Electroabsorption-modulated widely tunable DBR laser transmitter for WDM-PONs Liangshun Han, Song Liang,* Huitao Wang, Lijun Qiao, Junjie Xu, Lingjuan Zhao, Hongliang Zhu, Baojun Wang, and Wei Wang Key Laboratory of Semiconductor Material Sciences, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China * [email protected]

Abstract: We present an InP based distributed Bragg reflector (DBR) laser transmitter which has a wide wavelength tuning range and a high chip output power for wavelength division multiplexing passive optical network (WDM-PON) applications. By butt-jointing InGaAsP with 1.45µm emission wavelength as the material of the grating section, the laser wavelength can be tuned for over 13nm by the DBR current. Accompanied by varying the chip temperature, the tuning range can be further enlarged to 16 nm. With the help of the integrated semiconductor optical amplifier (SOA), the largest chip output power is over 30mW. The electroabsorption modulator (EAM) is integrated into the device by the selective-area growth (SAG) technique. The 3dB small signal modulation bandwidth of the EAM is over 13 GHz. The device has both a simple tuning scheme and a simple fabrication procedure, making it suitable for low cost massive production which is desirable for WDM-PON uses. ©2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (140.3600) Lasers, tunable.

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#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30368

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1. Introduction The introduction of wavelength division multiplexing (WDM) into passive optical networks (PONs) is a promising way to increase the capacity of optical access networks dramatically [1, 2]. Besides, WDM-PONs also have the other advantages such as transparency in protocol, security, and simplicity in electronics [3]. For the large number of optical network units (ONUs) in WDM-PONs, colorless transmitters are highly desired so that the costs related to the inventory and management of the transmitters can be reduced greatly. Many techniques have been proposed for the realization of colorless OUNs, including spectrum sliced broadband light source [4], reflective semiconductor optical amplifiers (RSOAs) [5] or reflective electro-absorption modulators (REAMs) with amplified spontaneous emission (ASE) seeding [6], and injection locked Fabry-Pérot (FP) laser diodes [7]. Another kind of light source for WDM-PONs is wavelength tunable laser, the application of which is considered as the easiest way to realize colorless ONUs [8]. WDM-PONs with tunable lasers have superior properties such as remarkable simplicity and reliability, and remote management of the wavelength plan of the network [8]. In this paper, we report the fabrication of InP based tunable distributed Bragg reflector (DBR) laser monolithically integrated with semiconductor optical amplifier (SOA) and electroabsorption modulator (EAM). Unlike the external-cavity lasers (ECL) [9, 10], the DBR lasers have no moving parts, thus the performance instability related to mechanical shock and

#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30369

vibration can be avoided. What is more, the DBR lasers have a smaller size and can be easily integrated with other optical components as in this paper. The DBR lasers are also more suitable for use in WDM-PONs than the multi-section sampled grating distributed Bragg reflector (SGDBR) lasers [11, 12] for the following reasons. First, the DBR lasers have a simpler structure than the SGDBR lasers, leading to a lower complicity of chip design and a smaller chip size (the length of the SGDBR laser can be twice of that of the DBR laser). Second, may be the most important, it takes hours to calibrate a single device because of the complex control needed for wavelength tuning of the SGDBR lasers [13], which is a bottle neck of the manufacturing process. By contrast, because of the simple structure of the DBR lasers, less parameters are needed for the control of the device, making the calibration of the device much more rapid. What is more, device with less control parameters will have weaker wavelength shift with aging time and simpler wavelength locking algorithms. All these characteristics of the DBR lasers help to promote the low cost mass production of the device, which is favored for WDM-PON use. Tunable DBR laser transmitters have been fabricated in several previous works [14–16], however, all having limited output power and tuning range. By comparison, our device has an over 16 nm total wavelength tuning range, larger than 30mW maximum output power and over 12 GHz modulation bandwidth of the integrated EAM. As far as the tuning range and output power are concerned, the device is the best of its kind to the best of our knowledge. These properties make our device suitable for use in the future dense wavelength-divisionmultiplexed time-division multiple access passive optical networks (DWDM-TDMA PONs) as reported in [17] and [18], featured with the utility of a wide range of wavelength, tens of kilometers of reach distance and larger than 10 Gb/s down/upstream bit rates. 2. Device fabrication Figure 1 shows the Schematic wafer structure and a top view optical microscope image of the fabricated device, which consists of a four section DBR laser followed by a 500µm SOA and a 150µm EAM. The four sections of the DBR laser are, as from left to right in Fig. 1, a 200µm rear mirror, a 100µm phase section, a 400µm gain section and a 50µm front mirror, whose electrode is connected with the front mirror. To minimize the facet reflections, the waveguide is curved from the beginning of the SOA to reach a final facet angle of 7 degree. The device is fabricated by a three step metal organic chemical vapor deposition (MOCVD) process, which is only half of what is needed for the fabrication of a similar device as reported in [14], helping to reduce the cost of the device. The first MOCVD step is a selective area growth (SAG) process. Before the material growth, SiO2 mask pairs are first formed on the substrate in the gain and SOA regions. The MQWs are sandwiched between two 100 nm InGaAsP separate confinement heterostructure (SCH) layers (λPL = 1.2 μm, PL stands for photoluminescence) lattice matched to InP and consist of 6 compressively strained InGaAsP wells ( + 1.1 × 10−2, λPL = 1.59 μm) and 7 tensile strained InGaAsP barriers (−3 × 10−3, λPL = 1.2 μm). The SAG process creates a PL wavelength difference of about 40nm between the modulator and the gain (SOA) sections as shown in Fig. 2. Then the InGaAsP SCH layers and the MQW layer in the DBR and the phase sections are selectively removed by reaction ion etching (RIE) with CH4/H2 gas mixture. The surface defects generated during the RIE process are cleaned by treating the wafer with H2SO4: H2O2: H2O = 3:1:1 solution for 50s. A 400nm InGaAsP layer (λPL = 1.45 μm) lattice matched to InP is then butt-jointed with the MQWs of the gain and SOA sections in the second MOCVD growth run. In DBR lasers, the effective refractive index of the material of the grating sections is decreased by the free carrier plasma effect and the Kramers–Kronig effect [19] when there is current injection in the section. Thus, tuning of the emission wavelength of the laser to shorter wavelengths can be realized by current injection. For material with a narrower bandgap, the changing rate of the index is higher and the total amount of the index change before the decrease of the index is saturated with the current injection is larger [19]. As a consequence both the wavelength tuning range and wavelength tuning rate are higher when material with a longer emission wavelength is used for grating sections as in our device. By fabricating tunable DBR lasers #224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30370

