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Abstract
A distributed feedback (DFB) laser array of twenty wavelengths with highly reflective and anti-reflective (HR-AR) coated facets is both theoretically analyzed and experimentally validated. While the HR facet coating enhances high wall-plug efficiency, it inadvertently introduces a random facet grating phase, thereby compromising the lasing wavelength's predictability and the stability of the single-longitudinal-mode (SLM). In this study, two key advancements are introduced: first, the precisely spaced wavelength is achieved with an error of within ±0.2 nm using the reconstruction-equivalent-chirp (REC) technique; second, the random grating phase on the HR-coated facet is compensated by a controllable distributed phase shift through a two-section laser structure. The SLM stability can be improved while the wavelength can be continuously tuned to the standard wavelength grid. The overall chip size is compact with an area of 4000 × 500 µm2. The proposed laser array has a light power intensity above 13 dBm per wavelength, a high side mode suppression ratio above 50 dB, and low relative intensity noise under -160 dB/Hz. These attributes make it apt for deployment in DWDM-based optical communication systems and as a light source for optical I/O.
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1. Introduction
Efficient, compact, manufacturable multi-wavelength laser sources (MLS) are attracting great attention and can be widely used in optical communication systems, laser radar, co-packaged optical interconnects, emerging artificial intelligence, machine learning systems, and high-performance optical computing [1,2]. Furthermore, the upcoming high-performance computing architecture requires a new type of general-purpose I/O to overcome the limitations imposed by electrical I/O. Ayar Labs offers a solution by integrating an in-package optical I/O chip, TeraPHY, with a multi-wavelength light source, SuperNova. The current version of this light source provides eight optical ports, with each port having eight distinct wavelengths. This is just the initial phase. By increasing the number of wavelengths and optical ports, the chip's throughput can be effectively doubled without sacrificing its energy efficiency. Additionally, due to the highly compact microring modulator array used in optical I/O, there is a significant requirement for precise wavelength accuracy.
Typically, the multi-wavelength laser source takes the form of a semiconductor multi-wavelength laser array (MLA), with Bragg gratings defined by electron-beam lithography (EBL). Due to the point-by-point scanning inherent to EBL, accumulative phase errors in Bragg gratings become unavoidable, leading to residual errors in lasing wavelengths. Such wavelength discrepancies diminish the overall yield of the laser array, a reduction that becomes exponentially pronounced as the number of lasers increases. To ensure wavelength precision, each laser is equipped with either an individual resistor heater for thermal tuning or a distributed Bragg reflector for current tuning [3]. However, introducing these tunable components not only complicates the fabrication process but also results in thermal crosstalk and additional power consumption.
Distributed feedback (DFB) lasers with high-reflection and anti-reflection (HR-AR) facet coating have stronger application value than DFB lasers with both anti-reflection (AR-AR) facet coating due to the higher output efficiency, lower threshold gain and more compact structure [4]. However, in such DFB lasers, at the HR-coated facet the random reflection phase caused from cleavage process can often destroy the single-longitudinal-mode (SLM) stability, or lead to lasing wavelength floating and aligning ITU-T standard difficultly [5,6].
To solve these problems, we reported an HR-AR-coated DFB laser array with twenty-wavelength. The superior wavelength accuracy is achieved by a two-step method. First, the spacing accuracy is preliminary guaranteed by the reconstruction-equivalent-chirp technique. Here, Bragg wavelengths of ∼90% of the DFB lasers are within ±0.2 nm [7]. Noted that such precision is very high because 0.1-nm error of grating period leads to ∼0.6-nm error of Bragg wavelength. The further exact wavelength spacing and single-longitudinal-mode (SLM) operation is achieved by distributed phase compensation, which is realized by a two-section laser structure. By changing the ratio of the currents injected into the two sections, the refractive indexes of the two sections are slightly changed oppositely. As a result, the random facet grating phase can be compensated and the SLM operation can be guaranteed. At the same time, the lasing wavelength can be tuned to align with the wavelength grid without a large fluctuation of output power. The proposed MLA is measured exact wavelength spacings of 100 GHz, light power intensity above 13 dBm per wavelength, high side mode suppression ratio (SMSR) above 50 dB, and low relative intensity noise under −160 dB/Hz.
