n0 = 1.1; \[Alpha] = 0.5*10^-3; \[Theta] = \[Pi]/4;
n[y_] := n0 - \[Alpha]y; sm = 3*10^3;
equ = {D[n[y[s]]*y'[s], s] == -\[Alpha], D[n[y[s]]*z'[s], s] == 0,
y[0] == 0, y'[0] == Sin [\[Theta]], z[0] == 0,
z'[0] == Cos[\[Theta]]};
sol = NDSolve[equ, {z, y}, {s, 0, sm}]
ParametricPlot [{z[s] y[s]} /. sol[[1]],
{s, 0, sm}, AxesLabel \[RightArrow] {"Z", "Y"}]
Clear ["Global`*"]

n0 = 1.1; \[Alpha] = 0.5/10^3;
\[Theta] = Pi/4;
n[y_] := n0 - \[Alpha]y;
sm = 3*10^3;
equ =
{D[n[y[s]]*Derivative[1][y][
s], s] == -\[Alpha],
D[n[y[s]]*Derivative[1][z][
s], s] == 0,
y[0] == 0,
Derivative[1][y][0] ==
Sin[\[Theta]], z[0] == 0,
Derivative[1][z][0] ==
Cos[\[Theta]]};
sol = NDSolve[equ, {z, y},
{s, 0, sm}]
ParametricPlot[{z[s], y[s]} /.
sol[[1]], {s, 0, sm},
AxesLabel -> {"Z", "Y"}]
Clear["Global`*"]

alphay没有定义