Tiny Dichroic Mirror Can Boost Frequency-Doubling Efficiency
Mirrors embedded in an AlGaAs/AlxOy waveguide can form a resonant cavity.
The second-harmonic conversion efficiency of gallium arsenide can be several times as great as that of conventional nonlinear crystals such as lithium niobate. Moreover, because semiconductor lasers also can be fabricated from the material, the possibility of integrating sources and frequency converters in a single monolithic chip becomes very enticing. A drawback is that nonlinear wavelength conversion — in this case, second-harmonic generation — requires significant interaction length between the fundamental wavelength and the second harmonic. In other words, ultracompact (~100 μm) integrated circuits, containing both a laser and a frequency doubler, cannot be readily fabricated.
Figure 1. In a conventional waveguide Bragg reflector, the reflectance of the fundamental wavelength (thin blue line) can be high, but the transmission of the second harmonic (thick broken red line) is terrible (a). In the Stanford scientists’ dichroic mirror, the transmission of the second harmonic and the reflectance of the fundamental are both high (b). The colored lines represent the same parameters in (a) and (b). The transmission (thin blue line) and loss (thick broken red line) of the fundamental wavelength indicate optimal performance at the ~1565-nm fundamental wavelength (c). In all three cases, these curves show the calculated optical characteristics of the reflector shown in the inset. The second-harmonic transmission values in (a) and (b) correspond to wavelengths equal to one-half those indicated on the abscissa.
But the effective interaction length can be multiplied in a short space by resonating either the fundamental or the second-harmonic wavelength. Recently, investigators at Stanford University in California demonstrated a tiny dichroic mirror embedded in a single-mode AlGaAs/AlxOy waveguide that reflects the fundamental with minimal disturbance to the second harmonic. They suggest that the concept could lead to efficient second-harmonic conversion in tiny resonant cavities that are part of the same ultracompact optical integrated circuit as the source laser.
Figure 2. The fabricated dichroic mirror consisted of a tapered AlGaAs/AlxOy waveguide and offset Bragg combs.
Reflectors in waveguides are typically fabricated by etching a periodic grating into the sides of the waveguide. Although this approach is fine for single-wavelength reflectors, it fails for dichroic mirrors. The same periodicity that reflects the fundamental wavelength forces the second harmonic into radiation modes, which causes unacceptably high loss.
The Stanford researchers sidestepped this problem by taking advantage of the size difference between the fundamental-wavelength and second-harmonic modes propagating in a waveguide. They fabricated a tapered waveguide with a grating structure etched into the material adjacent to the taper (Figure 2). The second-harmonic mode is significantly smaller than the fundamental-wavelength mode and can pass through the tapered section of the waveguide without interacting with the teeth of the grating. The larger fundamental-wavelength mode, however, is reflected.
They fabricated waveguides similar to the one shown in Figure 2, and they found that the resulting optical characteristics were in good agreement with the calculated performance (Figure 1). Spurious Fabry-Perot fringes introduced a significant amount of noise into the fundamental-wavelength reflectivity but could easily be defeated in a mature device (Figure 3).
Figure 3. Fabry-Perot fringes resulting from spurious reflections introduced noise to the experimental data (blue), but that data nonetheless agreed with the calculated maximum (broken red) and minimum (solid red) calculated reflectivity of the fundamental wavelength. The scientists deliberately removed the effect of the fringes from the calculated curves and fitted them to the experimental data.
Direct measurement of the second-harmonic transmission was difficult because the waveguide propagates multiple modes at that wavelength, resulting in high loss. The scientists estimated the second-harmonic transmission by comparing the second-harmonic power generated in a waveguide containing their mirrors with that generated in an ordinary waveguide. They concluded that the measured transmission was consistent with the calculation shown in Figure 2.
In subsequent experiments, the researchers pumped their AlGaAs/AlxOy waveguide with a tunable diode laser from Agilent Technologies Inc. of Santa Clara, Calif., and observed normalized, second-harmonic conversion efficiencies that they believe are 20 times greater than the best previously obtained. (“Normalized” conversion efficiency, in this case, is the ratio of second-harmonic power to the square of fundamental-wavelength power.)
Figure 4. The core of the waveguide at the top of the ridge resembles a sandwich, with Al0.5Ga0.5As (pink) serving as the bread and AlxOy (white) serving as the filling. The electric field of the fundamental-wavelength mode (a) is relatively undisturbed by the thin layer of AlxOy, but the electric field of the second harmonic (b) experiences a significant perturbation. The resulting change in the second harmonic’s effective refractive index exactly compensates for the material’s dispersion, and both wavelengths experience an effective refractive index of ~2.2219.
Because AlGaAs is isotropic and not birefringent, normal birefringence phase matching for second-harmonic generation is not possible in the material. The material can be poled to employ quasi-phase-matching, but the fabrication process is complex. Moreover, quasi-phase-matching requires longer interaction lengths, making it less desirable for compact integrated circuits.
The scientists instead created an artificial birefringence in the material, invoking a technique developed nearly a decade ago at Thomson CSF Laboratoire in France that introduces a geometric asymmetry into the waveguide. As in any waveguide, the effective index depends on both the index of the bulk material and on the waveguide geometry, so that the geometric asymmetry produces an effective birefringence. The Stanford scientists introduced a thin (90 nm) layer of AlxOy into the core of the waveguide, and the strong index contrast between that material and the surrounding Al0.5Ga0.5As produced an effective birefringence sufficient to phase match the second-harmonic generation (Figure 4).
Figure 5. The second-harmonic power generated from an 80-μm-long fundamental-wavelength resonator (black) was approximately 20 times greater than that generated from an equally long waveguide lacking any mirrors (blue). Even when compared with a 400-μm-long plain waveguide (red), the resonator offered an ∼50 percent enhancement. The noise evident in these data results from multiple, spurious Fabry-Perot reflections in the experimental setup.
In a comparison between second harmonic generated in an 80-μm-long plain waveguide and in an 80-μm-long, fundamental-wavelength resonator created with their dichroic mirrors, the scientists saw a factor of 20 enhancement resulting from the resonator (Figure 5). They believe that further refinements in their initial, proof-of-concept demonstration could produce even better results, and that eventually this approach could lead to frequency-doubling devices that are hundreds of times more efficient than the lithium niobate waveguides that are available commercially.
Optics Letters, Nov. 15, 2006, pp. 3285-3287. Optics Letters, Dec. 15, 2006, pp. 3626-3628.
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