1. Introduction

Photonic integrated circuit (PIC) is a device that integrates multiple photonic functions on a small chip and allows for accurate dimension control and massive production [1–3]. Similar to electronic integrated circuits, PICs can significantly reduce the system cost, size, weight, and operation power (CSWaP). The PIC technology has transformed many optical technologies which traditionally rely on tabletop systems and bulky components, such as optical interconnects, nonlinear optics, and quantum photonics, into a chip-scale platform [4–7]. This device and system miniaturization has successfully led to a wide range of practical applications in computing, sensing, spectroscopy, and communication. However, the traditional passive PIC platform lacks efficient gain media or source components.

Hybrid integration, which combines different material systems on a single chip, is a promising candidate to overcome the limitation of the traditional passive PIC platform [8,9]. By integrating III/V semiconductor chips with passive PICs, it is possible to enhance the functionality of PICs and create novel miniaturized laser systems. III/V semiconductor gain chips are generally favored for hybrid photonic integration due to their small footprint, low cost, and high electrical-to-optical conversion efficiency. There are various approaches to obtain hybrid integration, including direct epitaxy growth, edge coupling, and wafer/die bonding [10–13]. Hybrid integration by edge coupling provides a simple solution and avoids the material mismatch problem because the active chips and passive chips can be fabricated, optimized, thermal controlled independently [14]. In addition, by using silicon nitride material in this hybrid platform, the transparency window can be extended below 1 um, which makes it possible to combine visible and near infrared laser sources through the III/V-Si$_3$N$_4$ hybrid integration [15].

It has become increasingly important to scale up on-chip diode laser power for many emerging applications. Simply coupling a single mode diode laser to passive PICs usually cannot provide the desired power, coherence, and system compactness. Beam combining through hybrid photonic integration can overcome these challenges. There are two different types of beam combining techniques that can enhance the laser system power and brightness: coherent beam combining (CBC) and wavelength beam combining (WBC) [16–23]. Traditional beam combining systems are based on free space or fiber optical components, so they are usually bulky, complex, and not robust.

In this manuscript, we will show that passive optical components in the traditional beam combining systems can be completely replaced by different PICs and demonstrate hybrid integration of the PICs with III/V gain chips for on-chip power scaling. In addition, since the PICs can be fabricated by use of dielectric materials with very high optical damage threshold and low loss, it is feasible to create the high-power compatible PICs. We will first discuss the operation principle of the traditional beam combining systems and explain how we could use different PICs to replace the free space optical components in these systems. We achieve CBC by using a chip-scale tree-like coherent coupler array to cross-couple different lasers and realize WBC by replacing the diffraction grating with arrayed waveguide gratings (AWGs). We then discuss various methods to expand the waveguide mode size so that the PICs in the beam combining systems can handle high optical power. Finally, we show two proof of concept experiments in which the CBC and WBC systems are realized with watt-level on-chip power by use of III/V- Si$_3$N$_4$ hybrid integration. PIC based beam combining greatly reduces the CSWaP of the direct diode laser system and makes it more practical for many applications, such as integrated nonlinear optics, remote sensing, free space communication, infrared countermeasure and light detection and ranging (LIDAR).

2. Coherent beam combining and wavelength beam combining

For a single laser, high power operation often results in thermooptic/optical nonlinear effects that destroy the desired spatial coherence and degrade the output beam quality. Beam combining is developed to overcome this limitation to obtain higher power and brightness by combining multiple laser beams. Conventional beam combining systems are based on bulky free space or fiber optical components. There are two different types of beam combining systems: coherent beam combining (CBC) and wavelength beam combining (WBC). In CBC systems, all the lasers operate at the same wavelengths and are phase-locked [24–26]. CBC systems require accurate modal and/or phase control. While in WBC systems, different lasers, which operate at different wavelengths, are combined into a single beam [27–31]. WBC systems can be implemented either by the open loop configuration, where the wavelength of each laser in the array is pre-determined, or the closed loop configuration, where the wavelength control and beam combining are simultaneously obtained. The main advantage of WBC is that it does not require accurate phase control and can scale to many array elements.

