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Abstract
We report a novel structure that is capable of wide wavelength tuning in the distributed Bragg reflector laser diode (DBR-LD) with a single grating mirror. This device’s DBR section has two tuning elements, plasma, and heater tunings, which are implemented simultaneously on the top of a single waveguide by using an in-between dielectric layer. For the proposed structure, a three-dimensional thermal simulation was conducted. The results showed that the temperature profile within the waveguide is highly affected by the position of heater metal and thermal conductivity of the p-cladding layer. As a result, it is important to use a uniform temperature region in the DBR section for a wide tuning range and stable single-mode operation. For a 550-μm long DBR-LD with a 250-μm long DBR section, a tuning range of 26 nm (i.e., 7 nm for plasma tuning and 19 nm for heater tuning); an SMSR of more than 45 dB; and a peak power variation of less than ± 2.5 dB were obtained. From the comparisons of two DBR-LDs with only one tuning element, we confirmed that using the dielectric layer is a very effective way of achieving a wide tuning under the independent tuning operation.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There has been considerable interest in tunable lasers for use in next-generation broadband backhaul and radio access networks (RANs) based on wavelength division multiplexing (WDM) systems [1–3]. The use of tunable lasers in these networks offers a number of compelling advantages over wavelength-fixed laser applications from the viewpoint of the operating expenditure (OPEX) and system functionalities, such as an inventory cost reduction, dynamic wavelength allocation, and the realization of a software defined network (SDN). In particular, in WDM-based mobile front-haul networks [4], the development of tunable lasers capable of supporting multiple sub-channel wavelengths within individual CWDM channel bands has been substantially required. These light sources should also be cost-effective and compact, and have a good uniform performance over all sub-channels.
Among a large number of developments on low-cost tunable lasers [5–14], a distributed Bragg reflector-laser diode (DBR-LD) with a single grating mirror is considered to be the most promising for this application because of its compact size, easy construction, and good long-term reliability. In addition, compared with sampled grating (SG) [5,8] and digital super-mode (DS) DBR-LDs [9], this type of DBR-LD is preferred owing to a simple fabrication structure and easy wavelength calibration. However, for a conventional structure, the tuning range is limited to about 10 nm [10].
One method for extending the tuning range in this structure is to use a heat sink temperature [11,12]. This method is very simple and can provide nearly continuous wavelength tuning. However, the operating temperature is limited to a narrow range because the LD performance deteriorates as the temperature increases, and as a result, the tuning range is less than 5 nm under operation at 10 °C to 60 °C when considering a tuning ratio of about 0.1 nm/°C.
Another approach is to introduce a thin-film heater as an additional tuning element [13,14]. The wavelength tuning in this type of DBR-LD is achieved by a carrier injection into a P-i-N junction through an ohmic metal, which is called plasma tuning, and through Joule heating of the waveguide using a heater metal, namely thermal tuning. In principle, the plasma tuning blue shifts the Bragg wavelength through the well-known carrier-induced refractive index change, whereas the thermal tuning applies a red shift through a change in the refractive index of the thermal heating. This method appears to be attractive because a thin-film heater can be simply implemented by applying a metallic patterning process near the waveguide, and can offer low-noise tuning spectra [15], unlike with plasma tuning.
Although it was previously reported that a tuning range of over 20 nm is achieved, contrary to our expectations, the thermal tuning was at most 6 nm [14]. For this result, we note that the heater metal was designed for placement 10 μm away from the center of the waveguide to avoid electrical crosstalk from the ohmic metal. Because the thermal conductivity of InP used as a cladding layer is as low as 68 W/(m⋅K), which is approximately 20% that of Au, the heat generated by the heater metal has a narrow range of thermal spread. Therefore, we believe that this deviation can considerably reduce the effect of thermal tuning on the optical mode confined in the waveguide. In addition, this can cause lateral asymmetry in the thermal distribution, which can prevent optical guiding in the waveguide, and can consequently degrade the output power and spectral quality.
