ECEN 4616/5616 Optoelectronic Design

1. ECEN 4616/5616Optoelectronic Design Class website with past lectures, various files, and assignments: http://ecee.colorado.edu/ecen4616/Spring2014/ (The first assignment will be posted here on 1/22) To view video recordings of past lectures, go to: http://cuengineeringonline.colorado.edu and select “course login” from the upper right corner of the page. If anyone has trouble accessing either of these sites, please notify me as soon as possible.

2. Gaussian Optics Gaussian Optics refers to the use of a linear model to represent idealized optical lenses, mirrors, etc. We’ve already shown that the imaging and ray tracing equations are linear in the paraxial limit – how can they be modified to represent ideal finite-sized lenses? This is not an idle mathematical exercise, as most optical designs need to start with an ideal system which solves the desired problem, such as a simple, two-lens zoom camera lens: Simple zoom lens based on the combination of powers formula: The ideal lenses are then replaced with lens combinations designed to be as ideal as possible:

3. Gaussian Imaging Recall that the focal length of a (positive) lens is defined as the distance past the lens that an incident plane wave will converge to a point, and that the lens power is defined as the inverse of the focal length: Since the converging wave has a curvature just past the lens, this leads to the conclusion that the lens adds a curvature of K to an incoming wave: f K K l l’ This observation leads to the imaging equation: i o The imaging equation follows from the definition of focal length and power, hence already describes an ideal lens. (We showed that real lenses behave ideally in the paraxial limit, but the imaging equation applies to any lens combination designed to be approximately linear.)

4. The paraxial ray refraction equation, , however, was only shown to hold true in the paraxial approximation where the angles, sines and tangents are all approximately equal: K h u u’ l’ l What definition of u, u’ will allow the linear paraxial refraction equation to also describe finite-sized ideal lenses? i.e., the problem is to find “angle variables” such that:

5. Solve both equations for K, and equate: Imaging Equation: Refraction Equation: , , ⇒ ⇒ ⇒ Equating the input variables (unprimed) and the output (primed) variables independently, we see that: then the linear ray trace equation and the imaging equation will be true for finite size lenses (h finite). Hence, for Gaussian ray tracing (ideal lenses), we will use the tangents of the ray angles in the diffraction equation.

6. Summary of Gaussian Optics (Ideal Lens) Rules Lenses are represented by plane surfaces. All calculations about a lens are done in the lens’ local coordinates, where the origin is the lens’ intersection with the z-axis. Lens power, K, is the curvature that a lens imposes upon an incident wave. (Hence, in the drawing at right, K h u u’ l’ l • Sign Rules: • The (local) origin is at the intersection of the surface and the z-axis. • All distances are measured from the origin: Right and Up are positive • All angles are acute. They are measured from: • From the z-axis to the ray • From the surface normal to the ray • CCW is positive, CW is negative • Indices of refraction are positive for rays going left-to-right; negative for rays going right-to-left. (Normal is left-to-right.) 3) Rays are characterized by their positions and the tangents of the angles they make with the z-axis. (These are generally referred to as the ‘angle variables’) The change in the angle variable of a ray is the negative product of the lens power and distance from the axis: u’ – u = -hK. (Hence rays are bent towards the axis by lenses with positive power and away from the axis by lenses with negative power.)

7. Summary of Gaussian Optics (continued) Lens1 Lens2 l2 h1 l’2 Image calculation at a lens: u1 u’1 Ray refraction at a lens: u2 u’2 l1 h2 l’1 d1 Image transfer between lenses: Ray transfer between lenses:

8. Focal Points of a Lens f’ f Since lenses can work in either direction, they have two focal points. f is called the primary focal point and f’ the secondary focal point. While f and f’ follow the primed variable rule for positive lenses, they are on opposite sides for negative lenses: f’ f

9. Lenses: Real and Virtual Images A real image can be projected on a screen. Projection screen Real Object f f’ Real Image

10. Lenses: Real and Virtual Images A Virtual Image appears to come from somewhere where there are no actual rays. Real Object Virtual Image f f’ Observer

11. Lenses: Real and Virtual Images Real Object f’ f Virtual Image

12. Lenses: Real and Virtual Images Real Object Virtual Image f’ f Negative lenses only form virtual images from real objects.

13. Lenses: Real and Virtual Images Virtual Object Real Image f’ f A negative lenses can form a real image from a virtual object however.

14. Lenses: Real and Virtual Images A virtual object for one lens can be a real image from the previous lens in a system. Real image from Lens 1 and virtual object for Lens 2 L1 L2 f’2 f’ Real Object f1 Real image from Lens 2 and system.

15. Optical Devices: Consider a positive lens with an object less than one focal length behind it: The Simple Magnifier: What is the effective magnification? Let the object height = h, and assume the user places his/her eye as close to the lens as feasible. Then, the angle that the central ray makes is the angular extent of the image for the user: S1 S2 h a Object Virtual image f f’ We need to compare this to the maximum angle that the object can be viewed without using the lens. This is obviously dependent on how close someone can focus their eyes, so is not a universal constant. For the purpose of these kinds of calculations, however, it is usual to use -250 mm as the average close viewing distance achievable for most of the population (the older population may have to use reading glasses!). Hence the maximum angle the object can subtend to the unaided eye is: The effective magnification is therefore:

16. S2 S1 h a We will further assume that the distance, S2, of the virtual image is made to also equal -250 mm. This will give us slightly more magnification, since the object can be closer to the lens than for an object at infinity. Object Virtual image f f’ The imaging equation then gives us: The magnification is then: Note that, if the object is at the focal distance, then the object is at ∞, and the resulting magnification is:

17. n2’ In the last lecture, we found the combination of power formula by tracing rays through a pair of surfaces: u’1=u2 n1 u1 =0 n1’=n2 h1 h2 u’2 Refract at second element: Refract through first element: d1 BFL f K2 K1 Eliminating h1 and using where K is the equivalent power of the combination, we get: Transfer to second element: General formula for combination of two surfaces:

18. The “Lensmakers” Equation The ‘thin lens’ is an approximation, as any real lens will have a finite thickness. In the last lecture (1-17) we saw that, by applying the paraxial assumption to a surface separating materials of indices n, n’ that the power of a surface is: Where c is the curvature (inverse radius) of the surface, n the index before and n’ the index after the surface. c1 c2 To adapt the surface power combination formula to a thick lens in air, let: A “Thick Lens”: d1 Substituting for the powers and indices: We get the formula for the power of a thick lens: