1. Introduction

Despite the success of quantum-cascade lasers in the mid (MIR)- and far-infrared (FIR) spectral ranges [1], room-temperature continuous-wave operation at the wavelengths longer than 10 μm seems to be unattainable in the near future. Therefore, alternative approaches to achieve MIR (or FIR) radiation should be examined. Nonlinear optical three-wave interaction, accompanying by generation of difference frequency in these ranges can be regarded as an attractive approach [2,3]. To achieve an effective frequency-down conversion it is desirable to have a laser launching simultaneously two parent wavelengths. Moreover, the optimal device should emit both the wavelengths co-axially with total spatial overlap at high light power. In addition, an external cavity allowing for the use of a nonlinear crystal would be desirable. A dual-wavelength vertical external cavity surface-emitting laser (VECSEL) is an almost perfect device to fulfil these requirements. Intracavity difference frequency generation, instead, achieved by mixing of radiation from two separate lasers always needs complex equipment, including a servocontrol external cavity frequency [4]. In sharp contrast to this, in a dual-wavelength VECSEL, both wavelengths are modes of the cavity and remain enhanced by the cavity, irrespective of slight variations in resonance frequencies, thus enhancing the efficiency of nonlinear three-wave interaction.

Recently, an optically pumped dual-wavelength VECSEL was demonstrated by our group [5]. When designing a dual-wavelength semiconductor laser with wavelength separation tens of nanometers or more, two main difficulties appear. One of them is concerned with optical field interactions in the active region of the device; the other is related to possible mode competition for the gain. These problems are solvable in various ways [6–15]. The optical field interactions, mainly consisting of absorption of the shorter wavelength λ_s in the gain material of the longer wavelength λ_L, can be solved to some extent by having physically separated gain materials for λ_s and λ_L [6–8] or by using a wide-band semiconductor gain material, combined with a selective feedback from a grating and a slit mirror [9,10]. These lasers would include the use of complicated optics to produce a co-axial dual-wavelength beam. Approaches exploiting vertical coupled-cavity geometry offer somewhat more compact solutions [11–15]. However, it is necessary that the cavity coupling remains weak in order to diminish optical mode interaction, which means that the wavelength separation Δλ. = λ_L -λ_s > 30 nm would hardly be achievable [15]. An independent pump of the individual gain elements [6–8,13–15] or a specially designed multi-quantum well active region [9,10] have been used to alleviate the mode competition for the gain.

In our laser [5] the absorption of λ_s was reduced by placing the λ_L quantum wells (QWs) at the nodes of the λ_s standing wave pattern. This approach worked quite well, but due to slight unavoidable inaccuracies in growth of the laser structure and in modelling the device, residual absorption led to unstable operation of the laser under high-power emission [16]. The independent pumping of the QWs operating at λ_s and λ_L was achieved by dividing the active region into the subsections with the carrier blocking layers.

Recently, successful sum-frequency generation was obtained by our device [17], confirming the potential of dual-wavelength VECSEL for nonlinear frequency conversion.

A linearly polarized dual-wavelength operation at a multiple watt power level by an optically pumped VECSEL utilizing a tilted intracavity Fabry-Perot etalon and a Brewster window has been reported [18]. However, Δλ achieved was only about 2.1 nm.

As compared to edge emitting lasers, VECSELs have many attractive features which could be applied to their dual-wavelength counterparts. These features include good power scaling and high power operation in a circularly symmetric fundamental TEM₀₀ mode [19]. To overcome the instability problems of our previous laser and to expand Δλ, we demonstrate in this paper a dual-wavelength VECSEL that does not require high accuracy in locating the λ_L QWs or in modelling the device. Our new device has a separate gain region for λ_s and λ_L. The λ_L gain region is isolated by an optical long-wave-pass filter to block λ_s from entering into the λ_L QWs. The structure resembles coupled-cavity VECSEL [11,12] with a specially designed intermediate mirror. Both the beams share the same optical axis, making alignment of the laser simple, and the optics required is similar to that of any single-wavelength VECSEL.

2. Device design

The laser was designed to be used as a source for intracavity difference frequency generation with a quasi-phase-matched GaAs nonlinear crystal as in [2,3]. Therefore, the wavelengths were chosen to fit the transparency window of the nonlinear crystal, to ensure good carrier confinement for λ_S QWs, and to control lattice strain introduced in the λ_L QWs. The maximum Δλ is limited by the last two factors, whereas for this particular design, the minimum Δλ is limited by the slope of the intracavity filter. Within these limits, Δλ_max and Δλ_min, the two wavelengths can be chosen arbitrarily. In order to avoid absorption of λ_S in the λ_L QWs, the gain sections for λ_S and λ_L were spatially separated. The λ_S QWs were located close to the surface of the device, while λ_L QWs were situated deeper in the structure. A long-wave-pass filter, which consisted of alternating 81-nm thick AlAs and 72-nm Al_0.30GaAs layers, was placed between the two gain regions to prevent λ_S from penetrating into the λ_L QW region. The computed electric fields inside the layer structure are displayed in Fig. 1. The stop-band of the long-wave-pass filter was chosen to have λ_L at one of the reflection minima on the λ_L side of the filter.

