Vertical-external-cavity surface-emitting lasers (VECSELs) were realized for the first time [1] about two decades ago. These lasers have gained a lot of attention in recent years due to their wavelength flexibility and due to the open resonator [2], which enables the use of a variety of intracavity optical elements, such as spectral filters or frequency converting elements. In previous work on AlGaAs-based VECSELs, GaAs quantum wells (QWs) were used to address wavelengths around 850 nm [3–6]. The spectral range from 700 nm to 800 nm has been barely covered by optically pumped VECSELs with a gap around 765 nm. To address this wavelength range, different methods have already been applied. The use of InP quantum dots as laser active material opened first access to fundamental ${\mathrm{TEM}}_{00}$ emission from 716 nm to 755 nm [7] with VECSELs. Hereby, output powers in the sub-100 mW range were achieved. With Raman conversion of a red VECSEL [8], the wavelength range from 736.6 nm to 750.4 nm could be addressed. Also, a volume holographic grating stabilized VECSEL, solely operating at 780 nm and delivering a maximum output of 30 mW, was realized [9]. Further, with wafer-fused gain structures applying intracavity second-harmonic generation (SHG), the spectral range from 720 nm to 764 nm [10] and emission at 785 nm [11] could be realized, reaching an output power of 1.5 W and 1 W, respectively. The 780-nm-range was also addressed recently with a distributed-feedback laser [12] to realize narrow linewidth operation.

The spectral range between 700 nm and 800 nm is highly interesting for a variety of medical applications in the field of dermatology [13], thanks to the optimum penetration depth into human skin [14,15]. To this day, alexandrite lasers [16,17] are used to address these wavelengths of medical interest. Also in atomic physics [18], several useful energy transitions (rubidium Rb, 780 nm and potassium K, 766 nm [19]) lie on this spectral region. Furthermore, the new and efficient magnetically activated and guided isotope separation (MAGIS, [20]) method could strongly benefit from a 770 nm laser to excite the $4\mathrm{s}{{}^2\mathrm{S}}_{1/2}$ energy transition in potassium [20,21].

In this Letter, we present a VECSEL structure designed for fundamental laser emission in the long 700-nm wavelength regime. The architecture of the AlGaAs-based VECSEL gain chip as well as results of characterization are presented below. The VECSEL structure was grown on an undoped $2^{\prime \prime }$ GaAs $(100)\pm 0.5°$ wafer in a V80H-10 VG Semicon solid source molecular beam epitaxy (MBE) system at a growth temperature of about 575°C. A GaAs buffer layer was first grown on the substrate followed by a distributed Bragg reflector (DBR) consisting of 33.5 pairs of $\lambda /2$-thick AlAs and ${\mathrm{Al}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{As}$ layers to create a broadband reflectance of $R_{\mathrm{DBR}}>99.9\%$ centered at the design wavelength of 765 nm. The active region was grown directly on top of the DBR. The structure was designed to be resonant and completely symmetric. The refractive index as well as the simulated electric field intensity distribution at the design wavelength are plotted in Fig. 1. The active region consists of 12 ${\mathrm{Al}}_{0.12}{\mathrm{Ga}}_{0.88}\mathrm{As}$ QWs with a thickness of about 7 nm each, arranged in four groups of three QWs each. The QWs are separated by 8 nm thick layers of ${\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ and are also embedded with 12 nm of the same material. As cladding material separating the QW groups, we employ ${\mathrm{Al}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$, which has an absorption edge at $\sim 576\mathrm{nm}$ [22]. The active region was sandwiched between 10 nm thick AlInP and GaInP layers, which help in blocking the electron diffusion and act as a window layer at the semiconductor–heat spreader interface, respectively. No thinning or other processing steps were applied. The sample was cleaved into $2.7\times 2.7{\mathrm{mm}}^2$ pieces, which were liquid capillary bonded [23] to an uncoated $3\times 3{\mathrm{mm}}^2$ sized and 300 μm thick single crystalline intracavity diamond heat spreader. The heat spreader gain chip package is then further mounted to a thermo-electrically controlled copper heat sink, which is cooled via a water/glycol cooling system. The gain chip was incorporated in both a linear (see Fig. 2) and a v-shaped laser resonator (for details, see schematic Fig. 3), the former to allow optimized power performance, the latter to allow incorporation of a birefringent filter for spectral control. As a pump laser, we used a coherent Verdi V18 emitting at ${\lambda }_{\mathrm{pump}}=532\mathrm{nm}$. The collimated pump beam ($Ø=2.25\mathrm{mm}$) was focused on the VECSEL chip with an anti-reflection coated lens ($f=200\mathrm{mm}$). The angle of incidence $\beta$ of the pump beam was about 17.5° in both the linear and v-shaped resonators. This resulted in a $(18.3\pm 0.3)\%$ reflection of the incident pump beam on the diamond–air interface. The pump lens distance was kept at $(197\pm 1)\mathrm{mm}$, which results in a calculated pump spot diameter (short axis) of $D_{\mathrm{pump}}=(62.5\pm 2.8)\mathrm{\mu m}$. This pump spot diameter, and therefore the pump lens distance, was kept the same in all measurements presented in this Letter because it yielded the optimal performance. This means to adjust the laser resonator length to maximum output power while maintaining fundamental ${\mathrm{TEM}}_{00}$ mode. At this condition, the pump spot diameter is equal to or slightly larger than the mode diameter but still small enough not supporting higher order transverse modes. For the linear resonator, the length was adjusted to $\sim 99.8\mathrm{mm}$, which results in a calculated mode diameter of $\sim 64\mathrm{\mu m}$ on the VECSEL chip. In all measurements, the heat sink temperature $T_{\mathrm{hs}}$ was kept at 14°C. The spectrometer with which all spectral measurements presented in this Letter were taken, was a StellarNet BLUE-Wave with a resolution limit of 0.8 nm. Due to the relatively low resolution of the spectrometer, Fabry–Perot resonances caused by the plane-parallel intracavity diamond heat spreader are not visible in the measured spectra. Namely, at a laser wavelength of 765 nm, the calculated free spectral range is around 0.4 nm for the 300-m thick diamond.

