In recent years, the rapid advancements in big data, cloud computing, and artificial intelligence have significantly increased the demand for computing power^[1]. To address interconnect challenges between computational chips, such as central processing units (CPUs), graphics processing units (GPUs), and general term for heterogeneous/composite processing units (XPUs) (chip-to-chip interconnect), optical input/output (I/O) technology has emerged as a solution, enhancing communication performance between unit chips by replacing electrical I/O interfaces with optical signals^[2]. The distributed feedback (DFB) laser array, as a typical light source solution, has garnered interest from institutions like Ayar Labs and Intel due to its stable mode, simple operation, and high output power across various wavelengths^[3]. By routing multiple wavelengths to $M\times N$ ports, the number of wavelengths is increased, thereby enhancing the overall bandwidth^[4], as illustrated in Fig. 1. However, increasing the number of ports results in significant power splitting losses, necessitating the use of high-power DFB laser arrays to compensate for the power loss^[5].

During the operation of high-power semiconductor lasers, energy losses occur^[6], with approximately 30% of the electric energy being converted into thermal energy. Effective thermal management is critical for the proper operation of high-power DFB laser arrays. Insufficient heat dissipation can cause a substantial increase in the temperature of the active region, resulting in reduced electro-optical conversion efficiency, wavelength redshift, limited maximum output power, and decreased reliability^[7].

As the number of optical I/O signals increases due to enhanced functionalities and higher performance requirements of integrated chips, the chip interconnect technology in package assembly has become more complex and sophisticated. Currently, most interconnections between semiconductor chips and packaging are achieved through wire bonding technology^[8]. However, flip-chip bonding is becoming the preferred choice for chip-scale package (CSP) integration technologies due to its technical advantages over wire bonding^[9]. In addition to cost reduction, the flip-chip structure provides improved thermal management for the laser array^[10], especially in the reduction of thermal resistance, and reduces the junction temperature of the laser to ensure the power performance of the laser.

In this study, we propose a novel flip-chip bonding structure for DFB laser arrays. By reducing thermal resistance, this innovative design not only enhances the heat dissipation capabilities of high-power DFB laser arrays but also improves their output power. This innovative flip-chip bonding design places the active region of the chip close to the thermoelectric cooler (TEC), allowing for efficient heat removal and lower junction temperatures. By significantly reducing thermal resistance and improving heat dissipation characteristics, this approach effectively increases the output power of high-power semiconductor lasers.

The reconstruction equivalent chirp (REC) technology is utilized to introduce an equivalent $\pi$ phase shift within the laser cavity, enabling precise control of the grating phase. The anti-reflection (AR) and high-reflection (HR) coatings are applied to the front and rear facets of the laser array to enhance output power. Additionally, an asymmetric phase-shifted (APS) grating structure is designed to improve the yield of single longitudinal mode (SLM) lasers. Experimental results of the proposed laser array demonstrated an average adjacent channel wavelength spacing of 398.719 GHz, with a design deviation of only 1.281 GHz and a maximum wavelength deviation of only 1.579 GHz. Compared to the conventional p-side-up design, the flip-chip design reduced the laser junction temperature by 28% and increased the maximum output power by approximately 20%. At a bias current of 110 mA, all the channel wavelengths maintained a side mode suppression ratio (SMSR) above 50 dB, and the far-field pattern approached a circular mode profile with a divergence angle of $25.8°\times 30.1°$ and a Lorentzian linewidth of 3.28 MHz. At a bias current of 250 mA, the flip-chip 8-channel high-power DFB laser array achieved a maximum output power exceeding 80 mW. Overall, the proposed flip-chip bonded DFB laser array exhibits excellent performance and holds promising potential for applications in optical I/O technology.

Figure 2(a) illustrates the schematic epitaxial structure of the proposed laser array unit. The use of InAlGaAs multi-quantum wells (MQWs) as the gain material enhances high-temperature performance. The grating fabrication process employs the REC technology to streamline production and improve precise phase control of the grating^[11]. This approach addresses the long production time and high costs associated with traditional electron beam lithography (EBL) in grating fabrication.

Figure 2(b) illustrates the grating with an asymmetric $\pi$ phase shift, precisely designed using REC technology. AR and HR coatings are applied to the front and rear cavity surfaces of the laser array to enhance output power. Figure 2(c) shows the position of the $\pi$ phase shift within the cavity. However, the random phase shift from the HR coating facet will deteriorate the single-mode properties and increase the uncertainty of the lasing wavelength.

