We have coherently combined a high-power broad-area laser diode array by using a feedback loop closed off-axis external Talbot cavity. The off-axis feedback from two gratings provides transverse-mode control of broad-area lasers. The Talbot configuration of the external cavity implements diffractive coupling among laser diodes. Feedback from two gratings increases external cavity quality factor and spectrum selection capability. As a result, spatial coherence was improved and spectral linewidth was narrowed down. The high visibility of the far-field profile indicates that high spatial coherence was achieved. We also observed symmetric far-field profiles indicating that laser array was phase locked to in-phase and out-of-phase super-modes, respectively. Transition between these super-modes was observed by tuning one grating’s tilted angle.
©2010 Optical Society of America
Laser beam combination provides the means to achieve high power emission with an excellent beam quality and reduces thermal management requirements . Coherent beam combining of laser array results in very powerful coherent light source. Owing to high electro-optical conversion efficiency, compactness, and wavelength tunability, semiconductor lasers may have an extensive variety of applications for direct energy, free-space optical communication, and nonlinear optics processes such as high-efficiency blue-green light generation.
External cavity designs have been proposed and extensively studied. A widely used design to achieve coherent beam combining is the external Talbot cavity [2–5]. Talbot cavity induces self-imaged diffractive coupling between the lasers and provides the means to lock the phases of laser diodes. Coherent beam combining of low-power single-mode laser diodes in external Talbot cavity was successfully demonstrated [2–5]. However, the demonstrated emission power was limited due to power limitation of each individual single-mode laser diode. Increasing the power of an individual emitter and maintaining its spatial coherence is a plausible root to achieve high power coherent emission from laser diode arrays.
A tapered laser diode can provide near-diffraction-limited, high-power coherent emission due to combination of a single-mode ridge structure and a tapered amplifier structure . Coherent beam combining of a tapered laser diode array has been investigated by a number of groups [7,8]. Recently, an important effort to develop high-brightness slab-coupled optical waveguide laser (SCOWL) arrays was demonstrated . The SCOWL coherent beam combining with 7.2 W output power was achieved using an external Talbot cavity .
Broad-area laser diodes have high electric-optical efficiency and are capable of generating high power emission. The cost of broad-area laser diode is low due to relatively simple fabrication process. Low spatial and temporal coherence due to weak transverse mode selection of the broad emitter size is the primary drawback in the direct application of broad-area laser diodes. To improve the beam quality, the off-axis optical feedback was employed to enhance certain transverse mode for both single broad-area laser diode and broad-area laser diode array [11–18]. Two different center wavelength volume gratings were used to reduce the spectral linewidth of laser diode array . The off-axis external cavity setup was successfully used to demonstrate spectral beam combining in broad-area laser diode array [20,21]. However, coherent beam combining has not been demonstrated.
Recently, we successfully implemented coherent beam combing of a broad-area laser diode array by using a V-shape external Talbot cavity . A 9.0 W coherent emission with 0.1 nm spectral linewidth was demonstrated. In this paper we propose and implement a closed feedback loop off-axis external Talbot cavity to achieve coherent beam combining of a broad-area laser diode array with higher spatial coherence. Since two gratings form a closed feedback loop, we call this off-axis external cavity a closed V-shape external cavity. The feedback from the gratings provides better spectral selection and increases the cavity quality factor. As a result, the output beam shows higher spatial coherence and narrower spectral line-width. We observed the symmetric far-field profiles indicating that laser array was phase locked to out-of-phase and in-phase super-modes, respectively. The far-field profile transition between these super-modes was observed by carefully adjusting one grating’s tilted angle. When laser array is locked in the in-phase super-mode, the angle of the center lobe of the array far-field profile is approximately 1.5 mrad and the spectrum linewidth is 0.07 nm that reaches the resolution limit of our spectrum analyzer. The output power reaches 12.8 W at high injection current and a high-visibility, far-field pattern indicates that a high degree of spatial coherence was achieved.
A schematic design of the experimental setup is shown in Fig. 1 . Our laser diode array was manufactured by Jenoptics. It is comprised of 47 emitters with 100 μm emitter size and 200 μm spacing. The front facet of laser diodes is anti-reflection (AR) coated with 0.5% residual reflectivity and the rear facet is high-reflection (HR) coated. The center wavelength is around 808 nm.
