100 mW‑class O‑band multi‑wavelength distributed feedback laser array for optical input‑output technology

-- Translated from the article by Yue Zhang et al.

Abstract

We propose and experimentally demonstrate an O‑band multi‑wavelength high‑power DFB laser array for optical I/O technology. The proposed DFB laser array operates at a 400 GHz wavelength spacing, complying with the high‑power CW‑WDM standard. The reconstruction‑equivalent‑chirp technique is adopted to precisely control the grating phase and introduce an equivalent π‑phase shift inside the laser cavity. Anti‑reflection (AR) and high‑reflection (HR) thin‑film coatings are applied to the front and rear facets to boost output power. The equivalent π‑phase‑shift structure is placed close to the HR‑coated facet to improve the single‑longitudinal‑mode yield. Optical spectra are measured at a bias current of 150 mA, showing wavelength deviation below 1.958 GHz and a side‑mode suppression ratio (SMSR) exceeding 55 dB. Statistical analysis indicates that 94.8 % of channels achieve an SMSR above 40 dB, and 90.0 % of wavelength spacings deviate from the designed value within ±12.5 %. The laser delivers an output power of over 100 mA? No, over 100 mW at a bias current above 370 mA. The measured far‑field divergence angles of the laser are 25.5° × 29.9°, and the measured Lorentzian linewidth is 1.68 MHz at 150 mA bias current. Featuring uniform wavelength spacing, high output power and excellent single‑longitudinal‑mode stability, the proposed O‑band multi‑wavelength DFB laser array is promising as a multi‑wavelength laser source for optical I/O applications.

1. Introduction

Technologies such as cloud computing, artificial intelligence (AI), and the Internet of Things (IoT) are driving exponential growth in data‑center traffic and accelerating the demand for computing power[1]. These requirements are rapidly approaching the performance limits of electrical input‑output (I/O)[2]. Therefore, developing optical I/O to replace all‑electrical I/O is critical to meet the demands of high‑performance, data‑intensive computing. In 2015, Orcutt et al. first reported on‑chip optical I/O technology based on optical communication in microprocessors[3], which has since attracted attention from institutions including Ayar Labs and Intel[4]. Dense wavelength‑division multiplexing (DWDM) is an excellent solution to satisfy the high bandwidth‑density requirements of optical I/O links. Multi‑wavelength lasers (MWLs) matching the DWDM grid constitute an essential component of optical I/O technology, helping improve performance, efficiency and bandwidth scalability. Distributed‑feedback (DFB) laser arrays are typical multi‑wavelength laser sources, featuring simple operating mechanisms and high single‑wavelength output power[5].

Among these studies, the work from Ayar Labs stands out. The organization reported its SuperNova light source: an O‑band multi‑wavelength DFB laser array fabricated via parallel processing[6]. Its bandwidth is enhanced by increasing the number of wavelengths, and multi‑wavelength signals can be routed and scaled to M×N ports (Figure 17). Nevertheless, an increase in port count introduces substantial power splitting loss, so high‑power O‑band DFB laser arrays are required to compensate for such loss.

In this paper, we design and fabricate a parallel 8‑channel high‑power DFB laser array with 400 GHz wavelength spacing. To improve output power, anti‑reflection (AR) and high‑reflection (HR) coatings are deposited on the front and rear facets of the laser array, respectively. Meanwhile, the reconstruction‑equivalent‑chirp (REC) technique is adopted for precise grating‑phase control, and phase‑tuning technology is introduced to equalize the phase inside laser cavities. Furthermore, an asymmetric phase‑shift (APS) structure is designed to improve single‑longitudinal‑mode (SLM) yield. Test results show that the average wavelength spacing is 398.83 GHz, deviating from the designed value by merely 1.17 GHz, with a maximum wavelength deviation of only 1.958 GHz. Statistical analysis demonstrates that 94.8 % of channels achieve a side‑mode suppression ratio (SMSR) above 40 dB, and 90.0 % of wavelength‑spacing deviations fall within ±12.5 % of the target value. Experimental results reveal kink‑free output power exceeding 100 mW at a bias current of 370 mA; all lasers attain an SMSR higher than 55 dB. At a bias current of 150 mA, the far‑field exhibits a nearly circular mode profile with divergence angles of 25.5° × 29.9°, and the Lorentzian linewidth is 1.68 MHz. The proposed 8‑channel high‑power DFB laser array delivers excellent performance and possesses promising application potential in the field of optical input‑output (I/O) technology.

