Wavelength-division-multiplexing (WDM) technology has recently shown great potential in the metro and access networks. WDM networks are expected to be applied in next-generation high-capacity front-haul networks and passive optical networks^[1,2]. Wavelength tunable semiconductor lasers are very important components in WDM networks. The lasers with fixed wavelengths are desired to be replaced by tunable lasers for enhancing the network flexibility and reducing the system costs^[3,4]. Besides, high-speed modulation per wavelength is required for the increasing data traffic^[5]. Tunable lasers with different tuning mechanisms have been investigated and reported, including the tunable distributed Bragg reflector (DBR) lasers, the external-cavity tunable lasers (ECTLs), the Vernier-effect-based tunable lasers, and the multi-wavelength distributed feedback (DFB) laser arrays (MLAs)-based tunable lasers^[6–10]. The tunable lasers based on the MLAs are superior for the simple wavelength tuning mechanism and the stable single-mode operations. The conventional multi-wavelength laser arrays are placed in parallel, and an extra optical combiner is monolithically integrated for the one-waveguide light output^[11]. The optical power loss is introduced by the optical combiner, and the fabrication process becomes complex for the integration of the passive optical combiner. Thus, the lasers in the multi-wavelength laser arrays are expected to be arranged in series to avoid the use of the optical combiner. However, the single-mode stabilities are deteriorated by the grating crosstalk from the lasers placed in series, and the grating crosstalk becomes quite serious when the wavelength spacing is smaller than the width of the grating stopband, which is typically 3.2 nm in the DFB laser^[12]. However, a large temperature range is required to tune the wavelength of interest if the wavelength spacing of the laser arrays is too large. The large temperature tuning range may increase the power consumption of the thermoelectric cooler (TEC), cause a large difference in the laser output power, and accelerate the aging of the device.

Here, we demonstrate a high-speed directly modulated tunable in-series laser array (DMTL) with four integrated lasers, and the Bragg wavelengths of the four lasers are separated by 2.4 nm. A three-section design is utilized to reduce the interference from other lasers and obtain good single-mode properties. Based on the three-section structure, the electro-optic (EO) response bandwidth can be enhanced owing to the reduced modulated cavity length. Meanwhile, the photon intensity stays high along the modulated section owing to the $\pi$ phase shift cavity design, which is beneficial to a high resonant frequency. Besides, we utilize the reconstruction-equivalent-chirp (REC) technique to simplify the grating fabrication process and enhance the precise control of the grating phase. Therefore, the proposed DMTL shows the advantages of compact device size and a simple fabrication process. In addition, a semiconductor optical amplifier (SOA) is designed in front of the laser array to amplify the output power.

The key to realizing a high-speed DMTL is to increase the modulation bandwidth and reduce the crosstalk among the gratings of the in-series lasers. The relaxation oscillation frequency ($f_r$) of the lasers should be increased as high as possible, and the resistor-capacitor (RC) constant should be decreased to meet the high bandwidth requirement^[13–15]. The relaxation oscillation frequency can be expressed as follows^[16]: $$f_r=\frac{1}{2\pi }\sqrt{v_g{g}^{\prime }{S}_0/{\tau }_P},$$(1)where $v_g$ is the group velocity, $S_0$ is the steady-state photon density in the cavity, $g^{\prime }$ is differential gain, and ${\tau }_P$ is the photon lifetime. Reducing the active region length is an effective method to decrease the photon lifetime and increase the photon density in the cavity, thus improving the relaxation oscillation frequency^[17]. However, a short active region length for DFB lasers will reduce the optical feedback of the grating, which results in high threshold gain and finally deteriorates the direct modulation bandwidth. In this Letter, we propose a three-section $\pi$ phase-shifted directly modulated DFB laser, which includes a short central gain section (GS) and two side sections (SSs), as shown in Fig. 1(a). The simulated distribution of the photon intensity along the cavity is shown in Fig. 1(b). Photons are concentrated in the central GS owing to the $\pi$ phase shift. Therefore, the central GS is injected with a large current for lasing and modulation. The SSs share the same multiple quantum wells (MQWs) and grating period as the central GS. The SSs are injected with a small current to provide additional feedback. As a result, a low threshold gain of the GS is obtained owing to the additional feedback from the SSs. Moreover, the relaxation oscillation frequency is enhanced owing to the high differential gain and photon density of the GS. The shortened gain length also lowers the parasitic capacitance. A thick benzocyclobutene (BCB) layer is designed underneath the p electrode of the central GS to further decrease the parasitic capacitance.