2. Design and simulations
The lasing wavelength of distributed feedback (DFB) lasers is determined by both the position of the lasing mode in the grating stopband and the wavelength of the grating stopband. The lasing mode is at the middle of the grating stopband for a typical λ/4 phase-shifted DFB laser with AR-AR-coated facet. However, for a HR-AR-coated DFB laser, the phase of the HR-coated facet grating is random while cleaving the chip. This results in an uncontrollable position of the lasing mode in the grating stopband. We need to finely designed the wavelength of the grating stopband and the position of lasing mode in this stopband to guarantee a precise lasing wavelength [6].
We can obtain a precision wavelength of the grating stopband via the use of the REC technique. Complex grating structures can be equivalently achieved in higher-order sub-grating of the sampled Bragg grating (SBG), by using the REC technique [7]. Typically, we use the +1st order sub-grating as the oscillation channel, and the equivalent period Λ+1 can be derived from
where Λ0 is the seed grating period and P is the sampled period. In our device, Λ0 is 253.13 nm, and the corresponding Bragg wave is far from the central wavelength of the material gain, which is approximately 1620 nm. Based on Eq. (1), by changing P, a multi-wavelength laser array can be realized. The fabrication of SBG requires only one-step holographic exposure and one-step µm-scale lithography for the grating fabrication. As a result, the use of REC technique can dramatically reduce equipment costs and fabrication time compared to conventional electron beam lithography (EBL) [6]. The phase error of the grating drops by a factor of (P/Λ0 + 1)2, the value of which is usually a few hundred [8].We arranged the position of the lasing mode in the grating stopband finely by distributed phase compensation of the two-section structure. The effective refractive index will change by changing the current injected into a DFB laser due to the free-carrier plasma effects and Joule heat [9]. We keep the total current fixed while changing the ratio of the current injected into the two sections, which keeps the overall Joule heat and total power consumption constant. Therefore, an effective refractive index difference is introduced between the two sections and distributed phase compensation can be achieved.
Based on the Fresnel equation and coupled-mode theory [10,11], we calculated the threshold conditions of a 500-µm-long DFB laser when the grating phase (ϕHR) at the HR-coated facet was varied from 0 to 2π. The Bragg wavelength is set at 1550 nm. All the gain margin (ΔgthL), lasing wavelength λ, and threshold gain (gthL) are varied with ϕHR, as shown in Fig. 1. Here ΔgthL refers to the threshold gain difference between the most probable side mode and the dominant mode, which is used to evaluate SLM property. Stable SLM operation can be guaranteed when ΔgthL is larger than 0.25. As the result shows, the more the ϕHR is close to 0 or 2π, the worse SLM properties is obtained and the further lasing wavelength deviates from Bragg wavelength.
Fig. 1. Calculated threshold gain gthL, gain margin ΔgthL, and lasing wavelength λ when HR coated facet grating phase ϕHR is varied from 0 to 2π for an HR-AR coated DFB laser.
In our investigation of the two-section DFB laser with HR-AR coated facet, we consider the electrical isolation's impact on the modes negligible. It functions primarily as insulation, enabling different current densities in the two sections. The effective refractive indexes (neff) of both sections are considered as variables owing to varying the current density. The section with AR-coated facet is denoted as Section I and the section with HR-coated facet is denoted as Section II. The neff difference between Section I and II is denoted by Δn. The length ratio of the Section I to II is denoted by RL. Here, only the Δn is varied while the average effective refractive index by length keeps unchanged. The total length of Section I and II is 500 µm. The gain margin ΔgthL, threshold gain gthL, and lasing wavelength λ are calculated when RL equals to 1 and Δn is varied from −0.005 to 0.005, which is induced by a total current of 100 mA, and ϕHR is varied from 0 to 2π, as shown in Fig. 2. As shown in Fig. 2(c), for any ϕHR from 0 to 2π, one can always find a value of Δn to obtain a ΔgthL higher than 0.25 while the lasing wavelength is 1550 nm or other wavelengths near 1550 nm. Figure 2(b) shows that when the lasing wavelength is 1550 nm, the threshold gain gthL is relatively low, which means low threshold current.
Fig. 2. Calculated (a) gain margin ΔgthL, (b) threshold gain gthL, and (c) lasing wavelength λ when the length ratio RL equals to 1, the effective refractive index difference Δn is varied from −0.005 to 0.005, and HR coated facet grating phase ϕHR is varied from 0 to 2π for an HR-AR coated two-section DFB laser.
