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610 OPTICS LETTERS / Vol. 18, No. 8 / April 15, 1993 Bistability and optical control of a distributed-Bragg-reflector laser M. Margalit, R. Nagar, N. Tessler, G. Eisenstein, and M. Orenstein Advanced OptoelectronicsCenter, Department of Electrical Engineering Technion - Israel Institute of Technology, Haifa 32000, Israel U. Koren and C. A. Burrus AT&T Bell Laboratories,Holmdel, New Jersey 07733 Received November 3, 1992 We demonstrate bistable operation and optical control of a specially designed distributed-Bragg-reflector laser. Optically controlled turn-on and turn-off with 1-pJ pulses at 1.5 ,m and 2-pJ pulses at 1.3 Am, respectively, is demonstrated as well as all-optical flip-flop operation. Bistable operation of diode lasers is thought to be an important feature of advanced optical signal processing. It is also an attractive means for studying basic nonlinear phenomena in diode lasers. Over the years there have been demonstrations of many bistable diode lasers.1 - 6 These studies differ by the specific underlying nonlinear mechanism that governs the bistable operation, by the type of control used (electrical, optical, or a combination of the two), and by the required switching energies. In this Letter we describe a novel bistable diode laser that is based on a specially designed distributedBragg-reflector (DBR) structure. The laser exhibits both power and wavelength bistability and can be operated as an all-optical flip-flop. The mechanism governing the operation of the present laser results from an interplay between the wavelength of a lasing mode and the wavelength-dependent reflectivity function of the Bragg section. This interplay, coupled with conventional laser nonlinearities, gain saturation, and carrier- and thermal-induced frequencyshifting effects,7 results in bistable characteristics. The operation of the device relies on two properties of the specially designed two-section 1.5-pm DBR structure, depicted in Fig. 1(a): First, a 250,tm-long Bragg reflector consisting of a low-loss shallow grating that yields a reflection function bistable operation of the laser is the adjustment of the spectral overlap such that the single cavity mode is placed on the short-wavelength side (the blue side) of the Bragg-reflection function, at low injection levels. This particular type of mode-DBR overlap, together with conventional gain nonlinearities and carrier effects,7 causes a self-adjusting mechanism of the phase condition that leads to bistable characteristics. Exploitation of the index-of-refraction nonlinearity in a laser cavity for achieving bistability was reported before,'0 -' 2 where either a multimode laser and a dispersive external cavity or a forward-biased twosection laser'1 " 2 was used. Optical signals if3PM 1g - 0146-9592/93/080610-03$5.00/0 I I Output 2 - section (a) with a relatively narrow bandwidth of = 225 GHz FVHM. Second, a short (130 /tum) strained-layer quantum-well gain medium8 that defines a large modal spacing of =300 GHz. The optical coupling between the two sections is essentially lossless and reflectionless.9 This laser operates such that the Bragg reflector overlaps spectrally one cavity mode at most [Fig. 1(b)], thus it can be turned on and off by controlling the phase condition of the cavity. The latter can be accomplished since the cavity can be adjusted so that no frequency, where there is sufficient gain, satisfies the N x 2 fr phase condition necessary for lasing. The laser can be turned into and out of this condition by an electrical drive or by optical signal injection. The key point in obtaining 1B DBR laser A 225GHz Bragg '300GHz Cavity (b) 1. 1 550nm Fig. 1. (a) Schematic diagram of the bistable laser. (b) Conceptual description of mode spacing. The inset shows the mode-DBR overlap on the blue side of the Bragg-reflection function. © 1993 Optical Society of America April 15, 1993 / Vol. 18, No. 8 / OPTICS LETTERS branch of the LI bistable curve. For operation on the upper branch, a current increase results in the usual thermal red shift, whereas a current decrease causes a gain decrease and a mode blue shift. The latter is a combination of a carrier-number-induced red shift and the usual thermally induced blue shift, which dominates. At any current level on the upper trace of the hysteresis loop (Fig. 2), the mode is red shifted compared with the mode at the same current on the lower trace. The losses, which are determined by the mode position relative to the Bragg-reflection function, are reduced along the upper trace, and therefore the laser turns off at a current lower than the turn-on current. This spectral behavior along the hysteresis loop is shown in Fig. 3, where a wavelength domain hysteresis loop is shown. This loop, together with the fixed position (in wavelength) of the Bragg-reflection function, is responsible for the power _ 3?1- 0 2- 0. 1 17 15 21 19 23 25 bistability (mA) Current Gain Fig. 2. Measured bistable LI curve. 