Chapter 1 Overview
5.2 High-Q Hybrid Laser
5.2.1 Light-Current Characteristics (L-I)
Hybrid lasers were mounted on a temperature controlled stage, based on an elec- tronically controlled thermo-electric cooler (TEC). External pumping was provided by a laser diode driver. Continuous-wave lasing at room temperature (20◦C in this work) was achieved for all lasers tested. Lasers on a given chip were arrayed in four groups of three devices each. The period of the grating was lithographically tuned
SiO2 Si
MQW p-InGaAs
superlattice n-InP
n-side contact
p-side contact H implant+
Hybrid Laser
High-Q Si Resonator
Figure 5.6. High-Q hybrid laser device schematics.
between groups, thereby tuning the emission wavelength. All lasers were otherwise designed to be identical. The period was tuned in the range of a = 230–240 nm, in steps of 2.5 nm, translating to a respective wavelength tuning range of 1530–1575 nm, in increments of 15 nm. From optical spectrum measurements (see section 5.2.2), the peak of the gain spectrum under pumping was found to redshift from the nominal quantum well (QW) emission wavelength of 1550 nm to approximately 1575 nm. This is mainly due to the energy gap shrinking with carrier injection.
Figure 5.7 presents representative power versus current characteristics (L-I) for four lasers from a single chip. Each trace in this figure corresponds to one laser from each wavelength group. Lasing up to 45 nm away from the gain peak was attained. The particular devices featured relatively small slope efficiencies, as the resonators they were based on, were intentionally designed to be undercoupled (Qe>
Qi) to maximize Q for linewidth reduction. The average cavity length was 1.1 mm.
The measured power was collected from one end of the cavity by a large-aperture photodetector. The amount of collected power depended primarily on the insertion loss in the coupling into free space through the laser facet. The quality of the facet varied significantly from device to device, largely due to the coarse nature of the laser bar cleave, thus making even identical lasers to differ significantly in power output.
The observed L-I curves are subject to this randomness and, as a result, a reliable trend for the slope efficiency could not be derived.
0 20 40 60 80 100
0 0.5 1 1.5 2 2.5 3
Current (mA)
Power (mW)
1530 nm 1560 nm 1545 nm 1575 nm
T = 20 Co
Figure 5.7. L-I characteristics of four high-Q hybrid lasers from a single chip, with lithographically tuned emission wavelength.
The behavior exhibited by the black trace of figure 5.7 is representative of the thermal roll-off effect, often encountered in these hybrid lasers. As the active region heats up, carriers localized in the QWs become increasingly thermalized and escape confinement, thus reducing the recombination rate, manifesting itself as a power roll- off in the L-I curve. The onset of this thermal effect on the hybrid platform occurs at relatively lower pump currents, than for conventional all-III-V lasers. This is due to the poorer heat dissipating capability of the hybrid structure. Heat generated in the III-V medium (mostly on the p-side of it) has to diffuse downward, through the SOI part of the structure, toward the heat sink. The presence of the low thermal conductivity buried oxide layer creates a thermal buffer that slows down the heat
extraction from the III-V. As a result, hybrid lasers tend to heat up faster and to reach the thermal roll-off threshold sooner than conventional III-V lasers.
Simultaneously with the output power, the voltage drop across the laser was also recorded as a function of the pump current. Figure 5.8 presents representative I-V characteristics of measured hybrid lasers. All lasers from a single chip showed I-V dependence that followed one of the two traces of figure 5.8. The distribution of
0 20 40 60 80 100
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Current (mA)
Voltage (V)
T = 20 Co
Figure 5.8. I-V dependence of all twelve high-Q hybrid lasers from a single chip.
devices between the two traces did not seem to follow a trend with the emission wavelength. Overall, the electrical behavior was consistent, with minor deviations attributed to processing-induced variations across the chip.
While output power tended to vary significantly between devices, threshold cur- rents exhibited a much more consistent trend. Threshold currents for devices in the same emission wavelength group were extremely repeatable, while a clear trend with detuning from the peak of the gain spectrum was also established. Figure 5.9 shows L-I curves of three lasers with the same emission wavelength. Unlike the variation in
slope efficiency, all three feature the same threshold, in this case 35 mA at 1545 nm.
Similar consistency was observed for all four wavelength groups. Thresholds as low as 27 mA were measured for lasers with emission wavelength closest to the gain peak and cavity lengths of 1.1 mm. This low threshold for the given cavity length is attributed to the enhanced intrinsic Qand the undercoupled laser cavities.
0 20 40 60 80 100
0 0.5 1 1.5 2 2.5
Current (mA)
Power (mW)
T = 20 Co
λ = 1545 nm
Figure 5.9. L-I characteristics of three high-Q hybrid lasers with same emission wavelength.
Figure 5.10 presents the dependence of threshold current on the emission wave- length offset from the gain peak (λ−λo), with the latter, as mentioned above, observed at λo = 1575 nm. Thresholds as high as 45 mA were measured for the maximum de- tuning of 45 nm from the gain peak. The trend line (red line) of figure 5.10 corresponds to a double exponential fit and it appears to be modeling the trend closely over the specific wavelength range.
