The influence of structured delayed optical feedback (SDOF) on a broad area laser is investigated experimentally. SDOF is realized with a miniature-sized convex external mirror. The setup takes into account the small time scales involved in semiconductor laser dynamics by employing short external resonator lengths. Careful choice of the feedback parameters leads to a narrow single-lobe farfield even at high pump currents. The experimental results confirm earlier microscopic dynamic simulations by O. Hess et al. predicting that SDOF might be capable of stabilizing the emission of broad area lasers.
© 2000 Optical Society of America
In recent years high power semiconductor laser sources have become key elements in many of today’s photonic applications due to their small size and excellent efficiency. In fields like solid-state laser pumping or free-space optical communications the amount of coherent optical output power available is a central issue. In spite of considerable improvements in laser processing technologies, two major constraints continue to be an obstacle to applications of high power semiconductor lasers: On the one hand, the well-known phenomenon of catastrophic optical damage of the resonator facet is responsible for an upper limit of coherent output power. This problem may be circumvented by considerable enlargement of the lateral stripe width. Thus broad area lasers (BALs) having an emitter width (50 µm or more) of at least one order of magnitude larger than typical low-power single-stripe lasers and laser diode arrays (LDAs) are commonly used in high-power applications. On the other hand, due to the large size of the BAL laser stripe or the strong coupling between the emitters in LDAs characteristic strong intrinsic nonlinear interactions of the optical field with the active semiconductor medium, which are usually suppressed in low-power lasers, lead to strongly incoherent light emission. In recent years it has been revealed that these instabilities have their origin in complex spatio-temporal processes [1, 2, 3, 4, 5] which in turn are caused and determined by the microscopic spatio-temporal and spatio-spectral dynamics [6, 4, 7]. In the overall device behavior, these instabilities severely degrade the performance of high-power semiconductor lasers and consequently limit their number of applications. Clearly, if one does not want to resort to using a semiconductor laser as pump source in a solid-state laser system only, efficient and compact schemes for stabilization and control of the complex spatio-temporal dynamics are needed to gain stable laser output.
Consequently, various laser systems as well as control setups and schemes have been proposed for both LDAs [8, 9, 10, 11] and BALs in recent years. In free-running BAL systems, the intrinsic nonlinear interaction causes the lateral mode profile to break up into multiple filaments which dynamically migrate across the active layer on a ns time scale . At elevated pump currents a considerable part of the output power originates from higher order transverse modes. Experimental and technological approaches to control the emission of BALs involved modification of the laser facet by means of photolithography either to create an unstable resonator by giving the laser facet an inward bent curvature  or by modifying the reflectivity of the facet in the transverse direction . Further, phase conjugated feedback  or a modification of the lateral refractive index profile by junction heating  demonstrated their ability to control the lasing mode in BALs. A Fourier-optical 4f setup was used by the current authors to selectively excite the fundamental mode or a specific higher order mode of a BAL .
Other free-space external resonator concepts have only been studied theoretically so far. A Fourier-optical like setup was investigated by Champagne et al. via a standard beam propagation method . Recently, numerical simulations by Hess et al. [8, 18] gave evidence that structured delayed optical feedback, realized by using an external convex mirror, might possibly lead to a stabilization of the emission of BALs. Simulations of the spatio-temporal delay-dynamics of BAL on the basis of semiconductor Maxwell-Bloch equations shed light on the fundamental internal processes present in feedback-sustained coherent emission. It was predicted that spatially structured delayed optical feedback causes a coherent phase coupling process of the otherwise chaotic filaments.
In the current contribution we experimentally investigate the potential of spatially structured delayed optical feedback for stabilizing the emission of BALs. In our setup (Fig. 1) SDOF is realized with an external convex mirror which spatially selectively couples only paraxial rays back into the active layer. Accounting for the small time scales (≤10 ps) involved in semiconductor laser dynamics the external mirror is only millimeters away from the output facet of the BAL. The experimental results confirm the findings of the microscopic simulations showing that by SDOF the spatio-temporally chaotic filaments of the free-running BAL are coherently coupled leading to a narrow single-lobe farfield.
2 Structured Delayed Optical Feedback
2.1 Experimental setup
The experiments are performed with a highly antireflection (AR) coated (front facet reflectivity R 0<10-5) 811 nm, 1.2W Spectra Diode Labs (SDL)  AlGaAs BAL, having an emitter size of 1 µm×100 µm. The rear facet is highly reflection coated (rear facet reflectivity Rrear >95%).
As schematically depicted in Fig. 1 a Au coated glass rod with a radius of r̃ is employed as external mirror (reflectivity R 1≈98%). The highly AR coated output facet of the BAL is aligned onto the optical axis at a distance L in front of the convex cylindrical mirror. To comply with the small time scales involved in semiconductor laser dynamics very short distances L are applied to provide for short resonator round trip times. A micro-cylindrical lens is used for collimation of the fast axis of the laser output. At distances L used in the control setup, the laser threshold current Ith (which generally depends on L) is measured to be ≈400mA, which is the exact value for L=2370 µm and r̃=0.5 mm. (This BAL without AR coating showed a threshold current of 410mA ). In the experiments, the nearfield and farfield at the rear facet of the BAL are simultaneously monitored with a CCD camera. Thus here we will focus on experimental observation of the time averaged performance of the setup.
