1. Introduction

In recent years, single-frequency vertical-external-cavity surface-emitting lasers (VECSELs) have been intensively investigated owing to their potential to combine a high output power [1,2], a narrow linewidth [3–6] and a large frequency-tunability [6] in one device. Such lasers, also called semiconductor disk lasers (SDLs), are available for a broad spectral range between the ultraviolet [7] and the mid-infrared [8] and are versatile systems that attract the attention from a wide range of application areas, such as spectroscopy [9], metrology [10], optical free-space telecommunication and laser cooling [11].

The other advantage of VECSELs arises from the combination of a semiconductor laser chip with an external cavity, in which intra-cavity elements can be easily employed to access diverse operating conditions. For instance, birefringent filters (BRFs) inside the cavity can enforce single-frequency continuous wave (CW) operation with excellent beam quality [12], while the use of an intra-cavity etalon can promote a stable two-color emission [13, 14], which can be utilized for the generation of CW THz-radiation via frequency conversion in a nonlinear crystal [15]. Employing saturable absorber mirrors VECSELs can be driven in a mode-locked regime [16,17] with peak powers as high as 4.3 kW [18]. It is worth noting, that nowadays mode-locking even without the use of saturable absorbers is observed for such lasers [19–23].

A thorough thermal management allows for considerably high output powers up to 106 W in transverse and longitudinal multimode operation [1]. Besides high output powers, however, numerous applications require a high degree of coherence and stability of the light source. Thus, frequency noise reduction, thermal stability and cavity optimization become significantly important in order to improve the performance of single-frequency VECSELs beyond current limitations. Such passive stabilization techniques serve as the foundation for high-power lasers with a narrow linewidth, prior to employing active stabilization schemes [3,6,24].

In this work, we demonstrate a narrow-linewidth single-frequency VECSEL emitting at 1013 nm, with an output power of 23.6 W. To our knowledge, it represents the highest output power for single frequency VECSELs reported so far. Passive stabilization techniques and an optimized VECSEL design are employed in order to demonstrate a sub-100-kHz free-running linewidth in the microsecond domain and a linewidth of 407 kHz for a sampling time of 1 ms, both at 23.6 W. Moreover, we point out the main contributors to frequency noise that limit the long-time stability of our high-power single-frequency VECSEL.

2. VECSEL chip design and setups

An MOVPE-grown VECSEL chip is employed with a gain region consisting of 10 InGaAs quantum wells (QWs), equally spaced by GaAsP barrier layers. The QWs are arranged to overlap with the antinodes of the standing light field, which is often referred to as a resonant periodic gain (RPG) structure. 22.5 AlAs/GaAs layer pairs form the distributed Bragg reflector (DBR), which has a reflectivity higher than 99.9% at 1013 nm. In order to obtain high output power in single-frequency operation, the micro-cavity resonance at room temperature is initially detuned from the emission wavelength of the QWs for about 18 nm. This is a similar configuration as used in previous studies in order to provide an enhanced spectral overlap and thus optimized gain at high pump powers [1]. The chip is bonded to a diamond heat spreader via solid-liquid-interdiffusion bonding and the semiconductor substrate is removed by selective wet etching.

The VECSEL is optically pumped by an 808 nm fiber-coupled laser which delivers a maximum pump power of 120 W. As shown in Fig. 1, a V-shaped cavity is formed by a high-reflectivity concave mirror (radius of curvature R_C = 600 mm) and a plane output coupler together with the VECSEL chip in between. Such a V-configuration leads to a doubled round-trip gain for a given temperature and carrier density [25], as the photons will pass the gain region twice as often as in a cavity where the chip serves as an end-mirror. This design resembles the key difference to the previous works. Because of the enhanced gain, an output coupler with 5% transmission can be employed to achieve high output power. Moreover, the laser mode does not experience any astigmatism, which makes the output beam circular and more suitable for applications. The total optical length of the cavity is 140 mm and it results in a longitudinal mode spacing around 1070 MHz. The TEM₀₀ mode on the chip has a diameter about 630 µm. To ensure fundamental-transverse-mode operation, the pump-spot size is chosen to be about 20% smaller than the laser mode. A 10-mm-thick birefringent filter is placed at Brewster’s angle (α_B) to control the emitted longitudinal mode. For a minimum loss on the lasing mode, no further intracavity elements are employed. On the other hand, other modes are further suppressed by the relatively high loss induced by the 5%-transmission output coupler. The temperature of the chip is stabilized by the thermo-electric cooler (TEC), which is attached to the back side of the copper heat sink. During operation, the heat generated inside the semiconductor is transferred by the TEC and removed by a water-cooled copper plate. Here, very soft pipes are employed in order to minimize the mechanical vibration coupled from the water cooling system to the heat sink. Most of the optical components are directly mounted on a low-frequency-damping optical table via pedestal pillar posts, which are made of stainless steel and provide high stiffness. The laser setup is surrounded by a plastic housing, which has walls of 20 mm thickness. Additionally, acoustic foam is attached to the inner side of the housing. In this way, thermal fluctuation and acoustical noise originating from the environment are strongly reduced.

