PHP Mongodb aggregation not working with lookup stage

I am trying to execute MongoDB aggregation command with lookup stage, but the command does not seem to be working:

// col_1
{
    "_id" : 1,
    "sever_id" : 1,
    "question" : "What is option",
    "status" : "active",
    "no_of_options" : "2"
}

// col_2
{
    "_id" : 1,
    "col_1_id" : 1,
    "option" : "option 1",
    "votes" : 0
}
{
    "_id" : 2,
    "col_1_id" : 1,
    "option" : "option 2",
    "votes" : 0
}


$connection = get_mongodb_connection();
$command = new MongoDB\Driver\Command([
    'aggregate' => 'col_1',
    'pipeline' => [
        ['$match' => ['sever_id' => 1]],
        ['$lookup' => [
            "localField" => '_id',
            "from" => 'col_2',
            "foreignField" => 'col_1_id',
            "as" => 'options',
        ]],
    ],
    'cursor' => new stdClass,
]);
$cursor = $connection->executeCommand('testdb', $command);

this command is returning blank array in cursor.

It is pretty simple but not working. What am I doing wrong in this case ?