Chapter 4 Mode Selective Light Source with over 40 GHz Direct Modulation Bandwidth
4.2 Scope, design and fabrication of high speed mode selective active-MMI LD
In order to achieve the high speed mode selective light source, mode selectivity and multi-cavity must be integrated within single device. As described in Chapter 2, the core principle to achieve the controllable output mode is to set the separated mode propagation path within single device. On the other hand, to achieve high modulation bandwidth by utilizing multiple photon photon resonance, multiple cavities are necessary. Based on the two requirements described above, it is easy to design the preferred high speed mode selective laser diode.
4.2.1 Scope of the device
Main oscillating cavity region Inner oscillating cavity region Fundamental mode
oscillating path
First order mode oscillating path
Based on multi-cavity structure, the 1 by 3 cavity structure is proposed. Figure 3.1 shows the scope of the device. The red line shows the main oscillating path cavity of the fundamental mode. Green line shows the main oscillating cavity path of first order mode. Different from the proposed 1 by 2 MMI structure in Chapter 2, scheme shown in Fig 3.1 has three access waveguide ports [6-8] to introduce multiple photon photon resonance phenomena. As described in previous chapter, an inner reflection edge must be introduced to generate the inner oscillating cavity region when much more PPRs are desired. This requires the high-mesa structure for waveguide configuration.
On the other hand, the waveguide width must be arranged to permit the higher order mode existence. One thing must be noted is that, according to the previously experience in our group, practically fabricated waveguide with ~3.5µm does not permit the propagation of 1st order mode. Consequently, we designed the access waveguide width to be 4.5 µm to ensure the propagation of 1st order mode.
Furthermore, the access waveguide acting as the modulation section at right side should be electrically separated from the multimode section to ensure a high photon density inside the modulation region. The access waveguide at left side, which acts as mode selector must also be electrically separated from the multimode section to control the output mode itself. Thus, based on such ideals, the device could be designed and fabricated.
4.2.2 Design of the device
The device schematic view is shown in Fig. 3.2. The width of the waveguide is 4.5 µm to permit the propagation of 1st order mode. Total length of the device is designed to be ~490 µm. At right side of the device shown in the figure, the modulation section is electrically separated from the multimode section. Length of modulation section is set as 50 µm for a short modulation cavity length. The radio frequency signal is injected only into the modulation section. The multimode section together with the central access waveguide left side is the pumping section. Pumping current is injected into the pumping section, i.e., the multimode region and central access waveguide are connected together through a common current source when operating. The two propagation paths shown in Fig. 3.2 illustrate the oscillating path of fundamental mode and 1st order mode. As a controllable output mode is desired, the mode selector section consisting from two bending access waveguide are separated from the multimode section electrically. The two bending access waveguides, which act as mode selector region, are connected through different current source when operating, i.e., current is injected into the two waveguides individually.
0th mode path
1st mode path
Main pumping section
Modulation section 350 μm
50 μm 490 μm
MMI edge
Fig. 3.2. Schematic view of active-MMI LD 0th
mode selector 1st
order mode selector
The waveguide cross section configuration is high-mesa structure to introduce the inner reflecting mirror at MMI edge [9, 10]. Because of the existence of inner reflecting mirror at multimode region edge, similar to the analysis in previous section, at least another 3 inner oscillating cavities must exist inside the device. Furthermore, as the structure shown in Fig. 3.2 are quite similar to the device configuration in Chapter 3, much more oscillating cavities might be existing inside the device. Consequently, much more PPRs must exist in the small signal response result. Such hypothesis will be explained in the following device characteristic section.
As the device is the mode selective light source, the propagation condition of each mode must be discussed. Figure 3.3 shows the mode propagation condition in the device for both fundamental mode and first order mode. Access waveguide at right side is the common output
0 1
Optical Power
10
0
10
0 100 200 300 400 500
(a)
Propagation distance [μm]
Width [μm]
10
0
10
0 100 200 300 400 500
Propagation distance [μm]
Width [μm]
port of both fundamental mode and first order mode. As can be seen form the figure, for both fundamental mode and 1st order mode, including the common output port and the multimode section, two modes shares the common propagating region. On the other hand, access waveguides left side are the separated propagation path for 0th and 1st order mode. The Upper side bending access waveguide is the fundamental mode and downside the 1st order mode.
