Single-cladding antiresonant reflecting optical waveguide-type diode

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Nov 1, 1995 - A significantly simplified type of antiresonant reflecting optical waveguide (ARROW) laser, ... Efficient coupling of high optical power from diode.
November 1, 1995 / Vol. 20, No. 21 / OPTICS LETTERS

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Single-cladding antiresonant reflecting optical waveguide-type diode laser I. V. Goltser, L. J. Mawst, and D. Botez Department of Electrical and Computer Engineering, University of Wisconsin – Madison, 1415 Johnson Drive, Madison, Wisconsin 53706 Received June 5, 1995 A signif icantly simplif ied type of antiresonant ref lecting optical waveguide (ARROW) laser, which is also immune to gain spatial hole burning, is proposed for achieving high-power, single-spatial-mode operation. Modal calculations, which include two-dimensional analysis, confirm strong intermodal discrimination (12 – 16 cm21 ) between the fundamental and the f irst-order lateral modes. Above-threshold analysis shows that gain spatial hole burning has a negligible effect on the performance of this new type of ARROW device, which in turn dramatically enhances its single-mode, high-power capability by comparison with the conventional ARROW laser.  1995 Optical Society of America

Eff icient coupling of high optical power from diode lasers into single-mode fibers or planar waveguides is required for applications such as rare-earth-doped (f iber) amplifiers, frequency upconversion in f luoridebased fibers, and second-harmonic generation. The diode source needs to exhibit a stable, single-mode beam with a spot size closely matching that of the fiber or waveguide. Antiresonant ref lecting optical waveguide (ARROW) structures have been extensively studies1,2 as passive waveguides of large transverse spot size. The waveguiding structure of the conventional ARROW device consists of a low-index core region, clad on both sides by the first pair of a quarter-wave stack, which is antiresonant to the radiation leakage of the fundamental mode. As a result, low loss is obtained for the fundamental mode, but high-order modes are highly lossy. The concept has been extended to active devices such as ARROW-type lasers,3 – 5 which have demonstrated stable, single-spatial-mode operation to peak-pulsed powers as high as 0.5 W from relatively large (4–6 mm) emitting apertures. However, ARROW lasers need tight current conf inement to the low-index core region4; otherwise, because of gain spatial hole burning6 (GSHB), the first-order mode is easily excited at moderate drive levels above threshold.4,6 In this Letter we propose a new ARROWtype laser, which in contrast to a conventional ARROW device requires only one quarter-wave-type cladding region for effective ref lection and is not vulnerable to GSHB. The proposed laser thus has the potential to be a relatively simple source of reliable, stable, high s.0.3 Wd cw coherent power. The proposed structure and the corresponding effective (refractive) index conf iguration are shown schematically in Fig. 1. We consider an Al-free 0.98-mm-emitting laser structure for which quarterwave-type (lateral) cladding regions are formed by burying in InGaP material high-index passive GaAs (transverse) guides in close proximity to the separateconf inement-heterostructure (SCH) single-quantumwell (SQW) active region. Then the cladding regions support both even and odd transverse modes,7 which cause lateral antiguidance and guidance,8 respectively 0146-9592/95/212219-03$6.00/0

(i.e., the core effective index na1 is ,nb1 , the effective index of the even transverse mode, and . nb2 , the effective index of the odd transverse mode). More specif ically we consider that the SCH-SQW active region is made of a 10-nm-thick InGaAs active layer sandwiched between 100-nm-thick InGaAsP sEg ­ 1.62 eVd confinement layers. In the cladding regions, the spacing between the active region and the passive GaAs guide layer w is 0.15 mm, and the GaAs guide-layer thickness h is 0.16 mm. At the wavelength l ­ 0.98 mm, the effective indices of refraction are na1 ­ 3.241 in the core region and nb1 ­ 3.29 and nb2 ­ 3.22 in the cladding regions, corresponding to the even and odd transverse modes, respectively.

