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Single mode operation of broad-area diode lasers, which is the key to obtain high power, high brightness sources, is difficult due to highly nonlinear materials and strong coupling between gain and index. Conventional broad-area lasers usually operate with multiple modes and have poor beam quality. Laser bars usually consist of incoherently combined broad-area single emitters placed side by side. In this article, we have demonstrated a novel integrated laser architecture in which Bragg diffraction is used to realize simultaneous modal control and coherent combining of broad-area diode lasers. Our experimental results show that two 100μm wide, 1.3mm long InP broad-area lasers provide near-diffraction-limited output beam and are coherently combined at the same time without any external optical components. Furthermore, our design can be expanded to a coherently combined broad-area laser array that turns a laser bar into a coherent single mode laser with diffraction-limited beam quality.
© 2012 Optical Society of America
1. Introduction
High Power operation of semiconductor diode lasers requires the large emitting aperture to facilitate heat dissipation and overcome catastrophic optical damage (COD), and high brightness operation requires the single (waveguide) mode to generate a diffraction-limited, single-lobe far field. However, commonly used wide-stripe semiconductor lasers can not satisfy these requirements simultaneously, because the weak modal control based on index guiding is not robust against the index perturbation induced by current injection and nonlinear effects. Several approaches are proposed to accomplish both requirements, i.e., the single mode operation of broad-area diode lasers. One is based on unstable resonators, e.g., curved mirror or tapered diode lasers [1–5]. Through diffractive coupling between two mirror facets, these lasers can provide the large modal discrimination to obtain high power, near diffraction-limited output. Another approach proposed in recent years is slab-coupled optical waveguide lasers (SCOWL) [6–9]. In these structures, the optical mode is confined away from the active region, resulting in a small confinement factor and relatively large single-mode spot size. These designs can be incoherently combined using the wavelength beam combining to obtain higher output power with almost the same brightness as the single emitter [10–12]. It can satisfy the power requirement of many applications where the spectral brightness is not critical. However, these designs cannot be directly coupled as coherent laser arrays to obtain higher brightness without external cavities or phase control [13].
Depending on whether external components are needed, coherently combined laser arrays can be divided into two categories. The first category can be monolithically implemented. Several examples are evanescently coupled laser arrays [14, 15], chirped and Y-coupled laser arrays [16–18] and leaky wave coupled (anti-guided) laser arrays [19, 20]. These structures are only compatible with narrow stripe index-guided single-mode lasers or gain-guided lasers. Thus, the total width of conventional coherent diode laser arrays is limited less than a few hundred microns. The second category requires external cavities or phase control, including externally injection-locked laser arrays [21,22], Talbot cavity laser arrays [23,24], Self-Fourier cavity laser arrays [25] and master oscillator power amplifier (MOPA) arrays [26, 27]. Since external cavities and/or accurate phase control are needed, these systems are usually complex, bulky and not robust [10].
In this article, we demonstrate monolithic coherent beam combining of angled-grating board-area diode lasers. The angled-grating broad-area laser (α-DFB laser) [28–30] is used as the building element, where the grating and angled geometry provide the single waveguide mode operation and strong modal discrimination by filtering out other spatial modes. We use Bragg diffraction to directly couple two angled-grating broad-area lasers by overlapping their output facets (see Fig. 1). The overlapped region forms a triangular photonic crystal coupler that coherently combines the two lasers. In-phase operation is preferred since optical gain is provided at the coupling region. We fabricated and characterized the proposed laser device and the measured near field and far field profiles prove that our approach can obtain near-diffraction-limited beam quality and coherent beam combining at the same time for broad-area diode lasers. This work constitutes an important step toward a bar-scale single-mode laser by expanding our design into a coherently coupled laser array on a standard laser bar.
Fig. 1 Schematic of a coherently combined angled-grating laser. (a) L and W are the length and width of a single emitter, respectively. θ is the tilt angle of the grating. (b) The cross-section structure of a single emitter. (c) Planar geometry of the combined angled-grating laser. Two coherently combined emitters (the output from two legs in the coupled structure) constructively interfere in the far field.