To solve this problem, in the present study, we propose a novel tuning structure where the both tuning elements are implemented simultaneously on the top of a single waveguide. In this structure, the heater metal is placed at the center of the waveguide, and then isolated the waveguide electrically from the ohmic metal by inserting a dielectric layer between the two. To examine the effect of lateral deviation in heater metal on the waveguide heating and to confirm the thermal distribution within the waveguide for the proposed structure, we conducted three-dimensional thermal analyses for the LD chip-on-submount. The lateral and longitudinal thermal distributions within the waveguide were then examined. For experimental confirmation, we fabricated the DBR-LDs and evaluated their output power and tuning spectra.
The rest of this paper is organized as follows. The device structure and thermal analyses are described in Section 2 and 3, respectively. Section 4 presents the experimental results for the fabricated DBR-LDs. Finally, Section 5 summarizes the results of this research.
2. Device structure
A schematic mask view and the layer structure of the proposed DBR-LD are depicted in Figs. 1(a) and 1(b), respectively. The DBR-LD has a cavity of 550 μm in length, and the active (250 μm) and passive waveguides (300 μm) are monolithically integrated. The device simply consists of gain and DBR sections, and the 250-μm long DBR section contains two tuning elements, namely, plasma tuning operated by the current ITP through the ohmic metal, and heater tuning by ITH using the heater metal. Each tuning element was electrically isolated using a 200-nm thick SiNx layer between the two metal types on the top of the waveguide, which makes it possible to achieve an independent tuning operation.
Fig. 1 (a) A schematic mask view and (b) layer structure of tunable DBR-LD with two tuning elements in a DBR section. In mask view, IG, ITP, and ITH denote the currents for the gain, plasma tuning, and heater tuning, respectively.
The epitaxial layers were grown using metal-organic chemical vapor deposition (MOCVD). During the monolithic integration between the active and passive waveguides, a butt-joint coupling method is used [6,8]. The active region in the gain section contains two pairs of multiple quantum wells (i.e., four 6-nm thick and four 8-nm thick 0.8% compressively strained InGaAsP wells, and nine 10-nm thick 0.6% tensile-strained InGaAsP barriers) for broad gain [16] and 70-nm thick lattice-matched separate confinement hetero-structure (SCH) layers. The passive core has a 0.27-μm thick InGaAsP with a bandgap wavelength of 1.3 μm. The grating layer has a 25-nm thick InGaAsP with a wavelength of 1.3 μm, and its period and duty cycle were designed to satisfy the conditions for a lasing wavelength of near 1.55 μm and a coupling coefficient of about 30 cm−1 [17]. For the fabricated DBR-LDs, the length of the grating was implemented as 220 μm and shifted to 30 μm toward the DBR facet.
Among our waveguide fabrication tools, an EMBH processing technique was applied [18,19]. As for the metal structures, the heater metal of Cr(30 nm)/Au(170 nm) was a strip line 5 μm in width, and an 8-μm wide ohmic metal of Ti(30 nm)/Pt(50 nm)/Au(350 nm) was connected to the Au-plating pad metal through the via-hole next to the heater metal line. After the typical LD fabrication processes, the facet of the DBR section was anti-reflection (AR) coated using ion beam deposition of TiO2 and SiO2. A reflectivity of about 0.5% was obtained at 1.55 μm. Figure 2 shows a SEM image of the fabricated DBR-LD.
3. Thermal analysis of the proposed structure
Joule heating generated by the current injection through a resistive metallic wire changes the refractive index of the waveguide, and thus, tunes the Bragg wavelength in the DBR section. To examine the effect of the heater metal on the waveguide, we conducted three-dimensional thermal simulations for the chip-on-submount, where the LD was AuSn solder-pasted onto the AlN submount, using the electric current and heat transfer interface modules of the COMSOL multiphysics modeling software [20]. The designed structure was nearly the same as the fabricated structure. The waveguide had an etched-mesa buried hetero-structure (EMBH) [18], where the widths in the active and passive regions were 10 and 20 μm, respectively, and the lengths of the gain, isolation, and DBR sections were 250, 50, and 250 μm, respectively. In this simulation, the temperature of the submount bottom was set to 25 °C under an ambient temperature of 30 °C. The thermal conductivities of the InGaAs(P), SiNx, InP, Cr, Au, AuSn, and AlN used were 5, 20, 68, 93.7, 317, 251, and 285 W/(m⋅K), respectively.
