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---
# Frequency Domain Solver
---
This tutorial demonstrates Meep's [frequency-domain solver](../Scheme_User_Interface.md#frequency-domain-solver) which is used to compute the fields produced in a geometry in response to a [continuous-wave (CW) source](https://en.wikipedia.org/wiki/Continuous_wave). For details on how this feature works, see Section 5.3 ("Frequency-domain solver") of [Computer Physics Communications, Vol. 181, pp. 687-702, 2010](http://ab-initio.mit.edu/~oskooi/papers/Oskooi10.pdf). This example involves using the frequency-domain solver to compute the fields of a ring resonator which has been described in [Tutorial/Basics](Basics.md#modes-of-a-ring-resonator). We will verify that the error in the computed fields decreases monotonically with decreasing tolerance of the iterative solver.
Usage of the frequency-domain solver involves only two changes to the [original simulation](https://github.com/stevengj/meep/blob/master/scheme/examples/ring.ctl): (1) replace the Gaussian-pulse source with a [continuous source](../Scheme_User_Interface.md#source), and (2) turn on complex fields since, by default, real fields are used. Everything else remains unchanged.
Since the frequency-domain solver uses an [iterative method](https://en.wikipedia.org/wiki/Iterative_method), there are a couple of things we can do to improve its convergence: (1) use a non-zero smoothing width for the CW source (default is 0) to reduce the high-frequency oscillations produced by its abrupt turn on (which have slow group velocities and are absorbed poorly by [PML](../Perfectly_Matched_Layer.md)), and (2) increase the $L$ parameter of the [BiCGSTAB-L](https://en.wikipedia.org/wiki/Biconjugate_gradient_stabilized_method) iterative solver from the default of 2 to 10.
We will compute the fundamental mode at five different tolerance values chosen on a logarithmic scale. We will then plot the L2 norm of the error in the fields as a function of the tolerance. The simulation script is shown below.
```scm
(define-param n 3.4)
(define-param w 1)
(define-param r 1)
(define-param pad 4)
(define-param dpml 2)
(define sxy (* 2 (+ r w pad dpml)))
(set! geometry-lattice (make lattice (size sxy sxy no-size)))
(set! geometry (list
(make cylinder (center 0 0) (height infinity)
(radius (+ r w)) (material (make dielectric (index n))))
(make cylinder (center 0 0) (height infinity)
(radius r) (material air))))
(set! pml-layers (list (make pml (thickness dpml))))
(set-param! resolution 10)
(define-param fcen 0.118)
(define-param df 0.08)
(set! sources (list
(make source
(src (make continuous-src (frequency fcen) (fwidth df)))
(component Ez) (center (+ r 0.1) 0))))
(set! symmetries (list (make mirror-sym (direction Y))))
(set! force-complex-fields? true)
(define-param solve-cw-tol 1e-8)
(define-param solve-cw-maxiters 10000)
(define-param solve-cw-L 10)
(define (ez-mag r ez) (magnitude ez))
(init-fields)
(meep-fields-solve-cw fields solve-cw-tol solve-cw-maxiters solve-cw-L)
(in-volume (volume (center 0 0) (size (- sxy (* 2 dpml)) (- sxy (* 2 dpml))))
(output-epsilon)
(output-real-field-function "ez-mag" (list Ez) ez-mag))
(exit)
```
The results are shown in the figure below. The error in the fields decreases monotonically with decreasing tolerance of the frequency-domain solver. The inset shows the magnitude of the scalar E<sub>z</sub> field, computed using a tolerance of 10<sup>-12</sup>, superimposed on the ring-resonator geometry. Note the three-fold mirror symmetry of the field pattern (fundamental mode) and the faint presence of the point source.
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