Laser diodes are well-suited for generating short optical pulses in the nanosecond range, making them suitable for various applications such as free-space communication, spectroscopy, metrology, material processing, and light detection and ranging (LiDAR) systems. Improving the signal-to-noise ratio is crucial in LiDAR applications, particularly for the detection of objects at long distances under varying atmospheric conditions. One effective approach to achieve this is to utilize wavelength-stabilized lasers combined with narrowband optical filters. This combination helps enhance the detection capability by reducing noise and unwanted background signals, allowing for improved object detection even in challenging atmospheric conditions. DFB lasers are widely used in optical communication systems due to their wavelength stability, low noise, and narrow linewidth. However, achieving high peak powers requires high currents, which necessitates specially tailored driver electronics [1]. To address this issue, a method to reduce the required currents involves growing a stack of several laser diodes separated by tunnel junctions (TJs) on the expense of an increase voltage [2–6]. Furthermore, a significant advancement has been made by epitaxially stacking multiple active regions alternated with TJs in a single vertical waveguide core [7,8]. This layer stack allows a higher-order vertical mode to lase, with its nodes and antinodes positioned at the locations of the TJs and active regions, respectively. As a result, the impact of the large absorption in the highly doped TJs can be minimized ensuring a high slope efficiency. The concept of epitaxially stacking multiple active regions and TJs also enables stabilization of the emission wavelength using an internal Bragg grating [9–11] or the lateral mode using a ridge waveguide [12]. Bragg gratings can be implemented in a cavity either in a distributed Bragg reflector (DBR) or in a distributed feedback (DFB) configuration. In a DBR laser, the Bragg grating placed adjacent to one of the laser facets is electrically unbiased. Thus, it provides no gain but consumes a part of the chip area. Typically, the Bragg grating is placed on the rear facet, necessitating a high reflectivity. Fortunately, higher-order surface gratings exhibit a high modal reflection coefficient over wide parameter ranges if properly designed [13]. In DFB lasers, the Bragg grating extends across the entire length of the electrically driven cavity. Thus, a DFB configuration could potentially conserve a chip area. However, in order to obtain a high external differential efficiency, the reflectivity must be carefully optimized [13], which results in narrow suitable parameter ranges. While the manufacturing costs of DBR lasers are higher, the manufacturing tolerances of DFB lasers are lower. In previous publications [9–11] we reported on DBR broad area (BA) lasers with multiple active regions and TJs. In this paper, we present for the first time results of experimental investigations of DFB BA lasers.

The wafers used for fabrication of the DFB BA lasers were grown in a single metal–organic vapor phase epitaxy (MOVPE). The design of the epitaxial layer structure presented here is the same to that of the DBR lasers reported previously [9–11]. It comprises three single compressively strained InGaAs quantum wells as active regions and two GaAs TJs, which are embedded in an optical confinement layer of Al_0.45Ga_0.55As. This layer is sandwiched between cladding layers of n-Al_0.5Ga_0.5As and p-Al_0.75Ga_0.25As. Due to the fact that under pulse operation the electrical and thermal conductivities do not matter because of strongly reduced self-heating, we chose a relatively high Al content in all layers to maximize the bandgap energy, thereby reducing vertical leakage currents. The placement of the active regions and the TJs is carefully selected to align with the nodes and antinodes, respectively, of the third vertical waveguide mode. The period of the surface grating must be chosen sufficiently large to enable current injection between the etched grooves. Hence, a 40th Bragg-order grating with a period of 5513 nm was implemented. The grooves of the grating were defined by electron beam lithography and transferred into the semiconductor by reactive ion etching. To achieve high external differential efficiency, an etch depth of 1490 nm and a residual layer thickness (distance between the positions of top-most quantum well and bottom of the grooves) of 430 nm were chosen to keep the reflectivity small (<40%). Lateral optical and current confinements are achieved by reactive ion etching trenches into the epitaxial layer structure. The etch depth for the mesa is approximately 5400 nm, ensuring penetration through the bottom TJ (4722 nm deep) but not reaching the bottom quantum well (QW). The p-contact stripes with widths of w = 50 μm and w = 200 μm are defined by opening a deposited insulator (SiN) followed by the evaporation of metals (Ti–Pt–Au) to form an Ohmic contact. After wafer thinning and n-metallization, the wafer is cleaved into individual laser bars. To ensure high mode selectivity and slope efficiency, the facet designated as output facet is anti-reflection coated (residual reflectivity of less than 0.1%), while the other facet is high-reflection coated (95% reflectivity).

