The continuing increase of transmission rates at all levels of telecommunication networks and fiber-based radio-frequency photonic systems raises the demand for very high speed, low-cost optical transmitters. Much effort has been put into developing wide-bandwidth lasers and modulators over the past 10 years. To date, the largest reported bandwidth of directly modulated free-running semiconductor lasers at 1.55μm is 30GHz. On the other hand, external modulators operating at speeds of 40Gb/s are currently available commercially, and modulators operating at speeds in the 100GHz range are under development. Titanium indiffused lithium niobate (Ti:LiNbO3) electro-optic (EO) modulators and polymer EO modulators are the two most promising technologies. The drawback of Ti:LiNbO3 modulators, however, is their poor sensitivity, while the technology of polymer EO modulators is still very immature. Therefore, directly modulated laser sources with very high modulation bandwidths, exceeding 100GHz, are still highly desirable.
Injection locking has been actively researched for its potential to improve ultrahigh frequency performance of semiconductor lasers and to reach beyond the record values of modulation bandwidth achieved for free-running devices.1 Significant increases in the resonance frequency and modulation bandwidth have been achieved by injecting external light into diode lasers. So far, improved microwave performance has been observed in edge-emitting lasers with Fabry-Pérot cavity, distributed-feedback (DFB) lasers, and vertical-cavity surface-emitting lasers (VCSELs).1 The highest experimentally observed 3dB modulation bandwidth of ∼80GHz, by far exceeding those achieved in free-running devices, has been reported for injection-locked VCSELs.
Strong optical injection is crucial for reaching the ultimate limits of modulation bandwidth enhancement in injection-locked lasers. The smallest possible values for both cavity round-trip time and reflectivity of the mirror used for injection (maximizing the injection coupling rate coefficient) would be ideal for that application. The inherent design trade-off between these parameters, however, makes further optimization of both edge-emitting lasers and VCSELs for enhanced high-speed performance very problematic.
Figure 1. Schematic diagram of a strongly injection-locked whistle-geometry semiconductor ring laser that is monolithically integrated with a single-mode master distributed-Bragg-reflection (DBR) laser. DC: Direct current. RF: Radio frequency. WRL: Whistle geometry microring laser.
To overcome the limitations of VCSELs, we recently proposed a novel injection-locking scheme (see Figure 1) involving a distributed-Bragg-reflector (DBR) master laser monolithically integrated with a unidirectional whistle-geometry microring laser (WRL).2 Our approach allows for complete coupling of the master laser output into the ring laser, providing a dramatically increased injection coupling rate. We confirmed the advantage of the novel injection-locking scheme using numerical modeling. A greatly enhanced resonance frequency of up to ∼160GHz was predicted in numerical calculations for the strongly injection-locked ring laser.
Figure 2. Two strongly injection-locked cascaded WRLs are monolithically integrated with a single-mode master DBR laser.
Modulation frequency response of the free-running WRL (red curve) and that of the second injection-locked WRL (cascaded injection-locking scheme of Figure 2
) calculated for several values of positive frequency detuning (Δω2
) between the master laser and the second WRL. Frequency detuning between the master laser and the first injection-locked WRL (Δω1
) is 100GHz. Modulation frequency response is normalized to low-frequency response of the free-running ring laser. dB: Decibels.
Typical of all optical injection-locking schemes, however, the modulation response showed a very significant reduction in the modulation efficiency between low frequency and the resonance frequency, which limits the usefulness of the approach to narrowband applications. One possible way to overcome the low-frequency roll-off problem and to attain tailorable and broad modulation bandwidth is to use cascaded injection locking.3 Consequently, we modified our original injection-locking scheme (see Figure 1) to a cascaded system with two strongly injection-locked unidirectional WRLs (see Figure 2), where the modulated optical output of the first ring laser is used to injection-lock the second one. We modeled the dynamics of two strongly injection-locked cascaded microring lasers (monolithically integrated with a single-mode master DBR laser) by a system of rate equations written in terms of the photon numbers, phases, and total carrier numbers in the master DBR and microring slave lasers. Using numerical simulations, we calculated the modulation response of the second ring laser for a small-signal modulation applied to the first ring laser, assuming certain frequency detunings between the master DBR laser mode and the modes of the two ring lasers. We observed a notable enhancement of the modulation response in the cascaded scheme of Figure 2 as compared to that of the single strongly injection-locked ring laser of Figure 1. This was due to the second resonance peak occurring at a lower modulation frequency, corresponding to the frequency detuning between the master and the second ring laser (see Figure 3).
Our work demonstrates the possibility of overcoming the low-frequency roll-off problem and attaining tailorable and broad modulation bandwidth in injection-locked whistle-geometry semiconductor ring lasers. Our future work will involve combining this injection-locking scheme with multiple cascaded ring lasers, which should result in flat broadband modulation response.
Marek Osinski, Gennady A. Smolyakov
Center for High Technology Materials
University of New Mexico
1. E. K. Lau, L. J. Wong, M. C. Wu, Enhanced modulation characteristics of optical injection-locked lasers: a tutorial, IEEE J. Sel. Top. Quant. Electron. 15
, no. 3, pp. 618-633, 2009. doi:10.1109/JSTQE.2009.2014779
2. G. A. Smolyakov, M. Osinski, Rate equation analysis of dynamic response in strongly injection-locked semiconductor microring lasers, Proc. SPIE
7933, pp. 79330D, 2011. doi:10.1117/12.878806
3. X. Zhao, D. Parekh, E. K. Lau, H. K. Sung, M. C. Wu, W. Hofmann, M. C. Amann, C. J. Chang-Hasnain, Novel cascaded injection-locked 1.55-μm VCSELs with 66GHz modulation bandwidth, Opt. Express 15
, no. 22, pp. 14810-14816, 2007. doi:10.1364/OE.15.014810