Wavelength tuning in diode laser butt-coupling.

Wavelength tuning in diode laser butt-coupling. Yakov Sidorin a Pentti Karioja b and Dennis Howe a aoptical Sciences Center, niversity of Arizona, Tucson, AZ 8572 1-0094. bvtt Electronics, P.O.Box 1 100, FJN-90571, Oulu, Finland ABSTRACT Coherent optical feedback into a continuously operating diode laser causes optical output power fluctuations as well as a spectrum shift. This spectrum shift can be accounted for by solving the boundary value problem for the optical field'. We choose another method, in which the change of the nonlinear laser characteristics and the optical properties of the oscillation cavity versus the effective reflectance of Fabry- Perot etalon formed by the laser output mirror and an external reflector is described by a phenomenological model. We consider a high power laser diode with significant external feedback from the entrance facet of a bun-coupled fiber and explain the wavelength variations of up to 1 5imi versus laser-to-fiber separation that we observed experimentally. The longitudinal mode spectrum of the butt-coupled laser diode is characterized as well. Keywords: wavelength tuning, extremely short external cavity, butt-coupling, laser diodes, optical couplers. 1. NTRODCTON A tunable external cavity laser diode (ECLD) has some advantages over a solitary laser diode: it is readily operated in a single longitudinal mode, many longitudinal modes are accessible (phase-continuous tuning also possible), linewidth is reduced. Filters placed in the external cavity enable tuning across the wide gain bandwidth of the laser gain medium. All configurations for ECLDs have the goal of maximizing the optical feedback and the wavelength selectivity of the cavity to improve the ability to obtain singlemode oscillation without mode-hopping instabilities. n general, ECLD configurations are difficult to implement or they require many expensive components. From the point of view of integrated optics an attractive means of implementing an ECLD might be via a configuration similar to butt-coupling. Very few papers have been published on the subject of wavelength tuning of very short external cavity ECLDs that are realized via butt-coupling2'3. A possible explanation for the apparent lack of interest in this subject is that the butt-coupled ECLD configurations considered so far usually employed weak feedback (compared to the long external cavity configurations) and they do not allow for selective filtering to be implemented due to the short length of the external cavity. n this paper we present the results of strong feedback on the spectral behavior of a high power laser diode (HPLD) used in an extremely short external cavity configuration that is realized via butt-coupling. Such strong feedback is due to the facts that (i) HPLDs have low output facet reflectivity, (ii) the external reflector (i.e., the entrance facet of the coupled fiber) has the reflectance similar to that of the HPLD output facet and (iii) the separation between the HPLD and fiber is on the order of the Rayleigh range of the laser beam. 772 SPE Vol. 2994 0277-786X971$1O.OO

2. MEASREMENTS We studied butt-coupling of a A1GaAs Fabry-Perot HPLD that has low output facet reflectance of 4% (SDL 5400C5410C; absorption rate 2 cm, 750 pm cavity length, nominal wavelength 845 nm,rear facet reflectance 95%). Optical feedback was provided by a 1 m long single-mode silica fiber with an uncoated cleaved front facet that was precisely positioned with respect to the HPLD ( see Fig. 1). LASER FBER LENS BEAM OPTCAL SPECTRM DODE SPLTrER ANALYZER L f '<!? POSTONER A. zz:o%isssr :,, VOLTMETER DETECTOR RFSPECTRM Fig. 1. Experimental setup for measurement of spectral properties and intensity noise of a HPLD in a butt-coupling configuration. The fiber positioning can be varied in 20 nni increment. Reflections from the back end of the fiber were eliminated by angle-cleaving. The HPLD output and fiber entrance facets comprise a short external etalon (several pm in length)4. Angular misalignment was allowed to within 10 mrad. The diode drive current and temperature were kept constant during the experiment. The end facet of the fiber was imaged onto the receiver of an optical spectrum analyzer (HP70950A) to observe the variation of HPLD spectrum with a resolution of 0.08 nm. The variations of the output power were monitored with a photodiode. Measurements of the relative intensity noise (RN) were carried out via an RF-spectrum analyzer (HP8592B). 3. THE MODEL An extremely short external cavity diode laser and its equivalent are schematically shown in Fig. 2: here rk is the field reflectance of a kth boundary, d is the solitary HPLD cavity length and z is the external cavity length. Dielectric susceptibilities of diode laser material and external medium are assumed to be 6d and e, respectively. The effective field reflectance of the external etalon formed by the surfaces with reflectances r2and r3 characterizes the output coupling of the extremely short ECLD and can be written as4 1 r r11. (v, z) = r2 + C. (2pz)[r2r3 exp( ik2z)] (1) r2 p1 773