For the experimental characterization, the DFB BA lasers are integrated into temperature-controlled modules, as illustrated in Fig. 1. The module contains two electrical pulse drivers with final stages based on GaN transistors, which are well suited for high-power and high-current switching due to their minimal parasitic capacitance and low on-resistance [1,14]. To assemble the laser, it is initially soldered with the epitaxial side (p-side down) onto a CuW submount measuring of 4 mm × 1.5 mm. Subsequently, it is adhesively bonded into a module and wire-bonded to bond pads on the top of the electronic driver board. This assembly process of using multiple bond wires in parallel ensures low and reproducible inductances, thereby minimizing electrical losses and establishes a reliable thermal connection. The electrical pulse driver allows for precise control of the pulse width, frequency, and amplitude. The integrated gain-switched lasers with electronic drivers can generate pulse currents of up to 400 A and produce high pulse power. They are capable of generating optical pulses with durations ranging from as low as 3 ns to more than 10 ns. The repetition frequencies can be adjusted within the range between 1 kHz and 150 kHz.

 

Fig. 1. Nanosecond pulsed electronic driver integrated with a soldered and bonded 1 mm long DFB BA laser.

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First, the temporal pulse shape is investigated by coupling the optical pulses emitted by the laser into an integrating sphere. The optical signal is then converted into an electrical signal using a fast photodiode and subsequently analyzed with a real-time oscilloscope, revealing a full-width half-maximum (FWHM) of approximately 10 ns for lasers under test at a pulse current of 15 A, as shown in Fig. 2. The rise time, which corresponds to the transition from 10% to 90% intensity, is measured to be approximately 6 ns. The fall time corresponding to the transition from 90% to 10% intensity is approximately 5 ns.

 

Fig. 2. Optical pulse shape of 10 ns for BA DFB lasers with stripe widths of 50 µm and 200 µm at a pulse current of 15 A and a repetition frequency of 10 kHz.

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Second, the light–current characteristics of the lasers are investigated. The pulse power and current are determined by calculating the average optical power, pulse width, and repetition frequency, as described in Ref. [10]. Figure 3 compares the light–current characteristics of both DFB BA lasers under investigation.

 

Fig. 3. Comparison of light–current characteristics of DFB lasers with stripe widths of 50 µm and 200 µm at 10 ns pulse width, 10 kHz, and 25°C.

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For a stripe width of 200 µm, the pulse power reaches a maximum value of 100 W at a pulse current of 63 A. In contrast, the 50 µm wide laser achieves a pulse power of 26 W and is operated only up to a pulse current of 21 A. Thus, the maximum achievable pulse power increases with stripe width. In comparison, this finding emphasizes that a 1 mm long DFB BA laser with three active regions and a stripe width of 200 µm exhibits higher pulse power when compared to a 6 mm long DBR BA laser with a single active region and a stripe width of 50 µm, as shown in Ref. [14]. Furthermore, the slope of the light–current curve is higher for the 200 µm wide laser in comparison to the laser with the smaller stripe width. Several factors can contribute to a lower slope, such as leakage currents, internal losses, or mirror losses [15,16]. The specific underlying causes of the lower slope observed in Fig. 3 are yet to be determined. It should also be noted that a higher-order Bragg grating introduces additional losses as discussed in Ref. [9], reducing the slope efficiency of a DBR laser compared to a Fabry–Perot (FP) laser. Additionally, the maximum pulse energy measured for the laser with 50 µm stripe width amounts to 0.26 µJ, while for the 200 µm laser, it is approximately 1 µJ.

Next, to validate the successful wavelength stabilization of the laser, the optical spectrum is analyzed at different temperatures (see Fig. 4).

 

Fig. 4. Time-averaged optical spectra of DFB lasers with stripe widths of 50 µm and 200 µm operated at 25°C and 50°C with 10 ns pulses and 100 kHz repetition frequency at 16 A (20 W) and 30 A (54 W).

