A 128-channel linear array photodetector, with 25 μm pixel pitch, connected to a readout integrated circuit (ROIC) was developed for mid-infrared spectroscopic applications that use a wavelength-variable quantum cascade laser (QCL). The detector is composed of III-V semiconductor InAsSb, has sensitivity in the mid-infrared region, and is operated at room temperature.
The photovoltaic type of MIR detectors has a low shunt resistance, which causes high dark current. Therefore, a multi-series detector, which has a high total shunt resistance, is generally used for uncooled operation. However, in our newly developed detector array, a single element is employed as each channel’s detector to achieve high signal sensitivity. Also, a DC feedback (DCFB) mechanism is applied to the ROIC to draw out the detector’s high dark current.
The detector’s performance is evaluated using a pulsed QCL with an emission wavelength of 7-10 μm at room temperature. A reverse voltage is applied to the detector to improve the detector’s characteristics and allow it to respond to a QCL’s pulse width of 100 nsec. Although the reverse voltage increases the detector’s dark current, the DC feedback draws out the dark current up to 1 mA. The detector’s sensitivity is 1.5 A/W at 7 μm, the TIA’s gain is designed to be 1k ohm, so the total trans gain obtained is 1.5 V/mW. The detector’s noise input equivalent power is 200 nW. Therefore, a high signal-to-noise ratio can be achieved because the pulsed QCL can output a peak power higher than several tens of milliwatts.
In future radio access systems, base stations will be mainly accommodated in wavelength- and time-division multiplexing passive optical network (PON) based mobile backhaul and fronthaul networks, and in such networks, failed connections in an optical network unit (ONU) wavelength channel will severely degrade mobile system performance. A cost-effective in-service ONU wavelength channel monitor is essential to ensure proper system operation without failed connections. To address this issue, we propose a reflectometry-based remote sensing method that provides ONU wavelength channel information with the optical line terminal-ONU distance. The proposed method enables real-time monitoring of ONU wavelength channels without data signal quality degradation and is also able to determine if the ONUs are connected to the PON. Experimental results show that it achieves wavelength channel distinction with a high distance resolution (∼10 m). Additionally, with the method, the distance resolution for distinguishing the ONUs after the PON splitter is determined by the received signal bandwidth or the test light modulation speed rather than by the pulse width as in conventional optical time-domain reflectometry.
Many base stations are accommodated in TWDM-PON based mobile backhaul and fronthaul networks for future radio access, and failed connections in an optical network unit (ONU) wavelength channel severely degrade system performance. A cost effective in-service ONU wavelength channel monitor is essential to ensure proper system operation without failed connections. To address this issue we propose a reflectometry-based remote sensing method that provides wavelength channel information with the optical line terminal (OLT)-ONU distance. The method realizes real-time monitoring of ONU wavelength channels without signal quality degradation. Experimental results show it achieves wavelength channel distinction with high distance resolution.
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