a1709_1.pdf JTuA74.pdf
The Fabrication of Laser Array by Holographic Interference Lithography Chuli Chao1, 2, Chi-Yu Ni1, Rong Xuan1, 2, Hao-Chung Kuo3 1
Department of Electrophysic, National Chiao Tung University, Hsinchu 300, Taiwan 2 Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan 3 Department of Photonic and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan e-mail:
[email protected]
Abstract: We have developed a novel method to produce different grating periods in one chip and applied this in the fabrication for laser array. The result shows accurate controllability of lasing wavelength and low threshold currents. ©2007 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (140.2010) Diode laser arrays
1. Introduction Recent progress in the wavelength division multiplexing (WDM) technologies for broadband fiber optic communication systems has called for a multiple wavelength light source. Integration of lasers into a multi-wavelength laser array (MLA) has been presented as a promising approach to reduce the cost per wavelength. DFB laser array is key components for wavelength division multiplexing (WDM) systems in order to exploit the broad bandwidth of optical fibers [1]. In the case of conventional monolithic DFB laser array, the lasing wavelength is controlled by adjusting the grating period pitch in identical optical waveguides. This is usually realized by e-beam lithography [2], which is an expensive and low throughput fabrication technique. Use of holographic interference lithography can be realizable [3]. In order to realize a different grating pitch for each laser, both the holographic set-up (for the period) and the wafer (for the ridge spacing) should be moved. Going to a single lithography step requires proper localized holographic exposure. This paper describes the realization and use of a window mask that allows proper localized isolation. Due to the simple fabrication process, this device is suitable for mass production with high yield. In this paper, the fabrication of a four-channel laser array associated with asymmetric quantum wells is demonstrated. 2. Fabrication of four-channel laser array In this section, we will explain the fabrication process of DFB laser array. An asymmetric quantum wells structure on an n-InP (100) substrate was used as the initial wafer. The four different DFB grating patterns Λ were prepared on the same wafer. Four channel DFB laser cavities with the same structure. Stripe width was 2 μm and cavity length was 250 μm, respectively. At first, a 400 nm-thick SixNy layer was deposited and window with of 100 μm was made by using conventional lithography process, the window direction is perpendicular to the grating lines. After developing, the SixNy mask layer was etched by SF6 plasma reactive ion etching (RIE). For grating realization, we use holographic interference lithography. The grating was written directly on the surface of InP substrate on which photoresist is smeared. The thickness of photoresist is about 300 nm, as figure 1 (a). The grating were transferred into the surface of InP substrate by a wet chemical etching using HNO3 composition at 25℃, as figure 1 (b). Then SixNy layer was removed and grew again a new SixNy layer. The other grating pattern with different period was produced in the distance of 150 µm apart from the first pattern. This repetition could arbitrarily fabricate different grating patterns. The lasing wavelength is controlled at 10 nm spacing by changing the grating pitch using holographic interference lithography. In this paper, we made four different grating patterns and standard laser fabrication was followed.
a1709_1.pdf JTuA74.pdf
(a) (b) Fig. 1. (a) SEM image of grating profile, (b) SEM image of the cross section of the grating recorded on the InP substrate.
3. Results Fig. 2 (a) shows typical light-current characteristics of individual lasers in our laser array. The lasers were measured at room temperature under CW operation. As shown in the figure, all the lasers in the array exhibited similar performances. The threshold currents were between 12~27mA and slope efficiency was about 0.136 W/A. The average power was about 7.2 mW and the deviation of output power was merely 3.5 dB at the injection current of 80mA. Laser Array L-I curve
Laser Array Spectrum -40
10
-45
Power (mW)
8
Ch1 Ch2 Ch3 Ch4
6
-50
4
-55
Ch1(dBm)
-60
Ch2(dBm) Ch3(dBm)
-65
Ch4(um)
-70
2 -75
0
-80
0
20
40
60
Current (mA)
80
100
1530
1540
1550 Wavelength(nm)
1560
1570
(a) (b) Fig. 2 (a) Light-current characteristics of individual channels of the asymmetric quantum wells laser array, (b) Lasing spectra of four channels measured at room temperature.
Fig. 2 (b) shows typical lasing spectra of four-channel. The lasing wavelength of each channel is shown in figure. All the channels spacing were 10 nm. The results show that both the lasing wavelength and the channel spacing were well managed and accurately controlled. In addition, all the lasers revealed side-mode suppression ratio (SMSR) of over 30 dB. 4. Conclusions We have demonstrated that using holographic interference lithography is a great technique for generating DFB laser arrays with different grating periods. Not only can holographic interference lithography achieve submicron resolution over large exposure area with an effectively infinite depth of focus in one step, but also is well matched for the DFB laser array manufacturing. This fabrication technique has the flexibility in changing the grating period by change the exposure angle. Therefore, this grating fabrication technique may find wide application for the future generations of lightwave components, particularly for WDM components. 5. References [1] T. P. Lee, C. E. Zah, and R. Bhat, "Progress in strained quantum well lasers and DFB laser arrays for FITL and WDM applications," in OEC’94, (1994), 14D1-1. [2] P. N. Woolnought, P. Birdsall, P. J. O’Sullivan, A. J. Cockburn, and M. J. Harlow, "Fabrication of a four-channel DFB laser transmitter OEIC for 1550 nm operation," Electron. Lett., Vol. 29, No. 15, (1993), pp. 1388-1390. [3] H. Okuda, Y. Hirayama, H. Furuyama, J. Kinoshita, and M. Nakamura, "Five-wavelength integrated DFB laser arrays with quarter-wave-shifted structures," IEEE J. Quantum Electron., Vol. 23, No.6, (1987), pp. 843-847.