After a nearly 18-month period that saw Silicon Valley launch just a single IPO, two companies in the past month have entered the fray, one announcing its intention to go public and another witnessing stronger-than-anticipated demand in its first day of trading.

Observers say the move by Fortinet Inc. of Sunnyvale and the strong trading of Avago Technologies Ltd., a Singapore company with U.S. headquarters in San Jose, can be taken as signs that quality companies are ready to go public—and that market conditions are shifting to help that happen.

Don’t call it a flood. But you can call it a sign of life that’s been missing. The IPO window has been closed for so long that very mature companies with good revenue stories may now find it opening just a crack.

Networking companies, software companies, and players in the cleantech space may find special momentum in the current, cautious-but-optimistic atmosphere, experts say.

“My investment banking contacts are telling me that some of the institutional investors that buy IPOs are starting to tell them, ‘We’re interested in this segment again,’” said Steve Bochner, the newly appointed CEO at Palo Alto law firm Wilson Sonsini Goodrich & Rosati.

“It’s early days, for sure, but I could be sitting here saying, ‘Gee, there’s no IPOs,’ and that’s not the case,” Bochner said. “I’m encouraged by the little bit of momentum we see.”

U.S. companies raised $1.6 billion in 12 IPOs during the second quarter of this year—the first increase in IPO activity since the fourth quarter of 2007, according to a report from PricewaterhouseCoopers. Scott Gehsmann, a capital markets partner in PwC’s transaction services practice, said more companies will probably test the IPO waters in the latter part of the year.