issuance of NOC is not compulsory nor you can force your employer to give you one...some good employers who understands gave one but you must have a very good reason to convince them to..it is also not against the law if they refuse to do so...if you really want to go and you can't get NOC from them..then resign, wait for two years if you are still willing then return..if you can't do it..then pray that the law change so your wish could be granted.