4.2 - 0LO 4.1 LO E a 4.0 3.9 C 3.8 3.7 15 21 18 Gain 24 Current 27 30 (mA) Fig. 3. Wavelength bistability curve. Figure 2 shows the measured bistable light output power versus the injected current (LI curve) for a given current swing to the gain section and zero current to the Bragg section. The explanation of the bistable operation, with reference to the inset of Fig. 2, is as follows. Below threshold, 611 an increase in current causes a material gain increase and a simultaneous cavity loss increase owing to a small blue shift of the mode frequency such that its overlap with the Bragg-reflection bandwidth decreases. However, the net effect is an increase in gain so that eventually the laser reaches threshold. This operation of the laser at threshold is unstable since any excess carriers above the clamping level red shift the mode, thereby reducing the cavity losses and lowering the clamping level, which in turn causes generation of more excess carriers until the laser stabilizes. This process explains the discontinuity of the LI curve and the generation of the upper of Fig. 2. The effect of light injection on the hysteresis loop is depicted in Fig. 4. The experiment uses a diode laser emitting near 1.5 ,um as a source for the turnon signal and a second diode laser emitting near 1.3 Am for the turn-off signal. Both optical signals are coupled to the gain section as seen in Fig. 1(a). Figure 4(a) shows the effect of the 1.5-/Lmsignal. It is clear that it affects only the turn-on threshold of the hysteresis loop. The 1.5-,umsignal saturates the gain, which causes a modal red shift. This increases the mode-DBR overlap, thereby reducing the losses and turning the laser on at a lower level of injected current. In contrast, the 1.3-/,.m signal affects only the turn of threshold. This signal is absorbed in the cavity, which causes a carrier density increase with no significant thermal effects. The resultant blue mode shift increases the mode-DBR mismatch, enhancing the cavity losses and causing the laser to turn off at a current higher than the corresponding current without light injection. The results shown in Fig. 4 suggest the possibility of optically controlled turn-on and turn-off by using the 1.5- and 1.3-pumsignals, respectively. This enables one to operate the laser in a flip-flop mode. (b) (a) 4- 4 D 3- I3 S 3: 0 2- o2 0. 0. 1- fi 15 17 19 21 23 25 Gain Current (mA) 16 17 19 21 23 25 Gain Current (mA) Fig. 4. Effect of optical control signals on the hysteresis loop (the dashed lines represent the boundaries of the hysteresis loop with no optical injection). (a) Effect of the 1.5-,m signal, (b) effect of the 1.3-,um signal. 612 OPTICS LETTERS / Vol.18, No. 8 / April 15, 1993 also demonstrated and is depicted in Fig. 5. The 1.5um pulse turns the laser on, and the 1.3-/um pulse turns the laser off. The switching energies were 1 and 2 pJ for turn-on and turn-off, respectively. Finally, for comparison we examine the characteristics of a second laser that differs from the first one only in that its cavity mode overlaps the Braggreflection function on the long-wavelength side (red side), as in the inset of Fig. 6(a). 0 ON OFF- DBR Laser 0.27 0.30 0.32 0.35 0.37 0.40 T ("s) Fig. 5. Dynamic behavior of the optical flip-flop. (a) Figure 6(a) shows the measured LI curve of that laser which exhibits no bistability. Below threshold, a current increase reduces losses (owing to the mode blue shift) while also increasing the gain until the laser reaches threshold, which in this case is a stable one. Above threshold, a current decrease retraces the LI curve so that no hysteresis loop is obtained. Optical control of this device by injection of a 1.5-/Lm optical signal into the gain section was demonstrated. This causes a red shift (owing to gain saturation), which further increases the losses and increases the threshold. A measurement of the effect is shown in Fig. 6(b). The 1.5-,um optical signal can therefore turn the laser off (without latching to a second stable state), so it can serve as a wavelength-converting device. (b) References S 'S 01 30 01 1. I. H. White and J. E. Carrol, Electron. Lett. 19, 558 (1983). 4, 0o 2. T. Odagawa and S. Yamakoshi, Electron. Lett. 25, 1429 (1989). 0. 3. H. Shoji, Y. Arakawa, and Y. Fuji, J. Lightwave Technol. 8, 1630 (1990). 15 19 23 27 31 35 Gain Current (mA) 4. M. Okada, H. Kikuchi, K. Takizawa, and H. Fujikake, IEEE J. Quantum Electron. 27, 2003 (1991). 5. Y. Ozeki and C. L. Lang, IEEE J. Quantum Electron. 15 19 23 27 31 35 Gain Current (mA) Fig. 6. (a) Measured LI curve with no bistability. The inset shows the mode-DBR overlap on the red side of the Bragg-reflection function. (b) Effect of a 1.5-gm optical control signal. Such an optical flip-flopwas tested both statically and dynamically. For the static operation, the device was dc biased below threshold (18.5 mA), and we observed laser turn-on when the optical signal at 1.5 Iumwas applied. Subsequent removal of the 1.5,um signal left the laser turned on. Next we applied the 1.3-Aumoptical signal and observed laser turnoff. Removal of the 1.3-,gm signal left the laser in its off state. Dynamic operation of the flip-flop was 27, 1160 (1990). 6. M. J. Adams, Opt. Quantum Electron. 21, 15 (1989). 7. K. Peterman, Laser Diode Modulation and Noise (Kluwer Academic, Dordrecht, The Netherlands, 1988). 8. P. J. A. Thijs, in Digest of the IEEE Thirteenth InternationalSemiconductorLaser Conference(Institute of Electrical and Electronics Engineers, New York, 1992), paper A-1. 9. T. L. Koch and U. Koren, J. Lightwave 274 (1990). Technol. 8, 10. P. Glas and R. Muller, Opt. Quantum Electron. 14, 375 (1982). 11. F. S. Felber and J. H. Marburger, Appl. Phys. Lett. 28, 731 (1976). 12. A. N. Olsson, W. T. Tsang, and R. A. Logan, Appl. Phys. Lett. 44, 375 (1984).