<title>RF performance of optical injection locking</title>

Abstract

Coherent photonic systems promise novel functionality and/or improved performance compared to direct detection photonic systems, but have the disadvantage of being sensitive to optical phase noise. The most common approach to this problem is to force one laser to track the phase of the other with a phase locked loop (PLL), so that the phase noise of the lasers cancelsout of the RF heterodyne beat note. Although the PLL approach has been implemented for semiconductor lasers, the large linewidth of these lasers and the resulting large PLL loop bandwidth severely constrain the design and limit performance. This disadvantage of the PLL approach is particularly relevant for many applications, since semiconductor lasers are preferred for system insertion.An alternative approach for establishing coherence between two lasers is optical injection locking. Standard theory indicates that injection locking can act in the same way as a first order PLL with a bandwidth as high as several GHz, which is large enough to achieve state of the art noise levels (e.g. -130 dBcIHz @ 1 MHz offset) with semiconductor lasers. We present phase noise measurements on the beat note of two injection locked semiconductor lasers. Our results (phase noise @ 1 MHz offset as low as -125 dBcIHz) indicate that state of the art phase noise performance from injection locked lasers should be obtainable in practice. We found that it is necessary to length match the two paths in the experiment (master -> detector and master -> slave -> detector) to avoid excess noise due to delay decorrelation of the master, and also that environmental noise seems dominant at offsets < 20 kHz.

Polarization independent optical injection locking for carrier recovery in optical communication systems

“…Under locked conditions and within the locking range, the IL system transfers phase modulation, replicating phase information. At the same time, the amplitude modulations can be highly suppressed [19,20]. Consequently, the amplitude fluctuations at the output of the 3-dB coupler caused due to the low frequency phase modulation ( φ Δ ) on one arm of the module will be suppressed, whereas the required carrier phase information will pass through.…”

“…As discussed in Section 1, the polarization-locking module may generate amplitude fading in the signal, due to the interference in the 3-dB coupler in the module, which in turn may result in a loss of injected power and thus, the injection lock. Theoretically, these lowfrequency amplitude fluctuations are rejected by the IL [19,20]. To systematically test this, we added additional amplitude fluctuations into our system to verify the AM suppression capabilities of our IL set-up.…”

Characterization of a coherent optical RF channelizer based on a diffraction grating

“…A high degree of coherence between the signal beam and the LO comb is required to minimize the phase noise associated with the heterodyne RF signal. Continuous-wave (CW) injection locking of mode-locked lasers has been shown to establish phase coherence between the optical carrier of the signal and the optical frequency comb [9], [10], and we have previously characterized the RF performance of injection locking for this application [11]. The coherent optical channelizer concept is illustrated in Fig.…”

CW injection locking of a mode-locked semiconductor laser as a local oscillator comb for channelizing broad-band RF signals

“…This technique offers a large locking bandwidth whereas electronic phase-lock loops are limited by the electrical bandwidth, the linewidth of the lasers used, and the inherent loop delay [15]. The RF phase noise of the beat frequency of two injection-locked CW lasers has been measured to be as low as 125 dBc/Hz at 100 kHz offset [16], which is comparable to high-performance synthesizers. Recently, injection locking of mode-locked semiconductor lasers has spurned new interests in both temporal synchronization and spectral control of mode-locked lasers.…”