Theoretical and experimental study of a laser diode end-pumped passively <inline-formula><math display="inline" overflow="scroll"><mi>Q</mi></math></inline-formula>-switched <inline-formula><math display="inline" overflow="scroll"><msub><mi>Nd:YVO</mi><mrow><mn>4</mn></mrow></msub></math></inline-formula> laser with <inline-formula><math display="inline" overflow="scroll"><msup><mrow><mi>Cr</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup><mo>:</mo><mi>YAG</mi></math></inline-formula> saturable absorber