in the quantum efficiency of the laser. Indeed, after the forward bias ESD stress test an inhomogeneous surface layer morphology was observed in some cases, in contrast to the unstressed samples. zyxwvuts 1 .o zyxwvutsrqponmlkjihgf 2 zyxwvutsrqpo 08 zyxwvutsrqponmlkjihgfe 0.6 n a - $ zyxwvutsrqponmlkj 0.4 a 0 I 0.2 0 1 io zyxwvutsrqponmlkjihgfed 3 10 10 5 ; ., g 10 0 -9 10 10 -il 10'~ , I , , --,- ,- .T , , , I , , , current, mA a '$ ~_,__,_.,...,.. zyxwvutsrqpo r. ..,.- T..7 current, mA b Fig. 3 zyxwvutsrqponmlk Results of reverse bias ESD step stress test of proton implanted VCSEL a Optical power against current b Current-voltagecharacteristics before and at different steps . . . . . . .. . .. . before ESD _ _ _ _ _ -1200v -8OOV -1500V At increased forward bias stress amplitudes, the heating of the surface region induces propagation of near-surface defects, e.g. originating from the proton implantation [5], towards the active region. Measurements of the optical emission spectra during ESD tests did not show significant changes in the emission wavelength at the beginning of the degradation process. This indicates that the active layer is not affected in this stage of degradation. However, when the ESD stress test was continued to pulse amplitudes well above the damage threshold value, lasing was completely sup- pressed and a broad, LED-type, emission spectrum was observed. The development of the zyxwvutsrqpo P-I characteristics during negative bias ESD stress (Fig. 3a) is very similiar to the development observed during forward bias stress (Fig. zyxwvutsr h), also the quantum efficiency drops initially and only later does the lasing threshold current increase. In the development of the Z- V characteristics, additional to the sudden increase of the dark current after degradation there is also a monotonically decreasing breakdown voltage from an ini- tial value of -14.8V before degradation to a value of -13.0V after application of the -1500V ESD pulse. It can be concluded that, in particular, the positive bias stress ESD damage threshold values of proton implanted VCSELs are substantially lower than the values reported for 820 and 1320nm LEDs [6] and 1300nm Fabry-Perot laser diodes [7], devices fre- quently used in optical access network and LAN applications. 0 IEE 2000 Electronics Letters Online No: 20001152 DOI: 10.1049/el:20001152 H.C. Neitzert (Universita di Salerno, Dipartimento di Elettronica, I- 84084 Fisciano (SA), Italy) I August 2000 References 1 FIEDLER, U,, REINER, G., SCHNITZER, P., and EBELING, K.J.: 'Top surface-emitting vertical-cavity laser diodes for 10-Gbis data transmission', ZEEE Photonics Technol. Lett., 1996, 8, pp. 746-748 M.K., and MORGAN, R.A.: 'Reliability study of 850nm VCSELs for data communications'. Proc. 34th IEEE Rel. Symp., 1996, pp. 203- 210 CHENG, Y.M., HERRICK, R., PETROFF, P.M., HIBBS-BRENNER, M.K., and MORGAN, R.A.: 'Degradation mechanisms of vertical cavity surface emitting lasers'. Proc. 34th IEEE Rel. Symp., 1996, pp. 211-213 ESD STM5.1: 'Sensitivity testing, Human Body Model, component level'. ESD Association, Rome, 1998 2 HAWTHORNE, R.A., GUENTER, J.K., GRANVILLE, D.N., HIBBS-BRENNER, 3 4 5 JIANG, W., GAW, C., KIELY, P., LAWRENCE, B., LEBBY, M., and CLAISSE, P.R.: 'Effect of proton implantation on the degradation of GaAsiGaAlAs vertical cavity surface emitting lasers', Electron. Lett., 1997, 33, pp. 137-139 6 NEITZERT, H.c., and PICCIRILLO, A.: 'Sensitivity of multimode bidirectional optoelectronic modules to electrostatic discharges', Microelectron. Reliub., 1999, 39, pp. 1863-1871 7 TWU, Y., CHENG, L.s., CHU, s.N.G., NASH, F.R., WANG, K.w., and PARAYANTHAL, P.: 'Semiconductor laser damage due to human- body-model electrostatic discharge', J. Appl. Phys., 1993, 74, pp. 15 10-1 520 Femtosecond Yb:YCOB laser pumped by narrow-stripe laser diode and passively modelocked using ion implanted saturable-absorber mirror G.J. Valentine, A.J. Kemp, D.J.L. Birkin, D. Burns, F. Balembois, P. Georges, H. Bernas, A. Aron, G. Aka, W. Sibbett, A. Brun, M.D. Dawson and E. Bente The authors present, what they believe to be, the first femtosecond Yb:YCOB laser, pumped by a low-power, narrow- stripe laser diode. To facilitate modelocking, high-energy oxygen ion implantation of the saturable absorber is employed. 21Ofs pulse generation at 16mW average output power for 14OmW incident pump power is reported. Recently, we reported on the possibility of using low-power, nar- row-stripe AlGaInP laser diodes as the pump source for femtosec- ond Cr:LiSAF lasers [l]. This is a low-cost alternative to high- power broad-stripe laser-diode pump sources since the power available from narrow-stripe devices, though lower, is used more efficiently owing to their near-diffraction-limited beams. This tech- nique may be appropriate to laser systems for which higher power narrow-stripe laser diodes are available, e.g. 1 p ytterbium based lasers pumped at the zero-phonon line near 976 nm. In this spec- tral region, narrow-stripe InGaAs laser diodes are available with powers as high as 200mW. One promising ytterbium host which has recently received inter- est is the calcium rare-earth oxoborate matrix (i.e. Ca4YO(B03)3 (YCOB) or Ca4GdO(B03), (GdCOB)) [2]. Its strong crystal field minimises the quasi-three-level behaviour of Yb (due to the large splitting of the ground state) and broadens the emission band per- mitting ultrashort pulse generation. As a potential medium for the development of an efficient, low-cost, low-power laser system, Yb:YCOB appears to be very promising: when pumped at the zero-phonon line at 976 nm, the quantum defect is less than 7% [2] and its gain-cross-section upperstate-lifetime product is four times larger than Cr:LiSAF and ten times that of Yb:glass. Unfortunately, the small gain-cross-section and long upperstate- lifetime of Yb:YCOB contribute to a high threshold for stable con- tinuous wave (CW) modelocking [3]. Although a 9Ofs Yb:GdCOB laser was recently reported [4], it used a semiconductor saturable absorber mirror as the modelocking device which incorporated an absorber grown at low-temperatureto increase defects and hence reduce the absorber recovery time [3]. Because this device was prone to damage by a Q-switching Yb laser, our present work has concentrated on using saturable Bragg reflectors (SBR) grown at optimum temperature. These have high damage thresholds due to few defects but hence have a long absorber recovery time. This further increases the threshold for CW modelocking. In this Letter we report on a first experiment to assess the feasi- bility of modelocking an Yb:YCOB laser system pumped by one narrow-stripe laser diode. Ion implantation of a metal-organic chemical vapour deposition (MOCVD) grown SBR is utilised to increase the defect concentration and hence reduce the absorber recovery time to facilitate modelocking. This post-growth defect enhancement approach offers the possibility of 'tailoring' SBR absorbers to specific applications. Fig. 1 shows a schematic diagram of the cavity used, which is similar'to the laser from [l]. The folding mirror through which the pump was focused (Ml) was operated close to normal incidence. The astigmatism from the Brewster cut gain medium was compen- zyx ELECTRONICS LETTERS 14th September 2000 Vol. 36 No. 79 1621