Emerging silicon photonics-based optical I/O technology can realize the optical interconnection between chips^[1]. The performance of traditional electrical I/O technology is gradually approaching the limit, and it is difficult to meet the needs of high-density chip interconnection^[2]. Optical I/O technology is expected to replace traditional electrical I/O technology to achieve optical interconnection between chips using the characteristics of high bandwidth, low latency, and high capacity of photons^[3]. In optical I/O technology, a high-performance multi-wavelength laser (MWL) is a key device. The number of wavelengths of MWL is used to improve the bandwidth of the interconnection between chips, so the number of optical carriers and the power balance of the MWL are extremely important.

Various types of MWL sources have been proposed and reported including mode-locked lasers^[4], frequency combs^[5], and multi-wavelength distributed feedback (DFB) laser arrays^[6]. Among them, the control mode of frequency combs and mode-locked lasers is complex, and the degree of freedom of the wavelength adjustment is small. In contrast, the DFB laser array has the advantages of stable mode, simple operation mechanism, and uniform wavelength spacing^[7]. However, the combination of the DFB laser array output will be a challenge. Traditional passive combiners such as multi-mode interferometers (MMIs), waveguide array gratings (AWGs), and planar lightwave circuits (PLCs) need to be integrated with lasers, which will bring great process difficulty and coupling loss. To overcome this challenge, we propose a shallowly etched Y-branch waveguide, which shares the same epitaxial structure as the laser to achieve a monolithic integrated combiner.

However, the light confinement ability of this shallow etched waveguide structure is relatively weak. Therefore, a smaller waveguide spacing of the laser is required to obtain a larger curvature radius of the Y-branch and reduce the bending leakage loss. When multiple parallel DFB lasers are working at the same time, this smaller waveguide spacing will lead to greater thermal crosstalk, which will increase the wavelength spacing error. To achieve uniform wavelength spacing, it is necessary to adjust the working current of each laser, but it will also bring about problems such as uneven output power. Therefore, the traditional laser array needs to balance between accurate wavelength spacing and power equalization. To solve this problem, flip-chip bonding^[8], optical attenuator array^[9], and other schemes have been proposed. However, the flip-chip bonding and optical attenuator array will increase the difficulty of the process.

In this Letter, a monolithic integrated MWL with uniform wavelength spacing and power balance is proposed and experimentally verified. The proposed MWL consists of an eight-channel DFB laser array, eight integrated power equalizers (PEs), a cascaded Y-branch, and a semiconductor optical amplifier (SOA). Each of these lasers is connected in series with a PE, which is then combined through the Y-branch and finally output by the SOA amplification. The cascaded Y-branch shares the identical active layer with the DFB laser array, PEs, and the SOA, and it avoids using the complicated butt-joint or selective-area growth technique. By adjusting the operating current injected to the PEs and the DFB laser array, an eight-wavelength output with uniform wavelength spacing and balanced power is achieved. The reconstruction equivalent chirp (REC) technique is employed to simplify the grating fabrication and enhance the precise control of the grating phase^[10]. The experimental results show that the wavelength spacing of the proposed MWL is $100\mathrm{G}\pm 4.38\mathrm{G}$, and the maximum deviation of the intensity (MDOI) is 1.0 dB under a 25°C working environment. Compared with the traditional MWL structure, the wavelength spacing error is reduced from 0.32 to 0.035 nm, and the MODI is reduced from 3.8 to 1.0 dB. The side mode suppression ratio (SMSR) of each wavelength is above 40 dB. The overall output power of the MWL chip is above 25 mW when the current injected into the SOA is 150 mA. The proposed MWL maintains high mode stability, high wavelength spacing uniformity, and high power uniformity under different SOA and LD currents. To characterize the relative intensity noise (RIN) and high-speed modulation performance, each wavelength in the MWL was measured. The RIN of all wavelengths is less than $-138\mathrm{dB}/\mathrm{Hz}$, and a clear 25 Gb/s non-return-to-zero (NRZ) eye diagram is obtained in the external lithium niobate Mach–Zehnder modulator. The superior performance of the proposed MWL makes it a promising method for low-bit-error optical I/O links and high-density chip interconnection systems.

