Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Up to 168-Gb/s PAM-4 direct detection transmission with silicon
  travelling wave Mach-Zehnder modulator
18
The categories of this paper are: econ
3
The categories of this paper are: physics
10
The categories of this paper are: q-bio
22
The categories of this paper are: chao-dyn
24
The categories of this paper are: dg-ga
21
The categories of this paper are: adap-org
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
25
The categories of this paper are: patt-sol
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con