Dual-wavelength (DW) lasers are essential optical sources and difference-frequency generators and have been widely researched in the applications of optical sensing^[1,2], millimeter wave generation^[3], LiDAR, and THz communication^[4]. The traditional DW source consisting of two separately packaged lasers can partially satisfy these demands, but it suffers from the shortcomings of having low reliability, high power consumption, and large footprint^[5,6]. Benefiting from the advancement of semiconductor techniques and integrated photonics, monolithically integrated DW lasers are reported to address these problems^[7]. Among them, distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers are the most common optical sources with the advantages of single longitudinal mode, low threshold, and stable lasing wavelength^[8]. However, the fabrication of fine Bragg gratings in both relies on electron beam lithography, which suffers from stitching errors, proximity effects, and long-time writing^[9]. Recently, to simplify the fabrication processing and improve the accuracy of the lasing wavelength, the reconstruction equivalent-chirp (REC) technique, i.e. designing sampled grating, has been proposed^[10]. With the contributions of these, the monolithically integrated scheme becomes the main trend and provides an alternative approach for applications in various fields including communication, sensing, and microwave photonics.

Inspired by the traditional scheme, where two discrete lasers are cascaded by an optical fiber coupler, the waveguide combiners based on Y-branch and multimode interference are implemented for the DW laser to generate microwaves utilizing an optical heterodyne^[11,12]. The bottleneck of discrete components in size, weight, and power (SWaP) can be addressed by photonic integration^[13]. By utilizing the period-one (P1) phenomenon in the optical injection, two DFB lasers are cascaded monolithically in a series to generate a microwave signal^[14,15]. However, the additional supplementary means are necessary for most P1-based schemes such as an optical sideband injection by radio signal modulation^[16], a phase section^[17,18], and a distributed Bragg reflector^[19] between two lasers, which will significantly increase the fabrication complexity and cost of DW lasers. To address these issues, the dual-mode laser can simultaneously emit two lasing modes stably, while the fixed frequency spacing with its poor tunable range (several GHz) limits its application^[20,21]. Integrated monolithically with an amplified feedback cavity, the tuning range of the dual-mode laser can be expanded to only 15 GHz with the optical power fluctuating to around 50 dB, especially limiting the applications in sensing^[22].

In this paper, to the best of our knowledge, we first propose and demonstrate a monolithic tunable DW laser based on the chirped sampled grating. The device consists of three sections, i.e., front, mid, and rear sections, positioned on the chirp sampled grating with two equivalent π phase shifts. The impacts of the chirp rate and grating interference on the wavelength spacing are elaborately illustrated by the simulation. To verify the numerical investigation, the DW lasers with different index modulation amplitudes are implemented. As a reference, the tuning scope of a typical semiconductor optical amplifier (SOA)-based scheme is recorded and analyzed. By varying the injected current of each section, the wide tunable range of the wavelength spacing for the proposed DW laser is achieved and benefits from the structure of the three-section and chirp sampled grating. Such a monolithic DW laser with a wide tunable range from 59.50 to 116.25 GHz embodies the potential alternative as an optical source for integrated sensing and communication, radar systems, and microwave photonics.

In the dual-mode DFB laser with uniform grating, the partially shared resonant cavity for two modes results in mode competition, reducing the working stability of the two modes. To suppress grating interference and ensure stable two-mode emission, the linearly chirped Bragg grating and two π phase shifts are implemented, which broaden the stopband and enable the realization of dual lasing modes^[23]. The chirp grating with two π phase shifts works as the resonant cavity for the dual-mode DFB laser, where the periods at the π phase shift determine the lasing wavelengths of the two modes. Additionally, it is difficult for the single-electrode-based DW laser to achieve a wide tuning range of the wavelength spacing, while the multi-electrode scheme enables the flexible current injection. Therefore, the chirp grating and multi-electrode are crucial for DW lasers with tunable wavelength spacing.

