what is the loss function in char recognition using Tensorflow? I have code in Tensorflow using convolution neural network to recognize the characters in street view Text (SVT) data.

Since the label type is string, what should I use instead of tf.nn.sparse_softmax_cross_entropy_with_logits() in the loss function?

I cannot use tf.nn.sparse_softmax_cross_entropy_with_logits() because the labels her must be an int dtype??
The loss function is correct, you just need to convert categorical variables into numerical representations using one-hot vector encoding. Please take a look at this.