2.2 Experimental results
First experiments are carried out with an convex external mirror having a radius r ~=1 mm aligned onto the optical axis and without a microlens for collimating the fast laser axis. Due to the large radius of curvature and the missing collimation a pump current of Ip=3 Ith has to be applied to cause the laser to react to the external resonator. Typical nearfield and farfield patterns obtained with this setup are depicted in Fig. 2. The distance L is varied from 1200 µm (La) via 1400 µm (Lb) to 1600 µm (Lc) corresponding to round trip times of 8 ps, 9.3 ps, and 10.7 ps, respectively. All nearfields are strongly filamented and broad farfields sitting on a high background are acquired.
Further experiments are performed with an external mirror having a radius of r̃=0.5 mm. A micro-cylindrical lens (f=910 µm) is introduced into the setup for collimation of the fast axis of the laser output. By introducing the micro-cylindrical lens more light is coupled back into the BAL. This higher feedback leads to a substantial reduction of the background signal in the farfield measurements. Figure 3 shows scans of the nearfield (left) and farfield (right) intensity patterns of the BAL when the convex mirror is aligned onto the optical axis at at various distances L and a pump current of Ip=2 Ith. The distance L is varied from 1870 µm (L 1) via 2370 µm (L 2) to 3370 µm (L 3), corresponding to round trip times of 12.4 ps, 15.8 ps, and 22.5 ps, respectively. Larger distances L could not be realized due to geometrical restraints. The most stable laser output is obtained at a distance L 2=2370 µm. Therefore, at this distance, the dependence of the feedback on pump current is further investigated. As can be seen from Fig. 4 (right) increasing the pump current from 400mA (Ith) to 900mA (2.25 Ith) a narrow peak in the farfield emerges from the background. At Ip=2.25 Ith the farfield has a width of 1.3° (FWHM of the slow axis). However, at 1100mA (2.75 Ith) the farfield broadens and breaks up into multiple stripes. The corresponding measured nearfields exhibit strong filamentation at all pump currents (Fig. 4 (left)). Nevertheless, a certain pattern is observed when the farfields exhibit a single peak: The nearfield intensity at the edges of the active region is noticeably higher than in the middle where a local minimum is found and maxima are located at x=±10 µm on the laser facet. The single-lobe farfields corresponding to these nearfield intensity patterns indicate that the filaments are coherently coupled to a large extent.
In free-running BALs irregularly filamented nearfield patterns develop at high enough pump currents. The filaments are uncoupled generating a broad double-lobe farfield radiation pattern.
In the case of SDOF an unstable resonator configuration acting as a low pass filter is applied to stabilize the emission of BALs. Small feedback times (about 20 ps) in our SDOF setup provide for a coherent output signal that travels from the laser to the mirror and back on a time scale comparable with the time scales of characteristic dynamic processes within the active area of the BAL. However, the experiments reveal that the feedback parameters need to be carefully chosen. With an external convex mirror having a radius of curvature of r̃=0.5 mm and a resonator length of L=2370 µm the nearfields exhibit a certain pattern with local maxima at the edges of the active area and a minimum in the middle up to pump currents of 2.25 Ith. These patterns strikingly correspond to the nearfield patterns found in microscopic simulations of successfully applied SDOF . The simulated patterns show maxima at the nearfield edges and a local maximum in the middle. Both simulated and experimental nearfield patterns lead to a narrow single-lobe farfield since both their Fourier-transforms are similar. The narrow farfields indicate that a coherent phase coupling process of the filaments is induced by SDOF.
An increase of the pump current leads to a speedup of the spatio-temporal dynamics which increases filamentation . Our SDOF setup is able to provide for stabilizing feedback up to pump currents as high as Ip=2.25 Ith. At higher pump currents feedback of the external resonator is not strong enough to suppress irregular filamentation.
We have experimentally investigated the behavior of a broad area laser (BAL) under the influence of structured delayed optical feedback (SDOF). The SDOF setup is taking into account the interplay of the characteristic dynamic processes within the active region and the coherent spatially modified feedback-signal by using a miniature-size external resonator. Feedback times of some 10 ps are in the order of times scales typical for dynamic processes within the BAL. Upon application of SDOF with appropriately chosen feedback parameters multiple optical filaments which characterize the free-running BAL are coherently coupled.
The steady-state results of the spatial nearfield and farfield profiles obtained in the experiment confirm earlier dynamic simulations of SDOF on the basis of semiconductor Maxwell-Bloch delay equations . The results of the experiments reveal that structured delayed optical feedback anticipated in the simulations, indeed, allows for the stabilization of the BALs originally temporally and spatially chaotic states by inducing a coherent phase-coupling between the optical filaments. The filament-coupling by SDOF which results in a narrow single-lobe farfield is experimentally maintained up to pump currents of 2.25 Ith.
The authors would like to thank C. Simmendinger and O. Hess for very fruitful discussions concerning the dynamical processes present in BALs.
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