IOP PUBLISHING

SEMICONDUCTOR SCIENCE AND TECHNOLOGY

doi:10.1088/0268-1242/22/7/022

Semicond. Sci. Technol. 22 (2007) 802–805

40 Gbps electroabsorption modulated DFB laser with tilted facet formed by dry etching Joong-Seon Choe, Yong-Hwan Kwon, Jae-Sik Sim and Sung-Bock Kim Electronics and Telecommunications Research Institute, 161 Gajeong-dong, Yuseong-gu, Daejeon 305-700, Korea E-mail: [email protected]

Received 9 January 2007, in final form 22 May 2007 Published 8 June 2007 Online at stacks.iop.org/SST/22/802 Abstract We fabricated electroabsorption modulated distributed feedback (DFB) laser diodes for 40 Gbps application through the selective-area growth method. The facet reflection was not sufficiently removed by just depositing the anti-reflection coating so that low frequency resonance occurred by optical feedback from the facet to the DFB laser. A tilted facet formed by dry etching processes successfully reduced the optical feedback and the resonance in E/O response decreased significantly.

1. Introduction As the communication environment develops, the components as well as communication systems are required to work at higher bit rate, and a lot of research has been conducted into high speed optical devices, such as a distributed feedback (DFB) laser, electroabsorption modulator (EAM), electrooptic modulator and photodetector (PD) [1–4]. A directly modulated DFB laser is not suitable for long-haul optical communication because its wavelength chirp is relatively large. External modulators, operating in conjunction with DFB lasers, are widely used as sources of a modulated optical signal. However, much attention has recently been paid to the electroabsorption modulated DFB laser (EML), which is the monolithic integration of a DFB laser and an EAM. In view of device engineering, EML has advantage in coupling efficiency because there is no fibre coupling between the EAM and the DFB laser. EML helps to design more compact transponder owing to its small footprint, and improves the cost-effectiveness of the system by reducing the number of components. The EAM and DFB laser must have different active layers by their operation principles and therefore complicated processes and/or epitaxy technique are required for the integration [5–7]. The EAM and DFB laser should be integrated so that most of the laser output propagates into the EAM waveguide. While coupled on a common substrate, the EAM and DFB laser must be isolated electrically [8]. 0268-1242/07/070802+04$30.00

With insufficient electrical isolation, there can be leakage of the modulation signal to the DFB laser that can induce direct modulation of the DFB laser and frequency chirp, which deteriorates the transmission characteristics. Photocurrent from the EAM can also change the lasing wavelength if flown to the DFB laser. These problems can be solved by increasing isolation resistance. Generally electrical isolation as high as ∼105
can be achieved by trench etching or ion implantation between the EAM and the DFB laser. Modulated output also, if reflected back into the DFB laser, induces frequency chirp and relaxation oscillation that limit long-haul transmission at a high bit rate. Isolation of the modulated optical signal from the DFB laser is one of the key parameters in EML compared to the combination of the discrete DFB laser and external modulator where the optical isolator can remove the back-reflection effect. To suppress the optical feedback in EML, in addition to anti-reflection (AR) coating, InP window and/or tilted waveguide is widely used [9]. In this study, tilted facet was adopted at the end of straight waveguide through the dry etching process. Compared to the other methods mentioned above, the tilted facet by dry etching has advantages of simpler process, shorter device length, and less radiation loss.

2. Fabrication and measurement First, buffer, n+ -InP cladding, and 1.24Q InGaAsP grating layers were grown on a semi-insulating (1 0 0) InP substrate

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Figure 1. The structure of EML device with (a) parallel facet formed by mechanical cleaving and (b) tilted facet by dry etching processes.