which include only a gain section and a grating section [20], we proved that the tuning range of DBR lasers can be extended effectively by using butt-jointed long wavelength InGaAsP as the material of the grating section. A thin p-InP space layer, a InGaAsP (λPL = 1.2 μm) etch stop layer, a p InP cladding, and a p + InGaAs contact layer are then successively grown in the third MOCVD growth step after gratings are formed only in the InGaAsP layer of the DBR sections by holographic lithography and dry etching. A 3μm wide ridge waveguide is fabricated by wet etching, which is terminated above the InGaAsP etch stop layer. The p+ InGaAs layer in the separations between each two different sections of the device is removed and the separation regions are implanted with He+ to increase the electrical isolation. Ti/Au and Au/Ge/Ni are used as p and n electrode, respectively. Polyimide is formed under the contact pad of the EAM to reduce the parasitic capacitance. After cleaving into bars, the EAM output facet is anti-reflection (AR) coated. The rear facet is left uncoated.

Fig. 1. Schematic wafer structure (upper) and an optical microscope image (lower) of the device.

Fig. 2. PL spectra of the materials of different sections of the device.

3. Device characterizations and discussions The chip is mounted on an AlN heat sink and characterized at different temperatures. The optical output power of the device is measured by an integrating sphere from the EAM facet with the EAM unbiased. For wavelength tuning characterization, the light from the device is coupled into a single mode fiber and measured by an optical spectrum analyzer (OSA). The phase section is not injected in all the measurements presented here. Figure 3(a) plots the light output power as functions of the gain current and SOA current at 25 °C with the DBR section unbiased. As can be seen, the threshold current of the device is about 15mA. The light output power as functions of the SOA current and temperature are shown in Fig. 3(b). When the bias currents of the SOA section and the gain section are 200mA and 140mA, the output power of

#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30371

the device is above 20, 27 and 32 mW, respectively, for the temperatures of 40 °C, 25 °C and 10 °C. The output power of the device at 25 °C can be further increased to be above 30 mW as the SOA current is increased to about 240 mA. To the best of our knowledge, this is the largest output power that has been obtained from similar DBR transmitter chips, helping to improve the power budget and scalability of WDM PONs. For the DBR transmitters reported in [14] and [15], which have a similar structure as our device, the output power fitful for use is less than about 10mW. Without SOA, the maximum output power of the device reported in [16] is only around 5.5mW.

Fig. 3. (a) The light output power as functions of the gain current and SOA current at 25 °C, (b) The light output power as a function of the SOA current and temperature with a gain current of 140 mA, (c) The effect of the SOA injection on the property of the spectrum of the device at 40 oC. The DBR current is 0 mA.