2. Laser design and fabrication
Figure 1 shows a schematic of the coherently combined angled-grating laser. Our laser device is fabricated in an InP-based multiple quantum well (MQW) epitaxy wafer which is designed to emitter light around 1552nm. The epitaxial structure is shown in Table 1. The combined laser cavity consists of two sets of angled-gratings that tilt to the opposite directions with the same angle. The overlap area of the two gratings defines a two dimensional coupling region. The phase locking of two emitters is obtained by the wave coupling through Bragg diffraction in this overlap region. The resonance wavelength is chosen to be around 1550nm. To effectively couple two emitters and reduce modal loss in gratings [31, 32], the tilt angle θ is set to be 10°. Accordingly, the grating period can be calculated to be 1.368μm. The dimensions of a single emitter are 1.3mm × 130μm (L × W). The etch depth of 1.0μm is chosen to obtain a grating coupling coefficient around 0.1/μm. Light is confined by the gratings transversely and by total internal reflection vertically.
Table 1. Specifications of the InP-Based Epitaxy Wafer
The fabrication process consists of a series of steps of lithography, etching, planarization and metallization. First, a SiO2 layer is deposited by PECVD as a hard mask. Then the grating structure is defined by ebeam lithography. After two steps of dry etching, the gratings are transferred to the epiwafer. Next, the structure is planarized by spinning a layer of BCB and then it is etched back until the epiwafer surface exposes. After a SiO2 insulation layer is deposited and a contact window is opened, the p-side metal contact is deposited. Then the whole chip is thinned and n-side metal contact is deposited. After the chip is cleaved to the desired length, the laser diode is mounted and wired on a c-mount for measurement. Figure 2 shows the scanning electron microscope (SEM) pictures of the gratings and the packaged laser diode.
Fig. 2 Scanning electron microscope images of the combined angled-grating laser. (a) InP grating etching profile. (b) Device cross section after planarization and metallization. (c) Two dimensional coupling region. (d) Top view of the packaged laser diode.
3. Simultaneous modal control and coherent combining through Bragg diffraction
The cavity mode in a single angled-grating emitter can be obtained by solving the 2D coupled wave equations [33–35]. The calculated mode is a snake-like zigzag mode which consists of two planewave-like components in resonance with the grating, R1 and R2, as shown in Fig. 3(a). The angles between R1/R2 and the grating direction are both equal to θ, the grating tilt angle. The wavevectors of R1, R2 and the grating satisfy the resonance condition: k⃗R1 + k⃗G = k⃗R2, as shown in the inset of Fig. 3(a). The propagation direction of the R1 component is perpendicular to the facet and that of R2 is tilted. When R1 is reflected by the facet, it will be fed back to the cavity; but for R2, it will be lost. Therefore, once used as the laser cavity, the angled-grating resonator will self-adaptively select the cavity mode with the maximum R1 and the minimum R2 component at the facets. This mode profile is shown in Fig. 3(b) which is simulated by FDTD method. Through the large modal discrimination provided by the grating and strong spatial filtering provided by the angle geometry, angled-grating broad-area (>100μm) diode lasers can obtain stable single mode operation [28, 29, 34].
Fig. 3 Wave coupling and cavity modes in the single emitter and coupled emitter. (a) A single angled-grating emitter. R1 and R2 are two planewave-like components resonate with the grating. The phase matching condition between k-vectors is shown in the inset. (b) FDTD simulation result of a single angled-grating resonator. The solid arrows represent the R1 component and the dashed arrows represent the R2 component. (c) An on-chip combined angled-grating laser. Arrows in blue represent wave components in the left grating, while arrows in red represent wave components in the right grating. The inset shows the coupling between different wavevectors through the grating. (d) FDTD simulation result of a combined angled-grating resonator.