However, achieving such linearly chirped Bragg gratings and precise phase shifts is challenging with conventional electron beam lithography. With the benefits of the REC technique, the complexity of fabrication is declined and the precise control of the grating phase is improved. Thus, the chirp sampled grating (CSG) and two equivalent π phase shifts (π-EPSs) based on the REC technique are utilized for the DW laser. As for the sampled grating, the index modulation change Δn(z) can be derived as Δn(z)=Δn02∑m=−∞∞Fmexp[j2π(zΛ0+mzP−mΔPP)]+c.c.,(1)where Λ0 and Δn0 are the period and index modulation amplitude of the basic grating, Fm, P, and ΔP are the mth order Fourier coefficient, the period of the sampling structure, and the difference in the sampled grating period, respectively. Generally, the first order (m=1) sub-grating is used to design goal grating, and the π-EPS is introduced by controlling the length of ΔP (ΔP/P=0.5). The equivalent grating period Λ+1 of the +1st order sub-grating and its relationship with the Bragg wavelength λ+1 can be written as 1Λ+1=1Λ0+1P,(2)λ+1=2neffΛ+1,(3)where neff is the effective refractive index. Derived from Eq. (2), the relationship between differential variations of ΔΛ+1 and ΔP satisfies with ΔΛ+1=ΔP(P/Λ0+1)2.(4)

For the REC-based DFB laser with the sampled grating, the fabrication of basic grating and the sampled structure can be divided into two steps. The first is holographic exposure for basic grating and the second is a micrometer-level contact exposure for the sampling structure. By properly designing the epitaxial wafer, the Bragg wavelength is kept far away from the center of the gain region, where the Bragg wavelength of the first-order sub-grating is located to ensure emission. The period Λ0 of the basic grating is the constant, and the variation of the sampling period P can be negligible due to the narrow wavelength spacing (∼100GHz) in this paper, i.e., the relationship of ΔP and ΔΛ+1 can be approximated as linear. Thus, DFB lasers with fine nano-scale structures, including the chirp grating and phase shift can be equivalently realized by the REC technique with micron-lithography, significantly reducing the cost and complexity of fabrication.

The wavelength spacing, determined by the chirp rate and length of the sampled grating, is a vital parameter for the DW laser. The schematic diagram of the proposed DW laser with an SOA is depicted in Fig. 1(a). The sampled grating consists of three parts: front, mid, and rear sections. To avoid the discontinuity of the sampled grating and to provide single-mode lasing, the linear CSG serves as the resonant cavity, and the π-EPS is implemented between each section. Figure 1(b) illustrates the partial enlargement of the cross section of the DW laser chip. The sampling periods at EPS1 and EPS2 determine two Bragg wavelengths of the CSG, i.e., λ1 and λ2. The Bragg wavelength spacing is depicted as ΔλB, satisfying ΔλB=λ1−λ2. Using the REC technique, the Bragg spacing can be controlled precisely with the linearly increasing sampling period, as shown in Fig. 1(c).

However, considering the grating interference between each section, the mode wavelength spacing Δλm of the DW laser is slightly larger than the Bragg wavelength spacing ΔλB. This phenomenon exacerbates, especially when the Bragg spacing is less than 100 GHz, posing a major obstacle for designing the DW laser with a proper wavelength spacing. To further investigate the wavelength spacing with various grating parameters, the transmission spectrum of the DW laser with such a sampled grating is calculated by the transfer matrix method^[24]. By simply varying the rate of the sampling period, the Bragg spacing can be controlled precisely. In the simulation, the values of neff, Δn0, Λ0, and the length of each section are set at 3.183, 0.0050, 257.886 nm, and 200 µm. As shown in Fig. 2(b), the transmission spectra of the sampled grating for various cascaded schemes are represented by the orange, yellow, and blue curves, respectively. The average Bragg wavelengths of front + mid and mid + rear schemes are 1550.78 and 1551.59 nm, where the peaks of the transmission spectrum represent the corresponding resonant modes. Distinguishing between the simple superposition of the transmittance spectra for the front + mid and mid + rear sections, two modes of the total are located at 1550.73 and 1551.64 nm. The Bragg spacing at 0.80 nm is smaller than the mode spacing at 0.91 nm, where two modes separate from each other due to the grating interference.

