開いたものから md-dialog を開こうとしましたが、問題は、2 番目の md-dialog が開いたときに最初の md-dialog が閉じたことです。
// the controller of the first popUp 
class popUpOneController{
      constructor($mdDialog){
      this.$mdDialog=$mdDialog;
       .
       .
       .
       } 
  others methods codes......
// the second $mdDialog to be opened from the  first 
// this function will be executed by clicking  on a html  button
  openPopUp2(){
      // here we call a component into the $mdDialog
      this.$mdDialog.show({
            template: '<interlo-sibel  data-to-pass='+data+'  index-selectedElm='+index+' type='+type+' ></interlo-sibel>',
            targetEvent: event,
            escapeToClose: false
        })
   } 
   popUpOneController.$inject=['$mdDialog'];
 export default popUpOneController