 

Fig. 1. Design of the active region of the gain chip is shown in this figure. Refractive index as well as simulated electric field distribution is plotted versus cross-section distance in the chip.

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Fig. 2. Long-term exposure (30 s, Canon EOS 650D) photograph of the AlGaAs-VECSEL operated in the linear resonator. On the left side, one can see the HR external mirror; on the right side, the VECSEL chip (origin of the intracavity beam) is mounted.

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Fig. 3. Schematic drawing of the v-shaped cavity used for the tuning measurements (see Fig. 5). The beam profile (see Fig. 6) and the power transfer measurement at the fixed wavelength of 780 nm (see Fig. 7) were also taken in this configuration.

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A photograph of the operating AlGaAs-VECSEL in the linear resonator is shown in Fig. 2. The heat sink-mounted VECSEL chip is located on the right side. A highly reflective (HR) mirror with a radius of curvature of $r_{\mathrm{HR}}=100\mathrm{mm}$ and diameter of $1/2^{\prime \prime }$ located on the left side completes the linear resonator. The HR external mirror was used instead of an outcoupler to enhance the intracavity circulating power. The used 720 nm long-pass filter prevents over-exposure by visible wavelengths. Therefore, the pump laser beam is not visible. For optimized power performance characterization of the laser, we employed the linear cavity as shown in Fig. 2. It was operated with an outcoupler with a reflectivity of $R_{\mathrm{out},\mathrm{curve}}=97\%$ and a radius of curvature of $r_{\mathrm{out}}=100\mathrm{mm}$. Figure 4 shows the output power of the free-running VECSEL as a function of incident pump power for the linear cavity. A typical linear input–output behavior with a slope efficiency of ${\eta }_{\mathrm{diff}}=27.1\%$ can be seen. The laser threshold appears at a pump power of $P_{\mathrm{pump},\text{thr.}}=0.38\mathrm{W}$. A maximum output power of $P_{\mathrm{out},\max }=4.24\mathrm{W}$ was obtained. A thermal rollover could not be clearly identified, but it seems to appear at maximum pump power. Due to a limited amount of pump power (18 W) available in this work, a more detailed investigation was not possible. Measurements of the laser emission spectrum, plotted in the inset of Fig. 4, show the full span of thermal shift that could be observed during the input–output measurement. The laser emission peak maximum shifts from 765 nm, obtained at very low pump powers, to 773 nm at the highest power, and consequently the full width at half maximum (FWHM) increases from 3.2 nm to 4.9 nm. Taking into account the quantum efficiency ${\eta }_{\mathrm{quant}}={\lambda }_{\mathrm{pump}}/{\lambda }_{\mathrm{out}}$, this is not surprising, because about 30% of the absorbed pump power is directly transferred into heat.

 

Fig. 4. Power transfer measurement data of the free-running VECSEL, taken in a linear resonator. Inset: two typical laser spectra taken during the input–output measurement at low and high pump powers.

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Wavelength tuning measurements (see Fig. 5), the beam profile measurement (see Fig. 6), and the power transfer measurement at a fixed emission wavelength of 780 nm (see Fig. 7) were performed in the v-shaped resonator (see Fig. 3). Tuning measurements were performed via rotating the intracavity birefringent filter to different angles to obtain information about the spectral distribution of the residual gain bandwidth. It is delivered by the material gain influenced by sub-cavity, heat spreader, birefringent filter, and cavity resonances, including all losses in the present VECSEL system. Several exemplary emission spectra normalized to the corresponding output power are shown in Fig. 5. The measurements were performed for the v-shaped cavity with a HR folding mirror M1 ($r_{\mathrm{fold}}=75\mathrm{mm}$) and a planar outcoupler M2 ($R_{\mathrm{out},\mathrm{plan}}=97\%$). Incident pump power during the tuning measurement was $P_{\mathrm{pump},\text{inc.}}=9.6\mathrm{W}$. The thickness of the used birefringent filter was 0.5 mm, resulting in a free spectral range of $\sim 130\mathrm{nm}$ for laser wavelengths around 765 nm. A tuning range of 40.5 nm, i.e., from 747.5 nm to 788.0 nm, was attained. In Fig. 5 also, photoluminescence (PL) emission curves, centered at $\sim 757\mathrm{nm}$ and belonging to the two long-wavelength laser emission spectra, are part of the measured data. This is the case, as the spectra were measured from one of the low-power reflections from the birefringent filter’s surfaces. Due to the relatively low laser power, this reflected light contains also a more significant fraction of PL emission comparable to the laser peak intensity.