In addition, the three-section structure can help suppress the side modes to guarantee the single-mode operations. As shown in Fig. 1(b), the photons of the dominant lasing mode in the cavity are mainly distributed around the center of the cavity, which is different from the side modes. Therefore, the priority of the dominant lasing mode can be increased by the different current injections to the central GS and the SSs. Thus, the single-mode stabilities can be enhanced. Besides, a phase error of the grating in the whole cavity may destroy the single-mode properties, and the grating phase precision can be improved through the REC technique. The REC technique studied in previous work uses a sampled grating to introduce equivalent $\pi$ phase shifts and improve the precision of the grating phase^[18–20]. Usually, we use a $+1$st-order grating to provide grating feedback and select the lasing mode for its high equivalent coupling coefficient and its ability to introduce equivalent $\pi$ phase shift. The Bragg wavelength of the $+1$st-order grating (${\lambda }_{+1}$) is determined by the sampling period ($P$), and the variation of ${\lambda }_{+1}$ is derived as Eq. (2). The ${\lambda }_{+1}$ is not sensitive to the variation of the sampling period, and thus the grating fabrication tolerance is relaxed by the factor ${(P/{\mathrm{\Lambda }}_0+1)}^2$, with the value of several hundred. The schematic of the grating designed based on the REC technique is shown in Fig. 2(a), and the sampling period of the four integrated lasers is shown in Fig. 2(b): $$\mathrm{\Delta }{\lambda }_{+1}=\frac{1}{{(\frac{P}{{\mathrm{\Lambda }}_0}+1)}^2}2n\mathrm{\Delta }P.$$(2)

We adopted a standard ridged waveguide structure for the proposed in-series laser array for its simple fabrication process. The compressively-strained InAlGaAs MQW structure is utilized as the active layer for its excellent high-temperature performance. The p-InGaAsP is used as the grating layer. The thickness of the p-InGaAsP grating layer is 50 nm, and the photoluminescence (PL) wavelength of the p-InGaAsP grating layer is 1.36 µm. The coupling coefficient of the uniform grating is approximately $9300{\mathrm{m}}^{-1}$ to guarantee that the coupling coefficient of the $+1$st-order sub-grating is $3000{\mathrm{m}}^{-1}$. Owing to the grating design based on the REC technique, the fabrication of the grating only requires a holographic exposure and micrometer (µm)-level photolithography, avoiding the expensive electron beam lithography. The BCB is fabricated underneath the p electrode of the central GS to decrease the parasitic capacitance, thus improving the direct modulation bandwidth. Figure 3 shows the picture of the DMTL chip under the microscope. The four integrated lasers are all 550 µm long, and the GS is 275 µm long. The electrodes of the SSs are connected to simplify the measurement process. The SOA with 250 µm length is integrated to adjust the output power. The total size of the chip is $2500\mathrm{\mu m}\times 300\mathrm{\mu m}$. Both the front and rear facets are coated with anti-reflection films. The output waveguide of the SOA is tilted 7 deg to further decrease the facet reflection.

The proposed DMTL was bonded onto a high-frequency submount to facilitate device characterization. Figure 4(a) shows the fabricated DMTL chip on the submount (COS). AlN is used as the substrate for its high thermal conductivity. The ground coplanar waveguide (GCPW) structure is used to feed the RF signal. The 40 Ω TaN resistor is fabricated for impedance matching. In addition, the RF signals and bias currents are injected separately from two sides. The bias currents are injected into the lasers directly and do not pass the 40-Ω resistors, which effectively decreases the power consumption. The magnetic beads are bonded on the DC line of the substrate to prevent the RF signals.

The DMTL COS is placed on a copper carrier, and the temperature of the carrier can be adjusted by the TEC. We analyzed the lasing spectra using the optical spectrum analyzer (Yokogawa, AQ6370). The temperature of the TEC is set at 25°C first. The currents injected to the GS, SS, and SOA ($I_{\mathrm{GS}}$, $I_{\mathrm{SS}}$, $I_{\mathrm{SOA}}$) are 80, 30, and 30 mA, respectively. When one laser works, the front sections are injected with a small compensation current ($I_{\mathrm{com}}$) to avoid material absorption of the light, and $I_{\mathrm{com}}$ is set at 15 mA here. The optical spectra of the four lasers are shown in Fig. 4(b). The fitting slope with the lasing wavelengths of 2.47 nm represents the average wavelength spacing, which only deviates from the design by 0.07 nm. Besides, the side mode suppression ratios (SMSRs) of the lasing modes of the four lasers are 59.5, 52.8, 53.5, and 54.0 dB, respectively. Thus, precise wavelength spacing and good single-mode operations are obtained for all the four lasers.

We studied the impact of $I_{\mathrm{GS}}$ and $I_{\mathrm{SS}}$ on the lasing performance. We measured the optical spectra of the laser diode (LD1) as an example. As shown in Fig. 5(a), $I_{\mathrm{SOA}}$ and $I_{\mathrm{SS}}$ are both fixed at 30 mA, and good single-mode operations are kept when $I_{\mathrm{GS}}$ is adjusted from 30 to 110 mA. Figure 5(b) shows the optical spectra of LD1 as a function of $I_{\mathrm{SS}}$ when $I_{\mathrm{SOA}}$ and $I_{\mathrm{GS}}$ are fixed at 30 and 70 mA, respectively. The single-mode properties deteriorate when $I_{\mathrm{SS}}$ is above 50 mA.