In this Letter, the proposed MWL consists of an eight-channel DFB laser array, eight PEs, a cascaded Y branch, and an SOA, as shown in Fig. 1(a). The Y branch shares the same active layer with the DFB laser array, PEs, and SOA, avoiding the use of complex heterogeneous techniques such as the butt-joint technique and the selective area growth technique. The Y-branch and laser both adopt a shallow-etched ridge waveguide structure, so the optical confinement ability is relatively weak. It is necessary to design a smaller laser waveguide spacing (20 µm) to obtain a larger Y-branch curvature radius, thereby reducing the bending leakage loss. However, when the eight lasers are simultaneously lasing, due to the small waveguide spacing, there is a serious thermal crosstalk between different waveguides, resulting in the wavelength spacing deviating from the design value. Therefore, the injection current needs to be greatly adjusted to achieve a uniform wavelength spacing. However, this current adjustment scheme will lead to a large power difference between the channels. In order to solve this problem, we integrated eight PEs in front of the laser to balance the output power and achieve an eight-wavelength output with uniform power and uniform wavelength spacing.

In order to prepare laser arrays with low cost and high wavelength accuracy, we use the REC technology to simplify grating fabrication and enhance the precise control of the grating phase. The fabrication of the sampled grating requires only a common holographic exposure and µm-level photolithography^[11]. Compared with the traditional electron beam lithography (EBL) method^[12], the equipment cost and manufacturing time are reduced. For the sampled gratings, the index modulation change $\mathrm{\Delta }n(z)$ can be expressed as^[13]$$\mathrm{\Delta }n(z)=\frac{1}{2}s(z)\exp \left(j\frac{2\pi z}{{\mathrm{\Lambda }}_0}\right)+\text{c.c}.$$(1)Here, $s(z)$ is the periodic function of a sampling modulation, and ${\mathrm{\Lambda }}_0$ is the period of the uniform seed grating. According to the Fourier series expansion, $s(z)$ 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 and $F_m$ is the Fourier coefficient corresponding to the $m$th-order channel of the sampled grating. According to Eqs. (1) and (2), we can obtain $$\mathrm{\Delta }n(z)=\sum _m\frac{1}{2}{F}_m\exp \left[j(\frac{2\pi z}{{\mathrm{\Lambda }}_0}+\frac{2m\pi z}{P})\right]+\text{c.c}.$$(3)

Equation (3) shows that the sampled grating is actually a superposition of different orders of subgratings with different grating periods, and the grating period of the $m$th-order subgranting is given by $$\frac{1}{{\mathrm{\Lambda }}_m}=\frac{1}{{\mathrm{\Lambda }}_0}+\frac{1}{P}.$$(4)

Usually, the $+1$st-order subgrating is used as the laser cavity. 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. Different lasing wavelengths are realized by different sampling periods $P$ with the same uniform seed grating ${\mathrm{\Lambda }}_0$. By designing the sampled grating pattern, the equivalent phase shift can be introduced into the high-order subgrating. In this MWL, the equivalent $\pi$ phase shift ($\pi$-EPS) is introduced into the $+1$st subgrating to achieve grating phase matching and ensure stable single-mode operation of each wavelength laser, as shown in Fig. 1(a). Besides, the precision of the grating phase can be enhanced by a factor of ${(P/{\mathrm{\Lambda }}_0+1)}^2$ through the REC technique, typically exceeding two orders of magnitude higher than that of the traditional EBL technique^[10].

