1. Introduction

In recent years, with the increase in data usage for Internet and mobile communication, the development of wavelength division multiplexing (WDM) technology has attracted much attention. This technology uses multiple wavelengths to transmit multiple signals on a single optical fiber to expand transmission capacity. A wavelength-tunable semiconductor laser diode serves as a key optical component in WDM transmission systems as it generates multiple light wavelengths and transmits them through the same optical fiber at different wavelengths. In this regard, a cost-effective and widely tunable light source is essential for the operation of WDM networks. Furthermore, such light sources enable efficient inventory management.

Several types of wavelength-tunable semiconductor laser diodes have been investigated for application in WDM systems [1–8]. Representative types of semiconductor-based wavelength-tunable laser diodes include distributed Bragg reflector laser diode (DBR-LD), sampled grating DBR laser diodes (SG-DBR-LD), and super-structure grating DBR laser diodes (SSG-DBR-LD). Among them, a DBR-LD is the simplest structure that combines a gain region that generates lights and a DBR region that selects the wavelength. The wavelength of DBR-LD can be tuned by injecting current into the DBR region and/or applying heat to it [9–12]. The current injection method involves carrier injection at high speed and thus allows for fast wavelength tuning. However, it has limited tuning range due to its relatively small refractive index change. In comparison, the thermal tuning method has a relatively wider tuning range but consumes more energy as it needs to generate a large amount of heat for a wide range of wavelength tuning.

Typically, a thermo electric cooler (TEC) is attached under the chip mount and driven at a constant temperature to operate LD. In the case of a thermally-tuned DBR-LD, a heater is operated to tune the wavelength, and the heat generated from the DBR heater is transmitted to the bottom chip. This increases temperature of the LD chip, which consequently requires additional TEC driving. This additional TEC operation lowers the temperature of the LD chip and reduces the heat generated from the DBR heater, resulting in reduced efficiency of wavelength tuning. To solve this problem, a thermal isolation structure can be introduced to block heat transfer. This additional structure can maintain the temperature of the LD chip while preventing the TEC from taking away the heat from the DBR region, thereby improving wavelength tuning efficiency. Some studies have employed an InGaAs epitaxial sacrificial layer to fabricate a thermal isolation structure through selective wet etching to physically separate the DBR region and TEC [13,14]. This method involves growing InGaAs on the front of the wafer and removing all InGaAs, except for the DBR region. The epitaxial layer that corresponds to the device structure is then grown on top of the superficial layer, followed by the fabrication of the waveguide. This fabrication method requires further epitaxial growth of an InGaAs sacrificial layer. Nevertheless, the fabrication process of DBR-LD is complex due to the requirement of additional processes, such as the process of confining InGaAs to the DBR region and selective etching.

This study reports a technique for fabricating a DBR-LD with a wide wavelength tuning range using a single wet etching step, which is simpler than using an InGaAs sacrificial layer to create a thermal isolation structure. The proposed technique is similar to the general process for fabricating a typical DBR-LD but has an additional implementation of a thermal isolation structure using a single reverse mesa etching step. The etching rate and angle of the etched reverse mesa were used to calculate the etching time required to separate the substrate and DBR regions, resulting in a DBR-LD with a thermal isolation structure. Furthermore, this study describes the wavelength tuning range and operating characteristics of the DBR-LD with a thermal isolation structure using this wet etching technique.

2. Design and fabrication

Figure 1 shows a schematic diagram and cross-sectional view of the fabricated DBR-LD, which comprise a gain region that generates light, a DBR region that selects the wavelength, and a phase control (PC) region that allows fine wavelength tuning. This is similar to a typical three-section DBR-LD structure. A micro-heater is mounted on the top of the DBR waveguide to apply heat to the DBR grating and can be controlled by a current or voltage source to raise the temperature. Additionally, a trench structure was introduced around the DBR waveguide for thermal isolation. The gain region of the DBR-LD is 250-µm long, and the passive waveguide, which includes the phase control (PC) and DBR grating waveguide, is 300-µm long. Both are integrated using the butt-joint structure [15,16].

 

Fig. 1. (a) Schematic diagram and (b) cross-sectional view of the fabricated DBR-LD with a thin-film heater.