Our numerical simulations indicate that positioning the $\pi$ phase shift at a location one-fifth of the cavity length from the HR facet significantly improves the single-mode yield. This configuration yields a much higher SLM yield compared to that when the $\pi$ phase shift is conventionally centered within the laser cavity. The single-mode property can be evaluated using the threshold gain margin (TSM), defined as the threshold gain difference between the most probable side mode and the dominant mode. Stable single-mode operation is achieved when the TSM exceeds 0.25^[12]. As shown in Fig. 3, only 13% of the threshold gain margin exceeds 0.25 for the conventional center $\pi$ phase shift grating, while 90% of the threshold gain margin exceeds 0.25 for the proposed APS grating structure as the random phase from the HR coating varies from 0 to $2\pi$. Thus, the SLM yield is significantly enhanced by the proposed APS grating structure.

REC technology simplifies the grating fabrication process to only one step of traditional holography and one step of micron-level photolithography. For periodically sampled gratings, the refractive index modulation can be expressed as^[13]$$\mathrm{\Delta }n(z)=\frac{1}{2}s(z)\mathrm{\Delta }n\exp \left(j\frac{2\pi z}{{\mathrm{\Lambda }}_0}\right)+\text{c.c},$$(1)where ${\mathrm{\Lambda }}_0$ is the period of the seed grating, and $s(z)$ is the periodic function of a sampling modulation, which can be expressed as $$s(z)=\sum _m{F}_m\exp \left(j\frac{2m\pi z}{P}\right),$$(2)where $P$ is the sampling period, $m$ is the $m$th order Fourier series, and $F_m$ is the Fourier confidence. According to Eqs. (1) and (2), we can obtain $$\mathrm{\Delta }n(z)=\sum _m\frac{1}{2}\mathrm{\Delta }n{F}_m\exp (j\frac{2m\pi z}{P}+j\frac{2\pi z}{{\mathrm{\Lambda }}_0})+\text{c.c},$$(3)when $\pi$ phase shift is located at $z_0$ in a sampled grating, and the expression of the sampled grating-based REC is $$\mathrm{\Delta }n(z)=\{\begin{array}{ll}\frac{\mathrm{\Delta }{n}_0}{2}\sum _m{F}_m\exp (j\frac{2\pi z}{{\mathrm{\Lambda }}_0}+j\frac{2m\pi z}{P})+\text{c.c}& z\le z_0\\ \frac{\mathrm{\Delta }{n}_0}{2}\sum _m{F}_m\exp (j\frac{2\pi z}{{\mathrm{\Lambda }}_0}+\frac{2m\pi z}{P}-j\frac{2m\pi \mathrm{\Delta }P}{P})+\text{c.c}& z>z_0\end{array}.$$(4)

Usually, the $+1$st order subgrating is used as the laser cavity. The grating period of the $+1$st order sub-grating is derived as $$\frac{1}{{\mathrm{\Lambda }}_{\pm 1}}=\frac{1}{{\mathrm{\Lambda }}_0}\pm \frac{1}{P},$$(5)where ${\mathrm{\Lambda }}_{+1}$ is the grating period of the $+1$st order sub-grating. The Bragg wavelength of the uniform seed grating is designed far away from the gain region to avoid lasing, while that of the $+1$st order subgrating is in the center of the gain region. The equivalent $\pi$ phase shifts are introduced in the gratings to realize grating phase matching and ensure stable single-mode operations for each lasing wavelength. The precision of the grating phase can be enhanced by a factor of ${(P/{\mathrm{\Lambda }}_0+1)}^2$ through the REC technology, typically exceeding two orders of magnitude^[14,15].

The modal field and far-field divergence angle of the laser array units with the proposed epitaxial structure are simulated. In the design process, the number of quantum wells and barriers within the active region, along with the thickness of the p-SCH layer, are deliberately reduced. This adjustment expanded the mode profile in the vertical direction, thereby minimizing the vertical far-field divergence angle. The simulation results are shown in Fig. 4. Figures 4(a) and 4(b) illustrate the fundamental and higher-order mode field distributions, respectively. Figure 4(c) shows a nearly circular far-field mode profile, which facilitates coupling with single-mode fibers. Figure 4(d) reveals that the far-field divergence angles of the output waveguide are $25.2°\times 30.7°$.

