Chapter 4 Mode Selective Light Source with over 40 GHz Direct Modulation Bandwidth
4.4 Over 40 GHz bandwidth mode selective light source
property is confirmed on the design device. The corresponding spectrum of only 0th mode lasing condition is measured. Such result is shown in Fig. 3.11. As can be seen, the 0th mode mainly locates at shorter wavelength side of 1565 nm.
Similar evaluation was done also for 1st order mode lasing case shown in Fig. 3.12. Main pumping current is also set as 120 mA and 1st order mode selector region current as 10 mA.
The current in 0th mode arm is 0 mA. The corresponding near field pattern (NFP) is shown in Fig. 3.13. A clear only 1st order mode lasing output mode is confirmed. The corresponding spectrum of only 1st order mode lasing case is shown in Fug 3.14. As can be seen in the figure, the spectrum of 1st order mode is at longer side than that of 0th mode lasing case. This result indicates the two modes can be distinguished by analyzing the spectrum.
As such device is the mode selective light source, it is quite easy to manipulate the output mode by changing current injection condition. Figure 3.15 (a)~(e) illustrate the gradual change from 0th mode to 1st order mode.
As described in previous sections such device has more than 4 electrodes, thus the P-I
characteristic must be evaluated in different conditions. Figure 3.16 is evaluated P-I characteristic of the device when all electrodes are injected with current through a common current source. As can be seen from the figure, the threshold current is 84 mA. Such threshold current is quite large when compared with compact size lasers such as VSCELs. Considering the large active area which results from the multimode section as well as the wide access waveguide region, the threshold current density is, however, only 1.3 kA/m2. Such value is quite normal when compared with the conventional laser diodes. As all the electrodes are connected with current source, both fundamental mode and first order mode propagation path has carrier injection thus enough gain. Consequently, both fundamental mode and first order
40 30 20 10
0 0
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Current [mA]
Output Power [mW]
CW@25oC
mode competition
84 mA
600 700
Fig. 3.16. P-I characteristic evaluated when all electrodes are connected through common current source.
Fig. 3.17. Near field pattern of the device when both 0th mode and 1st order mode are lasing.
seen from the figure, when booth 0th mode and 1st order are lasing simultaneously, the overlapping between two modes results in such pattern shown in Fig. 3.17. It is necessary to note that, such pattern (both modes are lasing) is not all stable, i.e., the overlapping pattern between two modes does not always result in such pattern (for instance the pattern in 3.15 (b) and Fig 3.15 (c)). Furthermore, because of the existence of two modes, the mode competition is observed. In Fig. 3.16, at around 300~350 mA current range, the output power decrease with the increase of the total current. The two modes compete with each other for the limited gain, thus resulting in the decrease of output power.
Pumping current varies from 90 mA to 150 mA
8 6 4 2 0 0
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Current of 0th mode arm[mA]
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90 mA 100 mA 110 mA 120 mA 130 mA 140 mA 150 mA
Mode hopping
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Fig. 3.18. P-I characteristic of only 0th mode lasing case
On the other hand, the P-I characteristics of single mode lasing condition have also been evaluated. Figure 3.18 and Fig. 3.19 shows the P-I characteristic of only 0th mode and 1st order mode, respectively. The horizontal axis represents the current injected into 0th mode access waveguide in Fig. 3.18 (Same as shown in Fig. 3.19 for 1st order mode case). Different color in Fig. 3.18 and Fig 3.19 represent different pumping current in main pumping region, which consists of multimode and central access waveguide left side. As can be seen from Fig. 3.18 and Fig. 3.19, as the pumping current starts from 90 mA, the laser already started lasing even if the mode selector access waveguide has no current injection. Thus 0 mA is already above the threshold current. In Fig. 3.18 and 3.19, it is quite clear that the output power increase with the increase of injection current into the mode selector arm. Another point that is needed to be noted is that, the output power of 0th mode and 1st order mode is almost the same. This result indicates the fact that the current injection efficiencies for 0th mode and 1st order mode are the same. This is because of the improvement on the slit waveguide electrode process. Finally, as can be seen in Fig. 3.19, at 1st order mode injection current 20 mA and 80 mA range, the ripples on P-I characteristics has been confirmed. This is because of the mode competition. In more details, the output mode is 1st order mode below 20 mA. The output mode changes to, however, 0th mode when the current is within 20~23 mA. Such phenomenon is so-called mode hopping.
Similar phenomenon has also been confirmed at current range of 80~85 mA.
4.4.2 Over 40 GHz bandwidth mode selective light source
As proposed bandwidth enhancement scheme in Chapter 3, the multiple PPRs as well as the damping filling mechanism between CPR and PPR together leads to high direct modulation bandwidth. The small signal response of the device has been evaluated using vector network
Fig. 3.20. Small signal frequency response when both mode selector has no current injection
-3 dB
Frequency [GHz]
Frequency response [dB]
Fig. 3.21. Small signal frequency response when both mode selector has 10 mA current injection
Frequency response [dB]
Frequency [GHz]
analyzer (VNA). In all of the following results from Fig. 3.20~Fig. 3.22, different colors represent different pumping current (80~170 mA every 10 mA step) injected into the multimode region (pumping region). Frequency response (FR) shown in Fig. 3.20 was
Fig. 3.22. Small signal frequency response of only 0th order mode lasing case.
Frequency [GHz]
Frequency response [dB]
Fig. 3.23. Small signal frequency response of only 1st order mode lasing case.
Frequency [GHz]
Frequency response [dB]
is 0. On the other hand, the Fig. 3.21 shows the FR results when pumping current is injected.
Meanwhile, the bending arms has current injection of 10 mA. According to common sense, response in Fig. 3.21 should be stronger than that of Fig.3.20 simply because the current injected is more. The response result in Fig. 3.21, however, is weaker. One reason might be that, in Fig. 3.21, because the mode selector region has current injection, the 0th mode and 1st order mode might have enough gain, resulting in a mode competition between two modes.
Consequently, the output power itself might be lower than the case in Fig. 3.20.
The direct modulation bandwidth has been evaluated for single mode lasing condition. In Fig. 3.22 and Fig. 3.23, the frequency responses of 0th mode and 1st order mode are shown, respectively. As can be seen from the two figures, for both 0th mode lasing operation and 1st order mode lasing operation, the 3 dB direct modulation bandwidth is over 40 GHz. The bandwidth itself is, however, not limited by the device performance while by the evaluation range of the VNA as the VNA only support the measurement up to 40 GHz. Such high direct modulation bandwidth is achieved because of the existence of multiple PPRs. The PPR frequencies are observed at 4.5 GHz, 8.9 GHz, 11.9 GHz, 13.5 GHz, 16.9 GHz, 19.2 GHz, 22 GHz, 24 GHz, 29 GHz, 34 GHz, and 37 GHz. Such multiple PRPs prevent the damping at high frequency sufficiently. As a result, over 40 GHz direct modulation bandwidth on each mode has been confirmed. Such result indicates the potential of fabricating a single laser chip that than operate on Tb/s level.
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