Long wavelength Vertical Cavity Surface Emission Lasers (VCSELs) operating at 1.55 µm and above have known an increase interest in recent years. They have proven to be efficient transmitters for optical fiber communication networks [1–3]. VCSELs appear as the ideal source in optical telecommunication networks as they exhibit a low power consumption, a high spectral purity, a high coupling efficiency with optical fibers, and a strong potential in low cost fabrication. Quantum Wells (QW) are commonly used as a high gain medium in VCSELs, and are usually grown on conventional (001) oriented substrates. Despite these interests, QW-VCSELs suffer from the existence of two competiting orthogonal polarization eigenstates of emission [4,5]. This appears as a severe drawback, as numerous components of optical interconnect and networks, such as polarizers, quarter- and half-wavelength plates are polarization sensitive. The lack of a well-defined polarization selection mechanism leads to the coexistence, switching, or bistabillity of lasing modes. Under digital modulation operation, the polarization ratio reached during each pulse fluctuates randomly. This fluctuation is more pronounced at higher modulation frequencies and is a major source of Relative Intensity Noise (RIN) or polarization noise in laser devices, leading to a crippling bit error rate [6].

Polarization instability in QW-based VCSELs has been studied by many research groups. It has been shown that residual strains in the epitaxial structure may lead to slightly different refractive indexes along the two main crystallographic directions [110] and [1-10) [7]. There is though the coexistence of two orthogonal resonance eigenmodes. The two modes are in competition, and are driven by the overlap between those and the gain curve. Each mode can exist and switch one with each other, depending mainly on the input power excitation, the temperature-dependent cavity dilatation, or the parasitic optical feedback.

Later, theoretical models based on spin relaxation processes depict more precisely quantum mechanisms involved in the determination of the polarization state in VCSELs, and how it can be controlled [8–10].

Several groups have already demonstrated nearly polarization-stabilized VCSELs, by introducing an internal loss anisotropy, such as birefringent mirrors [11], elliptical cavities [12], and more recently birefringent gratings [13] and high-contrast sub-wavelength gratings [14]. Despite its clear interest, this last approach requires complex technological processing. Recently, we have proposed a process free approach based on the use of a strongly anisotropic gain medium constituted of elongated nanostructures, referred as quantum dashes (QDHs) These nanostructures have already proved to be efficient to control of polarization in GaAs-based VCSELs [15].We have thus demonstrated polarization-stable emission of QDH-based VCSELs on InP, along the [1–10] crystallographic direction under continuous wave (CW) operation with an extinction ratio over 30dB above threshold, and over 4 dB below threshold [16]. As a comparison, QW-based VCSELs exhibits random polarization states below and above threshold, with various extinction ratios.

In this letter, 1.55 µm emitting QDH- and QW-VCSEL are fabricated, and their performances are compared in terms of polarization stability when operating under a low-frequency modulation. We evidence a clear improvement by using QDH as an active medium to decrease the device RIN related to random polarization VCSEL output fluctuations.

VCSEL samples have been grown on (001) nominally oriented InP substrates. The active region consists of three sets of 6 QDH layers (or three sets of three InGaAs QWs lattice-matched on InP), each set being located at a stationary electric field maximum intensity position in the microcavity. More details about the QDH growth can be found in reference [16]. The micron-scale cavity is designed using sputter-deposited Bragg mirrors, based on two dielectric materials, amorphous silicon (a-Si) and amorphous silicon nitride (a-SiN_x). These two materials display a high refractive index difference (Δn = 1.9), which allows to benefit from a better reflectivity and thermal conductivity in comparison with the epitaxial Bragg mirrors counterparts available on InP substrate [17]. After the deposition of the bottom Bragg mirror including 6 a-SiN_x/a-Si periods, an Au–In eutectic bonding is performed to transfer VCSEL samples on silicon substrates. The top Bragg mirror is deposited after the removing of the InP substrate by a mechanical polishing and a selective chemical etching [17]. Both samples exhibit laser emission at room temperature around 1.55 µm under CW operation. Threshold densities of power are quite similar and close to 15 kW.cm⁻².

Dynamic polarization state in VCSEL emission is studied with a comparison between both the QW-VCSELs and QDH-VCSELs. An acousto-optical modulator (AOM) has been used to create a 1064 nm optical pumping signal with 1µs-long pulses, and a repetition period of 10µs. This pulse duration is short enough to overcome any thermal effect. Indeed, with a typical time of a few µs, thermal roll-over can cause a decrease in the output power over one pulse. In our case, this was not observed for pulses duration below 5µs. The 1064 nm laser beam is focused with a microscope objective on the VCSEL samples with a spot diameter of 10 µm. The VCSEL output signal is collected back by the same objective and measured with a fast InGaAs detector, and polarization-resolved measurements are extracted with the use of an optical polarizer presenting a 30 dB extinction factor. Figure 1 displays the average intensity of VCSEL optical output power as a function of the pump power, measured over 10 following pump pulses (referred in the following as the <P> characteristics). QW-based VCSEL exhibits a threshold power density of 20 kW.cm⁻², which is comparable to reported values obtained for similar devices [18]. In comparison, the QDH-VCSEL exhibits a reduced 13 kW.cm⁻² threshold power density, which may be linked to the reduced density of states related to the QDHs. Nevertheless, such direct comparison is rather difficult to make, as experimental thresholds are determined from total incident pump power, rather the real absorbed pump power. Indeed, the cavity reflection at the pump wavelength may vary from one VCSEL to another, inducing variations in total incident pump power threshold.