 

Fig. 1. Index profile (continuous line) of the dual-wavelength VECSEL along with the electric field of λ_S (dashed line) and λ_L (dotted line).

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The λ_S and λ_L gain sections contained six compressively strained In_0.14Ga_0.86As QWs and seven strained In_0.25Ga_0.75As QWs, respectively, situated in the anti-nodes of the cavity standing wave to ensure good coupling between the optical field and the QWs. To cancel lattice strain induced by the QWs, the top active region had two strain-compensating layers of GaAs_0.70P_0.30 of 20 nm in thickness (see Fig. 1). The bottom active region had two 10-nm and three 20-nm thick GaAs_0.70P_0.30 layers. Carrier transport across the GaAs_0.70P_0.30 layers was blocked by high potential-energy barriers, which were automatically induced by the band-gap discontinuity at the GaAs_0.70P_0.30/GaAs heterojunctions. The locations of the GaAs_0.70P_0.30 layers were chosen to equalize absorption of pump light per QW.

3. Device fabrication and measurement setup

The layer structure was grown by molecular beam epitaxy (MBE) on a 2” n-GaAs (100) substrate after growth-rate calibrations under similar growth conditions to those given in [4]. A close agreement between the simulated reflectance and the measured reflectance is seen in Fig. 2. The largest deviations appearing near the absorption peaks of the QWs and near the resonances inside the structure are due to slight uncertainties in absorption coefficients and a red-shift chosen for computer simulations.

 

Fig. 2. Measured (continuous line) and simulated (dashed line) reflectivity spectra of the dual-wavelength VECSEL structure.

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A 2.5 × 2.5 mm² chip of the gain material was capillary bonded to a natural diamond heat spreader. The V-shape VECSEL cavity setup with a 1 % output coupler was identical to the one published in [5]. The pump wavelength was 808 nm.

4. Results and discussion

The laser mount temperature was kept at 15 °C in all measurements. The output power (P_out) per laser beam was studied by means of a short-wave-pass filter. The threshold pump power was 2.3 W for λ_L and 5.4 W for λ_S (see Fig. 3). The maximum conversion efficiency of 10.8 % was achieved at the pump power of 16 W, corresponding to P_out ≈ 0.5 W and 1.2 W at λ_S and λ_L, respectively. The higher threshold and the lower slope efficiency for λ_S were likely due to the smaller number of the λ_S QWs and a weaker carrier confinement than those for λ_L.

The long wavelength, λ_L, reached a thermal roll-over condition before λ_S did so (which happened at the pump power of 19.4 W) because of higher thermal impedance caused by the optical long-wave-pass filter in the heat dissipation pathway. Therefore, the maximum simultaneous P_out was 0.75 W for λ_S and 1.38 W for λ_L at the pump power of 21.2 W.

Figure 4 shows the emission spectrum at the pump power of 17.7 W. The spectrum is made up of two peaks, one at λ ≈ 966 nm, the other at 1047 nm (record large Δλ ≈ 81nm for VECSELs). The beam quality factor M² was 1.38 and 1.51 in the lateral directions (at the pump power of 16 W), measured by a scanning slit device with 5-μm wide slits and a silicon detector.

 

Fig. 3. Output power versus pump power for λ_S (dashed line) and for λ_L (dotted line).

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The temporal behaviour of the laser was investigated by measuring intensities of λ_S and λ_L versus operational time with a 1-GHz silicon detector. Both emissions exhibited unstable operation not until for P_out > 300 mW, having an AC component of little more than 20 % of the average intensity. The period of the AC component was about 2.5 ns. When compared to our previous dual-wavelength laser, instabilities of the new laser occurred at about twice higher P_out than those of the previous laser [5]. We believe that the short-wavelength absorption by the deeper QWs of our earlier design is now solved quite satisfactorily. However, other reasons for intensity noise (e.g., incomplete overlapping of the pump and lasing beams in the rather long active region, or short wavelength feedback through the filter) may still be present in the new design, but in fact intensity noise has also been observed in a single-wavelength VECSEL in [20], i.e., this phenomenon is not related to dual-wavelength emission alone. A more precise laser design and alignment will likely eliminate noise and increase P_out of this first ever proof-of-concept device.

 

Fig. 4. Output spectrum of the dual-wavelength VECSEL.

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5. Conclusions

We have demonstrated a high-power optically pumped dual-wavelength laser designed particularly for intracavity difference frequency generation with a quasi-phase-matched GaAs nonlinear crystal by integrating feedback mirrors and gain regions of two VECSELs into a single monolithically grown semiconductor heterostructure chip. The maximum output power, which was simultaneously obtained for the two wavelengths at 15°C, was 0.75 W for λ_S ≈ 966 nm and 1.38 W for λ_L ≈ 1047 nm, using the pump power of 21.2 W. Both emissions were stable for P_out < 300 mW, being significantly more stable than what was obtained by our previous dual-wavelength laser.