where z is the length of external cavity, k =2itv C is the propagation constant for light in the external cavity and C LL (2pz) is the field overlap integral between the laser mode that appears (internal to the laser active medium) at the laser output facet and the one that has made p-round trips inside the external etalon. This etalon serves several purposes: laser mirror compound laser ri r2 r3 refl(z) d z a) b) Fig.2: a) External cavity diode laser; b) Equivalent scheme: reff (z) is the reflectance of the external etalon of the length z. 0? C)., 0? Fig. 3: Spectral distribution of the external etalon effective reflectance versus external etalon length. i) it acts as an output coupler for the butt-coupled HPLD; ii) it performs as a filter, selecting one or more of the solitary HPLD oscillation modes; iii) due to its adjustable length, which is on the order of the Rayleigh range of the HPLD output beam, it can provide a wide range of output reflectances that are "seen" by the HPLD lasing mode that is butt-coupled into the fiber. The effective power reflectance Reff(Z,?) = reff(z, X2 varies periodically in our experiments within 1 to 10% (see Fig. 3), although it can get as high as 16% when z < 1 tm. 774

The solitary HPLD has a -25 ma cw threshold and usually operates in a multi-longitudinal mode regime. Axial mode selection in our system with feedback is determined by the cavity loss and medium gain modulation that is a function of the external etalon length z.one can find the required gain condition for the ECLD realized via butt-coupling to be g(z,?) = am in[r1 reff(z, A)] d (2) where am the HPLD cavity internal loss. By varying the length z ofthe external etalon, we control the center frequencies ofthe etalon output coupler's reflection bands according to Eq.(l). Minima of radiative loss (defined as the second term in Eq.(2)) correspond to the external cavity reflection peaks; these shift in frequency as z is varied. Prospective laser oscillation occurs at the optical frequencies where the gain, as given by Eq.(2), is equal to its threshold value Jth due to some mechanism, the HPLD threshold current were changed, a new threshold gain level, together with a new oscillation frequency, would result through the influence of the carrier density corresponding to the new threshold current value. The dependence of the threshold current on external feedback is known to be5 'thr = Cthr[amd + 1fl[(RReff)21] (3) where the constant C depends on the structure and composition of the gain medium. Obviously, the threshold current (and therefore the carrier density in the HPLD active region and threshold gain) varies as we change z through its dependence on R(z). The calculated modulation of a threshold current versus zin our experimental system is presented in Fig. 4. 40 E 4) 0 4) 35 30 25 20 15 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 External Cavity Length (pm) Fig. 4: Threshold current modulation of a butt-coupled HPLD. An estimate of the gain spectral distribution for the SDL 5400C5410C HPLD active medium was carried out using certain assumptions about the laser structure which are based on guidance provided by the manufacturer. We used the traditional functional form of the gain spectrum6 775