The typical power-current-voltage ($PIV$) curve of LD1 is described in Fig. 6(a). $I_{\mathrm{SOA}}$ and $I_{\mathrm{SS}}$ are 10 and 30 mA, respectively. Figure 6(a) shows the threshold current is 19 mA, and the differential resistance of the central GS is 12.2 Ω. The performance of the SOA was also characterized. Figure 6(b) shows the output power as a function of $I_{\mathrm{SOA}}$, and the output power of all the lasers can reach 14 mW at $I_{\mathrm{SOA}}$ of 60 mA.

The tunable DFB laser arrays are superior for their simple wavelength tuning mechanism. The wavelength of interest can be obtained by selecting the working laser and adjusting the temperature of the TEC. Here, we tuned 12 channels with 100 GHz spacing for our proposed DMTL. $I_{\mathrm{GS}}$, $I_{\mathrm{SS}}$, and $I_{\mathrm{com}}$ are 80, 30, and 15 mA, respectively. We adjusted $I_{\mathrm{SOA}}$ to keep the output power of all the 12 tuned channels balanced. The optical spectra of the tuned 12 channels with 100 GHz spacing are shown in Fig. 7(a). The temperature variation of the TEC for tuning the 12 channels is only 16°C, which is mainly due to the precise and small wavelength spacing, and the thermal coefficient is approximately 0.11 nm/°C. The right of Fig. 7(b) shows the SMSRs of the tuned 12 channels, and all the SMSRs are above 50 dB, indicating quite good single-mode operations.

The EO response was analyzed using the 70 GHz vector network analyzer (Anritsu MS4647A). The temperature of the TEC is set at 25°C. $I_{\mathrm{SOA}}$ and $I_{\mathrm{SS}}$ are both 30 mA. Figure 8(a) shows the EO response of LD1 when $I_{\mathrm{GS}}$ is different. As is shown in Fig. 8(a), the 3 dB bandwidth is increased at a high $I_{\mathrm{GS}}$. The 3 dB bandwidth reaches 21.3 GHz at the bias current of 100 mA. The EO responses of the four lasers when the $I_{\mathrm{GS}}$ is 100 mA are all analyzed. As shown in Fig. 8(b), the 3 dB bandwidths of the four lasers are 21.3, 20.8, 21.4, and 20.7 GHz, respectively.

The 25 Gb/s non-return-to-zero (NRZ) large-signal modulation was performed with a pseudo-random bit sequence (PRBS) of $2^{31}-1$. The PRBS signal is generated by a 100G bidirectional encoder representation from transformers (BERT, Golight) and amplified by a 38 GHz electrical amplifier (SHF L810A). The peak-to-peak current ($I_{\mathrm{PP}}$) fed to the central GS is about 74 mA. $I_{\mathrm{SOA}}$, $I_{\mathrm{SS}}$, and $I_{\mathrm{GS}}$ are 30, 30, and 100 mA, respectively. The optical eye diagrams are analyzed by the optical sampling oscilloscope (Keysight N1092A). The left of Fig. 9 shows the back-to-back optical eye diagrams for all 12 channels. The dynamic extinction ratios of all 12 channels are above 4 dB. The right of Fig. 9 shows the optical eye diagrams of all 12 channels after 10 km transmission over a standard single-mode fiber, and clearly opened eye diagrams are obtained for all 12 channels. Besides, 50 Gb/s data per wavelength can be obtained through the four-level pulse amplitude modulation (PAM4). Figure 10(a) shows the electrical eye digrams of the 50 Gb/s PAM4 signals after the electrical amplifier. Channel 10 is selected as an example to conduct the modulation with the 50 Gb/s PAM4 signals. As shown in Fig. 10(b), a clearly opened PAM4 optical eye diagram is obtained for channel 10.

In this Letter, we demonstrated a 25 Gbaud/s DMTL. High output power ($>14\mathrm{mW}$) and high uniform wavelength spacing are obtained for the proposed tunable in-series laser array. With a temperature variation of only 16°C, 12 channels with 100 GHz spacing are tuned. Good single-mode operations with the $\mathrm{SMSRs}>50\mathrm{dB}$ are obtained for all 12 channels. The 3 dB EO response bandwidth is above 20 GHz for all four lasers at the bias current of 100 mA. NRZ data transmission of 25 Gb/s over a single-mode fiber of up to 10 km is obtained for the proposed device. Besides, 50 Gb/s data per wavelength is obtained through PAM4. The proposed DMTL shows the potential for a compact and low-cost light source for WDM systems, such as next-generation front-haul networks and passive optical networks.