An optical module (110;210) suitable for use in an Erbium doped fiber amplifier, EDFA, for amplifying optical signal, comprising: an optical fiber component, comprising: a first fiber, configured to input a signal light (Ls) having a first wavelength; a second fiber, configured to input a pump light (Lp) having a second wavelength; a third fiber, configured to output the pump light (Lp) and the signal light (Ls) together; a fourth fiber, configured to input an amplified signal light (Las) which amplified from the signal light (Ls) and a surplus of the pump light (Lp) output by the third fiber; and a fifth fiber, configured to output the amplified signal light (Las); a walk-off birefringent crystal adjacent to the optical fiber component; a collimating lens adjacent to the walk-off birefringent crystal and opposite to the optical fiber component; a first wave plate/plates and a second wave plate provided between the walk-off birefringent crystal and the collimating lens; a Faraday rotator adjacent to the collimating lens and opposite to the walk-off birefringent crystal; a wavelength division multiplexing, WDM, filter provided between the collimating lens and the Faraday rotator, the WDM filter being configured to pass the signal light (Ls) and the amplified signal light (Las), and reflecting the pump light (Lp); a mirror adjacent to the Faraday rotator and opposite to the WDM filter, the mirror reflecting most of the signal light (Ls) and the amplified signal light (Las); and an optical detecting component (50;52) adjacent to the mirror and opposite to the Faraday rotator; wherein the optical fiber component, the walk-off birefringent crystal, the first and the second wave plates (33 and 34, respectively), the collimating lens, the WDM filter, the Faraday rotator and the mirror are arranged and oriented with respect to each other so that the signal light (Ls) input from the first fiber and the pump light (Lp) input from the second fiber are reflected and coupled to the third fiber, the amplified signal light (Las) input from the fourth fiber is reflected and coupled to the fifth fiber, and the surplus of the pump light (Lp) input from the fourth fiber is reflected and dissipated, so that an isolator is formed which is configured to pass the signal light (Ls) and the amplified signal light (Las) in a forward direction, and isolate the signal light (Ls) and the amplified signal light (Las) in a reverse direction, the isolator including the walk-off birefringent crystal, the collimating lens, the first wave plate, the second wave plate and the Faraday rotator, wherein the optical detecting component (50;52) is configured to monitor a magnitude of the amplified signal light (Las), wherein the first wave plate/plates is/are aligned with the first fiber and the fourth fiber, and wherein the second wave plate is aligned with the second fiber. a first fiber, configured to input a signal light (Ls) having a first wavelength; a second fiber, configured to input a pump light (Lp) having a second wavelength; a third fiber, configured to output the pump light (Lp) and the signal light (Ls) together; a fourth fiber, configured to input an amplified signal light (Las) which amplified from the signal light (Ls) and a surplus of the pump light (Lp) output by the third fiber; and a fifth fiber, configured to output the amplified signal light (Las); a walk-off birefringent crystal adjacent to the optical fiber component; a collimating lens adjacent to the walk-off birefringent crystal and opposite to the optical fiber component; a first wave plate/plates and a second wave plate provided between the walk-off birefringent crystal and the collimating lens; a Faraday rotator adjacent to the collimating lens and opposite to the walk-off birefringent crystal; a wavelength division multiplexing, WDM, filter provided between the collimating lens and the Faraday rotator, the WDM filter being configured to pass the signal light (Ls) and the amplified signal light (Las), and reflecting the pump light (Lp); a mirror adjacent to the Faraday rotator and opposite to the WDM filter, the mirror reflecting most of the signal light (Ls) and the amplified signal light (Las); and an optical detecting component (50;52) adjacent to the mirror and opposite to the Faraday rotator; wherein the optical fiber component, the walk-off birefringent crystal, the first and the second wave plates (33 and 34, respectively), the collimating lens, the WDM filter, the Faraday rotator and the mirror are arranged and oriented with respect to each other so that the signal light (Ls) input from the first fiber and the pump light (Lp) input from the second fiber are reflected and coupled to the third fiber, the amplified signal light (Las) input from the fourth fiber is reflected and coupled to the fifth fiber, and the surplus of the pump light (Lp) input from the fourth fiber is reflected and dissipated, so that an isolator is formed which is configured to pass the signal light (Ls) and the amplified signal light (Las) in a forward direction, and isolate the signal light (Ls) and the amplified signal light (Las) in a reverse direction, the isolator including the walk-off birefringent crystal, the collimating lens, the first wave plate, the second wave plate and the Faraday rotator, wherein the optical detecting component (50;52) is configured to monitor a magnitude of the amplified signal light (Las), wherein the first wave plate/plates is/are aligned with the first fiber and the fourth fiber, and wherein the second wave plate is aligned with the second fiber.