The length ratio RL is also studied to optimize the structure when keeping overall length 500 µm. To evaluate the laser performance, the threshold gain gthL, gain margin ΔgthL, and tuning range Δλ are averaged when Δn is varied from −0.005 to 0.005 and ϕHR is varied from 0 to 2π. The length ratio RL is varied from 1/4 to 4, and the averaged gthL and ΔgthL are calculated as shown in Fig. 3(a). The result shows that when it comes to RL = 2, the lowest gthL and highest ΔgthL are obtained. Besides, to evaluate the SLM property of the structure, the SLM ratio RS is also defined as the proportion of the situations that ΔgthL is greater than 0.25. As shown in Fig. 3(b), the highest SLM ratio RS and tuning range Δλ is obtained when RL equals to 2. Therefore, we designed the lasers with RL = 2.
Fig. 3. Averaged (a) threshold gain gthL, gain margin ΔgthL, (b) tuning range Δλ, and the SLM ratio RS when the length ratio RL is varied from 1/4 to 4.
3. Fabrication
The laser structure is depicted in Fig. 4(a). In the device fabrication, an InAlGaAs separate-confinement heterostructure multi-quantum well (SCH-MQW) layer, a p-InGaAsP, and a n-InP layer grating layer were successively grown through the metal-organic chemical vapor deposition (MOCVD). Then, the sampled Bragg grating layer was formed by one-step holographic exposure and one-step µm-scale lithography. Then, a p-InGaAs contact layer and a p-InP cladding layer were successively regrown with MOCVD in the second epitaxial growth. A 2-µm-wide ridge waveguide was formed by etching two grooves on two sides. Shallow grooves are then etched on the ridge waveguide for electrical isolation between the two sections. These grooves, with a width of 5 microns, match the waveguide's width. A layer of silicon dioxide was deposited as the insulation layer, and the silicon dioxide on the top of ridge waveguide was etched for metal contact. Ti/Pt/Au electrodes are deposited with magnetron sputtering and lift-off on the p- and n-side. Subsequently, the wafer is cleaved into laser bars, and the bar facets are coated with AR and HR layers. The microscopic image of a twenty-channel DFB laser array with an HR-coated facet is shown in Fig. 4(b).
Fig. 4. (a) Schematic of the proposed two-section DFB laser with HR-AR coated. (b) Microscopic image of a fabricated twenty-channel DFB laser array. (SCH-MQW: separate-confinement heterostructure multi-quantum well; SBG: sampled Bragg grating).
4. Device characteristics
During the measurement, the temperature of the laser chip was controlled at 25 °C by a thermoelectric cooler (TEC). The laser chip is mounted on an AIN submount using AuSn solder, with the chip electrodes and submount electrodes interconnected by gold wires. First, we measured the output light power versus current for the 20- channel DFB laser array when the two sections have the same injection current density. Figure 5(a) shows that the threshold currents are varied from 22.2 to 25.8 mA, with an average of 24.1 mA. The output light power is saturated when the current is larger than 200 mA. The inset of Fig. 5(a) shows the current-voltage curve of a laser, which shows a differential resistance of approximately 6 Ω.
Fig. 5. (a) Output light power versus current for the 20-channel laser array when the two sections have the same injection current density. The inset shows the applied voltage versus current for a single laser. (b) Superimposed spectra, (c) wavelength, and SMSR when I1 was fixed at 80 mA and I2 was varied from 0 to 120 mA.
The currents injected into sections I and II are denoted by I1 and I2, respectively. Figure 5(b) shows the spectra when I1 was set as 80 mA and I2 was varied from 0 to 120 mA. With the increase of I2, lasing wavelength red-shifts from 1542.2 to 1545.5 nm, as shown in Fig. 5(c). In the situation when I2 equals to 14, 56, 100 and 118 mA, the mode-hopping happens owing to the cyclical variation of the phase compensation. When I2 changes from 60 to 98 mA, SMSR keeps above 50 dB and a continuous wavelength tuning range reaches to 1.85 nm.
More importantly, we measured the lasing spectra when total current was fixed at 120 mA. With the total current fixed and only the current injection proportion changed, the extra Joule heat and power fluctuation can be reduced. As shown in Fig. 6, with the increase of I1 and decrease of I2, lasing wavelength blue-shifts and the maximal continuously tunable range is 1.31 nm with the SMSR larger than 40 dB, which agrees with our theoretical prediction. Besides, the output power variation keeps within 3 dB with a current range of I2 from 24.7 to 113.6 mA. As a result, benefited from the high precision of Bragg wavelength by REC technique and the ability of slightly wavelength tuning, an MLA with accurate wavelength spacing can be obtained.
