Stable single mode operation of ...

Stable single mode operation of gratingsurfaceemitting laser arrays under frequencymodulated operation N. W. Carlson, D. P. Bour, G. A. Evans, R. Amantea, and S. K. Liew Citation: Appl. Phys. Lett. 57, 756 (1990); doi: 10.1063/1.103412 View online: http://dx.doi.org/10.1063/1.103412 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v57/i8 Published by the AIP Publishing LLC.

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Stable single mode operation of grating"'surface..emitting laser arrays under frequency~modufated operation N. W. Carlson, D. P. Sour, G. A Evans, R. Amantea, and S. K. Liew David Sarnoff Research Center, CN-5300, Princeton, New Jersey 08543-5300

(Received 20 February 1990; accepted for publication 12 June 1990) Direct frequency-modulated operation of ridge-guided linear grating-surface-emitting laser arrays is reported. Simultaneous far-field and spectral measurements of these devices at 1 GHz modulation rates show that both single longitudinal mode and single spatial mode operation are maintained.

Grating-surface-emitting (GSE) laser technology is very promising for providing monolithic single-frequency semiconductor diode laser sources that can produce the high-power, narrow-divergence, electronically steerable output beams necessary for many optical systems applications. 1-4 Some applications, such as free-space optical communications, require that the far-field pattern remain stable under modulated operation. In these instances, the electronic beam steering that can occur in GSE laser arrays would be undesirable. 2 Therefore, it is important to establish the set of operating conditions that provide simultaneous spectral and spatial mode stability under GHz modulation rate operation. In this letter we report on an experimental demonstration of the modal stability of linear GSE arrays under direct frequency modulated operation. Using a nonlinear network model for GSE arrays, the conditions for stability of the spatial mode of a GSE array are calculated. The coherent monolithic three-element linear arrays of GSE lasers were similar to those recently reported to have demonstrated single-mode operation with spectral linewidths as narrow as 290 kHz. 5 Figure 1 shows a schematic diagram of a linear GSE array as wen as the structure and composition of the epilayers. These GSE lasers were fabricated from organometallic vapor phase epitaxial layers grown on Alo.J5G~.g5As substrates. The processing steps and structure are described in more detail in Ref. 1. The gain sections consisted of 3-,um-wide ridge guides and were 300 pm long with 200-,um-Iong distributed Bragg reflector (DBR) sections on each end of the gain section. The Alo.lSGao.8SAs substrate on which these devices were grown was transparent to the 0.87 ,urn wavelength of the laser, so devices from the wafer could be mounted p side down (for better heat sinking) and the light coupled out by the grating was transmitted through the n substrate into air. There was no antirefiect coating on the n side and no high reflect coating on the p side, so only about half of the outcoupled light was collected. Linear GSE arrays were mounted on a BeO submount with patterned metal stripes so that the current to each gain section could be independently addressed. Single-element GSE lasers (one gain section with two DBR emitters) had thresholds in the range 30-60 rnA. In general, single element lasers operated in a single longitudinal mode at currents up to twice threshold. Single and multielement GSE lasers with a de bias were modulated at GHz frequencies while simultaneous 756

Appl. Phys. Lett. 57 (8), 20 August 1990

spectral and far-field measurements were made. Figure 2 shows the simultaneous measurements of far-field and spectral output at the dc bias operating point and at three different power levels of a 1.25 GHz sine wave signal that was used to modulate a single-element GSE laser. The spectral measurements were made with a 7.5 GHz freespectral range Fabry-Perot interferometer, and show that this laser is operating in a single longitudinal mode at the de bias point. When the power of the rf modulation is increased, the modulation sidebands increase in magnitude and higher order sidebands are also observed. At the highest rf power used, the component of the power spectrum at the carrier frequency is almost extinguished. Over the entire range of rf modulation power, no broadening of the carrier or modulation sidebands was measured to within the resolution of the Fabry-Perot interferometer (~50 MHz). The time-averaged far-field patterns corresponding to de operation and the various rf modulation levels are next to the respective power spectra in Fig. 2. Essentially no change in the far-field pattern was observed as the modulation level was increased. The peak intensity of the lobes, the visibility (noted as V in Fig. 2), and the angular divergence of the lobes all remain unchanged from their de values. Three adjacent gain sections were driven to form a coherent linear array. In order to determine that the three gain sections were locked in a single longitudinal mode, the spectral output of the array was monitored with a FabryPerot interferometer and the currents to the three gain GRINSCH·SQW Active Layer

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Far Field

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FIG. 2. Simultaneous measurement of the far-field and spectral outputs of a single-clement GSE laser as a function ofmoduJation level at 1.25 GHz.