For our proposed structure, the Bragg wavelength of the uniform grating is set at 1635 nm. The period of the uniform seed grating (${\mathrm{\Lambda }}_0$) is 257.886 nm. To realize eight wavelengths at 1542.4, 1543.2, 1544.0, 1544.8, 1545.6, 1546.4, 1547.2, and 1548.0 nm, according to Eq. (4), the sampling grating period ($P$) ranges from 4.189 to 4.495 µm. The cavity length of the eight lasers and PEs is 500 µm, and the $\pi$-EPS is introduced in the center of the cavity. The waveguide spacing between adjacent channels is 20 µm. The SOA, Y branch, PEs, and LDs in the MWL chip are isolated by an isolation slot. As shown in Fig. 1(b), the overall sizes of MWL with and without PEs are $3000\mathrm{\mu m}\times 500\mathrm{\mu m}$ and $2500\mathrm{\mu m}\times 500\mathrm{\mu m}$, respectively. To minimize light reflection, the waveguides of the SOA are tilted at an angle of 7 deg.

The proposed MWL is implemented with a ridge waveguide structure. A layered structure is grown through metal–organic chemical vapor deposition (MOCVD)^[14]. First, the $n\text{-}\mathrm{InP}$ buffer layer, $n\text{-}\mathrm{InAlGaAs}$ lower optical confinement layer, compressive strain InAlGaAs multiple-quantum-well (MQW) structure, $p\text{-}\mathrm{InGaAlAs}$ upper optical confinement layer, and $p\text{-}\mathrm{InGaAsP}$ grating layer are successively deposited on the $n\text{-}\mathrm{InP}$ substrate. Then, a sampling grating is formed using conventional holographic exposure combined with µm-level lithography technology. Subsequently, the $p\text{-}\mathrm{InP}$ cladding layer, $p\text{-}\mathrm{InGaAsP}$ etch-stop layer, and $p\text{-}\mathrm{InGaAs}$ contact layer are grown successively. The etch-stop layer p-InGaAsP of the waveguide is grown to define the position where the waveguide stops during wet etching, thereby ensuring the consistency and accuracy of the etching depth. The ridge waveguide with a width of 2 µm is formed by wet etching. Finally, the wafer is cleaved into bars, and an anti-reflection film with a reflectivity of less than 0.1% is coated at both ends to complete the laser preparation, as shown in Fig. 1 (b). Figure 1(c) presents a cross-sectional SEM view of the designed MWL.

We first study the spectral characteristics of the MWL without PEs when eight lasers are working at the same time. The temperature is controlled at 25°C by a thermoelectric cooler (TEC). The optical spectra are recorded and analyzed using the AQ6370 optical spectrum analyzer. When the currents injected into the SOA and the cascaded Y branch are 100 and 60 mA, respectively, and the current injected into the eight lasers is 60 mA, the measured spectrum is shown in Fig. 2(a). The lasing wavelengths are 1543.02, 1544.14, 1545.02, 1545.86, 1546.65, 1547.34, 1548.12, and 1548.55 nm, respectively. All eight lasers have stable single-mode operating characteristics. The SMSRs of eight wavelengths are more than 40 dB. The MDOI of the eight wavelengths is 3.8 dB. We then perform a linear fitting of the operating wavelengths of the MWL, and the results are shown in Fig. 2(b). The wavelength deviations are shown in the inset of Fig. 2(b). Under this scheme, the maximum wavelength deviation of the eight lasers is 0.32 nm. The results show that the wavelengths of LD1 and LD8 are below the fitting curve, while the wavelengths of LD4 and LD5 are above the fitting curve. This wavelength deviation mainly comes from the thermal crosstalk caused by the simultaneous operation of eight lasers. Since the heat generated by the laser will diffuse to both sides, the heat accumulation of the lasers located in the center of the array (LD4 and LD5) is significantly higher than that of the edges (LD1 and LD8), which increases the wavelength spacing error.

