In a 5- to 6-quart pan over high heat, bring broth, 2 cups water, and fettuccine to a boil; cook, stirring often, until pasta is tender to bite, about 7 minutes.
Meanwhile, rinse caviar in cold water and drain well in a fine strainer.
Blend cornstarch with 3 tablespoons water until smooth.
Stir cornstarch mixture, yogurt, and wasabi with pasta, then stir until boiling again. Add salmon and stir gently just until hot, 1 to 2 minutes.
Serve pasta in bowls and garnish with salmon caviar.