Depending on if external optical components are needed, coherent beam combining of diode laser arrays is divided into two categories. As shown in Fig. 1(a), the first category can be monolithically implemented, including: i) evanescently coupled laser arrays, where multiple closely spaced lasers are coupled to form a super transverse mode; ii) chirped and Y-coupled laser arrays; and iii) leaky wave coupled (anti-guided) laser arrays [32–34]. The main problem with these previously demonstrated coherent arrays is that they provide poor modal discrimination and have limited scalability. The second category, which requires external optical components, includes: i) master oscillator power amplifier (MOPA) arrays (left); ii) Talbot cavity laser arrays (middle); iii) self-Fourier cavity laser arrays (right), as shown in Fig. 1(b) [35–37]. These systems usually provide better performances. But since external components and/or phase control are needed, these systems are complex and expensive.

 

Fig. 1. (a) Coherent beam combining systems based on monolithic diode laser arrays (b) Coherent beam combining systems based on external cavities.

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Figure 2 shows the open-loop and close-loop WBC configurations. For the open-loop WBC shown in Fig. 2(a), the wavelengths of different lasers are pre-set so that they can be combined by use of several spectral filters such as volume Bragg gratings or thin film filters. For the close-loop WBC shown in Fig. 2(b), the key components include a reflective single-mode diode amplifier array, a transform lens, a diffraction grating, and a partially reflective output coupler. The operation of this WBC system can be understood as a grating spectrometer run in reverse. When a collimated multiwavelength optical beam propagating in the normal direction of the output mirror is incident on the diffraction grating, different wavelengths will be diffracted into different directions due to the dispersive nature of the grating. The transform lens focuses the beams from different propagation directions into different locations on its focal plane. Therefore, the combined system translates different wavelength components into different locations. Because the gain elements are reflective amplifiers, they must obtain the feedback from the output coupler to reach the lasing threshold. In this way, the array element will self-adaptively select its operation wavelength since the output mirror forces a coaxial propagation and thus combines the beams from different array elements.

 

Fig. 2. (a) Open-loop WBC (b) Close-loop WBC.

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2.1 PIC based coherent beam combining

Passive coherent beam combining systems obtain the mutual coherence through the optical cross-coupling among array elements. The cross-coupling results in common resonator modes (supermodes). Multiple supermodes are often supported and each supermode corresponds to a distinctive amplitude and phase distribution among the array elements. We need to select a single supermode as the lasing mode to obtain coherent beam combining. This means that the modal loss difference between the lowest-loss supermode and other supermodes, i.e., the modal discrimination, needs to be sufficiently large. For the traditional laser array shown in Fig. 1(a), all the supermodes have similar modal losses so that there is little modal discrimination. We can improve the coherent array modal discrimination and performance by using external cavities, such as the Talbot cavity and self-Fourier cavity. Different from the traditional laser arrays, the gain elements here are not coupled side by side. As shown in Fig. 1(b), the optical cross-coupling is obtained through the controlled diffraction in the external cavity, and it is carefully designed to increase the modal discrimination. The output beam from each individual gain element first diffracts in the external cavity and is then reflected and fed back to all the array elements. The relation between the initial output field distribution of the gain elements and the fed-back field can be described by a coupling matrix $K$:

 

Fig. 3. Converting a conventional free-space external-cavity based CBC array (a) to a hybrid integrated CBC array (b) on a single chip. We use the on-chip coupler array to replace the diffractive external cavity in free space.

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The composite mode of the coupled cavity is found by solving the eigenvalue problem of the transfer matrix of the CBC cavity [38]. Our configuration forces all the lasers to constructively interfere at the nonzero feedback port and destructively interfere at the zero feedback ports. There is only one supermode supported since the optical feedback is provided at a single output port and evenly distributed back to the array. This feedback mechanism results in uniform cross coupling between any two lasers in the array and leads to the maximum modal discrimination for coherent beam combining. For the 3 dB coupler design, multiple mode interferometers (MMI) or directional couplers (DC) are commonly used. But the MMI may cause unwanted back reflection to the laser array. Here we choose the DC as the 3 dB coupler in the hybrid integrated CBC system.

