{"id":406727,"date":"2025-01-23T06:54:44","date_gmt":"2025-01-23T06:54:44","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/difference-between-n_eff-and-group-index-in-single-frequency-simulation-fde\/"},"modified":"2025-01-23T08:09:02","modified_gmt":"2025-01-23T08:09:02","slug":"difference-between-n_eff-and-group-index-in-single-frequency-simulation-fde","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/difference-between-n_eff-and-group-index-in-single-frequency-simulation-fde\/","title":{"rendered":"difference between n_eff and group index in single-frequency simulation (FDE)"},"content":{"rendered":"<p>&lt;p&gt;&lt;p&gt;Hi,&lt;\/p&gt;&lt;div&gt;I simulate a simple waveguide made of silicon in FDE (with a single wavelength of 1064nm). And get the different modes that can propagate in a cross-sectional area defined by the simulation region.&lt;\/div&gt;&lt;div&gt;I would expect that for propagation of a single frequency &#8211; the phase velocity and the group velocity would be the same. Because there is no group of waves &#8211; there is a single wave with a wavelength of 1064nm.&lt;\/div&gt;&lt;div&gt;Why do I get a big difference (factor 2)?&lt;\/div&gt;&lt;div&gt;<img decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2025\/01\/23-01-2025-1737615222-mceclip0.png\" \/>&lt;\/div&gt;&lt;div&gt;In addition, I understand that in reality there is no single frequency but a finite bandwidth &#8211; and I would still expect it to be close and not twice.&lt;\/p&gt;&lt;p&gt;The reason I want to understand this is because (in general) I want to build a traveling wave electrode modulator .&lt;br&gt;So I want the propagation velocities of the optical and RF signals to be close. But I&#8217;m not sure which velocity to take in each of them (especially in the optical) &#8211; phase velocity or goup velocity&lt;\/p&gt;&lt;p&gt;&nbsp;&lt;\/p&gt;&lt;p&gt;thanks.&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-406727","topic","type-topic","status-publish","hentry","topic-tag-waveguide-2","topic-tag-group-velocity"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27833"],"_bbp_topic_id":["406727"],"_bbp_subscription":["502241","33648"],"_bbp_author_ip":["77.125.71.196"],"_bbp_last_reply_id":["408377"],"_bbp_last_active_id":["408377"],"_bbp_last_active_time":["2025-02-04 18:26:58"],"_bbp_reply_count":["3"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["2"],"_bbp_engagement":["502241","33648"],"_btv_view_count":["231"],"_bbp_topic_status":["unanswered"],"_edit_last":["502241"],"_bbp_revision_log":["a:1:{i:406737;a:2:{s:6:\"author\";i:502241;s:6:\"reason\";s:0:\"\";}}"]},"test":"haimshalpost-bgu-ac-il"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/406727","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":1,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/406727\/revisions"}],"predecessor-version":[{"id":406737,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/406727\/revisions\/406737"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=406727"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}