Sub-kHz Quantum Linewidth Semiconductor Laser On Silicon Chip

We report on a semiconductor laser on silicon platform with record-low sub-kHz quantum noise-limited linewidth, based on spontaneous emission control and optical loss reduction

Quantum control of phase fluctuations in semiconductor lasers

Few laser systems allow access to the light–emitter interaction as versatile and direct as that afforded by semiconductor lasers. Such a level of access can be exploited for the control of the coherence and dynamic properties of the laser. Here, we demonstrate, theoretically and experimentally, the reduction of the quantum phase noise of a semiconductor laser through the direct control of the spontaneous emission into the laser mode, exercised via the precise and deterministic manipulation of the optical mode’s spatial field distribution. Central to the approach is the recognition of the intimate interplay between spontaneous emission and optical loss. A method of leveraging and “walking” this fine balance to its limit is described. As a result, some two orders of magnitude reduction in quantum noise over the state of the art in semiconductor lasers, corresponding to a minimum linewidth of 1 kHz, is demonstrated. Further implications, including an additional order-of-magnitude enhancement in effective coherence by way of control of the relaxation oscillation resonance frequency and enhancement of the intrinsic immunity to optical feedback, highlight the potential of the proposed concept for next-generation, integrated coherent systems

High-coherence semiconductor lasers based on integral high-Q resonators in hybrid Si/III-V platforms

The semiconductor laser (SCL) is the principal light source powering the worldwide optical fiber network. The ever-increasing demand for data is causing the network to migrate to phase-coherent modulation formats, which place strict requirements on the temporal coherence of the light source that no longer can be met by current SCLs. This failure can be traced directly to the canonical laser design, in which photons are both generated and stored in the same, optically lossy, III-V material. This leads to an excessive and large amount of noisy spontaneous emission commingling with the laser mode, thereby degrading its coherence. High losses also decrease the amount of stored optical energy in the laser cavity, magnifying the effect of each individual spontaneous emission event on the phase of the laser field. Here, we propose a new design paradigm for the SCL. The keys to this paradigm are the deliberate removal of stored optical energy from the lossy III-V material by concentrating it in a passive, low-loss material and the incorporation of a very high-Q resonator as an integral (i.e., not externally coupled) part of the laser cavity. We demonstrate an SCL with a spectral linewidth of 18 kHz in the telecom band around 1.55 μm, achieved using a single-mode silicon resonator with Q of 10^6

Theory and observation on non-linear effects limiting the coherence properties of high-Q hybrid Si/III-V lasers

Hybrid Si/III-V is a promising platform for semiconductor narrow-linewidth lasers, since light can be efficiently stored in low loss silicon and amplified in III-V materials. The introduction of a high-Q cavity in silicon as an integral part of the laser's resonator leads to major reduction of the laser linewidth. However, the large intra-cavity field intensity resulting from the high-Q operation gives rise to non-linear effects. We present a theoretical model based on non-linear rate equations to study the effect of two-photon absorption and induced free-carrier absorption in silicon on the laser's performance. The predictions from this model are compared to the experimental results obtained from narrow-linewidth lasers fabricated by us. It is shown to have an effect on the linearity of the L-I curve, and to reduce the achievable Schawlow- Townes linewidth

Quantum control of phase fluctuations in semiconductor lasers

Few laser systems allow access to the light–emitter interaction as versatile and direct as that afforded by semiconductor lasers. Such a level of access can be exploited for the control of the coherence and dynamic properties of the laser. Here, we demonstrate, theoretically and experimentally, the reduction of the quantum phase noise of a semiconductor laser through the direct control of the spontaneous emission into the laser mode, exercised via the precise and deterministic manipulation of the optical mode’s spatial field distribution. Central to the approach is the recognition of the intimate interplay between spontaneous emission and optical loss. A method of leveraging and “walking” this fine balance to its limit is described. As a result, some two orders of magnitude reduction in quantum noise over the state of the art in semiconductor lasers, corresponding to a minimum linewidth of 1 kHz, is demonstrated. Further implications, including an additional order-of-magnitude enhancement in effective coherence by way of control of the relaxation oscillation resonance frequency and enhancement of the intrinsic immunity to optical feedback, highlight the potential of the proposed concept for next-generation, integrated coherent systems

Kicking the habit/semiconductor lasers without isolators

In this paper, we propose and demonstrate a solution to the problem of coherence degradation and collapse caused by the back reflection of laser power into the laser resonator. The problem is most onerous in semiconductor lasers (SCLs), which are normally coupled to optical fibers, and results in the fact that practically every commercial SCL has appended to it a Faraday-effect isolator that blocks most of the reflected optical power preventing it from entering the laser resonator. The isolator assembly is many times greater in volume and cost than the SCL itself. This problem has resisted a practical and economic solution despite decades of effort and remains the main obstacle to the emergence of a CMOS-compatible photonic integrated circuit technology. A simple solution to the problem is thus of major economic and technological importance. We propose a strategy aimed at weaning semiconductor lasers from their dependence on external isolators. Lasers with large internal Q-factors can tolerate large reflections, limited only by the achievable Q values, without coherence collapse. A laser design is demonstrated on the heterogeneous Si/III-V platform that can withstand 25 dB higher reflected power compared to commercial DFB lasers. Larger values of internal Qs, achievable by employing resonator material of lower losses and improved optical design, should further increase the isolation margin and thus obviate the need for isolators altogether

Coherent and Incoherent Optical Feedback Sensitivity of High-coherence Si/III-V Hybrid Lasers

We demonstrate that high-coherence Si/III-V hybrid lasers are much more robust than conventional III-V DFB lasers against both coherent and incoherent optical feedback by examining the frequency noise power spectral density of the lasers

Coherent and Incoherent Optical Feedback Sensitivity of High-coherence Si/III-V Hybrid Lasers

We demonstrate that high-coherence Si/III-V hybrid lasers are much more robust than conventional III-V DFB lasers against both coherent and incoherent optical feedback by examining the frequency noise power spectral density of the lasers

Narrow-Linewidth Oxide-Confined Heterogeneously Integrated Si/III-V Semiconductor Laser

We demonstrate a narrow-linewidth heterogeneously integrated silicon/III-V laser based on the oxide-confinement method. The laser achieves an output power of 4 mW and a linewidth of 28 kHz with a threshold current of 60 mA and a side mode suppression ratio of 50 dB at 1574 nm