Spetrosc 1988;131:405.

[17] Adler-Golden S, Lee J, Goldstein N. J Quant Spectrosc Radiat

Transfer 1992;48:527.

ARTICLE IN PRESSA. Ray et al. / Optics & Laser Technology 39 (2007) 359–367 367

[18] Ponsardin PL, Browell EV. J Mol Spectrosc 1997;185:58.

[19] Bruno A, Pesce G, Rusciano G, Sasso A. J Mol Spectrosc 2002;

215:244.

[20] Steyert DW, Wang WF, Sirota JM, Donahue NM, Reuter DC. J

Quant Spectrosc Radiat Transfer 2004;83:183.

[21] Rothman LS, Rinsland CP, Goldman A, Massie ST, Edwards

DP, Flaud JM, et al. J Quant Spectrosc Radiat Transfer

1998;60:685;

Merienne MF, Jnouvrier A, Hermans C, Vandaele AC, Carleer

M, Clerbaux C, et al. J Quant Spectrosc Radiat Transfer

2003;82:99;

Fally S, Coheur PF, Carleer M, Clarbaux C, Colin R, Jenouvrier

A, et al. J Quant Spectrosc Radiat Transfer 2003;82:119.

[22] HITACHI Optodevice Hand Book, 1989.

[23] Analog Devices Data Sheet, 1995.

[24] Hui AK, Armstrong BH, Wray AA. J Quant Spectrosc Radiat

Transfer 1977;19:509.

[25] Lesage P, Audoin C. Radio Science 1979;14:521.

[26] Salvade Y, Dandilker R. J Opt Soc Am A 2000;17:929.

[27] Ray A, Bandyopadhyay A, Ray B, Ghosh PN. IEE Proc

Optoelectron 2004;151:490.

[28] Press WH, Teukolsky SA, Vetterling WT, Flannery BP.

Numerical recipes in FORTRAN. The art of Scientific

computing. Cambridge: Cambridge University Press;

1992.

[29] Bevington PR. Data reduction and error analysis for the physical

sciences. New York: McGraw-Hill; 1969.

[30] Gharavi M, Buckley SG. Appl Spectrosc 2004;58:468.