Additionally, different to the 1 by 2 structure discussed in Chapter 2, the newly proposed device is based on 3 access ports configuration, thus the central straight waveguide has the issue of mode overlap. As can be seen in Fig. 3.3, both fundamental mode and first order mode has power distribution in the central straight access waveguide left side [11-15]. Considering the fact that too much over lapping, especially the upper and down side access waveguide might lead to serious mode cross talk issue, a proper MMI length is needed. Experimental results in Chapter 2 has shown the feasibility of mode selectivity active device. The structure of the device discussed in this chapter is, however, slightly different from the configuration discussed
A
B
C
MMI length [μm]
Normalized optical power [%]
0th mode total power
1st mode total power
in Chapter 2. Thus, it is still necessary to decide the MMI length carefully. When the main structure (1 by 3 MMI configuration) is fixed, the left issue is to determine the MMI length.
MMI length [μm]
0th – 1st order mode power ratio [dB]
Fig. 3.5. 0th mode to 1st order mode power ration as function of MMI length
A
B
C
Normalized optical power [%]
Fig. 3.6. Fundamental mode and first order mode power distribution of 1st ordermode injection condition. Horizontal axis is the MMI length.
MMI length [μm]
0th mode total power
1st mode total power
In order to decide the MMI length, the power distribution inside the device must be discussed. Similar to the discussion given in Section 2, we consider two cases: fundamental mode injection case and 1st order mode injection case. Figure 3.4 shows the fundamental mode injection result. The fundamental mode is injected into the device from the right side and power distributions of each mode in access waveguide A, B and C are discussed. In port A, B, and C, two different kind of modes exist: fundamental mode and first order mode. Thus, the power distribution of 0th mode and 1st order mode in each port are accumulated. The power in Fig 3.4 shows the total power of each mode in all three ports, i.e., blue line is the summation power of 0th mode in port A, B, and C. Similarly, orange line is summation power of 1st order mode in port A, B, and C. As can be seen from the figure, the MMI length largely affects the power distribution in the access waveguide ports. At the length of 285 µm, the fundamental mode reaches the maximum power and 1st order mode lowest power. The power ratio in dB scale is shown in Fig. 3.5. As can be seen, when MMI length is from 280 to 295 µm. Similar analysis was done when 1st order mode is injected. Figure 3.6 shows such result. The first order mode total power reaches maximum value. The 1st – 0th mode power ratio in dB scale is shown in
MMI length [μm]
1st – 0th mode power ratio [dB]
Fig. 3.6. 1st order mode to 0th mode power ration as function of MMI length
285µm. From results shown in Fig. 3.4~3.6, 285 µm MMI length has been confirmed for to reduce the mode crosstalk problem.
(d) BCB BCB
InGaAsP/InGaAsP MQW Substrate
p-InP n-InP
(a)
Substrate p-InP n-InP
Dry etching
(b)
Electrode
(e) (c)
High-mesa
(a): Active 7-layer MQW InGaAsP/ InGaAsP wafer.
(b): Dry etching process.
(c): Formation of High-mesa waveguide.
(d): BCB buried to form the side wall (e): Formation of electrode
~2 µm
4.2.3 Device fabrication
The device fabrication process is shown in Fig. 3.7. The 7-layer InGaAsP/ InGaAsP multiple quantum well active wafer is used. After the device pattern is determined, the mask was prepared. After the mask pattern was covered over the wafer, dry etching was done. The etching depth is ~2 µm, which is below the active layer. Such etching depth forms the so-called high-mesa waveguide. One of the properties of such waveguide is that, such structure forms the relatively strong reflection at the edge of waveguide. In the case of the active-MMI LD, the inner reflecting mirror is formed at the edge of the multimode region. After etching and formation of high-mesa waveguide, the polyimide material is buried to form the side wall.
After BCB process, the last step is the electrode formation. As described in Chapter 2, the slit waveguide is used for 1st order mode access waveguide. The slit waveguide electrode process is, however, improved when compared with the process discussed in Chapter 2.
The electrode pattern on slit waveguide is shown in Figure 3.8. Different from the electrode structure in Chapter 2, the slit gap is buried with BCB material to fill up the gap. The electrode is directly formed on top of the waveguide. Such process prevented the damage of the electrode. Thus, much more current injection efficiency is achieved than that of the structure in Chapter 2.
electrode
Slit structure