Surface emitting semiconductor lasers produce flexible femtosecond pulse trains

A laser-driven frequency comb is a device that generates rapid-fire pulses of radiation. Although the titanium:sapphire combs typically used in metrology and spectroscopy applications can emit pulse trains at ∼100MHz, many comb applications could benefit from considerably higher repetition frequencies (1–10GHz) with large spacing between the modes (resonant frequencies) of the laser. Systems with higher repetition frequencies offer increased power per mode and easy isolation of individual modes using diffraction gratings. Unfortunately, when mode-locked solid-state lasers are operated at these increased frequencies, complex instabilities arise because of the unavoidable retention of energy in the gain material. Optically pumped vertical external cavity surface-emitting lasers (VECSELs), with their thin (typically ∼1μm) gain region, combine many of the advantages of solid-state lasers with the short upper-level lifetime of semiconductor gain material, making them interesting as flexible sources of femtosecond pulses at multi-gigahertz repetition frequencies.

The parameter range of femtosecond mode-locked VECSELs is impressive. Pulse durations of 335fs at peak powers >300W have been demonstrated,¹ and power scaling can yield average output powers >1W.²Continued probing of the limits of the gain dynamics has recently yielded pulse durations of 107fs and groups of 60fs pulses, making VECSELs competitive with solid-state lasers.^{3, 4} Two new research directions have recently emerged: VECSELs with flexible repetition frequencies (tunable over many gigahertz) and femtosecond-pulse harmonically mode-locked VECSELs (with repetition frequencies up to 175GHz).

We developed two approaches toward increasing the flexibility of femtosecond mode-locked VECSELs. For tunable repetition frequency operation, the key challenge is ensuring that the pulse fluence (energy per unit area) on the active structures does not vary significantly with large changes in the cavity length. This issue is solved through cavity design: one arm of the cavity is nearly collimated. For harmonic mode locking, an intracavity diamond etalon provides feedback for selecting the repetition frequency while reducing the active region's temperature and permitting an increased average power.

The laser cavity shown in Figure 1 exploits the optical Stark effect and surface recombination, with quantum wells in the gain structure and semiconductor saturable absorber mirror (SESAM) to provide fast pulse shaping. A key feature of the optically pumped quantum well gain structure is its anti-resonant design, which increases the bandwidth of the spectral filter imposed by the finite gain bandwidth. The laser is nearly collimated between the curved high reflector and the output coupler, and there is a tight focus on the SESAM and a 60μm radius mode on the gain mirror to mode-match the pump spot. High-quality molecular beam epitaxy ensures growth accuracy.

Figure 1. Schematic diagram of the tunable repetition frequency laser cavity. The laser is tightly focused on the semiconductor saturable absorber mirror (SESAM), and its beam is nearly collimated between the concave high reflector (HR) and plane output coupler (OC).

Using this cavity, we demonstrated continuous repetition frequency tuning in the range 2.78–7.87GHz, as well as in an 8% tuning range just above 1GHz.^{5, 6} At repetition frequencies of 2.78–5.5GHz, this apparatus produces femtosecond pulses, slowly varying in duration. As shown in Figure 2, above 6GHz the peak intensity of the pulses on the SESAM is insufficient to access the optical Stark regime, and longer 2.3–2.5ps pulses are produced. The average output power was ∼40mW across the entire tuning range. Figure 3 shows the invariant performance seen over the narrower tuning range near 1GHz, with 450fs pulses at 56mW.

Figure 2. Pulse duration versus repetition frequency for a mode-locked vertical external cavity surface-emitting laser (VECSEL), tunable from 2.78 to 7.87GHz.

Figure 3. Autocorrelation and optical spectrum (inset) of a 450fs pulse VECSEL with 8% repetition frequency tuning near 1GHz. a.u.: Arbitrary units.

Harmonic mode locking can be achieved with no additional cavity elements using the SESAM structure substrate as a coupled external cavity.⁷However, significant restrictions prevent this technique from producing femtosecond pulses. Our alternative approach—see Figure 4—features an intracavity diamond heat spreader bonded to the gain mirror using liquid capillary bonding. In addition to enabling power scaling, this structure acts as a subcavity that selects the repetition frequency of the mode-locked pulse train. Tuning the laser cavity to a multiple of the diamond etalon optical thickness yields stable harmonic mode locking (see Figure 5). Our apparatus attained a combined output power of 300mW, repetition frequency of 175GHz, and pulse duration of 400fs.⁸

Figure 4. Schematic diagram of the VECSEL used for harmonic mode locking. The diamond heat spreader enables power scaling while acting as a subcavity for setting the repetition frequency.

Figure 5. Top: Autocorrelation of a 175GHz train of 400fs pulses. Bottom: Optical spectrum of the pulse train. The total average output power=300mW.

With their highly tunable repetition frequency, ultrahigh repetition frequency harmonic mode locking, and excellent pulse quality, femtosecond mode-locked VECSELs operating at and above gigahertz repetition frequencies have begun to match or outperform the power and pulse duration of classical solid-state lasers. Our work has already spurred interest from other research groups (a 625fs-pulse VECSEL continuously tunable at 6–11GHz was recently described by Hoffman et al.⁹), and we are continuing to explore the relative contributions of the optical Stark effect, carrier thermalization, and intrinsic absorber recovery time in the various regimes over which these devices can be operated.