I found that those bots often visit my site and got only several pages then left. How can attract bots to visit more pages and more deeply? any suggestions? Thanks!
Do you have links from other sites pointing to those pages? That seems to be the most effective way I have found.
The importance of a good internal linking structure can not be over emphasized. A site shouldn't have a tree like internal linking structure. The internal linking structure should be like a net or web, where pages are interconnected. My main site has around 19,000 webpages and over 1 million internal links. These aren't spammy internal links for the sake of having internal links. All links are about helping the user continue their quest for information. Each page has a right hand navigation menu that is categorized and links to all of the main pages for my site. Individual pages link to other relevant pages in the content of the page to allow the user to continue to "drill down" into a specific topic to get more information. The better your site is logically cross linked, the more likely it is that users will spend more time on your site and the easier time SEs will have indexing your site more deeply.
After setting up more links, I would use google sitemaps to aid the spiders. Try and setup some external links to individual articles/pages other than the front page too. Also the link in your sig points to site:www.optical-network.com, which should be http://www.optical-network.com