Method of controlling optical transmitter, and optical transmitter

1. A method of controlling an optical transmitter to generate a main signal having an output power at a predetermined value, and to generate an auxiliary signal, the optical transmitter including a wavelength tunable light source that outputs continuous wave light having a tunable wavelength, an optical modulator that outputs the main signal by phase modulation of the continuous wave light, a pump laser that outputs excitation light, an erbium-doped optical fiber amplifier (EDFA) that amplifies the main signal by using the excitation light, a variable optical attenuator (VOA) that attenuates the main signal amplified by the EDFA and outputs the attenuated main signal as the main signal generated by the optical amplifier, a pump laser controller that performs feedback control of the pump laser so that an output power of the excitation light is kept close to a first set value, and a VOA controller that performs feedback control of the VOA so that the output power of the attenuated main signal output from the VOA is kept close to a second set value, the method comprising:

controlling the optical transmitter while in normal mode operation to generate the main signal with the output power thereof set at the predetermined value by:

amplifying, by the EDFA, the main signal output from the optical modulator,

attenuating and outputting, by the VOA, the main signal amplified and output by the EDFA, and

maintaining the output power of the main signal output from the VOA at the predetermined value; and

controlling the optical transmitter while not in normal mode operation to generate the auxiliary signal by:

suspending phase modulation by the optical modulator so that the optical modulator outputs the continuous wave light from the wavelength tunable source;

disabling feedback control of the VOA performed by the VOA controller, and applying a constant control signal to the VOA;

disabling feedback control of the pump laser performed by the pump laser controller; and

controlling the pump laser to modulate the optical power of the excitation light to generate the auxiliary signal, the auxiliary signal being a low speed signal relative to the main signal.

2. The method of controlling an optical transmitter according to claim 1 , wherein

the auxiliary signal has a level of “1” and a level of “0,” and

the method further comprises:

setting the level “1” between an upper limit value and a lower limit value of the output power of the excitation light when the feedback control of the pump laser is performed by the pump laser controller, and

setting the level “0” to be smaller than the level “1” and larger than zero.

3. A method of controlling an optical transmitter to generate a main signal having an output power at a predetermined value, and to generate an auxiliary signal, the optical transmitter including a wavelength tunable light source that outputs continuous wave light having a tunable wavelength, an optical modulator that outputs the main signal by phase modulation of the continuous wave light, a pump laser that outputs excitation light, an erbium-doped optical fiber amplifier (EDFA) that amplifies the main signal by using the excitation light, a variable optical attenuator (VOA) that attenuates the main signal amplified by the EDFA and outputs the attenuated main signal as the main signal generated by the optical transmitter, a pump laser controller that performs feedback control of the pump laser so that an output power of the excitation light is kept close to a first set value, and a VOA controller that performs feedback control of the VOA so that the output power of the main signal output from the VOA is kept close to a second set value, the method comprising:

controlling the optical transmitter while in normal mode operation to generate the main signal with the output power thereof set at the predetermined value by:

amplifying, by the EDFA, the main signal output from the optical modulator,

attenuating and outputting, by the VOA, the main signal amplified and output by the EDFA, and

maintaining the output power of the main signal output from the VOA at the predetermined value; and

controlling the optical transmitter while not in normal mode operation to generate the auxiliary signal by:

suspending phase modulation in the optical modulator so that the optical modulator outputs the continuous wave light from the wavelength tunable light source;

setting the output power of the excitation light from the pump laser to a predetermined value; and

controlling the VOA to vary the attenuation by the VOA and modulating the output power of the output light of the EDFA to generate the auxiliary signal, the auxiliary signal being a low speed signal relative to the main signal.

4. The method of controlling an optical transmitter according to claim 3 , wherein

the attenuation of the VOA varied in the step of generating the auxiliary signal is set to a first attenuation or a second attenuation,

the first attenuation is set between an upper limit value and a lower limit value of attenuation when feedback control of the VOA is performed by the VOA controller, and

the second attenuation is set to be larger than the first attenuation.

5. An optical transmitter comprising:

a wavelength tunable light source that outputs continuous wave light having a tunable wavelength;

an optical modulator;

a pump laser that outputs excitation light;

an erbium-doped optical fiber amplifier (EDFA) that amplifies light output from the optical modulator by using the excitation light;

a variable optical attenuator (VOA); and

a controller configured to control the wavelength tunable light source, the optical modulator, the pump laser, the EDFA, and the VOA to generate a main signal and an auxiliary signal,

the controller generating the main signal by:

causing the optical modulator to phase modulate the continuous wave light from the wavelength tunable light source and output phase-modulated light,

causing the EDFA to amplify the phase-modulated light from the optical modulator and output amplified light, and

causing the VOA to attenuate the amplified light from the EDFA and output attenuated light as the main signal, and

the controller generating the auxiliary signal by:

causing the optical modulator to suspend phase modulation whereby the optical modulator outputs the continuous wave light from the wavelength tunable light source,

causing the pump laser to output intensity modulated light as the excitation light,

causing the EDFA to amplify the continuous wave light from the optical modulator by use of the intensity modulated light from the pump laser to output amplified, intensity-modulated light, and

causing the VOA to attenuate the amplified, intensity-modulated light from the EDFA to output attenuated, intensity-modulated light as the auxiliary signal,

wherein the controller causes the pump laser to vary the intensity of the excitation light laser between a first set value and a second set value smaller than the first set value.

6. An optical transmitter comprising:

a wavelength tunable light source that outputs continuous wave light having a tunable wavelength;

an optical modulator;

a pump laser that outputs excitation light set to a predetermined output power;

an erbium-doped optical fiber amplifier (EDFA) that amplifies light output from the optical modulator by using the excitation light;

a variable optical attenuator (VOA) that has a variable attenuation; and

a controller that controls configured to control the wavelength tunable light source, the optical modulator, the pump laser, the EDFA, and the VOA to generate a main signal and an auxiliary signal,

the controller generating the main signal by:

causing the optical modulator to phase modulate the continuous wave light from the wavelength tunable light source and output phase-modulated light,

causing the EDFA to amplify the phase-modulated light from the optical modulator and output amplified light, and

causing the VOA to attenuate the amplified light from the EDFA and output attenuated light as the main signal, and

the controller generating the auxiliary signal by:

causing the optical modulator to suspend phase modulation whereby the optical modulator outputs the continuous wave light from the wavelength tunable light source,

causing the pump laser to output intensity modulated light as the excitation light,

causing the EDFA to amplify the continuous wave light from the optical modulator by use of the intensity modulated light from the pump laser to output amplified, intensity-modulated light, and

causing the VOA to attenuate the amplified, intensity-modulated light from the EDFA to output variable attenuated, intensity-modulated light as the auxiliary signal,

wherein the controller causes the VOA to vary the attenuation of the amplified, intensity-modulated light between a first set value and a second set value larger than the first set value.

7. The optical transmitter of claim 5 , wherein the auxiliary signal is a low speed signal relative to the main signal.

8. The optical transmitter of claim 6 , wherein the auxiliary signal is a low speed signal relative to the main signal.