2.2 PIC based wavelength beam combining

We create a PIC based WBC system by replacing the diffraction grating in free space WBC systems with an arrayed waveguide grating (AWG). The AWG WBC system realizes the beam combining and wavelength control of the laser array elements at the same time. In addition, it can increase the spectrum utilization efficiency of the WBC system to improve the scalability. As shown in Fig. 4, the functions of the transform lens, diffraction grating, and output coupler in a free space WBC system are all integrated into the passive PIC chip and the diode lasers are directly coupled to the input waveguides of the AWG through mode matching. The AWG provides the same function as the diffraction grating, the input waveguide array provides the same function as the transform lens, and the output waveguide array with coatings or DBRs provides the same function as the output coupler. Through hybrid integration, the laser can only obtain the feedback from the output waveguide of the AWG, either through a DBR reflector or a straight cleaved facet, to reach the threshold. Because of the spectral filtering property of the AWG, different laser array elements operate at different wavelengths, corresponding to different passbands of the AWG. At the output waveguide, all the optical power from different laser array elements will be combined, just like the free space WBC system where the output coupler forces a coaxial propagation of different beams. Since there is no cross-coupling between the laser array elements, every wavelength channel can be operated (turned on/off) independently, which provides spectral control flexibility for the integrated WBC system as a multi-wavelength source. Due to the routing function of the waveguide, the specific location of each laser array element decouples from its operation wavelength, leading to the large array scalability and high spectrum utilization efficiency.

 

Fig. 4. Converting a conventional free space WBC laser system (a) to a PIC-based WBC laser system (b) on a single chip. We use the chip-scale AWG to replace the diffraction grating in free space.

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AWG is one of the most commercially successful PICs and is mainly used as the wavelength division multiplexer (WDM) for optical fiber communication systems. A chip-scale AWG consists of the input waveguides, two symmetric slab waveguides (free propagation region, FPR) connected by a waveguide array with a fixed length difference $\Delta L$, and the output waveguides. When an optical beam propagating through an input waveguide enters the first FPR, it diffracts in the chip plane since there is no lateral confinement. At the end of the first FPR, the diffracted beam is coupled into the waveguide array and propagates towards the second FPR. If the fixed length difference between two neighboring waveguides is the integer multiple of the center optical wavelength ($\Delta L=m\lambda _{wg}$, $m$ is the grating order), the optical field at the entrance of the second FPR will be the same as that at the end of the first FPR. Due to the symmetry, the beam will be focused into the matched output waveguide after it propagates through the second FPR. If the operation wavelength is different from the center wavelength, the fixed array length difference creates a phase gradient for the beams at the entrance of the second FPR. As a result, the beam will be focused into a different location at the end of the second FPR, i.e., a different output waveguide. Thus, when the AWG is configured as in Fig. 4, each input waveguide selects a different wavelength for beam combining at the output waveguide.

2.3 High power compatible PICs for beam combining

To improve the PIC power handling capability, there are various mode engineering methods to design large-mode-size PICs for high power applications. Here we focus on Si$_3$N$_4$ and Ge doped SiO$_{2}$ as the waveguide core material because they have a large transparency window and show three different waveguide designs in Fig. 5. Since the PICs for beam combining are purely passive, we only need to reduce the material absorption to prevent heat generation. To withstand high optical power and mitigate nonlinear effects, the mode size of the single mode waveguide needs to be expanded for beam combining applications. The first method to expand the mode size is to reduce the index contrast between the core and cladding, which is realized by controlling the germanium doping concentration in SiO$_{2}$ for the glass-on-Silicon waveguide. Here we compare the mode profile for the glass-on-Silicon waveguide and the 300 nm thick Si$_3$N$_4$ waveguide with SiO$_{2}$ cladding. The refractive index contrast between the core and cladding for the glass-on-Silicon platform is around 0.75% or lower [39], which efficiently enlarges the modal area centered in the core region. The modal profiles (fundamental TE mode) of the single mode waveguide for the glass-on-silicon waveguide and 300 nm Si$_3$N$_4$ waveguide at 1 um are shown in Fig. 6(a) and Fig. 6(b), respectively. It is shown that the modal area for the 300 nm Si$_3$N$_4$ waveguide is about 0.41 um$^{2}$, which is about one over seventy of the modal area 28.57 um$^{2}$ for the glass-on-silicon waveguide. This low index contrast technology not only benefits the improvement of the optical damage threshold and the power handling capability but also helps the reduction of waveguide propagation loss. The lowest propagation loss as reported is below 0.05 dB/cm (glass waveguides on Silicon) [39].

