We report the fabrication and properties of 850 nm wavelength AlGaAs/GaAs-based transceiver chips, in which vertical-cavity surface-emitting lasers (VCSELs) and photodiodes are monolithically integrated. Various types of devices allow half-and full-duplex bidirectional optical interconnection at multiple Gbit/s data rates over a single butt-coupled glass or polymer-clad silica optical fiber with core diameters of 100 or 200 µm. Whereas metal-semiconductor-metal (MSM) photodiodes are employed for these large-area fibers, we also investigate the integration of PIN-type photodiodes which appear more promising in combination with standard 62.5 or 50 µm core diameter graded-index multimode fibers. This interconnect solution based on two identical chips is attractive owing to lower volume, weight, and cost. Applications will be found in home, in-building, industrial, or automotive networks and potentially within computer clusters or central offices.
Vertical-cavity surface-emitting lasers (VCSELs) with single-mode, single-polarization emission at a wavelength of 894.6 nm have become attractive light sources for miniaturized Cs-based atomic clocks. So far, VCSELs used for these applications are single-mode because of small active diameters which has the drawbacks of increased ohmic resistance and reduced lifetime. By employing surface grating reliefs, enhanced fundamental-mode emission as well as polarization-stable laser oscillation are achieved. VCSELs with 5 μm active diameter show side-mode suppression ratios of 20 dB even at currents close to thermal roll-over with orthogonal polarization suppression ratios better than 20 dB at elevated ambient temperatures up to 100 °C.
Abstract:We report on the characterization and validation of customdesigned 894.6 nm vertical-cavity surface-emitting lasers (VCSELs), for use in miniature Cs atomic clocks based on coherent population trapping (CPT). The laser relative intensity noise (RIN) is measured to be 1 × 10 −11 Hz −1 at 10 Hz Fourier frequency, for a laser power of 700 μW. The VCSEL frequency noise is 10 13 · f −1 Hz 2 /Hz in the 10 Hz < f < 10 5 Hz range, which is in good agreement with the VCSEL's measured fractional frequency instability (Allan deviation) of ≈ 1 × 10 −8 at 1 s, and also is consistent with the VCSEL's typical optical linewidth of 20-25 MHz. The VCSEL bias current can be directly modulated at 4.596 GHz with a microwave power of −6 to +6 dBm to generate optical sidebands for CPT excitation. With such a VCSEL, a 1.04 kHz linewidth CPT clock resonance signal is detected in a microfabricated Cs cell filled with Ne buffer gas. These results are compatible with state-of-the-art CPT-based miniature atomic clocks exhibiting a short-term frequency instability of 2-3×10 −11 at τ = 1 s and few 10 −12 at τ = 10 4 s integration time.
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