Resonant-antiresonant coupled cavity VCSELs.

The wavelength tuning range of a tunable vertical-cavity surface-emitting laser (VCSEL) is strongly influenced by the design of the interface between the semiconductor cavity and the air cavity. A simplified model is used to investigate the origin of the dramatic differences in free spectral range (FSR) and tuning slope observed in semiconductor cavity dominant, extended cavity, and air cavity dominant VCSELs. The differences arise from the positioning of the resonant and antiresonant wavelengths of the semiconductor cavity with respect to the center wavelength. The air cavity dominant design is realized by designing an antiresonant semiconductor cavity, resulting in a larger tuning slope near the center of the tuning range and a wider FSR toward the edges of the tuning range. The findings from the simplified model are confirmed with the simulation of a full VCSEL structure. Using an air cavity dominant design, an electrically pumped laser with a tuning range of 68.38 nm centered at 1056.7 nm at a 550 kHz sweep rate is demonstrated with continuous wave emission at room temperature. This epitaxial design rule can be used to increase the tuning range of tunable VCSELs, making them more applicable in swept-source optical coherence tomography and frequency-modulated continuous-wave LIDAR systems. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Wavelength-swept lasers are important components in modern optical communications, light detection and ranging (LIDAR), optical coherence tomography (OCT), and high-resolution laser spectroscopy. The most important performance criteria are wavelength tuning ratio (Δλ/λ C ) and sweep speed. Vertical-cavity surface-emitting lasers (VCSELs) with tuning capability [1] have exhibited many desirable attributes including wafer-scale fabrication and testing, continuous and wide tuning, small footprint, and low power consumption. With a very short cavity (2-10 μm), the VCSEL's wavelength can be tuned by varying the optical thickness of some of the DBR layers or the optical cavity. This may be accomplished by varying the refractive index of some of the layers [2][3][4] or their physical thicknesses. The former approach has yet to experimentally result in a wide sweep range. Alternatively, using a microelectromechanical system (MEMS) to physically change the optical cavity length, a wide, continuous tuning range has been demonstrated [5]. Since the first MEMS-tunable VCSEL reported in 1995, many advances have been reported for center wavelengths (λ C ) of 850 nm, 980 nm, 1060 nm, 1310 nm, and 1550 nm [6][7][8][9][10][11][12][13]. Conventional MEMS-tunable VCSELs are designed with a high optical intensity concentrated in the semiconductor portion [5][6][7]. This configuration is referred to as semiconductor cavity dominant (SCD) design. The tuning ratio of a SCD design is limited to ~3.5% by the relatively small free spectral range (FSR).
To increase the tuning range, researchers have designed VCSELs with a λ C /4-thick antireflection (AR) layer with AR layer. This configuration was referred to as the extended cavity (EC) design in [8]. In this case, the semiconductor and air cavities are perfectly matched. They resonate as one cavity, as if the semiconductor cavity "extends" into the air region. Previously, a very large static tuning range of 102 nm centered at 1550 nm (Δλ/λ C = 6.6%) was reported for an electricallypumped EC VCSEL using electro-thermal tuning [9]. Limited by the thermal time constant, the tuning speed is shown to be relatively slow at 215 Hz with a smaller dynamic sweep range of 87 nm (Δλ/λ C = 5.6%). The EC design has also been implemented at a center wavelength of 1050 nm with a swept tuning range of 63.8 nm (Δλ/λ C = 6.1%) and a faster sweep rate of 240 kHz [10]. Both devices utilize dielectric distributed Bragg reflectors (DBR) with a high index contrast to minimize the effective length of the cavity, increasing the FSR at the cost of increased fabrication complexity due to additional deposition steps or multiple oxidation layers.
Recently, a third configuration called the air cavity dominant (ACD) design was reported, which forces the optical field to be confined more significantly in the air cavity at the center wavelength [11]. This design led to a record tuning ratio of 6.9% for an electrically-pumped VCSEL, while allowing more flexible choices of materials and thicknesses in the semiconductor-air coupling (SAC) region and the bottom DBR.