A pair of prism mirrors separates the laser beam into two different feedback paths. The right-side feedback path consists of cylindrical lenses CL1-CL3 and grating G1 while the left-side path consists of cylindrical lenses CL4-CL6 and grating G2. The optical axis of the feedback optics and the slow-axis of the laser array form a small angle θ = 2.38°. This angle is equal to half of the angle separating the two laser beams (see Fig. 1).
In the right-side feedback path, a telescope comprised of a fast-axis correction (FAC) lens (fg = 1.3 mm, NA = 0.5) and a cylindrical lens CL2 collimates the laser beam along the fast axis. The “smile” effect due to non-perfect manufacturing process could be reduced eight times by the fast-axis telescope in our external cavity configuration according to ray-tracking calculation [23,24]. The laser diodes are imaged on the focal plane of CL3 through the transform optics comprised of a pair of confocal cylindrical lenses CL1 and CL3. The laser beam is reflected by diffraction grating G1 which is arranged in the Littrow configuration. The blaze angle of G1 is about 18° and the first-order diffraction efficiency is more than 85%. The locked center wavelength of the laser array is determined by the grating tilting angle with respect to the laser diodes front facet. In the left-side feedback path, a telescope comprised of the FAC lens and CL5 collimates the laser beam along the fast axis. Another confocal cylindrical lens pair, CL4 and CL6, images the laser diodes onto the surface of the grating G2 (2000-line/mm, gold-coating) which is also mounted in the Littrow configuration. G2 reflects the first-order diffraction into laser diodes while coupling the zero-order out. The grating G2 reflects 20% of the incident light back to the laser array and couples the rest out of the cavity. G2 serves as an optical coupler for the entire array. The far-field profile is projected to the focal plane of cylindrical lens CL7 and recorded by a CCD camera. An optical spectrum analyzer with a sensitivity of 70 pm is used to measure the spectrum while a power meter behind the beam-sampler monitors the output power.
Inset 1 in Fig. 1 shows the external cavity for an individual broad-area laser diode. The effective external cavity is composed of gratings G1 and G2 (the AR-coated front-facet reflectivity of laser diode is much lower than reflectivity of gratings). G1 serves as an end mirror and a spectral selector while the HR coating rear facet serves as a beam folding mirror. G2 serves as both a spectral selector and an output coupler. The unfolded external cavity for the entire array is shown in Fig. 2 . The near-field distributions of laser diodes emission toward the right feedback path or toward the left feedback path are imaged on the CL3, CL6 focal plane, respectively. The distance between grating G1 and the CL3 focal plane equals the half-Talbot distance D = Z t/2, where Zt = 2d 2/λ is the Talbot distance, λ is the laser wavelength, and d is the array pitch. The diffractive coupling is achieved due to the Talbot self-image effect. G2 is located at the CL6 focal plane and provides non-coupled feedback as well as an output. The effective cavity round-trip is calculated from the formula L = 2(D) = Zt.
3. Results and discussions
Figure 3 shows the two different symmetric far-field profiles generated from the phase-locked laser diode array when G1 is positioned around the half-Talbot plane from CL3 focal plane and G2 is positioned at CL6 focal plane (effective round-trip of the external cavity equals Talbot distance). To characterize the degree of coherence in our laser system, we calculated the visibility of the far-field profile using the following expression: V = (Imax-Imin)/(Imax + Imin). The far-field profile clearly demonstrates coherent addition, and the visibility of the profile is higher than 97%, which significantly exceeds 80% visibility obtained in our earlier work . The full width at half maximum (FWHM) of the center lobe is about 1.2 mrad. The angle between two adjacent peaks is 4.4 mrad and is consistent with the theoretical angle spacing expected from a 200 μm-periodic coherent light source (λ/D = 0.808 μm/200 μm = 4.04 mrad). The FWHM of spectral linewidth is measured to be 0.07 nm, which is the resolution limit of our spectrum analyzer. The output power was approximately 1.5 W at the injection current of 34 A. The out-of-phase super-mode far-field profile is shown in Fig. 3a. One can observe a symmetric structure with two highest symmetric peaks in the far-field profile equally spaced around the center. This indicates that the out-of-phase super-mode was selected. When we tuned the tilting angle of G2 (rotate in optical table plane) by 2.2 mrad, the far-field profile showed a symmetric structure with the highest peak positioned at the center (see Fig. 3(b)), which indicates that the in-phase super-mode was selected. The change of the grating tilting angle 2.2 mrad is close to [7,8], .
