Laser diode based oxygen sensing: A comparison of VCSEL and DFB laser diodes emitting in the 762 nm region

INFRAREDPHYSICS &TECHNOLOGY ELSEVIER

Infrared Physics & Technology 38 (1997) 325-329

Laser diode based oxygen sensing: A comparison of VCSEL and DFB laser diodes emitting in the 762 nm region V. Weldon a,*, j. O ' G o r m a n a j.j. P6rez-Camacho a, D. McDonald a j. Hegarty a J.C. Connolly b, N.A. Morris b, R.U. Martinelli b, J.H. Abeles b a Optronics Ireland, Physics Department, Trinity College, Dublin 2, Ireland b David SamoffResearch Centre, Princeton, NJ, USA Received 4 November 1996

Abstract

The performance of VCSEL and DFB laser diodes for spectroscopic based high sensitivity oxygen sensing is compared. Detectivities of < 20 ppm m using the DFB laser diode and < 7 x 103 ppm m using the VCSEL were determined utilising wavelength modulation spectroscopy and harmonic detection. We assess factors influencing the relative performance of these devices, including spectral resolution, laser optical power and light current curve linearity. © 1997 Elsevier Science B.V. Keywords: VCSEL; DFB laser diodes; Oxygen; Sensing; Absorption spectroscopy

Gas detection based on direct optical absorption spectroscopy using near infrared laser diodes is a well established sensing technique [1-3]. For such sensing techniques, single frequency distributed feedback (DFB) laser diodes are preferred although gas detection using Fabry Perot lasers devices and external cavity lasers diodes has been investigated [4-6]. Vertical cavity surface emitting lasers (VCSELS) also provide narrow single frequency emission and have long been considered suitable for such sensing applications. However, objective comparison of the relative performance of VCSEL and DFB laser sources in these and many other application areas have, until recently, been precluded due to the lack of suitable structures with emission in the

* Corresponding author. E-mail: [email protected].

same wavelength regime. Recently DFB [7] laser diodes with emission around A = 761 nm have been developed and we have demonstrated high sensitivity oxygen ( 0 2) sensing using these devices [8]. In this paper we investigate and discuss some key device characteristics which influence the relative performance of the VCSEL and DFB laser diodes for low gas detection limits and also compare these devices for use in high sensitivity gas sensing applications. Fig. l(a) shows the measured emission wavelength (A) temperature dependence for a GaAs-A1GaAs DFB laser diode. Also shown is its emission wavelength dependence on drive current where the laser heat sink temperature, T, is held constant at T = 15°C. The wavelength emission tunes linearly with both temperature and current, I, at respective rates of AA/AT= 5.97 X 10 -2 n m / ° C and AA/A1 = 4.74 X 10 -3 n m / m A . In comparison

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V. Weldon et al. / Infrared Physics & Technology 38 (1997) 325-329

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Fig. l(b) illustrates the corresponding measured spectral tuning characteristics of the VCSEL ( A A / A T = 0.05 n m / ° C and AA/AI = 0.162 n m / m A ) which also exhibit good linearity. However, some evidence of mode hopping at 5.3 and 7.7 mA is observed when current tuning the VCSEL. Hence care was taken to avoid these current operating conditions when performing subsequent spectroscopic and detection limit measurements. We note, also, that A A/AI is much greater for the VCSEL device. This large tuning rate impacts the deplorabil-

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Temperature (°C) Fig. 1. (a) Measured emission wavelength versus temperature (T) in the range - 5 < T < 32°C at a constant drive current of 40 mA for the GaAs-A1GaAs DFB laser diode. Also shown is the emission wavelength versus current (1) in the range 19 < 1 < 50 mA at a constant temperature of 15°C. (b) Measured emission wavelength versus temperature (T) in the range - 5 < T < 32°C at a constant drive current of 7 mA for the A1GaAs VCSEL. Also shown is the emission wavelength versus current ( I ) in the range 4 < 1 < 10 mA at a constant temperature of 15°C.

