Long-wavelength VCSELs for gas sensing A. Sirbu*, A.Caliman *, V.Iakovlev ", A. Mereuta *, G. Suruceanu " and E. Kapon *" * Laboratory of Physics of Nanostructures, EPFL, 1015 Lausanne, Switzerland " BeamExpress, 1015 Lausanne, Switzerland 1
Outline VCSEL fabrication approach; Results at 1550 nm; Results at 2000 nm; CO2 sensing results; Summary. 2
Tunable laser diode spectroscopy, TLDS with long- wavelength VCSELs in the 1500-2000 nm range Gas Methane Ammonia Hydrogen sulphide Carbon Monoxide Carbon Dioxide Water Ethylene Hydrochloride Symbol CH 4 NH 3 H2S CO CO 2 H 2 O C 2 H 4 HCl Absorption line, nm 1650 1512, 1540 1578 1570 2004, 1953 2003, 1953 1617 1740 gas-cell sensing Typical VCSEL RIN= -130 db/hz corresponds to the shot noise of the photo-detector at ~3 µa that corresponds to 3 µw incident power; 1 mw SM output for optical path length of the order of 10 cm at specified wavelengths is OK; A cost-effective technology for building a wavelength inventory is important for large-scale applications. 3
4 regions on the wafer with different emission wavelengths Top mirror Cav.Eng. layer A InP active Cav.Eng. Layer B Bottom mirror - Emission at 1550 nm, 1560 nm, 1570 nm, 1580 nm - Regions with different output coupling: 20 and 17 pairs in the top DBR 4
10 4 Photoluminescence spectra of InAlGaAs quantum wells at different temperatures Off-set max Off-set min 8000 6000 4000 20 10 C step Emission wavelength: 1550-1580 nm 2000 110 0 1350 1400 1450 1500 1550 1600 1650 Wavelength, nm Gain medium: InAlGaAs 5 QWs, 1.1% compressive strain, grown by LP MOCVD 5
Overview of VCSELs characteristics (20 pairs top DBR) Voltage, V 6 5 4 3 2 1 l 1 1554 nm l 2 1563 nm l 3 l 4 1571 nm 1580 nm LIV, 20 C 0 0 0 5 10 15 20 Current, ma Similar LIV characteristics for different wavelengths 6 5 4 3 2 1 Power, mw Threshold Current, ma Maximal power, mw 8 7 6 5 4 3 2 1 08 7 6 5 4 3 2 1 0 1554 nm 1563 nm 1571 nm 1580 nm 1554 nm 1563 nm 1571 nm 1580 nm 0 20 40 60 80 100 Temperature, C Ith and Pmax variations with temperature 6
Spectral characteristics at 1560 nm Spectra at 2 mw output 10 db/div 3 ma 7 ma 11 ma 15 ma 19 ma 20 C Voltage, V 20 C 40 C 60 C 80 C 1560 1564 1568 Wavelength, nm 1560 1565 1570 1575 Wavelength, nm Single-mode emission in the full operation range; At low currents optical guiding is due to lateral refractive index variation, while at higher currents is due thermal lens; Wavelength tuning: 0.4 nm/ma; 0.15 nm/ºc 7
Impact of output coupling 8 7 6 ------- 17 pairs ------- 20 pairs Power, mw 5 4 3 2 1 0 0 5 10 15 20 25 30 35 40 Current, ma 0 C P max I th DP/DI 20 pairs 6.8 mw 3.5 ma 0.43 17 pairs 8 mw 5 ma 0.58 8
Wavelength inventory 20 19 18 SM operation with SMSR more than 40 db 60 db/div 17 16 15 14 13 12 11 10 0.3 nm/ma; 0.1 nm/ C 20 C 40 C 60 C 9 8 7 6 1'558 1'562 1'566 1'570 1'574 Wavelength, nm 9
Wavelength tuning LIV and spectral characteristics of 7 mm tj diameter device 0 C 15nm 37 ma 10 db/div 15 ma 25 ma 5 ma 1560 1565 1570 1575 1580 Wavelength, nm 15 nm tuning 10
1550 nm far-field field and line-width Far-field and line-width measurements performed at 2 mw output, RT FWHM=9 Frequency, MHz Lorentzian line-shape; n = 4.5 MHz Self-homodine line-width measurements performed by M.Grossenbacher, ED EPFL 11
Wavelength selection in the 2000 nm band 15 C V. Iakovlev, Internal report 12
Wavelength tuning Wavelength, nm Single mode emission in the 4-18 ma current range A. Mereuta et al. PTL 2008, IPRM-2008 Wavelength tuning range ~ 5 nm Wavelength tuning rates: λ/ I ~ 0.31 nm/ma / T ~ 0.14 nm/ C 13
CO 2 detection with LPN/BX VCSEL 1.8 1.7 1.6 2f 2f 2f 8 C 2f diode3248 wafer l070502b 1st DC 1st DC 1st DC 1st DC 0.095 0.09 1.5 0.085 1.4 0.08 1.3 1.2 0.075 1.1 0.07 5 5.5 6 6.5 7 7.5 Idc [ma] V.Iakovlev, Internal report Dashed lines ordinary room air Solid lines 10 mbar CO2 Optical path length - 10 cm 14
Two-species detection: CO 2 and H 2 O CO 2 H 2 O CO 2 15 cm CO 2 H 2 O V. Iakovlev, Internal report 15
Summary 1.5 mm VCSELs are produced with a broad range of wavelength inventory on the same wafer of 40 nm, high SM output power of 6.9 mw @ 0 C and 2.5 mw @ 80 C with line-width of 4.5 MHz and 15 nm of tuning range with current; Capability of fabrication of 2 mm VCSELs with 0.5 mw SM output; 2003-nm VCSELs detection of carbon dioxide (CO2) and water (H2O) molecules in air is demonstrated. 16