Self-compensating polarization modulator
A modulator unit for modulating the polarization of an optical signal includes a light source, a polarization-dependent phase modulator, and a reflector. The light source outputs an optical signal and emits it as the input signal directed at the phase modulator. The optical signal contains a first and a second polarization components having a first and a second polarization directions, respectively. The phase modulator modulates a first phase of the first polarization component in the first polarization direction and passes on the modulated input signal to the reflector. The reflector retroreflects the received optical signal towards the phase modulator and changes its polarization by 90°. The phase modulator modulates a second phase of the second polarization component of the retroreflected optical signal in the first polarization direction. The modulator unit outputs the modulated optical signal as the polarization-modulated output signal.
1 . A modulator unit for modulating the polarization of an optical signal, comprising:
a light source;
a polarization-dependent phase modulator;
a non-polarizing beam splitter arranged between the light source and the polarization-dependent phase modulator; and
a reflector;
wherein the light source is configured to output an optical signal and emit the output signal as an input signal to the non-polarizing beam splitter and in a direction of the polarization-dependent phase modulator, wherein the optical signal contains a first polarization component having a first polarization direction and a second polarization component having a second polarization direction before entering the non-polarizing beam splitter;
wherein the first polarization direction differs from the second polarization direction;
wherein the input signal to the non-polarizing beam splitter includes the first polarization component and the second polarization component, and the non-polarizing beam splitter is configured to pass on the input signal to the polarization-dependent phase modulator such that both the first polarization component and the second polarization component of the input signal pass though the polarization-dependent phase modulator in the same first direction towards the reflector,
wherein the polarization-dependent phase modulator is configured to modulate a first phase of the first polarization component of the input signal in the first polarization direction and to pass on the input modulated signal to the reflector;
wherein the reflector is configured to retroreflect the received optical signal in the direction of the polarization-dependent phase modulator as a retroreflected optical signal and at the same time to change polarization of the received optical signal, so that the first polarization component having the first polarization direction receives the second polarization direction and the second polarization component having the second polarization direction receives the first polarization direction, such that both the first polarization component and the second polarization component of the retroreflected signal pass though the polarization-dependent phase modulator in the same second direction away from the reflector, wherein the first direction towards the reflector is opposite the second direction away from the reflector;
wherein the polarization-dependent phase modulator is configured to modulate a second phase of the second polarization component of the retroreflected optical signal in the first polarization direction;
wherein the non-polarizing beam splitter is configured to guide at least a part of the retroreflected optical signal phase-modulated by the polarization-dependent phase modulator in a specified direction; and
wherein the modulator unit is configured to output the optical signal thus modulated as a polarization-modulated output signal.
2 . The modulator unit as claimed in claim 1 ,
wherein the polarization-dependent phase modulator includes a crystal, configured to be subjected to an electrical voltage and to change its index of refraction, by which the phase of the first polarization component of the optical signal is changed.
3 . The modulator unit as claimed in claim 2 ,
wherein the modulator unit is configured to vary the electrical voltage applied to the crystal over time.
4 . The modulator unit as claimed in claim 1 ,
wherein an absolute value of the first phase of the first polarization component of the input signal in the first polarization direction differs from an absolute value of the second phase of the second polarization component of the retroreflected optical signal in the first polarization direction.
5 . The modulator unit as claimed in claim 4 ,
wherein the polarization-dependent phase modulator is configured to change a difference between the first phase and the second phase over time.
6 . The modulator unit as claimed in claim 1 ,
wherein the light source is configured to output light with a defined optical mode.
7 . The modulator unit as claimed in claim 1 ,
wherein the modulator unit is configured to activate the light source so that the light source emits pulsed optical signals.
8 . An optical signal transmission link, comprising:
a modulator unit as claimed in claim 1 ; and
a receiver configured to receive optical signals;
wherein the modulator unit is arranged to emit the output signal in a direction of the receiver.
9 . A satellite comprising a modulator unit as claimed in claim 1 .