The typical effect of the current injection in the SOA section on the property of the spectrum of the device can be seen from Fig. 3(c). As the current in the SOA is increased, the emission wavelength is redshifted with a speed of 6.5 × 10−4nm/mA, which is caused by the thermal effect of the SOA section. The wavelength jump shown in the figure is attributed to the 0.02nm resolution of the OSA used for the measurements. As shown in Fig. 3(c), the side mode suppression ratio (SMSR) of the emission keeps being above 35dB with up to 200mA SOA current. As the current is further increased to be above 250mA, though the output power increases further, the SMSR drops rapidly due to the rising of FP modes originated from the parasitic cavity between the EAM facet and the 50µm front Bragg mirror, limiting the highest light power fitful for use. Besides the residue reflection of the EAM facet, the light feedback from the front grating also has an important effect. For a device with all the same structure but a 100µm front Bragg mirror, the largest output power with larger than 30dB SMSR is less than 20mW at 25 °C. Though the effect of the Bragg mirror is noticeable in our device, it should be relatively weaker than in the devices as in [14], where the only Bragg mirror is placed between the gain section and the SOA section. The high reflectivity of the mirror

#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30372

needed for the proper working of these devices imposes a need of a much lower reflectivity of the output facet than our device to achieve high output power with acceptable SMSR. By contrast the reflectivity of the front mirror can be quite low in our device, because mode selectivity is achieved by the rear mirror with a high reflectivity. It can be seen that the small parasitic cavity effects in the SOA section contribute greatly to the high output power of our device. Another factor that helps to increase the output power is the high quality of the MQW active material obtained by the SAG technique.

Fig. 4. (a) Laser emission wavelength and the corresponding SMSR (25 °C) as functions of the inject current of the DBR section and temperature. (b) The typical optical spectra obtained from the device. (c) The effect of the current injection into the DBR section on the output power. The currents of the gain and SOA sections are 100 and 150 mA, respectively.

Figure 4(a) shows the laser emission wavelength and the corresponding single mode suppression ratio (SMSR) as functions of the inject current of the DBR section and temperature. As can be seen, the emission is around 1.55 μm and the wavelength can be tuned for over 12 nm for a Bragg current of 100 mA. For the device, no apparent effect of the temperature on the wavelength tuning range is observed. Except in the regions where mode jump happens, the SMSR is higher than 30 dB for the whole tuning range. The wavelength tuning range of our device is noticeably larger than the DBR lasers fabricated by other than the butt-joint techniques, such as the quantum well intermixing and SAG techniques [21, 22], whose range is limited to be less than 10 nm. By using InGaAsP with 1.45µm PL wavelength as the material of the grating section, the tuning range of the DBR laser can be effectively enlarged, because the longer the emission wavelength of the material, the more rapidly the index of the material changes with the injected current [19]. As shown in Fig. 4(a), accompanied by varying the chip temperature for 30 degrees (from 10 °C to 40 °C), a total of 16.2 nm tuning range is achieved, which is the largest for DBR transmitters of the same kind to the authors’ knowledge. For example, with a similar device structure, the transmitters reported in [14] and [15] have less than 9.1 nm range of wavelength tuning. The emission wavelength of another EAM integrated DBR transmitter fabricated by SAG technique can be

#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30373

tuned for only 3.45nm [16]. Figure 4(b) shows the typical spectra obtained from the device, all having larger than 30dB SMSR. A wider tuning range of the light sources in PONs is always desired, because more data traffic can then be admitted to the networks [23]. The effect of the current injection into the DBR section on the output power is shown in Fig. 4(c). During the tuning process, the decrease of the output power of the laser due to the free carrier absorption is less than 1 dB.

Fig. 5. The static extinction curves of the integrated EAM at 40 °C (a) and 25 °C (b).

Fig. 6. The electrical to optical response of the modulator.

The static extinction curves of the integrated EAM of the device with up to 5 V reverse bias are shown in Fig. 5. During the measurements, the inject currents of the gain section and the SOA section are 130 mA and 180 mA, respectively. At 25 °C, the device provides 28–32 dB of total extinction for wavelengths from 1547 to 1557 nm. At 40 °C, the device provides 23–27 dB of total extinction for wavelengths from 1548 to 1558 nm. A 50-GHz network analyzer is used for the measurements of the small signal frequency response of the EAM at 25 °C. The obtained electrical to optical response of the modulator is shown in Fig. 6. The measured 3-dB frequency bandwidths are around 13 GHz, which is enough for up to 16 Gb/s modulation. With the integrated EAM the reach distance of the PONs can be greatly extended than the PONs with direct modulated light sources, while maintaining a compact size of the sources. Figure 7 shows the superimposed far field patterns observed from the EAM facet. The divergence angles (full width at half maximum (FWHM)) are 30.8 o and 52.8 o in the horizontal and vertical directions, respectively.

