Tremor Media is already the largest online video ad network in the U.S., and the company says it’s been profitable since 2009. Now the company just closed a large $40 million series D financing to accelerate its product R&D particularly its ad management system, and expand into “other media channels” (TV perhaps?).

The round was led by the Draper Fisher Jurvetson Growth Fund, with some money coming from DFJ proper and Triangle Peak Partners. Previous investors also put in more money, including Canaan Partners, Meritech Capital Partners and SAP Ventures. This round doubles the total capital Tremor has raised since launch to almost $80 million.

According to comScore, Tremor’s video ad network reaches a potential audience of 81.7 million people in the U.S., putting it ahead of YuMe, Advertising.com, BBE, and Break Media. Online video advertising is the fastest growing portion of online advertising, and Tremor needs to cement its position among the top video ad networks. That $40 million will certainly come in handy as it attempts to do that. It also could be a war chest for smaller acquisitions.