Fig. 6. (a) Superimposed spectra, (b) wavelength, SMSR, and output power when total current was fixed at 120 mA and the current injection proportion was varied.
First, to compare with the conventional one-electrode laser array, the lasing spectra is obtained when the injection current densities into all sections are the same. Figure 7(a) shows that the lasing wavelength are distributed in total chaos owing to the random grating phase in the HR facet when the total injection currents are 120 mA. Then the injection current proportion was changed to compensate the random phase and tune the wavelength with the total injection current fixed. Figure 7(b) shows that all the wavelengths are distributed orderly to meet the ITU-T grid with spacings of 100 GHz. The current pairs of I1-I2 of the twenty channels were 60-60, 56-64, 75-45, 89-31, 93-27, 35-85, 39-81, 66-54, 49-71, 100-20, 77-43, 69-51, 51-69, 82-38, 73-47, 44-76, 66-54, 48-72, 42-78, 40-80 mA, respectively. The SMSRs of all channels are above 50 dB. We also studied the power variation of the output light power for the twenty-wavelength laser array. Figure 7(c) shows that the output power variation of the twenty-wavelength laser array is 1.2 dB, lower than that before current tuning (1.4 dB).
Fig. 7. Superimposed spectra of the twenty-wavelength laser array (a) before and (b) after the tuning of current injection proportion. (c) Output power of all the channels.
A low relative intensity noise (RIN) is important for optical communication systems and high-performance optical computing. The laser output was coupled into a tapered single mode fiber, passed through an optical isolator, and then reached to a high-speed photodetector with a trans-impedance amplifier. Then, a 26.5 GHz bandwidth bias-Tee was used to divide the converted electrical signal into alternating current (AC) and direct current (DC) components. The AC signal was measured by a 43.5-GHz-bandwidth electrical spectrum analyzer (R&S FSW43) [12]. The average output voltage of the DC signal was detected by a digital multimeter. First, we measured the RIN when the total injection current is changed. Figure 8(a) shows that the RIN is decreased when the injection current is increased. It reaches to under −163 dB/Hz when the total current reaches to 120 mA. We also studied the RIN when the current injection ratio is changed with a fixed total current of 120 mA. Figure 8(b) shows that all the RIN is under −160 dB/Hz when I1 is varied from 30 to 100 mA. Besides, we also measured the 3-dB linewidth by delayed self-heterodyne interferometric technique [13]. The laser output was coupled into a tapered single-mode fiber, passed through an optical isolator, and then divided in two arms, one of them delayed by 25 km delay fiber and the other one frequency shifted by 80 MHz with acousto-optic modulator. Both arms were recombined to generate a beat frequency on the photodiode, which was then measured by the electrical spectrum analyzer. Figure 8(c) shows that the 3-dB linewidth is approximately 1.73 MHz, according to the fitting of the Lorentz curve.
Fig. 8. (a) Measured RIN spectra with varied total currents when current density injected into the two sections are the same, (b) measured RIN spectra when the current injection ratio is changed with total current fixed, and (c) measured 3-dB linewidth by the beat signal frequency spectrum and the fitted Lorentz curve.
5. Conclusion
In this paper, we proposed a twenty-wavelength, highly-reflective coated laser array with precise wavelength spacings of 100 GHz, aligning with the ITU-T standard. The random grating phase at the HR-coated facet is compensated by a two-section laser structure. The precision of Bragg wavelengths is guaranteed by the REC technique and its two sections structure. For the twenty-wavelength laser array, all the SMSRs are above 50 dB, output light power is above 13 dBm, and the RIN is under −160 dB/Hz when the total current is 120 mA. Noted that, only regular fabrication process is used without utilizing active-passive heterogeneous integration or time-consuming electron beam lithography. Consequently, our proposed multi-wavelength laser array offers a viable light source option for optical communication systems. Furthermore, the next-generation SuperNova by Ayar Labs requires an increase in the number of wavelengths to 16 for each optical I/O port. We can customize the laser array accordingly to meet these requirements. Our subsequent plan involves utilizing MMI couplers to merge the arrayed lasers, so it can apply to optical I/O by virtue of its exact wavelength spacings.
Funding
National Key Research and Development Program of China (2023YFB2806400, 2020YFB2205800); Chinese National Key Basic Research Special Fund (2018YFE0201200, 2018YFA0704402, 2018YFB2201801); National Natural Science Foundation of China (62004094, 61975075).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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