g(e21 ) = g (E21 )(f2 f1) (4) where E21 is the energy ofthe transition at frequency V21, gax(e21) is the maximum gain, (j j ) is the Fermi factor that depends on the injection level. We assumed a bulk AlxGa1xAs active layer (thickness -So. 1 pm, x 0.01 ), parabolic shaped transition bands and used tabulated optical characteristics for GaAs and A1GaAs7'8. We neglected the presence of light-holes, lineshape broadening and polarization dependence. Carriers were treated as an ideal Fermi gas in 3D6'9. Then it can be shown that the maximum of the threshold gain spectral distribution shifts as much as 15 urn in response to the change in threshold current that occurred in our experiment. This shift, combined with the periodic variation of the external etalon's effective reflectance, leads to the observed mode selection and wavelength tuning. 4. EXPERMENTALRESLTS AND COMPARSON WTH THE MODEL Figs. 5 and 6 present typical experimentally observed and calculated wavelength tuning curves as well as the spectral structure of light output that occurred at several z values for two different starting conditions (initial diode-to-fiber separations of 5 pm and 15 pm respectively). For specific ranges of z, the position of the external etalon's reflectance maxima and the shape of the lasing medium's gain spectrum collaborate to cause single-mode operation as in Fig. 5(u), provided that the local maximum of the external etalon's reflectance versus frequency is sufficiently narrow (otherwise multimode operation occurs, see Fig. 6(u)). t can be seen from Fig. 5 that the shallow slopes of the wavelength tuning curve are formed by the tuning of a single mode while the vertical regions and "dips" correspond to multimode operation. 5. CONCLSON n our butt-coupling experiments we demonstrated that by working with external cavity lengths on the order of the Rayleigh range of the laser beam it is possible to tune the working wavelength ofthe HPLD by about 15 nm, using only the variation of the external cavity length, while maintaining constant driving current and temperature.we consider the two reflectors that form the extremely short external cavity (i.e., the HPLD output facet and coupled fiber entrance facet) to comprise an etalon that acts as a tunable output coupler of the butt-coupled HPLD. The fact that the two mirrors that form the external etalon have similar reflectances and that they are closely spaced causes the net reflectance of the etalon to be highly modulated as the spacing varies. This large depth of modulation of etalon net reflectance is the key to the achievement of a large tuning range. An analytical model that includes the effect of multiple reflections in the external etalon, each attenuated by a modal overlap integral, and which is based on phenomenological principles, shows good agreement with the experimental results. Our experiments show that in general there exists a trade-off among the tuning range that can be achieved, coupling efficiency and the spectral purity of light. That trade-off may be an important issue for integrated optics application. 776

E 852 850 848 846 844 842 840 838 836 a 0.0 0.1 0.2 0.3 0.4,- d experiment model 0.5 0.6 0.7 0.8 0.9 1.0 Change n External Cavity Length (pm) i) -30 E -60-30 ii) -60 839 851 Wavelength 839 851 (nm) Fig. 5: i) Wavelength tuning curves, and ii) Spectra in butt-coudling experiment. nitial sedaration z0 ' 5 microns. 777

848 847 846 experiment model p. 845 844 i) 843 a 842 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Change in External Cavity Length (p.m) 0.9 1.0-30 'C 'C E -60 ii) -60 842 848 842 848 Wavelength (nm) Fig. 6: 1) Wavelength tuning curves in butt coupling experiment. 15 microns. z0 and ii) Spectra nitial separation 778

ACKNOWLEDGMENTS This work was supported by the Optical Data Storage Center, Optical Sciences Center, niversity of Arizona. REFERENCES 1. A. Olsson and C.L Tang, "Coherent optical interference effects in external-cavity semiconductor lasers", JEEEJ. Quantum Electron., QE-17, 1320-1323, 1981. 2. D.T. Cassidy, D.M. Bruce, and B.F. Ventrudo, "Short external cavity module for enhanced single-mode tuning of ngaasp and AlGaAs semiconductor diode lasers", Rev. Sd. nstrum., 62,2385-2388, 1991. 3. P.A. Ruprecht and J.R. Brandenberger, "Enhancing diode laser tuning with short external cavity", Opt.Commun., 93, 82-86, 1992. 4. Y. Sidorin and D. Howe, Diode laser- to-fiber butt-coupling: extremely short external cavity, submitted to Appi. Opt. 5. G.P. Agrawal and N.K. Dutta, Semiconductor Lasers, Chapter 2, 2nd Ed. (Van Nostrand Reinhold), New York, 1993. 6. C. Kittel, ntroduction to Solid-State Physics, 130-135 (John Wiley & Sons), New York, 1986. 7. Handbook of Optical Constants of Solids 1, 429-444 (Academic Press), 1985. 8. Hcrndbook of Optical Constants of Solids, 5 13-558 (Academic Press), 1991. 9. H. Haug and S.W. Koch, Quantum theory of the optical and electronic properties of semiconductors, Chapter 6 (World Scientific), 1990. Further author information: Y.S. (correspondence): E-mail:yakovu.arizona.edu; Telephone: 520-621-8263; Fax: 520-621-4358 P.K.: E-mail: pka@ele.vtt.ti D.H.: E-mail: dghoweburke.opt-sci.arizona.edu; Telephone: 520-621-4995; Fax: 520-621-4358; WWW: hup:www.opt-sci.arizona.edulodsc.html 779