ity of the technology in the general gas sensing arena. For example to achieve control over the VCSEL emission wavelength by current tuning, equivalent to that obtained here with the DFB laser diode, would require utilisation of higher resolution ultralow noise current sources [9], for driving VCSEL based monitoring systems. The current driver used for this work had a resolution of 0.05 mA which was determined by the digital to analogue convertor controlling the output of the current generator. Fig. 2 shows absorption spectra obtained using amplitude modulation and synchronous detection while temperature tuning the DFB laser diode and VCSEL emission wavelengths across the R and P rotational branches of the A band of 02 in the b I ~ - (v' = 0) ~ X 3~ ; (/p- = 0) electronic transition at pressures of 300 and 1000 mbar, respectively. We note that the VCSEL spectrum was obtained at an increased gas pressure of 1000 mbar to broaden the 02 absorption lines to achieve sufficient spectral resolution in the absorption spectrum when temperature tuning the VCSEL emission wavelength. The peak value of the individual absorption lines in the R branch was obtained by high resolution spectroscopy using current tuning of the DFB laser diode emission wavelength. This approach is not possible with the

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Fig. 3. (a) DFB laser diode light current curve taken at T = 15.5°C with a 32 m source to detector separation in air at atmospheric pressure. The otherwise linear characteristic is distorted by the O 2 absorption feature R7Q8. (b) VCSEL light current curve with a 32 m source to detector separation in air at atmospheric pressure. The characteristic is distorted by several O 2 absorption features.

VCSEL in the present set up since, given its comparatively large A A/A I, the achievable spectral resolution is limited by the resolution of our current driver (0.05 mA). This limitation is clearly illustrated by comparison of Fig. 3(a) and (b) which show DFB and VCSEL laser diode light current (L-l) curves obtained with a source to detector separation of 32 m in air (20.9% 0 2) at a pressure of 1000 mbar. With the DFB a single high contrast modulation of the laser (L-l) characteristic due to the DFB emission line passing through the R7Q8 absorption line of 02 can be seen. On the other hand the VCSEL wavelength tuning rate with current is such that over its 8 mA maximum operating range several absorption lines in the P branch are evident. The underlying superiority in the above threshold DFB laser diode (L-l) curve linearity is also clear. (L-l) curve linearity on a macroscopic scale is normally considered essential for low base-line noise when using

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sensitive signal recovery techniques such as wavelength modulation spectroscopy (WMS). We note however, that for the VCSEL (L-l) curve linearity may not be critical since A A / A I is relatively large. In these experiments the large A A / A I impacts the influence of characteristic linearity since the optimum wavelength modulation amplitude utilised in WMS, which is typically a few times the targeted absorption feature linewidth, can be achieved here, using a relatively small (0.1 mA) current modulation. Hence only non-linearities in the VCSEL (L-I) curve on a current scale < 0.1 mA will contribute to the WMS 2nd harmonic noise signal level. Indeed for non-pressure broadened absorption lines this required 0.1 mA current excursion is further reduced. Furthermore the small current modulation amplitude will reduce the emission amplitude modulation which is another factor limiting detection sensitivity in WMS. However, such potential advantages can only be realised with ultra low noise high resolution laser drivers. Spectral resolution is another important issue in the relative performance of DFB and VCSEL devices for these applications. Assuming a Lorentzian lineshape, with ( l / f ) noise not included, the emission linewidth (FWHM) of the DFB laser diode is 12 MHz (2.3 X 10 -5 nm) [7]. Consequently the spectral resolution is essentially determined by A A/AI, which for our experiment, with m}~mi n = 2.37 x 10 -4 nm (123 MHz) is significantly less than the measured linewidth (FWHM = 2.9 GHz) of an atmospheric pressure broadened absorption line. On the other hand the VCSEL tuning rate with current is about 34 times that of the DFB laser diode while the typical VCSEL spectral line width of 200 MHz (3.86 X 10 - 4 r u n ) is also greater. The major contribution to the spectral resolution is the minimum wavelength step of the VCSEL tuning which, in our experimental system, corresponds to a 0.1°C temperature increment and is 5 X 10 -3 nm (2.5 GHz). In high spectral resolution gas sensing the temperature dependence of the laser emission wavelength is often utilised in order to lock the device emission line to the target gas absorption feature. Implementation of such line locking techniques and o n / o f f resonance measurements are more complicated and demanding for the VCSEL due to its large A A/A I. For example, in our system using state of the art commercial

