Though if it is on or under the surface of the earth, “straight” will be a bend radius of around 6,370km. We don’t make a lot of buildings that deal with this but transcontinental or transoceanic cables certainly do. If someone designed a fiber that required absolutely no bend in order to work you’d have to use it in buildings or dig much deeper holes.
There was an encoding mechanism proposed about 10-15 years ago that used spirally polarized light to carry more channels, but it required the surface of the fiber to be polished to a much higher degree than existing cables in order for the light to go around bends properly.
If you’re using the planet as your “flat surface” then sure. If, however, you’re willing to deal with exiting the atmosphere at each end, you can use Real Straightness. But I don’t know anyone running a single segment for that distance.
I’m sure there are some microwave antennas still out there doing the Lord’s Work. At least in the Plains states where hills are low and putting antennas on two of them gives you some extra distance. How far do microwaves bend over the horizon?
Don't they generally do the opposite of bend over the horizon? Two towers that are observed (visible wavelength = tiny Fresnel zone) to have line-of-site can easily be obstructed (microwave = huge Fresnel zone).
If you're going out of the atmosphere may as well skip the fiber and just point lasers through the vacuum of space directly and reap the benefits of the faster speed of light through a vacuum vs glass!
A straight cable has an infinite radius, the more bend the smaller the radius