using metallorganic chemical vapour deposition. The grating was formed through holographic lithography patterning, followed by dry etching and slight chemical etching with HBr:H2 O2 :H2 O = 8:2:100 solution. Active layers were grown with SiNx selective-area growth (SAG) pattern on the surface. After the SAG pattern was removed, p-InP cladding and p+ -InGaAs contact layer growth finalized the epitaxy. The fabrication process begins with the ridge formation. In this study, the reverse mesa ridge structure was chosen in order to keep both the contact resistance and active region capacitance small [10]. Reverse mesa ridges comprising p-InP were formed along the [0 1 1] direction by dipping the wafers with SiNx stripe patterns in the HBr:H3 PO4 = 1:1 solution for 7 min. The mesa bottom widths were 2 µm and 1.5 µm for the DFB laser and EAM, respectively. Active and separate confinement heterostructure layers were etched by RIE to expose the n+ -InP contact layer. Outside of the active and ntype contact region, n+ -InP layer was etched away to minimize the area of the doped region beneath the ground-signal-ground (GSG) travelling-wave electrode that feeds the RF signal. p+ and n+ ohmic contacts were formed with Ti/Pt/Au and Cr/Au, respectively. Etched sidewalls were passivated with polyimide (PI-2723) so that the GSG electrode could be electrically connected from the top surface of the ridge to the substrate exposed after RIE processes. Electrical isolation between the EAM and DFB laser part was achieved by RIE to form a trench and measured to be over 100 k
. After the cleaving process, AR coating layers were deposited on the EAM facet. The resultant facet reflectivity was 0.5%. The device structure after fabrication is schematically shown in figure 1. As in figure 1, two kinds of devices were made: devices with (a) parallel and (b) tilted facet. Parallel facet is conventionally adopted as it can be made easily by the mechanical cleaving process. When an angle other than 0◦ is needed between waveguide and facet, a curved part is usually inserted along the waveguide to meet the facet with the tilt angle [9]. Devices with such structures have disadvantages of long device length and large radiation loss along the curved part. In this paper, etched facet was made during the three successive RIE processes—for separate confinement heterostructure region, n+ -InP, and isolation trench etching. The angle between the cleaving plane and etched facet was 7◦ in order to minimize the back-reflection into the waveguide [11]. Figure 2 shows the plan view microscope image of a device in a chip bar. The lengths of the DFB laser, EAM and isolation trench are 400 µm, 100 µm and 150 µm,

Figure 2. Plan view microscope image of an EML device in a chip bar. (This figure is in colour only in the electronic version)

Figure 3. Schematic diagram of the setup for measuring the E/O response characteristics of EML devices.

respectively. In this figure, the right GSG electrode of the EAM is for feeding the modulation signal. The modulation signal modulates the DFB laser output during the propagation along the EAM waveguide and exits the device through the left GSG electrode. To suppress electric reflection, the GSG electrode was designed to have a characteristic impedance near 50
. The electric reflection of the EML device during operation was less than −11 dB over the frequency range from dc to 40 GHz. Figure 3 shows the schematic diagram of the E/O response measurement setup. In an EML, a DFB laser operates under the CW condition and an EAM modulates the DFB laser optical output according to the signal from a port of a vector network analyser (VNA, Anritsu 37397C). A voltage source (Keithley 236) applies reverse bias to the EAM through a bias tee. The modulated output of the EML is coupled to a tapered single mode fibre that is connected to 40 Gbps receiver module [3]. The RF output of the receiver is measured at another port of VNA. The modulation signal is fed to the EAM through a GSG probe and terminates at 50
resistor that is attached to another GSG probe contacting the left electrode.

3. Device characteristics Through the photoluminescence experiment, the active layers grown by the SAG method were found to have transition wavelengths of 1.540 µm and 1.490 µm for the DFB laser and the EAM, respectively. After fabrication, the DFB laser lased at a threshold current of 23 mA and its lasing wavelength was 1.554 µm. Static extinction was about 15 dB at −3 V. From the E/O measurement, the 3 dB bandwidth of the device was found to be about 27 GHz. 803

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Figure 4. E/O response of parallel-faceted EML with AR coating. Inset is E/O response before AR coating was deposited. The curves are shifted vertically.

Figure 5. E/O response of EML with tilted facet after AR coating deposition. Curves are vertically shifted for the clarity.