Figure 3(a) shows the three-dimensional thermal distribution for the chip-on-submount at the heater current ITH of 100 mA, and Figs. 3(b) and 3(c) represents temperature cross-section contour maps in the DBR section under the same current when the lateral center position of heater metal is designed to be 0 and 5 μm, respectively. When the current is injected into the heater metal, the maximum temperature is shown at the surface of heater metal. The heat generated from the heater metal increases the temperature of the waveguide in the DBR section, and spreads out toward the bottom of the chip. In this simulation, the temperature profile depends highly on the position of metal and thermal conductivity of the p-InP. It was also found that, as the width of the waveguide decreases and its depth increases, the temperature increases.
Fig. 3 (a) 3D simulation results for the thermal distribution of DBR-LD chip-on-submount and temperature cross-section contour maps in DBR section when the lateral center position of heater metal is (b) 0 μm and (c) 5 μm at a heater current ITH of 100mA.
To see the temperature profiles near the core layer in Fig. 3(b) in detail, lateral and longitudinal thermal distributions of the core layer were examined, as shown in Figs. 4(a) and 4(b), respectively. As these Figs. show, as the heater current increases from 50 to 100 mA, the peak temperature within the waveguide increases from 31.5 °C to 51.5 °C. In the lateral distribution, the peak temperature appears at the core layer (with a width of 1.5 μm) and spreads with two different profiles. This is because the thermal conductivities of InGaAsP and InP are different, and consequently, this distribution enhances the optical confinement because the refractive index increases with the increase in temperature. Considering that the typical optical mode size (full width at an intensity of 1/e2) of a BH structure is about 5 μm, the optical mode will experience a temperature change of 1.7 °C (i.e., from 49.8 °C to 51.5 °C) at this condition. As for the longitudinal distribution, an isolation section of 50 μm in length with a trench depth of 0.3 μm seems to have a role in the thermal isolation between the gain and DBR sections for a relatively low heater current. However, it is necessary to increase the length of the isolation at a high heater current because of the significant temperature difference between the center and edge in the DBR section. From this result, for a wide tuning range and stable single-mode operation, it is desirable to shorten the length of the grating compared to the length of the heater metal, or shift the grating to the DBR facet side for the use of the Bragg condition in the uniform temperature region within the DBR section.
Fig. 4 (a) Lateral thermal distribution along the red dotted line of Fig. 3(b), and 3(b) longitudinal thermal distribution of the core layers of the DBR-LD.
4. Experiment results
Figure 5 shows the typical L-I-V characteristics of the fabricated DBR-LD chips with an operating temperature of 25 °C. The measurements were conducted under a continuous-wave (CW) operation of the gain current IG without the use of tuning currents. In the L-I curves, the threshold current and slope efficiency appear to be 6 ± 0.5 mA, and 0.2 ± 0.05 W/A, respectively. In the I-V curves, the threshold voltage is about 0.8 V, and the series resistance is estimated to be approximately 8 ± 0.5 Ω.
Fig. 5 Typical L-I-V characteristics for the fabricated DBR-LD chips. All measurements were conducted under the CW operation of the gain current IG without the use of tuning currents at a temperature of 25°C.
