Polarization effects in optically injected telecom lasers

Vertical-cavity surface-emitting lasers (VCSELs) operating at the important telecommunication wavelength of 1550nm are expected to play a key role in present and future optical networks. They offer significant inherent advantages over traditional in-plane semiconductor lasers. These include low fabrication costs, high coupling efficiency to optical fibres, and easy integration into 2D arrays. In addition, optical injection is a well-known technique for improving the performance of a semiconductor laser without modifying its design. Hence, in recent years considerable effort has gone into the study of optically injected 1550nm VCSELs.

Despite this, not much attention has been paid to the effects of different polarized injection. We analyzed the behavior of 1550nm VCSELs under the external injection of light with different polarization states. We identified various phenomena, including polarization switching (PS), polarization bistability (PB), injection locking (IL), and nonlinear dynamics, which could enable novel uses of these devices.

We first plotted a spectrum of a solitary 1550nm VCSEL: see Figure 1(a). The two modes, one lasing and one subsidiary, correspond to the two orthogonal polarizations of the fundamental transverse mode of the VCSEL. We refer to the lasing and subsidiary modes as parallel and orthogonal, respectively. We also plotted the spectrum after injection of an orthogonally polarized optical signal: see Figure 1(b). In this situation, the orthogonal mode locks to the externally injected signal, the parallel mode is suppressed, and PS occurs as the emitted polarization of the VCSEL switches from parallel to orthogonal.¹ Hysteresis may also occur associated with PS, and for those cases we also obtained PB.^1,2

Figure 1. Spectra of a 1550nm vertical-cavity surface-emitting laser in solitary conditions (a) and under orthogonal polarized injection (b).

Furthermore, we were able to use optical injection to induce IL in 1550nm-VCSELs. We measured locking for these devices on injection with arbitrary polarization and found important differences in the shape and extension of the locking bandwidth, depending on the polarization of the externally injected signal.^{3, 4} However, we observed stable IL in only a small region around the resonant wavelength of the device. Outside this region, a rich variety of nonlinear dynamics occurred.^{5, 6} We therefore extended our initial experimental analyses to study these nonlinear dynamics.

For both parallel- and orthogonally polarized injection, we identified the boundaries between regions of different dynamics such as limit cycle (or period 1, P1), period doubling (or period 2, P2), and chaos (C).^{5, 6} According to convention, we plotted these regions in the plane of injection strength versus frequency detuning to produce so-called stability maps: see Figure 2(a) for an example of a measured stability map for a 1550nm VCSEL under parallel polarized optical injection. Regions of different dynamics appear outside the IL range (light gray). These include period 1 dynamics (P1, blue), period doubling (P2, green), and chaos (C, red). We also plotted typical experimental radio frequency (RF) spectra for these dynamics: see Figure 2(b). P1 is characterized by the observation of single frequency periodic oscillations, and thus a single dominant peak appears in the spectrum. P2 is defined by the occurrence of periodic oscillations at two different frequencies (one being the half of the other). A broad and strongly irregular spectrum is characteristic of chaotic dynamics.

Figure 2. (a) Stability map. (b) Radio frequency spectra for P1 (limit cycle), P2 (period doubling), and C (chaos). P_in: Injection strength. Δf: Frequency detuning.

Finally, it is worth noting that the measured stability maps show interesting differences, depending on the case of polarized injection considered. Under parallel optical injection, similar results to those observed in edge-emitting devices were obtained, whereas a completely different shape, exhibiting novel and distinct features, was attained when the device was subject to orthogonally polarized injection.^{5, 6}

In summary, we have induced PS, PB, IL, and nonlinear dynamics in a VCSEL operating at 1550nm, which is an important wavelength for telecommunications. This rich variety of behavior holds out the promise of novel applications. We are now working toward the use of PS and PB in these devices for optical switching and optical signal-processing applications.⁷ Furthermore, we have recently proposed an optical neuron based on PS occurring in 1550nm VCSELs.⁸ We are also working on the use of the described nonlinear dynamics to develop VCSEL-based microwave frequency generators and chaotic sources for encrypted communication systems.