The limitation of the glass-on-Silicon waveguide is the large bending radius or the large footprint of the device due to very low index contrast, which prevents from realizing a large scale of integration of the PICs within the chip dimension of centimeters. The large footprint also requires expensive thermal stabilization control. Alternatively, we could choose to minimize the geometric size of the Si$_{3}$N$_{4}$ core to expand the mode by taking advantage of highly accurate nanofabrication processes. The widely used Si$_{3}$N$_{4}$ platform has a waveguide height of around 300 nm. By changing the Si$_{3}$N$_{4}$ layer thickness to 100 nm, we could have a 2 um wide single-mode waveguide, as shown in Fig. 6(c). The modal area is about 1.04 um$^{2}$ for the 100 nm Si$_{3}$N$_{4}$ waveguide, much bigger than the 300 nm Si$_{3}$N$_{4}$ waveguide. The confinement factor for the 300 nm and 100 nm Si$_{3}$N$_{4}$ waveguides is 75.40% and 26.11%, respectively. For the 300 nm Si$_{3}$N$_{4}$ platform, the single mode waveguide we use has a modal field about 0.437 um$^2$, while the modal field is about 1.163 um$^2$ within the 100 nm Si$_{3}$N$_{4}$ platform. The increased mode field area improves the power handling capability of the 100 nm Si3N4 platform. This field distribution also leads to a small propagation loss for the waveguide with the 100 nm core thickness. The following hybrid integrated CBC and WBC systems are all based on the 100 nm Si$_{3}$N$_{4}$ waveguide.

3. Results and discussions

We first demonstrate GaAs-Si$_{3}$N$_{4}$ chip-scale CBC through hybrid integration of four SOAs and a 1X4 DC array, as illustrated in Fig. 7. We use three 1x2 DCs to realize a 1x4 DC array. We utilize two groups of SOAs with 1x4 DCs to create the whole hybrid CBC setup. Each group contains two identical SOAs on an independent III/V chip. The injected current to each chip is evenly split for the two SOAs because they share a single metal contact. Each III/V chip is mounted on a stage with an individual current source and an active water-cooling system, which helps to maintain the performance of multiple SOAs. The TEC temperature is 18 $\circ$ for all the SOAs. The direct coupling between the SOAs with the input waveguides is shown in the inset of Fig. 7 (group 1 SOAs in red frame and group 2 SOAs in blue frame). To obtain the CBC operation for this hybrid laser system, we must turn on all the SOAs at the same time to avoid the loss at the zero-feedback ports of the DC array. When all the SOAs are coherently combined, they will destructively interfere at the zero-feedback ports and constructively interfere at the port with the cleaved facet. Experimentally, we use an IR camera to monitor the zero-feedback ports so that the CBC condition is maintained. The measurement results of the hybrid CBC system are illustrated in Fig. 8. It includes the LI curves of the individual SOA chip (only group 1 is turned on in the blue line) and the coherently combined lasers (both groups are turned on). The current shown in Fig. 8(a) is the driving current for one III/V chip with two SOAs, which means that the total injection current to the whole hybrid CBC system should be doubled and the injection current to one SOA should be halved. When four SOAs are coherently combined, the threshold current of the hybrid CBC laser system is about 260 mA (per SOA group). It should be pointed out that the threshold current value itself does not indicate the loss of hybrid integration since it strongly depends on the SOA waveguide dimension and material system. We maintain the maximum injection current about 800 mA for each chip to avoid rollover in the LI curve and active chip damage. The output power of the single chip-based hybrid laser and the coherently combined lasers is about 70 mW and 430 mW, respectively, when the injection current is about 800 mA. This result indicates that there is a 7.8 dB loss due to the zero-feedback ports if we only turn on one group of SOAs. It is clear that the CBC of all four SOAs can avoid the extra cavity loss induced by the coupler array since it leads to the destructive interference at the lossy ports. When the injected current is about 620 mA, the lasing spectrum of the coherently combined lasers is shown in Fig. 8(b), which shows multiple longitude modes inside the hybrid laser cavity. If a single longitudinal mode is desired, a grating or ring filter can be used to select a single longitudinal mode [40]. Thus, it is still possible to obtain the common cavity mode for CBC if we increase the number of SOAs in our hybrid system [41].