By varying the Bragg spacing of the sampled grating, the impact of the grating interference on the mode spacing Δλmode is elaborately investigated in Fig. 3(a). With the Bragg spacing varying from 0.2 to 0.8 nm in steps of 0.2 nm, the corresponding simulated Δλmode are 0.6048, 0.6768, 0.7824, and 0.9056 nm. Due to the restriction of the grating interference, it is difficult to narrow Δλmode below 0.4 nm (∼50GHz) by simply decreasing the Bragg spacing, which will limit the working range of the DW laser. As a reference, the index modulation amplitude Δn0 is decreasing to 0.0016, and the length of each is extended to 750 µm. The transmittance spectra with the Bragg spacing ranging from 0.16 to 0.28 nm are shown in Fig. 3(b), while the corresponding Δλmode are 0.208, 0.232, 0.260, and 0.292 nm. To further demonstrate the difference with Δn0 at 0.0050 and 0.0016, the relationship between the Bragg spacing and the mode spacing is simulated and analyzed in Fig. 3(c). The mode spacing decreases with Bragg spacing decreasing, but the difference between the Bragg spacing and mode spacing increases with high Δn0. As for the CSG with low Δn0, the total and Bragg spacing are almost the same, which will increase the length of the laser cavity.

The tunable range of the wavelength spacing is another crucial parameter for the DW laser. By varying the injected currents of the front and rear sections, the effective refractive index neff of the corresponding section will change. In this paper, the influence of the injected current on the mode wavelength spacing Δλm will be investigated by varying neff. In the simulation, Δn0 is set to 0.0050 and Bragg spacing is at 0.4 nm. As revealed in Fig. 4, with the increment of neff in the front and rear sections, the mode spacing Δλm of the DW laser decreases and increases, respectively. The simulation results indicate that the flexible tuning of the wavelength spacing can be realized by varying the injected currents in the front and rear sections, which is benefited from the three-section structure.

A ridge waveguide structure is implemented for the proposed DW laser with the CSG. By utilizing two-stage metal-organic chemical vapor deposition (MOCVD), the growth of epitaxial material is accomplished with multiple layers, consisting of an InAlGaAs multiple quantum well (MQW) structure and a p-InGaAsP grating layer. In addition, the CSG is fabricated by a conventional holographic exposure, which is combined with conventional photolithography and subsequent etching process. As illustrated in Fig. 5(a), the fabricated DW laser chips with total length and width at 1000 and 250 µm, while both of the front and rear facets are anti-reflection (AR) coated with reflectivity at 0.5‰. Additionally, the lengths of the SOA, TAR, front, mid, and rear sections are 250, 75, 225, 225, and 225 µm, respectively. An optical spectrum analyzer (OSA, Yokogawa AQ6370) is used to record the optical spectra of the proposed laser. The depth of basic grating of the DW laser is 60 nm, which represents high index modulation amplitudes Δn0. The frequency spacing, instead of the wavelength spacing, is used to estimate the tuning range of the proposed DW laser. Figure 5(b) demonstrates the PI characteristics of the proposed DW laser working at the 1550 nm band. The total current is the sum of the three-section structure, and the current of each section is equal to each other approximately. With the current of SOA (ISOA) and temperature at 40 mA and 25°C, the threshold currents and the maximum power of the DW laser are at around 32.4 mA and 24.33 mW.