 

Fig. 1 QW-VCSEL (a) and QDH-VCSEL (b) average optical output power versus optical pump power, measured along the [1–10] (triangles) and [110] (squares) polarization orientation at low frequency modulation (1µs, 100kHz). Circular dots display the characteristics without any polarization selection. Inserts present the output spectrum of the QDH (a) and QW (b) devices, measured at an incident power being twice the corresponding threshold.

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Figure 1(a) represents the polarization resolved <P> characteristics of the QW-VCSEL. Inserts present the output spectrum of both devices (in log scale) for an incident power being twice the threshold. Single mode operation is demonstrated for the QDH and QW devices respectively at 1.61 µm and 1.54 µm, as the free spectral range intensity ratio is higher than 40 dB in both cases. At low excitation power, the output emission is fundamentally polarized along the [110] axis. At incident power close to 30 mW, a polarization switch occurs, and the output emission becomes mainly polarized along the [1–10] direction. As mentioned previously, such behaviour has been classically observed for VCSEL based on QW grown on substrates with a usual orientation [5,7]. Note also that from a location to another on the VCSEL sample, polarization state may vary as well as the polarization switching threshold [5,7,16]. On the opposite, as shown on Fig. 1(b), the QDH-VCSEL exhibits a stable polarization state along the [1–10] direction over the whole range of operation, and this polarization state is not dependent from the location on the VCSEL sample [16]. The next step of the study is the observation over time, of the polarization state of both VCSELs, under a low-frequency optical modulation. Typical measured output oscillogrammes, above threshold, are presented in Fig. 2 , as function of the polarizer orientation (along [110] or [1–10]) for the QW- and QDH-VCSELs. Note also that measurements along the two orthogonal directions are not made simultaneously, but one after the other.

 

Fig. 2 Typical oscillogrammes of a QW-VCSEL (a,b) and a QDH-VCSEL(c,d), measured along the two main polarization axis [1–10] (black/left) and [110] (red/right).Blue dashes represent calculated interval of variation due to the fluctuations of the pumping laser. Measurements are performed at room temperature, under low frequency modulation (1µs, 100kHz).

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From these figures, it appears that the QW-VCSEL exhibits important intensity fluctuations from one pulse to another. This is observed on both of the two main polarization states. The QDH-VCSEL output intensity remains quite constant along the [1–10] polarization orientation, and there is no laser emission along the [110] direction. These observations are consistent with previous measurement under CW operation, depicting the control of the polarization state of the VCSEL when QDHs are used as an active medium. The slight intensity variation observed on Fig. 2(c) is mostly related to the slight fluctuations of the incident pump (about 1%).

To go further in the demonstration, and in order to quantify both dispersions of polarization-resolved intensity, we performed statistical measurements. These consisted in measuring the peak intensity distribution over 500 consecutive laser pulses, along one polarization orientation, for both VCSELs, as function of the incident power above threshold (P_th). Distributions of those intensities are presented in the Fig. 3(a) (and Fig. 3(b)) along the [110] polarization orientation for the QW-VCSEL (along [1–10] for the QDH-VCSEL respectively).

 

Fig. 3 Polarization-resolved output intensity dispersion curves, measured along the [110] direction for the QW-VCSEL (a), and along the [1–10] direction for the QDH-VCSEL (b), for increasing incident power above threshold (from 1.0 Pth up to 2.3 Pth). Each dispersion curve is calculated over 500 consecutive pulses, at low frequency modulation (1µs, 100kHz).

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From Fig. 3(a), it is clear that the QW-VCSEL dispersion width presents a large variation as function of the incident power, with a maximum at a pump power around the polarization switching regime observed previously (Fig. 1(a)). Indeed, when operating around the switch power (~2*Pth), the dispersion width is 6.4 times larger than the one measured at threshold. Thus This width increase of a factor superior to 2 while doubling the pump power indicates that excess intensity noise is introduced, related to an enhanced random polarization switching. In the case of the QDH-VCSEL (Fig. 3(b)), the dispersion width remains weaker than the QW-VCSEL, and increases slightly with incident power, being less than 2 times larger when the incident power doubles. As mentioned previously, this indicated that such reduced increase in QDH-VCSEL dispersion width is mainly connected to the fluctuations of the pump intensity. From this confrontation, we can affirm that QDH-VCSEL is much more stable than QW-VCSEL. The use of QDH as an active region for the VCSEL enables to increase the VCSEL intensity stability, corresponding to a significant reduction of the intensity noise.

Conclusion

In conclusion, we have carried out experiments under modulated excitation to compare directly the output polarization stability of VCSELs, as function of the active region nature, being QW- or QDH-based. In the case of the QW-VCSEL, we have shown the well-known polarization switching depending on the incident excitation power, which is not observed for the QDH-VCSEL. Polarization resolved intensity dispersion experiments carried out on both devices clearly evidenced excess noise induced by the random polarization switching on QW-VCSELs. In contrast, the QDH-VCSEL exhibits a constant relative intensity noise. This confrontation demonstrates a net superior stability in the output polarized emission when QDH is used as an active medium in the VCSEL. Further measurements of polarization noise at higher frequencies of modulation are under investigation for the QDH-VCSEL. These preliminary results tend to prove that QDH-VCSELs may be ideal candidates as highly reliable sources for polarization sensitive optical telecommunication links.