diff --git a/src/backscattering_analyzer/analyzer.py b/src/backscattering_analyzer/analyzer.py
index c345bc7..0ef5340 100644
--- a/src/backscattering_analyzer/analyzer.py
+++ b/src/backscattering_analyzer/analyzer.py
@@ -7,7 +7,7 @@ from backscattering_analyzer import (
     opt_compute_light,
     interpolate,
 )
-from numpy import loadtxt, pi
+from numpy import loadtxt, logspace, pi
 from scipy.io.matlab import loadmat
 from scipy.optimize import Bounds, minimize
 
@@ -180,7 +180,7 @@ class Analyzer:
             )
         )
 
-        bounds = Bounds(0, 1)
+        bounds = Bounds(1e-30, 1e-3)
         min_result = minimize(
             fit_compute_light,
             guess,
@@ -194,7 +194,7 @@ class Analyzer:
                 data,
                 reference,
             ),
-            method = 'TNC',
+            method="Powell",
             bounds=bounds,
         )
 
@@ -232,4 +232,25 @@ class Analyzer:
         _ = plt.legend()
         _ = plt.show()
 
+        """
+        as the minimization does not work (why ?), visual help
+        """
+        x = logspace(-50, 0, 1000)
+        y = [
+            fit_compute_light(
+                x[i],
+                factor_n,
+                coupling_n,
+                factor_d,
+                coupling_d,
+                self.settings.power_in,
+                self.settings.power_out,
+                data,
+                reference,
+            )
+            for i in range(len(x))
+        ]
+        _ = plt.loglog(x, y)
+        _ = plt.show()
+
         return min_result.x