By varying the injected current of each section, we first have a glance at the wavelength tunable range on the DW laser of the C band with high Δn0, as shown in Fig. 5(a). When the injected current ISOA, Ifront, Imid, and Irear are all 40 mA, the mode wavelength spacing of the DW laser with Bragg spacing at 20 GHz is 0.6375 nm (79.7 GHz). The spectra are consistent well with the numerical analysis results, verifying the effectiveness of the principle elaborated above. The red and blue curves sketch the peak wavelength (λ1 and λ2) of the two modes by recording the local maxima, and the difference value of the two modes is the vital performance parameter for the wavelength tunable DW laser. In Fig. 6(a), the frequency spacing narrows from 78.00 to 75.00 GHz with ISOA varying from 0 to 60 mA, which indicates the limited tuning range of the SOA-based scheme. Distinguishing with the SOA-based scheme, the directly injected current in the rear and front sections of the DW laser will cause a significant impact on the working wavelength, enhancing the tunable range of the DW laser. As shown in Fig. 6(b), the λ1 and λ2 increase simultaneously, but the thermal effect is stronger in the front section, which leads to the decrease of λ2−λ1, i.e., λ1 shifts faster than λ2. With Irear varying from 30 to 90 mA, the DW laser works stably in a dual-mode state, and the tuning frequency spacing ranges from 69.50 to 116.25 GHz. When Irear is over 81 mA, the competition between the two modes becomes intense, the mode of λ1 disappears, and the DW laser works in a single-mode state. Figure 6(c) illustrates that the tuning range of the DW laser covers from 59.50 to 80.75 GHz by varying Ifront from 30 to 79 mA.

Figure 7 illustrates the spectra of the DW laser with various Ifront and Irear, where two side modes are produced by four-wave mixing. With Irear increasing from 30 to 85 mA, the frequency spacing between the two modes enlarges, and the DW laser works at a single-mode state due to the mode competition finally. By varying Ifront from 30 to 80 mA, the frequency spacing between the two modes can be narrowed, and the laser works in a chaotic state with the optical injection enhancement. The frequency spacing can be extended to 116.25 GHz and narrowed to 59.50 GHz by setting Ifront and Irear at 79 and 80 mA, respectively. In brief, the proposed tunable DW laser with high Δn0 ranges from 59.50 to 116.25 GHz.

To further verify the impact of Δn0 on the initial wavelength spacing, the DW laser with the Bragg spacing at 20 GHz, the depth of basic grating at 25 nm, and each section at 750 µm is fabricated, working at the O band, whose epilayer wafer is same to Ref. [25]. The corresponding mapping diagram about the spectra of the DW laser and the wavelength shift by varying ISOA, Ifront, and Irear are shown in Fig. 8. The tuning range of varying the injected current in the SOA, rear section, and front section are from 25.70 to 19.89 GHz, 19.47 to 38.31 GHz, and 18.85 to 21.54 GHz, respectively. By sweeping ISOA, Ifront, and Irear, the total tuning range of the DW laser with low Δn0 is from 18.85 to 38.31 GHz. Consistent with the above simulation, the narrow wavelength spacing (∼20GHz) can be achieved by selecting a low Δn0, which is overlooked in previous research for the DW laser. Among them, mode hopping occurs due to the mode competition, when the current Ifront exceeds 115 mA. Additionally, the low Δn0 and long cavities lead to the deterioration of side-mode suppression. Due to the resolution limitation of the OSA and the high requirements for the measurement of the narrow wavelength spacing, there is little fluctuation in the lasing wavelength curve compared with the results in the C band.

In summary, we propose and experimentally demonstrate the tunable wavelength-spacing and monolithic DW DFB laser based on a CSG. The CSG with two equivalent π phase shifts is arranged for the DW laser, which consists of front, mid, and rear sections. By utilizing the transfer matrix method, we investigate the relationship between the mode spacing and Bragg spacing for the DW laser with different index modulation amplitudes Δn0. Consistent with numerical simulation, the experimental results indicate that the relatively high Δn0 limits the further narrowing of the wavelength spacing by simply decreasing the Bragg spacing, which was overlooked previously. The length of the DW laser becomes compact due to the existence of one common cavity. With the contribution of a three-section structure, the tuning range of the wavelength spacing is significantly extended by varying injected currents in each part. Compared with the poor performance of the typical SOA-based tuning method (only ∼5GHz), the proposed DW laser achieves a tunable range of up to 56 GHz. Based on the numerical simulation and experimental validation, the proposed scheme not only clarifies the mechanism in designing initial wavelength spacing but also provides a monolithic DW laser with a wide and flexible tuning range for various applications of fiber communication and optical sensing.