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Figure 2(a) shows a scanning electron microscopy (SEM) image of the fabricated DBR-LD. The fabrication process of the DBR-LD is almost identical to that described in previous reports [9,17], except for the thermal isolation structure fabrication. A layer of n-InGaAsP is grown on top of an n-InP substrate using a molecular organic chemical vapor deposition method, which would serve as the DBR diffraction grating layer. A DBR grating pattern is formed on the grating layer using electron beam lithography and etching. The period and duty cycle of the fabricated DBR grating and designed to satisfy a Bragg wavelength of 1.55 µm and a coupling coefficient of 30 cm⁻¹, respectively [18]. An active layer with a multi-quantum-well (MQW) structure is grown on top of the DBR grating. The MQW structure comprise seven 6 nm-thick InGaAsP wells and eight 8 nm-thick InGaAsP barriers. Figure 2(b) shows an SEM image of an integrated interface with an active layer in the gain region and a passive core layer using the butt-joint method. Here, the active layer in the rest of the area excluding the gain region was etched and removed to integrate the active layer in the gain region and passive core layer monolithically. The butt-joint interface was fabricated with a tilt of approximately 15° to the perpendicular to the propagation direction of the waveguide to minimize reflections at the interface and ensure wavelength stability. After the completion of the butt-joint, the epitaxial growth was carried out to create the pnp-InP current blocking layer, p-InP cladding layer, and p ⁺-InGaAs ohmic contact layer, thereby completing the epitaxial structure. The waveguide of device was fabricated in the form of an etched mesa buried heterostructure (EMBH) [19,20] that is 9 µm in width and about 4 µm in depth, enhancing the heat efficiency of the heater, as shown in Fig. 2(c). The micro-heater in the DBR region was fabricated with a length of 200 µm and a width of 6 µm using Pt metal deposited on a 200-nm-thick SiN dielectric film on top of the EMBH-shaped waveguide. The measured heater resistance at 25°C was ∼30 Ω. To create a thermal isolation structure in the DBR waveguide region, etching patterns of 20-µm wide were formed on both sides of the EMBH waveguide at a distance of 5 µm from its position. HBr solution was used to etch the InP layer beneath the EMBH waveguide into a reverse mesa (RM) shape [21]. As the etching progresses, the width of the bottom of the waveguide gradually decreases due to RM etching, eventually reaching a width of 0 µm. This separates the substrate and the waveguide, forming a thermal isolation structure. The use of RM etching with the HBr solution minimizes the undercut, allowing for the simple implementation of a thermal isolation structure in a single etching process without affecting the performance of the waveguide. Figure 2(d) and (e) show SEM images of the thermal isolation structure in the DBR waveguide region fabricated using RM etching.

 

Fig. 2. SEM images of the (a) the top view of fabricated DBR-LD, (b) a side-view of active–passive integration, (c) a cross-sectional view of the gain section with an EMBH. The width and height of the EMBH were fabricated to be 9 µm and 4 µm, respectively. Additionally, SEM images of the DBR section with an EMBH and RM ridge waveguide (RM-RWG) are shown in (d) cross-sectional and (e) top views.

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Figure 3 shows a schematic cross-sectional view of the fabrication process for trenches that are etched in a RM shape to achieve thermal isolation. This figure describes the above mentioned processes: a waveguide was fabricated in the EMBH shape, a mask pattern for etching was created on the bottom, and a RM was formed using HBr solution. Figure 4 shows the width of the RM bottom as a function of etching time. When etching InP using HBr solution, the vertical etch rate for the (100) crystal plane was measured to be 1.45 µm/min and the horizontal etch rate was 0.65 µm/min. In this experiment, the InP layer is etched by HBr solution at an angle of 64° in the (111) direction with respect to the (100) plane, as measured from the etch cross-section. This value was slightly larger than the previously reported values [21,22]. The width of the bottom of the waveguide etched with a RM is expected to reach 0 µm after ∼16 min of etching in our structure, as calculated from the values obtained from the measured angle of the etched surface. This is expected to result in the separation of the waveguide from the substrate.

