Photon-number amplification in a multijunction single-mode laser cavity

Recently, the possibility of readout and amplification of the photon number in a light beam with a sensitivity better than the shot-noise limit has been predicted and realized. At the heart of these experiments, which essentially are quantum versions of an optical repeater, is an intermediate step of optoelectronical conversion. Unlike optical beams, which most often carry shot noise, the quantum noise of an electrical current is readily below the shot-noise level. This allows the signal to be manipulated electrically with less noise than would be the case with (most) all-optical manipulation. The fundamental noise limit in the experiments is set only by the quantum efficiency of the detectors and light emitters, e.g., noise-free amplification is possible (and a <3-dB noise figure is realized). However, in practice a more severe limit is set by thermal and amplifier noise (as is the well-known case in most optical receivers). To circumvent this limit, the setup shown in Fig. 1 (proposed in Ref. 2 and experimentally realized in Ref. 5) can be used. In this case the detected current is fed through series coupled light emitters whose outputs are added. There is no electrical amplifier involved, and if a separate bias is used, a high-impedance drive can suppress the thermal noise. However, a drawback is that the output is not single mode (it can become single mode by subsequent detection and regeneration, as demonstrated in Ref. 5). To realize an amplified replica of the input signal directly in a single mode, we propose to use a configuration, as in Fig. 2, consisting of series coupled p-n junctions inside a laser cavity. This structure can be grown as either an edge-emitting or a surface-emitting laser (in the latter case with thin n⁺ p⁺ tunnel junctions, positioned at the standing wave nodes, to connect the p-n junctions). There are other potential advantages with this laser structure. Since the junctions are series coupled, the differential resistance will increase, making the impedance matching (usually to 50 Ω) less severe. In order to achieve a high conversion efficiency (and low noise), the laser should be operated high above threshold. Interestingly, the threshold current will decrease, roughly in proportion to the number of junctions, yet the threshold power is kept constant. This optoelectronic device, which is possible to fabricate with present OEIC technology, could find applications as a compact and ideally noise-free regenerator/wavelength converter/detector for use in optical networks and optical interconnects.