Coherent combining of two symmetrical angled-grating emitters that tilt to the opposite directions is obtained by overlapping them with each other at one facet, as shown in Fig. 3(c). The overlapped region forms a triangular 2D periodic structure (photonic crystal), which enables the cross coupling of two single emitters through Bragg diffraction. Outside the coupling region, the cavity modes are the same as that in a single angled-grating emitter. For simplicity, we only take into consideration of the first order Bragg diffraction in the coupling region. Denoted in Fig. 3(c), both components R1 and R′1 can be coupled into R2 and R′2, respectively, which means that part of energy in one emitter can be injected into the other one in the coupling region. The wavevectors of these four components should satisfy the following phase matching conditions:k⃗R1 + k⃗G1 = k⃗R′2, k⃗R1 + k⃗G2 = k⃗R2, k⃗R′1 + k⃗G1 = k⃗R′2, k⃗R′1 + k⃗G2 = k⃗R2, as shown in the inset of Fig. 3(c). Furthermore, R1 and R′1 should be in phase due to the optical gain provided in the coupling region which would suppress the out-phase interference. Because of the same wavevector selection mechanism in the single emitter, the wave components with the normal incident angle at two facets will be favored. FDTD simulations were carried out to show the preferred mode of a coherently combined laser cavity in Fig. 3(d). In this figure, the mode outside the coupling region is still snake-like just as same as that in a single angled-grating laser. In the coupling region, the wave components from two individual lasers constructively interfere, which means that they are efficiently coupled. Thus, mode control and coherent beam combining are simultaneously obtained through Bragg diffraction in this new laser cavity design.
4. Measurement results
The measurement results of the L-I curve, spectrum, near field and far field shown in this section are obtained in a cryostat system with the heat sink temperature set at 230K. In all the measurements, the lasers are electrically pumped in CW operation.
4.1. L-I curve and optical spectrum
Figure 4(a) shows the light-current curve of the combined angled-grating laser. The threshold current is around 700mA and the slope efficiency is about 0.12W/A. The relatively low slope efficiency indicates high optical loss in the cavity which is mainly caused by the roughness of the gratings induced during the dry etching process. As for the power combining efficiency, ideally, the output power of a coupled emitter should be twice output power of a single emitter when the injected current is doubled. However, thermal effects are more serious in the combined laser due to nearly doubled heat load. When the thermal effect is not obvious, for example at 1.2A, the output power of the coupled emitter is 78.57 mW. The output power of the single emitter is 44.74 mW at 0.6A. Thus the corresponding power combining efficiency is about 0.9. Figure 4(b) shows the optical spectrum of the same laser diode. The pump current is 1200mA which is about 1.7Ith. The peak wavelength is 1549.62nm close to the designed grating resonance wavelength. The inset shows a zoomed-in spectrum from 1544nm to 1548nm indicating a free spectrum range (FSR) of 0.22nm, in agreement with the cavity length of 1.3mm. The FSR suggests that the longitudinal modes are defined by two end facets. Longitudinal mode competition is observed around the peaks in the red circle due to the small modal gain difference between the adjacent longitudinal modes [34].
Fig. 4 Measurements of the light-current curve and optical spectrum. (a) L-I curves of the combined angled-grating laser (solid line) and the single emitter (dashed line). (b) Spectrum of the combined laser diode when the injection current is 1200mA. The inset is a zoomed-in spectrum in the black circle.
4.2. Near field and far field
Figures 5(a) and 5(b) show the near-field image and profile of the combined angled-grating laser. It is clear that there are two emitting regions along the facet. The difference between the intensities of the two regions comes from the nonuniformity in the wafer and induced by the fabrication processes. The distance between the two emitting regions (368μm) and the emitting width of each region (106μm) indicates that light indeed emits from the designed angled-grating areas. The area between two emitters is illuminated a little bit due to the current leakage. The far field of the same coupled laser is shown in Figs. 5(c) and 5(d). We compare the far field profile of the combined laser with an uncoupled single emitter. If two coupled emitters are coherently combined and in-phase, they will constructively interfere in the far field and the overall envelop of the interfered far field remains the same as that of a single emitter. The only difference is that within the overall envelop, interference patterns present. This is exactly our measurement results, as shown in Figs. 5(c)–5(f). The uncoupled single emitter was fabricated on the same chip with the combined laser. The grating parameters such as the period, duty cycle, and total width are also the same. It is clear that the overall envelop of the combined laser’s far field is very similar to that of the uncoupled single emitter. The fine interference patterns in Figs. 5(c) and 5(d) prove that two emitters are coherently combined. The two lobes in the far field comes from the degeneracy of two band-edge modes of the grating [31, 33] since the grating etched depth is bigger than the designed value. Single-lobe far field can be obtained by introducing a central defect or 2D photonic crystal structure [32, 36] or by decreasing the coupling strength of the gratings [33]. The FWHM divergence angles (1.6° for the single emitter and 2° for the combined laser) of these two lasers are still much smaller than a conventional broad-area laser (∼10°). The difference in the divergence angles between the single emitter and coupled emitters are mainly due to different near-field distribution induced by the non-uniformity of fabrication and current injection.