 

Fig. 3. Cross-sectional schematic diagram of the RM-RWG during its fabrication process. (a) EMBH structure, (b) SiN mask for wet etching, and (c) RM etching using the HBr solution. In this test, α and β were set to be 5 µm and 20 µm, respectively.

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Fig. 4. Width of the waveguide bottom with respect to the etch time. The solid line represents the values calculated using an angle measurement of the etched surface.

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3. Results and discussion

Figure 5(a) shows the output-power-gain applied-current voltage (L–I–V) characteristics of the fabricated DBR-LD. The corresponding measurement was conducted under a continuous-wave operation of the gain current with no tuning. Threshold currents measured at 25°C were approximately 6 mA and slope efficiencies were approximately 0.2 W/A. Figure 5(b) shows the output spectrum of the fabricated DBR-LD at a gain current of 60 mA. Its peak wavelength was approximately 1540 nm, and the side-mode suppression ratio (SMSR) was larger than 50 dB at 25°C.

 

Fig. 5. (a) L–I–V characteristics of the fabricated DBR-LD as a function of the gain current and (b) output spectrum at a gain current of 60 mA at 25°C.

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Figure 6 shows the lasing wavelength and SMSR characteristics of the output spectrum with respect to the DBR current for the structure etched for 17 min. The heater resistance, applied power, and current for the DBR section were approximately 30 Ω, 0.4 W, and 120 mA, respectively. As the current injected into the heater of the DBR-LD increased, the peak wavelength of the laser increased proportionally to the square of the current. Moreover, it can be seen that the SMSR of the output spectrum repeated periodically between 30 and 50 dB as the heater current was injected. The lasing wavelength was determined using the reflection spectrum of the grating and the cavity mode. When the DBR reflection spectrum shifted owing to the heater operation, the resonance-mode phase repeated periodically, resulting in the SMSR change and mode hoping. This indicates that the change period of the SMSR is related to the free spectral range. The maximum wavelength tuning range was approximately 40 nm when 120 mA heater current was applied. According to the Bragg condition, this shows an effective refractive index change of approximately 0.083 at a diffraction-grating period of 240 nm.

 

Fig. 6. Lasing wavelength (blue) and SMSR (red) of the output spectrum as a function of the DBR current.

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Figure 7 shows the superimposed output tuning spectra with different heater currents and the photoluminescence (PL) spectrum of the active medium. For the tuning spectra, the gain current was fixed at 60 mA and the heater tuning current was applied up to 120 mA. The lasing wavelength was changed from 1540.2 to 1580.7 nm with the DBR current range being up to 120 mA. When the lasing wavelength redshifted up to approximately 40 nm, the output power reduced by approximately 14 dB and PL intensity decreased about 8 dB. We believe that this decrease may be closely related to the gain bandwidth limit of the active medium. Another reason may be the distortion of the DBR reflection spectrum, which is also related to the SMSR degradation, with a nonuniform thermal distribution along the longitudinal direction.

 

Fig. 7. Superimposed tuning spectra of the fabricated DBR-LD (colored solid lines) and PL spectrum of the active medium (black-dashed line). For the tuning spectra, the gain current was fixed at 60 mA and the heater tuning current was applied up to 120 mA.

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4. Summary

In this paper, we discussed a technology that easily implements a thermal isolation structure through RM wet etching using a HBr solution. Using this technique, we developed a thermally tunable DBR-LD equipped with a thermal isolation structure by reducing the width underneath the DBR waveguide region through RM etching on both sides, ultimately separating the substrate and the waveguide. The DBR-LD achieved a wide wavelength tuning range and high tuning efficiency due to the thermal isolation structure that completely separates the DBR waveguide and substrate. This structure effectively prevents the heat generated from the heater on the upper side of the DBR from being transferred to the TEC mounted below the substrate. In the DBR-LD, the peak wavelength of the laser increased exponentially as the heater current increased. When a heater current of 120 mA was applied, the laser exhibited a wavelength tuning range of approximately 40 nm, which is approximately four times larger than the tuning range of a structure without thermal isolation (10 nm). These results are promising for designing low-cost, low-power-consumption transceivers covering the full C-band in WDM-based mobile fronthaul network application.