Figure 5 shows the schematic of a typical high-power laser diode package. The chip unit is first bonded to a base with a matching coefficient of thermal expansion (CTE) and then soldered to a highly thermally conductive heat sink, typically made of copper. Using COMSOL simulation, we analyzed the thermal effects in the active region under identical environmental conditions and consistent heat power. Figure 5(b) shows the simulation result of the heat flux distribution of the conventional p-side-up structure, while Fig. 5(c) shows the flip-chip structure. It can be observed that in the p-side-up structure, the active region is further from the underlying heat sink and dissipates heat upwards into the air, which has a lower thermal conductivity, leading to a temperature build-up that degrades laser performance. In contrast, the flip-chip structure places the active region closer to the heatsink base, allowing heat to dissipate more effectively, demonstrating a clear advantage.

As illustrated in Fig. 6, simulations are conducted to compare the steady-state central temperatures of the active regions in two different chip structures under identical environmental conditions and consistent thermal power. The flip-chip structure exhibited lower temperatures in the steady state. In the simulation, the temperature of the heat sink is maintained constant. Consequently, the junction temperature of the flip-chip structure is calculated to be lower, demonstrating its thermal dissipation advantage, and compared to the conventional p-side-up structure, it exhibits a lower thermal resistance.

Figure 7(a) shows a microscopic top view of the designed high-power DFB laser array chip. Figure 7(b) shows the scanning electron microscope (SEM) image of the ridge waveguide, which is fabricated using an integrated dry and wet etching process. The epitaxial structure is grown on an n-type InP substrate using metal–organic chemical vapor deposition (MOCVD). The active region of the InAlGaAs consists of tensile-strained quantum barriers and compressive-strained quantum wells^[16]. Gratings are fabricated using holography, UV exposure, and inductively coupled plasma (ICP) etching. Subsequently, cladding and InGaAs contact layers are grown on the grating layer. Ti/Pt/Au are deposited by magnetron sputtering as p-electrodes and n-electrodes, respectively. In the design, the n-SCH layer is thicker than the p-SCH layer, and the number of quantum wells is reduced to form an asymmetric waveguide structure. This design reduces internal losses, increases slope efficiency, and improves output power. The asymmetric waveguide structure also causes the optical eigenmode to expand towards the $n$-doped region vertically, thereby reducing the vertical beam divergence angle. The 8-channel DFB laser array has a total length of 500 µm, a total width of 2000 µm, and a laser unit spacing of 250 µm. The principle of the flip-chip bonding structure and the laser array chip prepared are demonstrated in Fig. 7(c).

The different bonded structures can be easily prepared using the Finetech INEPLACER®lambda 2 benchtop sub-micron flip-chip bonder. To facilitate the comparison of the differences between the two structures, laser array units with identical designs are fabricated on the same aluminum nitride (AlN) submount using both the conventional p-side-up structure [Fig. 8(a) left] and the innovative flip-chip bonding structure [Fig. 8(a) right]. Figure 8(b) shows the cross-section along the light emission direction. The active region of the flip-chip bonded laser array unit is positioned in closer proximity to the AlN submount, resulting in enhanced heat dissipation.

The performance of the laser array units with p-side-up and flip-chip structures is investigated at different temperatures, as shown in Fig. 9(a) and 9(b). Maintaining an injection current of 110 mA, the spectra for both structures are collected as the temperature of the semiconductor thermoelectric cooler at the chip’s bottom varied from 25°C to 60°C in 5°C intervals. It can be observed that at any given temperature, both structures maintained a side-mode suppression ratio (SMSR) of over 50 dB.

As illustrated in Fig. 9(c), the wavelength redshift for both structures is approximately 0.1 nm/°C under varying temperatures. Additionally, the flip-chip bonded laser units exhibited shorter wavelengths at the same temperatures compared to the conventional p-side-up configuration, indicating reduced thermal interference.

The characteristics of the p-side-up and flip-chip laser array units at varying injection currents under the same temperature conditions are investigated. As depicted in Figs. 10(a) and 10(b), the spectra are collected at 25°C with injection currents ranging from 50 to 250 mA in 50 mA intervals. The maximum wavelength shift induced by the current tuning in the p-side-up structure, as shown in Fig. 10(a), is 3.38 nm, while the flip-chip structure exhibits a shift of only 2.44 nm, resulting in a difference of 0.94 nm. This difference is attributed to the more efficient heat dissipation in the flip-chip design. Due to the wavelength being tuned by the temperature at a rate of 0.1 nm/°C, it can be calculated that the flip-chip structure reduces thermal resistance by approximately 28%.

The power-current curves of the laser array units with the p-side-up and flip-chip structures at 25°C, with injection currents ranging from 0 to 250 mA, are shown in Fig. 10(c). The flip-chip design achieves a kink-free output power of up to 80 mW, which is approximately 20% higher compared to the p-side-up design. Additionally, a slight reduction in threshold current is observed in the flip-chip design.

