Chapter 1 Overview
5.1 High-Q Si Resonators
Chapter 5
Experimental Results
In this chapter, we will present results of fabricated devices, passive Si resonators and hybrid Si/III-V lasers. In the timeline of the project, passive resonators on Si were developed first, to validate the grating design and analysis methodology. The quality of passive resonators was assessed directly by measuring their loaded quality factors, without the complication of the absorbing III-V medium of the hybrid structure. In this first part of the project, important parameters of the fabrication process were also tuned. Resonator design parameters were experimentally evaluated, serving as useful input for the design and fabrication of the hybrid laser resonators. Here, we will briefly present representative results of Si resonators before we move on to the experimental characterization of hybrid lasers. Details on the fabrication process of both resonators and lasers can be found in appendix A.
uniform with nominal period of 600 nm, designed to couple to free space beams at an angle of 30◦ and center wavelength around 1575 nm. The experimentally observed 3 db coupling efficiency bandwidth was 20–30 nm. The coupler pads were 10µm wide and connected to the main waveguide via “slow” tapers approximately 250µm long, to suppress the excitation of higher-order transverse modes.
Si
SiO
2L
yh
H
H
BOXz y
Figure 5.1. Schematic cross section of the SOI waveguide platform.
Figure 5.2. Top view schematic of a high-Q Si resonator with grating couplers.
Figure 5.3 shows different views of a fabricated high-QSi resonator, taken with a scanning electron microscope (SEM). Due to the sheer scale of the structure, a single frame image of the full structure could not be obtained. Instead, different sections of the grating (e.g., mirror, defect) are shown. Besides, the modulation of the grating in true scale is so slow, that is difficult to discern in any snapshot. Resonator lengths varied from approximately 500µm to slightly over 1 mm, using the length of the mirrors to tune the external loading (Qe).
Passive resonators were characterized by measuring their frequency response in transmission mode. An incoming wave of varying frequency is coupled via the grat- ing couplers to the waveguide’s fundamental guided mode and transmitted through the resonator. The transmitted light, coupled out through the second coupler, is col- lected at a photodetector and its intensity is plotted as a function of frequency (or
Figure 5.3. Scanning electron microscope (SEM) images of a fabricated high-Q Si resonator.
wavelength), revealing the transmission spectrum of the resonator. To be able to re- solve narrow-line resonances (e.g.,Q >2×105), the scanning resolution in frequency has to be sufficiently fine, at least a few times finer than the targeted resonance linewidths. This corresponds to wavelength scanning resolution better than 1 pm.
Frequency scanning using standard tunable laser sources relies on the piezo-activated tuning of the length of an external cavity, a process generally slow and with inherent hysteresis. Narrow-line resonances are susceptible to thermal drift over the timescale of one scan, thus making the accurate determination of the linewidth challenging.
To speed up data acquisition and improve accuracy, we implemented an electron- ically controlled frequency-sweeping scheme, in the configuration shown in figure 5.4.
A commercial tunable laser (Santec TSL-510) is externally modulated to produce a fast frequency sweep in time around a given frequency set point. The wavelength span of the sweep is limited by the maximum applied voltage to about 80 pm. For
HeNe SFL
APD OSC
PC
90/10
trigger
Figure 5.4. Frequency-sweeping configuration for the measurement of high-Q Si res- onators.
every set point, the swept frequency output of the tunable laser is coupled into the resonator and the transmission is monitored by an oscilloscope (OSC), with timing triggered by the sweep-generating module. To calibrate the frequency sweep, part of the laser output is transmitted through a Mach-Zehnder interferometer, the output of which is also monitored by the oscilloscope. This way, a connection between the res- onator’s transmission in time and its spectrum is established. We further locked the swept-frequency laser (SFL) via a feedback loop, thereby linearizing the sweep and making the calibration more straightforward. For more details on the implementation of a swept-frequency laser, the reader is referred to [171–173].
Due to the limited frequency scan window and in order to avoid the use of a pre- liminary broadband sweep,a priori knowledge of reference features in the resonator’s transmission spectrum was found to be useful. As one such feature, for example, serves the bandgap of the resonator, which is usually searched for at the beginning of every measurement. Then, expecting the resonant mode to be located near the low frequency band edge, the tunable laser is manually tuned until the resonance enters the scan window. The recorded oscilloscope signal is then grabbed by a com- puter (PC) and mapped from time onto frequency space. This mapping provides us with only a relative frequency scale for the transmission spectrum, which is, however, all we need to extract the resonance’s linewidth. An absolute frequency reference point, within a certain uncertainty range, is available through the laser’s frequency
setpoint. Scanning across a resonance in this fast manner provides a practically in- stant snapshot of the resonator’s transmission signature, removed from environmental perturbations. Furthermore, as the scanned light spends significantly reduced time on resonance, thermal effects are greatly suppressed.
To increase the power coupled into waveguides through the grating couplers, fibers terminated with focusing lens modules were used. Red light laser (HeNe) was also employed to facilitate alignment with the couplers. To obtain a linewidth as close as possible to the intrinsic, resonators were increasingly unloaded from the waveguides by increasing the mirror length. This comes at the expense of transmitted power, which decays exponentially with mirror length. For increasingly longer mirror sections the SNR at the output degrades significantly, setting a sensitivity-imposed limit on the highest loaded Q that can be resolved. To push this limit higher, a high-gain, low noise photodetector (Newport InGaAs 2153) was used, featuring femtoWatt level sensitivity.
Figure 5.5 shows a typical transmission spectrum of a high-Q resonance. The blue line corresponds to the experimental trace and the red one to the Lorentzian fit, from which the quality factor is extracted. Loaded Qs as high as 1 million were obtained. The showcased result of figure 5.5 corresponds to a resonator with photonic well parameters V = 200 GHz, Ld = 100µm, waveguide width Ly = 2.0µm, etch depth h = 100 nm and longitudinal hole diameter Wx = 105 nm. Given the degree of unloading of the particular resonator, the measured loaded Q represents a close approximation of the resonator’s intrinsic quality factor.