Fig. 1. Proposed structure and corresponding effective refractive-index configuration. For l ­ 0.98 mm, w ­ 0.15 mm, h ­ 0.16 mm, SCH-SQW active region made of a 10-nm-thick InGaAs quantum-well layer, and 100-nm-thick InGaAsP confinement layers, the calculated effective refractive indices are na1 ­ 3.241, nb1 ­ 3.29, and nb2 ­ 3.22. In the cladding regions the solid and dashed curves correspond to the even and odd transverse modes, respectively.  1995 Optical Society of America

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OPTICS LETTERS / Vol. 20, No. 21 / November 1, 1995

The two-dimensional code called MODEM,7 analogous to the model used by Amann9 and Hadley et al.,10 was used to obtain the lateral modes of the structure. In each region 10 transverse modes are considered: one supported 19 radiation modes in the core, and two supported 18 radiation modes in the cladding regions. Figure 2 shows the edge radiation losses for the lateral fundamental and first-order modes of the ARROW structure, with core width d ­ 6 mm, as a function of the cladding region width s. The fundamental (f irst-order) mode of the ARROW structure is def ined as a two-dimensional mode with one field-intensity main peak (two field-intensity main peaks and a null) residing in the core region. The losses reach minima at s ­ 1.8, 2.7, and 3.6 mm, which correspond to 3ly4, 5ly4, and 7ly4 layers (i.e., points of antiresonance). Actually, the values are 0.55 –0.6 mm larger than those from use of the effective index method, because we are dealing with dual-state Fabry –Perot resonators11 (i.e., to the 1.5p-, 2.5p-, and 3.5p-thick layers one has to add a phase shift related to the field overlap integral of the fundamental transverse mode in the core and the even transverse mode in the cladding region). For our particular choice of the parameters, the field overlap integral between the fundamental (transverse) mode in the core region and the even (transverse) mode in the cladding region is jC01 j2 ­ 0.4. As one can see from Fig. 2, the loss for the fundamental ARROW mode stays relatively low (1–3 cm21 ) over regions 0.4 mm wide around the antiresonance points. That is, just as for conventional ARROW devices, fabrication tolerances are relatively large. It also is worth noting that although we use only one quarterwave-type cladding region, the radiation losses are relatively small. That happens because the fundamental (transverse) mode (from the core) couples strongly to the odd (transverse) mode (jC02 j2 ­ 0.6), a lateral waveguiding conf iguration that is practically lossless,8 and relatively weakly to the even (transverse) mode (jC01 j2 ­ 0.4), which corresponds to lateral antiguidance. In turn the losses are smaller than those of the case of a pure antiguide7,9 (jC01 j2 ­ 1.0). Just as for ARROW structures, the losses for the firstorder lateral mode are relatively high (11–14 cm21 ), which provides the base for single-mode operation for the proposed device. For further study we consider only structures with antiresonance around the 3ly4 point, because there the intermodal discrimination is highest (12–16 cm21 ). We also have performed an above-threshold analysis (including carrier-induced index depressions, carrier diffusion and GSHB) of the behavior of the singlecladding ARROW-type laser. This above-threshold (one-dimensional) treatment has been developed previously for antiguided phase-locked arrays,12,13 and recently has been applied to conventional ARROW structures.6 Although the model is one-dimensional (i.e., the effective index method), we believe it provides the proper trends in above-threshold behavior for our proposed device, just as it did for conventional ARROW devices.6 We used the following parameters: d ­ 6 mm (the same parameters as those used in Figs. 1 and 2, except that only nb1 is considered to be the effective index in the cladding re-

gions), linewidth enhancement factor a of 2 and 4, gain cross section sg ­ 10216 cm2 , carrier diffusion length LD ­ 3 mm, mirror losses of 10 cm21 , and a uniform

Fig. 2. Cold-cavity radiation losses for the fundamental (solid curves) and first-order (dashed curves) lateral modes of the structure shown in Fig. 1 (with d ­ 6 mm) as a function of the width of the cladding region, s, calculated by two-dimensional analysis. The three minima correspond with antiresonances at 3ly4-, 5ly4-, and 7ly4-thick cladding regions

Fig. 3. (a) Cold-cavity radiation losses versus claddingregion width s for the fundamental (solid curve) and firstorder (dashed curve) lateral modes (d ­ 6 mm), calculated by the effective index method, for a structure near the 3ly4 antiresonance point. (b) Calculated fundamentalmode relative output power and first-order mode net gain Imsb1 d, as a function of the relative drive above threshold, for three values of the cladding-region width s; and for two values of a, the linewidth enhancement factor. Imsb1 d ­ 0 corresponds with the first-order mode’s reaching threshold.

