A new scheme for coherent tuning of laser light

Since the development of lasers, the use of the coherence of light has been studied intensively for many different applications. At high-intensity, it has been used to generate different frequencies ranging from soft x-rays to infrared.^1,2 Wide-bandwidth coherent light can generate ultra-short pulses and temporal and spatial solitons.³ With a very narrow bandwidth, it is critical for high-sensitivity detection of noisy signal in heterodyne or homodyne detection systems.⁴ Further, the interaction between coherent light and matter has long been of interest in atomic and semiconductor physics.^5,6 We have now demonstrated that the wavelength of a laser can be tuned using, and affecting, only the coherent light.⁷

The distributed-Bragg-reflector (DBR) laser fabricated in this experiment is shown in Figure 1. Tuning electrodes, triangular in shape, are located in front of the DBR section. The height of a triangular electrode is 8 μm and it has a 45° side angle. A total of 32 triangles occupy a 225 μm-long section. The other three sections, active, phase, and DBR, have structures of common DBR lasers. The laser was fabricated by using planar buried heterostructure (PBH) and butt-jointed active and passive sections.⁸ The wave-guide width is 2 μm. The active layers consist of seven multiple-quantum-well layers of InGaAs, with a target wavelength of 1550nm. The passive waveguide layer is bulk InGaAsP that has a bandgap wavelength of 1.3 μm, a thickness of 300nm, and is surrounded by InP cladding layers. The DBR gratings are formed wsing conventional holographic lithography, again with a target wavelength of 1550nm. A more detailed description of the fabrication process is available in the literature.^7,8

The principle of coherent tuning is as follows: Current injection into a triangular electrode changes the refractive index of the waveguide underneath it. A photon passing through the electrode is refracted and changes its wave vector component along the propagation direction by k = k₀cosθ, where k₀ is an original wave vector component and θ the refracted angle (typically 0° to 0.5°). If the light is spatially coherent, then each electrode adds to the refracted angle, producing a large change in the wave vector. If the light is spatially incoherent, then the refracted angle does not accumulate and averages out to zero, with no change of the wave vector.

The experimental results shown in Figure 2 prove clearly that coherent tuning is possible, at least for small angles. Figure 2(a) shows a 1.4nm tuning of laser wavelength, or 2.4° change of the wave vector, with a 50mA injection into the tuning electrodes: the current in the phase section was kept at zero. Figure 2(b) shows the spectra of electroluminescence and DBR stop band in the same condition as Figure 2(a) except for the active current injected below the lasing threshold. The wavelength of the DBR band does not shift noticeably. Figure 2(c) shows laser peaks with the same amount of current injection into the phase section as Figure 2(a), with the injection of current in the tuning section kept at zero. The laser peaks are confined within a range of, at most, two longitudinal modes, which is typical for DBR lasers. The current injection into the phase section excludes any possibility of tuning by a change of carrier density in the waveguides.

We confirmed these observations with tens of laser chips fabricated on the same substrate. The laser peaks are at the long wavelength edge of DBR stop band because the gain curve has a peak at the same edge, 1550nm. A simple estimation shows that 2.4° is quite close to the difference between the critical angle and the fundamental-mode angle for our waveguide. Figure 2 also shows that a 5mA injection induces a 0.8nm shift or a 1.84° change of the wave vector, proving that more than 20 electrodes are involved in the refraction.