 

Fig. 1 Schematic drawing of the experimental setup with boxed VECSEL, confocal scanning Fabry-Perot interferometer and frequency discriminator.

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3. Experimental results

A heat sink temperature of 16 °C allows for single-frequency operation, which is achieved at a fundamental transverse mode and a single longitudinal mode, up to a maximum output power of 23.6 W (Fig. 2).To maintain single-frequency operation at high powers, a careful tuning of the BRF is required. However, this tuning causes variations in the resulting output power. The linear fitting of the single-frequency output power, shown as the dashed line in Fig. 2, yields a slope efficiency of 44% and a laser threshold at about 15 W net input power. Neglecting the 30%-high reflection loss of the pump beam at the air-chip interface, a total optical-to-optical efficiency of 33% is achieved for an output power of 23.6 W at 71.2 W net input power.

 

Fig. 2 Output power as function of net input power.

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Single-frequency operation is confirmed via a self-made high-resolution confocal scanning Fabry-Perot interferometer (SFPI) which reveals a free spectral range (FSR) of 500 MHz [Fig. 3(a)]. The slightly asymmetrical shape of peaks in the SPFI spectra is due to misalignment and the difference between rise- and fall-time of the current amplifier employed in the SPFI. At an output power of 20 W, single-frequency operation which lasted for more than one minute without mode hopping was observed. As the net input power is increased above 70 W, the VECSEL starts to experience thermal roll-over [26]. If the chip is pumped stronger, side-peaks will arise and single-frequency operation will turn into multiple-longitudinal-mode operation, which is indicated by black squares in Fig. 2. The corresponding SFPI spectrum is shown in the inset of [Fig. 3(a)]. To demonstrate the fundamental-transverse-mode profile, a CCD camera image is recorded under single-frequency operation and is presented in [Fig. 3(b)], with Gaussian cross-sections in both dimensions.

 

Fig. 3 (a) Scanning Fabry-Perot interferometer spectrum at an output power of 23 W. The free spectral range (FSR) amounts to 500 MHz. Inset: multiple-longitudinal-mode spectrum at a net pump power of 73 W. (b) Output beam profile captured by a CCD camera representing TEM₀₀ mode operation. The intensity distributions of the horizontal and vertical cross-sections through the center of the spot are shown on top and right, respectively.

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The laser linewidth deduced from the interferometer measurement is 3.3 MHz, which is limited by the resolution of the SFPI. In order to measure the precise value of the linewidth, another Fabry-Perot cavity with a FSR of 750 MHz and a finesse of 34 was set up as a frequency discriminator (see Fig. 1). By coupling a part of the laser output beam into the reference cavity, it converts the frequency fluctuation of the free-running laser into an amplitude variation of the transmitted signal [27]. A DAQ card, which has a maximum sampling rate of 200 kSamples/s, is employed to record the transmitted signal. Since the laser linewidth exhibits a dependency on sampling time [4,6], it is necessary to deduce the resulting linewidth as a function of different sampling time windows (see Fig. 4). At an output power of 23.6 W, the free-running laser yields a linewidth of 88 kHz for a sampling time of 100 µs and 407 kHz for 1 ms. As the sampling time increases, all kinds of low frequency noise start to affect laser-linewidth broadening to a greater extent. As a result, the linewidth is deduced to be 1.78 MHz for a sampling time of 1 s. Although the graph in Fig. 4 does not provide a direct measure of the undesirable noise components, attribution is feasible via the indirect measure which is provided by sampling-time-dependent linewidth analysis. In this study, signatures of two main contributors to linewidth broadening in our systems are observed, well agreeing with previous studies considering noise mechanisms. The first contributor to linewidth broadening is attributed to the acoustical and mechanical resonance from the water cooling system, which corresponds to the considerably steep increase of linewidth in the millisecond domain, i.e. for sampling times ranging from approximately 1 ms to 5 ms (cf. blue colored area in Fig. 4). This is in good agreement with the literature [4,6]. Further broadening of the laser linewidth at longer time spans occurs mainly due to the second contributor, which we attribute to fast thermal drifts within the setup, one source of which is the thermal fluctuation induced by pump laser intensity fluctuation as identified in the literature [3,28] (cf. red colored area in Fig. 4). This illustrates, that a stable free running system is expected as long as aforementioned noise components – generally spoken: external disturbance – are suppressed.

 

Fig. 4 Laser linewidth as function of sampling time.

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4. Conclusion

To summarize, we presented a high-power single-frequency VECSEL operating at an emission wavelength of 1013 nm with a maximum output power of 23.6 W. The linewidth of the emission is determined to be in the sub-100-kHz range for short sampling times of 100µs, while a linewidth of 407 kHz is obtained at a sampling time of 1 ms. The study of the linewidth as function of the sampling time reveals that the stability of the free-running VECSEL is mainly limited by low frequency noise, which could be compensated by active-stabilization techniques in future studies [24]. With the help of improved thermal management techniques, we have the reason to believe that it is possible to push the single-frequency output power above 30 W.