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The emitted light is collected using an integrating sphere with an attached multi-mode optical fiber and is then injected into an optical spectrum analyzer. To ensure a high signal-to-noise ratio, the measurements are conducted at a repetition frequency of 100 kHz. The optical spectra are measured with a resolution bandwidth of 50 pm. Both DFB lasers provide an optical spectrum with a peak center wavelength of 886 nm at 25°C. The narrow optical spectra with a FWHM of less than 0.24 nm for the 50 µm laser and 0.19 nm for the 200 µm laser show a successful effect of the Bragg gratings integrated into the lasers. For comparison, the spectral widths represent a slight improvement in spectral bandwidth compared to a 6 mm long DBR laser with a 50 µm stripe width and only one active region (0.5 nm), as reported in Ref. [14]. However, at a temperature of 50°C, another peak is observed at 907 nm. The grating was originally designed to enforce lasing at a wavelength around 905 nm corresponding to the 40th Bragg order. The shorter wavelength peak at 888 nm is attributed to a reflection through the 41st Bragg order and the position of the gain maximum at the shorter wavelength. Consequently, further optimizations are required to enable lasing at the desired Bragg order. To analyze the temperature dependence of the laser emission, the optical spectrum is measured at two temperatures, 25°C and 50°C. This results in a shift of the peak wavelength by 1.6 nm, and the wavelength shift is determined to be approximately 64 pm/K.

Figure 5 shows the measured near-field intensity profiles in the lateral direction. To determine the lateral beam quality factor (M²), beam profiles are measured at pulse currents of 21 A (at 20 W, 10 ns, 10 kHz) for the laser with stripe width of 50 µm and 64 A (at 100 W, 10 ns, 10 kHz) for the 200 µm laser. The full widths of the near field based on the FWHM value and the second moment of the intensity distribution are 49 µm and 60 µm, respectively, for the 50 µm stripe width laser and 199 µm and 239 µm, respectively, for the 200 µm stripe width laser. The FWHM widths approximately correspond to the stripe widths.

 

Fig. 5. Measured lateral near field at a pulse current of 21 A (26 W) for the 50 µm wide laser and 64 A (100 W) for the 200 µm wide laser.

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In Fig. 6, the lateral and vertical far-field (FF) intensity profiles are presented. The vertical profiles observed in both lasers correspond to the third vertical mode, characterized by three intensity peaks. The second-moment FF angles are 54° vertically (fast-axis) and 12° laterally (slow-axis) for both lasers. The observed vertical profiles are in good agreement with the simulated ones, as referenced in [7].

 

Fig. 6. The measured distribution of the vertical FF intensity of 50 µm at 21 A (26 W) (black lines) and 200 µm at 64 A (100 W) (red lines) wide lasers. Vertical FFs (solid lines) and lateral FFs (dashed lines).

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Along both the lateral and vertical directions, the beam quality factor M² is determined using the second moments of near and far fields. The lateral beam quality factor is determined as M²_lat = 11.8, while the vertical factor is determined as M²_vert= 3.7 for the 50 µm wide laser. In comparison, a 6 mm long DBR laser with only one active region and a stripe width of 50 µm was reported to have a lateral beam quality factor of M²_lat = 9 in Ref. [14]. However, for the 200 µm wide laser, the lateral and vertical beam quality factors are approximately M²_lat = 41.4 and M²_vert = 3.9, respectively. These results indicate that the beam quality factor (M²) increases with the stripe width, caused by an increasing number of lateral lasing modes, guided due to the etched trenches having a lower effective index than the mesa.

Lastly, the brightness [17]

In summary, the combination of wavelength-stabilized lasers with multiple active regions and tunnel junctions offers significant advantages, particularly in applications where low currents, high output power, and reasonable beam quality are crucial. Distributed feedback broad area lasers with a cavity length of 1 mm, operating around 886 nm and driven by 10-ns-long pulses, achieved output powers of up to 100 W, a lateral beam quality factor of 41.4, and a brightness of 81 MW/cm²sr at a pulse current of 63 A. In contrast, the lasers with a stripe width of 50 µm have an improved beam quality factor of 11.8 but reach only an output power of 26 W. Due to the Bragg grating integrated into the active cavity, the spectral bandwidth of the laser emission measures only about 0.2 nm. The wavelength shift with temperature is similar to the one of the DBR lasers investigated previously, with a value of only 64 pm/K. These properties make these lasers suitable for use in automotive LiDAR systems, where their narrow optical spectrum, wavelength stability, and high side-mode suppression ratio are advantageous when utilizing narrowband optical filters. These successful results open up new possibilities for the application of DFB lasers in LiDAR systems.