#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30374

Fig. 7. The far field patterns observed from the EAM facet.

Fig. 8. Laser emission wavelength and the corresponding SMSR (25 °C) as functions of the inject current of the DBR section and temperature. The length of the DBR section is 170 µm.

It should be noted that the wavelength tuning range of the transmitter can be further increased. In the ridge waveguide structure of our device the inject current diffuse laterally in the DBR material freely, which leads to a lower tuning efficiency. As shown in Fig. 4(a), the 12 nm tuning range needs a 100 mA Bragg current. Because of the presence of the selfheating effect which counteracts the effect of inject current on refractive-index, larger current injection in the DBR section wouldn't increase the tuning range. A buried ridge structure [24] or buried heterostructure can be used for the fabrication of the device to increase the wavelength tuning range by promoting the current injection efficiency. Then, it is shown that a small value of κL, where κ and L are the coupling coefficient and the length of the reflector, respectively, leads to a wider range of wavelength tuning [25]. The tuning properties of a DBR laser with a shorter rear DBR section (170 µm) are shown in Fig. 8. As can be seen, the range of tuning by the DBR current is increased to be over 13nm. Consequently, a 16nm total range of wavelength coverage can be obtained within a smaller range of temperature from 15 °C to 40 °C, which helps to reduce the power consumption of the temperature controller. To increase the SMSR and avoid the irregular mode jumps during the tuning process as shown in Fig. 8, a reflector with large enough κL resulting from a smaller κ and larger L is helpful [25]. The phase section in our device as shown in Fig. 1 can be used to tune the emission wavelength in a small range, so that any wavelength within the tuning range can be obtained. From the device, 20 × 100GHz or 40 × 50-GHz spaced channels can thus be obtained. There is, however, a more simple way (for the characterization of the device) to get all the channels

#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30375

with 100 GHz or 50 GHz spacing within the tuning range of the device. By properly choosing the length of the gain section, a longitude mode spacing of 0.8nm or 0.4nm can be realized. Only two parameters are necessary to access all the different channels: the temperature and the DBR current. When the temperature is changed the tuning curve shifts continuously, the settings for different wavelengths can then be simply derived from the tuning characteristics at 10 °C and 40 °C, for example. The phase section can be omitted, which would further ease the characterization process, favoring cost-efficient mass production of the device. For practical use of our device as ONU light source, the most important issue is low operational expenditure and capital expenditure [26]. Form the above, the advantages of our device can be summarized as following: First, the size of the device is as small as 250µm × 1600µm, which is noticeably smaller than that of the DFB laser arrays or SGDBR lasers. Together with the simple fabrication procedure, a low cost of fabrication can be obtained. What is more, it is possible to adopt the transistor outline (TO)-CAN for the package of our device [27], which has a small footprint and is well known as a low cost technique. Then, in our device, an EAM is monolithically integrated, thus the need for a separate modulator is eliminated, helping to lower the coupling loss of light power between the laser and modulator and reduce the system size and cost. Compared with other techniques for realizing colorless OUNs, taking the carrier distribution scheme as an example [18, 26], in which integrated reflective EAM-SOA chips are used, the disadvantage of our device is the need for relatively complex characterization and precise wavelength control. There are, however, advantages of using our device such as higher power for up stream light and having no effects from the Rayleigh backscattering. 4. Conclusion In conclusion, a DBR laser monolithically integrated with SOA and EAM is fabricated by combing the SAG technique and the butt-joint technique. By using a bulk InGaAsP layer with 1.45µm emission wavelength as the material of the DBR section, an over 16nm wavelength tuning range is obtained. With the help of the SOA, the chip output power can be larger than 30 mW. The integrated EAM has a 3dB small signal modulation bandwidth of 13 GHz and a larger than 22 dB static extinction ratio. The simple tuning method and fabrication process make the device suitable for future WDM-PON applications. Acknowledgments The work was supported by the National “863” project (Grant Nos. 2013AA014502, 2011AA010303), the National Nature Science Foundation of China (NSFC) (Grant Nos. 61474112, 61274071, 61090392, 61006044), and the National 973 Program (Grant No. 2012CB934202).

#224572 - $15.00 USD Received 9 Oct 2014; revised 21 Nov 2014; accepted 21 Nov 2014; published 26 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030368 | OPTICS EXPRESS 30376

Electroabsorption-modulated widely tunable DBR laser transmitter for WDM-PONs.

We present an InP based distributed Bragg reflector (DBR) laser transmitter which has a wide wavelength tuning range and a high chip output power for ...
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