In this study, we reveal the origin of the increased tuning range of the ACD design and the impact of the design on threshold material gain. The swept operation of the device is demonstrated, exhibiting a swept tuning ratio of 6.5% at a sweep rate of 550 kHz. The high sweep rate is attributed to the lightweight high-contrast grating (HCG) used as the tunable mirror [14]. Figure 1 shows the schematic and the scanning electron microscopy (SEM) image of our 1060-nm ACD HCG tunable VCSEL. The device consists of a semiconductor portion, a top HCG mirror, and an air gap in between forming an air cavity. The semiconductor portion (starting from the top) includes a semiconductor-air coupling (SAC) region, two pairs of p-DBRs (Al 0.12 Ga 0.88 As high-index layer first, followed by Al 0.9 Ga 0.1 As low-index layer, Al 0.12 Ga 0.88 As high-index layer, and Al 0.98 Ga 0.02 As layer for oxidation), a 1λ C cavity with five quantum wells in the center, followed by 38.5 pairs of n-DBRs, all grown on an n-doped GaAs substrate. One can identify two longitudinally coupled cavities: one centered at the active cavity with quantum wells and a second centered at the air gap between the HCG and the semiconductor. As described in [11], the SAC region dictates the difference between the three designs: SCD, EC and ACD.  To understand the curvatures of the SCD and ACD resonance lines, we examine the two cavities: the air cavity defined by r 1 and r 2 , and the semiconductor cavity defined by r 2 and r 3 . The FP resonances for the semiconductor cavity, computed by removing r 1 from the transfer matrix simulation described above and plotted in blue in Figs. 2(c) and 2(d), are horizontal lines since they do not depend on the air cavity length. The FP resonances for the air cavity between r 1 and r 2 , computed by removing r 3 from the simulation and plotted in red in Figs. 2(c) and 2(d), are linearly proportional to the air cavity length. Coupling between the semiconductor cavity and air cavity occurs when the two families of lines intersect each other, marked with circles on Figs. 2(c) and 2(d). The FP resonances of the full structure follows these two families of lines but avoid the crossings as shown by the black curved traces. Figure 2(c) shows the case where n SAC = 1. The semiconductor cavity is in resonance at λ C = 1060 nm and thus a blue line is shown at the center wavelength 1060 nm. At an air gap ofλ C /4, r 1 directly touches the semiconductor cavity. The full structure resonance lines are coincident with the semiconductor cavity resonance lines. As the air gap increases, an avoided crossing causes the full structure resonance to follow the air cavity resonance. Eventually, the full structure resonance avoids a second crossing to switch back to following the semiconductor resonance line. Since there is a semiconductor resonance at λ C , the full structure resonance shows a low tuning slope at the center wavelength. This is the characteristic feature of an SCD design. Figure 2(d) shows the case where n SAC = n S . Due to the λ C /4 SAC refractive index, the FP wavelengths for the semiconductor cavity are shifted from those in the SCD case ( Fig. 2(c)). In this ACD case, the semiconductor cavity is in antiresonance at λ C , with the nearest FP modes located instead at 1130.7 and 997.6 nm. Again, the full structure resonance lines begin coincident with the semiconductor lines at an air cavity length of 0. The VCSEL resonance curves avoid the crossings between semiconductor and air resonances as air cavity length increases. Since the semiconductor cavity is in antiresonance at λ C , the full structure resonance follows the air cavity resonance, resulting in a large tuning slope. This represents the ACD case. A stronger coupling between the semiconductor and air cavities in either an ACD or SCD design pushes the black lines apart, approaching the tuning characteristic for the EC case, in which the cavities are perfectly coupled.

Underlying physics of tunable VCSELs
The mathematical origin of the semiconductor resonances lies in the phase of r 2 . If n SAC < n AR , then the interface between the semiconductor cavity and the SAC layer dominates r 2 . The reflection phase into the semiconductor cavity, ∠r 2 (λ C ), is zero, and the semiconductor cavity is in resonance at λ C . For the special case in which n SAC ≅n AR , the magnitude of r 2 is insignificant and the VCSEL cavity resonates as a unit. If n SAC > n AR , then the interface between the air cavity and the λ C /4 SAC layer dominates r 2 . The reflection phase ∠r 2 (λ C ) = π, the semiconductor cavity is in antiresonance at λ C , and the design is ACD. Note this description is very general and applies to more complex designs, such as that depicted in Fig.  1, which has two pairs of p-DBR between the 1λ C cavity and the SAC, and the SAC consists of a window (λ C /2) layer between the λ C /4 n SAC layer and air cavity.