The emission power of laser diode array with the out-of-phase supermode far-field profile is slightly higher than emission power of laser array with the in-phase super-mode far-field profile. We measured laser output power as a function of the injection current with the in-phase super-mode far-field profile. The measured L-I characteristic curves are shown in Fig. 4 . The squares-curve shows the L-I dependence for an array placed in the external closed V-shape cavity while the circles-curve shows the L-I dependence for a free-running array. We measured different values of the threshold currents and slopes for the laser diode array with or without the external cavity. For the closed V-shape cavity, the threshold current is 32 A and the output slope efficiency is 0.41 W/A. For the AR-coated broad-area laser array, the threshold current is approximately 40 A and output slope efficiency is 0.98 W/A. There is a crossover of the two curves at the value of 56 A. Different threshold currents and different L-I slopes can be explained by analyzing the laser system configurations. The closed-V-shape external cavity only enhances a certain off-axis broad-area laser diode mode, which is shown in the Inset 1 of Fig. 1. The enhanced off-axis lasing broad-area laser diode mode suppresses other broad-area laser diode modes. Consequently, the beam quality of individual broad-area laser diode has been greatly improved as shown in Ref [11–21]. The gratings G1 and G2 serve as cavity mirrors, which provide more feedback than AR-coated front-facet. As a result, the closed-V-shape external cavity reduces the laser threshold from 40 A to 32 A. The slope efficiency is reduced due to the coupling out efficiency reduction. A similar behavior of the L-I curves for the laser diode array with and without an external cavity was demonstrated in Ref [26,27]. By carefully designing the external cavity (optimizing the reflectivity of the gratings and the gain medium length), we can increase the slope efficiency and, accordingly, the coherent emission power. It is important to emphasize that although the slope efficiency of the AR-coated laser diode array without external cavity is higher, it only generates a broad-spectrum (2~3 nm), poor beam quality non-coherent emission.
Our closed V-shape cavity generates a high contrast coherent pattern from an AR coated broad-area laser diode array. A high visibility (>97%) coherent far-field pattern at low power (0.8 W, I = 32 A) is shown in Fig. 5a , and the narrow spectral linewidth (0.07 nm FWHM) with 50 dB side-mode suppression spectrum is shown in the Fig. 5b. The increased visibility indicates that a better spatial coherence is achieved. By comparing configurations of the closed-V-shape external cavity and our earlier experimental design, we found that the closed-V-shape external cavity provided more feedback and increased cavity quality factor. The feedback of two gratings provides more spectral control to the laser diode array, as a result, the spatial coherence is enhanced and spectral linewidth of laser diode array is further narrowed down from 0.1 nm to 0.07 nm.
Figure 6 shows the far-field pattern and the spectrum of a closed-V-shape external cavity laser diode array at high injection current (I = 60 A). The corresponding output power was 12.8 W. The visibility of the far-field profile was 84% while the spectral linewidth showed the same value as for low current operation (0.07 nm). Compared with the low injection current (I = 32 A), the visibility of far-field profile was reduced from 97% to 84%, and the far-field angle (FWHM) of the center lobe was increased from 1.2 mrad to 1.5 mrad. Far-field profile visibility decreased with the increase of injection current. Coherence deterioration may occur due to mode competition between intrinsic broad-area laser diode modes and external cavity modes. Further reduction of the front-facet AR coating reflectivity will increase the threshold of the free-running broad-area laser diode array and will shift the cross-over point of two L-I curves (see Fig. 4) towards the higher value of the driving current. Consequently mode competition will be reduced and high spatial coherence will be maintained at higher injection current.
In conclusion, we demonstrated high-power, coherent beam combing of a broad-area laser diode array with narrow spectral linewidth by using a closed-V-shape external Talbot cavity. The off-axis optical feedback from two gratings enhanced the feedback into the external cavity, increased the cavity quality factor and spectrum selection capability. The visibility of the far-field profile using a closed-V-shape external Talbot cavity was higher than the visibility obtained in our earlier experiment . We observed the far-field profile transition between the out-of-phase and in-phase super-modes by tuning the tilting angle of one of the gratings. The closed V-shape external cavity design is robust and offers the capability of high-power operation. By further optimizing design of the closed-V-shape-cavity, it will be possible to further increase the output power and maintain high visibility of the far-field profile.
This research was supported by the Office of Naval Research and also in part by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory. Oak Ridge National Laboratory is managed by UT-Battelle, LLC for the U.S. Department of Energy under Contract DE-AC05-00OR22725.
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