As shown in Fig. 11(a), at 25°C with an injection current of 110 mA per laser channel, the proposed laser array demonstrated uniform output power, with SMSR values all exceeding 50 dB. The wavelength range for the 8 channels is 1294.35 to 1310.94 nm with stable SLM operation. Figure 11(b) presents the linear fitting of the laser wavelengths for the 8 channels. The fitting results indicated an average wavelength spacing of 398.719 GHz (2.273 nm), with a maximum deviation of 1.579 GHz (0.09 nm) for the laser array.

Figure 12 illustrates the $P\text{-}I\text{-}V$ curves of the flip-chip bonded 8-channel laser array at 25°C, with an injection current range of 0 to 250 mA. The threshold currents of the lasers are all below 15 mA. The differential resistance for each channel is approximately 4 Ω, and the maximum slope efficiency of a single channel is 0.368 mW/mA. When the bias current reaches 250 mA, the maximum output power of the flip-chip bonded 8-channel laser array exceeds 80 mW and shows a good linear relationship in output power.

As shown in Fig. 13, the far-field divergence angles of the flip-chip bonded 8-channel laser array unit are measured at 25°C with injection currents of 110 and 150 mA. Figures 13(a) and 13(b) illustrate that the far-field divergence angles measured at injection currents of 110 and 150 mA are $25.8°\times 30.1°$ and $26.3°\times 29.5°$, respectively. These results are close to the simulated values. The nearly circular mode profile is well-suited for coupling with a single-mode fiber (SMF).

At a temperature of 25°C and a bias current ranging from 50 to 200 mA, the linewidth of the flip-chip bonded 8-channel array units is measured using the delayed self-heterodyne method^[17,18]. The emitted light from the flip-chip bonded array units is coupled into a fiber via a coupling system and then split into two paths. One path is delayed using a 25 km optical fiber, while the other path undergoes a slight frequency shift through an acoustic-optic modulator (AOM) before both are coupled into the fiber and detected by a photodetector (PD). The spectrum is subsequently recorded by a spectrum analyzer. A Lorentzian fitting for the measured spectra is performed to accurately evaluate the Lorentzian linewidths, which are shown in Fig. 14. It is found that the Lorentzian linewidth reaches its minimum of 3.28 MHz at a bias current of 150 mA.

The RIN directly affects the intensity modulation performance of DFB laser arrays^[19]. A low RIN is desired for the optical I/O technology. The laser output is first coupled into a tapered SMF via an optical isolator. The light is then directed to a high-speed PD equipped with a trans-impedance amplifier (TIA) to convert the optical signal into an electrical signal. Subsequently, a 26.5 GHz bandwidth bias tee is employed to separate the electrical signal into its direct current (DC) and alternating current (AC) components. The average DC voltage is measured using a digital multimeter, while the AC component, which represents the noise, is analyzed with a 43.5 GHz bandwidth electrical spectrum analyzer (R&S®FSW43). We investigated the RIN of the flip-chip bonded 8-channel laser array. As shown in Fig. 15, at an injection current of 110 mA and a temperature of 25°C, the RIN of all eight channels ranges from $-135.3$ to $-138.7\mathrm{dB/}\mathrm{Hz}$.

In this paper, a flip-chip bonded high-power 8-channel DFB laser array is proposed and experimentally demonstrated as an ideal solution for optical I/O applications. The use of InAlGaAs MQWs as the gain material enhances high-temperature performance, while the precise design of asymmetric $\pi$ phase-shifted gratings using the REC technology is detailed. AR and HR coatings are applied to the front and rear cavity facets of the laser array to increase output power. Experimental results indicate that the flip-chip laser array unit exhibits superior heat dissipation compared to the p-side-up structure, allowing for more efficient thermal release from the active region. It is calculated that the flip-chip design reduces the junction temperature of the laser unit by 28% and increases maximum output power by approximately 20% compared to the p-side-up configuration. Furthermore, at a bias current of 110 mA, with all channel wavelengths maintaining an SMSR above 50 dB, the far-field pattern approaches a circular mode profile with a divergence angle of $25.8°\times 30.1°$ and a Lorentzian linewidth of 3.28 MHz. At a bias current of 250 mA, the maximum output power of the flip-chip 8-channel high-power DFB laser array exceeds 80 mW. Additionally, the RIN for all 8 channels remains below $-135.3\mathrm{dB/}\mathrm{Hz}$. The proposed flip-chip DFB laser array demonstrates excellent performance, indicating its potential for applications in optical I/O technology.