For a typical tunable MEMS-VCSEL design, the air cavity length is chosen to be large enough to allow large tuning range with a maximum MEMS movement approximately 1/3 of the air gap. The FSR is thus the limiting factor in tunable VCSEL designs. As noted above, FSR is not constant with changing air cavity length. The range-limiting FSR is the shortest wavelength difference between the modes directly above and directly below the center wavelength, as these are the modes which are able to achieve threshold. In both ACD and SCD designs, the highest FSR is located near the intersections of the VCSEL cavity modes and the semiconductor cavity modes. Since the semiconductor cavity modes are off-center in an ACD VCSEL, the FSR is highest when the VCSEL resonance is far from the center of its tuning range. In contrast, the FSR of an SCD VCSEL is decreased as the VCSEL resonance moves away from the tuning center. The difference in FSR is illustrated in Fig. 2(b), which shows the VC that to obtain cavity with m words, it is be

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Experimental results
The device shown in Fig. 1 is fabricated using process described in [11]. The GaAs sacrificial layer is removed by selective wet etching to form the 1.32 µm air gap. The SAC region of the actual device uses a design resembling the case shown in Fig. 3(f), with a λ C /2 window layer composed of an InGaP etch stop and a GaAs contact layer on top of a λ C /4 low-index layer. Previously, with a combination of thermal, current and electrostatic tuning, single-mode continuous lasing across a 73-nm range was demonstrated [11]. With an optimized MEMS design, we obtain a continuous sweep by applying a DC tuning voltage of 31.5 V plus an AC tuning voltage of 10.0 V PP at the mechanical resonance frequency of 550 kHz, as shown in Fig. 6(a). Resonant excitation of the mechanical structure displaces the mirror further than the equivalent DC voltage, eliminating the need for a tuning voltage high enough to break down the semiconductor junction [16]. The full dynamic tuning range is 68.38 nm, spanning from 1022.46 nm to 1090.84 nm, which is a direct proof of the extended FSR by our ACD design and is close to the calculated tuning range of 76 nm. If the AC voltage is increased to displace the MEMS further, the next Fabry-Perot mode will begin to lase over the same range of wavelengths. This shows that the tuning range is FSR limited and not threshold limited.
The threshold current for each wavelength is determined using the swept spectrum. The DC tuning bias, AC amplitude, and AC frequency are set such that the movement of the mirror traces one period of the tuning curve. A series of DC currents, ranging from 0.1 mA to 4 mA in steps of 0.05 mA, is applied through the laser diode. For each DC current, the emission spectrum is measured. The threshold at each wavelength is then determined by numerically differentiating the spectral intensity with respect to laser diode current and locating the abrupt step corresponding to the threshold. The results of this measurement are shown in Fig. 6(b). For comparison, the threshold current is also measured at a series of DC tuning biases.
The shape of the measured threshold current plotted versus wavelength in Fig. 6(b) deviates from the shape of the simulated threshold material gain curve in Fig. 5(c) in several aspects. First, the minimum threshold is blue-shifted to 1040 nm due to differences in HCG dimensions caused by variation in the lithography and etch processes. The second deviation is the peak at 1075 nm found in both the AC and DC measurements. This peak corresponds to a transition between two transverse modes. Transverse mode suppression is achieved in nontunable oxide VCSELs by placing an oxide aperture near a longitudinal intensity node of the desired Fabry-Perot mode. In a tunable VCSEL, the position of the oxide layer with respect to the mode changes with wavelength, which can cause different transverse modes to dominate at different wavelengths. In the future, different transverse control mechanisms such as multiple oxide apertures, ion implantation, or buried heterostructure can be used to eliminate higher order transverse modes during tuning. Chirped QWs could also be used to reduce wavelength dependence in threshold current.

Conclusion
In summary, we investigate the mechanism behind the ACD configuration's large tuning range improvement over SCD and EC tunable VCSELs, finding that an antiresonance in the semiconductor cavity at the center wavelength is the cause for the high tuning slope and wide FSR. Our measurements of ACD devices confirm our theory of tuning ratio enhancement, demonstrating electrically pumped VCSELs with a high tuning ratio of 6.5% with resonant MEMS tuning at 550 kHz.