Figure 4 shows the frequency responses of an EML device with AR-coated cleaved parallel facet at several EAM bias voltages. Note that the curves have been vertically shifted for the sake of clarity. The current injected to the DFB laser was set at 70 mA, which produced the fibre-coupled optical power of about 1 mW with 0 V applied to EAM. Although frequency response is flat at high frequency regime, there exists a resonance at about 3 GHz. The resonance, originating from DFB laser relaxation oscillation, may occur by the leakage of the electric modulation signal or feedback of the modulated signal from the EAM facet [12]. In this case, it is caused mainly by the optical feedback, considering that the electric isolation resistance is high enough while the reflectivity of AR-coated facet is not sufficiently low [12]. The resonance behaviour was found to be strongly related to the combination of the EAM bias and the laser current. As in figure 4, at the fixed laser current the resonance peak changes in both magnitude and shape with the EAM bias. While the E/O response remains nearly unchanged at high frequency regime (>10 GHz), the peak magnitude changes from −8 dB (negative sign means dip) to 17 dB with the EAM bias. This phenomenon can be explained by the fact that as the EAM bias changes, the round-trip phase of the reflected light also changes in the EAM waveguide [13]. Comparing to the E/O response before AR coating, shown in the inset of figure 4, it is clear that the resonance peak decreased with AR coating on the facet. However, the significant magnitude of resonance even with AR coating shows that just depositing the AR coating layers does not suffice for avoiding the occurrence of the resonance unless the AR coating is so perfect that the resultant reflectivity is less than 0.01%. An InP window is usually adopted to further remove the optical feedback at the facet and was proven to be efficient for that purpose [9]. But the InP window structure requires the butt coupling process, which raises the production cost and increases the process difficulty especially in reverse mesa structures by selective wet etching. As described in section 2, some devices were fabricated with the facet tilted 7◦ through RIE processes (figure 1(b)). The same AR coating layers were deposited on both the tilted

facet and parallel facet devices. Because the AR coating was optimized for the parallel facet devices, the effect of reflectivity reduction is less when the facet is tilted. Figure 5 shows the E/O response of the EML with tilted facet after AR coating deposition at several EAM biases. The current injected to the DFB laser was set at 70 mA, as for the parallel-faceted EML, in order to rule out the effect of operation condition upon the device characteristics. In this figure, low frequency resonance is observable at some voltages. However, the magnitude is much smaller than that in the parallel-faceted EML. The resonance peak does not exceed 2.8 dB in magnitude over the bias range applied to the EAM. Additional suppression of the resonance can be expected if using AR coating optimized for the tilted facet. The signal is noisy over the full frequency range due to the low fibre coupling efficiency originating from the poor far field pattern. As the facet was made during several RIE processes, on the etched surface there exist steps formed during each RIE process. These steps cause beam scattering that deforms the far field pattern in the vertical direction. At high frequency over 30 GHz in figure 5 the signal is even noisier due to the low signal to noise ratio of VNA as well as the low coupling efficiency. The far field pattern can be improved by onestep facet formation using a deep etching apparatus such as inductively coupled plasma.

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4. Conclusion EML with tilted facet was fabricated and its E/O response was characterized. Compared with the parallel-faceted EML, the tilted-faceted EML was found to be immune to optical feedback from the EAM facet to the DFB laser. With the tilted facet, the resonance was efficiently suppressed by about 14 dB. The tilted facet in this study was made at the end of straight waveguide by the dry etching process and therefore it is superior to curved waveguide in view of the short device length and no radiation loss.

40 Gbps EML with tilted facet formed by dry etching

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[7] Johnson J E, Morton P A, Tanbuk-Ek T and Tsang W T 1995 Integrated electroabsorption modulators for WDM systems IEEE Lasers and Electro-Optics Society Annual Meeting (1995) vol 1, pp 124–5 [8] Salvatore R A, Sahara R T, Bock M A and Libenzon I 2002 Electroabsorption modulated laser for long transmission spans IEEE J. Quantum Electron. 38 44–476 [9] Park B H, Kim In, Kang B-K, Bae Y-D, Lee S-M, Kim Y H, Jang D H and Kim T-I 2005 Investigation of optical feedback in high-speed electroabsorption modulated lasers with a window region IEEE Photonics. Technol. Lett. 17 777–9 [10] Aoki M, Komori M, Tsuchiya T, Sato H, Nakahara K and Uomi K 1997 InP-based reversed-mesa ridge-waveguide structure for high-performance long-wavelength laser diode IEEE J. Sel. Top. Quantum Electron. 3 672–83 [11] Jaskorzynska B and Nilsson J 1991 Modal reflectivity of uptapered, tilted-facet, and antireflection-coated diode-laser amplifiers J. Opt. Soc. Am. B 8 484–93 [12] Suzuki M, Tanaka H, Akiba S and Kushiro Y 1988 Electrical and optical interactions between integrated InGaAsP/InP DFB lasers and electroabsorption modulators J. Lightwave Technol. 6 779–85 [13] Agrawal G P and Dutta N K 1993 Semiconductor Lasers 2nd edn (New York: Van Nostrand Reinhold)

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