sections were adjusted so that single longitudinal mode operation was observed on the scanning Fabry-Perot (adjacent longitudinal modes of this three-element array should be separated by about 27 GHz). The spatial coherence between the end elements when the array operated single mode was typically 80--90%. Once the range of single longitudinal operation was determined, a subsequent fine adjustment of the currents to the gain sections was done to optimize the far-field pattern. The array was maintained at this dc operating point while a capaciiive1y coupled 1 GHz signal was used to modulate only the center gain section. The three gain sections were biased at 1.5, 1.8, and 1.9 times their respective individual threshold currents, and at this operating point the three-element array had a cw power output of about 12 m W. Figure 3 shows the simultaneous measurements of far-field and spectral output at the de bias operating point and at three different r1' power levels that were used to modulate the center gain element of the array, As in the case of the single element, di.rect FM operation was observed with almost no change in the far-field pattern from what was measured in the de case, The measured full width half maximum of the farfield angular divergence (in Fig. 3) of 0.03" is instrument limited, so the actual far-field angular divergence is narrower. In the power spectra measurements, there is an increasing asymmetry that appears in the sidebands as the modulation level is increased. This indicates that the phase difference between the FM and AM components of the optical field is changing." Also, at an rf modulation power of + 5 dRm or more the array is no longer operating in a single longitudinal mode as evidenced by the additional structure that appears in the central region of the power spectrum. Even though the array is no longer operating in a single spectral mode, the time-averaged far-field pattern shows only a 5% reduction in the peak intensity of the dominant lobe and essentially no change in the visibility or angular divergence of the lobes. No broadening of the carrier or modulation si.debands is observed in these power 757

Appl. Phys. Lett., Vol. 57, No.8, 20 August 1990

FIG. 30 Simultaneous measurement of the far-field and spectral outputs of a coherent three-element linear GSE laser array as a fUllction of the 1 GHz modulation level applied io the center gain section. The contrast of the far-field pattern is given by C.

spectra measurements. However, at modulation power levels of about -+- 12 dRm or more broadening of the carrier and sidebands became noticeable, and at + 15 dBm the power spectrum was too broad to be resolved by the Fabry-Perot interferometer. In spite of this severe spectral instability, the time-averaged far-field peak intensity was reduced only by about 40% and the visibility dropped to only 58%. An understanding of the conditions under which it is possible to modulate a GSE array and maintain spectral and spatial mode stability can be obtained hy using a nonlinear network model to calculate the array properties above threshold. 7 These calculations have revealed that arrays with uniform power distributions in the gain sections show the greatest modal stability as the array is driven to higher operating levels. This occurs because the mode that produces the most uniform saturation of the gain distribution across the array will have the maximum discrimination against other modes. Also, if the current distribution to the array gain sections is varied in a symmetric fashion with respect to the center of the array, it is possible to tune the operating frequency without introducing a near-field wave front tilt that would steer the far-field beam. 2 An example of such a calculation is shown in Fig. 4 where the far field of a three-element array (with uniform power distribution to the gain sections) operating at 1.5 times threshold is shown. The solid trace is the far-field pattern calculated when the unsaturated gain to each active section is the same and the dotted trace is the calculated far field when the unsaturated gain to the center section is increased (in a quasi-static manner) by 12% and the unsaturated gain to the end sections was fixed. The total power output changed by only about 5%. This symmetric change of the unsaturated gain distribution of the array is equivalent to Carlson et al.

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FIG. 4. Nonlinear network model calculation of the far-field output of a uniformly pumped three-clement GSE array. The solid trace is for a uniformly pumped array. The dotted trace corresponds to the situation when only the unsaturated gain to the center gain sections is increased hy 12%.

changing the current to the center gain section while the currents to the end gain sections are held fixed. No beam steering occurs, but there is a change in the relative intensities of the side lobes and the peak intensity of the dominant lobe decreases by about 18%. These quasi-static calculations show that the spatial mode of a GSE array (operating above threshold in a single longitudinal mode) is stable with respect to small symmetric current changes to the gain sections. In order to calculate the amplitude

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Appl. Phys. Lett., Vol. 57, No.8, 20 August 1990

and frequency responses under dynamic conditions, it is necessary to do a small-signal analysis of the nonlinear network model results. 8 Both the experimental and theoretical results presented here show that it is possible to directly frequency modulate GSE laser arrays and maintain modal stability. Although more experiments are needed to measure the dynamic frequency response and the bit error rates when actual data are transmitted by a GSE array, these new results suggest that it may be feasible to use GSE technology in coherent communications systems. \ G. A. Evans, N. W. Carlson, J. M. Hammer, M. Lurie, J. K. Butler, S. L Palfrey, R. Amantea, L. A. Carr, F. Z. Hawry!o, E. A. James, C. J. Kaiser, J. B. Kirk, and W. F. Reichert. nmE J. Quantum Electron. 25, 1525 (1989). LN. W. Carlson, G. A. Evans, R, Amantea, S. L Palfrey, J. M. Hammer, M. Lurie, L. A. Carr, F. Z. Hawrylo, E. A. James, C. J. Kaiser, 1. B. Kirk, and W. F. Reichert, App!. Phys. Lett. 53, 2275 (I988j. J D. F. Walch, R. Parke. A. Hardy, W. Streifer, and D. R. Scifres, App!. Phys. Lett. 55, 813 (1989). 4K, Kojima, S. Noda, K. Mitsunaga K. Kyuma, and K. Hamanaka, AppL Phys. Lett. 50, 1705 (1987). 5N. W. Carlson, D. P. Bour, G. A. Evans, and S. K. Liew, IEEE Photon. Techno!. Lett. 2, 242 (1990). oS. Kobayashi, Y. Yamamoto, M. Ito, and T. Kimura, IEEE J. Quantum Electron. 18, 582 (1982). 7R. Amantea (unpublished). 8 R. J. Lang and A. Yariv, IEEE J. Quantum Electron. 21, 1683 (1985).

Carlson et al.

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