Optical single sideband transmitter

• US 20030189745A1
• Filed: 12/17/2002
• Published: 10/09/2003
• Est. Priority Date: 04/05/2002
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Kikuchi, Nobuhiko, Sugawara, Toshiki, Takeyari, Ryoji
• Granted Patent: US 6,850,713 B2
• Time in Patent Office: Days
• US Class Current: 359/237
• CPC Class Codes: H04B 10/505 using external modulation
• CPC Class Codes: H04B 10/5051 using a series, i.e. cascade, combination of modulators
• CPC Class Codes: H04B 10/5053 using a parallel, i.e. shunt, combination of modulators
• CPC Class Codes: H04B 10/50572 to control the modulating signal amplitude including amplitude distortion
• CPC Class Codes: H04B 10/506 Multiwavelength transmitters
• CPC Class Codes: H04B 10/508 Pulse generation, e.g. generation of solitons
• CPC Class Codes: H04B 2210/258 treating each wavelength or wavelength band separately
• CPC Class Codes: H04L 7/0008 Synchronisation information channels, e.g. clock distribution lines

Abstract

With the normal modulation method, it is difficult to construct a Hilbert transform device because it is complicated. To solve the problem, the present invention generates a single sideband modulated optical pulse train by driving a Mach-Zehnder optical modulator for optical pulse generation with a laser source'"'"'s sine wave clock signals rendered 90 degrees out of phase from each other. The generated pulse train is entered into an optical modulator, modulated with an NRZ (nonreturn-to-zero) data signal, and filtered by a narrow-band optical filter to obtain one of two sidebands.

Claims

1. An optical single sideband transmitter comprising a laser source, a first optical module, a second optical module, and an optical filter, wherein said transmitter acquires RZ (return-to-zero) modulated light that is single-sidebanded in the (1) and (2) order or in the (2) and (1) order indicated below:

(1) The laser light output from said laser source is entered into said first optical modulator, converted to a single-sidebanded periodic optical pulse train. Said periodic optical pulse train, which is the output from said first optical modulator, is entered into said second optical modulator and intensity modulated by a digital information signal. The resulting output is then single-sidebanded by said optical filter; and

(2) The laser light, which is the output from said laser source, is entered into said second optical modulator and subjected to NRZ (nonreturn-to-zero) intensity modulation by said digital information signal. Said NRZ intensity modulated optical signal, which is the output from said second optical module, is entered into said first optical module. Said first optical modulator converts said optical signal to a RZ optical signal with single sidebanded optical carriers. The resulting output is then single-sidebanded by said optical filter.

2. The optical single sideband transmitter according to claim 1, wherein a Mach-Zehnder optical modulator is used as said first optical modulator, and two sine wave electrical signals having the same bit rate as the digital information signal are phase shifted by 90 degrees from each other and applied to electrodes that are provided for two optical paths of said Mach-Zehnder optical modulator.

3. The optical single sideband transmitter according to claim 1, wherein an optical signal is wavelength division multiplexed.

4. The optical single sideband transmitter according to claim 1, wherein a lithium niobate (LN) optical modulator, polymer-based optical modulator, or semiconductor optical modulator is used as said Mach-Zehnder first optical modulator.

5. The optical single sideband transmitter according to claim 1, wherein a modulator integrated light source, which is obtained by integrating a semiconductor laser and an electro-absorption semiconductor optical modulator or Mach-Zehnder semiconductor optical modulator, is used to perform the roles of said laser source and second optical modulator;

or wherein the role of said second optical modulator is performed by directly converting the current of said laser source with an NRZ electrical signal.

6. The optical single sideband transmitter according to claim 1, further comprising one or more optical couplers, an optical detector, and a maximum control circuit,

wherein part of the optical signal output from said optical filter is branched by the optical coupler, converted to an electrical signal by the optical detector, and entered into the maximum control circuit, which controls the frequency of the optical signal output from a signal light source or said optical filter center frequency so as to maximize the intensity of said electrical signal.

7. An optical single sideband transmitter, comprising a plurality of laser sources differing in wavelength, a first optical modulator, an optical demultiplexer, one or more second optical modulators, and an optical filter to transmit an optical signal whose intensity waveform is subjected to RZ (return-to-zero) modulation by a digital information signal,

wherein laser light outputs, which are different in wavelength and generated from said plurality of laser sources, are multiplexed, entered into the first optical modulator, simultaneously converted to a single sideband modulated periodic optical pulse train, and demultiplexed into individual wavelengths by said optical demultiplexer, then the resulting optical pulse trains differing in wavelength are modulated with an information signal by the second optical modulator, and filtered by said optical filter to acquire and output one of two sidebands.

8. The optical single sideband transmitter according to claim 7, further comprising one or more optical couplers, an optical detector, and a maximum control circuit,

wherein part of the optical signal output from said optical filter is branched by the optical coupler, converted to an electrical signal by the optical detector, and entered into the maximum control circuit, which controls the frequency of the optical signal output from a signal light source or the center frequency of said optical filter so as to maximize the intensity of said electrical signal.

9. An optical single sideband transmitter, comprising a plurality of laser sources differing in wavelength, a first optical modulator, one or more second optical modulators, and one or more optical filters to transmit an optical signal whose intensity waveform is subjected to RZ (return-to-zero) modulation by a digital information signal,

wherein laser light outputs, which are different in wavelength and generated from said plurality of laser sources, are subjected to NRZ (nonreturn-to-zero) modulation with an information signal by respective second optical modulators appropriate for individual wavelengths, and filtered by an said optical filter to obtain one of two sidebands, then the resulting optical signals are multiplexed and entered into the first optical modulator, and simultaneously converted to a single sideband modulated optical RZ signal for output purposes.

10. The optical single sideband transmitter according to claim 9, wherein a modulator integrated light source, which is obtained by integrating a semiconductor laser and an electro-absorption semiconductor optical modulator or Mach-Zehnder semiconductor optical modulator, is used to perform the roles of said laser source and second optical modulator;

or wherein the role of said second optical modulator is performed by directly converting the current of said laser source with an NRZ electrical signal.

11. The optical single sideband transmitter according to claim 10, further comprising one or more optical couplers, an optical detector, and a maximum control circuit,

wherein part of the optical signal output from said optical filter is branched by the optical coupler, converted to an electrical signal by the optical detector, and entered into the maximum control circuit, which controls the frequency of the optical signal output from a signal light source or the center frequency of said optical filter so as to maximize the intensity of said electrical signal.