Fig. 5 Measurements of the near field and far field of the coupled emitter and single emitter. (a, b) The near field image and profile of the coupled emitter; (c, d) The far field image and profile of the coupled emitter; (e, f) The far field image and profile of a single emitter.
We extracted the distance between two emitters and the width of one emitter from the measured near-field in Fig. 5(a). We assumed that the two emitters were in phase and calculated the far-field pattern by use of the standard diffraction theory. The calculation result is shown in Fig. 6 in the red dashed line and agrees well with the measured result. The angular distance between two interference stripes in the measurement result is 0.246° and it is 0.234° in the calculation. Steady interference patterns are observed when we increase the pump current up to 2A, which proves that the combining approach is still effective under a high pump condition.
Fig. 6 Far-field profiles of the coherently combined laser and simulation result. Solid line presents the measurement result of the coherently combined laser and the dashed line is the simulation result.
The distinction of the fringes shown in Fig. 6 is low for a few possible reasons. One is the uneven outputs from two beams as shown in Fig. 5(b). Multiple longitudinal modes observed in the spectrum also reduce the visibility of the fringes due to the incoherent addition of different sets of interference patterns with different spacing of fringes. Another important reason is the strong background noise in the infrared vidicon camera we used.
5. Discussion
The performance of the laser in our demonstration is degraded by the strong thermal effect. As shown in Fig. 4(a), the thermal rollover is obvious when the injection current is higher than 1600mA and at the same time, in Fig. 4(b), the output wavelength (∼ 1550nm) indicates that the laser active region is already at room temperature even though the heat sink is set at 230K. There are two main reasons for thermal problems. The first one is that the active wafer used in this work has not been optimized for high power applications. The other one is the thermal management problem caused by p-side up bonding. The lasers have a relatively thin contact layer (∼300nm) and their facets are not coated so that p-side down bonding approach was not used. By using a p-side down approach and better heat spreader, the performance of our laser such as the output power will be improved. In addition, the slope efficiency can be improved by reducing the edge roughness of grating through an optimized dry etching process.
Though only two single emitters are coherently combined in this article, the proposed approach can be expanded to a 1D coherently coupled broad-area laser array. Figure 7 shows the schematic of such a coherent array on a laser bar. Strong optical coupling of the cavity modes in the overlap region between any two adjacent lasers leads to phase locking. For each individual laser, the phase accumulated in a round-trip has to be integer multiples of 2π. Therefore, all the emitting apertures along one side of the bar are in phase. Compared to conventional coherent laser arrays, the width of each individual emitter in our design is almost two orders of magnitude larger. In addition, the Bragg diffraction based combining mechanism is much more robust against nonlinear and thermal effects. The whole structure can be also considered as a folded zigzag cavity in which all the emitters on the bar are part of a super-laser and achieve mutual coherence. It should be pointed out that light of one emitter can be directly injected not only into the adjacent emitters through the coupling regions, but also into other emitters in the array. This implies that a large number of emitters can be coherently combined [10,37], which would lead to a bar-scale single mode diode laser with diffraction-limited output.
Fig. 7 Planar geometry of a zig-zag coherently combined laser array. Coupling regions are marked by red triangles. Coherently combined outputs (marked in black circle) constructively interfere in the far field.
6. Conclusion
In summary, we have demonstrated a new type of on-chip coherently combined angled-grating broad-area lasers. Simultaneous modal control and coherent combining are achieved through Bragg diffraction in the proposed laser. Our measurement results of the near field and far field show that two angled-grating broad-area lasers are coherently combined and provide near-diffraction-limited output without any external optical components. Our design can be expanded to a coherently combined broad-area laser array, which makes our laser architecture a promising candidate to obtain high power, high brightness bar-scale single mode diode lasers.
Acknowledgments
The authors acknowledge funding support from an ARO Young Investigator Award ( W911NF1110519) and DURIP Award ( W911NF1110312). Part of the work is also supported by a DARPA Young Faculty Award ( N6600110-1-4038). The authors also acknowledge the use of the Gatech Nanotechnology Research Center Facility and associated support services in the completion of this work.
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