content_image/396439.jpg content_image/396448.jpg content_image/396447.jpg content_image/396446.jpg
## Appendix B: Derivation of Eqs. 7a and 7b 

<img src='content_image/396439.jpg'>

From Eqs. 6 and 5a, 

<img src='content_image/396448.jpg'>

Noting that the phase has a stationary point at s = 0 [11], we expand it in Taylor’s series around s = 0 to obtain 

as stated in Eq. 7a. 

Similarly from Eqs. 6 and 5b, 

as stated in Eq. 7b. 

<img src='content_image/396447.jpg'>

<img src='content_image/396446.jpg'>