2. Device Design and Fabrication

A. Device Design

Figure 2 shows the schematic diagram of the epitaxial structure of the proposed DFB laser array. InAlGaAs multiple‑quantum‑well (MQW) is adopted as the gain material to achieve superior temperature characteristics. This 8‑channel high‑power DFB laser array is fabricated using the reconstruction‑equivalent‑chirp (REC) technique. In conventional laser arrays, gratings need to be fabricated by electron‑beam lithography (EBL), which increases manufacturing cost and time and poses challenges for cost reduction. As a method proposed in recent years, the REC technique can simplify the grating fabrication procedure for DFB lasers and their arrays, cut manufacturing cost and time, and improve grating precision, whereby equivalent phase shifts can be realized in high‑order sub‑gratings. During grating fabrication, only conventional lithography and one‑step micrometer‑scale lithography are required[8].

The grating structure is illustrated in Figure 3. The design of the equivalent π‑phase shift is based on the reconstruction‑equivalent‑chirp (REC) technique. First, a specially designed sampling process is applied to the uniform grating. For a periodically sampled grating, its refractive‑index modulation can be expressed as:

where \(s(z)\) is the periodic function of sampling modulation, and \(\Lambda_0\) is the period of the uniform seed grating. According to Fourier‑series expansion, \(s(z)\) can be expressed as:

where \(P\) is the sampling period, and \(F_m\) is the Fourier coefficient corresponding to the \(m\)-th‑order channel of the sampled grating. Combining Equation (1) and Equation (2), we obtain:

Equation (3) indicates that the sampled grating is essentially a superposition of multi‑order sub‑gratings with different grating periods, where the grating period of the \(m\)-th‑order sub‑grating is:

When a sampling‑period variation ΔP is introduced to \(s(z)\) at \(z_0\), the refractive‑index modulation of the \(m\)-th‑order sub‑grating changes as follows:

where the expression for the equivalent phase shift \(\theta\) is given by:

Normally, the +1st‑order sub‑grating is utilized as the laser resonant cavity. When ΔP is designed to be P/2, an equivalent π‑phase shift can be realized in the +1st‑order sub‑grating. Under this condition, the grating period of the +1st‑order sub‑grating can be derived as:

where \(P\) and \(\Lambda_0\) are the periods of the sampled grating and the uniform seed grating, respectively. Different lasing wavelengths can be achieved by employing the same uniform seed grating combined with varied sampling periods.

To verify the experimental results, we simulated the mode field and far‑field divergence angle of the DFB laser. In the design, the vertical mode‑field distribution is expanded by reducing the number of quantum wells and barriers in the active region and increasing the thickness of the p‑type SCH layer, so as to minimize the vertical far‑field divergence angle. The simulation results are shown in Figure 4: the far‑field divergence angles of the output waveguide are 23° in the horizontal direction and 29° in the vertical direction.

The structure is grown on an n‑type InP substrate by metal‑organic chemical vapor deposition (MOCVD). The InAlGaAs active region consists of tensile‑strained barriers and compressively‑strained quantum wells. The gratings are fabricated via holographic lithography and inductively‑coupled plasma (ICP) etching processes. Afterwards, an InP cladding layer and an InGaAs contact layer are sequentially grown over the grating layer. Ti/Pt/Au and Ti/Au, deposited by magnetron sputtering, are adopted for the p‑type and n‑type electrodes, respectively. The total length and width of this 8‑channel DFB laser array are 500 μm and 2000 μm, with a laser pitch of 250 μm. Anti‑reflection (AR) and high‑reflection (HR) coatings are deposited on the front and rear cavity facets of the lasers. Figure 5(a) presents the top‑view micrograph of the designed high‑power DFB laser array. The ridge‑waveguide structure of the high‑power DFB laser array is fabricated by a combination of dry and wet etching. Figure 5(b) shows the cross‑sectional scanning electron microscope (SEM) image of the DFB laser.

B. Device Fabrication

Figure 5(c) shows the appearance of the box‑packaged chip. A thermoelectric cooler (TEC) is integrated inside the package to guarantee stable operating temperature. Optical coupling between internal components and a single‑mode fiber (SMF) is realized via lens collimation, and an optical isolator is equipped to suppress reflection interference from outside the system.

3. Device Characterization

The DFB laser array was mounted p‑side up on an AlN substrate for testing. Figure 6 presents the optical spectra and side‑mode suppression ratio (SMSR) of the 8‑channel DFB laser array at 25 °C under a bias current of 150 mA. The spectral data were acquired and analyzed using an AQ6370 optical spectrum analyzer. As shown in Figure 6(a), the laser array covers a wavelength range of 1294.35‑1310.94 nm. Linear fitting of the laser wavelengths yields an average wavelength spacing of 398.83 GHz, with a deviation of merely 1.17 GHz from the designed value and a maximum wavelength deviation of only 1.958 GHz. This indicates that the lasers fabricated with the REC technique achieve precise wavelength control, and the well‑designed position of the π‑phase shift endows the lasers with excellent single‑longitudinal‑mode (SLM) yield. As illustrated in Figure 6(a), all channels of the laser array exhibit an SMSR above 55 dB with good output‑power uniformity.