To compensate for the wavelength error, we adjusted the operating current of eight lasers. When the currents injected into SOA and the cascaded Y branch are 100 and 60 mA, and the currents injected into eight lasers are 79, 58, 52, 51, 50, 60, 57, and 80 mA, respectively, the measured spectrum is shown in Fig. 2(c). The lasing wavelengths are 1543.77, 1544.56, 1545.32, 1546.11, 1546.92, 1547.74, 1548.53, and 1549.34 nm, respectively. The MDOI of the eight-wavelength output is 7.3 dB. We then performed a linear fitting of the operating wavelengths of the eight lasers, and the results are shown in Fig. 2(d). The wavelength deviations are shown in the inset of Fig. 2(d). Compared with the current before tuning, all wavelengths show a slight redshift. This phenomenon is attributed to the increase of the current of LD1 and LD8, the redshift of the wavelengths, and the decrease of the current of LD4 and LD5, which makes up for the redshift caused by temperature accumulation. Under this scheme, the maximum wavelength deviation of the eight lasers is 0.03 nm. However, the reduction of the wavelength spacing error is at the expense of power balance. In this tuning method, the maximum current difference between the lasers reaches 30 mA, causing the MDOI to increase to 7.3 dB, which sharply deteriorates the power balance of the MWL. Therefore, the traditional MWL cannot achieve accurate wavelength spacing and balanced output power at the same time.

In order to achieve uniform wavelength spacing and power equalization, we innovatively introduced a PE in front of each laser. This PE is a waveguide, which shares the same active region structure as the laser. By changing the injection current, the power of each channel can be adjusted. The spectral characteristics of the MWL with PEs are measured when eight lasers are working at the same time, and the test temperature is also controlled at 25°C. When the currents injected into the SOA and cascaded Y branch are 100 and 60 mA, respectively, the currents injected into the eight lasers are 70.5, 58.1, 52.5, 50.2, 49.8, 56.4, 59.7, and 72.3 mA, respectively, and the currents injected into the PEs are 5.5, 15.2, 23.6, 27.2, 28.8, 16.9, 14.3, and 3.3 mA, respectively, the measured spectrum is shown in Fig. 3(a). The lasing wavelengths of the MWL are 1544.47, 1545.32, 1546.11, 1546.87, 1547.64, 1548.41, 1549.20, and 1550.02 nm, respectively. The SMSRs of eight wavelengths are more than 44 dB. The MDOI of the eight-wavelength output is 1.0 dB. We then performed a linear fitting of the operating wavelengths of the eight lasers, and the results are shown in Fig. 3(b). The wavelength deviations are shown in the inset of Fig. 3(b). Under this scheme, the wavelength deviation of the eight lasers is less than 0.035 nm. As shown in Fig. 3(a), there are still small side lobes in the spectrum outside the eight main peaks, which are mainly attributed to the four-wave mixing effect of the eight wavelengths in the Y branch.

The PE performance with different biasing currents is also studied, as shown in Fig. 4(a). At the time, the operating currents of the SOA, cascaded Y branch, and LD are 100, 60, and 60 mA, respectively. The output power of each channel is recorded by the Thorlabs PM100A power meter. The results show that the output power of each channel increases with the increase of the current injected into the PE. Therefore, by tuning the current, the power balance of each channel can be achieved. It is worth noting that the eight-wavelength output of the MWL changes little with the injection current of the PE, as shown in Fig. 4(b). This tiny wavelength change can be easily compensated by the laser current without affecting the power balance.

In the following, we characterize the performance of the MWL with PEs. The lasing performance with different biasing currents is also studied. To maintain a uniform wavelength spacing, the current injected into the eight lasers increases in a current step of 6 mA. Figure 5 shows the effect of the total current of the eight lasers on the MWL spectrum. The results show that under the tuning of PEs, the eight-wavelength output has uniform wavelength spacing in the whole current range. The wavelengths redshift with the total current at a rate of 0.0096 nm/mA owing to Joule heating, which is shown in Fig. 6(a). As shown in Fig. 6(b), when the total current of the laser changes from 388 to 628 mA, the SMSRs of the eight wavelengths are all above 42 dB, and the standard deviation of the intensity (SDOI) is below 0.6 dB, indicating that the MWL has stable single-mode operation, uniform wavelength spacing, and balanced power.