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V. Weldon et aL / Infrared Physics & Technology 38 (1997) 325-329

instrumentation, the minimum current tuning/wavelength step (0.05 mA/8.1 × 10 -3 nm (4.1 GHz)) is somewhat greater than FWHM value of the atmospheric pressure broadened 02 absorption linewidth. However, as already noted the minimum spectral tuning step may be significantly reduced by using high resolution, low noise analogue current drivers

[9]. The influence of these device characteristics on gas sensing applications may be directly assessed by comparing their low detection limits. Targeting the R7Q8 absorption line in the R rotational branch, which has a line strength of 7.7 × 10 -24 c m 2 molecule -1 cm -~, we previously reported a detection limit of 20 ppm over a 1 m path length using a DFB laser diode [8] at atmospheric pressure. With a VCSEL source we now target the P9P9 absorption line in the P branch which has a line strength of 7.2 × 10 -24 c m 2 molecule- 1 cm- 1 [10] and achieve a low detection limit of 7 × 103 ppm m at a pressure of 1000 mbar. The detection limit was not directly measured but (similar to Ref. [8]) extrapolated from the magnitude of the WMS signal level obtained with room air (20.9% 02) over a path length of 0.15 m, assuming SNR = 1 at the detection limit. For comparison, we note that for the GaAs-AIGaAs DFB laser diode the typical output optical power when performing the detection limit measurements was approximately 15 roW, while in the case of the VCSEL it was approximately 0.5 mW. A laser modulation frequency of 600 Hz was used for the low detection limit measurements. Periodic oscillations, arising from widely spaced 6talon fringes, can be seen on the baseline of the spectra in Fig. 2. Interference fringes are a common occurrence in these optical sensing arrangements [11]. These 6talon fringes were present during these low detection limit measurements. This approach is acceptable in this case since the fringes are widely spaced in wavelength compared with the 02 absorption linewidth and the resultant interference fringe background was stable and reproducible in our laboratory set up. The sensitivity measured here is significantly worse than that achieved previously with an equivalent set up using the DFB device. This reduction in sensitivity might be attributable to a number of factors, the poor spectral resolution by current tuning, L - I curve linearity, laser output power and the relative intensity

noise (RIN) of the device. It is to be expected that VCSEL and DFB laser diode output power fluctuations, particularly RIN and its dependence on emission power and frequency will impact the achievable detectivity. A theoretical model, such as described in the literature [ 12], may be used to predict the affects of device characteristics on the detection sensitivity. In summary we have reported the first wide spectral measurement of the P branch in the A band of O 2 using VCSELS. We have also experimentally compared the VCSEL performance to DFB laser diodes with respect to their suitability for 0 2 sensing applications. The impact of characteristics such as tuning rate dependent resolution, output optical power and perhaps less importantly L - I curve linearity in determining the relative capability of the two devices for high sensitivity 0 2 sensing has been discussed. The need for ultra-low noise analogue current drivers for use with the VCSEL has been highlighted. To our knowledge this is the first experimental comparison of the relative merits of VCSEL and DFB technologies where single mode emission with high side mode suppression ratio is required.

Acknowledgements J.J.P.-C. acknowledges the financial support of the Spanish Ministry of Education.

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