Figure 6(b) shows the power‑current‑voltage (P‑I‑V) curves of the 8‑channel DFB laser array at 25 °C, with the laser bias current ranging from 0 to 500 mA. The test results reveal that the threshold current of all lasers is lower than 25 mA. The maximum slope efficiency reaches 0.364 mW/mA at a bias current of 100 mA, and the differential resistance is approximately 4 Ω. At a bias current of 370 mA, the 8‑channel laser array achieves kink‑free output power exceeding 100 mW, and the output power exhibits favorable linearity before thermal roll‑off occurs.

Figure 7(a) presents the optical spectra and side‑mode suppression ratio (SMSR) of a single laser element at a bias current of 150 mA over the temperature range from 25 °C to 65 °C. The laser wavelength varies from 1308.32 nm to 1312.09 nm, and stable single‑longitudinal‑mode (SLM) operation is maintained throughout. The wavelength tuning range induced by temperature variation is approximately 3.77 nm, corresponding to a thermal tuning coefficient of 0.09425 nm/°C. This high‑power DFB laser maintains an SMSR above 55 dB across the temperature range of 25 °C to 65 °C.

We also characterized the power‑current‑voltage (P‑I‑V) curves of the laser element over the temperature range of 25 °C to 65 °C, as shown in Figure 7(b). As the temperature increases, the threshold current rises gradually from 16 mA to 23 mA, while both the slope efficiency and the maximum kink‑free output power of the laser decrease progressively. At a bias current of 100 mA, the slope efficiency drops from 0.332 mW/mA to 0.281 mW/mA, and the saturation power decreases from 103 mW to 67 mW.

We further evaluated the single‑longitudinal‑mode yield and wavelength‑spacing yield based on 400 sets of measurement data. Figure 8(a) plots the measured side‑mode suppression ratio (SMSR), and Figure 8(b) illustrates the wavelength spacing obtained from statistical analysis of ten laser arrays. A total of 94.8 % of channels exhibit an SMSR above 40 dB, and 98.0 % achieve an SMSR above 35 dB, indicating a high single‑longitudinal‑mode yield. Meanwhile, 90.0 % of the wavelength spacings deviate within ±12.5 % from the designed value, demonstrating satisfactory wavelength‑spacing yield. In addition, thermal crosstalk induces wavelength shift in adjacent laser elements, as depicted in Figure 8(c). The maximum wavelength shifts of Laser #1 and Laser #4 originate from neighboring laser elements, reaching approximately 0.14 nm.

As illustrated in Figure 9, the far‑field divergence angles of the laser were measured at 25 °C under bias currents of 100 mA and 150 mA. As shown in Figure 9(a) and (b), the far‑field divergence angle is 25.6° × 29.9° at a bias current of 100 mA, and 25.5° × 29.9° at 150 mA. Such nearly circular mode profile is highly desirable for coupling with single‑mode fiber (SMF).

We measured the linewidth of the high‑power DFB laser using the delayed self‑heterodyne method at 25 °C with bias currents ranging from 50 mA to 200 mA. The laser light is coupled into an optical fiber via a coupling system and then split into two paths. One path undergoes time delay, while the other is slightly frequency‑shifted by an acousto‑optic modulator. The two optical paths are finally combined into a single‑mode fiber, and the results are recorded by an optical spectrum analyzer. Lorentzian fitting is performed on the measured spectra to accurately evaluate the Lorentzian linewidth. The results are presented in Figure 10: the Lorentzian linewidth ranges from 1.68 MHz to 2.89 MHz as the bias current varies from 50 mA to 200 mA.

4. Conclusion

In this paper, we fabricate an 8‑channel high‑power DFB laser array with equivalent π‑phase shift. The output power is enhanced by depositing anti‑reflection (AR) and high‑reflection (HR) coatings on the front and rear cavity facets, and an asymmetric phase‑shift structure is designed to improve the single‑longitudinal‑mode (SLM) yield. The test results show that the average wavelength spacing is 398.83 GHz with a deviation of only 1.17 GHz from the designed value. Statistical analysis demonstrates that 94.8 % of the measured side‑mode suppression ratio (SMSR) is higher than 40 dB, and 90.0 % of the wavelength spacings deviate within ±12.5 % from the target value. At a bias current of 150 mA, the laser achieves kink‑free output power over 100 mW with an SMSR above 55 dB. Under the same bias current of 150 mA, the far‑field divergence angle is 25.5° × 29.9°, and the Lorentzian linewidth is 1.68 MHz. Benefiting from its excellent performance, this 8‑channel high‑power DFB laser array serves as a potential candidate for optical input‑output (I/O) applications.