90 deg hybrid for a 9-10 GHz band

I am not used to working this high in frequency, and am not sure what the best way to design a 90 deg hybrid for operation over 9-10GHz would be. Would a simple branchline coupler work for this application?

A branchline hybrid is comprised of four quarterwave transformers. By it's very nature, it will be a narrowband structure. Depending on how much ripple you can afford in your thru and coupled paths, and return loss at the edges of your band, a single box hybrid may be sufficient. You can get more broadband by using a double box branchline, too. Read up on Microwaves101 for more info.
Branchline Couplers - Microwave Encyclopedia - Microwaves101.com

Depending on your implementation, a Lange coupler could also be used (Microwaves101, Coupler pages). They are more compact, but can take more time to initially design and refine. It's all a trade-off. Hope that helps.

That is a pretty high frequency for a branch line coupler--the junction effects at the 4 corners will kill you. Also, the line lengths are short with respect to their widths, so you have an aspect ratio problem getting the lines to work like transmission lines.

I would go with a lange coupler for microstrip, or a overlay stripline coupler. Either will have plenty of bandwidth.

What is your substrate material and substrate thickness?