This is a simulation of a single mode fiber. The sliders allow you to manipulate this simulation. The Core and Cladding Index sliders allow you to change the reflective index of the core or cladding. The lambda slider allow you to change the wavelength of the light. If you check wide range the maximum vales that you can set any of the sliders to will increase. The output is the Group velocity.
You may have noticed that if you increase the Core Index then the maximum value for which Cladding Index can be set will increase. This is because the Cladding Index can never be bigger than the Core Index. You may also have noticed that if you change some of the values then the Group Velocity might be outputted as Null. This is because this simulations assumes That 1.5 < V < 2.
Kasap, S. O. Optoelectronics and Photonics: Principles and Practices. Upper Saddle River, NJ: Prentice Hall, 2001. Print.