To facilitate heat sinking, as well as to perform temperature-dependent measure-
0 10 20 30 40 50 26
28 30 32 34 36 38 40 42 44 46
Offset Wavelength (nm)
Threshold Current (mA)
T = 20 Co
Figure 5.10. Threshold current as a function of offset emission wavelength from the gain peak (λo = 1575 nm).
ments, the same laser bar, results from which were presented above, was die bonded to improve thermal contact with the temperature-controlled stage. The chip was also flipped around and L-I measurements were retaken from the opposite laser facet. Sig- nificant discrepancies between measured output powers from the two facets of a given laser were found, confirming the variation in facet quality. Figure 5.11 shows L-I and I-V characteristics of a laser emitting at 1545 nm. In addition to the increased power output at 100 mA, compared to the output at the same pump current from the other facet, this time the laser could also be pumped harder, up to 150 mA, thanks to the reduced heating effect. A record-high, among the lasers tested in this work, output power of 9 mW was measured for this particular laser. This result shows promise for the potential of these lasers for high-power operation. With the use of a commercial- grade packaging process (e.g., dicing, polishing, AR-coating, fiber-coupling) can be
greatly improved. Furthermore, tuning the resonator loading (i.e., reduced undercou- pling) to increase slope efficiency, can also be used boost the output power.
0 50 100 1500
2 4 6 8 10
Power (mW)
0 50 100 150
0 0.5 1 1.5 2 2.5
Current (mA)
Voltage (V)
T = 20 Co
Figure 5.11. L-I and I-V characteristics of a die-bonded high-Q hybrid laser.
We tested the lasers’ light output characteristics as a function of temperature.
Tuning the TEC setpoint, we measured the power output as a function of the pump current at different temperature setpoints. This was done on the already die bonded chip. Figure 5.12 presents the change of a hybrid laser’s L-I characteristic with tem- perature in the range 10–70◦C. Continuous-wave lasing up to 70◦C was attained.
As carriers spread out in energy with increasing temperature, higher carrier densi- ties are needed to reach transparency (Ntr). Higher carrier densities in turn, lead to increased absorption (e.g., Auger absorption) and reduced carrier lifetime, particu- larly through the increase of nonradiative carrier recombination [175]. The decrease in slope efficiency with temperature has been found to be modeled closely by an in- crease in internal absorption losses, primarily between the split-off and heavy-hole valence bands [176–178].
Figure 5.13 presents the dependence of threshold current on temperature. Thresh-
0 50 100 150 0
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Current (mA)
Power (mW)
T
10 C < T < 70 Co o
Figure 5.12. L-I characteristic of a hybrid laser as a function of operating temperature.
old current (or density) grows exponentially with temperature and is often modeled as
Ith=Ioexp T
To
, (5.1)
whereTo is a characteristic temperature, measure of the threshold current’s sensitivity to temperature. The lowerTo, the more sensitive threshold current is to temperature.
As both the carrier density and gain to reach transparency increase with tempera- ture, so does the threshold current. This increase is slower at lower temperatures (e.g., 280–300 K), but quickly starts to grow at higher temperatures, as nonradiative pro- cesses (e.g., Auger recombination) and internal losses (e.g., intervalence absorption) increase with carrier density. In the case of the hybrid laser of figure 5.13, we split the measurement range in three regions of gradually increasing sensitivity to temperature.
The characteristic temperature extracted from fitting to equation (5.1) are 77 K, 42 K and 30 K for the temperature ranges 280–300 K, 300–330 K, and 330–350 K respec-
tively. These characteristic temperatures are typical of InGaAsP/InP QW lasers at 1.55µm.
280 300 320 340 360
2.5 3 3.5 4 4.5 5
Temperature (K) Ln[I th(mA)]
7 27 47 67 87
Temperature ( C)o
T = 42 Ko
T = 30 Ko
T = 77 Ko
Figure 5.13. Threshold dependence on temperature for a high-Q hybrid laser.
The emission wavelength was recorded on a wavemeter over the temperature range 10–70◦C and its dependence is presented in figure 5.14. Over the given range, the wavelength tunes by approximately 5 nm, a shift that corresponds to a wavelength sensitivity with temperature of about 0.089 nm/K. Although QW-based lasers typi- cally exhibit sensitivities on the order of 0.3 nm/K, this dependence is usually reduced in DFB lasers at the levels of 0.07 nm/K, due to the wavelength selectivity provided by the grating. Overall, the temperature-dependent characteristics seem to be III-V material dictated and to have carrier over to the hybrid laser.
280 290 300 310 320 330 340 1574
1575 1576 1577 1578 1579 1580
Temperature (K)
Wavelength (nm)
7 17 27 37 47 57 67
Temperature ( C)o
dλ
dT= 0.089 nm/K
Figure 5.14. Emission wavelength dependence on temperature for a high-Q hybrid laser.