All spectra were obtained by measuring the fiber-coupled power under the CW operation. Figure 6 shows the tuning spectra at an IG of 90 mA. The spectra show an SMSR of >50 dB with an average spacing of about 1.45 nm. In this measurement, the plasma current ITP and the heater current ITH were varied within the range of 0 to 50 mA, and 0 to 138 mA, respectively. From the I-V test of the heater metal, a resistance RH of 36 Ω was obtained. The lasing wavelength without the use of tuning currents was about 1557 nm (black line), and each tuning wavelength moves toward the opposite direction as expected. The wavelengths at an ITP of 50 mA and ITH of 138mA (corresponding to 4.97 V) were 1550 nm (the far left, cyan) and 1576 nm (the far right, dark blue). A total tuning range of 26 nm, and a side-mode suppression ratio (SMSR) of more than 45 dB, were shown, and a variation in peak power of less than ± 2.5 dB over the tuning range was obtained without adjusting the gain current. For comparison, we additionally prepared two types of DBR-LDs, one with only plasma tuning, and the other with only thermal tuning, and tested them. It was found that the tuning property of each DBR-LD with one tuning element is nearly the same as individual tuning property with two tuning elements. Based on this result, we confirmed that the isolation method using the dielectric layer makes it possible to achieve the independent tuning operation.
Fig. 6 Fiber-coupled output tuning spectra for the fabricated DBR-LD. The gain current was fixed at 90 mA, and the currents for the plasma and heater tunings were changed within a range of 50 and 138 mA, respectively.
Figure 7 shows the wavelength tuning and SMSR as a function of the tuning currents. As for the plasma tuning, according to the square-root-like carrier-density–current relationship, the tuning range becomes saturated with an increase in current, and the-mode hopping appears to be stair-like with a wavelength interval corresponding to the effective cavity. In contrast, for the heater tuning, the tuning range becomes parabolic with the increase in current according to the relationship between the linear heat and electrical power (i.e., ITH2RH). In the DBR-LD, a lasing mode is determined at the wavelength satisfying the Bragg condition (i.e., λB = 2neqΛ, where λB, neq, and Λ are the Bragg wavelength, the equivalent refractive index including the material dispersion, and the grating period, respectively) among the cavity modes, and the tuning range can be simply estimated from the change in neq, namely Δneq. According to this relation, a tuning range of 26 nm corresponds to a Δneq of 0.0055.
5. Summary
We proposed and demonstrated widely tunable DBR-LDs with plasma and heater tuning elements, both of which were implemented on a single waveguide and separated electrically by inserting a SiNx between the ohmic and heater metals. For the proposed structure, the lateral and longitudinal thermal distributions within the waveguide were examined. The results showed that, as the heater current increases, the temperature within the waveguide also increases. The results of lateral distribution indicate that the temperature profile within the waveguide is highly affected by the lateral deviation of heater metal and thermal conductivity of the p-cladding layer at a given waveguide structure. As for the longitudinal distribution, it was found that a 50 μm-long isolation section has a role in the thermal isolation for a relatively low heater current, and however, its length has to be increased at a high heater current. For the monolithic integration and waveguide fabrication of this type of LD, a butt-coupling method and etched mesa buried hetero-structure processing technique were adopted. For a 550-μm long DBR-LD with a 250-μm long DBR section, a tuning range of 26 nm (i.e., 7 nm for plasma tuning and 19 nm for heater tuning), an SMSR of more than 45 dB, and a peak power variation of less than ± 2.5 dB were obtained over the range of the plasma and heater currents (0 to 50 mA for the plasma current, and 0 to 135 mA (4.86 V) for the heater current) at a gain current of 90 mA. For this result, we think this large heater tuning with good SMSR is owing to the use of a uniform temperature region (by shortening the length of grating and shifting the grating to the DBR facet side) in the DBR section with the centrally located heater metal. To the best of our knowledge, this result is the largest value ever reported for a DBR-LD with a single grating mirror, and as for the thermal tuning of 19 nm, this value is more than three times compared to the previous work [14]. An additional improvement in the tuning performance is the subject of future investigations. In addition, the thermal efficiency and power consumption can be improved by optimizing the structures of the waveguide and heater metal. A study on realizing this type of tunable laser is ongoing.
Funding
Institute for Information and Communications Technology Promotion and the Government of Korea (MSIT) (Grant # 2017-00823).
Acknowledgements
This work was supported by Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korea government (MSIT) (2017-00823, Development of 25 Gb/s Tunable Laser and its related components for Multi Sub-channel CWDM applications).
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