We experimentally demonstrate the AWG based WBC by using the GaAs-Si$_{3}$N$_{4}$ platform, as illustrated in Fig. 9. Here we utilize 4 SOAs to improve the total output power of the hybrid WBC system. Similar to the CBC configuration, each active chip includes two same SOAs with a separation about 400 um. In our experiment, the AWG central wavelength is designed near 1 um and the FSR is about 9.6 nm with 8 input channels. The separation of the input channel waveguide of the AWG is 200 um to enable the alignment with the two SOAs on one chip at the same time. For the AWG layout, we have channel 1 to channel 4 along the bottom side, and channel 5 to channel 8 along the left side. The direct chip to chip coupling between the SOA array and AWG chip is illustrated in insets of Fig. 9. Due to the waveguide spacing difference (the SOA waveguide separation is 400 um and the AWG input waveguide separation is 200 um), we show the combined LI and spectrum results for the channel 1, 3 /5, 7 and channel 2, 4 /6, 8 for simplicity.

 

Fig. 5. (a) Glass-on-silicon waveguide with Ge doped SiO$_{2}$ core and SiO$_{2}$ cladding (b) Standard Si$_{3}$N$_{4}$ waveguide with SiO$_{2}$ cladding (300 nm thick core) (c) Thin core (100 nm) Si$_{3}$N$_{4}$ waveguide.

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Fig. 6. Modal profiles of (a) the glass-on-silicon waveguide with the 0.75% index contrast ratio, (b) the 300 nm Si$_3$N$_4$ waveguide (SiO$_{2}$ cladding), and (c) the 100 nm Si$_3$N$_4$ waveguide. The waveguide core sizes for three different waveguides are 6x6 um$^{2}$, 1.1x0.3 um$^{2}$, and 2.0x0.1 um$^{2}$, respectively.

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Fig. 7. The schematic of the on-chip coherent beam combining with PICs. The yellow parts are the SOAs. The light green parts are the tapers, waveguides, and directional coupler array. The blue frame inset shows the edge coupling between the one group (group 1) of two SOAs and the DC. The red frame inset shows the edge coupling between the other group (group 2) of two SOAs and the DC. The actual DCs array chip size is about 5x6 mm$^2$.

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Fig. 8. (a) The LI curve of the CBC system. The red line is the result with all 4 SOAs. The blue line is the result with two SOAs (group 1). (c) The normalized output spectrum of the coherent beam combining system. For the LI curve of the CBC with all 4 SOAs, the actual current of the whole hybrid CBC system should be doubled since there are two groups of SOA array.

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Fig. 9. The schematic of the AWG-based hybrid WBC system. The red frame inset shows the edge alignment of two SOAs (located at bottom) with the input waveguide (channel 1,3 and channel 2,4) of the AWG chip. The blue frame inset shows the edge alignment of two SOAs (located at side) with the input waveguide (channel 5,7 and channel 6,8) of the AWG chip. The actual AWG chip size is about 6x8 mm$^2$.

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The experiment results of the hybrid AWG-based WBC system are plotted in Fig. 10 and Fig. 11. The red line (CH1,3,5,7 SUM) in Fig. 10(a) is the output power sum of the single chip-based hybrid laser (channel 1, 3 + channel 5, 7) when each individual group SOAs are turned on. Since two SOAs on one chip share the same metal contact, we could only turn on/off them as a group. The blue line (CH1,3,5,7 WBC) is the wavelength combined LI when all the four SOAs (two chips) are turned on at the same time. The combined result shows good agreement with the summed result in Fig. 10(a). The current shown in Fig. 10(a) is the driving current for one active chip with two SOAs, similar to the CBC setup. The threshold of hybrid WBC laser system with four SOAs is about 210 mA (per SOA group). The slope efficiency of hybrid WBC laser system is about 0.68 W/A (total injection current should be doubled for the combined system), much higher than previously demonstrated hybrid lasers. The total WBC on-chip laser power is over 1 W when the injection current for each active chip is about 850 mA. The alignment between three chips (one passive chip with two active gain chips) with multiple waveguides introduces a small coupling loss, which results in the difference between the summed LI result and the wavelength combined LI result. All the LI curves show rollover due to the accumulated heat inside the SOA region when the driving current is over 900 mA. This rollover performance can be improved by use of advanced semiconductor gain chip structure design or bonding technique (p-side down bonding) in the future. In the spectral domain, the black dashed line (WBC) in Fig. 11(b) refers to the spectrum from the AWG output when all the 4 SOAs are turned on simultaneously, which perfectly overlaps with the blue line (channel 1,3) and purple line (channel 5,7) illustrated in Fig. 11(a). This result proves that the AWG-based hybrid integration successfully combines the outputs of different lasers in different wavelength bands. For the wavelength combined LI and spectrum results from channel 2, 4/ 6, 8, we can obtain the similar conclusion from Fig. 10(b), Fig. 11(c) and Fig. 11(d). The reason why we have 5 (or more) spectral lines in Fig. 11 is that the AWG has a FSR, which means the fifth spectral line comes from the next adjacent channel inside the neighboring FSR. The FSR can be increased to eliminate extra spectra lines. As for the crosstalk, we can suppress it by increasing the number of waveguides used in the array and optimizing the fabrication processing to reduce the sidewall roughness of the waveguide. An AWG with a larger FSR and less crosstalk will certainly improve the WBC laser system performance.

