IPhone : iOS-How to use NSOperationQueue in Swift

I'm using NSOperationQueue in my app,I write it with Objective C.And I'm currently converting my app to Swift.I wonder how can I use NSOperationQueue in Swift?

Here is my code on objective c

 -(NSOperationQueue*)operationQueue
{
    if(_operationQueue == nil) {
        _operationQueue = [NSOperationQueue new];
    }
    return _operationQueue;
}

And I already tried in swift

func operationQueue() -> NSOperationQueue{

    if(_operationQueue == nil){

        _operationQueue = NSOperationQueue.new()

    }

But it gives me error " NSOperationQueue is not convertible to 'MirrorDispotion'"