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pumping). That is, in the case of the single-cladding ARROW-type device, preferential current pumping of the core is not necessary for maintaining single-mode operation to high drive levels above threshold. The stability of the beam pattern is evident from the calculated near- and far-f ield patterns [Figs. 4(a) and 4(b)] where beam broadening does not occur as the drive increases from threshold to 10 times threshold. In conclusion, we propose here a new, singlemode, high-power ARROW-type diode laser. Twodimensional analysis conf irms strong discrimination between modes. The design can be implemented easily by using two-step metal organic chemical vapor deposition.3 – 5,15 Not only is the proposed device much simpler than the conventional ARROW laser, but it also is unaffected by GSHB. References

Fig. 4. Calculated near- and far-f ield patterns at threshold and 10 times threshold for the single-cladding ARROW laser analyzed in Fig. 3.

pumping profile. Figure 3(a) shows the cold-cavity loss as a function of the cladding region width. The minimum loss occurs indeed at a three-quarterwave-thick cladding layer (i.e., s ­ 1.3 mm), and the ratio of first-order to fundamental-mode losses at that point is 8, in agreement with the theoretical work by Nomoto et al.14 It is interesting to note that even for the two-dimensional case (Fig. 2) the ratio of firstorder to fundamental-mode losses at the 3ly4 point is still 8. In Fig. 3(b) the relative output power for the fundamental mode and the net gain, Imsb1 d, for the first-order (lateral) mode are shown as a function of the relative drive above threshold for three different points around antiresonance. As one can see, for all cases, the first-order mode is far from reaching threshold even at 10 times fundamental-mode threshold. Increasing the linewidth conf inement factor from 2 to 4, which enhances GSHB, does not affect the intermodal discrimination. The results compare quite well with the first-order mode’s reaching threshold at only about 3.5 times fundamental-mode threshold in the case of conventional ARROW lasers6 (no preferential

1. T. L. Koch, U. Koren, G. D. Boyd, P. J. Corvini, and M. A. Duguay, Electron. Lett. 23, 244 (1987). 2. T. Baba, Y. Kokubun, T. Sakaki, and K. Iga, J. Lightwave Technol. 6, 1440 (1988). 3. L. J. Mawst, D. Botez, C. Zmudzinski, and C. Tu, Appl. Phys. Lett. 61, 503 (1992). 4. L. J. Mawst, D. Botez, C. Zmudzinski, and C. Tu, IEEE Photon. Technol. Lett. 4, 1204 (1992). 5. L. J. Mawst, D. Botez, C. Zmudzinski, and C. Tu, Electron. Lett. 28, 1795 (1992). 6. L. J. Mawst, D. Botez, R. F. Nabiev, and C. Zmudzinkski, Appl. Phys. Lett. 66, 7 (1995). 7. D. Botez, Proc. Inst. Electr. Eng. Part J 139, 14 (1992). 8. Note that for the case na1 . nb2 the lateral (fundamental) mode centered in the core is, strictly speaking, of the leaky type. However, because for typical structures the radiation loss is negligible (e.g., for d ­ 6 mm, s ­ 1.85 mm, and l ­ 0.98 mm, the radiation loss value is 0.004 cm21 ), for all practical purposes the mode is guided. 9. M.-C. Amann, IEEE J. Quantum Electron. QE-22, 1992 (1986). 10. G. R. Hadley, D. Botez, and L. J. Mawst, IEEE J. Quantum Electron. 27, 921 (1991). 11. P. Yeh, C. Gu, and D. Botez, Opt. Lett. 17, 24 (1992). 12. R. F. Nabiev, P. Yeh, and D. Botez, Appl. Phys. Lett. 62, 916 (1993). 13. R. F. Nabiev and D. Botez, IEEE J. Select. Topics Quantum Electron. 1, 138 (1995). 14. M. Nomoto, S. Abe, and M. Miyagi, Opt. Commun. 108, 243 (1994). 15. C. Zmudzinski, D. Botez, L. J. Mawst, A. Bhattacharya, M. Nesnidal, and R. F. Nabiev, IEEE J. Select. Topics Quantum Electron. 1, 129 (1995).