 

Fig. 10. LI curves (a) of the wavelength combining result with AWG channel 1, 3 and channel 5, 7. LI curves (b) of the wavelength combining result with channel 2, 4 and channel 6, 8. The currents shown in (a) and (b) are the currents injected into each SOA chip. The total current should be doubled since there are two SOAs chips in the hybrid WBC system.

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Fig. 11. Lasing spectrum (a) when each SOAs group (channel 1, 3 and channel 5, 7) are turned on independently. Wavelength combining lasing spectrum (b) of the wavelength combining result with AWG channel 1, 3 and channel 5, 7. The injection current for both SOA group 1 and SOA group 2 is bout 320 mA when the optical spectra are obtained. Lasing spectrum (c) when each SOAs group (channel 2, 4 and channel 6, 8) are turned on independently. Wavelength combining lasing spectrum (d) of the wavelength combining result with AWG channel 2, 4 and channel 6, 8. The injection current for both SOA group 1 and SOA group 2 is about 330 mA when the optical spectra are obtained.

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Due to the imperfect power splitting ratio (ideally it should be 0.25:0.25:0.25:0.25) of the 1x4 DC array, the output power of the CBC system is lower than the WBC system. For the passive CBC technique that we used here, the efficient combining relies on the complete destructive interference at lossy ports. When the power splitting ratio is not ideal, the amplitude errors in each channel lead to incomplete destructive interference and total power loss. The measured 1x4 DC array is about 0.235:0.197:0.175:0.167 which leads to a low combining efficiency. On the contrary, the WBC system allows a higher fabrication tolerance for the AWG compared to the DCs array for the CBC system. Each wavelength channel inside the AWG is independent, which means the total output of the WBC is just the sum of all combined channels. For both the hybrid integrated CBC and WBC systems, we demonstrate efficient direct chip to chip coupling with more than one SOA in each gain chip. This result shows that we can align multiple waveguides simultaneously when we couple one passive chip with one active chip through hybrid integration. In our current demonstration, the separation between two SOA waveguides on the active chip is 400 um. Theoretically, if we reduce the SOA waveguide separation, it is possible to use more SOAs in one gain chip to scale up the CBC and WBC laser power with hybrid integration.

4. Conclusion

In this paper, we introduce the concept of the PIC-based beam combining with hybrid integration to reduce the SWaP of the diode laser combining system. We experimentally demonstrate the PIC-based CBC and WBC systems and obtain over 1 W on-chip laser power through the hybrid integration of four RSOAs with passive PICs. In the CBC demonstration, we utilize the asymmetric directional coupler array which introduces the asymmetric loss to select the super mode with the lowest loss inside the hybrid laser cavity. In the WBC experiment, we replace the bulky free-space components with an on-chip AWG to realize multiple wavelength channel lasers and beam combining. PIC-based CBC and WBC systems in the hybrid integrated III/V-Si$_{3}$N$_{4}$ platform are essential for the power scaling and SWaP reduction of future high power, high brightness direct diode laser systems and will enable a wide range of emerging applications in research and industry.