 

Fig. 5. Several exemplary emission spectra, taken for different angles of rotation of the intracavity birefringent filter, are shown normalized to the corresponding output power (v-shaped cavity; see Fig. 3).

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Fig. 6. Typical beam profile of the VECSEL, taken from the outcoupled laser beam in the v-shaped cavity. Additional vertical and horizontal cross sections including Gaussian fits are plotted.

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Fig. 7. Power transfer measurement performed in the v-shaped cavity and (inset) a typical spectrum, here taken at an incident pump power of $P_{\mathrm{pump}}=14.4\mathrm{W}$.

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Figure 6 shows a typical beam profile taken with an OPHIR Beamstar-FX-66-NT beam profile camera at an output power of 720 mW behind the planar outcoupler in the v-shaped cavity with included birefringent filter. A perfect Gaussian intensity distribution can be seen, as it can be expected from semiconductor disk lasers. The slight ellipticity of $1.26:1$ can be derived from the ${\mathrm{FWHM}}_{\mathrm{X}}$ and ${\mathrm{FWHM}}_{\mathrm{Y}}$ values given by the Gaussian fit functions applied on the intensity cross sections. Simulations of the beam propagation in the laser cavity with the open-source software “reZonator” deliver exactly the same ellipticity value. This is mainly due to the folding angle of $\alpha =22°$ (see Fig. 3). No mode jumps or non-${\mathrm{TEM}}_{00}$ beam profiles were observed in the well-adjusted cavities during the measurements.

Regarding applications, the suitability of the VECSEL to Rb spectroscopy [24,25] was preliminarily examined by performing power transfer investigations near the D1 absorption line of Rb, which lies at the wavelength of 780 nm. The VECSEL was spectrally adjusted with the birefringent filter to this wavelength. The measurement plotted in Fig. 7 shows more than 2.8 W output power at its maximum and a slope efficiency of ${\eta }_{\mathrm{diff}}=21.7\%$. This result shows that there is sufficient laser power available, even if further losses would be induced via frequency stabilization and spectral filtering. Such stabilization and spectral filtering are necessary to operate the VECSEL in the kilohertz linewidth range [26] with several hundreds of milliwatts, which are typically needed for spectroscopy applications. The laser threshold in the used v-shaped cavity occurs at a pump power of $P_{\mathrm{pump},\text{thr.}780}=4.32\mathrm{W}$, which is a high value compared to the measurement taken in the free-running configuration. This could be expected as the 780 nm emission is located quite at the long-wavelength edge of available gain referring to the tuning measurement (see Fig. 3). The most probable explanation is that semiconductor gain typically broadens with increasing charge carrier density, which rises with increasing pump power, while its maximum shifts slightly to higher energies [27]. A thermal red shift of the gain might also play a role here, which additionally supports long-wavelength operation. The v-shaped resonator with its folding mirror M1 and the birefringent filter as intracavity element create additional losses. This further increases the laser threshold. The VECSEL’s linewidth (FWHM), dictated by the transmission characteristic of the birefringent filter, was approximately at 2.7 nm during the whole measurement. A typical spectrum is plotted in the inset Fig. 7.

With this work, we demonstrated a direct emitting VECSEL that was designed to operate at wavelengths around 765 nm for the first time to our knowledge. We present fundamental characterization results of this all-AlGaAs-based double hetero-structure VECSEL. A maximum output power of 4.24 W was obtained as well as a tuning range of 40.5 nm from 747.5 nm to 788.0 nm. Fundamental transverse mode operation was always observed during VECSEL operation.

To further widen the access to the 700 nm to 800 nm range with VECSELs, slight variations in the aluminum content in the QWs would reduce emission wavelengths and enable a spectral coverage down to 700 nm [28]. In order to prevent unnecessary heating of the active region, the use of red diode lasers emitting at 640 nm or 675 nm as pump sources would be rational for future investigations. In terms of choice of material, it would also be useful to investigate, e.g., GaAsP-based gain structures and compare them with the present ones. GaAsP-based QWs have formerly been used to reach the 700-nm-regime [29–32]. Furthermore, another alternative to deliver 7XX-nm emission is represented by the AlGaInAs material system [32].