Charles d'Orleans wrote most of his poetry while a prisoner of war. Got me to thinking of Richard Lovelace's best known work, composed, of course, during his time in prison:

To Althea, from Prison

WHEN Love with unconfinèd wings
Hovers within my gates,
And my divine Althea brings
To whisper at the grates;
When I lie tangled in her hair
And fetter'd to her eye,
The birds that wanton in the air
Know no such liberty.

When flowing cups run swiftly round
With no allaying Thames, 10
Our careless heads with roses bound,
Our hearts with loyal flames;
When thirsty grief in wine we steep,
When healths and draughts go free—
Fishes that tipple in the deep
Know no such liberty.

When, like committed linnets, I
With shriller throat shall sing
The sweetness, mercy, majesty,
And glories of my King;
When I shall voice aloud how good
He is, how great should be,
Enlargèd winds, that curl the flood,
Know no such liberty.

Stone walls do not a prison make,
Nor iron bars a cage;
Minds innocent and quiet take
That for an hermitage;
If I have freedom in my love
And in my soul am free,
Angels alone, that soar above,
Enjoy such liberty.

(A very good audio bit can be found here.)