We further investigated the impact of the $I_{\mathrm{SOA}}$ on the performance of the MWL. Under the above current injection scheme, we record the spectra under different SOA currents, as shown in Fig. 7. The results show that, when the $I_{\mathrm{SOA}}$ is in the range of 50–150 mA, eight lasers can achieve stable single-mode output. The single-mode stability and power uniformity of the eight wavelengths are shown in Fig. 8(a). The SMSRs are all above 42 dB, and the SDOI is below 0.5 dB for the eight wavelengths, indicating stable single-mode operations and uniform output power for each wavelength. At the same time, we also record the change of the output power of the MWL chip with $I_{\mathrm{SOA}}$ when eight lasers are simultaneously lasing, as shown in Fig. 8(b). When $I_{\mathrm{SOA}}$ reaches 150 mA, the output power of MWL exceeds 25 mW.

In the optical I/O system, intensity modulation is usually used, so the RIN of the laser is a crucial technical index. Here we use the R&S^®FSW43 electrical spectrum analyzer to record the RIN of eight lasers, as shown in Fig. 9. During this test, the operating currents of SOA, cascaded Y branch, PEs, and LD are 100, 60, 20, and 60 mA, respectively. For convenience of measurement, the RIN is measured when each laser works alone. The results show that the RINs of all lasers are below $-138\mathrm{dB}/\mathrm{Hz}$. The measurement results show that the eight-wavelength output of the MWL has a low RIN, which is suitable for intensity-modulated optical signal carriers for on-chip optical I/O systems.

In addition, high-speed intensity modulation is performed on eight optical carriers of the MWL to evaluate the performance of the MWL as a high-density on-chip interconnect laser source. 25 Gb/s NRZ large-signal modulation was performed with a pseudo-random bit sequence (PRBS) of $2^{31}-1$. The PRBS signal is generated by a 100 G bidirectional encoder representation from transformers (BERT, Golight) and amplified by a 38 GHz electrical amplifier (SHF L810A). Using the same injection current scheme as above, the optical eye diagrams are analyzed by the optical sampling oscilloscope (Keysight N1092A). In this measurement, eight lasers work at the same time and then use the AWG to filter the output of eight wavelengths and couple them one by one to the lithium niobate Mach–Zehnder modulator for intensity modulation. Figure 10 shows the 25 Gb/s back-to-back optical eye diagrams for the eight wavelengths of the WML. The mask “25GBASE-LR_ER_TX Hit Ratio 5E-5” is embedded to evaluate the quality of the eye diagrams and mask margins of the MWL. A clear eye diagram was obtained at eight wavelengths. The dynamic extinction ratios of the eye diagram at eight wavelengths were 16.75, 14.08, 16.11, 15.44, 16.8, 14.79, 14.18, and 16.41 dB, respectively. The margins of the eye diagram at eight wavelengths were 40.8%, 40.9%, 38.7%, 44.0%, 45.4%, 43.9%, 40.6%, and 42.0%, respectively. This result shows that the proposed MWL can become a high-density on-chip optical chip interconnect laser source.

In this Letter, a monolithic integrated MWL with uniform wavelength spacing and power balance is proposed and experimentally verified. The proposed MWL chip consists of an eight-channel DFB laser array, PEs, a cascaded Y branch, and an SOA. By adjusting the operating current of the PEs and the laser array, an eight-wavelength output with uniform wavelength spacing and balanced power is achieved. The experimental results show that the wavelength spacing of the proposed MWL is $100\mathrm{G}\pm 4.38\mathrm{G}$, and the maximum power deviation is 1.0 dB under a 25°C working environment. Compared with the traditional MWL structure, the wavelength spacing error is reduced from 0.32 to 0.035 nm, and the MODI is reduced from 3.8 to 1.0 dB. The overall output power of the MWL is above 25 mW when the current injected into the SOA is 150 mA. The proposed MWL maintains high mode stability, high wavelength spacing uniformity, and high power uniformity under different SOA and LD currents. The RINs of all wavelengths of the proposed MWL are less than 140 dB/Hz, and a clear 25 Gb/s NRZ eye diagram is obtained in the external lithium niobate Mach–Zehnder modulator. The superior performance of the proposed MWL makes it a promising method for low-bit-error optical I/O links and high-density chip interconnection systems.